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Biomedical subjects

W M Johnston

Publications and source records attributed to W M Johnston.

At least 19 recordsLinked to original sources
PubMed1991

Fracture incidence on debonding of orthodontic brackets from porcelain veneer laminates.

The bonding force and the incidence of porcelain fracture for two porcelain systems, on removal of orthodontic brackets bonded to porcelain veneer laminates after thermocycling, were determined. A factorial design with a total sample size of 256 was used. The factors tested were (1) porcelain type (feldspathic [porcelain CE] or high alumina [porcelain VI]), (2) surface treatment (glazed or roughened), (3) priming agent (silane or alcohol), (4) bonding resin (macrofilled or microfilled), and (5) debonding time (30 minutes or 24 hours). An analysis of variance revealed that surface treatment, primer, resin type, and debonding time each contributed significantly to the debonding force. The probability of porcelain fracture was dependent on the porcelain type and the debonding force, with porcelain VI exhibiting a significantly higher probability of fracture than porcelain CE at the same debonding force. The etched surface of porcelain CE exhibited a more retentive microstructure than porcelain VI. The success of using bonded orthodontic brackets on porcelain veneer laminates depends on both the bond of resin to porcelain and the resistance of porcelain to fracture during debonding. For a given porcelain veneer, factors that provide a higher debonding force result in a higher probability of porcelain veneer fracture.

Acrylic Resins↗
PubMed1991

Duration of tooth color change after bleaching.

Tooth color change was monitored after a single, in-office bleaching technique using a colorimeter. Twenty young adults participated in the double-blind study at The Ohio State University College of Dentistry. Half of the participants had their maxillary anterior teeth bleached and half were controls. By one month, the large initial color change was considerably reversed and only noticed by one subject. After six months, the effect of bleaching was small, but still measurable.

Adult↗
PubMed1991

In vitro tooth color change with repeated bleaching.

The interest of patients in vital tooth bleaching has created a demand for treatment. However, few objective data are available to help determine the number of bleaching appointments necessary and the lightening that can be expected. The purpose of this study was to measure color changes in bleached extracted teeth and compare these to control groups. Group A was etched and bleached with 35% stabilized hydrogen peroxide. Group B was treated the same as group A, but the bleaching solution was replaced with distilled water. Group C was not treated, but was stored in water. Mean color difference after one treatment was 3.33 for group A, 1.67 for group B, and 0.48 for group C. After six treatments, the overall color difference was 3.82, 2.41, and 1.38 for groups A, B, and C, respectively (P less than .01). Color changes beyond those found after the first treatment were small, suggesting that there was little benefit in repeated bleachings under the conditions of this study.

Acid Etching, Dental↗
PubMed1989

Repair of glass ionomer restorative materials: flexure strength of specimens repaired by two methods.

This study evaluated the adhesion of newly mixed glass ionomer restorative materials to set materials. The flexure strength of specimens formed by the addition of new material to week-old material by using two different surface treatments was compared with the flexure strength of solid specimens that were 1 week old. All repaired specimens attained at least 44% of the average strength of unrepaired specimens of the same material. Repairs made with a combination of unlike materials were not significantly different from most repairs with like materials. For all materials collectively, repairs made with a 37% phosphoric acid pretreatment were significantly stronger than repairs made with a 40% polyacrylic acid pretreatment.

Acid Etching, Dental↗
PubMed1989

Colour measurements of all ceramic crown systems.

The objectives of this study were: (i) to determine variability among colour parameters of five different ceramic crown systems; and (ii) to measure the effect of using coloured luting agents on restoration colour. The crown systems studied were Cerestore, Dicor, Hi-Ceram, Renaissance, and Vitadur-N. Five crowns for each system were made according to manufacturer's instructions with the same nominal shade (Vita Lumin Vacuum A2) to fit an Ivorine central incisor tooth. Restoration thickness was adjusted to within +/- 0.1 mm (+/- 0.05 mm in the mid-facial area where colour measurements were to be made) with the aid of a dial calliper prior to glazing or, in the case of Dicor, surface staining. Where a core was part of the system this was fabricated to the minimum recommended thickness. The crowns were cemented using luting agents of five different colours in a randomly chosen sequence. The colour of each restoration/cement combination was measured three times using a small-area colorimeter (Minolta CR-121). The variance of each colour parameter (L*, a*, b*) was statistically compared for each crown system using an analysis of variance procedure, as was the effect of the cement. Observed differences were related to visual perception by using the colour difference formula. There were statistically significant differences among the variances of the crown systems and the cements, with significant interactions between crown systems and direction of colour and between cement and direction of colour. Restorations made with different ceramic crown systems had noticeably different colour despite having the same nominal shade. Changing the shade of the luting agent had a perceivable effect on Dicor crowns and, to a lesser extent, on Vitadur-N crowns but not on the other systems due, presumably, to the opacity of their core materials.

Color↗
PubMed1989

The detection of incipient caries with tracer dyes.

The purpose of this study was to determine the increase in color contrast produced by the use of a tracer dye in detection of incipient caries lesions with transillumination. Twenty-four caries-free first premolars were immersed in an acid gelatin for production of artificial incipient caries lesions. After the lesions had developed, these teeth were photographed by transillumination. Two photographs were taken of each tooth. The first photograph showed the lesion without dye. A blue tracer dye was then added and absorbed by the lesion, and a second photograph was taken. The data on the color difference were obtained by use of a reflectance colorimeter and showed a four-fold increase between the lesion and surrounding area with the dye. A two-way analysis of variance was used for the statistical interpretation. The color difference between the lesion without the dye and then with the dye was significant. The use of the blue tracer dye, therefore, significantly increased the contrast in the images of the artificial incipient lesions.

Benzenesulfonates↗
PubMed1989

Assessment of appearance match by visual observation and clinical colorimetry.

Judgments of appearance matching by means of the visual criteria established by the United States Public Health Service (USPHS) and by means of an extended visual rating scale were determined for composite resin veneer restorations and their comparison teeth. Using a colorimeter of 45 degrees/0 degrees geometry and the CIELAB color order system we used the color of the restorations and comparison teeth to calculate a color difference for every visual rating. Statistically significant relationships were found between each of the two visual rating systems and the color differences. The average CIELAB color difference of those ratings judged a match by the USPHS criteria was found to be 3.7. However, the overlap in ranges of the color differences for those comparisons rated matches and mismatches indicates the importance of other factors in appearance matching, such as translucency and the effects of other surrounding visual stimuli. The extended visual rating scale offers no advantages to the more broadly defined criteria established by the USPHS.

Color↗
PubMed1989

Performance assessment of colorimetric devices on dental porcelains.

The selection of an appropriate material that duplicates the appearance of natural tooth structure is very important in restorative dentistry. Photometric and colorimetric analysis techniques offer great potential as a tool for aiding in the duplication process. The degree to which these techniques will be useful depends on the accuracy and precision with which they can be applied to translucent as well as opaque surfaces. The purpose of this investigation was to evaluate the performance of three currently-available photometric devices. The performance capabilities of the instruments were tested on various shades of opaque and translucent dental porcelain surfaces. The performance tests were designed for evaluation of the accuracy and precision of the instrument relative to a well-studied reference instrument. CIELAB color difference metrics were used for the performance analysis. The results revealed that each of the photometric instruments evaluated was capable of producing color measurements with precision. However, the degree of accuracy with which the color measurements were made varied depending on the instrument used and the type of material surface being measured. A photo-electric tristimulus colorimeter showed the best overall performance on the porcelain surfaces, supporting its use as a valuable tool for evaluating color in dentistry.

Analysis of Variance↗
PubMed1989

Relative microhardness of glass ionomer restorative materials as an indicator of finishing time.

Microhardness was evaluated as an indicator of appropriate finishing intervals for glass ionomer restorative materials. The hardness of several materials was determined after the first hour of mixing. A material clinically documented to be satisfactorily finished after a delay of 15 minutes attained 29% of its 24-hour hardness during that interval. Samples of the other materials finished on reaching this relative hardness, which differed from the manufacturers' suggested finishing delays, showed no differences in surface characteristics, appearance, or surface roughness compared with specimens finished at the recommended times.

Aluminum Silicates↗
PubMed1989

Microabrasion of human enamel in vitro using hydrochloric acid and pumice.

The rubbing application of a hydrochloric acid-pumice mixture has been advocated for the removal of fluorotic-like areas of permanent teeth. However, there have been no previous reports as to the amount of enamel removed during this process. The objective of this project was to measure the amount of enamel lost during successive rubbing applications of an 18% HCl-pumice mixture. Nine extracted permanent molars were mounted in die stone, and each was subjected to 10 sequential 5-sec rubbing applications of an 18% HCl-pumice mixture. After rubbing for 5 sec, the teeth were rinsed with water for 10 sec and then thoroughly dried. Enamel loss was measured to the nearest micron utilizing a measuring microscope with 3-dimensional digital stage positioning. Measurements were taken at the consistent x,y coordinates of three points on the tooth surface after each application. The change in the z coordinate, representing the spatial change in the location of the enamel surface, was recorded for each point after each application on the tooth. Regression analysis revealed enamel loss of 12 microns after the initial application, and an average of 26 microns of enamel loss after each successive application. The regression coefficient was significant at P less than 0.001.

Dental Enamel↗
PubMed1989

The retention of titanium pins in high-copper amalgam and their influence on its fracture resistance.

The influence of titanium pins on the fracture resistance of amalgam and the retention of amalgam by titanium pins were compared to that of stainless-steel pins. The retention of stainless-steel pins embedded in amalgam to various depths was also evaluated. None of the pins had a significant influence on the compressive fracture resistance of amalgam. All pins embedded to standard depth in amalgam failed in tension, with titanium pins demonstrating the lowest resistance to tensile forces. Stainless-steel pins embedded 1.75 or 1.5 mm and loaded in tension caused failure in the amalgam, but at forces comparable to the retention of such pins in dentin.

Copper↗
PubMed1988

The effects of manipulative variables on the color of ceramic metal restorations.

Certain variables were tested for their influence on the color of ceramic metal restorations. The variables included firing temperature, condensation technique, modeling liquid, and brand of porcelain. In addition, the color of different nominal shades was compared. The color was measured with a Minolta CR-121 small-area colorimeter. The small (7 mm2) measuring area enabled readings to be made on actual crown specimens rather than disks. The results were expressed in the CIELAB color system enabling comparisons to be made related to visual perception. The following conclusions can be drawn from the study. 1. The small-area colorimeter is able to detect statistically significant and perceivable color differences between shades of porcelain. The variation in color parameters associated with making replications of a restoration is significantly greater than the error associated with making repeated color measurements of the same restoration. 2. Restorations made with the different brands of porcelain studied have noticeably different colors despite having the same nominal shade. 3. Color changes caused by the choice of modeling liquid were not statistically significant in this study. If the mean color differences are truly representative, using Rainbow or Carv-eze modeling liquids with Vita VMK porcelain may produce color changes that are just barely detectable. 4. The manipulative variables of firing temperature and condensation have little influence on the color of the restorations.

Color↗
PubMed1988

Direct bonding of orthodontic brackets to porcelain veneer laminates.

The forces required to debond orthodontic attachments from porcelain veneer laminates were studied in vitro. Brackets were bonded to 160 veneered bovine incisor teeth before the determination of the debond force. The independent variables studied were resin type, priming agent, porcelain surface preparation, and debonding time. The average debond forces were compared with those obtained by debonding brackets bonded to natural teeth by means of the acid-etch technique. The bond between the resin and the porcelain surface was found to be satisfactory for direct bonding of orthodontic attachments. Roughening the porcelain surface and using a silane primer required an average debond force comparable to that of the acid-etched enamel bond at 24 hours. However, it increased the risk for porcelain fracture during debonding. Roughened surfaces and surfaces with micro-fractures could be satisfactorily finished and polished with either a series of graded Ceramiste points or a diamond-impregnated polishing wheel followed by a diamond polishing paste.

Acrylic Resins↗
PubMed1986

Light scattering and gloss of an experimental quartz-filled composite.

For samples of polymethylmethacrylate with and without quartz filler, the inverse of the contrast-gloss ratio is shown to be related to surface roughness and to the optical scattering coefficient. This finding adds to the importance of optical scattering, which has been widely studied because of its relation to color and translucency of materials. Furthermore, optical scattering by composite fillers is shown to be linearly related to the concentration of the filler material within the range of concentrations studied. Quartz fillers were incorporated at concentrations from 5 to 20 weight percent and were short fibers or granular powder, with the granular particles ranging in median equivalent spherical diameter from 15 to 3.3 micron. The efficiency of optical scattering for the granular quartz filler increased as the size of the filler decreased.

Color↗
PubMed1978

Effect of hydrostatic tensile stress on the growth of Escherichia coli and Bacillus cereus.

The specific growth rates of Escherichia coli and Bacillus cereus were measured for growth media in a flask, a lens-plate arrangement simulating an isolated capillary space, and a lens-plate arrangement under hydrostatic tensile stress. The specific growth rates of the bacteria were the same for the flask and lens-plate arrangement without hydrostatic tensile stress, but were enhanced when the growth media were subjected to hydrostatic tensile stress. The enhanced specific growth rates reached steady values at a tensile stress of 40 pascals. The effect was observed up to tensile stresses of around 100 pascals. The maximum increase in specific growth rate was 25% for E. coli and 22% for B. cereus.

Bacillus cereus↗