J.H. Kinney

The dentin substrate: structure and properties related to bonding.

OBJECTIVES Dentin is a vital, hydrated composite material with structural components and properties that vary with location. These variations are reviewed along with alterations by physiological and pathological changes that allow classification into various forms of dentin. Structural characteristics and mechanical properties are reviewed and the limitations of our understanding of structure-property relationships for normal and modified forms of dentin are discussed with respect to their impact on dentin bonding. Recent progress in methods available to study dentin and its demineralization are emphasized with their promise to increase our understanding of dentin properties and structure. DATA SOURCES Recent microstructural studies, focusing on scanning electron microscopy, atomic force microscopy and X-ray tomographic microscopy are included. A review of fundamental studies with emphasis on microstructurally sensitive methods, and prior reviews of basic mechanical properties are included with discussion of their correlation to composition and structure. STUDY SELECTION AND CONCLUSIONS Emphasis in this work was placed on the major structural components of the tissue, including the collagen based organic matrix and its mineral reinforcement, the distribution of these components and their microstructural organization as related to mechanical properties and response to demineralization. Little information is included on biochemical and developmental studies or on non-collagenous proteins and other organic components for which limited understanding is available with respect to their role in structure-property relations and influence on bonding. In spite of the fact that the complexity of dentin precluded a comprehensive review, it is clear that local structural variations influence properties and impact nearly all preventive and restorative dental treatments. Much more work is needed in order to understand differences between vital and non-vital dentin, and dentin from extracted teeth. Although our knowledge is rudimentary in certain areas, increasingly sophisticated methods of studying dentin should provide the necessary information to model structure-property relations, optimize dentin bonding, and improve many aspects of preventive and restorative dentistry.

The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature.

The past 50 years of research on the mechanical properties of human dentin are reviewed. Since the body of work in this field is highly inconsistent, it was often necessary to re-analyze prior studies, when possible, and to re-assess them within the framework of composite mechanics and dentin structure. A critical re-evaluation of the literature indicates that the magnitudes of the elastic constants of dentin must be revised considerably upward. The Youngs and shear moduli lie between 20-25 GPa and 7-10 GPa, respectively. Viscoelastic behavior (time-dependent stress relaxation) measurably reduces these values at strain rates of physiological relevance; the reduced modulus (infinite relaxation time) is about 12 GPa. Furthermore, it appears as if the elastic properties are anisotropic (not the same in all directions); sonic methods detect hexagonal anisotropy, although its magnitude appears to be small. Strength data are re-interpreted within the framework of the Weibull distribution function. The large coefficients of variation cited in all strength studies can then be understood in terms of a distribution of flaws within the dentin specimens. The apparent size-effect in the tensile and shear strength data has its origins in this flaw distribution, and can be quantified by the Weibull analysis. Finally, the relatively few fracture mechanics and fatigue studies are discussed. Dentin has a fatigue limit. For stresses smaller than the normal stresses of mastication, approximately 30 MPa, a flaw-free dentin specimen apparently will not fail. However, a more conservative approach based on fatigue crack growth rates indicates that if there is a pre-existing flaw of sufficient size (approximately 0.3-1.0 mm), it can grow to catastrophic proportion with cyclic loading at stresses below 30 MPa.

Hardness and young's modulus of human peritubular and intertubular dentine

A specially modified atomic-force microscope was used to measure the hardness of fully hydrated peritubular and intertubular dentine at two locations within unerupted human third molars: within 1 mm of the dentine enamel junction and within 1 mm of the pulp. The hardness of fully hydrated peritubular dentine was independent of location, and ranged from 2.23 to 2.54 GPa. The hardness of fully hydrated intertubular dentine did depend upon location, and was significantly greater near the dentine enamel junction (values ranged from 0.49 to 0.52 GPa) than near the pulp (0.12-0.18 GPa). A Nanoindenter was used to estimate the Youngs modulus of dehydrated peritubular and intertubular dentine from the unloading portion of the load displacement curve. The modulus values averaged 29.8 GPa for the peritubular dentine (considered to be a lower limit), and ranged from 17.7 to 21.1 GPa for the intertubular dentine, with the lower values obtained for dentine near the pulp.

Glucocorticoid-treated mice have localized changes in trabecular bone material properties and osteocyte lacunar size that are not observed in placebo-treated or estrogen-deficient mice.

This study compares changes in bone microstructure in 6‐month‐old male GC‐treated and female ovariectomized mice to their respective controls. In addition to a reduction in trabecular bone volume, GC treatment reduced bone mineral and elastic modulus of bone adjacent to osteocytes that was not observed in control mice nor estrogen‐deficient mice. These microstructural changes in combination with the macrostructural changes could amplify the bone fragility in this metabolic bone disease.

A micromechanics model of the elastic properties of human dentine.

A generalized, self-consistent model of cylindrical inclusions in a homogeneous and isotropic matrix phase was used to study the effects of tubule orientation on the elastic properties of dentine. Closed-form expressions for the five independent elastic constants of dentine were derived in terms of tubule concentration, and the Youngs moduli and Poisson ratios of peri- and intertubular dentine. An atomic-force microscope indentation technique determined the Youngs moduli of the peri- and intertubular dentine as approx. 30 and 15 GPa, respectively. Over the natural variation in tubule density found in dentine, there was only a slight variation in the axial and transverse shear moduli with position in the tooth, and there was no measurable effect of tubule orientation. It was concluded that tubule orientation has no appreciable effect on the elastic behaviour of normal dentine, and that the elastic properties of healthy dentine can be modelled as an isotropic continuum with a Youngs modulus of approx. 16 GPa and a shear modulus of 6.2 GPa.

The Importance of Intrafibrillar Mineralization of Collagen on the Mechanical Properties of Dentin

It is widely held that the hardness and modulus of dentin increase in proportion to the mineral concentration. To test this belief, we measured hardness and modulus of normal dentin and an altered form of dentin without gap-zone mineralization in wet and dry conditions by AFM nanoindentation to determine if the modulus and hardness scale linearly with mineral concentration. Mineral concentrations in the mid-coronal location of the normal and altered dentins were 44.4 vol% and 30.9 vol%, respectively. Surrounding the pulp of the altered dentin was a region of higher mineralization, 40.5 vol%. The indentation modulus of normal dentin was 23.9 (SD = 1.1) GPa dry and 20.0 (SD = 1.0) GPa wet. In mid-coronal regions of the altered dentin, the indentation modulus was 13.8 (SD = 2.0) GPa dry and 5.7 (SD = 1.4) GPa wet. In the more mineralized regions of the altered dentin, the modulus was 20.4 (SD = 1.8) GPa dry and 5.3 (SD = 0.8) GPa wet; the properties of the altered wet dentin did not correlate with mineral concentration. The results of this study raise doubt as to whether mineral concentration alone is a sufficient endpoint for assessing the success or failure of remineralization approaches in restorative dentistry.

Effect of orientation on the in vitro fracture toughness of dentin: the role of toughening mechanisms☆

Toughening mechanisms based on the presence of collagen fibrils have long been proposed for mineralized biological tissues like bone and dentin; however, no direct evidence for their precise role has ever been provided. Furthermore, although the anisotropy of mechanical properties of dentin with respect to orientation has been suggested in the literature, accurate measurements to support the effect of orientation on the fracture toughness of dentin are not available. To address these issues, the in vitro fracture toughness of dentin, extracted from elephant tusk, has been characterized using fatigue-precracked compact-tension specimens tested in Hanks balanced salt solution at ambient temperature, with fracture paths perpendicular and parallel to the tubule orientations (and orientations in between) specifically being evaluated. It was found that the fracture toughness was lower where cracking occurred in the plane of the collagen fibers, as compared to crack paths perpendicular to the fibers. The origins of this effect on the toughness of dentin are discussed primarily in terms of the salient toughening mechanisms active in this material; specifically, the role of crack bridging, both from uncracked ligaments and by individual collagen fibrils, is considered. Estimates for the contributions from each of these mechanisms are provided from theoretical models available in the literature.

Viscoelastic properties of demineralized human dentin measured in water with atomic force microscope (AFM)‐based indentation

Using an atomic force microscope (AFM) with an attachment specifically designed for indentation, we measured the mechanical properties of demineralized human dentin under three conditions: in water, in air after desiccation, and in water after rehydration. The static elastic modulus (E(h)r = 134 kPa) and viscoelastic responses (tau(epsilon) = 5.1 s and tau(sigma) = 6.6 s) of the hydrated, demineralized collagen scaffolding were determined from the standard linear solid model of viscoelasticity. No significant variation of these properties was observed with location. On desiccation, the samples showed considerably larger elastic moduli (2 GPa), and a hardness value of 0.2 GPa was measured. Upon rehydration the elastic modulus decreased but did not fully recover to the value prior to dehydration (381 kPa).

Mechanical properties of mineralized collagen fibrils as influenced by demineralization

Dentin and bone derive their mechanical properties from a complex arrangement of collagen type-I fibrils reinforced with nanocrystalline apatite mineral in extra- and intrafibrillar compartments. While mechanical properties have been determined for the bulk of the mineralized tissue, information on the mechanics of the individual fibril is limited. Here, atomic force microscopy was used on individual collagen fibrils to study structural and mechanical changes during acid etching. The characteristic 67 nm periodicity of gap zones was not observed on the mineralized fibril, but became apparent and increasingly pronounced with continuous demineralization. AFM-nanoindentation showed a decrease in modulus from 1.5 GPa to 50 MPa during acid etching of individual collagen fibrils and revealed that the modulus profile followed the axial periodicity. The nanomechanical data, Raman spectroscopy and SAXS support the hypothesis that intrafibrillar mineral etches at a substantially slower rate than the extrafibrillar mineral. These findings are relevant for understanding the biomechanics and design principles of calcified tissues derived from collagen matrices.

Crack blunting, crack bridging and resistance-curve fracture mechanics in dentin: effect of hydration.

Few studies have focused on a description of the fracture toughness properties of dentin in terms of resistance-curve (R-curve) behavior, i.e., fracture resistance increasing with crack extension, particularly in light of the relevant toughening mechanisms involved. Accordingly, in the present study, fracture mechanics based experiments were conducted on elephant dentin in order to determine such R-curves, to identify the salient toughening mechanisms and to discern how hydration may affect their potency. Crack bridging by uncracked ligaments, observed directly by microscopy and X-ray tomography, was identified as a major toughening mechanism, with further experimental evidence provided by compliance-based experiments. In addition, with hydration, dentin was observed to display significant crack blunting leading to a higher overall fracture resistance than in the dehydrated material. The results of this work are deemed to be of importance from the perspective of modeling the fracture behavior of dentin and in predicting its failure in vivo.