A2 ◊Page A1 ENDO TRIBUNE Dental Tribune Middle East & Africa Edition | 2/2020 was observed after multiple canal in- strumentations. 20 The authors also found surprisingly high values of cyclic fatigue resistance and safe in vitro use in severely curved canals. In agreement with previous research- ers, Pedulla et al. reported higher values of fatigue resistance for Hy- Flex EDM fi les (COLTENE) even when compared with reciprocating fi les made from M-wire.21 Unfortunately, most of the available literature on bending stiffness and cyclic fatigue fracture resistance of NiTi rotary or reciprocating instruments concerns studies performed at room tempera- ture. However, room temperature is not a clinically relevant temperature. Current instruments are used at body temperature rather than room temperature. This makes most of the previous studies obsolete and their conclu- sions cannot be applied in the clini- cal practice. It seems that the trans- formation temperature (Af) of rotary or reciprocating NiTi fi les might al- ter their clinical behaviour at body temperature. Hulsmann et al. (2019) reported that environmental tem- perature has a 500 % impact on the lifetime of instruments.22 A transfor- mation temperature near body tem- perature can result in instruments that appear to be fl exible and fa- tigue-resistant at room temperature; however, at clinically relevant tem- peratures, the instruments become stiffer and less fatigue-resistant. The Af of HyFlex EDM was found to be close to 52 °C, far above body tem- perature. Af temperature analysis of EDM fi les revealed the presence of monoclinic martensite B19 structure and rhombohedral R-phase.23 There- fore EDM instruments are always in a rhombohedral R-phase and martensitic crystallographic state at clinically relevant temperatures. A martensitic structure at body tem- perature, like HyFlex EDM, will ex- ert superior fl exibility and fatigue fracture resistance. The extreme fl exibility and fatigue resistance of these fi les, combined with the lack of restoring force, render them ideal for use in the instrumentation of highly curved and complicated canals. HyFlex EDM Max Curve sequence EDM made feasible the use of a sin- gle-fi le enlargement approach with rotational movement. Most cases can be shaped quite quickly, effec- tively and safely by using a single 25/~ HyFlex EDM OneFile with short- stroke pecking movements, fre- quent fl ute cleaning and irrigation between the strokes. The OneFile has a tip size of 25 with a .08 taper. The taper is a constant .08 in the apical 4 mm of the instruments, but reduces progressively up to .04 in the coronal portion of the instrument. The fi le has three different cross-sectional zones over the entire length of the working part (rectangular in the api- cal part and two different trapezoi- dal cross sections in the middle and coronal parts of the instrument) to increase its fracture resistance and cutting effi ciency.21 Whenever larger apical preparations are required, three fi nishing HyFlex EDM fi les of constant taper can be used (40/.04, 50/.03 and 60/.02). For constricted and obliterated ca- nals, thin and long roots, curved ca- nals of more than 27° and S-shaped canals with a curvature of smaller than 5 mm in radius, single-fi le EDM shaping is not feasible. For these challenging cases, the HyFlex EDM Max Curve sequence was introduced for use with the TCA technique. With this combination, all those cases can be handled effectively and predict- ably. The new HyFlex EDM Max Curve set includes 15/.03, 10/.05 and 20/.05 fi les. Under the new concepts ÿPage A3 Fig. 3: HyFlex EDM Max Curve sequence and size breakdown. about 6 %, and after 100,000 defor- mations, it is about 4 %. Within this range, the memory effect can be ob- served.16 Besides stress-induced martensitic transformation, the lattice organisa- tion of NiTi alloys can be altered by altering the temperature. When a conventional NiTi austenitic mi- crostructure is cooled, it begins to change into martensite. The tem- perature at which this phenom- enon begins is called the martensite start temperature. The temperature at which martensite is again com- pletely reverted is the martensite fi nish When martensite is heated, it begins to change into austenite. The temperature at which this phenom- enon begins is called the austenite start temperature. At and above the austenite fi nish temperature (Af), the material will have completed its shape memory transformation and will display its super-elastic charac- teristics.18 Before 2011, the Af temperature for the majority of available NiTi instru- ments was at or below room tem- perature. As a result, conventional NiTi fi les were in the austenitic phase during clinical use, showing shape memory and super-elasticity. In 2011, controlled memory (CM) fi les were introduced by international dental specialist COLTENE. These fi les are manufactured utilising a unique thermomechanical process that controls the material’s memory, making the fi les extremely fl exible and fatigue-resistant without the shape memory and restoring force of other NiTi fi les. The Af transfor- mation temperature of CM fi les has been found to be clearly above body temperature. As a result, these fi les are mainly in the martensitic phase at body temperature.18 When the material is in its martensitic form, it is soft, ductile and without shape memory, and can easily be deformed, but will recover its shape and super-elastic properties upon heating over the Af temperature. Moreover, a hybrid martensitic mi- crostructure, like that used in the HyFlex CM fi les (COLTENE), is more likely to have a better fatigue resist- ance than an austenitic microstruc- ture is. At the same stress intensity, the fatigue crack propagation speed of austenitic structures is much faster than that of martensitic ones. A quantitative analysis based on the model of the fracture process zone showed that the martensite transfor- mation in the shape memory NiTi alloy caused a 47 % increase in the apparent fracture toughness.19 Very recently, CM thermomechani- cal processing was combined with an innovative machining procedure for the manufacture of rotary NiTi fi les. Electrical discharge machin- ing (EDM) results in instruments of increased surface hardness, cut- ting effi ciency and extreme fatigue resistance. In the fi rst published paper evaluating these fi les, a typi- cal spark-machined peculiar surface was reported and low degradation Figs. 4a–h: Tactile-controlled activation technique explaine