ABSTRACT
OBJECTIVES: This study compared the cyclic fatigue resistance of nickel-titanium (NiTi) files obtained in a conventional test using a simulated canal with a newly developed method that allows the application of constant fatigue load conditions. MATERIALS AND METHODS: ProFile and K3 files of #25/.06, #30/.06, and #40/.04 were selected. Two types of testing devices were built to test their fatigue performance. The first (conventional) device prescribed curvature inside a simulated canal (C-test), the second new device exerted a constant load (L-test) whilst allowing any resulting curvature. Ten new instruments of each size and brand were tested with each device. The files were rotated until fracture and the number of cycles to failure (NCF) was determined. The NCF were subjected to one-way ANOVA and Duncan's post-hoc test for each method. Spearman's rank correlation coefficient was computed to examine any association between methods. RESULTS: Spearman's rank correlation coefficient (rho = -0.905) showed a significant negative correlation between methods. Groups with significant difference after the L-test divided into 4 clusters, whilst the C-test gave just 2 clusters. From the L-test, considering the negative correlation of NCF, K3 gave a significantly lower fatigue resistance than ProFile as in the C-test. K3 #30/.06 showed a lower fatigue resistance than K3 #25/.06, which was not found by the C-test. Variation in fatigue test methodology resulted in different cyclic fatigue resistance rankings for various NiTi files. CONCLUSIONS: The new methodology standardized the load during fatigue testing, allowing determination fatigue behavior under constant load conditions.
Subject(s)
Androsterone , FatigueABSTRACT
The purpose of this study was to determine the effect of sodium hypochlorite and steam autoclaving on the cyclic fatigue of nickel-titanium endodontic files. Two types of files with a .06 taper and #30 were used, K3(R) (SybronEndo, Glendora, California, USA) and Hero642(R) (Micro-Mega, Besancon, France). The files were divided into 6 experimental groups containing 10 files each group depending the soaking time in 6% sodium hypochlorite solution and number of cycles of steam autoclave. After sterilization, a cyclic fatigue test was performed on each file, and the fracture time was recorded in seconds. The control group underwent the cyclic fatigue test only. After the test, the surface characteristics of the files were observed using scanning electron microscopy (SEM). All groups containing the Hero 642(R) files showed a similar cyclic fatigue fracture time. However, the cyclic fatigue fracture time with the K3(R) files was significantly shorter in groups which were treated with sodium hypochlorite than in the control group (P < 0.05). SEM revealed both Hero642(R) and K3(R) files to have significant corrosion on the file surface in groups treated with sodium hypochlorite, compared with the sharp and regular blades of the control group. K3(R) files showed more corrosion than the Hero642(R) files. Bluntness of the blades of the K3(R) file was observed in groups treated with steam autoclave. Although there was no obvious destruction on the surface of steam autoclaved Hero642(R) files, slight bluntness was observed. Sterilizing with a steam autoclave is much less destructive to K3(R) files than sodium hypochlorite. The longer time exposed to sodium hypochlorite, the more destructive pattern was shown on the blades of the files. Therefore, when using sodium hypochlorite solution, the exposure time should be as short as possible in order to prevent corrosion and increase the cyclic fatigue fracture time.