Prosthesis delivery system

A prosthesis delivery system comprises a sheath defining an axial direction; at least one tether movable axially relative to the sheath; and a holder movable axially within the sheath, configured to constrain radially the tether(s). Collapsing the prosthesis to a collapsed state in the sheath involves constraining radially with a holder the tether(s), attached to the prosthesis; and moving the tether(s) and the holder axially within the sheath such that the prosthesis is forced to collapse into the sheath and the holder is retracted into the sheath.

This application is a National Stage of International Application No. PCT/GB2011/001504, filed Oct. 19, 2011, which claims the benefit of GB Patent Application No. 1017921.6, filed Oct. 22, 2010, the contents of which are herein incorporated by reference.

The present invention relates to a prosthesis delivery system, a method for collapsing a prosthesis into a sheath and a method for delivering a prosthesis to a target position. More particularly, but not exclusively, the present invention relates to a method for delivery and recapture of a heart valve.

In the following a prosthetic collapsible heart valve will be used as an example of a percutaneously implantable prosthesis that needs to be delivered to an organ, in this case the heart. For brevity, the prosthetic heart valve will generally simply be referred to as a “heart valve”.

In recent years percutaneous implantation of heart valves has emerged as a valid alternative to surgical valve replacement. In this context, percutaneous refers to accessing the heart with a minimally invasive technique, as opposed to full open-heart surgery. Percutaneous techniques include endovascular implantation and thorasic-microsurgery. According to these techniques, access is done via needle-puncture of the skin, and does not require scalpel incisions to open the thorasic cavity and expose the heart. Another technique, known as surgical transapical access, to access the heart involves puncturing the apex of the ventricle so as to access the heart with minimal access surgery.

For percutaneous delivery of a heart valve, the valve must be collapsible to a compressed state such that it can be delivered e.g. through the venous or arterial system using a catheter and a guidewire, to the required position, and then expanded in situ into its normal operating state. In many cases known in the art, the support structure is essentially similar to a stent used for angioplasty.

PCT/GB 10/000627, which is herein incorporated by reference in its entirety, discloses a heart valve prosthesis comprising a support structure and a flow-control structure. The support structure comprises a framework deformable between an expanded state and a compressed state and vice versa. The support structure supports the flow-control structure. The flow-control structure is for permitting blood flow in an axial direction of the prosthesis, and for restricting blood flow in a direction opposite to the axial direction. At least one end of the support structure comprises a plurality of apexes of the framework of the support structure. The support structure is collapsible into the compressed state by pulling on the apexes, to enable it to be drawn into a sheath in the compressed state. The sheath has an inner radial dimension smaller than the radial dimension of the support structure in the expanded state.

This heart valve can be implanted by retrograde access or antegrade access. In both cases, the heart valve is collapsed to a compressed state and held within the sheath on the end of a guidewire that is inserted into the vascular system through a catheter. The heart valve can be inserted by surgical transapical access.

FIGS. 10 to 13show a sequence of snapshots of collapsing the prosthetic heart valve into a compressed state. InFIG. 10, loops of filiform material42pass through pairs of adjacent loops40of the support structure of the prosthesis10. The threads of filiform material42pass out through the sheath44. When the filiform material is pulled, the loops40are gathered together as shown inFIG. 11, and then the upper petal-like shapes of the prosthesis10collapse and can be withdrawn into the sheath44as shown inFIG. 12. In this state, the lower petal-like shapes have also become folded. On further pulling of the filiform material, the structure is completely withdrawn into the sheath44as shown inFIG. 13.

When the sheath44has been delivered, for example endovascularly on the end of a guidewire to the required implantation position, the reverse sequence ofFIGS. 10 to 13is performed and the sheath44is withdrawn away from the heart valve structure. The lower petal-like protrusions expand first as shown inFIG. 12and enable the heart valve to be initially correctly positioned, including rotational positioning. Further withdrawal of the sheath44allows the heart valve to self-expand as shown inFIGS. 11 and 10. In the event that the heart valve needs to be repositioned or retrieved entirely, the filiform material42can be pulled again to collapse the structure into its compressed state, either partially or fully. When the positioning is finalised, the filiform material42and the sheath44can be completely withdrawn via the reverse route through which access was obtained.

However, there are a number of problems with known systems for percutaneous delivery of prosthetic heart valves. One problem is achieving the compressed state of the heart valve in the sheath, the heart valve being both radially and axially compact to fit within the sheath so as to pass round tortuous bends in the vascular system when being delivered to the heart. In particular, it can be difficult to collapse the heart valve sufficiently such that it can be fully inserted into the sheath. Another problem is that the heart valve can catch on the rim of the sheath and jam when it comes into contact with the sheath, thereby preventing the heart valve from being fully inserted. A further problem is that while inserting the heart valve into the sheath, the heart valve may get damaged or destroyed.

The present invention seeks to alleviate, at least partially, some or any of the above problems.

According to an aspect of the present invention, there is provided a prosthesis delivery system comprising a sheath, at least one tether and a holder. The sheath defines an axial direction. The at least one tether is movable axially relative to the sheath. The holder is movable within the sheath, and is configured to constrain radially the at least one tether.

According to a further aspect of the present invention, there is provided a method of collapsing a prosthesis to a collapsed state in a sheath. The method comprises the steps of:

constraining radially with a holder at least one tether, attached to the prosthesis; and

moving the at least one tether and the holder axially within the sheath such that the prosthesis is forced to collapse into the sheath and the holder is retracted into the sheath.

According to a further aspect of the present invention, there is provided a method of delivering a prosthesis collapsed in a sheath to a target position. The method comprises the steps of:

retracting the sheath axially away from the prosthesis attached to at least one tether such that the prosthesis partially expands from the collapsed state, wherein the at least one tether to which the prosthesis is attached is constrained radially by a holder that is movable axially within the sheath;

positioning in the target position the prosthesis that is in a partially collapsed state, attached to the at least one tether; and detaching the at least one tether from the prosthesis.

FIG. 1depicts a schematic perspective view of an embodiment of the prosthesis delivery system. The apparatus comprises a sheath2, at least one tether4and a holder6. Preferably, the apparatus is connected to a guidewire8.

In particular,FIG. 1depicts an embodiment of the apparatus, comprising an internal tube (i.e. holder6), hosting one or more control thread loops (i.e. tethers4), a guidewire8for endovascular guidance, and an external sheath2to collapse fully the prosthesis10and contain it in the collapsed configuration.

In an embodiment, the delivery system of the present invention comprises a holder6(which may take the form of a tube) within a sheath2(which may take the form of a tube). The tethers4(which may comprise wires) are fed through the holder6and when the tethers4are pulled, the top loops of the prosthesis10(which may be a valve) draw together. Once in this position the prosthesis10can then be pulled into the sheath2and removed. The delivery system of the present invention has an advantage over delivery systems that do not have a holder6in that the risk of the top loops of the prosthesis10catching on the rim of the sheath2and jamming is reduced. Such undesirable catching and jamming can happen particularly if the top loops of the prosthesis10are not drawn together adequately when the prosthesis10is being pulled into the sheath2.

The invention will be described in further detail below.

The guidewire8helps to control the movement of the apparatus through the vasculature of a living being. The apparatus is for delivering a prosthesis to an organ of a living being. The living being may be a mammal and, in particular, may be a human. The apparatus is a size suitable for being inserted into the vasculature of the living being.

The sheath2is longitudinal in shape and defines an axial direction. The sheath2is for housing the prosthesis10when the prosthesis10is in a collapsed state. The sheath2is configured to move through the vasculature of the living being. The purpose of the sheath2is to protect the prosthesis10during its delivery from outside the living being to the organ. The prosthesis10is collapsed to a compressed state and held within the sheath2on the guidewire8that is inserted into the vascular system through a catheter (not shown).

The sheath2may take the form of a tube open at both ends. Preferably, the sheath2is substantially cylindrical, having a substantially constant diameter throughout its axial (i.e. longitudinal) length. The diameter of the sheath2is great enough such that the prosthesis10fits inside the sheath2when it is in the compressed state. However, when the prosthesis10is its expanded shape, the prosthesis10does not fit inside the sheath2. The sheath2has an inner radial dimension smaller than the radial dimension of the prosthesis10in the expanded state.

The diameter of the sheath2is small enough such that it can fit inside the vascular system of the living being. The prosthesis10is inserted into the sheath2and removed from the sheath2at one opening of the sheath2, which may be termed the lead end3of the sheath2.

The at least one tether4is movable axially relative to the sheath2. Preferably, the apparatus comprises a plurality of tethers4. For example, as depicted inFIG. 1, the apparatus may comprise three tethers4. The number of tethers4that may be used is not particularly limited and may be two, four or more. In the following description, for clarity it will be assumed that there is a plurality of tethers4.

The tethers4are configured to attach to the prosthesis10. For example, in the case of a prosthetic heart valve, which comprises a support structure and a flow-control structure, the tethers4are configured to attach to the framework of the support structure of the prosthesis10. The tethers4may extend along the length of the sheath2. The tethers4are disposed within the sheath2. Preferably, the tethers4are configured to be extendable beyond the lead end3of the sheath2. The tethers4are not integrally connected to the sheath2.

The tethers4are configured to attach to the prosthesis10at a lead end5of the tethers4. The tethers4may be configured to be controllable so as to be detachable from the prosthesis10, via the end of the tethers4that is opposite to the lead end5of the tethers4. The purpose of this is that when the prosthesis is attached to the delivery system via the tethers4and is in the correct place in the vascular system ready to be delivered, the prosthesis10can be left at the target position and the delivery system retrieved out of the body.

The detachment of the tethers4from the prosthesis10may be performed either by use of a guidewire attached the tethers4. Each tether4may have its own guidewire, or a plurality of tethers4may share a single guidewire. Alternatively, the detachment of the tethers4from the prosthesis10can be controlled by direct manipulation of the tethers4themselves. The tethers4can be directly manipulated to detach from the prosthesis10in the case that the tethers4extend from the lead end5to a position outside the body of the living being.FIG. 5depicts such a construction.

The tethers4may comprise an unclosed loop of filiform material. The loop is formed at the lead end5of the tethers4. The two ends of the unclosed loop of filiform material are at the opposite end (i.e. the control end) of the tethers4. In this case, the detachment of the tethers4from the prosthesis10can be performed by manipulation at the open ends of the unclosed loop of filiform material.

As will be described in further detail below in relation toFIGS. 6 to 9, the tethers4may comprise a hooked rib (i.e. a rib12with a hook13at its lead end). In this case, the hooked rib may be attached to a guidewire for controlling the detachment of the tether4from the prosthesis10.

The apparatus may comprise a mixture of tethers4that comprise unclosed loops of filiform material and tethers4that comprise hooked ribs.

The holder6is movable axially within the sheath2. The holder is configured to constrain radially the tethers4. The purpose of the constraining is to allow one end of the prosthesis10to be held by the tethers4in a collapsed state such that when the partially collapsed prosthesis10is fully inserted into the sheath2, the part of the prosthesis10that first enters the sheath2does not catch on the edges of the sheath2.

The holder6may hold the tethers4in a convergence region7. The convergence region7may be within an aperture at the lead end3of the sheath2when viewed in the axial direction. The purpose of the convergence region7is to allow one end of the prosthesis10to be collapsed in the convergence region7such that the collapsed end of the prosthesis10fits inside an aperture of the lead end3of the sheath2. For this reason, the convergence region7is within the internal region of the sheath2.

The holder6may encircle, or surround the tethers4when viewed in cross section. The holder groups, or collects the tethers4together. Preferably, the holder6forms a closed loop when viewed in cross section.

The radial constraint of the tethers4by the holder6allows a clearance between the outside of the collapsed end of the prosthesis10(which is attached to the tethers4) and the inner surface of the sheath2such that the collapsed end of the prosthesis10does not come into contact with the sheath2. Of course, when the prosthesis10is fully inserted into the sheath2, parts of the prosthesis10come into contact with the internal surface of the sheath2. However, when the insertion process begins, the prosthesis10can be partially collapsed such that the end of the prosthesis closest to the sheath2is collapsed and attached to the radially constrained tethers4such that the collapsed part of the prosthesis10does not catch on the sheath2as it is inserted. Subsequently, as the sheath2is forced around the rest of the prosthesis10(e.g. the uncollapsed part of the prosthesis10), the prosthesis10does not become jammed, but is forced to collapse by the pressure of the sheath2.

The convergence region7is a region just beyond the lead end of the holder6. The lead ends5of the tethers4constrain the collapsed end of the prosthesis10at the convergence region7. The convergence region7may be a convergence area, within the cross sectional area of the sheath2when viewed in the axial direction. The sheath2may have a cross sectional area consistent throughout its length. However, if the cross sectional area of the sheath2varies throughout its length, then the relevant cross sectional area is the cross sectional area of the sheath2at the lead end3of the sheath2. The lead end3is the end at which the prosthesis10enters and exits the sheath2.

FIGS. 2 to 4depict a method of collapsing a prosthesis10into the sheath2of the delivery system. InFIGS. 2 to 4, the tethers4comprise unclosed loops of filiform material. However, as mentioned above and depicted inFIGS. 6 to 9, the present invention may also be implemented with tethers4comprising hooked ribs12,13.

InFIG. 2, the loops of the tethers4are attached to the prosthesis10. The prosthesis may comprise a support structure having a framework with distal cells. The distal cells have apexes11. The support structure is collapsible from the fully expanded state into the compressed state by pulling on the apexes11. This enables the support structure to be drawn into the sheath2in the compressed state. The sheath2has an inner radially dimension smaller than the radial dimension of the support structure in the expanded state.

Each of the tethers4forms a loop that passes through a number of the distal cells of the framework of the prosthetic device. The tethers4are attached to the apexes11of the support structure of the prosthesis10at the lead end5of the tethers4. The tethers4may be looped around one or more of the apexes11. InFIG. 2, the two tethers4are depicted as being looped around three apexes11each.

The method of collapsing involves constraining radially the tethers4with the holder6. The holder6fits within the sheath2. Hence the radial constraint results in a clearance between the constrained section of the tethers4and the inner surface of the sheath2. The tethers4are attached to the prosthesis10. This state is depicted inFIG. 2.

Subsequently, the tethers4and the holder6are moved axially within the sheath2such that the prosthesis10is forced to collapse into the sheath2and the holder6is retracted into the sheath2. The resulting state is depicted inFIG. 4. The arrows inFIG. 4indicate the direction of movement of the sheath2relative to the holder6and the tethers4.

More particularly, the collapse of the device into the delivery system (depicted inFIGS. 2 to 4) may be achieved by retracting the tethers4into and/or through the holder6, which may be an internal tube, in order to group the distal edges of the prosthesis frame in the convergence region7. The dimensions of the convergence region7are smaller than the diameter of the external sheath2(seeFIG. 3). Hence, keeping the tethers4fixed in this position, the external sheath2is advanced relatively to the prosthesis so as to force the prosthesis to collapse into it (seeFIG. 4). This operation can be repeated once the prosthesis10has been fully deployed into the target position of the anatomical region. This process may be performed so as to achieve the complete retrieval of the prosthesis10.

Preferably, the method comprises the step of retracting the tethers4relative to the holder6so as to collapse partially the prosthesis10such that the collapsed end of the prosthesis10fits within an aperture at the lead end3of the sheath2. For example, according to the embodiment depicted inFIGS. 2 to 4, the step of moving the tethers4and the holder6axially relative to the sheath2is broken up into two stages.

In the first stage, the prosthesis10is partially collapsed such that one end of the prosthesis10fits within an aperture at the lead end3of the sheath2. In order to perform this first stage, preferably the tethers4are movable axially within the holder6. The tethers4are pulled such that they are retracted into/towards the sheath2. The arrows inFIG. 3indicate the direction in which the tethers4are pulled. The end of the prosthesis10closest to the sheath2collapses into the convergence region7because the tethers4are constrained radially by the holder6.

In the second stage, the tethers4together with the holder6are retracted into/through the sheath2. The collapsed end of the prosthesis10enters into the lead end3of the sheath2without touching the edges of the sheath2. Once the collapsed apexes11are retracted into the sheath2, other parts of the prosthesis2come into contact with the sheath2, thereby having the effect of forcing the rest of the prosthesis10to collapse into the sheath2.

Prior to the state depicted inFIG. 2, the lead ends5of the tethers4are attached to the prosthesis10. This step may be performed outside the body of the living being. However, as explained in relation toFIGS. 6 to 9, if a tether4comprising a hooked rib12,13is used, this step can be performed in the body of the living being.

As depicted inFIGS. 2 to 4, the holder6may comprise a tube that is substantially co-axial with the sheath2. The holder6does not necessarily have to take the form of a tube. In an embodiment, the holder6may comprise an annular ring. Preferably, the holder6is on a guidewire for endovascular guidance.

The holder6is different from the sheath2. The holder6fits inside the sheath2. When the prosthesis10is in the sheath2in the collapsed state, the holder6may be in the sheath2. Preferably, the holder is not attached to the sheath2. There may be a clearance gap between the outer surface of the holder6and the inner surface of the sheath2. Alternatively, the holder6may be attached to the sheath2provided that the holder6is movable axially relative to the sheath2.

The holder6may be movable relative to the sheath2between an extended position (as depicted inFIG. 2) and a retracted position (seeFIG. 4). In the extended position, a lead end of the holder6extends beyond a lead end3of the sheath2. In the retracted position, the lead end3of the sheath2extends beyond the lead end of the holder6.

Preferably, a lead end of the holder6is configurable to extend beyond a lead end3of the sheath2. The holder6may be initially positioned such that the convergence region7into which the collapsed apexes11are to be fit is extended beyond the lead end3of the sheath2. This allows the partially collapsed end of the prosthesis10to be collected away from the lead end3of the sheath2(seeFIG. 3) before the sheath2is forced around the rest of the prosthesis10. Hence, it is preferable that the lead end of the holder6is initially beyond the lead end3of the sheath2before being moved to be inside the sheath2(as shown inFIG. 4).

However, it is possible that the desired function, namely collecting the collapsed end of the partially collapsed prosthesis10such that it does not come into contact with the sheath2when the process of forcing the prosthesis10into the sheath2is begun, can be performed by having the lead end of the holder6not extending beyond the lead end3of the sheath2.

This is because as depicted inFIG. 3, the collapsed end of the prosthesis10forms a tapered shape, with the narrow section closest to the sheath2. As such, it is possible for the lead end of the holder6to be slightly within the sheath2, while still having the collapsed end of the prosthesis10to be initially inserted into the sheath2without touching the sheath2. However, if the lead end of the holder6is retracted too far behind the lead end3of the sheath2, then it would not be possible to collapse one end of the prosthesis10such that it does not come into contact with the sheath2.

When the prosthesis10is in the collapsed state within the sheath2as depicted inFIG. 4, it is ready to be delivered to a target position of an anatomical region, for example in the heart of a living being.

As mentioned above, the sheath2with the collapsed prosthesis10inside it is entered into the vascular system of the living being via a catheter. The sheath2is moved within the vascular system along the guidewire8.

The method of delivering the prosthesis10to the target position involves retracting the sheath2axially away from the prosthesis10that is attached to the tethers4. This results in the prosthesis10partially expanding from the collapsed state. This step is depicted in the transition fromFIG. 4toFIG. 3.

The tethers4to which the prosthesis10is attached are constrained radially by the holder6. As a result, the prosthesis10is attached to the tethers4in the partially collapsed state.

The delivery of the prosthesis10further involves positioning in the target position the prosthesis10that is in the partially collapsed state and attached to the tethers4. The position of the prosthesis10can still be controlled because it is still attached to the tethers4.

Once the prosthesis10is in the target position, the tension on the tethers4is relaxed, thereby allowing the prosthesis10to fully expand (subject to the constraints of the vasculature in which it is situated).

Hence in summary, the deployment of the prosthesis10is achieved by retracting the external sheath2, leaving the prosthesis10engaged only on the tethers4. In this stage, the prosthesis is still firmly fixed to the delivery system, and can be manoeuvred and repositioned. Releasing the tension from the tethers4, the prosthesis10deploys fully. If necessary, the collapse sequence of operations can be repeated to retrieve the prosthesis10.

The full expansion of the prosthesis10is achieved by simply relaxing the tension on the tethers4. After the release of the expansion of the prosthesis10, the delivery system can be moved away from the prosthesis10following the vascular route, keeping the tethers4connected to the prosthesis10. The valve positioning and haemodynamic performance can be checked with state of the art techniques (e.g. by fluoroscopy, echocardiography and/or aorthography) and, if necessary, the catheter can be readvanced and the prosthesis10safely recollapsed and repositioned (or completely removed and exchanged for another solution) by pulling the tethers4. This feature of allowing the prosthesis10to expand fully while still being connected to the tethers4means that it is not necessary for the delivery system to retrieve the valve to be kept in proximity of the prosthesis10. This reduces the possibility of the delivery system from interfering with the performance of the prosthesis10when these are verified.

Once the procedure is satisfactorily completed, the tethers4can be detached from the prosthesis10and extracted.

FIG. 5depicts the prosthesis delivery system during a process of detaching the tether4from the prosthesis10. In the embodiment depicted inFIG. 5, the tether4comprises an unclosed loop of filiform material. The loop can be detached from the prosthesis10by pulling on one of the open ends of the loop of filiform material. This unthreads the loop from the prosthesis10. Hence, when the positioning of the prosthesis10in the target position is correct, the tethers4may be removed from the apparatus by disengaging one of their terminations and retracting the other.

InFIG. 5, two tethers4are depicted at different stages of detachment. The tether4shown in the left hand side ofFIG. 4remains looped around apexes11of the prosthesis10. However, the tension has been released and one of its terminations is moving through the sheath2so as to pass through the apexes11, thereby detaching the tether4from the prosthesis10. An arrow is shown indicating the direction of movement of the other end of the tether4, namely away from the prosthesis10. The tether4shown in the right hand side ofFIG. 4is at a later stage of detachment and is no longer attached to the prosthesis10. By pulling the tether4in the direction of the arrow, the tether4is removed completely from the delivery system.

FIGS. 6 to 9depicts another embodiment of the present invention in which the tethers4comprise a hooked rib,12,13configured to be movable axially relative to the holder6. The hooked rib12,13is configured to move between a collapsed position and an expanded position. In the collapsed position, which is depicted inFIG. 6, the hooked rib12,13is held by the holder6in an elastically deformed state. In the expanded position, the hooked rib12,13extends radially beyond the holder6.

The hooked rib12,13comprises a hook13on the end of a rib12. The hook13may be formed integrally with the rib12. Alternatively, the hook13may be a separate member from the rib12, connected to the rib12. The hook13is designed to attach to the prosthesis10. As depicted inFIGS. 6 and 7, the prosthesis10may comprise a permanent wire14, which is suitable for the hook13to attach to.

The rib12is formed of an elastic material. In the collapsed position depicted inFIG. 6, the ribs12are constrained by the holder6. When the hooked ribs12,13are moved from the collapsed state to the expanded position (depicted inFIG. 7) the hooked ribs12,13extend beyond a lead end of the holder6. The hooked ribs12,13extend through the holder6. Due to the elastic nature of the ribs12, the ribs extend radially beyond the holder6. This allows the ribs to be attached by the hooks13to the prosthesis10even when the prosthesis10is in its fully expanded position.

More particularly, an embodiment of the present invention comprises a holder6, which may comprise an internal tube, hosting one or more ribs12made terminating with hooks13(and a guidewire), and an external sheath2to collapse fully the prosthesis10and contain it in the collapsed configuration. The prosthesis10may include a permanent wire passing through all distal cells of the framework of the support structure of the prosthesis.

Once the hooks13are attached to the prosthesis10in the expanded state, the process for collapsing the prosthesis into the sheath2is similar to the process for the embodiments in which the tethers4comprise a loop of filiform material as described above. The tethers4comprising the hooked ribs12,13are moved axially through the holder6so as to bring the lead end of the tethers4(i.e. the hooks13) towards the holder6so as to collapse partially the prosthesis10. This step is seen as a transition from the state depicted inFIG. 7to the state depicted inFIG. 8. The arrows inFIG. 8indicate the direction of movement of the tethers4relative to the holder6. Subsequently, the sheath2is moved axially relative to the tethers4and the holder6such that the prosthesis10is forced to collapse into the sheath2and the holder6is retracted into the sheath2. This results in the state depicted inFIG. 9.

To retrieve the prosthesis10, the hooked ribs12,13are advanced and expanded. The arrows inFIG. 7indicate the direction of advancement of the ribs12. The desired function of the ribs12to expand in this way may be achieved by making the ribs12from a superelastic material. The hooked ribs12,13engage their hooks13with the permanent wire14in one more points as depicted inFIG. 7. Then the ribs12are pulled back into the holder6, which may comprise a tube, thus retracting one or more portions of the permanent wire14of the prosthesis10. As a result, the permanent wire14forms a number of loops that reduce in dimension, grouping the distal edge of the prosthesis frame to the convergence region7, which is dimensionally smaller than the diameter of the external sheath2, as depicted inFIG. 8. Hence, keeping the tethers4fixed in this position, the external sheath2is advanced so as to force the prosthesis10to collapse into it, as depicted inFIG. 9.

An advantage of a delivery system that has tethers4comprising hooked ribs12,13is that it allows retrieval and repositioning of the prosthesis10after the implantation procedure has been completed.

The prosthesis delivery system of the present invention may be used in conjunction with any collapsible prosthetic device and is not limited to use with prosthetic heart valves. The average diameter of the prosthesis10when in the expanded state is preferably in the range of from 10 to 40 mm, more preferably, the average diameter of the prosthesis10is in the range of from 18 to 32 mm. The diameter of the prosthesis when in the compressed state is preferably less than 12 mm. A compressed prosthesis10of this size is suitable for transapical access, and more preferably, the diameter of the prosthesis when in the compressed state is less than 8 mm. The axial length of the prosthesis in the expanded state is preferably in the range of from 12 to 200 mm, more preferably from 15 to 55 mm. When in the radially compressed state, the axial length occupied by the prosthesis is increased relative to its expanded state because of the way that the structure folds, however the increase in axial length is less than 100%, preferably less than 80%, and can be as little as 20%.