Mechanical and Thermal Properties of Fluoride/Artificial Saliva-Exposed Nickel-Titanium Archwires

Decreased mechanical properties have been reported in Nickel-Titanium (NiTi) archwires after clinical use and in vitro exposure to fluoride preparations.

Objectives: The purpose of this study was to evaluate the mechanical property change of a NiTi archwire after exposure to a commercial fluoride preparation or artificial saliva, and to determine whether there is any coincident change in superelastic thermal characteristics as detected by differential scanning calorimetry (DSC).

Methods: Superelastic rectangular NiTi orthodontic archwires (Ormco) were stored at 37ºC in either a 1.1% sodium fluoride and acidulated phosphate gel (Phos-flur gel; Colgate Oral Pharmaceuticals) or phosphate buffered saline (PBS) for 1.5 hours. As-received wires served as a control.  Mechanical properties [elastic modulus (E) and yield strength (YS)] were determined via 3-point bending at 37°C (n=10) while thermal properties were analyzed with DSC at 10°C/min between -80°C and 100°C (n=5). Results were analyzed with ANOVA/Tukey (a=.05).

Results: The enclosed table displays the mean and standard deviation for each group/parameter. Several significant (p<.05) decreases in bending parameters with exposure to PBS or fluoride are noted, whereas only the cooling enthalpy of the PBS group was significantly lower than those of the control group.

Parameters		Control	PBS	Fluoride
Mechanical	Loading E (GPa)	60.9±0.9 A	57.6±1.8 B	59.0±1.1 B
	Loading YS (MPa)	956±53 A	889±22 B	934±30 A
	Unloading YS (MPa)	466±18 A	418±6.5 B	454±16 A
	Unloading E (GPa)	59.5±1.2 A	58.1±1.1 B	59.1±1.0 AB
Thermal	Austenite Finish Temp (°C)	23.0±0.8 A	23.0±0.5 A	22.9±0.7 A
	Heating Enthalpy (J/g)	9.5±3.6 A	11.6±2.0 A	11.0±1.7 A
	Cooling Enthalpy (J/g)	4.7±0.4 A	4.0±0.3 B	4.8±0.2 A

Different letters denote significant differences (p<0.05) in parameters among groups.

Conclusion: Differences in NiTi superelastic character as determined by DSC does not seem to account for the mechanical property change when NiTi is exposed to fluoride or artificial saliva.