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PubMed2002

Stiffness of the abdominal aorta in obese children.

Obesity is pathogenically related to clinical and subclinical disorders that contribute to the development of atherosclerotic plaques and their complications leading to onset of cardiovascular events. Arterial stiffness may be an indicator of early vascular changes signaling the development of vascular disease. The purpose of this study was to assess the stiffness of the abdominal aorta using transthoracic echocardiography in normotensive obese and hypertensive obese pediatric patients and a control group. The study group consisted of 25 healthy children (M/F: 13/12) as a control group (Group I), 25 normotensive obese children (M/F: 13/12) (Group II) and 25 hypertensive obese children (M/F: 14/11) (Group III). The mean ages were 12.1 +/- 1.8, 11.9 +/- 1.5 and 12.4 +/- 1.4 years, respectively. Aortic strain (S), pressure strain elastic modulus (Ep) and normalized Ep (Ep*) measurements were significantly different in the hypertensive obese group, and cholesterol levels and body mass index were higher in this group. These findings may be important in determining the relationship between obesity and cardiovascular risk factors at pediatric age.

Algorithms↗

PubMed1979

Separate determination of the pulsatile elastic and viscous forces developed in the arterial wall in vivo.

The viscoelastic behaviour of arteries in vivo is analyzed by separate representation of the purely elastic and the purely viscous properties, using natural pressure and diameter pulses of various dog arteries recorded under steady-state conditions. The circumferential wall stress (sigma) and the radius (r) of the mean wall layer are calculated as functions of time and the hysteresis of the sigma-r diagram is represented. The stress is regarded as the sum of an elastic stress (sigma el) which is a function of r, and a viscous stress (sigma vis) which is a function of dr/dt. Thus sigma el = sigma - sigma vis. Since the sigma el-r diagram must be free from hysteresis, the disappearance of the loop is the criterion that indicates that sigma el has been found. sigma vis is formulated as a second degree polynomial of dr/dt whose coefficients are determined using that criterion. The sigma el-r curve is always nonlinear and the elastic modulus increases with increasing radius. The sigma vis-dr/dt curve, too, is nonlinear. Its slope decreases with increasing dr/dt. The same applies to the wall viscosity (pseudoplastic behaviour). The nonlinear properties can be represented adequately by processing the experimental data in the time domain. Problems inherent in investigations based on the frequency domain, as reported in the literature, are pointed out.

Animals↗

PubMed1987

Experimental study in rat metatarsals to determine whether callus vascularity is an indicator of the mechanical strength of healing fractures.

Changes in the mechanical properties and percentage area of blood vessels of healing fracture callus were followed using rat metatarsals. By 24 weeks after fracture the mean ultimate tensile stress and elastic modulus were still less than half that of the contralateral unfractured bone, whereas the mean torsional modulus had almost reached that of the unfractured bone. The percentage area of blood vessels declined from five days post-fracture and showed no changes which coincided with the increases in mechanical strength or moduli. We conclude that studies of vascularity would not justify a prediction of the strength of a healing fracture.

Animals↗

PubMed1990

Muscle stiffness measured under conditions simulating natural sound production.

Isolated whole frog gastrocnemius muscles were electrically stimulated to peak twitch tension while held isometrically in a bath at 4 degrees C. A quartz hydrophone detected vibrations of the muscle by measuring the pressure fluctuations caused by muscle movement. A small steel collar was slipped over the belly of the muscle. Transient forces including plucks and steady sinusoidal driving were applied to the collar by causing currents to flow in a coil held near the collar. The instantaneous resonant frequencies measured by the pluck and driving techniques were the same at various times during a twitch contraction cycle. The strain produced by the plucking technique in the outermost fibers was less than 1.6 x 10(-4%), a strain three orders of magnitude less than that required to drop the tension to zero in quick-length-change experiments. Because the pressure transients recorded by the hydrophone during plucks and naturally occurring sounds were of comparable amplitude, strains in the muscle due to naturally occurring sound must also be of the order 10(-3%). A simple model assuming that the muscle is an elastic bar under tension was used to calculate the instantaneous elastic modulus E as a function of time during a twitch, given the tension and resonant frequency. The result for Emax, the peak value of E during a twitch, was typically 2.8 x 10(6) N/m2. The methods used here for measuring muscle stiffness are unusual in that the apparatus used for measuring stiffness is separate from the apparatus controlling and measuring force and length.

Acoustic Stimulation↗

PubMed2000

Stiffness, strength and healing assessment in different bone fractures--a simple mathematical model.

External fixators can only be removed safely when fractures have healed sufficiently to restore mechanical integrity to the bone. A bending stiffness of 15 N m/ degrees has been suggested as a means of estimating mechanical integrity. To examine whether this end point stiffness value can be applied to all fractures, the present study examined the degree of variability in predicted stiffness and strength that arises from variations in bone dimensions. Results imply that there is no common value for the end-point of bending stiffness in different bones. At an end point value of 15 N m/degrees, the maturity of the fracture repair tissue (represented by its elastic modulus) can vary 500-fold between an adult femur with a 0.5-mm gap to a child's mid diaphyseal tibia with a 1.0-mm gap. Fortunately, the strength does not vary by as large an extent as the modulus. However, even though two fractures each have reached a stiffness of 15 N m/degrees, a fracture in a bone of 50 mm diameter may exhibit only 60% of the strength of repair in a bone of 30 mm diameter. Therefore, caution should be exercised when using the bending stiffness as an end point indicator for different bones.

Adult↗

PubMed1986

Characterization in vitro of the biomechanical properties of anastomosed host artery-graft combinations.

An in vitro investigation is described in which the biomechanical properties of several host artery-graft combinations are characterized under realistic hemodynamic environments. Canine carotid arteries served as the host vessel and were anastomosed to one of the following graft materials: 4 mm I.D. thin-walled expanded polytetrafluoroethylene (e-PTFE), 6 mm I.D. thin-walled e-PTFE, modified human umbilical vein, autogenous foreleg vein, and carotid artery. A novel feature of the experimental design is the use of a pulsatile perfusion apparatus that simulates realistic normotensive or hypertensive hemodynamics, including pulse rate, perfusion pressure, and flow rate. Measurements of dynamic transmural pressure and vessel radial motion (determined with a helium-neon laser micrometer) were obtained during pulsatile perfusion. From these data calculation of the mismatch in diameter and elasticity modulus between the host carotid artery and various graft materials is made. The longitudinal propagation of the non-uniform radial displacement associated with the construction of the anastomotic junction is also calculated and presented. The present methodology is directly applicable to testing of other vascular substitutes and provides repeatable and reliable biomechanical data.

Animals↗

PubMed1988

Softening of thermoplastic polyurethanes: a structure/property study.

Softening of thermoplastic polyurethanes (TPU) in a simulated body environment (37 degrees C n-saline) was studied as a function of composition, structure and resultant morphology of these (AB)n type block copolymers. The structural variations were attempted by changing chemical composition and molecular weight of both hard A and soft B segments and their weight ratio in the polymer. In addition, the influence of bulk and/or surface modifiers, such as "reacted-in" polysiloxanes and fluorinated polyalkylether glycols, was also investigated. The degree of softening, expressed as a percentage decrease of the elastic modulus (5% tensile modulus) upon two hours exposure to the testing environment, is significant, reversible and depends on the ratio of hard to soft segment and the extent of microphase separation. Since these parameters can be selected during the polymer synthesis and processing into desirable shapes, the degree of softening can thus be controlled. This softening at body temperature represents one of the most notable performance advantages of these biomaterials.

Elasticity↗

PubMed1988

Age-related changes in the beagle spine.

Age-related changes were evaluated in the spines of beagle dogs by radiology, histology, and mechanical testing. Thirty healthy beagle dogs were divided into five groups having mean ages of 2-14 years. Radiographs were taken of intact spines at the time of death and of defleshed spines at necropsy. Cervical, thoracic, and lumbar segments were tested in compression to calculate peak stress, peak strain, and elastic modulus. Adjacent spine segments were graded grossly for osteophyte development, and sections of the intervertebral discs were evaluated histologically. Histologic evidence of disc degeneration and changes in the mechanical properties of the intervertebral disc joint preceded radiographic changes. Changes in the mechanical properties of the disc space were probably a result of the disc degeneration rather than the spondylitic lesions.

Aging↗

PubMedDate not supplied

An experimental study of the mechanical behavior of frozen arteries at low temperatures.

An experimental study of the mechanical response of frozen arteries to tensile stresses at low temperatures is presented. The Dynamic Mechanical Analyzer was used to perform the mechanical experiments. It was found that the frozen artery shows a kind of elastic-plasticity when the temperature is between -20 C and -40 C. And with the decrease of the temperature, the plasticity deformation decreases. Thus at the temperature of -120 C no plasticity deformation is observed before the artery's fracture and the tissue shows quite perfect elastic brittleness, both peripherally and axially. These kinds of mechanical characteristics help explain the fracture phenomena occurring during cryopreservation of the arteries. The mechanical properties, including elastic modulus and fracture strength, are also given. It is known that Cryoprotectant (CPA) used in cryopreservation is necessary in maintaining the tissue's biological functions. Our investigation of its effect on the artery's mechanical properties found that the existence of CPA can soften the tissue at low temperatures, thus may decrease the possibility of fractures during the cryopreservation.

Animals↗

PubMed1984

Measurements of the compressive properties of scleral tissue.

Tests were conducted on small samples of sclera taken from cattle eyes and human eyes to determine the mechanical properties of the tissue in compression. It was found that the elastic modulus for radial compressive stress of human sclera was more than a factor of 100 less than the modulus for circumferential tensile stress. The stress-strain relationship was found to be mildly nonlinear and exhibited hysteresis when cycled through a pattern of increasing-then-decreasing stress. Given sufficient time, however, the sample returned to its original state and, therefore, exhibited no permanent deformation. Measurements of Poisson's ratio yielded values of 0.46 to 0.50, indicating that these samples were essentially incompressible.

Aged↗

PubMed1993

Effect of crosslinking agent content, monomer functionality, and repeat unit chemistry on properties of unfilled resins.

Seven mechanical/physical properties were used to evaluate 10 unfilled resins: eight aromatic dimethacrylates and two urethane dimethacrylates. Physical property tests included compressive strength, Young's modulus in compression, uniaxial tensile strength, intrinsic yield point, toothbrush abrasion, Knoop hardness, and water sorption. Controlled changes were made in the following four material parameters: amount of crosslinking diluent present in the uncured monomer, functionality of the monomer, repeat unit chemistry of the monomer (urethane vs. aromatic structure) and mode of activation (chemical vs. visible light). Polymers containing a high concentration of crosslinking agent (50 wt%) were found to be tougher and to possess lower hardness than materials containing lesser amounts of crosslinking agent. This was attributed to the flexible nature of the triethylene glycol dimethacrylate crosslinking molecule. Exposure to water plasticized the highly crosslinked materials to the degree that the yield point and elastic modulus were effectively lowered. Most of the tested properties were unaffected by differences in functionality except resistance to toothbrush abrasion, which was enhanced for polymers derived from high functionality monomers. The urethane-based polymers sorbed substantially more water than the aromatic-based materials, which presumably resulted in lower values for surface hardness. However, the urethane resins were very tough, and excellent resistance to toothbrush abrasion was observed. Property differences caused by differences in activation mode were small, although the visible light materials did sorb more water.

Absorption↗

PubMed1990

Effect of magnesium ions on red cell membrane properties.

Spectrin forms aggregates in solution when incubated at relatively high concentrations (several millimolar) of divalent cations. According to the evidence of electron microscopy, aggregates of globular appearance and rather uniform size are cooperatively formed from spectrin dimers, no intermediate structures being seen. Inter-dimer chemical cross-linking of spectrin in intact red cell membranes is enhanced if magnesium ions at a concentration of 0.5 mM or more are present. On the other hand, the elimination of magnesium from the interior of intact cells causes no significant change in shear elastic modulus, measured by micropipette assays, nor is there any dependence of membrane filtration rate on intracellular free magnesium concentration in the range 0-1 mM. Magnesium-depleted cells are, however, converted into echinocytes within a short period, in which control cells, exposed to ionophore and external magnesium ions, remain completely discoid. Magnesium-depleted cells also undergo structural changes on heating below the temperature at which vesiculation sets in. These reveal themselves by the transformation of the cells to a unique characteristic shape, by grossly reduced filtrability, and by extensive agglutination of the cells when treated with a bifunctional reagent. Magnesium ions thus regulate the stability, but not to any measurable extent the gross elasticity, of the red cell membrane.

Cross-Linking Reagents↗

PubMed2006

A novel combinatorial approach for understanding microstructural evolution and its relationship to mechanical properties in metallic biomaterials.

The new generation of metallic biomaterials for prosthesis implantation (orthopedic and dental) typically have a Ti base with fully biocompatible alloying additions such as Nb, Ta, Zr, Mo, Fe and Sn. While the binary Ti-Ta and the ternary Ti-Nb-Ta systems are promising, the large composition space afforded by these systems offers tremendous scope in terms of alloy design via optimization of alloy composition and thermomechanical treatment. In the present paper a novel combinatorial approach has been developed for rapidly exploring the microstructural evolution and microstructure-microhardness (or elastic modulus) relationships in these systems. Using directed laser deposition, compositionally graded alloy samples have been fabricated and subsequently heat-treated to affect different microstructures in terms of the volume fraction and distribution of the alpha phase in the beta matrix as a function of composition. Subsequently, composition-specific indentation-based hardness and modulus information has been obtained from these graded samples, and the resulting data have been used to develop relationships between the composition, microstructure and mechanical properties. Such rapid combinatorial assessments can be very useful in optimizing not only the alloy composition but also the desired microstructure for achieving the best combination of properties for specific orthopedic or dental applications.

Alloys↗

PubMed2004

Plasma adrenomedullin and carotid atherosclerosis in atherothrombotic ischemic stroke.

OBJECTIVE: Adrenomedullin is known to exert anti-atherosclerotic actions by inhibiting proliferation and migration of smooth muscle cells in vitro. Here we examine the relationship between the plasma concentration of adrenomedullin and ultrasonographic characteristics of carotid arteries both in ischemic stroke and in the absence of cerebrovascular disease. METHODS: We studied 61 patients with atherothrombotic ischemic stroke in the chronic phase and 50 patients without any cerebrovascular disease. Intima-media thickness and vascular lumen diameters were evaluated by carotid ultrasonography. Plasma mature-adrenomedullin was determined by radioimmunoassay. RESULTS: Plasma mature-adrenomedullin in the patients with atherothrombotic ischemic stroke in the chronic phase (2.01 +/- 0.58 fmol/ml) was significantly higher than that in the patients without any cerebrovascular disease (1.24 +/- 0.18 fmol/ml, P < 0.001). With multiple regression analysis, plasma mature-adrenomedullin was found to be predicted by: stroke status (atherothrombotic ischemic stroke versus no cerebrovascular disease), diabetes status (yes/no), left ventricular ejection fraction, internal carotid artery intima-media thickness, and common carotid artery pressure strain elastic modulus (R = 0.79; F5,105 = 85.39, P < 0.0001). CONCLUSION: Plasma mature-adrenomedullin showed significantly positive associations with carotid atherosclerosis and atherothrombotic ischemic stroke, independent of systolic blood pressure.

Adrenomedullin↗

PubMed2001

Systolic blood pressure, pulse pressure and arterial stiffness as cardiovascular risk factors.

For many years systolic and diastolic blood pressure were the exclusive mechanical factors predicting cardiovascular risk in populations of normotensive and hypertensive individuals. However, if hypertension acts as a mechanical factor with deleterious consequences on the arterial wall, the totality of the blood pressure curve should be considered to evaluate the risk. The purpose of this review is to show that, in addition to systolic and diastolic blood pressure, other haemodynamic indexes with particular relevance for cardiac complications and that originate from pulsatile pressure should be taken into account, namely brachial pulse pressure and aortic pulse wave velocity. The main recent findings in normotensive and hypertensive populations are: (i) increased pulse pressure is an independent predictor of myocardial infarction, congestive heart failure and cardiovascular death, even in hypertensive patients undergoing successful antihypertensive drug therapy; (ii) increased aortic pulse wave velocity and increased carotid incremental elastic modulus are also both independent predictors of cardiovascular mortality, mainly in patients with end-stage renal disease and, to a lesser extent, in individuals with essential hypertension. Currently, increased pulse pressure and increased pulse wave velocity may be considered either as simple markers of an underlying vascular disease or as strong cardiovascular risk factors. The solution of this important question requires the development of specific intervention trials.

Arteries↗

PubMed1995

Stability of dental waxes following repeated heatings.

The flow and strength properties of dental waxes were examined following excessive and repeated heatings of the materials. For one product, the flow at 40 +/- 0.5 degrees C was reduced by 25.3% following heating above 200 degrees C. A decrease of the elastic modulus at 20 +/- 1 degree C by approximately 66% was observed in some cases after the heating temperature had been increased to 300 degrees C. Property variations were related to compositional changes, which were investigated by infrared spectoscopy and thermal analysis. Exposure of dental waxes to temperatures higher than 200 degrees C, particularly if it is repeated, may affect the composition and properties, resulting in inferior materials.

Dental Materials↗

PubMed2006

An EFE model on skin-sleeve interactions during arm rotation.

Skin and garment constitute a dynamic contact system for human body comfort and protection. Although dermatological injuries due to fabric actions during human body movement are common, there is still no general guidance or standard for measuring or evaluating skin/garment contact interactions, especially, during intense sports. A three-dimensional explicit finite element (EFE) model combined with Augmented Lagrange algorithm (ALA) is developed to simulate interactions between skin and fabric during rotation of the arm. Normalized effective shear stresses at the interface between skin and the sleeve during the arm rotation are provided to reflect the severity of the interactions. The effects due to changes in fabric properties, fabric-skin gap, and arm rotation rate are also illustrated. It has been demonstrated from our predictions that factors such as elastic modulus, friction coefficients, density of fabric, and the initial gap between skin and fabric influence significantly the shear stress and thus the discomfort and even injury potential to skin during intensive body movement such as sports and military. Thus this study for the first time confirms quantitatively that poorly chosen fabric with inappropriate garment design renders adverse actions on human skin.

Clothing↗

PubMed1993

Biomechanical wall properties of the human rectum. A study with impedance planimetry.

Biomechanical properties of the rectal wall were studied in 17 healthy adult volunteers (nine men and eight women). With impedance planimetry it is possible to obtain simultaneous measurements of pressure and rectal cross sectional area (CSA) during balloon inflations. Rectal distensions were done with an intraluminal balloon using specified pressures up to 40 cmH2O above baseline rectal pressure. Balloon inflation elicited a phase of rapid increase in rectal CSA followed by a phase of slow increase until a steady state was reached. Steady state occurred within 67 to 140 seconds with the shortest period at the highest distension pressures. Steady state rectal CSA values had a non-linear relation to increasing distension pressure. Rectal CSA values in women showed a tendency of being slightly higher than male values at all pressure steps with a significant difference at 3 and 5 cm H2O. Biomechanical parameters were calculated from rectal CSA pressure relations. Circumferential wall tension increased in a linear way. Rectal compliance decreased in a non-linear way with no further decline between 30 and 40 cmH2O. The pressure elastic modulus increased steeply until a distension pressure of 35 cmH2O with no further increase to 40 cmH2O. This suggests that rectal tone is reduced as the muscle fails to resist further distension at 35 cmH2O and higher pressures. Impedance planimetry offers new possibilities for investigation of anorectal physiology through the study of segmental biomechanical wall properties of the human rectum.

Adult↗