Abstract:
A secure stent for maintaining a lumenal opening constructed preferably as a tubular structure of NiTi material or bioabsorbable polymers. The circumference of the tube is preferably in the shape of a polygon in contrast to the circular or oval shape of a body lumen into which the stent is to be placed. The polygon shape and ribs provides interference with the lumen wall and resists stent migration. The diameter of the stent tube is configured with each end enlarged providing flanges for interference with a lumen wall. The central portion of the stent is also bulged out to an increased diameter to provide an enhanced lumen wall resistance to avoid migration. In addition, the locking feature of a ribbed structure prevents the stent from collapsing, and thereby maintains the lumen opening. The stent is preferably constructed from polymers, including bioabsorbable polymers, and/or super elastic materials. The bioabsorbable polymer construction aids removal by causing the tube diameter to collapse. Removal of the stent can therefore be accomplished by simply grasping the proximal end of the stent. Alternatively, a stent constructed entirely of bioabsorbable material will eventually be entirely absorbed, avoiding the need for removal. Alternatively, the stent can be preferably constructed of NiTi or other shape memory material and set in the desired shape at a high temperature. Installation is accomplished by cooling the stent to the malleable Martensite state and winding it on a small diameter mandrel of an insertion/removal tool. The compacted stent is then placed in a probe and inserted in a body lumen, whereupon it is heated to an Austenite state where it regains its spring tension, forcing it back toward the set shape. Removal is accomplished by cooling the stent to the malleable Martensite state and pulling it out. If the selected material is bioabsorbable, the stent generally does not have to be removed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to intralumenal stent devices for maintaining a lumenal opening in a human body, and more particularly to a urethral stent that is configured to keep open the lower urinary tract and other body lumens.  
           [0003]    2. Description of the Prior Art  
           [0004]    Various devices known as stents have been proposed, developed and used for placement in a human body to maintain a lumen opening. Typical applications include treating occlusions of blood vessels, and urethra blockages due to benign prostate hyperplasia. Problems that generally need attention in the design and use of stents include methods of insertion and removal, and prevention of stent migration. Most stents in the marketplace are constructed of a metallic coil of nitinol alloy or stainless steel. In U.S. Pat. No. 5,830,179 a stent is constructed as a coil of nitinol alloy. Nitinol is a member of a class of materials known to have “shape memory.” In practice, the wire is heated to a high temperature, wound on a mandrel or otherwise placed in a set position and cooled. The material stresses result in a “spring” tension built into the material to return to the set position as long as the material is above a certain temperature known as an Austenite state. In order to insert the stent in a body lumen, it is cooled, causing it to enter what is known as a Martensite state in which it is very malleable and can be wound on a small diameter mandrel. Once in position in the body lumen, the stent is heated, resulting in its entering back into the Austenite state, wherein the spring tension is restored, urging it back toward the set position. An alternate design uses outwardly flanged ends to provide increased resistance with the lumen wall.  
         SUMMARY  
         [0005]    It is therefore an object of the present invention to provide an improved stent that can be readily removed.  
           [0006]    It is a further object of the present invention to provide a stent that effectively resists migration after installation.  
           [0007]    It is another object of the present invention to provide a stent that has a coating for delivery of a treatment substance.  
           [0008]    Briefly, a preferred embodiment of the present invention includes a secure stent for maintaining a lumenal opening constructed preferably as a tubular structure of NiTi material or bioabsorbable polymer. The circumference of the tube is preferably in the shape of a polygon in contrast to the circular or oval shape of a body lumen into which the stent is to be placed. The polygon shape and ribs provide interference with the lumen wall and resist stent migration. The diameter of the stent tube is configured with each end enlarged providing flanges for interference with a lumen wall. The central portion of the stent is bulged out to an increased diameter to provide an enhanced lumen wall resistance to avoid migration. In addition, the locking feature of a ribbed structure prevents the stent from collapsing, and thereby maintains the lumen opening. The stent is preferably constructed from polymers, including bioabsorbable polymers, and/or super elastic materials. The bioabsorbable polymer construction aids removal by causing a reduction in the tube diameter as material is absorbed by body material. Attachment for removal of the stent can then be accomplished by simply grasping the proximal end of the stent. Alternatively, a stent constructed entirely of bioabsorbable material will eventually be entirely absorbed, avoiding the need for removal. Alternatively, the stent can be constructed of NiTi or other shape memory material and set in the desired shape at a high temperature. Installation is accomplished by cooling the stent to the malleable Martensite state and winding it on a small diameter mandrel of an insertion/removal tool. The compacted stent is then placed in a probe and inserted in a body lumen, whereupon it is heated to an Austenite state where it regains its spring tension, forcing it back toward the set shape. Removal is accomplished by cooling the stent to the malleable Martensite state and pulling it out. If the selected material is bioabsorbable, the stent generally does not have to be removed. 
       
    
    
     IN THE DRAWING  
       [0009]    [0009]FIG. 1 a  contains side and end views of a preferred embodiment of the stent of the present invention;  
         [0010]    [0010]FIG. 1 b  shows an alternate embodiment with a circular end view;  
         [0011]    [0011]FIG. 2 a  shows a stent with a hexagonal cross section of constant area;  
         [0012]    [0012]FIG. 2 b  shows a hexagonal stent with a concave central section;  
         [0013]    [0013]FIG. 2 c  shows a stent with a hexagonal cross section and convex/bulbous central section;  
         [0014]    [0014]FIG. 3 shows a stent formed from perforated, thin flat material;  
         [0015]    [0015]FIG. 4 a  is a view of flat, stepped material for forming a stent;  
         [0016]    [0016]FIG. 4 b  shows the stepped material formed in an expanded spiral;  
         [0017]    [0017]FIG. 4 c  shows the stepped material in a tight, compact form;  
         [0018]    [0018]FIG. 5 a  is a perspective view of an expanded stent constructed with narrow, flat protrusions;  
         [0019]    [0019]FIG. 5 b  shows the stent of FIG. 5 a  wound in a compact form;  
         [0020]    [0020]FIG. 6 a  shows a stent similar to FIG. 5 a  with a corrugated elongated protrusion;  
         [0021]    [0021]FIG. 6 b  shows the stent of FIG. 6 a  wound in a compact form;  
         [0022]    [0022]FIG. 7 a  shows sharply and evenly corrugated sheet material;  
         [0023]    [0023]FIG. 7 b  shows a stent wound from the corrugated material of FIG. 7 a;    
         [0024]    [0024]FIG. 7 c  shows a stent having an alternate wound form, and constructed from the material of FIG. 7 a;    
         [0025]    [0025]FIG. 7 d  shows a stent wound from material with alternating abrupt and tapered lengths;  
         [0026]    [0026]FIG. 7 e  illustrates a stent wound from a corrugated material with abrupt points separated by curved sections;  
         [0027]    [0027]FIG. 7 f  shows a stent wound from continuously curved corrugated material;  
         [0028]    [0028]FIG. 7 g  illustrates holes in stent material;  
         [0029]    [0029]FIG. 8 illustrates the use of a turn block to expand and contract the cross section of a stent;  
         [0030]    [0030]FIG. 9 shows a scissor-jack for expanding and contracting a stent;  
         [0031]    [0031]FIG. 10 illustrates the use of a biodegradable coating over a stent base;  
         [0032]    [0032]FIG. 11 is a list of bio-absorbable/biodegradable materials;  
         [0033]    [0033]FIG. 12 is a list of anti-microbial coating materials;  
         [0034]    [0034]FIG. 13 lists coating materials that can be used as lubricants;  
         [0035]    [0035]FIG. 14 is a list of drugs/pharmaceuticals, etc. for inclusion in a stent coating;  
         [0036]    [0036]FIG. 15 shows a stent base of smaller diameter with perforations through which a material can be ejected to secure the stent base to a body lumen wall;  
         [0037]    [0037]FIG. 16 illustrates a stent in the form of a balloon;  
         [0038]    [0038]FIG. 17 a  illustrates an endoscopic instrument for inserting a stent;  
         [0039]    [0039]FIG. 17 b  is an expanded view of a stent and ejection device in reference to FIG. 17 a;    
         [0040]    [0040]FIG. 18 shows a polycatheter and balloon device for inserting a stent; and  
         [0041]    [0041]FIG. 19 illustrates a simple stent installation tool. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0042]    A preferred embodiment of the present invention is illustrated in FIG. 1 a  wherein a tubular stent  10  formed from coiled wire  11  is shown in a longitudinal view  12  and an end view  14 . The present invention includes longitudinal variations in the stent cross section, the longitudinal direction defined by axis  16 . The stent  10  has a flared proximal end  18  and a flared distal end  20 . The middle portion  12  is bulged out. The combination of these variations in the cross section, i.e. variations in the distance of the tube wall from axis  16  as a function of distance along the axis  16 , including flared ends  18 ,  20  and the bulged midportion  22  results in a stent with an increased strength to retain a lumen wall, and an increase in resistance to stent migration/movement in a body lumen. The bulged central/midportion  22  is important in that it provides greater strength in resisting lumen wall pressure than a straight tube section would provide. The stent wall can be constructed from any of various biologically compatible materials, such as NiTi, stainless steel, and various biodegradable polymers. The benefit of the construction is that pressure on the rim  24  of the flares  18 ,  20  is transferred in part to bulbous midsection  22 , giving it greater strength. The end view  14  illustrates another feature of the present invention, showing the outline of the rim  24  of the distal end  20  of the flare. This hexagonal shape continues for the entire length of the stent  10 , varying in area from a maximum at the ends  18 ,  20  and in the middle  22  to a minimum contour  26  between the midsection and flared ends. The hexagonal shape is a preferred embodiment, but other irregular shapes are also included in the spirit of the present invention. The novel purpose of an irregular outline for a stent is to provide increased frictional contact with a typically round or oval shaped lumen wall. The pressure of the irregular shaped stent against the lumen wall causes the wall to expand and partially conform to the stent outline. The irregular shaped stent contour provides areas (for example at  28 ) of increased pressure, resulting in more resistance with the body lumen wall than would occur if the stent outline were round or oval, such as illustrated in the alternate embodiment of FIG. 1 b.    
         [0043]    [0043]FIG. 2 a  shows an embodiment utilizing only the irregular cross-section feature, without the variation in cross-section over the length of the stent. FIG. 2 b  illustrates the use of the irregular cross-section combined with flared ends. FIG. 2 c  shows a stent with only a bulged middle. FIG. 2 d  shows a stent  33  formed by slotting the stent material on each end  35 ,  37  and in the middle  39 , and then expanding the ribs  41  outward. The ribs  41  can be a flat ribboned shape or as generally shown, or they can be further configured as illustrated with rib end sections  43  and  45  that are bent outward longitudinally to provide a sharper rib shape, such as the ribs shown in FIGS. 6 b ,  7   c , etc.  
         [0044]    [0044]FIG. 3 shows the use of a thin sheet material  30  to form a stent  32 . The perforations  34  are optional. The slot  34  allows the stent  32  to be readily collapsed for insertion and removal.  
         [0045]    [0045]FIGS. 4 a - 4   c  illustrate a stent construction using a flat ribbon type of material that is cut in steps as shown in FIG. 4 a . The steps are therefore formed in the plane of the flat, sheet/ribbon material as distinguished from steps or corrugations that will be shown in subsequent figures of the drawing. When the material of FIG. 4 a  is wound on a mandrel, it has an expanded form as shown in FIG. 4 b . It can be heated and set in the expanded configuration of FIG. 4 b , and then compressed by further winding to a smaller configuration such as FIG. 4 c . The step lengths “d 1 ” determine the minimum circumference of the tightly wound stent as shown in FIG. 4 c . Each “turn” of the stent is spaced from the next by the distance d 2  which can be any value desired.  
         [0046]    [0046]FIGS. 5 a  and  5   b  illustrate another alternate stent  38  embodiment. Constructed from flat material of width “w”, it is bent, forming a plurality of short protrusions  40  of lengths hi and a single elongated protrusion  42  of length L. The protrusions  40  and  42  are joined with a radius R, allowing an open lumen  44  through the full length of the stent which is the width “w” of the flat material. The stent  38  is placed in a cylindrical shape as shown in FIG. 5 b  by winding the elongated protrusion  42  around the axis  46  of the stent lumen  44 , in the process folding/bending over the short protrusions  40  resulting in a compressed stent  38  of small diameter D for insertion into a body lumen. FIGS. 6 a  and  6   b  illustrate an alternate embodiment  48  of the same general type as shown in FIGS. 5 a  and  5   b . he elongated protrusion  50  has a corrugated side  52  that is included to increase contact resistance with a body lumen wall to reduce stent migration. FIG. 5 b  shows the compressed, wound state of the stent, clearly showing the corrugated side  52  facing outward. This figure also clearly illustrates bent shorter protrusions and a stent lumen  56  that are features in common with the stent  38  of FIGS. 5 a  and  5   b.    
         [0047]    A further alternate stent embodiment  58  is shown in its wound compressed state in FIG. 7 a . It is formed from a corrugated material  59  as shown in FIG. 7 b . Additional alternate stent embodiments constructed from corrugated sheet material are shown in FIGS. 7 a - 7   g . Shown in FIG. 7 a  is an evenly bent material  58  which can be wound to form stents  59  and  61  as shown in FIGS. 7 b  and  7   c.    
         [0048]    [0048]FIG. 7 d  shows a similar stent  63 , differing from stent  61  in that the sheet material is bent so as to provide abrupt ridges  65 , which interfere with each other to resist winding once the stent  63  is expanded, providing a self-locking feature.  
         [0049]    In fact, all of the stents of FIGS. 7 b - 7   f  provide a degree of resistance to compression/rewinding due to the resistance provided by interfering corrugations. Stent  75  of FIG. 7 f  provides the least resistance, having smoothly formed corrugations. Expansion is encouraged in the stent  61  design of FIG. 7 d  by the more gently sloping ramps  67 .  
         [0050]    The stent  69  of FIG. 7 e  uses ridges  71  separated by curved portions  73 . In FIG. 7 f  the stent  75  is constructed of continuously curved corrugations  77 . Any of the stents constructed of sheet material can also have holes, such as holes  79  in stent  81  of FIG. 7 g.    
         [0051]    The stents of FIGS.  1 - 7  are preferably constructed of a shape memory material and heat set in an expanded configuration in the Austenite state. In order to insert the stent in a body lumen, it is cooled to the Martensite state wherein the material becomes malleable, lacking resiliency. In this state, the material can be reformed to a compact state. In this compact state, it can be inserted into a body lumen. The stent is preferably placed on a mandrel that is part of an insertion tool prior to cooling and compacting.  
         [0052]    A preferred shape memory material is nitinol (NiTi), but the present invention includes the use of other shape memory materials that will be apparent to those skilled in the art. In addition, the stent material can be a biodegradable material, such as a biodegradable polymer. The stents can also be made from a combination of biodegradable and non-degradable materials. For example, in FIG. 7 f , the outer layer can be constructed from a biodegradable material, and the inner layer can be constructed of a non-biodegradable material. In this case, when the outer layer is absorbed, the inner layer can be removed.  
         [0053]    The stents can also be constructed from nitinol or other super elastic material, processed/heat-treated to what is known as a “super elastic” state. In this state the material retains its resiliency at lower temperatures, and can be used for a permanent stent installation. Removal would require use of a tool to cut or compress the stent.  
         [0054]    The stents of FIGS. 1 a  through  2   c , and FIGS. 4 b ,  5   a ,  6   a  and  7   b  are all shown in an expanded state. When they are constructed of a shape memory material and heat set in this expanded state, they can then be cooled and wound or otherwise compressed to minimize the size during insertion into body lumen. The shape of FIGS. 4 c ,  5   b , and  6   b  are all examples of a compressed stent in its Martensite state. After insertion in a body lumen, the temperature rises and the material returns to the Austenite state, regaining its resiliency, and causing a force against a body lumen wall in the effort to return to the original state. The stent  58  as shown in FIG. 7 a  can conceivably be further compressed for insertion by bending the protrusions.  
         [0055]    As mentioned above, the stents as disclosed herein can be made out of any bio-compatible material that will allow some method of insertion and removal from a body lumen. The stents of FIGS.  1 - 7  could be constructed of a permanently resilient material such as stainless steel, and could be collapsed with some difficulty for installation in a probe for insertion. However, removal in such a case would generally require a forceps. Constructing the stents of FIGS.  1 - 7  with a shape memory material as discussed above is preferred. After cooling the stent, it can be wound, folded, collapsed, etc. as required in order to be loaded into a probe lumen for transport into a body lumen. A push rod in back of the stent in the probe lumen can be used to eject the stent once the probe is in the desired location. As discussed above, the stent is then simply heated, by any of various means including body temperature or a warm saline solution to bring the stent back to the Austenite state wherein it regains its original resiliency. Removal is accomplished by injecting a cool saline solution to bring the stent back to the malleable Martensite state, whereupon it can be readily pulled out.  
         [0056]    The flared ends  18 ,  10  of FIG. 1 provide resistance with the body lumen wall, keeping the stent from moving in the lumen. The bulbous portion  22  is placed where maximum body lumen enlargement is required. The force of the body lumen on the stent portion  22  tends to cause the ends  18  and  20  to expand, which provides enhanced resistance with the lumen walls to avoid migration. The force of the ends  18  and  20  on the body lumen wall is also reflected back to provide resistance to compression of portion  22 .  
         [0057]    Other shapes for the circumference/cross-section of the stents are also included in the spirit of the present invention. For example, the polygon shape in FIG. 1 could be square, five-sided, an octagon as shown, etc., or other irregular shape to increase resistance between the stent and the body lumen wall. The present invention also includes a circular or oval circumference, as indicated in FIG. 1 b.    
         [0058]    An alternate method of collapsing and expanding a stent is illustrated in FIGS. 8 and 9. An expansion and contraction apparatus can be installed inside a stent that is constructed of sheet material. FIG. 8 shows a turn block  60  that can be activated to contract the diameter of a stent  62  for insertion in a body lumen. The turn block can then be applied to expand the stent against the lumen wall. When removal is required, the reverse procedure is applied. FIG. 9 shows a similar arrangement where a scissor apparatus  64  (similar to a small car jack) is used to expand and contract the diameter of a stent  66 .  
         [0059]    [0059]FIG. 10 illustrates another embodiment of a stent  68  that is designed for temporary use. NiTi or other biologically compatible material  70  is used to form a stent base. A coating of biodegradable material  72  is placed over the base  70 . The base can optionally also be made of biodegradable material. The base  70  can be of any desirable configuration that can be collapsed for insertion in a probe lumen for installation in a body lumen, including structures similar to those of FIGS.  1 - 7 . The expanded size of the base  70  is preferably small enough to clear the size of the body lumen into which it is to be placed, if it is not biodegradable, so that when the material  72  is absorbed by the body, the base  70  can be easily removed. A second coating  74 , or first coating if coating  72  is omitted, can be included. The coating  74  is generally for inclusion of some type of treatment substance, but can be for any purpose, including the purpose of providing interference with the body lumen walls. If coatings  72  and/or  74  are for determining the stent size, the selection of material  72  and thickness depend on how long the stent is to remain in the body. After the material has been sufficiently absorbed, the stent base can be removed by simply grasping it with an appropriate device through an endoscope lumen, for example. This procedure avoids the need to compress the stent for removal, although collapsible stents can also be used. If the stent base is biodegradable, it will eventually be absorbed, and may therefore not have to be removed.  
         [0060]    [0060]FIG. 11 lists various biodegradable materials that can be used to coat a stent, as indicated above. The stents, or stent bases, described above can be constructed from NiTi or a biodegradable polymer, or any other appropriate material known to those skilled in the art, such as stainless steel or any of various compatible polymers. As mentioned above, stents can be constructed entirely from biodegradable material. A number of these are listed in FIG. 11 and will be recognized by those skilled in the art as applicable to the construction of the stent designs disclosed herein. The benefit of using an all biodegradable material is to avoid the necessity of any removal of a stent.  
         [0061]    Coating materials for layer  74 , as discussed above in reference to FIG. 10, include, but are not limited to those listed in FIGS. 12, 13 and  14 . FIG. 12 lists anti-microbial coating materials to reduce the possibility of infection. FIG. 13 lists a selection of materials that reduce friction, i.e. for lubrication. FIG. 14 lists various drugs/pharmaceuticals, etc. as examples of potentially beneficial materials that can be applied to the stent to provide a localized treatment of body tissues.  
         [0062]    Another embodiment of the present invention is illustrated in FIG. 15 wherein a cylindrical stent base  76  includes a plurality of holes  78 . The stent base  76  is inserted in place in a body lumen. An injector probe, symbolically illustrated as item  80 , is then inserted inside the base  76 , and a bio-compatible material  82  is injected and forced out the holes  78  and against the wall of the body lumen (not shown). The material  82  would preferably be constructed to harden i.e. set-up quickly after injection.  
         [0063]    A still further embodiment of the present invention is illustrated in reference to FIG. 16 wherein an inflatable balloon is used as a temporary stent. The balloon  84  is shown in its uninflated state by solid lines  86 . The balloon has a lumen  88  therethrough. The walls of the balloon are constructed with variations of thickness to force expansion in desired directions. The wall  90  of the lumen  88 , and the walls of the flared end sections  92 ,  94  are thicker to avoid expansion. The wall  96  of the outside center portion is thinner to force expansion upon balloon inflation, the inflated state indicated by the dashed lines  98 . A self-sealing inflation port  100  is provided on a proximal end  102 . The distal end  104  is inserted first in the body lumen.  
         [0064]    A method and apparatus for inserting a stent is illustrated in FIGS. 17 a  and  17   b  utilizing an endoscopic instrument  106 . The instrument  106  has a telescope  108 , an irrigation/aspiration port  110 , and a slide trigger  112 . Section “A” of FIG. 17 a  is shown enlargened in FIG. 17 b . The endoscopic instrument  106  includes a probe  114 , in which is inserted a first tube  116  through which a telescope probe and/or instrument can be passed. A stent  118  is assembled over the first tube  116 . In order to eject the stent from the tube  116 , a second tube  120  is provided encircling the first tube  116 . The second tube  120  is linked to the slide trigger  112  and pressing the trigger  112  in toward handle  122  moves the second tube  120  to eject the stent from the probe  114 . With the stent in place in the body lumen, it will immediately expand if it is constructed from a permanently resilient material such as stainless steel. If it is constructed from a shape memory material such as Nitinal (N i T i ), it would have been cooled prior to insertion and compressed in the Martensite state. When it is in place in the body lumen, it expands when its temperature is raised, bringing it back into the Austenite state and regaining its resiliency. Removal of the shape memory material is accomplished by cooling the stent to bring it into the maleable Martensite state and then simply pulling it out.  
         [0065]    [0065]FIG. 18 illustrates another tool that can be used to insert a stsent, including a polycatheter  124  having two one way valves  126  and  128 . The catheter  124  has a probe  130  upon which is mounted a first balloon  132  supplied with air by way of valve  126  and a second balloon  134  supplied with air by way of valve  128 . A stent  136  is positioned around the second balloon  134 . The stent  134  shown in FIG. 18 is formed from flat ribbon material for ease of illustration, but any of the stents disclosed above in FIGS.  1 - 4 , as well as others that will be apparent to those skilled in the art can be installed. The ends  138 ,  140  of the stent material are releasably attached to the probe  130 . The method of attachment can be through use of an adhesive, or by way of other fragile connection.  
         [0066]    Insertion of the stent  136  is illustrated in FIG. 18, with the catheter inserted in a urinary tract  142  and the stent  136  positioned adjacent the prostate  144 . The first balloon  132  is positioned just inside the bladder  146  at this point in the procedure. Air is applied to the first balloon  132  through one way valve  126 , expanding it as shown by dashed lines  148  to contact the bladder wall, securing the catheter  124  in place. The second balloon is then inflated through valve  128 . As the balloon  134  expands, tension is placed on the attachment of the ends  138  and  140  until they break, freeing the stent  136  to expand against the wall  150  of the urinary tract  142 . If the stent is stainless steel or other permanently resilient material, it will immediately expand upon breaking the attachment ends  138  and  140 . If the stent is a shape memory material, it will expand after first being raised in temperature to the Austenite state, which may occur from body temperature or by injection of a heated solution into the urinary tract.  
         [0067]    [0067]FIG. 19 is a simplified sketch for illustrating some of the features of a stent insertion tool. The tool  152  includes a body probe  154  for insertion in a body lumen, and an installation probe  156 . An apparatus  158  includes a housing  160 , a spring  162 , and plate  164  attached to the probe  156 , all configured to apply a spring force to retain the probe  156  inside probe  154  during traversal of a body lumen. With the probe head  166  in place, an operator pushes on the button  168 , impelling the stent as explained above. Upon releasing the button, the spring  162  retracts the probe  156 .  
         [0068]    Although the present invention has been described above in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all such alterations and modifications as fall within the true spirit and scope of the invention.