Effect of socio-demographic status on dental caries in pupils by using a multilevel hurdle model

Yunes Jahani, Mohammad R. Eshraghian, Abbas R. Foroushani, Keramat Nourijelyani, Kazem Mohammad, Arash Shahravan, Mahin Alam

Adolescents, Youth and Schools Health Unit, Kerman University of Medical Sciences, Kerman, Iran.
Department of Epidemiology and Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran;.
Kerman Oral and Dental Diseases Research Center, Kerman University of Medical Sciences, Kerman, Iran.

DOI: 10.4236/health.2013.57150                

Abstract

Introduction: Dental caries is the most prevalent chronic disease among schoolchildren. We aimed to investigate the association between dental caries index and socio-demographic status in schoolchildren by using a multilevel hurdle model. Materials and Methods: A cross-sectional epidemiological study was carried out on 906 primary school pupils in Kerman, Iran in 2012. The subjects were selected through a stratified cluster random sampling. At first, the whole target area was stratified according to two geographic areas, the north and south area. Then each area was stratified according to gender. In the next step, several schools were considered as clusters, which were selected randomly in each geographic area and gender strata. All the schoolchildren in these schools were included in this study as samples. Twelve-year-old pupils were examined for dental caries. The dependent variable in this study was the dmft/DMFT index. Eight variables, including socio-demographic information, were collected. Multilevel negative binomial hurdle model was employed for data analysis. Results: The prevalence of caries-free pupils was 30.1% and the mean dmft/DMFT was 3.6 ± 2.2. Negative binomial part of the adjusted model showed that the dmft/DMFT adjusted rate for females was 1.36 (95% CI: 1.10-1.79) times higher than males. Also, the dmft/DMFT adjusted rate for overweight pupils was 0.86 (95% CI: 0.74-0.98) times lower than those with normal weight. Logistic part of the adjusted model showed that the posibility of caries-free state in overweight pupils was 1.95 (95% CI: 1.22-3.11) times higher than those with normal weight. In addition, pupils whose fathers and mothers were workers and housewives, respectively, and those with a high maternal age were at a greater risk for caries experience than others (p < 0.05). Conclusion: BMI, gender, parent’s job and mother’s age at delivery were factors effecting on dental caries in schoolchildren. These pupils need more attention to dental care.

Keywords

Multilevel Hurdle Model; Dental Caries; Socio-Demographic Status; Primary School Pupils; dmft/DMFT Index

1. INTRODUCTION

Dental caries is the most prevalent chronic disease among schoolchildren. It is a cumulative and progressive disease causing pain, infection and possible disfigurement, particularly in children [1]. The World Health Organization’s report on oral health provides an overview of global epidemiology of caries, confirming its international pandemic distribution [2]. Nearly 20% of 2 - 4- year-old children have clinically detectable caries, and by age 17, nearly 80% of young people have one or more teeth with caries [3]. The majority of studies within the last 10 years in Iran have shown that caries prevalence in pupils is 35% - 85% [4-7]. Since children are the most susceptible groups to tooth decay, determining dental caries indices and factors influencing it as one of the most important aims of various research studies in different countries [2,8].

Research has shown that tooth decay is a multi-factorial disease, which is under the influence of numerous factors such as parents’ education, parents’ job, size of the family and other socio-demographic factors [9-11]. Some studies have shown that variables such as BMI, gender and mother’s age at delivery are effective on dental caries [12-17]. Understanding the impact of sociodemographic predictors on dental caries could be useful for planning public health policies, thus leading to a better allocation of resources [18].

Statistical modeling plays an important role in understanding caries risk factors [19]. Some researcher found that caries counts are not generally approximated by a normal distribution and common analysis such as t-test and linear regression are not appropriate [10,19-21]. In addition, there are a high proportion of zero scores for dental caries indices. In order to correct results and interpretations, we used a multilevel negative binomial hurdle model. The hurdle model approach is a 2-part count regression method which deals with the phenomenon of excess zeros in data [19].

We aimed to investigate the association between dental caries index and socio-demographic factors in 12- year-old schoolchildren living in Kerman, the largest and most important city in southeast Iran, in 2012. We attempt to identify those children at greatest risk to dental caries formation.

2. METHODS

2.1. Data Set Examined

This cross-sectional study was carried out on 906 twelve-year-old schoolchildren in Kerman, Iran in 2012. In Iran, trained school nurses regularly examine the health information for pupils in primary schools. The dental examination was carried out by school nurses for detection of dental caries in accordance with World Health Organization criteria [22]. The dental examination was noninvasive and x-rays were not used. These health records were used and socio-demographic information was recorded for selected pupils in 2012. The subjects were selected through a stratified cluster random sampling. At first, the whole target area was stratified according to two geographic areas, the north and south area; the northern area generally has a better economic situation than the southern area. Then each area was stratified according to gender. In the next step, several schools were considered as clusters, which were selected randomly in each geographic area and gender strata; all the schoolchildren in these schools were included in this study as samples. The subjects were from ten boy schools and nine girl schools, representing 10% of the target population.

2.2. Measurements

In this research, the dmft/DMFT index was used as the dependent variable because it defines the condition of caries in children [22]. This index was calculated as the sum of total decayed (d/D), missing (m/M) and filled (f/F) teeth for primary and permanent dentitions, respectively.

Pupil’s BMI (Body Mass Index) was calculated as weight in kilograms divided by the square of the height in meters (kg/m²). Standardized percentile curves of BMI in Iran were used. According to these curves underweight condition is defined as under the 5th percentile curve, normal as between the 5th and 85th percentile, at risk of overweight as higher than the 85th and lower than the 95th percentile and overweight as higher or equal to the 95th percentile [23].

The categorical variables for pupils, studied here, included gender (boy/girl), family size (three, four and five or more) and BMI (underweight, normal, at risk of overweight and overweight). Parental data included father’s and mother’s education (high school degree or lower/ university), father’s job (employee, worker and self-employed), mother’s job (employee/housewife) and mother’s age at delivery (less than 35/more than 35 years).

2.3. Reliability and Validity

For assessing inter-examiner reliability (i.e. school nurses), three school nurses and twenty pupils were selected. Then, dental examinations were carried out by each school nurse on all the twenty pupils. The intraclass correlation (ICC) between the results of examinations (dmft/DMFT) for school nurses was 0.91, demonstrating a high agreement rate between school nurses. In addition, for assessing validity, a dentist carried out dental examinations on the same twenty pupils. Then the results were compared with those of school nurses. Repeated measures analysis of variance did not show any significant differences between the dentist and school nurses in relation to dental examinations (ρ = 0.26). In addition, Pearson’s correlation coefficients (ρ) between the dentist and each school nurse in relation to dental examinations was high (ρ = 0.92 for Examiner 1, ρ = 0.91 for Examiner 2 and ρ = 0.91 for Examiner 3), indicating that the results of examinations by school nurses were valid.

2.4. Statistical Analysis

Frequency and percentages of caries-free (dmft/DMFT = 0) pupils and categorical variables were reported. The mean ± SD for caries severity, dmft/DMFT index, among subjects with any caries was reported. A multilevel negative binomial hurdle model (MNBH) was used to investigate the effect of the variables evaluated on dental caries, which has recently been suggested as an approach that gives a better fit to these types of count data [19].

A backward elimination (B.E) approach was used to exclude independent variables from the adjusted model. The final model contained only factors that were significant at a significance level of p ≤ 0.05. The odds ratio (OR) and its 95% confidence interval were reported in relation to the logistic component of the model, corresponding to the caries-free state. The incidence rate ratio (IRR) and its 95% confidence interval were reported in relation to the truncated negative binomial component of the model, corresponding to caries severity, i.e. dmft/ DMFT index [19].

The Akaike information criterion (AIC) index was used to compare the model used here with Poisson hurdle model, as well as with Poisson and negative binomial regression model. Data were analyzed using programs written with SAS 9.2.

3. RESULTS

Table 1 shows different characteristics of the subjects. Of the 906 schoolchildren in this study, most were of normal weight. The majority of the pupils lived in a family of four persons. The majority of the parents was high school graduates or had less education and most were self-employed. The majority of mothers were housewives. In addition, most mothers had been under 35 years at delivery (childbirth).

The caries-free prevalence in the subjects was 30.1%. The mean dmft/DMFT among pupils with any caries was 3.6 ± 2.2 (range: 0 - 9). The caries-free prevalence and the mean dmft/DMFT in the present study are presented in Table 1.

Table 2 shows the results of fitting separate univariate multilevel NBH model between each independent variable and dmft/DMFT index. Logistic parts of these models showed that father’s and mother’s job, mother’s age at delivery and BMI were significant factors for the prevalence of caries-free state (p < 0.05). Other variables had no significant effect on the prevalence of caries-free state (p > 0.05). According to truncated negative binomial parts of the hurdle model, only gender and BMI had a significant effect on caries severity (p < 0.05). Other variables had no significant effect on caries severity (p > 0.05).

Conflicts of Interest

The authors declare no conflicts of interest.

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