Causes

The most common form, called "primary" OA, is the gradual breakdown of cartilage covering the ends of bones, mostly due to prolonged stress on the joint. Often this is attributed to age, but age is not the only reason. Cartilage in OA patients has an abundance of enzymes that break it down, regardless of age.

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Another form, called "secondary" OA, is a result of trauma to the tissues surrounding the joint. It can show up at younger ages than primary OA. For example, people who have jobs that require repeated bending and strain often get OA in the knee. High school football players who get knee or hip injuries can also develop OA. Secondary OA has not, however, been reliably linked to physical exercise, even jogging.