(Joanna Chikwe M.D., Farzan Filsoufi M.D.)
Bioprosthetic valves are described according to whether they are made of porcine aortic valves or made of pericardium. In this section, we describe bioprosthetic valves which are mounted on a metallic stent (Stented bioprostheses). Pericardium is usually bovine in origin, but may be porcine or equine, and pericardial valves are almost invariably stented. Bioprosthetic valves are also sometimes referred to according to the method of tissue fixation and treatments employed as first, second or third generation valves. Third generation valves are expected to offer improved freedom from structural valve degeneration.
All porcine and pericardial valves are now fixed in glutaraldehyde, which as described in the Historic Review section, was discovered in the 1960's to cross-link collagen fibers, and reduce tissue antigenicity, enzymatic degradation and cell viability. Glutaraldehyde fixation of porcine valves was initially performed at normal closing pressure (60-80mmHg), which led to some loss of the natural tissue architecture, eventually increasing the predisposition of porcine valves to calcify. To minimize the risk of this complication, zero pressure is now applied for porcine prostheses and low and zero-pressure fixation is used for pericardial prostheses. Despite this improvement in fixation methods, the valvular tissue still tends to calcify within 10 to 20 years after implant depending upon patient's age. The true cause and mechanisms of calcification have been and remain the subject of debate. The general belief is that glutaraldehyde is the culprit but that does not explain why valves not treated in glutaraldehyde such as homografts also calcify, nor why the patient's own tissues may calcify (e.g. calcified aortic valve stenosis or mitral annular calcification). In fact, all pathologic tissues tend to calcify under abnormal conditions of stress and biochemical environment with abnormal phosphocalcium metabolism and absence of living cells. These drawbacks are minimized by glutaraldehyde fixation associated with calcium mitigating agents using a variety of chemicals, the most effective being surfactant and other techniques decreasing the amount of phospholipids.
The first stented glutaraldehyde preserved porcine valve was homemade and clinically implanted by Carpentier in 1969, and manufactured by Edwards Laboratories in 1970. However, at Dr Carpentier's request, this valve was made widely commercially available only in 1972 after ensuring that the glutaraldehyde treatment represented a real improvement over his previous method of valve processing. At the same time, Warren Hancock, a former Edwards engineer founded his own laboratory in the early 1970's to develop a similar stented porcine valve first treated by formalin and then by glutaraldehyde. The Carpentier-Edwards porcine valve bioprosthesis, which featured a flexible stent and a supra-annular configuration, was made commercially available without restriction in 1971. Modifications in use include the SupraAnnular valve and the Duraflex low-pressure mitral valve. Hancock developed a model in which the porcine right coronary cusp was replaced with a leaflet from another porcine valve in an effort to improve the hemodynamic profile of his valve. This Modified Orifice model was released in 1976. The Hancock Modified Orifice II additionally employed a thinner stent and a lower profile making it particularly valuable for insertion in the mitral position where prominent struts were associated with left ventricular wall rupture. The Biocor stented bioprosthesis valve now marketed by St. Jude Medical is designed for suprannular implantation. Medtronic produces the Mosaic stented bioprosthesis valve which employs a "mosaic of technologies" including glutaraldehyde fixation, anticalcification treatment, zero-pressure fixation and a flexible stent.
Instead of an intact porcine aortic valve, pericardial valves are manufactured from bovine pericardium, and mounted on a stent for ease of implantation. The potential advantages included increased durability through the increased amount of collagen in pericardium, and improved hemodynamics resulting from the more symmetrical function of the leaflets. The earliest pericardial valves were the Ionescu-Shiley which had good initial function but high early failure rates.
In 1989, Carpentier developed a unique method of mounting the leaflets on an original stent which did not require stitching into the pericardium itself. This new bioprosthesis, the Carpentier-Edwards Perimount bioprosthesis, benefited from an improved tissue fixation technique, also developed by Carpentier. It gained FDA approval in 1991 and is widely used today in both the earlier version and the more recent Magna valve (which has a narrower sewing cuff and is designed for supraannular implantation). Another pericardial valve, the Mitroflow valve (Sorin Biomedica Cardio srl, Italy) was subsequently developed with a unique design whereby the stent is inside rather than outside the pericardial leaflets, resulting in an increased effective orifice area for a given annular diameter. It is therefore of particular use in the small aortic root.
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