Minimization of polymerization shrinkage effects on composite resins by the control of irradiance during the photoactivation process

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.

As vértebras cervicais como estimadoras do crescimento e desenvolvimento em pacientes portadores de fissura lábio-palatal / Cervical vertebrae as growth and development estimator in cleft lip/palate patients

A observação da análise da maturação das vértebras cervicais nas radiografias cefalométricas laterais, que são de rotina no diagnóstico ortodôntico, tem se mostrado um método alternativo na definição da fase de desenvolvimento ósseo e, consequentemente, tem contribuído na determinação da melhor época para se iniciar um tratamento que envolva crescimento de desnvolvimento dos pacientes. Vários métodos para esta avaliação foram propostos, dentre eles, Lamparsky, que relaciona as mudanças observadas no tamanho e forma das vértebras cervicais, modificado, posteriormente, por Hassel e Farman. Assim, a proposta deste trabalho enquadra-se na necessidade de se estimar a época em que ocorrem as alterações no crescimento e desenvolvimento craniofaciais em pessoas co fissuras lábio-palatinas. Para tanto foram avaliadas radiografias laterais cefalométricas e radiografias carpais de pacientes do Hospital de Reabilitação de Anomalias Craniofaciais (HRAC), masculino e feminino, com idade entre 7 e 17 anos, utilizando-se, para esta avaliação, os métodos de Greulich e Pyle (GP) e Hassel e Farman (HF). Após a análise dos dados obtidos, constatou-se um eficácia de 92% dos métodos, portanto, ambos podem ser utilizados para a avaliação do crescimento e desenvolvimento de fissuras lábio-palatinas...