Abstract
Introduction
Wear of polyethylene tibial inserts has been cited as being responsible for up to 25% of revision surgeries, imposing a very significant cost burden on the health care system and increasing patient risk. Accurate measurement of material loss from retrieved knee bearings presents difficult challenges because gravimetric methods are not useful with retrievals and unworn reference dimensions are often unavailable. Geometry and the local anatomy restrict in vivo radiographic wear analysis, and no large-scale analyses have illuminated long-term comparative wear rates and their dependence on design and patient factors. Our study of a large retrieval archive of knee inserts indicates that abrasive/adhesive wear of polyethylene inserts, both on the articular surface and on the backside of modular knees is an important contributor to wear, generation of debris and integrity of locking geometry.
The objective of the current study is to quantify wear performance of tibial inserts in a large archive of retrieved knees of different designs. By assessing wear in a large and diverse series, the goal is to discern the effect on wear performance of a number of different factors: patient factors that might help guide treatment, knee design factors and bearing material factors that may inform a surgeon's choice from among the array of arthroplasty device options.
Methods
An IRB approved retrieval database was queried for TKA designs implanted between 1997 and 2017. 1385 devices from 5 TKA designs were evaluated. Damage was ranked according to Hood's method, oxidation was determined through FTIR, and wear was determined through direct measurement of retrieved inserts using a previously established protocol. Design features (e.g. materials, conformity, locking mechanisms, stabilization, etc.) and patient demographics (e.g. age, weight, BMI, etc.) were cataloged. Multivariate analysis was performed to isolate factors contributing to wear, oxidation, and damage.
Results
Wear and oxidation were both found to scale with time in vivo in conventional and crosslinked polyethylene. Wear rate was also found to scale with time in vivo, but was not found to be a function of oxidation. Regression shows patient age and female sex to correlate negatively with wear rate. Polished trays, crosslinked polyethylene, and constrained knee designs are all correlated with decreased wear rates.
Discussion
While this study indicates that loosening and infection are predominant causes for TKA revision, wear related failure remains common. We believe this to be the largest existing comparative study of modern TKA wear rates. Insert wear is shown to correlate with several patient factors. Wear performance also varies significantly between knee designs, polyethylene material choice and tray surface finish. When compared to a historical standard for knee wear rates, all designs evaluated in the current study exhibited significant improvements in wear rates. Retrieval analysis can provide insight into implant and patient related factors that contribute to knee wear, with the goal of improving patient outcomes and best matching design decisions to patient populations.
