A 57-year-old woman who was referred to our hospital with a longstanding retromalleolar pain and local swelling in the right ankle. Ultrasound scans showed an accessory peroneal muscle. This muscle was identified as a peroneus quartus muscle. Punctum maximum for pain was the tuberculum peroneum radiating into her posterolateral ankle.
Ultrasound shows a muscle structure located posteromedially to the peroneal tendons. Its proximal insertion is from the peroneus brevis muscle, and the distal insertion is the trochlear process of the calcaneum (peroneal tubercle). The peroneus brevis has a normal shape, and the peroneus longus is regular in size and fibrillar pattern as well. The inferior and the superior peroneal retinaculum is normal in thickness and forms the convex roof for the three tendons. There is no tendon instability. The peroneal tubercle is a bit larger than normal, and the peroneus longus tendon is running over this tubercle but is not affected. The tubercle can be palpated as a local firm lump at the level of the lateral aspect of the calcaneum (see clinical picture below). The tubercle is painful probably because of the insertion of the peroneum quartus tendon into this tubercle or because of the narrowed osteofibrous tunnel by the accessory peroneal tendon in combination with the hypertrophied tubercle. Probably the inferior peroneum retinaculum induces impingement of the peroneal tendons.
The third accessory tendon in the common sheath for both the peroneals must be a peroneus quartus accessory tendon. The peroneus quartus lies posteromedial to the peroneals. The location of the distal myotendinous junction, the origin and the insertion onto the retrotrochlear eminence of the calcaneus and the sonographic appearance in the retro- and inferior malleolar area is commonly seen for a quartus.
The presence of an anomalous peroneus quartus results in crowding of the lateral ankle compartment, and the peroneus quartus muscle prevents the ankle for dorsiflexion and becomes tensile during walking. The quartus tendon in the inframalleolar region impinges the peroneus brevis and longus tendons in the osteofibrous tunnel of the inferior peroneal retinaculum.