Abstract:
The invention is a method for manufacturing a steerable catheter having a distal end, proximal end, an outer jacket, a pull wire and a central lumen. The central lumen is maintained in a circular shape without bulges diminishing the useful inter-diameter by using an outer jacket with an elliptical shape and uneven thickness to encase a pull wire. The pull wire friction is also reduced by using one wire of larger diameter to create the lumen for the pull wire of smaller diameter, thus reducing the friction on the pull wire and reducing the locking of the catheter body around the pull wire at bends preventing movement of the pull wire. The distal end of the catheter also has a smooth tip due to heat treating of steel wire braid reinforcement that does not require the addition of bulge forming additional material to control tip shape.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]    The present application is a continuation of application Ser. No. 09/833,324, filed 12 Apr. 12, 2001 for Process for Producing Steerable Sheath Catheters of Jin Shimada, which claims priority from Provisional Application No. 60/199,169 filed Apr. 24, 2000. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    This invention relates to catheters which are used to provide access into the human body. More particularly, the present invention is directed to steerable sheath catheters which are used to provide access into the human vasculature for delivery of additional tools, instruments, medications or fluids.  
           [0003]    Catheters have been commonly used in medical practice to reach locations inside the body otherwise unreachable without surgery. The body of a catheter is long and tubular and contains an inner lumen. A catheter has a distal end or tip which enters the patient as well as a proximal end that has a handle for control by the operator.  
           [0004]    The tip of the catheter is first inserted into a major vein, artery or other body cavity. The catheter is then further inserted and guided to the area of concern. The catheter is often used as a delivery method for other tools, such as balloons for performing angioplasty or a camera for performing endoscopy. As medical knowledge increases, the catheterization procedures have become more complicated and more exacting. The usefulness of catheters is largely limited by the ability to successfully manipulate the position and orientation of the catheter tip into small and tortuous vessels. Therefore the goals for a successful catheter design are to maximize the inner diameter while minimizing the outer diameter and maintaining control and flexibility of the catheter.  
           [0005]    One method of directing a catheter into position is through the use of a guide wire. First the guide wire is fed into position within the patient. Then the catheter is urged over the guide wire. However, it is not uncommon for the position of the catheter tip to become dislodged from the desired location as the guide wire is removed.  
           [0006]    To avoid this problem, other catheters known in the art, are guided into place without the use of guide wires. These catheters have sufficient pushability that the tip of the catheter can be directed from a proximal location without buckling or kinking. Unfortunately, such guide catheters tend to be more difficult to steer into position and the necessary stiffness can limit their placement in areas with sharp curves.  
           [0007]    Catheters with tips preformed into particular shapes specialized for specific applications are known in the art. The pre-shaping of the catheter may aid the placement of the tip in the desired location. However, the pre-shaping of catheters for particular applications requires a hospital to provide a wide array of catheter shapes and sizes for use. Another disadvantage to preformed catheters is that they do not allow the physician to adapt the catheter to account for any peculiarities of a patient&#39;s vascular system. A physician can attempt to reshape a catheter before use, by applying heat. However, such manual reshaping is not only time consuming but can compromise the lumen of the catheter, by causing the circular lumen to ovalize or flatten out as the catheter is bent, or even kink or seal at a bend destroying the catheter&#39;s usefulness.  
           [0008]    Steerable sheath catheters, the present invention being one example, are also directed into position from a proximal location. However, the tips of these catheters are steerable due to the action of one or more pull wires that are embedded along the length of the catheter body. Pre-forming of the catheter is not necessary because the operator can adjust the shape of the catheter or steer the tip as the catheter is directed into the body. Therefore these catheters are capable of use in a wider range of procedures than the specialized preformed catheters.  
           [0009]    A current method in the art used to manufacture steerable sheath catheters is to form the catheter on a mandrel using multiple layers: an inner liner, a layer of wire braid and an outer thermoplastic jacket. The inner liner is pulled over the mandrel and tightened down. The pull wire is laid axially along the inner liner, often within a groove present on the surface of the mandrel. The steel braid is pulled or woven over the inner liner and pull wire. After the steel braid is tightened down, the entire catheter is encased in a thermoplastic outer jacket. The outerjacket is then encased in heat shrink material and heated. The heat causes the thermoplastic jacket layer to flow, which when teamed with the pressure from the heat shrink material causes the thermoplastic outerjacket to flow into the steel braid consolidating the catheter into one unit. [5,669,920; 6,042,578; 5,527,325] 
           [0010]    The mandrel in this process usually has a longitudinal groove to facilitate the placement of the pull wire during the manufacturing process. The inner liner of the catheter is placed over the mandrel and is pushed into the groove. The pull wire is then laid in the groove on top of the inner liner. The steel braid and outerjacket can then be pulled easily over the mandrel without disturbing the pull wire. However, the use of this process results in the creation of a bulge in the central lumen. This reduces the useable diameter of the central lumen for the insertion of other instruments. In general, it is desirable to maximize the ratio of the inside diameter to the outer diameter of the tubular body of the catheter.  
           [0011]    Another problem in the current art is that by embedding the pull wire through the action of a thermoplastic polymer teamed with a heat shrink material or embedding the wire in the catheter body by spraying the outer jacket material over the wire is that the pull wire creates its own lumen. [6,030,371] Therefore the pull wire and its lumen are approximately equal in diameter. This creates three related difficulties. First, there is friction created between the walls of the lumen and the pull wire as an operator attempts to control the catheter by moving the pull wire. The friction increases the difficulty in operating the pull wire. Second, as the catheter is deflected (bent) through the movement of the pull wire, the steel braid embedded in the outer wall of the catheter is also pulled and flexed. As the steel braid flexes, the forces created can deform the lumen. This can cause the steel braid to lock down on the pull wire and its lumen. This greatly increases the friction and can prevent movement of the pull wire as its lumen is deformed from a circular shape into an ovular shape. The third problem is that as the pull wire is “locked down” in the bent catheter, the pull wire and catheter loses the ability to spring back to the original shape as the force on the pull wire from the operator at the proximal end is removed. Accordingly, there remains a need in the art for a catheter with a pull wire with reduced friction and reduced interference from the steel braid which would allow for easier control by the operator and would allow the catheter to spring back into its original shape.  
         BRIEF SUMMARY OF THE INVENTION  
         [0012]    The invention includes a method for manufacturing a steerable catheter having a distal end, a proximal end, an outer jacket, a pull wire and a central lumen having a maximized ration of inner diameter to outer diameter. The central lumen is maintained in a circular shape without bulges diminishing the useful inter-diameter by using an outer jacket with an elliptical shape and uneven thickness to encase a pull wire. The pull wire friction is also reduced by using one wire of larger diameter to create the lumen for the second pull wire of smaller diameter, thus reducing the friction on the pull wire and reducing the locking of the catheter body around the pull wire at bends preventing movement of the pull wire. The distal end of the catheter also has a smooth tip due to heat treating of steel wire braid reinforcement that does not require the addition of bulge forming additional material to control tip shape. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 A perspective view of the catheter.  
         [0014]    [0014]FIG. 2 A perspective view of the catheter body.  
         [0015]    [0015]FIG. 3 A cross-sectional view of the catheter assembly before lamination by heating.  
         [0016]    [0016]FIG. 4 A cross-sectional view of the catheter assembly after lumen-defining wire is removed.  
         [0017]    [0017]FIG. 5 A cross-sectional view of the catheter assembly after insertion of pull wire into the lumen. 
     
    
     DETAILED DESCRIPTION  
       [0018]    The present invention provides an improved catheter that maximizes the usable inner diameter and minimizes pull wire friction. The usable inner diameter is maximized compared to the outer diameter by modifying manufacturing techniques. The pull wire is not laid into a groove in the mandrel. Therefore there is no predisposition to form a bulge within the inner lumen of the catheter. Also, the shape of the outerjacket material has been modified from a tube with circular cross-section and uniform thickness to a tube with elliptical or ovular cross-section and uneven thickness. The outer jacket material is thicker at the position of the pull wire. When the outerjacket material is heated until it flows, the material will redistribute around the circumference of the catheter body due to the force of the heat shrink material. The result is an approximately circular catheter with the pull wire embedded within the outer jacket, not protruding into the inner lumen nor bulging out of the outer diameter.  
         [0019]    The outer diameter of the catheter is minimized at the tip by a novel treatment method used on the wire braid. The wire braid is formed on a disposable core. An end section of the wire braid is heat tempered and cooled before incorporation into the catheter body. The action of heat tempering the section of wire braid placed at the tip of the catheter releases the stress on the wire and reduces radial forces. Without heat tempering, the wires maintain the stress from being braided, the braid patterns provide radial pressure or outward force at the ends of the braid resulting in a distorted braid pattern. The invention prevents the problem encountered in the prior art of the ends of the wire braid flaring and protruding through the outer jacket of the catheter. In the prior art, the wire braid is contained either through the application of additional material or changing the manufacturing process of the catheter so that the outerjacket material is not heated until fluid. There are problems with both techniques. When additional material is added at the tip of the catheter, an undesirable bulge is formed in that area. If the manufacturing process is changed to deter wire protrusion by incompletely melting the outer jacket material, often the result is incomplete integration and lamination of the catheter, which can result in failure of the catheter.  
         [0020]    One alternative method in the prior art would be to heat treat the entire wire braid. However, there is a loss of radial force capacity and the possibility of increased kinking of the wire braid when the entire length of it is heat tempered. Also, the wire will not expand as desired to into the outerjacket material when the outer jacket material is liquified. The result is a less flexible catheter with possible increased interference between the pull wire and wire braid as well as problems with integrity of the lamination process.  
         [0021]    Another method in the prior art to prevent the wire braid from flaring out of the catheter body, is to place an additional piece of polyester around the end of the wire braid. The polyester has a higher melting point than the outer jacket material. Therefore, when the outer jacket material is liquified to allow it to flow into the wire braid, the wire braid remains contained and does not escape from the catheter. The problem with the addition of this additional polyester material is that an undesirable bulge is created on the distal end of the catheter where this additional material is added. An additional problem of poor bonding due to differences in the materials and their melting points can also be seen.  
         [0022]    To minimize pull wire friction, the present invention uses one wire in order to create a lumen and then removes that wire and replaces it with a smaller diameter pull wire in order to control the catheter. The benefits of having a pull wire with a smaller diameter than the lumen is to allow easier movement of that pull wire through the reduced friction of contact between the lumen and the pull wire. An additional benefit is that as the catheter is bent, there is additional space inside the lumen, so as the wire braid is pulled, thereby placing force onto the lumen, the pull wire will not become as easily locked down by the changed shape of the lumen. If the pull wire is the same size as the lumen as it is in prior art applications, the wire braid can lock down the pull wire and prevent its movement as the lumen is deformed from a circular shape into an ovular shape. This problem may require a catheter to have multiple pull wires just to allow the catheter to move in one plane. If a pull wire in the current art becomes locked down as the catheter is bent, operation of a pull wire on the opposite side is necessary to return the catheter to its original straight configuration. In the current invention, because the lumen is larger than the pull wire, forces placed on the lumen by the wire braid are much less likely to lock down the pull wire and preventing its free movement and control of the catheter. Therefore, the inventive catheter can return to its original straight configuration simply by releasing the force on the pull wire. In summary, the inventive catheter allows you to have a pull wire with free movement without any loss in internal or external space and also has increased flexibility because the braid will not lock up the pull wire.  
         [0023]    [0023]FIG. 1 illustrates the basic structure of a catheter generally indicated at  10  made in accordance with the present invention. The catheter  10  extends from a proximal end  11  to a distal end  13 . The proximal end  11  is handled by the operator, while the distal end  13  contains the catheter tip which is inserted into the body. The overall length of the catheter may be varied as necessary for various applications. Typical catheter lengths will be on the order of 20-60 inches, with a preferred length of 48 inches.  
         [0024]    The catheter  10  is generally tubular in shape and desirably includes a central lumen  12  as seen in FIG. 2. As is known in the art, alternative embodiments include more than one lumen or subdividing a large lumen into two or more separate lumens, such as in balloon angioplasty.  
         [0025]    The basic method of manufacture according to a first embodiment of the present invention will be described below and as seen in FIG. 3. The catheter components as they are assembled will be collectively referred to as a catheter assembly  26 . A ground mandrel  14 , which is preferably approximately 4 feet in length, is the first component of the catheter assembly  26 . The mandrel  14  has two ends named for reference the distal and the proximal ends. The inner liner  16  is placed on the mandrel  14 . The inner liner  16  is preferably an extruded Teflon® (polytetrafluoroethylene) tubing, which is available commercially. The inner liner  16  is knotted at one end (e.g. the distal end  13  shown in FIG. 1) and is fed on to the mandrel  14 . It is snugged down by pulling and knotted on the other end (e.g. the proximal end  11  shown in FIG. 1) also.  
         [0026]    A lumen defining wire  18 , is placed longitudinally along the inner liner  16 . The lumen defining wire  18 , is composed of stainless steel and is 0.010 inches in diameter. In the preferred embodiment, the lumen defining wire  18  is encased inside another preformed Teflon® tube before placement along the inner liner. In alternative embodiments the lumen defining wire  18  may be covered with other lubricious materials before placement, an example being coating the lumen defining wire with silicone.  
         [0027]    A wire braid  20 , which is either purchased separately or braided on site, is formed onto disposable core material in order to achieve the proper diameter. The wire braid  20  is preferably composed of φ0.003 high tensile stainless steel wire. The wire braid  20  is formed in a standard braid pattern with preferably approximately 16 wires at 45-60 PPI. Before the wire braid  20  is placed onto the catheter assembly  26 , one end is heat tempered with a torch or alternate heat source. The wire braid  20  is cooled, removed from the disposable core material and carefully slid over the catheter assembly  26 . It is necessary that care is taken not to disturb the position of the lumen defining wire  18 , which must remain straight. The end of the wire braid which has been heat treated or annealed terminates somewhat before the distal end  13  of the mandrel  14 . The untreated end of the wire braid  20  is knotted at the proximal end  11  of the mandrel  14 . Therefore, at the distal end  13  of the assembly both the inner liner  16  and the lumen defining wire  18  are exposed.  
         [0028]    An outer jacket  22  is slid over the catheter assembly  26 . The outer jacket  22  is a tube extruded from Pebax® before application to the catheter assembly  26 . Pebax® is a thermoplastic elastomer resin by the ATOCHEM Corporation of France. The outer jacket  22  is made of either single or multiple sections of tubing that are butted together over the catheter assembly  26  leaving the distal end of the wire braid  20  exposed. Different sections of the outer jacket  22  may have different softness/stiffness (tensile) characteristics in order to facilitate particular features in the catheter. For example, a bending region may have an outer jacket section that has greater softness than a region that will remain straight.  
         [0029]    In the preferred embodiment, a tube of heat shrink material  24  is placed over the top of the outer jacket  22 . The heat shrink material  24  is a fluoropolymer or polyolefin material. FIG. 3 displays a cross-section of the catheter assembly  26  before lamination of the materials by heating.  
         [0030]    Next, the entire catheter assembly  26  is laminated by heating until the outer jacket  22  liquefies. The heat shrink material  24  has a higher melt temperature that the outer jacket  22  and when it constricts the heat shrink material retains its tubular shape thereby forcing the liquefied outer jacket  22  into the wire braid and into contact with the lumen defining wire  18  and inner liner  16 . The catheter assembly  26  is cooled and the outer jacket  22  solidifies. The heat shrink material  24  is scored and cracked open in order to remove it. After removal, the outer jacket  22  becomes the outside layer of the catheter.  
         [0031]    In alternative embodiments the catheter assembly  26  may be laminated using alternative techniques. In one embodiment, after the outer jacket  22  is slid over the catheter assembly  26 , the catheter assembly  26  may be laminated using heat in conjunction with a press that has a mold for defining the final shape of the catheter. Another embodiment applies the outer jacket  22  by extruding the material over the catheter assembly  26 .  
         [0032]    Next the lumen defining wire  18  is removed from the distal end and removed from the catheter assembly  26 . A lumen  28  for placement of a pull wire  30  remains in the outerjacket  22  as displayed in FIG. 4. A permanent pull wire  30  is inserted into this lumen  28  from the distal end of the catheter assembly  26  as displayed in FIG. 5. At the end of the pull wire  30  is attached to a pull ring which is placed around the distal end of the catheter assembly  26 . The permanent pull wire has a diameter of preferably approximately 0.008 inches. The diameter of the permanent pull wire  30  is smaller than the lumen defining wire  18 . Alternate embodiments include coating the permanent pull wire  30  with Teflon®, marketed by E. I. duPont de Nemours and Company Corporation of Wilmington, Del., or other lubricious materials, such as silicones so that the wire is more easily moved within the lumen.  
         [0033]    After the permanent pull wire  30  and attached pull ring are in place, another section of outer jacket made from Pebax® is placed on the distal end of the catheter assembly  26 , over the exposed mandrel, permanent pull wire, pull ring and annealed wire braid. This distal end section of Pebax® material is also covered with heat shrink tubing and is heated until the Pebax® material is liquified. When liquified, this distal end section flows to connect with the main section of outer jacket and captures the pull ring of the permanent guide wire. After heating is complete and the catheter is cooled, the pull ring is secured by the Pebax® polymer and the heat shrink tubing is once again removed. The mandrel is removed from the completed catheter body which is ready for installation of a handle on the proximal end.  
         [0034]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.