A 3D model analysis of patellofemoral joint: effect of anterior translation and internal rotation of tibia

The objective of this study was to find out the effect of tibial position and orientation to patellofemoral kinetics, especially during deep knee flexion. The data about patellofemoral kinetics is necessary for calculating tibiofemoral joint force which is used to evaluate a design of knee prosthesis, however, there have been no data how it changes by tibial anterior translation or internal rotation which recent prostheses are aiming. We performed a three-dimensional model analysis with axioms of point contact and force equilibrium. The flexion angle was from 0° to 150° by four conditions. The neutral condition was based on the two-dimensional analysis using PS type of the prosthesis, without internal rotation and plenty femoral roll back. The other conditions were with over 20mm of tibial anterior translation, with 15° of tibial internal rotation, and both the translation and rotation during deep knee flexion. As a result, patellofemoral joint force reduced by tibial anterior translation and did not change by tibial rotation. Tensile force of patella tendon did not change by the tibial position/orientation. By rotating tibia internally, medial contact force increased and lateral reduced. Moreover, tibial anterior translation facilitated the dislocation of patellofemoral joint, and lateral side lifted off with tibial internal rotation. The contact force reduced rapidly before the lift off, and medial contact force increased by the lateral lift off. In this case, although the magnitude of the patella tendon force did not change, the direction of the force might change unignorably. Both tibial anterior translation and internal rotation might be necessary to be capable of deep knee flexion, however, it is also necessary to discuss the interaction between tibiofemoral and patellofemoral joints for evaluating the new prosthesis intending deep knee flexion.