Osteoarthritis (OA) is defined by the American College of Rheumatology as a "heterogeneous group of conditions that lead to joint symptoms and signs which are associated with defective integrity of articular cartilage, in addition to related changes in the underlying bone at the joint margins." Osteoarthritis is usually classified as primary or idiopathic when there is no obvious predisposing cause, and secondary when there is some clearly defined predisposing pathology . Idiopathic OA is the most common form of arthritis and is a debilitating progressive disease that affects 60% of men and 70% of women over the age of 65  with enormous socioeconomic costs, rivaling those of ischemic heart disease. As the Baby Boomers reach middle age and obesity is on the increase in the general population, OA will have an even greater impact on society in the future. Primary OA is a frustrating disease for both patient and clinician, because neither cause nor cure is known and, once started, the disease cannot be halted, even though individual rates of joint degradation can vary considerably . Understanding of the various aspects of the disease has advanced sufficiently, however, to give rise to cautious optimism that other treatments beside joint replacement may become available. This chapter provides an introduction to the pathogenesis of OA, concentrating on primary or idiopathic OA.
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