Reconsideration of enamel etching protocols for universal adhesives: Effect of etching method and etching time

Purpose: To evaluate whether different etching methods with reduced etching times would improve the enamel bonding efficacy of universal adhesives. Materials and Methods: Three enamel etching methods were evaluated - 1. phosphoric acid ester monomer etching (PPM); 2. phosphoric acid etching (PPA); and 3. polyalkenoic acid etching (PLA) - as were three universal adhesives: 1. BeautiBond Universal (BU); 2. Prime&Bond elect (PE); and 3. Scotchbond Universal Adhesive (SU). The shear bond strengths of the universal adhesives to ground enamel and ground enamel etched for 1, 5, 10, and 15 s with different etching methods were determined after 24 h and 10,000 thermal cycles. Surface roughness average (R_a) and surface area ratio (surface area:planar area) were measured using a confocal laser scanning microscope. Field-emission scanning electron microscope (FE-SEM) observations of enamel with different etching protocols were also conducted. Results: The bond strengths of universal adhesives to enamel subjected to PPA etching with maximum times of 1-15 s and PLA etching for 15 s were significantly higher than those to ground enamel. The bond strength to enamel subjected to PPM etching did not increase and was similar to that for ground enamel. PPA and PLA etching were effective at increasing the R_a and surface area ratio of enamel. Although both values were significantly higher than those of ground enamel, those subjected to PPA etching were significantly higher than those which underwent PLA etching. In addition, the R_a and surface area of enamel subjected to PPM etching were similar to those of ground enamel, regardless of the etching time. Conclusions: Phosphoric acid etching for < 1-15 s and polyalkenoic-acid etching for 15 s both improve the bonding of universal adhesives, the R_a, and the surface area ratio of enamel. However, phosphoric acid ester monomer etching was not effective, regardless of the etching time.