Peroneus tertius is an inconstant fourth anterior compartment muscle that dorsiflexes and everts the foot. When present its tendon passes through the fourth extensor compartment with the EDL before inserting on the dorsal fifth metatarsal. Its absence in 8-10% of individuals is a normal variant. It shares the dorsiflexion role with tibialis anterior, EHL, and EDL but uniquely adds eversion, bridging the anterior and lateral compartment functions.
| Origin | Anterior fibula lower third and interosseous membrane |
|---|---|
| Insertion | Dorsal surface of the fifth metatarsal base and shaft — absent in 8-10% of individuals |
| Nerve Supply | Deep peroneal nerve (L4, L5, S1) |
| Blood Supply | Anterior tibial artery |
| Actions | Ankle dorsiflexion; Foot eversion (unlike the other anterior compartment muscles) |
|---|
Peroneus tertius avulsion fractures at the fifth metatarsal dorsal base (distinct from the styloid peroneus brevis avulsion and the Jones diaphyseal fracture) are managed conservatively. The three-fracture classification of the fifth metatarsal base (Zone 1 avulsion, Zone 2 Jones, Zone 3 diaphyseal stress) is critical for management — only Zone 1 avulsions involving peroneus tertius reliably heal without surgery.
The peroneus tertius tendon, when present, is palpable on the dorsal foot lateral to the EDL tendons during resisted dorsiflexion-eversion.
Peroneus tertius avulsion fracture of the dorsal fifth metatarsal base healing predictably with protected weight bearing in a walking boot.
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