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

To the Editor,

I read the study by Park and colleagues [11] that examined several hundred explanted Articular Surface Replacement (ASR) metal-on-metal prostheses (DePuy Inc, Warsaw, IN, USA) with great interest.

One of the ASR’s unique features was its extremely low diametrical clearance [1]. However, DePuy Inc (Warsaw, IN, USA) appeared to have skewed their clearances to the lower tolerance limit for the production of their components for unknown reasons [9]. Low clearance leaves a press-fit device vulnerable to the effect of cup deflection and a critical reduction in the gap between the head and the cup [3], which can manifest as increased wear or component loosening [14]. Multiple studies and national joint registry reports have demonstrated a high incidence of these problems with ASR devices [2, 10, 12]. It is, therefore, critical to gain as much understanding of the mechanisms leading to the high revision rate of this prosthesis to avoid future problems.

I inquire as to why the authors decided not to report the clearances on the 555 devices they have measured? The authors generally found poor bony ingrowth on the cups they examined [11]. Would the authors agree that low clearance and lubricant starvation leading to excessive micromotion would be a good explanation for failure of bony ingrowth?

I note that the authors’ (S-HP, ZL, PAC, EE) institution received grants from DePuy Inc (Warsaw, IN, USA) in support of the research. A UK retrieval centre, also heavily funded by DePuy Inc, reported on explanted DePuy products [6, 13] but also chose not to report the dimensions of the components. One of the reasons given for this omission is that one cannot determine accurately the manufactured dimensions from the study of explants [5]. This is simply not true. The measurement and reporting of volumetric wear calculations (as low as 1 mm³ in the current study [11]) is dependent on the ability to successfully reverse engineer the original geometry. I made this point to Hothi and colleagues in previous correspondence [7] but have not, as yet, received a response.

Finally, I would also seek clarification on one comment made in the paper. The authors conclude: “The increased diameter and decreased coverage angle of the ASR design may have resulted in the observed high incidence of edge wear while perhaps decreasing the risk for impingement and dislocation” [11]. This is not accurate. The recessed nature of the articular surface means that the reduced articular coverage angle does not confer a greater ROM [8]. In fact, the unique design of the ASR bearing surface leaves it more vulnerable prone to edge wear while at the same time increasing contact stresses and not providing a greater ROM [4].