Stiffness in bending of a superelastic Ni-Ti orthodontic wire as a function of cross-sectional dimension

Superelasticity is a property used in orthodontics to initiate tooth movement in the first stage of orthodontic treatment. It is the aim of all clinicians to accomplish biological tooth movement, which implies the use of low, continuous force and requires archwire with low stiffness. In this study, 15 nickel-titanium archwires with three different cross-sectional dimensions were tested in three-point bending to determine the nature of forces in a loading and unloading cycle. The evolution of stiffness in bending as a function of wire size is discussed. The applied forces or stiffness dependence on cross-sectional size differs from the linear-elastic prediction because of the superelasticity property. We discuss the origin of the nonconventional profile of curves and the nature of reversible large deformation of these alloys. Martensitic transformation is at the origin of nonlinear elasticity. The stiffness decreases with increasing deflection, and this phenomenon is emphasized in the unloading process. The value of stiffness appears to vary with wire size but depends on the ratio of volume of martensitic transformation. During martensitic transformation, the rigidity (elastic modulus) of the alloy is nonconstant. These results and their understanding should allow a different approach of biomechanical considerations, ie, a large-size square wire does not produce necessarily high forces.