What Is the Frequency of Early Dissociation of Bipolar Cups and What Factors Are Associated With Dissociation?

backgroundDissociation between a prosthetic cup and femoral head is a rare complication of bipolar hemiarthroplasty that usually occurs during closed reduction of a dislocated bipolar cup. The dissociation usually results in reoperation because closed reduction is challenging. To our knowledge, no study has reported on the frequency of bipolar cup-femoral head dissociation, and risk factors for dissociation are unknown. QUESTIONS/PURPOSES: The purposes of this study were (1) to evaluate the frequency of dissociation of prosthetic femoral heads from bipolar cups during manual reduction of a dislocated bipolar hemiarthroplasty; and (2) to explore in a preliminary fashion any prosthesis or patient factors that may be associated with dissociation.

methodsFrom May 2003 to August 2016, 60 bipolar hemiarthroplasties were treated at three institutions as a result of early (within 3 years) dislocation after arthroplasty; of these, 55 could be identified and categorized in terms of the type of locking mechanism used, and these were the patients evaluated in this retrospective study. Because the index hemiarthroplasties were not performed exclusively at the three participating centers, we cannot comment on the indications for prosthesis selection in these patients (such as why some patients may have received components with single rather than double locking mechanisms). Manual reduction was attempted in all patients. Although 34 dislocations were manually reduced, 21 hips underwent surgical intervention because they could not be closed reduced (14 hips) or because of bipolar cup-femoral head dissociation (seven hips). We compared patients' demographics and prosthetic factors between the dissociation group (n = 7) and the nondissociation group (n = 48).

resultsBipolar cup-femoral head dissociation occurred in seven of 55 patients with dislocations (13%). Among the seven dissociated cups, six had a single polyethylene locking ring, whereas 20 of 48 nondissociation cups (42%) had the single locking ring (p = 0.044). We found no other patient or prosthesis factors associated with dissociation.

conclusionsBipolar cups with a single locking mechanism (rather than a double locking mechanism) may be associated with an increased risk of bipolar cup-femoral head dissociation. However, because we could not evaluate prosthesis indications in this study, and because our sample size was too small to control for potential confounding variables, future studies are needed to confirm this finding. All the same, because good alternatives are available, we do not recommend the use of this type of locking mechanism in bipolar hemiarthroplasty. LEVEL OF EVIDENCE: Level III, therapeutic study.