Since the 1980s, many studies [
10,
12] have investigated the effectiveness of PFS, most of which involved controlled experiments under ideal conditions. The retention rate of PFS and the reduction rate of dental caries were often used as an evaluation index for preventing dental caries. The first permanent molar was randomly selected on one side for PFS, while the same arch on the other side was chosen as the control group without intervention. However, research under ideal conditions may not truly reflect the effect of PFS in the population under real conditions. Moreover, few studies have explored the influence of ecological factors in PFS.
The purposes of this study are to evaluate the retention rate and effectiveness of PFS under non-ideal conditions and to explore the impact of ecological factors. This study is based on the work conducted by NODIPC, where PFS were used by dentists on a large scale on children aged 7–9 years in hospitals and dental clinics. In the busy daily work of the dentist, they could seal dozens of children’s teeth with PFS.
The retention of PFS and the prevalence rate of caries
The results indicated that the retention rate of PFS decreased over time. The full and partial retention rates were 81.6% and 13.4% after 12 months, respectively. In comparison, the full and partial retention rates were 75.1% and 14.2% after 24 months, respectively. Tu Rui et al. [
22] demonstrated that the annual incidence of caries of the first permanent molars of children aged 7 years in China was 6.75%. Under realistic conditions, our study indicated that the incidence of caries and DMFT of the first permanent molars sealed by PFS were 2.06% and 0.07 after 12 months and 4.46% and 0.16 after 24 months, respectively. The results also demonstrate that the implementation of PFS can effectively reduce the risk of caries.
We also studied the relationship between the retention of PFS and gender and region through a chi-square analysis. The results indicate there was a significant difference in the retention rate according to gender after 24 months and the retention rate of boys was higher. This result may be due to the physical growth differences between boys and girls. Further, the physical development of girls occurs earlier than in boys and the eruption time of the first permanent molars in girls are earlier. Accordingly, the risk of sealant loss in girls is higher than in boys [
23].
In 2017 and 2018, the retention rate of the occlusal surface was significantly higher than the retention rate of the buccal/palatal surface. The buccal groove of the first permanent molar has a simpler shape than occlusal surface, shallower in position, and not perpendicular to the occlusal surface, meaning it is easily contaminated by saliva during operation. Therefore, buccal/palatal sealant is not well retained and is easily lost. It reminds us that we should pay more attention to the groove of the buccal/palatal surface, isolate moisture, and standardize each step.
Anson concluded that the reasons for PFS loss were as follows: uncooperative children during the operation, not fully erupted teeth, difficulty in isolating moisture, and insufficient acid erosion. Therefore, standardized clinical operation is the key to the success of PFS [
24]. In addition, if a child is uncooperative during the operation, the etched tooth surface can be easily contaminated; hence, it is necessary to communicate this with the child before the operation (or postpone treatment for extremely uncooperative children). Moreover, the moisture insulation measures such as suction, cotton rolls, or rubber barriers can improve the quality of PFS.
It is worth noting that the retention rate in rural areas was higher than in urban areas in Shanxi after 24 months, which is inconsistent with some studies [
11,
25]. This phenomenon may be related to the type of organization implementing the project, such as if it is implemented in a public hospital or a private dental clinic. Among the 2201 teeth of urban children, 1507 teeth (68.5%) were sealed in private clinics and 694 teeth (31.5%) were sealed in public hospitals. Among the 1098 teeth of rural children, the proportions were 81.0% (889) and 19.0% (209), respectively. Compared with public hospitals, private dental clinics have relatively few channels to obtain customers. Accordingly, private dental clinics hope to obtain customer through this national project, meaning they are more motivated to implement the project and invest more labor and materials.
The results of the study also indicated there was no significant difference in the retention rate of PFS between the maxillary first permanent molars and the mandibular first permanent molars 12 and 24 months after PFS. This demonstrates that the retention rate is not related to a specific tooth arch, which is consistent with the results of other studies [
26,
27].
The spatial heterogeneity of the PFS retention rate
The spatial heterogeneity of the PFS retention rate was not clear after 12 months. Further, except for Datong City and Linfen City, the retention rates of other cities were > 90%. In comparison, the PFS retention rate after 24 months had greater stratified heterogeneity. Moreover, the PFS retention rate increased from north to south and the retention rates of Jincheng and Yuncheng in the southern region were both > 90%.
The study explored the interaction influence between the retention rate of PFS and eight ecological factors in the socioeconomic, medical resources, and natural environmental factors using the Geo-Detector model. The results indicate that natural environmental factors are important ecological factors that determine the spatial pattern after 12 months, while medical resources factors are important after 24 months. More specifically, FW plays a leading role in the spatial pattern of retention rate after 12 months and PMT after 24 months. We explored the interaction of ecological factors using the Geo-Detector model, where it was found that FW and NMI is the most influential interaction factor after 12 months while FW and PTI is the most influential after 24 months.
In the early 1980s, Shanxi Province of China was a major high-fluorine area with endemic fluorine poisoning. Although there was a significant decrease in FW after comprehensive prevention and control measures for reducing fluoride, some areas still have a higher FW than the national standard in Shanxi Province [
28]. Further, studies [
28,
29] have shown that tooth tissue will generate insoluble fluorapatite or calcium fluoride at finite concentrations of fluorine, which are deposited in the demineralized area of the tooth surface and promote remineralization. Hence, a small amount of fluorine has a preventive effect on caries. Research [
30] indicates that the clinical effect of PFS combined with fluoridization is more effective than PFS alone. Moreover, fluoridization has the effect of preventing the loss of PFS. However, continuous intake of a large amount of fluoride will cause fluorine poisoning, which can result in dental and skeletal fluorosis. Well-known facts are that dental fluorosis affects enamel structure, producing several porosities on its surface affecting, pit and fissures of teeth [
31]. This is important because patients with dental fluorosis are more likely to suffer losses after PFS. As shown previously, the uneven distribution of FW in Shanxi Province may cause the spatial heterogeneity of the PFS retention rate.
Shanxi Province is a vast territory with a large population, leading to uneven economic development among regions and unbalanced distribution of medical resources. Studies [
32] have demonstrated that the distribution of medical technicians across Shanxi Province differs significantly. Further, medical resources are mainly concentrated in the southern and central regions of Shanxi, where the economy is developing rapidly and the population is denser. Accordingly, there is a particularly higher level of dental care in the south of Shanxi. For example, Yuncheng City government actively trained dental professionals as early as the 1980s to solve the problem of denture care in rural areas. Further, they have continued to develop rural oral health services. After more than 20 years of practice in dental prevention and treatment, a three-level dental prevention network with non-public ownership as the main body has been established in counties, townships, and villages, which has formed a dental prevention model for poor rural areas [
21]. This imbalance of development has resulted in a limited number of medical institutions, a shortage of medical technicians, limited levels of medical technology, and a restricted approach to health services in the north. All of these factors ultimately cause a difference in people’s health across regions. This result suggests that Shanxi Province should strengthen medical and technician construction in the north, optimize the allocation of medical resources, and promote the balanced development of regional medicine.
Moreover, there are two main limitations of this study. The research period is not long enough and all samples in the study are from Shanxi Province. Although they are representative, they have obvious regional characteristics. More samples from other regions will be included in the future study.