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Results for “Elastic Modulus”

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PubMed1993

Mechanical implications of collagen fibre orientation in cortical bone of the equine radius.

Mechanical test specimens were prepared from the cranial and caudal cortices of radii from eight horses. These were subjected to destructive tests in either tension or compression. The ultimate stress, elastic modulus and energy absorbed to failure were calculated in either mode of loading. Analysis was performed on the specimens following mechanical testing to determine their density, mineral content, mineral density distribution and histological type. A novel technique was applied to sections from each specimen to quantify the predominant collagen fibre orientation of the bone near the plane of fracture. The collagen map for each bone studied was in agreement with the previously observed pattern of longitudinal orientation in the cranial cortex and more oblique to transverse collagen in the caudal cortex. Bone from the cranial cortex had a significantly higher ultimate tensile stress (UTS) than that from the caudal cortex (160 MPa vs 104 MPa; P < 0.001) though this trend was reversed in compression, the caudal cortex becoming relatively stronger (185 MPa vs 217 MPa; P < 0.01). Bone from the cranial cortex was significantly stiffer than that from the caudal cortex both in tension (22 GPa vs 15 GPa; P < 0.001) and compression (19 GPa vs 15 GPa; P < 0.01). Of all the histo-compositional variables studied, collagen fibre orientation was most closely correlated with mechanical properties, accounting for 71% of variation in ultimate tensile stress and 58% of variation in the elastic modulus. Mineral density and porosity were the only other variables to show any significant correlation with either UTS or elastic modulus. The variations in mechanical properties around the equine radius, which occur in close association with the different collagen fibre orientations, provide maximal safety factors in terms of ultimate stress, yet contribute to greater bending of the bone as it is loaded during locomotion, and thus lower safety factors through the higher strains this engenders.

Animals↗

PubMed2002

Dental resin composites containing silica-fused whiskers--effects of whisker-to-silica ratio on fracture toughness and indentation properties.

Dental resin composites need to be strengthened in order to improve their performance in large stress-bearing applications such as crowns and multiple-unit restorations. Recently, silica-fused ceramic whiskers were used to reinforce dental composites, and the whisker-to-silica ratio was found to be a key microstructural parameter that determined the composite strength. The aim of this study was to further investigate the effects of whisker-to-silica ratio on the fracture toughness, elastic modulus, hardness and brittleness of the composite. Silica particles and silicon carbide whiskers were mixed at whisker:silica mass ratios of 0:1, 1:5. 1:2, 1:1, 2:1, 5:1, and 1:0. Each mixture was thermally fused, silanized and combined with a dental resin at a filler mass percentage of 60%. Fracture toughness was measured with a single-edge notched beam method. Elastic modulus and hardness were measured with a nano-indentation system. Whisker:silica ratio had significant effects on composite properties. The composite toughness (mean+/-SD; n = 9) at whisker:silica = 2:1 was (2.47+/-0.28) MPa m(1/2), significantly higher than (1.02+/-0.23) at whisker:silica = 0:1, (1.13+/-0.19) of a prosthetic composite control, and (0.95+/-0.11) of an inlay/onlay composite control (Tukey's at family confidence coefficient = 0.95). Elastic modulus increased monotonically and hardness plateaued with increasing the whisker:silica ratio. Increasing the whisker:silica ratio also decreased the composite brittleness, which became about 1/3 of that of the inlay:onlay control. Electron microscopy revealed relatively flat fracture surfaces for the controls, but much rougher ones for the whisker composites, with fracture steps and whisker pullout contributing to toughness. The whiskers appeared to be well-bonded with the matrix, probably due to the fused silica producing rough whisker surfaces. Reinforcement with silica-fused whiskers resulted in novel dental composites that possessed fracture toughness two times higher than, and brittleness less than half of current dental composites.

Carbon Compounds, Inorganic↗

PubMed2005

Resuscitation-induced intestinal edema decreases the stiffness and residual stress of the intestine.

We have shown that acute edema impairs intestinal transit and we wanted to know whether this could be from changes in the physical characteristics of the intestine. Our hypothesis was that acute edema will change the physical characteristics of the intestine, which were measured by standardized engineering measures of elastic modulus, to determine stiffness and opening angle, and to determine residual stress. Rats were randomized to sham, mild edema (80 mL/ kg of normal saline resuscitation), and severe edema groups (80 mL/kg of normal saline resuscitation with intestinal venous hypertension). Segments of distal ileum were hung to a fixed point in a tissue bath and to a tensiometer and were stretched in increments of 1 mm, recording the new length and the corresponding force from the tensiometer to determine elastic modulus. Next, two transverse cuts were made yielding a 1- to 2-mm-thick ring-shaped segment of tissue and were then cut radially to open the ring. The opening angle was measured. Acute intestinal edema led to a decrease in transit, elastic modulus, and opening angle of the intestine in the absence of ischemic injury. Acute intestinal edema leads to a significant loss in stiffness and residual stress and is a plausible explanation for how acute edema impairs intestinal transit.

Animals↗

PubMed1993

[Mechanical adaptation of muscular arteries to acute increase of blood pressure in man. Contribution of the measurement of arterial wall thickness].

Although it was usually admitted that hypertension is associated with a reduction in conduit artery compliance and distensibility, recent experiments using newly developed high precision echo-tracking devices allowing simultaneous, non-invasive measurements of blood pressure and arterial diameter have suggested that compliance and distensibility, when calculated at equal transmural pressure, could indeed be maintained, or even increased, in hypertensive subjects. Such a maintained arterial compliance could be explained best by a decrease in wall stress despite the increase in arterial blood pressure, through either an increase in wall thickness or a decrease in arterial wall elastic modulus. Thus, the goal of the present study was to assess the role of the changes in these determinants of vessel compliance in the arterial response to an acute increase in arterial pressure in 7 healthy volunteers. Arterial pressure (AP, Finapress) as well as right radial artery internal diameter and wall thickness (Echo-Tracking, NIUS 2, Asulab) were measured continuously before and after a 2 min cold pressor test (CPT). The following parameters were then calculated at fixed (105 mmHg) pressure from the pressure/diameter curve: compliance (mm2/mmHg) and distensibility (mmHg-1) mid-wall stress (dynes/cm2) and incremental elastic modulus (dynes/cm2). During CPT, mean AP increased (from 83 +/- 4 to 106 +/- 8 mmHg; p < 0.01), compliance distensibility; and mean internal diameter (mm) decreased, mean wall thickness (mm) increased, whereas wall stress and elastic modulus remained unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Physiological↗

PubMed2003

Nanoindentation properties of compression-moulded ultra-high molecular weight polyethylene.

This paper investigates the elastic modulus and hardness of untreated and treated compression-moulded ultra-high molecular weight polyethylene (UHMWPE) tibial inserts of a total knee replacement (TKR) prosthesis. Investigations were carried out at a nanoscale using a Nanoindenter at penetration depths of 100,250 and 500 nm. The nanomechanical properties of surface and subsurface layers of the compression-moulded tibial inserts were studied using the untreated UHMWPE. The nanomechanical properties of intermediate and core layers of the compression-moulded tibial insert were studied using the cryoultrasectioned and etched UHMWPE treated samples. The cryoultrasectioning temperature (-150 degrees C) of the samples was below the glass transition temperature, Tg (-122 +/- 2 degrees C ), of UHMWPE. The measurement of the mechanical response of crystalline regions within the nanostructure of UHMWPE was accomplished by removing the amorphous regions using a time-varying permanganic-etching technique. The percentage crystallinity of UHMWPE was measured using differential scanning calorimetry (DSC) and the Tg of UHMWPE was determined by dynamic mechanical analysis (DMA). Atomic force microscopy (AFM) was used to assess the effect of surface preparation on the samples average surface roughness, Ra. In this study, it was demonstrated that the untreated UHMWPE samples had a significantly lower (p < 0.0001) elastic modulus and hardness relative to treated UHMWPE cryoultrasectioned and etched samples at all penetration depths. No significant difference (p > 0.05) in elastic modulus and hardness between the cryoultrasectioned and etched samples was observed. These results suggest that the surface nanomechanical response of an UHMWPE insert in a total joint replacement (TJR) prosthesis is significantly lower compared with the bulk of the material. Additionally, it was concluded that the nanomechanical response of material with higher percentage crystallinity (67 per cent) was predominantly determined by the crystalline regions within the semi-crystalline UHMWPE nanostructure.

Biocompatible Materials↗

PubMed2006

Drought tolerance of selected Eragrostis species correlates with leaf tensile properties.

BACKGROUND AND AIMS: Previous studies on grass leaf tensile properties (behaviour during mechanical stress) have focused on agricultural applications such as resistance to trampling and palatability; no investigations have directly addressed mechanical properties during water stress, and hence these are the subject of this study. METHODS: Critical (lethal) relative water contents were determined for three species of grass in the genus Eragrostis varying in their tolerance to drought. Measurements were taken for leaf tensile strength, elastic modulus, toughness and failure load under different conditions of hydration, and light microscopy and histochemical analyses were undertaken. KEY RESULTS: Leaf tensile strength of fully hydrated leaves for the drought-intolerant E. capensis, the moderately drought-tolerant E. tef and the drought-tolerant E. curvula correlated well with drought tolerance (critical relative water content). Eragrostis curvula had higher tensile strength values than E. tef, which in turn had higher values than E. capensis. Measurements on the drought-tolerant grass E. curvula when fully hydrated and when dried to below its turgor loss point showed that tensile strength, toughness and the elastic modulus all increased under conditions of turgor loss, while the failure load remained unchanged. Additional tests of 100 mm segments along the lamina of E. curvula showed that tensile strength, toughness and the elastic modulus all decreased with distance from the base of the lamina, while again the failure load was unaffected. This decrease in mechanical parameters correlated with a reduction in the size of the vascular bundles and the amount of lignification, as viewed in lamina cross-sections. CONCLUSIONS: The results confirm that leaf mechanical properties are affected by both water status and position along the lamina, and suggest a positive correlation between leaf internal architecture, tensile strength, cell wall chemistry and tolerance to dehydration for grasses.

Eragrostis↗

PubMed2004

Enhanced anticoagulant activity of enoxaparin in patients with ESRD as measured by thrombin generation time.

BACKGROUND: Patients with renal dysfunction who undergo systemic anticoagulation with enoxaparin are at increased risk for bleeding. Although there is decreased renal clearance of enoxaparin in this population, the clinical utility of monitoring antifactor Xa activity is controversial because it is weakly correlated to bleeding. The goal of this study was to investigate the role of other novel anticoagulation markers, such as thrombin generation time, platelet contractile force, and clot elastic modulus, while controlling for antifactor Xa activity in patients with and without renal dysfunction. METHODS: Thirty anticoagulant- and antiplatelet-naive subjects completed this trial (10 controls, 10 patients with chronic kidney disease, and 10 patients with end-stage renal disease [ESRD]). Blood samples were obtained and spiked ex vivo with increasing concentrations of enoxaparin antifactor Xa activity (0.25, 0.5, 1.0, and 3.0 IU/mL). Thrombin generation time, platelet contractile force, and clot elastic modulus were measured in each group at each antifactor Xa activity concentration. RESULTS: Subjects with ESRD had an approximately 50% greater anticoagulant effect, determined by thrombin generation time prolongation, than controls at antifactor Xa activity concentrations of 0.5 to 3.0 IU/mL. This may explain why subjects with ESRD with seemingly therapeutic antifactor Xa levels still experience adverse bleeding. There were no intergroup differences in platelet function, determined by platelet contractile force and clot elastic modulus. CONCLUSION: Antifactor Xa poorly predicts the degree of anticoagulation in patients with ESRD administered low-molecular-weight heparin (LMWH). Thrombin generation time may be a clinically useful anticoagulation monitoring tool to monitor LMWH therapy, especially in patients with renal dysfunction. Additional randomized prospective studies are needed to corroborate these findings.

Adult↗

PubMed1995

A model for the evaluation of mandibular bone response to implant materials.

An in vivo animal (goat) mandibular bone model has been evaluated for the cortical and trabecular bone response to biomaterials with different elastic modulus and/or different surfaces. The effect of the elastic modulus on the bone formation was also studied. The difficulties encountered in orientation of the implants probably could have been surmounted if pre-operative radiographs had been available. It was also established that there is a large variance in the amount of trabecular and cortical bone in the mandible of the goat 'bone model'.

Aluminum Oxide↗

PubMed1999

Three-dimensional microstructural analysis of human trabecular bone in relation to its mechanical properties.

The purpose of this preliminary study is to explore the relationship between elastic modulus, bone mineral density (BMD), and trabecular microstructure in three dimensions. Twenty cubes of trabecular bone were processed from two lumbar vertebrae obtained from one individual. The BMD of each cube was measured by dual-energy X-ray absorptiometry and peripheral quantitative computed tomography. Each cube was serially scanned by microcomputed tomography to produce three-dimensional data sets. By analyzing these data sets, three-dimensional trabecular microstructural indices of connectivity density and fractal dimension were calculated as well as histomorphometric parameters. The cubes were tested mechanically in a nondestructive manner for measurement of their elastic modulus. This preliminary study showed that: (1) bone mass index is correlated with mechanical properties, with coefficients of correlation ranging from 0.552 to 0.601; and (2) when controlling for BMD, no association could be detected between measures of structural complexity (connectivity density and fractal dimension) and elastic modulus in the craniocaudal direction of human vertebral bodies.

Biomechanical Phenomena↗

PubMed1996

Magnetic resonance imaging of trabecular bone structure in the distal radius: relationship with X-ray tomographic microscopy and biomechanics.

The contribution of trabecular bone structure to bone strength is of considerable interest in the study of osteoporosis and other disorders characterized by changes in the skeletal system. Magnetic resonance (MR) imaging of trabecular bone has emerged as a promising technique for assessing trabecular bone structure. In this in vitro study we compare the measures of trabecular structure obtained using MR imaging and higher-resolution X-ray tomographic microscopy (XTM) imaging of cubes from human distal radii. The XTM image resolution is similar to that obtained from histomorphometric sections (18 microns isotropic), while the MR images are obtained at a resolution comparable to that achievable in vivo (156 x 156 x 300 microns). Standard histomorphometric measures, such as trabecular bone area fraction (synonymous with BV/TV), trabecular width, trabecular spacing and trabecular number, texture-related measures and three-dimensional connectivity (first Betti number/volume) of the trabecular network have been derived from these images. The variation in these parameters as a function of resolution, and the relationship between the structural parameters, bone mineral density and the elastic modulus are also examined. In MR images, because the resolution is comparable to the trabecular dimensions, partial volume effects occur, which complicate the segmentation of the image into bone and marrow phases. Using a standardized thresholding criterion for all images we find that there is an overestimation of trabecular bone area fraction (approximately 3 times), trabecular width (approximately 3 times), fractal dimension (approximately 1.4 times) and first Betti number/ volume (approximately 10 times), and an underestimation of trabecular spacing (approximately 1.6 times) in the MR images compared with the 18-microns XTM images. However, even for a factor of 9 difference in spatial resolution, the differences in the morphological trabecular structure measures ranged from a factor of 1.4 to 3.0. We have found that trabecular width, area fraction, number, fractal dimension and Betti number/volume measured from the XTM and MR images increases, while trabecular spacing decreases, as the bone mineral density and elastic modulus increase. A preliminary bivariate analysis showed that in addition to bone mineral density alone, the Betti number, trabecular number and spacing contributed to the prediction of the elastic modulus. This preliminary study indicates that measures of trabecular bone structure using MR imaging may play a role in the study of osteoporosis.

Absorptiometry, Photon↗

PubMed2006

Mechanical analysis of chorion softening in prehatching stages of zebrafish embryos.

During early development, the chorion envelope of the zebrafish embryo undergoes a thinning process called "chorion softening," which has so far only been characterized chemically. In this study, a micromechanical force sensing system was used to characterize and quantitate mechanical modifications of the zebrafish embryo chorion during early development. Quantitative relationships between applied forces and chorion structural deformations were established at various embryonic stages. The measured penetration force into the chorion at the blastula stage was 1.3-fold greater than those at the prehatching stage. Furthermore, chorion elastic modulus values were determined by using a biomembrane elastic model. The elastic modulus of the chorion at the blastula stage was 1.66-fold greater than that at the prehatching stage, thus indicating that the chorion envelope become mechanically "softened" at the prehatching stage. The experimental results quantitatively describe "chorion softening," which is most likely due to proteolytic activities at the prehatching stage. Gradual chorion softening during embryonic development was also artificially achieved by treating blastula chorion with pronase, a proteolytic enzyme. The forces required to penetrate the pronase-treated chorion were similar to those at the prehatching stage. This similarity suggests that "chorion softening" may be induced by the release of protease from the embryos, and the chemical nature of the process involves proteolytic fragmentation of the ZP2 protein.

Animals↗

PubMed2004

Coronary flow velocity reserve and indices describing aortic distensibility in patients after coronary angiography.

BACKGROUND: The purpose of this study was to assess the elastic properties of the descending aorta and the coronary flow velocity reserve (CFR) in patients after coronary angiography. METHODS AND PATIENTS: We recruited 112 subjects with stable angina pectoris without a previous myocardial infarction: 17 consecutive patients with anatomically normal coronary arteries, 24 patients with non-significant coronary artery disease (CAD), 31 patients with significant left anterior descending coronary artery (LAD) disease and 40 patients with multivessel disease (MVD). Transoesophageal echocardiography (TEE) is useful for evaluation of the elastic properties of the descending aorta. The physical behaviour of vessels in response to an intraluminal force is described by the elastic modulus (E(p)) and Young's circumferential static elastic modulus (E(s)). Coronary flow velocities can be measured in the LAD under baseline conditions and during dipyridamole stress. The CFR was calculated as the ratio of the average peak diastolic flow velocity during hyperaemia to that at rest. RESULTS: The indices of aortic distensibility, CFR and mean CFR, were different in patients with LAD disease and in those with normal coronary angiograms. There were no further changes in these parameters in cases with MVD. In patients with non-significant CAD, the CFR, mean CFR and stiffness moduli lie between those for negative cases and those for patients with LAD disease/MVD. CONCLUSIONS: When there was significant stenosis of the LAD, the CFR was significantly decreased, while indices of aortic distensibility were increased as compared with the negative controls. Interestingly, not only the CFR, but also E(p) and E(s) displayed no further changes in cases with MVD as compared with LAD disease.

Adult↗

PubMed1989

The effects of single doses of X rays on the mechanical properties of pig skin in vivo.

Changes in the mechanical properties of pig skin have been studied in vivo, using a dermal extensometer, after irradiation with a single dose of 18 Gy of X rays. There was no significant change in the stiffness of irradiated skin, when compared with unirradiated skin, until 9 weeks after irradiation when the irradiated skin was significantly stiffer. This effect was also found at 12 and 15 weeks after irradiation. When the increase in skin thickness, as a consequence of oedema, was taken into account a significant increase in the unrelaxed elastic modulus of irradiated skin was only seen at 12 and 15 weeks after irradiation. There were no significant changes in force relaxation, after extension of the skin, over this time period. After the resolution of oedema, which was associated with a significant 20% reduction in the thickness of irradiated skin relative to unirradiated skin, the mechanical properties of irradiated skin were not markedly different from those of unirradiated skin. However, between 30 and 39 weeks after irradiation there was a further wave of dermal thinning, resulting in a total reduction in the thickness of irradiated skin relative to unirradiated skin of 26%. This was associated with a rapid rise in the skin stiffness and unrelaxed elastic modulus by approximately 65 and approximately 140%, respectively. It was only at these late times after irradiation that the force relaxation of the skin was modified significantly. At 9 and 12 weeks after irradiation the reduction in skin stiffness and the unrelaxed elastic modulus were dose related. Based on the percentage of fields showing a significant reduction in these biomechanical parameters, ED50 values of between 12 and 14.5 Gy were established. This would appear to be a sensitive method for assessing radiation-induced dermal changes since few gross changes are observed in this dose range.

Animals↗

PubMed1998

Does microdamage accumulation affect the mechanical properties of bone?

It has never been demonstrated that microcrack accumulation in bone leads to impaired mechanical properties. We hypothesized that microdamage accumulation is positively and linearly correlated with a reduction in bone's elastic modulus. We also tested the hypothesis that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices. Canine femurs (n = 26) were tested in four-point cyclic bending under load control until they had lost between 5 and 43% of their stiffness. Ten femurs were used as nonloaded controls. The loaded portion of the bone was stained en bloc with basic fuchsin to detect the presence of microdamage. The number of stained microcracks, their lengths and the area of damaged bone were measured under the microscope. Crack numerical density, surface density, mean crack length, and the percentage of damaged area were calculated. Significant microdamage accumulation was not detected until the bone had lost 15% of its elastic modulus. The relationship between crack density and stiffness loss was approximately quadratic, but the relationship between damaged area and stiffness loss was linear. There were significantly more microcracks in tensile cortices, but on average cracks were significantly longer in compressive cortices. We conclude that microcrack accumulation impairs the mechanical properties of bone by reducing its elastic modulus. We also conclude that damage accumulates more rapidly in tensile cortices, but crack growth is greater in compressive cortices.

Animals↗

PubMed2003

[Evaluation of bone structure and quality of ovariectomized rats by microcrack].

OBJECTIVE: To compare microcrack with bone mineral desity (BMD), bone histomorphometry and biomechanics parameters, and to investigate the potential of microcrack in the evaluation of bone biomechanical quality. METHODS: Eight 10-month-old Sprague-Dawley rats were served as baseline controls, and 90 10-month-old rats were randomly divided into A, B, and C groups. Each group comprised ovariectomized (OVX), 17 beta-estradiol treated [EST, 10 micro/(kg x d)] and sham-operated (SHAM) subgroups. Rats from groups A,B and C were killed at the 3rd, 15th and 21st week post-operatively. Total body and lumbar vertebral BMD were measured before being killed, and BMD of isolated lumbar vertebrae and tibiae were measured after killing. Bone histomorphometry of the proximal end of isolated right tibia was performed,and compression test was carried out on the isolated 5th lumbar vertebra (L5). After fatigue damage, the isolated 4th lumbar vertebra was stained by en bloc basic fuchsin staining, and microcrack density (Cr. Dn) and microcrack surface density (Cr. SDn) were de- termined on the bone tissue sections. Bone parameters in each subgroup of rats were observed at different time. RESULTS: (1) At the 15th and 21st week post-operatively, multi-part BMD, Cr. Dn and Cr. SDn were higher than those at the 3rd week. (2) At the 15th week, trabecular separation (Tb. Sp) increased, trabecular number (Tb. N) decreased, and the maximum loading level and elastic modulus of vertebra reached the peak. (3) At the 3rd week, Tb. Sp, Cr. Dn and Cr. SDn in the OVX subgroup were greater than those in the EST subgroup, while the percentage of trabecular area (TbTr) in the OVX subgroup was lower than that of the EST and SHAM subgroups. No changes of BMDs and biomechanic parameters were observed among the three subgroups. (4) At the 15th week, multi-part BMD and maximum loading level in the OVX and EST subgroups were lower than those in the SHAM subgroup, while elastic modulus, bone histomorphometry parameters, Cr. Dn and Cr. SDn had no change among the three subgroups. (5) At the 21st week, multi-part BMDs, Tb. N and TbTr in the OVX subgroup were smaller than those in the EST and SHAM subgroups. Tb. Sp, bone formation rate, mineral apposition rate, percent labeled perimeter,Cr. Dn and Cr. SDn in the OVX subgroups were greater than those in the EST and SHAM subgroups. Maximum loading level and elastic modulus of vertebra in EST and OVX subgroups were lower than those in the SHAM subgroup. There were no significant differences in all of these parameters Microcrack can be regarded as an alterative between the EST and the SHAM subgroup. Conclusion parameter in the evaluation of bone biomechanical quality.

Animals↗

PubMed1999

Assessment of arterial compliance by carotid midwall strain-stress relation in normotensive adults.

Examining left ventricular midwall as opposed to endocardial mechanics enhances understanding of left ventricular function in individuals with abnormal cardiac geometry. Accordingly, we used carotid ultrasound and applanation tonometry of arterial pressure to derive carotid midwall strain and its relation to carotid peak-systolic and end-diastolic stresses in 82 apparently normal, employed subjects (56 men, 26 women; median age, 47 years; 70% white; 21% overweight) with no evidence of coronary or valvular heart disease. Regression equations relating carotid luminal and midwall strain to the increment in carotid stress during systole (Deltacarotid stress) were used to predict strain for the observed Deltastress. Observed/predicted carotid luminal or midwall strain was calculated as a measure of carotid luminal or midwall strain for imposed stress, termed stress-corrected strain. Midwall carotid strain was similar in women and men but was negatively related to older age (r=-0.35, P=0.001) and higher body mass index (r=-0.31, P=0.005) and brachial and carotid blood pressure (r=-0.30 to -0.45, all P<0.01). The pulsatile change in arterial load, measured by Deltacarotid stress, was positively related to midwall strain (r=0. 44, P<0.001) more closely than was carotid luminal strain. Regression analyses revealed that carotid midwall strain was positively related to Deltastress, with additional negative relations to age and carotid diastolic diameter (all P<0.001). Stress-corrected carotid midwall strain was strongly and negatively correlated with midwall elastic modulus and Young's modulus (both r=-0.77, P<0.001), followed by elastic modulus (r=-0.74, P<0.001), midwall Young's modulus (r=-0.73, P<0.001), midwall stiffness index (r=-0.70, P<0.001), and stiffness index (r=-0.66, P<0.001). Thus, in normal adults, carotid midwall strain is unrelated to gender, is positively related to pulsatile carotid load as measured by Deltacarotid stress, and is negatively related to age, overweight, and standard measures of arterial stiffness.

Age Factors↗

PubMed1998

Changes in the fracture toughness of bone may not be reflected in its mineral density, porosity, and tensile properties.

Age-related changes in the skeleton often lead to an increase in the susceptibility of bone to fracture. Such changes most likely occur in the constituents of bone, namely, the mineral and organic phases, and in their spatial arrangement manifested as orientation and microstructure. In the past, however, bone loss or decline in bone mineral density has been considered to be the major contributing factor for the increased risk of bone fractures, and elastic modulus and ultimate strength have been commonly used to assess bone quality and strength. However, whether these properties provide sufficient information regarding the likelihood of bone to fracture remains debatable. Using a novel fracture toughness test, which measures the energy or stress intensity required to propagate a crack within a material, the objective of this study was to investigate if the mineral density and mechanical properties of bone can accurately predict bone fragility as measured by fracture toughness. Changes in fracture toughness (K(IC)), bone mineral density (BMD), elastic modulus (E), yield and ultimate strength (sigma y and sigma s), porosity (P0), and microhardness (Hv) of bone were examined as a function of age in a baboon model. With increasing age, the fracture toughness of bone decreased, and its microhardness increased. However, no significant changes were found in BMD, E, P0, sigma y, and sigma s as a function of age. In addition, simple regression analyses revealed no significant correlation between bone fracture toughness and the other parameters, except for microhardness of bone. The results of this study indicate that changes in bone fracture toughness may not be necessarily reflected in its mineral density, porosity, elastic modulus, yield strength, and ultimate strength.

Aging↗

PubMed1996

Effects of nitric oxide and sodium nitroprusside on the intrinsic elastic properties of pressurized rat coronary artery.

The study was designed to assess the influence of either nitric oxide (NO) or sodium nitroprusside and the absence of endothelium on the intrinsic elastic properties of coronary arteries from WKY rats. For this purpose, segments of the right interventricular coronary were mounted in an arteriograph where wall thickness and internal diameter were continuously monitored while intraluminal pressure was controlled in the absence of flow. To study the passive properties, pressure-diameter relationships were determined by measuring the corresponding internal diameter for each stepwise increase in intraluminal pressure. Thus, wall stress, strain and incremental elastic modulus (Einc) were assessed in the following experimental conditions: control, incubation with nitro-L-arginine methyl ester (L-NAME, 100 microM) or L-NAME + L-arginine (L-arg, 100 microM), incubation with sodium nitroprusside (SNP, 100 microM), endothelium removal (CHAPS). The Einc-stress relationship was not significantly different in the different experimental conditions, but values of Einc plotted as function of strain were significantly decreased after L-NAME incubation and partly reversed after L-arg addition. The same effect was observed after endothelium destruction but to a lesser extent. After SNP incubation, values of Einc were significantly decreased for small values of strain and increased for high values of this parameter. These results show that NO synthase inhibition induced, for a given strain, a decrease of elastic modulus in coronary arteries. It can be speculated that functional antagonism exerted by NO against spontaneous contractile tone was reduced. Thus, the smooth muscle cells were in a greater state of activation and probably more strongly involved in the intrinsic elastic properties of this preparation. However, an unexplained effect of NO on wall stiffness cannot be excluded. Conversely, SNP increased the initial diameter and induced an initial decrease in stiffness followed by a subsequent increase. After endothelium destruction, stiffness was significantly decreased compared to control conditions. It can be concluded that NO modulates the intrinsic elastic properties of the coronary arteries through smooth muscle cell relaxation. Furthermore, results with SNP support the hypothesis that the lower the state of activation of the smooth muscle cells, the higher the elastic modulus of the arterial wall in this coronary artery preparation.

Animals↗