US20050131520A1

US20050131520A1 - Compliant blood vessel graft

Info

Publication number: US20050131520A1
Application number: US10/987,313
Authority: US
Inventors: Peter Zilla; Nasser Rafiee; Deon Bezuidenhout; Thomas Franz; Mark Yeoman; Hellmut Bowles; Nareak Douk; Michael Wolf
Original assignee: Zilla Peter P.; Nasser Rafiee; Deon Bezuidenhout; Thomas Franz; Yeoman Mark S.; Hellmut Bowles; Nareak Douk; Wolf Michael F.
Current assignee: Kips Bay Medical Inc
Priority date: 2003-04-28
Filing date: 2004-11-12
Publication date: 2005-06-16
Also published as: US20090306764A1; US9517121B2

Abstract

A graft for replacement of a section of an artery and methods of making the graft. The graft comprises a flexible, resilient, generally tubular external support and a blood vessel segment, e.g., a vein segment, carried within and having an ablumenal surface in contact with and supported by the tubular support, the graft being capable of resilient radial expansion in a manner mimicking the radial compliance properties of an artery.

Description

CROSS REFERENCE

This application is a continuation-in-part of U.S. patent application Ser. No. 10/834,360, filed Apr. 28, 2004, which claims priority from U.S. Provisional Application Ser. No. 60/466,226 filed Apr. 28, 2003, both of which applications are incorporated herein by reference in their entireties.

FIELD OF THE INVENTION

This invention involves a graft involving a blood vessel segment and a supportive sheath chosen to provide the graft with mechanical compliance properties which resemble those of a healthy native artery.

BACKGROUND OF THE INVENTION

Various types of vascular prostheses are known or available. Commercially available synthetic vascular grafts in use are commonly made from expanded polytetrafluoroethylene (e-PTFE), or woven, knitted, or velour design polyethylene terephthalate (PET) or Dacron®. These prosthetic vascular grafts may have various drawbacks. When used for repairing or replacing smaller diameter arteries, these grafts may fail due to occlusion by thrombosis or kinking, or due to an anastomotic or neointimal hyperplasia (exuberant cell growth at the interface between artery and graft). Another problem may involve expansion and contraction mismatches between the host artery and the synthetic vascular prosthesis, which may result in anastomotic rupture, stimulated exuberant cell responses, and disturbed flow patterns and increased stresses leading to graft failure.
Problems also exist with the use of autologous saphenous vein grafts in these applications. Use of autologous saphenous vein grafts to bypass blockages in coronary arteries has become a well-established procedure. However, their success in the long term has been limited. In the coronary position, the literature reports a low (45-63%) patency of vein grafts after 10-12 years. It is believed that these failures result from remodeling of the implanted vein in response to greatly increased internal pressure, that is, as the vein is required to function as an artery. In general, arteries have substantial musculature and, although able to expand diametrically in response to increased internal pressure, are capable of withstanding normal arterial pressure variances. Veins, on the other hand, are not required to withstand arterial pressure variances and are relatively incapable of withstanding the higher arterial pressures without substantial bulging. In this regard, the nominal venous diameter seen under nominal venous pressure is seen to approximately double upon exposure to arterial pressure.
Increases in lumenal diameter of these magnitudes in vein segment implants are accompanied by increases in tangential stress. Tangential stress has been shown to be proportional to the lumenal radius-wall thickness ratio. In healthy arteries, this ratio remains constant across multiple species. However, this does not occur in veins. It is believed that a vein's smooth muscle cells increase their growth rate and secrete extra-cellular matrix components in response to such increases in tangential stress. This becomes a remodeling response, and is likely an attempt by the vein to reduce the lumenal radius-wall thickness ratio, and consequently the tangential stress. However, it appears that these reactions overcompensate in the veins, resulting in the phenomenon of neointimal hyperplasia yielding grossly thickened and stiff graft walls. As the dilation of the vein segment continues, the resulting mismatch between the vein and artery diameters may lead to disturbance of flow patterns, which may also favor the formation of thrombi.
Problems also exist when tubular prostheses are used as exteriorly accessible shunts to facilitate access to the circulatory system for, e.g., the administration of medicines and nourishment and for dialysis procedures.

SUMMARY OF THE INVENTION

It has now been found that a blood vessel segment such as a vein segment, if externally supported by an appropriate, flexible, radially-resiliently tubular support, can provide a valuable tubular prosthesis. A vein segment so supported can function in much the same fashion as the artery that is to be replaced. That is, it functions without undue bulging or aggravated mismatching phenomena leading to graft failure. Unless otherwise indicated, the term “compliance” means the ratio of the diameter change of a vessel as it expands in the radial direction in response to a given change in vessel pressure, and the values for compliance referred to below result from dynamic, in vitro testing. As described in greater detail below, the compliance of venous graft is largely dependent upon the compliance of the external, radially resilient support.
The invention in one embodiment, accordingly, relates to a flexible, resilient, generally tubular external support within which may be supported a blood vessel segment such as a vein segment to form a graft. The tubular support is capable of resilient radial expansion in a manner mimicking the compliance properties of an artery, and compliance figures in the range of 3 to 30%/100 mm Hg are appropriate. The tubular support may be formed of a knitted or woven fiber mesh that is so formed as to exhibit the needed compliance properties.
The invention in certain embodiments provides a venous graft for replacement of a section of an artery. The graft comprises a flexible, resilient, generally tubular external support and a vein segment carried within and having an ablumenal surface in contact with and supported by the tubular support, the venous graft being capable of resilient radial expansion in a manner mimicking the compliance properties of an artery. Compliance figures in the range of 3 to 30%/100 mm Hg are appropriate, although compliance values ranging up to 50%/100 mm Hg may be desired in some instances. The tubular support may take the form of a fiber mesh, such as a knitted, braided or woven mesh, the fibers of which may, if desired, be appropriately crimped to provide the required resiliency and compliance.
In other embodiments, the invention relates to a method for producing a venous graft for use, for example, in replacing a section of an artery. A segment of a vessel is provided, and is sheathed in a generally tubular support in supportive contact with the ablumenal surface of the vein segment. The support is sufficiently flexible and radially resilient as to provide the resulting graft with compliance properties mimicking the compliance properties of the artery to be replaced. Sheathing of the vessel segment within the tubular support may be accomplished by supporting the generally tubular support upon an exterior surface of an applicator having an internal passage within which is positioned the vessel segment, and removing the applicator to permit the tubular support to come into supportive contact with the ablumenal surface of the vessel segment. Axial dimensional changes in the tubular support may be controlled as necessary to provide the graft with the desired compliance properties mimicking arterial compliance properties.
Other embodiments of the invention relate to vessel grafts that include a flexible, resilient, generally tubular external support formed of a shape memory alloy, and a vessel segment carried within and having an ablumenal surface in contact with and supported by the tubular support. The shape memory support may be placed around a vessel segment when the shape memory material is in a first enlarged configuration. The tubular support comes into supportive contact with the ablumenal surface of the vessel when the support is transformed, as by a temperature increase or upon removal of an introducer tube over which the tubular support is supported, into a second configuration different from the first configuration. The shape memory support in its second configuration may exhibit superelastic properties and in any event is sufficiently flexible and resilient as to provide the venous graft with compliance properties mimicking the compliance properties of, for example, an artery. Compliance figures in the range of 3 to 30%/100 mm Hg are appropriate. The tubular support may take the form of a wire mesh made of shape memory alloy, such as a knitted or woven mesh, the wires of which may, if desired, be appropriately crimped to provide the required resiliency and compliance.
The invention is described hereafter primarily with respect to grafts that utilize veins that are received within a tubular support and that can function as replacements for arterial segments in, for example, coronary by-pass procedures, but the grafts of the invention may also utilize other vessels such as arteries, including treated vein and artery segments from donor animals such as vessels of porcine and bovine origin.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a pressure versus diameter graph typifying the characteristics of a native vein, native artery, a non-compliant stented vein, and a compliant stented vein;
FIG. 2 is a schematic cross-sectional view of an artery;
FIG. 3 is a representative pressure versus strain graph;
FIG. 4 is a pressure versus graft diameter graph;
FIG. 5 is a photograph of a tubular support in a first configuration, shown in an axially compressed and radially expanded configuration and supported on a plastic tube;
FIG. 6 is a photograph of the tubular support of FIG. 5 in an axially elongated and radially reduced configuration to conform to a vein outer diameter;
FIG. 7 is a side view of the graft of FIG. 6, showing a length-governing element;
FIG. 8 is a schematic view of braided elements;
FIG. 9 is a perspective view of a braided tubular support;
FIG. 10 is a schematic view of knitted elements;
FIG. 11 is a side view of a section of a knitted tubular support;
FIG. 12 is a view of angular pre-braiding crimped elements;
FIG. 13 is a perspective, schematic view of an angular pre-braiding crimped tubular support;
FIG. 14 is a view of rounded pre-braiding crimped elements;
FIG. 15 is a view of angular pre-knitting crimped elements;
FIG. 16 is a view of rounded pre-knitting crimped elements;
FIG. 17 is a broken-away, perspective view of a post-braiding crimped tubular support;
FIG. 18 is a broken-away, perspective view of a venous graft showing a portion with anti-fraying element;
FIG. 19 is a broken-away, perspective view of one embodiment utilizing an applicator for assembling a venous graft;
FIG. 20 is a broken-away, perspective view of the use of a modified applicator for assembling a venous graft;
FIG. 21 is a photographic, perspective view of a section of a knit tubular support;
FIG. 22A is a schematic cross-sectional view of an assembly device;
FIG. 22B is a schematic, prospective view of a step in the assembly of a vessel graft;
FIG. 23 is a schematic, prospective view of another step in the assembly of a vessel graft;
FIG. 24 is a schematic cross-section of an attachment of a vessel to a tubular support;
FIG. 25 is a schematic cross-section of another attachment of a vessel to a tubular support;
FIG. 26 is a schematic cross-section of yet another attachment of a vessel to a tubular support;
FIG. 27 is a schematic cross-section of the attachment of a vessel to a tubular support utilizing a sleeve;
FIG. 28 is a schematic cross-section of the attachment of a vessel to a tubular support utilizing an adhesive tape;
FIG. 29A is a cross section of a clip bearing an adhesive tape segment;
FIGS. 29B through D are schematic views showing stages in the application of an adhesive tape segment to a vessel graft;
FIG. 30 is a schematic view showing severance on a bias of the vessel graft also shown in FIG. 29D;
FIG. 31 is a schematic view of the attachment to an artery of a segment shown in FIG. 30; and
FIG. 32 is a photograph of a portion of a bioprosthetic access vessel graft.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

Applicants have recognized that significant deficiencies attend to the past methodologies and devices relating to the increased pressures experienced by vein grafts utilized in arterial positions. The increased pressures lead to excessive dilation of vein grafts in arterial circulation, leading to the development of intimal hyperplasia, which causes occlusion of the vessel.
Intimal hyperplasia is believed to be a primary reason for vein graft failure. In this context it is known that intact endothelium acts in a manner to protect against the proliferation of underlying vascular smooth muscle cells, known as VSMC. The intact endothelium also plays a role in VSMC contractile responses. The VSMC have also been shown to release factors with long term physiological effects on the endothelial cells, including maintenance of a non-proliferative state. By comparison, the pathogenesis of intimal hyperplasia in a vein graft may follow the sequence of dilatation under arterial pressure; overstretching to maximum capacity; disruption of borders of endothelial cells; rupture of internal elastic membranes; migration of smooth muscle cells into the intimal layer and resultant unbalanced proliferation; atrophy of media and further consolidation of stiffness; and graft arteriosclerosis with traumatic media necrosis and atrophy, as well as pathological surface and wall stress and strain. These phenomena may result in a decrease in vein graft patency within six years. Intimal hyperplasia may be observed in such grafts from about 16 months, while anastomotic intimal hyperplasia may occur at about 18 months, and arteriosclerosis may occur from about 45 months.
Others have attempted to overcome certain of these problems by use of metallic or polymeric external structures designed to arrest the dilation of the vein graft. FIG. 1 graphs blood pressure against vessel diameter, with D_Orepresenting the vessel diameter at zero pressure. As shown in this graph, lines 16, 18 represent the normal diastolic, i.e. low (80 mm Hg) and normal systolic, i.e. high (120 mm Hg) physiological blood pressure range for humans. Line 21 may represent the diameter of an artery (D_A) at 100 mmHg, and line 23 may represent the diameter of a vein (D_V) at the same pressure of 100 mmHg. An unstented native artery reacts to pressure loading as shown at line 32, and an unstented vein reacts to the same loading as shown at line 35. The use of known stents with vein grafts results in movement of line 35 in the direction shown by arrow 38, resulting in the approximate profile indicated at line 42 showing the response of a pressure loaded vein and non-compliant stent combination. Although this prevents over-dilation, and some advantage accrues, this may lead to further unhealthy sequelae. Also, to the extent that vein-stent combination devices may be shown to limit some of the dilation and intimal hyperplasia in the mid-graft region, they may not be able to prevent intimal hyperplasia at the anastomoses. This can be a significant problem for vein grafts that are transplanted into the arterial circulation vasculature. Prior attempts to resolve these problems fail to recognize the full implications of a vein being used in these situations. Accordingly, factors in the design of a vein-graft that may have a significant impact on its long term patency may have been missed.
One important factor in proper remodeling is that of proper cyclic stretch. Applicants are able to incorporate this concept into vein-stent grafts of the invention. In similar manner, the role of vascular endothelial growth factor (VEGF) in vascular smooth muscle cells may be very important to the design of a preferred arterial vein-stent graft. It is known that low concentrations of VEGF may play a role in preserving and repairing the arterial lumenal endothelial layer. Further, it is suggested that activation of the VEGF receptor KDR is affected by cyclic stretch. Applicants believe that the phenomenon of upregulation of VEGF expression by physiological stretching of vascular smooth muscle cells is one reason for redesigning a vein-stent graft which has improved, controllable cyclic stretch features.
A further consideration is the influence of tensile stress/strain on the structure and organization of smooth muscle cells during development and remodeling, particularly as to the orientation of such cells. In a larger topographical sense, this may also relate to the role of blood flow in the formation of focal intimal hyperplasia in known vein grafts, including inducement of eddy blood flow at locations of graft-host diameter mismatch.
These considerations and deficiencies can be addressed with the various structures and methodologies of the present invention in which a vein graft is provided that exhibits compliance properties mimicking those of healthy arteries. Radial expansion and contraction of the graft is permitted in a manner that mimics the radial expansion and contraction of an artery to at least closely approach the desired result in which the vein graft, its connections to adjacent arterial ends or stumps, and the adjacent arterial portions tend to expand and contract in a similar manner, to thereby substantially avoid anastomotic compliance mismatches. This is accomplished through the use of a flexible, resilient, generally tubular external support that engages the ablumenal surface of a vein segment carried within the support, the support being so fabricated as to functionally provide the graft with the compliance properties of an artery.
Compliance Properties
As noted earlier, compliance is the ratio of the diameter change of a vessel in the radial direction to a given change in vessel pressure, and the values for compliance referred to below result from dynamic, in vitro testing. Compliance values are reported here as percentage changes in the internal diameter of a vessel per a 100 mm Hg change in vessel pressure, as measured in the range of normal blood pressures, that is, from about 80 mm Hg to about 120 mm Hg. In the laboratory, it is convenient to measure compliance through the use of an elongated balloon structure over which a candidate tubular support is positioned. Distilled water at about 37° C. is pumped into the balloon to cause it to inflate, and the pressure within the balloon is cycled between 0 mm Hg and 140 mm Hg at a frequency of about 72 cycles per minute to mimic a normal pulsatile blood flow. The change in internal volume is measured between 0 mm Hg and 140 mm Hg to provide pressure/volume data. From this data is subtracted the pressure/volume data resulting from repeating the procedure with the balloon alone, and from the resulting pressure/volume data the percentage change in the internal diameter of the tubular support between 80 and 120 mm Hg can be calculated. It is convenient to express this radial compliance value as %/100 mm Hg.
The compliance of an implanted venous graft may be measured in vivo through the use of ultrasound techniques in which the vein graft is visualized in a cross-sectional view and the dimensional change of the vessel with varying blood pressure is recorded for at least one and usually a number of cardiac cycles. The cross-sectional lumenal area of the vein graft is measured for the smallest cross-sectional configuration and the largest cross-sectional configuration for one cardiac cycle. The smallest cross-sectional configuration of the vein graft lumen is associated with diastolic blood pressure whereas the largest cross-sectional configuration is associated with systolic pressure. The cross-sectional lumenal area values for diastolic and systolic blood pressure are used to calculate the lumenal diameter values and the vein graft compliance. Compliance values of a venous graft measured in vivo often are slightly larger that the compliance values measured in the laboratory, and the compliance values referred to herein are laboratory values resulting from the in vitro measurements described above.
FIG. 2 is a sectional representation of vascular tissue useful for illustrating the relation of the natural arterial structure with the prosthetic venous graft structure of the invention. The natural adventitial layer 95 of an artery 98 is comprised of two main tissue types that contribute to the mechanical properties of the natural artery, namely elastin and collagen. The mechanical properties of these two soft tissue components are described in Table I below:

TABLE I

Soft Tissue Elastic Modulus (Pa) Max Strain (%)

Elastin 4 × 10⁵ 130

Collagen 1 × 10⁹ 2-4
As shown in the above table, these two soft tissue types have large differences in mechanical properties. Elastin is very elastic, and collagen is very stiff in comparison. These two tissue types are combined in the adventitial layer to produce a non-linear elastic response. As shown in FIG. 3, the combined effect of the characteristics of elastin 101 and collagen 104 (having a greater role at higher strains) results in a non-linear response curve (shown loading at 135 and un-loading at 137) within the physiological pressure range of a natural artery between about 80-120 mm Hg. This characteristic of pulsatile expansion and contraction of arteries requires fine mechanical compliance of any prosthetic graft, i.e., a close mimicking by the prosthetic device of the mechanics and timing of the natural artery distending and reshaping under change in blood pressure.
From an engineering standpoint, the following relationships may be helpful from a design standpoint in producing venous stent grafts of the invention. $C_{d} = \frac{Δ D}{D_{diastolic} Δ P} \times 100 \times 100 mmHg$
in which C_dis compliance, P is blood pressure, ΔP is the difference between systolic and diastolic blood pressures, D is vessel diameter, and ΔD represents the diameter change between systolic and diastolic pressures.
The stiffness of blood vessels is stated as a stiffness index (β), and is a measure of the changes of curvature and diameter, stated as: $β = \frac{\ln \frac{P_{systolic}}{P_{diastolic}}}{\frac{Δ D}{D_{diastolic}}} = D_{diastolic} \frac{\ln P_{systolic} - \ln P_{diastolic}}{Δ D}$
A related characteristic of blood vessels is that of elastic modulus (K), which is considered a measure of stiffness, and is stated as: $K = \frac{V_{diastolic} Δ P}{Δ V} \propto \frac{D_{diastolic} Δ P}{Δ D} \propto \frac{1}{C}$
in which C is compliance, V_diastolicis the vessel volume per unit length at diastole, and ΔV is the difference in unit volumes between systole and diastole. In terms of diametric compliance, as an example, $K = D_{diastolic} \frac{P_{systolic} - P_{diastolic}}{D_{systolic} - D_{diastolic}} = D_{diastolic} \frac{Δ P}{Δ D}$
FIG. 4 shows that the Elastic Modulus (K), as defined in the above equations, is proportional to the secant S₁of the pressure-diameter curve PD₁, plotted on a linear scale (left y-axis in FIG. 4), between diastolic and systolic pressure. The slope, (P_syst−P_diast)/(D_syst−D_diast), of the secant S₁is a good approximation to the slope of the pressure-diameter curve PD₁in that pressure range. From the above equations for the Elastic Modulus (K) it can be appreciated that the Elastic Modulus (K) is not equal to the slope of the secant S₁but is proportional to the slope by a factor D_diastolic. Compliance (C_d) is approximately proportional to the Elastic Modulus (K) hence it is approximately proportional to the inverse of the secant S₁of the pressure-diameter curve PD₁between diastolic and systolic blood pressure.
The stiffness index (β) is proportional to the secant S₂of the pressure-diameter curve PD₂between diastolic and systolic blood pressure when the pressure-diameter curve is plotted on a logarithmic pressure scale (right y-axis in FIG. 4). The slope of the secant S₂is (in P_syst−1n P_diast)/(D_syst−D_diast) and is a good approximation to the slope of the pressure-diameter curve PD₂in that pressure range. It can be again appreciated, from the above equations for the Stiffness Index (β) that the Stiffness Index (β) is not equal to the slope of the secant S₂but is proportional to the slope by a factor D_diastolic.
Compliance data of natural human vessels is categorized by vessel type and by age of the vessel (i.e., age of patient). For example, a common carotid artery has about a 6.6%/100 mm Hg compliance value. The values for a superficial femoral artery and a femoral artery are 6-10%/100 mm Hg. A value for a saphenous vein, however, is about 4.4%/100 mm Hg, while an aorta ranges generally from about 20-50%/100 mm Hg, depending on the location. Also, the lengths of grafts according to location in the body must be considered, and substantial lengthwise variance in graft lengths is not uncommon. It is also known that the diameter of various arteries change over time, and this may have a significant impact on overall compliance values. Returning to FIG. 1, line 80 represents the pressure-diameter data that certain embodiments of venous grafts of the invention seek to emulate, wherein the compliance properties of a native artery (line 32) is closely mimicked.
Support Materials and Manufacture

The radially resilient support may be manufactured from any biologically acceptable material that possesses the ability to be shaped into a tubular structure having the required compliance. Polymeric fibers may be employed, such as polyurethanes, polyethylene terephthalate, polypropylene, and polytetraflouroethylene, and good results may be obtained through the use of wires of such metals as stainless steel and cobalt-chromium alloys. Wires made of shape memory alloys such as nitinol may be used to advantage. Shape memory elements or filaments may be made of one or more shape memory materials as exemplified in the following table, it being understood that this is not to be considered an exhaustive list. Also, any metal or metal alloy may be coated with a polymer for improved biocompatibility, recognizing that the polymer may or may not be biodegradable.



	ALLOYS	POLYMERS

	Ag—Cd	Two component system based on
		oligo(Σ-caprolactone)dimethacrylate and
		N-butyl acrylate
	Au—Cd	Polyurethanes
	Cu—Al—Ni	Polynorborenes
	Cu—Sn	Poly(ether ester)s consisting of
		poly(ethylene oxide) and poly(ethylene
		terephthalate) (EOET copolymers)
	Cu—Zn	Ethylene vinyl acetate copolymers
	Cu—Zn—Si	Polystyrene polybutadiene copolymer
	Cu—Zn—Sn
	Cu—Zn—Al
	In—Ti
	Ni—Al
	Ni—Ti
	Fe—Pt
	Mn—Cu
	Fe—Mn—Si

With respect to shape memory alloys, other design considerations include temperatures, different diameters and radial compliance, shape transformation dimensional changes, and wire thicknesses. Generally, shape memory alloys and shape memory polymers may have transformation temperatures which are below physiological temperatures, i.e., 37° C., to ensure self-righting responses. Preferably, transformation temperatures will also be above room temperature to ensure that the shape memory material reinforcing does not need to be refrigerated for storage purposes. Thus, the ideal shape memory transformation temperatures will likely be between 21° and 37° C. This transition may either be a two-way or a one-way directional transition, with a currently preferred embodiment including a two-way directional transition. The transition temperature range can either be a short, i.e. 0.5° C., or a long transition temperature range, i.e. 10° C., where the shape is proportionally regained over this temperature range. For example, for a desired temperature transition to be 100% complete at 25° C. but with it starting at 20° C., then this would yield a temperature range of 5° C. The changes in radial diameter due to the shape memory material experiencing transformation dimensional changes is preferably in a range of from 5% to 30%.
An embodiment of a tubular support utilizing a shape memory alloy is illustrated in FIGS. 5 and 6. FIG. 5 shows an arterial reinforcement tubular support 77 formed of one or more shape memory material elements 165. These elements are braided, but may also be knitted or woven, into a generally tubular structure designed for placement around a portion of a vein to produce an arterial graft. In this example, a shape memory alloy is employed because of its so-called “superelastic” properties rather than its ability to undergo temperature-induced phase changes, although some phase change from austenite to stress-induced martensite may occur. In FIG. 5, the braided tube is positioned on a hollow plastic straw as representing a vein segment, and has been compressed axially to produce an increase in diameter. By extending the braided tube axially, as shown in FIG. 6, the tube becomes reduced in diameter to provide support to the vein segment.
The shape memory braided material shown in FIGS. 5 and 6, if used also for its phase transformation properties, may be supplied in a first configuration (which may be in the martensite phase) which can be easily manipulated to receive a vein segment 86 within the structure, and a second configuration (shown in FIG. 6, which may be in the higher temperature austenite phase) which has a “remembered” narrower diameter configuration to provide support to the vein segment. The contact of inner surfaces 170 of the structure with ablumenal surfaces 175 of the vein segment 86 is shown also in FIG. 7. The resilience of shape memory materials can be controlled by altering compositions, tempering procedures, wire diameters, etc., so that a tubular support fashioned from this material may mimic (when combined with the minimal mechanical values of a vein segment) the compliance values of a host artery in order to optimize the venous graft-artery interaction. This aspect of compliance mimicking has components of expansion, recoil, timing, and tissue remodeling. In this example, the vein-stent compliance values are chosen to closely mimic those of a healthy native artery. Whereas the shape memory wires are shown as braided in FIGS. 5, 6 and 7, they may also be knit, and in fact the knit configuration appears to offer certain advantages.
Radially resilient tubular supports may be knit from metal wire, such as stainless steel and cobalt-chromium alloys. Metal wires ranging in diameter from about 25 to 150 micrometers are appropriate for knit supports with diameters in the range of 35 to 50 micrometers being particularly useful, although larger or smaller diameters may be employed as desired. For braided tubular supports, metal wires ranging in diameter from about 37 to about 170 micrometers are appropriate, although larger or smaller diameters may be employed.
Knitting procedures may be performed by known methods using, for example, a LX96 knitting machine manufactured by the Lamb Knitting Machine Corporation. Favorable radial compliance and tubular dimensional properties may result from knitting the tubular structure in a manner providing loops that alternate in the circumferential direction between larger and smaller loops, as shown in FIG. 21. In this Figure, smaller loops 250 are shown alternating circumferentially with larger loops 251. Such alternating loop sizes typically present themselves visually as longitudinal stripes extending axially along the tubular support, as the adjacent loops of each size align in the longitudinal axis. Each closed end of the loop may be either rounded or generally square-shaped or variations in between, and, the sides of the loop may turn outward, be parallel, or turn inward. The latter design has shown some advantage in acting like a spring and assisting in the stability of the overall dimensions of the tubular structure, and maintaining its compliance characteristics.
The knitted or braided tubular support may then be subjected to crimping to provide crimps extending, for example, about the circumference of the tubular support (that is, in the manner shown in FIG. 17). One way of doing this is through the use of an axially fluted mandrel that is inserted into the tube and is pressed outwardly against a wall of the tube to force the wall against a complementary shaped outer female mold to bend the knitted or braided wires and to form a circumferential crimp, the crimp resulting from each flute or raised ridge of the mandrel extending axially of the support.
A compliant venous graft using various metals or polymers for the tubular support may be provided in several ways. Embodiments may be advantageously provided in knitted form. FIGS. 8 and 9 show material 165 in a braided configuration, and FIGS. 10 and 11 show material 165 in a knitted configuration. Mechanical characteristics of the tubular support may be enabled by the type of shaping and relational structures formed on the elements making up the knit or braided structures. It is contemplated that a technique involving crimping of the material 165 to achieve angular crimps (shown in FIGS. 12 and 13), formed prior to the braid or knit construction, and rounded crimps (shown in FIG. 14) may provide acceptable results. Crimping techniques that may be appropriate with pre-knit configurations are shown in FIG. 15 (angular crimps) and FIG. 16 (rounded crimps). Another technique for achieving certain performance characteristics of braided or knitted shape memory materials 165 is to perform crimping after braiding or knitting, i.e. post-braiding or post-knitting. FIG. 17 shows one embodiment of material 165 formed in a braided configuration and having a post-braided crimping operation applied to form a crowned pattern to achieve desired crimp characteristics.
Crimp angle and pitch density may be important variables in certain embodiments of the current design of the tubular supports. It is understood, however, that certain advantages of this invention may still be achieved without use of crimping. Ranges of crimp angle are preferably from about 10° to 85° to the line of lay of the reinforcing wire or braid. The crimp size may vary from 0.01 to 5 mm in length. It is desired that the braid or helical wires have a pitch angle that may vary from about 5-85° to the axial length of the vein graft.
Applicants have identified certain crimping techniques that relate to crimping either before or after braiding or knitting. For example, in post-braid crimping the material braids are produced according to existing techniques, after which macroscopic longitudinal crimping is imparted to the tubular mesh using a post-braid crimping tool. However, according to the material and specific configuration of the stent, if the post-braid crimping of braided tubes does not achieve sufficient compliance then alternate methods are possible. One example is to effect pre-braid crimping, thereby setting the memory of a shape memory material in a crimped configuration and subsequently straightening the material before braiding. The crimp is thus induced upon exposure to physiological temperatures.
In certain embodiments, it is appropriate to provide for “jump” grafts, or “skip” grafts to communicate a stented graft with another vessel. To accommodate such grafts, an opening is made in the resilient, external tubular support of a compliant graft of the invention so that a portion of the vessel wall itself is exposed through the opening to enable a jump graft to be attached at that location. It is desirable to provide for such openings in the support wall prior to assembly of the compliant graft. When the tubular support is made of a shape memory alloy, such as nitinol, an opening in the mesh may be made by supporting the mesh on an appropriately shaped mandrel and gently moving the fibers forming the mesh away from what is to be the center of the opening. A pin or other support is placed in the opening to keep it open, the pin being held and supported by the mandrel. The tubular support, constrained in this shape, is subjected to a heat treatment, e.g., in the 500° C. range, tor a short period of time and then cooled. The resulting tubular support, in its austenite phase, exhibits the usual super elasticity associated with nitinol and other shape memory alloys, and the opening thus provided in the wall of the tubular support remains open and accessible for formation of a jump graft. By selection of an appropriately shaped and sized tubular support with a pre-formed access opening, a surgeon may produce a graft prosthesis having an opening in the wall of the tubular support positioned where desired for formation of a jump graft.
The external tubular support adjusts the mechanical and geometrical properties of the vein graft to match or mimic healthy arterial properties and therefore adapt to the arterial pressure of the host artery. Accordingly, this results in substantial matching of the lumen of the vein graft and the host artery, the substantial matching of compliance of the vein graft and the host artery, and substantial matching of the radius to wall thickness ratio (r/wt) of the vein graft to the host artery. As noted above, optimization of the vein-stent compliance should ensure that the vein-stent graft mimics the behavior of arteries regarding the non-linear stiffening with increasing diameter due to elevated blood pressure, “locking” at a maximum pressure, and then demonstrating dynamic recoil in a timely manner.
When venous grafts utilizing knit or braided tubular supports are cut at angles suitable for end-to-end anastomoses, either at generally right angles or in scallop-like shape, the ends of the supports may experience fraying (see, for example, FIG. 17). Certain methods and structure are helpful to eliminate such fraying. In one embodiment, adjustable rings 210 of bioabsorbable or biodegradable material are placed generally circumferentially around a portion of the material 165, and in contact with external surfaces 217, as shown in FIG. 18. The number of rings may be varied as needed. The location of the rings may be adjusted to the position of anastomoses where vein and tubular support need to be cut. The cut or suture may be carried out through the ring, and the ring may be absorbed or degraded over a predetermined time, as desired.
Another embodiment of a structure to prevent fraying of a knit or braided tubular support when it is cut is the use of polymer coating for the fiber mesh. This feature may also provide the benefit of preventing gluing of joints and contact zones of elements of the stent. However, use of the radially compliant tubular support as a reinforcing structure may advantageously involve bonding of the ablumenal surface of a vein segment to confronting internal surfaces of the support. This attachment or connection may be accomplished through the use of a glue or other material having adhesive or connecting characteristics. In one embodiment, a fibrin glue or other material having adhesive or connective characteristics may be sprayed on designated portions of the vein (as exemplified at 283 in FIG. 20) and/or the tubular support. Another embodiment includes placement of a material on designated portions of the lumenal surfaces of the tubular support so as to provide the characteristics of contact adhesion and/or bonding when these portions contact the vein. However, the glue or other material must not inhibit the function of the tubular support. It is desirable that the contact of the tubular support with the ablumenal vein segment surface be reasonably uniform along the length of the support, and that regions of much higher force of the support against the ablumenal wall of the vein be avoided.
Performing the anastomoses of small-diameter unsupported vein grafts in the coronary position is complicated by the tendency of the free end of the vein to collapse on itself, thereby obscuring the lumen and making it difficult for the surgeon to identify a suitable position for the placement of sutures. The application of an external tubular support (referred to sometimes herein as a stent) on a vein graft potentially further complicates the suturing, as the collapsed vein is situated inside (and at least partially obscured by) the non-collapsed stent material. By attachment of the ablumenal surface of the vein to the lumenal surface of the stent, however, the stent offers support to the vein to prevent it from collapsing, and this is particularly the case when stents formed from shape memory alloys are employed. As noted above, adjustable rings 210 and various adhesives may be employed to bond the confronting surfaces of the vessel and the supporting tubular structure together.
Attachment of the vessel surface to the tubular support stent can be achieved in various ways. In one embodiment, a covering gel is employed that attaches to the vessel wall and surrounds and entraps the stent wires, thereby attaching the stent to the vessel, this embodiment being schematically depicted in FIG. 24, the tubular support fibers being shown at 320, the vessel wall at 322 and the gel at 324. Examples of such gels are synthetic gels (such as modified polyethylene glycol, polyvinyl alcohol, acrylic gels, etc) and biological gels (such as fibrin, gelatin, and albumin glues). In another embodiment shown schematically in FIG. 25, an adhesive glue 326 such as a cyanoacrylate attaches the stent to the vessel ablumenal wall, the glue adhering to both the vessel and the stent.
In another embodiment, shown schematically in FIG. 26, individual fibers 320 of the stent are coated with an adhesive material 328 containing groups reactive to the vessel tissue. This material may be either directly applied on the metal wires (FIG. 26) or on a polymeric coating with which the wires are pre-coated. This material coating, applied optionally over a polymer coating on the wires, may also be employed in other embodiments, for example, those shown in FIGS. 24, 25, 27 and 28.
In a preferred embodiment, a sleeve is placed over the assembled stented vessel. As schematically depicted in FIG. 27, the sleeve 330 may primarily offer mechanical support for the stent to prevent fraying of cut edges, in the manner discussed above in connection with FIG. 18, while having minimal if any effect on the mechanical properties of the assembly, such as compliance. Referring to FIG. 28, a sleeve may comprise an adhesive tape 332 having an elastic backing material 334 bearing an adhesive material 336 having a consistency enabling it to penetrate between fibers of the tubular mesh and adhesively contact the ablumenal surface of the vessel. The adhesive may have the consistency of a gel. The elastic material may be a fabric, and may be formed from a variety of materials that are inherently elastic (e.g. polyurethane elastomers) or materials that are not elastic by themselves, but may act in an elastic fashion due to the fact that they are coated with or entrapped in an elastic gel-like material that constitutes the adhesive portion of the tape. Crimped fibers made from polyesters (PET) or degradable materials such polylactic acid (PLA) or polyglycolic acid (PGA) or copolymers thereof may also be used for the elastic material.
The adhesive material 336 should be of sufficient cohesive strength and adhesive strength to the vessel wall by virtue of mechanical interlocking and/or covalent chemical binding to attach the stent to the vessel during normal handling during implantation. As the vessel tissue contains both nucleophilic (amino, thiol and hydroxyl groups) and electrophilic (carboxyl) groups, the adhesive may employ a number of chemical groups capable of reacting with the vessel wall. Aldehydes, acyl chlorides, activated esters, isocyanates or carboxylic acids (plus activators such as carbodiimides) are examples of compounds capable of reacting with nucleophilic groups on the tissue, and alcohols and amines may be employed to react with the electrophilic carboxylic acid groups on the tissue (in the presence of an activator, e.g. a carbodiimide).
In general, the adhesive may be composed of synthetic polymers, in their swollen or unswollen states. Gel-like characteristics may be imparted by the adhesive material itself, or by swelling the material with a solvent or a plasticizing agent, such as water. Gels offer the advantages of having viscoelastic properties that may simulate the mechanics of vessels to some extent, and of being capable of deforming to facilitate contact and binding of the gel to the tissue through the gaps in the stents. In addition, gels may contribute to the strength of the bond between the vessel wall and the stent by mechanical interlocking. Adhesives may be non-degradable cross-linked materials such as polyethylene glycols, polyimines, polyacrylates (e.g., polyacrylic acid, polyacrylamides, poly(hydroxyethyl methacrylate), and co-polyacrylates. Degradable and/or resorbable adhesives may include multifunctional polyethylene glycols containing degradable end groups and/or crosslinked with degradable crosslinkers, and non-crosslinked or lightly crosslinked polyvinyl alcohol.
As noted above, the adhesive material desirably is functionalized with groups capable of reacting with the vessel tissue. For example, an adhesive may comprise crosslinked polyacrylic acid gels functionalized with aldehyde groups (e.g., via a diamine bridge) capable of reaction with tissue amines. As another example, a polyvinyl alcohol, the degree of hydrolysis, molecular mass and tacticity (and thus crystallinity) have been adjusted to render the adhesive slowely dissolvable in vivo can be appropriately functionalized with groups reactive toward tissue groups.
Although the adhesive tape 332 may be applied as desired to the stented vessel, a preferred method involving particular stented vessels having a supportive tubular structure derived from a shape memory material utilizes a pre-assembled clip comprising a disposable elastic metal clip and a segment of the adhesive tape, as schematically depicted in FIGS. 29 A through D. Referring to FIG. 29A, the metal clip 340 is shaped to provide an opening for reception of the stented vessel, and may, for example, be generally “C” shaped. The clip contains within it a section of the adhesive tape 332, the adhesive surface 336 facing the interior of the clip and the elastic backing adjacent the inside surface of the clip. As desired, the assembled clip may be positioned within the jaws of a pliers-like applicator 342 (FIG. 29B). In use, a section 344 of a stented vessel is inserted into the opening of the clip as shown in FIG. 29B. The jaws of the applicator 342 are closed on each other, the stented vessel becoming temporarily flattened as shown in FIG. 29C and the adhesive penetrating between fibers of the stent to adhesively contact the ablumenal surface of the vessel. The jaws of the applicator are opened, enabling the section of the stented vessel to resume its substantially circular cross-sectional configuration. The metal clip is removed, and excess tape is removed from the edges to provide the stented tube structure schematically shown in FIG. 29D.
An alternative method involves the employment of a tape dispenser that contains a continuous roll of the tape, sections of the tape being severed from the roll and applied to the stented vessel by hand.
The application of the tape to the stented vessel can be performed at a position required by the surgeon according to the implant position. FIG. 30 shows how a taped section 350 of a stented vessel may be cut to provide a biased opening that may be sutured to, for example, a coronary artery. The taped section is cut so that the resulting open end is configured to conform generally to the ablumenal surface of an artery or other vessel to which the stented vessel is to be attached. FIG. 30 shows a cut being made generally along the dashed lines to provide segments 352, 354. FIG. 31 shows how the bias-cut taped end of the segment 352 may be sutured to a length of artery 356, and it will be understood that the artery has a surgically prepared, axially extending slit (not shown) providing an opening through its wall about which the segment 352 is sutured to communicate the graft with the artery.
By appropriately cutting the taped portion, the end of the segment can be shaped to conform as needed to the external contour of various vessels to which it is to be attached. For example, a graft may be shaped to make an appropriately angled (e.g., 45 degrees) juncture with a coronary artery, the artery and the stented graft segment lying generally in the same plane. In another example, the stented graft may cross over a vessel to which it is to be attached. In this “cross-overjump” configuration, slit-like openings are provided in the taped section of the graft segment and the vessel, and the taped section and the vessel are sutured together about their confronting openings and to establish fluid communication through the openings. The slit-like openings may be made such that they are approximately parallel as they confront each other. In yet another example, the stented graft and a vessel may be positioned substantially parallel and in contact with each other. In this “longitudinal jump” configuration, slit-like, desirably axially extending openings are made in the taped section of the stented graft and in the vessel, and suturing is performed as above.
Applicants have further recognized the need for a device to facilitate assembly of the radially compliant tubular support and a vein segment. It is desirable that such an applicator should not obscure the stent lumen, and that it should allow for easy insertion of the vein. It is further recognized that a design whereby diameter is increased by length compression, as in a braided configuration, would allow easy slipping of the tubular support over a vein. FIG. 19 illustrates this concept in combination with an applicator 279 to apply the braided support 284 to a vein 86. This longitudinal braid contraction phenomena (shown earlier in FIGS. 5 and 6), and which must be carefully managed at the time of joining the vein to the stent, is likely quite useful to achieving the goals of an applicator 279, as noted above. This applicator may also facilitate placement of anti-fraying rings 210. In one embodiment, the method of using the applicator comprises the steps of: providing the means of longitudinally contracting a stent; holding the stent in the contracted position with increased stent diameter resulting; inserting a vein into the stent lumen; and distending the stent longitudinally while the vein is inserted simultaneously until the stent is slipped over the desired portion of the vein. Further design considerations must ensure that the stent will not be fixed to the vein in a longitudinally over-distended or contracted state, so as to ensure that the predetermined mechanical stent properties remain viable.
FIG. 20 shows an embodiment in which a tubular support 185 is received along the outer surface of an applicator 281 having an internal passage, and, while passing the vein segment 86 from within the applicator passage, the tubular support is drawn onto the ablumenal surface of the vein segment. The applicator here may be a thin walled tube resembling a soda straw.
FIGS. 22A and B show the end of a syringe 300 having a mechanism 302 for engaging and immobilizing the end of a blood vessel and the surrounding tubular support. The mechanism may be any mechanism adapted to connect to the ends of open tubes, and one such mechanism may include an outer tubular portion 301 within which is received an end portion 304 of a blood vessel, the mechanism including an axially movable internal tapered mandrel 305 that is inserted in the vessel. Axial movement of the mandrel (to the left in FIG. 22) pinches and captures the end of the blood vessel between the mandrel and an introducer sheath carrying a tubular support, the sheath and tubular support together being shown in FIG. 22A as 303.
Referring to FIG. 22B and 23, in one assembly method, the distal end 304 of the vessel is supported by the mechanism 302 with the proximal end of the vessel being attached to a cord 306. The tubular support 308 (FIG. 23) is carried exteriorly of and is supported by an introducer tube 310. A plug 312 sized to engage the interior wall of the introducer tube is attached to the proximal end of the vessel. The introducer tube 310, bearing on its outer surface the tubular support 308, is drawn over the cord 306, the plug and the vessel segment, the cord being maintained under tension to facilitate sliding the introducer tube over the vessel. The end of the tubular support is gripped by the mechanism 302 (FIG. 22A). Thereafter, the introducer tube is withdrawn proximally away from the mechanism 302. By frictionally engaging the interior surface of the introducer tube, the plug 312 exerts a gentle longitudinal tension on the proximal end of the vessel, causing the vessel to shrink somewhat in diameter. As the introducer tube is withdrawn, the tubular support 308 comes into contact with the ablumenal surface of the vessel. Once the introducer tube has been completely withdrawn, the gentle longitudinal tension on the vessel is relaxed and the vessel itself seeks to return to its original larger diameter to thereby more closely contact the tubular support.
The plug may be made of any appropriate material that frictionally engages the interior wall of the introducer tube as it is withdrawn, and a soft urethane rubber plug, for example, may be employed.
Although assembly of the stented graft has been described as occurring away from the vein or artery to which it will eventually become attached, in practice, such assembly can be affected by attaching one end of the harvested vessel segment to the artery or vein with a cord attached to its other end. The mesh tubular support is then urged gently over the ablumenal surface of the vessel while maintaining gentle tension on the cord until the support is positioned where desired.
It is desired that the support be applied to a vein at a predetermined length which is associated with a particular desired compliance. A length-defining support feature or system should ensure a predetermined support length. This is particularly true with respect to braided supports, and perhaps less important with knit supports in which radial resilience is less dependent upon the amount to which the support is extended axially.
In a braided support, and to a much lesser extend in a knit support, compliance and related mechanical properties are linked to the support length through the pitch angle. Imparting a change in length results in a change in pitch angle and compliance. However, the compliance of the support is a mandatory characteristic which is optimized, as noted above, to mimic the compliance of a normal healthy host artery. When applying a support to a vein segment, it is important to accurately accommodate the predetermined tubular support length, even after longitudinal contraction of the support for the attachment of the support to the vein.
With braided, and to a much lesser extent knit supports, axial support length may be controlled, for example, through the use of an axially extending, relatively inextensible element, (as for example the thread 78 in FIG. 7), that restrains the tubular support from unwanted axial extension. The thread may be woven through the support mesh and may be fastened, as by welding, to the various wires that the thread encounters along the length of the support so that as the support is stretched axially, the extent of axial elongation is controlled by the thread as the thread is drawn taut. Moreover, this feature may enable a length of the tubular support to be divided into portions of appropriate length, with the permitted axial extension of each portion controlled by the section of thread within it. As presently envisioned, a vein segment may be sheathed in a tubular support as discussed in detail above, with the intent of cutting out a smaller segment of the resulting venous graft for use in the surgical replacement of an artery, and the venous graft that is thus used will have vein and tubular support ends that are coextensive.

Various generally tubular external wire mesh supports were fabricated from metal wires by braiding and by knitting, some being crimped and others not, and the diametrical compliance of each was measured using the in vitro diametrical compliance testing outlined above. The measured compliance values were dependent upon many variables, including wire size, tightness of braid or knit, etc. The following values were obtained:



		Compliance
	Design	%/100 mmHg

A	Braided Non-crimped	0.9
B	Braided Crimped	5.6
C	Braided Crimped	1.8
D	Knitted Non-crimped	3.4
E	Knitted Crimped	7.9
F	Knitted Crimped	8.0
G	Knitted Non-crimped	10-21
H	Knitted Non-crimped	9-21
I	Knitted Non-crimped	16->30
J	Knitted Non-crimped	>30
K	Knitted Non-crimped	10-16
L	Knitted Non-crimped	21-29
M	Knitted Non-crimped	22-28
N	Knitted Non-crimped	>30
O	Knitted Non-crimped	10-15
P	Knitted Non-crimped	9-11
Q	Knitted Non-crimped	13-24
R	Knitted Non-crimped	>30

A surgical procedure is proposed for use of the graft disclosed herein. This procedure, which may also be viewed as a simple method for placement of a venous reinforcement structure, includes, in this example, application of the compliant external tubular support during the procedure of vein excision. In many instances, vein excision is considered a standard part of a surgical operation, which is usually done by an assistant at a time when the surgeon is still in the preparatory phase of the operation. When an autologous blood vessel such as a segment of the saphenous vein is harvested for use in accordance with the invention, it may contain side branches that need to be removed before the vein is enclosed within the tubular support. Closure of the opening that remains after removal of a side branch can be accomplished in several ways. Surgically placed sutures or small clips such as Liga clips may be used to ligate small sections of branches. However, when a branch has been removed flush with the ablumenal Surface of the vessel, it is desirable to close the resulting opening in the vessel by suturing to reduce interference with the tubular support. Purse string sutures are appropriate. To avoid undue narrowing of the lumen of the vessel when a purse string suture is pulled tight, the ends of the purse string suture are preferably tightened by pulling them in the longitudinal direction, i.e., axially of the vessel, rather than in a direction transverse to the longitudinal direction of the vein.
In one embodiment, an initial step includes dissection and freeing of a saphenous vein. The saphenous vein is dissected free in a standard way, leaving it in situ while ligating and cutting off its branches as discussed above. The second step involves testing for potential wall leaks of the vein. In order to test the isolated saphenous vein for potential leaks, it is normally cannulated distally and cold heparinised blood is injected while the proximal end is occluded. This inflation of the vein (using old techniques) with a syringe creates pressures of up to 350 mm of Hg and is often a main reason for traumatic damage of the vein wall. Therefore, a pressure limiting mechanism may be positioned between the vein cannula and the syringe. The external tubular support cannot be applied yet because leaks in the vein wall need to be freely accessible for repair. Therefore, no over-distention protection is placed around the vein yet, necessitating the limitation of the inflation pressure to a level suitable for detecting any leaks of concern but less than a level deemed to cause unacceptable damage, such as, for example, in one embodiment, 15 mm of Hg, the pre-maximal dilatation pressure for veins. The tissue remodeling functions of applicants' invention become more critical in view of the importance of leak testing and the reality of possible damage to the intimal layer in the vein during even the most carefully performed leak testing.
The next step involves assembling the harvested vein segment and an external tubular support of this invention. In this step, the tubular support (typified here as a knit support) is mounted on a tube or straw-like applicator within which is positioned the vein segment. See FIG. 20. The straw is then removed axially, leaving the support and vein in contact to form the venous graft. Over-extension of the tubular support is prevented using a length-limiting central thread or other means, as described above. As required, the vein segment is then inflated under arterial blood pressures of 120 mm of Hg, causing it to contact the tubular support inner lumenal surfaces. In certain embodiments, an adhesive securing the tubular support to the vein will ensure that the vein does not collapse during the surgical procedure when no internal pressure is applied. Again, it should be recognized (without limitation) that this is one of several ways to accomplish the above objectives.
The following sequence may occur at this or another time during the procedure. One of the external anti-fraying rings or cuffs is slid to the end of the vein, and a typical double-S-cutting line is used to prepare the end for the first anastomoses. The thin cuff prevents fraying of the tubular support and also gives the vein tissue and tubular support a uniformity which makes the surgical process of stitching the vein to the host artery in an end-to-side fashion much easier. Another thin anti-fraying ring is then slid down from the applicator to a position where either a side-to-side anastomoses for a sequential graft is being performed, or where the vein is being cut at an appropriate graft length. The half of the sliding cuff which remains on the original vein will make the process of the anastomoses to the proximal host artery much easier. In the case of a coronary artery bypass graft, for example, the end of the remaining vein protected by the other half of the cuff is used for the next distal graft anastomoses.
Although the invention has been described primarily in connection with the use of autologous blood vessels to form a stented graft as a replacement for diseased vessels, the invention has other uses as well. External stent structures can be used to allow repeated access to the vascular system for administration of medicines or nourishment or the administration of dialysis procedures. Exterior access grafts may be subject to frequent penetration by hypodermic needles, and it is desirable not only that the vessel walls of the graft retain their capacity to be punctured many times, but also that the wounds thus formed in the vessel wall self-seal to a large extent.
In accordance with the invention, an external stent structure 360 as shown in FIG. 32 may be produced from a blood vessel segment harvested from human or non-human animals, such as segments of bovine or porcine arteries, the vessels having been treated, as by known aldehyde fixing or decellularization. A flexible, resilient tubular support of the type described herein may be placed over the resulting venous or arterial vessel segment to provide a bioprosthetic access graft to enable ready access to the cardiovascular system. For this purpose, one may form a bioprosthetic device utilizing a suitably treated bovine or porcine artery or vein segment by placing the segment within an elastic, resilient tubular support sized to contact and squeeze against the ablumenal surface of the vessel when the vessel is filled with blood, as when it is being used as a shunt between an artery and a vein of a patient. In this manner, once a hypodermic needle has been inserted and withdrawn from the wall of the vessel, the elastic, resilient tubular support tends to exert a squeezing force on the vessel, forcing the edges of the wound to close upon one another.
FIG. 32 is a photograph showing an access graft formed from a decellularized porcine artery segment 362 contained within a knitted, resilient tubular support 364. This figure illustrates a puncture wound 366 formed by a syringe 368. The edges of the wound are brought together by the squeezing action imparted by the knitted support. It may be noted that the knitted support, or braided support if desired, may be made with loops or openings 370 that are sized to receive a hypodermic needle of the desired diameter. For hypodermic needles commonly used for dialysis procedures, for example, needle sizes, and hence the size of openings between fibers of the tubular support, may be in the range of about 1.8 and 2.1 mm.
As structures have become increasingly complex, not only in design but also in the range of material use, pure analytical methods have begun to fail in describing the behavior of such structures. Due to the scientific challenge of closely matching a vascular graft of the type described herein to a host, analytical methods are rendered somewhat obsolete. Development of a prosthetic vascular graft which mimics the mechanical requirements and dynamic compliance of a normal healthy artery is made possible, however, with certain old tools, particularly cut and try methods in which incremental changes are made to the material or structure of the tubular support to modify its compliance properties, and the resulting properties are used to guide further changes. Empirical data or constitutive equations and mathematical analyses may be used for certain features. Alternatively, the use of numerical modeling with such tools as, for example, Finite Element Models and Methods, relying on continuum mechanics, along with certain other tools makes this level of customization feasible.
While the embodiments of the invention described herein are presently preferred, various modifications and improvements can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated by the appended claims, and all changes that fall within the meaning and range of equivalents are intended to be embraced therein.

Claims

1. A flexible, resilient, generally tubular external support within which may be supported a vein segment to form a venous graft mimicking the compliance properties of a healthy artery, the tubular support being capable of resilient radial expansion in a manner providing compliance in the range of 3 to 30%/100 mm Hg.

2. The tubular external support of claim 1 in which said support comprises a knit wire mesh.

3. The tubular external support of claim 1 in which said support comprises a braided wire mesh including an element for controlling the degree of axial extension of the support.

4. The tubular external support of claim 3 in which said element comprises a thread attached to and carried internally of the support.

5. The tubular external support of claim 1 including a vein segment carried within the support, the vein segment having an ablumenal surface in contact with, and supported by, said tubular support.

6. A venous graft for replacement of a section of an artery, the graft comprising a flexible, resilient, generally tubular external support and a vein segment carried within and having an ablumenal surface in contact with and supported by the tubular support, the venous graft being capable of resilient radial expansion in a manner mimicking the compliance properties of an artery.

7. The venous graft of claim 6 wherein said tubular support is capable of said resilient radial expansion without significant axial dimensional changes.

8. The venous graft of claim 6 wherein the compliance of the graft ranges from 3 to 30%/100 mm Hg.

9. The venous graft of claim 6 wherein the external support comprises a generally tubular fiber mesh capable of expanding in diameter through resilient movement of wires of the mesh to accommodate radial expansion of the vein segment supported in it sufficient to provide the graft with said compliance.

10. The venous graft of claim 6 wherein said external support comprises a knit, tubular mesh capable of expanding radially to accommodate radial expansion of the vein segment supported in it within said compliance range.

11. The venous graft of claim 6 wherein said external support comprises a braided fiber mesh so configured as to exhibit radial expansion in said compliance range without significant reduction in the axial length of the support.

12. The venous graft of claim 11 wherein said fiber mesh is made of metal wire.

13. The venous graft of claim 12 wherein said metal wire is of stainless steel or of a cobalt chrome alloy.

14. The venous graft of claim 11 wherein said metal wire is a shape memory alloy.

15. The venous graft of claim 11 wherein said fiber mesh is polymeric.

16. The venous graft of claim 6 wherein the ablumenal surface of the vein segment is bonded to said tubular support.

17. The venous graft of claim 6 wherein the vein segment and the tubular support are axially coextensive.

18. The venous graft of claim 10 wherein said knitted tubular fiber mesh is made from crimped fibers.

19. The venous graft of claim 9 wherein said tubular fiber mesh is made from fibers that are crimped after having been formed into said tubular mesh.

20. A venous graft for replacement of a section of an artery, the graft comprising a flexible, resilient, knit wire mesh configured as a generally tubular external support and a vein segment carried within and having an ablumenal surface in contact with and supported by the tubular support, the support having resilient radial expansion and contraction characteristics that provide the venous graft with compliance properties mimicking those of an artery.

21. The venous graft of claim 1 wherein the compliance of the venous graft ranges from 3 to 30%/100 mm Hg.

22. Method of producing a venous graft for use in replacing a section of an artery, comprising providing a segment of a vein and sheathing the segment in a generally tubular support in supportive contact with the ablumenal surface of the vein segment, the support being sufficiently flexible and radially resilient as to provide the graft with compliance properties mimicking the compliance properties of an artery.

23. The method of claim 22 including the step of cutting said venous graft from a longer section of vein sheathed in said tubular support, the ends of the vein and tubular support of the venous graft being coextensive.

24. The method of claim 22 including the step of supporting said generally tubular support upon an exterior surface of an applicator having an internal passage within which is positioned the vein segment, and removing the applicator to permit the tubular support to come into supportive contact with the ablumenal surface of the vein segment.

25. The method of claim 22 including the step of supporting said generally tubular support upon an exterior surface of an applicator having an internal passage, and, while passing the vein segment from within the applicator passage, drawing the tubular support onto the surface of the vein segment.

26. The method of claim 22 wherein said generally tubular support is chosen to be sufficiently radially resilient as to provide the venous graft with a compliance ranging from 3 to 30%/100 mm Hg.

27. The method of claim 24 wherein said tubular support is axially resilient, the method including the step of controlling axial dimension changes of the tubular support as the support comes into supportive contact with the vein segment to provide the venous graft with said compliance properties.

28. The method of claim 27 including the step of providing the tubular support with an axially extending, relatively inextensible element that restrains the tubular support from unwanted axial extension.

29. The method of claim 28 wherein said inextensible element comprises a flexible fiber positioned within the tubular support and between the tubular support and the ablumenal surface of the vein segment.

30. The method of claim 28 wherein the flexible fiber is a wire joined to the interior of the tubular support.

31. The method of claim 22 including the step of bonding the ablumenal surface of the vein segment to the tubular support.

32. The tubular external support of claim 1 in which said a knit wire mesh support comprises loops that alternate in size circumferentially of the support.

33. The method of claim 22 including forming said tubular support from a knit wire mesh, wherein said mesh is formed with loops that alternate in size circumferentially of the support.

34. Method of producing a blood vessel graft comprising providing a segment of a blood vessel and sheathing the segment in a generally tubular support in supportive contact with the ablumenal surface of the vessel segment, the support being sufficiently flexible and radially resilient as to contribute to the compliance properties of the graft.

35. The method of claim 34 including supporting the generally tubular support on the exterior surface of an introducer tube, sliding the introducer tube over the vessel segment, and removing the introducer tube to enable the tubular support to contact the ablumenal surface of the vessel.

36. The method of claim 34 including affixing a cord to one end of the vessel segment, and maintaining tension on the vessel segment during sheathing of the vessel segment within the tubular support.

37. The method of claim 36 wherein the cord comprises a plug frictionally received within the introducer tube, the method including withdrawing the introducer tube from the vessel segment and tubular support, friction between the tube and plug maintaining the vessel in tension during such withdrawal.

38. A blood vessel graft comprising a flexible, fiber mesh configured as a generally tubular external support and a blood vessel segment carried within and having an ablumenal surface in contact with and supported by the tubular support, the support having resilient radial expansion and contraction characteristics contributing to the compliance properties of the graft.

39. The graft of claim 38 including an adhesive adhering the tubular support to the ablumenal surface of the vessel to resist collapse of the vessel within the support.

40. The graft of claim 38 including an adhesive tape positioned around a portion of the tubular support, the tape including an adhesive that extends through the fiber mesh support and into adhesive contact with the ablumenal surface of the vessel.

41. The graft of claim 39 wherein said adhesive at least partially embeds said fiber mesh and adhesively contacts the ablumenal surface of the vessel.

42. The graft of claim 39 wherein said adhesive is formed as a coating on the fibers of the fiber mesh.

43. Method of attachment of the graft of claim 40 to another blood vessel, comprising cutting said portion of the graft bearing said adhesive tape to conform to the wall of the other blood vessel, the adhesive tape maintaining the cut end of the graft open and preventing fraying of the cut end.

44. The method of claim 43 wherein the graft is cut on the bias.

45. Method of applying an adhesive tape to the exterior surface of a vessel graft that includes a vessel segment supported within a flexible, elastic fiber mesh tubular support, the method comprising compressing the graft between lengths of adhesive tape having an adhesive that penetrates the mesh to adhesively contact the ablumenal surface of the vessel, and allowing the graft to resume its tubular configuration.

46. The method of claim 45 including positioning the tape lengths in a deformable clip, inserting a segment of the stent in the clip between the tape lengths, deforming the clip to compress and flatten the stent, and removing the clip.

47. The graft of claim 37 wherein said vessel is of porcine or bovine origin.

48. A graft for replacement of a section of an artery, the graft comprising a flexible, resilient, knit wire mesh configured as a generally tubular external support and a blood vessel segment carried within and having an ablumenal surface in contact with and supported by the tubular support, the support having resilient radial expansion and contraction characteristics that provide the graft with compliance properties mimicking those of an artery.