Root surface strain during canal shaping and its influence on apical microcrack development: a preliminary investigation.

AIM: To determine the root surface strain (RSS) generated during root canal shaping and its effects on apical microcrack development. METHODOLOGY: Twenty-five extracted human mandibular premolars were selected and decoronated. The teeth were instrumented with either the ProTaper (PT) or WaveOne (WO) (Dentsply Maillefer) NiTi rotary systems (n = 10 per group) or used as controls (n = 5). Instrumented root canals were enlarged to ProTaper F4 (size 40, 0.06 taper) or using WaveOne LARGE (size 40, 0.08 taper) instruments according to the manufacturer's instructions. An electrical strain gage (KFG02-120-C1-16, Kyowa Dengyo, Tokyo, Japan) was fixed on the proximal root surface and connected to a strain amplifier via a bridge box in order to measure RSS. During canal shaping, the strain output of the amplifier was recorded. The instantaneous RSS induced by each instrument and the maximum RSSs were determined. All teeth were then stained with contrast media and imaged with micro-computed tomography (micro-CT) at an isotropic resolution of 10 µm to detect microcracks. The mean maximum RSS values (microstrain) and mean number of microcracks recorded for both groups were tested for statistical significance using Mann-Whitney U-test. Presence/absence of microcracks in both groups was compared by chi-square tests. RESULTS: Increased baseline RSS from strain accumulation during canal shaping was observed, with similar maximum RSS (mean ± SD) for PT (416.6 ± 185.1 µstrain) and WO (398.2 ± 163.8 µstrain) (P = 0.94). The interevaluator reliability for microcrack detection using micro-CT had a kappa value of 0.998. Compared to the PT group, there was a trend for fewer samples with microcracks in the WO group (P = 0.051). On the micro-CT images, apical microcracks were detected in 20 PT and 11 WO samples (P = 0.10). The microcracks were observed in the buccolingual direction in all WO and 81% of PT samples. No vertical root fractures were found. The maximum RSS obtained during canal shaping was poorly correlated with the number of microcracks found (R(2)  = 0.093). CONCLUSIONS: Based on these preliminary data, canal shaping appears to cause apical microcracks regardless of the type of rotary instrument motion. Contrast-enhanced micro-CT was able to identify microcracks in roots.

Heat treatment of nickel-titanium rotary endodontic instruments: effects on bending properties and shaping abilities.

AIM: To evaluate the bending properties and shaping abilities of nickel-titanium endodontic instruments processed by heat treatment. METHODOLOGY: K3 files were heated for 30 min at 400 °C (group 400), 450 °C (group 450) and 500 °C (group 500). Files that were not heat treated served as controls. A cantilever-bending test was used to evaluate changes in specimen flexibility caused by heat treatment. Curved root canal models were prepared. The times required for preparation, deformation and fracture were recorded. Pre- and postoperative images were superimposed. The amounts of resin removed from both the inner and the outer sides of the curvature in the apical 6 mm were determined. RESULTS: In the cantilever-bending test, load values of the control group and group 500 were higher than those of groups 400 and 450 at the elastic range (P<0.05). At the superelastic range, the bending load of the control group was the highest amongst all groups (P<0.05). Regarding shaping ability, in the control group, root canals at the apex were transported more to the outer side of the curvature compared with those of all heat-treated groups (P<0.05). Root canals of group 400 at 3 mm from the apex were transported less compared with those of other groups (P<0.05). No significant difference was found in working time amongst the groups. In group 450, there was no plastic deformation or fracture of the file. CONCLUSIONS: Heat treatment of files might improve their flexibility, making them more effective for preparation of curved canals.