Tendon sheath

A tendon sheath is a layer of synovial membrane surrounding a tendon. Its primary function is to reduce friction between the tendon and the surrounding tissues, allowing for smooth and efficient movement of the associated joint. These sheaths are particularly important where tendons pass over joints or through bony tunnels where friction is likely to be high.

The tendon sheath is essentially a double-layered tube, with the tendon running through the space between the two layers. The inner layer, also called the visceral layer, adheres directly to the tendon surface. The outer layer, or parietal layer, is in contact with the surrounding tissues. The space between these two layers is filled with synovial fluid, a viscous, lubricating fluid that minimizes friction during tendon movement.

Tendon sheaths are found most commonly in the hands, wrists, feet, and ankles. Areas with tight spaces or many tendons crossing bony prominences often have well-developed sheaths.

Inflammation of the tendon sheath, known as tenosynovitis, can result from overuse, repetitive motions, injury, or infection. This inflammation can cause pain, swelling, and limited range of motion. Common examples of tenosynovitis include De Quervain's tenosynovitis (affecting the tendons on the thumb side of the wrist) and trigger finger (affecting the tendons that flex the fingers).