The traditional CR prosthesis relies on the proprioception of the PCL and causes femoral roll back through ligament tension to achieve joint flexion and stability(4). The traditional CR prosthesis needs to retain high function of the PCL. However, it may be found that the PCL has lost tension long or after release during TKA, which leads to the instability of the prosthesis. In addition, in order to obtain excellent knee flexion and avoid restricting the rolling motion of the femur, the traditional CR prosthesis uses flat gasket. Wada et al(5). analyzed the kinematics of four types of prostheses in TKA and pointed out that the prostheses with flat gaskets were prone to displacement and wear. Daniilidis et al(6). found that compared with the flat gasket, the highly congruent gasket effectively reduced the contradictory forward displacement and non-physiological roll back, while the rotating platform avoids the limitation of the highly congruent front and rear lips on the femoral condyle roll back and tibial rotation during high flexion. At the same time, the lateral condyle roll back during flexion, which is more in line with physiological knee kinematics.
At present, numerous studies have reported that the use of highly congruent rotating platform prosthesis is conducive to joint stability(7). Peter et al(8). compared traditional CR prosthesis with highly congruent prosthesis, and found that there was no difference between the two groups in terms of function score, complications, etc., but the revision rates of traditional CR prosthesis was higher, among which the instability of knee joint was the most crucial reason.
Comparing the revision risk of the rotating platform TKA with that of the fixed platform TKA, Marek Lacko(9) proposed that the rotating platform TKA may increase the risk of revision risk at an early stage due to knee instability, mainly on account of improper surgical techniques, such as unbalance of flexor-extension gap and dislocation of components. In our study, neither sacrifice nor retain the PCL resulted in early revision due to joint instability, which is attributed to a suitable gap balance to improve joint stability. The deep disk design of the highly congruent gasket increases the contact area between the gasket and the femoral interface, which improves the congruent between the femoral component and the gasket. Even in the absence of the PCL, the protruded anterior lip of the gasket plays a role in limiting the excessive forward movement of the femoral condyle during flexion(10). At the same time, the pressure of the patella prevents the femur from moving forward and causes it to roll backward. The combination of the above two items effectively avoids the occurrence of bearing dislocation when the knee joint is over-extended or over-flexion.
The highly congruent gasket improves the stability of the prosthesis and overcomes the requirement of high function of PCL. The results of this study are consistent with those of Benjamin M. The congruence of the rotating platform is higher than that of the fixed platform. Therefore, in the TKA of the rotating platform with highly congruent, if the tension of the posterior cruciate ligament is lost during the operation, as long as the medial and lateral collateral ligaments are well balanced, the joint range of motion and stability will not be affected.
It should be noted that in patients without developmental articular deformity due to wear and tear, parallel flexion and extension gap can be obtained after the removal of osteophytes and osteotomies without the need for release or extension of the medial and lateral ligaments. In such patients, the coordination of the cruciate ligament and the collateral ligament is synchronous. Therefore, as long as the proper posterior tibial slope is ensured, postoperatively the tension of the posterior cruciate ligament is usually not higher than that of the collateral ligament, that is, the excessive backward rolling of the femur, and the release of the posterior cruciate ligament is not required. However, for patients with developmental extraarticular deformity, the medial collateral ligament may have a higher tension than the lateral collateral ligament after osteotomy. It is required that the tension on both sides of the gasket must be equal, otherwise the gasket will prolapse and wear will be accelerated. In this case, it is necessary to release and extend the ligament on the tight side. In addition, after placing the gasket matching the length of the collateral ligament, if the posterior tibial slope is unsuitable, the tension of the PCL will exceed that of the collateral ligament, which causing the bearing dislocation. In the early stage, we usually attributed the tight flexion space to insufficient release of the posterior cruciate ligament, rather than the posterior tibial slope. At the same time, it was not considered to distinguish the origin of varus and varus deformity, so we released the PCL resulting in PCL tension-free cases mostly occurred in the early studies. Then we use the gap balance method to balance the flexion gap. In the case of a fully tensioned balancer, the gasket thickness should be reduced to obtain the appropriate buckling clearance. If there is still spacer prolapse at this time, it is preferred to increase posterior tibial slope (usually 7°), and then to release the PCL.
The results of our study confirmed that postoperative HSS scores and knee ROM of the two groups were significantly improved compared with preoperative those, which indicated that both sacrifice and retention of PCL could improve knee function in CR highly congruent rotating platform TKA.
There are still some deficiencies in this study: the sample size is not large enough. PCL tension-free patients mostly exist in the early stage of the study. The follow-up time is not long enough, and long-term follow-up is beneficial to clarify the impact of PCL ligament sacrifice on survival of prosthesis.