日本人上顎切歯の形質に関する人類学的研究

Abstract

Much attention has been focussed on the morphology of human upper incisor teeth not only from anatomical viewpoint but also from practical dental prosthetic consideration. The physical traits of these teeth also provides a variety of significant problems of anthropology and their regressive phenomena, especially of lateral incisors have been studied most extensively from the evolutional viewpoint as these usually showed most marked signs of regression second to the wisdom teeth. The present author, therefore, undertook a series of measurement on the upper incisor teeth and examined the morphology of their crown and root and made comparative study on some of their physical traits. The study has yielded several facts of interest summarized as follows. 1. The labial surface of crown in central incisors generally showed ladder-like and inverted wide U-form, and that in lateral incisors generally showed inverted slender U-form. When viewed from the direction of incisal margin, the labial surface of both incisors showed a flattened surface in most cases, except that there were some 17% of central incisors showing concave surface that corresponded the so-called double-shovel form. It was confirmed further that the shape of labial surface view from the incisal margin showed no correlation with the contour of labial side view of the same teeth when tested by means of chi-square test, and this suggested that the labial surface morphology might have been produced in the above stated form as result of the developmental difference of individual ridges. 2. The contour of the lingual side view of the teeth showed generally a slender triangle in most cases, particularly with lateral incisors. This means that the constriction from incisal margin toward cervical direction in these teeth was more marked in the lingual side than in the labial side of the teeth. 3. The occurrence of the shovel-shaped incisors was considerably high with percent frequency of 56.8 in the central incisors and 66.9 percent frequency in the lateral incisors and this may be regarded as one of characteristic traits among mongoloid race that includes the Japanese. A positive correlation was found to exist between the above trait and the marginal ridge of the teeth, and just as in the case of double-shoveled trait, this shovel-shape trait was also considered to have resulted from accentuated growth of the marginal ridge. 4. The percent frequency for the spine of the incisors was respectively 28.1% in the central incisors and 29.2% in the lateral incisors, that for the occurrence of the foramen cecum was 7.8% in the central, 55.1% in the lateral incisors respectively, and that for the linguo-gingival fissure was respectively 21.9% in the central, 35.7% in the lateral incisors. The statistical analysis on the possible relations between these traits including the shovel-shaped incisors and the mesio-distal, labio-lingual width of the crown has revealed that although the occurrence of foramen cecum might be one of the tooth's regressive traits, it could not deserve to be concluded as such in any definite term. On the contrary, the linguo-gingival fissure and the shovel-shaped forms of the incisors my be considered to represent the primitive traits of the teeth, and occurrence of spine was not considered to have any phylogenetic significance. 5. Measurement of size done on the individual teeth showed that the lateral incisors suffered marked reduction in every items studied as compared with the central incisors, and the most reduced was the mesio-distal width of crown, followed by the labio-lingual width and length of crown and root of the teeth in this order. Comparison of the corner angles of incisal margin showed that there was no appreciable difference between both mesial and distal corner angles with the central incisors, while with the lateral incisors the one at the distal corner was significantly greater than the one at the mesial corner.