Abstract:
An internal formation for a conduit. The formation has a longitudinally extending member adapted to extend along an inside surface of at least a portion of the length of the conduit. The longitudinally extending member has an asymmetric profile in a direction transverse of the longitudinal axis of the member.

Description:
FIELD OF INVENTION 
     The invention relates to an internal formation for a conduit and especially, a formation for modifying the flow of a fluid within a conduit. 
     BACKGROUND OF THE INVENTION 
     Vascular prosthesis, such as vascular grafts and stents, have been known for a number of years. Recently, it has been proposed that a helical formation formed on the internal surface of the vascular prosthesis can be used to generate desirable flow characteristics in blood flowing through the prosthesis, by encouraging spiral flow of the blood and so helping to reduce turbulence and/or dead spots with the fluid flow. It is believed that generating a spiral flow pattern in the blood, and thereby reducing turbulence and dead spots, can help to reduce vascular disease. 
     In this context, the term “spiral flow” refers to fluid flow in which there is a spiral and/or helical flow component to the fluid flow. 
     Examples of helical formations in vascular prosthesis are disclosed, for example, in International (PCT) Patent Application Nos. WO 00/38591 and WO 02/098325, and UK Patent Application No. 2369797 each of which is incorporated herein by reference. 
     However, one of the disadvantages of having a helical formation in a conduit is that the formation itself can create turbulence as the blood flows past the formation. This may have the unintentional effect of reducing or removing the spiral flow properties from the blood that the helical formation is intended to generate. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the present invention, there is provided an internal formation for a conduit, the formation comprising a longitudinally extending member adapted to extend along an inside surface of at least a portion of the length of the conduit, the longitudinally extending member having an asymmetric profile in a direction transverse of the longitudinal axis of the member. 
     An advantage of providing a formation having an asymmetric profile is that it is possible to reduce the turbulent effects of the formation on a fluid flowing through the conduit. 
     Preferably, the longitudinally extending member extends helically along the length of the conduit, and preferably, extends helically along the internal side wall of the conduit. 
     Typically, a first surface of the longitudinally extending member is at least partially directed towards an inlet of the conduit and a second surface of the longitudinally extending member is at least partially directed towards the outlet of the conduit. 
     Preferably, the first surface intersects a diameter of the conduit at a smaller angle than the second surface intersects a diameter of the conduit. The first and second surfaces may be planar and/or curved. If one or both the surfaces are curved, they may be concave or convex, or a combination of concave and convex. 
     Typically, the angle that the first surface subtends with a diameter of the conduit extending through a portion of the profile closest to the centre of the conduit is less than 20°, preferably between 5° and 15°, and most preferably substantially 10°. 
     Typically, the first and second surfaces extend from the internal surface of the conduit towards each other and towards a central longitudinal axis of the conduit. The first and second surfaces are typically coupled together at an apex or by a third surface. Preferably, the third surface is a curved surface. 
     Preferably, the distance along the internal surface of the conduit from said diameter of the conduit to the point at which the second surface meets the internal surface of the conduit is substantially 0.25 of the internal width of the conduit. 
     In one example of the invention, the formation is for blood flow tubing, such as a vascular prosthesis. For example, a graft, a stent or a graft/stent combination. The formation may be in the form of an insert that is formed separately of the conduit and then mounted within the conduit, or may be integrally formed with the conduit, or may be formed by an elastic or non-elastic deformation of a side wall of the conduit. If the deformation is elastic, a former is retained around the conduit to maintain the internal formation. 
     Preferably, the formation is designed to effect spiral flow of a fluid, and typically a liquid, flowing through a conduit in which the insert is located. Where the conduit is blood flow tubing, the liquid is blood. 
     In accordance with a second aspect of the present invention, there is provided a conduit, the conduit including an internal formation in accordance with the first aspect of the invention. 
     The conduit may have one internal formation, or more than one internal formation. If there is more than one internal formation, the formations be in parallel around the conduit and/or in series along the conduit. 
     In addition, if there is more than one internal formation, the formations may be identical or may be different. If they are different, they may vary in height and/or the angle of the first and/or second faces. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of an internal formation in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic side view of a stent with the front half of the mesh body of the stent not shown for clarity; 
         FIG. 2  is a cross-sectional view of the stent of  FIG. 1  through a plane perpendicular to the longitudinal axis of the stent; and 
         FIG. 3  is a cross-sectional view of a graft through a plane perpendicular to the longitudinal axis of the graft. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a stent  1  having a cylindrical wire mesh body  2  and three internal formations in the form of inserts  3  mounted within the mesh body  2 . The inserts  3  may be formed from a plastic material and may be mounted within the mesh body by being moulded onto the mesh body  2  so that the some of the mesh is entrapped within the inserts  3 . The inserts  3  are spaced equidistantly around the inside of the mesh body  2  and are each in the shape of a helix so that the inserts  3  extend along and around the inside of the mesh body  2 . 
       FIG. 2  is a cross-sectional view through the stent  1  in a plane perpendicular to the longitudinal axis of the stent  1 , and looking in the direction of intended fluid flow through the stent  1 . The stent  1  has a diameter of 3.5 mm and the height of each insert  3  is 0.5 mm. Each insert  3  has two faces, a first face  4  facing against the fluid flow direction and a second face  5  facing with the fluid flow direction. The first face  4  is steeper than the second face  5  and is at an angle θ of substantially 10° to a diameter  7  of the stent  1  intersecting apex  6  of the inserts  3 . The angle of the second face  5  to the same diameter  7  is such that the distance x from the point at which the diameter intersects the internal surface of the mesh body  2  to the intersection of the second face  5  with the internal surface of the mesh body is approximately 0.25 of the internal diameter of the stent  1 . Hence, the profile of the inserts  3  is asymmetric, with the first face  4  of the inserts  3  being steeper than the second face  5 . 
       FIG. 3  shows a cross-sectional view of a graft  10  comprising a main body  11  having a deformation in the side wall of the body  11 , the deformation being in the form of a helical formation  12 . The helical formation  12  could be formed by any conventional means, such as described in UK Patent Application No. 2369797. Unlike the stent  1 , the graft  10  only has a single helical formation  12 . However, it is possible that the graft  10  could be provided with multiple helical formations  12 . 
     The helical formation  12  has a first face  13  and a second face  14  coupled together by a curved surface  16 . As shown in  FIG. 3 , the main, central section of the first face  13  is at angle of approximately 10° to a diameter  15  of the graft  10  that intersects the curved surface  16 . The main, central section of the second face  14 , if it is extrapolated down to the inside surface  17  of the graft  10 , intersects the surface  17  at a distance y from where the diameter  15  intersects the surface  17 . The distance y is approximately 0.25 of the internal diameter of the graft  10 . For example, if the graft  10  has an internal diameter of 8 mm, the distance y is 2 mm. 
     In use, the graft is orientated so that the blood flow is against the first face  13 . 
     The asymmetric profile of the inserts  3  and the helical formation  12  have the advantage of minimising turbulence in blood flow through the stent  1  and the graft  10  at typical blood flow rates and pressures. However, it is possible that similar or different asymmetric profiles could be used in certain blood flow applications, if desired or necessary. 
     Similar internal formations with asymmetric profiles could be used in other applications to reduce turbulence of fluid, and especially liquids, flowing within a conduit. However, for different fluid densities, flow rates and/or pressures, the angles of the first and/or second faces, and/or the height of the internal formation relative to the internal diameter of the conduit may be different.