Wear in retrieval studies – measurement, amount and

In retrieval studies, the implants retrieved in revision surgery are examined.

Retrieval studies are fundamental for understanding the causes of failure (Jacobs and Wimmer 2013, Hart et al. 2015). Retrieved components should be analyzed thoroughly, that is, inspected visually, microscopically, nanoscopically and measured for material loss (Pourzal et al. 2013). Retrieval analysis can then be combined with clinical patient data (for example, age, sex, BMI, follow-up time, allergological tests) imaging data, blood metal ion concentrations and histological analysis of the periprosthetic tissues to further the understanding of the etiology and pathogenesis of implant failure (Hart et al. 2015). Different methods have been used to estimate bearing surface wear, such as linear wear (maximum wear scar depth) and volumetric wear (total volume of the material lost from bearing surfaces) (Lord et al. 2011). Volumetric wear is considered primary as the total amount of material lost from the surface is of the utmost importance (Ilchmann et al. 2008). However, no single standard exists for the volumetric wear measurement of retrieved implants. Bills et al. described a method developed on the basis of an ISO standard for in vitro wear measurement (Bills et al. 2012). The volumetric

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wear was measured using a coordinate machine that probed the explanted implant surface and created a geometrical map of the surface. This map was then compared to a reference map of an unworn surface and the volumetric wear calculated using computer software. Similar geometrical methods have also been used in other studies (Morlock et al. 2008, Becker and Dirix 2009, Witzleb et al. 2009, Lord et al.

2011). However, Bills et al. reported significant measurement uncertainties, which make comparisons between studies challenging (Bills et al. 2012).

Dozens of retrieval studies have been performed over the last three decades (Tables 1 and 2). The reported sample sizes have mostly been small. The data are very heterogenous as some studies have only included data for one component, some studies have included data for both components separately, and some studies have combined the data for both components to calculate total wear. Furthermore, the follow-up time in these studies has been variable and various statistics have been used. However, as can be seen from Table 1, the mean/median wear volume in most studies has ranged between 10 and 100 mm³, and the mean/median volumetric wear rates have been between 2 and 20 mm³/year. In most studies, the mean/median linear wear has been between 10 and 100 m and the linear wear rate between 5 and 20 m/year (Table 2). Compared to the mean steady-state wear of 0.4 mm³/10⁶ cycles (comparable to a year of prosthesis use) reported in the simulator study meta-analysis by Kretzer et al., it becomes obvious that the real-world wear seen in the retrieval studies is several-fold higher than that reported in preclinical simulator studies (Kretzer et al. 2009). This highlights the importance of retrieval studies to assess the true performance of prostheses. There are no clearly defined boundaries for abnormal versus normal wear, but volumetric wear rates >

1 mm³/year and linear wear rates > 5 m/year are generally considered abnormal (Hart et al. 2012a, Sidaginamale et al. 2013, Cook et al. 2019). In most retrieval studies, however, the average values reported exceed these values (Tables 1 and 2).

It is therefore safe to state that most of the MoM hip replacements studied produce higher than expected and higher than acceptable amounts of wear debris.

Several factors associated with the high wear of implants have been discovered in retrieval studies, and these can be further categorized into implant-, patient- and surgeon-specific factors. Implant-specific factors include clearance, cup arc of cover and femoral head size (Underwood et al. 2011, Matthies et al. 2013a). Certain implant designs are more susceptible to high wear than others, especially the DePuy ASR hip resurfacing and the DePuy ASR XL THA (Ebramzadeh et al.

2011, Underwood et al. 2011). The ASR hip resurfacing has certain design differences compared with the older generation BHR, that is, reduced arc of cover

of the cup, smaller clearance and lower radius of the acetabular rim (Underwood et al. 2011). Low angle of cup cover, small head size and clearance reduce the

“contact patch to rim distance” (CPRD) of the cup and predispose the implant to edge-loading, which is a well-established cause of high wear (Morlock et al. 2008, Matthies et al. 2011, 2013a, Underwood et al. 2011). In a study of highly worn ASR acetabular cups, severe edge-loading was present in all components and on average constituted 58% of the total wear volume (Lu and Ebramzadeh 2019). A recognized patient-specific factor related to high wear is motion pattern (Mellon et al. 2013a). Cup positioning, namely inclination and anteversion, are factors defined as surgeon-specific. Suboptimal cup inclination angle is associated with high wear (Morlock et al. 2008, Hart et al. 2011a, Matthies et al. 2011, Cook et al. 2019).

Inclination angle has been shown to correlate with CPRD and edge-loading, which serves to explain the abnormal wear (Morlock et al. 2008, Matthies et al. 2011). In a similar manner, cup anteversion is also related to high wear through its effect on CPRD and the risk of edge-loading (Matthies et al. 2013a, Cook et al. 2019).

However, edge-loading leading to high wear may also occur in well-positioned implants (Matthies et al. 2011, Hart et al. 2013). A safe zone of 30-50 degrees inclination and 5-25 degrees of anteversion has been shown to reduce the dislocation rate in THA, later termed “Lewinnek’s safe zone” (Lewinnek et al.

1978). In relation to MoM hip replacements, it has been shown that inclination angle outside this safe zone is also related to high wear (Hart et al. 2011a, Matthies et al. 2011). The geometrics of surgeon- and implant-specific factors are illustrated in Figure 3.

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Figure 3. A 2-dimensional schematic drawing of the key surgeon- and implant-specific factors related to the wear behavior of hip replacement. Adopted from Matthies et al. 2013a.

*Clearance = RCUP - RHEAD

Table 1. Studies that have reported the volumetric wear of metal-on-metal hip replacements.

THA = Total Hip Arthroplasty, HR = Hip Resurfacing, SD = Small Diameter, LD = Large Diameter, 1st Gen = First generation, 2nd Gen = Second generation, 3rd Gen = Third generation.

Study Implant type,

subgroup Component N Wear volume (range)

mm³ Volumetric wear rate

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* Calculated from individual measurements given in the study

Table 2. Studies investigating the linear wear of metal-on-metal hip replacements. THA = Total Hip Arthroplasty, HR = Hip Resurfacing, SD = Small Diameter, LD = Large Diameter, 1^st Gen = First generation, 2^nd Gen = Second generation, 3^rd Gen = Third generation.

Study Implant type

& possible subgroups

Component N Linear wear (range)

m

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Hart et al. 2013 2^nd Gen HR Cup 276 - Median 4.5 (IQR 0.2

* Calculated from individual measurements given in the study.

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