title: Interface micromotions increase with less-conforming cementless glenoid components.
abstract: BACKGROUND
  The optimal degree of conformity between the glenoid and humeral components in total shoulder arthroplasty for best performance and durability is still a matter of debate. The main aim of this study is to evaluate the influence of joint conformity on the bone-implant interface micromotions in a cementless glenoid implant.
  
  
  METHODS
  Polyethylene inlays with different degrees of conformity (radial mismatch of 0, 2, 4, and 6 mm) were mounted on a cementless metal back and then implanted in a bone substitute. These glenoid components were loaded by a prosthetic humeral head during a force-controlled experiment. Normal-to-interface micromotions and bone substitute deformations were measured at different points of the interface. Rim displacement and humeral head translation were also measured. A finite element (FE) model of the experiments was implemented to estimate the normal- and tangent-to-interface micromotions in the entire bone-implant interface.
  
  
  RESULTS
  All measured variables increased with less-conforming PE inlays. Normal-to-interface micromotions were significantly larger (P < .05) when the radial mismatch was 6 mm compared with the fully conforming inlay. The FE model was in agreement and complemented the experimental results. FE model-predicted interface micromotions were already significantly larger when the radial mismatch was equal to 4 mm.
  
  
  CONCLUSIONS
  In a force-controlled experiment with a cementless glenoid component, a non-conforming PE inlay allows larger interface micromotions than a conforming inlay, reaching a magnitude that may hamper local bone ingrowth in this type of component. This is mainly because of the larger humeral head translation that boosts the effects of the so-called rocking-horse phenomenon.
authors.affiliation: Department of Orthopaedics, Leiden University Medical Center, Leiden, The Netherlands. d-suarez@javeriana.edu.co
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Suárez
authors.forename: Willem
authors.initials: W
authors.lastname: Nerkens
authors.forename: Edward R
authors.initials: ER
authors.lastname: Valstar
authors.forename: Piet M
authors.initials: PM
authors.lastname: Rozing
authors.forename: Fred
authors.initials: F
authors.lastname: van Keulen
mesh_terms: D019643:Arthroplasty, Replacement; D020342:Finite Element Analysis; D006801:Humans; D007595:Joint Prosthesis; D008954:Models, Biological; D009038:Motion; D011474:Prosthesis Design; D012785:Shoulder Joint / Q000601:surgery*; D013314:Stress, Mechanical