Patent Publication Number: US-8529618-B2

Title: Ostium support for treating vascular bifurcations

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 11/603,338 filed on Nov. 21, 2006, which is a Continuation-In-Part of U.S. patent application Ser. No. 11/249,138 filed on Oct. 12, 2005, which is a Continuation-In-Part of U.S. patent application Ser. No. 11/190,514 filed on Jul. 27, 2005, which is a Continuation-in-Part of U.S. patent application Ser. No. 11/076,448 filed on Mar. 9, 2005, which is a Continuation-in-Part of U.S. patent application Ser. No. 10/807,643 filed on Mar. 23, 2004, now U.S. Pat. No. 7,481,834, which claims the benefit of priority of U.S. Provisional Application No. 60/463,075, filed on Apr. 14, 2003, the full disclosures of which are incorporated in their entireties herein by reference. This application also is a Continuation-In-Part of U.S. patent application Ser. No. 10/965,230, filed on Oct. 13, 2004, the full disclosure of which is incorporated by reference in its entirety herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the present invention relate generally to medical devices and methods. More particularly, embodiments of the present invention relate to the structure and deployment of a prosthesis having a stent or other support structure and at least one, and in some implementations at least two fronds for deployment at a branching point in the vasculature or elsewhere. 
     Maintaining the patency of body lumens is of interest in the treatment of a variety of diseases. Of particular interest to the present invention are the transluminal approaches to the treatment of body lumens. More particularly, the percutaneous treatment of atherosclerotic disease involving the coronary and peripheral arterial systems. Currently, percutaneous coronary interventions (PCI) often involve a combination of balloon dilation of a coronary stenosis (i.e. a narrowing or blockage of the artery) followed by the placement of an endovascular prosthesis commonly referred to as a stent. 
     A major limitation of PCI/stent procedures is restenosis, i.e., the re-narrowing of a blockage after successful intervention typically occurring in the initial three to six months post treatment. The recent introduction of drug eluting stents (DES) has dramatically reduced the incidence of restenosis in coronary vascular applications and offers promise in peripheral stents, venous grafts, arterial and prosthetic grafts, as well as A-V fistulae. In addition to vascular applications, stents are being employed in treatment of other body lumens including the gastrointestinal systems (esophagus, large and small intestines, biliary system and pancreatic ducts) and the genital-urinary system (ureter, urethra, fallopian tubes, vas deferens). 
     Treatment of lesions in and around branch points generally referred to as bifurcated vessels, is a developing area for stent applications, particularly, since at least about 5%-10% of all coronary lesions involve bifurcations. However, while quite successful in treating arterial blockages and other conditions, current stent designs are challenged when used at a bifurcation in the blood vessel or other body lumen. Presently, many different strategies are employed to treat bifurcation lesions with currently available stents all of which have major limitations. 
     One common approach is to place a conventional stent in the main or larger body lumen over the origin of the side branch. After removal of the stent delivery balloon, a second wire is introduced through a cell in the wall of the deployed stent and into the side branch. A balloon is then introduced into the side branch and inflated to enlarge the side-cell of the main vessel stent. This approach can work well when the side branch is relatively free of disease, although it is associated with increased rates of abrupt closure due to plaque shift and dissection as well as increased rates of late restenosis. 
     Another commonly employed strategy is the ‘kissing balloon’ technique in which separate balloons are positioned in the main and side branch vessels and simultaneously inflated to deliver separate stents simultaneously. This technique is thought to prevent plaque shift. 
     Other two-stent approaches including Culotte, T-Stent and Crush Stent techniques have been employed as well. When employing a T-Stent approach, the operator deploys a stent in the side branch followed by placement of a main vessel stent. This approach is limited by anatomic variation (angle between main and side branch) and inaccuracy in stent positioning, which together can cause inadequate stent coverage of the side branch origin commonly referred to as the ostium or Os. More recently, the Crush approach has been introduced in which the side-vessel stent is deployed across the Os with portions in both the main and side branch vessels. The main vessel stent is then delivered across the origin of the side branch and deployed, which results in crushing a portion of the side branch stent between the main vessel stent and the wall of the main vessel. Following main-vessel stent deployment, it is difficult and frequently not possible to re-enter the side branch after crush stenting. Unproven long-term results coupled with concern regarding the inability to re-enter the side branch, malapposition of the stents against the arterial wall and the impact of three layers of stent (which may be drug eluting) opposed against the main vessel wall has limited the adoption of this approach. 
     These limitations have led to the development of stents specifically designed to treat bifurcated lesions. One approach employs a stent design with a side opening for the branch vessel which is mounted on a specialized balloon delivery system. The specialized balloon delivery system accommodates wires for both the main and side branch vessels. The system is tracked over both wires which provides a means to axially and radially align the stent/stent delivery system. The specialized main vessel stent is then deployed and the stent delivery system removed while maintaining wire position in both the main and side branch vessels. The side branch is then addressed using the kissing balloon technique or by delivering an additional stent to the side branch. Though this approach has many theoretical advantages, it is limited by difficulties in tracking the delivery system over two wires (See, e.g., U.S. Pat. Nos. 6,325,826 and 6,210,429 to Vardi et al.). 
     Notwithstanding the foregoing efforts, there remains a need for improved devices as well as systems and methods for delivering devices, to treat body lumens at or near the location of an Os between a main body lumen and a side branch lumen, typically in the vasculature, and more particularly in the arterial vasculature. It would be further desirable if such systems and methods could achieve both sufficient radial support as well as adequate surface area coverage in the region of the Os and that the prostheses in the side branches be well-anchored at or near the Os. 
     2. Description of the Related Art 
     Stent structures intended for treating bifurcated lesions are described in U.S. Pat. Nos. 6,599,316; 6,596,020; 6,325,826; and 6,210,429. Other stents and prostheses of interest are described in the following U.S. Pat. Nos. 4,994,071; 5,102,417; 5,342,387; 5,507,769; 5,575,817; 5,607,444; 5,609,627; 5,613,980; 5,669,924; 5,669,932; 5,720,735; 5,741,325; 5,749,825; 5,755,734; 5,755,735; 5,824,052; 5,827,320; 5,855,598; 5,860,998; 5,868,777; 5,893,887; 5,897,588; 5,906,640; 5,906,641; 5,967,971; 6,017,363; 6,033,434; 6,033,435; 6,048,361; 6,051,020; 6,056,775; 6,090,133; 6,096,073; 6,099,497; 6,099,560; 6,129,738; 6,165,195; 6,221,080; 6,221,098; 6,254,593; 6,258,116; 6,264,682; 6,346,089; 6,361,544; 6,383,213; 6,387,120; 6,409,750; 6,428,567; 6,436,104; 6,436,134; 6,440,165; 6,482,211; 6,508,836; 6,579,312; and 6,582,394. 
     SUMMARY OF THE INVENTION 
     There is provided in accordance with one aspect of the present invention, a prosthesis for placement at an ostium from a main body lumen to a branch body lumen. The prosthesis comprises a radially expansive support, the support configured to be deployed in at least a portion of the branch body lumen. An elongate flexible spiral frond extends axially from an end of the support, and is configured to be positioned across the ostium and into the main body lumen. At least one circumferential link is spaced axially apart from the support by the spiral fond. 
     At least a portion of the prosthesis may be provided with a drug coating, and at least a portion of the frond and the circumferential link may be provided without a drug coating. The frond may extend about the longitudinal axis of the prosthesis through at least about two full revolutions. 
     In accordance with another aspect of the present invention, there is provided a prosthesis for placement at an ostium opening from a main body lumen to a branch body lumen. The prosthesis comprises a radially expansible branch vessel support having a tubular body with a longitudinal axis. The support is configured such that at least a portion of the support can be deployed in the branch body lumen. A spiral frond extends axially from an end of the support, through at least about four complete revolutions about the longitudinal axis of the support, and is adapted to extend across the ostium. 
     In accordance with a further aspect of the present invention, there is provided a method for treating a bifurcation between a main lumen and a branch lumen. The method comprises the steps of providing a radially expansible scaffold, having a proximal end, a distal end, and a support structure on the distal end. A circumferential link is provided on the proximal end, and at least one spiral frond extends axially between the support structure and the circumferential link. The scaffold is translumenally navigated to a treatment site, and deployed at the site such that the support is in the branch lumen and the circumferential link is in the main lumen. 
     In accordance with a further aspect of the present invention, there is provided a method of supporting a vessel wall in the vicinity of an ostium between the main vessel and a branch vessel. The method comprises the steps of positioning at least one spiral frond across the ostium, the frond having a first end on a first, branch vessel side of the ostium and a second end on a second, main vessel side of the ostium. The first end of the frond is supported against the vessel wall on the first side of the ostium, and the second end of the frond is supported against the vessel wall on the second side of the ostium. 
     The supporting the first end step may comprise expanding a first expandable support structure against the vessel wall. The first expandable support structure may be attached to the first end of the frond. 
     The supporting the second end of the frond step may comprise expanding a second expandable support structure against the vessel wall. The second expandable support structure may be attached to the second end of the frond. Alternatively, the second support structure may be separate from the frond, and expanded such that the second end of the frond is entrapped between the second support structure and the vessel wall. 
     Further features and advantages of the present invention will become apparent from the detailed description of preferred embodiments which follows, when considered together with the attached drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a prosthesis constructed in accordance with the principles of the present invention. 
         FIG. 1A  is a detailed view of the fronds of the prosthesis of  FIG. 1 , shown with the fronds deployed in broken line. 
         FIG. 2  is a cross-sectional view taken along line  2 - 2  of  FIG. 1 . 
         FIGS. 2A-2E  are lateral views showing embodiments of a stent having fronds in a rolled out configuration.  FIG. 2A  shows an embodiment having serpentine-shaped fronds,  FIG. 2B  shows an embodiment having filament shaped fronds, while  FIG. 2C  shows an embodiment having filament shaped fronds with alternating shortened fronds.  FIGS. 2D and 2E  illustrate a nested transition zone configuration with two different stent wall patterns. 
         FIG. 2F  is a lateral view as in  FIG. 2D , with the added feature of a circumferential link to assist in maintaining the spacial orientation of the fronds. 
         FIG. 2G  is a lateral view as in  FIG. 2F , having a helical frond. 
         FIGS. 3A and 3B  are lateral and cross sectional views illustrating an embodiment of a stent having fronds and an underlying deployment balloon having a fold configuration such that the balloon folds protrude through the spaces between the fronds. 
         FIGS. 4A and 4B  are lateral and cross sectional views illustrating the embodiment of  FIGS. 3A and 3B  with the balloon folded over to capture the fronds. 
         FIGS. 5A-5C  are lateral views illustrating the deployment of stent fronds using an underlying deployment balloon and a retaining cuff positioned over the proximal portion of the balloon.  FIG. 5A  shows pre-deployment, the balloon un-inflated;  FIG. 5B  shows deployment, with the balloon inflated; and  FIG. 5C  post-deployment, the balloon now deflated. 
         FIGS. 6A-6B  are lateral views illustrating the change in shape of the cuff during deployment of a stent with fronds.  FIG. 6A  shows the balloon in an unexpanded state; and  FIG. 6B  shows the balloon in an expanded state, with the cuff expanded radially and shrunken axially. 
         FIGS. 6C-6D  are lateral views illustrating an embodiment of a cuff configured to evert upon balloon inflation to release the fronds. 
         FIGS. 7A-7B  are lateral views illustrating an embodiment of a tether for restraining the stent fronds. 
         FIGS. 8A-8B  are lateral views illustrating an embodiment of a proximally retractable sleeve for restraining the stent fronds. 
         FIGS. 9A-9B ,  10 A- 10 B and  11 A- 11 B illustrate deployment of a stent at an Os between a main blood vessel and a side branch blood vessel in accordance with the principles of the methods of the present invention. 
         FIGS. 12A-12H  are lateral and cross section views illustrating deployment of a stent having filament fronds an Os between a main blood vessel and a side branch blood vessel in accordance with the principles of the methods of the present invention. 
         FIGS. 13A-13C  illustrate side wall patterns for three main vessel stents useful in combination with the prosthesis of the present invention. 
         FIG. 13D  is an image of a deployed main vessel stent having a side wall opening in alignment with a branch vessel. 
         FIGS. 14A-14E  are a sequence of schematic illustrations showing the deployment of a vascular bifurcation prosthesis with linked fronds. 
         FIG. 15  is a schematic profile of a stepped balloon in accordance with one aspect of the present invention. 
         FIG. 16  is a balloon compliance curve for one embodiment of the stepped balloon in accordance with the present invention. 
         FIG. 17  is a schematic representation of a stepped balloon positioned within a vascular bifurcation. 
         FIG. 18  is a schematic representation as in  FIG. 17 , with a different configuration of stepped balloon in accordance with the present invention. 
         FIG. 19  is a first step in a deployment sequence using a stepped balloon and prosthesis in accordance with the present invention. 
         FIG. 20  is a second step in the deployment process disclosed in connection with  FIG. 19 , in which the prosthesis is positioned across the Os. 
         FIG. 21  is a third step in a deployment sequence, in which a prosthesis has been expanded utilizing a stepped balloon in accordance with the present invention. 
         FIG. 22  is a fourth step in a deployment sequence, in which the stepped balloon has been removed following deployment of the prosthesis across the Os. 
         FIG. 23  is a side elevational schematic view of a distal portion of a catheter in accordance with the present invention. 
         FIG. 23A  is a cross sectional view taken along the lines  23 A- 23 A in  FIG. 23 . 
         FIG. 23B  is a cross sectional view taken along the lines  23 B- 23 B in  FIG. 23 . 
         FIG. 24  is a side elevational view as in  FIG. 23 , with a modified catheter in accordance with the present invention. 
         FIG. 24A  is a cross sectional view taken along the line  24 A- 24 A of  FIG. 24 . 
         FIG. 24B  is a cross sectional view taken along the line  24 B- 24 B in  FIG. 24 . 
         FIG. 25  is a schematic view of a two guidewire catheter in accordance with the present invention, in position at a vascular bifurcation. 
         FIG. 26  is a schematic representation as in  FIG. 25 , showing the second guidewire advanced distally through the fronds. 
         FIG. 27  is a schematic view as in  FIG. 25 , with a modified two guidewire catheter in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Embodiments of the present invention provide improved prostheses and delivery systems for their placement within a body lumen, particularly within a bifurcated body lumen and more particularly at an Os opening from a main body lumen to a branch body lumen. The prostheses and delivery systems will be principally useful in the vasculature, most typically the arterial vasculature, including the coronary, carotid and peripheral vasculature; vascular grafts including arterial, venous, and prosthetic grafts such as a bifurcated abdominal aortic aneurysm graft, and A-V fistulae. In addition to vascular applications, embodiments of the present invention can also be configured to be used in the treatment of other body lumens including those in the gastrointestinal systems (e.g., esophagus, large and small intestines, biliary system and pancreatic ducts) and the genital-urinary system (e.g., ureter, urethra, fallopian tubes, vas deferens), and the like. 
     The prosthesis in accordance with the present invention generally comprises three basic components: a stent or other support, at least one frond extending from the support, and a transition zone between the support and the frond. These components may be integrally formed such as by molding, or by laser or other cutting from tubular stock, or may be separately formed and secured together. 
     The term “fronds” as used herein will refer to any of a variety of structures including anchors, filaments, petals or other independently multiaxially deflectable elements extending from the stent or other support structure, to engage an adjacent main vessel stent or other associated structure. These fronds can expandably conform to and at least partially circumscribe the wall of the main body vessel to selectively and stably position the prosthesis within the side branch lumen and/or optimize wall coverage in the vicinity of the ostium. Further description of exemplary frond structures and prostheses is found in co-pending application Ser. No. 10/807,643, the full disclosure of which has previously been incorporated herein by reference. Various embodiments of the present invention provide means for capturing or otherwise radially constraining the fronds during advancement of the prosthesis through the vasculature (or other body lumen) to a target site and then releasing the fronds at the desired deployment site. 
     The prostheses of the present invention are particularly advantageous since they permit substantially complete coverage of the wall of the branch body lumen up to and including the lumen ostium or Os. Additionally, the prostheses have integrated fronds which expandably conform to and at least partially circumscribe the wall of the main body vessel to selectively and stably link the prosthesis to the main vessel stent. The fronds may be fully expanded to open the luminal passage through the main branch lumen. Such complete opening is an advantage since it provides patency through the main branch lumen. Moreover, the open main vessel lumen permits optional placement of a second prosthesis within the main branch lumen using conventional techniques. 
     In a first aspect of the present invention, a prosthesis comprises a radially expansible support and at least one or often two or more fronds extending axially from an end of the support. The fronds are adapted to extend around, or “expandably circumscribe” a portion of, usually at least one-half of the circumference of the main vessel wall at or near the Os when the support is implanted in the branch lumen with the fronds extending into the main lumen. By “expandably circumscribe,” it is meant that the fronds will extend into the main body lumen after initial placement of the support within the branch body lumen. The fronds will be adapted to then be partially or fully radially expanded, typically by expansion of a balloon or other expandable structure therein, so that the fronds deform outwardly and conform to the interior surface of the main lumen. 
     The fronds will usually extend axially within the main vessel lumen for some distance after complete deployment. In certain embodiments, the contact between the fronds and the main vessel wall will usually extend both circumferentially (typically at least one frond may cover an arc equal to one-half or more of the circumference of the main vessel) and axially. 
     Deformation of the fronds to conform to at least a portion of the wall of the main body lumen provides a generally continuous coverage of the Os from the side branch lumen to the main vessel lumen. Further and/or complete expansion of the fronds within the main body lumen may press the fronds firmly against the main body lumen wall and open up the fronds so that they do not obstruct flow through the main body lumen, while maintaining patency and coverage of the side branch and Os. 
     Usually, the prosthesis will include at least two or three fronds extending axially from the end of the support. The prosthesis could include four, five, or even a greater number of fronds, but the use of three such fronds is presently contemplated for a coronary artery embodiment. The fronds will have an initial length (i.e., prior to radial expansion of the prosthesis) which is at least about 1.5 times the width of the prosthesis prior to expansion, typically at least about 2 times the width, more typically at least about 5 times the width, and often about 7 times the width or greater. The lengths will typically be at least about 2 mm, preferably at least about 3 mm, and more preferably at least about 6 mm. The frond length may also be considered relative to the diameter of the corresponding main vessel. For example, a prosthesis configured for use in a branch vessel from a main vessel having a 3 mm lumen will preferably have a frond length of at least about 7 mm and in some embodiments at least about 9 mm. 
     Embodiments of the present invention incorporating only a single frond are also contemplated. The single frond may extend axially from the branch vessel support as has been described in connection with multi frond embodiments. Alternatively, the single frond (or two or three or more fronds) may extend in a helical or spiral pattern, such that it wraps in a helical winding about the longitudinal axis extending through the branch vessel support. 
     The fronds may have a fixed width or a width which is expandable to accommodate the expansion of the support, and the fronds may be “hinged” at their point of connection to the support to permit freedom to adapt to the geometry of the main vessel lumen as the prosthesis is expanded. As used herein, “hinged” does not refer to a specific structure such as a conventional hinge, but rather to any combination of structures, materials and dimensions that permit multiaxial flexibility of the frond relative to the support so that the frond can bend in any direction and/or rotate about any axis to conform to the ablumenal surface of the expanded main vessel stent under normal use conditions. It is also possible that the fronds could be attached at a single point to the support, thus reducing the need for such expandability. The fronds may be congruent, i.e., have identical geometries and dimensions, or may have different geometries and/or dimensions. In particular, in some instances, it may be desirable to provide fronds having different lengths and/or different widths. 
     In another aspect of the invention, at least one of the of fronds has a loop or filament shape and includes a first expandable strut configured to be positioned at the Os in an expanded state and provide radial support to an interior portion of the main body lumen. The fronds can be fabricated from flexible metal wire, molded, laser cut or otherwise formed from tube stock in accordance with known techniques. The strut can be configured to be substantially triangular in the expanded state. Also, at least one of the fronds may be configured to be expandably deployed proximate a vessel wall by an expandable device such as an expandable balloon catheter. 
     In another aspect of the invention, a prosthesis delivery system comprises a delivery catheter having an expandable member and a prosthesis carried over the expandable member. The prosthesis has a radially expandable support such as a tubular stent and at least two fronds extending axially from the support. The system also includes a retainer for capturing the fronds to prevent them from divaricating from the expandable member as the catheter is advanced through a patient&#39;s vasculature. “Divarication” as used herein means the separation or branching of the fronds away from the delivery catheter. Various embodiments of the capture means prevent divarication by constraining and/or imparting sufficient hoop strength to the fronds to prevent them from branching from the expandable member during catheter advancement in the vasculature. 
     In one embodiment, the capturing means comprises a portion of the expandable member that is folded over the fronds where the folds protrude through axial gaps between adjacent fronds. In another embodiment, the capturing means comprises a cuff that extends over at least a portion of the fronds to hold them during catheter advancement. The cuff can be positioned at the proximal end of the prosthesis and can be removed by expansion of the expandable member to either plastically or elastically deform the cuff, break the cuff, or reduce the cuff in length axially as the cuff expands circumferentially. The cuff is then withdrawn from the target vessel. In yet another embodiment, the capturing means can comprise a tether which ties together the fronds. The tether can be configured to be detached from the fronds prior to expansion of the expandable member. In alternative embodiments, the tether can be configured to break or release upon expansion of the expandable member so as to release the fronds. 
     In an exemplary deployment protocol using the prosthesis delivery system, the delivery catheter is advanced to position the prosthesis at a target location in a body lumen. During advancement, at least a portion of the fronds are radially constrained to prevent divarication of the fronds from the delivery catheter. When the target location is reached, the radial constraint is released and the prosthesis is deployed within the lumen. 
     In various embodiments, the release of the fronds and expansion of the prosthesis can occur simultaneously or alternatively, the radial constraint can be released prior to, during, or after expanding/deploying the prosthesis. In embodiments where the radial constraint comprises balloon folds covering the fronds or a cuff or tether, the constraint can be released as the balloon is inflated. In alternative embodiments using a cuff or tether, the cuff/tether can be withdrawn from the fronds prior to expansion of the support. 
     Embodiments of the above protocol can be used to deploy the prosthesis across the Os of a branch body lumen and trailing into the main body lumen. In such applications, the prosthesis can be positioned so that the stent lies within the branch body and at least two fronds extend into the main body lumen. The fronds are then circumferentially deformed to conform to at least a portion of the main vessel wall to define a main vessel passage through the fronds. At least two and preferably at least three fronds extend into the main body lumen. 
     Radiopaque or other medical imaging visible markers can be placed on the prostheses and/or delivery balloon at desired locations. In particular, it may be desirable to provide radiopaque markers at or near the location on the prosthesis where the stent is joined to the fronds. Such markers will allow a transition region of the prosthesis between the stent and the fronds to be properly located near the Os prior to stent expansion. The radiopaque or other markers for locating the transition region on the prosthesis can also be positioned at a corresponding location on a balloon catheter or other delivery catheter. Accordingly, in one embodiment of the deployment protocol, positioning the prosthesis can include aligning a visible marker on at least one of the prosthesis, on the radial constraint, and the delivery balloon with the Os. 
     In various embodiments for deploying the prosthesis, the support is expanded with a balloon catheter expanded within the support. In some instances, the support and the fronds may be expanded and deformed using the same balloon, e.g., the balloon is first used to expand the support, partially withdrawn, and then advanced transversely through the fronds where it is expanded for a second time to deform the fronds. A balloon the length of the support (shorter than the total prosthesis length) can be used to expand the support, and then be proximally retracted and expanded in the fronds. Alternatively, separate balloon catheters may be employed for expanding the support within the side branch and for deforming the fronds against the wall of the main body lumen. 
     The fronds may expand radially in parallel with the support section of the prosthesis. Then, in a second step, the fronds may be folded out of plane as the main vessel stent or balloon is deployed. Deformation of the fronds at least partially within the main body lumen provides a generally continuous coverage of the Os from the side body lumen to the main body lumen. Further and/or complete expansion of the fronds within the main body lumen may press the fronds firmly against the main body lumen wall and open up the fronds so that they do not obstruct flow through the main body lumen. 
     The prosthesis may include at least one and in some embodiments at least three fronds extending axially from the end of the support. The fronds will have an initial length (i.e., prior to radial expansion of the stent) which is at least about 1.5 times the cross sectional width of the support prior to expansion, typically at least about 2 times the width, more typically at least about 5 times the width, and often about 7 times the width or greater. The lengths of the fronds will typically be at least about 2 mm, preferably at least about 3 mm, and more preferably at least about 6 mm, as discussed elsewhere herein additional detail. The fronds will usually have a width which is expandable to accommodate the expansion of the stent, and the fronds may be “hinged” or otherwise flexibly connected at their point of connection to the prosthesis to permit freedom to adapt to the geometry of the main vessel lumen as the stent is expanded. It is also possible that the fronds could be attached to the single point to the prosthesis, thus reducing the need for such expandability. Fronds may be optimized for particular bifurcation angles and orientations, such as by making the fronds for positioning closer to the “toe” of the bifurcation longer than the fronds for positioning closer to the carina or “heel” of the bifurcation. 
     The fronds are configured such that during deployment and main vessel stent passage and placement, the fronds allow for longitudinal elongation or compression. In particular, the fronds may elongate axially during main vessel stent deployment, in order to fully accommodate the main vessel stent as will be apparent from the disclosure herein. The longitudinal adjustability of the fronds enables the implant to axially elongate or contract by at least about 5%, and often at least about 10% of the total length of the implant, based upon its predeployed length. In some cases, the longitudinal adjustability of the fronds enables the implant to axially elongate or contract by at least about 20% to about 30% of the total length of the implant, based upon its predeployed length. In some cases, the longitudinal adjustability of the fronds enables the implant to axially elongate or contract by at least about 30% to about 50% or more of the total length of the implant, based upon its predeployed length. Thus, in an implant having an overall length of about 19 mm prior to deployment, the axial length of the implant as measured along the outside surface of the longest frond post-deployment may achieve an elongation of at least about 1.9 mm under normal deployment conditions. 
     Referring now to  FIGS. 1 and 2 , an embodiment of a prosthesis and delivery system  5  of the present invention for the delivery of a prosthesis to a bifurcated vessel can include a prosthesis  10  and a delivery catheter  30 . Prosthesis  10  can include at least a radially expansible support section  12  and a frond section  14  with one or more fronds  16 . The base of the fronds resides in a transition zone, described below. In various embodiments, the frond section  14  includes at least two axially extending fronds  16 , with three being illustrated. 
     Balloon catheters suitable for use with the prosthesis of the present invention are well understood in the art, and will not be described in great detail herein. In general, a catheter suitable for use for deployment of the prosthesis of the present invention will comprise an elongate tubular body extending between a proximal end and a distal end. The length of the (catheter) tubular body depends upon the desired application. For example, lengths in the area of from about 120 cm to about 140 cm are typical for use in a percutaneous transluminal coronary application intended for accessing the coronary arteries via the femoral artery. Other anatomic spaces including renal, iliac, femoral and other peripheral applications may call for a different catheter shaft length and balloon dimensions, depending upon the vascular access site as will be apparent to those of skill in the art. 
     The catheter shaft is provided with at least one central lumen, for an inflation media for inflating an inflatable balloon carried by the distal end of the catheter shaft. In an over the wire embodiment, the catheter shaft is additionally provided with a guidewire lumen extending throughout the entire length thereof. Alternatively, the prosthesis of the present invention may be deployed from a rapid exchange or monorail system, in which a proximal access port for the guidewire lumen is provided along the side wall of the catheter shaft distally of the proximal manifold, such as within about the distal most 20 cm of the length of the balloon catheter, or from a convertible system as is known in the art. 
     The catheter shaft for most applications will be provided with an approximately circular cross sectional configuration, having an external diameter within the range of from about 0.025 inches to about 0.065 inches depending upon, among other things, whether the target bifurcation is in the coronary or peripheral vasculature. Systems may have diameters in excess of about 0.25 inches and up to as much as about 0.35 inches in certain applications. Diameters of from about 1.5 mm up to as large as about 7 mm are contemplated for coronary indications. Additional features and characteristics may be included in the deployment catheter design, such frond retention structures discussed below, depending upon the desired functionality and clinical performance as will be apparent to those of skill in the art. 
     The radially expansible support section  12  will typically be expandable by an expansion device such as a balloon catheter, but alternatively it can be self expandable. The support section  12  may be formed using any of a variety of conventional patterns and fabrication techniques as are well-described in the prior art. 
     Depending upon the desired clinical result, the support section or stent  12  may be provided with sufficient radial force to maintain patency of a diseased portion of the branch lumen. This may be desirable in an instance were vascular disease is present in the branch vessel. Alternatively, the support section  12  may be simply called upon to retain the fronds in position during deployment of the primary vascular implant. In this instance, a greater degree of flexibility is afforded for the configuration of the wall pattern of the support section  12 . For example, support section  12  may comprise a helical spiral, such as a Nitinol or other memory metal which is deployable from an elongate deployment lumen, but which reverts to its helical configuration within the branch vessel. Alternative self expandable structures may be used such as a zig-zag series of struts, connected by a plurality of proximal apexes and a plurality of distal apexes and rolled into a cylindrical configuration. This configuration is well understood in the vascular graft and stent arts, as a common foundation for a self expandable tubular support. 
     In one implementation of the present invention, the prosthesis comprises an overall length of about 19 mm, which is made up of a stent having a length of about 9.6 mm, a targeted expanded diameter of about 2.5 mm and a plurality of fronds having a length of about 9.3 mm. 
     The fronds will usually have a width measured in a circumferential direction in the transition zone which is expandable from a first, delivery width to a second, implanted width to accommodate the expansion of the support, while maintaining optimal wall coverage by the fronds. Thus, although each of the fronds may comprise a single axially extending ribbon or strut, fronds are preferably configured to permit expansion in a circumferential direction at least in the transition zone with radial expansion of the support structure. For this purpose, each frond may comprise a single axially extending element, but often comprises at least two axially extending elements  66 A and  66 D, and optimally three or more axially extending elements, which can be spaced laterally apart from each other upon radial expansion of the prosthesis, to increase in width in the circumferential direction. The increased width referred to herein will differ on a given frond depending upon where along the length of the frond the measurement is taken. Fronds of the type illustrated herein will increase in width the most at the end attached to the support, and the least (or none) at the free apex end as will be appreciated by those of skill in the art. Circumferentially expanding at least the base of the frond enables optimal wall coverage in the vicinity of the ostium, following deployment of the prosthesis at the treatment site. In addition, multiple elements results in a greater surface area as a biological substrate or increased delivery of pharma agents. 
     In the illustrated embodiments, each of the fronds  16  has an equal width with the other fronds  16 . However, a first frond or set of fronds may be provided with a first width (measured in a circumferential direction) and a second frond or set of fronds may be provided with a second, different width. Dissimilar width fronds may be provided, such as alternating fronds having a first width with fronds having a second width. 
     In each of the foregoing constructions, radially symmetry may exist such that the rotational orientation of the prosthesis upon deployment is unimportant. This can simplify the deployment procedure for the prosthesis. Alternatively, prostheses of the present invention exhibiting radial asymmetry may be provided, depending upon the desired clinical performance. For example, a first frond or set of fronds may be centered around 0° while a second frond or set of fronds is centered around 180° when the prosthesis is viewed in a proximal end elevational view. This may be useful if the fronds are intended to extend around first and second opposing sides of the main vessel stent. Asymmetry in the length of the fronds may also be accomplished, such as by providing fronds at a 0° location with a first length, and fronds at 180° location with a second length. As will become apparent below, such as by reference to  FIG. 9A , certain fronds in the deployed prosthesis will extend along an arc which aligns with the axis of the branch vessel at a distal end, and aligns with the axis of the main vessel at a proximal end. The proximal ends of fronds of equal length will be positioned axially apart along the main vessel lumen. If it is desired that the proximal ends of any of the fronds align within the same transverse cross section through the main vessel lumen, or achieve another desired configuration, fronds of different axial lengths will be required as will become apparent to those of skill in the art. 
     Certain additional features may be desirable in the prosthesis and/or deployment system of the present invention, in an embodiment in which the rotational orientation of the prosthesis is important. For example, the catheter shaft of the deployment system preferably exhibits sufficient torque transmission that rotation of the proximal end of the catheter by the clinician produces an approximately equal rotation at the distal end of the catheter. The torque transmission characteristics of the catheter shaft may be optimized using any of a variety of structures which are known in the art. For example, a helical winding may be incorporated into the wall of the catheter shaft, using any of a variety of embedding techniques, or by winding a filament around an inner tube and positioning an outer tube over the winding, subsequently heat shrinking or otherwise fusing the tubes together. Bi-directional torque transmission characteristics can be optimized by providing a first winding in a first (e.g. clockwise) direction, and also a second winding in a second (e.g. counter clockwise) direction. The winding may comprise any of a variety of materials, such as metal ribbon, or a polymeric ribbon. Various tubular meshes and braids may also be incorporated into the catheter wall. 
     In addition, the rotational orientation of the prosthesis is preferably observable fluoroscopically, or using other medical imaging techniques. For this purpose, one or more markers is preferably provided on either the prosthesis, the restraint or the deployment catheter, to enable visualization of the rotational orientation. 
     The sum of the widths measured in the circumferential direction of the fronds  16  when the prosthesis is in either the first, transluminal navigation configuration or the second, deployed configuration will preferably add up to no more than one circumference of the stent portion of the prosthesis. In this manner, the width of the frond  16   s  at the level of attachment may be maximized, but without requiring overlap especially in the first configuration. The width of each frond  16  may generally increase upon deployment of the prosthesis to at least about 125%, often at least about 200%, and in some instances up to about 300% or more of its initial width, at least at the distal end (base) of the frond  16 . The proximal free end of each frond  16  may not increase in circumferential width at all, with a resulting gradation of increase in circumferential width throughout the axial length from the proximal end to the distal end of the frond. While portions of the fronds may expand as described above, in alternate constructions, the fronds may have a width that remains constant or substantially constant throughout the length of the frond as the prosthesis is deployed. 
     The fronds may be “hinged” as has been described at their point of connection to the support to permit freedom to adapt to the geometry of the main vessel lumen as the prosthesis is expanded. It is also possible that each frond is attached at a single point to the support, thus reducing the need for such expandability at the junction between the frond and the support. The fronds may be congruent, i.e., have identical geometries and dimensions, or may have different geometries and/or dimensions. Again, further description of the fronds may be found in co-pending application Ser. No. 10/807,643. 
     Fronds  16 , will usually extend axially from the support section  12 , as illustrated, but in some circumstances the fronds can be configured to extend helically, spirally, in a serpentine pattern, or other configurations as long as the configuration permits placement of the stent in a vessel such that the fronds extend across the Os. It is desirable, however, that the individual fronds be radially separable so that they can be independently, displaced, folded, bent, rotated about their longitudinal axes, and otherwise positioned within the main body lumen after the support section  12  has been expanded within the branch body lumen. In the schematic embodiment of  FIG. 1 , the fronds  16  may be independently folded out in a “petal-like” configuration, forming petals  16   p , as generally shown in broken line for one of the fronds in  FIGS. 1 and 2 . 
     In preferred embodiments, fronds  16  will be attached to the support section  12  such that they can both bend and rotate relative to an axis A thereof, as shown in broken line in  FIG. 1A . Bending can occur radially outwardly and rotation or twisting can occur about the axis A or a parallel to the axis A as the fronds are bent outwardly. Such freedom of motion can be provided by single point attachment joints as well as two point attachments or three or more point attachments. 
     Referring now to  FIG. 2A , an exemplary embodiment of a prosthesis  50  (shown in a “rolled out” pattern) comprises a support or stent section  52  and a frond section  54 . Support section  52  comprises a first plurality of radially expansible serpentine elements  56  which extend circumferentially to form a cylindrical ring having a plurality of open areas or cells  57  therein. The cylindrical rings formed by serpentine elements  56  are coaxially aligned along the longitudinal axis of the support section  52 , and, in the illustrated embodiment, alternate with a second plurality of cylindrical rings formed by radially expandable serpentine elements  58  defining a second set of smaller cells  59 . Strut coverage in the range of from about 10% to about 20%, and in some embodiments between about 16%-18% by area is contemplated. A plurality of spaced apart, axially extending struts  61  connect adjacent rings. The particular pattern illustrated for this structure is well-known and chosen to be exemplary of a useful prosthesis. It will be appreciated that a wide variety of other conventional stent structures and patterns may be equally useful as the support section of the prostheses of the present invention. See, for example,  FIGS. 2B-2F . 
     The wall patterns can be varied widely as desired to provide additional coverage, transition in axial stiffness, and accommodate various side branch angles with respect to the main vessel long axis as well as ostial geometries, i.e., diameter and shape. 
     The support section  52  is joined to the frond section  54  at a plurality of points  65  along a transition line or zone  60 . Individual fronds  16 , comprise a circumferentially expandable wall pattern. In the embodiment illustrated in  FIG. 2A , each frond comprises four curving elements  66  at the distal end of the transition zone  60 , which reduce in number to three and then to two in the axial (proximal) direction away from the stent  52 . The particular structures shown illustrate one example of a way to achieve circumferential expansion of the individual fronds as the prosthesis is expanded. This is accomplished since each frond is attached to three adjacent serpentine ring apexes  63  in the proximal most serpentine ring  56 . Thus, as these serpentine rings  56  are expanded, the circumferential distance between adjacent apexes  63  will increase, thereby causing each frond to “widen” by expanding in a circumferential direction. It would be possible, of course, to join each of the fronds  16  only at a single location to the prosthesis  52 , thus allowing the anchors to be deployed without radial expansion. Two or four or more points of attachment may also be used, depending upon the wall pattern and desired performance of the resulting prosthesis. The struts in the transition section are designed to “cup” with adjacent struts such that the gap formed within and between fronds in the expanded prosthesis is minimized. 
     The circumferentially expandable fronds are curved about the longitudinal axis of the prosthesis and have a number of hinge regions which increase their conformability upon circumferential expansion by a balloon, as described hereinafter. Such conformability is desirable since the fronds will be expanded under a wide variety of differing anatomical conditions which will result in different final geometries for the fronds in use. The final configuration of the fronds in the main vessel lumen will depend on a number of factors, including length of the fronds and geometry of the vasculature and will vary greatly from deployment to deployment. While the fronds together will cover at least a portion of the main vessel wall circumference, most fronds will also be deformed to cover an axial length component of the main vessel wall as well. Such coverage is schematically illustrated in the figures discussed below. 
     In other embodiments, prosthesis structure  50  can include four or five or six or more fronds  16 . Increasing the number of fronds provides an increased number of anchor points between a branch vessel stent and a main vessel stent. This may serve to increase the mechanical linkage between stent  10  and another stent deployed in an adjacent vessel. In various embodiments, fronds  16  can be narrower (in width) than embodiments having few fronds so as to increase the flexibility of the fronds. The increased flexibility can facilitate the bending of the fronds during stent deployment including bending from the branch body lumen into the main body lumen. 
     Referring now to  FIG. 2B , in various embodiments, fronds  16  can comprise thin filaments formed into loops  17 . An exemplary embodiment of a prosthesis structure  50  having a plurality of filament loops  17  is shown in  FIG. 2B  in a rolled out pattern. In various embodiments filament loops  17  can have at least one or two or more intra-filament connectors  18 ,  19  which extend in a circumferential direction to connect two adjacent filaments defining a filament loop  17 . Connectors  18 ,  19  preferably include at least one nonlinear undulation such as a “U”, “V” or “W” or “S” shape to permit radial expansion of the prosthesis in the vicinity of the fronds. (The intra-filament space may be crossed with a balloon catheter and dilated to larger diameters). 
     The illustrated embodiment includes a first intra-filament connector  18  in the transition area  60  for each frond  16 , and a second connector  19  positioned proximally from the first connector  18 . One or both of the first and second connectors  18 ,  19  can be configured to expand or otherwise assume a different shape when the fronds are deployed. At least five or ten or 20 or more connectors  18 ,  19  may be provided between any two adjacent filaments  66  depending upon the desired clinical performance. Also connectors  18 ,  19  can be continuous with frond loops  17  and have substantially the same cross sectional thickness and/or mechanical properties. Alternatively, connectors  18 ,  19  can have different diameters and/or mechanical properties (e.g. one or more of increased elasticity, elastic limit, elongation, stiffness etc.) and/or biological properties (surface finish, passivation, coatings, etc.). In one embodiment the distal connector  18  can be stiffer than the proximal connector  19  so as to allow more flexibility at the proximal tip of the fronds. 
     Connectors  18  and  19  can be further configured to perform several functions. First, to act as mechanical struts to increase the stiffness (e.g. longitudinal, torsional, etc) of the filament fronds  16 . Second, when the fronds are deployed, connectors  18  and  19  can be designed to assume a deployed shape which provides radial mechanical support (e.g. act as prosthesis) to the target vessel including at the OS. This is particularly the case for first connector  18  which can be configured to unfurl in the circumferential direction and assume a semi-triangular shape in its deployed state with an expansion axis (of the connected points of the triangle to fronds) substantially parallel to the radial axis of the vessel. This configuration of connector  18  serves to provide radial mechanical support as well as coverage at the OS in particular. Connector  18  can also be configured to assume other deployed shapes as well, such as semi-circular etc. The number and spacing and deployed shape of the connectors  18  can be configured to provide the same amount or density at the OS (e.g. number of struts per axial or radial length of tissue) as the stent region  52  of the prosthesis provides to the rest of the vessel. In general, by varying the dimensions and number of the filaments  66  and connectors  18  any of a variety of physical properties can be achieved. The connectors  18  and  19  and filaments  66  may be selected and designed to cooperate to provide maximum area coverage, and/or maximum mechanical radial force, or either objective without the other. The number of filaments can be in the range of from about 3 to about 30, with specific embodiments of 4, 6, 10, 20 and 25. 
     In various embodiments, the arrangement of the filaments fronds can be configured to provide several functions. First, as described above they can be configured to provide increased coverage and hence patency of the Os by having an increased number of mechanical support points in the Os and hence a more even distribution of force (e.g. radial force) on the fronds. Also, for embodiments of drug coated stents, including drug eluting stents they provide an increased amount of surface area for the elution of the drug. This in turn, serves to provide increased and/or more constant local concentration of the selected drug at the vessel wall and/or other target site. Other pharmacokinetic benefits can be obtained as well, such as a more constant drug release rate. For stents coated with anti-cell proliferative, anti-inflammatory and/or anti-cell migration drugs such as Taxol (paclitaxel), Rapamycin and their derivatives, the use of high filament type fronds serve as a means to reduce the incidence and rate of hyperplasia and restenosis. Similar results can be obtained with other drugs known in the art for reducing restenosis (e.g. anti-neo-plastics, anti-inflammatory drugs, etc.). Also in a related embodiment the filament fronds can be coated with a different drug and/or a different concentration of drug as the remainder of the stent. In use, such embodiment can be configured to provide one or more of the following: i) a more constant release rate of drug; ii) bimodal release of drug; iii) multi drug therapies; and iv) titration of drug delivery/concentration for specific vessels and/or release rates. As disclosed in additional detail below, the drug may be incorporated into a biostable, biodegradeable, or bioerodable polymer matrix, and may be optimized for long-term pharma release (prophylactic local drug delivery). 
     In general, in any of the embodiments herein, the prosthesis of the present invention can be adapted to release an agent for prophylactic or active treatment from all or from portions of its surface. The active agents (therapy drug or gene) carried by the prosthesis may include any of a variety of compounds or biological materials which provide the desired therapy or desired modification of the local biological environment. Depending upon the clinical objective in a given implementation of the invention, the active agent may include immunosuppressant compounds, anti-thrombogenic agents, anti-cancer agents, hormones, or other anti-stenosis drugs. Suitable immunosuppressants may include ciclosporin A (CsA), FK506, DSG(15-deoxyspergualin, 15-dos), MMF, rapamycin and its derivatives, CCI-779, FR 900520, FR 900523, NK86-1086, daclizumab, depsidomycin, kanglemycin-C, spergualin, prodigiosin25-c, cammunomicin, demethomycin, tetranactin, tranilast, stevastelins, myriocin, gliotoxin, FR 651814, SDZ214-104, bredinin, WS9482, and steroids. Suitable anti-thrombogenic drugs may include anti-platelet agents (GP IIb/IIIa, thienopyridine, GPIb-IX, ASA, etc and inhibitors for the coagulation cascade (heparin, hirudin, thrombin inhibitors, Xa inhibitors, VIIa Inhibitors, Tissue Factor Inhibitors and the like). Suitable anti-cancer (anti proliferative) agents may include methotrexate, purine, pyridine, and botanical (e.g. paclitaxel, colchicines and triptolide), epothilone, antibiotics, and antibodies. Suitable additional anti-stenosis agents include batimastat, NO donor, 2-chlorodeoxyadenosine, 2-deoxycoformycin, FTY720, Myfortic, ISA (TX) 247, AGI-1096, OKT3, Medimmune, ATG, Zenapax, Simulect, DAB486-IL-2, Anti-ICAM-1, Thymoglobulin, Everolimus, Neoral, Azathioprine (AZA), Cyclophosphamide, Methotrexate, Brequinar Sodium, Leflunomide, or Mizoribine. Gene therapy formulations include Keratin 8, VEGF, and EGF, PTEN, Pro-UK, NOS, or C-myc may also be used. 
     Any of the coatings described herein may be provided on either the lumenal surface, the ablumenal surface, or both, on the prosthesis disclosed herein. In addition, coatings may be provided on only the support portion, only the frond portion, only the transition portion, or any combination thereof, depending upon the desired clinical performance. In addition to the coatings described above, at least a portion of the prosthesis may be provided with a coating which renders the implant compatible for in vivo attachment and proliferation of cells on the surface thereof. This coating is preferably provided on the ablumenal surface of the implant, and may be omitted from the lumenal surface of the implant. In general, the coating may comprise a therapeutically effective amount of an antibody which reacts with an endothelial cell surface antigen, to facilitate cellular proliferation on the surface of the implant. Additional details of antibody coatings to promote endothelial cell adherence may be found in U.S. Pat. No. 7,037,332, entitled Medical Device with Coating that Promotes Endothelial Cell Adherence, issued May 2, 2006 to Kutryk, et al., the disclosure of which is incorporated in its entirety herein by reference. 
     Methods of preventing restenosis include inhibiting VSMC hyperplasia or migration, promoting endothelial cell growth, or inhibiting cell matrix proliferation with the delivery of suitable compounds from the prosthesis. Radiation, systemic drug therapy and combinations of the foregoing may also be used. The desired dose delivery profiles for the foregoing are in some cases reported in the literature, or may be optimized for use with the prosthesis of the present invention through routine experimentation by those of skill in the art in view of the disclosure herein. 
     Binding systems (e.g., chemical binding, absorbable and non absorbable polymeric coatings) for releasably carrying the active agent with the prosthesis are well known in the art and can be selected to cooperate with the desired drug elution profile and other characteristics of a particular active agent as will be appreciated by those of skill in the art. 
     In general, the drug(s) may be incorporated into or affixed to the stent in a number of ways and utilizing any biocompatible materials; it may be incorporated into e.g. a polymer or a polymeric matrix and sprayed onto the outer surface of the stent. A mixture of the drug(s) and the polymeric material may be prepared in a solvent or a mixture of solvents and applied to the surfaces of the stents also by dip-coating, brush coating and/or dip/spin coating, the solvent (s) being allowed to evaporate to leave a film with entrapped drug(s). In the case of stents where the drug(s) is delivered from micropores, struts or channels, a solution of a polymer may additionally be applied as an outlayer to control the drug(s) release; alternatively, the active agent may be comprised in the micropores, struts or channels and the active co-agent may be incorporated in the outlayer, or vice versa. The active agent may also be affixed in an inner layer of the stent and the active co-agent in an outer layer, or vice versa. The drug(s) may also be attached by a covalent bond, e.g. esters, amides or anhydrides, to the stent surface, involving chemical derivatization. The drug(s) may also be incorporated into a biocompatible porous ceramic coating, e.g. a nanoporous ceramic coating. The medical device of the invention is configured to release the active co-agent concurrent with or subsequent to the release of the active agent. 
     Examples of polymeric materials known for this purpose include hydrophilic, hydrophobic or biocompatible biodegradable materials, e.g. polycarboxylic acids; cellulosic polymers; starch; collagen; hyaluronic acid; gelatin; lactone-based polyesters or copolyesters, e.g. polylactide; polyglycolide; polylactide-glycolide; polycaprolactone; polycaprolactone-glycolide; poly(hydroxybutyrate); poly(hydroxyvalerate); polyhydroxy (butyrate-co-valerate); polyglycolide-co-trimethylene carbonate; poly(dioxanone); polyorthoesters; polyanhydrides; polyaminoacids; polysaccharides; polyphosphoesters; polyphosphoester-urethane; polycyanoacrylates; polyphosphazenes; poly(ether-ester) copolymers, e.g. PEO-PLLA, fibrin; fibrinogen; or mixtures thereof; and biocompatible non-degrading materials, e.g. polyurethane; polyolefins; polyesters; polyamides; polycaprolactam; polyimide; polyvinyl chloride; polyvinyl methyl ether; polyvinyl alcohol or vinyl alcohol/olefin copolymers, e.g. vinyl alcohol/ethylene copolymers; polyacrylonitrile; polystyrene copolymers of vinyl monomers with olefins, e.g. styrene acrylonitrile copolymers, ethylene methyl methacrylate copolymers; polydimethylsiloxane; poly(ethylene-vinylacetate); acrylate based polymers or copolymers, e.g. polybutylmethacrylate, poly(hydroxyethyl methylmethacrylate); polyvinyl pyrrolidinone; fluorinated polymers such as polytetrafluoroethylene; cellulose esters e.g. cellulose acetate, cellulose nitrate or cellulose propionate; or mixtures thereof. 
     When a polymeric matrix is used, it may comprise multiple layers, e.g. a base layer in which the drug(s) is/are incorporated, e.g. ethylene-co-vinylacetate and polybutylmethacrylate, and a top coat, e.g. polybutylmethacrylate, which is drug(s)-free and acts as a diffusion-control of the drug(s). Alternatively, the active agent may be comprised in the base layer and the active co-agent may be incorporated in the outlayer, or vice versa. Total thickness of the polymeric matrix may be from about 1 to 20μ or greater. 
     The drug(s) elutes from the polymeric material or the stent over time and enters the surrounding tissue, e.g. up to ca. 1 month to 10 years. The local delivery according to the present invention allows for high concentration of the drug(s) at the disease site with low concentration of circulating compound. The amount of drug(s) used for local delivery applications will vary depending on the compounds used, the condition to be treated and the desired effect. For purposes of the invention, a therapeutically effective amount will be administered; for example, the drug delivery device or system is configured to release the active agent and/or the active co-agent at a rate of 0.001 to 200 μg/day. By therapeutically effective amount is intended an amount sufficient to inhibit cellular proliferation and resulting in the prevention and treatment of the disease state. Specifically, for the prevention or treatment of restenosis e.g. after revascularization, or antitumor treatment, local delivery may require less compound than systemic administration. The drug(s) may elute passively, actively or under activation, e.g. light-activation. 
     A possible alternative to a coated stent is a stent containing wells or reservoirs that are loaded with a drug, as discussed by Wright et al., in “Modified Stent Useful for Delivery of Drugs Along Stent Strut,” U.S. Pat. No. 6,273,913, issued Aug. 14, 2001; and Wright et al., in “Stent with Therapeutically Active Dosage of Rapamycin Coated Thereon,” US patent publication US 2001/0027340, published Oct. 4, 2001, the disclosures of both of which are incorporated in their entireties herein by reference. 
     Wright et al, in U.S. Pat. No. 6,273,913, describes the delivery of rapamycin from an intravascular stent and directly from micropores formed in the stent body to inhibit neointimal tissue proliferation and restenosis. The stent, which has been modified to contain micropores, is dipped into a solution of rapamycin and an organic solvent, and the solution is allowed to permeate into the micropores. After the solvent has been allowed to dry, a polymer layer may be applied as an outer layer for a controlled release of the drug. 
     U.S. Pat. No. 5,843,172 by Yan, which is entitled “Porous Medicated Stent”, discloses a metallic stent that has a plurality of pores in the metal that are loaded with medication. The drug loaded into the pores is a first medication, and an outer layer or coating may contain a second medication. The porous cavities of the stent can be formed by sintering the stent material from metallic particles, filaments, fibers, wires or other materials such as sheets of sintered materials. 
     Leone et al. in U.S. Pat. No. 5,891,108 entitled “Drug Delivery Stent” describes a retrievable drug delivery stent, which is made of a hollow tubular wire. The tubular wire or tubing has holes in its body for delivering a liquid solution or drug to a stenotic lesion. Brown et al. in “Directional Drug Delivery Stent and Method of Use,” U.S. Pat. No. 6,071,305 issued Jun. 6, 2000, discloses a tube with an eccentric inner diameter and holes or channels along the periphery that house drugs and can deliver them preferentially to one side of the tube. Scheerder et al. in US patent publication US 2002/0007209, discloses a series of holes or perforations cut into the struts on a stent that are able to house therapeutic agents for local delivery. 
     Referring to the patent literature, Heparin, as well as other anti-platelet or anti-thrombolytic surface coatings, have been reported to reduce thrombosis when carried by the stent surface. Stents including both a heparin surface and an active agent stored inside of a coating are disclosed, for example, in U.S. Pat. Nos. 6,231,600 and 5,288,711. 
     A variety of agents specifically identified as inhibiting smooth muscle-cell proliferation, and thus inhibit restenosis, have also been proposed for release from endovascular stents. As examples, U.S. Pat. No. 6,159,488 describes the use of a quinazolinone derivative; U.S. Pat. No. 6,171,609, describes the use of taxol, and U.S. Pat. No. 5,176,981, the use of paclitaxel, a cytotoxic agent thought to be the active ingredient in the agent taxol. The metal silver is cited in U.S. Pat. No. 5,873,904. Tranilast, a membrane stabilizing agent thought to have anti-inflammatory properties is disclosed in U.S. Pat. No. 5,733,327. 
     More recently, rapamycin, an immunosuppressant reported to suppress both smooth muscle cell and endothelial cell growth, has been shown to have improved effectiveness against restenosis, when delivered from a stent. See, for example, U.S. Pat. Nos. 5,288,711 and 6,153,252. Also, in PCT Publication No. WO 97/35575, the monocyclic triene immunosuppressive compound everolimus and related compounds have been proposed for treating restenosis, via systemic delivery. 
     Any one or a combination of the frond section, support section and transition may comprise a bioabsorbable material, which will degrade or otherwise dissipate over time. The bioabsorbable implant may be a convenient platform for the elution of any of a variety of biologically active agents, such as those identified above. 
     Prostheses in accordance with the present invention may comprise any of a variety of bioabsorbable polymers, depending upon the desired performance. These may include poly(alpha-hydroxy acid) such as polylactide [poly-L-lactide (PLLA), poly-D-lactide (PDLA)], polyglycolide (PGA), polydioxanone, polycaprolactone, polygluconate, polylactic acid-polyethylene oxide copolymers, poly(hydroxybutyrate), polyanhydride, polyphosphoester, poly(amino acids), or related copolymers materials, each of which have a characteristic degradation rate in the body. For example, PGA and polydioxanone are relatively fast-bioabsorbing materials (weeks to months) and PLA and polycaprolactone are relatively slow-bioabsorbing material (months to years). 
     Bioabsorbable PLLA and PGA material are degraded in vivo through hydrolytic chain scission to lactic acid and glycolic acid, respectively, which in turn is converted to CO.sub.2 and then eliminated from the body by respiration. Heterogeneous degradation of semicrystalline polymers occurs due to the fact that such materials have amorphous and crystalline regions. Degradation occurs more rapidly at amorphous regions than at crystalline regions. This results in the product decreasing in strength faster than it decreases in mass. Totally amorphous, cross-linked polyesters show a more linear decrease in strength with mass over time as compared to a material with crystalline and amorphous regions. Degradation time may be affected by variations in chemical composition and polymer chain structures, and material processing. 
     Controlled release of a drug, via a bioabsorbable polymer, offers to maintain the drug level within the desired therapeutic range for the duration of the treatment. In the case of stents, the prosthesis materials may be selected to maintain vessel support for at least about two weeks or until incorporated into the vessel wall even with bioabsorbable, biodegradable polymer constructions. 
     Several polymeric compounds that are known to be bioabsorbable and hypothetically have the ability to be drug impregnated may be useful in prosthesis formation herein. These compounds include: poly-1-lactic acid/polyglycolic acid, polyanhydride, and polyphosphate ester. A brief description is provided below. 
     Poly-1-lactic acid/polyglycolic acid has been used for many years in the area of bioabsorbable sutures. It is currently available in many forms, i.e., crystals, fibers, blocks, plates, etc. These compounds degrade into non-toxic lactic and glycolic acids. There may, however, be several problems with this compound. The degradation artifacts (lactic acid and glycolic acid) are slightly acidic. The acidity can cause minor inflammation in the tissues as the polymer degrades. This same inflammation could be detrimental in coronary and peripheral arteries, i.e., vessel occlusion. Another potential problem associated with this polymer is the ability to control and predict the degradation behavior. It does not appear possible for the biochemist to accurately predict degradation time, which could be detrimental for a drug delivery device if dosing parameters need to be tightly controlled. 
     Other compounds which could be used are the polyanhydrides. They are currently being used with several chemotherapy drugs for the treatment of cancerous tumors. These drugs are compounded into the polymer which is molded into a cube-like structure and surgically implanted at the tumor site. 
     Polyanhydrides have weaknesses in their mechanical properties, due to low molecular weights. This drawback may make them difficult to process into a filament form such as for the fronds or transition section of the prostheses disclosed herein. Also, polyanhydrides have relatively poor solubility, making characterization and fabrication difficult. 
     A third class of compounds which may be preferred includes polyphosphate esters. Polyphosphate ester is a compound such as that disclosed in U.S. Pat. Nos. 5,176,907; 5,194,581; and 5,656,765 issued to Leong which are incorporated herein by reference. The polyphosphate esters have high molecular weights ( 600 , 000  average), yielding attractive mechanical properties. This high molecular weight leads to transparency, and film and fiber properties. It has also been observed that the phosphorous-carbon-oxygen plasticizing effect, which lowers the glass transition temperature, makes the polymer desirable for fabrication. 
     The basic structure of polyphosphate ester monomer is shown below. 
     
       
         
         
             
             
         
       
     
     where 
     P corresponds to Phosphorous, 
     O corresponds to Oxygen, 
     and R and R1 are functional groups. 
     Reaction with water leads to the breakdown of this compound into monomeric phosphates (phosphoric acid) and diols (see below). 
     
       
         
         
             
             
         
       
     
     It is the hydrolytic instability of the phosphorous ester bond which makes this polymer attractive for controlled drug release applications. A wide range of controllable degradation rates can be obtained by adjusting the hydrophobicities of the backbones of the polymers and yet assure biodegradability. 
     The functional side groups allow for the chemical linkage of drug molecules to the polymer. This is shown below. 
     
       
         
         
             
             
         
       
     
     The drug may also be incorporated into the backbone of the polymer. 
     
       
         
         
             
             
         
       
     
     The highly hydrolytically reactive phosphorous ester bond, the favorable physical properties, and the versatile chemical structure may make the polyphosphate esters a superior drug delivery system for a prosthesis. See U.S. Pat. No. 5,545,208 to Wolff, the disclosure of which is incorporated in its entirety herein by reference. 
     Use of multiple filaments per frond also provides for a more open structure of the fronds section  54  of the prosthesis to allow for an easier and less obstructed passage of a guide wire and/or the deployment balloon by and/or through the fronds (e.g., during un-jailing procedures known in the art). Similarly, use of the flexible filaments also allows the main vessel to track between fronds and engage the main vessel stent. In particular, the thinner frond filaments facilitate advancement of the fronds over the circumference and/or the length of a main vessel stent during deployment of the fronds or the main vessel stent. Moreover, the filaments can be configured to be easily withdrawn and then re-advanced again to allow for repositioning of either of the branch vessel stent. Other means for facilitating advancement of the main vessel stent between the fronds can include tapering the fronds and/or coating the fronds with a lubricous coating such as PTFE or silicone (this also facilitates release of the fronds from constraining means described herein). Finally, by having an increased number of filaments, the mechanical support of the Os is not compromised if one or more filaments should become pushed aside during the stent deployment. That is, the remaining filaments provide sufficient support of the Os to maintain it patency. In these and related embodiments, it may be desirable to have at least six loops  17  each comprising at least one filament looped back upon itself at its proximal limit to provide at least two elements per frond. 
     Various embodiments of the fronds can be configured to provide an increased amount of mechanical linkage between the fronds and the main vessel stent. In general, the frond design seeks to 1) track to site, 2) allow for advancement of MV Stent 3) increase frond-MV stent interaction and 4) frond MV wall interactions. Another means includes increasing the number of fronds to provide an increased number of anchor points between a branch vessel stent and a main vessel stent. This in turn provides an increased amount of mechanical linkage between the two stents such that they increasingly operate mechanically as one structure rather than two after deployment. This also serves to improve the spatial stability of the deployed stents within both vessels. That is, there is reduced movement (e.g., axial or radial) or reduced possibility of movement of one or both stents within their respective vessels. In particular, the linkage serves to provide radial strength of the structure in the ostium. 
     Referring now to  FIG. 2C  in an alternative embodiment of a prosthesis  50  having filament fronds  17 , one or two or more frond can be a shortened frond  16   s . That is a frond that is shortened in the longitudinal direction. In the illustrated embodiment, shortened fronds  16   s  and full length fronds  16  alternate around the circumference of the stent. The amount of shortening can range from 10% to 99%. In a preferred embodiment, fronds  16   s  are shortened by approximately slightly less than 50% in length from the length of un-shortened fronds  16 . Embodiments having shortened fronds, reduce the likelihood of resistance when the main vessel stent  150  is positioned. Shortened fronds  16   s  also can be configured to act more like point contacts on the main vessel stent  150  and should therefore be less likely to be swept towards the Os by deployment and/or misalignment of the main vessel stent and deployment balloon. Also, use of less material in the fronds tends to produce less displacement of the fronds even if the main vessel stent or balloon catches multiple fronds, and may produce a lower biological reaction (less foreign material). 
       FIGS. 2D and 2E  illustrate an alternative side wall patterns for the transition portion of the prosthesis of the present invention, on stents having two different side wall patterns. As described previously, the specific stent or other support structure configuration may be varied considerably within the context of the present invention. 
     In each of the embodiments of  FIGS. 2D and 2E , the struts  70  at the frond root (e.g. transition zone) are provided with an interdigitating or nesting configuration. In this configuration, as viewed in the flat, laid out view as in  FIGS. 2D and 2E , a plurality of struts  70  extend across the transition zone. A distal segment  72  of each strut  70  inclines laterally in a first direction, to an apex  74 , and then inclines laterally in a second direction to a point that may be approximately axially aligned with a distal limit of the distal segment  72 . The extent of lateral displacement of the strut between its origin and the apex  74  is greater than the distance between adjacent struts, when in the unexpanded configuration. In this manner, adjacent struts stack up or nest within each other, each having a concavity  78  facing in a first lateral direction and a corresponding convexity  80  in a second lateral direction. This configuration seeks to optimize vessel wall coverage at the ostium, when the stent is expanded. 
     The axial length of each frond is at least about 10%, often at least about 20%, and in some embodiments at least about 35% or 75% or more of the length of the overall prosthesis. Within this length, adjacent fronds may be constructed without any lateral interconnection, to optimize the independent flexibility. The axially extending component of the frond may be provided with an undulating or serpentine structure  82 , which helps enable the fronds to rotate out of the plane when the main vessel stent is deployed. Circumferential portions of the undulating fronds structure make the frond very flexible out of the plane of the frond for trackability. A plurality of connectors  84  are provided between parallel undulating filaments  86 ,  88  of each frond, to keep the frond from being overly floppy and prone to undesirable deformation. Each of the fronds in the illustrated embodiment has a broad (i.e. relatively large radius) frond tip  90 , to provide an atraumatic tip to minimize the risk of perforating the arterial or other vascular wall. 
     The interdigitating construction in the transition zone, as well as the undulating pattern of the frond sections both provides optimal coverage at the ostium, and provides additional strut length extension or elongation capabilities, which may be desirable during the implantation process. 
     It may also be desirable to vary the physical properties of the filaments,  86 ,  88 , or elsewhere in the prosthesis, to achieve desired expansion results. For example, referring to  FIG. 2E , each frond  16  includes a first filament  92 , attached at a first attachment point  94  and a second filament  96  attached at a second attachment point  98  to the stent. A third filament  100  and a fourth filament  102  are connected to the stent at an intermediate attachment point  104 . As illustrated, the transverse width of the third and fourth filaments  100  and  102  are less than the transverse width of the first and second filaments  92 ,  96 . The thinner filaments  100 ,  102  provide less resistance to expansion, and help maintain optimal coverage in the vicinity of the ostium upon expansion of the prosthesis. 
     In any of the embodiments described herein, the fronds may be considered to have a lumenal surface at least a portion of which will be in contact with an outside surface of the main vessel stent, and an ablumenal surface which will be pressed into contact with the vascular wall by the main vessel stent. The lumenal and ablumenal surfaces of the fronds may be provided with similar or dissimilar characteristics, depending upon the desired performance. For example, as described elsewhere herein, the frond and particularly the ablumenal surface may be provided with a drug eluting characteristic. 
     It may also be desirable to modify the lumenal surface of the frond, to enhance the physical interaction with the main vessel stent. For this purpose, the lumenal surface of the frond may be provided with any of a variety of friction enhancing surface characteristics, or engagement structures for engaging the main vessel stent. Friction enhancing surfaces may comprise the use of polymeric coatings, or mechanical roughening such as laser etching, chemical etching, sputtering, or other processes. Alternatively, any of a variety of radially inwardly extending hooks or barbs may be provided, for engaging the main vessel stent. Preferably, any radially inwardly extending hooks or barbs will have an axial length in the radial direction of no greater than approximately the wall thickness of the main vessel stent strut, to minimize the introduction of blood flow turbulence. Although a variety of main vessel stents are available, the inventors presently contemplate wall thicknesses for the struts of such main vessel stents to be on the order of about 0.003 to 0.0055 inches for native coronary indications. Any of the foregoing surfaces textures or structures may also be provided on the ablumenal surface of the main vessel stent, to cooperate with corresponding textures or structures on the fronds, to enhance the physical integrity of the junction between the two, and potentially reduce the risk for vessel perforation by fronds. 
     As will be described in additional detail in connection with the method, below, proper positioning of the prosthesis with respect to the bifurcation may be important. To facilitate positioning of the transition zone relative to the carina or other anatomical feature of the bifurcation, the prosthesis is preferably provided with a first radiopaque marker at a distal end of the transition zone and a second radiopaque marker at the proximal end of the transition zone. The proximal and distal radiopaque markers may take the form of radiopaque bands of material, or discreet markers which are attached to the prosthesis structure. This will enable centering of the transition zone on a desired anatomical target, relative to the ostium of the bifurcation. In general, it is desirable to avoid positioning the stent or other support such that it extends into the main vessel. A single marker may be used to denote the placement location of the transition zone. 
     Alternatively, the marker band or bands or other markers may be carried by the deployment catheter beneath the prosthesis, and axially aligned with, for example, the proximal and distal ends of the transition zone in addition to markers delineating the proximal and distal end of the prosthesis. 
     Although the prosthesis has been disclosed herein primarily in the context of a distal branch vessel stent carrying a plurality of proximally extending fronds, other configurations may be constructed within the scope of the present invention. For example, the orientations may be reversed such that the fronds extend in a distal direction from the support structure. Alternatively, a support structure such as a stent may be provided at each of the proximal and distal ends of a plurality of frond like connectors. This structure may be deployed, for example, with a distal stent in the branch lumen, a plurality of connectors extending across the ostium into the main vessel, and the proximal stent deployed in the main vessel proximal to the ostium. A separate main vessel stent may thereafter be positioned through the proximal stent of the prosthesis, across the ostium and into the main vessel on the distal side of the bifurcation. 
     In addition, the prosthesis has been primarily described herein as a unitary structure, such as might be produced by laser cutting the prosthesis from a tubular stock. Alternatively, the prosthesis may be constructed such as by welding, brazing, or other attachment techniques to secure a plurality of fronds onto a separately constructed support. This permits the use of dissimilar materials, having a variety of hybrid characteristics, such as a self expandable plurality of fronds connected to a balloon expandable support. Once released from a restraint on the deployment catheter, self expandable fronds will tend to bias radially outwardly against the vascular wall, which may be desirable during the process of implanting the main vessel stent. Alternatively, the entire structure can be self expandable or balloon expandable, or the support can be self expandable as is described elsewhere herein. In general, the proximal end of the fronds will contribute no incremental radial force to the prosthesis. The distal end of the fronds may contribute radial force only to the extent that it is transmitted down the frond from the support structure. 
     In each of the embodiments illustrated in  FIGS. 2A-2E , the fronds have been illustrated as extending between a first end which is attached to the support  52 , and a second, free end. In any of the frond designs disclosed herein, it may be desirable to provide a connection between the fronds in the vicinity of the free end. The connection may be accomplished in any of a variety of ways, such as providing a series of interfrond segments or connections, which, when deployed and expanded, form a circumferential ring which links the fronds. Alternatively, the circumferential link may be frangible, such that it maintains the spatial orientation of the fronds prior to a final expansion step, but is severed or otherwise releases the fronds upon final expansion. 
     The provision of a circumferential link at the proximal end of the fronds may provide a variety of benefits. For example, in an embodiment intended for balloon expansion at the treatment site, the circumferential link will assist in maintaining the crimped profile of the fronds on the balloon during transluminal navigation. The circumferential link may be configured with sufficient holding force that an outer sleeve such as those discussed in connection with  FIGS. 5 and 6  may be omitted. In addition, the provision of a circumferential link may provide sufficient radiopacity either by itself or by carrying separate radiopaque markers to permit visualization of the ends of the fronds. 
     Once at the treatment site, the circumferential link will assist in maintaining the spacing of the fronds and also in holding the proximal ends of the fronds open to facilitate advancement of the main vessel stent therethrough. The circumferential link may additionally assist in controlling the fronds if, during the procedure, it is determined to crush the fronds against a wall of the vessel. The circumferential link will assist in maintaining the fronds against the wall while a secondary strategy is employed. 
     The circumferential link may be provided by any of a variety of techniques which will be understood to those in the stent manufacturing arts. For example, the circumferential link may be formed integrally with the stent and fronds such as by laser cutting from tube stock. Alternatively, the circumferential link may be attached to previously formed fronds, using any of a variety of bonding techniques such as welding, brazing, adhesives or others depending upon the materials of the fronds and circumferential link. Although it may add to the wall thickness, the circumferential link may be interlocked with or crimped to the fronds. 
     The circumferential link may alternatively be a polymeric band or tubular sleeve. For example, a radially expandable tubular sleeve may be positioned around the outside surface of the fronds, or adjacent the lumenal surface of the fronds. A polymeric circumferential link may also be formed such as by dipping the fronds or spraying the fronds with a suitable polymeric precursor or molten material. Polymeric circumferential links may be permanent, severable, or may be bioabsorbable or bioerodeable over time. 
     One embodiment of a circumferential link is illustrated schematically in  FIG. 2F . In this embodiment, a circumferential link  120  is provided, which connects each adjacent pair of fronds together, to produce a circumferential link  120  which extends completely around the axis of the prosthesis. In this illustration, the circumferential link  120  thus comprises a discrete transverse connection between each adjacent pair of fronds. Thus, for example, a first segment  122  is provided between a first and a second frond. The first segment  122  is expandable or enlargeable in a circumferential direction. A first segment  122  has a first end  126  at the point of attachment of the first segment  122  to a first frond, and a second end  128  at a point of attachment between the first segment  122  and a second frond. The arc distance or the linear distance between the first end  126  and second end  128  measured in a plane transverse to the longitudinal axis of the prosthesis is enlargeable from a first distance for transluminal navigation, to a second distance following expansion of the fronds within the main vessel. To accommodate the radial expansion of the circumferential link  120 , the first segment  122  is provided with an undulating configuration having at least one and optionally  2  or three or more apex  130 , as will be understood in the art. In one embodiment, each adjacent pair of fronds is connected by a transverse segment (e.g.,  122 ,  124  etc.) and each of the transverse segments is identical to each other transverse segments. 
     Although the first segment  122  and second segment  124  are each illustrated in  FIG. 2F  as comprising only a single transversely extending filament, two or three or more filaments may be provided between each adjacent pair of fronds, depending upon the desired performance. As used herein, the term “circumferential link” does not limit the link  120  to only a single filament between adjacent fronds. For example, the circumferential link may comprise a stent or other support structure which is similar to the support structure  52 . 
     Referring to  FIG. 2G , there is illustrated a prosthesis in accordance with the present invention illustrating a spiral frond configuration. In general, the prosthesis  50  comprises a support section  52  and a frond section  54  generally as has been discussed previously. Depending upon the desired performance characteristics, a transition section  60  may be provided between the support section  52  and the frond section  54 . In the illustrated embodiment, the proximal end of the frond section  54  is provided with a circumferential link  120  as has been discussed. 
     The frond section  54  comprises a single frond  16 , in the illustrated embodiment in the form of a single wire or filament, which extends in a spiral configuration about the longitudinal axis of the prosthesis. The frond  16  may comprise a single filament as illustrated, or may comprise a more complex, fenestrated or multi-filament configuration as has been discussed herein. 
     The frond  16  is additionally formed into a sinusoidal configuration, alternately having concavities facing in a proximal direction and concavities facing distally. In the illustrated embodiment, the frond  16  is provided with a sinusoidal pattern having a plurality of generally oppositely facing substantially constant radius curves. The fronds  16  may alternatively comprise a plurality of “U” shaped or “V” shaped undulations, or other configuration depending upon the desired clinical performance. 
     In the illustrated embodiment, a single frond  16  extends in a spiral about the longitudinal axis of the prosthesis  50 . Alternatively, two fronds  16  or three fronds  16  or more may be utilized, each spiraling about the longitudinal axis, as will be apparent to those of skill in the art in view of the disclosure herein. 
     The spiral frond  16  will generally extend for at least two complete revolutions about the longitudinal axis of the prosthesis. The frond  16  will often extend through at least about 4 complete revolutions, and, in some embodiments, at least about 6 complete revolutions about the longitudinal axis of the prosthesis. The axial length of the frond section  54  in a helical frond embodiment will often be at least about 25% of the overall, unstretched length of the prosthesis. In certain embodiments, the axial length of the frond section  54  will be at least about 30% of the overall unstretched length of the prosthesis. 
     At the distal end of the frond section  54 , a connection is made to the support section  52 . In the illustrated embodiment, a plurality of connectors  61  is provided. Connectors  61  may conveniently be formed integrally with or attached to the support section  52  at any of a variety of locations, such as on one or more apexes  63 . The connectors  61  extend from the apex  63  to the frond  16 , such as on a distally convex curve or a distally concave curve of the frond  16 . One or two or three or four or more connectors  61  may be utilized to connect the frond  16  to the support section  52 . In the illustrated embodiment, each apex  63  on the proximal end of the support section  52  is provided with a unique connector  61 , for connection to the frond  16 . As can be appreciated by those of skill in the art, the axial length of the connectors  61  will enlarge progressively at progressive circumferential positions about the axis of the support section  52 , to accommodate the inclined angle of the distal most loop of frond  16 , which, due to its spiral configuration, resides on a transverse plane which is inclined at an angle θ with respect to the longitudinal axis of the prosthesis  50 . In spiral frond embodiments, the angle θ can be within the range of from about 95° to about 170°. In spiral frond embodiments, the angle θ can be within the range of from about 95° to about 135°. In spiral frond embodiments, the angle θ can be about 110°. 
     In the embodiment illustrated in  FIG. 2G , the proximal end of the frond  16  is provided with a circumferential link  120 . Circumferential link  120  may be connected to the frond  16  in a variety of ways, such as by providing one or more connectors  121 . Connector  121  is illustrated as connected to or formed with an apex  123  on the sinusoidal frond  16 , and also to a distally facing apex  125  on the circumferential link  120 . A second connector  128  is illustrated, similarly connected between a proximally facing convexity on the frond  16  and a distally facing convexity on the circumferential link  120 . In the illustrated embodiment, every other apex  125  on the circumferential link  120  is provided with a connector for connection to the frond  16 . Alternatively, every third apex  125  or every apex  125  may be connected to the frond  16  by a connector  121 . 
     In many ways the prosthesis illustrated in  FIG. 2G  may be utilized in a manner similar to other prostheses disclosed herein. However, the spiral configuration of fronds  16  provides a greater level of radial support than many of the other frond configurations disclosed herein. As a consequence, the spiral frond configuration may also be utilized as a provisional side branch stent while other configurations may lack sufficient radial force to be used in this manner. In a provisional stenting, the stent is placed in the side branch and then evaluated for whether a second stent is desirable. The spiral wound frond exhibits sufficient radial force that it may be used either alone or with a second stent. If a second stent is desired, it may be advanced through the side wall of the spiral frond  16  and into the main vessel as has been discussed. Alternatively, the spiral frond prosthesis may be left as a single stent, depending upon the desired clinical performance. 
     In addition, the spiral fronds support the cantilevered transition zone  60  such that radial strength is provided within the ostium of the bifurcation. 
     In general terms, the prosthesis  50  may be considered to be a tubular structure which comprises a plurality of axially extending fronds having a first radially expandable structure on a first end and a second radially expandable structure on a second end. The first radially expandable structure is a support structure  52  such as a stent, as has been described for positioning within the branch vessel. The second radially expandable structure comprises the circumferential link  120 . 
     Typically, the circumferential link  120  will provide significantly less radial force than the first support structure  52 , in view of its primary function to maintain the spacing and orientation of the fronds rather than providing support to the vessel wall. In a typical embodiment, the support structure  52  will have a first radial force, the circumferential link  120  will have a second, lesser radial force, and the fronds will contribute nothing or essentially nothing to the radial force of the structure. Alternatively, the circumferential link  120  may have a radial force which is approximately equal to the radial force of the support structure  52 , and possibly even in excess of the radial force of the support structure  52 , depending upon the desired clinical performance. The fronds may exhibit a radial force, which may be due mostly or entirely to the adjacent stent or circumferential link. 
     In an implementation of the invention intended for use in the coronary arteries, the stent portion may have a crush resistance or radial strength on the order of at least about 10 psi or 12 psi, and, often at least about 14 or 15 psi. The circumferential link may have a radial force or crush resistance of no greater than about 90%, often no greater than about 50%, and in some embodiments no greater than about 25% of the radial force or crush resistance of the branch vessel stent. Thus, in a prosthesis having a stent with a crush resistance of at least about 14 or 15 psi, the circumferential link might have a crush resistance of less than about 4 or 3 psi. The crush resistance in the fronds may be less than about 2 psi or less than about 1 psi, depending upon the length of the fronds, structure of the fronds, crush resistance of the adjacent structures, and other factors that may affect the cantilevered transfer of radial force from the adjacent stent or circumferential link. 
     Radial strength or crush resistance as used herein, may be determined in psi by constructing a radial strength test fixture. In general, the radial strength test fixture comprises a pressured chamber, adapted to allow the insertion of a flexible tube which can be sealed at each end to the walls of the chamber such that the exterior wall of the tubing is exposed to the pressure generated in the chamber while the central lumen of the tube is exposed to ambient atmospheric pressure. Any of a variety of thin walled flexible tubing may be utilized, such as a thin walled latex tubing, such that the inside diameter of the latex tubing may be approximately 10% less than the nominal expanded diameter of the stent. The stent is expanded within the tubing, such as by inflating an associated dilation balloon to its rated burst pressure or other pressure sufficient to expand the stent to its intended implanted diameter. The balloon may be deflated and the balloon catheter withdrawn. The tubing is mounted in the pressure chamber as described above. Air or other inflation media may be pumped into the pressure chamber to slowly increase the pressure within the chamber (for example at a rate of about 1 psi per second). Once any portion of the central lumen through the prosthesis has been reduced under pressure to less than or equal to 50% of its original lumen diameter, the pressure in the chamber is noted and considered to be the radial force or crush resistance of the prosthesis. 
     The second radially expandable structure (circumferential link) may also have a shorter axial length than the first radially expandable structure (stent). For example, in a coronary artery embodiment, the axial length of the stent may be at least 300% or 500% or more of the length of the circumferential link. 
     The fronds will have a length in the axial direction between the support  52  and the circumferential link  120  of generally in excess of about 2.5 mm or 3 mm, and in certain embodiments in excess of about 5 mm. At least some or all of the fronds may have a length in excess of about 8 mm, and, in one implementation of the invention intended for the coronary artery, the frond length is in the vicinity of about 9.4 mm. 
     The circumferential link may also have a smaller strut profile compared to the strut profile in the support  52 . For example, the cross sectional dimensions of a strut in the support  52  and/or the fronds may be on the order of about 0.003 inches by about 0.055 inches in an embodiment intended for coronary artery applications. In the same embodiment, the cross sectional dimensions through a strut in the circumferential link may be on the order of about 0.001 inches by about 0.003 inches. 
     The frond length may also be evaluated relative to the main lumen diameter. For example, in the coronary artery environment, diameters in the range of from about 2 mm to about 5 mm are often encountered. Frond lengths of at least about equal to the main vessel diameter (e.g. at least about 2 mm or 3 mm or 4 mm or greater) are contemplated. Fronds lengths of as much as 2 times or 3 times or 4 times or more of the diameter of the associated main vessel are also contemplated. 
     Deployment of the bifurcation prosthesis with linked fronds may be understood by reference to  FIGS. 14A-14E . In  FIG. 14A , the side branch guidewire  121 ′ has been positioned in the side branch and the main vessel guidewire  123 ′ has been positioned in the main vessel. The side branch stent is next deployed in the side branch, with the fronds extending across the ostium and into the main vessel. The circumferential link  120  may either self expand or be balloon expandable to provide a main vessel stent opening. See  FIG. 14B . 
     Referring to  FIG. 14C , the side branch wire is retracted from the side branch and advanced between the fronds into the main vessel. The main vessel wire  123  may be retracted at this point in the procedure. The main vessel stent is then advanced over the wire through the opening formed by the circumferential link, and through a space between adjacent fronds into the desired position. 
     Referring to  FIG. 14D , the main vessel stent is deployed to entrap the fronds against the vessel wall. The circumferential link is additionally trapped against the vessel wall. Post dilation to open the side wall opening into the branch vessel may optionally be accomplished, by retracting the side branch wire and readvancing it into the side branch. See  FIG. 14E . 
     Based upon the foregoing description, it will be apparent to those of skill in the art that the prosthesis of the present invention may be implanted in a variety of alternative manners. For example, the first support structure (described above as a side branch stent) may be positioned in the main vessel, distally (from the perspective of the delivery catheter) of the bifurcation with the fronds extending proximally across the opening to the side branch. The second support structure (referred to above as a circumferential link), if present, is positioned in the main vessel proximally of the side branch opening. A standard stent may then be positioned such that the distal end of the stent is within the side branch, and a proximal end of the stent is within the main vessel, such as within the circumferential link. 
     Referring now to  FIGS. 3A-8 , in various embodiments prosthesis/delivery system  205  can include a prosthesis with stent  210  and fronds  220  which are configured to be captured or otherwise radially constrained by the delivery system during advancement of the stent through the vasculature or other body lumen. As shown in  FIGS. 3A-3B , fronds  220  can be separated by axial gaps or splits  230  along the length of the frond structure. Splits  230  can have a variety of widths and in various embodiments, can have a width between 0.05 to 2 times the width of the fronds, with specific embodiments of no more than about 0.05, 0.25, 0.5, 1 and 2 times the width of the fronds. Fronds  220  can be configured to have sufficient flexibility to be advanced while in a captured mode through curved and/or tortuous vessels to reach the more distal portions of the vasculature such as distal portion of the coronary vasculature. This can be achieved through the selection of dimensions and/or material properties (e.g. flexural properties) of the fronds. For example, all or a portion of fronds  220  can comprise a resilient metal (e.g., stainless steel) or a superelastic material known in the art. Examples of suitable superelastic materials include various nickel titanium alloys known in the art such as Nitinol™. 
     Any of a variety of modifications or features may be provided on the fronds, to enhance flexibility or rotatability in one or more planes. For example, fronds may be provided with a reduced thickness throughout their length, compared to the thickness of the corresponding stent. The thickness of the frond may be tapered from relatively thicker at the distal (attachment) end to the proximal free end. Fronds may be provided with one or more grooves or recesses, or a plurality of wells or apertures, to affect flexibility. The specific configuration of any such flexibility modifying characteristic can be optimized through routine experimentation by those of skill in the art in view of the present disclosure, taking into account the desired clinical performance. 
     It is desirable to have the fronds captured and held against the delivery catheter or otherwise restrained as the stent is advanced through the vasculature in order to prevent the fronds from divaricating or separating from the prosthesis delivery system prosthesis. Capture of the fronds and prevention of divarication can be achieved through a variety of means. For example, in various embodiments the capture means can be configured to prevent divarication by imparting sufficient hoop strength to the fronds, or a structure including the fronds, to prevent the fronds from separating and branching from the deployment balloon as the balloon catheter is advanced through the vasculature including tortuous vasculature. In theses embodiments, the capture means is also configured to allow the fronds to have sufficient flexibility to be advanced through the vasculature as described above. 
     In an embodiment shown in  FIGS. 3A-4B , the fronds can be captured under the flaps  242  of a deployment balloon  241  of a delivery balloon catheter  240 . In this and related embodiments, the balloon  241  and stent  210  can be configured such that flaps  242  are substantially matched up or aligned with splits  230 . This can be achieved using alignment techniques known in the art (e.g., use of alignment fixtures) when the stent  220  is positioned over balloon  241 . The flap material will initially extend or protruded through the splits, but is then folded over onto one or more fronds  220  to capture those fronds. In an embodiment, this can be achieved by partially inflated and then deflated the balloon, with folding done after the inflation or deflation. Folding can be done by hand or using a capture tube or overlying sleeve known in the art. Also in an embodiment, folding can be facilitated by the use of one or more preformed folds  243 , also known as fold lines  243 . Folds  243  can be formed using medical balloon fabrication methods known in the art such as mold blowing methods known in the art. In an embodiment using folds  243 , folding can be achieved by inflating the balloon with the overlying fronds in place, so as to have the balloon flaps  242  protrude through splits  230 , then the balloon is deflated to have flaps  242  fold back over fronds  220  at fold lines  243 . 
     Once stent  210  is properly positioned at the target vessel site, balloon  241  is at least partially inflated which unfurls flaps  242  covering fronds  220  so as to release the fronds. Once released, deployment balloon  241  can also be used to expand or otherwise deform the fronds  220  to deploy them in the selected vessel as is described herein. Alternatively, a second balloon can be used to expand and deploy the fronds as is also described herein. 
     To avoid pinching the balloon material of balloon  241  between layers of stent metal during the stent crimping process in one embodiment, fronds  220  can be configured such that they do not overlap when crimped down to a smaller diameter. This can be achieved by configuring the fronds to be sufficiently narrow so that crimping the stent to a smaller diameter does not cause them to overlap, or through the use of a crimping fixture or mandrel known in the art. In various embodiments, fronds  220  can be configured to have a selectable minimum split width  230   w  between spits  230  after crimping. This can be in the range of 0.001 to about 0.2 inches with specific embodiments of 0.002, 0.005, 0.010, 0.025, 0.050 or 0.1 inches. 
     In another embodiment for using the delivery balloon catheter to capture the fronds, a section of a balloon  241  (not shown) can be configured to evert or fold back over a proximal portion of the stent and thus overly and capture the fronds. When the balloon is inflated, the overlying section of balloon material unfolds, releasing the fronds. The everted section of balloon can over all or any selected portion of the fronds. Eversion can be facilitated through the use of preformed folds described herein, in the case, the folds having a circumferential configuration. The folded section of balloon can be held in place by a friction fit or through the use of releasable low-strength heat bond or adhesive known in the art for bonding the balloon to the fronds. In one embodiment for positioning the everted section, the balloon is positioned inside the scaffold section of the stent and then partially inflated to have an end of the balloon protrude outside of the scaffold section, then the balloon is partially deflated and everted section is rolled over the fronds and then the balloon is fully deflated to create a vacuum or shrink fit of the balloon onto the fronds. 
     In various embodiments, fronds  210  can be captured by use of a tubular cuff  250  extending from the proximal end  241   p  of delivery balloon  241  as is shown in  FIGS. 5A-5C . In one embodiment, the cuff is attached to the catheter at or proximal to the proximal end  241   p  of the delivery balloon. In alternative embodiments, the cuff can be attached to a more proximal section of the catheter shaft such that there is an exposed section of catheter shaft between balloon and the cuff attachment point with the attachment point selected to facilitate catheter flexibility. Alternatively, the cuff is axially movably carried by the catheter shaft, such as by attachment to a pull wire which extends axially along the outside of or through a pull wire lumen within the catheter shaft, or to a tubular sleeve concentrically carried over the catheter shaft. In either approach, the cuff is positionable during translumenal navigation such that it overlies at least a portion of the fronds  220 . 
     After prosthesis  210  is positioned at the target vascular site, the stent region is deployed using the delivery balloon as described herein. The frond(s) can be released by withdrawal of the restraint. In most embodiments, the entire catheter assembly including the cuff or other restraint, balloon, and catheter shaft are withdrawn proximally to fully release the fronds. In alternative embodiment the cuff can be slidably withdrawn while maintaining position of the delivery balloon. This embodiment permits frond release prior to or after stent deployment. 
     Release of the fronds by the cuff can be achieved through a variety of means. In one embodiment, cuff  250  can be configured such that the proximal frond tips  220   t , slip out from the cuff when the balloon is deployed. Alternatively, the cuff may be scored or perforated such that it breaks at least partially open upon balloon deployment so that it releases fronds  220 . Accordingly, in such embodiments, cuff  250  can have one or more scored or perforated sections  250   p . In such embodiments, portions of cuff  250  can be configured to break open at a selectable inflation pressure or at a selectable expanded diameter. In one embodiment, the cuff material can be fabricated from a polymer that it is more plastically deformable in a radial direction than axially. Such properties can be achieved by extrusion methods known in the polymer arts so as to stretch the material axially. In use, such materials allow the cuff to plastically deform in the radial when expanded by the deployment balloon, and then to stay at least partially deformed when the balloon is deflated so as to still cover the fronds. An example of such a material includes extruded Low density Polyethylene (LDPE). Further description of the use of the cuff  250  and other capture means may be found in U.S. patent application Ser. No. 10/965,230 which is fully incorporated by reference herein. 
     In various embodiments, cuff  250  can be configured such that it plastically deforms when the balloon is inflated and substantially retains its “inflated shape”  250  is and “inflated diameter”  250   id  after the balloon is deflated is shown in  FIGS. 5B and 5C . This can be achieved through the selection of plastically deformable materials for cuff  250  (e.g. plastically deformable polymers), the design of the cuff itself (e.g. cuff dimensions and shape) and combinations thereof. For example, a cuff fixed to a catheter shaft and having the same approximate internal diameter as the deployed stent may be folded over the stent fronds to constrain them (using conventional balloon folding techniques). That cuff may be unfolded when the stent deployment balloon is inflated and the fronds released. The cuff can be withdrawn along the balloon and catheter. In an alternative embodiment of a folded-over cuff, the cuff is relatively inelastic and has an internal diameter approximately that of the deployed stent. 
     Also the cuff can be configured such that it shortens axially as it is expanded by the deployment balloon or other expansion device. This can be accomplished by selecting the materials for cuff  250  such that the cuff shrinks axially when it is stretched radially as is shown in  FIGS. 6A and 6B . Accordingly, in one embodiment, the cuff can be made of elastomeric material configured to shrink axially when stretched radially. 
     In another embodiment, all or a portion of the cuff can be configured to fold over or evert onto itself upon inflation of the balloon to produce an everted section  251  and so release the enveloped fronds as is shown in  FIGS. 6C-6D . This can be facilitated by use of fold lines  252  described herein, as well as coupled the cuff to the balloon catheter. In one embodiment the cuff can be coaxially disposed over the proximal or distal end of the balloon catheter or even slightly in front of either end. This allows the cuff to disengage the fronds yet remain attached to the balloon catheter for easy removal from the vessel. In use, these and related embodiments allow the fronds to be held against the balloon to be radially constrained or captured during stent advancement and then easily released before, during or after balloon inflation to deploy the stent at the target site. 
     In various embodiments, all or a portion of cuff  250  can be fabricated from, silicones, polyurethanes (e.g., PEPAX) and other medical elastomers known in the art; polyethylenes; fluoropolymers; polyolefin; as well as other medical polymers known in the art. Cuff  250  can also be made of heat shrink tubing known in the art such as polyolefin and PTFE heat shrink tubing. These materials can be selected to produce a desired amount of plastic deformation for a selected stress (e.g. hoop stress from the inflation of deployment balloon). In particular embodiments, all or a portion of the materials comprising cuff  250  can be selected to have an elastic limit lower than forces exerted by inflation of the deployment balloon (e.g., the force exerted by 3 mm diameter balloon inflated to 10 atms). Combinations of materials may be employed such that different portions of the cuff (e.g., the proximal and distal sections or the inner and outer surfaces) have differing mechanical properties including, but not limited to, durometer, stiffness and coefficient of friction. For example, in one embodiment the distal portion of the cuff can high a higher durometer or stiffness than a proximal portion of the cuff. This can be achieved by constructing the proximal portion of the cuff from a first material (e.g., a first elastomer) and the distal portion out of a second material (e.g. a second elastomer). Embodiments of the cuff having a stiffer distal portion facilitate maintaining the fronds in a restrained state prior to deployment. In another embodiment, at least a portion of an interior surface of the cuff can include a lubricous material. Examples of suitable lubricious materials include fluoropolymers such as PTFE. In a related embodiment, a portion of the interior of the cuff, e.g., a distal portion, can be lined with lubricous material such as a fluoropolymer. Use of lubricous materials on the interior of the cuff aids in the fronds sliding out from under the cuff during balloon expansion. 
     Referring now to  FIGS. 7A-7B , in another embodiment for restraining the fronds, a tether  260  can be placed over all or portions of fronds  200  so as to tie the fronds together. Similar to the use of cuff  250 , tether  260  can be released by the expansion of the balloon  241 . Accordingly, all or a portion of the tether can be configured to plastically deform upon inflation of balloon  241  so as to release the fronds. Alternatively, the tether can be configured to be detached from the fronds prior to expansion of the balloon. In one embodiment, this can be achieved via a pull wire, catheter or other pulling means coupled to the tether directly or indirectly. 
     In various embodiments, the tether can be a filament, cord, ribbon, etc. which would simply extend around the fronds to capture them like a lasso. In one embodiment the tether can comprise a suture or suture-like material that is wrapped around the fronds. One or both ends of the suture tether can be attachable to a balloon catheter  241 . In another embodiment, tether  260  can comprise a band or sleeve that fits over fronds  220  and then expands with expansion of balloon  241 . In this and related embodiments, tether  260  can also be attached to balloon catheter  241 . Also tether  260  can be scored or perforated so that a portion of the tether shears or otherwise breaks upon balloon inflation, thereby releasing the fronds. Further, the tether  260  can contain a radio-opaque other medical image visible marker  260   m  to allow the physician to visualize the position of the tether on the fronds, and/or determine if the tether is constraining the fronds. 
     Referring now to  FIGS. 8A-8B , in other embodiments of the delivery system  10 , the fronds can be constrained through the use of a removable sleeve  270  that can be cover all or a portion of fronds  220  during positioning of the stent at the target tissue site and then be removed prior to deployment of the fronds. In one embodiment, sleeve  270  can be slidably advanced and retracted over stent  210  including fronds  220 . Accordingly, all or portions of sleeve  270  can be made from lubricous materials such as PTFE or silicone. Sleeve  270  can also include one or more radio-opaque or other imaging markers  275  which can be positioned to allow the physician to determine to what extent the sleeve is covering the fronds. In various embodiments, sleeve  270  can be movably coupled to catheter  240  such that the sleeve slides over either the outer or inner surface (e.g., via an inner lumen) of catheter  240 . The sleeve can be moved through the use of a pull ire, hypotube, stiff shaft or other retraction means  280  known in the medical device arts. In one embodiment, sleeve  270  can comprise a guiding catheter or overtube as is known in the medical device arts. 
     Referring now to  FIGS. 9A-11B , an exemplary deployment protocol for using delivery system  5  to deliver a prosthesis ( 10 ) having a stent region ( 12 ) and having one or more fronds ( 16 ) will be described. The order of acts in this protocol is exemplary and other orders and/or acts may be used. A delivery balloon catheter  30  is advanced within the vasculature to carry prosthesis  10  having and stent region ( 12 ) and fronds  16  to an Os O located between a main vessel lumen MVL and a branch vessel lumen BVL in the vasculature, as shown in  FIGS. 9A and 9B . Balloon catheter  30  may be introduced over a single guidewire GW which passes from the main vessel lumen MVL through the Os O into the branch vessel BVL. Optionally, a second guidewire (not shown) which passes by the Os O in the main vessel lumen MVL may also be employed. Usually, the prosthesis  10  will include at least one radiopaque marker  20  on prosthesis  10  located near the transition region between the prosthesis section  12  and the fronds  16 . In these embodiments, the radiopaque marker  20  can be aligned with the Os O, typically under fluoroscopic imaging. 
     Preferably, at least one proximal marker will be provided on the prosthesis at a proximal end of the transition zone, and at least one distal marker will be provided on the prosthesis at the distal end of the transition zone. Two or three or more markers may be provided within the transverse plane extending through each of the proximal and distal ends of the transition zone. This facilitates fluoroscopic visualization of the position of the transition zone with respect to the Os. Preferably, the transition zone is at least about 1 mm and may be at least about 2 mm in axial length, to accommodate different clinical skill levels and other procedural variations. Typically, the transition zone will have an axial length of no more than about 4 mm or 5 mm (for coronary artery applications). 
     During advancement, the fronds are radially constrained by a constraining means  250   c  described herein (e.g., a cuff) to prevent divarication of the fronds from the delivery catheter. When the target location is reached at Os O or other selected location, the constraining means  250   c  is released by the expansion of balloon  32  or other constraint release means described herein (alternatively, the constraining means can be released prior to balloon expansion). Balloon  32  is then further expanded to expand and implant the support region  12  within the branch vessel lumen BVL, as shown in  FIGS. 10A and 10B . Expansion of the balloon  32  also partially deploys the fronds  16 , as shown in  FIGS. 10A and 10B , typically extending both circumferentially and axially into the main vessel lumen MVL. The fronds  16 , however, are not necessarily fully deployed and may remain at least partially within the central region of the main vessel lumen MVL. In another embodiment, the constraining means can be released after balloon expansion. 
     In another embodiment for stent deployment, after deploying stent  10 , the cuff or other constraining means  250   c  need not be removed but can remain in position over at least a portion of the fronds so as to constrain at least the tip of the fronds. See, e.g.,  FIG. 12A , discussed in additional detail below. Then a main vessel stent  150  is advanced into the main vessel to at least partially overlap the fronds as described above. This method provides a reduced chance that the frond-tips will caught in or on the advancing main vessel stent  150  because the fronds are still captured under the cuff. After placement of stent  150  balloon  32  together the 12 stent portion of the side branch prosthesis is deployed by inflation of 30 balloon. Prosthesis delivery system including cuff  250   c  are removed (by pulling on catheter  30 ) to release the fronds which when released, spring outward to surround a substantial portion of the circumference of the main vessel stent  150  and the delivery procedures continues as described herein. This approach is also desirable in that by having the cuff left on over the fronds, the frond-tips are constrained together resulting in more advancement of the main vessel stent  150 . This in turn can reduce procedure time and increase the accuracy and success rate in placement of the main vessel stent  150  particularly with severely narrowed, eccentric, or otherwise irregularly shaped lesions. In various embodiments, cuff  250   c  and/or proximal end of balloon  32  can have a selectable amount of taper relative to the body of the balloon to facilitate advancement of one or both of the main vessel stent  150  or stent  10  into the target tissue site when one device has already been positioned. Such embodiments also facilitate placement into severely narrowed vessels and/or vessels with irregularly shaped lesions. 
     Various approaches can be used in order to fully open the fronds  16 . In one embodiment, a second balloon catheter  130  can be introduced over a guidewire GW to position the second balloon  132  within the fronds, as shown in  FIGS. 11A and 11B . Optionally, the first catheter  30  could be re-deployed, for example, by partially withdrawing the catheter, repositioning the guidewire GW, and then advancing the deflated first balloon  32  transversely through the fronds  16  and then re-inflating balloon  32  to fully open fronds  16 . A balloon which has been inflated and deflated generally does not refold as nicely as an uninflated balloon and may be difficult to pass through the fronds. It will generally be preferable to use a second balloon catheter  130  for fully deforming fronds  16 . When using the second balloon catheter  130 , a second GW will usually be prepositioned in the main vessel lumen MVL past the Os O, as shown in  FIGS. 11A and 11B . Further details of various protocols for deploying a prosthesis having a stent region ( 12 ) and fronds or anchors, such as prosthesis  10 , are described in co-pending application Ser. No. 10/807,643. 
     In various embodiments for methods of the invention using prosthesis/delivery system  5 , the physician can also make use of additional markers  22  and  24  positioned at the proximal and distal ends of the prosthesis  10 . In one embodiment, one or more markers  22  are positioned at the proximal ends of the fronds as is shown in  FIGS. 9A and 9B . In this and related embodiments, the physician can utilize the markers to ascertain the axial position of the stent as well as the degree of deployment of the fronds (e.g., whether they are in captured, un-captured or deployed state). For example, in one embodiment of the deployment protocol, the physician could ascertain proper axial positioning of the stent by not only aligning the transition marker  20  with the Os opening O, but also look at the relative position of end markers  22  in the main vessel lumen MVL to establish that the fronds are positioned far enough into the main vessel, have not been inadvertently positioned into another branch vessel/lumen. In this way, markers  20  and  22  provide the physician with a more accurate indication of proper stent positioning in a target location in a bifurcated vessel or lumen. 
     In another embodiment of a deployment protocol utilizing markers  22 , the physician could determine the constraint state of the fronds (e.g. capture or un-captured), by looking at the position of the markers relative to balloon  30  and/or the distance between opposing fronds. In this way, markers  22  can be used to allow the physician to evaluate whether the fronds were properly released from the constraining means prior to their deployment. In a related embodiment the physician could determine the degree of deployment of the fronds by looking at (e.g., visual estimation or using Quantitative Coronary Angiography (QCA)) the transverse distance between markers  22  on opposing fronds using one or medical imaging methods known in the art (e.g., fluoroscopy). If one or more fronds are not deployed to their proper extent, the physician could deploy them further by repositioning (if necessary) and re-expanding balloon catheters  30  or  130 . 
     Referring now to  FIG. 12A-12I , an exemplary and embodiment of a deployment protocol using a deployment system  5  having a prosthesis  10  with fronds  16  will now be presented. As shown in  FIG. 12A , prosthesis  10  is positioned at Os opening O with catheter  30  such that the stent section  12  is positioned substantially in branch vessel BV with the fronds  16  extending into the Os O and in the main vessel lumen MVL. In this embodiment a second delivery catheter  130  containing a stent  150  has been positioned in the MVL prior to positioning of catheter  30 . Alternatively, catheter  130  can be positioned first and the branch vessel catheter  30  subsequently. In embodiments where catheter  130  has been positioned first, the proximal end of catheter  30  including fronds  16  can be positioned adjacent a proximal portion of balloon  132  of catheter  130  such that portions of captured fronds  16  and stent  150  are positioned side by side. Such alignment can be facilitated by lining up one or more radio-opaque markers (described herein) on the two catheters. 
     Next, as shown in  FIGS. 12B-12C , balloon  32  of catheter  30  is expanded. Then as shown in  FIGS. 12D-12E , catheter  30  together with cuff  250   c  is withdrawn from the vessel to uncover and release the fronds  16 . When deployed, the fronds  16  are positioned between the vessel wall and stent  150  and substantially surround at least a portion of the circumference of the main vessel stent  150 C/delivery system ( 130 ) as well as making contact with a substantial portion of inner wall Wm of main vessel lumen MVL. Preferably as shown in  FIG. 12E , the fronds are distributed around the circumference of the Wall Wm. Also as shown in  FIG. 12E  one of the fronds  16 A may bent back by stent  150 , but may not be contacting the vessel wall. 
     Then, as shown in  FIGS. 12F-12H , balloon  132  is expanded to expand and deploy stent  150  after which the balloon is deflated and catheter  130  is withdrawn. Expansion of stent  150  serves to force and hold fronds  16  up against the vessel wall in a circumferential pattern as is shown in  FIG. 12G . This essentially fixes the fronds in place between expanded stent  150  and the vessel wall. As such, the fronds may serve five functions, first, as an anchoring means to hold stent  12  in place in the branch vessel lumen BVL. Second they serve as a mechanical joining means to mechanically join stent  12  to stent  150 . Third, to provide stent coverage to prevent prolapse of tissue into the lumen as well as in the case of a drug coated stent to deliver agent. Finally, they also provide additional mechanical prosthesising (hoop strength) to hold open Os of the branch vessel. More specifically, the now fixed fronds  16  can be configured to serve as longitudinal struts to more evenly distribute expansion forces over a length of the vessel wall as well as distribute compressive forces over a length of stent  12 . 
     The prosthesis of the present invention, may be utilized in combination with either main vessel stents having a substantially uniform wall pattern throughout, or with main vessel stents which are provided with a wall pattern adapted to facilitate side branch entry by a guidewire, to enable opening the flow path between the main vessel and the branch vessel. Three examples of suitable customized stent designs are illustrated in  FIG. 13A through 13C . In each of these constructions, a main vessel stent  110  contains a side wall  112  which includes one or more windows or ports  114 . Upon radial expansion of the stent  110 , the port  114  facilitates crossing of a guide wire into the branch lumen through the side wall  112  of the main vessel stent  110 . A plurality of ports  114  may be provided along a circumferential band of the main vessel stent  110 , in which instance the rotational orientation of the main vessel stent  110  is unimportant. Alternatively, as illustrated, a single window or port  114  may be provided on the side wall  112 . In this instance, the deployment catheter and radiopaque markers should be configured to permit visualization of the rotational orientation of the main vessel stent  110 , such that the port  114  may be aligned with the branch vessel. 
     In general, the port  114  comprises a window or potential window through the side wall which, when the main vessel stent  110  is expanded, will provide a larger window than the average window size throughout the rest of the stent  110 . This is accomplished, for example, in  FIG. 13A , by providing a first strut  116  and a second strut  118  which have a longer axial distance between interconnection than other struts in the stent  110 . In addition, struts  116  and  118  are contoured to provide a first and second concavity facing each other, to provide the port  114 . 
     Referring to  FIG. 13B , the first strut  116  and second strut  118  extend substantially in parallel with the longitudinal axis of the stent  110 . The length of the struts  116  and  118  is at least 2 times, and, as illustrated, is approximately 3 times the length of other struts in the stent. Referring to  FIG. 13C , the first and second struts  116  and  118  are provided with facing concavities as in  FIG. 13A , but which are compressed in an axial direction. Each of the foregoing configurations, upon expansion of the main vessel stent  110 , provide an opening through which crossing of a guidewire may be enhanced. The prosthesis of the present invention may be provided in kits, which include a prosthesis mounted on a balloon catheter as well as a corresponding main vessel stent mounted on a balloon catheter, wherein the particular prosthesis and main vessel stent are configured to provide a working bifurcation lesion treatment system for a particular patient. Alternatively, prostheses in accordance with the present invention may be combined with separately packaged main vessel stents from the same or other supplier, as will be apparent to those of skill in the art. 
       FIG. 13D  is an image of a main vessel stent having a side opening, deployed such that the side opening is aligned with the branch vessel lumen. 
     In accordance with a further aspect of the present invention, there is provided a stepped balloon for use with the prosthesis disclosed herein. The stepped balloon may be utilized for the initial implantation of the prosthesis, or for reconfiguring a previously implanted prosthesis as will be apparent to those of skill in the art. 
     Referring to  FIG. 15 , there is illustrated a schematic side view of a distal end section of a catheter  150  having an elongate flexible tubular shaft  152  with a stepped balloon  154  mounted thereon. The dimensions, materials and construction techniques for the catheter shaft  152  are well understood in the art, and discussed briefly elsewhere herein. In general, shaft  152  has an axial length sufficient to reach from the desired percutaneous access point to the treatment site, and will typically include at least one inflation lumen for placing the stepped balloon  154  in fluid communication with a source of inflation media, as well as a guidewire lumen for either over the wire or rapid exchange guidewire tracking. 
     The stepped balloon  154  extends between a proximal end  156  and distal end  158 . The balloon  154  is necked down to the catheter shaft  152  at each of the proximal and distal ends, and secured to the shaft  152  using any of a variety of adhesives, thermal bonding, or other techniques well known in the art. 
     The stepped balloon is provided with a proximal zone  160  and a distal zone  162 , separated by a transition zone  164 . In the illustrated embodiment, the proximal zone  160  has a greater inflated diameter than the distal zone  162 . Alternatively, the relative dimensions may be reversed, such that the distal zone  162  has a greater inflated diameter than the proximal zone  160 , such as for use in a retrograde catheterization from the branch vessel into the main vessel. 
     The diameters and lengths of the proximal zone  160  and distal zone  162  may be varied considerably, depending upon the intended target site. In an implementation of the invention designed for use in the coronary artery, a proximal zone  160  may be provided with a diameter in the range of from about 3 mm to about 4 mm, and the distal zone  162  may have an inflated diameter in the range of from about 2 mm to about 3 mm. In one implementation of the invention, the proximal zone  160  has an inflated diameter of about 3.5 mm and the distal zone  162  has an inflated diameter of about 2.5 mm. In general, the inflated diameter of the proximal zone  160  will be at least 110% of the inflated diameter of the distal zone  162 . In certain implementations of the invention, the inflated diameter of the proximal zone  160  will be at least 125% of the inflated diameter of the distal zone  162 . 
     The proximal zone  160  has a working length defined as the axial length between a proximal shoulder  166  and a distal shoulder  168 . The working length of the proximal zone  160  is generally within the range of from about 5 to about 30 mm, and, in one embodiment, is about 9 mm. The working length of the distal zone  162  extends from a proximal shoulder  170  to a distal shoulder  172 . The working length of the distal zone  162  is generally within the range of from about 5 to about 20 mm, and, in one embodiment, is about 6 mm. In the illustrated embodiment, each of the proximal zone  160  and distal zone  162  has a substantially cylindrical inflated profile. However, noncylindrical configurations may also be utilized, depending upon the desired clinical result. 
     The configuration and axial length of the transition zone  164  may be varied considerably, depending upon the desired frond configuration and ostium coverage characteristics of the implanted prosthesis. In the illustrated embodiment, the transition zone  164  comprises a generally frustoconical configuration, having an axial length between proximal shoulder  170  of the distal zone  162  and distal shoulder  168  of the proximal zone  160  within the range of from about 1 to about 10 mm, and, in one embodiment, about 2.5 mm. 
     The transition zone of this balloon delineates the transition from one diameter to another. In one embodiment this transition zone may be 4 mm in length and ramp from 2.5 to 3.5 mm in diameter. This conical surface is used to mold or flare the ostium of the bifurcation from the smaller side branch to the larger main vessel. In this configuration this stepped balloon may be utilized for deploying the prosthesis. Used in this manner the leading and trailing surfaces are utilized to expand the device in the side branch and main vessel and the transition zone is used to flare the transition zone of the stent against the wall of the ostium. 
     The wall of the stepped balloon  154  may comprise any of a variety of conventional materials known in the angioplasty balloon arts, such as any of a variety of nylons, polyethylene terephthalate, various densities of polyethylene, and others known in the art. Material selection will be influenced by the desired compliancy and burst strength of the balloon, as well as certain manufacturing considerations. 
     The stepped balloon may be formed in accordance with techniques well known in the angioplasty arts. For example, stock tubing of the desired balloon material may be inflated under the application of heat within a Teflon lined capture tube having the desired stepped configuration. The proximal and distal ends may thereafter be axially stretched with the application of heat to neck down to a diameter which relatively closely fits the outside diameter of the elongate shaft  152 . 
     The balloon may be constructed such that it assumes the inflated stepped configuration at a relatively low inflation pressure. See, e.g., an exemplary compliance curve in  FIG. 16 . Alternatively, the balloon may be configured for sequential expansion, such as by allowing the distal zone  162  to inflate to its final outside diameter at a first pressure, to firmly position the branch vessel stent, and the proximal zone  160  only inflates to its final diameter at a second, higher inflation pressure, where a sequential deployment of the implant is desired. 
     In one embodiment the balloon is constructed such that it assumes the inflated stepped configuration upon inflation and retains this shape throughout its inflated working range (from initial inflation to rated burst pressure.) In this embodiment the balloon working range is from 1 ATM to 16 ATM at rated burst pressure. 
     In another embodiment the balloon is constructed such that it initially assumes the inflated stepped configuration within the lower pressures of its working range and trends to the same diameter at the higher pressures. The diameter of this balloon at higher pressure approximates that of the larger diameter in the stepped configuration. 
     In another embodiment the balloon is constructed such that it initially has a single diameter during the initial lower pressures of its working range and assumes its inflated stepped configuration at higher pressures. The diameter of the balloon at lower pressure approximates that of the smaller diameter in the stepped configuration. 
     Alternatively, the function of the stepped balloon  154  may be accomplished by providing two distinct balloons,  160 ′ and  162 ′. The proximal balloon  160 ′ may be inflated by a first inflation lumen (not illustrated) and the distal balloon  162 ′ may be inflated by a second inflation lumen (not illustrated) extending throughout the length of the catheter shaft, to separate inflation ports. Inflation may be accomplished simultaneously or sequentially, depending upon the desired clinical procedure. Alternatively, a proximal balloon  160 ′ and a distal balloon  162 ′ may be both inflated by a single, common inflation lumen extending throughout the length of the catheter shaft. 
     The stepped balloon  154  is preferably navigated and positioned within the vascular system under conventional fluoroscopic visualization. For this purpose, the catheter  150  may be provided with at least one radiopaque marker. In the illustrated embodiment, a first radiopaque marker  174  is provided on the catheter shaft  152 , at about the proximal shoulder  166 . At least a second radiopaque marker  176  is provided on the shaft  152 , aligned approximately with the distal shoulder  172 . Proximal marker  174  and distal marker  176  allow visualization of the overall length and position of the stepped balloon  154 . 
     In addition, a first transition marker  178  and second transition marker  180  may be provided on the shaft  152 , at a location corresponding to a transition zone on the prosthesis. Transition markers  178  and  180  thus enable the precise location of the prosthesis transition with respect to the ostium between the main vessel and branch vessel, as has been discussed elsewhere herein. Each of the markers may comprise a band of gold, silver or other radiopaque marker materials known in the catheter arts. 
     In one embodiment of the stepped balloon  154  intended for use in the coronary artery, the axial length of the balloon between the proximal marker  174  and distal marker  176  is approximately 19.5 mm. The length between the distal marker  176  and transition marker  180  is approximately 6.1 mm. The distance between the transition markers  178  and  180 , including the length of the transition markers, is about 4.5 mm. As will be apparent to those of skill in the art other dimensions may be utilized, depending upon the dimensions of the prosthesis and the target anatomy. 
     Referring to  FIGS. 17 and 18 , there is schematically illustrated two different configurations of a stepped balloon  154  in accordance with the present invention, positioned and inflated within a treatment site at a vascular bifurcation, with the prosthesis omitted for clarity. In each, a stepped balloon  154  is positioned such that a proximal zone  160  is inflated within a main vessel  182 . A distal zone  162 , having a smaller inflated diameter than proximal zone  160 , is positioned within the branch vessel  184 . The stepped balloon  154  has been positioned to illustrate the relative location of the transition markers  178  and  180 , with respect to the carina  186  of the bifurcation. 
       FIGS. 19 through 22  illustrate one application of the stepped balloon and prosthesis in accordance with the present invention. Referring to  FIG. 19 , there is illustrated a bifurcation between a main vessel  200  and a branch vessel  202 . A main vessel guidewire  204  is illustrated as positioned within the main vessel, and a branch vessel guidewire  206  is in position extending from the main vessel  200  into the branch vessel  202 . 
     A balloon catheter  208  carrying a prosthesis  210  is advancing along the branch vessel guidewire  206 . 
     Referring to  FIG. 20 , the catheter  208  has advanced to the point of positioning the prosthesis  210  across the ostium between the main vessel  200  and branch vessel  202 . 
     Referring to  FIG. 21 , a stepped balloon  212  carried by the catheter  208  has been inflated across the ostium into the branch vessel  202 .  FIG. 22  illustrates the implanted prosthesis  210 , after the balloon catheter  208  has been proximally retracted. 
     As can be seen from  FIGS. 21 and 22 , dilation of the stepped balloon  212  across the ostium enables expansion of the distal zone of the prosthesis in the branch vessel, the proximal zone of the prosthesis in the main vessel, and a transition zone of the prosthesis spans the ostium. The main vessel guidewire  204  may thereafter be proximally retracted to a point proximal to the prosthesis  212 , and distally advanced through the proximal portion and the fronds of the prosthesis. A main vessel stent may thereafter be positioned in the main vessel  200  as has been discussed elsewhere herein. 
     Referring to  FIGS. 23 and 24 , there is illustrated a schematic representation of a distal portion of a stepped balloon catheter in accordance with the present invention. In general, the catheter includes a primary guidewire lumen as is understood in the art, such as for tracking the guidewire which extends into the branch vessel. Unlike previous embodiments disclosed herein, the catheter of  FIGS. 23 and 24  includes a secondary guidewire lumen, such as for tracking the guidewire which extends into the main vessel lumen beyond the bifurcation. 
     Referring to  FIG. 23 , a catheter  220  extends between a proximal end  222  (not shown) and a distal end  224 . A balloon  226  is carried in vicinity of the distal end  224 , as is known in the balloon catheter arts. Balloon  226  may comprise a stepped balloon as has been described elsewhere herein, or a tapered balloon, or a conventional cylindrical angioplasty or stent deployment balloon, depending upon the desired performance. 
     The catheter  220  includes a guidewire lumen  228  which extends throughout the length of at least a distal portion of the catheter  220 , to a distal port  230  at the distal end  224  of the catheter  220 . In an embodiment intended for over-the-wire functionality, the first guidewire lumen  228  extends proximally throughout the length of the catheter, to a proximal manifold. In an alternate configuration intended for rapid exchange functionality, a proximal access port (not shown) provides access to the first guidewire lumen  228  at a point along the length of the catheter distal to the proximal end  222 . In general, rapid exchange proximal access ports may be within the range of from about 10 cm to about 30 cm from the distal end  224 . 
     As can be seen with reference to, for example,  FIG. 23A , the catheter  220  is additionally provided with an inflation lumen  232  which extends throughout the length of the catheter to the proximal end  222 . The distal end of inflation lumen  232  is in communication via an inflation port  234  with the interior of the balloon  226 , to enable placement of the balloon  226  in fluid communication with a source of inflation media. 
     Referring to  FIG. 23B , the catheter  220  is additionally provided with a second guidewire lumen  236 . Second guidewire lumen  236  extends between a proximal access port  238  and a distal access port  240 . The distal access port  240  is positioned proximally to the distal end  224  of the catheter  220 . In the illustrated embodiment, the distal port  240  is positioned on the proximal side of the balloon  226 . Generally, the distal port  240  will be no greater than about 4 cm, and often no greater than about 2 cm proximal of the balloon  226 . 
     The proximal access port  238  may be provided on the side wall of the catheter, such as within the range of from about 10 cm to about 60 cm from the distal end  224 . In one embodiment, the proximal access port  238  is within the range of from about 25 cm to about 35 cm from the distal end  224 . The proximal port  238  is preferably spaced distally apart from the proximal end  222  of the catheter  220 , to enable catheter exchange while leaving the main vessel guidewire in place as will be apparent in view of the disclosure herein. 
     As illustrated in  FIG. 23 , the second guidewire lumen  236  may be formed as an integral part of the catheter body. This may be accomplished by providing an initial  3  lumen extrusion having the desired length, and trimming away the wall of the second guidewire lumen  236  distally of the distal port  240  and proximally of the proximal port  238 . 
     Alternatively, the second guidewire lumen  236  may be separately attached to a conventional catheter shaft such as is illustrated in  FIG. 24 . In this construction, the second guidewire lumen  236  is defined within a tubular wall  237 , which may be a separate single lumen extrusion. The tubular wall  237  is positioned adjacent the catheter shaft, and bonded thereto using any of a variety of techniques known in the art, such as thermal bonding, adhesives, solvent bonding or others. Superior bonding and a smooth exterior profile may also be achieved by placing a shrink tube around the assembly of the catheter  220  and tubular wall  237 , and heating the shrink tube to shrink around and combine the two structures as is well understood in the catheter manufacturing arts. It may be desirable to place a mandrel within the second guidewire lumen  236  and possibly also the first guidewire lumen  228  and inflation lumen  232  during the heat shrinking process. 
     In use, the second guidewire lumen  236  enables control over the main vessel guidewire. Referring to  FIG. 25 , there is illustrated a two guidewire catheter  208  in position across a bifurcation from a main vessel  200  into a branch vessel  202 . The branch vessel guidewire  206  has been positioned in the branch vessel  202 , and the catheter  208  advanced into position over the wire into the bifurcation. The prosthesis  210  is illustrated in its expanded configuration, and the balloon has been deflated. 
     Prior to percutaneously introducing the catheter into the patient&#39;s vasculature, the main vessel guidewire  204  is positioned within the secondary guidewire lumen  236 , and the catheter and main vessel guidewire assembly is advanced as a unit along the branch vessel guidewire to the treatment site. 
     As seen in  FIG. 25 , the distal exit port  240  of the secondary guidewire lumen  236  is aligned such that the main vessel guidewire  204  is aimed down the lumen of the main vessel  200 . In the illustrated embodiment, the secondary guidewire lumen is attached to the outside of the step balloon. The stent is crimped onto the step balloon, and the exit of the secondary guidewire lumen is between the intermediate zone and the circumferentially extending link of the prosthesis. In the crimped configuration, the distal exit  240  of the secondary lumen  236  resides between two adjacent fronds. 
     Following deployment of the stent and deflation of the balloon as illustrated in  FIG. 25 , the main vessel guidewire  204  may be distally advanced into the main vessel beyond the bifurcation, in between the two adjacent fronds. See,  FIG. 26 . 
     Referring to  FIG. 27 , there is illustrated an embodiment similar to  FIG. 25 , except that the distal exit port  240  of the main vessel guidewire lumen  236  is positioned proximally of the balloon. The precise location of the distal exit  240  may be varied, so long as it permits direction of the main vessel guidewire distally within the main vessel beyond the bifurcation. In general, the distal exit  240  may be located within the axial length of the prosthesis as mounted on the catheter. 
     Following distal advance of the main vessel guidewire  204  into the main vessel distally of the bifurcation, the catheter  208  may be proximally withdrawn from the treatment site leaving the main vessel guidewire  204  in place. The catheter  208  may be removed from the main vessel guidewire  204  as is understood in the rapid exchange catheter practices, and a secondary catheter may be advanced down the main vessel guidewire such as to dilate an opening between the fronds into the main vessel beyond the bifurcation and/or deploy a second stent at the bifurcation as has been discussed herein. 
     In  FIGS. 25 through 27 , the catheter  208  is schematically illustrated as a construct of a separate main vessel lumen attached to a catheter body. However, in any of the foregoing catheters the body construction may be that of a unitary extrusion as has been discussed previously. 
     The stepped balloon of the present invention may be used in a variety of additional applications. For example, the distal lower diameter section of the device may be used to slightly open a small blood vessel then the system advanced to treat the index lesion with an appropriately sized catheter. In one embodiment the stepped balloon may function as a standard PTCA catheter for the treatment of advanced cardiovascular disease. Specifically in cases where only a small diameter balloon catheter is capable of crossing a diseased lesion, the smaller diameter leading portion of the step balloon may be used to predilate the lesion. The catheter would then be deflated and the larger diameter trailing segment advanced across the lesion. The larger diameter portion of the stepped balloon would then be used to dilate the diseased lesion to a larger diameter. In this way the stepped balloon functions as both a pre-dilation and final dilation catheter. 
     Although the present invention has been described primarily in the context of a prosthesis adapted for positioning across the Os between a branch vessel and a main vessel prior to the introduction of the main vessel stent, in certain applications it may be desirable to introduce the main vessel stent first. Alternatively, where the prosthesis of the present invention is used provisionally, the main vessel stent may have already been positioned at the treatment site. The main vessel stent may include a side branch opening, or a side branch opening may be formed by advancing a balloon catheter through the wall of the stent in the vicinity of the branch vessel. Thereafter, the prosthesis of the present invention may be advanced into the main vessel stent, though the side wall opening, and into the branch vessel, with the circumferential link positioned within the interior of the main vessel stent. In many of the embodiments disclosed herein, the circumferential link will expand to a diameter which is approximately equal to the expanded diameter of the branch vessel support. Thus, upon initial deployment of the prosthesis, the circumferential link may be expanded to a diameter which is less than the adjacent diameter of the main vessel. If the prosthesis of the present invention is positioned within a previously positioned main vessel stent, it may therefore be desirable to include a post dilatation step to expand the circumferential link up to the inside diameter of the main vessel stent and also to deform the fronds outwardly and rotationally to conform to the interior surface of the main vessel stent. 
     While the above is a complete description of the preferred embodiments of the invention, various alternatives, modifications, and equivalents may be used. Also, elements or steps from one embodiment can be readily recombined with one or more elements or steps from other embodiments. Therefore, the above description should not be taken as limiting the scope of the invention which is defined by the appended claims.