Reduced Articular Surface of One-piece Cups: A Cause of Runaway Wear and Early Failure

Abstract

Background Despite the clinical success of modern metal-on-metal articulations, concerns with wear-related release of metal ions persist. Evidence suggests metal ion release is related to the effective coverage of the head in the metal shell (the cup's functional articular arc). A recent study suggests a reduced functional articular arc is associated with increased ion release and the arc is a function of component design, size, and the abduction angle. Questions/purposes The purposes of this study were to (1) measure the functional articular arc in different sizes of currently available one-piece metal shells from several different manufacturers; and (2) compare the functional articular arc of these one-piece metal shells with the 1808 arc of conventional hip arthroplasty acetabular components.Methods We calculated the available articular surface arc for 33 one-piece metal cups using measurements of cup depth and internal cup radius. Results The arc of the articular surface varied among manufacturers and generally decreased with decreasing shell diameter. The mean functional articular arc was 160.5°± 3.6°(range, 151.8°-165.8°), which was less than the 180°arc of a conventional acetabular component. Conclusions Our data show certain cup designs are at higher risk for failure as a result of the decreased articular surface arc. This, along with analysis of abduction angles, supports the recent findings of bearing failure with vertically placed implants. Care must be taken when implanting these shells to ensure they are placed in less abduction to avoid edge loading and the potential for early bearing failure.

High Prevalence of Adverse Reactions to Metal Debris in Small-headed ASR™ Hips

“…Survival rates in our study also were worse than those reported by Langton et al [14]. We believe this is because our cohort included only patients with a small femoral head size, meaning that our patients were more prone to edgeloading as a result of a reduced functional arc [10]. Therefore, failure resulting from increased wear originating from the bearing surface instead of the taper is likely to be more prevalent in our cohort.…”

“…Functional arc of the cup increases with increasing head sizes [10] thus offering more cup coverage and decreasing the occurrence of edgeloading. Thus, especially in patients with large head implants in hip resurfacing, the prevalence of adverse reactions may be lower than in our current cohorts.…”

Risk of Impingement and Third-body Abrasion With 28-mm Metal-on-metal Bearings

“…Micrometer measurements of internal liner geometry provided cup-face angles (CFA), cup-bearing angles (CBA), ROM, and polar-impingement angles (Fig. 1A) [9,23]. Liners were replicated in silicon (Aquasil-LV TM ; Densply, Milford, DE, USA) and the replicas sectioned for scanning electron microscopy (SEM) analysis (Fig.…”

“…Many studies focused on cobalt-chromium (CoCr) alloys, cup designs ( Fig. 1), surgical positioning, and effects of ''edge loading'' [12,23,29,38,58,59,76]. Additionally, impingement, subluxation, and dislocation have always been unpredictable risk considerations [3,18,22,24,27,28,31,43,52,66,72,74,77,80].…”

Letter to the Editor: Do Complication Rates Differ by Gender After Metal-on-metal Hip Resurfacing Arthroplasty? A Systematic Review

“…This certainly applies to many women, but not all of them. Additionally, not all hip resurfacing devices have the same coverage of the head by the socket [2] and such differences in the small sizes can greatly affect the outcome of hip resurfacing arthroplasty. This is particularly important in our overly litigated society because every time a publication is made that suggests female gender is a risk factor, it reinforces the use of simplistic treatment algorithms and perfectly suitable female patients get denied the opportunity to be treated with hip resurfacing arthroplasty.…”