title: Contact mechanics of reverse total shoulder arthroplasty during abduction: the effect of neck-shaft angle, humeral cup depth, and glenosphere diameter.
abstract: BACKGROUND
  Implant design parameters can be changed during reverse shoulder arthroplasty (RSA) to improve range of motion and stability; however, little is known regarding their impact on articular contact mechanics. The purpose of this finite element study was to investigate RSA contact mechanics during abduction for different neck-shaft angles, glenosphere sizes, and polyethylene cup depths.
  
  
  METHODS
  Finite element RSA models with varying neck-shaft angles (155°, 145°, 135°), sizes (38 mm, 42 mm), and cup depths (deep, normal, shallow) were loaded with 400 N at physiological abduction angles. The contact area and maximum contact stress were computed.
  
  
  RESULTS
  The contact patch and the location of maximum contact stress were typically located inferomedially in the polyethylene cup. On average for all abduction angles investigated, reducing the neck-shaft angle reduced the contact area by 29% for 155° to 145° and by 59% for 155° to 135° and increased maximum contact stress by 71% for 155° to 145° and by 286% for 155° to 135°. Increasing the glenosphere size increased the contact area by 12% but only decreased maximum contact stress by 2%. Decreasing the cup depth reduced the contact area by 40% and increased maximum contact stress by 81%, whereas increasing the depth produced the opposite effect (+52% and -36%, respectively).
  
  
  CONCLUSIONS
  The location of the contact patch and maximum contact stress in this study matches the area of damage seen frequently on clinical retrievals. This finding suggests that damage to the inferior cup due to notching may be potentiated by contact stresses. Increasing the glenosphere diameter improved the joint contact area and did not affect maximum contact stress. However, although reducing the neck-shaft angle and cup depth can improve range of motion, our study shows that this also has some negative effects on RSA contact mechanics, particularly when combined.
authors.affiliation: Roth McFarlane Hand and Upper Limb Center, St. Joseph's Health Care, University of Western Ontario, London, Ontario, Canada.
authors.forename: G Daniel G
authors.initials: GD
authors.lastname: Langohr
authors.affiliation: Roth McFarlane Hand and Upper Limb Center, St. Joseph's Health Care, University of Western Ontario, London, Ontario, Canada.
authors.forename: Ryan
authors.initials: R
authors.lastname: Willing
authors.affiliation: Roth McFarlane Hand and Upper Limb Center, St. Joseph's Health Care, University of Western Ontario, London, Ontario, Canada.
authors.forename: John B
authors.initials: JB
authors.lastname: Medley
authors.affiliation: Roth McFarlane Hand and Upper Limb Center, St. Joseph's Health Care, University of Western Ontario, London, Ontario, Canada.
authors.forename: George S
authors.initials: GS
authors.lastname: Athwal
authors.affiliation: Roth McFarlane Hand and Upper Limb Center, St. Joseph's Health Care, University of Western Ontario, London, Ontario, Canada. Electronic address: jajohnso@uwo.ca.
authors.forename: James A
authors.initials: JA
authors.lastname: Johnson
mesh_terms: D019643:Arthroplasty, Replacement; D003198:Computer Simulation; D020342:Finite Element Analysis; D006801:Humans; D006811:Humerus / Q000503:physiopathology* / Q000601:surgery; D007595:Joint Prosthesis; D011474:Prosthesis Design; D016059:Range of Motion, Articular / Q000502:physiology; D012540:Scapula / Q000503:physiopathology*; D012785:Shoulder Joint / Q000503:physiopathology* / Q000601:surgery
keywords: FEA; RSA; Reverse total shoulder arthroplasty; contact mechanics; finite element analysis; glenosphere diameter; humeral cup constraint; neck-shaft angle