Abstract
In modern endodontics, nickel-titanium (NiTi) rotary instruments are used on a large scale for root canal shaping. Nevertheless, the separation of an instrument is a serious concern during shaping. The aim of this study is to determine and compare the torsional fracture characteristics of three types of NiTi endodontic instruments, each with different cross-section designs and movements performed during root canal shaping: Endostar E3 (Endostar, Poldent Co. Ltd., Warsaw, Poland); Reciproc R25 (VDW, Munich, Germany); and Protaper Next X2 (Dentsply Maillefer, Ballaigues, Switzerland). Fifteen instruments are used in this study, divided in three groups (n = 5): Group Endostar, Group Reciproc and Group Protaper. For testing, each instrument is used to shape five simulated root canals, following which its torsional stress to failure is measured. The fracture lengths of all three groups are roughly between 2 and 3 mm from the tip. Higher values of the moment of torsion in fracture, and smaller values of the maximum twisting angle are observed for Group Endostar, as well as closer to circular cross-sections. However, the values of the shear tension are similar for all three groups, because the disadvantage given by the fracture section shape for Groups Reciproc and Protaper is compensated either by size or by intrinsic properties of the instrument material. For the shear tension the Endostar values are insignificantly increased (Kruskal–Wallis test, p = 0.207), and in the case of the maximum twist angle the Protaper values are insignificantly increased (Kruskal–Wallis test, p = 0.287). Because of the instruments shape and conicity, the analysis had to be carried out separately with regard to the length of the fractured tip. Rules-of-thumb are extracted from the study for current practice: if a blockage of the first 2 to 3 mm part of the tip can be anticipated (by the excessive curving of the instrument), the handpiece must be adjusted to torque values that do not exceed 1.5 to 2.5 N · cm for Endostar and 1 to 2 N · cm for Reciproc and Protaper instruments.
Highlights
In modern endodontics NiTi rotary instruments are used universally for root canal shaping
How to chose between the various types of NiTi endodontic instruments on the market today, each with its own cross-sectional shape that behaves differently on stress distribution and fracture tendency? This is a question at which this study aims to contribute for an answer
The aim of the present work is to study the torsional fracture characteristics of these endodontic instruments manufactured with different techniques
Summary
In modern endodontics NiTi rotary instruments are used universally for root canal shaping. Compared to conventional stainless-steel files, they are more elastic and have a higher cutting quality. They allow us to achieve the tapered root canal shape with less canal transportation because of their super-elasticity [1]. Walia et al first introduced NiTi root canal files to solve the issue of the rigidity of stainless-steel instruments and to improve the preparation of curved root canals [2]. Unexpected fracture of NiTi rotary instruments during their use (i.e., inside the tooth) remains a current problem and has a negative influence on the treatment outcome [3,4,5,6,7]. Flexural fatigue appears to be more common in curved root canals, while torsional failure can occur in a straight root canal [12,13,14]
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