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).
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.
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).
