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May 5, 2021
Bioprosthetic Valves Bioprosthetic Valves

Bioprosthetic Valves

(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.

Porcine Valves

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.

Pericardial Valves

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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