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Adhesion testing of dentin bonding agents: A review
Dent Mater
11 :117-l
25, March, 1995
Adhesion testing of dentin bonding agents: A review David H. Pashleyl, Hidehiko Sanoz, Bernard Ciucchis, Masahiro Yoshiyamad, Ricardo M. Carvalhos ‘Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, Georgia, USA jDepartment of Operative Dentistry, Tokyo Medical and Dental University, Tokyo, JAPAN ,YDepartment of Restorative Dentistry and Endodontics, University of Geneva, Geneva, SWITZERLAND 4Department of Conservative Dentistry, Tokushima University, Tokushima, JAPAN 5Department of Operative Dentistry, Bauru School of Dentistry, University of Sho Paula, BRAZIL
ABSTRACT Adhesion testing of dentin bonding agents was reviewed starting with the adhesion substrate, dentin, the variables involved in etching, priming and bonding, storage variables and testing variables. Several recent reports attempting to standardize many of these variables were discussed. Recent advances in the development of new bonding systems have resulted in bond strengths on the order of 20-30 MPa. At these high bond strengths, most of the bond failure modes have been cohesive in dentin. As this precludes measurement of interfacial bond strength, new testing methods must be developed. One such new method, a microtensile method, was described along with preliminary results that have been obtained. The last decade has produced major advances in dentin bonding. The next decade should prove to be even more exciting.
HISTORICAL PERSPECTIVE Adhesion testing in dentin bonding studies has developed steadilysincethe pioneeringwork of Buonocore(1955). Many improvementsin bond testing have been developed over the last 40 years. The contributions of Bowen (1965), Kemper and Kilian (19761,Watanabe et al. (1987) and Retief (1991) helped to standardizebond testing methods. However,there are an enormousnumber of variablesthat can influencebond testing. These have been listed in Table 1 under the broad categories of substrate variables, etching variables, priming variables, bonding variables, storage variables and testing variables. With regard to substrates, much information has been generated using bovine teeth. However, the use of human teeth is preferable (Retief et al., 1990). Decreases in bond strength in deep dentin were reported by Causton (1984), Stanford et al. (19851,Mitchem and Gronas (1986) and by Tao and Pashley (1988). However, as bonding systems have become more hydrophilic,the sensitivityof bond strength to dentin depth has decreased (Prati and Pashley, 1992). As
superficial and deep dentin are so different structurally,it would seem desirable to screen dentin bonding agents with both types of substrates(Tagamiet al., 1990).
BONDING SUBSTRATES Occlusal dentin tends to give lower bond strengths than proximal or buccal dentin (Causton, 1987; 0ilo and Olsson, 1990). There is greater regionalvariabilityof dentin wetness (Prati and Pashley,1992) in occlusaldentin than in proximal or buccal dentin. As bonding agents improve, this regional sensitivityof bond strengthmay disappear The use of human unerupted third molars requires comment. While these teeth are conveniently collected for bond testing,it shouldbe emphasizedthatthey are much more permeable and wetter than erupted teeth (Pashley, unpublishedobservations).Clinically,most dentinbonding is done to previously restored teeth, carious teeth or abraded lesions, most of which contain sclerotic dentin. Sclerotic dentinis lessetchableDuke and Lindemuth,1991;Gwinnett, 1992;VanMeerbeeket ah, 1994) and its tubules are generally occludedby mineralcrystals.Thus,the bond strengthsto such dentin are thought to be lower than to unerupted third molars. Nara et al. (1994) reported lower bond strengths to sclerotic dentin compared to normal dentin. Clearly, such studiesneed to be expanded because scleroticdentin is one of the most common bonding substrates. More informationis needed on how to prepare and etch such dentin.
SPECIMEN HANDLING The surface preparation of dentin varies widely but few
careful comparisonshave been made. Some laboratoriesuse aluminum oxide abrasive paper while others use silicon carbide(Table 1). Grits vary from 320 to 1200. While the use of abrasive papers is convenient in the laboratory,clinically, dentin is never prepared with abrasive paper. Even when using high-speed handpieces, differences have been found
Dental Materials/March 1995 117
A.
Substrate 1. Human or bovine dentin? 2. Superficial, middle or deep dentin? 3. Occlusal, proximal, buccal? 4. Third molars or incisors? 5. Sanded surfaces? Grit? Method? AI,O,, SIC, 320,600,800 6. Dental burs? Diamond, carbide? Low-speed vs. high-speed; air-water? Reuse of teeth? Mount in plastic, stone, etc.?
C. 1. 2. 3. 4. 5. 6. E.
B.
or 1000 grit.
Primina Cover entire surface or apply within matrix How much primer? Passive or active? How long? Wash or evaporate? How long? Light-cure or not? Wet vs. dry? How wet, how dry?
Storage Water, isotonic saline, etc.? Room temperature or 37X? 100% RH or water? Preservatives? sodium azide, thymol, chloramine? Pulpal pressure or not, magnitude? Composition of fluid? Time. 24 hr.? months? years? Thermal stress? Temperatures, dwell time, number of cycles? a. Tooth flexure (load) tests. Magnitude, number of cycles?
1. 2. 3. 4. 5. 6. 7.
between the etchabilityof smear layerscreatedwith diamond burs us carbide burs (Pashley,unpublished observations). It would be desirable to prepare dentin in the laboratory with dentalbuts operatedin high-speedhandpieces(Tagamiet al., 1991). A related question involves the influence of surface roughness on bond strength since low-grit abrasives give rougher finishes than high-numbered grits. Mowery et al. (1987) reportedthat 600 grit Sic produced lower mean shear bond strengths of Scotchbond to dentin than did 60 grit Sic finished surfaces. Finger (1988) found little differencein the tensile bond strength of a number of bonding systems as a functionof dentin roughness. McInneset al. (1990) also found little effect from surface roughness on shear bond strength using Scotchbondm-30 (3M Dental Products,St. Paul, MN, USA). With regard to reusing teeth, there are several points to consider If one is testingthe sensitivityof a bonding systemto dentindepth,then determinationof bond strengthsto super& cial, middle and deep dentin is desirablebecause it permits use of repeated measures analysis of variance (wirier, 1971). However, there is always the danger that the previous debondingtest may have produced cracksin the substrate,or the previous bonding may have forced resin deep into the tubules,therebyalteringthe afIinityof resinsfor the substrate. Careful SEM analysis should provide answers to those questions. Ifthe previousbond test caused cohesivefailure of dentin,then the specimenis no longer usable. When teeth are 118 Pashley et allDentin adhesion testing
1. 2. 3. 4. 5. 6. 7. 0.
Etchina Etch or no etch? What type of etchant? How much etchant? Renewed? How long? Passive or active? Rinse? How long? Dry ? How long? Rewet? How much?
1. 2. 3. 4. 5. 6.
Bonding How much adhesive for how long? Spread with air? How thin, how dry? What diameter bonding area? Pack with pressure or no pressure? Pulpal fluid/pressure or not? Light-curing. How much light? How long?
D.
F. Testina 1. Shear vs. tensile 2. Stress rate 3. Immediate vs. 24 hr. vs. months. 4. Express dentin bonds in MPa or as % of enamel bond strength? 5. Microleakage vs. bond strength 6. Gap size vs. bond strength 7. Regional bond strengths vs. center 8. Gingival floor of Class V vs. occlusal floor of Class I cavities 9. Configuration factors. Flat surfaces vs. 3-D cavities.
mounted in plastic or stone, the dentin surface should be positioned so that it is always above the surface of the embedding material. This avoids smearing the embedding materialover the dentinsurfaceduring surfacepreparationof the dentin. However,the dentin surfacescan stillbe contaminated by substances leaching from the embedding material duringwater storageperiodsbetweensurfacepreparationand bonding procedures. Therefore,many investigatorsprefer to prepare the surface immediatelybefore bonding.
PULPAL FLUID Many embedded specimens have relatively dry pulp chambers (Table 1). Thus, even though the specimens are stored in water, there may be a gradient of water content throughout the dentin, which may lead to higher or lower dentinbonds, dependingupon the bonding system. Moisture effects have been repeatedly demonstrated(Mitchem, et al., 1988; Tao and Pashley,1989;Pratiet al., 1991) althoughmany of the most recentlydevelopedbonding systemsare much less sensitiveto the degree of wetness of dentin than were earlier bonding agents (Prati and Pashley, 1992; Alhabashy et al., 1993; PratietaZ.,1995). Some of the problemsassociatedwith attemptsto simulate the in. uiuo condition in. vitro (Pashley 1991)have been eliminatedby bondingin viva (Stewartetal., 199Oa;Mason et al., 1994) or by bonding in uiuobut aging in vitro(Stewartet aZ., 199Ob). One way to better simulate in uiuo conditionsis to fill the
pulp chamber with water, a physiological salt solution (Tao and Pashley,1988) or a 1:3 dilutionof plasma with physiological salt solution(Nikaido et al., 1995). This solutioncan also be placed at zero hydrostatic pressure or some positive pressure (Tao and Pashley, 1989) during either bonding or during storage or under both conditions (Table 1). Early studies using simulated pulpal pressures in vitro utilized 32 cm HO. More recentresearchindicatesthat normalpulpal pressures in intact pulps are about 12 cm HO in the cat Wongsavan and Matthews, 1992) and 14 cm HO in humans (Andrews et al., 1972; Ciucchi et al., 1995). These values increase in inflamed pulps Wan Hassel, 1971; Stem& et al., 1972;Tonder and Kvinnsland, 1983). When local anestheticsolutionscontainingvasoconstrictor agents are injected in patients, pulpal blood flow is temporarilyreduced about 60% (Pit&Fordet al., 1993; Odor et al., 1994). Since pulpal pressure is derived from perfusion pressure, reductions in pulpal blood flow should lead to reductionsin pulpal tissue pressure. This is the drivingforce that moves pulpal/dentinalfluid to the surface where it can diluteprimersand competewith resin monomersfor collagen or other constituents of dentin. Some investigators have conductedbonding studies where a pulpal pressure of 34 cm H,O was applied during bonding but not during storage (Mitchem et aZ., 1988). Others have bonded at zero pressure but stored under 32 cm HO (Tao and Pashley, 1988; 1989). Stillothers have bonded and storedunder 36 cm HO (Pratiet al., 1991; Prati and Pashley,1992). In view of what is known today bonding and storageof dentinspecimensshouldbe done at simulatedpulpal pressuresof 15-33 cm HO (Table 1). The highervalue simulatesprobablepulpalpressuresthat develop severalhours after cavity preparationdue to pulpal irritation (Pashleyet al., 1981). Thus far, the discussion has been limited to dentin as a bonding substrate. Much progress has been made over the last ten years in this area. Investigators should follow the standardspublishedby Rueggeberg(1991) and by Siiderholm (1991) during bonding studies. The next problem involves bonding methods and variables.
mineralized dentin subsurfaces occurring simultaneously Simplysubstitutinga high modulusresincompositefor a lower modulus material has been shown to significantlyincrease the apparent shear strengths(Ericksonet al., 1989) of bonds. True interfacialtesting,whether in shear or tension probably becomes a cleavage test as soon as the first crack begins to propagate from a defect, void or other source of stressconcentration. Quantitative bond testing involves determination of fracture toughness or the energy of fracture (O’Brien and Rasmussen,1984;Tam and Pilliar,1993). It has been applied both to the toughness of bulk materialsand to the inter-facial bondsbetweenadhesiveresinsandtoothstructure(Harashima et ab, 1988; Tam and Pilliar, 1993, 1994; Lin and Douglas, 1994).
FINITE ELEMENT ANALYSIS Finiteelementanalysis(FEA)has been usefulto predictstress distributionswithin teeth and at the interface of adhesives and dentin Wan Noort et al., 1989; 1991). This modeling requires knowledge of the strength of materials us. the strength of mineralizeddental tissues and the difXerencesin elastic moduli of materials us. dental tissues. Then threedimensional stress distributionwithin these structures can be calculatedduring various types of loading. Unfortunately, little information is available on the large stress gradients that develop at bonded interfaces because of a lack of knowledge of the modulus of elasticity of demineralized dentincollagen,resin-infiltrateddemineralizeddentinmatrix, resin tags, length of resin tags, etc. As this information becomes available,FEA may provide importantnew insight into the dynamics of bonded interfacesduring applicationof cyclicloadingor thermal gradientsand during other types of stressing.
FRACTURE MODElFRACTOLOGY
In early studies of dentin bonding, almost all bond failures wereadhesivein naturebecausethe bondsweremadeto smear layer-covereddentin. Careful SEM examinationof both sides of failed bonds made to smear layers revealedthat the failure was not adhesive but that the smear layer failed cohesively BONDING METHODS AND VARIABLES That is, both sides of the failed bond were covered by smear 0ilo’s (1987)reviewof debonding tests dividedthem into quali- layer particles (Tao and Pashley, 1988). Thus, there is a tative screeningtests and quantitativetests. This separation danger in classifying the mode of failure visually,whether the nakedeye or a dissectingmicroscope.Neithermethod was based upon the expectationsof the test. “Do we want a using quality type of test to study bond or no bond and the mode of has the resolution required to identify the mode of failure failure, or do we want to quantitate the bond in order to which is often microscopic in detail. However, such visual predict something about the load capacity and lifetimeof the classificationis helpful in providing overall descriptions of bond” (0ilo, 1987). The quality test should be well- obviousmodes of bond failure,especiallycohesivefracturesin standardized and easy to perform. It can be tensile, shear, adhesives,compositesor in dentin. It is of interest to note that different bonding materials torsion, cleavage, pull or extrusion (Haller, 1993) or Cpoint bending @ilo, 1987). The easiest to perform are shear tests. produce different modes of fracture. Burrow et al. (1993) The load can be distributed(as in lap-shearor blunt-endshear reported that most of the bond failures that they observed bar) or inter-facial(wire loop in shear). There is a strong were adhesive in nature for Superbond C&B but were tendency to develop a bending moment in most shear tests. primarily cohesive in dentin for Clear61 Liner Bond 2. Tensiletests,in theory,should developmore uniform stress Apparently there are large differencesin the stress distribudistributions if there is correct alignment between the tions at the dentin interfacethat lead to very differentmodes specimen and the adherent. However,stress distributionsin of fracture between these two materials, even though their such tests have been shown to be nonuniform (van Noort apparent tensile bond strengths were not statistically different(13.8 + 3.8 MPa for Super-bondD Liner us. 13.4 + et al., 1989; 1991). Presumably, this is due to complex combinations of elastic and plastic deformations of the 4.0 MPa for Clear-61Liner Bond 2) after 3 mon of water adhesives,resincomposites,demineralizeddentinsurfacesand storage(Burrow et al., 1993). Dental Materials/March1995 119
For systems that bond to dentin by creation of a hybrid layer, it is of interest to know whether the bond fails adhesively at the top of the hybrid layer,whether the hybrid layer fails cohesively or whether the failure occurs at the bottom of the hybrid layer where one would expect the steepest gradient in moduli of elasticitybetween the hybrid layer and the underlying mineralizeddentin (Burrow et al., 1993). As these are microscopicstructures,SEM examination is required. However, the size of conventional bond-dentin interfaces(ca. 3 mm diameter)is a very large area to examine by SEM at the magnificationsrequiredto resolvehybrid layer components (i.e., 10,000x). They are so large that one often sees many types of failure patterns in the same sample (Fig. 1). Similardetailed microscopicexaminationis required in analyzinginterfacialfailuresin fractology(Tam and Pill& 1993; 1994; Lin and Douglas, 1994). Fig. 1. Concentric fracture patterns at the interface of dentin bonded with Clearfil Liner Bond (from Burrow eta/., 1993); with permission.
STABILITY OF DENTIN BONDS While the developmentof early (i.e., 15 min) bond strengthis
a very important variable in evaluating a bond system, so is the long-term(i.e., 1 y) bond strength. That is, we would like to have early bond strengths in excess of 20 MPa which do not fall over the long term. Kiyomura (1987) reported that 4-METADIMA-TBBdentin bonds fell from 15 MPa to 6 MPa in 1 y and to 3 MPa in 5 y when samples were immersed in water at 37°C. Few long-termstudiesare done, and hence, it is unclear how stable the dentin bonds are of recently developed bonding systems. Burrow et al. (1993) recently publisheda longitudinalstudy of the effectsof storagetime on the tensile bond strengths of a number of dentin bonding systems. They reported that there was little change in the tensilebond strength of Superbond D Liner or Clear61Liner Bond 2 over a 1 y period, but that the Clear61Liner Bond System and Clear61 Photobond exhibited slow, progressive decreasesin bond strengthover a 1 y period. Presumably,those systems that successfullyinfiltratethe demineralizeddentinsurfaceto its completedepthshouldgive more stable bonds than those systems which have only infiltratedthe top half of the demineralizeddentin. However, the resins must also wet the collagen fibers and polymerize
Fig. 2. Cohesive fracture of large piece of dentin obtained during shear bond test-
ing. A. Fractured dentin (FD) on dentin side of failed bond. B. Fractured dentin piece on resin side (from Eick et a/., 1993); with permission.
120 Pashley et a/./Dentin adhesion testing
and Rodriguez (1962) &ported the ultimate ten& strength of human dentin was 52110 MPa (Table 2). I&man (1967) reported an ultimatetensile strength (UTS) of 37 2 11 MPa. Smith and Cooper (1971) reported a shear strength of
Mineralized Enamel
II.
III.
Dentin Dentin Dentin Dentin Dentin
(human) (human) (human) (human) (bovine)
Enamel Dentin, Dentin, Dentin, Dentin, Dentin, Dentin,
(human) at DEJ Superficial Middle Inner Deep Near Pulp
Demineralized Dentin (bovine) Dentin (bovine) Dentin (human) Dentin (human)
Strength 10.3 t 51.7+ 36.6 f 78.4? 93.8ill.l 90.6 f
84.1 19.3 + 5.4 ll.Of 5.8 14.7 + 5.9
Bowen & Rodriguez (1962) - tensile Craig (1993) - tensile Bowen & Rodriguez (1962) Lehman (1967) - tensile Watanabe et al. (In Press) shear Sano et al. (1994b) - tensile Sano et al. (1994b) tensile
93.2 f 13.9 138.3 131.5 90.3 78.5 49.1 45.1
-
Smith Smith Smith Smith Smith Smith Smith
28.0 26.0 29.6 105.6
-
Akimoto Sano et Sano et Sano et
f + ? *
2.6 (10) 10.3 (9) 10.9 (12) 13.3 (12) (6) 18.9 (8)
3.9 (5) 11 .O (10) 5.9 (10) 16.3 (10)
Resin-infiltrated demineralized dentin (RIDD) RIDD (AB2) 121.6 ? 20.3 (6) (SC-MP) 111.5~14.5(10) (SB) 117.6? 12.2 (6) (CLB) 102.6+ 3.7 (6) (PB) 57.6 f 16.4 (6)
0.26 i 0.12 0.25 * 0.07 13.7 * 3.4
3.6? 3.1 f 4.1 + 2.3 + 2.1 *
superficialdentin (done with a micropunch test) at 132 -c 28 MPa (Table 2). The chemical and physical propertiesof dentin have recently been reviewed by Marshall (1993a, 1993b) and Kinney et al. (1995).Watanabeet al. (In press) measured the shear strengthof human dentinbetween 78 f 13 MPa and 91.8 rf:12.7 MPa dependingupon locationand tubule orientation. Using a microtensile method, Sano et al. (199413) obtained a UTS of human dentin of 106 2 16 MPa (Table 2). Sano et al. (1995) reported that resin-infiltrateddemineralized dentin producedyield stressesthat were not statistically different from that of mineralized dentin, although the modulusof elasticitywas much lower(Table2). Akimoto(1991) measuredthe tensilestrengthof demineralizedbovine dentin matrix at 28.0 MPa (Table 2). Sano et al. (1994b) confirmed the high tensilestrengthof human demineralizeddentin. They reported the ultimate tensile strength of the demineralized dentin matrix was 29.6 * 5.9 MPa and that the modulus of elasticitywas 250 MPa (Table2). In a more recentpaper (&no et al., 19951,the authors suggest that the true strengthof the demineralized dentin matrix is closer to 90 MPa when calculated on the basis of the cross-sectional area of the collagen fibers rather than the total cross-sectionalarea of demineralizeddentinwhich is approximately70% water-30% organic matrix. Thus, the strength of the demineralized dentin matrix may be three times higher than the reported value of 29.6 MPa. All of thesevalues for the ultimatestrength of dentin are much greater than the 25-30 MPa values
0.8 0.7 0.8 0.3 0.5
San0 San0 San0 San0 San0
& Cooper & Cooper & Cooper & Cooper & Cooper & Cooper & Cooper
et et et et et
(1971) - shear (1971)- shear (1971) - shear (1971) -shear (1971) - shear (1971)- shear (1971) - shear
(1991) -tensile al. (1994b) - tensile al. (199413) - tensile al. (1994b)- tensile
al. (1995)
- tensile al. (1995) - tensile al. (1995) - tensile al. (1995) - tensile a/. (1995) tensile
associatedwith the cohesive failure of dentin that have been obtainedduring bond testing. This discrepancysuggeststhat the bonded specimensdevelop abnormal stress distributions during testing, leading to failures of the dentin substrate at stresses far below its ultimate strength. This notion was confirmed by Van Noort et al. (1989; 19911 using finite element analysis to demonstrate the nonuniform stress distributionswithin conventionallybonded specimens.
TESTING METHODS Testingmethodshave never been well standardizedalthough a number of importantrecommendationshave been made for both the substrate (Rueggeberg, 1991) and testing methods C&lo,1987;Siiderholm,1991). The latterreference,whilevery comprehensive,does not recommenda standardsurface area but rather provides for surface areas of 0.071, 0.196 or 0.785 cm2(diametersof 3,5 or 10 mm). As fracturestrengthis given per unit area, the surface area is extremelyimportant. It can be controlled with appropriate molds or jigs but one obtains differentstress distributionsif one coats dentin with an adhesive layer before or after positioning a matrix (Van Noort et al., 1991). Wang et al. (1972) obtained constant adhesive bond strengths between two adherents when the bonded surface area was varied. However,this was not done with a biologicalbonding substrate. The only reportthat this writeris awareofregardingvariationsin dentinbond strength withsurfaceareawas anA4DR abstractpublishedby Erickson Dental Materials/March
7995 121
Composite Dentin
A
TBS (MPal TBS=58.758-2.792 log SA (P=O.OOOl,
80
?=0.703)
I
Composite Dentin
0’
I
0
5
I
10
I
15
SA (mm21 Fig. 4.Tensile bond strength (TBS) of Clearfil Liner Bond 2 as a function of bonded surface area using a microtensile bond strength testing method. Open circles indicate specimens exhibiting cohesive failure in dentin while closed circles indicate adhesive failures. Fig. 3. Bonding procedure and specimen preparation for microtensile bond strength test. A, B. Preparation of tooth for bonding. C. Composite crown bonded to dentin. D. Preparation of bonded specimen for sectioning. E. Serial sectioning of specimen F. Individual slab shown in profile and full-view. G. Slab trimmed at bonded interface. H.Trimmed slab in special grips.
et al. (1989). Using bovine dentin and Scotchbond2 adhesive, they found significantlyhigher shear bond strengthsusing a wire loop compared to knife-edge or blunt shear bars. Further,the stifferP-30 gave higher shearbond strengthsthan did Silux. When they examined the effect of surfacearea, the highest bond strengths were obtained with the smallest surface area that they tested (0.096 cm?, but they were not statisticallysignificantlydifferent from those obtained with surface areas of 0.123 or 0.196 cm2. There is a tendency for large bonded surface areas to produce cohesive failures in dentin at relatively low bond strengths.Using bovine teeth and a bonding area of 0.238 cm2 (5.5 mm diameter hole), Perinka et al. (1992) reported an 82% cohesivefdure of dentinbonded with ClearIilLinerBond System at 9.2 f 4.4 MPa. When Erickson et al. (1989) used Scotchbond 2/p-30 to bond to bovine dentin using a surface area of 0.123 cm2(3.5 mm diameterhole), they obtained 80% cohesive fractures in dentin at a 16.9 -c 3.1 MPa. Recently, Sano et al. (1994a; 1994c) have reported that tensile bond strengthsare inverselyrelated(Fig. 4) to bonded surface area using very small areas (0.005-0.12 cm?. In that study,they found that as the cross-sectionalarea of bonded specimens was reduced, the number of cohesive failures of dentin fell to zero at about 2 n-n-&.Below 2 mm2,all failureswere adhesive in nature. In an effort to developbond tests of small areas, Sano et al. (1994a; 1994c) developedwhat can be calledthe “microtensile method”. In essence, this method involves bonding adhesive resins to the entire flat occlusalsurfaceof teeth which is then covered with a resin composite “crown”. After curing and storage in water, the specimen is vertically sectional into multipleserial sections(Fig. 3E) using a slow-speeddiamond 122
Pashley et a/./Dentin adhesion testing
saw (Buehler Ltd., Lake Bluff, IL, USA). The resultingslabs are composedof an upper half of resin compositeand a lower half of dentin (Fig. 3F). Using an ultra-finediamond bur, the cross-sectionedarea at the bonded interface was reduced to form an hour-glass shape to ensure maximum stress developmentat that region(Fig.3G). The bonded surfacearea that was tested was calculatedfrom the specimen thickness and width which was measured with a digital micrometer. The ends of the specimens were attached to microgripsof a Bencor Multi-T device (Danville Engineering, Danville, CA, USA) with cyanoacrylatecement. The devicewas then placed in a universaltestingmachine (Instron,Model #loll, In&on Corp., Canton, MA, USA) and stressed at 1 mm/min in tension. After testing,the modes of fractureof each specimen were determinedby examination in a dissectingmicroscope (B&L Stereomicroscope,Rochester,NY, USA) at 10x. These experimentswere begun using ClearhlLinerBond 2 (KurarayCompanyLtd.,Kyoto,Japan). When the tensilebond strength was plotted as a function of bond surface area, an exponential increase in bond strength was noted with decreasingsurface area (Fig. 4). At the largest surface areas (ea. 0.07-o.12 crnz),there were some cohesivefailuresof dentin atbond strengthsranging from 1520 MPa. However,at bonded surfaceareas below 0.02 cm2,all of the failureswere adhesive in nature by visual examination even though the bond strengths increased to 50-60 MPa. The highest adhesive failure of those bonds was 71 MPa. These resultsprovideadditionalevidencethat the intrinsic strengthof dentin is much higher than the 25-30 MPa values that are commonly measured in bond testing when dentin fails cohesively. In essence,this new testing method tends to produce higher bond strengths than conventional methods, the results are apparentlydependentupon surface area, and it producesprimarilyadhesivefailuresat surfaceareas below 0.02 cmz. Qualitativelysimilar results have been obtained with other adhesive systems. Thereasonforthisincreaseinbondstrengthwithdecreases in bonded surface area is probably due to the presence of
Advantaaes 1. More adhesive failures, fewer cohesive failures 2. Higher inter-facial bond strengths can be measured 3. Permits measurements of regional bond strengths 4. Means and variances can be calculated for single teeth 5. Permits testing of bonds made to irregular surfaces 6. Permits testing of very small areas 7. Facilitates SEM examination of the failed bonds since the surface area is approximately 1 mm*. Disadvantaaes 1. Labor intensive, technically demanding 2. Difficult to measure bond strengths < 5 MPa. 3. Requires special equipment 4. Samples are so small they dehydrate rapidly
defects or stressraisers at the bonded interfaceor within the substrate. According to the Griffith defect theory (Griffith, 19201, the tensilestrengthof a brittlematerialdecreaseswith increasing cross-sectionalarea. Larger specimens seem to contain more defectsthan smallerspecimens. The same may be true of bonded dentin surface areas. Adhesive bonding is not un5orn-imicroscopically The interfaces can contain air bubbles, phase separations, surface roughnesses and non uniform film thicknessesthat can lead to nonuniform stress distributions. Van Noort et al. (1989; 1991) used finite elementstressanalysesto demonstratethat tensileand shear bond strengthmeasurementswere highly dependentupon the geometry of the tested interface, the nature of the load application, the presence or absence of adhesive flash, etc. According to their analyses,the probabilitythat defects may dominate the propagationof fracture is very high because of nonun6ormity of the interfacialstressdistribution,especially at the edge of the interface. This may initiatefracturesat the defects,resultingin lower tensile bond strengthsthan might be measured in smaller samples. While no stress analyses have yet been done on the specimensused in the microtensile method,it is probablethat their smallsizehas led to improved stressdistributionsand hence to failuresof materialsthat are closerto their true ultimate strengths. This may also be why the present researchersobtained higher tensile strengthsfor dentin (Table 2) than were previously reported. Smith and Cooper (1971) also obtained higher shear strengthsof dentin in small samples relativeto larger samples. There are a number of advantages and disadvantagesto the use of the microtensile testing method (Table 3). The greatest advantage of the technique is that one can obtain exclusivelyadhesive bond failures of materialsif the bonded surface area is about 1 1111112. Using conventional bonding techniques, we are rapidly approaching the point where we will not be able to distinguish product improvements if the failures that occur in testing are within dentin rather than interfacial. Using the microtensile method, multiple specimenscan be obtained from a single tooth. If the crosssectionalarea of each specimenis the same, one can calculate a mean and standard deviation of the bond strength of a material to a single tooth. Another significantadvantage is that the bonded surface does not have to be flat. Small,
irregular surfaces can be evaluated after bonding under clinicalconditions(Table 3). Dr. Bernard Ciucchi (University of Geneva) is using this method to measure differencesin the regionalbond strength of Class II restorations.Dr.Hidehiko Sano and his colleagues (Tokyo Medical and Dental University) are using the microtensilemethod to measure the bond strength of resin adhesivesto excavatedcaries-affecteddentin. This is possible because the method can measurethe bond strengthsof small, irregularlyshaped areas. The disadvantagesof the method (Table 3) are that it is technicallydi%cult and labor intensive. When the interfacial region is selected,the cross-sectionalsurface area is adjusted with a diamond bur under copious air-water spray. If one uses too much lateralpressureon the bur,the bond will fail. If the bur is not concentric,it can producesufficientvibrationsto damage the bonded interface. Materials or regions that produce low bond strengths (aa. 5 MPa) oRen fail during trimming. The samples are so small that they must be kept moist to avoid drying effects. Care must be taken during sectioningto avoid heat production. A slow-speedsaw blade lubricated with water is required. Special equipment is required to ensure proper alignment of the specimens and application of uniform tensile stress without any torsion effects. The actual tensile forces that develop are generally less than 3-4 kg. Thus, a 5 kg straingauge is generallyused. Adhesion tests, while not perfect, have enabled the development of improved bonding systems and techniques. There was a real need for standards with regard to the substrate, the various steps in bonding and in testing. Unfortunately,now that standards have been promulgated, they may have to be modified because the newest products develop such high strength that there are too many cohesive failures in the substrate. To avoid this problem, a new microtensiletesting method has been developed that has a number of advantages (Table 3). This method permits bonding under clinicallyrelevantconditions. Bonds in excess of 70 MPa have been measured in the adhesivefailure mode. Presumably, new products will be developed which are intrinsicallystronger and which develop stronger bonds to mineralizedtooth structures. They should be simple to use, bond equallywell to enamel,superficialor deep dentin,and be relativelyinsensitiveto moisture. This should permit more uniform, consistent, dentin bonding which will be of significantbenefit to adhesive dentistry. In conclusion, significant improvements in bonding procedures and products have developed over the past ten years. Dentinbond strengthsare oftenhigherthan resinbonds made to acid-etched enamel. These higher dentin bond strengths develop nonuniform stress distributionsin dentin during in vitro testing, causing cohesive failures in the substrate rather than in the bonded interface. Thus, conventionalbond testing methods can no longer be used to detect further improvements in product development or bonding procedures. New testing methods need to be developed and standardizedto provide researcherswith the tools needed to make further advances in resin adhesion to dentin. More emphasis should be placed on bonding to scleroticcervical dentin and caries-affecteddentin to perfect the materials and techniques required to bond to these clinicallyrelevant substrates.
Dental Materials/March 1995 123
Grit&h AA (1920). The phenomena of rupture &d fldw”in solids. Phil Trans Roy Sot Lon (Series A> A221:168This work was supportedin part by grant DE06427 from the 198. National Institute of Dental Research and by the Medical GwinnettAJ (1992). Inter-facialmorphology of resin composCollege of Georgia Dental Research Center. The authors are ite and shiny erosivelesions. Am JDent 5: 315-317. gratefulto Shirley Johnston for secretarialsupport. Haller B, Hofmann N, KIaiber B, Pfannkuch A (1993). BestSndigkeit des komposit-dentinverbundes bei Presented at the 1994 ADM Annual Meeting, Bermuda, November 1994 kunshicher alterung.Dtsch Zahnarztl 2 48:100-104. Harashima I, Hirasawa T, Tomioka K, Okada J (1988). Address correspondence and reprint requests to: Fractographyof the bonding betweenlight-curedresin and David H. Pashley tooth substrates. Dent Mater 7:151-159. School of Dentistry Kemper K, Kihan R (1976). New test system of tensilebond Medical College of Georgia strengthtesting. J Dent Res 55: Abstr.No. 308. Augusta, Georgia USA 30912-1129 Kinney JH, Balooch M, Haupt DL, Marshall GW (1995). Mineral distributionand dimensionalchanges in human dentinduring deminerahzation.JDent Res 74:1179-1X34. REFERENCES Kiyomura M (1987). Bonding strengthof bovine dentin with 4-META/MMA-TBBresin: Long-term stabilityand influAkimot~ T (1991). Study on adhesionof MM&TBBO resin to ence of water. J Jpn Dent Mater 6:860-872. dentin. Jpn Sot Dent Mater Devices 10:42-54. AIhabashy A, Swi&EJ, Boyer DB, Denehy GE (1993). Effects Lehman ML (1967). Tensile strength of human dentin. J of dentinpermeabilityand hydrationon the bond strengths Dent Res 46:197-201. Lin CP, Douglas WH (1994). Failure mechanisms at the of dentin bonding systems. Am JDent 6:123-126. Andrews SA, Van Hassel HJ, Brown AC (1972). A method for human dentin-resin interface: A fracture mechanics approach. J Biomech 27:1037-1048. determiningthe physiologicbasis of pulp sensoryresponse. A preliminaryreport. 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The long term dura- McInnes PM, Wendt SL, Retief DH, Weinberg R (1990). bility of bond strengths to dentin. Bull Tokyo Med Dent Effect of dentin surfaceroughnesson shear bond strength. Dent Mater 6:204-207. Univ 40:173-191. Causton BE (1984). Improved bonding of composite restor- Mitchem JC, Gronas DG (1986). Effects of time atherextracative to dentine. Br Dent J 156:93-95. tion and depth of dentin on resin dentin adhesives. JAm Dent Assoc 113:285-287. Causton BE (1987). In vitro assessment of dentin bonding agents. In: Todays Dentist. Proceedings of International MitchemJC, TerkIaLG, Gronas DG (1988). Bonding of resin dentinadhesivesunder simulatedphysiologicalconditions. Symposium on Adhesion, Its Theory and Practice in ReDent Mater 4:351-353. storativeDentistry,Specialedition.London:CurrentMediMower-yAS, Parker M, Davis EL (1987). Dentin bonding: cal LiteratureLtd., 18-27. Effect of surface roughness on shear bond strength. Oper Ciucchi B, BouiIlaguetS, Holz J, PashIey DH (1995). DenDent 12:91-94. tinaIfluid dynamicsin human teeth, in uiuo. J Endodant Nara Y, Katsuyama S, Dogon IL ( 1994). A comparisonof the 21:191-194. adhesion of two new restorative systems to dentin with Craig RG (1993). Restorative Dental Materials, 9th ed. St. wedge-shaped defects. JDent Res 73:130, Abstr.No. 232. Louis: Mosby 64. Duke ES, LindemuthJS (1991). Variabilityof chnical dentin NikaidoT, Burrow MF,TagamiJ, TakatsuT (1995). Effect of pulpal pressure on adhesion of resin compositeto dentin: substrates. Am J Dent 4241-246. Bovine serum us. saline. Quintessence Int 26:221-226. EickJD,RobinsonSJ,ChappeIlRp Cobb CM, SpencerP ( 1993). The dentin surface- Its influenceon dentin adhesion,Part O’Brien WJ, Rasmussen ST (1984). A critical appraisal of dental adhesiontesting.In: MittaIKL ed. AdhesiveJoints, III. Quintessence Int 241571-582. New York Plenum PublishingCorp., 289-305. Erickson RL, GlasspooleEA (1994). Bonding to tooth structure: A comparison of glass-ionomerand composite-resin Odor TM, Pitt-FordTR, McDonaldF ( 1994). Effect of inferior alveolar nerve block anesthesia on the lower teeth. systems. J Esth Dent 6:227-244. Endoabnt Dent 7Faumatol 10:144-148. Erickson RL, GlasspooleEA, Retief DG (1989). Intluenceof test parameterson dentinbond strengthmeasurements.J 0ilo G (1987). Adhesion of dental materials to dentin: Debonding tests. In: ThylstrupA, Leach SA, Qvist V, eds. Dent Res 68:374, Abstx. No. 1543. Dentine and Dentine Reactionsin the OraICavity Oxford: Finger WJ (1988). Dentin bonding agents. Relevance of in IRL Press Ltd., 219-224. vitro investigations.Am JDent 1:184-188.
ACKNOWLEDGMENTS
124 Pashley et a/./Dentin adhesion testing
Oilo G, Olsson S (1990). Tensile bond strength of dentin adhesives: A comparison of materials and methods. De& Muter 6:138-144. Pashley DH (19911. Zn vitro simulations of in uiuo bonding conditions. Am JDent 4:237-240. Pashley DH, Nelson R, Pashley EL (1981). In uiuo fluid movement across dentin in the dog. Arch Oral BioZ26:707-710. Perinka L, Sano H, Hosoda H (1992). Dentin thickness, hardness and Ca-concentration US.bond strength of dentin adhesives. Dent Mater 8: 229-233. Pitt-Ford TR, Seare A, McDonald F (1993). Action of adrenaline on the effect of dental local anesthetic solutions.
Endodont Dent TFaumatol9:31-35. Prati C, Pashley DH (1992). Dentin wetness, permeability thickness and bond strength of adhesive systems. Am J
Dent 5:33-38. Prati C, Pashley DH, Montanari G (1991). Hydrostatic intrapulpal pressure and bond strength ofbonding systems.
Dent Mater 7:54-58. Prati C, Ferrieri P Galloni C, Mongiori R, Davidson CL (1995). Dentin permeability and bond quality as tieded by new bonding systems. J Dent 23:217-226. Retief DH, Mandras RS, Russell CM, Denys FR (19901. Extracted human US.bovine teeth in laboratory studies. Am J Dent 3:253-258. Retief DH (1991). Standardizing laboratory adhesion tests.
Am JDent 4:231-236.
Rueggeberg FA ( 1991). Substrate for adhesion testing totooth structure - Review of the literature. Dent Mater 7:2-10. Sano H, Shono T, Sonoda H, Takatsu T, Ciucchi B, Horner JA, Pashley DH (1994a). Tensile bond strength us surface area for dentin bonding - Evaluation of a method using limited surface areas. Jpn J Conserv Dent 37:882-887. Sano H, Ciucchi B, Matthews WG, Pashley DH (1994b). Tensile properties of mineralized and demineralized human and bovine dentin. JDent Z&s 73:1205-1211. Sano H, Shono T, Sonoda H, TakatsuT Ciucchi B, Carvalho R, Pashley DH (1994c). Relationship between surface area for adhesion and tensile bond strength - Evaluation of a microtensile bond test. Dent Mater 10:236-240. Sano H,TakutsuT Ciucchi B, Russell CM, Pashley DH (1995). Tensile properties of resin-infiltrated demineralized dentin. JDent Res 74:1093-1102. Smith DC, Cooper WF,G (1971). The determination of shear strength. A method using a micro-punch apparatus. Br
Dent J 130:333-337. Soderhohn K-JM (1991). Correlation of in uivo and in vitro performance of adhesive restorative materials: A report of the ASC MD156 Task Group on Test Methods for the Adhesion of Restorative Materials. Dent Muter 7:74-83. Stanford Jw Sabri Z, Jose S (1985). A comparison of the effectiveness of dentin bonding agents. Znt Dent J 35:139-144. Stem& A, Iversen J, Mjor IA (1972). Tissue pressure and histology of normal and inflamed tooth pulps in Macaque
monkeys. Arch Oral Bid 17:1501-1511. Stewart BL, Harcourt JK, Tyas MJ ( 199Oa). Comparisons of bond strengths to dentin in cavities restored before and &r extraction. J Dent Res 69:945, Abstr No. 99. Stewart BL, Harcourt JK, Tyas MJ ( 199Ob). Determination of adhesive bond strength to dentin. A new method. Aust
Dent J 35~454-458. Tagami J, Tao L, Pashley DH (1990). Correlation among dentin depth, permeability and bond strength of adhesive resins. Dent Mater 6:45-50. Tagami J, Tao L, Pashley DH, Hosoda H, Sano H (1991). Effects of high-speed cutting on dentin permeability and bonding. Dent Mater 7:234-239. Tam LE, Pilliar RM (1993). Fracture toughness of dentin resin-composite adhesive interfaces. J Dent Z&s 72:953-
959. Tam LE, Pilliar RM (1994). Fracture surface characterization of dentin-bonded in&facial fracture toughness specimens. J Dent Res 73:607-619. Tao L, Pashley DH (1988). Shear bond strength to dentin: Effects of surface treatments, depth and position. Dent
Mater 41371-378. Tao L, Pashley DH (1989). Dentin perfusion effects on the shear bond strengths of bonding agents to dentin. Dent
Mater 5:181-184. Tonder KJH, Kvinnsland I ( 1983). Micropuncture measurements of interstitial fluid pressure in normal and inflamed dental pulp in cats. J Endodont 9:105-109. Van Hassel HJ (1971). Physiology of the human dental pulp. Oral Surg 32:126-134. Van Meerbeek B, Braem M, Lambrechts P,Vanherle G (1994). Morphological characterization of the interface between resin and sclerotic dentin. J Dent 22:141-146. Van Noort R, Howard IC, Cardew G (1989). A critique of bond strength measurement. J Dent 17:61-67. Van Noort R, Cardew GE, Howard IC, Noroozi S (1991). The effect of local inter-facial geometry on the measurements of the tensile bond strength to dentin. J Dent Z&s 70:889-
893. Vongsavan N, Matthews B (1992). Fluid flow through cat dentin in uivo. Arch Oral Biol37:175-185. Wang T Ryan F, Schonhom S ( 1972). Effect of bonding defect on shear strength in tension of lap joints having brittle adhesive. JAppl Polym Sci 16:1901-1909. Watanabe LG, Lacy AM, Davis DR (1987). Shear bond strength: Single plane us. conventional lap shear. J Dent Res 67:383, Abstr. No. 2159. Watanabe LG, Nguyen T, Garner M, Kilbourne AM, Marshall SJ, Marshall GW (In Press). Dentin shear strength relative to tubule orientation. Effects of tubule orientation and intratooth location. Dent Mater (accepted). Winer BJ (1971). Statistical Principles in Experiment,al Design. 2nd ed. New York McGraw-Hill Book Company, 261-308.
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25, March, 1995
Adhesion testing of dentin bonding agents: A review David H. Pashleyl, Hidehiko Sanoz, Bernard Ciucchis, Masahiro Yoshiyamad, Ricardo M. Carvalhos ‘Department of Oral Biology, School of Dentistry, Medical College of Georgia, Augusta, Georgia, USA jDepartment of Operative Dentistry, Tokyo Medical and Dental University, Tokyo, JAPAN ,YDepartment of Restorative Dentistry and Endodontics, University of Geneva, Geneva, SWITZERLAND 4Department of Conservative Dentistry, Tokushima University, Tokushima, JAPAN 5Department of Operative Dentistry, Bauru School of Dentistry, University of Sho Paula, BRAZIL
ABSTRACT Adhesion testing of dentin bonding agents was reviewed starting with the adhesion substrate, dentin, the variables involved in etching, priming and bonding, storage variables and testing variables. Several recent reports attempting to standardize many of these variables were discussed. Recent advances in the development of new bonding systems have resulted in bond strengths on the order of 20-30 MPa. At these high bond strengths, most of the bond failure modes have been cohesive in dentin. As this precludes measurement of interfacial bond strength, new testing methods must be developed. One such new method, a microtensile method, was described along with preliminary results that have been obtained. The last decade has produced major advances in dentin bonding. The next decade should prove to be even more exciting.
HISTORICAL PERSPECTIVE Adhesion testing in dentin bonding studies has developed steadilysincethe pioneeringwork of Buonocore(1955). Many improvementsin bond testing have been developed over the last 40 years. The contributions of Bowen (1965), Kemper and Kilian (19761,Watanabe et al. (1987) and Retief (1991) helped to standardizebond testing methods. However,there are an enormousnumber of variablesthat can influencebond testing. These have been listed in Table 1 under the broad categories of substrate variables, etching variables, priming variables, bonding variables, storage variables and testing variables. With regard to substrates, much information has been generated using bovine teeth. However, the use of human teeth is preferable (Retief et al., 1990). Decreases in bond strength in deep dentin were reported by Causton (1984), Stanford et al. (19851,Mitchem and Gronas (1986) and by Tao and Pashley (1988). However, as bonding systems have become more hydrophilic,the sensitivityof bond strength to dentin depth has decreased (Prati and Pashley, 1992). As
superficial and deep dentin are so different structurally,it would seem desirable to screen dentin bonding agents with both types of substrates(Tagamiet al., 1990).
BONDING SUBSTRATES Occlusal dentin tends to give lower bond strengths than proximal or buccal dentin (Causton, 1987; 0ilo and Olsson, 1990). There is greater regionalvariabilityof dentin wetness (Prati and Pashley,1992) in occlusaldentin than in proximal or buccal dentin. As bonding agents improve, this regional sensitivityof bond strengthmay disappear The use of human unerupted third molars requires comment. While these teeth are conveniently collected for bond testing,it shouldbe emphasizedthatthey are much more permeable and wetter than erupted teeth (Pashley, unpublishedobservations).Clinically,most dentinbonding is done to previously restored teeth, carious teeth or abraded lesions, most of which contain sclerotic dentin. Sclerotic dentinis lessetchableDuke and Lindemuth,1991;Gwinnett, 1992;VanMeerbeeket ah, 1994) and its tubules are generally occludedby mineralcrystals.Thus,the bond strengthsto such dentin are thought to be lower than to unerupted third molars. Nara et al. (1994) reported lower bond strengths to sclerotic dentin compared to normal dentin. Clearly, such studiesneed to be expanded because scleroticdentin is one of the most common bonding substrates. More informationis needed on how to prepare and etch such dentin.
SPECIMEN HANDLING The surface preparation of dentin varies widely but few
careful comparisonshave been made. Some laboratoriesuse aluminum oxide abrasive paper while others use silicon carbide(Table 1). Grits vary from 320 to 1200. While the use of abrasive papers is convenient in the laboratory,clinically, dentin is never prepared with abrasive paper. Even when using high-speed handpieces, differences have been found
Dental Materials/March 1995 117
A.
Substrate 1. Human or bovine dentin? 2. Superficial, middle or deep dentin? 3. Occlusal, proximal, buccal? 4. Third molars or incisors? 5. Sanded surfaces? Grit? Method? AI,O,, SIC, 320,600,800 6. Dental burs? Diamond, carbide? Low-speed vs. high-speed; air-water? Reuse of teeth? Mount in plastic, stone, etc.?
C. 1. 2. 3. 4. 5. 6. E.
B.
or 1000 grit.
Primina Cover entire surface or apply within matrix How much primer? Passive or active? How long? Wash or evaporate? How long? Light-cure or not? Wet vs. dry? How wet, how dry?
Storage Water, isotonic saline, etc.? Room temperature or 37X? 100% RH or water? Preservatives? sodium azide, thymol, chloramine? Pulpal pressure or not, magnitude? Composition of fluid? Time. 24 hr.? months? years? Thermal stress? Temperatures, dwell time, number of cycles? a. Tooth flexure (load) tests. Magnitude, number of cycles?
1. 2. 3. 4. 5. 6. 7.
between the etchabilityof smear layerscreatedwith diamond burs us carbide burs (Pashley,unpublished observations). It would be desirable to prepare dentin in the laboratory with dentalbuts operatedin high-speedhandpieces(Tagamiet al., 1991). A related question involves the influence of surface roughness on bond strength since low-grit abrasives give rougher finishes than high-numbered grits. Mowery et al. (1987) reportedthat 600 grit Sic produced lower mean shear bond strengths of Scotchbond to dentin than did 60 grit Sic finished surfaces. Finger (1988) found little differencein the tensile bond strength of a number of bonding systems as a functionof dentin roughness. McInneset al. (1990) also found little effect from surface roughness on shear bond strength using Scotchbondm-30 (3M Dental Products,St. Paul, MN, USA). With regard to reusing teeth, there are several points to consider If one is testingthe sensitivityof a bonding systemto dentindepth,then determinationof bond strengthsto super& cial, middle and deep dentin is desirablebecause it permits use of repeated measures analysis of variance (wirier, 1971). However, there is always the danger that the previous debondingtest may have produced cracksin the substrate,or the previous bonding may have forced resin deep into the tubules,therebyalteringthe afIinityof resinsfor the substrate. Careful SEM analysis should provide answers to those questions. Ifthe previousbond test caused cohesivefailure of dentin,then the specimenis no longer usable. When teeth are 118 Pashley et allDentin adhesion testing
1. 2. 3. 4. 5. 6. 7. 0.
Etchina Etch or no etch? What type of etchant? How much etchant? Renewed? How long? Passive or active? Rinse? How long? Dry ? How long? Rewet? How much?
1. 2. 3. 4. 5. 6.
Bonding How much adhesive for how long? Spread with air? How thin, how dry? What diameter bonding area? Pack with pressure or no pressure? Pulpal fluid/pressure or not? Light-curing. How much light? How long?
D.
F. Testina 1. Shear vs. tensile 2. Stress rate 3. Immediate vs. 24 hr. vs. months. 4. Express dentin bonds in MPa or as % of enamel bond strength? 5. Microleakage vs. bond strength 6. Gap size vs. bond strength 7. Regional bond strengths vs. center 8. Gingival floor of Class V vs. occlusal floor of Class I cavities 9. Configuration factors. Flat surfaces vs. 3-D cavities.
mounted in plastic or stone, the dentin surface should be positioned so that it is always above the surface of the embedding material. This avoids smearing the embedding materialover the dentinsurfaceduring surfacepreparationof the dentin. However,the dentin surfacescan stillbe contaminated by substances leaching from the embedding material duringwater storageperiodsbetweensurfacepreparationand bonding procedures. Therefore,many investigatorsprefer to prepare the surface immediatelybefore bonding.
PULPAL FLUID Many embedded specimens have relatively dry pulp chambers (Table 1). Thus, even though the specimens are stored in water, there may be a gradient of water content throughout the dentin, which may lead to higher or lower dentinbonds, dependingupon the bonding system. Moisture effects have been repeatedly demonstrated(Mitchem, et al., 1988; Tao and Pashley,1989;Pratiet al., 1991) althoughmany of the most recentlydevelopedbonding systemsare much less sensitiveto the degree of wetness of dentin than were earlier bonding agents (Prati and Pashley, 1992; Alhabashy et al., 1993; PratietaZ.,1995). Some of the problemsassociatedwith attemptsto simulate the in. uiuo condition in. vitro (Pashley 1991)have been eliminatedby bondingin viva (Stewartetal., 199Oa;Mason et al., 1994) or by bonding in uiuobut aging in vitro(Stewartet aZ., 199Ob). One way to better simulate in uiuo conditionsis to fill the
pulp chamber with water, a physiological salt solution (Tao and Pashley,1988) or a 1:3 dilutionof plasma with physiological salt solution(Nikaido et al., 1995). This solutioncan also be placed at zero hydrostatic pressure or some positive pressure (Tao and Pashley, 1989) during either bonding or during storage or under both conditions (Table 1). Early studies using simulated pulpal pressures in vitro utilized 32 cm HO. More recentresearchindicatesthat normalpulpal pressures in intact pulps are about 12 cm HO in the cat Wongsavan and Matthews, 1992) and 14 cm HO in humans (Andrews et al., 1972; Ciucchi et al., 1995). These values increase in inflamed pulps Wan Hassel, 1971; Stem& et al., 1972;Tonder and Kvinnsland, 1983). When local anestheticsolutionscontainingvasoconstrictor agents are injected in patients, pulpal blood flow is temporarilyreduced about 60% (Pit&Fordet al., 1993; Odor et al., 1994). Since pulpal pressure is derived from perfusion pressure, reductions in pulpal blood flow should lead to reductionsin pulpal tissue pressure. This is the drivingforce that moves pulpal/dentinalfluid to the surface where it can diluteprimersand competewith resin monomersfor collagen or other constituents of dentin. Some investigators have conductedbonding studies where a pulpal pressure of 34 cm H,O was applied during bonding but not during storage (Mitchem et aZ., 1988). Others have bonded at zero pressure but stored under 32 cm HO (Tao and Pashley, 1988; 1989). Stillothers have bonded and storedunder 36 cm HO (Pratiet al., 1991; Prati and Pashley,1992). In view of what is known today bonding and storageof dentinspecimensshouldbe done at simulatedpulpal pressuresof 15-33 cm HO (Table 1). The highervalue simulatesprobablepulpalpressuresthat develop severalhours after cavity preparationdue to pulpal irritation (Pashleyet al., 1981). Thus far, the discussion has been limited to dentin as a bonding substrate. Much progress has been made over the last ten years in this area. Investigators should follow the standardspublishedby Rueggeberg(1991) and by Siiderholm (1991) during bonding studies. The next problem involves bonding methods and variables.
mineralized dentin subsurfaces occurring simultaneously Simplysubstitutinga high modulusresincompositefor a lower modulus material has been shown to significantlyincrease the apparent shear strengths(Ericksonet al., 1989) of bonds. True interfacialtesting,whether in shear or tension probably becomes a cleavage test as soon as the first crack begins to propagate from a defect, void or other source of stressconcentration. Quantitative bond testing involves determination of fracture toughness or the energy of fracture (O’Brien and Rasmussen,1984;Tam and Pilliar,1993). It has been applied both to the toughness of bulk materialsand to the inter-facial bondsbetweenadhesiveresinsandtoothstructure(Harashima et ab, 1988; Tam and Pilliar, 1993, 1994; Lin and Douglas, 1994).
FINITE ELEMENT ANALYSIS Finiteelementanalysis(FEA)has been usefulto predictstress distributionswithin teeth and at the interface of adhesives and dentin Wan Noort et al., 1989; 1991). This modeling requires knowledge of the strength of materials us. the strength of mineralizeddental tissues and the difXerencesin elastic moduli of materials us. dental tissues. Then threedimensional stress distributionwithin these structures can be calculatedduring various types of loading. Unfortunately, little information is available on the large stress gradients that develop at bonded interfaces because of a lack of knowledge of the modulus of elasticity of demineralized dentincollagen,resin-infiltrateddemineralizeddentinmatrix, resin tags, length of resin tags, etc. As this information becomes available,FEA may provide importantnew insight into the dynamics of bonded interfacesduring applicationof cyclicloadingor thermal gradientsand during other types of stressing.
FRACTURE MODElFRACTOLOGY
In early studies of dentin bonding, almost all bond failures wereadhesivein naturebecausethe bondsweremadeto smear layer-covereddentin. Careful SEM examinationof both sides of failed bonds made to smear layers revealedthat the failure was not adhesive but that the smear layer failed cohesively BONDING METHODS AND VARIABLES That is, both sides of the failed bond were covered by smear 0ilo’s (1987)reviewof debonding tests dividedthem into quali- layer particles (Tao and Pashley, 1988). Thus, there is a tative screeningtests and quantitativetests. This separation danger in classifying the mode of failure visually,whether the nakedeye or a dissectingmicroscope.Neithermethod was based upon the expectationsof the test. “Do we want a using quality type of test to study bond or no bond and the mode of has the resolution required to identify the mode of failure failure, or do we want to quantitate the bond in order to which is often microscopic in detail. However, such visual predict something about the load capacity and lifetimeof the classificationis helpful in providing overall descriptions of bond” (0ilo, 1987). The quality test should be well- obviousmodes of bond failure,especiallycohesivefracturesin standardized and easy to perform. It can be tensile, shear, adhesives,compositesor in dentin. It is of interest to note that different bonding materials torsion, cleavage, pull or extrusion (Haller, 1993) or Cpoint bending @ilo, 1987). The easiest to perform are shear tests. produce different modes of fracture. Burrow et al. (1993) The load can be distributed(as in lap-shearor blunt-endshear reported that most of the bond failures that they observed bar) or inter-facial(wire loop in shear). There is a strong were adhesive in nature for Superbond C&B but were tendency to develop a bending moment in most shear tests. primarily cohesive in dentin for Clear61 Liner Bond 2. Tensiletests,in theory,should developmore uniform stress Apparently there are large differencesin the stress distribudistributions if there is correct alignment between the tions at the dentin interfacethat lead to very differentmodes specimen and the adherent. However,stress distributionsin of fracture between these two materials, even though their such tests have been shown to be nonuniform (van Noort apparent tensile bond strengths were not statistically different(13.8 + 3.8 MPa for Super-bondD Liner us. 13.4 + et al., 1989; 1991). Presumably, this is due to complex combinations of elastic and plastic deformations of the 4.0 MPa for Clear-61Liner Bond 2) after 3 mon of water adhesives,resincomposites,demineralizeddentinsurfacesand storage(Burrow et al., 1993). Dental Materials/March1995 119
For systems that bond to dentin by creation of a hybrid layer, it is of interest to know whether the bond fails adhesively at the top of the hybrid layer,whether the hybrid layer fails cohesively or whether the failure occurs at the bottom of the hybrid layer where one would expect the steepest gradient in moduli of elasticitybetween the hybrid layer and the underlying mineralizeddentin (Burrow et al., 1993). As these are microscopicstructures,SEM examination is required. However, the size of conventional bond-dentin interfaces(ca. 3 mm diameter)is a very large area to examine by SEM at the magnificationsrequiredto resolvehybrid layer components (i.e., 10,000x). They are so large that one often sees many types of failure patterns in the same sample (Fig. 1). Similardetailed microscopicexaminationis required in analyzinginterfacialfailuresin fractology(Tam and Pill& 1993; 1994; Lin and Douglas, 1994). Fig. 1. Concentric fracture patterns at the interface of dentin bonded with Clearfil Liner Bond (from Burrow eta/., 1993); with permission.
STABILITY OF DENTIN BONDS While the developmentof early (i.e., 15 min) bond strengthis
a very important variable in evaluating a bond system, so is the long-term(i.e., 1 y) bond strength. That is, we would like to have early bond strengths in excess of 20 MPa which do not fall over the long term. Kiyomura (1987) reported that 4-METADIMA-TBBdentin bonds fell from 15 MPa to 6 MPa in 1 y and to 3 MPa in 5 y when samples were immersed in water at 37°C. Few long-termstudiesare done, and hence, it is unclear how stable the dentin bonds are of recently developed bonding systems. Burrow et al. (1993) recently publisheda longitudinalstudy of the effectsof storagetime on the tensile bond strengths of a number of dentin bonding systems. They reported that there was little change in the tensilebond strength of Superbond D Liner or Clear61Liner Bond 2 over a 1 y period, but that the Clear61Liner Bond System and Clear61 Photobond exhibited slow, progressive decreasesin bond strengthover a 1 y period. Presumably,those systems that successfullyinfiltratethe demineralizeddentinsurfaceto its completedepthshouldgive more stable bonds than those systems which have only infiltratedthe top half of the demineralizeddentin. However, the resins must also wet the collagen fibers and polymerize
Fig. 2. Cohesive fracture of large piece of dentin obtained during shear bond test-
ing. A. Fractured dentin (FD) on dentin side of failed bond. B. Fractured dentin piece on resin side (from Eick et a/., 1993); with permission.
120 Pashley et a/./Dentin adhesion testing
and Rodriguez (1962) &ported the ultimate ten& strength of human dentin was 52110 MPa (Table 2). I&man (1967) reported an ultimatetensile strength (UTS) of 37 2 11 MPa. Smith and Cooper (1971) reported a shear strength of
Mineralized Enamel
II.
III.
Dentin Dentin Dentin Dentin Dentin
(human) (human) (human) (human) (bovine)
Enamel Dentin, Dentin, Dentin, Dentin, Dentin, Dentin,
(human) at DEJ Superficial Middle Inner Deep Near Pulp
Demineralized Dentin (bovine) Dentin (bovine) Dentin (human) Dentin (human)
Strength 10.3 t 51.7+ 36.6 f 78.4? 93.8ill.l 90.6 f
84.1 19.3 + 5.4 ll.Of 5.8 14.7 + 5.9
Bowen & Rodriguez (1962) - tensile Craig (1993) - tensile Bowen & Rodriguez (1962) Lehman (1967) - tensile Watanabe et al. (In Press) shear Sano et al. (1994b) - tensile Sano et al. (1994b) tensile
93.2 f 13.9 138.3 131.5 90.3 78.5 49.1 45.1
-
Smith Smith Smith Smith Smith Smith Smith
28.0 26.0 29.6 105.6
-
Akimoto Sano et Sano et Sano et
f + ? *
2.6 (10) 10.3 (9) 10.9 (12) 13.3 (12) (6) 18.9 (8)
3.9 (5) 11 .O (10) 5.9 (10) 16.3 (10)
Resin-infiltrated demineralized dentin (RIDD) RIDD (AB2) 121.6 ? 20.3 (6) (SC-MP) 111.5~14.5(10) (SB) 117.6? 12.2 (6) (CLB) 102.6+ 3.7 (6) (PB) 57.6 f 16.4 (6)
0.26 i 0.12 0.25 * 0.07 13.7 * 3.4
3.6? 3.1 f 4.1 + 2.3 + 2.1 *
superficialdentin (done with a micropunch test) at 132 -c 28 MPa (Table 2). The chemical and physical propertiesof dentin have recently been reviewed by Marshall (1993a, 1993b) and Kinney et al. (1995).Watanabeet al. (In press) measured the shear strengthof human dentinbetween 78 f 13 MPa and 91.8 rf:12.7 MPa dependingupon locationand tubule orientation. Using a microtensile method, Sano et al. (199413) obtained a UTS of human dentin of 106 2 16 MPa (Table 2). Sano et al. (1995) reported that resin-infiltrateddemineralized dentin producedyield stressesthat were not statistically different from that of mineralized dentin, although the modulusof elasticitywas much lower(Table2). Akimoto(1991) measuredthe tensilestrengthof demineralizedbovine dentin matrix at 28.0 MPa (Table 2). Sano et al. (1994b) confirmed the high tensilestrengthof human demineralizeddentin. They reported the ultimate tensile strength of the demineralized dentin matrix was 29.6 * 5.9 MPa and that the modulus of elasticitywas 250 MPa (Table2). In a more recentpaper (&no et al., 19951,the authors suggest that the true strengthof the demineralized dentin matrix is closer to 90 MPa when calculated on the basis of the cross-sectional area of the collagen fibers rather than the total cross-sectionalarea of demineralizeddentinwhich is approximately70% water-30% organic matrix. Thus, the strength of the demineralized dentin matrix may be three times higher than the reported value of 29.6 MPa. All of thesevalues for the ultimatestrength of dentin are much greater than the 25-30 MPa values
0.8 0.7 0.8 0.3 0.5
San0 San0 San0 San0 San0
& Cooper & Cooper & Cooper & Cooper & Cooper & Cooper & Cooper
et et et et et
(1971) - shear (1971)- shear (1971) - shear (1971) -shear (1971) - shear (1971)- shear (1971) - shear
(1991) -tensile al. (1994b) - tensile al. (199413) - tensile al. (1994b)- tensile
al. (1995)
- tensile al. (1995) - tensile al. (1995) - tensile al. (1995) - tensile a/. (1995) tensile
associatedwith the cohesive failure of dentin that have been obtainedduring bond testing. This discrepancysuggeststhat the bonded specimensdevelop abnormal stress distributions during testing, leading to failures of the dentin substrate at stresses far below its ultimate strength. This notion was confirmed by Van Noort et al. (1989; 19911 using finite element analysis to demonstrate the nonuniform stress distributionswithin conventionallybonded specimens.
TESTING METHODS Testingmethodshave never been well standardizedalthough a number of importantrecommendationshave been made for both the substrate (Rueggeberg, 1991) and testing methods C&lo,1987;Siiderholm,1991). The latterreference,whilevery comprehensive,does not recommenda standardsurface area but rather provides for surface areas of 0.071, 0.196 or 0.785 cm2(diametersof 3,5 or 10 mm). As fracturestrengthis given per unit area, the surface area is extremelyimportant. It can be controlled with appropriate molds or jigs but one obtains differentstress distributionsif one coats dentin with an adhesive layer before or after positioning a matrix (Van Noort et al., 1991). Wang et al. (1972) obtained constant adhesive bond strengths between two adherents when the bonded surface area was varied. However,this was not done with a biologicalbonding substrate. The only reportthat this writeris awareofregardingvariationsin dentinbond strength withsurfaceareawas anA4DR abstractpublishedby Erickson Dental Materials/March
7995 121
Composite Dentin
A
TBS (MPal TBS=58.758-2.792 log SA (P=O.OOOl,
80
?=0.703)
I
Composite Dentin
0’
I
0
5
I
10
I
15
SA (mm21 Fig. 4.Tensile bond strength (TBS) of Clearfil Liner Bond 2 as a function of bonded surface area using a microtensile bond strength testing method. Open circles indicate specimens exhibiting cohesive failure in dentin while closed circles indicate adhesive failures. Fig. 3. Bonding procedure and specimen preparation for microtensile bond strength test. A, B. Preparation of tooth for bonding. C. Composite crown bonded to dentin. D. Preparation of bonded specimen for sectioning. E. Serial sectioning of specimen F. Individual slab shown in profile and full-view. G. Slab trimmed at bonded interface. H.Trimmed slab in special grips.
et al. (1989). Using bovine dentin and Scotchbond2 adhesive, they found significantlyhigher shear bond strengthsusing a wire loop compared to knife-edge or blunt shear bars. Further,the stifferP-30 gave higher shearbond strengthsthan did Silux. When they examined the effect of surfacearea, the highest bond strengths were obtained with the smallest surface area that they tested (0.096 cm?, but they were not statisticallysignificantlydifferent from those obtained with surface areas of 0.123 or 0.196 cm2. There is a tendency for large bonded surface areas to produce cohesive failures in dentin at relatively low bond strengths.Using bovine teeth and a bonding area of 0.238 cm2 (5.5 mm diameter hole), Perinka et al. (1992) reported an 82% cohesivefdure of dentinbonded with ClearIilLinerBond System at 9.2 f 4.4 MPa. When Erickson et al. (1989) used Scotchbond 2/p-30 to bond to bovine dentin using a surface area of 0.123 cm2(3.5 mm diameterhole), they obtained 80% cohesive fractures in dentin at a 16.9 -c 3.1 MPa. Recently, Sano et al. (1994a; 1994c) have reported that tensile bond strengthsare inverselyrelated(Fig. 4) to bonded surface area using very small areas (0.005-0.12 cm?. In that study,they found that as the cross-sectionalarea of bonded specimens was reduced, the number of cohesive failures of dentin fell to zero at about 2 n-n-&.Below 2 mm2,all failureswere adhesive in nature. In an effort to developbond tests of small areas, Sano et al. (1994a; 1994c) developedwhat can be calledthe “microtensile method”. In essence, this method involves bonding adhesive resins to the entire flat occlusalsurfaceof teeth which is then covered with a resin composite “crown”. After curing and storage in water, the specimen is vertically sectional into multipleserial sections(Fig. 3E) using a slow-speeddiamond 122
Pashley et a/./Dentin adhesion testing
saw (Buehler Ltd., Lake Bluff, IL, USA). The resultingslabs are composedof an upper half of resin compositeand a lower half of dentin (Fig. 3F). Using an ultra-finediamond bur, the cross-sectionedarea at the bonded interface was reduced to form an hour-glass shape to ensure maximum stress developmentat that region(Fig.3G). The bonded surfacearea that was tested was calculatedfrom the specimen thickness and width which was measured with a digital micrometer. The ends of the specimens were attached to microgripsof a Bencor Multi-T device (Danville Engineering, Danville, CA, USA) with cyanoacrylatecement. The devicewas then placed in a universaltestingmachine (Instron,Model #loll, In&on Corp., Canton, MA, USA) and stressed at 1 mm/min in tension. After testing,the modes of fractureof each specimen were determinedby examination in a dissectingmicroscope (B&L Stereomicroscope,Rochester,NY, USA) at 10x. These experimentswere begun using ClearhlLinerBond 2 (KurarayCompanyLtd.,Kyoto,Japan). When the tensilebond strength was plotted as a function of bond surface area, an exponential increase in bond strength was noted with decreasingsurface area (Fig. 4). At the largest surface areas (ea. 0.07-o.12 crnz),there were some cohesivefailuresof dentin atbond strengthsranging from 1520 MPa. However,at bonded surfaceareas below 0.02 cm2,all of the failureswere adhesive in nature by visual examination even though the bond strengths increased to 50-60 MPa. The highest adhesive failure of those bonds was 71 MPa. These resultsprovideadditionalevidencethat the intrinsic strengthof dentin is much higher than the 25-30 MPa values that are commonly measured in bond testing when dentin fails cohesively. In essence,this new testing method tends to produce higher bond strengths than conventional methods, the results are apparentlydependentupon surface area, and it producesprimarilyadhesivefailuresat surfaceareas below 0.02 cmz. Qualitativelysimilar results have been obtained with other adhesive systems. Thereasonforthisincreaseinbondstrengthwithdecreases in bonded surface area is probably due to the presence of
Advantaaes 1. More adhesive failures, fewer cohesive failures 2. Higher inter-facial bond strengths can be measured 3. Permits measurements of regional bond strengths 4. Means and variances can be calculated for single teeth 5. Permits testing of bonds made to irregular surfaces 6. Permits testing of very small areas 7. Facilitates SEM examination of the failed bonds since the surface area is approximately 1 mm*. Disadvantaaes 1. Labor intensive, technically demanding 2. Difficult to measure bond strengths < 5 MPa. 3. Requires special equipment 4. Samples are so small they dehydrate rapidly
defects or stressraisers at the bonded interfaceor within the substrate. According to the Griffith defect theory (Griffith, 19201, the tensilestrengthof a brittlematerialdecreaseswith increasing cross-sectionalarea. Larger specimens seem to contain more defectsthan smallerspecimens. The same may be true of bonded dentin surface areas. Adhesive bonding is not un5orn-imicroscopically The interfaces can contain air bubbles, phase separations, surface roughnesses and non uniform film thicknessesthat can lead to nonuniform stress distributions. Van Noort et al. (1989; 1991) used finite elementstressanalysesto demonstratethat tensileand shear bond strengthmeasurementswere highly dependentupon the geometry of the tested interface, the nature of the load application, the presence or absence of adhesive flash, etc. According to their analyses,the probabilitythat defects may dominate the propagationof fracture is very high because of nonun6ormity of the interfacialstressdistribution,especially at the edge of the interface. This may initiatefracturesat the defects,resultingin lower tensile bond strengthsthan might be measured in smaller samples. While no stress analyses have yet been done on the specimensused in the microtensile method,it is probablethat their smallsizehas led to improved stressdistributionsand hence to failuresof materialsthat are closerto their true ultimate strengths. This may also be why the present researchersobtained higher tensile strengthsfor dentin (Table 2) than were previously reported. Smith and Cooper (1971) also obtained higher shear strengthsof dentin in small samples relativeto larger samples. There are a number of advantages and disadvantagesto the use of the microtensile testing method (Table 3). The greatest advantage of the technique is that one can obtain exclusivelyadhesive bond failures of materialsif the bonded surface area is about 1 1111112. Using conventional bonding techniques, we are rapidly approaching the point where we will not be able to distinguish product improvements if the failures that occur in testing are within dentin rather than interfacial. Using the microtensile method, multiple specimenscan be obtained from a single tooth. If the crosssectionalarea of each specimenis the same, one can calculate a mean and standard deviation of the bond strength of a material to a single tooth. Another significantadvantage is that the bonded surface does not have to be flat. Small,
irregular surfaces can be evaluated after bonding under clinicalconditions(Table 3). Dr. Bernard Ciucchi (University of Geneva) is using this method to measure differencesin the regionalbond strength of Class II restorations.Dr.Hidehiko Sano and his colleagues (Tokyo Medical and Dental University) are using the microtensilemethod to measure the bond strength of resin adhesivesto excavatedcaries-affecteddentin. This is possible because the method can measurethe bond strengthsof small, irregularlyshaped areas. The disadvantagesof the method (Table 3) are that it is technicallydi%cult and labor intensive. When the interfacial region is selected,the cross-sectionalsurface area is adjusted with a diamond bur under copious air-water spray. If one uses too much lateralpressureon the bur,the bond will fail. If the bur is not concentric,it can producesufficientvibrationsto damage the bonded interface. Materials or regions that produce low bond strengths (aa. 5 MPa) oRen fail during trimming. The samples are so small that they must be kept moist to avoid drying effects. Care must be taken during sectioningto avoid heat production. A slow-speedsaw blade lubricated with water is required. Special equipment is required to ensure proper alignment of the specimens and application of uniform tensile stress without any torsion effects. The actual tensile forces that develop are generally less than 3-4 kg. Thus, a 5 kg straingauge is generallyused. Adhesion tests, while not perfect, have enabled the development of improved bonding systems and techniques. There was a real need for standards with regard to the substrate, the various steps in bonding and in testing. Unfortunately,now that standards have been promulgated, they may have to be modified because the newest products develop such high strength that there are too many cohesive failures in the substrate. To avoid this problem, a new microtensiletesting method has been developed that has a number of advantages (Table 3). This method permits bonding under clinicallyrelevantconditions. Bonds in excess of 70 MPa have been measured in the adhesivefailure mode. Presumably, new products will be developed which are intrinsicallystronger and which develop stronger bonds to mineralizedtooth structures. They should be simple to use, bond equallywell to enamel,superficialor deep dentin,and be relativelyinsensitiveto moisture. This should permit more uniform, consistent, dentin bonding which will be of significantbenefit to adhesive dentistry. In conclusion, significant improvements in bonding procedures and products have developed over the past ten years. Dentinbond strengthsare oftenhigherthan resinbonds made to acid-etched enamel. These higher dentin bond strengths develop nonuniform stress distributionsin dentin during in vitro testing, causing cohesive failures in the substrate rather than in the bonded interface. Thus, conventionalbond testing methods can no longer be used to detect further improvements in product development or bonding procedures. New testing methods need to be developed and standardizedto provide researcherswith the tools needed to make further advances in resin adhesion to dentin. More emphasis should be placed on bonding to scleroticcervical dentin and caries-affecteddentin to perfect the materials and techniques required to bond to these clinicallyrelevant substrates.
Dental Materials/March 1995 123
Grit&h AA (1920). The phenomena of rupture &d fldw”in solids. Phil Trans Roy Sot Lon (Series A> A221:168This work was supportedin part by grant DE06427 from the 198. National Institute of Dental Research and by the Medical GwinnettAJ (1992). Inter-facialmorphology of resin composCollege of Georgia Dental Research Center. The authors are ite and shiny erosivelesions. Am JDent 5: 315-317. gratefulto Shirley Johnston for secretarialsupport. Haller B, Hofmann N, KIaiber B, Pfannkuch A (1993). BestSndigkeit des komposit-dentinverbundes bei Presented at the 1994 ADM Annual Meeting, Bermuda, November 1994 kunshicher alterung.Dtsch Zahnarztl 2 48:100-104. Harashima I, Hirasawa T, Tomioka K, Okada J (1988). Address correspondence and reprint requests to: Fractographyof the bonding betweenlight-curedresin and David H. Pashley tooth substrates. Dent Mater 7:151-159. School of Dentistry Kemper K, Kihan R (1976). New test system of tensilebond Medical College of Georgia strengthtesting. 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