The aim of this study was to determine the relationship between number of present teeth (PT) and number of occlusal units (OUs). The data were obtained from a periodontal disease examination based on the health promotion law in Tokyo, Japan in 2005. Data from a total of 1,549 (524 male and 1,025 female) 60-year-old people were analyzed in this study. The number of OUs was counted by analyzing their dental charts. Any pair of opposing teeth of the same type was counted as one OU. The maximum number of OUs in a 28-tooth dentition was therefore 14. Our study revealed that the mean number of OUs decreased along with the number of PT. The OUs of the molars were lost first, followed by those of the premolars. The anterior OUs were last to be lost. The mean number of OUs was always lower than half the number of PT: 10.4 at 24 PT, 7.2 at 20 PT, 4.4 at 16 PT, and 1.3 at 10 PT. In the posterior region (premolars and molars), the mean number of OUs was 4.7 at 24 PT, 2.6 at 20 PT, and 1.0 at 16 PT. For molars, there were 1.4 OUs at 24 PT, 0.6 at 20 PT, and 0.4 at 16 PT. Participants with fewer than 20 PT had fewer than two OUs in the posterior region. This research demonstrates that the number of OUs can be estimated from the number of PT. This knowledge can be used to reveal more detailed information about the oral health status of a given population.
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