Formation of phosphate-containing calcium fluoride at the expense of enamel, hydroxyapatite and fluorapatite

Qualitative analysis on crystal growth of synthetic hydroxyapatite influenced by fluoride concentration

OBJECTIVE

This study was designed to analyze the crystal growth of synthetic hydroxyapatite (HA) particles in pH 7.0 supersaturated solutions with different fluoride concentrations by FE-SEM, FE-TEM, X-ray diffraction (XRD), and FTIR.

DESIGN

Eight groups of pH 7.0 calcium phosphate supersaturated solutions were prepared with different fluoride concentrations (0, 1, 2, 4, 8, 16, 32, and 64 ppm). Each solution was introduced into the reactive column containing the synthetic HA for 48 h. The resulting products were prepared for FE-SEM, FE-TEM, XRD, and FTIR.

RESULTS

The FE-SEM examination revealed various morphological changes of the crystals, with additional, less-ordered crystallites in experimental solutions containing more than 8 ppm of fluoride. FE-TEM examination showed an additional amorphous layer on the surface of the crystals with the presence of fluoride, whereas definite lattice structures completely reached the surface of the crystals without fluorides. XRD data showed that all crystals had the same patterns as the unreacted synthetic HA, regardless of fluoride concentration. With FTIR results, the intensity of the OH-libration mode decreased when adding fluoride, compared to that of pristine HA. The resulting crystals were considered to be partially fluoridated HA under room temperature and pH 7.0 supersaturated solutions.

CONCLUSION

Under the experimental conditions in this study, fluorides mainly react with the surface of the seed HA and have an impact on the growth of HA in a less effective manner as the concentration of fluoride increases.

Surface characteristics of a modified acidulated phosphate fluoride gel with nano-hydroxyapatite coating applied on bovine enamel subjected to an erosive environment

This study evaluated the antierosive effect applying a modified acidulated phosphate fluoride (APF) gel containing nano-hydroxyapatite (nHAp) on the enamel surface before erosion. After polishing, the exposed flat enamel surfaces (n = 7/group) from bovine incisors were treated with artificial saliva (S - negative control), orange juice (ERO), APF gel (positive control) and APF_nHAp gel. All samples were subjected to six cycles of demineralization (orange juice, pH ~ 3.5, 10 min) followed by remineralization (saliva, 37°C, 1 hr). The enamel surface morphology, topography, and inorganic composition were analyzed using scanning electron microscopy, roughness testing, and micro energy-dispersive X-ray fluorescence spectrometry, respectively. The mean (standard deviation) roughness values (Ra, μm) were S, 0.13 (0.05); ERO, 0.25 (0.07); APF, 0.22 (0.08); and APF_nHAp, 0.17 (0.04). Ra values were significantly higher after ERO (p < .01) and APF (p < .05) treatments than after S. The enamel surface morphology was altered by the treatments, except for the S specimens. The mineral content of the enamel showed a clear trend with Ca and P reduction in the order of APF < S < APF_nHAp < ERO and APF < S < ERO < APF_nHAp, respectively. We can conclude that APF gel increased mineral concentration on the enamel. Moreover, the APF_nHAp material modified the composition and morphology of the enamel surface.

Surface reactivity of octacalcium phosphate-derived fluoride-containing apatite in the presence of polyols and fluoride

The present study was designed to characterize co-precipitated fluoridated apatitic materials from octacalcium phosphate (OCP) precursor and to investigate their surface reactions with polyols including glycerol in the presence of fluoride ions. Laboratory-synthesized fluoridated apatite crystals (LS-FA) were obtained in a solution containing fluoride (F) from 25 to 500 ppm. LS-FAs and commercially available fluoroapatite (FA) and hydroxyapatite (HA) were characterized by physical techniques, such as X-ray diffraction. LS-FA obtained in the presence of 100 ppmF (100 ppm-LS-FA) had an apatitic structure, but its solubility was close to HA in a culture medium (α-MEM) despite the fact it contains over 3 wt % of F. 100 ppm-LS-FA, FA, and HA were then subjected to the human serum albumin (HSA) adsorption test at pH 7.4 (in a 150 mM Tris-HCl buffer) and the dissolution and re-mineralization experiments in the presence of xylitol, D-sorbitol, or glycerol, and F under acidic and neutral conditions. Adsorption affinity of HSA was estimated as highest for FA and lowest for LS-FA. LS-FA, FA, and HA were immersed in a lactic acid solution with the polyols and/or F ion-containing solution up to 200 ppm to analyze the dissolution behavior. LS-FA had the highest dissolution tendency in the conditions examined. Glycerol enhanced the dissolution of phosphate from apatite crystals in particular from LS-FA. The results suggest that the apatite crystals, obtained through the hydrolysis of OCP in the presence of F, provide a more reactive surface than FA or HA under physiological environments. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2235-2244, 2018.

The Application of in Vitro Models to Research on Demineralization and Remineralization of the Teeth: Reaction Paper

Despite the advantages of in situ model studies, in vitro models are most important to provide insight into the mechanism of dental caries, the mechanisms of fluoride action, and profile screening.

In this reaction paper following Dr. White's review, the emphasis is, first, on the role of mobile fluoride (FL) in fluoride efficacy, the formation of "CaF2-like" material as fluoride reaction product, and on fluoride reaction product localization. Second, mineral assessment techniques are discussed. Finally, the main differences between caries lesion formation in vitro and in situ are considered.

The role of water and mineral-collagen interfacial bonding on microdamage progression in bone

Microdamage would be accumulated in bone due to high-intensity training or even normal daily activity, which may consequently cause fragility fracture or stress fracture. On the other hand, microdamage formation serves as a toughening mechanism in bone. However, the mechanisms that control microdamage initiation and accumulation in bone are still poorly understood. Our previous finite element model indicated that different interfacial properties between mineral and collagen in bone may lead to distinct patterns of microdamage accumulation. Therefore, the current study was designed to examine such prediction and to investigate the role of water and mineral-collagen interactions on microdamage accumulation in bone. To address these issues, 48 mice femurs were divided randomly into four groups. These groups were dehydrated or treated with perfluorotripropylamine (PFTA) or NaF solution to change water distribution and mineral-collagen interfacial bonding in bone. After three-point bending fatigue tests, the types of microdamage (i.e., linear microcracks or diffuse damage) formed in bone were compared between different groups. The results suggested that (1) bone tissues with strong mineral-collagen interfacial bonding facilitate the formation of linear microcraks, and (2) water has little contribution to the growth of microcracks.

Uptake of fluoride from aqueous solution on nano-sized hydroxyapatite: examination of a fluoridated surface layer

Hydroxyapatite (Ca(10)(PO(4))(6)(OH)(2), HAP), both as a synthetic material and as a constituent of bone char, can serve as an effective and relatively inexpensive filter material for fluoride (F(-)) removal from drinking water in low-income countries. Fluoride uptake on HAP can occur through different mechanisms, which are, in principle, influenced by solution composition. Suspensions of HAP (2 g L(-1)) were equilibrated under controlled pH conditions (pH 6.5, 7.3, 9.5) at 25 °C for 28 d after the addition of different F(-) concentrations (0.5-7.0 mM). The reacted HAP solids were examined with Transmission Electron Microscopy (TEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray Photoelectron Spectroscopy (XPS), and Nano Secondary Ion Mass Spectroscopy (NanoSIMS). Fluoride uptake on HAP was dependent on pH, with the highest capacity at pH 6.5; the lowest uptake was found at pH 9.5. Under all experimental conditions, the thermodynamically stable mineral phase was fluorapatite, (Ca(10)(PO(4))(6)F(2), FAP). Fluoride uptake capacity was quantified on the basis of FTIR and XPS analysis, which was consistent with F(-) uptake from solution. The results of XPS and NanoSIMS analyses indicate that a fluoridated surface layer with a thickness of several nanometers is formed on nanosized HAP.

Influence of fluoride- or triclosan-based desensitizing agents on adhesion of resin cements to dentin

Effect of desensitizers on the bond strength of resin cements to dentin was evaluated. Intact premolars (N = 90) were embedded in polymethyl methacrylate; dentin surfaces were exposed, and they were randomly divided into two main groups of cements (Duolink (D), Variolink II (V); n = 45 per group) and then into three desensitizer subgroups (n = 15 per subgroup). Teeth in controls (C) were treated according to cements' adhesion protocols; the other two groups received either fluoride- [Aqua-Prep F (F)] or triclosan-based [Seal&Protect (T)] desensitizers. Ceramic disks (Empress 2) were adhered; specimens were thermocycled (×5,000 cycles, 5-55 ± 1°C, dwell time 30 s) and subjected to shear bond strength test (MPa ± SD) in a universal testing machine (crosshead speed 1 mm/min). Failure types were classified using scanning electron microscope. For V, application of both desensitizers (29.6 ± 7.8 and 22.8 ± 2.8 for F and T, respectively) did not present significantly different results than that of the VC (21.2 ± 2.3; p > 0.05, one-way ANOVA). In D, F (20.6 ± 2.4) showed significantly higher results (p < 0.05) than those in T (16.1 ± 3.9) and DC group (15.2 ± 2.3). V showed significantly higher results than D (p < 0.05, Bonferroni). F and T did not negatively affect the bond strength results with D and V. Adhesive failures were more frequent with both T (84%) and F (66%) in D; cohesive failures in the cement (88%) were more commonly observed with F in V. Both F and T desensitizers can be safely used prior to final cementation but F in combination with V seems to be more reliable, considering both the bond strength and the failure types.

Uniform partial dissolution of bone mineral by using fluoride and phosphate ions combination

Mineral content is one of the main predictors of the mechanical properties of bone tissue. The contribution of the bone mineral phase to the mechanical properties of bone has been investigated by reducing the mineral content of bone with different in vitro treatment techniques such as hydrochloric acid (HCl), ethylenedinitrilo tetraacetic acid (EDTA), and fluoride ion treatment. In this study, we propose a new treatment technique which combines fluoride and phosphate ions. Bovine femur specimens were used to determine the mechanical properties of cortical bone after different fluoride phosphate ion combination treatments. The treatment solutions, which contain different fluoride and phosphate ion concentrations, dissolved part of the bone mineral in a uniform manner throughout the bone samples. Dissolution by products, which precipitated in the bone tissue, contained calcium fluoride with phosphate ions (CaF(2)/P) and fluorapatite/fluorhydroxyapatite-type material (FAp/FHAp) and acted as filler. Depending on the fluoride and phosphate concentration in a treatment solution, the precipitated material's ratio of FAp/FHAp to total fluoride containing phase (FAp/FHAp + CaF(2)/P) in bone tissue also changed. High fluoride ion content in treatment solutions generated more CaF(2)/P type of precipitate, and low fluoride ion concentration generated more FAp/FHAp type precipitates as compared to high fluoride concentration treatments. These experiments show that phosphate ions are another important parameter of a treatment solution, in addition to ionic strength, pH, and the duration of treatment. In vitro, phosphate fluoride combinations partially dissolve bone mineral content in a wider range than fluoride treatment alone in a uniform manner. With this new technique one can control more precisely the partial dissolution of the bone mineral and mineral phase's contribution to mechanical properties of bone tissue.

Effect of bone mineral content on the tensile properties of cortical bone: experiments and theory

The effect of mineral volume fraction on the tensile mechanical properties of cortical bone tissue is investigated by theoretical and experimental means. The mineral content of plexiform, bovine bone was lowered by 18% and 29% by immersion in fluoride solutions for 3 days and 12 days, respectively. The elastic modulus, yield strength and ultimate strength of bone tissue decreased, while the ultimate strain increased with a decrease in mineral content. The mechanical behavior of bone tissue was modeled by using a micromechanical shear lag theory consisting of overlapped mineral platelets reinforcing the organic matrix. The decrease in yield stress, by the 0.002 offset method, of the fluoride treated bones were matched in the theoretical curves by lowering the shear yield stress of the organic matrix. The failure criterion used was based on failure stresses determined from a failure envelope (Mohr's circle), which was constructed using experimental data. It was found that the model predictions of elastic modulus got worse with a decrease in mineral content (being 7.9%, 17.2% and 33.0% higher for the control, 3-day and 12-day fluoride-treated bones). As a result, the developed theory could not fully predict the yield strain of bones with lowered mineral content, being 12.9% and 21.7% lower than the experimental values. The predicted ultimate stresses of the bone tissues with lower mineral contents were within +/- 10% of the experimental values while the ultimate strains were 12.7% and 26.3% lower than the experimental values. Although the model developed in this study did not take into account the presence of hierarchical structures, voids, orientation of collagen molecules and micro cracks, it still indicated that the mechanical properties of the organic matrix depend on bone mineral content.

Effect of fluoride application on tensile bond strength of self-etching adhesive systems to demineralized dentin

STATEMENT OF PROBLEM

It has been reported that the bond strength of composite to demineralized dentin is lower than that to sound dentin. This can be a problem in the success of so-called sealed restorations.

PURPOSE

The aim of this study was to evaluate the effect of fluoride application on the tensile bond strength of self-etching adhesive systems to demineralized dentin.

MATERIAL AND METHODS

One hundred twenty extracted bovine incisors were ground flat with 600-grit silicone carbide paper. Ninety teeth were then immersed in acetate buffer (pH 4.0) to form demineralized dentin. These teeth were randomly divided into 3 groups of 30 each: surfaces treated with fluoride solution (Group NF), surfaces treated with fluoride solution followed by a water rinse (Group RF), and control surfaces with no pretreatment (Group C). The remaining 30 teeth comprised a group with normal dentin surfaces (Group S). Each group was further divided into 3 subgroups of 10 each to test the bond strength test of Clearfil SE Bond, Unifil Bond, and Mac-Bond II. Tensile bond strengths (in MPa) were measured with a universal testing machine at a crosshead speed of 0.5 mm/min. Mean bond strengths were analyzed by 2-way analysis of variance and Fisher's PLSD (P =.05). SEM observations of the surfaces before and after priming and at resin-dentin interfaces in each group were performed. Elemental analysis of the dentin surfaces before priming was also carried out.

RESULTS

The bond strengths of the adhesives to demineralized dentin in Groups NF, RF, and C were significantly lower than that of the normal dentin in Group S (P <.05). The mean bond strengths of the 3 adhesives in Group NF were higher than those in Groups RF and C, but a significant difference was observed only when Clearfil SE Bond was used (P <.05). In SEM images the open dentinal tubules on the surface and the resin tag formation at the resin-dentin interface were apparent in Group NF but were not observed in Groups RF and C. On elemental analysis, considerable amounts of fluoride and calcium were detected in the surfaces of Group NF.

CONCLUSION

Within the limitations of this study, the surface treatment with fluoride solution supported the resin tag formation at the resin-dentin interface and slightly improved the bond strength of the self-etching adhesive systems tested to demineralized dentin. This result indicated that the fluoride treatment to demineralized dentin might contribute to the success of the sealed restoration.

Changing the structurally effective mineral content of bone with in vitro fluoride treatment

Bovine femur cortical bone specimens were tested in tension after being treated in vitro for 3 days with sodium fluoride solutions of different molarity (0.145, 0.5, and 2.0M). The treatments alter the mechanical properties of the bone samples with different degrees as compared to control samples (untreated). The mechanical properties of the treated samples have lower elastic modulus, yield and ultimate stress, acoustic impedance and hardness, and higher ultimate strain and toughness as compared to control samples. The observed effects were intensified with the increasing molarity of the treatment solutions. This study shows that the fluoride treatment can be used to investigate the composite behavior of bone tissue by altering the structurally important bone mineral content in a controlled manner.

Effect in vitro acidification on plaque fluid composition with and without a NaF or a controlled-release fluoride rinse

Plaque fluid ion concentration changes, especially fluoride, in response to the pH decrease associated with a cariogenic episode are important components of the caries process. A "controlled-release" (CR) fluoride rinse, based on the controlled release of fluoride in the presence of calcium, has been shown to form large fluoride reservoirs in resting plaque. In this study, the in vitro acid-induced release of fluoride, and other ions, was examined in 48-hour-fasted plaque fluid from subjects (n = 11) who received no rinse, or who used a 228-ppm CR or NaF fluoride rinse 1 hr before being sampled. After collection, the plaque was centrifuged to yield plaque fluid, acidified (0.1 microL of 0.5 mol/L HCl per milligram plaque), and then re-centrifuged before a second sample was obtained. Although previous studies indicated a higher plaque fluid fluoride after the new rinse relative to NaF, no statistically significant difference was observed here. Average fluoride release after acidification (average pH, 5.2) was statistically greater following the use of the CR rinse (153 micromol/L) compared with the NaF rinse (17 micromol/L). No fluoride release was seen in the no-rinse samples. The pH, free calcium, phosphate, acetate, propionate, and buffer capacity were not affected by the different amounts of fluoride deposited in the plaque. However, following acid addition, an increase in free calcium and phosphate was observed, which was also independent of the rinse. The large release of fluoride following acidification suggests that the new rinse may provide an improved cariostatic effect.

Review on fluoride, with special emphasis on calcium fluoride mechanisms in caries prevention

Low concentrations of fluoride have a beneficial effect on enamel and dentin de- and remineralization. After fluoride treatments, such as topical applications, rinses or dentifrices, salivary fluoride concentrations decrease exponentially in a biphasic manner to very low concentrations within a few hours. For treatments to be effective over periods longer than the brushing and the following salivary clearance, fluoride needs to be deposited and slowly released. Calcium fluoride (or like) deposits act in such a way, owing to a surface covering of phosphate and/or proteins, which makes the CaF2 less soluble under in vivo conditions than in a pure form in inorganic solutions. Moreover, due to the phosphate groups on the surface of the calcium fluoride globules, fluoride is assumed to be released with decreasing pH when the phosphate groups are protonated in the dental plaque.

Increased overnight fluoride concentrations in saliva, plaque, and plaque fluid after a novel two-solution rinse

Recent studies showed that salivary, plaque-fluid, and whole-plaque fluoride were significantly higher 120 min after subjects rinsed with a novel two-solution rinse than after they rinsed with a NaF rinse of the same fluoride concentration. In this study, the persistence of these increases was investigated overnight, a period of time that is more clinically relevant. Improved analytical techniques for the ultramicro determination of whole-plaque and plaque-fluid fluoride from the same sample are also described. Thirteen subjects abstained from toothbrushing for 48 hrs and rinsed for 1 min with a 12 mmol/L (228 ppm) NaF or the two-solution rinse before bedtime. Samples were then collected the following morning before breakfast: (1) Saliva samples were either clarified by centrifugation or acid-extracted with 1 mol/L HClO4; and (2) single-site molar plaque samples were centrifuged to obtain plaque fluid and/or extracted with 1 mol/L HClO4. Results showed that, compared with NaF, the two-solution rinse produced significantly higher fluoride concentrations in all samples: The concentration of fluoride in whole plaque and whole saliva following the new rinse exceeded concentrations found after the NaF rinse by factors of three and four, respectively, while in plaque fluid, the two-solution rinse produced about a two-fold increase over NaF values, which were near baseline levels. This increase, however, was only about 20% in centrifuged saliva. The increases in saliva and especially in plaque-fluid fluoride after the two-solution rinse indicate a greater remineralization potential, while the enhanced fluoride reservoirs found in plaque overnight after this rinse constitute a reserve that may release fluoride into the plaque fluid over an extended period of time.