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Contraction Stress Related to Degree of Conversion and Reaction Kinetics

¹ Department of Dental Materials, University of São Paulo, School of Dentistry, Av. Prof. Lineu Prestes, 2227, São Paulo, SP, 05508-900, Brazil; and
² Department of Biomaterials and Biomechanics, Oregon Health & Science University, School of Dentistry, 611 SW Campus Drive, Portland, OR 97201, USA;

*corresponding author, OHSU, Biomaterials and Biomechanics Dept., School of Dentistry, 611 SW Campus Dr., Portland, OR 97201, USA, bragar{at}ohsu.edu

	ABSTRACT

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Polymerization shrinkage of composites bonded to cavity preparations generates stress on the tooth/restoration interface. The purpose of this study was to verify the influence of degree of conversion and speed of polymerization reaction on contraction stress. We prepared experimental composites with different curing rates by varying the concentration of inhibitor (butylated hydroxytoluene). We verified the effect of degree of conversion by submitting one of the composites to different photo-activation times. Contraction stress was monitored for 10 minutes in a tensilometer. Fourier-transformed infrared spectrometry was used for assessment of the degree of conversion. Volumetric shrinkage was determined by means of a mercury dilatometer. Degree of conversion and volumetric shrinkage showed a non-linear relationship with energy density. Degree of conversion showed a pronounced influence on stress. Increased inhibitor concentration reduced curing rate and contraction stress in composites, without compromising the final degree of conversion.

KEY WORDS: composites • polymerization • contraction stress • volumetric shrinkage

	INTRODUCTION

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The polymerization of dimethacrylate-based composites is accompanied by substantial shrinkage. Clinically, composite strain is hindered by the confinement of the material bonded to the tooth, and, as a result, shrinkage manifests as stress. The resultant stress may damage the bonding (Davidson et al., 1984) or cause deflection of the surrounding tooth structure (Suliman et al., 1994; Meredith and Setchell, 1997). Polymerization contraction stress has been extensively studied since the early days of composite restorations (Bowen, 1967). Basically, the magnitude of the contraction stress is related to the configuration of the restoration, i.e., the ratio of bonded:unbonded surface area (Feilzer et al., 1987), the compliance of the material and the surrounding tooth structure (Alster et al., 1997), the composite's degree of conversion, and its conversion rate.

The volumetric shrinkage of currently used composites is in the range of 2 to 6% (Labella et al., 1999). There is a direct relationship between degree of conversion and shrinkage (Venhoven et al., 1993; Silikas et al., 2000). Therefore, for a given composite, a reduction in the final DC will lead to lower shrinkage and lower contraction stress. However, low degree of conversion might compromise some of the material's mechanical properties (Ferracane and Greener, 1986).

An alternative approach to the reduction of contraction stress without affecting final conversion is for the curing rate (DC/s) of the composite to be decreased. A polymer network developing at a slower rate may be able to yield to shrinkage forces through molecular rearrangements, delaying the stress build-up (Feilzer et al., 1990). The curing rate is proportional to the square root of the power density (PD = mW/cm²) (Odian, 1991). Reduction in curing rate may be achieved by modulation of the power density delivered in the early stages of the photo-activation period or by the use of a low-power density for extended exposure times (Sakaguchi and Berge, 1998; Rueggeberg et al., 1999). Bouschlicher and Rueggeberg (2000) reported that reduction of the curing rate, with its concomitant reduction of stress rate (MPa/s) obtained by power density modulation during photoinitiation, can result in decreased final stress while maintaining an equivalent degree of conversion. Varying the concentration of photo-initiators or inhibitor in experimental unfilled resins also has been shown to reduce curing rate while maintaining the same final conversion (Venhoven et al., 1996; Payne et al., 2001). But the effects of these variables on degree of conversion and contraction stress in filled resins still need to be determined.

The relative contributions of final conversion and conversion rate to the contraction stress in a given composite are unknown. This information would be useful for the development of new strategies to reduce contraction stress. Therefore, the aim of the present study was to verify the influence of degree of conversion and reaction speed on contraction stress development in resin-based composites.

	MATERIALS & METHODS

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Formulation of Experimental Composites
A series of experimental composites was prepared based on a 1:1 mixture by weight of Bis-GMA (2,2-bis[4-(2-hydroxy-3-methacryloxy propoxy)phenylene] propane; Esschem, Inc., Linwood, PA, USA) and TEGDMA (triethylene glycol dimethacrylate; Esschem, Inc.). Photosensitizer (Camphorquinone; Polysciences, Inc., Warrington, PA, USA) and co-initiator [DMAEMA, 2-(dimethylamino)ethyl methacrylate; Aldrich Chemical Co., Inc., Milwaukee, WI, USA] were used in a concentration of 0.5 wt% each. We prepared 4 materials by varying the amount of inhibitor (BHT, butylated hydroxytoluene; Sigma Chemical Co., St. Louis, MO, USA). The concentrations used were 0.05, 0.2, 0.5, and 1.0 wt%. Nanofiller (0.04 µm; OX-50, Degussa AM, Frankfurt, Germany) was added to the monomer mixture, representing 40% of the total weight of the composite.

Experimental Design
Two groups of experiments were devised. In each one, materials were tested for contraction stress, degree of conversion, and volumetric shrinkage. The same light unit was used in all tests (VIP, Bisco, Inc., Schaumburg, IL, USA), and the power density reaching the surface of the composite was determined with the use of a laboratory-grade power meter (PowerMax 5200, Molectron Detector, Inc., Portland, OR, USA) and a spectrometer (Ocean Optics, Inc., Dunedin, FL, USA) for each specific test apparatus. The variation in power density among the three experimental methods was approximately 10%. One series of experiments was conducted for verification of the influence of degree of conversion on the contraction stress. For such, the composite containing 0.2% BHT was subjected to different curing times (15, 30, 60, and 120 sec) under an average power density of 184 mW/cm² (energy densities: 2760, 5520, 11040, and 22080 mW/cm² x s or mJ/cm², respectively) so that the final conversion, but not the curing rate, would be varied. Another series of experiments was carried out for verification of the influence of different curing rates on contraction stress. We tested 4 experimental composites with different BHT levels (0.05, 0.2, 0.5, and 1.0 wt%) to vary the curing rate, but not the degree of conversion. In all the tests conducted in this series, the composites were irradiated for 60 sec under 258 mW/cm² (average among experimental set-ups), receiving a total energy density of 15480 mJ/cm².

Contraction Stress Test
The contraction stress test set-up was previously described in detail (Condon and Ferracane, 2000). Composite, 0.83 mm thick (C-factor = 3), was bonded between sandblasted and silane-treated glass stubs mounted in a tensilometer. The composite was light-cured from the top for the specified time through the glass rod. Contraction force was measured for 10 min, while the composite thickness was maintained and monitored by a feedback system equipped with a capacitance probe, with accuracy of ± 0.25 µm. Contraction force data were acquired at a rate of 0.5 sec^-1. We calculated the maximum stress by dividing the maximum contraction force by the area of the glass stub. The maximum stress rate was also determined as the greatest stress increase between two consecutive data points divided by the elapsed time. Five specimens were tested in each experimental group.

Degree of Conversion Assessment
Degree of conversion was determined by Fourier-transformed infrared spectrometry (Analect Instruments, Inc., Irvine, CA, USA) according to the standard baseline technique (Rueggeberg et al., 1990). A thin film of uncured composite (30 µm) was formed between a potassium chloride crystal and a clean sheet of cellophane. The tip of the light unit was placed at a 45° angle to the surface of the crystal, as close as possible to the composite without interfering with the infrared beam. During the 1st and 2nd min, spectra composed of 5 scans were acquired every 3 sec at a resolution of 4 cm^-1. Photoactivation was triggered after the first spectrum was obtained, and this first spectrum was used as the uncured reference. In the 3rd and 4th min, spectra (also composed by co-addition of 5 scans) were obtained every 10 sec. For the last 6 min, spectra were obtained every 30 sec by the co-addition of 20 scans. The ratio between the aliphatic (1640 cm^-1) and the aromatic (1610 cm^-1) carbon double bonds was used for calculating the degree of conversion for each specimen (Ferracane and Greener, 1986). Five specimens were tested in each group. The reaction time constant was calculated from the conversion curve. The time constant represents the time for the degree of conversion to reach a value of 0.632 of its final value in phenomena described by exponential growth curves (Watts and Cash, 1991).

Volumetric Shrinkage Measurements
Volumetric shrinkage was measured in a mercury dilatometer (ADA Health Foundation, Gaithersburg, MD, USA). Approximately 0.06 g of composite was placed on a sandblasted and silanated glass slide. The glass slide was then clamped to a glass column. The glass column was filled with mercury, and an LVDT probe (linear variable differential transducer) was positioned touching the mercury surface. The tip of the light unit was placed under the glass slide. Using previously input values of mass and density of the sample, we converted the LVDT readings to volumetric shrinkage values. Data acquisition occurred at the rate of 0.1 sec^-1. The initial shrinkage, corresponding to the value obtained at 12 sec, was recorded as an indicator of the initial reaction speed (Watts and Cash, 1991). Three specimens were tested in each group.

Results from the 3 experiments were analyzed by one-way analysis of variance and Tukey's test, with a global significance level of 0.05.

	RESULTS

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Longer curing times (similar power density but higher energy density) led to higher conversion, shrinkage, and stress (Table, Fig, 1A). A 38% increase in conversion (from 39% to 54%), between 15 and 120 sec, corresponded to a three-fold increase in stress (from 3.1 to 9.5 MPa). Between 60 and 120 sec, a non-significant increase in conversion led to a proportional increase in shrinkage, but a 34% increase in contraction stress. The kinetics parameters (maximum stress rate, polymerization time constant, and initial shrinkage) were similar for the 4 experimental groups (Table).

View larger version (15K): [in this window] [in a new window]   Figure 1. Average contraction stress curves obtained with different exposure times (A) and different inhibitor concentrations (B).

The regression curves between contraction stress and degree of conversion (A) and time constant (B) showed a very good fit with a second-order polynomial (Fig. 2). Contraction stress values showed an accentuated increase above 45% conversion. Contraction stress values were inversely related to the polymerization time constant.

View larger version (12K): [in this window] [in a new window]   Figure 2. Polynomial regression curve of contraction stress vs. degree of conversion (A) and contraction stress vs. time constant (B).

	DISCUSSION

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As expected, when the composite was irradiated at the same power density for different time periods, the kinetics parameters were statistically similar. Since the speed of the reaction was the same with all 4 exposure times, degree of conversion was the only factor responsible for differences in stress. Conversion and shrinkage at different exposure times did not show significant differences beyond exposure times of 60 sec. Above a certain energy density level, conversion and shrinkage no longer increased proportionally with energy density, due to the restricted mobility of the free radicals in the polymer network (Lovell et al., 1999). The smallest change in both shrinkage and conversion occurred between 60 and 120 sec, but this corresponded to the largest increase in stress. This non-linear correlation between degree of conversion and stress (Fig. 2A) suggests that, as the polymer network develops and modulus increases rapidly, the composite's capacity for viscous flow or its compliance decreases, and the material cannot yield to shrinkage forces. As a result, the stress increases dramatically with small increases in shrinkage or conversion.

For the group of materials with different reaction speeds, contraction stress was significantly reduced only between 0.5 and 1% BHT. This suggests that the effects of reduced curing rates on contraction stress are limited, and significant reductions in stress can be verified only after the curing rate drops below a certain threshold. Similar observations can be drawn from studies comparing different curing routines that modulated the power density during early stages of photoinitiation (Bouschlicher and Rueggeberg, 2000). Reduction in reaction speed by chemical inhibition occurs as the free radicals are terminated by reacting with the phenolic hydrogen of the BHT molecule (C₁₅H₂₄O). The phenoxy radicals may then inactivate another free radical by C-C or C-O coupling or by loss of another hydrogen atom to form a quinone, which may react further. Therefore, each inhibitor molecule can terminate two or more polymer chains (Moad and Solomon, 1995). The conversion proceeds at a reduced rate until the inhibitor is completely consumed. This extends what has been called the "pre-gel phase", where the shrinkage forces can be dissipated before the cross-linking reaches a certain point at which molecular displacement becomes impossible. After that, the shrinkage is likely to generate stress. The tendency for lower degree of conversion with higher BHT levels indicates that, in concentrations above 1%, the final conversion may be compromised.

Though different thicknesses of composite were used in the three tests, the linear correlation between shrinkage and degree of conversion (r = 0.90, p < 0.001) shows that the curing rate and extent, as determined on thin films in the Fourier-transformed infrared spectrometry, were indicative of those for the thicker samples. The actual curing rate could not be determined, because the highest rates of data acquisition possible in the spectrometry analysis and in the mercury dilatometer were not sufficient to follow the rapid change in values during the initial moments of the reaction. Spectrometry measurements showed substantial noise when spectra were acquired by co-addition of only 5 scans. A higher number of scans per spectrum would be necessary to improve the signal-to-noise ratio, but it was not possible due to the spectra acquisition rate required. Therefore, the shrinkage data were also useful to validate the results of degree of conversion over time (Venhoven et al., 1996).

The results of the present study confirmed the idea that contraction stress is related to degree of conversion and conversion rate. Clinically, it might be possible for the degree of conversion of a composite to be reduced to a certain extent for the reduction of contraction stress without affecting physical properties. However, the determination of the lower limit of conversion for the different materials and the clinical reproduction of those levels may be difficult tasks. On the other hand, from a manufacturing standpoint, the use of high inhibitor levels to reduce the reaction speed may be an effective approach to a significant decrease in the contraction stress without compromising the final conversion.

	ACKNOWLEDGMENTS

 
This study was supported in part by FAPESP (The State of São Paulo Research Foundation, grant 1999/11543-6) and by NIH/NIDCR (grant DE 07079). The authors also thank Esstech for providing the resin monomers.

Received August 6, 2001; Last revision December 14, 2001; Accepted December 18, 2001

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS & METHODS RESULTS DISCUSSION REFERENCES

 
Alster D, Venhoven BA, Feilzer AJ, Davidson CL (1997). Influence of compliance of the substrate materials on polymerization contraction stress in thin resin composite layers. Biomaterials 18:337–341.[Medline]

Bouschlicher MR, Rueggeberg FA (2000). Effect of ramped light intensity on polymerization force and conversion in a photoactivated composite. J Esthet Dent 12:328–339.[Medline]

Bowen RL (1967). Adhesive bonding of various materials to hard tooth tissues–VI. Forces developing in direct-filling materials during hardening. J Am Dent Assoc 74:439–445.[Medline]

Condon JR, Ferracane JL (2000). Assessing the effect of composite formulation on polymerization stress. J Am Dent Assoc 131:497–503.[Abstract/Free Full Text]

Davidson CL, De Gee AJ, Feilzer A (1984). The competition between the composite-dentin bond strength and the polymerization contraction stress. J Dent Res 63:1396–1399.[Abstract/Free Full Text]

Feilzer AJ, De Gee AJ, Davidson CL (1987). Setting stress in composite resin in relation to configuration of the restoration. J Dent Res 66:1636–1639.[Abstract/Free Full Text]

Feilzer AJ, De Gee AJ, Davidson CL (1990). Quantitative determination of stress reduction by flow in composite restorations. Dent Mater 6:167–171.[Medline]

Ferracane JL, Greener EH (1986). The effect of resin formulation on the degree of conversion and mechanical properties of dental restorative resins. J Biomed Mater Res 20:121–131.[Medline]

Labella R, Lambrechts P, Van Meerbeek B, Vanherle G (1999). Polymerization shrinkage and elasticity of flowable composites and filled adhesives. Dent Mater 15:128–137.[Medline]

Lovell LG, Newman SM, Bowman CN (1999). The effects of light intensity, temperature, and comonomer composition on the polymerization behavior of dimethacrylate dental resins. J Dent Res 78:1469–1476.[Abstract/Free Full Text]

Meredith N, Setchell DJ (1997). In vitro measurement of cuspal strain and displacement in composite restored teeth. J Dent 25:331–337.[Medline]

Moad G, Solomon DH (1995). The chemistry of free radical polymerization. London: Pergamon.

Odian G (1991). Principles of polymerization. 3rd ed. New York: John Wiley & Sons.

Payne MD, Ferracane JL, Sakaguchi RL (2001). Monitoring curing of composites with varied BHT levels using DMA and PhotoDSC (abstract). J Dent Res 80(Spec Iss):250.

Rueggeberg FA, Hashinger DT, Fairhurst CW (1990). Calibration of FTIR conversion analysis of contemporary dental resin composites. Dent Mater 6:241–249.[Medline]

Rueggeberg FA, Caughman WF, Chan DCN (1999). Novel approach to measure composite conversion kinetics during exposure with stepped or continuous light-curing. J Esthet Dent 11:197–205.[Medline]

Sakaguchi RL, Berge HX (1998). Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dent 26:695–700.[Medline]

Silikas N, Eliades G, Watts DC (2000). Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 16:292–296.[Medline]

Suliman A, Boyer DB, Lakes RS (1994). Polymerization shrinkage of composite resins: comparison with tooth deformation. J Prosthet Dent 71:7–12.[Medline]

Venhoven BAM, De Gee AJ, Davidson CL (1993). Polymerization contraction and conversion of BisGMA-based methacrylate resins. Biomaterials 14:871–875.[Medline]

Venhoven BAM, De Gee AJ, Davidson CL (1996). Light initiation of dental resins: dynamics of the polymerization. Biomaterials 17:2313–2318.[Medline]

Watts DC, Cash AJ (1991). Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 7:281–287.[Medline]

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