Defining hard and soft tissue asymmetry using three dimensional CBCT analysis

• Main Author: Reason, Marisa
• Other Authors: Briss, Davis
• Language: en_US
• Published: 2018
• Subjects: Dentistry
• Online Access: https://hdl.handle.net/2144/32947

INTRODUCTION: Asymmetry is a common occurrence in the craniofacial bones of humans and as Orthodontists, facial esthetics are a major concern in our daily practice1. However defining asymmetry often relies on a subjective perception or an index that is established using 2-dimensional photos2. To this day, most of the studies provide an arbitrary number to define a subject as asymmetric, but to the best of our knowledge, no study has analyzed subject's hard and soft tissue to find out where asymmetries originate in 3-dimensions. OBJECTIVES: The aims of the study are to 1) establish appropriate, reproducible soft tissue landmarks and their bony counterparts in CBCT images 2) evaluate the correlation between the skeletal and soft tissue landmarks 3) use the measurements to objectively define asymmetry. MATERIALS AND METHODS: Cone Beam Computed Tomography (CBCT) images of 60 human subjects seeking or undergoing orthodontic treatment (mean age=19.8 ± 11.6) were selected from a CBCT repository. The DICOM files were imported into InVivoDental5.3 software (AnatomageTM; San Jose, Calif.) for screening. Hard and soft tissue masks were created on all scanned images. Two groups were created using a symmetry index adapted from the method prescribed by Grammer and Thornhill2: symmetric (n=48, mean age 19.68) and asymmetric (n=12, mean age=19.92). 10 hard and soft tissue landmarks were identified on each scan in reference to established mid-sagittal, nasion-horizontal, and coronal planes. Linear measurements to the reference planes were recorded giving all landmarks an x-, y-, and z- coordinate. Differences in means and standard deviations of the symmetric and asymmetric groups were done using student t-tests or Wilcoxon rank sum test with a 5% significance level. Spearman correlations tests were done between hard and soft tissue landmarks in the symmetric and asymmetric groups respectively. RESULTS: The differences in the mean linear distances between the symmetric and the asymmetric groups in 14 hard and soft tissue points to their respective planes were found to be statistically significant: These points included Pog, Pog', Gn, Gn', GoRL, GoRL'. Spearman correlation test showed that the r-values for 15 hard and soft tissue pairs were statistically significant. CONCLUSION: Statistical significant differences exist in the linear measurements between hard and soft tissue points when comparing symmetric and asymmetric subjects. When we begin to compare the three planes, we see that significant bony asymmetries exist that are not visible to the human eye in 2-dimensions. Therefore, to define asymmetry a 3-dimensional analysis is needed to view where hard and soft tissue asymmetries originate.