Light curing of resin-based composites and adhesive systems

Light cured resin-based materials are predominantly used in current dental practice. Light curing protocols have changed over time following changes particularly in light-curing units (LCUs) since the photoinitiator system in these materials has remained virtually unchanged. Though there are attempts to modify the photoinitiator system, the most frequently used one is based on camphorquinone and a tertiary amine.

On the other hand, the LCU technology has been developing in several directions. LCUs comprise four different types of light sources: halogen, light-emitting diode (LED), plasma arc and laser. Halogen and LED LCUs are most often used in dental practice and studied in the dental literature. Light intensity and curing time have been identified as important parameters in monomer conversion which affect mechanical characteristics of the resultant polymer and subsequently its clinical performance. As light intensity has increased from about 500 mW/cm2 which is characteristic of the so-called ‘conventional’ LCUs to more than 700 mW/cm2 in the so-called ‘high-power’ LCUs, most manufacturers recommend shorter curing time. Consensus opinion in the current dental literature is that light energy density (light intensity multiplied by curing time) is a more important determinant of the degree of conversion of resin-based composites (RBCs) and adhesives than light intensity. It is currently recommended to cure adhesive systems for 20 s with LCUs operating at intensities of about 500 mW/cm2 and 10 s with LCUs operating at intensities of more than 700 mW/cm2. For RBCs, the recommended curing time is 40 s with the former LCUs and 20 s with the latter ones. The recommended thickness for each layer of RBCs in the incremental technique is still 2 mm.
Though many LCUs possess additional curing modes, such as soft-start or pulse in order to reduce polymerisation shrinkage of RBCs, there is no scientific evidence that these modes affect the long-term clinical performance of resin-based restorations.

It has been shown that maximum absorption range of camphorquinone is about 468 nm and therefore most LCUs, especially LED and plasma arc, have a very narrow emission range. However, the absorption range of co-initiators may be outside the emission range of such LCUs, thus, leading to insufficient conversion. Most recently, the so-called ‘poly-wave’ LCUs have been introduced on the market in an attempt to cover the absorption range of the entire photoinitiator system and produce maximum conversion for a given material. Future studies will show whether this new approach ensures such monomer to polymer conversion which would lead to better mechanical properties of RBCs and adhesives.

Studies have shown that increased curing distances lead to lower degree of conversion and it has recently been suggested that 6 mm may be a cut-off distance. However, it should be noted that various LCUs and materials may exhibit differences in curing efficiency at various distances. Therefore, as a general rule, the LCU tip should be placed as close as possible to the surface of RBCs and adhesives.

The superficial layer of RBCs and adhesives is insufficiently cured due to oxygen inhibition. It is removed by polishing RBCs but in adhesives, this layer serves as an intermediate zone enabling the formation of the RBC-adhesive bond. It is, therefore, important to use RBCs and adhesives with compatible chemical composition in order to achieve optimal RBC-adhesive bond by interaction of compatible monomers from both materials.