Abstract:
A catheter system for removably securing a stent which generally includes an inner tubular member and an outer tubular member with an expandable member disposed about the inner tubular member. A foamed cover is disposed between the expandable member and the inner tubular member. An expandable stent is crimped onto the expandable member and is retained in place by compressing the foamed cover. Used with a stent delivery system, the catheter system is inserted into a body lumen such as an artery. The stent is secured in place while advancing the system through tortuous body lumen passages. The stent is advanced to the desired location in the body lumen and implanted by inflating the expandable member and thereby expanding the stent into the body lumen. The stent is released from the system by deflating the expandable member and withdrawing the catheter system from the body.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to devices for the treatment of heart disease and particularly to endo-arterial prosthesis, which are commonly called stents. More particularly, the invention relates to catheter assemblies for removably securing the stent to the catheter during delivery through a body lumen. 
     Several interventional treatment modalities are presently used for heart disease including balloon and laser angioplasty, atherectomy and by-pass surgery. In typical balloon angioplasty procedures, a guiding catheter having a preformed distal tip is percutaneously introduced through the femoral artery into the cardiovascular system of a patient in a conventional Seldinger technique and advanced within the cardiovascular system until the distal tip of the guiding catheter is seated in the ostium. A guide wire is positioned within an inner lumen of a dilatation catheter and then both are advanced through the guiding catheter to the distal end thereof. The guide wire is first advanced out of the distal end of the guiding catheter into the patient&#39;s coronary vasculature until the distal end of the guide wire crosses a lesion to be dilated, then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient&#39;s coronary anatomy over the previously introduced guide wire until the balloon of the dilatation catheter is properly positioned across the lesion. Once in position across the lesion, the balloon, which is made of relatively inelastic materials, is inflated to a predetermined size with radiopaque liquid at relatively high pressure (e.g., greater than 4 atmospheres) to compress the arteriosclerotic plaque of the lesion against the inside of the artery wall and to otherwise expand the inner lumen of the artery. The balloon is then deflated so that blood flow can be resumed through the dilated artery and the dilatation catheter can be removed therefrom. Further details of dilatation catheters, guide wires, and devices associated therewith for angioplasty procedures can be found in U.S. Pat. No. 4,323,071 (Simpson-Robert); U.S. Pat. No. 4,439,185 (Lindquist); U.S. Pat. No. 4,516,972 (Samson); U.S. Pat. No. 4,538,622 (Samson, et al.); U.S. Pat. No. 4,554,929 (Samson, et al.); U.S. Pat. No. 4,616,652 (Simpson); U.S. Pat. No. 4,638,805 (Powell); U.S. Pat. No. 4,748,982 (Horzewski, et al.); U.S. Pat. No. 5,507,768 (Lau, et al.); U.S. Pat. No. 5,451,233 (Yock); and U.S. Pat. No. 5,458,651 (Klemm, et al.), which are hereby incorporated herein in their entirety by reference thereto. 
     One problem which can occur during balloon angioplasty procedures is the formation of intimal flaps which can collapse and occlude the artery when the balloon is deflated at the end of the angioplasty procedure. Another problem characteristic of balloon angioplasty procedures is the large number of patients which are subject to restenosis in the treated artery. In the case of restenosis, the treated artery may again be subjected to balloon angioplasty or to other treatments such as by-pass surgery, if additional balloon angioplasty procedures are not warranted. However, in the event of a partial or total occlusion of a coronary artery by the collapse of a dissected arterial lining after the balloon is deflated, the patient may require immediate medical attention, particularly in the coronary arteries. 
     A focus of recent development work in the treatment of heart disease has been directed to endoprosthetic devices called stents. Stents are generally cylindrically shaped intravascular devices which are placed within an artery to hold it open. The device can be used to prevent restenosis and to maintain the patency of a blood vessel immediately after intravascular treatments. In some circumstances, they can also be used as the primary treatment device where they are expanded to dilate a stenosis and then left in place. 
     One method and system developed for delivering stents to desired locations within the patient&#39;s body lumen involves crimping a stent about an expandable member, such as a balloon on the distal end of a catheter, advancing the catheter through the patient&#39;s vascular system until the stent is in the desired location within a blood vessel, and then inflating the expandable member on the catheter to expand the stent within the blood vessel. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel, holding open the passageway thereof. 
     However, retaining the position of the stent in the proper location on the expandable member while advancing the catheter through the body lumen has been found to be difficult. If the stent is dislodged from or moved on the expandable member the system will not correctly deliver the stent into the body lumen. This would require repeating the procedure. This delays insertion of the stent into the body lumen and lengthens the time of the stenting procedure. 
     Different methods have been attempted to maintain the position of the stent on the expandable member. One such method involves a protective sheath surrounding the catheter and stent assembly, which is retracted prior to inflation of the expandable member. The use of the sheath, however, increases the profile of the catheter assembly which must traverse narrow vessels and the sheath reduces the flexibility of the distal end of the delivery catheter. It would be an improvement to use a technique which does not increase the overall profile of the catheter assembly and maintains the flexibility of the delivery catheter. 
     Another method has been to remove the friction reducing coating on the expandable member in the location of the stent thereby allowing the catheter assembly&#39;s pre-coated surface to hold the stent in frictional contact. This method has not proven highly efficient in maintaining the stent in the desired location. 
     What has been needed and heretofore unavailable is a highly efficient means of maintaining a stent in a desired location on a stent delivery system without increasing the overall profile of the catheter assembly or compromising the flexibility. The present invention satisfies this need. 
     SUMMARY OF THE INVENTION 
     This invention is directed to an improvement in stent delivery systems for removably securing a stent onto an expandable member. Securing the stent is accomplished by tightly crimping the stent onto the balloon portion of a catheter wherein a deformable foamed material has been disposed. By compressing the deformable foamed material the stent is secured due to the increased frictional contact and interference between the stent and the compressed catheter assembly. 
     Stent delivery systems are typically composed of an elongated tubular member, or catheter, encompassed by an expandable member such as an inflatable balloon. The stent is located about the expandable member so that the two can be expanded together. The improvement of this invention includes introducing a compressible foamed cover into the space between the tube member and the expandable member. The foamed cover may be adhered to the elongated tube member, the expandable member, or injected into the space therebetween. The stent is then crimped onto the expandable member so that the foamed cover is compressed. The foamed cover may be compressed with sufficient force to form pockets of material within the foamed cover corresponding to gaps within the structure of the stent. The foamed cover may also extend beyond one or both of the longitudinal ends of the stent so that sufficient crimping force can be applied on the expandable stent to form ridges in the foamed cover located beyond the ends of the stent. 
     This invention can be used with the known configurations of stent delivery catheter assemblies including over-the-wire (OTW) intravascular catheters and rapid exchange (Rx) intravascular catheters. 
     The foamed cover may be composed of any soft polymer, such as polyurethane elastomers, polyamide block copolymers, polyester block copolymers and polyolefins. It is contemplated that the foamed cover is softer and more flexible than when in its solid form. The foam density, cell openness and cell size can be optimized to provide desired stent security and desired profiles. 
     The present invention results in a simplified method of inserting the stent into the body lumen. The catheter assembly is inserted into the body lumen without further steps being taken to secure the stent. The expandable member is inflated at the desired location expanding and implanting the stent within the body lumen. When the expandable member is then deflated, the stent is released and the remainder of the catheter assembly may be withdrawn leaving the stent implanted within the body lumen. 
     Other features and advantages of the invention will be come apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view depicting a stent delivery system depicting elements of the invention prior to crimping. 
     FIG. 2 is a cross-sectional view taken along lines  2 — 2  in FIG.  1 . 
     FIG. 3 is a cross-sectional view taken along line  3 — 3  in FIG.  1 . 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  in FIG.  1 . 
     FIG. 5 is a cross-sectional view of the corresponding area in FIG. 1 after crimping the stent onto the expandable member of the catheter. 
     FIG. 6 is a cross-sectional view taken along lines  6 — 6  in FIG.  5 . 
     FIG. 7 is a partial cross-sectional view depicting a stent delivery system configured as a rapid exchange catheter incorporating the invention. 
     FIG. 8 is a cross-sectional view taken along lines  8 — 8  in FIG.  7 . 
     FIG. 9 is a cross-sectional view taken along lines  9 — 9  in FIG.  7 . 
     FIG. 10 is a cross-sectional view taken along the lines  10 — 10  shown in FIG.  7 . 
     FIG. 11 is a plan view of a stent delivery system having a rapid exchange configuration and including elements of the invention. 
     FIG. 12 is a longitudinal cross-sectional view of a body lumen depicting an elevational view of a partially inserted stent delivery system. 
     FIG. 13 depicts a longitudinal cross-sectional view of a body lumen and an elevational view of a stent delivery system where the stent has been expanded. 
     FIG. 14 is a cross-sectional view taken along lines  14 — 14  in FIG.  13 . 
     FIG. 15 is a cross-sectional view depicting a partially withdrawn stent delivery system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 through 6 illustrate a catheter assembly with a removably secured stent which embodies the features of the invention. Generally, catheter assembly  8  includes inner tubular member  10  and outer tubular member  11  aligned about the inner tubular member. The outer tubular member has expandable member  16  incorporated within the member, located adjacent the distal end of the inner tubular member. The inner tubular member and the expandable member define annular space  22  between them. A foamed inner cover  24  is located in annular space  22 , preferably connected to outer surface  12  of inner tubular member  10 . An expandable stent  26  is positioned around expandable member  16 . When stent  26  is tightly crimped about outer surface  20  of expandable member  16 , inner surface  18  of the expandable member will compress foamed cover  24 . This configuration secures expandable stent  26  in the proper location while the stent is inserted into a body lumen by increasing the frictional contact between the expandable stent and outer surface  20  of the expandable member due to the compression of foamed cover  24 . This configuration presents an improvement to stent delivery systems by increasing the efficiency of delivering and implanting the stent in the body lumen. 
     Stent delivery systems are typically composed of inner tubular member  10  and outer tubular member  11  with inflatable means, such as expandable members, attached to the distal end. The expandable member  16  preferably is attached to or is one piece and formed from the outer member, and is aligned about the inner tubular member, thereby defining an annular space  22  between the two. This allows inflation fluid to inflate the expandable member. Crimping expandable stent  26  about the expandable member allows the two to be expanded together. The improvement of this invention is to removably retain the stent onto the expandable member by applying a foamed cover within the annular space. 
     Expandable stents  26  are generally composed of a lattice configuration which defines gaps  28  within the stent&#39;s body. In one embodiment of the invention, expandable stent  26  is crimped about expandable member  16  and foamed cover  24  with sufficient force to cause pockets of material  30  to form in the foamed cover and which correspond with gaps  28  in the stent. This pocketing further enhances the retention of the expandable stent by increasing the interference between the stent and the combination of the expandable member and the foamed cover. This configuration is most clearly illustrated at FIG.  6 . 
     Another embodiment of the invention is to extend the foamed inner cover  24  within the annular space beyond the location where one or both of longitudinal ends  42 , 44  of expandable stent  26  will be located on outer surface  20  of expandable member  16 . Alternatively a pair of spaced-apart foamed covers  24  may be employed and positioned at each of the longitudinal ends  42 , 44  of the expandable stent (not shown). plurality of longitudinally arranged foamed strips or foamed rings are also contemplated. 
     The expandable stent can then be crimped about the expandable member and the foamed cover  24  with sufficient force to cause ridges  46 , 48  to be formed beyond one or both ends of the stent. These ridges  46 , 48  further enhance the retention of the expandable stent by increasing the interference between the stent and the combination of the expandable member and the foamed cover. Another advantage of this embodiment is in increasing the diameter of the expandable member distal to the stent. The increased diameter just distal to the stent helps to hold the stent on the expandable member as the delivery catheter is advanced through sometimes tortuous body lumens. 
     This stent delivery catheter assembly can be configured by modifying commonly known stent delivery systems. On many of these systems, the expandable member  16  is an inflatable balloon. FIGS. 1 through 6 show the catheter assembly configured as an over-the-wire (OTW) intravascular catheter  34 . FIGS. 7 through 10 and FIGS. 11 through 15 show the catheter assembly configured as a rapid exchange (Rx) intravascular catheter  36 . The Rx configuration typically requires fluid inflation lumen  40  to be positioned within inner tubular member  10 , which allows inflation fluid access into annular space  22 , through port  60  for inflation of expandable member  16 . 
     The foamed cover  24  may be attached to outer surface  12  of inner tubular member  10  as shown in the figures. Another embodiment, not shown in the figures, is to inject the foamed cover into annular space  22  without attaching it to any surface. Another embodiment, not shown in the figures, is to attach the foamed cover onto inner surface  18  of expandable member  16 . When the catheter assembly is configured as an Rx intravascular catheter  36  and the foamed cover is attached to outer surface  12  of inner tubular member  10 , the foamed cover is configured so as not to obscure port  60  or the fluid communication between annular space  22  and fluid inflation lumen  40  of inner tubular member  10 . 
     The foamed inner cover  24  is intended to provide conformability to the stent/balloon profile as well as a gentle force to oppose the expandable member  16  when a stent  26  is crimped thereon. Moreover, the foamed cover  24  provides a locking force that facilitates keeping part of the expandable member  16  between stent struts. 
     The foamed cover  24  may be formed from generally available materials ncluding any soft polymer such as polyurethane and polyethylene. This material can be pre-formed into a thin-walled cylinder (about 0.010-0.030 inch wall thickness) and then placed upon inner tubular member  10 . This cylinder can be trimmed to the same length or slightly longer than the stent length. 
     The foam density, cell openness and cell size of the foamed inner cover  24  can be optimized to provide appropriate properties to serve the purposes of either stent security or profile. The foam density can be lowered to 20-80% of its solid density to offer a cushion for stent crimping. The cells can be formed either open or closed. The foamed cover  26  can have a thickness in the uncompressed state in the range of 0.002 to 0.02 inch for some applications such as for use in the coronary arteries, however, the size range would be much larger for use in saphenous veins or aortic applications. The amount of mass and profile added to the system can be calculated to optimize the overall system. 
     During manufacturing, the foamed cover  24  tube is first produced and measured. Its inner diameter should match the outer diameter of the inner tubular member  10 . The foamed cover tube is slid over the inner tubular member  10  either between the expandable member  20  markers or along the length of the whole expandable member  20 . One or two drops of adhesive can be applied to secure the cover on the inner tubular member  10 . After the expandable member  20  is bonded on the distal shaft, the inner member is inserted with the foamed cover  24  into the outer member. The inner member is then bonded at the distal end of the expandable member  20 . The foamed cover  24  should be located in the middle of the expandable member  20 . The expandable member  20  is then folded and the stent is slid over the expandable member  20 . 
     FIGS. 1 through 6 further illustrate a method for configuring a catheter assembly with a removably secured expandable stent  26 . FIG. 3 shows inner tubular member  10  having expandable member  20  aligned about it, thereby defining an annular space  22  therebetween. The first step in the method being providing foamed cover  24  within annular space  22  as described above. 
     The next step is to crimp the expandable stent onto outer surface  20  of expandable member  16  so that inner surface  18  of expandable member  16  compresses foamed cover  24 . The compressibility of the foamed cover allows for the generation of sufficient counterforce on the crimped stent without significantly increasing the profile of the crimped stent. Consequently, a counter-force is generated from the foamed cover  24 . FIGS. 5 and 6 most clearly illustrate the results of this step. A preferred method includes crimping the expandable stent with sufficient force to produce pockets of material  30  in foamed cover  24  corresponding to gaps  28  in the stent. Furthermore, it is preferable to crimp the expandable stent with sufficient force to produce ridges  46 , 48  in the foamed cover which extends beyond one or both ends of the stent. Later, where desired, heat and pressure can be applied to enhance crimping by locking the deformed profile of the balloon and foamed cover materials. 
     The present invention catheter assembly with a removably secured stent can be used for delivering and implanting a stent into a body lumen when used in combination with stent delivery systems. These systems include modified over-the-wire (OTW) catheters  34  and rapid exchange (Rx) catheters  36 . FIGS. 11 through 15 illustrate an exemplary use of the invention using an Rx catheter. The catheter assembly  36  with expandable stent  26  crimped firmly in place on expandable member  16  is inserted into a body lumen, e.g., a coronary artery. FIGS. 12 through 15 illustrate a typical situation in which the invention is used after an intravascular procedure has damaged a patient&#39;s arterial lining to such an extent that the lining needs support to prevent it from collapsing into the arterial passageway and obstructing blood flow through the artery. In these situations, as others, the stent can be delivered to, and implanted into, the location requiring support without the use of further means to secure the stent. 
     Catheter assembly  36  is advanced along guide wire  52 , which is already in position distal to the location requiring support. As illustrated in FIG. 13, expandable member  16  is inflated thereby expanding stent  26 . This may be accomplished, for example, by injecting fluid into annular space  22  under substantial pressure (4 to 10 atm is preferred). Expandable member  16  is then deflated leaving expandable stent  26  expanded and implanted in the desired location of the body lumen. The remainder of the catheter assembly, with the expandable member deflated and no longer in frictional contact with the expandable stent, is then withdrawn from the body lumen. 
     The dimensions of the intravascular catheter will generally follow the dimensions of intravascular catheters used in angioplasty procedures in the same arterial location. Typically, the length of a catheter for use in the coronary arteries is about 150 cm, the outer diameter of the catheter shaft is about 0.035 inch (0.89 mm), the length of the balloon is typically about 20 mm and the inflated diameter about 1 to about 8 mm. The lengths and diameters of the catheters for use in peripheral arteries, saphenous veins, and to treat aortic abdominal aneurysms would vary. 
     The materials of construction for the inner tubular member and expandable member may be selected from those used in conventional balloon angioplasty catheters, such as those described in the patents incorporated by reference. 
     While the present invention has been described herein in terms of delivering an expandable stent to a desired location within a patient&#39;s blood vessel, the delivery system can be employed to deliver stents to locations within other body lumens so that the stents can be expanded to maintain the patency of those body lumens. This invention may also be used to deliver and implant other intravascular prosthesis, such as tube grafts to treat abdominal aortic aneurysms. Various changes and improvements may also be made to the invention without departing from the scope thereof.