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

Aim: Purpose of this work was to assess by Electron Dispersive X-Ray analisys (EDX) the corrosion of endodontic rotary Nickel – Titanium (Ni-Ti) instruments due to a solution of sodium hypochlorite (NaOCl). Materials & Methods: Ni-Ti endodontic instruments 25.06 produced by eight different brands were analyzed. Eight Ni-Ti rotary file were randomly selected by a sample of 20 new instruments of every brand. A total of 64 instruments divided into eight groups, one for each brand, were tested in this study. K3, Mtwo, ProTaper, Race, NRT, Hero 642, Hero Shaker and Flexmaster files formed groups A, B, C, D, E, F, G, H respectively. Eight new niti files, one of every brand, didn't immersed in NaOCl solution were 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. 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 instrument was rinsed by bidistilled water for neutralize the effect of NaOCl, dried, registered with an ID number and stored in sterile cases. The surface of NiTi file tested was evaluated by SEM looking for pitting corrosion effects of NaOCl immersion with or without precipitate on the shaft, blade and tip of instruments. More eight new niti file, one of every brand, didn't immersed in NaOCl solution and were used as control group. 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) the detachment of many black and some white particles. EDX analysis proved that these particles consisted of Nichel to the black and Titanium oxide for white ones. Solutions of the remaining groups (B, C, D, F, H) there was no visible corrosion products. EDX analysis allowed a qualitative and semi-quantitative of corroded and not corroded areas of endodontic instruments surface after immersion in sodium hypochlorite. In particular in corroded areas of K3 (group A), EDX showed a disappearance of Ni and a drastic reduction of Ti compared to EDX of a not corroded areas of K3 instruments. Corroded areas of NRT and Hero Shaper files showed a reduction of Ni and Ti on the surface and the presence of impurities by tungsten in NRT files and precipitate formation of NaCl crystals at EDX evaluation. The remaining groups Mtwo, ProTaper, Race, Hero 642, Flexmaster, respectively, groups B, C, D, F, H showed no significant signs of corrosion such as Ni and Ti reduction at EDX evaluation. Conclusion: NiTi endodontic files are subject to corrosion after contact for 8 minutes with a solution of sodium hypochlorite at 5% and 37°C. EDX assessments showed that the K3, the NRT and Hero Shaper were those who underwent more signs of corrosion with loss of Ni and Ti while Mtwo, ProTaper, Race, Hero 642 and Flexmaster showed no significant signs of corrosion and a normal presence of Ni and Ti on surface of instruments at EDX evaluation in our experimental conditions. The Nichel and Titanium lost by corroded endodontic instrumens in sodium hypoclorite solution were considered signs of corrosion and changes the properties of the alloy of files that may be more susceptible to unexpected separation during its use.