Socioeconomic gradients in general and oral health of primary school children in Shiraz, Iran

Introduction

Health status is largely determined by social class and socio-economic status. Understanding the association between socio-economic status and health outcomes is of great importance for planning health promotion strategies. It is generally accepted that “there is a social gradient in health”^1–4. In the national trend, the gradient is the status of an individual in socio-economic hierarchy and shows that people at the top of social hierarchy have better health than those in lower levels⁵. In the international trend, the gradient indicates more affluent countries have better health outcomes compared to poorer countries⁶.

The effect of socio-economic status is shown on a wide range of health outcomes from drug misuse related diseases⁷ to age-specific mortality⁸. A study on 6 to 11 year-old children of the United States found that social class had positive association with minor and major physical disorders such as colds, infections, allergies, diabetes, and even epilepsy⁹. The relationship between household income and a range of health outcomes in children and adolescents has been shown in Britain¹⁰. A study in France has found a significant positive correlation between family income and some of the most important child health indicators such as anthropometric measurements¹¹.

In consistence with general health, social gradients are shown in oral health indicators. It seems that despite the remarkable improvements in the averages of oral health indices among communities in the last decades, inequities in oral health - mostly related to social inequalities - continue to exist^12,13.

A study on the relationship between work conditions and health inequalities in Switzerland has found clear social gradients for almost all adverse working status and the outcomes of oral health¹⁴. In a case-control study in Mexico, lower socioeconomic status was identified as a risk factor for non-syndromic orofacial clefts¹⁵. Another study on Chinese 5-year-old children found that there were notable gradients in carious primary teeth related to their household income¹⁶. A study on young children of Salem, Tamil Nadu, India, showed that those from lower socio-economic groups, especially whose parents' had lower education were more likely to suffer from Early Childhood Caries¹⁷. It was also shown that poor socio-economic background can be a strong predictor for poor oral hygiene¹⁸.

There has been limited evidence in Iran regarding the association between socio-economic status and general and oral health gradients, particularly among children. Therefore, this study was designed to assess the existence of social gradients in oral and general health of 8- to 11-year-old primary school children in three different socio-economic areas of Shiraz, a city in Southern Iran.

Material and methods

This cross-sectional study was conducted on 8- to 11-year-old children (third to fifth grade primary school children) of the city of Shiraz, Iran, in 2009. Approval and ethical permission were obtained from the National Ethical Committee in Medical Research of Iran (# 85/p/3/1095). Permission to enter the selected schools was then obtained from the Educational Head Office of the Province of Fars (# 17568/55). Based on the prevalence of Developmental Defects of Enamel (DDE) and a confidence level of 95%, a sample size of 335 was estimated appropriate for this study.

A three-stage cluster sampling approach was used to select the children. The main Educational Affairs Office of Shiraz had a category which divided the city into three areas based on socio-economic status: upper, middle, and lower social class areas. This kind of division seemed more appropriate for this study than any other division, such as geographically separated zones. One boys' and one girls' school (primary schools are segregated in Iran) were selected from each of the three socio-economically different areas. This was done by simple randomization using the list of primary schools in each area. Then, two classes were randomly selected in each school from the third, forth, and fifth grades. All children present in each selected class were included in the study, only if their parents provided written consent.

Although socio-economic differences between the three areas were well recognized by officials (Educational Head Office 2006), the number of parents having an occupation (any job), and the number of parents having a permanent occupation (as a job security indicator) were used as socio-economic related variables to compare the three areas and confirm the socio-economic difference between children chosen from these areas^19,20. Related information was obtained from schools' administration offices.

Height and weight are valuable indicators of present and past health status. Dividing the body height into trunk and leg length provides more precise information particularly on nutrition status of the early years of life^21,22. Standing height, leg length, weight, and BMI (Body Mass Index) were used in this study as indicators of general health.

To obtain children's height and weight, the tools and methods used by United Kingdom’s Medical Research Council were adopted²². For weight, a digital scale (Beurer Electronic Weight Scale PS 07, Germany) with accuracy of 100 grams was used. It was calibrated in each school. A wall mounted height meter with accuracy of millimetres was used for measuring children's standing height. A similar metre adjusted to the seat of a chair was used to measure the length of the upper trunk, from head to the chair’s seat (sitting height). The leg length was calculated by deduction of the sitting height from the standing height. BMI was calculated by dividing the weight by the square of the standing height. Identically calibrated tools were used for all cases in all schools.

Assessment of dental caries, DDE and gingival health determined the oral health status of children. Clinical intra-oral examinations were carried out using WHO screening criteria to record data on Decay, Missing, and Filled Teeth (dmft) for the primary dentition and DMFT for the permanent dentition²³. Permanent teeth were assessed from first molar to first molar in each arch. Therefore, second permanent molars, if present, were not assessed. DDEs of permanent teeth were recorded based on the Modified DDE Index²⁴. Gingival health and oral hygiene were assessed using the Gingival Index (GI)^25,26 and the Debris Index – Simplified (DI-S)²⁷. All examinations were carried out in classrooms using natural light, disposable mirrors and tongue blades.

Normality of distribution of each outcome variable was assessed by a histogram. All continuous variables had a distribution close to normal. The difference between sexes was analysed by a t-test. Two socio-economic variables of number of parents with any occupation and with a permanent occupation were compared among the three areas using one-way ANOVA. Anthropometric measurements, gingival health, and oral hygiene status were also assessed among the three areas by one-way ANOVA. Anthropometric measurements (linear regression), gingival health status, and oral hygiene status (quantile regression) were also compared among the three areas after adjusting for sex and age. DMFT, dmft, number of teeth needed treatment, number of permanent teeth with DDE and total number of DDEs in children’s permanent dentition were compared between areas using Poisson regression model after adjusting for sex and age, and number of permanent teeth present in the mouth. SPSS statistical package (version 14) and STATA statistical package (version 10 Intercooled) were used. The significance level was set at α=0.05.

Results

Consent forms were sent for 376 parents, out of which 335 accepted to participate (response rate = 89.1%). Table 1 shows the number of cases examined in each area by sex.

The number of parents with an occupation (P < 0.001) and with a permanent occupation (P < 0.001) significantly increased from the lower socioeconomic to the higher socioeconomic area (Table 1). These results confirm that the three areas were socio-economically different.

Girls were slightly heavier and taller, but had shorter leg length than boys. All anthropometric variables had distributions close to normal. Children in lower socio-economic neighbourhoods were shorter and lighter, and had shorter legs. Even after adjustments were made for sex and age, still all anthropometric variables: height (P < 0.001), weight (P < 0.001), BMI (P = 0.001) and leg length (P < 0.001) of children significantly increased as the socio-economic status of area increased. These findings demonstrated clear social gradient in the general health status among Shiraz primary school children.

The average number (± standard deviation) of present deciduous teeth was 6.2 (± 4.3). Number of permanent teeth present in the mouth ranged from four to 24 with an average of 16 fully erupted permanent teeth per child (Table 2). Number of present permanent teeth was significantly higher in girls (mean = 18.1) than in boys (mean = 13.6). The difference remained statistically significant after adjustment for age (P < 0.001).

Forty nine percent (164) of children had a DDE. There was no difference between the percentages of boys and girls having a DDE in permanent teeth. Table 2 shows the number of permanent teeth with DDE and the total number of DDEs in permanent teeth. Both variables increased as socio-economic status of area decreased. The trends were statistically significant for number of permanent teeth with DDE (P = 0.018) and the total number of DDEs in permanent teeth (P = 0.025) after adjusting for number of present permanent teeth in the mouth. The significance levels increased (P = 0.008 for both variables) after adjusting for age and sex in the regression model.

The average dmft (of the primary dentition) was 2.8 ± 2.5. Three quarters of the cases (75.2%) had a dmft of 1 or more. The average DMFT (of the permanent dentition) was 1.22 ± 1.5. Almost half of the sample (47.5%) had caries experience in their permanent teeth. DMFT increased from 0.7 in 8-year-olds to 1.4 in 11-year-olds. Although the mean DMFT was higher in girls (1.3) than in boys (1.0) (P = 0.02), the difference was not significant after adjustments for age and number of permanent teeth (P = 0.42).

There was no statistically significant trend in DMFT or dmft among areas. Children in the middle class area had the highest caries prevalence and highest need for treatment among all three areas. Although a clear path of social gradients was not observed, total number of teeth with treatment need was significantly lower in children in high socio-economic area compared with other children (P = 0.006). There was also a significant difference in mean number of deciduous teeth with treatment need between the high socio-economic area and the two other areas (P = 0.005) (Table 3).

A considerable number of children had gingival inflammation. Average GI score was 0.18 with only 82% of children being scored 0. The average DI-S was 0.52. The difference between the sexes was not statistically significant (P = 0.43 for GI and P = 0.44 for DI-S).

The gingival health significantly improved as the socio-economic status of area increased (P < 0.001). The significance did not change after adjustments for sex and age. There was also significant trend in oral hygiene status (P < 0.001). The number of children with dental plaque (P < 0.001), and also the average level of dental plaque in children (P < 0.001) decreased as the socio-economic status of area increased (Table 4).