d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e50–e62

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 review of “micro”bond strength test methods

Steve Armstronga,∗, Saulo Geraldeli a, Rodrigo Maiab, Luís Henrique Araújo Raposoc,Carlos José Soaresc, Junichiro Yamagawaa,1

a Department of Operative Dentistry, University of Iowa College of Dentistry, 229 Dental Science Bldg. S, Iowa City, IA 52242-1001, USAb Health Directory, Brazilian Air Force, Av. Marechal Câmara, 233 - 9◦ Andar – Castelo, Rio de Janeiro – RJ, CEP: 20020-080, Brazilc Department of Operative Dentistry and Dental Materials, Federal University of Uberlândia, Av. Pará 1720, Campus Umuarama –Uberlândia-MG, CEP: 38400902, Brazil

a r t i c l e i n f o

Article history:

Received 20 November 2009

Accepted 23 November 2009

Keywords:

Microtensile

Microshear

Finite element analysis

Bond strength

a b s t r a c t

The objective of this paper is to critically review the literature regarding the mechanics,

geometry, load application and other testing parameters of “micro” shear and tensile adhe-

sion tests, and to outline their advantages and limitations. The testing of multiple specimens

from a single tooth conserves teeth and allows research designs not possible using con-

ventional ‘macro’ methods. Specimen fabrication, gripping and load application methods,

in addition to material properties of the various components comprising the resin–tooth

adhesive bond, will influence the stress distribution and consequently, the nominal bond

strength and failure mode. These issues must be understood; as should the limitations

inherent to strength-based testing of a complicated adhesive bond joining dissimilar sub-

strates, for proper test selection, conduct and interpretation. Finite element analysis and

Dental adhesives comprehensive reporting of test conduct and results will further our efforts towards a stan-

dardization of test procedures. For the foreseeable future, both “micro” and “macro” bond

strength tests will, as well as various morphological and spectroscopic investigative tech-

niques, continue to be important tools for improving resin–tooth adhesion to increase the

service life of dental resin-based composite restorations.

emy

© 2009 Acad

1. Introduction to the review

Bond strength testing has been traditionally accomplishedby creating one test specimen per tooth or tooth surfacewhich is then loaded to failure in either shear (SBS), tensile

(TBS), or fracture-based manner. Currently, the most commonapproach is to load multiple test specimens from each toothin either a tensile (�TBS) or shear (�SBS) manner. The earliest

∗ Corresponding author. Tel.: +1 319 335 7211; fax: +1 319 335 7267.E-mail address: steven-armstrong@uiowa.edu (S. Armstrong).

1 Mr. Junichiro Yamagawa was a visiting scholar at the University ofand is an employee of the Tokuyama Dental Corporation.0109-5641/$ – see front matter © 2009 Academy of Dental Materials. Pudoi:10.1016/j.dental.2009.11.155

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

reference to the use of a ‘micro’ tensile device for biomechani-cal engineering was by Okuno who studied whisker-reinforcedmetal alloy [1]. Subsequently microtensile testing was usedto evaluate tensile strength of collagen and elastin fibers inporcine tricuspid leaflets [2], aortic valves [3], patellar groove

Iowa at the time of his participation in the writing of this paper

articular cartilage [4], and cortical bone [5].Sano et al. introduced microtensile testing to dentistry to

measure the ultimate tensile strength and modulus of elas-ticity of mineralized and demineralized dentin [6]. This test

blished by Elsevier Ltd. All rights reserved.

2 6

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are relatively brittle materials, therefore, the Griffith strength

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

ffered versatility that could not be achieved using conven-ional shear and tensile methods. This groundbreaking workas then applied to bond strength evaluations, such as; nor-al vs. caries-affected occlusal dentin [7], normal vs. sclerotic

ervical dentin [8], different regions of restored MOD prepa-ations [9], in vivo class V restorations [10], regions of occlusalentin [11], dentin depth and cavity configuration [12], andundreds of other dental biomaterials studies. “Micro” ten-ile bond strength (�TBS) lends itself to additional researchesigns that the “macro” tests do not, such as, the elimina-ion of tooth dependency through balanced designs [13], andas shown reduced test variance [14]. Simply using a smallerechanical bond strength test specimen in the laboratory

oes not eliminate the challenges that teeth, beyond theirelatively small size, present, i.e., structural heterogeneity,

echanical anisotropy, visco-elasticity, presence of dentinaluids, and simulation of their complex and harsh functionalnvironment.

A 1995 review [15] of dentin adhesion testing stated aumber of advantages for the �TBS approach: more adhesive

ailures and fewer cohesive failures, measurement of highernterfacial bond strengths, means and variances can be cal-ulated for single teeth; permits testing of irregular surfaces;ermits testing of very small areas and facilitates SEM/TEMxaminations of the failed bonds since the surface area ispproximately 1 mm2. Drawbacks cited were the labor inten-ity, technical demand, and dehydration potential of thesemaller samples. In an excellent follow up paper by Pashley etl. the versatility of microtensile methods over conventionalpproaches to evaluate clinically relevant sites and substratesas comprehensively detailed and critiqued [16].

Microtensile bond strengths tend to be much higher (often× to 4×) than that of macro TBS values because the defectoncentration in the small cross-sectional interfacial areas isower [14,17]. More recently, the �SBS test has become popu-arized as an alternative to the conventional SBS test [18,19].he �SBS test again obtains multiple specimens per toothut without the damage history due to specimen section-

ng, trimming and shaping that is required with �TBS testing.icroSBS, as with �TBS, has been advocated as a method for

egional mapping, but also is very conducive to depth profilingf different substrates.

Currently no broad agreement exists within the scientificommunity as to the appropriate conduct, usage and inter-retation of these tests and any attempts at standardizationf test methods have been difficult. The objective of this paper

s to critically review the literature regarding the mechanics,eometry, load application, preparation effects, and other test-ng parameters of these “micro” adhesion tests, outlining theirdvantages and limitations.

. Literature

.1. What makes a good bond strength test?

he realization of a widely accepted, validated, standard testethod for adhesive resin–tooth bond strength testing is

n elusive and controversial endeavor [20–22]. Although aonsensus or standard approach does not currently exist in

( 2 0 1 0 ) e50–e62 e51

dentistry, bond strength testing remains useful and necessaryfor the screening of new products and study of experimentalvariables. Therefore, the scientific community should striveto identify important testing parameters and journal editorsshould demand comprehensive reporting of these parametersso that critical interpretations of data and meaningful com-parisons of results can be achieved. Published studies, teststandards (see Appendix A), and technical reports [23] can bereferenced for important considerations regarding specimenfabrication, test conducting, and reporting. First; however,basic principles regarding the use of strength-based testingmust be understood.

Strength-based testing does not quantify an inherentmaterial property of the bond of restorative materials totooth structure. The measured bond strength and the fail-ure mode or debond pathway produced is dependent, amongother things, upon: flaws existing within or between mate-rials, specimen size and geometry, material properties ofeach component of the bonded assembly, and method ofload application. Adhesive resin–tooth bond strength testinginvolves two separate substrates and complicated interphasesor zones of interdiffusion between these components, allpossessing different material properties. Therefore, even ifa perfectly uniform tensile load could be applied across aresin-based-composite (RBC)–dentin adhesively bonded jointa non-uniform stress distribution will occur in the adhesivejoint. The ratio of Poisson’s ratio (�) to the Young’s modulus (E)would have to be equivalent for the: RBC, RBC–adhesive resin‘interface’, adhesive resin, adhesive resin–hybrid layer ‘inter-face’, hybrid layer, hybrid layer–dentin ‘interface’, and dentinto produce a uniform stress state [24]. In other words, the RBCand dentin substrates and the adhesive resin and their inter-diffusion zones under loading, would all have to either: (1) notdeform laterally, or (2) deform laterally in an equal manner,to permit a homogenous stress distribution. The ratio �:E ishigher for the adhesive and hybrid layer than the adherends,thus the lateral deformation in these regions will be greaterthan for the RBC and tooth structure, if the bond remainsintact under loading, a complex stress distribution will occurin the adhesive resin and hybrid layer. Clearly, a complexstress state exists in ‘macro’ and ‘micro’ shear and tensiletesting of the adhesive resin–tooth bonded joint.2 These priorcomments ignore the material property differences that com-monly exist within a single material when comparing bulk vs.surface regions. As pointed out at an earlier ADM meeting [25],it may be more appropriate to refer to these tests, regardlessof type of test or size of specimen, as simply “bond strengthtests”.

Smaller test specimens are ‘stronger’ than larger onesdue to the lower probability of having a critical sized defectpresent and aligned in a crack opening orientation relativeto the applied load. Tooth structure and dental adhesives

2 Also why both KIc and KIIc loading modes must be consideredin the determination of interfacial fracture toughness or alterna-tively, to use a fracture energy (Gc) approach, due to mode mixityacross RBC–tooth adhesive joints.

e52 d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e50–e62

Table 1 – Dentin SBS results using Ultradent jig loaded at 1 mm/min. Mold diameter is 2.38 mm for a cross-sectionalbonding area of 4.45 mm2. Groups identified by different letters are significantly different at p < 0.05 (unpublished data).

Specimen fabrication method SBS (MPa ± SD) Predominate failure mode

Adhesive system applied and light-cured before mold placementAdhesive system applied before and light-cured after mold placementAdhesive system applied and light-cured after mold placement

equation [26]:

�f =(

2E�

�co

)1/2

where �f is the fracture strength in uniform tension, E theplain strain elastic modulus, � the surface energy per unitarea, and co the starter crack size, applies. This reminds usthat the measured “bond strength” at failure will be dependentnot only upon the fracture strength but the presence of flaws[27]. Another way to think about this is that the resultant bondstrength data obtained from mechanical testing represents astatistical distribution of the flaws (discontinuity, defect, bub-ble, void, residual solvent, etc.) present in each test specimenleading to its failure. This concept lends itself nicely, not only,to the presentation and interpretation of data by probabilisticstatistical methods such as Weibull, but as an explanation asto why smaller specimens are stronger. Another way to saythis is “Strength values (whether from testing a monolithicspecimen or a bonded specimen) simply provide insight intothe stresses a particular material will support given the flawsize distribution” [27]. Griffith demonstrated that drawn glassfibers were “stronger” than ordinary glass, proving the vol-ume dependency of strength, as did da Vinci, four centuriesearlier, when testing wires of various lengths. This volumedependency of strength tells us that a smaller test specimenwill be less likely to have a larger flaw that leads to its fail-ure. Therefore a higher apparent “strength” is measured. Inconverse, the larger the size of the specimen, the more likelywill be the presence of a larger flaw that leads to its failure,and, therefore, a lower apparent strength. The adoption of theterm “microtensile” into our literature does not fully acknowl-edge these principles. Tensile testing should not be simplycategorized as those with a less than 1 mm2 (microtensile) orgreater than 1 mm2 cross-sectional testing region without fullconsideration of this volume dependency of strength. Never-theless, use of the term “microtensile” is firmly established inour literature.

3. Microshear bond strength testing

Shear bond strength testing with bonded cross-sectional areasof 1 mm2 or less is also referred to as ‘micro’ SBS [18]. Thisrelatively simple test permits efficient screening of adhesivesystems, regional and depth profiling of a variety of substrates,and conservation of teeth. Aqueous storage durability studiesare also possible with �SBS due to the relatively short diffu-sional distances (0.02–0.05 mm) from the cavosurface [28]. Thegeneral findings based upon FEA and failure mode analysis of

macroSBS testing; however, hold true for �SBS testing. TheseFEA findings being: (1) tensile stresses produced by the bend-ing moment at load application being responsible for fractureinitiation, (2) highly nonuniform stress distribution concen-

34.08 ± 2.45 A Cohesive in dentin 67%32.06 ± 1.36 B Cohesive in dentin 83%22.92 ± 1.28 C Apparently adhesive 83%

trated in the substrate, and (3) a nominally measured bondstrength that severely underestimates the true stress the spec-imen resisted at fracture [29–32].

In addition, Placido et al. concluded that �SBS results mayactually worse represent shear bond strength than the con-ventional macroSBS test [19]. Increased stress concentrationand tensile forces during shear load application were shownwhen factoring in the relatively thicker adhesive layer, fartherload application from the adhesive bond, and the use of lowermodulus flowable RBCs (to avoid the introduction of flaws inthe small molds required) common to �SBS tests. In contrast,McDonough et al., using three-dimensional FEA demonstratedthat the tensile forces during loading could be minimized byoptimizing specimen dimensions and load application loca-tion [33].

A significant advantage over �TBS methods is that the�SBS specimen is pre-stressed prior to testing only by moldremoval. Similar to macroSBS methods; however, the use ofthe mold for RBC placement can lead to the introduction offlaws and different stress concentrations upon shear load-ing [34]. Adhesive resin flash beyond the restoration marginwill be present if the adhesive is cured before mold place-ment and if the adhesive bond is formed within the mold agreater chance of uneven adhesive resin thickness, meniscusformation at the border and introduction of flaws are possible.These specimen fabrication methods should be thoroughlydescribed as they can significantly affect results (Table 1).Regardless, �SBS methods remain an especially useful testfor those substrates with properties such as glass ionomersor enamel, that make them particularly susceptible to thespecimen preparation effects and testing conditions of �TBStesting.

4. Microtensile bond strength testing

The microtensile bond strength test is calculated as the ten-sile load at failure divided by the cross-sectional area of thebonded interface. However this nominal strength is valid onlyif a state of uniform, uniaxial stress is present [35–37] withthe maximum tensile stress present and homogenously dis-tributed in the region of the smallest cross-sectional bondedarea [38]. Therefore, if experimental conditions are constant,it is expected that �TBS results for restorative material Abonded to substrate B from different laboratories would be inagreement. However, variable methods and parameters havebeen employed by the different laboratories all over the worldresulting in bond strength data that can hardly be comparedacross studies [38,39].

Several factors potentially contribute to this interlabora-tory variability and are nearly impossible to identify frompublished papers. Searching PUBMED (January–July 2009) withthe keyword “microtensile” resulted in more than 90 papers.

2 6 ( 2 0 1 0 ) e50–e62 e53

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(

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(

(

(

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(((((

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Fig. 1 – von Mises stress distribution. (A) Hourglass- andstick-shaped specimens with bond line perpendicular to

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

nly 10% of these papers described in any detail: the grip-ing device, specimen fixation method to this gripping device,echanical properties of the adhesive and the adherends

ested, or specimen shape. Different results could be obtainedith any change in these testing variables, making interlab-ratory comparisons of little value. Without full reporting ofethods, a simple rank listing of groups by load at breakageould be just as useful for comparisons.

Advantages of �TBS testing include [15,16]:

(1) Conservation of teeth,(2) Evaluation of regional bond strengths possible [11,40,41],(3) Evaluation of remaining dentin thickness effects possible

[42,43],(4) Evaluation of intra- and inter-tooth variability [9,44],(5) Evaluation of bond strength to various cavity walls in

restoration possible [45,46],(6) Evaluation of bond strength to intra-radicular dentin is

possible,(7) Conducive to evaluation of the effects of RBC polymeriza-

tion shrinkage stress [12,47],(8) More uniform loading may be possible due to less bend-

ing offset, relative to conventional tensile testing, due toalternative gripping method,

(9) Fewer cohesive failures in substrates,10) Bond strengths are higher than those measures from con-

ventional tensile and shear bond strength tests due to thedecreased number of defects in the substrate or at thebond interface (advantage?),

11) Additional research designs can be performed to accountfor tooth dependency, such as, multiple surfaces within acavity, various substrates within a tooth, durability test-ing by aqueous storage [13],

12) Accelerated environmental aging is feasible by aqueousstorage due to short diffusional distances [48],

13) Possible to evaluate very small surface areas when nec-essary, e.g., caries-affected dentin [7],

14) Can minimize shear effect by tensile testing a relativelyflatter region of tooth when not preparing surface, e.g.,unground enamel [49,50],

15) SEM fractography can be readily performed to determinethe mode of failure [51],

16) Clinically retrieved restorations can be evaluated[10,52,53],

17) Conducive to comprehensive examination of researchquestion, such as, mechanical, morphologic and chem-ical studies on same sample [54,55].

Limitations of �TBS testing include [15,16]:

1) Labor intensive, technically demanding,2) Difficult to measure very low bond strength (<5 MPa),3) Specimens easily dehydrate,4) Specimens easily damaged,5) Post-fracture specimens can be lost or damaged when

removing from active gripping devices that use glue,

6) Difficult to fabricate with consistent geometry, surface fin-

ish and damage history without aid of special equipment,7) Lack of consensus exists for conduct of test, reporting of

pre-test failures and fractures outside of the designated

load; (B) hourglass- and stick-shaped specimens withangled bond line.

testing region of specimen (applicable to all dental bondstrength testing).

4.1. Gripping devices

Surface preparation of dentin and enamel for �TBS testingshould produce a flat surface with a standardized, clinicallyrelevant smear layer. This specimen should then permit thetensile load to be applied perpendicularly to the adhesive bondline. Regardless of the method to produce this surface prepa-ration, such as, dental handpiece or SiC polishing paper, itis not uncommon to produce either a non-perpendicular ornon-flat surface or both. This can produce spurious �TBS datadue to: inaccurate bonded cross-sectional areas, incorpora-tion of additional flaws, and a non-normal load application.A non-normal load application, whether due to the specimenor gripping mechanism significantly alters the stress distribu-tion at the bonded interface [37,39]. Several specimen grippingdevices, both active and passive, have been developed in anattempt to properly apply a tensile load normal to the bondline [13,56,57], this will occur; however, only if the specimen’sbond line is properly aligned with its gripping surfaces (Fig. 1).

Test specimens are attached to the load train couplers ofmechanical testing machines by either active or passive grip-ping devices. Active gripping can be either mechanical or,most commonly in �TBS testing, via fast-setting glue. Thesespecimen-fixation procedures require careful manipulationand special test jigs [6,8,41,56–58]. Theoretically, these jigsshould assure that a pure tensile force is applied to the testspecimen. Bending forces can occur during load applicationdue to: non-parallel specimen alignment, bond line not per-pendicular to the specimen gripping surfaces, and/or, unevengripping forces [31,37,59].

In the original microtensile test set-up the specimens wereglued onto a Bencor Multi-T gripping device with quick set-ting cyanoacrylate gel that covered the entire surface of bothspecimen ends [6]. The Ciucchi’s jig was later introduced as

e54 d e n t a l m a t e r i a l s 2

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6 ( 2 0 1 0 ) e50–e62

a modification to this active gripping method [8]. Neither ofthese devices guarantees proper alignment because the spec-imen is glued to a flat gripping surface [57,59]. A groove parallelto the applied load was added in the so-called, Geraldeli’s jig,in an attempt to improve specimen alignment [56]. Later, theCiucchi’s jig was modified in a similar manner for the samereasons [57]. Relative to a flat faced active gripping device, anotched gripping device: requires less glue, reduces the pos-sibility of glue contamination on the bond line, and reducestest variability for both stick and dumbbell specimens [42,57].

Regardless, the use of glue for active gripping of micro-specimens is technically challenging and specimens mustcommonly be reglued during testing due to either specimenpull-out or glue detachment from the face of the grippingdevice. Additionally the particular glue used or unknownforces imparted upon the test specimen during curing of theglue may affect results (Table 2). Our laboratory previouslyused Jacobs’ chucks aligned in the upper and lower mem-bers of a displacement controlled testing machine to grip �TBSspecimens. The specimen was first fixed to an acrylic rod withRBC, trimmed, placed in the Jacobs chuck and lowered intoan empty Plexiglas rod below for fixation with cyanoacrylate.A fixed period of time was allowed for the cyanoacrylate toharden before tensile testing; during this time, compressiveforces then rapidly increasing tensile forces were recordedon the load cell. Not uncommonly the specimen had to bereglued multiple times before testing. These experiences,combined with the difficulty of forming the specimen notchmanually, lead us to eventually develop a self-aligning, glue-less, passive gripping device, i.e., “Dircks device” (Fig. 2). Thismethodology requires computer numerically controlled (CNC)machining to produce a dumbbell-shaped specimen withgeometry that permits uniform contact between the grippedsection of the specimen (neck region) and the grip faces ofthe Dircks device [60]. This passive gripping device consistsof two identical aluminum parts guided by two vertical stain-less steel alignment pins. A central notch or pocket, with aradius of curvature (0.6 mm) in the lateral shoulders matchesthat of the specimen. The main advantages of this deviceare: efficient and reproducible specimen alignment, simplemanipulation without the use of any glue, lack of specimendehydration, remaining dentin thicknesses as small as 1 mmcan be tested, and absence of preloading stresses possiblewith active gripping methods. Moreover, the dumbbell-shapedmicro-specimens used for this device have been shown topresent a better stress distribution than hourglass or stick-shaped specimens [61,62].

Poitevin et al., developed a �TBS testing device with top-bottom fixation to minimize stress concentrations [42,57]. Atrimmed dumbbell-shaped specimen is mounted by one endin a pin-chuck and lowered onto a horizontal table for fixa-tion of the opposite end with cyanoacrylate. This top-bottomset-up demonstrated slightly less variability, a higher numberof apparently interfacial failures, and a more homogeneousfracture surface, when compared to specimens tested using aflat jig. These results are attributed to a more perpendicular

alignment of the interface and a stress impact more com-pletely following the specimen’s long axis, i.e., less bendingoffset. They also demonstrated that the use of a notched jigdiminishes the intra- and inter-tooth variability, as compared

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e50–e62 e55

Fig. 2 – von Mises stress distribution (A) �TBS testing set-up of Dircks device; (B) dumbbell-shaped specimen tested in theD he aD

tpififSnsa

agl

Fd

ircks device showing homogeneous stress distribution in tircks and the dumbbell specimen (unpublished data).

o a flat jig. Regardless of active or passive gripping or thearticular gripping device, standardization of the intra- and

nter-tooth differences can be partially addressed by using axed number of specimens per tooth, minimizing regional dif-erences, and using a fixed number of teeth per group [63].oares et al., verified with FEA analysis that increasing theumber of faces of specimen fixation was directly related totress homogeneity, with the best distribution occurring whenll surfaces were attached [38] (Fig. 3).

The use of glue as a means of active gripping has thedvantage of being able to attach relatively fragile and non-eometric test specimens, but also has several potentialimitations that may affect results [38]:

(1) Non-static loads to the specimen during glue polymer-ization,

(2) Effects on specimen geometry dependent load distribu-tion [62],

ig. 3 – von Mises stress distribution in uTBS specimen geometrumbbell [38].

dhesive bond region; arrows indicate contact area between

(3) Some degree of specimen drying is required for attach-ment,

(4) Glue components may come into contact with thespecimen interface or testing region during application(Table 2),

(5) Glue may possess inadequate attachment strength, lead-ing to repeat testing or an inability to test,

(6) Fractures may occur near or in glue attachment, and notin the testing region (Fig. 4),

(7) Glues deform before rupture due to relatively low Young’smodulus [64],

(8) Time consuming,(9) Difficult to remove fractured specimens from device

without damage to or loss of the specimen,

(10) Difficult to remove remaining glue from device in prepa-

ration for next test,(11) Minimum substrate size required for glue application

[58].

ies when gripped on all surfaces (A) hourglass; (B) stick; (C)

e56 d e n t a l m a t e r i a l s 2

Fig. 4 – Post-tensile test resin-caries-affected-dentinspecimen in Geraldeli’s jig using Zapit cyanoacrylateshowing cohesive fracture in the dentin located at the

interface of specimen and glue.

Due to limitations with the use of glue, our laboratory uses,as described earlier, a non-gluing, passive gripping device.This; however, requires additional equipment, that is cost-prohibitive for wide adoption, to shape the specimen foruniform contact between the gripped section of the speci-men and the grip faces [18,47,60,65]. Active gripping methods,such as glue or clamps, are then required if tensile specimenscannot be precisely machined. If dumbbells are not feasible,the use of stick-shaped specimens due to the high stress con-centration factors found with notched hourglass geometriesis encouraged [38,39].

4.2. Specimen geometry and preparation effects

The first specimen design for �TBS proposed by Sano et al.was the hourglass shape. A sharp notch of the hourglass wasdesigned to concentrate stresses where the adhesive bondinterface is located, avoiding the undesirable fracture of theadherends near the glue [6]. Several specimen types have beensubsequently reported that can be categorized as; hourglass,slab (rectangular), stick (square), and dumbbell geometries,with cross-sectional shapes that are either square, rectangularor round. The specimen geometry has a significant influenceon homogeneity of stress and if stress concentrations cannotbe completely avoided, they should be at least minimized. Thestress concentration and pattern in the hourglass samples isvery different from the dumbbell and stick samples [38]. Thehourglass sample fails at lower stress compared to the stickor dumbbell, due to the large stress concentration induced inthe adhesive [36,39]. The stress concentrates at or very closeto the adhesive edges of the hourglass specimen indicatingwhere the initiation of failure is likely; different types of fail-ures for the hourglass specimen, compared to dumbbell andstick, have been reported [38,39].

A basic tenet of materials testing is to produce consistenttest specimens. For the hourglass-shaped specimen design,there are a large number of designs in the published literature[16], therefore, caution is advised in interpreting and compar-ing results [38,66]. The hourglass neck geometry should be

carefully analyzed since the radius of curvature of the notchhas a significant influence on stress concentration values [67].The use of a equipment specially designed for notch forma-tion results in more uniform geometries of consistent size

6 ( 2 0 1 0 ) e50–e62

with potentially less stress imparted to the specimen thanmanually trimming [18,65]. This equipment is most often usedto prepare dumbbell-shaped specimens [48,65,68]. Dumbbell-shaped specimens demonstrate peak stress concentration inthe radius of curvature or ‘neck’ (Fig. 2) which could lead tofailure in the non-gauge region during testing, especially prob-lematic for resin-enamel testing (Table 3) [62].

Through all phases of specimen fabrication, a carefulexamination should be carried out to identify any defectsthat, based on pre-determined criteria, would exclude thespecimen from testing. Appropriate magnification should beused to determine failure location (gauge or non-gauge) anddebond pathway (cohesive in substrates, apparently interfa-cial, or mixed). Any failures located outside of the gauge regionshould be treated as right-censored data points. These right-censored data points potentially had higher bond strengthsthan the recorded nominal strength and bias will occur ifexcluded. We have used dumbbell-shaped resin–dentin spec-imens in our laboratory for several years and have very rarelyobserved a fracture location entirely outside of the test gaugeregion. Those failures that occur in the specimen neck (radius)may be due to stress concentration (Fig. 2), gross voids betweenRBC increments or inadequate remaining dentin thickness.Significantly different fracture locations and failure modes arecommonly observed with the use of active gripping with glue(Table 2).

However, a higher incidence of non-gauge failure locationoccurs, as does pre-testing failures (PTF), when testing resin-enamel bonds with the passively gripped dumbbell specimen(Table 3). This may be due to the material property differencesin the enamel substrate as compared to dentin. When sub-jected to the stress concentration, as shown by FEA (Fig. 2),the higher modulus and lower toughness of enamel cannotadequately transfer the stress from the neck region to the testregion without failure.

Stress transference from the gripping region to the test-ing region (gauge) will improve directly with the notch radiusof curvature, also referred to as the ‘neck’. Teeth; how-ever, physically put limits on overall specimen geometry. Theobservance of failures within the gauge section verifies, fol-lowing St. Venant’s principle [69,70], that the notching waslocated far enough away from the middle of the specimenfor resin–dentin specimens. The bond line located anywherewithin the gauge section, theoretically, permits a suitabletest, however, the technical ability to place the bond line inthe gauge section during trimming also limits gauge sectionlength.

Due to lack of consensus of specimen design and therelatively higher incidence of PTFs during trimming, somelaboratories have recommended the notchless stick or ‘non-trimming’ technique [16,45]. Stick-shaped specimens aresimple to prepare and when compared to a dumbbell geometrywith a rectangular testing region had similar bond strengths,stress concentrations and failure locations; whereas an hour-glass geometry was found to be significantly different to thestick and dumbbell geometries and more sensitive to flaws

introduced during specimen preparation [39,71]. Sadek et al.,demonstrated that the use of the diamond saw producesdefects on corners of the sticks, resulting in high levels ofPTFs (35.4% for enamel and 18.2% for dentin) which could be

d e n t a l m a t e r i a l s 2 6 ( 2 0 1 0 ) e50–e62 e57

Table 3 – Enamel and dentin 24 h �TBS using five different bonding agents using the methods described in Sattabanasuket al. [60] (unpublished data).

Bonding agents

A B C D E

Unground enamela

Mean bond strength with PTFsb (MPa)[Coefficient of variation %]

31.0[34]

5.3[158]

41.9[38]

34.1[42]

11.9[121]

Pre-test failures 1/20 15/20 0/20 1/20 11/20Bond strength excluding PTFs (MPa)[Coefficient of variation]

32.6[24]

18.1[41]

41.9[38]

35.9[35]

25.3[45]

Non-gauge fracturec 4/20 0/20 5/20 2/20 2/20

Ground enamelMean bond strength with PTFsb (MPa)[Coefficient of variation %]

33.8[31]

3.7[232]

40.8[44]

37.7[26]

28.2[55]

Pre-test failures 0/20 18/20 0/20 0/20 4/20Bond strength excluding PTFs (MPa)[Coefficient of variation]

33.8[31]

28.4[31]

40.8[44]

37.7[26]

35.0[22]

Non-gauge fracturec 5/20 0/20 9/20 12/20 0/20

DentinMean bond strength with PTFsb (MPa)[Coefficient of variation %]

58.2[18]

38.3[46]

56.4[16]

55.9[23]

55.0[21]

Pre-test failures 0/30 2/30 0/30 0/30 0/30Bond strength excluding PTFs (MPa)[Coefficient of variation]

58.2[18]

41.0[36]

56.4[16]

55.9[23]

55.0[21]

Non-gauge fracturec 2/30 1/30 1/30 3/30 3/30

d en

ebond

ricttsstgttrnc

tpraqttatm

daola

a Surface area of unground enamel assumed to be equal to flat grounb PTFs included as 1 MPa.c All non-gauge fractures involved a portion of the gauge length in d

educed by diamond wire sectioning [72]. The relatively highncidence of PTFs observed can be explained by the stress con-entrations generated by the specimen preparation defects athe sharp corners and by the material property differences ofhe substrates. This can be improved by using a round cross-ectional testing region or maybe by chamfering the corners ofquare and rectangular specimens. The testing region geome-ry is rarely reported or discussed when a particular specimeneometry is recommended as being most appropriate for �TBSesting. Although producing a more favorable stress distribu-ion, fabricating a �TBS dumbbell-shaped specimen with aound testing region requires special equipment or computer-umerically controlled fabrication methods that are currentlyost-prohibitive for wide adoption [38].

Trimming is very technique sensitive and it induces addi-ional stress as reflected in the number of specimens that failrior to testing, especially in weaker bonds or specimens withelatively brittle behavior [42,66]. The operator’s experiencend manual skill will, therefore, influence the results and theuality of the study [36]. It is difficult to standardize freehandrimmed specimens to consistently produce the test region ofhe bonded interface, the results of which, can significantlyffect the stress distribution [38,68]. In addition, the action ofhe bur can be particularly aggressive as uneven cutting forces

ay be applied.As years of research and clinical experience have clearly

emonstrated, adhesion to acid-etched enamel is more reli-

ble than to dentin. However, owing to the intrinsic brittlenessf this tissue relative to dentin, it will fail under relatively

ower loading levels with the surface areas used in �TBSnd with greater variability (Table 3) [73]. Microtensile testing

amel and dentin.

pathway.

has been questioned as an appropriate trial for enamel [74],which is fragile, anisotropic, and has water content lower thandentin. Pre-testing microscopic analysis of prepared sticksrevealed a more frequent occurrence of microcracks in enamelthan in dentin, with microcracks most often located at theperiphery of the sticks, suggesting flaw introduction duringpreparation [72].

The thickness (<1.5 mm) of the specimen affects its abilityto survive the preparation procedures necessary for microten-sile testing. A relatively high incidence of PTFs (26%) duringhourglass trimming of microtensile dentin specimens to cre-ate a 0.5 mm wide bonding has been reported [45]. Phrukkanonet al., recommended never reducing the cross-sectional area atthe bonding interface to less than 1.1 mm2 due to an observedhigh pre-test failure rate [18]. This calls into question thedefinition and utility of the term “microtensile”. Shono etal., related the rate of PTFs using a trimming technique tothe mean tensile bond strength measured and reported thatgroups with bond strengths 13 MPa or lower were less likely tosurvive preparation [11]. The non-trimming technique impartsless cumulative damage to the specimen since bond strengthsas low as 5 MPa have been measured [16]. However, the non-trimming technique does not create a defined test region withuniform stress state.

4.3. Test speed

A human chewing-cycle lasts around 800 ms, with the clos-ing movement reaching duration of less than 400 ms [75]. Thistranslates into over 2000 mm/s, a value 500 times higher thanwhat may be employed in conventional bond strength test-

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ing [76]. However, such high rates are limited by the effectiveloading rate of testing equipment. Few in vitro studies havebeen carried out to address the influence of crosshead speedon �TBS [42,77,78]. These studies unanimously reported thatthere was no difference on microtensile bond strength inthe range of crosshead speeds evaluated (0.01–10.0 mm/min).According to Yamaguchi et al., the smaller specimens usedin microtensile testing contain a lower number of internaldefects and a more homogeneous distribution relative to“macro” testing; therefore, such samples exhibit indepen-dence of strain-rate which reduces the impact of speed onbond strength values [78]. However, the time–temperatureequivalence of viscoelastic materials tell us that at very lowor high testing speeds nominal bond strengths should reflecta strain-rate sensitivity. Poitevin et al., reported that thelower the speed, the greater the difference between stress atmaximum load and stress at breaking; regardless, they rec-ommended 1 mm/min crosshead speed because of a moreuniform stress-time pattern [42].

4.4. Role of FEA in the interpretation of “micro” bondstrength testing

Finite element analysis is a very powerful tool that can assistin our understanding of bond strength testing parameters andthe interpretation of test results. Studies using FEA have beenreferenced throughout this review with little explanation dueto space limitations. The nominal �TBS is not determinedsolely by adhesive bond properties. The initiation of fractureduring testing is due to the critical combination of stress anddefect size [24,79]. Defect size is, at least theoretically, control-lable and verifiable; however, stress distribution is determinedby mechanical properties of components, shape of the spec-imen, and the load magnitude and direction. The advantageof using FEA is that it allows separation of these parametersand their effects [38,39]. The combination of diverse materi-als and complex geometry makes stress distribution analysisin teeth very complicated [37]. Nevertheless, FEA of stressdistribution has been used to study the sensitivity of bondstrengths to specimen design, imperfections introduced dur-ing specimen fabrication, and changes in testing conditions[38,39,71]. Stress distribution in different experimental condi-tions can then be qualitatively related to the most probablesites of failure initiation [80]. This possibility does not existwithin mechanical tests alone. Some studies have used FEAonly to predict the parameters of the experimental condi-tions [38]; however, numerical models require experimentalvalidation and FEA results that have not been laboratory vali-dated should be viewed with reservation [37,39]. On the otherhand, experimental methods that do not take into consider-ation mechanical properties of the materials used and theexperimental test conditions should also be analyzed pru-dently. Fractographic microscopy of debonded specimens isalso highly encouraged for clarification of FEA predictions andmechanical test results.

Numerical analyses using FEA can satisfactorily deter-

mine states of stress and strain and energy release ratesor stress-intensity factors within a bonded joint [81]. How-ever, convergence of stress and strain produced by the finiteelement codes in the vicinity of boundary corners or mate-

6 ( 2 0 1 0 ) e50–e62

rial discontinuities, even with good mesh refinement, canoccur. A critical consideration is the parameters chosen toanalyze the FEA outcomes. Frequently von Mises criterion isused, without consideration of the type of the stress concen-tration, i.e., compressive or tensile. The Maximum PrincipalStress is proposed as a more appropriate parameter to ana-lyze �TBS testing because the maximum tensile stresses areshown isolated, and the stress concentration factor can becalculated as Kt, that is, ratio of maximum principal and nom-inal stresses. However, if only a punctual stress is recorded,this value may be influenced by mesh quality and bound-ary conditions. Therefore, it is recommended to use the top5% of test values to calculate quantitatively the MaximumPrincipal Stress parameters. Indeed the equivalent stresscan also be applied using the subroutine, when taking intoaccount that dental structures and most dental materials,being relatively brittle, fail more easily under tensile loads.The equivalent stress combines the six stress componentsfrom the three-dimensional stress state according to a rela-tionship known as the von Mises criterion [82]. Since dentinstrength in tension is approximately three times lower than incompression [83], the von Mises criterion can be modified toaccount for the fact that tensile stress components are morecritical than compressive [32], resulting in a more adequateanalytical parameter, called, Modified von Mises equivalentstress.

5. Summary

Before bond strength testing can be standardized we mustunderstand how the measured nominal bond strength isrelated to the local stress distributions generated during test-ing and, most importantly, how this information relates toclinical performance. As stated by Sudsangiam and van Noort,“Inherent in the process of standardization is the belief thatthe results derived from the bond strength test will have somevalidity and meaning as long as bond strength can be mea-sured consistently. This is highly questionable . . . becauseno amount of standardization will overcome inconsistencyproblems if a test is fundamentally flawed” [84]. Until suchtime that the relationship between a particular bond strengthtest and clinical performance is fully understood, more prag-matic goals may be the following: (1) adoption of universallyaccepted terminology and definitions, (2) standardized report-ing of specimen handling and fabrication, (3) inclusion ofpositive and negative controls during testing, (4) standardizedreporting of experimental set-up and test mechanics, and (5)full reporting of, or access to, a complete data set.

Thorough documentation and communication of howbond strength tests, whether “micro” or “macro”, are con-ducted will assist greatly in furthering our understanding ofthe strengths and limitations of various testing methods andshould help narrow the knowledge gap between the labora-tory and clinic (Appendix A). Stick-shaped specimens glued byall gripping surfaces and passively gripped dumbbell-shaped

specimens with round cross-sectional gauge lengths holdmuch promise, but more studies remain to be done beforeconclusions can be drawn. Bond strength tests, regardlessof type and size, remain useful as screening tools for new

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•

•

•

•

•

maaocaop

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

dhesive approaches and investigation of experimental vari-bles. These remain strength-based testing approaches of aomplicated adhesive joint with all inherent limitations [31],herefore, if ones goal is to measure an adhesive bond mate-ial property, a fracture mechanics approach, not without greatifficulty, may prove more successful.

ppendix A. Appendix3

“Uniaxially loaded tensile strength tests provide informa-tion on strength-limiting flaws from a greater volumeof uniformly stressed material. Therefore, because of theprobabilistic strength distributions of ceramics (adhesiveresin–tooth structure bonds), a sufficient number of spec-imens at each testing condition is required for statisticalanalysis” (in contrast to a sufficient number of measure-ments to account for any component of uncertainty arisingfrom a systematic effect or error).“Strengths obtained using different volumes or surfaceareas of material in the gauge sections will be different dueto these sizes, . . . resulting strength values can be scaled toan effective volume or surface area of unity.”“Results of tensile tests of specimens fabricated to standard-ized dimensions from a particular sized material and/orselected portions of a part may not totally represent thestrength and deformation properties of the entire full-sizeend product or its in-service behavior in different environ-ments.”“Surface preparation of test specimens can introduce fabri-cation flaws that may have pronounced effects on tensilestrength. Universal or standardized test methods of sur-face preparations do not exist. It should be understood thatfinal machining steps may or may not negate machiningdamage introduced during the early coarse or intermediatemachining. Thus specimen fabrication history may play animportant role in the measured strength distributions andshould be reported.”“Bending in uniaxial tensile tests can cause or pro-mote non-uniform stress distributions with maximumstresses occurring at the specimen surface leading to non-representative fractures originating at surfaces or neargeometrical transitions. . . . Similarly, fracture from surfaceflaws may be accentuated or muted by the presence of thenon-uniform stresses.”“The brittle nature of advanced ceramics (enamel, dentin,

RBC behaves in a brittle manner also) requires a uniforminterface between the grip components and the grippedsection of the specimen.”

3 The following guidelines are from ASTM C 1273: standard testethod for tensile strength of monolithic advanced ceramics at

mbient temperatures. Philadelphia, PA, 1995 (with commentsdded). Although this standard is not written for tensile testingf two substrates joined by an adhesive bond, the writersontend that the statements included below are applicable to thedhesive resin–tooth bond due to the inherently brittle behaviorf all components of the bonded assemble and for reasons statedreviously in the body of the paper.

( 2 0 1 0 ) e50–e62 e59

• “Gripping devices can be classed generally as those employ-ing active (e.g., glue) and those employing passive (e.g.,Dircks device) grip interfaces.◦ The important aspect of passive grip interfaces is uniform

contact between the gripped section of the specimen andthe grip faces.

◦ Moderately close tolerances are required for concentricityof both the grip and specimen diameters.”

• “Regardless of which type of coupler is used, alignment ofthe testing system must be verified at the beginning andend of a test series.”

• “Allowable Bending—analytical and empirical studies haveconcluded that for negligible effects on the estimatesof strength distribution parameters (for example, Weibullmodulus, m, and characteristic strength, ��) allowable per-cent bending should not exceed five.”

• “At a minimum, an autographic record of applied load vs.time should be obtained. Crosshead displacement of thetest machine may also be recorded but should not be usedto define displacement or strain in the gauge section espe-cially when self-aligning couplers are used in the loadtrain.”

• “Flat tensile specimens—. . . Disadvantages include the rel-atively small volume of material tested and the sensitivityof the specimen to small dimensional tolerances or distur-bances in the load train. It should be noted that the gaugesection of flat tensile specimens for tensile strength mea-surements are sometimes cylindrical. While this type ofgauge section adds to the difficulty of fabrication and there-fore the cost of the flat specimen if does avoid the problemof fractures initiating at corners of non-cylindrical gaugesections. Corner fractures may be initiated by stress con-centrations due to the elastic constraint of the corners butare more generally initiated by damage (chipping, etc.) thatcan be treated by chamfering the corners . . .”

• “Final finishing should be performed with diamond toolsthat have between 320 and 600 grit (35–14 �m). No less than0.06 mm per face should be removed during the final fin-ishing phase, and at a rate not more than 0.002 mm perpass.”

• “Generally, surface finishes on the order of average rough-nesses, Ra, of 0.2–0.4 �m is recommended to minimizesurface fractures related to surface roughness. However, insome cases the final surface finish may not be as impor-tant as the route of fabrication due to the generation ofsubsurface damage during the fabrication process.”

• “In many instances, the bulk of the material is removed in acircumferential grinding operation with a final, longitudinalgrinding operation performed in the gauge section to assurethat any residual grinding marks are parallel to the appliedstress.”

• “Generally, computer numerically controlled (CNC) fabrica-tion methods are necessary to obtain consistent specimenswith the proper dimensions within the required tolerances.A necessary condition for this consistency is the completefabrication of the specimen without removing it from the

grinding apparatus.”

• “Number of test specimens—use C1239 Practice forreporting uniaxial strength data and estimating Weibullparameters for advanced ceramics.”

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• “Load rate—for most advanced ceramics exhibiting linearelastic behavior, fracture is attributed to a weakest-link frac-ture mechanism generally attributed to stress-controlledfracture from Griffith-like flaws. . . .. Alternatively, selectstress rates to produce final fracture in 5–10 s to minimizeenvironmental effects when testing in ambient air.”

• “Note that results from specimens fracturing outside theuniformly stressed gauge section are not recommended foruse in the direct calculation of a mean tensile strength.Results from specimens fracturing outside the gauge sec-tion are considered anomalous and can be used only ascensored [right] tests, i.e., specimens in which a tensilestress at least equal to that calculated” by fractures occur-ring in the “uniform gauge section before the test wasprematurely terminated by a non-gauge section fracture.”

• “Fractographic examination of each failed specimen ishighly recommended to characterize the fracture origins.”

• Test report should include:◦ Tensile test geometry (include engineering drawing).◦ Type and configuration of the test machine.◦ Type and configuration of grip interface used (include

drawing as needed).◦ Type and configuration of load train couplers (include

drawing as needed).◦ Number of specimens tested validly (that is, fracture in

the gauge section). In addition report total of number ofspecimens tested to provide an indication of the expectedsuccess rate of the particular specimen geometry and testapparatus.

◦ All relevant material data (tooth substrate description,restorative materials, etc.).

◦ Description of the method of specimen preparationincluding all stages of machining.

◦ Testing environment: temperature and humidity.◦ Test mode (load, displacement, or strain control).◦ Test rate (load rate, displacement rate, or strain-rate.◦ Percent bending and corresponding average percent

strain.◦ Mean tensile strength, standard deviation and coefficient

of variation.◦ Estimates of strength distribution parameters (for exam-

ple, Weibull modulus and characteristic strength).◦ Pertinent overall specimen dimensions.◦ Average surface roughness.◦ Average cross-sectional dimensions.◦ Pre-load rate and value.◦ Fracture location relative to the gauge section midpoint.◦ Type and location of fracture origin (flaw) relative to the

front of the specimen as marked.

Added suggestions:

• Describe specimen fabrication and geometry in sufficientdetail, to include length and shape of gauge region, such as,cylindrical, square, square with chamfered corners.

• Report all modes of failure (locus in the adhesive bond

through which the failure propagates) with consistentnomenclature and definition.Cohesive in tooth structure adherend (enamel, dentin, etc.).Cohesive in restorative material adherend (RBC, RMGI, etc.).

6 ( 2 0 1 0 ) e50–e62

Adhesive joint or adhesive bond: defined from interface ofRBC and adhesive resin to the bottom of the hybrid layer(RBC–AR to BHL–dentin). Pocious recommends using thephrase “apparent” failure in adhesive joint or bond sincesophisticated surface chemistry analysis is required to con-firm a true “adhesive failure” between the adhesive systemand the adherend.Mixed failure: in adhesive joint and adherend(s).

• Encourage identification, if possible, of fracture origin. Thefracture origin is the flaw from which the strength-limitingcrack began to produce the debond pathway or mode of fail-ure. The debond pathway can be relatively easily identifiedwhen using adequate magnification; however, the true frac-ture “origin” requires fractographic expertise and, due to theviscoelastic nature of materials in the adhesive resin–toothstructure bonded joint, may be very difficult to preciselyidentify.

• Report and include all pre-test specimen failures as left-censored data with a defined value.

• Include non-gauge section or test region (if no gauge regionexists) failures as right-censored data.

• Report descriptive and inferential results using both normaland Weibull distribution statistics.

• Use random effects (ANOVA) and frailty effects (Weibull)for clustered data, i.e., multiply specimens from the sametooth. This accounts for tooth dependency while using spec-imen as the experimental unit.

• To reduce material property mismatch across the adhesivejoint, consider the use of a single adherend, such as, dentinbonded to dentin, RBC bonded to RBC.

• Additional terminology:

“Stick”: square or nearly square microtensile specimen(sometimes referred to as a “beam”).

“Slab”: rectangular microtensile specimen.“Notch”: any attempt at narrowing the specimen to con-

centrate stress in a test region.“Dumbbell”: a notched specimen with a defined radius of

curvature from the end or shoulder of the specimen to astraight gauge length that includes the adhesive joint.

“Hourglass”: a notched specimen with a defined radius ofcurvature from the end or shoulder of the specimen to theadhesive joint without a defined gauge length.

“Active gripping method”: mechanical fastening of speci-men to gripping device, such as glue or clamps.

“Passive gripping method”: specimen is placed in a testingdevice without the aid of glue or mechanical gripping; deviceshould self-align the specimen parallel to the tensile load.

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