Abstract:
A stent delivery balloon catheter system includes a pair of retainers which are anchored to the catheter to hold the stent against the balloon until the stent is to be deployed. Each retainer is made of filaments which are woven together to form a braided tube shaped structure. Further, each retainer has one end which is anchored to the catheter while the free end of the retainer overlaps a respective end of the stent. Upon inflation of the balloon for deployment of the stent, the balloon also urges against each retainer. This causes the woven tube structure of the retainer to expand, and thereby shorten. As the retainers shorten, they also withdrawn from the stent, and thereby release the stent for deployment.

Description:
FIELD OF THE INVENTION 
     The present invention pertains generally to stent delivery catheters. More particularly, the present invention pertains to stent retainers which are incorporated into a stent delivery system for the purpose of holding a stent against the balloon of a balloon catheter during its advancement into the vasculature of a patient. The present invention is particularly, but not exclusively, useful as a stent retainer which relies on a mechanical reconfiguration of the retainer to separate the retainer from the stent during its deployment. 
     BACKGROUND OF THE INVENTION 
     The introduction of a foreign object into the vasculature of a patient presents obviously complex problems. Specifically, both the object and its delivery system must be capable of being advanced into the vasculature for its intended purpose, without unduly traumatizing the patient. Further, this advancement must be accomplished accurately and with great precision. To this end, interventional systems are typically designed to be as smooth as possible, to have as small a profile as possible, and to be as easily controlled as possible. These design characteristics, however, do not directly address the different set of problems which are confronted when, after being advanced into the vasculature by a delivery system, an object is thereafter deployed or disengaged from the delivery system to remain in situ. 
     In order to deploy or disengage an object from a delivery system in the vasculature of a patient, it is necessary to somehow reconfigure the object and the delivery system for this purpose. The structures used for these functions, however must not be allowed to interfere with the aforementioned task of advancing the object into the vasculature. Furthermore, the structure used to deploy or disengage an object from its delivery system must not itself create problems which would traumatize the patient while the system is in the vasculature. In sum, it is important for a delivery system to function reliably and safely. It happens that several devices have been disclosed which are directed toward this result. 
     U.S. Pat. No. 3,902,501 which issued to Citron et al. for an invention entitled “Endocardial Electrode,” and which is assigned to the same assignee as the present invention, discloses an interventional device for the deployment of a medical electrode. Specifically, the invention disclosed by Citron et al. incorporates a shroud which is axially fixed in its position on the device. As so positioned, the shroud overlaps and holds the exposed ends of the electrode&#39;s tines against the electrode body until the tines are to be deployed. To deploy the tines, a balloon on the electrode body is inflated to withdraw the tines from the shroud. Another example of this same basic deployment scheme, but for a slightly different application, is provided by U.S. Pat. No. 4,950,227 which issued to Savin et al. for an invention entitled “Stent Delivery System.” According to the invention of Savin et al., a stent is positioned over the balloon of a balloon catheter. A pair of sleeves are axially fixed on the device with one end of each sleeve anchored to the catheter while the other end overlaps a respective end of the stent to hold the stent on the balloon. The result is that the sleeve is fixed at an axial location on the catheter. In an action similar to that disclosed for the device of Citron et al., when the balloon is inflated, the ends of the stent are withdrawn from their respective sleeve and the stent is thus deployed. 
     A difficulty not resolved by either the Citron et al. device or the Savin et al. device is the fact that the sleeves of Savin et al., like the shroud of Citron et al., are axially fixed and therefore substantially immobile. Stated differently, both of these devices rely solely on a change in the configuration of the object being deployed to disengage the object from the shroud or sleeves which hold the object against the balloon. Neither of these devices rely on a mechanical change in either the shroud or the stent for this disengagement. Thus, because the shroud (Citron et al.) or the sleeves (Savin et al.) do not mechanically withdraw axially from the tines or stent during deployment, less of the tines or stent can be covered by the respective shroud or sleeves than would otherwise be possible prior to deployment. Consequently, with less overlapping coverage, there is an increased risk of premature dislodgment of the object being deployed, and an increased risk of a system malfunction during deployment. 
     In light of the above it is an object of the present invention to provide a system for selectively holding a stent on a balloon delivery catheter which incorporates retainers that will mechanically shorten with an inflation of the balloon to facilitate disengagement of the stent from the system. Still another object of the present invention is to provide a system for selectively holding a stent on a balloon delivery catheter which effectively covers the projecting ends of a stent to protect the patient from snags, hang-ups or entanglements that might otherwise occur during advancement of the system into the vasculature of the patient. Yet another object of the present invention is to provide a system for selectively holding a stent on a balloon delivery catheter which includes stent retainers that revert to a low profile after stent deployment to facilitate withdrawal of the system from the vasculature of the patient. Another object of the present invention is to provide a system for selectively holding a stent on a delivery catheter which is relatively easy to manufacture, simple to use and comparatively cost effective. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, a system for selectively holding a stent on a balloon delivery catheter includes both a proximal retainer and a distal retainer which respectively interact with the proximal end and the distal end of the stent. During advancement of the system into the vasculature of a patient, and prior to deployment of the stent, the retainers are used to hold the stent on the catheter. During actual deployment of the stent, however, as the balloon is being inflated the retainers are designed to mechanically withdraw from the stent and thereby facilitate disengagement of the stent from the system. Finally, after deployment of the stent, the retainers collapse with the deflated balloon to provide a low profile for the system during its withdrawal from the patient&#39;s vasculature. 
     In accordance with the present invention a stent is initially positioned over the central, or working, portion of an elongated inflatable balloon. Each of the retainers is then positioned over a respective cone at the ends of the balloon and each retainer has one end which is anchored to the catheter, the other, unanchored end is then positioned to overlap an end of the stent. For example, the proximal end of the proximal retainer is anchored, or bonded, to the catheter, while the distal end of the proximal retainer overlaps the proximal end of the stent. Similarly, the distal end of the distal retainer is anchored to the catheter while the proximal end of the distal retainer overlaps the distal end of the stent. 
     The structure of both the proximal retainer and the distal retainer are essentially the same. Specifically, each retainer includes a first plurality of filaments which are woven together with a second plurality of filaments to create a braided tube. With this structure, when the retainer is mounted on the catheter, the first plurality of filaments are helically disposed with a positive pitch angle relative to the longitudinal axis of the catheter, and the second plurality of filaments are likewise helically disposed, but with a negative pitch angle. The overall result is that the retainer is axially movable whenever the retainer expands in response to an inflation of the balloon. More specifically, this movement is between a first configuration wherein the retainer has a first length, and a second configuration wherein the retainer has a shorter second length. During a transition from the first to the second configuration, both the positive and negative helical pitches are increased. Importantly, as indicated, the first length is longer than the second length. 
     In the operation of the system of the present invention, the stent with overlapping retainers is advanced into the vasculature of a patient. Once the stent is properly positioned for deployment in the vasculature, the balloon is inflated. Due to the inflation of the balloon, the stent is expanded by the working section of the balloon. At the same time, the retainers are expanded by the cones of the balloon. With this expansion, the helical pitch of filaments in both of the retainers is increased and the retainer is actually shortened in an axial direction. The combined effect of the expanding stent and the shortened retainers causes the retainers to withdraw from the stent and thereby disengage the stent from the system. Subsequently, upon deflation of the balloon, the retainers collapse with the balloon to facilitate withdrawal of the system from the vasculature, and thus leave the stent implanted at the desired position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
     FIG. 1 is a side elevational view of the major components at the distal end of the stent delivery system of the present invention with the components separated from each other for clarity; 
     FIG. 2 is a comparison view of the braid component of a stent retainer according to the present invention with the retainer shown in an elongated configuration and in a compressed configuration; 
     FIG. 3A is a view of the stent delivery system of the present invention inserted into a vessel within a stenosis in the vasculature of a patient prior to inflation of the balloon for deployment of the stent; 
     FIG. 3B is a cross-sectional view of the stent delivery system in the configuration shown in FIG. 3A; 
     FIG. 4A is a view of the stent delivery system shown in FIG. 3A with the balloon inflated to deploy the stent; 
     FIG. 4B is a cross-sectional view of the stent delivery system in the configuration shown in FIG. 4A; 
     FIG. 5 is a view of the stent delivery system shown in FIGS. 3A and 4A after the stent has been deployed and the balloon deflated for withdrawal from the vasculature, the stent is shown in cross-section for clarity; 
     FIG. 6 is a perspective view of a stent retainer of the present invention; 
     FIGS. 7A-H are each a cross sectional view of various embodiments for the stent retainer of the present invention as seen along the line  7 - 7  in FIG. 6; and 
     FIG. 8 is a perspective view of the stent delivery system of the present invention employing an alternate embodiment for the retainers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, a stent delivery balloon catheter system in accordance with the present invention is shown and generally designated  10 . As shown in FIG. 1 the system  10  includes an elongated inflatable balloon  12  of a type well known in the pertinent art, a stent  14 , a proximal retainer  16  and a distal retainer  18 . More specifically, the balloon  12  is an integral part of a catheter  20  which is formed with an inflation lumen. The system  10  also includes a fluid pump (not shown) which is in fluid communication with the balloon  12  via the inflation lumen. Thus, as intended for the present invention, the fluid pump can be activated to either inflate or deflate the balloon  12 . 
     As indicated in FIG. 1, the balloon  12  has a working section  22  which is intermediate a proximal cone  24  and a distal cone  26 . Although the balloon  12  is shown in an inflated configuration in FIG. 1, it is to be appreciated that the overall configuration of the balloon  12 , and specifically the configurations of working section  22  and cones  24 ,  26 , will be changed when the balloon  12  is deflated. As clearly set forth below, this change in configuration of the balloon  12  between an inflated and a deflated configuration is important to the operation of the system  10 . 
     During the assembly of the system  10 , the stent  14  is positioned over the working section  22  and located to surround the balloon  12 . Additionally, the proximal retainer  16  is positioned over the proximal cone  24  to surround the balloon  12 , and the distal retainer  18  is similarly positioned over the distal cone  26  of the balloon  12 . More specifically, in the initial assembly of the system  10  the balloon  12  is deflated. With the balloon  12  deflated, the stent  14  is positioned over the working section  22  and held against the balloon  12  in a manner well known in the pertinent art, such as by crimping. Once the stent  14  is in place on the balloon  12 , the retainers  16 ,  18  are properly attached to the system  10 . To do this, the proximal end  28  of proximal retainer  16  is anchored to the catheter  20  in a manner well known in the pertinent art, such as by heat bonding to the balloon  12 . Likewise, the distal end  30  of distal retainer  18  is anchored to the catheter  20 . It can be noted that the balloon  12  of the system  10  is, in all important respects, a conventional angioplasty balloon. No specific materials are required, and no specific dimension or configuration for the working section  22  or the cones  24 ,  26  are required. 
     FIG. 1 also shows that, due to their relative lengths and the selected anchor points where the retainers  16 ,  18 , are attached to the catheter  20 , when the stent  14  and the two retainers  16 ,  18  have been positioned on the balloon  12  there will be some overlap between these components of the system  10 . Specifically, the distal end  32  of proximal retainer  16  will overlap the proximal end  34  of stent  14  by a distance  36 . Similarly, the proximal end  38  of distal retainer  18  will overlap the distal end  40  of stent  14  by a distance  42 . The exact magnitude of the distances  36 ,  42  are a matter of design choice, but they should be selected with the capabilities of the respective retainers  16  and  18  in mind. An appreciation of these capabilities will, perhaps, be best obtained by reference to FIG.  2 . 
     In FIG. 2, the proximal retainer  16  is used as an example and is shown in two configurations. The configuration for retainer  16  shown in the upper part of the drawing corresponds to the condition wherein balloon  12  is deflated. On the other hand, the configuration for retainer  16 ′ which is shown in the lower part of the drawing corresponds to the condition wherein balloon  12  is inflated. The capability for retainer  16  to move between these two configurations is, in large part, due to the structure of the retainer  16 . As shown, the retainer  16  includes a first plurality of juxtaposed filaments  44 . Also shown is a second plurality of juxtaposed filaments  46  which are interwoven with the filaments  44  to create a braided tube-like structure. It is to be appreciated that in this braided condition, the filaments  44  and the filaments  46  each assume a generally helical configuration which can be characterized by a pitch angle. For this purpose, consider the filament  44   a  as an example of the first plurality of filaments  44 , and the filament  46   a  as an example of the second plurality of filaments  46 . When the balloon  12  is deflated, it will be seen that the filaments  44  establish a positive pitch angle +α relative to an axis  48  and the filaments  46  establish a negative pitch angle −α relative to the axis  48 . For purposes of the present invention, the axis  48  is taken to be the longitudinal axis defined by the retainer  16 ,  18 . Incidentally, when the retainers  16 ,  18  are mounted on the catheter  20 , axis  48  will also be the longitudinal axis of the catheter  20 . 
     When the balloon  12  is inflated, the filaments  44 ,  46  will move, the pitch angle of the filaments will change, and the retainer  16  will assume the general configuration shown in FIG. 2 for retainer  16 ′. For the retainer  16 ′ shown in FIG. 2, it is to be appreciated that, when the balloon  12  is inflated the filament  44   a  will establish a positive pitch angle +β and that the filament  46   a  will establish a negative pitch angle −β. Importantly, the pitch angles ±α are smaller than the pitch angles ±β. As a consequence of this change in configuration, the length  50  of the proximal retainer  16  (with balloon  12  deflated) is longer than the length  52  of the proximal retainer  16 ′ (with balloon  12  inflated). For the present invention, the filaments  44 ,  46  can be made of filaments, or groups of filaments (yarn), and be of any metallic or plastic material in either a monofilament or multi-filament configuration, or in general, any material deemed suitable for the application thus described herein, such as nylon monofilament, stainless steel wire, glass fibers or an elastomer impregnated with a material such as graphite for enhanced lubricity. A suitable elastomer for this purpose is a PolyEther Block Amide (PEBA) available under the name PEBAX®, obtainable from the Elf ATOChem Corporation, Philadelphia, Pa. (e.g. PEBAX 5533). In any case, the significance of the interactions between filaments  44 ,  46  during the shortening of the retainer  16 ,  18  may be best appreciated by considering the operation of the system  10 . 
     In the operation of the system  10  of the present invention, the catheter  20  is advanced over a guidewire  54  into a vessel  56  of a patient&#39;s vasculature. Specifically, the catheter  20  is advanced into the vasculature until the stent  14  is positioned across the stenosis  58 , or some type of obstruction or lesion in the vessel  56 , where it is to be deployed (see FIG.  3 A). Prior to deployment of the stent  14 , the dimensional relationships between the stent  14 , the proximal retainer  16  and the distal retainer  18  are, perhaps, best seen in FIG.  3 B. 
     Once the stent  14  is properly positioned, the balloon  12  is inflated to expand the stent  14 ′ to a configuration, as shown in FIG.  4 A. As is well known, this allows the stent  14 ′ to act as a support structure for maintaining a patency in the vessel  56 . For the present invention, and still referring to FIG. 4A, it should also be noted that the retainers  16 ′,  18 ′ have withdrawn from the stent  14 ′. Specifically, as previously disclosed with reference to FIG. 2, upon inflation of the balloon  12 , the retainers  16 ,  18  are mechanically shortened by more than the respective overlap distances  36 ,  42 . Thus, the retainers  16 ′,  18 ′ withdraw from the stent  14 ′. This separates the retainers  16 ′,  18 ′ from the stent  14 ′ so that they no longer act to hold the stent  14 ′ on the balloon  12 . With deployment of the stent  14 , the now-changed dimensional relationships between the stent  14 ′, the proximal retainer  16 ′ and the distal retainer  18 ′ are best seen in FIG.  4 B. After deployment of the stent  14 ′, the balloon  12  is deflated. 
     Upon deflation of the balloon (see FIG. 5) the expanded stent  14 ′ will retain its expanded configuration and will be separated from the system  10 . Thus, the expanded stent  14 ′ remains in situ. The retainers  16 ′  18 ′, however, no longer cooperate with the stent  14  and they therefore collapse with the balloon  12  as it returns to its deflated configuration. They do not reengage with the stent  14 ′ and, accordingly, the retainers  16 ,  18  release the expanded stent  14 ′ from the system  10 . As can then be appreciated with reference to FIG. 5, after balloon  12  has been deflated, the balloon  12  and the retainers  16 ,  18  are withdrawn from vessel  56  of the patient&#39;s vasculature. As will be appreciated by the skilled artisan, the collapse of the retainers  16 ,  18  can be caused by biasing the retainers  16 ,  18  toward their collapsed configuration. This can be accomplished in several ways, such as by specifically weaving the filaments  44 ,  46  in a manner to generate the bias, or by use of a biasing sleeve  60  which can be incorporated into the retainer  16 ,  18 . 
     In FIG. 6, an exemplary retainer  16  is shown for purposes of disclosing possible embodiments for the retainer  16  which incorporate a biasing sleeve  60 . As shown in FIG. 6, the retainer  16  is generally tubular shaped. With this shape the retainer  16  will have both an outer surface  62  and an inner surface  64 . As will be appreciated by the skilled artisan, either or both surfaces  62 ,  64  can be entirely or partially covered by a biasing sleeve. Further, it will be appreciated that the retainer  16  can be divided into longitudinal segments which may, or may not, include braided filaments  44 ,  46 . Several possibilities are shown in FIGS. 7A-H. 
     FIG. 7A illustrates an embodiment for the retainer  16  wherein there is only the filaments  44 ,  46  and no additional structure. For this embodiment, the collapsing bias of the retainer  16  must be provided by the filaments  44 ,  46  themselves. For the embodiment of retainer  16  shown in FIG. 7B, however, a biasing sleeve  60  is shown which covers the entire outer surface  62 . For this embodiment, as with all other embodiments which incorporate biasing sleeves, it is to be appreciated that the sleeves  60  not only bias the retainer  16 ,  18  to return to its original tubular shape, they also provide the patient with some protection against abrasions which may be caused the structure of the filaments  44 ,  46 . With the addition of a biasing sleeve  66  over the entire inner surface  64 , the retainer  16  assumes the embodiment shown in FIG.  7 C. For another embodiment, FIG. 7D shows that the sleeve  60  may extend past the proximal end  28  and past the distal end  32  of the retainer  16 . In FIG. 7E, the sleeve  60  is shown to be bifurcated in order to cover proximal end  28  with only a sleeve portion  60   a  and to cover distal end  32  with only a sleeve portion  60   b . The embodiment of FIG. 7F shows an embodiment wherein the sleeve  60  on the outer surface  62  is integral with the sleeve  66  on the inner surface  64 . For this embodiment, the filaments  44 ,  46  are effectively embedded between the sleeves  60 ,  66 . The embodiment of FIG. 7G is similar to that shown in FIG. 7D, with the difference being that the sleeve  66  on inner surface  64  extends beyond the proximal end  28  and the distal end  32 , and not vice versa. Finally, the embodiment for the retainer  16  shown in FIG. 7H shows an integral connection between sleeve portions  60 / 66   a  at the proximal end  28  of retainer  16 , and sleeve portions  60 / 66   b  at the distal end  32  of the retainer  16 . It will be appreciated that still other variations in the embodiments of retainers  16 ,  18  are possible. 
     FIG. 8 shows the system  10  of the present invention incorporating the particular embodiment for retainers  16 ,  18  that is depicted in FIG.  7 H. As shown in FIG. 8, each of these retainers  16 ,  18  has a segment  68  which includes braided filaments  44 ,  46 . For this particular embodiment, however, the segment  68  of the braided filaments  44 ,  46  is bonded between extensions  70  and  72  of the sleeve  66 . As can be appreciated by the skilled artisan, these extensions  70 ,  72  can be made of an elastomeric material, such as a PEBAX® material disclosed above. 
     While the particular Method Of Stent Retention To A Delivery Catheter Balloon-Braided Retainers as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.