Tendon of flexor hallucis longus

The flexor hallucis longus tendon is the distal tendinous extension of the flexor hallucis longus muscle, which originates from the posterior surface of the fibula. The tendon courses distally along the posterior aspect of the ankle, passes through a fibro-osseous tunnel beneath the sustentaculum tali of the calcaneus, and continues along the plantar aspect of the foot. It ultimately inserts onto the base of the distal phalanx of the great toe (hallux), enabling plantarflexion of the hallux at both the interphalangeal and metatarsophalangeal joints.

The course of the FHL tendon is anatomically divided into three zones:

• Zone 1: From the musculotendinous junction to the orifice of the tunnel beneath the sustentaculum tali (posterior ankle region).

• Zone 2: From the tunnel under the sustentaculum tali to the knot of Henry (midfoot), where the FHL crosses and may connect with the flexor digitorum longus (FDL) tendon.

• Zone 3: From the knot of Henry to its insertion at the base of the distal phalanx of the hallux (great toe).

The tendon is enveloped by a sheath, which in zone 2 consists of a proximal fibrous part and a distal fascial part, with the medial plantar nerve in close proximity, especially in the distal fascial segment. The FHL tendon’s distal insertion is typically eccentric and lateralized on the distal phalanx, a feature that may be accentuated in hallux valgus deformity.