title: The influence of radiographic viewing perspective and demographics on the critical shoulder angle.
abstract: BACKGROUND
  Accurate assessment of the critical shoulder angle (CSA) is important in clinical evaluation of degenerative rotator cuff tears. This study analyzed the influence of radiographic viewing perspective on the CSA, developed a classification system to identify malpositioned radiographs, and assessed the relationship between the CSA and demographic factors.
  
  
  METHODS
  Glenoid height, width, and retroversion were measured on 3-dimensional computed tomography reconstructions of 68 cadaver scapulae. A digitally reconstructed radiograph was aligned perpendicular to the scapular plane, and retroversion was corrected to obtain a true anteroposterior (AP) view. In 10 scapulae, incremental anteversion/retroversion and flexion/extension views were generated. The CSA was measured, and a clinically applicable classification system was developed to detect views with >2° change in CSA vs. true AP view.
  
  
  RESULTS
  The average CSA was 33° ± 4°. Intraobserver and interobserver reliability was high (intraclass correlation coefficient ≥ 0.81) but decreased with increasing viewing angle. Views beyond 5° anteversion, 8° retroversion, 15° flexion, and 26° extension resulted in >2° deviation of the CSA compared with the true AP view. The classification system was capable of detecting aberrant viewing perspectives with sensitivity of 95% and specificity of 53%. Correlations between glenoid size and CSA were small (R ≤ 0.3), and CSA did not vary by gender (P = .426) or side (P = .821).
  
  
  CONCLUSIONS
  The CSA was most susceptible to malposition in anteversion/retroversion. Deviations as little as 5° in anteversion resulted in a CSA >2° from true AP view. A new classification system refines the ability to collect true AP radiographs of the scapula. The CSA was unaffected by demographic factors.
authors.affiliation: Department of Orthopaedics, Orthopaedic Research Laboratory, University of Utah, Salt Lake City, UT, USA; Department of Orthopaedic Surgery, Kantonsspital Baselland, Liestal, Switzerland.
authors.forename: Thomas
authors.initials: T
authors.lastname: Suter
authors.affiliation: Department of Orthopaedic Surgery, Kantonsspital Baselland, Liestal, Switzerland.
authors.forename: Ariane
authors.initials: A
authors.lastname: Gerber Popp
authors.affiliation: Division of Epidemiology, Department of Internal Medicine, University of Utah, Salt Lake City, UT, USA.
authors.forename: Yue
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Division of Epidemiology, Department of Internal Medicine, University of Utah, Salt Lake City, UT, USA.
authors.forename: Chong
authors.initials: C
authors.lastname: Zhang
authors.affiliation: Department of Orthopaedics, Orthopaedic Research Laboratory, University of Utah, Salt Lake City, UT, USA.
authors.forename: Robert Z
authors.initials: RZ
authors.lastname: Tashjian
authors.affiliation: Department of Orthopaedics, Orthopaedic Research Laboratory, University of Utah, Salt Lake City, UT, USA; Department of Bioengineering, Scientific Computing and Imaging Institute, University of Utah, Salt Lake City, UT, USA. Electronic address: heath.henninger@utah.edu.
authors.forename: Heath B
authors.initials: HB
authors.lastname: Henninger
mesh_terms: D000328:Adult; D000368:Aged; D002102:Cadaver; D003710:Demography; D005260:Female; D006801:Humans; D021621:Imaging, Three-Dimensional; D008297:Male; D008875:Middle Aged; D056888:Patient Positioning; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D012540:Scapula / Q000000981:diagnostic imaging; D012680:Sensitivity and Specificity; D012737:Sex Factors; D012785:Shoulder Joint / Q000000981:diagnostic imaging* / Q000601:surgery; D014057:Tomography, X-Ray Computed
keywords: Critical shoulder angle; computed tomography (CT); demographics; digitally reconstructed radiograph; reproducibility; viewing perspective