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Biomedical subjects

B W Darvell

Publications and source records attributed to B W Darvell.

At least 19 recordsLinked to original sources
PubMed1992

Casting system effectiveness--measurement and theory.

Evaluating castability as a property of dental casting alloys is an unworkable concept, since any measure is demonstrably affected by several external factors. Casting System Effectiveness (CSE) is shown experimentally and theoretically to depend primarily upon the time taken for the advancing front to freeze. A spiral tube mold was used to measure casting length obtained under variations in casting temperature; this pattern is proposed as a standard measure of CSE. A Bernoulli 'free-fall' numerical model is shown to reproduce the principal features of such casting, with some evidence of viscosity limitation of the turbulent flow at long casting lengths. Direct measurement of melt velocities confirmed turbulent flow in the mold. A method is also suggested for assessing the influence of investment porosity on CSE.

Acceleration↗
PubMed1991

Strength of phosphate-bonded investments at high temperature.

There has been little reported on the strength of phosphate-bonded investments, and in particular the role of 'special liquids' in this context is poorly understood. Using a novel test design, sequential testing of an investment diaphragm using a variable casting metal mass, a measure of the effect of 'special liquid' on strength under actual casting conditions has been obtained. The use of special liquid was found to increase substantially the strength of the four products tested. This test may provide more useful data than the conventional compressive strength test, which may give misleading information, especially if performed at room temperature.

Dental Bonding↗
PubMed1990

Effect of humidity on calcium sulphate hemihydrate.

The conversion of calcium sulphate hemihydrate powders to dihydrate by reaction with atmospheric water vapour has been re-examined gravimetrically. Storage is usually said to be safe at less than '70 per cent RH', but this is shown to be an inadequate criterion (water vapour pressure being more useful in the context) and based on an inappropriate experiment. At 23 degrees C, greater than about 2.3 kPa water vapour pressure caused rapid reaction, and this may be taken as a more reliable critical value.

Calcium Sulfate↗
PubMed1989

Viscosity of dental waxes by use of Stokes' law.

The rheological properties of dental waxes control their fabrication procedures and are dominated by their temperature dependence. Indeed, it is their rapid change in deformability with temperature which enables them to be used. Some measure of flow rate is therefore needed for characterization and quality control. A modified Stokes' method is proposed which allows measurement of an analogue of Newtonian viscosity and is suited to routine use in the range 10(4)-10(10) Pa.s, and which leads naturally to a determination of the activation energy for the process. This activation energy is of the order of 700 kJ/mole at low stress.

Viscosity↗
PubMed1989

Centers of rotation during jaw movements.

Anatomists consider the articulation of movable joints to be complex, involving movable instantaneous centers of rotation (ICR). However, prosthodontists often treat the temporomandibular joint (TMJ) as a model of a simple hinge. The aim of this study was to examine the case for a movable ICR during habitual opening and closing jaw movements. Young, dentate subjects were examined with a kinesiograph. Jaw movements were performed and recorded. The center of rotation of each movement pattern was identified, and its location related to the position of the TMJ. The results showed that opening and closing jaw movements were predominantly non-coincident, with a movable ICR located at a variable distance and direction from the TMJ. There was no evidence to suggest that the TMJ functioned as a simple hinge during jaw movements.

Adult↗
PubMed1988

Tooth wear and the position of the mental foramen.

Variation in the position of the mental foramen with respect to the teeth ("apparent" position) seems to be associated with race, but in the determination of this position, factors which affect the disposition of the teeth plainly have a bearing on the results. The apparent position was investigated in a sample of southern Chinese skulls of known age which were classified by the degree of tooth wear. Controlling for "true" position and size of the mandible, a highly significant correlation between wear and position was found, age and tooth size having no significant contribution as additional explanatory variables. The effect of tooth wear on the apparent position of the foramen may partly explain racial variation.

Adolescent↗
PubMed1979

The inapplicability of formulae to determine vertical dimension.

The accuracy and precision of certain external cephalometric measurements were investigated on a subject presenting optimal conditions, using ten operators. Due to difficulties in identifying landmarks and tissue compressibility, the standard deviation of a measurement for ten replications ranged from 0 . 3 to 1 . 7 mm, while the "between operators" standard deviation was no better than 2 . 0 mm. Measurement accuracy was also poor: that of primary interest, subnasale-gnathion, had a bias of the order of 7 mm, judged by a radiographic determination. A minimum denture success rate of 97% is equated with a standard deviation of 1 mm in the design freeway space. A recently published regression equation for predicting occlusal vertical dimension has a minimum error of prediction of 5 . 8 mm and thus is considered an inadequate predictive tool. The errors demonstrated in facial measurements suggest that using formulae to predict occlusal vertical dimension is unlikely to produce clinically acceptable results.

Cephalometry↗
PubMed1978

The development of an artificial saliva for in vitro amalgam corrosion studies.

Published data, consideration of chemical and physical properties and the requirements of amalgam corrosion testing were used to formulate an artificial saliva. A previously described method was used to determine the relevance of a number of human saliva constituents. Some results of those tests are described. Practical problems are discussed, and necessary further developments outlined. A number of other applications are suggested.

Artificial Organs↗