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Adhesion of denture tooth porcelain to heat-polymerized denture resin
A d h e s i o n of denture tooth porcelain to heat-polymerized denture resin B a l d w i n W. M a r c h a c k , D D S , a Z h a o k u n Yu, b X i a o Y u Z h a o , P h D , c a n d S h a n e N. W h i t e , B D e n t S c , MA, M S d
University of Southern California, School of Dentistry, Los Angeles, Calif. Use of porcelain denture teeth m a y be desirable in m a n y clinical situations, i n c l u d i n g implant-supported prostheses. However, lack of space because of frameworks often precludes the use of conventional retention by diatorics and pins. Adhesion of porcelain denture teeth to denture resin could also stiffen and possibly strengthen dentures and decrease stain ingress b e t w e e n porcelain teeth and resin denture bases. Unlike previous studies that investigated the b o n d b e t w e e n conventional feldspathic metal-ceramic porcelain and bis-GMA based composite resin, this study investigated adhesion of denture tooth porcelain to p o l y m e t h y l methacrylate (PMMA). High-energy air abrasion, hydrofluoric acid etching, and the use of a general purpose b o n d i n g agent resulted in an i m p r o v e d bond strength of heat-cured denture PMMA b o n d e d to denture tooth porcelain. Silane coating did not improve bond strengths, and conventional air abrasion was no more effective than polishing w i t h 600-grit silicon carbide. Storage in water and artificial aging substantially decreased b o n d strengths. The strongest b o n d strengths w e r e achieved by a high-energy-abrasion + etching + multiple-purpose bonding-agent treatment, but a simpler etching + multiple-purpose bonding-agent treatment also produced reliable results. A laboratory technique was suggested. The role of surface treatment in the m e c h a n i s m of adhesion was e x a m i n e d w i t h s c a n n i n g electron microscopy. High-energy abrasion p r o d u c e d a slightly more detailed initial topography than conventional air abrasion, but after etching, the high-energy topography became m u c h more detailed. Surface topography alone did not account for all differences found. (J PROSTHETDENT 1995;74:242-9.)
S o m e dentists prefer porcelain denture teeth to resin- based teeth, 1"3 but they do not bond to polymethyl methacrylate (PMMA) denture base resin. 4 Porcelain denture teeth are normally joined to acrylic resin denture bases by mechanically retentive features such as metal pins or diatoric undercuts. Unfortunately, these retentive features often have to be adjusted or removed because of lack of space. Lack of space may be the result of implant frameworks and bars, pendulous tuberosities, overdenture abutments, or lack of interarch distance. Adhesion of porcelain denture teeth to acrylic resin could stiffen and strengthen dentures. Stresses are often concentrated around denture teeth and result in the fracture
Presented at the Academy of Prosthodontics meeting, Orlando, Fla., May 1994. aAssociate Clinical Professor, Advanced Prosthodontics, Department of Restorative Dentistry. bAssociate Clinical Professor, Department of Restorative Dentistry/Biomaterials. CResearch Associate, Department of Restorative Dentistry/Biomaterials. 4Assistant Professor and Director of Clinical Research, Department of Restorative Dentistry/Biomaterials. Copyright 9 1995 by The Editorial Council of THE JOURNALOF PROSTHETIC DENTISTRY.
0022-3913/95/$3.00 + 0. 10/H65513
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of denture bases. However, a strong bond would allow stress to be transferred from resin to tooth and thus decrease crack propagation and strengthen the dentures. Stain ingress to cracks between porcelain teeth and resin denture bases, and subsequent mouth odor, would be decreased by adhesion of acrylic resin to these teeth. Masticatory stresses are often localized to and concentrated around retentive pins and thus increase the probability of fracture. Distribution of stresses over a greater area by adhesion of the tooth to the resin would decrease the possibility of failure. Adhesion to porcelain teeth should be investigated. Silane coupling agents were shown to improve adhesion of denture teeth to PMMA in the 1960s, 5 but the adhesion of PMMA to denture tooth porcelain has not been studied comprehensively. Recently, adhesion of bis-GMA to feldspathic metal-ceramic porcelain has been enhanced by abrasion, etching, and bonding agents. Therefore, this study attempted to find a reliable method of producing durable adhesion of PMMA to denture tooth porcelain. MATERIAL AND METHODS Maxillary right central incisor porcelain denture teeth of the same mold and shade (Trubyte Bioform, mold 21J, shade B81, Dentsply International Inc., York, Penn.) were obtained. These teeth were made without pins so they
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shearing load
treated surface porcelain _ denture tooth
v
acrylic resin adhesive tape
Fig. 1. Schematic of test specimen.
could be ground to a flat surface without exposing metal and so that stresses would not be concentrated around the pins. The teeth were embedded in epoxy resin (Epoxy Resin, Hastings Plastics, Santa Monica, Calif.) contained in phenolic rings (Buehler Ltd., Lake Bluff, Ill.). The epoxy resin was cured for 72 hours at room temperature. Then the specimens were ground with a series of abrasives (carborundum grit sizes 80 through 600) for uniform flat polished porcelain surfaces (Fig. 1). A fully randomized block experimental design was used. Twenty-four groups were compared with respect to the factors of surface air abrasion (none, conventional, or high-energy), acid-etching (none or etched), silane coating (none or silane), and a bonding agent (none or bondingagent). The surfaces were abraded with either a laboratory sandblaster (Integral Systems Inc., Culver City, Calif.) (90 psi N2, 50 pm alumina), with a high kinetic energy sandblaster (KCP 2000, American Dental Technology, Troy, Mich.) (90 psi N2, 50 pm alumina), or not abraded. The specimens were then either acid etched (10% hydrofluoric acid, Ultradent Products Inc., Salt Lake City, Utah), or not etched. After the surfaces were abraded and/or etched or not treated, the areas for bonding were isolated by use of adhesive tape (Scotch Tape, 3M, St. Paul, Minn.) with a prepunched circular hole (4 mm diameter) (Fig. 1). A silicone wax mold-release agent was then carefully painted on the tape but not on the exposed denture tooth surfaces. The bond test areas were isolated to prevent excess flash from adhering to the surrounding denture tooth, which would produce inflated bond strengths. After isolation of the test area, the bonding procedures were initiated. The exposed tooth surfaces were then either treated with a silane coupling agent (Silane, Ultradent Products Inc.) according to manufacturer's recommendations or the surfaces were left untreated. The exposed tooth surfaces were then coated with a multipurpose bonding agent (Tenure S, Den-Mat, Santa Maria, Calif.), according to the manufacturer's instructions, or not coated. 6 Five specimens were made for each of the 24 possible abrasion/ etctdsilane/bonding agent groups. The treated specimens were then placed in polyvinyl si-
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loxane molds (Express, 3M) in the lower parts of brass denture flasks (Teledyne Hanau, Buffalo, N. Y.). The upper parts of the flasks contained polyvinyl siloxane buttonshaped molds (5 mm diameter, 2 mm high) that were centered over the samples. Five specimens were placed in each flask. Heat-polymerized PMMA denture resin (Lucitone 199, Dentsply International Inc.) was mixed according to the manufacturer's recommendations until it reached a doughy consistency. Pea-sized pieces were placed in the button-shaped molds and the flasks were closed slowly in a screw-type press (Teledyne Hanau). The specimens were then cured according to manufacturer's recommendations in a water-filled curing tank (Teledyne Hanau), and after 8 hours of curing, the specimens were carefully removed from the polyvinyl siloxane molds and placed in water at 37 ~ C for 16 hours. After storage in water, the specimens were mounted in a servohydraulic universal testing machine (Instron, Canton, Mass.), and the resin-denture tooth interface was tested in shear at a load rate of 0.05 cm/minute (Fig. 1). Failure loads were plotted with a chart recorder. Mean failure loads and their standard deviations were calculated for each abrasion/etch/silane/bonding-agent group. A four-way analysis of variance (ANOVA) was computed to determine whether the main effects of abrasion, etching, silane, and bonding agent, and their interactions had significant effects on bond strength (p < 0.05). If the abrasion had a significant effect, a multiple comparisons test (Tukey's honestly least significant difference method) was used to determine which of the three abrasion subgroups (conventional, high-energy, or none) were effective (p < 0.05). The main effects that significantly improved bond strength were then further evaluated with extended storage and artificial aging to compare the four most promising treatment combinations, with a sample size of seven specimens per group. Specimens were made as previously described, but stored for 7 days in 37 ~ C water after fabrication and then artificially aged by thermal cycling (2000 cycles from 5~ C to 50 ~ C with a dwell time of 30 seconds and a transfer time of 20 seconds). After storage and aging, the specimens were tested as previously described. The group means and their standard deviations were calcu-
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F i g . 3. T h r e e - d i m e n s i o n a l b a r c h a r t of stored a n d artificially aged m e a n b o n d s t r e n g t h s in M P a . S a m e k e y as Fig. 2. F i g . 2. T h r e e - d i m e n s i o n a l b a r c h a r t of 24-hour m e a n bond s t r e n g t h s in M P a . HA, H i g h - e n e r g y abrasion; CA, c o n v e n t i o n a l abrasion; E, etching; S, silane; B, bonding; NONE, no t r e a t m e n t . I. T w e n t y - f o u r - h o u r s h e a r bond s t r e n g t h s a n d s t a n d a r d d e v i a t i o n s (SDs)
Table
Abrasion
None
Bonding agent
Mean
Etching
Silane
-
-
-
-
+
16.7
-
+
-
01.9
0.4
-
+
+
+ + + +
+ +
+
14.3 10.8 21.5 12.4 18.9
2.0 4.2 3.8 1.2 3.8
-
-
-
-
+
-
+
-
13.6 14.6 09.2
-
+
+
12.7
+ +
+ +
10.4 09.4 14.3 16.5
3.8 1.6 2.0 1.4 2.6 2.0 3.4 3.4
06.3 21.9
2.0 1.8
+
(MPa)
SD
02.4
0.2 1.6
l a t e d and a o n e - w a y A N O V A w a s c o m p u t e d to d e t e r m i n e w h e t h e r t h e g r o u p s w e r e d i f f e r e n t (p < 0.05) a n d a m u l t i ple c o m p a r i s o n s t e s t d e t e r m i n e d w h i c h groups w e r e simi l a r (p < 0.05). To u n d e r s t a n d t h e influence of s u r f a c e t r e a t m e n t on t h e m e c h a n i s m of a d h e r e n c e , s c a n n i n g electron m i c r o g r a p h s (SEMs) w e r e m a d e of a b r a d e d a n d / o r etched d e n t u r e t o o t h surfaces. A s c a n n i n g electron microscope w i t h a n e n e r g y d i s p e r s i v e s p e c t r o m e t e r (EDS) ( C a m b r i d g e 360, C a m b r i d g e I n s t r u m e n t s , C a m b r i d g e , U. K.) w a s u s e d to m a k e t h e i m a g e s at a m a g n i f i c a t i o n of • w i t h a n e n e r g y of 10 k V a n d digitally store t h e m . T h e m i c r o g r a p h s w e r e l a t e r p r i n t e d on P o l a r o i d film (Polaroid, C a m b r i d g e , Mass.). RESULTS
Conventional
+ + + + High-energy
244
-
-
-
-
-
+
-
+
-
+
+
17.5
+ + + +
+ +
+ +
20.7 22.6 23.2 10.6
15.4
1.6
5.4 2.8 2.4 6.2 2.4
Twenty-four hour mean shear bond strengths and their s t a n d a r d d e v i a t i o n s a r e s h o w n in T a b l e I and Fig. 2. T h e y r a n g e d f r o m 1.9 M P a for no a b r a s i o n + no e t c h i n g + silane + no b o n d i n g - a g e n t to 23.2 M P a for h i g h - e n e r g y a b r a s i o n + e t c h i n g + silane + no bonding-agent. F o u r - w a y A N O V A r e v e a l e d t h a t abrasion, etching, a n d b o n d i n g a g e n t all significantly i m p r o v e d 24-hour bond s t r e n g t h s (p < 0.0001) (Table II). H o w e v e r , the silane coup l i n g a g e n t did n o t i m p r o v e 24-hour s h e a r bond s t r e n g t h s (p 0.58) (Table II). A m u l t i p l e c o m p a r i s o n s t e s t i n d i c a t e d t h a t h i g h - e n e r g y a b r a s i o n was s u p e r i o r a n d c o n v e n t i o n a l a b r a s i o n w a s s i m i l a r to no a b r a s i o n (p < 0.05). T h e m e a n s a n d s t a n d a r d d e v i a t i o n s of the four artificially aged g r o u p s a r e p r e s e n t e d in Fig. 3 and in T a b l e III. T h e y r a n g e d from 3.0 M P a for h i g h - e n e r g y a b r a s i o n + no
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Fig. 4. SEM of glazed surface of porcelain denture tooth as supplied by m a n u f a c t u r e r (original magnification • All micrographs were made from perpendicular views without stage tilt. All black magnification bars represent 10 pm.
F i g . 5. SEM of polished porcelain (original magnification
xlO00).
T a b l e IL Four-way ANOVA for 24-hour shear bond strength S o u r c e of variation
Main effects Abrasion (A) Etching (E) Silane (S) Bonding (B) Interactions A- E A. S A- B E -S E 9B S -B A. E 9S A. E 9B A. S - B E 9S 9B A.E.S.B
S u m of squares
Degrees of freedom
Mean square
618 416 003 665
2 1 1 1
309 416 003
240 032 592 005 356 168 312 214 364 001 002
2 2 2 1 1 1 2 2 2 1 2
e t c h + b o n d i n g agent to 13.4 MPa for high-energy abrasion + etching + bonding-agent. One-way ANOVA confirmed that differences among the groups existed (Table IV), and multiple comparisons testing demonstrated that all four groups were dissimilar (p < 0.05). Representative micrographs illustrated that different surface t r e a t m e n t s had distinct effects on surface topography (Figs. 4 through 10). The polished surface was uniformly flat, but not completely smooth. Some scratches produced by the silicon carbide abrasive were visible, and occasional defects were seen (Fig. 5), which suggests t h a t d e n t u r e tooth porcelain may have a greater crystalline component t h a n feldspathic metal-ceramic porcelain and t h a t polishing with silicon carbide produces noticeable
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F-ratio
Significance level
666
28.5 38.4 00.3 61.3
<0.0001 <0.0001 0.5830 <0.0001
120 016 296 005 356 168 156 107 182 001 001
11.1 01.5 27.3 00.5 32.8 15.5 14.4 09.9 16.8 00.1 00.1
<0.0001 0.2279 <0.0001 0.4976 <0.0001 0.0002 <0.0001 0.0001 <0.0001 0.7193 0.8873
T a b l e III. Artificially aged group m e a n shear bond
strengths High-energy abrasion
Etching
Bonding agent
(MPa)
SD
Yes No Yes Yes
No Yes Yes Yes
Yes Yes No Yes
3.0 6.3 8.7 13.4
1.2 1.7 1.7 .2.4
All groups were different from each other (p < 0.05).
subsurface damage. T h e polished and etched surface r e m a i n e d fairly flat, b u t m a n y small narrow grooves were clearly visible. These grooves were occasionally interrupted by insoluble crystalline components. Microcracks
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Fig. 6. SEM of specimen that was polished and etched with 10% hydrofluoric acid for 30 seconds (original magnification • Surface topography appears to be fairly flat, but many grooves 2 to 3 pm wide can be seen.
Fig. 8. SEM of specimen that was polished, conventionally air abraded, and etched (original magnification • 1000). Surface appears to be rough and uneven, but grooves are fewer than in Fig. 4 and are now 5 to 8 ~m wide.
Fig. 7. SEM of specimen that was polished and conventionally air abraded (original magnification • Surface appears to be rough and uneven.
Fig. 9. SEM of specimen that was polished and high- energy abraded. This surface topography appears to be rough and uneven. Appearance is fairly similar to that of conventionally abraded specimen in Fig. 6, but surface topography is more detailed and some small-sized debris typical of shattered crystals are present.
T a b l e IV. O n e w a y A N O V A for f a t i g u e d s h e a r b o n d s t r e n g t h s Source of variation
Sum of squares
Degrees of freedom
Mean square
Between groups Within groups Total (corrected)
406 096 503
03 24 27
135.5 004.0
produced by silicon carbide sandpaper may have allowed ingress of acid, thus producing the fine grooves. Conventional abrasion produced a rough and jagged uneven surface. When conventional abrasion was followed by etching, the surface remained rough and uneven, but became dominated by relatively few large rounded grooves. Highenergy abrasion produced a rough and uneven surface, but
246
F-ratio
Significance level
33.7
<0.0001
with more detail than by conventional abrasion. Some small-sized debris typical of shattered crystals was present. This suggests that the high-energy particles substantially damaged the surface. Whe n high-energy abrasion was followed by etching, the surface remained rough and uneven, but small narrow grooves were also produced. This resulted in two types of roughness of markedly different
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sizes. This "two-tier" roughness may account for the superior bond strengths achieved with this surface treatment. The greater velocity high-energy abrasive particles may not only knock out superficial grains, but may also produce narrow microcracks and shattered crystals. These cracks may have been too narrow to see at • magnification, but may have preferentially etched and thus produced the narrow grooves. DISCUSSION High-energy-abrasion, hydrofluoric acid-etching, and use of a general-purpose dentin-bonding agent all improved bond strength of heat-polymerized denture acrylic resin bonded to denture tooth porcelain. Therefore, these methods are recommended to improve the bond strength of denture acrylic resin bonded to porcelain denture teeth. However, many dentists and laboratories may not have access to high-energy abrasion equipment; but acid-etching and a bonding agent, which are readily available and inexpensive, still produced reliable bonds after artificial aging. Conventional abrasion did not effectively improve bond strength, possibly because the velocity of the abrasive particles was insufficient to cause sufficient roughness of the strong dense denture-tooth porcelain matrix. It is probable that denture tooth porcelain had greater crystalline components and a lesser glassy component, or higher density, than conventional feldspathic metal-ceramic porcelain, which would render it less susceptible to abrasion or etching. 79 In addition, conventional abrasion is known to be less effective than etching in the improvement of bond strength to feldspathic metal ceramic porcelain. 1~ Although etching of feldspathic metal-ceramic porcelain is known to produce a rough micromechanically retentive surface,S, 10-12no evidence for its effectiveness on bonding to denture tooth porcelain has been reported. This study used 10% hydrofluoric acid with a 30-second etch time. This etch time is sufficient to dissolve surface glass, but longer times would not dissolve the crystalline components unless stronger and much more dangerous acids were used. Treatment with a silane coupling agent was not effective in improving bond strength to heat-polymerized resin. Silane coupling agents react slowly to form stable covalent bonds between substrates. Heat-polymerization of denture base resin in a water bath may allow early water ingress that could cause hydrolysis, 13in contrast to the immediate autopolymerization or light-polymerization of bis-GMA based filled composite resins against ceramic restorations. In addition, unfilled PMMA is more prone to water sorption than highly filled and cross-linked bis-GMA based materials. Use of an autopolymerizing resin, or heat-polymerizing with dry heat or microwave energy, may negate this problem. Previous studies of the effect of silane treatment on bond strength to porcelain denture teeth were poorly controlled or used autopolymerized resin
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Fig. 10. SEMofspecimenthatwaspolished, high-energy abraded, and etched. Surface remains rough and uneven, but unlike conventionally abraded and etched specimen, many small grooves are present.
only.7, 14-16The deleterious effect of water storage and artificial aging on the silane coating of porcelain denture teeth has been demonstrated. 7,13,14, 17 Some prior studies on the adhesion of composite resin to feldspathic metal-ceramic porcelain demonstrated that silane coupling agents can improve bond strength, 18,19but other studies produced mixed results, 11,20-24 reported that bond strengths decrease after storage or artificial aging, 253z or demonstrated that silane was ineffective, s A multipurpose bonding agent significantly improved bond strengths (Tables I through III and Figs. 2 and 3). 6 The type of bonding agent used in this study consisted of a low-viscosity mixture of small surface-active monomers dissolved in acetone and alcohol. 6 Thus the monomers may penetrate into tiny irregularities and copolymerize with other more viscous resins to form a strong micromechanical bond to denture tooth porcelain. It is expected that chemically similar bonding agents would produce similar results. Artificial aging by water storage and thermocycling dramatically reduced bond strengths (Tables I and III, Figs. 2 and 3). Studies on the adhesion of composite resin by adhesive resins to feldspathic metal-ceramic porcelain also demonstrated that extended thermocycling decreased bond strength. 32-34 The SEMs of the glazed surface of porcelain denture teeth (Fig 4) were similar to previously published micrographs of denture teeth made by the same manufacturer.16 As expected, the surface of the 600-grit polished specimens (Fig 5) was of intermediate roughness between that of previously published micrographs of diamond paste-polished porcelain and 240-grit polished porcelain. 15,16 One previously published micrograph clearly shows the fine narrow grooves produced by polishing + etching (Fig 6) or by polishing + high energy abrasion + etching (Fig 10) in this study; however, that study did not identify the specific treatment that produced the fine grooves. 16
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The results clearly show that complex interactions between different surface treatments affect bond strengths (Table I). An interaction between high-energy abrasion and etching produced the most favorable micromechanically retentive surface, namely the two-tier roughness. However, surface topography is not the only factor influencing adhesion. For example, the difference in bond strengths produced by conventional and high-energy abrasion cannot be accounted for by their slight difference in surface detail produced by preparatory abrasion (Figs. 7 and 8). Therefore, changes in surface energy and wettability produced by different treatments may also be important. The negative control, no abrasion + no etching + no silane + no bonding agent group, recorded a 24-hour bond strength of 2.4 MPa, but this bond would not be expected to survive prolonged storage or aging. The bond strength values for the most favorable groups after storage and aging ranged from 3.0 to 13.4 MPa. These are lower than typically achieved for bonding bis-GMA based composites to porcelain veneers, 11, 18-34but may still be sufficient to retain denture teeth, strengthen denture bases, and prevent stain ingress around denture teeth. Differences in formulation, processing, microstructure, and surface energy between PMMA and bis-GMA resins and between feldspathic metal ceramic and denture tooth porcelains probably account for the lower bond strengths. In this experiment, low failure stresses produced adhesive bond failure, but internal cohesive cracking of the porcelain was noted at shear stresses of approximately 10 MPa. High failure stresses, in the order of 20 MPa or more, produced catastrophic cohesive failure and shattering of denture teeth. Intermediate failure stresses produced a mixture of adhesive and cohesive failure similar to that of a previous study. 35 Even after storage and aging, three porcelain teeth in the high-energy abrasion + etching + bonding-agent group failed cohesively. Therefore, this study underestimated shear bond strengths of the stronger groups. Adhesion can be examined in many different ways, but this study was successful in the comparative evaluation of different techniques and treatments. The specimens were tested in shear, but the thickness of the isolating tape and flexure of the PMMA buttons probably introduced complex tensile forces to the bond interface. This test configuration was more demanding than routine clinical situations where some mechanical engagement of the denture tooth would occur and where the larger bulk of resin would allow greater relief of strain, thus protecting the bond. The artificial aging test simulated oral conditions by storing the specimens in water to ensure full water sorption, and then subjecting them to rigorous mechanical forces because of repeated expansion and contraction caused by thermocycling. However, the exact relationship between normal usage and thermocycling is unknown, and the ex-
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MARCHACK ET AL
act bond strength necessary for clinical success is also unknown. 17 Normal chewing forces are unlikely to reach the failure pressures recorded for the best treatment groups. 36 However, clinical failures produced by slow crack growth, namely fatigue, could be produced by many repetitions of small chewing forces. The low variances and the complete randomized block design allowed intergroup comparisons to be made with confidence, despite the small sample size. The experimental methodology was reliable, because no bonds in any test groups failed spontaneously during storage or aging before testing. Adhesion between denture teeth and denture bases would stiffen the denture because the porcelain teeth have a much higher elastic modulus that the PMMA denture base material. The strength of the denture might also be increased because strength is not only dependent on microscopic surface flaws, but also on macroscopic discontinuities and decreased base thickness around teeth. Thus denture base fractures that involve the base and teeth would be expected to be decreased. However, bases also fracture through other susceptible areas such as fraenal notches, and these types of fractures would not be affected by adhesion. LABORATORY TECHNIQUE The following laboratory technique is suggested for situations where retentive elements have been lost, increased strength of the denture is needed, or enhanced prevention of stain ingress is desired. 1. Shape the denture teeth to their expected final form. 2. Abrade the denture teeth with a high-energy system if available. 3. Complete the wax-up. 4. Flask the denture. 5. Boil out, washing the exposed parts of the teeth with detergent and hot water. 6. Etch the exposed parts of the teeth with a 10 hydrofluoric acid gel for 30 seconds. 7. Wash vigorously and dry. 8. Paint two coats ofseparating medium on the stone, not on the teeth, and let dry fully. 9. Apply a multipurpose bonding agent to the exposed parts of the teeth and light cure. 10. Mix and pack with resin as normal. 11. Continue routine procedures. Although, on the basis of the results of this study, the above protocol is expected to produce the best adhesion, many other combinations of surface treatment and bonding agent will also improve bond strength (Tables I through III, Figs. 2 and 3). If the equipment or materials mentioned are not available, others may be substituted, but bond strengths may be lower. CLINICAL IMPLICATIONS This study showed that routine adhesive procedures may strengthen the bond between denture teeth and den-
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MARCHACK ET AL
ture base resin. This may strengthen and stiffen denture b a s e s a n d d e c r e a s e d e n t u r e t o o t h loss, s t a i n i n g r e s s , a n d m o u t h odor.
CONCLUSION Reliable methods of bonding porcelain denture teeth to heat-polymerized polymethyl methacrylate denture base resin have been identified, described, and their mechanism investigated. We t h a n k Drs. M a r k L a t t a a n d Ron Zentz of D e n t s p l y I n t e r n a tional Inc., York Division, for providing the porcelain d e n t u r e t e e t h u s e d in this study. We also t h a n k Drs. W i n s t o n Chee and T e r r y D o n o v a n for t h e i r advice. REFERENCES
1. Dirksen LC. Plastic teeth: their advantages, disadvantages and limitations. J Am Dent Assoc 1952;44:265-8. 2. Appelbaum M. Theories of posterior tooth selection: porcelain versus acrylic. Dent Clin North Am 1984;28:299-306. 3. Shimoyama K, Uchida T, Nagao M, Odagiri K, Shirasaki Y, Tateishi T. Mechanical properties of artificial teeth. Bulletin of Tokyo Medical and Dental University 1993;40:13-6. 4. Suzuki S, Sakoh M, Shiba A. Adhesive bonding of denture base resins to plastic denture teeth. J Biomed Mater Res 1990;24:1091-103. 5. Paffenbarger GC, Sweeney WT, Bowen RL. Bonding porcelain teeth to acrylic resin denture bases. J Am Dent Assoc 1967;74:1018-23. 6. 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. 7. Semmelman JO, Kulp PR. Silane bonding porcelain teeth to acrylic. J Am Dent Assoc 1968;76:69-73. 8. Sorensen JA, Engelman MJ, Torres TJ, Avera SP. Shear bond strength of composite resin to porcelain. Int J Prosthodont 1991;4:17-23. 9. Kern M, Neikes MJ, Strub JR. HafLfestigkeit des klebeverbundles auf In-Ceram nach unterschiedlicher Oberflachenkonditionierung. [Tensile strength of the bond to In-Ceram after varying models of surface conditioning.] Deutsche Zahnarztliche Zeitschrii~ 1991;46:758-61. 10. Suliman AH, Swift EJ Jr, Perdigao J. Effects of surface treatment and bonding agents on bond strength of composite resin to porcelain. J Prosthet Dent 1993;70:118-20. 11. Stangel I, Nathanson D, Hsu CS. Shear strength of the composite bond to etched porcelain. J Dent Res 1987;66:1460-5. 12. A1 Edris A, A1 Jabr A, Cooley RL, Barghi N. SEM evaluation of etch patterns by three etchants on three porcelains. J PROSTHET DENT 1990;64:734-9. 13. Steas A, Picard B, Ogolnik R. Adhesion of artificial teeth to polymethacrylate resins. [French] Rev Odontostomatol Paris 1990;19:61-7. 14. Meyerson RL. Effects of silane bonding of acrylic resins to porcelain on porcelain structure. J Am Dent Assoc 1969;78:113-9. 15. Eames WB, Rogers LB, Feller PR, Price WR. Bonding agents for repairing porcelain and gold: an evaluation. Oper Dent 1977;2:118-124.
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16. Wood DP, Jordan RE, Way DC, Galil KA. Bonding to porcelain and gold. Am J Orthod 1986;89:194-205. 17. Duhaney HN. A clinical and laboratory study of silane bonding of porcelain teeth to the auto-curing methyl methacrylate base [MS Thesis]. Boston: Boston University, 1970. 18. Hsu CS, Stangel I, Nathanson D. Shear bond strength ofresin to etched porcelain [Abstract]. J Dent Res 1985;64:296. 19. Stokes AN, Hood JA. Thermocycling, silane priming, and resin/porcelain interfaces--an electrical leakage study. Dent Mater 1989;5:369-70. 20. Newburg R, Pameijer CH. Composite resins bonded to porcelain with silane solution. J Am Dent Assoc 1978;96:288-91. 21. Ferrando JM, Graser GN, TaUents RH, Jarvis RH. Tensile strength and microleakage of porcelain repair systems. J PROSTHET DENT 1983;50: 44-50. 22. Lacy AM, Laluz J, Watanabe LG, Dellinges M. Effect of poreelain surface treatment on the bond to composite. J PROSTHET DENT 1988; 60:288-91. 23. Nathanson D. Dental porcelain technology. In: Garber DA, Goldstein RE, Feinman RA, eds. Porcelain laminate veneers. Chicago: Quintessence Publ Co, Inc, 1988:24-35. 24. Lu R, Harcourt JK, Tyas MJ, Alexander B. An investigation of the composite resin/porcelain interface. Aust Dent J 1992;37:12-9. 25. Gregory WA, Hagen CA, Powers JM. Composite resin repair of porcelain using different bonding materials. Oper Dent 1988;13:114-8. 26. Pratt RC, Burgess JO, Schwartz RS, Smith JH. Evaluation of bond strength ofsix porcelain repair systems. J I~OSTHETDENT1989;62:11-3. 27. Bailey JH. Porcelain-to-composite bond strengths usingfour organosilane materials. J PROSTHETDENT 1989;61:174-7. 28. Diaz-Arnold AM, Aquilino SA. An evaluation of the bond strengths of four organosilane materials in response to thermal stress. J PROSTHET DENT 1989;62:257-60. 29. Diaz-Arnold AM, Schneider RL, Aquilino SA. Bond stengths of intraoral porcelain repair materials. J PROSTHETDENT 1989;61:305-9. 30. Llobel A, Nicholls JI, Kois JC, Daly CH. Fatigue life of porcelain repair systems. Int J Prosthodont 1992;5:205-13. 31. Appeldoorn RE, Wilwerding TM, Barkmeier WW. Bond strength of composite resin to porcelain with newer generation porcelain repair systems. J PROSTHETDENT 1993;70:6-11. 32. Nowlin TP, Barghi N, Norling BK. Evaluation of the bonding of three porcelain repair systems. J PROS~-mTDENT 1981;46:516-8. 33. Cooley RL, Tseng EY, Evans JG. Evaluation ofa 4-META porcelain repair system. J Esthet Dent 1991;3:11-3. 34. Wolf DM, Powers JM, O'Keefe KL. Bond strength of composite to porcelain treated with new porcelain repair agents. Dent Mater 1992;8:15861. 35. Matsumura H, Kawabara M, Tanaka T, Atsuta M. A new porcelain repair system with a silane coupler, ferric chloride, and adhesive opaque resin. J Dent Res 1989;69:813-8. 36. HaraldsonT. Comparisons ofchewingpatternsinpatientswithbridges supported on osseointegrated implants and subjects with natural dentitions. Acta Odontol Scand 1983;41:203-8. Reprint requests to: DR. BALDWINW. MARCHACK PINCUSBIOMATERIALSRESEARCHLABORATORY U.S.C. SCHOOLOF DENTISTRY# 4112 LOS ANGELES,CA 90089-0641
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University of Southern California, School of Dentistry, Los Angeles, Calif. Use of porcelain denture teeth m a y be desirable in m a n y clinical situations, i n c l u d i n g implant-supported prostheses. However, lack of space because of frameworks often precludes the use of conventional retention by diatorics and pins. Adhesion of porcelain denture teeth to denture resin could also stiffen and possibly strengthen dentures and decrease stain ingress b e t w e e n porcelain teeth and resin denture bases. Unlike previous studies that investigated the b o n d b e t w e e n conventional feldspathic metal-ceramic porcelain and bis-GMA based composite resin, this study investigated adhesion of denture tooth porcelain to p o l y m e t h y l methacrylate (PMMA). High-energy air abrasion, hydrofluoric acid etching, and the use of a general purpose b o n d i n g agent resulted in an i m p r o v e d bond strength of heat-cured denture PMMA b o n d e d to denture tooth porcelain. Silane coating did not improve bond strengths, and conventional air abrasion was no more effective than polishing w i t h 600-grit silicon carbide. Storage in water and artificial aging substantially decreased b o n d strengths. The strongest b o n d strengths w e r e achieved by a high-energy-abrasion + etching + multiple-purpose bonding-agent treatment, but a simpler etching + multiple-purpose bonding-agent treatment also produced reliable results. A laboratory technique was suggested. The role of surface treatment in the m e c h a n i s m of adhesion was e x a m i n e d w i t h s c a n n i n g electron microscopy. High-energy abrasion p r o d u c e d a slightly more detailed initial topography than conventional air abrasion, but after etching, the high-energy topography became m u c h more detailed. Surface topography alone did not account for all differences found. (J PROSTHETDENT 1995;74:242-9.)
S o m e dentists prefer porcelain denture teeth to resin- based teeth, 1"3 but they do not bond to polymethyl methacrylate (PMMA) denture base resin. 4 Porcelain denture teeth are normally joined to acrylic resin denture bases by mechanically retentive features such as metal pins or diatoric undercuts. Unfortunately, these retentive features often have to be adjusted or removed because of lack of space. Lack of space may be the result of implant frameworks and bars, pendulous tuberosities, overdenture abutments, or lack of interarch distance. Adhesion of porcelain denture teeth to acrylic resin could stiffen and strengthen dentures. Stresses are often concentrated around denture teeth and result in the fracture
Presented at the Academy of Prosthodontics meeting, Orlando, Fla., May 1994. aAssociate Clinical Professor, Advanced Prosthodontics, Department of Restorative Dentistry. bAssociate Clinical Professor, Department of Restorative Dentistry/Biomaterials. CResearch Associate, Department of Restorative Dentistry/Biomaterials. 4Assistant Professor and Director of Clinical Research, Department of Restorative Dentistry/Biomaterials. Copyright 9 1995 by The Editorial Council of THE JOURNALOF PROSTHETIC DENTISTRY.
0022-3913/95/$3.00 + 0. 10/H65513
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of denture bases. However, a strong bond would allow stress to be transferred from resin to tooth and thus decrease crack propagation and strengthen the dentures. Stain ingress to cracks between porcelain teeth and resin denture bases, and subsequent mouth odor, would be decreased by adhesion of acrylic resin to these teeth. Masticatory stresses are often localized to and concentrated around retentive pins and thus increase the probability of fracture. Distribution of stresses over a greater area by adhesion of the tooth to the resin would decrease the possibility of failure. Adhesion to porcelain teeth should be investigated. Silane coupling agents were shown to improve adhesion of denture teeth to PMMA in the 1960s, 5 but the adhesion of PMMA to denture tooth porcelain has not been studied comprehensively. Recently, adhesion of bis-GMA to feldspathic metal-ceramic porcelain has been enhanced by abrasion, etching, and bonding agents. Therefore, this study attempted to find a reliable method of producing durable adhesion of PMMA to denture tooth porcelain. MATERIAL AND METHODS Maxillary right central incisor porcelain denture teeth of the same mold and shade (Trubyte Bioform, mold 21J, shade B81, Dentsply International Inc., York, Penn.) were obtained. These teeth were made without pins so they
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shearing load
treated surface porcelain _ denture tooth
v
acrylic resin adhesive tape
Fig. 1. Schematic of test specimen.
could be ground to a flat surface without exposing metal and so that stresses would not be concentrated around the pins. The teeth were embedded in epoxy resin (Epoxy Resin, Hastings Plastics, Santa Monica, Calif.) contained in phenolic rings (Buehler Ltd., Lake Bluff, Ill.). The epoxy resin was cured for 72 hours at room temperature. Then the specimens were ground with a series of abrasives (carborundum grit sizes 80 through 600) for uniform flat polished porcelain surfaces (Fig. 1). A fully randomized block experimental design was used. Twenty-four groups were compared with respect to the factors of surface air abrasion (none, conventional, or high-energy), acid-etching (none or etched), silane coating (none or silane), and a bonding agent (none or bondingagent). The surfaces were abraded with either a laboratory sandblaster (Integral Systems Inc., Culver City, Calif.) (90 psi N2, 50 pm alumina), with a high kinetic energy sandblaster (KCP 2000, American Dental Technology, Troy, Mich.) (90 psi N2, 50 pm alumina), or not abraded. The specimens were then either acid etched (10% hydrofluoric acid, Ultradent Products Inc., Salt Lake City, Utah), or not etched. After the surfaces were abraded and/or etched or not treated, the areas for bonding were isolated by use of adhesive tape (Scotch Tape, 3M, St. Paul, Minn.) with a prepunched circular hole (4 mm diameter) (Fig. 1). A silicone wax mold-release agent was then carefully painted on the tape but not on the exposed denture tooth surfaces. The bond test areas were isolated to prevent excess flash from adhering to the surrounding denture tooth, which would produce inflated bond strengths. After isolation of the test area, the bonding procedures were initiated. The exposed tooth surfaces were then either treated with a silane coupling agent (Silane, Ultradent Products Inc.) according to manufacturer's recommendations or the surfaces were left untreated. The exposed tooth surfaces were then coated with a multipurpose bonding agent (Tenure S, Den-Mat, Santa Maria, Calif.), according to the manufacturer's instructions, or not coated. 6 Five specimens were made for each of the 24 possible abrasion/ etctdsilane/bonding agent groups. The treated specimens were then placed in polyvinyl si-
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loxane molds (Express, 3M) in the lower parts of brass denture flasks (Teledyne Hanau, Buffalo, N. Y.). The upper parts of the flasks contained polyvinyl siloxane buttonshaped molds (5 mm diameter, 2 mm high) that were centered over the samples. Five specimens were placed in each flask. Heat-polymerized PMMA denture resin (Lucitone 199, Dentsply International Inc.) was mixed according to the manufacturer's recommendations until it reached a doughy consistency. Pea-sized pieces were placed in the button-shaped molds and the flasks were closed slowly in a screw-type press (Teledyne Hanau). The specimens were then cured according to manufacturer's recommendations in a water-filled curing tank (Teledyne Hanau), and after 8 hours of curing, the specimens were carefully removed from the polyvinyl siloxane molds and placed in water at 37 ~ C for 16 hours. After storage in water, the specimens were mounted in a servohydraulic universal testing machine (Instron, Canton, Mass.), and the resin-denture tooth interface was tested in shear at a load rate of 0.05 cm/minute (Fig. 1). Failure loads were plotted with a chart recorder. Mean failure loads and their standard deviations were calculated for each abrasion/etch/silane/bonding-agent group. A four-way analysis of variance (ANOVA) was computed to determine whether the main effects of abrasion, etching, silane, and bonding agent, and their interactions had significant effects on bond strength (p < 0.05). If the abrasion had a significant effect, a multiple comparisons test (Tukey's honestly least significant difference method) was used to determine which of the three abrasion subgroups (conventional, high-energy, or none) were effective (p < 0.05). The main effects that significantly improved bond strength were then further evaluated with extended storage and artificial aging to compare the four most promising treatment combinations, with a sample size of seven specimens per group. Specimens were made as previously described, but stored for 7 days in 37 ~ C water after fabrication and then artificially aged by thermal cycling (2000 cycles from 5~ C to 50 ~ C with a dwell time of 30 seconds and a transfer time of 20 seconds). After storage and aging, the specimens were tested as previously described. The group means and their standard deviations were calcu-
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F i g . 3. T h r e e - d i m e n s i o n a l b a r c h a r t of stored a n d artificially aged m e a n b o n d s t r e n g t h s in M P a . S a m e k e y as Fig. 2. F i g . 2. T h r e e - d i m e n s i o n a l b a r c h a r t of 24-hour m e a n bond s t r e n g t h s in M P a . HA, H i g h - e n e r g y abrasion; CA, c o n v e n t i o n a l abrasion; E, etching; S, silane; B, bonding; NONE, no t r e a t m e n t . I. T w e n t y - f o u r - h o u r s h e a r bond s t r e n g t h s a n d s t a n d a r d d e v i a t i o n s (SDs)
Table
Abrasion
None
Bonding agent
Mean
Etching
Silane
-
-
-
-
+
16.7
-
+
-
01.9
0.4
-
+
+
+ + + +
+ +
+
14.3 10.8 21.5 12.4 18.9
2.0 4.2 3.8 1.2 3.8
-
-
-
-
+
-
+
-
13.6 14.6 09.2
-
+
+
12.7
+ +
+ +
10.4 09.4 14.3 16.5
3.8 1.6 2.0 1.4 2.6 2.0 3.4 3.4
06.3 21.9
2.0 1.8
+
(MPa)
SD
02.4
0.2 1.6
l a t e d and a o n e - w a y A N O V A w a s c o m p u t e d to d e t e r m i n e w h e t h e r t h e g r o u p s w e r e d i f f e r e n t (p < 0.05) a n d a m u l t i ple c o m p a r i s o n s t e s t d e t e r m i n e d w h i c h groups w e r e simi l a r (p < 0.05). To u n d e r s t a n d t h e influence of s u r f a c e t r e a t m e n t on t h e m e c h a n i s m of a d h e r e n c e , s c a n n i n g electron m i c r o g r a p h s (SEMs) w e r e m a d e of a b r a d e d a n d / o r etched d e n t u r e t o o t h surfaces. A s c a n n i n g electron microscope w i t h a n e n e r g y d i s p e r s i v e s p e c t r o m e t e r (EDS) ( C a m b r i d g e 360, C a m b r i d g e I n s t r u m e n t s , C a m b r i d g e , U. K.) w a s u s e d to m a k e t h e i m a g e s at a m a g n i f i c a t i o n of • w i t h a n e n e r g y of 10 k V a n d digitally store t h e m . T h e m i c r o g r a p h s w e r e l a t e r p r i n t e d on P o l a r o i d film (Polaroid, C a m b r i d g e , Mass.). RESULTS
Conventional
+ + + + High-energy
244
-
-
-
-
-
+
-
+
-
+
+
17.5
+ + + +
+ +
+ +
20.7 22.6 23.2 10.6
15.4
1.6
5.4 2.8 2.4 6.2 2.4
Twenty-four hour mean shear bond strengths and their s t a n d a r d d e v i a t i o n s a r e s h o w n in T a b l e I and Fig. 2. T h e y r a n g e d f r o m 1.9 M P a for no a b r a s i o n + no e t c h i n g + silane + no b o n d i n g - a g e n t to 23.2 M P a for h i g h - e n e r g y a b r a s i o n + e t c h i n g + silane + no bonding-agent. F o u r - w a y A N O V A r e v e a l e d t h a t abrasion, etching, a n d b o n d i n g a g e n t all significantly i m p r o v e d 24-hour bond s t r e n g t h s (p < 0.0001) (Table II). H o w e v e r , the silane coup l i n g a g e n t did n o t i m p r o v e 24-hour s h e a r bond s t r e n g t h s (p 0.58) (Table II). A m u l t i p l e c o m p a r i s o n s t e s t i n d i c a t e d t h a t h i g h - e n e r g y a b r a s i o n was s u p e r i o r a n d c o n v e n t i o n a l a b r a s i o n w a s s i m i l a r to no a b r a s i o n (p < 0.05). T h e m e a n s a n d s t a n d a r d d e v i a t i o n s of the four artificially aged g r o u p s a r e p r e s e n t e d in Fig. 3 and in T a b l e III. T h e y r a n g e d from 3.0 M P a for h i g h - e n e r g y a b r a s i o n + no
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Fig. 4. SEM of glazed surface of porcelain denture tooth as supplied by m a n u f a c t u r e r (original magnification • All micrographs were made from perpendicular views without stage tilt. All black magnification bars represent 10 pm.
F i g . 5. SEM of polished porcelain (original magnification
xlO00).
T a b l e IL Four-way ANOVA for 24-hour shear bond strength S o u r c e of variation
Main effects Abrasion (A) Etching (E) Silane (S) Bonding (B) Interactions A- E A. S A- B E -S E 9B S -B A. E 9S A. E 9B A. S - B E 9S 9B A.E.S.B
S u m of squares
Degrees of freedom
Mean square
618 416 003 665
2 1 1 1
309 416 003
240 032 592 005 356 168 312 214 364 001 002
2 2 2 1 1 1 2 2 2 1 2
e t c h + b o n d i n g agent to 13.4 MPa for high-energy abrasion + etching + bonding-agent. One-way ANOVA confirmed that differences among the groups existed (Table IV), and multiple comparisons testing demonstrated that all four groups were dissimilar (p < 0.05). Representative micrographs illustrated that different surface t r e a t m e n t s had distinct effects on surface topography (Figs. 4 through 10). The polished surface was uniformly flat, but not completely smooth. Some scratches produced by the silicon carbide abrasive were visible, and occasional defects were seen (Fig. 5), which suggests t h a t d e n t u r e tooth porcelain may have a greater crystalline component t h a n feldspathic metal-ceramic porcelain and t h a t polishing with silicon carbide produces noticeable
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F-ratio
Significance level
666
28.5 38.4 00.3 61.3
<0.0001 <0.0001 0.5830 <0.0001
120 016 296 005 356 168 156 107 182 001 001
11.1 01.5 27.3 00.5 32.8 15.5 14.4 09.9 16.8 00.1 00.1
<0.0001 0.2279 <0.0001 0.4976 <0.0001 0.0002 <0.0001 0.0001 <0.0001 0.7193 0.8873
T a b l e III. Artificially aged group m e a n shear bond
strengths High-energy abrasion
Etching
Bonding agent
(MPa)
SD
Yes No Yes Yes
No Yes Yes Yes
Yes Yes No Yes
3.0 6.3 8.7 13.4
1.2 1.7 1.7 .2.4
All groups were different from each other (p < 0.05).
subsurface damage. T h e polished and etched surface r e m a i n e d fairly flat, b u t m a n y small narrow grooves were clearly visible. These grooves were occasionally interrupted by insoluble crystalline components. Microcracks
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Fig. 6. SEM of specimen that was polished and etched with 10% hydrofluoric acid for 30 seconds (original magnification • Surface topography appears to be fairly flat, but many grooves 2 to 3 pm wide can be seen.
Fig. 8. SEM of specimen that was polished, conventionally air abraded, and etched (original magnification • 1000). Surface appears to be rough and uneven, but grooves are fewer than in Fig. 4 and are now 5 to 8 ~m wide.
Fig. 7. SEM of specimen that was polished and conventionally air abraded (original magnification • Surface appears to be rough and uneven.
Fig. 9. SEM of specimen that was polished and high- energy abraded. This surface topography appears to be rough and uneven. Appearance is fairly similar to that of conventionally abraded specimen in Fig. 6, but surface topography is more detailed and some small-sized debris typical of shattered crystals are present.
T a b l e IV. O n e w a y A N O V A for f a t i g u e d s h e a r b o n d s t r e n g t h s Source of variation
Sum of squares
Degrees of freedom
Mean square
Between groups Within groups Total (corrected)
406 096 503
03 24 27
135.5 004.0
produced by silicon carbide sandpaper may have allowed ingress of acid, thus producing the fine grooves. Conventional abrasion produced a rough and jagged uneven surface. When conventional abrasion was followed by etching, the surface remained rough and uneven, but became dominated by relatively few large rounded grooves. Highenergy abrasion produced a rough and uneven surface, but
246
F-ratio
Significance level
33.7
<0.0001
with more detail than by conventional abrasion. Some small-sized debris typical of shattered crystals was present. This suggests that the high-energy particles substantially damaged the surface. Whe n high-energy abrasion was followed by etching, the surface remained rough and uneven, but small narrow grooves were also produced. This resulted in two types of roughness of markedly different
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sizes. This "two-tier" roughness may account for the superior bond strengths achieved with this surface treatment. The greater velocity high-energy abrasive particles may not only knock out superficial grains, but may also produce narrow microcracks and shattered crystals. These cracks may have been too narrow to see at • magnification, but may have preferentially etched and thus produced the narrow grooves. DISCUSSION High-energy-abrasion, hydrofluoric acid-etching, and use of a general-purpose dentin-bonding agent all improved bond strength of heat-polymerized denture acrylic resin bonded to denture tooth porcelain. Therefore, these methods are recommended to improve the bond strength of denture acrylic resin bonded to porcelain denture teeth. However, many dentists and laboratories may not have access to high-energy abrasion equipment; but acid-etching and a bonding agent, which are readily available and inexpensive, still produced reliable bonds after artificial aging. Conventional abrasion did not effectively improve bond strength, possibly because the velocity of the abrasive particles was insufficient to cause sufficient roughness of the strong dense denture-tooth porcelain matrix. It is probable that denture tooth porcelain had greater crystalline components and a lesser glassy component, or higher density, than conventional feldspathic metal-ceramic porcelain, which would render it less susceptible to abrasion or etching. 79 In addition, conventional abrasion is known to be less effective than etching in the improvement of bond strength to feldspathic metal ceramic porcelain. 1~ Although etching of feldspathic metal-ceramic porcelain is known to produce a rough micromechanically retentive surface,S, 10-12no evidence for its effectiveness on bonding to denture tooth porcelain has been reported. This study used 10% hydrofluoric acid with a 30-second etch time. This etch time is sufficient to dissolve surface glass, but longer times would not dissolve the crystalline components unless stronger and much more dangerous acids were used. Treatment with a silane coupling agent was not effective in improving bond strength to heat-polymerized resin. Silane coupling agents react slowly to form stable covalent bonds between substrates. Heat-polymerization of denture base resin in a water bath may allow early water ingress that could cause hydrolysis, 13in contrast to the immediate autopolymerization or light-polymerization of bis-GMA based filled composite resins against ceramic restorations. In addition, unfilled PMMA is more prone to water sorption than highly filled and cross-linked bis-GMA based materials. Use of an autopolymerizing resin, or heat-polymerizing with dry heat or microwave energy, may negate this problem. Previous studies of the effect of silane treatment on bond strength to porcelain denture teeth were poorly controlled or used autopolymerized resin
SEPTEMBER 1995
Fig. 10. SEMofspecimenthatwaspolished, high-energy abraded, and etched. Surface remains rough and uneven, but unlike conventionally abraded and etched specimen, many small grooves are present.
only.7, 14-16The deleterious effect of water storage and artificial aging on the silane coating of porcelain denture teeth has been demonstrated. 7,13,14, 17 Some prior studies on the adhesion of composite resin to feldspathic metal-ceramic porcelain demonstrated that silane coupling agents can improve bond strength, 18,19but other studies produced mixed results, 11,20-24 reported that bond strengths decrease after storage or artificial aging, 253z or demonstrated that silane was ineffective, s A multipurpose bonding agent significantly improved bond strengths (Tables I through III and Figs. 2 and 3). 6 The type of bonding agent used in this study consisted of a low-viscosity mixture of small surface-active monomers dissolved in acetone and alcohol. 6 Thus the monomers may penetrate into tiny irregularities and copolymerize with other more viscous resins to form a strong micromechanical bond to denture tooth porcelain. It is expected that chemically similar bonding agents would produce similar results. Artificial aging by water storage and thermocycling dramatically reduced bond strengths (Tables I and III, Figs. 2 and 3). Studies on the adhesion of composite resin by adhesive resins to feldspathic metal-ceramic porcelain also demonstrated that extended thermocycling decreased bond strength. 32-34 The SEMs of the glazed surface of porcelain denture teeth (Fig 4) were similar to previously published micrographs of denture teeth made by the same manufacturer.16 As expected, the surface of the 600-grit polished specimens (Fig 5) was of intermediate roughness between that of previously published micrographs of diamond paste-polished porcelain and 240-grit polished porcelain. 15,16 One previously published micrograph clearly shows the fine narrow grooves produced by polishing + etching (Fig 6) or by polishing + high energy abrasion + etching (Fig 10) in this study; however, that study did not identify the specific treatment that produced the fine grooves. 16
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The results clearly show that complex interactions between different surface treatments affect bond strengths (Table I). An interaction between high-energy abrasion and etching produced the most favorable micromechanically retentive surface, namely the two-tier roughness. However, surface topography is not the only factor influencing adhesion. For example, the difference in bond strengths produced by conventional and high-energy abrasion cannot be accounted for by their slight difference in surface detail produced by preparatory abrasion (Figs. 7 and 8). Therefore, changes in surface energy and wettability produced by different treatments may also be important. The negative control, no abrasion + no etching + no silane + no bonding agent group, recorded a 24-hour bond strength of 2.4 MPa, but this bond would not be expected to survive prolonged storage or aging. The bond strength values for the most favorable groups after storage and aging ranged from 3.0 to 13.4 MPa. These are lower than typically achieved for bonding bis-GMA based composites to porcelain veneers, 11, 18-34but may still be sufficient to retain denture teeth, strengthen denture bases, and prevent stain ingress around denture teeth. Differences in formulation, processing, microstructure, and surface energy between PMMA and bis-GMA resins and between feldspathic metal ceramic and denture tooth porcelains probably account for the lower bond strengths. In this experiment, low failure stresses produced adhesive bond failure, but internal cohesive cracking of the porcelain was noted at shear stresses of approximately 10 MPa. High failure stresses, in the order of 20 MPa or more, produced catastrophic cohesive failure and shattering of denture teeth. Intermediate failure stresses produced a mixture of adhesive and cohesive failure similar to that of a previous study. 35 Even after storage and aging, three porcelain teeth in the high-energy abrasion + etching + bonding-agent group failed cohesively. Therefore, this study underestimated shear bond strengths of the stronger groups. Adhesion can be examined in many different ways, but this study was successful in the comparative evaluation of different techniques and treatments. The specimens were tested in shear, but the thickness of the isolating tape and flexure of the PMMA buttons probably introduced complex tensile forces to the bond interface. This test configuration was more demanding than routine clinical situations where some mechanical engagement of the denture tooth would occur and where the larger bulk of resin would allow greater relief of strain, thus protecting the bond. The artificial aging test simulated oral conditions by storing the specimens in water to ensure full water sorption, and then subjecting them to rigorous mechanical forces because of repeated expansion and contraction caused by thermocycling. However, the exact relationship between normal usage and thermocycling is unknown, and the ex-
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act bond strength necessary for clinical success is also unknown. 17 Normal chewing forces are unlikely to reach the failure pressures recorded for the best treatment groups. 36 However, clinical failures produced by slow crack growth, namely fatigue, could be produced by many repetitions of small chewing forces. The low variances and the complete randomized block design allowed intergroup comparisons to be made with confidence, despite the small sample size. The experimental methodology was reliable, because no bonds in any test groups failed spontaneously during storage or aging before testing. Adhesion between denture teeth and denture bases would stiffen the denture because the porcelain teeth have a much higher elastic modulus that the PMMA denture base material. The strength of the denture might also be increased because strength is not only dependent on microscopic surface flaws, but also on macroscopic discontinuities and decreased base thickness around teeth. Thus denture base fractures that involve the base and teeth would be expected to be decreased. However, bases also fracture through other susceptible areas such as fraenal notches, and these types of fractures would not be affected by adhesion. LABORATORY TECHNIQUE The following laboratory technique is suggested for situations where retentive elements have been lost, increased strength of the denture is needed, or enhanced prevention of stain ingress is desired. 1. Shape the denture teeth to their expected final form. 2. Abrade the denture teeth with a high-energy system if available. 3. Complete the wax-up. 4. Flask the denture. 5. Boil out, washing the exposed parts of the teeth with detergent and hot water. 6. Etch the exposed parts of the teeth with a 10 hydrofluoric acid gel for 30 seconds. 7. Wash vigorously and dry. 8. Paint two coats ofseparating medium on the stone, not on the teeth, and let dry fully. 9. Apply a multipurpose bonding agent to the exposed parts of the teeth and light cure. 10. Mix and pack with resin as normal. 11. Continue routine procedures. Although, on the basis of the results of this study, the above protocol is expected to produce the best adhesion, many other combinations of surface treatment and bonding agent will also improve bond strength (Tables I through III, Figs. 2 and 3). If the equipment or materials mentioned are not available, others may be substituted, but bond strengths may be lower. CLINICAL IMPLICATIONS This study showed that routine adhesive procedures may strengthen the bond between denture teeth and den-
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ture base resin. This may strengthen and stiffen denture b a s e s a n d d e c r e a s e d e n t u r e t o o t h loss, s t a i n i n g r e s s , a n d m o u t h odor.
CONCLUSION Reliable methods of bonding porcelain denture teeth to heat-polymerized polymethyl methacrylate denture base resin have been identified, described, and their mechanism investigated. We t h a n k Drs. M a r k L a t t a a n d Ron Zentz of D e n t s p l y I n t e r n a tional Inc., York Division, for providing the porcelain d e n t u r e t e e t h u s e d in this study. We also t h a n k Drs. W i n s t o n Chee and T e r r y D o n o v a n for t h e i r advice. REFERENCES
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