Arndt Klocke

The fracture behaviour of dental enamel

Enamel is the hardest tissue in the human body covering the crowns of teeth. Whereas the underlying dental material dentin is very well characterized in terms of mechanical and fracture properties, available data for enamel are quite limited and are apart from the most recent investigation mainly based on indentation studies. Within the current study, stable crack-growth experiments in bovine enamel have been performed, to measure fracture resistance curves for enamel. Single edge notched bending specimens (SENB) prepared out of bovine incisors were tested in 3-point bending and subsequently analysed using optical and environmental scanning electron microscopy. Cracks propagated primarily within the protein-rich rod sheaths and crack propagation occurred under an inclined angle to initial notch direction not only due to enamel rod and hydroxyapatite crystallite orientation but potentially also due to protein shearing. Determined mode I fracture resistance curves ranged from 0.8-1.5 MPa*m(1/2) at the beginning of crack propagation up to 4.4 MPa*m(1/2) at 500 microm crack extension; corresponding mode II values ranged from 0.3 to 1.5 MPa*m(1/2).

Determination of the Elastic-plastic Transition of Human Enamel by Nanoindentation

OBJECTIVES/METHODS From a materials scientists perspective, dental materials used for tooth repair should exhibit compatible mechanical properties. Fulfillment of this criterion is complicated by the fact that teeth have a hierarchical structure with changing mechanical behavior at different length scales. In this study, nanoindentation with an 8 microm spherical indenter was used to determine the elastic/plastic transition under contact loading for enamel. RESULTS The indentation elastic/plastic transition of enamel at the length scale of several hundreds of hydroxyapatite crystallites, which are within one enamel rod, is revealed for the first time. The corresponding penetration depth at the determined indentation yield point of 1.6GPa and 0.6% strain is only 7 nm. As a consequence of the small depth it is decisive for the experiment to calibrate the indenter tip radius in this loading regime. The elastic modulus of 123GPa was evaluated directly by the Hertzian penetration and not by the unloading part of the indentation curve. SIGNIFICANCE We believe these data are also a valuable contribution to understand the mechanical behavior of enamel and to develop nanoscale biomimetic materials.

Crack arrest within teeth at the dentinoenamel junction caused by elastic modulus mismatch

Enamel and dentin compose the crowns of human teeth. They are joined at the dentinoenamel junction (DEJ) which is a very strong and well-bonded interface unlikely to fail within healthy teeth despite the formation of multiple cracks within enamel during a lifetime of exposure to masticatory forces. These cracks commonly are arrested when reaching the DEJ. The phenomenon of crack arrest at the DEJ is described in many publications but there is little consensus on the underlying cause and mechanism. Explanations range from the DEJ having a larger toughness than both enamel and dentin up to the assumption that not the DEJ itself causes crack arrest but the so-called mantle dentin, a thin material layer close to the DEJ that is somewhat softer than the bulk dentin. In this study we conducted 3-point bending experiments with bending bars consisting of the DEJ and surrounding enamel and dentin to investigate crack propagation and arrest within the DEJ region. Calculated stress intensities around crack tips were found to be highly influenced by the elastic modulus mismatch between enamel and dentin and hence, the phenomenon of crack arrest at the DEJ could be explained accordingly via this elastic modulus mismatch.

Thermally-induced structural modification of dental enamel apatite: Decomposition and transformation of carbonate groups

Dental enamel is mainly composed of non-stoichiometric hydroxyapatite with A- and B-type carbonate groups in OH and phosphate sites, respectively. Structural and chemical modifications of dental enamel apatite were studied using FTIR and XRD techniques after heat treatment in air for 1h from 300 to 1193 K. Both IR and XRD results show a high degree of crystallinity of apatite that is enhanced with increasing temperature. The loss of B-type and A-type carbonate was studied; the amount of B-type carbonate and the total carbonate content decrease on heating while the amount of A-type carbonate first decreases up to 573 K and then increases from 573 to 973 K. Almost 50 % of the carbonate ions were released from dental enamel with the formation of β-tricalcium phosphate phase (β-TCP) after heat treatment at 973 K for 1 h. The incorporation of CO 2 and cyanate species in dental enamel was observed in the temperature range of 273–973 K and 673–1073 K, respectively. The content of CO 2 in dental enamel increases from 473 K to a maximum near 773 K and decreases thereafter. The mechanism of the decomposition and transformation of carbonate groups at different sites in enamel apatite structure is discussed.

Bond strength with custom base indirect bonding techniques.

Different types of adhesives for indirect bonding techniques have been introduced recently. But there is limited information regarding bond strength with these new materials. In this in vitro investigation, stainless steel brackets were bonded to 100 permanent bovine incisors using the Thomas technique, the modified Thomas technique, and light-cured direct bonding for a control group. The following five groups of 20 teeth each were formed: (1) modified Thomas technique with thermally cured base composite (Therma Cure) and chemically cured sealant (Maximum Cure), (2) Thomas technique with thermally cured base composite (Therma Cure) and chemically cured sealant (Custom I Q), (3) Thomas technique with light-cured base composite (Transbond XT) and chemically cured sealant (Sondhi Rapid Set), (4) modified Thomas technique with chemically cured base adhesive (Phase II) and chemically cured sealant (Maximum Cure), and (5) control group directly bonded with light-cured adhesive (Transbond XT). Mean bond strengths in groups 3, 4, and 5 were 14.99 +/- 2.85, 15.41 +/- 3.21, and 13.88 +/- 2.33 MPa, respectively, and these groups were not significantly different from each other. Groups 1 (mean bond strength 7.28 +/- 4.88 MPa) and 2 (mean bond strength 7.07 +/- 4.11 MPa) showed significantly lower bond strengths than groups 3, 4, and 5 and a higher probability of bond failure. Both the original (group 2) and the modified (group 1) Thomas technique were able to achieve bond strengths comparable to the light-cured direct bonded control group.

Thermal behavior of dental enamel and geologic apatite: An infrared spectroscopic study

Abstract Teeth and bones consist of an apatite-type structure and such biogenic apatites usually occur in nano-crystalline form. Because of the small particle size in biological tissues, local structural details of biogenic apatite have still not been resolved in detail. Comparison of the phonon spectra of enamel apatite with those of inorganically formed apatite was carried out to improve our understanding of the vibrational behavior of biogenic apatite. In situ mid-infrared absorption spectra of dental enamel and geologic fluorapatite were recorded from 300 K to ca. 750 K. Lattice vibration modes were studied at low temperature in the infrared region of 150 to 650 cm-1. The IR excitations indicate that geologic apatite undergoes heavier thermal changes than enamel apatite at temperatures between 60 K and 300 K. In situ high-temperature IR spectra confirm the different thermal evolution of dental enamel and geologic fluorapatite. The P-O overtones or combinational vibrations and hydrous species of enamel apatite show two different thermal regions below and above 600 K. The thermal behavior in the region below 600 K corresponds to the loss of adsorbed and part of the lattice water, combined with an increase of structural OH groups. In the second thermal region (above 600 K), the similarity of thermal response of dental enamel to that of the geologic apatite from 300 K suggests the existence of a highly ordered system. This result may be explained by the dehydration and atomic rearrangements in the channels of enamel apatite structure below 600 K.

New Insights into Structural Alteration of Enamel Apatite Induced by Citric Acid and Sodium Fluoride Solutions

Attenuated total reflectance infrared spectroscopy and complementary scanning electron microscopy were applied to analyze the surface structure of enamel apatite exposed to citric acid and to investigate the protective potential of fluorine-containing reagents against citric acid-induced erosion. Enamel and, for comparison, geological hydroxylapatite samples were treated with aqueous solutions of citric acid and sodium fluoride of different concentrations, ranging from 0.01 to 0.5 mol/L for citric acid solutions and from 0.5 to 2.0% for fluoride solutions. The two solutions were applied either simultaneously or consecutively. The citric acid-induced structural modification of apatite increases with the increase in the citric acid concentration and the number of treatments. The application of sodium fluoride alone does not suppress the atomic level changes in apatite exposed to acidic agents. The addition of sodium fluoride to citric acid solutions leads to formation of surface CaF2 and considerably reduces the changes in the apatite P-O-Ca framework. However, the CaF2 globules deposited on the enamel surface seem to be insufficient to prevent the alteration of the apatite structure upon further exposure to acidic agents. No evidence for fluorine-induced recovery of the apatite structure was found.

Side effects of a non‐peroxide‐based home bleaching agent on dental enamel

Changes in the chemistry and structure of enamel due to a non-peroxide-based home bleaching product (Rapid White) were studied in vitro using attenuated total reflectance-infrared spectroscopy, Raman spectroscopy, electron probe microanalysis, flame atomic absorption spectroscopy, and total reflection X-ray fluorescence. The results revealed that the citric-acid-containing gel-like component of the bleaching system substantially impacts on the dental hard tissue. Enamel is affected on several levels: (i) the organic component is removed from superficial and deeper enamel layers and remnants of the bleaching gel are embedded in the emptied voids; (ii) cracks and chemical inhomogeneities with respect to Ca and P occur on the surface; and (iii) within a submicron layer of enamel, the Ca-O bond strength in apatite decreases, thus enhancing calcium leakage from the bleached enamel hard tissue.

Effect of Time on Bond Strength in Indirect Bonding

The purpose of this in vitro investigation was to determine the influence of a reduced time interval before debonding on shear bond strength of stainless steel brackets bonded with a custom base indirect technique. A total of 135 bovine permanent mandibular incisors was randomly divided into nine groups of 15 specimens each. Three base composite-sealant combinations were investigated: (1) Phase II base composite, Custom I.Q. sealant, (2) Phase II base composite, Maximum Cure sealant, and (3) Transbond XT base composite, Sondhi Rapid Set sealant. Shear bond strength was measured for three different debonding time intervals: (1) time of transfer tray removal as recommended by the manufacturer, (2) 30 minutes after bonding of the sealant, and (3) 24 hours after bonding of the sealant. For groups bonded with Maximum Cure or Sondhi Rapid Set sealants, no influence of debonding time on shear bond strength was found. The Custom I.Q. sealant groups showed significantly lower bond strength measurements when debonded at the recommended tray removal time, and the Weibull analysis indicated a higher risk of bond failure at clinically relevant levels of stress. All base composite-sealant combinations showed acceptable bond strength at 30 minutes and 24 hours after bonding of the sealant.

Magnetic Forces on Orthodontic Wires in High Field Magnetic Resonance Imaging (MRI) at 3 Tesla

Background:In a previous investigation we reported on magnetic forces in the static magnetic field of a 1.5 Tesla MRI system. The aim of the present investigation was to assess forces on orthodontic wires in a high field strength MRI system at 3 Tesla.Materials and Methods:Thirty-two different orthodontic wires (21 archwires, eight ligature wires and three retainer wires) were investigated in a 3 Tesla high field strength MRI system (Intera, Philips Medical Systems, Best, The Netherlands). Translational forces were measured by the deflection angle test (ASTM F2052-02), and rotational forces assessed on a 5-point qualitative scale.Results and Conclusion:Translational forces ranged between 43.5 mN and 136.1 mN for retainer wires and between 0.6 mN (Noninium®) and 208.4 mN (Orthos™ StaStainless Steel) for steel archwires. Translational forces were up to 53.8 times as high as gravitational forces for retainer wires and up to 54.5 times as high for steel archwires, associated with marked rotational forces for the most part. Archwires manufactured from nickel-titanium, titanium-molybdenum and cobalt-chromium and different ligature wires showed no or negligible forces in the magnetic field. Carefully ligated wires should not present a risk due to translational and rotational forces in the high field MRI system at 3 Tesla.ZusammenfassungHintergrund:In einer vorangegangenen Untersuchung wurde bereits über im statischen Magnetfeld eines 1,5-Tesla-MRT-Systems wirkende Kräfte berichtet. Ziel dieser Untersuchung war nun die Darstellung und Beurteilung der auftretenden Kräfte während Hochfeld-Magnetresonanztomographie bei 3 Tesla.Material und Methodik:Es wurden 32 verschiedene kieferorthopädische Drähte (21 Drahtbögen, acht Ligaturendrähte und drei Retainerdrähte) in einem 3-Tesla-Hochfeld-MRT-System (Intera, Philips Medical Systems, Best, Niederlande) untersucht. Die Messung der auf die Drähte wirkenden Translationskräfte erfolgte mittels Fadentest (ASTM F2052-02), die Evaluierung der Rotationskraft wurde qualitativ anhand einer 5-Punkte-Skala vorgenommen.Ergebnisse und Schlussfolgerung:Die Messungen der Translationskräfte auf die Retainerdrähte ergaben zwischen 43,5 und 136,1 mN und auf die Stahl-Drahtbögen zwischen 0,6 mN (Noninium ®) und 208,4 mN (Orthos™ Stainless Steel). Die ermittelten Translationskräfte betrugen damit bei den Retainerdrähten bis zum 53,8fachen und bei den Stahl-Drahtbögen bis zum 54,5-fachen der auf diese Objekte wirkenden Gravitationskräfte, verbunden mit größtenteils ausgeprägten Rotationskräften. Translations- und Rotationseinflüsse auf Nickel-Titan-, Titan-Molybdän- und Kobalt-Chrom-Drahtbögen sowie auf verschiedene Ligaturendrähte waren nur gering oder nicht vorhanden. Im 3-Tesla-Hochfeld-MRT-System ist bei einer sorgfältigen Befestigung von kieferorthopädischen Drähten nicht von einer Gefährdung durch magnetische Kräfte auszugehen.