On the critical parameters that regulate the deformation behaviour of tooth enamel

Influence of water and protein content on the creep behavior in dental enamel

The creep behavior of untreated and deproteinized dental enamel in dry and wet state was analyzed by nanoindentation with a spherical tip. Additionally, the influence of the loading rate was investigated. Dry untreated and deproteinized dental enamel only showed minor creep over 100 s and deproteinization did not affect the dry enamel's behavior significantly. With slower loading rates some creep already occurs during the loading period, such that the creep displacement during load hold is less than with faster loading rates. Wet untreated and deproteinized enamel showed significantly more creep compared to the dry samples. The differences between the untreated and deproteinized enamel were only minor but significant, revealing that water affects the creep behavior of biological materials such as enamel significantly. The proposed deformation mechanism of naturally porous enamel under compression is compaction of the HAP crystallites and fluid displacement within material underneath the indented area. STATEMENT OF SIGNIFICANCE: This study investigates the creep behavior of untreated and deproteinized dental enamel in dry and wet conditions. It is shown that while the protein content does not affect enamel's behavior significantly, the wet conditions lead to an increased creep in enamel. The proposed deformation mechanism of naturally porous enamel under compression is compaction of the HAP crystallites and fluid displacement within material underneath the indented area. Based on this observation a simple analytical model has been developed, aiming to deepen our understanding of the deformation behavior of biological materials.

Comparative Sample Preparation Using Focused Ion Beam and Ultramicrotomy of Human Dental Enamel and Dentine for Multimicroscopic Imaging at Micro- and Nanoscale

(1) Background: The aim of this study was to systematically compare TEM sections of mineralized human enamel and dentine prepared by focused ion beam (in situ lift-out) technique and ultramicrotomy through a combination of microscopic examination methods (scanning electron microscopy and transmission electron microscopy). In contrast with published studies, we compared the TEM preparation methods using the same specimen blocks as those for the ultramicrotomy and FIB technique. (2) Methods: A further evaluation of TEM sample preparation was obtained by confocal laser scanning microscopy and atomic force microscopy. In addition, ultramicrotome- and focused ion beam-induced artefacts are illustrated. (3) Results: The FIB technique exposed a major difference between non-decalcified enamel and dentine concerning the ultrastructural morphology compared to ultramicrotome-prepared sections. We found that ultramicrotomy was useful for cutting mineralized dentine, with the possibility of mechanical artefacts, but offers limited options for the preparation of mineralized enamel. FIB preparation produced high-quality TEM sections, showing the anisotropic ultrastructural morphology in detail, with minor structural artefacts. Our results show that the solution of artificial saliva and glutardialdehyde (2.5% by volume) is a very suitable fixative for human mineralized tissue. (4) Conclusions: The protocol that we developed has strong potential for the preparation of mineralized biomaterials for TEM imaging and analysis.

Adhesion and whitening effects of P11-4 self-assembling peptide and HAP suspension on bovine enamel

OBJECTIVES

This study evaluated the adhesion and whitening effects of a combination of P11-4 self-assembling peptide and hydroxyapatite (peptide-HAP) on bovine enamel.

METHODS

Forty-six caries-free bovine teeth were selected, and 40 teeth were randomly allocated to one of five groups (n = 8). First, the effects of application frequency, exposure time, and storage in saliva on the whitening effects of an experimental low-concentrated peptide-HAP suspension (0.5 wt% HAP; Curodont, Credentis) were evaluated and compared with a commercial bleaching agent (VivaStyle Paint on Plus, VS, Ivoclar Vivadent). Tooth color was measured using a spectrophotometer (Gretag MacBeth), and color changes ΔE were statistically analyzed. Second, the effects of peptide-HAP concentration (low versus high: 6.25% HAP; Curodont Protect), and its interactions with saliva and postapplication restaining, were investigated. Third, enamel surfaces (n = 2) were treated with low concentration peptide-HAP and high-concentration peptide-HAP in polymeric and monomeric forms (Curodont Protect & Curodont Repair, Credentis) and analyzed by SEM.

RESULTS

The ΔE of the low-concentration peptide-HAP suspension did not differ from that of VS. Application frequency, exposure time, and storage in saliva did not have any significant impact on whitening efficacy of the peptide-HAP suspension. Increasing the concentration of the suspension did not promote overall ΔE. SEM observations confirmed the presence of the newly generated peptide and HAP on the enamel surface.

CONCLUSIONS

The peptide-HAP suspension is a mild tooth whitener, and the adhesion of peptide-HAP to enamel is concentration dependent.

CLINICAL RELEVANCE

This peptide-HAP suspension is effective in offsetting discoloration caused by restaining after treatment.

Wear mechanism of human tooth enamel: The role of interfacial protein bonding between HA crystals

Human tooth enamel, the most mineralized tissue in body, contains less than 2 wt% protein. Consequently, the importance of the protein to enamel mechanical response has always been overlooked. In this study, the role of minor protein in providing enamel microstructure and mechanical performance, especially tribological properties, were studied using deproteinization treatment and nano-indentation/scratch technique. Via the change from the original to the deproteinated conditions, a nanostructure degeneration from the assembly of hydroxyapatite (HA) crystals into nano-fibers to crystal aggregation has been found between the high-wear-resistance and low-wear-resistance on the enamel surface. Correspondingly, an energy dissipation to cause a unit volume of wear on enamel surface decreases by 50%, and wear volume increases by 80%. With the presence of protein, the occurrence of enamel wear requires to break the interfacial protein bonding between the HA crystals in nano-fibers and the break dissipates considerable energy, which benefits the enamel to resist wear. Thus, the protein in enamel, although of a very low content, is essential to resisting wear by mediating the assembly of rigid HA crystals via interfacial protein bonding. Replicating functions of the protein component will be critical to the successful development of bio-inspired materials that are designed for wear-resistance.

Research of the role of microstructure in the wear mechanism of canine and bovine enamel

The relationship between the microstructure and tribological behavior of mammalian tooth enamel has not been fully understood. In this paper, the microstructure, mechanical properties, and tribological behavior of canine (carnivore) and bovine (herbivore) enamel are studied using scanning electronic microscopy and nano-indentation/scratch technique, aiming to reveal the contribution of enamel microstructure to its mechanical and tribological properties. Canine enamel has a microstructure of hard keyhole-like rods embedded in soft inter-rod enamel, and its surface exhibits high resistance against both micro-crack initiation and crack-induced delamination during friction and wear process. Bovine enamel with the microstructure consisting of the hydroxyapatite (HAP) nano-fibers in decussation has higher surface hardness and better capabilities of resisting wear and encumbering crack propagation, as compared to canine enamel. In sum, the soft inter-rod enamel in the canine enamel contributes to high load tolerance and then protects enamel surface from brittle damage, while the staggered arrangement of HAP nano-fibers benefits hard bovine enamel in crack propagation resistance and then help resist wear and fatigue. The findings suggest that there exists a self-adaptation in enamel microstructure and tribological performance of mammals with their feeding habits, which will promote and assist the bionic design of high-performance materials.

Hydration-induced nano- to micro-scale self-recovery of the tooth enamel of the giant panda

The tooth enamel of vertebrates comprises a hyper-mineralized bioceramic, but is distinguished by an exceptional durability to resist impact and wear throughout the lifetime of organisms; however, enamels exhibit a low resistance to the initiation of large-scale cracks comparable to that of geological minerals based on fracture mechanics. Here we reveal that the tooth enamel, specifically from the giant panda, is capable of developing durability through counteracting the early stage of damage by partially recovering its innate geometry and structure at nano- to micro- length-scales autonomously. Such an attribute results essentially from the unique architecture of tooth enamel, specifically the vertical alignment of nano-scale mineral fibers and micro-scale prisms within a water-responsive organic-rich matrix, and can lead to a decrease in the dimension of indent damage in enamel introduced by indentation. Hydration plays an effective role in promoting the recovery process and improving the indentation fracture toughness of enamel (by ∼73%), at a minor cost of micro-hardness (by ∼5%), as compared to the dehydrated state. The nano-scale mechanisms that are responsible for the recovery deformation, specifically the reorientation and rearrangement of mineral fragments and the inter- and intra-prismatic sliding between constituents that are closely related to the viscoelasticity of organic matrix, are examined and analyzed with respect to the structure of tooth enamel. Our study sheds new light on the strategies underlying Nature's design of durable ceramics which could be translated into man-made systems in developing high-performance ceramic materials. STATEMENT OF SIGNIFICANCE: Tooth enamel plays a critical role in the function of teeth by providing a hard surface layer to resist wear/impact throughout the lifetime of organisms; however, such enamel exhibits a remarkably low resistance to the initiation of large-scale cracks, of hundreds of micrometers or more, comparable to that of geological minerals. Here we reveal that tooth enamel, specifically that of the giant panda, is capable of partially recovering its geometry and structure to counteract the early stages of damage at nano- to micro-scale dimensions autonomously. Such an attribute results essentially from the architecture of enamel but is markedly enhanced by hydration. Our work discerns a series of mechanisms that lead to the deformation and recovery of enamel and identifies a unique source of durability in the enamel to accomplish this function. The ingenious design of tooth enamel may inspire the development of new durable ceramic materials in man-made systems.

Enamel Microcracks Induced by Simulated Occlusal Wear in Mature, Immature, and Deciduous Teeth

Enamel wear, which is inevitable due to the process of mastication, is a process in which the microcracking of enamel occurs due to the surface contacting very small hard particles. When these particles slide on enamel, a combined process of microcutting and microcracking in the surface and subsurface of the enamel takes place. The aim of this study was to detect microscopic differences in the microcrack behavior by subjecting enamel specimens derived from different age groups (immature open-apex premolars, mature closed-apex premolars, and deciduous molars) to cycles of simulated impact and sliding wear testing under controlled conditions. Our findings indicated that the characteristics of the microcracks, including the length, depth, count, orientation, and relation to microstructures differed among the study groups. The differences between the surface and subsurface microcrack characteristics were most notable in the enamel of deciduous molars followed by immature premolars and mature premolars whereby deciduous enamel suffered numerous, extensive, and branched microcracks. Within the limitations of this study, it was concluded that enamel surface and subsurface microcracks characteristics are dependent on the posteruptive age with deciduous enamel being the least resistant to wear based on the microcrack behavior as compared to permanent enamel.

A COMPUTATIONAL STRATEGY TO EXAMINE THE PROFILE EFFECTS OF MICROPRISM REGIONS ON THE OVERALL ANISOTROPY OF HUMAN ENAMELS

In this study, our attention is mainly on elaborating a computational strategy to effectively predict the influence of prism profiles on the overall anisotropic property of human enamels (HEs). At first, two distinct schemes are developed separately with the aid of the polynomial fitting technique and the general power functions to mathematically describe the practical irregular and simplified regular profiles of enamel prisms. Hereafter, two parametric piecewise formulas, which facilitate the definition of anisotropic material properties of finite elements at different locations and make the numerical simulation of HE microstructures consisting of irregularly shaped prisms feasible, are presented to describe the orientation of hydroxyapatite (HAP) crystallites embedded in microprisms. The effective anisotropic moduli over a representative unit cell (RUC) under the periodic displacement constraint is concisely introduced according to the micromechanics, and a computational strategy is established to calculate these moduli numerically. Finally, the evaluations in the open literature are employed to demonstrate the validity of the elaborated computational strategy, and more investigations are conducted and yield the conclusions such that the material property of the inter-prism regions as well as the prism shapes plays a crucial role in determining the overall anisotropy of HEs.

Enamel apatite crystallinity significantly contributes to mammalian dental adaptations

The monophyodont molar teeth, prismatic enamel and the complexity of enamel microarchitecture are regarded as essential dental apomorphies of mammals. As prominent background factors of feeding efficiency and individual longevity these characters are crucial components of mammalian adaptive dynamics. Little is known, however, to which degree these adaptations are influenced by the crystallographic properties of elementary hydroxyapatite crystallites, the only inorganic component of enamel. In a miniature pig where individual molars differ significantly in duration of their development and in enamel resistance to attrition stress, we found highly significant differences between the molars in the size of crystallites, amount of microstrain, crystallinity and in enamel stiffness and elasticity, all clearly scaled with the duration of tooth calcification. The same pattern was found also in red deer bearing different molar type. The results suggest that the prolongation of tooth development is associated with an increase of crystallinity, i.e. the atomic order of enamel hydroxyapatite, an obvious component of micromechanical property of mature enamel. This relation could contribute to prolongation of dental development, characteristic of mammals in general. The aspects of enamel crystallinity, omitted in previous studies on mammalian and vertebrate dental evolution, are to be taken in account in these topics.

Effect of tooth bleaching agents on protein content and mechanical properties of dental enamel

This study investigated the effect of two bleaching agents, 16% carbamide peroxide (CP) and 35% hydrogen peroxide (HP), on the mechanical properties and protein content of human enamel from freshly extracted teeth. The protein components of control and treated enamel were extracted and examined on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Marked reduction of the protein matrix and random fragmentation of the enamel proteins after bleaching treatments was found. The mechanical properties were analyzed with Vickers indentations to characterize fracture toughness, and nanoindentation to establish enamel hardness, elastic modulus and creep deformation. Results indicate that the hardness and elastic modulus of enamel were significantly reduced after treatment with CP and HP. After bleaching, the creep deformation at maximum load increased and the recovery upon unloading reduced. Crack lengths of CP and HP treated enamel were increased, while fracture toughness decreased. Additionally, the microstructures of fractured and indented samples were examined with field emission gun scanning electron microscopy (FEG-SEM) showing distinct differences in the fracture surface morphology between pre- and post-bleached enamel. In conclusion, tooth bleaching agents can produce detrimental effects on the mechanical properties of enamel, possibly as a consequence of damaging or denaturing of its protein components.

Viscoplastic response of tooth enamel under cyclic microindentation

Cyclic microindentations were performed on the occlusal surface and axial section of tooth enamel, using the Berkovich indenter. Under the action of a cyclic indentation load, the indenter continuously penetrated into the tooth enamel and reached a quasi-steady state at which the penetration depth per cycle was a constant. At the quasi-steady state, both the amplitude of the indentation depth and the penetration depth per cycle for the cyclic indentation of the axial section are larger than those for the indentation of the occlusal section under the same loading condition. The energy dissipation per cycle consists of two contributions; one is the plastic energy dissipated per cycle due to the propagation of the plastic zone underneath the indentation and the other is the energy dissipation due to the viscous flow during the cyclic indentation. Both the penetration depth and the plastic energy dissipated per cycle at the quasi-steady state are independent of the maximum applied load and increase with increasing the amplitude of the cyclic indentation load.

Uniaxial compressive behavior of micro-pillars of dental enamel characterized in multiple directions

In this work, the compressive elastic modulus and failure strength values of bovine enamel at the first hierarchical level formed by hydroxyapatite (HA) nanofibers and organic matter are identified in longitudinal, transverse and oblique direction with the uniaxial micro-compression method. The elastic modulus values (∼70 GPa) measured here are within the range of results reported in the literature but these values were found surprisingly uniform in all orientations as opposed to the previous nanoindentation findings revealing anisotropic elastic properties in enamel. Failure strengths were recorded up to ∼1.7 GPa and different failure modes (such as shear, microbuckling, fiber fracture) governed by the orientation of the HA nanofibers were visualized. Structural irregularities leading to mineral contacts between the nanofibers are postulated as the main reason for the high compressive strength and direction-independent elastic behavior on enamels first hierarchical level.

Finite element analysis of depth effect on measuring elastic modulus of a core-shell structure for application of instrumented indentation in tooth enamel

Tooth enamel is a complex structure, consisting of numerous enamel rods surrounded by a protein-rich sheath. Considering the possible effect of the protein-rich sheath on the indentation deformation of an enamel rod and the limitation of the Oliver-Pharr method in measuring the elastic modulus of the enamel rod, we used a finite element method to analyze the indentation deformation of an elastic-perfectly plastic cylinder surrounded by an elastic-perfectly plastic film. A concept of the threshold indentation depth was proposed, at which the percentage error of the measured modulus of the cylinder is ±10%. For the indentation depth less than the threshold indentation depth, the elastic modulus measured from the indentation test can be approximated as the intrinsic elastic modulus of the cylinder. The normalized threshold indentation depth strongly depends on the modulus ratio of the film to the cylinder and the ratio of the film thickness to the cylinder radius. The results can be used to guide the use of the Oliver-Pharr method in characterizing the mechanical properties of tooth enamel and bio-composites with core-shell structures.

X-ray microdiffraction, TEM characterization and texture analysis of human dentin and enamel

Human tooth is a complex bioceramic composite, which consists of enamel, dentin and the interface, the dentin-enamel junction (DEJ). The crystal properties and ultrastructure of the inorganic phase through the thickness of healthy human molar teeth were investigated using X-ray microdiffraction (μXRD), electron diffraction and transmission electron microscopy (TEM) techniques. The XRD data were analysed using the Le Bail profile fitting approach. The size and the texture of the crystallites forming enamel and dentin in the crown part of teeth were measured using both techniques and then compared. Results showed that the thickness of dentin crystallites was found to decrease towards the DEJ, whereas the thickness of the enamel crystallites increased from the DEJ towards the outer layers. It was demonstrated that enamel exhibited an increase of texture in 002 lattice planes from the DEJ towards the outer layers. Texture was also detected in 102 lattice planes. The texture effect in 002 planes at the scale of less than 1 μm was also demonstrated in dentin. The variation of lattice parameters as a function of the position within the thickness of dentin and enamel was also observed. The values of the nonuniform microstrain in the dentin and enamel crystallites were from 1.40 × 10(-6) % to 4.44 × 10(-5) %. The good correlation between XRD and TEM indicated that μXRD is a useful technique to study crystallography and microstructure of heterogeneous enamel and dentin. The observed gradient characteristics of texture and crystallite size in enamel and dentin maybe an evolutionary outcome to resist wear and fracture, thereby contributing to the excellent mechanical properties of teeth.

Structural motifs and elastic properties of hierarchical biological tissues - a review

Recent progress made in the field of hierarchical biological materials is reviewed with an emphasis on the staggering characteristics at the smaller structural scale of a number of tissues. We show by means of selected examples that the small-scale architecture, and particularly the degree of staggering and overlap, plays a critical role in the macroscopic elastic behavior of those tissues.

Mechanical Properties of Enamel Nanocomposite

For adult Indian premolar teeth, we report for the first time ever the simultaneous evaluations of nanohardness, Young's modulus, and fracture toughness of the enamel nanocomposite. The nanohardness and Young's moduli were evaluated from near the beginning of the middle enamel region to within ~10 μm of the dentino-enamel junction (DEJ) and in the dentin region using the nanoindentation technique. The fracture toughness from near the middle of the enamel region to near the DEJ zone was measured using the microindentation technique. The deformation was studied using scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM). The relative differences in the extents of biomineralization in the enamel and dentin regions were studied by the energy dispersive X-ray (EDS) technique. The variations of the toughness of the enamel as a function of the toughness of the protein matrix phase have been analyzed which showed that the predicted value of the toughness of the protein present in the nanocomposite was comparable to that of other bioproteins reported in the literature. Further, the work of fracture estimated from the measured value of toughness of the enamel nanocomposite agreed well with the experimental data reported in the literature.

Role of crystal arrangement on the mechanical performance of enamel

The superior mechanical properties of enamel, such as excellent penetration and crack resistance, are believed to be related to the unique microscopic structure. In this study, the effects of hydroxyapatite (HAP) crystallite orientation on the mechanical behavior of enamel have been investigated through a series of multiscale numerical simulations. A micromechanical model, which considers the HAP crystal arrangement in enamel prisms, the hierarchical structure of HAP crystals and the inelastic mechanical behavior of protein, has been developed. Numerical simulations revealed that, under compressive loading, plastic deformation progression took place in enamel prisms, which is responsible for the experimentally observed post-yield strain hardening. By comparing the mechanical responses for the uniform and non-uniform arrangement of HAP crystals within enamel prisms, it was found that the stiffness for the two cases was identical, while much greater energy dissipation was observed in the enamel with the non-uniform arrangement. Based on these results, we propose an important mechanism whereby the non-uniform arrangement of crystals in enamel rods enhances energy dissipation while maintaining sufficient stiffness to promote fracture toughness, mitigation of fracture and resistance to penetration deformation. Further simulations indicated that the non-uniform arrangement of the HAP crystals is a key factor responsible for the unique mechanical behavior of enamel, while the change in the nanostructure of nanocomposites could dictate the Young's modulus and yield strength of the biocomposite.

Stress-strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

The role of property gradients on the mechanical behavior of human enamel

In this study, the mechanical design principles of human enamel were evaluated using a hybrid experimental and computational approach. Nanoindentation was applied to evaluate the load-depth response of human enamel, and Vickers indentations were used to assess the damage behavior. An elastic-plastic numerical model was then developed to analyze the stress and strain distribution about the indentations, and to characterize energy dissipation about indents in three locations including inner, middle and outer enamel. Results confirm that enamel exhibits a gradient in its mechanical behavior. Outer enamel has a limited potential for energy dissipation by inelastic deformation, indicating that the ability of outer enamel to resist fracture is low. While inner enamel, the region close Dentin Enamel Junction (DEJ), possesses less resistance to penetration deformation, it has a much higher capacity to dissipate energy by inelastic deformation than outer enamel. The computational simulations identified that the gradients in mechanical properties of human enamel promote resistance to penetration, energy dissipation and mitigation of fracture, all critical performance requirements of human teeth.

Ceramic-like wear behaviour of human dental enamel

This paper reports a transmission electron microscopy (TEM) analysis of subsurfaces of enamel specimens following in vitro reciprocating wear tests with an enamel cusp sliding on a flat enamel specimen under hydrated conditions. The obtained results show that crack formation occurred in the wear scar subsurface. The path followed by these cracks seems to be dictated either by the histological structure of enamel or by the contact stress field. Moreover, the analysis of a set of enamel wear results obtained from the literature and application of fracture-based models, originally developed for ceramics, correlate well, confirming the similar wear processes taking place in these materials. This analysis also reveals a marked influence of coefficient of friction on the enamel wear rate: for a higher coefficient of friction value, enamel wear can be severe even under forces generated during normal operation of teeth.

Perturbed amelogenin secondary structure leads to uncontrolled aggregation in amelogenesis imperfecta mutant proteins

Mutations in amelogenin sequence result in defective enamel, and the diverse group of genetically altered conditions is collectively known as amelogenesis imperfecta (AI). Despite numerous studies, the detailed molecular mechanism of defective enamel formation is still unknown. In this study, we have examined the biophysical properties of a recombinant murine amelogenin (rM180) and two point mutations identified from human DNA sequences in two cases of AI (T21I and P41T). At pH 5.8 and 25 °C, wild type (WT) rM180 and mutant P41T existed as monomers, and mutant T21I formed lower order oligomers. CD, dynamic light scattering, and fluorescence studies indicated that rM180 and P41T can be classified as a premolten globule-like subclass protein at 25 °C. Thermal denaturation and refolding monitored by CD ellipticity at 224 nm indicated the presence of a strong hysteresis in mutants compared with WT. Variable temperature tryptophan fluorescence and dynamic light scattering studies showed that WT transformed to a partially folded conformation upon heating and remained stable. The partially folded conformation formed by P41T, however, readily converted into a heterogeneous population of aggregates. T21I existed in an oligomeric state at room temperature and, upon heating, rapidly formed large aggregates over a very narrow temperature range. Thermal denaturation and refolding studies indicated that the mutants are less stable and exhibit poor refolding ability compared with WT rM180. Our results suggest that alterations in self-assembly of amelogenin are a consequence of destabilization of the intrinsic disorder. Therefore, we propose that, like a number of other human diseases, AI appears to be due to the destabilization of the secondary structure as a result of amelogenin mutations.

Role of prism decussation on fatigue crack growth and fracture of human enamel

The role of prism decussation on the crack growth resistance of human enamel is evaluated. Miniature inset compact tension (CT) specimens embodying a section of cuspal enamel were subjected to Mode I cyclic or monotonic loads. Cracks were grown in either the forward (from outer enamel inwards) or reverse (from inner enamel outwards) direction and the responses were compared quantitatively. Results showed that the outer enamel exhibits lower resistance to the inception and growth of cracks. Regardless of the growth direction, the near-threshold region of cyclic extension was typical of "short crack" behavior (i.e. deceleration of growth with an increase in crack length). Cyclic crack growth was more stable in the forward direction and occurred over twice the spatial distance achieved in the reverse direction. In response to the monotonic loads, a rising R-curve response was exhibited by growth in the forward direction only. The total energy absorbed in fracture for the forward direction was more than three times that in the reverse. The rise in crack growth resistance was largely attributed to a combination of mechanisms that included crack bridging, crack bifurcation and crack curving, which were induced by decussation in the inner enamel. An analysis of the responses distinguished that the microstructure of enamel appears optimized for resisting crack growth initiating from damage at the tooth's surface.

Nanoscratch resistance of human tooth enamel treated by Nd: YAG laser irradiation

The purpose of this study was to investigate the friction behaviour of human tooth enamel irradiated by Nd:YAG laser. Forty specimens were randomly divided into four groups, and received the Nd:YAG laser irradiation with different energy densities (without irradiation; 20, 40, and 80 J/cm2). Subsequently, nano-scratch tests were conducted on the treated enamel surfaces, with progressive loads from 0.1 to 80 mN. The morphologies of the treated enamel surfaces and the scratch scars were observed via a field emission scanning electron microscope and an AMBIOS XP-2 stylus profilometer. The results indicated that the Nd:YAG laser promoted slight morphological alterations under a lower energy density (20 J/cm2), but different morphologies occurred under a higher energy density (40 and 80 J/cm2). The irradiated enamel surfaces had inferior nano-scratch resistance, and the nano-scratch resistance decreased as the energy density increased. Nd:YAG laser irradiation decreased the nano-scratch resistance of the enamel. The main damage mechanisms of the irradiated enamel under nano-scratch consisted of brittle delaminations and formation of cracks.

Structural Integrity of Enamel: Experimental and Modeling

Tooth enamel is the hardest tissue in the human body and is directly responsible for dental function. Due to its non-regenerative nature, enamel is unable to heal and repair itself biologically after damage. We hypothesized that with its unique microstructure, enamel possesses excellent resistance to contact-induced damage, regardless of loading direction. By combining instrumented indentation tests with microstructural analysis, we report that enamel can absorb indentation energy through shear deformation within its protein layers between apatite crystallites. Moreover, a near-isotropic inelastic response was observed when we analyzed indentation data in directions either perpendicular or parallel to the path of enamel prisms. An “effective” crystal orientation angle, 33°–34°, was derived for enamel microstructure, independent of the loading direction. These findings will help guide the design of the nanostructural architecture of dental restorative materials.

Mineralization potential of polarized dental enamel

Background

Management of human teeth has moved from a surgical to a more conservative approach of inhibiting or preventing lesion progression. Increasing enamel mineralization is crucial in this regard. A potential difficulty is the preferential mineralization of the outermost portion of the enamel that can prevent overall mineralization. We describe a strategy for increasing the mineralization potential of dental enamel.

Methodology/Principal Findings

Extracted human premolar teeth enamel (n = 5) were exposed to a high concentration of hydrogen peroxide with an energizing source. Samples were stored in artificial saliva at 37°C for 1 wk. A desktop X-ray micro-CT system was used to evaluate the mineral density of samples. Mineral distribution was polarized between the lower and the higher mineralized portion of enamel by charged oxygen free radicals due to activation of permeated hydrogen peroxide. The kinetics of energy absorption in the deeper enamel region demonstrated improvement of preferential mineralization into the region without restricting overall mineralization of the enamel. Subsequent increasing mineralization, even in the dense mineralized outer portion of enamel, was also achieved.

Conclusions/Significance

This increased mineralization may promote resistance to acidic deterioration of the structure. The present study is one of the primary steps towards the development of novel application in reparative and restorative dentistry.

On the R-curve behavior of human tooth enamel

In this study the crack growth resistance behavior and fracture toughness of human tooth enamel were quantified using incremental crack growth measures and conventional fracture mechanics. Results showed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve) with crack extension from the outer to the inner enamel, and that the rise in toughness is a function of distance from the dentin enamel junction (DEJ). The outer enamel exhibited the lowest apparent toughness (0.67+/-0.12 MPam(0.5)), and the inner enamel exhibited a rise in the growth toughness from 1.13 MPam(0.5)/mm to 3.93 MPam(0.5)/mm. The maximum crack growth resistance at fracture (i.e. fracture toughness (K(c))) ranged from 1.79 to 2.37 MPam(0.5). Crack growth in the inner enamel was accompanied by a host of mechanisms operating from the micro- to the nano-scale. Decussation in the inner enamel promoted crack deflection and twist, resulting in a reduction of the local stress intensity at the crack tip. In addition, extrinsic mechanisms such as bridging by unbroken ligaments of the tissue and the organic matrix promoted crack closure. Microcracking due to loosening of prisms was also identified as an active source of energy dissipation. In summary, the unique microstructure of enamel in the decussated region promotes crack growth toughness that is approximately three times that of dentin and over ten times that of bone.

Transmission electron microscope characterisation of molar-incisor-hypomineralisation

Molar-incisor-hypomineralisation (MIH), one of the major developmental defects in dental enamel, is presenting challenge to clinicians due, in part, to the limited understanding of microstructural changes in affected teeth. Difficulties in the preparation of site-specific transmission electron microscope (TEM) specimens are partly responsible for this deficit. In this study, a dual-beam field emission scanning electron microscope (FESEM)/focused ion beam (FIB) milling instrument was used to prepare electron transparent specimens of sound and hypomineralised enamel. Microstructural analysis revealed that the hypomineralised areas in enamel were associated with marked changes in microstructure; loosely packed apatite crystals within prisms and wider sheath regions were identified. Microstructural changes appear to occur during enamel maturation and may be responsible for the dramatic reduction in mechanical properties of the affected regions. An enhanced knowledge of the degradation of structural integrity in hypomineralised enamel could shed light on more appropriate management strategies for these developmental defects.