Osteoarthritis (aw-stee-oh-are-THRYE-tis), a degenerative joint disease, is the most common form of arthritis, afflicting some 20 million Americans. Osteoarthritis largely affects cartilage, causing it to fray, wear, ulcerate, and in extreme cases, to disappear entirely, leaving a bone-on-bone joint. Bone spurs can form at the edges of the joint, causing disability most often in weight-bearing joints, such as the knees, hips, and spine. This disease primarily occurs in older peopleusually after age 45but isn't necessarily an inevitable part of aging. Excessive or unusual wear of the joints results from obesity, poor posture, injury, strain from occupational or recreational activities, or a combination of these factors. In addition, genetic factors can predispose certain people to the disease. The most common symptoms are pain and stiffness in the morning that can last up to 20 minutes. Patients suffering from osteoarthritis may also experience brief stiffness after inactivity, such as riding in a car or sitting in a theater seat. It's sometimes difficult to distinguish where the pain originates because discomfort in one part of your body may be caused by osteoarthritis elsewhere. As the disease progresses, you may lose range of motion in the affected joints. The symptoms are usually relieved with a combination of rest and exercise, but your doctor may prescribe some form of medication for pain relief.