The peroneus quartus is an accessory peroneal muscle present in approximately 10-22% of individuals, arising from the posterior fibula or peroneus brevis and inserting variably around the lateral ankle. It occupies the retromalleolar groove alongside the peroneus brevis and longus, potentially causing overcrowding and peroneal tendon instability or split tears.
| Origin | Posterior fibula or peroneus brevis muscle belly in the lower leg |
|---|---|
| Insertion | Variable: retromalleolar groove of the fibula, the peroneal trochlea of the calcaneus, or the cuboid |
| Nerve Supply | Superficial peroneal nerve |
| Blood Supply | Peroneal artery perforators |
| Actions | Variable; may assist eversion; may occupy space in the retromalleolar groove and affect peroneal tendon stability |
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The peroneus quartus is identified on MRI as an additional muscle-tendon unit in the retromalleolar groove. It may cause peroneal tendon crowding, predisposing to peroneus brevis longitudinal split tears and superior peroneal retinaculum injury. In peroneal tendon surgery, the peroneus quartus must be identified and either retained or excised depending on its size and whether it contributes to tendon crowding. Its presence may explain some cases of unexplained lateral ankle pain after ankle sprain.
An accessory peroneus quartus occupying the retromalleolar groove alongside the peroneus longus and brevis creates a three-tendon crowding that predisposes to peroneus brevis longitudinal splitting; MRI identifies the supernumerary tendon and surgical treatment involves excision of the peroneus quartus combined with repair of any associated peroneus brevis split.
