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We studied the relationship of tooth agenesis with tooth size and its proportional variability (PV) based on mesiodistal crown diameters of 276 Japanese males including 49 individuals for reference. Tooth agenesis was classified into third molar agenesis, hypodontia, and multiple agenesis. In addition, third molar agenesis was classified into four types according to the number of congenitally missing third molars. PV was measured by standard deviation of log-transformed data. The size of remaining teeth was generally larger in the agenesis groups than in the reference group, and largest in individuals with hypodontia, followed by those with multiple agenesis and those with third molar agenesis. The findings suggest the existence of two types of tooth agenesis differing in nature, where remaining teeth tend to enlarge in the ‘moderate type’ and reduce in the ‘degenerative type.’ The former is dominant in (tooth agenesis of) the Japanese, whereas the latter seems to be more prevalent in European descendants, which is in accordance with recent findings in genetics. The ‘moderate type’ might be advantageous for survival in human microevolution because of its improved function and reduced risk of dentoskeletal discrepancies. The PV of tooth size was greater in the agenesis groups than in the reference group except for premolars and second molars. Among those with third molar agenesis, the greatest increase in PV was exhibited by those with all third molars missing, followed by those with two third molars missing. Among remaining teeth, canines and first molars tended to exhibit a greater increase of PV in agenesis groups, whereas their magnitude of PV did not exceed that of other teeth. These results can be explained by the genetic stability of canines and first molars and an increased variation due to common factors across remaining teeth associated with tooth agenesis.

Tooth size and its proportional variability in Japanese males with agenesis in permanent dentition