Load deflection characteristics of square and rectangular archwires

Aims: To determine and compare the relative stiffness of a large selection of commonly-used square and rectangular steel, super-tempered steel, NiTi, and TMA orthodontic archwires of various cross-sections, in order to provide the clinician with a useful, easy-to-consult guide to archwire sequence selection. Materials and methods: Twenty-four archwires of different cross-section shape, size and material were selected. Each type was subjected to a modified three-point bending test, performed in triplicate using an Instron 4467 dynamometer. Each sample was deflected by 1 mm, and the corresponding load recorded. The relative stiffness of each archwire sample was calculated, and samples were compared by material and by cross-section. Results: A considerable difference in resistance to deflection was revealed between all the tested archwires. As expected, the resistance to deflection of archwires of the same cross-section was found to increase with increasing stiffness of their construction material. Specifically, steel archwires can be as much as 8 times stiffer than NiTi archwires of the same shape and cross-section, and super-tempered steel archwires are invariably stiffer than traditional steel versions. Marked differences in resistance to deflection were also found between NiTi archwires made of the same material but with different shape characteristics. Conclusions: In archwires of the same cross-section, steel is always stiffer than TMA and NiTi, and super-tempered steel is always stiffer than conventional steels. In archwires of the same material, the stiffness increases with the cross-section, in particular with its height.