A mineral, mimetite Pb5(AsO4)3Cl, is one of the most insoluble minerals and continues to be considered a viable remedial strategy for immobilization of Pb and As from contaminated soils. It has been recognized that many well-known, naturally-occurring, and synthetic chelators strongly influence dissolution processes in near-surface geological environments. In this study, crystals of mimetite were observed in scanning electron microscopy (SEM) and atomic force microscopy (AFM) before and after dissolution in EDTA (ethylene diamine tetra-acetic acid) solution. Direct in situ observations at room temperature made in an AFM fluid cell revealed that the grain surface roughness has increased due to development of etch pits. Both hexagonal and prismatic walls developed dissolution features between 0.6 and 1.2 µm, respectively, during duration of the experiment. AFM observations suggest surface-controlled dissolution dominated step retrieval on both prismatic and hexagonal surfaces. SEM observations showed the development of rounded edges on hexagonal walls and elongated, oval etch pits on the prismatic wall. These results, representing early dissolution patterns on mimetite surfaces, might suggest that low pH conditions in soils containing organic acids similar to EDTA might contribute to remobilization of Pb and As from mimetite when applied to stabilization of these toxic metals in contaminated soils.

Eight dissolution models of calcium apatites (both fluorapatite and hydroxyapatite) in acids were drawn from the published literature, analyzed and discussed. Major limitations and drawbacks of the models were conversed in details. The models were shown to deal with different aspects of apatite dissolution phenomenon and none of them was able to describe the dissolution process in general. Therefore, an attempt to combine the findings obtained by different researchers was performed which resulted in creation of the general description of apatite dissolution in acids. For this purpose, eight dissolution models were assumed to complement each other and provide the correct description of the specific aspects of apatite dissolution. The general description considers all possible dissolution stages involved and points out to some missing and unclear phenomena to be experimentally studied and verified in future. This creates a new methodological approach to investigate reaction mechanisms based on sets of affine data, obtained by various research groups under dissimilar experimental conditions.

Minimally invasive caries-removal procedures remove only caries-infected dentin and preserve caries-affected dentin that becomes remineralized. Dental cements containing calcium phosphate promote remineralization. This study evaluated the in vivo remineralization capacity of resin-based calcium-phosphate cement (Ca-P) used for indirect pulp-capping. Carious and sound teeth indicated for extraction were randomly restored with the Ca-P base or without base (control), followed by adhesive restoration. Study teeth were extracted after three months, followed by elemental analysis of the cavity floor. Mineral content of affected or sound dentin at the cavity floor was quantified by electron probe micro-analysis to 100-mum depth. After three months, caries-affected dentin underneath the Ca-P base showed significantly increased calcium and phosphorus content to a depth of 30 mum. Mineral content of treated caries-affected dentin was in the range of healthy dentin, revealing the capacity of Ca-P base to promote remineralization of caries-affected dentin.

This in vitro study aimed to evaluate a pH-cycling model for simulation of caries-affected dentin (CAD) surfaces, by comparing the bond strength of etch-and-rinse adhesive systems on sound and artificially-created CAD. Dentin substrates with different mineral contents and morphological patterns were created by submitting buccal bovine dentin to the following treatments: (1) immersion in artificial saliva during the experimental period (sound dentin, SD), or (2) induction to a CAD condition by means of a dynamic pH-cycling model (8 cycles, demineralization for 3 h followed by mineralization for 45 h). The bond strength of Excite or Prime and Bond NT adhesive systems was assessed using the microtensile bond strength (microTBS) test. Dentin microhardness was determined by cross-sectional Knoop evaluations. Resin-dentin morphology after the treatments was examined by scanning electron microscopy. SD produced significantly higher microTBS than CAD for both adhesives evaluated, without differences between materials. CAD exhibited lower microhardness than SD. Morphological analysis showed marked distinctions between SD and CAD bonded interfaces. Under the conditions of this study, differences in morphological pattern and dentin mineral content may help to explain resin-dentin bond strengths. The proposed pH-cycling model may be a suitable method to simulate CAD surfaces for bonding evaluations.

The ultrastructural organization and chemical components of enamel crystallites in two deciduous teeth affected by RO, an uncommon developmental condition giving the tooth a ghost-like appearance, were investigated by scanning and transmission electron microscopy, X-ray diffraction, electron probe microanalysis and Fourier transform improved microspectroscopy. Pathologic mineralizations involving prismatic structures, which affected the size, shape and stoichiometric structure of crystallites and led to enhanced Mg/Ca and Na/Ca ratios and crystal defects, were observed. Local circulatory disorders may have caused this developmental anomaly.

New experimental data about surface processes of interaction between natural apatite and phosphoric acid solutions were obtained by scanning electron microscopy, Auger electron spectroscopy, and IR reflection spectroscopy. The interaction was found to occur nonstoichiometrically (incongruently) on the very thin surface layer of apatite. The experimental data obtained were compared and extended with results taken from literature. The following sequence of ionic detachment from the surface of apatite to a solution was suggested: first fluorine for fluorapatite or hydroxyl for hydroxyapatite, next calcium, and afterward phosphate. A new chemical mechanism of apatite dissolution was proposed as a result. The mechanism for the first time described the surface irregularity of the dissolution process at the nanolevel. A comparison between this new dissolution mechanism and earlier mechanisms described in the literature was made.

Mineralization and crystal deposition are natural phenomena widely distributed in biological systems from protozoa to mammals. In mammals, normal and pathological calcifications are observed in bones, teeth, and soft tissues or cartilage. We review studies on the adaptive apatite crystal formation in enamel compared with those in other calcified tissues (e.g., dentin, bone, and fish enameloids) and in pathological calcifications, demonstrating the adaptation of these crystals (in terms of crystallinity and orientation) to specific tissues that vary in functions or vary in normal or diseased conditions. The roles of minor elements, such as carbonate, magnesium, fluoride, hydrogen phosphate, pyrophosphate, and strontium ions, on the formation and transformation of biologically relevant calcium phosphates are summarized. Another adaptative process of crystals in biology concerns the recent development of calcium phosphate ceramics and other related biomaterials for bone graft. Bone graft materials are available as alternatives to autogeneous bone for repair, substitution, or augmentation. This paper discusses the adaptive crystal formation in mineralized tissues induced by calcium phosphate and related bone graft biomaterials during bone repair.

We evaluated the microstructural characteristics of newly formed bone tissue at the interface with cement. The bone-cement interfaces of the femoral components of nine hip prostheses retrieved after loosening were investigated by means of X-ray diffraction on microareas and microhardness. The bone far from the interface of two stable prostheses was used as a control. The newly formed bone adjacent to cement in the loosened prostheses showed a maturity degree lower than that of bone adjacent to cement in stable prostheses. The lattice parameters of bone apatite did not show significant variations as compared to the reference values. Bone trabeculae at the interface with loosened prostheses often showed an osteoid lining characterized by a strongly demineralized lamellar and haversian structure. Radioopaque cement particles are sometimes found in the trabecular bone tissue around the prosthesis.

The use of several calcium phosphate (Ca-P) materials for bone repair, augmentation, substitution and as coatings on metal implants has gained clinical acceptance in many dental and medical applications. These Ca-P materials may be of synthetic or natural origin, available in different physical forms (dense or macroporous, particles or blocks) and are used in bulk as coatings for metallic and non-metallic substrates or as components in composites, cements and bioactive glasses. Biodegradation or bioresorption of calcium phosphate materials implies cell-mediated degradation in vitro or in vivo. Cellular activity during biodegradation or bioresorption occurs in acid media; thus the factors affecting the solubility or the extent of dissolution (which in turn depends on the physico-chemical properties) of the Ca-P materials are important. Enrichment of the microenvironment due to the release of calcium and phosphate ions from the dissolving Ca-P materials affects the proliferation and activities of the cells. The increase in the concentrations of the calcium and phosphate ions promotes the formation of carbonate apatite which are similar to the bone apatite. The purpose of this invited paper is to discuss the processes of biodegradation or bioresorption of Ca-P materials in terms of the physico-chemical properties of these materials and the phenomena involved including the formation of carbonate apatite on the surfaces and in the vicinity of these materials. This phenomenon appears to be related to the bioactivity of the material and the ability of such materials to directly attach to bone and to form a uniquely strong material-bone interface.

High resolution transmission electron microscopy (Hr TEM) studies on biological and synthetic calcium phosphate have provided information on the dissolution process at the crystal level. The purpose of this study was to investigate the dissolution of ceramic hydroxyapatite (HA) after implantation using Hr TEM. Recovered HA ceramic implanted in bony and nonbony sites in animals and in periodontal pockets in humans were used for the study. For comparison, sections of human fluorotic enamel with caries and sections of shark enameloid previously exposed to 0.1 HCl were similarly investigated. Hr TEM studies demonstrated that in both the biological and ceramic apatites, the lattice and atomic defects were the starting points in the dissolution process. However, significant differences in the process of dissolution were observed: (1) biological apatite crystals showed preferential core dissolution whereas ceramic apatite crystals showed nonspecific dissolution at the cores and at the surfaces; (2) the dissolution of biological apatites appeared to consistently extend along the crystal's c-axis whereas dissolution of the ceramic HA did not appear to be correlated with the crystal's c-axis. The observed differences in crystal dissolution between biological and ceramic apatites may be attributed to the following: (1) the unique crystal/protein interaction present with biological apatites but absent in ceramic HA; (2) differences in defect distribution between biological and ceramic apatites which are due to the differences in the original of these defects; and (3) the longer morphological c-axis of biological apatites compared with that of ceramic apatites.(ABSTRACT TRUNCATED AT 250 WORDS)

The application of high resolution electron microscopy, computer image processing, and image simulation techniques to the investigation of synthetic nonstoichiometric apatites has provided new details of apatite crystal growth mechanisms. Under certain precipitation conditions, calcium-deficient apatites with distinct octacalcium phosphate (OCP)-apatite intergrowths have been observed. Apatite crystals with unit-cell thick overgrowths of OCP on their surfaces confirmed the stepwise hydrolysis crystal growth mechanism initially proposed by Brown (Nature 196:1048-1050). However, many crystals also contained a central two-dimensional OCP inclusion one to two unit cells thick, embedded in an apatite matrix. Similar planar defects have been observed in dental enamel, dentin, and bone crystals. We have developed a modified version of Brown's stepwise OCP hydrolysis apatite crystal growth mechanism to explain the formation of crystals with OCP central planar defects. The mechanism involves the nucleation of an OCP seed that grows until it reaches a critical size, rh, before OCP hydrolysis occurs. Apatite subsequently grows epitaxially on the OCP seed, thereby embedding it in the center of an apatite crystal. Apatite growth is facilitated by partial screw dislocations emanating from the planar defect.

Electron microscopy of the intact surface zone of white spot and brown spot carious lesions showed that in general their ultrastructure was similar. Their outermost crystalline surface consisted of small crystals similar to those in healthy enamel, crystals with central core dissolution, and rounded crystals. Below this, surface demineralization of enamel was observed as the enlargement of micropores, the central core dissolution of crystals, the formation of channels and the enlargement of spaces at prism boundaries. Remineralization of enamel was observed as the partial occlusion of voids, the rounding and enlargement of crystals, and some new needle-shaped crystals. Some other features indicated combined demineralization and remineralization. The occlusion of spaces at prism boundaries was a more common feature in brown spot lesions, whereas the pockets of rounded crystals were more common in white spot lesions. A relatively uniform distribution of needle-shaped crystals throughout the intact surface zone was a feature of some brown spot lesions only.

The present study aims to obtain further information about the crystallographic and structural alterations in the mineral phase of enamel with the onset of caries. For this purpose, X-ray microbeam diffraction analysis was carried out on ground sections prepared from natural white spot lesions. Electron spin resonance (ESR) analysis was also performed on block samples cut from white spot lesions and from undamaged enamel of the same teeth. The results of crystallinity measurements showed that enamel apatite in demineralized lesions was lower in crystallinity than the apatite in the surface layer and surrounding sound enamel. This X-ray diffraction study also revealed the presence of two non-apatitic minerals in the demineralized lesions. One type of the mineral is whitlockite, giving spotty rings. The nature of the other mineral, giving a single ring-like reflection, remains to be elucidated. A comparison of ESR spectra taken from the caries-attacked and undamaged enamel provided evidence that the former samples have a lower degree of alignment of apatite microcrystals. The occurrence of non-apatitic mineral phases and the observed difference in microcrystalline alignment may be the results of a remineralization process.

The crystallographic features of various histological sites found in enamel carious lesions were studied by means of X-ray microbeam diffraction. The most interesting finding was the crystallinity of apatite crystals in the intact surface layer covering the demineralized lesions was higher than that of the crystals in the subsurface lesions or unaffected areas. Acid-dissolution experiments using synthetic hydroxyapatite pellets showed that a well mineralized layer was produced on the pellet surface under the condition that no Ca and PO4 ions were added to the initial acidic solution. Furthermore, X-ray diffraction analysis revealed that half-line breadth values of 004, 222, and 310 reflections of apatite pellets decreased, and their integrated peak intensities increased with dissolution time. These findings are in agreement with the results obtained in the study of enamel carious lesions, leading to the conclusion that the intact surface layer is formed by deposition of the mineral ions from dissolving subsurface lesions, and that this process is accompanied by the improvement in crystallinity of apatite crystals, possibly due to growth of the crystals.