In this study, QLF technology, based on the autofluorescence of teeth, was employed to objectively and quantitatively assess microleakage in pit and fissure sealants. The autofluorescence variable ΔFmax exhibited the highest level of accuracy for detecting initial microleakage, enabling non-invasive identification of early-stage microleakage. Utilizing the QLF technique in clinical settings to monitor the condition of sealants and restorations allows for early-stage microleakage detection, thereby preventing the onset of severe secondary caries.
Various fluorescence variables can be obtained during the process of evaluating tooth autofluorescence using QLF technology. Among these, the parameter △Fmax, which reflects areas where tooth autofluorescence is most diminished and appears darkest, was selected for this study31. △Fmax showed a significant increase in correlation with rising histological scores at each stage, facilitating the differentiation of microleakage depth using △Fmax (Table 1, Fig. 3). Moreover, a statistically significant strong correlation was observed between △Fmax and histological scores (r = -0.72, P < 0.001). According to previous studies, △Fmax has been reported to non-destructively evaluate early caries lesions progressing into the sub-surface areas of pits and fissures on the occlusal surface of teeth32,33. Another study focusing on the detection of enamel cracks using QLF reported that ΔFmax has been proposed as an indicator for detecting enamel crack lesions, as the fluorescence appears more scattered and darker in the deepest part of the lesion34. It was found that ΔFmax, which represents the deepest part of the lesion, serves as a stable indicator for analysing areas with tooth cracks and it is less influenced by the selection of the Area of Interest (AOI)34. Based on these findings, △Fmax is a reliable parameter for evaluating sealant microleakage, exhibiting low inter-examiner variability and minimal susceptibility to the methodological influences.
According to the evaluation of the diagnostic accuracy of QLF technology for detecting microleakage around the pit and fissure sealants, fluorescence variable obtained from QLF technology were valid not only for determining the presence of microleakage but also for distinguishing its severity or progression level (AUC > 0.8). In particular, △Fmax exhibited the highest level of diagnostic accuracy (AUC 0.91) when distinguishing between cases of no microleakage (score 0) and all levels of microleakage (scores 1, 2, 3), achieving both high sensitivity and specificity. Sealants typically lack fillers allowing them to effectively flow into complex pit and fissure structures. As a consequence, sealants exhibit relatively lower physical strength and are highly susceptible to microleakage. Therefore, the early detection of sealant microleakage is even more critical than for other restorative materials. In this context, the effective detection of sealant microleakage—ranging from initial enamel outer-half microleakage (score 1) to deeper levels (scores 2 and 3)—using ΔFmax in this study is of significant clinical relevance.
When sealant microleakage extends beyond the inner half of the cross-sectional area (score 2 or 3), the risk of severe secondary caries development increases. In this study, diagnosing microleakage beyond the inner half exhibited high diagnostic accuracy (AUC 0.91) and sensitivity (0.95), but moderate specificity (0.68). These results are consistent with previous systematic review studies that compared various methods for detecting secondary caries in amalgam and resin composite restorations29. Particularly, when sealant microleakage extends beyond the inner half, it suggests that bacteria have already penetrated the sealant base, increasing the risk of partial sealant loss or the onset of secondary caries. In such clinical scenarios, where sealant reapplication can be performed without additional tooth removal, diagnostic methods like QLF—offering higher sensitivity over specificity—are deemed to have notable clinical utility from a preventive standpoint.
The resin components comprising the sealant typically contain radiopaque fillers to facilitate their identification in dental X-rays. The type and quantity of these fillers may vary depending on the manufacturer. In this study, Clinpro sealant (3M-ESPE, USA), was utilized. Upon application and curing, this sealant turned to an opaque off-white colour when visually observed and appeared darker under fluorescent observation compared to the natural tooth enamel (Fig. 3). In a previous research, when various types of resin composites were evaluated using QLF, there were significant differences in their autofluorescence patterns. Most resin composites exhibited brighter fluorescence than natural tooth fluorescence. However, some products displayed darker fluorescence than the tooth autofluorescence. This difference in fluorescence is believed to originate from variations in the fluorescent components included in the productes35. The dark fluorescence of the sealant material used in this study could have interfered with the detection of fluorescence loss due to microleakage, potentially affecting the interpretation of results in some cases. However, as reported in the previous study, using a sealant that exhibits brighter fluorescence than natural tooth enamel could enable more sensitive evaluations of fluorescence loss associated with microleakage.
QLF technology allows for the simultaneous assessment of changes in both tooth autofluorescence and red fluorescence produced by bacterial metabolites. However, because this study employed extracted teeth, it was challenging to evaluate the impact of red fluorescence originating from actual oral biofilm. In an oral environment, the red fluorescence from bacterial biofilms infiltrating the areas of sealant or restoration microleakage can indeed influence the fluorescence changes. Therefore, future research should consider evaluating not only the fluorescence changes caused by actual microleakage in the oral environment but also the fluorescence changes associated with bacteria-related microleakage.