CROSS REFERENCES TO RELATED APPLICATIONS 
     The present application is a nonprovisional patent application claiming benefit under 35 U.S.C. §119(e) from U.S. Provisional Application Ser. No. 60/748,107, filed on Dec. 6, 2005, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
    
    
     BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates generally to spine surgery and, in particular, to methods and apparatus for treating spinal stenosis. 
     II. Discussion of the Prior Art 
     Spinal stenosis is a narrowing of spaces in the spine which results in pressure on the spinal cord and/or nerve roots. This disorder usually involves the narrowing of one or more of the following: (1) the canal in the center of the vertebral column through which the spinal cord and nerve roots run, (2) the canals at the base or roots of nerves branching out from the spinal cord, or (3) the openings between vertebrae through which nerves leave the spine and go to other parts of the body. 
     Pressure on the lower part of the spinal cord, or on nerve roots branching out from that area, may give rise to pain or numbness in the legs. Pressure on the upper part of the spinal cord (neck area) may produce similar symptoms in the shoulders, or even the legs. The condition generally occurs in patients who are in their last decade or decades of life. 
     Laminectomy, which involves removing bone, the lamina, from the vertebrae, is the most common surgical treatment for spinal stenosis. Laminectomy enlarges the spinal canal, thus relieving the pressure on compressed nerves. Surgical burs, drills, punches, and chisels are used during the procedure. 
     Surgeons risk injuring the nerves or the spinal cord as they enlarge the spinal canal. In addition, elderly patients frequently have co-morbidities that increase the risk of laminectomy. Complications of laminectomy include increased back pain, infection, nerve injury, blood clots, paralysis, prolonged recovery, and death. 
     Lumbar fusion is frequently preformed in conjunction with laminectomy. Current fusion techniques require abrasion of large surfaces of bone. Bone bleeds during and after abrasion. Current fusion techniques increase the risk of spinal stenosis procedures. Fusion also prolongs patient recovery following spinal stenosis surgery. Furthermore, various fusion techniques require the severing and/or removal of certain soft tissue surrounding the spine, including but not limited to the supraspinous ligament, the intraspinous ligament, the ligamentum flavum, the posterior longitudinal ligament, and/or the anterior longitudinal ligament. 
     Increasingly, surgeons are looking for improved methods of effecting less invasive treatments for spinal stenosis. The device must be able to be safely and consistently implanted without excess damage to the patient. The present invention is directed at overcoming, or at least improving upon, the disadvantages of the prior art. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a surgical apparatus for treating spinal stenosis without the need for a laminectomy. Broadly, the invention resides in an apparatus configured for placement in an intraspinous space, (e.g. posteriorly to a spinal canal between a first spinous process and an adjacent second spinous process). In the preferred embodiment, the device permits spinal flexion while limiting spinal extension, thereby providing an effective treatment for treating spinal stenosis. The invention may be used in the cervical, thoracic, or lumbar spine. 
     The preferred embodiments teach a spinal apparatus configured for placement between adjacent vertebrae and adapted to fuse to a first spinous process. Various mechanisms, including shape, porosity, tethers, and bone-growth promoting substances may be used to enhance fusion. The tether may be a wire, cable, suture, allograft tissue, or other single or multi-filament member. Preferably, the device forms a pseudo-joint in conjunction with the non-fused vertebra. 
     The spinous process spacer of the present invention may be of bone or non-bone construction. In the bone embodiments, the spinous process spacer may be constructed from any suitable allograft, including but not limited to portions of clavicle, rib, humerus, radius, ulna, metacarpal, phalanx, femur, tibia, fibula, or metatarsal bone. In non-bone embodiments, the spinous process spacer may be any suitable construction, including but not limited to polyaryletherketone (PEEK) and/or polyaryletherketoneketone (PEKK). In either event, the spacer includes a slot or indent to receive a portion of a spinous process to enhance fusion. The device may contain one or more bone-growth promoting substances such as BMP1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 . . . n, demineralized bone matrix, allograft cancellous bone, autograft bone, hydroxyapatite, coral and/or other highly porous substance. 
     During insertion of the spinous process spacer of the present invention, it may become necessary to sever the supraspinous and interspinous ligaments. In such instances it may be desirable to include an overlay designed to extend from one of the first and second spinous processes to the other in order to restore the integrity and functional benefits of the supraspinous and/or intraspinous ligaments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many advantages of the present invention will be apparent to those skilled in the art with a reading of this specification in conjunction with the attached drawings, wherein like reference numerals are applied to like elements and wherein: 
         FIG. 1  is a side view of the spinous processes of a pair of adjacent vertebrae with a spinous process spacer according to one embodiment of the present invention inserted therebetween, illustrating in particular the spinous process spacer tethered to one spinous process during use; 
         FIG. 2  is a perspective view of the spinal process spacer assembly of  FIG. 1  including bone growth promoting material; 
         FIG. 3  is a perspective view of a spinous process spacer according to one embodiment of the present invention; 
         FIG. 4  is a side plan view of the spinous process spacer of  FIG. 3 ; 
         FIG. 5  is a front plan view of the spinous process spacer of  FIG. 3 ; 
         FIG. 6  is a top plan view of the spinous process spacer of  FIG. 3 ; 
         FIG. 7  is a front cross section view of the spacer of  FIG. 3 ; 
         FIG. 8  is a perspective view of a spinous process spacer according to an alternative embodiment of the present invention; 
         FIG. 9  is a side view of the spinous process spacer of  FIG. 8 ; 
         FIG. 10  is an end view of the spinous process spacer of  FIG. 8 ; 
         FIG. 11  is a side view of the spinous processes of a pair of adjacent vertebrae with a spinous process spacer according to one embodiment of the present invention inserted therebetween, illustrating the spinous process spacer tethered to one spinous process during use and further illustrating the disruption of the supraspinous ligament and the intraspinous ligament during insertion; 
         FIG. 12  is a side view of the affixed spinous process spacer of  FIG. 11 , illustrating the further use of an overlay spanning from one spinous process to the other, primarily covering the distal portions of the spinous processes; and 
         FIG. 13  is a side view of the affixed spinous process spacer of  FIG. 11 , illustrating the further use of an overlay spanning from one spinous process to the other and covering a significant portion of the spinous processes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. The spinal alignment system disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination. 
       FIG. 1  illustrates a perspective view of a spinous process spacer (“SPS”) assembly  10  of the present invention in use between the spinous processes of a pair of adjacent vertebrae in a human spine. The SPS assembly  10  includes a spacer  12 , a primary tether  14 , and two side tethers  15  (only one of which is shown in  FIG. 1 ). The spacer  12 , as illustrated in  FIGS. 4-8 , is generally cylindrical and includes a main chamber  16 , a pair of insertion tool apertures  18 , a fusion notch  20 , and a pair of tether lumens  22 . As will be described in greater detail below, according to a preferred embodiment the spacer  12  is coupled to only one spinous process (e.g. the superior spinous process  2  as shown in  FIG. 1 ). This is accomplished, by way of example only, by securing the primary tether  14  to the superior spinous process  2  (as a first step of affixation), and then using a pair of side tethers  15  to affix spacer  12  to the primary tether  14 . This step may be accomplished, by way of example only, by passing one side tether  15  through each of the tether lumens  22 , further passing the side tether  15  between the superior spinous process  2  and the primary tether  14 , and finally tightening each side tether  15  until the spacer  12  is generally transverse to the longitudinal axis of the spine. 
     The spacer  12  may be of bone or non-bone construction. The bone embodiment involves manufacturing the spacer  12  from a suitable allograft, including but not limited to clavicle, rib, humerus, radius, ulna, metacarpal, phalanx, femur, tibia, fibula, or metatarsal bone. The non-bone embodiment involves manufacturing the spacer  12  from suitable non-bone materials, including but not limited to polyaryletherketone (PEEK) and polyaryletherketoneketone (PEKK). In either event, the spacer  12  is designed to fuse to the superior spinous process  2  over time, resulting in what is called “hemi-fusion” in that the spacer  12  will be fused to only one spinous process. This may be augmented by disposing any number of suitable fusion-inducing materials  17  within the spacer  12  (as shown by way of example only in  FIG. 2 ), including but not limited to BMP1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 . . . n, demineralized bone matrix, allograft cancellous bone, autograft bone, hydroxyapatite, coral and/or other highly porous substance. 
     Although shown and described with regard to the superior spinous process  2 , it will be appreciated that the spacer  12  may also be coupled to only the inferior spinous process  4  without departing from the scope of the present invention. The spacer  12 , once positioned, serves to distract the interspinous process space, which advantageously restores foraminal height in stenotic patients and may also indirectly decompress the intervertebral space. 
     As depicted in  FIGS. 3-4 , the main chamber  16  extends through the lateral sides of the spacer  12 . The main chamber  16  may be provided in any of a variety of suitable shapes in addition to the generally cylindrical shape as shown, including but not limited to a generally oblong, triangular, rectangular shape and/or combinations thereof. The pair of insertion tool apertures  18  may be located on either the posterior or anterior side of the spacer  12  and extend a portion of the way through the spacer  12 . The fusion notch  20  includes a slot or indent to receive a portion of the superior spinous process  2  (or other vertebral structure) to enhance fusion. The fusion notch  20  may be located generally towards the middle portion of the top of the spacer  12 . The notch  20  helps center the spacer  12  relative to the superior spinous process. 
     As best shown in  FIG. 7 , the tether lumens  22  each extend at an angle through the top surface of the spacer  12  and into the main chamber  16 . Each tether lumen  22  may be provided in any of a variety of suitable shapes in addition to the cylindrical shape shown, including but not limited to oblong, triangular, rectangular and/or any combination thereof. The primary tether  14  and the side tethers  15  may comprise any number of suitable materials and configurations, including but not limited to wire, cable, suture thread (permanent and/or bioresorbable), allograft tissue and/or other single or multi-filament member. Suture thread may include any number of components capable of attaching to a spinous process, including but not limited to ordinary suture threads known to and used by those skilled in the art of wound closure. The tethers  14 ,  15  may be of any length necessary to effectively fuse the spacer  12  to the particular spinous process. 
     According to an alternative embodiment of the present invention shown in  FIGS. 8-10 , the spacer  12  may be provided with a second notch  21  opposite the fusion notch  20 . The second notch  21  is capable of resting on the inferior spinous process  4  during use, which may assist in maintaining the spacer  12  in a fully centered position relative to the inferior spinous process  4 . As best shown in  FIG. 8 , the fusion notch  20  may be further provided with slots  23  extending into the main chamber  16 . When the spacer  12  is coupled to the superior spinous process  2 , these slots  23  will establish direct communication between the fusion-inducing compounds provided within the main chamber  16  and the lower aspect of the superior spinous process  2 , which advantageously augments the ability of the spacer  12  to fuse to the superior spinous process  2  (particularly if the spacer  12  is constructed of non-bone materials). 
     During insertion of the spinous process spacer of the present invention, it may become necessary to sever the supraspinous and interspinous ligaments.  FIG. 11  illustrates a SPS assembly  10  attached to a superior spinous process  2  as described above. Supraspinous ligament  6  is illustrated having been severed during the insertion process. Intraspinous ligaments  7 ,  9  remain intact, while intraspinous ligament  8  (situated between superior spinous process  2  and inferior spinous process  4 ) is also severed. 
       FIG. 12  illustrates an alternative embodiment of the present invention, in which the SPS assembly  10  may further include an overlay  30  designed to extend between the superior and inferior spinous processes  2 ,  4  in order to restore the integrity and functional benefits of the supraspinous ligament  6 . By way of example only, overlay  30  may be any material suitable for restoring the structural and functional integrity of the supraspinous ligament  6 , including but not limited to a surgical mesh, textile, and/or embroidery. Exemplary textiles are shown and described in commonly owned U.S. Pat. No. 5,990,378 entitled “Textile Surgical Implants Anchors,” which is attached hereto as Exhibit A forming part of this disclosure, and commonly owned US Patent Application Publication No. 2004/0078089 entitled “Textile Prosthesis,” which is attached hereto as Exhibit B forming part of this disclosure. Anchors  32  may be used to secure the overlay  30  to the spinous processes  2 ,  4 . Preferably, anchors  32  are inserted into the distal portion of the spinous processes  2 ,  4 , however it is contemplated that anchors  32  may be inserted into any portion of the spinous process suitable to provide purchase. Optionally, side anchors  34  may be inserted into the side of the spinous processes  2 ,  4  to further secure the overlay  30  to the bone. Anchors  32  and side anchors  34  may be any device suitable for attaching the overlay  30  to the bone, including but not limited to pins, screws, nails, tacks, staples, and the like. 
       FIG. 13  illustrates a still further alternative embodiment of the present invention, in which the SPS assembly  10  may further include an overlay  36  designed to extend between the superior and inferior spinous processes  2 ,  4  in order to restore the integrity and functional benefits of the supraspinous ligament  6  and the intraspinous ligament  8 . By way of example only, overlay  36  may be any material suitable for restoring the structural and functional integrity of the supraspinous ligament  6 , including but not limited to a surgical mesh, textiles, and/or embroidery (including the exemplary textiles referenced above). Anchors  38  may be used to secure the overlay  30  to the spinous processes  2 ,  4 . Preferably, anchors  38  are inserted into the distal portion of the spinous process spacers  2 ,  4 , however it is contemplated that anchors  38  may be inserted into any portion of the spinous process suitable to provide purchase. Optionally, side anchors  40  may be inserted into the side of the spinous processes  2 ,  4  to further secure the overlay  36  to the bone. Anchors  38  and side anchors  40  may be any device suitable for attaching the overlay  36  to the bone, including but not limited to pins, screws, nails, tacks, staples, and the like. 
     Although shown as separate components, it is contemplated that overlays  30 ,  36  may be integrally formed with spacer  12  such that the overlay and spacer are inserted contemporaneously. 
     The spacer  12  according to the present invention may be constructed of allograft bone and formed in a generally cylindrical shape. The spacer  12  of the present invention may be provided in any number of suitable shapes and sizes depending upon a particular patient and the shape and strength characteristics given the variation from cadaver to cadaver. The spacer  12  may be dimensioned for use in the cervical and/or lumbar spine without departing from the scope of the present invention. The spacer  12  may be dimensioned, by way of example only, having a length ranging between 6-20 mm and a height ranging between 20-25 mm. 
     The SPS assembly  10  of the present invention may be introduced into a spinal target site through the use of any of a variety of suitable instruments having the capability to releasably engage the spacer  12 . In a preferred embodiment, the insertion tool permits quick, direct, accurate placement of the spacer  12  between an upper and lower spinous process. An exemplary insertion tool is shown and described in commonly owned U.S. Pat. No. 6,923,814 entitled “System and Method for Cervical Fusion,” which is attached hereto as Exhibit C forming part of this disclosure. 
     In order to use the SPS assembly  10  of the present invention in a treatment of spinal stenosis, a clinician must first designate the appropriate spacer size  12 . A clinician can utilize the SPS assembly  10  in either an open or minimally invasive spinal fusion procedure. In either type of procedure, a working channel would be created in a patient that reaches a targeted spinal level. After the creation of the working channel, the interspinous space would be prepared. After preparation a sizer instrument is used to determine the appropriate size of the spacer  12 . Then the spacer  12  is positioned and inserted into the prepared space between the spinous processes. The device forces the spinous processes apart. The spine flexes as the spinous processes are forced apart. The neuroforamina and the spinal canal are enlarged as the spine is flexed. The SPS assembly  10  holds the vertebrae in a flexed position. By way of example only, the SPS assembly  10  may be made from an allograft shaft of a long bone such as the humerus, tibia, fibula, radius, ulna, or femur. 
     Preparation of the inter spinous process space includes perforating the interspinous ligament between the superior and inferior spinous processes. The supraspinous ligament may be either severed or left intact and distracted out of the way if necessary. A key part of the preparation includes abrading the inferior portion of the superior spinous process where it will communicate with the fusion inducing materials  32  packed in the main chamber  16 . Abrading removes the hard cortical bone from the inferior surface of the superior spinous process and leaves bleeding bone which is better adapted for fusion. As new bone generates to heal the abraded portion it may grow into the main chamber  16 , fixing spacer  12  to the superior spinous process. In the event that the supraspinous ligament has been severed, it may be desirable to secure an overlay  36  to the superior and inferior spinous processes as described above. 
     When constructed from allograft, the spacer  12  may be manufactured according to the following exemplary method. If necessary, first use a belt sander to reduce any high spots or imperfections to standardize the shape of the bone. Cut the allograft bone to length using the band saw. Remove the cancellous material from the inner canal to create the main chamber  16 . Using calipers, measure the struts and create a size distribution of spacers  12 . Machine the insertion tool apertures  18 . Set-up a standard vice for holding the implant across its width on the mill. Use a 3/32″ ball end mill to create the insertion tool apertures  18  (same as cervical allograft implant). Insert the spacer  12  into the vice and tighten. Calculate the centerline of the 20 or 25 mm long spacer  12 . Create the holes 2.26 mm away from each side of the centerline (4.52 mm hole to hole distance). Create a notch  22  for the spinous process. Set-up the cervical allograft holding fixture that uses the insertion tool apertures  18  and vice to hold the spacer  12  across its width on the mill. Use a ¼″ flat end mill to create the notch  22 . Calculate the centerline of the 20 or 25 mm long spacer  12 . Insert the spacer  12  onto the fixture using the insertion tool apertures  18  and tighten the vice. This automatically verifies the correct sizing/spacing of the insertion tool apertures  18 . Measure the spacer  12  height. Calculate the cut depth to create the desired spacer  12  size. Cut the flat on the spacer  12  to the desired depth. Remeasure the spacer  12  to insure proper cut depth. Drill the angled lumens  22  in face of spacer  12 . Remove the spacer  12  from the cervical allograft fixture and tighten into the standard vice. Using a battery powered or corded drill with a 1/16″ drill bit, drill through the front face to the canal on both sides. Belt sand the face if needed to create a flat surface for the drill bit to engage the spacer  12 . 
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the invention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined herein.
This invention relates generally to spine surgery and, in particular, to methods and apparatus for treating spinal stenosis.