The muscle arises from variety of structures in the posterior compartment of the calf distal to the soleal line. Its tendon passes posterior to the flexor hallucis longus muscle (pink arrows) and the medial malleolus and inserts into the quadratus plantae muscle (blue arrow) or the flexor digitorum longus tendon. The presence of a flexor digitorum accessorius longus muscle has been associated with tenosynovitis of the flexor hallucis longus tendon.
The relationship of the tendon to the neurovascular structures (yellow arrows) of the ankle is also important, as compression of these structures can lead to tarsal tunnel syndrome. The flexor digitorum accessorius longus tendon is located posterior and superficial to the tibial nerve as it courses deep to the flexor retinaculum through the tarsal tunnel.
On axial MR images, the muscle is seen within the tarsal tunnel, typically superficial to the neurovascular bundle. At this point, the appearance may be similar to the peroneocalcaneus internus muscle. The flexor digitorum accessorius longus muscle, however, may contain fleshy fibers in the tarsal tunnel, which may help in differentiating the two. In addition, unlike the flexor digitorum accessorius longus muscle, the peroneocalcaneus internus muscle insert onto the calcaneus.
- Cheung YY, Rosenberg ZS, Colon E, Jahss M. MR imaging of flexor digitorum accessorius longus. Skeletal Radiol. 1999 Mar;28(3):130-7.
- Sookur PA, Naraghi AM, Bleakney RR, Jalan R, Chan O, White LM. Accessory muscles: anatomy, symptoms, and radiologic evaluation. Radiographics. 2008 Mar-Apr;28(2):481-99.