Overview
The carpus (ossa carpi) is the group of short bones between the forearm and the metacarpus. In equestrian language it is called the knee, although it corresponds to the human wrist; the anatomical knee of the horse is the stifle. The horse has seven or eight carpal bones in two rows, bound by numerous ligaments. They form three stacked joints: the antebrachiocarpal, which provides most of the flexion, the middle carpal, with moderate movement, and the carpometacarpal, which is almost immobile.
Bones
- Proximal row: from medial to lateral, the radial carpal bone (os carpi radiale), the intermediate carpal bone (os carpi intermedium) and the ulnar carpal bone (os carpi ulnare). Caudolaterally, the disc-shaped accessory carpal bone (os carpi accessorium) projects and is easily palpated.
- Distal row: the second, third and fourth carpal bones. The third carpal bone is the largest and bears most of the weight. The first carpal bone is small and inconstant.
- Carpal canal (canalis carpi): bounded laterally by the accessory carpal bone and palmarly by the flexor retinaculum; the digital flexor tendons pass through it inside the carpal sheath.
Clinical relevance
- Chip fractures: small fragments off the dorsal margin of the carpal bones, mainly the radial carpal, the third carpal, the intermediate carpal and the distal radius, in racehorses. They result from repeated loading in hyperextension and are removed arthroscopically.
- Slab fractures of the third carpal bone: run through the bone from proximal to distal on its dorsal aspect; they are repaired with screws.
- Carpal hygroma: subcutaneous fluid accumulation over the dorsal carpus from repeated knocks (for instance, against the stable door).
- Incomplete ossification of the cuboidal bones: in premature or dysmature foals the carpal and tarsal bones are still cartilaginous and can collapse under weight, leaving a permanent angular deformity; it is diagnosed radiographically and requires exercise restriction.