Spraining to the foot for a month and there is still swelling indicates that the injury is more serious, may be caused by sprains caused by the lateral ankle ligament injury, joint capsule tear as well as cartilage damage in the ankle joint, as follows: 1. lateral ankle ligament injury: sprain caused anterior talofibular ligament rupture, resulting in swelling of the soft tissues of the ankle joint, local hemorrhage, reduced joint stability, accompanied by ankle flexion, extension and rotation activities are limited, and pain is more pronounced when moving the ankle joint. 2. Tear of joint capsule: Tear of ankle joint capsule caused by sprain will lead to localized swelling, pain, ecchymosis, limitation of limb movement, and so on, and in serious cases, it may lead to dislocation. 3. Cartilage injury in the ankle joint: after cartilage injury, the joint part will be swollen, and the friction sound of the joint surface can be seen, the joint space can be reduced, and occasionally free bodies can be found, and the limitation of joint activities will become more and more obvious with the passage of time until the joint mobility is obviously reduced. Long-term swelling after sprains may also have other reasons, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, under the guidance of the doctor to give targeted treatment or treatment.