Journal of Dental Research http://jdr.sagepub.com/
Water Aging Reverses Residual Stresses in Hydrophilic Dental Composites J.W. Park and J.L. Ferracane J DENT RES 2014 93: 195 originally published online 22 November 2013 DOI: 10.1177/0022034513513905 The online version of this article can be found at: http://jdr.sagepub.com/content/93/2/195
Published by: http://www.sagepublications.com
On behalf of: International and American Associations for Dental Research
Additional services and information for Journal of Dental Research can be found at: Email Alerts: http://jdr.sagepub.com/cgi/alerts Subscriptions: http://jdr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav
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research-article2014
JDR
93210.1177/0022034513513905
RESEARCH REPORTS Biomaterials & Bioengineering
J.W. Park1* and J.L. Ferracane2 1
Department of Conservative Dentistry, College of Dentistry, Yonsei University, Seoul, Korea; and 2Department of Restorative Dentistry, Division of Biomaterials and Biomechanics, School of Dentistry, Oregon Health and Science University, Portland, OR, USA; *corresponding author, [email protected]
Water Aging Reverses Residual Stresses in Hydrophilic Dental Composites
J Dent Res 93(2):195-200, 2014
Abstract
Introduction
Dental composites develop residual stresses during polymerization due to shrinkage. These stresses may change with time because of relaxation and water sorption in the oral environment. This phenomenon is likely dependent on the composition of the materials, specifically their hydrophilic characteristics, and could result in deleterious stresses on restorative materials and tooth structure. The purpose of this experiment was to use the thin ring-slitting method to compare the residual stress generated within composite materials of varying hydrophilicity when aged in wet and dry conditions after polymerization. Water sorption, solubility, elastic modulus, and residual stresses were measured in 6 commercial composites/ cements aged in water and dry conditions. The self-adhesive resin cement showed the highest water sorption and solubility. All composites showed initial residual contraction stresses, which were maintained when aged dry. Residual stresses in 2 of the self-adhesive cements and the polyacidmodified composite aged in wet conditions resulted in a net expansion. This experiment verified that residual shrinkage stresses in dental composites can be reversed during aging in water, resulting in a net expansion, with the effect directly related to their hydrophilic properties.
S
KEY WORDS: expansion, water sorption, water solubility, dimensional change, ring-slitting method, self-adhesive resin cement. DOI: 10.1177/0022034513513905 Received August 25, 2013; Last revision October 29, 2013; Accepted November 1, 2013 A supplemental appendix to this article is published electronically only at http://jdr.sagepub.com/supplemental. © International & American Associations for Dental Research
everal modified resin composite products have been introduced for a variety of applications, including flowable resin composites, packable composites, polyacid-modified composites (compomer), conventional resin cements, self-adhesive resin cements, flowable resins for core build ups, and others (Prager, 1997; Bayne et al., 1998; Nash et al., 2001; Wiegand et al., 2007; Ilie and Hickel, 2009, 2011; Luhrs et al., 2010). For all of these materials, polymerization results in shrinkage that produces stress within the material when it is constrained during the curing reaction. Previously, Park and Ferracane (2005, 2006) used a ring-slitting method for demonstrating the presence and magnitude of residual stress (RS) within light-cured dental composites aged in dry conditions, with the magnitude of the stress being a function of the extent of shrinkage and the properties of the composites. These stresses may be a concern for dental restorations in terms of creating and maintaining adhesion to tooth structure (May et al., 2012; May and Kelly, 2013). RSs have also been shown to influence the properties of ceramics, where the location of RS can either increase strength (if compressive) or decrease it (if tensile) (Taskonak et al., 2008). Furthermore, the occurrence and direction of these stresses may be influenced by other factors, such as time and storage condition (i.e., water sorption). A composite that absorbed some water might demonstrate a relief of the contraction RS from polymerization. But a composite with a more hydrophilic nature might absorb a much greater amount of water, leading to a net expansion or essentially a complete reversal of RS. This possibility has not been studied to date, but there is anecdotal evidence of cracking of ceramic crowns luted with a certain resin cement, presumably resulting from expansion forces as a result of water sorption. In light of the increased use of more hydrophilic cements and restorative materials designed for enhanced adhesion to tooth structure, it is important to evaluate the presence and direction of RS in the many types of currently existing resin-based dental materials. The purpose of this study was to use the thin ring-slitting method to compare the RS generated within various types of composite materials after aging in wet and dry conditions after polymerization. The hypothesis was that the magnitude and direction of the RS would be a function of the hydrophilicity of the materials, which was expected to vary based on differences in composition.
Materials & Methods The materials used included 2 restorative composites, a dual-cure type flowable composite for core build-ups, a polyacid-modified composite, a dualcure resin cement, and a self-adhesive resin cement (Table 1).
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196
Park and Ferracane
J Dent Res 93(2) 2014
Table 1. Materials Used in the Study Type Restorative composite Restorative composite Flowable core-buildup composite Polyacid modified composite Dual-cure resin cement Self-adhesive resin cement
Materials
Code
Filler (%):Wt (Vol)
Lot
Manufacturer
Esthet-X HD (A2)a Spectrum TPH (A2)b Core·X Flowc Dyract eXtra (A2)d Calibra (Translucent)e SmartCem 2 (Translucent)f
EXHD STPH CXF DEX CAL SC2
76 (60) 77 (57) 70 (?) 73 (47) 65 (?) 69 (46)
1100311 1106001211 1108001728 1108001124 110520 080619
Dentsply Caulk, Milford, DE, USA Dentsply DeTrey, Konstanz, Germany Dentsply DeTrey Dentsply DeTrey Dentsply Caulk Dentsply Caulk
BHT, butylated hydroxytoluene; Bis-EMA, 2,2-Bis[4-(2-methacryloxyethoxy)phenyl]propane; Bis-GMA, 2,2-Bis[4-(2-hydroxy-3-methacryloxypropyl1-oxy)phenyl]propane; BPO, benzoyl peroxide; CQ, camphoroquinone; TCB-resin, tetracarboxylic acid hydroxyethyl methacrylate ester; TEGDMA, triethyleneglycol dimethacrylate; TPMTMA, trimethylolpropane trimethacrylate; UDMA; urethane dimethacrylate. All materials were used well within their stated expiration dates. a Composition—barium boron fluoroaluminosilicate glass, silicon dioxide, inorganic iron oxide, titanium dioxide, sodium alumino sulphosilicate; urethane modified Bis-GMA resin, polymerizable dimethacrylate resin, photoinitiators, photoaccelerators. b Composition—barium boron fluoroaluminosilicate glass, barium boron fluoroaluminosilicate glass, hydrophobic amorphous fumed silica, inorganic iron oxide, titanium dioxide; urethane modified Bis-GMA resin, polymerizable dimethacrylate resin, TEGDMA, photoinitiators, photoaccelerators UV absorber, organic fluorescing agent, BHT. c Composition—UDMA, BPO, CQ, silicon dioxide, barium boron fluoroaluminosilicate glass. d Composition—Bis-GMA, UDMA, carboxylic acid modified dimethacrylate resin, TEGDMA, TPMTMA, TCB-resin, BHT; silicon dioxide, strontiumalimino-sodium-fluoro silicate glass, stronsium fluoride. e Composition—Base: barium boron fluoroalumino silicate glass, Bis-GMA, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, other colorants are inorganic iron oxide. Catalyst: barium boron, fluoroalumino silicate glass, Bis-GMA, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, benzoyl peroxide. f Composition—Base: UDMA, di- and tri-methacrylate resin, phosphoric acid acrylate resin, polymerizable dimethacrylate resin, barium boron fluoroaluminosilicate glass, 95.8% glass filler (3.8 μm) and 4.2% aerosol (16 nm), titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide. Catalyst: Barium boron fluoroaluminosilicate glass, UDMA, dipentaerythritol pentaacrylate phosphate, polymerizable dimethacrylate resin, organic peroxide initiator, camphoroquinone, phosphene oxide photoinitiator, BHT.
Water Sorption and Solubility Disk-shaped specimens (15 mm in diameter × 1 mm thick) were made with an acrylic mold according to ISO 4049 (n = 5). The light- and dual-cured composites were irradiated in a light-curing unit (Triad II, Dentsply, Milford, DE, USA) for 160 seconds (80 seconds each from top and bottom). The self-cured material was mixed and left in the dark for 5 minutes. The top and bottom surfaces were ground gently with No. 600 SiC paper to remove the matrix-rich layer and adjust the thickness, and the dimensions were measured to 0.01 mm to calculate volume, V. The specimens were aged in a 37°C oven for 22 hours, then moved to 23°C for 2 hours and weighed to an accuracy of 0.1 mg. The specimens were maintained in a desiccator and reweighed every 24 hours until a constant mass, m1, was obtained. The specimens were immersed in water at 37°C for 7 days; excess moisture was blotted off; then, they were reweighed, m2. Specimens were placed in a desiccator and reweighed daily until a constant mass was achieved, m3. Water sorption (Wsp) and solubility (Wsl) were calculated with the following equations:
Wsp =
m2 − m3 V
Wsl =
m1 − m3 V
Elastic Modulus Square rods (25 × 2 × 2 mm) of each material were made by filling glass tubes and curing in the light-curing unit for 40 seconds from top and bottom, for the light- and dual-cured specimens.
The self-cured specimens were mixed and kept in the dark for 5 minutes before removal from the mold. Specimens were aged dry for 24 hours at 37°C. Elastic modulus (E, GPa) was determined in 3-point bending (20-mm span) via a universal testing machine (QTEST, MTS, Eden Prairie, MN, USA) at a crosshead speed of 1 mm/min according to ISO 4049 (n = 5).
Residual Stress A brass mold consisting of 2 concentric cylinders was used to produce ring-shaped specimens (inner diameter = 14.46 mm, outer diameter = 17.83 mm, height = 0.9 mm). The composites were packed into the mold to occupy the thin ring-shaped space between the 2 cylinders and then centrifuged in a mixing device at 3200 rpm for 60 seconds (DAC 150 Speedmixer, FlackTek, Landrum, SC, USA). All specimens were then placed into the light-curing unit at constant location and illuminated for a total of 160 seconds to maximize the polymerization as follows: 40 seconds inside the mixing cup, 40 seconds within the brass mold, and 80 seconds after removal from the mold. For the light-curing mode of CAL, only the base paste was used. For the self-cure mode, the mixed cement was packed into the mold and left for 5 minutes before removal; then, 2 reference points were marked with oil-based permanent marker and a surgical blade. The specimens were aged for 7 days at 37°C, half kept dry (n = 10) and half in water (n = 10). The marked composite rings were cut with a low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) after being fixed with double-sided tape between glass slides. The distance between the reference points was measured before cutting, immediately after (0 hour), and then at 1 hour and 24 hours.
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J Dent Res 93(2) 2014 197 Water Aging Reverses Residual Stresses The procedure for measuring RS is the same as that reported previously (Park and Ferracane, 2006), with the modifications shown in the Figure. The difference is that the original design investigated residual contraction stress, which might be relaxed by aging in water; therefore, the specimens were aged dry. Here, the ring specimens were also aged in water for 7 days. The expectation was that any original contraction stresses might be relaxed and replaced with expansion stresses due to water sorption. The RS was calculated as in our previous studies.
Statistical Analysis The water sorption and solubility of each composite were analyzed with 1-way analysis of variance (ANOVA) / Tukey multiple comparison test (α < 0.05). Differences in elastic modulus were assessed with 2-way ANOVA/Tukey test (α < 0.05). Differences in the RS measurements were assessed with 3-way ANOVA/Tukey test (α < 0.05).
Results Water Sorption and Solubility SC2 dual-cure had greater water sorption than any of the other composites (Table 2). The 2 conventional composites, EXHD and STPH, had the least water sorption. The CXF had equivalent water sorption to the CAL cement, but CAL self-cure was higher than the CXF material. SC2 and CAL light cure showed the highest solubility. There were no differences among the other materials.
Elastic Modulus The elastic modulus of dry-aged specimens were significantly higher (averaging 16%) than those aged in water (Table 2). Thus, the values for elastic modulus in the wet and dry states were used for calculating the expansion/contraction RSs in their respective aging conditions. Elastic moduli were 5.3 to 9.5 GPa in the wet state and 6.4 to 10.5 in the dry state (Table 2). There was a significant interaction in the analysis, but it was small in relation to the size of the main effects and therefore not considered further.
Residual Stress There were significant differences in RS based on material, storage condition, and time after slitting the ring (Table 3). All dryaged composites showed positive values (i.e., shrinkage stress), with SC2 showing the least values at all times. The RS was lower for the specimens aged in water for all composites, except EXHD at time 0, STPH at time 0 and 24 hours, and CAL dual cure and light cure at time 0. The RS for CAL light cure at 1 and 24 hours was significantly higher when aged in water. SC2 showed significant expansion stresses with time after aging in water (positive RS). DEX showed a slight expansion stress.
Discussion It has been shown that dental composites are left with internal RS as a result of the polymerization reaction, which is accompanied by significant shrinkage (Park and Ferracane, 2006). These
Figure. The modified experimental methods (right) compared to the original ring-slitting method (left). After the composite ring was made, it was removed from the mold and maintained in water for 7 days. This time allowed for both a relaxation of the residual contraction stresses and the possibility of a net expansion stress due to water sorption.
stresses may be relaxed with time, especially when the material is exposed to and absorbs water. In fact, it is possible for some resin systems to expand from water sorption, and this may produce significant and deleterious forces on adjacent materials, including the tooth structure (Ito et al., 2005). It is likely that the composition of the particular cement or restorative (i.e., its hydrophilicity) and its properties (e.g., stiffness) may predispose the material to this phenomenon. However, the presence of residual expansion stresses within resin-based composite restoratives and cements had not been demonstrated until this study. This concern is worthy of further investigation based on the expanded use of more hydrophilic resin-based materials, such as polyacid-modified composites, which are designed to absorb water for a possible secondary polymerizing reaction within the oral cavity (Jedynakiewicz and Martin, 2001; Nicholson, 2008), and self-adhesive resin cements, which are designed to be hydrophilic to interact with and adhere to tooth structure (Ferracane et al., 2011). Therefore, in this study, we evaluated the amount and change of the RS in various resin-based composite materials after aging in water. This study confirmed the lower water sorption for conventional composites as compared with polyacid-modified composite or self-adhesive resin cement. For example, the water sorption has been reported to be between 10 and 20 µg/mm3 for conventional composites, 19 µg/mm3 for polyacid-modified composite, and 31 μg/mm3 for self-adhesive cement (Janda et al., 2007; Marghalani, 2012). The 2 conventional composites, EXHD and STPH, had the least water sorption, likely because of their generally hydrophobic chemistry, higher filler contents, and lower diluent concentrations. Perhaps the slightly higher water sorption for CAL and CXF is due to their need to flow
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198
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J Dent Res 93(2) 2014
Table 2. Water Sorption and Solubility (µg/mm3) and Elastic Modulus (GPa) (n = 5) Elastic Modulus Material
Water Sorption
Water Solubility
EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure
13.31 13.78 16.07 16.07 17.23 17.18 18.26 18.44 37.37
2.29 1.71 1.63 1.45 1.84 5.99 2.74 1.60 6.37
± ± ± ± ± ± ± ± ±
0.67 0.45e 0.36d 0.36d 1.83bcd 0.85bcd 1.08bc 1.26b 0.65a e
± ± ± ± ± ± ± ± ±
Wet
0.95 0.36b 0.78b 0.51b 1.12b 0.72a 0.62b 0.98b 0.81a
8.28 9.36 9.37 9.50 8.31 5.34 8.28 9.70 6.97
b
± ± ± ± ± ± ± ± ±
Dry
0.26 0.67ab 0.77ab 0.72a 0.46bc 0.59e 0.42c 0.40a 0.18d c
9.24 12.37 10.40 10.46 9.14 6.44 10.40 11.85 9.47
± ± ± ± ± ± ± ± ±
0.74b 0.88a 0.66b 0.42b 0.63b 0.59c 0.72b 0.47a 0.32b
For elastic modulus, dry > wet at p ≤ .05 for all composites. Means in a column with the same superscript are not significantly different. See Table 1 for material codes. Table 3. Residual Stress (MPa) for Each Composite at 0, 1, and 24 Hours after Slitting the Ring (n = 10) Storage Condition for 7 Days Time: Material 0 hours EXHD STPH CXF: Dual cure CFX: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p** 1 hour EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p** 24 hours EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p**
Wet
Dry
1.72 ± 0.03 ± 0.25 ± 0.67 ± 1.29 ± 2.78 ± 0.78 ± –1.44 ± –2.29 ± < .001
1.10ab 1.04cd 0.41bc 0.73bc 0.40abc 1.90a 0.51bc 1.56de 1.14e
1.03 ± 0.54 ± 2.75 ± 1.23 ± 1.55 ± 2.75 ± 2.40 ± 1.74 ± 0.43 ± < .001
1.05cde 0.45de 0.62a 0.36cde 0.36bcd 1.32a 0.43ab 0.58abc 0.89e
2.50 ± 1.13 ± 0.48 ± 0.82 ± 1.45 ± 10.65 ± 0.09 ± –1.68 ± –3.67 ± < .001
0.77b 0.47bc 0.60bcd 0.46bc 0.81bc 3.16a 1.17cd 1.63de 1.75e
5.01 ± 2.20 ± 3.40 ± 2.75 ± 2.55 ± 4.40 ± 1.89 ± 2.64 ± 0.88 ± < .001
1.81a 1.38cde 0.70bc 0.53cd 0.24cd 1.35ab 0.43de 0.65cd 0.68e
4.02 ± 1.82 ± 0.16 ± 1.02 ± 1.48 ± 16.25 ± 1.28 ± –0.96 ± –5.10 ± < .001
0.72b 0.55bc 0.66c 0.62bc 0.91bc 5.94a 1.12bc 1.75c 1.96d
5.87 ± 2.57 ± 4.86 ± 3.74 ± 3.76 ± 7.97 ± 2.52 ± 4.05 ± 1.38 ± < .001
1.95b 1.49de 1.09bc 0.63cd 0.47cd 2.87a 0.54de 0.99bcd 0.94e
p*
Water Aging Reverses Residual Stresses in Hydrophilic Dental Composites J.W. Park and J.L. Ferracane J DENT RES 2014 93: 195 originally published online 22 November 2013 DOI: 10.1177/0022034513513905 The online version of this article can be found at: http://jdr.sagepub.com/content/93/2/195
Published by: http://www.sagepublications.com
On behalf of: International and American Associations for Dental Research
Additional services and information for Journal of Dental Research can be found at: Email Alerts: http://jdr.sagepub.com/cgi/alerts Subscriptions: http://jdr.sagepub.com/subscriptions Reprints: http://www.sagepub.com/journalsReprints.nav Permissions: http://www.sagepub.com/journalsPermissions.nav
>> Version of Record - Jan 17, 2014 OnlineFirst Version of Record - Nov 22, 2013 What is This?
Downloaded from jdr.sagepub.com at DALHOUSIE UNIV on July 1, 2014 For personal use only. No other uses without permission. © International & American Associations for Dental Research
research-article2014
JDR
93210.1177/0022034513513905
RESEARCH REPORTS Biomaterials & Bioengineering
J.W. Park1* and J.L. Ferracane2 1
Department of Conservative Dentistry, College of Dentistry, Yonsei University, Seoul, Korea; and 2Department of Restorative Dentistry, Division of Biomaterials and Biomechanics, School of Dentistry, Oregon Health and Science University, Portland, OR, USA; *corresponding author, [email protected]
Water Aging Reverses Residual Stresses in Hydrophilic Dental Composites
J Dent Res 93(2):195-200, 2014
Abstract
Introduction
Dental composites develop residual stresses during polymerization due to shrinkage. These stresses may change with time because of relaxation and water sorption in the oral environment. This phenomenon is likely dependent on the composition of the materials, specifically their hydrophilic characteristics, and could result in deleterious stresses on restorative materials and tooth structure. The purpose of this experiment was to use the thin ring-slitting method to compare the residual stress generated within composite materials of varying hydrophilicity when aged in wet and dry conditions after polymerization. Water sorption, solubility, elastic modulus, and residual stresses were measured in 6 commercial composites/ cements aged in water and dry conditions. The self-adhesive resin cement showed the highest water sorption and solubility. All composites showed initial residual contraction stresses, which were maintained when aged dry. Residual stresses in 2 of the self-adhesive cements and the polyacidmodified composite aged in wet conditions resulted in a net expansion. This experiment verified that residual shrinkage stresses in dental composites can be reversed during aging in water, resulting in a net expansion, with the effect directly related to their hydrophilic properties.
S
KEY WORDS: expansion, water sorption, water solubility, dimensional change, ring-slitting method, self-adhesive resin cement. DOI: 10.1177/0022034513513905 Received August 25, 2013; Last revision October 29, 2013; Accepted November 1, 2013 A supplemental appendix to this article is published electronically only at http://jdr.sagepub.com/supplemental. © International & American Associations for Dental Research
everal modified resin composite products have been introduced for a variety of applications, including flowable resin composites, packable composites, polyacid-modified composites (compomer), conventional resin cements, self-adhesive resin cements, flowable resins for core build ups, and others (Prager, 1997; Bayne et al., 1998; Nash et al., 2001; Wiegand et al., 2007; Ilie and Hickel, 2009, 2011; Luhrs et al., 2010). For all of these materials, polymerization results in shrinkage that produces stress within the material when it is constrained during the curing reaction. Previously, Park and Ferracane (2005, 2006) used a ring-slitting method for demonstrating the presence and magnitude of residual stress (RS) within light-cured dental composites aged in dry conditions, with the magnitude of the stress being a function of the extent of shrinkage and the properties of the composites. These stresses may be a concern for dental restorations in terms of creating and maintaining adhesion to tooth structure (May et al., 2012; May and Kelly, 2013). RSs have also been shown to influence the properties of ceramics, where the location of RS can either increase strength (if compressive) or decrease it (if tensile) (Taskonak et al., 2008). Furthermore, the occurrence and direction of these stresses may be influenced by other factors, such as time and storage condition (i.e., water sorption). A composite that absorbed some water might demonstrate a relief of the contraction RS from polymerization. But a composite with a more hydrophilic nature might absorb a much greater amount of water, leading to a net expansion or essentially a complete reversal of RS. This possibility has not been studied to date, but there is anecdotal evidence of cracking of ceramic crowns luted with a certain resin cement, presumably resulting from expansion forces as a result of water sorption. In light of the increased use of more hydrophilic cements and restorative materials designed for enhanced adhesion to tooth structure, it is important to evaluate the presence and direction of RS in the many types of currently existing resin-based dental materials. The purpose of this study was to use the thin ring-slitting method to compare the RS generated within various types of composite materials after aging in wet and dry conditions after polymerization. The hypothesis was that the magnitude and direction of the RS would be a function of the hydrophilicity of the materials, which was expected to vary based on differences in composition.
Materials & Methods The materials used included 2 restorative composites, a dual-cure type flowable composite for core build-ups, a polyacid-modified composite, a dualcure resin cement, and a self-adhesive resin cement (Table 1).
195 Downloaded from jdr.sagepub.com at DALHOUSIE UNIV on July 1, 2014 For personal use only. No other uses without permission. © International & American Associations for Dental Research
196
Park and Ferracane
J Dent Res 93(2) 2014
Table 1. Materials Used in the Study Type Restorative composite Restorative composite Flowable core-buildup composite Polyacid modified composite Dual-cure resin cement Self-adhesive resin cement
Materials
Code
Filler (%):Wt (Vol)
Lot
Manufacturer
Esthet-X HD (A2)a Spectrum TPH (A2)b Core·X Flowc Dyract eXtra (A2)d Calibra (Translucent)e SmartCem 2 (Translucent)f
EXHD STPH CXF DEX CAL SC2
76 (60) 77 (57) 70 (?) 73 (47) 65 (?) 69 (46)
1100311 1106001211 1108001728 1108001124 110520 080619
Dentsply Caulk, Milford, DE, USA Dentsply DeTrey, Konstanz, Germany Dentsply DeTrey Dentsply DeTrey Dentsply Caulk Dentsply Caulk
BHT, butylated hydroxytoluene; Bis-EMA, 2,2-Bis[4-(2-methacryloxyethoxy)phenyl]propane; Bis-GMA, 2,2-Bis[4-(2-hydroxy-3-methacryloxypropyl1-oxy)phenyl]propane; BPO, benzoyl peroxide; CQ, camphoroquinone; TCB-resin, tetracarboxylic acid hydroxyethyl methacrylate ester; TEGDMA, triethyleneglycol dimethacrylate; TPMTMA, trimethylolpropane trimethacrylate; UDMA; urethane dimethacrylate. All materials were used well within their stated expiration dates. a Composition—barium boron fluoroaluminosilicate glass, silicon dioxide, inorganic iron oxide, titanium dioxide, sodium alumino sulphosilicate; urethane modified Bis-GMA resin, polymerizable dimethacrylate resin, photoinitiators, photoaccelerators. b Composition—barium boron fluoroaluminosilicate glass, barium boron fluoroaluminosilicate glass, hydrophobic amorphous fumed silica, inorganic iron oxide, titanium dioxide; urethane modified Bis-GMA resin, polymerizable dimethacrylate resin, TEGDMA, photoinitiators, photoaccelerators UV absorber, organic fluorescing agent, BHT. c Composition—UDMA, BPO, CQ, silicon dioxide, barium boron fluoroaluminosilicate glass. d Composition—Bis-GMA, UDMA, carboxylic acid modified dimethacrylate resin, TEGDMA, TPMTMA, TCB-resin, BHT; silicon dioxide, strontiumalimino-sodium-fluoro silicate glass, stronsium fluoride. e Composition—Base: barium boron fluoroalumino silicate glass, Bis-GMA, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, other colorants are inorganic iron oxide. Catalyst: barium boron, fluoroalumino silicate glass, Bis-GMA, polymerizable dimethacrylate resin, hydrophobic amorphous fumed silica, titanium dioxide, benzoyl peroxide. f Composition—Base: UDMA, di- and tri-methacrylate resin, phosphoric acid acrylate resin, polymerizable dimethacrylate resin, barium boron fluoroaluminosilicate glass, 95.8% glass filler (3.8 μm) and 4.2% aerosol (16 nm), titanium dioxide, iron oxide, hydrophobic amorphous silicon dioxide. Catalyst: Barium boron fluoroaluminosilicate glass, UDMA, dipentaerythritol pentaacrylate phosphate, polymerizable dimethacrylate resin, organic peroxide initiator, camphoroquinone, phosphene oxide photoinitiator, BHT.
Water Sorption and Solubility Disk-shaped specimens (15 mm in diameter × 1 mm thick) were made with an acrylic mold according to ISO 4049 (n = 5). The light- and dual-cured composites were irradiated in a light-curing unit (Triad II, Dentsply, Milford, DE, USA) for 160 seconds (80 seconds each from top and bottom). The self-cured material was mixed and left in the dark for 5 minutes. The top and bottom surfaces were ground gently with No. 600 SiC paper to remove the matrix-rich layer and adjust the thickness, and the dimensions were measured to 0.01 mm to calculate volume, V. The specimens were aged in a 37°C oven for 22 hours, then moved to 23°C for 2 hours and weighed to an accuracy of 0.1 mg. The specimens were maintained in a desiccator and reweighed every 24 hours until a constant mass, m1, was obtained. The specimens were immersed in water at 37°C for 7 days; excess moisture was blotted off; then, they were reweighed, m2. Specimens were placed in a desiccator and reweighed daily until a constant mass was achieved, m3. Water sorption (Wsp) and solubility (Wsl) were calculated with the following equations:
Wsp =
m2 − m3 V
Wsl =
m1 − m3 V
Elastic Modulus Square rods (25 × 2 × 2 mm) of each material were made by filling glass tubes and curing in the light-curing unit for 40 seconds from top and bottom, for the light- and dual-cured specimens.
The self-cured specimens were mixed and kept in the dark for 5 minutes before removal from the mold. Specimens were aged dry for 24 hours at 37°C. Elastic modulus (E, GPa) was determined in 3-point bending (20-mm span) via a universal testing machine (QTEST, MTS, Eden Prairie, MN, USA) at a crosshead speed of 1 mm/min according to ISO 4049 (n = 5).
Residual Stress A brass mold consisting of 2 concentric cylinders was used to produce ring-shaped specimens (inner diameter = 14.46 mm, outer diameter = 17.83 mm, height = 0.9 mm). The composites were packed into the mold to occupy the thin ring-shaped space between the 2 cylinders and then centrifuged in a mixing device at 3200 rpm for 60 seconds (DAC 150 Speedmixer, FlackTek, Landrum, SC, USA). All specimens were then placed into the light-curing unit at constant location and illuminated for a total of 160 seconds to maximize the polymerization as follows: 40 seconds inside the mixing cup, 40 seconds within the brass mold, and 80 seconds after removal from the mold. For the light-curing mode of CAL, only the base paste was used. For the self-cure mode, the mixed cement was packed into the mold and left for 5 minutes before removal; then, 2 reference points were marked with oil-based permanent marker and a surgical blade. The specimens were aged for 7 days at 37°C, half kept dry (n = 10) and half in water (n = 10). The marked composite rings were cut with a low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) after being fixed with double-sided tape between glass slides. The distance between the reference points was measured before cutting, immediately after (0 hour), and then at 1 hour and 24 hours.
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J Dent Res 93(2) 2014 197 Water Aging Reverses Residual Stresses The procedure for measuring RS is the same as that reported previously (Park and Ferracane, 2006), with the modifications shown in the Figure. The difference is that the original design investigated residual contraction stress, which might be relaxed by aging in water; therefore, the specimens were aged dry. Here, the ring specimens were also aged in water for 7 days. The expectation was that any original contraction stresses might be relaxed and replaced with expansion stresses due to water sorption. The RS was calculated as in our previous studies.
Statistical Analysis The water sorption and solubility of each composite were analyzed with 1-way analysis of variance (ANOVA) / Tukey multiple comparison test (α < 0.05). Differences in elastic modulus were assessed with 2-way ANOVA/Tukey test (α < 0.05). Differences in the RS measurements were assessed with 3-way ANOVA/Tukey test (α < 0.05).
Results Water Sorption and Solubility SC2 dual-cure had greater water sorption than any of the other composites (Table 2). The 2 conventional composites, EXHD and STPH, had the least water sorption. The CXF had equivalent water sorption to the CAL cement, but CAL self-cure was higher than the CXF material. SC2 and CAL light cure showed the highest solubility. There were no differences among the other materials.
Elastic Modulus The elastic modulus of dry-aged specimens were significantly higher (averaging 16%) than those aged in water (Table 2). Thus, the values for elastic modulus in the wet and dry states were used for calculating the expansion/contraction RSs in their respective aging conditions. Elastic moduli were 5.3 to 9.5 GPa in the wet state and 6.4 to 10.5 in the dry state (Table 2). There was a significant interaction in the analysis, but it was small in relation to the size of the main effects and therefore not considered further.
Residual Stress There were significant differences in RS based on material, storage condition, and time after slitting the ring (Table 3). All dryaged composites showed positive values (i.e., shrinkage stress), with SC2 showing the least values at all times. The RS was lower for the specimens aged in water for all composites, except EXHD at time 0, STPH at time 0 and 24 hours, and CAL dual cure and light cure at time 0. The RS for CAL light cure at 1 and 24 hours was significantly higher when aged in water. SC2 showed significant expansion stresses with time after aging in water (positive RS). DEX showed a slight expansion stress.
Discussion It has been shown that dental composites are left with internal RS as a result of the polymerization reaction, which is accompanied by significant shrinkage (Park and Ferracane, 2006). These
Figure. The modified experimental methods (right) compared to the original ring-slitting method (left). After the composite ring was made, it was removed from the mold and maintained in water for 7 days. This time allowed for both a relaxation of the residual contraction stresses and the possibility of a net expansion stress due to water sorption.
stresses may be relaxed with time, especially when the material is exposed to and absorbs water. In fact, it is possible for some resin systems to expand from water sorption, and this may produce significant and deleterious forces on adjacent materials, including the tooth structure (Ito et al., 2005). It is likely that the composition of the particular cement or restorative (i.e., its hydrophilicity) and its properties (e.g., stiffness) may predispose the material to this phenomenon. However, the presence of residual expansion stresses within resin-based composite restoratives and cements had not been demonstrated until this study. This concern is worthy of further investigation based on the expanded use of more hydrophilic resin-based materials, such as polyacid-modified composites, which are designed to absorb water for a possible secondary polymerizing reaction within the oral cavity (Jedynakiewicz and Martin, 2001; Nicholson, 2008), and self-adhesive resin cements, which are designed to be hydrophilic to interact with and adhere to tooth structure (Ferracane et al., 2011). Therefore, in this study, we evaluated the amount and change of the RS in various resin-based composite materials after aging in water. This study confirmed the lower water sorption for conventional composites as compared with polyacid-modified composite or self-adhesive resin cement. For example, the water sorption has been reported to be between 10 and 20 µg/mm3 for conventional composites, 19 µg/mm3 for polyacid-modified composite, and 31 μg/mm3 for self-adhesive cement (Janda et al., 2007; Marghalani, 2012). The 2 conventional composites, EXHD and STPH, had the least water sorption, likely because of their generally hydrophobic chemistry, higher filler contents, and lower diluent concentrations. Perhaps the slightly higher water sorption for CAL and CXF is due to their need to flow
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198
Park and Ferracane
J Dent Res 93(2) 2014
Table 2. Water Sorption and Solubility (µg/mm3) and Elastic Modulus (GPa) (n = 5) Elastic Modulus Material
Water Sorption
Water Solubility
EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure
13.31 13.78 16.07 16.07 17.23 17.18 18.26 18.44 37.37
2.29 1.71 1.63 1.45 1.84 5.99 2.74 1.60 6.37
± ± ± ± ± ± ± ± ±
0.67 0.45e 0.36d 0.36d 1.83bcd 0.85bcd 1.08bc 1.26b 0.65a e
± ± ± ± ± ± ± ± ±
Wet
0.95 0.36b 0.78b 0.51b 1.12b 0.72a 0.62b 0.98b 0.81a
8.28 9.36 9.37 9.50 8.31 5.34 8.28 9.70 6.97
b
± ± ± ± ± ± ± ± ±
Dry
0.26 0.67ab 0.77ab 0.72a 0.46bc 0.59e 0.42c 0.40a 0.18d c
9.24 12.37 10.40 10.46 9.14 6.44 10.40 11.85 9.47
± ± ± ± ± ± ± ± ±
0.74b 0.88a 0.66b 0.42b 0.63b 0.59c 0.72b 0.47a 0.32b
For elastic modulus, dry > wet at p ≤ .05 for all composites. Means in a column with the same superscript are not significantly different. See Table 1 for material codes. Table 3. Residual Stress (MPa) for Each Composite at 0, 1, and 24 Hours after Slitting the Ring (n = 10) Storage Condition for 7 Days Time: Material 0 hours EXHD STPH CXF: Dual cure CFX: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p** 1 hour EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p** 24 hours EXHD STPH CXF: Dual cure CXF: Self-cure CAL: Dual cure CAL: Light cure CAL: Self-cure DEX SC2: Dual cure p**
Wet
Dry
1.72 ± 0.03 ± 0.25 ± 0.67 ± 1.29 ± 2.78 ± 0.78 ± –1.44 ± –2.29 ± < .001
1.10ab 1.04cd 0.41bc 0.73bc 0.40abc 1.90a 0.51bc 1.56de 1.14e
1.03 ± 0.54 ± 2.75 ± 1.23 ± 1.55 ± 2.75 ± 2.40 ± 1.74 ± 0.43 ± < .001
1.05cde 0.45de 0.62a 0.36cde 0.36bcd 1.32a 0.43ab 0.58abc 0.89e
2.50 ± 1.13 ± 0.48 ± 0.82 ± 1.45 ± 10.65 ± 0.09 ± –1.68 ± –3.67 ± < .001
0.77b 0.47bc 0.60bcd 0.46bc 0.81bc 3.16a 1.17cd 1.63de 1.75e
5.01 ± 2.20 ± 3.40 ± 2.75 ± 2.55 ± 4.40 ± 1.89 ± 2.64 ± 0.88 ± < .001
1.81a 1.38cde 0.70bc 0.53cd 0.24cd 1.35ab 0.43de 0.65cd 0.68e
4.02 ± 1.82 ± 0.16 ± 1.02 ± 1.48 ± 16.25 ± 1.28 ± –0.96 ± –5.10 ± < .001
0.72b 0.55bc 0.66c 0.62bc 0.91bc 5.94a 1.12bc 1.75c 1.96d
5.87 ± 2.57 ± 4.86 ± 3.74 ± 3.76 ± 7.97 ± 2.52 ± 4.05 ± 1.38 ± < .001
1.95b 1.49de 1.09bc 0.63cd 0.47cd 2.87a 0.54de 0.99bcd 0.94e
p*
