Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Specimen clamp – holder – or support
Reexamination Certificate
1998-09-17
2001-03-06
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Specimen clamp, holder, or support
C073S081000, C073S818000
Reexamination Certificate
active
06196069
ABSTRACT:
BACKGROUND
This invention relates generally to the manufacture of medical devices, and specifically to the testing of generally flat components of medical devices, e.g., the leaflets of mechanical heart valve prostheses. These valves have a generally rigid frame and at least one generally rigid leaflet operatively attached to the frame so that, in the closed position, the leaflet contacts adjacent leaflets and/or the valve frame thereby closing the valve and preventing the flow of blood therethrough. In one design, two semicircular hinged leaflets pivot within groves or other geometry contained in the orifice housing. In the open position, the leaflets separate from each other, and open radially outward toward the inner walls of a body lumen in which the valve is located. Examples of these designs are disclosed in U.S. Pat. Nos. 4,233,690; 4,689,046; 4,692,165; and 4,863,458; all of which are incorporated herein by reference.
One of the most common materials presently in use for forming these leaflets is Pyrolite® carbon. In one use, an inner graphite core is surrounded with Pyrolite® carbon, forming two outer layers. The formed Pyrolite® carbon pieces may have small flaws that are undetectable without a proof test. The proof test ensures that only the highest quality material is used in this medical device, preventing the chance for mechanical failure during the operational lifetime of the device.
A proof test is necessary to determine whether a planar, generally brittle material, e.g., a Pyrolite® carbon mechanical heart valve leaflet, contains flaws at or above a critical flaw size which might affect the operational life of the valve. The presence of flaws of such a critical size for Pyrolite® valve leaflets can be determined by exposing the leaflet to an appropriate stress field.
Prior art proof tests use a pair of mated cylindrical molds called “shoes” that apply a uniaxial stress field across the surface of a planar leaflet. These prior art tests using cylindrical-shaped shoes require four separate orientations on both flat surfaces to adequately proof test the leaflet. The leaflet to be tested is placed between the mated shoes, and a compressive load is placed on the leaflet by pressing the shoes together. The load is then relieved, allowing the shoes to be separated and the leaflet repositioned to measure leaflet stress/strain in another direction. This process is repeated until at least four separate orientations were tested. If the leaflet breaks or cracks, it is discarded. Based on the geometry of the shoes, this process concentrates stress distribution along a single axis, and requires load testing in multiple orientations—a time-consuming process. Also, placement location of the leaflet between the cylindrical shoes is critical, adding further to the complexity and time involved in the test.
The proof test of the present invention improves the stress distribution across the leaflet surface under test, reduces testing time by reducing the need for repositioning the leaflet and generally simplifies flat leaflet proof testing.
SUMMARY OF THE INVENTION
The present invention provides an apparatus for testing the strength of planar surfaces such as leaflets of mechanical heart valve protheses.
All flat leaflets to be utilized in the manufacture of artificial heart valves are proof tested by exposure to elevated tensile stresses across their surfaces. This is achieved by conforming the leaflet between a mated pair of spherical, convex and concave molds which are called “shoes”, and applying a compressive force to the mated shoes. These spherical shoes apply biaxial tensile stresses across the leaflet surface. A spherical shoe proof test provides a significant reduction in processing time and handling per leaflet when compared to a prior art cylindrical shoe proof test. Furthermore, a spherical shoe proof test provides a more uniform stress distribution across the surface of a flat leaflet, and placement of the leaflet between the shoes is not critical, since a uniform biaxial stress is applied during the test.
A key feature of the invention is the use of spherically-shaped mating shoes which results in biaxial stress testing of the leaflets. In one aspect of the invention, the three key steps in the test include: (1) determining the spherical shoe radius necessary to reveal defects greater than the critical flaw size; (2) placing the leaflet to be proof tested between mated spherically-shaped shoes; and (3) applying the predetermined loading so that the leaflet conforms to the spherical shape defined between the shoes. The radius of the surface of the mated shoes directly relates to the critical flaw size of the test material, which critical flaw size is a function of material composition and thickness. In one aspect of the invention, the test material is a pyrolitic carbon leaflet for a mechanical heart valve prosthesis.
In the method of practicing the invention, the planar, generally brittle material is inserted between two mating concave/convex shoes, and a compressive load is applied to the mated shoes so that the leaflet conforms to the shape of the shoes. If the leaflet does not crack or break during this process, it passes the proof test. Cracks or breaks are determined by (1) acoustical monitoring during loading, and (2) visual inspection after loading.
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MacNutt William P.
Strzepa Peter
Wiedenmeier Martin
Barrow Kenneth S.
Lyren Philip S.
Noori Max
Scott Timothy L.
Sulzer Carbomedics Inc.
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