title: Glenohumeral mismatch affects micromotion of cemented glenoid components in total shoulder arthroplasty.
abstract: BACKGROUND
  The "rocking horse" phenomenon is considered the main cause of glenoid component loosening by eccentric loading of the glenoid rim. This study aimed to investigate the influence of increasing glenohumeral implant mismatch on bone-implant interface micromotion in a cemented all-polyethylene pegged glenoid biomechanical model.
  
  
  METHODS
  Five glenoid sizes, 40 mm, 44 mm, 48 mm, 52 mm, and 56 mm, representing +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm glenohumeral mismatch, respectively, were cyclically loaded according to ASTM Standard F2028-08 at a constant frequency of 2 Hz to a size-dependent humeral head subluxation translation. Additional glenoid components were cyclically loaded to their subluxation translations at a constant humeral head rate of 4.4 mm/s. Component micromotion was characterized as compression, distraction, and superior-inferior translation measured by differential variable reluctance transducers.
  
  
  RESULTS
  During constant frequency tests, 52-mm and 56-mm glenoids were unable to complete cyclic testing because of catastrophic failure of the glenoid-implant interface and permanent glenoid deformation, probably due to increasing severity of testing parameters. When tested at a constant humeral head speed, 48-mm, 52-mm, and 56-mm glenoids had significantly increased glenoid distraction and glenoid translation at cycle 50,000 compared with cycle 1. Distraction and translation measurements for 52-mm and 56-mm glenoids were significantly greater compared with 40-mm, 44-mm, and 48-mm glenoids at 50,000 cycles.
  
  
  CONCLUSIONS
  In a biomechanical model, optimal glenohumeral mismatch in cemented pegged glenoid implants is multifactorial and has not been definitively established. However, our data suggest that a radial mismatch of less than +10 mm may decrease the risk of glenoid micromotion.
authors.affiliation: Department of Orthopaedic Surgery, Western Michigan University School of Medicine, Kalamazoo, MI, USA.
authors.forename: Vani J
authors.initials: VJ
authors.lastname: Sabesan
authors.affiliation: Department of Orthopaedic Surgery, Beaumont Health System, Royal Oak, MI, USA.
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Ackerman
authors.affiliation: Department of Orthopaedic Surgery, Western Michigan University School of Medicine, Kalamazoo, MI, USA.
authors.forename: Vinay
authors.initials: V
authors.lastname: Sharma
authors.affiliation: Department of Orthopaedic Surgery, Oakland University-William Beaumont School of Medicine, Rochester, MI, USA; Department of Orthopaedic Research, Beaumont Health System, Royal Oak, MI, USA.
authors.forename: Kevin C
authors.initials: KC
authors.lastname: Baker
authors.affiliation: Department of Orthopaedic Surgery, Oakland University-William Beaumont School of Medicine, Rochester, MI, USA; Department of Orthopaedic Research, Beaumont Health System, Royal Oak, MI, USA.
authors.forename: Michael D
authors.initials: MD
authors.lastname: Kurdziel
authors.affiliation: Department of Orthopaedic Surgery, Beaumont Health System, Royal Oak, MI, USA; Department of Orthopaedic Surgery, Oakland University-William Beaumont School of Medicine, Rochester, MI, USA. Electronic address: mwiater@beaumont.edu.
authors.forename: J Michael
authors.initials: JM
authors.lastname: Wiater
mesh_terms: D019643:Arthroplasty, Replacement / Q000295:instrumentation*; D001696:Biomechanical Phenomena; D001843:Bone Cements / Q000627:therapeutic use; D019544:Equipment Failure Analysis; D061165:Glenoid Cavity; D006801:Humans; D058430:Humeral Head; D007595:Joint Prosthesis; D008422:Materials Testing; D009038:Motion; D020959:Polyethylene / Q000627:therapeutic use; D011474:Prosthesis Design; D011475:Prosthesis Failure / Q000209:etiology*; D012785:Shoulder Joint / Q000601:surgery*
chemical_list: D001843:Bone Cements; D020959:Polyethylene
keywords: Total shoulder arthroplasty; basic science study; biomechanical testing; glenohumeral radial mismatch; glenoid loosening