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Several years ago, a few endodontic instrument manufacturers developed nickel-titanium (NiTi) files (Twisted File and GTX) made with proprietary processes beyond those used for conventional NiTi files. This trend has continued with the introduction of shape-memory NiTi files (Berzins 2011 IADR#1710). Resistance to chemical dissolution is important in endodontic files since the conjoint factors of stress and corrosion may limit the fatigue life of rotary instruments.
Objectives: The objective of this study was to evaluate the corrosion properties of two conventionally-manufactured superelastic NiTi files (ProFile and K3), two newer superelastic NiTi files manufactured with proprietary treatments (Twisted File and GTX), and two shape-memory NiTi files (CM wire and HyFlex).
Methods: Six endodontic files were investigated: Twisted File and K3 (SybronEndo), ProFile and GTX (Dentsply Tulsa Dental), CM Wire (DS Dental), and HyFlex (Coltene-Whaledent). All files (n=8/brand) were size 40 with a .04 taper. The distal 12 mm of the files was isolated and electrochemically tested in 2.5% sodium hypochlorite (NaOCl) at 35oC. The open circuit potential (OCP) was monitored for 1 hr followed by polarization resistance and cyclic polarization tests. OCP at 1 hr and log transformed corrosion current (Icorr) were compared using ANOVA/Tukey (p<0.05).
Results: The enclosed table displays the electrochemical results (mean ± standard deviation).
File | OCP (mV vs. SCE) | Icorr (nA) |
ProFile | 577±11 A | 2030±580 C |
K3 | 343±139 B | 73±54 A |
Twisted File | 368±93 B | 67±28 A |
GTX | 574±32 A | 370±270 B |
CM Wire | -1±65 C | 11500±4700 D |
HyFlex | -104±88 C | 440±720 AB |
Different letters denote a significant difference (p<0.05) between files within each parameter. Shape memory files exhibited decreased nobility. In terms of corrosion rate (Icorr), over 100-fold differences were observed between files.
Conclusion: Extreme differences in corrosion properties exist among NiTi endodontic rotary files.
Keywords: Alloys, Corrosion, Endodontics, Metals and Physical