Abstract:
A rotational ablation atherectomy device including a flexible drive shaft and a compressible burr that may be inserted and extracted from a patient using a catheter having a diameter that is smaller than the operational diameter of the burr. In one embodiment, the burr includes a nose portion coupled to the drive shaft and one or more flexible abrasive disks disposed rearwardly from the nose portion. The flexible disks are foldable to be slidably received within a catheter. In another embodiment, the burr includes a support member coupled to the drive shaft, the support member having a resilient panel that spirals outwardly, forming a generally cylindrical ablation surface. The flexible panel can be elastically urged toward the support member and slidably inserted into the catheter. In a third embodiment, the burr includes a plurality of struts that are coupled to the drive shaft. An elastically compressible body disposed between the struts permits the struts to flex inwardly to reduce the burr diameter. In another embodiment, the burr includes a plurality of flexible wires attached at proximal and distal ends to the drive shaft. An abrasive sheath is disposed over the wires. The wires can be bent inwardly to compress the burr and re-expanded by rotation of the burr. In another embodiment, the burr comprises a nose portion and a resilient shell having a compressible, larger diameter abrasive section disposed at the proximal end of the nose portion.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to medical devices in general and, in particular, to atherectomy devices for removing occluding material from a patient&#39;s vessels.  
         BACKGROUND OF THE INVENTION  
         [0002]    A number of vascular diseases, such as arteriosclerosis, are characterized by the buildup of deposits (atheromas) in the intimal layer of a patient&#39;s blood vessels. If the atheromas become hardened into calcified atherosclerotic plaque, removal of the deposits can be particularly difficult. Deposits in the vasculature can restrict the flow of blood to vital organs, such as the heart or brain, and can cause angina, hypertension, myocardial infarction, strokes, and the like.  
           [0003]    To treat such diseases, many invasive and noninvasive techniques have been developed. For example, cardiac bypass surgery is now a commonly performed procedure whereby an occluded cardiac artery is bypassed with a segment of a healthy blood vessel that is obtained from elsewhere in the body. While this procedure is generally successful, it is traumatic to the patient because the entire chest cavity must be opened to access the site of the occluded vessel. Therefore, the procedure is not often performed on elderly or relatively frail patients.  
           [0004]    As an alternative to cardiac bypass surgery, numerous atherectomy devices have been developed for removing such deposits in a less invasive manner. One such device that is particularly suited to removing calcified atherosclerotic plaque is an ablative rotational atherectomy device, such as that disclosed in U.S. Pat. No. 4,990,134 by Auth. Auth teaches using a small burr covered, or partially covered, with an abrasive cutting material, such as diamond grit, to remove the occluding deposit by ablation. A rotational atherectomy device practicing the Auth invention is sold by the assignee of the present invention under the trademark Rotablator™.  
           [0005]    To perform the atherectomy procedure, a guide catheter is inserted into the patient, frequently at the femoral artery, and advanced through the patient&#39;s vasculature until the distal end of the guide catheter is located near a target occlusion. A guide wire is then inserted through the guide catheter and advanced past the occlusion. An atherectomy device having a flexible drive shaft attached to a small abrasive burr is then advanced through the guide catheter and over the guide wire to the point of the occlusion. The burr is then rotated at high speed and advanced through the occlusion to remove the deposit. The ablative process produces particles that are sufficiently small such that they will not re-embolize in the distal vasculature. As the burr removes the occlusion, a larger lumen is thereby created in the vessel, thereby improving blood flow through the vessel.  
           [0006]    It is well recognized that the risk of certain patient complications increases with the size of the guide catheter through which minimally invasive devices are routed. Larger guide catheters require larger access holes in the femoral artery, creating the potential for patient complications, such as the sealing of the puncture site after completion of the procedure. Therefore, physicians generally wish to utilize the smallest possible guide catheter during a procedure. However, the smaller size guide catheters can only accommodate correspondingly smaller sized ablation burrs. Therefore, if a large vessel is to be treated, a larger burr and larger guide catheter must be used to successfully remove all of the occlusion from the patient&#39;s vessel.  
           [0007]    In addition, existing ablation burrs are rigid, having a fixed diameter, and may require undesirably large forces to traverse larger occlusions. Therefore, currently many procedures are performed using multiple passes through the occlusion with ablation burrs of increasing diameter. While these procedures have proven effective, the use of multiple devices for a single procedure adds both time and cost to the procedure. Expandable rotational ablation burrs have been developed, such as those disclosed in U.S. Pat. No. 6,096,054, which is assigned to the assignee of the present invention. It is sometimes desirable, however, that the ablation burr have a fixed, well-defined maximum operating diameter. Expandable ablation burrs may have a maximum operating diameter that is a function of the rotational speed of the burr, or otherwise not provide sufficient dimensional stability for specific applications.  
           [0008]    Given these desired operating characteristics, there is a need for an atherectomy device having a burr with a predictable, well-defined maximum operating diameter that can treat large occlusions without requiring multiple burrs and that can be routed to the occlusion site using a relatively small diameter guide catheter.  
         SUMMARY OF THE INVENTION  
         [0009]    The invention disclosed herein is an atherectomy device utilizing a compressible burr, whereby the compressible burr can be advanced to and withdrawn from the site of an occlusion using a guide catheter having a diameter that is smaller than the operational diameter of the burr. Because the compressible burr expands in situ to its operational maximum diameter, a single burr can be used to ablate moderately thick occlusions, eliminating the need to use multiple burrs with graduated diameters.  
           [0010]    According to a first embodiment of the invention, the atherectomy device includes an ablation burr attached to a drive shaft with a support member, the burr having at least one foldable, annular abrasive disk attached to the support member, and an abrasive nose member disposed forwardly from the support member, such that the ablation burr can fit within a guide catheter in a folded configuration.  
           [0011]    In one aspect of the first embodiment, the foldable, annular disk has a plurality of radial cuts that extend from the edge of the disk part way towards the center. The radial cuts divide the annular disk into a plurality of disk segments that facilitate folding of the disk.  
           [0012]    According to a second embodiment of the invention, the compressible burr comprises an elongate support member attachable to the drive shaft and a radially extending panel attached to the support member that extends in a spiral fashion outwardly from the support member. The panel is elastically compressible such that the panel can be elastically urged toward the support member.  
           [0013]    In one preferred aspect of the second embodiment the panel includes a decreasing-diameter proximal portion that provides a ramp whereby retraction of the burr into the catheter will urge the panel toward a compressed configuration.  
           [0014]    According to a third embodiment of the invention, the compressible burr comprises a hub fixedly attachable to the drive shaft having a plurality of flexible struts attached thereto. A compressible body substantially fills the volume created by the interior of the struts. The struts have an abrasive outer surface. The struts can flex inwardly to elastically compress the compressible body.  
           [0015]    In one preferred aspect of the third embodiment, the struts comprise a generally convex back portion that form an increasing diameter portion of the burr and a generally concave forward portion that form a decreasing diameter portion of the burr.  
           [0016]    According to a fourth embodiment of the invention, the compressible burr comprises a plurality of plastically deformable wires that are attached to the drive shaft in spaced-apart fashion at a distal end, and a flexible sheath having an ellipsoidal volume that encloses the plurality of wires. A portion of the outer surface of the flexible sheath is coated with abrasive particles, such as diamond particles, to produce an ablative surface. The plurality of wires can be deformed inwardly to decrease the diameter of the burr, and are selected to expand on spin-up of the burr, thereby inflating the sheath to its predetermined ellipsoidal shape, or designed to expand to size when released from a guide catheter, into which it may be withdrawn.  
           [0017]    According to a fifth embodiment of the present invention, the compressible burr comprises a nose portion having an ablative leading surface, wherein the nose portion is attached to the drive shaft, and a resilient shell extends proximally from the nose portion. The resilient shell includes a compressible center portion having an abrasive outer surface. In one preferred aspect of the fifth embodiment, the shell includes a back portion that slidably engages the drive shaft such that when the center portion is compressed the back portion can move proximally. In one version of the fifth embodiment, the shell includes a back portion that is attached to the drive shaft, and has an elongate member extending forwardly to the nose portion. The center portion is open in the back and coaxially surrounds the elongate member of the back portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:  
         [0019]    [0019]FIGS. 1A, 1B and  1 C illustrate a compressible atherectomy burr according to a first embodiment of the present invention;  
         [0020]    [0020]FIGS. 2A and 2B illustrate a compressible atherectomy burr according to a second embodiment of the present invention;  
         [0021]    [0021]FIGS. 3A, 3B, and  3 C illustrate a compressible atherectomy burr according to a third embodiment of the present invention;  
         [0022]    [0022]FIGS. 4A, 4B,  4 C, and  4 D illustrate a compressible atherectomy burr according to a fourth embodiment of the present invention; and  
         [0023]    [0023]FIGS. 5A, 5B,  5 C, and  5 D illustrate two compressible atherectomy burrs according to a fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    As explained in further detail below, the present invention is an atherectomy device having an ablation burr that can be compressed to a smaller diameter to facilitate insertion and removal of the ablation burr, but will expand to a fixed diameter during the atherectomy procedure. Referring now to the drawings, the compressible atherectomy burr of the present invention will be described.  
         [0025]    [0025]FIGS. 1A, 1B, and  1 C illustrate a first embodiment of an atherectomy burr according to the present invention, wherein the burr  100  is attached to the end of a flexible drive shaft  90  that is disposed within a guide catheter  80 . The burr  100  has a nose portion  102  with an abrasive leading surface  104 . The abrasive leading surface  104  may be formed by affixing abrasive particles to the nose portion  102  or by making the nose portion  102  from a hard material, such as stainless steel, and machining or otherwise affecting an abrasive topography onto the surface of a hard material.  
         [0026]    At least one annular flexible disk  110  is located behind or proximal to the nose portion  102 . Three flexible disks  110  are shown in the preferred embodiment. The flexible disks  110  are made of polyurethane or other tough, flexible polymer, and have a center hole  112  that is approximately equal in diameter to the diameter of the drive shaft  90 , and the flexible disks  110  slidably engage the drive shaft  90 . A plurality of cylindrical spacers  106  are slidably inserted between the flexible disks  110 , maintaining them in the desired spaced-apart relation. The flexible disks  110  are fixedly connected to the drive shaft  90  such that rotation of the drive shaft  90  will cause the flexible disks  110  to rotate correspondingly. The details of the connection between the flexible disks  110  and the drive shaft  90  are not critical to the present invention, and may be accomplished in a variety of ways. For example, the flexible disks  110  can be welded, brazed or glued to the drive shaft  90 , or attached to the cylindrical spacers  106 , which are then affixed to the drive shaft. Alternatively, the end portion of the drive shaft  90  could be provided with a keyed (noncircular) shape, and the center hole  112  made to match the keyed shape. Other methods of rotationally coupling the flexible disks  110  to the drive shaft  90  are well known in the art, and within the scope of the present invention.  
         [0027]    The flexible disks  110  include a forward surface  114  having an abrasive portion  116  that preferably extends generally to the outer edge of the flexible disks  110 . The abrasive portion  116  may be formed, for example, by affixing abrasive particles, such as diamond particles, to selected portions of the forward surface  114 . Diamond particles may be attached to the forward surface  114  with an adhesive or a plating process, for example. In the preferred embodiment, the flexible disks  110  include a plurality of radial slots  118  that extend from the outer edge of the disks  110  part way to the center hole  112 . The radial slots  118  divide the outer portion of the flexible disks  110  into a number of disk segments  120 . The radial slots  118  may optionally terminate with a small hole  122 , the small hole relieving the stress at the end of the slot  118  and decreasing the force required to bend the disk segments  120 .  
         [0028]    As seen most clearly in FIG. 1B, the flexible disks  110  are intended to deform, or fold over, to be slidably insertable into the guide catheter  80 . The guide catheter  80  may include an expanded or fluted portion  85  at its distal end to accommodate the burr  100  with the folded flexible disks  110 . The burr  100  can then be inserted to the location of the occlusion that is to be treated using a guide catheter  80  having a diameter that is smaller than the diameter of the unfolded burr  100 . The catheter  80  can then be pulled back (or the drive shaft  90  pushed forward), releasing the burr  100  and permitting the flexible disks  110  to unfold to their full diameter. It will be appreciated that the flexible disks  110  have a well-defined maximum diameter that will not be significantly effected by spinning the drive shaft  90  at high rotation speeds. After the atherectomy procedure is completed, the drive shaft  90  can be pulled back into the distal end of the guide catheter  80  to fold the flexible disks  110  in order to remove the burr  100  from the patient&#39;s vasculature.  
         [0029]    It may be desirable to coat the back surfaces  115  of the flexible disks  110  and/or an inner surface  83  of the guide catheter  80  with a hydrophilic coating, such as Hydropass™, available from Boston Scientific and described in U.S. Pat. No. 5,702,754. The hydrophilic coating attracts water molecules, thereby making the surfaces slippery, facilitating insertion and removal of the burr  100  into the catheter  80 . In addition, the hydrophilic coating may be beneficial during ablation since less torque may be transferred to a vessel wall if the burr stalls. In addition, the differential cutting ability of the burr may be enhanced due to the increased ability of the burr to slide over soft tissues.  
         [0030]    It will be appreciated that in addition to the advantages associated with insertion and removal of the burr  100 , there may be further advantages of the flexible disks  100  during the atherectomy procedure. For example, the abrasive portions  116  are nominally oriented forwardly in the treated vessel, avoiding or minimizing undesirable contact between the abrasive portion  116  and the vessel wall. As the abrasive disks  110  encounter hardened occlusions in the vessel, forward motion of the flexible drive shaft  90  will cause the flexible disks  110  to bend backwardly, rotating the abrasive portions  116  toward the occlusion, thereby naturally enhancing the ablative action at the location of the hardened occlusion. Although this embodiment has been described and illustrated with three flexible disks  110 , it will be appreciated that more or fewer flexible disks  110  may be used to accommodate the needs of a particular application, and would be within the scope of the present invention. It will also be appreciated that the flexible disk  110  could be made without the radial slots  118 , thereby increasing the stiffness of the flexible disk  110 , while still permitting it to deform into a folded condition for insertion and removal.  
         [0031]    A second embodiment of a compressible burr according to the present invention is shown in FIGS. 2A and 2B. The burr  200  includes a centrally located cylindrical portion  210  that is fixedly and generally coaxially connected to a drive shaft  90  such that rotation of the drive shaft  90  will cause the burr  200  to rotate. The drive shaft  90  is covered over a substantial portion of its length with a guide catheter  80 , which, in the preferred embodiment, includes a fluted portion  85 . Although other attachment mechanisms are possible, in the preferred embodiment the central cylindrical portion  210  includes a center hole (not shown) through which the drive shaft  90  is inserted and fixedly attached using any suitable adhesive.  
         [0032]    A thin panel of flap portion  220  extends radially outward from the central cylindrical portion  210  to form a generally circular cylindrical shell that partially surrounds the center cylindrical portion  210 . The outer edge  224  of the panel portion  220  is disposed radially away from the center cylindrical portion  210  to form an elongate gap  226  between the outer edge  224  and the center cylindrical portion  210 . The panel portion  220  is formed from a semi-rigid material, selected such that the panel portion  220  can be elastically compressed to close the gap  226 , thereby decreasing the diameter of the burr  200 .  
         [0033]    The panel portion  220  includes a forward segment  230  that has a constant axial cross section, and a back segment  232  that tapers radially inward. The taper of the back segment  232  provides a ramp such that when the drive shaft  90  is retracted into the catheter  80 , the tapered back segment  232  will slidably engage the lumen of catheter  80 . As the drive shaft  90  is pulled further back into the catheter  80 , the panel portion  220  will elastically compress thereby reducing the diameter of the burr  200  as it is pulled into the catheter  80 , for easier insertion and extraction of the burr  200 .  
         [0034]    The forward segment  230  of the panel portion  220  includes one or more abrasive sections  228  on its exterior surface, providing an ablative surface for the atherectomy procedure. The abrasive portion  228  may be formed, for example, by affixing abrasive particles, such as diamond particles, to selected portions of the outer surface. It may be desirable to coat the back segment  232  of the panel portion  220  and/or the inner surface  83  of the fluted portion  85  of the guide catheter  80  with a hydrophilic coating to facilitate the retraction of the burr  200  into the catheter  80 . As will be appreciated, the burr  200  is rotated such that the edge  224  trails the movement of the burr. In the embodiment shown in FIG. 2B, the burr  200  is always rotated clockwise. However, the burr could also be constructed to rotate counterclockwise as desired.  
         [0035]    It is contemplated that this second embodiment of a burr  200  might also incorporate features of other atherectomy burrs described herein. For example, a smaller, forwardly facing nose portion, such as the nose portion  102  shown in FIG. 1A, could be added to the front of the burr  200  to produce a guide hole. Moreover, the panel portion  220  could include a tapered forwardmost segment (not shown) similar to the back segment  232 , but facing forwardly, to facilitate engagement of the occlusion. In particular, a tapered forwardmost segment could taper to generally meet the widest portion of a nose portion, to produce a substantially continuous, increasing diameter, ablative surface. Alternatively, the burr could have a forward nose not contiguous with the flap.  
         [0036]    A third embodiment of a compressible burr according to the present invention is shown in FIGS. 3A and 3B. The burr  300  includes a rear hub  310  that is fixedly connected to a drive shaft  90  such that rotation of the drive shaft  90  will cause the burr  300  to rotate. The drive shaft  90  is covered over a substantial portion of its length with a guide catheter  80 , that optionally includes a fluted portion at its distal end.  
         [0037]    The burr  300  includes a plurality of flexible struts  320 , each strut having a back portion  322  that is fixedly attached to the rear hub  310 , a forward portion  324  that extends forwardly from the back portion  322 , and a folded back portion  326 , that extends backwardly from the distal end  325  of the forward portion  324 . The plurality of flexible struts  320  are equally spaced around the perimeter of the hub  310 , cooperatively defining a volume therebetween. As seen most clearly in FIG. 3C, which shows a side view of an individual strut  320 , the back portion  322  is preferably longitudinally convex and includes a proximally extending tab portion  323  for attachment to the drive shaft  90 . The forward portion  324  is preferably longitudinally concave. The outer surface of the forward portion  324  is coated with diamond particles  327  to provide an abrasive surface.  
         [0038]    A compressible body  330 , such as a hollow elastomeric bladder, is provided within the volume defined by the interior of the flexible struts  320 . The flexible struts  320  are preferably attached to the compressible body  330 , such that the compressible body  330  will generally maintain the flexible struts  320  in a spaced-apart configuration, while also permitting longitudinal flexure of the struts  320 .  
         [0039]    The burr  300  can be fabricated, for example, by stamping or wire electro-discharge machining, the flexible struts  320  from a suitable metal, then welding the flexible struts  320  at a proximal end  321  to the rear hub  310 . A liquid injection molding process can then be used to create the compressible body  330  from silicone, or some other suitable material. Finally, any particulate abrasive, such as diamond particles can be attached to the forward portion  324  of the flexible struts  320 .  
         [0040]    It will be appreciated that the burr  300  can be deformed to a compressed state, as shown in FIG. 3B. The compressed state has a smaller maximum diameter than the relaxed, expanded state (shown in FIG. 3A). For example, a “pull-in” sheath  340  can be provided that slidably fits within the guide of catheter  80 . When the drive shaft  90  is pulled backwardly, the burr  300  will be pulled against the pull-in sheath  340 , such that the back portion  322  of the flexible struts  320  engage the sheath  340 . Pulling the drive shaft  90  further will result in an inward force on the compressible body  330  from the back portions  322  of the struts  320 , thereby permitting the sheath  340 , and burr  300  to be retracted into the guide catheter  80 . It will be appreciated that other means of compressing and retracting the burr  300  are also possible, including the use of a fluted catheter, as discussed above.  
         [0041]    A fourth embodiment of a compressible burr according to the present invention is shown in FIGS. 4A, 4B, and  4 C. The burr  400  is fixedly connected to a drive shaft  90  such that rotation of the drive shaft  90  will cause the burr  400  to rotate. FIG. 4A shows a side view of the burr  400  connected to a drive shaft  90 , and FIG. 4B shows a cross-sectional view of the burr  400 , taken along a axial center plane. The drive shaft  90  is covered over a substantial portion of its length with a guide catheter  80  that optionally includes a fluted portion at its distal end. The burr  400  includes a plurality of elongate flexible members or wires  410  (four shown in FIG. 4C), each wire  410  having a distal end  412  that is attached to the drive shaft  90 , and a proximal end  414  extending proximally from the distal end  412  that is also attached to the drive shaft  90 . The wires  410  are preferably equally spaced around the perimeter of the drive shaft  90 , and may attach directly to the drive shaft  90  or attach through an intermediate hub (not shown) that connects to the drive shaft  90 .  
         [0042]    A resilient sheath  420 , having a generally football shape or ellipsoidal shape, encloses the wires  410 . The resilient sheath  420  is attached to the drive shaft  90 , and may optionally also be attached to one or more of the wires  410 . The sheath  420  is thin and sufficiently flexible to collapse, or fold in on itself, and strong enough to provide the working surface for the burr  400 . An abrasive coating  430 , such as a coating including diamond particles, is applied to the forward portion of the sheath  420  in the manner described below. The sheath  420  may be attached to the wires  410 , for example, by use of an appropriate adhesive inside the sheath  420 . The burr  400  may be spun while the adhesive is drying, to keep the adhesive at the outer surface for bonding the wires  410  to the sheath  420 .  
         [0043]    The plurality of wires  410  provide a support for the sheath  420 , maintaining it in an uncompressed configuration, as shown in FIG. 4A, during the atherectomy procedure. To facilitate insertion and removal of the burr  400  through the vasculature of the patient, the burr  400  can be compressed by bending the wires  410  inwardly, as shown in FIG. 4D. The burr  400  can then be inserted through the guide catheter  80  to the site of the occlusion. The wires  410  are selected to have sufficient flexibility that upon spin-up of the burr for the ablation procedure, the wires  410  are forced outwardly by centrifugal forces, returning the burr  400  to the uncompressed configuration.  
         [0044]    Alternatively, the wires  410  may be made from a resilient elastically deformable material formed to maintain the burr in the desired shape (which may or may not be ellipsoidal), the elastically deformable material being able to elastically compress sufficiently to allow the burr  400  to be inserted through the guide catheter  80 , then elastically springing out to the desired shape when it is no longer constrained by the guide catheter  80 . Another alternative is to use a so-called shape memory alloy, such as NiTi, for the wires  410 . A shape memory alloy wire  410  is deformable to allow the burr to be compressed, but has a selectable preferred shape to which it will return (generally upon being heated).  
         [0045]    Two variations of a fifth embodiment of a compressible burr according to the present invention are shown in FIGS. 5A and 5C. The burr  500   a,    500   b  is rotatably coupled to a drive shaft  90  such that rotation of the drive shaft  90  will cause the burr  500   a,    500   b  to rotate. The burrs  500   a,    500   b  include nose portions  510   a,    510   b  having abrasive leading surfaces  512   a,    512   b  that taper in the distal direction. The abrasive leading surface may be formed, for example, by affixing an abrasive material such as diamond particles to the leading surfaces  512   a,    512   b  or by machining or otherwise roughing the leading surfaces  512   a,    512   b  to create an abrasive topography. A resilient shell  520   a,    520   b  is attached to back surfaces  514   a,    514   b  of the nose, for example, by use of an adhesive. Each resilient shell  520   a,    520   b  is shown most clearly in FIGS. 5B and 5C. A shell  520   a,    520   b  is generally axisymmetric, and includes a collapsible center portion  524   a,    524   b  that, in its uncollapsed state, has a greater outer diameter than the nose portion  510   a,    510   b.    
         [0046]    The shells  520   a,    520   b  may be made from any appropriate resilient material. In the preferred embodiment, a polyurethane polymer is used that having a low elasticity, so that the center portion  524   a,    524   b  will not stretch when the burr is rotated at high speeds. The center portion  524   a,    524   b  is provided with an abrasive outer surface  525   a,    525   b,  at least over the forward part of the center portion  524   a,    524   b.  The abrasive outer surface may be formed by affixing diamond particles, or other abrasive particles, to the center portion  524   a,    524   b  as described below.  
         [0047]    In the first variation of the burr  500   a,  shown in FIGS. 5A and 5B, the shell  520   a  includes a generally cylindrical proximal portion  522   a  extending backwardly from the center portion  524   a,  that is disposed coaxially around the drive shaft  90 . The proximal portion  522   a  is preferably not affixed to the drive shaft  90 , so that it can slide proximally or distally, to facilitate compression of the center portion  524   a.  A distal portion  526   a  of the shell  520   a  extends forwardly from the center portion  524   a  and is fixedly attached to the back surface  514   a  of the nose portion  510   a.  The nose portion  510   a  is attached to the drive shaft  90  such that rotation of the drive shaft will cause a corresponding rotation of the nose portion  510   a.  The distal portion  526   a  may optionally also have an abrasive outer surface. The resilient center portion  524   a  can be collapsed into the guide catheter (not shown) for easier insertion and removal of the burr  500   a,  and will expand to its uncompressed state as it is released from the guide catheter.  
         [0048]    In the second variation of the burr  500   b  , shown in FIGS. 5C and 5D, the shell  520   b  is formed in two parts. A proximal portion  522   b  is made from a hard material such as stainless steel. The proximal portion  522   b  includes a generally cylindrical back section  521   b  that is fixedly connected to the drive shaft  90 , and a smaller-diameter, elongate forward section  523   b  that extends coaxially forward. The nose portion  510   b  is attached to the distal end of the elongate forward section  523   b.  The nose portion may include an abrasive outer surface, similar to that described above. The resilient center portion  524   b  is attached to the back surface  514   b  of the nose portion  510   b.  The resilient center portion  524   b  has a maximum diameter that is greater than the diameter of the nose portion  510   b  that can be collapsed into the guide catheter (not shown) for easier insertion and removal of the burr  500   b.    
         [0049]    It will appreciated that collapsing these burrs  500   a,    500   b  aids in insertion and removal of the burrs into the patient&#39;s vasculature by permitting the use of a guide catheter having a smaller diameter than the working diameter of the burrs  500   a,    500   b  . Additionally, during the atherectomy procedure, as the burrs are rotated in the proximity of an occlusion, the resilient center portions  524   a,    524   b  will flex to accommodate restricted passageways in the patient&#39;s vessels that are causes by the occlusion. The resilient center portions  524   a,    524   b  and, in particular, the abrasive surfaces  525   a,    525   b  will provide a gentle, outward pressure on the occlusion, facilitating the ablative removal of the occlusion during the procedure, and the burrs  500   a,    500   b  will expand to the desired, predetermined maximum radius as the occlusion is removed.  
         [0050]    In the various embodiments of the preferred embodiment described above, where abrasive particles are to be affixed to a polymeric burr element, any suitable method of affixing the particles may be used. For example, in the preferred embodiments, the abrasive is secured to the polymeric member by creating a thin base layer of silver using vacuum deposition techniques such as are well known in the art. Metalization of polymeric materials is discussed, for example, in U.S. Pat. No. 5,468,562 to Farivar, et al., and in the references cited therein. Once the silver base layer is applied to the polymeric member, a layer of metal such as nickel having a slurry of diamond particles disposed therein can be plated to the base layer using an electro- or electroless-plating method as is done with conventional burrs.  
         [0051]    In some instances, it may be desirable to etch or mask a portion of the polymeric member with a patter of dots or other shapes so that the base layer does not completely surround the polymeric member. If the abrasive is only plated to the etched pattern, it may allow the polymeric member to more easily expand, collapse, or otherwise flex, and also enhance the adhesive stability of the abrasive coating. In the preferred embodiments, abrasive dots or pads having a diameter of approximately 0.010 to 0.015 inches are used.  
         [0052]    In addition to electroplating, it is believed that other techniques could be used to secure the abrasive to the balloon, such as by using an adhesive or chemically bonding sites on the outer surface of the polymeric balloon to which metal ions such as copper, silver, gold, or nickel may bond. These sites may be bonded to the polymeric member using a high-vacuum plasma system or by incorporating chemicals (such as carbon, silver, etc.) with the polymer prior to fabrication of the polymeric member. Alternatively, it is believed that pulse cathode arc ion deposition could be used to incorporate bonding sites on the surface of the elastomer.  
         [0053]    While the preferred embodiment of the invention has been illustrated and described, it will be appreciated that various changes can be made therein without departing from the scope of the invention. It is therefore intended that the scope of the invention be determined from the following claims and equivalents thereto.