Search PubMed⌕ Search

Biomedical subjects

G W Marshall

Publications and source records attributed to G W Marshall.

At least 19 recordsLinked to original sources
PubMed1992

Gamma-1 to beta-1 phase transformation in retrieved clinical amalgam restorations.

A number of important reactions occur in clinical amalgam restorations. This investigation examined the extent of the gamma-1 to beta-1 transformation in a variety of amalgam restorations retrieved from clinical trials. Sixteen restorations of five brands of low Cu amalgams, 28 restorations of four brands of high Cu blended amalgams, and eight restorations of three brands of high Cu single particle amalgams were studied. The extent of the gamma-1 to beta-1 transformation was determined by the ratio of the integrated intensity of the 002 beta-1 peak to the 330 gamma-1 peak in x-ray diffraction patterns. These values were correlated with age, marginal deterioration grade, and residual Hg content. Restorations consistently contained more beta-1 than comparable laboratory controls. Blended high Cu amalgams generally contained more beta-1, which was correlated with better marginal grades. Beta-1 content was inversely related to residual Hg content. No other associations could be established. A model of the complex interactions between corrosion and transformations was developed.

Corrosion↗
PubMed1992

Dental amalgam: the materials.

The goal of this presentation is to describe the material as it is used clinically, explain why small quantities of Hg can be released, and suggest ideas for amalgams that do not release mercury. A set amalgam is a dynamic material that undergoes many microstructural changes during clinical use, related to both the elevated temperature and corrosion-prone environment in the mouth and mechanical forces applied to the restoration. Amalgams can be divided roughly into two groups by their copper content: low Cu (traditional) and high Cu. High-Cu amalgams generally perform better clinically, but all amalgams corrode to some extent in the mouth. Some corrosion is deemed to be a positive factor, because corrosion product deposition reduces leakage at the margins of restorations; that is, the restorations are partly self-sealing. One of the reasons cited for the improved clinical performance of high-Cu amalgams over low-Cu amalgams is that the corrosion-prone phase, gamma 2, is nearly eliminated in high-Cu amalgams. Future research should involve improvements in the clinical performance of dental amalgams, studies of the mercury release from various types of amalgams and the toxic potential of this exposure, and the development of new amalgam systems that reduce the mercury exposure. Although the longevity of modern amalgams is impressive, it is important for their stability to be increased both clinically and microstructurally. An amalgam should be developed with a stable microstructure that, once set, would not change during clinical use. Microstructural changes lead to clinical deterioration. A stable system would not corrode, and the matrix transformation gamma 1 to beta 1 would be prohibited.(ABSTRACT TRUNCATED AT 250 WORDS)

Copper↗
PubMed1991

The effects of storage after extraction of the teeth on human dentine permeability in vitro.

Freshly extracted third molars were stored for one week in one of four solutions: 70% ethanol, 10% formalin, distilled water with thymol and phosphate-buffered saline with thymol. Crown segments were prepared and initial permeability measurements taken. Each specimen was placed in a fresh preparation of its original solution, and permeability was measured over two periods: 1, 4, 6 and 8 days or 1, 8, 15 and 22 days. Ten crown segments were used for each solution for each time sequence. Permeabilities were lower for those specimens stored in ethanol and formalin than in water/thymol and saline/thymol, but most specimens showed increased permeability with time. Both storage solution and storage time had statistically significant effects (p less than 0.05), with significant differences between specimens stored in water/thymol and saline/thymol and those stored in ethanol and formalin. Thus, type of storage solution effects dentine permeability over time.

Analysis of Variance↗
PubMed1990

Metal ceramic compatibility: a review of the literature.

Metal ceramic restorations are popular in restorative dentistry. Although bonding mechanisms are understood, defining compatibility of alloy and porcelain has been illusive. Chemical compatibility implies a chemical bond between ceramic and alloy that resists stresses resulting from thermal and mechanical incompatibility without compromising esthetics. Research has attempted to predict and test compatibility with varying degrees of success. Despite the success of metal ceramic restorations over the years, compatibility remains undefined without standardized testing procedures. For optimal results, the dentist should communicate with experienced laboratory personnel to meet the specific needs of the patient.

Dental Alloys↗
PubMed1989

A controlled clinical study of amalgam restorations: survival, failures, and causes of failure.

The survival and modes of failure of amalgam restorations were investigated retrospectively. 2660 Class I or II lesions were restored and evaluated yearly or half-yearly for failures during the 30- to 84-month follow-up. Restorations with unacceptable margins were not counted as failures if no traces of secondary caries could be seen. 8% of the restorations were lost because of patient drop-out. Of the remaining restorations, 1% was replaced due to primary caries. Of the remaining number (2431), 9% failed because of all other reasons. The leading mode of failure was bulk fracture (4.6%), followed by tooth fracture (1.9%), and marginal ridge fracture (1.3%). For all other reasons, 0.8% of the restorations failed. Only two restorations were replaced because of secondary caries. The alloy selection in both conventional and high-copper categories significantly influenced the survival of the restorations for reasons directly related to the restoration.

Dental Amalgam↗
PubMed1989

Comparison of backscattered scanning electron microscopy and microradiography of secondary caries.

Carious lesions are usually studied using light microscopy and/or microradiography which require preparation of thin sections. Backscattered scanning electron microscopy (BSEM) has received little attention although it provides information similar to that obtained with microradiographs, with the potential for higher resolution. Recently, microscopes have been introduced that can be used to study wet or nonconducting specimens, offering techniques for studying specimens without desiccation or preparation of thin sections. This investigation sought to determine if secondary carious lesions have the same characteristics when studied by microradiography as when using the "wet" BSEM mode. Microradiographs were made of thin sections from restored teeth with secondary caries induced in an artificial caries system. The thin sections were also studied by BSEM with a partial pressure in the specimen chamber to prevent specimen charging. Comparisons of the lesion size and shape were made using the two methods. Lesion depth measurements in enamel were the same; lesions that penetrated into dentin appeared to be of similar size and shape, but lesion depths measured by BSEM were slightly greater (paired t-test, p less than .05). This was a result of cracks at the carious enamel-dentin interface that probably developed during storage of the samples. Variations in the surface enamel rod structure and the development of subsurface lesions were apparent. Several zones were also apparent in the carious dentin, demonstrating loss of dentinal tubule detail in the depth of the lesion, collapse of tubules, and hypermineralized regions near the advancing front of the lesion. Several additional samples of natural carious teeth were examined. They demonstrated the characteristic structural features of the carious process.(ABSTRACT TRUNCATED AT 250 WORDS)

Dental Caries↗
PubMed1988

Restorative dental materials: scanning electron microscopy and x-ray microanalysis.

Restorative dental materials include the materials used to repair damaged teeth and/or replace missing teeth. The definition could be extended to include a much broader group of dental materials, but this paper concentrates only on those materials used to restore the crown portion of damaged teeth. Auxiliary materials, materials for removable denture prostheses, and root canal materials are excluded. Progress and recent research utilizing SEM (Scanning Electron Microscopy), EDS (Energy Dispersive Spectroscopy), and some related methods are presented for the following dental materials: dental amalgam, enamel and dentin bonding agents, dental composites, dental cements, casting alloys for crowns and bridges, and ceramic-metal and ceramic restorative materials. In addition to the basic characterization of these materials by SEM/EDS techniques, examples of replication methods for SEM study of restorative materials in situ are discussed. Some examples are also presented in the direct use of SEM/EDS for characterization of changes and degradation of restorative materials which occur during clinical use.

Dental Materials↗
PubMed1985

Characteristics of amalgam restorations with variable clinical appearance.

A sample of 165 extracted or exfoliated teeth containing amalgam restorations were categorized as to the degree of marginal integrity of the amalgam. Microstructures of the samples representing the highest (grade 11) and lowest (grades 1 and 2) degrees of marginal deterioration were studied by SEM/EDS. The clinical evaluation and microscopic evaluation indicated that samples showing poor marginal integrity contained large quantities of tin-rich and tin-chlorine-rich corrosion products with little or no gamma-2 phase remaining. Obvious signs of fracture and cracking at the cavosurface were always present. Products containing calcium were extensively associated with corrosion products containing tin, and these were found predominantly at the tooth-amalgam interface. No copper-rich amalgams were found in this grouping. Amalgams with good marginal integrity had no microscopic cracks or fractures and generally little evidence of corrosion was found. However, one of six samples had extensive corrosion products, indicating that corrosion product formation is not the only factor contributing to marginal deterioration. After 6 years of clinical use, the degree of marginal deterioration did not appear to depend unerringly on time of use. Further work is necessary to define the microstructural characteristics of samples showing intermediate amounts of marginal deterioration and to determine if copper-rich amalgams consistently have small to moderate amounts of marginal deterioration.

Adolescent↗
PubMed1979

Time-dependent phase changes in Cu-rich amalgams.

Cu-rich amalgams represent a significant change in amalgam alloy formulations which may result in enhanced performance. Significantly, these systems reduce or eliminate the gamma2 phase. This work studied the phase changes which occurred from one hour to six months in 13 commercial amalgams by x-ray diffraction. This included 9 two-particle or blend systems and 4 single-particle systems. Gamma2 was found in all systems, but was reduced or eliminated on storage at 37 degrees C at rates which varied from system to system. During gamma2 elimination, the Cu6Sn5 content correspondingly increased, indicating that solid-state reaction occurs for up to six months in some systems.

Chemical Phenomena↗
PubMed1978

Enamel surface characteristics on removal of bonded orthodontic brackets.

Thirty-six premolars which were scheduled for routine extraction were bracketed with either N or G system adhesives. After one week the brackets were removed and the residual adhesive removed by low-speed green stone, a hand scaler, or an ultrasonic scaler. Half the teeth were given a final pumice, and then all were extracted. The SEM was used to evaluate the enamel surface, and quantitative assessments were made for the area covered with retained adhesive and for damage to the surface. All three removal techniques left appreciable amounts of retained adhesive 20 to 60 per cent coverage for G and 55 to 75 per cent for N adhesive. All detectable adhesive was removed by final pumicing. The detectably roughened area was over 50 per cent for the green stone treatment for both materials and appeared to introduce unnecessary roughness. The hand scaler and the particular ultrasonic scaler used in this study did not severely roughen the enamel for either material and appeared to be effective for initial adhesive removal. All three techniques introduced occasional gouges 10 to 20 micrometer in depth which could not be removed. All other roughness was generally smoothed by final pumicing. Thus, pumicing is a necessary final step with all removal procedures studied.

Dental Bonding↗
PubMed1977

Roll-over test.

One hundred randomly selected nulliparous, normotensive women were subjected to the roll-over test between 28 and 32 weeks' gestation. Twenty-five women had a positive test. Thirteen of those 25 women developed pre-eclampsia requiring magnesium sulfate therapy. Eight had transient hypertension during labor requiring no therapy. Four had no evidence of hypertension during pregnancy. A false-positive rate for the 25 women with a positive roll-over test was 16 per cent. Seventy-five women had a negative roll-over test. Sixty-eight of those women had no evidence of hypertension of pregnancy. Seven had evidence of transient hypertension during labor requiring no therapy. A false-negative test was present in 10 per cent of the 75 patients with a negative roll-over test. In no case did a patient with a negative test develop pre-eclampsia. The roll-over test is recommended as a routine test for every pregnant patient between 28 and 32 weeks' gestation for the early diagnosis of hypertension of pregnancy.

Blood Pressure↗