The flexor tendons that provide the tension to bend the fingers, run from the forearm. In the fingers, there is a complex system of " pulleys " called the flexor sheath. The tendons are lined by a layer of tissue called the tenosynovium. This allows them to glide smoothly through the various " pulleys ".

Tenosynovitis is an inflammation in the lining of the fluid-filled sheath (called the synovium) that surrounds a tendon (the cord that joins muscle to bone). It usually occurs with tendonitis and it is related to stenosing tenosynovitis

The synovium or synovial membrane is a thin, weak layer of tissue which lines the non-cartilaginous surfaces within the joint space, sealing it from the surrounding tissue.

The membrane contains a fibrous outer layer, as well as an inner layer that is responsible for the production of specific components of synovial fluid, which nourishes and lubricates the joint. The membrane is also responsible for the removal of undesirable substances from the synovial fluid. Small villi can sometimes be found on its surface.

It can become irritated and thickened in conditions such as rheumatoid arthritis. When this happens, the synovium can become more of a problem for the joint, and sometimes must be removed.