Reduction of Mechanical Properties of NiTi Wires Due to Clinical Use

Objective:  This project assessed the change in mechanical properties of shape memory (SM) and heat activated, superelastic, shape memory (HESM) orthodontic wires during clinical use.  The current NiTi-based orthodontic wires provide light, continuous forces, which promote tooth movement with few complications. Manufacturers claim these wires show no mechanical property degradation during clinical use, regardless of length of treatment or magnitude of stress developed in the wire. Only clinical data can accurately validate this claim.  Methods: Ten Nitinol Classic^TM (SM) and ten NiTi^TM (HESM), 0.016” diameter wires (Unitek, Inc) were investigated.  Each wire was cyclically tested in 3-point bend mode from 0 mm to 2 mm deflection to 0 mm and the load/deflection data was collected throughout the cycle.  These wires were disinfected and installed in patients.  After 4 weeks, the wires were removed, disinfected, and retested.  Then, these wires were disinfected and reinstalled in the same patients for another 4 weeks.  After the next 4 weeks, the wires were retrieved, disinfected, and reexamined.  The data were converted to stress and strain and the hysteresis energy was calculated by numerical integration of the area inscribed by the loading and unloading portions of the cycle. Paired ANOVA and Tukey-Kramer techniques determined statistical differences in maximum stress at 2 mm deflection between wire types and hysteresis energy within wire types. Results: Nitinol developed 41% greater stress (p < 0.001) than NiTi at 2 mm deflection. The hysteresis energy of Nitinol lessened by 5.4% (p<0.001) and 10.3% (p<0.0001) after 4 and 8 weeks, respectively. The hysteresis energy of NiTi diminished by 1.1% (ns) and 6.6% (p<0.0001) after 4 and 8 weeks, respectively. Conclusions: Nitinol Classic^TM is not superelastic. Both Nitinol Classic^TM and NiTi^TM show reduced mechanical properties through 8 weeks of clinical use.