Abstract
High levels of shrinkage stress caused by volumetric variations during the activation process are one of the main problems in the practical application of composite resins. Objective The aim of this study is to reduce the shrinkage stress and minimize the effects caused by composite resin volumetric variation due to the photopolymerization. In this way, this work proposes a systematic study to determine the optimal dimming function to be applied to light curing processes.Material and Methods The study was performed by applying mathematical techniques to the optimization of nonlinear objective functions. The effectiveness of the dimming function was evaluated by monitoring the polymerization shrinkage stress during the curing process of five brands/models of composites. This monitoring was performed on a universal testing machine using two steel bases coupled in the arms of the machine where the resin was inserted and polymerized. The quality of the composites cured by the proposed method was analyzed and compared with the conventional photoactivation method by experiments to determine their degree of conversion (DC). Absorbance measurements were performed using Fourier-transform infrared spectroscopy (FT-IR). A T-test was performed on DC results to compare the photoactivation techniques. We also used scanning electron microscopy (SEM) to analyze in-vitro the adhesion interface of the resin in human teeth.Results Our results showed that the use of the optimal dimming function, named as exponential, resulted in the significant reduction of the shrinkage stress (~36.88% ±6.56 when compared with the conventional method) without affecting the DC (t=0.86, p-value=0.44). The SEM analyses show that the proposed process can minimize or even eliminate adhesion failures between the tooth and the resin in dental restorations.Conclusion The results from this study can promote the improvement of the composite resin light curing process by the minimization of polymerization shrinkage effects, given an operational standardization of the photoactivation process.
Highlights
The composite resins emergence has provided higher aesthetic quality in dental restorations
Our results showed that the use of the optimal dimming function, named as exponential, resulted in the significant reduction of the shrinkage stress (~36.88% ±6.56 when compared with the conventional method) without affecting the degree of conversion (DC) (t=0.86, p-value=0.44)
The pulverized composite resins were maintained in a dark box until the Fourier-transform infrared spectroscopy (FT-IR) analysis
Summary
The composite resins emergence has provided higher aesthetic quality in dental restorations. Despite the chemical advances in resins for the minimization of the polymerization shrinkage, this problem persists and can allow micro-infiltrations in the adhesive interface, which harm the restoration aesthetics and, in some extreme cases, favor the growth of bacteria due to marginal leakage. Despite the chemical advances in resins for the minimization of the polymerization shrinkage, this problem persists and can allow micro-infiltrations in the adhesive interface, which harm the restoration aesthetics and, in some extreme cases, favor the growth of bacteria due to marginal leakage4 To achieve both efficiency and quality, various efforts have been made to reduce the effects of polymerization shrinkage. Studies have shown that the variation of some physical parameters can significantly change the resin shrinkage during the curing process, concluding that the radiant exitance (or emittance) of the curing device has a correlation with the volumetric contraction induction of photopolymerized composite resins. It has been observed a variation of shrinkage stress when the composite is polymerized under different conditions of exposure to the luminous source, being that some studies showed that the irradiance modulation during the photoactivation process has an influence on the shrinkage stress modulus
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