The tendon sheath is the sheath filled with synovial fluid (joint lubricant) that usually surrounds tendons in human and also animal bodies. The tendon sheath plays a supporting protective role in this process, but it is just as susceptible to various diseases as it is to wear and tear and injury.
What is the tendon sheath?
Infographic on the to the anatomy, location and areas of inflammation in tendon sheathitis. Click image to enlarge. The tendon sheath is the double-walled sheath filled with the body’s own synovial fluid (technically called synovial fluid or synovia) that usually completely surrounds a healthy vein. Here it assumes a protective and relief function. In addition to the possibility of sustaining injuries to the tendon sheath due to external influences, it is also susceptible to various overuse reactions or diseases: The best-known example is tendon sheath inflammation.
Anatomy and structure
The structure of the tendon sheath is fundamentally similar to that of the joint capsule. Thus, the construction of the tendon sheath likewise distinguishes between the two layers of the outer stratum fibrosum and the inner stratum synoviale. The outer stratum fibrosum (fibrous layer) consists of tight connective tissue that merges with the periosteum in the attachment area of the joint capsule. This connective tissue generally determines the mechanical strength of the entire joint and thus enables smooth movement. The stratum synoviale, on the other hand, represents the inner layer of the joint cavity, which consists of several layers of fluid (synovial cells). However, since there is no direct contact with the cells and membranes here, this layer does not represent a basic tissue (epithelium). However, the inner synovial layer forms a double lamella, which is connected externally on the one hand with the surrounding connective tissue and internally on the other hand with the tendon. Between these two lamellae is a space filled with the viscous synovial fluid. This lubricating layer prevents friction and keeps the tissue supple.
Functions and tasks
As already indicated, the function of the tendon sheaths in the body is to protect the tendons and surrounding tissues from excessive friction. This is because the construction of the various layers that make up the tendon sheath, as well as the synovial fluid it contains, ensure that both friction and external pressure and stress are reduced as much as possible. Therefore, the tendon sheaths are locally particularly strongly formed where the tendons run over the joints with increased tension. However, the stronger formation relates less to the strength of the layers than to the concentration of synovial fluid. This is because it is present in higher quantities at the heavily stressed areas. Incidentally, the body itself is responsible for the production of synovial fluid. The inner layer of the joint capsule (technically known as the membrana synovialis) is specifically responsible for this, producing both a thick lubricating film and the minimally thin lubricating layer.
Diseases and complaints
Because of its function and nature, however, the tendon sheath is highly susceptible to overstimulation, and not only because of the high stresses that occur during rapid, constant, or punishing movements. Typical pathological changes that can affect the tendon sheath are usually referred to in medicine as tendovaginopathies. This is because overuse or injury to the tendon sheath can be the cause of tendovaginitis (tendonitis). Tendovaginitis is not only one of the most common diseases of the tendon sheath, but also of the hands. It usually manifests itself in severe, stabbing and pulling pain and can occur anywhere tendon sheaths are present. However, as already indicated, tendon sheath inflammation occurs particularly often in the wrist area. Tendovaginitis is usually divided into five different types: serous, fibrinous, necrotizing, purulent, and phlegmonous.Serous tendovaginitis affects the lubricating film, fibrinous tendovaginitis affects the fibrous tissue, necrotizing inflammation causes parts of the tissue to die, purulent inflammation is a severe form of tendovaginitis, and phlegmonous tendovaginitis is proven to spread. Depending on the type of tendovaginitis, it may need to be treated with antibiotics, plenty of rest, cooling, and pain medications.
