Magnesium incorporation into primary dental enamel and its effect on mechanical properties

On elastoplastic behavior of porous enamel-An indentation and numerical study

The micro/nano pores in natural mineralized tissues can, to a certain extent, affect their responses to mechanical loading but are generally ignored in existing indentation analysis. In this study, we first examined the void volume fraction of sound and caries lesion enamels through micro-computed tomography (micro-CT). A Berkovich indentation study was then carried out to characterize the effect of porous microstructure on the mechanical behavior of the human enamels. The indentation tests were also modeled using the nonlinear finite element analysis technique to simulate indentation load-displacement curves, which showed reasonable agreement with the experimental measurements. From the simulation results, the extent of densification in the plastic zone was identified and the corresponding stress and contact pressure evolutions were quantified. Further, a conventional elastic-perfectly plastic material model without considering micropores was also developed to investigate the compaction effect of the porous structure. The simulation results reveal that conventional elastic perfect-plastic constitutive models become less reliable to model the mechanical behavior of carious lesion enamel with increasing loss of mineral content as it underestimates the yield stress and plastic energy dissipation. This study divulges the importance of compaction of porous enamel structure beneath the indented area. Note that understanding the effect of porous microstructures on plastic behavior is vital as the involved inelastic deformation mechanism associated with irreversible processes, such as wear and localized microcracking, has a significant bearing on wear and fatigue behavior of enamel. STATEMENT OF SIGNIFICANCE: Based on micro-CT and nano-indentation characterization, a numerical model was developed aiming to precisely describe the deformation behavior of naturally porous enamel. Inelastic properties and energy dissipation characteristics of porous enamel were investigated in detail. This work demonstrated that the existence of micro-pores in White Spot Lesions (WSLs) contributes to mechanical stability, which can mitigate the reduction in Young's modulus and fracture toughness resulting from loss of mineral components. The knowledge gained from this study can be used to explain the mechanisms related to irreversible processes, such as contact induced cracking and wear, and strengthen understanding of the mechanical behavior of porous mineralized tissues.

Antibacterial, Antibiofilm, and Tooth Color Preservation Capacity of Magnesium Oxide Nanoparticles Varnish (in vitro Study)

Purpose

Antibacterial and antibiofilm properties of magnesium oxide nanoparticles (MgONPs) mixture assessed against Streptococcus mutans (S. mutans), in addition to examining MgONPs varnish impact on the preservation of the tooth color and inhibition of methylene blue diffusion to the enamel.

Methods

MgONPs mixture was prepared in deionized water (DW), absolute ethanol (E), and rosin with ethanol (RE), named varnish. The antibacterial and antibiofilm capacities of MgONPs mixtures were tested by agar well diffusion, colony-forming unit (CFU), and biofilm inhibition microtiter methods in triplicate and compared to sodium fluoride varnish (NaF) and chlorhexidine mouthwash (ChX). A spectrophotometer was used to record basic tooth color. The artificial demineralization was initiated for 96 h. Then, experimental materials were applied to the corresponding group, and 10-day pH cycles proceeded. Then, the color was recorded in the same ambient environment. The methylene blue diffusion was evaluated by staining the samples for 24 h. After that, the diffusion test was calculated by a digital camera attached to the stereomicroscope.

Results

The agar well diffusion test expressed a significant inhibition zone with all MgONPs mixtures (p = 0.000), and maximum inhibition zone diameter associated with MgONPs-RE. The same finding was observed in the CFU test. Additionally, 2.5%, 5%, and 10% MgONPs-RE varnish showed strong biofilm inhibition capacity (p = 0.039) compared to NaF and ChX groups that inhibit biofilm formation moderately (p = 0.003). The study shows that the 5% MgONPs-RE varnish maintains basic tooth color with minimal methylene blue diffusion compared to NaF varnish (p = 0.00).

Conclusion

Evaluating MgONPs as a mixture revealed antibacterial and antibiofilm capacity against S. mutans with a higher effect of MgONPs-RE varnish. Also, examining the topical effect of MgONPs-RE varnish on the preservation of the tooth color after pH cycle challenges and methylene blue diffusion to enamel confirmed the high performance of MgONPs-RE varnish at 5%.

Complex Evaluation of Nanocomposite-Based Hydroxyapatite for Biomedical Applications

A magnesium-doped hydroxyapatite in chitosan matrix (MgHApC) sample was developed as a potential platform for numerous applications in the pharmaceutical, medical, and food industries. Magnesium-doped hydroxyapatite suspensions in the chitosan matrix were obtained by the coprecipitation technique. The surface shape and morphological features were determined by scanning electron microscopy (SEM). The hydrodynamic diameter of the suspended particles was determined by Dynamic light scattering (DLS) measurements. The stability of MgHApC suspensions was evaluated by ultrasonic measurements. The hydrodynamic diameter of the MgHApC particles in suspension was 29.5 nm. The diameter of MgHApC particles calculated from SEM was 12.5 ± 2 nm. Following the SEM observations, it was seen that the MgHApC particles have a spherical shape. The Fourier-transform infrared spectroscopy (FTIR) studies conducted on MgHApC proved the presence of chitosan and hydroxyapatite in the studied specimens. In vitro antimicrobial assays were performed on Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853, and Candida albicans ATCC 10231 microbial strains. The antimicrobial experiments showed that MgHApC exhibited very good antimicrobial properties against all the tested microorganisms. More than that, the results of the in vitro studies revealed that the antimicrobial properties of the samples depend on the incubation time. The evaluation of the sample's cytotoxicity was performed using the human colon cancer (HCT-8) cell line. Our results suggested the great potential of MgHApC to be used in future applications in the field of biomedical applications (e.g., dentistry, orthopedics, etc.).

Investigation on the physical properties and biocompatibility of zirconia-alumina-silicate@diopside composite materials and its in vivo toxicity study in embryonic zebrafish

Bioceramic materials have a wide range of applications in the biomedical field, such as in the repair of bone defects and dental surgery. Silicate-based bioceramics have attracted biomedical researchers' interest due to their bioactivity and biodegradability. In this study, extended the scope of ZAS utilization in bone tissue engineering by introducing calcium-magnesium-silicate (diopside, CMS) as an interface material aim to develop a machinable bioceramic composite (ZASCMS) by the sol-gel method. The physicochemical characterization, in vitro biological properties and in vivo zebrafish cytotoxicity study of ZAS-based composites as a function of CMS contents, 0, 25, 50, 75 and 100 wt%, were performed. Results showed that the as-prepared ZASCMS possessed porous architecture with well-interconnected pore structure. Results also revealed that the mechanical properties of ZASCMS composite materials were gradually improved with increasing CMS contents. The ZASCMS composites with more than 50 wt% CMS had the highest compressive strength and modulus of 6.78 ± 0.62 MPa and 340.10 ± 16.81 MPa, respectively. Regarding in vitro bioactivities, the composite scaffolds were found to stimulate osteoblast-like UMR-106 cell adhesion, growth, and proliferation. The antibacterial activity of the ZASCMS composite scaffolds was tested against Staphylococcus epidermidis (S. epidermidis) and Escherichia coli (E. coli) also exhibited an antibacterial property. Furthermore, the in vivo studies using embryonic zebrafish were exposed to as-prepared particles (0-500 μg mL-1) and showed that the synthesized ZAS, CMS and ZASCMS composite particles were non-toxic based on the evaluation of survivability, hatching rate and embryonic morphology. In conclusions, our results indicated that the synthesized composite exhibited their biological properties and antibacterial activity, which could well be a promising material with high potential to be applied in orthopaedic and dental tissue engineering.

Bioactive Glasses Containing Strontium or Magnesium Ions to Enhance the Biological Response in Bone Regeneration

The non-surgical treatments are being required to reconstruct damaged tissue, prioritizing our body's natural healing process. Thus, the use of bioactive materials such as bioactive glass has been studied to support the repair and restoration of hard and soft tissue. Thus, in this work Bioglass 45S5 was developed, adding 1 and 2%mol of SrO or MgO and the physical and biological properties were evaluated. The addition of MgO and SrO at the studied concentrations promoted the slight increase in non-bridging oxygens number, observed through the temperature shift in phase transitions to lower values compared to Bioglass 45S5. The insertion of the ions also showed a positive effect on Saos-2 cell viability, decreasing the cytotoxic of Bioglass 45S5. Besides the Ca/P ratio on the pellets surface demonstrating no evidence of higher reactivity between Bioglass 45S5 and Bioglass with Sr and Mg, micrographs show that at 24 h the Ca/P rich layer is denser than in Bioglass 45S5 after the contact with simulated body fluid. The samples with Sr and Mg show a higher antibacterial effect compared to Bioglass 45S5. The addition of the studied ions may benefit the biological response of Bioglass 45S5 in dental applications as scaffolds or coatings.

Enamel Fluoride Uptake Determined Using the Microbiopsy Technique and Time-of-Flight Secondary Ion Mass Spectrometry: A Pilot Study

This study evaluated the suitability of time-of-flight secondary ion mass spectrometry (ToF-SIMS) to assess enamel fluoride uptake (EFU) in comparison with the microbiopsy technique. Enamel specimens were exposed to equimolar solutions of fluoride prepared from sodium fluoride (NaF), stannous fluoride (SnF2), or amine fluoride (AmF). EFU was quantified by both techniques on the same specimens. EFU was found to be highest for samples treated with AmF, followed by SnF2 and NaF. Both methods yielded clearly interpretable, highly correlating (r = 0.95) data. ToF-SIMS can be considered a promising alternative to the microbiopsy technique for near-surface EFU assessment.

Acquisition and evaluation procedure to improve the accuracy of SAED

The achievement of this work is that fine tuning of experimental and evaluation parameters can improve the absolute accuracy and reproducibility of selected area electron diffraction (SAED) to 0.1% without using internal standard. Due to the proposed procedure it was possible to reach a reproducibility better than 0.03% for camera length between sessions by careful control of specimen height and illumination conditions by monitoring lens currents. We applied a calibration specimen composed of nanocrystalline grains free of texture and providing narrow diffraction rings. Refinements of the centre of the diffraction pattern and corrections for elliptic ring distortions allowed for determining the ring diameters with an accuracy of 0.1%. We analyze the effect of different error sources and reason the achieved absolute accuracy of the measurement. Application of the proposed evaluation procedure is inevitable in case of multicomponent nanocomposites or textured materials and/or having close diffraction rings where application of automated procedures is limited. The achieved accuracy of 0.1% without internal standard is approaching that of routine laboratory XRD, and reduction of instrumental broadening due to the elaborated evaluation procedure allows for separation of close reflections, provides more reliable ring width and thus improved input parameters for further nanostructure analysis as demonstrated on dental enamel bioapatite.

Gradient Magnesium Content Affects Nanomechanics via Decreasing the Size and Crystallinity of Nanoparticles of Pseudoosteodentine of the Pacific Cutlassfish, Trichiurus lepturus Teeth

The formation of biomaterials such as enamel, dentin, and bone is important for many organisms, and the mechanical properties of biomaterials are affected by a wide range of structural and chemical factors. Special dentins exist in extant aquatic gnathostomes, and many more are present in fossils. When a layer of compact orthodentine surrounds the porous osteodentine core in the crown, the composite dentin is called pseudoosteodentine. Using various high-resolution analytical techniques, including micro-computed tomography (micro-CT), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Raman spectroscopy, and nanoindentation, we analyzed the micro- and nanostructures, chemical composition, and mechanical properties of pseudoosteodentine in the Pacific cutlassfish, Trichiurus lepturus teeth. Nanoscale oval-shaped hydroxyapatite (HA) crystals were distributed in a disordered manner in the pseudoosteodentine, and a cross-sectional analysis showed that the mineral crystallinity and crystalline particle size of the outer orthodentine were greater than those of middle and inner osteodentine. Moreover, the outer orthodentine comprised a mixture of smaller crystals and larger, more mature crystals. The nano-hardness and nano-stiffness of outer orthodentine were significantly higher than those of middle and inner osteodentine along a radical direction. The hardness and stiffness of pseudoosteodentine were inversely proportional to its magnesium (Mg) content. These data are consistent with the concept that Mg delays crystal maturation. The crystal size, crystallinity, nano-hardness, and nano-stiffness of pseudoosteodentine all decreased commensurately with the increase of its Mg concentration. The pseudoosteodentine of T. lepturus also can be regarded as a functional gradient material (FGM) because its mechanical properties are closely related to its chemical composition and nanostructure. Special pseudoosteodentine may therefore serve as a design standard for biomimetic synthetic mineral composites.

Raman Analyses of Laser Irradiation-Induced Microstructural Variations in Synthetic Hydroxyapatite and Human Teeth

The microstructural and molecular-scale variations induced by laser irradiation treatment on human teeth enamel in comparison with synthetic hydroxyapatite (HAp) were examined through Raman microprobe spectroscopy as a function of irradiation power. The results demonstrated that laser irradiation could modify stoichiometry, microstructure, and the population of crystallographic defects, as well as the hardness of the materials. These modifications showed strong dependences on both laser power and initial nonstoichiometric structure (defective content of HPO4), because of the occurrence of distinct reactions and structural reconstruction. The reported observations can redirect future trends in tooth whitening by laser treatment and the production of HAp coatings because of the important role of stoichiometric defects.

Advanced materials for enamel remineralization

Dental caries, a chronic and irreversible disease caused by caries-causing bacteria, has been listed as one of the three major human diseases to be prevented and treated. Therefore, it is critical to effectively stop the development of enamel caries. Remineralization treatment can control the progression of caries by inhibiting and reversing enamel demineralization at an early stage. In this process, functional materials guide the deposition of minerals on the damaged enamel, and the structure and hardness of the enamel are then restored. These remineralization materials have great potential for clinical application. In this review, advanced materials for enamel remineralization were briefly summarized, furthermore, an outlook on the perspective of remineralization materials were addressed.

The design, construction and application of graphene family composite nanocoating on dental metal surface

Enhancement of the biological and mechanical properties of dental metals is important for accommodation with therapeutic schemes in different stomatological disciplines. Nanocoatings based on graphene family nanomaterials (GFNs) improve the topological structure and physicochemical properties of metal surfaces, endowing them with new properties while maintaining inherent mechanical properties. Nano-composite coatings, composed of GFNs with one or more type of polymer, metal, oxide, and inorganic nonmetallic compound, offer more matching modification schemes to meet multifunctional oral treatment requirements (e.g., anti-bacterial and anti-corrosive activity, osteogenesis and angiogenesis). This review describes recent progress in the development of GFN composite nanocoatings for the modification of dental metals, focus on biological effects in clinical settings. Underlying molecular mechanisms, critical modification schemes, and technical innovation in preparation methods are also discussed. The key parameters of GFN composite nanocoating surface modification are summarized according to effects on cellular responses and antibacterial activity. This review provides a theoretical reference for the optimization of the biological effects and application of GFN composite nanocoatings for dental metals, and the promotion of the environmentally friendly large-scale production of high-quality multifunctional GFN-based nanocoatings in the field of oral science.

Combination of Two Synchrotron Radiation-Based Techniques and Chemometrics to Study an Enhanced Natural Remineralization of Enamel

The limitations to assess dental enamel remineralization have been overcome by a methodology resulting from the appropriate combination of synchrotron radiation-based techniques on both, infrared microspectroscopy and micro X-ray diffraction, with the help of specific data mining. Since amelogenin plays a key role in modulating the mineralization of tooth enamel, we propose a controlled ion release for fluorapatite structural ions (Ca²⁺, PO₄^3–, and F^–, also including Zn²⁺) by using weak acid and weak base ion-exchange resins in the presence of amelogenin to remineralize the surface of etched teeth. This combination provides the necessary ions for enamel remineralization and a guide for crystal growth due to the protein. Remineralized tooth samples were analyzed by applying the indicated methodology. The synchrotron data were treated using principal component analysis and multivariate curve resolution to analyze the mineral layer formed in the presence and absence of amelogenin. The remineralizing treatment created a fluorapatite layer free of carbonate impurities and with a similar orientation to that of the natural enamel thanks to amelogenin contribution.

A Comparative EPR Study of Non-Substituted and Mg-Substituted Hydroxyapatite Behaviour in Model Media and during Accelerated Ageing

To assess the application potential of novel biomaterials, their behaviour in model media and upon sterilization should be investigated, as well as the stability related to their storage conditions. Such data are lacking for Mg-substituted HAP (Mg-HAP). Therefore, the changes in the local structure of non-substituted and Mg-HAP after irradiation and immersion in corrected simulated fluid and saline solution for 28 days were followed by electron paramagnetic resonance (EPR) spectroscopy for the first time. To better understand the stability of radical species induced by sterilization, EPR spectra of samples kept for 2 h at temperatures up to 373 K were recorded to provide an insight into the stability of the sample storage conditions by the accelerated aging method. Samples were characterized by PXRD, FTIR, SEM, EDS, AAS and TGA. Results confirmed that irradiation does not induce changes in the composition or the structure of any of the investigated materials. Fading or the complete disappearance of radical signals in the EPR spectra after immersion in both media was accompanied by the disappearance of other phases formed as a minor byproduct in the synthesis of substituted HAP, as confirmed by PXRD and FTIR analysis. Obtained results confirm the great potential of Mg-HAPs for biomedical applications, although closer attention should be given to the processes related to sample storage stability at different temperatures.

Mechanical properties, microstructure and chemical composition of naked mole rat incisors

The naked mole rat incisors (NMRI) exhibit excellent mechanical properties, which make it a good prototype for design and fabrication of bionic mechanical systems and materials. In this work, we characterised the chemical composition, microstructure and mechanical properties of NMRI, and further compared these properties with the laboratory rat incisors (LRI). We found out that (i) Enamel and dentin are composed of organic matter, inorganic matter and water. The ratio of Ca/P in NMRI enamel is higher than that of LRI enamel. (ii) The dentin has a porous structure. The enamel has a three-dimensional reticular structure, which is more complex, regular and denser than the lamellar structure of LRI enamel. (iii) Enamel has anisotropy. Its longitudinal nano-hardness is greater than that of transverse nano-hardness, and both of them are higher than that of LRI enamel. Their nano-hardness and elastic modulus increase with the increase in distance from enamel-dentin boundary. The nano-hardness of dentin is smaller than that of enamel. The chemical composition and microstructure are considered to be the reasons for the excellent properties of NMRI. The chemical composition and unique microstructure can provide inspiration and guide for the design of bionic machinery and materials.

A Qualitative and Comprehensive Analysis of Caries Susceptibility for Dental Fluorosis Patients

Dental fluorosis (DF) is an endemic disease caused by excessive fluoride exposure during childhood. Previous studies mainly focused on the acid resistance of fluorotic enamel and failed to reach a consensus on the topic of the caries susceptibility of DF patients. In this review, we discuss the role of DF classification in assessing this susceptibility and follow the “four factors theory” in weighing the pros and cons of DF classification in terms of host factor (dental enamel and saliva), food factor, bacteria factor, and DF treatment factor. From our analysis, we find that susceptibility is possibly determined by various factors such as the extent of structural and chemical changes in fluorotic enamel, eating habits, fluoride levels in diets and in the oral cavity, changes in quantity and quality of saliva, and/or oral hygiene. Thus, a universal conclusion regarding caries susceptibility might not exist, instead depending on each individual’s situation.