Material and methods

The inclusion criteria were as follows: (1) patients who were treated with multibracket appliance, (2) patients whose intraoral scan and CBCT scan at the pre- and midtreatment stages were available (which were acquired for orthosurgical treatment), and (3) patients whose midtreatment intraoral scans were obtained in the WSS and LSS methods. The exclusion criteria were as follows: (1) patients treated with lingual brackets, (2) patients treated using orthodontic bands, (3) patients who underwent morphologic changes of the crown via interproximal reduction or enameloplasty during the treatment, and (4) patients who had restorative or prosthodontic treatment during the treatment.

As a result, 13 adult patients were retrospectively recruited in this study. From the recruited subjects, a total of 208 teeth (26 each of central incisors, canines, second premolars, and first molars in the maxillary and mandibular dentition, respectively) were tested.

A power analysis using G∗Power software (version 3.1.3; Franz Faul University, Kiel, Germany) determined that a sample size of 26 teeth per group would provide a power of 91% with an α of 0.05 to detect significantly different root axis of 2.5° (an effect size of 0.71), on the basis of the previous reports. ^{, ,}

This retrospective study was reviewed and approved by the Institutional Review Board of the Chonnam National University Dental Hospital in Gwangju, South Korea (approval no. CNUDH-2019-024).

The individual 3D tooth model was constructed by merging the pretreatment intraoral-scan crown and the CBCT-scan root ( Fig 2 ). The intraoral scan was performed using a TRIOS scanner (3Shape, Copenhagen, Denmark) and processed into stereolithography (STL) format using OrthoAnalyzer (3Shape) software. The CBCT scan (Alphard Vega; Asahi Roentgen, Kyoto, Japan; 80 kVp; 5 mA; 0.39-mm voxel size; scan time, 17 seconds; and field of view, 200 mm × 179 mm) was performed, and the digital imaging and communication in medicine files were exported to the Invivo5 software (Anatomage, San Jose, Calif) for 3D volume rendering. In the “medical design studio” module of the program, the individual tooth, including the root, was segmented using the “sculpt” function by a single operator (H.H.C) and exported as individual STL files for each tooth type. The intraoral-scan crown and the segmented CBCT-scan teeth were exported into the reverse engineering program (Rapidform; 3D Systems, Rock Hill, SC). Crown registration was then performed to superimpose the 2 3D images. Segmented CBCT-scan teeth of the maxillary and mandibular arches were registered with their corresponding intraoral-scan arches. Finally, the CBCT-scan crown was removed from the superimposed images, followed by the merging of the remaining intraoral-scan crown and the allocated CBCT-scan root. Thus, the final 3D tooth model consisting of the intraoral-scan crown and CBCT-scan root was constructed ( Fig 2 ).

Diagram of the study design. Individual 3D tooth models were constructed at the pretreatment stage. The root axis at the midtreatment stage was estimated using the 3D tooth models in the whole- and lingual-surface intraoral scans methods. The estimation error was evaluated by measuring differences of the mesiodistal angulation and buccolingual inclination between the estimated roots and the CBCT-scan roots.

The direct intraoral scan (TRIOS) was performed at the midtreatment stage by the WSS and LSS methods ( Fig 3 , A ). The WSS was started from the occlusal surface of the second molar to the contralateral second molar along the occlusal surface, followed by the lingual and buccal surfaces. The orthodontic brackets, which were bonded on the buccal surface, were included in the scanning scope. For the LSS, the same procedure was followed, from the occlusal to the lingual surface, but was completed without scanning the buccal surface. The maxillary and mandibular arch scans were exported as independent STL files (OrthoAnalyzer).

The procedure of estimating the root axis using 3D tooth models in the whole- and lingual-surface intraoral scan method, and reorientation of the estimated roots onto the gold standard CBCT. (A) Intraoral scan was performed at the midtreatment stage by the whole-surface scan and lingual-surface scan methods. (B) The individual 3D tooth models were superimposed onto the intraoral scans to estimate root axis. (C) The intraoral scan-crowns with their superimposed 3D tooth models were assembled into single entity. (D) The assembled entity was reoriented onto the gold standard CBCT by crown registration.

The individual 3D tooth models were superimposed 1 by 1 onto the midtreatment intraoral scans obtained by the WSS and LSS methods ( Fig 3 , B ). Regional superimposition was performed, which commands the 3D tooth models to superimpose onto the intraoral scan using a defined region as a reference. When superimposing the 3D tooth models onto the WSS image, the whole crown surface except the bracket area was employed as reference. In contrast, only the lingual and occlusal surfaces were employed when superimposing onto the LSS image. After that, the maxillary and mandibular intraoral-scan crowns with their superimposed 3D tooth models were assembled as a single entity in each arch ( Fig 3 , C ).

Midtreatment CBCT scan was used as a gold standard to determine the real root axis ( Fig 2 ), which was obtained on the same day as the midtreatment intraoral scans. The individual tooth, including the root, was segmented by the same operator (H.H.C) who processed the pretreatment CBCT. All the segmented teeth in each maxillary and mandibular arch were assembled as a single entity maintaining their original position. The midtreatment intraoral scan, attached with the 3D tooth models, was reoriented in accordance with the gold standard CBCT by best-fit crown registration ( Fig 3 , D ). The accuracy of the crown registration was validated by measuring shell/shell deviation, which measures the absolute values of the 3D Euclidean distances between the surface points on the 2 images. The mean shell/shell deviations were 0.20 mm in the maxillary arch and 0.21 mm in the mandibular arch ( Table I ). Because the registration errors were less than the clinically acceptable value of 0.22 mm, the crowns of the midtreatment intraoral scan and the gold standard CBCT were considered to be in the same position, and subsequent measurements were conducted.

The mesiodistal angulation and buccolingual inclination were measured in the estimated root axis by the WSS and LSS methods, and the root axis of the gold standard CBCT ( Fig 4 ). First, the occlusal plane was constructed by connecting the mesiobuccal cusp tips of the right and maxillary left first molars and the midpoint of the maxillary right central incisor edge. Second, the root axis plane of each tooth type was constructed by connecting 3 points: 1 point in the root apex and 2 points in the crown ( Table II ). The distal angle between the occlusal plane and the root axis plane was measured for the mesiodistal angulation, and the lingual angle between the occlusal plane and the root axis plane was measured for the buccolingual inclination ( Fig 4 ). In each subject, once the occlusal plane was constructed, the same occlusal plane was used for the measurement of the estimated and CBCT root axis angle.

Measurements of the mesiodistal angulation and buccolingual inclination. The mesiodistal angulation and buccolingual inclination were measured at the distal and lingual angles in conjunction with the occlusal plane and the root axis plane, respectively. The rectangular planes represent the occlusal plane and the root axis plane.

One investigator (S.W.L) performed all measurements of the 13 subjects. To test intra- and interobserver reliability of the measurements, the measurement including the construction of the occlusal and the root axis plane was repeated by 2 investigators (S.W.L and H.H.C) with an interval of 3-weeks after the initial measurement. The intraclass correlation coefficient was calculated to test the intra- and interobserver reliability.

Results

The intraclass correlation coefficient values of mesiodistal angulation and buccolingual inclination ranged from 0.883 to 0.988, indicating good reproducibility of measurements in both intra- and interobserver reliability ( Table III ).

Table III

ICC values of the measurements for inter- and intraobserver reliability

Tooth type (n = 52 per each tooth type)	Interobserver ICC		Intraobserver ICC
	Mesiodistal angulation	Buccolingual inclination	Mesiodistal angulation	Buccolingual inclination
Central incisor	0.892	0.914	0.927	0.983
Canine	0.883	0.901	0.946	0.968
Second premolar	0.901	0.883	0.988	0.982
First molar	0.887	0.879	0.987	0.981

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