The effect of mode of polymerization and presence of fluoride on the shear bond strength of orthodontic resins to bovine enamel

Abstract

Decalcification around orthodontic brackets is sometimes observed during and after treatment. Fluoride-releasing orthodontic resins have been shown to arrest early enamel caries formation. However, some previous studies demonstrate the therapeutic advantage of fluoride release was undermined by the increased incidence of bond failures. This research investigated the shear bond strength values of stainless steel brackets bonded with no-mix, chemically cured and light-activated resins and their fluoride-releasing equivalents to etched bovine enamel. The shear testing for the first group was completed after 24 hours of storage in artificial saliva. Fluoride-releasing resins are known to release a significant amount of fluoride within the first few days after bonding. The adhesives in this project were evaluated for amount and duration of fluoride release, utilizing a clinically-relevant model. The fluoride concentration of the resins was determined by HMOS (hexamethyldisiloxane) diffusion and the results were compared to those reported by the manufacturer. The shear bond strength values for the no-mix, chemically cured and light-activated resins for the secondgroup was determined after allowing significant fluoride release in artificial saliva to occur. The results showed that storage time did not affect the shear bond strength of the light-activated or chemically cured resins, yet the no-mix resins demonstrated a near doubling of shear bond strength with time. If fluoride is not present in the resin, the chemically cured resin is significantly stronger than the light-activated or no-mix resins. If fluoride is present in the resin, the chemically cured resin shear bond strength is equivalent to the light-activated and both are greater than the no-mix resin. The presence of fluoride significantly increased the bond strength of the light-activated resin in the short-term, and after storage both the chemically cured and light-activated fluoridecontaining resins were significantly stronger than the nonfluoride- containing counterparts. Fluoride-releasing chemically cured and light-activated resins released significantly more fluoride than the no-mix resin, with the chemically cured resin releasing the most. Both the light-activated and chemically cured resins demonstrated an early burst of fluoride release that levelled off within 20 and 30 days, respectively. The minimal fluoride release seen with the no-mix resin may be due to the dilution of the components during mixing. For all the resins tested.,, the vast majority of the fluoride available for release remained in the resins.