The delay in straightening the knee after sprain is considered to be caused by long-term braking of the knee joint and meniscus injury. 1. Long-term braking of the knee joint: after knee sprain, due to the injury recovery process is in a long-term braking state, the joints can appear inside the tissue adhesion, contracture, resulting in patients with limited knee extension, manifested in the joints can not straighten the situation. 2. Meniscus injury: acute knee sprain, often caused by meniscus tear injury, the torn meniscus can affect the knee flexion and extension function, activity will appear pain, joint interlocking and other symptoms, thus causing the knee can not straighten the situation. The patient’s knee sprain delay can not straighten there may be other reasons, it is recommended to go to the hospital as soon as possible, clear cause for standardized treatment, so as not to cause adverse consequences.