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, wherein at least one pair adjacent radial strut members have unequal axial lengths.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority pursuant to 35 U.S.C. § 119 (e) to provisional application 60/584,454 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, wherein at least one pair of circumferentially adjacent radial strut members have unequal axial lengths.  
       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, generally uniform strut width, as well as struts with uniform axial lengths. 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. 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 and strut members having different axial lengths. The design produces near uniform stress and/or strain for a given loading condition with or without the presence of defects or imperfections. The design also allows for greater flexibility, conformability, and offers a smaller crimping profile.  
       SUMMARY OF THE INVENTION  
       [0012]     The present 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, wherein at least one pair of circumferentially adjacent radial strut members have unequal axial lengths. In one embodiment of the present invention the stent has 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 from a plurality of radial strut members, and one or more radial arc members connecting adjacent radial struts. At least one pair of adjacent radial strut members have unequal axial lengths from one another. In addition, 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.  
         [0013]     Another embodiment of the present invention includes a stent comprising 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 from a plurality of radial strut members, and one or more radial arc members connecting adjacent radial struts. At least one pair of the adjacent radial arc members have a different geometry from one another. In addition, 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.  
         [0014]     In still another embodiment of the present invention, the stent comprises one or more radial support members having at least one radial component, wherein at least one pair of circumferentially adjacent radial components has different geometry from one another. In addition, at least one radial component has non-uniform cross-sections to achieve near-uniform strain distribution along the radial component when the radial component undergoes deformation.  
         [0015]     The present invention also includes a stent comprising one or more members each having a plurality of components, wherein at least one pair of circumferentially adjacent components has different geometry from one another. In addition, at least one component has non-uniform cross-sections to achieve near-uniform strain distribution along the component when the component undergoes deformation.  
         [0016]     In still another embodiment of the invention the 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 pair of circumferentially adjacent radial strut members has unequal axial lengths from one another. The stent further comprises one or more substantially circumferentially oriented flex connectors connecting longitudinally adjacent hoop components. Each flex connector comprises a flexible strut, with the flexible strut being connected at each end by one flexible arc.  
         [0017]     Another embodiment of the invention comprises a stent having 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 from a plurality of circumferential hoop sections, where each hoop section is formed from a plurality of radial strut members, and a plurality of radial arc members connecting adjacent radial struts. At least one pair of circumferentially adjacent radial strut members has unequal axial lengths from one another. The stent further comprises one or more substantially circumferentially oriented flex connectors connecting longitudinally adjacent hoop sections at one radial arc per hoop section. The circumferential amplitude of the flexible connector is at least 1.5 times greater than the circumferential amplitude of the connected radial arc.  
         [0018]     In still another embodiment of the present invention, the 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 from a plurality of circumferential hoop sections, and corresponding points on longitudinally adjacent hoop sections are circumferentially displaced from one another. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]      FIG. 1  is a perspective view of an intraluminal stent in an unexpanded or crimped, pre-deployed condition.  
         [0020]      FIG. 2  is a perspective view of an intraluminal stent in the fully expanded condition.  
         [0021]      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.  
         [0022]      FIG. 3B  is a magnified detail view of a proximal hoop element according to one embodiment of the present invention.  
         [0023]      FIG. 3C  is a magnified detail view of a internal hoop element according to one embodiment of the present invention.  
         [0024]      FIG. 3D  is a perspective view illustrating the nesting of the flex connectors and hoop section components (radial arc and radial strut) after crimping.  
         [0025]      FIG. 3E  is a magnified detail view of a flex connector according to one embodiment of the present invention.  
         [0026]      FIG. 3F  illustrates a repeating sinusoidal wave pattern having alternating high and low amplitude pairs according to one embodiment of the present invention.  
         [0027]      FIG. 3G  illustrates a repeating sinusoidal wave pattern according to one embodiment of the present invention.  
         [0028]      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.  
         [0029]      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).  
         [0030]      FIG. 5A  is a magnified detail view of a stent section as typically found in the prior art.  
         [0031]      FIG. 5B  is a magnified detail view of a stent section according to one embodiment of the present invention.  
         [0032]      FIG. 5C  is a graphical representation of the strain experienced by stent sections at various points along the stent section. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The present invention describes an intraluminal medical device that will accommodate the device expansion into the wall of a vessel lumen, while maintaining near uniform stress and/or strain in the radial arcs when deployed. 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.  
         [0034]     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 and expansion member, such as by the expansion of a balloon.  
         [0035]     With reference to the drawing figures, like strut and arc members are represented by like reference numerals throughout the various different figures. By way of example, radial strut  108  in  FIG. 1  is equivalent to radial strut  308  in  FIG. 3 .  
         [0036]     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, pre-deployed state, while  FIG. 2  shows the stent  100  in the fully expanded state.  
         [0037]     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 D1 for insertion into a patient and navigation through the vessels, and a second diameter D2 for deployment into the target area of a vessel, with the second diameter being greater than the first diameter.  
         [0038]     The stent  100  structure comprises a plurality of adjacent hoops  106 ( a )-( e ) extending between the proximal and distal ends  102 ,  104 . In the illustrated embodiment, the hoops  106 ( a )-( e ) encompass various radial support members and/or components. In particular, the radial components that comprise the hoops  106 ( a )-( e ) include a plurality of longitudinally arranged radial strut members  108  (for example,  108   b   1 ,  108   b   2 ,  108   b   3  for hoop  106 ( b )) and a plurality of radial arc members  110  (for example,  110   b   1 ,  110   b   2  for hoop  106 ( b )) connecting adjacent radial struts  108 . Circumferentially 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.  
         [0039]     The stent  100  structure further comprises a plurality of flex connectors  114 , which connect longitudinally adjacent hoops  106 ( a )-( e ). 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 substantially circumferentially 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 “S” shaped pattern. The plurality of flexible arc members  118  have a substantially semi-circular configuration and are substantially symmetric about their centers.  
         [0040]     Each flex connector  114  has two ends. One end of the flex connector  114  is attached to one radial arc  110  ( 110   a ) on one hoop, for examples hoop  106 ( c ), and the other end of the flex connector  114  is attached to one radial arc  110  ( 110   a ) on a longitudinally adjacent hoop, for example hoop  106 ( d ). The flex connector  114  connects longitudinally adjacent hoops  106 ( a )-( e ) together at “flex connector to radial arc connection region”  117 .  
         [0041]      FIG. 3A  illustrates a stent  300  according to one embodiment of the present invention. The stent  300  is in its 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  and into a cylinder with the top points “C” joined to the bottom points “D”.  
         [0042]     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.  
         [0043]     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  ( 308   b   1 ,  308   b   2 ,  308   b   3 ) connected between radial arc members  310   b  ( 310   b   1 ,  310   b   2 ) form a closed, cylindrical, hoop section  306   b  (as shown within the dotted rectangle  312 ) in  FIG. 3A .  
         [0044]     A section of flex connectors  314  (as shown within the dotted rectangle  326 ) bridge longitudinally adjacent hoop sections  306 . Each set of flex connectors  314  can be said to consist of a multiplicity of substantially circumferentially oriented flexible struts  316 , with each flexible strut  316  being connected at each end by one flexible arc  318  forming an “S” flexible connector  314 .  
         [0045]     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 having alternating amplitudes. It should be noted that the amplitudes may repeat in some predetermined pattern. For example, the internal hoop sections ( 306 ( b ),  306 ( c ), etc.) have amplitudes that repeat in pairs.  FIG. 3G  illustrates a repeating sinusoidal wave pattern having alternating high and low amplitude pairs according to one embodiment of the present invention. For references purposes, an imaginary reference line  375  is drawn perpendicular to the longitudinal axis of the stent  300  midway between the extreme positive and negative peaks of the sinusoidal wave pattern. Progressing circumferentially along the internal hoop section, two consecutive relatively high amplitudes  361  are followed by two consecutive relatively low amplitudes  360 .  
         [0046]     Similarly, the end hoop sections ( 306 ( a ,  306 ( c )) have amplitudes that repeat in a 3 to 1 pattern. Specifically,  FIG. 3G  illustrates a repeating sinusoidal wave pattern according to one embodiment of the present invention. Progressing circumferentially along the end hoop section, one relatively low amplitude  371  is followed by three consecutive relatively high amplitudes  370 .  
         [0047]     Circumferentially adjacent flex connectors  314  are attached to longitudinally adjacent hoops  306  every two complete sinusoidal cycles. As a result, a given internal hoop section  306  has half the number of flex connector attachment points  317  as radial arcs  310 , which results in a more flexible stent.  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 . One of skill in the art would understand that other repeating cycles are contemplated by the present invention. For example, the circumferentially adjacent flex connectors  314  may be attached to longitudinally adjacent hoops  306  every three, four, etc. complete sinusoidal cycles, or in some defined pattern.  
         [0048]     Each “S” flex connector  314  is shaped so as to nest together into the circumferentially adjacent S 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 “S” flex connectors  314  overlap. As described earlier, the flex connector  314  configuration, where circumferentially adjacent flex connectors  314  are attached to longitudinally adjacent hoops  306  every two complete sinusoidal cycles, thereby enhancing the ability of circumferentially adjacent flex connectors  314  to nest during crimping.  
         [0049]     In addition, the present design, utilizes variable amplitude substantially sinusoidal patterns for nesting the hoop sections during crimping. That is to say, the unconnected radial arcs  310  ( 310   a   1 ,  310   b   1 ,  310   c   1 ) will nest within the transition region between the circumferentially adjacent medium length radial strut  308  and connected radial arc  310 .  FIG. 3D  is a perspective view illustrating the nesting of the flex connectors  314  and hoop section  306  components (radial arc  310  and radial strut  308 ) after crimping.  
         [0050]     Stent  300  illustrated in  FIG. 3A  is comprised of 13 hoop sections  306  connected by 12 sections of flex connectors  314 . The 13 hoop sections  306  include 2 end hoop sections (proximal hoop section  306   a  and distal hoop section  306   c ) and 11 internal hoop sections  306   b.    
         [0051]     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  306 ( b ) 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 . That is to say, a corresponding point on longitudinally adjacent hoop sections are circumferentially displaced from one another. This configuration allows for increased amplitudes of flex connectors, which enables greater stent flexibility during delivery, and greater conformability post deployment. Alternatively, the adjacent hoop sections  306  may be oriented in phase.  
         [0052]     As described above, each hoop section in the illustrated embodiment is comprised of radial struts  308  and radial arcs  310  arranged in a largely sinusoidal wave pattern having alternating amplitudes. Each repeating wave pattern forms a hoop element  322 . The hoop element repeats at each flex connector  314  forming the hoop  306 .  
         [0053]     In one embodiment of the invention, the substantially circumferentially oriented flex connectors  314  connect longitudinally adjacent hoop sections  306  at one radial arc  310  per hoop section. The circumferential amplitude of the flexible connectors  314  are at least 1.5 times greater, in the unexpanded and un-crimped condition, than the circumferential amplitude of the connected radial arc  310 . This allows for increased flexibility during delivery, and increased conformability in a deployed state. In addition to the foregoing, the flex connector  314  may have a circumferential amplitude greater than the axial length of the flex connector  314 . This allows for an increased number of hoops  306  and flex connectors  314  over a given length. This enables the stent  300  to have greater scaffolding, increased flexibility, and a more uniform curvature when bending.  
         [0054]     By way of example,  FIG. 3A  shows each hoop section  306  being comprised of 4 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 designing stents of larger and smaller diameter.  
         [0055]      FIGS. 3B and 3C  are magnified detail views of proximal hoop element  322   a  and internal hoop element  322   b  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. A distal end hoop element  322   c  (not shown in detail) is a mirror image of proximal end hoop element  322   a  and 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.  
         [0056]     As earlier described, hoop element  322  comprises a plurality of radial struts  308  and radial arcs  310  arranged in a largely sinusoidal wave pattern having varying amplitudes. To achieve the varying amplitude 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 lengths and radial arcs  310  of different geometries. A stent having radial struts of differing lengths is described in U.S. Pat. No. 6,540,775 to Fischell et al., dated Apr. 1, 2003 and is incorporated by reference in its entirety herein. 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 that 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.  
         [0057]     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 strain, and therefore stress, 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.  
         [0058]     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).  
         [0059]     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.  
         [0060]     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.  
         [0061]      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.  
         [0062]     Z 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 .  
         [0063]     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.  
         [0064]      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.  
         [0065]      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.    
         [0066]     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.  
         [0067]     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.  
         [0068]     A graphical representation comparing the strain experienced by the 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 .  
         [0069]     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 .  
         [0070]     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 .  
         [0071]     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 .  
         [0072]     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 (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). 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.    
         [0073]     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%.  
         [0074]     Returning again to  FIGS. 3A through 3G , 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.  
         [0075]      FIGS. 3B and 3C  show hoop elements  322  with tapered radial arcs  310  and radial struts  308 , according to one embodiment of the present invention.  
         [0076]     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 long and medium length radial struts,  308   a   1  and  308   a   2 , respectively, and two different radial arcs  310   a   1  and  310   a   2 . The differences in the two radial arcs may include, different geometries, such as different arc cross-sections; different arc radii; and different arc lengths. However, one of skill in the art would understand that other geometric differences are also contemplated by the present invention, and the identified differences should not be meant to limit the scope of the invention.  
         [0077]     The use of the terms “long”, “medium”, “short” or “different” are meant to describe relative differences between the various components and not to connote specific or equivalent dimensions.  
         [0078]      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 long, medium and short length radial struts,  308   b   1 ,  308   b   2 , and  308   b   3  respectively, and two different radial arcs  310   b   1  and  310   b   2 . The differences in the two radial arcs may include, different geometries, such as different arc cross-sections; different arc radii; and different arc lengths. However, one of skill in the art would understand that other geometric differences are also contemplated by the present invention, and the identified differences should not be meant to limit the scope of the invention.  
         [0079]     Radial arc  310   b   1  connects medium radial strut  308   b   2  to small radial strut  308   b   3 , and is not connected to flex connector  314 . Similarly, radial arc  310   b   2  connects medium radial strut  308   b   2  to long radial strut  308   b   1 , and is connected to flex connector  314 .  
         [0080]     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.  
         [0081]     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.  
         [0082]     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.  
         [0083]     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 subcombinations 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.  
         [0084]     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.