Title: Fatigue test for prosthetic stent
Abstract: An improved method for testing a stent for a prosthetic valve includes applying a load with a fluid against a stented test structure in a backward direction with the stented test structure substantially blocking the flow of the fluid. The stented test structure includes a stent and a flexible membrane extending within the lumen defined by the stent with the flexible membrane having a plurality of contours connecting to the stent along the scallops. A corresponding testing apparatus includes a cyclic pressure applicator, a conduit connected to the cyclic pressure applicator, and a stented test structure mounted within the conduit to receive cyclic fluid pressures from the cyclic pressure applicator. The stented test structure includes a stent and a flexible membrane extending within the lumen defined by the stent. The flexible membrane substantially blocks flow of the fluid in a backward direction and does not fully open upon application of the fluid pressure in a forward direction.
Patent Number: 6,881,224 Issued on 04/19/2005 to Kruse, et al.
| Inventors:
|
Kruse; Steven D. (Bloomington, MN);
Cai; Chad Q. (Woodbury, MN)
|
| Assignee:
|
St. Jude Medical, Inc. (St. Paul, MN)
|
| Appl. No.:
|
034694 |
| Filed:
|
December 28, 2001 |
| Current U.S. Class: |
623/2.11; 73/37; 73/865.6 |
| Intern'l Class: |
A61F 002//24 |
| Field of Search: |
623/2.11,2.13,913
73/865.6,37
|
References Cited [Referenced By]
U.S. Patent Documents
| 4343048 | Aug., 1982 | Ross et al.
| |
| 4778461 | Oct., 1988 | Pietsch et al. | 623/2.
|
| 5176153 | Jan., 1993 | Eberhardt | 128/897.
|
| 5272909 | Dec., 1993 | Nguyen et al. | 73/37.
|
| 5406857 | Apr., 1995 | Eberhardt et al. | 73/866.
|
| 5531094 | Jul., 1996 | More et al. | 73/1.
|
| 5571174 | Nov., 1996 | Love et al.
| |
| 5584878 | Dec., 1996 | Love et al.
| |
| 5662705 | Sep., 1997 | Love et al.
| |
| 5670708 | Sep., 1997 | Vilendrer | 73/37.
|
| 5899937 | May., 1999 | Goldstein et al. | 623/2.
|
| 5961549 | Oct., 1999 | Nguyen et al.
| |
| 6165216 | Dec., 2000 | Agathos.
| |
| 6171335 | Jan., 2001 | Wheatley et al.
| |
| 6245105 | Jun., 2001 | Nguyen et al.
| |
| 6562069 | May., 2003 | Cai et al. | 623/2.
|
| Foreign Patent Documents |
| WO02/065952 | Aug., 2002 | WO.
| |
Primary Examiner: McDermott; Corrine
Assistant Examiner: Sweet; Thomas J
Attorney, Agent or Firm: Westman, Champlin & Kelly, LLC
Claims
What we claim is:
1. A method for testing a stent for a prosthetic valve, the method
comprising applying fluid pressure against a stented test structure in a
backward direction with the stented test structure substantially blocking
the flow of the fluid, the stented test structure comprising a stent and a
flexible membrane extending within a lumen defined by the stent, wherein
the flexible membrane does not fully open upon application of fluid
pressure in a forward direction.
2. The method of claim 1 wherein the fluid is a liquid.
3. The method of claim 1 wherein the fluid is saline.
4. The method of claim 1 wherein the fluid pressure has a peak over a cycle
from about 60 mmHg to about 200 mmHg.
5. The method of claim 1 wherein:
the stent comprises a plurality of commissure posts, the flexible membrane
extending between the commissure posts; and
the commissure posts deflect inward approximately an equivalent amount as
the commissure posts deflect in a corresponding valve when subjected to a
pulse duplicator at a physiological condition.
6. The method of claim 1 wherein the fluid pressure is cyclic.
7. The method of claim 6 wherein the cyclic fluid pressure is approximately
periodic.
8. The method of claim 7 wherein the periodic fluid pressure has a
frequency from about 1000 to about 6000 cycles per minute.
9. The method of claim 1 wherein the stented test structure is mounted
within a conduit.
10. The method of claim 9 wherein the conduit is connected to a cyclic
pressure applicator.
11. The method of claim 1 wherein the flexible membrane comprises a
polymer.
12. The method of claim 11 wherein the polymer comprises polyurethane or
silicone.
13. The method of claim 11 wherein the polymer is cast around the stent to
form an integral unit.
14. The method of claim 1 wherein the flexible membrane comprises a tissue.
15. The method of claim 1 wherein the flexible membrane opens upon
application of the fluid pressure in the forward direction no more than
about 80 percent of the full open lumen at the edge of the stent
corresponding to the inflow edge of the prosthesis.
16. The method of claim 1 wherein the flexible membrane opens upon
application of the fluid pressure in the forward direction from about 1
percent and about 60 percent of the full open lumen at the edge of the
stent corresponding to the inflow edge of the prosthesis.
17. The method of claim 1 wherein the flexible membrane opens upon
application of the fluid pressure in the forward direction from about 5
percent to about 30 percent of the full open lumen at the edge of the
stent corresponding to the inflow edge of the prosthesis.
18. The method of claim 1 wherein the flexible membrane forms a seal
against flow in any direction through the stented test structure.
19. The method of claim 1 wherein the stent comprises a plurality of
commissure posts and scallops extending between the commissure posts, and
wherein the flexible membrane has a plurality of contours connecting to
the stent along the scallops.
20. The method of claim 19 wherein the flexible membrane is at least partly
sealed along edges between contours to restrict flow through the membrane.
21. The method of claim 19 wherein the stent has three commissure posts.
22. The method of claim 19 wherein at least one contour comprises a one way
portal that provides flow upon application of fluid pressure in the
forward direction and closes against fluid pressure in a backward
direction.
23. A testing apparatus comprising a cyclic pressure applicator, a conduit
connected to the pressure applicator, and a stented test structure mounted
within the conduit to receive cyclic fluid pressures from the pressure
applicator, the stented test structure comprising a stent and a flexible
membrane extending within the lumen defined by the stent, wherein the
flexible membrane substantially blocks flow of a fluid in a backward
direction and is constrained so as not to fully open in response to fluid
pressure in a forward direction.
24. The testing apparatus of claim 23 wherein the cyclic pressure
applicator cycles the fluid pressures at a frequency from about 1500 to
about 6000 cycles per minute.
25. The testing apparatus of claim 23 wherein the flexible membrane
comprises polymer cast around the stent to form an integral unit.
26. The testing apparatus of claim 23 wherein the flexible membrane opens
upon application of the fluid pressure in the forward direction no more
than about 80 percent of the full open lumen at the edge of the stent
corresponding to the inflow edge of the prosthesis.
27. The testing apparatus of claim 23 wherein the flexible membrane opens
upon application of the fluid pressure in the forward direction from about
5 percent and about 30 percent of the full open lumen at the edge of the
stent corresponding to the inflow edge of the prosthesis.
28. The testing apparatus of claim 23 wherein the flexible membrane forms a
seal against flow in any direction through the stented test structure.
29. A stented test structure comprising a stent and a flexible membrane
extending within the lumen defined by the stent, wherein the stent
comprises a plurality of commissure posts and scallops extending between
the commissure posts, and wherein the flexible membrane connects to the
stent along the scallops and is constrained to open no more than about 80
percent of the full open lumen at the edge of the stent corresponding to
the inflow edge of the prosthesis upon application of fluid pressure in a
forward direction.
30. The stented test structure of claim 29 wherein the flexible membrane
comprises a polymer.
31. The stented test structure of claim 29 wherein the polymer comprises
polyurethane or silicone.
32. The stented test structure of claim 29 wherein the polymer is cast
around the stent to form an integral unit.
33. The stented test structure of claim 29 wherein the flexible membrane
comprises a tissue.
Description
FIELD OF THE INVENTION
The invention relates to methods for the testing of stents, i.e., leaflet
support structures, used for valved prostheses, especially heart valve
prostheses. In particular, the invention relates to stent fatigue testing
using hydraulic forces.
BACKGROUND OF THE INVENTION
Physicians use various prostheses to correct problems associated with the
cardiovascular system, especially the heart. For example, the ability to
replace or repair heart valves with prosthetic devices has provided
surgeons with a method of treating heart valve deficiencies due to disease
and congenital defects. A typical procedure involves removal of the native
valve and surgical replacement with a prosthetic heart valve.
Prosthetic heart valve leaflets or occluders perform the function of
opening and closing to regulate blood flow through the heart valve.
Typically, heart valve leaflets must either pivot or flex with each cycle
of the heart to open and close. Heart valves function as check valves,
which open for flow in one direction and close in response to pressure
differentials.
Prostheses can be constructed from natural materials such as tissue,
synthetic materials or a combination thereof. Prostheses formed from
purely synthetic materials can be manufactured, for example, from
biocompatible metals, ceramics, carbon materials, such as graphite,
polymers, such as polyester, and combinations thereof. Heart valve
prostheses with purely synthetic materials can be manufactured with rigid
occluders or leaflets that pivot to open and close the valve, or flexible
leaflets that flex to open and close the valve.
Although mechanical heart valves with rigid pivoting occluders have the
advantage of proven durability through decades of use, they are associated
with blood clotting on or around the prosthetic valve and thromboembolism.
Blood clotting can lead to acute or subacute closure of the valve. For
this reason, patients with mechanical heart valves remain on
anticoagulants for as long as the valve remains implanted. Anticoagulants
have associated risks and cannot be taken safely by certain individuals.
Heart valve prostheses with flexible leaflets can be constructed with
tissue leaflets or polymer leaflets. In prostheses with flexible leaflets,
the leaflets function similarly to natural leaflets. While the leaflets
are flexible, they must have a well defined and stable configuration to
properly open and close the valve at each cycle in response to pressure
differentials. Also, the leaflets should be durable to provide stable
performance over many years of use.
Unlike mechanical valves, tissue based bioprostheses do not require the
long term use of anticoagulants due to a lower incidence of
thromboembolism. While tissue leaflets have desired flexibility and
acceptable hemodynamic performance, tissue leaflets can calcify after
implantation, which results in loss of flexibility, resulting in improper
closure and/or opening of the valve. For individuals with appropriate
indications, tissue-based heart valve prostheses provide practical
alternatives to mechanical heart valve prostheses.
Valve prostheses with polymer leaflets have the potential to overcome the
shortcomings of both tissue and mechanical valve designs. The polymers
incorporated into heart valve prostheses should provide long term stable
function to be suitable alternatives for tissue leaflets or mechanical
valve leaflets.
SUMMARY OF THE INVENTION
In a first aspect, the invention pertains to a method for testing a stent
for a prosthetic valve. The method includes applying a fluid pressure
against a stented test structure in a backward direction with the stented
test structure substantially blocking the flow of the fluid. The stented
test structure includes a stent and a flexible membrane extending within
the lumen defined by the stent. The flexible membrane does not fully open
upon reversal of the fluid pressure applied in a forward direction.
In another aspect, the invention pertains to a testing apparatus comprising
a cyclic pressure applicator, a conduit connected to the pressure
applicator, and a stented test structure mounted within the conduit to
receive cyclic fluid pressures from the pressure applicator. The stented
test structure includes a stent and a flexible membrane extending within
the lumen defined by the stent. The flexible membrane substantially blocks
flow of the fluid in a backward direction and does not fully open upon
reversal of the fluid pressure in a forward direction.
In a further aspect, the invention pertains to a stented test structure
including a stent and a flexible membrane extending within the lumen
defined by the stent. The stent includes a plurality of commissure posts
and scallops extending between the commissure posts. The flexible membrane
connects to the stent along the scallops and opens no more than about 80
percent of the full open lumen at the edge of the stent corresponding to
the inflow edge of the prosthesis upon application of fluid pressure in a
forward direction.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a stented heart valve prosthesis with
flexible leaflets in an open position.
FIG. 2 is a perspective view of a stented heart valve prosthesis with
flexible leaflets in a closed position.
FIG. 3 is a side view of the heart valve prosthesis of claim FIG. 2.
FIG. 4 is a sectional side view of the heart valve prosthesis of FIG. 2
taken along line 4--4 of FIG. 2.
FIG. 5 is a perspective view of a wire-form stent.
FIG. 6 is a perspective view of a sheath shaped stent.
FIG. 7 is a perspective view of a stented test structure that provides for
some fluid flow in the center of the valve in the forward direction.
FIG. 8 is a perspective view of a stented test structure that provides for
some fluid flow at three locations along seams between contours of a
flexible membrane in the forward direction.
FIG. 9 is a perspective view of a stented test structure with one-way
portals on the contours of the membrane of the structure.
FIG. 10 is a perspective view of a stented test structure with one-way
portals on the walls of the stent with one hidden portal shown in phantom
lines.
FIG. 11 is a fragmentary side view of a mount of a stented test structure
with a one way check valve mounted adjacent the stented test structure to
provide one-way flow past the mount.
FIG. 12 is a perspective view of a stented test structure that does not
provide for any flow through the stented test structure.
FIG. 13 is a perspective view of a mandrel with a stent mounted on the
mandrel in preparation for dip coating.
FIG. 14 is a perspective view of an embodiment of a testing apparatus that
does not provide for flow through the stented test structure.
FIG. 15 is a perspective view of another embodiment of a testing apparatus
that provides for optional flow through the stented test structure.
DETAILED DESCRIPTION OF THE INVENTION
A new fatigue testing procedure for stents of prosthetic valves, especially
heart valve prostheses, involves hydraulic testing using the stent in
association with a flexible membrane connected to the stent which
transfers a load from the fluid to the stent. The flexible membrane
generally differs from conventional leaflets in that the flexible membrane
does not fully open or does not open at all when fluid pressure is applied
to a stented test structure in a forward direction, which would correspond
with the direction from the inflow edge to the outflow edge of a
prosthetic valve mounted on the stent. Since the flexible membrane does
not fully open for fluid flow, the stented test structure can be cycled at
a higher frequency than a corresponding heart valve prosthesis. The
hydraulic testing loads transferred to the stent more closely approximate
the type of loading on the stent in vivo following implantation. Thus, a
more accurate evaluation of the fatigue of the stent can be performed.
Valved prostheses have flexible leaflets that open and close in response to
pressure changes. In particular, damaged or diseased native heart valves
can be replaced with valved prostheses to restore valve function. In
embodiments with flexible leaflets, the leaflets function similarly to
native tissue leaflets. Suitable heart valve prostheses can be designed as
a replacement for any heart valve, i.e., an aortic valve, a mitral valve,
a tricuspid valve, or a pulmonary valve. In addition, valved prostheses
can be used for the replacement of vascular valves, although for
convenience, the discussion herein focuses primarily on heart valve
prostheses. The patient can be an animal, especially a mammal, and
preferably is a human.
The flexible leaflets of the valve are configured to flex in response to
changes in blood flow. In particular, the valves generally function as one
way check valves that fully open to allow flow in a forward direction and
close in response to pressure applied in a backward direction. Similar to
leaflets of a prosthetic valve, the membrane substantially blocks fluid
flow when fluid pressure is applied in the backward direction, which is
generally opposite to the forward direction defined above. In other words,
the backward direction corresponds with the direction from the outflow
edge to the inflow edge of a prosthetic valve mounted on the stent. When
the valve closes in response to pressure differentials, the free edges of
adjacent leaflets contact in the closed configuration with the leaflets
extending across the lumen. In the closed position, the leaflets eliminate
or greatly reduce back flow through the valve. The contacting portion of
the leaflets is referred to as the coaptation region.
Heart valve prostheses with flexible leaflets generally have leaflets
formed from tissue or polymers. Tissue-based valved prostheses can include
harvested natural valves or components thereof, or tissue portions that
are assembled into appropriate valve components. The tissue can be
autograft tissue, allograft tissue or xenograft tissue. The tissue,
especially xenograft tissue, can be fixed using a crosslinking agent.
Polymer leaflets are formed from a thin film of flexible polymer. Suitable
polymers are biocompatible, in that they are non-toxic, non-carcinogenic
and do not induce hemolysis or an immunological response. Heart valve
prostheses formed from polymers preferably are non-thrombogenic. Relevant
mechanical properties of polymers include, for example, stiffness,
strength, creep, hardness, fatigue resistance and tear resistance.
Preferred polymers are durable in that they do not significantly lose
their flexibility and do not significantly lose their mechanical strength
following many years of use.
In a prosthetic valve with flexible leaflets, the leaflets are supported by
a support structure. In stentless valve embodiments, the support structure
is not sufficiently rigid to maintain the leaflet function of the valve
against the forces opening and closing the valve. In embodiments of
particular interest herein, the support structure includes a sufficiently
rigid component that maintains the leaflet function of the valve against
the forces opening and closing the valve. In particular, the support
structure is sufficiently rigid to prevent valve collapse against the
fluid pressures closing the valve. Valves with a rigid support structure
are termed stented valves, and the sufficiently rigid support is called a
stent. The stent provides a scaffolding for the leaflets. Since the stent
is rigid, only the base of the stent is necessarily attached to the
patient or other device. As a particular example, heart valve stents are
used to support leaflet components within a prosthetic heart valve.
Stents generally include commissure posts and scallops that extend between
commissure posts. The commissure posts support the free edges of the
leaflets while the scallops support the fixed edges of the leaflets. The
scallop shape allows the attachment of a leaflet with sufficient freedom
of motion to move from a fully closed position to a fully open position.
As long as the stent provides for the appropriate support of the leaflets
along the commissure posts and scallops, the stent can be formed with a
range of designs from, for example, a thin wire-form or a sheath shape
that fills in all or a portion of the regions between the scallops. In
addition to the stent, a sewing cuff, fabric cover and/or other
non-support elements can be included with the prosthesis to facilitate
implantation of the valve and/or handling of the valve.
The edge of the prosthesis at or near the lower portion of the stent below
the scallops forms the inflow edge of the valve. Generally, the stent, the
sewing cuff and/or other structure together form a generally annular
structure between the scallops and the inflow edge. The other edge of the
valve opposite the inflow edge is the outflow edge. Flow through the
corresponding prosthesis incorporating the stent goes through the central
lumen of the valve. The leaflets can be attached to the stent using any of
a variety of fastening approaches. For example, the leaflets can be
fastened using suture, staples, adhesives or the like. Polymer leaflets
can be formed around the stent to form an integral unit.
Valved prostheses are generally intended for long term use following
implantation. Replacement of heart valve prostheses requires a surgical
procedure with its associated risks, and failure of the prosthesis can
place the patient at risk. Thus, testing of the components of the
prosthesis is important to ensure that the valve will perform in a
satisfactory fashion following implantation. Furthermore, regulatory
agencies require testing as a part of the approval process for a
prosthesis. Testing can also be a significant aspect of the design process
for the production of improved prostheses with desirable performance
characteristics.
Testing may involve several different aspects. Durability testing for
leaflets can be performed by mounting a completed valve in an accelerated
wear tester. Separate fatigue testing of the stents can be used to
evaluate the performance of the stents independent of the leaflets.
Stents are subject to considerable loads after implantation of the
prosthesis due to fluid forces on the leaflets. The loads reach a maximum
when the prosthetic valve is in a closed position. The stent keeps the
valve from collapsing against the force of the fluid pressing against the
closed leaflets. One testing approach for the stent alone involves
mechanical contact with the tips of the commissure posts to deflect the
commissure posts to an amount observed under mock physiological
conditions. While the displacement of the post tip may be similar to that
observed in a pulse duplicator system, the application of the load to the
stent during this mechanical fatigue testing can be distinctly different
from the load transferred from the leaflets to the stent when a pressure
load is applied to a closed valve by the fluid. Maximum stress levels in
the stent under a physiological load transferred from the leaflets to the
stent can be significantly different than a point load applied to the tips
of the stent to deflect the stent tips a similar amount. The relationship
between the stress applied by mechanical post deflection and load transfer
through a membrane under fluid pressures can vary considerably with stent
design. Similarly, mechanical deformation of the stent tips generally does
not reproduce the distribution of the load. Thus, the load magnitude and
overall deformation of the stent resulting from tip displacement may not
correspond with the magnitude and distributions of the load transferred
through leaflets to the stent in vivo.
In contrast, the improved stent testing procedures described herein
involves a hydraulic process using the stent with a flexible material
membrane attached to the stent to transfer the load from the fluid to the
stent. The flexible membrane attached to the stent to perform the testing
of the stent may have different mechanical properties than leaflets of the
prosthesis, although the membrane generally has similar geometry and
elasticity as leaflets in a valve. Unlike valve leaflets, the testing
membrane does not fully open to allow flow through the valve lumen under
test conditions. The preferred testing approaches herein involve the
transfer of a load from a fluid to the stent that more closely approximate
in magnitude and/or in distribution the loads applied in the corresponding
prosthesis after implantation. The magnitude of the load may depend on the
frequency at which the fluid is cycled, although the fluid pressure can be
adjusted to compensate for these frequency dependant variations in the
magnitude of the load.
The test apparatus to perform the stent fatigue testing involves the
stented test structure, a mount for the stented test structure and a
hydraulic system or cyclic pressure applicator to cycle the stented test
structure with a fluid. The stented test structure includes the stent, a
flexible membrane within the central lumen of the stent and, optionally,
mounting structures, such as a sewing cuff, to facilitate the mounting of
the stented test structure on the testing apparatus mount. The mount
generally involves a tubular element or the like that surrounds the base
of the stent to direct fluid pressures within the central lumen of the
stented test structure such that there is little or no leakage of fluid
around the outer portion of the stented test structure. A pump or other
means of applying cyclic pressures can be used to apply the load to the
stent for the testing. Suitable cyclic pumps include, for example, a
Harvard-style piston pump, a cam actuated piston pump or other cyclic pump
designs. In other embodiments, oscillating centrifugal forces or a
continuous pump with periodic valve functions can be used to produce
cyclic fluid pressures.
The stented test structure maintains fluid on both sides of the flexible
membrane of the stented test structure. The fluid generally is liquid on
the downstream (backward pressure generating) side of the stented test
structure and may be liquid or gas, such as air at atmospheric pressure,
on the upstream side of the stented test structure. While the flexible
membrane may not allow flow, the orientation of the structure can be
referenced relative to the upstream and downstream directions of a
corresponding valve with leaflets mounted on the stent. For the
performance of the stent fatigue test, fluid pressure is applied in a
backward direction from the downstream side of the stented test structure
with a selected magnitude and temporal variation
While the flexible membrane of the stented test structure has a closed
configuration approximating the closed position of the leaflets of the
corresponding prosthetic valve, the flexible membrane in preferred
embodiments has significant differences from the leaflets. In particular,
the flexible membrane generally does not open to a fully open position
when appropriate fluid pressure is applied in a forward direction to
provide for flow through the central lumen. The flexible membrane can be
partially sealed to limit the flow and/or the material of the flexible
membrane can be sufficiently less flexible than the leaflet material so
that the flexible membrane does not open up fully at the pressures used
for testing. In some embodiments, the flexible membrane is sealed across
the valve lumen to prevent any significant flow through the stented test
structure as the fluid cycles the structure.
In preferred embodiments, the material used to form the flexible membrane
in the stented test structure has an elasticity, i.e., in plane
extensibility, comparable to the leaflet material of the valve prosthesis
such that the load transferred to the stented test structure is
approximately comparable in magnitude and in distribution as the load
transferred by the leaflets in the corresponding prosthesis. With respect
to leaflets, to achieve the desired flexibility of the leaflets and to
reduce resistance to forward flow, the leaflets are thin. However, since
the flexible membrane does not flex to a fully open configuration, the
membrane does not have to be as flexible as the leaflets. In general,
elasticity of a piece of membrane material is inversely proportional to
its elastic modulus and thickness, while its flexibility is inversely
proportional to the elastic modulus and the thickness to the third power.
If the membrane material has lower elastic modulus than the leaflet, it
can be made thicker to have similar elasticity to the leaflet, but it will
be less flexible.
While the leaflets of a prosthetic valve are necessarily durable, fatigue
testing of the stent inherently entails pushing the stent structure near
its limits. It is not desirable to have durability of the flexible
membrane compromise the testing of the fatigue properties of the stent.
Since the membrane does not have to be as flexible, the membrane can be
made more durable. Increased durability can be introduced through one or
more approaches. For example, the flexible membrane of the stented test
structure can include additional reinforcements and the like that do not
significantly alter the elasticity while improving the durability.
Furthermore, different materials can be used to form the flexible membrane
than are suitable for leaflets both because of the reduced need for
flexibility and because the flexible material does not need to be
biocompatible. Additionally, or alternatively, the material can be made
thicker as long as the elasticity is not inappropriately altered.
If the flexible membrane of the stented test structure is formed from
tissue, the tissue can be reinforced at stress points, such as the
commissure posts, to improve the durability of the material. In addition,
the tissue can be sutured or otherwise fastened across a portion or all of
the free edges of the coapting leaflets to keep the leaflets from fully
opening. In addition, somewhat thicker tissue material can be used without
significantly altering the transfer of the load to the stent.
In some preferred embodiments, the flexible membrane is formed from a
polymer. The polymer can be extruded, cast, molded, calendered or the like
to form the membrane. In some preferred embodiments, the polymer membrane
is formed by placing the stent onto a mandrel and then dip coating the
mandrel into an appropriate polymer liquid. The polymer is then solidified
on the stent to form the stented test structure. In this way, the polymer
membrane is formed as an integral unit with the stent and is removed from
the mandrel as a unit. The mandrel is shaped to correspond approximately
with the closed configuration of the corresponding valve prosthesis. The
top of the mandrel can include sharp edges at selected locations to
separate any free edges of the membrane at those selected locations to
provide for desired amounts of flow through the polymer structure.
Alternatively, the polymer can be cut along its top edge to form partial
free edges in the polymer membrane. Reinforcements can be included in the
polymer material to make the membrane more durable.
Thus, the stent testing approaches described herein can provide a more
accurate test of a stent in which the forces on the stent more closely
approximate the physiological forces on the stent during valve function.
At the same time, by forming the flexible membrane such that it does not
fully open, the testing can be done at high cycling rates within a
practical testing time for an appropriate number of cycles that is
comparable to the cycling times used with mechanical post deflection test.
The apparatuses for performing the stent testing can be constructed or
adapted from presently available components and apparatuses.
Valved Prostheses And Stents
The testing procedures described herein can be used for testing stents to
be incorporated into valved prostheses. In particular, the stented valves
can be used in artificial hearts, heart valve prostheses, valved vascular
prostheses or left ventricular assist devices.
While the embodiments of the valved prosthesis shown in the figures below
have three leaflets, valved prostheses can be constructed with different
numbers of leaflets, such as one leaflet, two leaflets, four leaflets or
more than four leaflets. The prosthesis may or may not have the same
number of leaflets as the natural valve that it is used to replace. The
number of leaflets dictates the structure of the corresponding stent since
the stent supports the edges of the leaflet.
Heart valve prostheses are suitable for the replacement of damaged or
diseased native heart valves. Mammalian hearts have four major valves.
With appropriate sizing and attachment, the prosthetic valves are suitable
for replacement of any of the heart valves. In particular, heart valve
prostheses for replacement of the mitral and tricuspid valves generally
include rigid stents although some embodiments allowing for chordae
attachment may be stentless. Similarly, embodiments of aortic valves and
pulmonary valves may also include stents.
Mammalian veins include valves that assist with blood circulation by
limiting the amount of back flow in the veins. Veins collect blood from
capillaries and are responsible for returning blood to the heart.
Generally, vascular valves are replaced as part of a vascular graft with
sections of conduit. The stented valve prostheses can be incorporated into
a vascular graft with a conduit for replacement of a venous valve or for
the replacement of an aortic or pulmonary heart valve. In addition, a
stented valve, as described herein, can be incorporated into a left
ventricular assist device.
An embodiment of a stented heart valve prosthesis with flexible leaflets is
shown in its fully open position in FIG. 1. Heart valve prosthesis 100
includes leaflets 102, 104, 106, support structure/stent 114 and sewing
cuff 116. Heart valve prosthesis 100 with closed leaflets is shown in
FIGS. 2-4. Leaflets 102, 104, 106 contact the respective adjacent leaflets
to close the valve.
Sewing cuff 116 is used to attach valve 100 to the patient's tissue annulus
or to other portions of a prosthesis. In embodiments of interest, support
structure/stent 114 is relatively rigid, such that the support structure
functions as a stent to maintain leaflet function with attachment to the
patient only at base 142 of stent 114. Stent 114 includes commissure
supports 108, 110, 112 and scallops 120, 122, 124 between the commissure
supports. Free edges 130, 132, 134 of leaflets 102, 104, 106,
respectively, join at the commissure supports 108, 110, 112. Attached
edges 136, 138, 140 of leaflets 102, 104, 106 also secure to the stent 114
along scallops 120, 122, 124. The base 142 of stent 114 generally has the
shape of a cylindrical ring or the like that forms the opening into the
valve at the upstream or proximal end 143 of the valve.
Sewing cuff 116 generally extends from base 142 of stent 114. Sewing cuff
116 facilitates the attachment of the heart valve prosthesis to the
patient or device. Sutures, staples and/or other fastening mechanisms are
passed through the sewing cuff to secure sewing cuff 116 to the patient's
tissue annulus, to a conduit prosthesis or to other portions of a
prosthesis. Sewing cuff 116 preferably extends outward from base 142 so
that the fastening mechanism can be conveniently passed through sewing
cuff 116 to attach the valve without significant risk of piercing leaflets
102, 104, 106.
For any of the prosthetic valve embodiments, suitable rigid materials for
the stent include, for example, rigid polymers, metals, ceramics, carbon
materials and combinations thereof. Suitable rigid polymers include, for
example, polyacetals, such as Delrin.RTM. and Celcon.RTM., polysulfones,
polyethersulfones, polyarylsulfones, polyetheretherketones, and
polyetherimides. Suitable metals include biocompatible metals, such as,
stainless steel, titanium, cobalt alloys, such as Elgiloy.RTM., a
cobalt-chromium-nickel alloy, and MP35N, a
nickel-cobalt-chromium-molybdenum alloy, and Nitinol.RTM., a
nickel-titanium alloy. Heart valve stents made from spring metals, such as
Elgiloy.RTM., exhibit good mechanical properties, such as strength and
fatigue endurance, and can have a smaller cross-section than corresponding
polymer stents. Composite metal/polymer heart valve stents are described
in copending and commonly assigned U.S. patent application Ser. No.
09/475,721 to Reimink et al., entitled "Medical Devices With
Polymer/Inorganic Substrate Composites," incorporated herein by reference.
In addition, stents can be produced from ceramic materials, such as
silicon carbides or metal carbides, hydroxyapatite and alumina. Suitable
stents can also be produced from carbons such as pyrolytic carbon,
turbostratic carbon and graphite. Composites suitable for stents that
advantageously combine pyrolytic carbon and carbides are described in
copending and commonly assigned U.S. patent application Ser. No.
09/460,140 to Brendzel et al., entitled "Pyrolytic Carbon and
Metal/Metalloid Carbide Composites," incorporated herein by reference.
Various embodiments of the stent structure are suitable for testing by the
methods described herein. Two exemplary embodiments are shown in FIGS. 5
and 6. Referring to FIG. 5, stent 240 has a wire-form structure. Stent 240
has three commissure posts 244, 246, 248 that point generally downstream.
Commissure posts are connected to adjacent posts by scallops 250, 252,
254. The wire-form structure can have various cross sectional shapes of
the wire, which may or may not vary at different locations. For example,
stent 240 may flatten out and thicken at the lower portion of the scallop
to provide additional support for a sewing cuff or the like that is
positioned near the inflow edge of the valve. A variation on this type of
stent structure with a metal base ring connected to the scallops is
described in U.S. Pat. No. 4,343,048 to Ross et al., entitled "Stent For A
Cardiac Valve," incorporated herein by reference.
A sheath-shaped stent 270 is shown in FIG. 6. Three commissure posts 272,
274, 276 extend generally upward around the circumference of the stent.
Commissure posts 272, 274, 276 are connected by scalloped portions 278,
280, 282. In contrast with the wire-form structure of the stent in FIG. 5,
stent 270 has a filled in structure under commissure posts 272, 274, 276
and scallops 278, 280, 282. Stent 270 has an annular ring structure 284 at
the inflow edge 286. The outer surface of stent 270 can be covered with a
fabric to facilitate implantation of the valve or for convenience of
handling.
Flexible leaflets are generally formed from tissue or polymers. Natural
tissues are derived from a particular animal species, typically mammalian,
such as human, bovine, porcine, seal or kangaroo. These tissues may
include a whole organ, a portion of an organ or structural tissue
components. Suitable tissues include xenografts, homografts and
autografts. Synthetic tissue formed from a synthetic matrix is also
possible tissue material. Tissues can be fixed by crosslinking. Fixation
provides mechanical stabilization, for example, by preventing enzymatic
degradation of the tissue. Glutaraldehyde, formaldehyde or a combination
thereof is typically used for fixation, but other fixatives can be used,
such as epoxides, diimides and other difunctional aldehydes. Particularly
suitable flexible polymer materials for the formation of flexible polymer
heart valve leaflets include, for example, polyurethanes, polydimethyl
siloxanes, polytetrafluoroethylenes, derivatives thereof and mixtures
thereof.
Sewing cuff 116 can be produced from natural material, synthetic material
or combinations thereof. Suitable natural materials for sewing cuff 116
include, for example, fixed/crosslinked tissue, such as bovine or porcine
pericardial tissue. Suitable synthetic materials for sewing cuff 116
include flexible polymers, generally woven into a fabric. Preferred
materials include, for example, polyesters, or polytetrafluoroethylene.
Stented Structure For Testing
To test the stents under more appropriate loads and load distributions, the
stents are assembled into stented test structures with a flexible material
membrane supported by the stent. The stented test structures are similar
to closed prosthetic valves with flexible leaflets. However, the stented
test structures differ from the prosthetic valves in that the stented test
structures do not open fully, if at all. The flexible membrane serves to
transmit fluid pressures into loads, i.e. stresses, on the stent. If the
flexible membrane has similar characteristics as corresponding leaflets
and is attached to the stent in a similar way, the peak loads and
distribution of loads on the stent are comparable to the peak loads and
distributions of loads on the stent during actual use of a corresponding
prosthesis.
By limiting the movement of the membrane such that it does not fully open
in response to fluid pressures, the membrane has a significantly faster
response time relative to standard valve leaflets with respect to closing
upon reversal of fluid pressures. Limitations on membrane movements also
reduce wear and dynamic stress relative to valve leaflets, such that the
membranes generally last longer than leaflets. The limitations on the
membrane opening can be structural, i.e., sealing of coaptation edges, or
mechanical, i.e., reducing the flexibility of the membrane to the point at
which the membrane does not fully open at the fluid pressures of the
testing even though the coaptation edges are not sealed.
Similar materials can be used for the flexible membrane as are used for
flexible leaflets in valved prostheses. Thus, the flexible membrane can be
formed from tissue or polymer materials. In preferred embodiments, the
flexible membrane has similar elasticity as corresponding flexible
leaflets such that the transferred peak load is approximately the same in
the prosthesis and the stented test structure. However, the flexible
membrane does not need to be formed from the same material as the
corresponding valve leaflets and does not need to be biocompatible since
the stented test structure is not implanted into a patient. For example,
it may be desirable to test a stent for a tissue-based prosthetic valve
with a polymer flexible membrane.
While the flexible membrane should be somewhat flexible to respond
appropriately to fluid pressures, the flexible membrane does not need to
be as flexible as the prosthetic leaflets since the motion of the membrane
is limited. Due to the reduced need for flexibility, a more durable
material can be used for the membrane as compared to prosthetic leaflets.
Thus, elasticity can be adjusted to be similar to the valve leaflets to
transfer the load in an approximately equivalent manner without particular
regard for the resulting flexibility of he material.
The material used for the flexible membrane should be durable since the
material is used for the testing of stent fatigue. In evaluating stent
fatigue, the stented test structure is cycled to simulate many years of
use. For a human patient, a heart valve cycles about 40 million times each
year, and the valve ideally remains functional over the remaining natural
expected lifetime of the patient. In some embodiments, the flexible
membrane can withstand at least about 400 million cycles and more
preferably at least about 600 million cycles without significant
structural deterioration. While the flexible membrane can be replaced
while testing one particular stent, it is convenient to be able to use the
same flexible membrane throughout the test of a single stent.
Tissue for forming a flexible membrane can be similar to the tissue used
for leaflets. Glutaraldehyde crosslinked tissue can be used without
concern regarding cytotoxicity or treatment with anti-calcification
agents. Since flexibility is less of a concern, somewhat thicker tissue
can be used. For example, bovine and equine pericardium is somewhat
thicker than porcine heart valve tissue. However, tissues tend to be
somewhat variable in mechanical properties.
In some preferred embodiments, the membrane is formed from polymers.
Generally, any flexible polymer can be used for the flexible membrane
including, for example, the same flexible polymers that are used for
flexible valve polymer leaflets. A flexible polymer used to form the
leaflets of heart valve prostheses is preferably a polymer that has
sufficient durability to withstand the repeated cycling required for
replacement heart valve use. Polyurethanes and silicone polymers are
suitable polymers for forming flexible membranes that generally can
achieve desired performance requirements. Since the membrane does not have
to be as flexible as leaflets, the membrane can be made thicker than
flexible polymer leaflets. Polymer membranes formed from polyurethane or
silicone generally have a thickness from about 100 microns to about 2000
microns, and more preferably, from about 200 microns to about 500 microns.
The thickness of the polymer can be adjusted to yield the approximate
elasticity/extensibility of the valve leaflets to be mounted on the stent,
whether tissue or polymer.
Additionally, or alternatively, the thickness of the polymer can be
increased to make the membrane less flexible such that the membrane opens
a desired amount or does not open at all at the fluid pressures of the
testing. The testing flexible membrane material can be more elastic so
that the membrane can be thicker to match the elasticity of the leaflet
while forming a stiffer, i.e., less flexible, membrane. If the membrane
stiffness is large enough, the center of the membrane can open at a
specific fluid pressure, such as shown in FIG. 7, while the remaining
portions of the coapting edges remain closed. At even greater stiffness,
the membrane does not open at all. Thus, the same effects can be achieved
by selecting the stiffness without physically sealing the edges of the
membrane.
Polymer membranes can include reinforcements to improve the durability of
the membrane. Reinforcements can be formed as a thickening of the polymer,
by the embedding of a reinforcing fiber or the like within the polymer or
by a combination of a reinforcing material and a thickening. In
particular, the stented test structure can include a reinforcement at the
"outflow" edge of the membrane and/or along the contour of the membrane.
It can be particularly advantageous to reinforce the "outflow" edge since
the edge is susceptible to tearing upon repeated cycling of the stented
test structure, and with the membrane, the closed portions of the membrane
are susceptible to tearing to open the passage through the membrane to a
larger area. In addition, reinforcements can be placed throughout the
contour of the membrane or at selected locations along the contour. For
example, reinforcement fibers can run parallel to the "outflow" edge or
perpendicular to the "outflow" edge. The "outflow" edge is clear even if
the stented test structure is sealed from flow since the membrane is
shaped like leaflets that function as a one-way check valve that would
open to provide flow in one direction and close to prevent flow in the
opposite direction. Thus, the "outflow" edge of the membrane is the edge
that corresponds in position to the outflow edge of the corresponding
leaflets. Polymer heart valve leaflets are described further in copending
and commonly assigned U.S. patent application Ser. No. 09/666,823 to Woo
et al., entitled "Valved Prostheses With Reinforced Polymer Leaflets,"
incorporated herein by reference.
Tissue membranes can also be reinforced to improve durability. For example,
tissue membranes can include an extra strip of tissue attached along the
coaptation edge to reduce tearing of the edge. The strips can be attached
with suture, staples, adhesives and the like. Non-tissue reinforcements
can also be added to a tissue membrane.
The full open lumen is evaluated as the area encompassed by the largest
inner circumference of the stent obtained by projecting the stent onto a
plane. The plane is selected as the plane that yields the largest inner
circumference of the stent. Generally, this plane is perpendicular to a
central axis of the stent. In embodiments of particular interest, the
flexible polymer opens only a fraction of the full open lumen when the
fluid pressure is directed to opening the stented test structure. In some
embodiments, the stented test structure opens between no more than about
80 percent of the full lumen, in other embodiments from about 1 percent to
about 60 percent, in additional embodiments, from about 3 percent to about
50 percent, in further embodiments from about 5 percent to about 30
percent and in still further embodiments, from about 5 percent to about 20
percent of the full lumen. A person of ordinary skill in the art will
recognize that ranges within theses explicit ranges are contemplated and
are within the scope of the present disclosure. In other embodiments, the
flexible membrane is completely closed to flow through the stented test
structure.
For any membrane material, the flexible membrane in its closed
configuration can approximate the shape of the closed leaflets in the
corresponding valve. If the flexible membrane approximates the shape of
the closed leaflets, the peak loads transferred from the fluid to the
stent through the flexible membrane will more closely approximate the peak
loads in the corresponding prosthesis in vivo. Then, the fatigue testing
provides a more accurate evaluation of the performance of the stent in a
prosthesis following implantation. The shape of the flexible membrane can
approximate the shape of the closed leaflets whether or not the flexible
membrane and the leaflets are formed from the same material. Similarly,
the number of curved contours on the membrane generally corresponds with
the number of leaflets in the corresponding valve. The orientation of the
stented test structure can be referenced to the orientation of a
corresponding valve manufactured from the stent. Thus, the forward
direction is the direction to which the commissure posts point since the
leaflets are supported by the commissure posts.
A first embodiment of a stented test structure is shown in FIG. 7. Stented
test structure 300 includes stent 114 which is the stent of the prosthesis
100 of FIG. 2. Structure 300 also includes a flexible membrane 302
attached to stent 114. In a closed position, flexible membrane 302 has a
similar appearance as the leaflets of FIG. 2. As shown in FIG. 7, flexible
membrane 302 is in its open position. Flexible membrane 302 has three
contour sections 304, 306, 308 that correspond to the leaflets of the
analogous prosthesis. Contour sections 304, 306, 308 meet adjacent
contours at seams 310, 312, 314. Each contour has a small free edge 316,
318, 320, respectively, at the center of membrane 302. Free edges 316,
318, 320 open in response to fluid pressures to provide some fluid flow.
The respective lengths of free edges 316, 318, 320 and seams 310, 312, 314
can be adjusted to provide the desired amount of forward flow through the
stented test structure as well as the response time with respect to
opening and closing of the membrane. The length of free edges 316, 318,
320 correlates in a predictable manner with the size of the opening in the
center of the membrane when the membrane is in an open position.
Another embodiment of a stented test structure based on stent 114 of FIG. 2
is shown in FIG. 8. Structure 330 includes a flexible membrane 332
attached to stent 114. Flexible membrane 332 has three contour sections
334, 336, 338 that correspond to the leaflets of the analogous prosthesis
in FIG. 2. A central seam 340 connects contour sections 334, 336, 338 at
the center of flexible membrane 332. Seams 342, 344, 346 connect adjacent
contour sections away from the center of flexible membrane 332. Contour
section 334 has free edges 348, 350. Similarly, contour section 336 has
free edges 352, 354, and contour section 338 has free edges 356, 358.
Free edges open in response to fluid pressure to allow limited flow through
the flexible membrane in the forward direction. The length of the free
edges can be adjusted to yield the desired amount of flow. As shown in
FIG. 8, the free edges form three openings along the edges of contour
sections 334, 336, 338, roughly symmetrically distributed around the
center of the contour section. More or fewer free edges can be included to
correspondingly increase or decrease the number of openings for flow. For
example, free edges 350, 352 can be eliminated such that there are only
two openings formed by free edges 348, 358 and 354, 356. In other
embodiments, additional free edges can form more than three openings when
fluid pressures tend to generate forward fluid flow through the stent. For
embodiments with three contours having more than three openings, there is
more than one opening between two adjacent contours. Furthermore, features
of the embodiments in FIGS. 7 and 8 can be combined to have free edges
that open in the center of the stented test structure and free edges that
open along the interfaces between adjacent contour sections. Overall, the
number of free edges in the flexible membrane can yield one, two, three,
four or more openings in the flexible membrane when appropriate fluid
pressures are applied.
In another embodiment shown in FIG. 9, stented test structure 360 is
completely sealed along coaptation edges 362, 364, 366. Instead, one-way
portals 368, 370, 372 are provided along contours 374, 376, 378,
respectively, where stresses are low. Portals 368, 370, 372 flex open to
provide forward flow through a hole smaller than the flap of material
forming the portal. In a further embodiment shown in FIG. 10, stented test
structure 361 includes portals 363, 365, 367 located on stent 369. Portals
363, 365, 367 similarly provide for one-way flow. Furthermore, one-way
flow can be provided around the mount of the stented test structure of the
testing apparatus. For example, as shown in the fragmentary sectional view
of FIG. 11, mount 371 supports stented test structure 373 within a conduit
375. Mount 371 includes a one-way check valve 377 that provides for flow
in the direction of the arrow when appropriate pressures are applied. In
the embodiments of the stented test structure in FIGS. 9-11, the membrane
of the stented test structure can be completely sealed, which will
increase the membrane's integrity, strength and cycle life.
In general, the purpose of providing one-way flow through or around the
stented test structure is to provide a more rapid equilibration of the
fluid from the upstream chamber to the downstream chamber in testing
apparatuses with a configuration having a fluid loop. The one-way flow
compensates for u
