Influence of photoirradiation conditions

The latest adhesive systems require fewer and simpler application steps and have shorter application times in clinical settings1), leading to time-saving options such as single-step self-etch adhesives2). Recent trend in adhesive systems is the use of the so-called universal adhesive which can be used with total-etch, self-etch, or selective-etch techniques3), and can also be used to bond to a variety of substrates4). These simplified adhesives consist of acidic functional, hydrophilic, hydrophobic monomers in addition to solvent, water, and fillers5). Water, in particular, plays essential role in ensuring the ionization of acidic functional monomers, while added organic solvents facilitate mixing between hydrophilic and hydrophobic components6). However, the incorporation of water and solvents into the adhesive may reduce its mechanical properties, decreasing bond durability7). Therefore, their removal from universal adhesives before photoirradiation is important for optimal bonds. Current simplified adhesives present high hydrophilicity even after polymerization, which may increase their solubility and water uptake compared with their conventional multistep equivalents8). Longterm water leaching into adhesives may further compromise their mechanical properties, affecting dentin bond durability9). These phenomena may depend on the polymerization rate of the adhesive, which relies on the total energy per unit area at the adhesive surface. This energy density, which is calculated by multiplying the light intensity by the total photoirradiation time during curing10), varies according to clinical situations such as cavity preparation, tooth position, and existing adjacent teeth11). It decreases when the distance between the light tip and cavity wall increases12). According to a previous study, such a reduction in energy density causes lower dentin bond strength in single-step self-etch adhesives, and adequate bond strength requires an energy density surpassing 4,000 mJ/cm2,13). In addition, the observed dentin bond strength of single-step self-etch adhesives was strongly related with the interfacial characteristics of cured adhesive, and these parameters were affected by the light intensity of the curing unit13). Polymerization kinetics also plays an important role in the mechanical properties of adhesives14). Functional monomers generate a highly crosslinked network upon polymerization15). A higher polymerization rate typically strengthens the mechanical properties of the adhesive layer, enhancing bond durability16). Polymerization depends on light intensity and photoinduced temperature increase17,18), which diminish when the distance between the light tip and the materials increases19,20). Therefore, longer photoirradiation times are recommended for tooth positions where photoirradiation is hard to achieve, such as the distal aspect of the molars and lingual aspect of the mandibular incisors21). Longer photoirradiation times can compensate for reduced light intensity in terms of energy density22). However, this assumes that a certain total energy density level produces enough free radicals in the adhesive. For instance, polymerization of adhesive might depend on reaching a certain energy density level of free radicals, or a certain temperature, and changes in light intensity and photoirradiation time could affect either of these factors. Further, the effects on polymerization of an adhesive could depend on the composition of the adhesives, particularly the nature of the photo initiators or functional monomers, and on the Influence of photoirradiation conditions on dentin bond durability and interfacial characteristics of universal adhesives