Regarding the design of the study, cross-over effect of fluoride on the effectiveness of sealant placement in fissure caries prevention might exist, the amount of which cannot be estimated in this study. Despite this, similar proportions of molars in the two sealant groups were exposed to a topical fluoride application method in the same mouth. In addition, both types of sealant used in this study are fluoride-releasing materials. Thus, the above mentioned factor was considered to be balanced between the two sealant groups. Such factor was considered as a confounding factor and included in the logistic regression modeling during data analysis. The results show that it did not have a significant effect on the outcome of this study. Accumulation of the caries-preventive effect may be raised when combined application of different topical fluoride regimes are provided . The fluoride released from the two sealants plus the parallel exposure to topical fluoride applied on the contra-lateral molars in the majority of the mouth may increase the caries-preventive effect of both the sealants. In this case, the difference in caries-preventive effect between the two sealants would become smaller and require a larger sample size to detect it. It should be noted that the small magnitude of difference, most likely less than 10%, may not be clinically significant.
In this study, attrition bias due to the subjects and molars lost to follow-up was considered not a major problem because the following two reasons. Firstly, no statistically significant differences were found between the children followed at 24 months and those lost to follow-up regarding their background and oral health behaviors as well as the distribution of the molars. This indicates that the subjects and the molars lost to follow-up in this study at the 24 months probably did not differ substantially from those followed. Secondly, there was no statistically significant difference between the two groups in the distribution of children and molars lost to follow-up. The risk of detection bias was not totally avoided in this study. First of all, it is a common problem for studies comparing ART sealant with resin sealant because of the distinctly different appearance of the two types of sealants. Secondly, the two examiners placed a small number of sealants in this study after they had finished all the baseline examinations and when the other two independent dentists were not available. The latter factor is probably not too significant as the first follow-up evaluation took place 6 months after the sealant placement.
Resin sealant placement in this study is regarded as the positive control because its effectiveness in fissure caries prevention has been well established . Over the 24-month study period, incidence of dentine caries in the molars receiving resin sealant was only 3.9%, corresponding to a cumulative molar survival rate of 96% (SE = 0.01). This finding is comparable to that in a similar study conducted in southern China  as well as those of other studies [23, 27, 28]. Noting the background that the prevalence of dental caries in 12-year-old children in the study site was 29.8%, with 66.7% of which nested in the permanent 1st molar , such a low incidence of dentine caries may be taken to reflect the effectiveness of the resin sealant in preventing fissure caries in this study.
There was no statistically significant difference regarding the 24-month dentine caries incidence rates as well as the molar survival rates between the two sealant groups in this study. Multivariate two-level logistic regression analysis (GEE modeling) which can account for the effects of confounding factors and data clustering was adopted . Results of the GEE modeling confirmed that the risks to develop dentine caries in the fissures of the molars in the two sealant groups did not significantly differ from each other. Therefore, the null hypothesis of this study cannot be rejected.
There is currently no systematic review specially targeted on the comparison between ART sealant and light-cured resin sealant in fissure caries prevention. A literature search yielded four comparable original studies [16, 30–32]. Oba et al. found 3-year caries incidence rates of around 10% in both groups of molars receiving ART sealant and resin sealant respectively . Low 2-year caries incidence rates (<2%) in both the ART sealant and resin sealant groups were found by Chen et al., again, no significant difference in their effectiveness in fissure caries prevention was found . In the third study, no caries was observed in molars receiving ART sealant over 2 years and this was significantly better than that in the resin sealant group which showed a 4% caries incidence rate over the same period . A recent long term study showed 4-year caries incidence rates in both the ART sealant and resin sealant groups of less than 4% with no significant difference in their effectiveness in fissure caries prevention been found between them .
The 24-month retention rate of the resin sealant in this study is lower than those commonly reported in other studies which are around 80% [6, 33]. The lower retention rate of resin sealants in this study may be related to the less-than-optimal operating conditions for its placement which was the compromised school setting instead of a well-equipped clinical environment. Under field conditions, ample illumination, good moisture control, and thorough cleaning of the pits and fissures cannot be guaranteed. Similar problems were encountered in another study where sealants were also provided in a school setting which found a 93.8% complete loss of resin sealant 3 years after placement . Improvement of the operation conditions would probably lead to a better retention of resin sealants. Despite this, retention of the resin sealants in this study was still significantly higher than that of the ART sealants. This is in line with what has previously been reported .
The 24-month retention rate of ART sealant in this study (52% full + partial retention) is generally lower than those reported in previous comparable studies conducted under similar field settings which ranged 50-72% over a longer 3-year period [31, 34–36]. It was found in previous studies that retention of ART sealant was influenced by the experience of different operators with experienced ones performed better than the inexperienced ones [15, 34, 35]. Probably operators in those studies received better training and being more experienced in ART sealant placement.
Although a significantly lower retention rate and shorter retention time than those of the fluoride-releasing resin sealant were found for the ART sealant in this study, the effectiveness of ART sealant in fissure caries prevention did not differ significantly from that of the resin sealant used in this study. This might be explained by the findings of Beiruti et al. that high-viscosity GIC sealants had a four times higher chance of preventing caries development in re-exposed pits and fissures of occlusal surfaces in first molars than resin sealant over a 1- to 3-year period . It is also in agreement with a long-term follow-up study on ART sealant that the drop of the effectiveness of ART sealant in fissure caries prevention lagged the fall of its retention . In that study, it was found that dentine caries in molars with complete loss of ART sealant was relatively infrequent. Probably there were some clinically undetectable glass-ionomer particles retained in the deeper parts of the fissure as observed by Frencken and Wolke  and these offered continuing protection against caries. It is reiterated that due to the difference in the materials used, the success of a fissure sealing method should finally be assessed by the outcome of dentine caries prevention rather than material retention .
In the final GEE modeling in this study, it was also found that presence of early caries in the occlusal surfaces of the molars before sealant placement and shorter retention of sealant on the molars significantly increased the risk of developing dentine caries in the pits and fissures. These findings are consistent with those of other studies [1, 6, 26, 38].
Comparing the placement of the two types of sealants, it is noted that the number and training of dental personal required as well as the time used are similar. However, the set-up and running of an ART sealant program for children in schools will be easier than those of a resin sealant program. It is because the equipment required for ART sealant placement is rather simple (only a few hand instruments) whereas that for resin sealant placement requires an electrically powered dental unit. Given that the two methods yield similar effectiveness in fissure caries prevention, their relative affordability, availability, and simplicity should be considered when making a choice between the two. ART sealant might be more appropriate than resin sealant for use in less developed areas or in outreach dental service programs. In addition, the use of ART sealant instead of resin sealant will prevent the harmful exposure of children to additional Bisphenol A (BPA) releasing materials since systematic review has shown that BPA can be released from resin-based pit and fissure sealant . BPA exhibits a variety of toxicity effects on human bodies and evidences on the relationship between BPA and some adverse human health conditions (e.g. obesity, diabetes, coronary heart disease, enamel defects) have been shown by various studies [40, 41]. The raised safety concern has forced FDI to release a policy statement which discouraged the use of BPA in dental materials .