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    The Use of Dual Mobility Implants in Total Hip Arthroplasty

    Mid-term data show that today’s dual mobility constructs can provide excellent stability and range of motion in primary and revision total hip arthroplasty when patients are at a higher risk for dislocation. Modern implant designs have successfully mitigated several major failure mechanisms in older systems.

    Authors

    Jeremy Loloi, MD; Eric Chen, MD; Ran Schwarzkopf, MD, MSc; and James Slover, MD, MS

    History

    Instability remains a significant etiology for total hip arthroplasty (THA) failure. Bozic et at [1] reviewed data from more than 51,000 THA revisions performed over a 1-year span and found that instability was the most common revision etiology at 22.5%, followed by loosening and infection.

    Instability is a multifactorial issue involving both patient and surgical factors, including component design. One type of component proposed to prevent instability is the dual mobility cup, the concept for which emerged in the mid-1970s, led by French surgeon Gilles Bousquet. The original design, which debuted in 1979, incorporated a 22.2-mm metallic head that articulated with a polyethylene liner, which in turn articulated with an acetabular shell. The shell was manufactured from stainless steel with plasma-sprayed alumina. It had a 3-point fixation design with 2 Morse taper pegs for impaction into the ischium and pubis. The liner was made from ultra-high-molecular-weight polyethylene (UHMWPE) that was gamma sterilized in air.

    Various modifications and improvements have been made to the mechanics, metallurgy, and materials of the original design:

    • Titanium and hydroxyapatite have replaced the alumina coating.
    • Flanges and modular shells were added for screw fixation.
    • Highly cross-linked UHMWPE has improved wear.
    • Larger femoral heads have added stability.
    • Anatomic designs have decreased anterior overhang.

    Dual mobility cups with an eccentric design include a slight offset between the centers of rotation of the inner and outer articulation surfaces, creating a self-centering effect that encourages the polyethylene to realign its center of rotation with a line created through the weight-bearing axis. This was done to combat the characteristic tilting of the intermediate component into a varus position with concentric designs. [2] The inner articulation retention mechanism underwent improvements in the late 1990s and early 2000s, leading to a decreased incidence of intraprosthetic dislocation.

    Biomechanics of Dual Mobility Constructs

    Due to the 2-articulation design, dual mobility constructs allow for increases in hip range of motion (ROM) until impingement occurs. In the first articulation, the head is engaged but mobile within the polyethylene liner and follows the typical mechanical behavior of a hard-on-soft bearing in a standard THA. However, if the femoral neck and the rim of the polyethylene liner come into contact, the second articulation – consisting of the back of the polyethylene liner and the metallic acetabular shell – begins to function. As the polyethylene liner articulates, effective ROM is increased until impingement of the femoral neck against the rim of the shell ultimately occurs. Most hip motions in daily life take place at the inner, smaller bearing, producing lower friction torques. The larger, outer bearing surface provides stability for more extreme ROM. [3]

    The head-liner complex effectively functions as a large femoral head, increasing the head-neck ratio and subsequently the jump distance before dislocation. An in vivo study by Guyen et al [4] showed that dual mobility cups with 22.2-mm and 28-mm femoral heads allow for significantly greater ROM than conventional implants with similar head sizes. They found that the arcs of motion provided by 22.2-mm or 28-mm inner head dual mobility constructs exceed those of 22.2-mm and 28-mm conventional head in all planes of motion. The study authors concluded that tripolar cups with either size head provided the same effect as using an ultra-large head construct. [4]

    Outcomes in Primary and Revision THA

    Data support the use of dual mobility in revision THA cases, particularly in patients with a higher risk of instability. Hartzler et al [5] retrospectively reviewed 355 revision THAs, which included 146 hips with dual mobility articulations and 209 hips with 40-mm large heads and conventional liners. At up to 4 years of follow-up, patients in the dual mobility group had improved survivorship rates free of instability and re-revision for instability, as well as decreased all-cause re-revision risk. These findings were present despite a selection bias to use dual mobility constructs in patients at highest risk for instability. [5]

    Robust data also support the use of dual mobility in primary THA as well. Vielpeau et al [6] retrospectively evaluated 668 THAs with dual mobility constructs, including 437 first-generation Bousquet-designed implants used in the 1980s and 231 second-generation implant designs used in the early 2000s. First-generation implants showed 85% implant survivorship at minimum 15-year follow-up, with only 5 dislocations (1.1%). Second-generation implants had survivorship of more than 99% at 5 years, with no reported dislocations. [6]

    Boyer et al [7] retrospectively reviewed 240 primary THAs in 205 patients younger than age 75 who had the original Bousquet dual mobility design. They found an 80% survivorship at 22 years of follow-up when acetabular cup and liner revision was used as the primary endpoint, with no dislocations. The survivorship rate was comparable to traditional uncemented and cemented implants. [7]

    Boyer et al [7] also evaluated age as a factor in these revisions and found that no patients older than age 70 at the time of surgery had undergone a revision procedure. That was not the case for younger patients: The highest rate of revision was found in patient younger than age 30 at the time of surgery, with 45% requiring cup/liner revision. This is comparable to rates from studies investigating the use of conventional implants in younger patients. The revision rate plateaued in the 50- to 55-year-old age group, with about 10% requiring cup/liner revision. Failures were due to wear and loosening, not dislocation. Given the advances in implant design, the authors recommended the use of dual mobility constructs in patients over age 60 or at higher risk of dislocation. [7]

    Although dual mobility articulation had an immediate impact by decreasing the rate of prosthetic dislocation in early studies, the unique design also brought about unique complications. Philippot et al [8] classified intraprosthetic dissociation (dislocation of the small articulation) as:

    • Type 1, without arthrofibrosis or cup loosening
    • Type 2, blockage of the liner due to extrinsic phenomena such as arthrofibrosis or ectopic ossification
    • Type 3, associated with acetabular cup loosening

    The rate of intraprosthetic dislocation had been reported as between 0.28% to 4% prior to the use of highly cross-linked polyethylene. [8-10] Loosening had been a chief mode of failure for the early generations of dual mobility implants. Improvements in acetabular cup coating and the advent of modular dual mobility designs allowing for screw fixation have largely addressed the issue of early loosening.

    Despite significant advances in polyethylene composition manufacturing, concerns remain regarding wear, with the thinking being that 2 articulations would equal double the wear. Combes et al [9] described an overall incidence of osteolysis of 7% in a follow-up of 3474 primary THAs using dual mobility cups with non-highly cross-linked polyethylene. However, the incidence increased to 13% in patients who were younger than age 50 at the time of surgery. [9]

    A validated radiostereometric analysis technique for wear measurement of dual mobility implants has been published, but no studies to date have utilized the technique. Although no in vivo data on wear in modern implants is available, a review article by Blakeney et al [3] suggests that unpublished data presented at conferences demonstrate that dual mobility wear rates with newer, conventional polyethylene bearings are no greater than in implants with fixed bearings.

    Recently, De Martino et al [11] included data on 12,844 primary THAs and 5064 revision THAs from 59 articles in a systematic review on dislocations associated with dual mobility implants. The overall dislocation rates for dual mobility constructs were 0.9% and 3% for primary and revision THAs, respectively, with 0.7% and 1.3% rates of intraprosthetic dislocation. [11]

    Harwin et al [12] evaluated 144 primary THAs using modular dual mobility cups and showed no dislocations, with 98.6% survivorship at 7 years of follow-up. The 2 reported failures were not associated with the dual mobility cups. Their cohort also demonstrated excellent patient-reported satisfaction. The study authors concluded that dual mobility articulations are a safe and effective option for primary THAs. [12]

    The cost-effectiveness of dual mobility implants has been evaluated. In a review article, Rudy et al [13] noted that dual mobility constructs can be cost-effective for use in primary THA if implant costs do not exceed the cost of conventional implants by $1023 and that such prices may be achievable via institutional cost-containment efforts. They concluded that for dual mobility implants to be cost-effective, the implant failure rate must be 10% to 20% less than the failure rate of conventional implants. Long-term data on failure mechanisms of modern dual mobility systems are needed to determine if these rates are feasible. [13]

    Conclusion

    Dual mobility constructs provide excellent stability and ROM for high-risk primary and revision THAs. Although long-term data on modern implants are not yet available, improvements in implant design have successfully mitigated several major mechanisms failure in older systems, with robust mid-term data on modern implants demonstrating excellent implant performance and patient-reported outcomes. These implants can be cost-effective with implementation of institutional cost-containment efforts. It is important to understand that instability is a multifactorial phenomenon and that routine use of dual mobility constructs is not a substitute for good surgical technique.

    Author Information

    Jeremy Loloi, MD; Eric Chen, MD; Ran Schwarzkopf, MD, MSc; and James Slover, MD, MS, are from NYU Langone Orthopedic Hospital, NYU Langone Health, New York, NY.

    Disclosures: The authors have no disclosures relevant to this article.

    References

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