Stair ascent kinematics affect UHMWPE wear and damage in total knee replacements

Investigations were performed to characterize the wear patterns of tibial inserts in a load-controlled knee simulator by incorporating both normal gait and clinically acquired stair climbing kinematics and load conditions. Two different tibial insert designs were evaluated: the NexGen Cruciate Retaining Augmentable and NexGen Legacy Posterior Stabilized inserts. Two test conditions were run: standard gait only, and gait with bouts of stair climbing at a ratio of 70:1. Gravimetric wear measurements, damage and contact area assessments, and kinematic evaluations were performed. The addition of stair climbing kinematics significantly affected the wear behavior. Regardless of design, wear rates for standard gait tests were significantly higher than those that included bouts of stair climbing. The damage modes seen in both test conditions were primarily burnishing with secondary scratching and pitting. At 2 Mc, the damage areas were not significantly different between the two designs, but the damage area with stair climbing was significantly larger than that with gait alone. The fact that even small bouts of an additional activity of daily living could markedly impact wear simulator results calls into question the usefulness of studies that rely solely on kinematics and load inputs assumed from level gait.