Effects of joint pathology on muscle.

The muscle wasting associated with joint damage may be highly selective; knee disorders produce quadriceps wasting but little change in the size of the hamstrings. This causes isolated quadriceps weakness, so predisposing to a position of knee flexion. Nociceptors and other receptors in and around the joint can have flexor excitatory and extensor inhibitory actions. At the knee, these receptors are likely to excite hamstrings and inhibit quadriceps. Although other actions could occur, quadriceps inhibition may be favored by a position of knee extension. Quadriceps inhibition will weaken voluntary contraction, reduce tone, and contribute to wasting of the muscle, further predisposing to a position of knee flexion. The potency of quadriceps inhibition may be considerable, even in the absence of perceived pain. A small, apparently trivial effusion (or even a clinically undetectable effusion) may cause important inhibition. In order to improve the orthopedist's ability to prevent flexion contracture of the injured or operated joint, he must look not only for ways of reducing joint pain, but also for ways of preventing activity in other joint afferents. For example, he must consider the possible effects of joint position, intraarticular pressure, suture-line tension, and afferent blockade.