Endodontic instruments after torsional failure: Nanoindentation test

Abstract

This study aimed to evaluate effects of torsional loading on the mechanical properties of endodontic instruments using the nanoindentation technique. ProFile (PF; size 30, taper 04; Dentsply Maillefer, Switzerland) and stainless steel (SS; size 30, taper 02; Mani, Japan) instruments were subjected to torsional test. Nanoindentation was then performed adjacent to the edge of fracture (edge) and at the cutting part beside the shank (shank). Hardness and elastic modulus were measured under 100-mN force on 100 locations at each region, and compared to those obtained from the same regions on new instruments. It showed that PF and SS instruments failed at 559 ± 67 and 596 ± 73 rotation degrees and mean maximum torque of 0.90 ± 0.07 and 0.99 ± 0.05 N-cm, respectively. Hardness and elastic modulus ranged 4.8-6.7 and 118-339 GPa in SS, and 2.7-3.2 and 52-81 GPa in PF. Significant differences between torsion-fractured and new instruments in hardness and elastic modulus were detected in the SS system used. While in PF system, the edge region after torsional fracture had significantly lower hardness and elastic modulus compared to new instruments. The local hardness and modulus of elasticity of endodontic instruments adjacent to the fracture edge are significantly reduced by torsional loading.

Torsional resistance of three ProTaper rotary systems

“…Cyclic fatigue occurs while the file rotates in a curved geometry, wherein the stress is at its highest at the area of maximum curvature, producing alternating compression and tension cycles until fracture [5, 6]. Torsional fatigue failure occurs when the torque resulting from contact between the file and canal wall exceeds the torsional strength of the file, or by twisting the file through its longitudinal axis at one end while the tip or another part of the file is locked in the canal [1, 4, 7]. The lifetime of NiTi files may be influenced by many factors, such as file geometry, metal surface treatment, thermal treatment, and metallurgic characterization of the NiTi alloys [8–11].…”

Fracture of nickel titanium rotary instrument during root canal treatment and re‐treatment: a 5‐year retrospective study

“…, Jamleh et al . , ). Fatigue of NiTi alloys mainly occurs as the result of repeated clinical use with stresses much lower than that required for fracture on a single force application and without obvious warning (Courtney , Gambarini , Svec & Powers , Yared , Wolcott et al .…”

Root dentine and endodontic instrumentation: cutting edge microscopic imaging

“…Therefore, NiTi alloy has become the preferred manufacturing alloy with its superelastic properties and lower hardness of around 3.17 + 0.2 GPa [16]. This has allowed better negotiation of curved canals and complex anatomies [26], and minimized transferring the applied stresses laterally into the dentine [19].…”

“…The physical properties of the machined endodontic files, however, vary depending on the alloy composition, size (diameter) and geometry of the file [15]. The elastic modulus for stainless steel files is around 340 + 30 GPa, and their hardness is around 6.5 + 0.5 GPa [16].…”

“…Developments in endodontic instrumentation have been driven by the need for improved canal preparation techniques in order to minimize unwanted dentine cutting that can affect the treatment outcome [25]. Therefore, NiTi alloy has become the preferred manufacturing alloy with its superelastic properties and lower hardness of around 3.17 + 0.2 GPa [16]. This has allowed better negotiation of curved canals and complex anatomies [26], and minimized transferring the applied stresses laterally into the dentine [19].…”