H.J. Wilson

Depth of cure of radiation-activated materials—effect of mould material and cavity size

A method for determining depth of cure of radiation-activated restorative materials has been developed. The effect of mould material and cavity size on depth of cure was determined using a digital penetrometer method. Values were obtained for three selected shades of three dentine products and, in addition, the same three shades of an enamel variant were assessed. All these values were obtained using a 4 mm diameter x 6 mm deep stainless steel mould.

Light curing unit effectiveness assessed by dental radiometers.

OBJECTIVES The purpose of this investigation was to assess the effectiveness of five commercially available hand-held dental radiometers and a computer-based experimental radiometer. METHODS Light intensity of five visible light activation units was determined using the dental radiometers. The influence of curing light intensity on depth of cure of a hybrid composite material was determined using a digital penetrometer. RESULTS The radiometers evaluated varied with respect to sensor aperture diameter, scale readings (analogue or digital) and the units of measurement (arbitrary or mW cm2). The experimental computer-based radiometer allowed continuous recording of intensity against time; thus the light output could be monitored over the entire irradiation period. CONCLUSIONS When light intensity readings were normalized with regard to a standardized light sensing device aperture of 4 mm diameter, a linear relationship was found between depth of cure and the logarithm of the intensity of the light. The results of this investigation support the use of dental radiometers for periodically monitoring visible light activation units.

The compatibility of tissue conditioners with denture cleaners and chlorhexidine.

Abstract Three tissue conditioners, Coe Comfort, Ivoseal and Visco-gel, were immersed in a number of denture cleaners and in two chlorhexidine preparations for 30 days. Changes in the surface of the conditioners were assessed visually at 3, 10, 20 and 30 days and changes in softness were determined by indentation tests on days 10 and 20. A marked deterioration was seen in Coe Comfort and Visco-gel after immersion in Steradent, and in Coe Comfort after immersion in Dentural. A significant reduction in softness occurred in Visco-gel specimens which had been immersed in Denclen and Valdent.

The relationship between transverse strength and testing methods for dental ceramics

Abstract Various factors which affect the strength of dental porcelain have been investigated. The variables studied include rate of loading, heat treatment, firing temperature, vacuum firing, specimen size, and span-to-depth ratio.

Some properties of dental ceramics

The rather exacting aesthetic requirements of dental restorations has largely dictated the composition of dental ceramic materials. The so-called dental porcelains have evolved from the traditional whiteware formula of feldspar, clay and quartz. One of the first variables which might be considered is that of chemical composition. It is, however, generally accepted that for ceramic materials small variations in composition are not critical. Since the early work of Watts in 1918, little interest has been expressed in the chemical composition of dental porcelain. The composition of traditional dental porcelains has evolved over a period of many years by practical trial and error, as discussed by Jones (1971). The development of dental porcelains advanced to the point where any major improvements in physical properties could only be achieved by major changes in composition and technique as advocated by McLean & Hughes (1965). The resulting alumina/glass composites which have been developed use a technique which involves the build up of an inner core which contains a high proportion of crystalline alumina (45-50 %) embedded in a glass matrix surrounded by a transparent outer veneer. This work compares the effect of such inter-related variables as composition, inclusion of filler, viscosity, texture, hardness, and strength of the fired product.

A simple abrasion test for composites

Abstract A simple abrasion test has been devised for evaluating composite and other restorative materials which have similar densities. The method grades the materials in order of their resistance to abrasion and the results for some composites are presented. The test makes use of a standard Wig-L-bug, the results being obtained by weight-loss.

An investigation of test-house variability in the mechanical testing of dental materials and the statistical treatment of results

The suitability of a test for standard specification testing depends inter alia on the ability to repeat the tests in a reproducible manner at a number of test centres. This work involved the investigation of three tests currently proposed as standard specification tests for dental materials. It was found that one test (compressive strength of cements) is inappropriate for inclusion in standards due to an unacceptable variation in test results between test centres. The treatment of results suggested in standards should place less emphasis on the mean value of a relatively small number of test specimens and the use of a simple form of probability theory in which, say, 80 per cent of specimens are required to achieve a certain pass level. In some standards there may be a need to increase the numbers of test specimens significantly in order to achieve a more meaningful and reliable result. The use of Weibull statistics can be adopted as a means of identifying tests which are suitable for inclusion in standard specifications, although it is doubtful that a test at this level of sophistication is required or desirable for inclusion in the standards themselves.

A radio-opaque denture material

Abstract The need for an acceptable radio-opaque denture material is highlighted and a new material, Opex, is described. The new material has good radio-opacity and physical and mechanical properties and may be handled as a conventional denture base material.

The mechanical strength and microstructure of all-ceramic crowns

Digital radiographs need additional metadata in order to be accurate when being converted to analog media. Resolution is a major reason of failures in proper printing or digitizing the images. This letter shortly explains the overlooked pitfalls of digital radiography and photography in dental practice, and briefly instructs the reader how to avoid or rectify common problems associated with resolution calibration of digital radiographs.

Clasp retention and composites: an abrasion study

An in vitro test for screening the abrasion resistance and abrasivity of composite resins when used to provide tooth undercuts for removable partial denture clasps is described. In the present study nine composite resins were tested against wrought stainless steel round clasps. The results indicated that the abrasion of any of the composites tested was unlikely to cause a noticeable loss of retention in the clinical situation. However, there was marked abrasion of the clasps by two of the composites which would be likely to be clinically relevant.