Research Article: Micro X-Ray Computed Tomography Mass Loss Assessment of Different UHMWPE: A Hip Joint Simulator Study on Standard vs. Cross-Linked Polyethylene

Date Published: January 20, 2017

Publisher: Public Library of Science

Author(s): Saverio Affatato, Filippo Zanini, Simone Carmignato, M. A. Pérez.


More than 60.000 hip arthroplasty are performed every year in Italy. Although Ultra-High-Molecular-Weight-Polyethylene remains the most used material as acetabular cup, wear of this material induces over time in vivo a foreign-body response and consequently osteolysis, pain, and the need of implant revision. Furthermore, oxidative wear of the polyethylene provoke several and severe failures. To solve these problems, highly cross-linked polyethylene and Vitamin-E-stabilized polyethylene were introduced in the last years. In in vitro experiments, various efforts have been made to compare the wear behavior of standard PE and vitamin-E infused liners. In this study we compared the in vitro wear behavior of two different configurations of cross-linked polyethylene (with and without the add of Vitamin E) vs. the standard polyethylene acetabular cups. The aim of the present study was to validate a micro X-ray computed tomography technique to assess the wear of different commercially available, polyethylene’s acetabular cups after wear simulation; in particular, the gravimetric method was used to provide reference wear values. The agreement between the two methods is documented in this paper.

Partial Text

Total hip arthroplasty (THA) is a successful orthopaedic standardized procedure to restore functionality on damaged hip due injury or arthritis [1]. As the number of successful operations has increased, the goal of developing alternate bearing surfaces has been to create a joint with decreased friction and wear rates but with increased strength. Moreover, techniques have become standardized and the average age of those receiving hip replacements has reduced. Since ideal bearing surfaces for THA are still being continuously sought, continue effort are made in order to obtain the “ideal bearing surface; the bearing surface should have superior wear characteristics and should be durable, bio-inert, cost-effective, and easy to implant [2].

The process of cross-linking has been utilized to improve the wear resistance of polyethylene. Some hip simulator studies have shown that the cross-linking process can reduce the type of wear that occurs in acetabular components by >95% [18, 23, 24]. Pre-clinical wear simulations are necessary to predict and quantify the consequent hip prostheses functionality. The aim of the present study was to validate a micro-CT based procedure to assess the wear of commercially available different configurations of polyethylene acetabular cups. In this light, the gravimetric quantification method of wear was considered the reference value. The two methods were found to be correlated and in agreement with mean difference in the order of 11%. The highest percentage difference (≅45%) was registered for the least worn component (XLPE_1). The differences are not only due to the uncertainty of CT wear measurements, but also to the uncertainty of the density value. Furthermore, although the gravimetric method is currently considered the gold standard, it is also unavoidably affected by uncertainties of measurements.

The results obtained from the approach presented in this work produced important improvements on the wear measurement techniques used for the total hip replacement. From the obtained results, it can be concluded that micro-CT and gravimetric measurements of wear are in agreement, provided that micro-CT dimensional measurements are performed under temperature control and that metrological performance verification and systematic errors correction are adopted to allow metrological use of micro-CT. Despite this adopted methodology for wear loss characterization requires further works in order to improve its reliability and accuracy, it has the potential to become a new reference method in wear assessment of UHMWPE components. In fact, the micro-CT method can be useful to determine the location of wear and to analyze how wear distribution for the investigated specimens change by the effect of different loads.