Abstract
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 adhesion 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 conventional ‘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 substrates, for proper test selection, conduct and interpretation. Finite element analysis and comprehensive reporting of test conduct and results will further our efforts towards a standardization of test procedures. For the foreseeable future, both “micro” and “macro” bond strength tests will, as well as various morphological and spectroscopic investigative techniques, continue to be important tools for improving resin–tooth adhesion to increase the service life of dental resin-based composite restorations.
1
Introduction to the review
Bond strength testing has been traditionally accomplished by creating one test specimen per tooth or tooth surface which is then loaded to failure in either shear (SBS), tensile (TBS), or fracture-based manner. Currently, the most common approach is to load multiple test specimens from each tooth in either a tensile (μTBS) or shear (μSBS) manner. The earliest reference to the use of a ‘micro’ tensile device for biomechanical engineering was by Okuno who studied whisker-reinforced metal alloy . Subsequently microtensile testing was used to evaluate tensile strength of collagen and elastin fibers in porcine tricuspid leaflets , aortic valves , patellar groove articular cartilage , and cortical bone .
Sano et al. introduced microtensile testing to dentistry to measure the ultimate tensile strength and modulus of elasticity of mineralized and demineralized dentin . This test offered versatility that could not be achieved using conventional shear and tensile methods. This groundbreaking work was then applied to bond strength evaluations, such as; normal vs. caries-affected occlusal dentin , normal vs. sclerotic cervical dentin , different regions of restored MOD preparations , in vivo class V restorations , regions of occlusal dentin , dentin depth and cavity configuration , and hundreds of other dental biomaterials studies. “Micro” tensile bond strength (μTBS) lends itself to additional research designs that the “macro” tests do not, such as, the elimination of tooth dependency through balanced designs , and has shown reduced test variance . Simply using a smaller mechanical bond strength test specimen in the laboratory does not eliminate the challenges that teeth, beyond their relatively small size, present, i.e., structural heterogeneity, mechanical anisotropy, visco-elasticity, presence of dentinal fluids, and simulation of their complex and harsh functional environment.
A 1995 review of dentin adhesion testing stated a number of advantages for the μTBS approach: more adhesive failures and fewer cohesive failures, measurement of higher interfacial bond strengths, means and variances can be calculated for single teeth; permits testing of irregular surfaces; permits testing of very small areas and facilitates SEM/TEM examinations of the failed bonds since the surface area is approximately 1 mm 2 . Drawbacks cited were the labor intensity, technical demand, and dehydration potential of these smaller samples. In an excellent follow up paper by Pashley et al. the versatility of microtensile methods over conventional approaches to evaluate clinically relevant sites and substrates was comprehensively detailed and critiqued .
Microtensile bond strengths tend to be much higher (often 2× to 4×) than that of macro TBS values because the defect concentration in the small cross-sectional interfacial areas is lower . More recently, the μSBS test has become popularized as an alternative to the conventional SBS test . The μSBS test again obtains multiple specimens per tooth but without the damage history due to specimen sectioning, trimming and shaping that is required with μTBS testing. MicroSBS, as with μTBS, has been advocated as a method for regional mapping, but also is very conducive to depth profiling of different substrates.
Currently no broad agreement exists within the scientific community as to the appropriate conduct, usage and interpretation of these tests and any attempts at standardization of test methods have been difficult. The objective of this paper is to critically review the literature regarding the mechanics, geometry, load application, preparation effects, and other testing parameters of these “micro” adhesion tests, outlining their advantages and limitations.
2
Literature
2.1
What makes a good bond strength test?
The realization of a widely accepted, validated, standard test method for adhesive resin–tooth bond strength testing is an elusive and controversial endeavor . Although a consensus or standard approach does not currently exist in dentistry, bond strength testing remains useful and necessary for the screening of new products and study of experimental variables. Therefore, the scientific community should strive to identify important testing parameters and journal editors should demand comprehensive reporting of these parameters so that critical interpretations of data and meaningful comparisons of results can be achieved. Published studies, test standards (see Appendix A ), and technical reports can be referenced for important considerations regarding specimen fabrication, test conducting, and reporting. First; however, basic principles regarding the use of strength-based testing must be understood.
Strength-based testing does not quantify an inherent material property of the bond of restorative materials to tooth structure. The measured bond strength and the failure mode or debond pathway produced is dependent, among other things, upon: flaws existing within or between materials, specimen size and geometry, material properties of each component of the bonded assembly, and method of load application. Adhesive resin–tooth bond strength testing involves two separate substrates and complicated interphases or zones of interdiffusion between these components, all possessing different material properties. Therefore, even if a perfectly uniform tensile load could be applied across a resin-based-composite (RBC)–dentin adhesively bonded joint a non-uniform stress distribution will occur in the adhesive joint. The ratio of Poisson’s ratio ( ν ) to the Young’s modulus ( E ) would have to be equivalent for the: RBC, RBC–adhesive resin ‘interface’, adhesive resin, adhesive resin–hybrid layer ‘interface’, hybrid layer, hybrid layer–dentin ‘interface’, and dentin to produce a uniform stress state . In other words, the RBC and dentin substrates and the adhesive resin and their interdiffusion zones under loading, would all have to either: (1) not deform laterally, or (2) deform laterally in an equal manner, to permit a homogenous stress distribution. The ratio ν : E is higher for the adhesive and hybrid layer than the adherends, thus the lateral deformation in these regions will be greater than for the RBC and tooth structure, if the bond remains intact under loading, a complex stress distribution will occur in the adhesive resin and hybrid layer. Clearly, a complex stress state exists in ‘macro’ and ‘micro’ shear and tensile testing of the adhesive resin–tooth bonded joint. 2
2 Also why both K Ic and K IIc loading modes must be considered in the determination of interfacial fracture toughness or alternatively, to use a fracture energy ( G c ) approach, due to mode mixity across RBC–tooth adhesive joints.
These prior comments ignore the material property differences that commonly exist within a single material when comparing bulk vs. surface regions. As pointed out at an earlier ADM meeting , it may be more appropriate to refer to these tests, regardless of type of test or size of specimen, as simply “bond strength tests”.
Smaller test specimens are ‘stronger’ than larger ones due to the lower probability of having a critical sized defect present and aligned in a crack opening orientation relative to the applied load. Tooth structure and dental adhesives are relatively brittle materials, therefore, the Griffith strength equation :
σ f = 2 E γ π c o 1 / 2
where σ f is the fracture strength in uniform tension, E the plain strain elastic modulus, γ the surface energy per unit area, and c o the starter crack size, applies. This reminds us that the measured “bond strength” at failure will be dependent not only upon the fracture strength but the presence of flaws . Another way to think about this is that the resultant bond strength data obtained from mechanical testing represents a statistical distribution of the flaws (discontinuity, defect, bubble, void, residual solvent, etc.) present in each test specimen leading to its failure. This concept lends itself nicely, not only, to the presentation and interpretation of data by probabilistic statistical methods such as Weibull, but as an explanation as to why smaller specimens are stronger. Another way to say this is “Strength values (whether from testing a monolithic specimen or a bonded specimen) simply provide insight into the stresses a particular material will support given the flaw size distribution” . Griffith demonstrated that drawn glass fibers were “stronger” than ordinary glass, proving the volume dependency of strength, as did da Vinci, four centuries earlier, when testing wires of various lengths. This volume dependency of strength tells us that a smaller test specimen will be less likely to have a larger flaw that leads to its failure. Therefore a higher apparent “strength” is measured. In converse, the larger the size of the specimen, the more likely will be the presence of a larger flaw that leads to its failure, and, therefore, a lower apparent strength. The adoption of the term “microtensile” into our literature does not fully acknowledge these principles. Tensile testing should not be simply categorized as those with a less than 1 mm 2 (microtensile) or greater than 1 mm 2 cross-sectional testing region without full consideration of this volume dependency of strength. Nevertheless, use of the term “microtensile” is firmly established in our literature.
2
Literature
2.1
What makes a good bond strength test?
The realization of a widely accepted, validated, standard test method for adhesive resin–tooth bond strength testing is an elusive and controversial endeavor . Although a consensus or standard approach does not currently exist in dentistry, bond strength testing remains useful and necessary for the screening of new products and study of experimental variables. Therefore, the scientific community should strive to identify important testing parameters and journal editors should demand comprehensive reporting of these parameters so that critical interpretations of data and meaningful comparisons of results can be achieved. Published studies, test standards (see Appendix A ), and technical reports can be referenced for important considerations regarding specimen fabrication, test conducting, and reporting. First; however, basic principles regarding the use of strength-based testing must be understood.
Strength-based testing does not quantify an inherent material property of the bond of restorative materials to tooth structure. The measured bond strength and the failure mode or debond pathway produced is dependent, among other things, upon: flaws existing within or between materials, specimen size and geometry, material properties of each component of the bonded assembly, and method of load application. Adhesive resin–tooth bond strength testing involves two separate substrates and complicated interphases or zones of interdiffusion between these components, all possessing different material properties. Therefore, even if a perfectly uniform tensile load could be applied across a resin-based-composite (RBC)–dentin adhesively bonded joint a non-uniform stress distribution will occur in the adhesive joint. The ratio of Poisson’s ratio ( ν ) to the Young’s modulus ( E ) would have to be equivalent for the: RBC, RBC–adhesive resin ‘interface’, adhesive resin, adhesive resin–hybrid layer ‘interface’, hybrid layer, hybrid layer–dentin ‘interface’, and dentin to produce a uniform stress state . In other words, the RBC and dentin substrates and the adhesive resin and their interdiffusion zones under loading, would all have to either: (1) not deform laterally, or (2) deform laterally in an equal manner, to permit a homogenous stress distribution. The ratio ν : E is higher for the adhesive and hybrid layer than the adherends, thus the lateral deformation in these regions will be greater than for the RBC and tooth structure, if the bond remains intact under loading, a complex stress distribution will occur in the adhesive resin and hybrid layer. Clearly, a complex stress state exists in ‘macro’ and ‘micro’ shear and tensile testing of the adhesive resin–tooth bonded joint. 2
2 Also why both K Ic and K IIc loading modes must be considered in the determination of interfacial fracture toughness or alternatively, to use a fracture energy ( G c ) approach, due to mode mixity across RBC–tooth adhesive joints.
These prior comments ignore the material property differences that commonly exist within a single material when comparing bulk vs. surface regions. As pointed out at an earlier ADM meeting , it may be more appropriate to refer to these tests, regardless of type of test or size of specimen, as simply “bond strength tests”.
Smaller test specimens are ‘stronger’ than larger ones due to the lower probability of having a critical sized defect present and aligned in a crack opening orientation relative to the applied load. Tooth structure and dental adhesives are relatively brittle materials, therefore, the Griffith strength equation :
σ f = 2 E γ π c o 1 / 2
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