Assessing the performance envelope of total knee replacement : an explicit finite element study

The purpose of this study was to examine: 1) The performance of TKR for a variety of activities, 2) The performance of TKR under the influence of eccentric loading, 3) The performance envelope of TKR under subject specific loading, and 4) The effect of misalignment on the performance of TKR. Two TKR designs: PFC Σ and PFC PLI knees (DePuy) have been examined in this thesis. Simulations were performed using dynamic finite element method. In the first study, the TKR produced larger kinematics and contact pressures when simulated using more demanding loads from stair descent and squatting activities. Stair ascent and level gait activities were also simulated. Inclusion of other loading activities in wear studies could help in understanding wear observed in vivo. In the 2nd study, the influence of varus/valgus knee on the performance of TKR was stimulated by offsetting the point of application of the axial load either medially or laterally. Medial eccentric loading was the most destructive for the TKR particularly when the collateral ligaments were not included. The femoral component rode up the anterior lip of the tibial insert and nearly subluxed. Lateral eccentric loading did not show significant kinematics variations as compared to medial eccentric loading, with or without the collateral ligaments. The femoral component did not ride up the anterior edge of the tibial insert. Both the medial and lateral eccentric loading should be avoided as they increased the contact stresses and would lead to accelerated wear. In the 3rd study, the simulations were performed by applying level gait loads obtained from 7 subjects and stair ascent loads from 9 subjects. Performance envelopes in the kinematics and contact pressures were obtained as a result of patient-to-patient variability. The kinematics for the PFC Σ and PLI designs were relatively insensitive to subject specific loading during level gait but greater variations during stair ascent.