Cyclic Fatigue

Background

In clinics, cyclic flexural fatigue of an instrument occurs when it rotates in a curved canal by repeated cycles of compressive and tensile stresses. Cyclic fatigue parameter represents mechanical resistance measures, whose improvements anticipate a better clinical performance when submitted to that specific stress reducing the chance of instrument fracture.

Aim

To evaluate the cyclic fatigue of NTendo Rainbow S rotary 25/0.06 by Nineten, India with traditional polishing technology compared to the same instrument but with an upgraded polishing technology.

Sampling

Based on the results of previous studies (1–4), a power calculation was performed using G*Power 3.1 (Heinrich Heine University, Dusseldorf, Germany) software indicating a minimum sample size of 10 instruments per group for the test.

Methods

Before the mechanical testing, instruments were examined under a stereomicroscope to ensure that no defects that could lead to deformation were present.

Cyclic Fatigue Test

Cyclic fatigue tests were performed using a custom device that allows to simulate an instrument working passively in a curved canal, as previously described (1–6). Instruments followed the rotation motion recommended by the manufacturer (400 RPM and 2.0 N.cm). Instruments were mounted on a 6:1 reduction handpiece (VDW/Sirona Dental Systems, Bensheim, Germany) powered by an electric motor (Reciproc Silver; VDW GmbH, Munich, Germany) and assembled on a tube model custom-made device (Figure 1).

The electric handpiece was mounted on a device to allow accurate and reproducible placement of each instrument within the simulated canal. The time to fracture (TTF) were recorded in seconds for each instrument using a digital chronometer and the experiment stop as soon as the fracture were detected visually and/or audibly. A digital microcaliper were used to determine the length of each fractured fragment length (FL).

Statistical Analysis

The Shapiro-Wilk and Levene tests were used to evaluate the assumption of normality and the equality of variance of data sets. Considering that results were normally distributed (p > .05), they were presented as means and standard deviations and statistically compared between groups using independent sample t-test. Significance level was set at p < .05 (SPSS v18.0 for Windows; SPSS Inc, Chicago, IL, USA).

Results

Cyclic Fatigue Test

Raw and consolidated data, as well as, statistical analysis regarding the number of cycles to failure of NT Rainbow S 25/0.06 with traditional polishing technology and NT Rainbow S 25/0.06 with upgraded polishing technology are presented in Table 1. The upgraded polishing technology instruments showed a higher time to fracture and a higher number of cycles to fracture when compared to traditional polishing technology instruments (p<0.0001). The mean and standard deviation of the time to fracture and number of cycles to failure are presented in Table 2.

Table 1. Time to fracture and number of cycles to failure

Table 2. Mean and standard deviation values

Remarks

According to the manufacturer NT Rainbow S 25/0.06 traditional and upgraded polishing technology instruments have exactly the same design regarding tip, taper, cross-sectional design and also the same manufacturing process and heat-treatment differing only in the polishing technology. The present results point to a clear advantage in terms of cyclic fatigue for the upgraded polishing technology instruments, with a statistically significant difference when compared to traditional polishing technology. Therefore, it is clear that the upgraded polishing technology improved the cyclic fatigue resistance and is recommended to be used in NT Rainbow S 25/0.06 instruments.

References

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