Patent Publication Number: US-6214029-B1

Title: Septal defect occluder

Description:
TECHNICAL FIELD 
     The present invention generally relates to devices for occluding septal defects or shunts in the heart or the vascular system, and particularly provides a low profile septal defect conforming device reversibly deliverable via catheter to a septal defect site. 
     BACKGROUND OF INVENTION 
     The term “septal defect” generally refers to a perforation or other type hole (i.e., a defect) which passes through a thin wall of muscle or other tissue (i.e., a septum) which divides or separates “areas” within the body. Such defects can occur, either congenitally or by acquisition, between chambers of the heart (i.e., atrium or ventricle) or the great vessels (interatrial and interventricular septal defects or patent ductus arteriosus and aortico-pulminatry window respectively), causing shunting of blood through the opening. 
     In the case of the atrium, the presence of a significantly large septal defect can cause blood to shunt across the defect from the right atrium to the left atrium and hence on to the left ventricle, aorta and brain. If the defect is not closed, the risk of stroke is increased. 
     Shunting of blood from the left to the right side can also have negative consequences. This can lead to death due to cardiac failure or hemoptysis. 
     In patients with significant sized ventricular septal defects or patent ductus arteriosus, there is shunting of blood from the high pressure left ventricle or aorta, into the right side chambers and pulmonary arteries which normally have much lower pressures. The torrential increase in flow at a high pressure can lead to cardiac failure and death, apart from the serious long-term complication of high pulmonary pressures which can cause a reversal of the direction of the shunt. 
     Atrial septal defects were initially corrected by open heart surgery which required the surgeon to open the chest of a patient and bypass the heart temporarily (e.g., by means of a mechanical heart or a “heart-lung machine”). The surgeon would then physically cut into the heart and suture small defects closed. In the case of larger defects, a patch of a biologically compatible material would be sewn onto the septum to cover (i.e., “patch”) the defect. 
     In order to avoid the morbidity, mortality and long recovery times associated with open heart surgery, a variety of transcatheter closure techniques have been attempted. In such techniques, an occluding device is delivered through a catheter to the septal defect site. Once the closure device is positioned adjacent the defect, it must be attached to the rest of the septum in a manner which permits it to effectively block the passage of blood through the defect. 
     One such closure device, as illustrated in U.S. Pat. No. 3,874,388 (King et al.), includes a pair of complex mechanical umbrellas, each having a plurality of arms extending radially from a central hub. The hubs of the two umbrellas are mechanically connected to one another and each umbrella includes a fabric covering over the arms, much like a common umbrella. The ends of each arm are provided with barbs which are anchored into the septum to hold the occluder in place. The complex umbrellas prove rather difficult to unfold after passage through a catheter, requiring an array of cables to deploy the arms. This makes proper placement of the device difficult, and the barbs on the arms prevent retraction or repositioning of the device once it is in place. Use of this device has been limited to adult patients because the device requires a large catheter, such as about 23 French (7.3 mm), for delivery. 
     Rashkind proposed a single-umbrella closure device capable of delivery through a 5 mm system which permitted use in children weighing at least about 20 kg. Similar to the King device, this umbrella utilizes barbed hooks on the ends of umbrella arms to ensure attachment to the septum, with the single umbrella being placed on the left side of the atrial septal defect. The barbs prevent disengagement of the device, and poorly centered or seated devices requiring open heart surgery for correction are common. 
     Due to the low success rate of previous devices, a “modified double-umbrella Rashkind occluder” in which the arms of the device are hinged to permit them to fold back against themselves was developed. A more compact collapsed condition and a less intrusive delivery as by an 11 French (3.7 mm) catheter were thereby facilitated. Furthermore, such a “clamshell” occluder did not include barbs at the end of the radial arms of the umbrella, allowing it to be readjusted and retrieved. Typically, this could be accomplished only once, and without subsequent redeployment due to damage or destruction of the device. Although arguably an improvement over heretofore known devices, such a device generally requires a complex loading jig for deployment and remains susceptible to moderately high shunting. 
     Sideris, in U.S. Pat. No. 4,917,089, proposed an occlusion device which combines a single umbrella with a separate anchoring device. Like the previous defect occlusion devices, Sideris&#39; invention utilizes an umbrella with a plurality of radially extending arms. A string connects the arms of this umbrella to a generally rhomboidally shaped anchor which includes an internal wire skeleton and a central, rhomboidally shaped piece of rubber. The string attached to the struts of the umbrella is affixed to the central rubber element of the anchor. The anchor is placed on the opposite side of the septum from the umbrella, and the length of the string limits movement of the occlusion device with respect to the septum. This style of occluder is difficult to deploy, and its overall bulkiness in the heart causes potential clot emboli due to protrusion into the atrial cavities. 
     Kotula et al., U.S. Pat. No. 5,725,552, provides a collapsible device comprising a heat-set woven metal fabric configured as a bell, hourglass, etc. for occluding an abnormal opening in a body organ. The device of Kotula et al. does not adequately “fill” the defect nor fit flat against, or readily conform to, the structures within the heart, thereby increasing the embolization potential with the use of such device. 
     Das, U.S. Pat. No. 5,334,217, teaches an occluder having paired disks, each of which comprises a membrane, and an elastically deformable frame carried about the periphery of each membrane. The disks are joined only at central portions of each membrane, thereby defining a conjoint disk. The Das device is intended to be self-centering within the defect. Since the ability to achieve defect conformity is limited due to the defined conjoint disk structure, residual shunting can occur. Furthermore, with such a device, the conjoint disk cannot uniformly apply and distribute a force to the “second” disk (e.g., as when the second disk follows the first disk into the catheter for purposes of retrieval. As a result, the occluder is caused to contort, resulting in non-symmetrical collapse, and the problems associated therewith. 
     All of the prior art devices described above suffer shortcomings. First, most of these systems (i.e., the occluder and delivery means) are mechanically complex and require a great deal of remote manipulation for deployment or retrieval, if the device is retrievable. This extensive remote manipulation, such as by applying tension to one or more cables in order to deploy the arms of an umbrella or to anchor the device in place, not only increases the difficulty of the procedure, but tends to increase the likelihood that the device will be improperly deployed. This can necessitate either retrieval, or repositioning so as to effectively occlude the defect and minimize the risk of embolization. 
     Second, all of these devices, except for Kotula and Das, essentially teach two separate members joined to each other at a single interface. With such device, when the left atrial member is opened, the central point tends to ride to the lower margin of the defect. Proper centering of the device is quite difficult, and when a self centering device as disclosed by Das is employed, it is at the cost of defect conformity. 
     It is desirable, therefore, to provide a simple, collapsible compact closure device which may be delivered through a small catheter. It is also highly advantageous to have such a device which can be readily reversibly deployed and retrieved with a minimum of remote manipulation and applied force. Further, a device which is self-centering and self-occluding, particularly one that possesses a defect conforming variable geometry to fill slit-like defects and patent foramen ovale, and one that can be released while still being tethered to the delivery mechanism to assure proper placement and function prior to release, would be superior to heretofore known devices. This is particularly true in view of the need to test for shunting of blood around the occluder device prior to release. 
     SUMMARY OF THE INVENTION 
     The present invention is a septal defect occluder which has first and second occluder panels. Each occluder panel includes a fabric support structure and fabric suspended from a perimeter thereof. The occluder panels are conjoined at a plurality of points which are located within an area bounded by the perimeter of each fabric support structure, as well as on the fabric, to thereby form a defect conforming region for the occluder. 
     The present invention is thus an improved device over structures known in the prior art. More specific features and advantages obtained in view of those features will become apparent with reference to the drawing figures and DETAILED DESCRIPTION OF THE INVENTION. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevational view of the septal defect occluder of the present invention deployed in a septal defect; 
     FIG. 2 is an end view of an occluder panel of the septal defect occluder of the present invention, particularly illustrating a center fabric attachment point; 
     FIG. 3 is an end view of the frame members of the device of the subject invention having two pairs of opposing inward eyelets; 
     FIG. 4 depicts a fabric sheet suitable for use with the invention; 
     FIG. 5 is an end view of two frame members of FIG. 3 arranged to form a fabric support structure having both internal and perimeter eyelets; 
     FIG. 6 depicts a first occluder half, corresponding to the fabric support structure of FIG. 5, showing the relationship between the fabric sheet of FIG.  4  and the underlying support structure; 
     FIG. 7 depicts a second occluder half, corresponding to the second fabric support structure of FIG. 5, showing the relationship between the fabric sheet of FIG.  4  and the underlying support structure; 
     FIG. 8 is a side view of the present invention being delivered within a catheter to a septal defect site; 
     FIG. 9 is a side view of the present invention being initially deployed within the septal defect, the second occluder half having expanded to conform to a portion of the defect; 
     FIG. 10 is a side view of the present invention deployed within the septal defect while under the control of tension imparting means; 
     FIG. 11 is a side view of the present invention being initially retrieved into the catheter from the septal defect site, the first occluder half being collapsed for catheter entry; and, 
     FIG. 12 is a side view of the present invention on its way to complete retrieval into the catheter from the septal defect site. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown generally in FIG. 1, a septal defect closure device  20  of the invention may be attached to the septum S (e.g., an atrial septum) to effectively conform to and block the defect, without protruding into atrial cavities and the like. As described in detail below, once the closure device  20  is in place, it becomes anchored to the septum and prevents the flow of blood through the atrial septum to the adjoining chambers of the heart. This will permit the heart to operate normally. 
     Referring now to FIGS. 1 and 2, the extremely low profile closure device includes first and second occluder panels  30 ,  60 . Each panel  30 ,  60  is generally round (e.g., circular, oval, elliptical etc.) so as to facilitate positioning, and minimizes chances of erosion and puncture. Each panel  30 ,  60  generally comprises a fabric support structure  32 ,  62  and fabric  33  suspended from a perimeter  34 ,  64  of the fabric support structures  32 ,  62 . The occluder panels  30 ,  60  are conjoined at a plurality of discrete points, located or positioned within the bounds of each of the fabric support structures  32 ,  62  (i.e., within an area bounded by each perimeter  34 ,  64  of the support structures  32 ,  62 ), as well as on the fabric  33  (which will be further explained with reference to FIGS. 2,  7  and  8 ). A defect conforming region  80  for the occluder  20  is thereby formed. The nature (i.e., structure, relationships therebetween and function) of the defect conforming region will be detailed hereinbelow, particularly with reference to FIGS. 1,  2  and  10 - 14 . At this point it may be said that the region  80  expandingly conforms to substantially completely and thoroughly satisfy the perimeter of the defect geometry. This stabilizes panels  30 ,  60  so that complete coverage of the defect from either direction is achieved. The defect is thereby occluded without even minimal shunting or distortion of the defect. 
     The fabric support structures  32 ,  62  of the occluder are generally flexible and elastically deformable, and include perimeter and traversing segments  36 ,  38 . Resilient fabric  33  (FIG. 4) is suspended or otherwise affixed to the perimeter segments  36  of the fabric support structures  32 ,  62 . As particularly shown in FIG. 2, the perimeter segments  36  of the fabric support structures  32 ,  62  extend substantially around the periphery  35  of the fabric  33 . The fabric  33  may be formed of a thin, flexible material which can be folded and pulled taut without being damaged. Elastic polymeric materials such as, for example, polyester knit, nylon, polypropylene, polytetrafluoroethylene (e.g., Teflon®), and expanded polytetrafluoroethylene (e.g., GoreTex®), as well as natural fabrics such as silk, are acceptable. 
     To accommodate the need of the fabric support structure to distort when retrieving the occluder  20  into the catheter, excess fabric can be provided. On an area basis relative to the support structure, an excess of fabric in the range, typically, of about 30-35 percent, and up to 50 percent, is sufficient. This range is required because the low stretch characteristics of the fabric prevent the support structure from collapsing in a manner suitable to get into the catheter. However, the 20 denier polyester knit is advantageous in that it possesses a low stretch character, is approximately 50% less bulky than known jersey style knit patterns which facilitates the use of smaller delivery catheters, and allows for the occluders to be retrieved into such catheters at forces that are not detrimental to either the catheter or the occluder (e.g., a 40 mm occluder may be pulled into a 12 French catheter using a reasonable peak force of about four pounds). A further advantage is that two complete fabric “patches” may be incorporated into the closure device (i.e., no need to remove material to reduce bulk), which thereby creates a device having a high reliability of successful closure. 
     The fabric  33  may be attached to their respective support structures  32 ,  62  by any suitable means. For instance, the fabric  33  may be directly attached to the support structures  32 ,  62  by means of an adhesive or the like, or the periphery  35  of the fabric  33  may be wrapped about each of the support structures  32 ,  62  and the peripheral edge attached to the rest of the fabric so as to essentially define a sleeve about each of the support structures  32 ,  62 . In the latter instance, the sleeve may fit the support structure relatively loosely so that the structure may move within the sleeve with respect to the fabric. The peripheral edge of the fabric may be affixed to the rest of the fabric sheet  33  in any suitable fashion such as by sewing. Preferably, though, the periphery of the fabric can be sewn to at least some portion of the perimeter segments  36  of the support structures  32 ,  62  using polyester, non-adsorbable suture. 
     Referring to FIG. 1, the fabric support members  32 ,  62  of the occluder panels  30 ,  60  are shown as being spaced from one another for purposes of the present explanation, but this is not the normal configuration (i.e., static condition) of the panels. In a static, non deployed condition, the fabric support structures of the device take a generally planar form, with the two fabric support structures  32 ,  62  generally abutting against, or closely proximate, one another. 
     Again referring to FIGS. 1 and 2, the occluder panels  30 ,  60  are conjoined at a plurality of discrete points, the points being selected to effectively link each of the fabric support structures  32 ,  62  together, as well as associate each sheet of fabric  33  carried thereby, so as to form the variably configurable defect conforming region  80 . With such arrangement, the resilient fabric  33  is not only inherently or indirectly positionable in response to the defect geometry, but also directly responsive vis-a-vis the conjoined support structures  32 ,  62 . 
     The conjoined points within the fabric support structures  32 ,  62 , which responsively link the opposing structures, comprise loops formed in the traversing segments  38  thereof, these loops defining internal eyelets  40  for the structures  32 ,  62 . The internal eyelets  40  of each of the structures  32 ,  62  are shown as being joined by suture (e.g., polyester, non-absorbable or other suitable material), and to some extent delimit the defect conforming region  80 , and serve to center the occluder  20  within the defect. The remaining point of conjointment comprises the union, at a single point, of the fabric of each of the fabric support structure so as to define a generally central fabric attachment point  42 . It is important that fabric  33  of each support structure  32 ,  62  be limitingly controlled via the union, however it is equally important that the fabric  33  remain substantially suspended for expansion during deployment, preferably exclusively about or by its periphery  35 . 
     As best seen in FIG. 2, the center attachment point  42  of the occluder  20  is preferably but not exclusively configured as a sutured cross stitch positioned in the center of the fabric  33 . Other attachment configurations or geometries are contemplated, to the extent that the center attachment point  42  maintains its functionality, namely that of control of the peripherally supported fabric, and generally contributing to a centering function for the occluder. Preferably the internal eyelets  40  are symmetrically oriented about the center fabric attachment point  42 . 
     In addition to internal eyelets  40  which are formed in the traversing segments  38  of each of the fabric support structures  32 ,  62 , the perimeter segments  36  of at least one (i.e., structure  62 ) of the fabric support structures  32 ,  62  include loops formed therein, thereby defining perimeter eyelets  44  for that particular support structure  62 . As best seen in FIGS.  1  and  8 - 12 , the perimeter eyelets  44  cooperate with urging means  46  carried by and or through a catheter  47  so as to aid in the symmetrical collapse of each of the fabric support structures  32 ,  62 , and the occluder panels  30 ,  60  thereby, during reversible retrieval of the device  20  into the catheter  47 . The perimeter eyelets  44  associated with the “catheter side” occluder panel  60  transmit and distribute deployment and retrieval forces imparted thereupon through the defect conforming region  80  and to the other occluder panel  30 . As will subsequently be discussed, the unique configuration of the fabric support structure components, and the relationships therebetween, provide numerous advantages (for example: symmetrical collapse of the occluder, less peak force for retrieval into a catheter for deployment, and heretofore unsurpassed sealing of narrow slit defects without the distorting defects typically associated with fixed geometry conjoint areas such as circumferential conjoint disks). 
     Referring now to FIGS. 3-7, the fabric support structures comprise cooperating frames  50 , each of which preferably resembles a “bowtie,” as best seen in FIG. 3. A more technical description for the frame geometry might be to characterize it as an octagon (i.e., a frame of eight legs or segments), particularly an octagon having a concave, rather than convex, “top” and “bottom” (i.e., ceiling and floor). Put yet another way, the frames resemble elongated hexagons whose long sides are “pinched” towards each other. The frames  50  may be generally characterized as having maximum and minimum dimensions and corresponding axes of maximum and minimum dimension  52 ,  54 . The above frame description is intended to be illustrative, not limiting, with alternating frame geometries satisfying the general characterization being possible. 
     The internal eyelets  40  of the fabric support structures  32 ,  62  are formed in each of the frames  50  along the axis of minimum dimension  54  (FIG.  3 ), as illustrated. The resilient internal eyelets  40  are generally disposed between adjacent ends of two legs or frame segments  51 , one end of the eyelet being attached to each leg  51 . The internal eyelets  40  are shown as lying generally in the same plane as the legs  51  and may extend generally outwardly of the periphery of each of the support structures  32 ,  62 , or may preferably extend inwardly of the periphery of the structures as shown in the figures. The eyelets are desirably formed to function as spring hinges. This will serve to ensure that the occluder panels  30 ,  60 , particularly the catheter side panel  60 , elastically return substantially to a plane-defining configuration even after they have been collapsed and delivered through a catheter. 
     Frames  50  of the device have internal eyelets  40 , as previously explained. The internal eyelets  40  of one fabric support structure (i.e.,  32 ) mate (i.e., align or register) with those of the other support structure (i.e.,  62 ) so as to thereby conjoin the occluder panels  30 ,  60  (FIG.  2 ). The perimeter eyelets  44  of fabric support structure  62  on the other hand are formed in its frames  50 A along the axis of maximum dimension  54  (FIG.  3 ). The perimeter eyelets  44  cooperatively engage urging means  46  so as to enable remote manipulation of the occluder  20  during retrieval. 
     Each fabric support structure  32 ,  62  comprises perpendicularly overlying frames, the axis of maximum dimension  52  of one frame  50  or  50 A substantially aligning with the axis of minimum dimension  54  of the other frame (FIG.  5 ). The somewhat oversized fabric  33  is shown in FIG. 6 underlaying the cooperating frames, the periphery thereof being sewn or otherwise affixed to those portions of the frames, which when configured as shown in FIG. 5, form a perimeter  34 ,  64  for each of the fabric support structures  32 ,  62 . It is again noted that the preferred fabric  33  contributes to an occluder  20  that has complete opposing fabric patches suspended by the fabric support structures  32 ,  62 , which in turn include a frame geometry and arrangement that generally reduce deployment and retrieval forces. In return, fabric and stitch wear and tear and frame “break through” (i.e., separation of the perimeter segments  36  from the fabric  33  upon expansion of the occluders  30 ,  60 ) are minimized. 
     Each frame  50  is preferably formed of a single elongate strand of wire W. As best seen in FIG. 3, each of the legs  51  may simply comprise a length of the wire, and the wire may be bent through greater than 360 degrees to define adjacent legs  51  and to form the loops or eyelets  40 ,  44 . The ends of the wire may be attached to each other in any secure fashion, such as by means of a weldment or a suitable biocompatible cementitious material. 
     The frames  50  should be formed of a flexible, elastically deformable material such as a metal, and the wire comprising the frame is formed of a superelastic material. One such material currently known in the art is a near-stoichiometric nickel/titanium alloy, commonly referred to as Nitinol or NiTi. Such superelastic materials may be elastically deformed to a much greater extent than most other materials, yet substantially fully recover their original shape when released. This permits the frame to be deformed sufficiently for insertion into, and passage through, a small-diameter catheter yet automatically elastically return to its initial shape upon exiting the catheter. 
     The frames are preferably manufactured with nitinol wire that can be wound around the pins of a forming die and subjected to heat treatment. Each device consists of four frames, two frames for each support structure. More particularly, each support structure  32 ,  62  comprises matchingly paired frame styles (i.e., as shown in FIGS. 6 and 7, occluder panel  30  has a pair of frames  50  whereas occluder panel  60  has a pair of frames  50 A). All eyelets  40 ,  44  can be made having generally a 0.030 inch inside diameter, and, as previously noted, be inward facing (i.e., directed toward the center fabric attachment point  42 ). The wire ends of each frame can be connected with a titanium hypo tube using a compression crimp. The titanium is more ductile than the nitinol, providing a reliable grip with excellent corrosion resistance. Alternately, the preferred shape of the frame may be cut out from a sheet of such superelastic material as a single block, by chemical etching, punching with a suitable punch and die, or any other appropriate forming method. 
     In order to enhance radiopacity so that the frame can be viewed remotely during deployment, the frame may be provided with a radiopaque coating, such as gold or platinum. For instance, the wire W may be plated with a thin layer of gold or platinum. In one particularly useful embodiment, a helically wound length of a thin radiopaque wire (not shown) is placed over the wire W; such core/coil structures are well known in the art. Alternatively, radiopaque marking bands (not shown), which are commercially available, may be employed. By placing one such band on each leg of the frame, a physician can remotely visualize the frame as a plurality of small bands; when the bands are appropriately spaced from one another on a monitor, the physician knows that the frame is properly deployed. 
     Referring now to FIGS. 8-10, the closure device  20  of the invention is shown being deployed to occlude a defect in a septum S. The first panel  60  (i.e., catheter side occluder panel) of the device  20  is positioned on one side of the defect while the second panel  30  is generally disposed on the other side. The frames  50  or  50 A of the fabric support structures  32 ,  62  are elastically biased toward the position shown in FIG.  2 . The defect conforming region  80  is positioned within, and expands so as to occlude the defect. Because the support structures  32 ,  62 , vis-a-vis their frames  50  or  50 A, are elastically biased toward their deployed configuration, they are biased generally toward one another and engage opposing sides of the septum about the defect. Since there are no compressive forces acting on the frames which might cause them to collapse, this serves to effectively hold the device in place and occlude the defect. The device is further shown in FIGS. 11 and 12 being retrieved from a septal defect site, as might be required in the event of inadvertent initial placement, size mismatch, or otherwise. 
     The fabric sheets  33  are formed of a relatively porous material (FIG.  4 ). While this may seem to contradict the purpose of the device, blood will tend to coagulate on the latticework provided by the porous material. Blood flow across the defect is usually substantially blocked after minimal time passage. If so desired, the conjoint portion of the device (or the entire device) may be treated with a thrombogenic agent to speed this natural process or may be impregnated with a biocompatible polymeric compound or the like to make it relatively impervious to fluids. 
     The primary purpose of using a porous fabric is to accelerate the process of permanently anchoring the device in place. The support structures hold the fabric tautly and in intimate contact with the surface of the septum S. This intimate contact between the septum and perimeter of the occluder permits ingrowth of collagen and fibrous tissue from the septum into the fabric. Over time, the membrane resting against the septum will become securely anchored to the septal wall and be covered by a layer of endothelial cells. 
     The design of this device is in stark contrast to the septal defect closure devices known in the art. As explained in detail above, prior art devices employ a mechanical umbrella of one design or another. The radially extending arms of the umbrella contact the septum and serve to space all but the peripheral edge of the umbrella away from the septum. Endothelial cells, collagen and fibrous tissue are therefore permitted to grow into only the very periphery of the umbrella. Thus, while a closure device of the invention essentially becomes an integral part of the septum, the complex mechanical structure of prior art devices does not enable as complete integration as the present invention. 
     The mechanical complexity of prior art devices also tends to markedly affect their durability. In the case of atrial or ventricular septal defects, for example, the heart obviously continues to beat after the device is in place. Since beating of the heart is accomplished by flexure of the heart muscles, the septum will flex to some degree with every beat of the heart. The radial arms must therefore flex with the septum with each and every time the heart beat. The number of cycles of this stress-inducing movement produces repeated stresses on the arms, which can eventually lead to mechanical failure and fracture of the arms. 
     When a closure device of the invention is deployed, the tension of the frame of the support structure opens the panel to occlude the defect. Since there are no radial arms to prop open the device, the occurrence of repeated flexion does not occur due to the beating of the heart or pressure differences between the cardiac chamber during the phase of contraction of the heart. To the contrary, any pressure difference would urge a frame and panel against the septum, more firmly occluding the defect. In addition, the superelastic material of the frame tolerates flexural stresses much better than the rigid steel arms of the prior art devices. The present device therefore will continue to flex with the septum without any significant effect on its structural integrity. 
     Although the foregoing has focused on application of the present invention to occlude atrial septal defects, the invention is not limited to occluding such defects. For instance, the instant closure device can be used to treat ventricular septal defects, patent ductus arteriosus or any other congenital or acquired orificial or tubular communications between vascular chambers or vessels. 
     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. Changes may be made in details, particularly in matters of shape, size, material, and arrangement of parts without exceeding the scope of the invention. Accordingly, the scope of the invention is as defined in the language of the appended claims.