Técnicas para reduzir os efeitos da contração de polimerização das resinas compostas fotoativadas / Techniques for reducing the effects of polymerization shrinkage of composites photoactivated resins

Introdução: uma complicação inerente a todas as resinas compostas é o stress gerado pela contração de polimerização. As resinas compostas da atualidade, após a polimerização, perdem entre 2% a 3% de todo o seu volume. Essa perda pode acarretar em alterações fortemente comprometedoras a nível micro e macroscópico. Objetivo: abordar a importância do uso de técnicas e manobras clínicas, que visam diminuir a contração de polimerização das resinas compostas, reduzindo seus efeitos na cavidade, para que se tenha um procedimento restaurador com elevado índice de sucesso clínico e boa aceitação pelos pacientes. Material e Métodos: foi realizada uma revisão da literatura por meio de uma busca bibliográfica nas seguintes bases de pesquisa online: PUBMED/MEDLINE, LILACS, BBO e SCIENCE DIRECT, através do rastreio de artigos relevantes publicados entre o período de 2000 a 2015. Resultados: comparando as formas de ativação dos compósitos, os compósitos fotopolimerizáveis possuem um menor escoamento e maior stress de contração se comparados a compósitos com ativação química, isso se dá devido à fotoativação que se destaca por ser uma reação rápida quando comparada a outros métodos, não dispondo de tempo para a resina se acomodar na cavidade e ter uma boa interação com o sistema adesivo previamente aplicado, levando a grande parte dos problemas clínicos das restaurações. Conclusão: questionamentos sobre os efeitos da contração de polimerização, ainda não foram completamente elucidados no meio científico, por isso algumas técnicas como a escolha de fotopolimerizadores a LED, manutenção da fase pré gel, manutenção do Fator C e técnica de inserção incremental, podem ser usadas para minimizar os efeitos dessa contração nas restaurações feitas com resina, afim de reduzir insucessos como, sensibilidade pós operatória, infiltração marginal e riscos de agressão pulpar levando a uma maior longevidade dos procedimentos restauradores com materiais resinosos. (AU)

Introduction: a complication inherent in all composite resins is the stress generated by the polymerization contraction. The present composite resins, after polymerization, lose between 2% and 3% of their entire volume. This loss can lead to strongly compromising micro and macroscopic changes. Objective: to address the importance of using technical and clinical maneuvers, which aim to reduce polymerization shrinkage of composite resins, reducing its effects in the cavity, in order to have a restorative procedure with high clinical success rate and good patient acceptance. Material and Methods: a review of the literature through a literature search in the following search online databases was performed: PubMed/MEDLINE, LILACS, BBO and SCIENCE DIRECT, through the screening of relevant articles published between 2000 to 2015. Results: comparing both activation of composites, the dental composites have a lower flow and higher stress of contraction compared to composites with chemical activation, this is the due to photoactivation that stands out for being a quick reaction when compared to other methods, not providing time for the resin to settle in well and have a good interaction with the previously applied adhesive system, leading to much of the clinical problems of restorations. Conclusion: questions about the effects of polymerization shrinkage have not been fully elucidated in the scientific community, so some techniques such as the choice of curing lights to LED, the pre gel phase maintenance, maintenance of Factor C and incremental insertion technique can It is used to minimize the effects of this contraction in restorations made with resin in order to reduce failures as post-operative sensitivity, microleakage and pulp assault risks leading to increased longevity of restorative procedures with resinous materials. (AU)

Resin-based luting agents and color stability of bonded ceramic veneers.

STATEMENT OF PROBLEM: The type of resin-based luting agent might influence the color stability of ceramic veneers. PURPOSE: The purpose of this study was to investigate the effects of resin-based agents and aging on the color stability of ceramic veneers bonded to enamel. MATERIAL AND METHODS: Ceramic disks were cemented to bovine enamel disks with 4 resin-based luting agents (n=10): dual-polymerizing cement (RelyX ARC), light-polymerizing cement (RelyX Veneer), flowable composite resin (Filtek Z350 Flow), or composite resin preheated for 30 minutes at 60°C (Filtek Z350 XT). CIE L*, a*, and b* color coordinates were measured 24 hours after cementation (baseline) with a color spectrophotometer and reevaluated after 10,000 and 20,000 thermal cycles. Color variation was calculated by using CIELab (ΔE*(ab)) and CIEDE2000 (ΔE00) methods. Then 95% confidence intervals were calculated for color variation means between baseline and 10,000 thermal cycles and between 10,000 and 20,000 thermal cycles. The 95% confidence intervals were also calculated for the means of individual color coordinates (L*, a*, and b*). RESULTS: The dual-polymerizing cement had the highest color variation among all luting agents. No significant differences were found in color variation among the light-polymerizing materials. All agents showed ΔE*(ab)>3.46 and ΔE00>2.25 after 20,000 thermal cycles. Variations in L*, a*, and b* coordinates were material dependent. The dual-polymerizing agent was yellowish and reddish after aging. CONCLUSIONS: The dual-polymerizing cement had higher color variation than the light-polymerized materials when used for bonding ceramic veneers to enamel. Flowable and preheated composite resins had similar color stability to that of light-polymerizing resin-based cement.

The effect of photopolymerization on the degree of conversion, polymerization kinetic, biaxial flexure strength, and modulus of self-adhesive resin cements.

STATEMENT OF PROBLEM: Understanding the effect of the degree of conversion on the mechanical properties of auto- and dual-polymerizing self-adhesive resin cements leads to a better estimation of their performance in different clinical scenarios. PURPOSE: The purpose of this study was to evaluate the effect of photopolymerization on the degree of conversion (DC) and polymerization kinetic of 4 dual-polymerized resin cements, 20 minutes after mixing, and its effects on the mechanical properties (biaxial flexural strength [FS] and modulus [FM]) after short-term aging. MATERIAL AND METHODS: Conventional (RelyX ARC and Clearfil Esthetic Cement) and self-adhesive resin cements (RelyX Unicem and Clearfil SA Cement) were applied to a Fourier infrared spectrometer to assess the DC (n=5) under the following 3 polymerization conditions: direct light exposure (dual-polymerizing mode), exposure through the prepolymerized disk, or autopolymerizing. The polymerization kinetic was recorded for 20 minutes. Then, disk-shaped specimens (n=11) were prepared to evaluate the effect of polymerization on the FS and FM in both extreme polymerization conditions (dual-polymerizing or autopolymerizing). Data were statistically analyzed by 2-way repeated measure ANOVA (DC) and by 2-way ANOVA (FS and FM), followed by the Tukey-Kramer post hoc test (α=.05). RESULTS: Autopolymerizing groups exhibited reduced DC means, whereas intermediate values were observed when resin cements were polymerized through the disk. All groups exhibited higher DC at the end of 20 minutes. The polymerization kinetic revealed a rising curve, and materials, when directly photopolymerized, reached a plateau immediately after light exposure. Regarding the flexural biaxial testing, most of the resin cements were affected by polymerization mode and differences among groups were product dependent. CONCLUSIONS: The resin cements achieved immediate higher DC and mechanical properties when photopolymerized. The total absence of photoactivation may still impair their mechanical properties even after short-term aging.

Hardness gradients of dual-polymerized flowable composite resins in simulated root canals.

STATEMENT OF PROBLEM: Information is lacking of the polymerization depth of dual-polymerized flowable composite resin foundation materials in simulated root canals. PURPOSE: The purpose of this study was to investigate the hardness gradients and the polymerization depth of dual-polymerized flowable composite resin foundation materials in simulated root canals. MATERIAL AND METHODS: Slots in steel split cylinders with 1 open end were filled with the following 6 materials: Luxa Core, Para Core, Clearfil DC Core, Multi Core Flow, Gradia Core, and Core-Flo DC. After filling, they were subjected to a light intensity of 1250 mWcm(-2) with a light-emitting diode light through their open ends for 20 seconds. The resulting specimens were stored in a light-proof box at 37°C, and the Knoop hardness gradients of each polymerized material were measured after 0.5 hour, 24 hours, and 120 hours. The surface readings were obtained in 1-mm intervals at 1 mm to 10 mm away from the open ends. The collected data were analyzed by 2-way ANOVA and the Student-Newman-Keuls test (α=.05). RESULTS: Before the Knoop hardness numbers of the 6 materials became stable, they decreased gradually in depth at each time point (P<.001). However, the depths at which they became stable differed. The Knoop hardness numbers of Luxa Core and Core-Flo DC reached stability at a depth of 3 mm, Para Core at 4 mm, and Clearfil DC Core, Multi Core Flow, and Gradia Core at 5 mm. Additionally, at 120 hours after exposure, the ratios of the Knoop hardness numbers at a depth of 5 mm to those at 1 mm were 63.08% for Luxa Core, 70.48% for Clearfil DC Core, 81.38% for Para Core, 80.49% for Gradia Core, 86.30% for Multi Core Flow, and 96.28% for Core-Flo DC. CONCLUSIONS: In simulated root canals, the flowable composite resin foundation materials tested had better polymerization under dual polymerizing than under chemical polymerizing, and their chemical-polymerized capabilities could determine the definitive polymerization depth.

Microhardness of light- and dual-polymerizable luting resins polymerized through 7.5-mm-thick endocrowns.

STATEMENT OF THE PROBLEM: The complete polymerization of luting resins through thick indirect restorations is still questioned. PURPOSE: The purpose of this study was to evaluate the degree of polymerization of light- and dual-polymerizable luting resins under thick indirect composite resin and ceramic endocrowns by means of Vickers microhardness measurements. MATERIAL AND METHODS: The Vickers microhardness measurements of a light-polymerizable microhybrid composite resin and a dual-polymerizable luting cement directly polymerized in a natural tooth mold for 40 seconds with a high-power light-emitting diode lamp (control) were compared with measurements after indirect irradiation through 7.5-mm-thick composite resin and ceramic endocrowns for 3 × 90 seconds. A test-to-control microhardness values ratio of 0.80 at a depth of 0.5 mm below the surface was assumed as the criterion for adequate conversion. RESULTS: For the Vickers microhardness measurements of a dual-polymerizable luting cement, no differences (P>.05) were found between Vickers microhardness control values and values reported after polymerization through composite resin and ceramic endocrowns. For The Vickers microhardness measurements (±SD) of a light-polymerizable microhybrid composite resin, control values were significantly (P<.05) higher (111 ±3.3) than those reported after polymerization through composite resin (100.5 ±3.8) and ceramic (99.7 ±2.3) endocrowns. However, the hardness values of The Vickers microhardness measurements of a light-polymerizable microhybrid composite resin polymerized through the endocrowns were approximately 10% to 12% lower than those of the control values. Two-way ANOVA showed the influence of the luting material on the Vickers microhardness values (P<.05). The effect of endocrown material was not significant (P>.05). CONCLUSIONS: Under the conditions of this in vitro study, Vickers microhardness values of the dual-polymerizable resin cement and the light-polymerizable restorative composite resin irradiated for 3 × 90 seconds with a high irradiance light-emitting diode lamp through 7.5-mm-thick endocrowns reached at least 80% of the control Vickers microhardness values, which means that both materials can be adequately polymerized when they are used for luting thick indirect restorations.

Influence of activation mode of resin cement on the shade of porcelain veneers.

PURPOSE: The aim of this study was to evaluate the influence of resin luting cement's activation mode in the final shade of porcelain veneers after accelerated artificial aging (AAA). MATERIALS AND METHODS: Porcelain veneers (IPS Empress Esthetic) were produced using a standardized shade (ET1) and thickness (0.6 mm). Twenty bovine teeth were collected, prepared, and divided into two groups: group I (n = 10)-light-cured group, only base paste was applied to the veneers; group II (n = 10)-dual-cured group, in which the same base paste used in group I and a transparent catalyst were proportionally mixed for 20 seconds and then applied to the veneers. The specimens were light-cured for 60 seconds each and were next subjected to AAA. They were submitted to color readings with a spectrophotometer in three instances: in the tooth surface (only the substrate), after the cementation and polymerization of the veneers, and after the AAA. The values of L*, a*, and b* were obtained and the total color change was calculated (∆E*). Values obtained were subjected to statistical analysis, with a significance of 0.05. RESULTS: There were no significant differences between dual- and light-cured modes considering ∆E*, L*, a*, and b* values obtained after aging (p > 0.05). Within the dual-cured mode there were no significant differences in ∆E*, L*, a*, and b* values (p > 0.05). CONCLUSION: No relevant differences were found between the two activation modes in color change. When submitted to aging, dual- and light-cured modes of the resin cement showed visually perceptible (∆E* > 1.0) color changes; however, within the threshold of clinical acceptance (∆E* > 3.3).

The effect of photoactivation time and light tip distance on the degree of conversion of light and dual-cured dentin adhesives.

BACKGROUND: The degree of conversion of dental adhesive is an important parameter since poor mechanical properties are related to low percentage of monomer-to-polymer conversion within resin-based materials. OBJECTIVES: To evaluate the influence of polymerization time and light guide distance on the degree of conversion (DC) of three contemporary dental adhesives. MATERIALS AND METHODS: The spectral data of ExciTE DSC, Single Bond ® , and Adper ® Prompt L-Pop were analyzed using FTIR spectroscopy after 20 s, 40 s, and 60 s of photoactivation times. Light tip distances were kept at 1, 3, and 6 mm during the exposures. STATISTICS: Data were analyzed using ANOVA and Tukey's test (α = 0.05). RESULTS: Within groups, greater DC values were found using a tip distance of 1 mm or a 60-s curing time for Single Bond ® (59%) and Adper ® Prompt L-Pop (65%). No statistically significant difference (P > 0.05) was found using either 1 mm or 3 mm tip distances after 20 s, 40 s, and 60 s of light curing time for Single Bond ® . ExciTE ® DSC showed the greatest DC values with light tip distances of 1 mm (90%) and 3 mm (89%), using 60 s of light curing. CONCLUSION: The self-etch adhesive Adper ® Prompt L-Pop could be applied in shallow cavity preparations and must be light cured for at least 40 s. The light-cured total-etch adhesive ExciTE ® DSC could be applied in every restorative scenario if the curing time is extended up to 60 s or if the tip distance is extended up to 3 mm.

Influence of power density and primer application on polymerization of dual-cured resin cements monitored by ultrasonic measurement.

We used ultrasonic measurements to monitor the influence of power density and primer application on the polymerization reaction of dual-cured resin cements. The ultrasonic equipment comprised a pulser-receiver, transducers, and an oscilloscope. Resin cements were mixed and inserted into a transparent mould, and specimens were placed on the sample stage, onto which the primer, if used, was also applied. Power densities of 0 (no irradiation), 200, or 600 mW cm(-2) were used for curing. The transit time through the cement disk was divided by the specimen thickness to obtain the longitudinal sound velocity. When resin cements were light-irradiated, each curve displayed an initial plateau of approximately 1,500 m s(-1), which rapidly increased to a second plateau of 2,300-2,900 m s(-1). The rate of sound velocity increase was retarded when the cements were light-irradiated at lower power densities, and increased when the primer was applied. The polymerization behaviour of dual-cured resin cements was therefore shown to be affected by the power density of the curing unit and the application of self-etching primer.