The flexor hallucis longus (FHL) muscle is attached to the middle two thirds of the posterior fibula and interosseus membrane. Its long muscle and tendon lie adjacent to the distal tibia. The fibular attachment carries a blood supply of importance when the ipsilateral fibula is used to graft defects in the tibial shaft. Fracture of the lower tibia and fibula can lead to selective traumatic contracture of the FHL, which in turn may cause a flexion contracture of the big and adjacent two toes. The cross connection of the FHL to the medial aspect of the flexor digitorum longus (FDL) in the mid-sole may result in clawing of the second and third toes. Lengthening of the tendon behind the lower tibia resolves the contracture.

Surgery which shortens the first ray, such as first metatarsal osteotomy or Keller’s procedure, slackens the pull of the FHL on the big toe, transferring it to the FDL via the cross connection. This in turn results in elevation of the big toe and usually in clawing of the second and third toes. The lesser toes supplied by the FHL can be identified by the extension tenodesis test. Distal tenotomies of the long flexor tendons to the second and third toes usually resolve the clawing and restore flexion of the big toe.

The FHL tendon passes through a strong flexor sheath at the level of the talus and calcaneus. In this tunnel attrition of the posterior aspect of the FHL tendon can occur, particularly in such people as ballerinas, in whom working on point causes kinking of the tendon where it enters the mouth of the tunnel. Crepitus can be felt behind the fully plantarflexed ankle with active movement of the big toe. Surgical release of the flexor sheath and repair of the tendon may be performed.