Principles of Fracture Treatment

CHAPTER 76  Principles of Fracture Treatment

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CHAPTER

Principles of Fracture Treatment Jörg A. Auer

Fractures are diagnosed in practically every bone of the horse and encountered at all ages. Fractures vary in clinical presentation and significance, ranging from exercise-induced fractures causing only relatively minor lameness, such as chip fractures of the carpus, to fractures causing a non–weight-bearing lameness, such as a transverse failure of the third metacarpal bone (MCIII). The management of intra-articular chip fractures and osteochondrosis lesions is discussed in Chapters 80 and 88, respectively. In this chapter, the nonsurgical and surgical principles of major fracture treatment are presented. Fracture treatment in the horse follows the same basic guidelines developed for humans1 and small animals.2,3 Many techniques can be derived from them, but some principles in the treatment of equine long bone fractures are unique.4,5 These differences are discussed in detail, with emphasis given to surgical fracture treatment, including external coaptation and internal fixation.

NONSURGICAL MANAGEMENT Some fractures heal sufficiently with nonsurgical management to allow the animal to return to an athletic career. Nonsurgical management techniques include stall rest and external coaptation.

Stall Rest Frequently, horses are admitted in a "fracture-lame" state without a visible or palpable fracture. According to the anamnesis, these patients can be found on pasture with the non– weight-bearing lameness, or they do not return to the stable in the evening with the other horses. The physical examination may reveal a small wound over a vestigial metacarpal or metatarsal bone, the radius, or the tibia. In the case of splint bone fracture, radiography may reveal a fracture or even multiple fractures. In selected cases, these fractures are amenable to nonsurgical management with a bandage alone. Additional information on the treatment of these fractures is found in Chapter 93.

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Other fractures amenable to management by stall rest include fractures of the deltoid tubercle, nonarticular patellar fractures, and fractures of the scapular spine. However, for the vast majority of fractures, nonsurgical management is not the treatment of choice and should not be advocated.

External Coaptation For a detailed description of the indications and applications of external coaptation devices, such as fiberglass casts and splints, please review Chapter 17. In this chapter, external coaptation is discussed as it pertains to fracture treatment. Splints The indications for limb splints as a sole means of fracture treatment are limited. More commonly, they are used as a form of emergency fixation (see Chapter 73). This type of external coaptation may be employed as treatment modality in fissure fractures of the diaphysis of the radius and tibia or as adjunct treatment to internal fixation of a fracture, either during the immediate postoperative period or as an intermediary step after removal of a cast. In acute fissure fractures of the radius and tibia, radiography may not show a fracture. Affected animals are usually reluctant to bear any weight on the limb. Initial management should include not only applying a splint bandage but also preventing the animal from lying down. It is frequently during the process of lying down or getting up that these fissure fractures evolve into complete fractures. Tying the horse with a short rope and providing a filled hay net to allow them to eat is an effective management method. Nonsteroidal anti-inflammatory agents are indicated to provide some comfort. However, the dosages should not be so high that they abolish the lameness completely. A complementary or independent management option involves applying a sling or rescue net (Figure 76-1).5 Usually the animals tolerate these slings very well. They allow the patient to rest their intact limbs for some time by lowering their abdomen into the sling. It is important that the sling be installed

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SECTION XII  MUSCULOSKELETAL SYSTEM Splint bandages should be changed every 3 to 4 days, and in hot and humid climates they should be changed more frequently. Caution should be used during the process because bearing weight on the limb without the splint or bandage could have deleterious effects on fracture healing. Caution: Fissure fractures penetrating a joint should be treated surgically. Casts

Figure 76-1.  An adult horse suffering from a fissure fracture of the tibia, supported by a rescue net. The net, which is applied relatively snugly to allow the horse to apply some weight to it, is tolerated very well.

Figure 76-2.  Craniocaudal (left) and lateromedial (right) radiographic views of a fissure fracture in the proximal metaphysis of the tibia of an adult horse.

External coaptation using fiberglass cast material as the primary treatment technique of a fracture may initially be considered as a conservative, less expensive type of treatment. However, soft tissue problems may require frequent cast changes, usually carried out under general anesthesia, which frequently increase the costs until they exceed that of a state-of-the-art internal fixation performed upon admission. Additionally, the advantages of early return to function, achieved through internal fixation, are lost. Cast materials selected for fracture treatment should be fiberglass because it allows the skin and the limb underneath the cast to breathe. Also, fiberglass weighs less, so the animal is more comfortable. Casts should be palpated daily and evaluated for hot areas. Any odor from the cast should be investigated, and weightbearing on the limb should be evaluated. Sudden changes in weight-bearing patterns, edema above the proximal cast end, and a foul odor or wet spots on the cast are signals of skin damage and possible necrosis underneath the cast. The same is true when hot areas develop in the fetlock region or at the dorsal aspect of the cast in the region of the proximal MCIII or third metatarsus (MTIII). All of these signs signal the need for cast removal. The first cast should be changed after 3 to 4 days, because during this time initial swelling has subsided, resulting in a loose cast that is ineffective in stabilizing the initial fracture and may even cause some additional damage to the soft tissues. Casts applied to foals should be changed at 10- to 14-day intervals and eliminated as soon as possible. In adult animals, longer intervals are tolerated. If the condition of the skin beneath the cast and the weight-bearing patterns permit, up to 5 or 6 weeks may be allowed before the cast is changed. Longer intervals reduce costs and, in most cases, are followed by better results. Some horses have thinner and more sensitive skin, which is more likely to be traumatized by a cast; in these cases, shorter intervals between cast changes may be required. Complications

such that the body has to be lowered only a few centimeters to allow it to rest in the sling. If the body has to be lowered too much, complete fractures of the bone may still occur. The Anderson Sling developed in California has also proved to work very well as a recovery system and for prolonged support of horses during their postfracture fixation period.6 If initially no fracture can be found on radiographs, additional images should be taken a few days later. Usually, at this point, fractures may be seen radiographically (Figure 76-2). Depending on the configuration of the fissure fracture and the width of the fracture gap, the management may be modified. When the fissure lines are small with a barely visible gap, no bandages are needed, but the animal is maintained in the sling for 2 to 3 weeks. If the fracture line is long and associated with a significant gap, the splint bandage should be maintained.

Nonsurgical management of fractures is associated with various complications (see Chapter 17). When internal fixation is not applied, the fracture fragments are not stable. The resulting callus formation often impinges on soft tissue structures or tendons and may prevent future athletic use. However, bone remodeling after fracture healing may reduce such a callus, eliminating impingement on the soft tissue structures. Skin trauma from casts and splints can be severe enough to endanger the outcome of fracture healing. It is important that the skin remain dry and healthy. Development of cast sores should be prevented whenever possible. Occasionally skin damage occurs at the time of injury or during the transport to the referral clinic, the latter as a result of inappropriate first aid. In thoses cases, special care has to be taken during cast application. If pressure sores do develop, casts must be changed



CHAPTER 76  Principles of Fracture Treatment

frequently and padded so that the pressure on the skin is redistributed. If infection develops underneath the cast, swelling causes increased pressure within the tissues. Because the skin cannot expand beyond the inner limitations of the cast, a compartment syndrome develops, resulting rapidly in tissue necrosis. Additionally, drainage from the limb accumulates within the cast, and the skin is damaged by enzymes. Hosing down the fiberglass cast with copious amounts of water on a daily basis prolongs its usefulness and postpones a necessary change, but an infected limb should be maintained under a cast only if no alternative exists. During the time that the limb is maintained in the cast, the joints are unable to move and the articular cartilage is poorly nourished. This results in loss of proteoglycans and subsequent degeneration of the cartilage. Additionally, the soft tissue structures surrounding the joint are not flexed and stretched, which causes them to become weak and inelastic. When prolonged external coaptation is used, these pathologic changes are exacerbated and are referred to as cast disease. If a foot is maintained under a cast while a limb fracture is allowed to heal, it is prevented from expanding during weightbearing and the structures underneath the hoof wall constrict. After the cast is removed and the limb is loaded once again, the foot expands, which causes pain for several days. Flexing of the joints after a prolonged fixation in one position causes pain and an initial unwillingness to bear weight. This, however, is overcome after a few days in most cases and can be facilitated with anti-inflammatory drugs.

SURGICAL MANAGEMENT Equine bone reacts to trauma and fracture with active new bone formation and subsequent remodeling. Many osteons are mobilized to facilitate remodeling of the cortex. However, fracture healing in the horse occurs at a slower pace than in most other animals, especially ruminants, small animals, and humans.7 Therefore any adjunct treatment that benefits bone healing is advantageous.

External Fixation External fixation using intraosseous or transosseous pins and clamps is common in humans8 and small animals but less so in the horse.8 This type of fixation is employed frequently as emergency treatment in open fractures or in severely comminuted fractures when anatomic reduction, reconstruction, and internal fixation of the fracture are not possible. External fixation techniques can be applied using three types of constructs: transfixation-pin casts, external fixators, or external skeletal fixation devices. Transfixation-Pin Casting Transfixation-pin casting was popularized around 1990.9 This efficient and relatively easily applied treatment method is indicated for comminuted fractures of the phalanges, the distal MCIII/MTIII, and breakdown injuries of the metacarpophalangeal joint.10 A retrospective study involving more than 50 horses and ponies treated by means of transfixation-pin casting showed that comminuted fractures had better results (86% healed) than simple fractures (23% healed).9 It is assumed that

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the micromotion between the fragments after fixation was distributed between more fragments in the comminuted fracture and therefore was decreased compared with simple transverse fracture, where distribution was not possible, leading to reduced strains and stresses exerted upon the bone fragments and the granulation tissue bridging them initially. Under aseptic conditions, two or three cross pins that are 4 to 6 mm in diameter are introduced in the metaphyseal region of the bone. The use of positive-profile pins (IMEX) is preferred.10 A 30-degree divergence of the pins in the frontal plane results in a stronger fixation and lower risk for postoperative fracture.11,12 The pins should be separated by 2 to 4 cm. A stab incision is made down to the bone and, using tissue protection, the predetermined-size hole is prepared. An effective method of heat control is to initially drill a smaller hole, followed by stepwise enlargement through larger drill bits. Simultaneous flushing with approximately 500 mL of cold sterile saline solution per drill hole, applied with a bulb syringe, is effective in reducing friction and thereby heat production. The saline lubricates the drill bit because of its special construction. Production of heat is associated with bone necrosis around the pin and its resultant loosening. A pin with a diameter that is 0.1 mm larger than the prepared hole (radial preload of 0.1 mm) provides the best pin holding strength with the least weakening of the bone surrounding the implant.1 If positive-threaded pins are used, appropriate threads have to be cut in the predrilled hole before pin insertion. Newer pin generations are equipped with a self-tapping device at the beginning of the threaded part. Care has to be taken to engage the threaded pin portion with both cortices. Protruding portions of the pins are cut off at a length of 3 to 5 cm from the limb, which allows their incorporation into the cast. After applying a double layer of stockinet, followed by a double layer of resin-impregnated foam padding (3M Corporation), a 5-mm layer of fiberglass cast is applied as described in Chapter 17. The ends of the pins can be covered by hoof acrylic and incorporated in an additional layer of fiberglass cast tape (Figure 76-3). An alternative approach involves applying dowels over the pin ends, fastened with set screws on the pins, just adjacent to the layer of casting tape. These dowels are subsequently covered by an additional solid layer of cast material. These two methods prevent migration of the pins after loosening, because they are fixed within the cast. It was shown in an in vitro study that the fixation of the pin within the cast plays a minor role.13 The fiberglass cast material appears to be the major determinant of axial stability. However, because it was an in vitro study, the long-term effect of such a fixation could not be evaluated. The originally proclaimed beneficial effect of incorporating the horizontal pins in a U-bar fastened around the distal limb9 has been abandoned because it could not be shown that it was more effective in supporting the loads.14 A combination of transfixation-pin casting and strategic lag screw placement across major fracture fragments to ensure anatomic reduction of intra-articular fractures may speed up fracture healing. Centrally threaded, positive-profile transfixation pins have greater resistance to axial extraction in the diaphysis than in the metaphysis of equine MCIII bone in vitro.15 Again, this observation resulted from an in vitro study and may lead to the conclusion that it is advantageous to introduce the pins into the diaphyseal region of the long bone. However, my experience is that diaphyseal pins lead to more complications, which include

SECTION XII  MUSCULOSKELETAL SYSTEM

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ring sequestrum formation and subsequent pathologic fractures (see later under "Complications"). A sleeve pin cast representing a modification of the external transfixation device16 has been developed. It consists of smooth pins with negative threads on either side over which tapered sleeves are applied and fixed with nuts aginst the bone; the pins were then incorporated in fiberglass cast material and tested in a servo-hydraulic material testing machine (MTS Bionix 858) (Figure 76-4).17 The results showed that the mean load to failure

b c d d'

e

for the tapered-sleeve transfixation-pin cast was significantly greater than that for a standard transfixation pin cast. This device has not been tested clinically yet. Recently a novel pin-sleeve cast (PSC) transfixation system was introduced.18 The system consists of a 45-mm long hollow cylinder of 8.2 mm outer diameter and 1 mm wall thickness that is implanted into the bone proximal and distal to the fracture (Figure 76-5, A). The inside surface of the cylinder contains two circular supports measuring 1.0 mm in width and 0.5 mm in thickness; they are mounted 5 mm from either end. A 5-mm diameter pin of 120 mm length with a smooth central portion is introduced through the cylinder, contacting it only at the circular supports (Figure 76-5, B). Each end of the pin contains metric screw threads of 15 mm length. The pin is incorporated

f

a

g

Figure 76-3.  Transfixation cast for the treatment of a comminuted fracture of the proximal phalanx, partially managed by strategically placed lag screws. a, Parallel inserted Steinmann pins (they differ in the orientation within the frontal plane); b, stockinet; c, custom foam; d, initial, and d′, second layer of fiberglass cast; e, hoof acrylic covering the Steinmann pin ends on the left side; f, custom-made dowels with set screw stabilizing the ends of the Steinmann pins on the right side; g, cortex screws placed in lag fashion to reduce fracture fragments.

A

B

Figure 76-4.  Schematic representation of a pin-sleeve cast. The sleeves (the same as shown in Figure 76-8) are mounted over a smooth pin containing negative threads at both ends. With the help of nuts, the sleeves are fixed to the bone and subsequently incorporated in a standard transfixation cast.

C

Figure 76-5.  Schematic representation of a novel pin-sleeve cast (PSC) transfixation system. A, Representation of the sleeve inserted into the bone and the central smooth pin tightened to the ring fixator (the pin is under tension). B, Close-up of the contact support of the pin within the intraosseous sleeve. C, During weight-bearing the pin is slightly bent without contacting the sleeve and surrounding bone, leading to minimal strain at the bone–sleeve interface.



CHAPTER 76  Principles of Fracture Treatment

(before introduction across the bone) into a 15-mm-wide hollow ring of 70- or 90-mm inside diameter through 5-mm holes. Using metric nuts, the pin is tightened to the outside of the circular ring around the limb, and in doing so an axial preload can be applied similar to an Ilizarov ring fixator. The rings are subsequently incorporated into a fiberglass cast. In vitro testing using calf metacarpal bone and an MTS testing machine (see earlier) and a 25 kN load cell (Type U-10M, 25kN, 250Nm) revealed that application of a pin preload significantly reduced strain measurement compared to the transfixation pin cast, which may prevent or retard pin loosening.19 Because the pin only has contact with the two support rings, the pin can bend somewhat without exerting additional strain on the bone (Figure 76-5, C). Given that pin loosening is the result of cyclic strain at the pin–bone interface over time (at the periosteal side at the proximal aspect and at the endosteal side at the distal aspect), the significant strain reduction at the cylinder–bone interface should be an improvement.19 Clinical tests will tell if the theoretical advantages can be transformed into empirical advantages. Advantages of transfixation-pin casting include no or only minimal load on the fracture site and minimal distraction and movement between the fragments. Also, the tissues are spared additional trauma and further disruption of the blood supply. All the disadvantages of external coaptation, such as the development of cast disease, osteoporosis, contracted feet, and tendon laxity, apply to this type of treatment. Pin tract infection with ring sequestrum formation can occur and is preceded by a sudden onset of lameness. This usually occurs in heavy horses (greater than 500 kg of body weight) after approximately 2 weeks and warrants immediate removal of the proximal transfixation pin. Another pin can be inserted at a different location to prolong unloading of the fracture, or, alternatively, the limb can be placed in a new cast without the most proximal pin. A new pin can be expected to form a similar sequestrum within the next 2 weeks, and the procedure can be repeated. If three pins are inserted initially, transfixation may be maintained for 6 to 8 weeks before simple casting is applied. At that time, a full-limb cast is applied to prevent rotation of the MCIII/MTIII. (Rotation can occur in a half-limb cast and may lead to a fracture across a pin tract.) External Skeletal Fixator Application of an external skeletal fixator allows immediate, although subnormal, weight-bearing on the limb. The Steinmann pins and Schanz screws (Steinmann pins with a threaded end) used with this fixation device cause minimal additional trauma to the injured soft tissue. Because in most cases no external coaptation is used, there is easy access to an open wound, which facilitates débridement and management. Application of an external fixator also allows additional correction and fracture alignment after the initial operation, whereas a fracture treated with internal fixation can be adjusted only through an additional surgery.20 However, this technique has not been very successful in equine patients. An external fixator uses transversely inserted (cross) Steinmann pins or Schanz screws proximal and distal to the fracture site. These pins are firmly connected to external rods through special clamps, which may be applied in many configurations.8 An in vitro study showed that the best results are achieved with an elaborate three-dimensional design, including three

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Steinmann pins proximal and distal to the osteotomy, inserted at different angles in the frontal plane and obliquely from proximal to distal, and connected to four external rods.20 Such a configuration is expensive and very heavy and therefore not practical, especially for horses. The Seldrill Schanz screw improves the bone-holding properties of Schanz screws. The design features a pin with a diameter that is 0.1 mm greater than the hole drilled for it (larger radial preloads result in microfractures and deformation of the bone surrounding the pin, with subsequent loosening).1 The Seldrill Schanz screw is manufactured of pure titanium and stainless steel and has a self-drilling, self-tapping tip.21 This implant is inserted through a stab incision without predrilling the bone or pretapping the hole, even in hard equine bone. The Seldrill Schanz screw contains a relatively thick core and thin threads and includes a portion with a built-in radial preload of 0.1 mm, immediately adjacent to the self-drilling, self-tapping tip. Because the sharp tip should not exit the opposite cortex, this implant is to be used in half splints (type I: a construct that contains one or two sidebars on one side of the bone), where the external tube or rod is located only on one side of the bone. Additionally, this device can be used in three-dimensional (type III: a construct that contains sidebars at three sides of the bone that are interconnected externally) configurations, where a half splint is connected to a full, bilateral splint (type II: a construct that contains a sidebar medially and laterally), which uses nonthreaded pins. A modification of an external fixator, the Pinless External Fixator (Synthes Inc.), is available for selected fractures in large animals.22 This device is manufactured in three sizes and configurations of clamps, which are applied over a bone without completely penetrating the cortex (Figure 76-6). The clamps are fastened through a connection rod, which is attached to an external fixator tube or carbon rod. This device is not rigid enough to support weight bearing of a large animal with a fractured limb, but it is effective in stabilizing mandible fractures in cattle and horses (see Chapter 102) and fractures of the tail in the horse. In vivo studies showed that the clamping force is maintained over several weeks while inducing only minor bony changes where the clamps contact the bone.23 Animals tolerate the device well. The advantage of this type of external fixator is the minimal damage to bone and tooth roots. The use of circular external skeletal fixators (CESFs) has become routine practice in small animal surgery for the management of developmental, traumatic, and degenerative orthopedic problems.24,25 The advantages of this system include immediate weight bearing, excellent mechanical properties, ability to stabilize short segments, ability to adjust the frames after their application, and preservation of joint mobility.26 The use of CESFs in large animals is mainly limited to cattle.24,27 Sporadic case reports are published where a CESF was applied to a long bone fracture in a horse (Figure 76-7).24 It is not very likely that this fracture fixation system will become very popular in horses in the future. External Skeletal Fixation Device The external skeletal fixation device (ESFD) was developed for horses with severely comminuted fractures of the phalanges, fractures of the distal MCIII/MTIII, and breakdown injuries of the metacarpophalangeal joint. The ESFD uses two or three transfixation pins in the intact bone proximal to the fracture,

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SECTION XII  MUSCULOSKELETAL SYSTEM

E

D

C

B

A

Figure 76-6.  One symmetric (A) and three asymmetric (B) titanium pinless fixator clamps are shown with connecting rods (C), clamps (D), and longitudinal rod (E). This configuration is used to treat mandibular fractures.

r d. 75

d. 0

A

B

C

Figure 76-7.  Application of a circular external skeletal fixator to distal MTIII fracture in an Arabian foal. A, The foal wearing the circular fixator. B, Radiographic view of the device in place immediately postoperatively. C, The healed fracture 75 days later. (Courtesy A. Ferretti, Legnano, Italy.)

and sidebars and a base plate. Weight-bearing forces are transmitted via the pins and sidebars around the fracture to the ground, allowing the animal to immediately bear full weight without loading the fracture.16,28-30 The original report described the device and 15 cases, only four of which survived longterm.28 Through a meticulous study of the design, an effective

ESFD transfixation system has been developed and is commercially available.16 Early complications included fractures through the pin tract while the device was worn or during recovery after implant removal. These complications were almost completely abolished with the latest generation ESFD.30 Removal of the device on the sedated, standing animal eliminates some



CHAPTER 76  Principles of Fracture Treatment

problems encountered after device removal, but the risk of fracture while wearing the device continues.30 Up to 90% of the stresses generated at the bone–pin interface contribute directly to pin bending, resulting in uneven stress distribution, with peak stresses concentrated at the outer bone cortex.31 The tapered-sleeve design was developed to reduce transcortical pin bending with weight-bearing. Large-diameter, tapered sleeves are applied over the transfixation pins (biaxially loaded in tension and shear) and are incorporated in a stronger, lighter frame (Figure 76-8).32 In vitro tests applying cyclic loading to this ESFD showed that significant increases in stiffness, reduced bending, and increases in load-to-failure-of-bone could been achieved with the tapered-sleeve design.32 Since bone failure occurs at a finite strain level, it appeared that the larger loads to failure indicated lower strains in the bone at the working stress level. To facilitate adjustments in tranfixation pin placement and their incorporation into the sidebar, a modular sidebar was developed consisting of several elements that could be assembled to the length needed for a specific fixation. The four elements fit on top of each other and are not separated by more than 10 mm. The modular components are connected using 4-mm-thick rubber tubing and sealed with duct tape. Once in place, the hollow connecting bar construct is filled with polymer that also enters the connecting elements.18 The ESFD was compared in an in vitro study to a transfixationpin cast and a modular sidebar construct (see later).33 The solid ESFD has a greater stiffness, higher yield and failure load, and a lower yield and failure displacement than the transfixationpin cast and the pin-sleeve cast. Mean cycles to failure for the transfixation-pin cast was 2996 ± 657 at a load of 16,000 N, and that for the solid ESF was 6560 ± 90 cycles at a load of 25,000 N. These results are encouraging and the new fixation

Figure 76-8.  Graphic representation of the external skeletal fixation device design.32 Two tapered sleeves are mounted and tightened on transosseous pins, which are then incorporated into the U-shaped apparatus, providing additional stability.

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device may prove to be a great alternative to transfixation-pin fixation. Aftercare Application of an external fixator or ESFD allows easy wound management of open fractures. Débridement should be performed under aseptic conditions, and every measure should be taken to prevent additional contamination. Broad-spectrum antibiotic coverage is indicated in any horse treated with external fixation during the entire time the device is in place. The skin around the cross pins should be cleaned daily with alcoholsoaked swabs and dried before reapplying the bandage. Internal fixation of the fracture may be considered once the infection has subsided and healthy granulation tissue has formed. At that time, the external fixation device is removed. Any sudden changes in weight-bearing patterns are indications for close scrutiny of the fracture and the fixation device. Radiographs should be taken at the onset of any complication and repeated in routine fashion at 2- to 4-week intervals without complications. Once fracture healing has occurred, the device is removed. The abrupt change in stability caused by removing the external fixator can be minimized through partial destabilization of the device. This is best achieved by moving the vertical connecting bars farther away from the limb. An alternative approach involves the strategic removal of one or two pins at 2- to 3-week intervals. After a few weeks in this configuration, the device is removed. Complications Loosening of the pins, infected pin tracts, and fracture through a pin hole are the main complications of external fixation in horses.14 Loosening of cross pins is the most frequent complication of external fixation.8,34 Weight-bearing on the affected limb causes osteolysis around the pins, followed by infection of the pin tract and subsequent loosening.34 Once a pin loosens, pain develops. A loose pin should be removed immediately, because it no longer serves a useful function, and infection around it will not subside as long as the pin remains in situ. Curetting and flushing of the tract facilitates cessation of draining within a few days. If drainage persists, the skin should be reopened, followed by an additional curettage of the pin tract. Removal of a loose pin destabilizes the fixation. Depending on the degree of fracture healing, it is important that pins be removed one at a time. Premature removal of the implants results in total instability. Removal of a loose transfixation pin is performed on the standing, sedated horse. If external coaptation was used, cast material immediately adjacent to the pin is removed and the pin is pulled out. The tract is flushed and the hole is filled with a surgical sponge soaked in an antiseptic solution and fixed in place with tape. If the pin is fixed within the cast through a dowel or a hoof acrylic pad (see Figure 76-3), a complete cast change under general anesthesia is necessary. Osteomyelitis may develop as an extension of pin tract infection or at the open ends of the fracture. Treatment of orthopedic infections is discussed in Chapter 85. When osteomyelitis is rampant and uncontrollable, euthanasia may be the only alternative. Soft tissue swelling occurs frequently in animals wearing an external fixator. This is a normal reaction to the implants, local

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SECTION XII  MUSCULOSKELETAL SYSTEM

insult, trauma, and controlled infection. Therefore adequate distance between the skin and the vertical bars and clamps has to be allowed to accommodate the swelling. This will prevent skin necrosis near contact areas of the clamps. Pathologic fracture is a common problem encountered in horses treated with the external skeletal fixation device or external fixator.30,35,36 Frequently, a doughnut-shaped cylinder of bone is walled off around the pin (Figure 76-9). This creates a relatively large defect in the bone and significantly reduces its strength. The bone within the fixator becomes weakened by osteoporosis (disuse atrophy depending on the duration of fixation), and bearing weight on the limb can result in failure through one of the pin tracts (see Figure 76-9). If the animal is not euthanized at once after failure, the fracture must be treated immediately by either internal fixation or a full-limb cast. Changing the arrangement of the Steinmann pins as well as the threads may limit the recurrence of this complication.35

Internal Fixation At the turn of the twentieth century, compression in rigid fixation was recognized as an important component of rapid fracture healing. Rigid internal fixation became an important step in reaching the goal of early return to full function for the fractured limb and the patient.37,38 The devastating effects of prolonged external coaptation—cast disease—were prevented when this goal was attained. Early return to function allows movement of the joints, associated nourishment of the articular cartilage, and prevention of proteoglycan loss. Additionally, disuse osteoporosis is prevented, and the soft tissues surrounding the fractured bone are maintained in physiologic condition.

B

A

Early return to function is achieved through anatomic reduction of the fracture and stable internal fixation.1 Internal fixation is achieved through opening of the skin. This may occur through a stab incision, as with intramedullary pinning, transcutaneous interlocking, and minimally invasive plating techniques, or through opening the skin over a greater distance, followed by separation of the soft tissues surrounding the fractured bones when bone plates are applied. Principles Fracture fragments fixed to each other under compression heal without callus formation.7 Precise anatomic reduction of the fracture is of paramount importance for this type of bone healing (primary union). It is also critical when the articular surface is involved, because if the reconstruction is not nearly perfect, osteoarthritis will develop (Figure 76-10). In the last 50 years, tremendous progress has been made in the art of equine fracture treatment. The greatest influence on this achievement can be ascribed to the AO Foundation. Founded in 1958 by four Swiss surgeons, the AO (Arbeitsgesellschaft für Osteosynthesefragen) quickly developed into a worldwide organization.1 In 1984, the AO Foundation was established, to which all the rights for royalty income were bestowed. Recently the three manufacturers that supplied surgeons with a large variety of sophisticated, high-quality implants (Mathys AG, Bettlach, Switzerland; Stratec Medical, Oberdorf, Switzerland [formerly Institut Strauman, Waldenburg]; and Synthes [USA], Paoli, PA) have merged to form a single worldwide company, Synthes, with its headquarters in Solothurn, Switzerland. For every implant sold, Synthes pays a certain amount of royalties to the AO Foundation to support teaching and the development of new instruments—a genuine approach to the improvement of fracture fixation. Techniques developed for human patients were quickly adopted by veterinarians, and many are applied in daily practice with good success.1-4 After the merger of the three companies, all patents were sold to Synthes, and with the interest from the investments, most of the activities of the AO Foundation can be offset. Additional income is generated through other sources. In 2008, the veterinary specialty within the AO Foundation, AOVET, was accepted

B

A

C

C

Figure 76-10.  A, A displaced articular fracture of the lateral condyle of Figure 76-9.  Three-dimensional reconstruction of a computed tomographic study of the third metacarpal showing ring sequestra (A) as a result of transfixation pinning, and a pathologic fracture (B) through the proximal pin tract or ring sequestrum. Periosteal new bone formation (C) surrounds the ring sequestra.

the distal third metacarpal. B, Inadequate reduction was achieved after interfragmentary compression via two cortex screws applied in lag technique. The articular surface is not congruent, which would lead to osteoarthritis. C, Adequate anatomic reduction was achieved before screw fixation, reestablishing the articular surface and normal bone-tobone contact.



CHAPTER 76  Principles of Fracture Treatment

as a fully funded specialty next to to AOTRAUMA, AOSPINE, and AOCMF (cranio-maxillo-facial). The goal of AOVET is to establish itself as the world leader in the treatment of musculoskeletal disorders of animals, including fractures. The implants, their function, and their application as discussed in this chapter are mainly those developed by the AO group. Included are screws, plates, pins, wires, and specially designed plates and nails. Only the instruments and implants used in equine fracture treatment are discussed. Approach to and Manipulation of Bone Before surgically approaching the bone, a careful diagnostic imaging study should be conducted that includes multiple radiographic views from a variety of angles. Ultrasonography, scintigraphy, and computed tomographic scans may aid standard radiography in the selection of the locations where implants are applied and the direction of their insertion. Potential interference of interfragmentary screws and plates with soft tissue structures must be considered. In articular fractures, reconstruction of the joint should be the deciding factor in the decision of how to approach the fracture and apply the implants. For some fractures, surgery should not be attempted. For example, a comminuted fracture of the radius with substantial defects in the caudal cortex has no chance to heal because continuous cycling of the implants eventually leads to implant failure. It is prudent to conduct a detailed discussion with the owners of equine patients about the chances for a successful surgical outcome, the potential complications, and the costs of the particular fracture repair before surgery. Availability of all implants and instruments needed at the time of surgery is an absolute prerequisite for a successful result. The approach to the bone should be carried out rapidly and carefully and with respect for Halsted’s principles of good surgical technique. Special attention should be paid to the blood supply, avoiding severance of major blood vessels. The periosteum should be maintained with the underlying bone whenever possible. Periosteum is stripped off the bone only immediately under selected implants, such as dynamic compression plates (DCPs). More recently developed implants, such as the limited contact dynamic compression plate (LC-DCP), the point contact fixator (PC-Fix), and the locking compression plate (LCP), are applied over the periosteum (see later). Massive dissection should be avoided, because it facilitates the accumulation of blood and serum. Planning the approach relative to the application of selected implants is of great importance for a successful outcome of the surgery. For example, the approach to the MCIII is made through longitudinal splitting of the common or lateral digital extensor tendon, which facilitates secure closure of the soft tissues and skin over the implants after the fracture is repaired.39,40 In human and small animal osteosynthesis, biological fracture fixation has become very popular.41 This technique abandons the dogma of anatomic reconstruction and accepts that proper axial and rotational alignment of the bone, despite incomplete reconstruction, followed by fixation of the fracture with strategically placed implants is preferred. Longer plates are used, providing better leverage conditions. Screws are inserted through the biomechanically most important holes, but not all holes in a plate are filled with screws. If possible, the plate is prebent (to conform to the shape of the contralateral intact bone), slid along the fractured bone through a small incision, and fixed with screws implanted through stab incisions. Such

1055

minimal fixation cannot be used successfully in horses. However, the principle of applying the implant by a minimally invasive technique is applicable to the horse, even if the implant has to be modified to meet the demands placed by the horse’s size. Instruments Basic instruments used for fracture treatment include a variablespeed air or electric drill with forward and reverse gears. To facilitate mechanical preparation of the threads in the bone, switching between forward and reverse should be easy. Drill bits and guides of different types and sizes are needed (Table 76-1). Drill guides allow application of concentric pressure, and they stabilize the drill bit, which prevents breakage. The bit is also less likely to slip off the bone surface, especially when it is obliquely applied. The guides protect tissues around the hole from frictional trauma generated by the drill bit and the tap. It has been shown that drilling at maximal speed (about 90 psi of air pressure) results in less heat production than drilling at low speeds.3 The drill bits should be sharp and should be exchanged frequently, because dull drill bits create heat in the hard and dense equine bone. This is especially true in adult animals. Continuous application of saline solution throughout the drilling procedure is important to reduce heat production.3 The drill bits are designed to allow penetration of the fluid along their outside perimeter, which facilitates lubrication and reduction of friction. However, not enough water can be flushed into the drill hole to effectively cool the drill bit. Frequent cleansing of the drill bit to remove the swath material is the single most important factor in reducing heat production. During drilling, axial pressure is applied to the drill bit without bending it. Bending causes the drill bit to become dull too rapidly because of interference with the drill guide. More importantly, the hole will be of a larger diameter than intended. Adequate but not excessive pressure should be applied. The instruments used for screw insertion are discussed later, in the paragraphs about the lag technique. Other instruments such as various drill guides, special reduction forceps, bone clamps, rongeurs, curets, osteotomes, and a mallet (for more information, see Chapter 11) aid in bone handling as well as in maintaining compression during fracture reduction and the insertion of the desired implants.27 The newly developed Equine Large Fragment Set consists of several trays: the instrument set containing the different instruments (Figure 76-11), the plate set containing the different plates selected by the veterinarian, a screw set containing cortex screws, and one screw set for locking head screws. Implants SCREWS Various types of screws serving different functions were developed by the AO together with Synthes (see Table 76-1). The parts of a screw include the head, shaft, core, and thread. Its attributes include pitch, shaft length, thread length, and total screw length. Screw types Cortex screws have a 0.7-mm thread width, and a thread length that depends on the screw length (see Table 76-1). A cortex screw does not contain a shaft portion and is referred to as a fully threaded screw. These types of screws are the most frequently

1.75 8 — Fully threaded

2.5 3.5

—

Cortical

1.25 6

—

Fully threaded

Thread hole Ø Tap Ø Screw shape

Cannulation guide pin

Type thread

Pitch Screw head diameter Special head design Thread length

— 3.0 Yes No Large (3.5 mm) hex

— 2.4 Yes No

Small (2.5 mm) hex

Shaft diameter Core diameter Self-tapping Self-drilling

Drive

Cortical

—

3.2 4.5

4.5 4.5

3.5 3.5

Screw Ø Glide hole Ø

4.5 mm Cortex

3.5 mm Cortex

Screw Name

Large (3.5 mm) hex

4.5 3.0 Yes No

Variable

—

1.75 8

Cortical

—

3.2 4.5

4.5 4.5

4.5 mm Shaft

Large (3.5 mm) hex

3

of length/ Fully threaded 3.1 2.7 Yes Yes 1

—

1.75 6.5

150 mm long /1.6 mm Ø Cancellous

4.5 4.5 None 3.2 4.5

4.5 mm Cannulated

TABLE 76-1.  Veterinary Large Animal Screw, Drill Bits, and Tap Chart

— 3.4 Yes Available (Europe only) T25 stardrive

Conical threaded Fully threaded

Conical threaded Fully threaded — 2.9 Yes Available (Europe only) T15 stardrive

Cortical narrow 1 6.6

—

—

Cortical narrow 0.8 5

3.2 None

4 None

4.0 mm Locking

2.8 None

3.5 None

3.5 mm Locking

— 4.4 Yes Available (Europe only) T25 stardrive

Conical threaded Fully threaded

Cortical narrow 1 6.6

—

4.3 None

5 None

5.0 mm Locking

Large (3.5 mm) hex

— 3.8 No No

Fully threaded

—

2 8

Cortical

—

4 5.5

5.5 5.5

5.5 mm Cortex

Large (3.5 mm) hex

4.5 3.0 No No

16 mm/32 mm/ Fully threaded

—

2.75 8

Cancellous

—

3.2 6.5

6.5 4.5

6.5 mm Cancellous

Large (4.0 mm) hex

4.8 4.5 Yes Yes

16 mm/32 mm/ Fully threaded

—

2.75 8.2

Cancellous

300 mm long /2.8 mm Ø

5 7.3 Optional

7.3 7.3

7.3 mm Cannulated

1056 SECTION XII  MUSCULOSKELETAL SYSTEM



CHAPTER 76  Principles of Fracture Treatment

1057

Figure 76-11.  Synthes Large Fragment Set: Instruments. The set contains all the drill bits, taps, drill guides, screwdrivers (hexagonal- and stardrive), T-handle, countersink, depth gauge, push-pull device, tension device, socket wrench, and torque limiting device that are needed to insert screws (in lag fashion) into bone as well as through, DCPs, LC-DCPs and LCPs. The instruments are arranged in 3 trays that fit on top of each other into main tray. Pictographs facilitate correct positioning of each instrument into the trays. (Courtesy Synthes Vet, West Chester, PA.)

Figure 76-12.  Schematic drawing of 7.3-mm cannulated screw with

applied in equine osteosynthesis. The shaft screw (see Table 76-1) is an exception and therefore a special cortex screw. The shaft screw contains a shaft portion of the same diameter as the outside diameter of the threads.42 The threads have the same geometry as the cortex screws. Shaft screws are available in various shaft and thread lengths. Because of the smooth shaft portion, these screws are ideally suited for lag screw fixation, especially through LC-DCPs, where up to 40 degrees of angulation relative to the long axis of the plate can be achieved. Cancellous screws have a wider thread diameter than the cortex screws and they have a different pitch (see Table 76-1). This screw is designed to improve holding power in soft cancellous bone, but it is only rarely used in equine internal fixation. Cannulated screws contain a central canal for a guide wire (see Table 76-1).43 The design resembles the cancellous screw, because it has a thinner shaft and a wider thread portion. The 7.3-mm cannulated screw contains a self-drilling and selftapping tip, as well as a reverse cutting device at the back end of the threads (Figure 76-12).44 An in vitro study using equine cadaveric femurs revealed that the 6.5-mm cancellous and the 7.3-mm cannulated screws vary in insertion properties (the 7.3-mm cannulated screw requires significantly greater insertion torques), but they have similar pullout properties in the mid, proximal, and distal metaphyses of foal femurs.45 Both screw types have greater holding power at the mid-diaphyseal location than at metaphyseal locations. Because of the overall similar holding power of 6.5-mm cancellous and 7.3-mm cannulated screws, it is unlikely that increasing the screw diameter

the guide pin inserted and half of the shaft removed. Insert: the reversecutting design of threads, which facilitates screw removal after healing of the fracture. (From Nixon AJ: Equine Fracture Repair. Saunders, Philadelphia, 1996.)

beyond 6.5 mm will provide increased holding power in foal bones. The use of the 7.3-mm cannulated screw should be considered for foal femoral fracture repair when greater accuracy is needed or when bone threads for the 6.5-mm cancellous screw have been stripped.45 Self-tapping cortex screws contain the same thread-cutting device at the tip as the tap, obviating one step of the standard screw insertion technique (see Table 76-1).46 These screws are popular in human surgery and are gaining more acceptance in equine surgery.47 An in vitro study revealed that the mechanical properties of regular and self-tapping 4.5-mm cortex screws are similar with regard to pullout strength from the adult equine MCIII and that the self-tapping cortex screws require less than half the total insertion time required by standard screws.47 Interestingly, bone failure and bone comminution during the pullout tests were more commonly associated with self-tapping screws.48 Locking head screws were introduced with the less-invasive stabilization system (LISS) and subsequently also applied in the locking compression plates by Synthes (Figure 76-13). The conical shape of the PC-Fix screw (which is no longer manufactured) served as a basis for the new design.49 The screw head was modified with a threaded profile, which complemented the one in the LISS plate hole. This design provided a stable angular

1058

SECTION XII  MUSCULOSKELETAL SYSTEM

Figure 76-13.  A self-tapping locking-head screw. Note the threads manufactured into the conical screw head. These interlock with complementary threads in the plate. The self-tapping ends are visible at the tip of the screw.

fixation of the screw–plate (fixator) junction: the screw head is self-centering in the hole, and it keeps the screw from backing out of the LISS and LCP fixator (see later). The pitch of the threads at the screw head is identical to that of the threads on the shaft. Because of the larger diameter of the screw head, the pitch seems larger than on the shaft. However, the threads on the screw head catch after turning only 180 degrees instead of 360 degrees in the shaft. This facilitates faster fixation of the screw head into the plate and reduces the development of plate deformation through tightening of the screw. It is important to remember that this screw must be at 90 degrees relative to the long axis of the plate. The screw was also adapted to the unique mechanical demands of an internal fixator. The core diameter of the screw was enlarged to resist the increased bending moments and higher shear forces induced by a fixator.47 This, plus the threaded screw–plate interface, allows the use of unicortical screw fixation in the diaphysis. The stability of unicortical fixation with locking head screws was established in a cadaveric biomechanical study.50 Unicortical screw fixation, in turn, allows the application of self-drilling or self-tapping screws, which was made possible by reducing the thread pitch and adding drill and tap sections to the screw tip (see Table 76-1).39 These design changes have additional benefits: screw length determination is no longer needed because all diaphyseal screws can be the same length; therefore screw lengths are not needed in increments of 2 mm, which results in a smaller inventory; predrilling and tapping are no longer needed, and the thread profile cut into the bone is more precise (because each screw is used once and therefore the drill and tap parts are sharp), which results in a better anchorage of the screw in the bone.49 In the horse, self-drilling and self-tapping screws are rarely used because of their high price, and screws are usually inserted bicortically. Therefore most of the advantages just listed do not apply to this species. However, self-tapping screws are used in the horse in bicortical applications. Currently, locking head screws for the variable axis plates are available. As the name says, these screws do not need to be introduced at a right angle relative to the long axis of the plate. This is possible because of the slightly curved surface along the axis of the screw head and the special configuration of the plate hole (see later). Other screws include the Herbert screw (Zimmer Orthopedics), an example of a self-contained compression screw.51 This

screw is fully threaded and contains threads not only over its entire length but also on the head. The head is wider than the rest of the screw and can be completely buried in the bone. It has been used in condylar fractures of MCIII/MTIII. Recently, a cannulated, tapered, variable-pitch, selfcompressing screw was developed (Acutrack Equine Screw). This screw is 45 mm long, and it has a diameter of 6.5 mm at its base that tapers to a diameter of 5.0 mm at its apex. Because of the tapered shape, no glide hole is needed. The screw is manufactured of titanium. Biomedical studies comparing Acutrack to 4.5-mm AO cortex screws inserted in lag fashion revealed that the screws had similar biomedical shear properties.52 The self-compressing action of the Acutrack Plus screw generated 65% of the compression pressure and 44% of the compressive force achieved with the 4.5-mm AO cortex screw.53 The overall pushout strength was higher in the Acutrack Plus screw.53 Simulated midbody fractures of the medial proximal sesamoid bone repaired with Acutrack self-compressing screws compared with 4.5-mm AO cortex screws showed mechanically comparable strengths.54 Both constructs were mechanically inferior to intact proximal sesamoid bones.54 A recent study compared compression pressures of the Synthes cortex screw and the Acutrack Plus screw in simulated equine MCIII lateral condylar fractures of varying fragment thicknesses. The results revealed that significantly lower compressive forces were achieved with the Acutrak Plus screws.55 Sizes The size of a screw is determined by the outside diameter of the threads. The standard screws for the horse are the 4.5- and 5.5-mm cortex screws. Pertinent data on each screw type as well as drill sizes needed are summarized in Table 76-1. The 5.5-mm screw was developed for compact equine bone. This screw has advantages over the 4.5-mm screw when used in adult horses.56-58 The 6.5-mm cancellous screw is available in three configurations: a 16-mm thread length, a 32-mm thread length, and fully threaded. Since the introduction of the 5.5-mm cortex screw, the 6.5-mm cancellous screw has diminished in importance, because the 5.5-mm screw can also be inserted when a 4.5-mm hole has been stripped. A stripped 5.5-mm hole can still be engaged by a 6.5-mm cancellous screw. Cannulated screws are manufactured in 3.5-, 4.5-, 7.0-, and 7.3-mm diameters, but only the latter two sizes are interesting for the equine surgeon, and they can be used if a 6.5-mm cancellous screw hole is stripped. The 3.5-mm cortex screw is used to achieve interfragmentary compression of certain fractures, such as third carpal bone slab fractures. It is also applicable for anatomic reduction of long bone fractures and interfragmentary compression of the fragments. The screw has such a small head that it can be completely buried in bone, which allows plating over the screw.30 This is a great advantage, especially if two plates have to be applied. The recently developed 5.0-mm locking-head screws are the strongest screws available for equine fracture treatment, because of their large core diameter (4.3 mm) (see Figure 76-13). The thread width is much smaller than standard cortex screws, but these screws are tightened into the thread hole within the combi-hole of the plate, facilitating solid fixation. These screws can be applied only through the LISS and the LCPs. They are available as nontapping; self-tapping; and self-drilling, selftapping screws. Only the first two types are of interest for equine



CHAPTER 76  Principles of Fracture Treatment

surgeons. The self-drilling, self-tapping screw is very expensive, and because of its design it is impossible to predetermine the depth of the screw hole. Also, in equine fracture treatment it is important to achieve screw purchase in both cortices. If a selfdrilling, self-tapping screw was inserted, the sharp self-drilling part of the screw would protrude out of the trans-cortex, which could produce soft tissue damage. The 4.0-mm locking head screw can be used with the 4.5/5.0 mm LCPs. They have the same head design as the 4.3-mm locking head screw but a thinner screw design. These screws are rarely used in equine osteosynthesis. The 3.5-mm locking head screws have a core diameter of 2.9 mm and are therefore stronger than the cortex screw of the same size. To ameliorate the excessive rigidity of the locking plates experienced in human surgery, a dynamic locking head screw (DLS) was recently developed. This screw type was tested in recent in vivo studies in sheep and resulted in additional callus formation around the osteotomy site.59,60 The DLSs are not justified for equine applications because they are expensive and the same problems are not encountered in the horse as in people. Additional smaller screws (2.7 mm, 2.4 mm, 1.5 mm, etc.) are available as regular cortex screws or in some instances as locking head screws. These implants may be applied with the corresponding plates for the repair of skull fractures (see Chapter 102). Most screws are available either with a hexagonal or a stardrive hole in the screw head, into which the corresponding screwdriver is inserted to power the screw into the bone. The preparation of the screw surface plays an important role in the holding power of the implant.56 Special surface preparation of stainless steel and titanium screws showed superior holding characteristics over the plain stainless steel and titanium screws.61 Functions Screws can be used as lag screws, position screws, and plate screws.62 There is a difference between a partially threaded cancellous or cannulated screw, which automatically produces a lag effect, and a cortex or shaft screw used in lag fashion.3 The cancellous and cannulated screws are inserted so that all of the threads pass the fracture line. Thus tightening of the screw provides interfragmentary compression (Figure 76-14, A).1-4 If the threads of a cancellous screw bridge the fracture gap, no compression can be achieved (see Figure 76-14, B). The same holds true for a cortex screw placed as a position screw (i.e., with no lag effect): its threads engage bone on both sides of a fracture, and because no glide hole was prepared in the near cortex, no interfragmentary compression is achieved. A plate screw lags the plate to the bone.1,2 The lag technique may, however, also be applied through a plate hole. In this case, the screws are inserted through the plate and they cross the fracture line.1-4 The lag technique is used to insert cortex screws so that they act in lag fashion. This technique is not necessary when using partially threaded screws (such as cancellous or cannulated screws) as lag screws when applied as discussed in later paragraphs. The cis-cortex or near cortex is drilled with a drill bit having the same diameter as the outside thread diameter of the screw (Figure 76-15, A). This is referred to as overdrilling. Therefore, at insertion of the screw, the threads do not engage bone in that cortex but glide through, so this portion of the hole is called the glide hole. The outside diameter of the insert drill

1059

A

B Figure 76-14.  Lag screw technique. A, A cancellous screw of the correct thread length. All threads are located past the fracture plane, allowing interfragmentary compression. B, Selection of a cancellous screw with too long a thread length. Threads are located on both sides of the fracture plane, preventing interfragmentary compression.

sleeve is the same as that of the glide hole, and the inside diameter is the same as that of the smaller drill bit, which has a diameter identical to that of the core of the screw. Insertion of this drill sleeve into the glide hole ensures concentric drilling of the trans-cortex and subsequent accurate reduction of the fracture (see Figure 76-15, B). The hole drilled through this sleeve across the trans-cortex or far cortex is referred to as the thread hole. To allow a greater contact area between the screw head and the bone, a depression is created at the near cortex using the countersink (see Figure 76-15, C). This decreases stress concentration at the screw head–bone interface. It is important to use the countersink in a 360-degree motion rather than in a to-andfro motion; otherwise, an imperfect indentation is cut, preventing proper seating of the head. Countersinking is especially important in screws inserted at nonorthogonal angles relative to the surface of the bone. Care is taken to insert the nozzle at the tip of the countersink axially into the glide hole and to remove the bone making contact with the instrument. If this is not done, tightening of the screw results in stress accumulation at the screw head–bone junction, and bending of the screw head will result. The depth gauge is then used to determine the total length of the screw, including the head (see Figure 76-15, D). Therefore, the length of the screw is measured to include the head. The depth gauge has a small hook at the end of its thin shaft that is inserted through the thread hole. By slightly tilting the instrument to one side, the hook catches the opposite cortex, and by sliding the movable portion toward the countersink depression, the exact length of screw is determined. In human and small animal osteosynthesis, 2 mm is added to the determined length to ensure that the tapered tip of the screw that does not engage the precut threads in the trans-cortex is positioned outside the bone. In equine bone, however, with the relatively thick cortices, the tapered tip is usually maintained within the bone, resulting in a loss of holding power of approximately 2 mm of screw. Because adequate bone stock is present

1060

SECTION XII  MUSCULOSKELETAL SYSTEM

Figure 76-15.  Lag technique, shown on a lateral condylar fracture of the distal third metacarpal.  A, The cis-cortex is overdrilled. B, The insert drill bit is placed into the glide hole and advanced past the fracture plane, and the concentric thread hole is drilled across the trans-cortex. C, A depression for the screw head is prepared with the countersink.  D, The required length of the screw is determined with the depth gauge. E, The threads are cut into the thread hole with the tap. F, The screw of predetermined length is inserted and solidly tightened with the hexagonal-tipped screwdriver. (From Nixon AJ: Equine Fracture Repair. Saunders, Philadelphia, 1996.)

A

B

C

D

E

F

to ensure secure fixation of the screw in the prepared hole, the screw that is used is either the exact length determined by the depth gauge or is 1 to 2 mm shorter. Using the tap sleeve to protect the soft tissues as well as to help guide the tap, the tap is inserted into the glide hole and the threads are cut into the thread hole (see Figure 76-15, E). The threads are cut by advancing the tap three half-turns clockwise, followed by one half-turn counterclockwise. The counterclockwise action allows transport of the swath material into the flutes of the tap and ensures precise cutting of the threads without interference of the swath material cut from previous threads. An experienced surgeon may tap the thread hole with the air drill (power tapping) to speed up the procedure. This is especially advantageous when many screws are to be inserted. It requires experience, however, or serious complications may arise, such as stripping of the thread holes, cross threading of the tap, and instrument breakage. If a self-tapping screw is used, this step is not necessary. The small AO air drill and the Colibri air drill (small battery-powered drill) are not powerful enough to insert a self-tapping screw without applying a to-and-fro "power-tapping" technique. The Synthes ComPact Air Drive II, however, has one third more power and easily inserts selftapping screws without to-and-fro movement. Once the hole has been tapped, it is flushed to clean out swath debris and to lubricate it. A screw of the predetermined length is inserted, using the hexagonal tipped or stardrive screwdriver (see Figure 76-15, F) either by hand or with an air drill. Final tightening is always carried out by hand. Care is taken to avoid excessive force, which may result in failure of the screw

head, or in stripping of the threads cut into the thread hole.47 This is not a common problem in dense equine bone. The shaft screw is inserted by applying the identical lag technique. However, care has to be taken that the glide hole is made slightly longer than the screw shaft. If the shaft is longer than the glide hole, no interfragmentary compression is achieved. In the lag screw technique, a lag screw (either a partially threaded cancellous screw or a cannulated screw) is inserted after using a drill bit of only one size across the entire bone. Threads are cut along the total length of the hole with the cancellous tap, and the lag screw is inserted. The threads in the cis-cortex should not be engaged by the screw threads but only those of the trans-cortex, allowing achievement of interfragmentary compression.1-4 Because the hardness of equine bone makes screw insertion difficult, it may be advisable to enlarge the ciscortex with a 4.5-mm-diameter drill bit after first drilling the entire hole with the 3.6-mm drill bit. Insertion of cannulated screws employs the same technique. However, the initial step involves placement of a guide wire in the desired location.44 A special drill sleeve allows insertion of parallel screws close together. It is advisable to predrill equine cortical bone with a small drill bit before inserting the guide wire to prevent bending it. All instruments are cannulated to accept the guide wire. The size of the drill bit depends on the size of the screw to be implanted and the size of the guide wire. Once the guide wire is in place, its correct position and depth is ensured through radiography. If necessary, adjustments are made at this time. The measuring device is then placed over the portion protruding out of the bone. The length of guide wire



CHAPTER 76  Principles of Fracture Treatment

1061

Figure 76-17.  Removal device for stripped 3.5-mm screw heads. An

Figure 76-16.  A capital femoral fracture is repaired with three cannulated screws. After the screws are inserted over a guide pin and tightened, the guide pins (which penetrated farther than the screws) are removed.

located in the bone is determined, and this determines the length of screw required. It is advisable to select a screw 3 to 5 mm shorter than the length of the guide wire inserted within the bone, to ensure secure seating of the wire throughout the implantation procedure. Subsequently, the cannulated drill bit is placed over the guide wire and the hole of predetermined length is prepared. The hole is tapped and finally the selected screw is inserted and firmly tightened (Figure 76-16). At the end, the guide wire is removed. The 7.3-mm cannulated screw has a self-drilling and selftapping tip. Therefore a screw of predetermined length is inserted without drilling a thread hole. Because of the initial insertion of the guide pin and the ability to select a screw of correct length, the danger of implanting a screw that is too long and protrudes from the opposite side of the bone is negligible. In equine bone, insertion occurs in the same manner as tapping, meaning that the screw is advanced three half-turns, followed by a half-turn in the opposite direction. Care has to be taken with power insertion.39 The position screw is used to maintain two pieces of bone at a certain distance apart and to prevent interfragmentary compression. This is achieved by drilling a hole of only one size (thread hole) across both cortices, followed by tapping. Only fully threaded cortex screws may be inserted as position screws. Because the threads catch in the cis- and trans-cortex, interfragmentary compression is prevented. No lag effect is achieved. It is advisable to apply a washer onto the bone surface to distribute the forces applied by the screw, because it is not possible to use the countersink. Its nozzle doesn’t fit into the drill hole without overdrilling it for 13 mm ( 1 2 inch). In some instances the latter is not possible because it would result in a lag effect. The plate screw is inserted by the technique described for the position screw. Any type of screw may be used in this manner. With the plate screw, the plate hole serves as a glide hole and allows compression of the plate onto the underlying bone, providing friction and stability.

intact hexagonal hole in the screw head is shown. It is smaller than the tip of the screw. However, once the hole in the screw head is stripped, the tip of the device (whose threads are oriented in the direction opposite to those in the screw) will fit.

Screw removal Cortex and locking-head screws are easily removed because of their fully threaded design. Similarly, shaft screws are easy to remove, because the shaft completely fills the glide hole. However, after a fracture has healed, a cancellous screw may be impossible to remove from hard equine bone, because during fracture healing, the precut threads in the cis-cortex fill in with solid bone. Removal of the screw requires the threads to cut their own way through the cis-cortex, a task for which they are not designed. This frequently results in the screw breaking, usually at the head–shaft or the shaft–thread junction. Therefore partially threaded cancellous screws should not be used when implant removal may be necessary at a later stage. Smaller cannulated screws have the same problem as cancellous screws. The 7.3-mm cannulated screw, however, contains a reversecutting edge at the caudal end of the threads, which facilitates recutting of the bone threads during screw removal (see Figure 76-13).43 Occasionally, the hexagonal indentation in the screw head is stripped during screw removal. This occurs if the screwdriver is improperly inserted in the hexagonal hole. Alternatively, if the hole is partially filled with tissue, the screwdriver cannot be inserted completely. Subsequent application of extraction force (counterclockwise motion on the screwdriver) may strip the hole within the screw head. This problem is mainly encountered with the hexagonal tipped 3.5-mm screw head and not in the stardrive head because of its smaller screw head. A special screw retrieval instrument has been designed for such situations (Figure 76-17). A shaft with a conical, threaded tip is inserted into the hexagonal indentation of the screw head. The threads of the tip have a reversed orientation compared with the screw threads. Therefore when the cone is tightened in the screw head with a counterclockwise motion, an extraction force is applied to the screw, allowing it to be easily removed. These screwretrieval devices are available for all sizes of screws. When a screw head is broken off, a special hollow drill bit is available to remove the bone surrounding the screw. Because it rotates counterclockwise, the threads on the inside of the hollow drill bit interlock with the screw, and advancing the drill

1062

SECTION XII  MUSCULOSKELETAL SYSTEM

A

B

Figure 76-19.  The Bagby plate, developed in 1958. A, A screw with a slanted screw head is inserted under load conditions into a larger plate hole, and through tightening of the screw, the axial compression is achieved across the fracture, similar to the dynamic compression plate principle shown in B.

Figure 76-18.  The Danis plate, developed in 1947. The plate screw is inserted into an oblong hole, and by tightening a smaller screw placed parallel to the long axis of the plate, the initial screw is displaced, providing axial compression of the fracture.

E

D

C

B

A

bit removes the broken screw. For screws of all sizes that have broken off, a special screw retrieval set has been developed. PLATES The first plates to contain an axial compression device were developed by Danis in 1947.63 They consisted of a plate with an oblong hole on one end. At the head of the plate, a compression screw could be introduced, which pushed the screw placed through the oblong plate hole toward the fracture line and in doing so provided axial compression to the fractured bone ends (Figure 76-18). Ten years later, Bagby introduced an impacting bone plate.64 The heads of the screws he designed had a conical underside. If the screw was inserted eccentrically into the plate hole, the conical underside made contact with the edge of the plate hole. By tightening the screw, the bone into which the screw was implanted was displaced toward the fracture site and thus induced axial compression (Figure 76-19). The first plates developed by the AO in 1958 contained round holes (Figure 76-20, A).63 Axial compression was applied with the help of a tension device (see later). Plate hole designs are shown in Figure 76-20 and described in the following paragraphs. The specific data on the different plates used routinely in equine osteo­ synthesis are summarized in Table 76-2. The dynamic compression plate (DCP) was considered the basic plate in equine fracture treatment for a long time. Recently there has been a move away from this plate toward limitedcontact dynamic compression plates (LC-DCPs) and locking compression plates (LCPs) (see later). Therefore, the detailed steps of plate application and discussion of the general principles of plate application in the horse are discussed under LC-DCP. Dynamic compression plate The 4.5-mm Dynamic Compression Plate is available in two plate widths. The narrow plate has holes arranged in a straight line, and the broad plate has holes offset to the left and right of the midline. The 3.5-mm broad plate, developed mainly for small animals, is manufactured from the same plate stock as the 4.5-mm narrow DCP. However, because of the smaller size of the plate holes, this basic plate is stronger than the narrow

Figure 76-20.  The plate holes designed for Synthes plates. A, The initially developed round hole. B, The dynamic compression plate (DCP) hole, which allows compression from one side. The screws can be angulated axially up to 25 degrees. C, The dynamic compression unit hole, which allows compression from either side and is used in the limited compression (LC)-DCP. The screws can be angulated axially up to 40 degrees. The plates have undercuts. D, Locking head plate holes used in the less-invasive stabilization system (LISS) plate, allowing only orthogonal insertion of the screws. The plates have undercuts. E, The combi-hole used in the locking compression plate allows the insertion of locking head screws—as shown here—and standard screws. The plates have undercuts and a pointed end that allows minimally invasive insertion through a small incision.

4.5-mm DCP and therefore may also be applied in foals.65 The holes in a DCP are designed to achieve dynamic compression with tightening of screws inserted in the "loading" position. The holes are machined according to the sliding spherical principle with an incline or slope pointing downward towards the central portion of the plate (see Figure 76-19, B).66 When a screw is inserted in the load position (offset 1  mm from the

Plate type

—

Hole spacing in plate midsection

—

—

No

18 DCU

13 DCU

No

Straight

Staggered

—

No

18 DCU

Staggered

4.5, 5.5, (6.5)

16

Yes

12 DCP

Straight

3.5, 4.0

25

Yes

16 DCP

Staggered

Staggered

—

—

16 16 2 round, rest DCP DCP No No

Staggered

13.5 4.2

Special (note 3)

Straight

Yes LO-LO 9 DCU-DCU 15

Yes LO-LO 9 DCU-DCU 15

13 13 Combi-hole Combi-hole

Straight

3.5, 4.0 3.5 LS

—

—

Yes

18 Combi-hole

LO-LO 13 LO-LO 13 DCU-DCU 20 DCU-DCU 20

Yes

18 Combi-hole

4.5, 5.5, (6.5) 4.5, 5.5, 5.0 LS (6.5) 4.0 / 5.0 LS Straight Staggered

—

17.5 6

Special

9 1

Special

LO-LO 13 DCU-DCU 20

Yes

18 Combi-hole

4.5, 5.5, (6.5) 4.0 / 5.0 LS Staggered

—

16

12 Oval, round w/ collar Yes

Straight

3.5, 3.5 LS

—

116 (6 holes) 188 (10 28 (2 holes) to 440 holes) to 148 (24 holes) to 440 (12 holes) (24 holes) Straight Straight Straight

17.5 5.2

Special (note 3)

Equine One-Third LCP Tubular LCP 4.5 Broad 5.5 Broad Plate

DCP, Dynamic compression plate; DCS, dynamic condylar screw; DCU, dynamic compression unit; DHS, dynamic hip screw; LC-DCP, limited-contact dynamic compression plate; LCP, locking compression plate; LO, locking.

No

Plate midsection

Hole Straight arrangement Hole spacing 13 Hole design DCU

4.5, 5.5, (6.5)

—

13.5 4.2

Standard

LCP LCP 3.5 Broad 4.5 (Note 4) Narrow

27 (2 holes) 94 (7 holes) 66 (3 holes) to 287 to 289 to 287 (22 holes) (22 holes) (16 holes) (note 5) Straight Straight Straight

11 3.4

Special (note 3)

LCP 3.5

(130° ALSO) 135°, (140°, 145°, 150°) — Barrel 25 mm Barrel 25 and — long 38 mm long 4.5, 5.5, (6.5) 4.5, 5.5, 4.5, 5.5, (6.5) 3.5, 4.0 (6.5) 3.5 LS

95°

3.5, 4.0

—

Straight

3.5, 4.0

—

Straight

—

Straight

—

19 5.8

Angled portion Screw size (mm)

16 5.4

Special (note 3)

Straight

16 4.8

Special (note 3)

Straight

12 3.6

Special (note 3) Special (note 3)

DHS Plate

Straight

17.5 5.2

Standard

DCP 3.5 Broad DCP DCS LC-DCP 4.5 Broad (Notes 1, 2) 4.5 Broad Plate

Plate angle

13.5 4.2

Standard

LC-DCP 4.5

103 (6 holes) 114 (6 holes) See Table 76-3 below to 359 to 370 (22 holes) (22 holes)

13.5 4.2

Standard

LC-DCP 3.5 Broad

28 (2 holes) 94 (7 holes) 34 (2 holes) 106 (6 holes) 86 (7 holes) to to 288 to 289 to 394 to 394 266 (22 holes) (22 holes) (22 holes) (22 holes) (22 holes)

Width (mm) Thickness (mm) Length (mm)

11 3.3

Standard

Name

Plate crosssection

LC-DCP 3.5

TABLE 76-2.  Standard and Special Plates Used in Large Animals

1064

SECTION XII  MUSCULOSKELETAL SYSTEM

TABLE 76-3.  DHS Plate Lengths Barrel Length

Barrel Angle

Plate Length

38 mm barrel

130° 135° 140° 145° 150° 130° 135° 140° 145° 150°

46 46 46 46 46 46 46 46 46 46

25 mm barrel

(2 (2 (2 (2 (2 (2 (2 (2 (2 (2

hole) hole) hole) hole) hole) hole) hole) hole) hole) hole)

to to to to to to to to to to

238 (14 hole) 333 (20 hole) 270 (16 hole) 270 (16 hole) 333 (20 hole) 110 (6 hole) 110 (6 hole) 110 (6 hole) 110 (6 hole) 110 (6 hole) Figure 76-21.  Limited-contact dynamic compression plate. The ends

center of the drill guide), the screw head contacts the plate at the top of the incline. During tightening, the screw head moves down the slope until it comes to rest at the bottom of the incline, just about in the center of the oval screw hole. Because the screw is introduced into the bone, screw movement toward the fracture line results in compression of the fractured bone ends. The center of the plate should be located over the fracture site, and this offset drilling can be carried out on either side of the fracture plane. Two screws on either side of the fracture can be used in the load position; using the plate holes alone will provide a maximum of 4  mm compression. Before tightening the second screw, the first screw on the same side of the fracture plane has to be loosened to achieve the additional 1  mm compression. Following tightening, the loosened screw is tightened again. Additional compression requires the external tension device. Limited-contact dynamic compression plate The DCP, up to now the workhorse for equine fracture treatment, was for a while replaced by the LC-DCP, especially in the United States. Studies in human medicine showed that the DCP caused osteoporosis under the plate, although this is not encountered in equine surgery (see later).1 This led to the development of biologically improved plates. In the conventional DCP, the plate holes provided the least resistance to failure. This problem was somewhat offset in equine fracture treatment by inserting screws through all plate holes. By designing a plate that contained at each cross-section along its entire length the same amount of metal, an implant of uniform bending stiffness was developed.62 To achieve this, half-moon–shaped pieces of metal were removed from the underside of the plate. This resulted in limited contact between bone and the plate, which led to the name of the plate: LC-DCP (Figure 76-21). The limited contact surface was welcomed in human surgery to fight the problem of osteoporosis developing under the plate.62 Extensive tests comparing the LC-DCP with the conventional DCP revealed that the LC-DCP had an equal bending stiffness and a 50% increase in the continuity of the bending stiffness.62 This reduces local stress concentration near fracture gaps. Additionally, the blood supply of the bone under the plate was significantly improved.67 Early mechanical tests conducted in the AO Research Institute in Davos revealed that the design of the LC-DCP provided increased resistance to cycling failure compared with the DCP.65 The undercuts of the plate allow the development of some callus bridges over the

are pointed, the screw holes are arranged in two slightly offset rows (top side of plate shown above) evenly distributed along the plate, and the underside of the plate (shown below) contains undercuts.

fracture gap, which led to a significant increase in stability, despite the fact the these bridges are small. The trapezoidal cross section allows the formation of shorter but stronger bone lamellae on either side. Also, it prevents the bone from growing over the plate. The dynamic compression unit (DCU) hole design in the LC-DCP, which allows axial compression to be applied from either side of the hole, replaced the conventional DCP hole design (see Figure 76-20). This allows the distribution of the plate holes evenly along the entire plate and obviated the need for a center in the plate. The DCU hole is also undercut at each end to allow the insertion of screws up to a 40-degree angle relative to the orthogonal direction. In the DCP hole, only a 25-degree angulation can be achieved.64 There are two plate widths; the narrow plate has holes arranged in a straight line, and the broad plate has alternating offset holes. The DCU holes are specially designed to allow dynamic compression as the screw is tightened. The holes are arranged according to the spherical gliding principle, with an incline, or slope, toward the center portion of the plate (Figure 76-22).65,66 This offset drilling can be carried out on either side of each hole and therefore on either side of the fracture line.65 The application of the LC-DCP requires the use of the special LC-DCP double drill guide identified by its undercuts on the handle, identical to the ones under the plate itself (Figures 76-23 to 76-26). (Note: the DCP drill guide should not be used with the LC-DCP.) An alternative to the LC-DCP drill guide is the universal drill guide, which contains a spring-loaded tip (see Figure 76-23). Pressing down on the drill guide places it near the center of the hole (see Figure 76-26, B). Placing the springloaded tip on the far end of the DCU hole relative to the fracture line (without pressing down on the drill guide) allows a 1-mm compression of the fracture line (see Figure 76-25, A). The technique of application is as follows. The fracture is reduced (Figure 76-27, A), and it is maintained in that configuration initially with pointed reduction forceps until one or two interfragmentary cortex screws, 3.5 or 4.5 mm in diameter, can be applied in lag fashion (see Figure 76-27, B). The plate is then contoured, overbent at the fracture site, and applied to the bone (see Figure 76-27, C). The first screw hole is drilled toward one end of the plate in neutral position (green LC-DCP drill guide



CHAPTER 76  Principles of Fracture Treatment

1065

Figure 76-22.  Design of the dynamic compression unit (DCU). Both sides of the hole are shaped like an inclined cylinder. Like a ball, the screw head slides down the incline. Because the screw head is fixed to the bone via the shaft, it can move only vertically relative to the bone. The horizontal movement of the head, as it impacts against the angled side of the hole, results in movement of the bone fragment relative to the plate and leads to compression of the fracture. With the DCU, compression can be achieved on either side, obviating the need for the plate to have a center, as in the dynamic compression plate.

Figure 76-23.  The limited-contact dynamic compression plate (LC-DCP) double drill guide (top) contains undercuts like those of the corresponding plate to distinguish it from the DCP double drill guide. The dark ring (green) represents the neutral guide, the light ring (yellow) the load guide. The universal drill guide (bottom) contains a spring-loaded guide for the thread hole and a larger guide for the glide hole. The universal drill guide is available for the different screw sizes.

or pressed-down universal drill guide). The screw is inserted but not completely tightened. This allows the plate to be pulled into a loaded position. The same can be achieved by drilling the initial hole through the load (yellow) LC-DCP guide or the universal drill guide placed at the far end of the hole, and maintaining the plate in the same position (see Figure 76-27, D). The hole for the second screw is drilled on the other side of the fracture line through a plate hole near the other end, using the load drill guide (see Figure 76-27, E), if additional compression is needed. Care is taken to ensure correct plate position before drilling the second hole. The hole is prepared for the screw, which is subsequently inserted. Interfragmentary compression is achieved through alternate tightening of the two screws (see Figure 76-27, F). More screws may be applied in the loaded position on either side of the fracture. A maximum of two screws can be placed under load conditions on either side of the fracture line. Therefore a maximal compression of 4 mm can be achieved. Before the second loaded screw is completely tightened, the first one on the same side has to be slightly loosened. This allows the additional compression to be applied. As

Figure 76-24.  The neutral LC-DCP drill guide inserted into a dynamic compression unit (DCU) hole (bottom). From below the plate, it can be seen that there is a gap between the right end of the DCU hole and the drill guide hole (top left). From the top, the arrow pointing toward the fracture line is visible (top right). In the DCP neutral guide, there is no arrow.

Figure 76-25.  The load LC-DCP drill guide inserted into a dynamic compression unit (DCU) hole (bottom). From below the plate, it can be seen that there is no gap between the right end of the DCU hole and the drill guide hole (top left). From the top, the arrow pointing toward the fracture line is visible (top right).

1066

SECTION XII  MUSCULOSKELETAL SYSTEM

A

B

Figure 76-26.  A, The universal drill guide placed into a dynamic compression unit (DCU) hole without applying pressure onto the guide. Viewed from below, the hole in the guide can be seen very close to the right end of the DCU hole. B, The universal drill guide placed into a DCU hole under pressure. The spring-loaded part of the guide is sticking out on top. From below, a gap can be seen between the hole in the guide and the right end of the DCU hole.

mentioned before, without this loosening, no additional compression is achieved and the two screws that are "compressed" toward each other are stressed. The remaining screws are implanted in neutral position. Any screw placed through a plate across a fracture line is introduced using the lag technique (see Figure 76-27, G). All the screws are finally tightened (see Figure 76-27, H). A cortex screw inserted through a plate in lag fashion perpendicularly across a fracture may not achieve the desired effect, because the threads in the glide hole cut into the cortex and prevent any gliding.65 To correct this undesirable effect, the shaft screw was developed. The shaft, which fills out the glide hole completely, does not cut into the cortex. Any cortex or shaft screw can be inserted under load (at the far end of the oblong plate hole) or in neutral position (at the center) of each plate hole. Under load, 1-mm centripetal displacement or compression of the fracture gap is implemented. In neutral position, 0.1-mm compression is achieved. If double plating is applied, only two screws are placed under load in the second plate, which in most cases is arranged 90 degrees to the first plate. Plates are contoured with the help of a plate-bending press to fit the surface of the bone. A perfectly contoured plate, however, compresses only the cortex immediately under the plate, whereas the opposite cortex remains decompressed (Figure 76-28, A). By slightly overbending the plates at the fracture site (see Figure 76-28, B), compression is achieved along the entire circumference of the bone (see Figure 76-28, C).1-3 Axial interfragmentary compression under a plate may also be implemented with the help of a tension device (Figure 76-29). The plate is applied to the bone with several screws in neutral position on one end of the fracture. The tension device is hooked into last hole on the other end of the plate and attached to the underlying bone through a unicortically applied 4.5-mm cortex screw. With the help of a wrench, the tension device is tightened, which pulls the plate toward the tension device and thus applies compression to the fracture site. Once adequate compression is applied, screws are inserted on the other side of the fracture through the plate in neutral position

and tightened. The tension device is subsequently removed and the remaining screws are inserted in empty plate holes. An in vitro study comparing the broad 4.5-mm DCP with the 4.5-mm LC-DCP revealed that the LC-DCP provided increased stability in static overload testing; however, it was significantly weaker in cyclic fatigue testing, which contradicted an earlier study that was not performed on cadaveric equine bone.68 The results of another in vitro study conducted by the same group comparing the 4.5-mm LC-DCP with the 5.5-mm LC-DCP in the same model used above are interesting.69 It showed that the 5.5-mm LC-DCP was superior in resisting static overload forces in palmarodorsal four-point bending. There was no significant difference in resisting static overload in torsion, but the 5.5-mm LC-DCP offered significantly less stability in cyclic fatigue loading. The 5.5-mm LC-DCP was previewed as the equine plate for fracture fixation and arthrodesis of the metacarpophalangeal joint. Because of the rapid gain in popularity of locking plates, the 5.5-mm LC-DCP was abandoned and replaced with a 5.5-mm LCP (see later). The stability of the fixation is derived from friction between the implants and the bone. A technique called plate luting has been developed to obtain 100% plate–bone contact by applying bone cement (methyl methacrylate) between the plate and the bone.70 This is achieved after all the screws of the plate are inserted. All the screws are then loosened, the plate is lifted off the bone, the soft bone cement is placed underneath it, and the screws of the plate are retightened, preferably with the power drill. Entrance of bone cement into the fracture line must be prevented because it retards or prevents bony union in that area. Once the screws are tightened, the soft cement fills the oblong plate holes around the screw heads and provides additional support, making the fixation extremely rigid.70-72 When only the oblong plate holes are filled with bone cement, a similar but lesser increase in strain protection occurs.72 Excess bone cement is rapidly and carefully removed. Plate luting is especially useful on bones with anatomically complex surfaces that make contouring of the plate difficult.72 The addition of gentamicin into the bone cement facilitates long-term release of this antibiotic and provides effective protection against postoperative infections (see Chapter 85). Plate luting is not used in humans and



CHAPTER 76  Principles of Fracture Treatment

1067

Stabilized fracture

Fracture reduction forceps

3.5-mm cortical screws placed in lag position

2.5-mm drill bit

A

B

E

F

C

D

G

H

Figure 76-27.  Repair of a simple oblique fracture of the third metacarpal with two cortex screws applied in lag technique combined with a broad LC-DCP as a neutralization plate. A, The large pointed reduction forceps maintains alignment of the fractured bone during implantation of the two 3.5-mm cortex screws. B, The two screws are implanted and the reduction forceps is removed. C, A 10-hole broad LC-DCP is applied to the dorsolateral aspect of the bone, distal from the two interfragmentary 3.5-mm cortex screws. The plate was overbent at the fracture site, allowing introduction of an aluminum template between the bone and the plate. D, A thread hole is drilled across the bone through the second most distal plate hole with the help of the yellow load drill sleeve. E, The screw is inserted but not completely tightened, followed by preparation of an identical hole at the opposite end of the plate. F, The second screw is inserted and both are alternately tightened, placing the fracture under axial compression. The remaining screw holes are prepared through the green neutral drill guide. G, A cortex screw is implanted in lag fashion across the fracture plane. H, All the screws are tightened. (From Nixon AJ: Equine Fracture Repair. Saunders, Philadelphia, 1996.)

B

A

C

Figure 76-28.  Application of a plate onto a bone. A, If a plate is perfectly contoured to the surface of the bone, a narrow gap develops at the fracture site opposite the plate after insertion and tightening of the screws. B, To overcome this problem, the plate is overbent (prestressed) about 1 mm, right at the fracture site. C, When the screws are reinserted, the entire circumference of the fracture is under compression.

1068

SECTION XII  MUSCULOSKELETAL SYSTEM

C

B

A

Figure 76-29.  A, The tension device is hooked in the last plate hole. B, The device is attached to the bone with a short screw. C, Twisting of the hexagonal screw head pulls the plate toward the left side and applies axial compression to the underlying fracture.

Figure 76-30.  Comminuted midshaft fracture of the third metacarpal

small animals because vascular necrosis of the bone develops under the plate, resulting in pathologic fractures after implant removal. This complication has not been reported in horses. With the use of locking plates, the importance of plate luting has decreased because the locking head screws effectively prevent micromovement of the screw heads within the oblong plate holes (see later). Screws should be inserted perpendicular to the surface of the bone. If a second plate is used, it should be positioned to allow the screw holes to be located between the screws of the other plate.39,73,74 This reduces the likelihood of inadvertent contact between the screws of the two plates. Every hole in a plate should be filled with a screw.39 Should a hole traverse a fracture line, the lag technique should be applied by overdrilling of the cis-cortex, and the screw should be directed so that it engages the opposite cortex next to the fracture line. Where no support can be achieved in a cortex, bone cement may be placed and the screw implanted. After the cement hardens, the screw will be solidly fixed. Application of 4.5-mm screws through a 4.5-mm plate allows 40 degrees of longitudinal angulation and 7 degrees of lateral angulation.65 Application of a 5.5-mm screw through the same hole allows only about 25 degrees of longitudinal angulation. The plates applied for fixation of a long bone fracture ideally extend over the entire length of the bone.4 Shorter plates must be staggered to ensure plate coverage of the total length of the bone. The most distal end of the proximal fragment in an oblique long bone fracture should be wedged between a plate and the opposing distal fragment. Therefore the configuration of a fracture dictates to a certain extent the location of the plates to be applied (not just the tension side of the bone; see later).39 Implants should be applied at a distance from severely bruised skin or areas of frank skin defects. Plate functions include compression, neutralization, tension band, and buttressing. The design of a plate does not dictate its function, because the same plate may be used for different functions, and a plate may serve more than one function at a time. The compression and neutralization functions are the most

showing a bone defect. The defect is filled with cancellous bone graft, and a broad 12-hole LC-DCP is applied over the defect in buttress function. For better visualization, the dorsally applied LC-DCP, routinely used in a clinical case, is not shown.

frequently applied. When a plate serves in compression function, two screws on either side of the fracture line are placed under a load.3 A plate serves a neutralization function after anatomic reconstruction and interfragmentary compression of a simple or comminuted fracture is accomplished by several screws placed in lag fashion.1-4,39 The various diverging shear, bending, and rotational torque forces exerted by these screws on the surrounding bone are neutralized by one or two plates. Such plates effectively bridge the proximal and distal aspects of the bone and protect the fixed fracture. The screws are inserted in the neutral position. Application of neutralization plates has to be planned ahead of time to avoid interfering with the reduction screws or eliminating options for their placement. Most plates applied satisfy both compression and neutralization functions. A plate applied in the tension band function transforms the tensile forces applied to the fractured bone underneath into compressive forces. The classic example of such a plate is one that is applied to fix an olecranon fracture. Because these plates are subjected mainly to tensile forces, they may be smaller than the plates used in compression or neutralization functions. Cortical bone defects that persist after anatomic reconstruction of the fracture cause instability of the repair because of stress concentration. Such areas need to be protected and bridged by an implant, maintaining length by preventing collapse of the fixation (Figure 76-30). A plate applied in such a fashion is called a buttress plate. It is advisable to fill the defect in the bone with a cancellous bone graft or bone replacement material. Any screw placed through the plate in the region of the defect should engage the trans-cortex. All the other principles for plate fixation are applied.



CHAPTER 76  Principles of Fracture Treatment

One-third tubular plate One-third tubular plates are very thin and, depending on the size of the hole, may be applied with a 3.5- or a 4.5-mm screw (see Table 76-2). The 3.5-mm plate is applied using standard technique in fracture treatment of proximal MCII, MCIV, MTII, and MTIV fractures in adult horses and of nondisplaced ulnar fractures in very young foals. T-plates T-plates are available for 4.5- and 5.5-mm screws as well as in a smaller version for the 3.5-mm screws. These plates are suited to areas where tension is applied without bending and sufficient space is not available for the application of a straight, regular plate. T-plates have been used for arthrodeses of the tarsometatarsal and distal intertarsal joints. The popularity of these plates has drastically decreased during the last few years. Dynamic condylar screw and dynamic hip screw system Dynamic condylar screw (DCS) and dynamic hip screw (DHS) plates are implant systems that were developed on the principles of the angled blade plate (ABP).75 The set contains the instruments, screws, and selected plates. These plates consist of a long lag screw with a 12.5-mm ( 1 2 -inch) thread width, a 25-mm (1-inch) thread length, and an 8-mm shaft diameter. The shaft is flattened on two opposing sides to prevent rotation of the screw after it is introduced into the barrel of the plate. The DCS or DHS is inserted at a predetermined angle (95 degrees for the DCS plate and 135 degrees [standard] for the DHS plate) (see Table 76-2). The most important step in the application of the DCS and DHS plate systems is the correct insertion of the 2.5-mm guide pin. Drill guides aid in the placement of the guide pin, which is best verified intraoperatively with an image intensifier. (Intraoperative radiographs, despite being more time consuming, are satisfactory as well.) Predrilling of the cortex with a 2.5-mm drill bit facilitates insertion of the guide pin. Care has to be taken that all of the four points on the base of the drill guide under the handle are in contact with the bone during drilling, even if the drill guide tip does not make contact with the underlying bone (Figure 76-31, A). Tilting of the drill guide with its tip down to bone results in false orientation of the DCS, which prevents the plate from making contact with the bone. This requires complicated adjustments of the plate angulations to make the plate contact the bone and prolongs the surgical procedure. If the plate–barrel junction is not in direct contact with the bone, the gap may be bridged with polymethyl methacrylate (PMMA). Once the guide pin is correctly placed, the subsequent steps are easily accomplished because all instruments are hollow and accept the guide pin. The measuring device is placed over the guide pin, and its depth in the bone is determined (see Figure 76-31, A). The triple reamer placed over the guide pin allows simultaneous drilling of the core hole, the portion for the plate barrel, and the beveled plate–barrel interface (see Figure 76-31, B). The reamer is assembled and adjusted to a length measuring 5 mm less than the pin portion located within the bone. This ensures persistence of the pin in its position throughout the screw insertion procedure. After preparing the screw hole, the threads are cut using routine technique (see Figure 76-31, C), and the screw of appropriate length is introduced, followed by application of the plate (see Figure 76-31, D). Once the shaft of the screw and the barrel are aligned, the barrel slides easily over the shaft and the plate

1069

position can be adjusted before impacting it onto the bone (see Figure 76-31, E). The barrel has the same inside diameter as the screw shaft cross section (8 mm and flattened at two opposite sides). The DCS plate has a barrel length of 25 mm, whereas the standard DHS has a barrel length of 38 mm (11 2 inches). A special version of the DHS plate has a 25-mm barrel length. After their implantation, the lag screw and the plate are joined with a connecting screw, making the two components work as one unit (see Figure 76-31, F). Tightening of the connecting screw creates interfragmentary compression, provided the lag screw has passed the fracture line. The DHS plate is the strongest plate available from Synthes.73 The DCS and DHS plates are versatile, rapidly implanted, and a real asset to large animal surgery, especially when treating long bone fractures in adult horses.76,77 The DCS system is useful in metaphyseal fractures of the MCIII or MTIII (Figure 76-32), the proximal radius, and even the femur. The DHS may be applied for arthrodesis of the metacarpophalangeal joint and in selected femoral fractures.75 Combined with 5.5-mm screws, these plates produce extremely strong fixations. Recently, the DCS/DHS plates were upgraded and equipped with combi-holes to allow insertion of locking head screws (see later, under "Locking Compression Plate"). Point contact fixator The point contact fixator (PC-Fix) plate, which is no longer manufactured, is mentioned here for historical purposes, because it represented the first type of internal fixator developed by Synthes.77-79 It was manufactured from pure titanium, and it had round, conical screw holes, evenly distributed along the plate, and a specially designed underside that represents a further development of the LC-DCP. One row of points is arranged along either side of the plate, with the points being located between two plate holes. Between two points, the plate is undercut in an arcuate pattern, similar to the LC-DCP but to a greater extent. The short unicortical screws contained a conical head that locked in the plate hole while being tightened, forming a solid unit between plate and screws, without applying any load onto the screw threads (Figure 76-33). The first clinical tests conducted with the PC-Fix were done on large animals, mainly cattle (Figure 76-34).80 The subsequently developed PC-Fix II was manufactured only as a 3.5-mm implant system. It had a slightly altered plate design and was applied with selftapping screws.81 A special plate-bending device was developed to prevent altering of the plate holes during bending. Development of this system led to the next generation of titanium implants: the LISS, which is described next. Less-invasive stabilization system The less-invasive stabilization system (LISS) consists of forged titanium plates, manufactured for the bones and sides of the bones (lateral versus medial aspect) to which it will be applied. The plates cannot be bent and are applied to the bone with self-cutting, self-tapping titanium screws of predetermined length. The shape of the plate is predetermined and forged accordingly.49,82 A guide system for transcutaneous insertion of the screws is mounted on the plate head (Figure 76-35, A). This bar also facilitates insertion of the plate through a small surgical incision at one end of the bone and subsequent advancement of the plate along the periosteum, therefore bridging the approximately reduced fracture. Once in place, the last plate hole is approached through a stab incision and connected

SECTION XII  MUSCULOSKELETAL SYSTEM

1070

a

c

b

A

B

a

b

c

c

d

C

a

b

e c

b d c

D

a

b

a

E

F

Figure 76-31.  Application of an LC-DCP and a dynamic condylar screw (DCS) plate to a radius fracture. A, The fracture is initially repaired with two interfragmentary 3.5-mm cortex screws, applied in lag fashion. Subsequently, a 14-hole LC-DCP is applied to the cranial bone surface (tension side) under compression. The guide pin (b) is applied through the special drill guide (a). The measuring device (c) applied over the guide pin allows determination of the length of the pin inserted in the bone (70 mm). B, The DCS triple reamer is assembled and set for the 65-mm drilling depth, which is 5 mm less than the pin length in the bone and ensures maintenance of the pin during DCS screw insertion. The triple reamer is placed over the guide pin (a) and the shaft hole for the DCS screw (b), the barrel hole for the plate (c), and the barrel–plate junction (d) are prepared. C, The DCS centering sleeve (c) is placed over the tap (b), which is subsequently placed over the guide pin (a). After inserting the centering sleeve into the barrel hole, the tap is advanced to the desired depth (65 mm). D, The DCS coupling screw (d) is placed through the T-handle and the DCS plate (e) is selected (12-hole) and connected to the DCS screw (b) of the desired length (60 mm). The centering sleeve (c) is applied over the coupling screw. After placing the assembly over the guide pin, the screw is inserted to a depth of 65 mm, which is marked on the centering sleeve as 5 mm. E, After tightening the screw and adjusting the horizontal bar of the T-handle parallel to the long axis of the bone, the DCS plate is seated over the shaft of the DCS screw with the help of the DCS impactor (a) and a mallet (not shown). Orientation of the instruments and implants is important, because the DCS screw (left insert) and the plate barrel (right insert) contain complementary parallel contours, which have to be aligned to allow sliding of the barrel over the screw shaft. F, The DCS compression screw is inserted through the plate barrel, inserted into the back end of the DCS screw, and tightened. This unites the three components (DCS screw, DCS plate, and connecting screw) into one unit. Insertion of all remaining screws and tightening of them completes the procedure.



CHAPTER 76  Principles of Fracture Treatment

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Figure 76-32.  Preoperative (A) and postoperative (B and C) radiographs of a metaphyseal of the third matacarpal fracture in an adult Icelandic pony. The fracture was repaired with a laterally applied DCS plate and a shorter dorsal DCP.

A

C

B

Figure 76-33.  Side view of PC-Fix. The arcs under the

90°

through the last hole in the aiming device with a drill sleeve, which is threaded into the plate. This establishes a solid frame, which maintains its angles during screw application and ensures that the screws are inserted orthogonal to the long axis of the plate so that they may be threaded into the plate holes. The remaining screws are inserted transcutaneously through stab incisions. An anatomic plate–bone interface is not important, because the screw heads interlock with the round, threaded plate holes, establishing a solid internal fixator. If the correct implant is selected, implantation of the system is efficient. This system is especially well suited to foals. A successful fixation has also been accomplished in a tibial fracture in a calf (see Figure 76-35, B and C).83 Locking compression plate The locking compression plate (LCP) is an implant system that combines the two treatment methods in one implant: compression plating and internal fixation.84 The LCP was developed to include the axial loading capabilities of the DCP and LC-DCP, the decreased plate–bone contact of the LC-DCP, and the

plate are easily recognized and lead to the points in contact with the bone. The unicortical screws are implanted perpendicular to the long axis of the plate, where the conical screw heads lock within the plate holes. This is an internal fixator. The plate is applied over the periosteum.

rigidity and stiffness of the LISS, where locking screws were first used.82,84 The goals were met by designing a combi-hole where either a standard screw or a locking screw can be inserted. It is not necessary to only apply locking head screws.85 An in vitro study comparing the application of two LCPs at right angles to each other with identical constructs using DCPs, LC-DCPs, and the clamp-rod internal fixators (CRIFs) in four-point bending showed that implanting two locking screws on either side of an oblique saw-cut across the artificial bone composite (Canevasit) provided significantly increased stiffness to the construct.86 Because the strength of a screw depends mainly on the core diameter and not thread width, the thicker core of the locking screws and the thin threads make the screw several times stronger than the conventional cortex screws.79 The LCP uses a combi-hole, which is a combination of a DCU and a LISS hole (see Figure 76-20). The surgeon may select the type of screw to be inserted at any given place—either a 4.5-mm or a 5.5-mm cortex screw (or even a cancellous screw) at an axial angle up to 40 degrees, or a 4.0-mm or 5.0-mm locking head screw with a thick core and thin threads and

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SECTION XII  MUSCULOSKELETAL SYSTEM

Figure 76-34.  Two PC-Fix plates are applied at right angles to each other in a comminuted and open third metacarpal fracture in a horse. Note the different configurations of the plates and the unicortical screws. With time, the empty spaces under the plate filled with bone.

A

additional threads at the screw head (Figure 76-36). The locking head screw, however, has to be inserted orthogonal to the long side of the plate. The original plate contained two beveled ends to allow insertion through a small incision and sliding of the plate along the periosteum of the fractured bone. Because the locking part of the combi-hole is positioned toward the center, insertion of a locking screw next to an articular surface left the bevelled tip extended over the joint. This led the veterinary specialists to modify the plate. Together with Synthes, the plate was redesigned with a pointed, bevelled tip at one end and a rounded edge at the other end. The plate has a slightly oval stacked hole, allowing insertion of either a locking head screw (the hole contains threads) or a cortex screw, the latter of which could be slightly angled away from the joint (Figure 76-37). A special tissue spreader has been developed to prepare the future plate bed. The thin beveled end separates the soft tissues from the periosteum. After the plate is contoured with the help of the intact contralateral bone, it can be fixed to the bone via a minimally invasive procedure. It is inserted through a small incision at one end of the bone, and the screws are then placed transcutaneously through stab incisions. By substituting cortex screws through some holes, costs can be significantly decreased without jeopardizing the stability and stiffness of the construct. Without application of the push-pull device (Figure 76-38) or standard cortex screws, both of which press the plate onto the surface of the bone, a gap of 2 mm will be present between the plate and the bone after its application. Therefore a basic decision has to be made at the onset of any LCP application regarding whether the plate has to be in close contact with the bone or not. In horses, it is desirable to have close contact between the bone and the plate to increase friction, which further stabilizes the construct. Once the fracture is anatomically reduced and stabilized by cortex screws placed in lag fashion, the first plate is positioned

B

C

Figure 76-35.  A, A less-invasive stabilization system (LISS) is inserted through a small proximal incision to repair a tibial fracture. B and C, Postoperative radiographs show the plate attached to the bone, but only the screws make contact with the bone. The fracture healed without problems.

with the plate holder, and the push-pull device is inserted at a slight angle through the DCU portion of the combi-hole (Figure 76-39, A). By turning the piston clockwise, the plate can be pressed onto the bone surface. At the same time, it temporarily fixes the plate to the bone. A second such device can also be applied through the stacked combi-hole on the other end of the plate, if desired. Next, all the strategic cortex screws are inserted and tightened to facilitate axial compression (if deemed necessary) and solid bone–plate contact at both ends of the bone



CHAPTER 76  Principles of Fracture Treatment

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Figure 76-38.  The push-pull device, which presses the LCP into close contact with the bone, resulting in increased stability of the construct. (Courtesy Synthes Vet, West Chester, PA.)

Figure 76-36.  A narrow LCP shown with a 4.5-mm cortex screw and hexagonal drive (right) as well as a 5.0-mm self-tapping locking screw with a stardrive (left). (Courtesy Synthes Vet, West Chester, PA.)

Figure 76-37.  A 10-hole veterinary LCP. The plate has one rounded end containing a stacked combi-hole through which either a locking head or a cortex screw can be inserted. The other end has a tapered and pointed tip that facilitates minimally invasive plate insertion. Note that the DCU parts of the combi-holes are arranged on both sides of the center of the plate toward the ends, whereas the threaded parts of the combi-holes point toward the center. (Courtesy Synthes Vet, West Chester, PA.)

(see Figure 76-39, B). After removal of the push-pull device and the plate holder, the second plate is applied at a 90-degree angle to the long axis of the first plate, using the same technique (see Figure 76-39, C). Next the locations where the locking screws will be implanted are selected to avoid contact with interfragmentary screws and the cortex screws of the other plate. Planning how screws are to be inserted is very important, even more so when LCPs are applied, because the locking screws must be inserted perpendicular to the plate. Screw position is different if a locking screw or a cortex screw is used through a combihole, which represents additional complexity when planning the surgery. Further complications arise if an original DCS containing mainly DCP holes and an LCP are combined, because the hole lengths and their spacing along the two plates differs. As mentioned before, the DCS is now available with combiholes. Once locking screws are applied, the plate is solidly fixed in its position. The LCP drill guide is carefully twisted into the threaded part of the combi-hole at a right angle to the long axis of the plate. To facilitate perpendicular insertion and subsequent solid engagement of the threads in the plate hole, the drill guide is placed onto the combi-hole and then twisted backward until a click is heard, which happens when the drill guide slips from the upper thread onto the one just below it. Then the drill guide is twisted forward to engage the threads of the combi-hole. When the drill guide is solidly seated, its position relative to the plate is evaluated once again, ensuring its perpendicular orientation. All the LCP drill guides provided in the set can be fixed

to the plates to speed up the procedure. Once their position perpendicular to the long axis of the plate and parallel to each other is confirmed, all the holes are drilled (see Figure 76-39, D). The drill guides are removed, and the screw lengths are determined using the depth gauge. The 4-N torque-limiting device is attached to the power drill followed by the insertion of the power attachment of the stardrive. By pressing the screwdriver into the stardrive indentation of the LCP screw in the rack, the correct screw is selected and inserted into the predrilled hole using a power-tapping technique. The screw is fully inserted until the torque-limiting device is idling in the plate hole, meaning that the 4-N insertion force has been reached. This precautionary step was initially introduced in human surgery to prevent cold welding between the titanium plate and screws. Because stainless steel is predominantly used in equine surgery, the danger of cold welding is circumvented. Nevertheless the use of the torque-limiting device is encouraged. Equine bone is hard, and if long screws are inserted, the 4 N threshold may be reached before the screw head threads are completely engaged in the plate. It is therefore prudent and good technique to complete final tightening with the hand screwdriver (see Figure 76-39, E). Once all locking screws in the first plate are implanted, the same procedure is repeated in the second plate (see Figure 76-39, F). Any empty plate holes in both plates are filled by applying a cortex screw through the DCU portion of the combihole at the angle necessary to prevent contact with screws in the other plate. Because the threads of the screw head and the threads in the plate are solidly intertwined, they form a unit. This prevents the screw head from moving within the plate hole if certain strains are applied, which significantly increases the stiffness of the construct. All the locking screws applied to a plate feel very solid, because the threads in the screw head engage the corresponding threads in the plate. This does not mean that the screw is solidly inserted in the bone underneath the plate. This is a new experience for the surgeon and must always be kept in mind. Under certain circumstances, it may be difficult to twist the drill guide perpendicularly into the threaded portion of the combi-hole, because major muscle bellies may be in the way. In these occasions, separate stab incisions through the muscle bellies are necessary to access the combi-hole at a right angle. The drill guides can be extended by threading one on top of the other, and in doing so, attain correct engagement in the plate and correct position for drilling. The LCP has, in a short period of time, established itself as the preferred plate of equine fracture fixation despite its higher costs, mainly caused by the screws.87 A recent study comparing 4.5-mm LCPs with 4.5-mm LC-DCPs confirmed the superior strength and stiffness of the LCP.88

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SECTION XII  MUSCULOSKELETAL SYSTEM

A

B

D

C

E

Figure 76-39.  Application of two LCPs to an oblique midshaft third metacarpal fracture. A, The fracture is reduced and stabilized by the two 3.5-mm cortex screws applied in lag fashion. A 10-hole broad veterinary LCP is applied to the bone with the plate holder and temporarily fixed in place with the push-pull device. By turning the piston clockwise (arrow), the plate is pressed onto the bone surface. B, To facilitate good plate–bone contact along the entire plate, cortex screws are implanted and tightened using plate screw technique at both ends and in the center near the fracture. Once in place, the lateral 11-hole narrow veterinary LCP is applied to the bone using the same technique. Note that the plate can be applied farther distad on the lateral aspect of the bone than on the dorsal aspect. C, Next the holes where locking screws are to be inserted are selected and the drill guide is twisted into the threaded portion of the combi-hole. Because the plate is solidly fixed to the bone, all four drill guides provided in the set are applied, followed by drilling all four holes. D, The locking head screws are inserted and tightened. The four drill sleeves for the locking head screws are then placed into selected plate holes, making sure that screws can be placed perpendicularly without interfering with previously inserted implants. E, All the remaining plate holes are filled with cortex screws inserted using the plate screw technique. Where indicated, lag technique is applied to increase interfragmentary compression.

Because of the rapidly increasing popularity of locking plates, a 5.5-mm LCP especially designed for equine fracture repair has been developed. Again, this plate was tested in an in vitro study against the 4.5-mm LCP.89 The 5.5-mm LCP was superior in resisting static overload in palmarodorsal four-point bending and cyclic fatigue testing. These results were superior to those achieved with the 5.5-mm LC-DCP. Taken together, these findings have established the 5.5-mm LCP as the premier equine plate for long bone fracture fixation and arthrodesis of the metacarpo- or metatarsophalangeal joint.

The human femoral LCP is ideal for lateral application to the equine radius. This bone has a slight craniocaudal curvature when viewd from the side. It is therefore not possible to apply a straight plate to its lateral aspect and span the entire bone. Either the middle holes are behind the bone or the proximal holes are in front of the bone. The human femoral LCP has a slight bend that matches the equine radius perfectly. The ideal combination is a 5.5-mm equine LCP applied cranially and a human femoral LCP applied laterally (Figure 76-40). The implants are available in stainless steel in all sizes (see Table 76-2).



CHAPTER 76  Principles of Fracture Treatment

A

1075

B

Figure 76-40.  Preoperative (A) and postoperative (B) radiographs of an oblique, spiral radial fracture. The fracture was repaired with a 17-hole 5.5 mm veterinary LCP cranially and a 17-hole 4.5-mm human femoral LCP (a slightly axially bent plate) laterally.

Figure 76-41.  A, The top portion of a variable angle LCP (VA-LCP) demonstrating the angles a screw can be inserted through the holes. B, Close-up side view of the screw head, depicting its threads on the rounded head and the stardrive design for the screw driver. (© by Synthes Inc, West Chester, PA.)

A

B

Because of the disadvantages posed by the need for locking head screws to be inserted perpendicular to the long axis of the plate, attempts were made to develop a locking mechanism that allows the insertion of locking head screws at various angles. Thus the variable angle LCP (VA-LCP) was created. Screws can be angled anywhere within a 30-degree cone around the central axis of the plate hole (Figure 76-41, A). The plate hole has a cloverleaf shape (see Figure 76-41, B) containing four threaded ridges, where the specially constructed screw head can interlock with the plate. The head of the variable angle locking screw is rounded to facilitate various angles within the locking hole (see Figure 76-41, C). A special double drill guide has been

developed that facilitates drilling of the variable angle screw holes on one side and fixed angle drilling on the other (Figure 76-42). The nozzle of the drill guide inserts coaxially into the central hole. In contrast to standard drill guides, which support the drill bit along its entire length, the funnel shape of the angle screw hole guide allows the drill bit to glide along the guide wall at a 30-degree angle in any direction selected (Figure 76-43). The first application of these holes was implemented in the 2.4-mm Variable Angle LCP Distal Radius System (Synthes Inc.) for humans. The plate is anatomically contoured to the volar aspect of the distal radius and is designated to address both simple and complicated fractures. If this plate hole design

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SECTION XII  MUSCULOSKELETAL SYSTEM

Figure 76-42.  Double drill guide: left for fixed-angle drilling, right for off-axis drilling. (© by Synthes Inc, West Chester, PA.)

Figure 76-43.  The off-axis drill guide is coaxially inserted into a plate hole. With the drill bit leaning onto the side-wall of the drill guide, total angles of 30 degrees can be achieved through the same plate hole in all directions. (© by Synthes Inc, West Chester, PA.)

is considered an improvement in human surgery, it will hopefully be applied to other locking plates. The LCP is the ideal plate for the veterinary surgeon. It can be applied in the same manner as a DCP with only cortex screws. Because the plate possesses the same type of undercuts as the LC-DCP, it can be used as such. In other words, the LCP fulfills all the desired demands and by purchasing only LCPs the veterinarian can reduce the armamentarium considerably. The price of the LCP is slightly higher than the DCP and LC-DCP and therefore costs are not a problem. So all the veterinary surgeon needs to decide is whether to apply cortex screws, locking head screws, or a combination thereof. Addendum Thorough knowledge of the basic principles of internal fixation and familiarity with the instruments and numerous implants are prerequisites for successful surgery. Everyone interested in treating long bone fractures should complete a basic and advanced AOVET course on internal fixation. CERCLAGE WIRE Cerclage wire is used frequently in humans and small animals.90,91 Tension band fixations with pins and wire are often carried out in dogs and cats. Cerclage wires are also applied around oblique long bone fractures in small animals. Also, this type of fixation has been used with small Steinmann pins to temporarily stabilize comminuted long bone fractures before

plate and lag screw application.3 Application of cerclage wire in equine long bone factures has not been successful because of a lack of stability and breakdown of the fixation. However, cerclage wires may be applied in a few situations. Proximal sesamoid fractures have been successfully treated with cerclage wires,92 but this technique was recently abandoned because of unsatisfactory long-term results. One frequent application of cerclage wire is growth retardation surgery, even though the wire is not applied in cerclage fashion (see Chapter 86). Cerclage wires have been used to treat nondisplaced ulnar fractures in foals.93,94 The wires are passed through holes placed in the frontal plane across the proximal and distal fragments of the ulna. A small loop created before entering the hole in the distal fragment allows even tightening on both sides of the bone after the wire is applied in figure-ofeight fashion, and the ends are twisted together. This type of fixation results in reduced trauma to the ulna and radius, and it prevents inadvertent fixation of the ulna to the radius, which can occur when using a plate in a young foal. (This may induce subluxation of the cubital joint.) A total of three or four figureof-eight wires are applied. Cerclage wire is used in arthrodesis of the fetlock joint.95 In comminuted fractures of the proximal sesamoid bones associated with complete breakdown of the suspensory apparatus, a tension band is inserted in figure-of-eight fashion through the palmar/plantar aspect of the metacarpophalangeal joint, before applying the dorsal plate (see Chapter 81).95 One frequent application of cerclage wires is their use as a tension band to manage mandibular and maxillar fractures (see Chapter 102). Wire tightening is carried out with utmost prudence. Initially, the wires are loosely twisted by hand. Then, with a pair of flat pliers, the two ends are grabbed, pulled at a right angle away from the bone, and, while decreasing the pulling force, evenly twisted around each other. Care must be taken not to twist one wire end around the straight end of the other wire (this type of fixation slips off under tension). Overtightening may result in wire breakage and breakdown of the fixation. CABLES Two types of cables have been introduced into orthopedic surgery as treatment modality for specific fractures. Ultrahighmolecular-weight polyethylene (UHMWPE) cable has been tested in an in vitro model for the repair of proximal sesamoid bone fractures and compared with the commonly used stainless steel cerclage wire (SSCW).96 The ultimate tensile strength of UHMWPE cable constructs was 34% greater than that of SSCW constructs. Fatigue strength was 2 to 20 times greater for UHMWPE cable constructs than for SSCW constructs. Separation of fragments was 153% less for limbs repaired by the cable construct compared with those repaired by the transfixed cerclage technique.96 These cables may also be beneficial in the use of fetlock breakdown injuries as a palmar figure-of-eight tension band. Another type of cable consists of multiple woven stainless steel, titanium alloy, or cobalt chromium alloy strands. It is available as 1.0- and 1.7-mm-diameter cables, consisting of a central bundle of 19 strands surrounded by eight outer bundles of seven strands each. It is designed to be used with all Synthes stainless steel and titanium plates. Specially designed positioning pins are used in empty plate holes, and the cable is threaded through an oblong hole in the pin. Once the cable is tightened,



CHAPTER 76  Principles of Fracture Treatment

the pin cannot move because it is pressed into the plate hole and therefore confined. A special tensioning device is used to tighten the cable before it is crimped with a cable crimper. Care should be taken to not exceed 50 kg of tension. Applying tension at levels higher than 50 kg may cause the cable to cut through soft or osteopenic bone (which is not a problem in horses). This product is used mainly in human orthopedic surgery in the management of periprosthetic fractures in elderly adults, where other internal fixation devices are not successful.97 Additionally, it is used as a tension band in the management of patellar fractures and olecranon osteotomies. Indications for these cables in horses are similar to those for the UHMWPE cables. Recently these implants were adopted for mandibular fractures (see Chapter 102). PINS Steinmann pins Steinmann pins are rarely used in fracture treatment in horses, mainly because they do not provide stability.97 They are used as transfixation pins in conjunction with external coaptation (see earlier discussion). Intramedullary application of Steinmann pins is used only in humeral fractures in foals.98 Multiple pins are introduced parallel to each other to fill the entire medullary space at its isthmus at the distal third of the bone. This "stacked-pin" method is presently the treatment of choice in these fractures. For additional stability, these pins may be encircled by cerclage wires and placed into the medullary cavity, possibly through a cortical defect or a drill hole. The wires are subsequently tightened. In cases with a cortical defect, the cerclage may also be applied intramedullarly. The advantage of the stacked-pin method is an increase in rotational and bending stability. A single pin provides no rotational stability. Collapse of the fracture along the single pin is a frequent complication. Application of multiple pins across a capital femoral physeal fracture has been advocated and has met with some success. Steinmann pins have also been used in the treatment of olecranon fractures in the very young foal in combination with a tension band made of multiple cerclage wires.96 Rush pins The Rush pin method of fracture treatment was popular before bone plating was introduced. Fracture fixation using these devices is an art.99 The slightly prebent pins are introduced obliquely into the distal fragment and advanced toward the opposite cortex. The tip, which is flattened on one side, slides off the opposite cortex and is redirected toward the cis-cortex. The pin length has to allow the tips to engage in the cis-cortex both proximally and distally to the fracture, providing fourpoint contact. Usually two pins are introduced, one from each side of the bone. When performed correctly, rotational stability is achieved with a minimum of implants and surgical trauma. The Rush pin fixation technique is not applicable to comminuted or open fractures. NAILS Intramedullary nails have a place in equine long bone fracture repair, but the ideal implant has not been developed despite recent efforts.10,43 Initially, intramedullary nails manufactured for human application were tried in equine fractures with mixed success.100 For example, in two reports, a solid titanium nail was inserted through the middle carpal joint after removing the

1077

articular cartilage of the middle carpal and carpometacarpal joints.101,102 The joints were fused to facilitate solid fixation. Transfixation was achieved with 4.5-mm screws inserted through the proximal aspect of the nail. Although good results were achieved, the fact that the joints had to be fused to facilitate healing of MCIII was undesirable. These fractures can heal with plating techniques without fusing a joint. A system of intramedullary interlocking nails (IINs) has been developed for equine humeral and femoral fractures (Figure 76-44).103-106 Comparison of this method of fracture repair to fracture plating has met with mixed results in experimental studies. A cadaveric in vitro biomechanical study on immature equine femurs revealed that a diaphyseal osteotomy fixed with two DCPs at 90 degrees to each other provided superior strength and stiffness compared with an IIN and a construct of an IIN and cranially applied DCP.104 In another in vitro study on osteotomized tibias, a construct composed of a 16-mm stainless steel nail with a wall thickness of 4 mm and four 8-mm interlocking screws was compared with a human unreamed femoral interlocking nail (UFN) and to tibias treated by means of double plating.107 The interlocking nails achieved similar loads until failure, but the plates demonstrated higher yield loads. In vitro cadaveric studies tested several interlocking nail configurations in MCIII and femurs in foals and in MCIII with and without a gap in adult horses.108-110 All constructs were weaker than the intact bone, and the parallel alignment of the holes for the interlocking screws were stronger than the offset screws.109 One study compared IIN with a combination of an IIN and a DCP and with two DCPs in a 1-cm gap model in foals, showing the double-plating construct to be closest to the intact bone in most aspects, followed by the combination and the IIN alone.107 Several application principles have been advocated in foals for the presently used equine IIN.111 The location and configuration of the fracture significantly affects the ultimate outcome of the repair. If possible, three interlocking screws should be inserted on either side of the fracture. Fractures near the epiphysis are less readily stabilized with an IIN, and the epiphyseal segment is at an increased risk for secondary fracture through the interlocking screw holes. In these instances, some type of supplemental fixation is desirable to decrease the potential of catastrophic failure of the fixation. In long oblique fractures, the IIN should be positioned to allow one or two interlocking screws placed in lag fashion across the fracture plane, if possible. In a nail–plate construct, the plate is applied 90 degrees to the interlocking screws. Whenever possible, bicortical screws are inserted through the plate. A lateral approach is used to expose the fractured humerus and femur. Before reduction, the fracture is débrided and the medullary cavity of the distal fragment is reamed. Reaming is accomplished with rigid reamers of increasing size to arrive at an ultimate hole diameter of 13 mm. This procedure destroys the intramedullary blood supply and slows healing. Unreamed intramedullary interlocking nails have not been successfully applied in living horses. Additional exposure is usually necessary to provide access to the proximal end of the bone for normograde reaming of the proximal segment. After both fragments are reamed, the fracture is reduced and held in that position with bone clamps. A nail of appropriate length is chosen, and with the targeting jig attached, the nail is inserted into the reamed medullary canal. Washers are used to prevent the conical heads of the cortex screws from penetrating the cortical bone.

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SECTION XII  MUSCULOSKELETAL SYSTEM out of six older foals with short oblique femur fractures also were successfully treated in this manner. In these foals, a DCP was applied to the cranial aspect of the bone in addition to an IIN.

A

B

C

Figure 76-44.  This long oblique humeral fracture (A) was treated with an intramedullary interlocking nail. To provide a greater screw–bone contact area, washers were used. The craniocaudal (B) and lateromedial (C) 2-month follow-up radiographs show progressive bone healing in the fracture gap. Bone length is maintained. (Courtesy J. P. Watkins, Texas A&M University.)

A successful result was achieved with an IIN in 5 of 10 foals with humerus fractures; the foals ranged in weight from 136 to 204 kg. They attained athletic soundness and performed their intended purpose without complications. Three out of three neonates with femoral fractures healed after IIN fixation. Four

AFTERCARE After fracture fixation with any of the internal fixation devices and techniques described here, overlying soft tissues and skin are closed in routine fashion. The use of of continuous suture patterns is advocated to reduce surgery time. Depending on the type of fixation and the technique of recovery from anesthesia, application of some type of external coaptation may be considered, because the animal has to be able to get up and place weight on the limb immediately after surgery. If a pool recovery is implemented, the limb and skin incision is in most cases protected only by a plastic adhesive sheet (Ioban 2) after applying cyanoacrylate superglue to the skin incision. This sheet is covered with elastic adhesive tape (Elasticon), which is exchanged for a regular bandage or splint bandage after successful recovery from anesthesia (see Chapter 21). External coaptation may consist of a fiberglass cast or a heavy splint bandage (see Chapter 17). Depending on the type of fixation, external coaptation is maintained for a few days to weeks. A fiberglass cast can be applied over a bandaged limb, and after the cast has cured, it can be split in half along the frontal plane. These "half shells" are reapplied using nonelastic adhesive tape. Such coaptation allows evaluation and, if necessary, wound management of the limb underneath. The unaffected contralateral limb should be protected and supported by a pressure bandage. In young foals, too much support is to be avoided to prevent temporary weakness of the flexor tendons. It is important to keep the patient comfortable with the help of moderate amounts of anti-inflammatory and analgesic drugs. These drugs should be used judiciously to prevent toxic reactions and to allow some residual amount of pain, so the patient will protect the injured limb. If a non–weight-bearing lameness lasts for a prolonged period, other problems develop in the healthy limbs, especially in foals. Therefore close observation of the patient is important until the animal starts to increase weight-bearing on the fractured limb. Application of a frog pad in adult horses may prevent the development of laminitis in the contralateral foot. Aside from postoperative infection (see Chapter 85), laminitis is the major complication that occurs after fracture repair. Again, administration of anti-inflammatory drugs in moderate amounts may prevent this problem. As early as possible, the animals should be allowed to walk. Although controlled exercise is advocated, free pasture exercise is discouraged. This is especially important if a weight-bearing fixation was performed and the animal is not placed in an external coaptation device. Patients with casts should not be exercised at all. Implant Removal Implants are foreign bodies and may have to be removed, particularly in young athletes. Therefore, implant removal depends on the use of the horse, the type of fracture treated, the type of implants employed, and the potential development of complications, including postoperative infection. Lag screws are not removed in horses unless they produce pain or bone reactions. The practice of removing lag screws in young racehorses has gained popularity. Frequently the reason for implant removal is



CHAPTER 76  Principles of Fracture Treatment

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MTIII, screws may be removed about 2 months postoperatively. Plate removal in foals may be carried out at an average of 4 to 6 months after fracture treatment. Staggered removal of the implants is advocated if two plates were applied, because it reduces the risk of refracturing the bone through one of the screw holes. (Filling the empty screw holes in the bone with a bone graft has been recommended in humans.79) An important reason for implant removal is infection surrounding the implants. An infection, once established around implants, persists and does not resolve until after the implants have been removed, even in the presence of broad-spectrum antimicrobials.38 Fractures can heal in the presence of infection if rigid internal fixation is maintained. However, it is frequently a race between loosening of the implants caused by the infection, and healing of the fracture. Once an infected fracture is healed, the implants are removed to allow resolution of the infection. If the implants are removed too early, before adequate healing of the fracture has occurred, refracture of the bone is likely. In one case, however, where titanium implants were applied to a comminuted open fracture of the proximal MTIII in a Thoroughbred foal, a postoperative infection resolved completely before implant removal (Figure 76-45).

A

REFERENCES

B

C

Figure 76-45.  A, A 2-month-old Thoroughbred colt was admitted with a comminuted fracture of the third metatarsal. B, The fracture was treated with two PC-Fix plates applied over the periosteum, which prevented adequate visualization and resulted in suboptimal fracture reduction. An infection developed, which was managed with parenteral broad-spectrum antibiotics, daily flushing of the surgical site, and local antibiotic application. Within a month, the infection had resolved. Bacterial cultures taken at the time of implant removal revealed no growth. C, The healed fracture after implant removal. The foal developed into a successful racehorse.

a request by the owner. Cerclage wire used for fracture fixation does not need to be removed unless the wire breaks, as it does frequently in the treatment of transverse fractures of the proximal sesamoid bones.92 Plates applied to long bones of horses should be removed in most cases. This is especially important in foals and if the animal enters or resumes an athletic career. Implants applied to the femur or olecranon and for arthrodeses purposes are left in place. This presupposes that no problems are encountered with the implants. The time to remove implants after the fracture has healed depends on the age of the animal, the type of fracture treated, and the implants used. In a condylar or stress fracture of MCIII/

1. Rüedi TP, Buckley RE, Moran CG: AO Philosophy and Evolution. p. 1. In Rüedi TP, Buckley RE, Moran CG (eds): AO Principles of Fracture Management. 2nd Expanded Ed. Thieme Verlag, Stuttgart, Germany, 2007 2. Houlton JEF, Dunning D: Perioperative Patient Management. p. 1. In Johnson AL, Houlton JEF, Vannini R (eds): AO Principles of Fracture Management in the Dog and Cat. Thieme Verlag, Stuttgart, Germany, 2005 3. Nunamaker DM: Basic Principles of Fracture Treatment. p. 5. In Fackelman GE, Auer JA, Nunamaker DM: AO Principles of Equine Osteosynthesis. Thieme Verlag, Stuttgart, Germany, 2000 4. Auer JA: Principleas of Fracture Treatment. p. 1000. In Auer JA, Stick JA (eds): Equine Surgery. 3rd Ed. Saunders Elsevier, St. Louis, 2006 5. Fürst AE, Keller R, Kummer M, et al: Evaluation of a new full-body animal rescue and transportation sling in horses: 181 horses (19982006). J Vet Emerg Crit Care 18:619, 2008 6. Hierzolzer G, Allgöwer M, Rüedi TH: Fixateur Externe-Osteosynthese. Springer-Verlag, Berlin, 1985 7. Schenk RK, Wilenegger H: Zum histologischen Bild der sogenannten Primärheilung der Knochenkompakta nach experimentellen Osteotomien am Hund. Experientia 19:593, 1963 8. Egger EL: Static strength evaluation of six external skeletal fixation configurations. Vet Surg 12:130, 1983 9. Nemeth F, Back W: The use of the walking cast to repair fractures in horses and ponies. Equine Vet J 23:32, 1991 10. Nixon AJ, Watkins JP, Auer JA: Principles of Fracture Treatment. In Nixon AJ (ed): Equine Fracture Repair. Blackwell, Wiley, Ames, IA In press 11. Taylor DS, Stover SM, Willits N: The effect of differing pin size on the mechanical performance of transfixation in the equine third metacarpal bone. Proc Vet Orthop Soc Lake Louise, Canada 20:2, 1993 12. McClure SR, Watkins JP, Bronson DG, et al: In vitro comparison of the effect of parallel and divergent transfixation pins on breaking strength of equine third metatarsal bones. J Am Vet Res 55:1327, 1994 13. McClure SR, Watkins JP, Hogan HA: In vitro evaluation of four methods of attaching transfixation pins into a fiberglass cast for use in horses. Am J Vet Res 57:1098, 1996 14. McClure SR, Watkins JP, Bronson DG, et al: In vitro comparison of the standard sort limb cast and three configurations of short limb transfixation casts in equine fore limbs. Am J Vet Res 55:1331, 1994 15. McClure SR, Hillberry BM, Fisher KE: In vitro comparison of metaphyseal and diaphyseal placement of centrally threaded, positive-profile transfixation pins in the equine third metacarpal bone. Am J Vet Res 61:1304, 2000 16. Nunamaker DM, Nash RA: A tapered-sleeve pin external skeletal fixation device for use in horses: Development, application, and experience. Vet Surg 37:752, 2008

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SECTION XII  MUSCULOSKELETAL SYSTEM

17. Elce YA, Southwood LL, Nutt JN, et al: Ex vivo comparison of a novel tapered-sleeve and traditional full-limb transfixation pin cast for distal radial fracture stabilization in the horse. Vet Comp Orthop Traumatol 19:93, 2006 18. Brianza S, Brighenti V, Bouré L, et al: In vitro mechanical evaluation of a novel pin-sleeve system for external fixation of distal limb fracture in horses: A proof of concept study. Vet Surg 39:601, 2010 19. Brianza S, Vogel S, Rothstock S, et al: Comparative biomechanical evaluation of a pin-sleeve transfixation system in calf cadaver metacarpal bones. Vet Surg 40:In print, 2011 20. Matter G: Mechanische Untersuchung verschiedener Fixateur externe Konfigurationen am Röhrenbein des Pferdes: Eine in vitro Studie Thesis, Zurich, Switzerland, 1993, University of Zurich 21. Frigg R: Development of new products at the AO ASIF Development Institute. Proc Am Coll Vet Surg, Large Animal Vet Symp, Chicago, 5:229, 1995 22. Lischer CJ, Fluri E, Kaser-Hotz B, et al: Pinless external fixation of mandible fractures in cattle. Vet Surg 26:14, 1997 23. Frigg R: The development of the pinless external fixator: From the idea to the implant—Introduction. Injury 23:S3, 1992 24. Ferretti A: The Application of the Ilizarov Technique to Veterinary Medicine. p. 551. In Bianchi-Maiocchi A, Aronson J (eds): Operative Principles of Ilizarov. Medi Surgical Vido, Milan, Italy, 1991 25. Marcellin-Little DJ, Ferretti A, Roe SC, et al: Hinged Ilizarov external fixation for correction of antebrachial deformities. Vet Surg 27:231, 1998 26. Lewis DD, Bronson DG, Samchukov ML, et al: Biomechanics of circular external skeletal fixation. Vet Surg 27:454, 1998 27. Aithal HP, Kinjavdekar P, Amarpal, et al: Management of tibial fractures using a circular external fixator in two calves. Vet Surg 39:621, 2010 28. Nunamaker DM, Richardson DW, Butterweck DM, et al: A new external skeletal fixation device that allows immediate weight bearing: Application in the horse. Vet Surg 15:345, 1986 29. Richardson DW, Nunamaker DM, Sigafoos RD: Use of an external skeletal fixation device and bone graft for arthrodesis of the metacarpophalangeal joint in horses. J Am Vet Med Assoc 191:316, 1987 30. Nunamaker DM, Richardson DW: External skeletal fixation in the horse. Proc Am Assoc Equine Pract 37:549, 1992 31. Huiskes R, Chao EYS, Crippen TE: Parametric analysis of pin-bone stresses in external fracture fixation devices. J Orthop Res 3:341, 1985 32. Nash RA, Nunamaker DM, Boston R: Evaluation of a tapered-sleeve transcortical pin to reduce stress at the bone-pin interface in metacarpal bones obtained from horses. Am J Vet Res 62:955, 2001 33. Nutt JN, Southwood LL, Elce YA, et al: In vitro comparison of a novel external fixator and traditional full-limb transfixation cast in horses. Vet Surg 39:594, 2010 34. Bignozzi L, Gnudi M, Masetti L, et al: Half pin fixation in 2 cases of equine long bone fractures. Equine Vet J 13: 64, 1981 35. Sullins KE, McIlwraith CW: Evaluation of 2 types of external skeletal fixation for repair of experimental tibia fractures in foals. Vet Surg 16:255, 1987 36. Rahn B: Personal communication, 1989. 37. Lambotte A: Chirurgie Opératoire des Fractures. Masson, Paris, 1913 38. Danis R: Théorie et Pratique de l’Ostéosynthèse. Masson, Paris, 1947 39. Bramlage LR: Long bone fractures. Vet Clin North Am Large Anim Pract 5:285, 1983 40. Auer JA: Surgical Equipment and Implants for Fracture Repair. In Nixon AJ (ed): Equine Fracture Repair. Blackwell, Wiley, Ames, IA, In press 41. Farouk O, Krettek C, Miclau T, et al: Minimally invasive plate osteosynthesis and vascularity: Preliminary results of a cadaver injection study. Injury 28(Suppl):S1, 1997 42. Rahm C, Ito K, Auer J: Lagscrew fixation of equine cannon bone fractures: A biomechanical comparative study of shaft and shaftless cortical screws. Vet Surg 29;564, 2000 43. Auer JA, Watkins JP: Instrumentation and techniques in equine fracture fixation. Vet Clin North Am Equine Pract 12:283, 1996 44. Fackelman GE, Auer JA: The AO/ASIF 7.3 mm cannulated screw in a model of the equine distal limb. Equine Pract 18:15, 1996 45. Johnson NL, Galuppo LD, Stover SM, et al: An in vitro biomechanical comparison of the insertion variables and pullout mechanical properties of AO 6.5-mm standard cancellous and 7.3-mm self-tapping, cannulated bone screws in foal femoral bone. Vet Surg 33:691, 2004 46. Baumgart FW, Cordey J, Morikawa K, et al: AO/ASIF selftapping screws (STS). Injury Suppl 24:S1, 1995 47. Schnewlin M, Auer JA: The AO/ASIF self-tapping 4.5 mm screws (STS). Proc Ann Symp Am Coll Vet Surg 5:231, 1995 48. Andrea CR, Stover SM, Galuppo LD, et al: Comparison of insertion time and pullout strength between self-tapping and nonself-tapping

AO 4.5-mm cortical bone screws in adult equine third metacarpal bone. Vet Surg 31:189, 2002 49. Frigg R, Appenzeller A, Christensen R, et al: The development of the distal femur less invasive stabilization system (LISS). Injury 32:SC24, 2001 50. Fankhauser C, Frenk A, Marti A: A comparative biomechanical evaluation of three systems for internal fixation of distal femur fractures. Orthop Res Soc Poster Presentation, 1999 51. Herthel DJ, Moody JL, Lauper I: The repair of condylar fractures of the third metacarpal bone and the third metatarsal bone using the Herbert compression screw in nine Thoroughbred racehorses. Equine Pract 19:6, 1995 52. Galuppo LD, Stover SM, Jensen DG, et al: A biomechanical comparison of headless tapered variable pitch and AO cortical bone screws for fixation of simulated lateral condylar fractures in equine third metacarpal bones. Vet Surg 30:332, 2001 53. Galuppo LD, Stover SM, Jensen DG: A biomechanical comparison of equine third metacarpal condylar bone fragment compression and screw pushout strength between headless tapered variable pitch and AO cortical bone screws. Vet Surg 31:201, 2002 54. Eddy AL, Galuppo LD, Stover SM, et al: A Biomechanical comparison of headless tapered variable pitch compression and AO cortical bone screws for fixation of a simulated midbody transverse fracture of the proximal sesamoid bone in horses. Vet Surg 33:253, 2004 55. Lewis AJ, Sod GA, Burba DJ, et al: Compressive forces achieved in simulated equine third metacarpal bone lateral condylar fractures of varying fragment thickness with Acutrack Plus screws and 4.5 mm AO cortical screws. Vet Surg 39:87, 2010 56. Yovich JV, Turner AS, Smith FW: Holding power of orthopedic screws in equine third metacarpal and metatarsal bones: Part I. Foal bone. Vet Surg 14:221, 1985 57. Yovich JV, Turner AS, Smith FW: Holding power of orthopedic screws in equine third metacarpal and metatarsal bones: Part 2. Foal bone. Vet Surg 14:230, 1985 58. Yovich JV, Turner AS, Smith RX, et al: Holding power of orthopedic screws: Comparison of self-tapped and pre-tapped screws in foal bone. Vet Surg 15:55, 1986 59. Pegel B: Der Einfluss verschiedener Konfigurationen von Osteosynthesen mit der LCP (Locking Compression Plate) auf die Stabilität und Knochenheilung nach Schrägosteotomien des Tibiaschaftes bei Schafen. Dissertation Vetsuisse Faculty, University of Zurich, Switzerland, 2011 60. Lagerpush N: Die Dynamisierung der winkelstabilen Plattenosteosynthese mit Hilfe der "Dynamic Locking Screw" (DLS)—Eine experimentelle Studie. Dissertation Vetsuisse Faculty, University of Zurich, Switzerland, 2010 61. Olmstead ML, Schenk R, Pohler O, et al: Bone screw holding power: The effect of surface character and metal type. Vet Surg 15:128, 1986 62. Nunamaker DM, Perren SM: Force measurements in screw fixation. J Biomech 9:669, 1976 63. Danis R: The operative treatment of bone fractures. J Int Chir 7:318, 1947 64. Bagby GW, Janes JM: An impacting bone plate. Mayo Clinic Proc 32:55, 1957 65. Perren SM, Allgöwer M, Brunner H, et al: Das Konzept der biologischen Osteosynthese unter Anwendung der Dynamischen Kopmressionsplatte mit limitiertem Kontakt (LC-DCP). Injury Suppl 22:1, 1991 66. Allgöwer M, Matter P, Perren SM, Rüedi T: The dynamic compression plate DCP. Springer-Verlag, Berlin, 1973 67. Monney G, Cordey J, Rahn B: Untersuchungen über die Blutzufuhr nach der Plattenosteosynthese mit DCP und LC-DCP. Injury Suppl 22:18, 1991 68. Sod GA, Hubert JD, Martin GS, et al: An in vitro biomechanical comparison of a limited-contact dynamic compression plate fixation with a dynamic compression plate fixation of osteotomized equine third metacarpal bones. Vet Surg 34:579, 2005 69. Sod GA, Mitchell CF, Hubert JD, et al: An in vitro biomechanical comparison of a 5.5 mm limited-contact dynamic compression plate fixation with a 4.5 mm limited-contact dynamic compression plate fixation of osteotomized equine third metacarpal bones. Vet Surg 37:289, 2008 70. Nunamaker DM, Bowmann KF, Richardson DW, et al: Plate luting: A preliminary report on its use in horses. Vet Surg 15:289, 1986 71. Turner AS, Cordey JR, Nunamaker DM, et al: In vivo strain patterns of the intact equine metacarpus and metatarsus following plate luting. Vet Comp Orthop Traumatol 3:84, 1990 72. Turner AS, Smith FW, Nunamaker DM, et al: Improved plate fixation of unstable fractures due to bone cement around the screw heads. Vet Surg 20:349, 1991

73. Auer JA, Watkins JP: Treatment of radial fractures in adult horses: An analysis of 15 cases. Equine Vet J 19:103, 1987 74. Sanders-Shamis M, Bramlage LR: Radius fractures in the horse: A retrospective study of 47 cases. Equine Vet J 18:432, 1986 75. Regazzoni P, Rüedi T, Allgöwer M: The Dynamic Hip Screw Implant System. Springer-Verlag, Berlin, 1987 76. Auer JA: Application of the dynamic condylar screw (DCS)-dynamic hip screw (DHS) implant system in the horse. Vet Comp Orthop Traumatol 1:18, 1988 77. Hunt DA, Snyder JR, Morgan JP, et al: Femoral capital physeal fractures in 25 foals. Vet Surg 19:41, 1990 78. Miclau T, Martin RE: The evolution of modern plate osteosynthesis. Injury 28(Suppl 1):A3, 1997 79. Tepic S, Perren SM: The biomechanics of the PC-Fix internal fixator. Injury 26(Suppl 2):B5, 1995 80. Auer JA, Lischer C, Kaegi B, et al: Application of the point contact fixator in large animals. Injury 26(Suppl 2):B37, 1995 81. Savoldelli D, Montavon P: Clinical handling: Small animals. Injury 26(Suppl):B47, 1995 82. Marti A, Fankhauser C, Frenk A, et al: Biomechanical evaluation of the less invasive stabilization system (LISS) for fixation of distal femur fractures. J Orthop Trauma 14:133, 2001 83. Auer JA: Internal fixators. Proc Eur Coll Vet Surg 13:202, 2004 84. Frigg R: Locking compression plate (LCP): An osteosynthesis plate based on the dynamic compression plate and the point contact fixator (PC-Fix). Injury 32(Suppl 2):B63, 2001 85. Wagner M, Frigg R: AO Manual of fracture management: Internal fixators—Concepts and Cases Using LCP and LISS. Thieme Verlag, Stuttgart, Germany, 2006 86. Florin M, Arzdorf M, Linke B, et al: Assessment of stiffness and strength of four different implants available for equine fracture treatment: A study on a 20 degree oblique long bone fracture model using a bone substitute. Vet Surg 34:231, 2005 87. Levine DG, Richardson DW: Clinical use of the locking compression plate (LCP) in horses: A retrospective study of 31 cases (2004-2006). Equine Vet J 39:401, 2007 88. Sod GA, Mitchell CF, Hubert JD, et al: In vitro biomechanical comparison of locking compression plate fixation and limited-contact dynamic compression plate fixation of osteotomized equine third metacarpal bones. Vet Surg 37:283, 2008 89. Sod GA, Riggs LM, Mitchell CF, et al: An in vitro biomechanical comparison of 5.5 mm locking compression plate fixation with a 4.5 mm locking compression plate fixation of osteotomized equine third metacarpal bones. Vet Surg 39:581, 2010 90. Blass CE, Caldarise SG, Torzin PA, et al: Mechanical properties of three orthopedic wire configurations. Am J Vet Res 46:1725, 1985 91. Hulse DA, Nelson J, Herron M: Cerclage, hemicerclage and tension band application. Texas Vet Med Assoc J 50:23, 1988 92. Martin BB, Nunamaker DM, Evans LH, et al: Tension band repair of mid body and large base sesamoid fractures in 15 horses. Vet Surg 20:9, 1991 93. Richardson DW: Ulnar Fractures. p. 416. In White NA, Moore JN (eds): Current Practice of Equine Surgery. 2nd Ed. JB Lippincott, Philadelphia, 1990

94. Nixon AJ: Fractures of the Ulna. p. 231. In Nixon AJ (ed): Equine Fracture Repair. Saunders, Philadelphia, 1996 95. Bramlage LT: Arthrodesis of the Fetlock Joint. p. 1064. In Mansman RA, McAlister GS (eds): Equine Medicine and Surgery, 3rd Ed. American Veterinary Publications Inc, Santa Barbara, CA, 1982 96. Rothaug PG, Boston RC, Richardson DW, et al: A comparison of ultra-high molecular weight polyethylene cable and stainless steel wire using two fixation techniques for repair of equine midbody sesamoid fractures: An in vitro biomechanical study. Vet Surg 31:454, 2002 97. Parvizi J, Venkat R, Rapuri JJ, et al: Treatment protocol for proximal femoral periprosthetic fractures. J Bone Joint Surg AM 78:8, 2004 98. Richardson DW: Ulna (Olecranon): Tension Band Wiring. p. 171. In Fackelman GE, Auer JA, Nunamaker DM (eds): AO Principles of Equine Osteosynthesis. Thieme Verlag, Stuttgart, Germany, 2000 99. Foerner JJ: The use of Rush pins in long bone fractures. Proc Am Assoc Equine Pract 23:223, 1977 100. Fröhlich D: Versuche zur intramedullären osteosynthese des metacarpus beim pferd. Thesis, University of Zürich, Switzerland, 1973 101. Herthel DJ, Lauper L, Rick MC, et al: Comminuted MCIII fracture treatment using titanium static interlocking intramedullary nails. Equine Pract 18:26, 1996 102. Herthel DJ: Application of the Interlocking Intramedullary Nail. p. 371. In Nixon AJ (ed): Equine Fracture Repair. Saunders, Philadelphia, 1996 103. Watkins JP: Intramedullary interlocking nail fixation in foals: Effects on normal growth and development of the humerus. Vet Surg 19:80, 1990 104. Watkins JP, Ashman RB: Intramedullary interlocking nail fixation in transverse humeral fractures: An in vitro comparison with stacked pin fixation. Proc Vet Orthop Soc 18:54, 1991 105. Radcliffe RM, Lopez MJ, Turner TA, et al: An in vitro biomechanical comparison of interlocking nail constructs and double plating for fixation of diaphyseal femur fractures in immature horses. Vet Surg 30:179, 2001 106. Nixon AJ, Watkins JP: Fractures of the Humerus. p. 242. In Nixon AJ (ed): Equine Fracture Repair. Saunders, Philadelphia, 1996 107. McDuffee LA, Stover SM, Taylor KT, et al: In vitro biomechanical investigation of an interlocking nail for fixation of diaphyseal tibial fractures in adult horses. Vet Surg 23:219, 1994 108. Fitch GL, Galuppo LD, Stover SM, et al: An in vitro biomechanical investigation of an intramedullary nailing technique for repair of third metacarpal and metatarsal fractures in neonates and foals. Vet Surg 30:422, 2001 109. Lopez MJ, Wilson DG, Trostle SS, et al: An in vitro biomechanical comparison of two interlocking-nail systems for fixation of ostectomized equine third metacarpal bones. Vet Surg 30:246, 2001 110. Galuppo LD, Stover SM, Aldridge A, et al: An in vitro biomechanical investigation of an MP35N intramedullary interlocking nail system for repair of third metacarpal fractures in adult horses. Vet Surg 31:211, 2002 111. Watknis JP: Personal communication, 2003