Reply to the Letter to the Editor: Five Hundred Fifty-five Retrieved Metal-on-metal Hip Replacements of a Single Design Show a Wide Range of Wear, Surface Features, and Histopathologic Reactions.

Abstract

To the Editor, We appreciate the opportunity to reply to Dr. Langton’s recent letter to the editor regarding our study [13]. The primary question Dr. Langton raises is why we decided against reporting head-cup clearance (difference between head and cup diameters) for the 555 retrievals. Unfortunately, there is only so much information that can be included in one research article. As it stands, the study included 26 graphs and photos and two tables, occupying a total of 18 pages of the Journal. Reporting clearance data in isolation would not be sufficient. We have been analyzing several variables available in our ASR (DePuy Inc, Warsaw, IN, USA) cohort including clearance and will be presenting these results in upcoming submissions for publication in peer-reviewed journals. As an example, we have a study, currently in press [10], that focuses on 42 ball and cup pairs, which had the largest amounts of volumetric wear (> 100 mm3 combined ball and cup wear) in our ASR collection. The range of clearance values in these 42 pairs was comparable to the remaining pairs which had < 100 mm3 of volumetric wear, with no statistically significant differences. While the presentation of our complete findings regarding clearance is beyond the scope of a letter to the editor, suffice to say we found that there was no correlation of wear against clearance in this cohort. In addition, we did not observe any correlation between clearance and bone attachment to the cup in our large cohort. The revision rates of the ASR reported by major joint registries are well known [1, 12] but to date, there are no straightforward explanations to universally account for all or the majority of these revisions. We disagree with Dr. Langton’s proposed explanation, that is, extremely low clearance has led to cup deformation, high wear, or component loosening in these retrievals. This hypothesis is not supported by the data in the present study’s large cohort of retrieved ASR pairs [13]. Dr. Langton invites us to comment on the correspondence between himself and the London Implant Retrieval Center [4-7]. In reading the original correspondence, Dr. Langton asked Hothi and colleagues [6] whether they could confirm or refute evidence that a substantial number of ASR components were produced out of tolerance with respect to the bearing diameters, resulting in extremely low clearance values. In his letter to us, rather than discussing tolerances, Dr. Langton states that the UK retrieval center “chose not to report the dimensions of the components.” Some clarification of terminology is in order. In their article, Hothi and colleagues [5] reported wear depth maps and volumetric wear. These are the primary quantities of interest that virtually all retrieval studies of metal-on-metal implants that report wear volume provide, including Dr. Langton’s own studies [8, 9]. These quantities are obviously calculated using unworn surfaces to estimate the initial dimensions of the bearing surfaces. However, the question of whether a retrieved implant was initially manufactured out of tolerance is a different question. Analyzing this question requires comparing the estimated initial dimensions, including diameter, sphericity and clearance, to the manufacturer’s specified tolerances. The errors involved in arriving at these estimations must also be kept in mind. These include the measurement accuracy, quality and size (surface area) of the unworn surface on each component, the ability to accurately identify the unworn area, and, last but not least, the choice of numerical method to calculate the wear volume [3, 11]. Given the best of all possible conditions, the accuracy of the estimation of wear volume can be as low as 1 mm3. Finally, Dr. Langton states that our conclusion that decreased cup coverage angle of the ASR may have reduced the risk of impingement “is not accurate.” Our conclusion is based on the lack of observable evidence for impingement or dislocation in our large retrieval cohort, which includes 532 total hip replacement retrievals and 23 resurfacing implants. Dr. Langton’s statement, that the ASR’s reduced articular coverage angle does not confer a greater ROM [8], is apparently based on laboratory measurements of hip resurfacing bearing couples, which are shown to have less ROM than total hip replacements [2]. Be that as it may, our conclusion regarding impingement is based on the observations and analysis of 555 ASR retrievals but clearly, reasonable minds can disagree.