The laser-Raman spectroscopic studies on human enamel and precipitated carbonate-containing apatites

An Exploration of Mineral Density, Elemental and Chemical Composition of Primary Teeth in Relation to Cord-Blood Vitamin D, Using Laboratory Analysis Techniques

Postnatally, severe vitamin D deficiency commonly results in rickets as well as potential defects in tooth mineralization. The effects of milder deficiency on oral health outcomes later in life are still unclear. This study used micro-computed tomography (μCT), energy dispersive X-ray analysis (EDX), and Raman spectroscopy to investigate mineral density, total density, and elemental composition of enamel and dentine in 63 exfoliated primary incisors from participants with known 25-hydroxyvitamin D levels (25-OHD) at birth. No differences in mineralization and chemical composition using μCT and EDX analysis were observed irrespective of 25-OHD status. Subtle structural differences were observed via Raman spectroscopy, with more crystalline enamel observed in those with sufficient 25-OHD at birth. Although subtle, the differences seen suggest further attention should be given to children with known milder levels of vitamin D deficiency in early life. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Exploration of utility of combined optical photothermal infrared and Raman imaging for investigating the chemical composition of microcalcifications in breast cancer

Microcalcifications play an important role in cancer detection. They are evaluated by their radiological and histological characteristics but it is challenging to find a link between their morphology, their composition and the nature of a specific type of breast lesion. Whilst there are some mammographic features that are either typically benign or typically malignant often the appearances are indeterminate. Here, we explore a large range of vibrational spectroscopic and multiphoton imaging techniques in order to gain more information about the composition of the microcalcifications. For the first time, we validated the presence of carbonate ions in the microcalcifications by O-PTIR and Raman spectroscopy at the same time, the same location and the same high resolution (0.5 μm). Furthermore, the use of multiphoton imaging allowed us to create stimulated Raman histology (SRH) images which mimic histological images with all chemical information. In conclusion, we established a protocol for efficiently analysing the microcalcifications by iteratively refining the area of interest.

Enamel Structure Defects in Kdf1 Missense Mutation Knock-in Mice

The Keratinocyte differentiation factor 1 (KDF1) is reported to take part in tooth formation in humans, but the dental phenotype of Kdf1 mutant mice has not been understood. Additionally, the role of the KDF1 gene in dental hard tissue development is rarely known. In this study, we constructed a Kdf1 missense mutation knock-in mouse model through CRISPR/Cas9 gene-editing technology. Enamel samples from wildtypes (WT) and Kdf1 homozygous mutants (HO) were examined using micro-computed tomography (micro-CT), scanning electron microscopy (SEM), an atomic force microscope (AFM) and Raman microspectroscopy. The results showed that a novel Kdf1 missense mutation (c. 908G>C, p.R303P) knock-in mice model was constructed successfully. The enamel of HO mice incisors appeared chalky and defective, exposing the rough interior of the inner enamel and dentin. Micro-CT showed that HO mice had lower volume and mineral density in their tooth enamel. In addition, declined thickness was found in the unerupted enamel layer of incisors in the HO mice. Using SEM and AFM, it was found that enamel prisms in HO mice enamel were abnormally and variously shaped with loose decussating crystal arrangement, meanwhile the enamel rods were partially fused and collapsed, accompanied by large gaps. Furthermore, misshapen nanofibrous apatites were disorderly combined with each other. Raman microspectroscopy revealed a compromised degree of order within the crystals in the enamel after the Kdf1 mutation. To conclude, we identified enamel structure defects in the Kdf1 missense mutation knock-in mice, which displayed fragmentary appearance, abnormally shaped prism structure, decreased mineral density, altered crystal ordering degree and chemical composition of the enamel layer. This may support the potential role of the KDF1 gene in the natural development of enamel.

METHODS FOR DIAGNOSING DENTAL CARIES LESIONS

Tooth decay (dental caries) commonly occurs throughout the world and is one of the most widespread infectious diseases of lifestyle, globally affecting all age groups; up to 90% schoolchildren and almost 100% adults in both developing and developed countries. When left untreated, it can lead to disease outbreaks resulting in advere-health and life-threatening conditions such as endocarditis or sepsis. Undoubtedly, basic measures are thus required in both dental and GP practice to ensure that dental caries are detected early. This article presents the various diagnostic methods used to identify these disease outbreaks.

Raman spectroscopy for early detection and monitoring of dentin demineralization

Early detection of dental caries and variations in composition/structure of both enamel and dentin represents an important issue in modern dentistry. Demineralization has been associated to teeth discoloration, development of caries, and formation of cavities.

OBJECTIVE

In this study, we systematically monitored the processes of demineralization/remineralization in dentin samples by means of three different spectroscopic techniques, namely, Raman spectroscopy, X-Ray Photo-electron spectroscopy (XPS), and X-Ray Diffractometry (XRD).

METHODS

Bovine dentin samples were first exposed to acidic solutions and their structure systematically monitored as a function of time and pH. Then, the samples were rinsed in artificial saliva to simulate remineralization.

RESULTS

The above three spectroscopic techniques provided quantitative structural information spanning from the nanometer to the millimeter scale of sample penetration depth. An irreversible level of demineralization was reached when dentin was exposed to pH 2 beyond a time threshold of 6h, successive treatments with artificial saliva being unable to restore the mineral fraction. On the other hand, short-term treatments at pH 5 and long-term treatments at pH 6 could partially or completely recover the dentin structure within one week of remineralization treatment.

SIGNIFICANCE

Two specific Raman parameters, namely, the bandwidth of the symmetric phosphate-stretching signal and the mineral-to-matrix intensity ratio, showed strong correlations with XPS and XRD data, and matched laser microscopy observations. Such correlations open the path to apply Raman spectroscopy in monitoring dentin demineralization in vivo and provide quantitative working algorithms for the prevention of oral caries.

FT-Raman spectroscopic characterization of enamel surfaces irradiated with Nd:YAG and Er:YAG lasers

Background. Despite recent advances in dental caries prevention, caries is common and remains a serious health problem. Laser irradiation is one of the most common methods in preventive measures in recent years. Raman spectroscopy technique is utilized to study the microcrystalline structure of dental enamel. In this study, FT-Raman spectroscopy was used to evaluate chemical changes in enamel structure irradiated with Nd:YAG and Er:YAG lasers.

Methods. We used 15 freshly-extracted, non-carious, human molars that were treated as follows: No treatment was carried out in group A (control group); Group B was irradiated with Er:YAG laser for 10 seconds under air and water spray; and Group C was irradiated with Nd:YAG laser for 10 seconds under air and water spray. After treatment, the samples were analyzed by FT-Raman spectroscopy.

Results. The carbonate content evaluation with regard to the integrated area under the curve (1065/960 cm^–1) exhibited a significant reduction in its ratio in groups B and C. The organic content (2935/960 cm^-1) area exhibited a significant decrease after laser irradiation in group B and C.

Conclusion. The results showed that the mineral and organic matrices of enamel structure were affected by laser irradiation; therefore, it might be a suitable method for caries prevention.

Vibrational algorithms for quantitative crystallographic analyses of hydroxyapatite-based biomaterials: I, theoretical foundations

The Raman spectroscopic method has quantitatively been applied to the analysis of local crystallographic orientation in both single-crystal hydroxyapatite and human teeth. Raman selection rules for all the vibrational modes of the hexagonal structure were expanded into explicit functions of Euler angles in space and six Raman tensor elements (RTE). A theoretical treatment has also been put forward according to the orientation distribution function (ODF) formalism, which allows one to resolve the statistical orientation patterns of the nm-sized hydroxyapatite crystallite comprised in the Raman microprobe. Close-form solutions could be obtained for the Euler angles and their statistical distributions resolved with respect to the direction of the average texture axis. Polarized Raman spectra from single-crystalline hydroxyapatite and textured polycrystalline (teeth enamel) samples were compared, and a validation of the proposed Raman method could be obtained through confirming the agreement between RTE values obtained from different samples.

Assessing Raman spectroscopy as a prescreening tool for the selection of archaeological bone for stable isotopic analysis

Stable isotope analyses for paleodiet investigations require good preservation of bone protein, the collagen, to obtain reliable stable isotope values. Burial environments cause diagenetic alterations to collagen, especially in the leaching of the organic bone content. The survival of bone protein may be assessed by the weight % collagen, % carbon and % nitrogen yields, but these values are achieved only after destructive chemical processing. A non-destructive method of determining whether bone is suitably preserved would be desirable, as it would be less costly than chemical processing, and would also preserve skeletal collections. Raman analysis is one such potential non-destructive screening method. In previous applications, Raman spectroscopy has been used to test both the alteration of the mineral portion of bone, as well as to indicate the relative amount of organic material within the bone structure. However, there has been no research to test the relationship between the Raman spectroscopic results and the survival of bone protein. We use a set of 41 bone samples from the prehistoric archaeological site of Ban Non Wat, Northeast Thailand, to assess if Raman spectroscopy analysis of the organic-phosphate ratio has a significant correlation with the weight % collagen, and carbon and nitrogen yields obtained by isotopic analysis. The correlation coefficients are highly statistically significant in all cases (r = 0.716 for collagen, r = 0.630 for carbon and r = 0.706 for nitrogen, p≤0.001 for all) with approximately or close to half of the variation in each explained by variation in the organic-phosphate ratio (51.2% for collagen, 39.6% for carbon, and 49.8% for nitrogen). Although the Raman screening method cannot directly quantify the extent of collagen survival, it could be of use in the selection of bone most likely to have viable protein required for reliable results from stable isotope analysis.

In vitro evaluation of enamel demineralization after several overlapping CO2 laser applications

This study aimed to evaluate the effects of repeated CO2 laser applications on the inhibition of enamel demineralization. Sixty-five human dental enamel slabs were randomly assigned to the following groups (n = 13): control (C), one application of the CO2 laser (L1), two applications of the CO2 laser (L2), three applications of the CO2 laser (L3), and four applications of the CO2 laser (L4). Enamel slabs were irradiated by a 10.6-μm CO2 laser operating at 5 J/cm(2). The slabs were subjected to a pH-cycling regimen and then analyzed by FT-Raman spectroscopy, energy-dispersive X-ray fluorescence spectrometry (EDXRF), cross-sectional micro-hardness, and scanning electron microscopy (SEM). Statistical analysis was performed using ANOVA and Tukey tests (p < 0.05). FT-Raman spectroscopy showed a reduced carbonate content for L1, L3, and L4 groups when compared to C (p < 0.05). The EDXRF data showed no statistical differences between the control and irradiated groups for calcium and phosphorus components (p > 0.05). Cross-sectional micro-hardness data showed a statistically significant difference between the control and all irradiated groups (p < 0.05), but no difference was found among the irradiated groups (p > 0.05) up to 30-μm depth. A tendency of lower demineralization occurred in deeper depths for L3 and L4 groups. The SEM results showed that with repeated applications of the CO2 laser, a progressive melting and recrystallization of the enamel surface occurred. Repeated irradiations of dental enamel may enhance the inhibition of enamel demineralization.

Pathological biominerals: Raman and infrared studies of bioapatite deposits in human heart valves

We studied pathological bioapatite from patients undergoing valvular replacement due to severe aortic and mitral stenosis. Three different types of mineralized human cardiac valves were analyzed. We used infrared and Raman spectroscopy to infer the presence of the carbonate group and evaluate the carbonate substitution in bioapatite structure. The Raman spectra showed that the pathological bioapatite is a B-type "carbonate-apatite" (CO(3)(2-) for PO(4)(3-)) similar to the major mineralized products derived from normal biomineralization processes occurring in the human body. Fourier transform infrared spectra (FT-IR) confirmed the B-type carbonate substitution (CO(3)(2-) for PO(4)(3-)) and showed evidence for the partial replacement of [OH] by [CO(3)] (A-type substitution). The carbonate content of the samples inferred by the spectroscopic measurements is in good agreement with the range of values estimated for biological apatite. On the contrary, the crystal size of the pathological apatite estimated using the percentage area of the component at 1059 cm(-1) of the infrared spectrum is in the nanometer range and it is significantly smaller than the crystal size of normal mineralized tissues.

Chemical composition and structure of peritubular and intertubular human dentine revisited

OBJECTIVE

Currently there is still a debate about whether peritubular dentine (PTD) is non-collageneous or collageneous tissue. The chemical composition and structure of human PTD and intertubular dentine (ITD) was re-visited in this study.

DESIGN

The dentine tubular region including ITD and PTD prepared from human third molars was in situ detected by means of micro-Raman spectroscopy (μRs) and atomic force microscopy (AFM).

RESULTS

From the μRs study, it was found that the mineral/matrix ratios (phosphate vs. CH(2)) in PTD were ∼3 times of those in ITD. For the mineral, the differences between PTD and ITD were small, but still detectable. For the organic matrix, the intensity ratios of amide III to CH(2) in ITD were ∼1.5 times of those in PTD, indicating the structural differences. In addition, there was a higher proline/hydroxyproline content in ITD than that in PTD. However, the overall Raman peak contour in the amide regions (I & III) was similar, indicating collagen might still exist in both the ITD and PTD. An in situ AFM observation of the dentinal tubular region during EDTA etching confirmed that dentine collagen ran across from the ITD into the PTD.

CONCLUSION

A phenomenon similar to that observed in the dentine-enamel junction is proposed to explain the above results. It is demonstrated that the μRs-AFM approach can be used to provide an insight into the structure of small dental tissues at the micron or sub-micron scale.

Carbonate assignment and calibration in the Raman spectrum of apatite

A series of apatites with varying carbonate levels was prepared in order to assign the carbonate bands and calibrate for Raman analysis of natural materials. Overlap of carbonate bands with phosphate peaks was resolved by curve fitting. A peak at 1,071 cm(-1) was assigned to a combination of the carbonate nu(1) mode at 1,070 cm(-1) with a phosphate nu(3) mode at 1,076 cm(-1). In addition, the carbonate nu(4) mode was identified in apatite samples with >4% carbonate. The carbonate nu(4) bands at 715 and 689 cm(-1) identify the samples as B-type carbonated apatite. The carbonate content of apatite was calibrated to a carbonate Raman band, and the method was used to determine the carbonate content of a sample of bovine cortical bone, 7.7 +/- 0.4%.

Laser-induced compositional changes on enamel: A FT-Raman study

Preventive effects of lasers on enamel caries have been documented in the past few decades. However, its specific mechanism remains uncertain.

OBJECTIVES

To evaluate the laser-induced change of enamel compositions, including carbonate and organic matters using Fourier-transform Raman spectroscopy (FT-Raman).

METHODS

Twelve windows (1 mm x 1 mm), created on six defect-free primary incisors, were characterized by FT-Raman microscopy (1024 nm) before and after Er:YAG laser treatment (Fidelis) with 5.1 J/cm(2)-2 Hz-5s. To assess the statistical significance of laser effects, Raman peaks assigned to nu(1) phosphate, type-A/B carbonates, and organic matters were evaluated with the paired-samples t-test.

RESULTS

The standardized intensity of type B carbonate decreased significantly from 0.117 to 0.106 (p=0.029), whereas the standardized intensity of carbonate A remained unchanged (p=0.467). Related to organic matters, the standardized intensity of peaks at 2940 cm(-1) and in the ranges of 1200-1600 cm(-1) decreased significantly, with p=0.005 and p<0.001, respectively. Revealing enamel crystallinity, the bandwidth of nu(1) phosphate on lased surfaces appeared to be unaltered after laser treatment (p=0.477).

CONCLUSIONS

Laser treatment may provide caries-preventive effect on enamel through reduction of carbonate and modification of organic matters.

MEPE is downregulated as dental pulp stem cells differentiate

Previous studies on dental pulp cell culture have described heterogenous mixtures of cells that differentiate into odontoblasts and form mineralized dentin.

OBJECTIVE

The aim of this study was to characterize the matrix extracellular phosphoglycoprotein (MEPE) expression by dental pulp stem cells (DPSC), related to cell differentiation.

DESIGN

DPSC differentiation to form mineralized nodules was characterized by Alizarin red staining and micro-Raman spectroscopy. Osteogenesis SuperArray analysis was used to broadly screen for osteogenesis-related genes altered by DPSC differentiation. Relative levels of expression of MEPE and DSP were determined by semiquantitative RT-PCR and Western blot.

RESULTS

Mineral analysis showed that as DPSC differentiated, they formed a carbonated hydroxyapatite mineral. Differentiation was initially marked by upregulation by Runx2, TGFbeta-related genes, EGFR and genes involved in collagen metabolism. ALP activity first increased, as DPSCs reached confluence but later decreased when cells further differentiated three weeks after confluence. MEPE was the only marker that was downregulated as DPSCs differentiated.

CONCLUSION

DPSC differentiation can be characterized by downregulation of MEPE as other markers of DPSC differentiation, such as DSP, are upregulated. Expression of MEPE related to DSP and can be used to monitor DPSC as they are used for studies of odontoblast differentiation, tissue engineering or vital pulp therapy. The downregulation of MEPE as DPSC differentiate, suggests that MEPE is an inhibitor of mineralization.

Detachment of titanium and fluorohydroxyapatite particles in unloaded endosseous implants

The shape, surface composition and morphology of orthopaedic and endosseous dental titanium implants are key factors to achieve post-surgical and long-term mechanical stability and enhance implant osteointegration. In this study a comparison was made between 12 titanium screws, plasma-spray-coated with titanium powders (TPS), and 12 screws with an additional coating of fluorohydroxyapatite (FHA-Ti). Screws were implanted in the femoral and tibial diaphyses of two mongrel sheep and removed with peri-implant tissues 12 weeks after surgery. The vibrational spectroscopic, ultrastructural and morphological analyses showed good osteointegration for both types of implants in host cortical bone. The portion of the FHA-Ti implants in contact with the medullary canal showed a wider area of newly formed peri-implant bone than that of the TPS implants. Morphological and EDAX analyses demonstrated the presence of small titanium debris in the bone medullary spaces near the TPS surface, presumably due to the friction between the host bone and the implant during insertion. Few traces of titanium were detected around FHA-Ti implants, even if smaller FHA debris were present. The present findings suggest that the FHA coating may act as a barrier against the detachment of titanium debris stored in the medullary spaces near the implant surface.

Raman spectroscopic studies of CO2 laser-irradiated human dental enamel

While the effects of carbon dioxide (CO2) laser radiation on the physical properties of human dental enamel are well characterized, little is known regarding laser-induced chemical changes. In this study, enamel was exposed to CO2 laser radiation to induce fusion and recrystallization, and the Raman spectra recorded using both dispersive and Fourier-transformed (FT) Raman spectroscopy. Spectra were compared to a heart-treated specimen of hydroxyapatite (HAP) and enamel. Laser irradiation induced chemical changes which differed from those induced by heat treatment. Comparing the Raman spectra of lased enamel to HAP and tricalcium phosphate (TCP), it is evident that CO2 laser irradiation of enamel causes the partial conversion of HAP to TCP. The effect of laser irradiation is not merely a simple local heating effect as previously thought, since simple heating of enamel leads to the formation of both TCP and Ca(OH)2, while laser treatment of enamel results in the formation of TCP but not Ca(OH)2.

Raman spectroscopy in dental research: a short review of recent studies

The Raman spectroscopic technique enables us to obtain vibrational (IR and far-IR) spectra of minerals by analyzing scattered light caused by (visible or near-visible) monochromatic laser excitation. The method possesses several advantages over IR absorption, including simple sample preparation, easy spectral/band analysis, and linear-response to mineral/chemical concentrations. In micro-Raman spectrometer systems, samples are positioned under an optical microscope, and specimens can be scanned with a lateral resolution (approximately 1 mm). In this paper, recent applications of micro-Raman spectroscopy and near-infrared Fourier transform Raman spectroscopy in the study of dental hard tissues and of calculus are reviewed. Special attention is given to mineral components in enamel, dentin, and calculus, and to calcium fluoride formed in/on enamel. The results from the use of an Ar(+)-laser/grating-based micro-Raman spectrometer show that: CaF2 formed in/on enamel by APF treatment is detectable and different from pure CaF2; and with the technique, the crystallite orientation in enamel can be determined. A Raman spectrometer based on Fourier transform and a diode-laser-pumped Nd:YAG laser (1.06 mm) can be used to obtain fluorescence-free Raman signals from biological materials, and identification of mineral components present in dental calculus is possible.

Orientational micro-Raman spectroscopy on hydroxyapatite single crystals and human enamel crystallites

Single crystals of synthetic hydroxyapatite have been examined by orientational micro-Raman spectroscopy. The observed Raman bands include the PO4(3-)/OH-internal and external modes over the spectral range from 180 to 3600 cm-1. The Raman-active symmetry tensors (A, E1, and E2) of crystal-class C6 have been assigned to these bands. Intensities of the Raman bands are invariant to the orientation of the a- or b-axis of a single crystal, but depend solely on the c-axis orientation. Changes in the spectral profile thus reflect changes in the c-axis orientation, which, with respect to the known excitation/observation coordinates, can be determined through the angular dependencies of Raman bands due to A and E2 symmetries. The useful bands are those of the PO4(3-)v4 vibrational mode, which contains relatively well-defined bands due to E2 (581 cm-1) and A (593 cm-1) symmetries. A preliminary Raman study on crystallites near the intact outer surface of human enamel has also been carried out. Strong orientational dependencies similar to those of hydroxyapatite single crystals have been observed among the bands in enamel spectra. This suggests that the crystallites in the region investigated are highly oriented. The results were consistent with an averaged orientation of the crystallite c-axis being perpendicular to the outer surface.

Raman spectra of human dental calculus

Raman spectra of human dental calculus have been observed for the first time by use of micro-Raman spectroscopy. The spectral features of calculus were influenced easily by heating caused by laser irradiation. Therefore, the measurements were carried out at relatively low power (5 mW, 1-micron spot size). The spectra could be characterized as phosphate vibrational bands due to the v1, v2, v3, and v4 modes. The overall spectral features did not resemble those of pure minerals such as brushite, octacalcium phosphate, and hydroxyapatite. There were spectral differences among mixed calculus particles obtained from 18 adults, probably due to variations in local mineral composition and differences among patients. However, the averaged spectral features did not vary significantly with formation period from 1 to 6 months. Freshly removed and stored (5-11 months) calculus also gave comparable Raman spectra. Measurements on a fractured sample indicated that Raman spectra at saliva and dentin interfaces are nearly identical, and major mineral constituents may not vary significantly along the growth axis of calculus.

Vibrational analysis by Raman spectroscopy of the interface between dental adhesive resin and dentin

The Raman microprobe technique was applied for analysis of the molecular components at the adhesive interface between 4-META/MMA-TBB resin and dentin. The Raman spectra showed that the 4-META molecules in monomer solution were mostly hydrolyzed into 4-MET molecules, which were then co-polymerized with MMA molecules to form resin and resin-reinforced dentin layers. On the basis of line analysis by the Raman microprobe, resin molecules were estimated to penetrate 6 microns into the dentin from the interface. Raman intensity studies indicated that the concentration of 4-MET molecular units in the resin-reinforced dentin was more than four times the concentration in the original monomer solution. This demonstrated the excellent infiltration ability of 4-MET monomer into dentin substrate in situ.

Mechanism of bone destruction due to middle ear cholesteatoma as revealed by laser-Raman spectrometry

In an investigation of the mechanism of bone destruction caused by chronic otitis media complicated with cholesteatoma, both the processes of demineralization and remineralization were studied in an animal model and clinically at the molecular level, using a laser-Raman spectrometer. From this investigation, it is proposed that the mechanism of bone destruction associated with cholesteatoma is a form of demineralization.

Micro-Raman line broadening in synthetic carbonated hydroxyapatite

Using a Raman microspectrometer, we have recorded Raman spectra of synthetically produced hydroxyapatite samples with varying carbonate contents. The apatites were produced from aqueous solutions at about 40 degrees C. From line-broadening values of the symmetric phosphate stretch, it was concluded that the carbonate substitution in these lattices has a maximum of 4.5 wt %. Both phosphate and hydroxyl ions seem to be involved in the carbonate substitution process.

Relationships between carbonate, sex and other macroelements in human whole enamel

Enamel samples were prepared from 40 teeth and CO3, P, F, Ca, Mg and Zn concentrations were determined. The carbonate concentration was higher in women than in men (P = 0.018), otherwise no sex dependence was found. Positive correlations were found between CO3 and Mg (P = 0.001). These relations should be taken into consideration when the solubility of enamel is studied.