Overview
The horse stands on a single digit, the third, and its metacarpus is dominated by the third metacarpal bone (os metacarpale III) or cannon bone, a long, straight and very strong bone, oval in cross-section. On its palmar side lie the second (medial) and fourth (lateral) metacarpal bones, called splint bones, which do not reach the fetlock. The first and fifth metacarpal bones are absent.
Key features
- Proximal end of Mc III: a broad articular surface for the distal row of the carpus. On the dorsomedial side, the metacarpal tuberosity receives the extensor carpi radialis.
- Mc II and Mc IV: proximally they articulate with the carpus and bear some weight; their shaft tapers and ends in a small button at the distal third of the cannon. An interosseous ligament binds them to Mc III.
- Palmar surface: between the two splint bones lies a channel filled by the suspensory ligament (m. interosseus medius), which arises from the proximal palmar surface of Mc III.
- Distal end: a single condyle with a sagittal ridge that articulates with the proximal phalanx and the two proximal sesamoid bones at the fetlock.
- Dorsal surface: subcutaneous, crossed by the extensor tendons; the cannon can be palpated along its whole length.
Clinical relevance
- Splints: exostoses between a splint bone and the cannon, from inflammation of the interosseous ligament or the periosteum. They are most common on Mc II of the thoracic limb in young horses, and usually stop hurting once they have consolidated.
- Splint bone fractures: of the distal end, often associated with suspensory desmitis; proximal ones are usually caused by kicks.
- Dorsal metacarpal disease (bucked shins): pain and inflammation of the dorsal surface of Mc III in young racehorses early in training, from insufficient adaptation of the bone to load.
- Condylar fractures: in racehorses, mainly of the lateral condyle; they start at the articular surface of the fetlock and are repaired with screws.