Material and methods

This research was accepted by the Research Ethics Committee of Shandong University Dental School (Protocol No. 20170702). Before participating in the study, all the patients and their legal guardians were notified of potential risks and provided written informed consent. The study was conducted according to the tenets of the Declaration of Helsinki for research involving human subjects.

The power calculation was performed in order to recruit the smallest sample size that would allow meaningful statistical analysis. It was calculated based on an α of 0.05 and a β of 0.2 to achieve the power of 80% and to detect the difference of 1 mm in mandibular alveolar bone linear height measurements between groups, with a 0.98-mm estimated standard deviation. The power analysis indicated a sample size of 11.8 in each group.

Sample selection was based on the following inclusion criteria: (1) the overjet was 6 to 7 mm with an Angle Class II molar relationship; (2) SNB angle was <75°, and ANB angle was >4°; (3) mandibular plane angle was <28°; (4) the crowding of the lower anterior arches were <4 mm. Exclusion criteria included arch spacing, tooth size anomaly, periodontal disease, and abnormal bone metabolism. A sample of 26 patients was randomly allocated to the TB or SGTB group. Patients were numbered according to the order of their first visit. Then the numbers of the patients were randomly allocated into 2 groups through the random numbers generated in SPSS (version 21.0; IBM, Armonk, NY). The allocation was performed by a staff member who was not involved in the trial. It was concealed from the participants and treating clinicians. After randomized allocation, the TB group comprised 13 patients (7 boys and 6 girls; mean age, 11.83 ± 1.17 years). The SGTB group comprised 13 subjects initially, but only 12 were evaluated in the final sample (7 boys and 5 girls; mean age, 12.00 ± 2.10 years).

In both groups, patients were told to wear appliances for 24 hours a day and see the doctor after 1 month for supervision and adjustments. The appliance would not be removed until a stable mandibular forward position was obtained. The treatment duration was 11.56 ± 1.73 months. After the TB or SGTB appliances were removed, the fixed appliance treatment began.

The TB appliance had the following basic components: the maxillary appliance incorporated a bite-block, a labial bow, a midline screw, 2 delta clasps placed upon the bilateral first permanent molars, 2 ball clasps placed between the premolars; the mandibular appliance incorporated a bite-block, 2 delta clasps placed upon the first premolars, 2 ball clasps placed between the central and lateral incisors. The maxillary and mandibular bite-blocks interlocked at a 70° angle with the mandible positioned forward, the maxillary and mandibular incisors were edge-to-edge with a 2-mm vertical separation.

Comparing with the TB appliance, there were several modifications of the SGTB appliance: (1) the appliance was semifixed with the maxillary component bonded to the maxillary teeth; (2) there was no labial bow on the maxillary component. Two brackets were embedded into the buccal facade of the upper occlusal planes; (3) there were 2 Adams clasps on the bilateral mandibular permanent first molars with lingual acrylic pads extending posteriorly. Two extra ball clasps were placed between the mandibular lateral incisors and canines bilaterally ( Fig 1 ).

A, A maxillary component of the TB appliance; B, mandibular component of the TB appliance; C, the mandible was guided forward by the TB appliance; D, maxillary component of the SGTB appliance; E, mandibular component of the SGTB appliance; F, the mandible was guided forward by the SGTB appliance.

Before treatment (T1) and immediately after the removal of the appliances (T2), CBCT images were acquired by the same researcher using the CBCT scanner (NewTom 5G, QR srl, Verona, Italy) at these settings: 5mA, 110 kV, exposure time of 10 seconds, voxel size of 0.30 mm, and the slice thickness of 0.3 mm. Then they were exported in the DICOM (digital imaging and communications in medicine) format.

All the CBCT images were transferred into Mimics software (version 19.0; Materialise, Leuven, Belgium). Primarily, thresholding based on Hounsfield Units was used to create the original mandibular masks (401 HU-2347 HU), the teeth masks (1420 HU-3580 HU), and the alveolar bone masks (148 HU-1988 HU). Secondly, they were precisely separated from neighboring tissues through the mask segment tools in Mimics, such as edit masks , and region growing . Thirdly, 3D virtual models of the mandible, the teeth, and the alveolar bone were reconstructed from their masks respectively. Finally, the surrounding alveolar bone models were separated from the alveolar bone models and were split as labial and lingual 1 through 5 cutting planes, which are described in Figure 2 .

The cutting plane 1 was created sagittally and divided the incisor equally. The cutting planes 2 and 3 were set parallelly at 2 mm bilateral to the cutting plane 1; the cutting plane 4 was set perpendicularly to the cutting plane 1 at the root apex. The cutting planes 2, 3, and 4 were created to separate the surrounding alveolar bone model from the alveolar bone model. The cutting plane 5 was set to cut the incisor coronally and equally and thus split the alveolar bone as labial and lingual ones. A, frontal view of the models with the 1, 2, 3, and 4 cutting planes are visible; B, lateral view of the models with the 4 and 5 cutting planes are visible; C, occlusal view of the models with the 1, 2, 3, and 5 cutting planes are visible.

In both groups, the mandible, the teeth, and the alveolar bone models of T2 were exported as stereolithography (STL) and imported into T1 CBCT data. Point registrations of the mandible were done by placing 4 pairs of landmark points (bilateral mandibular foramina, mental trigone, and genial tubercle) on T1 and T2 models; then, the T2 points could be moved to the specific locations on T1 points ( Figs. 3 , A and B ). Afterward, STL registrations were performed to improve accuracy, the minimal point distance filter was set as 0.10 mm ( Figs. 3 , C and D ). After registration, the stable structures during growth were checked to be precisely overlapped ( Figs. 3 , E – G ). Through the entire registration process, the teeth and the alveolar bone models were moved along with the mandibular models of T2, and the contours of them could be visualized respectively after registration.

A, frontal view in the point registration process: point P01 and P02 are on the bilateral mandibular foramina of the T1 mandibular model; point P01′ and P02′ are on the mandibular foramina of the T2 mandibular model. Point P03 and P03′ are on the mental trigone of the T1 and the T2 mandibular models respectively; B, posterior view in the point registration process: Point P04 and Point 04′ are on the genial tubercle of T1 and T2 mandibular models respectively; C, the T1 ( green ) and the T2 ( red ) mandibular models after point registration and before STL registration; D, the T1 ( green ) and the T2 ( red ) mandibular models after STL registration. E, F, G, contours of the mandibular models at coronal, transverse, and sagittal views of CBCT image. The stable structures during growth (contour of the chin below pogonion and inner contour of the cortical plate at the lower border of the symphysis) were precisely overlapped.

The measurements were performed by a single-blinded examiner. Sagittal slices where the mandibular left incisor was labio-lingually widest were chosen for alveolar bone linear measurements in all CBCT images. Using the tooth axis as a reference for measuring bone height and thickness has been used in previous studies, ^{, ,} and the surrounding alveolar bone is of clinical significance. Reference points, lines, and measurement variables are described in Tables I and II and Figure 4 , A . Since the thickness of the alveolar bone around mandibular incisors are inherently small at the midroot and crest level, the linear measurements tend to be underestimated. Therefore, we took the thickness measurements only at apex level and added the surrounding alveolar bone volume as measured variables. The volume of the surrounding labial and lingual alveolar bone models can be shown directly in Mimics ( Fig 4 , B ). The total surrounding alveolar bone volume was calculated by adding the labial and lingual volume together. After registration, the sagittal displacement of incisors were measured, which are described in Figures 4 , C and D .

A, reference points and lines used in linear measurements; B, the volume of the surrounding labial and lingual alveolar bone models can be shown directly by using the tool “3D Properties” in Mimics; C, drift distance of (1) the crown edge and (2) the root apex was measured perpendicular to the true vertical line; D, (3) angle of long axis.

Results

Of the 26 Chinese adolescents who were involved in this study, the TB group comprised 13 patients (7 boys and 6 girls, 11 in primary school and 2 in junior high school; mean age, 11.83 ± 1.17 years). The SGTB group comprised 13 subjects initially, but only 12 were evaluated in the final sample (7 boys and 5 girls, 9 in primary school and 4 in junior high school, mean age 12.00 ± 2.10 years) with a girl dropped out because of personal reasons (ie, she failed to complete the trail because she moved to another city). Both groups had similar ages and sex distribution at baseline. The cephalometric measurement of mandibular incisor proclination were (97.2 ± 8.7)° in the TB group and (97.3 ± 7.3)° in the SGTB group, with no significant difference between the groups. The baseline measured characteristics were similar and were reported as T1 data in Table III .

Table III

The measured variables of alveolar bone in the TB group and the SGTB group

Measurements	TB					SGTB
	T1		T2		P value	T1		T2		P value
	Mean	SD	Mean	SD		Mean	SD	Mean	SD
Tooth length	20.00	1.02	19.96	1.11	0.71 ∗	19.94	0.93	19.84	1.32	0.41 ∗
LABH	9.12	0.64	8.32	0.90	0.001 ∗ , ‡	9.98	0.67	9.67	0.68	0.01 ∗ , ‡
LIABH	8.65	0.57	8.02	1.13	0.06 ∗	8.91	0.91	8.87	1.35	0.91 ∗
LABT	4.02	0.68	4.13	1.02	0.63 ∗	3.79	1.02	3.63	1.16	0.4 ∗
LIABT	4.11	0.34	4.18	0.64	0.47 ∗	4.54	1.10	4.77	0.96	0.1 ∗
TABT	8.12	0.71	8.17	0.81	0.83 ∗	8.66	1.29	8.73	1.31	0.74 ∗
LV	108.88	31.93	96.34	35.10	0.009 † , ‡	133.86	46.56	126.98	51.40	0.028 † , ‡
LIV	127.91	26.11	134.23	21.81	0.36 ∗	199.13	51.38	200.98	51.09	0.64 †
TV	236.79	55.87	230.57	46.72	0.48 †	325.86	57.62	320.55	83.13	0.81 †

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