The fibulocalcaneus (peroneocalcaneus) internus muscle of MacAlister: Clinical and surgical implications

Abstract

A fibulocalcaneus (peroneocalcaneus) internus (PCI) muscle (of MacAlister) was identified and photographed in the left leg of a 78-year-old Caucasian female cadaver. This case study provides the first gross anatomical photo of this anomalous leg muscle and represents the first gross anatomical dissection of this muscle since 1914. The PCI muscle arose from the distal third of the fibula, posterior intermuscular septum of the leg, and flexor hallucis longus muscle. The PCI muscle inserted into the inferior surface of the medial calcaneus distal to the coronoid fossa, a small depression between the anterior tuberosity and the anterior apex of the sustentaculum tali. This insertion point differs from the historical literature, which commonly refers to the muscle inserting into the inferior surface of the sustentaculum tali of the calcaneus or simply distal to the sustentaculum tali into the medial aspect of the calcaneus. The PCI muscle is one of the least common muscular variants associated with the ankle joint, and it has been implicated in posterior ankle pain and impingement, as well as involvement in tarsal tunnel syndrome. This muscle is often confused with the flexor digitorum accessorius longus (FDAL) muscle. Additionally, this study describes ways to differentiate between the PCI and FDAL muscles in the lower leg. Radiologists and clinicians should be aware of this anomalous muscle when considering various diagnoses, interpreting radiographs, and pursuing surgical intervention to relieve posterior ankle symptoms.