A Rare Variation in the Pattern of Flexor Muscles of the Foot: Absence of Flexor Digitorum Brevis is compensated by Two Accessory Flexors to Digiti Minimi

Abstract

The tendon of the Flexor Digitorum Brevis (FDB) to the 5th toe was observed to be absent in the right foot of one 76 year old female cadaver. In addition to the missing muscle, two accessory muscles, which we named Accessory Flexor Digitorum Brevis (AFDB) pars cuneus and pars calcaneus according to their boney attachments, appear to compensate morphologically and biomechanically for the missing portion of FDB. The AFDB pars cuneus was found to attach to the inferolateral aspect of the 1st cuneiform, had a muscular belly which crossed superficial to the tendons of flexor digitorum longus (FDL) for the 2nd, 3th and 4th toes and had a slender tendon which accompanied the medial aspect of the FDL tendon into the fibrous digital sheath of the 5th toe. The AFDB pars calcaneus attached to the medial aspect of the calcaneus in common with the medial head of the Quadratus Plantae (QP). The AFDB pars calcaneus tendon crossed superficial to the medial and lateral heads of the QP and accompanied the lateral aspect of the FDL tendon into the fibrous digital sheath of the 5th toe. The tendons of the two accessory flexor muscles fused into a common tendon just proximal to the 5th metatarsophalangeal joint. This common tendon accurately replaced the missing FDB tendon, splitting and attaching to the sides of the middle phalanx of the 5th toe with the FDL tendon passing through the splitting terminal tendons. In this case two accessory flexor muscles appeared to compensate for the missing portion of the FDB muscle. This finding supports the hypothesis that biomechanical compensation may play a role in guiding myotomal cell migration during embryogenesis and may help to explain the occurrence of muscular variants.