Photo-activation of Resin Cements through Porcelain Veneers J.J. LINDEN, E.J. SWIFT, Jr.2, D.B. BOYER, and B.K. DAVIS1 College of Dentistry, The University of Iowa, Iowa City, Iowa 52242; and 'College of Dentistry, The Ohio State University, Columbus, Ohio 43210
The purpose of this study was to determine the effects of porcelain opacity, chemical catalyst, and exposure time on polymerization of light-activated resin-composite cements. Samples of microfill and hybrid composites, with and without catalyst (i.e., dual-cure and visible-light-activated), were polymerized by exposure to visible light through porcelain discs of different opacities. Microhardness testing (KHN) was used to compare degree of cure for each material at various exposure times. Porcelain opacity did not significantly affect hardness. However, the results indicated that a chemical catalyst and prolonged curing times might be essential for clinical success. J Dent Res 70(2):154-157, February, 1991
Introduction. Porcelain veneers have recently become a popular treatment modality for many esthetic dental problems, primarily discolored anterior teeth. The concept of esthetic porcelain facings originated during the 1930's and 1940's, when some actors used temporary porcelain veneers to alter the appearance of their teeth during performances (Garber et al., 1988). Several recent advances in dental bonding technology have led to the evolution of successful, permanently-cemented porcelain laminate veneers. The acid-etch technique for bonding resin to enamel, first introduced by Buonocore (1955), is the foundation for the modern porcelain veneer technique. Predictable bonding of resin composite to porcelain has also been developed through the use of hydrofluoric acid etching solutions and silane coupling agents (Newburg and Pameijer, 1978; Horn, 1983; Simonsen and Calamia, 1983; Calamia and Simonsen, 1984; Stangel et al., 1987; Lacy et al., 1988). Therefore, adhesion of porcelain to enamel can be accomplished by bonding with an intermediary layer of resin composite (Calamia, 1985; Garber et al., 1988). The composites specifically used for porcelain veneer cementation are activated by visible light, although some also include a chemical-cure component (Nixon, 1987; Garber et al., 1988). The degree of monomer conversion in a resin composite can be estimated by measuring its Knoop hardness (Rueggeberg and Craig, 1988; Chung, 1990). The hardness and degree of conversion of light-activated composites decrease with depth from the exposed surface. This effect is caused by attenuation of light by the composite (Cook, 1980; Council on Dental Materials, Instruments, and Equipment, 1985; McCabe and Carrick, 1989). The hardness of the resin is related to the intensity of the light source (Cook, 1980; Watts et al., 1984; Council on Dental Materials, Instruments, and Equipment, 1985; Fan et al., 1987). Exposure time also has an effect, since increased exposure time can compensate for reduced light intensity and increases the depth of cure (Cook Received for publication August 8, 1990 Accepted for publication October 31, 1990 2To whom correspondence and reprint requests should be addressed 154
and Standish, 1983; Council on Dental Materials, Instruments, and Equipment, 1985). The polymerization of light-activated resin composite beneath porcelain has been examined in only a few studies. Porcelain attenuates light to various degrees, depending on the thickness and shade of the porcelain (Brodbelt et al., 1980). Blackman et al. (1990) have reported that thick ceramic specimens prevent complete polymerization of resin cements. Some porcelain veneer texts and many clinicians recommend 60-second exposure times for adequate polymerization of resin-composite luting agents (Nixon, 1987). This recommendation is based on empirical clinical evidence rather than on data from controlled research. Strang et al. (1987) have reported that longer exposure times are probably required. Chan and Boyer (1989) recently recommended some exposure guidelines based on variations in porcelain thickness and shade. One factor not considered by Chan and Boyer was the opacity level of porcelain. Porcelain opacity is an important clinical consideration because veneers are often used to mask severely discolored teeth. A common method for fabricating maximum opacity veneers is to use only an opaque veneer porcelain, Ceramco IIG (Ceramco, Inc., Burlington, NJ). Veneers are made more translucent when this veneer porcelain is mixed with a standard crown and bridge porcelain, Ceramco II. A minimum opacity veneer is fabricated with an equal mixture, while a moderately opaque veneer is fabricated with a blend of 75% Ceramco IIG and 25% Ceramco II (Nixon, 1987). Depth of cure of the resin cement is generally not a concern with porcelain veneers, since a relatively thin resin layer is assumed. Also, thickness of porcelain is usually not a concern, because most veneers are 0.5-1.0 mm thick. Porcelain shade is frequently of little concern either, since most veneers are fabricated in relatively light shades, e.g., Vita A-1. Therefore, the porcelain opacity level could have a critical effect on polymerization of the underlying resin composite. The primary purpose of our study was to evaluate the effect of porcelain opacity on curing of composite when porcelain shade and thickness were held constant. We also examined the effects of resin composite type, curing method, exposure time, and time of measurement on resin polymerization.
Materials and methods. Refractory dies were used to make three porcelain discs, 16 in diameter and 0.75 mm thick, in a Vita A-1 shade. The discs were sanded to a flat surface by hand-grinding on wet 220-, 400-, and 600-grit silicon carbide paper. The final thickness of each porcelain disc was approximately 0.70 mm. The following porcelains were used to fabricate discs with different levels of opacity: (1) Ceramco IIG (opaque veneer porcelain) only, (2) a mixture of 75% Ceramco IIG with 25% Ceramco II translucent porcelain, and (3) a mixture of 50% Ceramco IIG with 50% Ceramco II translucent. The resin luting agents were placed in Plexiglass molds 5 mm in diameter and 0.75 mm thick. The molds were placed on a clear Mylar strip over dentin discs to provide proper reflectance. Another Mylar strip was placed over the composmm
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PHOTO-ACTIVATION OF RESIN CEMENTS
Vol. 70 No. 2
ite, with a porcelain disc placed on top. The composite was polymerized through the porcelain by a Prismetics visible-lightcuring unit (Caulk/Dentsply, Milford, DE) with a 13-mm tip at exposure times of 30, 60, 90, and 120 s (Fig. 1). The following resin composite luting agents were used: (1) Porcelite (Kerr/Sybron, Romulus, MI), a 67 wt% filled light-activated hybrid resin; (2) Porcelite with catalyst (dual-cure); (3) Heliolink (Vivadent USA, Inc., Tonawanda, NY), a 40% filled light-activated homogeneous microfill; and (4) Heliolink with catalyst (dual-cure). To reduce potential variation from composite shades, the lightest available shades ("whitish" Heliolink and untinted Porcelite) were used. Based on the various combinations of porcelain opacity, type of resin composite (by brand and polymerization method), and exposure time, there was a total of 48 experimental groups. Each group contained three samples. The surface hardness of each sample was measured with a Micromet II microhardness tester (Buehler, Ltd., Lake Bluff, IL) in three locations at 30 min and at 24 h after exposure. The samples were stored dry at room temperature before the second measurement. In addition, samples of the four composite/catalyst types were made with direct (i.e., not through porcelain) visiblelight exposure for five min. The microhardness of these samples represented the maximum attainable hardness for each composite/catalyst combination. Total transmission of the porcelain discs was measured with a Lambda 4B UV/VIS Spectrophotometer (Perkin-Elmer, Norwalk, CT), a dual-beam spectrophotometer with an integrating sphere. A wavelength of 470 nm was used because maximum absorption by camphoroquinone, the photo-initiator in resin composites, occurs at this wavelength (Cook, 1982).
transmission than the opaque disc. However, transmission of light at 470 nm was similar for all three discs (Table 1). The disc fabricated entirely with opaque porcelain had the lowest transmission percentage, 48.2%, at this wavelength. Mean hardness values for each of the four composite/catalyst groups are shown as a function of light transmission and exposure time in Figs. 3 and 4, respectively. The effects of measurement time are illustrated in Fig. 5. The data were subjected to an analysis of variance (ANOVA) that used the general linear models procedure of the SAS statistical software package (SAS Institute, Cary, NC). ANOVA showed significant hardness differences based on type of resin composite (p