title: Mechanical tradeoffs associated with glenosphere lateralization in reverse shoulder arthroplasty.
abstract: BACKGROUND
  Scapular notching in reverse shoulder arthroplasty occurs in up to 97% of patients. Notching is associated with decreased strength and reduced motion and may lead to long-term failure due to polyethylene wear. Many implant systems lateralize the glenosphere to address scapular notching, but the mechanical tradeoffs of lateralization have not been rigorously evaluated. We hypothesized that lateralization would decrease bony impingement but also decrease the mechanical advantage of the deltoid.
  
  
  METHODS
  Finite element models were created using the same implants with different amounts of glenoid lateralization: 5 mm of medialization to replicate glenoid erosion, as well as 2.5, 5, 7.5, and 10 mm of lateralization. Tests were performed with static and dynamic scapulae for motion in either the coronal or scapular plane. The angle of impingement between the scapula and the humeral polyethylene was recorded, as was the deltoid force required to elevate the arm.
  
  
  RESULTS
  Increasing lateralization decreased impingement while increasing the deltoid force required to elevate the arm. Differences were found between the static and dynamic scapulae, with the dynamic scapula model having increased humeral adduction before impinging. The impingement angle was also substantially affected by the bony prominences on the inferior scapula, showing how individual bony anatomy can affect impingement.
  
  
  CONCLUSIONS
  Lateralization is effective in increasing impingement-free range of motion but also increases the deltoid force required to perform identical tasks. In addition, impingement is determined by scapular motion, which should be included in all shoulder models.
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, USA. Electronic address: carolyn-hettrich@uiowa.edu.
authors.forename: Carolyn M
authors.initials: CM
authors.lastname: Hettrich
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, USA.
authors.forename: Vijay N
authors.initials: VN
authors.lastname: Permeswaran
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, USA.
authors.forename: Jessica E
authors.initials: JE
authors.lastname: Goetz
authors.affiliation: Department of Orthopaedics and Rehabilitation, University of Iowa, Iowa City, IA, USA.
authors.forename: Donald D
authors.initials: DD
authors.lastname: Anderson
mesh_terms: D019643:Arthroplasty, Replacement / Q000295:instrumentation / Q000379:methods*; D003198:Computer Simulation; D020342:Finite Element Analysis; D006801:Humans; D011474:Prosthesis Design; D017755:Prosthesis Fitting; D016059:Range of Motion, Articular; D012785:Shoulder Joint / Q000601:surgery*
keywords: Glenospere lateralization; deltoid force; finite element analysis; impingement; reverse shoulder arthroplasty; scapular notching