Abstract: Objective: To research the shape of the first permanent crowns, and to quantify and compare the three-dimensional morphological differences between crowns. Methods: One hundred pairs of Chinese juvenile dental plaster models were 3D optical scanned, including fifty pairs of males and fifty pairs of females. Principal component analysis was performed to assess the shape parameters. With the help of thin plate spline (TPS), the anatomical significance of each principal component was explored. Results: The first five principal components of the first upper and lower permanent molars comprised almost 70-80% of the molar shape variations, and the first two most important principal components were related to the mesiodistal-buccolingual ratios and the relative cusp positions. The tooth size was significantly correlated (r²=0.703), and the male molars were larger than the female molars (2.68% and 1.66% for the upper and lower molars, respectively), but no shape dimorphism was observed. Conclusion: The shape variations of upper and lower permanent molars were related to mesiodistal-buccolingual ratios, heights between teeth tips and fossea, and relative cusp positions. Distal cusps of lower permanent molar showed the highest variability. Sexual dimorphism was limited to size difference and the shape variation was weak.

Key words: Geometric morphology, Procrustes distance, Dental crowns, Shape analysis