Abstract:
The present invention is an access sheath comprising a tube or sheath with a passageway primarily for endoscopic procedures accessing the ureter through the bladder. The sheath has a bend limiting feature to limit the bend angle or bend radius. The sheath exhibits flexibility up to this limit at which point it becomes rigid. Excessive force is required to bend the sheath beyond the bend limit and would result in the kinking of the tubular frame.

Description:
This application claims priority to U.S. Provisional Application No. 61/126,060 filed May 1, 2008, which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to medical devices for surgical access. More particularly, the present invention relates to surgical access devices for introduction of endoscopic surgical instrumentation into the ureter. 
     BACKGROUND OF THE INVENTION 
     Access sheaths are commonly used to establish an accessible, minimally invasive passageway into the body to facilitate and expediate the insertion and removal of devices. Once access is established, devices can be passed through the access sheath and to the treatment site with increased speed and minimized patient trauma. For instance, typical kidney stone retrieval procedures require multiple insertions and removals of the stone basket and endoscope as successive stone fragments are captured. The access sheath protects the ureter from sharp points or jagged edges of the stone fragments as they are pulled from the ureter or kidney. The access sheath also provides the physician with an established pathway into the ureter avoiding the need to re-establish that path from the urethera through the bladder and into the ureter for each insertion of the endoscope. Thus the procedure is less traumatic to the patient while being easier and faster for the physician. 
     Due to the nature of their use, access sheaths need to be flexible to follow patient anatomy, provide a maximized working channel for the physician, and be robust enough to confidently endure manipulation. Prior art access sheaths have been constructed with a thin-walled polymer tube. While this construction allows for a flexible access sheath with a maximized working channel, devices of this type are susceptible to kinking, elongation, and ovalization. Kinking and ovalization may render the access sheath useless since instruments may no longer be able to pass through the access sheath to the target anatomy. Furthermore, kinking may cause trauma to the patient or damage to the instruments being used. 
     Some known prior art access sheaths, such as U.S. Pat. No. 7,005,026, solve the kinking problem by re-inforcing the wall with a wire or wires. In Applied Medical literature, the access sheath is shown tied in a knot to highlight the catheter&#39;s extreme kink resistance. While this makes the sheath more resistant to kinking, elongation and ovalization, it increases the thickness of the sheath wall. The increased wall thickness either reduces the working channel, increases the outside diameter or both. 
     Reduction of the working channel is undesirable for several reasons. In many procedures multiple instruments are needed to be placed at the target anatomy simultaneously, thus requiring a maximized working channel for their placement. Also, in a kidney stone retrieval procedure reducing the access sheath inner diameter may prohibit the extraction of larger stones that would otherwise be extractable through a larger working channel. 
     Increasing the outer diameter of the access sheath is also undesirable. As the diameter of the access sheath increases it dilates and distends the adjacent anatomy. For instance, in a urological procedure the access sheath can split the patient&#39;s ureter if the access sheath&#39;s outer diameter is too large. Similar trauma may be caused when entering other patient vasculature. 
     Another problem with kink resistant, reinforced walls is that they over-bend in the bladder when being pushed up the ureter for the initial placement or when repositioning in the middle of the procedure. This tendency to over-flex and loop into the bladder is common. The bladder is a big open space that does not provide any side support for the access sheath. Once it over bends in the bladder the tip can not be pushed into the ureter. A similar effect can be seen by pushing a straightened finger directly against a wall. It is easiest to push (transmit force to the wall) with a straight finger (0° bend) or a moderately bent finger (up to 90° bend). At 180° bend it is very difficult to place force on the wall. 
     Whereas manufacturers laud the ability of their access sheaths to bend 360°, it can be seen in the above text that what is needed is an access sheath which is flexible enough to accommodate anatomical bends while being pushable. In addition, an access sheath should accomplish this while maximizing the working channel. 
     SUMMARY OF THE INVENTION 
     The present invention is an access sheath comprising a tube or sheath with a passageway primarily for endoscopic procedures accessing the ureter through the bladder. The sheath has a bend limiting feature to limit the bend angle or bend radius. The sheath exhibits flexibility up to this limit at which point it becomes rigid. Excessive force is required to bend the sheath beyond the bend limit and would result in the kinking of the tubular frame. 
     In preferred embodiments, the sheath is a tubular frame or has a tubular frame that comprises the bend limiting feature that limits the bend angle or bend radius. Preferably, at least a portion of the tubular frame is formed from a rigid material having a slot or slots at different longitudinal locations to create a bend limit. 
     Current access sheaths have polymeric sheaths with wire-reinforced walls that are highly kink resistant. These sheaths easily bend more than 360° without kinking. This hyper-deflectibility decreases the ability of the access sheath to be advanced through open anatomy that does not support the sheath wall into a tight lumen offering resistance. In the case of ureteral access, the sheath must have some flex to match the turns as it passes from the urethra through the bladder and into the ureter. As the access sheath is inserted into the ureteral orifice, resistance is encountered. The longitudinal force applied at the proximal end of the access sheath will result in over-bending or looping of the access sheath&#39;s shaft in the bladder. A 180° bend renders advancement impossible. The procedure is delayed as the physician must retract the access sheath and retry placement of the sheath. 
     This extreme deflectibility of existing access sheaths also causes the access sheath to fully conform to the patient&#39;s anatomy. In some cases, the patient&#39;s anatomy may be tortuous. The fact that current access sheaths match the anatomical tortuosity causes the medical devices passing through it to follow the same tortuous path. It should be noted that patient anatomy has a fleshy flexibility which may be partly straightened to provide a more direct pathway. 
     The present invention overcomes these problems by providing a sheath with flexibility at low bends but which can not exceed a specific bend limit. The present invention exhibits the flexibility required to flex from the urethra, through the bladder and into the ureter. However, as the present invention flexes to its maximum bend radius or bend angle, the access sheath stiffens and allows a more effective longitudinal transmission of force. The more effective transmission of longitudinal force to the tip allows the invention to pass the ureteral orifice and enter the ureter with greater ease. The present invention&#39;s lower bend angle or bend radius also speeds the passage of instruments and protects them from breakage due to over bending. 
     In one embodiment, the present invention provides an access sheath with a rigid section and a bend limiting flexible section. As mentioned previously the bend limiting feature of the present invention allows navigation of the anatomy. The incorporation of a rigid section can create a straight passageway through highly compliant tissue such as a urethra. 
     In one embodiment, the present invention provides an access sheath having a thin wall. Conflicting demands made on medical devices include the desire to minimize the outer diameter while increasing the inner diameter. Decreasing the outer diameter of an access sheath is desired in order to minimize the trauma to the patient as the sheath enters and potentially enlarges the tissue through which it passes. In extreme cases, a vessel may be split due to an access sheath which is too large for the anatomy through which it passes. Meanwhile the inner diameter is desired to be as large as possible so that multiple pieces of medical instrumentation may be inserted or so that irrigation may be increased. The increase in inner diameter and decrease in outer diameter ultimately results in an optimized access sheath with the thinnest wall still able to perform all other required functions. Currently available access sheaths incorporate wire reinforced polymeric tubes to form the shaft of their devices. The invention incorporates a slotted tube which could be fabricated from a thin metallic wall having a decreased thickness. The invention&#39;s tubular frame provides for the possibility of thinner walls, yet is still able to perform all other required functions. 
     In one embodiment, the present invention provides an access sheath having a guidewire retention feature. Guidewires are commonly used in medical procedures. This includes guidewires with lubricious coatings that facilitate the easy advancement of the guidewire through patient anatomy. A typical problem faced by physicians is the tendency of guidewires to back-out of the patient during a procedure causing loss of access to the target anatomy. The present invention incorporates a guidewire retention feature into its hub which allows the guidewire to be retained without requiring an external retention method. 
     The present invention also provides an access sheath having a tubular frame and coating combination that can improve lubricity and change the flexibility. Lubricious coatings on the outside surface ease the insertion of the access sheath. Likewise, lubricious coatings on the inner surface of the access sheath allow for easy passage of other medical instruments as they access the target anatomy. Polymeric coatings within the slots or on the ID or on the OD of the tubular frame will change the flexibility of the sheath, which may be beneficial in some situations. 
     Further objects and advantages of preferred embodiments of the device described herein are such that preferred embodiments are safe, reliable, and easy to use. Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. 
         FIG. 1  is an isometric view of an embodiment of the present invention. 
         FIG. 2  is an isometric view of the dilator or obturator that is used in conjunction with the invention shown in  FIG. 1 . 
         FIG. 3  is an isometric view of the dilator or obturator from  FIG. 2  inserted into the access sheath shown in  FIG. 1 . 
         FIG. 4  is an enlarged isometric view of the proximal end of  FIG. 1  with a guidewire to show an embodiment of the present invention. 
         FIG. 5  is the same as  FIG. 4  showing the guidewire within the retention feature. 
         FIG. 6  is a side view of the access sheath showing the sheath in the straight or natural position and at its bend limit which is the curved position. 
         FIG. 7  is an enlarged isometric view of the distal end of the distal section of  FIG. 6 . 
         FIG. 8  is development of  FIG. 7  showing a part of the solid portion of the surface rolled out into a plane. 
         FIG. 9  is an enlarged view of a small portion of the slot shown in  FIGS. 1, 3, 6 and 7 . This is typically how the slot may look when the sheath is straight. 
         FIG. 10  is similar to  FIG. 9  but shows the possible slot configuration on the expansive side of the sheath at its bend limit in  FIG. 6 . 
         FIG. 11  is similar to  FIG. 9  but shows the possible slot configuration on the compressive side of the sheath at its bend limit in  FIG. 6 . 
         FIG. 12  is a longitudinal cross-sectional view showing another embodiment of the distal end of the distal section of  FIGS. 1, 6 and 7 . 
         FIG. 13  is a longitudinal cross-sectional view showing yet another embodiment of the distal end of the distal section of  FIGS. 1, 6 and 7 . 
         FIG. 14  is a longitudinal view showing another embodiment of the tubular frame or sheath shown in  FIGS. 1, 3, 6 and 7 . 
         FIG. 14 a    is an enlarged cross-sectional view of  FIG. 14  taken at line A-A showing an embodiment without coating (uncoated). 
         FIG. 14 b    is another embodiment of  FIG. 14 a    showing a coated outer diameter. 
         FIG. 14 c    is another embodiment of  FIG. 14 a    showing a coated inner diameter and a coated outer diameter. 
         FIG. 15  is a table showing the features of major currently marketed access sheaths compared to the invention. 
         FIG. 16  is a graph showing bending torque versus bend angle. It compares some of the widely distributed access sheaths with the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows an access sheath  10  that consists of a hub  14  and a tube or sheath  16 . The access sheath  10  generally consists of a distal section  20  which may be positioned within a patient, and a proximal section  22  with proximal end  23  into which other medical devices may be introduced. A hub  14  is located at the proximal section  22  to facilitate the insertion of medical devices. The tube or sheath  16  includes a tubular frame  18  that surrounds an internal passageway  26  leading from the hub  14  at proximal end  23  of the proximal section  22  to the opening  24  at the distal end  25  of the distal section  20 . The sheath length in  FIG. 1  is the distance between  23  and  25 . By the use of this passageway  26 , other medical devices may gain access to, and retrieval from, the interior of the patient. One use is the removal of stones from a patient&#39;s kidney. In that procedure an endoscope visualizes stones in the kidney so that they may be captured with a stone basket. The stone, stone basket and endoscope are then retracted through the access sheath  10 . The stone is removed from the basket so that the endoscope and basket may return to the kidney and repeat the procedure for additional stones. 
     In the distal section  20  of the access sheath  10 , one or more slots  28  are cut in a generally helical orientation through the wall of the tube. Details of the path of the slot  28  are too small to be conveyed in  FIG. 1 . Their form and function will be discussed later in greater detail. Generally the slots  28  allow limited flexure of the distal section  20  of the access sheath  10 . 
       FIG. 2  shows a dilator or obturator  30 . The dilator or obturator  30  generally consists of proximal end and a distal end with a tube or shaft in between. A knob  34  is located at the proximal end and a tapered tip  32  exists at the distal end. Between the distal and proximal ends there exists a tube or shaft which is somewhat longer than the tube  16  of the access sheath  10 , but with a smaller outer diameter. The dilator or obturator  30  may include an internal lumen for the passage of a guidewire. The dilator or obturator  30  may be attached to the access sheath  10  by the insertion of the dilator or obturator&#39;s tube or shaft through the passageway  26  of the access sheath  10 . 
       FIG. 3  shows a dilator or obturator  30  fully engaged with the access sheath  10 . When fully attached, the tapered tip  32  of the dilator or obturator  30  extends distal to the opening  24  to ease the insertion of the access sheath  10  into patient anatomy. Once the access sheath  10  is in the desired location, the dilator or obturator  30  may be removed to enable the use of the passageway  26  by other medical instrumentation. 
       FIGS. 4 and 5  are details of the hub  14  located at the proximal section  22  of the access sheath  10 . As mentioned above, the hub  14  facilitates the insertion of medical devices into the tube or sheath  16 . In facilitating this access, the hub  14  typically would take the shape of a funnel  50  with a large proximal opening which roughly decreases in diameter until the tube or sheath  16  is reached. 
     A common device used during medical procedures is a guidewire  48 . Built into the hub  14  is a guidewire retention feature  40 . The guidewire retention feature  40  consists of a first slot  42  and a second slot  44 . The second slot  44  is generally situated perpendicular to the first slot  42 . A detent  46  may also be included on the second slot  44 .  FIG. 4  shows a guidewire  48  as it may typically be placed in an access sheath. Note that in this configuration the guidewire  48  is free to move longitudinally through the passageway  26 . To retain the guidewire  48 , the shaft of the guidewire  48  is first passed through the first slot  42  of the hub  14 , then into the second slot  44  of the hub  14 . The frictional force resulting from the flexure of the guidewire  48  as it transverses the second slot  44  inhibits the longitudinal movement of the guidewire  48 . If present, the detent  46  inhibits the guidewire  48  from reentering the first slot  42 . Note that access to either the proximal or distal end of the guidewire  48  is not required to position the guidewire  48  into the guidewire retention feature  40 . Multiple guidewire retention features  40  may be included on the hub  14  in order to manage multiple guidewires  48 . Guidewire retention features  40  may be provided along the circumference of the hub  14  to allow for physician preference of guidewire  48  placement in the field of operation or to allow for right handed/left handed preferences. The guidewire retention feature  40  may also be employed to retain elongated flexible members other than guidewires. 
       FIG. 6  shows the restricted flexure or bend of the distal section  20  as previously mentioned. The longitudinal axis  60  or  62  of the tube or sheath  16  proceeds from the hub  14  and through the passageway  26 . The distal section  20  of the tube or sheath  16  includes one or more slots  28  cut in a generally helical pattern through the wall of the tube. The slot  28  allows a restricted flexure of the bending section  54  of the tube or sheath  16  from the longitudinal axis in the natural or straight position  60  to the longitudinal axis when bent  62 . It should be noted that the natural position does not necessarily have to be straight but could be manufactured with a preset curve. When viewed from above, the plane of flexure  63  is the paper that  FIG. 6  is printed on. Torque  65  is applied in this plane  63  at tip  25  which causes the bending section  54  of the access sheath  10  to deflect into bend angle  64  and bend radius  66  while the rigid section  56  of the access sheath  10  remains in the straight or natural position  60 . The longitudinal axis when bent  62  splits the distal section  20  into an expansive half  72  and a compressive half  70 . The nature of the slot  28  limits the amount of bend angle  64  and the bend radius  66  that the distal section  20  may exhibit. The bend radius limit of the longitudinal axis  60  or  62  is generally 3 to 4 inches or greater, preferably 6 inches or greater and optimally 12 inches or greater. The bend angle limit of the longitudinal axis  60  or  62  is generally 180° or less, preferably 90° or less and optimally 60° or less. It is noted that the nature of the slot  28  may allow multiple bend radii in potentially multiple planes. It is further noted that the slot  28  or multiple slots could extend over the entirety of the tube or sheath  16  for complete flexure of the access sheath  10 . 
       FIG. 7  is a detail of the bending section  54  of the tube. One or more slots  28  originate slightly proximal to the opening  24 . The slot or slots  28  continue in a generally helical orientation. The pitch of the generally helical orientation is defined by a longitudinal spacing  74 . Typically the slot pattern within tubular frame  18  shown in  FIG. 7  is formed using a laser to cut a slot in a rigid metallic tube or something that has a similar material or structure. The most common material being stainless steel or nitinol.  FIG. 8  shows a section of the tubular frame  18  if it was opened and flattened out. Cutting the pattern from flattened stock then rolling it up would be another method of producing the tubular frame. The pattern of the slot  28  is a repetition of a serpentine path which separates the tubular frame  18  into interlocking mortises  76  and tenons  80 . The mortise width  78  is less than the tenon width  82  to prevent the release of the mortise  76  from the tenon  80  when a tensional force is applied to the neighboring sections.  FIG. 9  shows that slot  28  separates solid portions  83  of the tubular frame. In addition slot  28  defines a mortise  76  and tenon  80 . Note that slot  28  itself has dimension. When a tension is applied as in  FIG. 10  to the neighboring solid portions  83 , the result is a gap width increase  86  which is limited as the gap width decrease  88  at the mortise  76  and tenon  80  tends towards zero assuming there is no coating to restrict the movement. When a compression is applied as in  FIG. 11 , the gap width increase  86  is limited as the corresponding gap width decrease  88  tends towards zero assuming there is no coating to restrict the movement. This limited gap width increase  86  and decrease  88  is multiplied through the plurality of the mortise  76  and tenons  80  to enable a cumulative effect upon the compressive half  70  and expansive half  72  and thus affecting a restrictive flexure or bend of the bending section  54  of the access sheath  10 . It is noted that the gap width  84  of the slot  28  could be varied where smaller or larger slots  28  would respectively allow increased or decreased bend radii  66  in bending sections  54 . Constantly varying thickness of slots  28  or the longitudinal spacing  74  could be used to create constantly varying bend radii  66 . 
       FIGS. 6 and 7  show the tubular frame  18  and the sheath or tube  16  being basically the same because it is easier to show that embodiment of the invention especially without any coating. Another embodiment would be to have the tubular frame occupy only a portion the longitudinal length of sheath  16  while the remainder of the sheath could be of another construction. The bend angle  64  would be taken only on the longitudinal axis  60  and  62  within the longitudinal length of tubular frame  18 . 
       FIGS. 12 and 13  are cross sectional details of the opening  24  of the access sheath  10  which shows several other embodiments of the invention. When a stone enters the inner diameter  92  of the passageway  26 , it may become wedged or jammed and occlude the passageway  26 . The creation of a reduced opening diameter  89  limits the size of stone entering the passageway  26  and ensuring that the stone will be able to fully navigate the passageway  26 . Furthermore, the mating of the dilator or obturator  30  to the access sheath  10  requires clearance between the inner diameter of the tube or sheath  16  and the outer diameter of the dilator or obturator shaft. It is desirable for the fit between the opening  24  and the shaft of the dilator or obturator  30  to be as small as possible to allow a gradual transition between the two items. The gradual transition allows for minimal trauma as the assembly is inserted into the patient anatomy. In  FIG. 12  a transition from the general inner diameter  92  of the tube or sheath  16  to the opening  24  is accomplished by the forming of the wall  94  causing a reduction from the outside diameter  90  to create an opening inner diameter  89  which is less than the tube&#39;s inside diameter  92 . The opening inner diameter  89  should approximate the outer diameter of the shaft of the dilator or obturator  30 . In  FIG. 13  the inner diameter reduction is accomplished by the addition of a sheath tip  93  onto the distal end  25  of the tube  16 . The sheath tip  93  could be a molded or machined item which is bonded, welded, or otherwise affixed to the distal end  25  of the tube  16 . The sheath tip  93  could be of a reduced durometer to create an atraumatic tip. 
       FIG. 14  shows a tube or sheath  16  exhibiting a slot  28  with longitudinal spacing  74  that varies along the generally helical path. Near the opening  24  is a first rigid section  56   a . The first rigid section  56   a  is followed by a first bending section  54   a  wherein the slot  28  has a first longitudinal spacing  74 . After a longitudinal distance the longitudinal spacing  74  of the slot  28  changes into a second longitudinal spacing  74  within a second bending section  54   b . After the bending sections, the tube or sheath  16  returns to a second rigid section  56   b . The changing of the longitudinal spacing  74  directly affects the number of mortise  76  and tenon  80  pairings along a longitudinal length of the tube. This creates bending sections  54   a  &amp;  54   b  with different bend radii  66 . The various bend radii  66  may be designed to match the desired maximum curvature as the access sheath  10  is inserted through, or rests within, the patient anatomy. In this embodiment the bend radius limit of the longitudinal axis is generally 4 to 5 inches or greater, preferably 9 inches or greater and optimally 18 inches or greater. The bend angle limit of the longitudinal axis is generally 140° or less, preferably 50° or less and optimally 30° or less. It is noted that the number of bending sections could be increased. Furthermore, the longitudinal spacing  74  could be constantly variable to create bending sections with a constantly varying limited flexure. It may also be considered that the longitudinal spacing  74  could vary in sectors of the circumference of the tube or sheath  16  in order to vary the limited flexure in different planes. 
       FIG. 14 a    is an enlarged cross-sectional view of  FIG. 14  taken at line A-A showing the tubular frame without coating which is an embodiment of the invention.  FIGS. 14 b  and 14 c    are cross sections of the tube or sheath  16  showing possible coatings of the tubular frame  18  which is another embodiment of the invention.  FIG. 14 a    shows a section of an uncoated tube or sheath  16  having a given outsider diameter  90 , inside diameter  92 , and wall thickness  96  defining the tubular frame  18 .  FIG. 14 b    shows a similar tubular frame  18 , or a different longitudinal section of the previous tubular frame  18 , wherein a coating  100  has been applied to the outer surface of the tubular frame  18 .  FIG. 14 c    shows a similar tubular frame  18 , or a different longitudinal section of the previous tubular frame  18 , wherein a liner  102  has been placed along the inner surface and a coating  100  has been applied to the outer surface of the tubular frame  18 . This coating  100  could be a polymeric jacket that would change the bending properties of the sheath especially if the material was within the slots. A similar configuration may incorporate a liner  102  applied to the inner surface of the tubular frame  18  but no coating is applied to the outer surface. The coating  100  and/or liner  102  may be lubricious to assist in the passage of the access sheath  10  into patient anatomy or the passage of medical devices through the passageway  26 . The coating  100  and/or liner  102  may be used to seal the slots  28  of the tube or sheath  16  thus limiting or eliminating the passage of fluid through the slot  28  from the inner diameter  92  of the tubular frame  18  to the outer diameter  90  of the tubular frame  18 . The coating  100  and/or liner  102  may also be used to affect the stiffness of the access sheath  10 . Wire reinforcement could also be incorporated to affect the bend properties of the access sheath. 
       FIG. 15  is a table listing physical attributes of access sheaths made by major manufacturers. Note that the invention is different from the other listed access sheaths in that the difference between the outer diameter  90  and inner diameter  92  is one French size as opposed to the typical two French sizes. This is also seen in the distal wall thickness  96  where the invention typically has a wall thickness  96  which is thinner than current competitive product. The wall is especially thin with the first listed configuration of the invention where the tubular frame  18  is uncoated or has a light surface coating such as a hydrophilic coating. The second listed configuration of the invention considers a tubular frame  18  with a coating  100  as is demonstrated in  FIGS. 4 b  and 4 c   . The sheath length is similar for all listed access sheaths. The sheath construction demonstrates that sheath construction among the major manufacturers is a stainless steel coil reinforced polymeric sheath. In contrast, the current invention incorporates the bend limiting tubular frame  18 . 
       FIG. 16  is a graph demonstrating the relationship between the bending torque  65  of an access sheath  10  and the resulting bend angle  64  shown in  FIG. 6 . Data was compiled by holding the proximal section  22  at the hub  14  horizontal while applying a perpendicular torque to a gage pin that was inserted into opening  24 . The resulting angle  64  at opening  24  from horizontal or natural position was recorded. As would be expected, all access sheaths required no torque to maintain the horizontal (0°) position. Raw data taken every 10° was used to compute the linear regressions plotted in  FIG. 16  for the three competitive products. ACMI UroPass demonstrated the most flexibility and the Cook Flexor demonstrated increased stiffness. The graph highlights the marked difference between the competitive product and the three bend limiting prototypes of the current invention. For all three prototypes, zero bending torque was experienced for several degrees after 0°. In the case of bend limiting prototype  3 , no torque was measured until an angle of 100° was reached. This lack of resistance is seen as the gap width  84  has free travel until the gap width decreased  88  reaches zero (assuming unimpeded slots). Once the gap width decreased and  88  reaches zero, the bending torque increases at a rate greater than that of the competitive product. This results in the current invention momentarily matching the bending torque of competitive product. For bend limiting prototype  1  this occurs at 15° to 20° of bend angle. For bend limiting prototype  2 , this occurs at 40° of bend angle. Bend limiting prototype  3  incorporates multiple bend sections  54  as illustrated in  FIG. 14 . The result of the multiple bend sections  54  is that the initial period of zero bend torque, and the rapid bending torque increase, is followed by period where the rate of increase in bending torque paralleled competitive product. For all three prototypes, when the gap width decreased  88  reached zero, the prototypes experienced very rapid increases in the amount of torque per degree of bend angle. The rate of change in bending torque is much higher later in the curve as compared to earlier sections of the prototype&#39;s curve. This is what is generally considered their limit. The rate of increase could be seen as asymptotic in nature; such that the bend limiting prototypes would never be able to reach the bend angle without failure or kinking commonly and easily achieved by competitive product. For the tested prototypes, maximum bend angles were approximately 30°, 50° and 140° to 150°. All three of these prototypes would have failed (kinked) before 180°. Prototypes  1  and  2  would have failed (kinked) most likely before 60° and definitely before 90°. 
     Another embodiment shown in  FIG. 16  is the level of bending torque that the access sheath prototypes (or tubular frame) can withstand within a bend angle of 180° or less. The maximum bending torque is generally 10 inch ounces or greater, preferably 20 inch ounces or greater and optimally 30 inch ounces or greater before kinking. 
     The prototypes tested were uncoated as is shown in  FIG. 14 a   . Had the prototypes been coated, as in  FIG. 14 b    or  14   c , the bending torque at lower bend angles would likely mimic competitive product until the gap width decreased  88  reaches a point at which an asymptotic like increase in bending torque would be observed. There would be a rapid drop when at the failure point in which a kink would result. 
     While preferred embodiments of the present invention relate to ureteral access sheaths for endoscopic procedures in the urinary system, several other applications are envisioned as well. Examples include the retrieval of biliary stones, gall bladder stones, or other objects or tissue during the course of an endoscopic or laparoscopic procedure. The present invention might also be useful for procedures using exceedingly small diameter catheters where pushability is required but the small dimension of the catheter structure makes the transfer of longitudinal force to the tip difficult. The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and it is therefore desired that the present embodiments be considered in all respects as illustrative and not restrictive.