© 2011 Wichtig Editore - ISSN 1120-7000 Hip Int ( 2011 ; : 06 ) 724 - 731 21 724 INTRODUCTION Despite the well-documented benefits of both metal-on- metal (MoM) hip resurfacing and large diameter total hip arthroplasty, recent data from the National Joint Registry (NJR) for England and Wales has shown higher than ex- pected revision rates; higher than for other bearing surface combinations (1). Of particular concern is the high rate (43%) of unexplained failure (2), which is thought to be as- sociated with adverse soft tissue reactions to metal wear debris (3-5). Several retrieval studies of current generation MoM hips have shown that edge wear, as a result of high cup inclination, is an important mechanism responsible for generating high levels of metal ions (6-9). This is one of the A retrieval analysis of explanted Durom metal-on-metal hip arthroplasties Ashley K. Matthies 1 , Johann Henckel 1 , John A. Skinner 2 , Alister J. Hart 1 1 Department of Musculoskeletal Surgery, Imperial College London, London - UK 2 Royal National Orthopaedic Hospital, Stanmore - UK ABSTRACT: Given the recent reports of high failure rates, an improved understanding of the mechanism of failure of large diameter metal-on-metal hip arthroplasties is essential. We present clinical data and tribological analysis of a consecutive series of 74 failed large diameter metal-on-metal hips, comparing the Durom (Zimmer) with the Birmingham hip resurfacing (BHR) (Smith and Nephew). We retrospec- tively analysed pre-, intra-, and post-operative clinical data and measured the linear wear and compo- nent form of the explanted components using a roundness measuring machine. A significantly higher proportion of hips in the Durom group failed as a result of acetabular loosening (p=0.001) and this was supported by evidence of reduced bone in-growth on the backside of the cup. Comparison of round- ness measurement revealed that the Durom hip was significantly lower wearing than the BHR (p<0.05) but the Durom femoral components were subject to significantly greater form errors (p<0.001), the pole of the head being flattened by up to 31 microns. Although the Durom hip is low wearing, reduced sphericity of the femoral component may have resulted in equatorial bearing, leading to an increased frictional torque at the cup-bone interface, preventing bone in-growth, and culminating in acetabular loosening. This supports recent clinical findings of high revision rates as a result of acetabular loosen- ing for the Durom metal-on-metal hip system. KEY WORDS: Metal-on-metal, Hip arthroplasty, Retrieval analysis Accepted: October 17, 2011 ORIGINAL ARTICLE reasons suggested to explain the higher metal ion levels and wear rates of the ASR hip system, which was designed with a cup articular arc angle lower than any other current generation design (10, 11). Therefore, comparison of differ- ent MoM hip designs will improve our understanding of fail- ure mechanisms and help identify design features that may improve ‘in vivo’ wear performance and clinical outcome. Three recent clinical studies have suggested that specific design features of the Durom acetabular component may increase its susceptibility to aseptic acetabular loosening due to reduced bone ingrowth at the cup-bone interface (12-14). In a series of 187 patients with 207 Durom MoM hips, Long et al reported a revision rate of 15% for ac- etabular loosening, with 72% of revisions demonstrating DOI: 10.5301/HIP.2011.8885