RO0111

I 35 clinical technique _ R-phase I roots1_2011 Torsionalstressismoredangerous.Itoccurswhen the tip or any other section of the file is locked inside the canal while the rest of the file is still rotating. This canleadtoinstantfileseparation.Torsionalstresscan occuranywhereinsidethecanalandcanbeevenmore dangerouswhenitiscombinedwithbendingstresses inside the curvature of the canal (Fig. 5). This can be the perfect trap for rotary files. Combining the two essential features of a file— security and flexibility—was not possible with the NiTi alloy available. Therefore, some manufacturers focused on the flexibility of the files while compro- mising security, and others focused on security while compromising flexibility. Intensive research was conducted into changing the crystalline struc- tureandobtainingadual-phasestructureasitsmain characteristic.Thedualphaseisamixofausteniteand theR-Phase.TheR-phasehastheadvantageofbeing abletocopewithincreasedpressurewhileincreasing thetorquevalue(Fig.6).Itappearsaftertheaustenite phase and still increases in torque until the ultimate deformation point. There is then a short martensite phase prior to separation. During the R-phase, there are actually two kinds of R-phases: R1 and R2. Ac- cording to thermal cycling tests under various con- stant stresses, the transformation strain of R2 is greaterthanthatofR1.Withconstantstressincreas- inglyapplied,thelatterstrainincreasesandtheformer decreases.However,thetotalstrainofbothR1andR2 isalmostconstantirrespectiveoftheamountofstress. R1 will start immediately after austenite, and then R2 willtakeoverinthemiddleofthetransitionareauntil theultimatedeformationpoint,wheremartensitewill be formed. With the R-phase, shaping curves will be much easier and safer as the files produced with the modified NiTi alloy are much more flexible than any otherfilewiththesamedimensions.Thisallowsthefile tocopewiththepressureofshapingthecurvessafely. As for the clinical applications, we must ensure that we do not place a very large file in a curve, such asa0.10or0.08taper,butpayattentiontothesever- ity of the curves, for two reasons: Fig. 4_Illustration of the bending stress to which a file is subjected inside the curve. Fig. 5_Illustration of the torsional stress and the load of stress that a file will take at the tip of the root canal or inside the curve, which will lead to file separation. Fig. 6_Graph showing the stress test result of a TF file using a torque metre. Fig. 7_M4 handpiece. Fig. 4 Fig. 5 Fig. 6 Fig. 7