Knee joint flexion activities produce a strange sound, commonly due to the following three reasons: First, due to degenerative changes in synovial tissue and inflammatory lesions, can lead to a decrease in the amount of synovial fluid secretion, increased friction between the articular cartilage and produce a strange sound when moving. Second, due to degenerative changes resulting in osteoarthritis, the articular cartilage surface can be exfoliated or even free, and the friction between the subchondral bone and each other can also produce a foreign noise. Third, it can be seen in synovial chondromatosis, due to synovial tissue degeneration and metaplasia to produce cartilage nodules, joint activity and synovial tissue friction and cartilage nodules stuck between the cartilage surface, also in flexion activities will produce a foreign sound.