Sample selection and ethical aspects
Forty-four human mandibular incisors were selected and stored in distilled water at room temperature until their use in this study. The selected teeth were donated by patients for the research and the donation term were signed. Digital radiographs were performed to evaluate if all teeth met the inclusion criteria. Teeth with complete root formation, absence of root curvature or dentin cracks/fractures, only one canal, no previous endodontic treatment or root resorption were included. The excess moisture was removed and the specimens were scanned at an isotropic resolution of 11.88 lm using a high-resolution microtomography scanner with aluminium and copper filters (SkyScan 1172; Bruker-microCT, Bruker, Aartselaar, Belgium) at 100 kV and 100 lA. A baseline micro-CT scan was performed using a according to the parameters used by Barreto .
This study was submitted and approved by Research Ethics Committee of Federal University of Rio Grande do Sul, Porto Alegre, Brazil (#68322017.8.0000.5347).
Root canal preparation and filling
Coronal opening was performed as usual using #1014 diamond burs (Fava Metalúrgica, São Paulo, Brazil) under water cooling. Next, canal scouting were performed with a size 10 and 15 K-file (Dentsply Maillefer, Ballaigues, Switzerland). Working length (WL) was established using a #15 K-flei (Dentsply Maillefer) wich was introduced into the canal until it became visible at the apical foramen and then subtracting 1 mm from this measurement.
All teeth were randomly divided into four experimental groups (n = 11) according to the instrument used for cervical preflaring and one control group: Control group - canals were prepared but without cervical preflaring; Gates Glidden group – burs size 2 and 3 (Dentsply Maillefer); WXN group – 25.07 instrument (Navigator; MEDIN, Nové Město na Moravě, Czech Republic); and Easy group – 25.08 instrument (ProDesign S; Easy Instrumentos Odontológicos, Belo Horizonte, Brazil).
Cervical preflaring in the Gates Glidden group was performed on a low-speed handpiece operating at 12.000 rpm, and a straight up-and-down motion was used up to 5 mm from the canal opening. Cervical preflaring in WXN and Easy groups were performed on an electric device operated at 350 rpm and torque of 2 N.cm. These instruments were used in an in-and-out motion.
The root canal preparation was performed with Wave One Gold Primary instruments (25.07) (Dentsply Sirona, York, EUA) in a reciprocating motion, using the speed and torque recommended by the manufacturer. Each instrument was used in five teeth. During canal preparation, the canals were irrigated with 5 mL of 2.5% of sodium hypochlorite (NaOCl) (Biodinâmica, Ibiporã, PR, Brazil) carried out with a syringe and 30-gauge needle (NaviTip, Ultradent, South Jordan, UT, USA) taken 1 mm short of the WL.
After instrumentation, the root canals were irrigated with 5 mL of 17% EDTA (Biodinâmica) for 3 min and rinsed with 2 mL distilled water and dried with absorbent sterile paper points. Root canal filling was performed using Tagger’s Hybrid technique. The Wave One Primary gutta-percha cones (Dentsply Sirona) were inserted into the canals to verify if they reach the WL and if they fit at the apical third. Subsequently, an epoxy-resin based sealer (Sealer Plus; MK Life, Porto Alegre, RS, Brazil) was mixed according to the manufacturer’s instructions and placed in the canal using a lentulo spiral. Finally, the gutta-percha cone was slowly inserted into the canal until it reached WL. Next, three FM gutta-percha cones were inserted passively. Finally, a McSpadden compactor #50 (Dentsply Maillefer) was coupled to a low-speed contra-angle and introduced passively into the root canal 4 mm short of the WL with forward-backward movements.
The excess of gutta-percha was removed with a heated plugger, and the coronal opening was cleaned with ethanol (Biodinâmica). Next, the coronal opening was etched with 37% phosphoric acid for 30 s, rinsed with 2 mL of distilled water for 30 s and restored using adhesive system (Scotchbond; 3 M ESPE, Saint Paul, MN, USA) and nanoparticulated composite resin (Filtek™ Z-350; 3 M ESPE). The restorative procedures were performed using the incremental filling technique. Each increment was light-cured for 40 s using a LED light source (Radii-cal, SDI, Bayswater, VC, Australia) with a power of 1200 mW/cm2.
All instrumentation and root canal filling procedures were performed by one single experienced operator and trained to use the instruments.
Analysis of root canal enlargement and dentin thickness
A new micro-CT scan was performed as described before and the reconstructed images obtained pre and post-preparation were geometrically co-registered with the preoperative data sets using the DataViewer software v.1.5.2 (Bruker-microCT), allowing quantitative comparison of the morphological parameter before and after preparation. For this step, the CTan v.1.12 software was used to evaluate the root canal volume and the dentin thickness pre and post root canal preparation in the cervical portion. The dentin thickness (mm) was evaluated at 3 mm and 5 mm from the cement-enamel junction and the canal volume (mm3) only in the first 5 mm.
Fracture resistance test
After restoration, the simulation of periodontal ligament was performed by teeth immersing in melted wax (Horus; Herpo ProdutosDentários, Petrópolis, RJ, Brazil) up to 1 mm below to the cementoenamel junction. After cooling, a 0.2 mm + 01 mm thick wax layer was obtained by coating the roots. Next, the specimens from all of the groups were embedded in plastic cylinders (16.5 mm inner diameter X 20 mm high) filled with a chemically cured acrylic resin (Dencrilay, Dencril, SP, Brazil) using the following steps: a) the specimen was fixed on a parallelometer, with the long axes of the teeth and cylinder parallel to each other and perpendicular to the ground, and b) the acrylic resin was prepared and poured inside the cylinder up to 1 mm below the cementoenamel junction.
After resin polymerization, the wax was removed from the root surface and the resin cylinder ‘sockets’ by using warm water for 2 s. The resin cylinders were filled with a polyether impression material (ImpregumTM Soft, 3 M ESPE) using a molding syringe. The teeth were re-inserted into their respective cylinder ‘sockets’, and any excess impression material was removed with a number 12 scalpel blade.
Fracture resistance tests were performed 48 h after removal of impression material. During this period, the specimens were kept in distilled water at 37 °C. Compressive loads were applied using a universal testing machine (EMIC DL 2000/700; São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min, applied on the palatal aspect of specimens at 135° along the long axis of the tooth.
The normality of the data was assessed by Shapiro Wilk Test. The two-dimensional and three-dimensional parameters were compared between and within groups using ANOVA and Tukey’s test (P < .05). The fracture resistance values were subjected to 1-way ANOVA and Tukey post-hoc tests. Statistical analysis was performed at a significance level of 5%.