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Common tendinous fibular sheath
A tendon sheath is a modified bursa that wraps around a tendon to protect it from friction on all sides. In the tight confines of the wrist, ankle, and digits, tendons often pass beneath fibrous bands called retinacula. The retinaculum is a connective tissue band that crosses over the tendons to keep them from being displaced upward when the muscle shortens and bends the joints. Because the retinaculum and bone create a fibro-osseous tunnel around the tendon, considerable friction can occur on all surfaces of the tendon at these locations. The fibrous membranes of tendon sheaths attach to the tendon on one side and the retinaculum and bone on the other. As the tendon moves through the tunnel the juxtaposed synovial membranes smoothly glide over each other with minimal friction.