Multilevel principal components analysis of three-dimensional facial growth in adolescents

Abstract Background and objectives: The study of age-related facial shape changes across different populations and sexes requires new multivariate tools to disentangle different sources of variations present in 3D facial images. Here we wish to use a multivariate technique called multilevel principal components analysis (mPCA) to study three-dimensional facial growth in adolescents. Methods: These facial shapes were captured for Welsh and Finnish subjects (both male and female) at multiple ages from 12 to 17 years old (i.e., repeated-measures data). 1000 “dense” 3D points were defined regularly for each shape by using a deformable template via “meshmonk” software. A three-level model was used here, namely: level 1 (sex/ethnicity); level 2, all “subject” variations excluding sex, ethnicity, and age; and level 3, age. The technicalities underpinning the mPCA method are presented in Appendices. Results: Eigenvalues via mPCA predicted that: level 1 (ethnicity/sex) contained 7.9% of variation; level 2 contained 71.5%; and level 3 (age) contained 20.6%. The results for the eigenvalues via mPCA followed a similar pattern to those results of single-level PCA. Results for modes of variation made sense, where effects due to ethnicity, sex, and age were reflected in modes at appropriate levels of the model. Standardised scores at level 1 via mPCA showed much stronger differentiation between sex and ethnicity groups than results of single-level PCA. Results for standardised scores from both single-level PCA and mPCA at level 3 indicated that females had different average “trajectories” with respect to these scores than males, which suggests that facial shape matures in different ways for males and females. No strong evidence of differences in growth patterns between Finnish and Welsh subjects was observed. Conclusions: mPCA results agree with existing research relating to the general process of facial changes in adolescents with respect to age quoted in the literature. They support previous evidence that suggests that males demonstrate larger changes and for a longer period of time compared to females, especially in the lower third of the face. These calculations are therefore an excellent initial test that multivariate multilevel methods such as mPCA can be used to describe such age-related changes for “dense” 3D point data

A new method for the 3D measurement of postoperative swelling following orthognathic surgery

Objective: To describe a new method for measuring facial\ud swelling following orthognathic surgery using a 3D laser scanning device.\ud \ud Design: Prospective clinical trial.\ud \ud Setting and Sample Population: University Dental Hospital,\ud Wales College of Medicine, Biology Life and Health Sciences.\ud Three subjects requiring bi-maxillary orthognathic surgery were recruited for the study.\ud \ud Experimental Variables: Laser-scanned images of the\ud subjects were obtained under a reproducible and controlled\ud environment with two Minolta Vivid 900 (Osaka, Japan) optical laser-scanning devices assembled as a stereo-pair. A set of left and right scanned images was taken for each subject and each scan took an average of 2.5 s. 3D laser scans were recorded over six time periods (T1 – re-surgical scan, postoperatively: T2 – 1 day, T3 – 1 week, T4 – 1 month, T5 – 3 months and T6 – 6 months).\ud \ud Outcome Measure: Facial scans from different time periods\ud were overlaid onto the baseline (T6) facial scan to determine the reduction and changes in swelling following orthognathic surgery. \ud \ud Results: The results showed that swelling could be accurately quantified following surgery. Furthermore, there was a significant reduction in the amount of swelling 1 month postoperatively. Furthermore, the facial morphology returned to approximately 90% of the baseline facial scan at 3 months.\ud \ud Conclusion: The 3D laser-scanning device and the method\ud described was a reliable and accurate measure of facial\ud swelling following surgery