title: The role of pectoralis major and latissimus dorsi muscles in a biomechanical model of massive rotator cuff tear.
abstract: BACKGROUND
  Superior migration of the humeral head after massive rotator cuff tear (mRCT) is thought to lead to cuff tear arthropathy. Previous biomechanical studies have demonstrated the ability of the pectoralis major and latissimus dorsi (PM/LD) muscles to resist this migration. This study examined the role of PM/LD muscles on glenohumeral joint forces and acromiohumeral contact pressures in a mRCT model.
  
  
  METHODS
  Six cadaveric shoulders were tested using a custom shoulder-testing system. Muscle insertions of the rotator cuff, deltoid, and PM/LD were preserved and used for muscle loading. Specimens were tested in 3 different humeral rotation positions at 0° abduction and 2 rotation positions at 60° abduction. Testing was performed for intact specimens, after supraspinatus removal, and after supraspinatus/infraspinatus/teres minor removal. PM/LD were loaded or unloaded to determine their effect. Humeral head kinematics, glenohumeral joint forces, and acromiohumeral contact area and pressure were measured.
  
  
  RESULTS
  For the mRCT condition at 0° abduction, unloading the PM/LD resulted in superior shift of the humeral head. Acromiohumeral contact pressures were undetectable when the PM/LD were loaded but increased significantly after PM/LD unloading. After mRCT, superior joint forces were increased and compressive forces were decreased compared with intact; loading the PM/LD resolved these abnormal forces in some testing conditions.
  
  
  CONCLUSIONS
  In mRCT, the PM and LD muscles are effective in improving glenohumeral kinematics and reducing acromiohumeral pressures. Strengthening or neuromuscular training of this musculature, or both, may delay the progression to cuff tear arthropathy.
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA.
authors.forename: Sean T
authors.initials: ST
authors.lastname: Campbell
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA; Department of Orthopaedic Surgery, University of California, Irvine, Irvine, CA, USA.
authors.forename: Kier J
authors.initials: KJ
authors.lastname: Ecklund
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA.
authors.forename: Eileen H
authors.initials: EH
authors.lastname: Chu
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA.
authors.forename: Michelle H
authors.initials: MH
authors.lastname: McGarry
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA; Department of Orthopaedic Surgery, University of California, Irvine, Irvine, CA, USA.
authors.forename: Ranjan
authors.initials: R
authors.lastname: Gupta
authors.affiliation: Orthopaedic Biomechanics Laboratory, VA Healthcare System, Long Beach, CA, USA; Department of Orthopaedic Surgery, University of California, Irvine, Irvine, CA, USA. Electronic address: tqlee@med.va.gov.
authors.forename: Thay Q
authors.initials: TQ
authors.lastname: Lee
mesh_terms: D000174:Acromion / Q000503:physiopathology; D000368:Aged; D000369:Aged, 80 and over; D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D058430:Humeral Head / Q000503:physiopathology; D007592:Joint Diseases / Q000503:physiopathology*; D008297:Male; D010369:Pectoralis Muscles / Q000503:physiopathology* / Q000601:surgery; D017006:Rotator Cuff / Q000503:physiopathology; D000070636:Rotator Cuff Injuries; D012782:Shoulder / Q000503:physiopathology*; D000070599:Shoulder Injuries; D012785:Shoulder Joint / Q000503:physiopathology; D064171:Superficial Back Muscles / Q000503:physiopathology* / Q000601:surgery
keywords: Massive rotator cuff tear; acromiohumeral pressure; latissimus dorsi; pectoralis major; rotator cuff tear arthropathy