A cadaveric case study of the fibulo calcaneus internus muscle as a rare accessory muscle of calf region

Abstract

The fibulocalcaneus (peroneocalcaneus) Internus (FCI) muscle which is also called as Peroneo calcaneus Internus (PCI) muscle (of MacAlister) is a rare variation of calf muscle with a low prevalence of <1%-3% in cadaveric dissection and imaging studies. The FCI arises from the posterior-medial aspect of the distal 1/3 of the fibula, descends posterior and lateral to the flexor hallucis longus (FHL) muscle. It than traverses the tarsal tunnel inferior to the sustentaculum tali of the Calcaneus, and get inserted into the plantar surface of the calcaneus. However, controversy exists concerning the exact location of the insertion site of FCI muscle. Recently the insertion of the FCI muscle has been described as distal to the coronoid fossa, a small depression between the anterior tuberosity and the anterior apex of the sustentaculum tali of the calcaneus. However, chronological descriptions described the FCI as inserting into either the sustentaculum tali itself or a small tubercle on the medical surface of the calcaneus distal to the sustentaculum tali. During routine dissection for under graduate students, about 56-year old Indian male, a Fibulocalcaneus (peroneocalcaneus) Internus (FCI) muscle was identified, which originated from the posterior medial surface of distal third of the fibula and tendon merges with 2 slip of flexor digitorum longus and get inserted on base of distal phalanx of second toe. Knowledge of the FCI muscle is important due its involvement with various ankle pathologies, including predisposing individuals to tarsal tunnel syndrome, FHL tenosynovitis, and posterior ankle impingement and pain. The FCI 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. This muscle is often confused with the flexor digitorum accessorius longus (FDAL) muscle. Additionally, this study describes ways to differentiate between the FCI 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 involvement to reduce symptoms of posterior ankle region.