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Biomedical subjects

K Asgar

Publications and source records attributed to K Asgar.

At least 37 records · Page 2Linked to original sources
PubMed1982

Mechanical properties of orthodontic wires in tension, bending, and torsion.

The mechanical properties of three sizes of stainless steel (SS), nickel-titanium (NT), and titanium-molybdenum (TM) orthodontic wires were studied in tension, bending, and torsion. The wires (0.016 inch, 0.017 by 0.025 inch, and 0.019 by 0.025 inch) were tested in the as-received condition. Tensile testing and stiffness testing machines along with a torsional instrument were used. Mean values and standard deviations of properties were computed. The data were analyzed statistically by analysis of variance using a factorial design. Means were ranked by a Tukey interval calculated at the 95 percent level of confidence. In tension, the stainless steel wires had the least maximum elastic strain or springback, whereas the titanium-molybdenum wires had the most. Higher values of springback indicate the capacity for an increased range of activation clinically. In bending and torsion, the stainless steel wires had the least stored energy at a fixed moment, whereas the nickel-titanium wires had the most. Spring rates in bending and torsion, however, were highest for stainless steel wires and lowest for nickel-titanium wires. A titanium-molybdenum teardrop closing loop delivered less than one half the force of a comparable stainless steel loop for similar activations.

Chemical Phenomena↗
PubMed1982

Tarnish and corrosion of noble metal alloys.

A factorially designed experiment was chosen to form alloys of a gold-palladium-silver-copper quaternary system. A total of 81 alloys for three levels of concentration of each of the four elements were made and tested for the degree of tarnish in in vivo and in vitro environments. The following conclusions may be drawn: 1. In both the in vitro and in vivo experiments, increasing the gold levels of concentration within an alloy decreased the degree of tarnish. 2. In both the in vitro and in vivo experiments, increasing the palladium levels of concentration within an alloy decreased the degree of tarnish with an influence greater than that produced by the gold content. 3. In the in vitro environment of 2% sodium sulfide solution, the degree of tarnish increased as the levels of concentration of silver to copper were increased. 4. In both the in vitro and in vivo experiments, the effect of the interaction of the four elements on the degree of tarnish and corrosion is directly related not only to the concentration of each element within the alloy, but also to their ratios.

Chemical Phenomena↗
PubMed1981

Inner surface roughness of complete cast crowns made by centrifugal casting machines.

Six variables that could affect the surface roughness of a casting were investigated. The variables were (1) type of alloy, (2) mold temperature, (3) metal casting temperature, (4) casting machine, (5) sandblasting, and (6) location of each section. It was determined that the training portion of a complete cast crown had rougher surfaces than the leading portion. Higher mold and casting temperatures produced rougher castings, and this effect was more pronounced in the case of the base metal alloy. Sandblasting reduced the roughness, but produced scratched surfaces. Sandblasting had a more pronounced affect on the surface roughness of the base metal alloy cast either at a higher mold temperature or metal casting temperature. The morphology and the roughness profile of the original cast surface differed considerably with the type of alloy used.

Crowns↗
PubMed1981

X-ray diffraction analysis of gamma 2 (Sn-Hg) phase in higher copper amalgams of varying mercury content.

Microstructures of high copper commercial amalgams containing varying amounts of mercury, ranging from 20% above to 25% below recommended values, were primarily investigated by x-ray diffraction. Mechanisms relating to the absence or presence of gamma 2 (Sn-Hg) phase in these amalgams were discussed in relation to the presence of copper and tin elements in their original alloys. The optimum mercury concentration in some amalgams was determined in order to keep them free of the gamma 2 (Sn-Hg) phase.

Chemical Phenomena↗
PubMed1979

The influence of heat treatments on several types of base-metal removable partial denture alloys.

Four removable partial denture alloys, Vitallium (Co-Cr alloy), Dentillium P.D. (Fe-Cr alloy), Durallium L.G. (Co-Cr-Ni alloy), and Ticonium 100 (Ni-Cr alloy), were evaluated in the as-cast condition and after heat treatment for 15 minutes at 1,300 degrees, 1,600 degrees, 1,900 degrees, and 2,200 degrees F followed by quenching in water. The following properties were determined and compared for each alloy at each heat treatment condition: the yield strengths at 0.01%, 0.1%, and 0.2% offsets, the ultimate tensile strength, the percent elongation, the modulus of elasticity, and the Knoop microhardness. The results were statistically analyzed. Photomicrographs were examined for each alloy and test condition. The following conclusions were made: 1. The "highest values" were exhibited by the as-cast alloy. 2. Heat treatment of the partial denture alloys tested resulted in reductions in strength, while the elongations varied. This study demonstrates that, in practice, one should avoid (a) prolonged "heat-soaking" while soldering and (b) grinding or polishing of the casting until the alloy is "red hot". 3. Durallium L.G. was the least affected by the various heat treatment conditions. 4. Conventional reporting of the yield strength at 0.2% offset, the ultimate tensile strength, and percent elongation are not adequate to completely describe and compare the mechanical behavior of alloys. The reporting of the yield strength at 0.01% offset, in addition to the other reported properties, will provide a more complete description of the behavior of the dental alloys.

Chromium↗
PubMed1978

Bond strength of three cements determined by centrifugal testing.

1. Each tooth preparation and crown should be used to test only one cement, since recementation will alter the bond strength associated with the second trial. 2. Recementation of a restoration should include a thorough cleaning of the preparation and the restoration. 3. Although zinc polycarboxylate cement possessed the highest bond strength, there were no statistically significant differences among the cements tested. 4. No correlation was found between bond strength and any individual property of compressive strength, tensile strength, or film thickness. 5. Values of ADA film thickness predicted the order of values of film thickness determined by a simulated clinical method.

Aluminum↗
PubMed1978

Physical properties of dental silver-tin amalgams with high and low copper contents.

Newly developed amalgams such as Tytin, Sybraloy, and Dispersalloy had low static creep, high compressive strength, and similar values for both diametral tensile strength and dimensional change in comparison with conventional amalgams such as Kerr Spheraloy, Caulk Spherical, Shofu Spherical, and Caulk 20th Century Micro Cut. The data for physical properties were evaluated statistically, and the amalgams were ranked according to the numerical position assigned to each property.

American Dental Association↗
PubMed1978

Microscopic observations of amalgams carved by different methods and subsequently polished.

Occlusal amalgam restorations placed in extracted deciduous molars were studied using an electron microscope. The fillings were carved only or both carved and burnished, and all were subsequently polished. Observations were made before and after polishing, and the results emphasized the effects of carving, burnishing, and polishing on the surface texture and marginal integrity of amalgam fillings.

Child↗
PubMed1977

Microstructure of dental amalgams containing high and low copper contents.

Microstructure of commercial dental alloys and their amalgams were studied primarily by x-ray diffraction, optical metallography, scanning electron microscopy, and x-ray energy dispersive spectroscopy. X-ray diffraction revealed more phases than normally reported in these materials. Presence of new phase was discussed and their formation mechanism understood. Some phases having interacting 2 theta degrees values with others were properly identified. Both new and conventional dental alloys and their amalgams, namely Tytin, Sybraloy, Dispersalloy, Kerr Spheraloy, Caulk Spherical, Shofu Spherical, and Caulk 20th Century Microcut were used to complete the investigation.

Chemical Phenomena↗
PubMed1976

Characterization of porosity of isostatically pressed and sintered nickel-base powdered metal.

Characterization of the pore structure of compacted and sintered parts made from a nickel-base powder was accomplished using the mercury porosimetry method. The theoretical density values for the sintered specimens varied from 56.3 to 96.7% which corresponds to a porosity of 43.7 to 3.3%. A maximum interconnecting median pore diameter of 21 mum resulted from a -80/+200 mesh powder compacted at 138 MN/m2 and sintered for 2 h at 1250 degrees C. Photomicrographs of the same sample showed that it had a maximum pore diameter of 200 mum. The interconnected pore volume decreased with decreasing particle size of the powder, increasing compaction pressure, and increasing sintering temperature. Mechanical properties of tensile strength, yield strength, elastic modulus and percentage elongation were correlated with the pore structure. Proper selection of particle size, compaction pressure, sintering times and sintering temperatures should permit parts with controlled porosity characteristics to be produced that possess adequate mechanical properties for application as implants.

Chemical Phenomena↗
PubMed1976

Evaluation of alternative alloys to type III gold for use in fixed prosthodontics.

A study was done to evaluate the fit of castings made from commercially available alloys promoted by their manufacturers as cheaper alternatives to type III gold for use in fixed prosthodontics. Two nickel-chromium alloys, two silver-palladium alloys, two gold-silver-palladium alloys, a 50% gold alloy, and a type III conventional gold alloy (used as a basis of comparison) were evaluated. After optimum conditions for waxing, investing, and casting were determined experimentally, four crowns (two full crowns and two three quarter crowns) were cast from each alloy. The castings were cleaned by pickling or sandblasting and placed on their respective dies. Only one casting was made from each die. Ten judges were asked to evaluate the marginal fit and completeness of each casting on its die. The judges rated each casting and the ratings were converted into rankings. Type III gold and Forticast tied and were ranked first; Paliney CB was ranked second; Alborium and Albacast tied and ranked third; Aurolite CB was ranked fourth; and Jelenko Experimental and Howmedica III tied and ranked fifth. It was found that the nickel-chromium alloys produced castings that had a tight fit and showed marginal discrepancies even under conditions designed to yield maximum expansion of the investment. These nonprecious metal castings were judged inferior in terms of fit when compared with castings made from noble metal-and precious metal-containing alloys. Existing investments and techniques were adequate for all castings except those made from the nonprecious alloys. It was concluded that low (approximately lambda 50%) gold alloys and palladium-silver-gold alloys that are commerically available can be used as substitutes for type III gold alloy without any loss inthe quality of casting fit.

Chromium Alloys↗
PubMed1975

Modified hygroscopic gold casting technique.

A modified hygrosopic gold casting technique has been described. It utilizes a dual-wax pattern with controlled amounts of water added to the setting investment to better influence both the location and amount of investment expansion. This technique has been used since 1970 with excellent clinical results.

Dental Casting Investment↗