J Korean Acad Conserv Dent.  2011 Jan;36(1):1-11. 10.5395/JKACD.2011.36.1.1.

Mechanical and geometric features of endodontic instruments and its clinical effect

• Kim HC

• KMID: 1446078
• DOI: http://doi.org/10.5395/JKACD.2011.36.1.1

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.

Keyword

Cross-section; Fracture; Geometry; Heat treatment; Mechanical response; NiTi rotary file

MeSH Terms

Dental Pulp Cavity
Dentin
Hot Temperature

Figure

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  Figure 1. Fracture modes of Nickel-Titanium rotary files. (a, b) Experimental files fractured from flexural fatigue. Figure (a) shows crack initiation zone (arrow) and fast fracture zone (box) which are common findings in cyclic fatigue fracture. (c, d) Experimental files fractured from torsional load. Figure (c) shows circular abrasion marks and skewed dimples at the center of rotation which are common findings in torsional fracture.

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  Figure 2. Non-cutting guiding tip of Nickel-Titanium rotary files. ProTaper retreatment file D1 has cutting tip.

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  Figure 3. Mechanical responses on bending force and torque for ProFile .06/30, ProTaper F3 and ProTaper Universal F3. (a) Bending moment needed to deflect the tip. (b) Torque required to rotate the file under 4 mm tip restriction. (c) von Mises stresses distribution under 2 mm deflection. (d) von Mises stresses distribution under 2.5 Nmm torsion load. This figure is reproduced from the International Endodontic Journal 2009;42:14-21.33

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  Figure 4. Schematic diagram showing a typical stress (torsional load) versus strain (distortion angle) relationship, with the various stages of deformation labeled. The calculated area under the stress-strain curve up to the point of fracture represents the toughness of the specimen.46,50

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  Figure 5. Various machined surface of NiTi rotary files before cyclic loading (left) and micro-cracks near the fatigue fracture area. Fine cracks that assumed an irregular path were noted in TF and RaCe, whereas ProTaper and Helix showed cracks running along the machining grooves. This figure is reproduced from the Jounal of Endodontics 2010;36:147-152.44

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  Figure 6. Representative von Mises stress distribution at the apical dentin during simulated shaping rotation with ProTaper at the working length: (a) the apical aspect and (b) the longitudinal section. Arrow indicates the apical constriction where maximum stress was located. The graph shows the cyclic stress profile during simulated shaping over a period of 1 second for all three rotary instruments in the location (a node of FE model) in which the highest von Mises stress was found (highest stress value for each file: ProTaper 386 MPa, ProFile 311 MPa, LightSpeed 108 MPa). This figure is cited from the Journal of Endodontics 2010;36:1195-1199.60

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  Figure 7. The difference of helical angle between K-file and reamer (upper). The different helical angle and pitch from GT file and FlexMaster are shown (lower).

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  Figure 8. Alternative pitch of RaCe and asymmetrical cross-section of Revo-S for reducing screw-in effect.

Reference

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