Overview
It is the deepest muscle of the caudal forearm. It has three heads, humeral (the largest), ulnar and radial, which join above the carpus in a common tendon. The tendon passes through the carpal canal and, at mid-cannon, receives its accessory ligament (the carpal or "inferior" check ligament), which arises from the palmar carpal ligament.
At the fetlock it passes through the ring of the superficial digital flexor and glides over the palmar surface of the proximal sesamoid bones, inside the digital sheath. In the pastern it widens and enters the hoof, where it passes over the flexor surface of the navicular bone (distal sesamoid), separated from it by the navicular bursa (bursa podotrochlearis), before inserting on the third phalanx.
Origin
- Humeral head: medial epicondyle of the humerus.
- Ulnar head: medial surface of the olecranon.
- Radial head: caudal surface of the radius.
Insertion
Flexor surface of the third phalanx (facies flexoria).
Action
Flexes all the joints of the digit and the carpus. With the limb planted, it supports the fetlock and the distal interphalangeal joint.
Innervation
Median and ulnar nerves (n. medianus, n. ulnaris).
Clinical relevance
- With its accessory ligament, which anchors it to the carpus, it is part of the stay apparatus of the thoracic limb.
- The tendon, navicular bone and navicular bursa form the podotrochlear apparatus. Their lesions are a major cause of palmar foot pain (navicular syndrome); magnetic resonance imaging often shows tendon lesions at the level of the navicular bone.
- Desmotomy of the accessory ligament (inferior check ligament) is used to correct flexural deformity of the distal interphalangeal joint ("club foot") in foals.
- In chronic laminitis with rotation of the third phalanx, tenotomy of this tendon reduces its pull on the bone; it is kept for severe cases that do not respond to other treatment.