What are the commonest causes of knee pain? A large number of diseases and injuries can damage the knee to the point where it becomes painful or will not function properly. Although a large number of diseases can cause knee pain, the following are the most important ones: osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis Osteoarthritis: This is an age-related 'wear and tear' type of arthritis. It usually occurs in people 50 years of age and older. Rheumatoid arthritis: This is a disease in which the synovial membrane that surrounds the joint becomes inflamed and thickened. This chronic inflammation can damage the cartilage and bone leading to pain and deformities. Post-traumatic arthritis: This can follow major fractures of the bones surrounding the knee or tears of the knee. In an arthritic knee the joint cartilage gets degenerated and the normally protected nerve endings in the bone gets exposed giving rise to severe pain. Also secondary effects like inflammation of synovium (soft tissue lining the joint), joint effusion, muscle spasm are present. Ultimately the joint end up in permanently deformed and a painful state restricting not only work and recreation, but also ordinary daily activities.