Injury, including sprain, strain, tendinitis, and torn cartilage, is the most common cause of knee swelling and inflammation. A sprain is a stretched or torn ligament that occurs because of a twisting or forceful impact on the knee. Strain also is the same thing and is usually caused by sudden twisting of the knee joint. Sometimes knee joint swelling is caused by inflammation of a tendon, and the condition is termed tendinitis. It is often a result of the overuse of the knee joint from activities, including cycling, jumping, and running.