Synovial joints are made up of five classes of tissues. These include bone, cartilage, synovium, synovial fluid, and tensile tissues composed of tendons and ligaments. The synovial lining in the bursae and tendon sheaths is similar to that within joints, with a slippery non-adherent surface allowing movement between planes of tissue. Synovial tendon sheaths line tendons only where they pass through narrow passages or retinacula, as in the palm, at the wrist, and around the ankle. Elsewhere the tendon lies in a bed of loose fibrous tissue. Bursae occur at sites of shearing in subcutaneous tissue or between deeper tissues such as muscle groups and fascia. Many bursae develop during growth but new or adventitious bursae can occur at sites of occupational friction.
A tendon (or sinew) is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension (Figure 1). Tendons are similar to ligaments and fasciae as they are all made of collagen, except that ligaments join one bone to another bone, and fasciae connect muscles to other muscles. Tendons and muscles work together. A bursa (plural bursae) is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of slimy fluid (similar in consistency to that of a raw egg white) (Figure 2). It provides a cushion between bones and tendons or muscles around a joint. This helps to reduce friction between the bones and allows for free movement. Bursae are filled with synovial fluid and are found around most major joints of the body such as the shoulder and the knee.