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Factors Affecting Photopolymerization Stress in Dental Composites
1 Department of Biomaterials and Oral Biochemistry, School of Dentistry, University of São Paulo, Av. Prof. Lineu Prestes, 2227 São Paulo, SP, Brazil, 09050-150; and Correspondence: * corresponding author, pfeifercs{at}gmail.com
Polymerization stress development results from the complex interplay of volumetric shrinkage, reaction kinetics, and viscoelastic properties. The objective of this study was to examine the relationships among volumetric shrinkage, degree of conversion, rate of polymerization (RPmax), and stress development for 2 model bis-GMA-based composites. Three irradiances were used—220, 400, or 600 mW/cm2—with exposure times adjusted to deliver the same radiant energy. Volumetric shrinkage was determined with a mercury dilatometer, degree of conversion and RPmax by differential scanning calorimetry (DSC), and polymerization stress with a low-compliance device (Sakaguchi et al., 2004b). Results indicated that polymerization reaction rate and shrinkage were not correlated. Irradiance was directly related to polymerization reaction rate and to stress development. The group with the highest stress/degree of conversion exhibited the lowest RPmax, so it can be assumed, within the limitations of this study, that the conversion was most closely related to stress development.
Key Words: dental composite • polymerization stress • reaction kinetics • irradiance • volumetric shrinkage
Restorative techniques with resin composites have evolved in an attempt to overcome the material’s greatest liability: polymerization shrinkage stress and its deleterious effect on marginal integrity. One often-used approach is based on the reduction of the polymerization reaction rate (Uno and Asmussen, 1991). Theoretically, in a polymer cured at slower rates, more time would be available for viscous flow and chain relaxation to occur (Sakaguchi et al., 2004b), postponing the onset of stress development and reducing its magnitude. Indeed, photoactivation protocols in which an initially low irradiance is used led to reductions in both stress development rate and final stress, compared with the conventional high irradiance routine, which can be due to the formation of a limited number of polymerization growth centers (Bouschlicher and Rueggeberg, 2000), which, in turn, may jeopardize crosslinking (Asmussen and Peutzfeldt, 2001) and conversion (Lu et al., 2005). The mechanisms involved in polymerization stress development are quite complex and may differ with each composite formulation (Sakaguchi et al., 2002), which may influence the efficacy of photoactivation protocols with modulated irradiance (Pfeifer et al., 2006). Composites presenting different volumetric shrinkage may show similar stress due to differences in elastic modulus or reaction rates (Sakaguchi et al., 2004a). For this reason, it is of interest to evaluate, in the same study, how different irradiances applied to composites with known composition affect polymerization stress development. Therefore, this study assessed polymerization rate and volumetric shrinkage as a function of composite formulation and irradiance, as an attempt to verify the contributions of these variables to polymerization stress development. Radiant exposure was kept constant throughout the study so that, for a given material, degree of conversion would not vary (Asmussen and Peutzfeldt, 2003a). The hypotheses tested were that: (1) volumetric shrinkage and degree of conversion do not vary with irradiance; and (2) polymerization stress is directly related to reaction rate.
Composite Preparation Two composites were mixed, with monomers used as received (Esstech, Linwood, PA, USA). Formulation B consisted of equal parts by weight of 2,2-bis[p-(2'-hydroxy-3'-methacryloxypropoxy)phenylene]propane (bis-GMA) and tri-ethylene glycol dimethacrylate (TEGDMA); formulation U had equal parts by weight of bis-GMA, TEGDMA, and 1,6-bis(methacryloxy-2-ethoxycarbonylamino)-2,4,4-trimethylhexane (UDMA). For both, 0.4 wt% of a tertiary amine (EDMAB - ethyl 4-dimethylaminobenzoate; Avocado, Heysham, England), 0.8 wt% of dl-camphoroquinone (Polysciences Inc., Warrington, PA, USA), and 0.1 wt% inhibitor (BHT - 2,6-di-tert-butyl-4-methylphenol; Sigma Aldrich, St. Louis, MO, USA) were added. Filler was introduced at 80 wt% (5% OX-50 - 0.04 µm; 75% silane-treated strontium glass ~2 µm; Bisco Inc., Schaumburg, IL, USA), with the aid of a mechanical mixer (DAC 150 Speed mixer, Flacktek, Landrum, SC, USA) for 2 min at 2400 rpm. All procedures were carried out under safe yellow light.
Sample Irradiation
Degree of Conversion and Rate of Polymerization
Polymerization Volumetric Shrinkage
Stress Development
Results from all tests were analyzed by two-way ANOVA and Tukey’s test for multiple comparisons (  = 0.05).
No significant interaction between irradiance and monomer blend was observed in any of the tests, except for maximum rate of stress development (RSmax, Table 2  ). Therefore, the averages for each experimental group are presented together with pooled averages corresponding to both main factors (Tables 1 and 2). Degree of conversion (Table 1) was significantly correlated only with irradiance (p < 0.001). High-irradiance groups presented statistically lower conversion values than both intermediate and low-irradiance groups, for the pooled data (Table 1). Maximum rate of polymerization (RPmax, Table 1) increased with the irradiance and was higher for the U series (p < 0.001 for both).
Polymerization stress (PSmax - Table 2  ) was influenced by both the monomer formulation and the irradiance (p < 0.001). B-series composites presented statistically higher stress than U-series. For RSmax, there was a significant interaction between the factors (p < 0.001). While no difference was observed between the irradiance levels for B-series, RSmax increased with irradiance for U-series (Table 2).
Volumetric shrinkage (VS - Table 2
Correlation plots between polymerization stress x RPmax (maximum reaction rate ) and polymerization stress x RSmax (maximum rate of stress development) were drawn (Fig. 2
Though the influence of degree of conversion, volumetric shrinkage, and stiffness, together with polymerization rate in stress development, has been well-documented (Condon and Ferracane, 1998, 2000; Braga and Ferracane, 2002), assessment of their relative contributions is very difficult, since properties and curing characteristics vary depending upon material composition (Feilzer and Dauvillier, 2003). In this study, we evaluated volumetric shrinkage and reaction rate in an attempt to identify the primary determinant of polymerization stress in two model composites. Ideally, all measurements should be taken in real time on the same specimen, to avoid any influence the specimen’s mass and geometry and the different test conditions would have on both degree of conversion and reaction rate. In this study, it was assumed that the data obtained with the DSC method are representative of the specimens used for the other tests, even though the reaction rate could have been slightly different in the different test set-ups. Radiant exposure and photoinitiator concentration were kept constant so that, for each material, degree of conversion, volumetric shrinkage, and elastic modulus were not expected to vary (Asmussen and Peutzfeldt, 2003a). In fact, at least for degree of conversion, this was true when irradiance levels of each composite were analyzed separately. When averages were pooled for the irradiance, the highest-irradiance group presented lower conversion values, providing evidence for the absence of complete reciprocity between irradiance and exposure, as previously reported by others (Musanje and Darvell, 2003; Peutzfeldt and Asmussen, 2005). Lower conversion with higher irradiance may be explained by a higher reaction rate (Lu et al., 2005), which causes the polymerization to become diffusion-limited at an earlier stage in conversion (Lovell et al., 1999). Consequently, degree of conversion is compromised. Moreover, it has been shown that short exposures (used with the highest irradiance level) may have a deleterious effect on final conversion (Peutzfeldt and Asmussen, 2005), which may be explained by the fact that the time required for the LED light to achieve full intensity may be several seconds (Tseng et al., 2007), leading to a significantly reduced final radiant exposure in these groups. Though reaction rate increased significantly with irradiance, corroborating the first hypothesis, the degrees of conversion at intermediate and low irradiances were equivalent. For these two groups, reciprocity between irradiance and exposure held true, in agreement with previously reported data (Asmussen and Peutzfeldt, 1999, 2003b). In spite of this apparent contradiction, it can be speculated that the exposure times used for the intermediate and low-irradiance levels (25 and 45 ± 3 sec, respectively), combined with exposures no higher than 500 mW/cm2, were able to provide a sufficient combination of irradiance and exposure time, as suggested previously (Halvorson et al., 2002). Both resin formulations reached similar conversions, although U-series presented a statistically higher reaction rate. Because the UDMA replaced some of the mobile TEGDMA as well as the more rigid Bis-GMA, this may account for the fact that the overall DC was not different for the two formulations. However, the enhanced ability for chain transfer in the UDMA system may account for the higher reaction rate (Stansbury and Dickens, 2001; Sideridou et al., 2002). The B-series had a statistically higher volumetric shrinkage compared with the U-series, which can be explained by the higher mole concentration of TEGDMA in the former. The molar equivalent of a 1:1 wt% bis-GMA/TEGDMA mixture is 1.8 TEGDMA molecules for each bis-GMA molecule (TEGDMA Mw = 286 g/mole; BisGMA: Mw = 512 g/mole). For the ternary mixture, the equivalent in moles is 1.8 molecules of TEGDMA and 1.1 UDMA for each bis-GMA molecule (UDMA: Mw = 470 g/mole). TEGDMA co-polymers have been reported to create much more dense networks compared with UDMA (Sideridou et al., 2003). Although the co-polymerization of dimethacrylates is not homogeneous, with a proposed tendency to form microgels (Elliott et al., 2001), the more flexible aliphatic units of TEGDMA may have favored enhanced molecule packing (Sideridou et al., 2003), leading to greater shrinkage. Composites in the B-series presented polymerization stress values higher than those in the U-series. This can be explained by the greater volumetric shrinkage presented by the former, as previously reported (Braga and Ferracane, 2002). This was true even considering that the U-series presented higher rates of reaction and polymerization stress development. Moreover, B-series composites presented a greater mole concentration of bis-GMA, probably increasing the elastic modulus (Asmussen and Peutzfeldt, 1998), which has been related to increased stress (Condon and Ferracane, 2000). Therefore, for the two composites studied, volumetric shrinkage and, potentially, the elastic modulus prevailed over reaction rate as the main stress determinants, as previously suggested by others (Lu et al., 2004). The fact that higher irradiance produced the lowest stress agrees with the conversion findings. It is also possible that the lower conversion shown by the high-irradiance group may have negatively affected elastic modulus. Interestingly, though, polymerization rate (RPmax) showed a positive correlation with stress development rate (RSmax) (R2 = 0.645), verifying that stress development occurs simultaneously with carbon double-bond conversion. Stress showed a trend to level off after 5 min, and all groups presented stress values very close to each other. Surprisingly, there was an inverse correlation between stress and RPmax or RSmax, rejecting the third hypothesis. This may be due to the low compliance of the stress test set-up, which may have led to stress relief and may have shadowed possible differences in stress development. Indeed, previous findings with highly rigid systems have reported a direct relationship between stress and the rate of stress development (Braga et al., 2002). In conclusion, when the two composites were compared, differences in reaction rate did not correlate with differences in shrinkage. The effect of elastic modulus on stress was not evaluated and might have played a role. The group with the highest stress exhibited the highest degree of conversion and the lowest reaction rate, so it can be assumed that the former was the governing factor for stress development.
The authors thank CAPES (Coordenação de Aperfeiçoamento de Pessoal em Nível Superior - grant BEX 0682/05-5) for financial support, Esstech for providing the resins, and Bisco for providing the fillers used in the experimental composites in this study. Received for publication April 25, 2007. Revision received July 11, 2008. Accepted for publication July 31, 2008.
Journal of Dental Research, Vol. 87, No. 11,
1043-1047 (2008)
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