Geometrical instruments – Gauge – With support for gauged article
Reexamination Certificate
2000-02-29
2002-04-30
Bennett, G. Bradley (Department: 2859)
Geometrical instruments
Gauge
With support for gauged article
C033S121000
Reexamination Certificate
active
06378221
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to systems and methods for measuring the thickness of sheet-like bio-materials and, in particular, to an improved pericardial tissue mapping and marking system and methods therefore, especially for measuring tissue to be used for making prosthetic heart valve leaflets.
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 natural valves. The two primary types of prosthetic heart valves known in the -art are mechanical valves and bio-prosthetic valves. Bio-prosthetic valves may be formed from an intact, multi-leaflet porcine (pig) heart valve, or by shaping a plurality of individual leaflets out of bovine pericardial tissue or other materials, and combining the leaflets to form the valve. The present invention provides systems and methods for assessing and preparing material for leaflets in bio-prosthetic valves.
The pericardium is a sac around the heart of vertebrate animals, and bovine (cow) pericardium is commonly used to make individual leaflets for prosthetic heart valves. The bovine pericardium is first harvested from the animal and then chemically fixed to crosslink collagen and elastin molecules in the tissue and increase the tissue durability, before being cut into leaflets. Various physical characteristics of the tissue may be examined before or after fixation.
One drawback faced by a patient having an implanted bio-prosthetic heart valve is the potential for calcification of the leaflets if the valve remains in place for an extended period of time (more than ten years). Calcification tends to make the leaflets less flexible. A significant amount of research has been accomplished in mitigating calcification of bovine pericardial leaflets to lengthen the useable life of the heart valve. Calcification may reduce the performance of the heart valve, and thus, the highest quality materials and design in the heart valve is required to forestall a failure of the valve from excessive calcium deposits.
One aspect of designing heart valves which is very important in improving their performance is the selection of the pericardial tissue used in the leaflets. In all heart valves, the natural action of the flexible heart valve leaflets, which seal against each other, or co-apt, is desirable. The difficulty in simulating the leaflet movement of an actual heart valve (especially a mitral valve) in a prosthetic valve is that the leaflets used are “inanimate.” There are no muscular attachments to the leaflets as in the natural valve, and the prosthetic leaflets must co-apt to function properly solely in response to the fluid pressures within the heart chambers. Indeed, natural coaptation of the leaflets in bio-prosthetic valves comprising a plurality of individual leaflets sewn together is particularly difficult, even when compared to inanimate but intact valves, such as harvested porcine valves.
Despite the drawbacks of artificial heart valve material, over twenty years of clinical experience surrounding implanted artificial heart valves has produced a proven track record of success. Research in extending the useful life of the bio-prosthetic valves continues, however. Much of this research involves the mechanical properties of fresh or fixed bovine pericardium.
A good discussion of the various physical properties of fixed bovine pericardium is given in Simionescu, et al, Mapping of Glutaraldehyde-Treated Bovine Pericardium and Tissue Selection For Bio-prosthetic Heart Valves, Journal of Bio-Medical Materials Research, Vol. 27, 697-704, John Wiley & Sons, Inc., 1993. Simionescu, et al., recognized the sometimes striking variations in physical properties of the pericardial tissue, even in the same pericardial sac. Their research mapped out areas in individual pericardial sacs and tested those areas for various properties to determine the optimum area on the tissue from which to cut heart valve leaflets. Simionescu, et al. measured the thickness of the pericardial sacs at 5 mm increments and plotted the resulting values on a paper template identical in shape and size to the sac. On other templates, parameters such as the suture holding power, fiber orientation, and shrinkage temperature were mapped. After superimposing all of the templates, optimum areas from which to cut leaflets were identified. Simionescu, et. al., utilized a manual thickness measuring tool similar to that described below with respect to FIG.
1
.
A number of steps in a typical commercial process for preparing pericardial tissue for heart valve leaflets is illustrated in FIG.
1
. First, a fresh pericardial sac
20
is obtained from a regulation slaughterhouse. The sac
20
is then cut open along predetermined anatomical landmarks, as indicated at
22
. The sac is then flattened at
24
and typically cleaned of excess fat and other impurities. After trimming obviously unusable areas, a window
26
of tissue is fixed, typically by immersing in an aldehyde to cross-link the tissue, and then quarantined for a period of about two weeks. Rough edges of the tissue window
26
are removed and the tissue bio-sorted to result in a tissue section
28
. The process of bio-sorting involves visually inspecting the window
26
for unusable areas, and trimming the section
28
therefrom. Subsequently, the section
28
is further cleaned as indicated at
30
.
The section
28
is then placed flat on a platform
32
for thickness measurement using a contact indicator
34
. The thickness is measured by moving the section
28
randomly around the platform
32
while a spindle
36
of the indicator
34
moves up-and-down at various points. The thickness at each point is displayed at
38
and recorded mentally by the operator. After sorting the measured sections
28
by thickness, as indicated at
40
, leaflets
42
are die cut from the sections, with thinner leaflets
42
generally being used for smaller valves, and thicker leaflets being used for larger valves. Of course, this process is relatively time-consuming and the quality of the final leaflets is dependent at several steps on the skill of the technician. Moreover, the number of leaflets obtained from each sac is inconsistent, and subject to some inefficiency from the manual selection process.
More recently, Baxter International Inc. has added a sophisticated leaflet selection method into its tissue valve manufacturing process. The method includes applying a load to each leaflet, as opposed to pericardial tissue in bulk, and recording the strain response. The results of the load test in combination with a droop test can be used to group similar leaflets. Such a method is disclosed in U.S. Pat. No. 5,961,549 to Huynh, issued Oct. 5, 1999, and entitled, “PROSTHETIC HEART VALVE LEAFLET SELECTION METHOD AND APPARATUS”. Although this method improves the quality of the resulting combination of leaflets, because of the existing inefficiencies in the process of supplying tissue from which to cut the leaflets, the subsequent filter of leaflet selection further reduces the total usable leaflet output such that costs are increased.
Despite much research into the characteristics of bovine pericardium and leaflets, there remains a need for a system and method for rapidly and reliably characterizing material, especially pericardial tissue, for use in fabricating heart valve leaflets.
SUMMARY OF THE INVENTION
The present invention provides a method of measuring the thickness of a bio-material sheet for use in bioprostheses, such as heart valves, grafts, and the like. The method involves mapping the thickness of the sheet and marking the sheet into areas or zones of similar thickness. The measuring, mapping, and marking steps can all be
Backer Steven Eric
Baeten Todd
Ekholm, Jr. Carl Roger
Gliniecki Robert
Graumlich Suzanne E.
Bennett G. Bradley
Condino Debra D.
Cumberbatch Guy L.
Edwards Lifesciences Corporation
James John Christopher
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