Elsevier

Wear

Volume 271, Issues 9–10, 29 July 2011, Pages 2379-2385
Wear

Short communication
Understanding the dual mobility concept for total hip arthroplasty. Investigations on a multiscale analysis-highlighting the role of arthrofibrosis

https://doi.org/10.1016/j.wear.2011.02.027Get rights and content

Abstract

In hip implants, UHMWPE (ultra high molecular weight polyethylene) liner wear is believed to be a key issue affecting the lifetime of the artificial joint. Dual mobility, a THA (total hip arthroplasty) concept where the liner moves inside the metallic shell, has become popular due to its low dislocation rate. To understand the tribological behavior of this particular implant, especially the role of the second mobility, 12 representative explants were selected from a bank of 250 explants. The implants used were Profil® femoral stem and Novae® metallic shell. The external surface, involved in the second mobility, was examined by 3D profilometry, SEM (scanning electron microscopy) and CMM (coordinate measuring machine). This study highlights a correlation between roughness and CMM evolutions and surgical parameters. A particular wear zone and a wear scenario were identified and validated according to the type of metal-back. A metal transfer between the metal back and the liner was isolated. CMM allowed to measure second mobility wear volume at a macroscopic scale. Thus a realistic wear mechanism has been suggested for this specific implant.

Introduction

Sir John Charnley introduced, for the first time, the use of polyethylene liner against a metallic femoral head, SS 316L [1]. This couple of materials became the Gold standard, i.e. the reference for the hip arthroplasty. This smooth (liner)–hard (head) couple was an alternative to others, as metal on metal, Mc Kee and Watson-Farrar [2] or ceramic on ceramic, Boutin [3]. This couple of materials became the Gold standard. The head diameter chosen was 22.2 mm, more than half the normal femoral head diameter, to decrease wear rates. In 1975, to overcome the early impingement issue, causing a mean dislocation rate of 5%, the dual mobility socket was introduced by Pr. Gilles Bousquet [4], [5], [6]. This concept was based on two articulating surfaces instead of one—between femoral head and UHMWPE liner, as well as between UHMWPE liner and metal back, Fig. 1. The second mobility exists because of a retentive chamfer maintaining head and liner together, thus allowing increasing head diameter while following Charnley 22.2 mm low friction philosophy.

The usual liner-head assembly is constituted only by the first mobility. This new implant enhanced both stability and survivorship because of decreasing dislocations [7], [8], [9], [10]. The follow-up studies of dual mobility concept exhibited similar behaviors to classical THA in prosthetic joint lifetime [11], [12], [13], [14], [15]. To prevent dislocation due to impingement, the dual mobility concept seems to be the most reliable treatment [16]. The second mobility is believed to be triggered when the femoral neck hits the liner. It lowers liner rim degradations and generation of wear debris. Dual mobility concept is concerned in 30% of THA in France, and is more and more investigated for the implants in US. Considering an extra friction surface, one might expect dual mobility to produce more wear debris than the classical artificial joint; the osteolysis mechanisms might be reinforced. The main key-points related to dual mobility are the understanding of the in vivo biomechanical behavior, the wear rate of UHMWPE involved in the second mobility and the material transfers due to wear and corrosion between the cup and the metal back.

In this study, the explant convex side was investigated for understanding the evolution of surface morphologies. The main aim was to analyse friction zones, wear orientation and total wear rates (from concave and convex sides). Finally, from these experiment results, including a specific treatment of data such as selecting the relevant roughness parameters, a tribological scenario might be determined for highlighting the main wear steps and understanding the in vivo course of the second mobility.

Section snippets

Explanted prostheses, implant selection

Since 1990, all explanted prostheses are stocked for scientific and legal purposes. The cleaning protocol consists in a curing with an antiseptic product, without sterilization process to avoid changes of the mechanical properties due to water absorption. Among the thousands of explants, over 250 dual mobility explants were available. A drastic selection of explants was practiced, based on clinical criteria, to get only the prosthetic elements without other complication than pure liner wear.

Worn volumes by CMM

The twelve explants were analysed using 3D CMM. The pristine liner was used as reference, calibrating the measurement of wear volume. Fig. 5 represents the liner volume, in white, according to the manufacturing minimal diameter. The red stripe corresponds to a zone where the measured diameter was inferior to this minimal manufacturing diameter. Apex was not measured on this experiment. As no wear stripe was found in this zone, we estimated unnecessary to add apex dimensional data. This wear

Conclusions and outlooks

The results from multi-scale analyses allowed suggesting a draft of a dual mobility function theory. The liner, when load is applied, is fixed within the metal back. Its position is now supposed to be roughly symmetric to the stem neck axis, because of the equatorial position of the wear stripe. Upon contact between liner and metal back during load, comes a strain on the liner which widens near the equator, which is the area where width is maximal, resulting in a liner blocking. Creep occurs

Acknowledgements

The authors wish to acknowledge Dr. M. Dursapt and Pr. H. Zahouani for the full-time access to the 3D profilometer at ENISE. Moreover the authors thank F. Blion for performing the 3D CMM measurements.

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