Overview
In the horse the interosseus medius has almost lost its muscle tissue: it keeps only residual muscle fibres and works as a very strong elastic ligament, the suspensory ligament of the fetlock. It is present in all four limbs.
It arises on the proximal palmar (or plantar) surface of the third metacarpal (or metatarsal) bone and runs down between the two splint bones (metacarpals II and IV), close against the cannon bone and deep to the flexor tendons. In the distal third of the cannon it splits into a medial and a lateral branch, which end on the proximal sesamoid bones. From each branch an extensor branch crosses the first phalanx obliquely towards dorsal and joins the tendon of the common digital extensor (or the long digital extensor in the pelvic limb).
The distal sesamoidean ligaments continue the line of pull from the sesamoids to the first and second phalanges. Interosseus, proximal sesamoids and sesamoidean ligaments together make up the suspensory apparatus of the fetlock.
Origin
- Thoracic limb: proximal palmar surface of the third metacarpal bone and distal row of carpal bones.
- Pelvic limb: proximal plantar surface of the third metatarsal bone and distal tarsal bones.
Insertion
Abaxial surfaces of the proximal sesamoid bones and, through the extensor branches, the digital extensor tendon.
Action
Supports the fetlock and prevents its hyperextension when the limb bears weight. When unloaded, it returns part of the stored elastic energy.
Innervation
Deep branch of the lateral palmar nerve (thoracic limb) or lateral plantar nerve (pelvic limb).
Clinical relevance
- It is the most important distal part of the stay apparatus: with the digital flexors and their accessory ligaments, it holds the fetlock without muscular effort while the horse stands.
- Suspensory desmitis is classified by location: proximal, body or branches. Proximal suspensory desmitis of the hind limb is common in dressage horses and responds worse to rest than its forelimb counterpart. It is localised by anaesthesia of the deep branch of the lateral palmar or plantar nerve and confirmed by ultrasonography.
- Branch lesions are often associated with fractures of the distal end of the splint bones, which lie next to them.
- In racehorses, failure of the suspensory apparatus with fracture of the proximal sesamoid bones is a catastrophic injury.
- Degenerative suspensory ligament desmitis mainly affects Peruvian Paso horses and older horses: the fetlock sinks progressively in several limbs. Its cause is not settled.