Aim 1: Evaluation of mandibular anterior alveolar bone before orthodontic treatment
Committee on Human Research (CHR) approval was obtained from the Institutional Committee on Human Research (IRB 10-00564). Pretreatment CBCT images of 75 non-growing individuals, 25 in three groups: low-angle (sella-nasion to mandibular plane (SN-MP) ≤28°), average-angle (30°–37°), and high-angle (≥39°), were analyzed. Consent to use individual CBCT data for research was obtained at the time of taking the CBCT. Buccolingual bone thickness was measured at the root apex, mid-root, and alveolar crest of the mandibular right central incisor. Inclusion criteria were non-growing, no orthodontic treatment before the initial CBCT scan, and no recorded craniofacial anomaly.
Using Anatomage Invivo5 CBCT software, sagittal slices were taken of the CBCT image through the middle of the root canal, at the midpoint of the long axis of the right central incisor (Fig. 1). From the sagittal slice of the center of the incisor, pretreatment measurements were taken for the buccolingual width at the apex and the alveolar crest (Fig. 2).
Reproducibility measurement
To test observer reliability, the measurements were repeated for ten subjects in each group, 1 month apart by the same investigator. The small difference in the measurements at the two observation times indicated reproducibility of the method.
Statistical analysis
The F test, or one-way analysis of variance (ANOVA), was used for statistical analysis. The means and standard deviations of each value were also calculated. When ANOVA was significant at 95 %, the Bonferroni correction was applied to verify where the statistically significant differences were correlated.
Aim 2: Evaluation of bony and tooth changes after orthodontic treatment
CHR approval (IRB 10-00564) covered this part of the study as well. Fifty-eight subjects seen at the orthodontic clinic between the dates of January 2005 to July 2012, and verified by superimposition of initial and final cephalograms as non-growing, were randomly chosen and categorized into low, average, and high mandibular angle skeletal patterns based on the SN-MP using the same criteria as in the control group described in aim 1.
Using the Invivo5, Anatomage software, superimpositions of the pretreatment and posttreatment scans were completed using five landmarks under the Point Registration Module of the software: mental foramen (left and right), nasion, and orbitale (left and right). The two volumetric images were then adjusted by the investigator to make the two images superimpose on the cranial base after using the Point Registration Superimposition Module. The superimpositions were then verified on the axial, sagittal, and coronal planes. The buccolingual bony changes were assessed at the apex, mid-root, and alveolar crest, and the root lengths before and after treatment were compared (Fig. 3) by first evaluating the pretreatment scan and then toggling to the posttreatment scan. It has been reported in the literature that tooth movement will on average cause a 10 % shortening of the roots [13–15]. In this study, any root resorption above 10 % was considered to be out of the norm and, thus, a negative sequela.
Intra-observer reliability
Thirty CBCT scans (ten from each skeletal pattern group) were measured 1 month apart to test reproducibility of measurements by the same observer. The average difference in the root length measurement between the two time points was 0.254 mm. The average difference in the thickness at the apex between the two time points was 0.21367 mm, and the average difference between the thicknesses from crest to crest was 2.24 mm.
The root length measurement was very reproducible. This reproducibility spans across SN-MP (increase in SN-MP does not change the reproducibility of the measurement), even though the alveolar morphology, as indicated in the findings of aim 1, changes. The alveolar bone thickness at the apex of the lower incisor can also be measured reliably by the same observer at two different time points. The total buccolingual thickness at the alveolar crest was not as reliable as the root length and width at the apex. This is probably due to the difficulty of locating the alveolar crest on the CBCT when the crest is thin.