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ePaper created 2016-01-20, 13:53:08 | version 1.38.0
I 43 industry report _ NiTi files I roots1_2015 _Two types of deformation Theuseoffilesintherootcanalunavoidablyleads todeformationofthefilematerial.Ifwetakeclassical rubber, probably the best known elastic deformable material, the issue is clear: if one stretches a rubber elasticband(Fig.1)itgenerallyreturnstoitsprevious shape by itself as soon as force is no longer applied. During this process, the applied energy escapes and this can be clearly measured at a physical level. Using a thermometer, the reversal of the stretching process shows a slightly increased temperature of the mate- rial (Fig. 2a). This type of deformation is elastic and completely reversible at the same time. Examination of the molecular structure of the rubber shows there arenochangestothemolecularbonds.Acomparable example for the elastic deformation of metals is the thin metal spiral children's toy, which, after an initial shove,runsdownstairsautomaticallyandrepeatedly several times without showing any visible signs of fatigue of the material (Fig. 2b). The second type of deformation is plastic defor- mation. This type of plastic deformation is well known to anyone who has ever been involved in a caraccidentwithdamagetothecar'sbodywork.The damage is usually irreversible, even if the dents have been repaired and resprayed. The reason is the changeinthemolecularstructureofthemetal(Fig.3) where the bonds change and the molecules diffuse (Fig.4).Incaseofarepeatcollisionwithsuchaprevi- ouslydamagedcar,thedamagedcardoororpassen- ger compartment will buckle much more readily as the new molecular structure has an extremely nega- tive effect on the overall stability of the material. It goes without saying that respraying the traces of an accidentaddsonlylittlemechanicalresistancetothe car; in dentistry, the differentiation between elastic, transient deformation and irreversible, plastic de- formation is at least equally important. A plastically deformed NiTi file will break too easily due to mate- rial fatigue, and plastic deformation can usually not bedetectedunderamagnifyingglassormicroscope, letalonewiththenakedeye,duetothehighbounce- backeffectofconventionalNiTimaterial.Minuteand also invisible microfractures that occur during the metal cutting manufacturing process can increase the risk of unexpected breaking of the instrument. Put into plain words, this means that the pseudo- elasticity of conventional NiTi files often masks ex- isting plastic deformation: visibly, the damaged file does not differ from an unused file, but the conse- quences during preparation can be serious. Until now, the treating dentist had no opportunity of checking the actual condition of the used instru- mentsbyhimself,eventheuseofdisposablefilesof- fers no guarantee, although it does increase safety somewhat. _Shape memory increases safety With the development of a new generation of NiTi files, COLTENE finally solved this quality control prob- lemwithaverysimpletrick.Meanwhileithasbecome possibletodifferentiatebetweentheelasticandplas- tic deformation of nickel-titanium alloys. To achieve this, the material used must possess a true shape memory.Ultimately,shapememoryisnothingelsebut ‘training’ the material to ‘memorise’ a certain shape underdifferentconditions.Afterdeformingthemate- rialbybendingorsimilarmeans,amaterialwithshape memory automatically returns to its original shape as soon as the external conditions are changed. Tem- perature or pressure deviations are examples of such changed parameters. Alternatively, one can induce a return to the original position through magnetism or via a simple chemical process. Applied to endodontic instruments, the practical advantages of this principle are soon evident: a NiTi file with a ‘controlled memory’ effect adopts the Fig. 2a_Energy escaping can be measured physically. Fig. 2b_Metal spiral children's toy. Fig. 2a Fig. 2b