###### Abstract  
The thickness of the tibial polyethylene (PE) insert is a critical parameter to ensure optimal soft-tissue balancing in the intraoperative decision-making procedure of total knee arthroplasty (TKA). However, there is a paucity of information about the kinetic response to PE insert thickness variations in the tibofemoral (TF) joint, and subsequently, the secondary effects on the patellofemoral (PF) biomechanics. Therefore, the purpose of this study was to investigate the influence of varying PE insert thickness on the ligament and TF compressive forces, as well as on the PF forces and kinematics, after a cruciate-retaining TKA. A previous patient-specific musculoskeletal model of TKA was adapted to simulate a chair-rising motion in which PE insert thickness was varied with 2 mm increments or decrements compared to the reference case (9 mm), from 5 mm up to 13 mm. Greater PE insert thickness resulted in higher ligament forces and concurrently increased the TF compressive force by 21% (13 mm), but slightly unloaded the PF joint with 7% (13 mm) while shifting the patella distally in the trochanter groove, compared to the reference case. Thinner PE inserts showed an opposite trend. Our findings suggest that the optimal PE insert thickness selection is a trade-off between the kinetic outcomes of the TF and PF joints. msculoskeletaletal model; polyethylene thickness; total knee arthroplasty; ligament forces; compressive forces; patellar kinematics +
Footnote †: journal: Applied  
variations on the soft-tissue tension and intraoperative kinematics based on knee laxity trials [10; 11]. Following this recommendation, some authors argue that the insertion of thicker PE components is likely to prevent accelerated wear and early surgical failure [12]. Nevertheless, the debate continues about the target PE insert thickness, with contradictory findings from earlier studies on the topic. [PERSON] et al. [13] demonstrated that implanted knees with a PE insert thickness of more than 14 mm performed similarly in the postoperative clinical evaluation, yielding no surgical failure due to aseptic loosening or tibial component instability as compared to those with a thickness under 14 mm. Contrarily, [PERSON] et al. [14] labeled tibial PE inserts thicker than 14 mm as potential contributors to posterolateral rotatory instability, leading to higher failure rates at mid- to long-term follow-up. One of the main limitations of these studies, however, was the failure to control for the variability in the depth of tibial resection, which might provide an inaccurate estimation of the actual effect of PE insert thickness variation on the surgical outcomes. From a biomechanical perspective, using thicker tibial PE inserts than the minimum recommended threshold (6-8 mm), while keeping the resection depth unchanged, can increase tension in the soft-tissue envelope; this can subsequently result in higher contact forces in the distal femur and tibial insert interface. The increased contact forces are also transferred to the fixation site, and this could, therefore, promote aseptic loosening of the prosthesis. A comprehensive evaluation of the kinetics after TKA is crucial to prevent higher than physiological mechanical stress and strain on the ligamentous structures as well as high load levels at the interface between bones and prosthetic components. This knowledge could lead to better surgical choices and increased implant longevity. Since the thickness of the tibial PE insert can be modified intraoperatively, the surgeon can customize the surgical plan by choosing a PE insert thickness that best replicates the natural knee kinematics and kinetics. Computational models of the musculoskeletal system represent a valuable tool in this intraoperative decision-making procedure. Musculoskeletal models can be personalized to patient-specific anatomical features and predict variations in the knee joint loads and kinematic patterns due to changes in implant design properties or surgical choices, such as the thickness of the tibial PE insert.