Tibialis accessorius is an anomalous muscle arising from the posterior leg and inserting with tibialis posterior at the navicular. Present in approximately 5-8% of individuals, it occupies space in the tarsal tunnel and is a cause of tarsal tunnel syndrome.
| Origin | Posterior medial tibia — variable, may arise from flexor digitorum longus or tibialis posterior |
|---|---|
| Insertion | Navicular tuberosity or medial cuneiform — joining the tibialis posterior insertion |
| Nerve Supply | Tibial nerve (L4, L5) |
| Blood Supply | Posterior tibial artery |
| Actions | Assists tibialis posterior — medial arch support; Accessory inversion and plantarflexion |
|---|
Tibialis accessorius in the tarsal tunnel reduces the space available for the tibial nerve, producing medial ankle pain and plantar numbness (tarsal tunnel syndrome). MRI identifies the anomalous muscle belly within the tunnel. Decompression of the tarsal tunnel must include release of the tibialis accessorius fascia. It can also produce a medial ankle prominence mimicking a soft tissue tumour.
May be palpable as a posteromedial ankle soft tissue mass that contracts during foot inversion.
Anomalous tibialis accessorius occupying the tarsal tunnel producing tibial nerve compression and plantar foot sensory symptoms, released during tarsal tunnel decompression surgery.
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