Biomedical subjects

Michael H Walter

Publications and source records attributed to Michael H Walter.

18 recordsLinked to original sources

PubMed2006

Apocarotenoid biosynthesis in arbuscular mycorrhizal roots: contributions from methylerythritol phosphate pathway isogenes and tools for its manipulation.

During colonization by arbuscular mycorrhizal (AM) fungi plant roots frequently accumulate two types of apocarotenoids (carotenoid cleavage products). Both compounds, C(14) mycorradicin and C(13) cyclohexenone derivatives, are predicted to originate from a common C(40) carotenoid precursor. Mycorradicin is the chromophore of the "yellow pigment" responsible for the long-known yellow discoloration of colonized roots. The biosynthesis of apocarotenoids has been investigated with a focus on the two first steps of the methylerythritol phosphate (MEP) pathway catalyzed by 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR). In Medicago truncatula and other plants the DXS2 isogene appears to be specifically involved in the AM-mediated accumulation of apocarotenoids, whereas in the case of DXR a single gene contributes to both housekeeping and mycorrhizal (apo)carotenoid biosynthesis. Immunolocalization of DXR in mycorrhizal maize roots indicated an arbuscule-associated protein deposition, which occurs late in arbuscule development and accompanies arbuscule degeneration and breakdown. The DXS2 isogene is being developed as a tool to knock-down apocarotenoid biosynthesis in mycorrhizal roots by an RNAi strategy. Preliminary results from this approach provide starting points to suggest a new kind of function for apocarotenoids in mycorrhizal roots.

Carotenoids↗

PubMed2006

Computer-aided analysis of the influence of digitizing and surfacing on the accuracy in dental CAD/CAM technology.

In dentistry, ceramic materials with high fracture resistance are needed for all-ceramic fixed partial dentures (FPDs). The sophisticated processing of advanced ceramics that can be used for such dental restorations demands the application of CAD/CAM technologies. These techniques necessitate digitizing of the prepared teeth or the planned restoration itself and surfacing of the acquired digital data before milling paths can be generated. As precision in fit is crucial for dental restorations, a computer-aided method for the quantitative and qualitative 3D analysis has been developed and applied. Factors influencing the obtainable precision in the application of CAD/CAM techniques were taken into consideration.

Computer-Aided Design↗

PubMed2005

Qualitative computer aided evaluation of dental impressions in vivo.

OBJECTIVES: Clinical investigations dealing with the precision of different impression techniques are rare. Objective of the present study was to develop and evaluate a procedure for the qualitative analysis of the three-dimensional impression precision based on an established in-vitro procedure. The zero hypothesis to be tested was that the precision of impressions does not differ depending on the impression technique used (single-step, monophase and two-step-techniques) and on clinical variables. METHODS: Digital surface data of patient's teeth prepared for crowns were gathered from standardized manufactured master casts after impressions with three different techniques were taken in a randomized order. Data-sets were analyzed for each patient in comparison with the one-step impression chosen as the reference. The qualitative analysis was limited to data-points within the 99.5%-range. Based on the color-coded representation areas with maximum deviations were determined (preparation margin and the mantle and occlusal surface). To qualitatively analyze the precision of the impression techniques, the hypothesis was tested in linear models for repeated measures factors (p < 0.05). RESULTS: For the positive 99.5% deviations no variables with significant influence were determined in the statistical analysis. In contrast, the impression technique and the position of the preparation margin significantly influenced the negative 99.5% deviations. SIGNIFICANCE: The influence of clinical parameter on the deviations between impression techniques can be determined reliably using the 99.5 percentile of the deviations. An analysis regarding the areas with maximum deviations showed high clinical relevance. The preparation margin was pointed out as the weak spot of impression taking.

Computer-Aided Design↗

PubMed2005

Survey of repellent use by service members arriving in Kuwait for Operation Iraqi Freedom 2.

The 8th Medical Brigade, deployed to Kuwait in January 2004 in support of Operation Iraqi Freedom provided theater-wide health service support and medical command and control. An historic "surge" of troops and equipment occurred during our deployment as 140,000 service members (SMs) redeployed out of theater and 110,000 arrived to take their place. The brigade's preventive medicine personnel conducted a questionnaire-based survey at military camps in northwestern Kuwait in mid-March to determine whether SMs had been provided with supplies and information to protect them against endemic vector-borne diseases. Of 1,200 questionnaires distributed, 870 (73%) were returned. In addition to military affiliation information, the survey contained specific questions related to the provision of n-diethyl-m-toluamide repellents, permethrin and treated uniforms, and medical threat briefings. On average, less than one-half of the SMs received these resources before their arrival in Kuwait. Significant differences of resource provision existed among service branches, components, and mobilization or deployment stations.

Humans↗

PubMed2004

Cloning, characterization, and immunolocalization of a mycorrhiza-inducible 1-deoxy-d-xylulose 5-phosphate reductoisomerase in arbuscule-containing cells of maize.

Colonization of plant roots by symbiotic arbuscular mycorrhizal fungi frequently leads to the accumulation of several apocarotenoids. The corresponding carotenoid precursors originate from the plastidial 2-C-methyl-d-erythritol 4-phosphate pathway. We have cloned and characterized 1-deoxy-d-xylulose 5-phosphate reductoisomerase (DXR), catalyzing the first committed step of the pathway, from maize (Zea mays). Functional identification was accomplished by heterologous expression of sequences coding for the mature protein in Escherichia coli. DXR is up-regulated in maize roots during mycorrhization as shown at transcript and protein levels, but is also abundant in leaves and young seedlings. Inspection of sequenced genomes and expressed sequence tag (EST) databases argue for a single-copy DXR gene. Immunolocalization studies in mycorrhizal roots using affinity-purified antibodies revealed a DXR localization in plastids around the main symbiotic structures, the arbuscules. DXR protein accumulation is tightly correlated with arbuscule development. The highest level of DXR protein is reached around maturity and initial senescence of these structures. We further demonstrate the formation of a DXR-containing plastidial network around arbuscules, which is highly interconnected in the mature, functional state of the arbuscules. Our findings imply a functional role of a still unknown nature for the apocarotenoids or their respective carotenoid precursors in the arbuscular life cycle.

Aldose-Ketose Isomerases↗

PubMed2004

CAD/CAM-machining effects on Y-TZP zirconia.

OBJECTIVES: Computer-Aided-Design/Computer-Aided-Manufacturing (CAD/CAM) techniques are gaining importance in fabricating crowns and fixed partial dentures (FPDs) made of yttria stabilized tetragonal zirconia polycrystals (Y-TZP). The specific aims of this study were (1) to test the hypothesis that surface flaws and microcracks are induced by the grinding of crowns (2) to analyze the material removal and the grinding-induced surface layer as well as to estimate the crack size caused by machining Y- TZP under conditions simulating the grinding of crowns. METHODS: Y-TZP disks and sectioned cylinders with polished separation planes were used for the analysis of the grinding procedure. While simulating the inner surface grinding of crowns, feed and cutting depth were varied (vw = 100, 75, 50 mm/min; ap = 0.1, 0.06, 0.02 mm). SEM was used for the quantitative assessment of the machined surface. RESULTS: While the crack length is not significantly influenced by the grinding parameters, the type of material removed varied with the cutting depth as well as with the feed. Grinding induced surface flaws and microcracks were detected at the internal top surface of the crowns. Half-cylinders machined under conditions simulating the inner surface grinding of crowns showed crack lengths between 2 and 15 microm. SIGNIFICANCE: Sectioned specimens with polished section planes are suitable for the analysis of the grinding process using the face and peripheral grinding procedure. The inner surface grinding of fixed restorations is the most challenging step of CAM of crowns and FPDs. Most important appeared to be the diamond tool, especially the number and shape of the active diamond grains.

Computer-Aided Design↗

PubMed2004

University of Alberta dental students' outreach clinical experience: an evaluation of the program.

In a 25-year-old satellite dental program, advanced dentistry and dental hygiene students provide highly supervised dental treatment for patients in government-funded clinics in 3 under-serviced communities in northern Alberta. Analysis of the performance data of the 2003 University of Alberta dentistry students in their fourth and final year was used to evaluate this program. The data showed that the undergraduate students did numerous diverse general dental procedures during the 2-week rotation, most of which were basic treatments. Students could participate in the program up to 3 times. In subsequent rotations, students tended to do fewer preventive measures, and more restorative treatment and dental extractions. Available grading data for all students indicated that treatment objectives were fulfilled in the vast majority of cases. Such community-based field experience may significantly add to the competence of students entering private practice.

Alberta↗

PubMed2003

Three Bacillus anthracis bacteriophages from topsoil.

Three Bacillus anthracis bacteriophages from Iowa topsoil are characterized as to latent period, morphology, structural proteins, DNA size, and restriction endonuclease digestion. Electron micrographs indicate that the three isolates include two members of the Myoviridae and one smaller phage belonging to the Podoviridae. Phages Nk and DB resemble Myoviridae phage SP50 in morphology, but host range studies, protein, and DNA analysis indicate that both differ from SP50. Phage MH is very similar to phage phi 29, but differs in terms of host range, structural protein, and DNA characteristics.

Bacillus Phages↗

PubMed2003

A new method for the computer-aided evaluation of three-dimensional changes in gypsum materials.

OBJECTIVES: The quantitative evaluation of the time- or process-dependent three-dimensional stability of dental materials is a common question in dentistry. An investigation procedure has been developed based on a CAD-surface model of a prepared upper canine, as well as a high-precision physical copy (metal master die). The specific aim of this study was to test this method's reliability. Additionally, the ability of the developed procedure to determine the three-dimensional stability of resin-reinforced gypsum master casts over time was investigated. METHODS: Ten duplicate dies of improved dental stone (esthetic-rock, dentona, Germany) were manufactured, and digitized 1, 3, 7, 28 and 56 days after pouring. A three-coordinate optical measuring device was used for the data acquisition. The three-dimensional accuracy of stone dies was determined by comparing the digitized data of the stone dies made from the metal master die to its CAD-surface model (Surfacer) Version 9.0. Imageware Inc., Ann Arbor Michigan, USA). To assess the procedure, test surfaces were created from the digitized data and compared with a reference. RESULTS: The mean deviation between the digitized point cloud and the test surface was less than 3 microm. During the 56 day examination period no significant three-dimensional changes in dimensional stability were found. SIGNIFICANCE: The procedure for the quantitative three-dimensional evaluation was shown to be suitable. Best-fit registration enabled a reliable alignment of the point cloud to the CAD-surface model. Alteration of three-dimensional accuracy over 6 weeks was insignificant and without clinical relevance.

Calcium Sulfate↗

PubMed2003

Arbuscular mycorrhiza: biological, chemical, and molecular aspects.

Mycorrhizas are the most important mutualistic symbioses on earth. The most prevalent type are the arbuscular mycorrhizas (AMs) that develop between roots of most terrestrial plants and fungal species of the Zygomycota. The AM fungi are able to grow into the root cortex forming intercellular hyphae from which highly branched structures, arbuscules, originate within cortex cells. The arbuscules are responsible for nutrient exchange between the host and the symbiont, transporting carbohydrates from the plant to the fungus and mineral nutrients, especially phosphate, and water from the fungus to the plant. Plants adapt their phosphate uptake to the interaction with the AM fungus by synthesis of specific phosphate transporters. Colonization of root cells induces dramatic changes in the cytoplasmic organization: vacuole fragmentation, transformation of the plasma membrane to a periarbuscular membrane covering the arbuscule, increase of the cytoplasm volume and numbers of cell organelles, as well as movement of the nucleus into a central position. The plastids form a dense network covering the symbiotic interface. In some of these changes, microtubules are most likely involved. With regard to the molecular crosstalk between the two organisms, a number of phytohormones (cytokinins, abscisic acid, jasmonate) as well as various secondary metabolites have been examined: (i) Jasmonates occur at elevated level, which is accompanied by cell-specific expression of genes involved in jasmonate biosynthesis that might be linked to strong carbohydrate sink function of AM roots and induced defense reactions: (ii) apocarotenoids (derivatives of mycorradicin and glycosylated cyclohexenones) accumulate in most mycorrhizal roots examined so far. Their biosynthesis via the nonmevalonate methylerythritol phosphate (MEP) pathway has been studied resulting in new insights into AM-specific gene expression and biosynthesis of secondary isoprenoids.

Carbohydrates↗

PubMed2002

Stimulation of carotenoid metabolism in arbuscular mycorrhizal roots.

Development of arbuscular mycorrhizal roots is correlated with accumulation of various isoprenoids, i.e. acyclic C(14) polyene 'mycorradicin' and C(13) cyclohexenone derivatives. We present data indicating a strong stimulation of carotenoid metabolism in such roots. Carotenoid profiling revealed mycorrhiza-specific accumulation of zeta-carotene in Zea mays and Medicago truncatula. Precursor accumulation after inhibition of phytoene desaturase (Pds) activity by norflurazon indicated an increased phytoene biosynthetic capacity in mycorrhizal roots of all species analyzed. Nicotiana tabacum plants transformed with a PDS promoter- GUS construct showed a cell-specific induction of PDS promoter activity in root cells containing arbuscules. Mycorradicin biosynthesis and, partially, mycorrhization were impaired in maize mutants deficient in carotenoid biosynthesis. These data indicate that (1) mycorradicin is probably synthesized via a C(40) precursor carotenoid, (2) carotenoid biosynthesis is induced in mycorrhizal roots, (3) induction occurs, at least partially, at the transcriptional level, and (4) that this may play a functional role during mycorrhization.

Carotenoids↗

PubMed2002

Two distantly related genes encoding 1-deoxy-d-xylulose 5-phosphate synthases: differential regulation in shoots and apocarotenoid-accumulating mycorrhizal roots.

Isopentenyl diphosphate, the universal precursor of isoprenoids, is synthesized by two separate routes, one in the cytosol and the other in plastids. The initial step of the plastidial pathway is catalysed by 1-deoxy-d-xylulose 5-phosphate synthase (DXS), which was previously thought to be encoded by a single-copy gene. We have identified two distinct classes of DXS-like cDNAs from the model legume Medicago truncatula. The deduced mature MtDXS1 and MtDXS2 proteins, excluding the predicted plastid-targeting peptides, are similar in size (72.7 and 71.2 kDa) yet share only 70% identity in their amino acid sequences, and both encode functional DXS proteins as shown by heterologous expression in Escherichia coli. Available DXS sequences from other plants can easily be assigned to either class 1 or class 2. Partial sequences of multiple DXS genes in a single genome may be found in the databases of several monocot and dicot plants. Blot analyses of RNA from M. truncatula, maize, tomato and tobacco demonstrate preferential expression of DXS1 genes in many developing plant tissues except roots. By contrast, DXS2 transcript levels are low in most tissues but are strongly stimulated in roots upon colonization by mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids. Monoterpene-synthesizing gland cells of leaf trichomes appear to be another site of DXS2 gene activity. The potential importance of DXS1 in many housekeeping functions and a still hypothetical role of DXS2 in the biosynthesis of secondary isoprenoids is discussed.

Amino Acid Sequence↗

PubMed2002

Effect of posterior crown margin placement on gingival health.

STATEMENT OF PROBLEM: The clinical impact of posterior crown margin placement on gingival health has not been thoroughly quantified. PURPOSE: This study evaluated the effect of posterior crown margin placement with multivariate analysis. MATERIAL AND METHODS: Ten general dentists reviewed 240 patients with 480 metal-ceramic crowns in a prospective clinical trial. The alloy was randomly selected from 2 high gold, 1 low gold, and 1 palladium alloy. Variables were the alloy used, oral hygiene index score before treatment, location of crown margins at baseline, and plaque index and sulcus bleeding index scores recorded for restored and control teeth after 1 year. The effect of crown margin placement on sulcular bleeding and plaque accumulation was analyzed with regression models (P<.05). RESULTS: The probability of plaque at 1 year increased with increasing oral hygiene index score before treatment. The lingual surfaces demonstrated the highest probability of plaque. The risk of bleeding at intrasulcular posterior crown margins was approximately twice that at supragingival margins. Poor oral hygiene before treatment and plaque also were associated with sulcular bleeding. Facial sites exhibited a lower probability of sulcular bleeding than lingual surfaces. Type of alloy did not influence sulcular bleeding. CONCLUSION: In this study, placement of crown margins was one of several parameters that affected gingival health.

Adult↗

PubMedDate not supplied

Efficacy and durability of Bacillus anthracis bacteriophages used against spores.

Antibiotics and vaccines help fight anthrax disease, but there are no anthrax spore control methods suitable for use in environments where humans are present. The work reported in this article indicates that bacteriophages may help reduce risk from anthrax spores. Dose-response studies demonstrated that higher concentrations of mixed Bacillus anthracis bacteriophages (3.5 x 10(8) plaque-forming units per milliliter) inhibited subsequent growth of bacteria when sprayed on B. anthracis spores. Phages also were tested for durability under conditions designed to simulate environments possibly encountered during mass phage production, storage, and use against anthrax spores. They remained infectious at temperatures from -20 degrees C to 37 degrees C, under filtration, aerosolization, and treatments with perspiration and blood. Phages were sensitive to temperatures over 55 degrees C and to desiccation. Ultraviolet light reduced spore viability more than phage infectivity under similar conditions. The potential for personal or environmental decontamination of anthrax spores with phages is discussed.

Anthrax↗

PubMedDate not supplied

Improving the fit of implant-supported superstructures using the spark erosion technique.

PURPOSE: The purpose of this study was to determine whether the passive fit of the implant-retained single-cast framework could be improved by spark erosion treatment. MATERIALS AND METHODS: An initial cast was produced in a transparent resin material. Five Branemark System implants were arranged in the interforaminal region, and abutments were placed on them with a torque of 20 Ncm. An impression was made using a standard impression technique described by Branemark. A corresponding master cast suitable for the spark erosion post-framework fabrication was produced. From this master cast, 12 frameworks were produced in a conventional single-cast procedure. Six of these were made of a high-gold alloy (Stabilor G); the other 6 were made of pure titanium (Biotan). These frameworks were then refined using the SAE Secotec Spark Erosion System. To measure the accuracy of the framework fit, the frameworks were measured before and after the spark erosion treatment using 2 different measurement methods-scanning electron microscopy to measure the gap widths (Sheffield test) and photoelastic stress analysis. RESULTS: The results of both measurement techniques correlated and demonstrated significant improvement in the accuracy of fit or in the passive fit for all 12 frameworks after spark erosion treatment. This improvement was statistically significant for the titanium frameworks. DISCUSSION: Dental practitioners and technicians should strive to achieve a precise passive fit of frameworks and superstructures to minimize additional stress at the interfaces of the prosthesis, abutment, and implant. CONCLUSION: The clinical use of the spark erosion technique to refine framework fit is recommended. (More than 50 references.)

Birefringence↗

PubMedDate not supplied

An innovative method for evaluation of the 3-D internal fit of CAD/CAM crowns fabricated after direct optical versus indirect laser scan digitizing.

PURPOSE: A procedure for 3-D analysis of the internal fit of fixed restorations has been developed. This study tested this method for investigating the fit of all-ceramic crowns. MATERIALS AND METHODS: Twelve data sets of a prepared maxillary canine were acquired by direct digitizing of the metal master die (CEREC 3 camera) and by digitizing gypsum dies after conventional impression taking (CEREC 3 scan), respectively. Using these data sets, 24 all-ceramic single crowns each were machined out of two glass-ceramics. The method is based on duplicated gypsum dies of the metal master, which were made for each crown. The space between the duplicate die and the internal surface of the respective crown was filled with a low-viscosity addition silicone. These silicone films (replicas) and their corresponding dies were digitized in the same measuring position. The internal fit was calculated and quantitatively and qualitatively analyzed. RESULTS: The mean and maximum positive deviations were 348 microm and 986 microm (camera), respectively, and 294 microm and 830 microm (scan), respectively, for Vitablocs Mk II and 332 microm and 920 microm (camera), respectively, and 307 microm and 852 microm (scan), respectively, for ProCAD. For both systems, the deviations were highest at the edges. CEREC scan yielded significantly better internal fit accuracy compared to the CEREC camera. CONCLUSION: Evaluation of the internal 3-D fit using the innovative method proved to be suitable. Indirect data acquisition using impression taking showed improved internal fit compared with the direct procedure. However, the differences between the data-acquisition techniques are small compared to their absolute values.

Calcium Sulfate↗

PubMedDate not supplied

Six-year clinical performance of all-ceramic crowns with alumina cores.

PURPOSE: The aim of the study was to evaluate the clinical performance of Procera Alumina AllCeram crowns (Nobel Biocare). MATERIALS AND METHODS: In 70 patients, 61 anterior and 46 posterior teeth were provided with single crowns and cemented with a glass-ionomer cement. RESULTS: Four patients were lost to follow-up. Six crowns had to be removed, all because of nonreparable fracture. At 6 years, the cumulative survival rate was 94.3% for all crowns, 96.7% for anterior crowns, and 91.3% for posterior crowns (survival = not removed). Most of the defects occurred within the first 1.5 years. CONCLUSION: The findings indicate a good clinical prognosis of both anterior and posterior Procera Alumina crowns.

Adult↗

PubMedDate not supplied

Metal-ceramic failure in noble metal crowns: 7-year results of a prospective clinical trial in private practices.

PURPOSE: The aim of this study was to evaluate the clinical performance of metal-ceramic crowns placed in 10 private practices. MATERIALS AND METHODS: In this prospective clinical trial, 95 patients were provided with 190 noble-metal metal-ceramic single crowns. RESULTS: The 7-year survival rates of the crowns were 95.5% (target event: any removal), 99.5% (target event: removal because of defective veneer), and 92.4% (target event: metal-ceramic complication of any kind). No significant explanatory variables for metal-ceramic complications could be detected by bivariate and multivariate testing. The consequences resulting from metal-ceramic defects were of minor clinical significance in most cases. CONCLUSION: The findings support previous claims that metal-ceramic restorations perform very well clinically, including in practices outside academic environments.

Adult↗