1973 年 15 巻 4 号 p. 103-112
As is shown by Figs. 1 to 4, those second premolars in which calcification was retarded at the initial examination (63 to 74 postnatal months) kept up the same trend in subsequent examination and their root formations were markedly less than normal, the tendency being more pronounced with boys. With boys, these teeth were found to have attained to relatively poor roots even at the final stage of our study (171 to 182 postnatal months). Therefore, it may be reasonably assumed that the same trend would be more or less carried into the subsequent stage of development even though we lack concrete data to substantiate the assumption. Generally speaking, the chronological assessment of the development of individual teeth should be compre-hensively made by taking into consideration such factors as the physical stature, weight , girdle of chest, bone age [9, 10, 11], sex maturity , as well as local factors including pathologic lesions of the root of precursor teeth [13, 14, 15] and effect of dental extraction . Concerning the amount of root formation and eruption of teeth, GRøN  maintains that the degree of root formation has closer bearings on the eruption of teeth than postnatal or bone age and when a root has been formed by 3/4, a tooth erupts clinically. According to HAVVIKKO , the bone eruption is distinguished from the clinical eruption of teeth and though admitting certain variations among teeth, a root has been formed by 1/2 at bone eruption and 3/4 at clinical eruption. The authors hold the view  that the average root at the period of normal replacement process is about 1/2 of what will be at perfection and roughly corresponds to the long diameter of crown. Even if normal calcification is retarded, the amount of root formation at bone eruption is more or less normal as long as the precursor suffers from no pathologic lesion of some kind. In other words, even with successors in which retarded calcification is observed the bone eruption will take place when the amount of root formed comes to correspond to the length of crown. This is equally true of the mandible and maxilla.
The mean period of bone eruption of calcification-retarded premolars is as late as 19 months in boys and 6 months in girls. This discrepancy is more pronounced in the mandible than in the maxilla..
Hitherto, the growth pattern of human teeth by MASSLER and SCHOUR  has been accepted as standard. According to this growth pattern, the crown of first premolar begins its calcification in 1 1/2 to 2 years reaching the perfection in 5 to 6 years, while that of second premolar begins in 2 to 2 1/2 years and reaches the perfection in 6 to 7 years. Therefore, both the beginning and perfection of first premolar is sooner by one year.
In our present study, however, not a few of the samples in the initial examination (5 yr. 3 mos. to 6 yr. 2 mos.) were observed to reveal mere traces of calcification of their roots. On the other hand, no retarded calcification of first premolars to such a great degree was noted. In addition, a large measure of variation was observed relative to the degree of calcification of upper and lower second premolars. It follows then that the above standard data do not necessarily represent reliable information in clinical environment. The study material of MASSLER and SCHOUR is not exactly known and they have drawn on the previous data by LOGAN and KRONFELD . The latter based their findings on the corpses of children and, for thisreason, they cannot be said to be consecutive or vertical in any sense. SATO and others [20, 21] pointed out that it was not proper to cite these data as if standard.
On the other hand, the vertical study by GARN, LEWIS and POLACHECK  established the existence of large individual variations in the eruption of mandibular premolars and molars, and also pointed lack of recognition of this fact at large. Other investigators including FANNING , HAVVIKKO , KANEDA