Dental Inclination and Thickness of The Alveolar Bone Around Incisors and Molars According to the Vertical Facial Type

Introduction: The teeth are incline differently according to the vertical skeletal typology and the corresponding alveolar bones do not always have the same thickness. However, the conflicting results in the literature, mainly related to the classification of patients according to vertical characteristics, creates a challenge for the clinician in determining the characteristic inclinations of teeth in a specific patient. The objective of this study was to find out dental inclination and thickness of the alveolar bone around incisors and molars in different facial type. Materials & Method: The sample included 51 CBCT radiographs (28 females and 23 males) and subdivided into three subgroups according to vertical facial skeletal pattern (22 normodivergent subjects, 06 hypodivergent subjects and 23 hyperdivergent subjects). Reformatted CBCT images were used to measure the thickness of the vestibular and lingual alveolar bone around the incisors and first molars at two levels of root length (apical and middle) and their inclination. The Kruskal-Wallis test for the comparison of more than 2 means was applied for statistical analysis. Result: Alveolar bone thickness in the apical region of the lingual side in hyperdivergent subjects was thicker than in the other two groups. The bone wall is thicker in the lingual than in the vestibular, regardless of the vertical skeletal typology. These varied inclinations were more pronounced anterior sector. Conclusion: Dental inclinations have close relationships with the alveolar bone thicknesses associated with them. The apex of the root of the lower incisor is closer to the lingual alveolar crest when it was inclined in the lingual direction. There is a significant association between bone thickness around the teeth and facial skeletal divergence.


INTRODUCTION
The usual clinical results searched by orthodontic treatments are architectural harmonization of the jaws and their dental arches, resulting in a facial aesthetic modification. This aesthetic impact is essential for both the patient and the practitioner. But, in addition to aesthetic, periodontal health and the alveolar bone limits are important factors in orthodontic treatment.
Age, gender and ethnic group are also important in establishing an appropriate orthodontic treatment plan; another important factor is the facial growth pattern and its several clinical characteristics. 1 Like the sagittal jaw relationships, vertical growth affects the thickness of the supporting bone. 2 Patients with long face usually have less supporting bones, which reduces the allowed movement available for teeth before reaching the anatomical limits.
In addition to the thickness of the alveolar bone, dental inclinations must be appropriate to ensure functional and aesthetic occlusion. The inclination of the teeth remains one of the six keys to Andrew's normal occlusion. 3 It is important not only for occlusal intercuspation but also for the aesthetics of the frontal smile. 4 Studies have shown variable results with regard to the buccolingual inclination of teeth with respect to the type of vertical growth. [5][6][7] Some authors have found that posterior teeth in hypodivergent patients were more lingually inclined than in hyperdivergent subjects. 2 Masumoto et al 6 found in subjects with normal occlusion that hypodivergent subjects had second molars more lingually inclined than hyperdivergent subjects. Grosso et al, 7 who classified subjects only by vertical facial type, demonstrated that the maxillary and mandibular molars of the hyperdivergent group were lingually inclined.
The heterogeneity of the results in the literature, mainly related to the classification of patients according to sagittal or vertical characteristics, creates a challenge for the clinician in determining the characteristic inclinations of teeth in a specific patient. 8 However, the information provided by conventional radiographic cephalometry is limited by its twodimensional (2D) nature with inaccuracy due to the deformation of anatomical structures. To overcome this disadvantage, CBCT offers the possibility of accessing volumetric data, which allows the morphology of the alveolar-dental complex and the 3D orientation of the teeth to be evaluated.
Hence the interest of our study, the objective of which was to evaluate tooth inclinations and alveolar bone thickness in black African subjects with the CBCT according to vertical facial type.  Fifteen randomly selected subjects were re-measured by the same examiner (K.B.E.) after 2 weeks to verify intra-examination reproducibility. A paired t-test was performed to verify the existence of significant differences from the initial measurements.

MATERIALS AND METHOD
The data were expressed as a mean and standard deviation. The Levene test was used to determine the equality of variances between samples. Finally, the Kruskal-Wallis non-parametric test was used. We considered a statistical level of signification of 5%.

RESULT
The sample included 51 untreated adult subjects, including 23 males (mean age 28.63 years ± 11.99 years) and 28 females (mean age 29.72 years ± 11.47 years) who met the inclusion criteria. No statistically significant differences (p = 0.569) were observed between the ages of the two gender.
The teeth are tilted differently according to the vertical skeletal typology. The various inclinations were more pronounced in the anterior sector ( Table 2). The is no significant difference in dental inclination of the first molars according to the vertical facial profile. The bone wall is thicker in the lingual than in the vestibular, regardless of the vertical skeletal typology (Table 3).

DISCUSSION
According to the results of our study, the teeth are tilted differently according to the vertical skeletal typology and the corresponding alveolar bones do not always have the same thickness. The varied inclinations were more pronounced in the anterior sector. Indeed, hypodivergent subjects showed a greater vestibular inclination of the maxillary and mandibular incisors compared to normal or hyper-divergent subjects.
Comparison of the results of our study with those of previous studies should be done with caution due to differences in sample selection and measurement techniques used.
Like Ross et al, 5 we found no statistically significant difference in the inclination of the first molars between the 3 different growth profile subgroups.
The absence of statistically significant differences could be due either to the limitations of the facial index used, which may not have chosen very extreme face types, or to the relatively small number of subjects in the horizontal pattern groups (n=6).
However, we found a greater tendency to lingual inclination in hyperdivergent subjects compared to horizontally growing subjects. This trend was observed by Grosso et al. 7 These authors had small groups for both short and long face types. Perhaps these represented the extremes of each type, and therefore a difference was highlighted.
The anterior area is more demanding in terms of aesthetics and treatment according to bone availability. 10 In any case, the lack of optimal bone availability around the teeth can be responsible for recurrence after orthodontic treatment. The 110.13° ± 9.76 inclination of the interincisal angle of our sample was much smaller than the interincisal angle of 131° reported in the literature. 11 This can be explained by the fact that  the subjects in our sample were all black Africans and therefore in bimaxillary dentoalveolar biprotrusion.
Using CBCT, the second finding of this study is that, in general, the bone wall is thicker in the lingual than in the vestibular, regardless of the vertical skeletal typology. This observation was relayed by Lee et al 12 in 2010.
However, these results are different from those reported in studies by Horner et al, 13 which showed that vestibular cortical bone tends to be thicker in hypodivergent patients than in hyperdivergent patients. This discrepancy with the results of our study could be attributed to the small number of selected hypodivergent.
When dental orthodontic movements are planned, the position and inclination of the teeth, as well as the thickness of the alveolar bone, should be considered as "diagnosis" variables. 14 In our study, we observed that the thickness of the vestibular alveolar bone at the apex was thinner than at the lingual side in hyperdivergent patients. This result is consistent with Eraydin's. 15 In general, a thorough assessment of the alveolar bone thickness of the teeth is necessary to determine the limits of possible dental movement during orthodontic treatment. Dental displacement is the basis of orthodontic therapy. It is the result between the application of a force to a tooth and the strength of its supporting tissues. Knowledge of alveolar bone volume can help orthodontists to better control the forces applied to avoid iatrogenic bone loss and bone fenestration.

CONFLICTS OF INTEREST
No potential conflict of interest relevant to this article was reported.