Abstract:
A spinal fixation device including two plates and a coupling element for coupling the plates in a fixed manner about adjacent spinous processes of the spine. Each plate is preferably equipped with integral spikes on the inwardly facing surfaces for pressing into the spinal processes and thereby augmenting the purchase between the plates and the spinous processes. Each plate contains a central aperture through which the coupling element passes in order to couple the plates together.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present application is a nonprovisional patent application claiming benefit under 35 U.S.C. § 119(e) from U.S. Provisional Application Ser. No. 60/898,818, filed on Jan. 31, 2007, the entire contents of which are hereby expressly incorporated by reference into this disclosure as if set forth fully herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    I. Field of the Invention 
         [0003]    The present invention relates generally to spinal surgery, and more particularly to devices for fusing adjacent spinous processes to stabilize the vertebral segment associated with the particular spinous processes. 
         [0004]    II. Discussion of the Prior Art 
         [0005]    The human spinal column is made up of two basic components, vertebrae (bone) and intervertebral discs (gel-like cushions that absorb pressure and prevent vertebrae from rubbing together). A number of vertebrae and intervertebral discs stack together to form a column that provides support and structure for the body while still allowing a large degree of motion and flexibility. The spinal column also serves to protect the spinal cord (a bundle of nerves linking the brain to the rest of the body) that runs through an opening formed in the center of the column. A pair of nerve roots exit the spinal column at each level through spaces formed between the vertebrae. Various traumatic events and degenerative conditions may result in undesirable motion or changes in disc height, both of which may cause chronic pain for the affected individual. The pain is generally caused when changes in disc height and improper motion allow adjacent vertebrae to impinge upon exiting nerve roots. The degree and treatment of pain varies by individual but in many instances the pain can be disabling and uncontrollable by non-invasive means, leaving surgery as the only viable option. Generally in such a case, two or more vertebrae are fused together, employing various instrumentation and methods to correct disc height and prevent improper movement of the vertebrae while fusion occurs, thereby eliminating or at least reducing the pain of the affected individual. 
         [0006]    While there are a variety of systems and methods for effecting spinal fixation while fusion occurs, one of the more common methods involves securing pedicle screws into the pedicles of the two or more adjacent vertebrae to be fixed. The challenge in this method is securing the pedicle screws without breaching, cracking, or otherwise compromising the pedicle wall, which may occur if the screw is not properly aligned with the pedicle axis. If the pedicle (or more specifically, the cortex of the medial wall, lateral wall, superior wall and/or inferior wall) is breached, cracked, or otherwise compromised, the patient may experience pain or neurological deficit due to unwanted contact between the pedicle screw and delicate neural structures, such as the spinal cord or exiting nerve roots. This may necessitate revision surgery, which is disadvantageously painful for the patient and costly, both in terms of recovery time and hospitalization. 
         [0007]    The present invention is directed to overcome one or more shortcomings encountered with current fixation devices and systems. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention relates to a spinal fixation device designed to be attached to adjacent spinous processes of the spine for immobilizing the adjacent spinous processes to promote fusion therebetween. The spinal fixation device may be used alone (that is, without any supplemental fusion devices, such as interbody fusion implants) or with supplemental fixation devices. In either event, the spinal fixation device allows fusion to occur between the adjacent spinous processes by maintaining them in an immobilized, locked relationship such that a boney bridge can form therebetween. The formation of the fusion bridge between the adjacent spinous processes may be augmented or facilitated by placing fusion-enhancing compounds between the spinous processes, including but not limited to autologous bone, allograft bone, bone morphogenic protein (BMP), and/or any number of suitable biomaterials. 
         [0009]    According to one embodiment of the present invention, the spinal fixation device includes two plates and a coupling element for coupling the plates in a fixed manner about adjacent spinous processes of the spine. Each plate is preferably equipped with integral spikes on the inwardly facing surfaces for pressing into the spinal processes and thereby augmenting the purchase between the spinous processes and the plates. Each plate contains a central aperture through which the coupling element passes in order to couple the plates together. 
         [0010]    The coupling element may be any number of devices capable of coupling the first plate to the second plate. In one exemplary embodiment, the coupling element may be a screw or bolt with one end threaded to engage a threaded aperture in one plate and the other end with a head dimensioned to engage with a respective region on the other plate and a driving tool. In another embodiment, the threaded end of the coupling element may be replaced with one having external ridges (as opposed to threads) to engage corresponding features in the aperture of one plate to prevent any backward motion once received through the aperture. This embodiment is advantageous in that the plates can be easily locked together and tightened by simply pushing the coupling element through one plate (with the head received within a corresponding region or recess of the first plate) and into the next (with the ridges locking at each point as the ridged section is advanced through the aperture of the second plate, the head may or may not be fully contained within the first plate). In either embodiment, the head may be constructed like a screw head with an internally disposed recess for receiving a driving element (e.g. hexalobe drive, Phillips screw driver, hex driver, etc. . . . ) or may be constructed without such an internally disposed recess and may instead be driven by an exteriorly placed driving element (e.g. wrench). 
         [0011]    The apertures may be provided in any number of different manners to help facilitate coupling the fixation element to the plates. For example, with the first embodiment of the coupling element (threaded screw or bolt), the aperture of one plate may be tapped with internal threads to engage external threads of the threaded section of the coupling element. With the second embodiment of the coupling element (ridged bolt), the aperture of one plate may be equipped with any number of suitable features, such as inwardly facing teeth or ridges, that engage with the ridges of the coupling element. 
         [0012]    Any number of suitable instruments may be provided to help facilitate the surgery, including but not limited to instruments for compressing and/or distracting the adjacent spinous processes prior to securing the plates (and thus immobilizing the spinous processes), as well as instruments to facilitate coupling the plates together such as drivers for tightening the coupling element to the plates or instruments for compressing the plates together. In one embodiment, the driving or compressing instrument may be equipped with a torque limiting mechanism that produces an audible (e.g. “click”) and/or and a tactile alert that lets the surgeon know he or she has applied optimal torque to the fixation element to fix the plates together. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a postero-lateral view of a portion of the spine with one example of a spinous process fixation system according to a first embodiment of the present invention implanted on adjacent spinous processes of a spine; 
           [0014]      FIG. 2  is a posterior view of the spinous process fixation system implanted on adjacent spinous processes as shown in  FIG. 1 ; 
           [0015]      FIG. 3  is a perspective view of the spinous process fixation system of  FIG. 1 ; 
           [0016]      FIG. 4A  is a plan view of the lateral surface of a first plate forming part of the spinous process fixation system of  FIG. 3 ; 
           [0017]      FIG. 4B  is a plan view of the medial surface of the first plate of  FIG. 4A ; 
           [0018]      FIG. 5A  is a plan view of the lateral surface of a second plate forming part of the spinous process fixation system of  FIG. 3 ; 
           [0019]      FIG. 5B  is a plan view of the medial surface of the second plate of  FIG. 5A ; 
           [0020]      FIG. 6  is a perspective view of one example of a coupling element forming part of the spinous process fixation system of  FIG. 3 ; 
           [0021]      FIG. 7  is a postero-lateral view of a portion of the spine with one example of a spinous process fixation system according to a second embodiment of the present invention implanted on adjacent spinous processes of a spine; 
           [0022]      FIG. 8  is a posterior view of the spinous process fixation system implanted on adjacent spinous processes as shown in  FIG. 7 ; 
           [0023]      FIG. 9  is a perspective view of the spinous process fixation system of  FIG. 7 ; 
           [0024]      FIG. 10A  is a plan view of the lateral surface of a first plate forming part of the spinous process fixation system of  FIG. 9 ; 
           [0025]      FIG. 10B  is a plan view of the medial surface of the first plate of  FIG. 9 ; 
           [0026]      FIG. 11A  is a plan view of the lateral surface of a second plate forming part of the spinous process fixation system of  FIG. 9 ; 
           [0027]      FIG. 11B  is a plan view of the medial surface of the second plate of  FIG. 9 ; and 
           [0028]      FIG. 12  is a perspective view of one example of the coupling element forming part of the spinous process fixation system of  FIG. 9 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    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 spinous process plate systems for spinal fusion disclosed herein boasts a variety of inventive features and components that warrant patent protection, both individually and in combination 
         [0030]      FIGS. 1-2  illustrate a spinous process fixation system  10  according to one embodiment of the present invention. The system  10  includes a first plate  12 , a second plate  14 , and a coupling element  16 . The system  10  is designed to be attached to adjacent spinous processes SP 1 , SP 2  of the spine for immobilizing the adjacent spinous processes SP 1 , SP 2  to promote fusion therebetween. The system  10  may be used alone (that is, without any supplemental fusion devices, such as interbody fusion implants) as shown in  FIGS. 1-2 . Alternatively, the system  10  may be used with supplemental fixation devices (not shown). In either event, the system  10  allows fusion to occur between the adjacent spinous processes SP 1 , SP 2  by maintaining them in an immobilized, locked relationship such that a boney bridge can form therebetween. The formation of the fusion bridge between the adjacent spinous processes SP 1 , SP 2  may be augmented or facilitated by placing fusion-enhancing compounds between the spinous processes (such as, e.g. between the plates  12 ,  14 ), including but not limited to autologous bone, allograft bone, bone morphogenic protein (BMP), and/or any number of suitable biomaterials. 
         [0031]    The specifics of the system  10  will now be described with reference to  FIGS. 3-6 . As shown in  FIGS. 3-4B , the first plate  12  includes a central body portion  50  extending between a pair of end portions  52 ,  54 . The central body portion  50  may have a generally curved perimeter and (as best viewed in  FIGS. 4A-4B ) has a width less than the width of the end portions  52 ,  54 . The increased width of the end portions  52 ,  54  is designed to present a relatively large footprint on the adjacent spinous processes SP 1 , SP 2 , which helps in establishing a robust engagement therewith while avoiding protrusion beyond the spinous processes SP 1 , SP 2 . Although generally “hook” shaped in the embodiment shown, one of ordinary skill in the art will appreciate that the end portions  52 ,  54  may be provided in any number of suitable shapes including but not limited to generally rectangular, generally triangular, and generally rounded. As shown in  FIG. 4B , this engagement may be augmented through the use of a plurality of spike elements  22  disposed on the medial facing surface of the end portions  52 ,  54 . These spikes  22  are designed to become embedded in the lateral surface of the spinous processes SP 1 , SP 2  when the system  10  is compressed in place as shown in  FIG. 1 . 
         [0032]    The first plate  12  includes a central aperture  18  dimensioned to receive a distal portion of the threaded screw  16  as shown in  FIGS. 1 and 3 . More specifically, the central aperture  18  preferably includes an internal threading feature capable of threadedly cooperating with the threads of the screw  16 . As will be described in greater detail below, the threaded engagement between the screw  16  and the first plate  12  allows the first plate  12  to be coupled to the second plate  14 . The first plate  12  may also, according to one embodiment, include a pair of attachment apertures  20  positioned on either side of the central aperture  18 . Each attachment aperture  20  is dimensioned to receive an extension element of an insertion tool (not shown). By way of example only, attachment aperture  20  may include, but not be limited to circular holes, indentations, or bosses that allow for the engagement of an insertion tool. The insertion tool may be used to hold and manipulate the plate  12  as needed to properly position it on the desired spinous processes SP 1 , SP 2 . 
         [0033]    The first plate  12  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The first plate  12  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, according to one embodiment, the width of the central body portion may range from 5 mm to 20 mm, the width of the end portions  52 ,  54  may range from 7.5 mm to 25 mm, the length of the central body portion  50  may range from 1 mm to 65 mm, the length of the end portions  52 ,  54  may range from 7.5 mm to 25 mm, and the thickness of the plate  12  may range from 1.5 mm to 15 mm. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  10  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0034]    As shown in FIGS.  3  and  5 A- 5 B, the second plate  14  includes the same general features as the first plate  12 . The central body portion  50  extends between the end portions  52 ,  54 . The central body portion  50  may have a generally curved perimeter and (as best viewed in  FIGS. 5A-5B ) has a width less than the width of the end portions  52 ,  54 . The increased width of the end portions  52 ,  54  is designed to present a relatively large footprint on the adjacent spinous processes SP 1 , SP 2 , which helps in establishing a robust engagement therewith while avoiding protrusion beyond the spinous processes SP 1 , SP 2 . Although generally “hook” shaped in the embodiment shown, one of ordinary skill in the art will appreciate that the end portions  52 ,  54  may be provided in any number of suitable shapes including but not limited to generally rectangular, generally triangular, and generally rounded. As shown in  FIG. 5B , this engagement may be augmented through the use of a plurality of spike elements  22  disposed on the medial facing surface of the end portions  52 ,  54 . These spikes  22  are designed to become embedded in the lateral surface of the spinous processes SP 1 , SP 2  when the system  10  is compressed in place as shown in  FIG. 1 . 
         [0035]    The second plate  14  includes a central aperture  24  dimensioned to receive a proximal end of the threaded screw  16  as shown in  FIGS. 1 and 3 . More specifically, as best shown in  FIG. 5A , the central aperture  24  includes a partially spherical surface  25  dimensioned to receive the partially spherical head  28  of the threaded screw  16  as will be described in detail in  FIG. 6 . The second plate  14  also, according to one embodiment, includes attachment apertures  20  positioned on either side of the central aperture  24 . Each attachment aperture  20  is dimensioned to receive an extension element of an insertion tool (not shown). By way of example only, attachment aperture  20  may include, but not be limited to circular holes, indentations, or bosses that allow for the engagement of an insertion tool. The insertion tool may be used to hold and manipulate the plate  14  as needed to properly position it on the desired spinous processes SP 1 , SP 2 . 
         [0036]    The second plate  14  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The second plate  14  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, according to one embodiment, the width of the central body portion may range from 5 mm to 20 mm, the width of the end portions  52 ,  54  may range from 7.5 mm to 25 mm, the length of the central body portion  50  may range from 1 mm to 65 mm, the length of the end portions  52 ,  54  may range from 7.5 mm to 25 mm, and the thickness of the plate  12  may range from 1.5 mm to 15 mm. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  10  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0037]    Referring now to  FIG. 6 , the coupling element  16  according to the first exemplary embodiment of the present invention is a threaded screw having a partially spherical head  28  and a shaft  30  extending therefrom with a threaded portion  26 . The shaft  30  of the screw  16  is dimensioned to be passed through the central aperture  24  of the second plate  14  and then onward through the central aperture  18  of the first plate  12  to the point where the threaded portion  16  threadedly engages the threads along the interior of the central aperture  18  of the first plate  12 . As this occurs, the partially spherical head  28  of the screw  16  will be drawn into the partially spherical surface  25  of the central aperture  24  of the second plate  14 . The partially spherical head  28  has a larger diameter than the inner periphery of the central aperture  24  such that the head  28  cannot pass through the aperture  24  but rather cooperates with the surface  25 . The screw  16  will thus draw the first plate  12  closer to the second plate  14  as the screw  16  is advanced through apertures  24 ,  18 . This rotation may be accomplished through the use of any number of suitable driving devices, including but not limited to a screwdriver or Allen wrench capable of cooperating with corresponding features within the head  28  (e.g. Hexalobe-head grooves  31  in the screw head  28  shown in  FIG. 6 ). 
         [0038]    The screw  16  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The screw  16  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, according to one embodiment, the width of the screw  16  may range from 3 mm to 10 mm, the length of the screw  16  may range from 15 nm to 50 mm, and the threaded portion  26  may have any number of suitable thread pitches. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  10  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0039]      FIGS. 7-8  illustrate a spinous process fixation system  110  according to another embodiment of the present invention. The system  110  includes a first plate  112 , a second plate  114 , and a coupling element  116 . The system  110  is designed to be attached to adjacent spinous processes SP 1 , SP 2  of the spine for immobilizing the adjacent spinous processes SP 1 , SP 2  to promote fusion therebetween. The system  110  may be used alone (that is, without any supplemental fusion devices, such as interbody fusion implants) as shown in  FIGS. 7-8 . Alternatively, the system  110  may be used with supplemental fixation devices (not shown). In either event, the system  110  allows fusion to occur between the adjacent spinous processes SP 1 , SP 2  by maintaining them in an immobilized, locked relationship such that a boney bridge can form therebetween. The formation of the fusion bridge between the adjacent spinous processes SP 1 , SP 2  may be augmented or facilitated by placing fusion-enhancing compounds between the spinous processes (such as, e.g. between the plates  112 ,  114 ), including but not limited to autologous bone, allograft bone, bone morphogenic protein (BMP), and/or any number of suitable biomaterials. 
         [0040]    The specifics of the system  110  will now be described with reference to  FIGS. 9-12 . As shown in FIGS.  9  and  10 A- 10 B, the first plate  112  includes a central body portion  150  extending between a pair of end portions  152 ,  154 . The central body portion  150  may have a generally curved perimeter and (as best viewed in  FIGS. 10A-10B ) has a width less than the width of the end portions  152 ,  154 . The increased width of the end portions  152 ,  154  is designed to present a relatively large footprint on the adjacent spinous processes SP 1 , SP 2 , which helps in establishing a robust engagement therewith while avoiding protrusion beyond the spinous processes SP 1 , SP 2 . Although generally “hook” shaped in the embodiment shown, one of ordinary skill in the art will appreciate that the end portions  152 ,  154  may be provided in any number of suitable shapes including but not limited to generally rectangular, generally triangular, and generally rounded. As shown in  FIG. 10B , this engagement may be augmented through the use of a plurality of spike elements  122  disposed on the medial facing surface of the end portions  152 ,  154 . These spikes  122  are designed to become embedded in the lateral surface of the spinous processes SP 1 , SP 2  when the system  10  is compressed in place as shown in  FIG. 7 . 
         [0041]    The first plate  112  includes a central aperture  118  dimensioned to receive a distal end of the ridged bolt  116  as shown in  FIGS. 7 and 9 . More specifically, as best viewed in  FIG. 10A , the central aperture  118  preferably includes a series ridges or flanges  119  capable of cooperating with ridges along the bolt  116 . As will be described in greater detail below, the ridged engagement between the bolt  116  and the first plate  112  allows the first plate  112  to be coupled to the second plate  114 . The first plate  112 , according to one embodiment, includes a rectangular boss anti-rotation feature  144  as shown in  FIGS. 10A and 10B . A corresponding feature shown as a rectangular channel  146  in coupling element  116 , engages the anti-rotation feature  144 . This embodiment limits the rotation of the first plate  112  and second plate  114  relative to each other about the axis of the ridged bolt  116 . Further, the first plate  112  also, according to one embodiment, includes a pair of attachment apertures  120  positioned on either side of the central aperture  118 . Each attachment aperture  120  is dimensioned to receive an extension element of an insertion tool (not shown). The insertion tool may be used to hold and manipulate the plate  112  as needed to properly position it on the desired spinous processes SP 1 , SP 2 . 
         [0042]    The first plate  112  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The first plate  112  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, the width of the central body portion may range from 5 mm to 20 mm, the width of the end portions  152 ,  154  may range from 7.5 min to 25 mm, the length of the central body portion  150  may range from 1 mm to 65 mm, the length of the end portions  152 ,  154  may range from 7.5 mm to 25 mm, and the thickness of the plate  112  may range from 1.5 mm to 15 mm. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  110  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0043]    As shown in FIGS.  9  and  11 A- 11 B, the second plate  114  includes the same general features as the first plate  112 . The central body portion  150  extends between the end portions  152 ,  154 . The central body portion  150  has a generally curved perimeter and (as best viewed in  FIGS. 11A-11B ) has a width less than the width of the end portions  152 ,  154 . The increased width of the end portions  152 ,  154  is designed to present a relatively large footprint on the adjacent spinous processes SP 1 , SP 2 , which helps in establishing a robust engagement therewith while avoiding protrusion beyond the spinous processes SP 1 , SP. Although generally “hook” shaped in the embodiment shown, one of ordinary skill in the art will appreciate that the end portions  152 ,  154  may be provided in any number of suitable shapes including but not limited to generally rectangular, generally triangular, and generally rounded. As shown in  FIG. 11B , this engagement may be augmented through the use of a plurality of spike elements  122  disposed on the medial facing surface of the end portions  152 ,  154 . These spikes  122  are designed to become embedded in the lateral surface of the spinous processes SP 1 , SP 2  when the system  110  is compressed in place as shown in  FIG. 7 . 
         [0044]    The second plate  114  includes a central aperture  124  dimensioned to receive a proximal end of the ridged bolt  116  as shown in  FIGS. 7 and 9 . More specifically, as best shown in  FIG. 11A , the central aperture  124  is a “truncated spherical” recess having straight sides  127  and semi-spherical end regions  129 . The straight sides  127  and semi-spherical end regions  129  are dimensioned to receive the generally straight sides and semi-spherical end regions of the head  128  of the ridged bolt  116  as will be described in detail below. The second plate  114 , according to one embodiment, includes retaining feature  148  that captures coupling member  116  inserted through central aperture  118 . The second plate  114  also, according to one embodiment, includes attachment apertures  120  positioned on either side of the central aperture  124 . Each attachment aperture  120  is dimensioned to receive an extension element of an insertion tool (not shown). The insertion tool may be used to hold and manipulate the plate  114  as needed to properly position it on the desired spinous processes SP 1 , SP 2 . 
         [0045]    The second plate  114  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The second plate  114  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, the width of the central body portion  150  may range from 5 mm to 20 mm, the width of the end portions  152 ,  154  may range from 7.5 mm to 25 mm, the length of the central body portion  150  may range from 1 mm to 65 mm, the length of the end portions  152 ,  154  may range from 7.5 mm to 25 mm, and the thickness of the plate  114  may range from 1.5 mm to 15 mm. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  110  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0046]      FIG. 12  illustrates an example of a coupling element  116  according to one embodiment of the present invention. As shown, the coupling element  116  is a ridged bolt having a “truncated spherical” shaped head  128  and a shaft  130  extending therefrom with a ridged portion  126 . The shaft  130  of the bolt  116  is dimensioned to be passed through the central aperture  124  of the second plate  114  and then onward through the central aperture  118  of the first plate  112  to the point where the ridged portion  116  matingly engages the ridges or flanges  119  along the interior of the central aperture  118  of the first plate  112 . As this occurs, the truncated spherical head  128  of the bolt  116  will be advanced into the central aperture  124  of the second plate  114 . The truncated spherical head  128  has a larger diameter than the inner periphery of the central aperture  124  such that the head  128  cannot pass through the aperture  124  but rather cooperates in a “keyed” fashion with the central aperture  124 . Specifically, straight sides  131  and semi-spherical portions  133  of the head  124  cooperate with the straight sides  127  and semi-spherical end regions  129 , respectively, of the aperture  124 . The bolt  116  will thus couple the first plate  112  to the second plate  114  as the bolt  116  is advanced axially into engagement with the ridges or flanges  119  of the first plate  112 . In one embodiment, the ridges or flanges  119  are dimensioned such that the ridges  126  of the bolt  116  pass relatively easily through the aperture  118  towards the lateral facing surface of the first plate  112  but relatively difficulty in the opposite direction. In this manner, the first plate  112  and second plate  114  will be coupled in a secure manner on adjacent sides of the spinous processes SP  1 , SP 2 . Any of a variety of tools may be used to remove the bolt  116  from engagement with the flanges  119  of the central aperture  118  so as to disengage the first plate  112  from the second plate  114 . 
         [0047]    The bolt  116  may be constructed from any of a variety to suitable materials without departing from the scope of the invention, including but not limited to titanium, polymeric materials (e.g. plastics) carbon fiber, and/or any other biologically acceptable material. The bolt  116  may also be provided having any number of suitable dimensions without departing from the scope of the invention. For example, in one exemplary embodiment, the width of the bolt  116  may range from 3 mm to 11 mm, the length of the bolt  116  may range from 15 mm to 50 mm, and the ridged portion  126  may range from 5 mm to 47 mm. It will be appreciated, however, that these dimensions are provided as examples of those that may be employed with the system  110  of the present invention and any number of suitable modifications may be made depending upon a variety of factors without departing from the scope of the invention. 
         [0048]    The embodiments described herein are intended to rigidly fix two spinous processes relative to one another. The devices  10 ,  110  may be implanted via a traditional “open” procedure or a minimally invasive procedure. In a minimally invasive procedure, the devices  10 ,  110  may be implanted generally posteriorly through a single incision (e.g. where the first plates  12 ,  112  and second plates  14 ,  114  are passed through the same incision) or multiple incisions (e.g. where the first plates  12 ,  112  are passed through one incision and the second plates  14 ,  114  are passed through a second incision). During a uni-portal introduction, the surgeon may pass both the first plate  12 ,  112  and the second plate  14 ,  114  into position on either side of adjacent spinous processes SP 1 , SP 2  at the same time. During a bi-portal introduction, the surgeon may first insert the first plate  12 ,  112  to engage one side of the spinous processes SP 1 , SP 2  and then insert the second plate  14 ,  114  against the spinous processes SP 1 , SP 2 . In either event, the surgeon can adjust the position of the end portions  52 ,  54 ,  152 ,  154  on the first plate  12 ,  112  and second plate  14 ,  114  so that the spike members  22 ,  122  are engaged into the spinous processes SP 1 , SP 2 . At this point, compression instrumentation may be applied to press the plates toward each other, whereupon the spikes enter the spinal processes SP 1 , SP 2 . Following the full seating of the plates on the spinal processes SP 1 , SP 2 , the screw  16  and bolt  116  are tightened using any number of suitable instruments. When the surgeon is satisfied with the degree to which the first plate  12 ,  112  and second plate  14 ,  114  are locked together (e.g. if a desired torque level is applied to the screw  16 ), then the site may be closed up, completing the stabilization procedure. 
         [0049]    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.