Effect of Time and Contamination on Early Resin-Composite Bond Strength

OBJECTIVES: To determine the effect of delay time between curing adhesive resin and resin composite placement, application of secondary acid-etch and adhesive layer, as well as surface contamination on shear bond strength.

METHODS: Facial enamel of 160 bovine incisors was flattened and crown sections mouthed in a resin ring. Enamel was acid etched followed by application and light-curing of a 4^th generation bonding agent (Optibond FL, Kerr). Teeth were divided into 4 groups: (1) (Control) immediate composite bonding, (2) dry storage, (3) exposed to 100% humidity, or (4) stored in a composite slurry. Within each group, half received a second etch-adhesive treatment before composite placement. Groups 2-4 incorporated time delays between initial etch-adhesive and subsequent treatments (second etch/adhesive or not) and composite bonding: 15, 30, 60min, 2.5 and 5h. Specimens were stored 24h (100% humidity, 37°C)  following composite bonding. The ring was debonded in shear in a universal testing machine. Peak loads were converted into stress (MPa). (n=5/parameter). Statistical analysis consisted of a 3-way ANOVA (excluding Immediate values). Data were then pooled within a given test parameter to include immediate values, and 1-way ANOVAs and Tukey post-hoc tests run.  Preset alpha of 0.05.

RESULTS: Excluding Immediate values, all factors significantly influenced bond strength (with no significant interaction): Delay P<0.001, Storage p=0.046, re-etch/adhesive p=0.033. Tables report mean (sd) strength (MPa).

Cells with similar letters in a Grouping column are not significantly different. With Immediate values included, strength decreased after 30m, rose at 2.5h, and declined at 5h. Exposure to slurry water significantly decreased strength, and re-etch/adhesive had no significant effect (p=0.163).

 CONCLUSIONS: When sequentially restoring adjacent enamel surfaces, it is best to immediately place composite prior to etching and bonding other teeth. Re-etching and second bonding agent application is not necessary in early bonding.