Objectives We examined the trend in the number of missing teeth in a Japanese community over a 10-year period and the potential associated explanatory factors. Design Prospective cohort study. Setting A population-based study conducted in 2007, 2012 and 2017 in Japan (Hisayama Study). Participants Residents of a Japanese community aged 40-79 years undergoing dental examination in 2007 (n=2665), 2012 (n=2325) and 2017 (n=2285). Outcome measures The number of missing teeth, periodontal condition, dental caries experience, dental plaque index and oral health behaviours were evaluated each year. The longitudinal analysis of variation in these factors were assessed using mixed models. Results The age-adjusted and sex-adjusted mean number of missing teeth decreased with time (6.80 in 2007, 6.01 in 2012 and 4.99 in 2017). The mean clinical attachment level (CAL), prevalence of periodontitis and dental plaque index decreased over the study period, while dental caries experience slightly increased. The level of oral health behaviour increased over time. Poisson mixed models showed that changes in mean CAL and dental caries experience were positively associated with the change in the number of missing teeth over time. Linear mixed models showed that changes in dental plaque index and no regular dental visit were positively associated with changes in mean CAL. Conclusions These findings suggest that a decreasing trend regarding the number of missing teeth in Japan might be associated with improvements in the periodontal condition due to changes in oral hygiene level and oral health behaviour.
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