Background

Prosthetic dislocation is one of the most common cause of implant failure after total hip arthroplasty (THA) [1]. The reported dislocation rate after primary THAs is 0.3-10% [2,3,4] and is much higher after revision THAs (5-30%) [5,6,7]. The cause of a dislocated prosthesis can be multifactorial, including both surgeon and patient related factors [8,9,10,11,12,13,14,15,16,17,18]. Several design changes have been made on the prosthesis to resolve this. Currently, dual mobility (DM) THA is one of the most successful designs to reduce the risk of dislocation [19]. The concept of DM was invented by Gilles Bousquet and Andrè Rambert in France in 1973 [19]. The design included Charnley’s low-friction principle and the theory of McKee and Watson-Farrar, which increased the femoral head-to-neck ratio, extending the “jumping” distance in order to prevent dislocations [20,21,22,23]. The first generation DM design was associated with higher aseptic loosening and intra-prosthetic dislocation (IPD) rate, which resulted from polyethylene wear, suboptimal fixation and surface coating of the acetabular component [24,25,26,27,28,29,30]. In the late 1990’s, a newer DM design was introduced with several modifications including modular design, shape, surface coating and highly cross-linked polyethylene to reduce the rate of aseptic loosening and IPD [31,32,33,34].

Compared with the fixed-bearing THA, several meta-analyses 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 1st generation and modern (2nd and 3rd 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 [41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115]. 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 Meta-analysis (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.

Fig. 1
figure 1

Preferred reporting items for systematic reviews and meta-analysis (PRISMA) flow diagram for the searching and identification of included studies

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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, intra-prosthetic 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.

Table 1 Characteristics of included studies
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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.

Table 2 Study assessment based on quality assessment tool for case series studies
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Table 3 Study assessment based on quality assessment tool for case control studies
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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 outcome domains (n=21), mixed etiologies (n=12), 1st generation DM designs (n=10), cemented liner to cup (n=9), cadaveric or in vitro studies (n=3), studies not written in English (n=3). After exclusion, a total of 119 studies were included [41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115, 120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163] (Figure 1). Of these studies, 45 were case-control studies while 74 were case series. Since the objectives of this study were to validate the risk factors and modes of failures in the modern dual mobility implants, we extracted only the dual mobility group but not the control group from the 45 case-control studies.

Baseline characteristics

This study included 30016 patients who had undergone DM-THA for primary and revision THA procedures. The mean age was 71.9 years (range, 19.2 to 87.6) and 63.2% of the patients were female. Mean follow-up duration in overall, primary, revision and recurrent dislocation group were 47.29 months (range, 3 to 152.4), 40.86 months (range, 3 to 152.4), 61.82 months (range, 6 to 87.6), and 35.23 months (range, 24 to 55), respectively. DM-THA was used in 19819 primary THA procedures, 9411 revision THA procedures and 786 revision THA procedures for recurrent dislocation.

Aseptic loosening

A total of 105 studies, including 28980 DM-THA procedures, reported the rate of aseptic loosening. The pooled rate was 1.6% (95% CI 0.008 – 0.032). The aseptic loosening rate in primary THA, revision THA and revision THA for recurrent dislocation were 0.9%, 2.2% and 2.4%, respectively (Table 4, Figure S1). A multivariate regression analysis revealed that a revision THA procedure for all causes (β=1.30, 95% CI 0.71 – 1.89), or for recurrent dislocation (β=1.18, 95% CI 0.26 – 2.10), carried a higher risk of aseptic loosening compared with a primary THA procedure (Table 5).

Table 4 Pooled event rate and clinical performance stratified by indications
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Table 5 Multivariate linear regression analysis
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Septic loosening

A total of 105 studies, including 28980 DM-THA procedures, reported septic loosening rates. The pooled rate was 1.6% (95% CI 0.007 – 0.037). The septic loosening rate in primary THA, revision THA and revision THA procedure for recurrent dislocation were 0.8%, 2.3% and 2.5%, respectively (Table 4, Figure S2). A multivariate regression analysis showed that both revision THA for all causes (β=1.85, 95% CI 1.26 – 2.44) and for recurrent dislocation (β=1.40, 95% CI 0.45 – 2.36) were at a higher risk of septic loosening, compared with a primary THA procedure (Table 5).

Extra-articular dislocation

A total of 113 studies, including 20447 DM-THA procedures, presented the extra-articular dislocation rate. The pooled rate was 1.2% (95% CI 0.006 – 0.025). The extra-articular dislocation rate in primary THA, revision THA and revision THA for recurrent dislocation were 0.6%, 1.3% and 2.5%, respectively (Table 4, Figure S3). Compared with a primary THA procedure, risk of dislocation was higher after revision THA procedures (β=1.02, 95% CI 0.30 – 1.73) (Table 5).

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 intra-prosthetic 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 intra-prosthetic dislocation (Table 5).

Periprosthetic fracture

A total of 100 studies, including 27731 DM-THA procedures, recorded the periprosthetic fracture rate. The pooled rate was 0.9% (95% CI 0.008 – 0.011). The periprosthetic fracture rates in primary THA, revision THA and revision THA for recurrent dislocation were 0.9%, 0.9% and 1.3%, respectively (Table 4, Figure S5). Revision THA procedure for all causes (β=0.93, 95% CI 0.23 – 1.62) was a risk factor for periprosthetic fracture (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 (Table 4, Figure S6). Younger age (β=-0.04, 95% CI -0.07 – -0.02), female sex (β=3.34, 95% CI 0.91 – 5.78), revision THA procedure for all causes (β=1.48, 95% CI 0.93 – 2.03) and for recurrent dislocation (β=1.08, 95% CI 0.24 – 1.92) were risk factors for implant failures (Table 5).

Functional outcome

We included 49 (N= 7086) and 21 (N= 2764) studies that evaluated the functional outcome using Harris hip score and Merle d’Aubigné score. The pooled Harris hip score and Merle d’Aubigné score were 84.87 (95% CI 78.99 – 90.76) and 16.36 (95% CI 15.20 – 17.53), respectively (Table 4, Figure S7, S8). Revision THA procedure for all causes (β=-9.44, 95% CI -15.17 – -3.72) and female sex (β=-4.10, 95% CI -8.17 – -0.03) were associated with lower functional scores. (Table 5).

Discussion

In this meta-analysis, we included 119 studies with 30016 primary and revision THA procedures using the modern DM design. At a mean follow-up of 47.3 months, the overall failure rate of modern dual mobility design was 4.2%. The most common failure modes include aseptic loosening (primary: 0.9%, revision for all causes: 2.2%, revision for recurrent dislocation: 2.4%), septic loosening (primary: 0.8%, revision for all causes: 2.3%, revision for recurrent dislocation: 2.5%), extra-articular dislocation (primary: 0.6%, revision for all causes: 1.3%, revision for recurrent dislocation: 2.5%), intra-prosthetic dislocation (primary: 0.8%, revision for all causes: 1.0%, revision for recurrent dislocation: 1.6%) and periprosthetic fracture (primary: 0.9%, revision for all causes: 0.9%, revision for recurrent dislocation: 1.3%). The multi-regression analysis revealed that revision THA procedures were associated with a higher risk of aseptic loosening, septic loosening, extra-articular dislocation, periprosthetic fracture, overall implant failure and lower Harris Hip scores. Interestingly, several risk factors that were identified for THA dislocation such as advanced age, female sex, posterolateral approach and increased BMI were not risk factors for extra-articular dislocation. On the other hand, younger and female patients were associated with higher risk of implant failure. 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 1st generation DM cup was less than 40 mm3/year, which was similar to wear rate of UHMWPE in fixed-bearing THAs (30–80 mm3/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 mm3/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 follow-up 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 evidence regarding the increased PE wear of modern DM articulation.

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 1st 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, fixed-bearing 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 cobalt-chromium (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 2nd 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 1st 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].

Our meta-analysis showed that the mid-term revision rates in primary and revision DM-THA were 2.3% and 5.5-6.0%, respectively. These results were comparable to the reported outcome of primary or revision, fixed-bearing THA [1, 38, 39, 60, 73, 98, 108, 183, 184]. In primary fixed-bearing THA, the mid-term and long-term revision rate ranged from 1.2-4.0% and 12.1-14.3%, respectively [1, 38, 60, 73, 98, 108, 183]. In revision fixed-bearing THA, the mid-term and long-term revision rates can be up to 5.3-13% and 27-45%, respectively [39, 184].

This meta-analysis revealed promising mid-term outcomes and a reduction in dislocation rate, but the long-term 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 2nd and 3rd 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.