The flexor digitorum accessorius longus (FDAL) is an anomalous muscle arising from the posterior leg and entering the tarsal tunnel alongside the main FDL. Present in approximately 6-8% of individuals, it is a significant cause of tarsal tunnel syndrome.
| Origin | Posterior tibia or fibula — variable |
|---|---|
| Insertion | FDL tendon or quadratus plantae — before the division into individual slips |
| Nerve Supply | Tibial nerve |
| Blood Supply | Posterior tibial artery |
| Actions | Assists FDL in toe flexion |
|---|
FDAL in the tarsal tunnel reduces the available space for the tibial nerve and its branches, causing tarsal tunnel syndrome. It is identified on MRI as an additional muscle belly in the tarsal tunnel. Surgical decompression of the tarsal tunnel must include release of the FDAL fascia for complete decompression.
Not palpable externally — identified by MRI in tarsal tunnel syndrome workup.
Anomalous FDAL occupying space in the tarsal tunnel causing tibial nerve compression and plantar foot numbness, released during tarsal tunnel decompression surgery.
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