Given the dynamic conflicts within CR-type implants within the body, where the knee joint's geometric characteristics lead to incongruent motion of the Femoral Condyle on the tibial plateau compared to what is required by the posterior cruciate ligament (PCL), this concept led to the emergence of PCL-sacrificing implants.

In knee joint replacements, if the PCL causes posterior rolling of the femoral condyle while the concave-shaped tibial plateau restricts the posterior rolling motion on it, it can result in impingement at the posterior joint surface, leading to flexion difficulties. Furthermore, PCL-retaining implants have relatively flat tibial plateau inserts, resulting in a smaller contact area at the tibiofemoral joint and exacerbating polyethylene wear.

The first modern knee joint posterior cruciate ligament-sacrificing implant was the Freeman-Swanson knee joint prosthesis, with the first surgery completed in March 1970. This joint was described as a "roller in a trough," and one of its design principles was not relying on the intact cruciate ligaments to maintain knee joint stability.

The most successful posterior-stabilized knee prosthesis was first used at the Hospital for Special Surgery (HSS) in New York in 1973. It was an early precursor to PCL-sacrificing implants. While the majority of cases that received this prosthesis replacement had good postoperative joint function, a small subset experienced posterior dislocation of the joint during flexion, especially in patients who had previously undergone patellar resection surgery. These knee joints, due to the elimination of posterior rolling of the femur, would dislocate on average at 90° of flexion. Later, this type of prosthesis evolved into the posterior-stabilized design.

The posterior-stabilized prosthesis is currently the most popular design among PCL-sacrificing implants. It features a central post in the tibial plateau insert and a cam-spine mechanism in the femoral condyle, simulating posterior femoral rolling to increase the range of joint flexion (as shown below).

Posterior-Stabilized——Prosthesis Design Principles

The intermediate pillar in the tibial plateau and the cam-spine device between the femoral condyles start to engage at 60° to 70° of flexion, guiding posterior rolling of the femoral condyles, thus increasing the range of motion. This design aids in maintaining joint stability after PCL removal. Additionally, the increased curvature of the platform and improved congruence of the tibiofemoral joint reduce polyethylene wear.

Reportedly, PS (Posterior-Stabilized) implants have a posterior rollback distance of approximately 7.7mm (physiological: 14.2mm). Simultaneously, an increased curvature of the tibial plateau enhances knee joint stability and reduces polyethylene wear. This design concept forms the foundation for modern posterior cruciate ligament (PCL) replacement-type implants. Advocates of PCL removal believe that this implant design and replacement concept are technically straightforward and capable of correcting fixed knee joint deformities. PCL removal can be easily performed in knee flexion, facilitating effective clearing of the posterior joint capsule. Tibial resection doesn't require consideration of PCL endpoints; achieving equal flexion and extension gaps is sufficient. Additionally, increasing the posterior slope of tibial resection significantly improves knee flexion. Bone thickness of tibial plateau resection can be easily managed by selecting the thickness of the tibial plateau polyethylene insert. After all resections and collateral ligament balancing are completed, the joint gap spacer should be inserted into the knee joint to check if the knee can fully extend, alignment of the knee joint in extension, and varus-valgus stability, among other factors. The same method can assess joint stability in flexion. Furthermore, due to the overall high congruence of joint surfaces, implant wear is lower compared to PCL-retaining implants.

Posterior-stabilized implants are suitable for knee joint replacement in cases with severe deformity and posterior cruciate ligament (PCL) damage but intact ligament structures around the joint. For severely deformed knee joints, meticulous soft tissue release before bone resection is crucial. This involves initially removing bone spurs near the collateral ligaments, preliminarily releasing contracted ligaments, and restoring the lower limb's alignment to near-normal before proceeding with bone resection. Unlike PCL-retaining implants, posterior-stabilized joint replacement surgery requires the removal of both the anterior and posterior cruciate ligaments. Additionally, it involves an increased femoral condyle resection to accommodate the implant's cam-spine structure within the intercondylar space.

The classic surgical approach for posterior-stabilized implants is the flexion-extension gap technique. Based on the principle of equal flexion and extension gaps, joint gaps are adjusted through ligament release and soft tissue balancing. First, the tibial plateau is resected, then, using this plateau as a reference, the distal femur is resected in extension, and the posterior femoral condyle is resected at 90° of flexion. This results in a rectangular gap with equal flexion and extension spaces. After PCL removal, it becomes easier to remove posterior bone spurs and the popliteus bone and release the posterior joint capsule, making soft tissue balancing more manageable than in PCL-retaining implants. During the entire balancing process, it's important to first expose and remove bone spurs and then decide whether further release is necessary based on the situation, to avoid excessive laxity after ligament release.

This section briefly introduces the flexion-extension gap balancing method (as shown in the figure below).

Initial - Ligament Balancing

A. After exposing the joint, start by removing bone spurs near the collateral ligaments of the femur and tibia, even within the intercondylar fossa, to release tension in the collateral ligaments. This exposes the intercondylar notch and reveals the true bone bed for better positioning.

B. After removing bone spurs and the initial release of the collateral ligaments, check that the extension gap is mostly balanced.

C. Similarly, the flexion gap is also balanced. Before opening the flexion gap, you can remove the PCL or cut it at its endpoint.

D. Expose the tibia for tibial resection. Then, using the tibial plateau resection surface as a reference, perform resections on the distal femur and posterior condyles to achieve equal rectangular extension and flexion gaps. Finally, complete the resection of the femoral anterior and posterior slopes and intercondylar areas. Nowadays, femoral side resections are often performed using a four-in-one design resection module, meaning that after the distal femur resection, the resections of the anterior and posterior condyles and slopes of the femur are done in one step using a four-in-one resection plate.

Secondary ——Ligament Balancing

A. After bone resection, test the extension gap again and release the corresponding contracted ligaments and joint capsule to obtain a rectangular extension gap.

B. Similarly, adjust the flexion gap to balance it and make it equal to the extension gap. Most knee joint replacement instruments come with gap blocks of appropriate thickness to measure and compare extension and flexion gaps.