The accessory flexor digitorum longus is an anomalous additional muscle belly arising from the posterior compartment of the lower leg, joining the FDL tendon at or proximal to Henry's node. It is present in approximately 6-8% of individuals and is one of the accessory muscles most commonly causing tarsal tunnel syndrome by occupying the tarsal tunnel and compressing the tibial nerve and its branches within the confined retinacular space.
| Origin | Variable: posterior fibula, tibia, flexor hallucis longus, or interosseous membrane in the posterior lower leg |
|---|---|
| Insertion | Joins the flexor digitorum longus tendon proximal to Henry's node or inserts independently on the FDL or flexor digitorum brevis |
| Nerve Supply | Tibial nerve |
| Blood Supply | Posterior tibial or peroneal artery branches |
| Actions | Assists flexion of the lesser toes; may contribute to toe flexion independently |
|---|
The accessory FDL is the most common accessory muscle causing tarsal tunnel syndrome: as the muscle belly descends to become tendinous at the ankle, it occupies space within the tarsal tunnel alongside the tibial nerve, FHL, FDL, and tibialis posterior. Exercise-induced enlargement of the muscle belly in the tarsal tunnel produces a dynamic compression pattern worsened by activity. Ultrasound or MRI identifies the accessory muscle belly; surgical fasciotomy or excision of the accessory muscle relieves the compression.
An accessory FDL muscle belly descending into the tarsal tunnel compresses the tibial nerve producing exercise-induced medial ankle pain and paresthesiae in the sole; ultrasound demonstrates the accessory muscle belly within the tarsal tunnel and surgical decompression with excision of the accessory belly provides lasting relief.
