## 4 Discussion
Our findings highlight the importance of preoperative planning of the appropriate tibial insert thickness, which considers both the thickness value available and the tibial resection depth and, additionally, support the conceptual premise that selection of a tibial insert that is either too thin or too thick can have a negative impact on the TF kinetics, leading to a sub-optimal solution. Varying the thickness of the PE insert revealed marginal secondary effects on the PF biomechanics. The quadriceps muscle force remained unchanged across PE insert thickness variations in the full arc of the knee range of motion. However, more thickness in the tibial PE component increased the quadriceps tendon-to-femur force in the range of 60\({}^{\circ}\)-90\({}^{\circ}\) of knee flexion. Contrary to our expectations, the PF compressive force slightly decreased with 2 mm and 4 mm greater PE insert thickness, compared to the reference case, and this change was more perceivable from mid-flexion to 90\({}^{\circ}\). This is in agreement with the findings of a previous study [32] and might be explained by quadriceps-femur load sharing as the quadriceps wraps around the distal femur in a flexed knee position. This mechanism is also reflected in the PF kinematics. The patella was consistently more distal with thicker PE inserts than in the reference case throughout the range of flexion and extension, as expected due to the joint line elevation, which subsequently results in patella baja. From a clinical point of view, unloading the PF joint could reduce patellar complications after TKA, especially anterior knee pain, which strongly correlates with postoperative patient dissatisfaction and impaired quality of life [33; 34; 35]. On the other hand, patella baja may have serious consequences on the overall function of the knee after surgery, such as limited range of motion because of patellar maltracking. Moreover, the distally displaced patella can impinge on the anterior part of the tibial PE insert or the tibial tray during flexion, potentially increasing wear [36]. Hence, surgeons should carefully assess the concomitant effects on the PE joint when pre-planning the desired PE insert thickness to ensure that the patella slides properly in the trochlear groove. To the best of our knowledge, this is the first musculoskeletal simulation study which explores the biomechanical consequences of tibial PE insert thickness variations on both the TF and PF joints after cruciate-retaining TKA. A major strength of this study is the use of a computational modeling approach to investigate the effect of tibial PE insert thickness in a highly controlled simulation environment, where all the other surgical variables, such as the tibial resection depth, remained unchanged. This overcomes an important limitation of clinical studies, in which confounding factors, such as the implant design, size, and alignment, are present and can potentially affect the surgical outcomes [9]. This study has several limitations. At first, the modeled ligament mechanical properties, such as stiffness and reference strain, were determined from the literature. Furthermore, the musculoskeletal simulation represented only a mechanically aligned cruciate-retaining TKA, meaning that the results cannot be extrapolated to other surgical techniques or implant designs. It is also noteworthy that the computational model simulated only one patient, disregarding the anatomical variability or other pathological conditions among different patients. Further research to overcome this limitation may be to characterize the effect of variability in the morphological knee joint phenotypes on the postoperative kinetics with varying tibial PE insert thicknesses. A promising methodology to explore this could be the combination of statistical shape modeling and musculoskeletal simulation.