Pathology of substitute heart valves: new concepts and developments

Poly-2-methyl-2-oxazoline–modified bioprosthetic heart valve leaflets have enhanced biocompatibility and resist structural degeneration

Bioprosthetic heart valves (BHV) fabricated from glutaraldehyde-fixed heterograft tissue, such as bovine pericardium (BP), are widely used for treating heart valve disease, a group of disorders that affects millions. Structural valve degeneration (SVD) of BHV due to both calcification and the accumulation of advanced glycation end products (AGE) with associated serum proteins limits durability. We hypothesized that BP modified with poly-2-methyl-2-oxazoline (POZ) to inhibit protein entry would demonstrate reduced accumulation of AGE and serum proteins, mitigating SVD. In vitro studies of POZ-modified BP demonstrated reduced accumulation of serum albumin and AGE. BP-POZ in vitro maintained collagen microarchitecture per two-photon microscopy despite AGE incubation, and in cell culture studies was associated with no change in tumor necrosis factor-α after exposure to AGE and activated macrophages. Comparing POZ and polyethylene glycol (PEG)–modified BP in vitro, BP-POZ was minimally affected by oxidative conditions, whereas BP-PEG was susceptible to oxidative deterioration. In juvenile rat subdermal implants, BP-POZ demonstrated reduced AGE formation and serum albumin infiltration, while calcification was not inhibited. However, BP-POZ rat subdermal implants with ethanol pretreatment demonstrated inhibition of both AGE accumulation and calcification. Ex vivo laminar flow studies with human blood demonstrated BP-POZ enhanced thromboresistance with reduced white blood cell accumulation. We conclude that SVD associated with AGE and serum protein accumulation can be mitigated through POZ functionalization that both enhances biocompatibility and facilitates ethanol pretreatment inhibition of BP calcification.

Noncalcific Mechanisms of Bioprosthetic Structural Valve Degeneration

Bioprosthetic heart valves (BHVs) largely circumvent the need for long‐term anticoagulation compared with mechanical valves but are increasingly susceptible to deterioration and reduced durability with reoperation rates of ≈10% and 30% at 10 and 15 years, respectively. Structural valve degeneration is a common, unpreventable, and untreatable consequence of BHV implantation and is frequently characterized by leaflet calcification. However, 25% of BHV reoperations attributed to structural valve degeneration occur with minimal leaflet mineralization. This review discusses the noncalcific mechanisms of BHV structural valve degeneration, highlighting the putative roles and pathophysiological relationships between protein infiltration, glycation, oxidative and mechanical stress, and inflammation and the structural consequences for surgical and transcatheter BHVs.

Ectopic mineralization in heart valves: new insights from in vivo and in vitro procalcific models and promising perspectives on noncalcifiable bioengineered valves

Ectopic calcification of native and bioprosthetic heart valves represents a major public health problem causing severe morbidity and mortality worldwide. Valve procalcific degeneration is known to be caused mainly by calcium salt precipitation onto membranes of suffering non-scavenged cells and dead-cell-derived products acting as major hydroxyapatite nucleators. Although etiopathogenesis of calcification in native valves is still far from being exhaustively elucidated, it is well known that bioprosthesis mineralization may be primed by glutaraldehyde-mediated toxicity for xenografts, cryopreservation-related damage for allografts and graft immune rejection for both. Instead, mechanical valves, which are free from calcification, are extremely thrombogenic, requiring chronic anticoagulation therapies for transplanted patients. Since surgical substitution of failed valves is still the leading therapeutic option, progressive improvements in tissue engineering techniques are crucial to attain readily available valve implants with good biocompatibility, proper functionality and long-term durability in order to meet the considerable clinical demand for valve substitutes. Bioengineered valves obtained from acellular non-valvular scaffolds or decellularized native valves are proving to be a compelling alternative to mechanical and bioprosthetic valve implants, as they appear to permit repopulation by the host's own cells with associated tissue remodelling, growth and repair, besides showing less propensity to calcification and adequate hemodynamic performances. In this review, insights into valve calcification onset as revealed by in vivo and in vitro procalcific models are updated as well as advances in the field of valve bioengineering.

A Standardized Dissection Protocol to Generate Aortic Valvular Scaffolds from Porcine Hearts

OBJECTIVE

To describe a particular harvesting procedure for isolating intact porcine aortic heart valve roots as potential sources for biologic scaffolds.

METHODS

Fresh porcine hearts were brought to the Tissue Engineering and Regenerative Medicine Laboratory at the University of Medicine and Pharmacy in Targu Mures. The aortic roots were extracted from the porcine hearts by anatomical dissection. For this purpose, we used a basic surgical instrument kit. This initial phase was the first step in obtaining acellular extracellular matrix as a biologic scaffold material.

RESULTS

Aortic roots were isolated with preservation of the ascending aorta as well as the intact aortic sinus and coronaries together with the adjacent myocardial tissue and anterior leaflet of the mitral valve. This approach allowed for safe mounting of roots into mounting rings for perfusion decellularization.

CONCLUSIONS

The described procedure is a feasible protocol for obtaining intact biological valvular scaffolds from porcine hearts. Reduced requirements regarding tools and personnel underline the easiness of aortic root harvesting using this particular procedure.

Paediatric nanofibrous bioprosthetic heart valve

The search for an optimal aortic valve implant with durability, calcification resistance, excellent haemodynamic parameters and ability to withstand mechanical loading is yet to be met. Thus, there has been struggled to fabricate bio-prosthetics heart valve using bioengineering. The consequential product must be resilient with suitable mechanical features, biocompatible and possess the capacity to grow. Defective heart valves replacement by surgery is now common, this improves the value and survival of life for a lot of patients. The recent paediatric heart valve implant is suboptimal due to their inability of somatic growth. They usually have multiple surgeries to change outgrown valves. Short-lived valve bio-prostheses occurring in older patients and younger ones who more usually need the replacement of its damaged heart with prosthesis led to a new invasive surgical interventions with an improved quality of life. The authors propose that nanofibre scaffold for paediatric tissue-engineered heart valve will meet most of these conditions, most particularly those related to somatic growth, and, as the nanofibre scaffold is eroded, new valve is produced, the valve matures in the child until adulthood.

Microstructural alterations owing to handling of bovine pericardium to manufacture bioprosthetic heart valves: A potential risk for cusp dehiscence

INTRODUCTION

Cross-linking and anti-calcification of prosthetic heart valves have been continuously improved to prevent degeneration and calcification. However, non-calcific structural deteriorations such as cuspal dehiscences along the stent still require further analysis.

MATERIAL AND METHOD

Based upon the previous analysis of an explanted valve after 7 years, a fresh commercial aortic valve was embedded in poly(methyl methacrylate) (PMMA) and cut into slices to ensure the detailed observation of the assembly and material structures. A pericardial patch embossed to provide the adequate shape of the cusps was investigated after paraffin embedding and appropriate staining. The microstructural damages that occurred during manufacturing process were identified and evaluated by light microscopy, polarized microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

RESULTS

The wavy collagen bundles, the key structure of the pericardium patch, were damaged to a great extent at suture sites along the stent and in the compressed areas around the stent post. The fixation of the embossed pericardium patch along the plots of the stent aggravated the microstructural modifications. The damages mainly appeared as the elimination of collagen bundle waviness and delamination between the bundles.

CONCLUSION

Considering the modes of failure of the explant, the damages to the collagen bundles may identify the vulnerable sites that play an important role in the cusp dehiscence of heart valve implants. Such information is important to the manufacturers. Recommendations to prevent in vivo cusp dehiscence can therefore be formulated.

Stabilisation of Collagen Sponges by Glutaraldehyde Vapour Crosslinking

Glutaraldehyde is a well-recognised reagent for crosslinking and stabilising collagens and other protein-based materials, including gelatine. In some cases, however, the use of solutions can disrupt the structure of the material, for example, by causing rapid dispersion or distortions from surface interactions. An alternative approach that has been explored in a number of individual cases is the use of glutaraldehyde vapour. In this study, the effectiveness of a range of different glutaraldehyde concentrations in the reservoir providing vapour, from 5% to 25% (w/v), has been explored at incubation times from 5 h to 48 h at room temperature. These data show the effectiveness of the glutaraldehyde vapour approach for crosslinking collagen and show that materials with defined, intermediate stability could be obtained, for example, to control resorption rates in vivo.

Hydrodynamic Assessment of Aortic Valves Prepared from Porcine Small Intestinal Submucosa

Infants and children born with severe cardiac valve lesions have no effective long term treatment options since currently available tissue or mechanical prosthetic valves have sizing limitations and no avenue to accommodate the growth of the pediatric patient. Tissue engineered heart valves (TEHVs) which could provide for growth, self-repair, infection resistance, and long-term replacement could be an ideal solution. Porcine small intestinal submucosa (PSIS) has recently emerged as a potentially attractive bioscaffold for TEHVs. PSIS may possess the ability to recruit endogenous cardiovascular cells, leading to phenotypically-matched replacement tissue when the scaffold has completely degraded. Our group has successfully implanted custom-made PSIS valves in 4 infants with critical valve defects in whom standard bioprosthetic or mechanical valves were not an option. Short term clinical follow-up has been promising. However, no hydrodynamic data has been reported to date on these valves. The purpose of this study was to assess the functional effectiveness of tri-leaflet PSIS bioscaffolds in the aortic position compared to standard tri-leaflet porcine bioprosthetic valves. Hydrodynamic evaluation of acute PSIS function was conducted using a left heart simulator in our laboratory. Our results demonstrated similar flow and pressure profiles (p > 0.05) between the PSIS valves and the control valves. However, forward flow energy losses were found to be significantly greater (p < 0.05) in the PSIS valves compared to the controls possibly as a result of stiffer material properties of PSIS relative to glutaraldehyde-fixed porcine valve tissue. Our findings suggest that optimization of valve dimensions and shape may be important in accelerating de novo valve tissue growth and avoidance of long-term complications associated with higher energy losses (e.g. left ventricular hypertrophy). Furthermore, long term animal and clinical studies will be needed in order to conclusively address somatic growth potential of PSIS valves.

Multiple-Step Injection Molding for Fibrin-Based Tissue-Engineered Heart Valves

Heart valves are elaborate and highly heterogeneous structures of the circulatory system. Despite the well accepted relationship between the structural and mechanical anisotropy and the optimal function of the valves, most approaches to create tissue-engineered heart valves (TEHVs) do not try to mimic this complexity and rely on one homogenous combination of cells and materials for the whole construct. The aim of this study was to establish an easy and versatile method to introduce spatial diversity into a heart valve fibrin scaffold. We developed a multiple-step injection molding process that enables the fabrication of TEHVs with heterogeneous composition (cell/scaffold material) of wall and leaflets without the need of gluing or suturing components together, with the leaflets firmly connected to the wall. The integrity of the valves and their functionality was proved by either opening/closing cycles in a bioreactor (proof of principle without cells) or with continuous stimulation over 2 weeks. We demonstrated the potential of the method by the two-step molding of the wall and the leaflets containing different cell lines. Immunohistology after stimulation confirmed tissue formation and demonstrated the localization of the different cell types. Furthermore, we showed the proof of principle fabrication of valves using different materials for wall (fibrin) and leaflets (hybrid gel of fibrin/elastin-like recombinamer) and with layered leaflets. The method is easy to implement, does not require special facilities, and can be reproduced in any tissue-engineering lab. While it has been demonstrated here with fibrin, it can easily be extended to other hydrogels.

Tissue-engineered fibrin-based heart valve with a tubular leaflet design

The general approach in heart valve tissue engineering is to mimic the shape of the native valve in the attempt to recreate the natural haemodynamics. In this article, we report the fabrication of the first tissue-engineered heart valve (TEHV) based on a tubular leaflet design, where the function of the leaflets of semilunar heart valves is performed by a simple tubular construct sutured along a circumferential line at the root and at three single points at the sinotubular junction. The tubular design is a recent development in pericardial (nonviable) bioprostheses, which has attracted interest because of the simplicity of the construction and the reliability of the implantation technique. Here we push the potential of the concept further from the fabrication and material point of view to realize the tube-in-tube valve: an autologous, living HV with remodelling and growing capability, physiological haemocompatibility, simple to construct and fast to implant. We developed two different fabrication/conditioning procedures and produced fibrin-based constructs embedding cells from the ovine umbilical cord artery according to the two different approaches. Tissue formation was confirmed by histology and immunohistology. The design of the tube-in-tube foresees the possibility of using a textile coscaffold (here demonstrated with a warp-knitted mesh) to achieve enhanced mechanical properties in vision of implantation in the aortic position. The tube-in-tube represents an attractive alternative to the conventional design of TEHVs aiming at reproducing the valvular geometry.

The tensile and viscoelastic properties of aortic valve leaflets treated with a hyaluronidase gradient

Purpose

When diseased, aortic valves are typically replaced with bioprosthetic heart valves (BPHVs), porcine valves or bovine pericardium that are fixed in glutaraldehyde. These replacements fail within 10-15 years due to calcification and fatigue, and their failure coincides with a loss of glycosaminoglycans (GAGs). This study investigates this relationship between GAG concentration and the tensile and viscoelastic properties of aortic valve leaflets.

Methods

Aortic valve leaflets were dissected from porcine hearts and digested in hyaluronidase in concentrations ranging from 0-5 U/mL for 0-24 hours, yielding a spectrum of GAG concentrations that was measured using the uronic acid assay and confirmed by Alcian Blue staining. Digested leaflets with varying GAG concentrations were then tested in tension in the circumferential and radial directions with varying strain rate, as well as in stress relaxation.

Results

The GAG concentration of the leaflets was successfully reduced using hyaluronidase, although water content was not affected. Elastic modulus, the maximum stress, and hysteresis significantly increased with decreasing GAG concentration. Extensibility and the radius of transition curvature did not change with GAG concentration. The stress relaxation behavior and strain-rate independent nature of the leaflet did not change with GAG concentration.

Conclusions

These results suggest that GAGs in the spongiosa lubricate tissue motion and reduce stresses experienced by the leaflet. This study forms the basis for predictive models of BPHV mechanics based on GAG concentration, and guides the rational design of future heart valve replacements.

Amniotic fluid-derived stem cells for cardiovascular tissue engineering applications

Recent research has demonstrated that a population of stem cells can be isolated from amniotic fluid removed by amniocentesis that are broadly multipotent and nontumorogenic. These amniotic fluid-derived stem cells (AFSC) could potentially provide an autologous cell source for treatment of congenital defects identified during gestation, particularly cardiovascular defects. In this review, the various methods of isolating, sorting, and culturing AFSC are compared, along with techniques for inducing differentiation into cardiac myocytes and endothelial cells. Although research has not demonstrated complete and high-yield cardiac differentiation, AFSC have been shown to effectively differentiate into endothelial cells and can effectively support cardiac tissue. Additionally, several tissue engineering and regenerative therapeutic approaches for the use of these cells in heart patches, injection after myocardial infarction, heart valves, vascularized scaffolds, and blood vessels are summarized. These applications show great promise in the treatment of congenital cardiovascular defects, and further studies of isolation, culture, and differentiation of AFSC will help to develop their use for tissue engineering, regenerative medicine, and cardiovascular therapies.

Effects of Leaflet Stiffness on In Vitro Dynamic Bioprosthetic Heart Valve Leaflet Shape

Advances in the development of replacement heart valves require a deeper understanding of the valve dynamics. In the present study, dynamic aortic valve (AV) leaflet geometries were quantified in vitro using a structured laser-light imaging system (Iyengar et al., ABME 29(11):963-973, 2001). Native AV leaflets were first imaged under simulated physiological flow conditions within a rigid glass conduit with simulated anatomic sinuses. Next, the valve/glass conduit combination was removed from the loop and immersed in a 0.625% aqueous glutaraldehyde solution at room temperature for 24 h to produce a bioprosthetic heart valve (BHV). The BHV leaflets were then re-imaged under identical flow conditions while kept in the same position in the glass conduit to minimize artifacts associated with removal/reinsertion of the valve. We observed that: (1) the native leaflet exhibited small, high frequency shifts in shape; (2) the BHV leaflet demonstrated a more stabile shape, as well as focal regions of prolonged, high curvature; (3) the BHV leaflet opened and closed faster by ~10 ms compared to native leaflet; (4) in both the BHV and native states, the AV opened from basal region leading to free edge (5) when closing, both the native and BHV close with both free edge and circumferential together. The high bending observed in the BHV leaflet correlated with known locations of tissue deterioration previously reported in our laboratory. Thus, in order to minimize leaflet tissue damage, methods of chemical modification utilized in BHVs that maintain leaflet flexibility are necessary to minimize the onset and progression of tissue damage. We conclude that leaflet stiffness can have a considerable effect on dynamic valve motion, and can induce deleterious bending behaviors that may be associated with tissue breakdown and valve failure. Moreover, these unique data can provide much needed quantitative information for computational simulation of heart valve leaflet stiffness on heart valve function.

Characterizing the collagen fiber orientation in pericardial leaflets under mechanical loading conditions

When implanted inside the body, bioprosthetic heart valve leaflets experience a variety of cyclic mechanical stresses such as shear stress due to blood flow when the valve is open, flexural stress due to cyclic opening and closure of the valve, and tensile stress when the valve is closed. These types of stress lead to a variety of failure modes. In either a natural valve leaflet or a processed pericardial tissue leaflet, collagen fibers reinforce the tissue and provide structural integrity such that the very thin leaflet can stand enormous loads related to cyclic pressure changes. The mechanical response of the leaflet tissue greatly depends on collagen fiber concentration, characteristics, and orientation. Thus, understating the microstructure of pericardial tissue and its response to dynamic loading is crucial for the development of more durable heart valve, and computational models to predict heart valves' behavior. In this work, we have characterized the 3D collagen fiber arrangement of bovine pericardial tissue leaflets in response to a variety of different loading conditions under Second-Harmonic Generation Microscopy. This real-time visualization method assists in better understanding of the effect of cyclic load on collagen fiber orientation in time and space.

Metal mesh scaffold for tissue engineering of membranes

Engineering of the membrane-like tissue structures to be utilized in highly dynamic loading environments such as the cardiovascular system has been a challenge in the past decade. Scaffolds are critical components of the engineered tissue membranes and allow them being formed in vitro and remain secure in vivo when implanted in the body. Several approaches have been taken to develop scaffolds for tissue membranes. However, all methods entail limitations due to structural vulnerability, short-term functionality, and mechanical properties of the resulted membrane constructs. To overcome these issues, we have developed a novel hybrid scaffold made of an extra thin layer of metal mesh tightly enclosed by biological matrix components. This approach retains all the advantages of using biological scaffolds while developing a strong extracellular matrix that can stand various types of loads after implantation inside the body.

Effects of cyclic flexural fatigue on porcine bioprosthetic heart valve heterograft biomaterials

Although bioprosthetic heart valves (BHV) remain the primary treatment modality for adult heart valve replacement, continued problems with durability remain. Several studies have implicated flexure as a major damage mode in porcine-derived heterograft biomaterials used in BHV fabrication. Although conventional accelerated wear testing can provide valuable insights into BHV damage phenomena, the constituent tissues are subjected to complex, time-varying deformation modes (i.e., tension and flexure) that do not allow for the control of the amount, direction, and location of flexure. Thus, in this study, customized fatigue testing devices were developed to subject circumferentially oriented porcine BHV tissue strips to controlled cyclic flexural loading. By using this approach, we were able to study layer-specific structural damage induced by cyclic flexural tensile and compressive stresses alone. Cycle levels of 10 x 10(6), 25 x 10(6), and 50 x 10(6) were used, with resulting changes in flexural stiffness and collagen structure assessed. Results indicated that flexural rigidity was markedly reduced after only 10 x 10(6) cycles, and progressively decayed at a lower rate with cycle number thereafter. Moreover, the against-curvature fatigue direction induced the most damage, suggesting that the ventricularis and fibrosa layers have low resistance to cyclic flexural compressive and tensile loads, respectively. The histological analyses indicated progressive collagen fiber delamination as early as 10 x 10(6) cycles but otherwise no change in gross collagen orientation. Our results underscore that porcine-derived heterograft biomaterials are very sensitive to flexural fatigue, with delamination of the tissue layers the primary underlying mechanism. This appears to be in contrast to pericardial BHV, wherein high tensile stresses are considered to be the major cause of structural failure. These findings point toward the need for the development of chemical fixation technologies that minimize flexure-induced damage to extend porcine heterograft biomaterial durability. (c) 2010 Wiley Periodicals, Inc. J Biomed Mater Res, 2010.

Bioengineering challenges for heart valve tissue engineering

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and enhances survival and quality of life for many patients. However, repairs of congenital deformities require very small valve sizes not commercially available. Further, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. It is believed that a tissue engineered heart valve can accommodate many of these requirements, especially those pertaining to somatic growth. This review provides an overview of the field of heart valve tissue engineering, including recent trends, with a focus on the bioengineering challenges unique to heart valves. We believe that, currently, the key bioengineering challenge is to determine how biological, structural, and mechanical factors affect extracellular matrix (ECM) formation and in vivo functionality. These factors are fundamental to any approach toward developing a clinically viable tissue engineered heart valve (TEHV), regardless of the particular approach. Critical to the current approaches to TEHVs is scaffold design, which must simultaneously provide function (valves must function from the time of implant) as well as stress transfer to the new ECM. From a bioengineering point of view, a hierarchy of approaches will be necessary to connect the organ-tissue relationships with underpinning cell and sub-cellular events. Overall, such approaches need to be structured to address these fundamental issues to lay the basis for TEHVs that can be developed and designed according to truly sound scientific and engineering principles.

Evaluation of hemolysis in patients with prosthetic heart valves

The primary mechanism and most common cause of hemolytic disease in patients with prosthetic heart valves are mechanical trauma to red blood cells and paraprosthetic valvular regurgitation, respectively. Presenting features in patients with this condition include anemia, congestive heart failure, fatigue, jaundice, dark urine, and a regurgitant murmur. Various laboratory studies can be utilized to diagnose hemolytic anemia and to assess the severity of hemolysis. Transthoracic echocardiography, transesophageal echocardiography, and Doppler studies including color Doppler are useful imaging methods to assess valve function. Treatment is usually medical (oral iron); however, in patients with paravalvular regurgitation, surgery is often required to correct the anemia.

In vivo biomechanical assessment of triglycidylamine crosslinked pericardium

While glutaraldehyde crosslinking is most often used to fabricate bioprosthetic heart valves (BHV) using heterograft tissues, it predisposes BHV to calcification and dramatically stiffens the heterograft tissues. Our group previously reported the synthesis and characterization of a novel epoxy-crosslinker, triglycidylamine (TGA). TGA pretreatment of BHV tissues compared to glutaraldehyde results in both calcification resistance in subdermal implants and improved leaflet compliance. In these prior studies, optimal calcification inhibition was noted with the combined use of TGA with mercapto-aminobisphosphonate (MABP). In the present study, we investigated the hypothesis that bovine pericardium cross-linked with TGA-MABP retains these beneficial biomechanical properties in vivo using a novel mitral valve anterior leaflet (MVAL) ovine valvuloplasty model. Bovine pericardial specimens were crosslinked with either glutaraldehyde or TGA-MABP, from which 1cm2 sections were implanted in the ovine MVAL after removal of the original tissue of the same size. An array of four sonomicrometry transducers were implanted on the corners and used to compute the complete in-surface strain tensor cardiac cycle over the cardiac cycle at 0 and 4 weeks. Following explant samples were fixed in formalin for histology studies. At 4 weeks both treatment groups experienced no dimensional changes in the unloaded state, indicating no shrinkage. When fully loaded during peak systolic ejection, TGA-MABP valvuloplasty patches were significantly more compliant, which did not change at 4 weeks. In contrast, the glutaraldehyde areal strain increased significantly by 4 weeks. Estimated implant stresses for both treatment groups, based on previously measured biomechanical properties [Connolly JM, Alferiev I, Clark-Gruel JN, Eidelman N, Sacks M, Palmatory E, et al. Triglycidylamine crosslinking of porcine aortic valve cusps or bovine pericardium results in improved biocompatibility, biomechanics, and calcification resistance: chemical and biological mechanisms. Am J Pathol 2005;166(1):1-13], were 40 and 250 kPa in the circumferential and radial directions, respectively, which are comparable to predicted BHV peak stress levels. We conclude that TGA-MABP crosslinked bovine pericardium, when subjected to in vivo BHV stress levels in a blood-contacting environment, maintains stable functionality.

Platelet responses to silicon-alloyed pyrolytic carbons

Pyrolytic carbon (PYC) containing approximately 7 wt % silicon is used in most clinical mechanical heart valves where it has demonstrated a high level of blood compatibility. The Si, present as SiC, is included since it is believed to enhance durability. However, it has been suggested that SiC reduces PYC blood compatibility. In the present study, PYC valve leaflets were prepared with low, conventional, and high levels of Si. The in vitro responses of human platelets to these materials were then quantified. Platelet responses were consistent with previous reports: Adherent platelets were extremely well spread, closely followed submicron contours, and formed very few aggregates or microthrombi-like structures. No significant differences with respect to the Si concentrations were observed for platelets adherent per unit area and the numbers of thrombi-like structures. Some differences were observed with platelet morphologies and the material surface covered with platelets, although these did not vary consistently with respect to Si concentration. These results indicate that lowering (or raising) the Si alloy concentration in PYC over a reasonable range (0.54-13.5 wt % as examined here) is unlikely to improve or otherwise alter the in vivo blood compatibility of this important clinical material.

An experimentally derived stress resultant shell model for heart valve dynamic simulations

In order to achieve a more realistic and accurate computational simulation of native and bioprosthetic heart valve dynamics, a finite shell element model was developed. Experimentally derived and uncoupled in-plane and bending behaviors were implemented into a fully nonlinear stress resultant shell element. Validation studies compared the planar biaxial extension and three-point bending simulations to the experimental data and demonstrated excellent fidelity. Dynamic simulations of a pericardial bioprosthetic heart valve with the developed shell element model showed significant differences in the deformation characteristics compared to the simulation with an assumed isotropic bending model. The new finite shell element model developed in the present study can also incorporate various types of constitutive models and is expected to help us to understand the complex dynamics of native and bioprosthetic heart valve function in physiological and pathological conditions.

Elastin calcification in the rat subdermal model is accompanied by up-regulation of degradative and osteogenic cellular responses

Calcification of vascular elastin occurs in patients with arteriosclerosis, renal failure, diabetes, and vascular graft implants. We hypothesized that pathological elastin calcification is related to degenerative and osteogenic mechanisms. To test this hypothesis, the temporal expression of genes and proteins associated with elastin degradation and osteogenesis was examined in the rat subdermal calcification model by quantitative real-time reverse transcription-polymerase chain reaction and specific protein assays. Purified elastin implanted subdermally in juvenile rats exhibited progressive calcification in a time-dependent manner along with fibroblast and macrophage infiltration. Reverse transcription-polymerase chain reaction analysis showed that relative gene expression levels of matrix metalloproteinases (MMP-2 and MMP-9) and transforming growth factor-beta1 were increased in parallel with calcification. Gelatin zymography showed strong MMP activities at early time points, which were associated with high levels of soluble elastin peptides. Gene expression of core binding factor alpha-1, an osteoblast-specific transcription factor, increased in parallel with elastin calcification and attained approximately 9.5-fold higher expression at 21 days compared to 3 days after implantation. Similarly, mRNA levels of the bone markers osteopontin and alkaline phosphatase also increased progressively, but osteocalcin levels remained unchanged. We conclude that degenerative and osteogenic processes may be involved in elastin calcification.

Quantitative Histological Examination of Bioprosthetic Heart Valves

BACKGROUND

The histological features that characterize infective endocarditis in bioprosthetic valves are not accurately defined. Moreover, bioprosthetic valves may have a noninfective, degenerative evolution associated with calcifications, vegetation-like lesions, and inflammatory infiltrates. Such histological findings may be misdiagnosed as infective endocarditis.

METHODS

Pathologic analysis of inflamed bioprosthetic valve tissues was conducted retrospectively for 21 patients who underwent surgical removal of a bioprosthetic valve because of suspected infective endocarditis and for 67 patients who underwent surgical removal of a bioprosthetic valve because of noninfective dysfunction. To better define the histological criteria for infective endocarditis, we used quantitative image analysis to compare these 2 groups of patients with respect to vegetations, calcifications, and patterns of inflammation.

RESULTS

Histologically, infective endocarditis in patients with bioprostheses was characterized by demonstration of microorganisms, vegetations, and neutrophil-rich, inflammatory infiltrates. Valve tissue specimens from patients whose bioprosthetic valves were removed because of noninfective complications showed, in 30% of cases, inflammatory infiltrates mainly composed of macrophages and lymphocytes. Inflammatory adherent thrombi that can occur to the surface of noninfective degenerative bioprostheses are differentiated because their vegetations have macrophage-rich content. A neutrophil surface area with a cutoff value of > or =1.5% of the total valve tissue surface area is highly specific (94%) for infective endocarditis.

CONCLUSIONS

When no microorganisms are detected and vegetations are not found in bioprosthetic valve tissues during the histological examination, a neutrophil-rich inflammation might better define the term "active endocarditis" in the Duke criteria and would allow differentiation between infective endocarditis and inflammatory, noninfective valve processes in patients with bioprosthetic valves.

Comparative study of the L-hydro process and glutaraldehyde preservation

Commercial bioprosthetic heart valves are commonly preserved in glutaraldehyde and are cytotoxic to host cells, preventing spontaneous endothelialization. The aim of this study was to demonstrate the potential for in vivo endothelialization of bioprostheses treated by the L-Hydro process which consists of mild extraction of antigenic substances and incorporation of antiinflammatory and antithrombotic agents. Seven stented porcine heart valves treated by the L-Hydro process and 3 glutaraldehyde-fixed porcine heart valves were implanted in the mitral position in juvenile sheep. The valves were evaluated by echocardiography, angiography, histology, and histochemistry. No hemodynamic differences were observed, but scanning and transmission electron microscopy showed nearly complete coverage by endothelial cells of all leaflets in the L-Hydro-treated valves after 5 months of implantation. The endothelial cells were in direct contact with the underlying collagen and expressed von Willebrand-related antigens. The surfaces of the glutaraldehyde-treated valves were covered by fibrin, macrophages, calcium, and thrombotic material; only sparse endothelial cells were observed and contact with the underlying tissue was incomplete. These data indicate that L-Hydro-treated porcine valves are capable of inducing spontaneous endothelialization.

Simulated Bioprosthetic Heart Valve Deformation under Quasi-Static Loading

For more than 40years, the replacement of diseased natural heart valves with prosthetic devices has dramatically extended the quality and length of the lives of millions of patients worldwide. However, bioprosthetic heart valves (BHV) continue to fail due to structural failure resulting from poor tissue durability and faulty design. Clearly, an in-depth understanding of the biomechanical behavior of BHV at both the tissue and functional prosthesis levels is essential to improving BHV design and to reduce rates of failure. In this study, we simulated quasi-static BHV leaflet deformation under 40, 80, and 120mmHg quasi-static transvalvular pressures. A Fung-elastic material model was used that incorporated material parameters and axes derived from actual leaflet biaxial tests and measured leaflet collagen fiber structure. Rigorous experimental validation of predicted leaflet strain field was used to validate the model results. An overall maximum discrepancy of 2.36% strain between the finite element (FE) results and experiment measurements was obtained, indicating good agreement between computed and measured major principal strains. Parametric studies utilizing the material parameter set from one leaflet for all three leaflets resulted in substantial variations in leaflet stress and strain distributions. This result suggests that utilization of actual leaflet material properties is essential for accurate BHV FE simulations. The present study also underscores the need for rigorous experimentation and accurate constitutive models in simulating BHV function and design.

Application of tissue-engineering principles toward the development of a semilunar heart valve substitute

Heart valve disease is a significant medical problem worldwide. Current treatment for heart valve disease is heart valve replacement. State of the art replacement heart valves are less than ideal and are associated with significant complications. Using the basic principles of tissue engineering, promising alternatives to current replacement heart valves are being developed. Significant progress has been made in the development of a tissue-engineered semilunar heart valve substitute. Advancements include the development of different potential cell sources and cell-seeding techniques; advancements in matrix and scaffold development and in polymer chemistry fabrication; and the development of a variety of bioreactors, which are biomimetic devices used to modulate the development of tissue-engineered neotissue in vitro through the application of biochemical and biomechanical stimuli. This review addresses the need for a tissue-engineered alternative to the current heart valve replacement options. The basics of heart valve structure and function, heart valve disease, and currently available heart valve replacements are discussed. The last 10 years of investigation into a tissue-engineered heart valve as well as current developments are reviewed. Finally, the early clinical applications of cardiovascular tissue engineering are presented.

Cyclic loading response of bioprosthetic heart valves: effects of fixation stress state on the collagen fiber architecture

Biologically derived, chemically modified collagenous tissues are being increasingly used to fabricate cardiac valve prostheses and as biomaterials in cardiovascular repair. A stress-free state during chemical modification has been shown to preserve the collagen fiber architecture of the native tissue, potentially preserving native mechanical properties and improving prostheses durability. However, it is not known if the native collagen fiber architecture is stable during long-term in vivo operation. To address this question, we obtained porcine aortic valves chemically treated at (i) 0 mmHg transvalvular pressure (with 40 mmHg aortic pressure) and (ii) 4 mmHg transvalvular pressure, then subjected the valves to 0, 1 x 10(6), 50 x 10(6), and 200 x 10(6) in vitro accelerated wear testing (AWT) cycles. The resulting changes in collagen fiber architecture were quantified using small angle light scattering analysis (SALS). SALS measurements indicated that collagen fibers in the 0 mmHg pressure-fixed leaflets became more aligned between 1 x 10(6) and 50 x 10(6) AWT cycles. In contrast, only minor changes (not statistically significant) in collagen fiber orientation occurred in the 4 mmHg pressure-fixed valvular tissue with cycling. It was also noted that although the 0 mmHg group was fixed without transvalvular pressure, distention of the root induced significant changes in collagen structure of the leaflets. Overall, our observations suggest that the native collagen fiber crimp of the 0 mmHg pressure-fixed leaflets were rapidly lost after only 50 x 10(6) AWT cycles (equivalent to approximately 1.6 patient years) and thus may not be maintained over a sufficient period of time to be clinically beneficial. Further, the collagen structure of the native aortic valve is exquisitely sensitive to dimensional change in the aortic root-independent of the presence of transvalvular pressure. Our findings also suggest that without in vivo remodeling, any collagenous tissue used to fabricate BHV may undergo similar degenerative, irreversible changes in vivo.

Involvement of matrix metalloproteinases and tenascin-C in elastin calcification

Elastin degeneration and calcification occur in many cardiovascular diseases, including medial arterial elastocalcinosis, atherosclerosis, and bioprosthetic heart valve mineralization. In the present study, we tested the hypothesis that the onset and progression of elastin-oriented calcification is associated with matrix remodeling and elastin degradation events. We studied whether aluminum ions inhibit elastin calcification by reducing elastin degradation and altering remodeling events. Subdermal implantation of pure elastin in juvenile rats resulted in a time-dependent calcification of elastin, reaching high levels 21 days after implantation. In situ hybridization showed that elastin calcification was associated with an up-regulation of matrix metalloproteinase (MMP) mRNA expression, specifically MMP-9 and MMP-2. Gelatin zymography demonstrated increased MMP-9 and MMP-2 enzyme activities in early stages of elastin calcification. Calcified elastin displayed a time-dependent pattern of tenascin-C (TN-C) and alkaline phosphatase (AP) expression. Pretreatment of pure elastin with aluminum ions prior to implantation resulted in complete inhibition of elastin calcification. Aluminum ion binding to elastin was found to protect elastin against MMP-mediated degradation in vitro. Noncalcified, explanted aluminum-pretreated elastin exhibited reduced activities of MMPs. TN-C expression in elastin implants exhibited a time-dependent pattern that was also affected by pretreatment of elastin with aluminum ions. In conclusion, elastin calcification is accompanied by matrix remodeling events, and the efficacy of aluminum pretreatment in inhibiting elastin calcification may be related in part to its effects on elastin remodeling.

Extracellular matrix degrading enzymes are active in porcine stentless aortic bioprosthetic heart valves

Glutaraldehyde-fixed porcine aortic valve tissues are widely used for heart valve replacement surgery in the form of bioprosthetic heart valves (BHVs). The durability of BHVs in the clinical setting is limited by tissue degeneration, mechanical failure, and calcification. BHVs rely on the putative ability of glutaraldehyde to render biologic tissues metabolically inert and fully resistant to enzymatic attack. In the present study, we detected and partially characterized the activity of collagen and elastin-degrading enzymes in unimplanted, glutaraldehyde-fixed porcine aortic cusp and wall tissues and compared enzyme activities with those extracted from fresh tissues. Active enzymes capable of degrading extracellular matrix were found to be present in soluble form as well as immobilized on glutaraldehyde-crosslinked tissue matrix. Total levels of collagenolytic activities were evaluated to approximately 0.25 microg of degraded collagen/mg of dry tissue/24 h for both glutaraldehyde-fixed wall and cusp tissues. A major finding of this study was the ability of soluble tissue enzymes to partially degrade glutaraldehyde-fixed collagen and particularly large amounts of glutaraldehyde-fixed elastin. These calcium-dependent gelatinases share many biochemical similarities with matrix metalloproteinases. These data strongly indicate that glutaraldehyde-fixed porcine valvular tissues are not metabolically inert and are not entirely resistant to enzymatic attack, thereby rendering BHVs vulnerable to biologic degeneration and subsequent chronic failure.

Biaxial mechanical response of bioprosthetic heart valve biomaterials to high in-plane shear

Utilization of novel biologically-derived biomaterials in bioprosthetic heart valves (BHV) requires robust constitutive models to predict the mechanical behavior under generalized loading states. Thus, it is necessary to perform rigorous experimentation involving all functional deformations to obtain both the form and material constants of a strain-energy density function. In this study, we generated a comprehensive experimental biaxial mechanical dataset that included high in-plane shear stresses using glutaraldehyde treated bovine pericardium (GLBP) as the representative BHV biomaterial. Compared to our previous study (Sacks, JBME, v.121, pp. 551-555, 1999), GLBP demonstrated a substantially different response under high shear strains. This finding was underscored by the inability of the standard Fung model, applied successfully in our previous GLBP study, to fit the high-shear data. To develop an appropriate constitutive model, we utilized an interpolation technique for the pseudo-elastic response to guide modification of the final model form. An eight parameter modified Fung model utilizing additional quartic terms was developed, which fitted the complete dataset well. Model parameters were also constrained to satisfy physical plausibility of the strain energy function. The results of this study underscore the limited predictive ability of current soft tissue models, and the need to collect experimental data for soft tissue simulations over the complete functional range.

Loss of chondroitin 6-sulfate and hyaluronan from failed porcine bioprosthetic valves

Explanted porcine bioprosthetic valves have a thinned spongiosa, partially because of an overall loss of glycosaminoglycans (GAGs). We measured the concentrations of specific GAG classes in explanted bioprosthetic valves (n = 14, implanted 12.0 +/- 4.7 years) compared with glutaraldehyde-fixed porcine controls. After extraction with NaOH, GAGs were analyzed using either a hexuronic acid assay or fluorophore-assisted carbohydrate electrophoresis to quantify the individual GAG classes. The total GAG concentration in explants was 198 +/- 95 pmol/mg wet weight-93% less than freshly fixed controls. Explants also contained altered proportions of the different GAG classes relative to controls. The proportions of hyaluronan and chondroitin/dermatan-6-sulfate were reduced from 39 to 7% and 34 to 18% of total GAGs, respectively. The predominant explant GAG class was chondroitin/dermatan-4-sulfate (proportion elevated from 14 to 70%). This GAG is commonly found in the collagen-associated proteoglycan decorin, which is likely well crosslinked by glutaraldehyde. Chondroitin-6-sulfate is commonly found in the water- and hyaluronan-binding proteoglycan versican, which is likely poorly crosslinked. The loss of versican and its associated water-binding capacity is consistent with the thinned spongiosa. The resultant compromise of hydration, compressive resistance, and viscoelasticity may be responsible for the deterioration of the bioprosthesis in vivo.

Collagen fiber disruption occurs independent of calcification in clinically explanted bioprosthetic heart valves

The durability of bioprosthetic heart valves (BHV) is severely limited by tissue deterioration, manifested as calcification and mechanical damage to the extracellular matrix. Extensive research on mineralization mechanisms has led to prevention strategies, but little work has been done on understanding the mechanisms of noncalcific matrix damage. The present study tested the hypothesis that calcification-independent damage to the valvular structural matrix mediated by mechanical factors occurs in clinical implants and could contribute to porcine aortic BHV structural failure. We correlated quantitative assessment of collagen fiber orientation and structural integrity by small angle light scattering (SALS) with morphologic analysis in 14 porcine aortic valve bioprostheses removed from patients for structural deterioration following 5-20 years of function. Calcification of the explants varied from 0 (none) to 1+ (minimal) to 4+ (extensive), as assessed radiographically. SALS tests were performed over entire excised cusps using a 0.254-mm spaced grid, and the resultant structural information used to generate maps of the local collagen fiber damage that were compared with sites of calcific deposits. All 42 cusps showed clear evidence of substantial noncalcific structural damage. In 29 cusps that were calcified, structural damage was consistently spatially distinct from the calcification deposits, generally in a distribution similar to that noted in porcine BHV subjected to in vitro durability testing. Our results suggest a mechanism of noncalcific degradation dependent on cuspal mechanics that could contribute to porcine aortic BHV failure.

Tissue characterization and calcification potential of commercial bioprosthetic heart valves

BACKGROUND

Tissue properties may contribute to intrinsic calcification of bioprosthetic heart valves. Phospholipids have been proposed as potential nucleation sites for calcification. Other tissue properties might also be important in calcification.

METHODS

Commercial and control bioprosthetic valve tissues were characterized by shrinkage temperature, moisture content, free amine content, phospholipid content, and calcification level after 90-day rat subcutaneous implantation as described.

RESULTS

Shrinkage temperature, moisture content, and free amine content were typical for glutaraldehyde-cross-linked tissues. Phospholipid and calcium levels varied considerably among valve types. There was a significant correlation between phospholipid levels and calcification (r = 0.63, p = 0.04). Sulzer Carbomedics Mitroflow and Toronto SPV valve tissues had significantly more calcification than other commercial bioprostheses in this study (p < 0.01). Carpentier-Edwards Duraflex, CE SAV, and CE PERIMOUNT valve tissues had significantly less calcification than Medtronic Mosaic in this animal model (p < 0.02).

CONCLUSIONS

Processes that reduce phospholipid levels are associated with reduced calcification in the rat subcutaneous model. Significant differences in calcification level were found among commercially available valves. The clinical significance of these results is unknown.

Fifteen-year clinical experience with the biocor porcine bioprostheses in the mitral position

BACKGROUND

Bioprosthetic valve use represents a crucial improvement in surgical treatment of mitral valve disease. The aim of this study is to determine the long-term durability of the Biocor porcine bioprosthetic mitral valve.

METHODS

Between 1985 and 1989, a total of 158 Biocor porcine bioprosthetic valves were placed in the mitral position, and long-term results of these patients were investigated retrospectively in 1999.

RESULTS

Thirty-day mortality was 4.4% (7 patients). Total follow-up was 1,499 patient-years. Actuarial survival was 83.66% +/- 3% at 5 years, 77.78% +/- 3.36% at 13 years (1.8% patient-year). Multivariate analysis demonstrated younger age, duration of implantation, congestive heart failure, and functional class to be significant predictors of late mortality. Actuarial freedom from valve-related mortality was 98.58% +/- 1% at 15 years (0.13% patient-year). Actuarial freedom from structural valve deterioration was 95.49% +/- 1.8% at 5 years, 70.2% +/- 4.12% at 10 years, and 64.82% +/- 5.34% at 13 years (2.6% patient-year). Actuarial freedom from structural valve deterioration-related reoperation was 98.43% +/- 1.1% at 5 years, 89.15% +/- 2.85% at 10 years, and 76.82% +/- 7.91% at 14 years. Multivariate analysis showed younger age and duration of implantation to be significant predictors of structural valve deterioration and its related reoperation.

CONCLUSIONS

By studying a 15-year time period, it is seen that this new generation porcine bioprosthetic valve should be considered an alternative for mechanical valves in selected patients.

Development and characterization of tissue-engineered aortic valves

Tissue-engineered aortic valves, known as recellularized heart valves, were developed by seeding human neonatal fibroblasts onto decellularized, porcine aortic valves. Recellularized heart valves were cultured up to 8 weeks in a novel bioreactor that imposed dynamic pulsatile fluid flow to expose the dermal fibroblasts to mechanical forces. Our data showed that, under static or dynamic flow conditions, dermal fibroblasts attached to and migrated into the decellularized, porcine valve scaffolding. The human cells remained viable as indicated by MTT viability staining. Gradual colonization of the decellularized porcine scaffolding by the human dermal fibroblasts was shown histologically by hematoxylin & eosin staining, immunocytochemically using a monoclonal antibody directed against prolyl-4-hydroxylase (an intracellular enzyme expressed by human fibroblasts synthesizing collagen), and quantitative digital image analyses. Thymidine and proline radiolabeled analog studies at 1, 2 and 4 weeks of individual leaflets cultured statically demonstrated that the human fibroblasts were mitotic and synthesized human extracellular matrix proteins, thereby supplementing the existing porcine matrix. The overall approach results in a heart valve populated with viable human cells. In the development of valves that perform in a similar manner as natural biological structures, this approach may present some unique benefits over current medical therapies.

Staphylococcus aureus, Platelets, and the Heart

Infective endocarditis (IE) caused by Staphylococcus aureus is serious, burgeoning frequency, and growing increasingly resistant to antibiotics. S. aureus IE is associated with high morbidity and mortality rates in nosocomial and community-acquired settings. S. aureus is the most common, most virulent IE etiologic pathogen. S. aureus IE pathogenesis depends upon complex interaction among the pathogen, platelets, plasma proteins, and vascular endothelial cells. S. aureus coordinates the expression of key virulence factors required for the specific pathogenic phases of IE. Platelets, now appear to play an important role in antimicrobial host defense against S. aureus IE and other endovascular infections. Platelet microbicidal proteins are believed to significantly contribute to the antimicrobial properties of platelets; however, abnormal disposition of native or prosthetic cardiac valves is an important risk factor in S. aureus IE establishment and severity. Thus, the need to define the molecular mechanisms of S. aureus pathogenesis and host defense against IE is urgent. Understanding these mechanisms will yield new approaches for the prevention and treatment of such life-threatening cardiovascular infections due to S. aureus.

Bioprosthetic heart valve leaflet motion monitored by dual camera stereo photogrammetry

Dual camera stereo photogrammetry (DCSP) was applied to investigate the leaflet motion of bioprosthetic heart valves (BHVs) in a physiologic pulse flow loop (PFL). A 25-mm bovine pericardial valve was installed in the aortic valve position of the PFL, which was operated at a pulse rate of 70 beats/min and a cardiac output of 5 l/min. The systolic/diastolic aortic pressure was maintained at 120/80 mmHg to mimic the physiologic load experienced by the aortic valve. The leaflet of the test valve was marked with 80 India ink dots to form a fan-shaped matrix. From the acquired image sequences, 3-D coordinates of the marker matrix were derived and hence the surface contour, local mean and Gaussian curvatures at each opening and closing phase during one cardiac cycle were reconstructed. It is generally believed that the long-term failure rate of BHV is related to the uneven distribution of mechanical stresses occurring in the leaflet material during opening and closing. Unfortunately, a quantitative analysis of the leaflet motion under physiological conditions has not been reported. The newly developed technique permits frame-by-frame mapping of the leaflet surface, which is essential for dynamic analysis of stress-strain behavior in BHV.

Carpentier-Edwards standard porcine bioprosthesis: a 21-year experience

BACKGROUND

The Carpentier-Edwards standard porcine bioprosthesis experience to 21 years has been evaluated to determine the influence of structural valve deterioration by valve position in various age groupings.

METHODS

From 1975 to 1988, 1,181 patients had the prosthesis implanted in 1,198 procedures. The mean age of the population was 57.9+/-12.5 years (range, 21 to 85 years). Aortic valve replacement was performed in 564 patients (47.8%); mitral valve replacement, 478 (40.5%); and multiple valve replacement, 132 (11.2%). Concomitant procedures were performed in 337 patients (28.5%), and 140 (11.9%) had previous operations.

RESULTS

The early mortality was 8.1% (97), only 0.4% (5) of which were valve-related. The total follow-up was 10,405 years (mean, 8.8+/-5.2 years). The late mortality was 5.7%/patient-year (591), with the valve-related component 1.6%/patient-year (168) with a 13% reoperative mortality (56). The linearized rate of structural valve deterioration was 3.8%/patient-year (395), with overall complications of 7.1%/patient-year (737). The overall survival at 20 years was 17.2%+/-3.1% (p < 0.05; aortic valve replacement greater than mitral valve replacement or multiple valve replacement). The freedom from structural valve deterioration was, at 18 years, 25.8%+/-2.8% overall, AVR 40.6%+/-4.2%, and MVR 8.5%+/-3.4% (p < 0.05, aortic valve replacement greater than mitral valve replacement or multiple valve replacement). The freedom from structural valve deterioration for aortic valve replacement was, at 15 years, for patients older than 70 years, 82.9%+/-9.1%; 61 to 70 years, 73.2%+/-4.7%; 51 to 60 years, 58.8%+/-5.4%; 41 to 50 years, 41.7%+/-8.2%; and 21 to 40 years, 25.5%+/-7.8%. The freedom from structural valve deterioration for mitral valve replacement was, at 15 years, for patients older than 70 years, 89.8%+/-7.6%; 61 to 70 years, 22.8%+/-6.3%; 51 to 60 years, 26.3% +/- 5.7%; 41 to 50 years, 11.7%+/-5.6%; and 21 to 40 years, 7.0%+/-4.7%.

CONCLUSIONS

The prosthesis is recommended for aortic valve replacement for patients older than 70 years and for patients 61 to 70 years (when extended longevity is not anticipated) and for mitral valve replacement for patients older than 70 years.

Comparison of different anticalcification treatments for stentless bioprostheses

BACKGROUND

New anticalcificant treatments have been developed because tissue calcification is a major contributing factor for bioprosthetic valve failure.

METHODS

Aortic valve leaflet and aortic root tissue samples from stentless bioprostheses treated with No-React (Biocor, Belo Horizonte, Brazil), AOA (Medtronic freestyle, Minneapolis, MN), and BiLinx (St. Jude Medical, St. Paul, MN) were compared to a control group by subcutaneous implantation in 60 male weanling Sprague-Dawley rats.

RESULTS

Calcium levels were in the range of 0.3 to 2.2 mg/g dry tissue at 3 and 12 weeks in all three treated aortic valve leaflet implants. The BiLinx treatment proved anticalcificant effectiveness on aortic root samples as well. There were statistically significant differences for valve leaflet tissue samples: No-React = AOA < BiLinx < < Control and for aortic root tissue samples: BiLinx < < AOA < Control = No-React.

CONCLUSION

Calcification of aortic valve leaflets was significantly reduced by all new anticalcificant treatments. Inhibition of cellular calcification (BiLinx) resulted in additional reduction of aortic root calcification. Maximum anticalcificant properties upon both leaflet and aortic root is important as these are considered a functional unit in stentless bioprostheses.

Aortic valve homografts in adults: a clinical perspective

Aortic valve replacement is a lifesaving measure in patients with severe aortic valve disease. In the United States, the most commonly used prostheses are the mechanical and bioprosthetic valves. With mechanical valves, long-term anticoagulation is necessary because of high thrombogenic potential. Bioprosthetic valves have a relatively high incidence of structural failure, especially in younger patients. Aortic valve homografts, derived from human heart donors or autopsy material, provide an alternative to mechanical or animal valves. The advantages of the homograft in comparison with the mechanical prostheses are the low incidence of thromboembolism without anticoagulation and lower valvular gradients in smaller sizes. Homografts are relatively resistant to endocarditis and are the valve of choice during active endocarditis. Their major mode of failure has been aortic regurgitation; however, recent advances in preservation and operative techniques have decreased this problem. Whether implantation of an aortic valve homograft should be the procedure of choice in subsets of patients remains controversial. Herein we review the history, techniques, results, complications, and current indications for aortic valve homografts.

Optimal bovine pericardial tissue selection sites. II. Cartographic analysis

In Part I of this work we used small-angle light scattering (SALS) to quantify the fiber architecture of 20 bovine pericardial sacs, along with corresponding tissue-thickness measurements, to determine optimal material selection sites. In order to determine the anatomic consistency of these sites, the fiber architecture and thickness data from all 20 sacs were averaged together using a cartographic analysis method that took advantage of the geometry of the prolate spheroid mold used to process the sacs. Optimal selection sites were determined based on a local criteria where all fiber preferred directions within a 2.54-cm circular area were within +/- 10 degrees. The largest contiguous area (LCA) for the entire BP sac was 20.54 cm2, located in the vicinity of the left ventricle of the heart. The LCA tissue thicknesses were also relatively uniform, further supporting the use of these areas. However, even within these optimal areas there was a +/- 20 degrees standard deviation in local fiber preferred directions, resulting in at best a 40 degrees spread in local preferred directions. The observed structural variability may be due to regionally heterogeneous physiologic loadings induced by the ligamentous attachments. These attachments may alter the regional fiber preferred orientation to support local mechanical loadings. Overall, given the inherent structural variability of the BP sac, we conclude that use of anatomic location alone will not consistently guarantee the selection of tissue specimens with a highly homogeneous and predictable fibrous structure. It is thus suggested that a direct fiber measurement presorting method be employed when selecting BP specimens for bioprosthetic applications where tissue structural homogeneity and uniformity is critical.