Abstract:
A graft has a seamless flow dividing structure. A method of manufacturing the flow dividing graft structure includes providing a first section of graft material having at least one side, a first end, and a second end. An opening is drawn out through the at least one side. A second section of graft material is coupled with the opening. An angled section is formed along the first section of graft material. The angled section provides a seamless division of flow supplied from the second section to the first section and directs the flow to each of the first and second ends of the first graft material. The resulting graft structure includes a main graft section. A branch graft section is coupled with the main graft section at an angled divider section. The angled divider section is seamless and is suitable for dividing flow through the flow dividing graft structure.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority to, and the benefit of, co-pending U.S. Provisional Application No. 60/335,937, filed Nov. 14, 2001, for all subject matter common to both applications. The disclosure of said provisional application is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to grafts suitable for replacing blood vessels, and more particularly to a method of manufacturing grafts resulting in vascular grafts of different configurations having seamless junctions with one or more branching graft legs.  
         BACKGROUND OF THE INVENTION  
         [0003]    Vascular grafts are routinely used to replace damaged or diseased blood vessels to restore blood flow. There are numerous configurations of vascular grafts available, some of which have one or more branches. Axillobifemoral and large diameter bifurcated grafts are examples of vascular grafts having one or more branches. In the case of axillobifemoral grafts, a side branch graft is attached to a main trunk section of the graft. In the case of large diameter bifurcated vascular grafts, a bifurcated section attaches to a larger diameter main trunk section of the graft.  
           [0004]    There are several known methods of manufacturing a branched vascular graft. One such method can be summarized as a suturing process, the result of which is depicted in FIG. 1A. The example graft is made by W. L. Gore &amp; Associates, Inc. as model #SB2001. The vascular graft  10  has a main trunk  12  section. The vascular graft  10  further includes a first branch  14  and a second branch  16 . The first branch  14  is sutured together with the main trunk  12  at a first intersection  18 . The second branch  16  is sutured together with the main trunk  12  at a second intersection  20 . The method for manufacturing the vascular graft  10  illustrated begins with the formation of the main trunk  12 . Each of the first branch  14  and the second branch  16  are formed separate from the main trunk  12 . The first branch  14  is then sutured on to the main trunk  12  at the first intersection  18  and the second branch  16  is sutured on to the main trunk  12  at the second intersection  20 . The sutures at the first intersection  18  and the second intersection  20  create small perforations, which would leak fluid, such as blood, passing through the vascular graft  10  unless sealed. Therefore, a sealant/adhesive  22  is applied to the exterior portion of the vascular graft  10  to seal the sutures and provide necessary reinforcement to the vascular graft  10 .  
           [0005]    [0005]FIG. 1B shows an internal view of the first intersection  18  and the second intersection  20  of FIG. 1A. Looking along the length of the main trunk  12 , the first intersection  18  is on the left side and the second intersection  20  is on the right side of the vascular graft  10 . The conventional method of manufacture results in a divider  13  positioned between each of the intersections  18  and  20 . The divider  13  directs the fluid flow into each of the branches  14  and  16 .  
           [0006]    A different known method of manufacturing a vascular graft is described in U.S. Pat. No. 6,203,735B1 to Edwin et al. (Edwin &#39;735). The method of shaping three-dimensional products involves manipulating an expanded polytetrafluoroethylene tubular body into a desired three-dimensional formation. The method includes radially expanding a longitudinally expanded polytetrafluoroethylene (ePTFE) tube to form a radially expanded ePTFE (rePTFE) tube, engaging the rePTFE tube circumferentially about a shaping mandrel. The assembly is heated to a temperature below the crystalline melt point temperature, or sintering temperature, of polytetrafluoroethylene to radially shrink the diameter of the rePTFE tube into intimate contact with the shaping mandrel. The assembly is then heated to a temperature above the crystalline melt point temperature of polytetrafluoroethylene to amorphously lock the microstructure of the shaped polytetrafluoroethylene body.  
           [0007]    [0007]FIG. 1C depicts yet another conventional configuration for a textile or fabric graft  24 . The graft  24  is made of a textile or fabric that is woven into the main trunk section  25  and legs  26  and  28 . The weaving process leaves a hole at the point of the divider  27 , which must be sewn together to seal the graft  24  and prevent leakage. This is often referred to as a seamless graft, but there is a small seam at the divider  27  location that must be sewn to prevent fluid leakage.  
         SUMMARY OF THE INVENTION  
         [0008]    There is a need for a seamless flow dividing graft structure and a corresponding method of making. The present invention is directed toward further solutions to address this need.  
           [0009]    In accordance with one example embodiment of the present invention, a method of manufacturing a flow dividing graft structure includes providing a first section of graft material having at least one side, a first end, and a second end. An opening is drawn out through the at least one side. A second section of graft material is coupled with the opening. An angled section is formed along the first section of graft material. The angled section provides a seamless division of flow supplied from the second section to the first section and directs the flow to each of the first and second ends of the first graft material.  
           [0010]    In accordance with further aspects of the present invention, the method includes expanding the first section of graft material prior to drawing out the opening. The method continues with placing the first section of graft material over a mandrel prior to drawing out the opening. The first section of graft material is restrained against the mandrel. The first section of graft material is shrink fit about the mandrel. Additional graft material is wrapped in a helix pattern about the first section of graft material and the mandrel. Additional graft material is then wrapped over the helix pattern and first section of graft material to form a second layer of graft material. The second layer of graft material is then restrained. The second layer of graft material, the helix, and the first section of graft material are heated. The opening drawn out through the at least one side includes drawing out a trunk, cutting a hole in the trunk, and removing the mandrel. The second graft section is installed on a second mandrel. First and second leg mandrels are installed onto the second mandrel. The first and second graft sections are heated. Additional graft material is wrapped around the second graft section. A cover of graft material is placed over the first and second graft sections. The cover of graft material is restrained. The cover of graft material is then shrink fit. The mandrel is removed to form the flow dividing graft structure.  
           [0011]    In accordance with further aspects of the present invention, the flow dividing graft structure is formed of a hydrophobic, biocompatible, inelastic material. The flow dividing graft structure can also be formed of a bioresorbable material. The angled section can be sufficiently narrow to enable a reduced flow resistance and a reduced flow turbulence. The angled section can be monolithic. The flow dividing graft structure can be suitable to simulate anatomical physiological fluid flow divider conditions of a normal flow dividing hollow organ within a patient.  
           [0012]    In accordance with another embodiment of the present invention, a method of manufacturing a flow dividing graft structure includes providing a section of graft material. The graft material is expanded, layered, and shrink fitted with additional graft material in a predetermined manner about a shape pattern to make one or more graft leg members seamlessly coupled with a main portion of the graft. The form is removed from the graft.  
           [0013]    In accordance with another embodiment of the present invention, a flow dividing graft structure is provided. The structure includes a main graft section. A branch graft section is coupled with the main graft section at an angled divider section. The angled divider section is seamless and is suitable for dividing flow through the flow dividing graft structure.  
           [0014]    In accordance with further aspects of the present invention, the flow dividing graft structure is formed of a hydrophobic, biocompatible, inelastic material. The flow dividing graft structure can also be formed of a bioresorbable material. The branch graft section can intersect with the main graft section to form the angled divider section which is sufficiently narrow to enable a reduced flow resistance and a reduced flow turbulence through the flow dividing graft structure. The angled divider section can be seamless. The angled divider section can be monolithic. The flow dividing graft structure can be suitable to simulate anatomical physiological fluid flow divider conditions of a normal flow dividing hollow organ within a patient.  
           [0015]    In accordance with another embodiment of the present invention, a flow dividing graft structure formed by providing a section of graft material, expanding, layering, and shrink fitting the graft material with additional graft material in a predetermined manner about a shape pattern to make one or more graft leg members seamlessly coupled with a main portion of the flow dividing graft structure, and removing the shape pattern from the flow dividing graft structure is provided. The flow dividing graft structure includes a seamless monolithic structure having a main section. At least one seamlessly coupled branch section extends from the main section. The flow dividing graft structure includes a seamless flow divider junction between the main section and the at least one seamlessly coupled branch section.  
           [0016]    In accordance with further aspects of the present invention, the flow dividing graft structure further includes a continuous monolithic junction and flow divider formed at each branch section having enhanced strength relative to conventional sewn seamed branch connection grafts.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The features and advantages of the present invention will become better understood with regard to the following description and accompanying drawings, wherein:  
         [0018]    [0018]FIGS. 1A, 1B, and  1 C are illustrations of vascular grafts produced according to conventional methods of manufacture;  
         [0019]    [0019]FIG. 2 is an illustration of a vascular graft resulting from the process of one aspect of the present invention;  
         [0020]    [0020]FIGS. 3A through 3C are a step-by-step illustration of a method of manufacture according to one aspect of the present invention;  
         [0021]    [0021]FIG. 4 is an illustration of another vascular graft resulting from the process of another aspect of the present invention;  
         [0022]    [0022]FIG. 5 is an illustration of an internal portion of a junction in accordance with the teachings of the present invention; and  
         [0023]    [0023]FIG. 6 is a table comparing experimental results of graft performance. 
     
    
     DETAILED DESCRIPTION  
       [0024]    An illustrative embodiment of the present invention relates to a vascular graft and corresponding method of making the vascular graft that is more efficient and results in a durable graft with seamless junctions. By “seamless” what is meant is a junction in which a seam is substantially imperceptible to fluid flowing through the graft, and does not contain holes or perforations from thread, sutures, or the like. The seamless junction differs from conventional grafts made of a fabric or textile that require a seamed connection between, for example, a main trunk section and a branch of a graft. Some conventional grafts address the holes of the seam with a reinforcement sewn over the seamed connection to cover the holes. Other conventional grafts may use a sealant on the exterior portions of the seam, preventing leakage through the holes formed by the seam, but leaving the seam and thread surface imperfections on the interior walls of the graft. None of the conventional solutions is seamless as intended by the teachings of the present invention.  
         [0025]    The embodiments utilize a process to produce vascular grafts having one or more branches without the use of sutures for connecting the one or more branches. Sealants or adhesives are also not required to reinforce or seal the branch junctions. The inventive method results in seamless junctions, or angled sections, between a main trunk portion of the graft and one or more branches. The seamless junction in branched grafts represents a significant improvement in overall quality and integrity of the junction(s). The inventive method provides the ability to tailor junction shape and angle, which can result in improved flow at locations within the graft where branches re-direct flow through the graft. The improved flow dynamics at the branch junctions improve the long term clinical performance of the branched graft structure. In addition, the present method provides for the creation of an anatomically accurate junction, to better simulate and support normal, physiologic flow characteristics.  
         [0026]    [0026]FIGS. 2 through 6, wherein like parts are designated by like reference numerals throughout, illustrate example embodiments of vascular grafts and a corresponding method of making according to the present invention, in addition to experimental test results. Although the present invention will be described with reference to the example embodiments illustrated in the figures, it should be understood that many alternative forms can embody the present invention. One of ordinary skill in the art will additionally appreciate different ways to alter the parameters of the embodiments disclosed, such as the size, shape, or type of elements or materials, in a manner still in keeping with the spirit and scope of the present invention.  
         [0027]    [0027]FIG. 2 illustrates a graft  30  resulting from the method of manufacture according to the teachings of the present invention. The graft  30  includes a main trunk  32  section. The main trunk  32  branches out into a first leg  34  and a second leg  36 , resulting in a bifurcated configuration. The main trunk  32  section represents a primary section, or starting point, from which other sections, such as legs  34  or  36 , can extend. The main trunk  32  section does not need to be larger than the legs  34  and  36 . Rather, the main trunk  32  section serves as a section that supports other sections. In the event a graft or structure made by the teachings of the present invention has a substantially symmetrical configuration with no clear primary section, the main trunk  32  section would be any one of the multiple sections making up the graft.  
         [0028]    The first leg  34  branches off the main trunk  32  at a, angled section or first junction  38  and the second leg  36  branches off the main trunk  32  at another angled section or second junction  40 . The first junction  38  and the second junction  40  are seamless transitions from the main trunk  32  section to each of the first leg  34  and the second leg  36 , forming a monolithic structure. The term “monolithic” is meant to indicate that the resulting structure is formed of layers of material that are fused or bonded chemically or physically in a manner that prevents subsequent separation of the layers. The layers become a single structure that is effectively monolithic.  
         [0029]    The main trunk  32  section maintains a larger diameter relative to the diameter of each of the first leg  34  and the second leg  36 . The size and dimensions of the main trunk  32  section and each of the first leg  34  and the second leg  36  can vary depending on the application for the graft. Some uses may require larger diameter configurations, while other uses may require smaller diameter configurations. Likewise, the diameter and length of the first leg  34  can differ from the diameter and length of the second leg  36 . The example graft  30  maintains dimensions of 18 mm×9 mm. One of ordinary skill in the art will appreciate that a graft with tapered dimensions can be constructed to better match patient anatomy or improve surgical technique. In addition, other dimensions for the graft  30  are possible, depending on a particular application.  
         [0030]    The main trunk  32  section and the first leg  34  and second leg  36  are all formed of a biocompatible flexible material, such as, for example, expanded polytetrafluoroethylene (ePTFE). The ePTFE material is a hydrophobic, biocompatible, inelastic material having a low coefficient of friction. Alternatively, the biocompatible material can be constructed from a bioresorbable material, such as polyglycolic acid polymers, polycaprolactone polymers, polylactic acid polymers, or copolymer combinations thereof. Any material can be used to form a vascular graft that is suitable as a substitution for vessels that carry or circulate fluids within a body, and is compatible with the process of the present invention for manufacture of the graft with seamless junctions.  
         [0031]    The method of the present invention can also form other types of grafts, such as axillofemoral, axillobifemoral, coronary arterial, bifurcated, and trifurcated configurations. The ePTFE can serve as the material to form these other types of grafts, in addition to other suitable materials, depending on the application of the graft.  
         [0032]    [0032]FIGS. 3A through 3C show a stepwise illustration of a method for manufacturing the graft  30  of FIG. 2, in addition to grafts of other configurations. The example illustrated herein forms the graft from ePTFE material, but other suitable materials can be utilized as understood by one of ordinary skill in the art. In addition, the method of the present invention can be executed by hand, by machine, or by combination of both hand and machine.  
         [0033]    The method begins with providing a length  42  of tubular ePTFE material at a diameter about equal to a desired diameter for the smallest of the legs being formed by the method (step  70 ). The length  42  of tubular ePTFE is expanded to a diameter approximately equal to a desired diameter for the main trunk  32  section (step  72 ). The expanded length  42  of tubular ePTFE is then placed over a two-piece mandrel  44  (step  74 ) which contains a ball insert  44 A and a bar insert  44 B. Restraining mechanisms  46  bind the ends of the expanded length  42 , and additional restraining mechanisms  48  bind portions of the expanded length  42  around a central portion of the mandrel  44  (step  76 ). A shrink fitting process shrinks the expanded length  42  onto the mandrel  44  with applied heat (step  78 ). The heat applied for ePTFE material can be in the range between 330 and 380 degrees Celsius, for a period of about four to ten minutes.  
         [0034]    The method continues with the removal of the restraining mechanisms  46  and  48  and the wrapping of additional ePTFE material  45  in a helix fashion about the length  42  on either side of the mandrel  44  and heat fused to the graft by a heat treatment in which heat is applied to the assembly in the range of 330 to 380 degrees Centigrade for a period of about four to ten minutes (step  80 ). A wrapping of additional ePTFE material  47  is then applied across the length  42  (step  82 ). The wrapped additional material  47  is restrained as in step  76 , and heat is applied in the range between 330 and 380 degrees Centigrade, for a period of about ten to twenty minutes. The heat causes the wrapped additional material  47  to shrink fit around the assembly (step  84 ).  
         [0035]    The additional wrap material utilized in the method of the present invention can be formed of a hydrophobic, biocompatible, inelastic material, such as ePTFE. Alternatively, the wrap material can be constructed from a bioresorbable material, such as polyglycolic acid polymers, polycaprolactone polymers, polylactic acid polymers, or copolymer combinations thereof.  
         [0036]    The restraining mechanisms  48  are removed and a trunk profile is created by drawing or pulling the ball insert  44 A out and away from the bar insert  44 B of the two piece mandrel  44  to create a trunk  52  profile, a first leg  54 , and a second leg  56  (step  86 ). A hole is cut in the trunk  52  profile and the ball insert  44 A is removed, followed by the removal of the bar insert  44 B through the hole in the first leg  54  or second leg  56  (step  88 ).  
         [0037]    A trunk section  58  is installed on to a bifurcate mandrel trunk tool  60  (step  90 ). The first leg  54  and trunk  52  are installed on to the bifurcate mandrel trunk tool  60  and a first bifurcate mandrel leg tool  62  (step  92 ). A second bifurcate mandrel leg tool  64  then slides through the second leg  56  and couples with the bifurcate mandrel trunk tool  60 , and the assembly is restrained and heat treated between 330 and 380 degrees Centigrade for a period of about ten to twenty minutes (step  94 ). A wrap  57  is installed around the bifurcate mandrel trunk tool  60  (step  96 ). The second bifurcate mandrel leg tool  64  is then removed and an ePTFE cover material  59 , prepared as in steps  86  and  88 , is placed on to the mandrel  44  (step  98 ). The second bifurcate mandrel leg tool  64  is re-installed and an ePTFE cover  66  is installed over the trunk section  58  (step  100 ). The entire assembly is restrained using restraining mechanisms  68  (step  102 ). The entire assembly is then shrink fit onto the bifurcate mandrel trunk tool  60  and leg tools  62  and  64  (step  104 ). The heat applied to the assembly ranges between 330 and 380 degrees Centigrade, for a period of about fifteen to thirty minutes.  
         [0038]    The first bifurcate mandrel leg tool  62  and the second bifurcate mandrel leg tool  64  are removed from the bifurcate mandrel trunk tool  60  and the first leg  54  and second leg  56 . The bifurcate mandrel trunk tool  60  is then removed (step  106 ). The desired bifurcated graft  30  remains.  
         [0039]    One of ordinary skill in the art will appreciate that the teachings of the present invention can result in the formation of grafts of a number of different configurations. For example, FIG. 4 illustrates a graft  110  having a single branch or leg  114  extending from a main trunk  112 . The graft  110  is made in accordance with the method of the present invention, thus there is a seamless junction  116  connecting the leg  114  with the main trunk  112 . The number, shape, size, location, and dimension of legs branching off the main trunk portion can vary as understood by one of ordinary skill in the art. The teachings of the present invention enable the design of a monolithic graft having seamless junctions and having one or more sections of predetermined dimensions as desired.  
         [0040]    [0040]FIG. 5 illustrates an internal view of the first junction  38  and the second junction  40  of FIG. 2. The view looks into the larger end of the main trunk  32 . Looking along the length of the trunk  32 , the first junction  38  is on the left side and the second junction  40  is on the right side of the graft  30 . The first junction  38  leads to the first leg  34 , and the second junction leads to the second leg  36 . The method of the present invention enables a divider  39  between each of the junctions  38  and  40  and the legs  34  and  36  to be narrow relative to other conventional grafts. The narrow characteristic of the divider  39  allows for a more efficient control of fluid flow through the graft  30 , and substantially reduces resistance to fluid flow and associated turbulence. The narrow divider  39  thus enables a relatively smoother flow at the transition from the trunk  32  to the legs  34  and  36 . The narrow divider  39  further provides for a more physiologically accurate flow characteristics through the graft  30 .  
         [0041]    The narrow divider  39  made in accordance with the teachings of the present invention is a seamless divider  39 . There are no perforations or threads from sutures. The divider  39  is a seamless and monolithic feature that can efficiently and effectively divide and distribute a fluid flowing past the divider  39 . The absence of a seam enhances the strength of the divider  39  and results in a more durable graft that is able to withstand relatively higher fluid pressures relative to conventional grafts.  
         [0042]    The inventive method of the present invention utilizes a process to produce products having one or more branches or legs without the use of sutures. The method thus results in a monolithic structure without seams. The size, shape, and the angle of the branches or legs can vary, and can be tailored for specific applications.  
         [0043]    The seamless monolithic structure also promotes improved flow dynamics. Anatomically correct flow characteristics can be reproduced in a graft made in accordance with the teachings of the present invention.  
         [0044]    Other know bifurcated grafts have developed kinks at the legs due to repetitive longitudinal force exerted on the legs by the beating aorta, and by external compression forces exerted by internal organs. The structure of the present invention significantly reduces graft kinking and abrasion of surrounding internal organs when implanted. The ePTFE is formed of a microstructure of nodes and fibrils that provide radial support integral to the graft wall. The microstructure provides the enhanced kink resistance and minimizes organ abrasion.  
         [0045]    Grafts made in accordance with the teachings of the present invention offer enhanced junction strength as well. For example, on a 16 mm×8 mm graft, junction strength can approach about 54 lbs. of pressure. This is a significant increase over other known graft devices, some of which are limited by the strength of sutures used to create the intersection or junction, in combination with adhesive or sealant.  
         [0046]    The teachings of the present invention provide for the enhanced junction strength in that the main trunk section and leg sections are formed such that the coupling of these sections occurs at locations other than major areas of stress concentration during use. In other words, one major area of stress caused by fluid flow is the divider  39 . In other conventional grafts, the divider includes perforations and threads from sutures which weaken the overall strength of the graft. The present invention makes use of a seamless junction and seamless divider  39  that enhance the strength of the graft because they contain no perforations.  
         [0047]    The increased junction strength is evidenced by trial experiments performed by Atrium Corporation of Hudson, N.H. and displayed in the table of FIG. 6. The table illustrates results obtain from tests performed on a prototype Atrium graft (Atrium graft) made in accordance with the teachings of the present invention and a sample graft made by W. L. Gore &amp; Associates, Inc. having model number SB2001 (Gore graft). Both grafts were 16 mm×8 mm thin wall grafts. The wall thickness (WT) in the trunk and leg portions was as indicted in the table. A tensile force was applied to each graft using a commercially available tensile test apparatus made by Instron Corp., which measures force to yield the material to failure. Evidence of material or junction failure was observed at different force values. The Atrium graft was able to withstand 54 lbs. of pressure at each junction, representing longitudinal tensile strength (LTS), while the Gore graft withstood 38 lbs. of pressure. The Atrium graft had a radial tensile strength (RTS) of 151 lbs. in the trunk and 138 lbs. in the leg, while the Gore graft had an RTS of 150 lbs. in the trunk and 124 lbs. in the leg. The Atrium graft had a suture retention strength (SRT) of 2.4 lbs. in the trunk and 1.7 lbs. in the leg, while the Gore graft had an SRT of 1.7 lbs. in the trunk and 1.3 lbs. in the leg. The water entry pressure (WEP) withstood by the Atrium graft was 279 mm Hg, while the WEP withstood by the Gore graft was 275 mm Hg. The experimental data suggests that the Atrium graft has a relatively greater strength in all areas measured relative to the sample Gore graft.  
         [0048]    Numerous modifications and alternative embodiments of the present invention will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode for carrying out the present invention. Details of the structure may vary substantially without departing from the spirit of the invention, and exclusive use of all modifications that come within the scope of the appended claims is reserved.