Salvaging failed metal-on-metal hip resurfacing with a polyethylene acetabular revision

Introduction

Background

Little is known about the results of revising a failed hip resurfacing arthroplasty (HRA) with another HRA. Revision to total hip replacement (THR) has been the common revision procedure. Revision resurfacing procedures pose several unique challenges that are not encountered in revision THR surgery. Revising hip resurfacing to THR often results in reducing the femoral head dimension, which can lead to instability. In addition, many patients remain committed to the resurfacing concept. They struggle with the recommendation of revision to THR and ask if their HRA can be revised to another HRA. If the femoral component of a resurfaced hip is stable and secure, then acetabular revision is an option just as it would be for THR. For patients presenting with acetabular loosening, instability, or a reaction from the metal-on-metal bearing surface, revision of only the acetabular component is an option. It is also possible to revise the acetabular component to another metal component, but the decreasing trust of metal-on-metal implants makes this an unlikely choice (1,2).

Increasingly, the field of resurfacing is moving back to polyethylene acetabular components (3-5). Acetabular revision presupposes that a reasonable match of the acetabular polyethylene to the retained femur is possible. However, until recently, there have been very few polyethylene resurfacing acetabular components available. Often, the closest match is 1 mm larger than the femur. Using an acetabular component that is smaller than the femoral component will not work due to the resulting instability and edge loading. The results of using components with an imperfect match are unknown. The size of any hip component is typically 0.1–0.2 mm different compared to its nominal label. For both hip replacement and resurfacing, the inner dimension of the acetabular component is usually 0.3–0.8 mm larger than the outer dimension of the femur. This is called the diametral clearance. The optimal clearance is not known. THR with diametral clearances of up to 1.8 mm has been successful (6). Larger clearance reduces the surface contact and allows joint fluid to enter the space between the femoral and acetabular components (6). Smaller diametral clearance increases the stability of the hip, but too small a clearance results in difficulty in movement and increased wear (6). In all circumstances, the femoral dimension is smaller than the acetabulum.

Rationale and knowledge gap

Finding a suitable acetabular prosthesis in revision HRA is a substantial challenge. Other challenges are: (I) accessing the acetabulum with the femur in the way is difficult; (II) only one acetabular component will fit the retained femur and the space to place an acetabular component in hip resurfacing is at a premium; and (III) achieving stable fixation of the acetabular component is difficult. Acetabular bone loss can be a crucial problem in the revision resurfacing setting. The stiffness of original metal resurfacing acetabular components causes stress shielding, and intraoperative component extraction may contribute to further bone loss. Also, adverse reactions to metal wear debris have been associated with bone and soft-tissue damage and a high complication rate during revision surgery (2,7,8).

Objectives

There are direct matches of polyethylene components available from manufacturers for approximately one-half of all metal resurfacing components. The match to the others will be imperfect by 1 mm. The hypothesis of this study is that acetabular only revision from metal to polyethylene is a reasonable option. A further hypothesis is that a polyethylene component that is not a perfect match to the femoral component can be successful. The purpose of this report is to evaluate the results of acetabular revision hip resurfacing surgery using components that either match the femoral component or are mismatched within 1 mm. This article is presented in accordance with the STROBE reporting checklist (available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-82/rc).

Methods

This cohort study analyzed information collected prospectively, as all patients undergoing HRA are enrolled in an HRA database. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Prior to data collection, the Institutional Review Board of the Swedish Medical Center, Seattle, WA, USA, approved the study (No. 5190S-11). The IRB approved data collection from all hip resurfacing procedures performed by the author for both primary and revision indications. Patients provided their written and verbal consent for study participation and publication of the results. The author performed 326 revision hip resurfacing procedures between 1999 and 2020. The inclusion criteria were all patients choosing a polyethylene acetabular component for revision of their failed metal hip resurfacing. Exclusion criteria were patients who chose revision to THR or metal acetabular revision.

Patients were revised to a THR if (I) they requested a THR or (II) the femoral component was not well positioned or there was femoral loosening/bone loss. Revision of the metal acetabular component to another metal acetabular component was not offered after 2017 and is not included in this study. Femoral only revision to a dual mobility component is offered if the metal acetabular component was well fixed, well positioned and not recalled. We offer femoral revision to dual mobility, both for femoral component failure and adverse reaction to metal wear debris cases.

Patients had three options for a metal reaction: (I) revision to THR, (II) femoral dual mobility, or (III) acetabular revision to polyethylene. We no longer offer the 4^th option of acetabular revision with metal.

All revision surgeries used acetabular implants that were chosen based on patient and surgeon preference and matched the existing metal femoral implants in place. The original metal femur was preserved in all cases. Metal-on-metal resurfacing manufacturers do not offer revision implants in either metal or polyethylene (9). Patients were placed into one of two groups based on whether their revision acetabular component matched their retained femoral component completely or was mismatched by 1 mm.

Implants used

All polyethylene acetabular implants were Food and Drug Administration (FDA) cleared and came from one manufacturer (BP Acetabular Cup, Endotec Inc., Fullerton, CA, USA).

The implants are used for THR, tripolar hip replacement, or HRA. All polyethylene acetabular components were ethylene oxide sterilized. The two-piece acetabular components consisted of a plasma-sprayed titanium shell with a cross-linked polyethylene liner. The polyethylene thickness was 4 mm and the acetabular shell thickness was 2 mm. The liners were made of GUR 1020 polyethylene, cross-linked by irradiation at 7.5 mRad. They did not have Vitamin E to protect against oxidation (Figure 1).

Figure 1 Components of the polyethylene acetabular implant. (A) Acetabular shell; (B) acetabular liner.

Outcome measures

The most validated joint specific patient-reported outcome measures are the Hip Disability Osteoarthritis and Outcome Score (HOOS) and the Harris Hip Score (HHS) (10,11). The University of California Los Angeles Activity Score was used to assess activity (12). The HOOS score has five subscales (activities of daily living, pain, symptoms, sports, and quality of life) totaling 100 points. The HHS also has 100 points divided equally into four domains (pain, function, absence of deformity, and range of motion). The more important outcomes, though, are the complications, re-revision need, and implant survivorship. Bias was controlled using patient-reported rather than surgeon-reported outcomes.

Also, a physiotherapist (Kate Moore), who was blinded to the procedure and the implants and not involved in the treatment, collected the examination data and outcomes of care of the patients. Patients were followed at 6 weeks, 6 months, 1 year, and then every 2 years after their resurfacing procedure.

Radiographic evaluations

Two blinded independent observers analyzed all the radiographs (John McDermott and Briana Arnold). The intra-observer and interobserver reliability were 0.96 and 0.94, respectively. Cup inclination and anteversion angles were measured. The radiographs were evaluated for radiolucent lines, osteolysis, and radiographic signs of osseointegration. The radiographs were also analyzed for signs of component wear, and signs of stress shielding, and impingement.

Retrieval analyses

Thirteen highly cross-linked polyethylene and retrieval specimens were collected (seven during revision, six postmortem) and were assessed using optical scanning and a digital coordinate measuring machine to determine wear, damage, and delamination.

Statistical analysis

Continuous variables are presented as means with standard deviations or 95% confidence intervals, and categorical variables are presented as frequencies and percentages. A paired t-test was used to evaluate improvement in outcome scores, and independent t-tests were performed to compare outcomes between the two study groups. Significance was set at P<0.05.

Results

There were 162 patients in the matched group and 164 patients in the mismatched group. The mean follow-up was 13 years (range, 5–26) for 153 patients in the matched group and 157 patients in the mismatched group. Three patients in each group were lost to follow-up, six patients in the matched group and four in the mismatched group died during the follow-up period. There were seven mismatched and six matched retrievals postmortem or during revision available for wear studies. In the matched group, their mean age was 53 years (range, 22–75), there were 84 men, 77 women, 1 nonbinary, and their mean body mass index (BMI) was 26 kg/m². In the mismatched group, their mean age was 54 years (range, 32–74), 85 were men, 78 were women, and 1 was nonbinary, with a mean BMI of 27 kg/m².

Indications for revision

For the matched group, the indications for revision were metal reaction 158, loosening 2, and instability 2; for the mismatched group, indications were metal reaction 160, loosening 2, and instability 2. The implant sizes used in the revisions are shown in Table 1.

Patient-reported outcomes

The outcomes scores for patients in the matched (n=162) and mismatched (n=164) groups were very similar. Mean HOOS scores did not differ significantly between the matched and mismatched groups [91 (range, 84–97) vs. 91 (range, 83–98); P=0.81]. The mean HHS scores were 92 (range, 86–96) in the matched group and 92 (range, 83–100) in the mismatched group, with no statistically significant difference (P=0.75). The mean UCLA Activity Scores for both groups were 7 (range, 4–10).

Complications

Re-revision was required for nine patients (5.9%) in the matched group and eight patients in the mismatched group (5.1%). In the matched group, re-revisions were performed for polyethylene wear (n=2), loosening (n=2), infection (n=2), and pain/seroma (n=2), and 1 for femoral avascular necrosis. In the mismatched group, re-revisions were performed for polyethylene wear (n=2), loosening (n=2), infection (n=2), and pain/seroma (n=2). The polyethylene wear revisions were at nine and 11 years in the matched group and at 10 and 12 years in the mismatched group. There was one revision at 14 years to a stemmed femoral component for progression of the osteonecrosis with subsidence of the femoral resurfacing component in the matched group; the acetabular shell and liner were maintained. There were no femoral neck fractures. There were four wound infections that were resolved with antibiotics and incision care. Femoral neck impingement presenting as a small notch was noted in 10 cases in the matched group and 11 in the mismatched group. There was femoral neck narrowing without significance in 10 cases in the matched group and 11 in the mismatched group. The average acetabular component inclination was 41° and the average anteversion was 15° in both groups (Figure 2).

Figure 2 Anteroposterior pelvic radiograph. (A) Metal-on-metal resurfacing that failed by acetabular loosening. (B) Revision to a polyethylene acetabular component.

Retrieval analysis

The wear analysis of the retrieved components was performed using precise digital calipers. The retrieval analysis showed the mean wear rate of the polyethylene components was 0.02 mm/year (range, 0.017–0.04 mm/year) for the 13 retrievals. There was no internal or rim cracking, scratching, burnishing, or delamination. The original machine markings were visible. There was no difference in the wear rate by size of the polyethylene liner. There also was no difference in the wear depending on whether the polyethylene was matched or mismatched. The wear occurred in the anterior/superior location in all retrievals.

Discussion

The hypothesis of this work is that acetabular only revision from metal to polyethylene is a reasonable option. A further hypothesis is that a polyethylene component that is not a perfect match to the femoral component can be successful. The purpose of this report is to evaluate the results of acetabular revision hip resurfacing surgery using components that either match the femoral component or are mismatched within 1 mm.

Key findings

Revision of failed metal-on-metal hip resurfacing to polyethylene was successful. It was equally successful whether the new acetabular component was a match to the retained femur or 1 mm larger in diametral clearance. The retrieval analyses did not find increased wear based on either the size of the implants or the accuracy of the match to the femoral component. The re-revision rate of less than 6% in this study is better than the results reported with revision to THR, for which failure rates of 7% to 38% have been reported (7,8).

The implants used in this study have undergone wear simulator testing, which showed excellent resistance to wear over 30 million cycles (13). This also included testing with 1–2 mm increased diametral clearance. No adverse wear was seen. Increased diametral clearance has also been used on occasion in the primary setting for both resurfacing and THR (BP Acetabular Cup, BioCore 9, Whippany, NJ, USA; Figure 3).

Figure 3 Anteroposterior pelvic radiograph of primary hip resurfacing with a 1 mm-increased diametral clearance.

Revision using mismatched components has been reported recently as a successful option in total shoulder replacement, particularly in reverse total shoulder replacement (14). Mismatched components with differences as much as 4 mm in the shoulder were shown to have successful outcomes at 5-year follow-up (15).

Strengths and limitations

There are limitations to this work: (I) one highly experienced surgeon performed all the procedures; (II) the operation is technically demanding, and the results may not be reproducible in other centers; and (III) a single implant design was used. This implant requires exacting manufacturing and insertion techniques (1). Hip resurfacing using other implants has not been as successful (10). Inevitably, there is bias in this study. The decision to continue with HRA even when the index procedure has failed indicates patient bias. Further, accepting mismatch reflects the author’s training and experience but not that of other surgeons (16). The patients in this study all had femoral head sizes larger than 40 mm. It is possible and even likely that smaller sizes would have a different outcome.

Failed hip resurfacing cases present a challenge. The patients are often in high demand both because of their hip function and because of psychological makeup. Many patients still want and need the high function they associate with hip resurfacing (17,18). Also, they have an ongoing interest in preserving their femoral bone. The most common revision option is conversion to THR. Surgeons may be hesitant to use implants from different manufacturers even when they are size-matched due to the off-label nature of such configurations. There are even more concerns when the implant dimensions do not match. There are also some procedural reasons to consider staying with HRA rather than conversion to THR: (I) decreased average operative time, (II) decreased blood loss, and (III) decreased implant costs (19). In addition, if the resurfacing can be revised to another resurfacing, then the original femoral bone and dimension are preserved.

Comparison with similar research

Whether hip resurfacing revision makes sense will depend on the complications and success of the procedure (17,18). Combining an acetabular implant from one manufacturer with a femoral component from a different manufacturer is an accepted but off-label practice in primary and revision THR. This occurs in 5% to 30% of revision procedures currently, but once a common practice in both primary and revision surgery. Typically, the dimensions match within a fraction of a millimeter and this poses no problems. The reasons for using implants from different manufacturers are: (I) the combination selected represents the best clinical choice, (II) the original implant manufacturer no longer supports their product with matching components or, in some instances, the manufacturer no longer exists, and (III) surgeon or patient preference, such as in this study (20,21).

There are reports of poor outcomes with mismatched THRs. In these reports, the acetabular component was larger than the total hip femoral component by 3–4 mm, and instability and adverse wear occurred. This is readily apparent both intraoperatively and on postoperative radiographs. These fail in a short time. Also, the mismatched literature reports are for THR in the 28–36 mm size range rather than 40 mm or more resurfacing sizes used in this study (16).

Explanation of findings

Biomechanically, there are several explanations to support why mismatched implants would theoretically yield equivalent properties to standard implant combinations. In polyethylene bearing couples, the clearance or size difference between the bearing surfaces is less crucial to fluid-film lubrication than it is with metal-on-metal implants. The clearances that have been used in polyethylene were 0.3–1.8 mm (6). Therefore, with polyethylene, the acetabular component can be larger than the femoral component without degrading the result. Factors such as soft-tissue condition, bone health, component positioning, and component fixation are more important.

Conclusions

Failed metal-on-metal resurfacing implants can be salvaged successfully by revision of the metal acetabular component to polyethylene. This is true when there is a direct size match between the acetabular polyethylene and when the acetabular polyethylene is 1 mm larger than the femur. The results obtained in this study are promising and provide another option for patients facing difficult revision surgery. The results of this study should be viewed in context with the limitations of this study. Further research in several centers with larger numbers of patients is necessary.

Acknowledgments

The author thanks Kate Moore for collecting preoperative and postoperative patient data, Roman Matusek for performing radiographic imaging and Briana Arnold and John McDermott, MD, for interpreting radiographic imaging.

Footnote

Reporting Checklist: The author has completed the STROBE reporting checklist. Available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-82/rc

Data Sharing Statement: Available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-82/dss

Peer Review File: Available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-82/prf

Funding: None.

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at https://aoj.amegroups.com/article/view/10.21037/aoj-2025-1-82/coif). The author has no conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Institutional Review Board of Swedish Medical Center (No. 5190S-11) and patients provided their written and verbal consent for study participation and publication of the results.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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