The study of dry bones and models in the laboratory can easily give the impression that the knee, elbow, or hip is a point at which one bone rubs against another. In life, however, the bones do not touch each other; rather, fluid and soft tissues separate them and hold them in proper alignment (fig. 9.6).
The bones of a synovial joint are separated by a joint (articular) cavity containing the synovial fluid. Synovial fluid is rich in albumin and hyaluronic acid, which give it a viscous, slippery texture similar to that of raw egg white.9 It nourishes the articular cartilages and removes their wastes, and it contains phagocytes that clean up tissue debris resulting from wear of the joint cartilages. The adjoining surfaces of the bones are covered with a layer of hyaline articular cartilage about 2 mm thick in young, healthy joints. The cartilages and synovial fluid make movements at synovial joints almost friction-free. A fibrous joint (articular) capsule encloses the cavity and retains the fluid. It has an outer fibrous capsule continuous with the periosteum of the adjoining bones and an inner synovial membrane of areolar tissue, which secretes the fluid.
In the jaw, sternoclavicular, and knee joints, cartilage grows inward from the joint capsule and forms a pad called a meniscus10 between the articulating bones (see ovi = egg
10men = moon, crescent + iscus = little
Figure 9.6 Structure of a Simple Synovial Joint.
Why is a meniscus unnecessary in an interphalangeal joint?
fig. 9.23). The meniscus absorbs shock and pressure, guides the bones across each other, reduces the chance of dislocation, and distributes force across the entire joint instead of at just a few points of contact.
The most important accessory structures of a synovial joint are tendons, ligaments, and bursae. A tendon is a strip or sheet of tough, collagenous connective tissue that attaches a muscle to a bone. Tendons are often the most important structures in stabilizing a joint. A ligament is a similar tissue that attaches one bone to another. Several ligaments are named and illustrated in our later discussion of individual joints, and tendons are more fully considered in chapter 10 along with the gross anatomy of muscles.
A bursa11 is a fibrous sac filled with synovial fluid, located between adjacent muscles or where a tendon passes over a bone (see fig. 9.19). Bursae cushion muscles, help tendons slide more easily over the joints, and sometimes enhance the mechanical effect of a muscle by modifying the direction in which its tendon pulls. Bursae called tendon sheaths are elongated cylinders wrapped around a tendon. These are especially numerous in the hand and foot (fig. 9.7).
Insight 9.1 Clinical Application
When synovial fluid is warmed by exercise, it becomes thinner (less viscous) and more easily absorbed by the articular cartilage. The cartilage then swells and provides a more effective cushion against compression. For this reason, a warm-up period before vigorous exercise helps protect the articular cartilage from undue wear and tear.
Because cartilage is nonvascular, its repetitive compression during exercise is important to its nutrition and waste removal. Each time a cartilage is compressed, fluid and metabolic wastes are squeezed out of it. When weight is taken off the joint, the cartilage absorbs synovial fluid like a sponge, and the fluid carries oxygen and nutrients to the chondrocytes. Lack of exercise causes the articular cartilages to deteriorate more rapidly from lack of nutrition, oxygenation, and waste removal.
Weight-bearing exercise builds bone mass and strengthens the muscles that stabilize many of the joints, thus reducing the risk of joint dislocations. Excessive joint stress, however, can hasten the progression of osteoarthritis by damaging the articular cartilage (see insight 9.4, p. 320). Swimming is a good way of exercising the joints with minimal damage.
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