Effect of surface defects and cross-sectional configuration on the fatigue fracture of NiTi rotary files under cyclic loading

Article information

Restor Dent Endod. 2004;29(3):267-272

Publication date (electronic) : 2004 May 31

doi : https://doi.org/10.5395/JKACD.2004.29.3.267

Yu-Mi Shin ¹, Eui-Sung Kim ¹, Kwang-Man Kim ², Kee-Yeon Kum ¹

¹Department of Conservative Dentistry, Yonsei University, Korea.

²Department of Dental Materials, Yonsei University, Korea.

Corresponding author: Kee-Yeon Kum. Department of Endodontics, Youngdong Severance Hospital, College of Dentistry, Yonsei University, Dokok-Dong, Kangnam-Gu, 146-92, Seoul, 135-270, Korea. Tel: 82-2-3497-3566, Fax: 82-2-3463-0551, kum6139@yumc.yonsei.ac.kr

Abstract

The purpose of this in vitro study was to evaluate the effect of surface defects and cross-sectional configuration of NiTi rotary files on the fatigue life under cyclic loading. Three NiTi rotary files (K3™, ProFile®, and HERO 642®) with #30/.04 taper were evaluated. Each rotary file was divided into 2 subgroups: control (no surface defects) and experimental group (artificial surface defects). A total of six groups of each 10 were tested. The NiTi rotary files were rotated at 300rpm using the apparatus which simulated curved canal (40 degree of curvature) until they fracture. The number of cycles to fracture was calculated and the fractured surfaces were observed with a scanning electron microscope. The data were analyzed statistically. The results showed that experimental groups with surface defects had lower number of cycles to fracture than control group but there was only a statistical significance between control and experimental group in the K3™ (p<0.05). There was no strong correlation between the cross-sectional configuration area and fracture resistance under experimental conditions. Several of fractured files demonstrated characteristic patterns of brittle fracture consistent with the propagation of pre-existing cracks.

This data indicate that surface defects of NiTi rotary files may significantly decrease fatigue life and it may be one possible factor for early fracture of NiTi rotary files in clinical practice.

Keywords: Surface defect; Cross-sectional configuration; Cyclic fatigue; Brittle fracture; Ductile fracture; NiTi rotary files

References

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(open-access) :

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Figure 1

Cross-sectional configuration of K3™, ProFile®, and HERO 642®.

Figure 2

The apparatus developed for fracturing NiTi rotary files under cyclic loading (Schneider's curvature: 40°).

Figure 3

A SEM image of surface defect (arrow) of K3™ (experimental group, ×150).

Figure 4

A SEM image of the fractured surface of HERO 642® file after cyclic loading: a control group (×150).

Figure 5

A higher magnification view of the rectangular area shown in Figure 4. Ultimate ductile fracture region that are typically characterized with microvoid formation and dimpling are seen (×1000).

Figure 4

A SEM image of the fractured surface of HERO 642® file after cyclic loading: a control group (×150).

Figure 6

SEM image of the fractured surface of ProFile® after cyclic loading (experimental group, ×150): The left square area shows a region of brittle fracture (BF) which transit to ultimate ductile failure (DF).

Figure 7

A higher magnification view of brittle fracture (BF) area shown in Figure 6. A region of brittle fracture is shown originating from the pre-existing surface defects (×500).

Figure 6

Table 1

Study Design of control and experimental groups.

Table 2

Mean number of cycles to fracture and statistical comparison between control and experimental group of each file system.

S: Statistically significant at p < 0.05 (Wilcoxon Rank Sum Test). NS: Non-significant

Number of cycles to fracture = Time to fracture (sec)×5 (300 rpm = 300 cycles/min)

Table 3

Statistical comparison between the number of cycles to fracture and the cross sectional configuration area at 5 mm level.

^†Statistically significant between cross sectional area (D₅) and number of cycles to fracture (Kruskal-Wallis test, p < 0.05)