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Biomedical subjects

A R Greenberg

Publications and source records attributed to A R Greenberg.

12 recordsLinked to original sources
PubMed1999

The effects of age and dietary restriction without nutritional supplementation on whole bone structural properties in C57BL/6J mice.

While caloric restriction is a proven means to extend longevity, its effects on bone are not well understood. This study examined the effects of dietary restriction without vitamin or mineral supplementation on bone in female 60- and 120-day-old C57BL/6J mice. Baseline controls were sacrificed at 60 or 120 days, while diet-restricted animals ate approximately 72.9-78.6% of the ad libitum fed animals for thirty days. 60-day-old ad libitum animals experienced normal growth with average increases of 6.4% in bone length, 23.5% in bone mass, 9.4% in %mineralization, 36.4% in maximum strength, 59.2% in stiffness, 22.3% in cortical thickness, 12.9% in %cortical area, and 11.3% in microhardness. Growth in 120-day-old ad libitum animals followed a trend but with more modest increases. Diet-restricted mice matured very little from baseline levels in 60-day-old animals. There were no significant changes from baseline levels in the parameters indicated above, except for a 8.3% decrease in %cortical area attributable to increased resorption. 120-day-old diet-restricted animals also evidenced little deviation from baseline levels except for significant decreases in %mineralization (2.1%) and %cortical area (6.7%). The effects of diet restriction on bone properties decreased with age. Bone from 60-day-old diet-restricted mice showed diminished mechanical and compositional properties, resulting from little growth and excess resorption. Bone from 120-day-old diet-restricted mice showed little growth and some resorption. Increased resorption, localized on the endosteal surfaces, likely minimized the negative impact of structural degradation of the long bones. Resorption may have also provided minerals to compensate for nutritional deficiencies.

Aging↗
PubMed1997

Response-to-stimulus interval does not affect implicit motor sequence learning, but does affect performance.

Nissen and Bullemer (1987) reported that implicit motor sequence learning was disrupted by the addition of a secondary task. They suggested that this effect was due to attentional load that the secondary task adds. Recently it has been suggested that the attentional load is not critical, but rather that the secondary task affects timing, either by lengthening or by making inconsistent the response-to-stimulus interval (RSI)--that is, the delay between when a subject makes a response and when the next stimulus appears. In six experiments we manipulated the RSI and found no support for these two hypotheses. An inconsistent RSI did not adversely affect implicit motor sequence learning. A long RSI did not affect learning, although under some conditions subjects did not express learning if the RSI was long. These results are interpreted as reflecting the effects of attention.

Female↗
PubMed1995

Material and compositional properties of selectively demineralized cortical bone.

Timed immersion in buffered ethylenediamine-tetraacetic acid (EDTA) was used to selectively alter the mineral content at each level in the cortical bone structural hierarchy. The effects on the mechanical behavior were investigated using a combination of experimental techniques which provide collectively a wide range of resolution (5 microns to 3 mm). Optical microscopy and histological analysis demonstrated a heterogeneous structure consisting of a mineralized tissue core surrounded by a layer of demineralized tissue (collagen) whose thickness varied depending on the immersion time. The mechanical behaviors of treated samples with (intact) and without (core) the surrounding demineralized layer were evaluated using three-point flexure. Overall, the intact specimens became significantly less brittle with increased immersion time in buffered-EDTA. For the core specimens, there was a systematic decrease in the elastic flexural properties (E, sigma e, epsilon e). The site-specific properties of the specimens were determined using microhardness testing, scanning acoustic microscopy, and wavelength dispersive analysis. The mineralization and site-specific properties of the mineralized cores were not significantly affected by buffered-EDTA immersion; however, histomorphometric analysis showed a decrease in the mineralized volume fraction via widening of the pre-existing vascular channels. The experimental hierarchy was effective in discerning site-specific property changes and the localized heterogeneities resulting from the buffered-EDTA treatment. Based on the results of this study, buffered-EDTA treatment can be used to facilitate the determination of material and physical properties of intact and demineralized tissues within a single cortical bone sample.

Animals↗
PubMed1993

Effects of rehydration state on the flexural properties of whole mouse long bones.

The effects of bone water content during dehydration and rehydration on the flexural properties of whole mouse femora were evaluated using three-point bending. The elastic and plastic flexural properties of the bones were determined on a dry mass normalized basis over dehydration times ranging from 0.25 to 48.0 hr; and (following complete dehydration) rehydration times ranging from 0.08 to 12.0 hr. Bones stored in physiological saline for times < 1 hr served as the control group. As expected, dehydration produced increased stiffness and strength along with decreased ductility. Upon rehydration, a statistically significant linear dependence of mechanical properties on recovered free water was obtained for all parameters except the maximum load. Elastic mechanical properties comparable to the controls were regained at differing rates and levels of recovered water content; however, after 3 hr of rehydration there were no statistically significant differences with respect to the control values. The results of this study indicate that the original flexural properties of whole mouse femora are preserved by air dehydration and can be recovered using appropriate saline rehydration intervals.

Animals↗
PubMed1992

The physical and mechanical effects of suspension-induced osteopenia on mouse long bones.

The present investigation addresses the extent of tail-suspension effects on the long bones of mice. The effects are explored in both sexes, in both forelimb and hindlimb bones, and in both diaphyseal and metaphyseal/epiphyseal bones. Two weeks of suspension provided unloading of the femora and tibiae and an altered loading of the humeri. Whole-bone effects included lower mass (approximately 10%) and length (approximately 4%) in the bones of suspended mice compared to controls. The geometric and material properties of the femora were considered along the entire length of the diaphysis and in the metaphysis/epiphysis portions as a unit. Geometric effects included lower cross-sectional cortical area (16%), cortical thickness (25%) and moment of inertia (21%) in the femora of suspended mice; these differences were observed in both distal and proximal portions of the femur diaphysis. The relative amount of bone comprising the middle 8 mm of the diaphysis was greater (3%) in the control mice than in the suspended mice. Significant mass differences between the group in the metaphysis/epiphysis were not observed. Material effects included lower %ash (approximately 2%) in the femora and tibiae as well as in the humeri of suspended mice compared to controls. With respect to the measured physical and material properties, suspension produced similar bone responses in male and female mice. The effects of suspension are manifested largely through geometric rather than through material changes.

Animals↗
PubMed1984

Influence of ionic environment of the stress relaxation behavior of an invertebrate connective tissue.

The rheological properties of an invertebrate connective tissue were measured in three different ionic environments. Short-term stress relaxation tests were conducted on sections of holothurian (Echinodermata) body wall immersed in isotonic monovalent and divalent salt solutions and deionized water. Using a reduced modulus format, the viscoelastic behavior over the experimental time scale was described by a two term Maxwell equation with empirically determined spring constants and relaxation times. In addition, equilibrium relaxation moduli (Ge) were estimated from the empirical relationship of Chasset and Thirion (1965, in Physics of Non Crystalline Solids, ed. Prins, North Holland). The experiments indicated that both relaxation times and equilibrium moduli decreased in the presence of monovalent and divalent inorganic ions whereby the effect of the Na+ was greater than that of the Ca++. The present findings are compared with those reported for vertebrate connective tissue.

Animals↗
PubMed1982

Comparison of the elastic properties of nickel-titanium and beta titanium arch wires.

The elastic strength, stiffness, and range of two titanium alloys are compared as a function of wire configuration. From the four beta titanium and eight nickel-titanium sizes available, all arch wire permutations were evaluated both in bending and in torsion. Results show that the stiffnesses of the two alloy compositions overlap substantially, except for those wires with the lowest and the highest stiffnesses--that is, the 0.016 and 0.018 inch nickel-titanium and the 0.017 by 0.025 and 0.019 by 0.025 inch beta titanium arch wires, respectively. To conclude, both "variable-cross-section" and "variable-modulus" orthodontics are illustrated within the context of an equivalent wire stiffness chart which includes conventional stainless steel arch wires.

Dental Alloys↗
PubMed1981

Dynamic mechanical properties of amalgams.

A recently developed dynamic mechanical analysis technique was utilized to determine the setting rates of dental amalgams. Via a compound parallel beam apparatus the dynamic mechanical properties of five amalgam materials were continuously measured at 37 degrees C within ca. 7 min after the start of trituration. Over a 15 hr period results showed that the modulus of elasticity (Ea) increased from 1.2 X 10(10) Pa to 6.5 X 10(10) Pa--the latter generally approaching those values reported via ultrasonic techniques. Moduli results via compression tests on microspecimens (3 X 2 X 1 mm) paralleled previous reliable literature data, nominally yielding values for Ea of 2-3 X 10(10) Pa. The extent of Ea scatter from both inter- and intrapreparer experiments appeared similar, although superior technique generally could be recognized by a systematic shift of the mean results to higher Ea's. In addition to modulus studies both compressive strengths (sigma max) and their corresponding strains (epsilon) were measured on microspecimens at a strain rate (epsilon) of 0.031 min-1, with a significant increase being noted in sigma max for the one material tested at over an order of magnitude increase in epsilon.

Dental Amalgam↗
PubMed1981

Effects of composition and cross section on the elastic properties of orthodontic wires.

General formulas for wire properties have been derived. These enable comparisons among conventional orthodontic alloys and latest archwire materials. For six common archwire sizes, physical property considerations indicate that Ni-Ti makes the most active "leveling" archwire, the B-Ti is a superior intermediate archwire where flexibility is required, and that S.S. and Co-Cr are the wires of choice for finishing and other applications where stability of form is required. These conclusions are based on the current understanding of equivalent force systems and the mechanical properties of each general classification. Whether these wires or some others are used in future patient care will depend on an uncertain and gloomy economic forecast. The present viewpoint is that the greatest needs are the formulation of empirical equations that better describe the action of triple-stranded archwire in bending and more accurate physical property data for all wires in both the tensile and the torsional states. Such additional information could be very useful to the practitioner in choosing the optimal archwire under changing conditions of cost and availability.

Chromium Alloys↗
PubMed1976

Polymer-ceramic composite for tooth-root implant.

A new polymer-ceramic composite suitable for tooth-root implants has been developed in this study. This material exhibited the desirable combination of good mechanical properties, controlled porosity, and ease of processing. A thermal processing technique was utilized to polymerize acrylic acid (AA) in the presence of either 0.3 or 0.05 mu alumina particles. Porosity and pore size distribution were influenced by the alumina particle size and the processing technique. For a 50 vol % AA solution, the composite had an average compressive strength of 18,000 psi and 38% porosity when 0.3 mu filler particles were used. In comparison, the 0.05 mu alumina-filled composite had an average compressive strength of 28,000 psi with a 15% porosity. Data on the physical and structural characteristics of the composite are presented in this study. Based on these results, the composite material shows good potential for use in tooth-root implants as well as other orthopedic implant applications.

Absorption↗