title: Factors affecting the stability of reverse shoulder arthroplasty: a biomechanical study.
abstract: BACKGROUND
  Despite the success of reverse shoulder arthroplasty (RSA) in treating patients with painful pseudoparalytic shoulders, instability is a common complication and currently the factors affecting stability are not well understood. The objective of this study was to investigate a number of factors as well as the interactions between factors to determine how they affect the stability of the prosthesis. These factors included: active arm posture (abduction and abduction plane angles), loading direction, glenosphere diameter and eccentricity, and humeral socket constraint.
  
  
  METHODS
  Force required to dislocate the joint, determined using a biomechanical shoulder simulator, was used as a measure of stability. A factorial design experiment was implemented to examine the factors and interactions.
  
  
  RESULTS
  Actively increasing the abduction angle by 15° leads to a 30% increase in stability and use of an inferior-offset rather than a centered glenosphere improved stability by 17%. Use of a more constrained humeral socket also increased stability; but the effect was dependent on loading direction, with a 88% improvement for superior loading, 66% for posterior, 36% for anterior, and no change for inferior loading. Abduction plane angle and glenosphere diameter had no effect on stability.
  
  
  CONCLUSIONS
  Increased glenohumeral abduction and the use of an inferior-offset glenosphere were found to increase the stability of RSA. Additionally, use of a more constrained humeral socket increased stability for anterior, posterior, and superior loading. These identified factor effects have the potential to decrease the risk of dislocation following RSA.
authors.affiliation: Mechanical and Materials Engineering, Queen's University, Kingston, ON, Canada.
authors.forename: Allison L
authors.initials: AL
authors.lastname: Clouthier
authors.forename: Markus A
authors.initials: MA
authors.lastname: Hetzler
authors.forename: Graham
authors.initials: G
authors.lastname: Fedorak
authors.forename: J Tim
authors.initials: JT
authors.lastname: Bryant
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Deluzio
authors.forename: Ryan T
authors.initials: RT
authors.lastname: Bicknell
mesh_terms: D019643:Arthroplasty, Replacement; D001696:Biomechanical Phenomena; D006801:Humans; D007593:Joint Instability / Q000503:physiopathology*; D008954:Models, Biological; D011474:Prosthesis Design; D012785:Shoulder Joint / Q000601:surgery*