Determine the cause of pain under the knee in general terms. The first cause may be a problem with the cartilage at the back of the knee. There is a layer of cartilage on the surface of the joint, which is white brittle bone, and normal articular cartilage is transparent cartilage, as colorless and transparent as ivory white. After damage to the cartilage occurs, it can be seen on the surface as chafing and erosion, and damage to the patellar cartilage can result in pain below the knee. The pain is mainly manifested in the process of squatting up, and the pain is aggravated in the process of climbing and going up and down stairs. The pain is not obvious in the process of walking on a flat road because the force on the articular surface of the patella is relatively small in the process of walking on a flat road. The second cause is the formation of chronic aseptic inflammation of the fulcrum of the patellar ligament below the patella, called patellar tendonitis, which is mainly seen in people who engage in prolonged strenuous exercise, or high intensity physical labor, and can be treated with conservative therapy. The less common fibrosis of the infrapatellar fat pad, also known as infrapatellar fat pad inflammation, occurs when the infrapatellar fat pad is impacted by the joint during knee extension, resulting in chronic aseptic inflammation, which is treated mainly by rest and reducing stress stimulation of the infrapatellar fat pad. The knee is known in folklore as the knock-knee, and in medicine as the patella, a round bone on the front side of the knee that protects the knee joint. The most common cause of knee pain is degenerative changes in the articular cartilage on the back side of the knee, mainly in women, because of physiological anatomical differences, women are more prone to abnormal patellar trajectory compared to men, which can cause eccentric loading of the patella, and women like to wear high heels, which can increase the stress on the knee. If knee pain occurs during walking up and down hills or climbing stairs or squatting, it can be further confirmed to be a degeneration of the cartilage on the back side of the patella, i.e. chondromalacia patellae. If the surface of the cartilage can be seen under direct vision as erosion, chafing, or even cartilage defects, if the condition is further aggravated, the cartilage may even fall off and form intra-articular free bodies in the joint cavity, causing conditions such as joint interlocking jams.