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The thermodynamic restrictions on the elastic coefficients of linear orthotropic elasticity and linear transversely isotropy elasticity are recorded and it is shown that previously reported data for the elastic orthotropic constants of bone satisfy these thermodynamic restrictions.
In modeling the anisotropic properties of hydroxyapatite (HAp), Katz found that two kinds of phenomenological relationships held among the elastic stiffness coefficients. Firstly, there are three linear combinations--(c11 + c22 + c33), (c44 + c55 + c66), (c12 + c13 + c23)--which arise naturally when computing the isotropic averages of anisotropic crystal systems over all possible spatial orientations. Secondly, the degree of elastic anisotropy in such crystal systems is characterized by two specific factors: (a) the ratio of the linear compressibility along the unique axis to that perpendicular to it, (c11 + c12 - 2c23)(c33 - c13); and (b) the ratio of the two shear moduli, c44/c66. There have been a number of experiments in recent years which have used either mechanical methods or ultrasonic techniques to measure the anisotropic elastic properties of bovine and human cortical bone. Analyses of data from these experiments show that the above relationships also play a significant role in characterizing the elastic anisotropy in bone.
An ultrasonic technique was used to measure both the elastic modulus (Young's modulus) of trabecular bone material and the elastic modulus of the cancellous structure. The average trabecular modulus, measured on specimens obtained from three human and one bovine distal femora, was 13.0 GPa (S.D. 1.47) and 10.9 GPa (S.D. 1.57), respectively. On human specimens the structural elastic modulus was found to be related to the structural (apparent) density raised to the 1.88 power. The elastic modulus from the bovine specimens showed a more linear relationship with the density of the cancellous structure (density raised to the 1.57 power).
A coronal plane model of a distributed elastic sole has been proposed and analyzed with respect to the effects of different medial-lateral elasticity distribution on pronation under quasi-static conditions. The distributed model consists of an array of linear vertical line springs. Under minimum energy assumption, the behavior of the top surface of the interface under resultant force and moment loading was shown to be equivalent to that of a rigid-body mechanism under the same loading. The model was then combined with a rigid-link model of the lower limb. Expressions that describe the relationship of the interface aggregate parameters with pronation and the center of pressure were obtained. These expressions were confirmed by an experiment in which the elastic distribution in the interface was systematically varied and the pronation angle and the center of pressure measured. The model has the potential of being a useful analytical tool in the design of elastic soles in running shoes.
Effects of canine hip replacement (with a porous-coated femoral component) on the material properties of surrounding cortical bone were evaluated. The hypotheses were: (1) after four months of implantation, mechanical properties of the cortex would change, and (2) a collared implant would be associated with smaller changes than a collarless design. Unilateral total hip arthroplasty was performed in 15 mixed-breed dogs. Nine received a collared and six received a collarless femoral component. Four months after implantation, longitudinal ultrasonic wave propagation velocities and bone mineral densities (from dual energy X-ray absorptiometry) were measured in harvested femora and used to calculate the axial elastic constitutive coefficients for the cortex surrounding the implants. Results showed no difference in bone elasticity or bone density between collared and collarless designs. Significant velocity decreases from control values (p < 0.0001) were noted in all implanted femora at four months. Bone mineral densities also displayed decreased values after four months of implantation (p < 0.0145). Elastic coefficients were consistently less after four months of implantation when compared to control values (p < 0.0001). This alteration in material properties would affect load transfer into the implanted femur via the increased disparity between implant and bone stiffnesses regardless of the component design. Significant differences in the elastic coefficients between implanted and control femora support hypothesis 1. However, no group differences were found between collared and collarless implantations; thus, the study does not support hypothesis 2.
Breath-holding subjects often exhibit involuntary contractions of respiratory muscles which are much stronger and faster than the efforts they would make during unrestricted breathing at the same level of CO2 and O2. To gain a better understanding of the genesis of these contractions, we compared them with the respiratory response to external elastic loading. Normal men rebreathed a mixture of 8% CO2 in oxygen against no load, elastic loads of 25 and 75 cm H2O/L, and held their breath, equivalent to an elastic load of 226 cm H2O/L. At iso-CO2, increasing loads led to progressively smaller tidal volumes, inspiratory flow rates and ventilation. However, respiratory muscles were progressively activated by the loads, as indicated by increasing occlusion pressure, so that inspiratory flow rate and ventilation were defended much better than could be expected if no neural compensation occurred. The pattern of respiratory muscle activity in breath-holding was qualitatively similar to that in elastic loading, and seemed quantitatively to be an extreme form of reaction to a large load. The reduction in inspiratory time and therefore of peak inspiratory pressure and ratio of inspiratory to total time with very large loads could be viewed as an adaptive response to limit respiratory muscle fatigue.
To further characterize the contribution of the stresses accumulated during inflation in viscoelastic elements of the lungs to the rate-dependence of pulmonary dynamic elastance, we analyzed the changes in the pressures measured at the airway opening and in subpleural air spaces during airway occlusions performed at constant inflation rates of 5, 10, 20, and 40 ml/(kg sec) in 13 anesthetized piglets (mean age = 7 days). The analysis was repeated after saline lavage of the lungs and during intravenous infusion of histamine in 7 and 4 of the piglets, respectively. Viscoelastic stresses dissipated as stress relaxation were solely responsible for the differences between dynamic and static elastance before and after lung lavage and for more than 40% of this difference during histamine infusion (the remainder probably being caused by ventilation inequalities). The viscoelastic contribution to dynamic elastance increased by more than two-fold after lung lavage and was independent of inflation rate and only minimally dependent upon inflation volume. Our results demonstrate that viscoelastic stresses are primarily responsible for the dynamic stiffening of piglet lungs at low rates of inflation. They also support the notion that viscoelastic and elastic stresses are coupled as the lungs inflate.
Elastic fibers are composed of a central core of elastin that is amorphous and electron-lucent in conventional transmission electron micrographs and peripheral microfibrils. A complex infrastructure within the amorphous elastin of mature rat aorta is made visible by fixation and staining with a glutaraldehyde-ruthenium red mixture in phosphate buffer or osmium-ruthenium red in cacodylate buffer. The infrastructure is composed of at least two interlacing but distinct elastic structural components; a framework of circumferentially orientated microfibrils and a three-dimensional meshwork of filaments that permeate the fiber. The latter resembles a reticulum that has previously been observed in freeze-fractured and negatively stained elastin and attributed to the supramolecular organization of elastin. Microfibrils also extend from the core of the elastic fiber into the surrounding matrix where they appear to function as anchoring fibers. These observations indicate that the elastic properties of the arterial wall are an integrated function of both elastin and microfibrils.
The detailed variation of the lumen diameter of common carotid arteries throughout the cardiac cycle was noninvasively measured using a pulse-echo ultrasonic technique in a population of 100 free-living, unrelated, human subjects between the ages of 6 and 25 years. The fractional diameter change (mean +/- SEM) during the cardiac cycle in the 50 male subjects (0.122 +/- 0.004) was found to be significantly (P less than 0.001) greater than the corresponding diameter change in the 50 female subjects (0.106 +/- 0.003). The pressure-strain elastic modulus, Ep, for the carotid arteries was also computed for these subjects by dividing the blood pulse pressure measured in the brachial artery by the fractional diameter change of the carotid. The male elastic modulus (67 +/- 3 kPa) was not significantly (P approximately 0.17) different from the female elastic modulus (62 +/- 2 kPa). Subject age and systolic blood pressure were able to explain approximately one-third of the variability in Ep. Reproductibility studies clearly demonstrated that only a small fraction of the remaining variability could be attributed to experimental technique or intrasubject variability. The results suggest that studies can now be safely performed on young human subjects to assess the effects of a variety of developmental, behavioral, and environmental factors on arterial elasticity. Such results could help to establish an important data base on arterial mechanics which might help in evaluating health promotion recommendations to societies.
The purpose of this study was to assess the elastic behaviour of calf pericardium used in the construction of cardiac bioprosthesis valve leaflets, sutured with different types of commercially available sutures: silk, Gore-Tex, Surgilene and nylon. Thirty-two samples (four series of eight samples each) were subjected to tensile strength testing to breakage. The breaking stress (MPa) ranged between 4.89 MPa for samples sutured with Gore-Tex and 5.22 MPa for those sewn with nylon. Three samples from each series were subjected to a stepwise stress test, involving increasing levels of stress followed by return to zero, to define the elastic limit (the cut-off point beyond which strain is no longer reversible). Analysis of the results provided the mathematical functions that govern the elastic behaviour (stress/strain) within the elastic range for each type of sutured sample. The series sutured with Surgilene presented the highest mean value (1.649 MPa). Finally, a statistical study was carried out to determine which series showed the greatest probability of having the least interaction between the thread and the pericardium. Allowing an interval of +/- 10%, Gore-Tex showed the best probability in this respect. However, real fatigue testing is necessary to definitively determine which is the best suture to use.
According to Koepchen, stimulation of different receptors within a complex network produces typical patterns of effector responses. We investigated whether the dynamical volume elasticity of the heart chamber is an independent component of cardiovascular control. The measurement of the dynamical volume elasticity was calculated by changes of the eigenfrequency of a modified catheter transducer system. The carotid baro- and chemoreflex affected end-diastolic pressure, contractility and calculated diastolic and systolic elasticity to different extents. In conclusion, cardiac volume elasticity is an independent element interacting with other parameters in cardiovascular control.
Elastography is a new ultrasonic imaging technique that produces images (elastograms) of the elastic properties of complaint tissue. To determine the Young's modulus it is necessary to measure or estimate any five of seven relevant variables. In elastography, the measured quantity is the normal strain component in the direction of the applied load, and the three normal components of stress may be estimated using the modified Love's analytical models while assuming a value close to 0.5 (incompressible) for Poisson's ratio. The distribution of Young's moduli can thus be computed and displayed in the form of two-dimensional images called elastrograms. The analytical models used for the estimation of the three normal components of stress assume that the target is semi-infinite and homogeneous in composition. The objective of this article is to determine some of the errors associated with the assumption of homogeneity of the target. Experiments using computer simulations were performed to study the efficiency with which elastograms display the contrast in the Young's modulus of a lesion or target, with respect to its background under certain conditions. It was observed (using the definition of contrast-transfer efficiency of elastography as the ratio of the elasticity contrast as measured from an elastogram, to the true contrast) that elastograms were consistently efficient in quantitatively depicting the elasticity contrast of hard lesions; however, they showed suboptimal contrast-transfer efficiency in cases of soft lesions in a hard background. In general, elastograms are efficient in displaying the elasticity contrast of hard or soft lesions which have a low contrast level with respect to the surroundings, irrespective of their size and location.
OBJECTIVES: Coronary lumen changes during and after excimer laser-assisted balloon angioplasty were measured by quantitative coronary angiography, and the results were compared with the effects of balloon angioplasty alone. BACKGROUND: Reduction of atherosclerotic tissue mass by laser ablation in the treatment of coronary artery disease may be more effective in enlarging the lumen than balloon angioplasty alone. METHODS: A series of 57 consecutive coronary lesions successfully treated by xenon chloride excimer laser-assisted balloon angioplasty were individually matched with 57 coronary artery lesions successfully treated by balloon angioplasty alone. The following variables were measured by quantitative coronary analysis: 1) ablation by laser, 2) stretch by balloon dilation, 3) elastic recoil, and 4) acute gain. RESULTS: Matching by stenosis location, reference diameter and minimal lumen diameter resulted in two comparable groups of 57 lesions with identical baseline stenosis characteristics. Minimal lumen diameter before excimer laser-assisted balloon angioplasty and balloon angioplasty alone were (mean +/- SD) 0.73 +/- 0.44 and 0.74 +/- 0.43 mm, respectively. Laser ablation significantly improved minimal lumen diameter by 0.56 +/- 0.44 mm before adjunctive balloon dilation. In both treatment groups, similar-sized balloon catheters (2.59 +/- 0.35 and 2.56 +/- 0.40 mm, respectively) were used. After laser-assisted balloon angioplasty, elastic recoil was 0.84 +/- 0.30 mm (32% of balloon size), which was identical to that after balloon angioplasty alone, namely, 0.82 +/- 0.32 mm (32%). Consequently, both interventions resulted in similar acute gains of 1.02 +/- 0.52 and 1.00 +/- 0.56 mm, respectively. Minimal lumen diameter after intervention was equal in both groups: 1.75 +/- 0.35 and 1.75 +/- 0.34 mm, respectively. The statistical power of this study in which a 25% difference in elastic recoil (0.2 mm) between groups was considered clinically important was 95%. CONCLUSIONS: In matched groups of successfully treated coronary lesions, xenon chloride excimer laser ablation did not reduce immediate elastic recoil after adjunctive balloon dilation or improve the final angiographic outcome compared with balloon angioplasty alone using similar-sized balloon catheters.
A new Japanese nickel-titanium (NiTi) alloy wire was developed by the Furukawa Electric Co., Ltd. of Japan. This wire was subjected to uniaxial tensile testing and a specially designed three-point bending test to determine the wire stiffness, and to evaluate spring-back, shape memory, and super-elasticity. The Japanese NiTi wire exhibited an unusual property termed "super-elasticity," which no other orthodontic wire has shown. This phenomenon was researched thoroughly. The wire delivered a constant force over an extended portion of the deactivation range. Among all the wires compared, Japanese NiTi alloy wire was the least likely to undergo permanent deformation during activation. The new alloy exhibited a specific stress-strain curve unlike those of the other tested materials. Stress remained nearly constant despite the strain change within a specific range. This unique feature is the manifestation of so-called super-elasticity. Heat treatment enabled the load magnitude at which super-elasticity is reflected to be influenced and controlled by both temperature and time. A unique and useful process was also developed so that an arch wire delivering various magnitudes of force for a given activation could be fabricated from the wire of the same diameter. The clinical application of wires of this new alloy should be more likely to generate a physiologic tooth movement because of the relatively constant force delivered for a long period of time during the deactivation of the wire. Japanese NiTi alloy should be considered an important material addition to clinical orthodontic metallurgy.
The aim of the current study was to find out whether there are sex-dependent differences in the relative number of elastic fibres in human heart valves. Twenty-six aortic valves, 26 mitral valves, 33 pulmonary valves and 28 tricuspid valves of both sexes were obtained at autopsy from newborn to 89-year-old patients who died of noncardiac diseases. The quantitatively morphometric investigations were carried out on conventionally stained (Resorcinfuchsin) histological sections. The results were qualitatively examined with immuno-histochemically marked (anti-elastin antibodies) histological sections. Earlier examinations by Leutert [1976. Z. Gesamte Inn. Med. 31, 97-104] showed that the atrioventricular valves have the following layers: endothelium, atrial fibroelastic tissue (S1), fibrous tissue, ventricular fibroelastic tissue (S2) and endothelium. In our study, the ventricular side of the semilunar valves corresponds to side S1, whereas the vessel side corresponds to side S2. Three regions of interest were examined on each side of the valves: base, mid and tip. The number of elastic fibres per measuring area for all four human heart valves was significantly higher (p < 0,001) in fibroelastic tissue of side S1 than in fibroelastic tissue of side S2. Neither on side S1 nor on side S2 were there significant gender-related differences in the relative number of elastic fibres per measuring area. The results suggest a characteristic distribution of the elastic fibre system which is not sex-dependent but closely related to the function of the heart valves.
BACKGROUND: The effect of raloxifene on aortic elasticity in healthy postmenopausal women is unknown. The purpose of the present study was to examine the effect of raloxifene on aortic elasticity and cardiovascular structure and function in healthy postmenopausal women. METHODS: A randomized, crossover, double-blind, placebo-controlled clinical trial was performed. Fourteen healthy postmenopausal women received treatment with raloxifene 60 mg daily and matching placebo for 8 weeks with an 8-week washout period in between the 2 treatment periods. Cardiovascular magnetic resonance imaging was used to assess ascending thoracic and abdominal aortic elasticity and cardiovascular structure and function (left ventricular volumes, ejection fraction, and mass and mitral annular displacement) before and at the end of each treatment period. RESULTS: Administration of raloxifene had no significant effect on either heart rate or systemic blood pressure. Raloxifene treatment was associated with a small decrease of the ascending aorta wall thickness (pretreatment 2.4 +/- 0.3 vs posttreatment 2.2 +/- 0.2 mm, P = .01). Consequently, there was an increase in the Young's elastic modulus after raloxifene treatment at the ascending thoracic aorta but not the abdominal aorta. There were no significant differences in aortic compliance or any cardiac indexes after raloxifene treatment. CONCLUSIONS: Raloxifene administration in healthy postmenopausal women over an 8-week period may decrease the aortic wall thickness but has no significant effects on aortic compliance or cardiac structure and function.
Multiphoton induced blue/green autofluorescence by near infrared femtosecond laser pulses has been used to selectively image intratissue elastic fibers in native and tissue engineered (TE) viable heart valves without any invasive tissue removal, embedding, fixation, and staining. Elastic fibers could be clearly distinguished from collagenous structures which emit ultraviolet/violet radiation when excited with intense ultrashort pulses due to second harmonic generation. Deep-tissue three-dimensional imaging of elastic fibers with submicron spatial resolution was performed by optical sectioning of heart valves using a multiphoton laser scanning microscope in connection with a tunable 80 MHz femtosecond laser source. The technology was used to diagnose extracellular matrix structures and cell resettlement of TE heart valves prior implantation. This novel non-invasive method opens the general possibility of high-resolution in situ imaging of elastic fibers, collagen structures and intracellular organelles in living intact tissues without staining.
Microenvironments appear important in stem cell lineage specification but can be difficult to adequately characterize or control with soft tissues. Naive mesenchymal stem cells (MSCs) are shown here to specify lineage and commit to phenotypes with extreme sensitivity to tissue-level elasticity. Soft matrices that mimic brain are neurogenic, stiffer matrices that mimic muscle are myogenic, and comparatively rigid matrices that mimic collagenous bone prove osteogenic. During the initial week in culture, reprogramming of these lineages is possible with addition of soluble induction factors, but after several weeks in culture, the cells commit to the lineage specified by matrix elasticity, consistent with the elasticity-insensitive commitment of differentiated cell types. Inhibition of nonmuscle myosin II blocks all elasticity-directed lineage specification-without strongly perturbing many other aspects of cell function and shape. The results have significant implications for understanding physical effects of the in vivo microenvironment and also for therapeutic uses of stem cells.