Abstract:
A surgical instrument and method for facilitating the capture of objects during surgery are provided. The surgical instrument includes a flexible tube defining a cable passageway and being configured for insertion of at least portion of the flexible tube into a body cavity of a patient. The surgical instrument further includes flexible cable slidably disposed in the cable passageway. A snare loop is attached to the cable end and both are adapted so that the snare loop can be selectively retracted or extended within the cable passageway by sliding the cable proximally or distally within the cable passageway. The loop member is adapted for selectively encircling and engaging at least a portion of an object in the body cavity. The system also comprises a tether, the tether attached to the loop member at one end and to the flexible tube at the other, so that extension of the flexible cable causes the snare loop to deform in a predetermined manner.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation-in-part of U.S. patent application Ser. No. 10/006,042 filed Dec. 7, 2001, the specification of which is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to surgical instruments and more particularly to instruments such as snares that may be used for grasping and removing material during surgery.  
         [0003]     During surgery, there is often a need for the collection and removal of tissue specimens such as polyps or other material from a body cavity of the patient. The removal of such tissue is often accomplished through the use of a snare device or slip-knotted suture loop that is inserted into the patient through a cannula. The snare device is generally mounted to the end of a cable slidably disposed within a flexible tube. The user of the snare device must maneuver the end of the flexible tube and cable through the body cavity, which may have a number of convolutions such as in the case of an intestinal cavity. The snare must then be maneuvered so as to surround the material to be grasped and then cinched around the material, typically using an external actuation mechanism.  
         [0004]     The maneuverability of the snare device is generally limited to motion resulting from extension or retraction of the snare from the end of the flexible tube, extension or retraction of the flexible tube and the cable/rod and, under certain circumstances, rotation of the flexible tube and the cable/rod. This limited maneuverability can make it difficult to position the snare around the material of interest, particularly in areas where there is little room for repositioning of the tube and/or cannula.  
       SUMMARY OF THE INVENTION  
       [0005]     There is accordingly a need for a surgical instrument snare with an additional manipulation capability that facilitates the maneuvering of the snare to encircle material within a body cavity of a patient.  
         [0006]     Accordingly, an embodiment of the present invention provides a surgical instrument for facilitating the capture of objects during surgery. The surgical instrument comprises a flexible tube having a proximal tube end and a distal tube end, the flexible tube defining a cable passageway and being configured for insertion of at least a distal portion of the flexible tube into a body cavity of a patient. The surgical instrument further comprises a flexible cable having a proximal cable end and a distal cable end. At least a portion of the flexible cable is slidably disposed in the cable passageway. A snare loop having a loop member is attached to the distal cable end. The snare loop and the cable are adapted so that the snare loop can be selectively retracted within the cable passageway by sliding the flexible cable proximally relative to the cable passageway. The snare loop and the cable are also adapted so that at least a portion of the snare loop can be selectively extended from the distal tube end by sliding the flexible cable distally relative to the cable passageway. The snare loop has a longitudinal loop diameter and is adapted for selectively encircling and engaging at least a portion of an object in the body cavity. The surgical instrument also comprises a first tether having a proximal first tether end and a distal first tether end defining a tether length dimension. The distal first tether end is attached to the loop member and the proximal first tether end is attached to the flexible tube.  
         [0007]     Other objects and advantages of the invention will be apparent to one of ordinary skill in the art upon reviewing the detailed description of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  is a partially sectioned view of a surgical instrument according to an embodiment of the invention;  
         [0009]      FIG. 2  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 1 ;  
         [0010]      FIG. 3  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 1 ;  
         [0011]      FIG. 4  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 1 ;  
         [0012]      FIG. 5  is a perspective view of a surgical instrument according to the invention with a portion of the instrument inserted into a body cavity;  
         [0013]      FIG. 6  is a perspective view of a surgical instrument according to the invention with a portion of the instrument inserted into a body cavity;  
         [0014]      FIG. 7  is a perspective view of a snare loop of a surgical instrument according to the invention illustrating a step in a sequence of encircling a polyp with the snare loop;  
         [0015]      FIG. 8  is a perspective view of a snare loop of a surgical instrument according to the invention illustrating another step in a sequence of encircling a polyp with the snare loop;  
         [0016]      FIG. 9  is a perspective view of a snare loop of a surgical instrument according to the invention illustrating another step in a sequence of encircling a polyp with the snare loop;  
         [0017]      FIG. 10  is a perspective view of a snare loop of a surgical instrument according to the invention illustrating another step in a sequence of encircling a polyp with the snare loop;  
         [0018]      FIG. 11  is a perspective view of a snare loop of a surgical instrument according to an embodiment of the invention;  
         [0019]      FIG. 12  is a side view of a the snare loop illustrated in  FIG. 11 ;  
         [0020]      FIG. 13  is a partially sectioned view of a surgical instrument according to an embodiment of the invention;  
         [0021]      FIG. 14  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 13 ;  
         [0022]      FIG. 15  is a partially sectioned view of a surgical instrument according to an embodiment of the invention;  
         [0023]      FIG. 16  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 15 ; and  
         [0024]      FIG. 17  is a perspective view of a snare loop of the surgical instrument illustrated in  FIG. 15 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The present invention provides a surgical instrument to allow a surgeon to remotely manipulate an automatically deforming snare loop and to maneuver the deformed snare loop in position around an object or tissue inside a body cavity of a patient. By “deform,” it is meant that the snare loop is oriented in more than one plane.  
         [0026]      FIGS. 1 and 2  illustrate a surgical instrument  100  according to one embodiment of the invention. The surgical instrument  100  includes a snare control module  110  that is used to control the deployment and retraction of a snare loop  150  from a flexible tube  132 . The snare loop  150  is formed from a resilient loop member  152 . The loop member  152  has first and second ends  154 ,  156  that are attached to a flexible cable  134  at its distal end  138 . The resilient loop member  152  may take the form of a wire, cable or band formed from metal or a resilient surgical plastic. The resilient loop member  152  may also be a suture material attached to a biasing member or material. The resilient loop member  152  may be formed from an electrically conductive material for use as a cauterization loop. The loop member  152  may be attached to the cable  134  in any conventional manner such as bonding or welding.  
         [0027]     The flexible cable  134  is slidably disposed within a flexible tube  132  formed from a polymer or other material suitable for use in surgical applications. The flexible tube  132  has an inside diameter sized to accommodate the flexible cable  134  and the snare loop  150 . In particular, the flexible tube  132  is sized so that the snare loop  150  can be easily retracted into the distal end  133  of the flexible tube  132  when the flexible cable  134  is moved proximally relative to the flexible tube  132  and extended out of and from the flexible tube  132  when the cable is moved distally relative to the flexible tube  132 .  
         [0028]     The snare loop  150  is formed so that when freed from the flexible tube  132 , it will form a substantially elliptical loop that can be used to encircle an object, although the snare loop may be formed into any geometrical shape. As the ends  154 ,  156  of the resilient loop member  152  are drawn into the flexible tube  132 , the loop member  152  is cinched to a smaller and smaller loop. If an object is encircled by the loop member  152 , the loop member  152  can be tightened around the object to secure the snare loop  150  to the object. If no object is encircled, the snare loop  150  may be drawn within the flexible tube  132 .  
         [0029]     The loop member  152  may be formed so as to include an extension  158  that extends outward from the distal end of the elliptical loop formed by the loop member  152 . Such an extension can be used to facilitate the capture and retrieval of certain objects.  
         [0030]     The snare control module  110  may be used to control the retraction and extension of the snare loop  150  from the flexible tube  132 . The snare control module  110  has a generally cylindrical body  112 , a control slide  122 , an actuation rod  128  and a tube connector  130 . The body  112  of the snare control module  110  is formed by two frame members  114 ,  116  and a proximal end portion  118 . The frame members  114 ,  116  and the end portion  118  may be integrally formed into a single body structure. The frame members  114 ,  116  define a central frame opening  117  that runs diametrically through the body  112 .  
         [0031]     The control slide  122  has a central portion  123  having a cable passageway  125  formed therethrough. The cable passageway  125  is sized to slidably accommodate the body  112  within the cable passageway  125 . This allows the control slide  122  to be reciprocated along the body  112 .  
         [0032]     The proximal end  135  of the flexible tube  132  is attached to the distal end of the body by a hollow, cylindrical connector  130 . Alternatively, the tube flexible  132  may have a flange (not shown) at its proximal end  135  that can be used to hold the proximal end  135  in place within the connector  130 . The connector  130  may be attached to the body  112  by internal threads configured to mate with external threads on the frame members  114 ,  116 .  
         [0033]     An actuation rod  128  is attached at one end to the proximal end  136  of the cable  134  and at its other end to an actuation rod pin  129  attached to the central portion  123  of the control slide  122 . The actuation rod pin  129  is positioned diametrically across the cable passageway  125  within the central frame opening  117  so that it does not impede the reciprocal motion of the control slide  122 . When the control slide  122  is moved toward the proximal end of the body  112 , the actuation rod  128  also moves in this direction, which, in turn, moves the proximal end  136  of the cable  134  proximally relative to the flexible tube  132 . When the control slide  122  is moved away from the proximal end of the body  112 , the actuation rod  128  moves the proximal end  136  of the cable  134  distally relative to the flexible tube  132 .  
         [0034]     The reciprocal motion of the control slide  122  can thus be used to control the deployment of the snare loop  150  from and the retraction of the snare loop  150  into the distal tube end  133 . Moving the control slide  122  distally causes the snare loop  150  to be extended from the flexible tube  132 . Moving the control slide  122  proximally causes the withdrawal of the snare loop  150  toward and into the flexible tube  132 .  
         [0035]     To facilitate one hand control of the reciprocal motion of the control slide  122 , finger rings  124 ,  126  may be attached to the central portion  123  of the control slide  122  and a thumb ring  120  may be attached to the proximal portion  118  of the body  112 .  
         [0036]     It will be understood by those having ordinary skill in the art that other mechanisms could be used in the snare control module  110  without departing from the scope and spirit of the present invention. Such mechanisms may include, for example, slide actuators without finger rings and actuators configured for use in electro-cautery.  
         [0037]     The surgical instrument  100  also includes a tether  160  that can be used to assist in maneuvering the snare loop  150 . The tether  150  is attached to the flexible tube  132  and the snare loop  150  so that extension of the snare loop  150  a predetermined distance from the distal tube end  133  causes the snare loop  150  to deform from a substantially planar condition as shown in  FIG. 2  to a curved three dimensional condition as shown in  FIG. 4 . As will be discussed, deforming the snare loop  150  in this manner serves to facilitate the positioning of the snare loop  150  for capturing body tissue such as polyps. The tether  160  of the surgical instrument  100  has a distal end  164  that is attached to the loop member  152  and a proximal end  162  that is attached to the flexible tube  132 . In the illustrated embodiment the proximal tether end  162  is attached to the flexible tube  132  at an attachment point  163  near the distal tube end  133 . The proximal tether end  162  may be attached to the flexible tube  132  by any suitable method such as bonding, welding or tying, and may be attached to the exterior surface of the tube  132  or to the interior surface. If attached to the interior surface of the tube  132 , a portion of the tether  160  would, of course, be disposed within the cable passageway  125 . The distal tether end  164  may be attached to the loop member  152  anywhere around the circumference of the snare loop  150 . In the illustrated embodiment, the distal tether end  164  is attached at the distal-most point on the loop member  152 . This positioning of the tether attachment is particularly useful in assisting the surgeon in maneuvering the snare loop  150  over an object such as a polyp.  
         [0038]     The tether  160  may be formed from any thread-like structure including but not limited to thread, wire, cable and chain. The tether  160  may be formed from any suitable material including but not limited to steel or other metal, polymeric materials such as nylon, and twisted cotton or other textile materials.  
         [0039]     The proximal tether end  162  may be attached at any point on the flexible tube  132  which is sufficient to create a tensile force on tether  160 . The tether has an operative length that is useful in determining a preferred tether length. The operative length dimension of the tether  160  is defined as the distance along the tether  160  from the attachment point  163  of the proximal tether end  162  to the attachment point of the distal tether end  164  less the distance of the attachment point  163  from the distal tube end  133 . When the attachment point  163  of the proximal tether end  162  is immediately adjacent the distal tube end  133 , the tether length and the operative ether length are the same.  
         [0040]     The operative tether length and the location of the tether attachment point  163  on the flexible tube  132  determine the degree to which the snare loop  15  is deformed for a given amount of extension of the snare loop  150  from the distal tube end  133 . For example, if the operative tether length is greater than the longitudinal diameter DL of the snare loop  150 , the snare loop  150  can be fully extended from the distal tube end  133  without deforming the snare loop  150  from its substantially planar condition (see  FIG. 3 ). If the snare loop  150  is extended beyond a predetermined distance X (where X is the operative tether length minus DL) from the distal tube end  133 , the snare loop  150  begins to deform (see  FIG. 4 ). If, however, the operative tether length is less than the longitudinal diameter DL of the snare loop  150 , the snare loop  150  will begin to deform even before it is completely extended from the distal tube end  133 .  
         [0041]     In either case, a predetermined condition wherein the snare loop  150  has adopted a desired three dimensional shape can be achieved by moving the flexible cable  134  (and thus, the snare loop  150 ) distally a predetermined distance relative to the flexible tube. The predetermined condition may be any degree of deformation deemed efficacious for a particular procedure.  
         [0042]     A desirable operative tether length can be expressed in terms of tether length ratio, which may be defined as the ratio of the operative tether length divided to the longitudinal diameter DL of the snare loop  150 . In instances where it is desirable for the snare loop  150  to begin to deform prior to complete deployment, it has been found that a satisfactory operative tether length ratio may be in a range from about 0.3 to about 0.7. A particularly satisfactory tether length ratio may be in a range from about 0.4 to about 0.6.  
         [0043]     Turning now to  FIGS. 5 and 6 , the flexible tube  132  and the snare loop  150  may be introduced into a body cavity  14  through the lumen  52  of a cannula  50 . The body cavity  14  is defined by first and second tissue walls  10  and  16 . As shown, the cannula  50  has been inserted through an opening  12  in the first tissue layer  10 . The distal end  133  of the flexible tube  132  may be passed through the cannula  50  into the cavity  14  with the snare loop  150  in a stowed position within the tube  132  as shown in  FIG. 4 . In this configuration, the distal tether end  164  of the tether  160 , which is attached to the snare loop  150 , is also withdrawn within the flexible tube  132 .  
         [0044]     The flexible tube  132  and the flexible cable  134  are formed so as to be sufficiently flexible to pass through any curves in the cavity  14  that may be encountered as the flexible tube  132  is inserted. Once the flexible tube  132  has been inserted and positioned near the area of interest, the control slide  122  of the snare control module  110  can be moved in the distal direction relative to the body  112  to cause the snare loop  150  to be extended as shown in  FIG. 6 . It will be understood that, depending on the relative sizes of the snare loop  150  and the cannula lumen  52 , it may be possible to insert the flexible tube  132  through the cannula  50  with the snare loop  150  already partially or completely deployed. In either case, the snare loop  150  and the tether  160  may be positioned within the cavity  14  as shown in  FIG. 6 .  
         [0045]      FIGS. 7-10  illustrate a sequence by which the snare loop  150  can be used to encircle a polyp  18  attached to the lower tissue wall  16 . Initially, the snare loop  150  is generally in its stowed position or, if deployed, is positioned near the distal tube end  133 . At this point, the tether  160  is in a slack condition (i.e., is not under tension). As shown in  FIG. 7 , the snare loop  150  may be at a level where it cannot easily be positioned over the top of the polyp  18 . This problem can occur as a result of the geometry of the cavity  14  and the stiffness of the tube  132 . The tether  160 , however, provides a way of deforming the snare loop  150  to overcome the problem. Further movement of the flexible cable  134  in the distal direction causes the snare loop to extend further from the distal tube end  133  until a critical length is reached where the tether  160  becomes taut. Moving the flexible cable  134  still further causes the snare loop  150  to deform as shown in  FIG. 8 . The movement of the flexible cable  134  may be accomplished using the control slide  122 . As shown in  FIG. 8 , extending the snare loop  150  by moving the control slide  122  distally relative to the body  112  of the snare control module  110  creates a tensile force as the tether  160  remains the same length while the extended portion of the snare loop  150  and cable  134 . This tensile force causes the snare loop  150  to deform so that the distal portion of the snare loop member  152  may be raised above the level of the polyp.  
         [0046]     When the snare loop  150  has been deformed into a desired deformed condition, the flexible tube  132  can then be translated further into the cavity  14  to position the snare loop  150  over the polyp  18  as shown in  FIG. 9 . The surgeon can then release the tension in the tether  160  by moving the flexible cable  134  proximally relative to the cable passageway  125 . Retracting the cable  134  and the snare loop  150  releases the tension in the tether  160 , which allows the snare loop  150  to resume its shape and encircle the polyp  18  as shown in  FIG. 10 . When releasing the tension in the tether  160 , it may be necessary for the surgeon to simultaneously advance the flexible tube  132  slightly to assure that the snare loop  150  properly encircles the polyp. The snare loop  150  can then be cinched around the polyp  18  by further moving the flexible cable  134  proximally relative to the cable passageway  125 . The snare loop  150  may be removed from the polyp  18  by reversing the above procedure.  
         [0047]     In some embodiments of the invention, the surgical instrument  100  may alternatively have a pre-deformed snare loop  1150  attached to the distal end  138  of the flexible cable  134 , as shown in  FIG. 11 . Unlike the snare loop  150  of  FIGS. 1-10 , the pre-deformed snare loop  1150  of  FIG. 11  is biased so as to adopt a three dimensional “deformed” condition when unconstrained. As a result, no tether is required. The snare loop  1150  of these embodiments is formed from a loop member  1152  comprising a memory retaining material that can be pre-formed to adopt a desired shape. Once so-formed, the loop member  1152  can be compressed so as to fit within a constraining space such as the cable passageway  125 . When subsequently removed from the constraining space the loop member  1152  re-adopts the pre-formed three-dimensional shape. The memory retaining material may be any medical-grade material that is sufficiently resilient and will provide the desired capture shape. Such materials may include shap metal alloys such as nitinol.  
         [0048]     Embodiments of the invention having the pre-deformed snare loop  1150  may be used in a substantially similar manner to the embodiments having the tethered snare loop  150 . At the time of manufacture, the snare loop  1150  is deformed such that the fully deployed snare loop  1150  has a pre-determined three dimensional shape intended to facilitate capture of bodily tissue. This shape may be similar to that achieved with the tether of previous embodiments. For example, as shown in the side view depicted in  FIG. 12 , the loop member  1152  may be pre-formed to adopt a continuous curve as viewed in a vertical, longitudinal plane. This curve is oriented so that the loop member  1152  bends continuously away from an axis  139  centered on the flexible cable  134  at its distal end  138 .  
         [0049]     While the pre-deformed snare loop  1150  is in a stowed position within the cable passageway  125 , the loop member  1152  presents a substantially linear shape as viewed in the vertical, longitudinal plane. However, when the flexible cable  134  is extended distally and the snare loop  1150  is deployed from the cable passageway  125 , the loop member  1152  begins to “self-deform.” Once completely unconstrained, the loop member  1152  returns to the desired pre-formed three dimensional shape. The instrument can then be used to capture tissue in substantially the same manner as described above. The pre-deformed loop  1150  is positioned adjacent the tissue so that retraction of the pre-deformed loop  1150  causes it to at least partially encircle the tissue. Further retraction causes the pre-deformed loop  1150  to be cinched around the tissue.  
         [0050]     A surgical instrument  200  according to another embodiment of the invention is shown in  FIGS. 13 and 14 . The surgical instrument  200  may include a snare control module  210  that is used to control the deployment and retraction of a snare loop  250  from a flexible tube  232 . The snare loop  250  is formed from a resilient loop member  252 . The loop member  252  has first and second ends  254 ,  256  that are attached to a flexible cable  234  at its distal end  238 . The materials and configuration of these features are similar to those of the previous embodiments.  
         [0051]     As before, the flexible cable  234  is slidably disposed within a flexible tube  232 . The flexible tube  232  has an inside diameter sized to accommodate the flexible cable  234  and the snare loop  250 . In particular, the flexible tube  232  is sized so that the snare loop  250  can be easily retracted into the distal end  233  of the flexible tube  232  when the flexible cable  234  is moved proximally relative to the flexible tube  232  and extended out of and from the flexible tube  232  when the cable is moved distally relative to the flexible tube  232 .  
         [0052]     The surgical instrument  200  includes a fixed tether  260  and one or more manipulable tethers  261   a ,  261   b  as illustrated in  FIGS. 13 and 14 . The fixed tether  260  is similar to the fixed tether  160  of the surgical instrument  100  of  FIGS. 1-10 . The proximal end of the fixed tether  260  is attached to the flexible tube  232  at an attachment point  263 . The distal end  264  of the fixed tether  260  is attached to the distal tip of the snare loop  250 . The fixed tether  260  provides a similar automatic deformation of the snare loop  250  as before.  
         [0053]     The manipulable tethers  261   a ,  261   b  provide the capability to manually maneuver snare loop  250  when the snare loop  250  has been deployed from the flexible tube  232 . The distal ends  265   a ,  265   b  of the manipulable tethers  261   a ,  261   b  are each attached to the snare loop  250 . As shown, the manipulable tethers  261   a ,  261   b  may be attached to opposite sides of the snare loop  250  so that the manipulable tethers  261   a ,  261   b  may be used as reins to steer the snare loop  250 . Although two manipulable tethers are shown, it will be understood by those of ordinary skill in the art that any number of manipulable tethers may be used without departing from the scope and spirit of the invention.  
         [0054]     The manipulable tethers  261   a ,  261   b  are slidably disposed through the cable passageway  225  of the flexible tube  232  along with the flexible cable  234 . The flexible tube  232  and, in particular, the may be sized so that the flexible cable  234  and the manipulable tethers  261   a ,  261   b  can be moved substantially independently within the cable passageway  225 . The manipulable tethers  261   a ,  261   b  extend out of the distal end  233  of the flexible tube  232  along side the flexible cable  234 .  
         [0055]     As shown in  FIG. 13 , the manipulable tethers  261   a ,  261   b  may be passed through the flexible tube  232  and through the connector  230  into the opening  217  between the frame members  214 ,  216 . The proximal ends  263   a ,  263   b  of the manipulable tethers  261   a ,  261   b  thus may be extended out from the body  212  of the snare control module  210  and may be secured in any suitable fashion or may be left as free ends for a surgeon to manipulate and tie off as desired. It will be apparent that pulling one of the manipulable tethers  261   a ,  261   b  in the proximal direction, will tug on the portion of the snare loop  250  to which that tether is attached, thereby causing an additional deformation of the snare loop  250  that can be used to assist in positioning the snare loop  250  for capture of tissue.  
         [0056]     The fixed tether  260  of the surgical instrument  200  operates in a manner similar to the tether  160  of the previous embodiments. The manipulable tethers  261   a ,  261   b  may be used to steer and deform the snare loop  250  from side to side either before or after the snare loop  250  is placed in its fully deformed condition. Accordingly, the fixed tether  260  and the manipulable tethers  261   a ,  261   b  may be used individually or in combination to maneuver the snare loop  250  as desired.  
         [0057]     It will be understood that the manipulable tethers may also be used in an instrument configuration wherein the tethers are not disposed through the tube  232  but are instead separately passed out of the body cavity through the cannula.  
         [0058]     A surgical instrument  300  according to another embodiment of the invention is shown in  FIGS. 15-17 . The surgical instrument  300  may include a snare control module  310  that is used to control the deployment and retraction of a snare loop  350  from a flexible tube  332 . The snare loop  350  is formed from a resilient loop member  352 . The loop member  352  has first and second ends  354 ,  356  that are attached to a flexible cable  334  at its distal end  338 . The materials and configuration of these features are similar to those of the previous embodiments.  
         [0059]     As before, the flexible cable  334  is slidably disposed within a flexible tube  332 . The flexible tube  332  has an inside diameter sized to accommodate the flexible cable  334  and the snare loop  350 . In particular, the flexible tube  332  is sized so that the snare loop  350  can be easily retracted into the distal end  333  of the flexible tube  332  when the flexible cable  334  is moved proximally relative to the flexible tube  332  and extended out of and from the flexible tube  332  when the cable is moved distally relative to the flexible tube  332 .  
         [0060]     The surgical instrument  300  includes an adjustable tether  360  that provides automatic deformation of the snare loop  350  as in the previous embodiments. As in previous embodiments, the adjustable tether  360  has a distal tether end  362  attached to the loop member  352  and a proximal tether end  364  attached to the flexible tube  332 . In this embodiment however, the proximal tether end  364  is attached to the flexible tube  332  by a tether adjustment member  377 . The tether adjustment member  377  is formed as a collar that is slidably mounted to the flexible tube  332 . The tether adjustment member  377  is constructed of a biologically inert material, such as the material used for the construction of the flexible tube  332 . The tether adjustment member  377  is configured so that it can be selectively placed in various positions along the length of the flexible tube  332 . This effectively changes the operative length of the adjustable tether  360 , which, in turn, changes the point at which deformation begins when the snare loop  350  is deployed from the flexible tube  332 . The tether adjustment member  377  thus allows the user to adjust the degree and timing by which the snare loop  350  is deformed as it is extended from the distal tube end  333 .  
         [0061]     The tether adjustment member  377  may include any suitable arrangement for selectively fixing its location such as, for example, a set screw (not shown).  
         [0062]      FIGS. 16 and 17  illustrate the effect of the positioning of the tether adjustment member  377 . As shown in  FIG. 16 , with the tether adjustment member  377  positioned a distance X1 from the distal tube end  333 , extension of the loop member  350  a distance L1 from the distal tube end  333  provides a significant deformation of the loop member  350 . As shown in  FIG. 17 , however, when the tether adjustment member  377  is positioned a distance X2 from the distal tube end  333  (X2 being less than X1), the snare loop  350  must be extended a greater distance L2 from the distal tube end  333  in order to achieve the same degree of deformation. Although the tether  360  is the same length in each case, the operative length of the tether  360  is changed, so that the snare loop  350  must be extended further before the tether  360  is placed in tension so as to deform the snare loop  350 . This allows the surgeon to adjust the instrument for different applications prior to inserting the flexible tube  332  into the cannula.  
         [0063]     It will be understood that the present invention may be applied to other forms of surgical instruments and snares. This includes snares with loops formed by a belt or wire having either or both ends passing out through a cannula. Such snares are typically cinched by applying tension to the free end (or ends) of the belt or wire. The tether of the present invention could easily be attached to the loops of such snares.  
         [0064]     Other embodiments and uses of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. The specification and examples should be considered exemplary only. The scope of the invention is limited only by the claims appended hereto.