Review Question - QID 219905

An intraprosthetic dislocation has occurred, as evidenced by the radiograph demonstrating eccentric positioning of the femoral head/stem (Figure B) and polyethylene liner dislodged in the soft tissue (Figure C).

Dual mobility (DM) implants in total hip arthroplasty (THA) have gained popularity for their ability to reduce the risk of dislocation, particularly in high-risk patients such as those with recurrent dislocations or multiple revision surgeries. The design includes a smaller femoral head articulating within a large polyethylene liner, which then articulates with the acetabular cup, creating two points of articulation. Intraprosthetic dislocation (IPD) is a known complication where the smaller femoral head disengages from the polyethylene liner, leading to implant failure. Recent advancements in materials and design aim to minimize the occurrence of IPD, but careful patient selection and surgical technique remain crucial.

Hartzler et al. reviewed their series of 355 patients undergoing revision THA with either a DM construct or a 40-mm femoral head. The authors found reduced dislocation, revision, and reoperation rates among the DM cohort compared with large femoral heads. The authors note that these findings are even more significant given the bias of using DM in the case of high-risk patients. They conclude that dual mobility indications should be judiciously expanded in the contemporary revision of THA.

Addona et al. reviewed their single surgeon series of DM implants placed during primary, revision, and conversion THA procedures. They report on dislocation events, emphasizing the rate of successful closed reduction, reoperation due to failure of closed reduction, and incidence of intraprosthetic dislocation (IPD). Among DMs, the dislocation rate was 4.55%, and the rate of IPD after closed reduction was 5/7 (71%), with all five requiring revision surgery to either another DM bearing or constrained liner. To reduce the high incidence of IPD, the authors advocate for regional or general anesthesia in the operating room and for the reduction to be performed under fluoroscopy.

Figure A demonstrates a periprosthetic dislocation of a DM liner as evidenced by the relatively small femoral head and surrounding polyethylene liner shadow. Figure B shows the post-reduction attempt with the head ball sitting eccentrically inside the acetabular cup. Figure C is post-reduction CT demonstrating the dislocated polyethylene liner sitting posteriorly in the soft tissue. Illustration A shows the initial periprosthetic dislocation with arrows indicating the liner in place. Illustration B shows the post-reduction CT with the liner highlighted in red. Illustration C shows another example of DM periprosthetic dislocation on the left with a post-reduction attempt and IPD on the right.

Incorrect answers:
Answers 2-4: The provided radiographs and CT scan do not show any evidence of liner, stem, or trochanteric fracture or acetabular component subsidence.
Answer 5: The post-reduction radiograph and CT demonstrate eccentric reduction with polyethylene dislodged into the posterior soft tissue.