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

Abstract

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: Twenty extracted human mandibular molars with severely curved mesial roots having two separate canals were prepared using XP-Endo Shaper files (Group A: XPS in mesio-buccal canals) and Hyflex CM files (Group B: HCM in mesio-lingual canals). Pre-instrumentation images of apical surface of each root were obtained with Environmental Scanning Electron Microscope (ESEM). Root surface strain (RSS) generated during canal preparation was measured as micro-strain (ìstrain) using electrical strain gauges fixed on apical third. Strain output was digitally recorded to analyze both instantaneous RSS and the maximum RSS. Mean maximum RSS values produced during canal preparation with both systems were tested for statistical significance using independent t-test. Post-instrumentation images were acquired to evaluate the presence/absence of apical microcracks. Examination was performed twice by three blinded examiners at 2-week intervals. Inter- and intra-evaluator reliability was analyzed using the Kappa statistic test. Association between the mean maximum RSS and development of apical microcracks was evaluated by linear regression. Results: 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). 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.