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L J Nunez

Publications and source records attributed to L J Nunez.

7 recordsLinked to original sources
PubMed1990

Observations on the elastic behavior of a synthetic orthodontic elastomer.

The study focused on changes in elasticity that accompany water storage of a synthetic orthodontic elastomer. We plotted loading and unloading curves to permit the direct measurement of instantaneous elastic recovery (IER) and permanent set (PS) and the calculation of delayed recovery (DR). We obtained baseline data by testing dry as received material. Comparable tests were performed on material that had been stored in water at 37 degrees C for one, seven, 14, 42, and 70 days. Gas chromatography-mass spectrometry was used for analysis of organic substances leached from the elastomer by water. A two-way ANOVA revealed that extension distance and water storage duration affected load requirement, IER, PS, and DR. The presence of leachable organic substances in 14-, 42-, and 70-day storage water was evidence of time-dependent matrix decomposition. Findings from tests of elastic performance and analysis of specimen storage water indicate that exposure of the elastomer to water leads first to weakening of noncovalent forces and subsequently to degradation.

Analysis of Variance↗
PubMed1976

Carcinogenic activity of a chlorinated polyether polyurethan.

On the basis of the results of an earlier study, a particular polyurethan sample (Y-238) was selected for further evaluation of its carcinogenic potential. This sample was subjected to physical and chemical tests for elucidation of its chemical structure, molecular weight, and molecular weight distribution. Additional biological tests were conducted on male NBR rats by implanting various quantities of the sample i.p., while others received an intrabronchus implant. Tumors, assessed histologically as malignant, were observed following both routes of implantation. The most common neoplasms of the pulmonary site was epidermoid carcinoma, while fibrosarcoma was the most common neoplasm in the peritoneal cavity. Data from the i.p. implantation suggested a dose-related incidence of cancers.

Animals↗
PubMed1975

Correlation of kinetic parameters and thermal behavior of segmented polyurethane elastomers with biological responses.

Kinetic studies of thermal degradation of 16 segmented polyether polyurethane samples, containing various amounts of 3,4-diaminotoluene and dibutyltin diacetate as additives, were carried out by thermogravimetry. From a single dynamic thermogravimetric experiment, the temperatures of initiation of degradation, 10, 25, and 50% (w/w) of degradation, as well as the activation energies for degradation, were determined. The activation energies were computed from the thermogravimetric curves using Broido's graphical approximation method, which applies to first-order decomposition kinetics. The results of stepwise multiple linear regression analysis indicate that the biological responses to elastomar samples, such as tissue culture, hemolysis, intramuscular implant, intradermal irritation, systemic toxicity, and histopathological rating, and the cumulative biological response index are highly correlated with thermal stability and kinetic measurements of the materials.

Animals↗
PubMed1975

Carcinogenesis from polyurethans.

Seventeen polyurethans, containing various substituent groups, and a polyethylene were implanted i.p. in groups of male black Bethesda rats and were evaluated for carcinogenesis over a 2-year period. Thirteen of the polyurethans and the polyethylene were similarly studied in females. Tumor development in these animals was expressed in terms of the incidence in the at-risk population, and the tumorigenic latent period was approximated for each sample. Twenty months after implantation, the relative tumorigenicity (area under the corrected cumulative tumor mortality versus time curve) in the males ranged from 0 (for the unimplanted controls) to 6.18 (for Y-238); for female rats this range was 0.29 (for unimplanted controls) to 5.72 (for Y-238). Estimated latent periods in the males ranged from 5 months (for Y-304) to 16 months (for Y-303), and 22.5 months for the unimplanted controls; for the females, the range was from 9 months (for Y-290) to 13.5 months (for Y-217), and 14 months for the unimplanted controls. The relative tumorigenicity of each sample was also compared to its in vitro activation energy for thermal decomposition. These data are discussed in terms of solid-state versus chemical carcinogenesis.

Animals↗
PubMedDate not supplied

Influence of amalgam, alloy, and Hg on the in vitro growth of Streptococcus mutans: III. Effect of specimen age and composition.

We will now summarize the conclusions from parts I, II and III of this study. A test procedure has been developed that provides a simple, quick, and nondestructive means of monitoring the in vitro growth of S mutans in the presence of amalgams and alloys. The spectrophotometric readings are related in a simple way to growth expressed as dry weight of bacteria and metabolic products. Results are expressed as growth relative to controls which represent bacteria growing under identical conditions but not in contact with metals. The %RA60 value that represents growth after 60 hours relative to controls is used as a measure of growth in the presence of alloys or amalgams. Spherical, fine cut, and dispersion alloys were studied as well as amalgams prepared from these alloys. The dispersion alloy inhibits growth less than the spherical alloy which in turn inhibits growth less than the fine cut alloy. The results for amalgams prepared from the alloys are reversed. At an aging time of two hours, dispersion alloy amalgams inhibit growth more than spherical alloy amalgams and fine cut alloy amalgams. Aging time of amalgams greatly influences the growth inhibition. Immediately after trituration growth is inhibited, but this inhibition is lost with aging. Different types of amalgams seem to lose growth inhibition at different rates. Hg composition in the range of 48 to 52% seems to have little effect on growth inhibition. It remains for clinical studies, which are concerned with the incidence of secondary caries associated with amalgams, to demonstrate that the age and nature of the amalgam are significant.

Dental Alloys↗