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

Benchtop characterization of the tricuspid valve leaflet pre-strains

The pre-strains of biological soft tissues are important when relating their in vitro and in vivo mechanical behaviors. In this study, we present the first-of-its-kind experimental characterization of the tricuspid valve leaflet pre-strains. We use 3D photogrammetry and the reproducing kernel method to calculate the pre-strains within the central 10×10 mm region of the tricuspid valve leaflets from n=8 porcine hearts. In agreement with previous pre-strain studies for heart valve leaflets, our results show that all the three tricuspid valve leaflets shrink after explant from the ex vivo heart. These calculated strains are leaflet-specific and the septal leaflet experiences the most compressive changes. Furthermore, the strains observed after dissection of the central 10×10 mm region of the leaflet are smaller than when the valve is explanted, suggesting that our computed pre-strains are mainly due to the release of in situ annulus and chordae connections. The leaflets are then mounted on a biaxial testing device and preconditioned using force-controlled equibiaxial loading. We show that the employed preconditioning protocol does not 100% restore the leaflet pre-strains as removed during tissue dissection, and future studies are warranted to explore alternative preconditioning methods. Finally, we compare the calculated biomechanically oriented metrics considering five stress-free reference configurations. Interestingly, the radial tissue stretches and material anisotropies are significantly smaller compared to the post-preconditioning configuration. Extensions of this work can further explore the role of this unique leaflet-specific leaflet pre-strains on in vivo valve behavior via high-fidelity in-silico models.

In-vivo heterogeneous functional and residual strains in human aortic valve leaflets

Residual and physiological functional strains in soft tissues are known to play an important role in modulating organ stress distributions. Yet, no known comprehensive information on residual strains exist, or non-invasive techniques to quantify in-vivo deformations for the aortic valve (AV) leaflets. Herein we present a completely non-invasive approach for determining heterogeneous strains - both functional and residual - in semilunar valves and apply it to normal human AV leaflets. Transesophageal 3D echocardiographic (3DE) images of the AV were acquired from open-heart transplant patients, with each AV leaflet excised after heart explant and then imaged in a flattened configuration ex-vivo. Using an established spline parameterization of both 3DE segmentations and digitized ex-vivo images (Aggarwal et al., 2014), surface strains were calculated for deformation between the ex-vivo and three in-vivo configurations: fully open, just-coapted, and fully-loaded. Results indicated that leaflet area increased by an average of 20% from the ex-vivo to in-vivo open states, with a highly heterogeneous strain field. The increase in area from open to just-coapted state was the highest at an average of 25%, while that from just-coapted to fully-loaded remained almost unaltered. Going from the ex-vivo to in-vivo mid-systole configurations, the leaflet area near the basal attachment shrank slightly, whereas the free edge expanded by ~10%. This was accompanied by a 10° -20° shear along the circumferential-radial direction. Moreover, the principal stretches aligned approximately with the circumferential and radial directions for all cases, with the highest stretch being along the radial direction. Collectively, these results indicated that even though the AV did not support any measurable pressure gradient in the just-coapted state, the leaflets were significantly pre-strained with respect to the excised state. Furthermore, the collagen fibers of the leaflet were almost fully recruited in the just-coapted state, making the leaflet very stiff with marginal deformation under full pressure. Lastly, the deformation was always higher in the radial direction and lower along the circumferential one, the latter direction made stiffer by the preferential alignment of collagen fibers. These results provide significant insight into the distribution of residual strains and the in-vivo strains encountered during valve opening and closing in AV leaflets, and will form an important component of the tool that can evaluate valve׳s functional properties in a non-invasive manner.

On the biomechanical role of glycosaminoglycans in the aortic heart valve leaflet

While the role of collagen and elastin fibrous components in heart valve valvular biomechanics has been extensively investigated, the biomechanical role of the glycosaminoglycan (GAG) gelatinous-like material phase remains unclear. In the present study, we investigated the biomechanical role of GAGs in porcine aortic valve (AV) leaflets under tension utilizing enzymatic removal. Tissue specimens were removed from the belly region of porcine AVs and subsequently treated with either an enzyme solution for GAG removal or a control (buffer with no enzyme) solution. A dual stress level test methodology was used to determine the effects at low and high (physiological) stress levels. In addition, planar biaxial tests were conducted both on-axis (i.e. aligned to the circumferential and radial axes) and at 45° off-axis to induce maximum shear, to explore the effects of augmented fiber rotations on the fiber-fiber interactions. Changes in hysteresis were used as the primary metric of GAG functional assessment. A simulation of the low-force experimental setup was also conducted to clarify the internal stress system and provide viscoelastic model parameters for this loading range. Results indicated that under planar tension the removal of GAGs had no measureable affect extensional mechanical properties (either on- or 45° off-axis), including peak stretch, hysteresis and creep. Interestingly, in the low-force range, hysteresis was markedly reduced, from 35.96±2.65% in control group to 25.00±1.64% (p<0.001) as a result of GAG removal. Collectively, these results suggest that GAGs do not play a direct role in modulating the time-dependent tensile properties of valvular tissues. Rather, they appear to be strongly connected with fiber-fiber and fiber-matrix interactions at low force levels. Thus, we speculate that GAGs may be important in providing a damping mechanism to reduce leaflet flutter when the leaflet is not under high tensile stress.

Changes in the mechanical properties of chemically treated bovine pericardium after a short uniaxial cyclic test

The mechanical behavior of calf pericardium, a biomaterial utilized in the manufacture of cardiac bioprostheses, in response to a short tensile cyclic test has been evaluated. The trial involved 120 samples cut longitudinally or transversely, subjected to 10 cycles until a stress of between 1 and 3 MPa was reached. Tests of hardness and tear propagation were performed, and the results were compared with a control series. The energy loss was also computed, and it was approximately 10-fold greater in the first cycle than the loss in the subsequent nine cycles. Despite this singularity, they correlated very precisely. The effect of the direction in which the tissue is cut on energy loss was not significant nor the difference between hardness prior to and after testing. The results of the tear propagation tests gave no statistical differences prior to and after testing. From the obtained results, it seems that the test carried out does not affect significantly the mechanical properties of calf pericardium.

On the in vivo deformation of the mitral valve anterior leaflet: effects of annular geometry and referential configuration

Alteration of the native mitral valve (MV) shape has been hypothesized to have a profound effect on the local tissue stress distribution, and is potentially linked to limitations in repair durability. The present study was undertaken to elucidate the relation between MV annular shape and central mitral valve anterior leaflet (MVAL) strain history, using flat annuloplasty in an ovine model. In addition, we report for the first time the presence of residual in vivo leaflet strains. In vivo leaflet deformations were measured using sonocrystal transducers sutured to the MVAL (n = 10), with the 3D positions acquired over the full cardiac cycle. In six animals a flat ring was sutured to the annulus and the transducer positions recorded, while in the remaining four the MV was excised from the exsanguinated heart and the stress-free transducer positions obtained. In the central region of the MVAL the peak stretch values, referenced to the minimum left ventricular pressure (LVP), were 1.10 ± 0.01 and 1.31 ± 0.03 (mean ± standard error) in the circumferential and radial directions, respectively. Following flat ring annuloplasty, the central MVAL contracted 28% circumferentially and elongated 16% radially at minimum LVP, and the circumferential direction was under a negative strain state during the entire cardiac cycle. After valve excision from the exsanguinated heart, the MVAL contracted significantly (18 and 30% in the circumferential and radial directions, respectively), indicating the presence of substantial in vivo residual strains. While the physiological function of the residual strains (and their associated stresses) are at present unknown, accounting for their presence is clearly necessary for accurate computational simulations of MV function. Moreover, we demonstrated that changes in annular geometry dramatically alter valvular functional strains in vivo. As levels of homeostatic strains are related to tissue remodeling and homeostasis, our results suggest that surgically introduced alterations in MV shape could lead to the long term MV mechanobiological and microstructural alterations that could ultimately affect MV repair durability.

Valvular endothelial cells and the mechanoregulation of valvular pathology

Endothelial cells are critical mediators of haemodynamic forces and as such are important foci for initiation of vascular pathology. Valvular leaflets are also lined with endothelial cells, though a similar role in mechanosensing has not been demonstrated. Recent evidence has shown that valvular endothelial cells respond morphologically to shear stress, and several studies have implicated valvular endothelial dysfunction in the pathogenesis of disease. This review seeks to combine what is known about vascular and valvular haemodynamics, endothelial response to mechanical stimuli and the pathogenesis of valvular diseases to form a hypothesis as to how mechanical stimuli can initiate valvular endothelial dysfunction and disease progression. From this analysis, it appears that inflow surface-related bacterial/thrombotic vegetative endocarditis is a high shear-driven endothelial denudation phenomenon, while the outflow surface with its related calcific/atherosclerotic degeneration is a low/oscillatory shear-driven endothelial activation phenomenon. Further understanding of these mechanisms may help lead to earlier diagnostic tools and therapeutic strategies.

A new method of estimating gauge length for porcine aortic valve test specimens

Porcine aortic valve (PAV) cusps are folded and wrinkled in the in vitro state. In the tensile testing of PAV specimens, estimating gauge length (the length at which a specimen starts to offer measurable resistance to load) is often difficult and subjective. We have therefore developed a new method for estimating the gauge length of such tissues. The method is based on the observation that the specimen's gauge length can be associated with a stationary point on the slope of its load-length curve if loaded from a wrinkled state, or a state of slight compression. We represented the load-length response of test specimens in the low-load, high-compliance region by a cubic function and determined the stationary point on the slope of the function using elementary calculus. The cubic function representation is fine-tuned by reducing or expanding an originally selected "test region" until the correlation coefficient of the cubic fit is maximized. The new method was applied to data obtained from the tensile testing of strips of heart valve tissue and was found to be objective, repeatable and robust.

European experience with the Mosaic bioprosthesis

OBJECTIVE

The purpose of this study was to prospectively evaluate the clinical and hemodynamic performance of the Mosaic bioprosthesis (Medtronic, Inc, Minneapolis, Minn).

METHODS

The stented porcine bioprosthesis combines the amino-oleic acid antimineralization treatment and the zero-pressure differential fixation technique for improved tissue durability. From February 1994 to May 1999, a total of 561 patients underwent valve replacement with the Mosaic bioprosthesis at 5 centers in Europe: 461 in the aortic and 100 in the mitral position. There were 261 women and 300 men; mean age at implantation was 70 years (range, 23-89 years). Mean follow-up was 2.9 years (range, 0-6.2 years), with a total follow-up of 1710.1 patient-years.

RESULTS

Postoperative mortality was 4.2% per patient-year, including a valve-related mortality of 0.4% per patient-year. The freedom from event rates in the aortic position at 5 years and in the mitral position at 4 years were, respectively, 96.6% +/- 1.1% and 94.9% +/- 3.3% for primary thromboembolism, 96.4% +/- 5.0% and 87.1% +/- 4.8% for antithromboembolic-related hemorrhage, 99.1% +/- 0.5% and 100% for thrombosed prosthesis, 98.8% +/- 1.2% and 100% for structural valve deterioration, 98.8% +/- 0.7% and 100% for nonstructural dysfunction, 98.4% +/- 0.6% and 94.4% +/- 3.8% for endocarditis, and 95.4% +/- 1.6% and 95.3% +/- 3.7% for explant and reoperation. Mean pressure gradient values at 5 years ranged from 7.5 to 15.9 mm Hg in the aortic position and at 4 years from 2.0 to 6.9 mm Hg in the mitral position across all valve sizes.

CONCLUSIONS

Clinical and hemodynamic performance of the Mosaic bioprosthesis were very satisfactory during the first 6 years after clinical introduction.

Biaxial mechanical properties of the natural and glutaraldehyde treated aortic valve cusp--Part I: Experimental results

To date, there are no constitutive models for either the natural or bioprosthetic aortic valve (AV), in part due to experimental complications related to the AV's small size and heterogeneous fibrous structure. In this study, we developed specialized biaxial testing techniques for the AV cusp, including a method to determine the local structure-strain relationship to assess the effects of boundary tethering forces. Natural and glutaraldehyde (GL) treated cusps were subjected to an extensive biaxial testing protocol in which the ratios of the axial tensions were held at constant values. Results indicated that the local fiber architecture clearly dominated cuspal deformation, and that the tethering effects at the specimen boundaries were negligible. Due to unique aspects of cuspal fiber architecture, the most uniform region of deformation was found at the lower portion as opposed to the center of the cuspal specimen. In general, the circumferential strains were much smaller than the radial strains, indicating a profound degree of mechanical anisotropy, and that natural cusps were significantly more extensible than the GL treated cusps. Strong mechanical coupling between biaxial stretch axes produced negative circumferential strains under equibiaxial tension. Further, the large radial strains observed could not be explained by uncrimping of the collagen fibers, but may be due to large rotations of the highly aligned, circumferential-oriented collagen fibers in the fibrosa. In conclusion, this study provides new insights into the AV cusp's structure-function relationship in addition to requisite data for constitutive modeling.

Assessment of pericardium in cardiac bioprostheses. A review

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.

Medtronic mosaic porcine bioprosthesis satisfactory early clinical performance

BACKGROUND

The Medtronic (Minneapolis, MN) Mosaic porcine bioprosthesis is an investigational prosthesis which incorporates zero-pressure fixation, aortic root predilation, low profile stent, and alpha oleic acid antimineralization treatment.

METHODS

From September 1994 to August 1996, 289 patients (mean age 70 years, range, 28 to 88 years) had 227 (78.5%) aortic valve replacements and 62 (21.5%) mitral valve replacements. Concomitant procedures were performed in 61.2% (139) of aortic valve replacements and 54.8% (34) of mitral valve replacements. Of the aortic valve replacement group 70 (30.8%) were in the 61 to 70 age group and 134 (59.0%) were 71 years or older. Of the mitral valve replacements, 23 (37.1%) were 61 to 70 years and 30 (48.4%) 71 years or older.

RESULTS

The early mortality, overall, was 4.2% (12 of 289); for aortic valve replacement it was 4.0% (9) and for mitral valve replacement it was 4.8% (3). The late mortality for aortic valve replacement was 2.6% per patient-year (3 events, 1.3% of total) and for mitral valve replacement it was 3.3% per patient-year (one event, 1.6% of total). The reoperative rate for aortic valve replacement was 3.0% per patient-year (4), while there were no mitral valve replacement reoperations. The freedom from major thromboembolism was 97.3%+/-1.6% for aortic valve replacement and 94.7%+/-3.0% for mitral valve replacement at 1 to 1.5 years. The freedom from reoperation was 96.7%+/-1.7% for aortic valve replacement; there was no reoperation for mitral valve replacement. There were no cases of structural valve deterioration. In the aortic position the mean systolic gradient was low, approximately 11 mm Hg, across all sizes (range 8 to 12 mm Hg at 3 months and 10 to 13 mm Hg at 12 months). In the mitral position the mean diastolic gradient was approximately 5 mm Hg (range, 2 to 6 mm Hg) for all sizes 25 to 31 mm at the early and 1 year follow-up echocardiographic assessment.

CONCLUSIONS

The early clinical performance and in vivo hemodynamics are encouraging.

The aortic valve microstructure: effects of transvalvular pressure

We undertook this study to establish a more quantitative understanding of the microstructural response of the aortic valve cusp to pressure loading. Fresh porcine aortic valves were fixed at transvalvular pressures ranging from 0 mmHg to 90 mmHg, and small-angle light scattering (SALS) was used to quantify the gross fiber structure of the valve cusps. At all pressures the fiber-preferred directions coursed along the circumferential direction. Increasing transvalvular pressure induced the greatest changes in fiber alignment between 0 and 1 mmHg, with no detectable change past 4 mmHg. When the fibrosa and ventricularis layers of the cusps were re-scanned separately, the fibrosa layer revealed a higher degree of orientation while the ventricularis was more randomly oriented. The degree of fiber orientation for both layers became more similar once the transvalvular pressure exceeded 4 mmHg, and the layers were almost indistinguishable by 60 mmHg. It is possible that, in addition to retracting the aortic cusp during systole, the ventricularis mechanically may contribute to the diastolic cuspal stiffness at high transvalvular pressures, which may help to prevent over distention of the cusp. Our results suggest a complex, highly heterogeneous structural response to transvalvular pressure on a fiber level that will have to be duplicated in future bioprosthetic heart valve designs.

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.

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.

New concepts in the design and use of biological prosthetic valves

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.

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.