Morphological Classification of Mandibular Dental Arch Forms by Correlation and Principal Component Analyses

Abstract

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_R, I1_L, I2_R, & I2_L), summits of the cuspids (C_R & C_L), buccal cusps of the premolars (P1_R, P1_L, P2_R, & P2_L), mesial buccal cusps of the first and second molars (M1_R, M1_L, M2_R, & M2_L), and the midpoint (A) of line I1_R-I1_L were designated as reference points. From A, let a vertical line intersected line M2_R-M2_L at reference point B. The line A-B intersected C_R-C_L at reference point E. We evaluated 1) the protrusion of the cuspids by ① angle I2_R-C_R-P1_R (∠R)+angle I2_L-C_L-P1_L (∠L); 2) the curvature of the anterior teeth by ② (A-B)/(C_R-C_L), ③ 180&orgm;-∠(C_R-A-C_L), and ④ (A-E)/(C_R-C_L); 3) the length to width ratio of the dental arch by ⑤ (A-B)/(M2_R-M2_L); 4) the degree of roundness of the mandibular arch by estimation of ⑥ (rθ₅-rθ₄)_R+(rθ₅-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, ⑤, and 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 dentitions 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.