Validity of bond strength tests: A critical review-Part II.

BACKGROUND: Macro-bond strength tests resulted in cohesive failures and overestimation of bond strengths. To reduce the flaws, micro-bond strength tests were introduced. They are the most commonly used bond-strength tests.
OBJECTIVE: Thus the objective of this review is to critically review the reliability of micro-bond strength tests used to evaluate resin-tooth interface. DATA COLLECTION: Relevant articles published between January 1994 and July 2013 were collected from Pubmed database, Google scholar and hand searched journals of Conservative Dentistry, Endodontics and Dental materials. DATA SYNTHESIS: Variables that influence the test outcome are categorized into substrate related factors, factors related to specimen properties, specimen preparation and test methodology. Impact of these variables on the test outcome is critically analyzed.
CONCLUSION: Micro-bond tests are more reliable than macro-bond tests. However, no standard format exists for reporting the bond strength tests which could lead to misinterpretation of the data and bonding abilities of adhesives.

INTRODUCTION

The rapid progress in dental adhesive technology has extensively influenced modern restorative dentistry. Despite the vast advances made in adhesive technology during the last 50 years, the bonded interface itself remains as a weakest point of an adhesive restoration.[1] Smaller test specimens are ‘stronger’ than larger ones due to the lower probability of presence of critical sized defects.[2] Thus micro-bond strength tests (bond area less than 3 mm2 ) were developed[3] and higher apparent ‘strength’ can be measured with more failures at the interface. Unlike the macro tests, which were discussed in previous article (Validity of bond Strength tests-part I: A critical review on macro-bond strength test methods) failures are adhesive rather than cohesive.[4]

MICRO-BOND STRENGTH TESTS

Micro-bond strength tests are categorized into three types: Micro-shear, micro-tensile and micro-push out tests based on the stresses exerted on the test specimens.

Micro-shear (μSBS) test

Shear bond strength (SBS) testing with bonded cross-sectional areas of 3 mm2 or less is referred to as ‘micro’ SBS.[56] It permits efficient screening of adhesive systems, regional and depth profiling of a variety of substrates, and conservation of teeth. A significant advantage over micro-tensile strength (μTBS) methods is that the μSBS specimen is pre-stressed prior to testing only by mold removal. However, the use of the mold for composite placement can lead to the introduction of flaws and different stress concentrations upon shear loading.[7]

The findings of Finite Element Analysis (FEA) reported uneven stress distribution by creating tensile stresses too.[89] A three-dimensional FEA[10] demonstrated minimized tensile forces during loading by optimizing specimen dimensions and load application location. Micro-shear test results may actually worse represent shear bond strength than the conventional macro-SBS test.[9] Area delimitation technique renders μSBS testing less questionable and should be considered as an important step during the application of the method.[11]

Micro-shear tests remain an extremely useful test for those substrates with properties such as glass ionomers or enamel that make them particularly susceptible to the specimen preparation effects and testing conditions of μTBS testing.[4]

Micro-tensile (μTBS) test

In micro-tensile test, further specimen processing or the actual preparation of the micro-specimens is required after the bonding procedure.[12] Advantages are that it involves better economic use of teeth (with multiple micro specimens originating from one tooth), the better control of regional differences (e.g. peripheral versus central dentin), the better stress distribution at the true interface,[3] ability to test irregular surfaces and very small areas and facilitates microscopic examinations of the failed bonds due to smaller areas. Drawbacks are the labor intensity, technical demand, dehydration potential of these smaller samples, difficulty in measuring bond strengths lower than 5MPa, difficulty in fabricating specimens with consistent geometry, easily damaged specimens and loss or fracture of post-fracture specimens.[1213] Micro-tensile bond test (μTBS) allowed additional research designs that the ‘macro’ tests did not, such as, the elimination of tooth dependency through balanced designs.[14]

Micro-push out (μPO) test

Micro-push-out test is a modification of push out test where the specimen thickness is less than or equal to 1 mm2. Micro-tensile bond strength test method is not appropriate for use with intracanal filling materials because of the high percentage of premature bond failures and the large variation in test results.[15] Micro-push out is more dependable than the micro tensile technique while measuring the bond strength of luted fiber posts.[16] A study[17] reported that a modified push-out approach and micro-tensile test revealed higher values than traditional pull-out and push-out methods. But this modified push-out approach needs more studies before making any conclusions.

VARIABLES INFLUENCING MICRO-BOND STRENGTH TEST RESULTS

Variables related to the clinical situation and the substrate treatment

Many variables related to clinical situation and substrate affect the micro-bond strengths [Figure 1].

Figure 1

Variables influencing micro-tests

Substrate related factors

Source of teeth

Unlike the contradictory results of macro-tests, bovine teeth were found to be better substitutes for human teeth than porcine teeth for micro-tests. In porcine teeth, the enamel prism orientation is different from that of human enamel.[18] Lopes[19] found many structural similarities between human and swine teeth and suggested use of porcine teeth. Morphological, chemical composition and physical property differences between human and bovine teeth must be considered when interpreting results obtained from any experiment with bovine teeth substrate.[20] For more predictable results, use of human teeth is recommended.

Substrate condition

Micro-tests have the added advantage of ability to test the bond strengths to various substrates like sclerotic, infected or affected carious dentin, hypoplastic enamel etc. Micro-tensile bond strength to caries-affected dentin[2122] and sclerotic dentin[2324] was lower when compared with that of normal dentin. This was due to decreased resin infiltration into dentinal tubules of caries-affected dentin by the mineral deposits.[25] Mechanical treatment with diamond bur or diamond paste[24] or conditioning with stronger acids may facilitate stronger bonding to sclerotic dentin.[23]

Dentin depth and permeability

Majority of studies showed decreased micro-tensile[2627] and micro-shear bond strengths[28] with increased dentin depths unlike the varied results of macro-tests. This may be due to increased permeability[29] and reduced percentage of solid dentin available for bonding. It is advisable to consider the type of adhesives while interpreting the results of bond strength tests.

Smear layer

Smear layer denseness, more so than thickness, may compromise bonding efficacy of adhesives, especially of self-etch systems.[30] Smear layer thickness did not affect the bond strengths in majority of the studies but the type of adhesive affected the bond strengths.[3132] Higher bond strengths were noted with etch and rinse systems where smear layer was removed[31] and with agitated self etching systems where smear layer was dispersed or dissolved.[33]

Enamel prism and dentinal tubule orientation

The effects of regional variations in tooth structure, such as the orientation of enamel prisms, orientation of dentinal tubules and the influence of cavity geometry on bond strength, are variously studied, in which the results were contradictory like macro-tests. This may be because of structural anisotropy; variation in enamel bonding sites and the type of adhesive.[283435]

Substrate location

Substrate location can be occlusal, cervical, buccal or gingival and root or crown. Micro-tensile bond strength to enamel[36] and dentin[353738] and micro-shear[28] bond strengths to dentin varied depending on the location of substrate similar to that of macro-tests. Majority of the authors concluded that results depended on the type of adhesive system.[3538]

Pulpal pressure

The presence of pulpal pressure resulted in a decrease of μTBS of various bonding systems while simulating in vivo conditions.[3940] Few studies reported reduction of μTBS was adhesive-dependent,[4142] which is similar to macro-bond strengths. One study,[43] reported no bond strength reduction when adhesives were applied to dentin supplied with water pressure.

Tooth donor age

Physical properties and morphological features of enamel and dentin vary with age progression. Age did not affect bond strength values of glass-ionomer based, all-in-one, single-step, self-etching adhesive system to dental hard tissues.[444546]

Storage conditions and time

Distilled water, saline, 0.05% saturated solution of thymol, 0.5% chloramines-T, 2% gluteraldehyde, 10% formalin solutions were studied as storage media for bond-strength tests.[4748] Zheng et al.[49] recommended use of frozen teeth at −20o C or storage of teeth in 1% chloramine at 4o C. Before using any teeth that were preserved in dry state, rehydration with distilled water for two weeks is recommended.[50]

Teeth that have been extracted for longer than six months could undergo degenerative changes in dentinal protein.[47] According to the ISO/TS 11405, teeth those were stored for one month, but not more than six months, after extraction should be used.[51]

Variables related to test specimen properties

Bonding area

Bond area depends on the specimen size; hence it is discussed in detail under the section of ‘specimen size’.

Elastic modulus of the resin composite

Mechanical properties of composite can affect the bond-strength test results. High elastic modulus of bonded composite, relative adhesive layer thickness and load application distance resulted in non-uniform stress distribution along bonded interfaces.[9] Mechanical properties of the composite affected the μTBS values due to uneven stress distribution.[52]

Variables related to specimen preparation for bond strength testing

Aging media

Various solutions like distilled water, artificial saliva and sodium hypochlorite (NaOCl) were used for aging.[47] No change in micro-tensile bond strength was observed after six months of water storage[53] whereas another study[54] observed sensitivity of resin-dentin bond to water degradation for four years. It was concluded that resin bonded to enamel protected the resin-dentin bond against degradation, while direct exposure to water for four years affected bonds.

Failure analysis of immersed test specimens in sodium hypochlorite (NaOCl) revealed a drop in μTBS correlated with specific dissolution of the hybrid layer, similar to in vivo failure patterns.[5556] Thus aging specimens in 10% NaOCl for 1 or 3 hours can be an alternative method for long-term water storage (6 or 12 months) micro-tensile bond strength studies.[57] Main drawback of this method is due to the non-specific properties; NaOCl also causes the mechanical properties of the dentin substrate itself to deteriorate.[58]

Aging time

Aging in artificial saliva for more than six months reduced bond strength.[56] One year water storage did not have any effect on micro-tensile bond strength[59] whereas three year storage reduced the bond strength.[60] Effect of aging time on micro-tensile bond strength varied with the medium used for aging and type of adhesive.[61]

Thermal cycling

Researchers reported diverse results though majority of them concluded that thermocycling reduced the bond strength to enamel and dentin.[566263646566676869] The effect of thermocycling depends on the chemical bonding potential of functional monomer, i.e. type of adhesive system.[64] Very few studies employed the thermocycling at a frequency of 500 cycles as recommended by ISO/TS specification. If it exceeds this frequency, bond strength was reduced in majority of these studies. Hence, for reliable results ISO/TS recommendations must be followed.

Mechanical cycling

Amount of load exerted while chewing and swallowing varies between 70N and 150N.[70] Load application of 500,000 cycles is equivalent to six months and 1,000,000 cycles is equivalent to one year of in vivo mastication.[71] Researchers used loads ranging from 50N to 125N at frequencies ranging from 0.5 Hz to 4 Hz.[62636872737475] Number of cycles ranged from 10,000 cycles to 500,000 cycles. Mechanical cycling resulted in mixed type of failures and bond strength was reduced in specimens when they were subjected to thermal and mechanical cycling.[687273]

Operator skill and technique sensitivity

Operator's skill in handling a material and/or using the test apparatus may affect the measured micro-shear bond strength. Operator skills may improve with repeated testing and material use.[76] In contrast to this, no statistically significant difference was observed in micro-push out bond strength values between the operators of different clinical experience.[77] Though adequate literature is not available regarding its effect on micro-tensile bond strength, operator skill can increase the reliability of test results.

Variables of influence related to test mechanics

Specimen size

The micro-specimen preparation protocols are more technique-sensitive. For the micro test methods ‘trimmed’ and ‘non-trimmed’ micro-specimens are prepared and both have their advantages and disadvantages.[5278] A reduction of bond strength was observed in enamel[36], when the bonding area was increased from 0.5 to 3.0 mm2. Similar phenomenon was observed in specimen sizes with 1.2 and 2.0 mm in diameter where an inverse linear relationship between specimen size and bond strength when tested either in tension or shear[5] and that cross-sectional shape (cylindrical or rectangular) has little effect on micro-tensile bond strength.[79]

Specimen geometry

Stick shaped, hour glass and dumbbell-shaped specimens are used for micro-tensile testing. Trimming the specimens at the interface to hourglass-shaped specimens better concentrates stress at the interface and may result in premature failures due to interfacial defects.[80]

Interfaces can be trimmed by free hand using a dental handpiece[381] or a plexiglass table on an Isomet saw (Buehler, Lake Forest, IL, USA) and trim the specimen under microscopic observation, using a device like table saw.[12] Use of a semi-automatic trimming of micro-specimens using a Micro Specimen Former (University of Iowa, Iowa City, IA, USA) is highly advisable to trim rectangular specimens into specimens with a circular cross-section.[82]

Dumbbell-shaped specimens distribute the stress more uniformly due to their cylindrical geometry and boundary conditions only if manufacturing process would be improved (like CAD-CAM® process) to reduce imperfections into the interface. Otherwise, stick-shaped, non-trimmed specimens sectioned with diamond wire are preferred for enamel specimens as they can be prepared in a less destructive, easier, and more precise way.[83]

Other factors such as specimen-jig attachment, specimen-loading speed and specimen alignment also influence the final outcome; and therefore, should be standardized within the test set-up.[84]

One major concern is the required number of individual teeth from which many micro-specimens can be prepared to be statistically sound.[85] An elegant way to handle this problem is to use every tooth as its own control.[82] As also recommended by ISO/TS No. 11405[51], another way would be to apply survival analysis like the Weibull model or Cox proportional hazard using the force that is required for bond failure.[86]

Gripping devices

Test specimens are attached to the load train couplers of mechanical testing machines by either active or passive gripping devices. A non-normal load application, either the specimen or gripping mechanism significantly alters the stress distribution at the bonded interface.[87] Several specimen gripping devices, both active and passive, have been developed in an attempt to apply a tensile load normal to the bond line by aligning specimen's bond line with its gripping surfaces.[848889] Active gripping can be either mechanical or through a fast-setting glue. Bending forces can occur during load application due to: Non-parallel specimen alignment, bond line not perpendicular to the specimen gripping surfaces, and/or, uneven gripping forces.[8790]

These specimen-fixation procedures require careful manipulation and special test jigs like Bencor multi-T gripping device and Ciucchi's jig. None of these gripping devices guarantees proper alignment because the specimen is glued to a flat surface.[84] In Geraldeli's jig and modified Ciucchi's jig, a groove parallel to the applied load was added in to improve specimen alignment.[88]

Dirck's device was introduced as self aligning, glue-less, passive gripping device.[30] It is less sensitive to human error than Geraldeli's, and produced a more uniform stress distribution at the dumbbell specimen adhesive layer than did the Geraldeli's device at the stick layer.[91] Poitevin et al.[2784], developed a μTBS testing device with top bottom fixation to minimize stress concentrations.

Cross-head speed

Cross-head speeds ranging from 0.01 mm/min to 10.00 mm/min were tested for their influence on micro-tensile bond strength test. Unlike controversial results of macro-tests, all the studies concluded that the influence of the cross-head speed might be negligible while measuring micro-tensile bond strengths.[2792] Poitevin et al. recommended a cross-head speed of 1 mm/min for more uniform stress-time pattern.[27]

Fractographic analysis includes classification of the interfacial phases, crack initiation, direction and pattern of crack propagation, energetics of the fracture (single event or fatigue; brittle or ductile), and the phases included along the fracture plane.[93]

Possible failure modes [Figure 2] of fractographic analysis are:

Figure 2

Fractographic analysis

Cohesive in dentin,

Cohesive in resin,

Adhesive (dentin–adhesive interface),

Adhesive (resin–adhesive interface),

Mixed (dentin–adhesive–resin with small portions of dentin),

Mixed (dentin–adhesive–resin with large portions of dentin).

Depending on the fracture path, cohesive failures and mixed failures with large portions of dentin and resin should be excluded in results.[94] To characterize the adhesive joints several surface analysis methods are available, including electron spectroscopy for chemical analysis (ESCA), secondary ion mass spectroscopy (SIMS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and atomic force microscopy (AFM).[95]

CLINICAL RELEVANCE

In a systematic review[96], the results of bond strength tests did not correlate with laboratory tests that evaluated the marginal seal of restorations such as micro leakage or gap analysis. Though there was no significant correlation between micro-tensile bond strength data and the clinical index, there existed a correlation between micro-tensile bond strength and 6-month water storage and marginal discoloration of Class V restorations. A significant, quite reasonable correlation was nevertheless found between the aged bond-strength data and the 5-year clinical data. Hence, besides measuring the ‘immediate’ bond strength of adhesives to enamel and dentin, measuring the ‘aged’ bond strength should be encouraged in order to predict the clinical effectiveness of adhesives.[82]

Bond-strength test results of specimens after 24 hours and 3 months of storage in water should be comparable with those of comparable adhesive systems with an acceptable, proven clinical record.[96] Non-invasive methods of bond evaluation like X-ray micro computed tomography (CT) and acoustic emission can be considered while testing adhesives.[97]

RECOMMENDATIONS FOR IMPROVING VALIDITY OF MICRO-TESTS

Specific recommendations are put forth for consideration while testing the adhesive strengths.

If traditional bond strength tests (shear, micro-shear, tensile, micro-tensile) are to be used, only adhesive failures or mixed failures with small (<10%) resin or dentin involvement should be considered for the bond strength calculation. This requires thorough microscopic evaluation (stereo and SEM) of the fractured surface.

Use of Weilbul statistics should be systematically applied to evaluate bond strength data to provide more information that is relevant. Studies should utilize a minimum of 30 non-cohesive failed specimens.

Fracture mechanics approach that includes estimation of fracture toughness or the strain energy release rate is encouraged.[94]

CONCLUSION

Bond strength testing can be used in the laboratory while developing newer adhesives but cannot be used solely as a means of predicting clinical performance.[98] Bonding effectiveness in the laboratory should be assessed by

Micro-tensile bond strength testing,

Sealing effectiveness testing using semi-quantitative marginal analysis or fully quantitative margin permeability measurement and possibly

Dynamic fatigue testing.[82]

Though a great diversity in laboratory testing of adhesives exists, validity of these tests can be improved by application of standardized protocols in test methodology and supplementing with the dynamic fatigue test results. The guidelines of ISO Technical Specification (No. 11405) on “Testing on adhesion to tooth structure” should be followed while assessing bonding efficacy.