Corrosion of endodontic NiTi instruments by sodium hypochlorite solution: a comparative SEM analisys

Aim: Purpose of this work was to assess the corrosion of endodontic rotary Nickel – Titanium (Ni-Ti) instruments due to a solution of sodium hypochlorite (NaOCl) by Scansion Electron Microscopy (SEM). Materials & Methods: Ni-Ti endodontic instruments 25.06 produced by eight different brands was analyze. Eight test groups were established, one for each brand. K3, Mtwo, ProTaper, Race, NRT, Hero 642, Hero Shaper and Flexmaster was groups A, B, C, D, E, F, G, H respectively. Eight Ni-Ti rotary file were randomly selected by a sample of 20 new instruments of every brand. Eight new niti file didn't immersed in NaOCl solution, one of every brand, was used as control group. Between instruments at multiple taper, as ProTaper, were used F2 because its caracteristics are closer to 25.06 dimension of other niti file. A total of 64 test instruments and eight control instruments were analyzed in this study. All Ni-Ti test files, new and never autoclavate, were totally immersed in a 5% solution of NaOCl (Niclor, OGNA) for 8 minutes in a glass container. Immediately every instruments was rinsed by bidistillated water for neutralize the effect of NaOCl, dried, registered with an ID number and stored in sterile cases. Surface NiTi file tested was evaluated by Scansion Electronic Microscopy looking for pitting corrosion effects of NaOCl immersion with or without precipitate on the shaft, blade and tip of instruments.. The NaOCl solutions used for immersion of all instruments were also analyzed looking for precipitate products by corrosion. Results: Solutions of sodium hypochlorite used for groups A (K3), E (NRT files) and G (Hero Shaper) showed the formation of corrosion products. In preparations of groups A, E and G there was a detachment of black particles present at the transition between blade and stem in group A and G, among the stem and shaft in Group E. Solutions of the remaining groups (B, C, D, F, H) there was no visible corrosion products. Evaluation by SEM showed signs of corrosion, like pitting, on the surface of the instruments of groups A, E, G immersed in sodium hypochlorite when compared with a same instrument of control group. In particular, at the level of the blades, all files K3 (group A) showed the formation of craters up to 400 micrometers in diameter, while the NRT files showed the formation of craters up to 200 micrometers in diameter due to localized corrosion. The blades of all Hero Shapers (Group G) show areas of pitting corrosion up to about 100 micrometers in diameter after immersion in a solution of sodium hypochlorite. The remaining groups Mtwo, ProTaper, Race, Hero 642, Flexmaster, respectively, groups B, C, D, F, H showed no significant signs of corrosion such as craters or cracks at SEM observation. Conclusion: NiTi endodontic files are subject to corrosion after contact for 8 minutes with a solution of sodium hypochlorite at 5% and 37°C. Among the files tested the K3, the NRT and Hero Shaper were those who showed more signs of pitting corrosion, while Mtwo, ProTaper, Race, Hero 642 and Flexmaster showed no significant signs of corrosion in our experimental conditions. Areas of pitting corrosion can be more or less narrow, but may be the starting point for an unexpected separation of the instrument during its use.