title: The effects of progressive lateralization of the joint center of rotation of reverse total shoulder implants.
abstract: BACKGROUND
  There has been a renewed interest in lateralizing the center of rotation (CoR) in implants used in reverse shoulder arthroplasty. The aim of this study was to determine the sensitivity of lateralization of the CoR on the glenohumeral joint contact forces, muscle moment arms, torque across the bone-implant interface, and the stability of the implant.
  
  
  METHODS
  A 3-dimensional virtual model was used to investigate how lateralization affects deltoid muscle moment arm and glenohumeral joint contact forces. This model was virtually implanted with 5 progressively lateralized reverse shoulder prostheses. The joint contact loads and deltoid moment arms were calculated for each lateralization over the course of 3 simulated standard humerothoracic motions.
  
  
  RESULTS
  Lateralization of the CoR leads to an increase in the overall joint contact forces across the glenosphere. Most of this increased loading occurred through compression, although increases in anterior/posterior and superior/inferior shear were also observed. Moment arms of the deltoid consistently decreased with lateralization. Bending moments at the implant interface increased with lateralization. Progressive lateralization resulted in improved stability ratios.
  
  
  CONCLUSIONS
  Lateralization results in increased joint loading. Most of that loading occurs through compression, although there were also increases in shear forces. Anterior/posterior shear is currently not accounted for in implant fixation studies, leaving its effect on implant fixation unknown. Future studies should incorporate shear forces into their models to more accurately assess fixation methods.
authors.affiliation: Department of Biomechanics, Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Oren
authors.initials: O
authors.lastname: Costantini
authors.affiliation: Department of Biomechanics, Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Daniel S
authors.initials: DS
authors.lastname: Choi
authors.affiliation: Department of Biomechanics, Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kontaxis
authors.affiliation: Department of Biomechanics, Sports Medicine and Shoulder Service, Hospital for Special Surgery, New York, NY, USA. Electronic address: gulottal@hss.edu.
authors.forename: Lawrence V
authors.initials: LV
authors.lastname: Gulotta
mesh_terms: D019643:Arthroplasty, Replacement / Q000379:methods*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D057645:Deltoid Muscle / Q000502:physiology* / Q000601:surgery; D006801:Humans; D007595:Joint Prosthesis; D016059:Range of Motion, Articular; D012399:Rotation; D012785:Shoulder Joint / Q000502:physiology* / Q000601:surgery*; D019415:Torque
keywords: Reverse total shoulder arthroplasty; biomechanical study; computer modeling; glenoid loading; implant design; lateralization; moment arms; muscle forces