The reinforcement ring may be an area of greater stiffness/crush resistance at an end portion of the stent. “Greater stiffness” here means having a stiffness/crush resistance greater than a portion of the stent adjacent the reinforcement ring. The reinforcement ring having greater stiffness may provide good inflow into the stent and through the vessel having the implant therein. While described herein as a “reinforcement ring,” the area of greater stiffness may be provided by an additional structure overlying the stent end (e.g., a “ring”) or may instead be an area where the strut structure is actually stronger, e.g. because the material forming the area of greater stiffness is inherently stiffer, a tighter cell structure, thicker struts or the like. For example, the reinforcement ring may have a different stent geometry, e.g., different strut width or is simply a fully-connected ring.

An exemplary embodiment of the reinforcement ring is illustrated in FIG. 7. As can be seen in FIG. 7, more of the ring struts making up the reinforcement ring are connected by flexible connectors/bridges to the adjacent ring than in the neighboring highly flexible segment.

Returning to the stent structure, as illustrated in FIG. 6, a length of stent 10 having length L0 includes high radial force segment 14 having a radial force and/or crush resistance RF1 and a flexibility F1 along the length L1 of the high radial/crush force segment 14. That is, a radial/crush resistive force RF1 of the high radial/crush force segment 14 is relatively greater than the remainder of the stent 10, and may be in the range of 0.75 to 1.00 N/mm, for example. The flexibility F1 of the high radial/crush force segment 14 may also be relatively lower than the remainder of the stent 10. Flexibility is evaluated/measured through angle of deflection. According to principles described herein, the high radial/crush force segment may be designed to withstand long term durability (fatigue) testing with a flexion range of 0-60 degrees.

The relatively high radial/crush force segment 14 is intended to be placed in a vessel in the region of the vessel prone to compression or crushing, such as pinching/crushing of the underlying left common iliac vein 1601 against the vertebra 1602 caused by May-Thurner syndrome, as illustrated in FIG. 3. The high radial/crush force segment has a diameter D1.

The length of stent L0 also includes a highly flexible segment 18, which has relatively greater flexibility than the high radial/crush force 14 segment along the length of the highly flexible segment 18. In addition, according principles of the present disclosure, the highly flexible segment 18 has a length L2, a diameter D2 and radial force, crush resistance RF2 and flexibility F2, where RF2<RF1 and F2>F1, such that the highly flexible segment is more flexible than the high radial/crush force segment 14. According to principles described herein, the highly flexible segment may be designed to withstand long term durability (fatigue) testing with a flexion range of 0-140 degrees. A radial resistive force RF2 of the highly flexible segment 18 may be in the range of 0.50 to 0.70 N/mm, for example.

The length of stent 10 may also include a transition segment 22 between the high radial/crush force segment 14 and the highly flexible segment 18, where the transition segment 22 has a length L3, a diameter D3 and radial force or radial resistive force (crush resistance) RF3 and flexibility F3, where RF1>RF3>RF2 and F1 and F2>F3>F1. The radial force or radial resistive force (crush resistance) RF3 and flexibility F3 of the transition segment 22 may vary over the length L3 of the transition segment 22 or may be constant along the length L3 of the transition segment 22.

Each of the high radial/crush force segment 14, transition segment 22 and highly flexible segment 18 has a different radial force, crush resistance and flexibility, which may be provided by different ring structures in each segment of the stent 10. As can be observed in FIG. 6, a high radial force segment 14 may have a cell structure that has relatively greater periodicity, may be formed of stiffer ring struts and flexible connectors, and/or may have a more closed cell structure or other structure to impart the desired radial force or crush resistance relative to the radial force or crush resistance of the highly flexible segment. For example, the strut geometry, thicker/wider struts provide more radial strength, number of apexes around the circumference of the stent/ring geometry can all drive radial force up or down, and the configuration/connection to the adjacent rings through the bridge connectors and more ring connectors can increase radial force. Similarly, the highly flexible segment 18 may have a cell structure that has relatively lesser periodicity, may be formed of relatively more flexible ring struts and flexible connectors, and/or have a more open cell structure. The transition segment may have a cell structure that transitions a geometry of the rings struts and flexible connectors of the high radial/crush force segment to a geometry of the highly flexible segment, or the transition segment may have a different cell structure than the high radial/crush force segment and the highly flexible segment. In an embodiment according to principles described herein, the stent having a high radial/crush force segment, a transition segment and a highly flexible segment may be cut from a single tube, such as nitinol, for example, but may also be formed by any other suitable means.

In the illustrated embodiment of FIG. 6, each of the segments of the stent has substantially the same diameter, such that D1≈D2≈D3. In another embodiment, the stent can be tapered such that D1>D2>D3. As described herein, one stent can treat a range of vein vessel diameters. The present stent structure may allow a single stent to treat multiple vessel sizes as the force exerted on the vessel remains fairly consistent over a range of diameters (3-4 mm). This is different than conventional stents in that most conventionally stents need to be specifically sized to the vessel they are treating (i.e., 0.5 mm-1.0 mm of oversizing). Thus, most conventional stents are offered in 2 mm increments (e.g., 10 mm, 12 mm, 14 mm, etc.). Adaptive diameter according to principles described herein simplifies sizing decisions for the doctor and allows a single stent to treat a long segment of vein, as the vein diameter generally reduces in diameter in the proximal direction.

It is contemplated that the length L2 of the highly flexible segment 18 will be greater than the length L1 of the high radial/crush force segment which will be greater than the length L3 of the transition segment.

An exemplary embodiment structure of a stent 110 according to principles of the present disclosure is shown in FIG. 8. As illustrated in FIG. 8, the diameter DS along the stent 110 at any given ring 112 is substantially the same (D1≈D2≈D3). In the embodiment illustrated in FIG. 8, each of the high radial/crush force segment (May-Thurner Syndrome “MTS” Section) 114, the transition segment (Transition Section) 122 and the highly flexible segment (Main Body Section) 118, has a similar cell pattern. In such case, the radial force or crush resistance RF of the segments may be varied by varying the thickness of the struts and/or flexible connectors 132 or the angular relationship of the struts with other struts and/or with the flexible connectors and/or the angulation of the flexible connectors themselves.

It should be noted that terms such as perpendicular, thickness, same, similar, and other dimensional and geometric terms should not be regarded as strict or perfect in their application. Instead, geometric and other dimensional reference terms should be interpreted based on their correspondence to accepted manufacturing tolerances and functional needs of the stent 110 on which they are employed. For example, the term “perpendicular” should be appreciated as affording a reasonable amount of angular variation due to manufacturing imperfections or the actual intentional curves cut or formed in the stent design 110. Also, any thickness, width or other dimension should be assessed based on tolerances and functional needs of the design rather than idealized measurements.

The thickness of the strut 128, on the other hand, is its depth in the radial direction which is generally perpendicular to the strut width measurement, as shown in FIG. 8. The strut thickness 128 normally corresponds to the wall thickness (outside diameter minus inside diameter) of the tube from which the stent 110 is laser cut after etching, grinding and other processing. But, embodiments of the stents disclosed herein are not necessarily limited to being laser-cut from a cylindrical tube with a predetermined wall thickness. They could also be formed or cut from flat sheets that are welded together at long edges to form a tube-like structure.

Each of the rings 112 is comprised of a plurality of ring struts 128 interconnected to form alternating peaks or apexes 120 and troughs 124. As shown in FIG. 8, each of the ring struts 128 is generally straight. In one embodiment shown in FIGS. 8-9, a stent 110 includes a plurality of rings 112 connected by a plurality of flexible connectors 132. The rings 112 are arranged in a spaced relationship along a long axis 116 of the stent 110. The connectors 132 extend between adjacent pairs of the rings 112. Each of the rings 112 and connectors 132 are comprised of a plurality of interconnecting struts. The dimensions and orientation of these struts are designed to provide flexibility and radial/crush stiffness according to principles of the present disclosure.

The exemplary hybrid stent 110 illustrated in FIG. 8 may be made of Nitinol tubing that is superelastic per ASTM F2063. The stent specification may further be as follows, post electropolishing: AF temperature of parts to be 19+/−10 degrees Celsius. The hybrid stent may be designed to treat a range of iliofemoral veins ranging in size from 12 mm to 20 mm. These dimensions are exemplary and a stent according to principles of the present disclosure are not so limited.

FIGS. 9A, 9B and 9C illustrate details of the strut and connector structure of the high radial/crush force segment 114 (FIG. 9A) and the highly flexible segment 118 (FIG. 9B) of the embodiment FIG. 8 at the locations indicated in FIG. 8. FIG. 9C is showing detailed dimensions of the eyelet 119 geometry in which a radiopaque (RO) marker will be inserted to aid the doctor with deployment location of the stent under fluoroscopy.

FIG. 9A illustrates ring struts 128a of the high radial/crush force segment 114. FIG. 9B illustrates ring struts 128b of the highly flexible segment 118.

As can be appreciated, foreshortening of the stent can be a particular problem for placement of a stent. In practice, stents with greater flexibility tend to foreshorten more. Accurate placement is ideal in all medical interventions, but it is of great interest in areas where the end that is first deployed is important. Such areas include at vessel bifurcations and branch vessels, so that the implant does not enter or interfere with the portion of the vessel that does not require treatment. Such a bifurcation is present at the inferior vena cava where it branches into right and left iliac veins, as described in more detail below.

As described herein, a stent according to principles described herein includes a high radial/crush force segment and a highly flexible segment. The high radial/crush force segment, with its stiffer structure, will have minimal foreshortening, and as a result, can allow for more accurate placement in the vessel into which it is implanted. FIG. 10 illustrates a rough placement of a stent according to principles of the present disclosure. FIG. 10 illustrates the inferior vena cava 1503 branching into the left common iliac vein 1504 and right common iliac vein 1505. It will be understood that the rough diagram illustrated in FIG. 10 represents the view looking down on a patient laying face-up (i.e., an anterior-poster view of the patient at the location of the bifurcation of the inferior vena cava 1503). For sake of simplicity, the abdominal aorta and its branching are not shown in FIG. 10, but are shown in FIG. 2, above. In an aspect described herein, peak-trough configurations (e.g., if used with the highly flexible segment) may not appreciably foreshorten.

As illustrated in FIG. 10, a multi-segment stent 10 according to principles described is placed in the left common iliac vein 1504. The high radial force segment 14 of the stent 10 may be allowed to extend into the iliac vein 1503, although the end of the high radial force segment is intended to be placed to be at the junction of the left common iliac vein 1504 and the iliac vein 1503. The highly flexible segment 18 extends away from the high radial force segment 14 and the transition segment 22 between the highly flexible segment 18 and the high radial/crush force segment 14.

To facilitate placement of the stent 10 at the junction of the left common iliac vein 1504 and the iliac vein 1503, the stent 10 may have a flared end 40 adjacent the high radial force segment 14, as illustrated in FIG. 11. The distal flared section is controlled by radius ‘r’. Exemplary flare sizes include 2.5 mm×5.0 mm and 5.0 mm×5.0 mm, but stent flares according to principles of the present disclosure are not so limited. The flared distal end of the stent may be used for placement of the stent at a bifurcation of two vessels such as the common iliac vein 1504 and the iliac vein 1503. The pre-loaded stent configuration on the delivery system described herein allows the distal flared section of the stent to be partially deployed from the delivery system allowing the operator to position the flared section of the stent at the bifurcation of two vessels. The delivery catheter is advanced central to the vessel bifurcation to be treated, in this case the left common iliac vein 1504. If radiopaque markers are provided on the implant, the operator can seat the partially deployed flare section of the stent at the bifurcation junction using the radiopaque markers. Once the central flared end of the partially deployed stent is in the appropriate deployment location and seated at the bifurcation junction the remainder of the stent can be deployed.

In an aspect of the present invention, a separate extension stent 50 may be included along with the stent 10. An embodiment of the separate extension stent 50 is illustrated in FIG. 12. As illustrated in FIG. 12, the separate extension stent 50 is tubular and may be a highly flexible segment similar to the highly flexible segment 18 in the hybrid stent 10 described above. In an aspect of the present disclosure, the separate extension stent 50 may comprise a plurality of rings 152, which comprise a plurality of ring struts 158 interconnected to form alternating peaks or apexes 160 and troughs 164. As shown in FIG. 12, each of the ring struts 158 is generally straight. The ring struts 158 may be connected to flexible connectors 162. The rings 152 are arranged in a spaced relationship along a long axis 116 of the stent 110. The flexible connectors 162 extend between adjacent pairs of the rings. The separate extension stent 50 may also include reinforcement rings on either or both ends of the tube. The dimensions and orientation of these struts are designed to provide flexibility and radial/crush stiffness according to principles of the present disclosure. Each of the rings 152 and connectors 162 comprises a plurality of interconnecting struts. The separate extension stent is made of an expandable material or a self-expandable material, such as Nitinol. The separate expansion stent 50 may be cut from a single tube, such as nitinol, for example, but could also be formed or cut from flat sheets that are welded together at long edges to form a tube-like structure.

An exemplary extension stent is illustrated in FIG. 13. The extension stent illustrated in FIG. 13 may be made of nitinol tubing that is superelastic per ASTM F2063. The stent specification may further be as follows, post electropolishing: AF temperature of parts to be 19+/−10 degrees Celsius. The extension stent may be designed to treat a range of iliofemoral veins ranging in size from 8 mm to 16 mm. These dimensions, as well as dimensions illustrated in the figures, are exemplary and a stent according to principles of the present disclosure are not so limited.

The separate extension stent 50 is placed in the left iliac vein 1504 adjacent the highly flexible segment 18 of the hybrid stent 10 and may overlap the end of hybrid stent 10, as illustrated in FIG. 14. The region of overlap in the illustration is indicted by reference number 200. The placement of the hybrid stent 10 and the separate extension stent 50 may be performed using the same delivery device at the same time. A second delivery catheter with pre-crimped extension stent may be introduced into the treatment vessel and approximate the proximal end of the previously deployed hybrid stent. The catheter with crimped extension stent would be inserted into the proximal end of the hybrid stent, positioned and the stent would be deployed utilizing the radiopaque markers on both stents to achieve appropriate overlap, e.g., 1 cm. In another aspect, the extension stent can be implanted as a stand-alone stent.

It should be noted that an extension stent as described herein may be used in combination with other stents as a “main stent”, besides the hybrid stent 10. In use, the extension stent can be used to allow for variation in placement.

In addition, the extension stent may include reinforcement rings where the reinforcement ring may be an area of greater stiffness/crush resistance at an end portion of the stent. “Greater stiffness” here means having a stiffness greater than a portion of the sent adjacent the reinforcement ring. The reinforcement ring having greater stiffness may provide good inflow into the stent and through the vessel having the implant therein. The reinforcement rings may make the extension stent easier to place with respect to the main stent, for example, by mitigating crushing of the ends as they are made to overlap. In addition, to facilitate placement, the ends of the extension stent and/or the stent to which it is to be placed adjacent can be coated with a polymer, such as urethane or PTFE. Also, the extension stent may include anchors, eyelets, radiopaque markers or other features to assist in placement of the extension stent. The extension stent may also be delivered with the main stent, or may be separately delivered to the vessel.

The extension stent may be delivered via an appropriate access site, (e.g. jugular, popliteal, etc.). The extension stent can be made to be “bidirectional”, such that it could be preloaded onto a delivery catheter without specific regard to the direction of the delivery (e.g., jugular, popliteal, etc.). E.g. the delivery can be made from above the treatment region or from below the treatment region. Such bidirectionality can be facilitated by the extensions stent geometry being symmetrical such that ends of the extension stent have the same geometry. The stent may be delivered by a coaxial delivery catheter. In another aspect of the present disclosure, a novel delivery device may include a cartridge that may be loaded onto a catheter and the hybrid sent also loaded on the catheter. The cartridge can be flipped by the operator for retrograde or anterograde. The stent may be preloaded onto the delivery catheter for the direction of the delivery (e.g., jugular, popliteal, etc.)

As can be appreciated, the actual stent ring geometry may vary from that disclosed herein, as long as the stent 10 includes a first section with a relatively higher radial force or crush resistance than a second section of the stent that has a relatively higher flexibility than the first section. It is also contemplated that the separate extension stent 50 have a flexibility similar to the highly flexible segment of the hybrid stent 10. Exemplary stent geometries for segments of the hybrid stent 10 and the extension stent 50 are taught in U.S. patent application Ser. Nos. 15/471,980 and 15/684,626, which are hereby incorporated by reference for all purposes as if fully set forth herein.

FIGS. 15A and 15B illustrate a “cut” (planar/flattened) view of an exemplary embodiment of a high radial/crush force segment of a stent in a compressed state according to principles of the present disclosure. FIG. 15A illustrates a stent geometry for the high radial/crush force segment 214 of a stent according to principles described herein. FIGS. 15A and 15B illustrate the exemplary high radial/crush force segment in a compressed state. FIG. 15B illustrates a magnified view of apices of the high radial/crush force segment of the embodiment illustrated in FIG. 15A according to principles of the present disclosure. The exemplary high radial/crush force segment 214 includes a plurality of rings 212 connected by a plurality of connectors 232. The rings 212 are arranged in a spaced relationship along a long axis of the stent high radial/crush force segment 214. The connectors 232 extend between adjacent pairs of rings 212. Each of the rings 212 is comprised of a plurality of interconnecting struts 228. The dimensions and orientation of these struts are designed to provide a relatively high radial/crush force such that the stent segment has a higher crush resistance than the adjacent transition segment or the highly flexible segment, (see FIGS. 6 and 8).

Each of the rings 212 is comprised of a plurality of ring struts 228 interconnected to form alternating peaks or apexes 240 and troughs 242. As shown in FIGS. 15A and 15B, each of the ring struts 228 is generally straight and has a main strut width 224 and a strut length 230. The main strut width 224 is the width of the strut in the circumferential direction but adjusted to be at about a right angle to the edge of the strut. In other words, the main strut width 224 is an edge to edge measurement corresponding to the outermost circumferential surface of the struts of the rings 212

Each of the connectors 232 itself is comprised a connector strut 234. In the present embodiment, the connector is a single connector strut 234, but the connector design is not necessarily limited to a single strut. As illustrated in FIG. 15A, an end 236 of each connector strut 234 connects to a respective ring strut 228. Each connector strut 234 in a plurality extends from its end that is connected to the respective ring strut 228 in a respective ring 212 toward an adjacent ring 212. The connector strut 234 extends in a direction neither parallel nor perpendicular to the longitudinal axis of the stent high radial/crush force segment 214. As illustrated in FIG. 15A, in this aspect of the high radial force segment in the illustrated embodiment, the connector connects 232 to a ring strut 228 in an adjacent ring that is offset in a latitudinal direction from the ring strut from which it extends. That is, as illustrated, the connector 234 connect from a ring strut 228 to a ring strut two places over from the one immediately adjacent—in the illustrated embodiment, there are two unconnected apices between two connected apices. In other words, in some embodiments, each ring strut 228 is not necessarily connected to another ring strut in an adjacent ring by a connector. It is possible, in another aspect, for each apex 240 to connect to an apex 240 in an adjacent ring 212. In some instances, those apices 240 may be connected by a connector 232 whose connection thereto is offset from the actual peak of the apex 240, as illustrated in detail in FIG. 15B. In some embodiments, the connectors 232 do not connect directly to or at the apexes 240 of the rings 212. Instead, they are offset somewhat along the length of the ring struts 228 to which they are connected. Also, in some embodiments, as illustrated in FIG. 15A, connectors 232 on either side of a ring 212 are “wound” in opposite directions (e.g., clockwise and counter clockwise directions) in a compressed configuration.

The connector struts 234—similar to the ring struts 228 of the exemplary embodiment—have a relatively constant width except where they connect to the rings 212. As with the ring struts 228 described above, the width of the connector struts 234 may enlarge somewhat as they merge into connections with the rings 212. As shown in FIG. 15B, for example, each of the connectors 232 also includes a main connector width 256 (between arrows) and an apex connector width 258 (between arrows). The main connector width 256 is the width of the connector strut 228, usually the minimum width or width expressing the area of highest flexibility, of the strut between the rings 212 and the connector apex 240. The apex connector width 258 is the width of the connector apex 240 somewhere along its bend, such as in the middle of the bend. In any case, the apex connector width 258 can be a structural expression of an area of high flexibility on the connector apex 240.

FIG. 16 illustrates an exemplary embodiment of a high radial/crush force segment of a stent in an expanded state according to principles of the present disclosure. As illustrated in FIG. 16, upon expansion of the compressed high radial force segment 228 illustrated in FIG. 15A, the rings 212 rotate such that the apexes of each ring 212 is circumferentially aligned between the apexes of the adjacent ring 212, through the design of the length of the connectors 232. In some embodiments, such as larger stents (e.g., outer diameters in the approximate range of 16-20 mm) the flexible connectors between and MTS section and a transition section may have “straight” connectors (in an expanded state), e.g. extending substantially in the axial direction of the stent. In other embodiments, such as smaller stents (e.g. outer diameters in the approximate range of 12-14 mm) may have angled stents in an expanded state, e.g. extending in a direction not substantially in the axial direction of the stent. While these embodiments are mentioned here, it is possible that any size stent may have “straight” connectors and/or “angled” connectors in the expanded state.

FIG. 17A illustrates a planar/flattened view of an exemplary embodiment of a flexible segment and an exemplary embodiment of a reinforcement segment of a stent in a compressed state according to principles of the present disclosure. FIG. 17B illustrates a magnified view of apices of the flexible segment of the embodiment illustrated in FIG. 17A according to principles of the present disclosure. FIG. 17C illustrates a magnified view of apices of the reinforcement rings of the embodiment illustrated in FIG. 17A according to principles of the present disclosure.

As illustrated in FIG. 17A, the exemplary flexible segment 318 includes a plurality of rings 312 connected by a plurality of connectors 332. The rings 312 are arranged in a spaced relationship along a long axis of the stent flexible segment 318. The connectors 332 extend between adjacent pairs of rings 312. Each of the rings 312 is comprised of a plurality of interconnecting struts 328. The dimensions and orientation of these struts are designed to provide a relatively higher flexibility such that the stent segment has a greater flexibility than the adjacent transition segment or the high radial/crush force segment, (see FIGS. 6 and 8).

Each of the rings 312 is comprised of a plurality of ring struts 328 interconnected to form alternating peaks or apexes 340 and troughs 342. As shown in FIGS. 17A and 17B, each of the ring struts 328 is generally straight and has a main strut width 324 and a strut length 330. The main strut width 324 is the width of the strut in the circumferential direction but adjusted to be at about a right angle to the edge of the strut. In other words, the main strut width 324 is an edge to edge measurement corresponding to the outermost circumferential surface of the struts of the rings 312

Each of the connectors 332 itself is comprised a connector strut 334. In the present embodiment, the connector 332 is a single connector strut 334, but the connector design is not necessarily limited to a single strut. As illustrated in FIG. 17A, an end 336 of each connector strut 334 extends from a trough 342 to an apex in an adjacent ring 312. In the exemplary embodiment, the connector strut 334 extends in a direction substantially parallel to the longitudinal axis of the stent flexible segment 318. As illustrated in FIG. 17A, in this aspect of the flexible segment in the illustrated embodiment, every fourth trough 342 is connected by a connector 332 to an apex 340 in an adjacent ring 312. That is, as illustrated, three adjacent troughs are not connected to the adjacent ring by a connector 332. In other words, in some embodiments, each trough is not necessarily connected to an apex 340 in an adjacent ring by a connector. It is possible, in another aspect, for each trough 342 to connect to an apex 340 in an adjacent ring 312. In some embodiments, although not shown, the connectors 332 may not connect directly to or at troughs 342 or the apexes 340 of the rings 312. Instead, they are offset somewhat along the length of the ring struts 328 to which they are connected. Also, in some embodiments, as illustrated in FIG. 15A, connectors 332 on either side of a ring 212 are “wound” in opposite directions (e.g., clockwise and counter clockwise directions).

The connector struts 334—similar to the ring struts 328 of the exemplary embodiment—have a relatively constant width except where they connect to the rings 312. The width of the connector struts 334 may enlarge somewhat as they merge into connections with the rings 312. FIG. 18 illustrates an exemplary embodiment of a flexible segment 318 of a stent in an expanded state according to principles of the present disclosure.

The exemplary reinforcement ring segment 426 includes a plurality of rings 412 connected by a plurality of connectors 432. The rings 412 are arranged in a spaced relationship along a long axis of the reinforcement rings 426. The connectors 432 extend between adjacent pairs of rings 412. Each of the rings 412 is comprised of a plurality of interconnecting struts 428. The dimensions and orientation of these struts are designed to provide a relatively greater radial force such that the stent segment has a higher crush resistance than the adjacent transition segment or the highly flexible segment, (see FIGS. 6 and 8). The present figures illustrate connectors 432 as horizontal with/parallel to the stent axial direction, but they could also be angled, and adjacent rings can be orientated such that apex-apex between rings are not aligned, as illustrated in FIG. 21. This connection pattern may be used at the end rings of the extension stent 50.

Each of the rings 412 is comprised of a plurality of ring struts 428 interconnected to form alternating peaks or apexes 440 and troughs 442. As shown in FIGS. 17A and 17C, each of the ring struts 428 is generally straight and has a main strut width 424 and a strut length 430. The main strut width 424 is the width of the strut in the circumferential direction but adjusted to be at about a right angle to the edge of the strut. In other words, the main strut width 424 is an edge to edge measurement corresponding to the outermost circumferential surface of the struts of the rings 312

Each of the connectors 432 itself may be comprised of a connector strut 434. In the present embodiment, the connector 432 is a single connector strut 434, but the connector design is not necessarily limited to a single strut. As illustrated in FIG. 17A, an end 436 of each connector strut 432 extends from an apex 440 to an apex 440 in an adjacent ring 412. In the exemplary embodiment, the connector 432 extends in a direction substantially parallel to the longitudinal axis of the stent reinforcement ring 412. As illustrated in FIG. 17A, in this aspect of the reinforcement segment 426 in the illustrated embodiment, every apex 440 is connected by a connector 432 to an apex 440 in an adjacent ring 412. In other words, in some embodiments, each apex is connected to an apex 440 in an adjacent ring by a connector 432. It is possible, in another aspect, for not all apices 440 to connect to an apex 440 in an adjacent ring 412. In some embodiments, although not shown, the connectors 432 may not connect directly to or at apexes 440 of the rings 412. Instead, they are offset somewhat along the length of the ring struts 428 to which they are connected.

The connector 432—similar to the ring struts 428 of the exemplary embodiment—have a relatively constant width except where they connect to the rings 412. The width of the connector struts 432 may enlarge somewhat as they merge into connections with the rings 412 or ring apices 440. FIG. 18 illustrates an exemplary embodiment of the reinforcement ring segment 426 of a stent in an expanded state according to principles of the present disclosure.

FIG. 17A also illustrates a transition portion where the flexible segment 318 connects to the reinforcement segment 426. As illustrated by the connection of the third ring from the right 512 in FIG. 17A, a connector 332 extends from a trough 340 from a ring 312 of the flexible segment 318 and connects to an apex 540a in the adjacent ring 512 (second ring from the right in FIG. 17A). On the opposite side of ring 512, each apex 540b of the ring 512 connects to apices 440 in adjacent ring 412. Accordingly, transition between the flexible segment 318 to the reinforcement segment 426 is accomplished.

A hybrid stent 510 having separate segments of varying radial/crush force and flexibility according to principles described herein may benefit from a smooth transition between segments. In an aspect of the present hybrid stent, the high radial/crush force segment may include rings along the length of the stent that are designed to rotate with respect to one another, and the transition and flexible region, where the stent opens more uniformly, may have no rotation. Thus, an aspect may allow for smooth transition between two adjacent regions/segments of the stent to address crimp and deployment issues that may result from the twist of the last ring of the high radial/crush force segment creating a twist in the adjacent transition/flexible segment region.