Secondary osteoarthritis can occur at any age in a previously damaged or congenitally abnormal joint. It is caused by "wear and tear" and most commonly affects the vertebrae, hips, and knees. There are two main theories for the cause of osteoarthritis - the biomechanical theory, which states that wear and tear leads to cartilage breakdown and bone changes, and the biochemical theory, which maintains that aging causes reduced cartilage maintenance and an excess of enzymes that degrade the cartilage. The primary symptom of osteoarthritis is thinning and fragmentation of the articular cartilage, which leads to joint stiffness, pain, and disability.