Abstract:
Apparatus and methods are described for treating cardiovascular diseases with an embolization coil placement system utilizing an embolization coil and a pusher device. The embolization coil includes a releasable attachment assembly with a deformable ring-shaped member for connecting the embolization coil to a hydraulic pressure actuated coil release assembly on the pusher device. The coil release assembly includes a coil mounting wire with a bulbous distal portion that extends distally from a proximal tubular portion of the pusher device. A small inflatable balloon is mounted on the pusher device surrounding the coil mounting wire and fluidly connected to the proximal tubular portion. The embolization coil delivery system is prepared for use by crimping the deformable ring-shaped member around the deflated balloon member and the mounting wire. The bulbous distal portion of the mounting wire retains the crimped ring-shaped member on the pusher device. A target site in the vascular system is catheterized with a microcatheter. Then, the pusher device with the embolization coil mounted on it is inserted into the microcatheter and advanced to the target site. Once the embolization coil is in position, the inflatable balloon is pressurized with a syringe or inflation device connected to a hub on the proximal tubular portion of the pusher device. As the inflatable balloon member expands, it deforms the ring-shaped member to release the embolization coil from the pusher device. The balloon member is then deflated and the pusher device is withdrawn. In this way, one or more embolization coils may be inserted to occlude the target site by creating a physical barrier to blood flow and encouraging thrombus formation.

Description:
FIELD OF THE INVENTION 
     The present invention concerns a medical apparatus for placing embolization coils at selected sites within a patient&#39;s cardiovascular system for treatment of cardiovascular diseases and vascular anomalies, such as aneurysms, arteriovenous fistulas and vascular shunts. In particular, the invention concerns a catheter system inducing embolization coils and an embolization coil placement device or “pusher” that uses a coil release assembly that is actuated by hydraulic pressure. 
     BACKGROUND OF THE INVENTION 
     One of the current treatments for cardiovascular diseases and vascular anomalies, such as aneurysms, arteriovenous fistulas and vascular shunts, utilizes embolization coils which are introduced into the vascular anomaly to occlude it by creating a physical barrier to blood flow and encouraging thrombus formation. Typically, the embolization coils are placed at the desired location utilizing a microcatheter and an embolization coil placement device, often called a “pusher”. First, the site to be treated is catheterized with the microcatheter using flow directed techniques and/or with the aid of a steerable guidewire. Then, after removing the guidewire from the microcatheter, the embolization coil to be deposited is inserted and advanced through the microcatheter with the aid of the pusher. Once the end of the pusher has reached the distal end of the catheter, the embolization coil is pushed completely out of the microcatheter into the intended site. This technique of pushing the embolization coil out of the microcatheter poses a certain number of problems. Positioning the proximal end of the embolization coil cannot be performed with precision and, once the coil has begun to exit the catheter, it is impossible to reposition or to retrieve the embolization coil. 
     Several techniques have been proposed to permit more precise and controlled placement of embolization coils: 
     U.S. Pat. Nos. 5,122,136, 5,354,295, 5,540,680, 5,569,245 and 5,743,905 describe different variations of a system of embolization coil placement that uses electrical energy for releasing an embolization coil. The embolization coil is welded to a pusher wire that is used to maneuver the coil into the desired position. The embolization coil remains attached to the pusher wire even when it is pushed beyond the distal end of the microcatheter, which allows the embolization coil to be repositioned or retrieved, if necessary. Once the embolization coil is in the desired position, an electric current is passed through the welded area to electrolytically detach the embolization coil. The electrolytic detachment process is relatively slow, precluding the possibility of instantaneous release of the embolization coil. The electrolytic detachment process may also release deleterious chemical byproducts. In addition, the need for an electrical energy source adds to the cost and complexity of the embolization coil placement system. 
     U.S. Pat. No. 5,108,407 describes an embolization coil detachment system using light energy delivered through an optical fiber. The embolization coil is bonded to the pusher with a heat sensitive adhesive. When the embolization coil is in the desired position, laser energy is directed through the optical fiber to break the attachment. The laser ablation of the adhesive bond may possibly release undesirable chemical byproducts. In addition, the need for a laser energy source and an optical fiber adds significantly to the cost and complexity of the embolization coil placement system. 
     Other systems using mechanical embolization coil detachment mechanisms also exist, for example U.S. Pat. Nos. 5,304,195 and 5,261,916. In general, these mechanisms allow retraction of the coil as long as they have not completely exited the catheter, but they release the coil as soon as the pusher exits the distal end of the catheter. 
     U.S. Pat. Nos. 5,725,534 and 5,234,437 describe mechanical embolization coil detachment mechanisms utilizing a screw thread or a helical coil on the pusher that screws and unscrews from a threaded counterpart on the embolization coil. Proper operation of the coil detachment mechanism depends on precise engagement and disengagement of the threaded parts, which may not always be reliable under difficult clinical conditions. 
     U.S. Pat. Nos. 5,312,415 and 5,350,397 describe embolization coil placement systems that use a frictional attachment mechanism for controlling the release of the embolization coil. A pusher wire is used to release the embolization coil from the frictional attachment mechanism. The need for a precise interference fit for proper engagement of the frictional attachment mechanism adds significantly to the cost of the embolization coils and the delivery system. 
     Although these previous devices and systems represent, for the most part, a significant advance in the treatment of vascular disease, there continues to be a great need for improved systems of embolization coil placement that overcome the difficulties and inconveniences attendant with the existing systems. 
     SUMMARY OF THE INVENTION 
     In keeping with the foregoing discussion, the present invention provides a system for placement of embolization coils that is simple, reliable and easily achieved using known manufacturing techniques. The system includes an embolization coil and a pusher device for embolization coil placement. The embolization coil is typically constructed of a helically-wound wire coil of a biocompatible metallic alloy wire, or alternatively of a biocompatible polymer or a metal and polymer composite. Additionally, the embolization coil may include fibers or other thrombogenic materials. A releasable attachment assembly extends from the proximal end of the helically-wound wire coil. The releasable attachment assembly is preferably in the form of a deformable ring-shaped member connected to the helically-wound wire coil by an extension member. The ring-shaped member may be a simple annulus or it may have a convoluted or Z-shaped configuration. 
     The pusher device has a proximal tubular portion connected to a hydraulic pressure actuated coil release assembly. The hydraulic pressure actuated coil release assembly has an embolization coil mounting wire that extends distally from the proximal tubular portion. The mounting wire has a larger diameter distal portion that may be spherical, ellipsoidal, cylindrical or bulbous in shape. A small inflatable balloon member is mounted on the pusher device surrounding the embolization coil mounting wire and fluidly connected to the proximal tubular portion. 
     The embolization coil delivery system is prepared for use by mounting an embolization coil on the hydraulic pressure actuated coil release assembly by crimping the deformable ring-shaped member around the deflated balloon member and the mounting wire. The enlarged diameter of the distal portion of the mounting wire retains the crimped ring-shaped member on the hydraulic pressure actuated coil release assembly of the pusher device. The target site for the embolization coil is catheterized using a combination of microcatheters, flow directed catheters, guiding catheters and/or steerable guidewires. Then, the distal end of the pusher device with the embolization coil mounted on it is inserted into the microcatheter and advanced to the target site. The embolization coil can be advanced and withdrawn and manipulated as necessary to achieved optimum placement within the target site. Once the embolization coil is satisfactorily positioned, the inflatable balloon member is pressurized using a fluid-filled syringe or inflation device. As the inflatable balloon member expands, it deforms the ring-shaped member to release the embolization coil from the pusher device. The balloon member is then deflated and the pusher device is withdrawn. These steps may be repeated as many times as necessary to achieve satisfactory occlusion of the target site by creating a physical barrier to blood flow and encouraging thrombus formation. 
     The embolization coils and pusher device of the embolization coil placement system may be provided as components of a complete catheterization kit that may also include a combination of microcatheters, flow directed catheters, guiding catheters, steerable guidewires, a syringe or inflation device and instructions for use according to the methods described herein. 
     The many advantages of the embolization coil placement system of the present invention include: a completely controllable detachment assembly that does not depend on the position of the embolization coil and pusher assembly relative to the delivery catheter; a simple hydraulically actuated system of embolization coil detachment that is nonelectrical and nonmechanical and that does not require additional equipment, such as an electrical source or laser source; instantaneous detachment of the embolization coils; and no release of secondary products due to material ablation or chemical degradation. These and other advantages will be readily apparent to one of ordinary skill in the art upon reading the following detailed description of the invention taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows an embolization coil delivery system constructed according to the present invention, including an embolization coil mounted on a pusher device having a hydraulic pressure actuated coil release assembly. 
     FIG. 2 shows the embolization coil delivery system of FIG. 1, with the balloon of the hydraulic coil release assembly inflated. 
     FIG. 3 shows the embolization coil delivery system of FIG. 1, with the balloon of the hydraulic coil release assembly deflated and with the pusher device withdrawn to release the embolization coil. 
     FIG. 4 is a lateral view of a ring-shaped coil attachment assembly on the proximal end of an embolization coil constructed according to the present invention, showing the coil attachment assembly in a closed position. 
     FIG. 5 is a proximal end view of the ring-shaped attachment assembly of FIG. 4 in the closed position. 
     FIG. 6 is a lateral view of the ring-shaped attachment assembly of FIG. 4 with the attachment assembly in an open position. 
     FIG. 7 is a proximal end view of the ring-shaped attachment assembly of FIG. 6 with the attachment assembly in the open position. 
     FIG. 8 is a lateral view of a Z-shaped coil attachment assembly on the proximal end of an embolization coil constructed according to the present invention, showing the coil attachment assembly in a closed position. 
     FIG. 9 is a proximal end view of the Z-shaped attachment assembly of FIG. 8 in the closed position. 
     FIG. 10 is a lateral view of the Z-shaped attachment assembly of FIG. 8 with the attachment assembly in an open position. 
     FIG. 11 is a proximal end view of the Z-shaped attachment assembly of FIG. 10 with the attachment assembly in the open position. 
     FIG. 12 is a lateral view of an alternate construction of a Z-shaped coil attachment assembly on the proximal end of an embolization coil, showing the coil attachment assembly in a closed position. 
     FIG. 13 is a proximal end view of the Z-shaped attachment assembly of FIG. 12 in the closed position. 
     FIG. 14 is a lateral view of the Z-shaped attachment assembly of FIG. 12 with the attachment assembly in an open position. 
     FIG. 15 is a proximal end view of the Z-shaped attachment assembly of FIG. 14 with the attachment assembly in the open position. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an embolization coil delivery system  100  constructed according to the present invention, including an embolization coil  102  mounted on a pusher device  120  having a hydraulic pressure actuated coil release assembly  130 . Preferably, the embolization coil  102  of the embolization coil delivery system  100  is constructed of a helically-wound wire coil  104  having a proximal end  106  and a distal end  108 . The helically-wound wire coil  104  will typically have a length of approximately 0.5-60 cm, an outer diameter of approximately 0.2-2 mm and an inner diameter of approximately 0.1-1.7 mm. The nominal outer diameter of the embolization coil  102  is typically given in inches, the most commonly used sizes being 0.010, 0.014, 0.018, 0.035 and 0.038 inches outer diameter. The length of the helically-wound wire coil  104  of the embolization coil  102  is shown truncated in the drawing figures for ease of illustration. Preferably, the helically-wound wire coil  104  is constructed of a biocompatible metallic alloy wire  116  suitable for permanent implantation, such as a stainless steel, cobalt, gold, platinum, tantalum or tungsten alloy. The wire  116  will typically have a diameter of approximately 0.05-0.15 mm. Alternatively, the embolization coil  102  may be constructed of a biocompatible polymer or a metal and polymer composite. If desired the helically-wound wire coil  104  can be formed with a space-filling geometry, such as a helical, spiral or random configuration. In addition, the embolization coil  102  may also include fibers or other thrombogenic materials to hasten occlusion of the target blood vessel after implantation of the embolization coil  102 . 
     Optionally, the distal end  108  of the helically-wound wire coil  104  may be smoothly rounded by welding, brazing or soldering. A releasable attachment assembly  110  extends from the proximal end  106  of the helically-wound wire coil  104 . In this exemplary embodiment of the embolization coil  102 , the releasable attachment assembly  110  includes a deformable annular ring-shaped member  114  connected to the helically-wound wire coil  104  by an extension member  112 . The deformable annular ring-shaped member  114  is preferably made from a malleable metal alloy that is easily deformed or expanded. The annular ring-shaped member  114  is shown in an unexpanded state in FIG.  1  and in an expanded state in FIGS. 2 and 3. The annular ring-shaped member  114  will preferably have an outer diameter similar to that of the helically-wound wire coil  104 , typically in the range of approximately 0.2-2 mm. The outer diameter of the annular ring-shaped member  114  in the expanded state will preferably be approximately 110-200 percent of the unexpanded diameter, typically in the range of approximately 0.22-4 mm. In one particularly preferred embodiment, the extension member  112  and the deformable annular ring-shaped member  114  are an extension of the biocompatible metallic alloy wire  116  that makes up the helically-wound wire coil  104 . Depending on the alloy chosen for wire  116 , the deformable annular ring-shaped member  114  may be heat treated to render it malleable and easily deformed. Alternatively, the deformable annular ring-shaped member  114  and/or the extension member  112  may be made of a separate wire joined to the helically-wound wire coil  104  by soldering, brazing, welding, adhesive or a mechanical attachment. 
     The pusher device  120  of the embolization coil delivery system  100  has a proximal tubular portion  122  extending proximally from the hydraulic pressure actuated coil release assembly  130 . The proximal tubular portion  122  may be constructed of metallic tubing, such as stainless steel hypodermic needle tubing or a nickel/titanium superelastic alloy, or it may be constructed of a rigid polymer, such as polyamide, or it may be an assembly or composite of metal and polymers. The proximal tubular portion  122  may be rigid or semi-rigid, or it may be constructed so that the rigidity varies along the length of the tubular portion  122 . The proximal tubular portion  122  will typically have a length of approximately  50 - 200  cm and an outer diameter of approximately 0.2-2 mm and has an inner lumen  124  with an inside diameter of approximately 0.1-1.7 mm. Preferably, a connector  134 , such as a standard female luer lock fitting, is attached to the proximal end of the proximal tubular portion  122  and in fluid connection with the inner lumen  124 . Alternatively, a detachable connector, such as a Touhy-Borst connector or a compression fitting may be removably attached to the proximal end of the proximal tubular portion  122 . 
     The hydraulic pressure actuated coil release assembly  130  has an embolization coil mounting wire or rod  126  that extends distally from the proximal tubular portion  122  of the pusher device  120 . The rod  126  is preferably made of a metal, such as stainless steel or a nickel/titanium superelastic alloy, and is attached to the proximal tubular portion  122  by soldering, brazing, welding, adhesive or a mechanical attachment. The rod  126  may be attached directly to the interior wall of the proximal tubular portion  122 , or the proximal tubular portion  122  may be machined to create a tongue portion  136  extending from its distal end for attachment of the rod  126 . On or near the distal end of the rod  126  is an expanded portion  128  having a larger diameter than the proximal portion of the rod  126 . The expanded portion  128  may be any convenient shape, such as spherical, ellipsoidal, cylindrical or any bulbous shape, and will preferably have an outer diameter similar to that of the embolization coil  102 , typically in the range of approximately 0.2-2 mm. The rod  126  proximal to the expanded portion  128  will typically have a diameter of approximately 0.1-1.5 mm. The rod  126  will typically have a length of approximately 0.1-50 cm. However, in an alternate construction, the rod  126  may run the full length of the pusher device  120  within the proximal tubular portion  122  and attach to the connector  134 , particularly if the proximal tubular portion  122  is constructed of a polymer tube. The rod  126  and the expanded portion  128  may be created from a single piece of metal wire by machining, such as by centerless grinding, swaging or stamping. Alternatively, a bead of material may be assembled onto the rod  126  by soldering, brazing, welding, adhesive bonding or mechanical attachment to create an expanded portion  128 . 
     Mounted on the distal end of the pusher device  120 , surrounding the rod  126  and the expanded portion  128 , is a small inflatable balloon member  132 . The inflatable balloon member  132  is shown in a deflated state in FIG.  1  and in an inflated state in FIG.  2 . The inflatable balloon member  132  is made of a polymer and can be dip molded from a polymer solution or blow molded from an extruded tube or a molded parison using known methods. Preferably, the inflatable balloon member  132  is made of a relatively low compliance polymer, such as polyamide, polyethylene terephthalate, polyethylene, polyolefin or polyvinyl chloride. Although it is less preferred, an elastic or high compliance inflatable balloon member  132  made of an elastomer, such as polyurethane, silicone or latex, may also be used, particularly if the annular ring-shaped member  114  is made of a soft and highly malleable metal alloy. The inflatable balloon member  132  may be spherical, ellipsoidal or cylindrical in shape and will typically have a length of approximately 2-20 mm and a wall thickness of approximately 0.005-0.050 mm. The inflatable balloon member  132  will preferably have an inflated diameter of approximately 110-200 percent of the diameter of the expanded portion  128  of the rod  126 , typically in the range of approximately 0.22-4 mm. The inflatable balloon member  132  has a proximal sleeve  138  that is attached to the distal end of the proximal tubular portion  122  and in fluid connection with the inner lumen  124 . The proximal sleeve  138  is adhesively bonded or heat bonded to the proximal tubular portion  122  to create a fluid tight connection. The inflatable balloon member  132  may be formed with a closed distal end  140 , as shown, by dip molding on an appropriately shaped mandrel. If the inflatable balloon member  132  is blow molded from an extruded tube, the distal sleeve (not shown) of the blow molded balloon member  132  can be heat sealed or adhesively sealed to create a closed distal end  140  on the balloon member  132 . The closed distal end  140  of the balloon member  132  may be adhesively bonded to the expanded portion  128  to stabilize its position on the pusher device  120 . 
     Alternatively, the inflatable balloon member  132  may be formed with a distal sleeve (not shown). In this case, the distal sleeve of the inflatable balloon member  132  may be adhesively bonded to the expanded portion  128  of the rod  126  or onto a distal extension (not shown) of the rod  126  that extends distally from the expanded portion  128 . 
     The embolization coil delivery system  100  is prepared for use by mounting an embolization coil  102  on the hydraulic pressure actuated coil release assembly  130  at the distal end of the pusher device  120 . The inflatable balloon member  132  is first deflated by drawing a vacuum on the luer lock connector  134 , then the inflatable balloon member  132  is wrapped or folded tightly around the embolization coil mounting rod  126 . While vacuum is held on the folded inflatable balloon member  132 , the annular ring-shaped member  114  is passed over the expanded portion  128  of the rod  126  while in an expanded state. Then, the annular ring-shaped member  114  is tightly crimped around the folded inflatable balloon member  132  to firmly attach the embolization coil  102  to the pusher device  120 . FIG. 1 shows the embolization coil delivery system  100  ready for use with the embolization coil  102  mounted on the pusher device  120 . The mounting step can be done in manufacturing so that the embolization coil delivery system  100  is packaged, sterilized and shipped to the end user with a pre-mounted embolization coil  102 . Additionally or alternatively, the end user may be furnished with separately packaged sterile embolization coils  102  for mounting on the pusher device  120  immediately prior to use. 
     Before use, the embolization coil delivery system  100  is prepped by attaching a fluid-filled syringe and a stopcock or other inflation device (not shown) to the luer lock connector  134 . If desired, the inflatable balloon member  132  may be vacuum prepped by drawing a vacuum with the syringe to evacuate as much air as possible from the balloon member  132 . However, it is important that the inflatable balloon member  132  not be pressurized while prepping the device, as this could lead to premature detachment or loosening of the embolization coil  102 . The target site for the embolization coil delivery system  100  is catheterized in the usual way using an appropriate combination of microcatheters, flow directed catheters, guiding catheters and/or steerable guidewires. The target site may be an aneurysm, an arteriovenous fistula or vascular shunt, a feeder artery to a vascular tumor, or any other vascular site or body lumen that is to be embolized or occluded. Then, the distal end of the pusher device  120  with the embolization coil  102  mounted on it is inserted into the microcatheter or guiding catheter and advanced to the target site. Once at the target site, the embolization coil  102  is maneuvered into the desired position with the aid of the pusher device  120 . Because the embolization coil  102  is firmly attached to the pusher device  120 , it can be advanced and withdrawn and manipulated as necessary to achieved optimum placement of the embolization coil  102 . If satisfactory positioning cannot be achieved or if complications arise, the embolization coil  102  can easily be withdrawn into the catheter and removed from the patient. 
     Once the embolization coil  102  is satisfactorily positioned within the target site, the inflatable balloon member  132  is pressurized using the syringe or inflation device. Hydraulic pressure expands the inflatable balloon member  132 , which in turn expands the deformable annular ring-shaped member  114  of the releasable attachment assembly  110 , as shown in FIG.  2 . To release the embolization coil  102 , the balloon member  132  is deflated by drawing a vacuum with the syringe and withdrawing the expanded portion  128  of the rod  126  from the expanded annular ring-shaped member  114 , as shown in FIG.  3 . 
     These steps may be repeated as many times as necessary to achieve satisfactory occlusion of the target site by delivering additional embolization coils  102  with the same pusher device  120 , or additional embolization coil delivery systems  100  with pre-mounted embolization coils  102  may be used. 
     The embolization coils  102  and pusher device  120  of the embolization coil placement system  100  may be provided as components of a complete catheterization kit that may also include a combination of microcatheters, flow directed catheters, guiding catheters, steerable guidewires and/or a syringe or inflation device. 
     Preferably, the embolization coil placement system  100  or a catheterization kit including the system  100  is supplied sterile in a protective package, along with instructions for use according to the methods described herein. 
     FIG. 4 is an enlarged lateral view of an embolization coil  102  similar to that shown in FIG. 1 with the releasable attachment assembly  110  shown in the unexpanded or closed position. The pusher device  120  is not shown in this view so that the construction details and operation of the releasable attachment assembly  110  can be better appreciated. The embolization coil  102  has a deformable ring-shaped member  114  that is shaped like an annulus connected to the proximal end  106  of the helically-wound wire coil  104  by an extension member  112 . The annular ring-shaped member  114  is in the unexpanded or closed position. FIG. 5 is a proximal end view of the embolization coil  102  of FIG. 4, also shown with the annular ring-shaped member  114  in the closed position. Preferably, the annular ring-shaped member  114  forms a complete circle that is smaller in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120  when crimped down in the closed position. 
     FIG. 6 is a lateral view of the embolization coil  102  of FIG. 4 with the attachment assembly  110  in an expanded or open position, similar to that shown in FIGS. 2 and 3. FIG. 7 is a proximal end view of the embolization coil  102  of FIG. 6, also shown with the annular ring-shaped member  114  in the open position. When expanded, the annular ring-shaped member  114  forms an enlarged C-shaped arc that is preferably larger in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120 . 
     FIG. 8 is an enlarged lateral view of an alternate construction of an embolization coil  150  according to the present invention. The releasable attachment assembly  152  of the embolization coil  150  is shown in the unexpanded or closed position similar to that shown in FIG.  1 . Again, the pusher device  120  is not shown in this view so that the construction details and operation of the releasable attachment assembly  152  can be better appreciated. The embolization coil  150  has a deformable ring member  154  that has a wave-like, convoluted or undulated configuration that can be described as W-shaped or Z-shaped, depending on how it is viewed. This exemplary embodiment of the embolization coil  150  shows only one possible configuration of the Z-shaped deformable ring member  154 , many other configurations are possible. The Z-shaped deformable ring member  154  is preferably made from a malleable metal alloy wire that is easily deformed or expanded. The Z-shaped deformable ring member  154  is preferably connected to the proximal end  156  of the helically-wound wire coil  158  by a pair of extension members  160 ,  162  by welding, brazing, soldering, adhesive or other known attachment techniques. The wire of the Z-shaped deformable ring member  154  may be an extension of the wire that makes up the helically-wound wire coil  158 . The Z-shaped deformable ring member  154  is in the unexpanded or dosed position in FIG.  8 . FIG. 9 is a proximal end view of the embolization coil  150  of FIG. 8, also shown with the Z-shaped deformable ring member  154  in the closed position. Preferably, the Z-shaped deformable ring member  154  forms a complete circle that is smaller in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120  when crimped down in the dosed position. 
     FIG. 10 is a lateral view of the embolization coil  150  of FIG. 8 with the attachment assembly  152  in an expanded or open position, similar to that shown in FIGS. 2 and 3. FIG. 11 is a proximal end view of the embolization coil  150  of FIG. 10, also shown with the Z-shaped deformable ring member  154  in the open position. When expanded, the Z-shaped deformable ring member  154  forms an enlarged circle that is preferably larger in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120 . 
     FIG. 12 is an enlarged lateral view of another alternate construction of an embolization coil  170  according to the present invention. The releasable attachment assembly  172  of the embolization coil  170  is shown in the unexpanded or closed position similar to that shown in FIG.  1 . Once again, the pusher device  120  is not shown in this view so that the construction details and operation of the releasable attachment assembly  172  can be better appreciated. The embolization coil  170  has a deformable ring member  174  that has a wave-like, convoluted or undulated configuration that can be described as W-shaped or Z-shaped, depending on how it is viewed. The Z-shaped deformable ring member  174  is preferably made from a thin-walled malleable metal alloy tube that is easily deformed or expanded. The metal alloy tube may be cut out to make the Z-shaped deformable ring member  174  using laser cutting, water jet cutting, abrasive cutting, photo etching or other known metal forming techniques. Alternatively, the Z-shaped deformable ring member  174  may be formed from a polymer or a metal and polymer composite. The Z-shaped deformable ring member  174  is preferably connected to the proximal end  176  of the helically-wound wire coil  178  by a pair of extension members  180 ,  182  by welding, brazing, soldering, adhesive or other known attachment techniques. This exemplary embodiment of the embolization coil  170  shows only one possible configuration of the Z-shaped deformable ring member  174 , many other configurations are possible. The Z-shaped deformable ring member  174  is in the unexpanded or closed position in FIG.  12 . FIG. 13 is a proximal end view of the embolization coil  170  of FIG. 12, also shown with the Z-shaped deformable ring member  174  in the dosed position. Preferably, the Z-shaped deformable ring member  174  forms a complete circle that is smaller in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120  when crimped down in the dosed position. 
     FIG. 14 is a lateral view of the embolization coil  170  of FIG. 12 with the attachment assembly  172  in an expanded or open position, similar to that shown in FIGS. 2 and 3. FIG. 15 is a proximal end view of the embolization coil  170  of FIG. 14, also shown with the Z-shaped deformable ring member  174  in the open position. When expanded, the Z-shaped deformable ring member  174  forms an enlarged circle that is preferably larger in diameter than the diameter of the expanded portion  128  on the rod  126  of the pusher device  120 . 
     As may be discerned from the various views of the releasable attachment assemblies in their expanded states, a further advantage of the present invention is that the deformable ring-shaped member may serve as an anchoring member for the embolization coil once deployed. The expanded ring-shaped member, if deployed to a sufficient diameter, may be used to anchor the embolization coil into the target vessel or to interlock multiple embolization coils together to prevent downstream migration of the coils from the intended treatment site. 
     While the present invention has been described herein with respect to the exemplary embodiments and the best mode for practicing the invention, it will be apparent to one of ordinary skill in the art that many modifications, improvements and subcombinations of the various embodiments, adaptations and variations can be made to the invention without departing from the spirit and scope thereof.