This experiment found that although the two construction methods recommended to retain the initial PCO, the two groups were different from each other in terms of the tightness of the flexion gap. If the negative correlation between the increase of PCO and the flexion angle is due to the tension of the flexion gap, then why does the decrease of PCO become a negative effect when PSA decreases? This may be because the increase of PSA has an adverse effect on rollback, which weakens the advantage of PCO in delaying impact, and causes more tension in the flexion gap and more pressure on the contralateral collateral ligament. The good rollback when PSA is reduced gives full play to the advantages of PCO retention. To better explain, we introduced cruciate-retaining(CR)prosthesis, which contrasted with PS rollback mechanism.
PSA, PCO and rollback in CR
In previous studies[12], the advantages of PCO recovery and PSA enhancement focus on the delay of impact. This theory has been widely recognized in CR prosthesis. Bellemans[13,9] found in their study that increasing PSA did indeed result in greater maximum flexion before the tibial gasket impacted the posterior femur, with a 1.7° flexion gain per degree of backward inclination. At the same time, PCO recovery was important because it also allowed greater flexion before impact. However, the rollback of CR is better due to the presence of the posterior cruciate ligament (PCL), which amplifies the effects of PCO and PSA. PCL effectively prevents posterior dislocation of the tibial plateau and optimizes rollback without excessive release. Research[14] proved that the increase of PSA in Cr was conducive to rollback, so that the increasing advantages of PCO and PSA at this time could be better reflected, and the two were complementary in the flexion gap. However, in PS, backward rolling is not as ideal as in CR, Banks[15] reported that CR had a greater posterior translation of the femur than PS during progressively increased activity.
PSA, PCO and rollback in PS
In PS prosthesis, the protective effect of PCL is lost and only depends on the interaction between the cam and the column to cause the backward roll. The increase of PSA not only cannot optimize the backward roll, but also has been proved many times to lead to abnormal forward roll. Piazza[16] show that the backward tilt of the tibial component shifts the interaction between the cam and the spine to a higher bending angle. As a result, the buckling angle increases and the range of action decreases, resulting in delay and shortage of backward roll. After computer simulation, Wang Zhi Wei [17] found that the increase of PSA delayed the cam binding by 38°, even much more than Piazza's delayed binding by 18°. Satoshi Hamai[18] directly compared the motion of PS and Cr when climbing stairs, and found that Cr showed good stability in the medium degree of flexion due to the presence of PCL. In PS, there was abnormal femoral forward roll and posterior impact of the tibial column, and PSA was significantly larger in the cases that appeared. It is worth mentioning that the cam and the column are not engaged when rolling forward at medium flexion angle. Piazza demonstrated that forward rolling occurred by simulating the change in cam-column contact with a high PSA. This may mean that anomalous forward roll caused by high PSA can occur in both cases when the cam is unengaged at a medium buckling angle and when the cam is engaged at a larger buckling angle. Ephrat Most[19] noted that reduced femoral rollback may be a limiting factor in knee reconstruction, and that the timing of cam post engagement must be combined with the joint geometry after TKA to enhance femoral rollback and increase the flexion range of the knee. The increased PCO during insufficient rollback exerts additional pressure on the collateral ligaments and affects flexion. In his study, Eisaku Fujimoto[20] raised the concern that too large PCO would make the flexion gap too tight and affect the flexion angle, and PCO should be properly repaired to achieve a better balance of the collateral ligament. In addition, it is necessary to consider that the flexion gap after PCL removal is larger [7]. A larger PSA means a larger PCO to balance the flexion gap, so the choice needs to be more careful. However, for the PSA decreasing group, the performance of backward roll would be better than that of the increasing group. In this case, the influence of PCO was similar to the delayed impact in CR, and PCO retention became a favorable factor. Therefore, the opposite effect of PCO was caused in the two groups.
To sum up, in the PS prosthesis, the selection of PSA may require the first guarantee of backward rolling, and then, according to the selected PSA, build a PCO that can delay the impact as much as possible without putting too much pressure on the collateral ligament. Roll-back optimization of PS is worthy of further study. From the practical point of view, conservative low PSA is suitable for most people. For a small number of individuals with small initial PSA, choosing 3° can avoid the occurrence of forward tilt. At the present stage, conservative low osteotomy may have optimization space, but it is relatively reasonable.
Differences in flexion-extension movement
In addition, we also found that the patellofemoral joint bouncing, delayed knee extension weakness, pain in front of knee joint were more common in the increased groups than in the decrease group. We think this is related to increased pressure on the patellofemoral joint and a decrease in the extensor arm.
Reviewing previous mechanical studies[21-23], good rollback can directly reduce the pressure of femur on patella, increase the angle between the extensor arm and the action line between quadriceps and patella ligament, and reduce the pressure on patellofemoral joint. This can effectively relieve muscle weakness and preoperative knee pain after joint replacement and reduce wear and tear of the prosthesis. However, the increase of PSA leads to poor backward roll, and the reduction of moment arm is difficult to achieve. Meanwhile, the complementary increase of PCO in the flexion gap is an adverse factor in patella pressure. [24] This may explain the higher incidence of patellofemoral joint bouncing, delayed knee extension weakness, and anterior knee pain in the increase group. At the same time, the increase of PSA has also been shown to lead to the excessive extension of the knee joint in the extension position, causing the femoral cam to impact the front of the tibial column, leading to the wear and deformation of the front column.[25] Hanjunlee[26] also suggests that care should be taken to match the tilt angle of the femur and tibia, as excessive extension could have a negative impact on the stability of the knee and quadriceps or hamstring fatigue. Therefore, from the perspective of flexion and extension movement, it is not recommended to increase PSA and PCO to reconstruct the flexion gap.
Limitations of the experiment
There are some limitations in this experiment. Firstly, the sample size of our study lost a lot due to the strict requirements on the type of prosthesis, lateral imaging data and PSM, but it was worth it for the scientific nature of the experiment and the comparability of grouped samples. The regression analysis of the reduction group showed that PCO was the biggest influencing factor of buckling angle, but it was limited by the sample size. The results were not statistically significant and require further study. Secondly, the retrospectively studied subjects were all Asian with PS prosthesis, which was not retroflexed and had non-single radial fixation platform. Whether the conclusions can be applied to other ethnic groups and prosthesis needs further study. Finally, there are bilateral operations in the data, and the flexion angle may be affected by the operation on the other side.