Verifying the Insertion of the Fibulocalcaneus (Peroneocalcaneus) Internus Muscle – A Rare Variant Leg Muscle

Abstract

The fibulocalcaneus (peroneocalcaneus) internus (PCI) muscle is a rare variant leg muscle with a low prevalence of <1%‐3% in cadaveric dissection and imaging studies. The PCI arises from the medial aspect of the distal third of the fibula, descends posterior and lateral to the flexor hallucis longus (FHL) muscle, traverses the tarsal tunnel inferior to the sustentaculum tali of the calcaneus, and inserts into the plantar surface of the calcaneus. However, controversy exists concerning the exact location of the insertion site of the PCI muscle. The insertion of the PCI muscle was recently described as being located distal to the coronoid fossa, a small depression between the anterior tuberosity and the anterior apex of the sustentaculum tali of the calcaneus. However, historical descriptions described the PCI 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 of a 53‐year‐old Caucasian male, a PCI muscle was identified, and it inserted into distal to the coronoid fossa on the plantar surface of the calcaneus. More specifically, it inserted into the plantar surface of the calcaneus in the space between the origin of the plantar talonavicular (spring) ligament and the plantar calcaneocuboid ligament. Knowledge of the PCI 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.