Accuracy of a surface-based fusion method when integrating digital models and cone-beam CT scans with metal artifacts


This retrospective study was reviewed and approved by the Institutional Review Board of Bundang Hospital, Seoul National University (B-1911/576-105) and the Institutional Review Board of Columbia University -British (H19-03765). A total of 20 class III skeletal adult patients (7 males and 13 females; age, 21.7 ± 4.0 years) were included, who were treated with single or double jaw orthognathic surgery at Bundang Hospital in Seoul National University from March 2016 to October 2019. All patients were selected according to the following inclusion criteria: (1) ANB

Image fusion procedure and measurement

All patients underwent CBCT (KODAK 9500, Carestream Health Inc., Rochester, NY, USA), which was obtained with a field of view of 200 mm × 180 mm, a voxel size of 0.2 mm, and exposure conditions of 80 kVp, 15 mA and 10.8 s. During CBCT scans, patients were asked to maintain an upright position. Their heads were positioned in which Frankfurt’s horizontal planes were parallel to the ground and stabilized by the ear rods. They were asked to keep their teeth in maximum intercuspation. All CBCT scans were saved as DICOM (Digital Imaging and Communications in Medicine) data files. DICOM data was converted to stereolithographic format, oriented and reconstructed using Geomagic software (Geomagic Qualify 2013®, 3D Systems, Morrisville, NC, USA) following reference plans. The horizontal plane (axial plane; X-plane) is the plane passing through the nasion, which is parallel to the Frankfurt horizontal plane (FH) passing through the left and right orbitals and the right part. The midsagittal plane (Y plane) is the plane passing through the nasion and the basion, while being perpendicular to the X plane. The coronal plane (Z plane) is perpendicular to the horizontal and midsagittal planes, defining the plane passing through the nasion (zero point; 0, 0 and 0) (Fig. 2).

In addition, simultaneously with CBCT acquisition for each patient, a conventional impression was taken with alginate (Aroma fine plus normal set, GC Corporation, Tokyo, Japan) to fabricate maxillary and mandibular dental casts. To produce digital models, surface images of the maxillary and mandibular casts and their maximum intercuspation were digitized in standard tessellation language (STL) format using a desktop model scanner (MD-ID0300, Medit Co, Seoul, Korea).

The digital molded images of the entire maxillary dentition were merged with the dental parts of the reconstructed CBCT images using Geomagic software. First, a point-based registration was performed using the cusp tips of the canines and the mesiobuccal cusps of the maxillary first molars in both images. Then, for more accurate integration, surface-based registration was performed (Fig. 3). The tips of the cusps or the occluso-buccal surfaces of the teeth above the bracket and the lingual surfaces of the teeth as the registration area were used with the best fit algorithm26.28. This procedure was performed twice two weeks apart by 2 examiners (a digital engineer and an orthodontist [J.-H.K.]).

picture 3

Workflow for studying the integration of digital maxillary models in CBCT scans. (a) Reconstructed CBCT image, (b) digital cast image, (vs) process of integrating maxillary digital molded images into dental parts of reconstructed CBCT images, (D) built-in skeletal models.

The 3D coordinate values ​​(x, y and z) of the cusps of the canines, the mesiobuccal cusps of the first molars and the contact points between the maxillary central incisors were measured in the coordinate system, which is constructed by X – , Y- and Z-plane through the nasion (zero point; 0, 0 and 0). The differences in the x, y or z coordinates of each tooth between two repeated fusions, measured by the 2 examiners, were evaluated13. In addition, intraclass correlation coefficients (ICC) were calculated to determine the intra- and inter-examiner reliability of the measurements of the 3D positions of the maxillary dentition after fusion by a digital engineer and an orthodontist, and between the 2 examiners.

statistical analyzes

Power analysis with correlation ρ H1 = 0.77, α = 0.05 and power (1 − β) = 0.80 showed a sample size requirement of 10 (G*Power v. Heinrich Heine University, Düsseldorf, Germany)29.

All statistical data were analyzed with SPSS software (Version 22.0, IBM, Armonk, NY, USA). Paired t-tests, Wilcoxon signed rank tests, ICC tests, and Bland-Altman analyzes were performed to assess differences and reproducibility between 3D positions (3D coordinates) of the maxillary dentition taken twice by each of the two examiners. Intra-rater reliability and inter-rater reliability were assessed using ICC as follows: ICC > 0.8/0.6/0.4/0.2 or ≤ 0.2 represents near perfect, substantial, moderate, poor or weak agreement, respectively.30. In addition, the significance level was set at P

Ethics approval

All procedures performed in studies involving human participants were reviewed and approved by the Institutional Review Board and in accordance with the 1964 Declaration of Helsinki and subsequent amendments or comparable ethical standards.

Informed consent

For this type of non-interventional retrospective clinical study, the Institutional Review Board of Bundang Hospital, Seoul National University and the Institutional Review Board of the University of British Columbia waived informed consent. .


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