• G Y Song, G Li, W H Lu, B Han, and T M Xu.
• Department of Orthodontics, Peking University School and Hospital of Stomatology, National Engineering Laboratory for Digital and Material Technology of Stomatology, Beijing Key Laboratory of Digital Stomatology, Beijing, P. R. China.
• Niger J Clin Pract. 2019 Dec 1; 22 (12): 1644-1653.

ObjectivesTo assess the validity of using the calibration ruler for correcting magnification of linear measurements and to explore and compare the vertical and horizontal magnification of four digital cephalometric units.MethodsAn acrylic box was imaged at seven sagittal positions using four digital cephalometric units: Orthopantomograph OC100, Orthopantomograph OC200, Sirona Orthophos CD, and Sirona Orthophos DS. The true linear lengths of the phantom, corrected, and uncorrected linear lengths on the images were measured and compared. The validity of measurements using the calibration ruler was assessed. The magnification values and distortion indices were calculated and compared among the four cephalometric units.ResultsFor linear measurements on the mid-sagittal plane and averaged linear measurements on bilateral symmetric sagittal planes, the bias 1.96 STD of the calibration ruler ranged from 1% to 2% for the four cephalometric testing units. For linear measurements on the single lateral sagittal plane, the bias 1.96 STD ranged from 3% to 6%. The vertical scanning charge-coupled device cephalometric unit produced the greatest distortion, ranging from 1.029 to 0.964.ConclusionThe metal millimeter calibration ruler is an accurate reference for linear measurement magnification correction. Because of unpredictability and machine specificity, the magnification and distortion of a cephalometric unit should be calibrated for the estimation of cephalometric measurement error.

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