US20110184508A2 - Improved tissue supporting devices - Google Patents

Abstract

A new multiple component stent arrangement which allows for initial self-expansion and subsequent deformation to a final enlarged size.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This application is a Continuation application from Ser. No. 09/427,291, filed on Oct. 26, 1999, which is a Continuation application from Ser. No. 08/737,492 filed on May 18, 1995, as a §371 of PCT/US95/06228 filed May 18, 1995, the contents of which is hereby incorporated by reference, now U.S. Pat. No. 6,582,461, which is a continuation-in-part of application Ser. No. 08/246,320, filed May 19, 1994. This application is also a continuation of application Ser. No. 09/172,590 filed on Oct. 14, 1998, now 6,451,052, which is a Division of application Ser. No. 08/737,492 filed on May 18, 1995, as a §371 of PCT/US95/06228 filed May 18, 1995, now U.S. Pat. No. 6,582,461, which is a continuation-in-part of application Ser. No. 08/246,320, filed May 19, 1994.
BACKGROUND OF THE INVENTION
• This invention relates to tissue supporting devices in general and most particularly to vascular stents for placement in blood vessels. A primary feature of the devices of this invention is that they are expandable within the body.
• In the past, such devices have been provided for implantation within body passageways. These devices have been characterized by the ability to be enlarged radially, often having been introduced into the desired position in the body as by percutaneous techniques or surgical techniques.
• These devices are either expanded mechanically, such as by expansion of a balloon positioned inside the device, or are capable of releasing stored energy to self-expand themselves within the body.
• The materials which have been used to make up these devices have included ordinary metals, shape memory alloys, various plastics, both biodegradable and not, and the like.
• This invention is concerned with the use of these materials in a new multiple component arrangement which allows for initial self-expansion and subsequent deformation to a final enlarged diameter in the body.
• Balloon expandable stents do not always expand uniformly around their circumference. As a result, healing may not take place in a consistent manner. If the stent is coated or covered, non-uniform expansion may tear the covering or coating. Additionally, long stents of this type may require long balloons which can be difficult to handle, difficult to size, and may not offer ideal performance in tortuous passages in blood vessels and the like.
• Thus, when addressing such issues, self-expandable stents have been thought to be generally more desirable. Unfortunately, one cannot control the degree of expansion and hence the degree of embedment in the vessel wall. It has been determined that a stent must be embedded to some degree to be clinically satisfactory.
• The stents of the present invention provide the best features of both of these types of stents without their drawbacks.
SUMMARY OF THE INVENTION
• The tissue supporting devices of this invention are generally cylindrical or tubular in overall shape and of such a configuration as to allow radial expansion for enlargement. They are often referred to herein in the general sense as “stents”. Furthermore, the devices are comprised of at least one component, element, constituent or portion which exhibits a tendency to self-expand the device to an expanded size and at least one other component, element, constituent or portion which is deformable so as to allow an external force, such as a balloon positioned within the body of the device, to further expand it to a final, larger desired expanded size. The terms “component”, “telement”, “constituent” and “portion” are often referred to herein collectively as “portion”.
• Preferably, the devices of the invention are made of metal and most preferably of shape memory alloys.
• In one embodiment, a first portion is a resilient spring-like metal for self-expansion and a second portion is a deformable metal for final sizing. In a more preferred shape memory embodiment, a first portion is a self-expanding austenitic one and a second is a martensitic one capable of deformation. In the case of shape memory embodiments the “portions” may be discrete or merely different phases of an alloy.
• The most preferred embodiment of the invention is a stent, preferably of shape memory alloy. The most preferred shape memory alloy is Ni—Ti, although any of the other known shape memory alloys may be used as well. Such other alloys include: Au—Cd, Cu—Zn, In—Ti, Cu—Zn—Al, Ti—Nb, Au—Cu—Zn, Cu—Zn—Sn, Cu—Zn—Si, Cu—Al—Ni, Ag—Cd, Cu—Sn, Cu—Zn—Ga, Ni—Al, Fe—Pt, U—Nb, Ti—Pd—Ni, Fe—Mn—Si, and the like. These alloys may also be doped with small amounts of other elements for various property modifications as may be desired and as is known in the art.
• The invention will be specifically described hereinbelow with reference to stents, a preferred embodiment of the invention although it is broadly applicable to tissue support devices in general.
BRIEF DESCRIPTION OF THE FIGURES
• FIG. 1 is a braided stent according to one embodiment of this invention.
• FIG. 2 is a graph showing the martensitic/austenitic temperature transformation curve and the superelastic area of a shape memory alloy.
• FIG. 3 is an end view of a layered stent having two discrete components according to one aspect of this invention.
• FIGS. 4aand4bare graphs showing the martensitic/austenitic temperature transformation curves of the layers in the stent ofFIG. 3.
• FIGS. 5aand5bare views of another embodiment of the invention comprised of alternating rings of shape memory alloy.
• FIG. 6 is a showing of a stent fragment of a braided version of a shape memory stent of this invention.
• FIG. 7 is a graph showing a temperature window for a shape memory alloy to be used in yet another stent version of this invention.
• FIG. 7ais a graph showing expansion of a stent with temperature.
• FIG. 7bis a graph of the same type, the stent having been cold-worked.
• FIG. 7cis a graph of the same type, the stent having had pseudoelastic prestraining.
• FIG. 7dis a graph of the same type, the stent having amnesia inducement.
• FIGS. 8-11 show various expandable configurations (closed and open) illustrated in fragment which may be used in the stents of this invention.FIGS. 9aand9bshow a preferred embodiment of an articulated stent.
• FIG. 12 shows another version of an expandable stent of the invention.
• FIG. 13 shows yet another version of a stent which may be used with the invention.
• FIG. 14 is a schematic showing of a braided stent made up of a plurality of strands.
• FIG. 15 is a detail of a single strand from the stent ofFIG. 14 showing that the strand is made up of a plurality of wires of two different types.
• FIG. 16 is a cross-sectional view taken along line16-16 ofFIG. 15 showing the two different types of wire.
DETAILED DESCRIPTION OF THE INVENTION
• Preferred embodiments of this invention are described below with particular reference to the accompanying drawing Figures.
• Referring first to the embodiment shown inFIG. 1, astent10 is shown comprised of braided orinterwoven metal strands12 and14.Strands12 are of a resilient spring-like metal such as spring steel, Elgiloy for example. Preferably,strands12 are spirally extending in the same direction, spiraling to the right as seen inFIG. 1.Strands14 are of a deformable or annealed metal such as stainless steel and are preferably spiraled in the opposite direction asstrands12, as shown inFIG. 1.
• Given such a stent construction of two components i.e.,strands12 and.14, it can be seen thatstent10 may be readily loaded on a catheter as by placing it over an uninflated balloon on a balloon catheter and compressing it tightly around the balloon and then placing a sheath over the stent to hold it in place during the transluminal placement procedure. Once in place, the sheath is removed, for example slid back, to expose the stent, allowing it to self-expand by force of theresilient strands12 to substantially assume a self-expanded shape/size. Some self-expansion may be restrained if held back bystrands14. To finally adjust the size of the stent, the balloon may be expanded by inflation from within the stent to exert an outward radial force on the stent and further enlarge it by stretching and deforming the deformable metal ofstrands14. This may be aided by building intostrands14, a series of readily deformable structures or means such as bends orkinks16 as shown inFIG. 1. It can be seen that a permanent adjustable size beyond the self-expanded size may be obtained with this embodiment. It is to be noted that many configurations other than braided may be readily devised to take advantage of this two component concept, including various of the subsequent configurations-described hereinbelow. Also, it should be noted that, although not preferred, the stent may be initially deployed without a balloon; the balloon following on a separate catheter.
• Referring now to subsequent features, other preferred embodiments of the invention will be described which make use of shape memory alloys and some of their unique properties, primarily their special types of deformation i.e., shape memory deformation in martensite and/or superelastic deformation in austenite.
• The term “superelasticity” is used to describe the property of certain shape memory alloys to return to their original shape upon unloading after a substantially deformation while in their austenitic state. Superelastic alloys can be strained while in their austenitic state more than ordinary spring materials without being plastically deformed. This unusually large elasticity in the austenitic state is also called “pseudoelasticity”, because the mechanism is nonconventional in nature, or is also sometimes referred to as “transformational superelasticity” because it is caused by a stress induced phase transformation. Alloys that show superelasticity also undergo a thermoelastic martensitic transformation which is also the prerequisite for the shape memory effect. Superelasticity and shape memory effects are therefore closely related. Superelasticity can even be considered part of the shape memory effect.
• The shape memory and superelasticity effects are particularly pronounced in Ni—Ti alloys. This application will therefore focus on these alloys as the preferred shape memory alloys. The shape memory effect in Ni—Ti alloys has been described many times and is well known.
• In near-equiatomic Ni—Ti alloys, martensite forms on cooling from the body centered cubic high temperature phase, termed austenite, by a shear type of process. This martensitic phase is heavily twinned. In the absence of any externally applied force transformation takes place with almost no external macroscopic shape change. The martensite can be easily deformed by a “flipping over” type of shear until a single orientation is achieved. This process is also called “detwinning”.
• If a deformed martensite is now heated, it reverts to austenite. The crystallographic restrictions are such that it transforms back to the initial orientation thereby restoring the original shape. Thus, if a straight piece of wire in the austenitic condition is cooled to form martensite it remains straight. If it is now deformed by bending, the twinned martensite is converted to deformed martensite. On heating, the transformation back to austenite occurs and the bent wire becomes straight again. This process illustrates the shape memory deformation referred to above.
• The transformation from austenite to martensite and the reverse transformation from martensite to austenite do not take place at the same temperature. A plot of the volume fraction of austenite as a function of temperature provides a curve of the type shown schematically inFIG. 2. The complete transformation cycle is characterized by the following temperatures: austenite start temperature (A_s), austenite finish temperature (A_f), both of which are involved in the first part (1) of an increasing temperature cycle and martensite start temperature (M_s) and martensite finish temperature (M_f), both of which are involved in the second part (2) of a decreasing temperature cycle.
• FIG. 2 represents the transformation cycle without applied stress. However, if a stress is applied in the temperature range between A_sand M_d, martensite can be stress-induced. Stress induced martensite is deformed by detwinning as described above. Less energy is needed to stress induce and deform martensite than to deform the austenite by conventional mechanisms. Up to about 8% strain can be accommodated by this process (single crystals of specific alloys can show as much as about 25% pseudoelastic strain in certain directions). As austenite is the thermodynamically stable phase at temperatures between A_sand M_dunder no-load conditions, the material springs back into its original shape when the stress is no longer applied.
• It becomes increasingly difficult to stress-induce martensite at increasing temperatures above A_f. Eventually, it is easier to deform the material by conventional mechanisms (movement of the dislocation, slip) than by inducing and deforming martensite. The temperature at which martensite can no longer be stress-induced is called M_d. Above M_d, Ni—Ti alloys are deformed like ordinary materials by slipping.
• Additional information regarding shape memory alloys is found in the following references, all of which are incorporated fully herein by reference:
• “Super Elastic Nickel-Titanium Wires” by Dieter Stockel and Weikang Yu of Raychem Corporation, Menlo Park, Calif., copy received November 1992;
• Metals Handbook, Tenth Edition, Vol. 2, Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, “Shape Memory Alloys” by Hodgson, Wu and Biermann, pp. 897-902; and,
• In Press,Titanium Handbook, ASM (1994), Section entitled “Structure and Properties of Ti—Ni Alloysby T. W. Duerig and A. R. Pelton.
• Since the most preferred shape memory alloy is Ni—Ti, the martensitic state of this alloy may be used to advantage in the two component concept of this invention.
• For example, with reference toFIG. 3, a layered construction may be provided in a stent30 (shown in end view) which is generally a hollow cylindrical or tubular body in shape but which may be formed in a wide variety of specific configurations or patterns to foster radial expansion of the body as exemplified inFIGS. 1, 5,6 and in subsequentFIGS. 8-11.
• Stent30 is comprised of at least twolayers32 and34, one of which 32 is a Ni—Ti alloy (50.8 atomic wt. % Ni, balance Ti, transition temperature of A_f=0° C.) and normally in the austenitic state, the other of which 34 is a Ni—Ti (49.4 atomic wt. % Ni, balance Ti, transition temperature A_f=60° C.) and normally in the martensitic state. Preferably, the inner layer is32 and the outer layer is34. However, this may be reversed and also a plurality of layers, alternating or otherwise, may be utilized in this particular embodiment.
• Stent30 is made to a fabricated size and shape (parent shape) which providesaustenitic layer32 its parent shape and size i.e., its superelastic high temperature shape and size. Obviously, in its as fabricated condition, the Ni—Ti alloy ofaustenitic layer32 is selected so as to have a transition temperature range between its austenitic and martensitic states which is lower than body temperature as to ensure that in the body and at body temperatures the austenitic state will always prevail.
• On the other hand,martensitic layer34 is of a Ni—Ti alloy having a transition temperature range significantly greater than body temperature so as to ensure that under body conditions the martensitic state will always prevail and the alloy will never transform to austenite in stent use. This is shown in the graphs ofFIGS. 4aand4bwhich demonstrate the relative transition temperatures oflayers32 and34, respectively for purposes of this invention. It can be seen from these graphs that the normal condition of layer32 (FIG. 4a) at body temperatures and higher is the austenitic state while the normal condition of layer34 (FIG. 4b) at body temperatures is martensitic.
• To manufacture the layered construction, one may make the austenitic portion with any standard metallurgical technique and vapor deposit the martensitic portion on its surface. Other manufacturing techniques such as diffusion bonding, welding, ion beam deposition, and various others will be apparent to those familiar with this art.
• Such a stent may be compressed or constrained (deformed to a small diameter) onto a balloon catheter as described for the previous embodiment and captured within a sheath. During the constrainment,austenitic layer32 may stress induce to a martensitic state. In the alternative, the stent may be cooled below the transition temperature oflayer32 to facilitate its deformation and constrainment.Martensitic layer34 merely undergoes deformation. Thus the stent may be “loaded” onto a balloon catheter. However, with temperature changes occurring up to body temperature,layer32 will remain martensite until the constraint is removed. When released in place in the body,stent30 will expand to a percentage of its self-expanded size and shape due to the transformation oflayer32 from martensite to austenite at which point the balloon may be used to radially expand the stent to a greater permanent diameter by deformingmartensitic layer34. On the other hand, initial deployment can take place without a balloon which may be separately inserted after deployment.
• The two component concept of the invention in the layered embodiment ofFIG. 3 requires both the martensitic and austenitic phase characteristics of shape memory alloy(s) in the twodiscrete components32 and34.
• Preferably, the stent is fabricated in such a way that theaustenitic layer32 tends to self-expandstent30 to a predetermined fabricated diameter (parent shape, also referred to herein as “shape memorized diameter”). Themartensitic layer34 tends to hold back this self-expansion, preventing full expansion. For example, the stent may only be able to self-expand to 75% of its full possible diameter (as determined by the austenitic layer). Therefore, expansion beyond 75% is accomplished by an applied external force, as by the balloon inside the stent. It is suggested that the stent not be expanded beyond its normal fabricated diameter for theaustenitic layer32 will have the tendency of making the stent diameter smaller as it tries to recover its fabricated diameter (parent shape). If the stent is subjected to a temperature above body temperature and above the transition temperature of the martensitic layer (which is clinically unlikely), the stent will self-expand to the fabricated diameter only. Depending on design size there are thus provided permanent stents capable of fulfilling any needed range of sizes with an adjustable sizing capability.
• As is known in the art, the desired properties of the shape memory alloys required for use in this invention may be obtained by alloy composition and working and heat treatment of the alloys, in various combinations or singly.
• Manufacturing techniques influence the phase characteristics of the material. Alloy composition, work history, and heat treatment all influence the final characteristics. At a specific operating temperature, say body temperature, the austenite phase material will have a transition temperature below body temperature (i.e., A_f=0° C.). The material is capable of taking high strains and recovering after the load is released. The martensite phase material will have a higher transition temperature than body temperature (i.e., A_f=60° C.), and is characteristically soft and pliable and retains the deformed shape after load removal. This martensite deformation is caused by detwinning, not the typical plastic deformation, or yielding, of crystal slip.
• With reference toFIGS. 5 and 6, other stent constructions are shown which are similar to the layered version ofFIG. 3 in so far as utilization of the two component concept of this invention is concerned.
• FIGS. 5aand5bshows a stent50 made up of alternatingexpandable rings52 and54 of austenitic and martensitic alloys, respectively, analogous tolayers32 and34 of theFIG. 3 embodiment.Rings52 and54 for example are interconnected bystrut members56 which may be of any material capable of rigidly holding the rings together. Other interconnector means may be used. As can be seen inFIG. 5b, the placement ofstrut members56 does not require them to take part in the radial expansion of the stent and they can therefore be of a relatively ordinary material such as stainless steel.
• Referring now toFIG. 6, a braided or interwoven construction is shown similar in construction to that of the embodiment ofFIG. 1. In this embodiment,strands62 extending to the right inFIG. 6 are an alloy in the austenitic state whereasstrands64 extending to the left inFIG. 6 are an alloy in the martensitic state.
• Referring now to the graph ofFIG. 7, it is demonstrated that the two component concept of the invention may be embodied in two phases, i.e., components of a single shape memory alloy and need not be in the form of two discrete components such as layers, members, wires, etc. In the graph ofFIG. 7, it can be seen that an alloy composition can be selected such that it has a phase transition temperature window that bounds the proposed operating temperatures of the stent, such as the normal body temperature range. Within this transitional window or zone, the material undergoes the phase transition and is effectively compositionally comprised of a ratio of austenitic to martensitic phase depending on the temperature of the stent. This ratio should be selected so as to provide sufficient radial force from the austenite phase while still allowing for further expansion of the martensite phase with a mechanical expansion means such as a balloon. Selecting body temperature as the operating temperature, a Ni—Ti alloy of about 50/50 atomic wt. % composition (range about 49/51%) will provide an acceptable “window” for this embodiment, the two components are the austenite and martensite phases of the nitinol.
• The method of making a stent may be described as follows. Age the shape memory material (Ni Ti) until body temperature falls somewhere within the transformation window. Therefore the stent will not fully recover to its high temperature shape at body temperature. An example of this technique is described below.
• A stent of tubular 50.8% Ni balance Ti was prepared having a 1.5 mm diameter. It was substantially all austenite at room temperature, the A_fbeing about 15-20° C. and therefore being superelastic at room temperature. The stent was cooled to below room temperature to form substantially all martensite and mechanically expanded to 4.7 mm in diameter. It was maintained at the 4.7 mm in diameter and heat treated at 500° C. for 30 minutes and water quenched. Finally, it was again cooled to below room temperature to form substantially all martensite and compressed to a diameter of 1.5 mm. After deployment and at body temperature the stent has a diameter of 3.5 mm. At about 70% of full expansion, i.e., about 40° C. it had a diameter of 4.5 mm and at 42° C. it had a fully expanded diameter of 4.7 mm.
• This method works fairly well, but due to the slope of the temperature versus diameter plot being extremely vertical at body temperature, a small change in body temperature, or manufacturing control, can have a large impact on the actual self expansion diameter. As can be seen fromFIG. 7, the slope of the line between A_fand A_sis rather steep with small changes in temperature leading to large changes in percent austenite and consequently large changes in diameter of a stent made of such an alloy.FIG. 7ashows a temperature versus diameter plot. Three methods of modifying the slope of the line on the temperature versus diameter graph are cold work, pseudoelastic prestraining, and amnesia inducement, illustrated inFIGS. 7b,7cand7d, respectively.
• Cold Work
• Residual cold work in nitinol draws out or masks the point of A_fon the diameter versus the temperature curve. This is seen by the sluggish increase in diameter as temperature increases in the last 20-30% of recover. By utilizing the effects of cold work, the effects of temperature change on diameter can be reduced in the last 20 to 30% of stent expansion. Shown inFIG. 7bis an example of a temperature versus diameter plot for a cold worked part.FIG. 7aabove shows an example of a part without cold work.
• Pseudoelastic Prestraining
• Utilizing the effects of pseudoelastic prestraining (S. Eucken and T. W. Duerig,ACTA Metal, Vol. 37, No. 8, pp 2245-2252, 1989) one can create two distinct plateaus in the stress-strain behavior. This difference in stress strain behaviors can be directly linked to two distinct A_ftemperatures for the two plateaus. By placing the transition between the two plateaus at the transition from self expanding to balloon expanding, i.e., 70%, one can control the characteristics of the stent at body temperature. The goal would be to place the A_ftemperature for the first plateau (from maximum compression to 70% expansion) below body temperature such that the stent has self expanding characteristics. The A_ftemperature for the second plateau would be above body temperature such that there is no additional self expansion in this region (70 to 100% expansion) a mechanical device, like a balloon, can then be used to custom size the stent between 70% and 100% of the high temperature shape. Results of such a technique is shown inFIG. 7c.
• Amnesia Inducement
• One of the characteristics of nitinol is cycle amnesia. This was also discussed about in the article referred to immediately above. As nitinol is cycled from its heat set shape as shown inFIG. 7d, there is an increase in the amount of amnesia to recover to the heat set shape with each cycle. As long as this amnesia is not caused by permanent plastic deformation, the amnesia can be removed by heating the part above M_d. This shows there is martensite left in the part after cycling which is preventing full recovery in the austenite phase (just above A_f). This presence of non recoverable martensite (below M_d) is what may be used for the balloon expansion region of the stent.
• FIGS. 8-11 represent examples of various expandable configurations (a=closed, b=expanded) which may be incorporated into the devices of this invention. The version shown inFIGS. 10aand10bmay be modified as shown inFIGS. 10cand10d(closed and open, respectively) by omitting portions (indicated at100 inFIGS. 10cand10d) as to render the stent flexible for articulation. This may be done to other of the structures as well to improve flexibility.
• Yet another version of a device incorporating the two component concept of the invention is shown inFIG. 12. In this embodiment, a fragment of astent110 is shown. The stent includes a self-expandingcomponent112 and a deformable, external forceexpandable component114.Self expanding component112 may be resilient spring-like metal such a stainless steel or it may preferably be a shape memory alloy in the austenitic state.Component114 may be any deformable metal or the like such as annealed stainless steel or preferably a shape memory alloy in the martensitic state. The two components may simply be mechanically, welded or bonded together. Functions and operations are as described hereinabove.
• Referring toFIG. 13 a version analogous to the embodiment ofFIG. 12 is shown in which the two component concept is again embodied as different zones or portions of a single metal material.
• As shown inFIG. 13, a stent120 (fragment showing) is of a self-expandingcomponent122 and adeformable component124, both of which may be a single metal as spring steel or austenitic Ni—Ti which has been appropriately treated with respect tocomponent124 as by localized heat treatment or the like to alter the characteristics of the material of the122 component so as to render it deformable or martensitic, depending on whether it is merely resilient or is austenitic. Again, function and operation are the same as with other embodiments.
• Referring now toFIGS. 14-16, a multi-strand braided stent is shown inFIG. 15. Eachstrand150 in the stent is a micro-cable. That is, each strand is made up of a plurality ofwires152 and154 as is seen inFIGS. 15 and 16. Each of thewires152 and154 consists of two different nitinol alloys as seen best inFIG. 16, or one nitinol and one ordinary metal such as stainless steel, platinum or tantalum. The latter two would provide enhanced radiopacity. Onenitinol alloy wire154 has an austenitic finish (A_f) temperature less than body temperature. Theother wire152 could be nitinol having an A_s(austenitic start) greater than body temperature. Also, it could be an ordinary metal. Additionally, one or more of the strands may be of a biodegradable material such as a plastic or may be of a material including an absorbable drug.
• Since the two alloys are stranded into micro-cable one does not have to engage in selective, discrete heat treating methods to produce both shape memory and martensitic effects.
• Radiopaque portions or coatings may be included on any parts of these stents as is known in the prior art.
• While this invention may be embodied in many different forms, there are described in detail herein specific preferred embodiments of the invention. This description is an exemplification of the principles of the invention and is not intended to limit the invention to the particular embodiments illustrated.
• The above Examples and disclosure are intended to be illustrative and not exhaustive. These examples and description will suggest many variations and alternatives to one of ordinary skill in this art. All these alternatives and variations are intended to be included within the scope of the attached claims. Those familiar with the art may recognize other equivalents to the specific embodiments described herein which equivalents are also intended to be encompassed by the claims attached hereto.

Claims (6)

1-34. (canceled)

35. A stent comprising:

a plurality of annular elements extending axially without engaging overlap, each annular element having a compressed state and an expanded state, wherein each annular element has a longitudinal dimension which is smaller in the radially expanded state than in the compressed state; and

at least one connecting member connecting adjacent of said annular elements, the connecting member having a longitudinal dimension and being configured to elongate the longitudinal dimension of the connecting member when the annular elements are transformed from the compressed state to the expanded state.

36. A tissue support device comprising:

a plurality of annular elements, each annular element having a compressed state and a radially expanded state, wherein each annular element has a longitudinal dimension which is smaller in the radially expanded state than in the compressed state; and

connecting members connecting adjacent of said annular elements, the connecting members having a longitudinal dimension and being configured so that the longitudinal dimension of the connecting members elongates longitudinally when the annular elements are transformed from the compressed state to the expanded state to compensate for the smaller longitudinal dimension of the annular elements in the expanded state.

37. A stent as inclaim 36 wherein the connectors are U-shaped.

38. A stent as inclaim 36 wherein the stent is formed of shape memory metal alloy material.

39-42. (canceled)

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