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Biomedical subjects

I Vesely

Publications and source records attributed to I Vesely.

At least 37 records · Page 2Linked to original sources
PubMed1998

The role of elastin in aortic valve mechanics.

Recent morphologic observations of elastin structures in aortic valves suggest that elastin is mechanically coupled to collagen. Since the mechanical stiffness of elastin is considerably lower than that of collagen, and aortic valves contain relatively little elastin, the mechanical importance of elastin in heart valve function is not clear. We have hypothesized that elastin acts to return the collagen fiber structure back to a resting configuration between loading cycles. The objectives of this research were therefore to elucidate the mechanical relationship between elastin and collagen structures within the aortic valve. To isolate elastin in a morphologically intact state, whole porcine aortic valve leaflets were digested in 0.1 N sodium hydroxide solution (NaOH) at a temperature of 75 degrees C for 45 min. Elastin structures from the fibrosa and ventricularis were tested mechanically, and their loading curves compared to those of the original leaflet layers and to whole cusps. The elastin structures generated very low forces, having an elastic modulus only 0.05% that of the whole tissue. The contribution of elastin to tissue mechanics was significant at low strains and differed between the fibrosa and the ventricularis. Elastin tended to dominate the distensibility curves of the radial ventricularis, but participated very little in the fibrosa. The low but significant tensions produced by the elastin structures of the aortic valve, together with previously observed elastin morphology as well as the measurable preload of elastin, suggest that the purpose of elastin in the aortic valve leaflet is to maintain a specific collagen fiber configuration and return the fibers to this state, once external forces have been released.

Animals↗
PubMed1998

Comparison of the compressive buckling of porcine aortic valve cusps and bovine pericardium.

BACKGROUND AND AIMS OF THE STUDY: Compressive buckling at sites of sharp leaflet flexure has been implicated as a mechanism of failure in porcine xenografts. The potential for such buckling to cause damage in new-generation pericardial valves, however, has not been examined. METHODS: Clinical-grade bovine pericardium fixed in 0.625% glutaraldehyde was cut into 5 mm-wide strips. Fresh porcine aortic valve leaflets were fixed flat in 0.625% glutaraldehyde and 5 mm-wide circumferential strips were cut. These tissues were bent to various curvatures, held bent with sutures, histologically processed, and sectioned and stained with hematoxylin and eosin. Images of the specimens were acquired by computer and the depth of compressive buckling, thickness of the specimen, and local curvature were measured. RESULTS: Porcine tissue showed a progressive increase in depth of buckling as both thickness and curvature increased, while bovine pericardium had minimal buckling at all curvatures. Porcine tissues buckled to a mean (+/- SEM) fractional depth of 0.23+/-0.012 while bovine pericardium buckled to only 0.09+/-0.006. CONCLUSIONS: These data suggest that the internal fibrous structure of bovine pericardium may tolerate high bending curvatures better than porcine aortic valve leaflets when stiffened and cross-linked with glutaraldehyde. This may explain the apparently good durability of current generation pericardial valves.

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PubMed1997

Viscoelasticity of dynamically fixed bioprosthetic valves. II. Effect of glutaraldehyde concentration.

OBJECTIVE: We have previously shown the benefits of dynamic fixation over conventional static fixation of bioprosthetic valves. In an attempt to increase the durability of bioprosthetic heart valves, we explored the benefit of low-concentration glutaraldehyde dynamic fixation. METHODS: Pig aortic valves obtained fresh from the abattoir and excised with the entire root were dynamically fixed in glutaraldehyde phosphate buffer solutions varying in concentration from 0.05% to 2.5%. Denaturation temperatures were measured and mechanical testing was performed at low (3 mm/sec) to high physiologic rates (30 mm/sec) at 37 degrees C in isotonic modified Hanks solution. RESULTS: When fixed dynamically in 0.05% glutaraldehyde solution for 24 hours, the tissue reached a degree of cross-linking (denaturation temperature = 82.8 degrees +/- 0.6 degree C) significantly higher than that obtained for 0.05% static fixation (denaturation temperature = 79.3 degrees +/- 0.9 degree C) (p < 0.05) but similar to that for conventional static fixation in 0.5% glutaraldehyde solution (denaturation temperature = 83.5 degrees +/- 0.3 degree C). After fixation in low-concentration glutaraldehyde (0.05%), final relaxation slopes and moduli in the circumferential direction were significantly higher than those for the statically fixed tissue but similar to those for the fresh tissue. However, both dynamic and static fixation had the effect of increasing tissue extensibility to similar extents in both directions, irrespective of glutaraldehyde concentration. CONCLUSIONS: Dynamic glutaraldehyde fixation of a porcine aortic valve at lower concentrations resulted in a better degree of cross-linking and a material with biomechanical properties that more closely mimic those of natural heart valve tissue.

Animals↗
PubMed1997

In vivo and in vitro models of calcification in porcine aortic valve cusps.

Both in vivo and in vitro models have been developed to study the initiation and progression of dystrophic calcification of bioprosthetic heart valves. Circulatory in vivo models have proven to be the most predictive of the success of a new valve designs or anticalcification schemes; however, these experiments are time consuming and expensive. An appealing alternative to circulatory implantation is the sub-cutaneous rat implantation model. This model is inexpensive and calcification occurs rapidly. Recent studies have shown, however, that some anticalcification methods work well in the subcutaneous model but are ineffective in the circulatory model. In vitro models would provide the most convenient method for testing new anticalcification strategies but, to date, no in vitro test system has been developed which produces calcification of rates and with morphology comparable with that in vivo models. We have also studied the effects of collagen damage and cell extraction on the calcification of porcine aortic valve cusps both in vitro and in the subcutaneous rat model, and found significant differences in the patterns of mineralization. The objectives of this paper therefore are to compare and contrast the different experimental protocols and procedures reported in the literature to better define the effects of different model systems on the calcification process.

Animals↗
PubMed1996

Dynamic glutaraldehyde fixation of a porcine aortic valve xenograft. I. Effect of fixation conditions on the final tissue viscoelastic properties.

Sixty porcine aortic valves were fixed under dynamic conditions at specific durations, pressures and vibration rates in a 0.5% glutaraldehyde phosphate buffer (pH 7.4, 0.2 M). Tensile relaxation tests were performed at low through high extension rates (0.3, 3 and 30 mm s-1) and tissue denaturation temperatures were determined by the hydrothermal isometric tension method. Conventional statically fixed valves and fresh valves were used as controls. No differences between dynamic and static treatment were observed at pulsation rates above those expected in the physiological range (i.e. above 1.2 Hz) or at higher pressures such as 30 mmHg. However, differences in both stress relaxation rates and denaturation temperatures were delineated in milder fixation conditions, i.e. at low pressures (< 4 mmHg) and low vibration rates similar to that of the normal heart beat (approximately 1.2 Hz). In these conditions the relaxation rate of the dynamically fixed tissue (-7.4 +/- 0.7% of stress remaining per log(s)) was similar to that of the fresh tissue (-6.7 +/- 1.2% log(s-1)) and significantly higher than the statically treated tissue (-3.9 +/- 1.7% log(s-1)). The rates of stress relaxation appeared to be strain rate dependent in both radial and circumferential directions when the tissues were strained at physiological rates during testing (> approximately 15000% min-1). Dynamically treated valves showed higher denaturation temperatures (mean +/- SD) (89.4 +/- 0.5 degree C) compared with the statically fixed (82.7 +/- 1.4 degrees C) or untreated (fresh) valves (65.5 +/- 0.8 degree C). The results suggest a higher degree of internal cross-linking owing possibly to enhanced penetration of the glutaraldehyde reagent and a greater accessability of reactive cross-linking sites on the collagen molecules. Better stress relaxation rates are likely associated with an increase in potential shearing between adjacent collagen fibres thus preserving the natural stress-reducing mechanism of the fresh, untreated valves. The dynamically treated valves therefore possess characteristics that may enable them to better resist long-term mechanical fatigue and in vivo degradation.

Animals↗
PubMed1996

Morphology of porcine aortic valve cusp elastin.

BACKGROUND AND AIM OF STUDY: While the structure and function of heart valve cusp collagen have been relatively well defined, the role and morphology of elastin remains poorly understood, despite the fact that it comprises up to 13% of the cusp dry weight. MATERIAL AND METHODS: The elastin structure of 24 hot-alkali-digested porcine aortic valve cusps was investigated with scanning electron microscopy. Elastin structures were categorized according to their morphology and a model of the distribution of these structures within the cusp was developed. RESULTS: The two main types of elastin observed, amorphous and fibrillar, were further categorized based on their morphology. Amorphous structures included continuous sheet, sheet with integrated fibers on the surface and sheet with fenestrations. Fibrillar structures identified were loose fibers, loose mesh/woven fibers and compact mesh. By imaging samples of digested fibrosa and ventricularis that had been microdissected apart, we were able to produce maps of the elastin structure in the two layers. The ventricularis contains a large continuous sheet of amorphous or compact mesh elastin that covers the entire layer. Elastin in the fibrosa is much more complex, consisting of large tubes that emerge from the aortic attachment and extend circumferentially across the cusp. The tubes, constructed of amorphous fenestrated sheet and loose mesh elastin, likely surround the large circumferential collagen bundles observed in the fibrosa. CONCLUSIONS: The elastin structures that we have identified help explain the measured mechanics of this tissue and suggest that collagen and elastin are highly integrated. As a result, we believe that elastin plays an important functional role in the cusp and that a full explanation of heart valve cusp mechanics must incorporate the contributions of both collagen and elastin.

Animals↗
PubMed1995

The hybrid xenograft/autograft bioprosthetic heart valve: in vivo evaluation of tissue extraction.

The major functional problem with bioprostheses is poor long-term durability. Bioprosthetic valves fail because of calcification and mechanical fatigue, both of which result from the glutaraldehyde fixation process. In an effort to develop a biologically active, non-cross-linked bioprosthetic valve, we devised a cellular extraction process. We tested the mechanical integrity of the processed valves and cultured both human and porcine cells on this material. To test the potential for calcification, we implanted strips of fresh, extracted, and glutaraldehyde-treated porcine heart valve tissue subcutaneously into 3-week-old Sprague Dawley rats for 21 days. We used atomic absorption spectroscopy to measure the extent of calcium accumulation and histopathologic assessment to evaluate the antigenic response. We found that the cell extraction process significantly reduced the propensity of the material to calcify in vivo (mean +/- standard deviation, 4.12 +/- 1.02 mg/g calcium extracted versus 10.75 +/- 3.9 mg/g calcium fresh versus 79.6 +/- 18.3 mg/g calcium glutaraldehyde fixed) but increased the antigenicity, as evidenced by increased cellular activity and resorption. Although they may reduce calcification, conventional detergent-based cell extraction techniques do not completely remove porcine aortic valve antigens and may in fact increase the antigenicity of the valve cusp material.

Animals↗
PubMed1995

Biaxial strain analysis of the porcine aortic valve.

The function of a bioprosthetic heart valve is determined largely by the material properties of the valve cusps. The mechanics of natural and bioprosthetic valve cusps have been studied extensively using uniaxial tensile testing. This type of testing, however, does not duplicate the natural biaxial loading condition. Whole-valve biaxial testing therefore is preferred. The objective of the present study was to investigate the heterogeneity of the valve cusps by mapping out the regional variability of the biaxial strain versus pressure relationship. Whole porcine aortic valves were mounted horizontally, submerged in physiologic saline solution at 37 degrees C, and pressurized in the range of 0 to 130 mm Hg of pressure. The ventricular side of the cusps were marked with black dots and the three-dimensional position of these dots was recorded together with the aortic pressure. By calculating the distance between the dots in the radial and circumferential directions in different regions, the local strain versus pressure relationship was determined. The results showed that the valve cusp material strained by 23% +/- 0.8% in the radial direction and 10.0% +/- 0.5% in the circumferential direction before lock-up. It was also found that while the valve cusp was highly anisotropic in the central region, the basal region was relatively isotropic, and the cusp as a whole was asymmetrical in its distensibility.

Animals↗
PubMed1995

Bioprosthetic valve tissue viscoelasticity: implications on accelerated pulse duplicator testing.

Most of our knowledge of heart valve mechanics has been gained from low strain-rate studies much lower than physiologic levels. Using a high-speed materials testing system, we compared the low and high strain-rate viscoelastic behavior of porcine aortic valve cusps at extension rates of up to 40 mm/s. Circumferential and radial strips were stretched and then held in their stretched configuration to measure their "stress-relaxation" behavior. During low strain-rate stretching, only 6% of the initial stress dissipated or relaxed after 1 second, whereas 25% of the stress dissipated during high strain-rate stretching. This considerable difference in stress relaxation suggests a rate-dependent viscoelastic behavior that has not been accounted for in valve design and may have important implications for accelerated pulse testing. Even though the valve cusp is loaded for only 0.4 seconds during each heartbeat, at least 15% of the stress may relax over that period. During accelerated pulse testing, however, sufficient time may not be available to allow the tissue fibers to relax back to their natural state before the subsequent loading cycle, leading to a higher baseline preload. In addition, because valve tissue is not given sufficient time to relax before the next cycle, pulse testing subjects the valves to lower-magnitude cyclic stresses than does physiologic loading. Because both the baseline preload and the magnitude of cyclic stresses may lead to early fatigue failure, accelerated wear testing may either overestimate or underestimate valve durability. Clearly, the mechanism of stress-induced failure of biologic tissues must be elucidated before too much validity is placed on pulse duplicator studies.

Animals↗
PubMed1995

Longitudinal and radial distensibility of the porcine aortic root.

The aortic root has been shown to be a highly distensible structure. The function of the aortic valve is intimately related to the expansion of the aortic root, and current nonexpansible stent designs may affect its performance. We therefore measured the radial and longitudinal expansion of the porcine aortic root as a function of pressure in both a static pressurization model and in an isolated working heart model. The radial and longitudinal expansion of the aortic root was measured using a custom-built digital sonomicrometer. Multiple ultrasonic crystals were sutured exterior to the commissures and along the length of the aortic root, and their separation was tracked at varying aortic pressures. In static testing, we found that commissural separation at zero pressure was 26% +/- 7% (mean +/- standard deviation) less than at 120 mm Hg, whereas the longitudinal distance between the base of the valve and the commissures decreased by 11% +/- 9%. Approximately one quarter of the total dimensional change occurred over the physiologic range of 80 to 120 mm Hg. In the isolated porcine heart model, we measured a greater distensibility than in the static tests. For example, at aortic pressures of 120/80 mm Hg (systolic/diastolic), the diameter of the aortic root would be 22% +/- 6% less at 80 mm Hg than at 120 mm Hg. The longitudinal dimensions would be 15% +/- 8% less at 80 mm Hg than at 120 mm Hg. We conclude that the aortic root contracts significantly when depressurized, as during valve replacement surgery, and that the in vivo distensibility of the aortic root is much greater that what is generally measured in vitro. These results suggest that dimensional changes in the implanted prosthetic valve and the recipient aortic root must be considered to achieve both optimal valve orifice and, in the case of distensible valves such as allografts, a proper valve cusp geometry.

Animals↗
PubMed1995

Aortic valve cusp microstructure: the role of elastin.

The aortic valve cusp is a three-layered structure, composed of differing amounts of collagen, elastin, and glycosaminoglycans. Little quantitative information is presently available on the amount, location, orientation, and overall structure of these constituents, particularly of elastin. We developed a technique to isolate aortic valve elastin in a morphologically intact state. Whole leaflets were digested in 0.1 N sodium hydroxide solution at a temperature of 75 degrees C. Both scanning electron microscopy and computerized three-dimensional reconstructions of serial sections showed a well-defined honeycomb or spongelike structure, suggesting that elastin forms a matrix that surrounds and links the collagen fiber bundles. This relationship between collagen and elastin is further supported by the naturally wavy configuration of the valve cusps, permitting elongations of 40%, even though collagen fibrils typically strain to 1% to 2% before fracture. Elastin likely acts to return collagen fibers back to their undeformed state, maintaining rest geometry.

Animals↗
PubMed1995

A comparison of macroscopic lipid content within porcine pulmonary and aortic valves. Implications for bioprosthetic valves.

Lipid droplets have been demonstrated within both explanted porcine bioprostheses and normal porcine aortic valves. Because of the increasing interest in pulmonary valves as an allograft or xenograft aortic valve substitute, we examined the incidence and distribution of such lipid deposits in 50 porcine aortic valves and 50 matched porcine pulmonary valves. All 300 cusps were removed with surgical scissors and, under a dissecting microscope, the ventricularis layer was removed to expose the spongiosal layer. Macroscopic extracellular lipid droplets analyzed by means of a dissecting microscope with an eyepiece grid and stereology point-counting techniques to provide an area-density average spatial probability map for each cusp. Only 8% of porcine aortic valves were free of lipid, with the distribution of the lipids being 52% +/- 14% right coronary cusp, 90% +/- 8% left coronary cusp, and 68% +/- 13% noncoronary cusp. Of the pulmonary valves, 60% were free of lipid, with the incidence of lipids being 26% +/- 12% left cusp, 6% +/- 7% right cusp, and 12% +/- 9% anterior cusp. Subsequently, lipid cluster samples underwent thin-layer chromatography, which showed them to be phospholipids, oleic acid (fatty acid), triglycerides, and unesterified cholesterol. One primary mode of bioprosthetic valve failure is leaflet calcification. The similarity of distribution within the spongiosal layer between leaflet calcification and intrinsic cusp lipids suggests that the observed lipids might act as a nucleation site for calcification. The substantially lower incidence of lipid in pulmonary valves therefore may represent a potential benefit when these valves are considered for use as aortic valve replacements.

Animals↗
PubMed1995

Porcine pulmonary and aortic valves: a comparison of their tensile viscoelastic properties at physiological strain rates.

Pulmonary valve autografts and allografts have been recently reported as being clinically effective for the replacement of the diseased aortic valve. While the biomechanics of the aortic valve have been widely studied, there is little information available about the mechanical properties of the pulmonary valve. We felt that it was necessary to investigate the mechanical properties of the pulmonary valve to determine if it is mechanically suitable as a long term replacement for the aortic valve. We employed physiological strain rate tensile testing to investigate the stress-strain and stress relaxation behaviour of 44 aortic and 40 pulmonary valve cusp strips cut in the radial and circumferential directions, in fresh and glutaraldehyde fixed states. Stress-strain and stress relaxation tests were performed on each test strip at extension rates of 40 mm/s, 4 mm/s and 0.4 mm/s. In all but one mechanical parameter, we found no difference between aortic and pulmonary valve tissues. The extensibilities and relaxation rates were similar, but the aortic valve tissue had a greater average modulus (p = 0.0005) than the pulmonary valve tissue (i.e. 7.41 MPa vs. 5.86 MPa respectively). Since bioprostheses are often constructed from materials with mechanical properties very different from those of the aortic valve, like pericardium, the slight difference between aortic and pulmonary valves is unlikely to affect the operation of the pulmonary valve in the aortic position. The pulmonary valve could therefore be considered mechanically acceptable as a replacement for the aortic valve.

Animals↗
PubMed1994

The distribution and morphology of aortic valve cusp lipids.

During dissections of porcine aortic valves, we noted clusters of spherical droplets, roughly 50 micrometers in size, within the central spongiosa of the valve cusps. Oil red-O staining of fresh tissues confirmed that the clusters were lipids, and thin layer chromatography showed that they consisted mainly of free cholesterol and triglycerides, with some cholesterol ester. The distribution of these clusters in 60 cusps from 20 porcine aortic valves was mapped using computerized morphometry. The ventricularis was peeled back to expose the spongiosa, and the spatial distribution of the lipid droplets was analyzed using a stereological point counting method. Computer averaging of lipid density images was done to obtain a mean distribution map of lipid occurrence. This distribution map suggests that porcine aortic valve lipids are found predominantly at the base of the valve cusps, and are conspicuously absent from the coaptation regions and the free edge. Moreover, 19/20 valves had lipids in the left coronary cusp and all 20 valves had lipids in at least two of the three cusps. Endogenous lipids are therefore ubiquitous in porcine aortic valves and may play a role in the calcification of xenograft bioprostheses.

Animals↗
PubMed1993

Natural preload of aortic valve leaflet components during glutaraldehyde fixation: effects on tissue mechanics.

The mechanics of glutaraldehyde-fixed aortic valve leaflets depend largely on the amount of stress present during fixation. Our previous work has suggested that even when the aortic valve is flaccid, the leaflet components are preloaded. We have, therefore, hypothesized that fixing valve leaflets in this naturally preloaded state will affect the function of their components, the fibrosa and the ventricularis. We have compared the elastic response of fibrosa and ventricularis fixed under 'low' and 'zero' tensile and compressive preload by testing 120 of these layers: (i) fresh, (ii) glutaraldehyde-fixed, and (iii) isolated from whole porcine aortic valve leaflets fixed while intact. In both the radial and circumferential directions, the fibrosa from intact-fixed valves was more extensible than the fresh (39.2 vs 29.2% strain to high modulus phase at p < 0.0122, and 12.7 vs 8.1% strain, at p < 0.0003, respectively). The ventricularis from intact-fixed valves, however, was less extensible than when fresh (35.4 vs 63.7% strain, at p < 0.00001 in the radial direction). The fibrosa must have, therefore, been fixed under compression and the ventricularis under tension, when fixed together in the intact aortic valve cusp. The tensile stresses in the intact-fixed ventricularis produced a greater circumferential elastic modulus than in separately fixed tissue (9.62 vs 4.65 MPa, at p < 0.00001), likely through a fibre recruitment process. Compressive stresses in the fibrosa produced a decrease in the elastic modulus both radially and circumferentially (from 3.79 to 2.26 MPa at p < 0.0023, and from 9.55 to 4.65 MPa at p < 0.00001, respectively). Fixing porcine aortic valves at even minimal tensile and compressive preload, such as that which occurs naturally, significantly alters both the extensibility and the elastic modulus of the valve leaflet components.

Animals↗
PubMed1993

Numerical simulation of leaflet flexure in bioprosthetic valves mounted on rigid and expansile stents.

Recent studies suggest that flexural stresses induced during the opening phase may be responsible for much of the mechanical failures of bioprosthetic heart valves. Sharp leaflet bending is promoted by the mounting of valves on rigid stents that do not mimic the systolic expansion of the natural aortic root. We, therefore, hypothesized that flexural stresses could be significantly reduced by incorporating a flexible or expansile supporting stent into the valve design. Using our own non-linear finite element code (INDAP) and the pre- and post-processor modules of a commercial finite element package (PATRAN), we simulated the opening and closing behaviour a trileaflet bovine pericardial valve. The leaflets of this valve were assumed to be of uniform thickness, with a non-linear elastic behaviour adapted from experimentally obtained bending stiffness data. Our simulations have shown that during maximal systolic valve opening, sharp curvatures are induced in the leaflets near their commissural attachment to the supporting stent. These areas of sharp flexure experience compressive stresses of similar magnitude to the tensile stresses induced in the leaflets during valve closure. By incorporating a stent with posts that pivot about their base, such that a 10% expansion at the commissures is realized, we were able to reduce the compressive commissural stressing from 250 to 150 kPa. This was a reduction of 40%. Conversely, a simple pliable stent with stent posts that deflect inward and outward under load did not achieve a significant reduction of compressive stresses. This numerical analysis, therefore, supports the theory that (i) high flexural and compressive stresses exist at sites of sharp leaflet bending and may promote bioprosthetic valve failure, and (ii) that proper design of the supporting stent can significantly reduce such flexural stresses.

Bioprosthesis↗
PubMed1993

Is zero-pressure fixation of bioprosthetic valves truly stress free?

Zero-pressure fixation has often been referred to as stress-free fixation, which implies that no leaflet stresses exist during the fixation process. The two aortic valve cusp layers, the fibrosa and the ventricularis, however, are believed to produce mutually opposing forces within the valve cusps. Residual stresses may therefore exist even during zero-pressure fixation. We first verified the presence of such internal stresses by separating the layers of pig aortic valve leaflets and measuring dimensional changes. In the 11 specimens examined, the fibrosa expanded radially by 30% +/- 13% (mean +/- standard deviation), whereas the ventricularis contracted by 12% +/- 4%. The ventricularis also contracted circumferentially by 13% +/- 3%. We measured the extensibility of 120 fresh and glutaraldehyde-fixed fibrosa and ventricularis components to investigate the mechanical effects of glutaraldehyde fixation under such internal stresses. We also tested the layers from leaflets that were fixed whole. We compared the extensibility of the fibrosa and the ventricularis, each fixed in the presence and absence of residual stresses, and found that, in the radial directions, the ventricularis from valve cusps that were fixed whole was less extensible than fresh ventricularis (35.4% versus 63.7% strain to high-modulus phase, p < 0.00001). The fibrosa from cusps that were fixed whole, however, was more extensible than fresh fibrosa (39.2% versus 29.5% strain, p < 0.0122 radially; 12.8% versus 8.2% strain, p < 0.0001 circumferentially). The ventricularis became less extensible because it was fixed under tension, and the fibrosa became more extensible because it was fixed under compression. This study therefore demonstrates the presence of residual tensile and compressive stresses in the ventricularis and fibrosa, even when the leaflets are relaxed. Zero-pressure fixation cannot therefore be considered truly stress free, in the engineering sense, because residual internal stresses affect collagen fiber crimp and change the extensibility of the fibrosa and the ventricularis.

Animals↗