title: How do deltoid muscle moment arms change after reverse total shoulder arthroplasty?
abstract: BACKGROUND
  Although many advantages of reverse total shoulder arthroplasty (RTSA) have been demonstrated, a variety of complications indicate there is much to learn about how RTSA modifies normal shoulder function. This study used a subject-specific computational model driven by in vivo kinematic data to assess how RTSA affects deltoid muscle moment arms after surgery.
  
  
  METHODS
  A subject-specific 12 degree-of-freedom musculoskeletal model was used to analyze the shoulders of 26 individuals (14 RTSA and 12 normal). The model was modified from the work of Holzbaur to directly input 6 degree-of-freedom humeral and scapular kinematics obtained using fluoroscopy.
  
  
  RESULTS
  The moment arms of the anterior, lateral, and posterior aspects of the deltoid were significantly different when RTSA and normal cohorts were compared at different abduction angles. Anterior and lateral deltoid moment arms were significantly larger in the RTSA group at the initial elevation of the arm. The posterior deltoid was significantly larger at maximum elevation. There was large intersubject variability within the RTSA group.
  
  
  CONCLUSIONS
  Placement of implant components during RTSA can directly affect the geometric relationship between the humerus and scapula and the muscle moment arms in the RTSA shoulder. RTSA shoulders maintain the same anterior and posterior deltoid muscle moment-arm patterns as healthy shoulders but show much greater intersubject variation and larger moment-arm magnitudes. These observations provide a basis for determining optimal implant configuration and surgical placement to maximize RTSA function in a patient-specific manner.
authors.affiliation: Department of Mechanical Engineering, University of Florida, Gainesville, FL, USA. Electronic address: walkerdr1986@gmail.com.
authors.forename: David R
authors.initials: DR
authors.lastname: Walker
authors.affiliation: Department of Orthopaedic Surgery, University of Florida, Gainesville, FL, USA.
authors.forename: Aimee M
authors.initials: AM
authors.lastname: Struk
authors.affiliation: Shoulder and Elbow Surgery Center, Funabashi Orthopedic Hospital, Chiba, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Matsuki
authors.affiliation: Orthopaedics and Sports Medicine Institute, University of Florida, Gainesville, FL, USA.
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: D001132:Arm / Q000503:physiopathology; D019643:Arthroplasty, Replacement; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D057645:Deltoid Muscle / Q000503:physiopathology* / Q000601:surgery; D005471:Fluoroscopy; D006801:Humans; D006811:Humerus / Q000503:physiopathology; D021621:Imaging, Three-Dimensional; D016059:Range of Motion, Articular / Q000502:physiology; D012540:Scapula / Q000503:physiopathology; D012782:Shoulder / Q000503:physiopathology* / Q000601:surgery; D012785:Shoulder Joint / Q000601:surgery*; D014057:Tomography, X-Ray Computed
keywords: Reverse; arthroplasty; computational; musculoskeletal; shoulder