Limitations of Study Method
The aim of this study was to test the null-hypothesis that the rate of losing complete retention of resin based fissure sealants, after an observation period of at least 24-months, does not predict caries manifestation on pits and fissures of permanent molar teeth significantly more accurately than any random values.
In order to meet this aim, the chosen study methodology focused on the data extracted from resin based fissure sealant trials, without taking aspects of internal trial validity under consideration. Such aspects, particularly related to attrition- and detection bias, may have affected the trial results and thus in turn, the data on which this study is based. Neither, however, was internal trial validity with specific focus on systematic errors/biases investigated during the systematic review by Kühnisch et al., 2012 , whose references (provided in its additional online content) constituted the main data source of this investigation (APPENDIX FILE 1). Consequently, this study is in line with the same assumptions concerning internal validity of the same included fissure sealant trials as those reviewed by Kühnisch et al., 2012 .
In this study, more than half of the available datasets were excluded (APPENDIX FILE 1). However, the exclusion of premolar teeth is in line with investigations into sealant effectiveness reported by other authors  and is based on the observation that caries develops less on premolar teeth than on molars . The exclusion of 24 datasets on the basis of their inclusion of the filled and extracted DMFT component appears justified, as the true reasons for the filling and extraction of formerly sealed teeth are unknown. Other reasons for data exclusion: lack of computable data reported, different numbers of teeth assessed for retention and caries and lack of caries assessment are clear indicators for non-relevance with regard to the study aim. The lack of 21 datasets from trials that remained untraceable in full copy through the available library- and Internet sources or directly from the authors remains a concern. However, these would have comprised just 19% of the total available data and whether some of the missing data would not have been excluded for other reasons too, if the trial reports could have had been traced, remains doubtful. Under such assumption, the lack of the missing data may not have significantly impacted the conclusion of this study.
Treatment effects on surrogate endpoints should reliably predict the effect of such treatment on the true clinical endpoint [21-25]. Hence, if the placement of resin-based sealants on pits and fissures of permanent molar teeth result in the retention/loss of the material after >24 months, this should reliably predict the absence/presence of caries manifestation beyond the play of chance. Within the context of this study, the number of teeth with/without completely retained sealants (nR+/nR-) should be able to predict reasonably the number of teeth that do not/do develop carious decay in pits and fissures (nC+/nC-). Such results form the true negative/positive (TN/TP) predictions. In comparison, the number of false negative/positive (FN/FP) results should not exceed between 2.5 – 10% of all predictions .
In this study, the test data for sealant retention (nR-) was obtained from clinical fissure sealant trials and compared to random values (nrand) in relation to the true clinical endpoint (= caries manifestation / nC-) and computed as median Diagnostic Odds ratios (DOR). It was expected, on the basis of current consensus concerning fissure retention [15-20], that the predictive DOR values from the test data would be significantly higher (more accurate) than those from the control. However, no statistically significant differences between the TP/TN and FP/FN dependent DOR values were found (Wilcoxon test: z = 0.56; p = 0.58; Sign test: z = 1.38; p = 0.17). The null-hypothesis (H0) could therefore not be rejected, thus no sufficient evidence in support of the alternate hypothesis (H1) was found. These results suggest that predictions based on the relationship of the retention rate of resin-based fissure sealants and caries manifestation are no more accurate than random guesses. Furthermore, the SROC curve (Fig. 2), based on such random values (median DOR 0.28), appears to indicate a higher (albeit inverted and still purely random) predictive accuracy than the sealant retention rate (DOR = 1.21; Fig. 1).
The lack of any significant difference in predictive accuracy between the retention rate and random values may not be surprising. Many factors other than the mere loss of complete sealant retention have a potential effect on caries manifestation in pits and fissures of permanent molar teeth. These factors may include: the position of the molar tooth in the mouth, the period between retention loss and follow-up, access to fluoride, oral hygiene and dietary habits, enamel structure, saliva factors or, possibly, factors that remain unknown. Manifestation of caries may also be related to sealed pits and fissures, if the partially lost sealants were misclassified as ‘completely retained’ or if pits and fissures still contain micro-remnants of sealant materials that provide caries protection but were classified as ‘loss of complete retention’.
Against this background, it may appear obvious that the retention rate alone is unable to account for the influence of other known and unknown factors that may influence caries manifestation. Nonetheless, the retention rate of fissure sealants has been accepted as valid surrogate for caries prevention [21-25]. Valid surrogate endpoints are defined as measurements or physical signs for use as substitutes for true clinical endpoints , while true clinical endpoints are defined as clinical, patient-relevant events of which the patient is aware, which the patient wants to avoid and which affect her/his quality of life . Caries manifestation in pits and fissures constitutes a true clinical endpoint that was utilized during first generation of fissure sealant trials [9-11]. In light of the inability of the retention rate to be an accurate predictor of caries in pits and fissures, further (randomised controlled) sealant trials should have retained this clinical endpoint as the outcomes measure while using resin-based sealant materials and sealant techniques, established during the previous trials, as gold standards against which new materials/techniques were to be compared.
The use of invalid surrogates that cannot sufficiently predict their clinical endpoints carries the danger of unexpected adverse effects in clinical trials, therapeutic uncertainties due to ambiguous evidence, as well as the risk of rejection of potentially useful therapies because they do not show benefits in line with the surrogate endpoints . Of these consequences, the latter is especially apparent with regard to glass-ionomer cement (GIC)-based fissure sealants , despite the fact that systematic reviews have found no difference between the caries-preventive effect of GIC and that of resin-based sealants [26, 27]. For these reasons, it has been repeatedly argued that: (i) biological outcomes should take precedence over mechanical ones; (ii) because sealants are placed in pits and fissures in order to prevent the onset of caries manifestation or to arrest it, the true outcome of such intervention should therefore be expressed in terms of how well such intervention has managed to achieve this objective [35, 36].