Chronological Changes in Retromolar Space Viewed From Radiographs in Japanese

Yamada H

Published on: 2023-06-28

Abstract

In Neanderthals, about 40,000 years ago, a retromolar space occupied a large area, and all three molars were erupted in an upright position [1]. In modern humans, however, the retromolar space is extremely narrow. The present study compared the amount of alveolar space posterior to the second molars (M₂-retromolar space) through the history of Japan. The radiographs of human skulls excavated from archaeological sites and panoramic radiographs of modern humans were used to investigate the M₂-retromolar space based on a Z point, which the intersection of the internal oblique line and the straight line connecting the most protrusion position of the first and second molars. The softening of the diet weakened the masticatory muscles (especially the masseter and medial pterygoid muscles), resulting in degeneration of the mandibular angle and widening of the mandibular angle. In addition, the development of the mandibular angle distal to the Z point was stagnant, and the M₂-retromolar became narrower as this area did not undergo distal directional growth movement at the time of the eruption of the third molar. Consequently, third molars today erupted in inclined or horizontal position. On a time scale, the M₂-retromolar space showed around 40% decreasing trend in the mesiodistal diameter of the second molars during the approximately 2,400 years from the beginning of the Yayoi era, to 1868, the end of the Edo era, and a similar 40% decrease during the 60 years from the 1930s to the 1980s. The latter showed a more rapid downward trend in the M₂-retromolar space over the period. The anterior shift of the dental arch due to wear between adjacent teeth did not cause the enlarged retromolar space.