Ageing effects of pH and thermocycling on the surface morphology of thermodynamic nickel titanium wires

AIM: Surface topography can affect the behaviour between an orthodontic wire and the slot of the bracket. Using atomic force microscopy (AFM), this study evaluated three-dimensionally the surface morphological alterations of two nickel titanium (NiTi) thermodynamic wires on ageing due to immersion in artificial saliva and thermocycling. MATERIALS AND METHOD: Fifty-two samples of each wire type [either NeoSentalloy NSA (3M Unitek, Monrovia, California, USA) or Thermalloy TA (Ormco Corporation, Glendora, California, USA) were divided into two equal groups, each dipped into artificial saliva containing 1500 ppm fluoride ions with a pH of either 5.5 or 3.5 for 30 days. These two pH groups were then further divided into two subgroups of 13 wires each, one of which was subjected to thermocycling immediately after the end of the immersion period. Data on the roughness, skewness, and superficial area of the wires were recorded by AFM before and after immersion (and thermocycling, when present). Statistical analysis included a Friedman test followed by Dunn’s post hoc test. RESULTS: The two NiTi wires showed a different start surface topography. A significant increase in surface roughness was recorded at the lower pH. TA wires showed greater superficial changes with respect to NSA wires at the lower pH. Thermocycling had no significant effect on superficial morphological variations of either NiTi wires. CONCLUSIONS: In spite of the similar chemical structures, the two NiTi wires showed differential corrosion resistance to low pH leading to variable surface roughness. Interestingly, surface roughness may increase the frictional forces between the slot and the wire itself implying slower arch-guided tooth movement. Further studies are required to determine any clinical implications.