Pain when the knee is bent and straightened is mainly caused by the following conditions: 1, long-term strain and wear of the knee cartilage, causing bony friction; 2, if there is no skeletal friction sound, but there is still pain, mainly consider the peripheral muscles and soft tissues of the knee joint strain, jamming or aseptic inflammation, causing knee bursitis or tendonitis; 3, pain when the anterior side of the knee is straightened, called patellofemoral arthritis; 4, pain on the medial or lateral side of the knee joint, mostly due to muscle and tendon strain in this area, causing collateral ligament strain or inflammation; 5, pain on the posterior side of the knee joint, probably due to bursa or hamstring strain, causing posterior stiffness, tension or pain when moving.