Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Heart valve – Flexible leaflet
Reexamination Certificate
1999-01-27
2003-02-25
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Heart valve
Flexible leaflet
Reexamination Certificate
active
06524339
ABSTRACT:
FIELD OF THE INVENTION
The invention pertains generally to medical method/devices and more particularly to bioprosthetic heart valves, such as cryopreserved, pre-trimmed human homograft valves which have sewing rings formed of natural tissue (e.g., pericardial tissue, dura mater, tendon sheath, etc..) affixed thereto prior to cryopreservation.
BACKGROUND OF THE INVENTION
Heart valve replacement surgeries have been performed in human beings for many years. In these surgeries, a patient's diseased or malfunctioning heart valve is removed and a prosthetic valve is surgically implanted in its place. The available types of prosthetic heart valves include mechanical valves (i.e., valves constructed of non-biological materials such as titanium, carbon or steel) and bioprosthetic valves (i.e., valves formed fully or partially of biological tissue).
A. Bioprosthetic Heart Valves
i. Heterografts vs. Homografts
Bioprosthetic valves include heterografts (also known as xenografts) as well as homografts (also known as allografts). Heterograft heart valves are formed of tissue that has been harvested from a non-human animal and subsequently implanted in a human recipient. Homograft heart valves are formed of valvular tissue that has been harvested from the heart of a human being and subsequently implanted in a human recipient.
Typically, heterograft heart valves are formed of tissue that has been harvested from the heart of an animal, such as a pig, and has been treated with a chemical fixative to preserve the tissue for subsequent implantation.
Typically, homograft heart valves are formed of tissue that has been harvested from cadaveric human donors, or from the explanted hearts of human heart transplant recipients whose ailing hearts had healthy valves despite the presence of cardiomyopathy or other cardiac pathology. The harvested homograft tissue is then treated chemically to kill any viruses or other microbes and subsequently cryopreserved (i.e., cooled to a very low temperature by immersion in liquid nitrogen) until the time of implantation. To date, commercially available homograft valves have typically been provided to the surgeon in a non-trimmed state (i.e., with a substantial amount of the donor's muscle tissue (e.g., cardiac septal muscle) affixed to the valve). Thus, prior to implantation, the homograft must be removed from the liquid nitrogen freezer used for the valve bank, thawed by the method recommended by the manufacturer, and then carefully trimmed of excess tissue. This trimming process is laborious and not particularly standardized. Also, this trimming process typically must be performed by a highly trained surgeon.
Stented vs. Unstented
Some bioprosthetic valves, known as “stented” bioprosthetic valves, incorporate a man-made stent or support frame upon which preserved allograft tissue is mounted and an annular sewing ring, formed of man-made materials (e.g., an annular nylon core covered with a knitted polyester sleeve), is formed about the inflow end of the valve to maintain the inflow end of the bioprosthesis in a non-collapsed “open” configuration and to provide a firm suture-holding structure around the valve to facilitate suturing of the valve to the annulus of the recipient. U.S. Pat. No. 4,759,758 (Gabbay) has purported to describe a stented bioprosthetic heart valve formed of a man-made stent having chemically preserved biological tissue (e.g., bovine pericardial tissue), mounted on the man-made stent to form the valve leaflets. Additionally, a quantity of preserved biological tissue or polyester (i.e., Dacron) that has been impregnated with collagen, is mounted about the base of the man-made stent to form a sewing ring thereon.
Examples of commercially available stented bioprosthetic valves include the Carpentier-Edwards®, PERIMOUNT™ Pericardial Bioprosthesis (Baxter Healthcare Corporation, Edwards CVS Division, P.O. Box 11150, Santa Ana, Calif. 92711-1150 as well as the Carpentier-Edwards® Porcine Bioprosthesis (Baxter Healthcare Corporation, Edwards CVC Division, P.O. Box 11150, Santa Ana, Calif. 92711-1150). Each of these valves are of the heterograft type.
Others, known as “stentless” bioprosthetic valves, do not include any manmade stent or support frame, and are formed entirely of preserved biological tissue, and do not include any “sewing rings” formed about their inflow ends.
Examples of commercially available stentless bioprosthetic valves of the heterograft type include the Edwards Prima™ Stentless Bioprosthesis (Baxter Edwards AG, Spierstrasse 5, CH-6848 Horw, Switzerland), the Medtronic Freestyle™ Aortic Root Bioprosthesis (Medtronic, Inc. 7000 Central Avenue NE, Minneapolis, Minn. 55432-3576) and the St. Jude Toronto™ Stentless Bioprosthesis (St. Jude Medical, Inc. One Lillehei Plaza, St Paul, Minn. 55117).
An example of a commercially available stentless bioprosthetic valves of the homograft type is the CryoValve™ cryopreserved aortic homograft (CryoLife Corporation, Atlanta, Ga.).
Stentless bioprosthetic valves may offer superior hemodynamic performance when compared to their stented counterparts, due to the absence of flow restrictions which can be created by the presence of a stent and/or sewing ring. Also, the stentless bioprosthetic valves may exhibit better post-implantation durability than the stented bioprosthetic valves, because they provide a more flexible structure which serves to dissipate stress during the cardiac cycle.
Stentless valves of the homograft type are particularly advantageous in that they exhibit excellent long-term durability and are completely devoid of synthetic or man-made components. The absence of such synthetic or man-made components has been demonstrated to minimize the likelihood of post-operative infection of homograft valves, even in patients who suffer from active endocarditis or other infectious processes within the thoracic cavity. However, the presently available homograft valves are associated with certain drawbacks, namely i) that they require a substantial amount of trimming by the surgeon prior to implantation and ii) the absence of a defined “sewing ring” about the inflow end can cause surgeons to experience difficulty in firmly sewing the inflow end of the homograft valve to the patient's native vale annulus.
B. Methods of Preserving Bioprosthetic Valves
Most bioprosthetic heart valves are formed at least partially of natural tissue that contains high concentrations of connective tissue proteins. Collagen, and to a lesser extent elastin, are the major connective tissue proteins which make-up the connective tissue matrix or framework of most biological tissues. The relative pliability or rigidity of each biological tissue is largely determined by its relative amounts of collagen and elastin and/or by the physical configuration (e.g., structural lattice) and confirmation of the connective tissue matrix.
At present, the natural tissue contained in most bioprosthetic heart valves is preserved, at the time of manufacture, by either chemical fixation (e.g., “tanning”) or by cryopreservation (e.g., cooling to a very low temperature by imersion in liquid nitrogen). Each of these tissue preservation techniques has certain advantages and disadvantages, as discussed more fully herebelow.
i. Chemical Fixation
The chemical fixation of biological tissues contained in bioprosthetic heart valves can be accomplished by contacting the tissue with one or more chemicals which will crosslink collagen and elastin molecules which are present within the tissue. Such crosslinking of the collagen and elastin serves to preserve the tissue so that it may be stored until it is needed for implantation in a patient. Examples of the types of biological tissues that are suitable for chemical fixation include cardiac valvular tissue, blood vessels, skin, dura mater, pericardium, ligaments and tendons. These anatomical structures typically contain connective tissue matrices, formed of collagen and elastin, and the cellular parenchyma of each tissue is disposed within and supported by its connective tissue matrix.
Each
Buyan Robert D.
Jackson Suzette J.
Stout, Uxa Buyan & Mullins, LLP
Willse David H.
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