Abstract
INTRODUCTION
Wear and polyethylene damage have been implicated in up to 22% of revision surgeries after unicompartmental knee replacement. Two major design rationales to reduce this rate involve either geometry and/or material strategies. Geometric options involve highly congruent mobile bearings with large contact areas; or moderately conforming fixed bearings to prevent bearing dislocation and reduce back-side wear, while material changes involve use of highly crosslinked polyethylene. This study was designed to determine if a highly crosslinked fixed-bearing design would increase wear resistance.
METHODS
Gravimetric wear rates were measured for two unicompartmental implant designs: Oxford unicompartmental (Biomet) and Triathlon X3 PKR (Stryker) on a knee wear simulator (AMTI) using the ISO-recommended standard. The Oxford design had a highly conforming mobile bearing of compression molded Polyethylene (Arcom). The Triathlon PKR had a moderately conforming fixed bearing of sequentially crosslinked Polyethylene (X3).
A finite element model of the AMTI wear simulation was constructed to replicate experimental conditions and to compute wear. This approach was validated using experimental results from previous studies.
The wear coefficient obtained previously for radiation-sterilized low crosslinked polyethylene was used to predict wear in Oxford components. The wear coefficient obtained for highly crosslinked polyethylene was used to predict wear in Triathlon X3 PKR components. To study the effect design and polyethylene crosslinking, wear rates were computed for each design using both wear coefficients.
RESULTS
Wear rates were significantly lower (69%) for the Triathlon fixed-bearing design compared to the Oxford mobile-bearing design (Fig 1, p<0.01). The FEA model predicted 46% of wear occurring at the back side of the mobile bearing (Fig 2). When wear was computed for the Triathlon PKR design using the wear coefficients used for the low crosslinked polyethylene, wear rates increased to 13.9 mg/million cycles.
DISCUSSION
We used a combined experimental and computational approach to quantify factors contributing to polyethylene wear after unicompartmental knee arthroplasty. To isolate the effect of crosslinking level and mobile-bearing design, we computed wear rates for both designs using the same wear coefficient obtained for low crosslinked polyethylene. Wear rates in the low crosslinked Triathlon PKR insert increased by more than 160% relative to those in the highly crosslinked Triathlon X3 PKR. The finite element method facilitates computation of relative back-side to front-side wear, which is challenging to obtain experimentally. The back-side wear Oxford mobile bearing was 46% of total wear. Major factors contributing to the difference in wear were back-side wear (46%) and increased crosslinking (63%) with the combined effect having an additive effect. Our FEA-predicted wear penetration rates (0.024 mm/million cycles) also compare well to in vivo studies, which reported penetration rates of 0.022 mm/year for Oxford bearings. A validated computer model is extremely valuable for efficient evaluation of wear performance and design development.
In summary, increasing conformity to increase contact area and reduce contact stress may not be the sole predictor of wear performance. A highly crosslinked polyethylene insert in a fixed-bearing design may provide the high wear performance of a mobile-bearing design without the increased risk for bearing dislocation.
