Oral Health Status in the Pars Cohort Study

1Department of MPH, Shiraz University of Medical Sciences, Shiraz, Iran 2Shiraz University of Medical Sciences, Shiraz, Iran 3Oral and Dental Disease Research Center, Department of Operative Dentistry, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran 4Non-Communicable Disease Research Center, Shiraz University of Medical Sciences, Shiraz, Iran 5Liver, Pancreatic and Biliary Diseases Research Center, Digestive Disease Research Institute, Tehran University of Medical Sciences, Tehran, Iran


Introduction
According to the World Dental Federation (Fédération Dentaire Internationale; FDI), oral health is multifaceted and includes the ability to speak, smile, smell, taste, touch, chew, swallow, and conveys a range of emotions through facial expressions with confidence and without pain, discomfort, and disease of the craniofacial complex. 1 Poor oral health, including periodontal disease and tooth loss, is common all over the world. In 2010, 2.3% of the world population (158 million people) were edentate. 2 Furthermore, dental caries is the most common oral disease worldwide and almost every person experiences caries during their life time. 3 According to the World Health Organization (WHO), oral disease stands as the fourth most expensive chronic disease to treat. 4 Poor oral hygiene can result in serious comorbidities including periodontitis, pain, speech and psychological problems, tooth decay, loss of tooth and discomfort in gums or teeth. 5 Epidemiologic studies have shown the association between periodontal disease, tooth loss and dental caries with an increased risk of chronic diseases such as cardiovascular disease and diabetes and also higher risk of cancer at different sites including lungs, oral cavity, esophagus, stomach and pancreas. 6 Due to the growing elderly population, the prevalence rates of oral and dental diseases are increasing. 2 Total or partial edentulism may be the onset of disability and mortality, particularly in the elderly. 7 Many factors affect oral health, including socioeconomic and environmental status, income, urbanization and access to health services. 8 Excessive and frequent consumption of sugar, inadequate fluoride exposure, and smoking and tobacco use are associated with increased incidence of dental caries and periodontal diseases. 9 Oral diseases pose the greatest burden on the poor and socially marginalized people. 10 In Iran, a developing country that has experienced an increase in the rate of older individuals, oral health has become a major public health issue. 11 According to different reports from epidemiological studies, [12][13][14] and mainly due to a lack in nationwide registries, data on oral health in many regions is scarce and largely missing, especially in middle-and low-income regions including southern Iran. Thus, in this study, we aim to determine the oral health status and its association with sociodemographic and economic variables in one of the largest cohort studies conducted in southern Iran.

Study Design and Settings
This is a cross-sectional study using data from the Pars Cohort Study (PCS). The PCS is an ongoing cohort study which started in 2012 in Valashahr district, a rural area consisting of Valashahr city and 93 villages in southern Iran, home to more than 40 000 inhabitants. In total, 9721 participants aged 40 to 75 years were invited and 9264 of them were recruited from 2012 to 2014. Potential participants were excluded if they were unwilling to participate or were a temporary resident. More details on the PCS design have been published previously. 15 The primary aim of the cohort study was to perform an in-depth investigation of non-communicable diseases. Accordingly, a list of more than 200 variables including socio-demographic, clinical and laboratory data were collected from each individual in the initial phase of the cohort. This research was approved by the Ethics Committees of both Tehran and Shiraz Universities of Medical Sciences. The purpose of the study was explained to the participants and informed written consents were obtained.

Oral Health Assessment
A structured questionnaire was designed which contained questions about oral health. First, the participants were asked if they had lost any permanent teeth, and if their answer was yes, the age at which the first permanent tooth was lost was inquired. Participants were also asked regarding removable partial or full dentures and whether they wore these dentures during the day. Frequency of tooth brushing was reported as never, once a day, twice a day, three times a day and other (which was less than once daily). Before starting the study, physicians were trained by a dentist. The physicians determined decayed, missing and filled teeth (DMFT) visually and used dental probes to assess cavitation.

Data Preparation
Data cleaning was done using appropriate techniques. 16 Inconsistent or erroneous data were identified and corrected. Data on age, sex and ethnicity (Persian, Turk and other ethnicities) were acquired. Educational level was categorized in 3 classes (illiterate, less than diploma and university degree). The multiple correspondence analysis (MCA) technique was applied and accordingly, the socioeconomic status (SES) of each participant was estimated. The participants were categorized into four classes (quartiles of estimated latent factor by MCA) including low, middle-low, middle-high, and high SES. 17 History of cigarette use, current use of cigarettes, use of tobacco products other than cigarettes and history of opium use were taken. For physical activity, the participants were classified into three groups (low, medium and high) based on their metabolic equivalent of task (MET) score. 18 Dental health indices including DMFT, missing teeth (MT), filled teeth (FT), decayed teeth (DT), and age at first tooth loss were categorized into four categories (quartiles), where distribution was inflated on a single value (i.e. 32 for MT, or zero for FT/DT), a separate category was defined for that value, and quartiles were defined after removing that value. Cases with tooth brushing twice or three times a day were put in a single category due to their small numbers. Finally, dental health indices for elderly people (65-75 years old) were also assessed.

Statistical Analysis
Data was described as frequency and percentage for descriptive variables and as mean and standard deviations and as median and interquartile range for quantitative variables.
Rate of DMFT and MT was calculated by dividing the value of each variable by 32 (the total number of teeth). The denominator for calculating the rate of DT or FT was the number of remaining teeth. Data analyses were done assuming a Poisson distribution for these rates. Because of a significant over-dispersion and also inflation of the distribution of dental health indices (i.e. DMFT, DT, MT, FT), data was analyzed using univariate and multivariable Generalized Negative Binomial regression (GNBReg). Four GNBReg models were fitted. Each model assessed the correlates of each of the four dental health indices. Crude and adjusted incidence rate ratios (IRR) and their 95% confidence intervals (CI) were estimated. Variables with likelihood ratio test P values of less than 0.1 were retained in the multivariate logistic regression model. P values less than 0.05 were considered significant. Data was analyzed using the Stata software (Release 11; StataCorp. College station, TX, USA)

Results
Overall, 9264 participants entered the final analysis. The participants' mean (SD) age was 52.6 (9.7) years and 53.8% of them were females. Other sociodemographic characteristics of participants are shown in Table S1 of Supplementary file 1.
In general, 4393 (47.4%) individuals did not brush their teeth at all, 2240 participants (24.2%) less than daily, 2037 participants (22.0%) brushed their teeth daily, and 592 participants (6.4%) brushed their teeth 2 or 3 times a day. Regarding ethnicity, 44.1% of Persians and 55.9% of non-Persian participants did not brush their teeth at all. Women showed a higher prevalence of tooth loss than men (mean 15.8; SD 10.9 vs. mean 14.5; SD 10.1, P < 0.001). Oral health determinants (DMFT and its subgroups, upper and lower denture use, frequency of tooth brushing and age at first tooth loss) are shown in Table 2.
In the negative binomial regression model, factors associated with tooth loss were: increasing age  Table S1 of Supplementary file 1).

Discussion
In our study, almost everyone had lost at least one tooth (96.8%) and less than one percent of the population had preserved all of their teeth. This represents the undesirable status of oral health in the studied population.
The prevalence of edentulism in our whole population was 19%. Peltzer and colleagues evaluated the prevalence of loss of all teeth among persons 50 years and older in 6 countries. 19 Their findings showed the prevalence rate in different countries to be at 3.0% in Ghana, 8.5% in South Africa, 9.0% in China, 16.3% in India, 18.0% in the Russian Federation and 21.7% in Mexico. According to a report by the National Center for Health Statistics, 20 approximately 19% of people aged 65 years or older in the United States were edentulous. In comparison, in our senior population, 43% had no natural teeth. Edentulous people tend to eat less fresh fruits and vegetables which results in deterioration in their nutrition and health. Furthermore, it leads to social isolation and loneliness due to embarrassment and loss of self-esteem. 21 The prevalence of dental caries reported in our total participants and in seniors was 68.9% and 50.3%, respectively, which is much more than that reported in other regions of the world. The prevalence of root caries among individuals within a senior age group (65-74-yearold) was 14.4% in Spain, 13 24.0% in Belgium, 13 28% in Germany, 13 28.4% in Turkey, 22 and 38.3% in Greece. 23 We found that women had more tooth loss than men (15.8% vs. 14.5%, P value < 0.001) which is similar to the figures of other reports. 24,25 This can be attributed to the bio-psychological conditions women face during their life; multiple births and breast feeding which remarkably affect their oral and dental health. 11 The rate of tooth loss (IRR: 1.52) and the rate of decayed teeth (IRR: 1:54) in our population increased with increasing age, which was expected as physiological  and pathological abrasions more often occur in calcified dental tissues with increasing age. 26 Besides, elderly people commonly use many medications due to different chronic conditions and diseases which result in dry mouth and hyposalivation and consequently trigger periodontal diseases and dental caries, the main causes of tooth extraction. 21,27 Rates of tooth loss were higher in Turks and minor ethnicities in our study. A higher rate of people of non-Persian ethnicity did not brush their teeth compared to people of Persian ethnicity (55.9% vs. 44.1%, P < 0.001). A significant difference has been reported in terms of oral health between the dominant ethnicity and minor ethnicities in other regions of the world, as well. 28 This may be due to the difference in medical insurance and different cultures and habits related to oral hygiene.
Reports have shown that being married is protective against tooth loss and periodontitis. Married people have more dental cleanings and significantly more dental visits than unmarried people. 29,30 However, we did not find any association between tooth loss or DMFT and marital status in our study.
Smokers in our study had more tooth loss (IRR: 1.17) and dental caries (IRR: 1.13), which is similar to other reports. 31,32 According to a cross-sectional study in Iran, among a group of participants who were opiate dependent, none of the participants had a healthy periodontium. 33 Besides the direct effects of drugs on oral health, individuals who have some form of drug dependency have many unhealthy behaviors including personal neglect of oral health care, high carbohydrate consumption and low priority for oral hygiene. Furthermore, drugs may suppress symptoms of tooth decay. 34 In another study by Reddy et al, it was shown that substance abusers had more missing teeth, periodontal destruction and gingival recession. 35 These studies were consistent with our results as we found that people who consumed opium had higher DMFT and more tooth loss.
In our study, individuals with higher physical activity had more tooth decay (IRR: 1.09). This could be explained as acute dehydration due to exercise and heat stress reduces saliva flow rate and causes saliva protein density and osmolality to increase. 36 In one study, Ferreira et al evaluated the effects of endurance training on dental erosion, caries and saliva. They found that one hour per week increase in endurance training was correlated with a rise in DMFT at 0.24 points. 37 This may be due to the fact that saliva protects against dental erosions due to its buffering and neutralizing characteristics on acidic food. 38 However, we did not find any association between physical activity and tooth loss. We found a higher prevalence of tooth loss and dental caries among illiterate participants which is similar to other reports; illiterate people or those with lower educational levels paid less attention to their oral health and had lower rates of visits to the dentists compared to people who finished high school or had higher education. 28,37,38 Multiple studies have shown that lower SES is significantly associated with a higher prevalence of tooth loss and dental caries, 25,[39][40][41] which is similar to our results. Furthermore, people in lower socioeconomic conditions prefer extraction rather than restoration of teeth. 25 This study has several limitations and strengths. This is a cross-sectional study and is limited by its nature and design. Causal relationships cannot be accurately assessed. Moreover, diagnosis of decay was based on clinical inspection and probe, and radiography images were not taken. One of the important strengths of this study is its large sample size with incorporation of a large variety of variables. Data were extracted from a population of individuals aged 40-75 years who belonged to a rural area which provides valuable information on the rural population of Iran. Tooth loss, dental caries and DMFT were assessed by trained physicians, which is much more reliable than individuals' self-report.
In conclusion, the present study showed that the prevalence of edentulism, partial edentulism and dental caries was high among the study population. The absence of large cohort studies focusing on oral health reinforces the demand for future studies. Some risk factors like cigarette and opium use, education and brushing of the teeth are modifiable; therefore, programs for increasing general awareness and improving healthy life style are needed. Furthermore, eradicating the causes of tooth loss and tooth decay should be established with special focus on older individuals, people with lower SES, illiterate participants, smokers and opiate consumers.