Peroneus quartus muscle: a case report and review of the literature.

The peroneus quartus is a supernumerary muscle of the lateral compartment of the lower leg. There are several variations of this accessory muscle, and its terminology is very confusing. The peroneus quartus is found with a frequency varying from 10 to 21.7% of observed individuals. It is rarely involved in pathologic processes of the foot and ankle. Only a few reports exist in the literature involving peroneus quartus in a "retromalleolar conflict" (lateral ankle stenosis), sometimes in association with longitudinal attrition and tears of the peroneus brevis. We present a novel case, together with a review of the clinical, anatomic, and radiologic references about this muscle, and we try to make order in classification and terminology of its different forms.