Tensile Bond Strength of Light-Activated Composite for Bonding Metal and Ceramic Brackets
Visible light-activated composite adhesives offer several advantages over conventional autopolymerizing adhesives such as extended working time, immediate ligation and easier cleanup. This study compared in vitro tensile bond strength and site of failure of a new light-activated adhesive and a commonly used two-paste adhesive for bonding ceramic and metal brackets. Manufacturer recommended light-activation times were evaluated using hardness as an indicator of cure. Light-activated composite specimens 0.3 mm in thickness were cured beneath metal brackets for 30 seconds and ceramic brackets for 10 seconds. Knoop hardness values were determined at various time intervals over a seven-day period. An evaluation of the hardness testing data indicated that manufacturer recommended cure times for both brackets were adequate. A significant increase in hardness over time also was noted for all specimens. Mean tensile bond strength comparisons were determined by dividing 88 human maxillary premolars into four groups of 22 specimens each. Each group had either metal or ceramic brackets bonded with either two-paste or light-activated adhesive. After bonding, specimens were thermocycled and stored in a humidor set at 37°C for seven days. Ceramic bracket specimens underwent further preparation to decrease bracket failures during debonding. At the end of seven days specimens were subjected to tensile stress using an Instron mechanical testing machine until failure occurred. No significant differences in tensile bond strength were found between light-activated and two-paste adhesives when similar brackets were used. Ceramic brackets bonded with either adhesive had significantly higher bond strengths than metal brackets bonded with the same adhesive. Nineteen ceramic brackets failed during debonding; however, tensile bond strengths of these specimens were not significantly different from those specimens where adhesive failure occurred. Specimens were viewed by light microscopy to determine percent bond failure at enamel surface. Groups using ceramic brackets tended to have a higher percent bond failure at enamel surface than did metal bracket groups. High bond strengths demonstrated by ceramic brackets coupled with the brittleness of the bracket itself requires special attention during debonding to avoid enamel damage. Early light-cured adhesives were not practical due to slow setting times plus their inability to cure beneath metal brackets. The adhesive tested in this study appears to have overcome these problems when manufacturer's recommended cure times are used.
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