Inability to straighten the leg may be caused by post-traumatic joint stiffness, congenital or acquired osteoarthritic deformity, etc. Treatment mainly includes physical therapy and medication. 1. Physiotherapy: For patients with post-traumatic joint stiffness or congenital osteoarthritic deformity, rehabilitation training is the mainstay, through which the muscle strength around the joints as well as the joint mobility can be improved; secondly, heat therapy, massage, physiotherapy and other methods can be chosen to increase the blood circulation and relieve the muscle tension. In addition, you can also wear individualized brace for correction. 2. Medication: Most of the patients with acquired osteoarthritic deformity have aseptic inflammation in the joints. Anti-inflammatory drugs such as meloxicam tablets and celecoxib can be taken orally to inhibit inflammation; sodium vitrate can also be injected into the joint cavities to increase the lubrication of the joints and minimize the wear and tear of the cartilage. The above drugs need to be used under the guidance of a doctor, avoid self-medication. There can be other reasons why the leg is not straight, so it is recommended to go to the hospital in time and have standardized treatment under the doctor’s guidance.