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PubMed2004

Cholesterol depletion increases membrane stiffness of aortic endothelial cells.

This study has investigated the effect of cellular cholesterol on membrane deformability of bovine aortic endothelial cells. Cellular cholesterol content was depleted by exposing the cells to methyl-beta-cyclodextrin or enriched by exposing the cells to methyl-beta-cyclodextrin saturated with cholesterol. Control cells were treated with methyl-beta-cyclodextrin-cholesterol at a molar ratio that had no effect on the level of cellular cholesterol. Mechanical properties of the cells with different cholesterol contents were compared by measuring the degree of membrane deformation in response to a step in negative pressure applied to the membrane by a micropipette. The experiments were performed on substrate-attached cells that maintained normal morphology. The data were analyzed using a standard linear elastic half-space model to calculate Young elastic modulus. Our observations show that, in contrast to the known effect of cholesterol on membrane stiffness of lipid bilayers, cholesterol depletion of bovine aortic endothelial cells resulted in a significant decrease in membrane deformability and a corresponding increase in the value of the elastic coefficient of the membrane, indicating that cholesterol-depleted cells are stiffer than control cells. Repleting the cells with cholesterol reversed the effect. An increase in cellular cholesterol to a level higher than that of normal cells, however, had no effect on the elastic properties of bovine aortic endothelial cells. We also show that although cholesterol depletion had no apparent effect on the intensity of F-actin-specific fluorescence, disrupting F-actin with latrunculin A abrogated the stiffening effect. We suggest that cholesterol depletion increases the stiffness of the membrane by altering the properties of the submembrane F-actin and/or its attachment to the membrane.

Animals↗
PubMed2005

[3D finite element analysis in consideration of the slide on residual ridge and the rigidity of mandibular complete denture].

PURPOSE: The motions and stress distributions of mandibular complete dentures during use were compared and investigated in terms of denture rigidity using 3D finite element method (FEM) stress analysis. METHODS: Four models for analysis of mandibular complete dentures with different elastic modulus (1. A resin base denture (C), 2. A resin base denture with reinforcement wire (R), 3. A metal base denture (M), 4. A metal plate denture consisting of a double structure (T)) and measured using 3D FEM. Modes were separated into two elastic bodies: the dentures and residual ridge. Analyses were executed under the same conditions total loads of 20 kgf and contact. RESULTS: 1. The total equivalent-potential strain of dentures was high beneath the load point for C and R (2.38 approximately 2.86 x 10(-3)epsilon at buccal-shelf of working side and 0.55 approximately 0.68 x 10(-3)epsilon at posterior residual ridge). The entire dentures barely distorted for M and T (0.98 approximately 1.17 x 10(-3)epsilon at buccal-shelf of working side and 0.21 approximately 0.25 x 10(-3)epsilon at posterior residual ridge). 2. In the main stress distribution of mucosa beneath denture bases, high loads were located beneath the loading point of dentures, at lingual distal areas of the working side in C and R. In M and T, increasing the rigidity of dentures reduced the stress beneath the loading point of dentures. Stress was distributed widely and evenly. CONCLUSIONS: When the rigidity of dentures is high, the strain of dentures decreases, the stress of the residual mucous membrane will evenly become distributed, and the distance of the balancing side from the denture border to the residual mucous membrane at the time of unilateral balanced position will decrease.

Dental Stress Analysis↗
PubMed1994

A parametric analysis of the contact stress in ultra-high molecular weight polyethylene acetabular cups.

It is well known that the wear factor for ultra-high molecular weight polyethylene (UHMWPE) sliding on metallic or ceramic counterfaces is largely independent of contact stress for modest loading conditions and sliding distances. However, it is now recognized that under more severe stress levels and with sliding distances comparable to those encountered in current replacement synovial joints, subsurface fatigue contributes to the volume of wear debris. Since the fatigue process is influenced by surface stress levels it is becoming increasingly important to limit the contact stress through design in order to minimize the volume of UHMWPE wear debris in implants. The contact pressure in UHMWPE acetabular cups has been predicted using both the simple elasticity analysis and the finite element method. It has been shown that the radial clearance between the femoral head and the socket is the dominant parameter in determining the contact stress. Thus, the radial clearance should be controlled so the contact half width is close to the femoral head radius (a total included angle of contact of 120 degrees) to minimize the contact pressure. There is little benefit to be gained by increasing the contact half width greater than the femoral head radius. This is consistent with the geometrical constraint of the anatomical position and the direction of loading. It has been shown that the radius of the femoral head has the most significant effect on the maximum contact pressure for these closely conforming contacts where the contact half width is close to the femoral head radius. The effect of the elastic modulus and the thickness of UHMWPE is relatively small under these contact conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetabulum↗
PubMed2000

Structural and rheological properties of methacrylamide modified gelatin hydrogels.

Dynamic shear oscillation measurements at small strain were used to characterize the viscoelastic properties and related differences in the molecular structure of hydrogels based on gelatin methacrylamide. Gelatin was derivatized with methacrylamide side groups and was subsequently cross-linked by radical polymerization via photoinitiation. The light treatment of methacrylamide gelatin solutions resulted in the production of hydrogel films with high storage modulus (G'). Mechanical spectra and thermal scanning rheology of the obtained hydrogels are described. The temperature scan of the network below and above melting point of gelatin allowed us to identify the respective contributions of chemical and physical cross-linkage to the hydrogel elastic modulus. The results indicate that the rheological properties of the gelatin-based hydrogels can be controlled by the degree of substitution, polymer concentration, initiator concentration, and UV irradiation conditions.

Acrylamides↗
PubMed1994

Custom impression trays: Part I--Mechanical properties.

Dimensional stability of custom impression trays is an important factor in determining the degree of accuracy achieved in forming a master cast. Such trays must remain stable over time and must not exhibit permanent deformation when a completed impression is removed from the oral cavity. Measurement of the mechanical properties allows comparison between various tray materials and is useful in interpreting data on stresses incurred during removal of the completed impression. In Part I of this three-part series, the various mechanical properties of five tray resins: one autopolymerizing polymethyl methacrylate, one light-polymerizing, and three brands of thermoplastic resins were recorded and compared. The thermoplastic resins studied in this investigation exhibited lower measured values for the strength and elastic modulus properties than the light-polymerizing resin and the autopolymerizing polymethyl methacrylate resin studied.

Dental Impression Technique↗
PubMed2002

Cancellous bone mechanical properties from normals and patients with hip fractures differ on the structure level, not on the bone hard tissue level.

Osteoporosis is currently defined in terms of low bone mass. However, the source of fragility leading to fracture has not been adequately described. In particular, the contributions of bone tissue properties and architecture to the risk or incidence of fracture are poorly understood. In an earlier experimental study, it was found that the architectural anisotropy of cancellous bone from the femoral heads of fracture patients was significantly increased compared with age- and density-matched control material (Ciarelli et al., J Bone Miner Res 15:32-40; 2000). Using a combination of compression testing and micro-finite element analysis on a subset of cancellous bone specimens from that study, we calculated the hard tissue mechanical properties and the apparent (macroscopic) mechanical properties. The tissue modulus was 10.0 GPa (SD 2.2) for the control group and 10.8 GPa (SD 3.3) for the fracture group (not significant). There were no differences in either the apparent yield strains, percentages of highly strained tissue, or the relationship between apparent yield stress and apparent elastic modulus. Hence, a difference in the tissue yield properties is unlikely. At the apparent level, the fracture group had a significantly decreased transverse stiffness, resulting in increased mechanical anisotropy. These changes suggest that bone in the fracture group was "overadapted" to the primary load axis, at the cost of fragility in the transverse direction. We conclude that individuals with a history of osteoporotic fractures do not have weaker bone tissue. Architectural and mechanical anisotropy alone renders their bone weaker in the nonprimary loading direction.

Aged↗
PubMed2005

Frictional evaluations of dental typodont models using four self-ligating designs and a conventional design.

Abstract: After a previous study using typodonts and three standardized archwire (AW) sizes, the frictional evaluations of four self-ligating brackets were directed toward the optimal AW-bracket system. Four participating manufacturers suggested three AWs, which were a representation of the three stages of orthodontic treatment, to be coupled with their respective self-ligating design. Four replicated typodont models were mounted with a self-ligating design, and a fifth model was mounted with a conventional design that served as a control. The first experiment evaluated the manufacturer-suggested AWs against the respective self-ligating design. Because no third-stage AWs could engage their respective designs, a second experiment was implemented to gain more detailed analyses of the designs. This experiment included any successful manufacturer-suggested AWs from the first experiment against the four self-ligating designs and the control design. All self-ligating designs performed with the efficiency and reproducibility associated with expectations. Specifically, self-ligation outperformed the conventional brackets when coupled with up to 0.020- x 0.020-inch wires. The clearance of the various AW sizes and alloys changed with malocclusion. Furthermore, the parameter that best correlated with drawing forces was the bending stiffness of the AW, which was directly associated with the nominal dimension of each wire. The best AW-bracket system can be selected, when taking into account the stiffness (elastic modulus and size of the AW) along with the amount of malocclusion present, once the treatment plan is determined.

Dental Alloys↗
PubMed1995

Modelling the effects of enamel anisotropy with the finite element method.

The finite element method was used to model an in vitro tooth loading system. The dentine was modelled as a linear elastic and isotropic material, while the enamel was modelled as an anisotropic material to take account of the biological variation in prism angulation. The elastic modulus values of enamel and dentine used in this analysis were altered in order to replicate the movement of the in vitro system. It was found that a dentine modulus of 15 GPa and an enamel modulus of 80 GPa in the principal prism direction gave the best replication of cuspal movement.

Anisotropy↗
PubMed2003

Material studies of lipid vesicles in the L(alpha) and L(alpha)-gel coexistence regimes.

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L(alpha)) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L(alpha) and gel phases. We show that the elastic modulus, K, and critical tension, tau(c), of L(alpha) vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15 degrees C to 45 degrees C, we measure decreases in both K and tau(c) of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L(alpha) vesicles including critical tensions that are 20-40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.

1,2-Dipalmitoylphosphatidylcholine↗
PubMedDate not supplied

The effects of digitization on the elastographic signal-to-noise ratio.

In elastography, the tissue under investigation is compressed and the resulting strain is estimated from the gradient of the displacement (time-delay) estimates. The displacements are typically estimated by cross-correlating the radiofrequency (RF) ultrasound signals of the pre- and postcompressed tissue. One of the parameters used to quantify the resulting quality of the elastogram is the elastographic signal-to-noise ratio (SNR(e)). For a uniformly elastic target (a single elastic modulus), the dependence of the SNR(e) on the applied strain has a bandpass characteristic that has been termed the strain filter. Theoretical expressions for the upper bound on the strain filter were developed earlier. Yet, simulated as well as experimental strain filters derived from uniformly elastic phantoms deviate from these upper bounds. The failure to achieve the upper bounds could be partially attributed to the fact that, in both simulations and experiments, the RF signals used to compute the TDEs are sampled and quantized. Using simulated models of uniformly elastic phantoms, a study of the dependence of the strain filter on the quantization and sampling rates was performed. The results indicated that the strain filter improves with both the sampling rate and the quantization, as expected. A theoretical analysis was done to incorporate quantization as a derating factor to the strain filter.

Elasticity↗
PubMed2006

Tensile behavior of cortical bone: dependence of organic matrix material properties on bone mineral content.

A porous composite model is developed to analyze the tensile mechanical properties of cortical bone. The effects of microporosity (volksman's canals, osteocyte lacunae) on the mechanical properties of bone tissue are taken into account. A simple shear lag theory, wherein tensile loads are transferred between overlapped mineral platelets by shearing of the organic matrix, is used to model the reinforcement provided by mineral platelets. It is assumed that the organic matrix is elastic in tension and elastic-perfectly plastic in shear until it fails. When organic matrix shear stresses at the ends of mineral platelets reach their yield values, the stress-strain curve of bone tissue starts to deviate from linear behavior. This is referred as the microscopic yield point. At the point where the stress-strain behavior of bone shows a sharp curvature, the organic phase reaches its shear yield stress value over the entire platelet. This is referred as the macroscopic yield point. It is assumed that after macroscopic yield, mineral platelets cannot contribute to the load bearing capacity of bone and that the mechanical behavior of cortical bone tissue is determined by the organic phase only. Bone fails when the principal stress of the organic matrix is reached. By assuming that mechanical properties of the organic matrix are dependent on bone mineral content below the macroscopic yield point, the model is used to predict the entire tensile mechanical behavior of cortical bone for different mineral contents. It is found that decreased shear yield stresses and organic matrix elastic moduli are required to explain the mechanical behavior of bones with lowered mineral contents. Under these conditions, the predicted values (elastic modulus, 0.002 yield stress and strain, and ultimate stress and strain) are within 15% of experimental data.

Biomechanical Phenomena↗
PubMed1998

Mechanical properties of human trabecular bone lamellae quantified by nanoindentation.

Improved preventive and therapeutic strategies for skeletal diseases such as osteoporosis rely on a better understanding of the mechanical properties of trabecular bone and their influence on cell mediated adaptation processes. The mechanical properties of trabecular bone are determined by composition as well as structural (trabecular architecture), microstructural (trabecular packets) and nanostructural (lamellae) organization. Density is the major predictor of the mechanical properties of trabecular structures and has been extended to the concept of fabric to include architectural anisotropy and improve even further the power of prediction. Recent advances in QCT and MRI technologies allow for precise assessment of 3D trabecular architecture and the mechanical consequences of structural changes can be increasingly well quantified by the means of computational methods. While single trabeculae have been tested using various techniques with contrasting results, little is known about the intrinsic mechanical properties of trabecular bone lamellae on which these computational methods rely. For instance, water and mineral content have a significant effect on the elastic, viscous, yield and postyield properties of bone tissue. In addition, collagen fiber orientation affects the mechanics of single remodeling units. Variations in composition and organization determined by age, accumulated damage or disease may therefore reduce the mechanical integrity of trabecular bone and deserve more attention. The aim of this work was to utilize a nanoindentation technique to quantify elastic modulus and hardness of human trabecular bone lamellae.

Aged↗
PubMed2001

Composites made of rapidly resorbable ceramics and poly(lactide) show adequate mechanical properties for use as bone substitute materials.

Porous composite materials made of poly(L, DL-lactide) and a ceramic component, alpha-tricalcium phosphate (alpha-TCP) or one of the rapidly resorbable glass ceramics, GB9N or GB14N, respectively, were developed to be used as bone substitutes. The present article describes the mechanical properties and the in vitro degradation characteristic of the different composite materials. The yield strength, the elastic modulus, and the molecular weight were measured after in vitro degradation up to 78 weeks. The initial strengths of the alpha-TCP composite (12.5 +/- 0.7 MPa) was higher than that of the GB9N and GB14N composites (8.3 +/- 0.2 MPa and 10.9 +/- 0.2 MPa, respectively). The initial elastic moduli of the composites were between 450 and 650 MPa. The mechanical properties remained constant until a degradation period of 26 weeks. Then they decreased continuously until they were completely lost at week 52. The molecular weight (M(w)) decreased steadily from 91,000 D in the case of the alpha-TCP composite and 78,000 D and 85,000 D in the case of the GB9N or GB14N composites, respectively, to about 10,000 D at week 78. It was concluded that the composites show adequate mechanical properties in the range of cancellous bone and a suitable degradation characteristic to be used as bone substitute materials.

Animals↗
PubMedDate not supplied

Decreased arterial elasticity associated with cardiovascular disease risk factors in the young. Bogalusa Heart Study.

Noninvasive ultrasonic examinations were performed in 1984 on a biracial sample of 109 10- to 17-year-old adolescents to determine whether elastic properties of the carotid arteries are associated with cardiovascular disease risk factors in the young. The subjects examined were in either the upper (high risk) or lower (low risk) race-, sex-, and age-specific tertile for both serum total cholesterol (TC) and systolic blood pressure (SBP) during a 1981-82 community survey. The pressure-strain elastic modulus (Ep), a measure of stiffness, for the carotid arteries was calculated by dividing the pulse pressure by the fractional diameter increase in the carotid artery during the cardiac cycle, as measured by ultrasonic techniques. Repeat studies on 20 randomly selected subjects demonstrated high reproducibility of the elasticity measurements (intraclass correlation coefficient = 0.84). The mean Ep in the high risk group was 5.1 kPa higher than in the low risk group, after controlling for race, sex, and age (one-sided p value = 0.03). Furthermore, a positive parental history of myocardial infarction was related to increased Ep levels (p less than 0.05), independently of race, sex, age, TC, and SBP. The results indicate that ultrasonic techniques can detect functional differences in the carotid arteries of children and adolescents that are associated with the risk of cardiovascular disease as adults.

Adolescent↗
PubMed2003

Elasticity imaging of atheroma with transcutaneous ultrasound: preliminary study.

BACKGROUND: Knowledge of the physical properties of atherosclerotic plaque is essential when evaluating its vulnerability in a clinical setting. Such knowledge, however, is still difficult to obtain with the various approaches developed to date. METHODS AND RESULTS: This article describes a noninvasive method for evaluating the regional elasticity (the elastic modulus in the circumferential direction) of tissue surrounding atherosclerotic plaque in which a novel phased tracking method is applied to measure minute changes in thickness of each of the multiple layers of the arterial wall during one heartbeat. By comparing the pathological findings with the distribution of elasticity, average elasticity of lipid and that of a mixture of smooth muscle and collagen fiber can be determined. On the basis of these reference parameters, each point is statistically categorized as lipid, mixture, or other. Thus, the plaque is electronically stained using transcutaneous ultrasound. By applying the method to the common carotid arteries, the presence of thin collagen fiber was clarified along the arterial axis for normal subjects, whereas soft inclusion of lipid was found for every plaque in subjects with hyperlipidemia. CONCLUSIONS: This novel method offers potential as a diagnostic technique for detection of plaque vulnerability with high spatial resolution.

Adult↗
PubMed1987

Characterization of Hexsyn, a polyolefin rubber.

Hexsyn is the Goodyear Tire and Rubber Company tradename for a polyolefin rubber synthesized from 1-hexene with 3-5% methylhexadiene as the source of residual double bonds for vulcanization. Under license from Goodyear, this same polymer has been manufactured by Lord Corporation for the hinge portion of finger joint prostheses using the tradename Bion. This rubber is currently licensed to the University of Akron and to the Cleveland Clinic Foundation for use in biomedical applications, and is being used primarily for biocompatible and highly fatigue resistant rubber components in ventricular assist and artificial heart systems. Results are presented from the physical, mechanical, and biological characterization of Hexsyn. Procedures are described for the synthesis, compounding, and post-molding extraction for Hexsyn. The physical testing of Hexsyn reported includes determinations of its density at 23 and 37 degrees C, initial hardness and hardness after aging in oxygen, blood, pseudoextracellular fluid and polyethylene glycol 600, typical molecular weights determined by gel permeation chromatography/low angle laser light scattering and intrinsic viscosity, thermal analyses by differential scanning calorimetry of Hexsyn gum, and vulcanized Hexsyn after exposure to blood and blood/fatigue conditions. Also reported are results of differential thermal analyses, thermomechanical analyses of virgin and annealed samples, and thermogravimetric analyses conducted in helium and in air. Dynamic mechanical analyses of Hexsyn include Clash-Berg and Rheovibron tests. Swelling was conducted to determine lot-to-lot and sheet-to-sheet variation for quality control and also a number of solvents were used so that the polymer-solvent interaction parameters could be determined. The permeability of Hexsyn to water, water vapor, and a variety of gases is reported. The permeability by contact angle measurements, refractive index, residual solvent analyses, migration of blood components into Hexsyn, melt rheology by Monsanto Rheometer, resistance to acids, and typical mold shrinkage for Hexsyn are reported. Mechanical testing of Hexsyn includes tensile strength, elongation, and tensile stress (modulus) at 23 and 37 degrees C and after conditions including exposure to blood, pseudoextracellular fluid, polyethylene glycol, oxygen, 100% relative humidity, and fatigue testing. Stress/strain calibration curves, flexural rigidity after aging in blood, tension set, compression set, stress relaxation, and the effect of repeated cycling on the elastic modulus are presented along with the results of Pico abrasion, skid resistance tests on wet concrete.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2000

Measurement of the elasticity of the actin tail of Listeria monocytogenes.

We report biophysical experiments performed on the bacterium Listeria monocytogenes, a model system to study actin-based motility. Using optical tweezers and electrophoresis experiments, we find that the bacterium is firmly attached to its tail, and we demonstrate that the tail responds as an elastic gel when deformed. We have measured its elastic modulus at a value of 10(3)-10(4) Pa, which is 10 times higher than the rigidity of the eukaryotic cytoplasm. These results demonstrate that the bacterium and its tail form a very robust system, consistent with the steadyness of the motion observed in vivo. We propose an elastic model for the propulsion mechanism which takes into account the connection and thus the interaction between the actin filaments. It provides a generic description of the various aspects of actin-tail based movements.

Actins↗