fig1 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 fig1 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 fig1 . 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 . fig2 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 . fig3 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 . fig4 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 . fig4 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 . fig6 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 fig6 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 . fig7 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 fig7 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 . fig8 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 . fig9 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 fig1 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 fig1 , 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 . fig6 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 . fig1 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 fig1 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 fig1 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 . fig1 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 . fig1 a is an enlarged cross - sectional view of fig1 taken at line a - a showing the tubular frame without coating which is an embodiment of the invention . fig1 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 . fig1 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 . fig1 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 . fig1 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 . fig1 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 fig4 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 . fig1 is a graph demonstrating the relationship between the bending torque 65 of an access sheath 10 and the resulting bend angle 64 shown in fig6 . 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 fig1 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 fig1 . 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 fig1 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 fig1 a . had the prototypes been coated , as in fig1 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 .