d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49

avai lab le at www.sc iencedi rec t .com

journa l homepage: www. int l .e lsev ierhea l th .com/ journa ls /dema

Adhesion to tooth structure: A critical reviewof “macro” test methods

Roberto R. Braga ∗, Josete B.C. Meira, Leticia C.C. Boaro, Tathy A. XavierUniversity of São Paulo School of Dentistry, Dept. of Dental Materials, Av. Prof. Lineu Prestes, 2227, São Paulo, SP 05508-000, Brazil

a r t i c l e i n f o

Article history:

Received 19 November 2009

Accepted 19 November 2009

Keywords:

Bond strength

Shear

Tensile

Dental adhesives

Finite element analysis

a b s t r a c t

Objectives. Bond strength between adhesive systems and the tooth structure is influenced by

a large number of variables, which makes the comparison among studies virtually impossi-

ble. Also, failure often times propagates into the dental substrate or the composite, deeming

the results questionable at best. In spite of the increased popularity gained by micro-tensile

and micro-shear tests, in vitro evaluations using specimens with relatively large bonding

areas remain frequent. This review focuses on aspects related to specimen geometry and

test mechanics of “macro” shear and tensile bond strength tests.

Methods. Besides information drawn from the literature, the effect of some parameters on

stress distribution at the bonded interface was assessed using finite element analysis (FEA).

Results. Bond strength tends to increase with smaller bonding areas and with the use of high

elastic modulus composites. Stress concentration at the bonded interface is much more

severe in shear compared to tension. Among shear methods, the use of the chisel shows the

highest stress concentration. Within the limits suggested by the ISO/TS 11405, crosshead

speed does not seem to influence bond strength values. Pooled data from currently avail-

able adhesives tested in either shear or tension showed 44% of adhesive failures, 31% mixed

and 25% cohesive in the substrate (tooth or composite). A comparative bond strength study

involving three adhesive systems revealed similarities between “macro” and “micro” coun-

terparts regarding material ranking, whereas “macro” tests presented a higher incidence of

cohesive failures.

Significance. Simplicity warrants “macro” bond strength tests an enduring popularity, in spite

of their evident limitations. From a mechanical standpoint, knowing the stress distribution

at the bonded interface and how it is affected by the materials and loading method used is

key to explain the results.

ed by Elsevier Ltd on behalf of Academy of Dental Materials. All rights

reserved.

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© 2009 Publish

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2. A survey of recent bond strength studies . . . . . . . . . . . . . . . .

3. Variables of influence related to specimen design . . . . . . . . . . . .

3.1. Bonding area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.2. Elastic modulus of the resin composite . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +55 11 30917840.E-mail address: rrbraga@usp.br (R.R. Braga).

0109-5641/$ – see front matter © 2009 Published by Elsevier Ltd on behdoi:10.1016/j.dental.2009.11.150

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alf of Academy of Dental Materials. All rights reserved.

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49 e39

4. Variables of influence related to test mechanics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e414.1. Type of loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e414.2. Crosshead speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e43

5. Incidence of cohesive failures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e436. A comparison between macro and micro bond strength tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e447. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e45

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Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

ver the years, clinicians have relied upon laboratory evalua-ions to choose which adhesive systems to use in their dailyractice. Though the validity of bond strength tests to pre-ict clinical performance of dental adhesives is questionable

1,2], recent evidence shows that clinical results can, to somextent, be estimated based upon laboratory results [3–5]. More-ver, mechanical testing of bonded interfaces has providedome valuable information in terms of identifying substrateariables [6,7] and helping define guidelines for applicationrocedures [8].

Until the mid-nineties, shear and tensile bond strengthests were performed exclusively in specimens with relativelyarge bonded areas, usually 3–6 mm in diameter (approxi-

ately 7–28 mm2). However, the validity of expressing bondtrength in terms of nominal (i.e., average) stress has beenuestioned due to the heterogeneity of the stress distribu-ion at the bonded interface [9–11]. Moreover, cohesive failuref both the composite and the dental substrate is a commonccurrence, precluding an accurate assessment of the interfa-ial bond strength [12]. The need for new methods to overcomehese limitations led to the use of specimens with small bond-ng areas (i.e., below 2 mm2), in the so-called micro-tensile and

icro-shear tests [13–15].In spite of the increased popularity of the “micro” bond

trength tests and the criticism endured by the conven-

ional tensile and shear methods, the number of articlessing “macro” tests published in recent years remainsigh, meaning that a lot of the available data on den-al adhesion still comes from mechanical tests performed

ig. 1 – Number of articles/year published on dentin and enamelww.scopus.com. Left: articles grouped by testing method, right

micro” (publications on bond strength of orthodontic brackets t

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in specimens with large bonded areas (Fig. 1). The pref-erence for conventional shear and tensile tests is justifiedbecause they are easy to perform, requiring minimal equip-ment and specimen preparation. However, though a lot ofinformation can be found on specimen geometry and othertesting variables for “micro” bond strength tests [16–18],the same is not true for the “macro” tensile and sheartests.

Reviews published in the past evidenced a concern withthe bonding substrate and specimen storage conditions, onlyminimally discussing the variables of the mechanical testitself [1,12,19–21]. The International Organization for Stan-dardization (ISO) technical specification ISO/TS 11405 [22],published in 1994 and last revised in 2003, reflects this ten-dency, describing with greater detail the characteristics andpreparation of the tooth substrate for the bonding procedure,and leaving aspects such as bonding area, testing assembliesor loading conditions more vague. As a result, a wide varietyof experimental protocols is found among researchers, withevident effect on the outcomes [23,24]. A systematic reviewwith meta-analysis identified composite type, bonding area,testing mode (shear, tensile or micro-tensile) and crossheadspeed as factors that significantly influence bond strength,along with several others related to the substrate, specimenstorage conditions and thermocycling [25]. The type of deviceused for load application was also shown to affect the results[26–29].

The purpose of this article was to review aspects related

to specimen geometry and test mechanics that may influence“macro” shear and tensile bond strength results. Finite ele-ment analysis (FEA) was used to assess the effect of selectedvariables on the stress distribution at the bonded interface.

bond strength between 1982 and 2008, according to: articles grouped by specimen dimensions, i.e., “macro” oro enamel were not included).

e40 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49

Fig. 2 – Three-dimensional models used for finite element analysis (left: shear; right: tension). Points A and B define the

diameter of the bonding area.

Specimen failure mode was discussed based on data retrievedfrom the recent literature. Also, a brief comparison between“macro” and “micro” bond strength tests was performed usingexperimental data obtained with three adhesive systems.

2. A survey of recent bond strength studies

In order to verify how researchers are currently performingtensile and shear bond strength tests, 100 studies publishedbetween 2007 and 2009 (shear: 74, tensile: 26) were surveyed[30–128]. Unsurprisingly, no consensus was found in any of theparameters observed.

Most studies used bonding areas between 3 and 4 mm indiameter (57%), while 4% did not include this information.These numbers do not differ from those reported 12 years ago,where the mean diameter among 50 studies was 3.97 mm and6% of the studies omitted this information [23]. Pre-formedrods were used in 31% of the tensile tests and none of theshear tests. Composite height varied between 2 and 5 mm.A description of the testing method was included in 66% ofthe studies. For shear loading, the knife-edge rod was usedin half of the studies describing the experimental assemblyutilized. All studies reported the crosshead speed, with 0.5and 1.0 mm/min being the most frequent values (46 and 41%,respectively), also in agreement with a recent survey [24].

Specimen failure mode analysis was present in 64% of thestudies, performed either under low magnification (10–50×)using stereomicroscopes (47%) or with the use of scanningelectronic microscopy (17%). Regarding sample size, 10 spec-imens per group were used in 59% of the studies, while 15%used between five and eight specimens and 26% used between

11 and 25 specimens per group. The highest coefficient of vari-ation found in each study averaged 36 ± 14% (minimum: 10%,maximum: 75%). This average is within the range mentionedin the ISO/TS 11405 as expected for these methods (20–50%).

3. Variables of influence related tospecimen design

3.1. Bonding area

The choice of bonding area used in “macro” tensile and sheartests is often made based on the substrate area available. TheISO/TR 11405 [22] does not identify a specific value, but it doesmention a clear delimitation of the bonding area as an impor-tant requirement and shows a diagram of a split mould with a3-mm diameter hole. The relationship between bonding areaand strength has received more attention with the develop-ment of micro-tensile and micro-shear tests. For specimenswith rectangular bonding areas between 0.25 and 11.65 mm2,tensile bond strength to dentin was shown to decrease asbonding area increased, following a logarithmic function [13].A similar trend was observed in enamel, with areas between0.5 and 3.0 mm2 [7]. Another study showed that specimenswith circular cross-section between 1.1 and 3.1 mm2 (1.2 and2.0 mm in diameter, respectively) presented an inverse linearrelationship between bonding area and strength when testedeither in tension or shear [14].

Such relationship between bonding area and strength isexplained by fracture mechanics, initially derived from a seriesof experiments and mathematical deductions performed byGriffith [129], from which he concluded that the strength of asolid elastic body is governed by the presence of microscopicflaws. A few decades later, Irwin [130] defined the parametersinvolved in crack propagation. Briefly, failure of the bondedinterface occurs when a crack propagates from a critical sizeflaw found in an area subjected to high tensile stresses. Thelarger the bonding area, the higher is the probability of a flaw

of critical size being present and, consequently, the lower isthe specimen’s bond strength.

Very few studies evaluated the influence of bonding areaon “macro” strength tests. The shear bond strength of a two-

2 6 ( 2 0 1 0 ) e38–e49 e41

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Table 1 – Maximum principal stress (�max) andmaximum shear stress (�max) at the dentin/compositeinterface, in MPa, observed in shear and tensile loadingas a function of composite elastic modulus (E, in GPa).

Composite E (GPa) Shear loading Tensile loading

�max �max �max �max

Fdr

d e n t a l m a t e r i a l s

tep etch-and-rinse system (Single Bond, 3M ESPE) did notary significantly in specimens with diameters between 2 mm3.1 mm2, 20 MPa) and 5 mm (19.6 mm2, 15 MPa), but it was sta-istically higher for specimens with 1 mm diameter (0.8 mm2,7 MPa) and lower for those with 6 mm diameter (28.3 mm2,MPa) [131]. Another study testing two three-step systemsnder tensile loading, Scotchbond Multi-Purpose (3M ESPE)nd Optibond (Kerr), showed statistically higher bond strengthor 2-mm diameter specimens (10.3 MPa and 15.7 MPa, respec-ively) compared to 4-mm diameter (7.0 MPa and 11.1 MPa,espectively) [132]. Though “macro” shear and tension testshow a trend for increased bond strength values with these of small bonding areas similar to what was observed

n micro-tensile specimens, the evidence in the literature isot sufficient to support a definite statement in terms ofhe influence of bonding areas on strength values. There-ore, the comparison among studies using different bondingreas must consider the specimen size when interpreting theesults.

.2. Elastic modulus of the resin composite

he use of stiffer composites may significantly increase bondtrength values. A recent study observed a weak but sta-istically significant correlation between dentin shear bondtrength and composite flexural properties [116]. A similarrend was observed in an earlier study evaluating the tensileond strength to dentin of an adhesive system associated withifferent composites [133].

The influence of composite elastic modulus on stress distri-ution at the bonded interface was studied by finite elementnalysis (FEA) using three-dimensional models representing3-mm diameter, 2-mm high composite cylinder (� = 0.25)

onded to a dentin disk (E = 18 GPa, � = 0.3) through a 50-�m

hick adhesive layer (E = 2 GPa, � = 0.3). A load was applied tohe composite either perpendicularly or parallel to the bondednterface in order to produce a nominal stress of 16 MPa. Dueo the symmetry in geometry and loading conditions, only

ig. 3 – Stress distributions (maximum principal stress, �max, anentin/composite interface according to the type of shear loadinight: wire loop. Load was applied at 0.2 mm from the bonded int

6 159 123 18.5 7.19 131 114 17.8 6.6

12 114 105 17.3 6.3

half of the model was represented (Fig. 2). MSC.PATRAN (MSCSoftware Corp., Santa Ana, CA, USA) was used for pre- andpost-processing, and MSC.Marc was used as processor.

In agreement with experimental data, stress concentra-tion at the bonded interface decreased as composite modulusincreased from 6 to 12 GPa, which is explained by the reductionin modulus mismatch between both materials. This effect,however, is much less pronounced for tensile loading (Table 1),as previously reported [9]. Also, the effect of composite elasticmodulus on bond strength seems to be dependent upon theadhesive system used [82].

4. Variables of influence related to testmechanics

4.1. Type of loading

Nominal bond strength values carry an underlying assump-tion that stresses are uniformly distributed across the bondedinterface. However, as displayed in Fig. 3, in the shear teststresses close to the loading area are much higher than thenominal shear value (16 MPa). Moreover, the choice of testingassembly has great influence on stress distribution. The use of

a knife-edge chisel causes severe stress concentration at theload application area, whereas the wire loop shows a betterstress distribution at the edge of the bonding area [10]. Stressconcentration with the use of the chisel may explain the small

d maximum shear stress, �max) at the dentin side of theg. Left: 0.2-mm knife-edge chisel; center: 2-mm flat rod;erface. Line A–B indicates the diameter of the bonding area.

e42 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49

Fig. 4 – �max/�max across half of the diameter of the bonded interface according to the type of shear loading, applied 0.2 mmto t

distant from the dentin/adhesive interface (left) or according

knife-edge chisel (right).

areas of composite cohesive failure close to the loading pointobserved experimentally [134]. Also, it may explain the sta-tistically lower bond strengths when the chisel is comparedagainst testing assemblies using larger contact areas betweenthe composite and the loading device, such as the Single-PlaneShear Test Assembly (SPSTA) [131], the Ultradent notched rodand the wire loop [27–29].

Another important aspect of shear tests is that closer to theinterface tensile stresses are actually higher than shear, sug-gesting that the former is responsible for failure initiation, aspreviously stated [9,135,136]. For the knife-edge chisel and thewire loop methods, shear stresses start to prevail over ten-sion at 0.3 mm from the load application area, while for theflat rod tensile stresses predominate up to 1.2 mm (Fig. 4, left).Therefore, the term “shear bond strength” would more appro-

priately refer to the loading mode, rather than the nature ofthe stress responsible for bonding failure.

The distance between the point of load application andthe bonded interface in shear tests also affects stress distri-

Fig. 5 – Stress distributions (maximum principal stress, �max, andentin/composite interfaces loaded in tension (left) or shear, usin(right). Line A–B indicates the diameter of the bonding area.

he point of load application using a 0.2-mm thick

bution. When load is applied up to 1 mm from the interface,tensile and shear stresses increase towards the bonded inter-face, explained by the Saint Venant principle (i.e., a generalizedstress concentration in areas close to the load applicationpoint) [136]. This trend for higher stress concentration withsmaller distances is supported by experimental evidenceshowing lower bond strength when the load was appliedat the interface as opposed to a 0.5 mm distance from thedentin substrate [134]. The ratio between maximum shearand maximum principal stress decreases as load applica-tion moves away from the interface, indicative of an increasein bending moment (Fig. 4, right). When load is appliedat distances beyond 1 mm from the interface (not shown),the increase in tensile stress as load application movesaway from the bonded interface becomes even more evident

[9,136].

In the tensile test, stresses are far more homogeneousacross the interface than in shear and, therefore, maxi-mum principal stress values are much closer to the nominal

d maximum shear stress, �max) at the dentin side ofg a 0.2 mm chisel applied at 0.2 mm from the interface

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49 e43

Fig. 6 – Average failure mode distribution (in %) accordingto bonding substrate and testing mode obtained from 37studies published between 2007 and 2009. Only data whereadhesive was applied following manufacturers’instructions were included. Numbers on top of eachcc

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Fig. 7 – Bond strength averages (in MPa) of the adhesivesystems according to the testing method used. Error bars

olumn indicate the number of publications used toalculate the averages.

trength. However, the area subjected stress levels close to theaximum is much larger than in the shear test (Fig. 5). The

eight of the composite cylinder in the tensile test does notnfluence stress distribution or magnitude if kept above 2 mm9].

.2. Crosshead speed

ue to the viscoelastic nature of polymers and compos-tes, it could be expected that bond strength tests wouldresent some strain rate sensitivity. Experimental obser-ations revealed that composite yield strength and elasticodulus increase at higher strain rates [137], possibly due to

econdary molecular processes. At high strain rates, the poly-er chains become stiffer and molecular mobility is reduced

138]. Determining the actual strain rate (unit: mm/mm s or−1) requires an extensometer and depending upon the spec-men geometry it may not be feasible. Therefore, specimenoading is usually expressed in terms of crosshead speed, in

m/min.Studies evaluating the influence of crosshead speed on

macro” shear and tensile bond strength are few and showontradictory results. Coincidentally, two of them used theame adhesive system and composite (Single Bond and Z100,M ESPE). However, bonding areas, specimen storage timesnd test assemblies were different. One study used theingle-Plane Shear Test Assembly (SPSTA) on specimens with-mm diameter bonding areas, finding no statistically signif-cant differences among crosshead speeds between 0.5 and0 mm/min [131]. The other study used a knife-edge steelod to test specimens with 3-mm diameter bonding area andound statistically higher bond strengths for those loaded at

.0 and 5.0 mm/min compared to 0.5 and 0.75 mm/min [139].

A similar scenario is observed with two studies that eval-ated the effect of crosshead speed on tensile bond strength.oth used similar bonding areas (3.6 and 4.0 mm in diame-

correspond to ±1 SD. In the same test, columns with thesame letter are not statistically different (p > 0.05).

ter) and while one did not report differences among crossheadspeeds ranging from 0.5 to 5.0 mm/min [140], the other foundstatistically higher bond strengths for crosshead speeds of 5.0and 10.0 mm/min compared to 0.1, 0.5 and 1.0 mm/min [141].

Such inconsistencies found for both shear and tensile testsmay be explained by differences among testing assembliesand/or the brittle nature of dental adhesives and composites.The comparison of crosshead speeds among testing assem-blies with different compliance levels is problematic and lessmeaningful than comparing load rates. Also, it has been ver-ified that at small strains, the strain rate variation had anegligible effect on the measured stress values of amorphouspolymers [142]. Nevertheless, it is important to emphasize thatamong the studies described, only one reported differences inbond strength within the crosshead speed range proposed inthe ISO/TS 11405 (0.75 ± 0.30 mm/min).

5. Incidence of cohesive failures

As mentioned above, reporting bond strength in terms ofnominal stress values is questionable due to the heteroge-neous stress distribution and also due to the occurrence ofcohesive failures both in the dental substrate and the resincomposite. Defining categories for classification of failuremodes of debonded specimens is a complicated task and,in some instances, the limit between mixed and cohesivefailure becomes merely subjective. Fig. 6 shows the failuremode distribution observed in 37 studies recently publishedaccording to the bonding substrate and the loading mode[31–34,37,39–41,43,45,64,73,76–80,84,86–88,95–99,101,102,108,109,117,118,120,121,123–125]. Overall, 44% of the specimensfailed exclusively along the bonded interface, while 31%presented mixed failures and 25% presented predominantly

(i.e., more than 75% of the bonding area) cohesive failureeither at the composite or the dental substrate.

Rather than an indication of strong bonding, cohesivefailure is explained by the mechanics of the test and the brit-

e44 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e38–e49

Fig. 8 – Failure mode distributions (in %) observed under 10–30× magnification for each adhesive system, according to thengle

testing method. Top, left: Adper SE Plus; top, right: Adper Si

bottom, right: pooled data.

tleness of the materials involved. In the shear test, tensilestress concentration in dentin near the crack tip causes thefailure to propagate into the substrate. Versluis et al. [134] ver-ified both experimentally and using a failure accumulationcomputer model a tendency for dentin failure to increase atlower crosshead speeds, thicker adhesive layers and movingthe point of load application away from the bonded interface.Dentin pull-out was also associated with high bond strengthvalues, although the correlation for individual specimens wasweak.

A recent study evaluating the tensile bond strength of fiveresin cements to dentin reported a strong positive correlationbetween strength values and the area of cohesive failure inresin observed in mixed mode failures, evaluated using scan-ning electron microscopy [31]. Bonding area has also beenassociated with the incidence of cohesive failures. Specimenswith rectangular cross-sectional areas larger than 7.17 mm2

built using a self-etching primer system (Clearfil Liner Bond 2,Kuraray) and loaded in tension showed exclusively cohesivefailures in dentin. Between 2.31 and 7.17 mm2, both cohesiveand adhesive failures were observed [13].

6. A comparison between macro and microbond strength tests

In order to verify how “macro” bond strength tests compare totheir “micro” counterparts in terms of material ranking, inci-dence of cohesive failures and data scattering, a three-step(Adper Scothbond Multi-Purpose), a two-step etch-and-rinse

Bond 2; bottom, left: Adper Scotchbond Multi-Purpose;

(Adper Single Bond 2) and a self-etch system (Adper SE Plus),all from the same manufacturer (3M ESPE, St. Paul, MN, USA),were tested for bond strength to superficial bovine dentin.Except for the micro-tensile specimens, teeth were embed-ded in PVC cylinders using self-cure acrylic, with the dentinsurface kept 2 mm above the embedding material. Dentinroughness was standardized using a 600-grit SiC paper. Formacro shear and tensile tests (n = 10), a 3-mm diameter areawas delimited with adhesive tape. After adhesive applicationfollowing manufacturers’ directions, a 3-mm height truncatedcone was built using a microhybrid composite (LLis, FGM Pro-dutos Odontológicos, Joinville, Brazil) with the smaller base(3-mm diameter) contacting the adhesive layer. Micro-shearspecimens (n = 5) were built by inserting the composite intosegments of Tygon tubing (Saint-Gobain Performance Plas-tics, Akron, OH, USA), four per tooth, with internal diameterof 0.76 and 0.4 mm in height, and photo-activating it in con-tact with the adhesive-coated dentin. For the micro-tensilespecimens (n = 5), teeth crowns were sectioned perpendicu-larly to the exposed dentin surface and the resulting dentinarea received the adhesive application. Such modification indentin tubule orientation was necessary to increase the dentinthickness available for specimen sectioning. A 4-mm highcomposite block was built onto the dentin surface. All spec-imens were stored in distilled water at 37 ◦C for 24 h priorto testing. For the micro-tensile test, stick-shaped specimens

0.8–1.0 mm2 (five per tooth) were cut using a diamond waferingblade immediately prior to testing. For macro- and micro-shear testing, a 200-�m knife-edge chisel placed in contactwith the dentin surface was used to debond the composite

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runcated cones/cylinders. Tensile test used a clamp that con-acted three-quarters of the truncated cone circumference.rticulated joints were used to assure that the bonded inter-

ace was perpendicular to the loading axis. For micro-tensileesting, Geraldeli’s jigs were used [143]. All tests were carriedut at a crosshead speed of 0.5 mm/min. Specimen failureode was assessed under a stereomicroscope at 10–30× mag-

ification. Results were submitted to one-way ANOVA/Tukeyest at a pre-set global significance level of 5%.

Bond strength results are displayed in Fig. 7. As discussedbove, the reduction in bonding area corresponded to anncrease in bond strength values. “Macro” tests displayed aimilar range of values, between 5.6 and 11.5 MPa. Both shearests ranked the adhesive systems similarly, while for the ten-ile tests there was an inversion between the two systems withhe highest bond strengths. Coefficients of variation rangedetween 28 and 36% for shear, 21 and 40% for micro-shear,2 and 38% for tension and 25 and 64% for the micro-tensileest. The higher scattering displayed by the self-etch systemn both “micro” tests may be explained by the association ofower bond strength with a more technique-sensitive speci-

en preparation.Failure mode distribution is shown in Fig. 8. Only the adhe-

ive system that achieved the lowest bond strength values inll testing modes did not present cohesive failures. For thether two systems, cohesive failures were more frequent withhe “macro” compared to the “micro” bond strength tests.ooled data revealed the incidence of cohesive failures of 45%or shear, 28% for tension, 13% for micro-shear and 12% for

icro-tensile test. It must be emphasized, though, that underigher magnification, the incidence of mixed and cohesive

ailures may increase for all testing modes.

. Summary

he need for tests capable of accurately assessing interfacialond strength is clear. With that in mind, some would arguehat the fracture mechanics approach is the optimal way tochieve “controlled” crack propagation at the bonded inter-ace to assess adhesion [144,145]. But this method is moreomplex and time consuming than others for measuring bondtrength to the dental substrate. Thus, in spite of their inher-nt shortcomings, “macro” bond strength tests will continueo be used for evaluating the adhesion of dental materials toooth structure due to their simplicity.

This review discussed aspects influencing stress distribu-ion that may impact dentin and enamel bond strength resultsnd failure modes of “macro” shear and tensile bond strengthests. Some of the information is likely applicable to “micro”ond strength tests as well. In terms of specimen design,he effect of the elastic modulus of the second substrate,ypically resin composite, on test results seems consistentmong experimental studies, as well as with FEA data, i.e., aigher modulus mismatch between substrates increases thetress concentration at the interface resulting in lower bond

trengths. The influence of bonding area, on the other hand,emains undefined, though at least a trend for increasingond strength values with the use of smaller bonding areasoes exist. Stress concentration is much more severe in speci-

( 2 0 1 0 ) e38–e49 e45

mens loaded in shear, compared to tension. Among the shearmethods, the use of the chisel as a loading device causesthe most severe stress concentration, which is also supportedby experimental findings showing lower bond strengths com-pared to other shear loading methods, such as the wire loopor the flat rod. Within the limits suggested in the ISO/TR11405, crosshead speed seems to have little influence on bondstrength results. Finally, in view of the many aspects affectingthe results of “macro” bond strength tests, in order to allowfor a more judicious comparison among studies researchersmust provide a thorough description of the specimen designand test configuration.

Acknowledgements

Authors would like to express their gratitude to FGM ProdutosOdontológicos and 3M ESPE for kindly donating the materialsused in the experimental section of this review.

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