Impact of Simulated Gastric Acid on Surface Roughness and Frictional Resistance of Orthodontic Archwires: An In vitro Study

Introduction: Acid regurgitation caused by gastro-oesophageal reflux disease can lead to decreases in intraoral pH below the critical pH for orthodontic appliances. Exposure of brackets and archwires to such acidic environments causes changes in the surface properties that affect the frictional forces generated during orthodontic sliding mechanics and reduce the clinical efficacy of the wires. Aim: This in vitro study aimed to determine the effects of simulated gastric acid with pH 1.2 and pH 4 on the surface topography, surface roughness, and frictional resistance properties of different types of orthodontic archwires. Materials and Methods: A comparative in vitro study evaluated a total of 135 samples of three commercially available archwires: Stainless Steel (SS), nickel titanium, and beta-titanium (n=45/group). Each archwire was divided into three subgroups (n=15) according to different pH solutions: distilled water (control; pH 6.7), pH 4, and pH 1.2. The samples in the two acidic pH subgroups were immersed in the respective solution for two minutes, rinsed with distilled water, and stored in distilled water at 37°C. The procedure was repeated 6 times a day for nine days with 24-hours interval in between each cycle. The surface topography of the wires was examined by scanning electron microscopy, while the surface roughness was inspected by noncontact surface profilometry. Finally, the frictional resistance was measured by a universal testing machine. One-way ANOVA and a Tukey’s post-hoc test were used for analysis, with values of p≤0.05 considered significant. Results: The results showed an increase in surface roughness of the SS (p<0.001) and beta-titanium (p=0.003) wires as the acidity of the solution increased. The SS wires showed the lowest frictional resistance in pH 1.2 solution among the wires used (p=0.005). No correlation was found between surface roughness and frictional resistance in this study. Conclusion: Increased acidity condition significantly affects the surface roughness of beta-titanium and SS wires, and has no effect on the frictional behaviour of the orthodontic wires expects in the SS wires. Thus, because the surface properties of the archwires can be affected by acidity in the oral cavity, it is recommended to change the orthodontic wires regularly throughout orthodontic treatment, especially in patients with gastro-oesophageal reflux disease.