Microstructural, microchemical, and mechanical changes associated with the clinical reuse of two nickel-titanium endodontic instruments

Abstract

Background: Nickel-titanium (NiTi) instruments have represented a great technological development that enabled endodontists conforming irregular-shaped root canals. Notwithstanding, the repeated use of these instruments may lead to the fracture without any prior visible warning signs. This study aimed to evaluate how multiple clinical instrumentation/sterilization cycles of two NiTi mechanized instruments can affect their microstructural, microchemical, and mechanical characteristics.

Materials and methods: In this observational descriptive study, a total of 140 NiTi instruments, 70 ProTaper Gold^® (PTG) and 70 WaveOne Gold^® (WOG) were analyzed. For each brand system, instruments were evaluated in the as-received condition (n = 10) and after one (n = 20), two (n = 20), and three (n = 20) instrumentation/sterilization cycles. Intraoperative instrumentation parameters were recorded for all used instruments. Afterward, the files were examined using scanning electron microscopy and energy-dispersive X-ray microanalysis. All of the instruments were tensile-fatigue tested until rupture in order to calculate the mechanical tensile strength and the maximum elongation percentage for the samples. Statistical analysis was completed using Chi-square, Kruskal-Wallis H-, or Mann-Whitney U-tests with a statistical significance set at P < 0.05.

Results: Significant increasing changes in surface topography (P < 0.05, Chi-square test) and chemical composition (P < 0.05, Kruskal-Wallis H-test) in both brand systems through instrumentation/sterilization cycles were detected. In addition, values of mechanical tensile strength and maximum elongation percentage increased significantly through instrumentation/sterilization cycles in the PTG group, whereas only the median values of mechanical tensile strength increased significantly in the WOG group (all P < 0.01, Kruskal-Wallis H-test).

Conclusion: Although multiple instrumentation/sterilization cycles may render NiTi instruments more flexible and fatigue resistant, the significant changes detected in their surface topography and chemical composition should preclude their repeated clinical use in the routine endodontic practice as prevention for breakage.

Keywords: Electron probe microanalysis; endodontics; instrumentation; scanning electron microscopy; tensile strength.

Figure 1

Unused (a) PTG and (b) WOG instruments (×23 magnification) showing a regular surface texture with milling marks than run perpendicular to the long axis of the files. At higher magnification (×1000), it can be noted that (c) PTG instruments possess sharp cutting edges and noncutting tips, whereas (d) WOG files show a morphology of cutting edges that appear rounded with a wrapped portion similar to a “wave curl” and have a tapering tip with a terminal rounded portion. Note that machining marks are shallower in WOG instruments when compared with those observed in PTG files. PTG: ProTaper Gold®; WOG: WaveOne Gold®.

Figure 2

Micrographs of PTG/WOG files showing surface defects: (a) WOG file showing unwinding after two instrumentation/sterilization cycles. (b) PTG file showing tip flattening after two instrumentation/sterilization cycles. (c) The tip of a WOG file resulted worn out after three instrumentation/sterilization cycles. (d) Crack lines running perpendicular to the axis of a PTG file after three instrumentation/sterilization cycles. (e) Cracks of cutting edge of a WOG file after three instrumentation/sterilization cycles. (f) Unused PTG file showing a surface micropit adjacent to the cutting edge. PTG: ProTaper Gold®; WOG: WaveOne Gold®.

Figure 3

High-magnification scanning electron microscopy images showing topographical changes of PTG and WOG instruments after use. (a) Blunt cutting edge in a PTG instrument after one instrumentation/sterilization cycle. (b) Cutting edge disruption in a WOG file after three instrumentation/sterilization cycles. (c) Presence of signs of corrosion on the surface of a PTG file after one instrumentation/sterilization cycle. (d) WOG instrument showing corrosion spots along the flat surfaces after three instrumentation/sterilization cycles. PTG: ProTaper Gold®; WOG: WaveOne Gold®.

Figure 4

Representative energy-dispersive X-ray spectrum of the surfaces of (a) new WOG and (b) used PTG files. The presence of NiTi on the noncorroded areas is clearly seen; while in the corroded zones an increase of oxygen and traces of other elements were detected. PTG: ProTaper Gold®; WOG: WaveOne Gold®.

Figure 5

Representative stress-strain curves of the instruments obtained in mechanical tensile tests. The lines show the stress produced by increasing tensile strain. As can be seen, the curves continue upward up to the final fracture. Note that stress-strain values were lower in the new instruments and higher after clinical use. (a) Unused PTG instrument. (b) Unused WOG instrument. (c) PTG instrument underwent two instrumentation/sterilization cycles. (d) WOG instrument underwent two instrumentation/sterilization cycles. PTG: ProTaper Gold®; WOG: WaveOne Gold®.