Abstract
In Japanese, the lower first molar erupted earlier than the lower central incisor before. The Japanese Society of Pedodontics (1988) reported contrary results for the eruption pattern. This phenomenon involves an important problem from a biological viewpoint, because the first erupted permanent tooth changed from the supplemental tooth (the first dentition) to the successional tooth (second dentition).
We investigated the eruption pattern of the central incisors and first molars in individuals to clarify the starting generation of this phenomenon. It was found that the “I” type, which the central incisor erupted earlier than the first molar, was increased in Japanese infants who were born in 1970's, and became 50% or more.
The comparison was made between the two previous studies (Okamoto,1934,1938; JSPD,1988)and among the pervious studies of other ethnic groups to discuss the change in the time of the eruption. The following conclusions were obtained: (1) The Japanese life style rapidly changed from traditional style to the European or North American styles from 1970 onward. This change of life style including eating habits is thought to be a possible factor of this phenomenon. (2) Early development of the secondary dentition directly caused this phenomenon. (3) We propose to add II B stage (exchange period of the incisors) in the Hellma's dental stage, if the "I" type will be more frequently found in Japanese children.
