Bovine jugular vein conduit: What affects its elastomechanical properties and thermostability?

The aim of this study was to compare the mechanical properties and thermal stability of the venous wall depending on the treatment method used, and, accordingly, on those structural changes in the tissue that this treatment causes. Bovine jugular vein walls (BJVWs) cross-linked with glutaraldehyde (GA), ethylene glycol diglycidyl ether (DE), and Contegra commercial conduit were evaluated using uniaxial stretching [with and without pre-conditioning (PreC)], differential scanning calorimetry, amino acid analysis, and attenuated total reflection infrared spectroscopy. Fresh BJVW was used as a control. It was shown that failure stress in non-PreC GA-treated and DE-treated materials was lower than that in fresh and Contegra counterparts. Contegra samples were the stiffest among the tested materials. Cyclic preloading leads to distortion of the mechanical behavior of this material, which is heterogeneous in composition and structure. The denaturation temperatures (T_d ) of all cross-linked BJVWs were higher than the T_d of the fresh vein. The microstructures of the tested BJVWs did not exhibit any differences, but the cross-linking density and hydration of the DE-vein were the highest. GA-cross-linking or DE-cross-linking and isopropanol exposure (Contegra) changed the protein secondary structures of the tested materials in different ways. We hypothesized that the protein secondary structure and hydration degree are the main causes of differences in the mechanical properties and thermal stability of BJVW.

Assessment of an Epoxy-Fixed Pericardial Patch with or without Ionically Bound Heparin in a Canine Model

In this study, an epoxy-fixed porcine pericardia I patch with or without ionically bound heparin was evaluated in a canine model as an alternative to the glutaraldehyde-fixed biological patch for clinical applications. To evaluate the effectiveness of this epoxy-fixed patch, a composite membrane composed of: an epoxy-fixed porcine patch with ionically bound heparin; a glutaraldehyde-fixed porcine patch with ionically bound heparin; an ePTFE polymeric patch; a polyester polymeric patch; an epoxy-fixed porcine patch without ionically bound heparin; and a glutaraldehyde-fixed porcine patch without ionically bound heparin was made. This membrane was assessed orthopically in a canine model. The early results (1-4 weeks post implant) revealed that the biological patches with ionically bound heparin had the mildest tissue reactions (inflammatory reaction, fibrosis, and adhesion) among all the test samples. However, by 12 weeks postoperatively, all the test samples had mild to severe tissue reactions. The order of tissue reactions with increasing severity was: the biological patches with ionically bound heparin, the biological patches without ionically bound heparin, and the polymeric patches. The results suggest that heparin may be used to reduce adhesion. Additionally, the epoxy-fixed tissue caused a relatively lower degree of inflammatory reaction than the glutaraldehyde-fixed tissue.

Contegra®conduit: current outcomes and concerns

A variety of congenital cardiac anomalies with severe right ventricular outflow tract (RVOT) obstruction or RVOT interruption require surgical reconstruction from the infundibulum up to the pulmonary artery bifurcation or even into the branches of the pulmonary arteries. Ideally, the conduit or valve required for such reconstruction has to be formed of autologous tissue that grows, resists infection, lasts for the life span of the patient and is readily available in all sizes. Such conduits, however, are not available and although several alternatives have been used, none of which are without potential drawbacks. Contegra valved bovine internal jugular vein conduit (Medtronic Inc., MN, USA) has recently emerged as a promising option for pediatric RVOT reconstruction and has been advocated for its 'off-the-shelf' availability in sizes ranging from 12 to 22 mm, surgical pliability and encouraging short- and mid-term success in experimental animal, as well as clinical studies. This review focuses on the current outcomes of Contegra conduit and highlights some of the major concerns related to the use of this conduit and strategies to tackle these concerns.

Percutaneous heart valve replacement: histology and calcification characteristics of biological valved stents in juvenile sheep

INTRODUCTION

Percutaneous techniques to replace the pulmonary valve are emerging as an alternative to congenital cardiac surgical procedures. Promising experimental and early clinical results have been reported so far, focusing on technical feasibility and valved stent function. The present study aimed to describe the micropathology after experimental percutaneous valve replacement.

METHODS

Self-expanding nitinol stents carrying a valved bovine jugular vein were transfemorally implanted into the pulmonary position of nine sheep. After 3 months of survival, macro- and micropathological examinations were carried out using standard staining techniques and immunohistochemistry. Additionally, calcification characteristics were determined by X-ray examinations and von Kossa stainings.

RESULTS

Six of nine animals survived the 3-month study time with good angiographic and echocardiographic function. All valves were grossly functional at the time of explantation. Slight fibrous overgrowth was seen at the inflow portions of two valved stents. No cuspal perforations or intracuspal hematomas were observed. Light microscopy proved the absence of cellular inflammatory infiltrates in any tissue samples. The myocardium directly proximal to the stent appeared structurally normal without calcification. The overall structure of the native pulmonary artery was well preserved with few mineral deposits spread diffusely throughout the wall distal to the stent. Massive calcification appeared in the bovine jugular-vein wall together with increased numbers of T lymphocytes. Neither calcific deposits in the cusps nor extrinsic mineralization was noted.

CONCLUSION

For the first time, micropathologic evaluation of percutaneously implanted heart valves is described. The results demonstrate that calcification of valved stents occurs in the wall portions without affecting the cusps. The cardiac structures in the vicinity had normal histology without inflammation.

Bovine Valved Xenograft in Pulmonary Position: Medium-Term Follow-Up With Excellent Hemodynamics and Freedom From Calcification

BACKGROUND

This study was designed to evaluate the outcome of Contegra xenograft valved conduit (Contegra, Medtronic Inc, Minneapolis, MN).

METHODS

From April 1999 to December 2003, 67 patients with a mean age of 16.1 +/-15.0 years (2 months to 53 years) and a mean weight of 39.7 +/- 27.1 kg (4 to 95 kg) were discharged after implantation of a Contegra conduit. The diagnosis contained the following: pulmonary valve replacement during Ross operation (n = 27), pulmonary valve regurgitation (n = 9), tetralogy of Fallot (n = 7), pulmonary atresia with ventricular septal defect (n = 7), double outlet right ventricle (n = 7), truncus arteriosus (n = 5), Taussig-Bing (n = 2), obstructed conduit (n = 2), and double discordance (n = 1). Conduit size was 14 mm in 2, 16 mm in 7, 18 mm in 12, 20 mm in 13, and 22 mm in 33 patients. Mean cardiopulmonary bypass was 155 +/- 48 min (65 to 337 min) and mean aortic cross clamping was 69 +/- 38 min (0 to 146 min). All patients underwent echocardiography, 23 of 67 (34%) patients had cardiac catheterization, and 23 of 67 (34%) patients had electrocardiograph-gated multislice computer tomography.

RESULTS

In a mean follow-up of 26.4 months (1 to 56 months) there was one late death (1 of 67 patients; 1.5% mortality) unrelated to the conduit. Five patients underwent reoperation; four were nonconduit-related and one was to replace a twisted conduit. Five patients underwent interventional cardiology; three were nonconduit-related and two were to stent a twisted or stenotic conduit. Echocardiography showed absent valve regurgitation in 30 of 67 (45%) patients, trivial in 21 of 67 (31%) patients, mild in 16 of 67 (24%) patients. The transconduit pressure gradient remained stable during follow-up, with peak pressure gradient 17 +/- 11 mm Hg and mean gradient 8 +/- 6 mm Hg. Internal diameters corresponded to 110% +/- 20% of the implanted diameter at level of proximal anastomosis, 112% +/- 18% at valve level, and 110% +/- 14% at distal anastomosis. Calcifications were not found, with the exception of a minimal (2.3 mm) parietal calcification.

CONCLUSIONS

The Contegra valved conduit provided excellent morphology and hemodynamics, and freedom from calcification in a medium-term follow-up.

Reconstruction of the right ventricular outflow tract with a bovine jugular vein graft fixed with a naturally occurring crosslinking agent (genipin) in a canine model

OBJECTIVE

This study was designed to evaluate a newly developed biologic valved conduit fixed with genipin used to reconstruct the right ventricular outflow tract in a canine model.

METHODS

Fresh bovine jugular veins with a retained native valve procured from a slaughterhouse were used as raw materials to fabricate the valved conduits. A naturally occurring crosslinking agent, genipin, was used to fix the procured jugular veins. The glutaraldehyde-fixed counterpart was used as a control. A canine model was used in the study.

RESULTS

Echocardiography revealed that the motion of the valvular leaflets in both the glutaraldehyde- and genipin-fixed conduits was satisfactory. The transvalvular pressure gradients of both studied groups were minimal. No endothelium-like cells were observed on the luminal surface of the conduit and the valvular leaflet for the glutaraldehyde-fixed group throughout the entire course of the study. In contrast, endothelium-like cells were observed on the entire surface of the genipin-fixed valved conduit retrieved at 6 months postoperatively in all the cases studied. There was no evidence of luminal fibrous peel in any the valved conduits studied. Degradation of valvular leaflet in one of the glutaraldehyde-fixed conduits was observed. In this particular case, thrombus formation was also observed on the surface of the valvular leaflet. On the other hand, no apparent degradation or thrombus formation was observed on the surfaces of the genipin-fixed valvular leaflet and conduit. A significantly more severe inflammatory reaction was observed for the glutaraldehyde-fixed conduit than for its genipin-fixed counterpart throughout the entire course of the study. The calcium contents of the samples before implantation and those retrieved at distinct implantation duration were minimal for both the glutaraldehyde- and genipin-fixed tissues.

CONCLUSION

Although further studies are necessary, the genipin-fixed valved conduit appears to have great potential in helping mitigate the complications observed in the commercially available conduits.

Acellular vascular tissues: natural biomaterials for tissue repair and tissue engineering

Various research groups around the world are actively investigating cardiovascular prostheses of biological origin. This review article discusses the need for such bioprosthetics and the potential role for natural tissues in cardiovascular applications such as cardiac valves and vascular grafts. Upon implantation, unmodified natural materials are subject to chemical and enzymatic degradation, seriously decreasing the life of the prosthesis. Therefore, methods such as glutaraldehyde and polyepoxide crosslinking treatments and dye-mediated photooxidation have been developed to stabilize the tissue while attempting to maintain its natural mechanical properties. Also, residual cellular components in a bioprosthetic material have been associated with undesired effects, such as calcification and immunological recognition, and thus have been the motivation for various decellularization processes. The effects of these stabilization and decellularization treatments on mechanical, biological and chemical properties of treated tissues have been investigated, specifically with regard to calcification, immunogenicity, and cytotoxicity concerns. Despite significant advances in the area of cardiovascular prostheses, there has yet to be developed a completely biocompatible, long-lasting implant. However, with the recent advent of tissue engineering, the possibility of applying selective cell seeding to naturally derived bioprosthetics moves us closer to a living tissue replacement.

Degradation potential of biological tissues fixed with various fixatives: an in vitro study

The purpose of this study was to investigate the in vitro degradation potential of porcine pericardia fixed with various aldehyde or epoxy compound (EC) fixatives, using bacterial collagenase and pronase. The fixatives investigated were formaldehyde (FA), glutaraldehyde (GA), monofunctional EC (EX-131), and multifunctional ECs (EX-810, EX-313, and EX-512). Fresh porcine pericardium was used as a control. The test samples were well immersed in a 20-U/mL collagenase solution or a 10-U/mL pronase solution and incubated at 37 degrees C at pH 7.5 for 24 h. The extent of degradation of each test sample was determined by measuring its increment in free amino group content and changes in collagen structure, denaturation temperature, and tensile stress after degradation. In general, the extent of tissue degradation with pronase was more notable than with collagenase. As observed with fresh tissue, the EX-131 EC fixed tissue radically disintegrated after either collagenase or pronase degradation, whereas the other test samples remained intact. The reason for this may reside in the more random molecular packing of the EX-131 EC-fixed tissue, which led to some loss in its helical integrity. This made penetration of enzymes into biological tissue easier. Of the multifunctional EC test groups, tissues fixed with tetrafunctional EC (EX-521) or trifunctional EC (EX-313) had relatively better resistance to degradation than those fixed with bifunctional EC (EX-810). The extent of degradation for the EX-313 or EX-512 EC fixed tissues was similar to that observed for the FA- or GA-fixed tissues. The results of this study indicated that the biological tissue fixed with monofunctional EC (EX-131) cannot resist bacterial collagenase or pronase degradation. However, resistance to degradation of the multifunctional EC (EX-313 or EX-152)-fixed tissues was comparable to that of the aldehyde (FA or GA)-fixed tissues. Therefore, of various EC fixatives, the EC with a greater number of functional groups should be chosen for tissue fixation to increase its resistance to enzymatic degradation.

In vitro study of enzymatic degradation of biological tissues fixed by glutaraldehyde or epoxy compound

The study, using bacterial collagenase, was to investigate the changes in characteristics of a collagen-rich tissue, porcine pericardium, fixed by glutaraldehyde or epoxy compound (ethylene glycol diglycidyl ether) during the course of degradation. Fresh porcine pericardium was used as a control. During degradation, the heat released by the reaction of collagenase with a test sample was monitored by a highly sensitive microcalorimeter. Also, the degree of degradation of each test sample was determined by measuring its increment in free amino group content and changes in denaturation temperature and tensile strength. Microcalorimetric analysis of collagenase degradation of fresh, epoxy-fixed, and glutaraldehyde-fixed tissues revealed that the heat released during degradation correlates well with the degree of tissue degraded. The cleaving of peptide bonds in biological tissue by collagenase degradation may increase its free amino group content and reduce its denaturation temperature and tensile strength. It was noted that the fresh tissue cannot resist bacterial collagenase degradation, while the glutaraldehyde-fixed tissue had a relatively better resistance to degradation than its epoxy-fixed counterpart.

Evaluation of an epoxy-fixed biological patch with ionically bound heparin as a pericardial substitute

In an attempt to develop an improved pericardial substitute, we undertook the development of an epoxy-fixed biological patch with ionically bound heparin. The study was to evaluate the cross-linking characteristics of this newly developed biological patch using its glutaraldehyde-fixed counterpart as a control. In addition, the feasibility of using this newly developed biological patch as a pericardial substitute was assessed in a canine model. In the study, it was observed that the epoxy-fixed biological patch appeared more similar to the native pericardium in colour and was more pliable than its glutaraldehyde-fixed counterpart. Also, both the epoxy- and glutaraldehyde-fixed biological patches had significant increases in fixation index and denaturation temperature as compared to the fresh one (p < 0.05). In the canine study, the epoxy-fixed biological patch with ionically bound heparin was found to have significantly less adhesion formation than those currently used clinically (p < 0.05).

Crosslinking characteristics of an epoxy-fixed porcine tendon: effects of pH, temperature, and fixative concentration

Injury to the anterior cruciate ligament (ACL) often results in functional instability within the knee joint and may induce a severe articular deterioration. Clinically, these patients may require an ACL reconstruction. In an attempt to develop an improved ligament prosthesis, a prototype xenograft--an epoxy-fixed porcine Achilles tendon--was developed. The study was intended to investigate the crosslinking characteristics of the epoxy-fixed porcine tendons fixed at different pHs (4.0, 7.4, 9.0, and 10.5), temperatures (4, 25, and 37 degrees C), and fixative concentrations (1, 2, or 4%). Samples of each group were taken out at various elapsed fixation periods. The crosslinking characteristics-fixation index, denaturation temperature, and moisture content-of each sample were determined. Additionally, the stiffness of the epoxy-fixed tendons under various fixation conditions was qualitatively compared. In the study it was noted that the color of porcine Achilles tendons remained natural after epoxy fixation. With increasing pH, temperature, or fixative concentration, the fixation indices and denaturation temperatures of the epoxy-fixed tendons increased. However, the fixed porcine tendons tended to be stiffer at a higher pH, temperature, or fixative concentration. The moisture contents of the epoxy-fixed tendons were relatively lower than the fresh ones, while they were approximately the same at different pHs, temperatures, and fixative concentrations. The implications of these findings for the epoxy-fixed porcine tendons in ACL reconstruction require further investigation.

Cross-linking characteristics of biological tissues fixed with monofunctional or multifunctional epoxy compounds

The cross-linking characteristics of biological tissues fixed by various epoxy compounds with different chemical structures (i.e. number of epoxide functional groups and backbone length) were investigated. Generally speaking, the tissues fixed with monofunctional fixatives were more pliable than those fixed with multifunctional fixatives. The fixation indices of the fixed tissues did not seem to be affected by the chemical structures of the fixatives. However, the number of functional groups and backbone length of the fixatives did play an important role in influencing the denaturation temperatures of the fixed tissues. In general, the denaturation temperatures of the multifunctional fixed tissues were higher than those of the monofunctional fixed tissues. Among the monofunctional fixed tissues, it was found that increasing the backbone length of the fixative decreased the denaturation temperature of the fixed tissue. This effect was present until the backbone length of the fixative became large enough.

Crosslinking characteristics of porcine tendons: effects of fixation with glutaraldehyde or epoxy

Anterior cruciate ligament (ACL) injuries, if left untreated, often produce significant disability in the athletically active population. Currently, autogenous tissue is the most commonly used substitute for ACL reconstruction because its immunogenicity is virtually nonexistent. However, the functional amount of autogenous tissue available for transplantation is limited. Additionally, this transplantation procedure may create a defect at the donor site, which can result in functional disability. To address these concerns, a prototype xenograft ligament prosthesis, epoxy-fixed porcine Achilles tendon, was developed. This study was intended to investigate the crosslinking characteristics of the epoxy-fixed porcine tendon. The fresh and glutaraldehyde-fixed porcine Achilles tendons were used as controls. Fresh porcine Achilles tendons procured from a slaughterhouse were used to fabricate the ligament prostheses. A 4% epoxy (ethylene glycol diglycidyl ether) solution or a 0.625% glutaraldehyde solution was employed to fix the porcine tendons. Samples of each group were taken out at various elapsed fixation periods. The crosslinking characteristics- denaturation temperature, moisture content, and fixation index-of each sample were then determined. In the study, it was learned that the crosslinking rate for the glutaraldehyde fixation was faster than that for the epoxy fixation. While the denaturation temperatures and the fixation indices for both studied groups were higher than for the fresh one, the denaturation temperature of the glutaraldehyde-fixed tendon was relatively higher than its epoxy-fixed counterpart. However, the fixation index and the moisture content for both studied groups were comparable. Also, it was noted that the epoxy-fixed tendon appeared more natural as compared to its glutaraldehyde-fixed counterpart. The implications of these findings for the epoxy-fixed tendon in the clinical ACL reconstruction require further investigation.

Studies on epoxy compound fixation

Bioprostheses derived from collagenous tissues have to be fixed and subsequently sterilized before they can be implanted in humans. Clinically, the most commonly used fixative is glutaraldehyde. However, the tendency for glutaraldehyde to markedly alter tissue stiffness and promote tissue calcification are well-recognized drawbacks of this fixative. To address the deficiencies with the glutaraldehyde-fixed tissue, a new fixative, epoxy compound, was used to fix biological prostheses. The study was undertaken to investigate the fixation rates and crosslinking densities of biological tissues fixed with various epoxy compounds. These epoxy compounds are different in their chemical structures. Glutaraldehyde was used as a control. The fixation rates and crosslinking densities of the fixed tissues were determined by measuring their fixation indices and denaturation temperatures, respectively. Generally, the epoxy-fixed tissues were more pliable than the glutaraldehyde-fixed one. Furthermore, the tissues fixed with monofunctional epoxy compound were more pliable than those fixed with multifunctional epoxy compounds. With increasing pH or temperature, the fixation rate of epoxy compound increased. However, the number of epoxide functional groups did not seem to affect the fixation rate of the epoxy compound. The fixation rate of glutaraldehyde was faster than that of epoxy compounds. Additionally, the crosslinking density of the glutaraldehyde-fixed tissue was greater than that of the epoxy-fixed counterparts. Moreover, it was noted that the denaturation temperatures of the tissues fixed with glutaraldehyde or multifunctional epoxy compounds were significantly higher than the fresh ones (p < 0.05), while that fixed with monofunctional epoxy compound stayed roughly the same throughout the entire fixation process (p > 0.05). The results obtained in this study may be used to optimize the fixation process for developing bioprostheses fixed with epoxy compounds.

Biological materials fixed with an epoxy compound: comparison of the effects with or without ionically bound heparin

Biological materials have been used as prosthetic devices such as heart valves, vascular grafts, and pericardial patches. These biological materials have to be fixed with crosslinking reagents and sterilized subsequently before they can be implanted in humans. Recently, a new crosslinking reagent, epoxy compound, has been used to fix bioprostheses. In this fixation technique, heparin may be ionically bound on the tissue surface. It has been shown that the amount of heparin bound to the tissue surface is proportional to the quantity of protamine impregnated in the biological tissues. However, it is not known if the impregnation of protamine will affect the crosslinking density of the biological tissues. This study was designed to compare the crosslinking densities of the epoxy compound fixed biological tissues with or without heparinization. Fresh porcine aortic valves procured from a slaughter house were first impregnated in various concentrations of protamine sulfate (0, 0.5, 1.0, or 1.5%) for about 30 min. The porcine aortic valves were then crosslinked in a 4% epoxy compound solution (Denacol EX-313). The porcine samples were taken out at various elapsed fixation periods: 18, 25, 48, 72, 96, and 120 h. Finally, the crosslinked porcine aortic valves were heparinized in a 0.5% sodium heparin solution for about 1 h. The crosslinking densities of the porcine leaflet and the aortic wall of each sample were determined by measuring their shrinkage temperatures. It was revealed that the impregnation of various concentrations of protamine did not seem to significantly alter the shrinkage temperatures of the porcine leaflet and the aortic wall throughout the entire fixation process (p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)