Restorative Dentistry & Endodontics

Screw-in forces during instrumentation by various file systems: Jung-Hong Ha, Sang Won Kwak, Sung-Kyo Kim, Hyeon-Cheol Kim; Restor Dent Endod 2016;41(4):304-309. Published online November 8, 2016; DOI: https://doi.org/10.5395/rde.2016.41.4.304

Abstract PDF PubReader ePub: Objectives
The purpose of this study was to compare the maximum screw-in forces generated during the movement of various Nickel-Titanium (NiTi) file systems.
Materials and Methods
Forty simulated canals in resin blocks were randomly divided into 4 groups for the following instruments: Mtwo size 25/0.07 (MTW, VDW GmbH), Reciproc R25 (RPR, VDW GmbH), ProTaper Universal F2 (PTU, Dentsply Maillefer), and ProTaper Next X2 (PTN, Dentsply Maillefer, n = 10). All the artificial canals were prepared to obtain a standardized lumen by using ProTaper Universal F1. Screw-in forces were measured using a custom-made experimental device (AEndoS-k, DMJ system) during instrumentation with each NiTi file system using the designated movement. The rotation speed was set at 350 rpm with an automatic 4 mm pecking motion at a speed of 1 mm/sec. The pecking depth was increased by 1 mm for each pecking motion until the file reach the working length. Forces were recorded during file movement, and the maximum force was extracted from the data. Maximum screw-in forces were analyzed by one-way ANOVA and Tukey's post hoc comparison at a significance level of 95%.
Results
Reciproc and ProTaper Universal files generated the highest maximum screw-in forces among all the instruments while M-two and ProTaper Next showed the lowest (p < 0.05).
Conclusions
Geometrical differences rather than shaping motion and alloys may affect the screw-in force during canal instrumentation. To reduce screw-in forces, the use of NiTi files with smaller cross-sectional area for higher flexibility is recommended.

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Mechanical and geometric features of endodontic instruments and its clinical effect: Hyeon-Cheol Kim; J Korean Acad Conserv Dent 2011;36(1):1-11. Published online January 14, 2011; DOI: https://doi.org/10.5395/JKACD.2011.36.1.1

Abstract PDF PubReader ePub: Abstract Introduction:
The aim of this paper is to discuss the mechanical and geometric features of Nickel-titanium (NiTi) rotary files and its clinical effects. NiTi rotary files have been introduced to the markets with their own geometries and claims that they have better ability for the root canal shaping than their competitors. The contents of this paper include the (possible) interrelationship between the geometries of NiTi file (eg. tip, taper, helical angle, etc) and clinical performance of the files as follows;
- Fracture modes of NiTi rotary files

- Non-cutting guiding tip and glide path

- Taper and clinical effects

- Cross-sectional area and clinical effects

- Heat treatments and surface characteristics

- Screw-in effect and preservation of root dentin integrity

- Designs for reducing screw-in effect

Conclusions:
Based on the reviewed contents, clinicians may have an advice to use various brands of NiTi rotary instruments regarding their advantages which would fit for clinical situation.

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Stress distribution for NiTi files of triangular based and rectangular based cross-sections using 3-dimensional finite element analysis: Hyun-Ju Kim, Chan-Joo Lee, Byung-Min Kim, Jeong-Kil Park, Bock Hur, Hyeon-Cheol Kim; J Korean Acad Conserv Dent 2009;34(1):1-7. Published online January 31, 2009; DOI: https://doi.org/10.5395/JKACD.2009.34.1.001

The purpose of this study was to compare the stress distributions of NiTi rotary instruments based on their cross-sectional geometries of triangular shape-based cross-sectional design, S-shaped cross-sectional design and modified rectangular shape-based one using 3D FE models.

NiTi rotary files of S-shaped and modified rectangular design of cross-section such as Mtwo or NRT showed larger stress change while file rotation during simulated shaping.

The stress of files with rectangular cross-section design such as Mtwo, NRT was distributed as an intermittent pattern along the long axis of file. On the other hand, the stress of files with triangular cross-section design was distributed continuously.

When the residual stresses which could increase the risk of file fatigue fracture were analyzed after their withdrawal, the NRT and Mtwo model also presented higher residual stresses.

From this result, it can be inferred that S-shaped and modified rectangular shape-based files were more susceptible to file fracture than the files having triangular shape-based one.

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