Effect of Heat-Treatment on Properties Of Cobalt-chromium Orthodontic Wires

Objectives: Different protocols appear in the literature with respect to heat-treating cobalt-chromium orthodontic (CoCr) wires. The objective was to determine the effect of variable heat-treatment time and method on the bending properties of CoCr wires.
Methods: Two tempers of CoCr ‘Elgiloy’ wires (Rocky Mountain Orthodontics), blue (B) and yellow (Y), were heat-treated for different durations. The groups (n=20/group/temper) included: 1) as-received (control); 2) brush-flame; 3) 480^oC for 5sec; 4) 480^oC for 10min; 5) 480^oC for 2hr; and 6) 480^oC for 5hr. Wire segments were tested by a three-point bend test. Stiffness/flexural modulus, percent recovery, and force values at select deflections were statistically compared using ANOVA/Bonferroni post-hoc test (p<0.05). A T-test compared the different tempers.
Results: Longer heat-treatment (2hr/5hr) increased % recovery, flexural modulus, and force values when compared to the as-received counterparts. Heat treatment for 10 minutes resulted in intermediate increases. Using a brush-flame technique reduced elastic recovery and resulted in greater bending variability.
Conclusions: Similar mechanical properties can be achieved in just 2 hours compared to the manufacturer recommended 5 hours of heat-treatment of Elgiloy wires. Ten minutes of heat-treatment, which may be more realistic in a busy orthodontic practice, can increase bending properties 50-75% compared to the 5-hour group. The brush-flame technique is not recommended due to inconsistent heating conditions resulting in varying bending properties.