THIRD CARPAL BONE SLAB FRACTURE REPAIR PATHOGENESIS AND DECISION MAKING

Fractures of the third carpal bone (C-3) include small osteochondral fragments (chip fractures), frontal slab fractures of the radial (most common) or intermediate fossae, or both, sagittal slab fractures, most commonly occurring in the radial fossa, and table surface collapse resulting in subchondral lucency of C-3. Small osteochondral fragments are the most common fracture type, and involve only the proximal articular surface. Slab fractures involve both the proximal and distal articular surfaces. It is important to differentiate true slab fractures from chip fractures, since surgical management and prognosis differ. In Thoroughbred (TB) racehorses right C-3 fractures are more common, whereas in Standardbreds (STB) the distribution is nearly equal. Number of fragments, degree of displacement, presence of comminution of C-3 or other associated carpal bone fractures, and amount of cartilage damage all affect surgical decision making and prognosis. Breed, sex, and value of affected horses affect often determine owner acceptance of surgical alternatives as well. Fractures of C-3 like those of the distal aspect of the third metacarpal/metatarsal bone, are now know to not be single-event catastrophic failure, but rather occur as the result of abnormal bony remodeling in response to race training. Sclerosis of the dorsal aspect of C-3 from impact loading is a normal adaptive response to training and results in thickened trabeculae in subchondral bone which in some horses can span the distance between the proximal and distal subchondral plates. Varying degrees of increased radiopacity (sclerosis) can be seen in the tangential (skyline) radiographic image of C-3. Sclerotic subchondral bone may induce abnormal shear forces in overlying cartilage and may be prone to ischemia or injury. Areas of resorption and necrotic subchondral bone may form when the remodeling process of resorption outpaces bone deposition, resulting in areas of bone loss, which appear radiolucent in radiographic images. These biomechanically inferior regions are thought to predispose C-3, and specifically the radial fossa to chip or slab fracture. This abnormal remodeling process is important to keep in mind, since even after surgery and rest, the process may recur when the horse returns to training, especially if conformational or other factors are constant. Bone scintigraphy is useful in identifying horses with areas of abnormal bony remodeling and early detection of subchondral injury, before overlying articular cartilage becomes damaged and more signs such as effusion and positive response to flexion occur. Diagnosis of C-3 fractures requires well-positioned and well exposed radiographic images. Importantly, the skyline radiographic image must be positioned to see the entire radial fossa without overlap from the radial carpal bone. Accurate determination of the distal extent of large C-3 chip fractures, best judged on the lateral or dorsolateral-palmaromedial oblique image is required to differentiate these from true slab fractures, since on the skyline radiographic image, this determination can be difficult. Medial C-3 corner chip fractures, subchondral lucency, and incomplete C-3 chip fractures need to be differentiated from sagittal slab fractures, since these conditions are best handled by arthroscopic evaluation without internal fixation. True sagittal slab fractures can be seen on the dorsomedial-palmarolateral (MLO) oblique radiographic image, but numerous images may be necessary to demonstrate the fracture line. Unusual L-shaped fractures of the radial fossa and the palmar aspect of C-3 can involve both sagittal and dorsal (frontal)plane. In many horses with displaced C-3 dorsal-plane slab fractures, a separate wedge-shaped osteochondral