A critical analysis of research methods and experimental models to study dentinal microcracks

Abstract

The purpose of this narrative review was to discuss the scientific milestones that led to the current understanding of the root dentinal microcrack phenomenon based on the interplay between the usage of micro-computed tomography (micro-CT) as an analytical tool alongside a close-to-mouth experimental model. In 2009, reports on the development of dentinal microcracks in extracted teeth after root canal preparation triggered an awareness of the potential for vertical root fractures (VRFs) of endodontically treated teeth could be developed from defects created by the mechanical stress of nickel-titanium preparation systems on dentine. This assumption was taken for granted, even though no cause-effect relationship had been scientifically demonstrated. Since then, several studies using the sectioning method with extracted teeth have been published and the large discrepancy amongst their outcomes soon become evident. Moreover, the high frequency of reported dentinal microcracks largely contrasted with the clinical incidence of VRFs, raising doubts on their methodological reliability. Using micro-CT technology, it was demonstrated by several studies that, in extracted teeth, dentinal defects already existed before the endodontic procedures, indicating that the initial reports framed a non-existing cause-effect relationship between canal preparation and dentinal microcracks. Although these new findings contributed to a better comprehension of this phenomenon, the misconception that microcracks were the starting point for VRFs was only surpassed with a new in situ approach using fresh cadavers. Surprisingly, microcracks were not identified in sound teeth. As a conclusion, dentinal microcracks in extracted teeth can be considered a non-natural occurrence observed only in a laboratory set-up as a consequence of dehydration and storage conditions. Thus, dentinal microcracks shall not be considered as the starting point for VRFs as they do not manifest in nonextracted teeth. Identifying dentinal microcracks as a laboratory phenomenon highlights the impact of recent scientific developments to disclaim the clinical relevance of laboratory-obtained outcomes.

Does Low-Taper Root Canal Shaping Decrease the Risk of Root Fracture? A Systematic Review

“…According to a recent narrative review, however, endodontic shaping may not be the cause of microcrack formation. In fact, these defects can be observed in extracted and stored teeth and may be due to procedural errors in the preparation of the experimental samples, rather than merely being due to the shaping [ 54 ]. Moreover, the studies that analyze microcrack formation showed discordant results, even when compared to each other [ 28 , 34 , 39 , 40 , 41 , 42 , 44 , 46 , 47 ].…”

“…The differences in the results of the studies included in this revision are, therefore, attributable to many factors and are not related to the taper of the instrumentation used, but instead are probably related to: The difference in the preparation motion [ 48 , 55 , 56 ]; The cross-sectional profile and blade design [ 10 , 63 , 64 ]; The type of alloy of the instrument used [ 54 , 64 , 65 , 66 ]. …”

In Vitro Qualitative Evaluation of Root-End Preparation Performed by Piezoelectric Instruments

“…The present in vitro study was performed on human teeth extracted for periodontal reasons. Some teeth presented cracks prior to the root-end preparation, which could have been present prior to extraction or may have occurred during the extraction maneuvers or during the shaping procedures [18,24]. In fact, in vitro preparation may cause cracks more often than in vivo because of the shock-absorbing capacity of the periodontal ligament and because of the dehydration occurring during the shaping procedure [7,25].…”

“…Great interest was placed on the dentinal microcrack phenomenon by clinicians, academics and researchers over the following years. In a recent narrative review conducted on crack formation, Versiani et al analyzed how the root dentinal microcracks observed in cross-sectional images of extracted teeth are not caused by canal-shaping procedures, and dehydration often causes cracking of the dentinal tissue, regardless of canal instrumentation [18].…”

Present status and future directions: vertical root fractures in root filled teeth

“…However, the results of ex vivo studies, as discussed previously, have been questioned, due to the presence of pre‐existing cracks in the sample selection (Arias & Peters, 2022; De‐Deus et al, 2014). The results of cadaver studies do not show any causal relationship between root canal shaping and the development of dentinal microcracks (Arias et al, 2014; Bahrami et al, 2017), and these results should be considered as more relevant due to absence of pre‐existing cracks (Versiani et al, 2021).…”