Silane bonding of porcelain denture teeth to acrylic resin denture bases Joseph P. Moffa, D.D.S., M.S.D.,* Wayne A. Jenkins, D.D.S., M.S.Ed.,** and Richard G. Weaver, D.D.S., M.S.D.*** United States Public Health Service Hospital, San Francisco, Cali/.
P o l y m e t h y l methact3,1ate , the most commonly used denture base material, does not bond directly to untreated porcelain denture teeth. Consequently, the retention of these teeth to the denture base material is currently achieved by mechanical means, such as metal pins and diatoric holes within the porcelain teeth. These retentive devices weaken the tooth and do not prevent the leakage of fluids along the tooth-resin interface. Silane coupling agents have been used successfully in industry to produce glassreirfforced resins and to achieve adhesive bonds between organic and inorganic materials. They are currently used to coat the silica glass and other reinforcing agents contained within direct-filling composite dental resins. Paffenbarger and co-workers/ Sernmelman and Kulp,'-' and Myerson :~ have suggested that silane coupling agents may also be used to treat porcelain denture teeth. The results of their laboratory studies indicate that a high tensile-strength bond is possible between dental porcelain and the polymethyl methacrylate denture resin by treating the teeth with a silane coupling agent such as gamma-methacyl°xpr°pyltrimeth°xysilane' Many potential advantages can be gained from the development of a direct bond between the porcelain tooth and the denture base material. Retention pins and This study was approved and supported in part by Bureau of Medical Services, Division of Hospitals and Clinics, Public Health Service Project Grant No. MY-70-19. Portions of this paper were presented at the Twenty-Ninth General Session of the International Association for Dental Research, Chicago, Ill. *Dental Research Coordinator, Division of Hospitals and Clinics, Bureau of Medical Services, United States Public Health Service, Department of Health, Education and Welfare. **Assistant Denta[ Research Coordinator, United States Public Health Service Hospital, San Francisco. ***Chief of the Education, Development and Research Section, Education Development Branch, Division of Dentistry, United States Public Health Service, Department of Health, Education and Welfare.
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diatoric holes would not be required, and a solid porcelain tooth would be stronger and conceivably less costly to manufacture. Because a direct bond would increase the effective cross-sectional area of the denture base and eliminate the tooth-resin crevice which is a frequent site of clinical fracture, the denture slmuld be strengthened. A direct bond of resin to porcelain should also eliminate the percolation of oral fluids around the teeth, minimize denture odor, and improve denture hygiene and esthetics. The objectives of this laboratory and clinical study were: (1) to evaluate the effect of silane-treated porcelain denture teeth upon the strength of commonly used heat-cured and self-cured acrylic resins and (2) to evaluate the clinical reliability of dentures made with these materials. MATERIALS AND METHODS
The silane-treated and control nontreated porcelain denture teeth used in this study were provided by the same manufacturer. "~ The silane-treated teeth were standardized to possess a minimum anchorage strength to resin of 18 pounds, as determined by the test method employed by Semmehnan and Kulp. ~ Mircolont and Cop), Cast:]: were the heat-cured and self-cured resins employed for both tim laboratory and clinical phases of this study. The transverse-strength specimens and dentures constructed with the heat-cured resin were processed by means of a layered silicone rubber-mold technique as described in detail by Marcroft and associates? This investing technique was selected for the following reasons- (1) The silicone layer would minimize the transmission of stresses to the porcelain teeth from the gypsunl mold during the processing procedures; (2) the surface layer of silicone rubber on the half of the flask which held the teeth would eliminate the need for a tinfoil substitute and tim possible contamination of the bonding surface of the silane-treated teeth; and (3) the silicone rubber would permit removal of the teeth from the flask so that all traces of wax might be eliminated by washing three times in a boiling solution of water and detergent and then by rinsing in clean, hot water. The heat-cured resin was mixed according to the manufacturer's directions, using three parts of powder to one part of liquid, by volume, and was processed at 163 ° F. for eight hours. The transverse-strength specimens and dentures constructed with the self-curing resin were processed in reversible hydrocolloid molds by the fluid-resin technique as described by Shepard. "~ This technique minimized the transmission of stresses to the porcelain teeth, eliminated the use of a tinfoil substitute, and facilitated the thorough cleansing of the teeth. The self-cured resin was mixed according to the manufacturer's directions using two parts of powder to one of liquid, by volume. The dentures were then processed at 100 ° F. for one hour, under 20 p.s.i, of air pressure. Laboratory phase. The laboratory phase of the study involved the determination of transverse strength upon specimens which were 6 by 12 by 75 ram. A 2 x 3 factorial experimental design was employed which evaluated the two types of resin (self-cured and heat-cured resins) and the three types of retention, including the silane-treated *Dentsply International, York, Pa. t'The Hygienic Dental Mfg. Company, Akron. Ohio. :]:Howmet Corp., Chicago, Ill.
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j. Prosthet. .|une,Dent. 1975
I ® Fig. I, Control and test transverse-strength specimens which contained two central incisor teeth. Dimensions were 6 by 12 by 75 ram. Fig. 2. Testing jig permitted three-point loading of specimens to test transverse strength.
T a b l e I. Mean transverse strength (p.s.i.)
Denture base materials --
Heat.cured resin Self.cured resin
Controls without teeth
! 1,500 (450)* 11,700 (450)
2,350 (350) 2,650 (350)
2,000 (400) 2,200 (300)
~Standard deviations in parentheses.
teeth with no pins or diatoric holes, the conventionally retained nontreated teeth with pins, and the control specimens which were solid bars of resin without teeth (Fig. 1). There were 10 replications for each treatment combina, tion for a total of 60 specimens. The specimens were randomly coded and stored in distilled water for one year. Prior to testing, they were thermal cycled 100 times, between 4 ° and 60 ° C. A testing jig was constructed which permitted three-point loading of the specimens (Fig. 2). They were oriented so that two central incisor teeth were located on the inferior or tensile portion of the specimen. Testing was done on an Instron testing machine ~ at a crosshead speed of 5 cm. per minute. T h e transverse strength was computed from the following formula: S--
3 WL 2 hd :~
where S = transverse strength, W = maximum load before fracture, L ~ distance between supports, b = width of specimen, and d -- thickness.of specimen. Clinical phase. The clinical phase of the study involved the assessment of clinical reliability of complete dentures made with heat-cured and self-cured resins, containing eittmr silane-treated or conventionally retained nontreated porcelain teeth, ~Instron Corporation, Carlton, Mass.
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Bonding of denture teeth to bases
Fig. 3. The transverse-strength specimens containing conventionally retained nontreated porcelain teeth failed in a similar manner to that observed clinically. Fig. 4. The transverse-strength specimens containing conventionally retained nontreated porcelain teeth failed at the porcelain-resin interface.
for eight patients. A randomized scheme was employed to determine which resin would be used and which arch would contain the silane-treated teeth. Since any grinding of the silane-treated teeth would remove the surface coating, all patients were carefully selected to assure that there was sufficient intermaxillary distance to obviate the need for grinding. The dentures were evaluated initially upon deflasking and polishing and again at the end of one year to determine the number of teeth fractured or lost and the extent of cracking, crazing, and leakage around the necks of the teeth. A penetrant dye* which fluoresces brightly under ultraviolet light was employed to aid in the evaluation of cracking, crazing, and leakage around the teeth. RESULTS
Laboratory phase. "Fhe mean transverse-strength values of specimens containing silane-treated teeth, nontreated conventionally retained porcelain teeth, and control heat-cured and self-cured resins are presented in Table I. These data were analyzed by a two-factor analysis of variance. When a significant F value was encountered, the Duncan new multiple range test was used to detect significant differences between the various treatment means. The 99 per cent/eve! of confidence was employed to signify statistical significance. No significant difference was found between the transverse strengths of fl~e heat-cured and self-cured resins. However, it was noted that conventionally retained porcelain teeth significantly decreased the transverse strength of both denture base materials by approximately 82 per cent. Bars containing conventionally retained nontreated teeth fractured in a manner very similar to the typical clinical nfidline denture fracture. The line of fracture extended through the resin and around the necks of the porcelain teeth (Fig. 3). A further examination of the fracture site revealed *Zyglo, Magnaflux Corporation, Chicago, Ill.
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J. Prosthet. Dent. June, 1975
Fig. 5. The transverse-strength specimen containing silane-treated porcelain teeth failed as a result of cohesive fracture of the porcelain. Fig. 6. A fluorescent penetrant dye demonstrates a moderate-to-severe craze pattern around the necks of the porcelain teeth in heat-cured resin.
Fig. 7. A minimal craze pattern was found around the necks of the porcelain teeth in self-cured denture resin when treated with a fluorescent penetrant dye. Fig. g. Mesiodistal fracture of posterior silane-treated porcelain teeth occurred with heat-cured denture resins. (Photographed with the aid of a penetrant a~ye and ultraviolet light.) that it occurred at the porcelain-resin interface (Fig. 4). There was no difference in the mode of fracture which occurred with the heat-cured and self-cured resins when conventionally retained porcelain teeth were used. Specimens which contained the silane-treated teeth produced a different type of failure pattern. In ever), instance, with both the heat-cured and self-cured resins, the line of fracture extended through both the resin denture base material and the porcelain tooth (Fig. 5). Failure occurred as a result of cohesive fracture of the porcelain, an indication that the bond strength between the silane-treated porcelain and the acrylic resin was greater than the tensile strength of the porcelain. In spite of this excellent bond, analysis of the data revealed that both the silane-treated teeth and the conventionally retained porcelain teeth significantly decreased the strength of the resins by approximately 82 per cent. Clinical phase. Immediately after the dentures were deflasked and polished, they were examined to evaluate crazing and processing-induced fractures. Examinations of
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Table U. N u m b e r of teeth fractured during processing
.[ _ A
Table III. N u m b e r of teeth lost during first year
i Heat-cured resins Self-cured resins
Dentures 0 10
._ 1 0
heat-cured dentures which had been treated with the fluorescent penetrant dye demonstrated a characteristic moderate-to-severe craze pattern in the resin around the necks of the teeth (Fig. 6). There appeared to be no difference in this pattern when the silane-treated and nontreated teeth were compared. Less crazing of the denture was experienced with the self-cured resins as compared to the heat-cured resins (Fig. 7). Once again, there was no signficant difference between the treated and nontreated teeth. The incidence of processing-induced tooth fracture is presented in Table II. Three out of four heat-cured dentures showed evidence of fractured silane-treated teeth. The typical fracture is shown in Fig. 8. The fractures occurred in a mesiodistal direction in posterior teeth, and both hah,es of the teeth were firmly attached to the underlying resin. There were no instances of this type of fracture with the self-cured resin denture base material. The number of silane-treated teeth lost in each denture during the first year of clinical service is shown in Table III. Only one out of four heat-cured acrylic resin dentures required replacement of a silane-treated tooth during this time period. This silane-treated tooth fractured, and the portions which remained were still firefly bonded to the heat-cured resin base material. In comparison, two of the four selfcured resin dentures required replacement of silane-treated teeth; of these dentures, one lost five and one lost 10 teeth. The teeth were not fractured but were lost as a result of failure of the porcelain-resin adhesive bond. DISCUSSION Clinically, it has been observed that denture fractures are usually found in areas which are subjected to high stresses, where the denture base material is thin in crosssection, and usually through the porcelain tooth/denture base interface. There is a lack of a true bond between untreated porcelain teeth and acylic resin denture base materials. If porcelain teeth could be bonded to the denture base by means of silane,
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J. Prosthet. Dent. June, 1975
this would increase the effective cross-sectional area and, theoretically, result in a stronger denture base. This hypothesis was tested in the laboratory phase of the study. The laboratory study confirmed the clinical observation that untreated porcelain teeth weaken the denture base. Although there was evidence of a bond between the heat-cured and self-cured resin denture base materials and the porcelain teeth, the silane treatment did not strengthen the denture base material. Apparently, the composite is weakened as a result of stress concentrations at the interface, the differential in the coefficients of thermal expansion of the porcelain and resin, and stresses induced within the porcelain as a result of polymerization shrinkage of the acrylic resin. The clinical phase of the study demonstrated that silane-treated teeth presented a serious problem of tooth fracture when a heat-cured resin was employed. Differences in the coefficients of thermal expansion of the porcelain and resin and the high temperature gradient involved in the heat-curing procedure were probably responsible for the high inciderice of tooth fracture. Although it appeared that the use of self-cured resin was a solution to the problem of silane-treated tooth fracture, the subsequent one-year evaluation demonstrated a high incidence of tooth loss due to a failure of the bond. SUMMARY AND CONCLUSIONS
A laboratory and clinical evaluation of silane bonding of porcelain denture teeth to heat-cured and self-cured acrylic resins was conducted. There was no significant difference in the transverse strength of these resins. Conventionally retained nontreated porcelain teeth significantly decreased tile strength of these resins by approximately 82 per cent. The silane bonding of porcelain teeth to these denture base materials did not effectively strengthen the acrylic resins. There was a greater incidence of fractured silane-treated teeth when dentures were processed with heat-cured as compared to self-cured acrylic resin. There was a greater incidence of loss of teeth due to failure of the silane bond with self-cured as compared to heat-cured resin. There was no observabledifference in the stain resistance between the silane-treated and conventionally retained nontreated porcelain denture teeth. The problems of tooth fractures induced by processing and loss of teeth due to bond failure, the lack of difference in strain resistance, and lack of evidence of strengthening of the denture base point to one conclusion. The present state of the art of silane treatment of porcelain denture teeth offers no advantage over conventionally retained porcelain denture teeth. The contribution of materials, supplies, and technical assistance from the Dentist's Supply Company, The Hygienic Dental Manufacturing Company, and the Howmet Corporation is gratefully acknowledged. References
1. Paffenbarger, G. C., Sweeney, W. T., and Bowen, R. S.- Bonding Porcelain Teeth to Acrylic Resin Denture Bases, J. Am. Dent. Assoc. 78: 1018-1023, 1969. 2. Semmelman, J. O., and Kulp, P. R.: Silane Bonding of Porcelain Teeth to Acrylic, Int. Assoc. Dent. Res. Abst. No. 356, March, 1967.
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3. Myerson, R. S.: Effect of Silane Bonding of Acrylic Resins to Porcelain on Porcelain Structure, J. Am. Dent. Assoc. 78: 113-119, 1969. 4. Marcroft, K. R., Tencato, R. L., and Hurst, W. W.: Use of a Layered Silicone Rubber Mold Technique for Denture Processing, J. PROSTH~T. DENT. I1: 657-664, 1961. 5. Shepard, W. L.: Denture Bases Processed From a Fluid Resin, J. PROSTHET. DEI';T. 19: 561-572, 1968. DRS. MOFFA AND JENKINS DEPARTMENT OF HEALTH, EDUCATION AND WELFARE UNITED STATES PUBLIC HEALTH SERVICE I-IosPITAL 15TH AVE. AND LAKE ST, SAN FRANEISCO~ CALIF, 94118
DR. WEAVER FEDERAL BLDG., RI~I. 204 7550 W~SCONStN AVE. BETHESDA, MD. 20014