title: Changes in Pelvic Incidence, Pelvic Tilt, and Sacral Slope in Situations of Pelvic Rotation.
abstract: METHODS
  Digitally reconstructed radiograph-based study.
  
  
  OBJECTIVE
  Using a computer-based method to determine what degree of pelvic rotation is acceptable for measuring the pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS). The effectiveness of a geometrical formula used to calculate the angle of pelvic rotation proposed in a previous article was assessed.
  
  
  BACKGROUND
  It is unclear whether PI, PT, and SS are valid with pelvic rotation while acquiring a radiograph.
  
  
  METHODS
  Ten 3-dimensionally reconstructed models were established with software and placed in a neutral orientation to orient all of the bones in a standing position. Next, 140 digitally reconstructed radiographs were obtained by rotating the models around the longitudinal axis of each pelvis in the software from 0 to 30 degrees at 2.5-degree intervals. PI, PT, and SS were measured. The rotation angle was considered to be acceptable when the change in the measured angle (compared with the "correct" position) was <6 degrees. The rotation angle (α) on the images was calculated by a geometrical formula. Consistency between the measured value and the set angle was assessed.
  
  
  RESULTS
  The acceptable maximum angle of rotation for reliable measurements of PI was 17.5 degrees, and the changes in PT and SS were within an acceptable range (<6 degrees) when the pelvic rotation increased from 0 to 30 degrees. The effectiveness of the geometrical formula was shown by the consistency between the set and the calculated rotation angles of the pelvis (intraclass correlation coefficient=0.99).
  
  
  CONCLUSIONS
  Our study provides insight into the influence of pelvic rotation on the PI, PT, and SS. PI changes with pelvic rotation. The acceptable maximum angle for reliable values of PI, PT, and SS was 17.5 degrees, and the rotation angle of the pelvis on a lateral spinopelvic radiograph can be calculated reliably.
authors.affiliation: *Department of Orthopaedic Surgery †Medical Imaging Department, Second Affiliated Hospital of Wenzhou Medical University, Wenzhou, China.
authors.forename: Hai-Ming
authors.initials: HM
authors.lastname: Jin
authors.forename: Dao-Liang
authors.initials: DL
authors.lastname: Xu
authors.forename: Jun
authors.initials: J
authors.lastname: Xuan
authors.forename: Jiao-Xiang
authors.initials: JX
authors.lastname: Chen
authors.forename: Kai
authors.initials: K
authors.lastname: Chen
authors.forename: Amit
authors.initials: A
authors.lastname: Goswami
authors.forename: Yu
authors.initials: Y
authors.lastname: Chen
authors.forename: Qiu-Yan
authors.initials: QY
authors.lastname: Kong
authors.forename: Xiang-Yang
authors.initials: XY
authors.lastname: Wang
mesh_terms: D001696:Biomechanical Phenomena; D005269:Femur / Q000000981:diagnostic imaging; D006801:Humans; D021621:Imaging, Three-Dimensional; D010388:Pelvis / Q000000981:diagnostic imaging / Q000502:physiology*; D012399:Rotation; D012447:Sacrum / Q000000981:diagnostic imaging / Q000502:physiology*