Influence of the Feldspathic Ceramic Thickness and Shade on the Microhardness of Dual Resin Cement
This study evaluated the microhardness of a dual resin cement under the influence of thickness and shade of a feldspathic ceramic. Ninety-five bovine incisors were selected; the crowns, with the roots removed, were embedded in a polystyrene resin and were randomly divided into 19 groups (n=5). On the buccal surface, a standardized cavity, 4.0 mm in diameter and 1.0 mm in depth, was prepared. Ceramic restorations (Noritake Ex 3) were manufactured with 4.0 mm diameter and 1, 2 and 4-mm thicknesses at shades A1, A2, A3, A3.5 and A4. A dual resin cement (Rely X-ARC) was inserted into the prepared cavity. A mylar strip was positioned over the prepared cavity, and light curing was performed for 40 seconds following the protocols: controls—without insertion of the restoration at distances of 0.0, 1.0, 2.0 and 4.0 mm. The remaining groups had the restorations positioned between the resin cement and light source during polymerization. The Vickers hardness test was performed on the cement layer with 50g of load application for 30 seconds, with 5 indentations for each sample. Two-way ANOVA (5 x 3) and Tukey test (α=0.05) were used to compare the results. The chemical curing of the dual resin cement was not sufficient to compensate for the energy attenuation promoted by the interposition of A3.5 and A4 ceramic material with 4-mm of thickness. The thickness had a greater influence on the cement micro-hardness than the ceramic restoration shade.SUMMARY
INTRODUCTION
The success of ceramic restorations depends on obtaining a strong, durable bond between the resin cement and dentin/enamel (Uctasli, Hasanreisoglu & Wilson, 1994). The magnitude of these bonds is directly proportional to an adequate cement polymerization. Polymerization is crucial for achieving optimal physical properties and satisfactory clinical performance of resinous materials. This process occurs through an additional reaction initiated by chemical and light cured activation (el-Mowafy, Rubo & el-Badrawy, 1999).
Inadequate cement polymerization under ceramic restorations is related to an insufficient amount of light radiation to activate monomers (Nathanson, 1987). Light intensity decreases in the function of restoration thickness and shade; therefore, ceramic material characteristics, such as optical translucency and refraction index, may determine the amount of light transmitted and, consequently, the degree of conversion of resin cements (Rasetto, Driscoll & von Fraunhofer, 2001).
Attenuation of the light activation energy promoted by the interposition of a ceramic material between the light source and resin cement is directly associated with thickness (Brodbelt, O'Brien & Fan, 1980; Strang & others, 1987; Chan & Boyer, 1989; Blackman, Barghi & Duke, 1990; Breeding, Dixon & Caughman, 1991; O'Keefe, Pease & Herrin, 1991; Uctasli & others, 1994; el-Badrawy & el-Mowafy, 1995; el-Mowafy & others, 1999; Lee & Um, 2001; Watanabe & others, 2002; Barghi & McAlister, 2003), shade (Strang & others, 1987; Chan & Boyer, 1989; Breeding & others, 1991; Cardash & others, 1993; Myers, Caughman & Rueggeberg, 1994; Tanoue & others, 2001; Watanabe & others, 2002; Barghi & McAlister, 2003) and opacity (Strang & others, 1987; O'Keefe & others, 1991; Uctasli & others, 1994; Watanabe & others, 2002) of the restorative material.
As the ceramic material becomes thicker, the effect of opacity increases, resulting in a greater influence on the resin cement polymerization (Uctasli & others, 1994). Similarly, the shade effect seems to be associated with restoration thickness. Since the ceramic layer reflects and refracts the light, decreasing the total energy that reaches the deepest restorative regions (Watanabe & others, 2002), an insufficient amount of energy, produced by light absorption through the ceramic, will result in inadequate polymerization of the setting material, which will become evident at the deepest restorative areas or at the adhesive interface.
In order to access the degree of cure of the resinous materials through the determination of resin surface hardness, a microhardness tester is usually used to produce microindentations distributed on the resin cement surface (Uctasli & others, 1994; Rasetto & others, 2001).
The hypothesis drawn from this study is that the interaction of ceramic restoration shade and thickness can enhance its negative influence on the resin cement polymerization. Therefore, the aim of this work was to evaluate the influence of feldspathic ceramic material thickness and shade on the microhardness of dual resin cement.
METHODS AND MATERIALS
Ninety-five freshly extracted bovine incisors were selected, cleaned and stored in 0.2% thymol solution. The roots were removed from the crown at the cementoenamel junction using a double faced diamond saw (#070, KG Sorensen, SP, Brazil). The samples were fixed on a wax plate, a PVC cylinder (Tigre, Joinville, SC, Brazil), 2.5 mm in diameter, was positioned around the sample; and polystyrene resin (Cromex, Piracicaba, SP, Brazil) was manipulated and poured into the cylinder. After resin polymerization, the buccal surface was ground with wet 180-, 320- and 600-grit SiC abrasive paper (Norton Abrasive, Campinas, SP, Brazil) in order to obtain a flat dentin surface exposure.
The teeth received standardized circular cavity preparations, 4.0 mm in diameter and 1.0 mm in depth, from a cavity preparation machine (Soares & others, 2003) using a #3053 diamond bur (KG Sorensen, SP, Brazil).
One technician, who employed a standardized technique in accordance with the manufacturer's instructions, made all the restorations. All feldspathic ceramic restorations (Noritake EX—3, Kizaeco, Japan) were fabricated with 4.0 mm in diameter, varying thicknesses—1, 2 and 4 mm with shades—A1, A2, A3, A3.5 and A4. Wax cylinders were prepared to serve as a guide for fabrication of the ceramic restorations. A refractory investment material (Ceramavest, All Ceramic Refractory, Cosmetex, USA) was proportioned in an analytical balance in the ratio of 30g of powder for 10.5g of liquid, manipulated in a vacuum environment for 45 seconds under light vibration and poured around wax cylinders (Inclusor Polidental). After investment crystallization, the wax was eliminated through washing with water at 100°C, and the system was inserted into a furnace (Phoenix, Quick Cool, Ceramco, Dentsply, Petrópolis, RJ, Brazil) at 1080°C for six minutes for refractory investment sintering. The investment cylinder was removed from the furnace and, after cooling, the ceramic was applied in layers and fired at 930°C (Phoenix–Quick Cool). After cooling, the ceramic restorations (Table 1 ) were divested using a diamond bur, #703 PM (KG Sorensen, SP, Brazil), following airborne particle abrasion with aluminum oxide at 2 bars of pressure, carefully removing particles next to the restorations.
In order to avoid additional lateral light exposue during cement polymerization, a metallic device 1-, 2- and 4-mm thick with a central hole, 4.0 mm in diameter, was fabricated (Figure 1). During light curing, the restoration was placed inside this metallic device and positioned over the tooth, with the restoration being placed exactly over the cement layer.
Citation: Operative Dentistry 31, 3; 10.2341/05-51
A dual resin cement (Rely X ARC, 3M ESPE, St Paul, MN, USA) (Table 1) was manipulated and inserted into tooth-prepared cavities. A mylar strip was positioned over the resin cement and light cured for 40 seconds using a halogen light source with an intensity of 600 mW/cm2 (XL 3000, 3M ESPE), according to the following protocol: control group—polymerization was performed directly over the cement (0 mm) and at distances of 1, 2 and 4 mm through use of the metallic device of adequate thickness; on the remaining group (Figure 2), each specific ceramic pastille was coupled to its metallic device and positioned over the cement before polymerization (Figure 3).
Citation: Operative Dentistry 31, 3; 10.2341/05-51
Citation: Operative Dentistry 31, 3; 10.2341/05-51
The samples were stored in a dark, dry location at 37°C for 24 hours. The resin cement cure degree was measured by means of Vickers microhardness test (Future Tech, Corp, Tokyo, Japan), with an application of 50g for 30 seconds and 5 indentations for each sample. The combinations among ceramic material shades and thicknesses and the control group resulted in 18 groups.
In order to analyze any possible interaction between factors in the study, statistical analysis was performed with two–way ANOVA (p<0.05)—5 x 3, following Tukey test with a general linear model procedure in Minitab 14 statistical software (Minitab Corporation, State College, PA, USA).
RESULTS
Table 2 presents the results of the two-way ANOVA, revealing that the effects of the independent variables and their interaction were significant (p<.0001).
Table 3 presents the means and standard deviation (SD) values of the Vickers hardness (VHN) of the groups. Data presented a normal, homogeneous distribution, enabling the factorial statistics analysis (5 x 3) and Tukey Test (p<0.05) application.
DISCUSSION
Bond strength among the ceramic restoration, resin cement and tooth structure is influenced by adequate cement polymerization. According to Cavel and others (1988), if an efficacious bond is obtained, the resin cement will offer clinical advantages in restoration strength and present extremely low solubility. On the other hand, inadequate polymerization could promote a decrease in the material's physical properties, in addition to causing pulp irritation and even devitalization due to the presence of residual monomers. In addition, insufficient hardening of the cement may lead to operative sensitivity due to washout of the unset cement material with subsequent gap development, which can predispose the tooth structure to plaque accumulation and recurrent caries (el-Mowafy & others, 1999).
The material under which the resin cement is inserted seems to exert a notable influence on the reached conversion degree (Matsumoto & others, 1986; Ferracane & others, 1986) and, according to Brodbelt and others (1980), light transmission through dental porcelain depends on the thickness of the restorative material. In accordance with the results of this study, the increase in ceramic thickness promoted a reduction in the degree of polymerization, principally for ceramic samples with a thickness of 4 mm. Placement of a ceramic restoration between the light source and resin cement changes the resin cement's degree of cure; Uctasli and others (1994) reported that, for all the resin cements tested, the increase in porcelain thickness and opacity decreased their hardness. Also, according to Brodbelt and others (1980), the ceramic material thickness will determine the amount of light that will reach the cement.
As the restoration thickness increases, there is an exponential decrease in transmitted light energy (Price, Murphy & Derand, 2000), which can be insufficient to cure a dual resin cement adequately when the restorative material is thicker than 2.5 mm. Lee and Greener (1994) related that this energy decreasing effect is promoted by photon absorption and scattering inside the ceramic body. Therefore, polymerization on deep regions of cavity preparations at the gingival floor in a proximal box, for example, relies more extensively on the self-curing component of resin cement (el-Mowafy & others, 1999). According to el-Mowafy and others (1999), ideally, dual cure resin cements should be capable of achieving a hardening degree through self-curing similar to or not much lower than that achieved through dual curing in order to ensure adequate cement polymerization in those areas where curing light is inaccessible. However, some authors reported that the self-curing component itself is not enough to ensure high hardness (Hasegawa, Boyer & Chan, 1991; Darr & Jacobsen, 1995; el-Badrawy & el-Mowafy, 1995; el-Mowafy & others, 1999). It was demonstrated in the results of this study that ceramic samples 4-mm thick in shades A3.5 and A4 promoted the lowest cement hardness levels.
Another important factor related to the degree of cement cure is the restorative material shade (Strang & others, 1987; Chan & Boyer, 1989; Breeding & others, 1991; O'Keefe & others, 1991; Barghi & McAlister, 2003). The ceramic shade markedly affected the amount of transmitted light and, as a consequence, the cement hardness of restorative material thicknesses greater than 2 mm was significantly reduced. Cardash and others (1993) showed that less energy reaches the composite when a ceramic restoration with a high chroma level is compared to the effect of a transparent glass. The pigments of the restoration are capable of absorbing light, negatively influencing the polymerization (Kanca, 1986; Barghi & McAlister, 2003). Barghi and McAlister (2003) reported that high chroma restorative materials promote an adverse effect on the hardness of the cement. According to Strang and others (1987), ceramic absorbs between 40% and 50% of the curing light, and shade does not significantly affect the amount of absorbed light for samples with thicknesses less than 1.5 mm. In addition, O'Keefe and others (1991) demonstrated that there is a greater influence of the restorative material thickness on light transmission than the restorative material shade and opacity with thicknesses higher than 1.5 mm. These findings agree with the results found in this study, because ceramic samples 1- and 2-mm thick in all shades did not significantly change hardness values and were similar to the control group and higher than the values found for samples 4-mm thick, except for shade A1.
Although dual resin cement presents a self-curing component, only the effective association with light cure can ensure clinically acceptable hardness values, principally in critical areas such as proximal boxes. In light of these results, it seems necessary to search for a way to compensate for the energy attenuation influenced by both the restorative material thickness and shade; an extended period of light curing can be used but more research is needed to confirm the suitability of this procedure.
CONCLUSIONS
In accordance with the applied methodology and after analysis of the results, it can be concluded that:
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Dual resin cement hardness after light curing depends on the interaction between thickness and shade of ceramic restoration.
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Ceramic restorations with thicknesses of 1-and 2-mm did not prejudice the cement hardness, irrespective of restoration shade.
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Ceramic restorations 4-mm thick showed that the increase in chroma saturation significantly decreased resin cement hardness.
Metallic device with three different thicknesses (1.0, 2.0 and 4.0 mm).
Experimental groups.
Restoration placed inside the metallic device just before polymerization.
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