Hardness And Microstructure Of Nickel-Titanium Endodontic Instruments

Hardness is an important property of endodontic files as it influences cutting efficiency. Lately, companies have altered manufacturing and processing variables to introduce nickel-titanium (NiTi) files of different bending and torsion properties with the latest generation of files being characterized as “shape memory” or “controlled memory”.  This implies these files may have a different proportion of phases (austenite/martensite) compared to conventional superelastic NiTi files that are predominantly austenite at room and oral temperature.  As a result, they may possess a different hardness and microstructure.

Objective: The objective was to compare the hardness and microstructure of six NiTi files currently on the market.

Method: The NiTi endodontic files tested were ProFile & GTX (Dentsply Tulsa Dental Specialties), K³ & Twisted File (SybronEndo Corporation), CM wire (DS Dental), and HyFlex (Coltene-Whaledent). The latter two are considered “shape-memory” files.  Files from each brand (n=5/brand) were embedded in resin and underwent metallographic preparations to assess the hardness and microstructure. Vickers microhardness (VHN in kg/mm²) was measured and statistically analyzed by one way ANOVA and Tukey’s HSD test (α=0.05). Microstructure was examined with an optical microscope after etching.

Result:

Vickers microhardness values are shown below.

FILES	VICKERS HARDNESS (VHN)
CM wire	255±10     a
HyFlex	264±11     ab
Twisted File	285±4       bc
ProFile	297±6       cd
K3	309±18     d
GTX	359±14     e

Different letters denote significant differences (p<0.05) between files. The “shape-memory” files possessed the lowest hardness values, whereas GTX was the hardest file tested. A martensitic lathe-like structure was a microstructural component in all files.  A slight variation in the proportion and size of the lathes was observed between brands, but this observation did not appear to correlate with microhardness results.

Conclusion:

The new “shape memory” endodontic files have lower hardness values compared to superelastic NiTi files.