Abstract
Introduction
In an effort to provide a TKA bearing material that balances resistance to wear, mechanical failure and oxidation, manufacturers introduced antioxidant polyethylene. In many designs, this is accomplished through pre-blending the polymer with the antioxidant before consolidation and radiation crosslinking. This study reports the wear performance (in terms of thickness change) of a hindered phenol (PBHP) UHMWPE from analysis of an early series of knee retrievals and explores these questions: 1) What is early-time performance of this new bearing material? 2) Is there a difference in performance between fixed and mobile bearings in this design? 3) How does quantitative surface analysis help understand performance at the insert-tray modular interface?
Methods
A series of 100 consecutive Attune™ knee inserts (DePuy Synthes, Warsaw, IN) received at revision by an IRB approved retrieval laboratory between September 2014 and March 2019 were investigated. In vivo duration was 0–52 months. Both the fixed bearing design (n=74) and the rotating platform mobile bearing design (n=26) were included. Dimensional change was determined by measurement of each insert and compared to the as-manufactured dimensions, provided by the manufacturer. The insert-tray interfaces under the loaded bearing zones were analyzed with light interferometry using an optical surface profiler (NewView™ 7300, Zygo, Middlefield, CT). Statistical analyses to explore relationships between measured variables were conducted using SPSS.
Results
Mean total through-thickness change of the inserts was 0.052 mm. Mean rate of thickness change for all inserts having in vivo duration > 12 months was 0.038 mm/year (fixed bearing 0.042, mobile bearing 0.029 mm/year). The rate of thickness change for all inserts showed a decreasing trend with duration that was not statistically significant, (rho -.244, p=.094); however, the mobile bearing cohort alone showed a significant decrease in thickness change rate with duration (rho= −.659; p=.014). Surface roughness (Sa) of the distal surface of the UHMWPE inserts under the bearing areas averaged 1.24 µm (range 0.12 – 8.53) and peak-to-valley height (PV) averaged 27.1 µm (range 4 – 95). Sa and PV both showed a decreasing trend with duration in vivo in the mobile bearing inserts, but that trend did not reach statistical significance (p= 0.05 criterion). Neither Sa nor PV showed correlation with measured thickness change.
Discussion
This study indicates that the rate of thickness change of a relatively new antioxidant cross-linked bearing material is very similar to other reported wear rates of crosslinked knee inserts. Lower wear rate of mobile bearing inserts compared to fixed bearings also is consistent with earlier published studies.
Direct comparison between quantitative thickness change and objective, quantitative surface metrology on the same series brings new information to the arena of measuring and reporting “wear” of UHMWPE and underscores the importance of the distinction between visual damage and actual thinning of the bearing. The systematic surface analysis of the modular interfaces showing that surface roughness (Sa) and total damage feature topography (PV) trend downward with in vivo duration of mobile bearings supports the hypothesis that relative motion at that interface may ‘polish out’ the surface topography over time.
For any figures or tables, please contact authors directly.
