Introduction: Tibial slope was shown to majorly affect the outcomes of Total Knee Arthroplasty (TKA). More slope of the tibial component could help releasing a too tight flexion gap in cruciate-retaining (CR) TKA and is generally associated with a wider range of post-operative knee flexion. However, an excessive tibial slope could jeopardize the knee stability in flexion. The mechanism by which tibial slope affects the function of CR-TKA is not well understood. Moreover, it is not known whether the tibial bone resection should be performed by referencing the anterior cortex (AC) of the tibia or the center of the tibial plateau (CP) and whether the choice of either technique plays a role. The aim of this study was to investigate the effect of tibial slope on the position of tibiofemoral (TF) contact point, knee ligament forces, quadriceps muscle forces, and TF and patellofemoral (PF) joint contact forces during squat activity in CR-TKA.
Methods: A previously validated musculoskeletal model of CR-TKA was used to simulate a squat activity performed by a 86-year-old male subject wearing an instrumented prosthesis [1,2]. Marker data over four consecutive repetitions of a squat motion were tracked using a motion optimization algorithm. Muscle and joint forces and moments were calculated from an inverse-dynamic analysis, coupled with Force-Dependent Kinematics (FDK) to solve knee kinematics, ligament and contact forces simultaneously. The tibial slope in the postoperative case was 0 degree and constituted the reference case for our simulations. In addition, eight additional cases were simulated with -3, +3, +6, +9 degrees of tibial slope, four of them simulating an AC referencing technique and four a CP technique.
Results: Compared to the reference case with no added slope, the total excursion of the tibiofemoral contact point increased on both medial and lateral side when more slope was added using the AC referencing technique, and decreased with negative slope. The total excursion of the contact point remained about unchanged when using the CP technique but the contact point shifted of about 1 mm more posteriorly, on the lateral side, and 0.7 mm, on the medial side, on average. In both AC and CP techniques the quadriceps forces, TF and PF contact forces decreased with more slope, but the PF contact forces were more drastically reduced using CP, with 3.5% less force every 3 degrees of added slope in flexion, on average. Medial and lateral collateral ligament became slack in flexion already with +6 degrees of slope when AC technique was used, whereas they always maintained some residual tension using the CP technique even at the highest slope.
Discussion and conclusion: Increasing the tibial slope affected substantially the knee function during squatting and the effects differed depending on the referencing technique. The CP referencing helps preserving the flexion gap and knee stability in flexion, by maintaining tension in both collateral ligaments. It also reduces the quadriceps forces and relieves the PF joint contact forces, which could potentially decrease pain in patients with a TKA and achieve a wider range of knee motion.