Patent Publication Number: US-2020289300-A1

Title: Asymmetrical stent with radially variable properties and method of using same

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to stents for use in supporting passageways within a live body, and more particularly to an asymmetrical stent constructed to produce a greater radial outward force in one region than in a second region of the stent. 
     BACKGROUND 
     There exist a number of disease states in which the deployment of a stent is part of accepted clinical protocol. In most of these instances, such as stents used to support partially blocked arteries, the stent is designed to produce substantially uniform outward radial forces around its circumference, so that the supported vessel resumes to a normal circular cross section. In other instances, the disease exhibits an asymmetry such that a uniform radial force stent may be used, but with a less than fully desirable outcome. For instance, in the case of an aortic dissection, the true lumen side and the false lumen side may result in an asymmetry in re-apposing the dissection flap to possibly produce excess stress on the aortic wall. Another example may be May-Thurner syndrome, in which one side of the iliac vein is compressed, while the other side of the vein may be unaffected. In still another example, a tumor or other abnormal tissue growth may tend to push on one side of a body passageway, tending to pinch the passageway closed, whereas the other side of the passageway may be relatively unaffected by the tumor growth. 
     The present disclosure is directed toward one or more of the problems set forth above. 
     SUMMARY 
     In one aspect, an asymmetrical stent includes a tubular framework that includes a first region exclusive of a second region, both of which extend less than completely around a longitudinal axis. The first region and the second region expand responsive to inflation of a balloon positioned within the tubular shaped framework. The first region is more resistant to expansion than the second region. The tubular shaped framework includes a first plurality of barbs and a second plurality of barbs that all point away from the longitudinal axis. The first and second plurality of barbs are on respective sides of a plane that includes the longitudinal axis and bisects the first region. 
     In another aspect, a method of using an asymmetrical stent includes positioning the asymmetrical stent at a treatment location. The asymmetrical stent is oriented so that a stiffer region faces a pre-determined portion of a lumen wall of a passageway. The asymmetrical stent is expanded until the asymmetrical stent contacts the lumen wall of the passageway. Then, both the asymmetrical stent and the passageway are further expanded by inflating a balloon. A portion of the asymmetrical stent moves along the lumen wall in a tangential direction perpendicular to a lumen centerline responsive to inflation of the balloon. The balloon is then deflated out of contact with the asymmetrical stent. The passageway and the asymmetrical stent shrink responsive to deflation of the balloon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a end view of a stent delivery system at a treatment location in a vessel; 
         FIG. 2  shows the asymmetrical stent of  FIG. 1  self expanded into contact with the vessel wall; 
         FIG. 3  shows both the asymmetrical stent and vessel being expanded by inflation of a balloon; 
         FIG. 4  shows the vessel and asymmetrical stent after the balloon of  FIG. 3  has been deflated and removed; 
         FIG. 5  is an enlarged schematic end view of the stent from  FIG. 4  showing external barbs and radiopaque markers; 
         FIG. 6  is a perspective view of the asymmetrical stent from  FIGS. 1-5 ; and 
         FIG. 7  is a perspective view of an asymmetrical stent according to another aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The concept of the present disclosure may be described as a stent that exerts different magnitudes of radial force in different regions around its circumference. These regions can then be aligned with desired anatomical locations. This disclosure describes a stent with variable radial stiffness in combination with microbarbs on the luminal surface of the stent. Although the present disclosure is generally applicable to self expanding stents, balloon expanded stents might also find potential applications consistent with the present disclosure. In any event, after expanding into contact with a passageway wall, such as via self expansion, a balloon may be used to over expand the stent and passageway to set the microbarbs, or other friction feature, to focus a greater outward radial force in a desired direction within the passageway. 
     There are a number of ways in which to vary the radial stiffness of a region of the stent structure. In regard to lattice or structure, stiffness could be modified by the pattern of closed verses open cells, strut length and/or width, strut thickness and/or strut spacing/density. Strut thickness could be modified by changing the thickness of some material, changing materials (nitinol, stainless steel, cobalt chrome, or other suitable material), and/or by grinding, etching, or electro-polishing away some of the material that makes up the stent. Yet another method could be to introduce the presence of a material coating on one region to alter the properties in that region relative to other regions around the circumference of the stent. The concept could also be accomplished by using differential application of heat treatment to the material in each of the different regions such that the final configuration provides variable radial stiffness. A combination of one or more of these strategies could be used to create two or more variable stiffness regions along the circumference of the stent, with the regions extending partially or fully along the axial length of the stent. A configuration of marker bands, such as radiopaque markers, could be used to enable a clinician to ascertain an orientation of the stent in vivo to aide in orienting the stent during a deployment procedure. 
     Microbarbs are added to the luminal side of the stent structure to oppose the passageway wall. The microbarbs may be arranged in longitudinal bands along the stent surface. The longitudinal band may be as small as a single row of barbs or cover the entire region or even the entire stent. A band of barbs may have a multi-row distribution, be distributed in a field area with no discernable rows, or some combination of both. The barbs may be oriented generally toward a single point on the circumference of the stent, which may be aligned with the anatomical orientation of interest to provide the greatest outward radial stiffness at the convergence. 
     Upon initial deployment, such as through self expansion, a stent according to the present disclosure will come to an equilibrium against the vessel or passageway wall with approximately uniform radial force along its circumference. In order to redistribute the forces biased against a region of the vessel wall, a balloon dilation of the stent could follow. As the balloon is inflated, the vessel and stent will both elastically expand. The stent will have a biased expansion with the region of low radial stiffness expanding more than the region of higher radial stiffness. As this occurs, the vessel wall will translate (i.e., move) tangentially across the surface of a portion of the stent to achieve an approximately uniform stress. The microbarbs are configured to allow this translation, but engage to prevent reverse translation. Upon deflation of the balloon, the microbarbs stop translation of the vessel over the stent surface back to the initial equilibrium state. As such, the tissue is biased in its loading, with a higher wall stress exerted in the region of greater radial force based upon the shear stress interaction between the barbs and the vessel wall. Those skilled in the art will appreciate that by inflating the balloon to different amounts of expansion, the net bias and radial force may be tuned to a specific application (e.g., re-apposition of a flap against a false lumen wall). 
     Referring now to  FIGS. 1-5 , an end view of an asymmetrical stent according to the present disclosure is shown being implanted in a passageway, such as a blood vessel to provide extra radial stiffness and support against one side of the vessel being compressed by some anatomical structure, such as tumor  19 .  FIG. 1  shows asymmetrical stent  20 , which in this example is a self expanding stent, held against expansion by a retractable sheath  18  of a type well known in the art. Asymmetrical stent  20  includes a first region  31  of higher radial stiffness and a second region  32  with a lower radial stiffness relative to the first region  31 . The radial stiffness within each region need not be uniform and may vary. These relative stiffnesses are shown schematically by the first region  31  having a thicker profile than the second region  32 . This difference is for illustrative purposes only, as the two regions  31  and  32  may have virtually indistinguishable thicknesses when viewed in an end-wise profile view, but could have different thicknesses without departing from the present disclosure. Asymmetrical stent  20  generally, and the first and second regions  31 ,  32  specifically, form a tubular shaped framework of a type well known in the art and not taught again here. In this example, the asymmetrical stent  20  is intended to be deployed in a manner that produces an extra radial outward force at treatment location  10  against the pressing tumor  19  growth outside of passageway  11 , which may be an artery, a vein or any other passageway in a living body in need of support by a stent. An enlarged view of the asymmetrical stent  20  is shown in  FIG. 5 , which includes rows of microbarbs  36 , that are omitted for clarity from the illustrations of  FIGS. 1-4 . 
       FIG. 2  shows asymmetrical stent  20  after self expanding into contact with the lumen wall  12  of vessel  11  responsive to movement release by the retractable sheath ( FIG. 1 ). If the procedure was stopped at this point, stent  20  may behave in passageway  11  similar to self expanding stents well known in the art in that the lumen wall  12  will be substantially supported with a nearly uniform radial outward force. The asymmetrical capability of stent  20  according to the present disclosure is activated by expanding both the stent  20  and the passageway  11  beyond the equilibrium point shown in  FIG. 2  with a balloon. When this occurs as shown in  FIG. 3 , the fact the first region  31  is more resistant to expansion than the second region  32  results in a portion of the stent  20  in the region where the first and second regions  31 ,  32  meet to move the stent along the lumen wall  12  in a tangential direction  14 . The magnitude  17  of the movement in tangential direction  14  is proportional to an inflation pressure in balloon  40 . Thus, those skilled in the art will appreciate that the magnitude  17  can be calibrated to the inflation pressure of balloon  40  so that the clinician may determine magnitude  17  based upon expected calibration data that could be determined experimentally. Thereafter as shown in  FIG. 4 , balloon  40  is deflated and both asymmetrical stent  20  and passageway  11  return to a new equilibrium that is similar in size or diameter to the equilibrium reflected in  FIG. 2  but with a different balance of forces such that the first region  31  places an extra radial outward force at the predetermined location  13  adjacent to tumor  19 , which is a greater radial outward force on the lumen wall  12  than by second region  32  directly opposite of location  13 . Although the residual displacement magnitude  17  may be equal above and below plane  25  as shown in  FIGS. 4 and 5 , the residuals may be different. 
     As best shown in  FIG. 5 , when balloon  40  is deflated, the first and second plurality of barbs  33 ,  34  engage the lumen wall  12  and prevent the asymmetrical stent  20  from moving in a reverse tangential direction  15  back toward the equilibrium position illustrated by  FIG. 2 . The microbarbs shown in  FIG. 5  are omitted from the views of  FIGS. 1-4  for clarity. Those skilled in the art will appreciate that a first plurality of barbs  33  and a second plurality of barbs  34  point generally away from the location  13  where the extra radial outward force acts, and to a lessor extent point away from longitudinal axis  22 . The orientation of each of the barbs  33 ,  34  point in a direction that is at an acute angle  27  with respect to a radius vector  28  that extends from longitudinal axis  22  through the respective barb  33 ,  34 . Those skilled in the art will appreciate that the acute angle  27  may vary for the barbs as they are located closer or further away from the location  13  where the extra radial outward force is needed. 
     Referring now in addition to  FIGS. 6 and 7 , the first region  31  and the second region  32  may be contiguous with one another as shown in the embodiment of  FIGS. 1-5 . However, in order to provide a less abrupt change between the increased stiffness of the first region relative to that of the second region  32 , the first and second regions  31 ,  32  may be separated by first and second transition regions  51 ,  52 , respectively as best shown in  FIG. 7 .  FIG. 7  shows an asymmetrical stent  120  in which the first and second regions  31  and  32  extend the full length of asymmetrical stent  120 . However, both regions  31  and  32  are separated by transition regions  51  and  52  so that the change in stiffness from that of the first region  31  to that of the second region  32  occurs in a more continuous and less abrupt manner than that associated with the embodiment of  FIGS. 1-5 .  FIG. 6  shows that the asymmetrical portion of asymmetrical stent  20  may have a length  24  that is less than the full length  23  of the asymmetrical stent  20  without departing from the scope of the present disclosure. Thus, an asymmetrical stent  20  according to the present disclosure may include a segment that has substantially uniform radial outward bias around its circumference, such as that associated with typical well known stents, but include a length portion  24  that includes the asymmetric features associated with that of the present disclosure. Furthermore, an asymmetrical stent  20  according to the present disclosure may include an asymmetric segment  24  that is sandwiched between conventional stent tubular frameworks on opposite ends as shown by the solid end dotted lines in  FIG. 6 . 
     All versions of an asymmetrical stent according to the present disclosure include a tubular shaped framework  21  that defines a longitudinal axis  22 . The tubular shaped framework  21  includes a first region  31  that exclusive of a second region  32 , both of which extend less than completely around the longitudinal axis  22 . The first region  31  and the second region  32  expand responsive to inflation of a balloon  40  positioned within the tubular shaped framework  21 . Those skilled in the art will appreciate that asymmetrical stent may self expand to assume a configuration that allows a balloon  40  to be positioned within the tubular shaped framework  21 . The first region  31  is more resistant to expansion than the second region  32 . The tubular shaped framework  21  also includes a first plurality of barbs  33  and a second plurality of barbs  34  that all point away from the longitudinal axis  22 . The first and second plurality of barbs  33 ,  34  are on respective sides of a plane  26  that include the longitudinal axis  22  and bisects the first region  31 , as best shown in  FIG. 5 . Although the first region  31  may be contiguous to the second region  32 , such as at two locations around the circumference of asymmetrical stent  20 , the two regions may be separated by a first transition region  51  and a second transition region  52 , as best shown in  FIG. 7 , in order to provide a more continuous transition between the different stiffness levels of the first and second regions  31 ,  32  respectively. Inclusion of transition regions  51  and  52  may be desirable in order to reduce the potential adverse effects from fatigue that the asymmetrical stent  20  may encounter during its working life. Although asymmetrical stent  20  is preferably self expanding in order to self expand to reach the equilibrium configuration associated with  FIG. 2 , those skilled in the art will appreciate that the stent may be balloon expanded, but still exhibit elasticity when over expanded by a balloon as shown in  FIG. 3 . The first region  31  may be contiguous with the second region  32  at two longitudinal lines  35  such that the first region  31  and the second region  32  together completely encircle the longitudinal axis  22 . In the case of the embodiment shown in  FIG. 7 , the first and second regions  31 ,  32  are not contiguous and are separated by transition regions  51  and  52  so that together the first region  31 , the second region  32 , the first transition region  51  and the second transition region  52  together completely encircle the longitudinal axis  22 . Although the first region  31  and likely the second region  32 , may extend the full length of the tubular shaped framework  21 , this disclosure also contemplates asymmetrical stents  20  in which the first region  31  extends less than a length  23  of the stent along longitudinal axis  22 , as best shown in  FIG. 6 . Although the first and second regions  31 ,  32  may have equal areas when in the equilibrium configuration associated with  FIG. 2 , one of the first region  31  and the second region  32  may have a smaller area than the other of the first and second regions  31 ,  32 .  FIG. 6  also shows a configuration in which the first region  31  has a smaller area than the second region  32 . The first and second plurality of barbs  33  and  34  may include at least one row of barbs  36  that extend along longitudinal axis  22  for a length on tubular shaped framework  21  that is about equal to the respective region  31  or  32 . Although not shown, the asymmetrical shape  20  may also include barbs in the non-asymmetric portion that flanks one or both sides of the asymmetrical first and second regions  31  and  32  as shown in  FIG. 6 . Preferably, the stent may include a plurality of rows of barbs  36  on opposite sides of the plane  26  that bisects the first region as shown in  FIG. 5 . Thus, the first and second plurality of barbs  33 ,  34  may include a band of barbs extending along the longitudinal axis  22  that is comprised or two or more rows of barbs  36 , or a field of barbs with no discernable rows. Also as best shown in  5 , in order to focus the excess radial outward force at the predetermined location  13  on the outer surface of asymmetrical stent  20 , the first plurality of barbs  33  may point in directions that diverge from the pointing directions  38  associated with the second plurality of barbs  34 . Finally, in order to assist the clinician in properly orienting the stent during an implantation procedure so that the predetermined location  13  is oriented properly within the patient passageway, the asymmetrical stent  20  may include a plurality of radiopaque markers  37  that are attached to the tubular shaped framework in a pattern that identifies an orientation of the first region  31  about the longitudinal axis  22  with imaging techniques well known in the art. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure finds general applicability to situations in which a passageway within a patient can benefit from asymmetric internal support so that more radial force is applied at one location of the lumen wall of the passageway verses a radial outward support opposite to that location. These circumstances might occur, for instance, when some other condition, such as a tumor, is pressing against the outside of a passageway on one side of the passage but not the other, so that excess radial outward force is necessary to push against the pinching tumor or other phenomenon. Other potential applications include supporting a vessel in a iliac vein responsive to May-Thurner syndrome. Although the present disclosure finds general applicability in any body passageway, the present disclosure finds particular application to supporting blood vessels, including both veins and arteries in the need of some asymmetrical support afforded by the asymmetrical stent of the present disclosure. 
     Referring again to  FIGS. 1-4 , a method of using an asymmetrical stent  20  according to the present disclosure includes positioning the asymmetrical stent  20  at a treatment location  10  as shown in  FIG. 1 . Passageway may be in a live body, or be an artificial passageway for teaching or demonstration purposes. Next, the asymmetrical stent is oriented so that a stiffer region  31  faces a predetermined portion  13  of a lumen wall  12  of a passageway  11 . This may be accomplished by the clinician appropriately rotating a delivery system while observing radiopaque markers  37  in a conventional manner. Next, the asymmetrical stent  20  is expanded until the stent  20  contacts the lumen wall  12  of the passageway  11 . This may be accomplished by withdrawing containment sheath  18  as shown in  FIG. 1  to allow the asymmetrical stent to self expand out into contact with the lumen wall  12  as shown in  FIG. 2  until achieving an equilibrium. Next, both the asymmetrical stent  20  and the passageway  11  are further expanded by inflating a balloon  40  as shown in  FIG. 3 . When this occurs, portions of the asymmetrical stent  20  move along the lumen wall  12  in a tangential direction  14  because the first region  31  is more resistant to expansion than the second region  32 . Next, the balloon  40  is deflated and moved out of contact with the asymmetrical stent shown in  FIG. 4 . The passageway  11  and the asymmetrical stent  20  elastically shrink back from the over expanded configuration of  FIG. 3  responsive to deflation of the balloon  40 , as shown in  FIG. 4 . However, barbs, or some other friction feature, that are attached to the tubular shaped framework  21  of the asymmetrical stent  20 , or incorporated onto a graft fabric, inhibit reverse movement of the asymmetrical stent  20  along the lumen wall  12  in a reverse tangential direction  15  responsive to deflation of the balloon. This phenomenon results in the engagement of the barbs with the passageway wall  12  in a shear stress interaction concentrating excess radial outward force at the predetermined location  13  that is located at about the center point of the first region  31 . Thus, the first and second plurality of barbs assist in directing a radially outward force from the stent  20  toward the predetermined portion  13  of the lumen wall  12  that is greater than a radially outward force on the lumen wall opposite the predetermined portion  13 . Those skilled in the art will appreciate that the distance of tangential movement  14  is proportional to the amount that the vessel or passageway  11  and  20  are overexpanded by balloon  40 , whose diameter is primarily responsive to the inflation pressure within the balloon. Thus, one may set a magnitude of the radially outward force at location  13  responsive to a predetermined inflation pressure of the balloon  40 . In addition, one may inflate the balloon  40  to a certain magnitude and ascertain whether the asymmetrical force is adequate. If not, the balloon may be re-inflated to a greater pressure to increase the asymmetrical force at the predetermined location  13 . Those skilled in the art will appreciate that because the movement is in one direction, one may not decrease the excess radial force. Thus, the clinician may step wise re-inflate the balloon to incrementally greater pressures until the asymmetrical stent  20  is properly configured with a desired radially outward force magnitude. 
     The present description is for illustrative purposes only, and should not be construed to narrow the breadth of the present disclosure in any way. Thus, those skilled in the art will appreciate that various modification might be made to the presently disclosed embodiments without departing from the full and fair scope and spirit of the present disclosure. Other aspects, features and advantages will be apparent upon an examination of the attached drawings and appended claims.