The Anatomy of the Tendon Sheath

A tendon sheath, which is a thin layer of tissue, surrounds each tendon in the body. The tendon sheath can also be called synovial lining or fibrous sheath.

Tendon sheaths help protect tendons from abrasive damage as they move. Synovial fluid, produced by the tendon sheath, maintains a barrier of moisture, which protects and lubricates tendons and their tendon sheaths. 

Overuse and traumatic injuries can cause pain or swelling of the tendon sheath, often requiring physical rest and/or additional treatment, such as medication, physical therapy, or surgery.

Anatomy

Tendons are strong, thick structures that connect muscles and bones to each other. They can withstand a degree of stretching and turning as the bones and muscles maneuver.

Synovium is connective tissue that lines the structures in the body’s joints, and a tendon sheath is a type of synovium that specifically lines tendons. 

Structure 

A tendon sheath is quite thin, but it is composed of a few layers of connective tissue—fibrous and synovial layers. The fibrous layer is supportive and protective; the synovial layer lines the tendons and produces synovial fluid.

Both of these layers are flexible and they move as the tendons move. Synovial fluid flows within the tissue layers of a tendon sheath.

Location 

Tendon sheaths are located around tendons, which are found in joints throughout the body, including the hands, arms, shoulders, legs, and feet.

Joints are complex, moveable structures composed of several types of tissue: bones, cartilage, muscles, tendons, and ligaments (which connect muscles to each other), as well as their supportive lining tissues, such as tendon sheaths. 

Function

The tendon sheaths protect the tendons when they are at rest and when they move. They prevent tendons from adhering to surrounding structures and they protect them from damage that could occur with repetitive movements.

Synovial fluid is a type of fluid that is constantly being produced by the synovial layer of a tendon sheath. This fluid nourishes the synovial tissue, and it also allows the tendons to move smoothly.

Associated Conditions

There are several conditions that can affect a tendon sheath. These ailments are associated with inflammation, which is swelling due to a proliferation of fluid and immune cells. Inflammation can occur in and around a tendon sheath as the result of injuries, overuse, or disease. 

Tenosynovitis

Tenosynovitis is inflammation of the tendon sheath. It usually becomes noticeable at a relatively early stage, when it can be treated.

This condition can affect one joint or more than one joint and can cause swelling and/or discomfort. It can be caused by overuse or by an injury.

Arthritis

Arthritis is the inflammation of a joint. It can involve any of the structures of a joint, including the tendon sheath. Osteoarthritis is inflammation caused by wear and tear, while rheumatoid arthritis is an autoimmune condition that occurs when the body’s immune system attacks its own joints.

Tendonitis

Tendonitis occurs when the tendon becomes inflamed. A tendon can be inflamed due to an overuse injury. This inflammation can also affect the tendon sheath.

Infection

An infection can contaminate a tendon and/or tendon sheath. This is not common and is usually due to severe trauma involving an open wound or an immune deficiency (a weak immune system). 

Fibrous Nodule

A fibrous nodule (also called a fibroma) is thickening of the tendon or the tendon sheath. It may cause swelling, a lump, or discomfort.

Giant-Cell Tumor

A giant-cell tumor of the tendon sheath is a benign (not dangerous) growth along the tendon sheath. It isn’t completely clear why they develop, but it is thought to result from overuse and wear and tear.

Rehabilitation

There are several treatments for conditions that affect the tendon sheath. Resting the affected joint and placing ice on it to reduce the swelling is often effective.

Anti-inflammatory medications, including over-the-counter oral (by mouth) medications or prescription-strength steroids, can reduce inflammation. Sometimes steroid injections are used to target the area of inflammation while avoiding systemic (whole body) effects.

If there is a severe injury, surgery may be necessary to repair the joint and allow the tendon sheath and other structures to heal. If you have had a major injury to your tendon and/or tendon sheath, you may also benefit from physical therapy or occupational therapy to build your strength and learn how to avoid movements that could cause further injury.