title: Cross-sectional geometry of the sacral ala for safe insertion of iliosacral lag screws: a computed tomography model.
abstract: OBJECTIVE
  To measure the dimensions of the narrowest portion of the sacral ala for safe insertion of iliosacral lag screws.
  
  
  METHODS
  Computed tomography (CT) model.
  
  
  METHODS
  Level One trauma center.
  
  
  METHODS
  Thirteen adult patients underwent pelvic CT imaging.
  
  
  METHODS
  Axial CT scans of intact pelves were reformatted in the sagittal plane at three-millimeter intervals from the first sacral body (S1 body) to the sacroiliac (SI) joint. Computer analysis and measurements of sacral geometry were used to determine the narrowest portion of the bony sacral ala. The maximum height, maximum width, and slope of the sacral ala through its geometric center in cross-section were measured.
  
  
  RESULTS
  The narrowest portion of the sacral ala in all patients was consistently located at the junction between the sacral body and the alar wings, termed the sacral pedicle, directly cephalad to the first sacral foramen. The average slope of the sacral ala at the sacral pedicle was 45.08 degrees (range 25 to 65 degrees). The average maximum height at the geometric center in cross-section was 27.76 millimeters, and the average width was 28.05 millimeters. However, outside the geometric center there was a sharp decrease in height and width of the sacral ala that was in large part determined by its relative slope.
  
  
  CONCLUSIONS
  Although the cross-sectional geometry of the sacral ala is highly variable among patients, there is ample space for iliosacral screws. To ensure safe insertion, iliosacral lag screws must be positioned in the geometric center of the sacral ala to avoid extraosseous placement.
authors.affiliation: Department of Orthopaedic Surgery, University of Louisville School of Medicine, Kentucky, USA.
authors.forename: F K
authors.initials: FK
authors.lastname: Noojin
authors.forename: A L
authors.initials: AL
authors.lastname: Malkani
authors.forename: L
authors.initials: L
authors.lastname: Haikal
authors.forename: C
authors.initials: C
authors.lastname: Lundquist
authors.forename: M J
authors.initials: MJ
authors.lastname: Voor
mesh_terms: D000328:Adult; D000368:Aged; D001863:Bone Screws; D003198:Computer Simulation; D005260:Female; D006801:Humans; D007085:Ilium; D008297:Male; D008875:Middle Aged; D012447:Sacrum / Q000033:anatomy & histology* / Q000000981:diagnostic imaging*; D014057:Tomography, X-Ray Computed