The flexor digitorum accessory longus (FDAL) muscle is one of the most commonly encountered anomalous muscles in the foot and ankle. Literature has documented the prevalence of the FDAL anywhere from 4% to 12%, based on cadaveric limb dissection. The variability of the origin, insertion, size, and location of the FDAL muscle can cause a wide array of foot and ankle pathologies, most notably, tarsal tunnel syndrome and flexor hallucis longus syndrome. Accessory musculature should be included in the list of differential diagnoses for foot and ankle pain until proven otherwise. This report presents a patient who exhibited pain localized to the medial malleolar region and was initially diagnosed with likely tarsal tunnel syndrome. On magnetic resonance imaging, a FDAL muscle was identified and shown to be impinging on the posterior medial anatomic structures. The patient underwent excision of the FDAL and is symptom free to date. The discussion of this case report can prompt foot and ankle surgeons to be more aware of this infrequent finding as well as treatment options. Level of Evidence: Therapeutic, Level IV.
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