Note: Descriptions are shown in the official language in which they were submitted.
~ ~6~5i7
I~e present invention relates to aliphatic poly-
amide surgical devices and to a method for producing the same.
The inven-tion is more particularly directed to thermall~
formed aliphatic polyamide surgical devices having improved
properties including flexibility or toughness and functional
integrity.
It is well known that in many and various surgi-
cal procedures synthetic devices, that i5, devices made froTn
non-biological materials, are very often implanted. Examples
of such techniques are in surgery wherein various stainless
steel or other metal clips are used to control bleeding in that
they are used to ligate various blood vessels during the
surgical procedure. Furthermore, in other surgical procedures,
~arious other metal rods, staples, clips, or sheets of material
are implanted for various supports or other reasons in the
surgical procedure. In rnost instances, these devices remain
in the patient for considerable periods of time, though in
some instances, they may be removed at some later date or
even rejected by the natural physiological function oE the
human body.
Even though these metal surgical devices cause
no harm from the medical viewpoint, it i5 often desired they
not be allowed to remain in the body as they greatly disrupt
the ability to subsequently utilize on the patient many of
the new diagnostic imaging procedures. The metal surgical
devices disrupt X-ray imaging, computerized axial tomography
imaging, ana other of the new types of diagnostic imaging
procedures.
Hence, it is desirable that these surgical devices
be replaced by plastic materials which do not have as disruptive
an effect on the diagnostic imaging procedures. However, in
trying to develop the plastic ~laterials to replace the metal
materials, it has been found that i~ is ~ery difficult to provide
a combination of strength, flexibility, and dimensional stabili-
ty in these plastic materials, to a deyree that compares to
the metal materials. Hence, the plastic materials have not
been readily accepted as substitutes for the metal materials.
This is especially true in the smaller devices such as ligating
clips and other t~pes of clips to either close off vessels
or connect material such as tissue. qhese clips are small and
have very small critical areas within the device that require
considerable strength, flexibility and functional integrity.
One class of material that has gained success in
various types of surgical devices is polyamide. Furthermore,
it is well known the toughness and various other physical
properties of polyamide can be improved by annealing. Techniques
and discussions on the annealing Qf polyamides (nylons) are
more fully described in the book entitled "~ylon Plastics" by
Melvin Kohan and Published by John Wiley and Sons in 1973.
Specifically, Chapter 17 of the book entitled "Treatment of
Processed Nylons" describes various techn;ques for annealing
nylon products.
Though it is known that annealing improves certain
properties of the polyamides, for various reasons injection
molded articles are not generally annealed. It has been found
that the standard annealing of polyamides as taught by the
art does not irnprove all of the properties required in injection
molded surgical devices, especially those properties of flexi-
bility and functional integrity, so important to the in vivo
use of the plastic surgical devices.
Accordingly, it is an object of the present in-
vention to improve the strength of thermally formed aliphatic
polyamide surgical devices.
It is a further object of the invention to improve
the flexibility and functional integrity of the aliphatic poly-
amide injection molded devices, especially clips used to close
off various small vessels during a surgical procedure.
In accordance with the present invention, there is
provided a method for improvin~ the 1n vivo properties of an
aliphatic polyamide, thermally formed, sur~ical device, which
comprises heating the device to a temperature of from about
60C. to 100C. in an environment essentially saturated with
moisture for a period of time sufficient to increase -the alpha-
crystallinity of the device to at least 15% whereby the -func-
tional integrity of the device is improved.
It has now surprisingly been found that ~y treat~
ing the ther~ally formed aliphatic polyamide surgical devices
with moist heat for a specific period of time, the crystalli-
nity of the polyamide and the functional integrity of the
thermally ~ormed surgical device will be improved. r~hus, by
trea~ing the thermally formed device at a temperature of at
least 60C. but less than 100~., and preferably from 80C-90C.,
in an environment essentially saturated with moisture such as,
for example, in hot water, preerably for a time of at least
10 minutes, the properties of the product important to its use
as a surgical device are substantially enhanced.
rrhe resultant new product which constitutes an-
other aspect of -the present invention comprises a thermally
formed aliphatic polyamide surgical device which has an ~-crys-
tallinity of at least 15% and a total crystallinity in excess of
25%.
rrhe surgical device of the invention is preferably
an injection molded device made of nylon-6.
57
Preferably, the ~-crystallinity is at least 1S% and
the total crystallinity is in excess of 30~O,
The new surgical dévices of the present invention,
at least in certain embodiments, have improved in vivo pro-
perties, that is, they maintain their strength and functional
integrity as they are utilized and implanted during the surgi-
cal procedure and later during and after the wound healing
process.
Preferred embodiments of the invention will now
be described in greater detail with reference to the accompany-
ing drawings, in which:
Figure 1 is an enlarged perspective view of a
ligating clip according to the invention,
Figure 2 i9 an enlarged perspective ~iew showing
the clip of Figure 1 in place closing off a blood vessel,
Figure 3 is an enlarged perspective view of an-
other surgical device according to the invention'
Figure 4 is an enlarged perspective view o~ yet
another surgical device according to the invention,and
Figure 5 is a cross-sectional view showing the
device of Figure 4 in place closing a wound.
Though the present invention is applicable to
many types of surgical devices made from thermally formed,
aliphatic polyamides, for the sake of clarity, it will be
described in detail in conjunction with what are known as
ligating clips made from aliphatic polyamides.
Referring to Figure 1, there is shown a ligating
clip 10 embodying the present invention. This clip is used to
ligate a blood vessel during various surgical procedures. T~e
clip comprises two leg members 11 and 12 joined at their
proximal ends by a hinge section 13. The leg members latch
--4--
or lock at their dis-tal ends 14 and 15, In Figure 2 there is
shown the clip o~ Figure 1 in its closed position closing off
the lumen of a blood vessel 16. The hinge area, which is
thinner than the remainder of the clip, should be flexible yet
strong. ~he body of the leg members should balance strength
and rigidity, and depending on the configuration of the distal
ends, they should have a certain degree of flexibility. All
in all, the clip in its entirety should have good dimensional
stability in that when the clip is closed it maintains the
closed position.
The surgeon, when placiny this clip, very often
has to place it in an area that he cannot see and, hence, it
is important that he feel the resistance of the hinged section
and the latching mechanism, that is; the -tactile feedback he
desires in the surgical device. The surgeon also bene~its from
having an audible click when the hook of leg member 11 deflects
and catches the opposite leg member 12.
Another thermally formed surgical device is shown
in Figure 3. This device is a 2-piece fastener 20 for closing
wounds and the like. The fastener comprises a staple 21 and
a receiver 22 ~or the staple. In Figure 4, there is shown
~et another thermally formed device 25 for closing wounds,
whethex they be in the skin or ~ascia or even in the muscle.
~lis device comprises a -thin extended section 26 which has
cross pieces 27 disposed at each end of the thin extended sec~
tion Using a suitable instrume~ which has a hollow needle
for holding the device, the needle is inserted through tissue
and the device used to close the wound with the extended
section 26 sp~nning the wound area and the cross pieces 27
gripping opposite sides of the wound area as shown in Figure 5.
LS;7
Other medical devices which are contemplated
within the present invention are solid products such as ortho-
pedic pins, clamps, screws and plates, clips, staples, hooks,
buttons and snaps, bone substitutes such as mandible pros-
thesis, needles, in~ra-uterine devices; various tubular
ducts such as ureter, cystic ducts, etc., surgical instruments,
vascular implants, couplers or supports, and vertebral discs,
as well as other similar devices.
The sur~ical devices of the present invention are
preferably produced by injection molding. The pol~amides are
injection molded as is well known in the art. It has been
found that during injection molding of nylon-6 mold tempera-
tures ranging -from about 40C. to 90C. can be used. Other
parameters used in the injection molding of nylon-6 are well
known.
In accordance with the present invention, i~jectio~
molded aliphatic po~yamide surgical clips are treated in the
presence of excess moisture to optimize the properties of the
injection molded clips, that is, to optimize them as far as
their intended use is concerned. It has been found that by
heating the clips at temperatures of at least 60C. but less
than 100C. and preferably from about 80C. to 90C~ in the
presence of excess moisture, preferably for a period of time of
at least 10 minutes, the desirable ln vivo properties of the
clips are improved. It is preferred that the clips be treated
in hot water although they may be treated in steam or ex-tremely
wet atmospheres for a period of time in order to obtain the
desired results~
~he following tests are used to determine the
properties of surgical clips.
Crystallinity
The crystallinity of the clip is a measurement
of the strength and functional integrity of the clip. X-ray
diffraction is a convenient method of determining the amount
and type of crystallinity in the clip. ~he X-ray crystallinity
data is obtained using a P~ILLIPS (trade mark) vertical gonio~
meter equipped with a graphite crystal monochrometer and scin-
tillation detector interfaced to a strip chart recorder~ CuKa
radiation is employed and a sample is moun-ted and run using
parafocusing geometry. The patterns obtained for a sample are
analyzed for alpha crystallinity, gamma crystallinity and amor~
phous content using a DU~ONT (trade mark) curve resolver.
Alpha cristallinity is a three dimensional
crystalline form which is stable into the melting temperature
whereas gamma crystallinity is a metastable, low ordered
structure which may be converted to the alpha form by a variety
of treatments~
Hinqe Strenqth
The hinge strength of the clip is the force
required to break the clip at the hinye area and is determined
by preconditloning the clip fox 16 hours at a relative
humidity of 60% and a temperature of 70F. The latching
mechanism at the distal end of the conditioned clip is cut
away and the cut ends of the leg members placed in the opposing
jaws of an INSTRON (trade mark~ tensiometer. The jaws are
steel faced~ Using a strain rate of 5 mrn/min. the jaws are
moved apart and the force necessary to break the hinge is
determined in kilograms.
Hinge Strength (In Vivo)
The in vivo strength of the clip is measured as
follows. Clips are separated without apparent bias into groups
t~
consisting of 10 clips each. ~ach group will correspond to
one hinge strength test interval. Special Long Evans rats,
weighing 150 to 300 grams are prepared for surgery, anes-
thesized, and 2 clips implanted in each rat. The clips are
implanted in the left and right posterior dorsal subcutis of
the rat~ At each postimplantation period, 5 rats are given
euthanasia and the clips carefully removed. The hinge strength
o~ the clips is determined by cutting away the latching mecha-
nism at the distal end of the clip and placing the cut ends
of the leg members in the opposing jaws of an Instron Tensio-
meter. The jaws are steel faced. Using a strain rate of
5 mm/min. the jaws are moved apart and the -Eorce necessary to
break the hinge is determined in kilograms.
Percent Extension
~ le percent extension of the clip is a measure of
the functional integrity and in part the dimensional stability
of the clip. The percent extension of the clip is equivalent
to apparent elongation of the clip determined from the Instron
measurements and is calculated by the expression:
Velocity X Length of Chart
Crosshead
% Extension = X 100
Ve]ocity X Gauge Length
Chart
Ihe following non-limiting examples further
illustrate the present invention.
Examples 1 throuqh 4
Nylon-6 resin, sold by Allied Chemical Corporation
as type 8207, having an ~~crystallinity and a total crystalli-
nity as shown in Table 1, is injection molded in the configu-
ration of the clip depicted in Figure 1. A multicavity mold in
~8--
~9~S~
an Arburg injection molding machine is used~ The mold tempera-
tures are as given in Table 1, and the nozzle temperatures is
approximately 240C, ~he other parameters of the injection
molding steps are similar to those generally used in the injec-
tion molding of nylon ~. Ihe molded par-ts are washed in hexane
and dried at ambient temperatures and 0.1 mm. pressure.
In Example 1, injection molded clips are tested
for hinge strength, percent extension, total crystallinity and
in vivo hinge strength. Some of the clips implanted in
animals to determine their efficacity open spontaneously when
implanted indicating their ]ack of functional integrity.
In Example 2, some of the clips from Example 1
are annealed in accordance with the present invention by
placing the clips in an atmosphere of steam for thirty minutes.
The annealed clips are tested for hinge strength, percent
extension, total crystallinity and in vivo hinge strength.
In Example 3, injection molded clips are tested for
hinge strength, percent extension, and total crystallinity.
Some of the molded clips are annealed in accordance with the
present invention by placing the clips in water at a temperature
of 80C. for the time indicated in Table lo The annealed clips
are tested for hinge strength, percent extension, and total
crystallinity.
In Example 4, injection molded clips are tested
for hinge strength, percent extension, and total crystallinity.
Some of the molded clips are annealed in accordance with the
present invention by placing the clips in water at a temperature
of 80C. for thirty minutes~ The annealed clips are sterilized
by subjecting the clips to a dosage of 2.5 megarads of ~Co
irradiation. The sterilized clips are tested for hinge
stren~th, percent extension, total crystallinity and in vivo
;i7
hinge strength. The clips are flexible when implanted in
animals, have excellent functional integrity and do not open
during use.
The parameters used in the e~amples as well as
the test results are provided in the following Table 1.
--10--
57
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S P~ -d O O d' d'
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H U~ 1-- 0 0 ~1 ) I i I
r~ (`1r I
~ e
~ t ~)
U
0
aa o N 0
~r ~
al u~
ta ~ U~ E~ ri E~ rl1 rl 1 ri
~ ~ ~ e ~ e ~ e ~ e ~ e a) e
o
C ~rl I ~ O ~ O ~ ~I o
-, ~
~ r-l r l r~lr lr l ~1
r ~
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h~ ~
r- C
a c~ n 0 0 0 0
ri r-lr1 r l ~(~) ~) (~
i U~ C
O
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a.- a) al
o ~I h
~I .1 . . o ~ , , , .
~ ~ ~ ~ ~ ~ ~ ~ h
U~ ~ ~0
a)
~ ~ h ~1
r~ ~ V r~ r~ r~ 3
o al o ~ ) O o o o 3
Q a
a ,~
,~ a
o
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5~
As may be seen from the above table, the annealed clip5
are not brittle, have excellent hinge strength which they
maintain in vivo. The annealing has no deleterious effect
on the percent extension in the clip while greatly increasing
crystallinity which results in an excellent balance in the
strength, flexibility and functional integrity of the clips.
It has alread~ been stated that annealing of thermally
formed polyamides is not a common practice. Moreover, it has
been found that existing annealing procedures for polyamides
do not improve those properties required in thermally -formed
surgical devices, especially those properties o~ ~lexibility
and functional integrity necessary for in vivo use of the
devices.
If polyamides are annealed at all, they are sometimes
heated in an oil bath to temperatures suitable for the type
of polyamide used (i. e., 130-la9C. for nylon-6~. Further-
more, it is well known that this p~actice is used to improve
lubricity o~ the parts due to imbibing of the annealing
medium for this reason. Moreover, this process has been found
to be additionnally deleterious due to discoloration of -the
annealed parts and the need o~ a costly oil removal process.
A second method of annealing polyamides is by heating
to elevated temperatures (i. e~, up to 50C. below the melt
temperature) in the presence of inert gas to avoid oY.idation
It has been found that heating thermally formed surgical
devices under nitrogen at elevated temperatures (i. e., 50-
150C.) for various time periods has little e~fect on the
desirable properties of these devices. Moreover, it has been
found that the annealing process is sometimes quite deleterious
to these devices.
;~ S~
The following example is illustrative of this point,
~xample 5
Clips as previously described are injection molded using
mold temperatures oE 86C. Molded clips are implanted in rats
and were found to be detached from the implant site and/or
opened when harvested after 14 days~ Other clips are annealed
under nitrogen Eor 1.5 to 24 hours at temperatures from 50-
150C. Properties of these clips are either not improved or
deleteriously a~fected as set ~orth in Table 2,
-13-
Table 2
~itrogen Hinge
Annealing Strength Extension
Parameters Device Appearance Kq. % Comment
None Clip Clear, Colorless 2.34 15 Gamma
Crystallinity
50 C/24 hours Clip Slightly Yellow
115C/17 hours Clip Slightly Yellow 2.00 8 __
130C/6 hours Bar Yellow -- -- Gamma, C~ystallini-
ty, Brittle
130C~12 hours bar Yellow -- -- Gamma, Crystallini-
ty, Brittle 'r~
140 C/3 hours bar Yellow -- -- Gamma, Crystallini-
ty, Brittle
140 C/6 hours Bar vellow Gamma, Cryst~ll;n;-
ty,Brittle
140C/6 hours Bar Yellow -- -- Gamma, Cryst~ll;ni-
ty, Brittle
150C~1.5 hour Bar Yellow -~ -- Gamma, Crystallini-
ty, Brittle
150C~3.0 hours Bar Yellow -- -- Increased alphaCrysta~linity,
Brittle
150C/3.0 hours Clip Yellow Could not -- Extra brittle
be tested
It rnay be seen that nitrogen annealing at elevated
temperatures causes discoloration and embrittlement as
illustrated by decreased percent extension at the breaking
o~ the clip. Test bars used in this experiment were of the
same thickness as clips.
The X-ray studies show that these polyamide clips and
bars as molded are at a low level of crystallinity mostly
o~ the gamma or metastable variety~ Only at 150C. after
three hours under nitrogen is any appreciable transition from
gamma to alpha seen. The bars and clips treated at these
conditions are extremely brittle and the clips were completely
non-functional, fracturing at the latch on closing.
The polyamide injection molded polymers treated in
accordance with the present invention have improved flexibi-
lity and ~unctional integrity which makes them especially
suitable as surgical devices~ ~urthermore, nylon-6 is readily
cobalt sterilizable so as to produce sterile surgical devices.
It should be pointed out that nylon-6 will become less
flexible if allowed to become completely dry. ~Ience, it may
be desirable in accordance with the present invention that
the nylon-6 surgical devices be packaged and maintained in an
environment where they axe at their moisture equilibrium to
remain flexible during storage and, hence, be suitable for use
in the various surgical procedures~
