In the present study, the prevalence of dental caries in children aged 18 months was 1.2%. Although few studies have been conducted in such a young population, the prevalence was thought to be lower than that in other countries (18 months of age: dmft = 2.8 ± 2.7 [17]; 1 year old: 1.9%, 2 years old: 10.7% [18]). Additionally, the prevalence was slightly lower than that of previous studies in Japan (3.3–4.0%) [13, 19]. The present results may be indicative of the oral health literacy of this city. However, ECC affects lifelong oral health into adolescence and adulthood [3], and it is problematic that some children have already experienced dental caries at 18 months of age in developed countries in the 21st century. Furthermore, these previous studies were small-scale compared with the present study, and large-scale surveys of young children in different regions are needed to clarify the status of ECC in both developed and developing countries [13, 17–19].
Because few studies have been conducted in such a young population, there is also a lack of research about the association between tooth eruption and dental caries [12]. In the present study, a greater number of erupted teeth was a significant factor associated with dental caries at 18 months of age. Earlier tooth eruption can be an important risk factor for dental caries in young children. The eruption timing of primary teeth is affected by both genomic factors and environmental factors [20–22]. Although the cause of increased dental caries risk associated with early tooth eruption could not be determined in the present study, our findings suggest an indirect relationship between the vulnerability of early erupting teeth, the early introduction of a mature diet including sugar-containing foods, and the difficulty of maintaining oral hygiene in young children. Longitudinal studies are needed to investigate up to what age earlier tooth eruption affects dental caries risk.
Cariostat® is a microbiological test that evaluates the acidogenic ability from sucrose of the aciduric bacteria in dental plaque, and is a simple test for identifying high-risk patients for dental caries [15]. Microbiological tests in some cross-sectional and longitudinal surveys show strong associations of dental caries with mutans streptococci and lactobacilli [15, 23]. In the present study, a Cariostat score (+++) was a significantly high-risk factor for dental caries. The acquisition and colonization of S. mutans are considered to occur at approximately 19–31 months after birth [24], and some studies have detected S. mutans in the oral cavity of infants younger than 12 months, albeit at low frequency [25]. The results of the present study suggest that bacteriological factors already exert a strong influence at 18 months of age. However, microbiological tests used alone were considered to have limited value in predicting dental caries in a previous study [23]. Nevertheless, previous studies are small-scale compared with the present study and did not include a population as young as 18 months of age. Microbiological tests can be effective in identifying children with high dental caries activity in large populations of children at an age with a low distribution of dental caries.
There are clear benefits of breastfeeding for a child in terms of health promotion and prevention of diseases such as severe lower respiratory infections, asthma, sudden infant death syndrome, and obesity [26]. It is also possible that early breastfeeding may protect against dental caries by delaying the introduction of sugar-containing foods [27]. However, breastfeeding and baby bottle use beyond 12 months, especially if frequent and/or nocturnal, are associated with ECC [27]. In the present study, breastfeeding up to 18 months of age was significantly related to dental caries. However, 97.2% of the children who continued breastfeeding up to 18 months of age had no caries in the present study, although the subsequent caries risk should be analyzed longitudinally. It is thought that breastfeeding is not a determinant factor but a high-risk factor of dental caries and leads to dental caries in combination with sugar consumption rather than directly leading to dental caries [27]. Therefore, rather than the components of breast milk, the habits and behaviors induced by prolonged breastfeeding may combine with sugar consumption, strongly influencing caries risk.
Birth order was a significant environmental factor for dental caries at 18 months of age in the present study. The association between birth order and dental caries has been investigated in recent studies and the present result was consistent with a previous study [28]. Birth order is thought to be related to oral health condition as well as general health condition [29]. Later-born children are likely to have more sibling influences and less parental attention, resulting in less individual care and earlier exposure to sugar-containing foods compared with first-born children [28]. These habitual eating characteristics were consistent with the habit of eating before bed and snacking frequency, both high-risk factors for dental caries in the present study.
In contrast, no significant associations of dental caries with commuting to daycare or guardians’ health condition were found in the present study, but it may be important for the long-term maintenance of a child’s oral health condition to evaluate the family environment.
The present study had several limitations. First, detailed daily intake of food and drink, especially those containing sugar, was not investigated. For example, juice intake in infants before the age of 1 year can be a serious ECC risk factor and should be avoided completely [30]. ECC caries risk assessment could have been more thoroughly evaluated by including these items in the interview sheet. Second, dental hygiene habits such as brushing frequency, fluoride use, and dental care patterns were also not investigated. These factors can depend on guardians and family and may be important factors. Third, dental caries is a chronic disease, and the present cross-sectional study has a certain level of limitation because there may be time discrepancies between dental caries development and risk factors, especially environmental factors. Nevertheless, we consider that dental caries distribution at 18 months, despite its low frequency, merits extensive investigation. The fundamental factors directly contributing to caries development, such as oral bacteria, diet, host, and time, were analyzed in a relatively well-balanced manner in the present study.