Tendon sheath of flexor hallucis longus

The tendon sheath of the flexor hallucis longus (FHL) is a complex anatomical structure that facilitates the gliding of the flexor hallucis longus tendon as it courses from the posterior ankle to the distal phalanx of the hallux. The sheath is divided into distinct zones based on anatomical landmarks and tissue composition:

• Zone 1: Located posterior to the ankle joint, from the musculotendinous junction to the orifice of the tunnel beneath the sustentaculum tali. This segment is closely associated with the retrotalar pulley, a fibrous structure that acts as a restraint and is histologically similar to other retinacula in the region. The retrotalar pulley is a key site for impingement and tenosynovitis, and its dimensions and relationship to the sustentaculum tali and posterior tubercles are highly variable.

• Zone 2: Extends from the tunnel under the sustentaculum tali to the knot of Henry. The sheath in this zone is subdivided into a proximal fibrous part (zone 2A) and a distal fascial part (zone 2B). The fibrous portion provides robust mechanical support, while the fascial segment is thinner and more pliable. The medial plantar nerve runs in close proximity to the sheath, especially in zone 2B, increasing the risk of iatrogenic injury during surgical intervention. The mean lengths of zone 2A and 2B are approximately 36 mm and 31 mm, respectively.

• Zone 3: Runs from the knot of Henry to the insertion at the base of the distal phalanx. The sheath here is thinner and less distinct, merging with the synovial environment of the toe flexors.

Throughout its course, the FHL tendon sheath is in direct contact with Kager’s fat pad and the posterior tibial neurovascular bundle, particularly in the deep ankle, which is relevant for both imaging interpretation and surgical approaches.