Aim: To investigate the fracture resistance of as-received and retrieved NiTi archwires.
Materials and methods: NiTi archwires (German Orthodontics, CA, USA) of various cross-sections were retrieved from orthodontic patients and brand-, type- and size-matched wires were included as controls. Specimens prepared from the selected wires were subjected to bending deformation at a loading frequency of 2 Hz and cyclic loading was applied either until fatigue failure or with a maximum number of loading cycles of 2 x 10(6). For each wire cross-section a minimum of five specimens were loaded; the cycle number of the fatigue fracture was recorded and the mean of the cycles was calculated from the five specimens to provide the corresponding point in the Wöhler diagram. Scanning electron microscopy was used to assess the fracture pattern of broken wires. The results of the fracture cycles of as-received and retrieved wires were analyzed with a general linear regression model with state (as-received vs. retrieved) and wire size serving as discriminating variables and Tukey's test at the 0.05 level of significance.
Results: Retrieved wires fractured at a significantly lower number of cycles compared to their as-received matches. The size of the wire played a role in determining fracture with larger cross-sections showing reduced fatigue failure properties and 0.30 and 0.035-mm specimens showing no fracture at the selected strain and number of cycles. The SEM investigations revealed evidence of smoother fractured surfaces in large-sized, rectangular cross-sectioned retrieved wires. Kaplan-Meier survival analysis with log rank test demonstrated the effect of aging and size in predicting survival of NiTi wires.
Significance: The extended life expectancy of NiTi archwires proposed in current treatment trends is associated with higher probability of fatigue fracture of wires. Large diameter and square or rectangular cross-sections possess an increased chance of failure relative to smaller diameter wires. A necessity may emerge to monitor patients to identify wire failures despite the commonly employed practice of increased time intervals between appointments in patients treated with NiTi wires.