Incision of the flexor retinaculum of the foot fails to relieve symptoms of tarsal tunnel syndrome due to presence of a variant flexor digitorum accessorius longus (FDAL) muscle
Authors: Snyder Sean B. , Maslowski Doug J. , Zdilla Matthew J. , Lambert H. Wayne 
Published: June 2021

Tarsal tunnel syndrome (TTS), or posterior tibial neuralgia, is a neuropathy associated with compressive entrapment of the tibial nerve as it travels deep to the flexor retinaculum of the foot, or within the tarsal tunnel. A patient with TTS often presents with paresthesia and numbness within the foot radiating to the big toe and first three toes, but these symptoms can also radiate proximally into the leg as far as the knee joint. Pain, burning, tingling, and electrical sensations extend over the plantar surface of the foot and the heel. Inflammation and swelling can result leading to compression of the tibial nerve. These symptoms are exacerbated with activity or when a physician applies pressure to, or taps upon, the compressed tibial nerve, which is called a positive Tinel's sign. Surgical incision the flexor retinaculum of the foot usually relieves these TTS signs/symptoms because it creates additional space for the impinged tibial nerve in this compressive entrapment neuropathy.

A 44‐year‐old competitive male athlete, who was diagnosed with TTS, failed to gain relief from the signs and symptoms of this neuropathy after incision of the flexor retinaculum of the foot. Subsequent MRI review revealed a large variant muscle in the posterior compartment of the distal affected leg, which was later identified as a flexor digitorum accessorius longus (FDAL). This variant muscle arose from the investing fascia of the flexor digitorum longus muscle medially, the transverse intermuscular septum posteriorly, the investing fascia of the flexor hallucis longus muscle medially. This FDAL also extended as far proximally as the middle of leg, and more specifically, the lower border of the gastrocnemius muscle. Though difficult to discern on the MRI, the FDAL appeared to attach to the tibia medially and fibula laterally. Throughout its course, the FDAL muscle was located immediately posterior and impinged upon the tibial nerve. Moreover, this FDAL followed its documented course distally in that it was located posterior to the medial malleolus and entered the tarsal tunnel (deep to the flexor retinaculum), where it continued to act as a space occupying lesion. Finally, the FDAL inserted onto the intersection of the flexor digitorum longus tendon and the quadratus plantae muscle in the second muscular layer of the sole of the foot.

To regain the quality of his once active lifestyle, it was necessary for this patient to undergo an addition surgery, using an open approach, in which this FDAL muscle was excised from its proximal attachment to just below the normal location of the flexor retinaculum of the foot. With the prevalence of the FDAL reported in 2–12% of cadaveric studies, this case emphasizes the importance of MRI review before tarsal tunnel surgery is performed and the importance of identifying variant leg muscles that may be causing the compressive entrapment neuropathy of the tibial nerve.

Support or Funding Information

WVU Initiation to Research Opportunities (INTRO) Program; NASA WV Space Grant Consortium [NNX10AK62H].

This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.