Article
Effect of Autoclaving Cycles on the Cyclic Fatigue Resistance of
Race and Race Evo Nickel-Titanium Endodontic Rotary Files:
An In Vitro Study
Rahaf A. Almohareb
1
, Reem Barakat
1,
*, Aroob Albakri
2
and Manal Altamimi
2
Citation: Almohareb, R.A.; Barakat,
R.; Albakri, A.; Altamimi, M. Effect of
Autoclaving Cycles on the Cyclic
Fatigue Resistance of Race and Race
Evo Nickel-Titanium Endodontic
Rotary Files: An In Vitro Study.
Metals 2021, 11, 1947. https://
doi.org/10.3390/met11121947
Academic Editor:
Francesco Iacoviello
Received: 9 November 2021
Accepted: 30 November 2021
Published: 2 December 2021
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4.0/).
1
Clinical Dental Sciences Department, College of Dentistry, Princess Nourah Bint Abdulrahman University,
Riyadh 11671, Saudi Arabia; raalmohareb@pnu.edu.sa
2
College of Dentistry, Princess Nourah Bint Abdulrahman University, Riyadh 11671, Saudi Arabia;
Aroob.ahmed.b@hotmail.com (A.A.); manalaltamimi@outlook.com (M.A.)
* Correspondence: rmbarakat@pnu.edu.sa; Tel.: +966-504440598
Abstract:
Objective: The aim of this study was to evaluate the influence of autoclave sterilization
on the resistance to cyclic fatigue of two nickel-titanium (NiTi) endodontic files of identical design
and taper, but with different NiTi alloy treatments: the newly introduced heat-treated Race Evo and
the electropolished Race files. Materials and methods: Fifteen Race (25/0.06) files and fifteen Race
Evo (25/0.06) files (n = 30 in total) were randomly assigned to five sub-groups each consisting of
three files of the same NiTi alloy treatment. One group served as a control with files unautoclaved.
The four remaining groups were sterilized in a steam sterilizer for 1, 3, 5, and 10 autoclave cycles,
respectively. Files then underwent cyclic fatigue testing in a simulated metal canal block. A scanning
electron microscope was used to inspect the surface of the fractured instruments. Statistical analysis
was conducted using independent t-test and multi-factorial analysis of variance with significance
set at a p value of
≤
0.05. Results: Both Race Evo and Race files showed no significant difference
between the different autoclaving cycles in terms of the number of cycles to fracture (p = 0.232
and p = 0.359). Despite rotating at a higher speed, the number of cycles to fracture of heat-treated
Race Evo files was significantly higher than that of Race files (p
≤
0.0001). Conclusion: Autoclave
sterilization has no significant effect on the resistance to cyclic fatigue of heat-treated Race Evo or
electropolished Race files. However, Race Evo files showed superior resistance to cyclic fatigue
irrespective of autoclaving cycles.
Keywords: autoclave; cyclic fatigue; heat treatment; NiTi; race; scanning electron microscope
1. Introduction
Nickel-titanium (NiTi) rotary files can provide faster and more predictable instru-
mentation of root canal systems. This alloy, which contains nearly 56% (wt) nickel and
44% (wt) titanium, grants them key properties, such as shape memory and super elasticity,
which have allowed them to revolutionize root canal instrumentation [
1
,
2
]. However,
these NiTi instruments have been plagued with a high risk of fracture related to either
torsional or flexural fatigue. A NiTi file rotating in a curved canal is subjected to a set of
continuous compressive and tensile stresses that results in its flexural (cyclic) fatigue [
3
].
Many factors influence cyclic fatigue: canal curvature, instrument dimensions, design,
and manufacturing process. Advancements in the manufacturing of the NiTi alloy itself,
such as thermal treatment [
4
] and surface treatment, have been suggested to enhance
fatigue resistance [5–7].
The NiTi alloy exists in three crystallographic phases: austenite, martensite, and R-
phase. The martensite microstructure shows superior resistance to cyclic fatigue. Heat-
treatment alters the temperature at which the microstructure of the NiTi alloy shifts from
one phase to another, adopting in the process the specific properties of that phase. As a
Metals 2021, 11, 1947. https://doi.org/10.3390/met11121947 https://www.mdpi.com/journal/metals
