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Impact of Curing Protocol on Conversion and Shrinkage Stress

¹ Department of Chemical & Biological Engineering, Engineering Center, ECCH 111, University of Colorado at Boulder, Boulder, CO 80309-0424, USA; and
² Department of Restorative Dentistry, University of Colorado Health Sciences Center, Denver, CO 80262, USA;

Correspondence: ^* corresponding author, Christopher.Bowman{at}colorado.edu

	ABSTRACT

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Since considerable shrinkage stress develops during the curing of dental composites, various soft-start photocuring protocols, aiming to lower stress but not compromise conversion, have been proposed. We hypothesized that utilizing soft-start photocuring will result in not only reduced stress, but also decreased conversion. We evaluated the impact of 3 protocols (soft-start, pulse, and standard) on the stress development and polymerization extent of an experimental composite. A novel set-up capable of simultaneous shrinkage stress, conversion, and temperature measurements on the same specimen was utilized. Analysis of the data shows that stress rises dramatically as a function of conversion in the vitrified state, and the utilization of soft-start or pulse curing results in specimens with reduced final conversion and shrinkage stress, compared with specimens cured according to the standard full-intensity protocol. Finally, this study demonstrates that the predominant reason for the reduced shrinkage stress attained with soft-start or pulse curing is a modest decrease in final conversion.

Key Words: shrinkage stress • conversion • curing protocol • dental composite

	INTRODUCTION

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Dimethacrylate-based dental restorative composites have become increasingly popular with the support of facile photopolymerization techniques. Highly crosslinked polymeric restorations with excellent tooth-like appearance are formed on command under ambient conditions. However, constrained shrinkage during polymerization densification generates polymerization shrinkage stress, which is one of the most significant concerns during the highly technique-sensitive clinical placement of composite restorations (Carvalho et al., 1996; Davidson and Feilzer, 1997). Although polymerization shrinkage has been significantly reduced in modern dental composite formulations, largely through the addition of inorganic filler, the induced shrinkage stress remains too high to allow direct filling to be applied in some large posterior restorations (Dietschi and Krejci, 2001).

Since the detrimental shrinkage stress tremendously weakens the performance and longevity of dental composites, numerous approaches have been proposed to decrease and minimize the shrinkage stress through manipulation of curing protocols and placement techniques. Among these approaches, soft-start curing (irradiation begins with a low-intensity, followed by a full-light intensity) and pulse curing (similar to soft-start curing except that a dark interval is included between the initial low-intensity and the following full-intensity curing) have attracted extensive investigations. One hypothesis for advocating these types of curing protocols is that the initial low-light intensity could facilitate a certain degree of polymer chain relaxation, such that a portion of the shrinkage stress relaxes while the system has not yet reached the vitrification stage. It has been claimed that the soft-start curing method partially relieves shrinkage stress, and achieves improved integrity of the composite/tooth interface, without compromising the final double-bond conversion or mechanical properties of the cured dental composite (Uno and Asmussen, 1991; Feilzer et al., 1995; Mehl et al., 1997; Lim et al., 2002). However, whether this approach significantly reduces shrinkage stress without decreasing the final conversion and mechanical properties is still under extensive debate, and the real benefit of soft-start curing has not being confirmed clinically (Friedl et al., 2000; Asmussen and Peutzfeldt, 2003; Soh and Yap, 2004).

One reason leading to this perplexity is that the interrelationships among double-bond conversion, polymerization rate, and polymerization shrinkage stress in composite dental restoratives are not well-understood. Since the double-bond conversion is related directly to the amount of polymerization shrinkage (Patel et al., 1987) and the material’s mechanical properties (Lovell et al., 2003; Steeman et al., 2004), it is crucial that one follow the real-time conversion while monitoring the shrinkage stress development of the same specimen. In evaluations of the effects of various curing protocols on conversion and shrinkage stress development, it can be misleading if the conversion measurement and shrinkage stress measurement are performed on different specimens, or the two measurements are performed at different times, since slight differences in sample size, configuration, or irradiation conditions can significantly diminish the validity of a direct correlation of the data from these two distinct experiments. Unfortunately, this aspect has been overlooked in most studies associated with curing protocol and shrinkage stress, not to mention in numerous other studies where the assessment of double-bond conversion was not performed at all.

In this investigation, we hypothesized that utilizing soft-start curing would result in not only reduced shrinkage stress, but also decreased double-bond conversion. The impact of three photocuring protocols (soft-start, pulse, and standard) on simultaneous stress and conversion development was examined with a novel experimental technique that is capable of measuring the real-time stress and conversion concurrently on the same specimen. The results were further elucidated regarding fundamental relationships among polymerization kinetics, network evolution, temperature change, and shrinkage stress development.

	MATERIALS & METHODS

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Materials
An experimental composite containing 70 wt% resin and 30 wt% silanized filler was used in this study. Since stress is predicated on the shrinkage of the resin component, the filler used was minimized for a clear indication of how various clinically used photo-curing protocols may affect the development of stress. The resin was composed of 2,2-bis[4-(2-hydroxy-3-methacryloxyprop-1-oxy)phenyl]propane (Bis-GMA, Esstech, Essington, PA, USA) and the reactive diluent triethyleneglycol dimethacrylate (TEGDMA, Polysciences, Warrington, PA, USA) at 70:30 weight ratio, along with 0.3 wt% camphorquinone (CQ, Aldrich Chemical, Milwaukee, WI, USA) as the visible-light initiator and 0.8 wt% ethyl 4-dimethylaminobenzoate (EDAB, Aldrich Chemical) as the co-initiator. A nanofiller (OX-50, Degussa AM, Frankfurt, Germany) was selected to maintain a relatively high degree of translucency in the uncured and cured composite materials. The filler was treated with 5 wt% silane coupling agent -methacryloxypropyl trimethoxysilane (Aldrich Chemical), before being blended with the Bis-GMA/TEGDMA resin. All chemicals were used as received.

Simultaneous Shrinkage Stress and Conversion Measurement
The shrinkage stress measurement device, referred to as a tensometer, was designed and fabricated at the Paffenbarger Research Center of the American Dental Association Health Foundation. This device is based on the cantilever beam theory that tensile force generated by a shrinking sample causes the cantilever of known beam constant to deflect. The shrinkage stress is then obtained by dividing the shrinkage force by the cross-sectional area of the disk-shaped sample (6.0 mm in diameter and 2.5 mm in thickness). The detailed description, experimental procedure, and characterization of the tensometer have been discussed in previous papers (Lu et al., 2004a,b).

In situ, real-time monitoring of the polymerization kinetics is enabled by means of near-infrared (NIR) spectroscopy, coupled with a fiber optic remote sensing technique. We obtained serial NIR spectra by configuring 2 optical fibers (wavelength from 350 to 2400 nm) to a Fourier transform-infrared spectrophotometer (Nexus 670, Nicolet Instrument, Madison, WI, USA) through a NIR fiberport. During the dynamic measurement of stress evolution, NIR signal was transmitted via fiber optic cables (1 mm diameter) through the diameter aspect of the specimen mounted in the tensometer. Conversion was monitored by real-time NIR in series collection mode, with a temporal resolution of 0.46 sec between spectrum collections. The series run collects peak area data from the region of 6232 to 6101 cm^–1, so that the decay of the methacrylate double-bond concentration during polymerization can be monitored. A detailed description of this simultaneous measurement technique has been discussed previously (Lu et al., 2004b), and illustrations are also available online as Appendices.

Curing Protocols
We used a halogen dental lamp, variable intensity polymerizer (VIP, BISCO, Schaumburg, IL, USA), to introduce the different curing protocols. The quartz rod ends were treated with silane bonding agent to ensure stable adhesion to the composite specimen during photopolymerization. Light intensity at the specimen interface of the lower quartz rod, which is used to conduct the curing light, was measured with a radiometer (Model 100, Demetron Research, Danbury, CT, USA). Three different curing protocols were investigated: STAN_irradiate at a constant light intensity of 450 mW/cm² for 60 sec; SOFT_start at 100 mW/cm² for the first 5 sec, followed immediately by another 60 sec of irradiation at 450 mW/cm²; and PULSE_start at 100 mW/cm² for the first 5 sec, wait for 2 min, then irradiate for another 60 sec at 450 mW/cm². All experiments were performed at ambient temperature (23 ± 1°C). The internal temperature of the composite during polymerization was monitored with an embedded K-type miniature thermocouple (diameter 0.127 mm; Omega Engineering, Stamford, CT, USA). For each set of experiments, 3 replicate runs were conducted. Results were analyzed by one-way analysis of variance (ANOVA) and Tukey’s HSD post hoc test, with a significance level of 0.05.

	RESULTS

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As revealed in conversion vs. time plots for specimens cured with the three protocols—STAN, SOFT, and PULSE (Fig. 1a)—polymerization proceeds rapidly, and specimens cured in STAN mode reached the highest final conversion (at 20 min from the start of irradiation), with an average of 67.5 ± 2.4%. Specimens cured with the SOFT protocol achieved an average of 61.9 ± 0.4% final conversion, while specimens cured in PULSE mode reached an average of 60.4 ± 0.7% final conversion. Between SOFT and PULSE curing protocols, no statistically significant difference for final conversion was found (Table).

View larger version (15K): [in this window] [in a new window]   Figure 1. Double-bond conversion vs. time (a) and shrinkage stress vs. time (b) for Bis-GMA/TEGDMA/OX-50 (49/21/30 by wt; Initiator, CQ 0.21 wt%; EDAB, 0.56 wt%) cured with 3 different curing protocols at T0 = 23°C: STAN, 450 mW/cm2 for 60 sec (—); SOFT, 100 mW/cm2 for 5 sec, followed by 450 mW/cm2 for 60 sec (– –); PULSE, 100 mW/cm2 for 5 sec, wait for 2 min, followed by 450 mW/cm2 for 60 sec (...). Vertical bar represents standard deviation. Estimate of variability is shown in the Table (n = 3).

As shown in real-time temperature vs. time profiles (Fig. 2), the STAN curing mode generates a significantly higher temperature increase (approximately 15°C), while the other 2 curing modes reach similar maximum temperature increases (approximately 10°C). Polymerization rate and shrinkage stress development rate have also been analyzed based on the derivative of double-bond conversion and shrinkage stress with respect to time. Specimens cured with STAN mode achieve not only the highest final conversion and shrinkage stress, but also the highest polymerization rate and shrinkage stress development rate (Table).

View larger version (13K): [in this window] [in a new window]   Figure 2. Sample temperature vs. time for Bis-GMA/TEGDMA/OX-50 (49/21/30 by wt; Initiator, CQ 0.21 wt %; EDAB, 0.56 wt%) cured with 3 different curing protocols at T0 = 23°C: STAN, 450 mW/cm2 for 60 sec (—); SOFT, 100 mW/cm2 for 5 sec, followed by 450 mW/cm2 for 60 sec (– –); PULSE, 100 mW/cm2 for 5 sec, wait for 2 min, followed by 450 mW/cm2 for 60 sec (...). Vertical bar represents standard deviation. Estimate of variability is shown in the Table (n = 3).

View larger version (14K): [in this window] [in a new window]   Figure 3. Shrinkage stress as a function of double-bond conversion for Bis-GMA/TEGDMA/OX-50 (49/21/30 by wt; Initiator, CQ 0.21 wt%; EDAB, 0.56 wt%) cured with 3 different curing protocols at T0 = 23°C: STAN, 450 mW/cm2 for 60 sec (—); SOFT, 100 mW/cm2 for 5 sec, followed by 450 mW/cm2 for 60 sec (– –); PULSE, 100 mW/cm2 for 5 sec, wait for 2 min, followed by 450 mW/cm2 for 60 sec (...). Each curve represents the average of 3 replicate runs (n = 3).

	DISCUSSION

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The significantly higher final conversion for specimens cured with STAN mode is attributed to the complex nature of photopolymerization behavior and the kinetics of dimethacrylate resins. One particularly important phenomenon is excess free volume (Kloosterboer, 1988; Anseth et al., 1995a). During photopolymerization of multi(meth)acrylate systems, the development of a crosslinked network is rapid, and gel point conversion is well below 10% (Macosko and Miller, 1976; Odian, 1991). For the microscopic free volume generated by chemical reaction to be converted into macroscopic volume shrinkage, the crosslinked polymer network must move cooperatively. However, when macroscopic volumetric shrinkage cannot keep up with the chemical reaction, a temporary excess of free volume is created which effectively increases the mobility of radicals and unreacted double-bonds. Therefore, higher degrees of conversion can be achieved, in comparison with those achieved in equilibrium volume systems (Anseth et al., 1995b). This phenomenon becomes more pronounced as the polymerization rate dramatically increases, and further enhanced conversion can be achieved. Other researchers have also observed that systems cured with higher incident light intensity achieve higher polymerization rates and final conversion (Maffezzoli and Terzi, 1998; Lovell et al., 1999).

Another aspect which affects conversion is the elevated temperature due to the exothermic polymerization as well as energy absorbed from the curing lamp. As shown in Fig. 2 and the Table, specimens cured with the STAN mode have greater temperature increases compared with specimens irradiated with the SOFT or PULSE mode. The higher temperature not only increases polymerization kinetics, but also enhances mobility of reacting species and postpones vitrification (Lange, 1999); hence, this non-isothermal process contributes to the increased polymerization rate and higher extent of polymerization.

We found that the predominant portion of the shrinkage stress did not start to develop until a much higher extent of polymerization was reached (Fig. 3). In these studies, more than 70% of the overall shrinkage stress was observed to develop over the last 15% of conversion. The significant increase of stress over conversion in the latter regime of polymerization is predominantly associated with the several-orders-of-magnitude increase of polymeric elastic modulus during the vitrification stage (Lange, 1999; Sakaguchi et al., 2002; Steeman et al., 2004).

Moreover, the direct correlation of real-time shrinkage stress and conversion from the same specimen clearly shows that, in the latter stage of polymerization, where shrinkage stress was concentrated, a small increase in conversion leads to a very significant increase of shrinkage stress. Not only will this small increase in conversion affect the final magnitude of stress developed in the forming network, but it is also critical when the effects of different curing protocols on stress evolution are investigated. It is therefore not surprising to observe that soft-start or pulse curing led to decreased shrinkage stress; however, significantly decreased final conversion was also produced. This finding is in contrast to other results, which indicated that reduced shrinkage stress can be achieved with soft-start curing, while the same final conversion can still be achieved (Uno and Asmussen, 1991; Lim et al., 2002). Furthermore, it should be noted that, in these previous studies, conversion values were obtained either from specimens different from those used for shrinkage stress evaluation, or from the same specimen (or a portion of it), but at a different time than the stress determination (Uno and Asmussen, 1991; Lim et al., 2002).

One major hypothesis supporting soft-start or pulse curing is that the initial lower light intensity would allow for polymer chain relaxation, so that shrinkage stress can be partly relaxed before the vitrification stage is reached. However, as illustrated in Fig. 3, for highly crosslinked, glassy Bis-GMA/TEGDMA systems, the majority of shrinkage stress develops during and after the vitrification stage, where viscosity approaches infinity, and elastic modulus is so high that stress relaxation is hardly observable. Consequently, any stress relaxation prior to the vitrification stage does not provide as much benefit to overall shrinkage stress reduction as expected. This finding was also supported by a separate investigation, where shrinkage stress relaxation phenomena were observed only prior to vitrification (Lu et al., 2004c). However, for this limited relaxation to be achieved, a much longer relaxation time is required, as compared with clinically practical time scales (Lu et al., 2004c).

In conclusion, the results from the simultaneous, in situ shrinkage stress ~ conversion measurements have confirmed our initial hypotheses, that utilizing low initial light intensity curing protocols resulted in specimens with both decreased double-bond conversion and shrinkage stress, compared with specimens cured with standard, full-intensity protocols. Specimens cured according to the conventional protocol also achieve the highest polymerization rate, shrinkage stress development rate, and temperature increase.

	ACKNOWLEDGMENTS

 
This research was financially supported by the NIH through grant #DE10959. The authors thank Esstech for providing Bis-GMA resin and Dr. Sheldon Newman for providing the curing lamp and radiometer.

	FOOTNOTES

 
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org.

Received for publication December 15, 2003. Revision received May 17, 2005. Accepted for publication June 1, 2005.

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Journal of Dental Research, Vol. 84, No. 9, 822-826 (2005)
DOI: 10.1177/154405910508400908


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This Article Abstract Figures Only Full Text (PDF) Data Supplement Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Saved Citations Download to citation manager Request Permissions Request Reprints Add to My Marked Citations Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Citing Articles via Scopus Google Scholar Articles by Lu, H. Articles by Bowman, C.N. Search for Related Content PubMed PubMed Citation Articles by Lu, H. Articles by Bowman, C.N. Pubmed/NCBI databases Compound via MeSH Substance via MeSH Hazardous Substances DB TRIETHYLENE GLYCOL DIMETHACRYLATE Social Bookmarking                         What's this?