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Introduction

Secondary caries is the most commonly reported reason for re-restoration of teeth, regardless of restorative material. This conclusion has been consistent in multiple narrative and systematic reviews on the clinical performance of dental restorations published over the last few decades .

For obvious ethical reasons, it is not feasible to conduct clinical trials to monitor the progress of initial secondary caries adjacent to restoration margins, with the objective of studying etiopathogenesis and/or to identifying potential prognostic factors. Prognostic factors are likely associated with the patient, the operator and restorative material, including the structure of the tooth-restoration interface following optimal, as well as suboptimal handling and placement of the restorative material .

Consequently, stakeholders with a strategy to decrease rates of secondary caries by improving restorative materials or material handling procedures are forced to statistically correlate as best as possible one, two or combinations of specific properties of existing restorative materials or handling procedures with reported rates of secondary caries in different clinical studies with various methodological qualities. Such statistics can be deceptive, because of a range of likely biases and possible confounding of both the independent and dependent variables in the majority of existing clinical studies.

Planning, conducting and reporting relevant outcomes of clinical comparative trials is logistically challenging, costly and potentially unpredictable if the study participant attrition is so high that adequate study power cannot be maintained. Moreover, correctly handled restorative materials placed under optimal conditions remain intact for an extensive time. Manufacturers may question a prioritizing of limited research funds to conduct clinical studies to evaluate the degrees of flawlessness of restorations monitored for anything less than 3 years. Moreover, to the author’s knowledge, there is no evidence that a satisfactory clinical performance after 1 or 2 years is predictive of good long-term performance. It is therefore debatable whether the results of any clinical study of less than 3 years should have any impact at all on consideration of change of existing material compositions or material handling procedures other than to reject materials exhibiting an unacceptably high early failure rate.

Stakeholders have therefore pursued alternative strategies to improve our understanding of how to develop innovative restorative materials which reduce the risk of secondary caries formation. Both academia and industry have designed numerous ingenious laboratory models and protocols for in situ experimental studies with the hoped objective of minimizing adhesion of cariogenic biofilms, preventing the occurrence of secondary caries adjacent to restorations, and elucidating the reasons for the deterioration of the restorative materials including the tooth-restoration interface.

Unfortunately, the correlation between microleakage around restorations or artificial caries-like lesions adjacent to restorations generated in vitro or in situ is poor versus tooth-restoration interface qualities measured in vitro or in vivo. It is also poor versus reported incidences of secondary caries observed in clinical efficacy or effectiveness studies . There are probably multiple reasons for the incongruence between experimental data and clinical observations of secondary caries. The objective of this review is to critically appraise the existing experimental research with a focus on etiopathogenesis or prevention of secondary caries, and to explore possible explanations for the seemingly poor correlation to clinical observations.