Smear layer removal efficacy of different irrigation techniques in conservatively instrumented root canals

Abstract Background Much attention has recently been drawn on minimally invasive endodontic systems. However, establishing effective irrigation methods is warranted to ensure the predictability of minimally invasive root canal instrumentation. This study aimed to compare the smear layer removal efficacy of different irrigation techniques in root canals instrumented with TruNatomy nickel-titanium rotary instruments. Methods In Experiment 1, 51 extracted human mandibular incisors were instrumented using TruNatomy Shaping Files up to Small (#20/0.04), Prime (#26/0.04), or Medium (#36/0.03) and irrigated with ethylenediaminetetraacetic acid (EDTA), sodium hypochlorite (NaOCl), and again with EDTA using syringe irrigation (SI) with the tip inserted 1 mm short of its binding position (n = 17). The smear layer on the root canal wall in the coronal, middle, and apical thirds was scored under scanning electron microscopy (SEM). In Experiment 2, 70 extracted mandibular incisors were instrumented using TruNatomy Small and irrigated with EDTA and NaOCl as in Experiment 1 using (1) conventional laser-activated irrigation (LAI) with an erbium-doped yttrium aluminum garnet laser unit at 30 mJ/10 Hz (LAI 30/10) or 70 mJ/20 Hz (LAI 70/20), (2) photon-induced photoacoustic streaming (PIPS) (20 mJ/15 Hz), (3) ultrasonic-activated irrigation, or (4) SI (n = 13). Five additional canals served as negative controls (no irrigation). The smear layer on the canal wall was scored under SEM images. Data were analyzed using the Kruskal-Wallis test and Mann-Whitney U test with Bonferroni correction (α = 0.05). Results In Experiment 1, the TruNatomy Small group showed significantly higher smear layer scores compared to the other groups in the apical and middle thirds (p < 0.05). In Experiment 2, the LAI 70/20 and PIPS groups showed significantly lower smear layer scores compared to the LAI 30/10 and SI groups in the apical third (p < 0.05). In the LAI 70/20 group, SEM images demonstrated crescent-shaped cavities, which are indicative of heat-induced injury from laser energy. Conclusions Conservative instrumentation using the TruNatomy Small reduced the cleaning efficacy of irrigation. However, irrigation using the PIPS technique, with the tip placed above the root canal orifice, demonstrated satisfactory smear layer removal without injury to the root canal wall.