ヒトの上顎小臼歯の形態, とくに歯冠と歯根との関係について

抄録

Human maxillary premolars were studied for their morphology with special regard to possible relation between individual parts of the teeth. Measurements were undertaken for the dimensions of the tooth crowns as well as for features of each part of occlusal surface of the crown, and results obtained were summarized as follows. 1) Dimension of Crowns The 1st premolars were considerably larger than the 2nd premolars with every items of measurement, especially with breadth of the crowns. The retrogressive indices as obtained for these teeth indicated that the retrogression was most pronouced in crown length, followed by crown breadth and crown thickness in order. 2) Height of Buccal and Lingual Cusps Cusps of both buccal and lingual sides of crown in the 1st premolar were higher than those in the 2nd premolar. Difference in the height of bilateral cusps was also greater in the 1st premolar than in the 2nd premolar where only a small difference was found. 3) Tooth Root The majority of the 2nd premolars had single root while the 1st premolars frequently showed dual or triple roots, and even when the latter teeth were single-rooted, longitudinal grooves were often observed on both mesial and distal surfaces of the root suggesting an incomplete separation of the root. On the basis of phylogenetic consideration, the human premolars would seem to have undergone successive retrogression from triple-rooted type down to dual-and single-rooted one. 4) Grooves of Occlusal Surface Most of the 1st premolars had their typical pattern of groove well maintained, but the 2nd premolars had lost some of the sub-grooves and showed generally flat appearance. Such pictures of occlusal surface grooves were shown to be closely associated with the stages of root separation in individual teeth. 5) Position of Cuspal Tips In the 1st premolars, the buccial cusp tip was located much closer to distal side and the lingual cusp tip closer to mesial side, and gave an asymmetrie picture against the axial line connecting the bucco-lingual sides. On the other hand, both tips of buccal and lingual cusps in the 2nd premolars were located slightly close to mesial side and appeared quite different from the 1st premolars. Due to the well established stability of morphology found in the 1st premolars, one may be able to recognize a close association between the location of both cuspal tips and the courses of occlusal grooves. such association, however, could not be observed in the 2nd premolars where malformed morphology was much frequent. This would suggest that the 1st premolars represented the fundamental morphology of these kinds of teeth and the 2nd premolars were of their deviated form. 6) Comparison of Diagrams Showing Mean Values of Measurement of Cuspal Surface A clear difference existed between the 1st and 2nd premolars in the location of buccal and lingual cusp tips by actual measurement in good agreement with macroscopical observation. when the 1st premolars were divided into 3 groups according to stages of ramification of the root for comparison, it was shown that their morphology resembled those of the 2nd premolars increasingly as the roots assumed characteristic form of the uniradicular tooth, and, on the other hand, as the ramification of roots became more apparent, so the buccal cusp tip was located increasingly toward distal side, eventually showing a markedly asymmetric picture agaist the bucco-lingual axis of the tooth. The above study would seem to suggest that the 2nd premolars are undergoing a series of degenerative processes including diminution of dimension, simplification of the pattern of occlusal surface grooves, reduction of root ramification and symmetrical location of the bucco-lingual cusp tips as compared to the morphology of the 1st premolars.