The most common clinical condition is chondromalacia patellae, which is a strain on the cartilage behind the patellae. It is caused by long-term squatting, or frequent knee flexion and extension, such as drivers in semi-flexion, frequent clutching and braking, or excessive exercise in tai chi, horse stance, dancing, or climbing hills, stairs and slopes. degenerative changes in the cartilage behind the patella, with chondral cracking, exfoliation, bone exposure, and subchondral bone sclerosis, resulting in pain behind the knee when standing. The treatment can be cartilage-nourishing drugs, such as glucosamine, sodium vitaminate joint injection, or chondroitin sulfate.