Accessory peroneal muscles
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Accessory peroneal muscles are a group of accessory muscles that can occur in the foot region as a normal variant in some individuals. The peroneal compartment is known as the lateral compartment of the leg.
Peroneus quartus muscle
Originally, several accessory muscles were distinguished in the peroneal compartment:
- peroneus quartus (PQ) (Otto)
- peroneus-calcaneus externum (Hecker)
- peroneus accessorius (White)
- peroneus digiti quinti (Testut)
This terminology has been simplified by summarizing all peroneal compartment variants under the definition of a peroneus quartus muscle as a muscle arising from the lower leg and inserting onto the lateral hind and midfoot. This explains the variable insertion sites of the PQ muscle:
- the retrotrochlear eminence of the calcaneus
- the metatarsal bone of the 5th toe
- the peroneal tendons
- the lateral retinaculum of the ankle
- the cuboid bone
The origin of the PQ muscle is the distal lateral portion of the fibula. It typically descends medial and posterior to the peroneal tendons.
The PQ muscle is usually asymptomatic. Occasionally, it may lead to crowding in the retromalleolar groove, predisposing to peroneus brevis tendon attrition and tear.
Peroneus tertius muscle
The peroneus tertius muscle is not a lateral (peroneal) compartment muscle, as it runs in the same compartment as the extensor digitorum longus, which is the anterior compartment of the leg. It arises from the lower third of the anterior surface of the fibula and the interosseous membrane and an intermuscular septum connecting it with the peroneus brevis muscle (septum of Otto). Its tendon passes under the superior and inferior extensor retinacula of the ankle and inserts into the dorsal surface of the base of the 5th metatarsal bone.
It may be associated with peroneus tertius syndrome, a rare cause of anterolateral ankle pain.
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