Hypodontia of permanent teeth was evaluated from orthopantomograms of 2072 apparently healthy pediatric patients at The Hospital of Nihon University School of Dentistry at Matsudo. The prevalence of congenitally missing teeth (CMT) was 8.7% in boys and 10.8% in girls, and 9.4% for both sexes combined. Most cases (67.8%) involved either one or two missing teeth. There were in total 574 CMT, and on average 2.8 teeth were missing per child. The most commonly absent tooth was the mandibular second premolar. On the other hand, no first molars were missing in any case. A high frequency of CMT mandibular incisors (18.82%) was observed, and this seems to be a characteristic peculiar to individuals of Asian ethnicity. Oligodontia (6 or more CMT excluding the third molar) ranged from 6 to 14 teeth, with a prevalence of 1.4% in general: 1.8% for girls and 0.9% for boys. Symmetry of CMT was predominant: 214 pairs for bilateral symmetry and 107 pairs for symmetry between two antagonistic quadrants. The distribution of CMT between maxillary and mandibular hypodontia in the right and left quadrants for boys and girls no had significant association (P < 0.05).

In a retrospective cross-sectional study, dental development of 1,620 healthy Japanese children, aged 3-15.9 years, were evaluated by the Demirjian method. A subset of 55 pantomograms were randomly chosen and reexamined. The intra-class correlation coefficient on maturity scores was 0.99. The Cohen's Kappa coefficient to agreement stage by teeth and overall agreement by stage resulted in average 0.85 and 0.82 respectively, both interpreted as "substantially reliable". The children were classified by sex and age. In each age group, dental age and chronological age was compared using paired t-test. Most of age groups were overestimated and had a significant difference. New standards for Japanese children were built using a logistic curve with the equation y‫ם1(/␤{*001ס‬e ‫(␣מ‬x‫מ‬x0))} as a basis. A residual analysis was made to verify if the curve was appropriate. The explained variance for the regression curve resulted in 94.25% for males and 95.07% for females. For both sexes, a linear regression line between chronological and estimated dental age showed 94% of association. The Demirjian standards were not suitable for the presentday Japanese children, so it was necessary to establish specific standard. the timing of observation 4). Second, methods 5,6) based on "absolute measurements of teeth" using radiographs have shown a good validity. However, they could be questioned because the measurement might be difficult to determine if definitive length of teeth is unknown 7) or because of the potential enlargement by radiographs 8). Third, methods based on "tooth mineralization", which was defined as an ongoing process from 1940 9). After that, several techniques to assess the DA by odontogenesis have been determined 10) because the tooth calcification demonstrated to be more controlled by genes and less influenced by environmental factors compared to other systems 11-13). The Demirjian method 14,15) is based on eight calcification stages that represent the formation of the seven left mandibular teeth, with exception of the third molar, from the first sign of tooth calcification to apex closure. A score is allocated for each stage