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The bite force of 2594 school children (1248 males and 1346 females) living in northern Japan was investigated during oral health examinations in May and June 2001, using a new type of occlusal force gauge. The subjects were recruited from a variety of educational institutes and comprised: 73 nursery (3-5 years old), 1019 primary (6-11 years old), 902 junior high (12-14 years old) and 600 high (15-17 years old) school children. The measuring apparatus consisted of a hydraulic pressure gauge, with a bite element encased in a plastic tube. The bite force was measured at the first molar or second primary molar in the children presenting in the permanent and primary dentitions, respectively. The findings revealed significant variations in bite force between children of different ages. The average bite force was 186.2 N in males and 203.4 N in females of nursery school children; 374.4 N in males and 330.5 N in females of primary school children; 514.9 N in males and 448.7 N in females of junior high school children; and 545.3 N in males and 395.2 N in females of high school children. The prevalence of malocclusion in the nursery school children was found to be less than 30 per cent, which contrasted with almost 70 per cent in the high school children.

An analysis of static occlusion including occlusal force and occlusal contact area in intercuspal position may be helpful but a dynamic analysis of occlusion should be regarded as more important for analyzing the stomatognathic system. Anthropological research has shown that the Japanese and the Mongolian populations share morphological and genetic similarities, although their dietary lives are not the same. The purpose of this study was to evaluate the relationship between environmental factors such as dietary life and stomatognathic function with a dynamic analysis of physiological tooth displacement. Ten clinically healthy subjects were recruited (mean age 24.8 +/- 1.0 years). The subjects were divided into two groups 1) Mongolian group: five Mongolians grown with a more or less natural texture diet and 2) Japanese group: five Japanese grown with a relatively soft diet. The displacement of the upper left first molar was measured during function using a three-dimensional tooth displacement transducer Type M-3. The tooth displacement in the Japanese group occurred mostly in an apicopalatal direction, but intruded basically parallel to the tooth axis in the Mongolian group. The stress-strain curve revealed that elastic socket deformation and viscous elements were more pronounced in the Japanese group. It was concluded that environmental factors such as dietary life could influence tooth displacement during function.

In order to examine the eruption order of the first two permanent teeth, kindergarten children 5 to 6 years old were examined in Hakone, Japan. Among a total of 817 children examined from 1976 to 1984, 349 were determined as I-type children, whose mandibular first incisor erupted earlier than the mandibular first molar, and 183 were as M-type children, whose first molar erupted earlier in the mandible. The mandibular I-type rate, i.e., the proportion of the I-type among a total of I- and M-type children, was 66% (349/532). In 1983-1984, the I-type rate was 70% among boys and 62% among girls, but the overall I-type rate did not differ significantly by the sex of the subject or by the year of examination. The I-type rate varied significantly with the season of subject's birth. Those born in October or November showed a significantly lower I-type rate (33%) than the other subjects (P less than 0.001), in spite of a shift of birth season with low I-type rate toward winter in 1983-1984. The difference of the I-type rate according to birth season suggests that the causes responsible for this change are primarily environmental and act at the prenatal or perinatal stage of life.