Joachim Enax

Ultrastructural organization and micromechanical properties of shark tooth enameloid

The outer part of shark teeth is formed by the hard and mineral-rich enameloid that has excellent mechanical properties, which makes it a very interesting model system for the development of new bio-inspired dental materials. We characterized the microstructure, chemical composition and resulting local mechanical properties of the enameloid from teeth of Isurus oxyrinchus (shortfin mako shark) by performing an in-depth analysis using various high-resolution analytical techniques, including scanning electron microscopy, qualitative energy-dispersive X-ray spectroscopy and nanoindentation. Shark tooth enameloid reveals an intricate hierarchical arrangement of thin (50-80nm) and long (>1μm) crystallites of fluoroapatite with a high degree of structural anisotropy, which leads to exceptional mechanical properties. Both stiffness and hardness are surprisingly homogeneous in the shiny layer as well as in the enameloid: although both tooth phases differ in structure and composition, they show almost no orientation dependence with respect to the loading direction of the enameloid crystallites. The results were used to determine the structural hierarchy of shark teeth, which can be used as a base for establishing design criteria for synthetic bio-inspired and biomimetic dental composites.

Characterization of crocodile teeth: Correlation of composition, microstructure, and hardness

Structure and composition of teeth of the saltwater crocodile Crocodylus porosus were characterized by several high-resolution analytical techniques. X-ray diffraction in combination with elemental analysis and infrared spectroscopy showed that the mineral phase of the teeth is a carbonated calcium-deficient nanocrystalline hydroxyapatite in all three tooth-constituting tissues: Dentin, enamel, and cementum. The fluoride content in the three tissues is very low (<0.1 wt.%) and comparable to that in human teeth. The mineral content of dentin, enamel, and cementum as determined by thermogravimetry is 71.3, 80.5, and 66.8 wt.%, respectively. Synchrotron X-ray microtomography showed the internal structure and allowed to visualize the degree of mineralization in dentin, enamel, and cementum. Virtual sections through the tooth and scanning electron micrographs showed that the enamel layer is comparably thin (100-200 μm). The crystallites in the enamel are oriented perpendicularly to the tooth surface. At the dentin-enamel-junction, the packing density of crystallites decreases, and the crystallites do not display an ordered structure as in the enamel. The microhardness was 0.60±0.05 GPa for dentin, 3.15±0.15 GPa for enamel, 0.26±0.08 GPa for cementum close to the crown, and 0.31±0.04 GPa for cementum close to the root margin. This can be explained with the different degree of mineralization of the different tissue types and is comparable with human teeth.

Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks

Teeth represent the hardest tissue in vertebrates and appear very early in their evolution as an ancestral character of the Eugnathostomata (true jawed vertebrates). In recent vertebrates, two strategies to form and mineralize the outermost functional layer have persisted. In cartilaginous fish, the enameloid is of ectomesenchymal origin with fluoroapatite as the mineral phase. All other groups form enamel of ectodermal origin using hydroxyapatite as the mineral phase. The high abundance of teeth in the fossil record is ideal to compare structure and composition of teeth from extinct groups with those of their recent successors to elucidate possible evolutionary changes. Here, we studied the chemical composition and the microstructure of the teeth of six extinct shark species, two species of extinct marine reptiles and two dinosaur species using high-resolution chemical and microscopic methods. Although many of the ultrastructural features of fossilized teeth are similar to recent ones (especially for sharks where the ultrastructure basically did not change over millions of years), we found surprising differences in chemical composition. The tooth mineral of all extinct sharks was fluoroapatite in both dentin and enameloid, in sharp contrast to recent sharks where fluoroapatite is only found in enameloid. Unlike extinct sharks, recent sharks use hydroxyapatite as mineral in dentin. Most notably and hitherto unknown, all dinosaur and extinct marine reptile teeth contained fluoroapatite as mineral in dentin and enamel. Our results indicate a drastic change in the tooth mineralization strategy especially for terrestrial vertebrates that must have set in after the cretaceous period. Possibly, this is related to hitherto unconsidered environmental changes that caused unfavourable conditions for the use of fluoroapatite as tooth mineral.

Die Mundhöhle als Ökosystem: Der sanfte Weg gegen Karies und Parodontitis?

Zahn‐ und Zahnfleischerkrankungen sind weit verbreitet und basieren primär auf der Anwesenheit bakterieller Biofilme. Die Charakterisierung von Biofilmen kann durch modernste Mikrobiomanalysen erfolgen, die Informationen über Bakterienzusammensetzung und ‐Diversität liefern können.

Reply to the ‘Comments on “Dental lessons from past to present: ultrastructure and composition of teeth from plesiosaurs, dinosaurs, extinct and recent sharks”’ by H. Botella et al., RSC Adv., 2016, 6, 74384–74388

The structure and composition of 13 fossilized tooth and bone samples aged between 3 and 70 million years were analysed. It was found that they all contained high amounts of fluoroapatite. This indicates that originally present hydroxyapatite had been converted to fluoroapatite during the diagenesis. Thus, the chemical analysis allows no conclusion with respect to the original composition of our fossil samples. Our results indicate that the diagenetic transformation of hydroxyapatite into fluoroapatite is at least partially dependent on microstructural characteristics of the original tissue such as the degree of porosity.

Composites of fluoroapatite and methylmethacrylate-based polymers (PMMA) for biomimetic tooth replacement

Synthetic composite materials that mimic the structure and composition of mammalian tooth enamel were prepared by mixing fluoroapatite rods (diameter 2-3 μm, thickness about 0.5 μm) and methylmethacrylate (MMA), followed by polymerization either during or immediately after ultracentrifugation, using either a tertiary amine/radical initiator for polymerization at room temperature or a radical initiator for thermal polymerization. This led to mineral-rich composites (mineral content between 50 and 75 wt%). To enhance the mechanical stability and the interaction between fluoroapatite and polymer matrix, small amounts of differently functionalized MMA monomers were added to the co-monomer mixture. Another approach was the coating of the fluoroapatite rods with silica and the polymerization in the presence of a siloxane-functionalized MMA monomer. The hardness of the composites was about 0.2-0.4 GPa as determined by Vickers indentation tests, about 2 times higher than the polymer matrix alone. The composites had a good resistance against acids (60 min at pH 3, 37 °C).

Overview of Calcium Phosphates used in Biomimetic Oral Care

Background: The use of biomimetic agents is an emerging field in modern oral care. Promising biomimetic substances for such applications are calcium phosphates, because their chemical composition is very similar to that of the mineral phase in human teeth, especially of natural enamel. Examples for their application include the remineralization of early caries lesions and repair of small enamel defects. Objective: This review provides an interdisciplinary view on calcium phosphates and their applications in biomimetic oral care. The aim of this work is to give an overview of in vivo and in situ studies comparing several calcium phosphates in preventive dentistry that can be used as a knowledge base for the development of innovative alternative oral care concepts. Methods: Books, reviews, and original research papers with a focus on in vivo and in situ studies were included. The databases PubMed® and SciFinder® were used for literature search. Calcium phosphates that are frequently utilized in oral care products are covered in this review and were used as search terms alone and together with the following key words: in vivo, in situ, caries, clinical study, and remineralization. From 13,470 studies found, 35 studies complied with the inclusion criteria and were used for this review. Results: Published in vivo and in situ studies demonstrate calcium phosphates’ potential in enamel remineralization. However, more studies are needed to further substantiate existing results and to extend and refine the application of calcium phosphates in modern oral care. Conclusion: Calcium phosphates represent an innovative biomimetic approach for daily oral care because of their high similarity to natural enamel that will broaden the range of future treatments in preventive dentistry.