Some electrochemical characteristics of human tooth enamel.

The fixed charge of enamel and the ratio of the diffusion coefficients of cations to anions of some common electrolytes within enamel were calculated from steady state electromotive forces developed in concentration cells across enamel caps of whole human teeth, sections of human enamel or synthetic hydroxyapatite membranes. The electrolytes were KCl, KF, NaCl, CaCl₂, KH₂PO₄ and K₂HPO₄. The results indicate that enamel behaves like an ion exchange membrane of which the fixed charge depends on the electrolytic environment and the sequence of exposure to various electrolytes.     

EMF measurements across hydroxyapatite membranes

An attempt was made to develop models applicable to the study of ionic transport in dental enamel. Membranes of hydroxyapatite, made by compression of the powdered salt, were mounted on the septum of concentration cells. Electrical potential differences, PD, were measured across the membranes using solutions of CaCl₂, NaCl, KCl, and potassium and sodium phosphates. With CaCl₂, the membrane acquired a net positive charge and the PD values were close to those expected for an ideal permselective membrane. High permselectivity and a net negative charge on the membrane were observed with solutions of Na₂HPO₄ and K₂HPO₄. The PD values obtained with NaCl and KCl solutions were close to those calculated for the junction potentials of the electrolytes; thus, for these uni-univalent salts the membrane behaved as an inert porous barrier. The permselective behaviour was interpreted in terms of sorption of Ca²⁺ and phosphate ions probably related to defects in the hydroxyapatite crystals. The lack of interaction with K⁺, Na⁺, and Cl^? was ascribed to the reduced ionic charge and the relatively large pore sizes of the membrane.When the membranes were mounted on an intraoral device and exposed to the oral environment prior to the PD measurements, the electrochemical behaviour of the membrane changed drastically. The PD values obtained were consistent with the acquisition of a net negative charge on the membrane surface. This negative charge decreased during the initial time (several hours) of measurements but persisted if saliva was added to the electrolyte solutions. A rapid adsorption of salivary proteins is thought to be responsible for the change of the membrane properties after intraoral exposure.It was concluded that any model developed to study ionic transport in dental enamel should include the interactions with the salivary constituents and the presence of the tooth pellicle.     

Raman spectra of human dentin mineral

Human dentin mineral has been investigated by using micro-Raman spectroscopy. Fluorescence and thermal problems were largely avoided by preparing dentin samples by grinding and ultrasonic agitation in acetone. The Raman spectral features were consistent with those of impure hydroxyapatite containing CO, and HPO₄. While spectral differences between enamel and dentin were clearly observable as changes in the bandwidth of the PO₄³ v^?1 band and the intensities of the OH^? CO₃^2- and HPO₄²bands, the technique could not detect spectral differences between coronal and root dentin. NaOCl, NaF and APF–gel treatments caused measurable changes in intensities of the bands due to CO² and HPO₄^?2; the results were found to be useful for band assignments. After NaOCl treatment, the Raman bands, presumably due to amide and HPO₄^?2, were lost, but the band intensity of the CO₃^?2 v, bands increased by 35–60%. This increase coincided with the appearance of a new broad band (250–300 cm^?1). The same treatment on enamel caused no increase in the CO₃² V, band intensity. This NaOCl–induced carbonate could be removed within 20 h in a 1000 ppm NaF solution. These findings indicate that the carbonate ions induced by the NaOCl treatment are presumably in or on the mineral surface. After 3 min of APF–gel treatment on NaOCl–pretreated dentin, the intensities of the hydroxyapatite phosphate bands dropped by approximately 20%, and newly formed CaF₂ and HPO₄ bands became observable.     

Alkaline phosphatase activity in the enamel organs of normal and rachitogenic rats.

A number of characteristics of alkaline phosphatase (AP) in the enamel organ were investigated in normal and D-deficient rats. From the tests of the substrate dependence it appeared that AP in the enamel organ from D-deficient rats had twice the activity compared with AP from normal rats. The apparent K_m value of AP from D-deficient rats was 4.6 × 10^?4M, significantly higher than the value in normal rats (3.3 × 10^?4M), pH optimum was 10.4 in both groups. Results from activation and/or inhibition with Mg²⁺, Ca²⁺, F^?, PO₄^3? and l-phenylalanine showed that the AP from the enamel organ has the same characteristics as AP from various other tissues.     

A mechanism for dental caries based on chemical processes and diffusion phenomena during in-vitro caries simulation on human tooth enamel.

Artificial carious lesions were produced in a range of lactate buffers containing disodium-methane-hydroxy-diphosphonate and the depths of the lesions were measured. The depth of the body of the lesion depended on the calculated concentration of un-ionized lactic acid (HL), and the innermost boundary depended on HL concn. and pH. Apparent diffusion coefficients of the order 10^?10cm²/s were calculated from the boundary movement. A mechanism is proposed for the caries process. Diffusion, predominantly of the un-ionized chemical species HL, H₃PO₄, CaHPO₄, Ca(H₂PO₄)₂ and CaL₂ occurs through the intercrystalline and interprismatic pores of enamel filled with water and protein. Dissolution takes place throughout the lesion by H⁺ and L^? as HL dissociates. The outer enamel surface is partly protected by reversible adsorption of suitable chemical species.     

Vibrational study of adsorbed phosphate ions on rhodium single crystal electrodes

The adsorption of H₂PO₄^? ions was studied on low Miller index rhodium single crystal electrodes by in situ FTIR spectroscopy. It is found that for Rh(1 0 0) and Rh(1 1 0), H₂PO₄^? ions are the major species at low potentials, but at higher potentials, some of the H₂PO₄^? ions undergo a potential induced deprotonation and probably there is a mixture of H₂PO₄^? and HPO₄^2? ions. On Rh(1 1 1) the deprotonation starts at very low potentials and at higher potentials the H₂PO₄^? is fully converted to HPO₄^2?. The behavior of the band center and of the band intensity with the applied potential was also analyzed. It is found that the adsorption increases from 0.08 V vs. a Pd–H₂ electrode up to 0.5 V and then it decreases when the OH starts to be adsorbed.     

Physical chemical effects of zinc on in vitro enamel demineralization

Objectives

Zinc salts are formulated into oral health products as antibacterial agents, yet their interaction with enamel is not clearly understood. The aim was to investigate the effect of zinc concentration [Zn²⁺] on the in vitro demineralization of enamel during exposure to caries-simulating conditions. Furthermore, the possible mechanism of zinc's action for reducing demineralization was determined.

Methods

Enamel blocks and synthetic hydroxyapatite (HAp) were demineralized in a range of zinc-containing acidic solutions (0–3565 ppm [Zn²⁺]) at pH 4.0 and 37 °C. Inductively coupled-plasma optical emission spectroscopy (ICP-OES) was used to measure ion release into solution. Enamel blocks were analysed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and HAp by X-ray diffraction (XRD) and neutron diffraction (ND).

Results

ICP-OES analysis of the acidic solutions showed a decrease in [Ca²⁺] and [PO₄³⁻] release with increasing [Zn²⁺]. FTIR revealed a α-hopeite (α-Zn₃(PO₄)₂.4H₂O)-like phase on the enamel surfaces at >107 ppm [Zn²⁺]. XRD and ND analysis confirmed a zinc-phosphate phase present alongside the HAp.

Conclusions

This study confirms that zinc reduces enamel demineralization. Under the conditions studied, zinc acts predominantly on enamel surfaces at PO₄³⁻ sites in the HAp lattice to possibly form an α-hopeite-like phase.

Clinical significance

These results have a significant implication on the understanding of the fundamental chemistry of zinc in toothpastes and demonstrate its therapeutic potential in preventing tooth mineral loss.     

The role of phosphate and carbonic acid-bicarbonate buffers in the corrosion processes of the oral cavity

The effect of phosphate buffer (NaH₂PO₄, Na₂HPO₄) and carbonic acid-bicarbonate system (NAHCO₃) on the corrosion behaviour of 3 dental amalgams was studied by means of anodic potentiodynamic polarization and atomic absorption spectrophotometry. All the compounds tested exhibited an inhibiting effect on the corrosion process of dental amalgam. The inhibiting effect of phosphate buffer was concentration-dependent. Furthermore, the results of this study suggest that phosphate buffer displays an aftereffect, that is, there was no need for the inhibitor to be constantly supplied.     

Surface free energy of enamel treated with sodium hexametaphosphate,calcium and phosphate

ObjectiveThis study evaluated the capacity of sodium hexametaphosphate (HMP) at different concentrations to alter the surface properties of dental enamel in order to increase calcium and phosphate adsorption.DesignBovine enamel blocks (4 mm × 4 mm, n = 144, 12/group) were divided: 0%; 0.25%; 0.5%; and 1% HMP, followed or not by application of solutions containing Ca or Ca-PO₄, totaling 12 groups. The treatments were performed for 2 min, and the surface free energy (mN/m) was calculated by measuring the contact angles of three probing liquids (deionized water, diiodomethane and ethylene glycol), which was used to determine the polar and nonpolar components of the enamel surface. Calcium (Ca), phosphate (PO₄) and HMP in the solutions treatment solutions were analyzed before and after treatment. The data presented normal and homogeneous distribution and then were subjected to ANOVA, followed by Student-Newman Keuls’ test (p < 0.05).ResultsThe higher the% of HMP in the solutions, the greater HMP adsorption and more electron-donor sites on enamel surface were achieved (p < 0.05). Also, Ca adsorption was higher with increasing% HMP in the solutions (p < 0.05), which in turn reduced electron-donor sites on enamel surface. Increased Ca and PO₄ adsorption occurred at 0.5% and 1% HMP after treatment with Ca-PO₄ solution, resulting in a less electron-donor sites on surface when compared to the other treatments (P < 0.05).ConclusionsHMP leads to a more electron-donor sites on enamel surface, what promotes greater adsorption of Ca and PO₄ ions.     

Effects of saliva contamination and phosphoric acid composition on bond strength

The purpose of this study was to determine the effects of a phosphoric acid gel and solution, and saliva contamination of etched enamel on the tensile bond strength (TBS) of a posterior composite resin to etched enamel. Ninety-six extracted human permanent maxillary canines were used in this study. The test system developed by Kemper and Kilian was used to determine the TBS of a posterior composite resin (P-10) to enamel. The ground enamel surfaces were etched with 37% H₃PO₄ gel for 60 sec and washed for 15 sec or 30 sec, and with 37% H₃PO₄ solution for 60 sec and washed for 15 sec or 30 sec, respectively. The ground enamel surfaces were also etched with H₃PO₄ gel for 60 sec, washed for 15 sec and rewashed for 15 sec after saliva contamination of the etched enamel for 15 sec, 60 sec and 60 min, respectively. Etched enamel surfaces contaminated with saliva for 15 sec were re-etched for 15 sec and washed for 15 sec. The test specimens were mounted in an Instron machine and subjected to a tensile load at 0.02 inch. min⁻¹. The force required to break a test specimen was recorded, and the bond strengths were calculated and expressed in MN.m⁻². The data were analyzed by ANOVA and Duncan's multiple range test at the 5% level of significance. The H₃PO₄ composition (gel vs solution) and wash times had no significantly different effect on TBS. Only a 60 min saliva contamination significantly reduced TBS. In this study the TBS was determined 24 h after specimen preparation. It is possible that salivary contamination of etched enamel may have an adverse effect on bond strength only after prolonged immersion of the test specimens prior to testing.     

Fluoride retention in human enamel after a single phosphoric acid and mixed phosphoric acid/SnF2 application in vitro

Pieces of enamel were treated with either an APF solution or a 1:1 mixture of APF/SnF₂ (APF: 1.23 per cent F^?, 0.1 M H₃PO₄; and SnF₂: 2 per cent) for 4 min at 37 °C followed by 30 min washing. There was an increase of 1640 parts/10⁶ in the fluoride concentration of APF-treated enamel. Most of the fluoride was, however, washed away within 24 h to a level of 230 parts/10⁶. After 7 × 24 h the F concentration was of the same order: 280 parts/10⁶. The initial increase in fluoride concentration was lower after a mixed topical application (1120 parts/10⁶) but after 24 h the fluoride concentration was 510 parts and after 7 × 24 h 370 parts/10⁶, although statistically not significant, higher than after a single APF-treatment. The higher F concentration may be due to the formation of more firmly-bound fluoride as a fluoridated hydroxyapatite. It is more probable that the CaF₂ layer is protected by insoluble Sn compounds.     

Investigation of anion adsorption on platinum electrodes in aqueous media by probe beam deflection

Probe beam deflection (PBD) was used to monitor concentration gradients adjacent to the platinum electrode surface in basic and acidic aqueous media. It was observed that the PBD technique can detect anion adsorption on platinum during a cyclic voltammetry experiment. Also, PBD can measure the potential-dependent extent of adsorption of various anions including ClO₄^?, H₂PO₄^? and Cl^? on the Pt electrode surface.     

Diffusion of small ionic species in human saliva,plaque fluid and plaque residue in vitro

The tracer diffusion of small ionic species was investigated in water, saliva, dental plaque fluid and in plaque residue packed by a range of g forces. Diffusion rate of NaCl was similar in water, saliva and plaque fluid. The rate of CaCl₂, NaH₂PO₄, Na₂HPO₄ and Na₂SO₄ was less in both saliva and plaque fluid. Diffusion rate of all ionic species tested was significantly less in plaque residue, most of all for CaCl₂. In plaque fluid, plaque residue and water, an inverse linear relationship was found between log diffusion coefficients and log molecular weights. Increasing the tortuosity of the diffusion path, by packing plaque residue, further reduced the diffusion rate of NaH₂PO₄ and NaHCO₃. In terms of three-dimensional diffusion, in plaque fluid, NaH₂PO₄, Na₂HPO₄ and CaCl₂ diffused a distance of 300 μm within 25 s, whereas in plaque residue the equivalent diffusion times were 1.5–5.7 min. The unique retardation of CaCl₂ in plaque residue implicated both non-specific and specific binding interactions.     

Pyruvic acid as an etching agent

Phosphoric acid at different concentrations has been extensively used as an etching agent to improve bonding of dental materials to enamel surfaces. Recently attention has been drawn to the possible use of polyfunctional organic acids as conditioning agents. The object of this investigation was to determine the optimal concentration of pyruvic acid as an etching agent. A commercial composite resin with an intermediary bonding system supplied with 37% H₃PO₄ as an etching agent was used as the control system. In addition, a comparative study was carried out to evaluate 37% H₃PO₄, 20% lactic acid and the optimal concentration of pyruvic acid as conditioning solutions. Etching enamel surfaces with 10% pyruvic acid resulted in the optimal tensile bond strength of the resin to etched enamel surfaces. The use of 10% pyruvic acid did not adversely affect the bond strength of the resin system when compared to enamel surfaces etched with 37% H₃PO₄ for the same time period. Significantly lower tensile bond strengths were recorded on enamel surfaces etched with 20% lactic acid. The rate and depth of etching obtained with 37% H₃PO₄ can be considerably reduced by using 10% pyruvic acid as the conditioning agent.     

Dual-functional adhesive containing amorphous calcium phosphate nanoparticles and dimethylaminohexadecyl methacrylate promoted enamel remineralization in a biofilm-challenged environment

ObjectiveThe cariogenic biofilm on enamel, restoration, and bonding interface is closely related to dental caries and composite restoration failure. Enamel remineralization at adhesive interface is conducive to protecting bonding interface and inhibiting secondary caries. This study intended to assess the remineralization efficiency of adhesive with dimethylaminohexadecyl methacrylate (DMAHDM) and nanoparticles of amorphous calcium phosphate (NACP) on initial caries lesion of biofilm-coated enamel.MethodsArtificial initial carious lesion was created via 72-hour immersion in demineralization solution and cariogenic biofilm was formed after 24-hour culture of Streptococcus mutans (S. mutans). Specimens were then divided into 4 groups: enamel control, enamel treated with NACP, DMAHDM and NACP+DMAHDM respectively. Samples next underwent 7-day cycling, 4 h in BHIS (brain heart infusion broth containing 1 % sucrose) and 20 h in AS (artificial saliva) per day. The pH of BHIS was tested daily. So did the concentration of calcium and phosphate in BHIS and AS. Live/dead staining, colony-forming unit (CFU) count, and lactic acid production of biofilms were measured 7 days later. The enamel remineralization efficiency was evaluated by microhardness testing and transverse microradiography (TMR) quantitatively.ResultsEnamel of NACP+DMAHDM group demonstrated excellent remineralization effectiveness. And the NACP+DMAHDM adhesive released a great number of Ca²⁺ and PO₄^3- ions, increased pH to 5.81 via acid neutralization, decreased production of lactic acid, and reduced CFU count of S. mutans (P < 0.05).SignificanceThe NACP+DMAHDM adhesive would be applicable to preventing secondary caries, strengthening enamel-adhesive interface, and extending the lifespan of composite restoration.     

Minimum nutritional requirements for cellular growth and extracellular slime polysaccharide production by the human dental plaque bacterial mutant Actinomyces viscosus T14Av

The requirements were examined in chemically-defined media. Magnesium sulphate, potassium phosphate and adenine were essential for cellular growth and sodium bicarbonate and inositol further stimulated growth. The addition of sodium phosphate to this medium stimulated both cellular growth and slime production. In contrast, ammonium sulphate inhibited slime production. Biotin and thiamine stimulated cellular growth and nicotinic acid was essential for slime production. Glutamine, alanine and cystine were essential for cellular growth. A minimal medium which supported both good cellular growth and slime production for strain T14Av contained adenine, sodium bicarbonate, MgSO₄, inositol, glucose, KH₂PO₄, Na₂HPO₄, biotin, thiamine, nicotinic acid, arginine, glutamine, alanine and cystine.     

Ion release and enamel remineralizing potential of miswak,propolis and chitosan nano-particles based dental varnishes

Purpose

Natural products can have an important role in caries control through their inherent biological abilities.

A histological and biochemical study of the effect of vitamin C-deficiency on induction of amelogenesis in hamster molars in vitro

Second maxillary molars of hamsters were cultured in the presence or absence of 250μg/ml vitamin C for periods up to 12 days. At various days, cultured expiants were studied with histological and biochemical methods to investigate effects of vitamin C-deficiency on matrix production and mineralization in vitro. As biochemical parameters for protein synthesis and mineralization, uptake and incorporation of [³H]-proline, ⁴⁵Ca and ³²PO₄ were used. To discriminate between synthesis of collagenous and non-collagenous proteins digestion of the [³H]-proline-labelled material by purified collagenase and its degree of hydroxylation were measured. Histologically, in control explants cultured with vitamin C, normal dentinogenesis, amelogenesis and mineralization in vitro were observed. In the vitamin C-deficient expiants, odontoblasts produced an abnormal predentine matrix and de-differentiated. Eventually, this matrix mineralized aspecifically. In the presence of this abnormal matrix, ameloblasts failed to differentiate, which suggests that a cell-matrix type of interaction is involved in the differentiation of the pre-ameloblasts. Biochemically, in the vitamin C-deficient explants protein synthesis and collagen synthesis were reduced by about the same extent; the in-vitro produced collagens appeared under-hydroxylated and could, in degraded form, easily be extracted in formic acid. An increase of [³H]-proline solubility in formic acid in the control explants, however, paralleled enamel matrix production and was attributed to the solubilization of proline-rich enamel matrix proteins. The production of acid insoluble phosphate was not affected by vitamin C-deficiency. The uptakes of ⁴⁵Ca and ³²PO₄ were retarded and the molar Ca:PO₄ uptake ratio was lower, reflecting the histologically observed aspecific mineralization.     

Calcium phosphate formation in gelatin matrix using free ion precursors of Ca2+ and phosphate ions

ObjectivesHydroxyapatite (HAp)/gelatin (GEL) nanocomposite has been developed as a bone substitute. The nanocomposite formation in the GEL matrix is greatly affected by the reaction between Ca²⁺ and phosphate ions. The mineralization of GEL macromolecules was investigated through a co-precipitation of calcium phosphates (Ca-P) by using free ions of Ca²⁺ and phosphate ions, Pi. The purpose of this study was to prepare a dense HAp/GEL nanocomposite through a free ion production process.MethodsFree ionic calcium, Ca²⁺, was produced through electrodialysis process using a cation membrane (CMV). Triprotic acid ions were diffused through an anion membrane (AMV) from an aqueous solution of H₃PO₄. The HAp/GEL nanocomposite was prepared by the co-precipitation process. As a reference material for comparison, Ca(OH)₂ and H₃PO₄ were used for the preparation of a HAp/GEL nanocomposite.ResultsThe dense compact body of dried Ca-P/GEL nanocomposite was obtained through the fine chemical reaction of Ca²⁺ and Pi. The free calcium ion Ca²⁺, diffused from the CMV of the cation reactor greatly affected the formation of Ca–P phase. Phosphate ion species diffused through the AMV in the anion reactor definitely influenced the reaction with Ca²⁺. For the formation of the Ca–P phase in the GEL matrix, the organic–inorganic interaction was analyzed using FT-IR. The crystal growth of HAp in the GEL matrix increased with the increase of GEL from XRD, FT-IR and TEM.SignificanceThe mineralization reaction in GEL macromolecules was critically influenced by the free ions of Ca²⁺ and inorganic phosphate ions, Pi. The interaction between Ca²⁺ and Pi in the GEL matrix was very fine compared to the HAp/GEL nanocomposite prepared from Ca(OH)₂ and H₃PO₄ with the GEL. The dense compact body of HAp/GEL nanocomposite was obtained for an artificial bone application.     

A comparative study of three etching solutions: effects on enamel surface and adhesive-enamel interface

The chemical treatment of enamel surfaces is an approach in obtaining increased bonding of dental materials to tooth surfaces. Three etching solutions commonly used at present are 50% H₃PO₄, 50% H₃PO₄ attenuated with 7% zinc oxide and 50% citric acid. The effects of these three conditioning solutions on normal and polished enamel surfaces and on the interfaces between untreated enamel and etched enamel surfaces were studied by scanning electron microscopy. The variable etching pattern observed with each of the acids made a comparative study difficult. This was further complicated by the dependence of the etching action on prism orientation on polished enamel surfaces. Additional factors that have to be considered are the presence of prismless enamel and perikymata. Surface profile recordings of polished and acid-etched polished enamel surfaces provided some quantitative measure of the etching action of the conditioning solutions. The 50% citric acid solution had the mildest etching action while 50% H₃PO₄ and 50% H₃PO₄ attenuated with 7% zinc oxide elicited comparable responses.