Discovery of a Flexor Digitorum Accessorius Longus (FDAL) Muscle – Implications for Tarsal Tunnel Syndrome

Abstract

The flexor digitorum accessorius longus (FDAL) is a variant posterior leg muscle with a reported incidence of 2-8% of lower limbs. The FDAL can originate from many structures in the lower limb, including the tibia fibula, posterior intermuscular septum, or any muscle within the deep posterior leg compartment.1 The FDAL typically travels through the tarsal tunnel before inserting in the flexor digitorum longus (FDL) or the quadratus plantae. In this case, we present a FDAL variant that originates from the flexor hallucis longus (FHL) and the neurovascular bundle containing the tibial nerve and posterior tibial artery and vein and inserts distally into the FDL. We observed this variant travel superficial to the neurovascular bundle within the tarsal tunnel, closely associated with the tibial nerve. After it traversed the tarsal tunnel, the FDAL variant dove deep to the neurovascular bundle before inserting into the FDL. Because of the size and proximity of the FDAL to the tibial nerve, a person with this accessory muscle may be predisposed to symptoms of tarsal tunnel syndrome.