Lack of leg extension after posterior cruciate ligament reconstruction can be caused by various reasons, the common reasons include intercondylar fossa impingement, quadriceps muscle weakness, knee adhesion and so on. 1. Intercondylar fossa impingement: When the intercondylar fossa is augmented or reconstructed after surgery, the bone channel of the femur end or the tibia end is positioned in front of the femur end, which causes the ligament and intercondylar fossa to impinge. 2. Weakness of quadriceps muscle: straightening the leg is mainly accomplished by pulling the quadriceps muscle in front of the thigh. If the quadriceps muscle is weak, even if there is no problem in the knee joint, it is difficult to straighten the leg after surgery. 3. Adhesion in knee joint: If effective rehabilitation exercise is not carried out in time after the operation, adhesion in the knee joint may be caused, which may make the leg unable to be straightened. There are other reasons why the leg cannot be straightened after posterior cruciate ligament reconstruction, so it is recommended to follow up with the doctor in time to find out the reason and then treat the problem so as to avoid delaying the condition.