Abstract:
According to aspects described herein, there is disclosed an improved hot balloon catheter delivery system for shape memory tubular stents is disclosed. An outer sheath is provided on the delivery system that facilitates stent placement on a balloon during delivery through an endoscope. In one embodiment, the stent may be positioned between radiopaque markers/electrodes and the balloon may be heated. The stent may be prevented from moving proximally relative to the catheter as the delivery system is tracked through the patient&#39;s anatomical passageways. The outer sheath may be positioned to ride over the proximal cone of the hot balloon catheter and abut the proximal end of the stent to facilitate proper alignment of the balloon for deployment. Upon deployment of the stent in an appropriate position, the sheath may be pulled away from the stent to expose the proximal cone of the balloon and thereby allow balloon and stent expansion.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/021,064 filed on Jan. 15, 2008. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosed technologies generally relate to stent delivery systems and more particularly to an outer sheath addition to hot balloon catheter delivery systems for shape memory tubular stents. 
       BACKGROUND 
       [0003]    A stent is generally a longitudinal tubular device formed of biocompatible material, which is useful to open and support various lumens in the body. Stents are used in various bodily passageways, such as in the coronary or peripheral vasculature, esophagus, trachea, bronchi, colon, biliary tract, urinary tract, prostate, brain, as well as in a variety of other applications in the body. These devices are implanted within the passage to open and/or reinforce collapsing or partially occluded sections. While stents and other intraluminary prosthesis are foreign objects to the human body, they are designed to remain within a body passageway for prolonged periods or even indefinitely. 
         [0004]    Stents generally include a flexible configuration. This configuration allows the stent to be inserted through curved vessels or lumens (natural or artificial) and curved instruments, such as an endoscope. Such stents are generally delivered using a flexible elongate catheter. The catheters are designed to resist kinking and may be developed with various levels of stiffness. The stents are designed and configured to radially compress for intraluminary catheter implantation. Once properly positioned adjacent the damaged vessel or lumen, the stent is radially expanded so as to support and reinforce the vessel or lumen. Radial expansion of the stent may be accomplished by inflation of a balloon or other expanding means attached to the catheter or the stent may be of the self-expanding variety which will radially expand once deployed. 
         [0005]    Tubular shaped structures, which have been used as intraluminary vascular stents, have included helically wound coils which may have undulations or zigzags therein, slotted stents, ring stents, braided stents and open mesh wire/filament stents, to name a few. Additionally, super-elastic and shape memory materials using plastic and/or metals have also been used to form stents. 
         [0006]    Diagnosis and treatment in patients of biliary strictures caused by malignant or benign biliary disease is treatable. A patient with biliary disease typically suffers from an obstruction in the pancreaticobiliary ductal system. Numerous diseases can cause the inability of bile flow, however, the presence of gallstones and/or strictures appear the most prevalent reason. For benign strictures, stenting is one possible solution. In particular, tubular stents are used in bile, pancreatic and hepatic ducts where a solid stent is needed and where in-growth can occur due to proliferating oncologous cells. 
         [0007]    Solid stents are often mounted on a balloon catheter and delivered non-invasively, such as through an endoscope, and deployed by expanding the stent to a larger diameter. For example, U.S. Pat. No. 5,191,883 to Lennox et al. and U.S. Pat. No. 6,830,559 to Schock describe balloon catheter delivery systems. Alternatively, the stent is slipped off a catheter with no expansion. For example, see U.S. Pat. No. 5,334,185 to Giesy et al. These stents are typically designed to withstand drastic deformations as it is conveyed through the twists and turns of the tortuous anatomy of a patient&#39;s gastrointestinal system. 
         [0008]    One drawback, however, is that as the delivery system is tracked through the endoscope, the stent may slide out of its intended position. In particular, a stent may slide proximally and may no longer be disposed between radiopaque markers on and/or the electrodes used to expand the balloon located on the delivery system. Thus, once out of position, the physician may find it difficult to accurately position the stent. Also, the stent may be difficult to properly expand if improperly positioned on the balloon. 
         [0009]    Additionally, even when the stent remains properly positioned on the delivery system, removal of the balloon after delivery can sometimes be problematic. After the stent has been fully expanded, the balloon is generally deflated before being removed. While or after being deflated, however, the balloon may get stuck to the stent. Sticking can occur due to anatomical curvatures in the delivery location which pinch or trap the balloon, stent to balloon material tackiness, a balloon wing fold getting hung-up and/or other reasons. During removal, if the balloon is stuck to the stent it can slightly pull the stent out of place or even completely out of position. 
         [0010]    Therefore, there is a need for improved delivery systems for stents deployed using an expansion member, such as a balloon catheter. Such a system preferably overcomes the shortcomings set forth above while being relatively easily manufactured and use. 
       SUMMARY 
       [0011]    An improved hot balloon catheter delivery system for shape memory tubular stents is disclosed. An outer sheath is provided on the delivery system that facilitates stent placement on a balloon during delivery through an endoscope. In one embodiment, the stent may be positioned between radiopaque markers/electrodes and the balloon may be heated. The stent may be prevented from moving proximally relative to the catheter as the delivery system is tracked through the patient&#39;s anatomical passageways. The outer sheath may be positioned to ride over the proximal cone of the hot balloon catheter and abut the proximal end of the stent to facilitate proper alignment of the balloon for deployment. Upon deployment of the stent in an appropriate position, the sheath may be pulled away from the stent to expose the proximal cone of the balloon and thereby allow balloon and stent expansion. 
         [0012]    One aspect of the disclosed technology includes an apparatus for delivering an expandable stent into a patient through a body lumen. The apparatus includes a flexible catheter, an expansion member, a control assembly and a sheath member. The flexible catheter is for delivering the stent. The catheter may have a stent delivery portion at a distal end thereof. The expansion member may be secured to the stent delivery portion of the catheter. The expansion member may be adjustable between a collapsed state and an expanded state. The expandable stent is mounted on and circumferentially surrounds at least a portion of the expansion member, whereby adjustment of the expansion member from the collapsed state to the expanded state causes radial expansion of the stent relative to a longitudinal axis of the stent. The control assembly may actuate the expansion member between the collapsed and the expanded states. The sheath circumferentially surrounds a portion of the catheter for limiting movement of the stent in relative to the catheter during delivery. 
         [0013]    Another aspect of the disclosed technology includes an assembly for delivering a stent into a patient through a body lumen. The assembly includes an expandable stent, a flexible catheter, an expansion member, a control assembly and a sheath. The flexible catheter is for delivering the stent. The expansion member may be secured to a distal end of the catheter. The expansion member may be adjustable between a collapsed state and an expanded state. Also, the stent may be mounted on at least a portion of the expansion member, whereby adjustment of the expansion member from the collapsed state to the expanded state causes radial expansion of the stent relative to a longitudinal axis of the stent. The control assembly may actuate the expansion member between the collapsed and the expanded states. Also, the sheath circumferentially surrounds a portion of the catheter for limiting movement of the stent relative to the catheter. 
         [0014]    Additionally, the expansion member may comprise a balloon. The balloon can include two opposed ends that when inflated during delivery are wider than a central portion of the balloon. Further, the sheath can be moveable along a length of the catheter. The sheath can be securable relative to the catheter for limiting the stent movement. Also, a distal end of the sheath can abut a proximal end of the stent. Further a distal end of the sheath can be radially wider than a central portion of the sheath. A distal end of the sheath can flare radially outwardly and/or include one or more slits passing through a wall of the sheath. 
         [0015]    Yet another aspect of the disclosed technology includes a method for delivering an expandable stent into a patient through a body lumen. The method comprises providing a flexible balloon catheter for delivering the stent. The catheter includes an expandable balloon, a control assembly and an outer sheath. The balloon may be secured to a distal end of the catheter and may be adjustable between a collapsed state and an expanded state. The control assembly may adjust the expansion member between states. Additionally, the method comprises mounting a stent on the balloon in the collapsed state. A distal end of the sheath may abut a proximal end of the stent for limiting movement of the stent relative to the catheter. Additionally, the method comprises inserting the flexible catheter and stent into a patient through a body lumen, while maintaining a fixed relative position between the stent and the catheter. Then, the control assembly is activated to expand the stent. Thereafter, the balloon catheter may be withdrawn once the stent has expanded. The method can further include the step of advancing the sheath distally inside the stent prior to withdrawing the catheter. 
         [0016]    These and other embodiments, features, and advantages of the disclosed technologies will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a perspective view of one aspect of a stent delivery system according to the disclosed technology. 
           [0018]      FIGS. 2A-D  are sectional views of one embodiment the distal end of the stent delivery system during progressive phases of releasing and delivering a stent according to the disclosed technology. 
           [0019]      FIG. 3  is a perspective view of one embodiment the distal end of the moveable sheath according to the disclosed technology. 
           [0020]      FIG. 4  is a perspective view of one embodiment the distal end of the moveable sheath according to an alternative embodiment of the disclosed technology. 
           [0021]      FIG. 5  is a perspective view of one embodiment the distal end of the moveable sheath according to another alternative embodiment of the disclosed technology. 
           [0022]      FIG. 6  is a perspective view of one embodiment the distal end of the moveable sheath according to another alternative embodiment of the disclosed technology. 
           [0023]      FIG. 7  is a perspective view of one embodiment the distal end of the moveable sheath according to another alternative embodiment of the disclosed technology. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    The invention relates to an apparatus, system and method for insertion and/or delivery of a stent in a bodily passageway. The invention allows the clinician or operator to more accurately deliver the stent by minimizing drift or a shift of position of the stent while passing through body lumens prior to delivery. The assembly is suited for medical applications (for example, endoscopic therapy) including in the biliary tract, the gastrointestinal tract, the urinary tract, and the respiratory tract. In particular, one embodiment of the invention is directed to an assembly, system and method for delivering an intraluminary prosthesis, such as a stent in a biliary duct. Additionally, an assembly in accordance with the disclosed technology could be used in the vascular system, urogenital tract, neurological system (e.g., in the brain) and in the cardiovascular system (e.g., in the heart) and artificially created lumens. Reference herein to bodily passageways or lumens may be to vessels or passageways in any of the aforementioned tracts and systems or elsewhere in the body. While discussed below primarily in connection with delivering shape memory stents, the apparatus and systems of the disclosed technology can also be used to deliver other types of stents and prostheses (e.g., stent-grafts). Further, it should be understood that stents, as referred to herein, are not limited to tube like structures. 
         [0025]    It should be noted that references herein to the term “distal” are to a direction away from an operator of the subject invention, while references to the term “proximal” are to a direction towards the operator of the subject invention. Accordingly, when the terms “distal” and “proximal” are used herein in the context of an assembly device that is being deployed within a body, such as a human body, by an operator, the term “distal” refers to a location within the body that is further within the body than a location that is “proximal” to the operator. 
         [0026]    With reference to the drawings,  FIG. 1  illustrates an apparatus  100 , which is a catheter delivery system used in some embodiments for shape memory tubular stents. Alternatively, the apparatus  100  can be used with non-shape memory stents that soften with the application of heat or whose shape is manipulated in other ways. Apparatus  100  includes a flexible catheter  10 , a sheath member  20 , an expansion member  30  and control elements  40  for activating and deactivating the expansion member  30 . The middle section of the catheter  10  is removed from the illustration in order to show more detail of the proximal and distal ends of the apparatus  100 . At the distal end, mounted on the expansion member  30  is an expandable stent  50  intended for insertion within a patient. 
         [0027]    The elongate catheter  10  may be a flexible tube that is inserted into a body lumen or duct and is capable of reaching a target stent delivery location. Also, the catheter  10  may include at least one inner passage allowing drainage/injection of fluids and/or access by surgical instruments or wiring. The proximal end of the catheter  10  may be provided with openings  12  communicating with one or more passages running through the catheter  10 . In this way, an inner passage can be dedicated to fluid flow, while other passages can be dedicated to wires or other control elements for the expansion member  30  at the distal end. 
         [0028]    The sheath member  20  may surrounds and extends along a substantial portion of the catheter  1 O. At its proximal end the sheath member  20  may include a handle  28  which may allow an operator to manipulate and control the sliding movement of the sheath  20  relative to the catheter  10 . The handle  28  and inner catheter  10  may include locking features (not illustrated) for at least temporarily securing those two elements relative to one another. Sheath  20  may be designed for entry into the body, and transluminal travel to a selected site in a body passageway. To this end, the sheath can be pliable in the sense that it readily bends to accommodate curved or serpentine luminal passages, yet has sufficient axial stiffness to allow control of the transluminal travel from the proximal end of the sheath. An inner sheath lumen can run longitudinally through the length of the sheath  20 . 
         [0029]    Also, sheath  20  can be preferably formed of a body compatible material. Desirably, the biocompatible material is a biocompatible polymer. Examples of suitable biocompatible polymers include, but are not limited to, polypropylene (PP), polytetrafluoroethylene (PTFE), polyethylene terephthalate (PET), high density polyethylene (HDPE) and the like. Additionally, the sheath  20  could have layers, coverings, films, coatings, and the like, desirably a polymeric covering, disposed over the outer or inner surfaces to improve interaction with the body lumen and the inner elements discussed below. Further, the sheath  20  or select portions thereof could be clear, opaque, translucent, colored and/or non-transparent as desired. 
         [0030]    Additionally, it should be understood that the materials of the sheath  20  as well as any component filaments of the sheath  20  can be further enhanced with coverings, films, coatings, surface modifications and/or structures and other materials and techniques. A covering may be composed of polymeric material and/or silicone. The covering may also comprise any plastic or polymeric material, including a somewhat hard but flexible plastic or polymeric material. The covering may be transparent or translucent, desirably substantially or partially transparent. Furthermore, the covering may be constructed of any suitable biocompatible materials, such as, but not limited to, polymers and polymeric materials, including fillers such as metals, carbon fibers, glass fibers or ceramics. Useful covering materials include, but are not limited, polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, including expanded polytetrafluoroethylene (ePTFE), fluorinated ethylene propylene, fluorinated ethylene propylene, polyvinyl acetate, polystyrene, poly(ethylene terephthalate), naphthalene dicarboxylate derivatives, such as polyethylene naphthalate, polybutylene naphthalate, polytrimethylene naphthalate and trimethylenediol naphthalate, polyurethane, polyurea, silicone rubbers, polyamides, polyimides, polycarbonates, polyaldehydes, polyether ether ketone, natural rubbers, polyester copolymers, styrene-butadiene copolymers, polyethers, such as fully or partially halogenated polyethers, and copolymers and combinations thereof. The coating or coatings may be on the entire sheath  20  or portions thereof. 
         [0031]    The expansion member  30  can be preferably a balloon element that can be inflated and deflated in a controlled manner in order to deploy the stent  50 . Examples of balloons used as expansion members  30  are described in U.S. Pat. No. 5,191,883 to Lennox et al. (Lennox) and U.S. Pat. No. 6,830,599 to Schock. The activation of the expansion member  30  from a contracted state to an inflated/expanded state and vise-versa may be regulated by control elements  40 . Only a distal portion of the overall control elements  40  are illustrated in the drawings. In particular, the two elements  40  illustrated in  FIGS. 1-2  represent electrodes for activating the expansion member  30 . At least one electrode is needed for this manner of delivery. Such electrodes are preferably part of a contemporary hot-balloon catheter control system as described in Lennox. However, it should be understood that other balloon catheter techniques or even other mechanical equivalents can be used as an expansion member with control elements in accordance with the disclosed technology. 
         [0032]    Additionally, the elements  40  also may include radiopaque markers. The radiopaque markers can either be incorporated with the electrodes described above or disposed in proximity thereto. Preferably, a pair of electrodes and a pair of markers are included in the distal end of the catheter  10 . The electrodes can also be radiopaque, thus eliminating the need for separate markers. The markers  40  can assist the physician or operator in visualizing the relative position of the delivery portion of the apparatus  100  with relation to the target delivery location. Such markers  40  can be formed from radiopaque materials, such as metallic-based powders or ceramic-based powders, particulates or pastes which may be incorporated into the distal catheter material. For example, the radiopaque material may be blended with the polymer composition from which the polymeric filament is formed, and subsequently fashioned into a portion of the catheter  10 . Alternatively, the radiopaque material may be applied only to the surface of the catheter  10 . As a further alternative, the radiopaque material may be attached to the stent (for example, attached or clipped-on mechanically or chemically through adhesives, etc.) Various radiopaque materials and their salts and derivatives may be used including, without limitation, bismuth, barium and its salts such as barium sulfate, tantalum, tungsten, gold, platinum and titanium, to name a few. Additional useful radiopaque materials may be found in U.S. Pat. No. 6,626,936, which is herein incorporated by reference. Metallic complexes useful as radiopaque materials are also contemplated. The markers  40  may be the only portion made radiopaque or the entire distal portion of the catheter  10  can be made radiopaque. Further, the distal portion of the catheter  10  may have an inner core of tantalum, gold, platinum, iridium or combination of thereof and an outer member or layer of nitinol to provide a composite filament for improved radiocapicity or visibility. Alternatively, portions of the catheter  10  may designed for improved external imaging under magnetic resonance imaging (MRI) and/or ultrasonic visualization techniques. MRI is produced by complex interactions of magnetic and radio frequency fields. Materials for enhancing MRI visibility include, but are not limited to, metal particles of gadolinium, iron, cobalt, nickel, dysprosium, dysprosium oxide, platinum, palladium, cobalt based alloys, iron based alloys, stainless steels, or other paramagnetic or ferromagnetic metals, gadolinium salts, gadolinium complexes, gadopentetate dimeglumine, compounds of copper, nickel, manganese, chromium, dysprosium and gadolinium. To enhance the visibility under ultrasonic visualization the area represented by markers  40  of the disclosed technology may include ultrasound resonant material, such as but not limited to gold. 
         [0033]    Also illustrated in  FIG. 1  is the stent  50 . The stent  50  can be formed of plastics, metals or other materials, it is preferred that a biocompatible construction is employed. Useful synthetic biocompatible polymeric materials include, but are not limited to, polyesters, including polyethylene terephthalate (PET) polyesters, polypropylenes, polyethylenes, polyurethanes, polyolefins, polyvinyls, polymethylacetates, polyamides, naphthalane dicarboxylene derivatives, silks and polytetrafluoroethylenes. The polymeric materials may further include a metallic, a glass, ceramic or carbon constituent or fiber. Useful and nonlimiting examples of bioabsorbable or biodegradable polymeric materials include poly(L-lactide) (PLLA), poly(D,L-lactide) (PLA), poly(glycolide) (PGA), poly(L-lactide-co-D,L-lactide) (PLLA/PLA), poly(L-lactide-co-glycolide) (PLLA/PGA), poly(D,L-lactide-co-glycolide) (PLA/PGA), poly(glycolide-co-trimethylene carbonate) (PGA/PTMC), polydioxanone (PDS), Polycaprolactone (PCL), polyhydroxybutyrate (PHBT), poly(phosphazene) poly(D,L-lactide-co-caprolactone) PLA/PCL), poly(glycolide-co-caprolactone) (PGA/PCL), poly(phosphate ester) and the like. Useful biocompatible materials include but are not limited to biocompatible metals, biocompatible alloys, biocompatible polymeric materials, including synthetic biocompatible polymeric materials and bioabsorbable or biodegradable polymeric materials, materials made from or derived from natural sources and combinations thereof. Useful biocompatible metals or alloys include, but not limited to, nitinol, stainless steel, cobalt-based alloy such as Elgiloy, platinum, gold, titanium, tantalum, niobium, polymeric materials and combinations thereof. These alloys may also be doped with small amounts of other elements for various property modifications as may be desired and as is known in the art. Further, the stent  50  may include materials made from or derived from natural sources, such as, but not limited to collagen, elastin, glycosaminoglycan, fibronectin and laminin, keratin, alginate, combinations thereof and the like. Alternatively, a portion of the stent  50  may comprise a resilient spring-like metal for self-expansion, while a second portion is a deformable metal for final sizing. It should be understood that shape memory elements may be discrete or merely different phases of an alloy. 
         [0034]    Referring to  FIG. 2A-2B , apparatus  100  is illustrated in various stages of deployment of stent  50 .  FIG. 2A  illustrates the distal end of the apparatus  100  with the stent  50  mounted thereon for delivery. Preferably, the configuration illustrated may be maintained while the physician inserts and tracks the apparatus  100  to the desired location within the patient. The expansion member  30 , in the form of an expandable balloon may be secured at two ends  30   a  and  30   b  to the catheter  10 . The expansion member  30  is illustrated in a deflated/collapsed state. The expansion member  30  may be positioned to cover and contain the electrodes  40 , which are part of the control assembly for actuating the expansion of the balloon  30 . The location of each electrode  40  may include radiopaque markers. In this way, the area between the two markers  40  represents the delivery portion of the catheter. Thus, by mounting the stent  50  over the delivery portion, the physician can use the markers  40  to track and accurately position the stent  50  in the patient. 
         [0035]    Also illustrated in  FIG. 2A  is sheath member  20  disposed radially adjacent and axially surrounding the stent  50 . Prior to insertion within the patient, distal sheath end  20   a  may be positioned to abut the proximal stent end  50   a,  as shown. Alternatively, a distal portion of the sheath  20  can be configured to hold and axially surround the stent  50 , expansion member  30  and the distal end of the catheter  10 . In this alternative design, the sheath  20  can then be pulled back to the position illustrated in  FIG. 2A , just prior to expansion of the stent  50 . Once positioned as illustrated in  FIG. 2A , preferably either design allows the sheath handle  28  to be secured relative to the catheter  10  in order to limit axial movement of the stent  50  relative to the delivery portion. 
         [0036]      FIG. 2B  illustrates the expansion member  30  expanded, thereby expanding the stent  50  to a desirable radius R. It should be noted that while radius R, illustrated in  FIG. 2B , is preferably larger than the smaller compressed radius r, illustrated in  FIG. 2A , the actual dimensions can vary from those illustrated in the drawings. In other words, the dimensions illustrated are for exemplary purposes only, and the actual dimensions would be designed and configured to conform to the intended stent application. The control assembly  40  is carefully regulated to ensure the requisite expansion dimensions for the stent  50 . During expansion, it may be desirable to use a balloon  30  that expands into a dog-bone shape, as illustrated, in order to further maintain the stent  50  generally centered between the markers  40 . Alternatively, a cage, basket or other device could be used to expand the stent  50 . 
         [0037]      FIG. 2C  illustrates the apparatus  100  after the stent  50  has been expanded and the expansion member  30  has been partially contracted or deflated. Also illustrated is an example of when expansion member  30  can get caught or stuck on the stent  50 . To assist in separating the expansion member  30  from the stent  50 , the sheath  20  can be advanced distally into the expanded stent  50 . This may help pull the expansion member  30  away from the stent  50  without moving the stent  50  out of position. Once the sheath  20  is advanced distally beyond the stent or when the physician is confident the expansion member  30  is not stuck to the stent  50 , the entire delivery system can be pulled back, as illustrated in  FIG. 2D , or completely withdrawn. In this way, the stent  50  should remain in its intended delivery location and the apparatus  100  can be safely removed. Additionally, prior to removal the sheath  20  could be used to deliver relatively hot, cold or thermally neutral fluids. 
         [0038]      FIGS. 3-7  illustrates various embodiments of the distal end of the sheath member  20 .  FIG. 3  illustrates a sheath member  20  with a continuous circular cross-section.  FIG. 4  illustrates a sheath member  21  whose distal end  21   a  bulges radially outward. The bulging end  21   a  can further assist in separating the stent  50  from the expansion member  30  after delivery. 
         [0039]      FIG. 5  illustrates a sheath member  22  whose distal end  22   a  flares radially outwardly, like a funnel or cone. A flared distal end  22   a  can alternatively be used as an anchor against the proximal end of the expanded stent  50 . Thus, the sheath  22  can prevent further stent movement as the expansion member  30  is pulled back inside the sheath  22 . It should be understood that the pitch and length of the flared distal end  22   a  can vary as suited for the particular stent and delivery conditions. 
         [0040]      FIGS. 6 and 7  illustrate further alternative sheath members  23 ,  24  whose distal ends include cutouts or slits  23   a,    24   a.  The slit  23   a  may have a rounded end while the slit  24   a  may have a straight taper. However, it should be understood that slits with other shapes could also be advantageous. The slits  23   a,    24   a  can further assist in separating the expansion member  30  from the expanded stent  50 . Additionally, the slits  23   a,    24   a  can used to maintain a desired rotational relationship between the stent  50  and the delivery system  100 . In other words, in addition to preventing the stent from sliding axially in the proximal direction relative to the catheter  10 , the slit can prevent the stent from rotating relative to the catheter  10  via the expansion member  30 . Distal end of sheath may be rounded to assist forward movement. 
         [0041]    Further, it should be understood that further techniques can be used in conjunction with the apparatus, system and methods described above, to ensure and facilitate accurate stent delivery. For example, altering balloon materials, applying a lubricant between the balloon and stent and/or leaving out pre-shrinking when mounting the stent on the balloon, can reduce sticking between the stent and the balloon. Similarly, changing how the balloon is folded into its compressed state, such as reducing or reorienting the wing folds of the balloon, can further assist in this regard. Sheath may have varying stiffness, varying reinforced layers, reinforcement, varying wall thickness, varying diameters, varying cross-sectional geometry. 
         [0042]    While various embodiments of the present invention are specifically illustrated and/or described herein, it will be appreciated that modifications and variations of the present invention may be effected by those skilled in the art without departing from the spirit and intended scope of the invention.