The most common proprietary thermal treated NiTi alloys are known as M-Wire, Gold Technology, Blue Technology (Dentsply, Tulsa, OK, USA) controlled memory wire or CM-Wire (Coltene, Cuyahoga Falls, OH, USA) and R-phase wire (SybronEndo, Orange, CA, USA) ( 16). Thermally treated NiTi alloys are characterized by a higher percentage of martensitic phase that is more flexible and resistant to fatigual stresses rather than the austenitic phase. Mechanical properties of nickel-titanium endodontic instrumentation like fatigue resistance, flexibility, cutting efficiency and canal centering ability have been improved since the introduction of thermal treatments ( 13- 15). Static and dynamic tests could be performed to investigate resistance to bending and cyclic fatigue lifespan: dynamic tests are usually similar to the static ones, with the adjunction of an axial movement that simulates a clinical approach ( 10- 12).
The mechanical properties of endodontic instruments have been investigated using different techniques: bending resistance tests and flexural cyclic fatigue resistance tests are usually performed by the use of artificial canals with different curvatures at room or body temperature, due to the impossibility to obtain standardization with natural extracted teeth ( 5- 9). The application of less stressing movements as such as alternate or reciprocating motion rather than continuous movement, reduced significantly the risk of intracanal leakage of the endodontic instrument ( 4). Unfortunately, with the use of mechanical devices, the intra-operative fracture of an endodontic instrument has become a more common accident due to the increased torsional and cyclical fatigue stresses ( 1- 3). The introduction of nickel-titanium alloy and the use of mechanical instrumentation in endodontics improved the quality of root canal treatment with less time consuming procedures. Differences among groups were statistically evaluated with an analysis of variance test ANOVA (significance level was set at p<0.05).
All instruments were inserted at the same length (18mm) and then rotated in the same reciprocating motion until fracture occurred: the time was stopped as soon as the fracture was visible and video-recorded with a 1/100 sec chronometer.
The same artificial root canal with a 90 degrees angle of curvature and 2 mm radius of curvature was used for all the tested instruments and the WOG counter-clock wise reciprocating motion with an engaging angle of 150° and a disengaging angle of 30° at 300 rpm, was selected for the test. A mobile support for the electric handpiece and a stainless-steel block containing the artificial canals were used. 40 new NiTi instruments with a length of 25 mm were tested: 20 Wave One Gold Medium (WOG), tip size 35 and variable taper (Dentsply Maillefer, Ballaigues, Switzerland) and 20 Edge One Fire (EOF) (EdgeEndo, Albuquerque, New Mexico) tip size 35 and the same variable taper.
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