As outlined in our January 16, 2013 editorial, “The Case Report Redefined with JBJS Case Connector,” one important role of the JBJS Case Connector is to alert the orthopaedic community about a potentially problematic device or therapy. When two or more such cases with similar mechanisms appear, we will identify the procedure or implant as a “watchable” intervention.
While this system is not statistically conclusive and may or may not be supported by other published case reports or registry data, the intention is to sharpen the focus of clinicians on the potential for similar problems and thereby enhance clinical outcomes and patient safety. The “Watch” designation may also encourage others to report related difficulties and enlist the orthopaedic community to either demonstrate that these are isolated, unrelated cases or sharpen the focus further by rigorously evaluating the intervention. Where appropriate, we may identify brand, model, or implant-lot specifics.
The dual-mobility approach to total hip arthroplasty is intended to provide improved stability along with two additional characteristics that are absent in many fixed-implant designs: increased range of motion and low component wear. Surgeons seem especially interested in dual-mobility devices for patients who need revision for recurrent dislocation, and that is the most common application in Europe, where dual-mobility prostheses have been used since the 1970s.
However, these devices are new to the orthopaedic community in the U.S. The first U.S. Food and Drug Administration (FDA)-approved dual-mobility hip implant was introduced by Stryker in February 2011 and is indicated for both primary and revision applications. Recent reports of early intraprosthetic dislocation have prompted us to issue this “Watch.”
Dual-mobility components essentially consist of a small metal femoral head embedded in a larger polyethylene ball, both of which are mobile within a metal acetabular cup. This is a reversal of the usual material configuration in total hip implants, in which the harder surface is the ball and the softer surface is the socket.
The dual-mobility design presents an advantage in range of motion (ROM): when the small femoral head reaches its ROM limit, the larger polyethylene ball can move in the cup. The claim for less wear stems from the lower torque forces generated by the small metal head that is encased in the larger polyethylene head, and the stability comes from the greater jump distances conferred by the large-diameter outer ball. With the polyethylene-on-metal interface, dual-mobility systems provide stability without the metal-on-metal contact that has caused so much concern in recent years.
Several European studies evaluating dual-mobility designs have confirmed high levels of implant stability. Philippot et al. identified “no early or late instability” among 384 cases that were followed for a mean of fifteen years1. Vielpeau et al. tallied only five dual-mobility devices that required revision for dislocation among 668 cases2. Three of these dislocations occurred after ten years or more, and the authors ascribed the two “early” dislocations (at four and six years postimplant) to “technical errors.” Vielpeau et al. concluded that “dual mobility can be recommended for patients over seventy years of age and for younger patients with high risk of dislocation.”
These studies also divulged a complication of dual-mobility systems that is now starting to emerge in the U.S.: intraprosthetic dislocation. The most common manifestation of this complication occurs when the metal femoral head separates from the surrounding polyethylene ball. This can leave the polyethylene component aberrantly positioned in the hip joint and the metallic femoral head in direct contact with the metal acetabular cup.
In a recent case series analyzing eighty-one cases of intraprosthetic dislocations among nearly 2000 dual-mobility hip replacement procedures in France from 1985 through 1998, Philippot et al. divided these dislocations into three types: “pure” intraprosthetic complications without arthrofibrosis or cup loosening, dislocation secondary to blocking of the liner, and dislocation associated with cup loosening3. Of note, these cases represent relatively late dislocations: the mean times to failure of these three types of dislocations were eleven, eight, and nine years after surgery, respectively.
In contrast to late intraprosthetic dislocations, two cases of early intraprosthetic dislocation of dual-mobility components appear in this edition of JBJS Case Connector. In both cases, surgeons chose a mix-and-match strategy to minimize surgical complexity and bone loss and to maximize hip stability. Despite these sound clinical objectives, both cases suggest that mixing dual-mobility components with components from manufacturers of non-dual-mobility systems may increase the risk of adverse events.
It should be noted that the mixed-component approach to dual-mobility devices is an off-label use, but it has appeared in the literature4. The FDA 510(k) clearance and the manufacturer’s surgical guidance for the dual-mobility components used in these cases explicitly instruct against their use with any component from another manufacturer.
Ward et al. report on the intraprosthetic dislocation of a dual-mobility implant where the polyethylene component completely dissociated from the metal head and lodged itself in the gluteal soft tissue. Because this adverse event occurred within two months of the device’s implantation, wear probably did not cause the intraprosthetic dislocation. The patient was an eighty-seven-year-old woman with dementia who had previously undergone two revision hip arthroplasties.
In this case, in order to match a retained femoral stem that had remained well fixed since its original implantation eleven years earlier, the surgeon used a properly sized femoral head from a different company than the maker of the polyethylene component. To use the dual-mobility device as labeled, the surgeons would have had to revise the well-fixed stem, increasing surgical complexity, bone loss, and potential risk to the patient.
While suggesting that the “mismatch of components is a potential etiology for the dissociation of the polyethylene component,” the authors go on to say that there are no known differences between cobalt-chromium heads of the same size from two different manufacturers. They report that the components were coupled in the operating room and that the bearing functioned properly during intraoperative stability testing. They surmise that the metal and polyethylene components had separated during an attempt at closed reduction of the dislocation when the patient had first presented with persistent instability and clunking. They also suggest that early intraprosthetic dislocations may be “generalizable to dual-mobility bearings and not related to the products of specific companies.”
This patient eventually received a constrained polyethylene liner and a metal femoral head manufactured by the maker of the femoral stem. After eight weeks, she was ambulatory with a walker and had not experienced additional dislocations.
In another case report in this edition of Case Connector, Riviere et al. implanted a dual-mobility bearing to revise a large-diameter metal-on-metal implant in a forty-two-year-old man who had an adverse reaction to metal debris and had a pseudotumor in the hip joint. Surgeons left the well-fixed monoblock cup in place to preserve bone and minimize surgical complexity. Six months after revision, the patient was doing well functionally, and the blood chromium and cobalt levels were substantially lower after one year.
However, at fourteen months postrevision, the patient presented with acute, intense groin pain. Radiographs revealed dislocation, and there was considerable metal debris in the joint. Closed reduction was unsuccessful, and during revision surgery, extensive wear of the mobile polyethylene component was apparent, with loose polyethylene particles in the joint and scratches on the dome of the OXINIUM head. Surgeons replaced the existing acetabular cup with a modular trabecular metal cup and implanted a ceramic modular bearing.
In this case, the authors speculate that the mobile polyethylene ball and existing acetabular cup were incompatible in shape and design. They recognize that the matched suprahemispherical dual-mobility cups are well designed for dual-mobility polyethylene bearings, but that the monoblock metal-on-metal cups provide an articular surface that is less than hemispherical and therefore not well suited for use with dual-mobility designs. Thus, they recommend against the use of dual-mobility components with cups designed for metal-on-metal bearings.
One irony about that recommendation is that surgeons are hoping that dual-mobility designs will be a viable option when revising metal-on-metal implants. They may eventually prove to be just that, but as these two cases suggest, early failures seem to arise when all components are not replaced simultaneously.
In another recently published case of intraprosthetic dislocation with the FDA-approved dual-mobility device, Banzhof et al. reported on a sixty-three-year-old woman with a long history of recurrent dislocations following a primary total hip arthroplasty in 20025. The most recent revision (a hemispherical cup and liner and 36.5-mm metal head) had followed an uneventful postoperative course for seven years until she had fallen and experienced two posterior dislocations in quick succession. During subsequent surgery, surgeons retained the original cup but lined it with the dual-mobility metal cup liner. Surgeons also implanted a dual-mobility polyethylene outer head (42 mm) and a metal inner head (28 mm).
Two months later, the hip spontaneously dislocated. After two unsuccessful attempts at closed reduction, radiographs revealed dissociation between the outer polyethylene head and the inner metal head. During surgical revision, surgeons found the metal head sitting in the cup liner and the polyethylene head positioned in the psoas sheath, with no signs of damage to either component. The cup liner was well fixed to the shell, but there was a slight separation of the locking mechanism. Subsequently, surgeons revised the acetabular component with a conventional titanium shell and also used a conventional liner and femoral head. One year later, the patient was walking without assistance, was pain free, and had not experienced any additional dislocations.
In this case, the authors attributed the separation of the metal and polyethylene heads to “impingement of the outer polyethylene head on the edge of the acetabular component” during the attempts at closed reduction. The authors recommend general anesthesia and use of muscle relaxants when attempting closed reduction of dual-mobility implant dislocations.
Cases of early intraprosthetic dislocation in dual-mobility devices are not restricted to revision procedures. In a case from the United Kingdom, Mohammed and Cnudde reported on a 70-year-old man who received a dual-mobility implant during a primary arthroplasty three years prior to presenting with insidious onset of hip pain6. Radiographs revealed an eccentric location of the metal head in the metal socket, abnormal position of the polyethylene liner, and heterotopic ossifications. Computed tomography images revealed a large cyst; aspiration of the cyst yielded 70 mL of black fluid that tested negative for infectious organisms.
While performing a single-stage revision arthroplasty, surgeons noted widespread black staining of all periprosthetic soft tissues. The metal head, dislodged from the polyethylene liner, had eroded through the metal socket. The components were extracted, and the metal head was found to be relatively unscuffed, with only minor wear to the convex surface of the polyethylene liner.
To avoid the numerous complications from metallosis, these authors encourage a “high index of clinical suspicion” for intraprosthetic dislocation in people who have dual-mobility devices and present with hip pain.
Until we know more about the exact etiology of both early and late intraprosthetic dislocation in dual-mobility hip implants, surgeons should consider intraprosthetic dislocation as a diagnosis in people who present with acute hip pain and have one of these implants, and they should use general anesthesia when attempting closed reductions of intraprosthetic dislocations.
Most importantly, until additional studies confirm whether, why, and in whom the mix-and-match approach to dual-mobility components is problematic, surgeons would be advised to adhere strictly to FDA and manufacturer guidance when implanting such devices, or to select an alternative implant design.
Case Connector would like to hear from surgeons who have had experience, either positive or negative, with these newly available prostheses.
F. The use of a dual-articulation acetabular cup system to prevent dislocation after primary total hip arthroplasty: analysis of 384 cases at a mean follow-up of 15 years. Int Orthop.
C. The dual mobility socket concept: experience with 668 cases. Int Orthop.
F. Intraprosthetic dislocation: a specific complication of the dual-mobility system. Clin Orthop Relat Res.
Van der Bracht
J. Revising the well-fixed, painful resurfacing using a double-mobility head: a new strategy to address metal-on-metal complications. J Arthroplasty.
JV. Femoral head dislodgement complicating use of a dual mobility prosthesis for recurrent instability. J Arthroplasty.
P. Severe metallosis owing to intraprosthetic dislocation in a failed dual-mobility cup primary total hip arthroplasty. J Arthroplasty.