Biomedical subjects
R van Noort
Publications and source records attributed to R van Noort.
Shear vs. tensile bond strength of resin composite bonded to ceramic.
Since the mode of failure of resin composites bonded to ceramics has frequently been reported to be cohesive fracture of either ceramic or resin composite rather than separation at the adhesive interface, this study was designed to question the validity of shear bond strength tests. The reasons for such a failure mode are identified and an alternative tensile bond strength test evaluated. Three configurations (A, conventional; B, reversed; and C, all composite) of the cylinder-on-disc design were produced for shear bond strength testing. Two-dimensional finite element stress analysis (FEA) was carried out to determine qualitatively the stress distribution for the three configurations. A tensile bond strength test was designed and used to evaluate two ceramic repair systems, one using hydrofluoric acid (HF) and the other acidulated phosphate fluoride (APF). Results from the shear bond strength tests and FEA showed that this particular test has as its inherent feature the measurement of the strength of the base material rather than the strength of the adhesive interface. In the tensile test, failure invariably occurred in the adhesive layer, with HF and APF showing a similar ability to improve the bond of resin composite to ceramic. It is concluded that the tensile bond strength test is more appropriate for evaluating the adhesive capabilities of resin composites to ceramics.
[Best food for dogs with diabetes mellitus].
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Tensile peel failure of resin-bonded Ni/Cr beams: an experimental and finite element study.
Tensile bond strength data for resin-bonded bridge cements would predict failure loads in excess of 1000 N when based on available bonded surface area. Such loads are unlikely to be encountered clinically, yet these bridges frequently fail. Loading conditions resulting in a peeling action at the adhesive interface may provide a more probable explanation for failure rather than being attributable to poor clinical or laboratory technique. This hypothesis was investigated by bonding grit-blasted Ni/Cr alloy beams of different thicknesses to a block of the same alloy using a resin-bonded bridge cement, leaving half the length of the beam free. Beams were pulled off the block with a peeling action by applying a tensile load to the free end of the beam and load at failure recorded. Fractured surfaces were examined under SEM. A two-dimensional finite element model of a section through the peel test arrangement was used to produce data of the stress distribution in the adhesive layer. Load at failure was closely correlated to beam thickness. Using this data and known tensile bond strength values it was possible to predict the loads necessary to cause adhesive failure of the beam, which were found to correlate closely with the experimental measurements. It was concluded that design and the stiffness of the retainers can have a potentially profound effect on the adhesive quality of resin-bonded bridges.
Dental materials: 1992 literature review.
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Clinical relevance of laboratory studies on dental materials: strength determination--a personal view.
The measurement of strength of materials is undertaken in a laboratory primarily to determine its relationship to the microstructural features of the material, as the understanding it brings will allow the production of stronger materials. Problems arise when attempts are made to derive some sort of clinical significance from these data without due regard to the functional aspects of the device from which the material is made. Three examples are presented which seek to highlight that: (1) strength, however desirable, is not necessarily the most important consideration; (2) great care has to be exercised in interpreting strength data; and (3) the most appropriate properties are considered for each individual dental application of the material.
[Injection against itching].
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Control of bacteria in dental water supplies.
A simple device which releases chlorhexidine acetate into dental unit water supplies is described. The device comprised methyl methacrylate resin containing 40% by weight of chlorhexidine powder, cast into a rod shape and placed in a reservoir. After initial cleaning by flushing with hypochlorite, five dental units were fitted with the device and five units acted as controls. The device successfully kept four units virtually free of bacteria over a 3-month period but effective cleaning of the tubing within the units was found to be essential for success. Sustained release of chlorhexidine offers one means of tackling the problem of contaminated water in dental units, without resorting to complex pumping or metering systems.
Fabrication, characterization and fracture study of a machinable hydroxyapatite ceramic.
In this study the preparation of a machinable hydroxyapatite from mixtures of a fine, submicrometer powder and either a coarse powder composed of porous aggregates up to 50 microns or a medium powder composed of dense particles of 3 microns median size is described. These were characterized using X-ray diffraction, transmission and scanning electron microscopy and infra-red spectroscopy. Test-pieces were formed by powder pressing and slip casting mixtures of various combinations of the fine, medium and coarse powders. The fired test-pieces were subjected to measurements of firing shrinkage, porosity, bulk density, tensile strength and fracture toughness. The microstructure and composition were examined using scanning electron microscopy and X-ray diffraction. For both processing methods, a uniform interconnected microporous structure was produced of a high-purity hydroxyapatite. The maximum tensile strength and fracture toughness that could be attained while retaining machinability were 37 MPa and 0.8 MPa m1/2 respectively.
Dental materials: 1991 literature review.
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A prospective study of the survival of chemically activated anterior resin composite restorations in general dental practice: 5-year results.
The principals of 26 general dental practices agreed to use six chemically activated resin composite restorative materials to restore Class III and Class V lesions and record information concerning their performance over a period of 5 years. The information collected was analysed by actuarial methods to assess the clinical longevity and reasons for replacement as perceived by the dentists operating in the General Dental Service in England. At the end of 5 years, 14 dentists provided sufficient returns for their data to be considered suitable for analysis. The database consisted of 2399 Class III and 1093 Class V restorations. The overall probability of survival at 5 years of Class III and Class V restorations was 62.9% and 71.8% respectively. The difference in performance between the six restorative materials was small, with the probability of survival varying from 70.4 +/- 2.9% to 56.3 +/- 2.9% for the Class III restorations and 78.6 +/- 3.7% to 67.7 +/- 4.2% for the Class V restorations. The main reasons for replacement were general surface discoloration, secondary caries and fracture. The chemically activated composite restorative materials available at the time of initiating this study produced comparable performances in general dental practice when used without enamel and dentine bonding techniques. This suggests that more general practice-based clinical studies are needed to determine whether or not improvements in materials and techniques are effectively transferred to the general practice situation.
The tensile bond strength of new and rebonded stainless steel orthodontic brackets.
The study investigated the effect on the tensile/peel bond strength of the variables associated with the bracket base, the enamel surface, and the type of adhesive when both new and used brackets were rebonded to a previously bonded enamel surface. The tensile/peel bond strength was firstly evaluated for three different types of stainless steel orthodontic bracket/base combinations. The cast integral base gave a significantly lower bond strength than the foil-mesh and photo-etched bases. Following debonding, a group of new brackets were bonded to the teeth using a chemically-activated or a light-cured adhesive. The old adhesive had been removed from the enamel by either a hand scaler or a tungsten-carbide bur. The rebonded new brackets demonstrated a small, but statistically significant fall in bond strength. No differences were found between the enamel preparations or the adhesives. A further group of previously debonded brackets were rebonded to the same teeth. The bracket bases were prepared by either smoothing with a green stone or heating in a bunsen flame followed by sandblasting and electropolishing. Highly significant falls in bond strength were obtained with all the bases. No significant differences were found between the two methods of bracket preparation.
The effects of enamel anisotropy on the distribution of stress in a tooth.
Enamel is thought to have highly anisotropic stiffness characteristics, because of its prismatic structure. It is probable that the enamel is stiffer in the prism direction compared with a direction perpendicular to it. The prisms are thought to run approximately perpendicular to the enamel-dentin junction. The curvilinear anisotropy that will result can readily be modeled by TOMECH, a finite element program developed at the University of Sheffield, since curvilinearity of mechanical properties is available as an automated feature of this program. The patterns of stress due to an external load were investigated in two-dimensional abstract models, and in a model of a mandibular second premolar, for both anisotropic and isotropic enamel. Results were compared with the commercial code ANSYS and good agreement obtained. Enamel with anisotropic properties was found to have a profoundly different stress distribution under load when compared with models with isotropic enamel. For isotropic enamel, the load path is directed through the stiff enamel shell, while for anisotropic enamel, the load path is directed into the dentin, as the load path follows the stiff direction of the enamel prisms. Thus, if enamel is indeed anisotropic, its function differs greatly from that suggested in previous hypotheses. Enamel with anisotropic material characteristics would provide a hard-wearing protective surface-coating while simultaneously diverting the load away from this brittle, low-tensile-strength phase, thus reducing the potential for tooth fracture.
Controlled intraoral delivery of hydrocortisone sodium succinate.
Controlled release of hydrocortisone sodium succinate from a device consisting of a drug containing a core of polymethyl methacrylate was achieved by the use of a design incorporating impermeable barriers to drug diffusion and the use of a semipermeable membrane. Initial in vivo study showed sustained intraoral delivery of hydrocortisone sodium succinate during a 6-day period.
Dental materials: 1990 literature review.
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[Exocrine pancreatic insufficiency in dogs].
Pancreatic exocrine insufficiency is the most common cause of maldigestion in dogs. This is usually caused by irreversible atrophy of the pancreas which subsequently requires life-long substitution therapy. The pathophysiology, symptoms and diagnosis are briefly reviewed in the present paper. The Trypsin-like-immunoreactivity test is recommended for establishing the diagnosis. Finally, treatment and possible causes of the failure of therapy are discussed.
[Dogs with persistent diarrhea].
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Dental materials: 1989 literature review.
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