Metacarpal Disease, Dorsal
Basic Information 
Epidemiology
Genetics and Breed Predisposition
Thoroughbred racehorses, racing Quarter Horses, and racing Arabians
Clinical Presentation
History, Chief Complaint
• Horses with bucked shins present with an acute onset of bilateral soft tissue swelling, heat, and sensitivity over the dorsal aspect of the metacarpi, with an associated lameness or stiffness. Very often this occurs after the first race or first speed work (a “breeze”) at near-racing distances. Some horses have a more gradual onset of these signs, with exacerbation after a race or breeze.
• Horses with an acute dorsal cortical stress fracture of MC-III typically exhibit moderate to severe lameness after a high-speed work (breeze) or race.
Physical Exam Findings
• In the acute stage, horses with bucked shins exhibit variable degrees of soft tissue swelling, heat, and sensitivity on palpation over the dorsal diaphyses of MC-III. When viewing the metacarpi from the lateral aspect, a distinct dorsal convexity is often seen. Firm pressure in this location often elicits a painful response. Lameness ranges from grade 1 to 3 (American Association of Equine Practitioners scale). These horses exhibit a bilateral stiff or choppy forelimb gait at a trot (sometimes mimicking a foot or carpal lameness). When one metacarpus is more severely affected than the other, a distinct head nod may be recognized. After a short period of rest and antiinflammatory treatment, there is generally considerable improvement.
• In horses with acute stress fractures, soft tissue swelling and signs of inflammation are commonly more focal and overlie a corresponding bony knot or periosteal irregularity (hard swelling or exostosis) along the dorsal or dorsolateral diaphysis of MC-III. Firm pressure at the fracture site consistently elicits a painful response and often exacerbates lameness. Acutely, lameness is moderate to severe (typically grade 3 to 4). With a brief period of rest (a few days to a few weeks), horses generally walk comfortably but remain lame at a trot in-hand (grade 2 to 3). Horses with chronic fractures may exhibit only mild lameness at a trot in-hand.
Etiology and Pathophysiology
• Both components of DMD are the result of maladaptive remodeling associated with repetitive strain of the metacarpus.
• Stress fractures are fatigue fractures resulting from accumulated high-strain cyclic loading.
• The majority of horses with dorsal cortical stress fractures of MC-III have experienced clinical bucked shins in the 6 to 12 months before fracture.
• In the United States, the majority of these fractures involve the left forelimb; this is attributed to the counterclockwise direction of racing.
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