Análise da dimensão fractal em mandíbula: correlação entre tomografia computadorizada de feixe cônico, radiografia periapical digital e microtomografia computadorizada

Abstract

Fractal Dimension (FD) stands for the complexity level and occupation rate of a space by a fractal object. The aim of this study was to identify whether there is a correlation between the FD values obtained in X-ray images with different resolutions: Cone-Beam Computed Tomography (CBCT), Microtomography (μCT), and Digital Periapical Radiography (DPR). A prospective study was conducted including healthy individuals older than 18 years, who required bone manipulation for third molar extraction, and who had a CBCT exam prior to the surgery. A bone fragment was collected in the retromolar region, which was radiographed in digital Xray equipment. Subsequently, the fragments were processed and embedded in methyl methacrylate, then scanned in μCT. Regions of Interest (ROIs) were created separately in the cortical and trabecular parts for evaluation: 1) CBCT: 6 ROIs (25 x 25 pixels) in three parasagittal sections; 2) DPR: 2 ROIs (64 x 64 pixels); 3) μCT: 6 ROIs (90 x 90 pixels). Two processing methods were assessed for FD calculation: one through binarization (FDc) and the other through a method based on the use of grayscale images (FDg), using the ImageJ software. In these ROIs, in addition to the FD, data from the histogram regarding the standard deviation and mean values of the grayscale values for DPR and CBCT were also evaluated, and for μCT, microarchitectural parameters of the cortical bone and trabecular bone were assessed. Shapiro-Wilk normality test was performed, followed by Spearman or Pearson correlation tests (significance level of 5%). There was a negative correlation between FDc and FDg in the cortical part in the CBCT (p 0.01, r-0.61). Regarding μCT results, there was no correlation between FD and the parameters analyzed in the trabecular part, and in the cortical, there was a tendency for a negative correlation of FDg with the bone surface (p 0.05, r-0.48). Among the microarchitectural parameters, there was a significant correlation between bone volume (BV), being negative with bone surface (BS) and trabecular surface (Tb.Sp), and positive with the trabecular thickness (Tb.Th) for trabecular portion, and for cortical there was a strong and positive correlation of porosity with OS and pore numbers (P.n), and moderate and negative correlation with bone mineral density (BMD). In the DPR there was no correlation between FD and histogram data in the trabecular, and there was only in the DFg with the standard deviation of the pixel values (p 0.004, r-0.66). Among the microarchitectural parameters, there was a significant correlation between bone volume (BO), being negative with the bone surface (BS) and trabecular surface (Tb.Sp), and positive with the trabecular thickness (Tb.Th) for the trabecular portion; and for cortical there was a strong and positive correlation of porosity with OS and pore numbers (P.n), and moderate and negative correlation with bone mineral density (BMD). In the DPR there was no correlation between FD and histogram data in the trabecular, and there was only in the DFg with the standard deviation of the pixel values (p 0.004, r-0.66). In the CBCT in the trabecular part, FDc correlated with the mean pixel value (p 0.03, r0.74), and the FDg correlated with the standard deviation of the pixel values (p 0.03, r0.73). In conclusion, there was no correlation between the FD values in the three modalities evaluated. FDc value was related to the mean pixel’s value, while the FDg was related to the variation range of the pixel values in the grayscale.