Overview
In cattle the metacarpus is dominated by the large metacarpal bone or cannon bone (ossa metacarpalia III et IV), the result of fusion of metacarpals III and IV, which belong to the two weight-bearing digits. The only remnant of the others is a small metacarpal V on the lateral side. The cannon bone is straight, flattened dorsopalmarly and covered almost only by skin, tendons and vessels, with no muscle bellies.
Key features
- Proximal end: a flat articular surface for the distal row of the carpus. On the dorsomedial side, the tuberosity of metacarpal III receives the extensor carpi radialis.
- Dorsal longitudinal groove (sulcus longitudinalis dorsalis): runs down the dorsal surface and marks the line of fusion of the two bones; it carries vessels. The palmar surface has a similar, shallower groove.
- Proximal and distal metacarpal canals (canalis metacarpi proximalis and distalis): pass through the bone at the ends of the dorsal groove and carry vessels between the dorsal and palmar surfaces.
- Metacarpal V (os metacarpale V): a small, rod-like bone applied to the lateral side of the proximal end.
- Distal end: two separate condyles, each with a sagittal ridge, which articulate with the proximal phalanx and the two proximal sesamoid bones of their digit at the fetlock. Between them lies the intercapital notch (incisura intercapitalis).
Clinical relevance
- Fractures in calves: the metacarpus is one of the bones most often fractured in calves, frequently from excessive traction during assisted calving. They may be diaphyseal or run through the distal physis (physeal fractures).
- Treatment: closed fractures heal well with external coaptation in a cast. Because the bone has little soft tissue cover, open fractures are common and worsen the prognosis.
- Examination: being subcutaneous, the outline of the cannon bone and its condyles is easy to palpate.