RO0111

34 I I clinical technique _ R-phase _Every dentist and endodontist dreams that all rootcanalscouldbestraightandlarge.Unfortunately, in reality they are often everything but. On a daily basis,wearefacedwithnewchallengesincurvedand narrow root canals. While NiTi rotary files have changed our world significantly, our first experiences were not so encouraging. We were faced with file separation owing to anatomical considerations of the root-canal system and our poor knowledge of the files, the alloy and the way to use the files properly. Muchresearchhasbeenconductedinordertobet- terunderstandthepropertiesofthealloyandthefiles. Researchershaveusedatorquemetretostudythefiles (Fig. 1). However, we realised that different data was required with regard to stressed or used files. Further research demonstrated that the yield, which is the point beyond the austenite phase, is stable regardless of the dimension and the size of the file (Figs. 2 & 3). The major difference is in the torque value and the behaviourofthemartensitephase.Deformationinthe martensite phase is irreversible and leads to file sepa- ration.Somefiledeformationcanbeseenclinicallyand itisasafetypropertyoffilesthattheyhavealongerde- formationperiodunderstressinthemartensitephase. There are two types of clinical stresses on a file in curved canals: bending and torsional stress (Figs. 4 &5).Bendingstressoccurswhenthefilepassesinside a curve and is subjected to stresses at an equal level: stretching on the outer part of the curve and com- pression of the alloy structure from the internal part ofthecurve.Thiskindofstresscannotbreakafile,but it can lower its torsional stress by at least 30% from the first few seconds that the file is active inside the curve. Fig. 1_Graph showing the stress test result of a K3 file conducted with a torque metre. It shows the austenite phase, yield point and the martensite plateau before file separation. Figs. 2 & 3_Graphs showing the difference between stress cycle #1 (Fig. 2) and 10 (Fig. 3). The yield is fairly stable and the austenite phase decreased in torque value. roots1_2011 R-phase advantages in shaping curves Author_ Dr Philippe Sleiman, Lebanon Fig. 1 Fig. 2 Fig. 3