Abstract
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.
Highlights
Because of superior flexibility and higher torsional fracture resistance of nickel–titanium (NiTi) instruments compared to those of stainless-steel (SS) instruments, NiTi instruments are widely chosen for the root canal treatment [1]
As NiTi instruments make contact with the dentinal wall to cut root dentin during root canal shaping, stress may be generated inside the NiTi instrument and a reactive force may be generated toward the root dentin
Four file systems made of different alloys and with different kinetic movements were tested: Two continuous rotation systems (ProTaper Universal F2 (PTU; Dentsply Sirona, Ballaigues, Switzerland) and ProTaper Gold F2 (PTG; Dentsply Sirona) and two reciprocating systems (WaveOne Primary (WOP; Dentsply Sirona) and WaveOne Gold Primary (WOG; Dentsply Sirona))
Summary
Because of superior flexibility and higher torsional fracture resistance of nickel–titanium (NiTi) instruments compared to those of stainless-steel (SS) instruments, NiTi instruments are widely chosen for the root canal treatment [1]. Geometric features and heat treatment could influence the mechanical properties [4,5,6] and clinical performances [6,7,8,9,10,11] of NiTi instruments such as the incidence of file separation and stress distribution through the dentinal wall during root canal shaping. The fluted cutting blades of NiTi instruments have a spiral form in the longitudinal aspect. This helical configuration is essentially necessary for the cutting and removal of infected dentin, but this may cause an unwanted apical driving power (the ”screw-in” effect). The screw-in effect is clinically defined as the tactile sensation of the instrument within the operator’s hand being pulled into the root canal apically
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