Tendon Sheaths in the Foot
A tendon sheath is a membrane that wraps around a tendon, which allows the tendon to stretch and prevents it from adhering to the overlying fascia. This sheath also produces a fluid, known as synovial fluid, which keeps the tendon moist and lubricated. Tendon sheaths consist of two layers: a fibrous layer, made of tight collagenous tissue, and a synovial layer. The synovial part of the tendon sheath consists of a visceral and parietal layer separated by synovial fluid. There are also fibrous bands, known as retinacula, which make a tunnel around the tendons. The tendon sheaths are located between these two structures and thus prevent friction between them. This article will talk about the tendon sheaths within the foot in detail, followed by any relevant clinical pathology.
Anterior to the ankle, there are three sheaths covering four of the tendons of the foot.
The first sheath encloses the tibialis anterior tendon and extends from the proximal aspect of the superior extensor retinaculum to the part of the inferior extensor retinaculum where it divides into two limbs.
The second sheath is a common sheath for the tendons of the fibularis tertius and extensor digitorum longus muscles. The sheath extends from the level of the malleoli to the base of the fifth metatarsal.
The third sheath, for the extensor hallucis longus tendon, begins just distal to the origin of the second sheath and extends to the base of the first metatarsal bone.
There are three sheaths located posteromedial to the ankle for the tendons of the tibialis posterior, flexor hallucis longus and flexor digitorum longus muscles. The sheath enclosing the tibialis posterior tendon runs from a point 4 cm superior to the medial malleolus and ends just proximal to where the tendon to the tuberosity of the navicular attaches. The second sheath, enclosing the flexor hallucis longus tendon, extends from the level of the medial malleolus to the base of the first metatarsal. The sheath of flexor digitorum longus extends just above the malleolus and terminates at the navicular.
Another sheath, posterolateral to the ankle, encloses the tendons of the fibularis longus and brevis muscles. The sheath is single at the proximal part of the tendons but it becomes double at the distal end. It extends proximally and distally for about 4 cm from the tip of lateral malleolus.