Risk factors and modes of failure in the modern dual mobility implant. A systematic review and meta-analysis

Background The aims of this meta-analysis were to: (1) validate the outcome of modern dual mobility (DM) designs in patients who had undergone primary and revision total hip arthroplasty (THA) procedures and (2) to identify factors that affect the outcome. Methods We searched for studies that assessed the outcome of modern DM-THA in primary and revision procedures that were conducted between January, 2000 to August, 2020 on PubMed, MEDLINE, Cochrane Reviews and Embase. The pooled incidence of the most common failure modes and patient reported outcomes were evaluated in patients who have received: (1) primary THA, (2) revision THA for all causes or (3) for recurrent dislocation. A meta-regression analysis was performed for each parameter to determine the association with the outcome. The study design of each study was assessed for potential bias and flaws by using the quality assessment tool for case series studies. Results A total of 119 studies (N= 30016 DM-THAs) were included for analysis. The mean follow-up duration was 47.3 months. The overall implant failure rate was 4.2% (primary: 2.3%, revision for all causes: 5.5%, recurrent dislocation: 6.0%). The most common failure modes were aseptic loosening (primary: 0.9%, revision for all causes: 2.2%, recurrent dislocation: 2.4%), septic loosening (primary:0.8%, revision for all causes: 2.3%, recurrent dislocation: 2.5%), extra-articular dislocation (primary:0.6%, revision for all causes:1.3%, recurrent dislocation:2.5%), intra-prosthetic dislocation (primary:0.8%, revision for all causes:1.0%, recurrent dislocation:1.6%) and periprosthetic fracture (primary:0.9%, revision for all causes:0.9%, recurrent dislocation:1.3%). The multi-regression analysis identified younger age (β=-0.04, 95% CI -0.07 – -0.02) and female patients (β=3.34, 95% CI 0.91–5.78) were correlated with higher implant failure rate. Age, gender, posterolateral approach and body mass index (BMI) were not risk factors for extra-articular or intra-prosthetic dislocation in this cohort. The overall Harris hip score and Merle d’Aubigné score were 84.87 and 16.36, respectively. Level of evidence of this meta-analysis was IV. Conclusion Modern dual-mobility designs provide satisfactory mid-term implant survival and clinical performance. Younger age and female patients might impact the outcome after DM-THA. Future research directions should focus on, (1) long-term outcome of modern dual-mobility design, including specific concerns such as intra-prosthetic dislocation and elevated metal ion, and (2) cost-effectiveness analysis of dual-mobility implant as an alternative to conventional THA for patients who are at high risk of dislocation. Supplementary Information The online version contains supplementary material available at 10.1186/s12891-021-04404-4.

Conclusion: Modern dual-mobility designs provide satisfactory mid-term implant survival and clinical performance. Younger age and female patients might impact the outcome after DM-THA. Future research directions should focus on, (1) long-term outcome of modern dual-mobility design, including specific concerns such as intra-prosthetic dislocation and elevated metal ion, and (2) cost-effectiveness analysis of dual-mobility implant as an alternative to conventional THA for patients who are at high risk of dislocation.
Compared with the fixed-bearing THA, several metaanalyses have validated a lower dislocation rate using DM articulation in both primary [35][36][37] and revision THA procedures [36][37][38][39]. Despite the established efficacy of DM articulation in preventing dislocation, it is with clinical importance to validate the overall implant survival and failure modes of this unique design. These studies could only provide results of inferential statistics rather than descriptive statistics with regard to the outcome after DM-THA because the included studies represented only a small number of DM-THA used in primary and revision THA procedures [36][37][38][39]. To our knowledge, the most recent and comprehensive systematic review discussing the outcome after DM-THA was conducted by Darrith et al. [40] The authors reviewed studies published from 2007 to 2016, including 54 studies with 14345 primary and revision THA procedures. They reported the overall failure rate (primary: 2.0%, revision: 3.4%) and incidence of common failure modes including aseptic loosening (primary: 1.3%, revision: 1.4%), extra-articular dislocation (primary: 0.46%, revision: 2.2%) and intra-prosthetic dislocation (primary: 1.1%, revision: 0.3%). However, this review included a mixture of the 1 st generation and modern (2 nd and 3 rd generations) DM designs. Several important modes of implant failure such as septic loosening and periprosthetic fracture were not analyzed in this review. Moreover, the number of articles regarding the outcome of modern DM-THA have doubled since 2016 . Therefore, an up-to-date meta-analysis is essential to validate the outcome of modern DM-THA. Our primary objective was to identify the overall implant failure rate and several common failure modes including aseptic loosening, septic loosening, extra-articular dislocation, intra-prosthetic dislocation and periprosthetic fracture. The secondary objective was to determine risk factors predisposing to implant failure and the functional performance of these patients after surgery.

Methods
We completed a comprehensive search on PubMed, MEDLINE, Cochrane Reviews and Embase for studies that reported outcome in patients who had undergone dual mobility total hip arthroplasty (DM-THA) published from the earliest record to August, 2020. The search was completed in accordance to the Preferred Reporting Items for Systematic Reviews and Metaanalysis (PRISMA) statement. The following terms were used in variable combinations: total hip arthroplasty, total hip replacement and dual mobility. Two authors (FYP, SWT) independently searched and screened the titles and abstracts for relevant studies. If there was disagreement, a third author (HHM) was consulted for a consensus. The bibliographies of the included studies were manually reviewed for relevant references. The search strategy is shown in Fig. 1.
We included original articles written in English that validated the outcome in patients who had undergone DM-THA for all kinds of indications including primary THA, revision THA or recurrent dislocation. We excluded review articles, letter to the editor, expert opinion, biomechanical studies, articles not written in English, study period earlier than 2000 or studies in which data were not obtainable. The included studies must contain at least one of the primary (e.g. overall implant failure rate, failure modes including aseptic loosening, septic loosening, extra-articular dislocation, intraprosthetic dislocation and periprosthetic fracture) or secondary outcome domains (e.g. functional scores). Two authors (FYP, SWT) examined all relevant studies and obtained data from the texts. If none of the above outcome domains can be obtained from the study, then we will exclude the study. For comparative studies (e.g. hemiarthroplasty or THA vs DM-THA), we extracted data from the DM-THA group if possible. If there was uncertainty regarding the data from the study, we contacted the authors for clarifications.
Two authors (FYP, SWT) examined all relevant studies and extracted data using a predetermined form. The primary aim was to determine the overall implant failure rate and failure modes including aseptic loosening, septic loosening, extra-articular dislocation, intra-prosthetic dislocation and periprosthetic fracture. We further validated these rates stratified by indications including primary THA, revision THA for all causes or for recurrent dislocation. The secondary aim was to identify risk factors for implant failures and to evaluate the functional outcome using Harris hip score [116] and Merle d'Aubigné score [117]. We recorded the first author, year, study design, number of THA procedures, indications, age, follow-up duration, implant brand and outcome parameters in Table 1.
Two authors (FYP, SWT) independently evaluated the methodological quality of the included studies using the NIH Quality Assessment Tool for Case Series Studies and Case Control Studies [118,119]. To assess the quality of case series study, the highest score on this scale is 9. A score between 7 and 9, 4 and 6, less than 4 were defined as "good", "fair" and "poor", respectively. For the quality of case control study, the highest score on this scale is 12. A score between 8 and 12, 5 and 7, less than 5 were defined as "good", "fair" and "poor", respectively. If there were disagreement, we consulted a third author (HHM). (Tables 2 and 3) Of the 119 included studies, the methodological quality was considered "good" in 72 (60.5%) studies and "fair" in 47 (39.5%) studies.

Statistical analysis
A meta-analysis of proportions was conducted using the Freeman-Tukey analysis under random-effects model to determine pooled estimates with a 95% confidence interval (CI). A random-effects model was used for differences among studies such as age, sex, surgical approaches, body mass index, indications for THA procedure, implant brand and methodology. A standard multivariate linear regression analysis (β) was performed to determine potential factors for implant failure or improved functional outcome. We completed all analyses with the Comprehensive Meta-Analysis (CMA) software, version 3 (Biostat, Englewood, New Jersey, USA) and significance was defined as p < 0.05.

Results
We identified 1123 studies according to our search strategy. We removed 714 duplicate records and 232 studies after reading the title and abstract. Another 58 studies were excluded after reading the full text as the studies did not meet the inclusion criteria: studies on different
Was the study population clearly and fully described, including a case definition?
Were the cases consecutive?
Were the outcome measures clearly defined, valid, reliable and implemented consistently across all Quality of the cohort study (score) 1. Was the study question or objective Was the study population clearly and fully described, including a case definition?
Were the subjects comparable?
Were the outcome measures clearly defined, valid, reliable and implemented consistently across all Quality of the cohort study (score) Was the study population clearly and fully described, including a case definition?
Were the cases consecutive?
Were the outcome measures clearly defined, valid, reliable and implemented consistently across all Quality of the cohort study (score) Was the study population clearly and fully described, including a case definition?
Were the outcome measures clearly defined, valid, reliable and implemented consistently across all Were the statistical methods welldescribed?
Quality of the cohort study (score) Were the outcome measures clearly defined, valid, reliable and implemented consistently across all Were the outcome measures clearly defined, valid, reliable and implemented consistently across all . Was the length of follow-up adequate?
Were the statistical methods welldescribed?
Quality of the cohort study (score) Quality of the cohort study (score) "Good" was defined as a total score of 7-9; "fair" as a score 4-6, and "poor" as a score of less than 4.
Was the study population clearly specified and defined?
3. Did the authors include a sample size justification?
Were controls selected or recruited from the same or similar population that gave rise to the cases (including the same timeframe)?
Were the definitions, inclusion and exclusion criteria, algorithms or processes used to identify or select cases and controls valid, reliable, and implemented consistently across all study participants?
Were the cases clearly defined and differentiated from controls?
Was the study population clearly specified and defined?
3. Did the authors include a sample size justification?
Were controls selected or recruited from the same or similar population that gave rise to the cases (including the same timeframe)?
Were the definitions, inclusion and exclusion criteria, algorithms or processes used to identify or select cases and controls valid, reliable, and implemented consistently across all study participants?
Were the cases clearly defined and differentiated from controls?
If less than 100 percent of eligible cases and/or controls were selected for the study, were the cases and/or controls randomly selected from those eligible?
Was the study population clearly specified and defined?
3. Did the authors include a sample size justification?
Were controls selected or recruited from the same or similar population that gave rise to the cases (including the same timeframe)?
Were the definitions, inclusion and exclusion criteria, algorithms or processes used to identify or select cases and controls valid, reliable, and implemented consistently across all study participants?
Were the cases clearly defined and differentiated from controls?
If less than 100 percent of eligible cases and/or controls were selected for the study, were the cases and/or controls randomly selected from those eligible?

Intra-prosthetic dislocation
A total of 113 studies, including 20447 DM-THA procedures, reported the intra-prosthetic dislocation rate. The overall rate was 1.0% (95% CI 0.007 -0.015). The intraprosthetic dislocation rate in primary THA, revision THA and revision THA for recurrent dislocation were 0.8%, 1.0% and 1.6%, respectively (Table 4, Figure S4). None of the factors including age, female sex, posterolateral approach, BMI or indication have led to intraprosthetic dislocation (Table 5).

Overall implant failure
A total of 105 studies, including 27873 DM-THA procedures, recorded the implant failure rate. The pooled rate was 4.2% (95% CI 0.021 -0.081) at a mean follow-up of 45.8 months. The implant failure rates in primary THA, revision THA and revision THA for recurrent dislocation were 2.3%, 5.5% and 6.0%, respectively (  Y= Yes, N= No; The maximum possible score on this scale is 12. "Good" was defined as a total score of 8-12; "fair" as a score 5-7, and "poor" as a score of less than 5. and for recurrent dislocation (β=1.08, 95% CI 0.24 -1.92) were risk factors for implant failures (Table 5).

Discussion
In this meta-analysis, we included 119 studies with 30016 primary and revision THA procedures using the modern DM design. In terms of functional outcome, the patients were satisfied with their postoperative function based on the improved Harris hip score and Merle d'Aubigné score. Dislocation is one of the common causes of THA implant failure and can be caused by many factors [8]. In current literature, the known risk factors include advanced age, female patients [9,10], obesity [11,12], previous hip surgeries [13], posterolateral surgical approach [14,15], THA for acute fractures, patients with neurological diseases [16], and patients with abductor weakness [17,18]. The dual mobility design increases femoral head-to-neck ratio and jump distance to improve stability [20][21][22][23]. Therefore, we can anticipate decreased dislocation rates for the DM design in primary and revision  THA. Even after revision THA due to recurrent instability, the dislocation rate was only 2.5%, which was much lower than the reported dislocation rate after primary THAs and revision THAs, which ranged from 0.3% to 10% [2][3][4] and 5% to 30% [5][6][7], respectively. In addition, a multivariate analysis revealed that older age, female patients, posterolateral approach and BMI were not risk factors for dislocation after DM-THA. Based on the difference in risk factors for dislocations, we can assume that the DM design can effectively overcome some of the shortcomings of previous THA designs. Nevertheless, optimization of component position and restoration of soft tissue tension are paramount to prevent dislocation in both primary and revision THA procedures. Despite these improvements, there are still some concerns with the DM design, including increased wear of the acetabular liner [164], increased risk of aseptic loosening [30] and intra-prosthetic dislocation [30].
The two-articulation design creates two surfaces for plastic deformation and wear, which theoretically leads to a higher wear rate than fixed-bearing THA. The inner, small articulation dominates the majority of movement and follows the Charnley's low-friction principle with a small-diameter head to reduce wear [20]. The motion between the outer shell and acetabular component occurs in extreme angle when femoral neck abuts the PE liner and creates a homogenous wear over the liner [40]. Using plain radiographs or implant retrieval analysis, several studies aimed to assess the volumetric difference in wearing of DM articulations and fixed-bearing THA [165][166][167][168][169][170][171][172]. Interestingly, the wear rate of ultra-high molecular weight polyethylene (UHMWPE) bearing in the 1 st generation DM cup was less than 40 mm 3 /year, which was similar to wear rate of UHMWPE in fixed-bearing THAs (30-80 mm 3 /year at 15 to 21 year follow up) [165][166][167][168][169]. In vitro simulation study for modern generation DM cup, using highly cross-linked polyethylene (HXLPE), reported lower wear rate in DM cup compared to fixed-bearing THA (1.2 vs. 2.7 mm 3 /million cycles, respectively) [170]. In another study performed by Laende et al., the wear rate of modern generation DM cups with HXLPE at 3 years followup was 0.02 mm/year in DM cup, which was similar to non-dual mobility constructs (0.00 to 0.06 mm/year) [69,171]. In contrast, Deckard et al. recorded the wear rate was two times higher for modern-generation DM cup with HXLPE than the fixed-bearing THA (0.27mm/year and 0.11 mm/year, respectively) [172]. The in vitro simulation or retrieval studies have validated reasonable wear rates of DM articulation using either UHMWPE or HXLPE [165][166][167][168][169][170]. The results from studies using plain radiographs to estimate the wear rate were controversial, which is considered less accurate than the retrieval or simulation studies [171,172]. Currently, there is limited The non-porous alumina-coated surface, tripod anchoring system of acetabular component and polyethylene wear have been associated with a higher aseptic loosening rate in the first-generation DM implants [24,29,31]. Several changes have been made in modern dual mobility designs, including [1] to replace UHMWPE with HXLPE to reduce wear [33,34]; (2) to add bevelled edges (or chamfer) in polyethylene (PE) inserts to lower femoral neck impingement and wear [32]; (3) press-fit fixation by bilayer coating of porous titanium and hydroxyapatite to enhance osseointegration on the outer surface [31]; (4) modular metal liner design to facilitate supplementary screw fixation. The long-term overall survival and aseptic loosening rate of the primary THAs using 1 st generation DM implants were 85-95.4% and 3-8.3%, respectively [24][25][26][27][28]. In this study, the primary THAs using modern generations DM implants are associated with a better overall survival (97.7%) and a lower aseptic loosening rate (0.9%). This pooled aseptic loosening rate was comparable to that of primary, fixedbearing THA from several registries, which ranged from 0.7-1.1% at 5 to 16 years [1,173,174].
The modern, modular design has an additional cobaltchromium (CoCr) liner inserted into a titanium acetabular component allowing supplementary screw fixation to enhance primary stability. However, the metal-on-metal interface between CoCr liner and titanium cup is at risk of fretting corrosion and remains a concern [175][176][177]. Metal ions can further lead to advance local tissue reaction (ALRT) and implant loosening [178]. The first study regarding metal ions was conducted by Matsen Ko et al., which revealed 21% of the patient had elevated serum chromium levels [179]. Other studies reported that serum ion levels (cobalt, chromium or titanium) was elevated in 9.3-23% of the patients [47,111]. On the other hand, some studies have noted that this elevation was not associated with clinical adverse events including instability, loosening or need of revision [64,67,72]. In summary, the current evidence suggests there is a slight elevation of serum ion level but this does not negatively affect the implant survival.
Intra-prosthetic dislocation (IPD) is a rare complication of DM design, which occurs as a result of retentive failure of the inner articulation. Long-term, homogenous PE wear or impingement at extreme range of motion between neck and PE liner leads to loss of PE retentive rim and IPD [180,181]. The incidence of IPD ranged from 0.7%-4.3% in first generation of DM cup and [29,30] modifications have been made to the 2 nd generation DM implants. These changes include a thinner, more polished femoral neck to reduce impingement with the liner and the use of HXLPE to reduce wear during contact [32]. In this study, we noted a lower IPD rate with the modern design in primary THA and revision THA was 0.8% and 1.0% respectively, which is much lower than the 1 st generation [29,30]. Another form of IPD has been observed in modern generation DM implants, which often occurs in the short-term. This form of IPD results from a secondary decapsulation of the liner followed by reduction for dislocation [182]. During close reduction of a dislocated DM-THA, impingement occurs between the PE liner and the posterior edge of the acetabular component. The excessive loading during reduction maneuver may "decapsulate" the femoral head from PE liner. Therefore, the reduction should be performed gradually under general anesthesia to reduce excessive muscle tension [29].
This meta-analysis revealed promising mid-term outcomes and a reduction in dislocation rate, but the longterm implant survival of modern DM-THA is still lacking. For revision THA procedures, younger age and female patients were associated with a higher risk of implant failure. Younger patients have been established as a risk factor for failure after primary THAs. However, whether female sex is a risk factor remains controversial [185][186][187][188]. This can be attributed to the representativeness of the study cohort, follow-up duration and type of implant. Although female patients have been associated with increased risk of dislocation, aseptic loosening, periprosthetic fracture and overall implant failure after primary THA [187,188], the same was not seen in DM-THA aside from overall implant failure. Potential confounders and inadequate follow-up duration are important considerations when interpreting this result.
We should recognize several limitations. First, we only included studies which the full text was available in English. In addition, due to the nature of our research question, the level of evidence of the included studies was low (III or IV). Second, we included studies that reported outcome of modern DM (the 2 nd and 3 rd generation) implants over a time span of 12 years between 2008 to 2020. Modern DM-THA implants were developed in the 1990s, and the studies about modern DM-THA implants were mostly conducted after 2000. We could only analyze factors that were clearly described in the studies, including age, sex, surgical approach, BMI and indication for hip arthroplasty. Factors such as surgeons' experience, patient activity level or implant designs could have affected the outcome but were unavailable and thus were not analyzed. Therefore, we considered articles that were conducted after 2000. Third, the protocol of this meta-analysis has not been registered, which can have a risk for reporting bias. Fourth, we did not include grey literature or unpublished studies in this work. Nonetheless, this review provides an updated review regarding the outcome of modern DM implants and factors that might affect the outcome.

Conclusions
In conclusion, the mid-term implant survival of modern dual-mobility design was satisfactory. Aseptic loosening continues to be the most common failure mode after DM-THA. Younger age and female sex were correlated with implant failure.