title: Excessive glenohumeral horizontal abduction as occurs during the late cocking phase of the throwing motion can be critical for internal impingement.
abstract: BACKGROUND
  The objective of this study was to determine the effects of increased horizontal abduction with maximum external rotation, as occurs during the late cocking phase of throwing motion, on shoulder internal impingement.
  
  
  OBJECTIVE
  An increase in glenohumeral horizontal abduction will cause overlap of the rotator cuff insertion with respect to the glenoid and increase pressure between the supraspinatus and infraspinatus tendon insertions on the greater tuberosity and the glenoid.
  
  
  METHODS
  Controlled laboratory study.
  
  
  METHODS
  Eight cadaveric shoulders were tested with a custom shoulder testing system with the specimens in 60 degrees of glenohumeral abduction and maximum external rotation. The amount of internal impingement was evaluated by assessing the location of the supraspinatus and infraspinatus articular insertions on the greater tuberosity relative to the glenoid using a MicroScribe 3DLX. Pressure in the posterior-superior quadrant of the glenoid was measured using Fuji prescale film. Data were obtained with the humerus in the scapular plane and 15 degrees , 30 degrees , and 45 degrees of horizontal abduction from the scapular plane.
  
  
  RESULTS
  At 30 degrees and 45 degrees of horizontal abduction, the articular margin of the supraspinatus and infraspinatus tendons was anterior to the posterior edge of the glenoid and less than 2 mm from the glenoid rim in the lateral direction; the contact pressure was also greater than that found in the scapular plane and 15 degrees of horizontal abduction. Conclusion Horizontal abduction beyond the coronal plane increased the amount of overlap and contact pressure between the supraspinatus and infraspinatus tendons and glenoid.
  
  
  CONCLUSIONS
  Excessive glenohumeral horizontal abduction beyond the coronal plane may cause internal impingement, which may lead to rotator cuff tears and superior labral anterior to posterior (SLAP) lesions.
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, California, and the University of California, Irvine, USA.
authors.forename: Teruhisa
authors.initials: T
authors.lastname: Mihata
authors.forename: Michelle H
authors.initials: MH
authors.lastname: McGarry
authors.forename: Mitsuo
authors.initials: M
authors.lastname: Kinoshita
authors.forename: Thay Q
authors.initials: TQ
authors.lastname: Lee
mesh_terms: D000328:Adult; D001484:Baseball; D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D008297:Male; D008875:Middle Aged; D009068:Movement / Q000502:physiology; D019534:Shoulder Impingement Syndrome / Q000209:etiology*; D000070599:Shoulder Injuries; D012785:Shoulder Joint / Q000502:physiology