Effect of temperature on cyclic fatigue resistance of heat-treated nickel-titanium files in simulated S-shaped root canals

Abstract

BACKGROUND: Heat-treated nickel-titanium files have gradually become the mainstream instruments in clinical applications due to their excellent flexibility and good root canal shaping ability. However, root canal preparation of S-shaped curved root canals is a challenging aspect of clinical root canal treatment, and instrument separation is prone to occur in complex root canals. OBJECTIVE: To compare the cyclic fatigue resistance of five heat-treated nickel-titanium files in simulated S-shaped root canals at different temperatures. METHODS: Twenty samples each of Hyflex CM, M3, Plex, Hyflex EDM, and R-phase nickel-titanium files were selected. A double-curved metal simulated S-shaped root canal was used. The cyclic fatigue resistance of the five files was tested at room temperature (24 °C) and heated to 65 °C using a heating mantle, with 10 samples per file per temperature. The number of cycles to fracture and the length of the fractured fragment were recorded. The fracture surfaces were examined by scanning electron microscopy. RESULTS AND CONCLUSION: (1) At both temperatures, the five files exhibited similar fracture patterns: first fracture occurred in the apical curvature, followed by fracture in the coronal curvature. All fracture surfaces showed typical features of cyclic fatigue. (2) At room temperature: Hyflex EDM had the highest number of cycles to fracture in the apical segment, Plex had the lowest; Hyflex CM had the highest in the coronal segment, M3 had the lowest; TF had the longest apical fragment, Hyflex EDM had the shortest; Plex had the longest coronal fragment, Hyflex CM had the shortest. (3) At 65 °C: TF had the highest number of cycles to fracture in the apical segment, M3 had the lowest; Hyflex EDM had the highest in the coronal segment, Plex had the lowest; Plex had the longest apical fragment, M3 had the shortest; Hyflex CM had the longest coronal fragment, Plex had the shortest. (4) When the temperature increased to 65 °C, except for the TF group, the number of cycles to fracture in both apical and coronal segments was significantly lower than at room temperature. With increasing temperature, the apical fragment length increased for Hyflex CM, Plex, and Hyflex EDM, and the coronal fragment length increased for Hyflex CM and M3. These results indicate that Hyflex EDM and R-phase TF files exhibited superior cyclic fatigue resistance in simulated S-shaped root canals at both room temperature and heating, and heating to 65 °C may decrease the cyclic fatigue resistance of Hyflex CM and Hyflex EDM files.

1. Introduction

In recent years, heat-treated nickel-titanium files have been widely used in clinical practice due to their excellent flexibility and superior root canal shaping ability. CM nickel-titanium files are a representative example. Through heat treatment, the phase transformation temperature of CM files can be increased to approximately 60 °C [1]. At room temperature, the nickel-titanium alloy is predominantly in the martensitic phase, which is softer than the high-temperature austenitic phase. Since the martensitic phase lacks shape memory properties, CM files can be pre-bent, which is extremely advantageous for preparing root canals with sharp curves or ledges. Meanwhile, another type of heat-treated file, the R-phase nickel-titanium file, has a phase transformation temperature of about 18 °C [2-3]. At room temperature, it is predominantly austenitic, and when subjected to stress in curved root canals, it undergoes stress-induced martensitic transformation. The unique feature of R-phase files is that the heat treatment introduces an intermediate R-phase transformation during stress-induced martensitic transformation, which significantly reduces the shear modulus, resulting in high flexibility and suitability for curved root canal preparation [4].

S-shaped root canals are a common and challenging condition in clinical root canal treatment. It has been reported that in mandibular first molars with a separate distolingual root, approximately 60% of the distolingual canals are S-shaped [5]. During preparation of S-shaped root canals, the complex and variable canal anatomy leads to diverse stress patterns on continuously rotating nickel-titanium files. The repeated phase transformations (in R-phase files) and atomic phase movements in the martensitic phase (in CM files) cause dislocation accumulation and microcrack formation, leading to instrument fatigue fracture [6]. Furthermore, when sodium hypochlorite solution is heated above 60 °C for root canal irrigation, it may induce phase transformations in heat-treated nickel-titanium files, thereby affecting their cyclic fatigue resistance [7].

Currently, research on the cyclic fatigue resistance of different heat-treated nickel-titanium files in S-shaped root canals is insufficient. TOPUOLU et al. [8] tested files made of CM wire in an S-shaped root canal model and found that they had better durability than M wire files. However, studies on R-phase files under similar conditions have not been reported. This study aims to compare the cyclic fatigue resistance of five heat-treated nickel-titanium files in simulated S-shaped root canals at different temperatures.