Lachiewicz, Paul F. MD; Watters, Tyler Steven MD
Dislocation is a serious and frequent complication following primary and revision total hip arthroplasty (THA). A review of the National Inpatient Sample database of revision THAs performed from October 1, 2005, through 2006 found hip instability to be the most common cause of revision, accounting for 22.5% of the 51,345 revision arthroplasties performed.1
In an effort to reduce the risk of dislocation after THA, a surgeon may use a large femoral head (eg, 36, 38, and 40 mm) that articulates with highly cross‐linked polyethylene.2 Constrained acetabular components have been used in specific circumstances, such as in patients with cognitive disorders or with abductor muscle insufficiency, and when larger femoral heads have been unsuccessful.3 A specific concern with these constrained devices is the risk of failure of different locking mechanisms and the increased stress produced at the implant‐bone interface, leading to increased wear and loosening.
Although dual‐mobility components have been used in Europe for >25 years, interest has been renewed recently in the United States in the use of new dual‐mobility components for difficult primary hip arthroplasty, revision of failed large‐head metal‐on‐metal hip arthroplasty, and revision for recurrent hip instability.
Design Concepts of Dualmobility Articulations
The concept of a dual‐mobility THA was first developed by Bousquet in 1974.4,5 This was an uncemented hemispherical acetabular component with a plasma‐sprayed alumina coating (porosity, 50 to 100 μm) and a highly polished stainless steel inner bearing surface. For mechanical fixation, the component was equipped with an upper anchor clip through which a 4.5‐mm screw was inserted into the ilium. Additionally, two stainless steel pins were pressed into two holes in the socket. This metal acetabular component was “attached” to a modular 22.25‐mm femoral head (following the model of the Charnley low‐friction arthroplasty prosthesis) using a bipolar retention polyethylene connecting piece (Figure 1). In a consecutive series of 144 primary THAs, 5 were removed for infection, 3 were revised for a fractured femoral stem, and 1 was revised early for dislocation. In the 135 cups available for follow‐up, this dual‐mobility component had a reported survival of 95.37% (95% confidence interval [CI], 91.95% to 98.79%) at 10 to 12 years, with 85 hips surviving to 10 years.4 Two revisions were performed to address recurrent dislocation, one in the second year for acetabular component retroversion and one at 10 years because of polyethylene wear. Three patients had groin pain resulting from radiographic loosening/migration, and 12 had notable radiolucent lines, with likely loose components.
A second report from the same institution provided data on 384 primary THAs (79 titanium and 305 stainless steel acetabular components) with a mean follow‐up of 15.3 years (range, 12 to 20 years).5 There were 13 aseptic loose acetabular components (3.3%), 14 intraprosthetic dislocations (3.6%), and 7 revisions for polyethylene wear (1.8%), for a total failure rate of 8.7%. Intraprosthetic dislocation occurred between the small femoral head and the mobile polyethylene insert, usually as the result of severe inner polyethylene wear. Of the 14 intraprosthetic dislocations, 9 were pure (presumably due to wear), 2 were associated with loosening of the acetabular component, and 3 were related to fibrosis and jamming of the second articulation. The mean time to intraprosthetic dislocation was 8.9 years (range, 4 to 16 years). Using surgical revision for aseptic loosening as the end point, the survival rate was 96.7% (95% CI, 94.6% to 98.8%) at 15 years and 95.9% (95% CI, 93.5% to 98.2%) at 18 years.5 To our knowledge, this particular implant was never used in North America.
The current generation of THA components has been referred to alternately as dual‐mobility sockets, unconstrained tripolar cups, and mobile‐bearing hips. These devices have a large polyethylene head that articulates with a polished acetabular component, with an additional smaller femoral head that is snap‐fit into the polyethylene. Currently, three North American manufacturers produce these devices. Multiple other designs are available only in Europe.
The ADM (Anatomic Dual Mobility, Stryker Orthopaedics, Mahwah, NJ) has a chrome‐cobalt alloy acetabular shell that is coated with both plasma spray titanium and hydroxyapatite. The shell ranges in size from 46 to 64 mm. There are anterior psoas tendon and inferior obturator foramen cutouts, and the inner surface is polished. No adjunctive fixation of the shell is possible. This dual‐mobility component has a 28‐mm cobalt‐chromium alloy or ceramic femoral head. On a back table and using a bearing press, the femoral head is introduced into a large (40‐ to 58‐mm), highly cross‐linked polyethylene liner (X3, Stryker) that articulates with the polished shell (Figure 2).
The MDM (Modular Dual Mobility, Stryker) component has an acetabular shell with titanium plasma spray coating and multiple holes for adjuvant screw fixation. Shells range in size from 42 to 64 mm. A modular polished cobalt‐chromium alloy articular surface component fits into the shell and articulates with a large (36‐ to 58‐mm), highly cross‐linked (X3) polyethylene liner that has a 28‐mm metal or ceramic femoral head within it (Figure 3). In both the ADM and the MDM components, the large polyethylene ball is reduced into the metal acetabular articular surface.
The E1 Active Articulation device (Biomet, Warsaw, IN) has a onepiece, high‐carbon, cobalt‐chromium alloy acetabular component with titanium porous plasma spray coating and rim fixation fins. No adjuvant screw fixation is possible. Using a bearing press, a 28‐mm femoral head is introduced into the highly crosslinked, vitamin E‐stabilized polyethylene bearing (E1, Biomet), which is then reduced into the metal shell (Figure 4).
The goal with these devices is to provide for greater range of motion and jump distance (ie, vertical or inferior head displacement required for dislocation) than is possible with a single‐piece large femoral head. The inability for adjuvant screw fixation with some of these designs makes their use in revision hip arthroplasty problematic. However, a metal dualmobility component without screw holes could be cemented into a wellfixed retained acetabular shell.
Surgeons also have the option of constructing a different unconstrained tripolar component intraoperatively. A standard or enhanced porous‐surface modular acetabular component receives a 40‐mm highly cross‐linked polyethylene liner.6 This articulates with a 40‐mm outer‐diameter hip fracture‐type bipolar component with a 22‐, 26‐, or 28‐mm innerdiameter femoral head (Figure 5). However, there are two major disadvantages of this construct. First, the inner polyethylene bearing is fabricated of conventional polyethylene. Second, this inner bearing is much thinner than in the dual‐mobility designs.
There is a paucity of independent (ie, non‐industry) basic science studies of dual‐mobility components. In vitro range of motion to impingement of one tripolar hip implant in an automated hip simulator has been evaluated and compared with in vitro range of motion to impingement with conventional implants.7,8 Tripolar components with 22.2‐ and 28‐mm femoral heads provided increased flexion, adduction, and external rotation compared with conventional 22.2‐ and 28‐mm femoral heads.7 At 90° of flexion and 40° of adduction, there was an increase of 45.2° in internal rotation with the tripolar components compared with the conventional 22.2‐mm femoral head and of 27.5° compared with the conventional 28‐mm femoral head.
A potential biomechanical disadvantage of dual‐mobility devices is that excessive motion may result in impingement of the femoral neck or the femoral component itself against the large outer polyethylene bearing, resulting in motion at the second articulation site. This could lead to increased polyethylene wear and intraprosthetic dislocation. It is also unclear whether, after fibrous tissue replacement of the hip hematoma postoperatively, motion will continue to occur at both articulations or at only the larger outer polyethylenemetal surface.
To our knowledge, no publication in the English‐language literature has reported the wear rate of a large polyethylene head against a polished metal surface. This new construct is the opposite of what THA surgeons have used with current bearing technology, in which the harder surface is the femoral head and the softer surface is the acetabular component. Such flipping of the bearing surfaces, as in dual‐mobility devices, may result in increased wear rates. This is a notable concern.
Indications and Results
The original designer of the first dual‐mobility THA apparently used this device in all primary THAs.4,5 A more recent study reports on the use of the POLARCUP Dual Mobility System (Smith & Nephew Orthopaedics AG, Rotkreuz, Switzerland), which has recently become available in the United States.9 This system has a stainless steel cup with hydroxyapatite coating and a 22‐ or 28‐mm inner femoral head. One study reported no dislocations in 150 primary hips managed with this device at a mean follow‐up of 6.2 years.9 However, two cups were revised for loosening.
It has been suggested that any dual‐mobility component is indicated only in patients at high risk for dislocation, that is, in patients aged >75 years, in women aged >70 years, after prior hip surgery, in patients with neuromuscular disorders or cognitive dysfunction, and in those with preoperative American Society of Anesthesiologists scores ≥3.10 One nonrandomized retrospective study compared the rate of dislocation of dual‐mobility cup THA with conventional THA in patients treated for femoral neck fracture via a posterior approach.11 At 1‐year follow‐up, there were 8 dislocations in 56 patients with conventional hips (28‐and 32‐mm heads) compared with no dislocations in 42 consecutive patients treated with the dual‐mobility component. However, this study was limited because posterior capsular repair was not performed in either group, and the cohorts were consecutive rather than randomized. Hamadouche et al12 recently reported on a retrospective review of 168 consecutive primary hips with a dual‐mobility socket after a minimum 5‐year follow‐up. Four hips were revised for intraprosthetic dislocation resulting from fatigue damage and wear of the mobile insert at the capture area (2.4%).
The greatest utility for dual‐mobility components may be revision THA, in particular, revision for recurrent dislocation. Two studies have evaluated the prevention of dislocation with a dual‐mobility design after revision THA performed for a variety of reasons, including recurrent dislocation.13,14 One study retrospectively evaluated 85 revisions (including five hips with recurrent dislocation) with substantial acetabular bone loss, which was defined as segmental in 21 hips, combined segmental and cavitary in 50 hips, severe segmental in 14 hips, and pelvic discontinuity in 3 hips.13 Using a cemented dualmobility component with a 22‐mm inner head and a polyethylene head ≥40 mm, there was only one dislocation at a mean follow‐up of 3 years (range, 2 to 5 years).
A different study retrospectively reviewed 163 revisions performed for various reasons, including recurrent dislocation in 26 hips and reimplantation after infection in 33 hips.14 Four different types of dual‐mobility components were implanted. There were six early dislocations (3.7%), all of which were successfully managed with closed reduction without recurrence. No dislocations occurred in patients who underwent revision for recurrent dislocation. The mean follow‐up was 5 years (range, 2 to 9 years). The overall rate of revision was 6.7%.
Three European studies have evaluated the results of dual‐mobility components used specifically to manage recurrent dislocation,15‐17 and three North American studies have evaluated unconstrained tripolar components (ie, metal bipolar heads reduced into polyethylene acetabular liners) used in the management of recurrent dislocation6,18,19 (Table 1). The European studies included 51,15 47,16 and 5017 patients managed for recurrent dislocation with a different dual‐mobility component in each study, with a mean follow‐up of 4, 4.3, and 8 years, respectively. The success rate (ie, no dislocation) was 94.5%,15 96%,16 and 98.3%.17 The three North American studies are much smaller, with 30,6 8,18 and 1119 unconstrained tripolar components, with mean follow‐ups of 3.2, 4.2, and 6.5 years, respectively. In the largest series, there were three dislocations, two of which required rerevision, for an overall success rate of 93.3%.6 There were no reported problems of loosening of the acetabular component or of excessive polyethylene wear in all three North American studies, but the follow‐up times were relatively short. No randomized studies have compared the results of dual‐mobility or unconstrained tripolar components with those of large femoral heads articulating with highly cross‐linked polyethylene components.
Dual‐mobility components have been used in Europe for more than two decades, and interest in this concept has been renewed recently in North America. Dual‐mobility components offer an additional articulating surface, with the goal of improving range of motion and stability of THA. The fabrication of several types of highly cross‐linked polyethylene has permitted large polyethylene femoral heads to articulate with a polished inner surface acetabular component. There are no data published to date on the wear of these components, which is of great concern because of the reports of intraprosthetic dislocation. Dualmobility components may have their greatest utility in revision THA, especially in cases of revision for recurrent dislocation. The early rates of success of older dual‐mobility components used to manage recurrent dislocation are encouraging, ranging from 90% to 98% at short‐ to midterm follow‐up. These components may be an alternative to constrained components in select patients with recurrent dislocation. Because there are no long‐term studies or registry data on these new dual‐mobility components, caution is advised in their routine use in primary and revision THA.
The authors would like to thank Mr. Stephen Perlman for his assistance with the literature search.
Evidence‐based Medicine: Levels of evidence are described in the table of contents. In this article, all references are level IV studies.
References printed in bold type are those published within the past 5 years.