Overview
The horse stands on digit III, made up of three phalanges. The joints of the digit are the metacarpophalangeal (fetlock), the proximal interphalangeal (pastern) and the distal interphalangeal (coffin) joints, the last one inside the hoof. The proximal and distal sesamoid bones, together with the flexor tendons and the suspensory ligament, form the system that supports the fetlock and digit during weight-bearing.
Phalanges
- Proximal phalanx (phalanx proximalis): the long pastern bone, the longest; it articulates with the condyle of Mc III and with the proximal sesamoid bones.
- Middle phalanx (phalanx media): the short pastern bone, short and cuboidal; its distal end lies partly inside the hoof.
- Distal phalanx (phalanx distalis): the coffin bone (pedal bone), which mirrors the shape of the hoof. It has a porous parietal surface attached to the wall by the laminae, a concave solar surface, the dorsal extensor process for the common digital extensor and the flexor surface for the deep digital flexor tendon. Its palmar processes carry the ungular cartilages, palpable above the heels.
Sesamoid bones
- Proximal sesamoid bones (ossa sesamoidea proximalia): two, on the palmar side of the fetlock. They receive the branches of the suspensory ligament and give origin to the distal sesamoidean ligaments; between them, the intersesamoidean ligament forms the channel through which the flexor tendons pass.
- Distal sesamoid or navicular bone (os sesamoideum distale): palmar to the distal interphalangeal joint. The deep digital flexor tendon glides over its flexor surface before inserting on the coffin bone, separated from it by the navicular bursa.
Clinical relevance
- Navicular syndrome (palmar foot pain): degeneration of the navicular bone, the navicular bursa, the deep digital flexor tendon and the ligaments supporting the bone. It mainly affects the thoracic limbs, often both, and is one of the most common causes of chronic lameness.
- Rotation and sinking of the coffin bone: in laminitis, separation of the laminae lets the coffin bone rotate or sink inside the hoof.
- Fractures of the proximal phalanx: mainly sagittal, in racehorses; fractures of the proximal sesamoid bones may come with failure of the suspensory apparatus.
- Ringbone and sidebone: the first is osteoarthritis of the pastern or coffin joint; the second, ossification of the ungular cartilages, is common in draught horses and rarely causes lameness.