The patient presents with repeated ankle injuries and pain over the lateral ankle.
MRI images show an accessory muscle posteromedial to the peroneus brevis and longus tendons and anterolateral to the Achilles tendon. The signal characteristics are identical to muscle and tendon on all pulse sequences.
Soft tissue masses of the foot and ankle include ganglion cysts, hemangiomas, lipomas, nerve sheath tumors, plantar fibromatosis, soft tissue chondroma, giant cell tumor of the tendon sheath, synovial sarcoma, liposarcoma, and malignant fibrous histiocytoma.
No treatment is required if the patient is asymptomatic. If the patient is symptomatic, nonoperative treatment using orthoses and physical therapy can be attempted. If conservative management is unsuccessful, surgical excision can be performed.
The MRI appearance of this finding is diagnostic because its signal and appearance are identical to the other muscles on all pulse sequences.
The accessory peroneus quartus muscle lies posterior and medial to the peroneus brevis and longus tendons. With respect to the Achilles tendon, it lies anterior and lateral. This muscle may present as a mass or incidental finding on MRI. It occurs in 13-15% of people. Most often, this muscle is asymptomatic; however, in some people it may be associated with lateral ankle pain or ankle joint instability. It may also predispose an individual to subluxation of the peroneal tendons due to its mass effect within the peroneal retinaculum.
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