Detoxification process for glutaraldehyde-treated bovine pericardium: biological, chemical and mechanical characterization.

BACKGROUND AND AIMS OF THE STUDY: Glutaraldehyde cross-linked bovine pericardium was post-treated with homocysteic acid in order to bond unreacted glutaraldehyde groups, and preserved in aldehyde-free solution to reduce the toxicity and calcification potential of the tanned tissue. The study aim was to verify the efficacy and preservation of chemical and mechanical properties of pericardial tissue treated in this way. METHODS: Detoxification efficacy was tested by measuring free aldehyde groups and by in vitro cultivation of human endothelial cells on pericardial samples. Material stability tests included mechanical characterization, shrink temperature measurement, pronase digestion resistance and amino acid analysis after acid hydrolysis. The effect of treatment on fatigue resistance was evaluated by monitoring changes in permeability and compliance caused by 15 x 10(6) cycles of pulsatile stress. RESULTS: Detoxified samples showed negligible glutaraldehyde residues and significantly improved endothelial cell proliferation compared with conventionally treated samples. Chemical and mechanical properties were similar in detoxified and glutaraldehyde-treated samples. Tissue fatigue behavior was not modified by homocysteic acid treatment. CONCLUSION: Homocysteic acid treatment effectively reduces toxicity but does not affect the stability of glutaraldehyde cross-linked pericardium.

Wear patterns in the Sorin Bicarbon mechanical heart valve: a clinical explant study.

BACKGROUND AND AIMS OF THE STUDY: Mechanical heart valve prostheses are subject to wear due to impact and friction between the occluder(s) and valve housing. Reference data on the extent of wear in vivo is lacking. Therefore, the aim of this study was to describe the wear pattern in Sorin Bicarbon valves explanted from humans for comparison with the findings established in an earlier in vitro study. METHODS: We investigated eight valves explanted from seven patients, which had been functioning for more than six months and were returned to the Sorin Company for investigation. The protocol comprised: (i) initial visual inspection on receipt; (ii) stereomicroscopy after cleaning and disassembly; (iii) computed planimetry of the worn areas in the housing; (iv) pivot surface profilometry; and (v) scanning electron microscopy. RESULTS: All wear depths and exposed areas of titanium were less than that predicted from in vitro figures. No valves were explanted due to mechanical failure and none of the patients had reported peripheral embolic events which could be attributed to valve wear. The extent of wear was comparable with that of other valves. CONCLUSIONS: This descriptive study did not indicate any association between the Bicarbon valve wear characteristics and clinical complications. Clinical studies on valve performance are of paramount importance for elucidating this issue. The establishment of an independent body for studying explanted valves in collaboration with the manufacturers may promote the publication of findings which are of interest for the entire medical community.

Wear assessment in bileaflet heart valves.

Sorin Biomedica has accumulated 20 years of experience in designing and manufacturing heart valves as well as in the development of turbostratic carbons, both as bulk coating (pyrolytic carbon, PyC) and as a thin film (Carbofilm). In designing a bileaflet valve, where hinges represent the most critical elements since wear occurs mainly at these points, we took into account: a) theoretical considerations on the geometry (flat-to-flat vs. curved-to-flat) of the coupling elements undergoing impact wear; b) kinematic coupling (sliding, rotation, rolling) between the moving parts in relation to friction wear; c) experimental wear rates of different material couples (PyC/PyC, metal/PyC) assessed by paying attention to test artifacts due to particle contamination; and d) wear mechanisms involving brittle fracture mechanics for bulk PyC and ductile fracture mode for metals. On the basis of the above evaluations a Carbofilm coated titanium alloy housing and PyC leaflets were developed for the Sorin Bicarbon valve. A flat-to-flat coupling between hinge stops and mating pivot surfaces for reducing the impact wear and a rolling action aimed at minimizing the friction wear were designed for the hinge. The Bicarbon long term durability has been assessed by accelerated wear tests conducted in comparison with clinically accepted bileaflet prostheses (CarboMedics and St. Jude Medical). The main results for the Bicarbon valve were, wear rates slightly lower or comparable to those found in the reference valves and wear morphology free from microfractures, while deep cracks associated with higher stress concentrations were detected on the PyC components of the reference valves. No mechanical failure or loss of functionality occurred up to 2,100 million cycles (equivalent of 52.5 years).

Design and performance characteristics of the Pericarbon Stentless valve.

The Pericarbon Stentless valve (PS) in an unstented bioprosthesis made of glutaraldehyde tanned bovine pericardium. Its design concept and chemical fixation process are based on those developed for the stented Pericarbon valve, which has been in clinical use for eight years. The two-sheet design and the special suture pattern with which the two pieces of pericardium are attached aim at avoiding stress concentration during function. The resulting cylindrical structure is a relatively low-profile valve conduit, which can be further trimmed by the implanting surgeon to the individual patient's specific anatomical requirements. The main advantage of the stentless concept is optimum hemodynamic characteristics, inherent in the high external/internal diameter ratio. The unique design and material selection of the PS valve offer the following advantages: Free-hand insertion similar to that with homograft aortic valves. Absence of any fabric cover or reinforcement providing a maximum effective orifice area and reduced risk of infection. Wide range of available sizes from 15 mm to 29 mm. Special methods have been developed for laboratory testing of the PS. These, and the in vitro results are presented in detail, with particular emphasis on the hydrodynamic performances under different flow conditions. Clinical implantation of the Pericarbon Stentless valve has already been started, and early results in the aortic position show good hemodynamic function and a low rate of complications.

Relationship between some design characteristics and wear in the Bicarbon heart valve prosthesis.

The hinge design of a new bileaflet valve (Bicarbon) is presented in relation to the long-term durability characteristics. Theoretical considerations supported by experimental findings, lead to the identification of two different wear mechanisms acting in bileaflet valves, i.e. impact and friction wear. Impact wear, caused by the collision between the hinge stops and inflow surface of the leaflet pivots, is the predominant phenomenon; it is mostly influenced by the design of the coupling elements. The wear due friction is significantly less important, given a proper kinematic coupling and a thorough washout. These different mechanisms can significantly affect the wear resistance and long-term reliability of bileaflet valves; therefore, they have been properly taken into account in designing the new valve model. The safety of the overall material and design solutions adopted in the Bicarbon valve has been confirmed by extensive accelerated durability tests resulting in no mechanical failure or loss of functionality.

Stentless porcine and pericardial valve in aortic position.

Fifty-seven patients underwent aortic valve replacement with a stentless glutaraldehyde-fixed bioprosthesis; 27 received a porcine aortic valve and 30 had a bovine pericardial valve. Two groups of 30 patients each who had aortic valve replacement with a tilting-disc mechanical valve or a stented porcine bioprosthesis served as controls. There were no differences in sex, body surface area, valve lesion, and valve size among the four groups. Results were assessed on a Doppler-based determination of maximum velocity across the valve, aortic valve area, and degree of valve regurgitation. Velocity across the valve was significantly less with stentless pericardial valves than with stentless porcine valves, stented bioprostheses, and mechanical valves. Stentless valves had a significantly larger aortic valve area when compared with stented valves. Mild central aortic insufficiency was detected more often with stentless pericardial than with stentless porcine bioprostheses (p = 0.04). Stentless valves showed a higher incidence of complete atrioventricular block when compared with stented valves (p = 0.04). Long-term studies are now warranted to assess the durability of both types of stentless valves.