Internal photo-activation of a dental composite using optical fibers: a holographic, thermographic and Raman study

The present study aimed to introduce a novel two-step photo-activation protocol of dental resin-based composites (RBC) using optical fibers. The objective was to investigate the effect of the proposed modality on polymerization shrinkage, considered as the main disadvantage of RBCs, by measuring tooth model deformation using real-time digital holographic interferometry (DHI). Simultaneous real-time monitoring of RBC temperature rise was conducted, using infrared thermography (IRT). Degree of conversion was measured at various depths, immediately after curing and after 24 h of dark storage, using Raman spectroscopy. Standardized tooth models made of dental plaster with a class II mesial-occlusal-distal cavity were used as molds for RBC fillings. The proposed two-step curing was designed as follows: in the first step–optical fibers connected to a commercial dental LED curing unit were inserted into the dental filling to cure the RBC from within; in the second step–fibers were extracted, remaining voids were filled with the RBC, and final conventional curing was performed. The final deformation value, defined as the sum of the measured values after the first and second curing step, was significantly lower (on average 36%) compared to conventional curing, as measured by DHI. Meanwhile, no influence of the photo-activation protocol on DC was revealed. IRT was demonstrated as a simple and useful method for coupling with DHI, providing additional essential information on polymerization reaction dynamics.