History and Background

The early days of the modern total hip arthroplasty (THA), in the 1960s and early 1970s, were primarily dominated by two different design philosophies. Sir John Charnley’s design was based on the “low friction” principle – using a monobloc femoral stem with a small head to articulate with a large polyethylene acetabular component to reduce volumetric wear(1). The McKee-Farrar prosthesis, on the other hand, used a metal-on-metal articulation with a large femoral head, allowing for a larger range of motion due to increased clearance between the neck of the implant and the acetabular liner(2). In 1974, Gilles Bousquet and André Rambert introduced the concept of a “dual mobility” implant, aimed at combining the advantages of the Charnley and McKee-Farrar designs(3). This was achieved by having a small metallic femoral head articulating with a large polyethylene liner (i.e. low friction); the polyethylene liner, in turn, was free to move within a metal hemispherical acetabular component, thereby providing the increased range of motion(4).

             Instability is consistently listed as the first or second most common reason for revision THA based on national registry data from across the world, responsible for up to 25% of revision surgeries(5–8). Patients can be predisposed to dislocation following THA for a range of reasons, including but not limited to: history of prior surgery, compromised abductors, history of spinal fusion, neuromuscular diseases, cognitive impairment, component mal-positioning, and smaller femoral head sizes(9). As THA volumes continue to rise(10), and the patient population trends towards an increase in weight and medical complexity(11), instability continues to be a major issue facing THA surgeons worldwide. Dual mobility components represent one of the tools that may help in the prevention and management of instability following THA.

            Despite a history that extends back nearly as far as the modern THA itself, there has been a recent proliferation of literature on the topic of dual mobility implants. Nearly half of PubMed publications on the topic have been produced in the last four years (Figure 1). This recent literature boom covers a broad range of topics, but a few themes have emerged, and one of the most hotly discussed and debated topics at recent conferences has been the use of dual mobility implants in primary THA(12). There is little doubt that dual mobility implants serve an important purpose in revision THA, though the specific indications may be debated; dual mobility implants in primary THA may be useful in a variety of scenarios, and according to some authors, may even be considered as a routine primary THA implant(13).

Figure 1 – PubMed results by year for dual mobility literature

Specific Indications for Dual Mobility Cups in Primary THA

Femoral Neck Fractures

Femoral neck fractures are a major global public health concern, with up to 2.2 million femoral neck fractures per year globally, a rate that is projected to continue to increase as populations around the world continue to age(14,15). Displaced femoral neck fractures in the elderly can be treated with hip hemiarthroplasty or THA, with the recent HEALTH trial demonstrating no important differences between these two treatment strategies in mortality, revision surgery rates, or functional outcome at 2 years post-op (16). Nonetheless, some patients with a femoral neck fracture undergo THA, either because of a contra-indication to hemiarthroplasty (e.g. pre-existing osteoarthritis), or based on a decision made by the surgeon and patient for a variety of reasons, which may include age, activity level, and other considerations(17). These patients, however, are at an elevated risk of dislocation compared to those undergoing primary THA, with rates of about 5% based on the health trial(16). Thus, dual mobility acetabular components may be considered in this population.

There is a paucity of Level 1 Evidence on this particular use of dual mobility implants. A randomized controlled trial (RCT ) comparing conventional THA and dual mobility implants in patients with displaced femoral neck fractures found that up to 1 year after surgery, patients in the dual mobility group had significantly higher functional scores, better range of motion, and no patients had dislocations in either group(18). There is also at least one RCT currently underway to compare dual mobility versus conventional THA in patients with femoral neck fractures(19). Interestingly, one pilot study on this topic found that a large definitive trial on this topic would not be feasible, at least in the context of the United Kingdom healthcare system(20).

In lieu of sufficient Level 1 Evidence, we turn our attention to the remaining available evidence for potential hypotheses on this topic. Albanese et al. conducted systematic review and meta-analysis of eighteen studies on the use of dual mobility implants in femoral neck fractures. (21). They reported a cumulative dislocation rate of 1.2%, and that it compared favourably to hemiarthroplasty and THA in terms of revision surgery rates(21). Interestingly, an economic analysis found dual mobility THA to be cost-effective compared to conventional THA in patients with displaced femoral neck fracture(22)

Overall, dual mobility implants may represent an important intermediate option between THA and hemiarthroplasty, with the potential to reduce dislocation rates (one of the primary concerns with THA in the context of femoral neck fractures), while providing an acetabular reconstruction option, when necessary, in these patients.

Reduced Spinopelvic Mobility

Consideration of spinopelvic mobility and parameters in THA is another topic of great interest in recent years, with review articles published on the topic in high impact orthopaedic journals in recent years(23,24). Patients with reduced spinopelvic mobility, either due to degenerative disease or spinal fusion, have a decreased ability to rotate their pelvis as they go from sitting to standing and vice versa, which can lead to a higher risk of dislocation. These patients can be defined in a number of different ways, including the presence of degenerative lumbar spine disease, the presence of abnormal sinopelvic parameters (pelvic incidence, lumbar lordosis, and pelvic tilt), or the presence of instrumented fusion of the thoracolumbar spine, possibly including lumbopelvic fixation in some cases(25).

There is a lack of Level 1 Evidence in this population– namely, there are no recent high quality RCTs that compare dual mobility implants with standard THA in patients with reduced spinopelvic mobility. A large, prospective cohort study by Vigdorchik et al. proposed a novel, simplified, hip-spine classification system for patients undergoing THA. (26). Group 1 patients had normal spinal alignment, while Group 2 patients had a flatback deformity; Group 2 patients were further divided into those with normal spinal mobility (Group 2A) and those with reduced mobility (Group 2B). In their study, all patients in Group 2B, and those with more than three levels of lumbar spine fusion, underwent THA with a dual mobility component(26). Dislocation-free survivorship was 93.2% in Group 2B at a mean follow-up of 4.6 years(26). A study of the Australian Orthopaedic Association National Joint Replacement Registry (AOANJRR) found that among 227 patients with adverse spinopelvic mobility who underwent THA with a dual mobility component, there were no dislocations at two-year follow-up(27)

Dagneaux et al. performed a case control study of 2728 patients who underwent THA and analyzed risk factors for instability – they found that the use of a dual mobility partially compensated for the higher risk of instability among patients with adverse spinopelvic parameters. Overall, the evidence for the routine use of dual mobility implants is limited in quality and quantity; nonetheless, there is both a biomechanical rationale and some potential signal in the early results which warrants further study of the use of dual mobility implants in this population. (28). Two retrospective cohort studies also found no cases of dislocation in patients with reduced spinopelvic mobility who underwent THA with a dual mobility cup at a mean follow-up of about 3 years(29,30)

High Demand Patients

An interesting indication proposed for the use of dual mobility cups in primary THA is in patients who are high demand – which may include patients who are young and/or active, or those with occupations, hobbies, or activities of daily living which require a normal or close to normal range of motion. While most other indications focus on patients who are generally at lower functional levels, some authors suggest that dual mobility cups may also confer a benefit in high demand patients(31). Again, there is no recent Level 1 Evidence available on this population.

A prospective cohort study of 43 active patients under 65 years of age reported that the use of a dual mobility cup in primary THA resulted in elevated but stable titanium levels that were at the high end of normal range, and no significant increase in cobalt or chromium levels.(32). A retrospective study of 91 primary THAs with dual mobility cups in patients younger than 55 demonstrated survival free of aseptic loosening to be 98.9% at mean follow-up of 9.8 years(33). Similarly, in a retrospective cohort of 204 primary THAs performed for avascular necrosis of the hip (mean age 42.5 years), a cumulative survival of 100% was reported at 5 years with no episodes of dislocation(34). Finally, an interesting and important consideration in the selection of implants for THA – Agarwala et al. emphasized the importance of dual mobility cups providing near-normal range of motion in their patients, as squatting and sitting cross-legged was an important part of daily life for patients in their region(35)

Other Indications

A range of other indications have been proposed for the use of dual mobility cups in total hip arthroplasty, including abductor deficiency, hip dysplasia, tumour, neuromuscular disease, and cognitive impairment(36). These indications either lack much evidence to support their use, or are often discussed in the context of other indications (e.g. cognitive impairment and femoral neck fractures). Thus, a thorough discussion of each of these indications is beyond the scope of this OE Original(37). 

So…should every primary THA get a dual mobility cup?

A few things are clear from the discussion above – first, the evidence on dual mobility cups in primary THA is limited in both quality and quantity, with very little in the way of Level 1 (or even Level 2) Evidence. Second, there does appear to be some signal in the early evidence for the use of dual mobility cups in some specific populations, including femoral neck fractures and those with reduced spinopelvic mobility. Third, there is a biomechanical rationale to explain why dual mobility cups, at least in theory, may help to prevent dislocations in patients at high risk of dislocation(38).

Nonetheless, there are some concerns with dual mobility implants which are important to consider. Intraprosthetic dislocations occur when the small femoral head dislocates from the large polyethylene liner.   (39) – this is a complication unique to dual mobility implants, and occurs at a rate of 2-4% at long-term follow-up – though these rates are based on cups which were implanted 17 to 22 years ago(40–43). Treatment essentially always involves revision surgery, and can predispose patients to risk of future instability(39). As well, some dual mobility designs involve a metal-on-metal implant surface (between the acetabular component and a metal liner which then articulates with the large polyethylene head)(44). While it is unclear if this represents a clinically relevant concern, there are measurable metal ion levels in these patients, including in asymptomatic patients with well-functioning implants(44). Finally, though the femoral head actually implanted on the trunnion is small in diameter, the effective size of the combined femoral head component is quite large. Some studies have raised concerns about the impact of this on taper-trunnion corrosion due to increased rotational torque placed on the neck and trunnion of the femoral component(4)

Overall, dual mobility cups have a long history in THA, and will continue to serve an important role in revision THA. Their role in primary THA is receiving more attention in recent literature, and there may be some signal for its use in specific populations. Nonetheless, further study is certainly needed, and there is not nearly enough evidence available to recommend for or against the use of dual mobility implants in routine primary THAs for all patients.

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