No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Time Dependence of Fluoride Uptake in Hydroxyapatite
Abstract
Fluoridation of enamel is believed to provide an effective tool to protect teeth from caries, but there is still little information on the timescale of fluoride uptake. In this study highly compressed pellets of hydroxyapatite are used as first-order model systems to approximate the mineral component of natural enamel for investigations on the time-dependence of fluoride uptake. We found that both, the overall amount of fluoride as well as the mean thickness of the fluoridated surface layer cannot be extended to any values just by increasing the application time of a fluoride containing agent. Instead both parameters start to become constant on a timescale of about 3 minutes. The present results as obtained on a synthetic model ‘tooth’ show that the timescale to provide the maximum amount of fluoride possible is of the same order of magnitude as that in usual daily practice in dental care when applying toothpastes or mouth rinses.
... This is in line with earlier reports where teeth were treated with fluoride-containing toothpastes [36,39,40]. The uptake of fluoride from toothpastes into a thin surface layer was reported to occur within a few minutes [41]. The fact that the immersion in acidic fluoride-free medium and also in water did not significantly change the fluoride content underscores the generally low fluoride concentration in teeth as reported earlier (see Ref. [14] for a recent overview of the literature). ...
... Consequently, chemical and crystallographic nature of the fluoride-rich surface layers may well be different because the analyzed samples were basically not identical. Earlier studies by X-ray photoelectron spectroscopy have shown that the penetration depth of incorporated fluoride is only a few tens of nanometers [32,37,41,45], well in line with our results. Consequently, the fluoride-rich surface layer comprises only about 0.1 to 0.2 vol% of the abraded sample used for X-ray diffraction. ...
Toothpastes and mouth rinses contain fluoride as a protective agent against caries. The aim of this study was to determine the degree of fluoride-uptake by human tooth mineral during immersion into fluoride-containing aqueous solutions as different pH. Human teeth were immersed in fluoride-containing solutions to assess the extent of fluoride incorporation into tooth enamel. A total of 16 extracted teeth from 11 patients were immersed at 37 °C for one minute into aqueous fluoride solutions (potassium fluoride; KF) containing either 250 ppm or 18,998 ppm fluoride (1-molar). Fluoride was dissolved either in pure water (neutral pH) or in a citrate buffer (pH 4.6 to 4.7). The elemental surface composition of each tooth was studied by energy-dispersive X-ray spectroscopy in combination with scanning electron microscopy and X-ray powder diffraction. The as-received teeth contained 0.17 ± 0.16 wt% fluoride on average. There was no significant increase in the fluoride content after immersion in 250 ppm fluoride solution at neutral or acidic pH values. In contrast, a treatment with a 1-molar fluoride solution led to significantly increased fluoride concentrations by 0.68 wt% in water and 9.06 wt% at pH 4.7. Although such fluoride concentrations are far above those used in mouth rinses or toothpastes, this indicates that fluoride can indeed enter the tooth surface, especially at a low pH where a dynamic dissolution-reprecipitation process may occur. However, precipitations of calcium fluoride (globuli) were detected in no cases.
... Fluoride is also known to be found in nearly all of the world's water supply, from bottled water to natural sources, such as lakes and oceans. Additionally, in its most prevalent forms, it is found in tap water and the dental office, which is assumed by nearly all of the world's population because fluoride is said to make teeth healthier and stronger [2]. However, what many fail to acknowledge is how fluoride actually works to accomplish such tasks as tooth improvement and maintenance and how it works by physiological mechanisms [3]. ...
... Fluoride works by interacting with the surface enamel of teeth [2]. Enamel is the hardest and most highly mineralized substance in the human body and is made up of approximately 97% calcium and phosphate and the crystalline structure is known as hydroxyapatite [4]. ...
Several benefits and problems arise from water fluoridation in developed countries. Evidence shows support for and against water fluoridation, while some tests remain inconclusive. A safe, ingestible range was established to be between 3 and 4 milligrams of fluoride per day for the average adult. While overall results were generally inconclusive, the future of water fluoridation is questionable as it seems to slowly be removed from major countries around the world.
... Fluoride induces remineralization due to its high affinity toward calcium phosphate without being incorporated into the enamel itself [76]. This has been shown experimentally [77][78][79] and theoretically by molecular dynamics simulations [80,81]. Multiple exposures to fluoride are likely to enhance this small effect during the many pH changes a tooth experiences each day [39]. ...
The natural remineralization of enamel is of major importance for oral health. In principle, early erosions (demineralization) induced by acidic beverages and foods as well as initial caries lesions can be covered and remineralized by the deposition of calcium phosphate, i.e., tooth mineral. This remineralization effect is characterized by the presence of calcium and phosphate ions in saliva that form hydroxyapatite on the enamel surface. Although it is apparently a simple crystallization, it turns out that remineralization under in vivo conditions is actually a very complex process. Calcium phosphate can form a number of solid phases of which hydroxyapatite is only one. Precipitation involves the formation of metastable phases like amorphous calcium phosphate that convert into biological apatite in a number of steps. Nanoscopic clusters of calcium phosphate that can attach on the enamel surface are also present in saliva. Thus, remineralization under strictly controlled in vitro conditions (e.g., pH, ion concentrations, no additives) is already complex, but it becomes even more complicated under the actual conditions in the oral cavity. Here, biomolecules are present in saliva, which interact with the forming calcium phosphate mineral. For instance, there are salivary proteins which have the function of inhibiting crystallization to avoid overshooting remineralization. Finally, the presence of bacteria and an extracellular matrix in plaque and the presence of proteins in the pellicle have strong influences on the precipitation on the enamel surface. The current knowledge on the remineralization of the enamel is reviewed from a chemical perspective with a special focus on the underlying crystallization phenomena and the effects of biological compounds that are present in saliva, pellicle, and plaque. Basically, the remineralization of enamel follows the same principles as calculus formation. Notably, both processes are far too complex to be understood on a microscopic basis under in vivo conditions, given the complicated process of mineral formation in the presence of a plethora of foreign ions and biomolecules.
... Zavalo-Alonso et al. characterized the external structure, roughness, and absolute depth profile of fluorotic and healthy enamel, confirming a positive association between fluorosis severity, enamel surface roughness, and absolute depth profile at the nanometer scale [27]. Furthermore, Zeitz et al. and Faidt et al. demonstrated increased resistance of hydroxyapatite against acids due to fluorination of the material [28][29][30]. Furthermore, Hannig et al. successfully utilized the contact mode of SFM to image and identify pellicle structures on enamel [31,32]. ...
Scanning force microscopy (SFM) is one of the most widely used techniques in biomaterials research. In addition to imaging the materials of interest, SFM enables the mapping of mechanical properties and biological responses with sub-nanometer resolution and piconewton sensitivity. This review aims to give an overview of using the scanning force microscope (SFM) for investigations on dental materials. In particular, SFM-derived methods such as force–distance curves (scanning force spectroscopy), lateral force spectroscopy, and applications of the FluidFM® will be presented. In addition to the properties of dental materials, this paper reports the development of the pellicle by the interaction of biopolymers such as proteins and polysaccharides, as well as the interaction of bacteria with dental materials.
... HAp samples therefore still have their value as a substitute for natural enamel in dental research, as they provide a reproducible and consistent surface chemistry, which allows controlled alterations in roughness or fluoride content. 45,80,81 The development of natural enamel, in contrast, is a highly complex process, which is among others influenced by the individual organic content of enamel and external factors, 82 which may lead to larger variations. The nonhydroxyapatite parts of enamel, the ionic substitutes in the mineral component, and/or the crystal orientation, however, seem to make the difference in reaching the lowest force values measured in this study. ...
Research into materials for medical application draws inspiration from naturally occurring or synthesized surfaces, just like many other research directions. For medical application of materials, particular attention has to be paid to biocompatibility, osseointegration, and bacterial adhesion behavior. To understand their properties and behavior, experimental studies with natural materials such as teeth are strongly required. The results, however, may be highly case-dependent because natural surfaces have the disadvantage of being subject to wide variations, for instance in their chemical composition, structure, morphology, roughness, and porosity. A synthetic surface which mimics enamel in its performance with respect to bacterial adhesion and biocompatibility would, therefore, facilitate systematic studies much better. In this study, we discuss the possibility of using hydroxyapatite (HAp) pellets to simulate the surfaces of teeth and show the possibility and limitations of using a model surface. We performed single-cell force spectroscopy with single Staphylococcus aureus cells to measure adhesion-related parameters such as adhesion force and rupture length of cell wall proteins binding to HAp and enamel. We also examine the influence of blood plasma and saliva on the adhesion properties of S. aureus. The results of these measurements are matched to water wettability, elemental composition of the samples, and the change in the macromolecules adsorbed over time on the surface. We found that the adhesion properties of S. aureus were similar on HAp and enamel samples under all conditions: Significant decreases in adhesion strength were found equally in the presence of saliva or blood plasma on both surfaces. We therefore conclude that HAp pellets are a good alternative for natural dental material. This is especially true when slight variations in the physicochemical properties of the natural materials may affect the experimental series.
... At the surface (first few nm), the fluoride content was equivalent to a fluoride substitution of about 62% in hydroxyapatite. The authors postulated a different rate of fluoride uptake as a function of the crystallographic face of a given hydroxyapatite crystal (but without providing experimental evidence for this assumption) [56]. ...
Dental erosion is a common problem in dentistry. It is defined as the loss of tooth mineral by the attack of acids that do not result from caries. From a physico-chemical point of view, the nature of the corroding acids only plays a minor role. A protective effect of fluorides, to prevent caries and dental erosion, is frequently claimed in the literature. The proposed modes of action of fluorides include, for example, the formation of an acid-resistant fluoride-rich surface layer and a fluoride-induced surface hardening of the tooth surface. We performed a comprehensive literature study on the available data on the interaction between fluoride and tooth surfaces (e.g., by toothpastes or mouthwashes). These data are discussed in the light of general chemical considerations on fluoride incorporation and the acid solubility of teeth. The analytical techniques available to address this question are presented and discussed with respect to their capabilities. In summary, the amount of fluoride that is incorporated into teeth is very low (a few µg mm⁻²), and is unlikely to protect a tooth against an attack by acids, be it from acidic agents (erosion) or from acid-producing cariogenic bacteria.
... F-HA showed good apatite-like layer deposition ability [67], can promote cell adhesion and protein adsorption, and improved ALP activity in cell culture [68]. It can obviously prevent the decrease of bone mineral density caused by osteoporosis [69,70], and inhibit bacteria growth [71,72]. ...
Hydroxyapatite (HA) is widely used in bone tissue engineering for its bioactivity and biocompatibility, and a growing number of researchers are exploring ways to improve the physical properties and biological functions of hydroxyapatite. Up to now, HA has been used as inorganic building blocks for tissue engineering or as nanofillers to blend with polymers, furthermore, various methods such as ion doping or surface modification have been also reported to prepare functionalized HA. In this review, we try to give a brief and comprehensive introduction about HA-based materials, including ion-doped HA, HA/polymer composites and surface modified HA and their applications in bone tissue engineering. In addition, the prospective of HA is also discussed. This review may be helpful for researchers to get a general understanding about the development of hydroxyapatite based materials.
... However, the influence of the conditioning films on each surface is remarkably similar. HAp samples therefore still have their value as substitute for natural enamel in dental research, as they provide a reproducible and consistent surface chemistry, which allows controlled alterations in roughness or fluoride content [43,63,64]. The development of natural enamel, in contrast, is a highly complex process, which is among others influenced by the individual organic content of enamel and external factors [65], which may lead to larger variations. ...
Research into materials for medical application draws inspiration from naturally occurring or synthesized surfaces, just like many other research directions. For medical application of materials, particular attention has to be paid to biocompatibility, osseointegration and bacterial adhesion behavior. To understand their properties and behavior, experimental studies with natural materials such as teeth are strongly required. The results, however, may be highly case-dependent because natural surfaces have the disadvantage of being subject to wide variations, for instance in their chemical composition, structure, morphology, roughness, and porosity. A synthetic surface which mimics enamel in its performance with respect to bacterial adhesion and biocompatibility would, therefore, facilitate systematic studies much better. In this study, we discuss the possibility of using hydroxyapatite (HAp) pellets to simulate the surfaces of teeth and show the possibility and limitations of using a model surface. We performed single-cell force spectroscopy with single Staphylococcus aureus cells to measure adhesion-related parameters such as adhesion force and rupture length of adhesins binding to HAp and enamel. We also examine the influence of blood plasma and saliva on the adhesion properties of S. aureus . The results of these measurements are matched to water wettability, elemental composition of the samples and the change in the macromolecules adsorbed over time. We found that the adhesion properties of S. aureus were similar on both samples under all conditions: Significant decreases in adhesion strength were found equally in the presence of saliva or blood plasma on both surfaces. We therefore conclude that HAp pellets are a good alternative for natural dental material. This is especially true when slight variations in the physicochemical properties of the natural materials may affect the experimental series.
... Thereby, the application of the fluorides was performed with a solution to ensure that mechanical brushing with a toothbrush had no additional effect. A recent study showed that an prolonged application time doesn't lead to a thicker surface layer or an increased number of pellicle bound fluoride ions 33 . Therefore, the used application time of the fluorides was 1 min in the present study. ...
The present clinical-experimental study aims to examine the effect of pure experimental fluoride solutions and stannous chloride on the initial oral bioadhesion under in situ conditions. After 1 min of pellicle formation on bovine enamel slabs, 12 subjects rinsed with 8 ml of the fluoride test solutions (NaF, Na2PO3F, AmF, SnF2,) with 500 ppm fluoride concentration each for 1 min. Additionally, rinsing without a solution (control) and rinsing with 1563 ppm SnCl2 solution took place for 1 min. Afterwards, fluorescence microscopy took place to visualize bacterial adhesion and glucan formation (8 h oral exposition) with DAPI and ConA and the BacLight method. TEM was performed to visualize the pellicle ultrastructure together with EDX to detect stannous ions. The rinsing solutions with pure SnF2 and SnCl2 reduced significantly the initial bacterial colonization (DAPI). While, NaF and Na2PO3F showed no significant effect compared to the control. There was no significant difference between AmF, SnF2 and SnCl2. All tested experimental solutions showed no reducing effect on the glucan formation. Considerable alterations of the pellicle ultrastructure resulted from rinsing with the Sn-containing solutions. SnF2 appears to be the most effective type of fluoride to reduce initial bacterial colonization in situ. The observed effects primarily have to be attributed to the stannous ions’ content.
... 316L Stainless steels (SS) have been widely used in various biomedical applications due to their good corrosion resistance, high mechanical strength and low price [1][2][3]. However, serious pitting corrosion of 316L SS is commonly observed, which leads to the destruction of materials by toxic ions released into the body from the 316L SS implant, and the metal is likely encapsulated by fibrous tissue [4][5][6][7]. Researchers extensively reported that modification/coating of 316L SS by inorganic biomaterials such as calcium phosphate, hydroxyapatite (HA) and bio glass coatings prevents the release of ions. They also show excellent strength, bone growth, crack roughness and highly smooth finish of surface [8,9]. ...
This work elucidates the antibacterial and corrosion protection efficiency of Mg substituted hydroxyapatite/multiwalled carbon nanotube composite coated 316L SS implant. A novel spray deposition method was adopted for the composite coating. The fabricated HA/MWCNT and Mg-HA/MWCNT films were characterized by X-ray diffraction, Fourier transform infrared spectroscopy and Field emission scanning electron microscopy. Mg addition played a significant role to decrease the crystallinity and increase the surface area of the HA/MWCNT composite. The vibrational spectra show shift in the P–O stretching vibration of PO4³⁻ group at 1127 cm⁻¹ after Mg substitution. The morphology of Mg-HA/MWCNT coating revealed spherical shaped apatite crystals with interconnected network like microstructure which is beneficial for bone tissue engineering. The Mg-HA/MWCNT nanocomposite revealed a better antibacterial activity against E.coli with a high zone of inhibition of 16 mm. The electrochemical behaviour of the Mg-HA/MWCNT/SS in simulated body fluid demonstrated an increased polarization resistance with decreased current density (icorr). Compared to bare HA/CNT coated implant, the Mg ions substituted implant revealed superior corrosion resistance with a lower corrosion penetration rate. Thus, the bioactive Mg-HA/MWCNT composite coated 316L SS implant developed with superior antibacterial and anticorrosion properties is considered as the potential aspirant for orthopaedic applications.
The primary objective of this study was to analyze the effects of fluoride treatment with cyclic variations in pH on the structure of stoichiometric hydroxyapatite (HAp), calcium-deficient HAp (CDHAp), and carbonated HAp (CHAp) powders. The structures of HAp, CDHAp, and CHAp before and after fluoride treatment were investigated using X-ray diffraction, Fourier-transform infrared, Raman, and nuclear magnetic resonance spectroscopic analyses. The fluoride treatment with cyclic variations in pH increased the calcium deficiency in HAp and CHAp but decreased in CDHAp. During fluoride treatment, fluoridated CDHAp or fluoridated calcium-deficient CHAp was formed on the surface of the HAp samples via dissolution and crystal growth, accompanied by the selective elution of component ions and partial substitution of OH⁻ groups in the HAp hexagonal lattice with F⁻ ions. No evidence of the formation of Ca(OH)2 and OH⁻ groups outside the HAp crystal lattice was obtained. A new perspective on the formation of structured water at the surface termination of the OH columns (disordered region), with possible interactions with adsorbed water molecules or nonspecifically adsorbed F⁻ ions was provided. The top surface of the fluoridated CDHAp consisted of an amorphous fluoride-rich hydrated layer, which included calcium phosphate and CaF2.
Introduction:
The aim of this in vitro study was to assess the suitability of high-resolution time-of-flight secondary ion mass spectrometry (ToF-SIMS) for visualizing cross-sectional changes in human enamel microstructure and chemical composition during treatment and remineralization cycling of artificially generated caries lesions underneath an artificial plaque.
Methods:
Treatments consisted of exposure to twice daily toothpaste/water slurries prepared from 0, 1,100, and 5,000 μg/g fluoride (F) NaF/silica toothpastes. In addition, treatments with slurries prepared from 1,100 μg/g F SnF2/silica toothpastes were done using 44Ca in the remineralization solution to allow for differentiation of newly formed mineral and exploration of incorporated metal dopants using ToF-SIMS. Complementary microhardness, scanning electron microscopy, and high-resolution transmission electron microscopy (HR-TEM) investigations were performed on enamel cross sections.
Results:
HR-TEM was used for the first time to determine the change in crystallinity during remineralization revealing distinct microstructural zones within one lesion. Chemical mapping using ToF-SIMS demonstrated that the distribution of F, while observed primarily in the new mineral phase, was widespread throughout the lesion with 44Ca substantially limited to the remineralizing mineral. Both penetrated the inter-rod spaces of the sound enamel illustrating how acid damage propagates into the native mineral as the caries lesion deepens. HR-TEM examination revealed different regions within the lesion characterized by distinct micro- and ultrastructures. Importantly, HR-TEM revealed a return of crystallinity following remineralization. F dose-response observations verified the ability of these high-resolution techniques to differentiate remineralization efficacy.
Conclusion:
The collective results provided new insights such as the visualization of F or calcium penetration pathways, as well as new tools to study the caries process.
The relationship between crystal facet and anticorrosion performance is studied by controlling the microstructure of hydroxyapatite (HAP). HAP in shape of rectangular plate exposing (002) facets promotes it exerting anticorrosion activity. The displacement reaction occurs between OH⁻ groups on (002) facet and corrosive Cl⁻, which reduces the corrosion rate. At the same time, anion exchange forms a stable chlorapatite to further enhance shielding effect of coatings. Moreover, self-supporting mechanism is proposed to explain the conversion between tetracalcium phosphate (TTCP) and HAP is benefited to making up for HAP consumed in the anticorrosion process. The rectangular plate-like HAP also prolongs the diffusion path of corrosive substances. Therefore, HAP with bare (002) facet raises the impedance by 110.3% compared with common HAP with exposed (300) facet. This research provides a direction for reducing the consumption and effective use of materials in the coatings industry.
Magnesium and its alloys are biodegradable materials with great potential for biomedical development; however, their high rate of degradation in biological environments limits the widespread application of these materials. In order to improve the corrosion resistance of magnesium alloy, a functional calcium phosphate coating was prepared on Mg–3Zn–0.5Zr–0.5Sr alloy by microarc oxidation (MAO) combined with chemical deposition of fluoridated hydroxyapatite (FHA). A dense calcium–phosphorus coating 6 μm thick composed of needle-shaped fluoridated hydroxyapatite formed on the surface of the MAO layer. The MAO-FHA coating exhibited good mineralization ability to induce hydroxyapatite deposition on its surface during degradation testing in simulated bodily fluids.
The etching behavior of polycrystalline synthetic hydroxyapatite samples has been evaluated to explore the protective impact of fluoride on a tooth-like model system. Etching rates before and after fluoridation with a NaF solution at pH 6 were determined by atomic force microscopy. Despite a very low F-concentration of ca. 0.2 at-% in the hydroxyapatite surface, a very strong effect on the acid-resistance can be observed. Depending on the crystal orientation, etching in a NaAc buffer at pH 4.5 was completely inhibited for at least 5 minutes. The major part of the surface withstood etching even for more than 23 minutes. These results give new insights into how the amount of incorporated fluoride in hydroxyapatite correlates with its protective impact.
There is no topically applicable low concentration fluoride delivery device available for caries prevention. This study was aimed to assess the use of a low concentration (1450 ppm) fluoride strip as an effective fluoride delivery system against enamel demineralization. The enamel surface composition and calcium-deficient hydroxyapatite or toothpaste treatments were investigated using X-ray photoelectron spectroscopy. In vitro enamel demineralization was assayed using a pH cycling model and the dissolution of calcium ions from the treated specimens was quantified using ion chromatography. After 24-hr fluoride-strip treatment, the enamel was covered with a CaF2 layer which showed a granular morphology of 1 μm in size. Below the CaF2 layer was a region of mixed fluorapatite and CaF2. Fluoride infiltrated extensively in enamel to produce highly fluorinated fluorohydroxyapatite. In comparison, low-fluoride-level fluorinated fluorohydroxyapatite was formed on the enamel specimen exposed to toothpaste. The treatments with the fluoride strip as short as 1 hr significantly inhibited enamel demineralization. The fluoride strip was effective for topical fluoride delivery and inhibited in vitro demineralization of enamel by forming CaF2 and fluoride-containing apatites at the enamel surface. It exhibited the potential as an effective fluoride delivery device for general use in prevention of caries.
This study aimed to: determine the Mg and CO3 distribution in the outer (surface), middle, and inner (closest to the enamel-dentin junction, EDJ) layers of human enamel; and determine the factors affecting the incorporation of Mg into synthetic apatites and the consequence of such incorporation on the properties of the apatites. Results demonstrated that the concentrations of Mg, CO3, and organic components increased from the surface to the inner layers close to the EDJ and a difference in crystallinity from the outer to the inner layers. Initial results indicated that the extent of dissolution of the inner layer enamel is greater than that in the outer or surface enamel. Results on synthetic apatites showed the following: (1) Limited Mg incorporation into apatite was dependent on solution [Mg/Ca] molar ratio, temperature, pH, and the presence of CO3 or fluoride (F); (2) incorporation of Mg causes reduction in crystallinity and an increase in the extent of dissolution of the apatite; (3) the negative effect of Mg on the properties of apatites is synergistic to that of CO3 and antagonistic to that of F; and (4) exposure to acid of Mg-containing apatites causes the dissolution of Mg-rich apatite and precipitation of Mg-poor apatite. The observed decrease in the [Mg/Ca] of enamel and synthetic apatites after acid exposure may explain the observed 'preferential loss' of Mg and CO3 in the initial stages of caries.
Hydroxyapatite substrates are common biomaterials, yet samples of natural teeth do not meet the demands for well-defined, highly reproducible properties. Pellets of hydroxyapatite were produced via the Field Assisted Sintering Technology (FAST) as well as via pressureless sintering (PLS). The applied synthesis routes provide samples of very high density (95% - 99% of the crystallographic density) and of very low surface roughness (lower than 1 nm when averaged per 1 µm(2)). The chemical composition of the raw material (commercial HAP powder) as well as the crystalline structure is maintained by the sintering processes. These specimens can therefore be considered as promising model surfaces for studies on the interactions of biomaterial with surfaces of biological relevance, as demonstrated for the adsorption of BSA proteins.
Hydroxy apatite ceramic blocks of varying density have been prepared from a commercial powder. The elastic properties, fracture
toughness, strength and sub-critical crack growth of these materials have been investigated. Young's modulus for the nearly
fully dense material is 112 GPa while the compressive strength is about 800 MPa. For the same material the strength and fracture
toughness under dry conditions are 115 MPa and 1.0 MPa m1/2, respectively. Substantial slow crack growth was found under these conditions. Under wet conditions the values for strength
and fracture toughness drop to about 75% of their “dry” values. In this case very serious slow crack growth is present.
A combination of electronic structure calculations and interatomic potential-based methods are employed to study the uptake and segregation of fluoride ions from solution into the hydroxyapatite lattice. The hydroxyapatite potential model derived for this work accurately reproduces experimental properties and relative enthalpies of formation and is further validated by Density Functiontional Theory calculations of fluoride defects in bulk hydroxyapatite. Calculations of solid solutions of fluor- and hydroxy-apatite show that hydroxy groups are easily replaced by fluoride ions on thermodynamic grounds (ΔEx=−0.4 to −6.4 kJ mol−1), forming sheets of fluoride in the a/b plane. Molecular Dynamics simulations of the incorporation of fluoride into hydroxyapatite show that fluoride ions are also easily incorporated from solution into the surface of hydroxyapatite (ΔE=−193 kJ mol−1), but they do not segregate into the bulk crystal beyond approximately 10 Å. The hydroxy-groups, which remain aligned in the c-direction in the bulk material, due to the formation of pairs of OH− groups, are found to reverse randomly in the surface region when a vacuum interface is introduced. This surface region shows considerable relaxation and reconstruction of the calcium and phosphate sub-lattices, leading to the disintegration of the OH− pairs and hence their re-orientation. However, when the vacuum is filled with water, the surface relaxation is less extensive and the hydroxy groups remain aligned, although surface OH− groups dissolve into the water. When surface OH− groups are replaced by fluoride ions, this fluoride remains in the crystal lattice and further anchors surface calcium ions. As such this surface fluoride prevents the onset of apatite dissolution, although the fluorapatite layer is only superficial.
Electron spectroscopic depth profiles of the chemical composition of bovine incisors after fluoride treatment show that interactions are restricted principally to a hyperfine outermost layer of only 0.05 μm. In this zone, high fluoride concentrations of up to 20% are correlated with drastic surface enamel degradations. A mixed layer of calcium phosphates and calcium fluorides is formed.
The mechanisms of action of fluoride have been discussed controversially for decades. The cavity-preventive effect is often traced back to effects on demineralization. However, an effect on bacterial adhesion was indicated by indirect macroscopic studies. To characterize adhesion on fluoridated samples on a single bacterial level, we used AFM force-spectroscopy with bacterial probes to measure adhesion forces directly. We tested the adhesion of Streptococcus mutans, Streptococcus oralis and Staphylococcus carnosus onto smooth, high density hydroxyapatite surfaces, pristine and after treatment with fluoride solution. All bacteria species exhibit lower adhesion forces after fluoride treatment of the surfaces. These findings suggest that the decrease of adhesion properties is a further key factor for the cariostatic effect of fluoride besides the decrease of demineralization.
Dental caries has declined in the 40 years since fluoridated toothpastes were introduced. Much has been learned about why fluoride is so effective and how this knowledge can be used to optimise programmes for caries prevention. Fluoride works through enhancing the remineralisation of early stages of caries and by inhibiting demineralisation, which would lead to dental caries. Remineralisation involves the deposition of calcium phosphates from saliva to rebuild partly dissolved enamel crystallites. When fluoride is incorporated the dissolution of these reinforced crystallites will be reduced during a subsequent sugar-induced and bacteria-mediated acid attack. Fluoride works primarily when it is present in the oral cavity. Based on our understanding of the fluoride mode of action the following advice can be given from clinicians to their patients: The fluoride concentration in oral products is related to efficacy but the concentration does not necessarily need to be high to be efficacious. Fluoride availability throughout the day is important; this can be achieved when fluoride products are used as part of the daily hygiene routine (F-brushing or rinsing). Alternatively, when fluoride is provided in the drinking water or through professionally applied F-varnishes or gels, the patient will benefit without requiring daily compliance to its use. The latter methods are particularly effective as additional treatments in high caries individuals.
High-resolution gold-valence-band photoemission spectra were obtained by the use of monochromatized Al Kα radiation and a single-crystal specimen. After background and scattering corrections were made, the results were compared directly with broadened theoretical density-of-states functions. The following conclusions were drawn: (i) Relativistic band-structure calculations are required to fit the spectrum. (ii) Both the Korringa-Kohn-Rostoker calculation of Connolly and Johnson and the relativistic-augmented-plane-wave calculation by Christensen and Seraphin give density-of-states results that (after broadening) follow the experimental curve closely. (iii) Of the theoretical functions available to date, those with full Slater exchange agree best with experiment (perhaps because of a cancellation of errors). Fractional (2/3 or 5/6) exchange gives d bands that are too wide. (iv) Eastman's 40.8-eV ultraviolet photoemission spectrum is similar to the x-ray spectrum, suggesting little dependence on photon energy above 40 eV. (v) Both (ii) and (iv) imply an absence of strong matrix-element modulation in the photoemission spectrum of gold.
Aim:
The study aimed to investigate the effect of a customary fluoride solution, containing sodium fluoride and amine fluoride, on initial biofilm formation on enamel and dentin in situ compared directly to chlorhexidine.
Methods:
Bovine enamel and dentin specimens were mounted on maxillary splints carried by 9 subjects. After 1 min of pellicle formation, rinses with tap water (control), chlorhexidine (meridol med CHX 0.2%, GABA) and a fluoride mouthrinse (elmex, GABA) were performed for 1 min. Subsequently, the slabs were carried for another 8 h. The adherent bacteria were determined by DAPI staining, live-dead staining and determination of colony-forming units after desorption; glucan formation was visualized with concanavalin A. Additionally, energy-dispersive X-ray spectroscopy (EDX) analysis of the in situ biofilm layers was conducted, and contact angle measurements were performed. Statistical evaluation was performed by means of the Kruskal-Wallis test followed by the Mann-Whitney U test (p < 0.05).
Results:
In the control group, significantly higher amounts of adherent bacteria were detected on dentin (4.8 × 10(6) ± 5.4 × 10(6) bacteria/cm(2)) than on enamel (1.2 × 10(6) ± 1.5 × 10(6) bacteria/cm(2), DAPI). Chlorhexidine significantly reduced the amount of adherent bacteria (dentin: 2.8 × 10(5) ± 3.4 × 10(5) bacteria/cm(2); enamel: 4.2 × 10(5) ± 8.7 × 10(5) bacteria/cm(2)). Rinses with the fluoride solution also significantly reduced bacterial adherence to dentin (8.1 × 10(5) ± 1.5 × 10(6) bacteria/cm(2)). Fluoride could not be detected by EDX analysis of the biofilms. Fluoride mouthrinsing did not influence the wettability of the pellicle-covered enamel surface.
Conclusion:
In addition to the reduction of demineralization and antibacterial effects, fluorides inhibit initial biofilm formation on dental hard tissues considerably, especially on dentin.
The paper focuses on the dependence of microstructure and elastic properties of sintered hydroxyapatite on the processing parameters. Several specimens were sintered in conventional furnace at various temperatures. Elastic moduli were measured ultrasonically and information about the microstructure was recovered from these data and then verified by analysis of microphotographs. It was obtained that the average shape of pores becomes more round as the sintering temperature increases. That leads, in particular, to higher fracture toughness of the material since the stress concentration near pores is reduced.
Molecular dynamics simulations of the incorporation of fluoride into hydroxyapatite show that fluoride ions are easily incorporated from solution into the surface of hydroxyapatite, but although the formation of a fluorapatite film prevents the onset of apatite dissolution, this fluoride does not segregate into the bulk material but remains at the surface. These findings significantly increase our understanding of the role of fluoride in stabilizing hydroxyapatite in tooth enamel and suggest that only repeated exposure to fluoride will have a lasting effect on the tooth enamel structure and its resistance toward dissolution and dental decay.
Reproducible high resolution spectra of nonconducting minerals of interest in biological calcification have been obtained by x‐ray photoelectron spectroscopy, utilizing a technique we call biased referencing, a combination of gold decoration and electron charge neutralization of the samples. Minerals examined were hydroxyapatite [Ca 1 0 (PO 4 ) 6 (OH) 2 ], fluorapatite [Ca 1 0 (PO 4 ) 6 F 2 ], calcium carbonate (CaCO 3 ), octacalcium phosphate [Ca 8 H 2 (PO 4 ) 6 ∙ 5H 2 O], monetite (CaHPO 4 ), brushite (CaHPO 4 ∙ 2H 2 O), calcium pyrophosphate (Ca 2 P 2 O 7 ), and an amorphous calcium phosphate synthesized at neutral pH (Ca/P=1.47). Samples were pressed into thin wafers onto which a small gold dot was affixed by vacuum deposition. Biased referencing provided calibration of sample core level ionizations (O 1s, Ca 2p, P 2p, F 1s, and C 1s) to the Fermi level of Au 4f 7 / 2 set at a conductive value of 84.0 eV. Measured binding energies represent initial data for these gold‐decorated insulators and may be used to compare data similarly generated from mineralized biological tissues.
Atomic subshell photoionization cross sections and asymmetry parameters are calculated with the Hartree-Fock-Slater one-electron central potential model (dipole approximation) for all elements Z = 1–103. The cross-section results are plotted for all subshells in the energy region 0–1500 eV, and cross sections and asymmetry parameters are tabulated for selected energies in the region 10.2–8047.8 eV. In addition, more detailed graphs are given for the 4d (Z = 39–71) and 5d (Z = 64–100) subshell cross sections in the vicinity of the Cooper minimum. These data should be particularly useful for work based on spectroscopic investigations of atomic subshells using synchrotron radiation and/or discrete line sources.
Structural and chemical changes that arise from fluoridation of hydroxyapatite (Ca(5)(PO(4))(3)OH or "HAp"), as representing the synthetic counterpart of tooth enamel, are investigated by X-ray photoelectron spectroscopy (XPS). Elemental depth profiles with a depth resolution on the nanometer scale were determined to reveal the effect of fluoridation in neutral (pH = 6.2) and acidic agents (pH = 4.2). With respect to the chemical composition and the crystal structure, XPS depth profiling reveals different effects of the two treatments. In both cases, however, the fluoridation affects the surface only on the nanometer scale, which is in contrast to recent literature with respect to XPS analysis on dental fluoridation, where depth profiles of F extending to several micrometers were reported. In addition to the elemental depth profiles, as published in various other studies, we also present quantitative depth profiles of the compounds CaF(2), Ca(OH)(2), and fluorapatite (FAp) that were recently proposed by a three-layer model concerning the fluoridation of HAp in an acidic agent. The analysis of our experimental data exactly reproduces the structural order of this model, however, on a scale that differs by nearly 2 orders of magnitude from previous predictions. The results also reveal that the amount of Ca(OH)(2) and FAp is small compared to that of CaF(2). Therefore, it has to be asked whether such narrow Ca(OH)(2) and FAp layers really can act as protective layers for the enamel.
A combination of electronic structure calculations and interatomic potential-based methods are employed to study the uptake and segregation of fluoride ions from solution into the hydroxyapatite lattice. The hydroxyapatite potential model derived for this work accurately reproduces experimental properties and relative enthalpies of formation and is further validated by Density Functiontional Theory calculations of fluoride defects in bulk hydroxyapatite. Calculations of solid solutions of fluor- and hydroxy-apatite show that hydroxy groups are easily replaced by fluoride ions on thermodynamic grounds (∆Ex=–0.4 to –6.4 kJ mol–1), forming sheets of fluoride in the a/b plane. Molecular Dynamics simulations of the incorporation of fluoride into hydroxyapatite show that fluoride ions are also easily incorporated from solution into the surface of hydroxyapatite (∆E=–193 kJ mol–1), but they do not segregate into the bulk crystal beyond approximately 10 Å. The hydroxy-groups, which remain aligned in the c-direction in the bulk material, due to the formation of pairs of OH– groups, are found to reverse randomly in the surface region when a vacuum interface is introduced. This surface region shows considerable relaxation and reconstruction of the calcium and phosphate sub-lattices, leading to the disintegration of the OH– pairs and hence their re-orientation. However, when the vacuum is filled with water, the surface relaxation is less extensive and the hydroxy groups remain aligned, although surface OH– groups dissolve into the water. When surface OH– groups are replaced by fluoride ions, this fluoride remains in the crystal lattice and further anchors surface calcium ions. As such this surface fluoride prevents the onset of apatite dissolution, although the fluorapatite layer is only superficial.
Defect sites on bone minerals play a critical role in bone remodeling processes. We investigated single crystal hydroxyapatite (100) surfaces bearing crystal defects under acidic dissolution conditions using real-time in situ atomic force microscopy. At defect sites, surface structure-dependent asymmetric hexagonal etch pits were formed, which dominated the overall dissolution rate. Meanwhile, dissolution from the flat terraces proceeded by stochastic formation of flat bottom etch pits. The resulting pit shapes were intrinsically dictated by the HAP crystal structure. Computational modeling also predicted different step energies associated with different facets of the asymmetric etch pits. Our microscopic observations of HAP dissolution are significant for understanding the effects of local surface structure on the bone mineral remodeling process and provide useful insights for the design of novel therapies for treating osteoporosis and dental caries.
In 1983/1984, a study of bone mass and fractures was begun in 827 women aged 20-80 years in three rural Iowa communities selected for the fluoride and calcium content of their community water supplies. The control community's water had a calcium content of 67 mg/liter and a fluoride content of 1 mg/liter. The higher-calcium community had water with a calcium content of 375 mg/liter and a fluoride content of 1 mg/liter. The higher-fluoride community's water had 15 mg/liter of calcium and 4 mg/liter of fluoride naturally occurring. In 1988/1989, a follow-up study characterized the 684 women still living and available for study. Residence in the higher-fluoride community was associated with a significantly lower radial bone mass in premenopausal and postmenopausal women, an increased rate of radial bone mass loss in premenopausal women, and significantly more fractures among postmenopausal women. There was no difference in the 5-year relative risk of any fracture in the higher-calcium community versus the control community; however, the relative risk was 2.1 (95% confidence interval (CI) 1.0-4.4) in women in the higher-fluoride community compared with women in the control community. There was no difference in the 5-year risk of wrist, spine, or hip fracture in the higher-calcium community versus the control community; however, the 5-year relative risk for women in the higher-fluoride community, compared with women in the control community, was 2.2 (95% CI 1.1-4.7). Estimates of risk were adjusted for age and body size.
After in vitro treatments of bovine enamel with NaF/Zonyl FSC and NH4F in-depth profiles of the chemical composition of the outermost zone of surface enamel were obtained with electron spectroscopy. The most important finding of the study was that after the treatments with nearly neutral as well as slightly acidic solutions of NH4F the ammonium cation was found on the enamel surfaces. Similarly a Zonyl FSC layer was detected on the enamel surfaces after the NaF/Zonyl FSC treatment, while sodium ion was never detected on the enamel surfaces. The implication is that the sorption of these cations plays a role in fluoride-enamel interactions.
After in vitro treatment of surface enamel (bovine incisors) with aqueous solutions of various fluorides (pH 4.0, 5.0, 6.0) the elemental composition of the outermost zone was investigated by electron spectroscopy. In acidic solution (pH 4.0 and 5.0) calcium fluoride is the main reaction product. It precipitates on surface enamel and in prism pits. The exception is pH 4.0 stannous fluoride solution which has no degrading effect on surface enamel, a complex tin compound being precipitated. The nearly neutral applications (pH 6.0) had very little effect on the composition of surface enamel.Copyright © 1988 S. Karger AG, Basel
The effect of tooth-bound fluoride (F) on enamel caries formation was investigated under the condition that loosely bound F was essentially absent. Eighteen thin enamel sections, prepared from the lingual or buccal surfaces of extracted human molars, were embedded in acrylic resin with the enamel surfaces exposed. The sections were placed in a pH 7 remineralizing solution (RS; 1.2 mmol/l Ca, 0.72 mmol/l P, 30 mmol/l KCl, 50 mmol/l HEPES) for 5 days, and were randomly divided into 3 groups: (1) control group that received no treatment, (2) acidulated phosphate fluoride (APF) group that received 5 cycles of a 4 min treatment with APF gel followed by immersion in the RS for 2 days (RS changed daily) and (3) dicalcium phosphate dihydrate (DCPD) - APF group that received 5 cycles of a 4-min pH 2.1 DCPD-forming solution followed by 4 min APF gel and then placed in the RS for 2 days. After the treatment cycles, the sections were washed in a constant composition F titration system to remove loosely bound F. An in vitro model, which consisted of cycles of de- (6 h) and remineralization (18 h) each day for 5 days, was used to produce caries-like lesions in the specimens. The DeltaZ (mineral loss) values, measured by quantitative microradiography, of the lesions formed in the three groups were (mean +/- standard deviation; n = 6) 91.2+/-12.3 microm for the control group, 41.3+/-10.1 microm for the APF group and 21.2+/-4.8 microm for the DCPD-APF group. The same system produced lesions in untreated shark enamel with a mean DeltaZ of 4.4+/-0.3 microm (n = 12). One-way fixed-effects ANOVA indicated that mineral loss was significantly different among the different groups (p<0.05). The results showed that enamel resistance to lesion formation increased with increasing tooth-bound F content. Shark enamel was much more resistant to demineralization than human enamel.
Electron and atomic force microscopy techniques have been applied to characterize both the in vitro deposition intensity and the microstructure of the KOH-soluble fluoride precipitates on human dental enamel. The study was focused on the effects of amine fluoride, sodium fluoride and sodium monofluorophosphate having a fluoride concentration of 0.1% F in acidulated and aqueous solutions. Under certain conditions, fluoride globules were formed within an initiation time of less than 20 s. This result supports the potential significance of this process for the cariostatic action of fluorides during dentifrice use. The deposition intensity seems to be dependent on the availability of Ca and F ions on the dental surface. A nanocrystalline calcium fluoride-like microstructure was revealed, with an additional phosphorus and oxygen incorporation as a function of the treatment time.
Using laser Raman microprobe spectroscopy, we have characterized the degree of hydroxylation and the state of atomic order of several natural and synthetic calcium phosphate phases, including apatite of biological (human bone, heated human bone, mouse bone, human and boar dentin, and human and boar enamel), geological, and synthetic origin. Common belief holds that all the studied phases are hydroxylapatite, i.e., an OH-containing mineral with the composition Ca10(PO4)6(OH)2. We observe, however, that OH-incorporation into the apatite crystal lattice is reduced for nanocrystalline samples. Among the biological samples, no OH-band was detected in the Raman spectrum of bone (the most nanocrystalline biological apatite), whereas a weak OH-band occurs in dentin and a strong OH-band in tooth enamel. We agree with others, who used NMR, IR spectroscopy, and inelastic neutron scattering, that-contrary to the general medical nomenclature-bone apatite is not hydroxylated and therefore not hydroxylapatite. Crystallographically, this observation is unexpected; it therefore remains unclear what atom(s) occupy the OH-site and how charge balance is maintained within the crystal. For non-bone apatites that do show an OH-band in their Raman spectra, there is a strong correlation between the concentration of hydroxyl groups (based on the ratio of the areas of the 3572 deltacm(-1) OH-peak to the 960 deltacm(-1) P-O phosphate peak) and the crystallographic degree of atomic order (based on the relative width of the 960 deltacm(-1) P-O phosphate peak) of the samples. We hypothesize that the body biochemically imposes a specific state of atomic order and crystallinity (and, thus, concentration of hydroxyl) on its different apatite precipitates (bone, dentin, enamel) in order to enhance their ability to carry out tissue-specific functions.
This paper discusses the concept of critical pH for dissolution of enamel in oral fluids. The critical pH does not have a fixed value but rather is inversely proportional to the calcium and phosphate concentrations in the solution. The paper also discusses why teeth dissolve in acid, why remineralization of white-spot caries lesions is possible and why remineralization of teeth eroded by acid is not possible.
The aim of this study was to develop a molecular-based structural model of human teeth after fluoridation with a commonly used amine fluoride, which is highly significant for understanding the effectiveness of topical fluoridation.
This multi method study used XPS, MAS-NMR and Raman-spectroscopy measurements in order to analyze powdered synthetic hydroxylapatite (HAp), powdered human enamel samples and human enamel pieces treated with amine fluoride (Elmex) fluid) in vitro.
The results lead to a complete structural characterization of the fluoridation products. A three layer composition of calcium hydroxide, calcium fluoride and an apatite species was identified.
The top surface CaF(2) layer acts as a fluoride reservoir and covers a layer of antimicrobial effective Ca(OH)(2). Ca(OH)(2) is a well-known therapeutic agent. However, up to now Ca(OH)(2) has not been described as a reaction product after topical fluoridation. Below the Ca(OH)(2) layer an acid resistant apatite species (FAp) was detected which reached directly into the bulk enamel HAp species. The three layer composition identified helps to understand the influence of fluoride application in the pathogenic mechanisms of tooth decay. Each component in this newly suggested structure model has a specific function, which explains how topical fluoridation of enamel reduces dental caries and influences its pathogenic mechanisms.
Mottled Teeth: An Endemic Developmental Imperfection of the Enamel of the Teeth Heretofore Unknown in the Literature of Dentistry
- G V Black
- F S Mckay
Black, G. V.; McKay, F. S. Mottled Teeth: An Endemic
Developmental Imperfection of the Enamel of the Teeth Heretofore
Unknown in the Literature of Dentistry. Dent. Cosmos 1916, 58, 129−
156.
Geographical Distribution of the Mottled Teeth in Japan
- T Masaki
- K Mumura
Masaki, T.; Mumura, K. Geographical Distribution of the Mottled
Teeth in Japan. Shikwa Gakuho 1931, 36, 875−893.
- C Zeitz
- T Faidt
- S Grandthyll
- H Haḧl
- N Thewes
- C Spengler
- J Schmauch
- M J Deckarm
- C Gachot
- H Natter
- M Hannig
- F Muller
- K Jacobs
Zeitz, C.; Faidt, T.; Grandthyll, S.; Haḧl, H.; Thewes, N.;
Spengler, C.; Schmauch, J.; Deckarm, M. J.; Gachot, C.; Natter, H.;
Hannig, M.; Muller, F.; Jacobs, K. Synthesis of Hydroxyapatite
Substrates: Bridging the Gap between Model Surfaces and Enamel.
ACS Appl. Mater. Interfaces 2016, 8, 25848−25855.
- K Kwon
- E Wang
- A Chung
- N Chang
- E Saiz
- U Choe
- M Koobatian
- S Lee
Kwon, K.; Wang, E.; Chung, A.; Chang, N.; Saiz, E.; Choe, U.;
Koobatian, M.; Lee, S. Defect Induced Asymmetric Pit Formation on
Hydroxyapatite. Langmuir 2008, 24, 11063−11066.