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Biomedical subjects

N K Sarkar

Publications and source records attributed to N K Sarkar.

At least 19 recordsLinked to original sources
PubMed1997

Influence of dentinal adhesives and a prefabricated post on fracture resistance of silver amalgam cores.

PURPOSE: Failures of silver amalgam cores for endodontically treated teeth have been attributed to the use of posts. The use of adhesive agents has been reported to improve the strength of silver amalgam restorations. The purpose of this study was to test the effects of two adhesive agents on the fracture resistance of silver amalgam cores with and without a post. MATERIAL AND METHODS: Sixty mandibular molars were restored with the use of silver amalgam, with or without one of the adhesive agents and/or a tapered prefabricated post. A universal testing machine, with a constant load rate of 0.5 mm/minute, was used to determine the force in kilograms at failure. RESULTS: The use of either adhesive agent with silver amalgam significantly increased the force at failure, and neither adhesive agent was significantly better than the other. Silver amalgam with a post and without an adhesive agent had the least resistance to fracture. CONCLUSIONS: The results indicated that endodontically treated mandibular molars restored with silver amalgam are more resistant to fracture when an adhesive agent is used.

Analysis of Variance↗
PubMed1997

Melatonin action on thyroid activity in the soft-shelled turtle, Lissemys punctata punctata.

Adult female turtles (Lissemys punctata punctata) were treated with pineal indoleamine melatonin (100 micrograms/100 g) or the antithyroid agent, methylthiouracil (100 micrograms/100 g) or melatonin together with methylthiouracil (100 micrograms of each drug/100 g) for 12 days. Melatonin alone inhibited thyroid activity as evidenced by reduction in the gland weight, follicular epithelial cell height, thyroid peroxidase, and plasma thyroxine levels. Methylthiouracil caused hyperplasia of the gland, although it inhibited thyroid activity and reduced thyroid peroxidase and plasma thyroxine levels. Melatonin together with methylthiouracil produced changes similar to those obtained with melatonin alone. The results indicate that melatonin probably exerts inhibitory effects influences on both thyrotropin release from the pituitary and the activity of the thyroid itself in turtles.

Animals↗
PubMed1997

Haematological changes in buprenorphine-treated mice.

Studies were carried out in adult male Swiss mice to determine whether different haematological parameters like total counts of red and white blood cells, differential counts of white blood cells, haematocrit value and blood haemoglobin level were affected by the synthetic opioid analgesic, buprenorphine, which has currently been known to be abused in several countries by the heroin addicts as a cheap substitute for heroin. The mice were daily given an intraperitoneal injection of buprenorphine (300 micrograms/kg) for 60 consecutive days. A severe leucopenia accompanied by decreases in lymphocyte and monocyte counts, an increase in neutrophil count, a decrease in haematocrit value, a rossette-like adherence of red blood cells to a few neutrophils and platelet satellitism were observed at the advanced stages of treatment. The abnormalities, however, reverted to normal within 45 days following withdrawal of the drug. The necessity of periodic monitoring of the blood picture in human abusers of the drug is suggested.

Agranulocytosis↗
PubMed1996

Photothermal effects on ovarian growth and function in the soft-shelled turtle Lissemys punctata punctata.

Investigations were carried out to find out the role of higher and lower ambient temperatures together with short and long photoperiods on ovarian growth and secretion of steroids (estradiol-17 beta and progesterone) in the adult female soft-shelled turtle Lissemys punctata punctata during the preparatory, recrudescent, and quiescent phases of its seasonal reproductive cycle. In each phase, females were treated with 3 degrees C higher and 3 degrees C lower than the mean highest and lowest ambient temperatures in association with short and long photoperiods for short-term (4 week) and long-term (8 weeks or more) durations. Short-term high-temperature treatment significantly stimulated ovarian growth and secretion of estradiol-17 beta during the preparatory phase. Eight-week treatment failed to exercise further stimulation over 4-week treatment, whereas 10-week treatment caused regressive changes of ovary (atresia of follicles). Both short- and long-term high-temperature treatments caused degeneration of ovarian follicles during the recrudescent phase. Low-temperature treatment slowed down ovarian growth and secretion in both the preparatory and recrudescent phases. In the quiescent phase, low- as well as high-temperature treatments exercised little influence over the refractory ovarian growth and secretion. Photoperiod appeared to have no effect on female reproduction in any phase. Thus, high temperature is suggested to have a triggering role on ovarian growth and secretion of estrogen at the early preparatory phase, but once the ovarian function sets in, high temperature seems to have a regressive rather than stimulatory effect on ovarian growth and function.

Aging↗
PubMed1995

Polyamines exert regulatory control on mycobacterial transcription: a study using RNA polymerase from Mycobacterium phlei.

We have used purified RNA polymerase from Mycobacterium phlei to study the role of polyamines in mycobacterial transcription. Both initiation and elongation phases of the process were affected biphasically by polyamines. Interaction of polyamines with DNA template plays an important role in transcription modulation. Our studies emphasize that polyamines can exert a regulatory control on mycobacterial transcription.

DNA-Directed RNA Polymerases↗
PubMed1995

Effect of a clamp on toothbrush bristle deterioration.

The purpose of this study was to evaluate, in vitro, the effectiveness of a device to prevent toothbrush deterioration. Ten toothbrushes from each of four brands, Oral-B 40, Py-co-pay Softex 12.0 row, Butler #330, and Oral-B 20, were divided into control and experimental groups. Toothbrushes were fixed into a machine that simulated toothbrushing by producing a reciprocating motion 41.5 mm in length at a rate of 380 strokes/min. Bristles slid over a ceramic disc that had a hardness similar to dental enamel. Pressure (450 g) was applied at a right angle to the toothbrush while it moved back and forth in a slurry of toothpaste. The pressure was calibrated with load cells. Toothbrushes were run for 15 minutes followed by a drying cycle for a total of four hours, or approximately equivalent to brushing twice daily for six months. During the drying cycle the bristles of the toothbrushes in the experimental group were compressed 3/4 their length from the brush head with a clamp (Gregory Patent #4884311). Bristle deterioration was evaluated based on Wear Index (WI), a quantitative means of ranking brushes in various stages of deterioration. Data on WI and WR showed that toothbrushes in the experimental group had significantly (< or = 0.005) less deterioration. Improvement in wear resistance was dependent on size and configuration of the bristles. Compressing the bristles between brushings retains the non-matted shape of the bristles and reduces their deterioration.

Analysis of Variance↗
PubMed1994

In vitro torque-deformation characteristics of orthodontic polycarbonate brackets.

The purpose of this study was to investigate the torque-deformation characteristics of the following four types of polycarbonate brackets: (1) pure polycarbonate, PPC (anterior Miura, RMO, Denver, Colo.), (2) ceramic reinforced polycarbonate, CRPC (Silkon bracket, American, Sheboygan, Wis.), (3) metal slot reinforced polycarbonate, MRPC (Plastic bracket, Tella Tech, Miami, Fla.), and (4) metal slot and ceramic reinforced polycarbonate, MCRPC (Spirit, Ormco, Glendora, Calif.). A stainless steel bracket, (Mini Diamond, Ormco, Glendora, Calif.), was used as a control. Ten brackets of each type were tested. Each bracket was bonded to a porcelain tooth and engaged in a torquemeter. The tooth-bracket assembly was made stationary by embedding it in die stone. Torsion was applied to the bracket at 4 degrees per minute and the resultant torque (grams.centimeters) and deformation (degree) were measured. For optimum labiolingual tooth movement for a maxillary incisor at 175 grams . centimeters, the amount of angular deflection necessary for the different polycarbonate brackets was the following: (a) 15 degrees for MRPC, (b) 17 degrees for MRPC, (c) 24 degrees for CRPC, and (d) > 30 degrees for PPC. The amount of deformation at this deflection was the least for MRCP followed by MCRPC, CRCP, and PPC. When compared with the stainless steel bracket, all polycarbonate brackets showed significantly (p < 0.0001) higher deformation and lower torque. Within the polycarbonate group, there was a significant difference (p < 0.0001) between each bracket for both measurements. The MRPC produced the highest torque and lowest deformation values followed by the MCRPC, CRCP, and PPC. It appears that only the metal slot reinforced brackets are clinically capable of torquing teeth sufficiently.

Analysis of Variance↗
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↗