Dental fluorosis and caries incidence in rural children residing in a high fluoride area in the dry zone of Sri Lanka

iA study was conducted to establish the prevalence land severity of dental fluorosis and to investigate iits relationship to dental caries in 222,12-14 year ,old children who have been life long residents in a rural area in the dry zone of Sri Lanka where the drinking water contains an above-optimal fluo­ ride concentration. Dental fluorosis was assessed using the WHO criteria. Samples of water used for drinking by them were analysed for fluoride levels. The results revealed that 97% of the chil­ dren were affected with dental fluorosis. Three per cent was completely free of dental fluorosis while about 20 per cent had extensive fluorosis (scores of 3 and 4). Fluoride levels in the drinking water samples varied from 0.21-9.8 ppm. Higher fluorosis scores were observed in children drink­ ing water with higher fluoride content. The preva­ lence of caries increased as the degree of fluoro­ sis .increased. The mean DMFT was 0.43 in chil­ dren showing no fluorosis but increased up to 1.65 in children showing a fluorosis score of 3.


Introduction
It is well documented that excess fluoride intake during the period of tooth formation causes den tal fluorosis (1,2). The fluoride appears to affect the activity of ameloblasts, especially during the late secretion and early maturation of enamel (3). The period of susceptibility for an individual may spread up to the time of formation of the crowns of third molar permanent teeth, the period of con cern being the first 7-8 years of life.
Possible sources of fluoride known to be strong enough to cause fluorosis include drinking water, inadvertent swallowing of fluoridated toothpaste, dilution of concentrated and pow dered infant formula with fluoridated water and possible dietary practices of excessive consump tion of tea or fish.
Endemic dental fluorosis has been reported from different regions of the world where excessive amounts of fluoride are contained in the drink ing water (4,5). In Sri Lanka too endemic dental fluorosis has been reported, especially in the North Central Province (NCP), where the preva lence rate reported is about 51-77% (6). Dental fluorosis prevalent in the NCP of Sri Lanka is at tributed to water-bone fluorides.
The purpose of this study was to establish the prevalence and severity of dental fluorosis and to investigate its relationship to dental caries in a group of 12-14 year old children residing in an area in the dry zone of Sri Lanka where the drink ing water contains high fluoride concentrations.

Selection of the sample
The sample selected for the present study belongs to a community living in a rural hamlet known as Eppawela in the North Central Province of Sri Lanka. This area located in the dry zone, has a mean maximum annual temperature of 32° C. As there is no pipe-borne water supply to this area, all families residing in this area obtain their drink ing water exclusively from wells.
The children included in the present study were selected from the secondary schools of the area. A sample of classrooms with children aged 12-14 years was selected using a simple random sam pling technique. All the children in the selected classrooms who were present on the day of the investigation constituted the study sample. How ever the children who were not life long residents of this area were excluded from the study.
The oral cavities of the children were examined under natural daylight to determine the dental fluorosis and prevalence of caries. The teeth were cleaned before examiniation. Severity of dental fluorosis was assessed using the modified Dean's index (7) with a range from normal enamel (score 0) to severe fluorosis (score 4). A score was as signed to each tooth present in the oral cavity. The fluorosis score assigned to an individual corre sponds to the more severe score assigned for two teeth.
Dental caries levels were evaluated following the WHO criteria (1987). Examinations for dental fluo rosis were carried out by one examiner (D.N.) and those for dental caries by another (M.S.C). An interexaminer variability, therefore did not exist. The intraexaminer variability was assessed by re examination of 20% of the sample each day dur ing the study. An intraexaminer level of agree ment between 0.82-0.87 and 0.79-0.86 was ob tained for dental fluorosis and dental caries re spectively The fluoride concentrations of drinking water samples collected from the children were analysed using the ion specific electrode method.

Results
A total of 222,12-14 year old children participated in the survey. The majority of the children be longed to the low socio-economic group. 52.7 per cent of children studied were females, 53.2 per cent were 12 years old, 28.8 per cent were 13 years old and 18 per cent were 14 years old.
As there were no significant differences in dental fluorosis scores between males and females or be tween different age groups, the results are pre sented for the total sample. Table 1 shows the distribution of dental fluorosis according to the Dean's classification. The results reveal that a mere 3 percent was completely free of dental fluorosis and showed normal tooth sur faces. 83.34 percent of the sample had a fluorosis score of 1 or above. Approximately 20 percent of the sample showed extensive fluorosis (score of 3 or above), whereas 66 percent showed mild fluo rosis. The prevalence of severe fluorosis with pit ting and chipping off enamel was seen in 5.4% of the sample.   Fluoride levels in the samples of drinking water collected varied from 0.21 to 9.8 ppm. Table 3 shows the percent distribution of fluorosis scores in the sample according to the water fluoride con centration. Higher dental fluorosis scores are ob served among the children whose water contained a high fluoride content.   Table 2 Deepthi Nanayakkara, Malkanthi Chandrasekera and W. R. Wimalasiri

Discussion
All families living in Eppawela obtain their drink ing water exclusively from wells. Due to the scar city of water, many tube wells drawing water from a depth of 36-75M have been constructed in convenient locations for the villagers to ob tain their water. Analysis of water samples from the wells showed the fluoride concentration in the ground water to vary from 0.21 to 9.8 ppm.
The fluoride content was much greater in the water samples obtained from tube wells, one be ing as high as 9.8 ppm.
The prevalence of fluorosis in the present study, 97 percent, is much greater than the values re ported in previous studies done in Sri Lanka (8,9). In these studies tube well water as a source of excessive fluorides is unlikely, as these wells were constructed much later than the period of devel opment of the permanent dentition of the sub jects studied. Widespread use of fluoridated toothpaste was also not prevalent at that time. The appearance of higher dental fluorosis scores in children consuming water with low fluoride concentrations suggests that there are other con tributory factors. The data showed that 64 per cent of the sample to be using fluoridated toothpaste. The extensive use of fluoridated toothpaste could be another factor that increased the prevalence of fluorosis. Samples of food items commonly con sumed by these children and vegetables grown in their own gardens were analysed to find the fluo ride content. The data revealed most samples, es pecially the green leaves, pulses, certain vegetables etc grown in this area to contain high fluoride con tents. The most commonly used beverage in this community is tea, which is known to contain high contents of fluoride. It has been estimated two to four cups of tea a day would on an average provide 0.6-1.2 mg of fluoride (13). This could be another contributory factor. The hot and dry climate prevailing in this area may influence the quantity of water consumed and thereby contribute towards fluorosis.
Since the high fluoride level in the drinking water bears no beneficial role in the reduction of dental caries and is a major contributory factor towards the aesthetically unacceptable dental fluorosis, methods of defluoridation need to he promoted in these areas to remove fluorides from drinking water. Until such times wells containing low lev els of fluorides need to he identified for the villag ers to obtain drinking water and health education programmes regarding the aetiology and preven tion of the 'stains' present in their teeth, need to be initiated.