Patent Publication Number: US-2022218371-A1

Title: Snare device with anti-skewing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 16/528,814, inventor Eric D. Mathews, filed Aug. 1, 2019, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to snare devices and relates more particularly to a novel snare device. 
     There are many situations in which it may be desirable to capture or to retrieve an object from a confined space. For example, in the field of medicine, it may be desirable, in some instances, to capture and to remove a blood clot from a blood vessel. To this end, a number of snare devices have been devised. 
     One example of a snare device is disclosed in U.S. Pat. No. 6,500,185 B1, inventors Mathews et al., which issued Dec. 31, 2002, and which is incorporated herein by reference. According to U.S. Pat. No. 6,500,185 B1 (hereinafter “the &#39;185 patent”), the snare device has a longitudinally extending support that defines an axis. The support has a flexible distal section and a rigid proximal section. The flexible distal section, which is in the form of a coiled wire, has a compressed state, in which it defines a first path relative to the axis. A core-wire extending along the axis has a relaxed state in which it defines a second path relative to the axis. An anchor disposed on the flexible distal section and attached to the core-wire causes the flexible distal section to follow the same path. An actuator engaged to a proximal end of the core-wire enables a surgeon to selectively apply a tensile force thereto. This tensile force causes the core-wire and the flexible distal section to transition together between the first path and the second path. 
     According to the &#39;185 patent, a suitable outer overall diameter for the support of the snare device is approximately 0.014 inch, which renders the device suitable for general intravascular use. In addition, according to the &#39;185 patent, a suitable material for forming the coiled wire support is a platinum alloy, which is radiopaque, thereby enabling a surgeon to track the position of the snare device within a body. 
     Another example of a snare device is disclosed in U.S. Patent Application Publication No. US 2006/0229638 A1, inventors Abrams et al., which was published Oct. 12, 2006, and which is incorporated herein by reference. According to U.S. Patent Application Publication No. US 2006/0229638 A1 (hereinafter “the &#39;638 publication”), device and methods for removing a foreign object from a body lumen are disclosed. A retrieval device in accordance with an exemplary embodiment includes an elongated member including a flexible collector element and a core-wire that can be engaged by a physician to actuate the collector element between a first position and a second position with the body. The collector element may comprise a coiled section including a coiled flat ribbon of rectangular cross-section adapted to assume a substantially straight shape in the first position and an expanded shape forming one or more helically oriented loops in the second position. According to the &#39;638 publication, stainless steel is a suitable material for the elongated member including the collector element. 
     One problem that has been noted with snare devices of the types discussed in the &#39;185 patent and in the &#39;638 publication is a phenomenon sometimes referred to as “skewing.” More specifically, where the flexible distal section of the support is in the form of a coiled wire, as the coiled wire transitions from the first path to the second path, some of the turns of the coiled wire may be subjected to inwardly-directed radial forces. If these forces are sufficiently great, they can cause one or more of the turns to be radially displaced inwardly or “skewed” relative to their neighboring turns. Such skewing is undesirable for at least a couple of reasons. For one thing, a skewed turn of the coiled wire may come into contact with the core-wire in such a way as to inhibit the ability of the core-wire to transition properly from its first path to its second path. In addition, a skewed turn of the coiled wire may result in permanent misalignment of the turns of the coiled wire, thereby impairing the ability of the coiled wire to transition properly between the first and second paths. 
     In U.S. Pat. No. 6,652,536 B2, inventors Mathews et al., which issued Nov. 25, 2003, and which is incorporated herein by reference, some different approaches to the above-described problem of skewing in a snare device are disclosed. According to U.S. Pat. No. 6,652,536 B2 (hereinafter “the &#39;536 patent”), one approach is to dimension the diameter of the lumen of the coiled wire small enough relative to the outer diameter of the core-wire so as to reduce the likelihood that one or more turns of the coiled wire migrate radially inwardly to an extent that skewing may occur. At the same time, the diameter of the lumen of the coiled wire is dimensioned to be large enough relative to the outer diameter of the core-wire to reduce the likelihood that the coiled wire and the core-wire bind with each other as the snare device is in use. 
     Another approach that is disclosed in the &#39;536 patent is to position a spacer coil around the core-wire, the spacer coil being made of, or having a portion made of, a radiopaque material, such as platinum. The spacer coil can either be in contact with the core-wire or be separated therefrom by a clearance that is small enough to reduce the likelihood that turns of the spacer coil, themselves, become radially displaced relative to the core-wire. The spacing between the coiled wire and the spacer coil is selected to be small enough to reduce the likelihood that a turn of the coiled wire may become radially displaced to an extent sufficient to allow adjacent turns of the coiled wire to come into contact with one another, thereby causing the intervening turn to be permanently skewed out of alignment. However, the spacing between the spacer coil and the coiled wire is also selected to be large enough to reduce the likelihood that the spacer coil and the coiled wire bind with each other as the device is in use. 
     Although the approaches disclosed in the &#39;536 patent may ameliorate the problem of skewing to a certain extent, the present inventor also believes that these approaches may be impractical or insufficient in many cases. For example, the use of a spacer coil increases the distal stiffness of the device, particularly when compressed/tensioned, thereby making the device more difficult to deliver through a tortuous lumen to an obstruction that requires snaring. Additionally, the increased friction between the spacer coil and the support coil inhibits the device from transitioning from its straight configuration to its relaxed coiled configuration. Moreover, the use of a spacer coil increases the manufacturing cost and complexity of the device and also introduces some potential problems, such as the skewing of the spacer coil, itself. Therefore, the present inventor believes that there is a need for an alternative approach to minimizing the occurrence and effects of skewing. 
     Other documents of interest may include the following, all of which are incorporated herein by reference: U.S. Pat. No. 7,058,456 B2, inventor Pierce, which issued Jun. 6, 2006; U.S. Pat. No. 6,620,172 B1, inventors Dretler et al., which issued Sep. 16, 2003; and U.S. Patent Application Publication No. US 2009/0209987 A1, inventors Mathews et al., which was published Aug. 20, 2009. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a novel snare device or similar instrument. 
     According to one aspect of the invention, there is provided an instrument, such as snare device, the instrument comprising: (a) a longitudinally extending support defining an axis, the support having an uncompressed state of comparatively greater length and a compressed state of comparatively lesser length, the support comprising a flexible distal section, the flexible distal section comprising a coiled filament of round cross-sectional shape and defining a first path relative to the axis when in the compressed state, the flexible distal section having a tensile strength, a filamentary cross-sectional area, and a break load, wherein the break load of the flexible distal section is equal to the tensile strength of the flexible distal section multiplied by the filamentary cross-sectional area of the flexible distal section; (b) a core-wire extending along the axis and anchored to the flexible distal section of the support, the core-wire having a relaxed state of comparatively lesser length and a tensioned state of comparatively greater length, the core-wire comprising a proximal portion terminating at a proximal end of the core-wire and a distal portion terminating at a distal end of the core-wire, the distal portion of the core-wire defining a second path relative to the axis when in the relaxed state, wherein the second path differs from the first path and includes a looped shape, wherein the distal portion of the core-wire has a filamentary diameter and a filamentary cross-sectional area, wherein the distal portion of the core-wire also has an upper plateau stress in response to a tensile force applied thereto, wherein a plateau force of the distal portion of the core-wire is equal to the upper plateau stress multiplied by the filamentary cross-sectional area of the distal portion of the core-wire, wherein the distal portion of the core-wire has a length, and wherein the filamentary diameter of the distal portion of the core-wire is substantially uniform over the length of the distal portion of the core-wire in the relaxed state; and (c) an actuator secured to a proximal end of the core-wire and to a proximal end of the support to selectively apply both a tensile force to the core-wire and a compressive force to the support, the tensile force causing the core-wire to transition from its relaxed state to its tensioned state, the compressive force causing the support to transition from its uncompressed state to its compressed state; (d) wherein the break load of the flexible distal section of the support is greater than the plateau force of the distal portion of the core-wire. 
     In a more detailed feature of the invention, the support may terminate distally at a distal end, and the distal end of the core-wire may be anchored to the distal end of the support. 
     In a more detailed feature of the invention, the support may terminate distally at a distal end, and the distal end of the core-wire may be anchored to the support at a distance proximal to the distal end of the support. 
     In a more detailed feature of the invention, the looped shape may be generally circular. 
     In a more detailed feature of the invention, the looped shape may comprise a conical helix. 
     In a more detailed feature of the invention, the looped shape may comprise a cylindrical helix. 
     In a more detailed feature of the invention, the looped shape may comprise a proximal cylindrical helix and a distal conical helix. 
     In a more detailed feature of the invention, the proximal portion of the core-wire may have a first strain in response to a tensile force, the distal portion of the core-wire may have a second strain in response to the tensile force, and the first strain may be less than the second strain. 
     In a more detailed feature of the invention, the core-wire may comprise a superelastic material, the first strain may be in an initial elastic region, and the second strain may be in a superelastic region. 
     In a more detailed feature of the invention, the core-wire may be a one-piece structure comprising a nickel-titanium alloy, the proximal portion of the core-wire may have a filamentary diameter of comparatively greater dimension, and the distal portion of the core-wire may have a filamentary diameter of comparatively lesser dimension. 
     In a more detailed feature of the invention, at least a portion of the core-wire may be coated with a lubricious coating. 
     In a more detailed feature of the invention, the first path may be a straight line. 
     In a more detailed feature of the invention, the support may further comprise a proximal portion, and the proximal portion of the support and the distal portion of the support may form a one-piece structure. 
     In a more detailed feature of the invention, the support may further comprise a sleeve, and the sleeve may be disposed around at least one of the proximal portion of the support and the distal portion of the support. 
     In a more detailed feature of the invention, the actuator may comprise a handle and a slide, the slide may be slidably mounted on the handle to be selectively moved proximally and distally, the proximal end of the core-wire may be coupled to the handle, and the proximal end of the support may be coupled to the slide. 
     In a more detailed feature of the invention, the actuator may further comprise an anchor, the proximal end of the core-wire may be secured to the anchor, and the anchor may be rotatably mounted on the handle, whereby tension applied to the core-wire may be adjusted by rotating the anchor. 
     In a more detailed feature of the invention, the instrument may further comprise a spacer coil mounted around the distal portion of the core-wire and interior relative to the support. 
     In a more detailed feature of the invention, actuation of the actuator may cause the support to transition from the uncompressed state to the compressed state and the core-wire to transition from the relaxed state to the tensioned state, and cessation of actuation of the actuator may cause the support to transition from the compressed state to the uncompressed state and the core-wire to transition from the tensioned state to the relaxed state. 
     In a more detailed feature of the invention, the support may have a wind direction, the core-wire may have a wind direction, the wind direction of the support may be one of left-handed and right-handed, and the wind direction of the core-wire may be the other of left-handed and right-handed. 
     In a more detailed feature of the invention, when the support is in the uncompressed state, the support may comprise turns that may be spaced apart from one another in an area immediately proximal to a distal joinder of the core-wire and the support. 
     In a more detailed feature of the invention, the support may have a wind direction, the core-wire may have a wind direction, the wind direction of the support may be one of left-handed and right-handed, the wind direction of the core-wire may be the other of left-handed and right-handed, and, when the support is in the uncompressed state, the support may comprise turns that may be spaced apart from one another in an area immediately proximal to a distal joinder of the core-wire and the support. 
     For purposes of the present specification and claims, various relational terms like “top,” “bottom,” “proximal,” “distal,” “upper,” “lower,” “front,” and “rear” may be used to describe the present invention when said invention is positioned in or viewed from a given orientation. It is to be understood that, by altering the orientation of the invention, certain relational terms may need to be adjusted accordingly. 
     Additional objects, as well as features and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are hereby incorporated into and constitute a part of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. These drawings are not necessarily drawn to scale, and certain components may have undersized and/or oversized dimensions for purposes of explication. In the drawings wherein like reference numerals represent like parts: 
         FIG. 1  is a fragmentary perspective view of a first embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in its coiled state; 
         FIG. 2  is a fragmentary perspective view of the snare device of  FIG. 1 , the snare device being shown in its straightened state; 
         FIG. 3  is an enlarged fragmentary section view of a proximal portion of the snare device of  FIG. 1 , the snare device being shown in its coiled state; 
         FIG. 4  is an enlarged fragmentary section view of a proximal portion of the snare device of  FIG. 1 , the snare device being shown in its straightened state; 
         FIG. 5  is an enlarged fragmentary section view of a distal portion of the snare device of  FIG. 1 , the snare device being shown in its coiled state; 
         FIG. 6  is an enlarged fragmentary section view of a distal portion of the snare device of  FIG. 1 , the snare device being shown in its straightened state; 
         FIG. 7  is a fragmentary side view of the core-wire shown in  FIG. 6 , the inner-core wire being shown in its straightened state; 
         FIG. 8  is a distal end view of the core-wire shown in  FIG. 6 , the inner-core wire being shown in its straightened state; 
         FIG. 9  is a proximal end view of the support shown in  FIG. 6 , the support being shown in its compressed state; 
         FIG. 10  is a distal end view of the support shown in  FIG. 6 , the support being shown in its compressed state; 
         FIG. 11  is an enlarged perspective view of the end cap shown in  FIG. 1 ; 
         FIG. 12  is a partly exploded perspective view of the actuator shown in  FIG. 1 ; 
         FIG. 13  is a cross-section of a conventional snare device having a permanently skewed turn of its coiled wire support; 
         FIG. 14  is a graphic representation of a stress-strain curve for a core-wire like that shown in  FIG. 5 ; 
         FIG. 15  is an enlarged fragmentary section view of a second embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in a straightened state; 
         FIG. 16  is an enlarged fragmentary section view of a third embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in a coiled state; 
         FIG. 17  is an enlarged fragmentary section view of a fourth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in a coiled state; 
         FIG. 18  is an enlarged fragmentary section view of a fifth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in a coiled state; 
         FIG. 19  is an enlarged fragmentary section view of a sixth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in a coiled state; 
         FIGS. 20( a ) and 20( b )  are enlarged fragmentary section views of a seventh embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in its coiled state and its straightened state, respectively; 
         FIG. 21  is an enlarged fragmentary section view of an eighth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being shown in its coiled state; and 
         FIG. 22  is a photo showing a portion of the skewed support of the snare device of Comparative Example A. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     NITINOL™ nickel-titanium alloy is often used in medical devices in its superelastic state, where the austenite finish temperature (A f ) is less than room or body temperature. When a tensile force is applied to a NITINOL™ wire, the material may be elongated as much as 8% and then return to nearly its original length. In fact, when the tensile force applied causes the wire to reach its loading plateau (˜2% elongation), there is a small amount of hysteresis that is experienced in the wire when the force is released. In order to incur no hysteresis in the wire length, the tensile force must be somewhat less than the amount required to reach the loading plateau. When a wire is processed so that one segment is smaller in diameter than another segment, both states (i.e., hysteresis and no hysteresis) may be achieved when applying a certain tensile force. 
     Additionally, superelastic NITINOL™ may be shaped using heat to form complex geometries. In this application, the distal section of a NITINOL™ wire may be shaped into forms that are advantageous for medical procedures. Such shapes may include a cylindrical helix, a conical helix, and other rounded shapes that have smooth transitions. Sharp bends in the shape are generally not conducive to the embodiments described herein. 
     Superelastic NITINOL™ is often used to access locations within the human body. When a NITINOL™ core-wire is placed within a coil and attached at a distal location, it is possible to apply a tensile force to the core-wire while also applying a compressive force to the coil. In this application, the NITINOL™ core-wire may have a distal section and a proximal section, where the distal section may be made smaller in diameter than the proximal section. The distal section may also be set into a complex shape as described above, and the distal section of the coil may follow the complex shape, thereby resulting in gap spacing between the coils. An actuator may be positioned proximally, attaching to both the core-wire and the coil in order to apply a tensile force to the core-wire and a compressive force to the coil. 
     When the actuator applies tensile and compressive forces to the core-wire and the coil, respectively, the core-wire may begin to translate within the coil, the distal coil gaps may begin to close, and the distal complex shape may move from complex to straight as the coils may become stacked in their fully compressed state. The distal core-wire may be constrained within the compressed coil and may be mostly, though not completely, straight. The applied force may tension the distal, smaller core-wire portion onto its loading plateau while the proximal portion may be tensioned an amount that does not reach its loading plateau but may remain fully elastic with no hysteresis. 
     While actuated, if the distal portion experiences an external force that impacts the compressed coil, the coil may translate radially or become skewed. When this occurs, individual turns of the coil may become positioned inside and outside the original outer diameter and may grab onto the core-wire. The device then may become stuck and not translatable. The present inventor has discovered that, if the break load, relating to the ultimate tensile strength of the coil, exceeds the tensile force required for the distal core-wire to reach the loading plateau, skews will not occur or will occur with a dramatically reduced frequency. In the devices of the present invention, the coil may be selected so that its break load is greater than the distal core-wire loading plateau force. This provides sufficient headroom in the event that the complex shape of the core-wire requires a tensile force exceeding the loading plateau to be applied (where the elongation is between 6-8%). 
     Because of these force requirements and limitations, the actuator may then be tuned to apply only the tensile/compression forces that are required to compress the coil which straightens the device. If the actuator applied a force greater than required for straightness, there would be a risk of unwanted core-wire hysteresis and coil skewing that prevents the usefulness of the device. 
     Thus, the present invention is directed at addressing the problem of skewing that commonly occurs in snare devices of the type described above, as well as in similar instruments. As noted above, when such devices transition from a coiled state to a straightened state, if the support comprises a coil and if the coil is subjected to inwardly-directed radial forces of sufficient magnitude, one or more of the turns of the coil may become radially displaced inwardly or “skewed” relative to their neighboring turns. Such skewing of the coil is undesirable as it may cause the coil wire to bind with the core-wire, thereby impairing the ability of the core-wire to transition properly from its coiled state to its straightened state. In addition, such skewing of the coil wire may result in permanent misalignment of the turns of the coil, thereby impairing the ability of the coil to transition properly between its coiled state and its straightened state. 
     As noted above and as will be discussed further below, according to the present invention, it has been found that, unexpectedly, the aforementioned problem of skewing may be ameliorated when the distal portions of the coil and the core-wire (i.e., the respective portions of the coil and the core-wire that transition between coiled and straightened states) are selected such that the following relationship exists therebetween: 
       Break Load (distal portion of coil) &gt;Plateau Force (distal portion of core-wire)    
     wherein the break load of the distal portion of the coil is equal to the tensile strength of the distal portion of the coil multiplied by the filamentary cross-sectional area of the distal portion of the coil, wherein the distal portion of the core-wire has a filamentary cross-sectional area and also has an upper plateau stress in response to a tensile force applied thereto, and wherein the plateau force of the distal portion of the core-wire is equal to the upper plateau stress multiplied by the filamentary cross-sectional area of the distal portion of the core-wire. 
     Referring now to  FIGS. 1 through 6 , there are shown various views of a first embodiment of a snare device, the snare device being constructed according to the teachings of the present invention and being represented generally by reference numeral  11 . Details of snare device  11  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from one or more of  FIGS. 1 through 6  and/or from the accompanying description herein or may be shown in one or more of  FIGS. 1 through 6  and/or described herein in a simplified manner. 
     Snare device  11  may comprise a core-wire  13 , a support  15 , an end cap  17 , and an actuator  19 . 
     Core-wire  13 , which is also shown separately in  FIGS. 7 and 8 , may be an elongated, flexible, inhomogeneous filamentary structure that is capable of transitioning from a relaxed state to a tensioned state when subjected to a threshold tensile or pulling force. In the present embodiment, core-wire  13  may comprise a proximal portion  21  and a distal portion  23 . Proximal portion  21  may terminate proximally at a proximal end  25  of core-wire  13 , and distal portion  23  may terminate distally at a distal end  27  of core-wire  13 . Proximal portion  21  and distal portion  23  may be contiguous with one another or may be interconnected by a short intermediate portion  29 . In the present embodiment, proximal portion  21  may be characterized by having a first strain in response to a given tensile force (which first strain may be uniform along the entire length of proximal portion  21 ), and distal portion  23  may be characterized by having a second strain in response to the same tensile force (which second strain may be uniform along the entire length of distal portion  23 ), wherein the first strain of proximal portion  21  is less than the second strain of distal portion  23 . As a result of the disparity in the first strain and the second strain, when core-wire  13  is subjected to a tensioning force, distal portion  23  tends to stretch more than does proximal portion  21 . In the present embodiment, the first strain of proximal portion  21  may be in the initial elastic region (i.e., completely reversible length, no permanent elongation) whereas the second strain of distal portion  23  may be in the superelastic region (i.e., some permanent elongation). 
     Proximal portion  21  of core-wire  13  may have a relaxed state in which it is substantially straight in overall shape, and distal portion  23  of core-wire  13  may have a relaxed state in which it includes a looped or coiled shape. In the present embodiment, the looped or coiled shape of distal portion  23 , in the relaxed state, may be a generally circular shape. In other embodiments, which are discussed further below, the looped or coiled shape of distal portion  23  may comprise a conical helix, a cylindrical helix, other rounded shapes, or combinations thereof. When core-wire  13  is pulled to its tensioned state, distal portion  23  may lengthen, and its looped or coiled shape may substantially straighten. (To the extent that proximal portion  21  may also lengthen, such lengthening may be minor or negligible as compared to that experienced by distal portion  23 .) 
     In the present embodiment, core-wire  13  may be a unitary or one-piece structure made of a super-elastic and shaped-memory material. Such a material has the property that, when deformed and heated past a critical temperature, it “remembers” its deformed state. When cooled and subjected to further deformation, such a material springs back to this remembered shape. A suitable super-elastic material from which core-wire  13  may be manufactured is a nickel-titanium alloy commonly sold under the trade name NITINOL™. In the case of the aforementioned nickel-titanium alloy, the critical temperature is in the neighborhood of 700-1020 degrees Fahrenheit. 
     Where, as described above, core-wire  13  is a unitary structure made of a super-elastic, memory-shaped material, the difference in yield forces between proximal portion  21  and distal portion  23  may be achieved by providing proximal portion  21  with a first filamentary cross-sectional diameter d 1 , which may be substantially uniform along the entire length of proximal portion  21 , and by providing distal portion  23  with a second filamentary cross-sectional diameter d 2 , which may be substantially uniform along the entire length of distal portion  23 , wherein second filamentary cross-sectional diameter d 2  is smaller than first filamentary cross-sectional diameter d 1 . (Intermediate portion  29  may uniformly taper in filamentary cross-sectional diameter from proximal portion  21  to distal portion  23 .) Where, as in the present embodiment, proximal portion  21  of core-wire  13  has a greater filamentary cross-sectional diameter than does distal portion  23  of core-wire  13 , a given tensioning force applied to core-wire  13  results in more stress being applied to distal portion  23  of core-wire  13  than to proximal portion  21  of core-wire  13 . As a result, distal portion  23  experiences more strain, and hence greater elongation, than does proximal portion  21 . 
     In the present embodiment, both proximal portion  21  of core-wire  13  and distal portion  23  of core-wire  13  may be generally circular in filamentary cross-sectional shape; however, it is to be understood that proximal portion  21  of core-wire  13  and distal portion  23  of core-wire  13  need not be circular in filamentary cross-sectional shape and may possess other filamentary cross-sectional shapes. 
     As noted above, in those embodiments in which core-wire  13  is a unitary wire made of a single material, different lengths of core-wire  13  may have different filamentary cross-sectional diameters. For example, proximal portion  21  and distal portion  23  may have different filamentary cross-sectional diameters from one another. The ratio of the filamentary cross-sectional diameters of proximal portion  21  and distal portion  23  of core-wire  13  may depend on the material properties of core-wire  13 . The ratio may be selected such that a suitable differential strain can be achieved with only a modest exertion of force by an operator. The respective filamentary cross-sectional diameters of proximal portion  21  and distal portion  23  of core-wire  13  may be such that the tensile force applied by an operator will be insufficient for core-wire  13  to lose the memory of its remembered shape. In general, this means that the tensile force must be such that the distal portion  23  is elongated by less than 8% of its relaxed length, and preferably within 2% to 7% of its relaxed length. 
     There exist a variety of methods for manufacturing a core-wire  13  having two or more portions that differ in their yield forces. In one method, a continuous wire made of a shaped-memory metal is ground to a smaller filamentary diameter to form the distal portion  23 . The distal portion  23  is then heat-set to the desired shape. To achieve actuation of the core-wire  13 , there must be a sufficient difference in the yield force of the proximal portion  21  and the yield force of the distal portion  23 . This may be achieved by ensuring that the ratio of the filamentary diameter of the proximal portion  21  to that of the distal portion  23  is about 1.35 or greater. For a core-wire  13  having a non-circular cross-section, this may be achieved by ensuring that the ratio of the area of the proximal portion  21  to that of the distal portion  23  is about 1.8 or greater. 
     The actual transition from one state to another can be viewed as a wave traveling along core-wire  13 . The direction in which this wave travels can be controlled by controlling the taper of the transition between proximal portion  21  and distal portion  23 . In the case of a taper as shown in  FIGS. 5 and 6 , the wave travels from proximal portion  21  to distal portion  23  when the operator tensions on core-wire  13 . Conversely, when the operator releases core-wire  13 , the wave again travels from proximal portion  21  to distal portion  23 . 
     Alternatively or additionally, core-wire  13  may be formed from a unitary length of material by a local heat-treatment to change the yield stress in the heat-treated region. This may be done, for example, by applying heat locally to the distal portion while masking the heat from the proximal portion. In some cases, the application of localized heat to form the distal section may eliminate a need to grind core-wire  13 . In other cases, the application of localized heat may reduce the ratio of the proximal portion filamentary diameter to the distal portion filamentary diameter. Although, in the present embodiment, core-wire  13  may be a unitary structure, other methods exist of providing core-wire  13  with portions with different yield strengths. For example, core-wire  13  may be made by joining together two dissimilar materials having different yield strengths. More specifically, for example, core-wire  13  may be formed by joining together a proximal portion  21  made of stainless steel, MP35N® nickel-chromium-cobalt alloy (SPS Technologies LLC, Jenkintown, Pa.), or the like and a distal portion  23  made of NITINOL™ nickel-titanium alloy or the like. The aforementioned proximal and distal portions may be joined together by one or more of welding (e.g., butt, seam, lap, resistance, friction), soldering, brazing, or one or more adhesives. 
     Also, although core-wire  13  is described above as consisting of or comprising a super-elastic material, it is to be understood that core-wire  13  need not include a super-elastic material. For example, core-wire  13  may consist of or comprise a non-super-elastic material, such as a high tensile stainless steel, where distal portion  23  of core-wire  13  is significantly smaller in filamentary cross-sectional diameter than proximal portion  21  of core-wire  13  (e.g., filamentary cross-sectional diameter ratio of proximal portion  21  to distal portion  23  is greater than 3:1) and the strain/elongation of distal portion  23  does not exceed the yield point. 
     In addition, although not shown, core-wire  13  may be coated with a lubricious coating to reduce friction between core-wire  13  and support  15 . An example of a composition or material that may be used to form the lubricious coating may be polytetrafluoroethylene (PTFE). The lubricious coating may be applied along the entire length of core-wire  13  or may be applied only to a portion thereof, for example, only to proximal portion  21 . Alternatively and/or additionally, core-wire  13  may be coated with other types of materials, such as a hydrophilic and biocompatible composite material, examples of which include polyvinyl pyrrolidone (PVP), polyacrylamide, and hyaluronic acid. 
     Support  15 , which is also shown separately in  FIGS. 9 and 10 , may be an elongated tubular structure of generally circular overall cross-section comprising a proximal portion  31  and a distal portion  33 . In the present embodiment, support  15  may be in the form of a flexible coiled wire or filament. The coiled wire or filament of support  15  preferably has a round filamentary cross-sectional shape, such as a circular filamentary cross-sectional shape. A round filamentary cross-sectional shape is desirable as the edges of the round wire coil tend to line up more consistently when compressed, providing greater column strength when placed under compression. 
     As will be discussed further below, support  15  may be inserted over core-wire  13  so that proximal portion  31  may be generally disposed over much of proximal portion  21  of core-wire  13  and so that distal portion  33  may be generally disposed over much of distal portion  23  of core-wire  13 . 
     Proximal portion  31  may terminate proximally at a proximal end  35  of support  15 , and distal portion  33  may terminate distally at a distal end  37  of support  15 . In the present embodiment, proximal portion  31  and distal portion  33  may be contiguous with one another, and support  15  may be a unitary or one-piece structure. (Alternatively, as disclosed, for example, in U.S. Pat. Nos. 6,500,185 and 6,652,536, support  15  may be a multi-piece structure wherein proximal portion  31  is a cannula and distal portion  33  is a coiled wire.) In the present embodiment, support  15  may have a uniform filamentary cross-sectional diameter d 3  along its entire length, may have a uniform overall cross-sectional diameter d 4  along its entire length, and may possess a uniform tensile strength along its entire length. 
     As noted above, support  15  may be a coiled wire and may comprise a segmented structure capable of articulation between its constituent segments or turns  39 . Preferably, support  15  has an equilibrium compressed state, in which it defines a straight path corresponding to that shown in  FIG. 2 , and a non-equilibrium uncompressed state, in which proximal portion  31  follows a generally straight path and distal portion  33  follows a coiled path corresponding to that shown in  FIG. 1 . In the present embodiment, when support  15  is in its equilibrium compressed state, all or substantially all of turns  39  may be in contact with their neighboring turns  39 . By contrast, when support  15  is in its non-equilibrium uncompressed state, turns  39  of distal portion  33  may be in contact with their neighboring turns  39  along an inner radius  36  but may be separated from one another along an outer radius  38  (as seen, for example, in  FIG. 5 ). Thus, when support  15  is compressed, outer radius  38  may be comparatively shorter in length (and may be equal in length to inner radius  36 ) whereas, when support  15  is uncompressed, outer radius  38  may be comparatively longer in length (and may be longer in length than inner radius  36 ). 
     Support  15  may consist of or comprise a strong coilable material, such as a stainless steel, a titanium alloy, a chromium-cobalt alloy (e.g., MP35N® cobalt alloy, L-605 cobalt-chromium-tungsten-nickel alloy, etc.), or any other suitable strong coilable metal or other material. Support  15  may be made of one or more materials that are safe for use during magnetic resonance procedures. Support  15  may be coated with a hydrophilic and biocompatible composite material, such as polyvinyl pyrrolidone (PVP), polyacrylamide or hyaluronic acid. A suitable outer overall diameter of support  15  for general intra-vascular use may be approximately 0.014 inches. 
     To enable a surgeon to track the position of snare device  11  within a body, support  15  may be made of, or may include a portion made of, a radiopaque material such as platinum, tungsten, iridium, tin, gold, silver, or an alloy thereof. Alternatively, support  15  may be made of a coilable material coated with a radiopaque coating. Support  15  may comprise a close wound coil, with or without preload, or it may comprise an open wound coil. 
     In addition, although not shown, support  15  may further comprise a sleeve (or other type of cover) disposed around the outside of proximal portion  31  and/or a sleeve (or other type of cover) disposed around the outside of distal portion  33 . Such sleeves may be, for example, a unitary structure covering both proximal portion  31  and distal portion  33 . In some cases, a sleeve covering proximal portion  31  may be made of one or more materials possessing good lubricity, such as polytetrafluoroethylene (PTFE), or may be made of one or more materials, such as polyethylene terephthalate (PET) or urethane, to which a lubricious coating is applied. Alternatively and/or additionally, the sleeve may be coated with a hydrophilic and biocompatible composite material of the type discussed above in connection with core-wire  13  and support  15 . In some cases, the sleeve may have properties conducive to protecting the coiled wire of support  15  from laser radiation (e.g., laser radiation from a YAG laser), which may be desirable where snare device  11  may be used in procedures involving the use of such lasers (e.g., stone lithotripsy). In some cases, a sleeve covering distal portion  33  may comprise materials having both highly elastic properties and low durometer, such as silicone or urethane, and/or may comprise materials having good lubricious properties, such as PTFE. Such a sleeve may serve to fill spaces between adjacent turns  39  of distal portion  33  of support  15  when snare device  11  is in its coiled state. By filling such spaces, snare device  11  may be better able to capture desired materials (e.g., stone fragments from lithotripsy, blood clots, foreign bodies) when in its coiled state. The sleeve, which may be attached to distal portion  33  of support  15 , may have an inner diameter larger than the outer overall diameter of support  15 , whereby snare device  11  in its coiled state may expand within the sleeve to create a channel suitable for restoring blood flow. The sleeve may be of a constant size or may be highly elastic and expandable or blood permeable. This arrangement may act as a temporary stent or an embolic protection device. 
     End cap  17 , which is also shown separately in  FIG. 11 , may be a generally hemispherically-shaped member made of a rigid material, such as stainless steel or the like. In some cases, as in the present embodiment, end cap  17  may be hollow and may include an open bottom end  40 - 1  and an open top end  40 - 2 . In other cases, as shown in other embodiments below, end cap  17  may be solid. End cap  17  may be used to mechanically couple core-wire  13  and support  15  at their respective distal ends. For example, in the present embodiment, distal end  27  of core-wire  13  may be fixed within top end  40 - 2  of end cap  17  by welding, soldering, brazing, or one or more epoxies or adhesives, and distal end  37  of support  15  may be fixed to open bottom end  40 - 1  of end cap  17  by welding, soldering, brazing, or one or more epoxies or adhesives. 
     Actuator  19 , which is also shown separately in  FIG. 12 , may comprise a handle  41 , an anchor  43 , and a slide  45 . 
     Handle  41 , which may be made of a rigid, durable material, such as a suitable polymer or metal, may be an elongated structure. In the present embodiment, handle  41  may be shaped to include a pair of longitudinally extending cavities, namely, a proximal cavity  47  and a distal cavity  49 . Proximal cavity  47  may extend distally from a proximal end  51  of handle  41 , and distal cavity  49  may extend proximally from a distal end  53  of handle  41 . Proximal cavity  47  and distal cavity  49  may be axially aligned with one another and may be contiguous, whereby the distal end of proximal cavity  47  may open into the proximal end of distal cavity  49 . In the present embodiment, proximal cavity  47  and distal cavity  49  may be generally cylindrical in cross-sectional shape, with proximal cavity  47  having a comparatively smaller diameter and with distal cavity  49  having a comparatively larger diameter. Proximal cavity  47  and distal cavity  49  may be appropriately dimensioned so that proximal portion  21  of core-wire  13  may be freely disposed therewithin. 
     Handle  41  may be shaped further to include a cavity  55 . Cavity  55 , which may be arranged to be transverse relative to the longitudinal axis of handle  41 , may extend inwardly from a side  57  of handle  41  and may intersect with and pass through proximal cavity  47 . 
     Anchor  43 , which may be made of a rigid, durable material, such as a suitable polymer or metal, may be an elongated, barrel-shaped structure. Anchor  43  may be appropriately dimensioned to be snugly mounted, yet rotatably adjustable, within cavity  55  of handle  41 . A transverse channel  59  may be provided in anchor  43  so that proximal end  25  of core-wire  13  may be secured to anchor  43  (for example, by being inserted through channel  59  and then tied around anchor  43 ). A slot  61  may be provided in an outer end of anchor  43  and may be shaped to receive a flat-head screwdriver or similarly shaped tool (not shown) that may be used to rotate anchor  43  within cavity  55 . As can be appreciated, by rotating anchor  43  within cavity  55 , the amount of tension that is applied to core-wire  13  may be adjusted. 
     Slide  45 , which may be made of a rigid, durable material, such as a suitable polymer or metal, may be an elongated, hollow structure. Slide  45  may be appropriately dimensioned to be slidably mounted within distal cavity  49  of handle  41  so as to move proximally and distally therewithin. Slide  45  may be shaped to include a proximal end  65 , a distal end  67 , and a longitudinal cavity  69  that may extend from proximal end  65  to distal end  67 . Cavity  69  may be appropriately dimensioned to permit core-wire  13  to extend freely therethrough. Proximal end  35  of support  15  may be fixedly secured to distal end  67  of slide  45  by welding, soldering, brazing, adhesive or other suitable means. A grip  70  may be fixedly mounted on slide  45  for use in moving slide  45  back and forth within handle  41 . A stop  71  may be mounted in an opening  72  on slide  45  and may be movable within a track  73  formed in handle  41 . Stop  71  may be arranged for contact with a set screw  74  mounted in an opening  75  on handle  41  to define a range of movement of slide  45  back and forth relative to handle  41 . 
     To summarize, in the present embodiment, proximal end  25  of core-wire  13  may be secured to anchor  43 , which, in turn, may be coupled to handle  41 , and distal end  27  of core-wire  13  may be secured to end cap  17 . Proximal end  35  of support  15  may be secured to slide  45 , which, in turn, may be slidably mounted on handle  41 , and distal end  37  of support  15  may be secured to end cap  17 . Alternatively stated, proximal end  35  of support  15  may be movable proximally and distally relative to proximal end  25  of core-wire  13  whereas distal end  27  of core-wire  13  may be fixed relative to distal end  37  of support  15 . 
     In view of the above, when slide  45  is at its most proximal position, proximal end  35  of support  15  is also at its most proximal position. As a result, the compressive force applied to support  15  is at its relative minimum, and, consequently, a distally-directed force applied to distal end  37  of support  15  is also at its relative minimum. In addition, due to the coupling of core-wire  13  and support  15  at end cap  17 , the distally-directed tensioning force applied to distal end  27  of core-wire  13  is also at its relative minimum. With the tensioning of core-wire  13  thus at its relative minimum, core-wire  13  assumes its relaxed state, with distal portion  23  of core-wire  13  forming a looped or coiled shape. With core-wire  13  assuming a looped or coiled shape, distal portion  33  of support  15 , which is constrained to follow the shape of distal portion  23  of core-wire  13 , assumes a correspondingly looped or coiled shape. While in this looped or coiled shape, distal portion  33  of support  15  is uncompressed. 
     By contrast, when slide  45  is at its most distal position, proximal end  35  of support  15  is also at its most distal position. As a result, the compressive force applied to support  15  is at its relative maximum, and, consequently, the distally-directed force applied to distal end  37  of support  15  is also at its relative maximum. In addition, due to the coupling of core-wire  13  and support  15  at end cap  17 , the distally-directed tensioning force applied to distal end  27  of core-wire  13  is also at its relative maximum. With the tensioning of core-wire  13  thus at its relative maximum, core-wire  13  stretches, particularly distal portion  23  of core-wire  13 . As distal portion  23  of core-wire  13  stretches, it straightens. In this straightened state, core-wire  13  no longer forces support  15  to adopt a looped or coiled shape. As a result, distal portion  33  of support  15  is allowed to revert to its equilibrium compressed state in which it follows a straight path. Thus, the movement of slide  45  from its proximal position to its distal position serves both to apply a tensile force to core-wire  13  and to apply a compressive force to the support  15 , the tensile force causing core-wire  13  to transition from its relaxed state to its tensioned state, the compressive force causing support  15  to transition from its uncompressed state to its compressed state. When support  15  is in its compressed state and follows a straight path, core-wire  13  is largely constrained by support  15  to follow a generally straight path, despite exhibiting some minor degree of flexure or undulation within support  15  (see  FIG. 6 ). 
     In use, snare device  11  may be transformed from its coiled state to its straightened state by moving slide  45  from its most proximal position to its most distal position. With snare device  11  thus transformed to its straightened state, snare device  11  may then be inserted into a patient to a desired location, such as to a location where end cap  17  of snare device  11  may be positioned to a point just beyond an object to be snared. Then, snare device  11  may be transformed back to coiled state from its straightened state by moving slide  45  back to its most proximal position from its most distal position, whereby the object may be snared by the coiled or looped portion of snare device  11 . Because of its coiled or looped shape, snare device  11  may be particularly well-suited for grasping a medical instrument to guide it to a desired location. For example, snare device  11  could be inserted through an implanted abdominal aortic aneurysm (AAA) endovascular aneurysm repair (EVAR) stent into the second iliac artery and adjacent to a guidewire that has been placed in the femoral artery. Deploying the circular or looped shape around the aforementioned guidewire to ensnare it, snare device  11  could then be withdrawn to guide the guidewire into the EVAR stent. Other possible uses for snare device  11  may be found in U.S. Pat. Nos. 6,500,185 and 6,652,536. 
     As noted above, as support  15  transitions from its uncompressed state to its compressed state, turns  39  of support  15  may experience varying radial forces. These forces may cause one or more turns  39  to become radially displaced or “skewed” relative to core-wire  13 . If the radial displacement is not too large, support  15  will not contact core-wire  13 , and the tension within support  15  will restore the radially displaced turns  39  to their equilibrium aligned positions upon removal of the radial force. However, as shown in  FIG. 13 , if the radial displacement is too large, as may be the case for a snare device like snare device 10 of U.S. Pat. No. 6,500,185, normally non-neighboring turns  85   a  and  85   c  may be caused to become adjacent to one another, permanently misaligning an intervening turn  85   b . If the permanently misaligned turn  85   b  extends far enough inwardly to contact core-wire  87 , core-wire  87  may be impaired from straightening. 
     In accordance with the present invention, it has unexpectedly been found that the aforementioned problem of skewing may be ameliorated if distal portion  33  of support  15  and distal portion  23  of core-wire  13  are characterized by the following relationship: 
       Break Load (distal portion 33 of support 15) &gt;Plateau Force (distal portion 23 of core-wire 13)    
     wherein the break load of distal portion  33  of support  15  is equal to the tensile strength of distal portion  33  of support  15  multiplied by the filamentary cross-sectional area of distal portion  33  of support  15 , wherein distal portion  23  of core-wire  13  has a filamentary cross-sectional area and also has an upper plateau stress in response to a tensile force applied thereto, and wherein the plateau force of distal portion  23  of core-wire  13  is equal to the upper plateau stress multiplied by the cross-sectional area of distal portion  23  of core-wire  13 . To minimize the likelihood that skewing occurs anywhere along the length of distal portion  33  of support  15 , the foregoing relationship (i.e., the break load of distal portion  33  of support  15  is greater than the plateau force of distal portion  23  of core-wire  13 ) should apply to the entire length of distal portion  33  of support  15 . 
     As an example of the foregoing relationship, distal portion  33  of support  15  may have a tensile strength of 322,000 psi and a filamentary cross-sectional area of 9.62×10 −6  in 2 , and distal portion  23  of core-wire  13  may have a filamentary cross-sectional area of 3.28×10 −5  in 2  and an upper plateau stress of 81,000 psi. As a result, the break load of distal portion  33  of support  15  is 3.10 (i.e., 322,000 psi times 9.62×10 −6  in 2 ), which is greater than the plateau force of 2.66 (i.e., 81,000 psi times 3.28×10 −5  in 2 ) for distal portion  23  of core-wire  13 . 
     It is to be understood that the above values are merely illustrative and that, as long as the break load of distal portion  33  of support  15  is greater than the plateau force of distal portion  23  of core-wire  13 , the tensile strength of distal portion  33  of support  15 , the filamentary cross-sectional area of distal portion  33  of support  15 , the filamentary cross-sectional area of distal portion  23  of core-wire  13 , and the plateau stress of distal portion  23  of core-wire  13  may fall anywhere within broad ranges of values. For example, and without limitation, the tensile strength of distal portion  33  of support  15  may be 300,000-420,000 psi, the filamentary cross-sectional area of distal portion  33  of support  15  may be 4.91×10 −6 -1.06×10 −4  in 2 , the filamentary cross-sectional area of distal portion  23  of core-wire  13  may be 1.80×10 −5 -4.13×10 −4  in 2 , and the upper plateau stress of distal portion  23  of core-wire  13  may be 70,000-90,000 psi. Additional examples falling within these ranges are presented below. 
     By comparison, if the plateau force for the distal portion of the core-wire is greater than the break load of the distal portion of the support coil, the support coil may be expected to skew. For example, if the distal portion of the core-wire were to have an upper plateau stress of 84,000 psi and a filamentary cross-sectional area of 2.04×10 −5  in 2  and if the support coil were to consist of a platinum alloy filament having a maximum ultimate tensile strength of about 180,000 psi and a filamentary cross-sectional area of 4.91×10 −6  in 2 , the support coil may be expected to skew when the snare device is straightened by tensioning the core-wire and compressing the support coil. This is because the plateau force of the distal portion of the core-wire would be 1.72 lbs, which would be greater than the break force of 0.88 lbs for the support coil. In a similar fashion, if the distal portion of the core-wire were to have an upper plateau stress of 84,000 psi and a filamentary cross-sectional area of 2.04×10 −5  in 2  and if the support coil were to consist of a stainless steel wire filament having a maximum ultimate tensile strength of about 340,000 psi and a filamentary cross-sectional area of 4.91×10 −6  in 2 , the support coil may be expected to skew when the snare device is straightened by tensioning the core-wire and compressing the support coil. This is because the plateau force of the distal portion of the core-wire would be 1.72 lbs, which would be greater than the break force of 1.67 lbs for the support coil. 
     Also, as alluded to above in connection with the &#39;638 publication, if the support coil has a rectangular filamentary cross-section, the support coil may be expected to skew. This is because the present inventor believes that a rectangular cross-sectional shape of the support coil will permit variable alignment at the support coil edges. 
     Another important aspect of the design of the snare device of the present invention is that the distal portion of the core-wire have a reasonably uniform filamentary diameter over its entire length in its relaxed state. For example, where a continuous wire that is made of a shaped-memory metal is ground to a smaller filamentary diameter to form the distal portion of the core-wire, the filamentary diameter of the distal portion of the core-wire should be reasonably uniform over its entire length. A typical barrel grind tolerance of ±0.0002 inch is reasonable for diameters larger than 0.003 inch. If the distal grind diameter were to decrease substantially as one moves distally along the distal portion of the core-wire, the tensile force associated with the loading plateau required to overcome the larger, more proximal diameter would result in the smaller diameter exceeding its loading plateau and elongating beyond the 8% recoverable strain. As a result, the smaller diameter would be plastically deformed and elongated, and the device would lose its shape in the relaxed position. 
     On the other hand, if the distal grind diameter were to increase substantially as one moves distally along the distal portion of the core-wire, the mechanism would not be able to transition completely from the relaxed state to the tensioned/compressed state. The tensile force that would be required to overcome the larger diameter would also elongate the unground core-wire, resulting in hysteresis along the entire length. Moreover, the extended length of the core-wire would prevent the support from ever being fully compressed, and the handle stroke would be insufficient to accommodate this movement. 
     Another aspect of the design of the snare device of the present invention is the curvature of core-wire  13 . It is generally known that NITINOL™ nickel-titanium alloy has a largely recoverable engineering strain, e, of 6-8%. Experimentally, the NITINOL™ core-wire is heat-set to 9% strain, according to the equation e=1/(2R/t+1), where R equals the bend radius and t equals the wire thickness. Because a straight configuration of the device is defined by the compressed coil, the core-wire does not have to be completely straight when placed under tension and, instead, may have residual bends even when the device is straight (see  FIG. 6 ). However, if the NITINOL™ core-wire has any sharp bends, the device will tend to skew in these locations since the compression force to overcome the tensioned wire shape will be focused at these points. Skewing from sharp bends may be mitigated against by increasing the ratio of coil wire break load vs core-wire plateau force. 
     Referring now to  FIG. 14 , there is graphically shown a stress-strain curve for an embodiment of core-wire  13  in which core-wire  13  is made of NITINOL™ nickel-titanium alloy, proximal portion  21  is circular in cross-sectional shape and has a uniform filamentary cross-sectional diameter of 0.012 in, and distal portion  23  is circular in cross-sectional shape and has a uniform filamentary cross-sectional diameter of 0.0079 in. As can be seen from the aforementioned stress-strain curve, distal portion  23  exhibits an upper plateau stress (i.e., an upper region of approximately constant stress in the stress-strain curve) of approximately 82,000-84,000 psi. 
     Referring now to  FIG. 15 , there is shown an enlarged fragmentary section view of a second embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  111 . Details of snare device  111  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 15  and/or from the accompanying description herein or may be shown in  FIG. 15  and/or described herein in a simplified manner. 
     Snare device  111  may be similar in most respects to snare device  11 . One difference between snare device  111  and snare device  11  may be that, whereas snare device  11  may comprise a core-wire  13  whose distal end  27  is anchored to end cap  17 , snare device  111  may comprise a core-wire  113  whose distal end  127  is not anchored to end cap  17 . Instead, distal end  127  of core-wire  113  may be anchored to an anchoring element  131  disposed on distal portion  33  of support  15  at an intermediate point that is distal to the coiled or looped shape of core-wire  113  and is proximal to distal end  37  of support  15 . A safety wire  133  may extend distally from anchoring element  131  to end cap  17  to prevent the support  15  from unravelling. This arrangement isolates any tensile force applied to core-wire  113  to points proximal to the aforementioned intermediate point and results in a coiled or looped shape having a floppy and atraumatic distal tip. Such an atraumatic tip may be advantageous because it enables the instrument to be maneuvered in constricted regions without a significant risk of perforating or otherwise damaging surrounding structures. 
     Referring now to  FIG. 16 , there is shown an enlarged fragmentary section view of a third embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  211 . Details of snare device  211  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 16  and/or from the accompanying description herein or may be shown in  FIG. 16  and/or described herein in a simplified manner. 
     Snare device  211  may be similar in most respects to snare device  11 . One difference between snare device  211  and snare device  11  may be that snare device  211  may further comprise a spacer coil  221 . Spacer coil  221  may have a proximal end attached to a proximal spacer  223  mounted on intermediate portion  29  of core-wire  13  and may have a distal end attached to a distal spacer  225  mounted on core-wire  13  proximate to distal end  27 . Spacer coil  221  may comprise or consist of a radiopaque material, such as platinum. However, other materials may also be used depending on the specific application of the instrument  211 . Other tubular structures may be used in place of spacer coil  221  to enclose core-wire  13 . For example, baffles, bellows or any such flexible and compressible tube having dimensions as described below can also be used. Spacer coil  221  may consist of a single wire formed into a coil or may comprise two or more wires formed into a coil. 
     Spacer coil  221  may be in contact with core-wire  13  or may be separated therefrom by a clearance that is small enough to minimize the likelihood that articulating spacer-coil segments or turns  229  of spacer coil  221  may, themselves, become radially displaced relative to core-wire  13 . This clearance is preferably determined by the dimensions of spacer coil  221 . In one embodiment, the clearance is selected to be less than the radius of the windings or turns that make up spacer coil  221 . 
     In addition, spacer coil  221  and support  15  may be dimensioned relative to one another to minimize the skewing of support  15  and, additionally, to minimize the likelihood that spacer coil  221  and support  15  may bind with each other during use. To minimize the likelihood that turns of support  15  may protrude into openings formed between turns of spacer coil  221 , the pitch angle of turns  229  of spacer coil  221  is preferably selected to be different from the pitch angle of turns  39  of support  15 . Such a difference can be achieved by winding spacer coil  221  and support  15  in opposite directions. In one embodiment, these two pitch angles are at right angles to each other. However, any difference in pitch angle will reduce the likelihood of penetration. 
     Referring now to  FIG. 17 , there is shown an enlarged fragmentary section view of a fourth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  311 . Details of snare device  311  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 17  and/or from the accompanying description herein or may be shown in  FIG. 17  and/or described herein in a simplified manner. 
     Snare device  311  may be similar in most respects to snare device  11 . One difference between snare device  311  and snare device  11  may be that snare device  311  may comprise a core-wire  313 , instead of core-wire  13 . Core-wire  313  may differ from core-wire  13  in that core-wire  313  may comprise a distal portion  323  that may include a conical helix (i.e., larger turns proximally and tapering smaller turns distally) when in its relaxed state. Such a conical helix may be used like a basket and may be useful for such purposes as capturing kidney stone fragments during lithotripsy (retropulsion). Although not shown, a blood permeable sack or sock (such as in U.S. Patent Application Publication No. 2009/0209987 A1) may be attached to the distal region  35  of support  15  to enable device  311  to be used as an embolic protection device. 
     Referring now to  FIG. 18 , there is shown an enlarged fragmentary section view of a fifth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  411 . Details of snare device  411  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 18  and/or from the accompanying description herein or may be shown in  FIG. 18  and/or described herein in a simplified manner. 
     Snare device  411  may be similar in most respects to snare device  11 . One difference between snare device  411  and snare device  11  may be that snare device  411  may comprise a core-wire  413 , instead of core-wire  13 . Core-wire  413  may differ from core-wire  13  in that core-wire  413  may comprise a distal portion  423  that may include a cylindrical helix when in its relaxed state. Such a cylindrical helix may be useful for such purposes as forming a temporary stent to open an otherwise occluded or constricted lumen in a person&#39;s body. For example, snare device  411  could be used to open blood vessels to restore blood flow, to open gastrointestinal vessels to allow the flow of food, bile, etc., and to open a ureter to restore the flow of urine. 
     Referring now to  FIG. 19 , there is shown an enlarged fragmentary section view of a sixth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  511 . Details of snare device  511  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 19  and/or from the accompanying description herein or may be shown in  FIG. 19  and/or described herein in a simplified manner. 
     Snare device  511  may be similar in most respects to snare device  11 . One difference between snare device  511  and snare device  11  may be that snare device  511  may comprise a core-wire  513 , instead of core-wire  13 . Core-wire  513  may differ from core-wire  13  in that core-wire  513  may comprise a distal portion  523  that may include, when in its relaxed state, a combination of a proximal cylindrical helix  525  and a distal cone  527 . Such a shape may be useful as a snare to capture objects, such as blood clots, gallstones, and other foreign bodies. 
     Although the approaches described above do significantly ameliorate the problem of skewing in devices of the type described herein, such devices sometimes encounter some difficulty in transitioning from a straightened configuration, in which the support is compressed and the core-wire is tensioned, to a looped or coiled configuration, in which the support is uncompressed and the core-wire is relaxed and assumes a looped or coiled shape. As discussed above in detail, when the device is in its straightened configuration, the device may be used to gain access to a site in need of treatment. By contrast, when the device is in its looped or coiled configuration, the device may be used to provide treatment to the site, such as by securing or retrieving a mass of some sort. If the device cannot properly transition from a straightened configuration to a looped or coiled configuration after gaining access to a site, treatment of the site (e.g., removal of a mass) cannot be performed as completely as desired. 
     Without wishing to be limited to a particular theory or explanation as to why difficulties sometimes occur in transitioning from a straightened configuration to a looped or coiled configuration, the present inventor offers the following: When the device is in its straightened configuration, the coils of the support are usually compressed, thus defining the path. However, as the device is relaxed, the coils of the support are no longer actively compressed, and the core-wire is no longer placed in tension. As a result, the non-straight configuration of the core-wire should then define the shape of the device. During this transition, the motion of return typically begins at the proximal end of the device and propagates towards the distal end. Often, the distal portion of the looped or non-straight configuration does not fully return to the original, relaxed state but remains somewhere between the straight configuration and the looped configuration, where the compressed coil still provides some definition to the shape. Typically, this is primarily due to frictional forces between the coils of the support and the core-wire. 
     To further an understanding of this problem,  FIGS. 20( a ) and 20( b )  show another embodiment of a snare device constructed according to the present invention, the snare device being represented by reference numeral  611 . As can be seen by comparing snare device  611  of  FIGS. 20( a ) and 20( b )  to snare device  11  of  FIGS. 5 and 6 , snare devices  611  and  11  may be similar in many respects. To some extent, snare device  11  may be regarded as an idealized embodiment of a snare device of the present invention (which idealized embodiment may be attained or approximated in some cases) whereas snare device  611  may be regarded as an alternative or variant embodiment of such a snare device that may be found in practice (which alternative embodiment may be attained or approximated in some cases). Accordingly, parts of snare device  611  that correspond to parts of snare device  11  are identified by reference numbers that exceed those of snare device  11  by  600 . 
     A principal difference between snare devices  11  and  611  is that, in snare device  11 , distal portion  23  of core-wire  13  is shown as not contacting any part of distal portion  33  of support  15 . By contrast, in snare device  611 , distal portion  623  of core-wire  613  is shown as contacting at least some turns  639  of support  615 . Such contact between distal portion  623  and support  615  while snare device  611  is in a straightened configuration (as in  FIG. 20( b ) ) may cause distal portion  623  to frictionally engage or bind to support  615  and, in so doing, may impede or prevent snare device  611  from transitioning completely from a straightened configuration to a looped configuration. As noted above, such a failure to transition completely from a straightened configuration to a looped configuration is undesirable. (It may be noted that, although distal end  627  of core-wire  613  is not shown in  FIGS. 20( a ) and 20( b )  as centered within end cap  617 , distal end  627  may be centered within end cap  617 .) 
     To ameliorate the above problem, the present inventor has identified certain modifications to the snare device that may be made. One such modification is to place a slight stretch in the support just proximal to the joint between the core-wire and the support. What this modification may accomplish is to provide a spring force to the compressed coils of the support so that, when the core-wire tension is released, the spring force is also released. Consequently, as the coils of the support reopen, they tend to slide along the core-wire and overcome the frictional forces holding the snare device in a semi-straight orientation. Preferably, the gap stretch is no greater than approximately one coil width, and one-half of a coil width is often sufficient to overcome the aforementioned frictional forces. In most cases, the length of the stretch only needs to be up to 4 times the coil diameter to overcome the frictional forces. This distal coil stretch may aid in the device returning to its fully relaxed configuration. 
     Another modification is to choose opposing wind directions for the core-wire loop and the coils of the support. When heat-setting a NITINOL™ core-wire into a helix or looped shape, a form tool is generally used to hold the core-wire in place before it is placed into a heat zone. This helix has a wind direction, as do the coils of the support. When the core-wire is placed inside a coiled support, if the wind direction of the coiled support and the core-wire are the same, the angles are somewhat similar, and the device may resist returning to a looped configuration when at rest. However, when the wind directions of the core-wire and the coiled support are opposite (e.g. left vs. right), the angles are more different, and the two members may slip more easily in relation to each other, thereby allowing the looped configuration to return more completely. 
     Referring now to  FIG. 21 , there is shown an enlarged fragmentary section view of an eighth embodiment of a snare device constructed according to the teachings of the present invention, the snare device being represented generally by reference numeral  711 . Details of snare device  711  that are discussed elsewhere in this application or that are not critical to an understanding of the invention may be omitted from  FIG. 21  and/or from the accompanying description herein or may be shown in  FIG. 21  and/or described herein in a simplified manner. 
     As can be seen, snare device  711  may include both of the modifications discussed above for promoting a more complete transitioning from a straightened configuration to a looped configuration. More specifically, snare device  711  may comprise a core-wire  713  and a support  715 . Core-wire  713  may be identical to core-wire  13 . Support  715  may be similar in many respects to support  15  but may differ from support  15  in the following two respects: First, whereas support  15  may comprise a coiled wire having a right-handed wind direction (see  FIG. 5 , where turns  39  are angled from right to left moving downwardly), support  715  may comprise a coiled wire having a left-handed wind direction (i.e., turns  739  are angled from left to right moving downwardly). Such a left-handed wind direction is opposite to the right-handed wind direction of core-wire  713 . Second, whereas turns  39  of support  15  are closely packed or compressed in the area proximal to the distal joinder of core-wire  13  and support  15  (even when snare device  11  is in its looped configuration), turns  739  of support  715  are spaced apart in an area  740  proximal to the distal joinder of core-wire  713  and support  715 . (It may be noted that, although the distal end of core-wire  713  is not shown in  FIG. 21  as centered within the end cap, the distal end of core-wire  713  may be centered within the end cap.) 
     It should be noted that, whereas snare device  711  is shown as differing from snare device  11  in both of the respects discussed above, snare device  711  may differ from snare device  11  in only one or the other of these two respects. Moreover, the problems discussed above and the modifications thereto may apply to any of the snare devices of the present invention. 
     The following examples are provided for illustrative purposes only and are in no way intended to limit the scope of the present invention: 
     Example 1 
     A snare device like that of  FIGS. 1-6  of the present application was constructed. The core-wire of the snare device was made of NITINOL™ nickel-titanium alloy, with the proximal portion of the core-wire being circular in cross-sectional shape and having a uniform filamentary cross-sectional diameter of 0.0075 in and with the distal portion of the core-wire being circular in cross-sectional shape and having a uniform filamentary cross-sectional diameter of 0.0051 in. Consequently, the filamentary cross-sectional area of the distal portion of the core-wire was 2.04×10 −5  in 2 . The upper plateau stress for the distal portion of the core-wire was 84,000 psi. As a result, the plateau force for the distal portion of the core-wire was 1.72 lbs. 
     The support of the subject snare device was in the form of a coil made of a stainless steel wire of round cross-section having the highest tensile strength that is commercially available, i.e., 420,000 psi (Fort Wayne Metals, Fort Wayne, Ind.). The support had a uniform overall outer diameter of 0.0135 in and a uniform overall inner diameter of 0.0085 in. Consequently, the filamentary cross-sectional diameter of the support was 0.0025 in, and the filamentary cross-sectional area of the support was 4.91×10 −6  in 2 . As noted above, the ultimate tensile strength of the support was 420,000 psi. As a result, the break force for the support was 2.06 lbs, which is greater than the plateau force of 1.72 lbs. Table 1 below summarizes some of the above-noted values. 
     
       
         
           
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Core-wire 
                   
               
            
           
           
               
               
               
            
               
                   
                 Distal 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Distal 
                 Portion 
                 Distal 
                 Support 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Distal 
                 Portion 
                 Upper 
                 Portion 
                   
                   
                 Ultimate 
                   
               
               
                 Portion 
                 Cross- 
                 Plateau 
                 Plateau 
                 Filamentary 
                   
                 Tensile 
                 Break 
               
               
                 Diameter 
                 Sectional 
                 Stress 
                 Force 
                 Diameter 
                 Cross-Sectional 
                 Strength 
                 Force 
               
               
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
               
               
                   
               
               
                 0.0051 
                 2.04 × 10 −5   
                 84,000 
                 1.72 
                 0.0025 
                 4.91 × 10 −6   
                 420,000 
                 2.06 
               
               
                   
               
            
           
         
       
     
     For the above-described embodiment, the support coil column strength was found to be sufficient to resist skewing when compressed by the tensile force applied by the core-wire. 
     Example 2 
     A snare device similar to that of Example 1, but larger in diameter, was constructed. The core-wire of the snare device was made of NITINOL™ nickel-titanium alloy, with the proximal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0121 in and with the distal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0082 in. Consequently, the filamentary cross-sectional area of the distal portion of the core-wire was 5.28×10 −5  in 2 . The upper plateau stress for the distal portion of the core-wire was 85,000 psi. As a result, the plateau force for the distal portion of the core-wire was 4.5 lbs. 
     The support of the subject snare device was in the form of a coil made of a stainless steel wire of round cross-section having an ultimate tensile strength of 340,000 psi. The support had a uniform overall outer diameter of 0.0226 in and a uniform overall inner diameter of 0.0136 in. Consequently, the filamentary cross-sectional diameter of the support was 0.0045 in, and the filamentary cross-sectional area of the support was 1.59×10 −5  in 2 . As noted above, the ultimate tensile strength of the support was 340,000 psi. As a result, the break force for the support was 5.4 lbs, which is greater than the plateau force of 4.5 lbs. Table 2 below summarizes some of the above-noted values. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Core-wire 
                   
               
            
           
           
               
               
               
            
               
                   
                 Distal 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Distal 
                 Portion 
                 Distal 
                 Support 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Distal 
                 Portion 
                 Upper 
                 Portion 
                   
                   
                 Ultimate 
                   
               
               
                 Portion 
                 Cross- 
                 Plateau 
                 Plateau 
                   
                   
                 Tensile 
                 Break 
               
               
                 Diameter 
                 Sectional 
                 Stress 
                 Force 
                 Filamentary 
                 Cross-Sectional 
                 Strength 
                 Force 
               
               
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
                 Diameter (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
               
               
                   
               
               
                 0.0082 
                 5.28 × 10 −5   
                 85,000 
                 4.5 
                 0.0045 
                 1.59 × 10 −5   
                 340,000 
                 5.4 
               
               
                   
               
            
           
         
       
     
     For the above-described embodiment, the support coil column strength was found to be sufficient to resist skewing when compressed by the tensile force applied by the core-wire. 
     Example 3 
     A snare device similar to that of Example 2, but larger in diameter, was constructed. The core-wire of the snare device was made of NITINOL™ nickel-titanium alloy, with the proximal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0148 in and with the distal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0098 in. Consequently, the filamentary cross-sectional area of the distal portion of the core-wire was 7.54×10 −5  in 2 . The upper plateau stress for the distal portion of the core-wire was 77,700 psi. As a result, the plateau force for the distal portion of the core-wire was 5.9 lbs. 
     The support of the subject snare device was in the form of a coil made of a stainless steel wire of round cross-section having an ultimate tensile strength of 344,000 psi. The support had a uniform overall outer diameter of 0.0263 in and a uniform overall inner diameter of 0.0163 in. Consequently, the filamentary cross-sectional diameter of the support was 0.0050 in, and the filamentary cross-sectional area of the support was 1.96×10 −5  in 2 . As noted above, the ultimate tensile strength of the support was 344,000 psi. As a result, the break force for the support was 6.8 lbs, which is greater than the plateau force of 5.9 lbs. Table 3 below summarizes some of the above-noted values. 
     
       
         
           
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Core-wire 
                   
               
            
           
           
               
               
               
            
               
                   
                 Distal 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Distal 
                 Portion 
                 Distal 
                 Support 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Distal 
                 Portion 
                 Upper 
                 Portion 
                   
                   
                 Ultimate 
                   
               
               
                 Portion 
                 Cross- 
                 Plateau 
                 Plateau 
                 Filamentary 
                   
                 Tensile 
                 Break 
               
               
                 Diameter 
                 Sectional 
                 Stress 
                 Force 
                 Diameter 
                 Cross-Sectional 
                 Strength 
                 Force 
               
               
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
               
               
                   
               
               
                 0.0098 
                 7.54 × 10 −5   
                 77,700 
                 5.9 
                 0.0050 
                 1.96 × 10 −5   
                 344,000 
                 6.8 
               
               
                   
               
            
           
         
       
     
     For the above-described embodiment, the support coil column strength was found to be sufficient to resist skewing when compressed by the tensile force applied by the core-wire. 
     Comparative Example A 
     A snare device similar to that of Example 2, but whose core-wire has a distal portion of larger diameter, was constructed. The core-wire of the snare device was made of NITINOL™ nickel-titanium alloy, with the proximal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0121 in and with the distal portion of the core-wire having a uniform filamentary cross-sectional diameter of 0.0090 in. Consequently, the filamentary cross-sectional area of the distal portion of the core-wire was 6.36×10 −5  in 2 . The upper plateau stress for the distal portion of the core-wire was 85,000 psi. As a result, the plateau force for the distal portion of the core-wire was 5.4 lbs. 
     The support of the subject snare device was in the form of a coil made of a stainless steel wire of round cross-section having an ultimate tensile strength of 340,000 psi. The support had a uniform overall outer diameter of 0.0226 in and a uniform overall inner diameter of 0.0136 in. Consequently, the filamentary cross-sectional diameter of the support was 0.0045 in, and the filamentary cross-sectional area of the support was 1.59×10 −5  in 2 . As noted above, the ultimate tensile strength of the support was 340,000 psi. As a result, the break force for the support was 5.4 lbs, which is equal to the plateau force of 5.4 lbs. Table 4 below summarizes some of the above-noted values. 
     
       
         
           
               
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Core-wire 
                   
               
            
           
           
               
               
               
            
               
                   
                 Distal 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Distal 
                 Portion 
                 Distal 
                 Support 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Distal 
                 Portion 
                 Upper 
                 Portion 
                   
                   
                 Ultimate 
                   
               
               
                 Portion 
                 Cross- 
                 Plateau 
                 Plateau 
                   
                   
                 Tensile 
                 Break 
               
               
                 Diameter 
                 Sectional 
                 Stress 
                 Force 
                 Filamentary 
                 Cross-Sectional 
                 Strength 
                 Force 
               
               
                 (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
                 Diameter (in) 
                 Area (in 2 ) 
                 (psi) 
                 (lbs) 
               
               
                   
               
               
                 0.0090 
                 6.36 × 10 −5   
                 85,000 
                 5.4 
                 0.0045 
                 1.59 × 10 −5   
                 340,000 
                 5.4 
               
               
                   
               
            
           
         
       
     
     For the above-described device, after many actuations, the device eventually skewed.  FIG. 22  is a photo showing the skewed support of this device. 
     The embodiments of the present invention described above are intended to be merely exemplary and those skilled in the art shall be able to make numerous variations and modifications to it without departing from the spirit of the present invention. For example, features from one or more of the embodiments described above may be combined in various permutations. All such variations and modifications are intended to be within the scope of the present invention as defined in the appended claims.