Note: Descriptions are shown in the official language in which they were submitted.
1 BACKGROUND OF THE INVENTION
This inv~ntion relates yenerall~ to cardiovascular
prostheses foxmed of synthe-tic fabrics. More particularly, it
relates to heart valve replacements comprising fabrics supporked
on frameworks and forming a txileaf]et configuration. -
The principal objects of the invention are to provide
heart valve replacements having a useful life o~ many flex cycles
to dup`licate as closel~ as possible the mechanical properties o~
natural heart valves, and to avoid the introduction of blood
clotting conditions.
The replacement of heart valves with prostheses has
become a standard surgical technique. However, the prostheses
currently in use do not entirely satisfy the above objecks.
Currently, most prosthetlc heart valves rely for closure on ~he
sealing of a ball or a flap against a gasket rlng. With this
construction the ball or flap is sîtuated within the flow
channel when lifted aWa~ fxom the gasket in the ~low configuration.
This is disadvantageous in two important respects. First, the
pressure drop across the valve during the open or Elow condition
~0 is greater than the pressure drop across the natural valve ~hich
causes a slight, but continuous and cumulative overload on the
heart~ Second, the presence o~ the ball or ~lap cxeates regions
of turbulent flow tending to damage the red blood cells.
With the ~oregoing disadvantages in mind, research has
been directed to developing leaflet valves more closel~ approxi~
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1 mating the structure and functions of the human valve. The
latter comprises thin, ~lexible membranes that fold natural
into the surrounding blood vessel in the open con~iguration,
thus causing a minimum of disturbance to the flowing blood. In
the closed conf~guration the leaflets form three contiguous
pouches that are held in close and leak-proof contact by the
pressure of the blood. As a result of the extreme lightness and
flexibility of the leaflets the valve has a short response time,
passing quickl~ fxom the fully closed to the fully open state,
10 with the result that there is little energy loss in the flowing ~-~
blood and a minimum of undesirable regurgitation.
These functional characteristics of the human valve
result from the composite structure of the natural l~aflet. This
comprises an arrangement of bundles of collagen fibers embedded
in a softer tissue material. The composite structure~gives the
leaflet good load bearing capacity , a high resistance to tear
and suficient softness and flexibility to make a good seal in
the closed configuration. At the peak of the pressure pulse, the
leaflet withstands a load exceeding 150 gm/cm along a line
therein normal to the load.
Heart valve tissue is also anisotropic in its elastic
propertiest that is, the load-deformation characteristic in one
direction is di~fexent fxom that in another direction. It has
been ~ound useful to define two particular directions for
purposes of this description. These directions are parallel
and perpendicular to the ~ree edge of the leaflet, and corres-
pond respectively to the circumferential and radial directions
commonly referred to in the literature.
In the direction parallel to the free edge the natural
leaflet extends very readily with increased load until an
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1 elongation of ten to twelve persent is reached at a load of one
to two grams per centimeter of leaflet width. upon further
increase in load the resistance to ~urther elongation increases
greatly. In the direction perpendicular to the free edge the
region of easy exten~ion with increased load continues to
approximately 20 percent elongation, at ~hich the load is about
2 gm/cm. Upon further increases in load the resistance to
further elongation, though greater than in the initial region,
is not as high a~ it is in the parallel direction.
A recent development involves the use of stabilized pig
heaxt valves as replacements for failing human valves. These
valves embody some of the characteris~ics of human valves
discussed above. ~owever, the collection, grading, sterilizing,
fixing and storing of pig valves is complicated and costly. In
consequence, a clear need has been realized for a trileaflet
heart va].ve made entirely from synthetic materials.
SUMMARY OF T~IE INVENTION
According to this invention, a replacement heart valve
2Q is formed of flexible fabric leaflets inserted into a frame of
three-lobed configuration, this frame supporting the leaflets
in proper orientation. The leaflets each consist essentially
of a textile of filaments having a substantial number of
open interstitial spaces in the range of 20 to 40 microns
evenly distributed throughout the fabric.
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1 Preferably, the fabrics are woven in the form
of ribbons hav.ing a selvage of uncut yarns, this selvage forming
the free edge of the heart valve leaflet. ..
To thls end, in one of its aspects, the invention
provides a heart valve prosthesis comprising in combination,
a frame having a plurality of mutually spaced, generally
parallel legs, each leg comprising a pair of rod portions ~ .
connected at one end thereof, each rod portion diverging at
its other end in the form of a lobe connected with a rod
portion of another leg, the lobes forming an aperture there-
between, and
a plurality of flexible porous fabric leaflets each consist-
ing essentially of a textile of filaments having a substantial
number of open interstitial spaces in the range of 20 to 40
microns evenly distributed throughout the fabric, each leaflet
being inserted between the rod portions of two of said legs,
being sealed to said legs and to the interconnecting lobe and
having a free edge extending between said legs, the free
edges of said leaflets being deflectable into mutual contact
~0 for sealing said aperture, the stretch characteristic of each
leaflet having a region of easy elongation up to at least ten ~:~
percent in directions both. parallel and perpendicular to its
free edge. : .
In another of its aspects, ~he invention provides a
fabric for a cardiovascular prosthesis comprising woven multi- .
filament polymeric yarns extending in first and second directions
forming an angle therebet.ween, the yarns in each of said
directions having crimps similarly oriented w;th respect to
the plane o.f the fabric and being bloomed in their interstices
39 to form a substantial number of open interstitial spaces in the
range of 20 to 40 microns evenly distributed throughout the
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1 Eabric, -the s-tretch characteristic of -the ~abric having a region
of easy elonyation up to at least ten percent in at least one
of said direc-tions.
lrhe foregolny and other features oE the invention are
clescribed more fully in the following description with reference
to the appended drawings.
_:RIEF DESC~IPI`ION OF THE DR~WINGS
Fig. 1 shows the main frame of ~he preferred form o~
the heart valve.
Fig. 2 shows a fabric ri~bon in the configuration formed
by inserting it into the main frame between the rod portions of
its legs.
Fig. 3 shows the second frame.
Fig. 4 is a top plan view of the frame shown in Fig. 1.
Fig. 5 is a top plan view of the frame shown in Fig. 3.
Fig. 6 shows the partially fabricat~d heart valve with
the fabric inserted into the main frame and cut open preparatory
to cementing thereto.
Fig. 7 is a developed view of the partially constructed
heart valve, corresponding to Fig. 6.
Fig. 8 is a cross-sectional view taken on line 8-8 in
Fig. 7.
Fig. 9 illustrates a flat braicled fabric pattern.
Fig. 10 i~s a photograph showing a plain woven nulti-
filament polymeric fabric prior to compressive shrinking.
Fig. 11 is a photograph showing tke fa~ric of Fig. 9
after compressive shrinking in the warp and weft directions.
DETAILED DESCRIPTION OF THE INVENTION
Referring to the drawings, Fig. lQ shows one form of
starting fabric 12 having warp yarns 14 and weft yarns 16. The
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1 warp and weft ~arns are each ~ormed of untwisted filaments 18 -~
of a polyester, for example, polyethylene terephthalate. For
illustration, the fabric may be woven with approximately 100
yarns per inch in each direction, the ~arns being about 30
denier. Each yarn contains 30 filaments 18, each filament having
a diameter of about 10 microns.
Because the ~arns have no twist, as shown in the woven
form of Fig. 10 they have a flattened configuration. Consequently,
the fabric is only about 3 to 4 filament-diameters thick. Also,
as ~urther explained below, the fabric is preferabl~ woven with
at least one selvage having no cut yarns. For this reason the
fabric is advantageousl~ ~oven in the form of a ri~bon, although
th.is is not a necessit~.
Commercially available polyester yarn generally contains
a number of impurities that ma~e it potentially damaging to the
body if used in implanted prosthetic devices. These impurities
include residual catalyst from the polymerization process,
oligomers, antioxidants and-other stabilizers, delusterant and
surface finishes. Accordingly, it is preferred to employ a pure
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~O polymeric material.
As shown in Fig. 10, the yarns 14 and 16 are Woven so
as to provide interstices 20. ~hese interstices have a role in
the subse~uent shrinking ~nd crimping pxocess steps described
belowt which are perPormed on the ~abric o~ ~ig. 10 to produce -~
the bilatexally crimped form shown in Fig. 11. In successive
steps, the fabric is shrunk in two directions by compacting it
while subject to in-plane compressive stress, each step being
substantially as described for example in U.S. Patent No.
3,001,262 dated September 26, 1961, to Charles Schwabe Parker and
Alexander Melville. A machine may be used similar to that
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1 described in U.S. Patents Nos~ 2,765,513 and 2,765,514 both
dated October 9, 1956, and hoth to Richard R. Walton. The
shrinking steps produce crimps in the yarns and cause them to
bloom or spread out in the interstices o~ the woven pa-ttern.
In oraer to produce the desired easy stretch character-
istics in both directions, smilar to those of the natural valve
leaflet, the same woven fabric is compacted in both the warp
and weft yarn directions sequentially. In this respect, the ,
present process is in contrast to the technique described in the
above patent ~o Parker et al, wherein the yarns of a first woven
fabric are crimped in the ~arp direction, the fabric is then
unravelled and,the warp yarns are used as the weft or filling
yarns of a subse~uentl~-woven fabric, the latter still later
being crimped in the warp direction to yive a two~wa~ stretch
chaxac-teristic. As a result of the present process the crimps in
the yarns in both the warp and weft directions are formed and '`
heat set to lie in planes generally parallel with the fabric,
and'the yarns are bloomed in the interstices to form spaces
of varying s-izes and orientations b~tween the filaments. A
substantial number of these latter spaces have dimensions in
a preferred range of 20'to ~0 microns and are produced'
'in an even dist~ibution throughout the fabric~ ' ' '
Thus ~he completed ~abric is relatively thin as compared with
the Einal fabric of Parke~ et al wherein the crimps and bloomed
filaments of the final weft or filling yarns are necessarily
displaced and reoriented duxing the second weaving relative to
the plane of the fabric and'the interstitial locations in the
final weave.
~.he des;ra~lillty of a thin fabric over a thicker one .
3~ has been recognized as the result of the functional charac-teristics
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1 desired, in particular the flexibility and stretch character
istics, the non-thrombogenic property and the capability of with-
standing many millions of flex cycles without fatigue failure.
Non-thrombogenic propertles are imparted to the fabric described
herein so that the flowing b]ood is in contac~ only with
naturally-occurring, compatible surfaces. Thus the filaments
in the fabric become analogous to the bundles of collagen
fibers in the natural leaflet, and provide a textile scaffold
or lattice onto which the body can deposit tissue to prov~de the
memhrane function of the leaflet and the desired sealing
properties. Thus the fabric becomes completely embedded in a
layer of living tissue that is thin enough to be nourished by
diffusion processes.
For effective use as a heart valve leaflet the fabric
must have a region of very easy extension up to elongation
levels in the range of 10 percent to 20 percent. In particular,
as will be evident from the following description of Figs. 1
to 8, each leaflet in the closed configuration is subjected
- to bending over a-relatively sharp radius along a line per-
pendicular to its free edge. Along this line there is a region
~ of high stress concentration.
An additional advantage of the present method is
illustrated by Fig. 11 wherein it wilL be noted that the
individual yarns are bloomed in the interstices ~0; that is,
the filaments are opened up locally in these interstices to
provide a distributed matrix of smaller interstices of
varying siæes and orientations. In contrast, the precursor
fabric such as that of Fig. 10 has a regular configuration
of spaced holes of relatively larger, fixed dimensions.
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1 The fabric manufac-tured according to the present
process has been tested for tissue ingrowth, and has been
shown to have performance superior in this respect to other
available fabrics. It can be produced with the specific
elongation characteristics needed for the leaflet application,
and moreover such elongation characteris~ics may differ in
two directions~ such as the warp and weft directions.
The following is a more detailed description of the
creping or compacting method employed according to this
invention. Using the machine described in the above-mentioned
patents Nos. 2,765,513 and 2,765,514, a ribbon of the woven
fabric shown in Fig. 10, approximately one and one-half
inches wide and six inches long is placed between two sheets
of paper and passed through the bite between a top roll and
a bottom roll in the warp-wise direction. The top roll
has a surface speed of 5.3 ft/sec and the bottom roll has a
surface speed of 1.1 ft/sec. During this compacting
run the warp threads are shrunk or compacted to form crimps,
these crimps being forced by the applied pressure of the rolls
to lie in planes generally parallel with the fabric. At
the same time the filaments of the warp threads are
spread apart in the interstices between the weft
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1 yarns; that is, the yarns ~loom in these interstices, thereby
formin~ spaces of var~ing sizes and orientations, as shown in
Fig. 11.
Upon completion of this compacting step the fabric is
removed from the paper sheets.
A second compacting run is then performed in a sub-
stantially identical manner to that described above, except that
the fabric piece is passed through the-bite of the rolls in the
weft-wise direction.
If desired, each of the above-described compacting runs ~ ~-
may be repeated, in which case the ribbon is preferably rotated
through 90 degrees after each run.
The compaction steps in the two directions may be varied
as to number and degree of compaction to produce the desired
load-elongation characteristic in each o~ the two directions,
thus approximating the corresponding characteristics o~ the
naturaL leaflet. In an~ case, the compaction steps are such as
to produce very eas~ extension up to elongatlonal levels of
10 to 20 percent.
After the series o~ compacting runs, the fabric is heat
set while in an unstressed state at a temperature below the
fusion temperature.For polyethylene terephthalate a tempexature
of 410 degrees F may be used, ~or example. The heat setting is
prePerably performed in a circulating hot air oven, and a t~pical
time duration is one and one-half minutes
In some cases it is desirable to treat the ~a~n prior to
weaving in order to prevent damage to the filaments during the
weaving process. ~ suitable method of treatment is a coating of
5 percent solution of polyvin~l alcohol. After weavlng, this
additive is wash~d out of the fabric in an aqueous wash.
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1 We turn next to a des'cription of the pre~erred ~orm of
trileaflet heart valve replacement. Referring to Fig. 1, there
is shown a main frame 22 comprisiny a single length of 0.1 cm
diameter round polypropylene rod bent into a form having three
mutually equidistant, generall.~ parallel legs 24, 26 and 28, each
leg comprising a pair of rod portions slightly spaced apar~, the
rod portions being connected at one end.. and diverging at the ~'
other end. The diverging rod portians form three lobes 30, 32
and 34. The connected ends of the rod portions.in each pair
form bights 36, 38 and 40. Fig. 4 is a top plan view of the main ~ ,
frame 22.
A second frame 42 (Figs. 3 and 5) is formed o~ a single
length of 0.1 cm diameter round polypropylene rod,bent into a .,
form having three lobes 44, 46 and 48 generally congruent with
the lobes 30, 32 and 34 so as to fit in close contact therewith
as shown in ~lg. 7.
The assembly is started by threading a bilaterally
crimped'and compacted ribbon S0, produced by the method described
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' above and appearing as in Fig. 11, through the three pairs of legs .
so as to produce the con~iguration shown in Fig. 2. The frame
22 is shown in Fig. 1 in exploded relation to Fig. 2 for clarity
of illustration.' The upper selvage has no unaut yarns and ~orms
the free e~ge~ 52, 54 and 56 of va'lve leaflets.
Thus a double layer o~ the ~abric is passed through each
pair o~ rod portions forming one of the legs 24, 26 and 28. It
is nece~sary to attach the fabric firmly to these legs, and also
to the connecting lobes 30, '32 and 34. To facilitate this
attachment, the ~abric is preferably cut lengthwise externally
of each leg as shown in Fig. 6.
Referring to Fig. 6 adhesive such as polyurethane
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1 dissolved in tetrahydrofuran is applied to attach the fabric tQ
each of the legs as follows. Flaps such as 58 and 60 are spread
apart and the adhesive is applied at the external point of
juncture of the ~laps where they enter between the rod portions,
in a continuous line extending bet~7een points a and b. The
adhesive material reaches to the external surfaces of the frame
by penetration through ~he fabric flaps along this line; that is,
the adhesive contacts the roa portions of the frame only on their
outer surfaces. The leaflets comprise only those portions of the
fabric on the inside of the framer and these portions are not
penetrated by the adhesive. Thus local stiffening and resultant
flex failure caused by such adhesive penetration is avoided.
The above method of adhesive application also
distributes the stresses of flexure evenly along the margins of
the leaflets and avoids excessi~e stress concentrations. These
margins are permitted to moVe upon each flexure over the rounded
contours of the surfaces of the rod portions that are loca~ed on
the inside of the frame, and that are not penetrated by the
adhesive. ;
The attachment of the ~abric to the lobes 30, 32 and 34
is next accomplished by first placing the second frame 42 adja-
cent those lobes with the fabric pieces passing therebetween as
shown in Figs. 7 and 8. ~dhesive 61 is then applied through the
fabric and to khe surfaces of both the main frame 22 and the
second frame 42, in a continuous line extending between the
points b of the respective legs and connecting these three points.
As in the previous step, the adhesive material preferably does
not penetrate any portion of the leaflet material lying within
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1 the main frame 22tand remains out of contac-t with blood passing
through the valve.
The foregoing steps essentially complete the fabrication
of the leaflet portions of the valve. The remaining steps of
fabrication are for the purpose of facilitating the suturing
of the prosthesis within the blood vessel The excess fabric
a~ailable on the outside of the ~rame can be rolled and consoli-
dated along the junction line between the main and second frames
to pro~ide attachment points or sti~ches during surgical
insertion.
The frame material is preferably polypropylene, although
other materials have also been employed with success. Pol~pro-
pylene has excellent flex endurance and chemical stability, but
is dif~icult to attach b~ adhesive to other matexials. To
facilitate adhesion, the main and second frames 22 and 42 ma~ be
encapsulated with polyuxethane b~ multiple dip co~ting. ~he
resulting encapsulated frame components have proved to demon-
strate the desired characteristics of polypropylene without
structural failures or breakdowns at the adhesive junctures.
Valves employing the fabric described a~ove have been
tested in an accelerated fatLgue tester to asse~s t~eir lon~-
term endurance characteristic~. Fatigue failures so induced have
generall~ occurred Ln the region of greatest fabric flexure,
tha~ is, along a line in each leaflet that is perpendîcular to
its free edge and substantiall~ equidistant between the contiguous
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legs. The failures have generall~ occurred b~ breakdown of the
ilaments in the yarns running parallel to the free edge of the
leaflets. As a means of providing greater fabric strength along
the last-mentioned lines, woven fabrics ma~ be provided with a
3 greater number of load-bearing yarns in this direction.
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1 However, there is ~ limit to the increase that is
possible using the plain woven patkern of Fig~ lO without
seriously disturbing the geometry o~ the fabric interstices.
An alternative fabric construction pattern having
improved strength against such fatigue failure is illustrated
in Fig. 9. The fabric shown is a flat braided ribbon 62 com-
prising 3 sets of yarns, namely, a first diagonal set 64, a
second diagonal set 66 and a longitudinal inlaid set ~8. The yarns
~n each of the three sets are pre~erably multifilament untwisted
yarns similar to those shown in Fig. lO. The ribbon 62 is braided
on a conventional flat braidin~ machine. It will be noted that
each`selvage has uncut yarns and one of those becomes the free
edge of each leaflet. Thus fraying of the free edges of the leaf-
lets is avoided as in the example described above. In this embodi-
ment both of the sets 64 and 66 perform the load bearing function
of a single set of yarns in the earlier-described ~abric. The
result is that a greater number of yarns have a significant
component of load bearing capacity oriented parallel to the free
edge.
The fabric 62 o~ Fig. 9 is preferably formed by braiding
the yarn sets 6~ and 66 with inlaid longitudinal ~arns 68 in a
well-known manner, thus producing a type of triaxial fabric.
Such flat braided fabrics have an additional advantage over
conventionally woven fabrics, in that they are inherentl~ highl~
extensible in the cross machine direction, that is, in the
direction perpendicular to the yarns 68. Such fabrics make it
possible to produce a two-way stretch characteristic with any
desired combination of stretch capabilities by compacting in the
direction of the yarns 68 only.
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