Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The inventive article described herein is a synthetic prosthesis for
replacement or repair of ligaments or tendons.
DESCRIPTION OF THE PRIOR ART
The generally accepted method of repair of ligaments and tendons is
through the use of tissue transplanted to the defect site from elsewhere in
the body. This method of repair often fails due to a number of factors,
including insufficient strength of the transplanted tissues, dependence of the
transplanted tissue on revascularization for viability, and inadequate
strength of attachment or fixation of the transplanted tissue.
A great need exists for a prosthetic device to replace damaged ligaments
and tendons, and there have been a number of previous attempts at providing
such devices. However, there is no prosthesis today which is widely accepted.
Among the reasons for failure of prosthetic devices are inadequate tensile
strength, lack of adequate fixation, deterioration of the device due to
mechanical stresses, and deterioration of the prosthesis/tissue interface.
Previous methods of attachment to bone and soft tissues which have been
attempted include:
U.S. Pat. Nos. 3,971,670, 4,127,902, 4,129,470, 3,g92,725, and 4,149,277.
These patents teach attachment through tissue ingrowth into porous
surfaces of the prosthetic device.
U.S. Pat. Nos. 3,613,120, 3,545,008, and 4,209,859. These patents teach
methods of tissue attachment to porous fabrics with various methods of
maintaining apposition to the repaired tissue.
U.S. Pat. Nos. 3,896,500, 3,953,896, 3,988,783, and 4,301,551. These
patents teach attachment to bone by means of rigid mechanical devices
such as screws~ threads or other devices.
SUMMARY OF THE INVENTION
In accordance with the inventions, as broadly described herein, the
prosthesis is made up of multiple porous strands of polytetrafluoroethylene
(PIFE) formed from concentric loops of a continuous filament. Immediate
postoperative attachment of the device is provided by integral eyelets formed
from adhered, gathered loop ends, which can be affixed directly to bony
tissue. This initial attachment is augmented and finally made redundant as
tissue grows into the porous strand material providing permanent attachment of
the prosthesis.
To achieve the foregoing objects an~ in accordance with the present
invention, as broadly described herein, the method for making a tensile
load-bearing tissue prosthesis of the type having a plurality of parallel
longitudinally adjacent strands connec~ed to at least one eyelet, the eyelet
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~2~15~
being for the initial attachment of the prosthesis to tensile force-applying
bone tissue, comprises forming a plurality of elongated concentric loops from
a continuous filament of the desired strand material until the desired number
of parallel strands are obtained, the concentric loops defining a projected
elongated area, and gathering the loop ends at one elongated area end to form
the eyelet, the method including the step of securing the gathered loop ends
against ungathering.
Preferably, the method includes the further steps of imparting a twist to
the loop strands about the longitudinal axis of the prosthesis.
This invention will be furth`er understood by reference to the d~awings
which are given for illustration only and are not intended to limit the scope
of the invention but wnich are to be read in conjunction with the
specifications.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a graph showing log stabilized tissue ingrowth and log
maximum strand thickness of the prosthesis of the present invention, as a
function of characteristic interstitial dimension;
Figure ~ is a photomicrograph of the PTFE material used in the
construction of the prosthesis of Example B;
Figure 3 is d photomicrograph of the PTFE material used in the
construction of the prosthesis of Example C;
Figure 4 is a photomicrograph of the material of Figure 3 laterally
s~retched to provide for measurement of characteristic interstitial
dimension;
Figure 5 shows a sch~matic perspective view of one prosthesis constructed
in accordanc with the present invention;
Figure 6 depicts schematically the implantation of an anterior cruciate
ligament prosthesis not constructed in accordance with the present invention;
Figure 7 depicts schematically a stage in one method of construction of a
prosthesis of the present invention;
Figure 8 shows a schematic perspective view of another prosthesis
constructed in accordance with the present invention;
Figure 9 depicts schematically a stage in another method of construction
o~ a prosthesis of the present invention;
Figure 10 depicts schema~ically a perspective view of yet another
prosthesis constructed in accordance with the present invention; and
Figures llA, B~ and C depict the implantation into a knee joint of the
prosthesis of Figure 8 into a knee joint as an anterior cruciate ligament
prosthesis.
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DESCRIPTION OF THE P~EFERRED EMBODIMENTS
The inventive article described herein is a synthetic prosthesis for
replacement or repair of ligaments or tendons. The prosthesis is made up of
multiple strands of porous PTFE. The porosity of the strands is characterized
by interconnecting void space throughout. Strand dimensions are small enough
to permit tissue gro~th in and through the entire strand~ The percent void
space, or porosity, is greater than 30%, which allows mechanical attachment of
tissue in the interstitial spaces of the prosthesis to provide s~fficient
attachment strength. This degree of porosity is a requirement only for those
sections of the device which are intended to be anchored through tissue
fixation. Porosity, as used here, is defined as;
% Porosity = [1 _ ~2 3 100
where: ~2 = density of porous material
~1 = density of solid PTFE making up the solid content of the porous
material. For PTFE which has never been sintered ~1 = 2.3
gm/cm3 and for materialsr which have been sintered a value of
2.2 gm/cm3 is used for JV1, although this value can actually
vary somewhat depending on sintering and cooling conditions.
Immediate postoperative attachment of the device is provided by eyelets
which are attached directly to bony tissue. This initial attachment is
augmented and finally made redundant as tissue grows in~o the porous strand
material, providing permanent attachment of the prosthesis to tissue. Tissue
can easily grow between and among the strands since they are not attached to
each other nor held together tightly. However, the depth to which tissue can
grow into each strand is governed by the dimensions of the interconnected void
corridors or pathways through the porous microstructure. The complex
intercommunicating void space is formed by the solid PTFE matrix. In some
cases, the matrix is made up of large solid nodes interconnected by long
flexible, relatively inelastic fibrils. Although the nodes may present rigid
inflexible structures to ingrowing tissue, the fibrils can be bent and pushed
aside by penetrating tissue. Other microstructures of this invention have
much smaller nodes which appear merely as connection points for the fibrils.
In both cases, the strength of the f~brils in tension is very high, and
although they can be bent by tissue, they cannot be stretched significantly.
The microstructures of this invention can be characterized by a mean
interstitial dimension which can be used to predict the depth of tissue
ingrowth. Short fibril lengths impede and bottleneck tissue invasion. Thus,
for porous strands having short fibril lengths, the overall strand dimension
must itself be small enough so that ingrowth and attachment will occur
throughout the entire strand.
The methods used here to characterize the fibril length of a particular
microstructure rely on visual examination of that microstructure. Photographs
at a convenient magnification can be provided through scanning electron
microscopy or, in some cases, light microscopy. The microporous PTFE
materials of this invention can vary sufficiently in their microstructure so
that different techniques of measuring the characteristic interstitial
dimension must be used. Strand fibers such as those made by the process
~3-
described in Example B possess a microstructure ~hich can clearly be
characterized by nodes interconnected by fibrils. The characteristic
interstitial dimension for materials of this type can be determined through a
direct measurement of the spacing between nodes. This measurement is taken
along a line placed in the direction of strength orientation (Figure 2). A
large enough number of measurements must be taken so that the node spacing is
adequately characterized. The mean node spacing thus provided is used to
characterize the interstitial space and thereby predict the depth of ingrowth
into that microstructure.
In strand material which has been manufactured by a stretching process
such as is described in U.S. Pat. No. 3,962,153, or the products of U.S. Pat.
No. 4,187,390, the nodes of PTFE can be smaller and much less defined. In
highly stretched products made according to these patents, node spacing
becomes very large and fibrils are packed together. The sintering step in
production of these materials causes the bundles of fibrils to coalesce and
form secondary attachment points. For this reason, the microstructure of such
materials is not readily apparent even under magnification. In determining
the characteristic interstitial dimension of these materials, it is necessary
to measure the distance between fibril suspension points rather than measuring
the fibril length (i.e., node spacing). The interstitial dimensions of these
materials can be observed if samples are prepared for microscopy by slightly
stretching the materia1 at right angles to its direction of strength
orientation. Upon stretching the sample 10% in the lateral direction, with
the sample restrained from shrinking in the longi'cudinal direction, the points
at which fibrils are connected become apparent under microscopic examination.
The distance between fibril connections is then measured at all obvious gaps
created between fibril bundles. This measurement is taken in the direction of
strength orientation. As with the method described previously for node
spacing, the number of measurements of fibril suspension distance must be
sufficient to characterize interstitial dimensions of the microstructure.
Figure 3 shows how material of this type appears without lateral
stretching as compared to Figure 4 which is a micrograph of the same material
with 10% lateral stretching. This lateral stretching, which is used only to
characterize the microstructure of the material, represents a temporary
structural reorientation. A force placed on the material in the longitudinal
direction causes a return to the original lateral dimension and a restoration
of the original microstructure. As previously described, it is believed that
the fibrils composing this microstructure are pushed aside by ingrowing
tissue. The method of measuring the characteristic interstitial diMension for
materials of this type is shown in Figure 4. Having once determined the
characteristic interstitial dimension by the techniques described, the proper
strand dimensions can be determined.
Figure 1 presents the relationship between characteristic interstitial
dimension and depth of ingrowth of tissue into the microporous strands of the
articles of this invention. The abscissa of Figure 1 refers to the ultimate
depth to which tissue may penetrate a microstructure of indicated
characteristic interstitial dimension regardless of implant time. The
relationship is derived from numerous experimental observations of various
kinds of implanted devices, all of which were composed of porous PTFE which
had been manufactured according to the teachings o~ U.S. Pat. No. 3,953,566,
U.S. Pat No. 3,962,153 or as described in Example B.
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The maximum strand thickness ~hich would allow tissue penetration through
the entire strand is approximately two times the tissue penetration depth.
The maximum strand thickness is presented as the right-hand ordina~e of ~igure
1. Thickness, as used here, refers to the appropriate minor cross-sectional
dimension of a strand, e.g., the diameter of a strand of circul~r
cross-section or the thickness of a strand of rectangular cross-section. In
general, combinations of characteristic interstitial void dimension and strand
thickness which fall underneath the curve are preferred because they allow
complete tissue penetration across the strand cross-section in a shorter time
interval. These preferred combinations may be determined from the following
relationships:
ln (strand diam) < 2.28 x 10-2 (CID) - 4.36
for CID > 7u, < 120u
ln (strand diam) < 6.98 x 10-2 (CID) - 9.94
for CID > 120u
where: CID = characteristic interstitial dimension (microns)
ln = natural logarithm
strand diameter is expressed in inches
The depth of tissue penetration into the microporous structure decreases
radically as characteristic interstitial dimension falls below 10 microns.
This decrease is due to the fact that in structures with this characteristic
spacing and below, only a small number of the interstitial pathways are large
enough to admit a single cel1 of the desired type. At characteristic
interstitial dimensions of 120u and greater, substantia1 vascu1arization
accompanies tissue ingrowth and allows for a greatly increased de?th of
penetration. We believe that this creates a slope increase in the
re1ationship of interstitial dimension and depth of tissue penetration as
sho~n in Figure 1.
A major requirement for a successfu1 ligament or tendon prosthesis is
that of adequate strength. In many situations prosthetic materials used to
rep1ace these natura1 structures are subjected to very high tensi1e loads.
The strength of the prosthesis must in some cases be many times that of the
peak 10ad to which it wi11 be exposed to compensate for the mechanical
properties of the prosthesis which are time-dependent.
From a mechanical strength standpoint, one of ordinary skill in the art
would realize that the number of individual strands needed for a particular
application will depend on several factors. These include: the individual
strand cross-sectional area; the tensile strength of the individual strand;
and the tensile force requirement for that particu1ar application, including
any safety factors for creep strain limitations. The individual strands used
in this invention can be constructed using the processes described in U.S.
Pat. No. 3,953,566, U.S. Pat No. 3,962,153 or following Example B. It is
desirable to use a high matrix tensile strength material in order to minimize
the overall physical dimensions of the device and thereby minimize the size of
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dri11 holes placed in the bone to mount the device. Matrix tensile strength
refers to the strength of the polymer in a porous specimen and is used as
defined in ~.S. Pat No. 3,953,566.
In the preferred form of this invention:
--The strand material is porous PTFE with a matrix tensile strength
greater than 20,000 psi, a porosity greater than 30~O~ and a
microstructure characterized by intercommunicating pathh~ays formed by
the boundaries of nodes and fibrils. - -
--Strand dimensions and characteristic interstitial dimensions of the
microstructure are selected such that tissue ingrowth throughout the
strand takes place in a rapid fashion.
--Each strand and the finished construction possess sufficient strength
necessary to meet the mechanical requirements of the particular
application.
--The parallel strands result from multiple loops formed from a
continuous filament of the strand material.
--The ends of the multiple loops are gathered and formed into at least
one eyelet for attaching the article to bone tissue.
--The uniformity of strand loading of the prosthesis under tensile force
is enhanced through:
1. Minimizing differences in loop length used to form the parallel
strands.
2. Compression of the loop strands in the eyelet segments to
provide strand-to-strand adhesion.
--The prosthesis also includes means for distributing the tensile load
among the strands as it passes around a radius, said means including:
1. A t~ist in the strand bundle about its longitudinal axis.
2. A loose strand braid.
--Although the ligament prosthesis embodiment in Figure 10 is shown with
a pair of opposing eyelets formed from elongated loops, the present
invention also encompasses a single eyelet 324 formed in the loops
gathered for attachment to bone. The loops at the other end 316 remain
ungathered or splayed to provide attachment to soft tissue such as
muscle tissue, as by suturing (see Figure 5). In this latter case, the
closed loop ends provide additional resistance to possible strand
slippage past the sutures. The single eyelet embodiment of this
prosthesis 310, could find use in the repair or replacement of tendons.
--6--
EXAMPLE A
This example demonstrates a prosthetic device which did not achieve
satisfactory system strength because the strand thickness was too large for
the interstitial dimension which characterized its microstructure (Figure 1).
The strand thickness (diameter) was 0.26 inches, porosity of the strand was
approximately 80%, and the characteristic interstitial dimension was about 78
microns. This interstitial dimension was determined as shown in Figure 2.
The prosthesis was used to replace the anterior cruciate ligament of a dog~by
routing the material through drill holes in the tibia and femur. Four holes
were drilled in the tibia 2 and femur 4 such that the prosthesis strand 6
formed a loop of material with two strands in the position of the original
liqament (Figure 6). Initial fixation was provided by tying the ends of the
strand together in a knot 8 to form a continuous loop. Ingrowth and formation
of tlssue within the interstices of the microporous material were expected to
augment the initial fixation strength and to distribute stresses to the
surrounding tissue. Each of the strands crossing the knee joint possessed a
tensile strength of about 550 pounds. The combined strength of these two
strands was then 1,100 pounds. After having been implanted for 260 days, the
knee joint was explanted.
~ rill holes were placed in the tibia and femur for mounting into tensile
test clamps. After removal of all supporting collateral structures about the
knee, the femur was distracted from the tibia along the axis of the prosthetic
ligament at a constant rate of 500mm per minute until failure. The length
spanning the intra-articular space between bone tunnels represented that
portion of the prosthesis placed under tensile load during the test, due to
tissue attachment to the prosthesis in the bone tunnels. The failure mode of
the system was rupture of the prosthetic device at the leve1 of exit from the
bone tunnels. Surprisingly, this rupture took place at a value of only 200
lbs. Through histologicàl inspection, we discovered that this reduction in
strength was related to the restriction of bony ingrowth to generally less
than lmm depth into the prosthesis. With a strand of this diameter and
characteristic interstitial dimension, attachment takes place only at a
circumferential ring of material on the periphery of the device. This reduced
area then becomes the only load-bearing material of the prosthesis as a
tensile force is initially applied. Failure occurs in this circumferential
ring of material first and then progresses through the central portion of the
prosthesis.
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~2~
EXAMPLE B
The experience cited in Example A led to the discovery that tissue
ingrowth must penetrate thrDugh~ut the cross-section of the strand in order to
provide adequate long-term system strength. Accordingly, a device was
constructed using a strand of similar porosity and characteristic interstitial
dimension but with a much smaller diameter. The strand material used to
construct the anterior cruciate ligament prosthesis of this example was made
as fol lows:
PTFE dispersion powder ("Fluo~ CD 123~ resin produced by ICI America) was
blended with 130cc of "ISOPAR K" odorless solvent (produced by Exxon
Corporation) per pound of PTFE, compressed into a pellet, and extruded
into a 0.108 inch diameter rod in a ram extruder having a 96:1 reduction
ratio in a cross-section from the pellet to the ex~ruded rod.
The extruded rod still containing Isopar K was immersed in a container of
Isopar K at 60~C and stretched to 8.7 times its original length between
capstans with an output velocity of about 86.4 ft/min. These capstans
were ab~ut 2.8 inches in diameter with a center-to-center distance of
about 4.~ inches. The diameter of the rod was re~uced from about 0.108
inch to about 0.047 inch by this stretching. The Isopar K was then
removed from this stretched material.
The stretched rod was then pulled through a circular densification die
heated to 300C. The opening in the die tapered at a 10U angle from
about 0.050 inch to 0.025 inch and then was constant for about 0.025 inch
length. The output velocity of the material exiting the die ~as 7.2
ft/min.
The stretched rod was then heated to 300~C through contact with heated,
driven capstans and stretched 4 1/2 fold (350%) with an output velocity
of 6.5 ft/min. These capstans had a diameter of 2.75 inches and a
center-to-center distance of 4.5 inches.
Finally, the rod was restrained from shrinking and exposed to about 367~C
in an air oven for 30 seconds.
In the finished form, ~he fiber made with this process possessed the
following characteristics:
Diameter - 0.02~ inches
Matrix Tensile Strength = 74,000 psi
Porosity = 80 . 8X
Characteristic Interstitial Dimension = 74u
As illustrated in Figure 7, prosthesis 10 was constructed on two steel
spsols 42, 44 which were mounted on a rack (not shown). The spools were
supported on studs 46, 48 spaced 14cm fr~m center line to center line. These
steel spools were threaded to allow demounting of one flange. The strand of
PTFE material was passed around these two spools 80 times so that a total of
160 strands connected the two spools. The two free ends of the fiber were
* T rA~elnArk
" .
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~Z056(~
tied together with multiple square knots. One spool was demounted from the
stud~ rotated through 180~ and remounted on the stud, thus imparting a
one-half twist about the longitudinal axis of the construction. The
construction was then wrapped with a thin film of PTFE a total of 25
revolutions each at three locations. This film was manufactured according to
the teachings of U.S. Pat. No. 3,962,153 and had the following
characteristics:
Width = 0.375"
Thickness = 0.00025"
Longitudinal matrix tensile strength = 70,000 psi
Porosity = 84%
The bundle of strands was wrapped with this thin film at two points 28,
30 adjacent to the spools 42, 44, thereby forming eyelets 24, 26, at the ends
of the construction (Figure 8). A central portion 38 was also wrapped with
fiIm. The two spools were then demounted from the studs and placed on a rack
constructed of thin metal wire designed to prevent rotation and longitudinal
shrinkage. This rack was then exposed to 375C in an air oven for six
minutes. After cooling, the spools were demounted from the ends of the
construction. The position occupied by the spools provided eyelets through
which this ligament prosthesis construction can be attached to bone with
screws or other suitable means of fixation. All areas which had been wrapped
with film had become compressed during the heating treatment due to film
shrinkage, thereby providing strand-to-strand cohesion. During the previously
described heating cycle, some fiber-to-fiber attachment in the unwrapped
regions also took place. These fibers were then individually separated using
a metal pick. The construction then comprised 160 microporous PTFE strands
connecting two eyelets of somewhat densified material. Prosthesis 10 included
a 180~ twist along the tensile load direction to better distribute the tensile
load among strands 20. PTFE tape wrap 38 surrounding strands 20 and
positioned approximately midway between ends 14, 16 of prosthesis 10 serYes to
maintain the twist by securing strands 20 against untwisting during
implantation. As with PTFE wraps 28, 30, wrap 3~ is intended to be positioned
outside of the bone contact area so as not to inhibit tissue ingrowth into
strands 20.
A device prepared in the manner just descri~ed was implanted into the
knee of a sheep to replace the excised host anterior cruciate ligament (see
Figures llA, B and C). This implantation was accomplished through the
placement of one 1/4" drill hole in both the tibia and femur. The placement
of the hole in the tibia was along the axis of the previously removed natural
anterior cruciate and exited at the insertion site. The placement of the
femoral drill hole began at the lateral distal femoral surface proximal to the
fe~oral epicondyle. The tunnel was angled such that the exit hole was created
just proximal to the lateral femoral condyle on the popliteal surface of the
femur. The prosthesis 10 was routed from the femoral exit site through the
intercondylar space, across the intra-articular space, and through the tibial
tunnel. The eye~ets 24, 26 and wrapped segments 28, 30 at the ends of the
construction were positioned to be to the outside of the drilled bone tunnels.
The placement of the wrapped segment 38 at the center region of the
construction was in the intra-articular space. rhe prosthesis 10 was then
anchored to bone with self-tapping orthopedic screws 32, 34 placed through the
eyelets 24, 26. The knee joint was determined to be stable immediately after
the operation.
_g_
After three months implant time, the knee was removed fro~ the ani~al and
drill holes placed in the tibia and femur into which clamps we~e mounted to
provide for tensile testing along the axis of the ligament construction.
After removal of muscle tissue and severing of all supporting collateral
structures about the knee, the femur was distracted from the tibia at a
constant rate of 500mm per minute until failure. System failure took place at
642 lb. The failure took place in the ligament prosthesis at the eyelet
secured to the femur. Rupture took place as the 10ad exceeded the fixation
provided by tissue ingrowth into the intra-osseous segments and was
transferred to the fixation screw. Device failure was related to an unwinding
of the strand material through the eyelet segments after several strands had
failed. Histologic inspection of this sample showed tissue ingrowth among and
into the strands. Tissue ingrowth had proceeded completely through the
diameter of some strands. We anticipate that with longer implant times the
majority of strands would have shown complete and thorough ingrowth.
--10-
S6~
EXAMPLE C
The thin film of expanded PTFE used as strand materia1 in this embodiment
was obtained from W. L. Gore and Associates, Inc., Fibers Division, Three Blue
Ball Road, Post Office Box 1010, Elkton, Maryland, 21921, under the part
number Y10383. This film had the following properties:
Width = 0.25"
Thickness = 0.0010"
Matrix Tensile Strength = 93,400 lb/in2
Porosity = 50%
Characteristic Interstitial Dimension = 11.0u
The mode of failure of the prosthetic ligament as described in Example B
led to the observation that improved strand-to-strand cohesion was desirable
in the eyelet region. Accordingly, a construction method somewhat modified
from that of Example B was employed, for prosthesis 110 shown in Figure 10.
Four spools 141, 142, 143, 144 were mounted on a rack (not shown) on
which the spools were supported by studs 145, 146, 147, 148 positioned so as
to form a 9cm x 5cm rectangle (Figure 9). These steel spools were threaded to
allow demounting on one flange. The strand of PTFE material was passed around
these four spools a total of 60 times. The strand of thin PTFE material was
twisted about its longitudinal axis 20 times during each complete
circumference around the four spools. The free ends of the continuous strand
were then tied together at a point midway along one of the 5cm bundle sides.
A 3.5cm segment midway in each of the 5cm sides at point 150, 152 was
then compressed in a sizing die to a rectangular cross section of 0.058" x
0.150". Durlng compression, this sizing die was heated to 360C and
immediately allowed to cool. This precompression stage is necessary to
facilitate the placement of the construction in an eyelet cnmpression die.
The central 1 inch segment of the precompressed region was then wrapped with
25 revolutions of a thin film of PTFE 160. This film was manufactured
according to the teachings of U.S. Pat. No. 3,962,153 and had the following
characteristics:
Width = 0.375"
Thickness = 0.00025"
Longitudinal Matrix Tensile Strength = 70,000psi
Porosity = 84%
The device was then demounted and placed on a two-post rack with two
steel pins placed 14cm center to center (similar to figure 7) with the
precompressed, film wrapped segments centered about the pins. With reference
to Figure 10, the two parallel precompressed segments were gathered adjacent
to each pin and wrapped at points 128, 130 a total of 25 time~ with thin film
of expanded PTFE of the type described above. Both ends 114, 116 of the
construction were placed into the final eyelet-forming die. The eyelets 124,
126 were then compressed within the die to a specific gravity estimated by
calculation to be 2.2, heated immediately to ~60C for 10 minutes, and then
cooled. Following removal from the eyelet-forming dies, the construction was
remounted on d two-post rack with a 180 twist about the longitudinal axis of
~2~
the device. A 0.4" segment 138 midway between the eyelets 124, 126 was then
wrapped tightly with 25 circumferential layers of the thin film described
previously and compressed in a heated cylindrical die to a specific gravity
calculated to be 2.2 It was held compressed at 360C for ten minutes and
then cooled and removed from the die. Referring to Figure 10, the
construction then comprised 120 microporous PTFE strands 120 connecting two
eyelets 124, 126 formed of densified strand material and multiple layers of
PTFE film. The purpose of the compressed eyelet area was to maintain
integrity of the remaining strands should one or rnore strands be severed.
This densification also provides for more uniform strand loading under tensile
forces. The purpose of the 180 twist in the strand bundle was to provide for
more uniform strand loading as the implanted bundle passes around the radius
of the femoral condyle in the intercondylar space. The purpose of the
compressed segment in the center of the strand bundle was to help preserve the
1/2 twist during implantation. We believe that a loose braid in the strands,
oriented about the longitudinal axis, will also serve to distribute the
tensile forces among the strands and could be substituted for the 180 twist.
One of ordinary skill in the art would know how to construct a loose strand
braid.
The device prepared in the manner just described was implanted into the
knee of a sheep to replace the excised host anterior cruciate ligament. This
implantation was accomplished using the techniques previously described in
Example B (see Figure 11). After six months implant time, the reconstructed
knee will be removed and tensile tested in the manner previously described.
The anticipated result of this test is that the tensile strength of the system
will be at least 600 pounds unless unrelated bone failure occurs prior to
failure of the ligamentous reconstruction. It is further antic~pated that
this level of tensile strength will be achieved through the presence of tissue
attachment to the individual fibers contained within the bone tunnels. Upon
histologic inspection, it is anticipated that at six months post-implantation,
substantial tissue formation will be observed among and penetrating into the
individual strands.
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