Arthritis is a condition that occurs in various joints in the body, especially in the knees, hips, and spine. It can affect any joint, but the shoulder is affected infrequently. When arthritis occurs, the cartilage that covers the ends of the bones making up the joint breaks down and often flakes off into the joint. The joint becomes swollen and stiff, and the lining tissue of the joint (the synovium) becomes overgrown. Frequently, spurs will develop around the margins of the joint and can, sometimes, break off inside. The pain can vary from mild to very severe, depending upon many factors, including the severity of the disease, the type of arthritis (most are wear and tear, or degenerative arthritis, but some are caused by rheumatoid disease) and the activity level.