eet of adhesive luting tions on film thickness hane
University of Southern California, School of Dentistry, Los Angeles, Calif. and Research Institute of Stomatology, Beijing Medical University, Beijing, China This study investigated the effect of luting agent-dentinal surface interactions on the film thicknesses of new adhesive luting agents. The method was in compliance with American National Standards Institution/American Dental Association (ADA) Specification No. 8 for zinc phosphate cement. In the control groups the luting agents were placed between two glass plates, as described in ADA Specification No. 8, but in the test groups the luting agents were positioned between a glass and a dentinal plate. The materials selected were zinc phosphate cement, glass ionomer cement, polycarboxylate cement, and a resinous cement with a dentinal bonding agent. A two-way analysis of variance was performed, and t tests were computed to compare glass with the dentinal plate within each material. Zinc phosphate and glass ionomer cements exhibited a significant decrease in film thickness when measured in contact with dentin, as compared with contact with the glass plate. However, polycarboxylate cement and the resinous cement with its dentin bonding agent showed a slight nonsignificant increase when contacting dentin, as compared with the glass plate. An explanation was offered and suggestions were made regarding future research.(J PROSTIIETDENT1992;68:49-52.)
inc phosphate cement has been the principal luting agent for castings, but recently several new classes of adhesive luting agents (ALAS) have been deve1oped.l These luting agents include glass ionomer cements, polycarboxylate cements, and various resinous cements that are commonly Bis-glycidylmethacrylate @is-GMA) derivatives with some inorganic filler.2-4 Surface interactions between ALAs and the tooth may be more critical than those for zinc phosphate cements, because many of these materials are purported to interact chemically and physically with tooth structure. When compared with zinc phosphate cements, the ALAS have demonstrated less marginal microleakage and greater retention.ss 6 The recorded film thicknesses of many ALAS have been comparable to those of zinc phosphate cement.7 However, these film thicknesses have been measured according to American National Standards Institute/American Dental Association (ANSI/ ADA) specifications that positioned the luting agent between two glass p1ates.s Jorgensen9 has reported that the film thickness of zinc phosphate is dependent upon hydraulics and surface roughness. This study investigated the effect of dentinal surface interactions on the film thickness of new adhesive luting agents.
aResearch Assistant and Professor, Department of Restorative Dentistry, USC School of Dentistry. bAssociate Professor, Department of Dental Materials, Research Institute of Stomatology, Beijing Medical University.
The experimental method was in broad compliance with ANSI/ADA Specification No. 8 for zinc phosphate cement, section 126.96.36.199 Tests were conducted at a room temperature of 23’ C and with a relative humidity of 67 9%) and all materials were manipulated exactly as described by the manufacturer. The encapsulated glass ionomer cement was mixed in an amalgam triturator (Capmix, ESPE, Seefeld/ Oberbay, Germany. Before testing, two experienced researchers calibrated their techniques until uniformity was confirmed, but one investigator then made all the test specimens. In the control group, a portion of the standard mix of cement was placed between two flat round glass plates of uniform thickness, with a surface area of 2 cm2. The same pair of plates was used in an identical orientation for each measurement. Before each specimen was made, the plates were cleaned with water and acetone, and were allowed to volatilize. At the termination of each manufacturer’s recommended mixing time, a load of 15 kg was applied vertically to the plates. In the experimental group the cement was secured between a glass slab and a disk of mature bovine dentin. The bovine dentin was freshly extracted and stored in water at 37’ C. The dentin was then finished using with 600 grit silicon carbide paper, with new dentinal specimens for each measurement. An electronic gauge (Minicon Electronic Gage, Seimitsu, Tokyo, Japan), with an accuracy of 0.5 pm and a scale range of 300 pm, was used for measurements that were recorded to the nearest micron. All eight measurements were initiated 10 minutes
I. Means and standard deviations* of film thicknessesin microns
Flecks Z.P. Thin film cement with Tenure Ketac-Cem Durelon
to glass (2.7)
Z.P., Zinc phosphate. *Values in parentheses.
after mixing beganand the instrument wasrecalibrated at the 15 kg load on the cleanedglassplates before each recording. The materials tested were: zinc phosphate cement (Flecks Zinc Phosphate, Keystone, Cherry Hill, N.J.); resinouscement with a dentin bonding agent (Den-Mat Thin Film Cement and Tenure, Den-Mat, Santa Maria, Calif.); aglassionomercement (Ketac-Cem Caplet, ESPE, Seefeld/ Qberbay, Germany); and a polycarboxylate cement (Durelon, ESPE). The meanfilm thicknessand its standard deviation were calculatedfor eachALA for glassand dentinal plate groups. A two-way analysisof variance (ANOVA) wascalculatedto determine if significant differences between test groups were related to material or substrate surface. t Tests were computed for each material to establishwhether the glass and dentinal plate groups were statistically different. RESULTS Mean film thicknessesand their standard deviations are listed in Table I and Fig. 1. A two-way ANOVA revealed
Thin Film Cement with Tenure
1. Film meansand standard deviations in microns. I, Standard deviation.
that the material and the material-substrate interaction did exert a significant influence on film thickness (p < 0.0001)but the substrate alone did not (p = 0.6). Table II lists the results of the t tests, examining each material individually. Both the zinc phosphate cement group and the glass ionomer cement group had significantly (p < 0.05) lower film thicknesseswhen tested contacting one dentinal surface than against a glasssurface. DISCUSSION The results indicated that when tested against dentin and not glass,someluting agents(zinc phosphateand glass ionomer cements)exhibited significantly lower film thicknesses.The polycarboxylate and resinouscementsdid not display a statistically significant increase,but did record a tendency toward increasedfilm thicknesses. The two-way ANOVA did not identify an overall statistical significance resulting from the substrate surface because some of the materials decreased while others increased,cancelingeachother’s effects. In retrospect, the two-way ANOVA should not have beenapplied to the substrate surface variable. The dentinal groupsgenerally exhibited greater variance than the respective glassgroups (Table I, Fig. 1). This was expected, becauseit is improbable that extensive dentinal surfacescould be prepared as uniformly as polished glass surfaces.With more specimensthis variance might be sufficiently small to allow discrimination between glassand dentin within more materials. ADA Specification No. 8 recommendstwo glassplates as the substrate surfacesfor determination of film thickness. This experimental designhas been questioned becauseit poorly simulates cementation on dentin, but unfortu-
mately there is no alternative to ADA Specification No. 8. Any test specification must be clearly definable, predictable, easy to reproduce, and have a wide range of tolerance for investigator error. The dentinal specimens in this study were difficult to obtain, because only one or two specimens were obtained from each cow mandible. Many bovine molar and premolar teeth were ground flat, only to be discarded because of an inadequate surface area or thickness of satisfactory dentin. Despite our being highly discriminating and careful, the dentinal samples resulted in greater standard deviations than the glass plate samples. Adhesion to dentin is unpredictable and is influenced by a wide range of variables.lO A similar protocol should be performed in another laboratory to confirm these results and determine if the findings apply merely to specific materials or to the entire classification of materials. A greater number of samples in each group could verify the tendency in this study for the film thicknesses of some materials to increase when contacting a dentinal surface. It is also strongly recommended that the effect of ALA-alloy surface interactions be investigated. The diminished film thickness of the glass ionomer and zinc phosphate cements against dentin can offer clinical advantages in the seating of cast restorations. Unfortunately, the data relating to the seating of cast restorations originated from the response of zinc phosphate cement to metal dies. Therefore it is suggested that this body of knowledge be extended to include various ALAS and their behavior, as compared with zinc phosphate cement on dentinal tooth preparations. The mechanism by which the film thicknesses was decreased is unknown. Zinc phosphate and polycarboxylate cements both contain a large amount of zinc oxide powder, and some of this powder remains as incompletely dissolved particles in the set cement. Resinous and glass ionomer cements both contain ceramic particles, so it is unlikely that the filler component offers an explanation. All of the materials tested contain or involve treatment with acid. Zinc phosphate cements contain phosphoric acid, while glass ionomer and polycarboxylate cements contain polyacrylic acids. The first stage of the Tenure dentin bonding agent involves application of 2.5% nitric acid to remove the smear layer, which is then carefully washed away with the acid. Therefore the mere application of acid, or smear layer removal, is probably not the sole mechanism. One possible explanation is related to the initial acidity of the unset cement. Ketac Cem glass ionomer cement has a pH of 1.5 at 1 minute after mixing, while Flecks zinc phosphate has a pH of 2.1, and Durelon has a pH of 3.4.11Perhaps this initial acidity allowed the aqueous zinc phosphate and glass ionomer cement solutions to increase the wetting and solubilization of the moist dentin surface, thus reducing film thickness. The pH of Ketac Cem glass ionomer cement rises
Tests dentin versus glass Mean difference
Flecks Z.P. Thin film cement + Tenure Ketac-Cem Durelon Abbreviations
Paired t value
as in Table I.
to 3.4 in approximately 8 minutes, and the pH of Flecks zinc phosphate rises to this level in approximately 14 minutes.ll Although the superficial smear layer was removed with nitric acid in the Tenure system, the dentin was only exposed to acid for 30 seconds before rinsing. The resinous cements in the Tenure system, N-phenyl glycyl-glycidylmethacrylate (NPG-GMA) and polymethyl-(PMDM), are only slightly hydrophilic compared with the aqueous ALAS. The glass ionomer and zinc phosphate cements sustained a sufficiently low pH for a suitable period to affect “self etching,” and combined with their aqueous nature, encouraged a low film thicknesses. Some dentists have reported sensitivity after cementation of castings with glass ionomer cement and several explanations have been postulated. These include moisture contamination of the unset glass ionomer cement matrix, desiccation of the tooth, water sorption by the setting matrix from the tooth, low initial pH, and hydraulic pressure in the dentinal tubules. Several of these possible mechanisms may be related to a “self-etching” phenomenon, but they require additional research. Proportioning and mixing times of glass ionomer cements must conform exactly to the manufacturer’s recommendations, castings must not be overfilled, and water must be removed from the tooth surface but definitely without removing water from within the dentinal structure. It is also crucial to protect the excessive unset cement with a varnish around the margins, because this protects the luting cement from moisture during the set.
SUMMARY The film thicknesses of zinc phosphate and glass ionomer cements were significantly reduced when tested in contact with a dentinal surface instead of a glass plate. An explanation is offered, and additional studies of this phenomenon are encouraged. The specific effect of this research on the seating of a fixed prosthesis should also be investigated. REFERENCES Fleck H. The chemistry of oxyphosphates. Dent Items 1902;24:906-17. 2. Smith DC. A new dental cement. Br Dent J 1968;125:381-92. 1.
3. Wilson AD, Kent BE. The glass ionomer: a new translucent dental filling material. J Appl Biotech 1971;21:213-21. 4. Bowen RL, Cobb EN, Rapson JE. Adhesive bonding of various materials to hard tooth tissues: improvement in bond strength to dentin. J Dent Res 1982;61:1070-6. 5. White SN, Sorensen JA, Kang SK, Caputo AA. Microleakage of new crown and fixed partial denture luting agents. J PROSTHET DENT 1992;
9. Jorgensen KD. Factors affecting the film thickness of zinc phosphate cements. Acta Odontol Stand 1960;18:479-90. 10. Rueggeberg FA. Substrate for adhesion testing to tooth structure-review of the literature. Dent Mater 1991;7:2-10. 11. Charlton DG, Moore BK, Swartz ML. Direct surface pH determinations of setting cements [Abstract]. J Dent Res 1991;70:567. Abstract #2406. Reprint
6. Tjan AJ, Li T. Retention and fit of crowns cemented with an adhesive resin [Abstract]. J Dent Res 1990;69:123. 7. White SN, Yu Z. Film thickness of new adhesive luting agents. J PROSTHET DENT (In press; 1992) 8. Certification programs of the Council on Dental Materials, Instruments and Equipment. Chicago: American National Standards Institution/ American Dental Association. Specification No. 8 1977.
DR. SHANE N. WHITE DIRECTOR OF RESTORATIVE’RESEARCH DEPARTMENT OF RESTORATIVE DENTISTRY USC SCHOOL OF DEKTISTRY
925 WEST 34~~ ST No. 4367 Los ANGELES, CA 90089.0641
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