Labial Surface Pattern on Permanent Upper Incisors of the Japanese

Abstract

The labial surface pattern of the upper central and lateral incisors in Japanese dentitions was studied. The materials used in this study were the 560 plaster casts of the upper and lower jaws of the Japanese living in the Chubu District of Japan, obtained with alginate impression material. The following results were obtained :1. The labial surface pattern of the upper central and lateral incisors are classified into three major groups according to the degree of development of the ridges and the number of the grooves.The First Type :The teeth that are markedly developed shovel-shape . Two markedly developed labial mesial and distal marginal ridges and one faint central labial ridge are seen on the labial surface. Between the two marginal ridges are formed a trapezoid or rectangular labial fossa which runs vertically and parallel to these ridges. This fossa extends as far as the cervical third. In those which have well developed central ridges, the labial fossa is divided into two parts-the mesial and distal fossa.The Second Type :Two low ridges not well developed (mesial and distal ridges) are formed in the place corresponding to that of the mesial and distal fossa in the First Type. The central ridge is more developed than in the First Type. Consequently, the labial fossa, being shallower and smaller, almost disappears or only leaves its mark at most.The Third Type :At the result of the labial marginal ridges either disappearing or combinig with the mesial and distal ridgees, we can see only three ridges on the labial surface.They are separated by two grooves, bur there is no labial fossa. The text book shows this type as the normal pattern of the incisors.2. There are many variations between these standard types. The important point is that in the upper incisors the labial surface pattern and the lingual surfacepattern are basically the same, the difference being in nothing more than the degree of development of each ridge. This especially of importance in considering the phylogeny of the teeth.3. Present authors classified the incisors into a four point scale according to the degree to which double shoveling existed. The percentage frequency of the double shovel-shaped upper incisors is shown in table 1. Of the upper central incisors of the Japanese 50.5% are double shovelled in marked and semi degree. Those which are not double shovelled are only 12.1%. As to the upper lateral incisors, however, the case is just contrary. The double shovelled ones decrease and the non double shovelled ones increase in number.4. Concerning the development of the double shovel-shape and their frequency, a strong correlation was found existing between the central incisors and the lateral incisors.5. The mesiodistal diameter of the tooth crown of the upper central and lateral incisors which are double shovelled is apparently larger than that of the one whose incisors are not double shovelled. Between them there is a significant difference.