Knee pain is usually caused by arthritis. There are three common types of arthritis: rheumatoid, traumatic, and osteoarthritis. When you injure your knee, it can cause traumatic arthritis. This is a condition that develops over time as damaged ligaments and or bones cause further damage to the knee structure and break down the articular cartilage. Rheumatoid arthritis causes the synovial membrane to produce too much fluid. The fluid overfills the joint space, thickens, and becomes inflamed c