Secular change in the Japanese occlusion: The frequency of the overbite and its association with food preparation techniques and eating habits.

Abstract

It has been taken for granted that an overbite and a related overjet is the ideal normal human condition. However, the form of the overbite is extremely recent in origin. This study has recorded evidence for the shift from the form of an edge-to-edge bite to an overbite in the dentitions of prehistoric and modern populations of 11 cultural periods and 4 ethnic groups in the Japanese Islands, spanning 6000 years. An edge-to-edge bite was the common occlusal form during the entire Jomon Period, the Yayoi Period, and the Kofun Period. Subsequently, the frequency of overbite accompanied by overjet (scissors-bite) increased to 50% in the Late Kofun-Early Historic Period. Later, it increased to 83.3% in the Kamakura Period, 76.9% in the Muromachi Period, 89.7% in the Edo Period, and 81.7% in the Modern Period. As demonstrated by the above percentages, a change occurred in the Late Kofun-Early Historic Period and a big shift occurred around the Kamakura Period. Also, the full and functional edge-to-edge bite is more common among the Epi-Jomon, Okhotsk people, and the Sakhalin Ainu who were primarily hunter/gatherer/fishers. The same tendency is found among the historic Hokkaido Ainu. The modern Hokkaido Ainu however, whose life style was forcibly changed to the Japanese way after the Meiji Period, display an overbite. In addition, more than 3mm overjet (mean) is characteristic after the Kamakura period. Both overbite and overjet depths are greatest during the Edo Period. Also, more diversity of occlusal form (i.e., deep overbite, extreme overjet, openbite, and underjet) is observed in the Edo and Modern Periods. This study shows that food preparation and eating are strongly associated with the appearance and development of the overbite and related overjet. The trend in Japanese cooking developments has been towards the introduction of cutting, slicing and simmering techniques that are associated with the use of chopsticks. This removes the incisor's function of immobilizing and cutting food. With such food preparation techniques and eating behavior, food was softened and reduced in size further before insertion in the mouth. The consequence was the development of the overbite. Furthermore, such cooking technology and eating habits induced lower tooth crowding, and this resulted in the reduction of the lower dental arch and caused the more marked overbite and overjet in the subsequent population.* *Originally published in <italic>DAI</italic> Vol. 61, No. 3. Reprinted here with corrected abstract.