Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Heart valve – Combined with surgical tool
Reexamination Certificate
2000-09-01
2002-10-01
Isabella, David J. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Heart valve
Combined with surgical tool
C623S901000, C623S909000, C623S913000, C033S511000
Reexamination Certificate
active
06458155
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to an apparatus and method for manufacturing bioprosthetic heart valves and, more particularly, to a sizer and method of sizing fresh donor heart valves to facilitate the fabrication of bioprosthetic heart valves.
BACKGROUND OF THE INVENTION
Prosthetic heart valves are used to replace damaged or diseased heart valves. In vertebrate animals, the heart is a hollow muscular organ having four pumping chambers: the left and right atria and the left and right ventricles, each provided with its own one-way valve. The natural heart valves are identified as the aortic, mitral (or bicuspid), tricuspid and pulmonary valves. Prosthetic heart valves can be used to replace any of these naturally occurring valves, although repair or replacement of the aortic or mitral valves is most common because they reside in the left side of the heart where pressures are the greatest.
Where replacement of a heart valve is indicated, the dysfunctional valve is typically cut out and replaced with either a mechanical valve, or a tissue valve. Tissue (e.g., xenograft) valves are often preferred over mechanical valves because they typically do not require long-term treatment with anticoagulants. Although so-called stentless valves are available, the most widely used tissue valves include some form of stent or synthetic leaflet support. The most common tissue valves are constructed with an intact, multi-leaflet donor valve, or with separate leaflets cut from bovine (cow) pericardium, for example. The most common intact valve used for stented and stentless valves is the porcine (pig) aortic valve, although other porcine valves and valves from other animals (e.g., equine or marsupial donors) have been used. The present invention is not limited to the preparation of porcine valves, though existing bioprosthetic heart valves on the market are nearly exclusively made from porcine valves, and thus the description herein will focus on such valves.
In a typical prosthetic valve fabrication process, the fresh porcine heart is first harvested in a certified slaughterhouse from the animal, weighed, and sorted into various valve size ranges by means of either estimating sized by eye based on the flattened aortic width, or by heart weight to valve size correlation. Of course, this correlation is a very rough estimate, with actual valve sizes differing quite a bit within similarly-sized porcine hearts. The aortic valve and surrounding tissue (hereinafter termed the “aortic valve isolation”) is then severed from the porcine heart. Because of its proximity to the aortic valve, the pulmonary artery remains connected to the aortic valve isolation. A cross-section of the aortic valve isolation can be seen in
FIG. 4
in the context of the sizer and method of sizing of the present invention.
At this stage, a large number of aortic valve isolations are packed in ice and shipped from the slaughterhouse to the prosthetic valve manufacturing facility. At the manufacturing facility, the aortic valve isolation is further sorted by valve size by technicians trained to estimate such valve size using their fingers. That is, the orifice diameter of the aortic valve annulus is estimated by insertion of one or more fingers through the inflow end of the aortic valve isolation. Because of the rough nature of the heart weight to valve size estimation, a large proportion of valves are rejected at this stage, resulting in wasted inventory and shipping costs.
It should be noted that the aortic valve annulus defines the narrowest opening through the valve, and is the reference dimension for implantation purposes. That is, the annulus diameter of the human patient is measured using conventional surgical sizers to determine the orifice size of the replacement bioprosthetic valve. Conventional sizers for measuring the human valve annulus typically comprise a series of incrementally-sized cylindrical elements marked with the corresponding outside diameter in mm. Most sizer sets include cylindrical elements that range from a low of 19 mm to a high of 33 mm, in 2 mm increments, and a common handle for manipulating the sizers. Some sizers for measuring the human valve annulus are shaped, or include flanges or other stepped features to also provide a measurement of the aortic root adjacent to the annulus. The aortic root is that part of the valve anatomy between the annulus and the convex sinuses of the ascending aorta, and has a generally scalloped appearance with the valve leaflets being attached along alternating arcuate cusps and upstanding commissures around its border. In any event, the primary measurement derived from conventional surgical sizers is the annulus diameter determined by finding which sizer fits properly in the annulus based on tactile feedback.
Following the estimation of the porcine aortic valve annulus diameter by the finger measurement technique, the fresh valve is then trimmed and chemically fixed to render it biologically inert for implantation purposes. The trimming procedure typically involves cutting away the pulmonary artery and surrounding muscle tissue from the inflow end of the valve. What is left is a generally tubular valve element having a small amount of tissue on the inflow side of the annulus, with the internal leaflets being enclosed and protected by the tubular ascending aorta. Chemical fixation may be accomplished using a variety of techniques and chemicals, though the most common procedure used involves supporting the tubular valve element on at least the ascending aorta or outflow portion with a fixation insert, immersing the assembly in a bath of fixing solution (e.g., glutaraldehyde), and either flowing fixing solution through the valve element or maintaining a predetermined pressure differential across the leaflets during the fixation process. See, for example, U.S. Pat. No. 4,372,743 to Lane, which describes maintaining a low pressure differential across the leaflets of between 1-4 mm Hg.
The use of fixation inserts is also quite effective in shaping the valve during the fixation process. For example, U.S. Pat. No. 5,197,979 to Quintero, et al. describes inserts having three outwardly convex regions for shaping the valve sinuses. More recently, U.S. Pat. No. 6,001,126 to Nguyen, et al. discloses inserts having a plurality of pin holes in the two convex regions corresponding to the coronary sinuses that enable coronary artery shaping plugs or mandrels to be mounted thereon. Whichever type of insert is used, the ultimate size of the fixed valve is influenced, at least in the sinus regions, by the insert. Preferably, the relative size of the annulus and sinus regions is identical to the human aortic valve being replaced. It is therefore very important to begin with a donor valve having an accurately sized annulus.
The fixation process causes some shrinkage in the tissue. Therefore, the sizing of fresh tissue provides only an estimate of the annulus size of the fixed tissue. The amount of shrinkage depends on the chemicals used, the duration of fixation, the pressure differentials within the valve, any heating that is applied, and other less significant factors. Because of these variables, fixed porcine aortic valves are sized once again using a caliper and/or a sizing stent to sort the valves into mounting sizes.
Another consideration for proper valve sizing is the dynamic expansion and contraction experienced in use after implantation. One study by Hansen, entitled Longitudinal and Radial Distensibility of the Porcine Aortic Root (Department of Electrical Engineering, the University of Western Ontario, London, Ontario, June 1994) showed that the aortic root might contract radially up to 25%, and longitudinally up to 12% when heart is arrested and the aortic root is under no pressure. The study suggests sizing the bioprosthetic replacement valve approximately 30% greater in diameter than the native aortic root at zero pressure to accommodate the expected expansion.
It is thus apparent that an accurate and reliable means for estimating, from the fresh valve,
Lam Hung
Nguyen Son Van
Ton-That Cuong
Chattopadhyay Urmi
Condino Debra D.
Cumberbatch Guy L.
Edwards Lifesciences Corporation
Isabella David J.
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