title: Navigated femoral anteversion measurements: general precision and registration options.
abstract: BACKGROUND
  Intraoperative reduction of femoral fractures can result in rotational malalignment. Navigation modules allow fluoroscopy-based femoral anteversion (AV) measurements; however, their clinical feasibility has not been fully evaluated. An important technical consideration when obtaining navigated femoral rotational alignment is the necessity for orthogonal navigated fluoroscopic images.
  
  
  METHODS
  The current investigators hypothesized that there would be a critical imaging angle between the fluoroscopic images which optimized the acquisition of accurate rotational measurements. Following initial testing in plastic femora, 14 intact human cadaveric femora were subjected to CT and navigated fluoroscopic assessment of AV. The navigated registration process included the following test series: Standard perpendicular AP and lateral imaging at a 90 degrees angle; planar rotation of the lateral fluoroscopy position perpendicular to the axis of femoral neck and angled to the AP position at 75 degrees, 60 degrees, and 50 degrees; inlet and outlet imaging of the axis of the femoral neck at angles of 10 degrees, 20 degrees, and 30 degrees. For all setups, the difference between the AV angle of the navigation system and the CT-measured angle was calculated.
  
  
  RESULTS
  Results revealed no relevant differences between CT and navigated determination of plastic femora (1.5 degrees). No significant deviations of cadaver femora with 90 degrees angle imaging (mean 2.00 degrees, range 0.00 degrees-4.33 degrees), 75 degrees angle imaging (mean 1.95 degrees, range 0.00 degrees-3.33 degrees) and 60 degrees imaging (mean 2.00 degrees, range 0.00 degrees-3.33 degrees) were found. However, significant deviations were found for the navigated 50 degrees angled imaging technique (mean 5.02 degrees, range 2.33 degrees-7.67 degrees).
  
  
  CONCLUSIONS
  These data demonstrated that the femoral AV angle can be calculated accurately by using two navigated fluoroscopic images when a minimum angle of 60 degrees is utilized between the angled fluoroscopic images for the registration process. A difference of <60 degrees does not allow for accurate navigated measurements.
authors.affiliation: Trauma Department, Hannover Medical School, Carl Neubergtsr. 1, 30625 Hannover, Germany.
authors.forename: Mustafa
authors.initials: M
authors.lastname: Citak
authors.forename: Daniel
authors.initials: D
authors.lastname: Kendoff
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Pearle
authors.forename: Padhraig F
authors.initials: PF
authors.lastname: O'Loughlin
authors.forename: Christian
authors.initials: C
authors.lastname: Krettek
authors.forename: Tobias
authors.initials: T
authors.lastname: Hüfner
authors.forename: Musa
authors.initials: M
authors.lastname: Citak
mesh_terms: D005269:Femur / Q000033:anatomy & histology* / Q000000981:diagnostic imaging*; D005272:Femur Neck / Q000000981:diagnostic imaging; D005471:Fluoroscopy; D006801:Humans; D007091:Image Processing, Computer-Assisted; D007432:Intraoperative Period; D012399:Rotation; D014057:Tomography, X-Ray Computed; D014102:Torsion Abnormality / Q000517:prevention & control