The effects of lubrication on the static frictional resistance of orthodontic brackets

Abstract

Abstract Background: Difficulties are experienced with the collection and storage of freshly harvested human saliva to use as a lubricant for the laboratory testing of the frictional resistance of orthodontic brackets. In order to overcome these difficulties, researchers have suggested the use of saliva substitutes due to their ease of storage and consistency of properties throughout testing. Others have criticized the use of artificial saliva and prefer the dry state. The present study aimed to compare the effects of human saliva and an artificial saliva (Saliva Orthana®) with the dry state for the static frictional resistance testing of orthodontic brackets. Methods: The static frictional resistance and the lubrication effect of human saliva, Saliva Orthana® and the dry state were investigated using upper central incisor stainless steel brackets and 0.019 x 0.025 inch stainless steel wires in an Instron® Universal Testing Machine. Static frictional resistance was measured 100 times for each lubrication state. The ‘wettability’ of each lubricant was determined by measuring the contact angle against a stainless steel surface using the CAM 200 Optical Contact Angle Meter. Distilled water acted as a control. The viscosity of each lubricant and their Newtonian or non-Newtonian fluid behaviour under stress was measured using a Brookfield Digital Rheometer Model DV-III+. Results: The differences in static frictional resistance between the three lubricants when examined as a group did not reach statistical significance (p = 0.059). The difference between human saliva and Saliva Orthana® was considered to be of weak statistical significance and clinical relevance (Means: 0.917 N; 0.819 N: p = 0.053). Human saliva and the dry state revealed very similar mean frictional values (Means: 0.917 N; 0.875 N: p = 0.932). The contact angle tests indicated a statistically significant difference between the lubricants with Saliva Orthana® having the smallest angle and therefore the highest ‘wettability’. Human saliva had the highest initial viscosity and behaved as a non-Newtonian fluid, contrasting with Saliva Orthana® and distilled water, both of which behaved as Newtonian fluids. Conclusion: The current results indicate that artificial saliva is not an ideal alternative to human saliva for friction testing in the laboratory. The results therefore support the proposal that, when human saliva is not available, it may be preferable to test orthodontic frictional resistance in the dry state.