Chapter 102The Bucked-Shin Complex
Etiology, Pathogenesis, and Conservative Management
Conditions of fatigue failure of bone and inadequacy of bone modeling and remodeling of the third metacarpal bone (McIII) in the racehorse are part of a condition known as bucked shins or dorsal metacarpal disease.1,2 Bucked shins start in young healthy racehorses, usually Thoroughbreds (TBs) and Quarter Horses, but occasionally Standardbreds (STBs), that undergo intense training for racing, usually as 2-year-olds, while the skeleton is still immature and in the growth phase (Figure 102-1, A). The true incidence of bucked shins is unknown and may vary geographically, but reports range from 30% to 90%. A North American questionnaire cited an incidence of 70%.1 Stress fractures (dorsal cortical or saucer fractures) usually occur some months after initial signs of bucked shins and may be a potentially life-threatening injury if a horse is raced or exercised at speed (see Figure 102-1, B). The diagnosis of bucked shins is easy and often made by the trainer or owner. The history of sudden tenderness or soreness of the left McIII (in North America) or both the McIIIs after high-speed work or the first race, or soreness developing the day after, are cardinal signs of early bucked shins. Horses with severe disease manifest acute lameness and extreme sensitivity to palpation of the dorsal cortex of the McIII and are unwilling to train or race. All gradations of pain or disability may be seen. Swelling and tenderness may suggest new bone proliferation. Radiology is helpful to determine the amount of periosteal new bone formation, which determines prognosis. Large accumulations of periosteal new bone on the dorsal or dorsomedial surface of the McIII suggest a serious imbalance between exercise and bone fatigue and may portend actual stress fractures that will be seen on the dorsolateral aspect of the McIII some months later.