人類小臼歯々冠の形態学的研究

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Morphology of the premolar crowns was studied on the plaster models of upper and lower jaws taken from 683 Japanese residents of both sexes in Kyushu district. The results were summarized as follows. 1. In the values of measurement for width and breadth of the crowns, those for the male were greater than those for the female, and as to individual teeth, those for the first premolar were greater than those for the second in the upper jaw, while in the lower jaw the relation between the two was reversed. 2. Measurement of the width and breadth indices of the crowns showed that the crowns of premolar teeth had in general greater bucco-lingual, and smaller mesio-distal diameters. However, comparison of both jaws in this respect revealed that the crowns of the premolars in upper jaw had more flattened mesio-distal diameter than those in lower jaw. 3. With respect to the number of cusps in the lower premolars, it was found that bicuspid occupied more than 90% of the first premolars and in contrast tricuspid occupied more than 50% of the second premolars, clearly indicating decreasing trend of bicuspid in the second premolars. 4. The shapes in which the occlusal grooves occur in the premolars of both jaws were classified into 5 types, A, B, C, D, E, according to the degree of degeneration observed. Following the classification, the type A represented the standard form of the occlusal groove in human premolars and the remaing types represented in the order successively degenerated forms, with the type E coming as most degenerated form. Analysis of frequency distribution of these types in every teeth of jaws disclosed that in the upper first premolar Type A predominated, followed by Type B, and the remaining forms rarely occurred. In the upper second premolars, Type C was predominant, followed by Type B, and Type A was the rarest. With premolars of lower jaw, Type E occupied the majority of the first premolars with small occurrence of Type C, and the remaining types were hardly observed. In the second premolars Type C was predominantly seen, followed by Type B and Type A in the order. 5. Occurrence of the marginal tubercle in the premolars of upper jaw was observed in the majority of the first premolars, but only in as few as 22% of the second premolars. 6. The position of the lingual cusp was measured as to whether it was located more distally or mesially against the bucco-lingual median line of the crown or it was on this line. The results showed that the majority of the teeth in both jaws was located mesially. However. the teeth having the cusps centered or located mesially occurred more frequently in lower jaw than in upper jaw. 7. Development of the disto-lingual cusps in the lower premolars was weaker in the first and considerably greater in the second. 8. Development of the crossing ridge was observed in the majority of the first premolars, but hardly observed in the second premolars in lowere jaw. 9. Caninisation of the lower first premolars was observed in approximately 50%. It has been a generally accepted concept that human teeth were undergoing degenerative process from backward teeth. This concept would lead to the assumption that the second of the premolars show more advanced stage of degeneration than the first. The results of the present study, especially, in the reduction in the number of occlusal cusps, the changing types of the occlusal groove, the frequency distribution of the marginal tubercle in the upper premolars, the caninisation in the lower premolars and the relative magnitude of individual teeth, furnished, on the whole, apparently conflicting evidences between the teeth of both jaws, and these evidences showed more advanced degeneration in the second premolars than in the first premolars with upper jaw, and not with lower jaw, where conversely the first premolars apparently suffered more advanced degeneration.
九州歯科学会の論文

人類小臼歯々冠の形態学的研究

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