Tendon sheath of extensor hallucis longus

The tendon sheath of the extensor hallucis longus is a synovial structure that envelops the tendon of the extensor hallucis longus muscle as it courses from the anterior compartment of the leg into the dorsum of the foot. This sheath begins proximally just above the superior extensor retinaculum and extends distally to the level of the first metatarsophalangeal joint. The sheath is composed of an inner synovial layer, which produces synovial fluid for lubrication, and an outer fibrous layer, which provides mechanical support and prevents bowstringing of the tendon beneath the superior and inferior extensor retinacula.

As the tendon of the extensor hallucis longus passes deep to the superior extensor retinaculum and inferior extensor retinaculum, the sheath is reinforced and facilitates smooth gliding. Distally, the sheath blends with the dorsal capsule of the first metatarsophalangeal joint and the extensor hood apparatus. Anatomical variations are common: accessory tendons such as the extensor hallucis capsularis or additional slips may share the main synovial sheath or have partial sheaths, especially near the first metatarsophalangeal joint.