Search PubMed⌕ Search

Biomedical subjects

D G Nelson

Publications and source records attributed to D G Nelson.

50 records · Page 3Linked to original sources
PubMed1984

High-resolution electron microscopy of octacalcium phosphate and its hydrolysis products.

The hydrolysis and dehydration products of synthetic octacalcium phosphate (OCP) were studied using X-ray diffraction, infrared spectroscopy, chemical analysis, and high-resolution electron microscopy (HREM). A "collapsed OCP" phase, identified by a characteristic 16.5 A reflection in its X-ray diffraction pattern, was observed when OCP was dehydrated. High resolution electron microscopy of the hydrolyzed and partially hydrolyzed reaction products also revealed local contrast features with an approximate 16.5 A periodicity. These features were consistent with a collapse of the OCP crystal structure and subsequent formation of epitaxial intergrowths of OCP and hydroxyapatite. Chemical analysis and X-ray diffraction of these samples were similar to previously reported calcium-deficient apatites. The hydrolysis of OCP to form calcium-deficient apatites is a reaction pathway which may be of importance in understanding the crystallographic changes occurring during the early stages of bone, calculus, and dental enamel formation.

Apatites↗
PubMed1984

Holistic and analytic modes of processing: the multiple determinants of perceptual analysis.

With the use of the criterion of privileged axes, a set of experimental studies establishes that analytic processing of saturation and brightness, prototypical integral dimensions, does sometimes occur. Still, it is less frequent and less successful than analytic processing of separable dimensions. The studies identify several factors that influence whether or not analytic processing occurs. Among them are the following: (a) stimulus factors, in addition to the choice of dimensions (e.g., the magnitude of dimensional differences); (b) task factors (e.g., the degree to which analytic processing is encouraged by instructions or by implicit task demands, the amount of time available for processing); and (c) subject factors (e.g., the amount of experience that the perceiver has had with the stimuli in tasks that encourage stimulus analysis).

Adolescent↗
PubMed1984

Crystallographic structure of enamel surfaces treated with topical fluoride agents: TEM and XRD considerations.

The crystallographic structure of surface coatings produced by three topical fluoride agents deposited by a single application on intact human enamel surfaces was investigated by light- and dark-field transmission electron microscopy, as well as by electron and x-ray diffraction. The agents studied were an acidic silane fluoride lacquer, a neutral NaF lacquer, and an APF gel. The smallest coherently diffracting particles were CaF2 microcrystallites 4-15 nm in diameter. Large apatite-like crystals, approximately 1 micron in length, were also observed in the outer surface coating produced by the silane fluoride lacquer.

Animals↗
PubMed1983

A high-resolution electron microscope study of synthetic and biological carbonated apatites.

A series of synthetic carbonated apatites and human dental enamels characterized by chemical analysis, infrared spectroscopy, and X-ray diffraction was studied using high-resolution transmission electron microscopy. Steps on the surfaces of apatite crystals, often only a few unit cells high and occasionally one unit cell high, were observed by their Fresnel diffraction contrast. Highly substituted synthetic carbonated apatites appeared to have more irregular and finer-textured surface features than materials with less carbonate substitution. The surface features of enamel apatite crystal were also irregular, but surface steps were less frequently aligned in crystallographic directions. Complex strain fields due to radiation damage centers were observed in some crystals and the fine structure of dislocations and grain boundaries in synthetic apatites was resolved at high magnification. Experimental lattice-image contrasts, in favorable circumstances, could be matched to computer-simulated images and were found to contain detail at near atomic resolution, around 2.0-2.5 A.

Adult↗
PubMed1983

Morphology of enamel surfaces treated with topical fluoride agents: SEM considerations.

Three topical fluoride agents deposited surface coatings of different morphology and thickness on intact human enamel surfaces. The agents studied were an acidic silane fluoride lacquer, a neutral NaF lacquer, and an APF gel. Each agent reacted with the enamel surface differently, producing its own distinctive etching pattern. The smallest particles observed in the surface coatings were from 20 to 30 nm in diameter and appeared to have indistinct morphologies. They often agglomerated into spherical globules ranging from 1-3 micron in diameter.

Acidulated Phosphate Fluoride↗
PubMed1982

Paracrystalline disorder of biological and synthetic carbonate-substituted apatites.

Paracrystalline disorder, a type of lattice imperfection which results in a loss of long-range crystalline order, was assessed in a range of biological and synthetic apatites using x-ray diffraction data. Carbonate substitution in the apatite lattice increased the amount of paracrystalline disorder in synthetic precipitated apatites, whereas trace amounts of fluoride and, to a lesser extent, zinc and strontium partially offset the disorder induced by carbonate. Paracrystalline disorder was significantly reduced in synthetic apatites prepared by high-temperature solid-state reaction. Human dental enamel was less disordered than either dentin or bone.

Apatites↗
PubMed1982

Preparation, analysis, and characterization of carbonated apatites.

A range of synthetic carbonated apatites were characterized using infrared and Raman spectroscopy, X-ray diffraction, and chemical analysis and compared to stoichiometric hydroxyapatite and dental enamel. Synthetic apatites were prepared by aqueous precipitation and by solid-state reaction at high temperatures. A method is described for the carbonate determination of apatite samples using gas chromatography. This study demonstrates that carbonate exists in two crystallographically distinct sites in the apatite crystal structure and is consistent with a carbonate-for-phosphate substitution mechanism, where four carbonate groups replace three phosphate groups. Sodium and hydroxyl ions are shown to be involved with the substitution of carbonate ions in the apatite structure. Observed sodium/carbonate and calcium/phosphorus molar ratios confirm this mechanism. Substitution of this type may explain the predominant mode of carbonate substitution in biological apatites.

Apatites↗
PubMed1981

The influence of carbonate on the atomic structure and reactivity of hydroxyapatite.

A series of synthetic carbonated and non-carbonated apatites was investigated using high resolution transmission electron microscopy, and their dissolution behavior was tested under sink conditions, using the rotating disc method. n-Beam lattice images of the apatite samples were of a resolution greater than has been previously reported, and revealed an increase in crystalline disorder with the incorporation of carbonate in the apatite structure. This resulted in an increased rate of dissolution and a decreased activation energy for dissolution for the carbonated apatites. Thus, carbonate increased the reactivity of hydroxyapatite.

Carbonates↗
PubMedDate not supplied

Thoracoscopic assisted pneumonectomy.

A lung carcinoma with tumor involving more than one lobe or in close proximity to the mainstem bronchus often requires pneumonectomy for surgical cure. Inflammation, bulky tumors, and dense adhesions limit the operative field of vision and may result in the abandonment of procedures with potential for complete extirpation. This case illustrates the utility of thoracoscopy in visualization of the hilum and other neurovascular structures in a patient with a proximal tumor and dense intrathoracic adhesions. Successful resection was made possible by use of combined open and thoracoscopic modalities.

Biopsy, Needle↗