Abstract:
Medical devices for the treatment of spinal conditions are described herein. The medical device of this invention includes a spacer that is disposed between adjacent spinous processes and has a layer of a soft or compliant material. The layer is preferably thicker along those portions of the spacer directly contacting the adjacent spinous processes and is preferably thinner or non-existent adjacent to the anterior portion of the support member. This preferred asymmetry of the compliant layer allows the spacer to be seated between spinous processes as anteriorly as possible.

Description:
BACKGROUND 
       [0001]    This invention relates generally to the treatment of spinal conditions, and more particularly, to the treatment of spinal stenosis using devices for implantation between adjacent spinous processes. 
         [0002]    The clinical syndrome of neurogenic intermittent claudication due to lumbar spinal stenosis is a frequent source of pain in the lower back and extremities, leading to impaired walking, and causing other forms of disability in the elderly. Although the incidence and prevalence of symptomatic lumbar spinal stenosis have not been established, this condition is the most frequent indication of spinal surgery in patients older than 65 years of age. 
         [0003]    Lumbar spinal stenosis is a condition of the spine characterized by a narrowing of the lumbar spinal canal. With spinal stenosis, the spinal canal narrows and pinches the spinal cord and nerves, causing pain in the back and legs. It is estimated that approximately 5 in 10,000 people develop lumbar spinal stenosis each year. For patients who seek the aid of a physician for back pain, approximately 12%-15% are diagnosed as having lumbar spinal stenosis. 
         [0004]    Common treatments for lumbar spinal stenosis include physical therapy (including changes in posture), medication, and occasionally surgery. Changes in posture and physical therapy may be effective in flexing the spine to decompress and enlarge the space available to the spinal cord and nerves—thus relieving pressure on pinched nerves. Medications such as NSAIDS and other anti-inflammatory medications are often used to alleviate pain, although they are not typically effective at addressing spinal compression, which is the cause of the pain. 
         [0005]    Surgical treatments are more aggressive than medication or physical therapy, and in appropriate cases surgery may be the best way to achieve lessening of the symptoms of lumbar spinal stenosis. The principal goal of surgery is to decompress the central spinal canal and the neural foramina, creating more space and eliminating pressure on the spinal nerve roots. The most common surgery for treatment of lumbar spinal stenosis is direct decompression via a laminectomy and partial facetectomy. In this procedure, the patient is given a general anesthesia as an incision is made in the patient to access the spine. The lamina of one or more vertebrae is removed to create more space for the nerves. The intervertebral disc may also be removed, and the adjacent vertebrae may be fused to strengthen the unstable segments. The success rate of decompressive laminectomy has been reported to be in excess of 65%. A significant reduction of the symptoms of lumbar spinal stenosis is also achieved in many of these cases. 
         [0006]    Alternatively, the vertebrae can be distracted and an interspinous process device implanted between adjacent spinous processes of the vertebrae to maintain the desired separation between the vertebral segments. Such interspinous process implants typically work for their intended purposes, but some could be improved. Where the spacer portion of the implant is formed from a hard material, point loading of the spinous process can occur due to the high concentration of stresses at the point where the hard material of the spacer contacts the spinous process. This may result in excessive subsidence of the spacer into the spinous process. In addition, if the spinous process is osteoporotic, there is a risk that the spinous process could fracture when the spine is in extension. 
         [0007]    Thus, a need exists for improvements in certain current interspinous process devices. 
       SUMMARY OF THE INVENTION 
       [0008]    The interspinous process implant of this invention includes a spacer that is disposed between adjacent spinous processes and has a layer of a soft or compliant material. Such a layer minimizes the high stress concentration between the spacer and the spinous process and thus improves the point loading characteristics of the spacer on the spinous process. This minimizes subsidence and also reduces the risk of fracture. The durometer of the layer is chosen to provide a sufficient cushion for the spinous process without minimizing the distraction capability of the spacer. Preferably, the compliant layer is located around the spacer such that the layer is thicker along those portions of the spacer directly contacting the adjacent spinous processes and is thinner adjacent to the anterior portion of the spacer. This asymmetry of the compliant layer allows the spacer to be seated between spinous processes as anteriorly as possible. Alternatively, the compliant layer may be located symmetrically (i) about the entire spacer, or (ii) such that the layer is located only along those portions of the spacer adapted to be directly in contact with the spinous processes, or (iii) such that the compliant layer is thicker along the superior and inferior portions of the spacer but such that there is also a thin layer around the anterior and posterior portions of the spacer, or (iv) about entire implant. 
         [0009]    In an alternative embodiment, a layer of soft or compliant material can be located within the spacer of the interspinous process implant as a separate core, which may have various cross sections, such as a circle or rectangle. As with the compliant layer described above, the durometer of the material can be adjusted in such a way so as to minimize the point loading on the spinous process and allow the core to take up some of the load. Again, this would minimize subsidence and reduce the risk of fracturing the spinous process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a side perspective view of one embodiment of an interspinous process implant shown in a collapsed configuration which may include the spacer of this invention; 
           [0011]      FIG. 2  is a cross-sectional perspective view of the implant of  FIG. 1  taken along line  2 - 2 ; 
           [0012]      FIG. 3  is a side perspective view of the implant of  FIGS. 1 and 2  shown in a deployed configuration; 
           [0013]      FIG. 4  is cross-sectional perspective view of the implant of  FIG. 3  taken along line  4 - 4 ; 
           [0014]      FIG. 5  is a cross-sectional view of the implant of  FIG. 1  similar to the view shown in  FIG. 2  but with a compliant layer disposed around the spacer; 
           [0015]      FIG. 6  is a schematic cross-sectional view of one embodiment of the spacer of this invention disposed between adjacent spinous processes; 
           [0016]      FIG. 7  is a schematic cross-sectional view, similar to the view of  FIG. 6 , of yet another embodiment of the spacer of this invention; 
           [0017]      FIG. 8  is a schematic cross-sectional view, similar to the view of  FIG. 6 , of still another embodiment of the spacer of this invention; 
           [0018]      FIG. 9  is a schematic cross-sectional view of an implant, similar to the view of  FIG. 6 , of a further embodiment of the spacer of this invention; 
           [0019]      FIG. 10  is a cross-sectional perspective view, similar to the view shown in  FIG. 5 , of another embodiment of the spacer of this invention; 
           [0020]      FIG. 11  is another cross-sectional view of the embodiment of the spacer of this invention shown in  FIG. 10  taken along line  11 - 11 ; 
           [0021]      FIG. 12  is a cross-sectional view, similar to the view of  FIG. 11 , of yet another embodiment of the spacer of this invention; 
           [0022]      FIG. 13  is a perspective view of still another interspinous process implant that may incorporate the spacer of this invention; and 
           [0023]      FIG. 14  is a perspective view of yet another interspinous process implant that may incorporate the spacer of this invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    As used in this specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, the term “a member” is intended to mean a single member or a combination of members, “a material” is intended to mean one or more materials, or a combination thereof. Furthermore, the words “proximal” and “distal” refer to directions closer to and away from, respectively, an operator (e.g., surgeon, physician, nurse, technician, etc.) who would insert the medical device into the patient, with the tip-end (i.e., distal end) of the device inserted inside a patient&#39;s body first. Thus, for example, the implant end first inserted inside the patient&#39;s body would be the distal end of the implant, while the implant end to last enter the patient&#39;s body would be the proximal end of the implant. 
         [0025]    As used in this specification and the appended claims, the term “body” means a mammalian body. For example, a body can be a patient&#39;s body, or a cadaver, or a portion of a patient&#39;s body or a portion of a cadaver. 
         [0026]    As used in this specification and the appended claims, the term “parallel” describes a relationship, given normal manufacturing or measurement or similar tolerances, between two geometric constructions (e.g., two lines, two planes, a line and a plane, two curved surfaces, a line and a curved surface or the like) in which the two geometric constructions are substantially non-intersecting as they extend substantially to infinity. For example, as used herein, a line is said to be parallel to a curved surface when the line and the curved surface do not intersect as they extend to infinity. Similarly, when a planar surface (i.e., a two-dimensional surface) is said to be parallel to a line, every point along the line is spaced apart from the nearest portion of the surface by a substantially equal distance. Two geometric constructions are described herein as being “parallel” or “substantially parallel” to each other when they are nominally parallel to each other, such as for example, when they are parallel to each other within a tolerance. Such tolerances can include, for example, manufacturing tolerances, measurement tolerances or the like. 
         [0027]    As used in this specification and the appended claims, the term “normal” describes a relationship between two geometric constructions (e.g., two lines, two planes, a line and a plane, two curved surfaces, a line and a curved surface or the like) in which the two geometric constructions intersect at an angle of approximately 90 degrees within at least one plane. For example, as used herein, a line is said to be normal to a curved surface when the line and the curved surface intersect at an angle of approximately 90 degrees within a plane. Two geometric constructions are described herein as being “normal” or “substantially normal” to each other when they are nominally normal to each other, such as for example, when they are normal to each other within a tolerance. Such tolerances can include, for example, manufacturing tolerances, measurement tolerances or the like. 
         [0028]    In one embodiment of the interspinous process implant of the invention, the implant includes a spacer that defines a longitudinal axis and is configured to be implanted at least partially into a space between adjacent spinous processes. The implant also has a first retention member and a second retention member. An axial force is exerted along the longitudinal axis such that each of the first retention member and the second retention member plastically expand in a direction transverse to the longitudinal axis. When plastically expanded, each of the first retention member and the second retention member has a greater outer perimeter than an outer perimeter of the support member. The implant configuration is shown in more detail in U.S. Patent Application Publication No. 2007/0225807, the entire contents of which are hereby expressly incorporated herein by reference. Although the interspinous process implant spacer of this invention is described specifically in connection with the configuration shown in U.S. Patent Application Publication No. 2007/0225807, it is to be understood that the invention described herein can be used in connection with other configurations for an interspinous process implant. For example, the invention described herein can be used in connection with the various interspinous process implants having a relatively hard spacer shown in U.S. Patent Application Publication Nos. 2008/0039859 and 2008/0086212, the entire contents of which are hereby expressly incorporated herein by reference. See also  FIGS. 13 and 14 . 
         [0029]      FIGS. 1-4  illustrate an interspinous process implant  10  that may incorporate the spacer of this invention. Implant  10  can be moved between a collapsed configuration, as shown in  FIGS. 1 and 2 , and a deployed configuration, as shown in  FIGS. 3-4 . Implant  10  includes a spacer  101 , a distal portion  102 , and a proximal portion  103 . Implant  10  defines a series of openings  105  disposed between distal portion  102  and spacer  101 , and proximal portion  103  and spacer  101 . Implant  10  includes a series of tabs  106 , a pair of which are disposed opposite each other, along the longitudinal axis of implant  10 , on either side of each opening  105 . Implant  10  also includes wings  107  that may be deployed so they extend radially from implant  10  when it is in the deployed configuration. As illustrated best in  FIGS. 3-4 , the arrangement of openings  105  and tabs  106  affect the shape and/or size of wings  107 . In some embodiments, the opposing tabs  106  can be configured to engage each other when implant  10  is in the deployed configuration, thereby serving as a positive stop to limit the extent that wings  107  are deployed. In other embodiments, for example, the opposing tabs  106  can be configured to engage each other during the deployment process, thereby serving as a positive stop, but remain spaced apart when implant  10  is in the deployed configuration (see, for example,  FIGS. 3-4 ). In such embodiments, the elastic properties of wings  107  can cause a slight “spring back,” thereby causing the opposing tabs  106  to be slightly spaced apart after tabs  106  have been moved to deploy wings  107 . 
         [0030]    As illustrated best in  FIG. 1 , when implant  10  is in the collapsed configuration, wings  107  are contoured to extend slightly radially from remaining portions of implant  10 . In this manner, wings  107  are biased such that when a compressive force is applied, wings  107  will extend outwardly from spacer  101 . Wings  107  can be biased using any suitable mechanism. For example, wings  107  can be biased by including a notch in one or more locations along wing  107 . Alternatively, wings  107  can be biased by varying the thickness of wings  107  in an axial direction. In addition, wings  107  can be stressed or bent prior to insertion such that wings  107  are predisposed to extend outwardly when a compressive force is applied to implant  10 . In such embodiments, the radius of wings  107  is greater than that of the remaining portions of implant  10  (e.g., the remaining cylindrical portions of implant  10 ). Preferably, wings  107  adjacent the proximal portion of implant  10  are designed to be predisposed to extend outwardly under less force than wings  107  adjacent the distal portion of implant  10 . This arrangement causes the proximal wings to deploy first and thus facilitates the proper location of implant  10  between the desired spinous processes. 
         [0031]    Preferably, implant  10  includes an outer compliant layer  300  located on an outer surface of spacer  101  in the areas where spacer  101  contacts an inferior portion of a superior spinous process and a superior portion of an inferior spinous process. See  FIGS. 6 through 9 . Alternatively, compliant layer  300  can be located about the entire surface of implant  10  along the entire axial length of implant  10 , or along the distal portion  102  and along spacer  101 , or along the proximal portion  103  and along spacer  101 . Compliant layer  300  may be formed from materials that may have a Modulus of Elasticity (MOE) that is particularly matched with the vertebral members along which implant  10  is located. For example, the difference of the MOE of compliant layer  300  and these vertebral members is not great than about 30 GPa. In other embodiments, the difference is less, such as not greater than about 15 GPa, not greater than about 5 GPa, or not greater than about 1 GPa. Specific examples of the material for compliant layer  300  can include silicone, polyaryletheretherketone (PEEK), polyurethane, and rubber. Other materials may also be used. 
         [0032]    Compliant layer  300  is applied to the outer surface of spacer  101  in such a way that compliant layer  300  has its greatest thickness in the areas where spacer  101  will contact the spinous processes. See  FIGS. 6 through 9 . In  FIG. 6 , compliant layer  300  is substantially uniformly disposed around most of the circumference of spacer  101  except along the anterior side of spacer  101 . In  FIG. 7 , compliant layer  300  is disposed along the superior and inferior side of spacer  101 . In  FIG. 8 , compliant layer  300  is disposed around the entire circumference of spacer  101 , but the thickness is minimized along the anterior and posterior portions of spacer  101 . In  FIG. 9 , compliant layer  300  is disposed completely and substantially uniformly around the circumference of spacer  101 . Preferably, compliant layer is between about 0 and 20 mm thick in these areas. Compliant layer  300  should have a minimal thickness in the area that is disposed along the anterior portion of spacer  101  when spacer  101  is located in the patient between adjacent spinous processes. See, for example,  FIG. 8 . Alternatively, compliant layer  300  can be non-existent in this area. See  FIGS. 6 and 7 . In yet another embodiment, compliant layer  300  may be located substantially symmetrically around the circumference of spacer  101 . See  FIGS. 8 and 9 . Where there is no layer  300  along the anterior portion of spacer  101 , it can be implanted between adjacent spinous processes as anteriorly as possible. This ensures that spacer  101  (i) is able to provide maximum distraction/spacing between adjacent spinous processes with minimal size, (ii) minimizes the potential for unwanted posterior migration of the implant, and (iii) provides the best potential outcome for the patient. See, for example,  FIGS. 6 and 7 . Compliant layer  300  can be applied in many different ways. For example, compliant layer  300  may be molded over appropriate portions of implant  10 , it may be formed as a separate member and placed over implant  10 , or it may be applied by chemically coating implant  10 . 
         [0033]    Spacer  101  also includes a central body  201  disposed within a lumen  120  defined by spacer  101 . Central body  201  is configured to maintain the shape of spacer  101  during insertion, to prevent wings  107  from extending inwardly into a region inside of spacer  101  during deployment and/or to maintain the shape of spacer  101  once it is in its desired position. As such, central body  201  can be constructed to provide increased compressive strength to spacer  101 . In other words, central body  201  can provide additional structural support to spacer  101  (e.g., in a direction transverse to the axial direction) by filling at least a portion of the region inside spacer  101  (e.g., lumen  120 ) and contacting the walls of spacer  101 . This can increase the amount of compressive force that can be applied to spacer  101  while allowing it to still maintain its shape and, for example, the desired spacing between adjacent spinous processes. In some embodiments, central body  201  can define a lumen  120 , while in other embodiments, central body  201  can have a substantially solid construction. As illustrated, central body  201  is fixedly coupled to spacer  101  with a coupling portion  203 , which is configured to be threadedly coupled to the distal portion of spacer  101 . The distal end of coupling portion  203  of central body  201  includes an opening  204  configured to receive a tool that is designed to deform the distal end of coupling portion  203 . In this manner, once central body  201  is threadedly coupled to spacer  101 , coupling portion  203  can be deformed or peened to ensure that central body  201  does not become inadvertently decoupled from spacer  101 . In some embodiments, an adhesive, such as a thread-locking compound can be applied to the threaded portion of coupling portion  203  to ensure that central body  201  does not inadvertently become decoupled from spacer  101 . Although illustrated as being threadably coupled, central body  201  can be coupled to spacer  101  by any suitable means. In some embodiments, for example, central body  201  can be coupled to spacer  101  by, for example, a friction fit. In other embodiments, central body  201  can be coupled to spacer  101  by an adhesive. Central body  201  can have a length such that central body  201  is disposed within lumen  120  along substantially the entire length of spacer  101  or only a portion of the length of spacer  101  or along a portion of the length of spacer  101  and a portion of proximal portion  103  and/or a portion of distal portion  102 . 
         [0034]    The proximal portion of central body  201  preferably includes cavity  202  configured to receive a portion of an insertion tool, not shown. Such an insertion tool is similar to the tool shown and described in commonly assign U.S. Patent Application Publication No. 2007/0276493, the entire contents of which are hereby expressly incorporated herein by reference. 
         [0035]      FIG. 10  illustrates an interspinous process device according to another embodiment of the invention. In the embodiment shown in  FIG. 10 , an inner core  400  is located in cavity  202 . Inner core  400  is formed from the same types of material as described above in connection with coating  300 . As shown in  FIG. 11 , inner core  400  may be formed as a cylinder having a generally circular cross section, although the cylinder could have other cross sections as well, such as a polygon or other symmetrical or unsymmetrical geometric shape. In the foregoing examples, inner core  400  is located within cavity  202  such that inner core is completely surrounded by central body  201 . Alternatively, the inner core may extend across the diameter of lumen  120  such that central body  201  is disposed along the superior and inferior sides of inner core  400 ′. See for example,  FIG. 12 . In this embodiment, inner core  400 ′ may have a generally rectangular cross section. Alternatively, the inner core could be arranged within lumen  120  so that central body is disposed along the distal and proximal sides of the inner core. As with the embodiment shown in  FIG. 11 , the cross section of inner core  400 ′ may take various geometric shapes. Other configurations may be used for the inner core as long as the inner core takes up some of the load on the implant when the spine is in extension. 
         [0036]    In use, once implant  10  is positioned on a suitable insertion tool, implant  10  is inserted into the patient&#39;s body and disposed therein such that spacer  101  is located between adjacent spinous processes. Thereafter, the insertion tool is used to move central body  201  axially towards the proximal portion of spacer  101  while simultaneously maintaining the position of the proximal portion of spacer  101 . In this manner, a compressive force is applied along the longitudinal axis of spacer  101 , thereby causing spacer  101  to fold or bend to deploy wings  107  as described above. Similarly, to move spacer  101  from the deployed configuration to the collapsed configuration, the insertion tool is actuated in the opposite direction to impart an axial force on the distal portion of spacer  101  in a distal direction, moving the distal portion distally, and moving spacer  101  to the collapsed configuration. 
         [0037]    Although shown and described above without reference to any specific dimensions, in some embodiments, spacer  101  can have a cylindrical shape having a length of approximately 34.5 mm (1.36 inches) and a diameter between 8.1 and 14.0 mm (0.32 and 0.55 inches). In some embodiments, the wall thickness of spacer  101  can be approximately 5.1 mm (0.2 inches). 
         [0038]    Similarly, in some embodiments, inner core  201  can have a cylindrical shape having an overall length of approximately 27.2 mm (1.11 inches) and a diameter between 8.1 and 14.0 mm (0.32 and 0.55 inches). 
         [0039]    In some embodiments, the shape and size of openings  105  located adjacent the distal portion  102  can be the same as that for the openings  105  located adjacent the proximal portion  103 . In other embodiments, the openings  105  can have different sizes and/or shapes. In some embodiments, the openings  105  can have a length of approximately 11.4 mm (0.45 inches) and a width between 4.6 and 10 mm (0.18 and 0.40 inches). 
         [0040]    Similarly, the shape and size of tabs  106  can be uniform or different as circumstances dictate. In some embodiments, for example, the longitudinal length of tabs  106  located adjacent proximal portion  103  can be shorter than the longitudinal length of tabs  106  located adjacent distal portion  102 . In this manner, as spacer  101  is moved from the collapsed configuration to the deployed configuration, tabs  106  adjacent distal portion  102  will engage each other first, thereby limiting the extent that wings  107  adjacent distal portion  102  are deployed to a greater degree than wings  107  located adjacent proximal portion  103 . In other embodiments, the longitudinal length of tabs  106  can be the same. In some embodiments, the longitudinal length of tabs  106  can be between 1.8 and 2.8 mm (0.07 and 0.11 inches). In some embodiments, the end portions of opposing tabs  106  can have mating shapes, such as mating radii of curvature, such that opposing tabs  106  engage each other in a predefined manner. 
         [0041]    Although illustrated as having a generally rectangular shape, wings  107  can be of any suitable shape and size. In some embodiments, for example, wings  107  can have a longitudinal length of approximately 11.4 mm (0.45 inches) and a width between 3.6 and 3.8 mm (0.14 and 0.15 inches). In other embodiments, the size and/or shape of wings  107  located adjacent proximal portion  103  can be different than the size and/or shape of tabs  106  located adjacent distal portion  102 . Moreover, as described above, wings  107  can be contoured to extend slightly radially from spacer  101 . In some embodiments, for example, wings  107  can have a radius of curvature of approximately 12.7 mm (0.5 inches) along an axis normal to the longitudinal axis of spacer  101 . 
         [0042]    In some embodiments, wings  107  and spacer  101  are monolithically formed. In other embodiments, wings  107  and spacer  101  are formed from separate components having different material properties. For example, wings  107  can be formed from a material having a greater amount of flexibility, while spacer  101  can be formed from a more rigid material. In this manner, wings  107  can be easily moved from the collapsed configuration to the deployed configuration, while spacer  101  is sufficiently strong to resist undesirable deformation when in use. 
         [0043]      FIG. 13  shows another interspinous process implant  1000  that may incorporate the spacer  101  of this invention. Implant  1000  includes a first wing  1010 , a spacer  101  and a lead-in and distraction guide  1100 . Alternatively, implant  1000  may include no lead-in and distraction guide. Implant  1000  may include a second wing  1020  that may be fixed to implant  1000  or may be removably attached thereto. For more a more detailed description, see the disclosure of U.S. Application Publication No. 2008/0039859. As mentioned above, the entire disclosure of that document is hereby expressly incorporated herein by reference. Compliant layer  300  is located around the spacer of  FIG. 13  in a similar fashion as described in connection with the previous embodiments of this invention. 
         [0044]      FIG. 14  shows yet another interspinous process implant  2000  that may incorporate the compliant layer of this invention. Implant  2000  has a generally H-shaped configuration wherein the cross-bar  2010  of the H is the spacer  101  of this invention. Compliant layer  300  is preferably located along the superior and inferior portions of cross-bar  2010 . 
         [0045]    Spacer  101  can be constructed with various biocompatible materials such as, for example, titanium, titanium alloy, surgical steel, biocompatible metal alloys, stainless steel, Nitinol, plastic, polyetheretherketone (PEEK), carbon fiber, ultra-high molecular weight (UHMW) polyethylene, biocompatible polymeric materials, etc. The material of spacer  101  can have, for example, a compressive strength similar to or higher than that of bone. In one embodiment, spacer  101 , which is placed between the two adjacent spinous processes, is configured with a material having an elastic modulus higher than the elastic modulus of the bone, which forms the spinous processes. In another embodiment, spacer  101  is configured with a material having a higher elastic modulus than the materials used to configure the distal and proximal portions of the implant. For example, spacer  101  may have an elastic modulus higher than bone, while proximal portion  103  and distal portion  102  have a lower elastic modulus than bone. In yet another embodiment, spacer  101  can be configured with material having a higher elastic modulus than inner core  201 , e.g. a titanium alloy material or Nitinol, while inner core  201  can be made with a polymeric material. Alternatively, spacer  101  can be configured with a material having a lower elastic modulus than inner core  201 , e.g. spacer  101  can be made with a polymeric material while inner core  201  is made with a titanium alloy material. 
         [0046]    While various embodiments of the invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. The foregoing description of the various interspinous process implants is not intended to be exhaustive or to limit the invention. Many modifications and variations will be apparent to the practitioner skilled in the art. It is intended that the scope of the invention be defined by the following claims and their equivalents.