Patent Publication Number: US-2005137454-A1

Title: Shape lockable apparatus and method for advancing an instrument through unsupported anatomy

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
      The present application is a non-provisional of U.S. patent application Ser. No. 10/173,227 (Attorney Docket No. 021496-002300US), filed Jun. 13, 2002, the full disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION  
     Field of the Invention  
      The present invention relates to apparatus and methods for placing and advancing a diagnostic or therapeutic instrument in a hollow body organ of unsupported anatomy, while reducing patient discomfort and risk of injury.  
      The use of the colonoscope for examining the interior of the large intestine or colon is well-known. In general, a physician performing an examination or treatment of the colon inserts a colonoscope into the anus and then advances the colonoscope into the colon. A complete examination requires the phy Apparatus and methods are provided for placing and advancing a diagnostic or therapeutic instrument in a hollow body organ of a tortuous or unsupported anatomy, comprising a handle, an overtube, a distal region having an atraumatic tip. The overtube may be removable from the handle, and have a longitudinal axis disposed at an angle relative to the handle. The overtube may be selectively stiffened to reduce distension of the organ caused by advancement of the diagnostic or therapeutic instrument. The distal region permits passive steering of the overtube caused by deflection of the diagnostic or therapeutic instrument while the atraumatic tip prevents the wall of the organ from becoming caught or pinched during manipulation of the diagnostic or therapeutic instrument.sician to advance the colonoscope into the colon, negotiate the sigmoid colon, and left and right colic flexures up to the cecum. Advancement of the colonoscope is generally accomplished by manipulation of a steerable tip of the colonoscope, which is controlled at the proximal end of the device by the physician, in addition to torquing and pushing the scope forward or pulling it backward.  
      Problems regularly occur, however, when negotiating the colonoscope through the bends of the colony such as at the sigmoid and left and right colic flexures. These problems arise because the colon is soft and has unpredictable fixation points to the viscera of the abdomen, and it is easily distensible. Consequently, after the steerable tip of the colonoscope is deflected to enter a new region of the colon, the principal direction of the force applied by the physician urging the proximal end of the device into the patient&#39;s colon is not in the direction of the steerable tip. Instead, the force is directed along the axis of the colonoscope towards the preceding bend(s), and causes yielding or displacement of the colon wall.  
      The loads imposed by the colonoscope on the colon wall can have a myriad of possible effects, ranging from patient discomfort to spastic cramp-like contractions of the colon and even possible perforation or dissection of the colon. Consequently, the colonoscope cannot be advanced as far as the cecum in up to one-sixth of all cases.  
      To address some of these difficulties, it is known to employ a guide tube that permits a colonoscope to be advanced through the rectum. One such device is described in U.S. Pat. No. 5,779,624 to Chang. An alternative approach calls for inserting the colonoscope through a curved region, and then mechanically actuating the portion of the device in the curved region to cause it to straighten, as described in U.S. Pat. No. 4,601,283 to Chikama.  
      Many patients find the operation of such previously-known devices unpleasant because the sigmoid portion of the colon is forced into an almost rectilinear shape by the guide tube. Due to the stiffness of the guide tube, careless handling of the guide tube presents a risk of injury to the colon.  
      Other previously-known apparatus and methods use an overtube having variable rigidity, so that the overtube may be inserted through curved anatomy in a flexible state, and then selectively stiffened to resist bending forces generated by passing a colonoscope through the overtube. One example of such a device is described in U.S. Pat. No. 5,337,733 to Bauerfiend. The device described in that patent comprises inner and outer walls having opposing ribs spaced apart across an air-filled annulus. The ribs are selectively drawn together to intermesh, and form a rigid structure by evacuating the annulus.  
      Another previously-known endoscopic device for delivering aneurysm clips within a hollow organ or vessel is described in U.S. Pat. No. 5,174,276 to Crockard. The device described in that patent includes a conduit formed from a multiplicity of elements that are capable of angulation relative to one another, and which becomes rigid when subjected to a tensile force. The device is described as being particularly useful in neurosurgery, where the variable rigidity of the device is useful for providing a stable platform for neurosurgical interventions, such as clipping an aneurysm.  
      While previously-known apparatus and methods provide some suggestions for solving the difficulties encountered in advancing diagnostic or therapeutic instruments through easily distensible body organs, few devices are commercially available. Although the precise reasons for this lack of success are uncertain, previously-known devices appear to pose several problems.  
      For example, the devices described in the Bauerfiend and Crockard patents appear to pose a risk of capturing or pinching tissue between the endoscope/colonoscope and the distal end of the overtube or conduit when the scope is translated. Also, neither device provides any degree of steerability, and must be advanced along the pre-positioned scope. In addition, the bulk of the proximal tensioning system described in Crockard is expected to interfere with manipulation of the endoscope. Other drawbacks of previously-known devices may be related to the complexity or cost of such devices or the lack of suitable materials. In any event, there exists an un-met need for devices to solve this long-felt problem in the field of endoscopy and colonoscopy.  
      In view of the foregoing, it would be desirable to provide apparatus and methods for facilitating placement of diagnostic or therapeutic instruments within easily distensible hollow body organs, such as the esophagus or colon.  
      It further would be desirable to provide apparatus and methods that permit a diagnostic or therapeutic device to be advanced into a hollow body organ, and which facilitates passage of the device through tortuous anatomy without requiring straightening of organ passageways already traversed.  
      It also would be desirable to provide apparatus and methods for facilitating placement of diagnostic or therapeutic instruments within easily distensible hollow body organs that include means for reducing the risk that tissue will become inadvertently pinched between the sheath apparatus and the advancing or withdrawing instrument, or caught as the diagnostic or therapeutic instrument is maneuvered through the hollow body organ.  
      It still further would be desirable to provide apparatus and methods that provide a low-cost, single use, easily manufacturable guide for inserting a diagnostic or therapeutic instrument in a hollow body organ.  
      It yet further would be desirable to provide apparatus and methods that provide a low-cost, easily manufacturable guide for inserting a diagnostic or therapeutic instrument in a hollow body organ, wherein a portion of the apparatus is disposable after a single use and a remaining portion of the device is re-usable.  
      Still further, it would be desirable to provide a device having a selectively locking shape for inserting a diagnostic or therapeutic instrument in a hollow body organ, but which facilitates manipulation of a proximal end of the diagnostic or therapeutic instrument.  
      It additionally would be desirable to permit multiple diagnostic or therapeutic devices to be positioned in a hollow, unsupported organ, so that at least one of the devices may be withdrawn and repositioned while the other devices are retained in place.  
     BRIEF SUMMARY OF THE INVENTION  
      In view of the foregoing, it is an object of the present invention to provide apparatus and methods for facilitating placement of diagnostic or therapeutic instruments within easily distensible or unpredictably supported hollow body organs, such as the esophagus or colon.  
      It is a further object of the present invention to provide apparatus and methods that permit a diagnostic or therapeutic device to be advanced into a hollow body organ, and which facilitates passage of the device through tortuous anatomy without requiring straightening of organ passageways already traversed.  
      It also is an object of the present invention to provide apparatus and methods for facilitating placement of diagnostic or therapeutic instruments within easily distensible hollow body organs that include means for reducing the risk that tissue will become inadvertently pinched or caught as the diagnostic or therapeutic instrument is maneuvered through the hollow body organ.  
      It is a still further object of the present invention to provide apparatus and methods that provide a low-cost, single use, easily manufacturable guide for inserting a diagnostic or therapeutic instrument in a hollow body organ.  
      It is another object of this invention to provide apparatus and methods that provide a low-cost, easily manufacturable guide for inserting a diagnostic or therapeutic instrument in a hollow body organ wherein a portion of the apparatus is disposable after a single use and a remaining portion of the device is re-usable.  
      Still further, it is an object of the present invention to provide a device having a selectively locking shape for inserting a diagnostic or therapeutic instrument in a hollow body organ, but which facilitates manipulation of a proximal end of the diagnostic or therapeutic instrument.  
      It is yet another object of the present invention to permit multiple diagnostic or therapeutic devices to be positioned in a hollow, unsupported organ, so that at least one of the devices may be withdrawn and repositioned while the other devices are retained in place.  
      These and other objects of the present invention are attained by providing apparatus comprising a proximal handle, an overtube coupled to the proximal handle and having a distal region, and an atraumatic tip disposed on the distal region. The apparatus includes a main lumen extending between the handle, overtube and atraumatic tip, through which a diagnostic or therapeutic instrument, such as an endoscope or colonoscope, may be translated.  
      The handle extends from the patient, e.g., through the mouth or anus, where it can be manipulated by the physician. The handle preferably comprises means for selectively locking the shape of the overtube. In this manner the overtube may be shape locked to assist one or more diagnostic or therapeutic instruments to negotiate the tortuous or unsupported anatomy of a hollow body organ, rather than distending the wall of the organ. The proximal handle may form part of a single use, disposable apparatus, or may be separable from the overtube and reusable. The overtube preferably is angled relative to a working axis of the handle, so that the handle does not interfere with manipulation of the diagnostic or therapeutic instrument inserted through the overtube.  
      An overtube constructed in accordance with the principles of the present invention may comprise a multiplicity of selectively-tensionable nested elements, a series of interconnected links surrounded by a selectively actuable clamping mechanism, a tubular member comprising a multiplicity of helical links formed from a material having variable durometer and surrounded by a clamping mechanism, or a thermo-responsive polymer or alloy: The overtube may include any of a number of aids for facilitating passage of the diagnostic or therapeutic instrument through the main lumen, including a lubricious liner, rails or rollers.  
      The atraumatic tip of the present invention preferably is configured to reduce the risk of capturing or pinching tissue between the overtube and a diagnostic or therapeutic instrument that is selectively translated through the overtube. This is preferably accomplished by the atraumatic tip applying a radially-outwardly directed load to the wall of the hollow body organ in the vicinity of the distal region where the diagnostic or therapeutic instrument exits the apparatus.  
      In addition, the distal region of the overtube preferably includes a flexible portion that permits a steerable tip of a diagnostic or therapeutic device disposed within the distal region to deflect the distal region of the overtube in a desired direction. This permits the overtube to be readily advanced together with the steerable tip of the diagnostic or therapeutic device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and the following detailed description of the preferred embodiments, in which:  
       FIG. 1  is a schematic view of a human colon illustrating a common difficulty encountered in advancing a colonoscope beyond the sigmoid colon;  
       FIG. 2  is a side view of illustrative apparatus of the present invention;  
       FIG. 3  is a side-sectional exploded view of nestable elements of a first embodiment of an overtube suitable for use in the apparatus of  FIG. 2 ;  
       FIG. 4  is a side-sectional view of a distal region of the apparatus of  FIG. 2  constructed in accordance with principles of the present invention;  
       FIG. 5  is a side-sectional view of an illustrative arrangement of a mechanism suitable for use in the handle of the apparatus of  FIG. 2 ;  
       FIG. 6  is a side-sectional view of the detail of a wire clamping system suitable for use in the handle of  FIG. 5 ;  
       FIG. 7A-7C  are schematic views of a method of using the apparatus of the present invention;  
       FIG. 8  is a schematic view of an alternative step In the method of using the apparatus of the present invention;  
       FIG. 9  is a side view of an alternative embodiment of the apparatus of the present invention;  
       FIGS. 10A and 10B , are a side-section view of an alternative element suitable for use in the overtube of  FIG. 2  and a roller element suitable for use with the element of  FIG. 10A , respectively;  
       FIGS. 11A and 11B  depict the use of lubricious rails in the overtube of the apparatus of FIGS.  2  or  9  to facilitate passage of a diagnostic or therapeutic device through the main lumen;  
       FIG. 12  is a schematic view of the lumen of the overtube of the present invention depicting the use of multiple devices;  
       FIGS. 13-18  depict side-sectional views of various alternative embodiments of an atraumatic tip constructed in accordance with the present invention;  
       FIGS. 19A-19C  are, respectively, a side-sectional view of an alternative embodiment of an overtube suitable for use in the present invention having a multiplicity of interconnected links surrounded by a clamping sleeve, and cross-sectional views of portions of the sleeve;  
       FIG. 20  is a side-sectional view of a further alternative embodiment of an overtube constructed in accordance with the present invention having a spiral bladder to actuate the clamping links;  
       FIG. 21  is a side-sectional view of another alternative embodiment of an overtube of the present invention having thermally-actuable bands;  
       FIGS. 22A and 22B  are side-sectional views of a yet further alternative embodiment of an overtube of the present invention comprising a series of helical links having regions of different durometer; and  
       FIG. 23  is a side-sectional view of yet another alternative embodiment of an overtube having thermally regulated stiffness. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 1 , problems associated with previously-known apparatus and methods for inserting and advancing a diagnostic or therapeutic instrument into a hollow body organ having tortuous or unsupported anatomy, illustratively, patient&#39;s colon C, are described. Colon C includes sphincter muscle SM disposed between anus A and rectum R. Rectum R is coupled via the rectosigmoid junction RJ to sigmoid colon SC. Sigmoid colon SC joins descending colon DC, which in turn is coupled to transverse colon TC via left colic flexure LCF. Transverse colon TC also is coupled by right colic flexure RCF to ascending colon AC and cecum CE, which receives waste products from the small intestine.  
      As illustrated in  FIG. 1 , colonoscope  10  having steerable distal tip  11  is typically inserted through anus A into rectum R, and then steered through rectosigmoid junction RJ into sigmoid colon SC. As depicted in  FIG. 1 , distal tip  11  of colonoscope  10  is advanced through sigmoid colon SC and deflected into descending colon DC. Further urging of the colonoscope by the physician can cause region  12  of the colonoscope to bear against and cause displacement of the rectosigmoid junction RJ, as illustrated by dotted lines  12 ′ and RJ′ in  FIG. 1 .  
      Such distension may result in patient discomfort or spasm, and if unnoticed, could result in injury to the colon. The potential for movement of colonoscope to cause distension, discomfort or spasm is also great where the colonoscope must negotiate left colic flexure LCF and right colic flexure RCF, and results in a large portion of such examinations terminating before the physician can advance distal tip  11  to cecum CE.  
      The present invention provides apparatus and methods for placing a diagnostic or therapeutic instrument through the tortuous or unpredictably supported anatomy of a hollow body organ, such as the esophagus or colon, while reducing the risk of distending or injuring the organ. Apparatus constructed in accordance with the present invention permits an endoscope or colonoscope to be readily advanced into a patient&#39;s tortuous or unsupported anatomy by selectively shape-fixing an overtube portion of the apparatus, while also preventing tissue from being captured or pinched between the overtube and scope.  
      Referring now to  FIG. 2 , apparatus  20  of the present invention is described. Apparatus  20  comprises handle  21 , overtube  22 , and distal region  23  having atraumatic tip  24 . Handle  21  includes lumen  25  that extends from Toughy-Borst valve  26  through overtube  22 , distal region  23  and atraumatic tip  24 . Lumen  25  is configured to facilitate passage of a standard commercially available colonoscope, such as colonoscope  10 , therethrough. Toughy-Borst valve  26  may be actuated to releasably lock colonoscope  10  to apparatus  20  when colonoscope  10  is inserted within lumen  25 . As described hereinafter, overtube  22  is configured so that it can be selectively transitioned between a flexible state and a rigid, shape-fixed state by actuator  27  disposed on handle  21 .  
      In  FIG. 3 , illustrative embodiment of overtube  22  comprises a multiplicity of nestable elements  30 . For purposes of illustration, nestable elements  30  are shown spaced-apart, but it should be understood that elements  30  are disposed so that their adjacent surfaces  31  and  32  coact. Each of nestable elements  30  has central bore  33  to accommodate colonoscope  10 , and preferably three or more tension wire bores  35 . When assembled as shown in  FIG. 2 , nestable elements  30  are fastened with adjacent surfaces  31  and  32  disposed in a coacting fashion by a plurality of tension wires  36  that extend through tension wire bores  35 .  
      In a preferred embodiment, adjacent surfaces  31  and  32  of each nestable element  30  are contoured to mate with the next adjacent element, so that when tension wires  33  are relaxed, surfaces  31  and  32  can rotate relative to one another. Tension wires  36  are fixedly connected to the distal end of overtube  22  at the distal ends and to a tensioning mechanism disposed within handle  21  at the proximal ends. When actuated by actuator  27 , tension wires  36  impose a load that clamps adjacent surfaces  31  and  32  of nestable elements  30  together at the current relative orientation, thereby fixing the shape of overtube  22 .  
      When the load in tension wires  36  is released, tension wires  36  provides for relative angular movement between nestable elements  30 . This in turn renders overtube  22  sufficiently flexible to negotiate a tortuous path through the colon. When the tensioning mechanism is actuated, however, tension wires  36  are retracted proximally to apply a clamping load to the nestable elements. This load prevents further relative movement between adjacent elements  30 , and stiffens overtube  22  so that any distally directed force applied to colonoscope  10  causes distal tip  11  to advance further into the colon, rather than cause overtube  22  to bear against the wall of the colon. The shape-fixed overtube absorbs and distributes vector forces, shielding the colon wall.  
      Referring now to  FIG. 4 , an illustrative embodiment of distal region  23  and atraumatic tip  24  is described. Distal region  23  comprises flexible, kink-resistant coil  41  encapsulated in flexible layer  42 . Layer  42  preferably comprises a soft elastomeric and hydrophilic coated material, such as silicon or synthetic rubber, and extends through bores  33  of nestable elements  30  to form liner  43  for lumen  25 . Layer  42  extends to handle  21  at the proximal end, and at the distal end terminates in enlarged section  44  that forms atraumatic tip  24 .  
      Layer  42  preferably joins with or is integrally formed with flexible elastomeric cover  45  which encapsulates nestable elements  30  in annular chamber  46 . Cover  45  provides a relatively smooth outer surface for overtube  22 , and prevents tissue from being captured or pinched during relative rotation of adjacent nestable elements  30 .  
      In accordance with one aspect of the present invention, colonoscope  10  may be positioned with its distal tip  11  disposed in distal region  23 , so that deflection of steerable distal tip  11  imparts an angular deflection to distal region  23  and atraumatic tip  24 . To ensure that there is no gross relative motion between colonoscope  10  and apparatus  20 , Toughy-Borst valve  26  is tightened to engage apparatus  20  to the colonoscope. In this manner, colonoscope  10  and distal region  23  may be simultaneously advanced through the colon, with the distal tip of the colonoscope providing a steering capability to apparatus  20 . Apparatus  20  therefore may be advantageously advanced together with colonoscope  10  when overtube  22  is in the flexible state, reducing relative motion between apparatus  20  and colonoscope  10  to those instances where overtube  22  must be shape-locked to prevent distension of the colon.  
      Still referring to  FIG. 4 , terminations  47  of tension wires are described. Terminations  47  illustratively comprise balls welded or molded onto the ends of tension wires  36  that ensure the tension wires cannot be pulled through tension wire bores  35  of the distalmost nestable element  30 . This ensures that the nestable elements cannot come loose when overtube  22  is disposed within a patient.  
      Alternatively, terminations  47  may comprise knots formed in the ends of tension wires  36 , or any suitable fastener that prevents the tension wires from being drawn through the tension wire bores of the distal-most nestable element. Advantageously, cover  45  provides additional assurance that all of nestable elements  30  can be safely retrieved from a patient&#39;s colon in the unlikely event of a tension wire failure.  
      Referring now to  FIGS. 2 and 5 , tension wires  36  within overtube  22 , liner  43  and lumen  25  extend from distal region  23 , through overtube  22 , and to handle  21 . Within handle  21 , each tension wire  36  passes through wire lock release  51  fixedly attached to handle  21 , and wire lock  52  disposed on slide block  53 . Each tension wire  36  terminates at wire tension spring  54 , which maintains tension wires  36  in light tension even when overtube  22  is in the flexible state. The degree of tension provided by wire tension springs  54  is not sufficient to clamp adjacent nestable elements  30  together, but on the other hand does not let gaps form between adjacent nestable elements, and helps to manage the tension wire take up or slack as overtube  22  makes various bends.  
      Slide block  53  is keyed to slide along rail  55  disposed between limit blocks  56  and  57 , and comprises a rigid block having a bore through which rail  55  extends and an additional number of bores as required for the number of tension wires  36  employed. Rack gear  58  is fixedly coupled to slide block  53 . Rack  58  mates with pinion gear  59 , which is in turn driven by bi-directional pawl  60  coupled to actuator  27 . Pinion gear  59  may be selectively engaged by either prong  61  or  62  of bidirectional pawl  60 , depending upon the position of selector switch  63 .  
      If prong  61  is selected to be engaged with pinion gear  59 , a squeezing action applied to actuator  27 , illustratively hand grip  64 , causes rack  53  to move in the D direction in  FIG. 5 , thereby applying tension to tension wires  36 . Repeated actuation of hand grip  64  causes slide block  53  to move progressively further in direction D, thereby applying an increasing clamping load on nestable elements  30 . Any slack lengths of tension wires  36  extending below slide block  53  are taken up by wire tension springs  54 . As discussed in greater detail below with respect to  FIG. 6 , wire locks  52 , which are affixed to slide block  53 , engage and retract tension wires  36  concurrently with movement of slide block  53  in the D direction.  
      If prong  62  is instead chosen by selector switch  63  to engage pinion gear  59 , repeated actuation of hand grip  64  causes slide block  53  to translate in direction U, thereby relaxing the tensile load applied by tension wires  36  to nestable elements  30 . Repeated actuation of hand grip  64  causes slide block  53  to advance in direction U until wire lock releases  51  engage wire locks  52 , releasing all tension from tension wires  36  except that provided by wire tension springs  54 . This action permits the clamping forces imposed on nestable elements  30  to be progressively reduced and render overtube  22  progressively move flexible, until when wire lock releases  51  engage wire locks  52 , the overtube is returned to its most flexible state.  
      Referring to  FIG. 6 , wire lock  52  and lock release  51  are described in greater detail. Wire lock  52  includes jaws  65  disposed within collet  66 . Collet  66  includes a tapered conical bore  67 . Jaws  65  have ramped exterior surfaces  68  and teeth  69 , and are biased against the surface formed by the tapered conical bore by springs  70 . Teeth  69  are configured to engage tension wire  36  under the bias force of springs  70 . When slide block  53  is moved in direction D (see  FIG. 5 ), jaws  65  engage and grasp tension wire  36  and retract the tension wire in direction D.  
      To disengage teeth  69  from tension wire  36 , e.g., when it is desired to allow overtube  22  to return to a flexible state, slide block  53  is actuated as described previously to move in direction U. Further actuation of slide block  53  towards limit block  56  and wire lock release  51  causes wire lock release  51  to extend into tapered conical bore  67  and push jaws  65  backward against the bias of springs  70 . Once tension wires  36  are freed from jaws  65 , overtube  22  returns to its most flexible state.  
      Referring to  FIGS. 7A-7C , a method of using apparatus  20  is described. Colonoscope  10  and overtube  22  my be inserted into the patient either simultaneously or by first backloading the overtube onto the colonoscope. To perform simultaneous insertion, colonoscope  10  is introduced into lumen  25  of handle  21  until distal tip  11  of the colonoscope is disposed in distal region  23 . Toughy-Borst valve  26  is actuated to lock apparatus  20  to colonoscope  10 . As one unit, colonoscope  10  and overtube  22  are inserted into rectum R of the patient, and navigated about rectosigmoid junction RJ. As discussed previously, steerable distal tip  11  may be used to impart angular deflection to flexible tip  24  to steer tip  24  about tortuous curves, such as rectosigmoid junction RJ. Once distal tip  11  and tip  24  have been negotiated past rectosigmoid junction RJ, the current shape of overtube  22  is locked in the manner discussed above to provide a rigid channel through which colonoscope  10  may be further advanced into the colon without distending rectosigmoid junction RJ. Once distal tip  11  of colonoscope  10  is negotiated past sigmoid colon SC, overtube  22  is released from its rigid state and advanced along colonoscope  10  until it too traverses sigmoid colon SC. Again, the current shape of overtube  22  is locked to provide a rigid channel for advancement of colonoscope  10 . To negotiate the remainder of the colon, such as left colic flexure LCF and right colic flexure RCF, the preceding steps may be repeated. In this manner, colonoscope  10  and overtube  22  may be navigated through the tortuous curves of the colon without distending the colon, and thereby causing discomfort, spasm or injury.  
      Alternatively, rather than simultaneously inserting both colonoscope  10  and overtube  22  into the patient, apparatus  20  first may be backloaded onto the colonoscope. First, overtube  22  is threaded onto colonoscope  10  and positioned proximal distal tip  11 , as shown in  FIG. 8 . Colonoscope  10  then is inserted into rectum R of the patient and advanced around rectosigmoid junction RJ. Overtube  22  is advanced along colonoscope  10  into rectum R of the patient, using colonoscope  10  as a guide rail to negotiate rectosigmoid junction RJ. Once overtube  22  traverses rectosigmoid junction RJ to the position shown in  FIG. 7A , the shape of overtube  22  is locked to provide a rigid channel through which colonoscope  10  may be further advanced into the colon. To negotiate the remainder of the colon, the steps discussed in reference to  FIGS. 7B-7C  may be performed.  
      With respect to  FIG. 9 , an alternative embodiment of handle  21  is described. Like handle  21  of  FIG. 5 , handle  71  also embodies a ratchet-type tension mechanism, but in this embodiment overtube  22  may be separated from handle  71 , thereby permitting handle  71  to be sterilized for repeated use. Handle  71  comprises housing  72  having actuator  73  that engages teeth  74  disposed along the length of rod  75 , which defines working axis W of handle  71 . Push knob  76  is affixed to the proximal end of rod  75  so that when pawl  77  is released, rod  75  may be pushed in a distal direction. Pawl  77  engages teeth  74  of rod  75  to prevent distally-directed motion of rod  75 . Spring  78  biases pawl  77  against teeth  74  of rod  75 , to provide a one-way ratchet effect when actuator  73  is squeezed.  
      As in the embodiment of  FIG. 5 , tension wires  36  extend through wire lock releases  79 , wire locks  80 , and are coupled to wire tension springs  81 . Wire locks  80  are affixed to block  82 , which translates within housing  72  responsive to movement of rod  75 . Wire locks  80  and wire lock releases  79  operate in the same manner as described with reference to  FIG. 6 .  
      In operation, squeezing actuator  73 , illustratively a hand grip, causes fork  83  to move rod  75  in a proximal direction so that pawl  77  captures the next distalmost tooth  74 . This movement also causes wire locks  80  to engage and grasp tension wires  36  and retract the tension wires proximally. Further actuation of actuator  73  causes overtube  22  to stiffen in the manner previously described. Spring  78  retains pawl  77  in continuous engagement with teeth  74 , thereby preventing rod  75  from moving in the distal direction.  
      When it is desired to make overtube  22  more flexible, pawl  77  is released and knob  76  pushed in the distal direction so that wire locks  80  engage wire lock releases  79 . As described above, this releases tension wires  36  from wire locks  80  and permits overtube to assume its most flexible state.  
      In accordance with one aspect of the present invention, overtube  22  of the embodiment of  FIG. 9  may be replaceably removed from yoke  84  of handle  71 . In addition tension wires  36  further may comprise connectors  85  that permit the tension wires to be disconnected. Such a configuration permits the overtube to be removed and discarded after a single use, while the handle may be sterilized and reused.  
      Yoke  84  is also configured to position overtube  22  so that longitudinal axis L of the overtube is angularly displaced from working axis W by a predetermined angle β. This arrangement prevents handle  71  from interfering with advancement of colonoscope  10  into lumen  25 .  
      In accordance with yet another aspect of the present invention, overtube  22  includes atraumatic tip  86  that comprises a soft foam-like material. Atraumatic tip  86  not only facilitates advancement of overtube  22  in traversing tortuous anatomy, but also serves to retain the organ wall a safe distance away from the opening through which the colonoscope is reciprocated by radially expanding the organ wall in the vicinity of the tip, as described hereinbelow with respect to  FIG. 14A . Accordingly, atraumatic tip  86  reduces the potential for tissue to be caught or pinched in lumen  25  when the colonoscope is manipulated.  
      With respect to  FIGS. 10A and 10B , an alternative structure is described to facilitate movement of a colonoscope within lumen  25  of overtube  22 . In particular, instead of using inner lining  43  as depicted in  FIG. 4 , some or all of nestable elements  30  may include roller bearings  87  that are received in insets  89  formed in nestable elements  30 . Bearings  87  may be disposed on ring  88  to facilitate assembly of the device.  
       FIGS. 11A and 11B  depict a further alternative embodiment, in which lubricious flexible rails  90  are disposed within bore  33  of nestable elements  30 . Rails  90  span the length of lumen  25 , and reduce contact between the colonoscope and the interior of the overtube, thereby facilitating movement of the colonoscope through overtube  22 .  
      In accordance with another aspect of the present invention, the diameter of lumen  25  preferably is configured to facilitate simultaneous passage of more than one diagnostic or therapeutic instrument therethrough. As shown in  FIG. 12 , lumen  25  may be dimensioned to permit auxiliary devices AD, such as for aspiration, biopsy, or additional lighting, to be advanced alongside colonoscope  10 . For example, if lumen  25  has a diameter of 13 mm and colonoscope  10  has an outer diameter of 10 mm, auxiliary device AD, such as a catheter, having a diameter of between 3F to 9F may be advanced through the remaining space within lumen  25 . Advantageously, this permits auxiliary devices AD to be successively placed within the patient&#39;s colon to perform additional diagnostic or therapeutic procedures without the need to remove colonoscope  10  and overtube  22  therefrom.  
      Referring to  FIG. 13 , an alternative embodiment of a distal region suitable for use in the overtube of the present invention is described. Distal region  100  is similar in construction to distal region  23  of the embodiment of  FIG. 4 , but has flexible coil  101  embedded in only the proximal portion of elastomeric layer  102 . Atraumatic tip  102  at the distal end of distal region  24  may further enhance the steerability of the overtube  22  when the steerable tip of the colonoscope is disposed therein.  
       FIGS. 14-18  illustrate additional configurations of atraumatic tips suitable for causing “tenting” of the wall of the hollow body organ. As used herein, tenting refers to the tendency of the atraumatic tip to be deflected radially outward in the vicinity of the tip of the overtube. This reduces the risk that the wall of the organ will become pinched or caught between the colonoscope and the entry to overtube  22  when the colonoscope is retracted within the overtube.  
       FIG. 14A  shows atraumatic tip  24  in the form of an inflatable donut-shaped balloon  110  affixed to distal region  23  of overtube  22 . Inflation lumen  111  extends from the handle through overtube  22  to provide fluid communication between balloon  110  and an inflation source, such as a syringe (not shown). As illustrated in  FIG. 14B , when balloon  110  is inflated, the wall of the colon radially deflects around balloon  110 . Thus, when colonoscope  10  is retracted into lumen  25 , it is less likely that the wall of the colon will be pinched or potentially dissected between overtube  22  and colonoscope  10 .  
       FIG. 15  depicts a further alternative embodiment of atraumatic tip  24 , comprising soft membrane  120  covering shape memory alloy petals  121 . Petals  121  preferably comprise loops of shape memory alloy wire, e.g., nickel titanium alloy, and extend radially outward in the proximal direction near the distal opening into lumen  25 , so that the proximal end of membrane-covered petals causes the “tenting” effect described hereinabove. The shape memory alloy may be activated to adopt a pre-formed shape when exposed to body temperature, and returned to a contracted state by flushing overtube  22  with cold water or air. Alternatively, petals  121  may be mechanically extended or retracted, or self-expanding.  
       FIG. 16  depicts a further alternative embodiment of atraumatic tip  24 . In the embodiment of  FIG. 16 , petals  130  covered by soft elastomeric membrane  131  extend distally from distal region  23  to form funnel-shaped element  132 . Atraumatic tip  24  provides a similar tenting effect to that described for the preceding embodiments.  
       FIGS. 17-18  provide further alternative configurations for atraumatic tip  86  of the embodiment of  FIG. 9 . Tip  140  preferably comprises a foam or soft elastomer, and may be affixed to distal region  23  of overtube  22  using a suitable biocompatible adhesive.  FIG. 18  depicts an alternative shape for a foam or soft elastomer bumper  150 , which includes a proximally-extending flange  151 . Of course, one of ordinary skill in the art will recognize that other configurations may be used in accordance with the principles of the present invention to form atraumatic tips that cause localized tenting of the colon wall, and these atraumatic tips may be used with the passively-steerable distal regions of the embodiments of  FIGS. 4 and 13 .  
      With respect to  FIGS. 19-23 , alternative embodiments of overtube  22  are described. Unlike overtube  22  of the above-described embodiments, which comprised a multiplicity of nestable elements that are clamped with a plurality of tension wires, the embodiments of  FIGS. 19-23  use alternative clamping mechanisms. In particular, the following embodiments comprise a plurality of links that may be stiffened by the use of compressive sleeves that compress individual links disposed along the length of the overtube.  
      Referring now to  FIGS. 19A-19C , a first alternative embodiment of the overtube of the present invention is described. Overtube  160  comprises a multiplicity of alternating spool links  161  and clamp links  162 . Each spool link  161  and clamp link  162  has a bore disposed therethrough to accommodate a standard colonoscope. Spool link  161  comprises rounded edges  163  disposed on its distal and proximal ends that are contoured to permit limited rotatable engagement with one of two contoured grooves  164  disposed within the bore of clamp link  162 . Accordingly, clamp link  162  comprises a greater outer diameter than spool link  161 . Each clamp link  162  also has through-wall split  168  longitudinally disposed to permit a reduction in the diameter of clamp link  162  when the clamp link is compressed, as discussed hereinafter.  
      Still referring to  FIGS. 19A-19C , a first embodiment of a compressive sleeve comprising inflatable sleeve  165  having first compressive portions  166  and second compressive portions  167 . Sleeve  165  is configured so that the inner diameters of second compressive portions  167  are smaller than those of first compressive portions  166  when sleeve  165  is inflated. Second compressive portions  167  may be disposed to engage clamp links  162 . Thus, when inflatable sleeve  165  is inflated by an inflation source (not shown) coupled to the handle, second compressive portions  167  compress against clamp links  162  to shape-fix overtube  160 . In  FIGS. 19B and 19C , cross sectional views of first compressive portions  166  and second compressive portions  167 , respectively, are shown when sleeve  160  is in its inflated state.  
       FIG. 20  illustrates an alternative embodiment of a compressive sleeve that also comprises an inflatable bladder. Unlike inflatable bladder  160  of  FIGS. 19A-19C , spiral bladder  170  has a constant inner diameter. Spiral bladder  170  preferably is helically disposed around the overtube. Accordingly, when bladder  170  is inflated, clamp links  162  are compressed onto spool links  161  to stiffen the overtube.  
       FIG. 21  depicts a further embodiment of a compressive sleeve  180 , comprising discontinuous hoops  181  made of shape memory alloy (e.g. nickel titanium alloy). Each hoop  181  includes gap  182 , which is spanned by spring  183 . Each hoop  181  is electrically connected to neighboring hoops  181  via insulated wires  184 , so that a serial electrical circuit is established. When hoops  181  are energized, they undergo a phase transition that causes the hoops to contract into a preformed shape that is diametrically smaller than the non-energized shape. Since hoops  181  may be disposed about clamp links  162 , contraction of hoops  181  may be used to apply a clamping load that compresses links  162  onto spool links  161  to stiffen the overtube.  
      Springs  183  contribute to structural integrity when hoops  181  are in their non-energized state. To energize and thereby contract hoops  181 , an electrical current may be run through wires  184 . To return hoops  181  to their non-contracted state and thereby return the overtube  22  to its flexible state, hoops  181  may be flushed with cold water or air. Of course one of ordinary skill in the art will recognize that hoops  181  also may be individually energized, thus requiring a parallel circuit.  
      With respect to  FIGS. 22A-22B , a still further alternative embodiment of an overtube suitable for use in the present invention is described. This embodiment comprises helical links  190  that are formed from an integral strip  191  having regions of different durometer, e.g., rigid material  192  and soft material  193 . When strip  191  is helically wound, helical links  190  are formed having rigid portions  194  and soft portions  195 . Rigid portions  194  provide structural integrity to the overtube, while soft portions  195  provide flexibility.  
      Helical links  190  are disposed within compressive sleeve  196 , which includes first compressive portions  197  and second compressive portions  198 . Compressive sleeve  196  is identical in structure and operation to that described in  FIGS. 19A-19C , except that second compressive portions  198  are aligned with, and apply a clamping force to, rigid portions  194  of helical links  190 . It will of course be understood that an overtube in accordance with the principles of the present invention could alternatively be formed using helical links  190  and either of the clamping systems described with respect to  FIGS. 20 and 21 .  
      With respect to  FIG. 23 , a still further embodiment of an overtube suitable for use in the apparatus of the present invention is described. Overtube  200  comprises a heat-softenable polymer layer  201 , (e.g., Carbothane®, a proprietary urethane-based polymer available from Thermedics Polymer Products, Woburn, Mass.), having wire  202  embedded within it. Wire  202  is coupled at the handle to- an energy source, so that by passing an electric current through wire  202 , sufficient resistive heating occurs to soften the polymer layer  201 , rendering it sufficiently flexible to negotiate tortuous or unsupported anatomy. When electrical energy is not supplied to wire  202 , no resistive heating of the wire or the polymer layer occurs, and the overtube instead cools and stiffens. Wire  202  serves the dual purpose of providing kink resistance and electric heating.  
      Still referring to  FIG. 23 , yet another alternative embodiment of an overtube suitable for use in the present invention comprises a soft elastomeric polymer layer  201  having a shape memory alloy wire  202  embedded within layer  201 . In this embodiment, the shape memory alloy is selected to have a martensite transition temperature above body temperature. When wire  202  is heated to a temperature above body temperature such as by passing an electric current through it, the wire transitions into the austenitic phase, and becomes stiffer, thereby shape locking the overtube. When application of the electric current ceases, wire  202  cools back into the martensitic phase, and renders the overtube flexible.  
      While preferred illustrative embodiments of the invention are described above, it will be apparent to one skilled in the art that various changes and modifications may be made therein without departing from the invention. The appended claims are intended to cover all such changes and modifications that fall within the true spirit and scope of the invention.