| Sign In to gain access to subscriptions and/or personal tools. |
Contraction Stress Determinants in Dimethacrylate Composites
1 Dept. of Biomaterials and Oral Biochemistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227, 05508-000 São Paulo, SP, Brazil; and Correspondence: * corresponding author, rrbraga{at}usp.br
The influence of composite organic content on polymerization stress development remains unclear. It was hypothesized that stress was directly related to differences in degree of conversion, volumetric shrinkage, elastic modulus, and maximum rate of polymerization encountered in composites containing different BisGMA (bisphenylglycidyl dimethacrylate) concentrations and TEGDMA (triethylene glycol dimethacrylate) and/or BisEMA (ethoxylated bisphenol-A dimethacrylate) as co-monomers. Stress was determined in a tensilometer. Volumetric shrinkage was measured with a mercury dilatometer. Elastic modulus was obtained by flexural test. We used fragments of flexural specimens to determine degree of conversion by FT-Raman spectroscopy. Reaction rate was determined by differential scanning calorimetry. Composites with lower BisGMA content and those containing TEGDMA showed higher stress, conversion, shrinkage, and elastic modulus. Polymerization rate did not vary significantly, except for the lower value of the 66% TEGDMA composite. We used linear regressions to evaluate the association between polymerization stress and conversion (R2 = 0.905), shrinkage (R2 = 0.825), and modulus (R2 = 0.623).
Key Words: polymerization stress • resin composite • degree of conversion • shrinkage • elastic modulus
Longitudinal clinical studies have identified the loss of interfacial integrity as one of the main causes for replacement of composite restorations (Brunthaler et al., 2003; Forss and Widström, 2004). The polymerization of dimethacrylate-based composites, characterized by volumetric shrinkage and increase in elastic modulus, is considered a primary source of interfacial stresses (Dauvillier et al., 2000; Ferracane, 2005). Polymerization stress of commercial composites has been found to be directly related to their filler content (Condon and Ferracane, 2000; Kleverlaan and Feilzer, 2005), suggesting that stress would be determined largely by their elastic modulus. However, similar stress values found among low- and regular-viscosity composites suggested that shrinkage may also play a role (Braga et al., 2003). Since both shrinkage and elastic modulus increase with the degree of conversion (Venhoven et al., 1996; Sakaguchi et al., 2002; Navarrete et al., 2005; Dewaele et al., 2006), a strong relationship was verified between the latter and stress (Braga and Ferracane, 2002; Calheiros et al., 2004; Lu et al., 2004). Finally, polymerization rate of reaction has also been shown to modulate stress, since a slower cure would allow for viscous flow and/or chain relaxation, accommodating part of the shrinkage and reducing stress (Bouschlicher and Rueggeberg, 2000; Lim et al., 2002). The bulk of the literature reports data drawn from commercial composites (Condon and Ferracane, 2000; Chen et al., 2001; Kleverlaan and Feilzer, 2005), precluding an accurate analysis of the influence of the resin matrix composition on stress development. One exception found that, among 3 BisGMA (bisphenylglycidyl dimethacrylate):TEGDMA (triethylene glycol dimethacrylate) formulations, those with higher TEGDMA concentration showed higher stress values, which were associated with higher shrinkage (Feilzer and Dauvillier, 2003). Perhaps the addition of other low-viscosity monomers with reduced shrinkage would subsequently reduce stress production. The purposes of the present study were: (1) to evaluate the effects of BisGMA, TEGDMA, and BisEMA (ethoxylated bisphenol-A dimethacrylate) contents on polymerization stress, degree of conversion, volumetric shrinkage, elastic modulus, and maximum rate of reaction of experimental composites; and (2) to investigate the association between polymerization stress and the other variables. The hypotheses were that: (1) on average, the tested variables decrease with BisGMA and increase with TEGDMA content; and (2) stress is directly related to the expected values of other variables.
Composite Formulations Nine formulations were prepared containing 33, 50, or 66 mol% of BisGMA (2,2-bis[4-(2-hydroxy-3-methylacryloxypropoxy)-phenyl]propane; Esstech, Essington, PA, USA) associated with TEGDMA (2-methyl-2-propenoic acid; Esstech), BisEMA (2,2-bis[4-(2-hydroxy-3-methylacryloxyethoxy)-phenyl]propane; Sigma-Aldrich Inc., St. Louis, MO, USA), or both monomers in equal parts. The photoinitiatior system was composed of camphorquinone (Sigma-Aldrich) and 2-(dimethylamino)ethyl methacrylate (Sigma-Aldrich), 0.5 wt% each. Inhibitor (butylated hydroxytoluene or 2,6-di-tetra-butyl¬-4-methylphenol; Sigma-Aldrich) was also added at 0.5 wt%. Silanated silica (OX-50; Degussa, Americana, Brasil) was added to mixtures at 40 wt%.
Polymerization Stress
Volumetric Shrinkage
Flexural Modulus
where C is the load (in Newtons) corresponding to the displacement d (in mm), L is the distance between the supports (6 mm), b is the specimen’s width, and h is its height (both in mm).
Degree of Conversion
Maximum Rate of Polymerization
Statistical Analysis
Mean values and standard deviations are displayed in the Table  . Statistically significant interactions between BisGMA concentration and co-monomer content (p < 0.01) were detected for all response variables, excepting conversion. Stress and shrinkage increased with co monomer concentration, except for the composite containing 66% TEGDMA, which presented statistically lower shrinkage than the material with 50% TEGDMA. Composites containing TEGDMA developed statistically higher stress and shrinkage than those formulated with BisEMA. The replacement of half of the TEGDMA by BisEMA significantly reduced mean stress only for the composites with 33% BisGMA, while mean shrinkage was reduced in composites containing 50% and 66% BisGMA.
Mean conversion decreased significantly with BisGMA content (p < 0.001) and was higher for composites containing TEGDMA compared with those containing BisEMA (p < 0.001). Composites containing both TEGDMA and BisEMA reached mean conversions statistically similar to those containing only BisEMA, and also to those containing only TEGDMA for the 33% and 50% BisGMA composites. Reaction rate was similar among groups, except for the composite containing 66% TEGDMA, which presented a statistically lower mean value. Composites containing TEGDMA presented statistically higher elastic modulus than those formulated with BisEMA. The substitution of half of the TEGDMA by BisEMA caused significant reductions in modulus for the composites with 50% and 66% BisGMA.
Results for regression analyses are shown in the Fig
In the present study, we defined formulations to assess the influence of BisGMA and low-viscosity monomers (TEGDMA and BisEMA) contents on polymerization stress and its determinants. Inorganic fraction was kept constant at a relatively low level, with the purpose of minimizing its influence on the evaluated properties. Ideally, all measurements should have been taken in real time and in the same specimen, to avoid any influence of size and geometry on polymerization of specimens used in the different tests. However, since this was not possible, it was assumed that the conversion and reaction rate data reported represent those of all the specimens tested. Regression analyses revealed that, in general, stress increased with conversion, shrinkage, and modulus. The coefficient of determination found between stress and conversion (R2 = 0.905) suggests that the latter may express the combined effects of shrinkage and modulus, both directly related to conversion (Venhoven et al., 1996; Sakaguchi et al., 2002). Moreover, in agreement with a previous report (Feilzer and Dauvillier, 2003), based on the coefficients of determination, stress was more closely related to shrinkage than to flexural modulus. A different study, however, detected a strong linear correlation between stress and tensile modulus, along with an inverse correlation with shrinkage (Kleverlaan and Feilzer, 2005). Such disparity can be credited, in part, to the broader range of modulus and shrinkage values considered in that study. Also, differences in compliance between testing systems used in both studies must be taken into consideration. The influence of composite stiffness on stress development is more clearly observed in highly rigid systems (Gonçalves et al., unpublished observations). A stronger correlation with in vitro microleakage results was obtained with stress data obtained with the use of poly(methyl methacrylate) rods, rather than glass rods, as substrate (Boaro et al., unpublished observations), which suggests that the influence of elastic modulus on stress development may be overestimated in low-compliance systems. The maximum rate of reaction did not vary significantly among the materials, except for the composite with the highest TEGDMA content. It is likely that, at such high concentration, TEGDMA greatly reduced resin viscosity, causing the onset of autoacceleration to be postponed, which, along with the possible increase on the rate of termination (kt), may help explain the lower RPmax found for this group (Lovell et al., 1999). Nevertheless, the fact that the composite presenting the highest stress showed the lowest rate value suggests that the influence of rate of reaction on stress development is overshadowed by the other variables. Therefore, the second hypothesis cannot be fully accepted. The reduction in conversion observed with higher BisGMA concentrations is explained by the high viscosity conferred by the hydroxyl groups, and by stiffness due to the phenyl rings, since both reduce the mobility of the reactive species (Sideridou et al., 2002; Dickens et al., 2003; Atai and Watts, 2006). The higher viscosity of Bis-GMA compared with TEGDMA increases the reactivity of BisGMA-rich mixtures and accelerates the initial conversion, more rapidly achieving the point where both chain propagation and termination become diffusion-controlled (Lovell et al., 1999), ultimately limiting the final conversion. As for the influence of the co-monomer on conversion, though both TEGDMA and BisEMA have much lower viscosities compared with BisGMA (Sideridou et al., 2002), the use of TEGDMA resulted in increased conversion compared with BisEMA or with the combination of both co-monomers. This suggests that the difference in viscosity between the 2 monomers was sufficient to affect the polymerization behavior of the mixture. At each BisGMA level, composites containing TEGDMA or TEGDMA+BisEMA presented statistically higher shrinkage than formulations containing only BisEMA. Besides increasing the mobility of the reaction medium (and therefore increasing conversion), due to its small molecular size and lower steric hindrance compared with those of BisGMA and BisEMA, the presence of TEGDMA facilitated the interaction among the reacting molecules, allowing for the formation of a more densely packed network (Cook and Moopnar, 1990; Sideridou et al., 2003). Nevertheless, this effect was not noticeable for the composite containing 66% TEGDMA, which showed lower shrinkage than would be expected based on conversion values. It is likely that the TEGDMA-rich mixture may have undergone more severe primary cyclization, which was shown to increase conversion, but may compromise the network formation, reducing crosslinking density (Elliott et al., 2001). Composites containing only BisGMA and BisEMA, in contrast, presented the lowest shrinkage, reflecting their low conversion. Moreover, due to its high molecular weight compared with that of TEGDMA, the presence of BisEMA increased the distance between polymer chains (Sideridou et al., 2003), which also contributed to the lower shrinkage. Flexural modulus values were situated within a relatively narrow range, between 1.3 and 2.7 GPa, verifying that the composition of the organic phase had much less impact on composite stiffness than the inorganic fraction (Labella et al., 1999). BisGMA:BisEMA composites presented statistically similar elastic moduli regardless of the ratio. Considering their similar structure, this finding suggests that intermolecular attraction via hydrogen bonding, expected to increase with BisGMA concentration, does not play a significant role in defining this property. This also was verified in the formulations containing the 3 monomers. The highest elastic modulus values were produced by the BisGMA:TEGDMA composites, again, with the 66% TEGDMA material as the exception. Besides the effect of primary cyclization, TEGDMA concentrations above 50% were shown to reduce elastic modulus, due to the increased flexibility of the molecule (Asmussen and Peutzfeldt, 1998). The results of this study suggest that, in composites with the same inorganic content, the composition of the organic matrix plays an important role in determining the polymerization stress as a direct result of its influence on conversion and, in turn, on shrinkage and elastic modulus. Further investigations are being conducted to determine the combined effects of variations in both organic and inorganic fractions, since the latter are expected to cause a more acute effect on composite elastic modulus and shrinkage.
The authors thank FAPESP (04/059750), CNPq, and CAPES (BEX 0682/05-5) for funding this project, Esstech and Degussa for kindly donating the monomers and the filler, and, finally, Julio Singer, Professor of Biostatistics (Dept. of Statistics, University of São Paulo), for his expert advice. This paper is based on a thesis submitted to the graduate faculty, University of São Paulo, in partial fulfillment of the requirements for the Master’s degree in Dental Materials. Received for publication October 19, 2007. Revision received January 3, 2008. Accepted for publication January 16, 2008.
Journal of Dental Research, Vol. 87, No. 4,
367-371 (2008) This article has been cited by other articles:
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
 |
|
|
 |
|
Sign In to gain access to subscriptions and/or personal tools.
TOP
ABSTRACT
INTRODUCTION
