ortho - the international C.E. magazine of orthodontics

ortho1_201230 I I technique_ archwire Author_Alberto Teramoto, DDS SENTALLOY: The story of superelasticity _Part I: History and basic concepts Introduction Since the days of Angle, many technological advances in archwires have enhanced our specialty, increased our efficiency, reduced our chair time, and as a result, increased our profitability. However, be- cause of the great number of nickel-titanium alloys that actually exist, it is important to understand the historicalbackgroundaswellasbasicconceptsabout them in order to visualize and recognize the clinical potentialtheyhaveinorthodontics.Althoughnickel- titaniumalloysappeartobethesame,therearemany smalldifferencesintheircompositionandmanufac- turing process, which inevitably make the difference between ordinary and extraordinary NiTi archwires. ThebeginningofNITINOL Nickel-titanium alloys have been found to be the mostusefulofallshape-memoryalloys(SMAs):They are metals that demonstrate the ability to return to some previous shape or size when subjected to an appropriate thermal procedure. In other words they “remember”theiroriginalshapes.Othershapemem- oryalloysincludecopper-zinc-aluminum-nickeland copper-aluminum-nickel, but they do not possess the combined physical and mechanical properties of nickel-titanium alloys. NiTiisuniquebecauseoftheforcelevelsexpressed whenheated,itscorrosionresistance,itsbiocompat- ibility, the ease with which the TTR can be set and the reasonable cost of fabricating a precise alloy. A metallurgist,Dr.WilliamJ.Buehler,doingresearchat Naval Ordnance Laboratory (NOL) in White Oak, Md., discovered the unique shape memory properties of this alloy. NITINOL is an acronym used to describe a genericfamilyofnickel-titaniumalloys.Itrepresents the two main elements of this alloy — nickel and titanium(NiTi)—andcontainsareferencetowhereit was devoloped in NOL, Naval Ordnance Laboratory.1 In 1958, Buehler was looking for a change in his professional career. An aerodynamics project at NOL wassearchingfortheappropriatematerialforthere- entry nose cone of the SUBROC missile. Jerry Persh, theprojectmanager,putBuehlertoworkassembling known property data on selected elemental metals and alloys that might be feasible. Early in the developmental stages, secondary researchonnickel-titaniumalloysledtoasignificant application by Raychem Corporation. The produced a product called Cryofit, which was a hydraulic line coupler for the U.S. Navy’s F-14 aircraft. However, this was just the beginning of a wide range of new and exciting applications in medicine, dentistry and diverse engineering areas. Buehler retired from NOL in1974butremainedinvolvedinthedevelopmentof NITINOL until 2005, at which time he moved to New Bern, N. C. HowNITINOLworks Exactlywhatmadethesemetals“remember”their original shapes was in question after the discovery of the shape-memory effect. George Kauffman (Department of Chemistry of University of Fresno) Fig. 1_Metallurgist William J. Buehler Fig. 2_A multicrystaline metal sample of NITINOL. Each pattern represents a different grain of random size, shape and orientation of the atom lattice. The blow-up on the right shows the structure of the austenite phase of the NITINOL atomic lattice called ‘body-centered cubic. Fig. 3_The cubes are intertwined such that the each corner is in the center of another cube. The distance of the center of a cube from a corner is shorter than the distance to a neighboring corner. Thus the ‘nearest neighbors’ of each nickel atoms (white balls) are titanium atoms (blue balls), not other nickel atoms and vice versa. Fig. 2 Fig. 3Fig. 1