Overview
In the horse, the deep digital flexor of the pelvic limb combines two muscles that lie on the caudal surface of the tibia, deep to the gastrocnemius and the superficial digital flexor:
- Lateral digital flexor: the larger one, made of the m. flexor hallucis longus and the m. tibialis caudalis. Its tendon is the main tendon.
- Medial digital flexor (m. flexor digitorum longus): smaller, with a long, thin tendon that runs down the medial side of the hock.
The main tendon passes over the sustentaculum tali of the calcaneus, through the tarsal canal, inside the tarsal sheath. The medial tendon joins it in the proximal metatarsus. From there it runs down the plantar surface of the metatarsus, passes through the manica flexoria of the superficial digital flexor at the fetlock and ends on the third phalanx, as in the thoracic limb.
Its accessory ligament (tarsal check ligament) is thin or absent. The model draws the muscle as one part, without a separate medial head.
Origin
- Lateral digital flexor: caudal surface of the tibia, lateral condyle of the tibia and fibula.
- Medial digital flexor: lateral condyle of the tibia.
Insertion
Flexor surface of the third phalanx (facies flexoria).
Action
Flexes the digit and extends the hock. With the limb planted, it supports the fetlock and the distal interphalangeal joint.
Innervation
Tibial nerve (n. tibialis).
Clinical relevance
- With the interosseus medius and the superficial digital flexor, it is part of the stay apparatus of the pelvic limb.
- Distension of the tarsal sheath, above the hock and on both sides, is known as thoroughpin. It is usually idiopathic and causes no lameness, but it can follow tendon injury, infection or fractures of the sustentaculum tali.
- Fractures and osteitis of the sustentaculum tali, often from wounds on the medial hock, damage the tendon inside the tarsal canal and cause severe lameness.