When I went to New York Hospital for Special Surgery, one of the top-ranked orthopedic hospitals in the United States, I visited the surgery of Professor Chitranjan S. Ranawat, former president of The Knee Society and former president of the American Association of Hip and Knee Surgeons (AAHKS). Ranawat’s surgery and learned a lot of surgical techniques. Using his surgical approach, he was able to maximize the mobility of the knee joint and achieve the mobility to resolve the “hyperflexion knee” in a normal person (Figure 1, Figure 2). 1. The distal femoral osteotomy is at least as thick as the prosthesis. If the osteotomy is too small, it will cause tension in the lateral support band during knee flexion and pain during knee flexion, resulting in limited flexion. 2, The anterior femoral condyle osteotomy is flush with the anterior cortex. If the osteotomy is too small, it will lead to patello-femoral filling, tension and pain in the supporting band during knee flexion, resulting in limitation of flexion. 3, After the osteotomy of the distal femur and proximal tibia is completed, a gap block is placed, and the medial side can be opened 1-2mm under the stress of external rotation, which can ensure straightening and not too loose. 4. Flexion gap = extension gap. Before osteotomy of the anterior and posterior femoral condyles, put in the osteotomy plate, flex the knee at 90° and measure the flexion gap with a steel ruler (Figure 3), so that the size of the flexion gap can be known in advance before osteotomy. The Spacerblock is then placed and the knee is turned internally and externally to observe the degree of medial and lateral joint gap opening, which is 1-2 mm (Figure 4). Depending on the difference in the size of the medial and lateral gaps, the external rotation angle is adjusted and the osteotomy plate is moved anteriorly or posteriorly. As the posterior cruciate ligament is removed with the posterior cruciate ligament replacement prosthesis, the flexion gap increases by 3mm-7mm. If the flexion gap is not measured before osteotomy, it may lead to excessive flexion gap and flexion instability. 5, pay attention to the removal of bone and bone superfluous outside the posterior femoral condyle prosthesis, these extra bone and bone superfluous that hinder flexion and extension. 6, of course, in addition to the surgery to do a good job, rehabilitation is also important, there are also some techniques to improve the flexion mobility of the knee joint, here is not introduced one by one. 2 months after knee replacement, the knee joint is fully straightened 2 months after knee replacement, the knee joint flexion mobility is close to normal.