Our KM-survival probability of 93.5% at six years FU (95%-CI: 88.8-95.3) is not compliant with the three year entry NICE benchmark. Longer FU is needed to compare our results with the full 10-year benchmark. Of the non-revised patients, there were only four patients with implant failure based on their HHS score. The combined endpoints of revision (n = 16) and HHS score < 70 points (n = 4), resulted in 20 failed prosthesis (7.1%). Since no other studies on MoM hip resurfacing have combined implant survival and Patient Reported Outcome scores to define implant performance, we cannot compare this result to other studies. We were able to identify all failure modes, including those from patients revised in other hospitals. Most frequent reasons for revision were fracture of the femoral neck (n = 7) and aseptic loosening (n = 5). All cases of aseptic loosening occurred relatively early and involved only the uncemented acetabular component. We think that insufficient seating of the acetabular component, which might occur due to deformation of the relatively thin cup during the impaction procedure, may have caused these early revision cases. In our series we have not observed any signs of ARMD during revision surgery, although post revision surgery two patients revised for persistent pain had histopathological evidence of adverse local tissue reaction (ALTR) to metal debris. Neither have we observed any signs of ARMD with diagnostic ultrasound scans in patients who were post-operatively symptomatic. We cannot completely rule out the presence of ARMD in our series, but since we observed two cases of ALTR, future follow-up will include routine metal ion analysis. Our complete FU, our detailed information on revision cases and the excellent clinical scores at the time of final FU are in contrast to other designs of HRA, of which failure rates of 25% for ARMD after six years FU are reported . Risk factors for ARMD are the inclination angle of the acetabular cup, implant design, small component sizes and occurrence of neck narrowing. Steep inclination angles and an acetabular cup with less than hemispherical coverage result in a small contact patch area (CPA), which increases the wear rate. Another risk factor is component size, with small sizes resulting in more friction, releasing more metal debris [18–20]. In our series there were 81 patients with such risk factors, but no ARMD was observed in any of these patients, neither with a diagnostic ultrasound scan nor during revision surgery. The critical inner bearing surface of the ReCap has a coverage arc ranging from 155–164 degrees from smallest to largest component which is similar to other designs with a larger CPA such as the Birmingham Hip Resurfacing design (Smith and Nephew PLC, London, UK), the Conserve plus (Wright Medical Technology, Inc., Arlington, USA) and the Cormet resurfacing design (Corin Group PLC, Cirencester, UK). Our findings on ARMD are in line with several other studies. Malviya found a 0.15% incidence of pseudotumors using the Birmingham Hip Resurfacing (BHR) . Beaulé et al. found a 0.1% prevalence of pseudotumors with MoM resurfacing after surveying nine Canadian Academic centers . Glyn-Jones et al. extensively studied the risk factors for pseudotumor formation in a large series of hip resurfacings. Gender and age had a significant independent influence on the revision rate for pseudotumour formation, and the incidence increased with time, with a mean time to pseudotumour revision of 3.5 years (1 to 8.3 years) . In the series presented by Steffen et al., there were three revision cases possibly related to metal debris. Two of these cases were revised around two years post-operatively, the other one at 5.6 years after surgery . These mean times to pseudotumour revision are within the maximum follow-up time of our case series (6.3 years), but we will have to stay alert on ARMD occurrence with longer follow-up.
Grammatopoulos reported a mean 10.1% neck narrowing in patients revised for pseudotumours. In our cohort the mean percentage of neck narrowing was considerably lower (2.3%), although individual cases had greater neck narrowing. We did observe neck narrowing in three out of the nine patients who were revised > 6 months after index surgery, but these three patients had less than 10% neck narrowing. Neck narrowing data from our cohort is supplementary to the observations by Gross and Liu. They also report < 1% revisions for adverse wear and based on their report and on data from our cohort we believe that the risk for adverse wear using this resurfacing design is low. Gross did report a lower revision rate compared to our study (3.4% versus 7.3%) but in his series the learning curve was avoided since the surgeon had performed 400 hip resurfacings before the presented series was started . As noted in the study by Gross, we also now have begun recommending routine metal ion tests in all our patients.
Strong points of our study are its prospective study design, a large consecutive study cohort, limited lost to FU and comparison to an objective benchmark. There is detailed FU on all revised patients including those revised in other hospitals, and both clinical outcome scores and radiological FU were analysed. Another advantage is that this study was conducted in a general district hospital rather than a design institution. Our study also has limitations: FU time is limited and there is no control group. We also have to bear in mind that the NICE-benchmark is applied to an OA population of all ages, and literature describes higher revision rates in younger patients [26–28]. Metal-ion levels were not obtained and there were no diagnostic ultrasounds made to check for ARMD in non-symptomatic patients. Compared to published literature, our study reports the clinical results on more patients with longer FU using the ReCap Hip Resurfacing system than any other study. Gagala et al. studied 25 patients (mean FU 11 months, range: 10–20) and found good short-term clinical results without significant complications . Baad-Hansen et al. conducted a radiostereometric analysis (n = 23). There was no statistically significant translation or rotation of the femoral component observed after two years FU . The absence of any revisions in these series might be due to the small number of patients and the short FU. In the evaluation of risk factors for early failure with HRA, the Australian Arthroplasty Register reported on 137 procedures between 1999 and 2008 using the ReCap hip resurfacing system . Their cumulative percent revision rate of 7.6% at three year FU using this system was worse than our implant survival at three years. A possible explanation might be that those 137 procedures were done by a large number of orthopedic surgeons in an extended period of time, limiting the individual expertise using this system. However, despite further enquiry, no more details could be provided by the Australian Arthroplasty Register.
Regarding patient selection, in our series the RR for revision was slightly higher for female and for older patients, although statistically the difference was not significant. Patients with smaller component sizes had a higher risk for revision, but this was also not statistically significant. This is in line with several other publications which show a significantly higher risk for revision in female patients, older patients, and in patients with small components [5, 29–32]. The possible absence of ARMD in our series might explain the equal risk for revision in patients with small or large component sizes. Looking at diagnosis, literature reports that the best HRA results are obtained with OA [5, 33]. In our series, only patients with this diagnosis were included.