The study sample was selected from a database comprising over 11,000 patients who had and completed orthodontic treatment between 1997 and 2005 at Peking University School and Hospital of Stomatology. All the records were anonymized and de-identified prior to analysis. This retrospective cephalometric study was approved by the Institutional Ethics Committee (NO.: PKUSSIRB-201626016).
The inclusion and exclusion criteria were as follows: (1) Han Chinese ethnicity; (2) fixed-appliance orthodontic treatment with extraction of 2 premolars in upper and/or lower arches,respectively; (3) mean distance of the lingual movements of point UIE and LIE were more than 3 mm in maxilla and mandible, respectively. (4) availability of the pre- and post-treatment lateral cephalograms which were of sufficient quality for identifying the relevant landmarks, all taken by the same X-ray machine; (5) lack of any significant medical history; (6) no craniofacial congenital malformation such as cleft lip and palate and syndromic disease; and (7) no need for orthognathic surgery.
Sample-size calculation was based on the significant results from a similar study by Lund et al., who studied alveolar bone changes after orthodontic treatment (the difference of means 0.2, and the difference of the standard deviations 0.9). PASS 2019 was used for sample size calculation, with with the established significance level and power of the test set at 0.05 and 80% respectively, the sample size would be 161.
In the maxilla, pre-treatment (T0) and post-treatment (T1) cephalograms of 477 individuals (217 class I and 260 class II/1, 337 females and 140 males) who met the selection criteria were included in this study. The patients ranged in age from 11 to 34 years, with an average age of 14.69 years.
In the mandible, 226 subjects (121 class I and 105 class II/1, 154 females and 72 males) aged from 11 to 35 years (mean age 15.03 years) qualified for the retrospective analysis.
The treatment protocol was standardized using an MBT (McLaughlin, Bennett, Trevisi) pre-adjusted appliance (Hangzhou Shinye Orthodontic Products; Hangzhou, China) with 0.022-inch slots. Maximum anchorage mechanics were planned for all patients and all patients were experienced space closure with sliding mechanics and light forces. Cephalograms were taken before treatment (T0) and immediately after treatment (T1).
To control for magnification, all lateral cephalograms were taken with the same cephalostat with the consistent object-film distance. After the cephalograms were scanned, cephalometric landmarks were located three times each by three senior residents who had undergone calibration training and were blinded to the study objectives. The points with higher dispersion were automatically detected by a customized software and were checked by the same resident. The average of the nine locations of each landmark was used in subsequent calculations by the customized cephalometric software. The cephalometric landmarks and reference planes are shown and explained in Fig 1 and Table 1.
The ABW of the labial, palatal/lingual and total alveolar crest were determined at the level of the center of resistance of the central incisors, which in this study was defined as a point located on the long axis of the tooth at a distance of 1/3 of the root length when measured from the alveolar crest (Fig. 2). UIR and LIR were used to represent the center of resistance of the central incisors in the maxilla and mandible respectively. A line passing through the center of resistance and parallel with the AC line (a line that connects the labial and palatal /lingual alveolar crest points) was used as the reference line (the observed level for alveolar bone width measurements). To ensure the consistency of the observed level, the distance between the AC line and the reference line on the pre-treatment cephalogram was recorded and then transferred to the post-treatment cephalogram. At this level, the labial (anterior), palatal/lingual (posterior), and total alveolar bone width was assessed in the maxilla (ABWL1, ABWP1, and ABWT1) and mandible (ABWL2, ABWP2, and ABWT2) at T0 and T1 respectively.
The ABH was measured as the vertical distance from both the labial and palatal/lingual side of the alveolar crest to the palatal plane in the maxilla (ABHL1 and ABHP1) and to the mandibular plane in the mandible (ABHL2 and ABHP2) (Fig.2).
The changes in the position of the upper incisor and point A were measured by superimposing the T0 and T1 lateral cephalograms on the palatal plane at the anterior nasal spine point (ANS) (Fig. 3a). On this superimposition, a horizontal line passing through the sella, parallel with the Frankfort plane, was drawn to form a horizontal reference line. A line perpendicular to the horizontal reference line, passing through the sella, was used as the vertical reference line. Three points on the most prominent upper central incisor- the incisal edge point (UIE), the center of resistance (UIR) and the apex of the root (UIA) were selected to be measured to reflect the position change of the upper incisor. The changes in the position of the lower central incisor and point B were measured by superimposing the T0 and T1 lateral cephalograms on the mandibular plane at the Gnathion (Fig. 3b). The antero-posterior and vertical changes in the position of the lower incisor and point B were determined using the same horizontal and vertical reference lines described above. LIE, LIR and LIA, the counterparts of UIE, UIR and UIA were used to reflect the position change of the lower incisor. Pre-treatment lateral cephalometric radiographs were traced with black lines, while post-treatment cephalograms were traced with gray lines.
All measurements were conducted by two trained examiners. The intraclass correlation (ICC) was 0.96. The average measurements were used for analysis. Since the data showed a normal distribution, t tests were used. The paired t-test was used to evaluate the bony changes resulted from incisor retraction. The one-sample t-test was used to evaluate the changes in the position of the incisor and points A and B. Pearson’s correlation analysis was used to analyze the correlations between the amount of incisor movement and the position changes of points A and B in the horizontal direction. The statistical analyses were performed with SPSS 27.0 (IBM, Armonk, NY), with a significance level of 0.05.