title: Linear and angular measurements of computer-generated models: are they accurate, valid, and reliable?
abstract: BACKGROUND
  Traditional advanced imaging modalities such as CT and MRI are limited in their ability to perform accurate linear distance and angular measurements regardless of anatomical orientation. The construction of 3D models has been used to perform anthropometric analyses as well as in the reconstruction of rapid prototypes. We hypothesized that such measurements would be precise to within 2 mm or 2 degrees of measurements performed with a coordinate measurement machine (CMM). We also hypothesized that there would be a high degree of interobserver reliability with these measurements.
  
  
  METHODS
  Multiple aluminum screws were implanted in various positions in three foam pelvises which were subsequently scanned by CT and rendered as 3D models using a commercially available software package (Mimics). Linear and angular measurements were performed using a CMM machine, the software package, and a dial caliper or goniometer. The deviation of the measurements from the CMM data was compared using ANOVA. The interobserver reliability of both the manual and computer-generated measurements was calculated.
  
  
  RESULTS
  The mean difference between the CMM distances and those measured manually and with the software was 2.12 +/- 1.20 mm and 1.57 +/- 1.05 mm, respectively. The mean difference between the CMM angular measurements and the angular measurements performed manually and with the software was 4.07 +/- 4.70 degrees and 1.62 +/- 1.32 degrees, respectively. In all cases, the manual measurements were significantly less accurate (p < 0.0001) and there was a high degree of interobserver reliability.
  
  
  CONCLUSIONS
  Computer-generated measurements taken from three-dimensionally reconstructed models are more accurate than manual measurements and are within 2 mm and 2 degrees of measurements performed with a CMM. These measurements have high interobserver reliability.
authors.affiliation: Department of Orthopaedic Surgery, University of California-Davis Medical Center, 4860 Y Street #3800, Sacramento, CA 95817, USA. amir.jamali@ucdmc.ucdavis.edu
authors.forename: Amir A
authors.initials: AA
authors.lastname: Jamali
authors.forename: Christopher
authors.initials: C
authors.lastname: Deuel
authors.forename: Aimee
authors.initials: A
authors.lastname: Perreira
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Salgado
authors.forename: John C
authors.initials: JC
authors.lastname: Hunter
authors.forename: E Bradley
authors.initials: EB
authors.lastname: Strong
mesh_terms: D000465:Algorithms; D006801:Humans; D021621:Imaging, Three-Dimensional; D008953:Models, Anatomic; D008954:Models, Biological; D015588:Observer Variation; D010387:Pelvimetry / Q000379:methods*; D015203:Reproducibility of Results; D014057:Tomography, X-Ray Computed