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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↗
PubMed1984

An in vivo method for the biological evaluation of metal implants.

The peritoneal cavity of the rat was used as an implantation site in order to study the quantitative, cellular response and the qualitative, histopathological response to three metals (Ag, Sn, Cu). The effects of the metals on the cells were correlated with the cellular concentrations of the metal as determined by chemical analysis. Small variations in the cell population and a minimal foreign body reaction was observed with an implanted control material (silicone polymer). Large increases in the number of cells and an intense foreign body reaction was observed with Cu implants. Decreases in the number of cells were seen with Sn and Ag implants, but only Sn elicited a foreign body reaction. Implantation of Ag failed to elicit a foreign body reaction. Significant concentrations of all three metals were detected in the retrieved cells.

Animals↗
PubMed1984

Surface resistance to abrasion of preformed laminate resin veneers.

Preformed laminate resin veneers should be adapted closely to the teeth to reduce the thickness of the composite layer. Minimal voids at the composite, veneer, and tooth junctional margin will result. Composite resins are difficult to finish, but laminate veneer can be highly polished. Poorly finished veneer surfaces appear to abrade to a smoother surface with toothbrushing while poorly finished composite surfaces remain rough. To reduce material loss from abrasion, the veneer restoration should be highly polished. Polishing the veneer at the gingival margin is particularly important to gingival health. Only rough veneer surfaces will be improved by toothbrushing abrasion. Special instructions should be given to patients with laminated veneers to reinforce proper oral hygiene and to discourage use of highly abrasive dentifrices. Continued monitoring of plaque accumulation should minimize problems related to oral hygiene.

Adolescent↗
PubMed1983

Corrosion product formation sequence on Cu-rich amalgams in various solutions.

A single particle and four blended Cu-rich amalgam systems were immersed in 37 degrees C solutions for 1-20 months in order to determine the characterization and sequence of corrosion product formation. X-ray diffraction and SEM/EDS were used to characterize the products. The same sequence of formation occurred in all systems in Ringer's and 0.1% Ringer's solutions. The times at which each product formed varied with the brand of the amalgam and the concentration of the solution. The initial products were ZnSn(OH)6 in Zn-containing systems and SnO2 in most other systems. Cu2O formed next, followed by CuCl2 X 3Cu(OH)2. Immersion in 1% Na2S yielded only HgS on all brands. A combination of 1% Na2S and Ringer's solution yielded CaSn(OH)6 after 2 months and Cu2O at later periods. Artificial saliva resulted in a retardation of corrosion product formation and only limited amounts of a Sn-rich product could be found after 20 months. Interactions of the various components appear to alter the nature and rate of corrosion product formation on these systems and additional systematic investigations are necessary to understand the influence of these interactions on corrosion.

Copper↗
PubMed1982

Enamel surface evaluations after clinical treatment and removal of orthodontic brackets.

Methods for the removal of resins attached to acid-etched enamel are receiving increased attention. An evaluation method which permitted convenient statistical comparisons of the effectiveness of various removal procedures was used. The method was based on ranking of SEM photomicrographs of epoxy replicas of teeth from clinical cases. Forty-five orthodontically treated teeth were evaluated after removal of brackets and a heavily filled bonding resin, using three basic procedures. Eleven raters ranked photomicrographs on the basis of the apparent smoothness of the replicated enamel surface. There was no significant difference in the judgment of smoothness by the raters. Significant differences were found between the three removal techniques. Enamel roughness decreased with removal technique in the following order: (1) hand scaler, (2) twelve-fluted carbide bur, and (3) ultrafine bur. A final pumicing treatment was also found to be beneficial, although it did not remove any deep scratches.

Composite Resins↗
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↗