The presence of a variant leg muscle, flexor digitorum accessorius longus (FDAL), causing tarsal tunnel syndrome, or posterior tibial neuralgia

Abstract

The flexor digitorum accessorius longus (FDAL) muscle is a variant muscle located in the posterior compartment of the distal leg. It typically originates in the distal third of the leg and can have a variable proximal attachment to the tibia, fibula, tibia and fibula, the posterior intermuscular septum, or any of the musculature in the deep posterior compartment of the leg. The FDAL then courses posterior to the medial malleolus, enters the the tarsal tunnel (deep to the flexor retinaculum), and ends by inserting onto either the flexor digitorum longus tendon and/or the quadratus plantae muscle, which are located in the second muscular layer of the sole of the foot. The prevalence of the FDAL has been reported in 6% of asymptomatic individuals based on MRI studies or 2–12% of cadaveric studies.The presence of the FDAL has been identified as a cause of tarsal tunnel syndrome, a compression neuropathy of the tibial nerve and/or its branches that become compressed as they course deep to the flexor retinaculum. As the FDAL traverses the tarsal tunnel, it lies immediately superficial to the neurovascular bundle and can act as a space‐occupying lesion, entrapping the tibial nerve and causing numbness, pain, and paresthesias in the foot, ankle, and toes. This variant muscle is best visualized through the use of magnetic resonance imaging (MRI). While there are conservative treatments for tarsal tunnel syndrome, most often surgical intervention is needed to remove the FDAL, and the success of this operative procedure is often based on the extent of damage to the tibial nerve prior to surgery. This presentation will include a patient case and show examples of the FDAL muscle following cadaveric dissection and within MRI studies. Radiologists and surgeons should be aware of presence of the FDAL muscle when considering the etiology of tarsal tunnel syndrome.Support or Funding InformationThe research of Jordan V. Swearingen and Kyle D. Miller was supported by the West Virginia University Initiation to Research Opportunities (INTRO) Program.