Case Report

Acute Intraprosthetic Dissociation of a Dual-Mobility Hip in the United States

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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.



Take-Home Points

  • AIPD of DM-THA is defined by dissociation within 1 year of implantation resulting from component impingement or closed reduction maneuvers.
  • This is a distinct entity from “late” IPD (>1 year) from implantation as this is associated most often with polyethylene wear, component loosening, and arthrofibrosis.
  • A history of DM dislocation followed by subjective “clunking,” instability, and a series of more frequent dislocations should raise concern for AIPD.
  • Classic radiographic findings of AIPD include eccentric hip reduction and soft tissue radiolucency (ie, halo sign) from dissociated polyethylene component.
  • Treating practitioners of AIPD should consider closed reduction with general anesthesia and sedation in the operating room to limit risk of dissociation.

Dual-mobility (DM) components were invented in the 1970s and have been used in primary and revision total hip arthroplasty (THA) in Europe ever since.1 However, DM components are most commonly used in the treatment of recurrent hip instability, and early results have been promising.2 In DM-THAs, a smaller (22-mm or 28-mm) metal femoral head snap-fits into a larger polyethylene ball (inner articulation), which articulates with a highly polished metal shell (outer articulation), which is either implanted directly in the acetabulum or placed in an uncemented acetabular cup. The 2 articulations used in these devices theoretically increase hip range of motion (ROM) and increase the inferior head displacement distance (jump distance) required for dislocation.3

However, this DM articulation with increased ROM may also cause chronic impingement of the femoral component neck or Morse taper against the outer polyethylene bearing, resulting in polyethylene wear and late intraprosthetic dissociation (IPD) (separation of inner articulation between femoral head and polyethylene liner). In 2004, Lecuire and colleagues4 reported 7 cases of IPD occurring a mean of 10 years after implantation during the period 1989 to 1997. In 2013, Philippot and colleagues5 reported that 81 of 1960 primary THAs developed IPD a mean of 9 years after implantation. These IPD cases were attributed to polyethylene wear or outer articulation blockage caused by arthrofibrosis or heterotopic ossification. Reports of acute IPD (AIPD), however, are rare. In 2011, Stigbrand and Ullmark6 reported 3 cases in which the DM prosthesis dislocated within 1 year after implantation. It was suggested that the inner metal head dissociated from the larger polyethylene component after attempted closed reduction for dislocation (separation of larger polyethylene component from acetabulum or acetabular liner).

DM components were unavailable to surgeons in the United States until 2011. The first US Food and Drug Administration (FDA)-approved DM device was the MDM (Modular Dual Mobility, Stryker). To our knowledge, 2 cases of AIPD with this prosthesis have been reported.7, 8 As with the cases in Europe, closed reduction was the suspected cause, but there was no explanation for the initial dislocation event.

In this article, we present the case of a nondemented man who developed AIPD of a THA with the MDM component and a 28-mm femoral head with a skirted neck (StelKast). His operative findings suggest a poor head-to-neck ratio caused by a larger diameter femoral neck or a skirted prosthesis, or a forceful reduction maneuver, may predispose DM components to AIPD. The patient provided written informed consent for print and electronic publication of this case report.

Case Report

In 2012, a 63-year-old man with a history of drug abuse underwent left primary THA. Seven posterior dislocations and 3 years later, the acetabular component was revised to the MDM prosthesis; the well-fixed StelKast femoral component was retained (Figure 1).

Figure 1.

Within 3 months after revision surgery, the left hip dislocated 3 times in 1 week, when the patient bent over to retrieve an object on the ground. The first 2 dislocations were treated with closed reduction under conscious sedation at an outside emergency department.

Figure 2.
Shortly after, the patient, with complaints of left hip pain and clunking, was seen by a physician assistant, but the treating team did not notice the eccentric reduction on radiographs. The third dislocation was treated with closed reduction under conscious sedation in the emergency department at our institution (Figure 2). Postreduction radiographs still showed the eccentric reduction, and a radiolucent halo was visible superior to the greater trochanter (Figure 3).
Figure 3.

With the patient’s erythrocyte sedimentation rate and C-reactive protein level both normal, a second revision was performed. During surgery, the polyethylene head was found beneath the gluteus maximus (Figure 4).
Figure 4.
Gross inspection revealed a small amount of eccentric polyethylene wear and metal debris of the inner articulation (Figure 5). As the abductor muscles were intact, it was decided to proceed with revision to a larger DM component and to downsize the femoral head to a skirtless component (Table, Figure 6).

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