The possible causes of left knee pain on stairs include knee trauma, knee bone spurs, osteoarthritis, chondromalacia patella, etc. 1. Knee trauma: trauma to the left knee joint may cause injuries to the meniscus, anterior and posterior cruciate ligaments, medial and lateral collateral ligaments, etc., and in severe cases, localized fractures can occur, and the phenomenon of pain in the left knee going up and down the stairs can also occur. 2. Bone spurs in the knee joint: it may be due to the slow degeneration and aging of the human knee joint, resulting in bone spurs in the knee joint, if the patient has bone spurs in the left knee joint, it may cause pain in the left knee going up and down the stairs. 3. Osteoarthritis: It may be related to heredity, patient’s age, inflammation, trauma and other factors, resulting in osteoarthritis, which will affect the joint between the patella and the femur, causing degeneration and aseptic inflammation of the joint. Wear and tear pain will occur when walking up and down stairs. 4. Chondromalacia patella: Chondromalacia patella is a disease caused by chronic injury to the cartilaginous surface of the patella. There is wear and tear damage to the articular cartilage between the patella and the femur, and this condition will cause pain and discomfort when the pressure on the joint increases when going up and down stairs. Left leg knee pain on the stairs may also have other reasons, it is recommended to go to the regular hospital in a timely manner, improve the relevant examination, clear cause, under the guidance of the doctor for targeted treatment or treatment.