Fluoroapatite/polymer-composite materials based on the microstructure of shark teeth and dulling liquids for intraoral tooth visualization

Ziel der Arbeit war die Entwicklung von biomimetischen synthetischen Fluorapatit/Polymer-Dentalkompositen für die Anwendung als Zahnfüllungsmaterial. Überdies wurden Mattierungsdispersionen entwickelt, um von Zähnen digitale 3D-Modelle zu erzeugen. Als biologische Modellsysteme wurden Haifisch- und Krokodilzähne chemisch, strukturell und mechanisch analysiert. Der Zahnschmelz (Enameloid) der Zähne von rezenten Haifischen besteht aus Fluorapatit, Ca5(PO4)3F, wobei der Fluoridgehalt mit 3,1 Gew% nahezu so hoch ist wie in geologischen Fluorapatit-Kristallen (3,64 Gew%). Enameloid zeigt verglichen mit Dentin eine höhere Kristallinität des Mineralanteils. Die Fluorapatit-Kristallite im Enameloid sind sehr dünn (50-80 nm) und sehr lang (> 1 μm), und ihn Bündeln organisiert. Diese Bündel lassen sich in drei Typen einteilen, die sich in ihrer Orientierung unterscheiden: Es gibt umlaufende, radiale und axiale Kristallitbündel. Jedes Kristallitbündel, unabhängig von seiner Orientierung, ist umhüllt von einer dünnen Schicht einer organischen Matrix. Den äußersten Teil des Enameloids bildet der sogenannte "shiny layer". Dieser besteht im unbehandelten Zustand aus ungeordneten polyedrischen Kristalliten. Obwohl es innerhalb des Enameloids eine starke strukturelle Anisotropie gibt, sind die lokalen mechanischen Eigenschaften weitgehend isotrop. Vickers-Mikrohärtemessungen und Nanoindentationsexperimente zeigen, dass Enameloid etwa sechsmal härter ist als Dentin. Die Härte von Haifischzähnen (Fluorapatit) und humanen Zähnen (Hydroxylapatit) ist vergleichbar, obwohl Fluorapatit als reines Mineral härter ist als Hydroxylapatit. Im Gegensatz zu rezenten Haifischzähnen haben Dentin und Enameloid von fossilen Haifischzähnen eine vergleichbare chemische Zusammensetzung (Fluorapatit). Die Enameloid-Mikrostruktur der fossilen Haifischzähne ist sehr gut erhalten und vergleichbar mit der in rezenten Haifischzähnen. Im Dentin fossiler Haifischzähne sind mittels Rasterelektronenmikroskopie mineralische Substanzen zu erkennen, die im Dentin rezenter Zähne nicht vorhanden sind. Dies könnte auf einen Rekristallisationsprozess während der Diagenese zurückzuführen sein. Krokodilzähne bestehen aus einem nanokristallinen Hydroxylapatit. Rasterelektronenmikroskopische Aufnahmen zeigen, dass die Enamelschicht verglichen mit humanen Zähnen und Haifischzähnen sehr dünn ist (100-200 μm). Die Kristallite im Enamel sind alle senkrecht zur Zahnoberfläche angeordnet, wobei es keine definierten Kristallitbündel gibt. Durch Thermogravimetrie und Synchrotron-Mikrocomputertomographie wurde ermittelt, dass Enamel den größten Mineralanteil besitzt, gefolgt von Dentin und dem Wurzelzement. Enameloid rezenter Haifischzähne wurde als Modellsystem für die Entwicklung biomimetischer Dentalkomposite ausgewählt, weil es, im Vergleich zu Hydroxylapatit, aus dem säureresistenteren Fluorapatit besteht. Zudem kann Fluorapatit synthetisch gut in unterschiedlichen Kristallit-Morphologien dargestellt werden. Zur Darstellung von biomimetischen Kompositen wurde Fluorapatit in unterschiedlichen Kristallit-Morphologien und -Größen hydrothermal synthetisiert. Kombiniert wurden diese Kristallite mit verschiedenen Methacrylat-Monomeren. Die Komposite wurden entweder durch Ultrazentrifugation des Fluorapatits in einer flüssigen Comonomermischung basierend auf Methylmethacrylat und anschließender thermisch initiierter Polymerisation mit Dibenzoylperoxid als Radikalstarter dargestellt, oder durch eine Polymerisation während der Ultrazentrifugation bei Raumtemperatur mit dem Zusatz eines tertiären Amins. Rasterelektronenmikroskopische Aufnahmen zeigen, dass ein kompaktes Material vorzugweise mit dem Amin-Zusatz erhalten wurde. Die Vickers-Härte der Komposite ist höher (0,3-0,4 GPa) als die der reinen Polymermatrix (0,2 GPa), wobei Haifischzähne noch härter sind (Dentin: 0,5-0,7 GPa; Enameloid: 3-4 GPa). Thermogravimetrische Analysen zeigen, dass der Mineralanteil der synthetischen Komposite bei ca. 60-70 Gew% liegt. Mattierungsdispersionen wurden auf Basis von Titandioxid (Anatas) und Stärkekleister bzw. Polyvinylpyrrolidon als Haftmittel entwickelt. Ein dichter Mattierungsfilm auf unterschiedlichen dentalen Materialoberflächen wurde mittels Rasterelektronenmikroskopie gezeigt. Analysen des getrockneten Mattierungsfilms mit Thermogravimetrie und energiedispersiver Röntgen-spektroskopie zeigen, dass der organische Anteil mit Polyvinylpyrrolidon als Haftmittel höher ist als bei der Verwendung von Stärkekleister. Erste klinische Analysen mit einer 3D-Kamera zeigen, dass geeignete Modelle erzeugt werden konnten. Zur Konservierung der Mattierungsdispersionen eignet sich Chlorhexidin, das bereits kommerziell in vielen antiseptischen Mundspülungen verwendet wird. Zusammenfassend behandelt die vorliegende Arbeit zwei Aspekte der modernen zahnmedizinischen Materialforschung. Ein Thema war die Charakterisierung von natürlichen Modellsystemen, d.h. Haifisch- und Krokodilzähnen, mit verschiedenen Analysemethoden. Die daraus resultierenden Ergebnisse können genutzt werden, um innovative und biomimetische Zahnersatzmaterialien zu entwickeln. Zudem wurden Mattierungsdispersionen dargestellt, die die Grundlage für moderne 3D-Abbildungstechniken sind. Dies kann dazu genutzt werden, um schnelle und exakte Zahnabdrücke zu erhalten.The aim of this study was the development of synthetic biomimetic fluoroapatite/polymer-composites, designed for their application as tooth restoration material. Additionally, dulling liquids for digital imaging of teeth were developed. Shark teeth and crocodile teeth were chosen as model systems, and thoroughly analyzed with various analytical methods. The outer layer of recent shark teeth (enameloid) consists of fluoroapatite, Ca5(PO4)3F, with a fluoride content of 3.1 wt%, which is nearly as high as in geological fluoroapatite crystals (3.64 wt%). The enameloid layer shows a higher crystallinity of the mineral phase compared to dentin. Enameloid consists of thin (50-80 nm) and long (> 1 μm) crystallites of fluoroapatite which are hierarchically organized in bundles. Three types of bundles with different orientations are present within the enameloid: circumferential, radial and axial crystallite bundles. Each crystallite bundle, regardless of its orientation, is covered by a thin layer of organic matrix. The outermost layer of the enameloid consists of the so-called "shiny layer", which is composed of randomly oriented solid polyhedral crystallites. Although there is a strong structural anisotropy within the enameloid, the local mechanical properties are widely isotropic. Vickers microhardness tests and nanoindentation experiments indicated that enameloid is about six times harder than dentin. The hardness of shark teeth (fluoroapatite) and human teeth (hydroxyapatite) is comparable, although, as pure mineral, fluoroapatite is harder than hydroxyapatite. In contrast to recent shark teeth, enameloid and dentin of fossilized shark teeth show similar chemical compositions, i.e., nearly stoichiometric fluoroapatite. The enameloid microstructure of fossilized shark teeth has been well preserved and is comparable to the microstructure of recent shark teeth. Scanning electron micrographs of dentin of fossilized shark teeth showed that mineral substances are present, which may be due to recrystallization processes during diagenesis. Crocodile teeth consist of nanocrystalline hydroxyapatite. Scanning electron micrographs showed that the enamel layer is very thin (100-200 μm), in contrast to thicker layers founds in human and shark teeth. The crystallites of the enameloid are oriented perpendicularly to the tooth surface, and no defined crystallite bundles can be observed. Enamel has the highest mineral content, followed by dentin and cementum as shown by thermogravimetry and synchrotron microcomputer tomography. Shark tooth enameloid was selected as a model system for the development of biomimetic dental composites because it consists of fluoroapatite which has a higher acid stability than hydroxyapatite. Additionally, fluoroapatite can be readily synthesized in various crystallite morphologies. For the preparation of biomimetic composites, fluoroapatite was hydrothermally synthesized in different crystallite morphologies and sizes. The crystallites were combined with different methacrylate monomers. The composites were then prepared using either of two routines. Firstly, through ultracentrifugation of fluoroapatite in a liquid comonomer mixture (based on methyl methacrylate), followed by a thermally initiated polymerization using dibenzoyl peroxide as a radical initiator. Alternatively, composites were synthesized via polymerization during ultracentrifugation at room temperature, with the second routine, utilizing the addition of a tertiary amine. Scanning electron micrographs showed that a compact material was achieved more effectively with the second routine, utilizing the addition of a tertiary amine. The composites showed higher Vickers hardness values (0.3-0.4 GPa) compared to the pure polymer matrix (0.2 GPa), whereas shark teeth are even harder (dentin: 0.5-0.7 GPa; enameloid: 3-4 GPa). The mineral content of the synthetic composites was 60-70 wt% as determined by thermogravimetry. Dulling liquids based on titanium dioxide (anatase) and a starch paste or polyvinylpyrrolidone were prepared. The resulting matting films densely covered different dental materials as shown by scanning electron microscopy. Furthermore, the dried matting films were analyzed by thermogravimetry and energy dispersive X-ray spectroscopy. In matting films which contained polyvinylpyrrolidone as adhesive the organic content is higher than with adhesives based on starch paste. First clinical analyses using 3D imaging showed that suitable models could be produced. Additionally, chlorhexidine can be used as a preservative because it is already found in commercially produced antiseptic mouthwashes. This work has covered two important aspects of modern dental material science: First, a thorough analysis of natural model systems (i.e., shark teeth and crocodile teeth) can foster research for innovative dental materials with improved durability and stability. Second, efficient matting films which are a prerequisite for modern, computer-based 3D imaging were developed. This can be used to obtain faster and more exact dental impressions

The composition of the dental pellicle: an updated literature review

BackgroundThe dental pellicle is a thin layer of up to several hundred nm in thickness, covering the tooth surface. It is known to protect the teeth from acid attacks through its selective permeability and it is involved in the remineralization process of the teeth. It functions also as binding site and source of nutrients for bacteria and conditioning biofilm (foundation) for dental plaque formation.MethodsFor this updated literature review, the PubMed database was searched for the dental pellicle and its composition.ResultsThe dental pellicle has been analyzed in the past years with various state-of-the art analytic techniques such as high-resolution microscopic techniques (e.g., scanning electron microscopy, atomic force microscopy), spectrophotometry, mass spectrometry, affinity chromatography, enzyme-linked immunosorbent assays (ELISA), and blotting-techniques (e.g., western blot). It consists of several different amino acids, proteins, and proteolytic protein fragments. Some studies also investigated other compounds of the pellicle, mainly fatty acids, and carbohydrates.ConclusionsThe dental pellicle is composed mainly of different proteins, but also fatty acids, and carbohydrates. Analysis with state-of-the-art analytical techniques have uncovered mainly acidic proline-rich proteins, amylase, cystatin, immunoglobulins, lysozyme, and mucins as main proteins of the dental pellicle. The pellicle has protective properties for the teeth. Further research is necessary to gain more knowledge about the role of the pellicle in the tooth remineralization process

Ca2+ release and buffering effects of synthetic hydroxyapatite following bacterial acid challenge

Background Synthetic particulate hydroxyapatite (HAP; Ca-5(PO4)(3)(OH)) is used as ingredient in oral care products but its effects on cariogenic biofilms are not clear yet. The primary mode of action of HAP may be acting as a calcium phosphate reservoir when deposited in oral biofilms and release Ca2+ and (hydrogen) phosphate ions upon bacterial acid challenge. The aim of this in vitro study was to test this hypothesis by investigating release of Ca2+ ions and potential buffering effects from HAP upon bacterial acid challenge in planktonic cultures and biofilms of Streptococcus mutans. Methods Planktonic cultures of S. mutans were grown in BHI broth with 1% sucrose or with additional 5% HAP or 5% silica for up to 48 h. Separately, biofilms of S. mutans were grown in BHI for 72 h in total. After 24 h of this biofilm culture, either BHI alone or BHI with additional 0.5% HAP or 0.5% silica was added. After 48 h, BHI with 1% sucrose was added to allow bacterial acid formation. Ca2+ release was determined colorimetrically and pH measurements were performed using a pH electrode. For statistical analysis, non-parametrical procedures were applied (n >= 10; Mann-Whitney U test; alpha = 0.05). Results Relevant release of Ca2+ was only evident in planktonic cultures or biofilms with HAP but not in both other groups (p <= 0.001). In suspended biofilms with HAP, median pH was 4.77 after 72 h and about 0.5 pH units higher as compared to both other groups (4.28 or 4.32, respectively; p <= 0.001). Conclusions Under the tested conditions, synthetic HAP releases Ca2+ ions upon bacterial acid challenge and may also show some buffering capacity but further studies are needed to investigate whether the concentrations tested here can also be reached clinically in dental biofilms

Caries-preventing effect of a hydroxyapatite-toothpaste in adults: a 18-month double-blinded randomized clinical trial

BackgroundDental caries is a worldwide challenge for public health. The aim of this 18-month double-blinded, randomized, clinical trial was to compare the caries-preventing effect of a fluoride-free, hydroxyapatite toothpaste (test) and a toothpaste with sodium fluoride (1450 ppm fluoride; positive control) in adults.MethodsThe primary endpoint was the percentage of subjects showing no increase in overall Decayed Missing Filled Surfaces (DMFS) index. The study was designed as non-inferiority trial. Non-inferiority was claimed if the upper limit of the exact one-sided 95% confidence interval for the difference of the primary endpoint DMFS between test and control toothpaste was less than the predefined margin of non-inferiority (Δ ≤ 20%).ResultsIn total, 189 adults were included in the intention-to-treat (ITT) analysis; 171 subjects finished the study per protocol (PP). According to the PP analysis, no increase in DMFS index was observed in 89.3% of subjects of the hydroxyapatite group and 87.4% of the subjects of the fluoride group. The hydroxyapatite toothpaste was not statistically inferior to a fluoride toothpaste with regard to the primary endpoint.ConclusionHydroxyapatite was proven to be a safe and efficient anticaries agent in oral care.Clinical trial registrationNCT04756557

Prevention of Caries and Dental Erosion by Fluorides&mdash;A Critical Discussion Based on Physico-Chemical Data and Principles

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 &micro;g mm&minus;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

Tooth Whitening with Hydroxyapatite: A Systematic Review

A steadily increasing public demand for whiter teeth has resulted in the development of new oral care products for home use. Hydroxyapatite (HAP) is a new ingredient to whiten teeth. This systematic review focuses on the evidence of whether HAP can effectively whiten teeth. A systematic search using the PICO approach and PRISMA guidelines was conducted using PubMed, Scopus, Web of Science, SciFinder, and Google Scholar as databases. All study designs (in vitro, in vivo) and publications in foreign language studies were included. Of the 279 study titles that the searches produced, 17 studies met the inclusion criteria. A new “Quality Assessment Tool For In Vitro Studies” (the QUIN Tool) was used to determine the risk of bias of the 13 studies conducted in vitro. Moreover, 12 out of 13 studies had a low risk of bias. The in vivo studies were assigned Cochrane-based GRADE scores. The results in vitro and in vivo were consistent in the direction of showing a statistically significant whitening of enamel. The evidence from in vitro studies is rated overall as having a low risk of bias. The evidence from in vivo clinical trials is supported by modest clinical evidence based on six preliminary clinical trials. It can be concluded that the regular use of hydroxyapatite-containing oral care products effectively whitens teeth, but more clinical trials are required to support the preliminary in vivo evidence

A Critical Review of Modern Concepts for Teeth Whitening

Besides prevention of caries and periodontitis, an increasing number of oral care products focus on teeth whitening. The aim of this review is to summarize and discuss frequently used whitening agents and their efficacy from a chemical viewpoint. Therefore, a comprehensive literature survey on teeth whitening agents and products was conducted. The current whitening methods are analyzed and discussed from a chemist&rsquo;s viewpoint. Frequently used whitening agents are abrasives (mechanical removal of stains), antiredeposition agents (prevention of deposition of chromophores), colorants (intended to lead to a white color), proteases (degradation of proteins), peroxides (oxidation of organic chromophores), and surfactants (removal of hydrophobic compounds from tooth surface). In-office bleaching using peroxides is effective, but side effects like tooth sensitivity or a damage of the natural organic matrix of enamel and dentin may occur. The applicability of abrasives in teeth whitening is limited due to potential tooth wear, especially when toothpastes with high RDA values are used. The effect of other whitening agents in vivo is often unclear because of a shortage of placebo-controlled clinical trials

On the Application of Calcium Phosphate Micro- and Nanoparticles as Food Additive

The human body needs calcium and phosphate as essential nutrients to grow bones and teeth, but they are also necessary for many other biochemical purposes (e.g., the biosynthesis of phospholipids, adenosine triphosphate, ATP, or DNA). The use of solid calcium phosphate in particle form as a food additive is reviewed and discussed in terms of bioavailability and its safety after ingestion. The fact that all calcium phosphates, such as hydroxyapatite and tricalcium phosphate, are soluble in the acidic environment of the stomach, regardless of the particle size or phase, means that they are present as dissolved ions after passing through the stomach. These dissolved ions cannot be distinguished from a mixture of calcium and phosphate ions that were ingested separately, e.g., from cheese or milk together with soft drinks or meat. Milk, including human breast milk, is a natural source of calcium and phosphate in which calcium phosphate is present as nanoscopic clusters (nanoparticles) inside casein (protein) micelles. It is concluded that calcium phosphates are generally safe as food additives, also in baby formula

Overview on Adjunct Ingredients Used in Hydroxyapatite-Based Oral Care Products

Hydroxyapatite, Ca5(PO4)3(OH), is a biomimetic active ingredient, which is used in commercial oral care products such as toothpastes and mouthwashes worldwide. Clinical studies (in vivo) as well as in situ and in vitro studies have shown the preventive effects of hydroxyapatite in various field of oral care. In some products, hydroxyapatite is combined with other active ingredients, to achieve an additional antibacterial effect or to promote gum health. This review analyzes the efficacy of six selected natural and nature-inspired ingredients that are commonly used together with hydroxyapatite. These additional actives are either antibacterial (lactoferrin, xylitol, and zinc) or promote gum health (allantoin, bisabolol, and hyaluronic acid). A systematic literature search was performed, and all studies found on each ingredient were analyzed. In summary, all analyzed ingredients mentioned in this review are well described in scientific studies on their beneficial effect for oral health and can be used to expand the preventive effect of hydroxyapatite in oral care products