Pain when bending the knee is mainly due to the following reasons: First, injury to the knee joint, such as a sprain or strain, can cause pain when bending the knee joint. Second, it may be due to damage to the ligaments, joint capsule, or meniscus inside the knee, which may cause pain when the knee is flexed or extended. Third, the pain may be caused by osteoarthritis of the knee joint due to aging and degeneration of the bones of the knee joint. Fourth, it may be due to chondromalacia of the patella that causes pain when the knee is bent. If the pain occurs when the knee is bent, the patient can apply local heat and massage or take oral chondroprotective medications such as glucose sulfate to relieve the pain.