After dislocating the knee and resetting it, it cannot be straightened actively, but can be straightened passively, which may be ligament rupture, tendon rupture, infrapatellar fat pad injury and so on. You should consult a doctor to find out the cause of the disease. 1. Ligament rupture: for patients with knee dislocation, ligaments tend to rupture, because the stability of the joint is better under normal circumstances, once the dislocation occurs, it will cause great damage to the ligaments, so that the ligaments are completely ruptured, losing the protection of the joints, which will lead to the dislocation of the knee joint. This leads to a situation where the knee is dislocated and reset, the active stretching is not straight, and the passive can be straightened. 2. Tendon rupture: dislocated knee may cause tendon rupture, tendon injury contraction of muscles, can feel the skin near the contraction, but can not pull the joint to produce normal joint activity. It shows the symptom that after the knee dislocation is reset, the active extension cannot be straightened, but the passive extension can be straightened. 3. Infrapatellar fat pad injury: knee dislocation if caused by infrapatellar fat pad injury, will lead to knee pain and even affect the flexion and extension activities, knee dislocation reset, active extension can not straighten, passive can straighten the symptoms. After the knee dislocation reset, active extension is not straight, passive can straighten, there may be other reasons, please ask the doctor to clarify the specific reasons, if necessary, the corresponding treatment.