The present application is a Continuation of copending U.S. patent application Ser. No. 08/499,423 filed Jul. 7, 1995.
FIELD OF INVENTION
This invention relates to the field of interior liners for pipes and tubes and particularly to liners for blood conduits.
BACKGROUND OF THE INVENTION
There exists a need for a liner to provide a new interior surface lining for pipes and tubes in various applications. A liner having a smaller circumference than the inner circumference of the tube or pipe intended to be lined could be easily located axially within that pipe or tube. If such a liner were circumferentially distensible by the application of an internal pressure it could be expected to conform to the topography of the inner surface of the pipe or tube during use even if that surface were rough and irregular. Alternatively, an inflatable balloon could be used to circumferentially distend the liner to cause it to conform to the interior surfaces of the tube being lined. The ends of the liner could be affixed to the interior surface of the lined pipe or tube by various known mechanical fastening means; in some instances it may not require fastening, particularly at the downstream end. Such a liner would be of even greater utility if it were made from a highly chemically inert material.
Particularly useful applications of such a concept would be as an interior liner for prosthetic vascular grafts or natural vessels. For example, the liner could be installed within arteriovenous grafts cannulated by dialysis needles for kidney dialysis. Such grafts presently have a useful life expectancy often limited by the number of times they can be cannulated due to damage caused to the graft wall by the needles. Repeated cannulation in the same region results in fluid leakage through the graft. Once excessive leakage occurs, the graft is abandoned or bypassed. If it were possible to extend the life of the graft by providing it with a new interior lining surface, the graft could continue to be used for cannulation by dialysis needles and the patient would be spared the additional trauma and disfigurement resulting from implanting an entirely new graft. Such a liner may also inhibit tissue growth that often leads to unacceptable narrowing of the flow cross section. It might be useful for providing a smoother flow surface for anastomoses of vascular grafts or living blood vessels including graft-to-blood vessel anastomoses. The liner could also be used to provide additional strength to weak or damaged blood vessels or vascular grafts, or to intentionally occlude side tributaries in living blood vessels. Further, the inner surfaces of diseased vessels could be lined subsequent to enlarging the flow channel via balloon angioplasty, thrombectomy, or by other means.
Various published documents describe the use of porous PTFE vascular grafts as interior liners for blood conduits. See, for example, Marin M L et al., “Transluminally placed endovascular stented graft repair for arterial trauma,” J Vasc Surg 1994; 20:466-73; Parodi J C, “Endovascular repair of abdominal aortic aneurysms and other arterial lesions,” J Vasc Surg 1995; 21:549-57 and Dake M D et al., “Transluminal placement of endovascular stent-grafts for the treatment of descending thoracic aortic aneurysms,” New England Journal of Medicine 1994; 331:1729-34. U.S. Pat. Nos. 5,122,154 to Rhodes and 5,123,917 to Lee describe similar applications. These documents typically describe the use of GORE-TEX□ Vascular Grafts or Impra□ Grafts as intraluminal grafts or interior liners for blood conduits. These commercially available porous PTFE vascular Grafts have specific disadvantages as interior liners.
GORE-TEX Vascular Grafts are porous PTFE tubes having a helical wrap of a reinforcing film that substantially prevents circumferential distension. The Impra Grafts do not have such a reinforcement and so may be circumferentially distended, however, these grafts will recoil significantly on release of the distending force and therefore must be retained in place by the use of mechanical means such as balloon expandable metal stents. Also as a result of the lack of a reinforcing layer, these grafts continue to circumferentially distend with exposure to increasing pressure and so do not have a second circumference at which the circumference stabilizes and does not substantially further distend with increasing pressure.
The disadvantages of presently available vascular graft materials for use as intraluminal grafts are well documented. For example, in a paper entitled “Endovascular Femoropopliteal Bypass: early Human Cadaver and Animal Studies” (Ann Vasc Surg 1995; 9:28-36), Doctor Ahn writes in describing the effectiveness of presently available intraluminal graft materials, “However, before this idea can be translated to broad clinical use, multiple problems still need to be resolved and/or avoided. The current study clearly shows the importance of a proper size match between the graft and the artery.” There is clearly a need for more effective intraluminal graft materials that are circumferentially distensible in order to conform smoothly to vessel walls without allowing retrograde dissection due to substantial recoiling of the graft following circumferential distension.
SUMMARY OF THE INVENTION
The present invention is an interior liner for tubes, pipes and blood conduits comprising a tubular form circumferentially distensible and conformable whereby the first circumference of the interior liner (the initial circumference of the liner at zero pressure) may be distended by the application of pressure causing the first circumference to be increased to a larger circumference. The qualities of being circumferentially distensible under pressure and conformable allow the interior liner to be placed into another pipe or tube and be circumferentially distended under pressure until the interior liner is smoothly conforming without gross wrinkles to the interior surface of the other pipe or tube even if that surface represents a rough, irregular, damaged or otherwise non-uniform topography. The use of a porous polymer to construct the interior liner enhances its ability to conform.
For applications in which the pipe, tube, or blood conduit to be lined may not have adequate strength to resist expected normal fluid operating pressures, the interior liner of the present invention is preferably provided with a self-limiting circumference whereby it is circumferentially distensible up to a second circumference beyond which it will not substantially distend if used within the designed range of operating pressures. Pressures approaching the burst pressure of the interior liner are necessary to cause further substantial circumferential distension beyond the second circumference. The circumference can, however, be expected to grow in response to creep (time-dependent plastic deformation). This self-limiting feature is useful for lining weakened pipes, tubes or blood conduits whereby the liner itself is capable of withstanding the normal fluid operating pressure of the lined system.
Blood conduits include living blood vessels (veins and arteries) and vascular grafts of both prosthetic and natural materials. Vascular grafts of natural materials include, for example, materials of human umbilical components and materials of bovine origin.
In another embodiment, the interior liner of the present invention has minimal recoil after being circumferentially distended so that it remains proximate with all interior surfaces of the pipe, tube or blood conduit to which it has been fitted. Minimal recoil is considered to mean recoiling diametrically (or circumferentially) in an amount of 14 percent or less and more preferably 10 percent or less from a diameter to which the liner has been circumferentially distended by an amount of 25 percent, with the recoiled diameter measured 30 minutes following the release of the circumferentially distending force.
Particularly for applications relating to use as a liner for blood conduits, it is preferred that the interior liner have a second circumference beyond which it is not readily distensible and minimal recoil. For many of these applications, it may also be preferred that the liner have a wall thickness of 0.25 mm or less.
The term circumference is used herein to describe the external boundary of a transverse cross section of the article of the present invention. For any given amount of distension, the circumference is the same whether the article is wrinkled, folded or smooth.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 describes a perspective view of the construction of an interior liner according to the present invention having a layer of helically-wrapped porous PTFE film applied in a single direction over the outer surface of a longitudinally extruded and expanded porous PTFE tube.
FIG. 2 describes a perspective view of the construction of an interior liner according to the present invention having two layers of helically-wrapped porous PTFE film applied in opposing directions over the outer surface of a longitudinally extruded and expanded porous PTFE tube.
FIG. 3 describes a perspective view of the construction of an interior liner according to the present invention having two layers of helically-wrapped porous PTFE film applied in opposing directions. No separate substrate porous PTFE tube is used beneath the film.
FIG. 4 shows a flow chart that describes a process for making a preferred interior liner of the present invention.
FIG. 5 describes an interior liner secured to a blood conduit by an expandable stent.
FIG. 6 describes a cross section of an interior liner of the present invention used in the repair of an arteriovenous vascular graft.
FIGS. 7A and 7B describe a method of anastomosing the interior liner to a blood conduit using sutures.
DETAILED DESCRIPTION OF THE INVENTION
The interior liner of the present invention is made is preferably made from porous PTFE and most preferably porous PTFE having a microstructure of nodes interconnected by fibrils made as taught by U.S. Pat. Nos. 3,953,566 and 4,187,390, both of which are herein incorporated by reference. When comprised of porous PTFE, the interior liner has additional utility because of the chemically inert character of PTFE and has particular utility as a liner of blood conduits including living arteries and veins, vascular grafts and various repairs to blood conduits, particularly including the lining of anastomoses. The porosity of the porous PTFE can be such that the interior liner is substantially impervious to leakage of blood and consequently does not require preclotting. For use as a blood conduit liner, the interior liner may preferably have a very thin wall thickness such as in the range of 0.10 to 0.25 mm and may be made to be even thinner; U.S. Pat. No. 4,250,138 describes a method of manufacturing porous PTFE tubes having such very thin wall thicknesses. Alternatively, the interior liner can be made to have wall thicknesses of greater than 0.25 mm if that were to be desirable for some applications.
The interior liner is preferably made to have a second circumference beyond which the circumference of the liner will not distend significantly unless the normal system operating pressure is substantially exceeded. For example, in the case of an interior liner intended for use as a blood conduit liner, pressures in excess of twenty-five times normal human systolic blood pressure (120 mm Hg) may be required to cause the interior liner of the present invention to substantially increase in circumference beyond its second circumference. One embodiment of the blood conduit interior liner would, for example, have an initial inside diameter of about 3.5 mm prior to circumferential distension. This small initial diameter allows for easy insertion into blood conduits. The second circumference of this embodiment would correspond to a diameter of, for example, 8 mm, so that the liner would be most useful for lining blood conduits having inside diameters of up to about 8 mm. The second circumference for this embodiment, corresponding to a diameter of 8 mm, prevents further distension of the circumference of the blood conduit under virtually all normal operating conditions. The second circumference is established by the presence of a thin film tube of helically wrapped porous PTFE film. The film tube can be bonded to the outer surface of a substrate tube of porous PTFE. This substrate tube is preferably made by longitudinal extrusion and expansion whereby a seamless tube is created; alternatively, the substrate tube may be made from a layer of porous PTFE film oriented substantially parallel to the longitudinal axis of the tube and having a seam in this same direction. The helically wrapped porous PTFE film is comprised primarily of fibrils which are oriented in a substantially circumferential direction around the outer surface of the substrate tube thereby restraining and limiting the second circumference of the resulting interior liner. The helically wrapped porous PTFE film is preferably wrapped in opposing directions with respect to the longitudinal axis of the tube. Such an interior liner may also be made from helically wrapped porous PTFE film wrapped helically in opposing directions without the use of a substrate tube.
Conversely, the interior liner may be made so as not to have a second circumference for applications not requiring additional circumferential strength.
The resistance of the interior to circumferential distension by pressure can be varied. For example, an interior liner can be made having a very thin wall thickness in order to be capable of being distended by blood pressure alone which may allow for relatively simple installation of the liner. Alternatively, the interior liner may be made to require a greater distending force to cause it to conform to the interior surface of a blood conduit, such as a distending force supplied by the inflation of a balloon catheter. Such balloon catheters are used conventionally to increase the diameter of balloon expandable metal stents during implantation of such stents into blood conduits as well as to increase the flow cross section in partially occluded living blood vessels. An interior liner requiring such a higher distending force is the result of the use of a substrate tube having a greater wall thickness, the use of more helically wrapped film around the exterior surface of the substrate tube, or both.
Previously available porous PTFE tubes that allow any appreciable amount of circumferential distensibility under pressure also recoil significantly when the pressure is removed and so require mechanical support such as stents along their entire length to hold them against the interior surface of a blood conduit. For most blood conduit applications it is preferable that the liner not recoil. Various embodiments of the present invention provide an interior liner that allows substantial circumferential distensibility without appreciable recoil which in turn allows for relatively easy insertion and deployment into a blood conduit, maximizes available cross sectional flow area by conforming uniformly to the interior surface of the blood conduit, and minimizes fluid accumulation between the liner and the blood conduit.
The percentage recoil of an interior liner is determined with the use of a tapered metal mandrel having a smooth, polished exterior surface. A suitable taper is 1.5 degrees from the longitudinal axis. Preferably the mandrel is provided with incremental diameter graduations at intervals whereby the inside diameter of a tube may be determined by gently sliding a tube onto the smaller diameter end of the mandrel and allowing the tube to come to rest against the tapered mandrel surface and reading the appropriate graduation. Alternatively the inside diameter of the tube may be measured by viewing the tube and mandrel, fitted together as previously described, using a profile projector measurement system. Using either a graduated mandrel or a profile projector, percentage recoil of an interior liner is determined by first measuring the initial diameter of the liner. The liner is then gently slid further onto the tapered mandrel with a minimum of force until a diameter increase of 25% is obtained. This increased diameter is considered to be the distended diameter. The liner is then pushed from the mandrel, avoiding the application of tension to the liner. After waiting at least 30 minutes to allow the liner to recoil, the recoil diameter is determined using the tapered mandrel by performing the same procedure as used to measure the initial diameter. Percentage recoil is then determined using the formula:
Minimal recoil is considered to be 14 percent or less and more preferably 10 percent or less.
In one embodiment of the present invention, circumferential distension results in some degree of twisting along the length of the liner. For applications requiring maximum conformability to irregular surface topography, alternative embodiments are described which do not twist along their length during circumferential distension.
The conformability and distensibility of the interior liner allow it to effectively line blood conduits even when the interior topography is irregular and non-uniform. Relatively tortuous blood conduits, acutely curved conduits and tapered conduits may be provided with a relatively smooth lining. The blood conduit liner having a second circumference is anticipated to be useful to repair aneurysms including aortic aneurysms and otherwise weak blood conduits. The interior liner is expected to be generally useful to provide a new flow surface to previously stenosed vessels, particularly in veins anastomosed to arteriovenous vascular grafts and in peripheral vessels such as those in the legs. It is also expected to be useful for the repair of arteriovenous access vascular grafts that have been cannulated by dialysis needles to the extent that their further use is jeopardized. The conformable quality of the inventive interior liner can provide such vascular grafts with a new blood flow surface and thereby allow their continued use. The conformability of the liner also allows it to provide a new, smoother flow surface for anastomoses and other blood conduit dissections and may consequently reduce the risk of intimal hyperplasia at the distal end of the graft. The interior liner may be installed without any distal anastomosis thereby reducing the risk of anastomotic hyperplasia. The liner may also be used to occlude side vessels if desired, such as for the conversion of veins to arteries during in situ bypass procedures. The liner may be useful in such procedures to smooth the remnants of removed venous valves or even to hold venous valves open and thereby obviate the need to remove them at all.
The interior liner may be provided in bifurcated or Y-graft configurations to allow lining of, for example, branched blood conduits. The tubular liner may also be cut into sheets if a sheet material such as an implantable repair patch is needed that requires distensibility or conformability.