U.S. patent application number 16/249844 was filed with the patent office on 2019-10-31 for prosthetic venous valve devices and associated methods.
The applicant listed for this patent is Brigham Young University. Invention is credited to Anton E. Bowden, Brian D. Jensen, Ryan Packer.
Application Number | 20190328511 16/249844 |
Document ID | / |
Family ID | 68291908 |
Filed Date | 2019-10-31 |
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United States Patent
Application |
20190328511 |
Kind Code |
A1 |
Packer; Ryan ; et
al. |
October 31, 2019 |
PROSTHETIC VENOUS VALVE DEVICES AND ASSOCIATED METHODS
Abstract
A prosthetic venous valve device is disclosed and described,
having a valve base including a cylindrical shape with a lumen
configured for axial blood flow, the valve base further including
an anterograde end and a retrograde end, a pair of flexure pivots
coupled to opposite sides of the valve base at the anterograde end,
and a pair of leaflets opposingly positioned with respect to one
another and each pivotally coupled to one of the pair of flexure
pivots, the pair of leaflets being separated from one another in a
default open position, wherein the pair of leaflets are
structurally configured to pivot from the default open position
toward one another to close the prosthetic venous valve to limit
retrograde venous blood flow under normal physiologic venous
conditions.
Inventors: |
Packer; Ryan; (Provo,
UT) ; Jensen; Brian D.; (Provo, UT) ; Bowden;
Anton E.; (Provo, UT) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Brigham Young University |
Provo |
UT |
US |
|
|
Family ID: |
68291908 |
Appl. No.: |
16/249844 |
Filed: |
January 16, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62617725 |
Jan 16, 2018 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61L 27/50 20130101;
A61F 2250/001 20130101; A61L 27/08 20130101; A61L 2430/20 20130101;
A61L 2400/12 20130101; A61F 2/2403 20130101; A61F 2/2475 20130101;
A61F 2/2406 20130101; A61F 2230/0069 20130101 |
International
Class: |
A61F 2/24 20060101
A61F002/24; A61L 27/08 20060101 A61L027/08; A61L 27/50 20060101
A61L027/50 |
Claims
1. A prosthetic venous valve, comprising: a valve base having a
cylindrical shape with a lumen configured for axial blood flow, the
valve base further including an anterograde end and a retrograde
end; a pair of flexure pivots coupled to opposite sides of the
valve base at the anterograde end; and a pair of leaflets
opposingly positioned with respect to one another and each
pivotally coupled to one of the pair of flexure pivots, the pair of
leaflets being separated from one another in a default open
position, wherein the pair of leaflets are structurally configured
to pivot from the default open position toward one another to close
the prosthetic venous valve to limit retrograde venous blood flow
under normal physiologic venous conditions.
2. The prosthetic venous valve of claim 1, wherein each leaflet
further comprises a pair of leaflet flaps, with each leaflet flap
coupled along a bottom edge of the leaflet on opposite sides of the
flexural pivot.
3. The prosthetic venous valve of claim 1, wherein the pair of
leaflets are structurally configured to pivot from the default open
position toward one another to close the prosthetic venous valve at
a retrograde hydrostatic pressure of from about 15 mmHg to about 25
mmHg at the anterograde end.
4. The prosthetic venous valve of claim 1, wherein the pair of
leaflets are structurally configured to pivot from the default open
position toward one another to close the prosthetic venous valve at
a retrograde hydrostatic pressure of about 20 mmHg at the
anterograde end.
5. The prosthetic venous valve of claim 1 having a maximum shear
rate of less than or equal to about 2,100 s.sup.-1 for anterograde
venous blood flow.
6. The prosthetic venous valve of claim 1 having a maximum shear
rate of less than or equal to about 1,100 s.sup.-1 for anterograde
venous blood flow.
7. The prosthetic venous valve of claim 1 having a maximum shear
rate of less than or equal to about 300 s.sup.-1 for anterograde
venous blood flow.
8. The prosthetic venous valve of claim 1, wherein the valve base,
the pair of flexural pivots, and the leaflets are made of a
biocompatible material.
9. The prosthetic venous valve of claim 8, wherein the
biocompatible material is carbon-infiltrated carbon nanotubes
(CI-CNTs).
10. The prosthetic venous valve of claim 1, wherein the valve base
further comprises an adjustment seam to allow the valve base to be
sized to fit a given vein.
11. The prosthetic venous valve of claim 1, wherein the pair of
leaflets are aligned with the cylindrical shape of the valve base
when in the default open position.
12. A prosthetic venous valve, comprising: a valve base having a
cylindrical shape with a lumen configured for axial blood flow, the
valve base further including an anterograde end and a retrograde
end; a pair of flexure pivots coupled to opposite sides of the
valve base at the anterograde end; a pair of leaflets opposingly
positioned with respect to one another and each pivotally coupled
to one of the pair of flexure pivots, the pair of leaflets being
positioned against one another at a center of the valve base in a
default closed position, wherein the pair of leaflets are
structurally configured to pivot from the default closed position
away from one another to open the prosthetic venous valve to allow
anterograde venous blood flow under normal physiologic venous
conditions.
13. The prosthetic venous valve of claim 12, wherein each leaflet
further comprises a pair of leaflet flaps, with each leaflet flap
coupled along a bottom edge of the leaflet on opposite sides of the
flexural pivot.
14. The prosthetic venous valve of claim 12, wherein the pair of
leaflets are structurally configured to pivot from the default
closed position away from one another to open the prosthetic venous
valve at an anterograde hydrostatic pressure of from about 3 mmHg
to about 8 mmHg at the retrograde end.
15. The prosthetic venous valve of claim 12, wherein the pair of
leaflets are structurally configured to pivot from the default
closed position away from one another to open the prosthetic venous
valve at an anterograde hydrostatic pressure of about 5 mmHg at the
retrograde end.
16. The prosthetic venous valve of claim 12 having a maximum shear
rate of less than or equal to about 2,100 s.sup.-1 for anterograde
venous blood flow.
17. The prosthetic venous valve of claim 12 having a maximum shear
rate of less than or equal to about 1,100 s.sup.-1 for anterograde
venous blood flow.
18. The prosthetic venous valve of claim 12 having a maximum shear
rate of less than or equal to about 300 s.sup.-1 for anterograde
venous blood flow.
19. The prosthetic venous valve of claim 12, wherein the valve
base, the pair of flexural pivots, and the leaflets are made of a
biocompatible material.
20. The prosthetic venous valve of claim 19, wherein the
biocompatible material is carbon-infiltrated carbon nanotubes
(CI-CNTs).
21. The prosthetic venous valve of claim 12, wherein the valve base
further comprises an adjustment seam to allow the valve base to be
sized to fit a given vein.
22. The prosthetic venous valve of claim 12, wherein the pair of
leaflets are aligned with the cylindrical shape of the valve base
when in the open position.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 62/617,725, filed on Jan. 16, 2018, which is
incorporated herein by reference in its entirety.
BACKGROUND
[0002] Chronic venous disease (CVD) is a very common problem in the
medical field. Varicose veins affect more than 25 million adults in
the United States, and more than 6 million are affected with
advanced venous diseases. Varicose veins occur when veins become
dilated and overfilled with blood. They appear purple or red in
color and are often painful. Individuals with varicose veins often
have symptoms of aching, burning, pressure, heaviness, or weakness
in the legs after standing or sitting for a long period of time.
Chronic venous insufficiency (CVI) is a condition that affects the
venous system of the lower extremities. CVI is characterized by
constant venous hypertension, which leads to pain, edema, skin
changes, and ulcers in the affected individual. These individuals
not only suffer the physical effects of the disease but also endure
the psychological ailments caused by undesired color changes and
bulging of the skin. In severe cases of CVI involving deep vein
thrombosis and pulmonary embolism, death can occur.
[0003] Current treatments for CVI treat the symptoms of the disease
rather than its source. Generally, doctors will first apply
compression therapy, and if needed follow that with laser ablation,
sclerotherapy, or stripping. These treatments remove the affected
vein from the venous system but do not treat the source of the
problem. A properly functioning venous system will return blood to
the heart by means of the interaction of a central pump, a pressure
gradient, a peripheral venous pump, and competent venous valves.
Often the main source of the problem is venous valve dysfunction.
The purpose of venous valves is to direct blood back toward the
heart and impede reverse flow. In the case of CVI, the valves do
not close properly and thus cause hypertension and blood pooling in
the lower limbs. When the individual's symptoms become serious
enough, the source of the problem can be resolved by performing
valve reconstruction surgery. The long-term success of such
methods, however, can be low depending on the type of surgery
performed, and in some cases the entire venous valve may be
destroyed, which makes surgery very challenging or even impossible
in some cases.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 illustrates a prosthetic venous valve device in
accordance with an example embodiment;
[0005] FIG. 2 illustrates a prosthetic venous valve device in
accordance with an example embodiment;
[0006] FIG. 3 illustrates modeling data of deflection results in
accordance with an example embodiment;
[0007] FIG. 4 illustrates modeling data of stress results in
accordance with an example embodiment;
[0008] FIG. 5 illustrates modeling data of a mesh of fluid domain
in accordance with an example embodiment;
[0009] FIG. 6A illustrates modeling data of velocity profiles at
the planes of symmetry in accordance with an example
embodiment;
[0010] FIG. 6B illustrates modeling data of velocity profiles at
the planes of symmetry in accordance with an example
embodiment;
[0011] FIG. 7 illustrates modeling data of shear rate throughout
the vein in accordance with an example embodiment;
[0012] FIG. 8 illustrates modeling data of shear rates near the
inlet of the valve in accordance with an example embodiment;
and
[0013] FIG. 9 illustrates modeling data of a region of low shear
rates in accordance with an example embodiment.
DESCRIPTION OF EMBODIMENTS
[0014] Although the following detailed description contains many
specifics for the purpose of illustration, a person of ordinary
skill in the art will appreciate that many variations and
alterations to the following details can be made and are considered
included herein. Accordingly, the following embodiments are set
forth without any loss of generality to, and without imposing
limitations upon, any claims set forth. It is also to be understood
that the terminology used herein is for describing particular
embodiments only, and is not intended to be limiting. Unless
defined otherwise, all technical and scientific terms used herein
have the same meaning as commonly understood by one of ordinary
skill in the art to which this disclosure belongs. Also, the same
reference numerals in appearing in different drawings represent the
same element. Numbers provided in flow charts and processes are
provided for clarity in illustrating steps and operations and do
not necessarily indicate a particular order or sequence.
[0015] Furthermore, the described features, structures, or
characteristics can be combined to in any suitable manner in one or
more embodiments. In the following description, numerous specific
details are provided, such as examples of layouts, distances,
network examples, etc., to provide a thorough understanding of
various embodiments. One skilled in the relevant art will
recognize, however, that such detailed embodiments do not limit the
overall concepts articulated herein but are merely representative
thereof. One skilled in the relevant art will also recognize that
the technology can be practiced without one or more of the specific
details, or with other methods, components, layouts, etc. In other
instances, well-known structures, materials, or operations may not
be shown or described in detail to avoid obscuring aspects of the
disclosure.
[0016] In this application, "comprises," "comprising," "containing"
and "having" and the like can have the meaning ascribed to them in
U.S. patent law and can mean "includes," "including," and the like,
and are generally interpreted to be open ended terms. The terms
"consisting of" or "consists of" are closed terms, and include only
the components, structures, steps, or the like specifically listed
in conjunction with such terms, as well as that which is in
accordance with U.S. patent law. "Consisting essentially of" or
"consists essentially of" have the meaning generally ascribed to
them by U.S. patent law. In particular, such terms are generally
closed terms, with the exception of allowing inclusion of
additional items, materials, components, steps, or elements, that
do not materially affect the basic and novel characteristics or
function of the item(s) used in connection therewith. For example,
trace elements present in a composition, but not affecting the
compositions nature or characteristics would be permissible if
present under the "consisting essentially of" language, even though
not expressly recited in a list of items following such
terminology. When using an open-ended term in this written
description, like "comprising" or "including," it is understood
that direct support should be afforded also to "consisting
essentially of" language as well as "consisting of" language as if
stated explicitly and vice versa.
[0017] As used herein, the term "substantially" refers to the
complete or nearly complete extent or degree of an action,
characteristic, property, state, structure, item, or result. For
example, an object that is "substantially" enclosed would mean that
the object is either completely enclosed or nearly completely
enclosed. The exact allowable degree of deviation from absolute
completeness may in some cases depend on the specific context.
However, generally speaking the nearness of completion will be so
as to have the same overall result as if absolute and total
completion were obtained. The use of "substantially" is equally
applicable when used in a negative connotation to refer to the
complete or near complete lack of an action, characteristic,
property, state, structure, item, or result. For example, a
composition that is "substantially free of" particles would either
completely lack particles, or so nearly completely lack particles
that the effect would be the same as if it completely lacked
particles. In other words, a composition that is "substantially
free of" an ingredient or element may still actually contain such
item as long as there is no measurable effect thereof.
[0018] As used herein, the term "about" is used to provide
flexibility to a given term, metric, value, range endpoint, or the
like. The degree of flexibility for a particular variable can be
readily determined by one skilled in the art. However, unless
otherwise expressed, the term "about" generally provides
flexibility of less than 1%, and in some cases less than 0.01%. It
is to be understood that, even when the term "about" is used in the
present specification in connection with a specific numerical
value, support for the exact numerical value recited apart from the
"about" terminology is also provided.
[0019] As used herein, a plurality of items, structural elements,
compositional elements, and/or materials may be presented in a
common list for convenience. However, these lists should be
construed as though each member of the list is individually
identified as a separate and unique member. Thus, no individual
member of such list should be construed as a de facto equivalent of
any other member of the same list solely based on their
presentation in a common group without indications to the
contrary.
[0020] Concentrations, amounts, and other numerical data may be
expressed or presented herein in a range format. It is to be
understood that such a range format is used merely for convenience
and brevity and thus should be interpreted flexibly to include not
only the numerical values explicitly recited as the limits of the
range, but also to include all the individual numerical values or
sub-ranges encompassed within that range as if each numerical value
and sub-range is explicitly recited. As an illustration, a
numerical range of "about 1 to about 5" should be interpreted to
include not only the explicitly recited values of about 1 to about
5, but also include individual values and sub-ranges within the
indicated range. Thus, included in this numerical range are
individual values such as 2, 3, and 4 and sub-ranges such as from
1-3, from 2-4, and from 3-5, etc., as well as 1, 1.5, 2, 2.3, 3,
3.8, 4, 4.6, 5, and 5.1 individually. This same principle applies
to ranges reciting only one numerical value as a minimum or a
maximum. Furthermore, such an interpretation should apply
regardless of the breadth of the range or the characteristics being
described.
[0021] Reference throughout this specification to "an example"
means that a particular feature, structure, or characteristic
described in connection with the example is included in at least
one embodiment. Thus, appearances of phrases including "an example"
or "an embodiment" in various places throughout this specification
are not necessarily all referring to the same example or
embodiment.
[0022] The terms "first," "second," "third," "fourth," and the like
in the description and in the claims, if any, are used for
distinguishing between similar elements and not necessarily for
describing a particular sequential or chronological order. It is to
be understood that the terms so used are interchangeable under
appropriate circumstances such that the embodiments described
herein are, for example, capable of operation in sequences other
than those illustrated or otherwise described herein. Similarly, if
a method is described herein as comprising a series of steps, the
order of such steps as presented herein is not necessarily the only
order in which such steps may be performed, and certain of the
stated steps may possibly be omitted and/or certain other steps not
described herein may possibly be added to the method.
[0023] The terms "left," "right," "front," "back," "top," "bottom,"
"over," "under," "lower," "upper," and the like in the description
and in the claims, if any, are used for descriptive purposes and
not necessarily for describing permanent relative positions. It is
to be understood that the terms so used are interchangeable under
appropriate circumstances such that the embodiments described
herein are, for example, capable of operation in other orientations
than those illustrated or otherwise described herein. As used
herein, comparative terms such as "increased," "decreased,"
"better," "worse," "higher," "lower," "enhanced," and the like
refer to a property of a device, component, or activity that is
measurably different from other devices, components, or activities
in a surrounding or adjacent area, in a single device or in
multiple comparable devices, in a group or class, in multiple
groups or classes, or as compared to the known state of the art.
For example, a data region that has an "increased" risk of
corruption can refer to a region of a memory device which is more
likely to have write errors to it than other regions in the same
memory device. A number of factors can cause such increased risk,
including location, fabrication process, number of program pulses
applied to the region, etc.
[0024] An initial overview of embodiments is provided below, and
specific embodiments are then described in further detail. This
initial summary is intended to aid readers in understanding the
disclosure more quickly and is not intended to identify key or
essential technological features, nor is it intended to limit the
scope of the claimed subject matter.
[0025] The primary function of the venous system is to return blood
to the heart. Gravity and hydrostatic pressure oppose returning
venous flow when an individual is in an upright position. To
overcome this deficit, the body uses a system of venous valves and
a peripheral pump mechanism (powered by the muscles of the lower
limbs) to overcome the forces of gravity. Blood circulates through
the arteries due to dynamic pressure that originates from the
heart. However, the majority of this dynamic pressure is dissipated
by the capillaries. Hydrostatic pressure in the veins is produced
by the weight of the blood column below the right atrium. Venous
return from the lower extremities is achieved by pushing blood with
the lower extremity muscle pumps, while the venous valves prevent
retrograde flow of the blood moving downward.
[0026] The main cause of chronic venous insufficiency (CVI) results
from venous valves becoming incompetent and thus failing to seal
properly, resulting in venous reflux and distal venous
hypertension. Currently there are few options for fixing
incompetent or failed venous valves. In some cases, the valve can
be reconstructed surgically, a process known as valvuloplasty,
which has about a 70% success rate. If the valve cannot be
repaired, valve transplants from other parts of the patient's
venous system can be performed, but this procedure is successful
less than 50% of the time.
[0027] One potential solution to treat the underlying source of CVI
is an implantable prosthetic venous valve that would restore proper
function of the damaged venous valve. Many attempts have been made
to make such a prosthetic venous valve, but none have been
sufficiently successful for use in the medical field. Some of the
primary problems with previous prosthetic venous valves include
their competency, patency, thrombogenicity, biocompatibility, and
issues related to incorrect sizing.
[0028] Prosthetic venous valves can generally be categorized into
two groups: bioprosthetic and mechanical valves. Bioprosthetic
valves are valves made of organic material from other animals or
humans. Mechanical valves are created from man-made materials. In
some cases, bioprosthetic valves have been designed using tissue
derived from human umbilical veins, allografts of valves from
animals such as dogs or sheep, or a combination of metal stents and
valves from dogs or cows. Unfortunately, tests using these valves
have shown them to have significant inflammatory responses, leading
to generally poor performance in animal tests.
[0029] Extensive research has also been done on mechanical venous
valves. In some cases, such valves have been constructed from
platinum or pyrolytic carbon-covered titanium, polyether urethane,
and poly(vinyl-alcohol) (PVA) cryogel. Work with these valves has
shown that the three main causes for valve failure are
biomaterial-induced thrombosis, very high shear stress rate
platelet activation and aggregation, and very low shear stress rate
coagulation. Although progress has been made using mechanical
venous valves, no valve is yet commercially available for
implantation due at least to these aforementioned problems.
[0030] The primary problems that have thus prevented the medical
acceptance and use of prosthetic venous valves include their
biocompatibility, thrombogenicity, correct sizing, and durability.
A prosthetic venous valve design that overcomes these problems
should have certain characteristics that are similar to a native
venous valve. Regarding leaflet operation, naturally closed
leaflets should open at a physiologically appropriate hydrostatic
pressure such as, for example, 5 mmHg. Naturally open leaflets
should close at a physiologically appropriate hydrostatic pressure
such as, for example, 20 mmHg. Additionally, the opening and
closing of the leaflets should be accomplished without exceeding
the ultimate strength of the leaflet material. As another example,
a prosthetic venous valve should be constructed of a biocompatible
material that does not cause a foreign body reaction and is
resistant to cell buildup on the valve structure. In yet another
example, a prosthetic venous valve should be anti-thrombogenic.
Coagulation can occur above the maximum physiological venous wall
shear rate of 3,500 s.sup.-1. Additionally, blood stagnation can
occur below the minimum physiological venous wall shear rate of 46
s.sup.-1. To avoid thrombus formation, a prosthetic venous valve
design should thus have shear rates above 46 s.sup.-1 and below
3,500 s.sup.-1.
[0031] The presently disclosed technology overcomes these problems
though a novel prosthetic venous valve design that is biocompatible
and that minimizes thrombus formation and growth. Such a prosthetic
venous valve can thus be utilized to treat secondary venous
incompetence for patients with CVI or to prevent CVI from
developing. The prosthetic venous valve can be made from a
biocompatible material such as, for example, infiltrated carbon
nanotubes (I-CNTs), and can be structurally designed and configured
to function in a similar manner to a native venous valve. As one
example, thrombus formation can be minimized by utilizing a
prosthetic venous valve geometry that reduces the wall shear rates
in the vein, which have been shown to correlate to thrombus growth.
The present prosthetic venous valve has shear rates that are well
within the range between 46 s.sup.-1 and 3,500 s.sup.-1. In some
examples, the prosthetic venous valve can have a maximum shear rate
of less than or equal to about 2,100 s.sup.-1 for anterograde
venous blood flow. In other examples, the prosthetic venous valve
can have a maximum shear rate of less than or equal to about 1,100
s.sup.-1 for anterograde venous blood flow. In yet another example,
the prosthetic venous valve can have a maximum shear rate of less
than or equal to about 300 s.sup.-1 for anterograde venous blood
flow. Furthermore, the prosthetic venous valve implanted into a
vein can thus induce valve leaflet operation under normal venous
conditions in a similar manner to a native venous valve without
material or structural failure. As such, the presently disclosed
prosthetic venous valve can direct venous blood to the heart and
reduce venous hypertension that results from reflux of blood to the
lower extremities. Such will allow the body to heal more
effectively and the function of the venous system can be
restored.
[0032] The present prosthetic venous valve can be configured to
have a default open state or a default closed state. For the
default open state, the prosthetic venous valve closes under normal
physiologic venous conditions. For example, at a retrograde
hydrostatic pressure of from about 15 mmHg to about 25 mmHg at the
anterograde end, a prosthetic venous valve in an open state,
including a default open state configuration, will close. As
another example, at a retrograde hydrostatic pressure of about 20
mmHg at the anterograde end, a prosthetic venous valve in an open
state, including a default open state configuration, will close. As
a further example, at an anterograde hydrostatic pressure of less
than or equal to about 5 mmHg at the retrograde end, a prosthetic
venous valve in a closed state, including a default closed state
configuration, will open. As another example, at an anterograde
hydrostatic pressure of from about 0.001 mmHg to about 5.00 mmHg at
the retrograde end, a prosthetic venous valve in a closed state,
including a default closed state configuration, will open. As yet
another example, at an anterograde hydrostatic pressure of about 5
mmHg at the retrograde end, a prosthetic venous valve in a closed
state, including a default closed state configuration, will open.
It is additionally noted that the prosthetic venous valve can be
sufficiently structurally similar to the native venous valve so as
to not significantly interfere with the fluid dynamics of the vein.
The present disclosure provides prosthetic venous valves that mimic
the opening and closing dynamics of natural venous valves and
greatly reduce thrombogenic issues through low wall shear rates and
the minimization of regions of stagnant blood. The prosthetic
venous valve is additionally made using a material or materials
that do not promote thrombus formation. It is generally noted that
the present prosthetic venous valve design can be used in a variety
of locations and is not limited to the examples described
herein.
[0033] One example of a prosthetic venous valve is shown in FIG. 1,
which can include a valve base 102 and a pair of opposingly
positioned leaflets 104. The valve base 102 can have a cylindrical
shape with a lumen configured for axial blood flow. Anterograde and
retrograde axial blood flow directions are indicated by the arrows
at each end of the prosthetic venous valve. Additionally, the
anterograde arrow signifies the anterograde end of the prosthetic
venous valve and the retrograde arrow signifies the retrograde end
of the prosthetic venous valve. Similarly, the anterograde end of
the valve base 102 is opposite to the retrograde end of the valve
base 102 and is shown as the anterograde edge 112. It is noted that
the example shown in FIG. 1 is in the default open state with the
leaflets 104 positioned apart from one another, which are aligned
with the cylindrical shape of the valve base when in this default
open position.
[0034] Each leaflet 104 can be pivotally coupled to the valve base
102 by a leaflet connection segment, also described as a flexural
pivot 108. The prosthetic venous valve is shown in an open position
in FIG. 1, with the leaflets 104 positioned away from one another.
This open position can include, as described above, a prosthetic
venous valve configured to have a default open state, as well as an
open position of a prosthetic venous valve configured to have a
default closed state. When the prosthetic venous valve closes, the
leaflets 104 move toward one another in a general direction
indicated by the arrows labeled "close." The closure of the
leaflets 104 blocks at least a portion of the anterograde end of
the valve base 102, thus blocking at least a portion, if not all,
of the retrograde blood flow when in use. Additionally, each
leaflet 104 can include a pair of leaflet flaps 106 extending
downward from opposite lower sides of the leaflet 104 on either
side of the flexural pivot 108. The leaflet flaps 106 can function
to further block retrograde blood flow when the prosthetic venous
valve is closing or in the closed state. The flexural pivot 108 can
be formed from a portion of the valve base 102, as shown in FIG. 1,
or the flexural pivot can be formed separately from the valve base
and coupled thereto (not shown). While flexural pivot can begin at,
or attach to, the anterograde edge 112, the flexural pivot 108
shown in the example of FIG. 1 extends from within the valve base
102 below the anterograde edge 112. In this case, the portion of
the flexural pivot 108 located within the valve base 102 is defined
by flexural pivot cuts 110. These flexural pivot cuts 110 can
extend upwardly to the bottom edge of the leaflets 104. In this
example, an upper portion of the flexural pivot cuts 110 can extend
around a portion of the circumference at the bottom edge of the
leaflets 104 and is referred to herein as the circumferential
flexural pivot cut 114. In the example shown in FIG. 1, the portion
of the flexural pivot cuts 110 above the anterograde edge 112 of
the valve base defines one edge of each leaflet flap 106. The
circumference around the bottom edge of the leaflets 104 at the
circumference flexural pivot cuts 114 similarly defines the
boundary between the leaflets 104 and the leaflet flaps 106.
[0035] In one example the leaflets 104 and the leaflet flaps 106
are coupled to the valve base 102 solely through the flexural
pivots 108. This arrangement allows the pivot flexibility of the
leaflets 104 to be adjusted relative to the valve base 102 through
alteration of the physical configuration and/or positioning of the
flexural pivot 108. In some cases, such can include altering the
thickness of the flexural pivot or a portion of the flexural pivot.
In other cases, the size and/or shape of the flexural pivot 108 can
be altered to affect leaflet flexibility. For example, the width of
the flexural pivot 108 can be varied at the apex, the base, and/or
at any point therebetween. Altering the width of the flexural pivot
108 can involve increasing or decreasing the distance between the
flexural pivot cuts 110, altering the shapes of the flexural pivot
cuts 110, and the like. The bottom edge of the flexural pivot cuts
110 in the valve base 102 can be varied in a circumferential
direction to increase or decrease the area of material coupling the
valve base 102 to the flexural pivot 108, which can also affect the
flexibility provided by the flexural pivot 108. Similarly, the
circumferential flexural pivot cuts 114 can be varied in a
circumferential direction to increase or decrease the area of
material coupling the leaflets 104 to the leaflet flaps 106. Such
variation can affect the flexibility of the leaflets 104 relative
to the leaflet flaps 106. In yet other cases, the position of the
flexural pivot 108 relative to the valve base 102, as well as the
extent and positioning of the coupling therebetween, can be altered
to affect leaflet flexibility. For example, further variation can
be achieved by altering the distance between the anterograde edge
112 of the valve base 102 and the points where the flexural pivot
cuts 110 begin, by varying the distance from the bottom edges of
the leaflet flaps 106 and the circumferential flexural pivot cuts
114, and the like.
[0036] The leaflet flexibility thus affects the behavior of the
leaflets 104 relative to one another when under fluid pressure,
which is thus a factor affecting the opening and closing dynamics
of the prosthetic venous valve. Other features that affect leaflet
flexibility, either in the leaflets themselves or in the flexural
pivot, can also be included that can affect the overall
functionality of the prosthetic venous valve in a fluid
environment. Any such feature is thus considered to be within the
present scope.
[0037] The prosthetic venous valve can have various physical
dimensions, which can vary depending on the location into which the
prosthetic venous valve will be implanted, the physical
characteristics of individual patients, and the like. In one
example, the intended location for implantation of a prosthetic
venous valve is in the common femoral vein where a majority of
venous valve problems occur. In this case, the common femoral vein
has an average inside diameter of about 12 mm, which can provide
the appropriate diameter sizing for prosthetic venous valve design.
In some examples, the length of the prosthetic venous valve can be
sufficiently long to provide stability, but not so long as to
interfere with the fluid dynamics of the vein. In one example, the
length of the prosthetic venous valve can be between 24 mm and 48
mm. Native vein valves tend to be about twice as long as the
diameter of the vein. The length of the valve base, for example,
can be sufficiently long to allow secure attachment in the vein and
to support the leaflets for proper function. In one non-limiting
example, the valve base can be from about 12 mm to about 24 mm long
with a valve leaflet length of about 15 mm to about 25 mm long. The
thickness of the valve base and leaflet walls can be as thin as
possible while still allowing proper support and function of the
prosthetic venous valve. In one example, these thicknesses can
range from about 0.18 mm to about 0.32 mm, or in another example
about 0.27 mm. The flexural pivot can additionally have a wide
range of physical characteristics and dimensions depending on the
design of the prosthetic venous valve and the desire flexibility of
the leaflets. In one non-limiting example, however, the flexure
pivot can have an average length of about 8 mm and an average width
of about 2.22 mm.
[0038] FIG. 2 shows another example of a prosthetic venous valve
device, which can include a valve base 202 and a pair of opposingly
positioned leaflets 204. The valve base 202 can have a cylindrical
shape with a lumen configured for axial blood flow.
[0039] Anterograde and retrograde axial blood flow directions are
indicated by the arrows at each end of the prosthetic venous valve.
Additionally, the anterograde arrow signifies the anterograde end
of the prosthetic venous valve and the retrograde arrow signifies
the retrograde end of the prosthetic venous valve. Similarly, the
anterograde end of the valve base 202 is opposite to the retrograde
end of the valve base 202 and is shown as the anterograde edge 212.
It is noted that the example shown in FIG. 2 is in the default open
state with the leaflets 204 positioned apart from one another,
which are aligned with the cylindrical shape of the valve base when
in this default open position.
[0040] Each leaflet 204 can be pivotally coupled to the valve base
202 by a flexural pivot 208. The prosthetic venous valve is shown
in an open position in FIG. 2, with the leaflets 204 positioned
away from one another. This open position can include, as described
above, a prosthetic venous valve configured to have a default open
state, as well as an open position of a prosthetic venous valve
configured to have a default closed state. When the prosthetic
venous valve closes, the leaflets 204 move toward one another in a
general direction indicated by the arrows labeled "close." The
closure of the leaflets 204 blocks at least a portion of the
anterograde end of the valve base 202, thus blocking at least a
portion, if not all, of the retrograde blood flow when in use.
[0041] The flexural pivot 208 can be formed from a portion of the
valve base 202, as shown in FIG. 2, or the flexural pivot can be
formed separately from the valve base and coupled thereto (not
shown). While flexural pivot can begin at, or attach to, the
anterograde edge 212, the flexural pivot 208 shown in the example
of FIG. 2 extends from within the valve base 202 below the
anterograde edge 212. In this case, the portion of the flexural
pivot 208 located within the valve base 202 is defined by flexural
pivot cuts 210. These flexural pivot cuts 210 can extend upwardly
to the anterograde edge 212. In this example, the flexural pivot
208 extends upwardly past the anterograde edge 212 to couple with
the associated leaflet 204.
[0042] In one example the leaflets 204 are coupled to the valve
base 202 solely through the flexural pivots 208. As described in
the example of FIG. 1, this arrangement allows the pivot
flexibility of the leaflets 204 to be adjusted relative to the
valve base 202 through alteration of the physical configuration
and/or positioning of the flexural pivot 208. In some cases, such
can include altering the thickness of the flexural pivot or a
portion of the flexural pivot. In other cases, the size and/or
shape of the flexural pivot 208 can be altered to affect leaflet
flexibility. For example, the width of the flexural pivot 208 can
be varied at the apex, the base, and/or at any point therebetween.
Altering the width of the flexural pivot 208 can involve increasing
or decreasing the distance between the flexural pivot cuts 210,
altering the shapes of the flexural pivot cuts 210, and the like.
The bottom edge of the flexural pivot cuts 210 in the valve base
202 can be varied in a circumferential direction to increase or
decrease the area of material coupling the valve base 202 to the
flexural pivot 208, which can also affect the flexibility provided
by the flexural pivot 208. In yet other cases, the position of the
flexural pivot 208 relative to the valve base 202, as well as the
extent and positioning of the coupling therebetween, can be altered
to affect leaflet flexibility. For example, further variation can
be achieved by altering the distance between the anterograde edge
212 of the valve base 202 and the points where the flexural pivot
cuts 210 begin.
[0043] As has been described, the prosthetic venous valve is
designed to open, close, and limit retrograde blood flow in a
manner similar to that of a native venous valve. It is noted that
some retrograde blood flow is normal in healthy individuals, and
thus is part of normal venous valve operation. During native venous
valve closure, for example, there is a cessation of anterograde
blood flow followed by a brief interval of retrograde blood flow.
Generally, retrograde blood flow that lasts less than 0.5 seconds
in the upright position is considered healthy, while retrograde
blood flow that lasts longer than 0.5 seconds is considered
pathologic reflux.
[0044] Regarding native venous valve function, such is believed to
occur according to four phases. The first phase is the opening
phase, where the native leaflets move from a closed position at the
center of the vein toward the sinus wall. On average this stage
lasts for about 0.3 seconds when the individual is in a horizontal
position. The second phase is the equilibrium phase, where the
leading edges of the native leaflets remain suspended in the
flowing stream and undergo self-excited oscillation, which
resembles the motion of a waving flag. The third phase is the
closing phase. As the axial velocity of blood passing through the
valve decreases, the pressure on the luminal side of the native
leaflet decreases. The native leaflets then start moving toward the
center of the vein as the pressure on the mural side increases and
the pressure on the luminal side decreases. This action generally
takes about 0.4 seconds for an individual at rest and is somewhat
shorter when foot movements are being performed. The fourth phase
is the closed phase, in which the native leaflets are maintained
against one another in a closed position. The prosthetic venous
valve of the present disclosure has been designed to thus imitate
these phases of the native venous valve under normal physiologic
conditions.
[0045] The prosthetic venous valves according to the present
disclosure can be made from any biocompatible material that is
capable of being formed into an appropriate valve structure having
the physical and geometric characteristics of a native venous
valve. In one example, the biocompatible material can be an
infiltrated CNT material. In some cases, the CNT material can be
disposed on an underlying substrate. Such an underlying substrate
can be a temporary support that is fully removed from the finished
prosthetic venous valve in some cases, or at least a portion of the
underlying support can remain as part of the finished prosthetic
venous valve in other cases. As will be recognized in the art,
there are a variety of techniques to manufacture CNTs, such as arc
discharge, laser ablation, plasma torch, and chemical vapor
deposition (CVD), to name a few. The present scope is not limited
by the technique of making or preparing CNTs, or by the particular
technique of infiltration of the resulting CNTs. In one
non-limiting example using MEMS manufacturing processes, however, a
mask can be made with a detailed 2-dimensional geometry that
matches the desired 2-D geometry of a prosthetic venous valve. The
CNTs can be subsequently grown vertically, thus extruding the
2-dimensional geometry defined by the mask into a 3-dimensional
pattern of CNTs. Thus, in one aspect, the CNT pattern making up the
body of the prosthetic venous valve can be grown from a support
substrate, either by this or another technology, with or without
using a mask. In another aspect, the CNTs can be grown or otherwise
produced on a separate substrate, removed, and subsequently
deposited on the support substrate in a molded fashion to form the
pattern of CNTs.
[0046] The CNTs formed or otherwise deposited on the support
substrate can then be infiltrated by an infiltrant material to bind
the CNTs together into a CNT layer. As the CNT pattern prior to
infiltration corresponds to the desired structure and geometry of a
prosthetic venous valve, the resulting CNT layer has the desired
structure and geometry of the prosthetic venous valve. Any material
capable of being used to infiltrate CNTs that is also biocompatible
over the long term can be used as an infiltrant material. Various
infiltrant materials can be utilized, including, without
limitation, carbon, pyrolytic carbon, carbon graphite, various
polymers, and combinations thereof.
[0047] The support substrate for growing the CNT layer can have a
variety of configurations. In one example, the support substrate
can be a rod or tube having an outside diameter to provide a
desired inside diameter of the resulting prosthetic venous valve.
In another example, the outside diameter of the rod or tube can
have an outside diameter that is greater than the desired inside
diameter of the resulting prosthetic venous valve, where the excess
of material can be utilized in an overlapping fashion to create a
size adjustment seam. CNTs can be deposited onto the outside of the
support rod or tube in a pattern and thickness to create the
structures that make up the prosthetic venous valve having an
appropriate wall thickness. Following infiltration of the CNT
pattern to form a self-supporting CNT layer, the support rod or
tube can be removed from the CNT layer, resulting in a prosthetic
venous valve.
[0048] In another example, the support substrate can be a planar
substrate upon which a planar 2-D pattern is outlined. In some
cases, a mask can be applied to the planar substrate, where the
planar 2-D pattern includes unmasked portions of the planar
substrate. CNTs can be grown or otherwise deposited according to
the 2-D pattern and extruded to a 3-D pattern of CNTs. Depending on
the physical properties of the mask, the CNTs can be grown
exclusively in the unmasked portions on the planar substrate or the
CNTs can be grown across the mask and the unmasked portions on the
planar substrate, where the CNTs on the mask are removed with the
mask expose the 3-D pattern of CNTs. It is noted that the 3-D
reference to the pattern of CNTs on the planar substrate refers to
the thickness of the layer of CNTs extending therefrom. The pattern
of CNTs can then be infiltrated with an infiltrant material to form
a self-supporting CNT layer. The CNT layer can then be rolled or
otherwise formed into its desired cylindrical shape, either with
the planar support still attached or following removal of the
planar support. The axial seam can then be fixed together at a
desired diameter or the axial seam can be formed into an adjustment
seam to allow size adjustment to a desired diameter prior to
implantation of the prosthetic venous valve. In some examples, the
axial seam can be positioned along the valve base at a location
that does not extend through either of the pair of leaflets. In
cases where it remains coupled to the CNT layer during forming of
the cylindrical shape, the planar substrate can subsequently be
removed.
[0049] In another example, a support substrate that is either in a
cylindrical or a planar configuration can have CNTs deposited or
formed thereon in a continuous layer without a pattern. Either
prior to or following infiltration to form a self-supporting CNT
layer, regions of the CNT material can be ablated, etched, or
otherwise removed to form the appropriate CNT pattern.
[0050] It is generally intended that, in the various examples
described above, the support rod or tube be fully removed from the
CNT layer to avoid biocompatibility issues arising from any support
rod or tube material remaining in the prosthetic venous valve. It
is to be understood that in some situations a portion of the
support material may remain in the finished prosthetic venous valve
device, provided the amount is sufficiently small to avoid
biocompatibility issues over the long-term use of the device.
EXAMPLES
[0051] The following examples pertain to specific embodiments and
point out specific features, elements, or steps that can be used or
otherwise combined in achieving such embodiments.
[0052] Prosthetic Venous Valve Modeling:
[0053] Design Environment
[0054] The geometric designing of the prosthetic venous valve was
done in Siemens NX 10.0. The NX files were then uploaded to ANSYS
for structural analysis. Only half of the valve was modeled due to
symmetry and to decrease the computational time in the structural
analysis. The venous valve shape was modeled as a thin sheet in NX.
The element used in ANSYS was SHELL181, which best represented the
valve since its thickness was very small compared to its length.
The thickness of the valve was specified in ANSYS. Prior
experiments show a Young's modulus and ultimate strength for carbon
infiltrated CNT (CI-CNT) of 10 GPa and 153 MPa, respectively. The
valve was meshed in ANSYS using 30 element divisions for each line
of the model. Displacement constraints were set on the model so
that the valve walls did not move. Finally, a pressure was applied
to the leaflets of the valve, which represented the hydrostatic or
dynamic pressure of blood in the body, depending on whether the
model was being tested for opening or closing.
[0055] Valve Design
[0056] For the prosthetic venous valve design, the manufacturing
and hemodynamics were taken into consideration. CNTs are primarily
grown on flat surfaces, with some experimental studies done on
cylindrical surfaces made of stainless steel. To make growth and
manufacturing easier the device can be made on a cylindrical steel
rod. The diameter of the prosthetic venous valve was chosen as 12.7
mm, since the closest steel rod dimensions to the common femoral
vein is a 0.5-inch diameter rod. The basic design, comprises a
cylindrical section that would be fixed in the vein (the valve base
with valve leaflets connected to the cylindrical section by short
flexible segments (the flexure pivots). To reduce backflow around
the flexible segments, flaps were inserted in the design to
decrease the amount of open space that blood might pass through
(leaflet flaps). The design was optimized to allow the valve to
fully close with a hydrostatic pressure of 20 mmHg, with stresses
lower than the material's ultimate strength. This objective was
achieved using a flexible segment width of 2.22 mm and a flexible
segment length of 8 mm. The leaflet length was 20 mm. The valve
thickness was 0.27 mm, which is commonly achieved for CNT growth.
The CI-CNT prosthetic venous valve produced a deflection of 6.85 mm
and had a maximum von Mises stress of 117.17 MPa (see FIGS. 3 and
4). 6.85 mm of deflection allows the leaflets to move just past the
midpoint of the vein and completely seal. The deflection is
possible since the maximum von Mises stress is below the ultimate
strength of CI-CNTs.
[0057] Model Reliability
[0058] In the real world the mechanical properties of materials can
vary greatly even if the material samples are manufactured with the
same conditions and tolerances. This is especially true for the
manufacturing of CI-CNTs since the creation process is complex. The
manufacturing of CI-CNTs involves depositing several thin films on
the surface of the substrate and growing the nanotubes in a furnace
with flowing gases. This multi-step process can result in different
mechanical properties of the CI-CNTs if the film thicknesses, time
in the furnace, or flow of the gases are different between
production runs. It is important to design for the mechanical
variability of CI-CNTs to ensure the venous valve will function
properly despite differences between each manufactured sample.
[0059] To test for this scenario the final design was analyzed
subsequently with a 20% reduction and a 20% increase in the
material's Young's modulus. The deflection decreased to 5.66 mm as
the Young's modulus was increased. This change in turn reduced the
maximum stress to 114.9 MPa. On the other hand, decreasing the
modulus by 20% caused the deflection to increase to 8.67 mm and the
maximum stress to increase to 120.69 MPa. This reliability study
shows that there is no danger of the prosthetic failing under a
variable modulus, but the concern regards the proper sealing of the
valve leaflets.
[0060] Fluid Dynamic Analysis:
[0061] Model Creation
[0062] A computational analysis was performed on the prosthetic
venous valve design to assess the hemodynamic changes it could
create after implantation. The fluent solver in ANSYS Workbench
18.2 was used for the analysis. First the valve design was modeled
in AutoCAD 2018 and then uploaded into the Fluid Flow (Fluent)
module in the ANSYS Workbench. Since the valve was symmetrical on
both sides, only one fourth of the valve was used for the fluid
model to reduce the computational time to solve the simulation. The
fluid domain was created by filling in the center of the vein with
a solid and extracting the solid prosthetic venous valve design. 60
mm of the vein was included after the outlet of the valve to aid in
visualizing how the valve would influence exiting flow dynamics.
The prosthetic valve was modeled in the open position to determine
the maximum velocity and shear rates in the vein. The vein was
modeled as 12.7 mm in diameter and 100 mm long.
[0063] Initial Conditions
[0064] Before the simulation was performed the initial inlet flow
condition was obtained. The inlet conditions depend on the material
characteristics of blood and the dynamics it has at the inlet of
the prosthetic valve. In a worst-case scenario, the highest shear
rates would be caused by an inlet flow profile that would
drastically change near the vein wall. This scenario would be very
similar to a fully developed profile. The peak flow rate in the
femoral vein can be up to 1600 mL/min. To determine the distance
the flow needs to travel before it becomes fully developed the
Reynolds number and viscosity of blood need to be obtained. For
Newtonian fluids the viscosity is a linear relationship between the
shear stress and shear rate given by Equation I:
.tau..sub.rz=.mu.(.differential.v.sub.z(.theta.r) (I)
Blood can act like either a Newtonian or a non-Newtonian fluid. For
shear rates below 50 s.sup.-1, blood acts like a yield-pseudo
plastic fluid and the shear stress is not a linear function of the
shear rate. At higher shear rates, above 50 to 80 s.sup.-1, the
viscosity of blood is constant, and the Newtonian equation
correctly models its behavior. In larger vessels, the wall shear
rate nearly always exceeds 100 s.sup.-1, and the average shear rate
nearly always exceeds 80 s.sup.-1. Since the common femoral vein is
a deep vein it was assumed that the shear rate would be above 80
s.sup.-1 and the viscosity of blood would be constant.
[0065] The Reynolds number can be calculated by using Equation
II:
Re=(.rho.vD)/.mu. (II)
where .rho., v, and .mu. represent the density, average velocity,
and viscosity of blood, respectively, and D represents the diameter
of the vein. The density and viscosity of blood at high shear rates
are 1056 kg/m.sup.3 and 0.00345 Pas. The average velocity of blood,
v, can be calculated by using the flow rate through the common
femoral vein, given by Equation III:
v=Q/A (III)
where Q represents the flow rate (1600 mL/min), and A represents
the cross sectional area of the vein (1.27.times.10.sup.-4
m.sup.2). These equations resulted in an average velocity and
Reynolds number of 0.21 m/s and 819.75, respectively.
[0066] This shows that the flow through the common femoral vein is
laminar. When flow is laminar the entrance length, or distance
required for fully developed flow, can be calculated by using
Equation IV:
.sub.entrance=0.05ReD (IV)
This equated to an entrance length of 521 mm. This inlet condition
was captured by modeling a 12.7 mm vein that was 600 mm long. Only
one fourth of the vein was used due to symmetry and to reduce the
computational cost of the problem. The velocity at the entrance of
the vein was set at 0.21 m/s and a relative mesh density study was
performed until the maximum velocity changed less than 1%. This
velocity profile was then saved and used as the inlet profile for
modeling the valve.
[0067] Meshing
[0068] Once the inlet velocity components were saved and the
prosthetic venous valve model was uploaded to ANSYS Workbench the
fluid domain was meshed. An adaptive medium fine mesh was used with
a growth rate of 1.2. A relative mesh density study was performed
to determine an acceptable mesh for the fluid analysis. The results
are shown in Table 2. The element and defeature sizes were
decreased from one test to the next. The defeature size removes
small geometric features that are smaller than the specified size.
Since the smallest feature in the model had a size of approximately
0.3 mm, the initial defeature size of the mesh was set at 0.3 mm
and the initial element size was 1.5 mm. The maximum velocity of
the blood was noted after each test of the mesh study to quantify a
sufficiently fine mesh. The mesh was considered accurate for the
simulation once the maximum velocity of the blood changed less than
1% between the mesh tests. The final mesh size had an element size
of 0.375 mm, a defeature size of 0.075 mm, 38,093 nodes, and
181,312 elements. The final mesh is shown in FIG. 5.
TABLE-US-00001 TABLE 2 Prosthetic Mesh Study Mesh Element Size
Defeature Size Maximum Velocity Density (mm) (mm) (m/s) 1 1.5 0.3
.38063 2 0.75 0.15 .38978 3 0.5 0.1 .39588 4 0.375 0.075 .39591
[0069] CFD Results
[0070] The fully developed peak flow rate through the valve
resulted in an increase of the maximum velocity from 0.379 m/s to
0.395 m/s at the centerline of the vein (see FIGS. 6A and 6B). The
velocity profile throughout the vein was undisturbed with no
reversed flow.
[0071] The maximum shear rate in the vein was 225.1 s.sup.-1. This
occurred at the inlet of the venous valve where the inner diameter
of the vein decreased due to the valve's thickness (see FIGS. 7 and
8). There was also a smaller region of high shear rates at the
corner of the top flap (visible in FIG. 7). Although these shear
rates are higher relative to the other shear rates throughout the
valve, they are still far below 3500 s.sup.-1. This indicates that
the prosthetic venous valve would be far less likely to form blood
clots due to elevated shear rates.
[0072] The minimum shear rate experienced throughout the vein was
0.123 s.sup.-1. This primarily took place around the borders of the
valve leaflets and between the leaflet flaps (see FIG. 9). The
lowest shear rates occurred near the top and bottom of the leaflet
flaps, which had smaller open areas for blood to pass through.
Opening and closing of the valve will help increase the shear rates
in these regions. Overall, the average shear rates in the areas
around the borders of the leaflets and leaflet flaps were
approximately 65 s.sup.-1. Since the average shear rates in these
areas is above 46 s.sup.-1, thrombosis formation would likely not
occur.
[0073] Model Validation
[0074] To ensure that the CFD shear rate results were correct and
in the appropriate range a theoretical wall shear rate in the
common femoral vein was calculated. Assuming the blood in the vein
is a Newtonian fluid, the flow is steady and laminar, and the vein
is straight and inelastic, Poiseuille's law may be applied to
determine the wall shear rate, given by Equation V:
.gamma.=(32Q)/(.pi.d.sup.3) (V)
where Q is the flow rate through the vein, d is vein diameter, and
.gamma. represents the wall shear rate. Using the peak flow
experienced in the femoral vein, 1600 mL/min, and a vein diameter
of 12.7 mm, the wall shear rate would be 132.6 s.sup.-1. This shows
that in a common femoral vein without any obstructions or venous
valve the wall shear rate would be close to 132.6 s.sup.-1. The
theoretical shear rate validates the CFD shear rate since the CFD
result is slightly above the theoretical value. This slight
increase is to be expected due to the inclusion of the prosthetic
venous valve.
* * * * *