Three-dimensional Geometric Morphology For The Classification Of Youth First Permanent Molar Crowns

[Objective]Understanding the morphological characteristics of teeth may help to improve computer-aided design processes.Previous studies have primarily used two-dimensional(2D)images,which provide limited information.This study used three-dimensional(3D)geometry to measure the shape of tooth crowns and to classify them according to the morphological characteristics of the crowns.[Methods]1.Three-dimensional optical scans were conducted on 100 pairs(50 male,50 female)of Chinese juvenile dental plaster models.A total of 225 and 246 morphometric landmarks and semilandmarks were placed on the upper and lower first permanent molar surfaces,respectively.MorphoJ and Morphologika 2 v2.5 software were used to perform a Generalized Procrustes Analysis(GPA)on the 3D landmark coordinates.After the model had been scaled,translated,and rotated,the 3D coordinates of all of the crowns were unified to obtain their mean shape.To assess the sizes of the crowns,the centroid sizes(CSs)were calculated.Differences in size by study group and by sex were measured using analysis of variance(two-way ANOVA).2.PCA was performed on the Procrustes coordinates.The analysis was conducted in shape space and in form shape space.The main PC was used to evaluate the shape variability and to determine the evaluation index.Geomagic Qualify software was used to measure and calculate the value of each evaluation index.In order to ensure the accuracy of the weight coefficient,all of the measurement data were normalized3.K-means clustering and UPGMA clustering methods were used to cluster the crowns respectively.Scatter plots of the principal components(PCs)of the shape variables were used to compare the clustering effects,and the optimal clustering was selected optimally.At the same time,based on the evaluation index of the main component,three-dimension measurement analysis was performed on each type of crown and the weight coefficients were calculated.The deviation chromatograms of the different types of crown average models were compared and analyzed to obtain the crown quantification standard[Results]1.Measurement error was estimated by the Procrustes distance between repeated digitizations.The analysis was performed separately for Ml and M2.The average error of the extents of the shape space over all 100 specimens was calculated at 4.1%(range:0.75%to 5.3%)for M1 and 4.4%(range:1.7%to 5.2%)for M22.The first five PCs contributed more than 50%of the shape variation.The largest variations in the shape of Ml and M2 were the size of the crown and the relative positions of the tips of the teeth,but the distolingual cusp of Ml and the distal cusp of M2 had dominant variability.The first permanent molars of male subjects were larger than those of female subjects(M1:2.68%;M2:1.66%).Sex dimorphism of the molars was manifested primarily in size;there was no shape dimorphism by sex.The upper and lower molar sizes were significantly correlated(r2=0.703),and the upper molars were larger relative to the lower molars3.The Scatter plots showed that the maxillary molars use either K-means or UPGMA clustering methods,and the major component coordinates of different categories have large overlaps.The lower molar classification was clear and definite.UPGMA clustering can be used to screen the similarity of the crown through the phylogenetic tree.Crowns with large variations can be removed.Therefore,the UPGMA clusters of the same category have higher polymerization and more uniform classification.In this experiment,the classification effect of UPGMA group clustering is better than that of K-means clustering group.4.Based on the anatomic significance of the molars represented by the first five PCs,the evaluation indexes were determined,the various groups of UPGMA were measured and analyzed.These ratios were significantly different in the groups of upper molars.However,the differences among the lower molar groups were related to the volume and the distance between the cusp tips and the fossa pits.From the sum of the weight of the UPGMA cluster,a certain gradient in the classification of upper and lower molars was found.The sum weight of male molars was higher than that of females,which shows that male molars were bigger than female molars.[Conclusions]1.The most significant morphological difference among tooth crowns was size;the sex difference in shape was not significant.The distolingual cusp of the upper first molar and the distal cusp of the lower first molar showed the largest variation in relative size,but they had little effect on the shape of the crown.The shape variation of the crown was influenced by a variety of PC parameters.By adjusting the main PCs of the crown,the shape of the crown can be adjusted.2.This experiment was an attempt to quantitatively analyze crown 3D morphology.The results have proved that the UPGMA clustering method based on3D geometric morphometry is feasible for the classification of crown morphology.Using the 3D geometric morphology method to extract the characteristic information of the upper and lower first permanent molar crowns,and then using the UPGMA cluster analysis method to classify the crowns.Relatively ideal dental crown classification and quantification criteria can be obtained,as well as the specific contour shape of each category,It also can provid a more intuitive reference for crown restoration.