Tensile Bond Strength of Resin Composite Bonded to Ceramic. A. Della Bona* and R. van Noort. Department of Restorative Dentistry, University of Sheffield, ...
J Dent Res 74(9): 1591-1596, September, 1995
Shear vs. Tensile Bond Strength of Resin Composite Bonded to Ceramic A. Della Bona* and R. van Noort Department of Restorative Dentistry, University of Sheffield, S10 2TA, Sheffield, United Kingdom; *to whom correspondence and reprint requests should be addressed, at The University of Passo Fundo, School of Clinical Dentistry, Department of Restorative Dentistry, 817 Teixeira Soares St., 99010-080, Passo Fundo, RS, Brazil
Abstract. Since the mode of failure of resin composites bonded to ceramics has frequently been reported to be cohesive fracture of either ceramic or resin composite rather than separation at the adhesive interface, this study was designed to question the validity of shear bond strength tests. The reasons for such a failure mode are identified and an alternative tensile bond strength test evaluated. Three configurations (A, conventional; B, reversed; and C, all composite) of the cylinder-on-disc design were produced for shear bond strength testing. Two-dimensional finite element stress analysis (FEA) was carried out to determine qualitatively the stress distribution for the three configurations. A tensile bond strength test was designed and used to evaluate two ceramic repair systems, one using hydrofluoric acid (HF) and the other acidulated phosphate fluoride (APF). Results from the shear bond strength tests and FEA showed that this particular test has as its inherent feature the measurement of the strength of the base material rather than the strength of the adhesive interface. In the tensile test, failure invariably occurred in the adhesive layer, with HF and APF showing a similar ability to improve the bond of resin composite to ceramic. It is concluded that the tensile bond strength test is more appropriate for evaluating the adhesive capabilities of resin composites to ceramics. Key words: tensile, shear, bond strength, finite element.
Received August 25, 1994; Accepted July 5, 1995
Introduction The dental profession still has no universally accepted bond strength test for resin composites bonded to ceramic, despite the great amount of research papers on this topic over the last decade (Stangel et al., 1987; Lacy et al., 1988; Nicholls, 1988; Tjan and Nemetz, 1988; Diaz-Arnold et al., 1989; Lu et al., 1992). Tensile, flexural, and shear tests (Nicholls, 1988; Bailey, 1989; Della Bona and Northeast, 1994) have been used to measure the resin-ceramic bond strength, with the shear bond strength test being the most popular. A notable feature of some recent publications (DiazArnold et al., 1989; Sorensen et al., 1991; Della Bona and Northeast, 1994) is the observation that the failure mode is often cohesive within the ceramic base rather than at the adhesive interface, on the basis of which it has been suggested that the bond strength exceeds the cohesive strength of the ceramic. This ignores the nature of the stresses generated and their distribution within the adherence zone, which can have a profound influence on the mode of failure. Finite element stress analysis (FEA) has been used to study the sensitivity of bond strengths to specimen design and changes in testing conditions (Anusavice et al., 1980; van Noort et al., 1989; van Noort et al., 1991; Shiau et al., 1993). These studies show that there is a need for a more critical approach to the design of appropriate tests for evaluating the bond strength of resin composite to ceramic if the desire for a standardized test procedure is to be achieved. For this objective to be accomplished, a careful examination of bond strength tests is mandatory for correct interpretation of the bond strength data. The contention of this study is that the shear bond strength test is inappropriate and inadequate for the in vitro assessment of resin composite bonded to ceramic, since failure occurs in the ceramic base and not at the adhesive interface. It is hypothesized that this mode of failure is due to the generation of high tensile stresses within the sample base as a consequence of the non-uniform stress distribution generated in the shear test arrangement. The aims of this study were to 1591
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examine a variety of shear bond strength test arrangements and to assess, by FEA, the effect of the stress distribution on the shear bond strength and failure mode. A tensile bond strength measurement technique for ceramic bonded to resin composite is proposed as a more suitable alternative.
Materials and methods The materials used consisted of a feldspathic ceramic (Vita VMK68, Vita Zahnfabrik, Bad Sackingen, Germany), an adhesive and a hybrid resin composite (Prisma Universal Bond3 and Prisma APH, Dentsply Limited, DeTrey Division, Weybridge, Surrey, UK), solutions of 9.6% hydrofluoric acid gel (HF) and 4% acidulated phosphate fluoride (APF), plus a silane coupling agent (Mirage Dental Systems, Chameleon Dental Products Inc., Kansas City, KS, USA).
Shear bond strength sample preparation Three configurations of the same specimen design were produced with identical adhesive interfaces in terms of both geometry and surface area. The first followed the conventional configuration (Group A), consisting of a ceramic base to which a resin composite cylinder was bonded. In the second configuration (Group B), the materials were reversed such that the specimen base was made of resin composite bonded to a ceramic cylinder. The third Group (C) was made solely out of resin composite, thus being devoid of any adhesive interface. To produce the conventional configuration (Group A), we made ten ceramic discs 10 mm in diameter and 3 mm in height. Vita VMK68 A2 dentin shade powder (batch #1485) was mixed with the Vita Modeling Liquid (batch #56331) and condensed into the mold by means of a brush and vibration (Aranda and Barghi, 1988; Evans et al., 1990). The green bodies were sintered according to the manufacturer's recommendations in a Vita Vacumat 200 oven (number 93468, Vita Zahnfabrik, H. Rauter GmbH & Co., Germany). The ceramic discs were embedded in epoxy resin (Metset type FT, Buehler, Coventry, UK), allowed to set for 24 hours, and ground flat by means of 400- and 600-SiC paper. All specimens were ultrasonically cleaned in distilled water for 5 min and dried. The ceramic surface was etched with HF for 2 min, washed thoroughly for 2 min under running tap water, and airdried. Silane bond enhancer was applied by a clean brush and allowed to dry fully. The specimen was then placed in a holding jig, where the resin composite cylinder was built up through a hole (R = 3 mm, h = 4 mm) located centrally in a disk-shaped silicone rubber mold. A thin layer of Prisma Universal Bond 3 was applied with a brush and light-cured with a Visilux 2 light-curing unit (Dental Products Division/3M, St. Paul, MN, USA) for 10 sec. Prisma AP.H (universal shade) was applied in two increments, with each one cured for 20 sec. The silicone rubber mold was removed and the sample cured for an extra 40 sec from different lateral directions. For the reversed configuration (Group B), two silicone rubber molds were made from machined acrylic resin models (Fig. 1). Ten Vita VMK68 A2 dentin ceramic rods (R = 3 mm) were made following the manufacturer's instructions. All rods were reduced to a height of 4 mm, had one surface flattened by 400- and 600-SiC paper on a special holding device, were ultrasonically cleaned in distilled water, and were dried. HF and silane coupling agent were applied to the flattened ceramic surface following the same procedures as described above. The ceramic rod was then placed
3 mm
all composite specimen
.I4 mm 10 mm Acrylic resin model 1
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Slo rbe Silicone rubber mould 1
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reversed specimen
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20 mm Acrylic resin model 2
Silicone rubber mould 2
embedded sample ready for shear bond
sirengih tesling
Figure 1. Acrylic resin models (1 and 2) and their respective silicone rubber molds. Mold 1 was used to build the reversed and allcomposite specimens. All specimens were embedded in epoxy resin in mold 2. in silicone rubber mold 1, and a thin layer of adhesive was applied to the prepared ceramic surface by brush. This layer was lightcured for 10 sec, and Prisma AP.H was packed into the mold in two 1.5-mm-thick increments, cured for 20) sec each. The sample was carefully removed from mold 1, and an additional 40 seconds' curing was applied to the resin composite base from different directions. The sample was then placed into mold 2 (Fig. 1), epoxy resin was poured into it, and it was allowed to set for 24 hours. The all-composite samples (Group C) were made solely out of Prisma AP.H which was placed incrementally down the central hole of mold 1 (Fig. 1) and gradually built up to create a cylinderon-disc specimen. This was carefully removed from the mold, and 40 seconds' additional curing time was applied from different lateral directions. The specimen was then placed in mold 2 (Fig. 1), embedded in epoxy resin, and allowed to set for 24 hours. Samples from all groups were stored in distilled water at 37°C for 3 days before shear bond strength testing occurred.
Shear bond strength test A Lloyd M5K universal testing machine
(I.J. Lloyd Instruments Ltd., Warsash, UK), with the knife edge placed as close as possible to the junction between the base and the cylinder, was used for testing. A cross-head speed of 0.5 mm/mmi was used and the maximum load recorded for each specimen. The nominal shear bond strength was calculated by P/A, where P is the load at failure and A is the cross-sectional area of the cylinder. The fracture surface of each specimen was examined under x40 magnification so that the mode of failure could be determined. The data were analyzed by one-way analysis of variance (ANOVA). Finite element stress analysis (FEA) The stress distributions for the three shear test configurations were determined from a two-dimensional plane-strain computer model of a central section with dimensions identical to those of the experimental samples (FINEL, Babcock Plower Ltd., London). The nodes at the sides and bottom of the base material were constrained in both the x and y directions. The Ploisson's ratio (pi) and elastic modulus (E) for the ceramic and resin composite were p = 0.30, E = 83 GPa and p = 0.25, E = 8 GPa, respectively
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Figure 2. A couple of grooved metal rods and a set of stainless steel clamps containing a three-point gripping system at one end and a holding mechanism for the testing machine at the other end. The three round-tip screws were designed to fit into the specimen groove and tightly grip it during tensile bond strength testing.
(Sakaguchi et al., 1992). A shear load of 10 N was applied parallel to the base at a position 0.2 mm above the surface of the base.
Tensile bond strength test design and sample preparation The tensile bond strength specimen design consists of two rod specimens of uniform cross-section, bonded together on their ceramic surfaces and pulled apart in the universal tester. A special set of clamps was fabricated in stainless steel containing a threepoint gripping system for the specimens at one end and a holding mechanism for the testing machine at the other end (Fig. 2). Forty Ni/Cr rods (R = 4 mm) were made with Talladium-V alloy (Talladium Inc., Valencia, CA, USA). A groove was cut around each metal rod at a distance of 2 mm from one of its ends to accommodate the three round-tip screws of the clamps. The non-grooved end of the metal rod was grit-blasted with 50 pm aluminum oxide and ultrasonically cleaned in distilled water. After metal degassing, Vita VMK68 opaque (batch #1222) and dentin ceramic (batch #1485) were applied and fired according to the manufacturer's instructions. By 600-grit SiC paper used on a special grinding device, the ceramic surface was flattened, resulting in a ceramic extension to the metal rod measuring 2 mm in height and 3.45 mm in diameter. Specimens were ultrasonically cleaned in distilled water, divided into two groups of 20 at random, and treated as follows: Specimens in group I had their ceramic surfaces treated with 9.6k/ HF for 2 min, washed thoroughly under running tap water, and dried with oil-free air. Silane coupling agent was applied and allowed to evaporate. A thin layer of Prisma Universal Bond 3 was brushed onto a pair of treated specimens and light-cured for 10 sec. Prisma AP.H was applied to the surfaces, the screw of the alignment jig was tightened to produce a thin resin composite layer (Fig. 3) and light-cured for a total of 120 sec from different directions (Blackman et al., 1990). No attempt was made to control the thickness of the adhesive layer other than by the amount of pressure being applied by the screw. We relied essentially on the film-thickness characteristics of the resin composite, and since the same procedure was used throughout, we assumed that this would not have varied greatly. The excess resin material was cut down to the ceramic level by resin composite polishing burs. Samples were stored in distilled water at 37°C for 3 days before undergoing tensile bond strength testing
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Figure 3. Twin specimens were placed in the alignment jig, and the adhesive complex layer was light-cured from different directions in a Lloyd M5K tester at a cross-head speed of I mm/mim. Specimens in group 2 were treated following the same procedures as used above, except for the application of 4% AI'F instead of HF for 2 min. Since just two groups of data were present, the independent Student's t test was selected for the statistical analysis. Selected specimens were further examined under the scanning electron microscope (Philips SEM 501, Eindhoven, The Netherlands).
Results The values for the nominal shear bond strengths of the three specimen configuration groups are presented in Table 1. The nominal shear bond strength for Group A was significantly different from those of Groups B and C (P < 0.001). The results for Group B and Group C were not significantly different at P < 0.001. Cohesive fractures of the ceramic base and interfacial adhesive failures occurred in equal proportions in Group A. In group B, 80% of failures were cohesive fractures of the resin composite base, and for group C, all failures were cohesive fractures of the resin composite base. The cohesive fracture path is shown schematically in Fig. 4, with the fracture running in an arc from the surface of the base at a point to the left of the point of load application and under the adhesive interface. The contours of the vertical ((a,v) and horizonital ((ar) stresses resulting from an applied shear load are shown in Figs. 5 and 6, respectively, for the FEA model of the conventional configuration. As expected, the pattern of stress in this shear bond strength test specimen design is highly non-uniform. A notable feature is that the maximum tensile stress in the vertical direction (ar ) occurs at the adhesive interface nearest the point of loaY application, as reported previously (van Noort et al., 1989), which is the result of a bending moment as suggested by Shiau ct al. (1993). Yet the highest tensile stress was found in the horizonital direction and not in the adhesive layer or the cylinder but at the surface of the base just to the left of the cylinder (Table 2). The stress distributions for the other two configurations were found to be very similar in terms of where the maximum tensile stresses occurred, although there were slight differences in the stress patterns. Also, the relative values for the maximum tensile stresses were different (Table 2), due to
((r5,,)
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Table 1. Shear bond strength data (MPa)
Number
Group A - Conventional B - Reversed C - All-composite
Mean
SD
Range
8.25 - 13.64 15.06 - 23.07 15.36 - 24.16 The vertical line denotes no significant difference between Groups, as determined by ANOVA (P < 0.001), with a post hoc Tukey's honestly significant difference (HSD) test. 10.37 18.02 21.82
10 10 10
1.99 2.52 2.45
Composite cylinder Applied load ____ __
_
I_I1_L_
Discussion The shear bond strength test arrangement has been the most
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Start of fracture
the different elastic moduli assigned to the base and the cylinder in the three configurations. The nominal tensile bond strengths for the HF- and APFetched experimental groups are presented in Table 3. No significant difference was found between the nominal tensile bond strengths of Group 1 and Group 2 (P < 0.05). All specimens in both Groups fractured within the adhesive interface complex. The fracture was always at or near the adhesive/ceramic interface and never in the bulk of the resin composite or the ceramic. Surface flaws exist in all materials, whereas interfacial flaws can arise only when two materials are stuck together. From the example in Fig. 7, areas of the etched ceramic surface can be identified by their scalloped appearance, together with regions of resin fracture where resin tags have formed in the etched ceramic surface. This suggests that failure is governed by interfacial as opposed to surface or bulk flaws in the ceramic or the resin composite. The adhesive layer thickness may have an influence on the measurement of bond strength if failure occurs from surface flaws or flaws within the bulk of the resin-composite. This is a factor which may need further exploration. There were no cohesive fractures of the ceramic, yet this was the dominant mode of failure for the shear bond strength test.
_
Fracture line
Ceramic base
i
Figure 4. A schematic representation of the mode of failure for the shear bond strength arrangement due to high horizontal tensile stresses at the surface of the base (thin arrow), exceeding the tensile vertical stresses at the adhesive interface.
common laboratory technique for evaluating adhesives for resin-bonded ceramic restorations and ceramic repair systems. It has been shown that shear bond strength measurements are very sensitive to the method of application of the adhesive and design of the testing arrangement (Anusavice et al., 1980; van Noort et al., 1989; van Noort et al., 1991). These factors can lead to false interpretation of the resultant bond strength data. This study shows that significant differences in shear bond strengths are obtained for different sample configurations when an identical geometric design and an identical adhesive interface are used. Use of a ceramic base resulted in a shear bond strength which was nearly half that obtained with a resin composite base, even though the bonded areas were identical. It gives some credence to the expression: "Imagine a bond strength value, and a test arrangement can be designed to produce such a result." The fact that a virtually identical shear bond strength was obtained for the all-composite sample compared with the
59 6
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,
t 2: 3 t 5: S: 7: 8: 9: 10: 11: 12: 13:
3.16 2119 123 0.74 8.26 -0.21 -0.69 -1.66 -2.14 -2.62 -3.11 -3.59 -4.55
6
3
IIfIla
x
7
a
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S I
Figure 5. FEA close-up contour plot of the vertical stress distribution (a for the conventional shear test arrangement (configuration A). The black arrow shows the point of load application. The numbers correspond to the stress values (MPa) in the legend.
yo)
Figure 6. FEA close-up contour plot for the horizontal stress distribution (uxx) for the conventional shear test arrangement (configuration A). The black arrow shows the point of load application. The numbers correspond to the stress values (MPa) in the legend.
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Table 2. Maximum tensile stresses for each computer model (MPa)
Table 3. Tensile bond strength data (MPa)
Configuration
Group
Stress Direction
A - Conventional
(r1 aXx
13 Reversed
(Ty
C - All-composite
(T_ (ryy
Maximum Value 3.16 9.75 3.21 8.15 1.95 8.77
composite base with a ceramic cylinder shows that the test does not actually assess the quality of the adhesive bond. This is consistent with observations of the modes of failure, which were predominantly cohesive fractures of the base material. Notwithstanding the lack of sophistication of the FEA models used, which could be much improved by mesh refinement and development of a 3-D model, the qualitative results obtained clearly indicate that all three configurations developed tensile surface stresses within the base very close to the cylinder-base interface edge nearest to the applied shear load. Since these surface tensile stresses ((u>x) were considerably higher than the interfacial vertical tensile stresses (u ) by approximately a factor of three, it is highly probable that fracture was initiated from the surface of the base, as confirmed by the many cohesive fractures observed and as suggested by Anusavice et al. (1980). The shear bond strengths determined in this study are therefore governed by the resistance of the base to surface tensile stresses, i.e., the base material's tensile strength, rather than the strength of the adhesive interface. This will be the case for as long as the adhesive interface is sufficiently strong to resist its local stressing conditions. Apparent differences in bond strength values when the same ceramic base is used can arise simply because of differences in its surface finish. When a ceramic base is grit-blasted, it will have a different resistance to
Figure 7. A scanning electron microscope view of the fracture surface of a tensile bond strength specimen from the HF-etched group. The scalloped appearance is typical of an etched ceramic surface (b), and areas of resin fracture (a) are also readily identifiable where resin tags have formed in the etched ceramic surface. Bar = 5 microns. The original magnification is 5000.
I - 9.6% HF etchant 2 - 4% APF etchant
Number
Mean
+
SD
10 10
13.46 13.43
2.18 4.46
Range 10.84 -17.73 8.49 - 21.62
The groups do not show a difference at the 0.05 significance level, as determined by independent Student's t test.
fracture from the surface tensile stresses than if it were etched or polished. Another contributory factor will be the differences in (uyy and T xx due to differences in the elastic moduli causing changes in the stress distribution, such that if a base or cylinder with a different elastic modulus is used, one could get yet another bond strength value, which again may have nothing to do with the adhesive interface. Thus, it is reasonable to suggest that the surface tensile stresses of the base material offer the best possible explanation for the shallow cohesive fractures of the ceramic base as observed in these and other shear bond strength tests. Though cohesive failures starting near the load application point were most frequent, some failures at the adhesive interface did occur in Groups A and B. This is related to the statistical nature of the strength characteristics of the materials used, all of which are highly brittle. Surface flaws, internal material flaws-in either the base material, the adhesive layer, or the cylinder-and large interface flaws can all play an important role in determining failure sites. Sometimes failure will occur at sites of relatively low local stress merely because there is a particularly large flaw so oriented in a stress field as to be ideal to cause fracture. Possible sites from which failure may start are therefore highly unpredictable, since this will depend on flaw size and distribution in relation to the stress distribution. Thus, statistically, one would expect some fractures to occur at the adhesive interface if there happens to be a flaw of a substantial size and ideal orientation to propagate. It is noteworthy that for the all-composite arrangement (Group C), none of the failures occurred in the interface region, where of course there would be no interfacial flaws, since there was no adhesive. These findings easily justify the search for a more representative bond strength test with a simpler geometric design and consequently a more uniform stress distribution. Nicholls (1988) described a tensile bond strength arrangement similar to the one designed for this study. In the present study, all failures for the tensile bond strength test occurred within the adhesive interface complex, regardless of the bonding procedure. This means that the data obtained provide a more representative measurement of the tensile bond strength of the bonding area rather than the strength of a particular base material, as probably happens during the shear bond strength test. Although stress inhomogeneities due to geometry are avoided, the interfacial stress should not be assumed to be uniformly tensile. FEA work has shown that some non-uniformity remains due to the changes in elastic moduli (Aivazzadeh et al., 1988). While these are considerably less than those occurring in the shear test arrangement, it is by no means suggested that the tensile bond strength test is ideal; other tests should be considered equally for their suitability (Tam
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and Pilliar, 1993: Degrange et al., 1994). Etching techniques have been widely used to improve the bond strength of ceramic to resin composite (Calamia et al., 1985; Sorensen et al., 1991; Della Bona et al., 1993). HF and APF are the most commonly used etchants for this purpose. Although no significant difference (P < 0.05) was found between the tensile bond strengths for specimens etched with 9.6% HF and those of specimens etched with 4% APF, the data in the group with 4% APF showed a wider spread than in the one etched with 9.6% HF. This suggests that HF etching may well produce a more reliable and consistent result. However, this could not be confirmed, since the sample size was too small for a Weibull analysis. Comparison of these results with those of other studies is difficult for several reasons. Clearly, a comparison with shear bond strength data would be invalid, since one would not be comparing like with like. The lack of a standardized tensile test procedure, plus the non-existence of previous reports on tensile bond strengths where 4% APF etchant was used, also makes such a task impossible. Even when a very similar test arrangement was used (Nicholls, 1988), the materials and research protocol were different. Nevertheless, ceramic repair systems provide some ability to bond to ceramic, although the mode of failure for the tensile bond strength tests indicates that the adhesive interface still represents the weak link in the system. Until such time that the tensile bond strength required for a clinically acceptable ceramic repair has been determined, its measurement can be used as only a crude ranking parameter, such that, for the present, long-term clinical performance of the ceramic repair materials remains the ultimate test. Within the limitations of the present in vitro study, it can be conduded that shear bond strength data in the cylinder-on-disc experimental design are governed by the cohesive strength of the base material used and not by the bond strength of the adhesive interface. Also, the cohesive failure of the base of the specimen is an inherent feature of the geometry of the shear bond strength test arrangement. Therefore, the shear bond strength procedure is inadequate as a means of assessing the quality of the adhesive bond of resin composite to ceramic, and perhaps the time has come for it to be abandoned in favor of a test which more genuinely measures the quality of the adhesive interface. Failure within the adhesive interface complex for the tensile bond strength test is evidence that this type of test arrangement is more appropriate for evaluating the bond strength of resin composite to ceramic.
Acknowledgments This work was supported in part by the Department of Restorative Dentistry at the University of Sheffield and is based on Dr. Della Bona's thesis, which was submitted to the graduate faculty, The University of Sheffield School of Clinical Dentistry, in partial fulfillment of the requirements for the MMedSci degree.
References Aivazzadeh S, Bichara M, Ghazal A, Verchery G (1988). Special mixed finite elements for interfacial stress analysis of adhesively bonded joints. In: Adhesively bonded joints: testing, analysis and design. Johnson WS, editor. Ann
J Dent Res 74(9) 1995 Arbor, MI: ASTM, pp. 133-144. Anusavice KJ, DeHoff PH, Fairhurst CW (1980). Comparative evaluation of ceramic-metal bond tests using finite element stress analysis. I Dent Res 59:608-613. Aranda R, Barghi N (1988). An evaluation of four techniques for condensation of three opaque porcelains. J Prosthet Dent 60:29-31. Bailey JH (1989). Porcelain-to-composite bond strengths using four organosilane materials. J Prosthet Dent 61:174-177. Blackman R, Barghi N, Duke E (1990). Influence of ceramic thickness on the polymerization of light-cured resin cement. J Prosthet Dent 63:295-300. Calamia JR, Vaidyanathan J, Vaidyanathan TK, Hirsch SM (1985). Shear bond strength of etched porcelains (abstract). J Dent Res 64:296. Degrange M, Charrier JL, Attal JP, Asmussen E (1994). Bonding of luting materials for resin-bonded bridges: clinical relevance of in vitro tests. i Dent 22(Suppl 1):s28-s32. Della Bona A, Northeast SE (1994). Shear bond strength of resin bonded ceramic after different try-in procedures. J Dent 22:103-107. Della Bona A, Barghi N, Garbin CA (1993). Etching porcelain with APF: etching time vs. concentration (abstract). J Dent Res 72:187. Diaz-Arnold AM, Schneider RL, Aquilino SA (1989). Bond strengths of intraoral repair materials. J Prosthet Dent 61:305-309. Evans DB, Barghi N, Malloy CM, Windeler AS (1990). The influence of condensation methods on porosity and shade of body porcelain. J Prosthet Dent 63:380-389. Lacy AM, LaLuz J, Watanabe LG, Dellinges M (1988). Effect of porcelain surface treatment on the bond to composite. J Prosthet Dent 60:288-291. Lu R, Harcourt JK, Tyas MJ, Alexander B (1992). An investigation of the composite resin/porcelain interface. Aust Dent J 37:12-19. Nicholls JI (1988). Tensile bond of resin cements to porcelain veneers. J Prosthet Dent 60:443-447. Sakaguchi RL, Cross M, Douglas WH (1992). A simple model of crack propagation in dental restorations. Dent Mater 8:131-136. Shiau JY, Rasmussen ST, Phelps AE, Enlow DN, Wolf DR (1993). Analysis of the "shear" bond strength of pretreated aged composites used in some indirect bonding techniques. J Dent Res 72:1291-1297. Sorensen JA, Engelman MJ, Torres TJ, Avera SP (1991). Shear bond strength of composite resin to porcelain. Int J Prosthodont 4:17-23. Stangel I, Nathanson D, Hsu CS (1987). Shear strength of the composite bond to etched porcelain. J Dent Res 66:1460-1465. Tam LE, Pilliar RM (1993). Fracture toughness of dentin/resincomposite adhesive interfaces. J Dent Res 72:953-959. Tjan AHL, Nemetz H (1988). A comparison of the shear bond strength between two composite resins and two etched ceramic materials. Int JProsthdont 1:73-79. van Noort R, Noroozi S, Howard IC, Cardew G (1989). A critique of bond strength measurements. J Dent 17:61-67. van Noort R, Cardew GE, Howard IC, Noroozi S (1991). The effect of local interfacial geometry on the measurement of the tensile bond strength to dentin. J Dent Res 70:889-893.
