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Biomedical subjects

N K Sarkar

Publications and source records attributed to N K Sarkar.

At least 37 records · Page 2Linked to original sources
PubMed1991

Microstructural characterization of a foil crown system.

In this study, the microstructural nature of the Renaissance crown system has been characterized in an effort to understand its reported properties and performance. Intergranular cavitation was characteristic of the foil surface. The brazed area joining various pleats was porous and exhibited an equiaxial grain structure. Particles of the interfacial alloy sintered with each other and bonded to the foil. Flow of porcelain into intergranular grooves of the foil and within the porous interfacial alloy network was observed. Tags that formed as a result contributed to the micromechanical retention of porcelain on the foil. No evidence of foil-porcelain chemical bonding was found, nor was there any indication of substantial interlayer diffusion.

Crowns↗
PubMed1990

An in vitro study of the effect endodontic access preparation and amalgam restoration have upon incisor crown retention.

Endodontic access preparation leads to a significant reduction in crown retention. This study sought to determine whether this retention can be regained after access. Eighteen extracted human maxillary incisors had PFM crowns fabricated. Crowns were cemented, retention measured, recemented, access preparations cut, and retention once again measured. Then, crowns were recemented and accesses restored with dental amalgam and new retention measured. This format allowed each crown to be used as a control of itself. For crowns cemented with zinc phosphate cement and restored with amalgam, a 126% increase over original retention was measured. For crowns cemented with polycarboxylate cement and restored with amalgam, a 237% increase over original retention was measured. For crowns cemented with polycarboxylate cement and restored with amalgam, a 237% increase over original retention was measured. These results suggest that recementing crowns secondary to endodontic access and restoring the access with amalgam regains and even surpasses the original retention.

Cementation↗
PubMed1990

Fluoride release from glass-ionomer cements in de-ionized water and artificial saliva.

Most of the data reported on the release of fluorides from glass-ionomer cements are based upon measurements made in de-ionized water. De-ionized water does not represent the complex chemistry of the oral environment. In this study, an artificial saliva solution was used, and the data on the fluoride ions released into it from four glass-ionomer cements were compared with those obtained in de-ionized water. The glass-ionomer cements used were: Ketac-Fil (ESPE), Ketac-Silver (ESPE), Fuji-II (GC), and Miracle Mix (GC). Ten disc samples (2 cm x 0.1 cm) of each cement were prepared. Each was individually suspended in either de-ionized water or artificial saliva (five samples in each medium) and stored for 24 h at 37 degrees C, then transferred to fresh media for a total of 60 days. Collected media were buffered with TISAB, and fluoride was measured by a specific electrode. Artificial saliva was prepared according to Fusayama et al. (1963). The data obtained showed that: (1) glass-ionomer cements released more fluoride in de-ionized water than in artificial saliva; (2) Ketac-Fil released 20% more fluoride in saliva than did Fuji-II, the latter releasing 49% more fluoride than Ketac-Fil in de-ionized water; and (3) conventional glass ionomers released more fluoride than did meta-reinforced ones in both media.

Cermet Cements↗
PubMed1989

Microscopic examination of embrittled pin-retained base metal casting.

Differential thermal analysis, optical microscopy, scanning electron microscopy, and energy dispersive radiographic analysis were used to examine and analyze two failed platinum-palladium-gold pin-retained nickel-chromium castings. The microstructure and chemical composition of the pin-casting interface were different from those of the pin and the cast alloy. Porosity was also observed at the interface. The observations with these samples suggest that the pin melted during casting. The embrittlement and failure of plantium-palladium-gold pins cast with nickel-chromium alloy have been attributed to this melting phenomenon. Further study is necessary to determine whether these observations are valid for a large number of samples.

Chemical Phenomena↗
PubMed1989

An in vitro study of the effect endodontic access preparation has upon the retention of porcelain fused to metal crowns of maxillary central incisors.

Endodontic therapy often requires cutting access preparations through existing restorations. The purpose of this study was to determine whether endodontic access preparation affected the retention of porcelain fused to metal crowns of maxillary central incisors. Retentive forces required to displace porcelain fused to metal crowns of maxillary central incisors were measured using an Instron constant displacement rate testing machine. Using a paired t test analysis, a significant difference was found between crown retention before and after endodontic access preparation.

Crowns↗
PubMed1988

Mechanism of improved corrosion resistance of Zn-containing dental amalgams.

The aim of this study was to determine the mechanism of improved corrosion resistance of Zn-containing dental amalgams. Two Zn-containing conventional amalgams, their Zn-free counterparts, and three experimental amalgams (SnHg, ZnHg, and SnZnHg) were evaluated by the potentiodynamic polarization technique in 1% NaCl solution. The main difference between the two types of amalgams was found in their respective breakdown potentials at which passivity was destroyed. The breakdown potential of Zn-containing amalgams was about 200 mV more positive than that of the Zn-free amalgams. The improved stability of the Zn-containing amalgams has been attributed to the formation of a previously reported Zn stannate passive film which, according to the polarization data, is more resistant to the aggressive chloride ion than tin hydroxide that forms on Zn-free amalgams. The formation of Zn stannate was not found to affect the oxygen reduction reaction, the major cathodic reaction involved in the corrosion of dental amalgams.

Chemical Phenomena↗
PubMed1987

The microstructural basis of creep of gamma 1 in dental amalgam.

A previous study has shown that the addition of small amounts of tin to gamma 1 leads to the formation of a SnHg intergranular precipitate. The purpose of this study was to determine the effect of this precipitate on the creep of gamma 1. Two groups of samples were investigated. Group I included a series of gamma 1 samples with 0, 1, 2, and 3 wt% Sn fabricated by a sintering technique at 55 degrees C and 125 mN/m2 for 72 h. Group II contained the same samples as in Group I that were further annealed at 60 degrees C for 2000 h. Annealing was carried out to allow gamma 1 grain growth and equilibrium distribution of Sn at grain boundaries. Creep tests were conducted according to the American Dental Association Specification 1. The microstructures of all samples were characterized by scanning electron microscopy. The creep of gamma 1 was found to be controlled by the intergranular precipitate when the Sn concentration was above 1%. For a specific concentration of Sn, i.e. the intergranular precipitate, creep was dependent on the distribution of Sn and not on the gamma 1 grain size. The more nearly continuous precipitates found in annealed samples favoured higher creep.

Chemical Phenomena↗
PubMed1986

Heat treatment, microstructure and corrosion of a low-gold casting alloy.

The aim of this study was to understand the relationship between heat treatment, microstructure and corrosion of a low-gold casting alloy. Potentiostatic and potentiodynamic polarization techniques were used to evaluate the chloride corrosion resistance of the alloy in the following conditions: (A) as cast, (B) bench cooled, (C) homogenized, (D) homogenized and aged for 2 h, and (E) homogenized and aged for 4 h. The microstructure of each sample, before and after corrosion, was examined by optical and/or scanning electron microscopy. In general, heterogeneous structures containing dendritic segregations and precipitates were found to be more prone to corrosion than the single phase solid solution structure produced by homogenization. On the basis of the potentiodynamic polarization data, the samples were ranked as follows: C greater than D greater than B greater than A greater than E, C being the most corrosion resistant structure.

Corrosion↗
PubMed1986

Solid solubility of tin in silver-mercury gamma 1 in dental amalgams.

The solid solubility of Sn in gamma 1 has been studied by examining a series of gamma 1 samples containing 0.25-2.0 wt% Sn by scanning electron microscopy and energy-dispersive X-ray analysis. It has been observed that the addition of Sn to gamma 1 leads to the formation of mainly an intergranular Sn-Hg compound. On the basis of this observation it has been concluded that the solubility of Sn in gamma 1 is virtually nil.

Chemical Phenomena↗
PubMed1985

Study of ascorbate status in murine and human leukaemias.

Since mice can synthesize ascorbic acid but man cannot, the ascorbate status in murine and human leukaemia was compared. The decline in plasma ascorbate concentration in both cases indicates that vitamin C deficiency occurs in malignancy. Analysis of tissue ascorbate values in mice also indicated that an enhanced rate of utilization of this vitamin occurs during malignancy, as does an increased rate of excretion, and both events may be responsible for vitamin C deficiency. The hepatic ascorbate values suggest an endeavour by the animals to compensate for the loss through increased synthesis and storage of the vitamin, at least in the early stages of the disease.

Animals↗
PubMed1985

Role of gallium in alloy-porcelain bonding.

With scanning electron micrographs and semiquantitative x-ray analysis, the role of gallium in alloy-porcelain bonding was studied. The insight gained from this study should be of interest to both dentists and laboratory technicians making ceramometal restorations.

Chemical Phenomena↗
PubMed1984

The in vivo corrosion of Dispersalloy.

The in vivo corrosion of Dispersalloy amalgam restorations was characterized by scanning electron microscopy and energy dispersive X-ray analysis. It was observed that the corrosion of CuSn resulted in pitting on the restoration surface. Corrosion of Sn-rich grain boundaries led to intergranular separation of gamma 1 grains. The tin released through corrosion reacted with Ca, P, Cl and O forming various insoluble complexes on the external surface, at the tooth-amalgam interface, and within the interior of the restoration. The demineralization of the amalgam-adjacent margin has been attributed to corrosion.

Chemical Phenomena↗
PubMed1983

The chloride corrosion behaviour of four orthodontic wires.

Potentiodynamic cyclic polarization of four orthodontic alloys (wires), namely Permachrome, Elgiloy, a beta-titanium alloy and Nitinol in a 1% NaCl solution within -500 mV and +300 mV (SCE) indicated the first three alloys to be passive whereas breakdown of passivity was observed on Nitinol. The SEM examination of the pre- and post-polarized alloy surfaces provided evidence which was consistent with the electrochemical measurements, in that the first three alloys exhibited no appreciable corrosion damage whereas pitting corrosion was observed on Nitinol. The results obtained from X-ray analysis of the pitted surface indicated that this pitting could be due to selective dissolution of nickel from Nitinol.

Chemical Phenomena↗