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Buchanan RM, Sacks MS. Interlayer micromechanics of the aortic heart valve leaflet. Biomech Model Mechanobiol 2013; 13:813-26. [PMID: 24292631 DOI: 10.1007/s10237-013-0536-6] [Citation(s) in RCA: 35] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/04/2013] [Accepted: 10/09/2013] [Indexed: 10/26/2022]
Abstract
While the mechanical behaviors of the fibrosa and ventricularis layers of the aortic valve (AV) leaflet are understood, little information exists on their mechanical interactions mediated by the GAG-rich central spongiosa layer. Parametric simulations of the interlayer interactions of the AV leaflets in flexure utilized a tri-layered finite element (FE) model of circumferentially oriented tissue sections to investigate inter-layer sliding hypothesized to occur. Simulation results indicated that the leaflet tissue functions as a tightly bonded structure when the spongiosa effective modulus was at least 25 % that of the fibrosa and ventricularis layers. Novel studies that directly measured transmural strain in flexure of AV leaflet tissue specimens validated these findings. Interestingly, a smooth transmural strain distribution indicated that the layers of the leaflet indeed act as a bonded unit, consistent with our previous observations (Stella and Sacks in J Biomech Eng 129:757-766, 2007) of a large number of transverse collagen fibers interconnecting the fibrosa and ventricularis layers. Additionally, when the tri-layered FE model was refined to match the transmural deformations, a layer-specific bimodular material model (resulting in four total moduli) accurately matched the transmural strain and moment-curvature relations simultaneously. Collectively, these results provide evidence, contrary to previous assumptions, that the valve layers function as a bonded structure in the low-strain flexure deformation mode. Most likely, this results directly from the transverse collagen fibers that bind the layers together to disable physical sliding and maintain layer residual stresses. Further, the spongiosa may function as a general dampening layer while the AV leaflets deforms as a homogenous structure despite its heterogeneous architecture.
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Affiliation(s)
- Rachel M Buchanan
- Center for Cardiovascular Simulation, Institute for Computational Engineering and Sciences, Department of Biomedical Engineering, The University of Texas at Austin, Austin, USA
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2
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Brody S, Pandit A. Microarchitectural characterization of the aortic heart valve. ADVANCES IN EXPERIMENTAL MEDICINE AND BIOLOGY 2004; 553:167-86. [PMID: 15503455 DOI: 10.1007/978-0-306-48584-8_13] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/19/2023]
Affiliation(s)
- Sarah Brody
- National Centre for Biomedical Engineering Science and Department of Mechanical and Biomedical Engineering, National University of Ireland, Galway, Ireland
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3
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Abstract
Cardiac valve bioprostheses are assessed in terms of their present and future clinical utility. The problems concerning durability basically involve early failure due to tears in the valve leaflets and late failure mainly associated with calcification of the biological tissue. New strategies for selection and chemical treatment of the biomaterials employed are analyzed, and the available knowledge regarding their mechanical behavior is reviewed. It is concluded that the durability of these devices, and thus their successful use in the future, depends on the knowledge of the interactions among the different biomaterials of which they are composed, the development of new materials, and the engineering design applied in their construction.
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Affiliation(s)
- J M García Páez
- Servicio de Cirugía Experimental Clínica Puerta de Hierro, Madrid, Spain
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4
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Abstract
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.
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Affiliation(s)
- I Vesely
- Department of Biomedical Engineering, The Cleveland Clinic Foundation, Lerner Research Institute, OH 44195, USA.
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Duncan AC, Boughner D, Vesely I. Viscoelasticity of dynamically fixed bioprosthetic valves. II. Effect of glutaraldehyde concentration. J Thorac Cardiovasc Surg 1997; 113:302-10. [PMID: 9040624 DOI: 10.1016/s0022-5223(97)70327-1] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
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.
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Affiliation(s)
- A C Duncan
- J.P. Robarts Research Institute, University of Western Ontario, London, Canada
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Duncan AC, Boughner D, Vesely I. Dynamic glutaraldehyde fixation of a porcine aortic valve xenograft. I. Effect of fixation conditions on the final tissue viscoelastic properties. Biomaterials 1996; 17:1849-56. [PMID: 8889064 DOI: 10.1016/0142-9612(96)00006-3] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
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.
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Affiliation(s)
- A C Duncan
- J.P. Robarts Research Institute, University of Western Ontario, London, Canada
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Abstract
The use of glutaraldehyde as a fixative in bioprostheses and drug delivery matrices is reviewed. The chemistry of glutaraldehyde cross-linking and its effect on the biological performance of a number of bioprostheses such as tissue heart valves, vascular grafts, pericardial patches, tendon grafts and drug delivery matrices are examined.
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Affiliation(s)
- A Jayakrishnan
- Polymer Chemistry Division, Sree Chitra Tirunal Institute for Medical Sciences and Technology, Trivandrum, India
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Vesely I. New concepts in the design and use of biological prosthetic valves. Cardiovasc Pathol 1995; 4:287-91. [PMID: 25851091 DOI: 10.1016/1054-8807(95)00055-a] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/07/1995] [Accepted: 06/07/1995] [Indexed: 10/16/2022] Open
Abstract
The natural aortic valve is a structure that has thus far eluded all attempts at duplication with synthetic materials. Real success in the replacement of the aortic valve has come about primarily through the use of biological devices, such as the porcine aortic valve xenograft. In the future, bioprostheses based more closely on the natural aortic valve may ultimately succeed where synthetic approaches have failed. Some recent advances in the design and development of bioprosthetic heart valves, such as the absence of a stent and the better preservation of the valve's natural biomechanical properties, show considerable promise in improving the long term durability of these devices. With a greater understanding of the structure/function relationship of the aortic valve at the micromechanical level, the future of bioprostheses may be even more biologically oriented than it is today.
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Affiliation(s)
- I Vesely
- Department of Biomedical Engineering, Research Institute, The Cleveland Clinic Foundation U.S.A
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Abstract
All types of contemporary cardiac valve substitutes suffer deficiencies and complications that limit their success. Mechanical and bioprosthetic valves are intrinsically obstructive, especially in small sizes. Mechanical valves are associated with thromboembolic problems; the chronic anticoagulation used in virtually all mechanical valve recipients causes hemorrhage in some. Calcification limits the success of porcine and pericardial bioprostheses, allograft valves, and the yet experimental trileaflet polymeric prostheses. The predominant mechanism of calcification in porcine, pericardial, and allograft valves is cell mediated, being nucleated at the membranes and in organelles of the transplanted cells. In polymeric leaflet valves, calcification is both extrinsic (in adherent thrombus) and intrinsic (subsurface and acellular in the solid elastomer). Nevertheless, except for a few notable exceptions, contemporary mechanical valves are durable. Other important potential complications of prosthetic and bioprosthetic valves include paravalvular leak, endocarditis, or extrinsic interference with function. Moreover, aortic valvular allografts undergo progressive noncalcific degeneration, tearing, sagging, and/or retraction. Studies of retrieved long-term cryopreserved allograft explants demonstrate severe degeneration, with distortion of normal architectural detail, loss of endothelial and deep connective tissue cells, and variable inflammatory cellularity. Thus, they are morphologically nonviable valves, whose structural basis for function seems primarily related to the largely preserved collagen, and they are unlikely to have the capacity to grow, remodel, or exhibit active metabolic functions. Since calcification intrinsic to the cusps is the major pathologic process necessitating bioprosthetic valve reoperations, efforts to prevent formation of mineral deposits are active.(ABSTRACT TRUNCATED AT 250 WORDS)
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Affiliation(s)
- F J Schoen
- Department of Pathology, Brigham and Women's Hospital/Harvard Medical School, Boston, Massachusetts 02115
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Krucinski S, Vesely I, Dokainish MA, Campbell G. Numerical simulation of leaflet flexure in bioprosthetic valves mounted on rigid and expansile stents. J Biomech 1993; 26:929-43. [PMID: 8349718 DOI: 10.1016/0021-9290(93)90055-j] [Citation(s) in RCA: 69] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
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.
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Affiliation(s)
- S Krucinski
- Department of Mechanical Engineering, McMaster University, Hamilton, Ontario, Canada
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Flomenbaum MA, Schoen FJ. Effects of fixation back pressure and antimineralization treatment on the morphology of porcine aortic bioprosthetic valves. J Thorac Cardiovasc Surg 1993. [DOI: 10.1016/s0022-5223(19)33860-7] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
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Schoen FJ, Levy RJ, Hilbert SL, Bianco RW. Antimineralization treatments for bioprosthetic heart valves. J Thorac Cardiovasc Surg 1992. [DOI: 10.1016/s0022-5223(19)34619-7] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 10/25/2022]
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Vesely I, Krucinski S, Campbell G. Micromechanics and mathematical modeling: an inside look at bioprosthetic valve function. J Card Surg 1992; 7:85-95. [PMID: 1554981 DOI: 10.1111/j.1540-8191.1992.tb00779.x] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
A major contributing factor in the degeneration of glutaraldehyde-treated porcine xenograft bioprostheses is tearing of the valve cusps near their commissural attachment to the supporting stent. We have been examining aortic valves at the micromechanical level, and have developed several sensitive techniques to evaluate the biomechanical changes produced by the glutaraldehyde fixation process. Additionally, we have developed a mathematical modeling technique that stimulates valve function during the entire cardiac cycle. Our micromechanical tests have shown that compressive buckling is common to all fixed tissues, occurs at physiological bending curvatures, and is likely to be the primary mode of mechanical failure of bioprosthetic valves. We have also shown that existing glutaraldehyde fixation techniques inhibit the natural internal shearing of the valve cusps, and disable the interaction of the fibrosa and the ventricularis. With our modeling technique, we have shown that flexural stresses are indeed concentrated near the valve commissures, and that appropriate modifications of the supporting stent can reduce flexural deformations. With these new, more revealing techniques at hand, prospective valve designs can be better evaluated prior to large scale animals and clinical testing.
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Affiliation(s)
- I Vesely
- John P. Robarts Research Institute, London, Ontario, Canada
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15
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Abstract
The elastic response of aortic valve cusps is a summation of its fibrous components. To investigate the micromechanical function of valve leaflet constituents, we separated the fibrosa and the ventricularis from fresh and glutaraldehyde-fixed leaflets and tested them individually. The ventricularis was stiffer circumferentially than radially (7.41 kPa vs 3.68 kPa, p less than 0.00001) and was more extensible radially (62.7% vs 21.8% strain to high modulus phase, p less than 0.00001). The fibrosa was also stiffer circumferentially than radially (13.02 kPa vs 4.65 kPa, p less than 0.0008), but had uniform extensibility. Glutaraldehyde fixation did not affect the circumferential elastic modulus of the fibrosa, but reduced its radial modulus from 4.65 kPa to 2.32 kPa (p less than 0.0078). The elastic modulus of the ventricularis remained unchanged. Fixation also reduced the extensibility of the ventricularis circumferentially (from 21.8% to 15.2% strain, p less than 0.018), but not radially, and increased the radial extensibility of the fibrosa from 27.7% to 46.1% (p less than 0.0048). These data show that while the ventricularis contains a large amount of elastin, the amount of radially oriented collagen is similar to that of the fibrosa. The fibrosa, by itself, has the same extensibility in both directions (about 23% strain), but can extend much more radially when connected to the rest of the leaflet because it is attached to the ventricularis in a highly folded configuration. The two layers therefore complement each other during aortic valve function, and become detrimentally altered by fixation in glutaraldehyde.
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Affiliation(s)
- I Vesely
- John P. Robarts Research Institute, University of Western Ontario, London, Canada
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