Biomedical subjects

R J Mitchell

Publications and source records attributed to R J Mitchell.

At least 55 records · Page 3Linked to original sources

PubMed1995

A comparison of infrared- and torch-soldering of Au-Pd and Co-Cr metal-ceramic alloys using a high-fusing solder.

PURPOSE: The purpose of this study was to compare the bond strengths and fracture modes of soldered joints formed by infrared and gas-oxygen torch heating of a high-fusing, gold-based solder in two metal-ceramic alloys. MATERIALS AND METHODS: Pairs of half-dumbbell-shaped specimens of either an Au-Pd or a Co-Cr alloy were positioned with a 0.25-mm gap and were joined using a high-fusing, gold-based solder by either gas-oxygen torch heating or infrared heating. The resulting specimens were subjected to a heat treatment that simulated ceramic firing. Each specimen was fractured in tension at a loading rate of 0.5 cm/min, and its bond strength was measured. The halves of the specimen were rejoined using the other heating method and were heat treated, and the specimen's bond strength was again measured. Fractured cross-sections were examined at a magnification of 40x to determine the following: the fracture mode (adhesive, cohesive, or mixed); the percentage of adhesive fracture; the presence or absence of voids, porosities, or flux inclusions; and the percentage of the cross-section that was discolored. RESULTS: Three-factor analyses of variance showed that neither the heating method, the particular specimen tested, nor the order of testing significantly affected the bond strength (P < .05). For each alloy, significantly fewer infrared-heated joints than torch-heated joints contained voids, porosities, or flux inclusions visible at 40x (chi 2 test, P < .05). All fractures were either entirely cohesive in the solder or mixed cohesive and adhesive. When infrared heating was used, entirely cohesive fractures occurred more frequently in the Au-Pd alloy specimens than in the Co-Cr alloy specimens (chi 2 test, P < .05). The coefficients of variation of the bond strengths for the infrared-heated joints were smaller than those for the torch-heated joints. CONCLUSIONS: Although the two heating methods produced solder joints that had strengths that were not significantly different, infrared-heated joints showed less scatter in bond strengths. It was suggested that, in the hands of most technicians, fewer infrared-heated joints would contain defects visible at a magnification of 40x. The presence of such defects may increase the probability of in vivo failure caused by cyclic stresses.

Analysis of Variance↗

PubMed1995

Expression of plasminogen activator-inhibitor-1 (PAI-1) during cellular remodeling in proliferative glomerulonephritis in the rat.

Pericellular proteolysis involves the plasminogen activator/plasmin system and plays an important role in cell remodeling involving cell migration and extracellular matrix turnover. Studies in this laboratory have previously characterized a model of proliferative glomerulonephritis induced by Habu snake venom (HSV) in the rat that involves cell migration, proliferation, and extracellular matrix accumulation. Because plasminogen activator-inhibitor-1 (PAI-1) has been used as a marker for cell migration as well as matrix accumulation, we were interested in examining the temporal and spatial expression and cellular sources of PAI-1 mRNA and translated protein over the course of HSV-induced proliferative glomerulonephritis. The results showed a highly localized and progressive expression of PAI-1 mRNA and translated protein by in situ hybridization and immunohistochemistry at the margins and periphery of glomerular lesions 8 and 24 hr after HSV. The expression of PAI-1 in glomerular lesions localized to the same sites as mesangial cell marker proteins, desmin and Thy-1.1, indicating that mesangial cells synthesize this important regulator proteolysis. Few cells expressed PAI-1 in the central aspects of glomerular lesions at later time intervals (48 and 72 hr) when cell proliferation and expression of extracellular matrix (fibronectin protein and mRNA) were maximal. Therefore, the expression of PAI-1 in this model was associated more with early events related to cell migration than with proliferation or extracellular matrix synthesis. These observations support the hypothesis that the plasminogen activator/plasmin system is involved in cell migration in early remodeling during glomerular disease.

Animals↗

PubMed1995

Expression of alternatively spliced fibronectin variants during remodeling in proliferative glomerulonephritis.

Fibronectin (Fn) plays an important role in tissue remodeling during embryogenesis, wound repair, and vascular disease, and is thought to regulate cellular processes such as cell adhesion, migration, proliferation, and differentiation through specialized domains within the molecule. In addition, Fn can be alternatively spliced at three regions: extradomains EIIIA, EIIIB, and a variable segment V, potentially giving rise to functionally distinct variants of the molecule. We have previously shown a sequential expression of cellular Fn first by platelets, followed by macrophages, then mesangial cells in habu snake venom-induced proliferative glomerulonephritis (Am J Pathol 145: 585-597, 1994). These studies examined the cellular sources and glomerular localization of Fn in general but did not distinguish between the various alternatively spliced isoforms. In this study, we examine by in situ hybridization and immunohistochemistry the temporal expression and cellular sources of EIIIA, EIIIB, and V in a model of proliferation glomerulonephritis that has cell migration, proliferation, and extracellular matrix synthesis as features of tissue remodeling. Macrophages were the first cells to express Fn mRNA showing an EIIIA+, EIIIB-, and V95+ pattern beginning at 8 hours after habu snake venom injection. Migrating mesangial cells at the margins of early lesions (8 and 24 hours) did not overexpress mRNA encoding these Fn variants, but immunofluorescence microscopy revealed V95 and EIIIA protein at the margins of lesions. EIIIB was absent in lesions at this time. At 48 hours and peaking at 72 hours after habu snake venom injection, mesangial cells in central aspects of glomerular lesions expressed abundant mRNA and protein for V95 and EIIIA. EIIIB mRNA and protein was slight in the mesangium at these times. Parietal epithelial cells, particularly adjacent to glomerular lesions, also expressed abundant mRNA and protein for all three variants throughout the course of the disease, beginning at 24 hours after habu snake venom injection. Expression of mRNA and protein for all three isoforms declined by 2 weeks after habu snake venom injection. These studies show that migrating mesangial cells do not require their own synthesis of Fn and suggest that they might rely on exogenous sources of Fn, particularly V95+ and EIIIA+ forms. Commencement of enhanced expression of EIIIA and EIIIB mRNA and protein by resident glomerular cells coincided with the temporal course of cell proliferation, acquisition of alpha-smooth muscle cell actin phenotype, and matrix synthesis, suggesting that Fn isoforms have specific functions during the course of glomerular remodeling.

Alternative Splicing↗

PubMed1994

Hydroxyapatite orbital implants. Scanning with technetium-99m MDP.

Porous hydroxyapatite spheres are an ideal prosthetic device for orbital implantation because they are incorporated into soft tissues. Once vascularized, an eye prosthesis can be coupled to the sphere by a peg placed within a central motility hole. This hole should not be drilled while the sphere is avascular because of the risk of infection. Radionuclide scanning with Tc-99m methylene diphosphonate has been used to assess implant vascular ingrowth because radiophosphonate deposition within the sphere parallels vascularization. Using this technique, the authors examined the hydroxyapatite implants of 15 patients 6 months following insertion. Results showed that complete vascularization is best defined by planar imaging rather than SPECT. While the relative intensity of implant activity may be an important feature, uniformity of activity is probably more significant.

Durapatite↗

PubMed1994

Polymorphisms of the gene coding for the cholesteryl ester transfer protein and plasma lipid levels in Italian and Greek migrants to Australia.

The relation between TaqI restriction fragment length polymorphisms (RFLPs) of the cholesteryl ester transfer protein (CETP) gene and plasma lipid and lipoprotein phenotypes was investigated in a sample of Italian and Greek migrants of both sexes, age 40-69 years. Italians display significantly higher mean triglyceride and lower mean high-density lipoprotein (HDL) cholesterol levels than Greeks. Greek females have significantly higher HDL cholesterol than Greek males, and Italian females have significantly higher low-density lipoprotein (LDL), HDL, and total cholesterol than Italian males. The differences in RFLP allele frequencies between the two ethnic groups and sexes are insignificant. Multivariate analyses show that in the Greek sample the TaqI B RFLP of the CETP gene has a highly significant effect on HDL cholesterol levels regardless of sex and that the TaqI A polymorphism has a significant effect on HDL levels in females but modulates LDL cholesterol concentrations in males. Among Italians, with the sexes considered separately or combined, no such effects of the CETP TaqI polymorphisms are detected. Kruskal-Wallis tests detected associations between the TaqI B polymorphism in all Greek samples but not in the Italian samples. Genotype CETP*B2 exhibits significantly higher HDL cholesterol concentrations than either of the other two TaqI B genotypes, but there is no evidence of a dosage effect of the *B2 allele. These data suggest that associations between the CETP gene and lipid phenotypes can be population specific. Further, they suggest that such associations are mediated in some way by gender.

Adult↗

PubMed1994

DNA polymorphisms at the lipoprotein lipase gene and their association with quantitative variation in plasma high-density lipoproteins and triacylglycerides.

Lipoprotein lipase (LPL) plays a critical role in the metabolism of lipoproteins because this enzyme hydrolyzes the triacylglycerides in chylomicrons and very low density lipoproteins. This process influences the production of high-density lipoprotein (HDL), which takes up tissue cholesterol for transport to the liver for excretion. Accordingly, LPL qualifies as a candidate gene for understanding lipid metabolic disorders and atherosclerosis. Studies on the relationship between genetic variation at the LPL locus and lipid phenotypes have produced equivocal results to date. To help clarify this issue, we investigated 144 outwardly healthy male Mediterranean migrants (from Italy and Greece), age between 40 and 70 years and resident in Australia, for associations between two common LPL restriction site polymorphisms and the following lipid and lipoprotein phenotypes: total plasma cholesterol, low-density lipoprotein (LDL), high-density lipoprotein (HDL), and triacylglycerides. A series of analysis of variance tests, controlling for age, body mass index, and ethnicity, showed that the HindIII polymorphism at the LPL locus is significantly associated with both triacylglyceride and HDL cholesterol concentrations in this sample. The PvUII polymorphism, however, showed no association with any lipid. Kruskal-Wallis tests confirmed the significance of the associations between the HindIII RFLP and both HDL (p = 0.008) and triacylglycerides (p = 0.03). When the sample was subdivided into subjects who exhibited primary hypertriacylglyceridemia and normolipidemics, a significant difference was observed in the frequency of HindIII (p < 0.05) but not PvuII genotypes. HindIII heterozygotes (H1,H2) were least and H2,H2 individuals were most at risk for triacylglyceridemia. Examination of the normolipidemic sample revealed some evidence for an independent effect of the PvuII polymorphism on both LDL cholesterol and total cholesterol levels.

Adult↗

PubMed1993

The effect of fatigue on the shear bond strength of resin bonded to porcelain.

PURPOSE: The objective of this study was to determine whether fatigue cycling affects the shear bond strength of a resin that is initially strongly bonded to porcelain. MATERIALS AND METHODS: Thirty-five disks of a feldspathic/leucite porcelain were fired on a phosphate-bonded investment material. Each disk was etched with hydrofluoric acid and treated with a silane solution and an adhesive. Resin cement composite cylinders were applied to the treated porcelain and light cured. All specimens were stored for 1 week in distilled water at 37 degrees C and then thermocycled for 1000 cycles in water between 2 degrees C and 50 degrees C. Fifteen of the specimens were randomly selected as control specimens. The remaining were subjected to fatigue cycling. A shear load was applied parallel to the bonded surface and cycled in a square wave between 0 and 26 N at 2 Hz for 27,500 cycles. All specimens were fractured in shear at 1.27 mm/min, and the shear bond strength was determined. RESULTS: Fifteen percent of fatigued specimens and 6.7% of the control specimens showed at least partial adhesive failure. All other specimens in both groups failed cohesively in the porcelain. There was no significant difference in the mean bond strengths of the fatigued and control groups (analysis of covariance, P > .05). CONCLUSIONS: For the conditions investigated, cyclic fatigue did not reduce the bond strength of this resin/porcelain system.

Analysis of Variance↗

PubMed1993

Two Y-chromosome-specific restriction fragment length polymorphisms (DYS11 and DYZ8) in Italian and Greek migrants to Australia.

The part of the Y chromosome not involved in recombination has been found to exhibit an extremely low frequency of DNA restriction fragment length polymorphisms (RFLPs) compared with either the X chromosome or autosomes. Also, the few Y-chromosome-specific RFLPs that have been identified have rarely been examined in more than one population. In this study two Y-chromosome-specific RFLPs at loci DYS11 and DYZ8 are examined in Italian and Greek migrants to Australia. The frequency of the rarer (8.5-kb) TaqI allele at DYS11 was 21% in Italians and even greater (34%) in Greeks. There is an inverse relationship between the frequency of the 8.5-kb allele and latitude on the Italian mainland; the regional variation (based on subject's birthplace in Italy) was significant (p < 0.01). The incidence of the 8.5-kb allele in southern Italy may reflect Greek colonization during pre-Roman times when this region was part of Magna Graecia. The frequency of the variant TaqI allele (7, 4 kb) at the DYZ8 locus is much higher in both Greeks and Italians (31% in each) than in Germans (5%), the only previously examined population. DYZ8 shows considerably less variation than DYS11 across the regional divisions of both Greece and Italy. The present findings, when added to the few other data available, indicate that these two Y-chromosome-specific loci are useful markers for investigating population affinities through the paternal line. Also, heterogeneity at these two loci (and added to that at the DYS1 locus) suggests that Mediterranean populations, compared with other groups, exhibit a high level of diversity of Y-chromosome-specific RFLPs.

Adult↗

PubMed1993

Effects of dentin age and bonding on microgap formation.

This study compared the effectiveness of dentin adhesives in extracted molars from younger (16-26) and older (58-87) human subjects. Extracted molars were selected to be restored with one of two dentin bonding agents (n = 22 per group). Opposite sides of each molar were ground flat and a 3 mm x 3 mm cylindrical preparation was milled into each surface. The opposing surfaces of each molar were then randomly restored with either a dentin bonding system that adhered to the dentin smear layer or one which bonded to the dentin after the smear layer was removed. All samples were examined at x150 to determine both the maximum gap in micrometers and extent of the gap in degrees. There were significantly shorter (P < 0.05) microgaps in older dentin with a bonding system that removed smear layer. This bonding system also produced shorter gaps in older dentin than in younger dentin. However, there was no statistically significant difference in the percentage of gap-free margins between age groups.

Adolescent↗

PubMed1992

Ethnicity and blood group polymorphisms in the population of Melbourne, Australia.

In an investigation of the extent of genetic variation in Melbourne, Australia, blood samples were collected from 3 of the largest ethnic groups comprising the present population; 251 Australian born of Anglo-Irish descent, 270 Greek born and 239 Italian born. Each sample was analysed for 5 red cell antigen systems, ABO, MNS, RH, KEL and FY. The Australian born sample was more similar to the Italians than the Greeks except for KEL R matrix and genetic distance analyses indicated that the Greek immigrants were similar to Greeks in Greece, but that Italian immigrants to Melbourne were not as close to a Southern Italian sample as their origins would suggest.

Alleles↗

PubMed1991

Etiology of temporomandibular disorders.

Temporomandibular (TM) disorders include a variety of conditions and disorders of the masticatory muscles and TM joints. As such, it is not appropriate or possible to discuss etiology in terms of a unified theory, as some authors and clinicians have attempted to do in the past. Recent literature reflects this awareness, and most authors now define etiology and treatment of these disorders in terms consistent with their medical counterparts. As with other musculoskeletal disorders, injuries, as well as microtrauma through parafunctional and excess functional stresses, seem to play major roles in etiology.

Humans↗

PubMed1990

Clinical acceptability of orthodontic retainers fabricated from stored alginate impressions.

The purpose of this study was to determine the clinical acceptability of orthodontic retainers fabricated from stored alginate impressions. In the laboratory, alginate impressions were made of a standard dentoform. In the clinic, maxillary alginate impressions were made of the teeth of 20 dentate adults. In each case, sets of stone models were made either immediately or after the impressions had been stored for 24 or 72 hours at high humidity. The linear dimensions of the three sets of dentoform models were compared with one another and with the standard dentoform. No differences were found. Claspless acrylic retainers were fabricated on the models from the adult subjects. The retainers were positioned in the subjects' mouths and evaluated for snap fit, stability, cervical margin fit, and self-retention. With the use of these criteria, 95% of the retainers made from impressions that were poured immediately were clinically acceptable. When impressions were stored 24 and 72 hours, the percentages of acceptable retainers were 80% and 90%, respectively. The differences in percentage of clinically acceptable retainers were not statistically significant. When impressions are stored at high humidity, a large percentage of clinically acceptable orthodontic retainers can be made from models poured as long as 72 hours after the impression is made.

Adult↗

PubMed1990

Migration and genetic structure in Northumberland.

To understand the genetic variation that occurs among regions of northern England, we estimated migration from places of birth and residence in the last two generations for a sample of 1367 families in Northumberland. There has been an increase in kinship among regions, compatible with the increased mobility of recent decades, but the kinship patterns suggest that any regional gene frequency differences have remained relatively undiluted. Comparison of kinship and geographic distance between regions indicates that geographic location is an important determinant of genetic structure.

Blood Grouping and Crossmatching↗

PubMed1990

An immunologic and genetic study of asthma in workers in an aluminum smelter.

The cause or causes of asthma among employees in aluminum smelters is unknown. We attempted to ascertain whether such workers who developed asthma differed in respect to indices of immunological function and certain genetic markers. Data were collected from 33 asthmatic and 127 nonasthmatic potroom workers. Asthmatic workers had significantly lower mean serum levels of immunoglobulin (Ig)M; however, mean levels of IgG and IgA, median levels of IgE, the capacity for recall of delayed type hypersensitivity, levels of immune complexes, and frequency of antinuclear or other autoantibodies did not differ from values for nonasthmatic workers. Asthma was found to develop on a background of atopy in 21 workers (64%), whereas there were no features of atopy in 12 workers (36%). Cigarette smoking had independent effects on immunological function. In respect to genetic markers, there was a higher frequency among asthmatic workers of the alpha-1-anti-trypsin deficiency phenotype MS, but the frequency of blood groups, Gm allotypes, or human leucocyte antigen types was similar. The study established that the profile of immune function, or genetic markers tested, did not differ essentially for workers in an aluminum smelter who did or did not develop asthma; however, there was an indication of heterogeneity in causation, as judged by "atopy-related" and "non-atopy-related" groups in the asthma population.

Adult↗

PubMed1990

Examining population structure through the use of surname matrices: methodology for visualizing nonrandom mating.

The analysis of nonrandom mating using the frequency of marital isonymy indirectly measures the degree of population structure. However, population structure is the result of all matings in a population. Difficulties with large surname matrices have resulted in data being summarized into a single statistic or collapsed into brief tables, with considerable loss of information. By using sophisticated computer graphing procedures and displays, it is possible to directly analyze the mating structure of a community. If P is a vector of proportions for each male surname i (i = 1, 2, 3, ..., n), Q a similar vector of female surnames j(j = 1, 2, 3, ...,m), then the expected frequency matrix E of each possible mating is P x Q. The difference D between the observed frequency matrix O and the expected matrix is O-E. The D matrix is graphed with the x axis containing the male surnames, the y axis the female surnames, and the z axis the difference values dij. Negative values represent negative nonrandom mating and positive values positive nonrandom mating. From 5417 marriages (1840-1963) in the Midlands of Tasmania, those between spouses having 1 of 194 core names were extracted. We analyze these marriages utilizing the new technique and examine the surface of the graph and statistical analysis of its finer structure. Among the results was the demonstration of frequency-dependent selection of surnames. This finding has significant implications for microevolution of human populations, as surnames have existed for possibly 700 years.

Computer Simulation↗

PubMed1989

Effect of a hydraulic jet prophylaxis system on composites.

Because air-powered devices that remove surface stain from teeth are commercially available, recent literature concerning the effect of the hydraulic jet prophylaxis system on oral tissues and restorative materials was reviewed. A study designed to compare the volume loss by different types of composites abraded by this system revealed a relatively high rate of abrasion for the small-particle composite compared with composites containing larger filler particles.

Acrylic Resins↗

PubMed1989

Effect of cementation method on the retention of anatomic cast post and cores.

This study determined the influence of the cementation method on the retention of anatomic posts. Three cementation methods were used for each post/tooth combination: (1) cement on post only, (2) cement in canal only, and (3) cement on post and in canal. The mean retentive force for method 2 was significantly higher than for method 1. The mean force for method 3 was not significantly different from either method 1 or method 2.

Cementation↗

PubMed1989

Fracture resistance of teeth restored with Class II composite restorations.

Teeth with large mesio-occlusal-distal cavity preparations fracture more easily than intact teeth. An intracoronal restoration capable of increasing tooth fracture resistance is desired. This in vitro study compared the fracture resistance of maxillary premolars restored with enamel bonding and dentin bonding. The effects of the type of curing system (chemical or photochemical) and marginal preparation (beveled or not) were also evaluated. Premolars restored with large MOD composite resin restorations were approximately twice as strong as the unrestored prepared tooth. However, all restored teeth were no more than half as strong as the intact tooth. Dentin bonded restorations were not significantly stronger than enamel bonded restorations.

Acid Etching, Dental↗