Patent Description:
The present disclosure relates generally to implantable medical devices, and more specifically to mechanisms for adjusting the diameter of implantable medical devices and associated methods thereof.

Implantable medical devices such as stents, stent-grafts, valves, and other intraluminal devices are used in a variety of medical procedures including to maintain, open, or adjust various body passageways or body lumens to maintain, prevent, and/or adjust fluid flow therethrough. Such devices may be implanted in various locations within the body of a patient including in the vascular system, coronary system, urinary tract, and bile ducts, among others.

In some instances, the size of the medical device required may change over time. For example, devices implanted in children may need to be removed and replaced with larger diameter devices as the child grows. In other scenarios, it may be beneficial to implant a larger diameter device and incrementally decrease the diameter, for example, to seal off a defect or slow fluid flow to a certain, afflicted area such as an aneurysm. It may also be beneficial to increase or decrease the size of a body lumen to adjust the rate of fluid flow therethrough such as during dialysis or instances of heart or kidney failure.

The diameters of implantable medical devices are often not easily adjustable or customizable, and many devices do not permit intravenous or percutaneous diametric adjustments. Current practices often require replacement of the device with a new, differently sized device altogether, which may require further operation and/or invasive procedures, causing added risk, stress and discomfort to the patient.

<CIT> is directed to methods and apparatuses relating to an expandable valve that may be implanted within the body, for example, at a semilunar position of the heart. The valve may include an expandable valve frame disposed about a number of valve leaflets and may be implanted in young patients. The diameter of the expandable valve frame may be suitably increased to accommodate patient growth where the valve is stretched to expand along with the valve frame. In some embodiments, an expandable valve is disposed within a covered stent having expandable cuffs attached at opposite ends of the stent. The expandable cuffs of the device may be surgically attached to separate regions of tissue to provide fluid communication between the tissue regions. In an embodiment, an expandable cuff attached at one end of a stent is sutured to an outer wall of a right ventricle and an expandable cuff attached at the opposite end of the stent is sutured to an outer wall of the main pulmonary artery, providing a passageway for fluid (i.e., blood) to flow between the right ventricle and the main pulmonary artery, through the valve. The implantable device may be expandable to accommodate growth in young patients. For some instances, the shape of the device may be manipulated to suit placement of the expandable valve conduit within the body.

<CIT> is directed to an eyeball clamping ring having a curved and elastic element where its ends are linked together and preferably overlap. One end has a positive shape with a plug and the other end has a matching negative shape with a socket.

<CIT> is directed to a heart valve prosthesis having a frame that includes at least one outwardly extending annular flange adjacent to a suturing ring and a band comprising an inwardly extending annular flange that is capable of engagement with the annular flange of the frame. A tortuous path is formed between the annular flange of the frame and the annular flange of the band that can engage and restrain a suture placed between them. One or more split portions or tab portions on the band are adapted to lock the band in engagement with the flange of the frame. Such a device can be used to more easily replace a heart valve by advancing a sutures through a suturing ring on the valve and the valvular rim such that they are brought outward of the flange extending from the frame and then advancing the band over the valve and over the sutures at a proximal, outflow end of the valve until the band engages the flange and captures the sutures between the band and the flange.

<CIT> is directed to a lumen support stent for use in an artery or any body lumen. The stent is formed from a plurality of ladder elements having elongated ribs bowed to define a circumferential arc of the tubular member and end rungs affixed to the elongated ribs. The elongated ribs of adjacent ladder elements are substantially parallel to one another and slidably engaged by the end rungs of adjacent ladder elements. Sliding of the end rungs along the circumferential arc defined by the engaged ribs creates a variable circumferential distance between the end rungs of adjacent ladder elements. Consequently, the stent has a first diameter in which the circumferential distance between end rungs of adjacent ladder elements is collapsed, and a variable second diameter in which the circumferential distance between end rungs of adjacent ladder elements is expanded.

The invention relates to a mechanism for adjusting the diameter of an implantable medical device according to claim <NUM>; and to a method of adjusting the diameter of this medical device according to claim <NUM>.

Refinements of the mechanism and the method are described in the respective dependent claims.

Various aspects of the present disclosure relate to adjustment mechanisms for adjusting diameters of implantable medical devices. Examples of implantable medical devices can include stents, stent-grafts, valves, and devices for occlusion and/or anastomosis, among others. In certain examples, the implantable medical devices may be configured to adjust (e.g., increase and/or decrease) the size of a particular artificial or natural body lumen, passageway, and/or conduit to promote, restrict, or otherwise adjust fluid flow therethrough. For reference, the term "lumen" should be read broadly to include any of a variety of passages, such as those associated with the vasculature, biliary tract, urinary tract, lymph system, reproductive system, gastrointestinal system, or others.

In certain instances, it may be beneficial to adjust the diameter of implantable medical devices after implantation inside the body of a patient. For example, in certain applications where the size of the body lumen increases or decreases over time, it may be beneficial to increase and/or decrease the diameter of the device to fit the changing size of the body lumen. In other instances, it may be beneficial to gradually reduce or restrict the flow of blood to a certain area, such as slowing blood flow to an aneurysm, adjusting urine flow during and/or after dialysis, and restricting and/or decreasing blood flow during heart or kidney failure.

In the above examples, it may also be beneficial to be able to adjust the implantable medical devices without additional, invasive procedures. Procedures such as these can impart added stress and discomfort on the patient. Therefore, a device that reduces potential, additional burden on the patient and/or medical provider would be desirable.

<FIG> shows a diametric adjustment mechanism for an implantable medical device, according to some embodiments. The diametric adjustment mechanism <NUM> includes a track <NUM> defining a series of diametric setpoints <NUM>, a rider <NUM> engaged with the track <NUM> and selectively movable along the track <NUM> between the series of diametric setpoints <NUM>, and a biasing element <NUM> to promote movement of the rider <NUM> in a certain direction along the track <NUM>. In some embodiments, the diametric adjustment mechanism <NUM>, also referred to herein simply as the adjustment mechanism <NUM>, is coupled to an implantable medical device <NUM> (<FIG>). The series of diametric setpoints <NUM> includes at least two setpoints, for example, a first diametric setpoint <NUM> (<FIG>) and a second diametric setpoint <NUM> (<FIG>), but may include more setpoints as desired. As the rider <NUM> moves along the track <NUM> between the series of diametric setpoints <NUM>, the diameter D of the adjustment mechanism <NUM> is either increased or decreased depending on the direction in which the rider <NUM> moves along the track <NUM>. For example, the rider <NUM> could move in a clockwise direction around the track <NUM> or in a counter-clockwise direction around the track <NUM> depending on the configuration of the track <NUM>. As the diameter D of the adjustment mechanism <NUM> is increased or decreased, the diameter d (<FIG>) of the implantable medical device <NUM> also increases or decreases.

In some embodiments, the series of diametric setpoints <NUM> correspond to a series of stop points spaced along the track <NUM> configured to keep the rider <NUM> at a certain location along the track <NUM> until a biasing force imparted by the biasing element <NUM> on the rider <NUM> is overcome. In other words, each of the stop points keep the adjustment mechanism <NUM> at a respective, desired diameter D until the biasing force is overcome and the rider <NUM> moves to the subsequent stop point. The biasing force can be overcome by application of a diametric force (e.g., a radial force applied in a radially outward direction from the longitudinal axis A of the adjustment mechanism <NUM>), a magnetic force (e.g., applied externally through the skin of a patient), or any other applied force that exceeds the biasing force and to cause the rider <NUM> to move along the series of diametric setpoints <NUM>. In some examples, the diametric force is an expansion force imparted on an interior of the implantable medical device <NUM> using a balloon catheter, although other methods of imparting an expansion force upon the adjustment mechanism <NUM> are also contemplated.

In various embodiments, the series of diametric setpoints <NUM> can be any of a series of notches, steps, grooves, bends, curves, crooks, or any other configuration capable of keeping the rider <NUM> at a certain location along the track <NUM>. In some examples, the series of diametric setpoints <NUM> may include portions that are flat, upwardly angled, or otherwise inflected as compared to the rest of the track <NUM> so that the rider <NUM> may sit, rest, or lodge at the respective one of the series of setpoints <NUM> until the biasing force is overcome, as shown in <FIG>.

In some embodiments, the series of diametric setpoints <NUM> includes a first diametric setpoint <NUM> and a second diametric setpoint <NUM>. The first diametric setpoint <NUM> corresponds to a first diameter D<NUM> of the adjustment mechanism <NUM> and the second diametric setpoint <NUM> corresponds to a second diameter D<NUM> of the adjustment mechanism <NUM>. Thus, moving the rider <NUM> between the first diametric setpoint <NUM> and the second diametric setpoint <NUM> causes the diametric adjustment mechanism <NUM> to increase and/or decrease from the first diameter D<NUM> to the second diameter D<NUM> and, in turn, causes the implantable medical device <NUM> to also increase or decrease from a first device diameter d<NUM> to a second device diameter d<NUM>.

The track <NUM> can include additional setpoints, as desired, for adjusting the diameter D of the adjustment mechanism <NUM>. For example, the track <NUM> can include an intermediate diametric setpoint <NUM> located between the first diametric setpoint <NUM> and the second diametric setpoint <NUM>. Similar to the first and second diametric setpoints <NUM>, <NUM>, the intermediate diametric setpoint <NUM> corresponds to an intermediate diameter Di of the diametric adjustment mechanism <NUM>, the intermediate diameter Di being between the first diameter D<NUM> and the second diameter D<NUM>. Additional diametric setpoints may allow for incremental adjustment of the adjustment mechanism <NUM> and/or the implantable medical device <NUM> between any number of diameters as desired. For example, in certain instances, a larger number of smaller, incremental diametric adjustments may be necessary or beneficial where, in other instances, fewer, larger adjustments may be desired.

In some embodiments, the track <NUM> defines a stepped path <NUM>, as shown in <FIG>, with each of the series of diametric setpoints <NUM> spaced along the stepped path <NUM>. In some embodiments, the first diametric setpoint <NUM> is located at a first step location <NUM> along the stepped path <NUM> and the second diametric setpoint <NUM> is located at a second step location <NUM> along the stepped path <NUM>. As discussed above, moving the rider <NUM> between the first diametric setpoint <NUM> (e.g., the first step location <NUM>) and the second diametric setpoint <NUM> (e.g., the second step location <NUM>) causes the adjustment mechanism <NUM> to increase and/or decrease from the first diameter D<NUM> to the second diameter D<NUM>. In some embodiments, the first step location <NUM> may be near a first end <NUM> of the track <NUM> and the second step location <NUM> may be near a second end <NUM> of the track <NUM>. However, the first and second step locations <NUM>, <NUM> can be located anywhere along the track <NUM> as desired.

In some embodiments, the track <NUM> defines a first adjustment path P<NUM>. The first adjustment path P<NUM> may be, for example, between the first diametric setpoint <NUM> and the second diametric setpoint <NUM>. For example, the rider <NUM> can move along the first adjustment path P<NUM> to adjust the adjustment mechanism <NUM> between the first diameter D<NUM> and the second diameter D<NUM>. In some embodiments, the first adjustment path P<NUM> may also be between the first diametric setpoint <NUM> and the intermediate diametric setpoint <NUM>. For example, the rider <NUM> can move along the first adjustment path P<NUM> from the first diametric setpoint <NUM> to the intermediate diametric setpoint <NUM> prior to moving to the second diametric setpoint <NUM>.

In some embodiments, the track <NUM> also defines a second adjustment path P<NUM>. For example, after moving from the first diametric setpoint <NUM> to the intermediate diametric setpoint <NUM>, the rider <NUM> may then move along the second adjustment path P<NUM> from the intermediate diametric setpoint <NUM> to the second diametric setpoint <NUM>. In various embodiments, the track <NUM> may define a third adjustment path, a fourth adjustment path, or any number of adjustment paths between each setpoint of the series of diametric setpoints <NUM> as desired.

Although the adjustment mechanism <NUM> is described above and shown in <FIG> to decrease in diameter as the rider <NUM> moves along the first adjustment path P<NUM> and the second adjustment path P<NUM>, the mechanism <NUM> can also be configured to increase in diameter as the rider <NUM> moves along the first and second adjustment paths P<NUM>, P<NUM>. For example, as the rider <NUM> moves along the first adjustment path P<NUM> from the first diametric setpoint <NUM> to the intermediate diametric setpoint <NUM>, the diameter D of the adjustment mechanism <NUM> may increase (i.e., from a smaller diameter to a larger diameter), and may further increase as the rider <NUM> moves along the second adjustment path P<NUM> from the intermediate setpoint <NUM> to the second diametric setpoint <NUM>.

The track also defines a return path <NUM> between the second diametric setpoint <NUM> and the first diametric setpoint <NUM>. The return path <NUM> allows for diametric adjustment of the adjustment mechanism <NUM> from the second diameter D<NUM> to the first diameter D<NUM>. The return path <NUM> is located adjacent and may be located substantially parallel to the stepped path <NUM>. In some embodiments, the return path <NUM> may be substantially straight such that the rider <NUM> can move continuously and uninterrupted from the second diametric setpoint <NUM> to the first diametric setpoint <NUM>. In some embodiments, the return path <NUM> allows for return of the rider <NUM> to its original location (e.g., the first step location <NUM>) so that in use the rider <NUM> remains continually engaged with the track <NUM>.

In some embodiments, the track <NUM> may define a continuous loop, as shown in <FIG>. This allows the rider <NUM> to move along the track <NUM> (e.g., between the series of diametric setpoints <NUM>) without disengaging from the track <NUM>. For example, the rider <NUM> can move along the stepped path <NUM> from the first diametric setpoint <NUM> to the second diametric setpoint <NUM> and then move along the return path <NUM> from the second diametric setpoint <NUM> to the first diametric setpoint <NUM> without disengaging from the track <NUM>.

In various examples, the rider <NUM> and the track <NUM> are complementary features that are configured to remain slidably coupled during diametric adjustment. In some embodiments, the rider <NUM> may be a projection, groove, or other feature capable of slidably engaging with the track <NUM>. The track <NUM> may define a depression or relief feature capable of receiving the rider <NUM>, or a raised rail or other feature on which the rider <NUM> traverses. For example, the track <NUM> can include at least one of a groove, a channel, a notch, an indentation, a rail, or any other feature capable of receiving or otherwise engaging with the rider <NUM>.

In some embodiments, the diametric adjustment mechanism <NUM> includes a biasing element <NUM>, as shown in <FIG>. As discussed above, the biasing element <NUM> promotes movement of the rider in a certain, desired direction along the track <NUM>. For example, the biasing element <NUM> may bias or promote movement of the rider <NUM> toward the second diametric setpoint <NUM> when the rider <NUM> is at the first diametric setpoint <NUM>, or toward the first diametric setpoint <NUM> when the rider <NUM> is at the second diametric setpoint <NUM>. In some embodiments, the biasing element <NUM> is configured to maintain the rider <NUM> at the first diametric setpoint <NUM> until the biasing force of the biasing element <NUM> is exceeded by the diametric force (i.e., from the catheter balloon, the magnet, or a bodily function such as, for example, a heartbeat) to move the rider <NUM> to the second diametric setpoint <NUM>, at which movement of the rider <NUM> is halted until the biasing force of the biasing element <NUM> is again exceeded by the diametric force.

<FIG> shows the diametric adjustment mechanism <NUM> coupled to the biasing element <NUM>, according to some embodiments. In some embodiments, the biasing element <NUM> is formed of a resilient material capable of imparting a bias on the rider <NUM>, as discussed above. In other words, the biasing element <NUM> promotes movement of the rider <NUM> in a certain direction along the track <NUM>. For example, where the capability to incrementally adjust an increasing diameter is desired, then the biasing element <NUM> may be configured to bias the adjustment mechanism <NUM> toward the smaller diametric setpoint. In turn, wherein the capability to incrementally adjust from a larger diameter to a smaller diameter is desired, then the biasing element <NUM> may be configured to bias the adjustment mechanism <NUM> toward the smaller diametric setpoint (e.g., biasing the rider <NUM> in a direction along the track <NUM> from the first diametric setpoint <NUM> toward the second diametric setpoint <NUM>). In some embodiments, the biasing element <NUM> is formed of a substantially flat sheet of material capable of overlapping itself to form a generally tubular or cylindrical shape, as is shown in <FIG>. In some embodiments, the biasing element <NUM> may include a length L extending from a first portion <NUM> to a second portion <NUM>, and a width W, the width W being a dimension perpendicular to the length L. The biasing element <NUM> also includes an outer surface <NUM>, an inner surface <NUM> (<FIG>), and a longitudinal axis A. In some embodiments, the track <NUM> is coupled to the outer surface <NUM>, near the second portion <NUM>, of the biasing element <NUM> and the rider <NUM> is coupled to the inner surface <NUM>, near the first portion <NUM>, of the biasing element <NUM> such that, when the biasing element <NUM> is folded over itself and overlaps (e.g., the first portion <NUM> overlaps the second portion <NUM>), the rider <NUM> engages the track <NUM> so that the rider <NUM> is slidably coupled to the track <NUM>.

<FIG> show the adjustment mechanism <NUM> coupled to an implantable medical device <NUM> in the form of a tubular implant. As shown, the adjustment mechanism <NUM> can be coupled to the implantable medical device <NUM> such that, when the adjustment mechanism <NUM> changes from the first diameter D<NUM> to the second diameter D<NUM>, the implantable medical device <NUM> also changes from the first device diameter d<NUM> to the second device diameter d<NUM>. As discussed above, examples of implantable medical devices <NUM> in the form of tubular implants may include stents and stent-grafts, among other tubular, cylindrically-shaped devices (e.g., heart valves, vascular filters, anastomosis devices, occluders, and others).

<FIG> shows the diametric adjustment mechanism <NUM> coupled to the implantable medical device <NUM> in an expanded configuration or, in other words, at the first diameter D<NUM>. As shown, the rider <NUM> is located at the first diametric setpoint <NUM> (e.g., at the first step location <NUM>). The first diametric setpoint <NUM> corresponds to the first diameter D<NUM> of the adjustment mechanism <NUM> and the first device diameter d<NUM> of the implantable medical device <NUM>.

As shown, the biasing element <NUM> can be a cylindrical member <NUM>, also described as a collar <NUM>, configured to surround an outer surface or a portion of the exterior surface of the implantable medical device <NUM>. In some embodiments, the collar <NUM> has elastic properties that impart the bias on the adjustment mechanism <NUM>. As discussed above, when the biasing force is overcome by the diametric force, the rider <NUM> moves, for example, from the first diametric setpoint <NUM> to the second diametric setpoint <NUM>, adjusting the diameters of the adjustment mechanism <NUM> and implantable medical device <NUM> as described above.

<FIG> shows the adjustment mechanism <NUM> at the intermediate diameter Di. The rider <NUM> is located at the intermediate diametric setpoint <NUM>. As discussed above, the intermediate diametric setpoint <NUM> corresponds to an intermediate diameter Di of the diametric adjustment mechanism <NUM> and an intermediate device diameter di of the implantable medical device <NUM>. As shown, movement of the rider <NUM> along the track <NUM> or, in some examples, along the stepped path <NUM>, facilitates adjustment of the implantable medical device <NUM> from the first device diameter d<NUM> to the intermediate device diameter di.

<FIG> shows the adjustment mechanism <NUM> in a compressed configuration or, in other words, at the second diameter D<NUM>. As shown, the rider <NUM> is located at the second diametric setpoint <NUM> (e.g., at the second step location <NUM>). The second diametric setpoint <NUM> corresponds to the second diameter D<NUM> of the adjustment mechanism <NUM> and the second device diameter d<NUM> of the implantable medical device <NUM>.

In some embodiments, a method of adjusting the diameter d of the implantable medical device <NUM> includes imparting a first diametric force on the diametric adjustment mechanism <NUM>. The first diametric force moves the rider <NUM> from the first diametric setpoint <NUM> to the second diametric setpoint <NUM> along the stepped path <NUM> according to a first configuration as can be seen in <FIG> (configured to transition from a larger diameter to a smaller diameter under the first diametric force, which is a compressive force) or according to a second configuration as can be seen in <FIG> (configured to transition from a smaller diameter to a larger diameter under the first diametric force FD1, which is an expansion force).

According to <FIG>, in some embodiments, the first diametric force may be a constrictive or compressive force configured to alter the diameter d of the implantable medical device <NUM>. In some examples, the diametric force is imparted by a manual force applied through the skin (e.g., by hand) or a force applied using one or more transcatheter devices (e.g., a balloon catheter or other device capable of diametric adjustment). The diametric force may also be applied (whether internally or externally) as a magnetic force that interacts with the rider <NUM> and "forces" or moves the rider <NUM> along the track <NUM> between any setpoints of the series of diametric setpoints <NUM> as desired. The method also includes imparting a second diametric force on the diametric adjustment mechanism <NUM>. In some examples, the second diametric force releases the rider <NUM> from the second diametric setpoint <NUM> and allows movement of the rider <NUM> from the second diametric setpoint <NUM> to the first diametric setpoint <NUM> along the return path <NUM> (e.g., as a result of the biasing force).

As shown in <FIG>, in some embodiments, the first diametric force is an expanding force. The expanding force may be imparted on an interior of the implantable medical device <NUM> in a variety of ways such as with a balloon catheter, as discussed above.

<FIG> show the adjustment mechanism <NUM> and implantable medical device <NUM> as an incremental expansion force is applied. As shown in <FIG>, the rider <NUM> is positioned at the first diametric setpoint <NUM> and the implantable medical device <NUM> is at the second (e.g., reduced) device diameter d<NUM>. <FIG> shows the adjustment mechanism <NUM> after a first diametric force has been applied. As shown, the rider <NUM> is positioned at the intermediate diametric setpoint <NUM> and the implantable medical device <NUM> is at the intermediate device diameter di. <FIG> shows the adjustment mechanism <NUM> after a second diametric force has been applied. The rider <NUM> is positioned at the second diametric setpoint <NUM> and the implantable medical device <NUM> is at the first (e.g., expanded) device diameter d<NUM>.

The first and second diametric forces and are described above as both compressive and expansion forces, the first and second diametric force and can be any of a variety of forces capable of overcoming the biasing force and moving the rider <NUM> along the track <NUM> from the first diametric setpoint <NUM> to the second diametric setpoint <NUM> and vice versa. In some examples, the diametric force is imparted by a manual force applied through the skin (e.g., by hand) or a force applied using one or more transcatheter devices (e.g., a balloon catheter or other device capable of diametric adjustment). The diametric force may also be applied (whether internally or externally) as a magnetic force that interacts with the rider <NUM> and "forces" or moves the rider <NUM> along the track <NUM> from the first diametric setpoint <NUM> to the second diametric setpoint <NUM>, from the second diametric setpoint <NUM> to the first diametric setpoint <NUM>, and/or between any setpoints of the series of diametric setpoints <NUM> as desired. For example, the rider <NUM> optionally includes a ferromagnetic material upon which an internal or external magnet may act. It should be understood that any other types of diametric forces FD may be used, as desired, to impart an applied force on the rider <NUM> and overcome the biasing force to move the rider <NUM> along the track <NUM>.

The adjustment mechanism <NUM> is optionally employed in a variety of applications. For example, the adjustment mechanism <NUM> is optionally employed to control flow through an intrahepatic portosystemic shunt device (e.g., in association with devices such as W. Gore & Associates Inc. 's product sold under the trade name "GORE® VIATORR® TIPS Endoprosthesis. " In other examples, the adjustment mechanism <NUM> is employed in an arteriovenous access application (e.g., to control flow through a fistula or graft, for example). In still further examples, the adjustment mechanism <NUM> is employed to control flow through a prosthetic valve (e.g., heart valve). In still further examples, the adjustment mechanism is employed to control flow in an aorta of a patient to control flow into the renal arteries of the patient (e.g., by controlling a diameter of a portion of an aortic stent graft). Although a few examples are provided, it should be understood that any of a variety of applications are contemplated. Methods of using the adjustment mechanism include a one-time adjustment, multiple adjustments, and adjustments of any frequency or periodicity (e.g., an adjustment per minute, hour, day, week, year, or per every heart beat).

In some examples, the adjustment mechanism <NUM> may be configured to adjust the diameter of a medical device with each of the patient's heartbeats. For example, the adjustment mechanism <NUM> may have many small diametric setpoints that each require a small biasing force to adjust. Therefore, with each heartbeat, the mechanism <NUM> may incrementally increase in diameter until reaching its full diameter, at which point the mechanism <NUM> may reset to its minimum diameter and repeat the cycle. The mechanism <NUM> may also be configured to incrementally decrease in diameter until reaching its minimum diameter, at which point the mechanism <NUM> may reset to its maximum diameter and repeat. Such continuous increasing or decreasing of the adjustment mechanism <NUM> may prevent the patient's body from adjusting to a new pressure, flow, or other property created by the presence of the adjustment mechanism <NUM>.

Claim 1:
A diametric adjustment mechanism (<NUM>) for an implantable medical device (<NUM>), the diametric adjustment mechanism (<NUM>) comprising:
a track (<NUM>) defining a series of diametric setpoints (<NUM>) including a first diametric setpoint (<NUM>) and a second diametric setpoint (<NUM>), a first adjustment path (P<NUM>) defined from the first diametric setpoint (<NUM>) to the second diametric setpoint (<NUM>), and a return path (<NUM>) defined from the second diametric setpoint (<NUM>) to the first diametric setpoint (<NUM>), the return path (<NUM>) being adjacent to the first adjustment path (P<NUM>);
a rider (<NUM>) engaged with the track (<NUM>) such that the rider (<NUM>) is selectively movable along the track (<NUM>) from the first diametric setpoint (<NUM>) to the second diametric setpoint (<NUM>), and from the second diametric setpoint (<NUM>) to the first diametric setpoint (<NUM>); and
a biasing element (<NUM>) biasing the rider (<NUM>) toward the first diametric setpoint (<NUM>) when the rider (<NUM>) is at the second diametric setpoint (<NUM>).