## 4 Discussion  
The most important findings in the present study were that a larger thickness of the tibial PE insert elevated the ligament forces throughout the range of knee flexion and extension and, consequently, increased the TF compressive force across the mediolateral compartment; in contrast, varying the PE insert thickness had little effect on the PF biome  
Figure 4: Peak forces around the knee extensor mechanism at varying PE insert thicknesses during a chair-rising simulation. From left to right: quadriceps muscle force, quadriceps tendon-to-femur force, and patellofemoral (PF) compressive force. Figure 5: Reference frames used to express the PF kinematics (left). Muscles and ligaments of the TF and PF joints are hidden in this model view. Patellar kinematic profiles at varying PE insert thicknesses during a chair-rising simulation (right). chanics, although it indicated an unexpected slight decrease in the PF joint loading. These results confirm our hypothesis and suggest that there is a trade-off in the kinetic behavior between the TF and PF joint structures with regard to varying PE insert thickness. Tibial PE insert thickness variation had a marked effect on the ligament forces and strain patterns, as expected because of overstuffing the TF joint space. With thicker PE inserts relative to the reference case, the MCL, LCL, and PCL forces increased considerably both in flexion and extension, although the changes in the PCL were more distinct after mid-flexion. The observed higher LCL forces over the MCL might be related to the larger stiffness value assigned to the MCL individual bundles (3000 N) based on literature experimental evidence [26]. Another fact worth noticing is that the PCL was engaged at lower flexion angles with thicker PE inserts, which may be due to the larger joint distraction in extension. On the other hand, thinner PE inserts (5 mm, 7 mm) slackened the MCL and LCL almost entirely from 60\({}^{\circ}\) to 90\({}^{\circ}\), due to the larger flexion and extension gap. This is an unfavorable scenario, as slackening the collateral ligaments may destabilize the knee in flexion. The PCL, unlike the other two ligaments, received more tension in the flexed knee with PE inserts thicker than 9 mm, reaching a peak strain at about 10% when using a 13 mm component. This is, however, well below the yield strain of 14% as reported in the literature, above which there might be a ligament injury as a result of overstretching [22]. Similar to the ligaments' kinetic behavior, the compressive force in the TF joint increased substantially at varying PE insert thicknesses, particularly at 90\({}^{\circ}\) of knee flexion with a 13 mm PE component. Considering that the quadriceps muscle force was roughly the same across PE insert thickness cases, this increase can be solely attributed to the summation of increased joint ligament forces. The magnitude of the TF compressive force found in this study is consistent with earlier observations [27], reporting an average peak of about four times BW during rising from a chair without the aid of arms in a natural knee. This supports our reported values with 9 mm (3.7 BW) or 11 mm (4 BW) thicknesses and raises the concern that using thicker PE inserts, such as 13 mm (4.5 BW) or more, could greatly elevate the joint loads on the PE surface, leading to destructive wear [28]. Surgeons should consider this surgical option relative to the resected bone on the tibial plateau, bearing in mind that a thicker PE insert also requires a lower level of tibial resection, which is associated with posteromedial bone failure and early aseptic loosening [29]. In addition, deeper tibial resections may lead to the removal of a considerable part of the tibial PCL attachment, which can result in a reduced femoral rollback and anteroposterior instability during flexion [30]. A recent cadaveric study proposed a PE insert thickness of 10 mm with a posterior slope of 5\({}^{\circ}\) to preserve more than 50% of the tibial PCL attachment site [31].