title: How sensitive is the deltoid moment arm to humeral offset changes with reverse total shoulder arthroplasty?
abstract: BACKGROUND
  Reverse total shoulder arthroplasty commonly treats cuff-deficient or osteoarthritic shoulders not amenable to rotator cuff repair. This study investigates deltoid moment arm sensitivity to variations in the joint center and humeral offset of 3 representative reverse total shoulder arthroplasty subjects. We hypothesized that a superior joint implant placement may exist, indicated by muscle moment arms, compared with the current actual surgical implant configuration.
  
  
  METHODS
  Moment arms for the anterior, lateral, and posterior aspects of the deltoid muscle were determined for 1521 perturbations of the humeral offset location away from the surgical placement in a subject-specific musculoskeletal model with motion defined by subject-specific in vivo abduction kinematics. The humeral offset was varied from its surgical position ±4 mm in the anterior/posterior direction, ±12 mm in the medial/lateral direction, and -10 to 14 mm in the superior/inferior direction.
  
  
  RESULTS
  The anterior deltoid moment arm varied in humeral offset and center of rotation up to 20 mm, primarily in the medial/lateral and superior/inferior directions. The lateral deltoid moment arm varied in humeral offset up to 20 mm, primarily in the medial/lateral and anterior/posterior directions. The posterior deltoid moment arm varied up to 15 mm, primarily in early abduction, and was most sensitive to humeral offset changes in the superior/inferior direction.
  
  
  CONCLUSIONS
  High variations in muscle moment arms were found for all 3 deltoid components, presenting an opportunity to dramatically change the deltoid moment arms through surgical placement of the reverse shoulder components and by varying the overall offset of the humerus.
  
  
  METHODS
  Basic Science Study; Computer Modeling.
authors.affiliation: Department of Mechanical Engineering, University of Florida, Gainesville, FL, USA.
authors.forename: David R
authors.initials: DR
authors.lastname: Walker
authors.affiliation: Department of Mechanical and Aerospace Engineering, University of Florida, Gainesville, FL, USA.
authors.forename: Allison L
authors.initials: AL
authors.lastname: Kinney
authors.affiliation: Orthopaedics and Sports Medicine Institute, University of Florida, Gainesville, FL, USA. Electronic address: wrightw@ortho.ufl.edu.
authors.forename: Thomas W
authors.initials: TW
authors.lastname: Wright
authors.affiliation: Department of Mechanical Engineering, University of Florida, Gainesville, FL, USA.
authors.forename: Scott A
authors.initials: SA
authors.lastname: Banks
mesh_terms: D000072228:Arthroplasty, Replacement, Shoulder; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D057645:Deltoid Muscle / Q000503:physiopathology*; D006801:Humans; D006811:Humerus / Q000503:physiopathology* / Q000601:surgery; D016059:Range of Motion, Articular; D012399:Rotation; D012785:Shoulder Joint / Q000503:physiopathology* / Q000601:surgery; D000072227:Shoulder Prosthesis
keywords: Deltoid components; deltoid muscle; humeral offset; joint center; muscle moment arm; reverse total shoulder arthroplasty