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J C Mitchell

Publications and source records attributed to J C Mitchell.

At least 37 records · Page 2Linked to original sources
PubMed1996

X-ray diffraction studies of oxidized high-palladium alloys.

OBJECTIVE: The purpose of this study was to use x-ray diffraction (XRD) to obtain new information about the oxide layers on four representative oxidized high-palladium alloys. METHODS: Cast specimens of two Pd-Cu-Ga alloys and two Pd-Ga alloys, with both polished and etched surfaces and air-abraded surfaces, were subjected to oxidation procedures recommended by the manufacturers. The specimens were analyzed by x-ray diffraction using CuK alpha radiation, and the peaks were compared to appropriate Joint Committee on Powder Diffraction Standards (JCPDS). RESULTS: The surface preparation procedure had a profound effect on the phases present in the oxide layers. For the specimens that had been polished and etched, CuGa2O4 and beta-Ga2O3 were detected on the 79Pd-10Cu-9Ga-2Au alloy, whereas SnO2 and CuGa2O4 were detected on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy. The oxide layers on both Pd-Cu-Ga alloys contained Cu2O1 and the oxide layer on the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy may contain beta-Ga2O3. The principal phase in the oxide layers on both Pd-Ga alloys that had been polished and etched was ln2O3, which exhibited extreme preferred orientation. No other phase was detected in the oxide layer on the 85Pd-10Ga-2Au-1Ag-1 ln alloy, whereas beta-Ga2O3 was found in the oxide layer on the 75Pd-6Ga-6Au-6Ag-6.5ln alloy. For the air-abraded specimens, beta-Ga2O2 was not present in the oxide layers on the Pd-Cu-Ga alloys, and beta-Ga2O3 was the major phase in the oxide layers on the Pd-Ga alloys. Palladium oxide(s) in varying amounts were detected for both surface preparations of the Pd-Cu-Ga alloys and for the air-abraded Pd-Ga alloys. Except for the 76Pd-10Cu-5.5Ga-6Sn-2Au alloy, the oxide layer caused minimal change in the lattice parameter of the palladium solid solution compared to that for the as-cast alloy. SIGNIFICANCE: Knowledge of the phases found in the oxide layers on these high-palladium alloys is of fundamental importance for interpreting differences in the adherence of dental porcelain to the metal substrates under static and dynamic conditions, and may provide guidance in the development of new high-palladium alloys with improved metal-ceramic bonding.

Copper↗
PubMed1996

Metallurgical structure and microhardness of four new palladium-based alloys.

PURPOSE: This investigation compared the Vickers hardness and microstructures of four recently marketed, palladium-based alloys for metal-ceramic restorations. MATERIALS AND METHODS: Wax patterns simulating copings for maxillary central incisors were invested in a fine-grained, carbon-free, phosphate-bonded investment. Following burnout, the palladium alloys were fused with a gas-oxygen torch, centrifugally cast, and bench-cooled. Representative castings were embedded in transparent metallographic resin and sectioned to yield two mirror-image specimens. The specimens were evaluated in either the as-cast condition or following heat treatment simulating the firing cycles for Vita VMK porcelain. Vickers hardness measurements (n = 50) were made using a 1-kg load, and photomicrographs of polished and etched specimens were obtained with a scanning electron microscope. RESULTS: The measured values of microhardness for the as-cast alloys were in excellent agreement with values reported by the manufacturer. The hardness in the as-cast condition was significantly greater for the Pd-Cu-Ga-In alloy, compared with the other three alloys, which did not contain copper. For the three high-palladium (> or = 75 wt%) alloys, there were small (4%-8%) decreases in hardness following heat treatment, whereas a larger decrease (13%) in hardness occurred for the Pd-Ag-In-Sn alloy after heat treatment. The porcelain firing cycles caused microstructural homogenization for all four alloys, and the relatively thick near-surface oxidation region in the Pd-Cu-Ga-In and Pd-Ag-In-Sn alloys was not observed in the two heat-treated Pd-Ga-Ag-In-Au alloys. CONCLUSIONS: The multiphasic microstructures of these alloys may have some significance for the in vitro and clinical corrosion behavior and the metal-ceramic bond strength. The hardness for the three high-palladium alloys may be controlled by submicroscopic precipitates that remain unaltered by heat treatment. The significantly greater hardness for the Pd-Cu-Ga-In alloy may cause greater difficulty for finishing castings in the dental laboratory compared with the other three alloys studied. The strengthening mechanism for the Pd-Ag-In-Sn alloy has significant temperature dependence, which might be exploited to achieve optimum mechanical properties.

Analysis of Variance↗
PubMed1995

X-ray diffraction studies of as-cast high-palladium alloys.

OBJECTIVES: The purpose of this study was to use x-ray diffraction (XRD) to investigate four representative high-palladium alloys in the as-cast condition and obtain new information about the metallurgical phases. METHODS: Two Pd-Cu-Ga alloys and two Pd-Ga alloys were cast into plate-shaped specimens (20 mm x 1.5 mm) which were bench-cooled. Polished and etched specimens were analyzed using two different x-ray diffractometers and CuKa radiation. RESULTS: All four alloys exhibited strong XRD peaks for the face-centered cubic (fcc) palladium solid solution matrix, and variations in the lattice parameter were consistent with alloy compositions. Comparison of the relative peak intensities for the alloys and the pure palladium powder standard indicated that the as-cast microstructures had preferred crystallographic orientation. Because the two Pd-Cu-Ga alloys contained appreciable amounts of near-surface lamellar interdendritic or eutectic constituents, it was possible to determine previously unreported XRD peaks for the Pd2Ga phase in these alloys. Low-intensity peaks in the Pd-Ga alloys were attributed to small amounts of secondary phases observed in the microstructures. SIGNIFICANCE: Knowledge of the metallurgical structures of these high-palladium alloys can be important for interpretation of microstructural observations, mechanical properties, corrosion behavior and clinical performance.

Dental Alloys↗
PubMed1994

Induction of high grade astrocytoma (HGA) by protons: molecular mechanisms and RBE considerations.

Protons of a specific energy, 55 MeV, have been found to induce primary high grade astrocytomas (HGA) in the Rhesus monkey (Macaca mulatta). Brain tumors of this type were not induced by protons of other energies (32-2,300 MeV). Induction of HGA has been identified in human patients who have had radiation therapy to the head. We believe that the induction of HGA in the monkey is a consequence of dose distribution, not some unique "toxic" property of protons. Comparison of the human experience with the monkey data indicates the RBE for induction of brain tumors to be about one. It is unlikely that protons cause an unusual change in oncogenic expression, as compared to conventional electromagnetic radiation.

Animals↗
PubMed1994

Performance of engine-driven rotary endodontic instruments with a superimposed bending deflection: V. Gates Glidden and Peeso drills.

A laboratory study was performed on Gates Glidden and Peeso drills to determine the incidence of shaft fracture when a bending deflection was superimposed on the rotating drills. Samples of sizes #1 to #6 stainless steel Gates Glidden drills, sizes #1 to #6 stainless steel and carbon steel-type P Peeso drills, and sizes #009 to #023 carbon steel-type B-1 Peeso drills from each of two manufacturers were evaluated with a unique apparatus that applied a 2-mm bending deflection while rotating the instruments. The apparatus did not restrict movement of the bur head during rotation. The test drills were rotated at 2500, 4000, and 7000 revolutions per minute, and the number of revolutions at failure was recorded. Scanning electron microscopic observations established that the stainless steel Gates Glidden and Peeso drills failed by ductile fracture, whereas the carbon steel Peeso drills failed by brittle fracture. Instrument fracture was always near the handpiece shank with this test, and the length of the fractured drills was measured from the working tip. It is recommended that this additional test be adopted to determine fatigue properties of engine-driven rotary endodontic instruments in establishing international performance standards.

Dental High-Speed Equipment↗
PubMed1993

Activity of yeast FLP recombinase in maize and rice protoplasts.

We have demonstrated that a yeast FLP/FRT site-specific recombination system functions in maize and rice protoplasts. FLP recombinase activity was monitored by reactivation of beta-glucuronidase (GUS) expression from vectors containing the gusA gene inactivated by insertion of two FRTs (FLP recombination targets) and a 1.31 kb DNA fragment. The stimulation of GUS activity in protoplasts cotransformed with vectors containing FRT inactivated gusA gene and a chimeric FLP gene depended on both the expression of the FLP recombinase and the presence and structure of the FRT sites. The FLP enzyme could mediate inter- and intramolecular recombination in plant protoplasts. These results provide evidence that a yeast recombination system can function efficiently in plant cells, and that its performance can be manipulated by structural modification of the FRT sites.

Base Sequence↗
PubMed1993

Histologic evaluation of periodontal attachment apparatus following the insertion of a biodegradable copolymer barrier in humans.

The purpose of this study was to histologically evaluate the effectiveness of polylactide:polyglycolide 50:50 (DL-PLGA) as a barrier to prevent epithelial migration and to promote new connective tissue attachment. Mucoperiosteal flaps were performed on 17 human teeth. DL-PLGA membrane was placed over the roots and alveolar bone. DL-PLGA was not placed over control teeth. Ninety days following surgery, block sections were obtained and processed for histologic evaluation. All specimens revealed a mean length of epithelial attachment within physiologic limits and recession of the gingival margin. There was little or no reattachment of the connective tissue. It is concluded that this copolymer barrier did not prevent epithelial migration nor enhance connective tissue attachment to human roots with severe horizontal bone loss and active periodontal disease.

Adult↗
PubMed1992

Measurement of bile acid synthesis in man by release of 14CO2 from [26-14C]cholesterol: comparison to isotope dilution and assessment of optimum cholesterol specific activity.

Bile acid synthesis can be measured as release of 14CO2 from [26-14C]cholesterol divided by cholesterol specific activity, but this method has not been validated in human subjects. We made twelve comparisons of this CO2 method to standard isotope dilution in six normal subjects and found a mean discrepancy of 6%. Linear regression analysis of one value with respect to the other revealed a correlation coefficient of 0.83 (P less than 0.01), a Y-intercept close to zero (-4.98) and a slope close to 1 (1.06), suggesting good correspondence between the two methods. To assess the potential for error arising from use of serum cholesterol to estimate specific activity of cholesterol used for bile acid synthesis, we compared synthesis measured using serum free cholesterol specific activity to that measured using bile cholesterol specific activity, which is known to be near isotopic equilibrium with the precursor pool used for bile acid synthesis. Synthesis calculated in these two ways differed by less than 10%. The data indicate that the CO2 method using either serum or bile cholesterol specific activity provides a valid estimate of bile acid synthesis in man.

Bile Acids and Salts↗
PubMed1991

A review of the USAF/NASA proton bioeffects project: rationale and acute effects.

Ionizing radiation represented one of the important hazards facing the first manned lunar mission. A combined USAF/NASA project was conducted from 1963 through 1969 to estimate the relative biological effectiveness (RBE) of the radiations of space. Approximately 2000 primates (Macaca mulatta) and 5000 mice were irradiated with protons and electromagnetic radiations. The proton energies studied were selected to be representative of the proton spectrum in space. Much of the project was concerned with the use of cyclotrons for proton irradiations and with dosimetry. Biological measurements included clinical findings, physiological changes, hematological changes, histopathology, and mortality. When allowance was made for variation of response as a consequence of depth-dose distribution, the RBE for protons was approximately 1. This was anticipated from earlier theoretical studies and radiation therapy in humans with high-energy charged-particle beams.

Acute Disease↗
PubMed1991

Role of cholesterol synthesis in regulation of bile acid synthesis and biliary cholesterol secretion in humans.

We used lovastatin, a specific inhibitor of HMG-CoA reductase, to study the role of cholesterol synthesis in regulation of both bile acid synthesis, measured by release of 14CO2 from [26-14C]cholesterol, and biliary cholesterol secretion, measured by standard marked perfusion techniques, in humans. Six volunteers were studied in each of four periods: a) control; b) 6-10 hours after a single 40 mg oral dose of lovastatin to study acute effects; c) after 5-6 weeks of lovastatin 40 mg orally twice a day to study steady-state effects; and d) 24 h after cessation of chronic lovastatin. Mean bile acid synthesis fell to 69% of control (P less than 0.01) after single-dose lovastatin and remained at 83% of control after 5-6 weeks on lovastatin (P less than 0.05). After withdrawal of lovastatin, mean bile acid synthesis was 88% of control (NS). Mean biliary cholesterol secretion did not change after single-dose lovastatin (103% of control), but fell to 81% of control during chronic lovastatin treatment (P less than 0.05). After withdrawal of lovastatin, mean cholesterol secretion remained at 80% of control (P less than 0.05). These data suggest that in humans cholesterol synthesis is an immediate regulator of bile acid synthesis. Cholesterol synthesis also regulates biliary cholesterol secretion, but the effect is not immediate and therefore may be indirect.

Aged↗
PubMed1991

Effects of lovastatin on biliary lipid secretion and bile acid metabolism in humans.

Lovastatin, an inhibitor of HMG-CoA reductase, lowers cholesterol saturation of bile. To determine the mechanism of this effect and further define the role of cholesterol synthesis in regulation of biliary lipid metabolism, we studied ten human volunteers in a control period and again after 5-6 weeks on lovastatin, 40 mg b.i.d. Mean sterol production from acetate in mononuclear leukocytes fell from 1.18 to 0.84 pmol/min per 10(6) cells on lovastatin (P less than 0.02). Concomitantly there was reduction in mean biliary secretion of cholesterol from 143 to 96 mumol/h (P less than 0.02). On lovastatin, mean pool size of bile acids by the Lindstedt method fell from 3193 to 2917 mumol (one-sided P = 0.05) and mean pool size by the one-sample method fell from 5158 to 4091 mumol (P less than 0.002). Lovastatin had no effect on mean fractional turnover rate of either cholic acid (0.77 vs. 0.74 day-1) or chenodeoxycholic acid (0.51 vs. 0.54 day-1). Mean total bile acid synthesis was lower on lovastatin (1443 vs. 1240 mumol/day), but the difference did not quite achieve statistical significance. In humans, inhibition of cholesterol synthesis by lovastatin lowers biliary cholesterol saturation by reducing cholesterol secretion into bile. Bile acid pool size, and perhaps bile acid synthesis, are also reduced by this inhibition.

Adult↗
PubMed1990

Methylchloroisothiazolinone-methylisothiazolinone reactions in patients screened for vehicle and preservative hypersensitivity.

More than 1100 patients were tested with methylchloroisothiazolinone-methylisothiazolinone, 100 ppm, in aqueous and petrolatum-based patch test materials from 1985 to 1987 by members of the North American Contact Dermatitis Group. Thirteen reactions to the aqueous materials and 10 to the petrolatum-based materials were observed. Irritant reactions were infrequent, and about half the reactions were deemed relevant. From 1984 to 1985, patch tests with this substance at a concentration of 250 ppm in petrolatum were conducted. Thirteen persons were identified as allergic, but three others were sensitized by the patch test procedure. Sensitization as not observed in tests with aqueous or petrolatum-based substance at a concentration of 100 ppm, and this concentration appears to be the best compromise between safety and sensitive detection of allergy. Use tests are helpful but not infallible as a guide in establishing relevance with methylchloroisothiazolinone-methylisothiazolinone. Wash-off products are frequently well tolerated by patients with positive reactions to this substance.

Adult↗
PubMed1989

Physical chemical evidence of structural weakness in coronary arterial calcification.

Physical-chemical properties of calcified human coronary arteries were obtained through 857 electron probe microanalyses of samples of endarterectomy tissue from 12 patients and by X-ray and electron diffraction from separate samples. The calcific phase exhibited a weight percentage calcium to phosphorus ratio of 2.5 (SEM 0.15), a density of 2.95 (0.02) g.cm-3, and a crystalline unit cell volume of 530 (0.5) A3, or 53 (0.05) nm3. The phase has been characterised as a phosphorus deficient dahllite variant of apatite in which the crystal structure has been weakened by the incorporation of water molecules and tetrahedral hydroxyl groups in isomorphic substitution conforming with the McConnell-type defect. Isomorphic substitution within the apatitic cell appears sufficient to suggest that the crystal structure of the mineral deposit is weakened to the extent that means might be sought to attack the early formation of the pathological deposition.

Apatites↗
PubMed1988

A sensitive assay for 6-thioguanine-resistant lymphocytes.

We have defined variants (V) resistant to 6-thioguanine (TG) by their ability to divide at least once during a 72-h incubation in a medium containing 0.2 mM TG. We blocked cytokinesis by adding cytochalasin B (CB) after 30 h in culture. The cells that had undergone nuclear division were identified by their content of 2 or more nuclei. The long incubation period allowed slow growing V to be counted. As a result we scored an order of magnitude more V than have been reported in assays using the conventional 30-40 h culture times. In the gamma-ray dose range of 0-0.5 Gy we scored 80 V per 1000 surviving lymphocytes per Gy--a result some two orders of magnitude larger than has been reported previously.

Adult↗