Osteoarthritis is the most common form of arthritis, affecting some 27 million Americans. It usually occurs with age and is sometimes called degenerative joint disease. It may affect knees, hips, neck, lower back and fingers as the cartilage breaks down in joints, causing pain, swelling and stiffness. Sufferers may hear a clicking sound when bending a joint. Age, obesity, joint injuries, overuse and heredity all are risk factors.