When the MoMHRs were divided by the sex of the patient (Table III), a multivariate Cox model analysis of the female group demonstrated that small femoral head size (p = 0.014) and implantation of a Cormet design (p = 0.009) significantly increased revision risk. However, only 18 Cormet designs were implanted in this series. The multivariate model for the male group identified Conserve (p = 0.002) and ReCap (p = 0.015) MoMHRs as the only factors significantly increasing revision risk.
Prevalence and Risk Factors
Of the 1,429 MoMHRs, 111 (7.8%) in 95 patients were revised because of pseudotumor at a mean of 6.1 years (range, 0.5 to 15.5 years). Pseudotumor accounted for 61.7% of all revisions (Table I). Risk factors for pseudotumor-related revisions are summarized in Table II and were sex-specific.
The pseudotumor-related revision rate for all MoMHRs was 14.0% (95% CI = 11.0% to 17.7%) at 15 years. Univariate analyses demonstrated that factors significantly increasing the risk of pseudotumor-related revision were female sex (p < 0.001; Fig. 2), small femoral head size (p < 0.001), young age at the time of the MoMHR (p = 0.011), and Conserve (p = 0.009) and ReCap (p = 0.007) implants. All factors remained significant in the multivariate model (Table III).
When the MoMHRs were divided by the patient’s sex (Table III), a multivariate Cox model showed young age to be the only covariate significantly increasing pseudotumor-related revision risk in the female group (p = 0.019). Small femoral head size was not significantly associated with pseudotumor-related revision in females (p = 0.189). The multivariate model identified Conserve (p = 0.001) and ReCap (p = 0.001) implants as the only factors significantly increasing pseudotumor-related revision risk in the male group.
We believe this to be the first independent study providing information on revision risk into the second decade following MoMHR. The prevalences of revision for all causes (12.6%) and for pseudotumor (7.8%) were high, with 15-year revision rates of 19.5% and 14.0%, respectively. The predictors of all-cause revisions (small femoral head size and non-BHR implants) differed from the predictors of pseudotumor-related revisions (female sex, small femoral head size, young age, and Conserve and ReCap implants). Furthermore, the predictors were sex-specific; implant design was the only predictor of all-cause and pseudotumor-related revisions in males whereas, in females, small femoral head size was the only predictor of all-cause revisions and young age was the only factor predicting pseudotumor-related revision.
To our knowledge, 15-year outcomes have been reported in only one study, by a surgeon involved in the design of a commonly used hip resurfacing implant27. That study showed an overall revision rate of 4.2% following 1,000 BHRs. Our inferior rates are likely related to numerous factors, including patient selection, surgical experience, and implant design, with only 45% of our cohort having a BHR. Our 10-year all-cause revision rate of 15.3% (95% CI = 13.2% to 17.8%) is also higher than the 9.7% to 12.6% recorded in registries1,2, although registries may underreport revisions21.
Although pseudotumor was the most common indication for revision (62% of all revisions), other large cohort studies with shorter follow-up have identified femoral neck fracture, component loosening, and osteonecrosis as primary modes of MoMHR failure10,11,17. The prevalence and rates of revision due to pseudotumor have increased substantially compared with the 1.8% prevalence and 4% rate of pseudotumor-related revision at 8 years noted in an early report5. Regular surveillance of MoMHRs from 2010 to 2012 and onward is an important contributory factor7-9,28; therefore, pseudotumor may become a more frequent revision indication compared with other modes of failure. However, registries may not accurately reflect pseudotumor-related revision rates for some time, given that pseudotumor did not become a revision indication until 20091 coupled with potential underreporting of MoMHR revisions21.
Predictors of all-cause revision risk were small femoral head size (8% increased risk per 2-mm decrease in head size) and non-BHR implants (1.6 to 3.0 times increased risk). Our findings support those from two large studies that also demonstrated small head size to be a more important predictor of all-cause revision than female sex11,18. Small head size makes patients more prone to femoral component loosening and femoral neck fracture18. Furthermore, small components are sensitive to malposition, which can lead to edge loading, increased wear, and pseudotumor14,15. Our findings regarding implant design confirm those from joint registries1,2,10, designing surgeons27,29, and independent centers15,30,31, and support using the BHR in appropriately selected patients. Although others have reported hip dysplasia and osteonecrosis as risk factors for MoMHR failure10,11,27, this was not observed in our study.
Predictors of all-cause revision were sex-specific. Small femoral head size was the most important predictor in females, and implant design was the only predictor in males. “Small” femoral head size differs between males (≤48 mm) and females (≤42 mm). Diametrically smaller components have lower tolerances for positioning error and are at increased risk of revision11,14,15,18. Therefore our data suggest that small head size is a significant predictor of revision in females regardless of implant design. However, there is a slightly greater margin for error when positioning small components in males, so males may have a good outcome with a small component if an established MoMHR design is used. Despite the BHR being the best-performing MoMHR device worldwide1,2,10, our data support recent recommendations to not implant small femoral heads in females19.
Pseudotumors have predominantly been reported in females3,4,10,17. Our study showed that pseudotumor-related revision was predicted by female sex (2 times increased revision risk) and young age (2% increase in revision risk per year younger). Our data suggest that, in contrast to all-cause revision, pseudotumor-related revision is more strongly predicted by female sex than by small femoral head size. Females have an increased risk of pseudotumor for various reasons. First, they have larger native head-neck ratios, which can be considerably reduced following MoMHR, thereby increasing the risk of edge loading and high wear32. Using smaller implants in females, who more frequently have abnormal acetabular anatomy10,11,27, increases the risk of malposition, which can also cause high wear14,15. Females also have an increased risk of metal allergy16, which may contribute to pseudotumors. Increased hip movement and flexibility as well as specific gait patterns in females33,34 may cause impingement and/or edge loading with subsequent pseudotumor formation. Such kinematics may be increasingly important in young active individuals, therefore explaining why young age is associated with pseudotumors in females.
In addition to MoMHRs, stemmed MoM total hip replacements (THRs) and, more recently, some non-MoM designs have had high short-term failure rates1,2,35,36. Pseudotumors develop after stemmed THRs as a result of wear and/or corrosion at modular junctions—namely, the femoral head-taper junction35,36. This problem may have become more apparent because of modifications to stem design and the use of larger femoral heads. Although patients with these devices also need regular surveillance28,37, it is important to note that, because of clear design differences, the risk factors identified in our MoMHR cohort cannot be extrapolated to stemmed MoM and non-MoM implants. It is therefore recommended that studies similar to ours be performed for patients with stemmed devices so that these patients can be risk-stratified for surveillance.
It is important for the orthopaedic community to learn lessons from the problems experienced with MoM bearings and modular THRs. After preclinical testing, the introduction of any new technology must undergo a rigorous and transparent process. This should include usage by small groups of experts, ideally as part of prospective randomized trials and independently controlled surveillance programs. Surgeons using new technologies during the early stages have a responsibility to report problems in a timely fashion to the manufacturers and to independent authorities7,9. The results of this initial experience must be interpreted in combination with registry data1,2 before widespread introduction of new technologies. Adopting this approach will allow innovation to continue while ensuring patient safety.
The strengths of our study include the large sample size and longer follow-up compared with previous reports5,10-12,17,18. Systematic methods, including contacting other centers, were used to identify all revisions. All revision indications were retrospectively confirmed using operative and histopathological findings, allowing us to identify revisions that were due to pseudotumor before this diagnosis was established3,4. In contrast, registries1,2,11-13, which are substantially limited by a lack of histopathological data, did not recognize pseudotumors until 2009 and underreport revisions21. Another strength of our study is that we employed robust survival analysis methodology, including sex-specific analysis. Also, the generalizability of our findings was improved by the fact that procedures were performed at a non-designing center with numerous surgeons using a range of common implants.
A limitation of our study and subsequent cohorts is the potential for surveillance bias. Because surgeons were unaware of pseudotumors before 20073, this complication was underreported. However, our retrospective review identified pseudotumors in patients who underwent revision for other indications prior to 2007. Pseudotumors were more likely to be recognized after 2007, with patient recalls in 201022 and 20127, which included asymptomatic individuals38,39. The increased awareness of pseudotumors and regular patient surveillance are likely to further inflate the reported pseudotumor prevalence and revision rates. Our data support an increasing pseudotumor revision rate over time, with pseudotumor-related revision risk being 2.44 times higher for MoMHRs implanted after 2007 compared with those implanted before 2007 (hazard ratio [HR] = 2.44, 95% CI = 1.51 to 3.95; p < 0.001) (Fig. 3). Furthermore, revision indications have changed over time. Surgery was initially performed only in symptomatic patients with large lesions3, but in light of poor short-term outcomes following revision40 our indications now include mildly symptomatic patients with smaller pseudotumors.
It remains unclear why pseudotumor prevalence and revision rates are continuing to increase into the second decade following MoMHR, rather than plateauing as one would expect given that the poorly performing designs and the prostheses implanted with technical errors were revised early after the procedures. Possible explanations for the continued increase in pseudotumor-related revision rates include closer patient surveillance, different revision indications, extended follow-up, or a combination of these factors, but clearly additional work is needed to explain why pseudotumor-related revisions are continuing to be identified with long-term follow-up.
Traditional metal-on-polyethylene THRs may fail as a result of aseptic loosening both early (because of technical errors) and late (due to wear), and it is possible that pseudotumors develop both early and late following MoMHRs for similar reasons. Revision of MoMHRs complicated by pseudotumor have proved technically challenging because the lesions can be invasive, involve neurovascular structures, and cause substantial bone and soft-tissue destruction3,40. This has led to poor short-term outcomes following pseudotumor-related revisions, with high complication (50%) and rerevision (38%) rates as well as inferior functional outcomes compared with those following primary THRs40,41. Another concern is that the outcomes of revisions of metal-on-polyethylene THRs appear to be more favorable than those following pseudotumor-related revisions. The 10-year implant survival rate following revision of THRs due to late aseptic loosening was reported to be 84% in a large cohort, with patients with surviving implants having good functional outcomes42. Similar outcomes have been demonstrated in other studies following revision THR43,44. Given that many MoMHR implants may still require revision because of pseudotumor and the substantial difference between the outcomes following revisions of MoMHRs and those following revision of THRs, regular surveillance of patients with MoMHRs is important coupled with a low threshold for considering revision surgery. Indeed, there is already evidence that such a strategy may improve patient outcomes following MoMHR revision45.
Our study had other recognized limitations. The findings may not apply to other MoMHR designs. Also, revision was used to define failure as it represents a problem serious enough to warrant surgery. However, some patients who did not undergo revision may have had radiographic evidence of failure or asymptomatic pseudotumors39. These additional cases might eventually increase our reported prevalence and rates of revision, but the natural history of asymptomatic pseudotumors remains uncertain46. It is not clear whether all such lesions eventually require revision46, as the frequency of asymptomatic pseudotumors is similar between patients with MoM implants and those with non-MoM implants47. An additional limitation is that, although our survival analysis was robust, there is a potential for residual confounding. Finally, our findings could have been influenced by some patients not completing the postal questionnaire and the possibility that some underwent revision abroad.
In conclusion, this study of a large cohort of MoMHRs showed a high prevalence and rate of all-cause and pseudotumor-related revisions at up to 15 years following primary arthroplasty. Predictors of revision differed between all-cause and pseudotumor-related revisions, and were sex-specific. These factors must be appropriately weighted and incorporated into current worldwide follow-up recommendations7-9,28 for risk-stratifying patients with MoMHRs for surveillance.
A table showing patient and implant factors for surviving MoMHRs in patients who completed the follow-up questionnaire versus those who did not complete the questionnaire is available with the online version of this article as a data supplement at jbjs.org.
NOTE: The authors thank the team involved in the patient recall, including the radiology department and the surgeons performing the primary and revision operations.
Investigation performed at the Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Sciences, Nuffield Orthopaedic Centre, University of Oxford, Oxford, United Kingdom
Disclosure: Arthritis Research U.K. provided one of the authors with funding in the form of a Clinical Research Fellowship. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work.
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