Supernumerary Muscles in the Leg and Foot: A Review of Their Types, Frequency, and Clinical Implications

Variations of the ankle anatomy are infrequent and exist as supernumerary muscles and tendons. Often understudied and overlooked, their presence can cause many complications of the lower extremity. These muscles, although often asymptomatic, can cause great pain and complications such as tenosynovitis, tarsal tunnel syndrome, lateral ankle instability, and ankle pain when they impinge on the normal anatomy of the patient. The goals of this study were to examine the prevalence of common variants such as the peroneus quartus, peroneus digiti quinti, accessory soleus, flexor accessorius digitorum longus, peroneocalcaneus internus, and tibiocalcaneus internus and to understand the morphological and clinical significance of these accessory muscles and their interactions with surrounding muscles. A literature review of anatomically and clinically based articles was conducted to evaluate the supernumerary muscles. Phrases such as accessory muscles were used to refine the search. Supernumerary muscles have an incidence of 3% to 62%, depending on the muscular compartment and the specific muscle. Although conservative treatments such as orthoses and nonsteroidal anti-inflammatory drugs may help alleviate some pain, surgical treatment yields higher success rates. Identification of supernumerary muscles is mainly by magnetic resonance imaging, but they can be detected by other methods, such as ultrasound. Due to the wide implications of these supernumerary muscles, this study attempted to create a reference guide for physicians to use in aiding their diagnoses and therapeutic plans. Although supernumerary muscles are typically asymptomatic, knowledge of these muscles is beneficial for clinicians in aiding their treatment plans.