Abstract:
This invention relates generally to expandable intraluminal medical devices for use within a body passageway or duct, and more particularly to an optimized stent having asymmetrical strut and loop members and the method for designing and optimizing said strut and loop members in a continuously variable fashion. In one embodiment of the invention the resulting stent includes one or more members each having at least one component. The component has non-uniform cross-sections to achieve near-uniform stress distribution along the component when the component undergoes deformation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority pursuant to 35 U.S.C. § 119 (e) to provisional application 60/584,375 filed on Jun. 30, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to expandable intraluminal medical devices for use within a body passageway or duct, and more particularly to an optimized stent having asymmetrical strut and loop members and a method for designing and optimizing said strut and loop members in a continuously variable fashion.  
       BACKGROUND OF THE INVENTION  
       [0003]     The use of intraluminal prosthetic devices has been demonstrated to present an alternative to conventional vascular surgery. Intraluminal prosthetic devices are commonly used in the repair of aneurysms, as liners for vessels, or to provide mechanical support and prevent the collapse of stenosed or occluded vessels.  
         [0004]     Intraluminal endovascular prosthetics involve the percutaneous insertion of a generally tubular prosthetic device, such as a stent, into a vessel or other tubular structure within the vascular system. The stent is typically delivered to a specific location inside the vascular system in a low profile (pre-deployed) state by a catheter. Once delivered to the desired location, the stent is deployed by expanding the stent into the vessel wall. The expanded stent typically has a diameter that is several times larger than the diameter of the stent in its compressed state. The expansion of the stent may be performed by several methods known in the art, such as by a mechanical expansion device (balloon catheter expansion stent) or by self-expansion.  
         [0005]     The ideal stent utilizes a minimum width and wall thickness of the stent members to minimize thrombosis at the stent site after implantation. The ideal stent also possess sufficient hoop strength to resist elastic recoil of the vessel. To fulfill these requirements, many current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting connectors or undulating longitudinal connecting connectors.  
         [0006]     The circumferential sets of strut members are typically formed from a series of diagonal sections connected to curved or arc sections forming a closed-ring, zig-zag structure. This structure opens up as the stent expands to form the element in the stent that provides structural support for the vessel wall. A single strut member can be thought of as a diagonal section connected to a curved section within one of the circumferential sets of strut members. In current stent designs, these sets of strut members are formed from a single piece of metal having a uniform wall thickness and generally uniform strut width. Similarly, the curved loop members are formed having a generally uniform wall thickness and generally uniform width.  
         [0007]     Although the geometry of the stent members may be uniform, the strain experienced by each member under load is not. The “stress” applied to the stent across any cross section is defined as the force per unit area. These dimensions are those of pressure, and are equivalent to energy per unit volume. The stress applied to the stent includes forces experienced by the stent during deployment, and comprises the reactive force per unit area applied against the stent by the vessel wall. The resulting “strain” (deformation) that the stent experiences is defined as the fractional extension perpendicular to the cross section under consideration.  
         [0008]     During deployment and in operation, each stent member experiences varying load along its length. In particular, the radial arc members are high in experienced loading compared to the remainder of the structure. When the stent members are all of uniform cross-sectional area, the resultant stress, and thus strain, varies. Accordingly, when a stent has members with a generally uniform cross-section, some stent members will be over designed in regions of lesser induced strain, which invariably results in a stiffer stent. At a minimum, each stent member must be designed to resist failure by having the member size (width and thickness) be sufficient to accommodate the maximum stress and/or strain experienced. Although a stent having strut or arc members with a uniform cross-sectional area will function, when the width of the members are increased to add strength or radio-opacity, the sets of strut members will experience increased stress and/or strain upon expansion. High stress and/or strain can cause cracking of the metal and potential fatigue failure of the stent under the cyclic stress of a beating heart.  
         [0009]     Cyclic fatigue failure is particularly important as the heart beats, and hence the arteries “pulse”, at typically 70 plus times per minute—some 40 million times per year—necessitating that these devices are designed to last in excess of 10 8  loading cycles for a 10-year life. Presently, designs are both physically tested and analytically evaluated to ensure acceptable stress and strain levels are achievable based on physiologic loading considerations. This is typically achieved using the traditional stress/strain-life (S-N) approach, where design and life prediction rely on a combination of numerical stress predictions as well as experimentally-determined relationships between the applied stress or strain and the total life of the component. Fatigue loading for the purpose of this description includes, but is not limited to, axial loading, bending, torsional/twisting loading of the stent, individually and/or in combination. One of skill in the art would understand that other fatigue loading conditions can also be considered using the fatigue methodology described as part of this invention.  
         [0010]     Typically, finite-element analysis (FEA) methodologies have been utilized to compute the stresses and/or strains and to analyze fatigue safety of stents for vascular applications within the human body. This traditional stress/strain-life approach to fatigue analysis, however, only considers geometry changes that are uniform in nature in order to achieve an acceptable stress and/or strain state, and does not consider optimization of shape to achieve near uniform stress and/or strain along the structural member. Nor is it known that such optimization has ever been attempted to be done in a continuously variable fashion in accordance with the present invention. By uniformity of stresses, a uniformity of “fatigue safety factor” is implied. Here fatigue safety factor refers to a numerical function calculated from the mean and alternating stresses measured during the simulated fatigue cycle. In addition, the presence of flaws in the structure or the effect of the propagation of such flaws on stent life are usually not considered. Moreover, optimization of the geometry considering flaws in the stent structure or the effect of the propagation of such flaws has not been implemented.  
         [0011]     What is needed is a stent design where the structural members experience near uniform stress and/or strain along the member, thereby maximizing fatigue safety factor and/or minimizing peak strain, and analytical methods to define and optimize the design, both with or without imperfections. One such resulting design contemplates stent members with varying cross-sections to produce a near uniform stress and/or strain for a given loading condition with or without the presence of defects or imperfections.  
       SUMMARY OF THE INVENTION  
       [0012]     This invention relates generally to expandable intraluminal medical devices for use within a body passageway or duct, and more particularly to an optimized stent having asymmetrical strut and loop members and a method for designing and optimizing said strut and loop members in a continuously variable fashion.  
         [0013]     In accordance with the present invention the method disclosed allows one to define a multitude of designs with utilization of a numerical methodology performed in a continuously variable fashion. More particularly, by inputting representative geometric, and material values in a continuously variable fashion with limited constraints, solving for resultant values, comparing these resultant values with target values, modifying where appropriate and repeating the process when required until the desired relationship between the resultant and target values is achieved and an optimized design in accordance with the present invention can be defined. For example, several of these designs that may result using this method are identified below.  
         [0014]     In one embodiment of the invention, the resulting stent comprises one or more hoop components having a tubular configuration with proximal and distal open ends defining a longitudinal axis extending there between. Each hoop component is formed as a continuous series of substantially longitudinally oriented radial strut members and a plurality of radial arc members connecting adjacent radial struts. At least one radial arc member has non-uniform cross-sections to achieve near-uniform strain distribution along the radial arc when the radial arc undergoes deformation.  
         [0015]     Another embodiment of the present invention results in a stent having one or more flex connectors having at least one flex component. The flex component is designed to have non-uniform cross-sections to achieve near-uniform strain distribution along the flex component when the flex component undergoes deformation.  
         [0016]     Similarly, another embodiment of the invention results in a stent which comprises one or more radial support members having at least one radial component. The radial component is designed to have non-uniform cross-sections to achieve near-uniform strain distribution along the radial component when the radial component undergoes deformation.  
         [0017]     In still another embodiment of the invention, the resulting stent comprises one or more members where each member has at least one component. The component is designed to have non-uniform cross-sections to achieve near-uniform strain distribution along the component when the component undergoes deformation.  
         [0018]     In still another embodiment of the invention, the resulting stent comprises a plurality of hoop components having a tubular configuration with proximal and distal open ends defining a longitudinal axis extending there between. Each hoop component is formed as a continuous series of substantially longitudinally oriented radial strut members and a plurality of radial arc members connecting adjacent radial struts. At least one radial arc member has non-uniform cross-sections to achieve near-uniform strain distribution along the radial arc when the radial arc undergoes deformation. The stent further comprises one or more longitudinally oriented flex connectors connecting adjacent hoop components. Each flex connector comprising flexible struts, with each flexible strut being connected at each end by one flexible arc.  
         [0019]     Another resulting stent when utilizing the present invention includes one or more hoop components having a tubular configuration with proximal and distal open ends defining a longitudinal axis extending there between. Each hoop component is formed as a continuous series of substantially longitudinally oriented radial strut members and a plurality of radial arc members connecting adjacent radial struts. At least one radial arc member has non-uniform cross-sections to achieve near-uniform stress distribution along the radial arc when the radial arc undergoes deformation.  
         [0020]     Still another medical device when utilizing the present invention comprises a stent including one or more flex connectors having at least one flex component. The flex component has non-uniform cross-sections to achieve near-uniform stress distribution along the flex component when the flex component undergoes deformation.  
         [0021]     The present invention also contemplates defining a stent having one or more radial support members, including at least one radial component. The radial component has non-uniform cross-sections to achieve near-uniform stress distribution along the radial component when the radial component undergoes deformation.  
         [0022]     Another resulting stent design according to the present invention comprises one or more members each having at least one component. The component has non-uniform cross-sections to achieve near-uniform stress distribution along the component when the component undergoes deformation.  
         [0023]     Still another resulting stent according to the present invention includes a plurality of hoop components having a tubular configuration with proximal and distal open ends defining a longitudinal axis extending there between. Each hoop component is formed as a continuous series of substantially longitudinally oriented radial strut members and a plurality of radial arc members connecting adjacent radial struts. At least one radial arc member has non-uniform cross-sections to achieve near-uniform stress distribution along the radial arc when the radial undergoes deformation. The stent further comprises one or more longitudinally oriented flex connectors connecting adjacent hoop components. Each flex connector comprising flexible struts, with each flexible strut being connected at each end by one flexible arc.  
         [0024]     Still another resulting stent according to the present invention comprises one or more hoop components having a tubular configuration with proximal and distal open ends defining a longitudinal axis extending there between. Each hoop component is formed as a continuous series of substantially longitudinally oriented radial strut members and a plurality of radial arc members connecting adjacent radial struts. At least one radial arc member has a non-uniform profile to achieve near-uniform strain distribution along the radial arc when the radial arc undergoes deformation.  
         [0025]     The present invention also contemplates defining a stent having one or more flexible connectors connecting adjacent hoop components. Each flexible connector is formed as a continuous series of substantially longitudinally oriented flexible strut members and a plurality of flexible arc members connecting adjacent flexible struts. At least one flexible arc member has a tapered profile to achieve near-uniform strain distribution along the flexible arc when the flexible arc undergoes deformation.  
         [0026]     Another object in accordance with the present invention is a method for defining optimized geometry of improved stents in the presence of a disrupted continuum. Such disruptions may be material or geometric in nature.  
         [0027]     Another object of the present invention is a method in which the comparison of the resultant and target values maximizes the difference between the two sets of values.  
         [0028]     Yet another object of the present invention is a method in which the comparison of the resultant and target values minimizes the difference between the two sets of values.  
         [0029]     Another object of the present invention is a method in which the comparison of the resultant and target values maximizes the ratio of the two sets of values.  
         [0030]     Another object of the present invention is a method in which the comparison of the resultant and target values minimizes the ratio of the two sets of values.  
         [0031]     Another object of the present invention is a method in which multiple comparisons of the resultant and target values are accommodated such as comparing multiple resultant values to a single target value or multiple resultant values to multiple target values or a resultant value to multiple target values or any combination thereof.  
         [0032]     Another object of the present invention is a method in which the comparison of the resultant and target values predicts the useful fatigue life of the stent.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is a perspective view of an intraluminal stent in an unexpanded or crimped, pre-deployed condition according to one embodiment of the present invention.  
         [0034]      FIG. 2  is a perspective view of an intraluminal stent in the fully expanded condition according to one embodiment of the present invention.  
         [0035]      FIG. 3A  is a front view illustrating a stent in its crimped, pre-deployed state as it would appear if it were cut longitudinally and then laid out into a flat in a 2-dimensional configuration according to one embodiment of the present invention.  
         [0036]      FIG. 3B  is a magnified detail view of a proximal hoop element according to one embodiment of the present invention.  
         [0037]      FIG. 3C  is a magnified detail view of an internal hoop element according to one embodiment of the present invention.  
         [0038]      FIG. 3D  is a magnified detail view of a distal hoop element according to one embodiment of the present invention.  
         [0039]      FIG. 3E  is a magnified detail view of a flex connector according to one embodiment of the present invention.  
         [0040]      FIG. 3F  is a magnified detail view of a tapered radial arc according to one embodiment of the present invention.  
         [0041]      FIG. 4A  is a graphical representation of the stress-intensity range (difference in stress intensity factors across the fatigue loads) along the Y-axis versus the length of the discontinuity along the X-axis.  
         [0042]      FIG. 4B  is a graphical representation of Fatigue Life of the stent (along the Y axis) as a function of the discontinuity size (along the X axis)  
         [0043]      FIG. 5A  is a magnified detail view of a stent section as typically found in the prior art.  
         [0044]      FIG. 5B  is a magnified detail view of a stent section according to one embodiment of the present invention.  
         [0045]      FIG. 5C  is a graphical representation of the strain experienced by stent sections at various points along the stent section.  
         [0046]      FIG. 6  shows the process steps of inputting the representative geometric, material and boundary condition inputs and solving the numerical representation.  
         [0047]      FIG. 7  shows the process flow including the steps of the inputting, solving, comparing, and modifying as well as the resolving after modification of the representative inputs. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0048]     The present invention describes an intraluminal medical device that is capable of expanding into the wall of a vessel lumen and physiological loading, while maintaining near uniform stress (uniform fatigue safety factor) and/or strain in one or more of the device components during use. For the purpose of this description, “use” may include the delivery, deployment and post deployment (short and long term) state of the device. An intravascular stent will be described for the purpose of example. However, as the term is used herein, intraluminal medical device includes but is not limited to any expandable intravascular prosthesis, expandable intraluminal vascular graft, stent, or any other mechanical scaffolding device used to maintain or expand a body passageway. Further, in this regard, the term “body passageway” encompasses any duct within a mammalian&#39;s body, or any body vessel including but not limited to any vein, artery, duct, vessel, passageway, trachea, ureters, esophagus, as well as any artificial vessel such as grafts.  
         [0049]     The intraluminal device according to the present invention may incorporate any radially expandable stent, including self-expanding stents and mechanically expanded stents. Mechanically expanded stents include, but are not limited to stents that are radially expanded by an expansion member, such as by the expansion of a balloon.  
         [0050]     With reference to the drawing figures, like parts are represented by like reference numerals throughout the various different figures. By way of example, radial strut  108  in  FIG. 1  is similar or equivalent to radial strut  308  in  FIG. 3 .  
         [0051]     Referring to  FIGS. 1 and 2 , there is illustrated perspective views of a stent  100  according to one embodiment of the present invention.  FIG. 1  illustrates the stent  100  in an unexpanded or crimped, pre-deployed state, while  FIG. 2  shows the stent  100  in the fully expanded state.  
         [0052]     The stent  100  comprises a tubular configuration of structural elements having proximal and distal open ends  102 ,  104  and defining a longitudinal axis  103  extending there between. The stent  100  has a first diameter D 1  for insertion into a patient and navigation through the vessels, and a second diameter D 2  for deployment into the target area of a vessel, with the second diameter being greater than the first diameter.  
         [0053]     The stent  100  structure comprises a plurality of adjacent hoops  106 ( a )-( d ) extending between the proximal and distal ends  102 ,  104 . In the illustrated embodiment, the hoops  106 ( a )-( d ) encompass various radial support members and/or components. In particular, the radial components that comprise the hoops  106 ( a )-( d ) include a plurality of longitudinally arranged radial strut members  108  and a plurality of radial arc members  110  connecting adjacent radial struts  108 . Adjacent radial struts  108  are connected at opposite ends in a substantially S or Z shaped pattern so as to form a plurality of cells. The plurality of radial arc members  110  have a substantially semi-circular configuration and are substantially symmetric about their centers.  
         [0054]     The stent  100  structure further comprises a plurality of flex connectors  114 , which connect adjacent hoops  106 ( a )-( d ). Each flex connector  114  comprises one or more flexible components. In the embodiment illustrated  FIGS. 1 and 2 , the flexible components include one or more longitudinally oriented flexible strut members  116  and a plurality of flexible arc members  118 . Adjacent flexible struts  116  are connected at opposite ends in a substantially N shaped pattern. The plurality of flexible arc members  118  have a substantially semi-circular configuration and are substantially symmetric about their centers.  
         [0055]     Each flex connector  114  has two ends. One end of the flex connector  114  is attached to one radial arc  110  on one hoop, for examples hoop  106 ( c ), and the other end of the flex connector  114  is attached to one radial arc  110  on an adjacent hoop, for example hoop  106 ( d ). The flex connector  114  connect adjacent hoops  106 ( a )-( d ) together at flex connector to radial arc connection regions  117 .  
         [0056]      FIG. 3A  illustrates a stent  300  according to one embodiment of the present invention. The stent  300  is in its crimped, pre-deployed state as it would appear if it were cut longitudinally and then laid out flat in a 2-dimensional configuration. It should be clearly understood that the stent  300  depicted in  FIG. 3A  is in fact cylindrical in shape, similar to stent  100  shown in  FIG. 1 , and is only shown in the flat configuration for the purpose of illustration. This cylindrical shape would be obtained by rolling the flat configuration of  FIG. 3A  into a cylinder with the top points “C” joined to the bottom points “D”.  
         [0057]     The stent  300  is typically fabricated by laser machining of a cylindrical, Cobalt Chromium alloy tube. Other materials that can be used to fabricate stent  300  include, other non-ferrous alloys, such as Cobalt and Nickel based alloys, Nickel Titanium alloys, stainless steel, other ferrous metal alloys, refractory metals, refractory metal alloys, titanium and titanium based alloys. The stent may also be fabricated from a ceramic or polymer material.  
         [0058]     Similar to  FIG. 1 , the stent  300  is comprised of a plurality of cylindrical hoops  306  attached together by a plurality of flex connectors  314 . By way of example, a plurality of radial strut members  308   b  are connected between radial arc members  310   b  to form a closed, cylindrical, hoop section  306   b  (as shown within the dotted rectangle  312 ) in  FIG. 3A .  
         [0059]     A section of flex connectors  314  (as shown within the dotted rectangle  326 ) bridge adjacent hoop sections  306 . Each set of flex connectors  314  can be said to consist of three longitudinally oriented flexible struts  316 , with each flexible strut  316  being connected at each end by one of four flexible arc members  318  forming a “N” shaped flexible connector  314  having two ends. Each end of the N flex connector  314  is attached to curved radial arc members  310  at strut flex connector attachment points  317 .  
         [0060]     In the illustrated embodiment, each hoop section  306  is comprised of radial struts  308  and radial arcs  310  arranged in a largely sinusoidal wave pattern. Each flex connector is attached to the adjacent hoop  306  every complete sinusoidal cycle, such that the number of N flex connectors  314  in the set of N flex connectors  326  is one-half of the total number of radial arc members  310  in the hoop section  306 .  FIG. 3E  depicts a detail of a typical flex connector  314  having a longitudinally oriented flexible strut  316  connected at each end to a flexible arc  318 .  
         [0061]     Each N flex connector  314  is shaped so as to nest together into the adjacent N flex connector  314  as is clearly illustrated in  FIG. 3A . “Nesting” is defined as having the top of a first flexible connector inserted beyond the bottom of a second flexible connector situated just above that first flexible connector. Similarly, the bottom of the first flexible connector is inserted just below the top of a third flexible connector that is situated just below that first flexible connector. Thus, a stent with nested individual flexible connectors has each individual flexible connector nested into both adjacent flexible connectors; i.e., the flexible connector directly below and the flexible connector directly above that individual flexible connector. This nesting permits crimping of the stent  300  to smaller diameters without having the “N” flex connectors  314  overlap.  
         [0062]     Stent  300  illustrated in  FIG. 3A  is comprised of 9 hoop sections  306  connected by 8 sections of flex connectors  314 . The 9 hoop sections  306  include 2 end hoop sections (proximal hoop section  306   a  and distal hoop section  306   c ) and 7 internal hoop sections  306   b.    
         [0063]     The internal hoop sections  306   b  are connected at opposite ends by the sections of flex connectors  314  in a defined pattern to form a plurality of closed cells  320 . The end hoop sections ( 306   a  and  306   c ) are connected at one end to the adjacent internal hoop section by a section of flex connectors  314 , and similarly form a plurality of closed cells. Adjacent hoop sections  306  may be oriented out of phase, as illustrated in  FIG. 3A . Alternatively, the adjacent hoop sections  306  may be oriented in phase. It should also be noted that the longitudinal length of the end hoop sections ( 306   a  and  306   c ) may be of a different length than the longitudinal length of the internal hoop sections  306   b . In the embodiment illustrated in  FIG. 3A , the end hoop sections ( 306   a  and  306   c ) have a shorter longitudinal length than the internal hoop sections  306   b.    
         [0064]     As described above, each hoop section in the illustrated embodiment is comprised of radial strut members  308  and radial arc members  310  arranged in a largely sinusoidal wave pattern. Each repeating wave pattern forms a hoop element  322 . The hoop element repeats at each flex connector  314  (in a given set of flex connectors  326 ) and forms the hoop  306 .  
         [0065]     By way of example,  FIG. 3A  shows each hoop section  306  being comprised of 5 hoop elements  322 . However, the number of repeating hoop elements  322  is not meant to limit the scope of this invention. One of skill in the art would understand that larger and smaller numbers of hoop elements may be used, particularly when providing stents of larger and smaller diameter.  
         [0066]     Moreover, the geometric features of the hoop or the entire structure for that matter, may be continuously varied. Values such as length, width, thickness, diameter, spacing of features, location of arc centers, number of radii in an hoop section, radius of gyration, area, volume, section modulus, bending Moment of Inertia, Torsional moment of Inertia, etc, or other dimensional or derived geometric values of which one or more or all may be modified as required in order to optimize the resultant value as compared to the target value.  
         [0067]      FIGS. 3B through 3D  are magnified detail views of proximal hoop element  322   a , internal hoop element  322   b , and distal hoop element  322   c , respectively, according to an embodiment of the present invention. The proximal end hoop element  322   a  is attached to the flex connector  314  along its distal end. The distal end hoop element  322   c  is attached to the flex connector  314  along its proximal end.  FIG. 3C  illustrates a typical internal hoop element  322   b  attached to adjacent flex connectors  314  along its proximal and distal ends.  
         [0068]     As earlier described, hoop element  322  comprises a plurality of radial struts  308  and radial arcs  310  arranged in a largely sinusoidal wave pattern. To achieve uniform stress and/or strain in each element of the wave pattern, the hoop elements  322  are, in general, comprised of radial struts  308  and radial arcs  310  of varying dimensions within each hoop element  322 . This design configuration includes radial struts  308  having different cross-sectional areas. In addition, the proximal and distal end hoop elements  322   a  and  322   c  are of a different configuration than the internal hoop elements  322   b . Accordingly, the radial arcs  310  and radial strut  308  members that are part of the internal hoop element  322   b  may be a different dimension than the corresponding strut on the proximal or distal end hoop elements  322   a  and  322   c  respectively. The proximal and distal hoop elements  322   a  and  322   c  are mirror images of one another.  
         [0069]     The intravascular stent must be circumferentially rigid and possess sufficient hoop strength to resist vascular recoil, while maintaining longitudinal flexibility. In typical sinusoidal and near sinusoidal designs, the radial arcs experience areas of high stress and/or strain, which are directly related to stent fatigue. However, the stress and/or strain experienced along the length of the radial arc is not uniform, and there are areas of relatively low stress and/or strain. Providing a stent having radial arcs with uniform cross-sectional results in areas of high maximum stress and/or strain and other areas of relatively low stress and/or strain. The consequence of this design is a stent having lower expansion capacity as well as lower fatigue life.  
         [0070]     The stent design according to the present invention has been optimized around stress (fatigue safety factor) and/or strain, which results in a stent that has near uniform strain, as well as optimal fatigue performance, along the critical regions of the stent. Optimal fatigue performance is achieved by maximizing the near uniform fatigue safety factor along the stent. Various critical regions may include the radial arcs  310  and/or radial struts  308  and/or flexural arcs  318  and/or flexural struts  316 . In a preferred embodiment the critical region includes the radial arc  310 . One method of predicting the stress and/or strain state in the structure is finite element analysis (FEA), which utilizes finite elements (discrete locations).  
         [0071]     This design provides a stent having greater expansion capacity and increased fatigue life. Where initial stress and/or strain was high, material was added locally to increase the cross-sectional area of the radial arc  310 , and thereby distribute the high local stress and/or strain to adjacent areas, lowering the maximum stress and/or strain. In addition, changing the geometry of the cross-section may also result in similar reductions to the maximum stress and/or strain. These techniques, individually or in combination (i.e. adding or removing cross-sectional area and or changing cross-sectional geometry) are applied to the stent component, for example, radial arc  310 , until the resultant stress and/or strain is nearly uniform. Another benefit of this design is a stent having reduced mass.  
         [0072]     The scope of this invention includes fracture-mechanics based numerical analysis in order to quantitatively evaluate pre-existing discontinuities, including flaws in the stent structure, and thereby predict stent fatigue life. Further, this methodology can be extended to optimize the stent design for maximum fatigue life under the presence of discontinuities. This fracture-mechanics based approach according to the present invention quantitatively assesses the severity of discontinuities in the stent structure including microstructural flaws, in terms of the propensity of the discontinuity to propagate and lead to in vivo failure of the stent when subjected to the cyclic loads within the implanted vessel. Specifically, stress-intensity factors for structural discontinuities of differing length, geometry, and/or position of the discontinuity within and upon the stent structure are characterized, and the difference in the stress intensities associated with the cyclic loads are compared with the fatigue crack-growth thresholds to determine the level of severity of the discontinuity. Experimental data for fatigue crack-growth rates for the stent material are then used to predict stent life based on the loading cycles required to propagate the discontinuity to a critical size.  
         [0073]      FIG. 4A  is a graphical representation of the stress-intensity range (difference in stress intensity factors across the fatigue loads) along the Y-axis versus the length of the discontinuity along the X-axis. The solid line  480  represents the threshold stress intensity range depicted as a function of discontinuity length. This threshold stress range is characterized for the given stent material. For a given stent design, discontinuities of differing length, geometry, and/or position of the discontinuity within and upon the stent structure are numerically analyzed by introducing them into and/or onto the stent structure, and the stress intensity ranges are computed for the fatigue loads in question. By way of example, the dotted points  481 - 485  in  FIG. 4A  represent the calculated stress intensity ranges for various discontinuity lengths. If these points  481 - 485  fall below the threshold stress intensity curve  480  for a given discontinuity length, the discontinuity is considered unlikely to propagate during stent use, and in particular use during the long term post deployment state. Conversely, if the points  481 - 481  fall on or above curve  480 , the discontinuity is more likely to propagate during use.  
         [0074]     A more conservative approach can be achieved by numerically integrating the fatigue crack propagation relationship for the given stent material between the limits of initial and final discontinuity size. This approach disregards the existence of threshold stress intensity range and is therefore considered more conservative. The numerical integration results in predictions of finite lifetimes for the stent as a function of discontinuity size.  FIG. 4B  is a graphical representation of Fatigue Life of the stent (along the Y axis) as a function of the discontinuity size (along the X axis), and is characterized by curve  490 .  
         [0075]     Curve  490  is compared to the design life of the stent, curve  491 , for additional assessment of stent safety. If the predicted fatigue life  490  for a given discontinuity size is greater than the design life  491 , stents with these discontinuities are considered safe. Conversely, if the predicted fatigue life  490  for a given discontinuity size is less than or equal to the design life  491 , stents with these discontinuities are considered more susceptible to failure during use.  
         [0076]      FIGS. 5A through 5C  may be used to compare the strain experienced by the stent according to one embodiment of the present invention to a typical prior art stent configuration.  FIG. 5A  shows a magnified detail view of a radial arc  510   a  and adjacent radial struts  508   a  (hereinafter stent section  530   a ) for a prior art stent. As can be seen in the illustrated section  530   a , the radial arc  510   a  has a uniform width along its entire length.  
         [0077]      FIG. 5B  shows a similar magnified detail view of a radial arc  510   b  and adjacent radial struts  508   b  (hereinafter stent section  430   b ) for a stent according to one embodiment of the present invention. Unlike the prior art stent section  530   a  shown in  FIG. 5A , the radial arc  510   b  has a non-uniform width to achieve near uniform strain throughout the radial arc  510   b.    
         [0078]     In this description, strain optimization is being described for the purpose of example. However, one of skill in the art would understand that this method may also be applicable to optimize the stress state as well.  
         [0079]     For comparative purposes, the strain at five position points ( 1  through  5 ) along each illustrated stent section  530  was measured for a given expansion diameter. Position point  1  is located along the radial strut  508 . Position points  2  and  4  are located at each root end of the radial arc  510 , where the radial arc  410  connects to the radial strut  508 . Position point  3  is located along the radial arc  510  at or near the apex or radial midpoint.  
         [0080]     A graphical representation comparing the strain experienced by the prior art stent section  530   a  to the strain experienced by the stent section  530   b  for a given expansion diameter is illustrated in  FIG. 5C . The strain experienced by the prior art stent is identified in the graph by curve C 1 , having non-uniform strain, with the strain position points designated by a diamond shape. The total strain experienced by the prior art sent section  530   a  is the area under the curve C 1 .  
         [0081]     The strain experienced by the stent according to one embodiment of the present invention is identified in the graph by the curve C 2 , having improved strain, with the strain position points designated by a square. The total strain experienced by the prior art sent section  530   b  is the area under the curve C 2 . Since both stent sections  530   a  and  530   b  experience the same expansion, the total strain is the same. That is to say, the area under the curve C 1  is the same as the area under the curve C 2 .  
         [0082]     It should be noted that the illustrated strains and loading are exemplary, and not meant to depict actual conditions or results. Instead, the illustrated strains are used for comparative purpose to demonstrate the effect of load on stent components having different geometries.  
         [0083]     Turning to  FIG. 5C , the strain experienced by the prior art stent is relatively low at position points  1  and  2 , reaching a strain of approximately 8 at the root of radial arc  510   a  (position point  2 ). The strain then increases dramatically to a maximum strain of approximately 50% at position point  3 , i.e. the apex of radial arc  510   a . The experienced strain is substantially symmetric about the apex of the radial arc  510 , dramatically decreasing to a strain of approximately 8 at the root of the radial arc  510   a  (position point  4 ), and nearly 0% at the radial strut  508   a , position point  5 .  
         [0084]     In comparison, the strain for the stent section  530   b  is relatively low at position points  1 , but increases more uniformly between position points  2  and  3 , reaching a strains of approximately 18% at the root of the radial arc  510   b  (position point  2 ) and 35% at the apex of radial arc  510   b  (position point  3 ). Similar to curve C 1 , curve C 2  is substantially symmetric about position point  3 . As can be interpreted from  FIGS. 5A through 5C , by modifying the material cross-section (adding or subtracting material) from the radial arc root (position points  2  and  4 ) the induced strain was increased. This decreases the induced strain at the radial arc apex (position point  3 ) since the total strain experienced by the section remains unchanged. Further, by modifying the cross-sectional area in an unconstrained or limited constrained fashion (in this case by adding or subtracting material) along the apex of radial arc  510   b  (position point  3 ), the induced strain is decreased. This automatically increases the induced strain at the radial arc  510   b  roots (position points  2  and  4 ).  
         [0085]     Modifications other than geometric, include material inputs such as mechanical properties. For example, strength is an inherent property of a material which depends on the choice, treatment and processing conditions of the said material, examples include the ultimate tensile strength, shear strength, the yield strength, the fatigue strength, the compressive strength, other common mechanical properties of materials include % elongation, ductility, shear modulus of rupture, hardness, Modulus of Elasticity, Modulus of rigidity, poisson&#39;s ratio, density and the endurance limit to name a few. Each of these may be modified, on a continuously variable basis, as required in order to achieve the desired result.  
         [0086]     These modifications can be done individually as described, or in combination, iteratively, to develop a stent section  530   b  having improved near uniform strain along the radial arc  530   b.    
         [0087]     The methodology employed in accordance with the present invention involves inputting a numeric representation of a starting continuum. The representation of this continuum includes geometric inputs such as dimensions of features, material inputs such as mechanical properties and boundary conditions which may include a combination of loading and/or displacements. Solving for the resultant values, such as a representative stress or strain state and comparing these values either directly or through derived relationships to target values and then, if necessary, modifying said representative inputs and repeating the process in order to approach the desired relationship between the resultant values and the target values. One such method in accordance with the present invention includes making an improved stent comprising using a numerical methodology to maximize fatigue safety factors in the stent structure utilizing a disrupted material continuum by first providing a stent, defining an initial stent geometry, followed by selecting a material for the stent, then quantifying the material properties of the stent and applying the properties to the stent. This is followed by meshing the stent geometry into two dimensional and three dimensional forms, applying loads and boundary conditions to the stent. Such as applying loads and boundary conditions to the stent structure to model deformation encountered during manufacturing which are static, or applying loads and boundary conditions to the stent structure to model deformations encountered during stent deployment which are also static, or applying loads and boundary conditions to the stent structure within a physiological revelent model, which may be static or dynamic loads (fatigue). This is followed by solving for displacement stresses and strains at a discrete point along the stent, predicting the critical values, such as the maximum strains in the stent structure and/or the minimum fatigue safety factor in the stent structure and then comparing the predicted critical values with benchmark values, which may include evaluating the strains for the initial stent geometry by comparing the maximum strain in the stent to the strain that the stent material can accommodate without failure, and/or evaluating the fatigue safety factor for the initial stent geometry under a cyclic load;  
         [0088]     An additional method in accordance with the present invention employs a numerical methodology to maximize fatigue safety factors in the stent structure utilizing a disrupted material continuum by introducing a discontinuity into the stent wherein said discontinuity may have various lengths, various geometry, occur in various locations along or within the stent; or may be material in nature such as the presence of a carbide or differing material properties in an localized region. One then predicts the stress-intensity factors along the interface between the discontinuity and the stent material, and calculates the difference in stress-intensity factors along the interface between the discontinuity and the stent material for the cyclic load. One can then experimentally characterize a fatigue crack growth threshold stress intensity factor for the stent material, and compare the difference in stress-intensity factors (SIF) with the fatigue crack growth threshold stress intensity factor (TSIF). If the difference in SIF is less than TSIF than the discontinuity is considered unlikely to propagate, whereas if the difference in SIF is greater than or equal to TSIF than the discontinuity is considered likely to propagate. Alternatively one may experimentally characterize the fatigue crack growth rate, in order to predict finite life for the stent as a function of discontinuity size and compare with required stent life (RSL). The fatigue crack growth rate for the stent material may be expressed as a function of discontinuity size; and may be determined by numerically integrating the fatigue crack growth relationship for the stent material between limits of initial and final discontinuity size. If the predicted finite life (PFL) is greater than RSL than the stent is considered safe from a fatigue perspective, whereas if the PFL is less than the RSL than the stent is considered unsafe (from a fatigue perspective).  
         [0089]     One advantage of having near uniform strain is that the peak strain (shown at position point  3 ) is greatly reduced. As a result, the stent may be expanded to a larger expansion diameter and still be within safe operating levels of induced strain. For example, the stent represented by curve C 2  could be increased in diameter until the peak strain at position point  3  is increased from 35% to 50%.  
         [0090]     The stent  300  according to one embodiment of the present invention is laser cut from a thin metallic tube having a substantially uniform wall thickness. To vary the cross-section of the stent components, particularly the radial arcs  310 , the components have been tapered, with larger widths in areas of high loading to achieve near uniform stress and/or strain. It should be understood that the taper does not have to be uniform, which is to say of a consistently changing radius. Instead, the width of the radial arc  310  is dictated by the resultant stress and/or strain experienced by the radial arc  310  at various locations along its length.  
         [0091]      FIGS. 3B through 3D  show hoop elements  322  with tapered radial arcs  310  according to one embodiment of the present invention.  
         [0092]     Turning to  FIG. 3B , a proximal hoop element  322   a  is shown according to one embodiment of the present invention. The hoop element  322   a  is comprised of two radial struts,  308   a   1  and  308   a   2 , and two different radial arcs,  310   a   1  and  310   a   2 . The radial struts  308   a   1  and  308   a   2  are shown having different profiles in the illustrated embodiment, but this should not be interpreted to limit the scope of the invention. Other embodiments may have identical or near identical radial strut profiles.  
         [0093]     Radial arc  310   a   1  connects radial strut  308   a   2  to radial strut  308   a   1 , and is not connected to flex connector  314 . Because the radial arc  310   a   1  is not connected to the flex connector  314 , the radial arc  310   a   1  experiences near proportioned loading, and thus has a substantially symmetrical geometry (with radial strut ( 308   a   1  or  308   a   2 ) connection points  315   a  having substantially equal cross-sections) to maintain near uniform stress and/or strain throughout. The approximate midpoint of the radial arc  310   a   1  according to the illustrated embodiment experiences slightly higher loading than the radial arc  310   a   1  connection points  315   a . To accommodate the higher loading and maintain near uniform stress and/or strain throughout the radial arc  310   a   1 , the midpoint of the radial arc  310   a   1  is thicker (has a greater width) than the radial arc to radial strut connection points  315   a.    
         [0094]     Conversely, radial arc  310   a   2  is directly connected to flex connector  314 , and experiences unbalanced loading. To maintain substantially uniform stress and/or strain through the radial arc  310   a   2 , the arc  310   a   2  has a substantially asymmetrical geometry, with radial strut ( 308   a   1 ,  308   a   2 ) connection points ( 313   a ,  317   a ) respectively, having substantially unequal cross-sections. Because the radial arc  310   a   2  to flex connector  314  connection point  317   a  has a large cross-section, the connection point  319   a , located immediately adjacent thereto, may have a slightly smaller width to maintain substantially uniform stress and/or strain. The approximate midpoint of the radial arc  310   a   2  according to the illustrated embodiment experiences slightly higher loading than the radial arc  310   a   2  connection points  313   a  and  319   a . To accommodate the higher loading and maintain near uniform stress and/or strain throughout the radial arc  310   a   2 , the midpoint of the radial arc  310   a   2  is thicker (has a greater width) than the radial arc to radial strut connection points  313   a  and  319   a.    
         [0095]      FIG. 3C  shows an internal hoop element  322   b  according to one embodiment of the present invention. The hoop element  322   b  is comprised of radial struts,  308   b   1 , and  308   b   2 , and radial arcs  310   b   1  and  310   b   2 . Each radial arc ( 310   b   1 ,  310   b   2 ) connects radial strut  308   b   1  to radial strut  308   b   2 . Each radial arc ( 310   b   1 ,  310   b   2 ) is also connected to flex connector  314  near the connection point with radial strut  308   b   2 . Because the radial hoop element  322   b  is substantially symmetrical, the radial arcs ( 310   b   1 ,  310   b   2 ) experiences near proportioned loading, and thus have substantially symmetrical geometry connection points  315   b ,  313   b , and  319   b  (having substantially equal cross-sections) to maintain near uniform stress and/or strain. The approximate midpoints of the radial arcs  310   b   1 ,  310   b   2  according to the illustrated embodiment experience slightly higher loading than the radial arcs  310   b   1 ,  310   b   2  connection points  315   b ,  313   b , and  319   b . To accommodate the higher loading and maintain near uniform stress and/or strain throughout the radial arcs  310   b   1 ,  310   b   2 , the midpoints of the radial arcs  310   b   1 ,  310   b   2  are thicker (have greater width) than the radial arc to radial strut connection points  315   b ,  313   b , and  319   b.    
         [0096]      FIG. 3D  illustrates a distal hoop element  322   c  according to one embodiment of the present invention. As earlier described, the distal hoop element  322   c  is a mirror image of the proximal hoop element  322   a  shown in  FIG. 3B . As such, the loading and resultant geometry of the strut members are similar.  
         [0097]     A distal hoop element  322   c  is shown according to one embodiment of the present invention. The hoop element  322   c  is comprised of two radial struts,  308   c   1  and  308   c   2  and two different radial arcs  310   c   1  and  310   c   2 .  
         [0098]     Radial arc  310   c   1  connects radial strut  308   c   2  to radial strut  308   c   1 , and is not connected to flex connector  314 . Because the radial arc  310   c   1  is not connected to the flex connector  314 , the radial arc  310   c   1  experiences near proportioned loading, and thus has a substantially symmetrical geometry (with radial strut ( 308   c   1  or  308   c   2 ) connection points  315   c  having substantially equal cross-sections) to maintain near uniform stress and/or strain through. The approximate midpoint of the radial arc  310   c   1  according to the illustrated embodiment experiences slightly higher loading than the radial arc  310   c   1  connection points  315   c . To accommodate the higher loading and maintain near uniform stress and/or strain throughout the radial arc  310   c   1 , the midpoint of the radial arc  310   c   1  is thicker (has a greater width) than the radial arc to radial strut connection points  315   a.    
         [0099]     Conversely, radial arc  310   c   2  is directly connected to flex connector  314 , and experiences unbalanced loading. To maintain substantially uniform stress and/or strain through the radial arc  310   c   2 , the arc  310   c   2  has a substantially asymmetrical geometry, with radial strut ( 308   c   1 ,  308   c   2 ) connection points ( 313   c ,  317   c ) respectively, having substantially unequal cross-sections. Because the radial arc  310   c   2  to flex connector  314  connection point  317   c  has a large cross-section, the connection point  319   c , located immediately adjacent thereto, may have a slightly smaller width to maintain substantially uniform stress and/or strain. The approximate midpoint of the radial arc  310   c   2  according to the illustrated embodiment experiences slightly higher loading than the radial arc  310   c   2  connection points  313   c  and  319   c . To accommodate the higher loading and maintain near uniform stress and/or strain throughout the radial arc  310   c   2 , the midpoint of the radial arc  310   c   2  is thicker (has a greater width) than the radial arc to radial strut connection points  313   c  and  319   c.    
         [0100]     The stent design according to the present invention may also be optimized around minimizing maximum stress and/or strain to obtain a stent that has near uniform stress and/or strain at each point along the flex connectors  314 . This design will provide a more flexible stent, having flex connector sections of smaller cross-section where the initial measured load and stress and/or strain were low. The aforementioned criteria (i.e. adding or removing cross-section) is applied to the flex connector  314  until the resultant stress and/or strain is nearly uniform. The resultant stress or stress state is a result of the application of loads and may include for example, shear stress, torsional stress, principal stress, maximum stress yield stress, compressive stress, tensile stress, etc, when comparing these resultant values to the desired target value, the comparision may be direct or indirect as in incorporating the resultant values with experimentally determined values in a derived or pre-determined relationship.  
         [0101]     The radial struts  308  experience relatively low stress and/or strain compared to the flex connectors  314  and radial arcs  310 , so tapering of the struts  308  is typically not necessary to minimize maximum stress and/or strain for fatigue resistance. However, increasing the cross-section of the radial struts  308  as illustrated in  FIGS. 3A through 3D  makes the struts  308 , and thus the stent  300 , more radio-opaque. This enhances the visibility of the stent during fluoroscopic procedures. Increasing the cross-section of the struts  308  may also include shaping or adding a shape to the strut to increase the strut size. In one embodiment a bulge shape  309  is added to the stent strut  308 . However, one of skill in the art would understand that the type of geometric shape added to the strut  308  is not meant to limit the scope of the invention.  
         [0102]     As  FIG. 6  illustrates, the inputting step ( 601 ) is followed by the solving step ( 602 ) and includes inputting representative geometric, material and boundary condition inputs which initially define the continuum which may be with or without a disruption such as a crack, flaw, fissure, void or any geometric or material discontinuity. The numerical representation is then solved in order to determine one or more resultant values at one or more locations within the continuum defined by said representative inputs.  
         [0103]      FIG. 7  illustrates the additional process steps of comparison ( 703 ) and modifying ( 704 ) which when coupled with inputting ( 701 ) and solving (or re-solving on subsequent iterations) ( 702 ) allow one to define a stent geometry in a continuously variable fashion in both disrupted and non-disrupted continuums.  
         [0104]     In addition to the embodiments described above, therapeutic or pharmaceutic agents may be added to any component of the device during fabrication to treat any number of conditions. Having radial struts  308  with increased widths, added shapes, or gradually increasing profiles will allow the stent to carry more agent.  
         [0105]     Therapeutic or pharmaceutic agents may be applied to the device, such as in the form of a drug or drug eluting layer, or surface treatment after the device has been formed. In a preferred embodiment, the therapeutic and pharmaceutic agents may include any one or more of the following: antiproliferative/antimitotic agents including natural products such as vinca alkaloids (i.e. vinblastine, vincristine, and vinorelbine), paclitaxel, epidipodophyllotoxins (i.e. etoposide, teniposide), antibiotics (dactinomycin (actinomycin D) daunorubicin, doxorubicin and idarubicin), anthracyclines, mitoxantrone, bleomycins, plicamycin (mithramycin) and mitomycin, enzymes (L-asparaginase which systemically metabolizes L-asparagine and deprives cells which do not have the capacity to synthesize their own asparagine); antiplatelet agents such as G(GP) II b /III a  inhibitors and vitronectin receptor antagonists; antiproliferative/antimitotic alkylating agents such as nitrogen mustards (mechlorethamine, cyclophosphamide and analogs, melphalan, chlorambucil), ethylenimines and methylmelamines (hexamethylmelamine and thiotepa), alkyl sulfonates-busulfan, nirtosoureas (carmustine (BCNU) and analogs, streptozocin), trazenes-dacarbazinine (DTIC); antiproliferative/antimitotic antimetabolites such as folic acid analogs (methotrexate), pyrimidine analogs (fluorouracil, floxuridine, and cytarabine), purine analogs and related inhibitors (mercaptopurine, thioguanine, pentostatin and 2-chlorodeoxyadenosine {cladribine}); platinum coordination complexes (cisplatin, carboplatin), procarbazine, hydroxyurea, mitotane, aminoglutethimide; hormones (i.e. estrogen); anticoagulants (heparin, synthetic heparin salts and other inhibitors of thrombin); fibrinolytic agents (such as tissue plasminogen activator, streptokinase and urokinase), aspirin, dipyridamole, ticlopidine, clopidogrel, abciximab; antimigratory; antisecretory (breveldin); anti-inflammatory: such as adrenocortical steroids (cortisol, cortisone, fludrocortisone, prednisone, prednisolone, 6α-methylprednisolone, triamcinolone, betamethasone, and dexamethasone), non-steroidal agents (salicylic acid derivatives i.e. aspirin; para-aminophenol derivatives i.e. acetominophen; indole and indene acetic acids (indomethacin, sulindac, and etodalac), heteroaryl acetic acids (tolmetin, diclofenac, and ketorolac), arylpropionic acids (ibuprofen and derivatives), anthranilic acids (mefenamic acid, and meclofenamic acid), enolic acids (piroxicam, tenoxicam, phenylbutazone, and oxyphenthatrazone), nabumetone, gold compounds (auranofin, aurothioglucose, gold sodium thiomalate); immunosuppressives: (cyclosporine, tacrolimus (FK-506), sirolimus (rapamycin), azathioprine, mycophenolate mofetil); angiogenic agents: vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF); angiotensin receptor blockers; nitric oxide donors; anti-sense oligionucleotides and combinations thereof; cell cycle inhibitors, mTOR inhibitors, and growth factor receptor signal transduction kinase inhibitors; retenoids; cyclin/CDK inhibitors; HMG co-enzyme reductase inhibitors (statins); and protease inhibitors.  
         [0106]     While a number of variations of the invention have been shown and described in detail, other modifications and methods of use contemplated within the scope of this invention will be readily apparent to those of skill in the art based upon this disclosure. It is contemplated that various combinations or sub combinations of the specific embodiments may be made and still fall within the scope of the invention. For example, the embodiments variously shown to be cardiac stents may be modified to treat other vessels or lumens in the body, in particular other regions of the body where vessels or lumen need to be supported. This may include, for example, the coronary, vascular, non-vascular and peripheral vessels and ducts. Accordingly, it should be understood that various applications, modifications and substitutions may be made of equivalents without departing from the spirit of the invention or the scope of the following claims.  
         [0107]     The following claims are provided to illustrate examples of some beneficial aspects of the subject matter disclosed herein which are within the scope of the present invention.