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P W Brown

Publications and source records attributed to P W Brown.

At least 19 recordsLinked to original sources
PubMed2001

Characterization of wollastonite-reinforced HAp--Ca polycarboxylate composites.

The effects of wollastonite on the mechanical properties and in vitro behavior of hydroxyapatite-Ca polyacrylate composites were studied. Powder mixtures of tetracalcium phosphate, poly(acrylic-co-itaconic), and wollastonite fibers (< or =75% by weight) were hot-pressed for 30 min at 300 degrees C and 60 kpsi. Tensile strengths, elastic moduli, and microstructures of the composites were investigated. The tensile strengths of these composites were improved by the addition of wollastonite fibers, whereas the elastic moduli decreased. The highest value of tensile strength (approximately 155 MPa) was achieved by the addition of 40% wollastonite. Composites were immersed in simulated body fluid (SBF) for up to 14 days and then in 1.5 SBF for a week. The changes in the concentrations of Ca, Si, and P ions and the pH of these solutions indicate bioactivity. An evaluation of the microstructures of the composites after SBF immersion indicated that apatite layers had formed on the surfaces of the composites.

Acrylates↗
PubMed2001

Chemically formed HAp-Ca poly(vinyl phosphonate) composites.

The formation of biocompatible organic-inorganic composites by reactions between tetracalcium phosphate (Ca4(PO4)2O, TetCP) and the biomedical polymer poly(vinyl phosphonic acid) (PVPA) is described. Composites were prepared by hot pressing mixtures of these powders at 80 kpsi and 300 degrees C for 30 min. Composite formation was investigated depending on the proportions of reactants and the processing route used. Two inorganic phases were produced as a result of the acid-base reaction between TetCP and PVPA: hydroxyapatite (Ca10(PO4)6(OH)2, HAp) and anhydrous dicalcium phosphate (CaHPO4, DCPA). The later phase preferentially formed at lower TetCP/PVPA ratios while the amount of HAp increased with increasing TetCP/PVPA ratio. The reactions appear to start with the softening of the polymer when heated to T > Tg. The flowing polymer surrounds the TetCP grains permitting the TetCP to initially form DCPA crystallites in a matrix of the Ca salt of the polymer. When H2O is added prior to pressing, the DCPA produced reacts with the remaining TetCP forming HAp.

Biocompatible Materials↗
PubMed2001

Herniography: a prospective, randomized study between midline and left iliac fossa puncture techniques.

AIM: To determine whether an optimal site of injection exists for herniography. MATERIALS AND METHODS: This was a prospective, randomized study of 93 consecutive patients who were referred for herniography over a period of 9 months. Patients underwent either a left iliac fossa (LIF) or midline puncture. Parameters assessed included initial adequate needle placement, complications, pain scores and body mass index (BMI). The groups were compared using Chi-squared test for categorical data, Student's t-test for continuous data and the Mann-WhitneyU-test for skewed data, withP < 0.05 considered statistically significant RESULTS: Four complications were encountered (4%), and these were equally distributed between the two groups. Adequate initial positioning of the needle was similar in both groups. The volume of local anaesthetic used was correlated with discomfort using a pain scale: a volume of >6 ml resulted in significantly more pain. More frequent initial adequate needle placement was observed in thin patients (BMI < 45 kg/m(2)) with experienced operators. Conversely, increased body mass index resulted in more difficult needle placement. CONCLUSION: Herniography is a safe procedure with few complications. There was no significant difference comparing the midline and LIF approaches.Nadkarni, S.et al. (2001). Clinical Radiology56, 389-392.

Anesthetics, Local↗
PubMed2001

Sclerosing encapsulating peritonitis: a case series from a single U.K. center during a 10-year period.

Sclerosing encapsulating peritonitis (SEP) is a serious complication of peritoneal dialysis (PD). Previous reports place the prevalence of SEP at 0.54%-7.3%. We estimated the prevalence of SEP in our unit to be 1.4% over the period 1989-1999. We here present the 6 identified cases. All of the patients presented with small-bowel obstruction; hemorrhagic ascites was identified in 3 cases. All 6 patients experienced ultrafiltration inadequacy, and 5 were treated with glucose polymer (icodextrin; duration of treatment: 1 month-2.5 years). Peritoneal dialysis was stopped at the time of diagnosis in 2 cases. In the other 4 cases, PD had been withdrawn some time prior to the SEP being diagnosed (2 weeks-5 years). Five of the patients have died; the 6th currently uses hemodialysis.

Adult↗
PubMed2000

Characterization of bioactive glass-reinforced HAP-polymer composites.

The effect of bioactive glass on the mechanical properties of hydroxyapatite-Ca polyacrylate composites was studied. Powder mixtures of tetracalcium phosphate (TetCP), poly(acrylic-co-itaconic) and bioactive glass (up to 50% by weight) were hot pressed for 30 min at 300 degrees C and 40 kpsi. Tensile strengths, elastic moduli, and microstructures of the composites produced were investigated. Results showed the mechanical properties of these composites were enhanced by the addition of bioactive glass. The highest values of tensile strength and elastic modulus were achieved with the addition of 10% bioactive glass. Composites were immersed in SBF for up to 10 days, then in 1.5 simulated body fluid (SBF) for a week. The changes in the concentrations of Ca, P, and Si ions of these solutions were measured. The microstructures of these composites after SBF immersion were also evaluated. Concentrations of Ca, P, and Si increased with the time of immersion in SBF owing to the formation of an apatite layer on their surfaces as found by SEM with energy-dispersive spectroscopy attachment.

Biocompatible Materials↗
PubMed2000

Low temperature formation of calcium-deficient hydroxyapatite-PLA/PLGA composites.

Hydroxyapatite-biodegradable polymer composites have been formed by a low temperature chemical route. Precomposite structures were prepared by combining alpha-Ca(3)(PO(4))(2) (alpha-tricalcium phosphate or alpha-TCP) with poly(L-lactic) acid and poly(DL-lactide-co-glycolide) copolymers. The final composite structure was achieved by in situ hydrolysis of alpha-TCP to Ca(9)(HPO(4))(PO(4))(5)OH (calcium deficient hydroxyapatite or CDHAp) either in solvent cast or pressed precomposites. Hydrolysis was performed at 56 degrees C-a temperature slightly above the glass transition of the polymers. The effects of polymer chemistry, composite formation technique, and porosity on hydrolysis kinetics and degree of transformation were examined with isothermal calorimetry, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, and scanning electron microscopy. Calorimetric data and XRD analyses revealed that hydrolysis reactions were inhibited in the presence of the polymers. Isothermal calorimetry indicated the extent of the alpha-TCP to CDHAp transformation in 24 h to be 85% in the solvent cast composites containing PLGA (85:15) copolymer; however, XRD analyses suggested almost complete reaction. The CDHAp formation extent was 26% for the pressed composites containing the same polymer. In the presence of NaCl as a pore generator, 81% transformation was observed for the pressed composites. This transformation occurred without any chemical reaction between the polymer-inorganic components, as determined by Fourier transform infrared spectroscopy. Minimal transformation to CDHAp occurred in composites containing poly(L-lactic) acid.

Biocompatible Materials↗
PubMed2000

Calcium-deficient hydroxyapatite-PLGA composites: mechanical and microstructural investigation.

The microstructural and mechanical properties of composites composed of calcium deficient hydroxyapatite (CDHAp) and poly(lactide-co-glycolide) (PLGA) have been investigated. The composites were formed by hydrolysis of alpha-tricalcium phosphate (alpha-TCP) to CDHAp in pressed precomposite compacts of alpha-TCP-PLGA-NaCl. The differences in hydrolysis of alpha-TCP-PLGA-NaCl for two compositions of 80:10:10 wt % and 60:20:20 wt %. were monitored by isothermal calorimetry and X-ray diffraction. The microstructural evolution and variance in final composite microstructure after hydrolysis at 37 degrees C, 45 degrees C, and 56 degrees C were examined by scanning electron microscopy. HAp-PLGA composite formed from the alpha-TCP-PLGA-NaCl (80:10:10) precomposites at 37 degrees C developed a tensile strength of 13.3 +/- 0.9 MPa, a flexural strength of 24.8 +/- 1.7 MPa, and Young's modulus of 2.8 +/- 0.3 GPa. These values were 12.00 +/- 0.2 MPa, 36.1 +/- 2.1 MPa, and 5.5 +/- 0.8 GPa for the precomposite composition 60:20:20. All these mechanical properties showed a variation with hydrolysis temperature and composition. The differences in mechanical properties were related to the final microstructures of the composites, which are governed by the morphological changes in the polymer structure at its glass transition temperature and the extent of cement-type formation of CDHAp by hydrolysis of alpha-TCP.

Biomechanical Phenomena↗
PubMed2000

An evaluation of mechanical property and microstructural development in HAP-Ca polycarboxylate biocomposites prepared by hot pressing.

A hot-pressing technique was used to prepare composites anticipated to be biocompatible. Ca(4)(PO(4))(2)O (TetCP) was reacted with an acrylic-itaconic copolymer (CoP) in the absence of a solvent to form composites comprised of Ca(10)(PO(4))(6)(OH)(2') (hydroxyapatite, or HAp) and the Ca polyalkenoate salt. The effect of temperature, pressure, and hot-pressing time on the mechanical properties and microstructure of the composites were studied. Results showed that both tensile strength and elastic modulus increased when temperature and time were increased. When the compaction pressure was increased, these properties initially increased but decreased at high pressures. These variations in the mechanical properties were correlated with the microstructure of these composites. The mechanism of the reaction was also studied. Reaction starts when the copolymer is heated to above its T(g) permitting it to flow and react with the TetCP grains. The COOH groups on the polymer are neutralized by Ca(2+) ions liberated from the TetCP. At the end of reaction, a network of the Ca polyalkenoate salt is formed in which HAp crystals are embedded.

Biocompatible Materials↗
PubMed2000

Designing an ambulatory clinical practice for outcomes improvement: from vision to reality--the Spine Center at Dartmouth-Hitchcock, year one.

Development of a new program for diagnosis and treatment of spine-related problems provided a unique opportunity to design and implement a new model for delivery of health care incorporating outcomes measurement and improvement. Key features include: application of microsystem thinking and interdisciplinary practice; integration of a uniform outcomes measurement tool, the Dartmouth Clinical Value Compass; and touch pad technology for data collection. This, for the first time, provided clinically meaningful point-of-service data and aggregated information for improvement. A further advantage was the ability to integrate a clinical research program within this microsystem. A multisite clinical research trial, the Spine Patient Outcomes Research Trial (SPORT), modeled on the Spine Center microsystem and funded by The National Institute of Arthritis, Musculoskeletal and Skin Diseases and the Office of Research on Woman's Health, the National Institutes of Health, and the National Institute of Occupational Safety and Health, the Centers for Disease Control and Prevention, is currently underway. The significant problems we face today cannot be solved by the same level of thinking that created them.

Ambulatory Care↗
PubMed2000

Influence of diet on the hematology and serum biochemistry of zinc-intoxicated mallards.

Changes in hematological and serum biochemistry parameters in female zinc (Zn)-dosed farm-raised mallards (Anas platyrhynchos) fed four different diets were examined. Sixty ducks received an average dose of 0.97 g of Zn in the form of eight, 3.30-mm diameter shot pellets containing 98% Zn and 2% tin, and another 60 ducks were sham-dosed as controls. Fifteen ducks from each of the two dosing groups were assigned to one of four dietary treatments: corn only, corn with soil, commercial duck ration only, or commercial duck ration with soil. Shot-pellet dissolution rates ranged from 7 mg/Zn/day to 27 mg/Zn/day. Regardless of diet, the Zn dose resulted in mortality; incoordination; paralysis and anorexia; decreased body, liver, pancreas, gonad, and gizzard weight; increased kidney weight; and macroscopic lesions. Zn-dosed ducks had a lower mean erythrocyte packed cell volume (PCV), higher mean reticulocyte count, and a greater number of individuals with immature and/or abnormal erythrocytes, than did control mallards. Mean total leucocyte counts were higher in Zn-dosed ducks than in controls. Zn-dosed ducks that had soil available had higher leucocyte counts than those without soil. Zn-dosed ducks were characterized by a marked heterophilia and relative lymphopenia. In Zn-dosed ducks, the mean lymphocyte count was highest in those provided a commercial duck ration, and lowest in those fed corn. In control ducks, the mean lymphocyte count was highest in ducks fed corn, and lowest in those provided soil along with a commercial duck ration. Zn-dosed mallards had higher serum aspartate aminotransferase and amylase levels, and lower alkaline phosphatase activities than control ducks. Serum phosphorus and uric acid concentrations were higher, and calcium, glucose, and total protein levels lower, in Zn-dosed ducks than in control ducks. Diet did affect serum calcium, phosphorus, total protein, and uric acid concentrations. Differences in erythrocyte and leucocyte parameters, serum enzyme activities, and metabolite concentrations were associated with dose and diet effects. Diets high in protein and other organic matter and calcium and phosphorus did not prevent or substantially alleviate Zn toxicosis in farm-raised mallard ducks.

Animal Feed↗
PubMed1999

Hydrolysis of alpha-tricalcium phosphate in NaF solutions.

Controlling the incorporation of Na and fluoride in apatite-based dental restoratives and analogs of bone formed under physiologic conditions may be desirable in controlling their long-term in vivo responses. Formation of these analogs was investigated. Hydrolysis of alpha-tricalcium phosphate (alpha-TCP) was carried out at physiologic temperature in NaF solutions and the solids product(s) were analyzed. alpha-TCP hydrolysis was complete within 48 h and the only solid products identified were apatites. Changes in solution chemistry were determined at 48 h and 4 weeks. Na and Ca concentrations were determined by spectrometric methods, P and F concentrations were determined by ion chromatography; pH values were obtained. The extent of fluoride and sodium incorporation in the apatites formed varied depending on the NaF concentration of the solution. Both increased with increasing NaF concentration. Apatite compositions ranged from Ca9.06(HPO4)0.94(PO4)5.06OH1.06 for hydrolysis in water to Ca9.28Na0.56(HPO4)(x)(PO4)(6-x)F0.920Hy for hydrolysis in 0.1 M NaF solution. The proportions of PO4, HPO4 and OH in Na- and F-containing apatites are indeterminate but obey the equation (y = constant+x). The (Ca+Na)/P ratio of the apatite formed by alpha-TCP hydrolysis in 0.1M NaF approaches 1.67 indicating the vacant Ca sites become almost completely filled by the Na ions. The surface areas of these apatites initially remain constant with increasing NaF concentrations; at elevated concentrations apatite surface areas decrease with increasing NaF concentration. This is consistent with the removal of Na from solution by its incorporation in the apatites, not by its adsorption onto their surfaces.

Adsorption↗
PubMed1999

Primary aortojejunal fistula: a case report.

Primary aortoenteric fistulae (AEFs) are extremely rare vascular entities, with fewer than 250 cases reported in the world medical literature as of 1996. Incidence is less than 1 per cent, with a mortality ranging from 33 to 85 per cent. Atherosclerosis remains the most common etiology, accounting for more than two-thirds of the cases reported. Other etiologies include carcinoma, ulcers, gallstones, diverticulitis, appendicitis, and foreign bodies. Early diagnosis is crucial for survival and mandates recognition of the typical "herald bleed." Additional findings on initial presentation frequently include flank pain, abdominal pain, hematemesis, melena, and an abdominal mass. More than 80 per cent of primary AEFs involve the duodenum, with the overwhelming majority located in the third or fourth portion. Successful management of primary AEF requires a high index of suspicion for diagnosis and prompt surgical intervention for survival.

Aortic Diseases↗
PubMed1998

Fluoride uptake by hydroxyapatite formed by the hydrolysis of alpha-tricalcium phosphate.

Although there is interest in forming synthetic analogs of hard tissues at physiologic temperature, significant gaps in knowledge exist with respect to the mechanisms by which precursor solids convert to apatites and also with respect to the apatite compositions that may be formed. In this study calcium-deficient HAp [Ca9(HPO4)(PO4)5OH] was prepared by hydrolysis of tricalcium phosphate (TCP), alpha-Ca3(PO4)2. The kinetics of HAp formation were studied as a function of temperature by isothermal calorimetry. TCP hydrolyzed completely within about 12 h, and the hydrolysis reaction evolved 133 kJ/mol of HAp formed. Although the kinetics of hydrolysis exhibited a strong temperature dependence, the mechanistic path taken appeared independent of temperature. The fluoridation of hydroxyapatite compositions having Ca/P ratios higher than 1.59 previously has been investigated. However, little work has been done on the fluoridation of more calcium-deficient hydroxyapatite. Ca9(HPO4)(PO4)5OH was formed at temperatures between 37.4 degrees and 55 degrees C to vary its morphology. These preparations then were reacted in NaF solution and the kinetics of fluoride incorporation studied. Solution chemical analyses were used to determine the amounts of fluoride incorporated. The extent of hydroxyl replacement by fluoride ranged from 17 to 72% and correlated with the surface area of the parent HAp.

Calcium Phosphates↗
PubMed1998

Formation of calcium-deficient hydroxyapatite from alpha-tricalcium phosphate.

This study investigated the factors influencing the kinetics of Ca9HPO4(PO4)5OH (calcium deficient hydroxyapatite or CDHAp) formation from alpha-Ca3(PO4)2 (alpha-TCP). The kinetics of CDHAp formation were investigated by isothermal calorimetry at constant temperatures ranging between 30 and 75 degrees C and by changes in pH at 37.4 and 70 degrees C. The calorimetric curves were characterized by two reaction peaks. Activation energies were calculated for the events resulting in these peaks. Values obtained were 48.4 and 67.7 kJ mol(-1), respectively, indicating nucleation and growth mechanisms for both events. Temperature had a significant effect on the growth rate as indicated by a decrease in surface area (26.5-15.0 m2 g(-1)) of the CDHAp with increasing temperature (30-75 degrees C). A linear relationship between hydrolysis temperature and CDHAp surface area was observed. The morphology of the CDHAp was plate-like and the crystallites became more regular as the reaction temperature was increased. A rapid elevation in pH upon mixing with water indicated the synthesis method initially used did not entirely eliminate slight compositional variations within the alpha-Ca3(PO4)2. Rapid elevation in pH retarded subsequent reaction. This effect was eliminated by increasing the duration of high-temperature firing during alpha-TCP synthesizing.

Biocompatible Materials↗
PubMed1998

Out on a limb.

Explore the source record for details and available documents.

Hand↗
PubMed1998

Comparative neuropathology of Kuru with the new variant of Creutzfeldt-Jakob disease: evidence for strain of agent predominating over genotype of host.

The three major influences on the phenotype of the transmissible spongiform encephalopathies are believed to be strain of agent, route of infection and host genotype. We have compared the pathologic profiles and genotypes of the new variant of Creutzfeldt-Jakob disease (vCJD) and kuru. The comparison reveals that there are distinct lesional differences particularly in the prion protein (PrP) load and distribution as seen by immunohistochemistry. The clinico-pathologic phenotypes and the genotypes of these two diseases are sufficiently different to suggest that the strain of agent may play a greater role than any presumptive common route of peripherally acquired infection.

Adolescent↗
PubMed1998

Hydrolysis of CaHPO4 in sodium fluoride solutions at 37.4 degrees C.

A possible chemical process occurring during caries reversal is conversion of acidic calcium phosphates to apatite. The role of fluoride in this process is of particular interest. The effects of fluoride on the rate of CaHPO4 hydrolysis at 37.4 degrees C were studied by a multimethod analysis involving X-ray diffraction, analyses of variations in solution chemistry, and observation of microstructural evolution. Hydrolysis in low NaF concentrations results in the formation of discrete fluorapatite crystals on the surfaces of the CaHPO4 crystallites. At CaHPO4/NaF molar ratios from approximately 9:1 to 10:2, fluorapatite formed in approximately 3 h as the only crystalline product and complete hydrolysis of CaHPO4 occurred; a pH value as low as 2.4 was attained with solution species being predominantly sodium phosphate. At NaF concentrations beyond those which result in pH minima, fluorapatite and CaF2 are the crystalline products. At 0.6 M NaF, pseudomorphs composed of fluorapatite and CaF2 crystals form without developing morphologies characteristic of individual fluorapatite and CaF2 crystals. CaHPO2 can hydrolyze completely to fluorapatite and CaF2 within a few hours depending on NaF concentration and liquid-to-solids ratio.

Apatites↗