A Comparative Study of the Fatigue Resistance of Four Orthodontic Alloys

Abstract

This investigation was designed to compare the fatigue resistance of four orthodontic alloys and to determine whether their surface characteristics had any influence on their fatigue lives. Stainless steel, Blue Elgiloy, TMA, and Nitinol were evaluated using a round (.018") and a rectangular (.016" x .022") size of each type of alloy. Fifty specimens of each size were tested, making the total sample size 400. The wire samples were evaluated using a cantilever bend test in which each specimen was stressed cyclically at the defection of .625 inch in either direction. All of the wires were evaluated in their untreated condition, with the exception of Blue Elgiloy, which was heat-treated prior to testing at 900° Fahrenheit for seven minutes in a burnout furnace. One specimen from each group was viewed under the scanning electron microscope to evaluate its surface structure and each alloy presented a unique microstructure. Stainless steel was relatively smooth. Blue Elgiloy was slightly rougher in appearance, Nitinol appeared dimply and crater-like, and TMA looked eroded or striated. It was suspected that the alloys which had a rough surface would be less resistant to fatigue. On the other hand, due to the differences in modulus of elasticity of each alloy, the amount of stress applied to the different materials varied accordingly and the effect of surface roughness on fatigue resistance was not great enough to overcome the effect of stress. Since TMA and Nitinol had relatively low moduli of elasticity, they were subjected to less stress and were thus more fatigue-resistant than either stainless steel or Blue Elgiloy. Nitinol sustained the most cycles to fracture in both the round and rectangular samples. In the round sample, TMA had the next best fatigue life, and stainless steel and Blue Elgiloy had comparable fatigue lives. With the rectangular samples, there were no significant differences between TMA, stainless steel, and Blue Elgiloy. Therefore, although Nitinol and TMA cost more than stainless steel or Elgiloy, their high flexibility, large working ranges, and favorable fatigue properties make them attractive substitutes for the conventional stainless steel or Elgiloy wires. TMA and Nitinol provide a new dimension in controlling the magnitude of forces used for tooth movement, and Burstone40 has stated that "the variable-modulus concept gives the orthodontist one more tool in the efficient design and use of his appliance."