Statistical shape analysis-based determination of optimal midsagittal reference plane for evaluation of facial asymmetry

•We proposed a midsagittal reference plane to evaluate facial asymmetry.•The proposed plane is the landmark-based midsagittal reference plane.•Statistical shape analysis determines the landmarks constituting the plane.•The landmarks are nasion, anterior nasal spine, and posterior nasal spine.•This plane was compatible with a symmetric midsagittal plane.

IntroductionThe purpose of this study was to determine, by statistical shape analysis of original and mirrored skeletal landmarks, the optimal landmark-based midsagittal reference plane for evaluation of facial asymmetry.MethodsThe study sample comprised 69 patients with facial asymmetry (36 men, 33 women; mean age, 23.0 ± 4.1 years). All landmarks were obtained with cone-beam computed tomography using a 3-dimensional coordinate system. For identifying the landmark-based midsagittal reference plane, the 3 landmarks nearest to the symmetric midsagittal reference plane were selected by ordinary and generalized Procrustes analyses. To verify the 3-landmark-based midsagittal reference plane's compatibility with the symmetric midsagittal reference plane, asymmetry measurements were calculated and tested for each.ResultsThe 3 nearest landmarks (nasion, anterior nasal spine, and posterior nasal spine) were selected for the 3-landmark-based midsagittal reference plane. The averages of the sums of the squared Euclidean distance and the squared Procrustes distance differences between the 2 configurations and shapes fabricated by the symmetric and landmark-based midsagittal reference planes, respectively, were calculated as 0.121 ± 0.241 mm and 1.69 × 10−6 ± 3.25 × 10−6. The testing results for the symmetric and landmark-based midsagittal reference planes were almost the same.ConclusionThe results indicated that a 3-dimensional midsagittal reference plane constructed of nasion, anterior nasal spine, and posterior nasal spine could be a valuable tool for the evaluation of patients with facial asymmetry.