Almost everyone in their seventies is creaky in the joints. The beautiful smooth joint surfaces of their youth have simply been worn away by continuous use. The more heavily used joints, which have been under most pressure, such as hips, knees and the joints between the vertebrae, have ragged, rough surfaces which are very obvious in X-ray pictures. In an attempt to repair and stabilize the joints, cells move in and make tough fibrous tissue which stiffens the joints. Damaged cells produce the familiar inflammatory response, which spreads to surrounding tissue, making it tender. The pain may be referred to distant areas, so osteoarthritis of the hip may be reported as pain in the upper leg and knee. As was described for the twisted ankle, a chronic defensive tactic to encourage recovery is to immobilize the joint. Eventually, unused muscles wither.
As with the other conditions described, the natural history of local tissue damage spreads its effect, including pain, to nearby and distant structures by a linked series of separate mechanisms. This is nowhere more clear than in osteoarthritis, in which the disorder starts locally in a joint but spreads to nearby tissue while the nervous system, attempting to immobilize the joint, generates an abnormal gait and posture, which in turn stresses other joints.
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