Biomedical subjects

D W Holcomb

Publications and source records attributed to D W Holcomb.

5 recordsLinked to original sources

PubMed1988

Influence of preparation conditions on the composition of type B carbonated hydroxyapatite and on the localization of the carbonate ions.

It is shown how certain aspects of the composition and structure of carbonated apatites depend strictly on preparation conditions, for example, excess of phosphate or calcium ions in the reaction medium, CO32- concentration, pH, ammonia added or not. Depending on those conditions, either one or the other of the two proposed mechanisms of introduction of carbonate ions into the B sites is dominant. The mechanisms are (1) replacement of a phosphate ion by a carbonate ion with the formation of three vacancies, one in a phosphate oxygen site and one each in the neighboring Ca2+ and OH- sites; and (2) replacement of a phosphate ion by a carbonate accompanied by a hydroxyl ion. Whether mechanism (1) is observed to dominate over mechanism (2), or vice versa, is accounted for by the relative concentrations of the various ions in the reaction medium. The number of vacancies is decreased by the presence of either, or both, excess calcium ions or ammonia in the reaction medium. A structural-chemical mechanism is advanced for the view that, with the smallest CO32- content, the A sites are favored but with increasing carbonate content the B sites become favored and the A-site content becomes less than it is when the total carbonate content is less.

Carbonates↗

PubMed1985

Infrared determination of the degree of substitution of hydroxyl by carbonate ions in human dental enamel.

About 11 +/- 1% of the carbonate ions in a human tooth enamel specimen (dense fraction, sp. g. greater than 2.95) were found to be in the A-sites, replacing hydroxyl ions. The determination was made with a difference infrared spectrometry technique utilizing both tooth enamel and a reconstituted biological apatite with a known amount of replacement of hydroxyl by carbonate ions.

Calcium Phosphates↗

PubMed1984

Role of acid phosphate in hydroxyapatite lattice expansion.

Questions remain about which subcomponents of human tooth enamel (TE) are responsible for its crystallographic alpha axis being nearly 0.02A longer than that of pure hydroxyapatite (OHAp) and contracting to that of OHAp on heating. From infrared spectroscopic and X-ray diffraction studies of a synthetic OHAp containing HPO4 and "structural" H2O, it has been concluded that HPO4 expands the alpha axis at the rate of approximately 0.0015A/wt% but that this accounts for substantially less than one-half of the total observable contraction. The remaining, more than one-half of the alpha axis change, may be only partially ascribable to "structural" H2O and partially to P2O7 (formed from the HPO4), coming out of solid solution in the apatite. Some 90% of the HPO4 observed with infrared is lost in the 160-240 degrees temperature range and more than one-half of the P2O7 observed as a separate phase is developed above that temperature and continues to increase all the way up to the 500 degrees C, the limit of the experiments. The loss of HPO4 is accompanied by reduction of disorder or variety in the structural OH ion sites, consistent with the view that initially some of the PO4 groups neighboring the OH ions were actually HPO4 groups.

Calcium Phosphates↗

PubMed1982

Variability of hydroxyapatite preparations.

Hydroxyapatite synthesized by various "standard" ways exhibits marked differences with preparation method. Specimens were prepared with two precipitation methods, a reflux method, a hydrothermal method, a high-temperature (1000 degrees C) solid-state reaction method, and by conversion of chlorapatite at 1000 degrees C. They were compared in detail by use of several techniques, the major ones being x-ray diffraction including Rietveld structure refinements, quantitative i.r. analyses, and deuteration kinetics studies. At least some of the specimens differed with respect to each of the approximately 14 properties measured. The major lattice parameter differences could be largely accounted for by structurally incorporated H2O, CO2-3, and + O2- for 2(OH)-. Deuterizability was used as an indicator of ease of diffusion along the X-ion channels, a property that may be related to dissolution kinetics. The differently prepared specimens differed in deuterizability by at least two orders of magnitude. The high-temperature preparations, which were monoclinic, deuterated little at 110 degrees C, even in 1000 h. The precipitated and reflux specimens deuterated readily. There were general indications of correlation between ease of diffusion and features providing passing sites for the diffusing species, e.g., OH- disorder, vacancies, and distortions in the walls of the X-ion channels (mostly by CO3 for PO4), and possibly OH- vacancies. Correlation of structural H2O, present in the aqueous preparations, with ease of diffusion is still ambiguous.

Deuterium↗

PubMed1980

Thermal decomposition of human tooth enamel.

Further insight into human tooth enamel, dense fraction (TE), has been obtained by following the change and loss of CO3(2-), OH-, structurally incorporated H2O, Cl-, and, indirectly, HPO4(2-) after TE had been heated in N2 or vacuum in the range 25-1000 degrees C. Quantitative infrared spectroscopic, lattice parameter, and thermogravimetric measures were used. Loss of the CO3(2-) components begins at much lower temperature (e.g., 100 degrees C) than previously recognized, which has implications for treatments in vitro and possibly in vivo. CO3(2-) in B sites is lost continuously from the outset; the amount in A sites first decreases and then increases above 200 degrees to a maximum at approximately 800 degrees C (greater than 10% of the possible A sites filled), where it is responsible for an increase in a lattice parameter. A substantial fraction of the CO3(2-) in B sites moves to A sites before being evolved, apparently via a CO2 intermediary. This implies an interconnectedness of the A and B sites which may be significant in vivo. No loss of Cl- was observed at temperatures below 700-800 degrees C. Structural OH- content increases approximately 70% to a maximum near 400 degrees C. Structurally incorporated water is lost continuously up to approximately 800 degrees C with a sharp loss at 250-300 degrees C. The "sudden" a lattice parameter contraction, approximately 0.014 A, occurs at a kinetics-dependent temperature in the 250-300 degrees C range and is accompanied by reordering and the "sharp" loss of approximately 1/3 of the structurally incorporated H2O. The hypothesis that structurally incorporated H2O is the principal cause of the enlargement of the a lattice parameter of TE compared to hydroxyapatite (9.44 vs 9.42 A) is thus allowed by these experimental results.

Carbon Dioxide↗