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Biomedical subjects

S N White

Publications and source records attributed to S N White.

At least 19 recordsLinked to original sources
PubMed1997

Relationship between static chemical and cyclic mechanical fatigue in a feldspathic porcelain.

OBJECTIVES: The goal of this study was to determine if static chemical and cyclic mechanical fatigue are independent, or if they interact to produce greater than additive strength loss in a feldspathic porcelain. METHODS: A blunt indentation technique was used to investigate the response of a feldspathic dental porcelain to cyclic mechanical fatigue and static chemical fatigue. All specimens were fabricated in a dry inert environment and then mechanically fatigued by cyclic loading and strength-tested in dry inert nitrogenous, ambient or wet environments. A series of experiments were performed to evaluate the effects of chemical and mechanical fatigue, and their interaction on strength loss; to determine the effects of, and interaction between, the factors of cyclic fatigue environment and strength test environment on strength; to ascertain if the type of environment during strength testing influenced specimen strength; and to distinguish between chemical damage caused by exposure to moisture alone and stress corrosion damage resulting from the strength testing environment, using a pair of two-way analysis of variance, a single one-way analysis of variance and a t-test (p < 0.05). RESULTS: These experiments indicated that both static chemical fatigue and cyclic mechanical fatigue significantly reduced specimen strength, but they did not interact to produce greater than summative effects. It was also learned that chemical fatigue was not detected on initial exposure to moisture and that it occurred to a small extent during mechanical fatigue cycling, and primarily occurred during strength testing through a stress-corrosion phenomenon. Micrographs visually evaluated the effects of mechanical and chemical fatigue on surface contact damage. SIGNIFICANCE: As both static chemical and cyclic mechanical fatigue influenced porcelain strength, they should both be considered in future evaluations. However, because they largely acted independently, they can be studied separately.

Aluminum Silicates↗
PubMed1997

Effect of fit adjustment on CEREC CAD-CAM veneers.

PURPOSE: To measure and compare the adaptation of CEREC CAD-CAM porcelain veneers with and without fit adjustment. MATERIALS AND METHODS: Two groups of porcelain veneers were fabricated using the CEREC CAD-CAM technique for standardized preparations on 10 artificial teeth. Mesial preparation contacts were broken, but distal contacts remained intact. One group was adjusted using a disclosing spray to improve fit, another group made for the same teeth was not adjusted before cementation. The veneers were cemented in a standardized manner to their teeth. The veneers and their attached cement were retrieved, embedded in epoxy resin and sectioned twice to produce eight sections. Each section was measured at three defined points from the external surface inwards. RESULTS: Three-way ANOVA disclosed that fit adjustment was relatively ineffective (P approximately 0.05), but that measurement section location, measurement point location and their interaction significantly affected fit (P < 0.05). Restoration of the broken proximal contact, or simulated diastema closure did not compromise fit. Incisal margins ranked worst for fit. Surface measurement point locations ranked worse than internal points. The fit of these CEREC CAD-CAM veneers was not substantially different from previously published fit values for conventional porcelain veneers.

Acrylates↗
PubMed1996

Strength of porcelain fused to titanium beams.

The purposes of this study were to measure strengths of layered porcelain fused to titanium beams, determine failure modes, and investigate the porcelain-titanium interface. A three-point flexural test and formulas derived especially for this purpose were used. The strength of layered porcelain-ceramic beams was limited by the cohesive tensile or compressive strengths of the porcelain, not by the porcelain-titanium interfacial bond, namely, the porcelain failure occurred at lower loads than did failure of the porcelain-titanium interface. The porcelain-titanium bond strength was at least 26 MPa. Scanning electron microscopy and energy-dispersive x-ray spectroscopy demonstrated that the bond was limited by delamination of a thin titanium-titanium oxide interface. Theoretic curves that describe effects of relative layer and total thickness on the force-bearing capacity of model beams were plotted. These curves indicated that porcelain-titanium prostheses should be made as thick as is practical, but the relative thickness of the porcelain and titanium layers would be less important.

Analysis of Variance↗
PubMed1996

Fatigue of resin cement-base metal alloy bond strength.

PURPOSE: Strong durable bonds between resin cements and metal alloys are critical to the success of resin-bonded, resin-veneered, or resin-retained prostheses. However, few comprehensive, comparative evaluations of materials or the fatigue effects of thermal cycling have been reported. The rate of strength loss may be a more important predictor of long-term success than bond strength. The purpose of this study was to investigate the effects of artificial aging by thermal cycling and resin cement type on the bond strengths to a base metal alloy. MATERIAL AND METHODS: This study investigated the effect of the number of thermal cycles (0, 1, 10, 100, 1,000, and 10,000) on the bond strengths of nine fixed prosthodontic resin cements. Specimens were assigned randomly to thermal cycle number/cement type test groups. Cylinders of a base metal alloy were bonded in an end-to-end configuration. One end of each bonded specimen was insulated, and the specimen was thermal cycled. Then, the bonds were tested in shear and bond strengths calculated. RESULTS: Two-way ANOVA revealed that the effects of cement type, the number of thermal cycles, and their interaction all significantly affected bond strength (p < .0001). Multiple range analysis showed that some cements had significant trends to lose bond strength with thermal cycling (p < .05), while others did not (p > .05). CONCLUSIONS: Both the type of resin cement and the amount of thermal cycling influenced bond strength to a base metal alloy. Some materials displayed more rapid loss of bond strength than others.

Analysis of Variance↗
PubMed1996

Biocompatibility of posterior restorative materials.

Biocompatibility of dental materials is an important consideration for the patient, clinician, laboratory technician and manufacturer. This paper examines biocompatibility testing methods and the biocompatibility of posterior restorative materials, including amalgam, casting alloys, resin composites, dentin bonding agents, cements, porcelains and ceramics.

Acrylic Resins↗
PubMed1995

Tensile bond strength of polyvinyl siloxane impressions bonded to a custom tray as a function of drying time: Part I.

Time-dependent bond strength studies of two polyvinyl siloxane impression materials to acrylic resin disks with their respective adhesives were studied to determine the optimal time for maximum bond strength. Six groups were tested with varying adhesive dry times of 0, 7, 15, and 60 minutes and 8 and 24 hours before testing. The results indicated that the bond strength of the adhesive increased at least twofold from time zero to 7 minutes adhesive dry time and peaked at 60 minutes for one of the materials and at 8 hours for the other. Bond strengths increased rapidly to the 15-minute test interval and then seemed to plateau. Both materials exhibited decreased adhesive bond strengths at 24 hours.

Acrylic Resins↗
PubMed1995

Adhesion of denture tooth porcelain to heat-polymerized denture resin.

Use of porcelain denture teeth may be desirable in many clinical situations, including implant-supported prostheses. However, lack of space because of frameworks often precludes the use of conventional retention by diatorics and pins. Adhesion of porcelain denture teeth to denture resin could also stiffen and possibly strengthen dentures and decrease stain ingress between porcelain teeth and resin denture bases. Unlike previous studies that investigated the bond between conventional feldspathic metal-ceramic porcelain and bis-GMA based composite resin, this study investigated adhesion of denture tooth porcelain to polymethyl methacrylate (PMMA). High-energy air abrasion, hydrofluoric acid etching, and the use of a general purpose bonding agent resulted in an improved bond strength of heat-cured denture PMMA bonded to denture tooth porcelain. Silane coating did not improve bond strengths, and conventional air abrasion was no more effective than polishing with 600-grit silicon carbide. Storage in water and artificial aging substantially decreased bond strengths. The strongest bond strengths were achieved by a high-energy-abrasion + etching + multi-purpose bonding-agent treatment, but a simpler etching + multiple-purpose bonding-agent treatment also produced reliable results. A laboratory technique was suggested. The role of surface treatment in the mechanism of adhesion was examined with scanning electron microscopy. High-energy abrasion produced a slightly more detailed initial topography than conventional air abrasion, but after etching, the high-energy topography became much more detailed. Surface topography alone did not account for all differences found.

Analysis of Variance↗
PubMed1995

In vivo marginal adaptation of cast crowns luted with different cements.

Large marginal crown discrepancies are deleterious. Although previous studies have examined casting seating in vitro, few have evaluated the effects of different luting cements on in vivo seating. Hence this in vivo study compared marginal discrepancies created by different cements on cast crowns. Tooth preparations were completed according to acceptable procedures on previously intact human molars that were severely periodontally compromised and scheduled for extraction. Castings were made by conventional techniques. The castings were randomly assigned to the following luting agents: zinc phosphate cement, resin-modified glass-ionomer cement, and the same resin-modified glass-ionomer cement with a dentinal bonding agent. After 6 months the teeth were carefully extracted, embedded, and sectioned, and vertical and horizontal discrepancies were measured. Analysis of variance revealed that all cements resulted in similar discrepancies. Vertical discrepancies were considerably greater than horizontal discrepancies, and the distributions of marginal discrepancies were skewed towards upper ranges.

Analysis of Variance↗
PubMed1995

Microleakage through dentin after crown cementation.

This study investigated the relationship between type of luting cement for artificial crowns and microleakage through dentinal tubules. Standardized preparations were made on intact human premolars, and crowns were made in a base metal alloy using conventional techniques. The castings were randomly assigned to the following luting agent groups: zinc phosphate (ZP), polycarboxylate (PC), glass ionomer (GI), phosphate ester composite resin (GMA/PE), and a composite resin with a NPG-GMA dentin bonding agent (GMA/NPG). Then they were cemented in a standardized manner. The specimens were artificially aged, stained, sectioned, and microleakage occurred through dentinal tubules toward the pulp measured. The rank in order from least to most (best to worst) leakage was GMA/NPG, GI, GMA/PE, PC, and ZP. Material GMA/NPG recorded significantly less leakage than all other materials. Therefore, the results of this study suggest that material GMA/NPG may reduce pulpal sensitivity and pathosis.

Analysis of Variance↗
PubMed1995

Effect of dynamic loading methods on cement film thickness in vitro.

PURPOSE: Reduced cement flm thicknesses can improve crown seating and decrease marginal discrepancies. Improved marginal adaptation has the potential to reduce plaque accumulation, periodontal disease, and cement dissolution. Studies have indicated that dynamic seating methods can reduce seating discrepancies associated with zinc phosphate and resin cements. However, other types of cements and other dynamic techniques have not yet been studied or compared, nor has the mechanism for improved seating been fully explained. Therefore, the purpose of this study was to investigate the effect of a variety of loading methods on the film thicknesses of current types of crown and fixed partial denture cements compressed between glass plates. MATERIALS AND METHODS: This study investigated the effect of three dynamic loading methods on film thickness of six representative fixed prosthodontic cements. These included zinc phosphate, resin-modified glass ionomer, encapsulated glass ionomer, adhesive composite resin, composite resin, and polycarboxylate. The method was derived from American Dental Association specifications for cement film thickness. In control groups, the cements were placed between two glass glass plates and statically loaded with a 15-kg weight. The test groups were initially similarly loaded, and then for 30 seconds further subjected to simulated repeated patient opening and closing, vibrations from an electromallet, or an ultrasonic device. RESULTS: Mean film thicknesses ranged from 7.4 micrometers for polycarboxylate / ultrasound up to 40.3 micrometers for composite resin / static. Two-way analysis of variance revealed that the effects of material type and cementation method and their interaction all significantly affected film thickness (P < .0001). Multiple range analysis showed that dynamic methods were generally superior to static loading and that the ultrasonic method was the best overall. CONCLUSIONS: The different dynamic loading methods all significantly decreased cement film thicknesses between glass plates. The ultrasonic method was the most effective. The type of cement used also influenced film thickness. Composite resins were more affected than other materials.

Analysis of Variance↗
PubMed1995

Moisture susceptibility of resin-modified glass-ionomer materials.

The purposes of this experiment were to determine if resin-modified glass-ionomer cements are less sensitive to moisture than are conventional glass-ionomer cements, to investigate the effects of barrier coatings, and to study the effects of different setting environments. Discoid specimens of a variety of resin-modified glass-ionomer materials and a conventional glass-ionomer cement control were stored in different environments and were protected with different barrier coatings. The diametral tensile strengths of the specimens were determined and analyzed with three-way analysis of variance. Resin-modified glass-ionomer cements are less sensitive to moisture than is the conventional glass-ionomer cement control. Drier environments produced stronger resin-modified glass-ionomer specimens. Use of a fissure sealant as a barrier coating increased overall specimen strength, and the individual materials differed in strength.

Absorption↗
PubMed1995

On the cutting edge: marginal integrity.

This paper reviews a number of recent University of Southern California School of Dentistry clinical and laboratory research studies on marginal integrity. Marginal integrity was comprehensively investigated in many different ways. A series of studies examined cement physical properties, casting seating and microleakage, from basic laboratory investigations to multicenter clinical trials. New strong adhesive cements were shown to have many important advantages, but they have a short clinical track record and appeared to be less user-friendly than the more traditional zinc phosphate cement. Casting adjustment techniques markedly improved crown seating. Conventional and new technologies for crown fabrication were evaluated in-vivo with respect to marginal integrity. Although science and art have progressed tremendously, the perfect margin was not achieved.

Cementation↗
PubMed1994

Luting cement-metal surface physicochemical interactions on film thickness.

Low film thickness is critical to the clinical success of cemented castings. This study investigated the effect of luting agent-metal physico-chemical surface interactions on film thicknesses of representative luting agents. Control group luting agents were placed between two glass plates, as described by American Dental Association specifications 8, 61, and 66, and test group luting agents were positioned between glass and metal plates. The materials selected were zinc phosphate cement, polycarboxylate cement, glass ionomer cement, glass ionomer-composite resin hybrid cement and a resinous cement, with a type III gold alloy, a noble metal ceramic alloy, and a base metal ceramic alloy. A two-way analysis of variance and follow-up tests were done. The effects of the type of metal surface, type of cement, and their statistical interaction significantly affected film thickness (p < 0.0001). The type of cement had a greater affect on film thickness than the type of metal. A glass ionomer cement produced lower overall film thicknesses than other cement types, and a noble metal ceramic alloy created lower overall film thicknesses than other types of metal. American Dental Association specifications for cement film thickness did not accurately reflect normal cement use.

American Dental Association↗
PubMed1994

In vivo microleakage of luting cements for cast crowns.

Standardized tooth preparations were completed on previously intact human molars in vivo, and castings were made with a precious metal ceramic alloy by conventional techniques. The castings were randomly assigned to the following luting agents: zinc phosphate, composite resin-glass ionomer hybrid, and a composite resin-glass ionomer hybrid with a dentinal bonding agent and were cemented in a standardized manner to periodontally compromised molars. After 6 months the teeth were carefully extracted, stained, embedded, and sectioned, and the in vivo microleakage was measured. ANOVA disclosed significant differences between groups, and a multiple comparisons test revealed that the zinc phosphate group leaked significantly more than other cement groups.

Adult↗