The inability to bend the leg is caused by strain injury to the ligaments and joint capsule of the knee joint. After strain injury to the ligaments and joint capsule of the knee joint, it will cause edema of the ligaments and joint capsule and obvious pain during flexion and extension activities, thus affecting the flexion and extension activities of the knee joint. Injury to the meniscus of the knee joint. After the injury to the meniscus, the knee joint will squeeze the injured meniscus during flexion and extension activities, thus causing significant knee pain and causing the patient to be unable to bend the leg. Third, in patients with degenerative osteophytes in the knee joint, the enlarged bone will stimulate the surrounding ligaments and joint capsule when the leg is flexed and extended, which will also cause significant pain and thus affect the bending of the leg.