title: Effects of the humeral tray component positioning for onlay reverse shoulder arthroplasty design: a biomechanical analysis.
abstract: BACKGROUND
  Recent shoulder prostheses have introduced a concept of a universal humeral stem component platform that has an onlay humeral tray for the reverse total shoulder arthroplasty (RTSA). No studies have reported how humeral tray positioning can affect the biomechanics of RTSA.
  
  
  METHODS
  The Newcastle Shoulder Model was used to investigate the biomechanical effect of humeral tray positioning in the Biomet Comprehensive Total Shoulder System (Biomet, Warsaw, IN, USA) RTSA. Five humeral tray configuration positions were tested: no offset, and 5 mm offset in the anterior, posterior, medial, and lateral positions. Superior and inferior impingement were evaluated for abduction, scapular plane elevation, forward flexion, and external/internal rotation with the elbow at the side (adduction) and at 90° of shoulder abduction. Muscle lengths and moment arms (elevating and rotational) were calculated for the deltoid, the infraspinatus, the teres minor, and the subscapularis.
  
  
  RESULTS
  Inferior impingement was not affected by the humeral tray position. There was less superior impingement during abduction, scapular plane elevation, and rotation with the shoulder when the tray was placed laterally or posteriorly. The subscapularis rotational moment arm was increased with a posterior offset, whereas infraspinatus and teres minor rotational moment arms were increased with an anterior offset. Very little change was observed for the deltoid elevating moment arm or for its muscle length.
  
  
  CONCLUSIONS
  Positioning the humeral tray with posterior offset offers a biomechanical advantage for patients needing RTSA by decreasing superior impingement and increasing the internal rotational moment arm of the subscapularis, without creating inferior impingement.
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA; The Leon Root, M.D. Motion Analysis Laboratory, Hospital For Special Surgery, New York, NY, USA. Electronic address: julien.berhouet@gmail.com.
authors.forename: Julien
authors.initials: J
authors.lastname: Berhouet
authors.affiliation: The Leon Root, M.D. Motion Analysis Laboratory, Hospital For Special Surgery, New York, NY, USA.
authors.forename: Andreas
authors.initials: A
authors.lastname: Kontaxis
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA.
authors.forename: Lawrence V
authors.initials: LV
authors.lastname: Gulotta
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA.
authors.forename: Edward
authors.initials: E
authors.lastname: Craig
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA.
authors.forename: Russel
authors.initials: R
authors.lastname: Warren
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA.
authors.forename: Joshua
authors.initials: J
authors.lastname: Dines
authors.affiliation: Department of Sports Medicine and Shoulder, Hospital For Special Surgery, New York, NY, USA.
authors.forename: David
authors.initials: D
authors.lastname: Dines
mesh_terms: D019643:Arthroplasty, Replacement / Q000295:instrumentation / Q000379:methods*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D057645:Deltoid Muscle; D006801:Humans; D006811:Humerus / Q000601:surgery; D007595:Joint Prosthesis; D016059:Range of Motion, Articular; D012399:Rotation; D017006:Rotator Cuff; D012785:Shoulder Joint / Q000601:surgery*
keywords: Reverse shoulder arthroplasty; humeral tray component; impingement; muscles length; muscles moment arm; onlay design