Experimental and numerical analysis of patello-femoral contact mechanics in TKA

In total knee replacement, patello-femoral issues are responsible for up to 50% of all revisions. Nevertheless, the mechanics of the patello-femoral joint have not been investigated as well as the tibio-femoral joint. In this study we analyzed the effect of the patellar height and the tibial positioning on the pressure distribution and contact force in the patello-femoral joint. The height of the patella was defined using the Blackburne-Peel Index. Five different patella positions were simulated ranging from a normal to patella alta situation. The tibial component was put in five different positions antero-posteriorly. A loaded squat, up to 120 degrees flexion, was analyzed using a knee simulator equipped with a pressure sensitive sensor fixed to the patellar component. Pressure distribution, contact area and contact force were continuously recorded during the test. A dynamic musculoskeletal modeling software was used to simulate numerically the same activity. The output parameters of the numerical simulation were the same as in the experimental activity. The experimental activity showed very good repeatability. The results of the numerical simulation are in agreement with the outcome of the experiments. The result of this study shows that in general the patello-femoral contact force increases as a function of flexion angle, until the quadriceps tendon wraps around the femoral component, in this way unloading the patella. A patella alta position induces wrapping at higher flexion angles leading to higher maximum contact forces. A normal patella position induces an earlier wrapping leading to lower maximum contact forces. For the position of the baseplate, a more anterior position leads to higher maximum contact forces and pressures while a more posterior position gives rise to lower forces and pressures. Overall, the contact pressures are quite high and close to the yield strength of UHMWPE. Both the contact areas and the contact forces increase significantly during squatting.