Minimally invasive medical device deployment and retrieval system

The present invention provides a medical device retrieval system comprising a working element carried by a flexible, elongate shaft, the working element having a proximal profile and the shaft extending proximally from the working element and a retrieval cover slidably carried along the shaft of the medical device, the cover having a deployed configuration and being capable of being compressed into a compressed configuration for deployment, yet resiliendy substantially return to the deployed configuration; the cover in its deployed configuration having a radially reduced proximal portion, a distally open distal end defining a distal opening having a maximum dimension at least as great as the maximum dimension of the proximal profile of the working element of the medical device, and an elongate internal recess defined between the proximal portion and the distal end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 8 - 12 schematically illustrate the operation of one embodiment of the present invention. FIG. 8 illustrates certain operative portions of the medical device retrieval system 10 of the invention in a fully deployed state. As noted above, the retrieval system of the invention is intended to be used in connection with a medical device having a working element carried by flexible, elongate shaft. In these drawings, the medical device is typified as a vascular trap similar to the vascular trap 250 of FIG. 11 , but omitting the tethers 290 . The working element of this medical device is a basket 12 , which may be substantially as outlined above in connection with the description of the basket 270 . The shaft in this design may simply comprise a guidewire 14 . While the construction and operation of the basket 12 may be substantially the same as that outline for the baskets shown in FIGS. 1 - 5 , it is generally preferred that the proximal band 13 of this basket be attached to the guidewire while the distal band 15 be permitted to slide along the guidewire. Hence, when the basket 12 is released from a delivery catheter and the basket is allowed to achieve a radially expanded configuration, the distal end of the collapsed device ( 272 in FIG. 1A ) will slide approximately toward the proximal end ( 274 ) of the collapsed device. It should be recognized that the medical device can be varied as desired. For example, the medical device used in connection with the present retrieval system could instead by a balloon catheter, wherein the working element would be the balloon portion of the catheter and the shaft would comprise the body of the catheter extending proximally of the balloon. The other elements of the retrieval system 10 generally comprise a retrieval sheath 20 and a cover 30 . It is to be understood that these drawings are intended merely for illustrative purposes and are not drawn to scale. In actual operation, the retrieval sheath 20 and the shaft 40 of the cover would likely be much smaller. These elements are simply drawn larger to make the various components easier to see in the attached illustrations. The cover 30 includes a radially expandable body 31 carried by a shaft 40 . The body has a proximal portion 32 which is radially compressed into close proximity with the shaft 40 and is desirably attached directly thereto. A tubular wall 34 extends distally from the proximal portion and terminates in a distally open end 36 . The body 31 defines a recess 38 within which the working element of the medical device may be retracted, as explained more fully below. The majority of the length of this recess is defined by the generally tubular wall 34 . This radially expandable body 31 can be formed of any suitable material. As explained more fully below, it is preferred that this body be capable of being collapsed within the retrieval sheath 20 for deployment, radially expand into a deployed configuration, yet be readily collapsed by the retrieval sheath to tightly engage the working element of the medical device. Any material which achieves this function may be used. In one embodiment (not shown), the body 31 is formed of a flexible plastic material, which may be reinforced with one or more flexible metal hoops or the like to bias the tubular plastic member into a funnel-like configuration. The illustrated embodiment is shown as comprising a series of flexible metal wires. As explained in some detail in Intemational Publication No. WO 96/01591, such a radially expandable device may be made rather conveniently utilizing a metal fabric having strands formed of a material which is both resilient and which can be heat treated to substantially set a desired shape. Materials such as elgiloy, hastelloy, incoloy, certain grades of stainless steel and shape memory alloys. Of these materials, shape memory alloy such as nitinol are particularly preferred. In one useful embodiment, the radially expandable body 31 is formed using the techniques outlined in WO 96/01591, starting with a metal fabric comprising both nitinol and platinum. For example, the fabric may be a generally tubular fabric formed of 48 wires having a diameter on the order of about 0.0015 inches and a pic rate of about 80-100 pics per inch. Of the 48 wires used to form this metal fabric, a relatively small percentage of the wires (e.g. 4-6 wires) may be formed of platinum or some other relatively radiopaque material to enhance visibility of the device on a fluoroscope without unduly affecting the resiliency of the fabric. If so desired, the wires can be coated with a therapeutic agent or with an antithrombogenic material. For example, the wires may be coated with heparin or with a known platelet-deactivating drug, e.g., a 2B-3A antagonist. This radially expandable body 31 is carried by a axially slidable shaft 40 . This shaft may take the form of a metallic hypotube, such as that discussed in connection with the embodiment of FIGS. 3 - 5 . More preferably, though, the shaft 40 comprises a flexible plastic material of the type that is commonly used in forming medical catheters. If friction of this shaft 40 with the retrieval sheath 20 and/or the shaft 14 of the medical device is anticipated to present a problem, this shaft 40 of the cover may be formed of polytetrafluoroethylene or another suitable low-friction material. The radially expandable body 31 may be attached to the shaft 40 in any suitable manner. Presumably, the ends of the wires defining the body 31 could be simply cast into the plastic defining the flexible shaft 40 . However, the embodiment shown in the drawings is somewhat easier to make, utilizing a pair of marker bands 46 and 48 to attach the body to the shaft by clamping the proximal end about the exterior of the sheath. Forming these clamps of a radiopaque material will make it easier to track the position of the cover 30 as it is deployed. In the illustrated embodiment, the cover comprises an exterior layer and an interior layer of the metal fabric, much like the basket 270 described above in connection with FIGS. 1 - 5 . In this configuration, the proximal marker band 46 may be used to clamp the exterior layer of the metal fabric to the exterior of the shaft 40 while the distal marker band 48 is used to clamp the interior layer of the fabric to the shaft, which the shaft 14 of the medical device is received, thereby permitting the cover 30 to track that shaft for deployment. The shaft 40 shown in FIGS. 8 and 9 extends distally beyond the distal marker band 48 such that the distal tip 42 of the shaft is received within the recess 38 of the cover. Not only will this make manufacturing easier, but it will reduce the likelihood that any guidewire or other device passing through the lumen 44 of the shaft 40 will get caught up in the metal fabric defining the radially expandable body 31 . The retrieval sheath 20 may simply take the form of a standard medical catheter, with a tip as described below. This sheath has a generally tubular wall defining a lumen 24 within which the shaft 14 of the medical device and the shaft 40 of the cover may be slidably received. The differences in the diameters of these three elements 20 , 40 and 14 are exaggerated in FIGS. 8 and 9 to illustrate operation of the device. In reality, these diameters would likely be substantially closer than those shown. The distal tip 22 of the retrieval sheath 20 may be beveled to produce a smoother tip. (The advantage of this tip construction will be highlighted below in connection with the discussion of FIGS. 7, 13 and 14 .) If so desired, a marker band 26 may be incorporated into the wall of the retrieval sheath 20 adjacent the distal tip 22 . This will help an operator visualize the relative position of the retrieval sheath 20 , the cover 30 and the basket 12 during operation. FIG. 9 is a schematic cross sectional view of the device illustrated in FIG. 8 prior to deployment of the cover. In operation, the medical device will typically be put in place first. As outlined above in connection with FIGS. 1 and 2 , the basket 12 may be positioned distally of a particular treatment site and the treatment device (e.g. a balloon catheter or an atherectomy device) can be guided over the shaft 14 of the trap to perform the intended procedure. In the use of the retrieval system of the invention with such a trap, one would typically deploy the retrieval sheath 20 and the cover 30 after the basket 12 has been in place for some time rather than deploying all three elements at substantially the same time. It should be understood, though, that simultaneous deployment may be appropriate in other circumstances, such as when a cover 30 and retrieval sheath 20 are used in connection with a Foley catheter or the like. Whereas FIG. 8 illustrates the cover in its deployed configuration, FIG. 9 illustrates the cover in a compressed configuration which is suitable for deployment. Even in its compressed configuration, it can be seen that the body 31 of the catheter generally includes a radially reduced proximal portion 32 , an elongate tubular wall 34 and a distally open distal end 36 which defines the distal-most portion of the cover. This is indirect contrast to the structure shown in FIGS. 3 and 4 , which show the cover 340 of that device in its collapsed state. In this collapsed state, the cover 340 has a distal segment 352 and a proximal segment 354 , both of which are generally tubular in shape and lie proximate the exterior surface of the guide wire 310 . Once this cover is deployed as shown in FIG. 5 , though, the cover must invert on itself to position the distal section 352 generally within the proximal section 354 to define a distal lip 358 of the cover. This distal lip 358 is merely an intermediate point along the longer, axially expanded configuration of the device when it is collapsed, as shown in FIGS. 3 and 4 . There are a number of advantages of the structure of the present cover 30 over the mechanically more complex design of FIGS. 3 - 5 . In the cover 340 of FIGS. 3 - 5 , the cover must invert on itself before it can be used to enclose the basket 320 . The resilient nature of the metal fabric used to form the cover 340 will tend to resiliently draw the distal hypotube 342 proximally toward the proximal control hypotube 344 once the constraint of the deployment catheter C has been removed. The walls of the vessel can hinder complete inversion of the cover 340 , though. In particular, if the inner diameter of the vessel within which the cover is to be deployed is significantly smaller than the outer diameter of the fully deployed cover, the cover may take on a sausage-like configuration, with the distal and proximal segments 352 , 354 of the cover expanding into engagement with the wall of the vessel, but being unable to expand sufficiently to allow the distal hypotube to invert the distal segment 352 so that it may be received within the proximal section 354 . In such a circumstance, the cover will not define a suitable recess for receiving the basket 320 therein. The design shown in FIGS. 8 - 12 does not require that the radially expandable body 31 invert on itself to reach its fully deployed configuration. Instead, the recess 38 will always remain in place. Deployment of the body 31 distally beyond the distal tip 22 of the retrieval sheath will simply allow this recess to expand to a size wherein it may readily receive the working element of the medical device with which the cover is used. While FIGS. 8 and 9 schematically illustrate the structure of the device and its various elements, FIGS. 10 - 12 are intended to schematically illustrate the manner in which the cover 30 may be used to retrieve a basket 12 which is full of emboli or other particular material. In FIG. 10 , the deployment catheter C (discussed above in connection with FIGS. 1 - 5 ) is shown extending into the lumen of the vessel and terminating proximally of the position of the basket 12 . The cavity of the basket 12 is filled with emboli E. If one were to simply pull the guidewire 12 proximally, this will tend to evert the generally umbrella-shaped basket 12 , raising the possibility that the emboli E could be dumped into the bloodstream of the vessel. In FIG. 10 , the retrieval sheath 20 is positioned proximally of the basket 12 , leaving a space between the distal tip 22 of the sheath 20 and the basket 12 . In this Figure, the cover 30 is still within the lumen 24 of the retrieval sheath 20 , much as in the configuration shown in cross section in FIG. 9 . Once the retrieval sheath, with the cover retained therein, is properly positioned, the shaft 40 of the cover 30 may be advanced distally with respect to the sheath 20 . This may be accomplished either by holding the sheath 20 stationary and advancing the shaft 40 of the cover distally or by holding the shaft 40 of the cover relatively stationary and withdrawing the retrieval sheath 20 proximally to expose the readily expandable body 31 beyond the distal tip 22 of the sheath 20 . When the body 31 of the cover exits the distal end of the retrieval sheath 20 , it will tend to resiliently substantially return to the configuration schematically illustrated in FIG. 8 . Unlike the cover 340 of FIGS. 3 - 5 , the body 31 of the present invention will begin to radially expand into its final shape as soon as the distal end 36 clears the distal tip 22 of the sheath 20 . Accordingly, there is no need to deploy the cover 30 so that even the proximal marker band 46 is positioned distally of the distal tip 22 of the retrieval sheath as shown in FIG. 8 . Instead, the proximal portion 32 of the body 31 may remain within the lumen of the retrieval sheath 20 , as suggested in FIG. 11 , without compromising operation of the cover 30 . FIG. 11 illustrates the device wherein the cover has been sufficiently deployed to define a recess large enough to receive the body of the basket 12 therein. To achieve the configuration shown in FIG. 11 , the shaft 14 of the vascular trap is withdrawn proximally, drawing the basket 12 within the enclosure 38 . As noted above in connection with FIG. 8 , the presently preferred embodiment of such vascular trap employs a proximal band 13 which attaches a proximal end of the metal fabric defining the basket directly to the shaft of the guidewire 14 while the distal connector 15 is allowed to slide along the length of the shaft 14 . Accordingly, when the operator pulls proximally on the guidewire 14 , this will tend to elongate the trap and cause it to evert. In the absence of aspiration or a cover 30 , this could present some difficulties. Prior to withdrawing the shaft 14 proximally, the distal end 36 of the cover is desirally brought immediately adjacent the basket 12 . In a preferred embodiment, the distal end 36 of the body 31 of the cover defines a distal opening having a maximum dimension which is at least as great as the maximum dimension of the proximal profile of the basket 12 , i.e., the maximum dimension of the proximal projection of the deployed basket. If the vessel is large enough, this would permit the cover to simply slide around the basket 12 without significantly stressing the basket and causing it to collapse in any way. More likely than not, though, there will be insufficient clearance between the basket 12 and the wall of the vessel to permit the cover to readily slide between the vessel and the basket. Accordingly, the distal end of the cover will typically be brought into engagement with a surface of the basket 12 . This will form between the cover and the basket and enclosure that includes both the cavity of the basket and the recess 38 of the cover. This movement of the cover distally into engagement with the medical device may be achieved either by actually physically moving the cover distally in an absolute sense, or simply withdrawing the basket 12 toward the cover which will effectively move the cover distally with respect to the medical device. FIG. 11 illustrates the relative positions of the elements of the invention if the operator continues to withdraw the guidewire 14 proximally after the cover initially engages the surface of the basket 12 . The basket has started to evert into a more oblong shape rather than the umbrella-shape shown in FIG. 10 . Nonetheless, the emboli still are retained within the enclosure defined by the cover and the basket. In one preferred embodiment, the body 31 of the cover is at least as long as the working element of the medical device which is to be retrieved therewith. This permits the working element to be entirely enclosed by the cover during the retrieval process, enhancing the likelihood of a successful retrieval without inadvertent dumping of the matter captured by the medical device back into the patient's body. While the cover can be little longer than the working element of the medical device, it is anticipated that the cover may be significantly longer than that working element. This will permit an operator greater flexibility in using the device without adding unduly to the cost. FIG. 12 schematically illustrates the next stage of the method of removing the medical device from the patients vascular system. In this view, the retrieval sheath has been moved distally with respect to the cover. As suggested above, this may be achieve either by moving the retrieval sheath distally along the cover or by withdrawing the cover (and, optimally, the medical device) proximally while holding the retrieval sheath 20 stationary. Urging the retrieval sheath distally with respect to the cover urges the cover to collapse about the medical device received therein. This causes the cover to tightly engage the surface of the medical device, helping better encase any particular matter received within the enclosure and limit the likelihood that it may spill back into the patient's vascular system. It also presents the device with a radially reduced profile, making it easier to withdraw the device from the patient's body without undue trauma. Looking at the device in FIG. 12 , the system has a particular configuration which is unique to the present invention. In this configuration, the working element of the medical device is completely retained within the recess 38 of the body 31 of the cover such that the distal end 36 of the cover 30 is positioned distally beyond the distal end of the working element 12 . In FIG. 12 , at least a proximal length of the basket 12 and the body 31 of the cover are retained within the lumen of the retrieval sheath 20 . This retrieval sheath radially compresses the proximal length of the cover such that an intermediate portion of the generally tubular wall 34 of the body 31 tightly engages a surface of the basket 12 . If so desired, the cover 30 and basket 12 may be further retracted so that they are both completely enclosed within the lumen of the retrieval sheath 20 prior to withdrawing the device from the patient's vessel. This is not necessary for effective operation of the current device, though, and may be left up to the physician's choice during the procedure. It should also be noted that the configuration shown in FIG. 12 may be further collapsed by withdrawing the basket 12 , cover 30 and retrieval sheath 20 proximally into the deployment catheter C, thereby further encasing the emboli and making it easier to withdraw the device from the vascular system. FIG. 7 illustrates one problem which could be encountered in deploying a medical device retrieval system 10 of the invention across a vascular obstruction. The vascular obstruction in FIG. 7 is typified as a stent 4 having a stenotic lesion 6 partially occluding the lumen thereof, but this is selected merely for illustration. Much the same problem could also be encountered with a variety of other vascular obstructions. The illustrated deployment sheath 20 has a blunt distal tip 22 ′. Due to the curvature of the vessel where the stent is located, the retrieval sheath tends to drift upwardly toward the outside of the curve rather than easily tracking the shaft 14 of the medical device through the center of the vessel. This problem becomes even more pronounced if the retrieval sheath is made stiffer, such as by incorporating metallic braid into the wall of the sheath, to improve pushability. In some instances, it can take undue time and effort to manipulate the distal tip of the retrieval sheath to clear the obstruction. In addition, use of excess force or movement of the sheath to clear the obstruction risks displacing the working element (not shown) of the medical device from the treatment site where it has been deployed. FIGS. 13 and 14 illustrate two proposed solutions to ameliorate these deployment difficulties. A first solution is illustrated in FIG. 13 while FIG. 14 illustrates another improvement which may be used alone or in conjunction with the device of FIG. 13 . Turning first to FIG. 13 , the retrieval sheath 20 shown therein includes a deployment stylet 70 slidably received in the lumen 24 thereof. This stylet has a lumen 75 within which the shaft 14 of the medical device is received, permitting the stylet to slide along that shaft 15 with the retrieval sheath 20 . The stylet 70 is provided with an elongate tubular body 72 and a tapering distal tip 74 . In use, the body 72 of the stylet desirably extends along the entire length of the retrieval sheath so that the proximal end of the sheath (not shown) extends proximally beyond the proximal end of the retrieval sheath so an operator may selectively control the stylet independently of the guide wire and of the retrieval sheath. The distal tip 74 of the stylet tapers from its proximal end 76 to its distal end 78 . At its proximal end, the distal tip has an outer diameter which approximates the diameter of the lumen 24 of the retrieval sheath at the distal end 22 thereof. As illustrated, it is not intended that the stylet 70 completely fill the lumen 24 of the sheath as that would lead to undue friction in moving the stylet relative to the sheath. The outer diameter of the sheath at the proximal end 76 of the tip 74 need only be close enough to the diameter of the distal lumen of the sheath 20 to avoid a sharp, traumatic change in diameter which would be likely to catch on vascular obstructions and hinder deployment of the sheath 20 in the vessel. The transition from the distal tip 74 of the stylet to the outer diameter of the sheath 20 can be further eased by proving the distal tip 22 of the sheath 20 with a beveled distal end. The distal end 78 of the stylet's distal tip 74 has an outer diameter which more closely approximates the outer diameter of the medical device shaft 14 . It is not expected that this distal end 78 be infinitely thin and track directly against the surface of the shaft 14 . Again, it is sufficient that the distal end 78 of the stylet be close enough to the diameter of the shaft 14 of the medical device to avoid a sharp, traumatic change in diameter which would be likely to catch on vascular obstructions and hinder deployment of the sheath 20 in the vessel. When the stylet is deployed such that its distal tip 74 extends distally beyond the distal tip 22 of the retrieval sheath, the stylet provides a transition between the shaft 14 of the medical device and the distal end of the retrieval sheath 20 . This makes it easier to track the shaft 14 and guide the device into position across a vascular obstruction. FIG. 13 illustrates the stylet positioned such that the proximal end 76 of the distal tip 74 is positioned immediately adjacent the distal tip 22 of the retrieval sheath, but this is not necessary. If the body 72 of the stylet has a substantially constant diameter over the relevant length, the stylet can be moved distally relative to the sheath 20 such that the body extends beyond the distal end of the sheath. This will not case any undue problem as the outer diameter of the body is desirably substantially the same as the outer diameter of the proximal end 76 of the distal tip. Use of the retrieval sheath 20 with the stylet 70 can be varied. If so desired, one can use the stylet in each and every deployment of the retrieval system of the invention. However, as outlined below, use of the stylet adds an additional step to the retrieval process and its use may be reserved for those circumstances where the operator either expects to encounter a vascular obstruction or has already encountered such an obstruction. In use, the stylet 70 and the cover 30 are exchangeable for one another, i.e., either the stylet or the cover may track along the shaft 14 within the lumen 24 of the retrieval sheath, but both cannot be used at the same time. Instead, one must be removed and replaced with the other. If the operator anticipates a vascular obstruction (or he or she wants to avoid exchanging devices twice if an obstruction is encountered), he or she can initially deploy the sheath 20 with the stylet. This may be accomplished by positioning the stylet 70 with respect to the sheath 20 such that the distal tip 74 of the stylet extends distally beyond the distal tip 22 of the sheath. Optimally, both the stylet and the sheath are advanced together along the shaft 14 until the distal tip 22 of the sheath is in a desired position with respect to the working element of the medical device. (In most circumstances, this will be at a location wherein the distal tip of the sheath is near the working element, but spaced proximally therefrom, as discused above in connection with FIG. 11 .) Once the sheath is in position, the stylet 70 may be exchanged for the cover 30 . This may be done in much the same fashion that catheters are exchanged in a typical balloon angioplasty procedure or the like. In most circumstances, an exchange wire will be attached to the proximal end of the shaft 14 of the medical device and the stylet 70 can be retracted proximally onto the exchange wire. Thereafter, the exchange wire can be disconnected and the cover may be advanced along the shaft 14 through the lumen 24 of the retrieval sheath. Using the marker band 26 of the retrieval sheath and the marker band 13 of the basket 12 (for example), any final adjustments to the position of the sheath with respect to the working element of the medical device can be made prior to deployment of the cover. The cover may then be moved distally with respect to the sheath 20 , either by distally advancing the cover or proximally retracting the sheath. As noted above, this permits the body 31 of the cover to radially expand into a deployed configuration wherein the distal end remains distally open and the enclosure is radially expanded. The cover may then be moved distally with respect to the working element of the medical device and into engagement with a surface of the medical device to form therebetween an enclosure. Optimally (but not necessarily, depending on the configuration of the medical device and the shape of the cover), the cover is advanced further with respect to the working element until the entire working element is effectively received in the recess 38 of the cover. Thereafter, the retrieval sheath is moved distally with respect to the cover to urge the cover to collapse about the working element and tightly engage the surface of the working element to retain any debris in the enclosure. FIG. 14 illustrates another improvement of the sheath 20 of the invention. In this embodiment, a distal length 21 of the sheath 20 is bent at an angle with respect to the body of the sheath. If a vascular obstruction is encountered, this distal bend will permit the operator to clear the obstruction by reorienting the sheath so that the distal tip 22 thereof is spaced toward the center of the vessel and away from the obstruction, whereupon the sheath can be further advanced. An angle of between about 5 and about 30° is believed to be sufficient for most purposes without unduly interfering with the proper deployment and retrieval of the cover 30 . The length of the distal length 21 can be varied as needed. In most circumstances, it is envisioned that the distal length 21 will be 5 cm or less, with a length of 1 cm to 3 cm being most likely. As noted previously, the sheath 20 of FIG. 14 with its bent distal length 21 may be used instead of or in conjunction with the stylet 70 shown in FIG. 13 . While a preferred embodiment of the present invention has been described, it should be understood that various changes, adaptations and modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.