The peroneus tertius is an inconstant muscle, absent in approximately 8 to 10 percent of people, that arises from the lower fibula anterior surface as part of the extensor digitorum longus group and inserts on the fifth metatarsal dorsum. Unlike the peroneus longus and brevis which are posterior compartment evertors, the peroneus tertius is an anterior compartment muscle that combines dorsiflexion with eversion. Its absence causes no functional deficit because the EDL and peroneal muscles compensate.
| Origin | Anterior surface of the lower fibula and adjacent interosseous membrane (distal to the extensor digitorum longus) |
|---|---|
| Insertion | Dorsum of the base of the fifth metatarsal |
| Nerve Supply | Deep peroneal nerve (L5, S1) |
| Blood Supply | Anterior tibial artery |
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The combination of dorsiflexion and eversion it produces helps resist inversion forces during landing and direction changes, and its anterior compartment location gives it a slightly different mechanical advantage than the peroneal evertors behind the lateral malleolus.
Peroneus tertius avulsion fractures involving the fifth metatarsal dorsum are distinct from styloid avulsions of the peroneus brevis and Jones fractures, and generally heal uneventfully with conservative management. Its absence is a normal anatomical variant that does not affect clinical assessment of ankle dorsiflexion or eversion.
When present, the peroneus tertius tendon is palpable on the dorsal foot lateral to the EDL tendons during resisted ankle dorsiflexion with eversion, running toward the fifth metatarsal base.
Avulsion of the tendon from the fifth metatarsal dorsum during forced plantarflexion-inversion, producing dorsal lateral foot pain at the fifth metatarsal base managed conservatively with immobilisation.
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