In this school-based cross-sectional study of adolescents’ population from the four regions in Kazakhstan, we found the overall prevalence of dental caries of 74% out of which 44% of subjects had obvious decayed teeth experience, while only 6% had missing teeth and 24% had their teeth treated (filled). The mean DMFT index was 2.48 among 12-year-olds and 3.9 among 15-year-olds. There were large differences of dental caries experience between the regions, with all indicators (decayed, missed and filled teeth) being less favourable in Semey region. Most associations of having no caries with covariates were in expected direction, the caries prevalence experience in our population was associated with higher age, geographic region, ethnicity, material deprivation frequent consumption of sugar-added drinks, age of start brushing the teeth and dental attendance pattern. Age, region, ethnicity, dietary habits and dental attendance pattern were found to be significantly associated with the predicted degree of caries experience among those with caries experience.
The prevalence of dental caries in our study was higher than in many developed (4, 22, 23) and some developing countries (12), but our results were very similar to other post-soviet countries. The prevalence of caries among 12-year-olds in Kazakhstan (73.8%) was roughly analogous to Moldova (77.5%), Russia (77.5%) and Georgia (68.9%) [(26–28)], and slightly lower compared to Lithuania (85.5%), Belarus (85%) and Latvia (91.2%) [(29–31)]. The 15-year-olds from Kazakhstan had higher prevalence of caries (77.9%) compared to another Central Asian country Uzbekistan (68%) (32), but lower than Moldova (86.2%), Russia (91.8%), Georgia (82.3%) and Armenia (90.8%) (26–28, 33). These similarities might be explained by the common challenges in transition period after the dissolution of the Soviet Union, when the access to public dental services for children dropped dramatically and dental healthcare became mostly private. Concurrently, the rapid increase in the availability of sugar-sweetened beverages and refined carbohydrate foods largely contributed to dental caries experience (34, 35).
The results of our study should be interpreted in view of several limitations. Firstly, the cross-sectional study design may not be appropriate way to estimate the causal relationships, yet the age of starting to brush the teeth or parent socio-demographic factors is likely to precede the development of the outcome, however we are less confident about the remaining covariates. Secondly, the response rate in this study was moderate (68%), however many recent studies in both developed and developing countries reported roughly the same percentage (8, 9, 23, 36). The moderate response rates may lead to both under- and overestimation of caries prevalence, but usually non-responders in dental health studies have poorer oral health than responders (37). The response rates were slightly higher in special schools, but the difference was not sufficient to introduce the selection bias. Third, adjustment for potential confounders may be incomplete; for example, the sources of fluoride were not identified in this study, which could have modified the results (38), urban-rural differences would also be another interesting aspect to consider in the future research.
The inclusion of two cities in each geographic area in this study was made intentionally to represent more affluent and larger urban centres as well as smaller peripheral cities, for example Nur-Sultan (the capital city) and Kokshetau in the North of Kazakhstan, and Oskemen (regional administrative centre) and Semey in the East of Kazakhstan region. The odds of observing no caries experience was significantly lower for adolescents who resided in the East-Kazakhstan region, with the lowest odds in Semey city. Besides, residing in Semey city contributed significantly to the amount of caries; in the count part of NBH model the adjusted rate ratio was 1.41, while the difference between the capital city and regional administrative centre Oskemen was rather modest, 1.06. The lower levels of caries experience and caries severity in particular in the capital city and neighbouring Kokshetau could be attributed to a wealthier population in this region compared to East-Kazakhstan, with better access to dental health services and more sophisticated system of private dental health care, the similar trend was observed in other developing countries (39). Further, the level of treatment (filled teeth) in both Nur-Sultan and Oskemen region were very high compared to their counterparts Kokshetau and Semey, while the proportion of missing teeth in these smaller peripheral settings exceeds that in large urban centres more than twice. The reduced public provision of dental services in less developed settings might increase the treatment needs (34), also the lack of oral health awareness among parents and poor health seeking behaviours might be another cause (34, 40).
Our findings on association of caries experience and its degree with oral health behaviours were consistent with previous research (36, 41, 42). Many recent studies showed strong association of dental caries experience with a frequent consumption of sugar-added foods and drinks, the behaviours that usually acquired in early childhood (43, 44). In our study frequent consumption of sugar-added drinks were strongly associated with lower prevalence of carries-free individuals, though it had opposite effect on caries severity in zero-truncated negative binomial model. On the other hand, frequent consumption of sugar-added foods showed its strong effect on caries severity, while no association was found with caries prevalence.
Although the tooth brushing is considered to be highly effective (5, 36, 42) and widely available and affordable method associated with prevention of dental caries (45), it wasn’t associated with both caries experience and caries severity in our study. By contrast, the association of dental caries experience with age when children were accustomed to brush the teeth was significant and benefited those who adapted the habit earlier in life, this was reported previously in the literature (5, 42). Interestingly, the regular dental attendance pattern was strongly associated with the higher DMFT which undoubtedly may suggest that dental services are focused on treatment in the studied region rather than prevention, as stated in some studies (11, 39). Besides, the rare dental visits and lower treatment level were mostly observed in unmarried parents, parents with lower education level and highly deprived group with limited access to private dental care especially in less advantaged regions.
Disparities in adolescent’s health according to socio-economic status are often reported (14, 15, 23). Our study results of the effect of material deprivation on oral health inequalities was observed only in individuals who were disease free. The difference in prevalence of caries free individuals for each level of material deprivation wasn’t incremental, with smaller difference between highly deprived and less deprived groups. This may suggest that there are some similarities in adverse oral health behaviours between these two groups (14, 34, 39), however, as already mentioned, the level of treatment was much lower in highly deprived group. The caries experience was also varying among study ethnic groups with almost three-fold increase of caries-free individuals among Russians compared to Kazakhs in the fully-adjusted model, which may reflect the differences in lifestyle and particularly in nutrition, since the traditional Kazakh diet includes high consumption of energy-dense foods; on the contrary the caries severity was marginally (9%) higher in Russians compared to Kazakhs, in addition, the treatment component of DMFT was also much lower among ethnic Russians.