title: Is there a rationale to use a dual mobility poly insert for failed Birmingham metal-on-metal hip replacements? A retrieval analysis.
abstract: BACKGROUND
  Previous studies showed poor outcomes for patients undergoing revision of failed metal-on-metal total hip arthroplasty (MoM-THA) and resurfacing (RS) with an increased risk of dislocation. Dual mobility inserts are an option to retain the acetabular component and change the metal-on-metal bearing to plastic-on-metal. The current study analyzes the rationale for the off-label use of a dual mobility poly insert (MDM X3, Stryker, Mahwah, NJ) in a Birmingham metal shell (Smith & Nephew, Memphis, TN).
  
  
  METHODS
  Based on retrievals from the implant database the study compared the clearance between 20 BHR shells, 31 MDM poly inserts and 24 ADM acetabular components of different sizes. The radial clearance was calculated for each possible combination of implants [n = 81 (MDM/BHR) and n = 119 (MDM/ADM)].
  
  
  RESULTS
  An MDM mobile bearing poly insert in an ADM shell has an average clearance of 0.314 mm (SD 0.031) compared to 0.234 mm (SD 0.030) in a BHR shell (p < 0.01). The minimal clearance is 0.246 and 0.163 mm, respectively. 30.9 % of the MDM/BHR clearances were within the range of the MDM/ADM bearing and 88.9 % had a clearance of more than 0.2 mm.
  
  
  CONCLUSIONS
  Clearances of the MDM poly insert in a BHR shell are reduced, and although the majority of combinations appear safe, the indication needs to be made on an individual base carefully considering alternative treatment options.
authors.affiliation: Hospital for Special Surgery, 535 East 70th Street, New York, NY, 10021, USA, rennerl@hss.edu.
authors.forename: Lisa
authors.initials: L
authors.lastname: Renner
authors.forename: Martin
authors.initials: M
authors.lastname: Faschingbauer
authors.forename: Friedrich
authors.initials: F
authors.lastname: Boettner
mesh_terms: D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation*; D006622:Hip Prosthesis; D006801:Humans; D011474:Prosthesis Design; D012086:Reoperation