Effect of Diode Laser Irradiation of Bonding Agents Before Curing Versus Standard Bonding Protocol on the Shear Bond Strength Between Resin Cement and Dentin /

Abstract

It has been more than half a decade since Buonocore and later Nakabayashi developed the concepts behind the most popular branch in restorative dentistry, dental adhesion. Owing to its ability to create intimate contact between the tooth and the restoration, adhesion theoretically eliminates marginal gaps, reduces micro-leakage, and prevents secondary caries. With the spread of the adhesive concept, came the development of resin cements, enabling bonding of indirect tooth-colored restorations through micro-mechanical interlocking instead of the conventional luting method employed by acid-based cements. Adhesive bonding, through appropriate surface treatment protocols, not only provides superior bond quality, but also increases the flexural properties of ceramic restorations. Flexural properties such as modulus of elasticity and strength prevent de-bonding of cemented restorations during function. However, due to the nature of adhesive bonding, it has been proven that the quality of adhesion is proportional to the performance of the bonding agent. Unfortunately, the performance of contemporary bonding agents is still flawed when it comes to dentin. Unlike enamel, which has about 96%wt inorganic material, dentin has a more complex heterogeneous structure, composed of about 70%wt inorganic material and up to 30%wt organic content and water. The dentinal structure is further complicated by its structural variability according to location, as it changes not only with differences in depth, but also from region to region within the same tooth. Moreover, the dentinal composition and structure constantly changes throughout life according to local factors such as age, trauma, and caries. This complex structure renders it challenging to achieve a reliable bond to dentin, and due to its hydrophilic nature, nano-leakage occurs at some point within the hybrid layer, resulting in eventual bond deterioration and failure. According to the recent literature, self-etch adhesive systems (6th, 7th, and 8th generation adhesives), also called simplified bonding systems, seems to outperform the etch and rinse adhesive systems in dentinal bonding. However, numerous studies still reported water-related bond deterioration after aging. The presence of HEMA, water, and ethanol, as well as the differences in molecular weights between acidic and cross-linking monomers have been blamed for the breakdown of adhesive interfaces created by simplified bonding systems. In light of these problems, improvements in bond formulations are constantly being attempted through diverse laboratory studies and clinical trials. But till now, manufacturers have yet to produce an adhesive system or technique that prevents dentin bond deterioration by time. Dental lasers have recently been gaining popularity, and their use have been incorporated into multiple restorative procedures, including dental adhesion. SE Gonçalves et al (1999) suggested an adhesive protocol in which a soft tissue laser irradiated on an uncured adhesive before polymerization resulted in a statistically significant increase in shear bond strength to dentin. The use of laser energy on the bonding agent before its curing has since been evaluated in further studies and has been further hypothesized to increase bond penetration, help evaporate the solvent, increase its degree of conversion (DC), and increase the modulus of elasticity of the hybrid layer. Some authors, however, obtained negative results, finding the laser to have no effect whatsoever on the bond strength. Others found an increase in immediate bond strength that dropped dramatically after aging. Nevertheless, achieving sustainable high dentin bond strengths is the ultimate goal of adhesive dentistry. The idea of incorporating laser energy to reach this goal is interesting as it is debatable. Therefore, in an attempt to further asses the validity of laser use in dentinal bonding, the aim of this study is to evaluate the effect of a soft tissue laser irradiated on uncured bonding agents on the dentin bond strength.