Phase transformation behaviour and bending property of twisted nickel–titanium endodontic instruments
XM. Hou1,2, Y. Yahata1, Y. Hayashi1, A. Ebihara1, T. Hanawa3 & H. Suda1
Pulp Biology and Endodontics, Department of Restorative Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan; 2Department ofEndodontics, Capital Medical University School of Stomatology, Beijing, China; and 3Department of Metallurgy, Division of Biomaterials, Institute of Biomaterials and Bioengineering, Tokyo Medical and Dental University, Tokyo, Japan
Hou XM, Yahata Y, Hayashi Y, Ebihara A, Hanawa T, Suda H. Phase transformation behaviour and bending
property of twisted nickel–titanium endodonticinstruments. International Endodontic Journal, 44, 253–258, 2011.
Aim To investigate the relationship between phase transformation behaviour and bending property of nickel–titanium endodontic instruments manufactured by a twisting process. Methodology The phase transformation behaviour and bending property of Twisted Files (TF; SybronEndo, Orange, CA, USA) and K3 (SybronEndo) with.06 taper and size30 tip were investigated. K3 was used as control group. Phase transformation behaviour was estimated by differential scanning calorimetry (DSC). Transformation temperatures were calculated from the DSC curve.
Bending load of the instruments was measured by cantilever-bending test at 37 °C. Data were analysed by Student’s t-test and Mann–Whitney U-test. Results The phase transformationtemperatures of TF were signiﬁcantly higher (P < 0.05) than those of K3. The bending load values were signiﬁcantly lower for TF than that of K3 (P < 0.05), both in the elastic and super-elastic ranges. Conclusions The new method of manufacturing NiTi instruments by twisting coupled with heat treatment might contribute to the increased phase transformation temperatures and superior ﬂexibility. Keywords:bending property, differential scanning calorimetry, manufacturing process, nickel–titanium alloy, Twisted File.
Received 24 June 2010; accepted 8 October 2010
Nickel–titanium (NiTi) instruments were ﬁrst applied in the ﬁeld of endodontics in 1988 (Walia et al. 1988). Since then, they have played an increasingly important role in endodontics. It is known that the super-elasticityand shape memory effect of these instruments are derived from martensitic transformation. At a temper-
Correspondence: Dr Arata Ebihara, Pulp Biology and Endodontics, Department of Restorative Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 5-45 Yushima 1-chome, Bunkyo-ku, Tokyo, 113-8549, Japan (Tel.: +81 (3) 5803 5494; fax: +81 (3) 5803 5494;e-mail: email@example.com).
ature above the transformation temperature range, NiTi alloy is composed of austenite whilst at a lower temperature it consists of martensite (Huang et al. 2003). A decrease in temperature can trigger phase transformation from austenite to martensite (termed martensitic transformation), and vice versa (reverse transformation). This phenomenon could also betriggered by stress (Daly et al. 2007). When temperature is higher than the transformation temperature and the NiTi alloy is composed of austenite, loading and unloading can cause forward martensitic transformation (i.e. stress-induced martensitic transformation) and reverse transformation (Miyazaki & Otsuka 1989, Thompson 2000). Furthermore, the rhombohedral (R-) phase transformation, alsothermo-elastic, precedes the
ª 2010 International Endodontic Journal
International Endodontic Journal, 44, 253–258, 2011
Phase transformation behaviour and bending property Hou et al.
martensitic transformation under certain conditions (Miyazaki & Otsuka 1986). This reversible thermoelastic martensitic transformation is the main reason for increased ﬂexibility of NiTi instruments over...