Acute Intraprosthetic Dissociation of a Dual-Mobility Hip in the United States
After decades of use in Europe, dual-mobility (DM) components for total hip arthroplasty (THA) were approved by the US Food and Drug Administration (FDA) in 2011 for use in the United States. DM-THAs are designed with an inner articulation between the femoral head and a larger polyethylene insert, and an outer articulation between the mobile polyethylene and a highly polished metal insert, to increase motion and minimize impingement. Intraprosthetic dissociation (IPD), defined as separation of the femoral head from the inner polyethylene articulation, is usually caused by polyethylene wear, and occurs 3 to 10 years after implantation. Early recognition of this complication is important for appropriate treatment. Late chronic IPD is caused by polyethylene wear, blocked motion of the outer bearing, or acetabular loosening. Acute IPD (AIPD), which occurs within 1 year after implantation, is rare and poorly understood. Only 2 cases of this early complication have been reported in the United States. The exact mechanism of injury is unknown, but AIPD may be associated with closed reduction maneuvers or neck impingement (large-diameter femoral neck, femoral head with skirted neck). In this article, we report the case of a nondemented 63-year-old man who developed AIPD 3 months after implantation of a DM component for recurrent dislocation.
Discussion
Recurrent dislocation and instability accounts for 22.5% of THA revisions in the United States.9 Until 2011, options for managing recurrent dislocation in the United States included modular component exchange, component revision for malposition, and use of constrained components.10 
In 1974, Bousquet first reported use of the DM prosthesis in primary THA; the prosthesis allowed increased stability without sacrificing motion or fixation.1 However, longer-term studies of DM components disclosed a new complication, IPD. In 2004, Lecuire and colleagues4 reported 7 cases of IPD occurring a mean of 10 years after implantation of the Bousquet prosthesis. 
AIPD, which occurs within 1 year after implantation, has been reported much less often than late IPD. Stigbrand and Ullmark6 reported 3 cases of AIPD that developed within 7 months after implantation of Amplitude and Advantage (Zimmer Biomet) DM prostheses.
This unusual complication apparently is not confined to a specific implant or region. Since the MDM component was introduced in the United States, 2 more cases of AIPD have been identified (Table). Banzhof and colleagues7 reported the case of a 68-year-old woman who, 2 months after the MDM was placed for recurrent instability, dislocated the component while rising from a seated position. Her IPD most likely resulted from a closed reduction. The affected hip eventually required closed reduction in the operating room. Postreduction radiographs showed the characteristic eccentric appearance; a halo, also visible in the soft tissues, corresponded with the dissociated radiolucent polyethylene liner. The authors attributed the early failure to an eccentrically seated metal liner that separated the locking mechanism. The MDM component was revised to a conventional THA, with the femoral head upsized and length added.
Ward and colleagues8 reported the case of an 87-year-old woman who had a conventional THA revised to an MDM component for recurrent instability. Two months after surgery, this patient, who had dementia, experienced 2 posterior dislocations while rising from a chair. Closed reduction in the emergency department seemed successful, but later she presented to the surgeon’s office with symptoms of instability and clunking, complaints similar to our patient’s. Radiographs showed an eccentric reduction caused by IPD, and the MDM component was revised to a constrained liner. Adding a MDM component to a retained DePuy (DePuy Synthes) femoral stem and head is considered “off-label use,” which, the authors proposed, may have been related to the AIPD in their patient’s case. However, one manufacturer’s femoral component and head are often mated with another manufacturer’s acetabular component to allow for a less complex revision. Our recommendation for surgeons is that, before proceeding with this treatment option, they investigate each component’s exact dimensions to ensure there are no subtle size differences that could cause problems. For example, a 28-mm head diameter that is actually 28.2 mm may affect mating properties, with the inner polyethylene articulation causing AIPD to develop.
Other cases of earlier IPD have been described, but they do not fit the APID definition given in this article. Riviere and colleagues14 reported the case of a 42-year-old man who, because of a previous adverse reaction to metal debris, underwent revision to a DM polyethylene ball in a retained BHR (Birmingham Hip Resurfacing) acetabular shell (Birmingham Hip, Smith & Nephew). Unfortunately, IPD occurred 14 months after surgery. Banka and colleagues15 reported the case of a 70-year-old woman who underwent revision to a DM cup for recurrent instability, but they did not specify the length of time between implantation and IPD and did not offer an explanation for the complication. Finally, Odland and Sierra16 reported the case of a 77-year-old man, with previous intertrochanteric and pelvic fractures, who underwent revision to a DM cup with retention of a Waldemar femoral component (Waldemar Link). He spontaneously developed IPD with ambulation 2 years after surgery.
Certainly, our patient’s presentation course is similar to other patients’. Within 3 months after revision to the MDM component, his left hip dislocated 3 times in 1 week. We contend his AIPD resulted from closed reduction, with the polyethylene dislodged from the femoral head with contact on the acetabulum. A larger or skirted neck may increase impingement during normal activity and thereby widen the polyethylene opening excessively and/or reduce the polyethylene ball ROM to impinge during the relocation maneuver. In this case, dissociation was noted only after the third dislocation. Pathognomonic eccentric positioning of the head in the acetabulum and, less commonly, the halo sign were evident on postreduction radiographs. Optimal treatment for AIPD of a DM component is controversial. Choices are limited to a constrained liner or, if possible, repeat DM with larger components. For recurrent dislocation, our patient underwent revision to an MDM component, but a femoral head with a skirted neck was used in an attempt to increase soft-tissue tension. During the second revision, minor eccentric wear of the inner articulation of the polyethylene component (consistent with impingement) was noted, and wear was visible on inspection of the outer articulation. We think his AIPD resulted from femoral neck impingement of the skirted head against the polyethylene ball.
AIPD is a discrete entity, with sudden failure of a DM component within 1 year after implantation. AIPD is characterized by dissociation of the femoral head from the inner articulation, resulting from impingement or closed reduction. More studies are needed to determine which patients with DM components are at highest risk and which treatment is most appropriate. We recommend taking extra care when reducing hips with this articulation and adopting a low threshold for general anesthesia use in the presence of paralysis.
Am J Orthop. 2017;46(3):E154-E159. Copyright Frontline Medical Communications Inc. 2017. All rights reserved.
