**4. Discussion**

The findings of the present study indicate that overall levels of dental caries and tooth loss among Australian adults have considerably declined over the past three decades. For example, the severity of dental caries experience and complete tooth loss among Australian adults has decreased by nearly 27% and 72%, respectively, from 1987–88 to 2017–18. In general, this decline in dental caries experience has been reflected in all three components of the DMFT index, showing overall reductions of 46%, 22% and 24% in the mean number of decayed, missing and filled teeth over 30 years since 1987–88. In contrast, the periodontal status of Australian adults has substantially deteriorated between 2004–06 and 2017–18, with an overall increase in the prevalence of moderate or severe periodontitis by nearly 31%. This deterioration is evident across all age groups, in particular with the almost 65% increase in the proportion of Australian adults aged 15–34 years who have moderate or severe periodontitis.

Several factors may explain improvements in dental caries experience, as well as tooth retention, that have been observed among Australian adults over the past three decades. Nearly 90% of Australians have access to fluoridated drinking water, while almost 97% of Australian children and adults brushed their teeth daily using a fluoridated toothpaste [20]. There has been consistent evidence to sugges<sup>t</sup> that community water fluoridation alongside widespread use of fluoridated toothpaste in Australia has played the most important role in preventing dental caries [21,22]. Prevention of dental caries in turn has led to increased retention of teeth, given that dental caries is regarded as the main cause of tooth loss. Furthermore, there has been a notable shift in dental treatment strategies, from high-extraction versus low-restoration to low-extraction versus high-restoration, which may have also contributed to improved tooth retention over the past three decades. These findings have consistently shown that Australian adults who usually visited only for a dental problem had higher levels of dental caries and tooth loss than those who visited for a dental check-up. For example, the severity of dental caries (as denoted by the mean DMFT) and the prevalence of complete tooth loss, respectively, were 1.31 and 6.8 times higher among Australian adults who usually visited only for a dental problem than for their counterparts who visited for a dental check-up. This finding concurs with what has been reported previously, indicating an association between improved oral health and favourable dental visiting patterns, including visiting for a dental check-up [23,24].

The NSAOH 2017–18 report has used several independent variables, such as year level of schooling, highest qualification attained, eligibility of public dental care and dental insurance, as socioeconomic indicators of the study population. Accordingly, the present findings have revealed that poor oral health has consistently been associated with lower levels of socioeconomic status. For instance, the prevalence of untreated dental decay was 1.22 times and 1.58 times higher among persons who had Year 10 or less of schooling and those who were dentally uninsured than their counterparts with Year 11 or more years of schooling and those with dental insurance. Likewise, the prevalence of complete tooth loss was 5.26 times and 3.82 times higher among individuals with Year 10 or less schooling and those who were without dental insurance, as opposed to their counterparts. These findings are consistent with those of previous studies, where more socially advantaged individuals presented with much improved oral health levels than those who were worse-off, and, consequently, supported the existence of socioeconomic inequalities in oral health [25,26].

Deterioration in periodontal health in Australian adults, which has been observed between 2004–06 and 2017–18, could be mainly ascribed to increased tooth retention. While the overall proportion of edentulous persons declined from 6.4% to 4%, the mean number of missing teeth due to pathology dropped from 4.6 to 4.4 during this period. Consequently, both the increase in the proportion of dentate adults as well as the number of retained teeth pose a greater vulnerability for periodontal disease. Our findings were consistent with those of previous studies where a strong association between age and periodontitis was observed; the older the individuals, the higher the prevalence of periodontal disease [10]. Associations between socioeconomic variables and periodontal disease, on the other hand, were similar to those seen with regard to dental caries and tooth loss. Accordingly, the prevalence of moderate or severe periodontitis was consistently higher among Australian adults in the lower socioeconomic strata. This is consistent with previous studies [10] and provides further evidence for the presence of socioeconomic disparities in oral health.

Employing a nationally representative sample of Australian adults and using a standardized examination protocol, as well as rigorous epidemiological survey methods, were

some of the main strengths of the study. Other strengths included having both the interviewers and oral examiners adequately trained to ensure the quality control of the study (high intra-class correlation coefficient values were obtained indicating a high level of interexaminer reliability and agreement), and the instruments used were based on previous studies, enabling comparisons to be made across the series of national surveys. Whilst the cross-sectional nature of the study did not warrant ascertaining cause–effect relationships, the present study could not represent Indigenous Australians in sufficient numbers. This, in turn, has resulted in creating small cell counts and relative standard errors of at least 25% in regard to Indigenous group/subgroup analyses, so interpretation of these results should be made with caution. Moreover, the use of partial recording protocols in the study could have contributed to flaws in estimating the prevalence of periodontitis. Despite such limitations, the findings showed that the overall oral health status, including the experience of dental caries, periodontal disease and tooth loss, was poorer in Indigenous Australians than in their non-Indigenous counterparts with regard to virtually all independent variables assessed. These findings are consistent with the previous studies, which were conducted among Indigenous groups in both Australia and elsewhere, indicating that Indigenous populations are among the most socioeconomically disadvantaged communities in the world [27–29]. It may be challenging for survey instruments and sampling methods employed in conventional population-level oral health surveys to capture the true picture of Indigenous populations and, accordingly, the need for implementing unique study methodologies for such populations has been highlighted [29].
