A computer model to simulate patellar biomechanics following total knee replacement: the effects of femoral component alignment.

OBJECTIVE The objective of this study is to analyze the biomechanics of the patellar component following total knee replacement. More specifically we investigated the effect of displacing the femoral component of an Insall-Burstein II total knee replacement on the patellar tracking and patello-femoral contact pressures. DESIGN We used a validated computer simulation of the knee joint to virtually insert the femoral component with the following four types of placements: (1) no misplacement, (2) 5 degrees of internal rotation, (3) 5 degrees of external rotation and (4) 5 degrees of flexion rotation. The patellar 3D tracking and patello-femoral contact pressures were computed for each femoral component placement as a function of knee flexion angle. BACKGROUND Complications at the patello-femoral joint are the among most frequent following total knee replacement. RESULTS Femoral component placement unevenly affected the associated patellar tracking: a 5 degrees internal rotation tilted and rotated the patella laterally by about 5 degrees throughout knee flexion. A 5 degrees external rotation of the femoral component had less effect on patellar tracking. A rotation of 5 degrees in flexion primarily caused patellar rotation (5-10 degrees lateral rotation). Femoral component malalignment had only minor effects on the peak pressure distributions at the patello-femoral interface. CONCLUSION These results suggest that femoral component positioning primarily affects patellar tracking, with a possible threat for patellar subluxation under external rotation of the femoral component. RELEVANCE Precise alignment of the prosthetic components is difficult to control during total knee replacement due to the lack of precise anatomical landmarks in the human knee joint. Consequently, the position of each prosthetic component may differ from the ideal one suggested by the manufacturer. Improper alignment of the prosthetic components during total knee replacement may lead to premature implant failure.