Abstract:
An apparatus and method are disclosed for measuring the desired length of a prosthetic device which is to be implanted in a body cavity of a patient. The apparatus generally includes a helically coiled stent formed of a resiliently-deformable material, a plunger which is connected to the proximal end of the stent, a sheath which slides over the plunger and stent when the plunger and sheath are used to insert and removably deploy the stent into the body cavity, and a scale for measuring an indication of the length of the stent once removably deployed in the body cavity. Proximal movement of the sheath to partially deploy the stent causes a length to be indicated on the scale. According to the method of the invention, the helically coiled stent of the apparatus is placed and partially deployed within the body cavity by use of the plunger and sheath of the apparatus. Once the stent bridges the body cavity, the scale of the apparatus is used to determine the length of the deployed stent. The apparatus is then removed from the body cavity, and a stent similar to the stent of the apparatus is cut to the length indicated on the scale. In a second embodiment of the apparatus and method, the stent of the measuring apparatus is substantially non-porous to allow occlusion of side vessels extending from the body cavity. The patient is then monitored for any ill effects resulting from the occlusion.

Description:
This application is a division of U.S. patent application Ser. No. 08/466,934 filed Jun. 6, 1995 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates broadly to a measuring and testing apparatus for measuring the size of a stent in a body vessel and for determining the effect of the stent on surrounding tissue and organs. More particularly, this invention relates to an apparatus and a method for measuring the length of a tubular braided stent needed for use in a blood vessel, and to stent loading and deployment apparatus and methods. The invention also relates to methods for determining whether a stent, when deployed, will block important connecting vessel, and whether such blockage would be detrimental to the patient. 
     2. State of the Art 
     Transluminal prostheses are well known in the medical arts for implantation in blood vessel, biliary ducts, or other similar organs or the living body. These prostheses are commonly known as stents and are used to maintain, open, or dilate tubular structures or to support tubular structures that are being anastomosed. When biocompatible material are used as a covering or lining for the stent, the prosthesis is called a stent-graft or endoluminal graft. If used specifically in blood vessels, the stent-graft is known as an endovascular graft. A stent may be introduced into the body by stretching it longitudinally or compressing it radially, until its diameter is reduced sufficiently so that it can be fed into a catheter. The stent is delivered through the catheter to the site of deployment and then released from the catheter, whereupon it self-expands. The contraction to stretching ratio and radial pressure of stents can usually be determined from basic braid equations. A thorough technical discussion of braid equations and the mechanical properties of stents is found in Jedweb, M. R. and Clerc, C. O., “A Study of the Geometrical and Mechanical Properties of a Self-Expanding Metallic Stent—Theory and Experiment”,  Journal of Applied Biomaterials;  Vol. 4, pp. 77-85 (1993). In light of the above, it becomes evident that a stent must possess certain elastic and compression qualities. 
     A typical state of the art stent, such as disclosed in U.S. Pat. No. 4,655,771 to Wallsten or in U. K. Patent Number 1,205,743 to Didcott, is shown herein in prior art FIGS. 1,  1   a,    2 , and  2   a.  Didcott and Wallsten disclose a tubular body stent  10  composed of wire elements  12 , each of which extends in a helical configuration with the centerline  14  of the stent  10  as a common axis. Half of the elements  12  are wound in one direction while the other half are wound in an opposite direction. With this configuration, the diameter of the stent is changeable by axial movement of the ends  9 ,  11  of the stent. Typically, the crossing elements form a braid-like configuration and are arranged so that the diameter of the stent  10  is normally expanded as shown in FIGS. 1 and 1 a.  The diameter may be contracted by pulling the ends  9 ,  11  of the stent  10  away from each other as shown by the arrows  16 ,  18  in FIG.  2 . When the ends of the body are released, the diameter of the stent  10  self-expands and draws the ends  9 ,  11  of the stent closer to each other. 
     The fact that stents undergo various dimension changes from their compressed form to their uncompressed form, results in complications in placement. Placement of a stent having any degree of elongation and radial force as a result of compression is very difficult for several reasons. First, the stent, depending on its pitch angle, may have to be pushed out of the catheter over a long distance. This may be extremely difficult in light of the increased friction forces and various bent sections encountered in the catheter as it traverses a tortuous path. Second, the stent may conversely shrink significantly in length as its diameter expands, thereby rendering it difficult to accurately place it in a vessel. Third, plaque, thrombus or other protrusions or inclusions in the blood vessel lumen may alter the diameter of the stent which consequently alters the length of the stent. The importance of extreme accuracy in placement of an endovascular graft (EVG) will be appreciated by those knowledgeable in the art. For example, in aneurysmal vessel disease, such as that encountered in the abdominal aorta where the distance between the renal arteries and the aneurysm is quite short (less than 3 cm), misplacement of an EVG over the renal arteries or only in the aneurysm can prove fatal. 
     Proper placement of the stent becomes impossible where the stent is too long or too short for the body cavity in which it is being deployed. In order to be effective, the dimensions of a vessel must be known very accurately and the stent must be tailored to match the specifications of the vessel. 
     Several difficulties arise, however, when trying to determine the proper stent length needed for any particular cavity. One such problem, especially present with the self expanding stent design such as described by Wallsten and Didcott, is that it is often difficult to predict exactly to what length the stent should be cut in order to properly fit within a particular blood vessel. For example, when deploying an EVG in an aortic aneurysm, the distal end of the stent may reside in the aneurysmal area if the stent is cut too short in length, thereby not sealing the aneurysm and causing potential problems, such as rupturing of the aneurysm. On the other hand, if the EVG is cut too long, the distal end of the EVG can extend into one of the iliac arteries which will lead to clotting of the contralateral iliac artery. Also, if deployed in a vessel with multiple branching, and EVG which is too long may inadvertently cover an arterial branch, thereby occluding the branch and starving the organ which it is intended to nourish. 
     It is known to presently approximate the deployment length of an EVG stent by using various angiographical techniques (x-ray examinations of blood vessels or lymphatics following the injection of a radiopaque substance). In particular, this is done by injecting radiopaque dye into a vessel and photographing the dye with an X-ray machine as it moves through the vessel. A shortcoming of this method, however, is that angiography usually produces only two-dimensional views of the vessels being examined which are limited by the plane in which the x-ray is taken. As a result, angiograms often fail to reveal the presence of tortuous paths of the examined vessel which may be going in and out of the plane of the angiogram. In addition, the EVG may expand in the area of the aneurysm, depending on the fibrin (the insoluble protein end product of blood coagulation, formed from fibrinogen by the action of thrombin in the presence of calcium ions) content in the aneurysm, and contract in the narrow areas of the aneurysm, thus rendering any prediction of the necessary stent size difficult. It is also known to use Computerized Tomography (CT) scans and the like to show arterial diameters from which the desired deployment stent length can be extrapolated. The prediction of stent deployment length based solely upon slices of diameter, as well as the non-predictability of the fibrin content in an aneurysm, however, limit the accuracy of CT scans. Other more novel methods for visualizing vessels include spiral CT scan and intravascular ultrasound (IVUS). Besides sharing some of the same disadvantages of angioscopy and CT scans, the spiral CT scan provides an image of the outside of the blood vessel only, and therefore fails to show the inside of the vessel where plaque and thrombus accumulate and where the stent is to be placed. The IVUS suffers from not visualizing the compressibility of fibrin and not providing a readout of vessel diameter and length. Another disadvantage shared by the aforementioned apparatus, is that they only provide instantaneous views of the vessel, and may therefore not be accurately representative of the vessel diameter during systole or diastole of the vessel. 
     Another problem encountered with stenting, especially with coated stenting (EVG deployment), is that branch arteries are often occluded. For example, when correcting an aortic aneurysm, an EVG is deployed between the neck of the proximal portion of the aneurysm below the renals to the bifurcation, or in the case of a bifurcated EVG, to the iliac arteries or beyond. As a result, the EVG may occlude arteries such as the lumbar arteries, intercostal arteries and even the mesenteric artery. In general, occlusion of these arteries is not detrimental to the patient as the mesentery and the spinal chord are fed by other collateral arteries. In a small number of patients, however, blockage of these arteries can result in paraplegia. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which provides accurate results. 
     It is also an object of the invention to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which is easy to use. 
     It is another object of the invention to provide a method for temporarily blocking a branch of a body vessel and determining if this blockage is detrimental to the patient. 
     It is a further object of the invention to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which includes a stent made from a resiliently deformable material. 
     It is another object of the invention to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which includes a plunger and a sheath for introducing and placing a stent in a body vessel. 
     It is an additional object of the invention to provide an apparatus for temporarily blocking a branch vessel where the apparatus includes a stent made from a resiliently deformable material which is coated with another resiliently deformable material which is capable of blocking a branch vessel. 
     A further object of the invention is to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which includes a calibrated scale. 
     Another object of the invention is to provide an apparatus and method for measuring the length of a stent or endovascular graft in a body vessel which includes a hollow catheter. 
     An additional object of the invention is to provide an apparatus and method for loading and deploying a length of stent or endovascular graft which was measured according to methods of the invention. 
     According to the invention, an apparatus for measuring the desired length of a prosthetic device which is to be implanted in a predetermined body cavity of a patient generally includes a helically coiled stent formed of a resiliently-deformable material with or without a coating, a plunger and sheath for inserting the stent into the body cavity and removably deploying the stent in the body cavity, and a measurement device for measuring an indication of the length of the stent once deployed in the body cavity. The apparatus may be constructed with a catheter having a lumen which accommodates a guide wire, thereby facilitating guiding the apparatus into the body cavity, and with a dilator tip to facilitate maneuvering of the catheter through the vasculature. 
     In the preferred embodiment of the invention, the proximal end of the stent is attached to the distal end of the plunger, the proximal end of the plunger is marked with a scale which is calibrated proportionally to the length of stent when in the compressed state, and the sheath is translatably adjustable over the plunger. Thus, movement of the sheath relative to the plunger deploys the stent from the sheath such that the stent is free to expand in the vessel in which it is being removably deployed. The amount of movement of the sheath relative to the plunger can be measured on the scale. The reading on the calibrated scale represents the “at rest” or fully uncompressed length of the stent being deployed by the sheath and plunger. 
     Other preferred aspects of the invention include a sheath to plunger lock or stop at the proximal end of the sheath which contains a threaded hub and a compressible O-ring, and a plunger to catheter lock at the proximal end of the plunger which also includes a threaded hub and a compressible O-ring. The threaded hub and compressible O-ring of the sheath to plunger lock are used to prevent unintentional motion of the sheath relative to the plunger, as well as serving as a hemostasis valve during an interventional surgical procedure. The threaded hub and compressible O-ring of the plunger to catheter lock serves as an additional hemostasis valve. If desired, a radiopaque medium can be dispensed at the distal end of the hollow catheter to permit the user to monitor the progress of the apparatus. 
     In further accord with the objects of the invention, a method of measuring the desired length of a prosthetic device which is to be implanted in a body cavity of a patient using the measuring apparatus of the invention is provided. According to the method of the invention, the helically coiled stent of the apparatus is placed and deployed within the body cavity via the placement means of the apparatus. Once sufficient length of the stent is deployed within the body cavity to span the desired length, the measuring device of the apparatus is used to determine the length to which the stent is to be cut. The apparatus is then removed from the body cavity, and the stent of the apparatus, or an equivalent stent, is cut to the measured length. 
     In the preferred method of the invention, a guide wire is first maneuvered through the body cavity where a stent is to be deployed until it reaches a point slightly beyond the deployment site. The sheath of the apparatus is then fully extended over the stent of the apparatus such that the stent is completely compressed within the sheath. The apparatus is then threaded along the guide wire via the hollow inner catheter of the apparatus until properly positioned within the body cavity. The user can monitor the progress of the compressed stent by use of a fluoroscope and radiopaque media which is carried and disseminated alongside the apparatus as it travels through the patient. In addition, the catheter and stent are themselves preferably radiopaque, thereby further aiding visualization under fluoroscopy. Once in position, the sheath of the apparatus is retracted while holding the plunger stationary. The portion of the compressed stent which is uncovered by the sheath deploys within the body cavity by expanding radially and decreasing in length. Retraction of the sheath continues until the user determines via fluoroscopy that the area of the body cavity to be bridged by the stent is fully bridged. At that point, i.e., once the appropriate length of stent has been deployed, the position of the stop of the sheath relative to the scale is read. Since the scale is calibrated, the values obtained will correspond directly to the length of the uncompressed stent which is required to bridge the body cavity. After the measurement has been taken, the sheath is re-extended over the stent, thus compressing it once again for easy removal from the body cavity. A separate stent is then prepared to the indicated length, and may be deployed in the body cavity by any known means in the art. Alternatively, the stent used to measure the cavity can be used by cutting it from the measuring device to the indicated length and placing it in the body cavity accordingly. 
     According to yet other aspects of the invention, a detachable hub is secured onto the proximal end of the inner catheter, and the plunger to catheter lock is made removable. Using this arrangement, the stent length measurement is conducted as summarized above. Once the measurement is read, the measuring device is entirely removed from the body, the proximal detachable hub is removed, the detachable plunger to inner catheter lock is removed, and the distal end of the catheter is pulled until the catheter is removed from the hollow plunger. The plunger connected to the stent is then pulled proximally until the stent is removed from the sheath. The stent is then marked from its distal end to the required length, and the proximal end of the plunger still connected to the stent is inserted into the sheath to plunger lock until the proximal end of the plunger sticks out of the distal end of the sheath. The proximal end of the plunger is pulled out of the distal end of the sheath until the stent is pulled through the sheath and out of the distal end of the sheath to the marking. The stent is then cut proximal of the marking such that the stent in the sheath is of the desired size, and the plunger containing the remaining end of the stent can be discarded; or alternatively the remaining portion of the stent can be severed from the plunger so that the plunger can be reused. With the stent loaded, the introducer system is preferably reassembled with the detachable hub, the detachable plunger to catheter lock, and a new or reused plunger. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a broken side elevation view of a prior art stent expanded in a non-stressed position; 
     FIG. 1 a  is a cross sectional view along line  1 A— 1 A of FIG. 1; 
     FIG. 2 is a broken side elevation view of a prior art stent stretched and contracted; 
     FIG. 2 a  is a cross sectional view along line  2 A— 2 A of FIG. 2; 
     FIG. 3 is a broken transparent side view of the endovascular measuring apparatus of the invention with its sheath retracted; 
     FIG. 4 is a broken transparent side view of the endovascular measuring apparatus of the invention when partially inserted within a body cavity and with its sheath fully extended; 
     FIG. 5 is a broken transparent side view of the endovascular measuring apparatus of the invention when fully inserted within a body cavity and with its sheath fully extended; 
     FIG. 6 is a broken transparent side view of the endovascular measuring apparatus of the invention deploying the stent within a body cavity such that the stent partially bridges the body cavity; 
     FIG. 7 is a broken transparent side view of the endovascular measuring apparatus of the invention deploying the stent within a body cavity such that the stent fully bridges the body cavity; 
     FIG. 8 is a view similar to FIG. 7 illustrating the measuring apparatus with a non-porous stent deployed in a body cavity having branching vessels; 
     FIG. 9 is an enlarged view similar to FIG. 3 of a detachable proximal hub used in conjunction with a method of the invention for deploying the measured stent; and 
     FIG. 10 is an enlarged view of similar to FIG. 3 of a detachable hemostasis valve used on conjunction with a method of the invention for deploying the measured stent. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The entire disclosure of U.S. patent application Ser. No. 08/466,934 filed Jun. 6, 1995 is expressly incorporated by reference herein. 
     Turning now to FIG. 3, the endovascular measuring apparatus  100  of the invention broadly includes a hollow plunger  102 , a wire stent  104 , a hollow sheath  106 , and a hollow inner catheter  108  attached to a hub  109 . The plunger  102  has a proximal end  110  with a first locking hemostasis valve  112  and a distal end  116  which is affixed to the proximal end  118  of the stent  104 . The hemostasis valve  112  includes an O-ring  113 , and a locking cap  114 . The lumen (not shown) of the hollow plunger  102  is dimensioned such that it can slide freely over the body of the hollow inner catheter  108 . The hollow inner catheter  108  serves as a guide for a guidewire  144  and as a tether to hold a soft flexible hollow dilator tip  148  in place at the distal end  146  of the catheter  108 . The tip  148  can be adjusted relative to the distal end  116  of the plunger  102  by sliding the inner catheter  108  within the plunger  102 . 
     Once the tip  148  is adjusted to accommodate the compressed stent  104 , the inner catheter  108  is locked into place by tightening the cap  114  onto a threaded portion  117  of the first locking hemostasis valve  112 . The cap  114  is effectively a locking mechanism which compresses the O-ring  113 , thereby fixing or locking the plunger  102  relative to the inner catheter  108  and the tip  148 . The body  120  of the plunger  102  contains a calibrated scale  122  having, e.g., fifty major divisions  124  spaced at calibrated intervals. The scale  122  is calibrated to adjust for the longitudinal length contraction and diameter expansion experienced by the particular stent  104  when being decompressed; i.e., the ratio of the length of the stent when in the sheath to the length of the stent when uncompressed. 
     The proximal end  118  of the wire stent  104  is affixed to the distal end  116  of the plunger  102  by any desirable means such as by heat fusing, insert molding, or gluing with epoxy. The body  128  of the wire stent  104  when uncompressed has a diameter larger than that of the plunger  102  and of the sheath  106 . 
     The distal end  130  of the sheath  106  is open, and the sheath  106  has a diameter slightly larger than that of the body  122  of the plunger  102  so as to be translatable along the plunger body. The sheath  106  is further translatable over the stent  104  due to flexible and deformable characteristics of the stent  104 . It will be appreciated that when the sheath  106  is positioned over the wire stent  104 , the stent  104  contracts and elongates in a manner similar to that discussed in the Background of the invention and shown at  132 . 
     The proximal end  131  of the sheath  106  is attached to a second hemostasis valve  133  which is preferably provided with external threads  135 . A second threaded cap  138  containing a second compressible O-ring  140  is screwed onto the proximal end of a second locking hemostasis valve  133 . The second threaded cap  138  mates with the threads  135  of the second locking valve  133  to reversibly fasten the sheath  106  to the plunger  102 . The O-ring is used both to prevent inadvertent slippage of the sheath  106  relative to the plunger  102  by acting as a friction-locking mechanism, and to serve as a hemostasis valve during interventional surgical procedures. By pulling the first locking valve  112  away from the second locking valve  133  (or pushing the sheath  106  relative to the plunger  102 ), the wire stent  104  can be pulled into the sheath  106  and compressed. Conversely, by pushing the first locking valve  112  toward the second locking valve  133  (or pulling the sheath  106  relative to the plunger  102 ), the distal end  126  of the wire stent  104  can be released and will expand towards its relaxed uncompressed configuration until (and if) constrained by the blood vessel in which it is being deployed. It will be appreciated that the second locking valve  133  can be positioned and will lock anywhere along the body  120  of the plunger  102 , thus providing the user with a means to control the length of stent  104  to be deployed. By reading the scale  122  at the location of the proximal-most end  142  of the second locking valve  133 , the length of stent required for deployment within the body cavity  202  at any given time can be determined. In particular, since the scale  122  is preferably calibrated to the ratio of the length of the stent  104  when compressed in the sheath  106  to the length of the stent  104  in its uncompressed state, the reading provided on the calibrated scale will inform the practitioner as to the length of uncompressed stent required to bridge any cavity in any path, regardless of the state that the stent will assume when deployed in the cavity. 
     Still referring to FIG. 3, it is noted that both the first and second locking hemostasis valves  112 ,  133  are preferably provided with flushing lines  115 ,  137 . The lines  116  and  137  permit the spaces between the concentric hollow sheath  106 , hollow catheter  108 , and hollow plunger  102  to be flushed with heparinized saline during the insertion procedure. It is also seen that the hollow catheter  108  extends from the proximal hub  109  past the open distal end  126  of the stent  104 . The catheter  108  has an interior lumen (not shown) dimensioned for following a guide wire  144  into the body cavity  202  (see FIG. 4) of a patient. The distal end  146  of the catheter  108  is coupled to the hollow dilator tip  148 . The hollow catheter  108  and dilator tip  148  are capable of transporting a radiopaque contrast medium (not shown) used for fluoroscopic viewing. 
     The plunger  102  and the sheath  106  of the apparatus  100  can be made from any durable biocompatible material such as nylon, polyurethane, Teflon®, polyester, PVC, polyethylene, polypropylene, etc., or various combinations of the above, with or without radiopaque fillers such as barium sulfate or bismuth subcarbonate. The dilator tip  148  can be formed of the same materials as the plunger  102  and sheath  106 , but is preferably formed of a softer durometer material such as Shore  80 A polyurethane or Pebax nylon with a radiopaque filler or a radiopaque marking band. The measuring apparatus  100  of the invention can be made disposable or reusable. The lumen (not shown) of the inner catheter  108  or the annular space  150  between the sheath  106  and plunger  102  can be used to inject radiopaque contrast media into the vessel to assist in placement of the apparatus  100  as discussed above. The stent  104  material can be of the same material and of similar geometry as would be used in an EVG, or it may be of a more radiopaque material such as tungsten, stainless steel, gold and the like. The apparatus  100  can be used in virtually any cavitous area of the body such as the urethra, esophagus, biliary duct, blood vessels, etc. or in any surgically made duct or shunt such as those made in the liver during transjugular intrahepatic portosystemic shunt procedures. 
     Referring now to FIGS. 4-7, the apparatus  100  of the invention is seen with reference to the method of the invention. According to the method of the invention, the measuring apparatus  100  of the invention is initially placed in its fully axially extended position (see FIG.  4 ), with the sheath  106  covering the entire length of the wire stent  104  which is in turn fully compressed. In this configuration, the second locking valve  133  of the sheath  106  is at its furthest distance from the first locking valve  112  of the plunger  102 , and is aligned with the scale  122  such that the proximal most end  142  of the stop coincides with the “0” mark  204  on the scale  122 . The tip  148  is adjusted to fit into the sheath  106  by loosening the first locking valve  112  and pulling the inner hollow catheter  108  proximally such that the stepped proximal end  143  of the tip  148  fits into the sheath  106  and the distal end  116  of the plunger  102  abuts the proximal end  118  of the compressed stent  104 . Tile distal end  206  of the guide wire  144  is located sufficiently past the body cavity  202  to allow proper placement of the measuring apparatus  100 . When positioning the measuring apparatus  100 , the distal ends of the stent  104  and sheath  106  should typically be located slightly past the distal neck  208  of the body cavity  202  in which the stent  100  is to be deployed (see FIG.  5 ). This is done to compensate for the tendency of the stent  104  to contract in length when going from its compressed configuration in the sheath  106  to its deployed configuration in the vessel  202 . It should be noted that the flexible hollow dilator tip  148  at the distal end  146  of the catheter  108  is radiopaque. Thus, a user may monitor the progress and placement of the measuring apparatus  100  by means of a ti fluoroscope (not shown). 
     Once the measuring apparatus  100  is properly positioned within the body cavity  202  (as in FIG.  5 ), the sheath  106  is slowly retracted (see FIG. 6) by first loosening the cap  138  on the second locking valve  133  and then, while holding the plunger  102  stationary, pulling the sheath  106  backwards. As the sheath is retracted, the distal end  126  of the stent  104  is released and expands back towards its uncompressed configuration until it engages the distal neck  208  of the cavity  202 . It will be appreciated that, as the distal end  126  of the stent  104  has an at rest uncompressed diameter greater than the distal neck  208  diameter of the body cavity  202 , the distal end  126  of the stent exerts pressure on the distal neck  208  when it is deployed, causing the distal end  126  of the stent  104  to be locked into place. As mentioned above, the overall length of the stent  104  decreases when it goes from its compressed configuration to its less compressed deployed configuration. It is thus important that the user position the distal end  126  of the stent  104  sufficiently past the distal neck  208  of the body cavity  202  to compensate for this shrinkage. It will be noted, however, that should the practitioner discover after the sheath  106  has been retracted that the distal end  126  of the stent  104  is not positioned far enough into the distal neck  208  of the body cavity  202 , the practitioner need only re-extend the sheath  106  fully over the stent  104  and repeat the above steps of positioning. 
     As indicated by FIG. 7, the sheath  106  is further retracted until the user determines, via fluoroscopy, that the stent  104  is sufficiently deployed so as to bridge the length of the body cavity  202 . As shown in FIG. 7, the length of stent  104  as retractably deployed must be slightly longer than the length of the body cavity  202 . In this manner, the proximal end  718  of the length of retractably deployed stent  104  and the distal end  126  of the stent are positioned respectively within the proximal and distal necks  210 ,  208  of the body cavity  202 . Once the desired length of stent  104  is retractably deployed, the proximal most end  142  of the second locking valve  133  is used as an indicator on the scale  122  of the plunger  102 . As discussed above, the scale  122  is calibrated such that the indicated number  702  represents the uncompressed length of stent needed to fully bridge the body cavity  202 . In this particular case, the scale  122  indicates 27 mm, signifying that a stent having an at rest, uncompressed length of 27 mm must be used to properly bridge the body cavity  202  which may be, e.g., 20 mm long. 
     Once the measurement is taken, the sheath  106  is re-extended over the stent  104  (as in FIG.  5 ), thus re-compressing it, and the entire measuring apparatus  100  is withdrawn from the body cavity  202  and the patient. The stent  104  may then be detached from the measuring apparatus  100  by cutting it with, for example, scissors, or a new stent or covered stent (not shown) having the same properties and pitch angle as the stent  104  of the measuring apparatus  100 , and having an at rest uncompressed length equal to or proportional to the recorded measurement, may be obtained. In the above example, a 27 mm stent of the same diameter and geometry would thus be obtained. This stent is then inserted into the body cavity  202  for deployment via any known means in the art. As the measurement method of the invention has already determined the proper stent length, the user is only left with the task of properly placing the stent within the body cavity  202 . 
     Turning now to FIG. 8, a second embodiment of the apparatus  300  of the invention is seen. In this embodiment, the stent  304  of the measuring apparatus  300  is coated with a microporous or non-porous elastomeric membrane. The apparatus  300  has particular advantageous use where the body cavity  301  has several branching vessels  302 ,  303  and a saccular aneurysm  308 . With the measuring apparatus  300  deployed inside the body cavity  301  as shown, the organs and tissues (not shown) fed by the branch vessels  302 ,  303  can be monitored to determine if they are suffering harmful effects as a result of the blocking of the branch vessels  302 ,  303  caused by the non porous stent  304 . For example, if the branch vessels  302 ,  303  were to represent arteries which nourish the spinal chord, the lower extremities of the patient can be tested and monitored to determine if blocking of these arteries causes paraplegia in the patient. Should such a determination be made, the coated stent can either be cut shorter so as to not block the branch vessels, or the procedure terminated altogether. Similarly, when proceeding to bridge an aortic aneurysm, the measuring apparatus can be used with a coated stent to determine whether there is a back flow from, for example, a lumbar artery into the aneurysm, which if not occluded can lead to rupture of the aneurysm. If a back flow is detected, interventional blockage of the lumbar artery with an occlusion device may be required prior to stenting the aorta. 
     In accord with yet another aspect of the invention, a detachable hub and detachable hemostasis valve for use in conjunction with methods for loading and deploying a stent or stent-graft are seen in FIGS. 9 and 10. In particular, a detachable hub  310  for use on the endovascular measuring apparatus  100  of FIGS. 3-8 (in lieu of hub  109 ) is seen in FIG. 9, having a cap  312  which screws onto threads  314 , an O-ring  316 , a lumen  317 , and a proximal handle  318  having a luer lock  320  capable of connection to a hemostasis valve or the like. The inner catheter  315  is fed through the lumen  317  of the detachable hub  310  and locked into place by tightening the cap  312  onto the threads  314 , thereby compressing the O-ring  316 . Similarly, the detachable hemostasis valve  410  of FIG. 10 is intended to replace the valve lock  112  of FIGS. 3-8. The detachable hemostasis valve  410  includes a body portion  412  having proximal threads  414  and distal threads  416 , distal and proximal caps  418 ,  420 , a lumen  422 , distal and proximal O-rings  424 ,  426 , and a flush port  430 . The inner catheter  108  and plunger  120  pass through the lumen  422 , and when in place, the distal cap  420  can be tightened on the distal threads  416  to compress the distal O-ring  424  and lock the valve onto the plunger  120 . Similarly, the proximal cap  418  can be tightened on the proximal threads  414  to compress the proximal O-ring  426  to lock onto the inner catheter  108 . The flush port  430  can be used to enable flushing of the annular space between the plunger  120  and the inner catheter  108  with, e.g., heparinized saline. 
     With the detachable hub  310  and lock  410  as provided in FIGS.  9  and  10 , the method of measuring a desired stent length can be carried out as described above with reference to FIGS. 3-8. However, in accord with another aspect of the invention, after the measurement, the provided apparatus can be used for loading and deployment of the measured stent or stent-graft. In particular, after the desired stent length has been measured, the entire measuring apparatus is removed from the body of the patient. Preferably, all lumens of the apparatus are then flushed with heparinized saline. The detachable hub  310  (FIG. 9) is then detached an removed, and the detachable lock  410  is detached and removed. With the hub  310  and lock  410  removed, the dilator tip  148  is grabbed an pulled distally, such that the inner catheter  108  is removed completely from the hollow plunger  120 . Then, the stent  104  is pulled through and entirely out of the sheath  106 . Using a waterproof, sterile, felt-tipped pen or the like, or any other desired mechanism, the stent of stent-graft  104  is marked to the desired length from its distal end  126  (e,g., 27 mm from the distal end of the stent). With the stent marked, the proximal end of the plunger  102 , still connected to the stent  104 , is inserted into the sheath, and through the plunger lock  133  until the proximal end  120  of the plunger sticks out of the distal end of the sheath  106 ; i.e., the plunger is inserted backwards through the sheath. The proximal end of the plunger sticking out to the distal end of the sheath is then pulled such that the stent or stent-graft  104  is pulled into the sheath and out of the distal end of the sheath to the mark. The stent  104  is then cut at, or just proximal to the marking such that the remaining stent (with the marking) with the plunger can be discarded, and the stent in the sheath properly loaded. With the sheath loaded, the introducer system is reassembled by inserting the catheter  108  through the sheath and stent, if desired, by providing a plunger to push out the stent or stent-graft  104  when properly located, and, if desired, by reattaching the hub  310  to the catheter, and the lock  410  to the plunger and catheter. It will be appreciated that the plunger utilized with the loaded sheath can be a new plunger used for deploying the stent  104 , or the remaining portion of the stent utilized in the initial measurements with the excess stent removed from the plunger. 
     The loading and deployment method of the invention as set forth above have numerous advantages. It will be appreciated that since the stent is loaded by pulling the stent with the plunger, there is less opportunity for the stent wires to scrape and perforate the wall of the sheath. In addition, funnels usually required to load the stent are eliminate, and the stent loading operation is simple. Further, the stent or stent-graft being utilized is the same unit which was used as the measuring devise, thereby rendering the system less expensive. 
     There have been described and illustrated herein several embodiments of a tubular braided stent and a method of manufacturing the stent of the invention. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while particular stent designs have been disclosed for use with the apparatus of the invention, it will be appreciated that other designs may work as well. For example, while a stent having a homogeneous pitch angle throughout has been disclosed, a stent with a different body and end pitch angle can also be used as disclosed in copending U.S. patent application Ser. No. 08/388,612, or continuously varying hyperbaloidal stents can be used. Furthermore while a particular mechanism for adjusting and locking the sheath relative to the plunger and a similar method for locking the plunger relative to the inner catheter has been disclosed, it will be understood that other mechanisms or no mechanisms may be used as well. Also, while a particular type of scale has been disclosed, it will be recognized that any other suitable scales could be used. For example, although a metric scale has been disclosed, an English system scale or any other measurement system scale could also be used. In addition, although a scale has been disclosed printed along the plunger body, the scale may instead include electronic measuring means coupled to an LCD readout. Furthermore, although the scale has been disclosed as having a particular calibration, any other calibration could be used. For example, although the scale has been calibrated to account for the contraction experienced by the stent when in an uncompressed configuration, the scale may be calibrated in any other fashion or may be uncalibrated. When uncalibrated, the practitioner can either conduct the necessary mathematics in order to determine the length of uncompressed stent to use, or can cut a stent in its compressed state in a sheath the same diameter as the sheath of the apparatus. In fact, if desired, no scale or calibration is necessarily required on the plunger, as the plunger can be marked by the practitioner during use, and measured afterwards. Although this measuring apparatus has been described for use with a self-expanding stent of the Wallsten or Didcott configuration, it will be appreciated that the measuring apparatus can be calibrated for use with other devices such as balloon expandable Palmaz or Gianturco stents and the like. The apparatus may also be used to acquire exact measurements of body cavities for data collection and subsequent use for other procedures such as bypass surgery, electrophysical mapping, endoscopic surgery, etc. Moreover, while a particular configuration for the dilator tip has been disclosed, it will be appreciated that other configurations or no dilator tip could be used as well. Furthermore, while a particular monitoring means has been described for use with the apparatus, it will be understood that any monitoring means can be similarly used. In particular, while the monitoring means were described to be fluoroscopy, other means such as radioscopy and CT scans may also be used. In addition, while a particular method of measuring the deployment length of a stent in a body cavity using the apparatus of the invention has been disclosed, it will be understood by those skilled in the art that details may be altered without changing the nature of the method. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided apparatus and method of the invention without deviating from their spirit and scope as so claimed.