Classification of Mandibular Dental Arches by Correlation and Principal Component Analyses

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To evaluate the morphology of dental arches, 53 (male: 29, female: 24) paired casts having normal dentitions and occlusion were selected from 396 (age: 18 to 26 years old; male: 257, female: 139) sets of dental study models. The mandibular dentitions were preliminarily classified as square, round-square, round and round V-shaped arches based on the conventional morphological descriptions. Midpoints of the incisor edge (I1(subscript R), I1(subscript L), I2(subscript R), I 2) the curvature of the anterior teeth by ② (A-B)/(C(subscript R)-C(subscript L)), ③ 180°-∠(C(subscript R)-A-C(subscript L)), and (A-E)/(C(subscript R)-C(subscript L)); 3) the length to width ratio of the dental arch by ⑤ (A-B)/(M2(subscript R)-M2(subscript L)); 4) the degree of roundness of the mandibular arch by estimation of ⑥ (rθ5-rθ4)R+(rθ9-rθ4)L; and 5) an item ⑦ for the differentiation of type Ⅰ and type Ⅱ round-square arches by relating the bilateral contour and position of break line P1-P2-M1-M2 (ⅰ) to line P1-M2 (ⅱ). The data of items ①, ②, ③, ④, ⑤and ⑥ were further standardized and summarized into three essential principal components: 1) the curvature of the anterior teeth, 2) the curvilinear contour of the dental arch, and 3) the length-to-width ratio of the dental arch. The results indicated that: 1) 36 cases (67.9%) of the mandibular denth ions were round-square arches which showed no prominent principal component. 11 cases (20.8%) were square arches and 6 cases (11.3%) were round V-shaped arches; no round arches was found in mandibular dentitions. 2) Statistical analysis indicated significant differences of items ③, ④ and ⑥ in various mandibular arches (Student's t-test). 3 )By examination of the three principal components, significant differences of item ⑤ between the round V-shaped arches and square and round-square mandibular arches were evident (Student's t-test). The present study elucidated that morphology of the mandibular arch was determined by a parameters representing the curvature of anterior teeth (composed of items ②, ③ and ④), and another parameter (item ⑥) representing roundness of the mandibular arch.