Overview
Its belly lies on the caudal forearm, between the flexor carpi ulnaris and the deep digital flexor, and contains much tendinous tissue. Above the carpus it receives its accessory ligament (lig. accessorium, the radial or "superior" check ligament), which arises from the caudomedial radius. The tendon passes through the carpal canal inside the carpal sheath and runs down the palmar cannon as the most superficial tendon.
At the fetlock it forms a ring (manica flexoria) through which the deep digital flexor tendon passes, and both run inside the digital sheath. Distal to the fetlock it splits into two branches, one on each side, that end on the first and second phalanges.
Origin
Medial epicondyle of the humerus and, through the accessory ligament, the caudomedial surface of the radius.
Insertion
Distal end of the first phalanx and proximal end of the second phalanx, on both sides.
Action
Flexes the digit and the carpus. With the limb planted, it supports the fetlock and prevents its hyperextension.
Innervation
Ulnar nerve (n. ulnaris).
Clinical relevance
- Thanks to the accessory ligament, the tendon is anchored to the radius and supports the fetlock without muscle contraction: it is part of the stay apparatus of the thoracic limb, together with the deep digital flexor and the interosseus medius.
- Superficial digital flexor tendinopathy ("bowed tendon") is one of the most common injuries of racehorses and sport horses. It mostly affects the middle third of the cannon, which bulges behind. It is diagnosed by ultrasonography, rehabilitation takes many months and re-injury is common.
- Desmotomy of the accessory ligament (superior check ligament) has been used in the treatment of tendinopathy and of some flexural deformities of the fetlock.
- Distension of the carpal sheath, proximal to the carpus, may be due to lesions of this tendon or to an osteochondroma of the distal radius rubbing on it.