Discovery of a Bilateral Fibulocalcaneus Externum Invertus Muscle: A Novel Fibularis Quartus Variant

Abstract

The fibularis (peroneus) quartus is a variant muscle found within the lateral leg compartment in ~11.5% of cadaveric specimens, though recent studies have shown a higher prevalence of ~20.9 – 21.9%. The proximal muscular attachments can originate from the fibula, fibularis brevis, fibularis longus, and/or the intermuscular septi that form the boundaries of the lateral leg compartment, but it then descends to reside posterior to the lateral malleolus of the fibula. The distal attachment of the fibularis quartus varies greatly, and fibularis quartus muscular variants are often named according to location of its distal attachment. The most common distal insertion site of a fibularis quartus muscle is the lateral surface of the calcaneus into either the fibular trochlea or the retrotrochlear eminence. Therefore, this variant of the fibularis quartus inserting into the lateral surface of the calcaneus is called the fibulocalcaneus externum. During routine cadaveric dissection of a 79‐year‐old Caucasian male, a rare and novel variation of the fibularis quartus muscle was discovered. Although this variant is seen arising from the fibula, fibularis brevis, and the posterior intermuscular septum, descending posterior to the lateral malleolus, and attaching distally to the lateral surface of the calcaneus, it is unique in that the muscle is inverted, with the tendonous part being more proximal and the muscular portion arising distally. This novel muscle was found unilaterally in the left leg via dissection initially with its distal head inserting on the fibular trochlea of the calcaneus, but ultrasonographic imaging verified its bilateral presence. The subsequent dissection of the right leg revealed a similar inverted muscle, but this variation split into two distal muscle bellies inserting into the retrotrochlear eminence posteriorly and the fibular trochlear anteriorly. Given its anatomy, we have named this rare fibularis quartus variant the fibulocalcaneus externum invertus muscle. Like most fibularis quartus variants, the fibulocalcaneus externum invertus muscle can cause ankle instability, ankle locking, chronic lateral ankle pain, and peroneal compartment syndrome, especially due to acting as a space‐occupying lesion within the superior peroneal tunnel due to its distal muscle belly/bellies. While it is unknown if this person had any clinical signs and/or symptoms, clinicians, surgeons, and/or radiologists should be aware of the fibulocalcaneus externum invertus when assessing radiological imaging or preparing for ankle surgeries in and around the lateral leg compartment.