title: Inclination-dependent changes of the critical shoulder angle significantly influence superior glenohumeral joint stability.
abstract: BACKGROUND
  The critical shoulder angle combines the acromion index and glenoid inclination and has potential to discriminate between shoulders at risk for rotator cuff tear or osteoarthritis and those that are asymptomatic. However, its biomechanics, and particularly the role of the glenoid inclination, are not yet fully understood.
  
  
  METHODS
  A shoulder simulator was used to analyze the independent influence of glenoid inclination during abduction from 0 to 60°. Spindle motors transferred tension forces by a cable-pulley on human cadaveric humeri. A six-degree-of-freedom force transducer was mounted directly behind the polyethylene glenoid to measure shear and compressive joint reaction force and calculate the instability ratio (ratio of shear and compressive joint reaction force) with the different force ratios of the deltoid and supraspinatus muscles (2:1 and 1:1). A stepwise change in the inclination by 5° increments allowed simulation of a critical shoulder angle range of 20° to 45°.
  
  
  RESULTS
  Tilting the glenoid to cranial (increasing the critical shoulder angle) increases the shear joint reaction force and therefore the instability ratio. A balanced force ratio (1:1) between the deltoid and the supraspinatus allowed larger critical shoulder angles before cranial subluxation occurred than did the deltoid-dominant ratio (2:1).
  
  
  CONCLUSIONS
  Glenoid inclination-dependent changes of the critical shoulder angle have a significant impact on superior glenohumeral joint stability. The increased compensatory activity of the rotator cuff to keep the humeral head centered may lead to mechanical overload and could explain the clinically observed association between large angles and degenerative rotator cuff tears.
authors.affiliation: Department of Orthopaedic Surgery and Traumatology, University of Berne, Insel Hospital, Switzerland. Electronic address: bmoor@gmx.ch.
authors.forename: B K
authors.initials: BK
authors.lastname: Moor
authors.affiliation: School of Engineering, Institute of Mechanical Systems, ZHAW Zurich University of Applied Sciences, Switzerland.
authors.forename: R
authors.initials: R
authors.lastname: Kuster
authors.affiliation: Division of Trauma Surgery, Zurich University Hospital, Zurich, Switzerland.
authors.forename: G
authors.initials: G
authors.lastname: Osterhoff
authors.affiliation: School of Engineering, Institute of Mechanical Systems, ZHAW Zurich University of Applied Sciences, Switzerland.
authors.forename: D
authors.initials: D
authors.lastname: Baumgartner
authors.affiliation: Division of Trauma Surgery, Zurich University Hospital, Zurich, Switzerland.
authors.forename: C M L
authors.initials: CM
authors.lastname: Werner
authors.affiliation: Department of Orthopaedic Surgery and Traumatology, University of Berne, Insel Hospital, Switzerland.
authors.forename: M A
authors.initials: MA
authors.lastname: Zumstein
authors.affiliation: Division of Trauma Surgery, Zurich University Hospital, Zurich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Bouaicha
mesh_terms: D002102:Cadaver; D019245:Compressive Strength / Q000502:physiology; D005260:Female; D006801:Humans; D058430:Humeral Head / Q000502:physiology; D007593:Joint Instability / Q000503:physiopathology*; D008953:Models, Anatomic; D018482:Muscle, Skeletal / Q000502:physiology; D017006:Rotator Cuff / Q000502:physiology; D033081:Shear Strength / Q000502:physiology; D012785:Shoulder Joint / Q000503:physiopathology*; D020069:Shoulder Pain / Q000503:physiopathology