11h10 Novel Approach to Orbital Hard- and Soft- Tissue Symmetry Analysis: A Pilot Validation

Introduction: Facial asymmetry with minimal deviations from the midsagittal reference plane is often considered acceptable and natural. Despite orbital volume measurement has been successfully applied for orbital symmetry assessment it can hardly explain involvement of the particular orbital hard- and soft-tissue structures in facial asymmetry. Accurate and reproducible surface-based orbital hard- and soft-tissue symmetry analysis has not been previously reported. 

Aim of the study: To validate a novel approach to surface-based symmetry analysis of the orbital hard- and soft tissues based on the qualitative and quantitative evaluation.

Materials and methods: Head CT scans of eight healthy subjects were selected, and automated segmentation of the skull was performed using the CNN-based online cloud tool (Virtual Patient Creator, ReLu BV, Leuven, Belgium). Segmentation of the orbital soft-tissue structures was performed using semi-automated pixel intensity-based techniques followed by the final refinement, mirroring, and surface-based global registration until reaching the least point-to-point distance between true and mirrored models (Mimics Innovation Suite, Materialise N.V.,Leuven, Belgium). Morphological symmetry of the registered models was analyzed using color-coded deviation maps.

Results: There was no significant difference between global and local symmetry parameters for hard-tissue structures, and high correlation of the symmetry parameters of the entire bone volume and orbital walls was confirmed. Periorbital soft-tissue volume demonstrated significantly lower degree of symmetry comparing to intra-orbital soft-tissue structures. Correlation between the particular orbital walls symmetry was significantly higher than between soft-tissue structures reflecting the dynamic and independent behavior of the orbital soft-tissue components. Strong structural anatomical connections between the orbital walls can additionally support these findings. Strong inter-group negative correlations between some hard- and soft-tissue structures allow to hypothesize the potential mechanism of the intra- and periorbital soft tissue compensation for bone structures asymmetry.

Conclusion: The suggested novel approach to orbital hard- and soft-tissue symmetry analysis provides a valuable comprehensible information on differential involvement and contribution of particular anatomical structures to orbital and periorbital asymmetry, and demonstrates high accuracy, reliability and reproducibility.