Abstract: Objective： To evaluate the accuracy of optical coherence tomography (OCT) in detecting root cracks after root canal instrumentation using histological gold standard. Methods： Twenty complete extracted human mandibular incisors that were free of caries, calculus, and root treatment were chosen and accessed coronally with a diamond bur, then mounted in resin blocks with alginate impression material using simulated periodontal ligaments, and the apex was exposed 3 mm. The teeth were stored in water at room temperature. Then the teeth were then instrumented to the major apical foramen (AF) at #30/0.09 using ProTaper Universal rotary nickel titanium system(Dentsply Maillefer, Ballaigues, Switzerland), irrigation with 1% sodium hypochlorite (NaOCl) by using a 26gauge needle followed after each instrument. The apical root was scanned with 360° of rotation by Swept-Source Optical Coherence Tomography （SS-OCT） (wavelength: 1 310 nm, scan rate: 20 kHz, axial resolution: 16 μm) with driving device（stepper motor and lifting platform）. The reconstruction images of axial planes 1, 2 and 3 mm from the apex were examined and the root cracks were blindly diagnosed by two observers. The horizontal section was performed at 1, 2 and 3 mm from the apex using low speed disc saw (Leica SP1600, Wetzlar, Germany). The presence of cracks was noted under an optical stereomicroscope (ZOOM-630E) with a cold light source using as gold standard to evaluate the accuracy of OCT in detecting root cracks after root canal instrumentation. Results： After canals instrumentation with ProTaper Universal rotary nickel titanium system to #30/0.09, root cracks were detected in 9 of 20 teeth by histological examination. Crack lines were observed on 13 of 60 horizontal sections and cracks on 12 of the 13 sections were detected by OCT. No cracks were observed in the other 47 of the 60 horizontal sections，none of which was misdiagnosed by OCT. The overall accuracy rate for detection of root cracks with OCT was 0.983, the sensitivity was 0.923, the specificity was 1.000, the positive predictive value (PPV) was 1.000 and the negative predictive value (NPV) was 0.979. Conclusion： OCT may be a promising nondestructive imaging method for diagnosing root canal cracks after canal instrumentation.

Key words: Optical coherence tomography (OCT), Root canal preparation, Root crack