Bond Strength of Composite to CO₂ Laser Ablated Enamel

Objectives: Enamel surfaces ablated by pulsed CO₂ laser have the advantage of an acid-resistant, thermally-modified outer layer. However, that layer may interfere with adhesion to composite restorative materials. This study investigated the influence of ablated enamel surface topography on the shear bond strength to composite. Methods: Enamel blocks(n=64), 3.5x3.5x2-mm, were prepared from non-carious extracted human teeth. A CO₂ laser operating at 9.3-µm wavelength, 300-Hz pulse repetition rate, incident fluence of 13 and 42J/cm², and spot size of 420-µm ablated the enamel with water-spray. The laser was rapidly scanned using computer-controlled galvanometers with different scanning patterns to produce surfaces of varying topography. Composite was bonded to enamel and bond strength was measured via single-plane shear test. All groups were bonded to Z-250 composite with Single Bond 2 bonding resin. Six groups were prepared with 10+ samples per group, including a non-laser-irradiated, acid-etched control group. Three groups (L-groups) were scanned at varying overlap between each laser pulse (100, 200 and 450-µm respectively) to produce increasingly rough surfaces and all were acid-etched before bonding. Two groups (R-groups) were laser-ablated patterns that formed retention holes cut of varying densities (holes every 500 and 250-µm). Those groups were not acid-etched. Groups were compared using Analysis of Variance (ANOVA) with a Tukey–Kramer post hoc multiple comparison test. Results: Groups L100(34.1±4.64MPa,n=12), L200(34.9±5.1MPa,n=12) and L450(33.4±5.5MPa,n=10) all achieved bond strengths in the same statistical group as the control(36.6±4.9MPa,n=10). The R-group samples yielded much lower bond strengths - R500(5.8±3.1MPa,n=10) and R250(15.3±3.9MPa,n=10). Conclusions: Bond strengths to composite of the laser-ablated and acid-etched samples were statistically equal to control, indicating the laser does not undermine bond strength. Non-acid etched samples yielded significantly lower bond strengths, but were much higher than non-etched samples in past studies due to the retention holes. Support: UCSF School of Dentistry International Dentist Program and NIH/NICDR Grant R01-DE019631