3D analysis of soft tissue variations in maxillofacial surgeries

In the last years, 3D technologies have acquired an increasingly central role in medical practice to improve diagnosis and imaging procedures. One of the fields in which 3D methodologies have found an important application is the support to maxillo-facial surgery where they seem to be an effective tool to plan more accurately operations and to predict the possible outcomes of the surgery, even the undesired ones. The aim of this thesis, carried out in collaboration with the maxillofacial department of the San Giovanni Battista Hospital in Turin, was to investigate the soft tissue variations in patients who underwent orthognathic surgeries and factors that influenced these variations. The study involved 17 patients who underwent Le Fort I, Bilateral Sagittal Split Osteotomy (BBSO) and Genioplasty surgeries. Their cone-beam computed tomography images were acquired before and after the surgery, obtaining, at the end of several elaboration steps of DICOM images, the pre- and post-operative 3D models of hard and soft tissue. The 3D models were imported in Matlab and twenty hard and soft tissue landmarks were placed manually, referring to the definitions given by Swennen. Subsequently, it was calculated the soft tissue thickness (STT), defined as the distance between the hard tissue landmarks and the corresponding on soft tissue. The first analysis regarded the estimate of STT post-operative variation focusing on identifying zones in which the STT changes were emphasised. It was also evaluated the difference between the STT of contralateral landmarks to examine if facial symmetry changes were significant after the surgery. Contextually, it was assessed the variation of the interalar distance, index of nose width modification. The statistically significant differences between pre- and post-operative data were assessed using the paired and the two-sample t-test (p< 0.05). Afterward, the correlation between the STT and BMI and sex was examined, since literature suggests they could influence facial STT. The correlation between BMI and STT was evaluated considering three subsets of landmarks that were supposed to differ in the quantity of adipose tissue stored in the zone. It was also investigated if there are some facial zones in which the STT is particularly related to sex. The statistical differences in the data were determined using the two-sample t-test or the Wilcoxon test (p< 0.05). To conclude this part, it was examined the simultaneous effect of sex and BMI on STT using the two-way ANOVA. The last part of the work regarded the description of the post-operative soft tissue movements using parameters or graphical tools. A vectorial map of soft tissue movement was calculated, describing each vector in spherical coordinates. For each patient, six statistical moments were calculated starting from the modulus and the direction of the vectors: mean, variance skewness, kurtosis, modified mean and modified skewness. A cross-correlation parameter was calculated to evaluate how the movements of the landmarks are related to each other. The thesis investigated deeply the 3D models database in terms of soft tissue variations, extracting information that could be useful for the development of an accurate system to predict the post-operative soft tissue variation and to describe, quantitatively, the success of the surgery in terms of facial appearance.