The current role of metal-on-metal hip replacements

2.10.1 Guidelines by authorities

Despite the wear-related problems in MoM hips, the majority of the national and international authorities have not recommended discontinued use of MoM hip resurfacings or stemmed THAs so far, but instead emphasize careful patient selection and follow-up (MHRA 2012, TGA 2012, Health Canada 2012, FDA 2012, Hannemann et al. 2013). Some of the national orthopaedic associations have advised against the use of all MoM hip replacements, or specifically large diameter THAs. The British Hip Society has advised against the use of large diameter (≥36 mm) MoM THAs, but not against small head MoM THAs or resurfacings (British Hip Society 2012). The Finnish Arthroplasty Association has advised against the use of all MoM hip

replacements (Finnish Arthroplasty Association 2015), and the Dutch Orthopaedic Association has advised against the use of all large diameter (≥36 mm) MoM replacements until further information about safety is available (Verheyen and Verhaar 2012).

2.10.2 MoM hip resurfacings

A minimum of 10-year clinical results have been reported for only two resurfacing designs: namely the BHR and Conserve plus. There are several studies reporting survivorship of BHR, both from the designing (Matharu et al. 2013, Daniel et al. 2014) and independent centers (Murray et al. 2012, Holland et al. 2012, Van Der Straeten et al. 2013, Reito et al. 2014b, Mehra et al. 2015). For Conserve plus, only the designing center has reported results with follow-up of more than 10 years (Amstutz et al. 2010). All the studies on BHR have reported worse implant survival figures for females (74% to 92%

at 10 to 15 years) compared with males (93% to 100%) (Murray et al. 2012, Holland et al. 2012, Van Der Straeten et al. 2013, Matharu et al. 2013, Daniel et al. 2014, Reito et al. 2014b, Mehra et al. 2015). Three out of six studies including multivariable analysis identified small femoral head size as a risk factor for failure (Holland et al. 2012, Murray et al. 2012, Matharu et al. 2013).

In registry data of NJR and AOANJRR, the overall revision rates are 13% at 10 years (NJR. 2014) and 11.2% at 13 years (AOANJRR 2014), with point estimates of individual resurfacing brands varying from 9% to 31% (10-year) and 4% to 24% (7-year), respectively.

Based on the higher revision rates compared to conventional THAs in registry data, the risk for ARMD and the lack of studies showing significantly improved functional outcome compared to conventional THAs, the discontinued use of MoM resurfacings has been suggested (Dunbar et al.

2014). However, that article has been criticized for only accounting for registry data, and omitting analysis of the excellent results of some resurfacing brands in specific subgroups (Matharu et al. 2015c). Some have suggested that the use of MoM resurfacings can be continued, but that the indications should be refined to include mainly male patients with OA, and that only head sizes of 46 mm and above should be used (Cadossi et al. 2015, Matharu et al. 2015c).

Also, the importance of thorough patient councelling about unique failure types of resurfacings (neck fracture, etc.), risk for ARMD and unknown

long-term effects is emphasized (Matharu et al. 2015c). The use of MoM resurfacings in women has been questioned (Murray et al. 2012).

2.10.3 MoM total hip arthroplasties

NJR data has described overall revision rates of 22% at 10 years (NJR 2014) for MoM THAs. In Australian registry data, the results of small head sizes are better (10-year revision rate 5.7% for ≤28 mm and 6.5% for 30-32 mm) compared to large sizes (10-year revision rate 12.9% for 36-40 mm and 25.5%

for >40 mm), with 7-year revision rates of individual component brands ranging between 4.3% and 37.2% in large head size MoM THAs (AOANJRR 2014). A study based on analyses from the NJR registry advised against using stemmed large diameter stemmed MoM hip replacements (Smith et al. 2012c).

However, this advice has been criticized for lumping together all the stemmed MoM devices and patient groups, as there are differences in implant designs and results for certain patient groups (Amstutz and Le Duff 2012).

Only one center has reported clinical results with follow-up exceeding 10 years for large diameter MoM THAs. In that study, 12-year implant survival of 87% (95% CI, 84% to 90%) was reported, with female gender, young age and high acetabular inclination as risk factors for failure (Lombardi Jr et al.

2015). Early and mid-term results have varied from catastrophic (51% survival at 6 years (Langton et al. 2011a) and 38% seven years (Reito et al. 2013)) to excellent between large diameter head size THA brands (Engh et al. 2010, Barrett et al. 2012). Even though a high prevalence (20%) of pseudotumors has been reported in small head size MoM THAs as well (Hwang et al. 2014), excellent 11 to 19-year survival figures (Grubl et al. 2007, Nikolaou et al. 2011, Randelli et al. 2012, Hwang et al. 2013) have been presented accompanied by registry data showing a low revision rate in small head size MoM THAs (AOANJRR 2014).

Of the most recent studies, several authors have reported that they have discontinued the use of large diameter MoM THAs (Levy and Ezzet 2013, Lombardi Jr et al. 2015), and some have stated that clearly longer-term results are needed (Saragaglia et al. 2015). The small head size MoM THA is considered to be a viable option by some institutions, although the potential risk of ARMD has raised some concerns (Hwang et al. 2013).

2.10.4 Comparison to National Institute for Health and Care Excellence criteria

According to United Kingdom National Institute for Health and Care Excellence (NICE) criteria, primary hip replacements should have a revision rate of 10% or less at 10-year follow-up (NICE 2000). In the NJR Annual Report 2014, 10-year revision rates of both hip resurfacings (13%) and stemmed MoM THAs (22%) exceeds the NICE benchmark. For MoM THAs, this is true both in males and females in all age groups. Unfortunately, NJR does not separate small and large diameter MoM THAs. However, among hip resurfacings, the revision rate is acceptably below the NICE criterion for males in all age groups, whereas females have a revision rate approximately double the NICE standard in all age groups. Of specific resurfacing brands, BHR and Durom (recalled) have revision rates within 10% at 10 years, whereas ASR, Cormet and Conserve plus clearly have a higher percentage.

(NJR. 2014). In the Australian registry Annual Report 2014, revision rates of hip resurfacings performed for primary OA as well as small head MoM THAs operated on for any reason are within the NICE criteria, whereas the revision rate of large diameter THAs is clearly higher. Also, AOANJRR data shows acceptable revision rates for males in all age groups, whereas revision rates of females in all age groups are unacceptably high. Males with head size >50 mm have an acceptable revision rate, whereas males with small components and females with any size components do not. BHR is the only one of the four resurfacing brands with 10-year follow up that fullfils the criterion. Among THAs, large head sizes (≥36 mm) have a higher revision rate than the NICE standard both in females and males, and in all age groups. The Pinnacle MoM acetabular system is the only large head MoM THA with a 10-year revision rate below the NICE benchmark. (AOANJRR 2014).

Based on registry data, the use of resurfacings in certain patients and small head diameter THAs in all patients can be rationalized, but the use of large diameter components is more problematic. It has been stated that “the era of one device fits all has long gone” (Amstutz and Le Duff 2012), and some centers have chosen to continue the use of some hip resurfacing and small diameter THA brands at least in certain patient groups, even though the risk for ARMD remains and the long-term systemic effects of accumulating metal debris is unknown (Migaud et al. 2012, Cadossi et al. 2015, Matharu et al.

2015c).