Abstract
INTRODUCTION
Studies of retrieved TKR components demonstrate that in vivo wear on the articular surface of polyethylene liners exhibits a much higher variability on their in vitro counterparts. Only one study has attempted to validate a patient-specific model of wear with a clinically retrieved component. The purpose of this study is to investigate the relationship between observed TKR contact conditions during gait and measured volume loss on retrieved tibial components.
METHODS
Eleven retrieved ultra-high molecular weight polyethylene (UHMWPE) cruciate-retaining tibial liner components from ten separate patients (implantation time = 8.6±5.6 years) had matching gait trials of normal level walking for each knee. Volume loss on retrieved components was calculated using a coordinate measuring machine and autonomous reconstruction. Motion analysis of normal level walking gait had been conducted between 1986 and 2005 for various previous studies and stored in a consented Human Mechanics Repository, ranging from pre-operative to long-term post- operative testing. Contact location between the femoral component and the tibial component on the medial and lateral plateaus were calculated throughout stance. A previously validated and fine-tuned parametric numerical model was used to calculate TKR contact forces for each gait trial. Vertical contact forces and contact paths on the medial and lateral plateaus were input as normal force and sliding distance to a simplified Archard equation for wear with material wear constant averaged from literature (2.42 × 10−7 mm3/Nm) to compute average wear per gait cycle. Wear rates were calculated using linear regression, and Pearson correlation examined correlations between modeled and measured wear.
RESULTS
Secondary motions at the knee from gait testing showed distinct grouping between trials of each patient. Three components demonstrated severe polyethylene delamination and were excluded from wear rate analyses. Calculated wear rates for measured and modeled volume loss showed excellent agreement on total surface (15.9 vs. 16.4 mm3/year), medial sides (11.4 vs. 11.7 mm3/year) and lateral sides (4.4 vs. 4.7 mm3/year) and were not significantly different. Volumes were significantly correlated between measured and modeled wear for the total part (r=0.758, p=0.017) and on the medial side (r=0.780, p=0.012), but not for the lateral side (r=0.482, p=0.154).
DISCUSSION
Measured wear rates were comparable to a previous study of a large population of retrieved MGII components. Medial wear volumes for six of eight mild wearing components were closely tracked by their modeled counterparts. Because the Archard equation produces wear volumes that are linearly related to time in situ, deviations from linear predictions arise from patient-specific variations in contact forces and tibiofemoral pathways during normal walking gait. As suggested by the results of the current study, these variations in gait between patients result in meaningful differences to the wear of the UHMWPE component. Despite many assumptions, this study demonstrates the feasibility of a patient-specific model of wear using a rare population of gait-matched retrievals.
