Posterior approach and dislocation rate: a 213 total hip replacements case-control study comparing the dual mobility cup with a conventional 28-mm metal head/polyethylene prosthesis.

INTRODUCTION Dislocation is a frequent complication of total hip arthroplasties (THA) especially in older patients, especially when using a posterior approach. In these cases, dual mobility (DM) cups developed by Gilles Bousquet in 1975 can be indicated to reduce this complication risk. HYPOTHESIS Dual mobility cups reduce the rate of dislocation in primary total hip arthroplasty using posterior approach in a single-surgeon series. AIM Test this hypothesis in a controlled study to compare the rate of dislocation in primary total hip arthroplasties done in patients over 50 years old either with a dual mobility cup or a conventional metal-on-polyethylene 28-mm diameter head. PATIENTS AND METHODS Two consecutive series of primary total hip replacements were performed by a single surgeon using a posterolateral approach. The piriformis tendon was left intact. The DM series included 105 patients who underwent arthroplasty between January 2005 and June 2007 with a dual mobility cup (60 women and 45 men, mean age 76.6±5.65 years old [53-93]). The control series (S series) included 108 patients who underwent arthroplasty (56 women and 52 men, mean age 74.2±5.9 years old [53-87]) with a conventional 28-mm polyethylene cup between January 2003 and June 2005. All hip replacements included a 28-mm metal-polyethylene cup and a 12-14-mm Morse taper. Both groups were comparable for gender, diagnosis, body mass index, type of anesthesia and ASA score distribution. All patients included in this series had a minimum follow-up of 1 year. RESULTS There were no dislocations in the DM series and five early dislocations (before the third month) in the S series for a rate of 4.63%. Although the rate of dislocation was higher in the S series (4.63% vs 0%), the difference was barely significant (P=0.0597). DISCUSSION This study comparing the incidence of dislocations after THA with conventional or dual mobility cups, shows that even using a posterior approach and in older patients, dual mobility cups increase stability with no postoperative dislocations. Although results are barely significant, a larger series should confirm the benefit of this implant. In this series, morbidity was not increased with dual mobility cups. LEVEL OF EVIDENCE Level III: retrospective case-control study.