The purpose of this study was to comparatively evaluate the effects of Twin-block (TB) appliance and sagittal-guidance Twin-block (SGTB) appliance on alveolar bone around mandibular incisors in growing patients with Class II Division 1 malocclusion, using cone-beam computed tomography.
The sample consisted of 25 growing patients with Class II Division 1 malocclusion (14 boys and 11 girls, mean age 11.92 ± 1.62 years) and was randomly distributed into the TB group (n = 13) and the SGTB group (n = 12). The treatment duration was 11.56 ± 1.73 months. Pretreatment (T1) and posttreatment (T2) cone-beam computed tomography scans were taken in both groups. Height, thickness at apex level, and volume of the alveolar bone around mandibular left central incisors were measured respectively on labial and lingual side, using Mimics software (version 19.0; Materialise, Leuven, Belgium). Based on the stable structures, 3-dimensional (3D) registrations of T1 and T2 models were taken to measure the sagittal displacement of incisors. Intragroup comparisons were evaluated by paired-samples t tests and Wilcoxon tests. Independent-samples t tests and Mann-Whitney U tests were used for intergroup comparisons.
In both groups, alveolar bone height and volume on the labial side of the incisors significantly decreased after treatment ( P <0.05). Lingual alveolar bone height, lingual and total alveolar bone volume, labial, lingual and total alveolar bone thickness showed no significant difference between T1 and T2 ( P >0.05). In both groups the incisors tipped labially and drifted to the labial side. Compared with the TB group, less labial alveolar bone loss, less incisor proclination and crown edge drift were found in the SGTB group ( P <0.05).
Labial alveolar bone loss around mandibular incisors was observed after both types of appliances treatment in growing patients with Class II Division 1 malocclusion. Less labial alveolar bone loss, less incisor proclination, and crown edge drift were found in the SGTB group than in the TB group during treatment.
Twin-block (TB) and sagittal-guidance Twin-block (SGTB) appliances caused labial alveolar bone loss around mandibular incisors.
Less labial alveolar bone loss was found in the SGTB group than that in TB group.
The incisors drifted and tipped labially after the TB and SGTB treatment.
Less incisor proclination and crown edge drift were found in the SGTB group.
The Twin-block (TB) appliance is a well-accepted functional appliance in correcting Class II Division 1 malocclusion with mandibular retrognathia. In the course of treatment, the patients’ mandibles are guided forward by the inclined plane of the bite-blocks. Condylar growth is stimulated, midfacial complex growth is restricted, and the growth pattern is modified, , thus patients could achieve favorable dentofacial relationships. , However, undesirable labial tipping of mandibular incisors was observed after TB appliance treatment, and it can cause periodontal consequences. Furthermore, the inherently thinner layer of bony support around mandibular incisors is more liable to sustain iatrogenic damage. The alveolar bone in the mandibular anterior region is crucial to the stability of incisors, periodontal health, and acceptable esthetics, therefore, it is an essential consideration during treatment.
To eliminate the potential side effects (ie, incisor proclination, and periodontal risks) of the TB appliance treatment, numerous types of modified TB appliance have appeared. Gill et al reported a progressively activated TB appliance with the mandible forward 3 mm every time until a normal overjet was achieved. Nevertheless, the authors concluded no significant difference in mandibular incisor proclination, which corresponds to the conclusions of De Vincenzo et al. Van der Plas et al demonstrated that the TB appliance with acrylic capping does not have a significant inhibition on mandibular incisor flaring.
The sagittal-guidance Twin-block (SGTB) appliance has been used as a modified TB appliance in China in recent years. As a type of modified appliance, the indication of the SGTB appliance was the same as the classic TB appliance, but constructive modifications were done : the SGTB appliance had maxillary component bonded to the maxillary posterior teeth. It guaranteed quicker adaptation and more orthopedic outcomes. The 2 brackets embedded into the upper occlusal planes allowed the fixed appliance treatment to be conducted simultaneously and result in shortened treatment time. The mandibular component consisted of extra clasps placed upon the mandibular first permanent molars, which produced better retention. The researchers claimed mild proclination of the mandibular incisors in the case report. So we assume that by using the TB or SGTB appliance, the alveolar bone around mandibular incisors might be affected differently, which has not been quantitatively evaluated so far.
Cone-beam computed tomography (CBCT) has been shown to have acceptable validity and reliability, not only in alveolar bone linear measurements, but also in volumetric measurements. , It has been used widely in alveolar bone assessment. The purpose of this study was to comparatively evaluate the effects of TB appliance and SGTB appliance on the alveolar bone around mandibular incisors in growing patients with Class II Division 1 malocclusion, using CBCT. The null hypothesis was that the 2 types of appliances cause similar effects on alveolar bone around mandibular incisors.
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 ).
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 .
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.
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 .
|Reference points and lines||Definition|
|3||Labial alveolar crest|
|4||Lingual alveolar crest|
|6||A line perpendicular to the long axis at the root apex|
|7||A line parallel to the long axis at the labial alveolar crest|
|8||A line parallel to the long axis at the lingual alveolar crest|
|9||The point of intersection of the line perpendicular to the axis of the incisor, with the labial contour of the symphysis|
|10||The point of intersection of the line perpendicular to the axis of the incisor, with the lingual contour of the symphysis|
|11||The line connecting point 2 and point 9|
|12||The line connecting point 2 and point 10|
|Tooth length||The length of line 5|
|LABH||Labial alveolar bone height: the length of line 7|
|LIABH||Lingual alveolar bone height: the length of line 8|
|LABT||Labial alveolar bone thickness: the length of line 11|
|LIABT||Lingual alveolar bone thickness: the length of line 12|
|TABT||Total alveolar bone thickness: the length of line 6|
|LV||Labial surrounding alveolar bone volume|
|LIV||Lingual surrounding alveolar bone volume|
|TV||Total surrounding alveolar bone volume|
Statistical analysis was performed using SPSS (version 21.0; IBM) software package. All measured variables were described by the mean and the standard deviation. Shapiro-Wilk test was used to assess normal distribution. For normal distribution data, the intragroup comparisons were analyzed using paired-samples t test, and independent-samples t test was used for intergroup comparisons. In case of abnormal distribution, Wilcoxon and Mann-Whitney U tests were used for intragroup and intergroup comparisons, respectively. A P value <0.05 was considered to be statistically significant.
The intraexaminer reliability was determined by performing the measurements for each CBCT image on 2 separate occasions by one examiner at a 2-week interval. The intraclass correlation coefficients were calculated, then the mean of the 2 measurements was used in statistical analysis. The errors of measurements were calculated using the Dahlberg formula : measurement error = √ Σ d 2 /2n (where d indicates deviations between the 2 measurements, and n indicates number of paired objects).
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 .
|T1||T2||P value||T1||T2||P value|
|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 †|