Bursae and Tendon Sheaths

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Chapter: Anatomy and Physiology for Health Professionals: Support and Movement: Articulations

1. What factors are most important in stabilizing synovial joints? 2. Describe the components of a synovial joint.

Bursae and Tendon Sheaths

The bursae and tendon sheaths are closely related to the synovial joints. They act similar to lubricated ball bearings, reducing friction during joint activ-ity between nearby structures. Bursae are flat fibrous sacs that have synovial membranes lining them. They contain a thin film of synovial fluid and are located where bones, ligaments, muscles, skin, or tendons rub together. A tendon sheath can be understood as a lengthened bursae wrapping totally around a tendon that is subjected to friction. They are common in areas such as the wrist, where several tendons are tightly crowded inside narrow canals.

Joints must be stabilized to avoid becoming ­dislocated during stretching and compression. Joint stability relies on articular surface shapes, number and position of ligaments, and also on muscle tone. Artic-ular surfaces actually play just a minor role. When they are larger and well fitting or when sockets are deep, joint stability is greatly improved. For example, the hip’s ball and socket joint is very stable because of the shape of its articular surfaces.

Bones are united, and excessive or undesirable motions are prevented by the capsules and ligaments of synovial joints. Generally, the more ligaments that exist, the stronger the joint. Sometimes, excessive ten-sion on the ligaments causes them to stretch, which is a condition that remains. Ligaments stretch approx-imately 6% of their length, until they break. When a joint is mostly braced by ligaments and this occurs, the joint becomes highly unstable.

Muscle tendons crossing joints are often the most important factors concerning stability. Muscle tone keeps these tendons tense. It means that the muscle, when relaxed, has low levels of contractile activity. This keeps muscles healthy and ready for stimulation. Muscle tone is vital for reinforcing areas such as the foot arches and the joints of the knees and shoul-ders. Articulations of the appendicular skeleton are described in TABLE 8-3.

1. What factors are most important in stabilizing synovial joints?

2. Describe the components of a synovial joint.

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