Fracture Biology, Biomechanics, and Internal Fixation

ADVANCES IN RUMINANT ORTHOPEDICS

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FRACTURE BIOLOGY, BIOMECHANICS, AND INTERNAL FIXATION Steven S. Trostle, DVM, MS, and Mark D. Markel, DVM, PhD

This article was written as a basic discussion of fracture biology, the biomechanics of fracture repair, and internal fixation as it relates to fracture repair in ruminants. 16, 26, 33-35, 39 Much of what we apply in ruminant orthopedics is derived from equine studies because of similar animal size and weight. Our understanding and knowledge of fracture biology, biomechanics, and internal fixation continues to evolve. The first section on fracture biology33, 34 discusses (1) both the macro- and microstructure and function of bone, (2) fracture healing, and (3) mechanisms of fracture union. In the second section on biomechanics, we discuss (1) basic biomechanical terminology, (2) the biomechanical behavior of bone, and (3) the influence of geometry on biomechanical behavior. The third section of this chapter presents the biomechanical and clinical applications of internal fixation techniques in ruminant orthopedics.

FRACTURE BIOLOGY

Macrostructure and Function of Bone

The skeleton provides numerous functions in the body including the protection of internal organs, the provision and facilitation of muscle

From the Comparative Orthopaedic Research Laboratory, Department of Surgical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin

VETERINARY CLINICS OF NORTH AMERICA: FOOD ANIMAL PRACTICE VOLUME 12 • NUMBER 1 • MARCH 1996

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action and body movement, the source of sites for muscle attachment, and the storage of calcium and phosphorous. 34, 39 Bone is living connective tissue made rigid by the orderly deposition of minerals and is the principle component of the skeleton. The unique structural characteristics of bone allow it to perform many roles. Bones of the appendicular skeleton are generally long cylindrical structures with narrow predominantly cortical midportions. The flared, enlarged ends of long bones diminish the stresses acting on the articular surfaces by distributing loads over a larger cross-sectional area. The immature long bone is classified into four distinct regions (Fig. 1). The physis, present at one or both ends of the bone, separates the epiphysis and the metaphysis. The central region of bone between the metaphyses is the diaphysis. The physis or growth plate is responsible for the majority of the long bone growth in young animals through a process called endochondral ossification. With maturation, the physis is obliterated and the entire expanded end of the bone is represented by the metaphysis, which is composed of trabecular (cancellous or spongy) bone surrounded by cortical and dense subchondral bone. The diaphysis is a hollow tube of cortical bone with a central cavity containing the major arterial blood supply to the bone and fatty marrow. The entire long bone, except in regions surfaced by articular cartilage or where ligaments, tendons, or joint capsules attach, is covered by periosteum. 4 The periosteum has two layers, an outer fibrous layer and an inner osteogenic layer. The outer fibrous layer is permeated by blood vessels and nerve fibers and acts in a supportive capacity. The inner

Epiphysis Growth plate Metaphysis

Diaphysis

111·.-1----

Medullary cavity

Articular cartilage

Figure 1. Illustration of an immature bovine femur demonstrating the different regions of a long bone.

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osteogenic layer provides the osteogenic cells necessary for fracture healing and is responsible for appositional growth before skeletal maturity. The osteogenic layer of the periosteum is thick, highly vascular, and firmly adhered to the bone, but with maturity becomes thin and loosely attached to the bone. Two types of bone are found in the mature skeleton at the macroscopic level. Hard, compact cortical bone is present in the shafts of long bones. Cancellous or trabecular bone, which is composed of a network of fine, interlacing partitions called trabeculae, encloses the cavities within bone that contain either hematopoietic or fatty marrow. Cancellous bone is present in most of the axial skeleton and in the ends of the long bones. Microstructure and Function of Bone

Three principal components of the microstructure of bone exist.34,39 These three components are intimately associated with each other to allow for a rapid response to the mechanical and homeostatic requirements of the body. These components include the cells, the organic extracellular matrix, and the inorganic portion of bone. Morphologically, the surfaces of bone are covered with osteoblasts and osteoclasts, which, in an antagonist manner, are responsible for constant bone turnover through simultaneous bone formation and resorption, respectively.lO Osteocytes, the third major cellular component of bone, reside within bone tissue and communicate with adjacent osteocytes and osteoblasts via channels called canaliculi. Osteoblasts, which originate from fibroblastic osteoprogenitor and mesenchymal cells, cover the majority of bone surfaces and are responsible for the formation of the organic matrix (osteoid) of bone. Osteoblasts produce the majority of the organic components of bone, including collagen, proteoglycans, and other noncollagenous proteins. Osteocytes are a portion (10 % ) of the osteoblastic population that become enclosed in matrix. Osteocytes have numerous cytoplasmic processes that extend and fill the canaliculi of bone between other osteocytes and osteoblasts to form an intricate transport and communication system within the bone. This interconnection of deeply embedded osteocytes and surfacelining osteoblasts regulates the flow of mineral ions from the extracellular space surrounding the osteoblasts and osteocytes. Osteoclasts are the cell type responsible for the majority of bone resorption. lO Osteoclasts are large, multinucleated cells on or near bone surfaces, residing within concavities called Howship's lacunae, which are the active sites of bone resorption. The area of contact between osteoclasts and bone consists of two regions: the ruffled border and a sealing zone. The ruffled border is composed of finger like membranous folds that extend various distances into the cytoplasm and are responsible for bone resorption. The sealing zone is characterized by a very dense homogenous cytoplasm that surrounds the ruffled border. The

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cytoplasmic membrane in the sealing zone is in close apposition to the bone and isolates the ruffled border, preventing leakage of lysosomal enzymes and hydrogen ions, thereby permitting these substances, which are produced by the osteoclasts, to be concentrated at the site of resorption. The organic matrix of bone acts as a supporting structure for the deposition and crystallization of inorganic salts. The composition of bone, by weight, is 21 % organic matrix, 71 % inorganic material, and 8% water. Approximately 950/0 of the organic matrix is collagen, with type I collagen the predominant collagen type found in bone. Because of its unique ultrastructure, collagen is exceedingly strong in tension. The remaining 5% of organic matrix is ground substance. The predominant constituents of ground substance are proteoglycans and glycosaminoglycans. Proteoglycans are high-molecular weight substances which, with their component acidic glycosaminoglycans, provide flexibility and resilience to the connective tissue matrix. l l Glycosaminoglycans also serve as the cementing substance between layers of mineralized collagen fibers in lamellar bone. The mineral portion of bone consists primarily of calcium and phosphate, mainly in the form of small crystals resembling synthetic hydroxyapatite crystals with the composition Cal0(~04)6(OHh.l0 Bone mineral crystals are extremely small, being 25 to 75 A in diameter and 200 A in length. The composite structure of hydroxyapatite and the organic matrix is responsible for the mechanical strength of bone. The fundamental unit of bone at the microstructural level is the osteon or Haversian system (Fig. 2). At the center of each osteon is a small channel, called a Haversian canal, that contains blood vessels, nerve fibers, and lymphatic-type channels. The osteon is composed of a concentric series of layers, or lamellae, of mineralized bone surrounding a central canal. Along the boundaries of each lamella are small spaces known as lacunae, each of which contains individual osteocytes. Canaliculi radiate from these lacunae connecting adjacent lamellae before ultimately reaching a Haversian canal. Cell processes extend from the osteocytes into the canaliculi, allowing nutrients from the blood vessels in the Haversian canal to reach the osteocyte. At the periphery of each osteon is a cement line, a narrow area of cementlike ground substance composed primarily of glycosaminoglycans. The cement line is the weakest portion of the bone's microstructure. Two distinct types of osteons are present in lamellar bone: primary and secondary osteons. Primary osteons form during appositional bone growth when the bone is increasing in diameter.14 Secondary osteons form during the continuous process of remodeling that occurs throughout life. 14, 15, 43 This process is initiated by the osteoclastic resorption of bone via a structure called a cutting cone and results in the connection of longitudinally oriented tubular cavities. Osteoblasts on the surface of the cutting cone then deposit successive layers of lamellae with an orderly fiber orientation. The newly formed cylinders of bone are called secondary Haversian systems or secondary osteons. Secondary osteons

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Circumferential lamellae

Endosteum

Haversian systems

Periosteum (split)

Branches of periosteal blood vessels

Volkmann's canals 11 :; ,i

:,{

.il

ii,

.ii III iii;:

'I

Figure 2. Illustration of the microstructural arrangement of a long bone depicted without the marrow cavity. Secondary osteons are apparent as the structural units of cortical bone. In the center of the osteons are the Haversian canals, which form the main branches of the circulatory network. Each osteon is bounded by a cement line and consists of lamellae, which are concentric rings composed of mineral matrix surrounding the Haversian canal. Along the boundaries of the lamellae are small cavities known as lacunae, each of which contains a single bone cell, or osteocyte. Radiating from the lacunae are tiny canals, or canaliculi, into which the cytoplasmic processes of the osteocytes extend. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

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consist of concentric sheets of lamella ted bone. Unlike primary osteons, these osteons are always bounded by cement lines that are formed where osteoclastic activity ceases and osteoblastic bone formation resumes. Lamellar bone, which is composed of osteons, is not the only type of bone within the body.14 The other type, called woven bone, is the first bone to appear in embryonic development and in the repair of fractures. Woven bone is characterized by coarse fiber bundles, approximately 30 f.Lm in diameter, which are randomly arranged in an interlacing fashion. Woven bone serves only as a temporary structure except in special locations such as the dental alveolus and osseous labyrinths and is gradually replaced by lamellar bone. In contrast to woven bone, lamellar bone consists of fine fiber bundles, 2 to 4 f.Lm in diameter, that are arranged irregularly in parallel, concentric curving sheets. Fracture Healing

Fracture healing results in the reconstitution of the original structure and material properties of the affected bone. The fracture healing process parallels fetal and newborn skeletal development. Bone has the unique ability to heal completely after a fracture, thereby returning to its original tissue structure and associated mechanical properties. Both local and systemic factors influence fracture healing. Local factors include degree of trauma, vascular injury, type of bone affected, degree of bone loss, degree of immobilization, infection, degree of contamination, and local pathologic conditions. Systemic factors include age, hormones, functional activity, nerve function, and nutritional status. Phases of Fracture Healing

Fracture healing is a continuum of overlapping events. For discussion purposes, fracture healing is described in three distinct phases: inflammation, reparation, and remodeling. 37 Bone reacts to fracture within a few hours by uniform periosteal cell activity. This initial cellular reaction is a fundamental response of bone to any injury and is called the primary callus response. Inflammatory Phase

The inflammatory phase is critical for the reparative phase of fracture healing. Similar to soft-tissue wounds, the inflammatory phase usually occurs over the first 2 to 3 weeks after an injury. Impairment of the inflammatory phase causes compromised tissue healing. 28 The cellular mechanisms necessary for repair and the processes protecting the healing tissue from infection are activated during the inflammatory phase. Briefly, the cellular response is regulated by chemical messengers such as kinins, complement factors, histamine, serotonin, prostaglandins, and leukotrienes. The coagulation cascade is also activated and contri-

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butes fibrin and fibrinopeptides. Collectively, they mediate the inflammatory reaction by causing vasodilation, migration of leukocytes, and chemotaxis along with platelets, which contribute growth factors initiating angiogenesis and mesenchymal cell proliferation. Locally at the site of injured tissue, granulocytes ingest and destroy bacteria, but do not contribute to repair. Macrophages and, to a lesser extent, lymphocytes aid in the destruction of bacteria and also stimulate repair by releasing angiogenetic factor( s) and other cell growth factors.28 Reparative Phase

The reparative phase overlaps and follows the inflammatory phase and may take from 2 to 12 months to be completed. During the reparative phase, the configuration of fracture healing is highly susceptible to mechanical factors such as interfragmentary motion. The inherent histologic course of fracture healing (without immobilization) begins with interfragmentary stabilization through periosteal and endosteal callus formation. 24 Interfragmentary stabilization restores continuity, and bone union occurs by intramembranous and endochondral ossification. Remodeling Phase

The remodeling phase occurs during and succeeds the reparative phase. A vascular and necrotic bone is replaced by Haversian remodeling. Minor mal alignment of fragments may be corrected by remodeling of the fracture site or by functional adaptation. This is particularly true in young animals with remaining bone growth potential. When loading bone, convex surfaces carry a positive charge and attract osteoclasts, whereas concave surfaces are negatively charged and attract osteoblasts. Subsequently, bone is removed from convex surfaces and laid down on concave surfaces. This phenomena tends to realign the bone after malunion. Fracture remodeling does not correct torsional deformities. In the inflammatory phase of healing, external callus tissue consists of primitive mesenchymal cells, fibroblasts, macrophages, and blood vessels. The origin of periosteal callus cells is controversial, though most investigators feel that the cambium layer of the periosteum is a major source of cells with both osteogenic and chondrogenic potential. The blood vessels of periosteal callus originate from surrounding extraskeletal tissues (muscles) and the medullary cavity. Whether invading vascular endothelial cells have osteogenic or chondrogenic potential is unknown. Angiogenesis involves the migration and proliferation of "endothelial cells, and this process can be stimulated by so-called angiogenetic growth factors.22 Maintenance of a hypoxic tissue gradient seems essential for the angiogenesis of healing tissue. Angiogenesis may be controlled by macrophages that produce angiogenic factors under hypoxic conditions.3D Both the fracture callus and the medullary cavity have low tissue oxygen tension during external callus formation. 2

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Mechanisms of Fracture Union

Fracture union is described in two patterns: primary (direct) bone healing and secondary (indirect or spontaneous) bone healing. Direct healing is associated with the contact of fracture fragments and healing occurs by Haversian remodeling. Indirect healing (healing with periosteal and endosteal callus) refers to the fact that intermediate fibrous tissue or fibrocartilage is formed initially and is subsequently replaced by bone. Haversian remodeling serves to revascularize necrotic fracture ends and reconstitute the fracture gap. For Haversian remodeling to occur, anatomic alignment of the fracture must be exact, fixation must be rigid, and blood supply must be sufficient. Induction and growth of the osteons (osteon remodeling) is not completely understood. The induction and proliferation of undifferentiated periosteal callus tissue is the first critical step in fracture healing by external callus. Callus formation is suppressed by rigid immobilization and minimal fracture motion. The formation of callus is dependent on numerous humoral factors. The induction and proliferation periods of periosteal callus that occur during the inflammatory and reparative phases have a limited duration. Primitive callus tissue shows a rapid chondrogenic transformation during the reparative phase. Whether cells with chondrogenic potential are derived from specific periosteal prechondrogenic cells or represent chondrocytes differentiated from primitive mesenchymal cells through signals created in the environment is unknown. The next critical step in achieving union of a fracture is the establishment of a bony bridge between the fragments. This involves the joining of osseous tissue with the entire system becoming immobile at least momentarily. During this stage of healing, inadequate fracture immobilization may cause the development of a hypertrophic nonunion by the persistence of fibrous tissue. 13 Interfragmentary motion is also important to fracture union, and changes in interfragmentary gap widths dictate the tissue formed within the gap. Interfragmentary strain is defined as the ratio between the relative displacement of the fracture ends and the initial gap width. 42 If interfragmentary strain is between 10% to 100%, only granulation tissue forms in the gap. As the fracture become more stable and inter fragmentary strain decreases to between 2% and 10%, fibrocartilage forms. When interfragmentary motion is 2% or less, bone begins to form in the gap. In endochondral ossification, bone matrix replaces the mineralized cartilage matrix. This process involves vascular invasion of the mineralized fibrocartilage. The basic cellular and biochemical changes of this process are not completely known. Current thoughts are directed at understanding the roles of collagenous and noncollagenous proteins and as growth factors in this process. During the ossification process of external callus, the total amount of calcium per unit volume of callus increases approximately fourfold. Hydroxyproline (an indicator of total collagen content) increases twofold

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while the breaking strength of the callus in tensile testing increases threefold. 3 Radiographic size of external callus is a poor predictor of fracture strength. 41 The re-establishment of fracture strength and stiffness appears more related to the amount of new bone joining the fracture fragments and less to the overall amount of uniting callus.9 BIOMECHANICS

Treatment of long bone fractures of large animals is laden with difficulties. Repair of adult long-bone fractures requires working at the mechanical limits of fracture fixation devices. 35 Precarious stability accompanied by poor fracture healing increases the risk of fixation failure before fracture union occurs. The demand for the ruminant orthopedic patient to be fully weight bearing and ambulatory in the immediate postoperative period is in stark contrast to human or small animal orthopedic repairs. Large animal surgeons must therefore understand biomechanics as it relates to fractures and their repair. Basic Biomechanical Terminology

The two most important mechanical properties of bone are strength and stiffness. 16, 33, 35, 39 These mechanical characteristics can be best determined for structures such as bone by examining the behavior of the structure when it is subjected to externally applied forces called loads. 39 When a structure such as bone is loaded, it causes a change in dimension or a deformation of the structure. When a load of known direction is placed on the structure, the deformation of that structure can be quantified and plotted on a load-deformation curve, which provides essential information regarding the structure's mechanical properties. A typical load-deformation curve for bone is illustrated in Figure 3. 35, 39 The initial linear portion of the curve, called the elastic region, is a measure of the elasticity of a structure. When a structure is loaded only through the elastic region of the curve, the structure returns to its original shape when the load is removed (nonpermanent deformation). When the structure no longer returns to its original shape when the load is removed (permanent deformation) the structure is said to yield. As the load exceeds the yield point, the structure exhibits plastic behavior. In the plastic region, the structure deforms to a much greater extent for a given load (the structure is less stiff) than it does in the elastic region of the curve. Progressively increasing the load results in the structure failing at some point. This load is the ultimate failure point on the curve. Three parameters for determining the strength of a structure are reflected in the load-deformation curve: (1) the load that the structure can sustain before failing, (2) the deformation that it can sustain before failing, and (3) the energy that it can store before failing. The strength of the structure in terms of load and deformation, or ultimate strength,

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Ultimate failure point

t

Deformation Figure 3. Load-deformation curve of a viscoelastic structure such as bone. Several important mechanical parameters can be determined from this curve. The ultimate failure point is the load at which the bone fractures. The stiffness of the structure is the slope of the initial elastic region of the curve. The area underneath the curve defines the energy the bone stores as it is loaded. On fracture, this energy is released into the bone and surrounding soft tissues. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

is indicated on the curve by the ultimate failure point. The energy storage is equal to the area under the curve. Energy to failure (the area under the curve up to the ultimate failure point) is an important concept to understand because the more energy a bone absorbs before fracturing, the greater is the soft tissue damage caused by the release of this energy during fracture propagation. The stiffness of the structure is defined as the slope of the elastic region of the curve, with a steeper slope indicating a stiffer structure. The load-deformation curve is useful for determining the mechanical properties of whole structures such as an entire bone, and is important for the understanding of fracture repair and the response of the repair to stresses. 39 This type of biomechanical testing characterizes the structural properties of the bone, which are dependent on its geometry. It does not characterize the local material properties of bone, independent of its geometry. To determine the local properties of a structure, testing conditions must be standardized. Such standardization is useful for defining the material properties of tissues. These data are crucial to our understanding of fracture repair methods, because we must know the relative material properties of the fracture fixation method in comparison to bone. Stress and strain are two other concepts important to a basic under-

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standing of biomechanics. Stress is the force per unit area that develops on a plane surface within a structure in response to an externally applied load. Normal stress is the intensity of the internal force perpendicular to a plane passing through a point in the body. Tensile stress is positive and compressive stress is negative. Commonly used units are psi or N/m2 (Pa). (1 MPa = 106 Pa = 1 N/mm2 ~ 145 psi.) Shear stress is the intensity of the internal forces parallel to a plane passing through a point in the body, and is expressed as force per unit area. Strain is defined as a localized change in dimension that develops within a structure in response to externally applied loads. The two basic types of strain are linear strain, which causes a change in the length of the specimen, and shear strain, which causes a change in the angular relationships within the structure. Linear strain is a measure of localized linear deformation (Le., lengthening or shortening) of a line oriented in a certain direction divided by its original length at a point in or on the structure. Linear strain is dimensionless and is expressed as a percentage. Shear strain is measured as the amount of angular deformation (')') of a right angle lying in the plane of interest in the sample, is expressed in radian, and is also dimensionless. Stress and strain can be determined in bone specimens by machining a standardized specimen (most commonly a cylinder or cube) and loading the specimen to failure. The results of this testing can be plotted as a stress-strain curve similar in appearance to a load-deformation curve (Fig. 4.) As in load deformation curves, loads in the elastic region do not cause permanent deformation, though once the yield point is exceeded some deformation remains after the load is removed. The strength of the material is defined as the ultimate failure stress, and the stiffness of the curve is obtained by dividing the stress at any point in the elastic portion of the curve by the strain at that same point. The stress-strain ratio (slope) is defined as Young's modulus and is applied when a sample is tested in compression or tension. When a sample is tested under pure shear forces, the slope of the linear region of the curve defines the shear modulus. The units of these moduli are the same as for stress, and higher moduli are indicative of stiffer materials. Mechanical properties differ between cortical and cancellous bone. Cortical bone is stiffer than cancellous bone but fails at a lower ultimate strain. Cancellous bone fails at approximately 75% strain whereas cortical bone fails at approximately 2% strain. Cancellous bone also has the ability to store more energy before failure than does cortical bone because of its porous structure. Cortical bone tends to be a fairly brittle material and can sustain only limited strain before fracture. Cancellous bone is a more ductile material because it can deform to a much greater degree before fracture. Bone has limited ability to deform elastically. When tested in tension, bone yields by debonding of osteons at the cement lines, the weakest portion of cortical bone. Bone, as a structure, does not respond similarly to loads in different orientations. For example, bone is stronger in compression than in tension. This phenomenon of possessing direc-

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Ultimate stress

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - Plastic region

Yield stress

Yield strain

Ultimate strain

Strain Figure 4. Stress-strain curve of a machined bone sample tested in compression. The slope of the elastic region of the curve is defined as Young's modulus in tension and compression, and as shear modulus when the specimen is subjected to pure shear forces. As in the load-deformation curve, if the specimen is loaded beyond the yield point, permanent deformation will occur. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

tional properties is called anisotropy. A material that exhibits neither structural orientation nor property dependence on the orientation direction within the material is said to be isotropic. The relationship between loading patterns and the mechanical properties of bone throughout the skeleton is extremely intricate. Generally, bone strength and stiffness are greatest in the direction in which loads are most commonly imposed. Biomechanical Behavior of Bone

The biomechanical response of bone to the forces it is subjected to depends on many factors, including the material properties of the bone tissue, the geometry of the bone, the loading mode applied (torsion, tension), the loading rate, and the frequency of loading (single cycle versus fatigue).33,35 Loading Modes

In normal daily activities, forces and moments are applied to bone in various directions, producing tension, compression, bending, shear, torsion, and combined loading (Fig. 5).33

FRACTURE BIOLOGY, BIOMECHANICS, AND INTERNAL FIXATION

Unloaded

Shear

Tension

Compression

Torsion

31

Bending

Combined loading

Figure 5. Illustration of the various loading modes as might occur in the bovine rgetacarpus. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

Tension

During tensile loading, equal and opposite loads are applied at the ends of a structure, resulting in tensile stresses and strains within the structure. Tensile stress can be thought of as many small forces acting away from the plane of interest. Maximal tensile stress occurs on a

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plane perpendicular to the applied load. When subjected to tension, the structure lengthens and narrows with failure occurring around the osteon by debonding of the cement line and pulling out of osteons. Clinically, tensile fractures occur as avulsion fractures at the traction physes of young animals such as in the olecranon or calcaneus, and as patellar fractures in older animals. These fractures are usually transverse in orientation, corresponding to the plane of maximal tensile stress. Compression

During compressive loading, equal and opposite loads are applied at the ends of a structure resulting in compressive stresses and strains within the structure. Compressive stress can be thought of as many small forces acting toward the plane of interest. Maximal compressive stress occurs on a plane perpendicular to the applied load. Under compression, the structure shortens and widens with failure occurring obliquely through osteons. The oblique orientation of the fracture corresponds to the plane of maximal shear stress (45 degrees to the orientation of the compressive load) because bone as a material is strongest in compression, is next strongest in shear, and lastly is weakest in tension. Clinically, pure compressive fractures are rare in ruminants but are the principle cause of Y-type fractures of the distal humerus and femur and some vertebral body fractures. Bending

In bending, loads are applied to a structure that causes it to bend about an axis. When a bone is loaded in bending, it is subjected to a combination of tension and compression. Tensile stresses act on one side of the neutral axis with compressive stresses acting on the opposite side (Fig. 6). The farther from the neutral axis, the greater the magnitude of these stresses. Clinically and experimentally, bending may be caused by three (three-point bending) or four (four-point bending) forces. Because bone is weakest in tension, the failure begins on the tensile surface of the bone. The fracture travels from the tensile surface of the bone to the compressive surface transversely until shear forces acting on a 45-degree plane become sufficiently high to cause a butterfly component on the compressive side of the bone (Fig. 7). A typical three-point bending fracture in ruminants would be associated with fractures of the metacarpal and metatarsal bones during assisted calving with an extraction device. Torsion

In torsion, a load is applied to a structure causing it to twist around an axis, resulting in a torque produced within the structure (Fig. 8). When a structure is subjected to torsion, shear stresses are distributed over the entire structure. As in bending, the magnitude of these stresses

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Figure 6. Illustration of a cross-section of the bovine femur subjected to bending, showing the distribution of stresses around the neutral axis (solid line). Tensile stresses act on the dorsal surface of the bone and compressive stresses act on the palmar surface. The stresses are highest on the periosteal surface of the bone and lower near the neutral axis. The tensile and compressive stresses are unequal because the bone is asymmetrical. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

is proportional to their distance from the neutral axis (usually the central axis of rotation). Therefore, for bone, the periosteal surface is subjected to the highest shear stresses when it is loaded in torsion. Shear stresses are maximal, parallel, and perpendicular to the neutral axis of the structure (the long axis of the bone). Maximum tensile and compressive stresses act on a plane 45 degrees to the neutral axis. This factor becomes important when examining the configuration of fractures that result from torsional loads. When a bone is loaded in torsion, the bone first fails in shear with the formation of the initial crack parallel (along the long axis of the cortex) to the neutral axis. A second crack then propagates along the plane of maximal tensile stress causing a spiral fracture to occur (see Fig. 7). Clinically, these torsional fractures are associated with spiral, mid-diaphyseal fractures of the humerus and femur in younger animals. Combined Loading

Combined loading is the most common loading pattern that occurs during daily activity. Loading of bone in an in vivo setting is complex because bones are subjected to multiple indeterminate loads and because their shapes are irregular. Combined loading is difficult to reproduce

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8

c

D

E

Figure 7. Illustration of typical long bone fracture morphology, corresponding to the type of external load applied to the long bone. The fracture pattern may vary depending on the magnitude of the composite loading mode involved. In compression and bending (A), the bone initially fails in tension (small arrows), and the fracture propagates toward the compression surface of the bone resulting in a large butterfly fragment. In pure bending (B), the bone again initially fails in tension (small arrows) and the fracture propagates toward the compressive surface with a smaller butterfly fragment than seen with combined bending and compression. In torsion (e), the bone fails in a spiral pattern with local shear and tension as the source of failure. In compression (D), the bone fails obliquely due to a combination of shear and compressive forces. In pure tension (E), the bone fails transversely. (Adapted from Markel MD: Fracture biomechanics. In Nixon AJ (ed): Equine Fracture Repair. Philadelphia, WB Saunders, 1992, p. 798.)

experimentally and research is being conducted to advance our knowledge and understanding of combined loading. Bone Fatigue

Bone is stiffer and sustains higher loads before failure when loads are applied at higher rates. Bone stores higher energies before failure at faster loading rates. The energy associated with trauma is dependent on the second power of the loading rate. When bone does fracture, the stored energy is released resulting in even more severe comminution and trauma to the surrounding tissue. Fractures may occur secondary to a single incident as might occur during recovery from anesthesia or by repeated applications of a load at lower magnitude. A fracture caused by repeated loads produced by

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Figure 8. Illustration of a cross-section of the distal bovine femur loaded in torsion, demonstrating the distribution of shear stresses around the neutral axis. The magnitude of the stresses are highest on the periosteal surface of the bone and lowest near the neutral axis. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

a few repetitions of high loads or by many repetitions of lower loads is called a fatigue fracture. In ruminants, most fractures are associated with trauma and are believed to be a single incident. Fatigue fractures are more common in animals such as horses that undergo repetitive continuous exertional activity. Influence of Geometry on Biomechanical Behavior

The mechanical behavior of bone is influenced by its geometry.35 In tension and compression, the load to failure and stiffness of bone are proportional to the cross-sectional area of the bone. The larger the area, the stronger and stiffer the bone. In an axial loading test of a structure with an unknown cross-sectional area (A), and material elastic modulus (E), the slope of the linear portion of the load deformation curve is defined as the axial stiffness (AE) of the structure. In bending, the cross-sectional area and distribution of the bone tissue around the neutral axis affects bone's biomechanical behavior. The quantity that takes into account these two factors is called the moment of inertia (I). A large area moment of inertia result is a stronger, stiffer bone. A third factor, the length of the bone, also influences strength and stiffness in bending. The longer the bone, the greater the magnitude of bending moment caused by the application of the force. Because of their lengths, the long bones of the skeleton are subjected to higher bending moments and therefore to high tensile and compressive forces. In a bending test of a specimen of unknown elastic modulus (E) and cross-sectional area moment of inertia (I), the slope of the linear portion of the load deformation curve provides a measure of bending

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resistance. This parameter is defined as the bending stiffness or flexural modulus of the structure. Factors affecting bone strength and stiffness in torsion are the crosssectional area and the distribution of bone around the neutral axis. The quantity that takes into account these two factors is termed the polar moment of inertia (Jo). The larger the polar moment of inertia, the stronger and stiffer the bone. In a torsional specimen with unknown shear modulus (G) and polar moment of inertia (Jo), the slope of the linear portion of the torque rotation curve provides a measure of torsional stiffness. The load a cylinder experiences when loaded under torsion is termed torque. Stress Risers

Geometric irregularities such as holes, notches, corners, and sudden changes in material properties may produce high, localized stress in structures under loading. The ratio of true maximum stress caused by these stress risers to the nominal stress calculated at the point by mechanical formulas is termed the stress concentration factor. The weakening effect of stress risers is particularly prominent under torsional loading. In sheep femora, the total decrease in bone strength in torsion was less than 5% for a defect equal to 10% of the bone diameter, 34% for a defect 20% of the bone diameter, and 62% for a 50% defect. 19, 27 For rectangular defects the length of the defect is the predominate factor in reducing the ultimate torque. BIOMECHANICAL AND CLINICAL APPLICATIONS OF METHODS OF INTERNAL FIXATION

Simple forms of orthopedic treatment have been used in the past to treat long-bone fractures in ruminants. Three major factors have advanced ruminant orthopedics. First, as discussed earlier in this chapter, our basic knowledge of bone physiology and fracture repair has grown enormously. Secondly, the size of the implants and equipment available to treat animals has increased to the point that mechanical support can be provided to large animals. Thirdly, clients demand treatment of conditions once thought of as irreparable, as the economic value and potential of selected ruminants has increased via advanced biotechnology in the form of gamete physiology. Selection of Cases for Internal Fixation

No defined guidelines exist to determine when internal fixation should be selected to repair orthopedic conditions in ruminants. 17, 20, 48 The major advantages of internal fixation are that it provides rigid stabilization of the fracture and immediate, functional use of the limb. 7

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This point cannot be overemphasized, because ruminant orthopedic patients are generally weight bearing and ambulatory in the early if not in the immediate postoperative period. The disadvantages of internal fixation are the associated costs of the general anesthesia, implants, and equipment required to apply internal fixation devices. Aside from economic issues, internal fixation requires a basic understanding of the principles of orthopedic fixation and firsthand knowledge of the implants and equipment used, which generally requires advanced training or extensive experience to develop proficiency. Internal fixation should be considered for long-bone fractures proximal to the carpus and tarsus of ruminants because these bone are difficult to stabilize with other forms of external coaptation or fixation. Highly comminuted fractures distal to the carpus and tarsus may benefit from the rigid stability offered by internal fixation.

Methods of Internal Fixation Plates

Several basic principles are important in fracture fixation using bone plates. 6, 12,35 These include bone properties, plate material and geometry, screw-bone interface, number of screws, screw material and tension, plate-bone interface, placement of the plate relative to loading, and compression between fragments (Fig. 9). The bending stiffness of a bone plate is related to the third power of the plate thickness and is directly proportional to the elastic modulus of the plate. Therefore, plate rigidity can be changed more by plate thickness than by plate modulus. The mechanical properties of bone affect the behavior of the plate-bone system. For example, less stiff bone increases the load-sharing contribution of the plate. Loads can be transmitted between plate and bone through the bone screws and through friction-type forces between the plate surface and bone. In large animal orthopedics, a bone plate is generally a load-sharing device, with some of the load supported by the plate and some load passing between bone fragments. Subjected to bending loads, a plated bone can take on a bending open (compressive surface) or bending closed (tensile surface) configuration. The placement of the plate relative to the loading direction determines the proportion of the load supported by the plate. The plate/bone composite is far more stiff in the bending closed position than in the bending open position. Plating provides the most rigid form of internal fixation used in ruminant orthopedics. Application of bone plates should be performed on the tension surface whenever possible because of the biomechanical advantages previously noted. In younger, lighter-weight animals one plate may be used to achieve stabilization. Often, in larger animals, two plates are used to obtain adequate stabilization of the fracture. The surgeon should contour the plate(s) to ensure that the entire surface of

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TROSTLE & MARKEL

Plate material and geometry

Bone properties

Plate - bone interface

Screw- bone interface

Compression between fragments

Number of screws Plate placement relative to loading Screw material and tension

Figure 9. Illustration of a bovine femur repaired with a dynamic compression plate demonstrating the factors that affect the stability of plated fractures. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

the plate is in contact with the bone (bone-plate interface). Every effort should be made to optimize the bone-plate interface because this provides for a more rigid and stable fracture repair and enhances the likelihood of a positive outcome. Plate(s) should span from the proximal to the distal metaphysis to enhance the bone-plate interface and minimize stress concentration at the ends of the bone. When two plates are used they should be placed 90 degrees to one another and end in staggered fashion so as to prevent stress concentration at the end of the bone and facilitate easier screw placement. The standard plates used to repair fractures of the long bones are the 4.5-mm narrow and the 4.5-mm broad dynamic compression plates, however other specialty plates are available (Table 1). The oval shape of the holes in dynamic compression plates allows them to be used to compress the fracture fragments. 6 Narrow plates have holes aligned in a straight line while the holes in broad plates are staggered. Narrow plates have a smaller width (12 mm versus 16 mm) and thickness (3.6 mm versus 4.5 mm) than do broad plates. Recently the use of dynamic condylar screw (DCS) and dynamic hip screw (DHS) plates has been reported in long bone fracture repair. 5 The DCS and DHS plates are of identical width to the 4.5-mm broad plates, but are thicker at 5.6 mm and 5.8 mm, respectively. Other plates such as the angled-blade plate,

ANDARD AND SPECIAL PLATES USED IN RUMINANT ORTHOPEDIC SURGERY

m)

Narrow DCP

Broad DCP

DCS Plate

DHS Plate

Angled Blade Plate

Cobra Head Plate

Condylar Buttress Plate

3.6 12

4.5 16

5.6 16

5.8 19

5.4 16

5.0 16

5.0 16

29 [2] to 263 [16] Straight

103 [6] to 359 [22] Offset

92 [5] to 204 [12] Offset

46 [2] to 270 [16] Offset

100 [6] to 260 [16] Offset

170 [8] to 218 [11] Offset

158 [7] to 285 [15] Offset

Straight

Straight

95°

nt 135°, 140°, 95° Curved end Curved end 145°, 150° 25- or 38-mm 38-mm long Angled portion 6 holes in the 6 holes in the long barrel for barrel for has U-shaped "Cobra" head Y-shaped head of the 12.5-mm 12.5-mm cross-section of the plate diameter DCS diameter DHS plate

namic compression plate; DCS = dynamic condylar screw; DHS = dynamic hip screw; H = horizontal; V = vertical. om Auer JA: Principles in Fracture Treatment. In Auer JA (ed): Textbook of Equine Surgery. Philadelphia, WB Saunders, 1992, p. 826.

2

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the Cobra-head plate, the T-plate, and the semi-tubular plate have been used in special circumstances. 6, 25, 29, 38 Recently, the use of bone plate luting has been recommended to augment fracture repair in large animal surgery.40,46 Bone plate luting involves the placement of a bone cement (polymethylmethacrylate) between the plate and bone and screw head and plate. To lute a bone plate, all the screws in the plate must be loosened and the plate lifted off the bone. The bone cement is then placed between the plate and the bone and the screws are immediately retightened. Bone cement should be prevented from entering the fracture site because this may delay or prevent healing. After retightening the screws, the bone cement is also placed in the unoccupied space in the screw holes of the plate. The two proposed mechanisms behind plate luting are that plate luting enhances the bone-plate interface and that it decreases shear stress at the screw head in the plate. Bone plate luting is not a substitute for poor contouring of plates. The additional use of antimicrobials in the bone cement may provide sustained local antimicrobial activity. Comminuted fractures should be converted to two-part fractures by attaching the fragments to the parent bone with lag screws. The use of 3.5-mm screws is recommended for this procedure because the heads can be sufficiently countersunk such that they do not interfere with bone plate application. Ideally, if one plate is used, it should be placed on the tension surface of the bone and loaded in compression. Plates placed on a tension surface should be prestressed before application to ensure transcortical compression and stability. If a second plate is used, it may be loaded in compression or in neutralization. Screws should be placed in every hole of a bone plate to maximize the bone-implant interface. Lag screw principles may be used within the bone plate and should be performed with large comminuted fragments. Screws

Two basic types of screws (cancellous and cortical) are used in ruminant orthopedic surgery (Table 2).6 The parts of a screw include the head, shaft, core, thread, pitch, shaft length, thread length, and total screw length, which differ among the different screw sizes. Cortical screws are completely threaded and have a relatively thin thread width. Cancellous screws are available in various thread lengths and have a wider thread diameter and pitch than do cortical screws. Screws are also commonly classified by their diameter. In general, 4.5-mm and 5.5-mm cortical and 6.5-mm cancellous screws are used in ruminant orthopedics. The 5.5-mm cortical screws have superior strength characteristics in adult equine third metacarpal and metatarsal bone when compared to the 4.5-mm cortical and 6.5-mm cortical screws. 52 The 6.5-mm cancellous screws are available in three different thread lengths (16 mm, 32 mm, and completely threaded). The 3.5-mm cortical screw also has been used to achieve interfragmentary compression because the head is small enough that it may be completely count-

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Table 2. SCREWS USED IN RUMINANT ORTHOPEDIC SURGERY Screw Type

Screw diameter (mm) Glide hole drill bit diameter (mm) Thread hole drill bit diameter (mm) Tap diameter (mm) Thread pitch Thread length

Cortical

Cortical

Cancellous

Cannulated

3.5

4.5

5.5

6.5

7.0

3.5

4.5

5.5

4.5

4.5

2.5

3.2

4.0

3.2

4.5

3.5

4.5

5.5

6.5

7.0

1.25 Entire

1.75 Entire

2.0 Entire

2.75 16 mm, 32 mm, entire

2.75 16 mm, 32 mm, entire

Cortical

Screw shape

J J

J

III III

Adapted from Auer JA: Principles in Fracture Treatment. In Auer JA (ed): Textbook of Equine Surgery. Philadelphia, WB Saunders, 1992, p. 826.

ersunk in the adult bovine cortex, allowing a plate to be applied over the screws. The use of a 7.0-mm cannulated screw system has been advocated for the repair of slipped capital femoral physeal fractures in adult bulls. sO The 7.0-mm cannulated screws were biomechanically compared to 5.5mm cortical and 6.5-mm cancellous screws in a femoral head fracture model. 51 Results of this study demonstrated that 7.0-mm cannulated screws had greater holding power than did 6.5-mm cancellous screws and were similar to 5.5-mm cortical screws. The cannulated screw system also offers added flexibility when screw placement is critical. Ruminant neonatal bones have a low bone density and thin bony cortices and their ability to support and sustain internal fixation devices is a primary concern. 21 Recent studies have evaluated the holding power of screws in neonatal bones.B, 30,31 Neonatal femurs showed no difference in holding power between 4.5-mm cortical, 5.5-mm cortical, and 6.5-mm fully threaded cancellous screws. 30 However, direct correlation existed between holding power and cortical thickness. 30 In reference to metacarpal and metatarsal bones of neonatal calves, 6.5-mm fully threaded cancellous screws had greater holding power than did either 5.5-mm cortical or 4.5-mm cortical screws, particularly in the metaphysis. B,31 No difference in holding power was noted between 4.5-mm and 5.5-mm cortical screws in the diaphysis or metaphysis. 31 Others have recommended the use of washers and bolts with screws to prevent bony failure at the screw-bone, screw-implant interfaces of neonatal bone. l ,20

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Screw placement should engage both the near (cis) and far (trans) cortices to achieve optimal stability. Interfragmentary compression may be achieved by use of screws both within and outside the bone plate. 6 Ideally, three screws should be proximal and three screws distal to the fracture fragments to achieve security. The use of power assistance in both tapping and placement of the screws does not alter the pullout strength when proper technique is utilized. 23 Power assistance can greatly reduce the surgical and anesthetic time and subsequent risk of complications. Intramedullary Pins and Interlocking Nails

Intramedullary devices have several advantages in fracture treatment, including restoration of bony alignment and recovery of early weightbearing in young, light-weight animals. 35 These devices are intended to stabilize a fracture by acting as an internal splint, forming a composite structure in which both the bone and the rod contribute to fracture stability. This load-sharing property of rods is fundamental to their design and should be recognized when they are used for fracture treatment. Several material and structural properties of intramedullary rods alter their axial, bending, and torsional rigidities. These include crosssectional geometry, rod length, the presence of a longitudinal slot, and the elastic modulus of the material. The cross-sectional geometry of the rod significantly affects all rigidities. In general, the overall rigidity of intramedullary rods increases with rod diameter because the moment of inertia is approximately proportional to the fourth power of the rod radius. The unsupported length of intramedullary fixation describes the distance between implant-bone contact at the proximal and distal segments of bone. This distance shortens as the fracture heals. In the initial stages of fracture healing, the unsupported length of a rod is important. For the unsupported length in bending, interfragmentary motion is proportional to the square of the unsupported length. Therefore, a small increase in unsupported length can lead to a larger increase in interfragmentary motion. With torsional loading, the unsupported length is determined by the points at which sufficient mechanical interlocking occurs between bone and implant to support torsional loads. For simple rod designs without proximal or distal locking mechanisms, little resistance to torsion may exist, and the concept of unsupported length is not applicable. Clinically, the placement of multiple pins (stack pinning) is recommended to increase intramedullary contact and enhance torsional and bending stability. The most commonly used intramedullary pins are Steinmann pins, which range in size up to 6.35 mm (0.25 inches). Intramedullary pins are available with either trocar, chisel, or trocar-threaded tips. Threaded pins are recommended for the repair of neonatal bone, which is less dense and therefore more prone to allowing migration. 44, 45 Intramedul-

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43

lary pins can be applied by a hand-driven or power-assisted device. Single or multiple stack pins may be used and placed in either normograde or retrograde fashion depending on the nature of the fracture. Intramedullary pins should be secured in the subchondral epiphysis, and care should be taken not to introduce or maintain intramedullary pins through the articular surface because of ensuing degenerative joint disease. In general, intramedullary pins should be used for diaphyseal fractures of relatively straight bones. Intramedullary pins are contraindicated for the repair of long oblique, spiral, or comminuted fractures without the use of devices that augment the primary repair and prevent overriding or rotation of the fracture fragments. Devices commonly used include cerclage wire and screws. 44 Recently intramedullary pins have been "tied in" with external skeletal fixators to provide additional stability.45 Interlocking nails (Fig. 10) have been used in human surgery to repair fractures of the femur, humerus, and tibia. Similar to intramedullary pins, interlocking nails provide stiffness in bending. The use of both proximal and distal locking with screws can prevent axial displacement of the bone along the rod and provide enhanced torsional rigidity. For

Material and structural properties of the rod

Proximal fixation mechanism

Implant - bone contact (reamed I unreamed)

Distal locking mechanism

Figure 10. Illustration of a bovine femur repaired with an interlocking, intramedullary nail demonstrating the important factors in intramedullary fracture fixation. (Adapted from Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, pp. 3-29; with permission.)

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interlocking nails, the unsupported length is typically determined by the distance between the proximal and distal locking points. Interlocking nails have been used for the repair of humeral and femoral fractures in foals. 49 A multi-institutional clinical trial is currently being performed to investigate the use of interlocking nails for longbone fracture repair in small and large animals. 18 A cadaveric, biomechanical study demonstrates promise for the use of interlocking nails in repair of diaphyseal femoral fractures in calves. 47 The strength of repairs using interlocking nails may depend to a large extent on the quality of the surrounding bone. References 1. Ames KA: Comparison of methods of femoral fracture repair in young calves. J Am Vet Med Assoc 179:458, 1981 2. Aro H, Eerola E, Aho AJ, et al: Tissue oxygen tension in externally stabilized tibial fractures in rabbits during normal healing and infection. J Surg Res 37:202, 1984 3. Aro H, Eerola E, Aho AJ: Determination of callus quantity in 4-week-old fractures of the rat tibia. J Orthop Res 3:101, 1985 4. Arnoczky SP, Wilson JW: The connective tissues. In Whittick WG (ed): Canine Orthopedics. Philadelphia, Lea & Febiger, 1990, p. 21 5. Auer JA: Application of the dynamic condylar screw (DCS) and dynamic hip screw (DHS) implant system in the horse. Veterinary Comparative Orthopedics and Traumatology 1:18, 1988 6. Auer JA: Principles in fracture treatment. In Auer JA (ed): Textbook of Equine Surgery. Philadelphia, Saunders, 1992, p. 812 7. Auer JA, Stiner A, Islein C, et al: Internal fixation of long bone fractures in farm animals. Veterinary Comp Orthopedic Traumatology 6:36, 1993 8. Bilkslager AT, Bowman KF, Abrams CF, et al: Holding power of orthopedic screws in large metacarpal and metatarsal bones of calves. Am J Vet Res 55:415, 1994 9. Black J, Perdigon P, Brown N, et al: Stiffness and strength of fracture callus: Relative rates of mechanical maturation as evaluated by a uniaxial tensile test. Clin Orthop 182:278, 1984 10. Boskey AL: Current concepts of physiology and biochemistry of calcification. Clin Orthop 167:225, 1981 11. Boskey AL: Connective tissues of the musculoskeletal system. In Slatter DH (ed): Textbook of Small Animal Surgery. Philadelphia, Saunders, 1985, p. 1926 12. Bramlage LR: Long bone fractures. Vet Clin North Am Large Anim Pract 5:285-310, 1983 13. Brown SA, Gillet NA, Broaddus TW: Biomechanics of fracture fixation by plastic rods and transverse screws. In Perren SM, Schneider E (eds): Biomechanics: Current Interdisciplinary Research. Boston, Martinus Nijhoff Publishers, 1984, p. 475 14. Carter DR, Hayes WC, Schurman DJ: Fatigue life of compact bone, II: Effects of microstructure and density. J Biomech 9:211, 1976 15. Carter DR, Spengler DM: Mechanical properties and composition of cortical bone. Clin Orthop 135:192, 1978 16. Chao EYS, Aro HT: Biomechanics of fracture fixation. In Mow VC, Hayes WC (eds): Basic Orthopaedic Biomechanics. New York, Raven Press, 1991, p 293 17. Crawford WH, Fretz PB: Long bone fractures in large animals. Vet Surg 14:295-302, 1985 18. Dueland RT, Johnson KA: Interlocking nail fixation of diaphyseal fractures in the dog: a multicenter study of 1991-1992 cases. Vet Surg 22:377, 1993 19. Edgerton BC, An KN, Morrey BF: Torsional strength reduction due to cortical defects in bone. J Orthop Res 8:851, 1990

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20. Ferguson JG: Management and repair of bovine fractures. Compend Continu Educ Pract Vet 4:S128, 1982 21. Ferguson JG: Special considerations in bovine orthopedics and lameness. Vet Clin North Am Food Anim Pract 1:131, 1985 22. Glaser BM, D'Amore PA, Seppa H, et al: Adult tissues contain chemoattractants for vascular endothelial cells. Nature 288:483, 1980 23. Gillis JP, Zardiackis LO, Gilbert JA, et al: Holding power of cortical screws after power tapping and self tapping. Vet Surg 21:362, 1992 24. Ham AW: A histological study of the early phases of bone repair. J Bone Joint Surg (A) 12:827, 1930 25. Hamilton GF: Fracture repair of long bones in cattle. In Proceedings of the 12th World Congress of Diseases in Cattle, Netherlands, 1982, p. 772 26. Hipp JA, Cheal EJ, Hayes WC: Biomechanics of fractures. In Browner BO, Jupiter JB, Levine AM, et al (eds): Skeletal Trauma. Philadelphia, Saunders, 1992, pp. 95-126 27. Hipp JA, Edgerton Be, An KN, et al: Structural consequences of transcortical holes in long bones loaded in torsion. J Biomech 23:1261, 1990 28. Hunt TK: Can repair processes be stimulated by modulators (cell growth factors, angiogenetic factors, etc.) without adversely affecting normal processes? J Trauma 24:S39, 1984 29. Kirker-Head CA, Fackleman GE: Use of the Cobra head bone plate for distal long bone fractures in large animals: A report of four cases. Vet Surg 18:227, 1989 30. Kirpenstein J, Roush JK, St-Jean G, et al: Holding power of orthopedic screws in femora of young calves. Veterinary Comp Orthopedic Traumatology 6:16-20, 1993 31. Kirpenstein J, St-Jean G, Roush JK, et al: Holding power of orthopedic screws in metatarsal and metacarpal bones of young calves. Veterinary Comparative Orthopedics and Traumatology 3:100-103, 1992 32. Knighton DR, Hunt TK, Scheuenstuhl H, et al: Oxygen tension regulates the expression of angiogenesis factor by macrophages. Science 221:1283, 1983 33. Markel MD: Fracture biology and mechanics. In Auer JA (ed): Textbook of Equine Surgery. Philadelphia, Saunders, 1992, p. 798 34. Markel MD: Bone structure and the response of bone to stress. In Nixon AJ (ed): Equine Fracture Repair. Philadelphia, WB Saunders, 1994, pp. 3-9 35. Markel MO: Fracture biomechanics. In Nixon AJ (ed): Equine Fracture Repair. Philadelphia, Saunders, 1994, pp. 10-18 36. Markel MO: Fracture healing. In Nixon AJ (ed): Equine Fracture Repair. Philadelphia, Saunders, 1994, pp. 19-29 37. McKibbin B: The biology of fracture healing in long bones. J Bone Joint Surg Br 60:150-162, 1978 38. Nemeth F: Treatment of supracondylar fractures of the femur in large animals. In Proceedings of the 12th World Congress of Diseases in Cattle, the Netherlands, 1982, p 757 39. Nordin M, Frankel VH: Biomechanics of bone. In Nordin M, Frankel VH (eds): Basic Biomechanics of the Musculoskeletal System. Philadelphia, Lea & Febiger, 1989, p. 3 40. Nunamaker OM, Richardson DW, Butterwick OM: Mechanical and biological affects of plate luting. J Orthop Trauma 5:138, 1991 41. Panjabi MM, Walter SD, Karuda M, et al: Correlations of radiographic analysis of healing fractures with strength: A statistical analysis of experimental osteotomies. J Orthop Res 3:212, 1985 42. Perren SM: Physical and biological aspects of fracture healing with special reference to internal fixation. Clin Orthop 138:175, 1979 43. Pritchard JJ: General morphology of bone. In Borune GH (ed): The Biochemistry and Physiology of Bone, volt. New York, Academic Press, 1991, p. 293 44. St-Jean G, DeBowes RM, Hull BL, et al: Intramedullary pinning of femoral diaphyseal fractures in neonatal calves: 12 cases (1980-1990). J Am Vet Med Assoc 200:1372, 1992 45. St-Jean G, DeBowes RM, Rashmir AM, et al: Repair of a proximal femoral fracture in a calf using intramedullary pinning, cerclage wire and external fixation. J Am Vet Med Assoc 200:1701, 1992

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46. Trostle SS, Wilson DC, Hanson PD, et al: Management of a radial fracture in an adult bull. J Am Vet Med Assoc 206:1917, 1995 47. Trostle SS, Wilson DG, Dueland RT, et al: In vitro biomechanical comparison of solid and tubular interlocking nails in neonatal bovine femora. Vet Surg 24:235, 1994 48. Tulleners EP: Management of bovine orthopedic problems, Part I: Fractures. Compend Contin Educ Pract Vet 8:S69, 1986 49. Watkins JP: Intramedullary interlocking nail fixation in foals: Effects of normal growth and development of the humerus. Vet Surg 19:80, 1990 50. Wilson DG, Crawford WH, Stone We, et al: Fixation of femoral physeal fractures with 7.0 mm cannulated screws in five bulls. Vet Surg 20:240, 1990 51. Wilson DG, Ulm MJ: Holding power of orthopaedic screws in bovine femoral heads: A comparison of 7.0 mm cannulated screws to 5.5 mm cortical and 6.5 mm cancellous screws. Veterinary Comparative Orthopedics and Traumatology 6:160, 1993 52. Yovich JV, Turner AS, Smith FW: Holding power of orthopedic screws in equine third metacarpal and metatarsal bones, Part II: Adult horse bone. Vet Surg 14:230, 1985

Address reprint requests to Steven S. Trostle, DVM, MS Comparative Orthopedic Research Laboratory Department of Surgical Service University of Wisconsin School of Veterinary Medicine 2015 Linden Drive West Madison, WI 53706