title: Should the acetabular cup be medialised in total hip arthroplasty.
abstract: BACKGROUND
  During total hip arthroplasty (THA), medialisation of the cup decreases the lever arm of body weight and is considered to be beneficial. Theoretically it should be compensated by an equivalent increase of the femoral offset in order to maintain global offset.
  
  
  OBJECTIVE
  We investigated via a numerical model the forces on each bundle of the abductor muscles and the loading forces at the head-cup interface. We compared THA with and without medialisation of the cup and with and without restoration of the global offset.
  
  
  METHODS
  A digital finite element model of the hip joint with THA was constructed. Monopodal weightbearing was simulated with the cup positioned so that the hip centre of rotation was left unchanged. Forces in the abductors and pressure on the prosthetic head were measured and then compared with three other configurations: 1. Medialisation of the cup by 5 to 15 mm with reproduction of the native femoral offset or; 2. with an increased femoral offset compensating for the cup medialisation and; 3. no medialisation of the cup with a femoral offset increased.
  
  
  RESULTS
  Medialisation of the cup decreased stresses on the head-cup interface and on the abductor muscles, even when the global offset was not restored. Anatomical placement of the cup did not give the best results. The optimal outcome in terms of stress was observed when the cup was medialised but global offset restored. Any increase of global offset significantly increases the length and the tension in the anterior bundles of the abductor muscles.
authors.affiliation: Santy Orthopaedic Centre, 24 Av Paul Santy, Lyon, France. bonnin.michel@gmail.com
authors.forename: Michel P
authors.initials: MP
authors.lastname: Bonnin
authors.forename: Pooler H A
authors.initials: PH
authors.lastname: Archbold
authors.forename: Luca
authors.initials: L
authors.lastname: Basiglini
authors.forename: Tarik A
authors.initials: TA
authors.lastname: Selmi
authors.forename: David E
authors.initials: DE
authors.lastname: Beverland
mesh_terms: D000077:Acetabulum / Q000601:surgery*; D019644:Arthroplasty, Replacement, Hip / Q000295:instrumentation / Q000379:methods*; D020342:Finite Element Analysis; D006621:Hip Joint / Q000601:surgery*; D006622:Hip Prosthesis; D006801:Humans; D008954:Models, Biological; D018482:Muscle, Skeletal