ortho - the international C.E. magazine of orthodontics

ortho1_201236 I I technique_ archwire differential scanning calorimeter (DSC) and X-ray diffraction. Although these tests do not necessarily reflect the clinical situations to which wires are usually subjected, they provide a basis for comparison of these wires with others NiTi wires. And in all of the tests,SENTALLOYhasproveditsefficiencyastheonly biologically correct archwire. Some of this examples are next. A)Three-pointbendingtest In order to demonstrate the difference between the first NITINOL wire (3M Unitek) and the super- elastic nickel-titanium alloy (SENTALLOY) in 1986, a three-point bending test was introduced by Miura.5 This test was designed to clarify the relationship between the loading and deflection by determin- ing the nature of the force being delivered during orthodontic treatment. This method is acceptable to demonstrate the springback properties. During cantilever bending, the wires of good springback property will increase the length and the angle of the specimens, so a superelastic-like property appears even if the wire does not possess this feature. Instead, a three-point bending test was designedbecausethiswouldaccuratelydifferentiate the wires that do not possess superelastic features. At the same time, the three-point bending test actually simulates the application of wire force on the teeth in the oral cavity. The deformation of NiTi alloys is induced with martensitic transformation; this can be reversed by heating the alloy to return to the austenite phase and is transformed by revers- ing back to the previous shape; this is produced by temperature. Materials Wirespecimenof0.016roundwireswasselected: stainless-steel, Co-Cr-Ni, work-hardened and NiTi SENTALLOY. In order to simulate oral cavity environ- mentthewiresandthesteelpolesweresetinacham- berat37°C.Themidpointofthewirewasdeflected2 mmatspeedof0.1mm/min,underapressurefroma metal pole 5 mm in diameter (Figs. 18, 19). Findings Both stainless-steel and Co-Cr-Ni wires showed a linear relationship when the amount of deflection was 2 mm and the load was around 1300 g (Fig. 20). As the deflection was removed, both of them showed a permanent deformation. NITINOL load de- flectioncurvewasalmostlinear;whenthedeflection of 2.0 mm was reached the load was 790 g (Fig. 21). When SENTALLOY wire load-increasing ratio was 2.0 mm, the load was 650 g. However, when the de- flectionwasdecreased1mmfrom1.6to0.6mm,the load was decreased by only a small amount, namely, values around 250–350 g (Fig. 22). By evaluating the test results, SENTALLOY wire showed superelastic property and was physiologi- cally compatible to the tooth movement because it provided continuous force for a long period of time during deactivation. B)Tensiletest According to Miura, superelasticity can be pro- duced by stress, not by temperature difference, and is called stress-induced martensitic transformation. Uniaxial tensile testing was performed all specimens were stretched using an Instrom universal testing machine. Fig. 24 Fig. 25 Fig. 26 Fig. 27 Fig. 28 Fig. 29