In-vitro evaluation of the material characteristics of stainless steel and beta-titanium orthodontic wires

Introduction: The exact composition and material properties of the metal alloys used in orthodontics are usually not identified by or even available from manufacturers. This makes meaningful comparisons between wires impossible and is unacceptable with regard to biocompatibility issues. The aim of this study was to investigate the material characteristics of contemporary stainless steel (SS) and beta-titanium (beta-Ti) wires, also known as titanium-molybdenum alloy (TMA), for comparison. Methods: Twenty-two different SS and beta-Ti wires, preferably straight wires sized 0.43 X 0.64 mm, (0.017 x 0.025 in) were tested blindly for wire dimensions, chemical compositions, bending and tensile properties, and surface characteristics. Results: Four chemical compositions were found for the beta-Ti wires: titanium-11.5, molybdenum-6, zirconium-4.5 tin; titanium-3, aluminum-8, vanadium-6, chromium-4, molybdenum-4, zirconium; titanium-6, aluminum-4, vanadium, and titanium-45 niobium. The SS wires were of AISI type 304 or the nickel-free variant BioDur 108. All beta-Ti wires showed high surface roughness values. TMA 02 significantly had the highest E-modulus, TMA 02 and TMA 11 had the highest 0.2% yield strength, TMA 02 had the highest hardness, and TMA 12 was the most ductile wire of the beta-Ti wires. All SS wires showed high 0.2% yield strength, SS 10 significantly had the lowest E-modulus and was the most ductile wire, and SS 08 significantly showed the lowest hardness values of all SS wires. Conclusions: Significant differences were found between SS and beta-Ti wires, but there was little or no difference between the mechanical and physical characteristics tested in each subgroup. However, the morphological analysis clearly demonstrated that the finishing phase (annealing, polishing) of the wires' production process lacks the quality one would expect with regard to good mechanical properties and biocompatibility issues. Accurate specifications are urgently needed concerning the quality of orthodontic wires on the market.