Overview
The digit of the pelvic limb repeats the structure of the thoracic one: three phalanges, two proximal sesamoid bones at the fetlock (metatarsophalangeal joint) and a distal sesamoid or navicular bone plantar to the coffin joint. The differences are of shape and proportion: the hind pastern is usually somewhat shorter and more upright, and the hoof is narrower and more pointed, with a steeper dorsal wall.
Phalanges
- Proximal phalanx (phalanx proximalis): the long pastern bone; it articulates with the condyle of Mt III and with the proximal sesamoid bones.
- Middle phalanx (phalanx media): the short pastern bone, short and partly inside the hoof.
- Distal phalanx (phalanx distalis): the coffin bone. It is narrower and longer than that of the thoracic limb, with a more concave solar surface. It has the dorsal extensor process, the flexor surface for the deep digital flexor tendon and the plantar processes, which carry the ungular cartilages.
Sesamoid bones
- Proximal sesamoid bones: two, on the plantar side of the fetlock; they receive the branches of the suspensory ligament and, with the intersesamoidean ligament, form the channel for the flexor tendons.
- Navicular bone (os sesamoideum distale): plantar to the distal interphalangeal joint; the deep digital flexor tendon glides over it, separated by the navicular bursa.
Clinical relevance
- Navicular syndrome: much less common than in the thoracic limb, because the pelvic limb bears less weight.
- Osteoarthritis of the pastern joint (ringbone): common in the pelvic limbs of horses that make sharp turns and sudden stops; in advanced cases it is treated by arthrodesis of the joint, after which many horses return to work.
- Fractures of the proximal phalanx and proximal sesamoid bones: in racehorses and sport horses, as in the thoracic limb.
- Laminitis: affects the thoracic limbs more often and more severely, but in severe forms it can involve all four.