Correlation between root surface strain and apical micro-cracks produced during canal preparation with thermally treated single or multi-file NiTi systems

Aim: To determine the apical root surface strain (RSS) generated during shaping with thermally treated file NiTi systems, and its association with the development of apical microcracks. Methodology: Increased baseline RSS from strain accumulation during canal shaping was observed in both groups. The mean ± SD maximum RSS recorded with XPS and HCM were 165.71 ± 86.57, and 132.14 ± 97.26 respectively with no statistical difference between them (p>0.05). Post-instrumentation microcracks were observed in only two canals prepared by XPS (10%) versus one canal prepared by HCM (5%), and this difference was also statistically non-significant (P>0.05). The inter-evaluator reliability for microcrack detection using ESEM had a Kappa value of 0.98 (p<0.001), while the intra-evaluator reliability had a Kappa value of 0.99 (p<0.001). The maximum RSS obtained during canal shaping was poorly correlated with the number of microcracks found (R2=0.090). Results: Increased baseline RSS from strain accumulation during canal shaping was observed in both groups. The inter-evaluator reliability for microcrack detection using ESEM had a Kappa value of 0.98 (p<0.001), while the intra-evaluator reliability had a Kappa value of 0.99 (p<0.001). The maximum RSS obtained during canal shaping was poorly correlated with the number of microcracks found (R2=0.090). Conclusions: Within the limitations of this study, canal shaping by the two systems was confirmed to strain the tooth structure. However, the measured RSS was poorly correlated with the development of apical microcracks. Hence, from the clinical context, both systems can be safely used to prepare severely curved root canals.