As the use of nickel-titanium (NiTi) file systems for root canal therapy has become popular; hence, knowledge and understanding of the characteristics of NiTi files is essential for dentists. Unintended sudden fracture can occur during root canal shaping, and it is important to understand the conditions that may cause instrument fractures. Torque is defined as the force required to rotate the NiTi file and can be considered of as a parameter for the stress generated. The endodontic engine maintains a constant rotational speed by adjusting torque regardless of the root canal conditions. The process of root canal shaping by rotary instruments is a series of actions that requires torque and generates stress to both the teeth and the NiTi instruments. The generated stress may induce the strain accumulation on NiTi instrument and the canal wall and lead to the development of microcrack in the instrument and dentinal wall. Therefore, understanding of torque and stress generated is important to prevent the fractures to the instrument and the teeth. This stress has been measured using various experimental approaches, including microcrack observation by using a microscope or computed tomography, attaching strain gauges to the teeth, and finite element analysis. This review focuses on the stress generated to the teeth and the instrument during instrumentation under various experimental conditions. The factors related to torque generation are also discussed.

Ha JH, et al. Restor Dent Endod. 2016 Nov;41(4):304-309. https://doi.org/10.5395/rde.2016.41.4.304

This study compared the maximum screw-in forces of various instruments during their movements. Forty simulated canals in resin blocks were randomly divided into four groups (n = 10): ProTaper Universal F2, ProTaper Gold F2, WaveOne Primary, and WaveOne Gold Primary. To standardize a lumen size, all artificial canals were prepared with ProTaper Universal F1. The rotation speed was set at 350 rpm with an automated 4 mm pecking motion at a speed of 1 mm/s. The pecking depth was increased by 1 mm for each pecking motion until the file reached the working length. During instrumentation, screw-in forces were automatically recorded by customized software. Maximum screw-in forces were analyzed by one-way ANOVA and Tukey’s post hoc comparison with the significance level at 0.05. WaveOne Gold files generated the lowest maximum screw-in forces, followed by ProTaper Gold, WaveOne, and ProTaper Universal (p < 0.05). Under the condition of this study, heat-treated nickel–titanium (NiTi) files with smaller cross-sectional area, fewer contact points, and reciprocating movements resulted in a lower screw-in effect.

The purpose of this randomized controlled clinical trial was to assess the short-term clinical effectiveness of sealer-based root canal obturation using calcium-silicate-based sealer (CSBS) and epoxy-resin-based sealer (ERBS). A total of eighty patients (eighty teeth) were enrolled and seventy-two patients (seventy-two teeth) were randomly assigned into four different sealer groups: AH Plus (AH, Dentsply Sirona), ADseal (AD, Meta Biomed), CeraSeal (CS, Meta Biomed), or EndoSeal TCS (ES, Maruchi). At the first visit, root canal treatment was performed using ProTaper. Next, the NiTi file system and passive ultrasonic irritation was applied with 2.5% NaOCl. At the second visit, the teeth were obturated with a sealer-based obturation technique using the assigned sealer. The quality of filling obturation was evaluated with postoperative radiographs in terms of the presence of voids and sealer extrusion. The patients were recalled at 1-week, 1-month, and 3-month intervals, and pain levels were measured using visual analog scales (VAS), and clinical examination performed with percussion and palpation. Radiographs were also taken. The results were statistically compared by using the χ² test or Fisher exact test. Among the 80 teeth included, 72 teeth received allocated intervention and 71 teeth were included in the analysis (98.6% recall). There was no significant difference among the sealers in void and sealer extrusion (p > 0.05). Postoperative pain was not indicated prominently regardless of the type of sealer. For a 3-month follow-up period, all cases were maintained successfully except for one case from AD. According to the findings, there was no significant difference between CSBS and ERBS in terms of postoperative pain or the healing process, and the variation in filling quality appears to be linked to the properties of each product.