Samples collection
1076 CBCT images taken from January 2013 to April 2018 were collected from Department of oral and maxillofacial radiology, Peking University School and Hospital of Stomatology. Inclusion and exclusion criteria are shown in Table 1. The sample included 675 females and 401 males (6.0 to 21.0 years old) and CBCT images were taken as regular examination for different purposes of diagnosis.
9 patients were chosen from April 2019 to September 2019, from Department of orthodontics, Peking University School and Hospital of Stomatology. The sample included 5 females and 4 males (9.7-13.8 years old). Treatment plans included traditional tooth-borne RME as initial step. Inclusion and exclusion criteria are shown in Table 1.
Biomedical Ethics Committee of Peking University School and Hospital of Stomatology has approved this study (Protocol No: PKUSSIRB-20180739147).
Morphological stage of midpalatal suture and cervical stage
CBCT images were taken with NewTom VGi (Quantitative Radiology, Verona, Italy), at 2.81 mA, 110 kV, 3.6-second exposure, 15*15 cm field of view, with axial slice thickness of 0.3 mm, and isotropic voxels. Patients sat upright with natural head position and jaws immobilized using a chin holder, keeping Frankfort plane horizontal to ground. Teeth were occluded at intercuspal position, with facial muscles relaxed.
Head orientation: Firstly in horizontal view, put anterior nasal spine (ANS)-posterior nasal spine (PNS) line as axis line. Then in coronal view, put septum line as axis line. In this way, sagittal orientation is already identified (Figure 1).
Observation plane: (1) Subjects with medial thickness of hard palate, observe the central plane of hard palate from upper to lower cortical bone lamella. (2) For subjects with obviously thicker hard palate, observe upper 1/3 and lower 1/3 planes of full thickness instead only the central plane [13]. (3) For subjects with overarching palatal vault, observe central plane of anterior 1/2 and posterior 1/2 instead only one central plane.
CVM stage was recorded according to CVM method by Baccetti et al [10, 11].
Dental casts measurements
9 patients took CBCT before traditional tooth-borne RME (T1) to record their MPS stage. Then maxillary dental cast was obtained and later Hyrax-type RME was delivered with 1-2 turns a day (0.25 mm per turn) until the orthodontist decided to stop activating (Figure 2). Maxillary dental cast was obtained immediately after active expansion ended (T2).
Dental casts were scanned with 3Shape R700 3D scanner (3Shape, Copenhagen, Denmark), saved as three-dimensional digital files (.STL format). Digital casts were then measured with 3D Slicer 4.10.2 digital imaging system (open source: https://www.slicer.org/). Accuracy of digital cast size from original plaster cast is no larger than 20 micrometers (Mitutoyo measurer, Mitutoyo Corp., Tokyo, Japan).
Diagram of digital dental cast, landmarks identification and measurements are shown in Figure 3. Lingual point on the maxillary first molar is located at the junction of the lingual groove with the palatal mucosa. Arch width at first molar level is defined as the distance between the lingual points of bilateral maxillary first molars. Arch width at second premolar, first premolar and canine level are also defined in this way [4]. Angulation of maxillary first molars defined by measuring the angle formed by the intersection lines passing through the mesio-buccal and mesio-lingual cusps. Angulation less than 180° indicates the possibility of molars’ buccal tipping, while larger than 180° implies the possibility of molars’ lingual tipping [4].
When analyzing expansion effects, total effect and average effect per activation were calculated.
Average effect of per activation = total effect
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Total activation times
Statistical analysis
SPSS 19.0 (IBM Corp., Armonk, New York, USA) was used for Spearman correlation analysis.
Python 3.6 software (Mac, Continuum Analytics 5.3.0,Anaconda Inc. New York, USA) was used for diagnostic test analysis. Diagnostic efficacy indexes include sensitivity, specificity, positive likelihood ratio (PLR) and area under ROC curve (AUC). PLR >10 or <0.1 indicates significantly better diagnostic efficacy. AUC reflects comprehensive performance of sensitivity and specificity [18].