Ultrastructural observations and growth of occluding crystals in carious dentine

Ultrastructural Analysis of Er:YAG Lased Bovine Dentin Contaminated by Cariogenic Bacteria

Objective: The purpose of this study is to investigate the crystal structure of bacteria-contaminated bovine dentin after Er:YAG laser irradiation at various energy densities from macroscale, microscale, and nanoscale. Background: Er:YAG laser can change the morphology and chemical components of dentin. Few preliminary researchers investigate the laser effect on crystal in dentin tissue. Methods: Twenty dentin specimens from bovine incisors were cocultured with S. mutans (UA 159) and divided into four groups with diverse Er:YAG laser irradiation energy (0, 6.37, 12.73, 19.11 J/cm²). The ultrastructure of dentin before and after laser irradiation was investigated with nanoanalytical electron microscopy. X-ray diffraction provided the information of lattice parameters in dentin. The morphology of dentin was observed by scanning electron microscopy. High-resolution transmission electron microscope images and selected-area electron diffraction patterns were obtained for characterizing crystal domain size, structure, and microenvironment of dentin. Results: The combination of these methods disclosed that there exist mineralized, demineralized, and remineralized dentin in the bacteria-invaded dentin and can be feasibly recognized using morphological features. Laser treatments influence hydroxyapatite (HAp) crystals in dentin tissue in different ways: needle HAp in mineralized dentin tissue keeps intact with laser irradiation of no higher than 19.11 J/cm²; laser irradiation improves the crystallinity of lamella HAp by domain growth and rearranges its growth orientations. Conclusions: We report an unprecedented presence of remineralization zone consisting of lamella HAp crystals with distinct high-index planes. These findings have broad implications on the role of laser operation in driving biomineralization and shed new insights into a possible relationship between laser irradiation and remineralization.

Magnesium whitlockite - omnipresent in pathological mineralisation of soft tissues but not a significant inorganic constituent of bone

Whitlockite is a calcium phosphate that was first identified in minerals collected from the Palermo Quarry, New Hampshire. The terms magnesium whitlockite [Mg-whitlockite; Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂] and beta-tricalcium phosphate [β-TCP; β-Ca₃(PO₄)₂] are often used interchangeably since Mg-whitlockite is not easily distinguished from β-Ca₃(PO₄)₂ by powder X-ray diffraction although their crystalline structures differ significantly. Being both osteoconductive and bioresorbable, Mg-whitlockite is pursued as a synthetic bone graft substitute. In recent years, advances in development of synthetic Mg-whitlockite have been accompanied by claims that Mg-whitlockite is the second most abundant inorganic constituent of bone, occupying as much as 20-35 wt% of the inorganic fraction. To find evidence in support of this notion, this review presents an exhaustive summary of Mg-whitlockite identification in biological tissues. Mg-whitlockite is mainly found in association with pathological mineralisation of various soft tissues and dental calculus, and occasionally with enamel and dentine. With the exception of high-temperature treated tumoural calcified deposits around interphalangeal and metacarpal joints and rhomboidal Mg-whitlockite crystals in post-apoptotic osteocyte lacunae in human alveolar bone, this unusual mineral has never been detected in the extracellular matrix of mammalian bone. Characterisation techniques capable of unequivocally distinguishing between different calcium phosphate phases, such as high-resolution imaging, crystallography, and/or spectroscopy have exclusively identified bone mineral as poorly crystalline, ion-substituted, carbonated apatite. The idea that Mg-whitlockite is a significant constituent of bone mineral remains unsubstantiated. Contrary to claims that such biomaterials represent a bioinspired/biomimetic approach to bone repair, Mg-whitlockite remains, exclusively, a pathological biomineral. STATEMENT OF SIGNIFICANCE: Magnesium whitlockite (Mg-whitlockite) is a unique calcium phosphate that typically features in pathological calcification of soft tissues; however, an alarming trend emerging in the synthetic bioceramics community claims that Mg-whitlockite occupies 20-35 wt% of bone mineral and therefore synthetic Mg-whitlockite represents a biomimetic approach towards bone regeneration. By providing an overview of Mg-whitlockite detection in biological tissues and scrutinising a diverse cross-section of literature relevant to bone composition analysis, this review concludes that Mg-whitlockite is exclusively a pathological biomineral, and having never been reported in bone extracellular matrix, Mg-whitlockite does not constitute a biomimetic strategy for bone repair.

Three-dimensional observation and analysis of remineralization in dentinal caries lesions

The remineralization mechanism in dental caries lesions is not completely understood. This study reports on ultrastructural and chemical changes observed within arrested caries lesions. Carious human teeth were observed using scanning electron microscopy (SEM) and focused-ion-beam (FIB)-SEM. The crystals detected in the caries lesions were characterized by transmission electron microscopy (TEM), along with chemical element mapping using energy-dispersive spectroscopy (EDS)-STEM. FIB-SEM 3D reconstructions revealed a severely damaged dentin surface abundantly covered by bacteria. Although the dentin tubules were clogged up to a depth of 100 μm, bacterial invasion into dentin tubules was not observed. TEM crystal analysis and EDS-STEM revealed the presence of Ca and P, as well as of Mg within the HAp crystals deposited inside the dentin tubules. It was concluded that extensive remineralization with deposition of Mg-HAp crystals had occurred in dentin tubules of caries-arrested dentin. Understanding the natural remineralization process is thought to be helpful for developing clinical biomimetic remineralization protocols.

Multiscale micromechanical modeling of the elastic properties of dentin

The paper focuses on multiscale modeling of the elastic properties of dentin. It is modeled as a hierarchical structure consisting of collagen fibers and hydroxyapatite particles at the lower level. Different concentrations of hydroxyapatite in this tissue correspond to peritubular and intertubular dentins. Then, the overall material is modeled as intertubular dentin matrix containing parallel cylindrical holes (the tubules) surrounded by layers of peritubular dentin. At each microstructural level, the model accounts for anisotropy of the constituents. The model predictions are compared with experimental data available in literature.

Oral Function Improves Interfacial Integrity and Sealing Ability Between Conventional Glass Ionomer Cements and Dentin

The aim of this study was to investigate if load cycling affects interfacial integrity of glass ionomer cements bonded to sound- or caries-affected dentin. A conventional glass ionomer, Ketac Bond, and a resin-modified glass ionomer (Vitrebond Plus), were applied to dentin. Half of the specimens were load cycled. The interfaces were submitted to dye-assisted confocal microscopy evaluation. The unloaded specimens of sound and carious dentin were deficiently hybridized when Ketac Bond was used. Ketac Bond samples showed an absorption layer and an adhesive layer that were scarcely affected by fluorescein penetration (nanoleakage), in sound dentin. Nevertheless, a higher degree of micropermeability was found in carious dentin. In Ketac Bond specimens, load cycling improves the sealing capability and remineralization at the cement-dentin interface as porosity and nanoleakage was reduced. In contrast, samples treated with Vitrebond Plus exhibited a Rhodamine B-labeled absorption layer with scarce nanoleakage in both sound and carious unloaded dentin. The adhesive layer was affected by dye sorption throughout the porous cement-dentin interface. Samples treated with Vitrebond Plus had significant increases in nanoleakage and cement-dye sorption after load cycling. Within the limitations of an in vitro study, it is expected that conventional glass ionomers will provide major clinical efficacy when applied to carious-affected or sound dentin.

The Difference of Structural State and Deformation Behavior between Teenage and Mature Human Dentin

Objective. The cause of considerable elasticity and plasticity of human dentin is discussed in the relationship with its microstructure. Methods. Structural state of teenage and mature human dentin is examined by using XRD and TEM techniques, and their deformation behavior under compression is studied as well. Result. XRD study has shown that crystallographic type of calcium hydroxyapatite in human dentin (calcium hydrogen phosphate hydroxide Ca9HPO4(PO4)5OH; Space Group P63/m (176); a = 9,441 A; c = 6,881 A; c/a = 0,729; Crystallite (Scherrer) 200 A) is the same for these age groups. In both cases, dentin matrix is X-ray amorphous. According to TEM examination, there are amorphous and ultrafine grain phases in teenage and mature dentin. Mature dentin is stronger on about 20% than teenage dentin, while teenage dentin is more elastic on about 20% but is less plastic on about 15% than mature dentin. Conclusion. The amorphous phase is dominant in teenage dentin, whereas the ultrafine grain phase becomes dominant in mature dentin. Mechanical properties of human dentin under compression depend on its structural state, too.

Comparative Physicochemical Analysis of Pulp Stone and Dentin

INTRODUCTION

Odontoblasts are responsible for the synthesis of dentin throughout the life of the tooth. Tooth pulp tissue may undergo a pathologic process of mineralization, resulting in formation of pulp stones. Although the prevalence of pulp stones in dental caries is significant, their development and histopathology are poorly understood, and their precise composition has never been established. The aim of the present study was to investigate the physicochemical properties of the mineralized tissues of teeth to elucidate the pathologic origin of pulp stones.

METHODS

Areas of carious and healthy dentin of 8 decayed teeth intended for extraction were analyzed and compared. In addition, 6 pulp stones were recovered from 5 teeth requiring root canal treatment. The samples were embedded in resin, sectioned, and observed by scanning electron microscopy and energy-dispersive spectroscopy. X-ray diffraction was performed to identify phases and crystallinity. X-ray fluorescence provided information on the elemental composition of the samples.

RESULTS

Pulp stones showed heterogeneous structure and chemical composition. X-ray diffraction revealed partially carbonated apatite. X-ray fluorescence identified P, Ca, Cu, Zn, and Sr within dentin and pulp stones. Zn and Cu concentrations were higher in pulp stones and carious dentin compared with healthy dentin.

CONCLUSIONS

Pulpal cells produce unstructured apatitic mineralizations containing abnormally high Zn and Cu levels.

Submicron-to-nanoscale structure characterization and organization of crystals in dentin bioapatites

The aim of this study was to ascertain the crystal morphology and to assess the ultrastructure and texture changes of sound (SD) and caries-affected dentin (CAD) after being restored with Zn-free and Zn-containing amalgam.

Biomimetic remineralization of dentin

OBJECTIVES

Remineralization of demineralized dentin is important for improving dentin bonding stability and controlling primary and secondary caries. Nevertheless, conventional dentin remineralization strategy is not suitable for remineralizing completely demineralized dentin within hybrid layers created by etch-and-rinse and moderately aggressive self-etch adhesive systems, or the superficial part of a caries-affected dentin lesion left behind after minimally invasive caries removal. Biomimetic remineralization represents a different approach to this problem by attempting to backfill the demineralized dentin collagen with liquid-like amorphous calcium phosphate nanoprecursor particles that are stabilized by biomimetic analogs of noncollagenous proteins.

METHODS

This paper reviewed the changing concepts in calcium phosphate mineralization of fibrillar collagen, including the recently discovered, non-classical particle-based crystallization concept, formation of polymer-induced liquid-precursors (PILP), experimental collagen models for mineralization, and the need for using phosphate-containing biomimetic analogs for biomimetic mineralization of collagen. Published work on the remineralization of resin-dentin bonds and artificial caries-like lesions by various research groups was then reviewed. Finally, the problems and progress associated with the translation of a scientifically sound concept into a clinically applicable approach are discussed.

RESULTS AND SIGNIFICANCE

The particle-based biomimetic remineralization strategy based on the PILP process demonstrates great potential in remineralizing faulty hybrid layers or caries-like dentin. Based on this concept, research in the development of more clinically feasible dentin remineralization strategy, such as incorporating poly(anionic) acid-stabilized amorphous calcium phosphate nanoprecursor-containing mesoporous silica nanofillers in dentin adhesives, may provide a promising strategy for increasing of the durability of resin-dentin bonding and remineralizing caries-affected dentin.

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.

Remineralization of artificial dentinal caries lesions by biomimetically modified mineral trioxide aggregate

Fluoride-releasing restorative materials are available for remineralization of enamel and root caries. However, remineralization of dentin is more difficult than remineralization of enamel due to the paucity of apatite seed crystallites along the lesion surface for heterogeneous crystal growth. Extracellular matrix proteins play critical roles in controlling apatite nucleation/growth in collagenous tissues. This study examined the remineralization efficacy of mineral trioxide aggregate (MTA) in phosphate-containing simulated body fluid (SBF) by incorporating polyacrylic acid and sodium tripolyphosphate as biomimetic analogs of matrix proteins for remineralizing caries-like dentin. Artificial caries-like dentin lesions incubated in SBF were remineralized over a 6 week period using MTA alone or MTA containing biomimetic analogs in the absence or presence of dentin adhesive application. Lesion depths and integrated mineral loss were monitored with microcomputed tomography. The ultrastructure of baseline and remineralized lesions was examined by transmission electron microscopy. Dentin remineralization was best achieved using MTA containing biomimetic analogs regardless of whether an adhesive was applied; dentinal tubules within the remineralized dentin were occluded by apatite. It is concluded that the version of MTA employed in this study may be doped with biomimetic analogs for remineralization of unbonded and bonded artificial caries-like lesions in the presence of SBF.

Increased matrix metalloproteinase-2 and bone sialoprotein response to human coronal caries

BACKGROUND

It has been suggested that host matrix metalloproteinase-2 (MMP-2) present in dentin may be involved in caries progression, however, its response to caries is not known. Bone sialoprotein (BSP) has been implicated in dentin mineralization and MMP-2 modulation.

OBJECTIVE

To identify and compare the distribution of MMP-2 and BSP in healthy human coronal dentin and those with early caries.

METHODS

Freshly extracted 3rd molars and premolars with and without early caries were fixed, demineralized and subjected to immunohistochemistry using a monoclonal anti-MMP-2 antibody and monoclonal anti-BSP antibody with an avidin-biotin complex method. Immunoreactivity was visualized with 3,3'-diaminobenzidine substrate and observed under light microscopy.

RESULTS

Immunohistochemical analysis revealed that MMP-2 and BSP are not detected in the tubule lumens of healthy dentin. However, intense immunoreactivity for MMP-2 and BSP was detected in association with the full length of the caries-affected dentinal tubules. The MMP-2 and BSP at the dentino-enamel junction appeared unaltered.

CONCLUSION

The results indicate that MMP-2 and BSP may be actively secreted by odontoblasts in response to carious insult. MMP-2 and BSP accumulation in the caries-affected dentinal tubules may indicate their potential involvement in the host defense mechanism which results in calcification of regions affected by the carious process.

Characterization of the chemically deposited hydroxyapatite coating on a titanium substrate

Bioactive hydroxyapatite (HA) coating on titanium (Ti) implant can be used as a drug delivery device. A controlled release of drug around the implant requires the incorporation of drug into the coating material during the coating process. HA coating was prepared using a two-step procedure in conditions suitable for simultaneous incorporation of the protein-based drug into the coating material. Monetite coating was deposited on Ti substrate in acidic condition followed by the transformation of the monetite coating to HA. X-ray diffraction (XRD) confirmed the formation of the monetite phase at the first step of the coating preparation, which was transformed into HA at the second step. Fourier transform infrared spectroscopy demonstrated typical bands of a crystallized carbonated HA with A- and B-type substitution, which was confirmed by the XRD refinement of the structural parameters. Scanning electron microscope was used to observe the morphology of monetite and HA coatings. Adhesion of the coatings was measured using a scratch tester. The critical shearing stress was found to be 84.20 ± 1.27 MPa for the monetite coating, and 44.40 ± 2.39 MPa for the HA coating.

Application of polychromatic µCT for mineral density determination

Accurate assessment of mineral density (MD) provides information critical to the understanding of mineralization processes of calcified tissues, including bones and teeth. High-resolution three-dimensional assessment of the MD of teeth has been demonstrated by relatively inaccessible synchrotron radiation microcomputed tomography (SRµCT). While conventional desktop µCT (CµCT) technology is widely available, polychromatic source and cone-shaped beam geometry confound MD assessment. Recently, considerable attention has been given to optimizing quantitative data from CµCT systems with polychromatic x-ray sources. In this review, we focus on the approaches that minimize inaccuracies arising from beam hardening, in particular, beam filtration during the scan, beam-hardening correction during reconstruction, and mineral density calibration. Filtration along with lowest possible source voltage results in a narrow and near-single-peak spectrum, favoring high contrast and minimal beam-hardening artifacts. More effective beam monochromatization approaches are described. We also examine the significance of beam-hardening correction in determining the accuracy of mineral density estimation. In addition, standards for the calibration of reconstructed grey-scale attenuation values against MD, including K(2)PHO(4) liquid phantom, and polymer-hydroxyapatite (HA) and solid hydroxyapatite (HA) phantoms, are discussed.

Slanted orientations of dentine tubules on remineralized dentine surfaces

Dentine carious lesions can be remineralized under optimal conditions, while the surface characteristics of the caries-attacked area may play an important role in the remineralization process. To understand such a surface mechanism, we examined the microstructures of the remineralized area pretreated with different methods. It was found that dentinal tubules on the remineralized surface orientated differently from intrinsic dentine tubules, with the specific alignment angle determined by different surface treatments. Various surface treatments included in this study were 37% phosphoric acid treatment (the etched group), 37% phosphoric acid etching followed by the application of 10% sodium hypochlorite treatment (the deproteinized group), and untreatment (the control group). These findings are helpful for understanding the non-restorative repair of dentine lesions and the remineralization process of the caries-affected dentin surface.

Changes in stiffness of resin-infiltrated demineralized dentin after remineralization by a bottom-up biomimetic approach

This study examined changes in elastic modulus, mineral density and ultrastructure of resin-infiltrated dentin after biomimetic remineralization. Sixty demineralized dentin beams were infiltrated with Clearfil Tri-S Bond, One-Step or Prime&Bond NT. They were immersed in simulated body fluid (SBF) for 1 week to maximize water sorption before determining the baseline elastic moduli. For each adhesive (N = 20) half of the beams remained immersed in SBF (control). The rest were immersed in a biomimetic remineralization medium. The elastic moduli were measured weekly for 15 additional weeks. Representative remineralized specimens were evaluated by X-ray microtomography and transmission electron microscopy (TEM). The elastic moduli of control resin-infiltrated dentin remained consistently low, while those immersed in the biomimetic remineralization medium increased by 55-118% after 4 months. X-ray microtomography of the remineralized specimens revealed decreases in mineral density from the beam surface to the beam core that were indicative of external mineral aggregation and internal mineral deposition. Interfibrillar and intrafibrillar remineralization of resin-sparse intertubular dentin were seen under TEM, together with remineralized peritubular dentin. Biomimetic remineralization occurs by diffusion of nanoprecursors and biomimetic analogs in completely demineralized resin-infiltrated dentin and proceeds without the contribution of materials released from a mineralized dentin base.