Patent Publication Number: US-2007123890-A1

Title: Tissue retrieval devices and methods

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims benefit of U.S. provisional application Ser. No. 60/733,849 filed Nov. 4, 2005, and entitled “Harpoon Tissue Retrieval Device”, which is hereby incorporated herein by reference in its entirety. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      Not Applicable.  
     BACKGROUND  
      1. Field of the Invention  
      The present invention relates generally to minimally invasive methods, devices and systems for treating spinal disorders using imaging guidance. More particularly, the present invention relates to devices and methods to reduce stenosis and increase the cross-sectional area of the spinal canal available for the spinal cord. Still more particularly, the present invention relates to devices and methods to percutaneously excise portions of an enlarged ligamentum flavum.  
      2. Background Information  
      The vertebral column (spine, spinal column, backbone) forms the main part of the axial skeleton, provides a strong yet flexible support for the head and body, and protects the spinal cord disposed in the vertebral canal, which is formed within the vertebral column. The vertebral column comprises a stack of vertebrae with an intervertebral disc between adjacent vertebra. The vertebrae are stabilized by muscles and ligaments that hold the vertebrae in place and limit the movements of the individual vertebra.  
      As illustrated in  FIG. 1 , each vertebra  10  includes a vertebral body  12  that supports a vertebral arch  14 . A median plane  210  generally divides each vertebra  10  into two substantially equal lateral sides. Vertical body  12  has the general shape of a short cylinder and is anterior to the vertebral arch  14 . The vertebral arch  14  together with vertebral body  12  encloses a space termed the vertebral foramen  15 . The succession of vertebral foramen  15  in adjacent vertebra  10  along the vertebral column define the vertebral canal (spinal canal), which contains the spinal cord  28 .  
      Vertebral arch  14  is formed by two pedicles  24  which project posteriorly to meet two laminae  16 . The two laminae  16  meet posteriomedially to form the spinous process  18 . At the junction of pedicles  24  and laminae  16 , six processes arise. Two transverse processes  20  project posterolaterally, two superior articular processes  22  project generally superiorly and are positioned superior to two inferior articular processes  25  that generally project inferiorly.  
      The vertebral foramen  15  is generally an oval shaped space that contains and protects the spinal cord  28 . Spinal cord  28  comprises a plurality of nerves  34  surrounded by cerebrospinal fluid (CSF) and an outermost sheath/membrane called the dural sac  32 . The CSF filled dural sac  32  containing nerves  34  is relatively compressible. Posterior to the spinal cord  28  within vertebral foramen  15  is the ligamentum flavum  26 . Laminae  16  of adjacent vertebral arches  14  in the vertebral column are joined by the relatively broad, elastic ligamentum flavum  26 .  
      In degenerative conditions of the spine, narrowing of the spinal canal (stenosis) can occur. Lumbar spinal stenosis is often defined as a dural sac cross-sectional area less than 100 mm 2  or an anterior-posterior (AP) dimension of the canal of less than 10-12 mm for an average male.  
      The source of many cases of lumbar spinal stenosis is thickening of the ligamentum flavum (e.g., ligamentum flavum  26 ). Spinal stenosis may also be caused by subluxation, facet joint hypertrophy, osteophyte formation, underdevelopment of spinal canal, spondylosis deformans, degenerative intervertebral discs, degenerative spondylolisthesis, degenerative arthritis, ossification of the vertebral accessory ligaments and the like. A less common cause of spinal stenosis, which usually affects patients with morbid obesity or patients on oral corticosteroids, is excess fat in the epidural space. The excessive epidural fat compresses the dural sac, nerve roots and blood vessels contained therein, and results in back, leg pain and weakness and numbness of the legs. Spinal stenosis may also affect the cervical and, less commonly, the thoracic spine.  
      Patients suffering from spinal stenosis are typically first treated with exercise therapy, analgesics, and anti-inflammatory medications. These conservative treatment options frequently fail. If symptoms are severe, surgery is required to decompress the spinal cord and nerve roots.  
      In some conventional surgical procedures to correct stenosis in the lumbar region, an incision is made in the back, and the muscles and supporting structures are stripped away from the spine, exposing the posterior aspect of the vertebral column. The thickened ligamentum flavum is then exposed by removal of a portion of the vertebral arch (e.g., vertebral arch  14 ), often at the laminae (e.g., laminae  16 ), covering the back of the spinal canal (laminectomy). The thickened ligamentum flavum ligament can then be excised by sharp dissection with a scalpel or punching instruments, such as a Kerison punch that is used to remove small chips of tissue. The procedure is performed under general anesthesia. Patients are usually admitted to the hospital for approximately five to seven days depending on the age and overall condition of the patient. Patients usually require between six weeks and three months to recover from the procedure. Further, many patients need extended therapy at a rehabilitation facility to regain enough mobility to live independently.  
      Much of the pain and disability after an open laminectomy results from the tearing and cutting of the back muscles, blood vessels, supporting ligaments, and nerves that occurs during the exposure of the spinal column. Also, because the spine-stabilizing back muscles and ligaments are stripped and detached from the spine during the laminectomy, these patients frequently develop spinal instability post-operatively.  
      Less invasive techniques offer the potential for reduced post-operative pain and faster recovery compared to traditional open surgery. Percutaneous interventional spinal procedures can be performed with local anesthesia, thereby sparing the patient the risks and recovery time required with general anesthesia. In addition, there is less damage to the paraspinal muscles and ligaments with minimally invasive techniques, thereby reducing pain and preserving these important stabilizing structures.  
      Various techniques for minimally invasive treatment of the spine are known. Microdiscectomy is performed by making a small incision in the skin and deep tissues to create a portal to the spine. A microscope is then used to aid in the dissection of the adjacent structures prior to discectomy. The recovery for this procedure is much shorter than traditional open discectomies. Percutaneous discectomy devices with fluoroscopic guidance have been used successfully to treat disorders of the disc but not to treat spinal stenosis or the ligamentum flavum directly. Arthroscopy or direct visualization of the spinal structures using a catheter or optical system have also been proposed to treat disorders of the spine including spinal stenosis, however these devices still use miniaturized standard surgical instruments and direct visualization of the spine similar to open surgical procedures. These devices and techniques are limited by the small size of the canal and these operations are difficult to perform and master. In addition, these procedures are painful and often require general anesthesia. Further, the arthroscopy procedures are time consuming and the fiber optic systems are expensive to purchase and maintain.  
      Hence, it remains desirable to provide simple methods, techniques, and devices for treating spinal stenosis and other spinal disorders without requiring open surgery. It is further desired to provide a system whereby the risk of damage to the dural sac containing the spinal nerves may be reduced.  
     SUMMARY OF THE PREFERRED EMBODIMENTS  
      In accordance with at least one embodiment of the invention, a device for retrieving an excised tissue segment comprises an elongate body having a central axis and an outer surface. In addition, the device comprises at least two prongs extending from the body in a direction substantially parallel to the central axis. Each prong includes a fixed-end coupled to the body, a free-end distal the body, an inner surface, and an outer surface substantially contiguous with the outer surface of the body. Further, the device comprises a space extending between the inner surfaces of the prongs that accommodates excised tissue.  
      In accordance with another embodiment of the invention, a method for treating stenosis in a spine of a patient comprises inserting a tissue excision device percutaneously into the patient. The tissue excision device comprises a distal cutting end and a through bore. In addition, the method comprises positioning the distal cutting end of the tissue excision device adjacent the region of interest. Further, the method comprises excising a tissue segment from the region of interest with the tissue excision device. Still further, the method comprises inserting a tissue retrieval device percutaneously into the bore of the tissue excision device. The tissue retrieval device comprises an elongate body having a central axis and an outer surface and at least two prongs extending from the body in a direction substantially parallel to the central axis. Moreover, the method comprises advancing the prongs towards the excised tissue segment within the bore of the tissue excision device. In addition, the method comprises grasping the excised tissue segment between the prongs of the tissue retrieval device. Further, the method comprises removing the excised tissue segment from the bore of the tissue excision device.  
      In accordance with another embodiment of the invention, a kit for performing a procedure on a spine comprises a volume of a contrast medium adapted to be inserted into the epidural space by the insertion member and expanded so as to compress a portion of the thecal sac and provide a safety zone within the epidural space. In addition, the kit comprises a tissue excision device. Further, the kit comprises a tissue retrieval device.  
      Thus, embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments, and by referring to the accompanying drawings.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more complete understanding of the disclosure, reference is made to the accompanying drawings, wherein:  
       FIG. 1  is cross-section of the spine viewed from the space between two vertebrae, showing the upper surface of one vertebra and the spinal canal with the dural sac and a normal (un-stenosed) ligamentum flavum therein;  
       FIG. 2  is cross-section of the spine viewed from the space between two vertebrae, showing the upper surface of one vertebra and the spinal canal with the dural sac and a thickened (stenosed) ligamentum flavum therein;  
       FIG. 3  is an enlarged cross-section of a vertebral foramen, showing a safety zone created by compression of the dural sac;  
       FIG. 4  is the cross-section of  FIG. 3 , showing a tissue excision device positioned in the ligamentum flavum according to an ILAMP procedure;  
       FIG. 5  is the cross-section of  FIG. 3 , showing a tissue excision tool positioned in the ligamentum flavum according to an alternative MILD procedure;  
       FIG. 6  is a partial cross-section of the lumbar portion of the vertebral column taken along lines  6 - 6  of  FIG. 1 ;  
       FIG. 7  is the cross-section of  FIG. 6 , showing the orientation of an imaging tool relative to the vertebral column;  
       FIG. 8  is the cross-section of  FIG. 6 , showing the orientation of a tissue excision device relative to the vertebral column;  
       FIG. 9  is a perspective view of the distal portion of an embodiment of a tissue retrieval device;  
       FIG. 10  is a cross-sectional view of the tissue retrieval device of  FIG. 9 ;  
       FIG. 11  is an enlarged cross-sectional view of the free-end of the tissue retrieval device of  FIG. 9 ;  
       FIGS. 12 and 13  are sequential schematic cross-sectional views showing the tissue retrieval device of  FIG. 9  retrieving an excised tissue segment from the tissue excision device of  FIG. 4  or  FIG. 5 ;  
       FIG. 14  is a perspective view of another embodiment of a tissue retrieval device; and  
      FIGS.  15  is a perspective view of the tissue retrieval device of  FIG. 14  including a tissue ejector.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The following discussion is directed to various embodiments of the invention. Although one or more of these embodiments may be presently preferred, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and not intended to intimate that the scope of the disclosure, including the claims, is limited to that embodiment.  
      For purposes of this discussion, the x-, y-, and z-axes are shown in  FIGS. 1, 3 ,  6 ,  7 , and  8  to aid in understanding the descriptions that follow. The x-, y-, and z-axes have been assigned as follows. The x-axis is perpendicular to the longitudinal axis of the vertebral column and perpendicular to the coronal/frontal plane (i.e., x-axis defines anterior vs. posterior relationships). The y-axis runs substantially parallel to the vertebral column and perpendicular to the transverse plane (i.e., y-axis defines superior vs. inferior relationships). The z-axis is perpendicular to the longitudinal axis of the vertebral column and perpendicular to the median/midsagittal plane (i.e., z-axis defines the lateral right and left sides of body parts). The set of coordinate axes (x-, y-, and z-axes) are consistently maintained throughout although different views of vertebrae and the spinal column may be presented.  
      It is to be understood that the median/midsagittal plane passes from the top to the bottom of the body and separates the left and the right sides of the body, and the spine, into substantially equal halves (e.g., two substantially equal lateral sides). Further, it is to be understood that the frontal/coronal plane essentially separates the body into the forward (anterior) half and the back (posterior) half, and is perpendicular to the median plane. Still further, it is to be understood that the transverse plane is perpendicular to both the median plane and coronal plane and is the plane which divides the body into an upper and a lower half.  
      The Spinal Canal and Spinal Stenosis  
      Referring again to  FIG. 1 , vertebral foramen  15  contains a portion of the ligamentum flavum  26 , spinal cord  28 , and an epidural space  27  between ligamentum flavum  26  and spinal cord  28 . Spinal cord  28  comprises a plurality of nerves  34  surrounded by cerebrospinal fluid (CSF) contained within dural sac  32 . Nerves  34  normally comprise only a small proportion of the dural sac  32  volume. Thus, CSF filled dural sac  32  is somewhat locally compressible, as localized pressure causes the CSF to flow to adjacent portions of the dural sac. Epidural space  27  is typically filled with blood vessels and fat. The posterior border of the normal epidural space  27  generally defined by the ligamentum flavum  26 , which is shown in its normal, non-thickened state in  FIG. 1 .  
       FIG. 2  illustrates a case of spinal stenosis resulting from a thickened ligamentum flavum  26 . Since vertebral foramen  15  is defined and surrounded by the relatively rigid bone its volume is essentially constant. Thus, thickening of ligamentum flavum  26  within vertebral foramen  15  can eventually result in compression of spinal cord  28 . In particular, the thickened ligamentum flavum  26  may exert a compressive force on the posterior surface of dural sleeve  32 . In addition, thickening of ligamentum flavum  26  may compress the blood vessels and fat occupying epidural space  27 .  
      Compression of spinal cord  28 , particularly in the lumbar region, may result in low back pain as well as pain or abnormal sensations in the legs. Further, compression of the blood vessels in the epidural space  27  that houses the nerves of the cauda equina may result in ischemic pain termed spinal claudication.  
      In order to relieve the symptoms associated with a thickened or enlarged ligamentum flavum  26 , methods, techniques, and devices described herein may be employed to reduce the compressive forces exerted by the thickened ligamentum flavum on spinal cord  28  and the blood vessels in epidural space  27  (e.g., decompress spinal cord  28  and blood vessels in epidural space  27 ). In particular, compressive forces exerted by the thickened/enlarged ligamentum flavum  26  may be reduced by embodiments of a minimally invasive ligament decompression (MILD) procedure. In some embodiments, the MILD procedure may be performed percutaneously to reduce the size of ligamentum flavum  26  by excising portions of enlarged ligamentum flavum  26 . In particular, in some embodiments of the MILD procedure, the ligamentum flavum  26  is accessed, cut and removed ipsilaterally (i.e., on the same side of vertebral arch  14 ) by a percutaneous caudal-cranial approach. Such an embodiment of the MILD procedure may be described hereinafter as Ipsilateral Approach MILD Procedure (ILAMP).  
      Creation of a Safety Zone  
      As shown in  FIGS. 1 and 2 , ligamentum flavum  26  is posteriorly apposed to spinal cord  28  within vertebral foramen  15 . Thus, placement of tools within ligamentum flavum  26  to excise portions of ligamentum flavum  26  creates a risk of for inadvertent damage to the spinal cord  28 , dural sac  32 , and/or nerves  34 . Thus, in preferred embodiments of the procedures described herein, prior to insertion of tissue excision devices into the ligamentum flavum  26 , a gap is created between ligamentum flavum  26  and spinal cord  28  to provide a safety zone between ligamentum flavum  26  and spinal cord  28 .  
      Referring now to  FIG. 3 , an enlarged cross-sectional view of a vertebral foramen  15  within a vertebra (e.g., vertebra  10 ) is illustrated. Vertebral foramen  15  includes epidural space  27  and spinal cord  28  containing nerves  34  and CSF within dural sac  32 . Further, a thickened/enlarged ligamentum flavum  26  extends into vertebral foramen  15 . To reduce the risk of damage to dural sac  32  and spinal cord  28 , a safety zone  40  is created between ligamentum flavum  26  and dural sac  32  in the manner described below.  
      As previously described, spinal cord  28  comprises nerves  34  surrounded by CSF and is contained within dural sac  32 . Since more than 90% of the volume of dural sac  32  in the lumbar region is filled by CSF, dural sac  32  is highly compressible. Thus, even when stenosis is causing compression of spinal cord  28 , in most cases it is possible to temporarily compress spinal cord  28  further. Thus, according to preferred embodiments, dural sac  32  is further compressed in the region of interest by injecting a fluid or medium into epidural space  27  to create safety zone  40 . The fluid may be injected into the epidural space  27  with an insertion member, such as a needle. The presence of the injected fluid comprising safety zone  40  gently applies an additional compressive force to the outer surface of dural sac  32  so that at least a portion of the CSF within dural sac  32  is forced out of dural sac  32  in the region of interest, resulting in safety zone  40  between dural sac  32  and ligamentum flavum  26 .  
      According to some embodiments, dural sac  32  is compressed by injecting a standard radio-opaque non-ionic myelographic contrast medium or other imagable or non-imagable medium into epidural space  27  in the region of interest. This is preferably accomplished with a percutaneous injection. Sufficient injectable fluid is preferably injected to displace the CSF out of the region of interest and compress dural sac  32  to at least a desired degree. The injected medium is preferably substantially contained within the confines of epidural space  27  extending to the margins of the dural sac  32 . The epidural space is substantially watertight and the fatty tissues and vascularization in epidural space  27 , combined with the viscous properties of the preferred fluids, serve to substantially maintain the injected medium in the desired region of interest. This method for protecting spinal cord  28  column may be referred to hereinafter as “contrast-guided dural protection.” 
      Referring now to  FIGS. 4 and 5 , once safety zone  40  has been created, a tissue excision tool or device  100  may be inserted into ligamentum flavum  26 . More specifically, the distal cutting end  101  of tissue excision device  100  is inserted into ligamentum flavum  26  in preparation for excising portions of enlarged ligamentum flavum  26 . Tissue excision device  100  may comprise any suitable device, tool or instrument for decompressing an enlarged ligamentum flavum  26  and relieving spinal stenosis caused by the enlarged ligamentum flavum. A variety of suitable tissue excision devices, including distal cutting ends, are disclosed in U.S. Application Ser. Nos. 11/193,581, 11/461,036, 60/733,754, 60/733,552, and 11/461,045, each of which is hereby incorporated herein by reference in its entirety.  
      Although tissue excision device  100  is shown as directly accessing ligamentum flavum  26  in  FIGS. 4 and 5  (i.e., without guidance from a cannula or portal), it should be appreciated that tissue excision device  100  may alternatively percutaneously access ligamentum flavum  26  via a cannula or other portal device. For instance, in some embodiments, tissue excision device  100  may be guided by and advanced through a cannula toward the ligamentum flavum  26 .  
      In the embodiment illustrated in  FIG. 4 , distal cutting end  101  of tissue excision device  100  is inserted and positioned in ligamentum flavum  26  on the same side (ipsilateral) of median plane  210  as tissue excision device  100  percutaneously accesses the patient. Consequently, tissue excision device  100  does not cross median plane  210 . However, in the embodiment illustrated in  FIG. 5 , distal cutting end  101  of tissue excision device  100  is positioned in ligamentum flavum  26  on the opposite side of median plane  210  as tissue excision device  100  percutaneously accesses the patient. Consequently, in this embodiment, tissue excision device  100  crosses median plane  210 .  
      While it is preferred that distal cutting end  101  of tissue excision device  100  remain within ligamentum flavum  26  as shown, the presence of safety zone  40  reduces the likelihood that dural sac  32  will be damaged, even if distal tip  101  of device  100  breaks through the anterior surface of ligamentum flavum  26 .  
      Because the present techniques are preferably performed percutaneously, certain aspects of the methods described herein may be facilitated by imaging. Imaging windows (e.g., a fluoroscopic window of access-FWA) may be employed to aid in performance of all or part of the procedures described herein. For instance, an imaging window may be employed to aid in insertion of tissue excision device  100  into ligamentum flavum  26  as shown in  FIGS. 4 and 5 . The methods and procedures described herein may be aided by any suitable imaging technology including, without limitation, 2D fluoroscopy, 3D fluoroscopy, CT, MRI, ultrasound or with direct visualization with fiber optic or microsurgical techniques. Stereotactic or computerized image fusion techniques are also suitable. Fluoroscopy is currently particularly well-suited to the techniques disclosed herein since fluoroscopic equipment is relatively safe and easy to use, readily available in most medical facilities, and relatively inexpensive.  
      In an exemplary procedure using direct biplane fluoroscopic guidance and local anesthesia, epidural space  27  is accessed for injection of contrast media adjacent to the surgical site. If the injected medium is radio-opaque, as are for example myelographic contrast media, the margins of expanded epidural space  27  will be readily visible using fluoroscopy or CT imaging. Thus, safety zone  40  created by the contrast-guided dural compression techniques can reduce the risk of damage to dural sac  32  and spinal cord  28  during MILD procedures to remove or displace portions of ligamentum flavum  26  and/or laminae  16  in order to treat spinal stenosis.  
      Injectable Medium  
      If desired, the injected fluid or medium can be provided as a re-absorbable water-soluble gel, so as to better localize safety zone  40  at the site of surgery and reduce leakage of this protective layer from the vertebral/spinal canal. The gel is preferably substantially more viscid and/or viscous than conventional contrast media. In general, a preferred viscid and/or viscous gel tends to remain localized at the desired site of treatment since it does not spread as much as standard liquid contrast media that are conventionally used in epidurography. This may result in more uniform compression of dural sac  32  and less leakage of the contrast medium out of the vertebral/spinal canal. In addition, preferred embodiments of the gel are re-absorbed more slowly than conventional contrast media, allowing for better visualization during the course of the surgical procedure(s).  
      A standard hydrophilic-lipophilic block copolymer (Pluronic) gel known in the art or other suitable gel may be employed as the injectable medium. The gel preferably has an inert base. In certain embodiments, the gel material is liquid at ambient temperatures and can be injected through a small bore, such as a  27  gauge needle. The gel then preferably becomes viscous when warmed to body temperature after being injected. The viscosity of the gel can be adjusted through the specifics of the preparation. The gel or other fluid is preferably sufficiently viscid and/or viscous at body temperature to compress and protect dural sac  32  in the manner described above and to remain sufficiently present in the region of interest for at least about 30 minutes. Thus, in some embodiments, the injected gel attains a viscosity that is two, three, six or even ten times that of the fluids that are typically used for epidurograms.  
      In certain embodiments, the injected medium undergoes a reversible change in viscosity when warmed to body temperature so that it can be injected as a low-viscosity fluid, thicken upon injection into the patient, and be returned to its low-viscosity state by cooling. In these embodiments, the injected medium is injected as desired, thickens upon warming, but can be removed by contacting it with a heat removal device, such as an aspirator that has been provided with a cooled tip. As a result of localized cooling, the gel reverts to its initial non-viscous liquid state and can be easily suctioned up the cooled needle or catheter.  
      In some embodiments, a contrast agent can be included in the gel itself, so that the entire gel mass is imagable. In different embodiments, the contrast agent may be injected first, followed by the desired amount of gel, or vice versa. In the embodiments in which the contrast agent and gel are injected separately, the contrast agent tends to be captured on the surface of the expanding gel mass, so that the periphery of the gel mass is imagable.  
      An example of a suitable injectable medium, including a contrast agent, having the desired properties is Omnipaque® 240 available from Nycomed, N.Y., which is a commercially available non-ionic iodinated myelographic contrast medium. Other suitable injectable media will be known to those skilled in the art. Because of the proximity to spinal cord  28  and spinal nerves  34 , it is preferred not to use ionic media in the injectable medium. The preferred compositions are reabsorbed relatively rapidly after the procedure. Thus any residual gel compression on dural sac  32  after the MILD procedure dissipates relatively quickly. For example, in preferred embodiments, the gel would have sufficient viscosity to compress dural sac  32  for thirty minutes, and sufficient degradability to be substantially reabsorbed within approximately two hours.  
      The injected medium may further include one or more bioactive agents. For example, medications such as those used in epidural steroid injection (e.g. Depo medrol, Celestone Soluspan) may be added to the epidural gel to speed healing and reduce inflammation, scarring and adhesions. The gel preferably releases the steroid medication slowly and prolongs the anti-inflammatory effect, which can be extremely advantageous. Local anesthetic agents may also be added to the gel. This prolongs the duration of action of local anesthetic agents in the epidural space to prolong pain relief during epidural anesthesia. In this embodiment, the gel may be formulated to slow the reabsorption of the gel.  
      The above-described injected mediums and gels may also be used for epidural steroid injection and perineural blocks for management of acute and chronic spinal pain. Thrombin or other haemostatic agents can be added if desired, so as to reduce the risk of bleeding.  
      In some embodiments, the gel may also be used as a substitute for a blood patch if a CSF leak occurs. The gel may also be used as an alternative method to treat lumbar puncture complications such as post-lumbar puncture CSF leak or other causes of intracranial hypotension. Similarly, the gel may be used to patch postoperative CSF leaks or dural tears. If the dural sac were inadvertently torn or cut, then gel could immediately serve to seal the site and prevent leakage of the cerebral spinal fluid.  
      Ipsilateral Approach for MILD Procedure (ILAMP)  
      Once safety zone  40  has been created, the margins of epidural space  27  are clearly demarcated by the injected medium and may be visualized radiographically if an imageable medium or contrast agent has been used. As mentioned above, percutaneous procedures can then be performed on ligamentum flavum  26  and/or surrounding tissues, with reduced potential for injuring dural sac  32  and spinal cord  28 .  
      A variety of suitable techniques and devices may be employed to reduce the size of the thickened/enlarged ligamentum flavum  26 , thereby decompressing spinal cord  28  as well as blood vessels contained within the epidural space  27 . Examples of suitable decompression techniques include without limitation, removal of tissue from ligamentum flavum  26 , laminectomy, laminotomy, retraction and anchoring of ligamentum flavum  26 , or combinations thereof. In some embodiments, a portion of the enlarged ligamentum flavum  26  is excised using tissue excision device  100  as best shown in  FIGS. 4 and 5 .  
      Accessing ligamentum flavum  26  with a tissue excision device  100  may present challenges. For instance, in some conventional approaches to correct stenosis caused by an enlarged ligamentum flavum  26 , an incision is made in the back of the patient and then the muscles and supporting structures of the vertebral column (spine) are stripped away, exposing the posterior aspect of the vertebral column. Subsequently, the thickened ligamentum flavum  26  is exposed by removal of a portion of vertebral arch  14 , often at lamina  16 , which encloses the anterior portion of the spinal canal (laminectomy). The thickened ligamentum flavum  26  can then be excised by sharp dissection with a scalpel or punching instruments. However, this approach is usually performed under general anesthesia and typically requires an extended hospital stay, lengthy recovery time and significant rehabilitation. As another example, some MILD procedures access ligamentum flavum  26  percutaneously by boring a hole through the vertebral arch  14  of vertebra  10 , often through a lamina  16 . A cannula and/or device  100  may be passed through the bore and/or anchored to the bore to access ligamentum flavum  26  for excision. While such a MILD approach is less invasive and reduces recovery time compared to the procedure just described, such an approach requires the additional step of boring a hole in the posterior of the vertebra  10  of interest. Thus, in some cases it will be preferable to employ a MILD procedure that percutaneously accesses ligamentum flavum  26  without the need to cut or bore through the vertebra.  
      Referring now to  FIG. 6 , a partial cross-sectional lateral view of a segment of a vertebral column  80  is illustrated. The segment of vertebral column  80  illustrated in  FIG. 6  includes three vertebrae  10   a ,  10   b , and  10   c . Each vertebrae  10   a ,  10   b ,  10   c  includes a vertebral body  12   a ,  12   b ,  12   c , that supports a vertebral arch  14   a ,  14   b ,  14   c , respectively. Vertical body  12   a ,  12   b ,  12   c  is anterior to vertebral arch  14   a ,  14   b ,  14   c , respectively. Each vertebral arch  14   a ,  14   b ,  14   c  together with vertebral body  12   a ,  12   b ,  12   c , respectively, encloses a vertebral foramen  15   a ,  15   b ,  15   c . The succession of vertebral foramen  15   a ,  15   b ,  15   c in adjacent vertebrae  10   a ,  10   b ,  10   c  define vertebral canal  81  (spinal canal) that runs along the length of vertebral column  80 . Vertebral canal  81  contains the spinal cord (not shown in  FIG. 5 ).  
      As previously described, each vertebral arch  14   a ,  14   b ,  14   c  includes two pedicles that project posteriorly to meet two lamina  16   a ,  16   b ,  16   c , respectively. In  FIG. 6 , one pedicle has been removed from each vertebrae  10   a ,  10   b ,  10   c  and thus, only the cross-section of one lamina  16   a ,  16   b ,  16   c  for each vertebrae  10   a ,  10   b ,  10   c , respectively, is shown. The two lamina meet posteriomedially to form the spinous process  18   a ,  18   b ,  18   c , respectively.  
      Lamina  16   a ,  16   b ,  16   c  of adjacent vertebrae  10   a ,  10   b ,  10   c  are connected by ligamentum flavum  26  (shown in cross-section). The relatively elastic ligamentum flavum  26  extends almost vertically from superior lamina to inferior lamina of adjacent vertebrae. In particular, ligamentum flavum  26  originates on the inferior surface of the laminae of the superior vertebra and connects to the superior surface of the laminae of the inferior vertebra. For instance, ligamentum flavum  26  originates on the inferior surface of lamina  16   a of superior vertebra  10   a  and connects to the superior surface of lamina  16   b  of the inferior vertebra  10   b . Thus, ligamentum flavum  26  spans an interlaminar space  82 . Interlaminar space  82  is generally the space between laminae of adjacent vertebrae in spinal column  80 .  
      Still referring to  FIG. 6 , each lamina  16   a ,  16   b ,  16   c  comprises a relatively broad flat plate of bone that extends posteromedially and slightly inferiorly from pedicles  24   a ,  24   b ,  24   c , respectively. Along the length of vertebral column  80 , the lamina  16   a ,  16   b ,  16   c  overlap like roofing shingles, with each lamina substantially parallel to and at least partially overlapping the adjacent inferior lamina. Further, the adjacent substantially parallel laminae are separated by the intervening ligamentum flavum  26  and interlaminar space  82 . For instance, lamina  16   a  is substantially parallel to and partially overlaps adjacent inferior lamina  16   b  and is separated from lamina  16   b  by ligamentum flavum  26  and interlaminar space  82 .  
       FIG. 7  illustrates vertebral column  80  as it may be oriented with the anterior side positioned down and posterior back surface  85  positioned upward, as may be encountered during a spinal procedure or surgery. In addition, in the embodiment illustrated in  FIG. 7 , ligamentum flavum  26  is thickened or enlarged, resulting in spinal stenosis. In particular, the anterior portions of enlarged ligamentum flavum  26  extend partially into spinal canal  81 , potentially exerting compressive forces on the spinal cord (not shown) that resides within spinal canal  81 .  
      As previously discussed, to relieve compressive forces on the spinal cord and hence relieve the associated symptoms of spinal stenosis, portions of ligamentum flavum  26  may be excised. However, to percutaneously excise portions of ligamentum flavum  26  via minimally invasive techniques, the innate structure of vertebral column  80  and each vertebra may present significant imaging challenges. For instance, lateral imaging windows/views of ligamentum flavum  26  substantially in the direction of the z-axis may be obscured by the various processes of the vertebrae (e.g., transverse processes, superior articular processes, inferior articular processes), the laminae of the vertebra, etc. Further, some anterior-posterior (A-P) imaging windows/views of ligamentum flavum  26  substantially in the direction of the x-axis may also be obscured by the laminae. In particular, in the A-P radiographic imaging planes substantially in the direction of the x-axis, the posterior edges of parallel laminae overlap and obscure ligamentum flavum  26  and interlaminar space  82 , particularly the anterior portions of ligamentum flavum  26  and interlaminar space  82  closest to spinal canal  81 . However, with an imaging window/view in a plane substantially parallel to the X-Y plane, at an angle θ generally in the direction of arrow  83 , and slightly lateral to the spinous process, interlaminar space  82  and ligamentum flavum  26  may be viewed with less obstruction from neighboring laminae. In other words, imaging windows/views generally aligned with arrow  83  ( FIG. 7 ) allow a more direct view of interlaminar space  82  and ligamentum flavum  26  from the posterior back surface with minimal obstruction by the vertebrae, and more specifically the laminae.  
      Typically, the long axes of the substantially parallel laminae (e.g., laminae  16   a ,  16 ,b,  16   c ) and interlaminar spaces (e.g, interlaminar spaces  82 ) are generally oriented between 60° and 75° relative to posterior back surface  85 . Thus, preferably the imaging means (e.g., x-ray beam, fluoroscopy tube, etc.) is positioned generally in the direction represented by arrow  83 , where θ between posterior back surface  85  and the imaging beam is substantially between 60° and 75°. In other words, the imaging means is positioned substantially parallel to the surface of the laminae. The resulting imaging window/view, termed “caudal-cranial posterior view” hereinafter, permits a clearer, more direct, less obstructed view of interlaminar space  82  and ligamentum flavum  26  from the general posterior back surface  85 . The caudal-cranial posterior view permits a relatively clear view of interlaminar space  82  and ligamentum flavum  26  in directions generally along the y-axis and z-axis. However, the caudal-cranial posterior view by itself may not provide a clear imaging window/view of interlaminar space  82  and ligamentum flavum  26  in directions generally along the x-axis. In other words, the caudal-cranial posterior view by itself may not provide a clear imaging window or view that can be used to accurately determine the posterior-anterior depth, measured generally along the x-axis, of a device across the ligamentum flavum  26 .  
      Thus, in preferred embodiments, an additional imaging window/view, termed “caudal-cranial posterior-lateral view” hereinafter, is employed to provide a clearer, unobstructed view of interlaminar space  82  and ligamentum flavum  26  in directions generally along the y-axis and z-axis. The caudal-cranial posterior-lateral view is generated by orienting an imaging means generally at an angle θ relative to posterior back surface  85  of the patient and also angling such imaging means laterally in an oblique orientation, revealing a partial lateral view of interlaminar space  82  occupied by ligamentum flavum  26  on the anterior side of the lamina and posterior to the underlying dural sac (not shown) and spinal cord (not shown).  
      By employing at least one of the caudal-cranial posterior view and the caudal-cranial posterior-lateral views, relatively clear imaging windows/views of the interlaminar space  82  and ligamentum flavum  26  in directions along the x-, y-, and z-axes may be achieved.  
      Referring now to  FIG. 8 , vertebral column  80  and a tissue access instrument  105  including a distal end  106  are illustrated. Tissue access instrument  105  may comprise a tissue excision device (e.g., tissue excision device  100 ), a cannula, a catheter, or other portal. Once unobstructed imaging windows/views of interlaminar space  82  and ligamentum flavum  26  are established in the manner previously described, tissue access instrument  105  is employed to percutaneously access interlaminar space  82  and ligamentum flavum  26 . More specifically, using images of the interlaminar space  82  and ligamentum flavum  26  obtained from the desired direction(s), (e.g., caudal-cranial posterior view and the caudal-cranial posterior-lateral view), tissue access device  105  may be employed to penetrate the skin and soft tissue in the posterior back surface  85  of the patient. In preferred embodiments, the skin entry point for tissue excision device  100  is between 5 and 10 cm inferior (caudal to) the posterior surface of the interlaminar space  82  of interest. For instance, if the portion of ligamentum flavum  26  between lamina  16   a  and lamina  16   b  is the area of interest, then tissue excision device  100  may be inserted into the patient&#39;s back about 5 to 10 cm inferior to posterior surface  84  of interlaminar space  82 .  
      Referring still to  FIG. 8 , tissue access device  105  is preferably initially inserted into the posterior tissue and musculature of the patient generally parallel to the longitudinal axis of spinal column  80 . In other words, the angle β between the posterior back surface  85  and tissue access device  105  is preferably between 0° and 10° when tissue access device  105  is initially inserted. Further, tissue access device  105  is preferably inserted into the posterior tissue and musculature of the patient on the same side (ipsilateral) of the median plane as the area of interest (e.g., the targeted portion of ligamentum flavum  26 ), as best seen in  FIG. 4 . Once tissue access device  105  is inserted into the posterior tissue and musculature of the patient, tissue access device  105  then may be oriented 5° to 90° relative to the posterior back surface  85  in order to create a trajectory across ligamentum flavum  26  in the area of interest. It is to be understood that once tissue access device  105  is inserted into the patient&#39;s posterior back surface  85 , the ends of tissue access device  105  (e.g., distal end  106 ) are free to pivot about the insertion location in posterior back surface  85  in the general direction of the y-axis and the z-axis, and may be advanced posteriorly or anteriorly generally in the direction of the x-axis.  
      Once inserted into the posterior tissue and musculature of the patient, tissue access device  105  can be positioned to provide a trajectory across interlaminar space  82  in the area of interest, generally towards the anterior surface of the lamina superior to the area of interest. For example, if interlaminar space  82  between lamina  16   a  and lamina  16   b  is the area of interest, tissue access device  105  is positioned to provide a trajectory that will allow a cutting instrument to be inserted across interlaminar space  82  between lamina  16   a  and lamina  16   b  towards the anterior surface of lamina  16   a  (superior lamina).  
      By switching between the caudal-cranial posterior view and the caudal-cranial posterior-lateral view, or by viewing both the caudal-cranial posterior view and the caudal-cranial posterior-lateral view at the same time, tissue access device  105  can be advanced to ligamentum flavum  26  in the area of interest with more certainty than has heretofore been present. Once distal end  106  of tissue access device  105  has reached ligamentum flavum  26 , portions of ligamentum flavum  26  may be excised with a tissue excision device (e.g., tissue excision device  100 ) so as to relieve pressure on the spinal nerves. If tissue access device  105  comprises a tissue excision tool, it may be inserted into ligamentum flavum  26  to excise portions of ligamentum flavum  26 . However, if tissue access device  105  comprises a cannula or portal, tissue access device  105  will be positioned adjacent or slightly within the ligamentum flavum  26  in the region of interest and a tissue excision device may be advanced through, and guided by, tissue access device  105  toward ligamentum flavum  26 . In some embodiments, excision can be performed generally from posterior to anterior across interlaminar space  82  and then laterally along the anterior portion of ligamentum flavum  26  if desired. The actual depth of distal end  106  of tissue access device  105  (or any tissue excision device passing through tissue access device  105 ) in the general direction of the x-axis may be adjusted with guidance from the caudal-cranial posterior-lateral view and appropriate retraction/advancement of tissue access device  105  and appropriate adjustment of tissue access device  105  between 5° and 90° relative to the posterior back surface  85 .  
      Referring now to  FIG. 4 , the tip of an exemplary tissue excision device  100  is shown schematically within ligamentum flavum  26 . Tissue excision device  100  may be the same device as tissue access device  105 , or may be a tool passed through tissue access device  105  if tissue access device  105  is a cannula or portal. In particular, tissue excision device  100  has accessed ligamentum flavum  26  according to the ILAMP method previously described. Thus, device  100  is positioned to excise portions of ligamentum flavum  26  on the same lateral side of median plane  210  as device  100  is percutaneously inserted. In other words, in the view shown in  FIG. 4 , device  100  is inserted into the body on the right side of median plane  210  and enters ligamentum flavum  26  on the right side of median plane  210  to excise portions of ligamentum flavum  26  on the right side of median plane  210 . In  FIG. 4 , device  100  does not cross median plane  210 .  
       FIG. 5  illustrates an embodiment of an alternative MILD method in which exemplary tissue excision device  100  is positioned to excise portions of ligamentum flavum  26  on the opposite lateral side of median plane  210  as device  100  is percutaneously inserted. More specifically, tissue excision device  100  is inserted into the body on the rights side of median plane  210 , enters ligamentum flavum  26  on the right side of median plane  210 , but is positioned to excise portions of ligamentum flavum  26  on the left side of median plane  210 . In  FIG. 5 , device  100  crosses median plane  210 .  
      In the manner described, portions of the ligamentum flavum can be excised by a percutaneous MILD procedure. In particular, with the approach described and as best illustrated in  FIGS. 4 and 6 , ligamentum flavum  26  can be accessed, and portions thereof removed via the interlaminar space on the same lateral side (ipsilateral) of median plane  210  as the entry point for instrument  101  (e.g., a cannula, a tissue excision tool, etc.). This approach may sometimes hereinafter be referred to as an Iplsilateral Approach MILD Procedure (ILAMP).  
      Tissue Excision Devices  
      In general, tissue excision device  100  is employed to excise relatively small portions of the stenosed or enlarged ligamentum flavum. By excising several small portions of the ligamentum flavum, the enlarged ligamentum flavum may be decompressed, thereby relieving pressure imposed on the spinal cord and the associated pain and other symptoms. Tissue excision device  100  typically includes a body having a central bore and a distal tip or cutting end adapted to cut a tissue segment (e.g., a segment of an enlarged ligamentum flavum). The cutting end is percutaneously inserted into the patient in the manner previously described and advanced toward the region of interest (e.g., enlarged ligamentum flavum). Once the cutting end of the tissue excision device  100  is positioned adjacent the region of interest, it may be employed to cut segments of tissue from the region of interest. The excised tissue segments are typically retained within the central lumen or bore of tissue excision device  100 .  
      Since the surgical procedures described herein are performed adjacent sensitive tissue (e.g., nerves of the spinal cord), they are preferably performed delicately and with minimal movement of the tools and devices near the sensitive tissues (e.g., tissue excision device  100 ). Thus, it may be desirable to minimize repositioning of tissue excision device  100 , especially the distal tip or cutting end of tissue excision device  100 . As used herein, the term “distal” refers to positions that are relatively closer to the region of interest (e.g., the thickened portion of the ligamentum flavum to be decompressed). To further this objective, in some embodiments, tissue excision device  100  may be substantially maintained within or adjacent the area of interest (e.g., enlarged ligamentum flavum) while making repeated excisions of portions of the tissue in the region of interest. In other words, in such embodiments, tissue excision device  100  may not be completely withdrawn from the area of interest and reinserted into the area of interest between each separate excision.  
      In cases when the distal tip or cutting end of tissue excision device  100  is maintained within the area of interest (i.e., not removed from the patient between each excision), the excised tissue segments may build up within the bore of tissue excision device  100 . Excessive build-up of excised tissue within tissue excision device  100  may inhibit or detrimentally impact continued cutting. Thus, between each excision by tissue excision device  100 , or at any desired time or interval, excised tissue segments within the bore of tissue excision device  100  are preferably retrieved and removed.  
      It is to be understood that tissue excision device  100  may comprise any suitable device capable of being employed to safely excise small portions of an enlarged ligamentum flavum. A variety of suitable tissue excision devices are disclosed in U.S. Application Ser. Nos. 11/193,581, 11/461,036, 60/733,754, 60/733,552, and 11/461,045, each of which is hereby incorporated herein by reference in its entirety.  
      Tissue Retrieval Devices  
      As previously described, in some embodiments, it may be desirable to retrieve and remove excised portions of the ligamentum flavum  26  from the distal cutting end  101  of tissue excision device  100 . Referring now to  FIGS. 9 and 10  an embodiment of a tissue retrieval device  200  is illustrated. Tissue retrieval device  200  comprises a cylindrical or tubular body  210  and a distal end or portion  260  extending from body  210 . Body  210  and distal portion  260  are coaxially aligned, sharing the same central axis  250 . In this embodiment, a through bore  240  is provided in body  210 . Thus, body  210  has an inner surface  211  at a radius R 1  and an outer surface  212  at a radius R 2  as best seen in  FIG. 10 .  
      In the embodiment shown in  FIGS. 9 and 10 , body  210  and distal portion  260  have a substantially circular cross-section. However, it should be appreciated that in different embodiments, body  210  and distal portion  260  may have any suitable shape and cross-section including, without limitation, circular, oval, or polygonal. The geometry and shape of tissue retrieval device  200  is preferably compatible with the tissue excision device that it is used in conjunction with. In other words, tissue retrieval device  200  is preferably shaped and configured to be slidingly disposed within, and advanced within, the tissue excision device towards the excised portion of the ligamentum flavum cut and retained within the distal portion of the tissue excision device  100 .  
      Referring still to  FIGS. 9 and 10 , in this embodiment, distal portion  260  comprises two prongs  270  extending from body  210  in a direction substantially parallel to axis  250 . As used herein to describe this arrangement, prongs  270  may be described as extending “axially” or in an “axial” direction from body  210 . However, it should be appreciated that in other embodiments, prongs  270  may be curved relative to axis  250 . For instance, prongs  270  may be curved radially inward (i.e., towards central axis  250 ) to form pincers. In this embodiment, prongs  270  are opposite one another and uniformly spaced about 180° apart.  
      Each prong  270  has a fixed-end  270   a  integral with body  210 , a free-end  270   b distal fixed-end  270   a , an inner surface  271 , and an outer surface  272 . Each fixed-end  270   a is fixed to body  210  such that prongs  270  do not move rotationally or translationally relative to body  210 .  
      Inner surface  271  of each prong  270  is positioned substantially at the same radius as the inner surface  211  of body  210  (i.e., radius R 1 ) and is contiguous with inner surface  211  of body  210 . In addition, outer surface  272  of each prong is positioned substantially at the same radius as outer surface  212  of body  210  (i.e., radius R 2 ) and is contiguous with outer surface  212  of body  210 . As used herein, the term “contiguous” may be used to describe a relatively smooth connection without substantial breaks. Thus, each prong  270  is positioned substantially equidistant from axis  250 . Although prongs  270  shown in  FIG. 9  are each substantially the same and positioned substantially the same distance from axis  250 , it should be understood that in other embodiments, one or more prongs (e.g., prongs  270 ) may be different in shape, length, geometry, and/or spaced at different distances from axis  250 . As will be described in more detail below, a void or space  280  extending radially between prongs  270  accommodates excised tissue grasped by distal portion  260 .  
      Referring now to  FIGS. 10 and 11 , a gripping surface  273  extends radially inward from inner surface  271  of each prong  270  proximal free-end  270   b , thereby defining a shoulder  274 . As best seen in  FIG. 11 , gripping surface  273  is oriented at an angle θ relative to inner surface  271  of each prong  270 . In this embodiment, angle θ between inner surface  271  and gripping surface  273  is about 90°, however, in other embodiments, angle θ may be more or less than 90°. Further, although angle θ is substantially the same in each prong  270 , it should be appreciated that angle θ may be the same or different for one or more prongs  270 . As will be explained in more detail below, gripping surface  273  engages and grasps excised tissue segments that are retained within space  280  between prongs  270 . To enhance the grasping of excised tissue segments within space  280 , angle θ is preferably less than or equal to 90°.  
      Referring still to  FIGS. 10 and 11 , free-end  270   b  of each prong comprises a tip  275  that is sharpened as a result of a beveled surface  276  that extends radially inward between outer surface  272  and gripping surface  273 . As best seen in  FIG. 11 , an angle λ between beveled surface  276  and outer surface  272  is preferably between 15° and 45°. In addition, beveled surface  276  and gripping surface  273  intersect at an angle α and form a corner or barb  277  that grasps tissue retained within space  280  between prongs  270 . Angle α between beveled surface  276  and gripping surface  273  is preferably between 15° and 75°. It should be appreciated that the “sharpness” of barb  277  may be enhanced by decreasing angle α and decreasing angle θ. In some embodiments, one or more prongs  270  may include multiple barbs along its inner surface  211  between fixed-end  270   a  and free-end  270   b  that enhance the ability of the one or more prongs  270  to grasp tissue. Further, in some embodiments, a portion of inner surface  271  of one or more prongs  270  may be texturized or roughened to further improve the ability of prongs  270  to grasp excised tissue. Such texturing may include without limitation diamond knurling, sand blasting, bead blasting, plasma etching, or combinations thereof  
      The proximal end of tissue retrieval device  200  preferably includes a handle (not shown) to facilitate insertion and removal of tissue retrieval device  200  into and out of a tissue excision device. The handle may be constructed from any suitable material including machined metal or molded from plastic.  
      Referring now to  FIG. 12 , an excised tissue segment  99  is retained within a lumen or bore XX within tissue excision device  100 . In particular, tissue excision device  100  is percutaneously inserted into the patient, with its distal cutting end (not shown) inserted first, and advanced toward the region of interest (e.g., enlarged ligamentum flavum). As previously described, tissue excision device  100  may be inserted and advanced through a cannula already positioned percutaneously through the patient. The distal cutting end of tissue excision device  100  is then positioned adjacent the region of interest. The distal cutting end is then employed to excise tissue segment  99  from the region of interest (e.g., excise a segment of enlarged ligamentum flavum). Excised tissue segment  99  is cut from the region of interest and disposed within a lumen or bore  110  of tissue excision device  100  adjacent the distal cutting end.  
      At this point, tissue segment  99  may be retrieved and removed from tissue excision device  100  by tissue retrieval device  200 . Specifically, distal portion  260  of tissue retrieval device  200  is slidingly disposed coaxially within bore  110  and advanced through bore  110  in the direction of arrow  291  towards excised tissue segment  99 . Tissue retrieval device  200  is preferably percutaneously inserted and advanced within tissue excision device  100 . To ensure sliding engagement between tissue excision device  100  and tissue retrieval device  200 , the inner radius R 3  of tissue excision device  100  defining bore  110  is preferably slightly greater than the outer radius R 2  of distal portion  260  and prongs  270 . Thus, distal portion  260 , including prongs  270 , are advanced within bore  110  towards the segment of tissue segment  99 .  
      Referring now to  FIGS. 12 and 13 , sharpened tips  275  of prongs  270  first contact excised tissue segment  99  as distal portion  260  is advanced in the direction of arrow  291 . Sharpened tips  275  and prongs  270  slide between tissue segment  99  and tissue excision device  100  as distal portion  260  is urged in the direction of arrow  291 . In particular, beveled surfaces  276  engage and slide across excised tissue segment  99 . As prongs  270  are advanced, excised tissue segment  99  moves towards and at least partially into space  280  between prongs  270 .  
      Prongs  270  are preferably advanced until at least a portion of segment of excised tissue  99  contacts inner surface  271  and engages barbs  277  and gripping surface  273 . Once barbs  277  and gripping surface  273  engage tissue segment  99 , distal portion  260 , including prongs  270 , is retracted and removed from tissue excision device  100  in the direction of arrow  292 . As distal portion  260  is withdrawn, barbs  277  and gripping surface  273  engage tissue segment  99 , and pull excised tissue segment  99 , thereby retrieving and removing excised tissue segment  99  from tissue excision device  100 . Once tissue retrieval device  200  has been completely removed from tissue excision device  100 , space  280  may be emptied by removing tissue segment  99  from between prongs  270 . This process may be repeated to retrieve and remove additional tissue excised by tissue excision device  100 .  
      In the manner described, excised tissue segment  99  may be retrieved and removed from a tissue excision device  100  by tissue retrieval device  200 . The devices and methods described do not require the removal or repositioning of tissue excision device  100  within the region of interest. Rather, tissue excision device  100  is maintained in position while tissue retrieval device  200  is employed to grasp, retrieve, and remove excised tissue segment  99  from tissue excision device  100 .  
      Referring now to  FIGS. 14 and 15 , an alternative embodiment of a tissue retrieval device  400  is illustrated. Tissue retrieval device  400  functions substantially the same as tissue retrieval device  200  previously described with the exception that tissue retrieval device  400  includes more than two prongs. Specifically, tissue retrieval device  400  comprises a body  410  and a distal portion  460  extending axially from body  410  that includes three prongs  470 . Each prong  470  includes a fixed-end  470   a  integral with body  410  and a free-end  470   b  including a sharpened tip  475 , beveled surface  476 , and barbs  477  for grasping a segment of tissue.  
      In addition, in this embodiment, tissue retrieval device  400  also comprises a tissue ejector  480  slidingly disposed within a through bore  440  in body  410  ( FIG. 15 ). Tissue ejector  480  includes a plunger  481  coupled to an ejection shaft  482 . Plunger  481  and ejection shaft  482  slide axially within bore  440  of body  410 . As depicted in  FIG. 15 , plunger  481  is shaped and configured to fit between prongs  470 . Plunger  481  preferably slidingly contacts the inner surface  411  of body  410  and the inner surface  471  of prongs  470 . Thus, plunger  481  preferably has an outer radius R 4  that is substantially the same or slightly less than the inner radius R 1  of body  410  and prongs  470 . Once tissue retrieval device  400  has retrieved a tissue segment from a tissue excision device and has been completely removed from the tissue excision device, the tissue segment between prongs  470  may be removed from tissue retrieval device  400  by advancing ejector shaft  482  and plunger  481  toward the tissue segment. As plunger  481  engages the tissue segment, plunger  481  will force the tissue segment from between prongs  470 , thereby ejecting the tissue segment from tissue retrieval device  400 . In some embodiments, ejector  480  may be controlled by a multi-function tool. Embodiments of suitable multi-function tools for ejecting a tissue segment from a tissue retrieval device (e.g., tissue retrieval device  400 ) are disclosed in U.S. application Ser. No. 11/461,045, which is hereby incorporated herein by reference in its entirety.  
      Embodiments of the tissue retrieval device described herein (e.g., tissue retrieval device  200 , tissue retrieval device  4300 , etc.) may comprise any suitable material(s) for use in surgical instruments. Such materials include, without limitation, 400 series stainless steel, 17 series stainless steel, or 300 series stainless steel. The invention may be fabricated by any suitable means including, without limitation, machining, laser-cutting, electromechanical deposition, and electro-polishing. As previously described, in some embodiments, the inner surface of one or more of the prongs (e.g., prongs  270 ) may be textured to enhance engagement with the tissue. The textured surface may be diamond knurled, sand blasted, bead blasted, plasma etched, or media blasted.  
      Although the embodiments described herein disclose tissue retrieval devices (e.g., tissue retrieval device  200 ) having prongs (e.g., prongs  270 ) that are uniformly spaced apart, it should be understood that the prongs of a tissue retrieval device may likewise be spaced non-uniformly spaced.  
      In addition, the body and/or distal portion of the tissue retrieval devices described herein may be formed from any suitable hollow body including, without limitation, a hypotube, cannula, or catheter. Alternatively, the body, the distal portion, and/or any bores of the tissue retrieval devices described herein may be machined. As previously mentioned, the prongs (e.g., prongs  270 ) of the tissue retrieval devices described herein are preferably integral with the body (e.g., body  210 ) of the tissue retrieval device (e.g., tissue retrieval device  200 ). However, it should be understood that the prongs of a tissue retrieval device may alternatively be distinct components that are mechanically coupled to the body. In such alternative embodiments, the prongs may be coupled to the body by any suitable means including, without limitation, welding, pins, or combinations thereof.  
      Embodiments of devices (e.g., tissue excision devices, tissue retrieval devices, etc.), methods, and systems disclosed herein may take several forms and may be used according to the ILAMP method described above, or used according to alternative MILD procedures (e.g., MILD procedure schematically illustrated in  FIG. 5 ). One such alternative MILD procedure is disclosed in U.S. application Ser. No. 11/193,581, which is hereby incorporated herein by reference in its entirety.  
      In addition, the methods and procedures disclosed herein may be facilitated by a kit for performing a spinal procedure (e.g., percutaneous decompression of enlarged ligamentum flavum). Such a kit preferably includes the basic components employed in one or more of the methods disclosed herein. For instance, in an embodiment, the kit preferably includes an insertion member (e.g., cannula) for accessing the epidural space, a contrast medium to create a safety zone, a tissue excision device to cut tissue segments, and an embodiment of the tissue retrieval device (e.g., tissue retrieval device  200 ,  400 ) to retrieve and remove the excised tissue segment from the tissue excision device. Depending on the application fewer or more components may be included in the kit.  
      While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims.