Discovery of a Fibulotalocalcaneus (Peroneotalocalcaneus) Muscle – Rare Variant of Fibularis Quartus

The fibularis (peroneus) quartus muscle is a variant muscle located in the lateral leg compartment that, when present, resides posterior to the fibularis longus and brevis muscles. Only found in approximately 11.5% of legs, the fibularis quartus arises from the lower one‐third of the fibula and descends to wrap posteriorly around the lateral malleolus. Confusion in the nomenclature arises due to the variations seen in regards to the distal insertion point of this variant leg muscle. The fibularis quartus can insert into the foot at a variety of locations, leading to many variations in the naming of the subtypes of this muscle. If the fibularis quartus muscle inserts into the retrotrochlear eminence and fibular (peroneal) trochlea of the calcaneus, it is called the fibulocalcaneus externus, which are the most common insertion sites. More distally, the fibularis quartus can insert into the cuboid bone, which is called the fibulocuboideus, though this variant is seen less frequently. Even further into the foot, a variation of the fibularis quartus may insert into the fifth metatarsal of the foot, changing its name to the fibularis digiti minimi (quinti). In this study, we examined the lateral leg compartment to see if a fibularis quartus may be present. In three of 20 legs (15%), a fibularis quartus muscle was identified. The first two examples were the more common fibulocalcaneus externus variant, in that this muscle inserted into the calcaneus. The third example was unique in that its distal tendon split to insert onto the calcaneus as well as the talus bone. The rare example of a fibulotalocalcaneus (peroneotalocalcaneus) muscle has only been reported once previously. This rare variant of the fibularis quartus is only the second known example of this exceedingly rare fibularis quartus variant. This finding is important clinically due to the presence of a fibularis quartus muscle being related to chronic lateral ankle pain and ankle instability. It is also important for radiologists, surgeons, and clinicians to be aware of these lateral leg muscle variants when looking at radiological imaging or in advance of surgical procedures in the lateral leg or foot. Support or Funding Information T. Walley Williams Summer Anatomy Research Fellowship; WV Research Challenge Fund [HEPC.dsr.17.06]; WVU Research Apprenticeship Program (RAP)