Measurement of the Dentin Wall Thickness of the Maxillary Central Incisor in Relation to the Stage of Root Development: A Pilot Study.

Objective: The aim of this study was to determine the average dentin wall thickness (DWT) of the maxillary central incisor (MCI) required for performing finite element analysis (FEA) models of root development. Material and methods: A total of 137 intraoral periapical radiographs of MCI in children aged 7 to 11 years were examined and then classified into 5 groups according to root development stages, which included 1/2 of root development (S1), 3/4 of root development (S2), more than 3/4 of root development (S3), complete development with wide-open apex (S4) and complete development with closed apex (S5). DWT was measured at three reference (horizontal) lines: at a distance of 1 mm from the apex (M), 4 mm from the apex (L) and at the cervical line (K). The distal dentin wall thickness (M1, L1, and K1), the pulp thickness (M2, L2, and K2), the mesial dentin wall thickness (M3, L3, and K3), and the apex thickness (N) were measured using the diagnostic software Soredex Scanora 5.1.2.4. Statistical analysis compared the values of the parameters K, L, and M between developmental stages (multivariate ANOVA) and the linear correlations between the parameters (Pearson's correlation analysis). All analyses were performed at significance level α = 0.05. Results: There were statistically significant differences between the developmental stages for parameters L and M, while no significant differences were found for parameter K. Most of the correlations between the parameters were statistically significant, with the values of the Pearson correlation coefficient R > 0.6 considered practically significant. All parameters on the same reference line for distal and mesial dentin wall thickness and for pulp thickness correlated well with each other (R = 0.46 - 0.68), but there was no statistically significant correlation with total root thickness on the same reference line (parameters K, L, or M), except for parameter K3 (R = 0.42). Conclusion: Despite the limitations of this study, the mean values of the selected parameters for the 5 groups of developmental stages of the maxillary central incisor could be used to model dentin wall thickness using finite element analysis.

Finite element model of load adaptive remodelling induced by orthodontic forces.

Many hypotheses have been formulated to explain orthodontic tooth movement. However, none of them can satisfactorily explain how small static orthodontic forces can induce bone remodelling. Our hypothesis assumes that small orthodontic forces do not lead to bone remodelling response directly, but rather indirectly by offsetting the tooth, leading to changes in bone loading during chewing that far exceed the changes caused by the orthodontic force alone. We developed a finite element model of a tooth with the periodontal ligament and the surrounding bone, and calculated the changes in strain energy density caused by the chewing force alone, the orthodontic force alone, and the combined action of the chewing and orthodontic forces. The results (average strain energy density 0.0005-0.01 kPa) demonstrate that the orthodontic loading alone does not produce strain energy density values in a range that is expected to induce remodelling (0.016-1.6 kPa). However, when the chewing and orthodontic forces are applied together, the highest values for the average strain energy density (0.02-0.99 kPa) as well as large changes in the bone tissue strain energy density (31.19-166.65%) are obtained. We conclude that the proposed hypothesis can indeed explain the observed bone remodelling that agrees with Wolff's law.