Joints.
Last updated Wednesday, January 19, 2005
Figure 1 - Tissues of a joint About jointsMotion and function of joints Human bones join with each other in a variety of ways to serve the
functional requirements of the musculoskeletal system. Foremost among
these needs is that of purposeful motion. The activities of the human
body depend on effective interaction between normal joints and the
neuromuscular units that drive them. The same elements also interact
reflexively to distribute mechanical stresses among the tissues of the
joint. Muscles, tendons, ligaments, cartilage, and bone all do their
share to ensure smooth function (see figure 1). In this role, the
supporting elements both unite the abutting bones and position the
joints in the optimal relationship for low-friction load-bearing. Two
important characteristics of normal joint function are stability and
lubrication.Cartilage The cartilage covering our joint surfaces is called "articular
cartilage." Normally, it is a smooth, well-lubricated surface that
offers less frictional resistance than that of an ice skate gliding on
ice.
Normal cartilage is very durable and somewhat elastic providing a
shock absorber for our joints. Articular cartilage does not have a
blood supply. Rather, it gets it oxygen and nutrients from the
surrounding joint fluid. When a joint is loaded, the pressure squeezes
fluid, including waste products out of the cartilage and when the
pressure is relieved, the fluid seeps back in together with oxygen and
nutrients. Thus, the health of cartilage depends on it being used.
Unfortunately, once it is injured, cartilage has a limited ability to
repair itself.
Damaged or abnormal cartilage loses it resistance to wear. The two
joint surfaces grate one on the other and shed particles of cartilage
which further contribute to joint surface wear. As the joint mechanics
deteriorate, the rate of wear increases. The process may continue until
most of the joint cartilage is gone. Bone spurs seem to be the body's
attempt to provide more joint surface, however, because these bone
spurs are not covered by normal cartilage, the affect is not helpful.
The wearing of cartilage may produce deformities such as bowed legs or
stiff spines. Loose pieces of bone and cartilage may break off and
cause joints to "lock".
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