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Extensor Tendon Injury
Chapter
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Extensor Tendon Injury Jane C. Boswell and Michael C. Schramme The extensor tendons of the carpus and digit and the respective synovial sheaths are vulnerable to injury because they lie relatively unprotected in the subcutaneous fascia on the dorsal surface of the limb. This chapter discusses lameness caused by injuries and disorders of the extensor tendons of the carpus and digit. Injuries of the extensor tendons of the hock (gastrocnemius and calcaneal tendons) are discussed elsewhere (see Chapter 80).
ANATOMY
References on page 1320
The extensor carpi radialis (ECR) muscle is the largest extensor muscle of the forelimb and has a prominent muscle belly on the cranial aspect of the antebrachium. The ECR muscle extends the carpus, smoothes carpal joint movement by dampening oscillations as the hoof strikes the ground, and flexes the elbow.1,2 The extensor carpi radialis tendon (ECRT) extends through most of the muscle belly and appears on the surface of the muscle in the middle of the antebrachium. The tendon passes through the middle groove at the distal end of the radius, over the dorsal aspect of the carpus, and inserts on the metacarpal tuberosity on the dorsal proximal aspect of the third metacarpal bone.1 As the tendon passes over the carpus, it is bound by the extensor retinaculum and enveloped by a synovial sheath, which extends from 8 to 10 cm proximal
to the carpus, to the level of the middle carpal joint. Distal to the sheath the ECRT is attached to the carpometacarpal joint capsule. There is usually a small bursa beneath the tendon at the level of the third carpal bone.1,3 The common digital extensor (CDE) muscle is a compound muscle with three heads (humeral, radial, and ulnar) that lies lateral to the ECR muscle on the craniolateral aspect of the radius and acts to extend the digit and carpal joints and to flex the elbow. The main common digital extensor tendon (CDET) appears on the surface of the muscle in the middle of the antebrachium and passes distally through the lateral groove on the cranial distal aspect of the radius and over the capsule of the carpal joints. As the tendon passes distally over the dorsal aspect of the metacarpal region, it courses medially and reaches the dorsal midline just proximal to the fetlock. At the level of the distal aspect of the proximal phalanx the tendon becomes wider as it is joined by extensor branches of the interosseous tendon (suspensory ligament). The CDET inserts on the extensor process of the distal phalanx and the dorsal surface of the proximal extremities of the proximal and middle phalanges. The CDET is enveloped by a synovial sheath as it passes over the carpus. The sheath extends from about 8 cm proximal to the carpus to the proximal end of the metacarpal region.1,3 A bursa is present between the tendon and the dorsal pouch of the fetlock joint capsule.1,3,4 A small tendon also arises from the smaller head (the radial head) of the CDE muscle. This tendon runs through the synovial sheath of the principal tendon and then passes laterally to fuse with the lateral digital extensor tendon (LDET) or may continue separately between the CDET and LDET to the fetlock. A small tendon may also arise from the ulnar head of the CDE muscle, which either fuses with the principal tendon or inserts on the joint capsule dorsal to the fetlock joint.
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The lateral digital extensor (LDE) muscle is smaller than the other extensor muscles and is situated caudal to the CDE muscle. The action of the LDE muscle is to extend the digit and carpus. The LDET arises at the level of the distal third of the antebrachium and passes distally through the groove on the lateral styloid process of the radius and over the lateral aspect of the carpus. The tendon becomes larger and flattens distal to the carpus as it joins the tendon of the radial head of the CDE muscle and a strong band from the accessory carpal bone. The LDET runs lateral to the CDET and gradually inclines toward the dorsal aspect of the metacarpal region and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx.1 A synovial sheath envelops the LDET as it passes over the carpus. It begins 6 to 8 cm proximal to the carpus and extends to the proximal end of the metacarpal region.1,3 At the fetlock a small bursa lies between the tendon and the joint capsule.1 The extensor carpi obliquus is a small muscle that extends the carpus. The tendon of the extensor carpi obliquus arises at the distal end of the radius and courses distally, cranially, and medially over the ECRT and then passes through the oblique groove at the distal end of the radius to insert on the base (head) of the second metacarpal bone. The tendon is enveloped by a synovial sheath.1 In the hindlimb, the long digital extensor (LoDE) muscle is situated superficially on the craniolateral aspect of the limb and acts to extend the digit and flex the hock. The tendon of the muscle begins in the middle of the muscle belly and passes distally over the dorsal aspect of the hock, where the tendon is bound by the extensor retinacula and enveloped by a synovial sheath. The sheath extends from slightly proximal to the level of the lateral malleolus of the tibia to the proximal third of the metatarsal region. The long digital extensor tendon (LoDET) is joined by the LDET about 10 cm distal to the tarsus. In the angle of this union the extensor digitorum brevis also joins the principal tendon of the LoDE.1 Distal to this point the arrangement of tendons is the same as in the forelimb. The LDE muscle lies on the lateral surface of the crus caudal to the LoDE muscle. The LDET runs through the entire length of the muscle belly and emerges at the level of the distal third of the tibia. The tendon descends through the groove on the lateral malleolus of the tibia, where it is bound by the extensor retinacula, and usually blends with the LoDET. In some horses the tendon does not insert on the LoDET but passes separately lateral to the LoDET and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx, like the corresponding tendon in the forelimb.1 The tendon is surrounded by a synovial sheath that extends from 4 to 6 cm above the lateral malleolus of the tibia to the proximal third of the metatarsal region.1,3
DIAGNOSTIC TECHNIQUES Many disorders of the extensor tendons, particularly acute injuries, may be diagnosed by careful clinical examination, gait analysis, and palpation. Diagnostic perineural or intrathecal analgesia may be useful in horses with chronic injuries to the extensor tendons to determine the importance of clinical findings. Synoviocentesis of distended tendon
sheaths or bursae also may be helpful to distinguish between tenosynovitis caused by injury or infection. Plain radiography is of little value in identifying soft tissue injuries of the extensor tendons unless radiodense foreign material or bone fragments are present. Radiology may be useful to evaluate mineralization within the tendons or the synovial sheaths, enthesopathy, and associated osseous abnormalities. Contrast radiography may demonstrate a penetrating tract, intrathecal adhesions, or synovial fistulae between the extensor tendon sheaths and carpal joints.5,6 Ultrasonography is a safe, noninvasive method of evaluating the extensor tendons, the synovial sheaths, and the bursae. A 7.5- or 10-MHz transducer and standoff pad are used to image the extensor tendons and the respective tendon sheaths. Careful evaluation of transverse and longitudinal images allows the clinician to determine the extent of tendon damage, the presence of foreign bodies within the tendon or tendon sheaths, synovial membrane hyperplasia, and intrathecal adhesions.7
CONDITIONS AFFECTING THE EXTENSOR TENDONS Laceration of the Digital Extensor Tendons
Lacerations of the extensor tendons commonly occur in the metacarpal or metatarsal regions of the limbs because of their superficial location at these sites (see Chapter 81). Extensor tendon lacerations are more common in the hindlimb8-11 and are frequently associated with considerable soft tissue damage and opening of the extensor tendon sheaths. Transection of an extensor tendon below the carpus or tarsus leads to reduced ability to extend the digit, which results in an exaggerated, rapid flip of the hoof at the end of the swing phase of the stride, intermittent knuckling over at the fetlock, or tripping at the walk. The horse, however, will bear weight in a normal posture if the foot is placed flat on the ground. This gait abnormality is more obvious in horses with hindlimb injuries and when the laceration is near the fetlock because remaining peritendonous fascial attachments provide some support to the distal part of the tendon in more proximal injuries. Lacerations with transection of the extensor tendons proximal to the carpus, and at or just proximal to the tarsus, are also common. Transection of the CDET and ECRT commonly occurs proximal to the carpus and often results in pain on flexion of the carpus. Transection of the LoDET proximal to or at the tarsus results in greater tarsal flexion during the swing phase of the stride and intermittent knuckling of the digit. Within a few days of transection of an extensor tendon the horse learns to adapt its gait, and tripping and knuckling of the fetlock become less frequent. The diagnosis of extensor tendon laceration is often apparent from the gait deficit. If necessary, the diagnosis may be confirmed by exploration of the wound with a sterile, gloved finger after aseptic preparation and wound lavage. Plain and contrast radiography should be used to evaluate concomitant joint or bone damage and to help to identify foreign bodies. After aseptic preparation, wounds over the extensor tendons should be debrided and lavaged. It is important to
debride any exposed or obviously devitalized bone and, where possible, to cover bone with skin to reduce the risk of sequestrum formation. Primary apposition of the lacerated tendon is unnecessary even if a large gap has formed between the distracted tendon ends. Progressive ultra sonographic evaluations show that fibrous tissue develops between the transected tendon ends; this tissue gradually becomes more organized and regains the linear arrangement of collagen along the line of the original tendon. This fibrous tissue provides an adequate mechanical link between the tendon ends, allowing extensor function of the digit to return.12 Although lacerations of extensor tendons may heal without external coaptation, wound healing is facilitated and exuberant granulation tissue formation is controlled if limb immobilization is used during the first 3 to 6 weeks after injury. Immobilization may be achieved using a polyvinylchloride (PVC) splint or cast. Full-limb PVC splints may be applied to the dorsal or palmar aspect of the forelimb to prevent knuckling of the fetlock and carpal flexion. In the hindlimb, a distal limb splint applied to the dorsal or plantar aspect of the limb can be used. Application of a shoe with a toe extension may help facilitate flat foot placement and prevent tripping or knuckling over of the fetlock in the early postinjury period. Casts provide an inexpensive and efficient means of immobilizing the limb and tendon ends and are especially useful when extensor tendon lacerations occur with extensive, contaminated wounds, in which primary wound closure cannot be achieved. After removal of the cast or splint, the horse should remain confined to a box stall for another 4 to 6 weeks. During this period the toe should be trimmed short so that it does not catch to cause knuckling and disruption of the organizing fibrous tissue. A controlled exercise program of in-hand walking exercise may be initiated at this time to strengthen the tendon and improve gliding function. Passive range of movement exercises may also be beneficial. If no signs of knuckling are present after 10 to 12 weeks of controlled exercise, a gradual return to athletic use may begin. The prognosis for return to athletic function after extensor laceration is good.8-11 In a retrospective study of 156 horses with extensor tendon lacerations, 74% returned to soundness and 60% of sports horses returned to previous activity at the same, or higher, level.10 In this study there was a significant influence on positive outcome if primary closure of the wound was performed, with horses 2.6 times more likely to return to soundness compared with those horses in which wound closure was not performed. Complications in that study included wound infection and dehiscence, exuberant granulation tissue, and sequestrum formation. Stringhalt may also occur as a complication of lacerations or injury to the extensor tendons in the dorsoproximal aspect of the metatarsal region. It is speculated that stringhalt may result from adhesion formation, which decreases the ability of the long or LDETs to stretch during hock flexion, or from abnormalities in the myotatic reflex, which governs extensor muscle tone and relaxation.13
Rupture of the Common Digital Extensor Tendon
Rupture of the CDET occurs in foals and may be present at birth or may develop within the first few weeks of life. This
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Fig. 77-1 • Fluctuant swelling over the dorsolateral aspect of the carpus in a foal with rupture of the common digital extensor tendon (CDET). The injury shows the typical transverse depression in the sheath where the CDET has ruptured (solid arrow) and the bulge in the sheath where the transected distal part of the tendon is situated (open arrow).
condition may be primary or secondary to flexural deformities of the carpus or metacarpophalangeal joint that result in increased tension in the CDET.14 Rupture of the CDET may be an inherited condition because Arabians and Quarter Horses were overrepresented in one retrospective study.15 As rupture of the CDET always occurs within the synovial sheath,14 affected foals have a characteristic fluctuant swelling over the dorsolateral aspect of the carpus at the level of the distal carpal joints (Figure 77-1). Palpation reveals fluid distention of the tendon sheath, and the ruptured ends of the CDET can usually be identified. Affected foals may have a normal stance but often stand with the carpus slightly flexed (over-at-the-knee appearance) and frequently knuckle forward on the fetlock. Most affected foals, however, quickly accommodate the gait and learn to flip the distal aspect of the limb during the swing phase of the stride. Diagnosis of rupture of the CDET is based on careful palpation and is confirmed by ultrasonography. Radiography of the carpus should be performed to rule out cuboidal bone malformation. In foals with primary injuries without associated flexural deformities, treatment usually involves box rest and the application of well-padded bandages to support the carpus and prevent abrasion of the dorsum of the fetlock.
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In foals that exhibit frequent knuckling over on the fetlock, or foals with concurrent flexural deformities of the carpus, a PVC splint should be applied to the palmar aspect of the limb, extending from the elbow to the fetlock. For foals with concurrent cuboidal bone malformation, tube casts from the elbow to the fetlock should be used to provide carpal support. Foals with splints usually walk comfortably, and sufficient fibrosis has usually developed after 2 to 4 weeks to allow removal of the splints. However, the foal should be maintained in padded bandages for several more weeks to maintain pressure on the area of tendon rupture to minimize the resultant blemish. It has been suggested that the ruptured ends of the CDET are unlikely to reunite but adhere to the tendon sheath and that the LDET assumes the function of the CDET.15 However, anecdotal evidence indicates that the tendon ends may rejoin in some horses.16 Consequently, foals with primary rupture of the CDET, without concurrent cuboidal bone malformation, usually have an excellent prognosis for athletic performance with minimal blemish. Foals with concurrent cuboidal bone malformation or flexural deformities have a more guarded prognosis for athletic performance.17,18
RUPTURE OF THE EXTENSOR CARPI RADIALIS TENDON Rupture of the ECRT is a rare condition of adult horses.15 Partial rupture may also occur rarely, particularly in horses that are used for jumping.19 Complete and partial rupture of the ECRT usually occurs within the synovial sheath on the dorsal aspect of the carpus. The injury is believed to result from trauma associated with repeated overflexion of the carpus.15 Partial rupture of the ECRT was reported to be a consequence of tendon damage caused by exostoses on the distal aspect of the radius.19 Acute rupture of the ECRT is characterized by pain and distention of the tendon sheath. Extension and flexion of the carpus are limited, which may result in dragging of the toe.20 In horses with chronic injuries of the ECRT, distention of the tendon sheath persists but is usually painless unless an associated inflammatory tenosynovitis occurs. Complete rupture of the ECRT results in excessive carpal flexion during limb protraction and consequently a higher arc of foot flight in the affected limb. Limb extension may be characterized by a double movement as the carpus suddenly snaps into extension.21,22 Atrophy of the ECR muscle may be evident in horses with chronic injuries. Partial rupture of the ECRT may be associated with mild lameness and restricted carpal flexion.19 Diagnosis of rupture of the ECRT may be confirmed by careful palpation, ultrasonography, and contrast radiography.6,7 Plain radiography should be used to evaluate associated osseous lesions. Horses with acute, complete rupture of the ECRT may be treated surgically. Tenosynovectomy with primary suturing of the tendon ends, or substitution of the extensor carpi obliquus tendon by tendon anastomosis, has been described.15 Other authors, however, believe that surgical apposition of the tendon ends is unnecessary and advocate tenoscopy, which allows characterization of the extent of the injury, debridement of the damaged tendon ends, and lavage of the sheath. After surgery the limb should be
immobilized in a tube cast or splint for 2 to 4 weeks. Once the cast and sutures have been removed, passive range-ofmovement exercises should be instituted to prevent the formation of intrathecal adhesions and to reduce carpal joint capsule fibrosis, which result in a limited range of carpal flexion. The prognosis for return to athletic performance for horses with complete rupture of the ECRT is unfavorable. The prognosis for return to athletic performance for horses with partial rupture of the ECRT is guarded to fair, but tenolysis and partial tenosynovectomy may benefit some horses.15,20
Trauma to the Extensor Carpi Radialis Tendon
Single episode trauma, such as a kick injury, or repetitive trauma from hitting fixed cross-country fences or pawing at a door, may result in a primary injury of the ECRT, often accompanied by tenosynovitis, with or without adhesion formation between the tendon and tendon sheath. Such injuries may be accompanied by new bone formation on the distal aspect of the radius. The ECRT is usually palpably enlarged, with associated distention of the tendon sheath. Ultrasonographic examination confirms enlargement of the tendon and determines the extent of injury to the tendon and the presence of adhesion formation. Although horses with mild injuries often respond well to medical management, those with severe injuries may require en bloc surgical removal of the injured tendon and the synovial lining of the tendon sheath. Cosmetic results are often poor, but the prognosis for athletic function is reasonable.
Enthesopathy of the Insertion of the Extensor Carpi Radialis Tendon
In the racing Standardbred, tearing of the ECRT attachment at its insertion on the proximodorsal aspect of the third metacarpal bone can cause lameness, and these tears may play a role in the cause of fractures of the dorsomedial aspect of the proximal end of the third metacarpal bone (see Chapter 37).23
CONDITIONS AFFECTING THE EXTENSOR TENDON SHEATHS Tenosynovitis Idiopathic Tenosynovitis
Idiopathic tenosynovitis may be defined as swelling with synovial effusion but without active inflammation, pain, or lameness.15 Idiopathic tenosynovitis of the extensor tendon sheaths has been reported in foals at birth.24 In adults the condition tends to develop insidiously. The pathogenesis in newborn foals is unknown, but in adult horses the pathogenesis is presumed to be caused by chronic trauma.15 However, distention of the LoDE sheath in hindlimbs often occurs bilaterally, is unassociated with any history of trauma, and is of unknown cause. Ultrasonographic examination usually reveals anechogenic fluid within the sheath. Occasionally, hypoechoic lesions are identified within the enclosed tendon, the clinical significance of which is unknown because lameness is rarely present. As idiopathic tenosynovitis usually is not associated with pain or lameness, treatment is unnecessary unless the
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owner is concerned about cosmesis. Treatment by injection of corticosteroids (10 to 20 mg triamcinolone acetonide or 40 to 80 mg methylprednisolone acetate) and pressure bandaging often only provides a temporary resolution of tendon sheath effusion.15 Anecdotal reports suggest that intrathecal injection of 4 to 15 mg of atropine sulfate, alone or with corticosteroids and hyaluronan, may cause permanent resolution of the effusion. Atropine sulfate is purported to reduce the production of synovial fluid from synoviocytes; however, its anticholinergic effects mean that its use has been associated with transient signs of mild colic and depression. The Editors have never recognized any side effects of intrathecal injection of atropine.
Acute Traumatic Tenosynovitis
Acute tenosynovitis is characterized by a rapidly developing effusion of a tendon sheath, accompanied by heat, pain, and variable lameness. Acute tenosynovitis of the extensor tendon sheaths is often caused by trauma, such as a fall or hitting a jump with the carpus.19 This injury is common in event horses, in which it often is not associated with lameness.16 Diagnosis of acute tenosynovitis is based on clinical signs and ultrasonography. Ultrasonography is useful to evaluate concurrent tendonitis of the extensor tendons and to differentiate acute tenosynovitis from other conditions that are associated with soft tissue swelling over the dorsal aspect of the carpus, including hygroma, cellulitis, synovial hernia, and effusion of the carpal joints (see Chapter 38). Horses with acute tenosynovitis of an extensor tendon sheath are treated by rest, cold hydrotherapy, and nonsteroidal antiinflammatory drugs (NSAIDs). Aspiration of fluid and injection of corticosteroids are reserved for horses that do not respond to more than 1 week of conservative treatment.25
Infectious Tenosynovitis
Infectious tenosynovitis is characterized by dramatic synovial effusion, heat, pain, swelling, and severe lameness. Contamination from a penetrating wound is the most common cause of infectious tenosynovitis, but infection also may arise from hematogenous spread of bacteria and iatrogenic infection.26 Infection of the CDET sheath has been reported as a complication of hemicircumferential periosteal transection.27 After penetration of a tendon sheath, severe lameness does not become evident unless infection becomes established. If the tendon sheath is open and draining, then the lameness may be less severe.28 Once infection is established, rapid and aggressive treatment is necessary to prevent synovial hyperplasia, fibrosis of the tendon sheath, intrathecal adhesion formation, and damage to the extensor tendons. Prompt recognition of infectious tenosynovitis is essential for a successful outcome. Diagnosis of infectious tenosynovitis is based on clinical signs and must be confirmed by synovial fluid aspiration and analysis. Infected synovial fluid is typically turbid and has a low viscosity, an elevated total nucleated cell count (>30 × 109/L, with more than 90% neutrophils), and a total protein concentration of more than 40 g/L. Contrast radiography (Figure 77-2) or ultrasonography may be useful to confirm a communication between a penetrating wound and the tendon sheath, especially if horses are examined soon after injury.
Fig. 77-2 • Lateromedial radiographic image of a carpus. Dorsal is to the left. Positive-contrast medium has been injected into the extensor carpi radialis tendon sheath and antebrachiocarpal joint in a horse that had sustained a deep puncture wound over the dorsal aspect of the carpus. Radiopaque contrast agent is present near the skin surface (solid arrow), outside the tendon sheath (small arrows), confirming penetration of the extensor carpi radialis sheath. The antebrachiocarpal joint (open arrow) was not involved.
The primary aim of treatment of horses with infectious tenosynovitis is the rapid elimination of bacteria and rapid return of the normal synovial environment. This is best achieved by wound debridement, lavage of the tendon sheath with copious amounts of sterile isotonic fluids, and the provision of bactericidal levels of appropriate antibiotics within the sheath. Appropriate systemic and intrathecal antibiotic therapy should be initiated immediately. The presence of Enterobacteriaceae most commonly is associated with tendon sheath infection caused by a penetrating wound, whereas staphylococci are most commonly identified as the cause of iatrogenic infections of a tendon sheath.26 The most effective combination of antibiotics for the treatment of infectious tenosynovitis is amikacin and cephalosporin (>85% effective).29 However, the cost of these drugs may be prohibitive, and other drug combinations, such as penicillin and gentamicin, may be considered. The initial selection of anti biotic should be altered according to bacterial susceptibility if a positive culture is obtained from the synovial fluid. Wound debridement and lavage of the affected tendon sheath with copious amounts of sterile isotonic fluids are important to reduce concentrations of bacteria and inflammatory mediators. Early in the infection, effective lavage may be achieved by through-and-through lavage using
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large-bore needles or arthroscopic egress cannulas. In more established infectious tenosynovitis, leukocytes and fibrin accumulate within the sheath, necessitating tenoscopic debridement. If satisfactory debridement and lavage have been achieved, primary closure of the sheath may be performed. The horse should be monitored closely for lameness, and repeated synovial fluid samples should be taken at 2- to 3-day intervals to monitor for return of infection. Alternatively the sheath may be left open, or a closed suction drain system may be inserted into the sheath for further elimination of inflammatory mediators and fibrin. The drain should be left in place for 3 to 5 days, but careful management is important to prevent ascending suprainfection. Systemic antibiotic therapy should be continued for at least 2 weeks after the resolution of clinical signs. Once the infection has been eliminated, intrathecal injection of corticosteroids or hyaluronan may be used, with physiotherapy, to reduce adhesion formation and restore the normal gliding movement of the tendon within its sheath. However, both drugs may cause immunosuppression within the sheath and may potentiate dormant infection. Early return to controlled exercise is also important to reduce intrathecal adhesion formation. If chronic infectious tenosynovitis becomes established, exploration of the tendon sheath and radical synovectomy may be required (see the following discussion). Successful outcomes have been reported after complete resection of the intrasynovial part of the CDET in horses with chronic infectious tenosynovitis and tendonitis.30,31 The prognosis for return to soundness after infectious extensor tenosynovitis is more favorable than for infectious flexor tenosynovitis,26,28 possibly because extensor tendons are non–weight bearing compared with flexor tendons. The prognosis for return to soundness is generally good after early surgical intervention and appropriate antibiotic therapy for horses with infectious extensor tenosynovitis.
Chronic Tenosynovitis
Chronic tenosynovitis of the extensor tendon sheaths is characterized by persistent synovial effusion, fibrous thickening of the sheath, and subcutaneous edema.32,33 Chronic tenosynovitis results in variable, sometimes severe, lameness, restricted carpal flexion, and a gait characterized by circumduction of the affected limb during protraction.33 However, in some horses there is no gait abnormality. Chronic tenosynovitis commonly arises after penetrating injuries to the carpal extensor sheaths, which may result in the inoculation of foreign material or bacteria into the synovial cavity and establishment of an infectious or noninfectious chronic tenosynovitis. The condition commonly occurs in horses jumping natural fences because of penetration of the sheath by thorns.33 Chronic tenosynovitis also may occur after acute tenosynovitis and may be associated with partial tendon rupture.22,32 Chronic inflammation of the tendon sheath causes granulomatous proliferation of the synovium, connective tissue deposition in the fibrous capsule, and fibrous adhesion formation between the tendon and its sheath, resulting in restriction of movement and pain on carpal flexion.32,33 Diagnosis of chronic tenosynovitis is based on clinical signs of effusion and thickening of the affected sheath (Figure 77-3) and restricted carpal flexion. Radiography of the carpus often reveals palisading new bone formation on
Fig. 77-3 • Chronic distention of the tendon sheath of the extensor carpi radialis in a horse with chronic tenosynovitis.
the craniodistal ridges of the radius adjacent to the affected sheaths (Figure 77-4) and entheseous new bone formation on the dorsal aspect of the carpal bones.33 Synovial fluid aspirates vary from serosanguineous to turbid, with an increased nucleated cell count and total protein concentration. Ultrasonography may demonstrate tendon damage, synovial hypertrophy, foreign bodies, and intrathecal adhesions. Conservative treatment with antibiotics, analgesics, intrathecal corticosteroids, and symptomatic physiotherapeutic procedures such as bandaging, cold hosing, massage, and forced exercise is usually unrewarding.19,22,32,33 Good results, however, have been reported after both tenoscopic and open surgical intervention.19,33,34 Tenoscopic examination of the sheaths of the ECRT, CDET, and LDET allows radical synovectomy of the hyperplastic synovium, removal of intrathecal adhesions, and debridement of damaged tendon and is associated with a good prognosis for resolution of lameness. The cosmetic appearance of most distended extensor tendon sheaths after tenoscopic surgery can be substantially improved, although most have some residual fibrosis.34 The prognosis for soundness after open surgical exploration and radical synovectomy has been reported to be good,33 although the resultant scar may cause a substantial blemish. After surgery, an intensive program of physiotherapy involving manual flexion and extension of the carpus and an ascending program of in-hand walking exercise should be initiated. Early passive motion to stimulate cavitation and reformation of the synovial lining and prevent adhesion formation is considered an essential part of the treatment.33 Initially, carpal flexion is resented, and administration of NSAIDs and sedatives may be required.
Fig. 77-4 • A craniolateral-caudomedial oblique radiographic image of the distal aspect of the radius in a horse with chronic tenosynovitis of the extensor carpi radialis tendon sheath. There is palisading new bone formation (arrows) on the craniomediodistal ridge of the radius adjacent to the tendon sheath.
Full flexion of the carpus should be achieved by 30 days after surgery.
Osteochondromatosis
Synovial osteochondromatosis of the ECRT sheath has been reported.35 Synovial osteochondromatosis is characterized by the formation of multiple small osseous bodies within a synovial-lined structure. The cause is unclear but it may be associated with trauma. Clinical signs include swelling on the dorsal surface of the carpus characterized by multiple, firm subcutaneous nodules and crepitus during joint movement, and lameness is usually evident. Diagnosis is confirmed by radiography and ultrasonography.35 No information is available for the surgical treatment of osteochondromatosis of the extensor tendon sheaths in horses, but partial synovectomy and endoscopic removal of the osteochondral bodies was successful in other synovial cavities in horses and other species.36
Intersynovial Fistulae
Intersynovial fistulae are uncommon, but they have been documented between the CDET sheath and antebrachiocarpal joint,37 the CDET sheath and middle carpal joint,15 and the ECRT sheath and middle carpal joint.5 The cause of these fistulae is unclear, but they are considered to be traumatic in origin and may occur in horses with carpitis or carpal fractures.5,37 Typically, horses with intersynovial fistulae have chronic lameness and distention of the affected tendon sheath. Synovial fluid can be massaged from the joint to the tendon sheath. Diagnosis may be confirmed by contrast radiography6 and intrasynovial analgesia. Surgical treatment is advocated in the management of intersynovial fistulae,15 but little information concerning
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Fig. 77-5 • Prominent soft tissue swelling over the dorsal aspect of the fetlock region in the hindlimb of a horse with severe lameness. This is a supratendonous infectious bursitis and should not be confused with infectious arthritis of the metatarsophalangeal joint. Arthrocentesis of the metatarsophalangeal joint through a dorsal approach may result in iatrogenic infectious arthritis.
the prognosis of affected horses is available because reports of this condition are rare. Use of tenoscopic approaches to close fistulae between the carpal joints and extensor tendon sheaths has been largely unsuccessful.34 Surgical treatment involving a combination of endoscopically assisted synovectomy and limited open exposure of the fistula, and closure of the fibrous layers of the joint and tendon sheath are generally recommended.34
Infectious Bursitis
Infectious bursitis may occur in any of the bursae associated with the extensor tendons over the dorsal aspect of the fetlock. The condition is seen most commonly in horses that jump natural obstacles (e.g., event and National Hunt horses). Typically these horses have swelling over the dorsal aspect of the fetlock (Figure 77-5) and mild-to-severe lameness. Infectious bursitis may affect the subtendonous bursae or more commonly affects an acquired subcutaneous (supratendonous) bursa on the dorsal aspect of the LoDET in the fetlock region.38 Occasionally both bursae may communicate around the lateral or medial aspect of the LoDET. Some horses have a severe, non–weight-bearing lameness. This condition frequently is confused with infectious arthritis of the metacarpophalangeal or metatarsophalangeal joints. Diagnosis is confirmed by ultrasonographic examination or contrast radiography6 and synoviocentesis. Synoviocentesis from the palmar or plantar pouch of the adjacent metacarpophalangeal or metatarsophalangeal joint should be used to rule out joint infection. Treatment consists of surgical drainage and debridement and appropriate antimicrobial therapy. The prognosis for return to soundness is good.
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Extensor Tendon Injury Jane C. Boswell and Michael C. Schramme The extensor tendons of the carpus and digit and the respective synovial sheaths are vulnerable to injury because they lie relatively unprotected in the subcutaneous fascia on the dorsal surface of the limb. This chapter discusses lameness caused by injuries and disorders of the extensor tendons of the carpus and digit. Injuries of the extensor tendons of the hock (gastrocnemius and calcaneal tendons) are discussed elsewhere (see Chapter 80).
ANATOMY
References on page 1320
The extensor carpi radialis (ECR) muscle is the largest extensor muscle of the forelimb and has a prominent muscle belly on the cranial aspect of the antebrachium. The ECR muscle extends the carpus, smoothes carpal joint movement by dampening oscillations as the hoof strikes the ground, and flexes the elbow.1,2 The extensor carpi radialis tendon (ECRT) extends through most of the muscle belly and appears on the surface of the muscle in the middle of the antebrachium. The tendon passes through the middle groove at the distal end of the radius, over the dorsal aspect of the carpus, and inserts on the metacarpal tuberosity on the dorsal proximal aspect of the third metacarpal bone.1 As the tendon passes over the carpus, it is bound by the extensor retinaculum and enveloped by a synovial sheath, which extends from 8 to 10 cm proximal
to the carpus, to the level of the middle carpal joint. Distal to the sheath the ECRT is attached to the carpometacarpal joint capsule. There is usually a small bursa beneath the tendon at the level of the third carpal bone.1,3 The common digital extensor (CDE) muscle is a compound muscle with three heads (humeral, radial, and ulnar) that lies lateral to the ECR muscle on the craniolateral aspect of the radius and acts to extend the digit and carpal joints and to flex the elbow. The main common digital extensor tendon (CDET) appears on the surface of the muscle in the middle of the antebrachium and passes distally through the lateral groove on the cranial distal aspect of the radius and over the capsule of the carpal joints. As the tendon passes distally over the dorsal aspect of the metacarpal region, it courses medially and reaches the dorsal midline just proximal to the fetlock. At the level of the distal aspect of the proximal phalanx the tendon becomes wider as it is joined by extensor branches of the interosseous tendon (suspensory ligament). The CDET inserts on the extensor process of the distal phalanx and the dorsal surface of the proximal extremities of the proximal and middle phalanges. The CDET is enveloped by a synovial sheath as it passes over the carpus. The sheath extends from about 8 cm proximal to the carpus to the proximal end of the metacarpal region.1,3 A bursa is present between the tendon and the dorsal pouch of the fetlock joint capsule.1,3,4 A small tendon also arises from the smaller head (the radial head) of the CDE muscle. This tendon runs through the synovial sheath of the principal tendon and then passes laterally to fuse with the lateral digital extensor tendon (LDET) or may continue separately between the CDET and LDET to the fetlock. A small tendon may also arise from the ulnar head of the CDE muscle, which either fuses with the principal tendon or inserts on the joint capsule dorsal to the fetlock joint.
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The lateral digital extensor (LDE) muscle is smaller than the other extensor muscles and is situated caudal to the CDE muscle. The action of the LDE muscle is to extend the digit and carpus. The LDET arises at the level of the distal third of the antebrachium and passes distally through the groove on the lateral styloid process of the radius and over the lateral aspect of the carpus. The tendon becomes larger and flattens distal to the carpus as it joins the tendon of the radial head of the CDE muscle and a strong band from the accessory carpal bone. The LDET runs lateral to the CDET and gradually inclines toward the dorsal aspect of the metacarpal region and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx.1 A synovial sheath envelops the LDET as it passes over the carpus. It begins 6 to 8 cm proximal to the carpus and extends to the proximal end of the metacarpal region.1,3 At the fetlock a small bursa lies between the tendon and the joint capsule.1 The extensor carpi obliquus is a small muscle that extends the carpus. The tendon of the extensor carpi obliquus arises at the distal end of the radius and courses distally, cranially, and medially over the ECRT and then passes through the oblique groove at the distal end of the radius to insert on the base (head) of the second metacarpal bone. The tendon is enveloped by a synovial sheath.1 In the hindlimb, the long digital extensor (LoDE) muscle is situated superficially on the craniolateral aspect of the limb and acts to extend the digit and flex the hock. The tendon of the muscle begins in the middle of the muscle belly and passes distally over the dorsal aspect of the hock, where the tendon is bound by the extensor retinacula and enveloped by a synovial sheath. The sheath extends from slightly proximal to the level of the lateral malleolus of the tibia to the proximal third of the metatarsal region. The long digital extensor tendon (LoDET) is joined by the LDET about 10 cm distal to the tarsus. In the angle of this union the extensor digitorum brevis also joins the principal tendon of the LoDE.1 Distal to this point the arrangement of tendons is the same as in the forelimb. The LDE muscle lies on the lateral surface of the crus caudal to the LoDE muscle. The LDET runs through the entire length of the muscle belly and emerges at the level of the distal third of the tibia. The tendon descends through the groove on the lateral malleolus of the tibia, where it is bound by the extensor retinacula, and usually blends with the LoDET. In some horses the tendon does not insert on the LoDET but passes separately lateral to the LoDET and inserts on the eminence on the dorsal proximal aspect of the proximal phalanx, like the corresponding tendon in the forelimb.1 The tendon is surrounded by a synovial sheath that extends from 4 to 6 cm above the lateral malleolus of the tibia to the proximal third of the metatarsal region.1,3
DIAGNOSTIC TECHNIQUES Many disorders of the extensor tendons, particularly acute injuries, may be diagnosed by careful clinical examination, gait analysis, and palpation. Diagnostic perineural or intrathecal analgesia may be useful in horses with chronic injuries to the extensor tendons to determine the importance of clinical findings. Synoviocentesis of distended tendon
sheaths or bursae also may be helpful to distinguish between tenosynovitis caused by injury or infection. Plain radiography is of little value in identifying soft tissue injuries of the extensor tendons unless radiodense foreign material or bone fragments are present. Radiology may be useful to evaluate mineralization within the tendons or the synovial sheaths, enthesopathy, and associated osseous abnormalities. Contrast radiography may demonstrate a penetrating tract, intrathecal adhesions, or synovial fistulae between the extensor tendon sheaths and carpal joints.5,6 Ultrasonography is a safe, noninvasive method of evaluating the extensor tendons, the synovial sheaths, and the bursae. A 7.5- or 10-MHz transducer and standoff pad are used to image the extensor tendons and the respective tendon sheaths. Careful evaluation of transverse and longitudinal images allows the clinician to determine the extent of tendon damage, the presence of foreign bodies within the tendon or tendon sheaths, synovial membrane hyperplasia, and intrathecal adhesions.7
CONDITIONS AFFECTING THE EXTENSOR TENDONS Laceration of the Digital Extensor Tendons
Lacerations of the extensor tendons commonly occur in the metacarpal or metatarsal regions of the limbs because of their superficial location at these sites (see Chapter 81). Extensor tendon lacerations are more common in the hindlimb8-11 and are frequently associated with considerable soft tissue damage and opening of the extensor tendon sheaths. Transection of an extensor tendon below the carpus or tarsus leads to reduced ability to extend the digit, which results in an exaggerated, rapid flip of the hoof at the end of the swing phase of the stride, intermittent knuckling over at the fetlock, or tripping at the walk. The horse, however, will bear weight in a normal posture if the foot is placed flat on the ground. This gait abnormality is more obvious in horses with hindlimb injuries and when the laceration is near the fetlock because remaining peritendonous fascial attachments provide some support to the distal part of the tendon in more proximal injuries. Lacerations with transection of the extensor tendons proximal to the carpus, and at or just proximal to the tarsus, are also common. Transection of the CDET and ECRT commonly occurs proximal to the carpus and often results in pain on flexion of the carpus. Transection of the LoDET proximal to or at the tarsus results in greater tarsal flexion during the swing phase of the stride and intermittent knuckling of the digit. Within a few days of transection of an extensor tendon the horse learns to adapt its gait, and tripping and knuckling of the fetlock become less frequent. The diagnosis of extensor tendon laceration is often apparent from the gait deficit. If necessary, the diagnosis may be confirmed by exploration of the wound with a sterile, gloved finger after aseptic preparation and wound lavage. Plain and contrast radiography should be used to evaluate concomitant joint or bone damage and to help to identify foreign bodies. After aseptic preparation, wounds over the extensor tendons should be debrided and lavaged. It is important to
debride any exposed or obviously devitalized bone and, where possible, to cover bone with skin to reduce the risk of sequestrum formation. Primary apposition of the lacerated tendon is unnecessary even if a large gap has formed between the distracted tendon ends. Progressive ultra sonographic evaluations show that fibrous tissue develops between the transected tendon ends; this tissue gradually becomes more organized and regains the linear arrangement of collagen along the line of the original tendon. This fibrous tissue provides an adequate mechanical link between the tendon ends, allowing extensor function of the digit to return.12 Although lacerations of extensor tendons may heal without external coaptation, wound healing is facilitated and exuberant granulation tissue formation is controlled if limb immobilization is used during the first 3 to 6 weeks after injury. Immobilization may be achieved using a polyvinylchloride (PVC) splint or cast. Full-limb PVC splints may be applied to the dorsal or palmar aspect of the forelimb to prevent knuckling of the fetlock and carpal flexion. In the hindlimb, a distal limb splint applied to the dorsal or plantar aspect of the limb can be used. Application of a shoe with a toe extension may help facilitate flat foot placement and prevent tripping or knuckling over of the fetlock in the early postinjury period. Casts provide an inexpensive and efficient means of immobilizing the limb and tendon ends and are especially useful when extensor tendon lacerations occur with extensive, contaminated wounds, in which primary wound closure cannot be achieved. After removal of the cast or splint, the horse should remain confined to a box stall for another 4 to 6 weeks. During this period the toe should be trimmed short so that it does not catch to cause knuckling and disruption of the organizing fibrous tissue. A controlled exercise program of in-hand walking exercise may be initiated at this time to strengthen the tendon and improve gliding function. Passive range of movement exercises may also be beneficial. If no signs of knuckling are present after 10 to 12 weeks of controlled exercise, a gradual return to athletic use may begin. The prognosis for return to athletic function after extensor laceration is good.8-11 In a retrospective study of 156 horses with extensor tendon lacerations, 74% returned to soundness and 60% of sports horses returned to previous activity at the same, or higher, level.10 In this study there was a significant influence on positive outcome if primary closure of the wound was performed, with horses 2.6 times more likely to return to soundness compared with those horses in which wound closure was not performed. Complications in that study included wound infection and dehiscence, exuberant granulation tissue, and sequestrum formation. Stringhalt may also occur as a complication of lacerations or injury to the extensor tendons in the dorsoproximal aspect of the metatarsal region. It is speculated that stringhalt may result from adhesion formation, which decreases the ability of the long or LDETs to stretch during hock flexion, or from abnormalities in the myotatic reflex, which governs extensor muscle tone and relaxation.13
Rupture of the Common Digital Extensor Tendon
Rupture of the CDET occurs in foals and may be present at birth or may develop within the first few weeks of life. This
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Fig. 77-1 • Fluctuant swelling over the dorsolateral aspect of the carpus in a foal with rupture of the common digital extensor tendon (CDET). The injury shows the typical transverse depression in the sheath where the CDET has ruptured (solid arrow) and the bulge in the sheath where the transected distal part of the tendon is situated (open arrow).
condition may be primary or secondary to flexural deformities of the carpus or metacarpophalangeal joint that result in increased tension in the CDET.14 Rupture of the CDET may be an inherited condition because Arabians and Quarter Horses were overrepresented in one retrospective study.15 As rupture of the CDET always occurs within the synovial sheath,14 affected foals have a characteristic fluctuant swelling over the dorsolateral aspect of the carpus at the level of the distal carpal joints (Figure 77-1). Palpation reveals fluid distention of the tendon sheath, and the ruptured ends of the CDET can usually be identified. Affected foals may have a normal stance but often stand with the carpus slightly flexed (over-at-the-knee appearance) and frequently knuckle forward on the fetlock. Most affected foals, however, quickly accommodate the gait and learn to flip the distal aspect of the limb during the swing phase of the stride. Diagnosis of rupture of the CDET is based on careful palpation and is confirmed by ultrasonography. Radiography of the carpus should be performed to rule out cuboidal bone malformation. In foals with primary injuries without associated flexural deformities, treatment usually involves box rest and the application of well-padded bandages to support the carpus and prevent abrasion of the dorsum of the fetlock.
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In foals that exhibit frequent knuckling over on the fetlock, or foals with concurrent flexural deformities of the carpus, a PVC splint should be applied to the palmar aspect of the limb, extending from the elbow to the fetlock. For foals with concurrent cuboidal bone malformation, tube casts from the elbow to the fetlock should be used to provide carpal support. Foals with splints usually walk comfortably, and sufficient fibrosis has usually developed after 2 to 4 weeks to allow removal of the splints. However, the foal should be maintained in padded bandages for several more weeks to maintain pressure on the area of tendon rupture to minimize the resultant blemish. It has been suggested that the ruptured ends of the CDET are unlikely to reunite but adhere to the tendon sheath and that the LDET assumes the function of the CDET.15 However, anecdotal evidence indicates that the tendon ends may rejoin in some horses.16 Consequently, foals with primary rupture of the CDET, without concurrent cuboidal bone malformation, usually have an excellent prognosis for athletic performance with minimal blemish. Foals with concurrent cuboidal bone malformation or flexural deformities have a more guarded prognosis for athletic performance.17,18
RUPTURE OF THE EXTENSOR CARPI RADIALIS TENDON Rupture of the ECRT is a rare condition of adult horses.15 Partial rupture may also occur rarely, particularly in horses that are used for jumping.19 Complete and partial rupture of the ECRT usually occurs within the synovial sheath on the dorsal aspect of the carpus. The injury is believed to result from trauma associated with repeated overflexion of the carpus.15 Partial rupture of the ECRT was reported to be a consequence of tendon damage caused by exostoses on the distal aspect of the radius.19 Acute rupture of the ECRT is characterized by pain and distention of the tendon sheath. Extension and flexion of the carpus are limited, which may result in dragging of the toe.20 In horses with chronic injuries of the ECRT, distention of the tendon sheath persists but is usually painless unless an associated inflammatory tenosynovitis occurs. Complete rupture of the ECRT results in excessive carpal flexion during limb protraction and consequently a higher arc of foot flight in the affected limb. Limb extension may be characterized by a double movement as the carpus suddenly snaps into extension.21,22 Atrophy of the ECR muscle may be evident in horses with chronic injuries. Partial rupture of the ECRT may be associated with mild lameness and restricted carpal flexion.19 Diagnosis of rupture of the ECRT may be confirmed by careful palpation, ultrasonography, and contrast radiography.6,7 Plain radiography should be used to evaluate associated osseous lesions. Horses with acute, complete rupture of the ECRT may be treated surgically. Tenosynovectomy with primary suturing of the tendon ends, or substitution of the extensor carpi obliquus tendon by tendon anastomosis, has been described.15 Other authors, however, believe that surgical apposition of the tendon ends is unnecessary and advocate tenoscopy, which allows characterization of the extent of the injury, debridement of the damaged tendon ends, and lavage of the sheath. After surgery the limb should be
immobilized in a tube cast or splint for 2 to 4 weeks. Once the cast and sutures have been removed, passive range-ofmovement exercises should be instituted to prevent the formation of intrathecal adhesions and to reduce carpal joint capsule fibrosis, which result in a limited range of carpal flexion. The prognosis for return to athletic performance for horses with complete rupture of the ECRT is unfavorable. The prognosis for return to athletic performance for horses with partial rupture of the ECRT is guarded to fair, but tenolysis and partial tenosynovectomy may benefit some horses.15,20
Trauma to the Extensor Carpi Radialis Tendon
Single episode trauma, such as a kick injury, or repetitive trauma from hitting fixed cross-country fences or pawing at a door, may result in a primary injury of the ECRT, often accompanied by tenosynovitis, with or without adhesion formation between the tendon and tendon sheath. Such injuries may be accompanied by new bone formation on the distal aspect of the radius. The ECRT is usually palpably enlarged, with associated distention of the tendon sheath. Ultrasonographic examination confirms enlargement of the tendon and determines the extent of injury to the tendon and the presence of adhesion formation. Although horses with mild injuries often respond well to medical management, those with severe injuries may require en bloc surgical removal of the injured tendon and the synovial lining of the tendon sheath. Cosmetic results are often poor, but the prognosis for athletic function is reasonable.
Enthesopathy of the Insertion of the Extensor Carpi Radialis Tendon
In the racing Standardbred, tearing of the ECRT attachment at its insertion on the proximodorsal aspect of the third metacarpal bone can cause lameness, and these tears may play a role in the cause of fractures of the dorsomedial aspect of the proximal end of the third metacarpal bone (see Chapter 37).23
CONDITIONS AFFECTING THE EXTENSOR TENDON SHEATHS Tenosynovitis Idiopathic Tenosynovitis
Idiopathic tenosynovitis may be defined as swelling with synovial effusion but without active inflammation, pain, or lameness.15 Idiopathic tenosynovitis of the extensor tendon sheaths has been reported in foals at birth.24 In adults the condition tends to develop insidiously. The pathogenesis in newborn foals is unknown, but in adult horses the pathogenesis is presumed to be caused by chronic trauma.15 However, distention of the LoDE sheath in hindlimbs often occurs bilaterally, is unassociated with any history of trauma, and is of unknown cause. Ultrasonographic examination usually reveals anechogenic fluid within the sheath. Occasionally, hypoechoic lesions are identified within the enclosed tendon, the clinical significance of which is unknown because lameness is rarely present. As idiopathic tenosynovitis usually is not associated with pain or lameness, treatment is unnecessary unless the
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owner is concerned about cosmesis. Treatment by injection of corticosteroids (10 to 20 mg triamcinolone acetonide or 40 to 80 mg methylprednisolone acetate) and pressure bandaging often only provides a temporary resolution of tendon sheath effusion.15 Anecdotal reports suggest that intrathecal injection of 4 to 15 mg of atropine sulfate, alone or with corticosteroids and hyaluronan, may cause permanent resolution of the effusion. Atropine sulfate is purported to reduce the production of synovial fluid from synoviocytes; however, its anticholinergic effects mean that its use has been associated with transient signs of mild colic and depression. The Editors have never recognized any side effects of intrathecal injection of atropine.
Acute Traumatic Tenosynovitis
Acute tenosynovitis is characterized by a rapidly developing effusion of a tendon sheath, accompanied by heat, pain, and variable lameness. Acute tenosynovitis of the extensor tendon sheaths is often caused by trauma, such as a fall or hitting a jump with the carpus.19 This injury is common in event horses, in which it often is not associated with lameness.16 Diagnosis of acute tenosynovitis is based on clinical signs and ultrasonography. Ultrasonography is useful to evaluate concurrent tendonitis of the extensor tendons and to differentiate acute tenosynovitis from other conditions that are associated with soft tissue swelling over the dorsal aspect of the carpus, including hygroma, cellulitis, synovial hernia, and effusion of the carpal joints (see Chapter 38). Horses with acute tenosynovitis of an extensor tendon sheath are treated by rest, cold hydrotherapy, and nonsteroidal antiinflammatory drugs (NSAIDs). Aspiration of fluid and injection of corticosteroids are reserved for horses that do not respond to more than 1 week of conservative treatment.25
Infectious Tenosynovitis
Infectious tenosynovitis is characterized by dramatic synovial effusion, heat, pain, swelling, and severe lameness. Contamination from a penetrating wound is the most common cause of infectious tenosynovitis, but infection also may arise from hematogenous spread of bacteria and iatrogenic infection.26 Infection of the CDET sheath has been reported as a complication of hemicircumferential periosteal transection.27 After penetration of a tendon sheath, severe lameness does not become evident unless infection becomes established. If the tendon sheath is open and draining, then the lameness may be less severe.28 Once infection is established, rapid and aggressive treatment is necessary to prevent synovial hyperplasia, fibrosis of the tendon sheath, intrathecal adhesion formation, and damage to the extensor tendons. Prompt recognition of infectious tenosynovitis is essential for a successful outcome. Diagnosis of infectious tenosynovitis is based on clinical signs and must be confirmed by synovial fluid aspiration and analysis. Infected synovial fluid is typically turbid and has a low viscosity, an elevated total nucleated cell count (>30 × 109/L, with more than 90% neutrophils), and a total protein concentration of more than 40 g/L. Contrast radiography (Figure 77-2) or ultrasonography may be useful to confirm a communication between a penetrating wound and the tendon sheath, especially if horses are examined soon after injury.
Fig. 77-2 • Lateromedial radiographic image of a carpus. Dorsal is to the left. Positive-contrast medium has been injected into the extensor carpi radialis tendon sheath and antebrachiocarpal joint in a horse that had sustained a deep puncture wound over the dorsal aspect of the carpus. Radiopaque contrast agent is present near the skin surface (solid arrow), outside the tendon sheath (small arrows), confirming penetration of the extensor carpi radialis sheath. The antebrachiocarpal joint (open arrow) was not involved.
The primary aim of treatment of horses with infectious tenosynovitis is the rapid elimination of bacteria and rapid return of the normal synovial environment. This is best achieved by wound debridement, lavage of the tendon sheath with copious amounts of sterile isotonic fluids, and the provision of bactericidal levels of appropriate antibiotics within the sheath. Appropriate systemic and intrathecal antibiotic therapy should be initiated immediately. The presence of Enterobacteriaceae most commonly is associated with tendon sheath infection caused by a penetrating wound, whereas staphylococci are most commonly identified as the cause of iatrogenic infections of a tendon sheath.26 The most effective combination of antibiotics for the treatment of infectious tenosynovitis is amikacin and cephalosporin (>85% effective).29 However, the cost of these drugs may be prohibitive, and other drug combinations, such as penicillin and gentamicin, may be considered. The initial selection of anti biotic should be altered according to bacterial susceptibility if a positive culture is obtained from the synovial fluid. Wound debridement and lavage of the affected tendon sheath with copious amounts of sterile isotonic fluids are important to reduce concentrations of bacteria and inflammatory mediators. Early in the infection, effective lavage may be achieved by through-and-through lavage using
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large-bore needles or arthroscopic egress cannulas. In more established infectious tenosynovitis, leukocytes and fibrin accumulate within the sheath, necessitating tenoscopic debridement. If satisfactory debridement and lavage have been achieved, primary closure of the sheath may be performed. The horse should be monitored closely for lameness, and repeated synovial fluid samples should be taken at 2- to 3-day intervals to monitor for return of infection. Alternatively the sheath may be left open, or a closed suction drain system may be inserted into the sheath for further elimination of inflammatory mediators and fibrin. The drain should be left in place for 3 to 5 days, but careful management is important to prevent ascending suprainfection. Systemic antibiotic therapy should be continued for at least 2 weeks after the resolution of clinical signs. Once the infection has been eliminated, intrathecal injection of corticosteroids or hyaluronan may be used, with physiotherapy, to reduce adhesion formation and restore the normal gliding movement of the tendon within its sheath. However, both drugs may cause immunosuppression within the sheath and may potentiate dormant infection. Early return to controlled exercise is also important to reduce intrathecal adhesion formation. If chronic infectious tenosynovitis becomes established, exploration of the tendon sheath and radical synovectomy may be required (see the following discussion). Successful outcomes have been reported after complete resection of the intrasynovial part of the CDET in horses with chronic infectious tenosynovitis and tendonitis.30,31 The prognosis for return to soundness after infectious extensor tenosynovitis is more favorable than for infectious flexor tenosynovitis,26,28 possibly because extensor tendons are non–weight bearing compared with flexor tendons. The prognosis for return to soundness is generally good after early surgical intervention and appropriate antibiotic therapy for horses with infectious extensor tenosynovitis.
Chronic Tenosynovitis
Chronic tenosynovitis of the extensor tendon sheaths is characterized by persistent synovial effusion, fibrous thickening of the sheath, and subcutaneous edema.32,33 Chronic tenosynovitis results in variable, sometimes severe, lameness, restricted carpal flexion, and a gait characterized by circumduction of the affected limb during protraction.33 However, in some horses there is no gait abnormality. Chronic tenosynovitis commonly arises after penetrating injuries to the carpal extensor sheaths, which may result in the inoculation of foreign material or bacteria into the synovial cavity and establishment of an infectious or noninfectious chronic tenosynovitis. The condition commonly occurs in horses jumping natural fences because of penetration of the sheath by thorns.33 Chronic tenosynovitis also may occur after acute tenosynovitis and may be associated with partial tendon rupture.22,32 Chronic inflammation of the tendon sheath causes granulomatous proliferation of the synovium, connective tissue deposition in the fibrous capsule, and fibrous adhesion formation between the tendon and its sheath, resulting in restriction of movement and pain on carpal flexion.32,33 Diagnosis of chronic tenosynovitis is based on clinical signs of effusion and thickening of the affected sheath (Figure 77-3) and restricted carpal flexion. Radiography of the carpus often reveals palisading new bone formation on
Fig. 77-3 • Chronic distention of the tendon sheath of the extensor carpi radialis in a horse with chronic tenosynovitis.
the craniodistal ridges of the radius adjacent to the affected sheaths (Figure 77-4) and entheseous new bone formation on the dorsal aspect of the carpal bones.33 Synovial fluid aspirates vary from serosanguineous to turbid, with an increased nucleated cell count and total protein concentration. Ultrasonography may demonstrate tendon damage, synovial hypertrophy, foreign bodies, and intrathecal adhesions. Conservative treatment with antibiotics, analgesics, intrathecal corticosteroids, and symptomatic physiotherapeutic procedures such as bandaging, cold hosing, massage, and forced exercise is usually unrewarding.19,22,32,33 Good results, however, have been reported after both tenoscopic and open surgical intervention.19,33,34 Tenoscopic examination of the sheaths of the ECRT, CDET, and LDET allows radical synovectomy of the hyperplastic synovium, removal of intrathecal adhesions, and debridement of damaged tendon and is associated with a good prognosis for resolution of lameness. The cosmetic appearance of most distended extensor tendon sheaths after tenoscopic surgery can be substantially improved, although most have some residual fibrosis.34 The prognosis for soundness after open surgical exploration and radical synovectomy has been reported to be good,33 although the resultant scar may cause a substantial blemish. After surgery, an intensive program of physiotherapy involving manual flexion and extension of the carpus and an ascending program of in-hand walking exercise should be initiated. Early passive motion to stimulate cavitation and reformation of the synovial lining and prevent adhesion formation is considered an essential part of the treatment.33 Initially, carpal flexion is resented, and administration of NSAIDs and sedatives may be required.
Fig. 77-4 • A craniolateral-caudomedial oblique radiographic image of the distal aspect of the radius in a horse with chronic tenosynovitis of the extensor carpi radialis tendon sheath. There is palisading new bone formation (arrows) on the craniomediodistal ridge of the radius adjacent to the tendon sheath.
Full flexion of the carpus should be achieved by 30 days after surgery.
Osteochondromatosis
Synovial osteochondromatosis of the ECRT sheath has been reported.35 Synovial osteochondromatosis is characterized by the formation of multiple small osseous bodies within a synovial-lined structure. The cause is unclear but it may be associated with trauma. Clinical signs include swelling on the dorsal surface of the carpus characterized by multiple, firm subcutaneous nodules and crepitus during joint movement, and lameness is usually evident. Diagnosis is confirmed by radiography and ultrasonography.35 No information is available for the surgical treatment of osteochondromatosis of the extensor tendon sheaths in horses, but partial synovectomy and endoscopic removal of the osteochondral bodies was successful in other synovial cavities in horses and other species.36
Intersynovial Fistulae
Intersynovial fistulae are uncommon, but they have been documented between the CDET sheath and antebrachiocarpal joint,37 the CDET sheath and middle carpal joint,15 and the ECRT sheath and middle carpal joint.5 The cause of these fistulae is unclear, but they are considered to be traumatic in origin and may occur in horses with carpitis or carpal fractures.5,37 Typically, horses with intersynovial fistulae have chronic lameness and distention of the affected tendon sheath. Synovial fluid can be massaged from the joint to the tendon sheath. Diagnosis may be confirmed by contrast radiography6 and intrasynovial analgesia. Surgical treatment is advocated in the management of intersynovial fistulae,15 but little information concerning
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Fig. 77-5 • Prominent soft tissue swelling over the dorsal aspect of the fetlock region in the hindlimb of a horse with severe lameness. This is a supratendonous infectious bursitis and should not be confused with infectious arthritis of the metatarsophalangeal joint. Arthrocentesis of the metatarsophalangeal joint through a dorsal approach may result in iatrogenic infectious arthritis.
the prognosis of affected horses is available because reports of this condition are rare. Use of tenoscopic approaches to close fistulae between the carpal joints and extensor tendon sheaths has been largely unsuccessful.34 Surgical treatment involving a combination of endoscopically assisted synovectomy and limited open exposure of the fistula, and closure of the fibrous layers of the joint and tendon sheath are generally recommended.34
Infectious Bursitis
Infectious bursitis may occur in any of the bursae associated with the extensor tendons over the dorsal aspect of the fetlock. The condition is seen most commonly in horses that jump natural obstacles (e.g., event and National Hunt horses). Typically these horses have swelling over the dorsal aspect of the fetlock (Figure 77-5) and mild-to-severe lameness. Infectious bursitis may affect the subtendonous bursae or more commonly affects an acquired subcutaneous (supratendonous) bursa on the dorsal aspect of the LoDET in the fetlock region.38 Occasionally both bursae may communicate around the lateral or medial aspect of the LoDET. Some horses have a severe, non–weight-bearing lameness. This condition frequently is confused with infectious arthritis of the metacarpophalangeal or metatarsophalangeal joints. Diagnosis is confirmed by ultrasonographic examination or contrast radiography6 and synoviocentesis. Synoviocentesis from the palmar or plantar pouch of the adjacent metacarpophalangeal or metatarsophalangeal joint should be used to rule out joint infection. Treatment consists of surgical drainage and debridement and appropriate antimicrobial therapy. The prognosis for return to soundness is good.