In Revision THA, Is the Re-revision Risk for Dislocation and Aseptic Causes Greater in Dual-mobility Constructs or Large Femoral Head Bearings? A Study from the Australian Orthopaedic Association National Joint Replacement Registry : Clinical Orthopaedics and Related Research®

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CLINICAL RESEARCH

In Revision THA, Is the Re-revision Risk for Dislocation and Aseptic Causes Greater in Dual-mobility Constructs or Large Femoral Head Bearings? A Study from the Australian Orthopaedic Association National Joint Replacement Registry

Hoskins, Wayne MBBS (Hons), FRACS, PhD1,2; Rainbird, Sophia PhD3; Dyer, Chelsea BSc (Maths&CompSc)4; Graves, Stephen E. MBBS, DPhil (Oxon), FRACS (Orth), FAOrthA3,5; Bingham, Roger MBBS, FRACS2

Author Information

1Faculty of Medicine, Dentistry and Health Sciences, the University of Melbourne, Parkville, Australia

2Traumaplasty Melbourne, East Melbourne, Australia

3Australian Orthopaedic Association National Joint Replacement Registry, Adelaide, Australia

4South Australian Health and Medical Research Institute, Adelaide, Australia

5Clinical and Health Sciences, University of South Australia, Adelaide, Australia

W. Hoskins, Faculty of Medicine, Dentistry and Health Sciences, Building 181, University of Melbourne, Grattan St., Melbourne, Victoria 3010, Australia, Email: [email protected]

Each author certifies that there are no funding or commercial associations (consultancies, stock ownership, equity interest, patent/licensing arrangements, etc.) that might pose a conflict of interest in connection with the submitted article related to the author or any immediate family members.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research® editors and board members are on file with the publication and can be viewed on request.

Ethical approval for this study was not sought because the AOANJRR is approved by the Commonwealth of Australia as a federal quality assurance activity under section 124X of the Health Insurance Act, 1973. All AOANJRR studies are conducted in accordance with ethical principles of research (the Helsinki Declaration II).

This work was performed at the Australian Orthopaedic Association National Joint Replacement Registry, which is based at the South Australian Health and Medical Research Institute, Adelaide, South Australia.

Clinical Orthopaedics and Related Research 480(6):p 1091-1101, June 2022. | DOI: 10.1097/CORR.0000000000002085

Abstract

Background 

Dislocation is one of the most common causes of a re-revision after a revision THA. Dual-mobility constructs and large femoral head bearings (≥ 36 mm) are known options for mitigating this risk. However, it is unknown which of these choices is better for reducing the risk of dislocation and all-cause re-revision surgery. It is also unknown whether there is a difference between dual-mobility constructs and large femoral head bearings according to the size of the acetabular component.

Questions/purposes 

We used data from a large national registry to ask: In patients undergoing revision THA for aseptic causes after a primary THA performed for osteoarthritis, (1) Does the proportion of re-revision surgery for prosthesis dislocation differ between revision THAs performed with dual-mobility constructs and those performed with large femoral head bearings? (2) Does the proportion of re-revision surgery for all aseptic causes differ between revision THAs performed with dual-mobility constructs and those performed with large femoral head bearings? (3) Is there a difference when the results are stratified by acetabular component size?

Methods 

Data from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) were analyzed for 1295 first-revision THAs for aseptic causes after a primary THA performed for osteoarthritis. The study period was from January 2008—when the first dual-mobility prosthesis was recorded—to December 2019. There were 502 dual-mobility constructs and 793 large femoral head bearings. There was a larger percentage of women in the dual-mobility construct group (67% [334 of 502]) compared with the large femoral head bearing group (51% [402 of 793]), but this was adjusted for in the statistical analysis. Patient ages were similar for the dual-mobility construct group (67 ± 11 years) and the large femoral head group (65 ± 12 years). American Society of Anesthesiologists (ASA) class and BMI distributions were similar. The mean follow-up was shorter for dual-mobility constructs at 2 ± 1.8 years compared with 4 ± 2.9 years for large femoral head bearings. The cumulative percent revision (CPR) was determined for a diagnosis of prosthesis dislocation as well as for all aseptic causes (excluding infection). Procedures using metal-on-metal bearings were excluded. The time to the re-revision was described using Kaplan-Meier estimates of survivorship, with right censoring for death or database closure at the time of analysis. The unadjusted CPR was estimated each year of the first 5 years for dual-mobility constructs and for each of the first 9 years for large femoral head bearings, with 95% confidence intervals using unadjusted pointwise Greenwood estimates. The apparent shorter follow-up of the dual-mobility construct group relates to the more recent increase in dual-mobility numbers recorded in the registry. The results were adjusted for age, gender, and femoral fixation. Results were subanalyzed for acetabular component sizes < 58 mm and ≥ 58 mm, set a priori on the basis of biomechanical and other registry data.

Results 

There was no difference in the proportion of re-revision for prosthesis dislocation between dual-mobility constructs and large femoral head bearings (hazard ratio 1.22 [95% CI 0.70 to 2.12]; p = 0.49). At 5 years, the CPR of the re-revision for prosthesis dislocation was 4.0% for dual mobility constructs (95% CI 2.3% to 6.8%) and 4.1% for large femoral head bearings (95% CI 2.7% to 6.1%). There was no difference in the proportion of all aseptic-cause second revisions between dual-mobility constructs and large femoral head bearings (HR 1.02 [95% CI 0.76 to 1.37]; p = 0.89). At 5 years, the CPR of dual-mobility constructs was 17.6% for all aseptic-cause second revision (95% CI 12.6% to 24.3%) and 17.8% for large femoral head bearings (95% CI 14.9% to 21.2%). When stratified by acetabular component sizes less than 58 mm and at least 58 mm, there was no difference in the re-revision CPR for dislocation or for all aseptic causes between dual-mobility constructs and large femoral head bearings.

Conclusion 

Either dual-mobility constructs or large femoral head bearings can be used in revision THA, regardless of acetabular component size, as they did not differ in terms of re-revision rates for dislocation and all aseptic causes in this registry study. Longer term follow-up is required to assess whether complications develop with either implant or whether a difference in revision rates becomes apparent. Ongoing follow-up and comparison in a registry format would seem the best way to compare long-term complications and revision rates. Future studies should also compare surgeon factors and whether they influence decision-making between prosthesis options and second revision rates. Nested randomized controlled trials in national registries would seem a viable option for future research.

Level of Evidence 

Level III, therapeutic study.

Introduction

Prosthesis dislocation is one the most common causes of re-revision after revision THA [2, 3, 24]. Dual-mobility constructs and large femoral head bearings (≥ 36 mm) are two known options for reducing this risk [14]. Constrained acetabular liners are a valid option, but they are usually reserved for more complex reconstructions or patients deemed to be at high risk for dislocation [18]. The American Joint Replacement Registry has reported dual-mobility use in 30.6% of revision THAs in 2018, an increase from 19.5% in 2012 [13]. Although this rise in use may be warranted, the increase has occurred despite a lack of clinical research with large patient numbers and long-term follow-up of the outcomes of dual-mobility constructs [2, 32, 37, 39]. The prosthesis option that reduces the risk of prosthesis dislocation and proportion of all-cause re-revision surgery is not known. Most comparative studies in THA have compared the outcomes of dual-mobility constructs with those of THA performed with a variety of femoral head sizes [16, 22, 29, 33, 35, 45, 54]. Systematic reviews and meta-analyses have found a benefit to dual-mobility constructs over fixed-bearing implants, inclusive of all femoral head sizes [23, 37, 39, 40, 53], but this may not be the case when solely comparing large femoral head bearings [37].

Although there are comparative studies of dual-mobility constructs and large femoral head bearings [6, 9-11, 18, 21, 26, 43, 48], few are multisurgeon and multi-institution, and only a few studies have evaluated these constructs in the setting of revision THA [6, 10, 18, 26, 48]. The long-term survivorship and associated complications of dual-mobility constructs are uncertain when compared with those of fixed bearings, which have benefited from larger numbers and longer term use [49]. Additionally, if there is a difference between dual-mobility constructs and large femoral head bearings, it is not known whether this difference is more apparent with larger acetabular component sizes because the head-to-acetabular component ratio decreases with fixed-bearing implants. Biomechanically, the benefits of dual-mobility constructs in reducing dislocation and improving the arc of motion before impingement may only be evident with acetabular component sizes larger than 56 mm [8, 27].

We therefore used data from a large national registry to ask: In patients receiving revision THA for aseptic causes after a primary THA performed for osteoarthritis, (1) Does the proportion of re-revision surgery for prosthesis dislocation differ between revision THAs performed with dual-mobility constructs and those performed with large femoral head bearings? (2) Does the proportion of re-revision surgery for all aseptic causes differ between revision THAs performed with dual-mobility constructs and those performed with large femoral head bearings? (3) Is there a difference when the results are stratified by acetabular component size?

Patients and Methods

Study Design and Setting

This study used data from the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR). The AOANJRR began collecting data on September 1, 1999 and includes information on almost 100% of arthroplasties performed in Australia since 2002. Registry data are validated against patient-level data provided by each of the State and Territory Health Departments in Australia. A sequential multilevel matching process is run monthly to search for all primary and revision arthroplasties as well as any missing data recorded in the AOANJRR that involved the same side and joint for the same patient, thus enabling each revision to be linked to the primary procedure. Data were also matched by the Australian Institute of Health and Welfare’s National Death Index to obtain information on the patient’s date of death. After cross-checking data, the registry can obtain information on more than 98% of joint procedures performed in Australia [3]. The AOANJRR also records the reasons for revision and the type of revision THA.

Participants

The study population was all patients with data recorded in the AOANJRR with a first revision THA performed for aseptic causes after a primary THA for a diagnosis of osteoarthritis. The study period was January 1, 2008 to December 31, 2019, because this corresponded with the first use of dual-mobility prostheses in Australia. We identified all dual-mobility constructs and all large femoral head bearings (≥ 36 mm). Primary prostheses using metal-on-metal bearing surfaces were excluded because of their known higher proportion of revision.

Descriptive Data

The study group consisted of 1295 patients who underwent a first revision THA (Supplementary Fig. 1; https://links.lww.com/CORR/A701). There were 502 dual-mobility constructs and 793 large femoral head bearings. Across the two groups, most patients were women (57% [736 of 1295]), and the mean age was 66 ± 12 years. There was a larger percentage of women in the dual-mobility construct group (67% [334 of 502]) versus the large femoral head bearing group (51% [402 of 793]), but this was adjusted for in our statistical analysis (Table 1). Patient age was similar for the dual-mobility construct group (67 ± 11 years) versus the large femoral head bearing group (65 ± 12 years). The distributions of American Society of Anesthesiologists (ASA) class and BMI were similar between patients with dual-mobility constructs and those with large femoral head bearings (Table 1). The mean follow-up duration was shorter for dual-mobility constructs (2 ± 1.8 years) versus large femoral head bearings (4 ± 2.9 years). The large femoral head bearing group consisted mostly of 36-mm heads (90% [710 of 793]) (Table 2).

Table 1. - Summary of first revision primary conventional THA (primary diagnosis of osteoarthritis; all-cause revision, first revision of THA [femoral and acetabular], and acetabular only)
Parameter Dual mobility (n = 502) Head size ≥ 36 mm (n = 793)
Age in years 67 ± 11 65 ± 12
BMI in kg/m2a
 Underweight: < 18.5 1.2 (5) 0.5 (2)
 Normal: 18.5-24.9 21 (84) 16 (63)
 Preobese: 25.0-29.9 36 (146) 39 (159)
 Obese class 1: 30.0-34.9 24 (97) 25 (101)
 Obese class 2: 35.0-39.9 11 (45) 14 (57)
 Obese class 3: ≥ 40.0 7 (26) 6 (25)
Women 67 (334) 51 (402)
ASA class at first revisionb
 1 5 (23) 8 (48)
 2 44 (211) 45 (261)
 3 46 (223) 44 (257)
 4 5 (24) 3 (15)
Data are presented as mean ± SD or % (n).
aExcludes 485 procedures with unknown BMI at first revision. Total procedures accounted for within the dual-mobility group = 403; total procedures accounted for within the head size ≥ 36 mm group = 407.
bExcludes 233 procedures in patients with an unknown American Society of Anesthesiologists (ASA) score. Total procedures accounted for within the dual-mobility group = 481; total procedures accounted for within the head size ≥ 36 mm group = 581.

Table 2. - Head size of first revision THA (n = 793)
Head size in mm % (n)
36 90 (710)
38 1 (6)
40 6 (50)
42 0.1 (1)
44 3 (23)
48 0.4 (3)
The head sizes shown above are for the group of head sizes ≥ 36 mm.

Primary and Secondary Study Outcomes

The primary outcome measure was the second-revision cumulative percent revision (CPR) for prosthesis dislocation. The secondary outcome measure was the re-revision CPR for all aseptic causes (excluding infection). Results were stratified by acetabular component sizes < 58 mm and ≥ 58 mm, set a priori on the basis of biomechanical [7, 23] and other registry data [17], which have suggested that this size may produce differences in dislocation rates.

Ethical Approval

The AOANJRR is approved by the Commonwealth of Australia as a federal quality assurance activity under section 124X of the Health Insurance Act, 1973. All AOANJRR studies are conducted in accordance with ethical principles of research (the Helsinki Declaration II).

Statistical Analyses

The time to the re-revision was determined using Kaplan-Meier estimates of survivorship, with right censoring for death or closure of the database at the time of analysis. The percentage of deaths was low, and so the Kaplan-Meier analysis was deemed adequate because the few deaths would not result in a large number of competing events for revision. Death was considered as a competing risk and a second analysis run using a competing-risks estimator; as expected, there was no significant change in results.

The unadjusted CPR was estimated each year of the first 5 years for dual-mobility constructs and for each of the first 9 years for large femoral head bearings, with 95% confidence intervals using unadjusted pointwise Greenwood estimates. The CPR is displayed until the number at risk for the group reaches 40, unless the initial number for the group is less than 100, in which case the CPR is reported until 10% of the initial number at risk remains. The apparent shorter follow-up of the dual-mobility construct group relates to the more recent increase in dual-mobility numbers recorded in the registry, with data collection commencing in 2008 for both groups (the year of the first recorded dual-mobility prosthesis). We tested proportionality for the Cox model. If the hazard ratio is proportional over the entire time of observation, the ratio of hazards between groups will not vary over time. The analytical comparisons of revision rates using the proportional hazard model are based on all available data.

We used hazard ratios with Cox proportional hazard models, adjusting for age, gender, and femoral fixation, to compare the proportion of revision between groups. The assumption of proportional hazards was checked analytically for each model. If the interaction between the predictor and the log of time was statistically significant in the standard Cox model, then we estimated a time-varying model. Timepoints were selected based on the greatest change in hazard, weighted by a function of events. Timepoints were iteratively chosen until the assumption of proportionality was met, and HRs were calculated for each selected time period. If no time period was specified, then the HR was proportional during the follow-up period. All tests were two-tailed at the 5% level of significance. The analysis was performed using SAS version 9.4 (SAS Institute Inc).

Results

Re-revision Surgery for Prosthesis Dislocation

There was no difference in the proportion of re-revisions for prosthesis dislocation between dual-mobility constructs and large femoral head bearings (hazard ratio 1.22 [95% CI 0.70 to 2.12]; p = 0.49) (Fig. 1). At 5 years, the CPR for prosthesis dislocation for dual-mobility constructs was 4.0% (95% CI 2.3% to 6.8%) and that for large femoral head bearings was 4.1% (95% CI 2.7% to 6.1%) (Supplementary Table 1; https://links.lww.com/CORR/A702).

F1
Fig. 1:
This graph shows the cumulative percent second revision for a diagnosis of dislocation by first THA revision group. A color image accompanies the online version of this article.

Re-revision Surgery for All Aseptic Causes

There was no difference in the proportion of all aseptic-cause second revisions between dual-mobility constructs and large femoral head bearings (HR 1.02 [95% CI 0.76 to 1.37]; p = 0.89) (Fig. 2). At 5 years, dual-mobility constructs had a CPR of 17.6% for all aseptic-cause second revisions (95% CI 12.6% to 24.3%), and the CPR for large femoral head bearings was 17.8% (95% CI 14.9% to 21.2%) (Supplementary Table 2; https://links.lww.com/CORR/A703).

F2
Fig. 2:
This graph shows the cumulative percent second revision for all aseptic causes by first THA revision group. A color image accompanies the online version of this article.

The most common reasons for a re-revision were infection, dislocation, and component loosening (Fig. 3), and these were the same for dual-mobility constructs and large femoral head bearings (Table 3). With longer follow-up, infection remained the most common reason for the re-revision, with component loosening being the second most common.

F3
Fig. 3:
These graphs show the cumulative incidence of re-revision. A color image accompanies the online version of this article.
Table 3. - Reason for the re-revision THA by the first revision group
Dual mobility Head size ≥ 36 mm
Revision diagnosis Type of revision as a percentage of the primary procedures (n = 502) Type of revision as a percentage of revision procedures (n = 55) Type of revision as a percentage of the primary procedures (n = 793) Type of revision as a percentage of the revision procedures (n = 116)
Infection 4.4 (22) 40 (22) 4.4 (35) 30 (35)
Prosthesis dislocation 3 (15) 27 (15) 3.4 (27) 23 (27)
Loosening 2.2 (11) 20 (11) 3.3 (26) 22 (26)
Fracture 0.4 (2) 3.6 (2) 1.0 (8) 6.9 (8)
Other 1.0 (5) 9.1 (5) 2.5 (20) 17 (20)
Number of revisions 11 (55) 100 (55) 15 (116) 100 (116)
Data in the table are presented as % (n).

Risk of Re-revision Surgery Stratified by Acetabular Component Size

When stratified by acetabular component sizes < 58 mm and ≥ 58 mm, there was no difference between dual-mobility constructs and large femoral head bearings in the re-revision CPR for prosthesis dislocation (HR 1.37 [95% CI 0.69 to 2.70]; p = 0.37 and HR 0.97 [95% CI 0.36 to 2.56]; p = 0.95, respectively) or for all aseptic causes (HR 1.00 [95% CI 0.71 to 1.41]; p = 0.99 and HR 1.09 [95% CI 0.62 to 1.94]; p = 0.76, respectively).

When stratified by acetabular component sizes < 58 mm and ≥ 58 mm, there was no difference in the CPR for all aseptic-cause re-revisions between dual-mobility constructs and large femoral head bearings (Fig. 4) or between the respective articulation for the different acetabular component size (Supplementary Table 3; https://links.lww.com/CORR/A704).

F4
Fig. 4:
This graph shows the cumulative percent second revision for all aseptic causes by acetabular component size and first THA revision group. A color image accompanies the online version of this article.

Discussion

After revision THA, dislocation is a leading cause of re-revision. Both dual-mobility constructs and large femoral head bearings can reduce this risk, but there are few large, comparative studies of the two prostheses. It is unknown which approach—dual-mobility constructs or large femoral head bearings—is associated with a lower rate of repeat revision surgery for prosthesis dislocation or for all aseptic causes. Studying this question with adequately large patient numbers to allow adjusting for confounders and sufficiently complete follow-up is difficult outside of national joint replacement registries. In this study, performed in the context of a large national registry, we did not identify any difference in the proportion of re-revisions for prosthesis dislocation or all aseptic causes when dual-mobility constructs and large femoral head bearings were compared (adjusted for age, gender, and femoral fixation). There was also no difference when procedures were stratified by acetabular component size < 58 mm or ≥ 58 mm for the same two groups. When surgeons perform a revision THA, either a dual-mobility construct or a large femoral head bearing are suitable options, and this is true for all acetabular component sizes. Based on these findings, future research should investigate factors behind surgeon decision-making or determine if a selection bias is present. Long-term follow-up is required to assess whether complications develop with either implant or whether a difference in re-revision rates becomes apparent.

Limitations

There are several limitations to this study. First, and most importantly, there is a potential for selection bias. Dual-mobility constructs might have been used more selectively in patients who were at a greater risk of having subsequent prosthesis dislocation or by surgeons with a higher complication profile. Because this was a national registry study, we were unable to assess the complexity of each procedure or determine the surgeons’ indication for the use of each implant. Dual-mobility constructs might be more beneficial than large femoral head bearings in more complex procedures. This may explain why the American Joint Replacement Registry reported that the use of dual-mobility constructs was greater in patients with a diagnosis of dysplasia [13]. Selection bias might also be minimized because data collection commenced at the same time point for each prosthesis, was recorded over the same time period, and the Kaplan-Meier estimates of revision and Cox proportional HRs account for time-dependent variation. Complications have been reported to decrease with more recent surgery [20], and this is why we had the same data collection start point. The dual-mobility construct group and the large femoral head bearing group were similar regarding patient age, ASA class, and BMI, but there were gender differences that we adjusted for in our statistical analysis. We do not believe this influenced results, and gender has not been shown to be a risk factor for dislocation [42]. Regardless, we adjusted the results for gender, along with age and femoral fixation. Women are also more likely to have smaller acetabulum sizes than men, but we still found no difference when stratifying by acetabular component size.

Although we adjusted for confounders, there may be other confounding variables that we did not record or adjust for, including surgical approach [19]; surgeon factors such as experience, volume, and training [25]; implant factors such as femoral offset [51] and component positioning [44]; patient factors such as spinopelvic conditions [4], patient comorbidities, ASA class, and BMI [31, 46]. Although ASA and BMI are now recorded by the AOANJRR, data collection for these did not commence in 2008; adjusting for them would have reduced the number of patients available for analysis. There was also no difference in ASA or BMI between groups at baseline, and we do not believe adjusting for these would change the results. Confounders related to surgical technique or performance and patient factors such as spinopelvic conditions and specific medical conditions are not recorded by the AOANJRR and cannot be analyzed in this national registry. Including additional potential confounding variables and risk factors for dislocation either through expanded data collection or data linkage may provide the opportunity to assess their impact more comprehensively.

Another limitation is that we included 36-mm femoral head sizes with 40-mm and 44-mm head sizes, and we have assumed the same results for re-revision caused by dislocation or all aseptic causes. Femoral head sizes of 40 mm and 44 mm are used less frequently than 36-mm heads [12]. A 36-mm femoral head size accounted for 90% of the study group, and there were not enough 40-mm or 44-mm femoral heads for an adequately powered statistical analysis. An ideal head-to-acetabular component ratio might exist; however, we found no difference between dual-mobility constructs and large femoral head bearings for acetabular component sizes < 58 mm and ≥ 58 mm for prosthesis dislocation or for all aseptic causes of re-revision, so equally this may not be true. Finally, we chose our secondary outcome measure to be all aseptic causes of second revision because we do not believe the articulation influences infection proportions. Infection can skew the results because of patient selection bias, as others have found [38].

Re-revision Surgery for Prosthesis Dislocation

Dislocation is a major contributor to re-revision surgery after revision THA. It is unknown which prosthesis best reduces this risk. After controlling for the confounding variables of age, gender, and femoral fixation, we found there was no difference in the proportion of re-revision for dislocation between dual-mobility constructs and large femoral head bearings. This suggests that either option is warranted, and future research should better ascertain factors that drive surgeons’ prosthesis selection choices. Our results are consistent with the results of several other retrospective, comparative studies of revision THA [6, 18, 48], although a benefit of dual-mobility constructs has been shown [10]. Chalmers et al. [6] performed a single-institution study of a single dual-mobility implant used when revising a hemiarthroplasty to THA. It was underpowered to detect a difference between dual-mobility constructs (n = 16) and large femoral bearings (n = 13). Hoskins et al. [18] performed a national registry study only including major first revisions for a diagnosis of THA dislocation. No difference was found between dual-mobility constructs (n = 265) and large femoral head bearings (n = 387). Stevenson et al. [48] performed a single-institution study of two dual-mobility implants used for isolated first acetabular revisions. They also included smaller head sizes and constrained liners but found no difference between dual-mobility constructs (n = 48) and large femoral head bearings (n = 99). The study by Hartzler et al. [10] was a single-institution study of a single dual-mobility implant used for all causes of first revision THA. There was a 15% loss of follow-up at 2 years leaving 126 dual-mobility constructs and 176 40-mm head bearings for comparison. There was one re-revision for dislocation in the dual-mobility group and 10 in the 40-mm head group, a difference they identified as statistically significant. Given that ours was a registry study, we were only able to compare dislocations that resulted in revision and did not include dislocations managed nonoperatively. There has been some benefit shown for dual-mobility constructs in reducing prosthesis dislocation that was not treated with re-revision surgery [10], but this has not been performed in all studies evaluating first-revision THA [6, 48] or primary THA [11, 26, 42].

It remains unclear whether there is a short- or long-term difference between dual-mobility constructs and large femoral head bearings in revision THA. Ongoing follow-up and comparison in a registry format would seem the best way to compare long-term complications and revision rates. Future registry studies of dislocation may benefit from the inclusion of additional potential confounding variables and risk factors either through expanded data collection or data linkages. Future studies should also compare surgeon factors and whether they influence decision-making between prosthesis options and second revision rates. Prospective study designs would seem warranted, and if well designed, would seem the best way to address confounders. To date, no published prospective study has compared the two prosthesis options for primary THA or revision THA, although we are aware of one ongoing study [50]. However, given how uncommon the events of interest are and how long they take to accrue, prospective studies may still be limited particularly with restrictions in length of follow-up that typically occur. Nested randomized controlled trials in national registries would seem the best way to take the strengths of each study design to best answer the research question.

Re-revision Surgery for All Aseptic Causes

There was no difference in the proportion of re-revision for all aseptic causes between dual-mobility constructs and large femoral head bearings, consistent with other retrospective, comparative studies on revision THA [6, 18, 48]. Not all studies have produced similar results, and dual-mobility constructs have shown some benefit [10]. These published studies are limited by size and/or follow-up, and a difference may become apparent with larger, better-designed studies and longer follow-up. Given our study findings that there is no difference in re-revision for dislocation or aseptic causes between dual-mobility constructs and large femoral head bearings, until longer follow-up is available or there is evidence to the contrary, either option may be acceptable. Surgeons will need to weigh the potential and known risks and benefits of each prosthesis option when making a surgical decision. Large femoral head bearings have been used for a longer period than dual-mobility constructs and have a long-term complication profile that has been better described. Although highly crosslinked polyethylene is commonly used, it remains unknown whether large femoral head sizes will have long-term consequences on polyethylene wear rates, component loosening, or trunnionosis [30], or what the effects will be of the higher friction amounts that large femoral heads produce [1, 34].

The most common causes for second revision in this study were infection, prosthesis dislocation, and component loosening, with component loosening increasing for both types of prostheses with longer follow-up duration. This will need to be closely monitored in the future. It is uncertain how polyethylene wear or metal ion generation will be affected over time with dual-mobility constructs [7, 28, 36, 47], given that fretting [14] and trunnionosis have been demonstrated [52]. These concerns are particularly relevant because dual-mobility is mostly used in younger, more active patients in some countries [13], and new complications are being identified after the more routine use of dual-mobility constructs [41]. Each manufacturer’s dual-mobility construct may produce different long-term results, and the true influence of inner and outer head size is not yet known [15]. Despite the concerns for the increased generation of metal ions with dual-mobility constructs (and large femoral heads), only one case or metal-related pathologic finding was recorded for dual-mobility constructs, and three were recorded for large femoral head bearings in this study. These may be underreported but it appears that this would not be a short-term issue. We also do not know about patients who may have had elevated metal ions levels who did not undergo re-revision, and we do not have long-term follow-up data. There also is a unique complication with dual-mobility constructs—intraprosthetic dislocation—which may be related to polyethylene wear of the outer head. This has been documented to occur late, more so with smaller outer femoral head diameters, and it could potentially increase in the future with certain dual-mobility constructs and different manufacturers’ polyethylene bearings [15].

Risk of Re-revision Surgery Stratified by Acetabular Component Size

There was no difference between dual-mobility constructs and large femoral head bearings for re-revisions for prosthesis dislocation or all aseptic causes when stratified by acetabular component sizes < 58 mm and ≥ 58 mm. Many of the benefits of dual-mobility constructs have been found in biomechanical studies [5], which historically have not always translated into clinical practice. Several nonclinical studies have suggested that the benefits of dual-mobility constructs over fixed-bearing implants would be more likely in acetabular component sizes ≥ 58 mm [8, 27]. A registry study on primary THA found dual-mobility components reduced the revision risk for dislocation with acetabular components smaller than 58 mm but not with larger acetabular component sizes or when acetabular component size was not stratified [17], which is similar to our findings. We also do not know whether there was a difference between different inner and outer head sizes in dual-mobility constructs, and future research should investigate this, rather than just the outer acetabular component size. Likewise, we also do not know whether there is a difference in revision rate for large femoral head sizes for different acetabular component sizes, and whether femoral head size should increase in proportion to the outer acetabular component size.

Conclusion

Using data from the AOANJRR, we identified no difference in the re-revision rate for prosthesis dislocation or for all aseptic causes between dual-mobility constructs and large femoral head bearings after a primary THA performed because of osteoarthritis and revised for aseptic causes. No difference remained when patients were stratified by acetabular component sizes < 58 mm and ≥ 58 mm. Therefore, either prosthesis option may be indicated in revision THA. Ongoing follow-up and comparison in a registry format would seem the best way to compare long-term complications and revision rates. Expanded data collection or data linkages may better assess for confounding variables and risk factors for dislocation. Future studies should also compare surgeon factors and whether they influence decision-making between prosthesis options and second revision rates. Although future prospective studies may be the best design to overcome issues with confounding variables, given how uncommon dislocations are, and how long they take to accrue, prospective studies may still be limited particularly with restrictions in length of follow-up that typically occur. Nested randomized controlled trials in national registries would seem a viable option for future research.

Acknowledgment

We thank the AOANJRR, the source of data used in this publication, for their generous assistance provided during the development of the study methods and data analysis.

References

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