Root surface strain during canal shaping and its influence on apical microcrack development: a preliminary investigation.
OBJECTIVE
To determine the root surface strain (RSS) generated during root canal shaping and its effects on apical microcrack development.


METHODS
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.
Endodontics, College of Dentistry, King Saud bin Abdulaziz University for Health Sciences, National Guard Health Affairs, Khashm Al An, Riyadh, Saudi Arabia.
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http://orcid.org/0000-0001-9504-2921
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Jamleh
Endodontics, School of Dentistry, West Virginia University, Morgantown, WV, USA.
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http://orcid.org/0000-0002-6364-8440
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Komabayashi
Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
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Ebihara
Cariology and Operative Dentistry, Department of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
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M
Nassar
Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
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Watanabe
Yoshioka Dental Clinic, Hiroshima, Japan.
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T
Yoshioka
Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
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K
Miyara
Department of Pulp Biology and Endodontics, Division of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan.
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H
Suda
D001641:Bicuspid / Q000000981:diagnostic imaging / Q000601:surgery; D003762:Dental Instruments; D003799:Dental Stress Analysis; D006801:Humans; D066298:In Vitro Techniques; D018915:Root Canal Preparation / Q000295:instrumentation / Q000379:methods*; D014082:Tooth Fractures / Q000000981:diagnostic imaging*; D014092:Tooth Root / Q000293:injuries; D055114:X-Ray Microtomography
continuous rotary motion; contrast media; micro-CT; microcrack; reciprocating rotary motion; surface tooth strain