ortho - the international C.E. magazine of orthodontics

ortho1_2012 I 31 technique_ archwire I Fig. 4_Crystals with good neighbors. A, B and C) NITINOL wire is treated at high temperature to set the parent shape. D) When NITINOL is cooled, the phase changes from austenite to martensite because martensite crystals are slightly flexible, they can deform to accommodate bending of the wire. E) They remain attached to neighboring crystals. F) Martensite crystals revert to their undeformed shape, and wire magically unbends. Fig. 5_Dr. George F. Andreasen joined the Iowa dentistry faculty in 1963 and was professor and chairman of the department of orthodontics at the University of Iowa (1965–1975). Fig. 6_Dr. Fujio Miura, professor and chairman of the First Department of Orthodontics of the Tokyo Medical and Dental University from 1962–1991. Fig. 7_Dr. Fujio Miura, left, and Dr. Masakuni Mogi, head of the Group of Dental Materials, first department of orthodontics (TMDU). Fig. 8_SENTALLOY, super elastic nickel-titanium alloy. describes this process as follows: In a non-memory metal, the strain of deformation is absorbed by re- arrangement of the crystals, and it is impossible to getthecrystalsbackintotheoriginalposition.Onthe other hand, in an alloy such as NITINOL the crystals stay in place: The atoms within the metal crystals rearrange themselves and the distorted objects re- vert to its original shape. There is no visible change in shape of the metal; all the changes occur at the atomic level.2 NITINOL had phase changes while still a solid; these phase changes are named martensite (low temperature) and austenite (higher temperature). The range of transition temperature (TTR) varies for differentcompositionsfromabout-50°Cto166°Cby varyingthenickeltitaniumratioorternaryalloywith smallamountsofothermetallicelements.Underthe transition temperature, NITINOL is in the martensite phase.Inthemartensitephase,thisalloycanbebent into various shapes; the crystal structure is disor- dered body-centered cubic. To fix the “parent shape” (austenite phase), the metal must be held in position and heated to about 500°C. The high temperature “causes the atoms to ar- rangethemselvesintothemostcompactandregular pattern possible” resulting in a rigid cubic arrange- mentknownastheaustenitephase;thecrystalstruc- ture becomes that of an “ordered” cubic, frequently called a cesium chloride (CsCl) structure. Above the transition temperature, NITINOL reverts from the martensite to the austenite phase, which changes it back into its parent shape. NITINOL is a conglomeration of tiny regions of singlecrystals,calledgrains,allofrandomsize,shape and orientation (Fig. 2). In the austenite phase the atoms of the grains adopt an atomic structure in which each nickel atom is surrounded by eight tita- niumatomsatthecornersofthecubeandeachtita- nium atom is likewise surrounded by a cube of nickel atoms(Fig.3).Inthemartensitephase,whenthewire coolsbelowitsTTR,thegrainschanges,whichmeans thatthenickelandtitaniumatomsassumeadifferent and more complex three-dimensional arrangement (Fig. 4). NITINOLinorthodontics Another early application and probably the most important for the orthodontic world was the intro- ductionofNITINOLintoorthodonticsasanarchwire. In 1968, Dr. George F. Andreasen (Fig. 5) read Fig. 4 Fig. 6Fig. 5 Fig. 7 Fig. 8 Fig. 9 Fig. 13Fig. 12Fig. 11Fig. 10