Patent Publication Number: US-9839510-B2

Title: Stent-grafts with post-deployment variable radial displacement

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the US national stage of International Application PCT/IL2012/000148, filed Apr. 4, 2012, which claims priority from US Provisional Application 61/528,242, filed Aug. 28, 2011, and U.S. Provisional Application 61/553,209, filed Oct. 30, 2011, both of which are assigned to the assignee of the present application and are incorporated herein by reference. 
    
    
     FIELD OF THE APPLICATION 
     This present application relates generally to prostheses and surgical methods, and specifically to tubular prostheses, including endovascular stent-grafts, and surgical techniques for using the prostheses to maintain patency of body passages such as blood vessels, and treating aneurysms. 
     BACKGROUND OF THE APPLICATION 
     Endovascular prostheses are sometimes used to treat aortic aneurysms. Such treatment includes implanting a stent or stent-graft within the diseased vessel to bypass the anomaly. An aneurysm is a sac formed by the dilation of the wall of the artery. Aneurysms may be congenital, but are usually caused by disease or, occasionally, by trauma. Aortic aneurysms include abdominal aortic aneurysms (“i”), which form between the renal arteries and the iliac arteries, and thoracic aortic aneurysms (“TAAs”), which may occur in one or more of the descending aorta, the ascending aorta, and the aortic arch. 
     SUMMARY OF APPLICATIONS 
     Some applications of the present invention provide endovascular stent-grafts characterized by high physiological compliance. Such high physiological compliance minimizes the effect of the stent-grafts on the pulse profile of a blood vessel in which the stent-grafts are implanted, such as the aorta. Large-caliber arteries, in particular the aorta, provide the majority of arterial vascular compliance. The aorta and the branching large blood vessels thus act as a mechanical capacitor that expands during systole and contracts during diastole. Conventional endovascular stent-grafts often comprise substantially non-compliant graft materials, and even when such stent-grafts utilize relatively flexible medical fabrics, the stent-grafts are usually essentially fully expanded under diastolic arterial pressure. As a result, conventional stent-grafts substantially do not radially pulsate with the systolic cycle. Therefore, implantation of conventional stent-grafts results in the replacement of a large portion of a compliant aorta with a non- or low-compliant prosthesis. Conventional endovascular stent-grafts thus generally change the aortic pulse profile. Such a reduction in the overall vascular compliance may have deleterious cardiovascular effects, by increasing the load of the heart and/or decreasing the effectiveness of propagation of the systolic pulse from the heart into the smaller-caliber vasculature. 
     Some applications of the present invention provide endovascular stent-grafts that have beneficial effects on the peripheral vascular load, while using well-proven, gold-standard, medical-grade textiles and metallic alloys. The stent-grafts are configured to provide mechanical compliance that maintains (or, in some cases, even restores) the native, healthy, physiological compliance of the arterial segment in which the stent-grafts are implanted. 
     In some applications of the present invention, an endovascular stent-graft comprises a generally tubular body, which is configured to assume a radially-compressed delivery state and a radially-expanded deployment state. The body comprises a flexible stent member and a tubular fluid flow guide. The tubular flow guide comprises a graft material, which is generally non- or minimally-elastic. The body includes a compliance-restoration body portion that extends axially along a portion of the body. When the body is in the radially-expanded deployment state, the compliance-restoration body portion is (a) characterized by a greatest diastolic outer radius when the body is internally pressurized by fluid having a pressure of 80 mmHg, typically by blood during diastole in an adult human, and (b) radially expandable to a greatest systolic outer radius when the body is internally pressurized by fluid having a pressure of 120 mmHg, typically by blood during systole in an adult human. The greatest systolic outer radius is typically at least 5% greater than the greatest diastolic outer radius, such as at least 10%, e.g., 15%, greater than greatest diastolic outer radius. This increase in outer radius at greater internal pressure occurs because the stent is heat-set to have a diameter that is substantially (e.g., 5%-20%) less than the graft&#39;s fully-expanded (i.e., without folds) diameter, and the stent has the appropriate radial compliance such that the entire stent-graft substantially changes its radius between a state in which the stent-graft is internally pressurized by a nominal hydrostatic diastolic pressure and a state in which the stent-graft is internally pressurized by a nominal hydrostatic systolic pressure. 
     For example, the stent member may comprise a highly elastic (e.g. flexible stainless steel) or a superelastic (e.g. Nitinol) alloy that is heat-set to have a first relaxed outer diameter, e.g., 23 mm, along the compliance-restoration body portion when the body is not internally pressurized by fluid. The tubular flow guide is configured to have a greater, second outer diameter, e.g., 30 mm, when internally pressurized to systolic pressure, e.g., 120 mmHg. When pressurized by fluid having a diastolic pressure, e.g., 80 mmHg, the compliance-restoration body portion assumes a diastolic outer diameter that is slightly larger than the first relaxed outer diameter, e.g., between 26 and 27 mm. When pressurized by the fluid having systolic pressure, e.g., 120 mmHg, the compliance-restoration body portion assumes a systolic outer diameter equal to the second outer diameter, e.g., 30 mm, as limited by the diameter of the non-compliant graft material. For some applications, proximal and distal end-portions of the stent-graft have respective relaxed outer diameters that are greater than the first relaxed outer diameter, which may help provide good fixation and sealing with the blood vessel wall. 
     In contrast, conventional thoracic aortic stent-grafts often comprise a Nitinol stent skeleton having a heat-set diameter of 32 mm and a tubular woven PET graft cylinder having a diameter of 30 mm, sewn to the stent skeleton. Such conventional stent-grafts may have a relaxed diameter of 30 mm, and do not allow for further expansion during systole. Conventional stent-grafts thus do not provide the radial compliance provided by some applications of the present invention. 
     In some applications of the present invention, a variable-length endovascular stent-graft comprises a generally tubular body, which comprises a fluid flow guide and a plurality of structural stent elements attached to at least a portion of the fluid flow guide. The body includes a variable-length section that extends axially along a portion of body. The body, including the variable-length section, is configured to assume a radially-compressed delivery state and a radially-expanded deployment state. The variable-length section, while radially-expanded in the deployment state, is configured to enable a change in an axial length thereof of at least 5 mm. 
     This change in axial length enables the stent-graft to accommodate any elongation of the blood vessel between the ends of the stent-graft that may occur after implantation of the stent-graft. Such elongation often occurs after implantation of a stent-graft. Because the stent-graft excludes the aneurysm from the blood circulation, the aneurysm thromboses, decomposes and shrinks, causing the blood vessel to become longer and narrower. Typically, such elongation occurs over a long period of time, and the stent-graft provides long-term accommodation of the elongation. Such accommodation decreases the risk of the stent-graft becoming dislodged, and decreases the risk of endoleak. Alternatively or additionally, this change in axial length provides axial compliance for reducing vascular resistance, similar to the radial compliance described above. 
     When the body is in the radially-expanded deployment state, the variable-length section is configured to assume an axially-shortest state thereof, in which state typically one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of a greatest outer diameter of the fluid flow guide along the variable-length section when in its axially-shortest state; for some applications, the variable-length section has no structural-stent-element-free portions when in its axially-shortest state. Typically, the variable-length section is configured such that the structural stent elements thereof do not undergo plastic deformation as the axial-length changes. 
     Typically, the body is configured such that elasticity of graft material of the fluid flow guide provides less than 5% of a change in an axial length of the variable-length section. In other words, the change in the axial length of the variable-length section is not primarily enabled by stretching of the graft material of the fluid flow guide. 
     There is therefore provided, in accordance with an application of the present invention, apparatus including an endovascular stent-graft, which includes a generally tubular body, which body (a) is configured to assume a radially-compressed delivery state and a radially-expanded deployment state, and (b) includes: 
     a flexible stent member; and 
     a tubular fluid flow guide, which includes a graft material, and is attached to the stent member, 
     wherein the body includes a compliance-restoration body portion, which extends axially along a portion of the body, and which includes a portion of the stent member and a portion of the fluid flow guide, 
     wherein, when the body is in the radially-expanded deployment state, the compliance-restoration body portion is (a) characterized by a greatest diastolic outer radius when the body is internally pressurized by fluid having a pressure of 80 mmHg, and (b) radially expandable to a greatest systolic outer radius when the body is internally pressurized by fluid having a pressure of 120 mmHg, and 
     wherein the greatest systolic outer radius is at least 5% greater than the greatest diastolic outer radius, such as at least 10% greater than the greatest diastolic outer radius. 
     For some applications, the fluid flow guide of the compliance-restoration body portion is shaped so as to be expandable to a maximum greatest outer radius equal to the greatest systolic outer radius of the compliance-restoration body portion, such that the compliance-restoration body portion is limited by the fluid flow guide from assuming an outer radius that is greater than the maximum greatest outer radius. 
     For some applications, the stent member is heat-set to cause the compliance-restoration body portion to assume the greatest diastolic outer radius when the body is internally pressurized by the fluid having the pressure of 80 mmHg. 
     For any of the applications described above, when the body is in the radially-expanded deployment state, the compliance-restoration body portion may be characterized by a greatest relaxed outer radius when the body is not internally pressurized by fluid, which greatest relaxed outer radius is no more than 95% of the greatest diastolic outer radius. For some applications, the stent member is heat-set to cause the compliance-restoration body portion to assume the greatest relaxed outer radius when unconstrained. 
     For any of the applications described above, the graft material may include a woven graft. 
     For any of the applications described above, the graft material of the portion of the fluid flow guide may be at least partially folded when the body is in the radially-expanded deployment state and is internally pressured by the fluid of having the pressure of 80 mmHg. 
     For any of the applications described above, the fluid flow guide, if not attached to the stent member, may be configured to assume first and second perimeters when internally pressurized by fluid having a pressure of 80 and 120 mmHg, respectively, the second perimeter being no more than 10% greater than the first perimeter. 
     For any of the applications described above, the fluid flow guide, if not attached to the stent member, may be configured to assume first and second perimeters when internally pressurized by fluid having a pressure of 80 and 120 mmHg, respectively, the second perimeter being between 0.5% and 5% greater than the first perimeter. 
     For any of the applications described above, the stent member may include a plurality of structural stent elements that are indirectly connected to one another by the fluid flow guide. 
     For any of the applications described above, the stent member may include a plurality of interconnected structural stent elements. 
     For any of the applications described above, the greatest systolic outer radius may be no more than 30% greater than the greatest diastolic outer radius. 
     For any of the applications described above, the greatest diastolic outer radius may be between 7.5 mm and 25 mm, when the body is in the radially-expanded deployment state. 
     For any of the applications described above, the greatest systolic outer radius may be between 8.5 mm and 30 mm, when the body is in the radially-expanded deployment state. 
     For any of the applications described above, the body may further includes distal and proximal portions, longitudinally between which the compliance-restoration body portion is disposed, 
     when the body is in the radially-expanded deployment state, the distal and proximal portions may be (a) characterized by greatest diastolic distal- and proximal-end-portion radii, respectively, when the body is internally pressurized by fluid having a pressure of 80 mmHg, and (b) radially expandable to greatest systolic distal- and proximal-end-portion radii, respectively, when the body is internally pressurized by fluid having a pressure of 120 mmHg, 
     the greatest systolic distal-end-portion outer radius may be less than 2% greater than the greatest diastolic distal-end-portion outer radius, and 
     the greatest systolic proximal-end-portion outer radius may be less than 2% greater than the greatest diastolic proximal-end-portion outer radius. 
     For any of the applications described above, the body may further include distal and proximal portions, longitudinally between which the compliance-restoration body portion is disposed, and respective greatest radii of the distal and the proximal portions may be each at least 5% greater than a greatest relaxed outer radius of the compliance-restoration body portion, when the body is unconstrained in the radially-expanded deployment state. 
     For any of the applications described above, the body may further include distal and proximal portions, longitudinally between which the compliance-restoration body portion is disposed, and respective greatest radii of the distal and the proximal portions may be each at least 5% greater than the greatest diastolic outer radius, when the body is in the radially-expanded deployment state. 
     For any of the applications described above, the body may further include distal and proximal portions, longitudinally between which the compliance-restoration body portion is disposed, and a greatest outer radius of the distal portion, when unconstrained, may be between 2 and 10 mm greater than the greatest systolic outer radius, when the body is in the radially-expanded deployment state. 
     For any of the applications described above, the graft material may include a material selected from the group of materials consisting of: Polyethylene terephthalate (PET), Dacron, Polytetrafluoroethylene (PTFE), ePTFE, and a combination of two or more of these materials. 
     For any of the applications described above, the stent member may include a superelastic alloy. 
     For any of the applications described above, the stent-graft may be configured to self-expand from the delivery state to the deployment state. 
     For any of the applications described above: 
     the flexible stent member may include a plurality of structural stent elements attached to at least a portion of the fluid flow guide, 
     the body may include a variable-length section, which extends axially along a portion of the body, and which includes one or more of the structural stent elements and a portion of the fluid flow guide, 
     the body, including the variable-length section, may be configured to assume the radially-compressed delivery state and the radially-expanded deployment state, 
     wherein, when the body is in the radially-expanded deployment state, the variable-length section may be configured to:
         enable a change in an axial length thereof of at least 5 mm, and   assume an axially-shortest state thereof, in which state the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of a greater outer diameter of the fluid flow guide along the variable-length section when in its axially-shortest state.       

     There is further provided, in accordance with an application of the present invention, apparatus including an endovascular stent-graft, which includes a generally tubular body, which body includes: 
     a fluid flow guide, which includes a graft material; and 
     a plurality of structural stent elements attached to at least a portion of the fluid flow guide, 
     wherein the body includes a variable-length section, which extends axially along a portion of the body, and which includes one or more of the structural stent elements and a portion of the fluid flow guide, 
     wherein the body, including the variable-length section, is configured to assume a radially-compressed delivery state and a radially-expanded deployment state, 
     wherein, when the body is in the radially-expanded deployment state, the variable-length section is configured to:
         enable a change in an axial length thereof of at least 5 mm, and   assume an axially-shortest state thereof, in which state the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of a greatest outer diameter of the fluid flow guide along the variable-length section when in its axially-shortest state.       

     For some applications, the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions, when the variable-length section is in the axially-shortest state when the body is in the radially-expanded deployment state. 
     For some applications, the body is configured such that elasticity of the graft material provides less than 5% of the change in the axial length of the variable-length section. 
     For some applications, the variable-length section is configured such that the structural stent elements thereof do not undergo plastic deformation during the change in axial length. 
     For some applications, the variable-length section is configured such that the enabled change in the axial length is no more than 10% of a greatest outer diameter of the fluid flow guide along the variable-length section when the variable-length section is in the axially-shortest state. 
     For any of the applications described above, the variable-length section may be configured such that the enabled change in the axial length is equal to at least 10% of a greatest outer diameter of the fluid flow guide along the variable-length section when the variable-length section is in the axially-shortest state. 
     For any of the applications described above, a surface coverage ratio of the one or more of the structural stent elements of the variable-length section on the fluid flow guide may be equal to at least 5% when the variable-length section is in the axially-shortest state when the body is in the radially-expanded deployment state. 
     For any of the applications described above, when the body is in the radially-expanded deployment state, the variable-length section may be configured to undergo the change in the axial length in response to a change in fluid pressure within the fluid flow guide. 
     For any of the applications described above, when the body is in the radially-expanded deployment state, the variable-length section may be configured to undergo an increase in the axial length, and not a decrease in the axial length. 
     For any of the applications described above, when the body is in the radially-expanded deployment state, the variable-length section may be configured to cyclically undergo an increase in the axial length that alternates with a decrease in the axial length. 
     For any of the applications described above, the variable-length section may be configured to undergo (a) an increase in the axial length in response to an increase in fluid pressure within the fluid flow guide, and (b) a decrease in the axial length in response to a decrease in the fluid pressure within the fluid flow guide. 
     For any of the applications described above, the variable-length section may be configured to undergo (a) an increase in the axial length in response to a decrease in fluid pressure within the fluid flow guide, and (b) a decrease in the axial length in response to an increase in the fluid pressure within the fluid flow guide. 
     For any of the applications described above, the variable-length section may be configured such that during a 5 mm change in the axial length, an average wall thickness of the graft material changes by no more than 15%. 
     For any of the applications described above, the variable-length section may be configured to assume a folded position at least when the variable-length section is in the axially-shortest state, in which folded position a first longitudinal subsection of the fluid flow guide is radially sandwiched between second and third longitudinal subsections of the fluid flow guide. 
     For any of the applications described above, the variable-length section may be configured such that in the folded position the second longitudinal subsection radially surrounds the first longitudinal subsection, and at least one of the one or more of the structural stent elements of the variable-length section is attached to the second longitudinal subsection. For some applications, the variable-length section is configured such that in the folded position the first longitudinal subsection radially surrounds the third longitudinal subsection, and at least one of the one or more of the structural stent elements of the variable-length section is attached to the third longitudinal subsection. For some applications, the variable-length section is configured such that none of the structural stent elements of the body is attached to the first longitudinal subsection. 
     For any of the applications described above, the variable-length section may be shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-outward bulge at least partially around a perimeter of an axial site on the variable-length section, which radially-outward bulge includes the one or more of the structural elements of the variable-length section. For some applications, the variable-length section is configured such that a radial dimension of the bulge decreases as the axial length of the variable-length section increases. For some applications, the variable-length section, when in an axially-longest state, is not shaped so as to define the bulge. 
     For any of the applications described above, the variable-length section may be shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-inward indentation at least partially around a perimeter of an axial site on the variable-length section, which indentation includes the one or more of the structural elements of the variable-length section. For some applications, the variable-length section is configured such that a radial dimension of the indentation decreases as the axial length of the variable-length section increases. For some applications, the variable-length section, when in an axially-longest state, is not shaped so as to define the indentation. 
     For any of the applications described above, the variable-length section may be configured such that when the variable-length section undergoes the change in the axial length, a proximal end of the variable-length section rotates with respect to a distal end of the variable-length section. 
     For any of the applications described above, the variable-length section may be shaped so as to define, at least when the variable-length section is in the axially-shortest state, at least one single-sided helix, which includes the one or more of the structural elements. For some applications, the variable-length section is configured such that a step size of the at least one-single-sided helix increases as the axial length of the variable-length section increases. 
     For any of the applications described above, the variable-length section may be shaped so as to define, at least when the variable-length section is in the axially-shortest state, at least one right-handed helix and at least one left-handed helix, which helices include the one or more of the structural elements. For some applications, the variable-length section is configured such that respective step sizes of the right- and left-handed helices either both increase, or both decrease as axial length of the variable-length section increases. 
     For any of the applications described above: 
     the body may include a compliance-restoration body portion, which extends axially along a portion of the body, and which includes a portion of the structural stent elements and a portion of the fluid flow guide, 
     when the body is in the radially-expanded deployment state, the compliance-restoration body portion may be (a) characterized by a greatest diastolic outer radius when the body is internally pressurized by fluid having a pressure of 80 mmHg, and (b) radially expandable to a greatest systolic outer radius when the body is internally pressurized by fluid having a pressure of 120 mmHg, and 
     the greatest systolic outer radius may be at least 5% greater than the greatest diastolic outer radius. 
     There is still further provided, in accordance with an application of the present invention, a method including: 
     providing an endovascular stent-graft, which includes a generally tubular body, which includes a flexible stent member, and a tubular fluid flow guide, which includes a graft material, and is attached to the stent member, wherein the body includes a compliance-restoration body portion, which extends axially along a portion of the body, and which includes a portion of the stent member and a portion of the fluid flow guide; 
     transvascularly introducing the stent-graft into a blood vessel of a human subject while the body is in a radially-compressed delivery state; and 
     thereafter, transitioning the body to a radially-expanded deployment state in the blood vessel, in which state the compliance-restoration body portion is characterized by (a) a greatest diastolic outer radius when the body is internally pressurized by blood of the subject during diastole, and (b) a greatest systolic outer radius when the body is internally pressurized by blood of the subject during systole, which greatest systolic outer radius is at least 5% greater than the greatest diastolic outer radius. 
     For some applications, transitioning the body to the deployment state includes allowing the body to self-expand. 
     For some applications, the greatest systolic outer radius is at least 10% greater than the greatest diastolic outer radius. 
     For some applications, providing the stent-graft includes providing the stent-graft in which the fluid flow guide of the compliance-restoration body portion is shaped so as to be expandable to a maximum greatest outer radius equal to the greatest systolic outer radius of the compliance-restoration body portion, such that the compliance-restoration body portion is limited by the fluid flow guide from assuming an outer radius that is greater than the maximum greatest outer radius. 
     For some applications, providing the stent-graft includes providing the stent-graft in which the fluid flow guide, if not attached to the stent member, is configured to assume first and second perimeters when internally pressurized by fluid having a pressure of 80 and 120 mmHg, respectively, the second perimeter being no more than 10% greater than the first perimeter. 
     For some applications: 
     the body further includes distal and proximal portions, longitudinally between which the compliance-restoration body portion is disposed, 
     when the body is in the radially-expanded deployment state, the distal and proximal portions are characterized by (a) greatest diastolic distal- and proximal-end-portion radii, respectively, when the body is internally pressurized by the blood during diastole, and (b) greatest systolic distal- and proximal-end-portion radii, respectively, when the body is internally pressurized by the blood during systole, 
     the greatest systolic distal-end-portion outer radius is less than 2% greater than the greatest diastolic distal-end-portion outer radius, and 
     the greatest systolic proximal-end-portion outer radius is less than 2% greater than the greatest diastolic proximal-end-portion outer radius. 
     For some applications, the greatest systolic outer radius is no more than 30% greater than the greatest diastolic outer radius. 
     For some applications: 
     the flexible stent member includes a plurality of structural stent elements attached to at least a portion of the fluid flow guide, 
     the body includes a variable-length section, which extends axially along a portion of the body, and which includes one or more of the structural stent elements and a portion of the fluid flow guide, 
     the body, including the variable-length section, is configured to assume the radially-compressed delivery state and the radially-expanded deployment state, 
     after the body is transitioned to the radially-expanded deployment state, the variable-length section is configured to enable a change in an axial length thereof of at least 5 mm, and 
     if the variable-length section assumes an axially-shortest state thereof while the body is in the radially-expanded deployment state, the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of a greatest outer diameter of the fluid flow guide along the variable-length section when in its axially-shortest state. 
     There is additionally provided, in accordance with an application of the present invention, a method including: 
     providing an endovascular stent-graft, which includes a generally tubular body, which includes a fluid flow guide, which includes a graft material, and a plurality of structural stent elements attached to at least a portion of the fluid flow guide, wherein the body includes a variable-length section, which extends axially along a portion of the body, and which includes one or more of the structural stent elements and a portion of the fluid flow guide, 
     transvascularly introducing the stent-graft into a blood vessel of a human subject while the body, including the variable-length section, is in a radially-compressed delivery state; and 
     thereafter, transitioning the body to a radially-expanded deployment state in the blood vessel, in which state the variable-length section is configured to enable a change in an axial length thereof of at least 5 mm, 
     wherein, if the variable-length section assumes an axially-shortest state thereof while the body is in the radially-expanded deployment state, the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of a greatest outer diameter of the fluid flow guide along the variable-length section when in its axially-shortest state. 
     For some applications, if the variable-length section assumes an axially-shortest state thereof while the body is in the radially-expanded deployment state, the one or more of the structural stent elements are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions. 
     For some applications, providing the stent-graft includes providing the stent-graft in which the body is configured such that elasticity of the graft material provides less than 5% of the change in the axial length of the variable-length section. 
     For some applications, providing the stent-graft includes providing the stent-graft in which the variable-length section is configured such that the structural stent elements thereof do not undergo plastic deformation during the change in axial length. 
     For some applications, providing the stent-graft includes providing the stent-graft in which the variable-length section is configured such that the enabled change in the axial length is no more than 10% of a greatest outer diameter of the fluid flow guide along the variable-length section when the variable-length section is in the axially-shortest state. 
     For some applications, when the body is in the radially-expanded deployment state, the variable-length section is configured to undergo the change in the axial length in response to a change in fluid pressure within the fluid flow guide. 
     For some applications, when the body is in the radially-expanded deployment state, the variable-length section is configured to undergo an increase in the axial length, and not a decrease in the axial length. 
     For some applications, when the body is in the radially-expanded deployment state, the variable-length section is configured to cyclically undergo an increase in the axial length that alternates with a decrease in the axial length. For some applications, the variable-length section is configured to undergo (a) an increase in the axial length in response to an increase in fluid pressure within the fluid flow guide, and (b) a decrease in the axial length in response to a decrease in the fluid pressure within the fluid flow guide. 
     For some applications, the variable-length section is configured such that during a 5 mm change in the axial length, an average wall thickness of the graft material changes by no more than 15%. 
     For some applications, the variable-length section is configured to assume a folded position at least when the variable-length section is in the axially-shortest state, in which folded position a first longitudinal subsection of the fluid flow guide is radially sandwiched between second and third longitudinal subsections of the fluid flow guide. 
     For some applications, the variable-length section is shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-outward bulge at least partially around a perimeter of an axial site on the variable-length section, which radially-outward bulge includes the one or more of the structural elements of the variable-length section. 
     For some applications, the variable-length section is shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-inward indentation at least partially around a perimeter of an axial site on the variable-length section, which indentation includes the one or more of the structural elements of the variable-length section. 
     For some applications, the body includes a compliance-restoration body portion, which extends axially along a portion of the body, and which includes a portion of the structural stent elements and a portion of the fluid flow guide; and after the body is transitioned to the radially-expanded deployment state, the compliance-restoration body portion is characterized by (a) a greatest diastolic outer radius when the body is internally pressurized by blood flow of the subject during diastole, and (b) a greatest systolic outer radius when the body is internally pressurized by blood of the subject during systole, which greatest systolic outer radius is at least 5% greater than the greatest diastolic outer radius. 
     The present invention will be more fully understood from the following detailed description of applications thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are schematic illustrations of an endovascular stent-graft, in accordance with an application of the present invention; 
         FIGS. 2A and 2B  are schematic illustrations of another configuration of the endovascular stent-graft of  FIGS. 1A and 1B , in accordance with an application of the present invention; 
         FIGS. 3A-C  are schematic illustrations of yet another configuration of the endovascular stent-graft of  FIGS. 1A and 1B , in accordance with an application of the present invention; 
         FIG. 4  is a schematic illustration of still another configuration of the endovascular stent-graft of  FIGS. 1A and 1B , in accordance with an application of the present invention; 
         FIG. 5  is a graph that schematically illustrates the stent-graft caliber of a compliance-restoration body portion of the endovascular stent-graft of  FIGS. 1A-4  vs. internal fluid pressure, in accordance with an application of the present invention; 
         FIG. 6  is a schematic illustration of a variable-length stent-graft, in accordance with an application of the present invention; 
         FIG. 7  is a schematic illustration of the variable-length stent-graft of  FIG. 6  coupled to a bifurcated fixation stent-graft, in accordance with an application of the present invention; 
         FIGS. 8A and 8B  are schematic illustrations of the variable-length stent-graft of  FIG. 6  in exemplary axially-shorter and axially-longer states, respectively, in accordance with an application of the present invention; 
         FIGS. 9A and 9B  are schematic illustrations of additional variable-length stent-grafts, in accordance with respective applications of the present invention; 
         FIGS. 10A and 10B  are schematic illustrations of the variable-length stent-graft of  FIG. 9A  in exemplary axially-shorter and axially-longer states, respectively, in accordance with an application of the present invention; 
         FIG. 11  is a schematic illustration of yet another variable-length stent-graft, in accordance with an application of the present invention; 
         FIGS. 12A and 12B  are schematic illustrations of the variable-length stent-graft of  FIG. 11  in exemplary axially-shorter and axially-longer states, respectively, in accordance with an application of the present invention; and 
         FIG. 13  is a schematic illustration of another stent-graft, which combines certain features of the stent-grafts of  FIGS. 1A-B  and  6 , in accordance with an application of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF APPLICATIONS 
       FIGS. 1A and 1B  are schematic illustrations of an endovascular stent-graft  20 , in accordance with an application of the present invention. Stent-graft  20  comprises a generally tubular body  22  having a central longitudinal axis  23 . Body  22  is configured to assume (a) a radially-compressed delivery state, typically when the body is initially positioned in a delivery catheter, and (b) a radially-expanded deployment state, upon deployment from the delivery catheter. Both  FIGS. 1A and 1B  show the stent-graft with body  22  in its radially-expanded deployment state. Body  22  is shown during diastole of an adult human in  FIG. 1A , and during systole of the adult human in  FIG. 1B . 
     Body  22  comprises a flexible stent member  24 , and a tubular fluid flow guide  26 . The fluid flow guide is attached to the stent member, such as by suturing or stitching. The flexible stent member may be attached to an internal and/or an external surface of the fluid flow guide. Optionally, a portion of the structural stent elements may be attached (e.g., sutured) to the internal surface of the fluid flow guide, and another portion to the external surface of the fluid flow guide. Flexible stent member  24  comprises a plurality of structural stent elements  28 , which are either indirectly connected to one another by the fluid flow guide (as shown), and/or interconnected with one another (configuration not shown). For some applications, structural stent elements  24  comprise a metal. Alternatively or additionally, the structural stent elements comprise a self-expanding material, such that body  22  (and, optionally, stent-graft  20 ) is self-expandable. 
     Alternatively or additionally, the structural stent elements comprise one or more metallic alloys, such as one or more superelastic metal alloys, a shape memory metallic alloy, and/or Nitinol. 
     Fluid flow guide  26  comprises a graft material, i.e., at least one biologically-compatible substantially blood-impervious flexible sheet. The flexible sheet may comprise, for example, a polyester, a polyethylene (e.g., a poly-ethylene-terephthalate), a polymeric film material (such as a fluoropolymer, e.g., polytetrafluoroethylene), a polymeric textile material (e.g., woven polyethylene terephthalate (PET)), natural tissue graft (e.g., saphenous vein or collagen), Polytetrafluoroethylene (PTFE), ePTFE, Dacron, or a combination of two or more of these materials. The graft material optionally is woven. The graft material of fluid flow guide  26  is generally non- or minimally-elastic. 
     Typically, stent-graft  20  is configured to self-expand from the delivery state to the deployment state. For example, stent member  24  may be heat-set to cause stent-graft  20  to self-expand from the delivery state to the deployment state. 
     Body  22  includes a compliance-restoration body portion  34 , which extends axially along a portion of body  22 , and which comprises a portion of stent member  24  and a portion of fluid flow guide  26 . When body  22  is in the radially-expanded deployment state, as shown in  FIGS. 1A and 1B , compliance-restoration body portion  34  is:
         characterized by a greatest diastolic outer radius R D  when body  22  is internally pressurized by fluid having a pressure of 80 mmHg, typically by blood during diastole in an adult human, as shown in  FIG. 1A  (and also by dashed lines in  FIG. 1B ); and   radially expandable to a greatest systolic outer radius R S  when body  22  is internally pressurized by fluid having a pressure of 120 mmHg, typically by blood during systole in an adult human, as shown in  FIG. 1B . For some applications, compliance-restoration body portion  34  expands to greatest systolic outer radius R S  even when the body is internally pressured by fluid having a pressure of greater than 120 mmHg.       

     Greatest systolic outer radius R S  is typically at least 5% greater than greatest diastolic outer radius R D , such as at least 10% greater than greatest diastolic outer radius R D , e.g., at least 15% greater than greatest diastolic outer radius R D . Alternatively or additionally, greatest systolic outer radius R S  is no more than 30% greater than the greatest diastolic outer radius R D . This increase in outer radius at greater internal pressure occurs because the stent is heat-set to have a diameter that is substantially (e.g., 5%-20%) less than the graft&#39;s fully-expanded (i.e., without folds) diameter, and the stent has the appropriate radial compliance such that the entire stent-graft substantially changes its radius between a state in which the stent-graft is internally pressurized by a nominal hydrostatic diastolic pressure and a state in which the stent-graft is internally pressurized by a nominal hydrostatic systolic pressure. 
     For some applications, fluid flow guide  26  of compliance-restoration body portion  34  is shaped so as to be expandable to a maximum greatest outer radius R M  equal to greatest systolic outer radius R S  of compliance-restoration body portion  34 , such that the compliance-restoration body portion is limited by the fluid flow guide from assuming an outer radius that is greater than the maximum greatest outer radius R M . (The outer radius might not otherwise be limited by stent member  24 , which is typically highly compliant and deformable, e.g., initially highly elastically deformable and subsequently, plastically deformable.) Typically, maximum greatest outer radius R M  of fluid flow guide  26  is greater than the greatest diastolic outer radius of stent member  24 , such as at least 5%, at least 10%, or at least 20% greater than greatest diastolic outer radius R D  of stent member  24 . During diastole, the inward compressive force applied by stent-member  24  is countered by the outward force applied by the internally pressurizing fluid (typically, diastolically-pressurized blood). These opposing forces jointly retain the fluid flow guide (and thus the entire compliance-restoration body portion  34 ) at greatest diastolic outer radius R D . 
     For some applications, when body  22  is in the radially-expanded deployment state: (a) greatest diastolic outer radius R D  is at least 7.5 mm, no more than 25 mm, and/or between 7.5 mm and 25 mm, and/or (b) greatest systolic outer radius R S  is at least 8.5 mm, no more than 30 mm, and/or between 8.5 mm and 30 mm. 
     As mentioned above, the graft material of fluid flow guide  26  is generally non- or minimally-elastic. Therefore, when compliance-restoration body portion  34  is internally pressured by diastolic pressure, and is thus characterized greatest diastolic outer radius R D  (which is less than maximum greatest outer radius R M  of fluid flow guide  26 ), the graft material of the compliance-restoration portion is at least partially folded. In other words, during diastole, the outer radius of compliance-restoration body portion  34  is less than the maximum outer radius of the fluid flow guide (though the actual circumference of the fluid flow guide remains essentially the same, so the graft material of the fluid flow guide must assume small folds to accommodate this state). (The actual circumference is to be understood as measuring the actual length of the fabric&#39;s wall if the fabric were to be flattened to remove any folds, invaginations, or bulges caused by the radial contraction of the fluid flow guide.) 
     Fluid flow guide  26 , if not attached to stent member  24  (e.g., before completion of manufacture of stent-graft  20 ), is configured to assume first and second perimeters when internally pressurized by fluid having a pressure of 80 and 120 mmHg, respectively. For some applications, the second perimeter is no more than 10% greater than the first perimeter, such as no more than 5% greater than the first perimeter. Alternatively or additionally, for some applications, the second perimeter is between 0.5% and 5% greater than the first perimeter. 
     For some applications, body  22  further includes distal and proximal portions  40  and  42 , longitudinally between which compliance-restoration body portion  34  is disposed. When body  22  is in the radially-expanded deployment state, distal and proximal portions  40  and  42  are:
         characterized by greatest diastolic distal- and proximal-end-portion radii R DD  and R PD , respectively, when body  22  is internally pressurized by fluid having a pressure of 80 mmHg, typically by blood during diastole in an adult human, as shown in  FIG. 1A ; and   radially expandable to greatest systolic distal- and proximal-end-portion radii R DS  and R PS , respectively, when body  22  is internally pressurized by fluid having a pressure of 120 mmHg, typically by blood during systole in an adult human, as shown in  FIG. 1B .       

     For some applications, greatest systolic distal-end-portion outer radius R DS  is less than 2% greater than greatest diastolic distal-end-portion outer radius R DD , and/or greatest systolic proximal-end-portion outer radius R PS  is less than 2% greater than greatest diastolic proximal-end-portion outer radius R PD . In other words, the radii of distal and proximal portions  40  and  42  change only slightly, or not at all, during the transition between diastole and systole. (In contrast, the outer radius of compliance-restoration body portion  34  changes substantially (e.g., by at least 5%), during the transition between diastole and systole, as described above.) 
     Reference is still made to  FIGS. 1A and 1B , and is additionally made to  FIGS. 2A and 2B , which are schematic illustrations of another configuration of endovascular stent-graft  20 , in accordance with an application of the present invention. Both  FIGS. 2A and 2B  show the stent-graft with body  22  in its radially-expanded deployment state. Body  22  is shown during diastole of an adult human in  FIG. 2A , and during systole of the adult human in  FIG. 2B . 
     In a first configuration, as shown in  FIGS. 1A and 1B , stent-graft  20  is configured such that:
         greatest diastolic outer radius R D  is approximately equal to (e.g., within +/−20% of) greatest diastolic distal-end-portion outer radius R DD  and/or greatest diastolic proximal-end-portion outer radius R PD ; and   greatest systolic outer radius R S  is greater than (e.g., at least 5% greater than, such as at least 15% greater than) greatest systolic distal-end-portion outer radius R DS  and/or greatest systolic proximal-end-portion outer radius R PS .       

     In a second configuration, as shown in  FIGS. 2A and 2B , stent-graft  20  is configured such that:
         greatest diastolic distal-end-portion outer radius R DD  is greater than (e.g., at least 5% greater than, such as at least 10% greater than) greatest diastolic outer radius R D , and/or greatest diastolic proximal-end-portion outer radius R PD  is greater than (e.g., at least 5% greater than, such as at least 10% greater than) greatest diastolic outer radius R D , and   greatest systolic outer radius R S  is approximately equal to (e.g., within +/−20% of) greatest systolic distal-end-portion outer radius R DS  and/or greatest systolic proximal-end-portion outer radius R PS .       

     In a third configuration (not shown), stent-graft  20  is configured such that:
         greatest diastolic outer radius R D  is less than (e.g., at least 10% less than, such as at least 15% less than) greatest diastolic distal-end-portion outer radius R DD  and/or greatest diastolic proximal-end-portion outer radius R PD ; and   greatest systolic outer radius R S  is greater than (e.g., at least 10% greater than, such as at least 15% greater than) greatest systolic distal-end-portion outer radius R DS  and/or greatest systolic proximal-end-portion outer radius R PS .       

     Reference is now made to  FIGS. 3A-C , which are schematic illustrations of yet another configuration of endovascular stent-graft  20 , in accordance with an application of the present invention. This configuration may be implemented in combination with any of the three configurations described immediately hereinabove with reference to  FIGS. 1A-B  and  2 A-B. All of  FIGS. 3A, 3B, and 3C  show stent-graft  20  with body  22  in its radially-expanded deployment state. In this configuration, when body  22  is in its radially-expanded deployment state, compliance-restoration body portion  34 , when not internally pressured by fluid, is characterized by a greatest relaxed outer radius R R  that is less than greatest diastolic outer radius R D  (and thus also less than even greater greatest systolic outer radius R S ). This greatest relaxed outer radius R R  occurs only during manufacture and does not occur in vivo (except perhaps briefly during the implantation procedure), because upon deployment and radial expansion of body  22  to its radially-expanded deployment state, the body is internally subjected to at least diastolic blood pressure. 
     In  FIG. 3A , body  22  is shown when it is not internally pressured by fluid. In this non-pressurized state, compliance-restoration body portion  34  is characterized by a greatest relaxed outer radius R R  that is no more than 95% of greatest diastolic outer radius R D , such as no more than 90% of greatest diastolic outer radius R D . Typically, stent member  24  is heat-set to cause compliance-restoration body portion  34  to assume greatest relaxed outer radius R R  when unconstrained, i.e., when no forces are applied to the compliance-restoration body portion by a delivery tool, walls of a blood vessel, or otherwise. As used in the present application, including in the claims, an “unconstrained” element is an element to which no forces are applied by a delivery tool, walls of a blood vessel, or otherwise. 
     In  FIG. 3B , body  22  is shown internally pressurized by fluid having a pressure of 80 mmHg, typically by blood during diastole in an adult human. In this diastolically-pressurized state, compliance-restoration body portion  34  is characterized by greatest diastolic outer radius R D . As described hereinabove with reference to  FIGS. 1A-B , during diastole, the inward compressive force applied by stent-member  24  is countered by the outward force applied by the internally pressurizing fluid (typically, diastolically-pressurized blood). These opposing forces jointly retain the fluid flow guide (and thus the entire compliance-restoration body portion  34 ) at greatest diastolic outer radius R D . 
     In  FIG. 3C , body  22  is shown internally pressurized by fluid having a pressure of 120 mmHg, typically by blood during systole in an adult human. In this systolically-pressurized state, compliance-restoration body portion  34  is characterized by greatest diastolic outer radius R S . 
     Reference is again made to  FIG. 3A . For some applications, when body  22  is unconstrained in the radially-expanded deployment state, distal and proximal portions  40  and  42  are characterized by greatest relaxed distal- and proximal-end-portion radii R DR  and R PR , respectively, when body  22  is not internally pressurized by fluid. For some applications, when body  22  is in the radially-expanded deployment state, greatest relaxed distal-end-portion outer radius R DR  is at least 5% greater than greatest relaxed outer radius R R  of compliance-restoration body portion  34 , and/or greatest relaxed proximal-end-portion outer radius R PR  is at least 5% greater than greatest relaxed outer radius R R . Such greater radii may help provide proper fixation to and sealing with the blood vessel wall. 
     Reference is now made to  FIG. 4 , which is a schematic illustration of yet another configuration of endovascular stent-graft  20 , in accordance with an application of the present invention.  FIG. 4  shows the stent-graft with body  22  in its radially-expanded deployment state during systole, with distal and proximal portions  40  and  42  shown by way of illustration in a radially-expanded state while unconstrained, i.e., while no forces are applied to these portions by a delivery tool, walls of a blood vessel, or otherwise. The shape of compliance-restoration body portion  34  during diastole is also shown by dashed lines. 
     In this configuration, stent-graft  20  is configured such that:
         a greatest unconstrained distal-end-portion outer radius R DU  is greater than greatest systolic outer radius R S , such as at least 5% greater than, e.g., as at least 15% greater than, and/or at least 2 mm greater than, no more than 10 mm greater than, or between 2 and 10 mm greater than greatest systolic outer radius R S ; and/or   a greatest unconstrained proximal-end-portion outer radius R PU  is greater than greatest systolic outer radius R S , such as at least 5% greater than, e.g., as at least 15% greater than, and/or at least 2 mm greater than, no more than 10 mm greater than, or between 2 and 10 mm greater than greatest systolic outer radius R S .       

     In addition, greatest systolic outer radius R S  is greater than greatest diastolic outer radius R D  (e.g., at least 3% greater than, such as at least 5% greater than). For some applications, each of greatest unconstrained distal- and proximal-end-portion radii R DU  and R PU  is at least 5 mm, no more than 20 mm, and/or between 10 and 30 mm, e.g., at least 11 mm, no more than 25 mm, and/or between 11 and 25 mm. 
     In this configuration, endovascular stent-graft  20  may be deployed in an aneurysmatic blood vessel, such as an aneurysmatic aorta, such as described hereinabove with reference to  FIGS. 1A-3C . Alternatively, endovascular stent-graft  20  may be deployed in a non-aneurysmatic blood vessel. In either case, the stent-graft may be configured to provide (passive, that is, by means of storage of mechanical energy) counterpulsation, by being configured to absorb and store blood pressure during systole and release and apply blood pressure during diastole. Counterpulsation is a technique for assisting the circulation by decreasing the afterload of the left ventricle and augmenting the diastolic pressure. Counterpulsation increases stroke volume by decreasing afterload, reduces heart workload, and maintains or increase coronary perfusion. Stent-graft  20  may thus be used, for example, for treating sclerotic disease in order to restore radial compliance of a blood vessel. 
     Reference is now made to  FIG. 5 , which is a graph that schematically illustrates the stent-graft caliber of compliance-restoration body portion  34  vs. internal fluid pressure, in accordance with an application of the present invention. Stent-graft caliber of compliance-restoration body portion  34  is expressed as a percentage of maximum graft caliber (i.e., the graft caliber at burst pressure). As can be seen, while the caliber of distal and proximal portions  40  and  42  of body  22  of stent-graft  20  remain the same as internal fluid (e.g., blood) pressure increases and decreases, the caliber of compliance-restoration body portion  34  varies between about 70% and 100% of the maximum graft caliber as the internal fluid pressure changes. 
     In contrast, conventional stents-grafts that comprise polyethylene terephthalate (PET) or polytetrafluoroethylene (ePTFE, available under the trademark Gore-Tex®) maintain approximately 100% and 95%, respectively, of their maximum graft caliber as internal pressure varies in a physiologically-normal range. It is noted that even conventional stent-grafts that comprise graft material that allows a 5% change in graft caliber cannot increase by at least 5% in radius, because the stent elements of conventional stent-grafts are heat-set to hold the stent-graft in its maximum graft caliber even when not pressurized by physiological blood pressure from the inside. 
     Reference is now made to  FIGS. 6-12B , which are schematic illustrations of a variable-length endovascular stent-graft  90 , in accordance with respective applications of the present invention. 
     Reference is made to  FIG. 6 , which is a schematic illustration of a variable-length stent-graft  100 , in accordance with an application of the present invention. Variable-length stent-graft  100  is one implementation of variable-length-stent graft  90 , described herein with reference to  FIGS. 6-12B . Stent-graft  100  comprises a generally tubular body  110 , which comprises a fluid flow guide  112  and a plurality of structural stent elements  114  attached to at least a portion of the fluid flow guide. Body  110  includes a variable-length section  120  that extends axially along a portion of body  110 . Body  110 , including variable-length section  120 , is configured to assume a radially-compressed delivery state and a radially-expanded deployment state. Typically, body  110  is configured to self-expand from the delivery state to the deployment state upon being deployed from a delivery catheter. Body  110  is shown in  FIGS. 6-12B  in the radially-expanded deployment state. 
     Variable-length section  120 , while radially-expanded in the deployment state, is configured to enable a change in an axial length thereof of at least 5 mm, such as at least 8 mm, e.g., at least 10 mm, and/or no more than 30 mm, e.g., no more than 25 mm. Alternatively or additionally, variable-length section  120  is configured such that the enabled change in the axial length is equal to at least 10%, e.g., at least 20%, and/or no more than 30%, e.g., no more than 10%, of outer diameter D of the fluid flow guide along the variable-length section when in its axially-shortest state (“greatest diameter” means the diameter at the longitudinal site having the greatest diameter). It is noted that even though the entire length of the stent-graft somewhat changes as the length of the variable-length section changes, the variable-length section should not be construed as including the entire length of the stent-graft. Instead, variable-length section  120  is to be understood as being that portion of the stent-graft that actually facilitates the change in axial length, as labeled in  FIGS. 6-12B . 
     When body  110  is in the radially-expanded deployment state, variable-length section  120  is configured to assume an axially-shortest state thereof. Typically, at least when variable-length section  120  is in this axially-shortest state, one or more of structural stent elements  114  are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5%, e.g., greater than 10%, of greatest outer diameter D of the fluid flow guide along the variable-length section when in its axially-shortest state; for some applications, the variable-length section has no structural-stent-element-free portions when in its axially-shortest state. Typically, variable-length section  120  is configured such that structural stent elements  114  thereof do not undergo plastic deformation as the axial length changes. 
     Fluid flow guide  112  comprises a graft material, i.e., at least one biologically-compatible substantially blood-impervious flexible sheet. The flexible sheet may comprise, for example, a polyester, a polyethylene (e.g., a poly-ethylene-terephthalate), a polymeric film material (such as a fluoropolymer, e.g., polytetrafluoroethylene), a polymeric textile material (e.g., woven polyethylene terephthalate (PET)), natural tissue graft (e.g., saphenous vein or collagen), Polytetrafluoroethylene (PTFE), ePTFE, Dacron, or a combination of two or more of these materials. The graft material optionally is woven. Typically, the graft material is not accordion-pleated. For some applications, structural stent elements  114  comprise a metal. Alternatively or additionally, the structural stent elements comprise a self-expanding material, such that body  110  (and, optionally, stent-graft  90 ) is self-expandable. Alternatively or additionally, the structural stent elements comprise one or more metallic alloys, such as one or more superelastic metal alloys, a shape memory metallic alloy, and/or Nitinol. 
     For some applications, as shown in the blow-up in  FIG. 6 , variable-length stent-graft  100  is configured to define at least one generally tubular foldable section  130 . For these applications, variable-length section  120  is configured to assume a folded position at least when variable-length section  120  is in the axially-shortest state. When the variable-length section is in which folded position, a first longitudinal subsection  132  of fluid flow guide  112  is radially sandwiched between second and third longitudinal subsections  134  and  136  of fluid flow guide  112 . The variable-length section is typically configured to gradually unfold as the axial length thereof increases. In the folded position, second longitudinal subsection  134  radially surrounds the first longitudinal subsection  132 , and first longitudinal subsection  132  radially surrounds third longitudinal subsection  136 , such that first longitudinal subsection  132  is radially sandwiched between second and third longitudinal subsections  134  and  136 . As a result, second subsection  134  at least partially longitudinally overlaps with both first and third longitudinal subsections  132  and  136 . 
     For some applications, at least one of structural stent elements  114  is attached to second longitudinal subsection  134 . Alternatively or additionally, for some applications, at least one of structural stent elements  114  is attached to third longitudinal subsection  136 . Further alternatively or additionally, for some applications, an average surface coverage ratio of structural stent elements  114  on fluid flow guide  112  along first subsection  132  is no more than 20%, such as no more than 10%, of the greater of (a) an average surface coverage ratio on fluid flow guide  112  along second longitudinal subsection  134  and (b) an average surface coverage ratio on fluid flow guide  112  along third longitudinal subsection  136 . For some applications, variable-length section  120  is configured such that none of structural stent elements  114  of body  110  is attached to first longitudinal subsection  132 . This lower average surface coverage ratio (such as no surface coverage) provides greater evertibility to first longitudinal subsection  132 , thereby enabling the transition of foldable section  130  from the folded state to the unfolded state. During this transition, first longitudinal subsection  132  is everted, i.e., turned inside-out. 
     Alternatively or additionally, the average surface coverage ratio of structural stent elements  114  on fluid flow guide  112  along first longitudinal subsection  132  is not necessarily no more than 20%. The greater evertibility of first longitudinal subsection  132  compared to second and third longitudinal subsections  134  and  136  may be provided by:
         configuring the structural stent elements along the first longitudinal subsection to be softer and/or thinner than the structural stent elements along the second and/or the third longitudinal subsections; and/or   configuring the structural stent elements along the first longitudinal subsection to be longitudinally short, e.g., as simple circles disposed circumferentially around the stent-graft. Optionally, the structural stent elements extend around less than 360 degrees of the circumference of the stent-graft, i.e., are circumferentially incomplete, in order to increase the evertibility of the first longitudinal subsection.       

     For some applications, a first subgroup of structural stent elements  114  is attached (e.g., sutured) to second longitudinal subsection  134 , and a second subgroup of structural stent elements  114  is attached (e.g., sutured) to third longitudinal subsection  136 . For some applications, one of the first and second subgroups of structural stent elements  114  is attached (e.g., sutured) to an internal surface of fluid flow guide  112 , and the other of the first and second subgroups is attached (e.g., sutured) to an external surface of fluid flow guide  112 . 
     For some applications, as shown in  FIG. 6 , structural stent elements  114  are arranged as a plurality of generally circumferential bands. Longitudinal adjacent ones of the bands may or may not be joined to one another. For some applications, one or more of the circumferential bands is attached (e.g., sutured) to fluid flow guide  112  along second longitudinal subsection  134  (either to an external surface and/or to an internal surface thereof), and one or more of the circumferential bands is attached (e.g., sutured) to fluid flow guide  112  along third longitudinal subsection  136  (either to an external surface and/or to an internal surface thereof). Optionally, in addition, one or more of the circumferential bands is attached to fluid flow guide  112  along first longitudinal subsection  132  (either to an external surface and/or to an internal surface thereof). 
     For some applications, a surface coverage ratio of the one or more of structural stent elements  114  of variable-length section  120  on fluid flow guide  112  is at least 5%, such as at least 10%, when variable-length section  120  is the axially-shortest state when body  110  is in the radially-expanded deployment state. 
     For some applications, such as shown in the blow-up of  FIG. 6 , foldable section  130  comprises exactly three subsections, in which case the foldable section may be considered a triple-collar section. For other applications, foldable section  130  comprises more than three subsections, such as described with reference to  FIG. 7  of above-mentioned U.S. Provisional Application 61/553,209, which is incorporated herein by reference. 
     Reference is now made to  FIG. 7 , which is a schematic illustration of variable-length stent-graft  100  coupled to a bifurcated fixation stent-graft  150 , in accordance with an application of the present invention. In this configuration, bifurcated fixation stent-graft  150  facilitates long-term anchoring of variable-length stent-graft  100  at a bifurcation. For example, as shown, bifurcated fixation stent-graft  150  may comprise a bi-iliac self-expandable stent that is deployed in the iliac arteries, in order to facilitate long-term anchoring of stent-graft  100  at the aorto-iliac bifurcation. Stent-grafts  100  and  150  may be deployed and/or coupled to each other using techniques described in one or more of the following patent application publications, all of which are assigned to the assignee of the present application and are incorporated herein by reference: (a) PCT Publication WO 08/107885, (b) PCT Publication WO 2011/007354, (c) PCT Publication WO 2010/150208, e.g., with reference to  FIGS. 14D-E ,  20 B, and/or  21 B thereof, and (d) US Patent Application Publication 2011/0208289. 
     Reference is now made to  FIGS. 8A and 8B , which are schematic illustrations of variable-length stent-graft  100  in exemplary axially-shorter and axially-longer states, respectively, in accordance with an application of the present invention. Stent-graft  100  is configured to assume a plurality of axially-shorter states (including the axially-shortest state described hereinabove with reference to  FIG. 6 ).  FIG. 8A  shows stent-graft  100  in one of these axially-shorter states, in which variable-length section  120  has axial length Ls, and variable-length section  120  is in the folded position, as described hereinabove with reference to the blow-up of  FIG. 6 .  FIG. 8B  shows stent-graft  100  in one of the axially-longer states, in which variable-length section  120  has axial length L L , and variable-length section  120  is nearly entirely unfolded. For some applications, the stent-graft may assume a slightly longer state, in which it is entirely unfolded (state not shown). 
     Reference is now made to  FIGS. 9A-B  and  10 A-B, which are schematic illustrations of variable-length stent-grafts  200  and  202 , in accordance with respective applications of the present invention. Variable-length stent-grafts  200  and  202  are respective implementations of variable-length-stent graft  90 , described herein with reference to  FIGS. 6-12B . Stent-grafts  200  and  202  comprise generally tubular body  110 , which comprises fluid flow guide  112  and structural stent elements  114  attached to at least a portion of the fluid flow guide. Body  110  includes variable-length section  120  that extends axially along a portion of body  110 . Body  110 , including variable-length section  120 , is configured to assume a radially-compressed delivery state and a radially-expanded deployment state. Body  110  is shown in  FIGS. 9A-B  and  10 A-B in the radially-expanded deployment state. 
     Variable-length section  120  of stent-grafts  200  and  202  are configured to change length by means of rotation of a proximal end  206  of variable-length section  120  with respect to a distal end  208  of variable-length section  120 . Such rotation causes structural elements  114  to twist. Variable-length section  120  shortens as structural elements  114  twist, and the graft material circumferentially folds around the more acutely curved structural elements. 
     For some applications, variable-length section  120  is configured to cyclically undergo an increase in the axial length that alternates with a decrease in the axial length. For some applications, the length of the variable-length section increase and decreases every heartbeat. For some applications, variable-section  120  shortens during diastole and lengthens during systole. 
     For some applications, as shown in  FIGS. 9A and 10A -B, variable-length section  120  of stent-graft  200  is shaped so as to define, at least when the variable-length section is in the axially-shortest state, at least one single-sided helix  204 , which comprises the one or more of structural elements  114 . For some applications, variable-length section  120  is configured such that a step size of the at least one-single-sided helix increases as the axial length of the variable-length section increases. 
     For some applications, as shown in  FIG. 9B , variable-length section  120  of stent-graft  202  is shaped so as to define, at least when the variable-length section is in the axially-shortest state, at least one right-handed helix and at least one left-handed helix, which helices comprise the one or more of structural elements  114 . For some applications, variable-length section  120  is configured such that respective step sizes of the right- and left-handed helices either both increase, or both decrease, as axial length of the variable-length section increases. 
       FIGS. 10A and 10B  show variable-length stent-graft  200  of  FIG. 9A  in exemplary axially-shorter and axially-longer states, respectively.  FIG. 10A  shows stent-graft  200  in one of its axially-shorter states, in which variable-length section  120  has axial length L S . Typically, variable-length section  120  is relaxed in the axially-shorter state.  FIG. 10B  shows stent-graft  200  in one of its axially-longer states, in which variable-length section  120  has axial length L L . The one or more of structural stent elements  114  are configured to be elastically deformed at least when variable-length section  120  is in an axially-shortest of the axially-shorter states, such that the stent elements protrude radially outwardly (as shown in  FIG. 10A ) or radially inwardly (configuration not shown). Typically, the portions of these stent elements that protrude are attached (e.g., sutured) to fluid flow guide  112 . 
     For configurations in which the stent elements protrude radially outward, variable-length section  120  is shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-outward bulge  210  at least partially around a perimeter of an axial site on variable-length section  120 . Radially-outward bulge  210  comprises the one or more of structural elements  114  of variable-length section  120 , and, typically, a portion of the graft material of fluid flow guide  112 . Typically, the variable-length section is configured such that a radial dimension of the bulge decreases as the axial length of the variable-length section increases. Typically, variable-length section  120  is configured such that structural stent elements  114  thereof do not undergo plastic deformation as the axial length changes. 
     Typically, when variable-length section  120  is in an axially-longest state (for example, as shown in  FIG. 10B , and  FIGS. 9A and 9B ), the stent elements that define the bulge generally do not protrude radially outwardly or radially inwardly. The variable-length section, when in an axially-longest state, thus is not shaped so as to define bulge  210 . Typically, structural elements  114  are relaxed in the axially-shortest state. For some applications, a surface coverage ratio of the one or more of structural stent elements  114  of variable-length section  120  on fluid flow guide  112  is at least 5%, such as at least 10%. 
     When body  110  is in the radially-expanded deployment state (as shown in  FIGS. 9 and 10A -B), variable-length section  120  is configured to assume an axially-shortest state thereof (which might, for example, be the state shown in  FIG. 10A ). In this state the one or more of structural stent elements  114  are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of greatest outer diameter D of the fluid flow guide along the variable-length section when in its axially-shortest state. In this configuration, the variable-length section typically has no structural-stent-element free axial portions, when the variable-length section is in the axially-shortest state. 
     Reference is made to  FIGS. 11 and 12A -B, which are schematic illustrations of a variable-length stent-graft  300 , in accordance with an application of the present invention. Variable-length stent-graft  300  is one implementation of variable-length-stent graft  90 , described herein with reference to  FIGS. 6-12B . Stent-graft  300  comprises generally tubular body  110 , which comprises fluid flow guide  112  and structural stent elements  114  attached to at least a portion of the fluid flow guide. Body  110  includes variable-length section  120  that extends axially along a portion of body  110 . Body  110 , including variable-length section  120 , is configured to assume a radially-compressed delivery state and a radially-expanded deployment state. Body  110  is shown in  FIGS. 11 and 12A -B in the radially-expanded deployment state. 
       FIGS. 12A and 12B  show variable-length stent-graft  300  in exemplary axially-shorter and axially-longer states, respectively.  FIG. 12A  shows stent-graft  300  in one of its axially-shorter states, in which variable-length section  120  has axial length L S . Typically, variable-length section  120  is relaxed in the axially-shorter state.  FIG. 12B  shows stent-graft  300  in one of its axially-longer states, in which variable-length section  120  has axial length L L . The one or more of structural stent elements  114  are configured to be elastically deformed at least when variable-length section  120  is in the axially-shortest state, such that the stent elements protrude radially outwardly (as shown in  FIG. 10A ) or radially inwardly (configuration not shown). Unlike the configuration shown in  FIGS. 9 and 10A -B, in the configuration shown in  FIGS. 11 and 12A -B the portions of the stent elements that protrude radially outward are not attached (e.g., sutured) to fluid flow guide  112 . Typically, the graft material of fluid flow guide  112  along the variable-length section folds as the variable-length section axially shortens, as shown in  FIG. 12A . 
     For configurations in which the stent elements protrude radially outward, variable-length section  120  is shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-outward bulge  310  at least partially around a perimeter of an axial site on variable-length section  120 . Radially-outward bulge  310  comprises the one or more of structural elements  114  of variable-length section  120 . Typically, the variable-length section is configured such that a radial dimension of the bulge decreases as the axial length of the variable-length section increases. Typically, variable-length section  120  is configured such that structural stent elements  114  thereof do not undergo plastic deformation as the axial length changes. 
     Bulge  310  radially protrudes less when the variable-length section is in an axially-longer state than when in an axially-shorter state. Typically, variable-length section  120  is relaxed in the axially-shorter state. For some applications, a surface coverage ratio of the one or more of structural stent elements  114  of variable-length section  120  on fluid flow guide  112  is at least 5%, such as at least 10%. 
     When body  110  is in the radially-expanded deployment state (as shown in  FIGS. 12A and 12B ), variable-length section  120  is configured to assume an axially-shortest state thereof (which might, for example, be the state shown in  FIG. 12A ). In this state the one or more of structural stent elements  114  are arranged along the variable-length section such that the variable-length section has no structural-stent-element-free axial portions having axial lengths greater than 5% of greatest outer diameter D of the fluid flow guide along the variable-length section when in its axially-shortest state. In this configuration, the variable-length section typically has no structural-stent-element free axial portions, when the variable-length section is in the axially-shortest state. 
     Reference is made to  FIGS. 9-10B and 11-12B . Alternatively, for some applications, variable-length section  120  is shaped so as to define, at least when the variable-length section is in the axially-shortest state, a radially-inward indentation at least partially around a perimeter of an axial site on the variable-length section. The indentation comprises the one or more of structural elements  114  of variable-length section  120 . The variable-length section is configured such that a radial dimension of the indentation decreases as the axial length of the variable-length section increases. Typically, the variable-length section, when in an axially-longest state, is not shaped so as to define the indentation. 
     Reference is again made to  FIGS. 6-12B . Typically, body  110  is configured such that elasticity of the graft material of fluid flow guide  112  provides less than 5%, such as less than 3% of a change in an axial length of variable-length section  120 . In other words, the change in the axial length of variable-length section  120  is not primarily enabled by stretching of the graft material of the fluid flow guide. As a result, an average wall thickness T of the graft material (labeled in  FIGS. 6, 9, and 11 ) does not decrease, or decreases only slightly, as the axial length increases. For some applications, during a 5 mm change in axial length of variable-length section  120 , average wall thickness T of the graft material changes by no more than 15%, such as by no more than 10%. 
     Reference is still made to  FIGS. 6-12B . For some applications, when body  110  is in the radially-expanded deployment state, variable-length section  120  is configured to undergo an increase in the axial length, and not a decrease in the axial length. For some applications, such an increase occurs gradually after implantation, such as over months or years, enabling variable-length endovascular stent-graft  90  to accommodate the gradual lengthening of the aorta that sometimes occurs. Such accommodation decreases the risk that the stent-graft might become dislodged and decreases the risk of endoleak. 
     Reference is still made to  FIGS. 6-12B . For some applications, when body  110  is in the radially-expanded deployment state, variable-length section  120  is configured to cyclically undergo an increase in the axial length that alternates with a decrease in the axial length. For some applications, the length of the variable-length section increase and decreases every heartbeat. This repeated change in axial length provides axial compliance for reducing vascular resistance, similar to the radial compliance described above. For some applications, variable-section  120  shortens during diastole and lengthens during systole. 
     Reference is still made to  FIGS. 6-12B . For some applications, when body  110  is in the radially-expanded deployment state, variable-length section  120  is configured to undergo the change in axial length in response to a change in fluid pressure within fluid flow guide  112 , such as every heartbeat as pressure increases and decreases during systole and diastole. This repeated change in axial length provides axial compliance for reducing vascular resistance, similar to the radial compliance described above. Therefore, for these applications, the variable-length section typically is not configured to lock upon elongation. 
     For some applications in which structural stent elements  114  comprises a shape memory alloy, such as Nitinol, the spring-like properties of the alloy enable this repeated change in length of the variable-length section. Typically, variable-length section  120  is configured to undergo (a) an increase in the axial length in response to an increase in fluid pressure within the fluid flow guide, and (b) a decrease in the axial length in response to a decrease in the fluid pressure within the fluid flow guide. Alternatively, variable-length section  120  is configured to undergo (a) an increase in the axial length in response to a decrease in fluid pressure within the fluid flow guide, and (b) a decrease in the axial length in response to an increase in the fluid pressure within the fluid flow guide. 
     Reference is again made to  FIG. 7 . For some applications, stent-graft  90  further comprises one or more fixation members  330 , such as barbs or hooks, located proximally and/or distally to variable-length section  120 . The fixation members are configured to provide secured positioning between at least an end of the stent-graft and an internal circumference of the lumen. (Although fixation members  330  are shown only in  FIG. 7 , they may also be provided in the other configurations of stent-graft  90  described herein with reference to  FIGS. 6 and 8A-12B .) For some applications, the fixation members are configured to provide secured positioning between at least an end of the stent-graft and an internal circumference of at least a branch of the lumen (e.g., a renal artery branching from the aorta). For example, the fixation members may comprises atraumatic arms that are configured to extend into the branch. For some applications, fixation members are configured to provide secured positioning between at least an end of the stent-graft and an external or internal circumference of another tubular stent-graft, e.g., are configured to allow telescopic anchoring of stent-graft  90  inside another stent-graft. 
     Reference is now made to  FIG. 13 , which is a schematic illustration of a stent-graft  400 , in accordance with an application of the present invention. Stent-graft  400  combines certain features of stent-graft  20 , described hereinabove with reference to  FIGS. 1A-5 , and certain features of stent-graft  90 , described hereinabove with reference to  FIGS. 6-12B . Stent-graft  400  includes both compliance-restoration body portion  34  and variable-length section  120 . Stent-graft  400  thus provides both the axial and radial compliance described hereinabove. 
     Stent-graft  400  may implement the configuration of compliance-restoration body portion  34  described hereinabove with reference to  FIGS. 1A-B  (as shown in  FIG. 13 ), or any of the other configurations of compliance-restoration body portion  34  described hereinabove with reference to  FIGS. 2A-4 . Similarly, stent-graft  400  may implement the configuration of variable-length section  120  described hereinabove with reference to  FIGS. 6, 7, and 8A -B (as shown in  FIG. 13 ), or any of the other configurations of variable-length section  120  described hereinabove with reference to  9 - 12 B. 
     As used in the present application, including in the claims, “tubular” means having the form of an elongated hollow object that defines a conduit therethrough. A “tubular” structure may have varied cross-sections therealong, and the cross-sections are not necessarily circular. For example, one or more of the cross-sections may be generally circular, or generally elliptical but not circular, or circular. 
     The scope of the present invention includes embodiments described in the following applications, which are assigned to the assignee of the present application and are incorporated herein by reference. In an embodiment, techniques and apparatus described in one or more of the following applications are combined with techniques and apparatus described herein:
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     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.