Clinical implications of novel variants of the fibularis (peroneus) quartus muscle inserting onto the cuboid bone: peroneocuboideus and peroneocalcaneocuboideus

Two variants of the fibularis (peroneus) quartus muscle were identified and photographed in the legs of a 70-year-old white male cadaver. A rare peroneocuboideus (fibulocuboideus) muscle (as described by Chudzinski) and a novel peroneocalcaneocuboideus (fibulocalcaneocuboideus) muscle was found in the right and left leg, respectively. The latter muscle has not been previously reported and was termed "peroneocalcaneocuboideus" on the basis of its origin and insertions. Also, the distal attachment of both muscles inserted onto the distal lip of the peroneal sulcus of the cuboid bone, which differs from the historical data. The insertion of the peroneocuboideus muscle was previously described as being at the tuberosity of the cuboid bone or, simply, the lateral surface of the cuboid. Therefore, the present case study provides the first gross anatomic photographs of these variant leg muscles along their entire length, identifies a novel fibularis quartus variant, and describes a new insertion site for the peroneocuboideus muscle. Throughout our report, the historical data are reviewed to list the prevalence and describe the clinical implications of the fibularis quartus muscle and its variants. The presence of variant fibularis quartus muscles has been known to cause lateral ankle pain and stenosis, ankle instability, fibular tenosynovitis, subluxation of the fibular (peroneal) tendons, and longitudinal splitting of the fibularis brevis tendon in radiologic and case studies. Therefore, surgeons, radiologists, and clinicians should be aware of these variant muscles when considering various diagnoses, interpreting radiographs, and pursuing surgical intervention to relieve lateral ankle pathologic features.