Tibialis accessorius is a rare accessory muscle variant of the medial lower leg, arising from the lower tibia and inserting into the navicular or accessory navicular bone. It passes through or adjacent to the tarsal tunnel and may be mistaken for a hypertrophied tibialis posterior or a soft tissue tumour on imaging.
| Origin | Posterior tibia in the lower leg, in continuity with or adjacent to tibialis posterior |
|---|---|
| Insertion | Navicular, medial cuneiform, or accessory navicular |
| Nerve Supply | Tibial nerve |
| Blood Supply | Posterior tibial artery |
| Actions | Assists plantarflexion and inversion, supplementing tibialis posterior function |
|---|
Tibialis accessorius is encountered incidentally on MRI or during medial ankle surgery. It can contribute to tarsal tunnel syndrome and medial ankle pain. Its relationship to the accessory navicular makes it clinically relevant in painful accessory navicular syndrome. Excision is curative when it causes symptoms.
May be palpable as an anomalous cord or prominence in the posteromedial lower leg, distal to the medial gastrocnemius.
Medial ankle and lower leg pain from tibialis accessorius causing tarsal tunnel crowding or symptomatic accessory navicular traction, managed by surgical excision.
