Bonding performance of universal adhesive systems to enamel - Effects of the acidic composition

Objectives: To evaluate the self-etch bonding potential of universal adhesive systems with varying acidic compositions by analyzing the wettability properties, topographical change, and microshear bond strength (µSBS) to enamel.

Methods: Eight universal adhesives were tested: All-Bond Universal (Bisco), Ambar Universal (FGM), Gluma Bond Universal (Kulzer), OptiBond Universal (Kerr), Peak Universal Bond (Ultradent), Prime&Bond Universal (Dentsply), Singlebond Universal (3 M ESPE), and Tetric N-Bond Universal (Ivoclar). Bovine incisors were prepared and treated with each adhesive according to the manufacturer's instructions. The following tests were carried out: contact angle to measure the adhesives' wetting ability; optical profilometry to evaluate the topographical changes obtained with adhesives' application; energy dispersive X-ray spectroscopy (EDX) to quantify the removal of inorganic elements from enamel; and µSBS to verify the bonding potential of adhesives to enamel. Statistical analyzes included ANOVA, Kruskal-Wallis, Weibull analysis, Pearson correlation, and chi square test (α=0.05).

Results: Significant differences were found in contact angles immediately after adhesive application and after 30 s, with Gluma Bond Universal and Tetric N-Bond Universal showing the lowest (the best wetting ability) and the highest values (the lowest wetting ability), respectively. EDX revealed varying degrees of Ca and P reduction, with Gluma Bond Universal resulting in the greatest Ca and P reduction from enamel (∼65 % and ∼62 % reduction, respectively. Topographical analysis indicated significant changes in roughness, mainly for Gluma Bond Universal. The µSBS mean values ranged from 14.7 MPa (Ambar Universal) to 26.8 MPa (Tetric N-Bond Universal). OptiBond Universal and Tetric N-Bond Universal exhibited the highest resin-enamel bonds. Weibull analysis was performed to better understand the bonding reliability of the tested adhesives, with OptiBond Universal demonstrating increased modulus and characteristic strength, performing as one of the most interesting compositions for enamel bonding. Adhesives based on carboxylic acids were more acidic than adhesives containing phosphate-based monomers (p = 0.033), and the removal of Ca and P elements from enamel was more intense with the use of adhesives based on carboxylic acids (p = 0.029). There was a strong relationship between the Sz values of adhesively treated enamel and the baseline contact angle (wettability) of adhesives (R² = -0.771; p = 0.025).

Significance: The study highlights significant variability among universal adhesives in terms of their interaction with enamel. Adhesives based on GPDM (OptiBond Universal) and 10-MDP (Tetric N-Bond Universal) demonstrated the best bonding performance, suggesting they may be preferable in clinical settings where strong and reliable enamel bonds are critical. The presence of 10-MDP was not significantly associated with improved dental bonds. The present findings provide valuable information for dentists in selecting adhesives that optimize clinical outcomes in enamel substrate using the self-etching adhesive approach.