An unusual variation of the flexor digitorum accessorius longus muscle—its anatomy and clinical significance

In this article we describe a variant accessory muscle found in the deep posterior compartment of the leg in a 96 year-old female human cadaver. The flexor digitorum accessorius longus was found bilaterally, originating by two heads from the shafts of the tibia and fibula. The two heads of the muscle were observed to be subequal in size and none of the fleshy fibers of the muscle entered the tarsal tunnel. This stands in contrast with previous descriptions of accessory digital flexors, which are typically asymmetrical or single-headed, and which almost always introduce fleshy fibers into the tarsal tunnel. This variant structure has the potential to entrap the tibial nerve and compress the posterior tibial vessels either proximal to the talocrural joint or within the tarsal tunnel, and may be implicated in clinical scenarios involving imaging, surgery, compartment syndromes, or deep-vein thromboses.