Light curing units: effect on composite resins polymerization and thermal behaviour of dental tissue: an in vitro study

Proper rehabilitation is linked to good characteristics of the material used and the observance of the protocol, infact a good polymerization is the basis of an equally good rehabilitation with dental composites. The portion of the composite sufficiently cured is not negligible (1) (2), a non uniform irradiance distribution from a light-curing unit (LCU) can cause locally inadequately cured and/or over-cured areas inhomogeneus cured composite can determine monomer elution or internal shrinkage stresses, with consequent reduction of the lifetime of dental resin based composite (RBC) restorations. (3). An incorrect polymerization changes the characteristics of the restorative material that can not meet the desired performance. Also the light curing at distance can reduce mechanical properties and could affect long-term durability of the composite restorations. (4). A good solution could be the low-viscosity bulk-fill materials that shows lower shrinkage force than a conventional flowable resin composite at high levels of degree of conversion up to 4-mm incremental thickness (5). The distance of the lamp from the deepest layer of the reconstruction, especially in the class II preparations can be 6,3mm from the upper surface (6); the resin applied in this area is exposed to the biofilm that increases its solubility (7). Another very important aspect is the wavelength of absorption of the activator: the most common is the camphorquinone (CQ) with a wavelength of 468nm. However, many new composites using different initiators, for example the phenylpropanedione (PPD) (398nm), monoacylphosphine oxide (Lucirin TPO) (381nm), bisacylphosphine oxide (Irgacure 819) (370nm). For these materials, polymerization with LED curing units of earlier generation is questionable, due to narrower spectra of the emitted wavelengths (8). The clinician has a lot of information available to reach a good degree of polymerisation, but, are not always known interactions of the lamps by polymerization with the composite used. Analyzing the trend of the change in temperature during the polymerization, it was noted that the transparency of the dental resin composite changes in a way proportional to the forward of the level of polymerization (9). This data is very important because if it were possible to measure the level of transparency it might knows the degree of polymerization. The international standards (ISO 4049, ISO 10650) are supported routine methods of measuring the degree of polymerization. For evaluating composite polymerization there are various techniques which are divided into two groups: direct and indirect. Direct tests are applied to determine the degree of conversion, an example it’s a Fourier transform infrared (FTIR) spectroscopy. This method requires the use of equipment not always readily available, making the researcher's work more difficult, are expensive, complex and time consuming. The indirect tests are inexpensive and easier to conduct: surface hardness is defined as resistance to surface indentation and it is an indirect method for measuring the degree of polymerization. The purpose of this study will be to understand the thermal and kinetics behavior of composite resins during polymerization.