Overview
As in the thoracic limb, the equine metatarsus is dominated by the third metatarsal bone (os metatarsale III) or cannon bone, which carries all the weight, flanked by the vestigial second (medial) and fourth (lateral) metatarsal bones. Mt III is longer than Mc III and rounder in cross-section. The first and fifth metatarsal bones are absent.
Key features
- Proximal end: articulates with the third tarsal bone and, through the splint bones, with the first-and-second and fourth tarsal bones at the tarsometatarsal joint.
- Mt II and Mt IV: taper distally and end in a small button. The lateral one (Mt IV) is longer and thicker than the medial one.
- Dorsolateral groove: between Mt III and Mt IV runs the dorsal metatarsal artery III, the main artery of the distal pelvic limb, whose pulse is palpable here.
- Plantar surface: holds the suspensory ligament (m. interosseus medius) between the two splint bones, arising from the proximal part of Mt III.
- Distal end: a condyle with a sagittal ridge that articulates with the proximal phalanx and the proximal sesamoid bones at the fetlock.
Clinical relevance
- Proximal suspensory desmitis: common in the pelvic limbs of sport horses; the ligament is enclosed between the splint bones and the fascia, and the prognosis with conservative treatment is worse than in the thoracic limb.
- Fractures of Mt IV: mainly from kicks, as it is exposed on the lateral side.
- Bone sequestra: the dorsal surface of the cannon, covered only by skin and tendons, can develop sequestra after wounds or blows.
- Condylar fractures: less common than in the metacarpus, but with the same pattern in racehorses.