Method: Three-dimensional stereophotogrammetric images of 182 unrelated adults (118F/64M) were soft-tissue landmarked using 3dMD Software and configurations were translated to the origin via the landmark subnasale. The left configuration was reflected over the X-Y plane and the Euclidean distance between each pair of 9 bilateral landmarks was calculated to measure landmark asymmetry without rotational standardization. To compute the standardized asymmetry scores, the landmarks nasion, subnasale, and sublabiale were chosen to define the mid facial plane of the translated configurations. Standardization of the configurations to the X-Y plane at Z=0 and the X-Z plane at Y=0 utilized iterative methods of rotation to determine optimal vertical position of the mid-facial plane. Similar reflections and calculations of the Euclidean distance were performed for the standardized data. Standardized and non-standardized landmark asymmetry scores were examined for differences using the Wilcoxon Signed Rank test to determine the impact of standardization on the estimate of asymmetry.
Result: Significant differences existed between the unstandardized and the standardized asymmetry scores for endocanthion (Wilcoxon p<0.0001), exocanthion (p=0.0286), subalare (p=0.0383), crista philtri (p<0.0001), and chelion (p=0.0407). For these landmarks, the estimate of asymmetry was significantly larger in the unstandardized data compared to the standardized data.
Conclusion: Insufficient rotational standardization of facial landmark data may result in an inaccurate estimate of bilateral landmark asymmetry, particularly in landmarks close to the mid-facial plane.
Keywords: Asymmetry, Growth & development, Human and Methodology