Inequalities of caries experience in Nevada youth expressed by DMFT index vs. Significant Caries Index (SiC) over time

For many years, the World Health Organization (WHO) global goal for year 2000 for dental caries of no more than an average of 3 DMFT (decayed, missing, filled teeth) at 12 years of age has been used as a global yardstick for oral health program success [1]. Decades ago the WHO developed oral disease surveillance systems to monitor dental caries in children. The first global map with DMFT data on 12-year-olds was presented in 1969. This map indicated high prevalence of caries in industrialized countries and generally low values in the developing countries [1]. Although dental caries prevalence in industrialized countries has declined significantly since the early 1970s, oral diseases, including caries, remain a major public health challenge [2‐4]. In 2007 the WHO reported that 60-90% of school children worldwide have dental caries [5]. Traditional dental care remains a significant economic burden for many countries, where 5-10% of public health expenditure relates to oral health [5]. In US children, the recently reported prevalence of dental caries was approximately 60% in ages 12 to 19, with a reported 20% having untreated tooth decay [4]. Childhood dental caries has been reported to be the most prevalent infectious disease in our nation - 5 times as common as asthma and 7 times as common as hay fever [6]. A review of progress towards meeting the Healthy People 2010 Objectives for Oral Health, noted that 11 of the objectives have shown little or no progress [6]. In 2004, most American children reported good oral health, but subsets suffered a higher level of oral disease, primarily children living in poverty and some racial/ethnic minority populations [7]. Seventy-eight (78) percent of 17-year-olds had experienced tooth decay. One in four American children was born into poverty, suffering twice as much tooth decay as their more affluent peers and more likely having no access to oral health care [7]. Children from families without medical insurance were 2.5 times less likely than insured children to receive dental care and 3 times more likely than insured children to have unmet dental needs [7]. The use of DMFT scores to establish severity and prevalence of caries is an accepted practice in the dental community and has continuously been used to assess prevalence in the National Health and Nutrition Examination Survey (NHANES) [4]. An analysis conducted by the WHO found that there was a skewed distribution of caries prevalence in many countries; a significant proportion of 12-year-olds still had high or even very high DMFT values even though a proportion was totally cariesfree [8]. This polarization of the caries picture has the effect of making the mean DMFT value less meaningful as a population descriptor in that it does not accurately reflect the burden of disease [9]. Thus it may lead to the incorrect conclusion that the caries situation for the whole population is under control, whereas in reality population subgroups still suffer from high caries rates. The Significant Caries Index (SiC) was introduced in 2000 to bring attention to the individuals with the highest caries values in each population [8, 10‐12]. The aims of this study were to analyze the prevalence and severity of dental caries in Nevada youth over a period of eight years and to compare its expression by means of DMFT and SiC; analyze the caries trends in the population and their underlying factors; and determine whether Nevada youth were at risk for significantly high levels of dental caries and should be specifically targeted for oral health interventions.

Demographics of those screened are shown in Table 1. During the eight years of survey the annually examined populations varied between 6,400 and 10,900 individuals. Apart from the first survey year the younger group (13-15 years old) stabilized at around 2/3 of the ones examined, whereas the older group (16-19 years old) comprised around 1/3. Throughout the surveys the gender balance was almost 50:50. There was a downward trend in the proportion of adolescents of White background, whereas both Blacks and Hispanics showed increasing proportions of the examined populations. The relative distribution of the study groups according to residence varied throughout the period with Clark County residents consistently underrepresented (Clark County comprises around 72% of Nevada's population). During the first 4 years of survey insurance status was relatively stable across the entire period with around 2/3 reporting insured status and 1/3 not insured.

The mean DMFT values by year is detailed in Table 2. Overall, the DMFT index stayed around 3 DMFT at the beginning and at the end of the survey period. However, during the period some decrease took place, which reverted to an increase towards the end of the survey years. This trend was reflected for the two age groups as well with the younger age group at a significant lower level than the older. Both age groups had significantly higher DMFT score than the comparable national samples of the same age illustrated by the NHANES figures in the table. Females had consistently higher DMFT than males and Hispanic examinees were consistently higher than Whites and Blacks. Residents of the fluoridated Clark County were consistently lower than residents of the rest of the State, and insured residents exhibited less caries experience than the non-insured group.

In order to assess the polarization in the caries picture over time, Figure 1 illustrates the proportion of the examinees who were cariesfree. Overall, in year 1 28% were cariesfree and around 32% were cariesfree in year 8. But it was evident that the younger and the older groups diverged over time with an increase in the cariesfree younger persons from around 30% to around 35% and a decrease among the older persons, from 26% to 22%. Conversely, this figure also illustrates that the prevalence of dental caries in the younger group was around 65% and around 77% in the older group.

The variation of cariesfree individuals over time in relation to racial background and county of residence respectively are illustrated in Figure 2 and Figure 3. It is interesting to note that the white population reflected a general increase in proportion of cariesfree individuals (from around 29% to around 38%), whereas the two other groups started out around 25%, improved somewhat during the survey years, but ended up around 25-26% in year 8 (Figure 2). According to county of residence a dramatic difference was evident in that the proportion of cariesfree individuals in fully fluoridated Clark County varied around 40-80%, whereas only around 10-20% in all other counties (not fluoridated) were found to be cariesfree.

The SiC indices by survey year is detailed in Table 3. It comprises the 30% of the examined population with the highest DMF values and thus, the mean caries values were considerably higher than the corresponding DMF values ranging from an initial 8.86 DMF to 6.01 in the middle of the period and then climbing up to 7.0 DMFT.

The SiC Index was significantly higher than mean DMFT Index when comparing each demographic variable within each survey year (p < 0.001). Females, older adolescents (16-19 year olds), minority groups, those living in areas where the municipal water supply is not fluoridated, and those without dental insurance had higher mean DMFT scores. SiC Indices were significantly higher across the board, with the results paralleling the trends in the mean DMFT scores

In order to assess the relative impact of the independent demographic variables on the probability of ending up in the high caries group we performed a bivariate logistic regression analysis. The resulting Wald Statistics as shown in Table 4 indicated that each of the independent variables was highly statistically significant. The pattern of the logistic regression analysis changed little from one year to the next. The probability of being in the SiC group, i.e. being among the 30% of individuals with the highest DMFT scores was related to being older (OR between 1.6-1.9), being female (OR around 2.0), being Hispanic (OR around 2.0) or Black (OR between 1.4-1.8), residing in a non-fluoridated community (OR between 1.8-2.8) or being non-insured (OR between 2.0 - 2.4)(Table 5).

This study investigated the caries prevalence and severity trends in Nevada youth using both the mean DMFT index and the SiC Index. The use of DMFT has been an accepted practice for assessing the prevalence and severity of caries in a population [4]. However the epidemiologic changes in the dental caries picture during the last 2-3 decades, have made it increasingly evident that mean DMFT values do not capture the polarized caries development with a more skewed distribution of caries [8]. Mean DMFT values are an average of all members of the population, irrespective of the distribution of the severity of the disease within the population.

The series of sequentially conducted epidemiologic studies demonstrated that downward trends were found in both mean DMFT scores and SiC Indices across all demographics compared (age, sex, race group, whether residing in fluoridated area, and dental insurance status) between year 1 and year 6. Since year six, there has been an upward trend in all demographics. An increasing divergence in caries prevalence was found between younger and older adolescents and between individuals of white and all other racial backgrounds. The mean DMFT values did not accurately reflect the skewed distribution of dental caries in Nevada youth leading to incorrect conclusions that the caries rate for the statewide population is under control. The data indicate there is a large proportion of adolescents in Nevada with poor oral health status, which only seems to improve very slowly. Decreasing caries prevalence in all countries is important to promote overall general health. The probability of ending up in the high caries group was consistently related to being older, being female, being Hispanic or Black, residing in a non-fluoridated community, and not having insurance cover.

Findings from this study should aid in two ways. At the community level, the study strongly points to the importance of retaining and expanding the community fluoridation program as an effective preventive measure. At the individual level the study identifies the need for more targeted efforts to reach children early with a focus on females, Hispanics and Blacks, and uninsured children.