GB2108135A - Polyamide surgical device and method for producing the same - Google Patents

Abstract

Thermally formed aliphatic polyamide surgical devices having an alpha crystallinity of at least 15% and a total crystallinity in excess of 25%, are obtained by heating them at 60 DEG C to 100 DEG C in an environment essentially saturated with moisture (preferably in hot water), to improve their strengths.

Description

vessel; Figure 3 is an enlarged perspective view of another surgical device in accordance with the present invention; Figure 4 is an enlarged perspective view of yet another surgical device in accordance with the present invention; and Figure 5 is a cross-sectional view showing the device of Figure 4 in place closing a wound.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE PRESENT INVENTION 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 of the present invention. The clip is used to ligate a blood vessel during various surgical procedures. The clip comprises two leg members 1 1 and 12 joined at their proximal ends by a hinge section 13. The leg members latch or lock at their distal ends 14 and 1 5. In Figure 2 there is shown the clip of 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. The 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 placing 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 benefits from having an audible click when the hook of leg member 1 1 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 for the staple. In Figure 4, there is shown yet another thermally formed device 25 for closing wounds, whether they be in the skin or fascia or even in the muscle. This device comprises a thin extended section 26 which has cross pieces 27 disposed at each end of the thin extended section. Using a suitable instrument 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 spanning the wound area and the cross pieces 27 gripping opposite sides of the wound area as shown in Figure 5.

Other medical devices which are contemplated within the present invention are solid products such as orthopedic pins, clamps, screws and plates; clips, staples, hooks, buttons and snaps; bone substitutes such as mandible prosthesis; needles; intra-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 surgical devices of the present invention are preferably produced by injection molding. The polyamides are injection molded as is well known in the art. I have found that during injection molding of nylon-6 mold temperatures ranging from about 400C to 900C can be used. Other parameters used in the injection molding of nylon-6 are well known.

In accordance with the present invention, injection molded aliphatic polyamide 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. I have found that by heating the clips at temperatures of at least 6000 but less than 10000 and preferably from about 800C to 900C in the presence of excess moisture for a period of time of at least 1 0 minutes, improves the desirable in vivo properties of the clips. It is preferred that the clips be treated in hot water although they may be treated in steam or extremely wet atmospheres for a period of time in order to obtain the desired results.

The 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. The X-ray crystallinity data is obtained using a Phillips vertical goniometer equipped with a graphite crystal monochrometer and scintillation detector interface to a strip chart recorder. CuKa radiation is employed and a sample is mounted and run using parafocusing geometry. The patterns obtained for a sample are analyzed for alpha crystallinity, gamma crystallinity and amorphous content using a DuPont Curve Resolver.

Alpha crystallinity 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.

HINGE STRENGTH The hinge strength of the clip is the force required to break the clip at the hinge area and is determined by preconditioning the clip for 16 hours at a relative humidity of 60% and a temperature of SPECIFICATION Polyamide surgical device and method for producing the same The present invention relates to aliphatic polyamide surgical devices and more particularly. to thermally formed aliphatic polyamide surgical devices having improved properties including flexibility or toughness and functional integrity.

BACKGROUND OF THE INVENTION It is well known that in many and various surgical procedures synthetic devices, that is, devices made from 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, various other metal rods, staples, clips, or sheets of material are implanted for various supports or other reasons in the surgical procedure. In most 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 of the human body.

Even though these metal surgical devices cause no harm from the medical viewpoint, it is 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, and 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 materials to replace the metal materials, it has been found that it is very difficult to provide a combination of strength, flexibility, and dimensional stability in these plastic materials, to a degree that compares to the metal materials. Hence, the plastics 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 types of clips to either close off vessels or connect material such as tissue. These 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 of polyamides (nylons) are more fully described in the book entitled "Nylon 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 techniques 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. I have found that the standard annealing of polyamides as taught by the art does not improve all of the properties required in injection molded surgical devices, especially those properties of flexibility and functional integrity, so important to the in vivo use of the plastic surgical devices.

SUMMARY OF THE PRESENT INVENTION I have discovered a new process which improves the strength of thermally formed aliphatic polyamide surgical devices. Furthermore, my new and improved process improves the flexibility and functional integrity of the aliphatic polyamide injection molded devices, especially clips used to close off various small vessels during a surgical procedure. Often the new surgical devices of the present invention, at least in certain embodiments, have improved in vivo properties, that is, they maintain their strength and functional integrity as they are utilized and implanted during the surgical procedure and later during and after the wound healing process.

In accordance with the present invention, I have discovered that by treating the thermally formed aliphatic polyamide surgical devices with moist heat for a specific period of time, it will improve the crystallinity of the polyamide and the functional integrity of the thermally formed surgical device. I have found that by treating the thermally formed device at a temperature of at least 600C but less than 1 00 C, and preferably from 800C-900C, in an environment essentially saturated with moisture, and preferably in hot water, for a time of at least 10 minutes, the properties of the product important to its use as a surgical device are substantially enhanced.

The new product of the present invention comprises aliphatic, polyamide, thermally formed, surgical devices, preferably nylon-6, injection molded, surgical devices, which have an ct-crystallinity of at least 15% and preferably at least about 20% with a total crystallinity of 25% and preferably a total crystallinity in excess of 30%.

DESCRIPTION OF THE DRAWINGS Figure 1 is an enlarged perspective view of a ligating clip of the present invention.

Figure 2 is an enlarged perspective view showing the clip of Figure 1 in place closing off a blood 700 F. 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 Tensiometer. The jaws are steel faced. Using a strain rate of 5 mm/min. the jaws are moved apart and the force necessary to break the hinge is determined in kilograms.

HINGE STRENGTH (IN VIVA) The in vivo strength of the clip is measured as follows. Clips are separated without apparent bias into groups consisting of 10 clips each. Each group will correspond to one hinge strength test interval.

Special Long Evans rats, weighing 150 to 300 grams are prepared for surgery, anesthetized, 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 of the clips is determined by cutting away the latching mechanism at the distal end of the clip and placing the cut ends of the leg members in the opposing jaw of an Instron Tensiometer. The jaws are steel faced. Using a strain rate of 5 mm/min the jaws are moved apart and the force necessary to break the hinge is determined in kilograms.

PERCENT EXTENSION The 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 Crosshead x Length of Chart %Extension= x 100 Velocity Chart x Gauge Length The following specific examples are illustrative of the present invention.

EXAMPLES 1 THROUGH 4 Nylon-6 resin, sold by Allied Chemical Corporation as type 8207, having an a-crystallinity and a total crystallinity as shown in Table 1, is injection molded in the configuration of the clip depicted in Figure 1. A multi-cavity mold in an Arburg injection molding machine is used. The mold temperatures are as given in Table 1, and the nozzle temperature is approximately 2400 C. The other parameters of the injection molding steps are similar to those generally used in the injection molding of nylon-6. The molded parts 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 efficacy open spontaneously when implanted indicating their lack 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 800C for the time indicated in Table 1. 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 800C for thirty minutes. The annealed clips are sterilized by subjecting the clips to a dosage of 2.5 megarads of 60Co irradiation. The sterilized clips are tested for hinge strength, percent extension, total crystallinity and in vivo 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 examples as well as the test results are provided in the following Table 1.

TABLE 1 in Vivo Hinge Molded Clips Annealed Clips Strength (Kg) Days Past Mold Hinge Exten- % Crystallinity Hinge Exten- % Crystallinity Implantation Example Temp. Strength sion Con- Strength sion No. C Kg % alpha gamma ditions Kg % alpha gamma 0 10 14 21 1 86' 2.34 15 7 15 -- -- -- -- -- 1.87 1.09 -- - 2 86' 2.34 15 7 15 steam 2.63 22 29 3 2.17 1.45 -- - 30 min.

2 86' 2.37 15 7 16 steam 2.31 28 29 4 1.89 1.45 -- - 30 min.

3 50 2.25 38 9 13 water 2.43 36 24 5 -- -- -- - 15 min.

3 50 2.25 38 9 13 water 2.42 34 25 2 -- -- -- - 15 min.

3 50 2.25 38 9 13 water 2.65 33 28 3 -- -- -- - 15 min.

4 50 2.25 38 9 13 water 2.44 26 31 4 2.12 -- 1.82 1.0 30 min.

1. Clips were brittle.

2. Tested without pre-conditioning.

As may be seen from the above table, the annealed clips 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 already been stated that annealing of thermally formed polyamides is not a common practice. Moreover, I have found existing annealing procedures for polyamides do not improve those properties required in thermally formed surgical devices, especially those properties of flexibility 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., 1 30-1 490C for nylon-6). Furthermore, it is well known that this practice is used to improve lubricity of the parts due to imbibing of the annealing medium for this reason. Moreover, this process has been found to be additionally deleterious due to discoloration of the annealed parts and the need of a costly oil removal process.

A second method of annealing polyamides is by heating to elevated temperatures (i.e., up to 500C below the melt temperature) in the presence of inert gas to avoid oxidation.

I have found that heating thermally formed surgical devices under nitrogen at elevated temperatures (i.e., 50-1 500C) for various time periods has little effect on the desirable properties of these devices. Moreover, I have found that the annealing process is sometimes quite deleterious to these devices.

The following example is illustrative of this point.

EXAMPLE 5 Clips as previously described are injection molded using mold temperatures of 860C. 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 for 1.5 to 24 hours at temperatures from 50-1 500 C. Properties of these clips are either not improved or deleteriously affected as set forth in Table 2.

TABLE 2 Nitrogen Hinge Annealing Strength Extension Parameters Device Appearance Kg % Comment None Clip Clear, 2.34 15 Gamma Crystallinity Colorless 5O0C/24 hrs. Clip Slightly - Yellow 11 50C/1 7 hrs. Clip Slightly 2.00 8 Yellow 1300C/6 hrs. Bar Yellow - - Gamma Crystallinity, Brittle 13000/12 hrs. Bar Yellow - - Gamma Crystallinity, Brittle 140 C/3 hrs. Bar Yellow - - Gamma Crystallinity, Brittle 1400C/6 hrs. Bar Yellow - - Gamma Crystallinity, Brittle 14O0C/6 hrs.Bar Yellow - - Gamma Crystallinity, Brittle 1500C/1.5 hrs. Bar Yellow - - Gamma Crystallinity, Brittle 1 5000/3.0 hrs. Bar Yellow - - Increased Alpha Crystallinity, Brittle 1 5000/3.0 hrs. Clip Yellow Could not - Extra Brittle be tested It may be seen that nitrogen annealing at elevated temperatures causes discoloration and embrittlement as illustrated by decreased percent extension at the breaking of 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 of the gamma or metastable variety. Only at 1 5000 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 flexibility and functional integrity which makes them especially suitable as surgical devices.

Furthermore, 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. Hence, 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 are at their moisture equilibrium to remain flexible during storage and, hence, be suitable for use in the various surgical procedures.

Having now described the present invention and certain specific embodiments, it should be apparent to those skilled in the art that there may be various modifications and alterations which may be made to the present invention without departing from the scope of the present invention. I only desire to be limited by the scope of the claims appended hereto.

Claims (17)

1. A method for improving the in vivo properties of an aliphatic polyamide, thermally formed, surgical device comprising heating the device to a temperature of from about 6000 to 10000 in an environment essentially saturated with moisture for a period of time sufficient to increase the alpha crystallinity of the device to at least 1 5% whereby the functional integrity of the device is improved.

2. A method according to Claim 1 wherein the device is heated for at least 10 minutes.

3. A method according to Claim 1 or 2 wherein the device is an injection molded device.

4. The method according to Claim 1,2 or 3 wherein the surgical device is a ligating clip comprising two leg members connected at their proximal ends by a resilient hinge and having locking means disposed at the distal ends of the leg members.

5. The method according to Claim 1, 2, 3 or 4 wherein the polyamide is nylon-6.

6. The method according to Claim 1,2,3,4 or 5 wherein the temperature is from 8000 to 9000.

7. The method according to Claim 1,2,3,4,5 or 6 wherein the molded device is heated in water.

8. A thermally formed aliphatic polyamide, surgical device having an a-crystallinity of at least 1 5% and a total crystallinity in excess of 25%.

9. A device according to Claim 8 which is an injection molded device.

10. An injection molded surgical device in accordance with Claim 9 comprising a ligating clip having two leg members connected at their proximal ends by a resilient hinge section and containing a locking mechanism at their distal end.

11. A surgical device in accordance with Claim 8, 9 or 10 wherein the polyamide is nylon-6.

12. A surgical device according to Claim 8, 9, 10 or 11 having an a-crystallinity of at least 20%.

13. An injection molded surgical device according to Claim 8, 9, 10, 11 or 12 having a total crystallinity of at least 30%.

14. A surgical device according to any of Claims 8-1 3 comprising a wound closure device having a thin flexible extended section having a cross piece disposed at each end of said section.

15. A method according to Claim 1, substantially as indicated in the foregoing Examples section.

16. A device obtained by a method according to any of Claims 1 to 7, or Claim 15.

17. A device according to Claim 8, substantially as described with reference to Figs. 1 and 2, Fig. 3, or Figs. 4 and 5 of the accompanying drawings.