title: The effect of humeral version on teres minor muscle moment arm, length, and impingement in reverse shoulder arthroplasty during activities of daily living.
abstract: BACKGROUND
  External rotation can be compromised after reverse total shoulder arthroplasty (RTSA). A functional teres minor (TM) is relatively common in patients with posterosuperior tears of the rotator cuff, and its function should be enhanced for better postoperative results. The aim of this study was to investigate how the version of humeral fixation can affect the TM rotational moment arm and muscle length as well as impingement after RTSA.
  
  
  METHODS
  A 3-dimensional shoulder model was used to describe RTSA. Four humeral fixation versions were tested: +20°, 0°, -20°, and -40° (+, anteverted; -, retroverted). TM rotational moment arm and length as well as impingement-free range of motion were calculated for a set of 3 simple clinical motions: (1) scapula plane abduction (0°-150°); (2) internal/external rotation with the arm in adduction; and (3) internal/external rotation with the arm in abduction. Six common activities of daily living were also evaluated.
  
  
  RESULTS
  An anteverted fixation maximized TM moment arms, but it also resulted in very short muscle length (compared with normal) and increased inferior impingement. In contrast, 40° humeral retroversion resulted in the longest TM muscle length, but it also showed the smallest moment arms and increased anterior impingement in some of the activities of daily living.
  
  
  CONCLUSIONS
  Even if TM external rotation moment arm is higher in RTSA than in a normal shoulder, the decreased length could impair its force generation. The 0° and 20° retroversion was the optimum compromise between sufficient TM length and moment arm with minimum impingement.
authors.affiliation: Department of Orthopaedic and Trauma Surgery, Campus Bio-Medico University, Trigoria, Rome, Italy.
authors.forename: Alessandra
authors.initials: A
authors.lastname: Berton
authors.affiliation: Sports Medicine and Shoulder Service, Orthopedics Department, Hospital for Special Surgery, New York, NY, USA.
authors.forename: Lawrence V
authors.initials: LV
authors.lastname: Gulotta
authors.affiliation: Department of Orthopaedic and Trauma Surgery, Campus Bio-Medico University, Trigoria, Rome, Italy.
authors.forename: Stefano
authors.initials: S
authors.lastname: Petrillo
authors.affiliation: Department of Orthopaedic and Trauma Surgery, Campus Bio-Medico University, Trigoria, Rome, Italy.
authors.forename: Pino
authors.initials: P
authors.lastname: Florio
authors.affiliation: Department of Orthopaedic and Trauma Surgery, Campus Bio-Medico University, Trigoria, Rome, Italy.
authors.forename: Umile Giuseppe
authors.initials: UG
authors.lastname: Longo
authors.affiliation: Department of Orthopaedic and Trauma Surgery, Campus Bio-Medico University, Trigoria, Rome, Italy.
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Denaro
authors.affiliation: Leon Root Motion Analysis Laboratory, Rehabilitation Department, Hospital for Special Surgery, New York, NY, USA. Electronic address: KontaxisA@HSS.edu.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kontaxis
mesh_terms: D000203:Activities of Daily Living; D019643:Arthroplasty, Replacement / Q000379:methods*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D006801:Humans; D006811:Humerus / Q000601:surgery*; D016059:Range of Motion, Articular; D012399:Rotation; D017006:Rotator Cuff / Q000502:physiology*; D012540:Scapula; D012782:Shoulder / Q000601:surgery; D012785:Shoulder Joint / Q000601:surgery*
keywords: Reverse shoulder arthroplasty; external rotation; humeral version; moment arm; muscle length; teres minor