This study evaluated the 2-year incidence of new caries in the first permanent molar teeth of 7–8 year-old children according to the CAST index and its risk factors. In the two-year follow-up, more than half of the children developed new caries in permanent first molar teeth (incidence) considering enamel and dentine lesions. The children with a low socio-economic status and those whose mothers had high school/diploma education (no university degree) developed a significantly larger number of new caries compared to the comparison group. There was no significant association between new caries development and parents' oral health related knowledge, number of children in the household, child’s birth order, gender, and frequency of sugary snacks per day and oral hygiene status of children according to OHI-S index.
Studies reporting caries incidence have reported either the incidence rate ratio (IRR) or the incidence rate per person-time at risk. A review study by Hummel et al. in 2019 found that at the general public level, the caries incidence rate is a promising statistic for predicting future caries increments. In this study, a meta-analysis of 32 studies including children and adolescents up to 21 years of age showed that a pooled caries incidence rate of 0.11 (0.09–0.13) per person-year at risk. In this review, considering D3 lesion showed that the annual increment ranged from 0.06 to 0.73 for DMFT. This meta-analysis showed that the pooled caries incidence rate and increment of DMFS and DMFT indexes were influenced by the method of individual studies .
The results of our study confirmed the findings of other studies on the impact of the socio-economic status on oral health. In our study, living district and mother’s education had a significant association with increase in number of dental caries. Most of the children with new caries development were from non-affluent districts or their mothers had high school education or diploma. This finding is in line with the results of a longitudinal study of 12-year-old children by Oritz et al., with two years of follow-up that showed a higher incidence of caries in children was associated with low socio-economic background .
The recent review study by Schwendicke et al.  reported that social position defined as parental educational/occupation or income has impact on oral health. Those with low level of social position have greater odds of having any caries lesions or caries experience . It might be inferred that the impact of socio-economic status on caries development might be related to the multifactorial nature of tooth decay. Parent of children from low socio-economic status usually have other priorities rather health that make them more vulnerable. One potential reason for this finding could be having predisposing risk factors as children with low socio-economic status may have inappropriate diet or limited access to health care services.
The present study showed that the mother’s education was associated with new caries development in children. Children whose mothers had high school education or diploma had more new caries compared with the children of mothers with an academic education. Our finding was in line with the results of an Iranian survey study conducted by Ghaseminpoor et al., also found that the mother’s education level was negatively related to permanent teeth caries in 12-year-old children . Also, the previous national survey on Iranian children in 2004 reported by Bayat-Movahed et al., showed that there is a significant relationship between caries and province, city of residence, family income, and parents' education level .Moreover, Edasseri et al., studied children aged 8–10 years with a 2-year follow-up in 2017 and found that the high education level (more than high school education) of one or both parents has a protective role against caries incidence . However, longitudinal studies conducted by Peres et al., in children aged 6–18 years and Llena et al., in 10-year-olds showed that the educational level of mothers was not associated with caries development [22, 23].
We found no association between new caries development and gender, father’s education, number of children in the household, oral health related knowledge of parents, sugary snack consumption and oral hygiene status of children according to the OHI-S index. A longitudinal study of children aged 9–11 years with a three-year follow-up period conducted by Melo et al., in Brazil found that the incidence of caries at the occlusal surface of the first permanent molars was significantly associated with previous caries history and plaque accumulation on this surface. However, there was no significant relationship between the incidence of caries and the child's gender, family income level, and mother's education .
There was no association between two or more sugary snack consumption and caries incidence measured by CAST index in the present study. Llena et al., conducted a five-year retrospective cohort on 10-year old children and found that the predictors of caries development in the first permanent molars were consumption of sugary drinks, frequency of brushing, caries history in deciduous and molar incisor hypomineralization . On the other hand, the results of a systematic review by Moynihan and Kelly in 2014 showed a relationship between caries prevalence and free sugars intake more than 10% of energy intake .
Our study findings was consistent with the results of a 4-year follow-up study of Chankanka et al. in 2011 reported no association between caries incidence and sugary food or drinks consumption . In our study sugar consumption was not measured according to some detail indices such as the energy intake and therefore no relationship could be found. Besides, there may be under-reporting of sugary snack consumption in our study as this is unacceptable due to wide health messages on the negative impact of having too much sugar on general health. Moreover, the delivering of topical fluoride at the community level including fluoride toothpaste and twice yearly application of fluoride varnish may influence the impact of sugar consumption on caries development.
There was no association between OHI-S as a measure of oral hygiene and new caries development in the present study. No relationship was found between OHI-S and DMFT in the study of 11–14 years children by Rehman et al.  whereas studies conducted by Oyedele et al., on 8–14 year-old children showed that poor oral hygiene was associated with a higher chance of developing caries .
The results of the present study showed no relationship between oral health knowledge of parents and caries incidence, which is in line with the result of studies by Maharani et al., and Saied-Moallemi et al., also Babaie et al., that found no association between parents’ oral health related knowledge and caries prevalence in children using DMFT and CAST index, respectively [12, 14, 29].
In the present study, gender was not related to new caries development, which is in line with the results of longitudinal studies by Oritz et al., Edasseri et al. and Peres et al. [6, 7]. Moreover, being the only child or the first child in the family did not increase caries increment, which was consistent with the findings of the study by Al-Meedani et al., that found no association between caries prevalence and birth order . However, Floyan et al., and Oyedele et al., found that having a sibling was associated with caries prevalence [28, 31].
Our study has some strength in terms of study design and measurement. The present study is the first study in this field reporting a more detailed index of caries including enamel and dentine lesions. It is also important that a limited number of children experienced severe complications of dental caries including swelling or sinus tract related to infectious pulp involvement and extracted teeth due to caries in the permanent first molars, which confirms a mean (SE) DMFT of 1.84 (0.03) in 12-year-old children previous reported by the Ministry of Health and Medical Education . As a representative sample across the capital city was used, provide a picture on caries incidence of the permanent dentition in children.
In our study, the impact of possible risk factors and risk indicators on the caries incidence of the permanent molars were assessed including oral hygiene status and snacking pattern. Other main risk factors including topical and systemic fluoride intake and saliva characteristics were not assessed directly. In our study, all children had access to public water in Tehran containing about 0.3–0.6 ppm fluoride . Fluoridated toothpaste is available in the local market and fluoride varnish delivered by the Ministry of Health and Medical Education semi-annually.
Considering the multi-factorial nature of caries, possible risk indicators were considered including the socio-economic status, parents' education, number of children in the household and child birth order. In our study, previous caries was not considered as predisposing factor. As, those risk factors and risk indicators that related to previous dental caries experience, are highly correlated with present status and considering the “co-linearity” problem of independent variables we did not entered it in our final data analysis.
However, this study had some limitations. In our study, the presence of caries may be reported higher than studies that use the DMFT index reporting cavitated dentine lesions. Therefore the risk factors affecting caries should be interpreted with caution . The snacking pattern of the children was assessed using a three-day food record, which has some limitations. There might be under-reporting of dietary intake of sugary items as the limitation of this tool was previously discussed by Thompson . It is recommended that two tools be used at the same time to increase the validity. Another weakness of this study was loss to follow-up. According to the design of the education system in Iran, schools may have different campuses for year 4–6. Therefore, some children move to other schools at higher grades. Our study as a longitudinal study on a representative sample of children using a comprehensive tool for caries assessment in children provides valuable information. As conducting longitudinal studies in school-aged children is challenging since these studies are expensive, time-consuming, and prone to attrition.