Midsagittal Plane First: Building a Strong Facial Reference Frame for Computer-Aided Surgical Simulation

ABSTRACT Purpose A facial reference frame is a 3-dimensional Cartesian coordinate system that includes 3 perpendicular planes: midsagittal, axial and coronal. The order in which one defines the planes matters. The purposes of this study were to determine: 1) what sequence (axial-midsagittal-coronal versus midsagittal-axial-coronal) produced more appropriate reference frames, and 2) whether orbital or auricular dystopia influenced the outcomes. Materials and Methods This was an ambispective cross-sectional study. Fifty-four subjects with facial asymmetry were included. The facial reference frames of each subject (outcome variable) were constructed using 2 methods (independent variable): axial-plane-first and midsagittal plane-first. Two board-certified orthodontists together blindly evaluated the results using a 3-point categorical scale based on their careful inspection and expert intuition. The covariant for stratification was the existence of orbital or auricular dystopia. Finally, Wilcoxon signed rank tests were performed. Results The facial reference frames defined by the midsagittal plane-first method was statistically significantly different from ones defined by the axial-plane-first method (P=0.001). Using midsagittal plane-first method, the reference frames were more appropriately defined in 22 (40.7%) subjects, equivalent in 26 (48.1%) and less appropriately defined in 6 (11.1%). After stratified by orbital or auricular dystopia, the results also showed that the reference frame computed using midsagittal plane-first method was statistically significantly more appropriate in both subject groups regardless of the existence of orbital or auricular dystopia (27 with orbital or auricular dystopia and 27 without, both P Conclusion The midsagittal plane-first sequence improves the facial reference frames compared with the traditional axial-plane-first approach. However, regardless of the sequence used, clinicians need to judge the correctness of the reference frame prior to diagnosis or surgical planning.