Abstract:
Medical devices for the treatment of spinal conditions are described herein. The medical device includes a main body that is adapted to be placed between the L5 vertebra and the sacrum so that the main body acts as a spacer with respect to the L5 vertebra and the sacrum to maintain distraction therebetween when the spine moves in extension. Channels are formed in the lower portion of the main body and allow a fixation device to extend through each channel at different angles. A locking mechanism is disposed in the channels to lock the fixation devices in each channel with respect to the device in a desired orientation.

Description:
BACKGROUND 
       [0001]    This invention relates generally to devices for the treatment of spinal conditions, and more particularly, to the treatment of various spinal conditions that cause back pain. Even more particularly, this invention relates to devices that may be placed between adjacent spinous processes to treat various spinal conditions. For example, spinal conditions that may be treated with these devices may include spinal stenosis, degenerative disc disease (DDD), disc herniations and spinal instability, among others. 
         [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 and other spinal conditions. The principal goal of surgery to treat lumbar spinal stenosis 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 and an incision is made in the patient to access the spine. The lamina of one or more vertebrae may be partially or completely removed to create more space for the nerves. 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]    The failures associated with a decompressive laminectomy may be related to postoperative iatrogenic spinal instability. To limit the effect of iatrogenic instability, fixation and fusion may also be performed in association with the decompression. In such a case, the intervertebral disc may be removed, and the adjacent vertebrae may be fused. A discectomy may also be performed to treat DDD and disc herniations. In such a case, a spinal fusion would be required to treat the resulting vertebral instability. Spinal fusion is also traditionally accepted as the standard surgical treatment for lumbar instability. However, spinal fusion sacrifices normal spinal motion and may result in increased surgical complications. It is also believed that fusion to treat various spinal conditions may increase the biomechanical stresses imposed on the adjacent segments. The resultant altered kinematics at the adjacent segments may lead to accelerated degeneration of these segments. 
         [0007]    As an alternative or complement to the surgical treatments described above, an interspinous process device may be implanted between adjacent spinous processes of adjacent vertebrae. The purposes of these devices are to provide stabilization after decompression, to restore foraminal height, and to unload the facet joints. They also allow for the preservation of a range of motion in the adjacent vertebral segments, thus avoiding or limiting possible overloading and early degeneration of the adjacent segments as induced by fusion. The vertebrae may or may not be distracted before the device is implanted therebetween. An example of such a device is the interspinous prosthesis described in U.S. Pat. No. 6,626,944, the entire contents of which are expressly incorporated herein by reference. This device, commercially known as the DIAM® spinal stabilization system, is designed to restabilize the vertebral segments as a result of various surgical procedures or as a treatment of various spinal conditions. It limits extension and may act as a shock absorber, since it provides compressibility between the adjacent vertebrae, to decrease intradiscal pressure and reduce abnormal segmental motion and alignment. Due to its design, this device also stabilizes motion in flexion. Thus, the DIAM® device provides stability in all directions and maintains the desired separation between the vertebral segments all while allowing motion in the treated segment. 
         [0008]    Although currently available interspinous process devices typically work for their intended purposes, they could be improved. For example, where the spacer portion of the implant is formed from a hard material to maintain distraction between adjacent vertebrae, 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. In addition, because of the human anatomy and the complex biomechanics of the spine, some currently available interspinous process devices may not be easily implantable in certain locations in the spine. 
         [0009]    The spine is divided into regions that include the cervical, thoracic, lumbar, and sacrococcygeal regions. The cervical region includes the top seven vertebrae identified as C1-C7. The thoracic region includes the next twelve vertebrae identified as T1-T12. The lumbar region includes five vertebrae L1-L5. The sacrococcygeal region includes five fused vertebrae comprising the sacrum. These five fused vertebrae are identified as the S1-S5 vertebrae. Four or five rudimentary members form the coccyx. 
         [0010]    The sacrum is shaped like an inverted triangle with the base at the top. The sacrum acts as a wedge between the two iliac bones of the pelvis and transmits the axial loading forces of the spine to the pelvis and lower extremities. The sacrum is rotated anteriorly with the superior endplate of the first sacral vertebra angled from about 30 degrees to about 60 degrees in the horizontal plane. The S1 vertebra includes a spinous process aligned along a ridge called the medial sacral crest. However, the spinous process on the S1 vertebrae may not be well defined, or may be non-existent, and therefore may not be adequate for supporting an interspinous process device positioned between the L5 and S1 spinous processes. 
         [0011]    Thus, a need exists for an interspinous process device that may be readily positioned between the L5 and S1 spinous processes without the reliance upon an S1 spinous process. Moreover, there is a need to provide an interspinous process device that can provide dynamic stabilization to the instrumented motion segment and not affect adjacent segment kinematics. 
       SUMMARY 
       [0012]    A spinal implant is described herein that is particularly adapted for placement between the spinous processes of the L5 vertebra and the S1 vertebra. The implant includes an upper saddle defined by a pair of sidewalls joined by a bottom wall. The upper saddle sidewalls may flare slightly outwardly away from the sagittal plane toward the top of the implant while the upper saddle bottom wall of the saddle may be concavely curved. In addition, the surfaces forming the upper saddle sidewalls and the upper saddle bottom wall extend in a direction, from the front of the implant to the rear of the implant, which is generally parallel to the sagittal plane. The implant also includes a lower saddle defined by a pair of sidewalls joined by a top wall. The lower saddle sidewalls flare outwardly away from the sagittal plane toward the bottom of the implant. In addition, the surfaces forming the lower saddle sidewalls extend in a direction, from the front of the implant to the rear of the implant, outwardly away from the sagittal plane. The lower saddle top wall may be concavely curved. In addition, the surface forming the lower saddle top wall extends in a direction, from the front of the implant to the rear of the implant, toward the top of the implant. 
         [0013]    The spinal implant described herein has outer sidewalls that extend on either side of the implant from the upper portion of the implant to the lower portion of the implant. The outer sidewalls flare outwardly away from the sagittal plane from the upper portion of the implant to give the implant a generally triangular-like shape. The wider bottom portion of the implant allows two lower lobes to be defined along the bottom portion of the implant adjacent to either side of the lower saddle. The lower lobes each define a channel extending through the thickness of the implant. The channels allow a fixation device to extend therethrough to engage the pedicles of the inferior vertebra and thus fix the implant in the desired location. The channels may extend at an angle of about 60 degrees away from the sagittal plane toward the 
         [0014]    The channels may be formed so that they allow the fixation device to extend through them at varying angles. This allows the surgeon to maneuver the fixation device along varying trajectories to engage the target pedicle at the desired location. This may be necessary given variations in the anatomy of different patients. In addition, a locking mechanism may be included in association with the channel to allow the trajectory of the fixation device to be fixed with respect to the spine to ensure that the implant maintains the desired orientation in the spine upon implantation. The locking mechanism may be a bushing disposed between the fixation device and the internal walls of the channel. In one orientation, the bushing may allow free rotation of the fixation device with respect to the channel and thus the implant. The shape of the bushing and the cross-section of the channel where the bushing is located are complementary to allow rotation of the bushing with respect to the channel and thus the implant. In another orientation, the bushing may lock the fixation device with respect to the channel and thus the implant. This locking feature may be achieved by using a fixation device that squeezes the bushing between the channel sidewall and the fixation device when the fixation device locks the implant to the spine. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a front perspective view of one embodiment of a lumbar-sacral implant allowing variable angle fixation; 
           [0016]      FIG. 2  is a rear perspective view of the embodiment of the lumbar-sacral implant shown in  FIG. 1 ; 
           [0017]      FIG. 3  is a schematic, partial cross-sectional view of the left portion of the embodiment of the lumbar-sacral implant shown in  FIG. 1  including a fixation device extending therethrough; 
           [0018]      FIG. 4  is a side elevation view of the lumbar-sacral implant shown in  FIG. 1  and including a fixation device extending therethrough; 
           [0019]      FIG. 5A  is a bottom cross-sectional view of another embodiment of the lumbar-sacral implant shown in  FIG. 1  taken along line  5 A- 5 A with a bushing disposed in each channel of the implant; 
           [0020]      FIG. 5B  is a schematic, partial cross-sectional view of the left portion of a further embodiment of the lumbar-sacral implant shown in  FIG. 1  showing a fixation device with a further embodiment of the bushing and channel of the implant; 
           [0021]      FIG. 5C  is a schematic, partial cross-sectional view of a left portion of a still further embodiment of the lumbar-sacral implant shown in  FIG. 1  showing a fixation device and a different configuration for the bushing and channel of the implant wherein the fixation device is not fully seated in the bushing; 
           [0022]      FIG. 5D  is a schematic, partial cross-sectional view of the left portion of the embodiment of the lumbar-sacral implant shown in  FIG. 5C  showing the fixation device locked with respect to the implant by the bushing disposed in the channel of the implant; 
           [0023]      FIG. 5E  is a schematic, partial cross-sectional view of a left portion of yet another embodiment of the lumbar-sacral implant shown in  FIG. 1  showing a fixation device with another configuration for the bushing and channel of the implant wherein the fixation device is not fully seated in the bushing; 
           [0024]      FIG. 5F  is a schematic, partial cross-sectional view of the left portion of the embodiment of the lumbar-sacral implant shown in  FIG. 5E  showing the fixation device locked with respect to the implant by the bushing disposed in a channel of the implant; 
           [0025]      FIG. 6  is a front elevation view of a lumbar-sacral implant similar to the one shown in  FIG. 1  mounted on a spine but wherein the implant has a two-piece configuration; 
           [0026]      FIG. 6A  is a side, cross-sectional view of the lumbar-sacral implant shown in  FIG. 6  illustrating the two-piece configuration; and 
           [0027]      FIG. 7  is a side elevation view of the lumbar-sacral implant shown in  FIG. 1  mounted on a spine. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    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, and “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 device end first inserted inside the patient&#39;s body would be the distal end of the device, while the device end last to enter the patient&#39;s body would be the proximal end of the device. 
         [0029]    As used in this specification and the appended claims, the terms “up”, “upper”, “top”, “down”, “lower”, “bottom”, “front”, “back”, “rear”, “left”, “right”, “side”, “middle” and “center” refer to portions of or positions on the implant when the implant is oriented in its implanted position, such as shown in  FIGS. 6 and 7 . 
         [0030]    As used in this specification and the appended claims, the term “axial plane” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into upper and lower parts. As shown in the FIGS., the axial plane is defined by the X axis and the Z axis. As used in this specification and the appended claims, the term “coronal plane” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into front and back parts. As shown in the FIGS., the coronal plane is defined by the X axis and the Y axis. As used in this specification and the appended claims, the term “sagittal plane” when used in connection with particular relationships between various parts of the implant means a plane that divides the implant into left and right parts. As shown in the FIGS., the sagittal plane is defined by the Y axis and the Z axis. 
         [0031]    As used in this specification and the appended claims, the term “body” when used in connection with the location where the device of this invention is to be placed to treat lumbar spinal stenosis, or to teach or practice implantation methods for the device, 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. A “body” may also refer to a model of a mammalian body for teaching or training purposes. 
         [0032]    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. Thus, 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. 
         [0033]    As used in this specification and the appended claims, the terms “normal”, “perpendicular” and “orthogonal” describe 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, perpendicular or orthogonal to a curved surface when the line and the curved surface intersect at an angle of approximately 90 degrees within a plane. Thus two geometric constructions are described herein as being “normal”, “perpendicular”, “orthogonal” or “substantially normal”, “substantially perpendicular”, “substantially orthogonal” to each other when they are nominally 90 degrees to each other, such as for example, when they are 90 degrees to each other within a tolerance. Such tolerances can include, for example, manufacturing tolerances, measurement tolerances or the like. 
         [0034]    A spinal implant  10  is described herein that is particularly adapted for placement between the spinous processes of the L5 vertebra and the S1 vertebra. However, it is to be understood that even though the following description of implant  10  is provided with reference to the L5 spinous process and the S1 spinous process, implant  10  may be used between other adjacent spinous processes and the discussion of the L5 spinous process may be interpreted to include any superior spinous process and the S1 spinous process may be interpreted to include the adjacent inferior spinous process. Such an implant is disclosed, for example, in U.S. patent application Ser. No. (Docket No. P0039292.00) filed on even date herewith, entitled Viscoelastic Lumbar-Sacral Implant and naming Eric C. Lange et al. as inventors, the entire contents of which are hereby expressly incorporated herein by reference. 
         [0035]    Implant  10  includes an upper saddle  20  defined by a pair of sidewalls  21   a  and  21   b  joined by a bottom wall  22 . The upper saddle sidewalls may flare slightly outwardly away from the sagittal plane toward the top of implant  10  while upper saddle bottom wall  22  may be concavely curved. The left upper saddle sidewall  21   a  is shown in the FIGS. It is to be understood that the right upper saddle sidewall may be a mirror image of left upper saddle sidewall  21   a . Implant  10  may have a variable radius, which may be from about 3.0 mm on the ventral face  12  to about 2.0 mm on the dorsal face  45 . This allows implant  10  to engage the L5 spinous process, which is usually thicker at the base. Upper saddle  20  may be oriented at about a 10 degree angle in the sagittal plane. The angle could be as large as about 20 degrees. The surfaces forming the upper saddle sidewalls and upper saddle bottom wall  22  may be generally parallel to the sagittal plane. This configuration for upper saddle  20  allows upper saddle  20  to receive the spinous process of an L5 vertebra therein. The height of the upper saddle sidewalls may be chosen so that the upper saddle sidewalls prevent the upper portion of implant  10  from moving laterally out of engagement with the spinous process of the L5 vertebra. The upper saddle sidewalls may extend between ⅓ and ½ of the base of the spinous process so they engage the lamina by about 2 to 3 mm. The upper saddle sidewalls may or may not have a constant cross-section. This allows upper saddle  20  to accommodate the variable thickness of the spinous process. Implant  10  also includes a lower saddle  30  defined by a pair of sidewalls joined by a top wall  32 . Left lower saddle sidewall  31   a  is shown in the FIGS. It is to be understood that the right lower saddle sidewall may be a mirror image of left lower saddle sidewall  31   a . Lower saddle  30  has a configuration to provide clearance of implant  10  over the S1 spinous process. As such, lower saddle  30  would not engage the spinous process of the S1 vertebra. The lower saddle sidewalls flare outwardly away from the sagittal plane toward the bottom of implant  10 . 
         [0036]    Implant  10  may also define a curved passage that extends between the outer sidewalls of implant  10 . The curve of this passage may be defined by a radius of curvature of about 20 millimeters where the openings to the passage, which are located on either side of implant  10 , are closer to the top of implant  10  than the nadir of the passage. Right side opening  85   b  is shown in the FIGS. It is to be understood that a left opening also exists on the left side of implant  10 . Other radii of curvature may also be used to define the passage. The nadir of the passage may be substantially aligned in the sagittal plane with the bottom most portion of upper saddle bottom wall  22  and the uppermost portion of lower saddle top wall  32 . A tether  90  may extend through the passage. The curve of the passage facilitates tether  90  being threaded through the passage with a standard curved surgical needle. 
         [0037]    The upper saddle sidewalls flare out and have a variable angle. The angle starts at about 40 degrees at the upper portion of upper saddle  20  and varies so that the angle is about 25 degrees at about the lowermost portion of upper saddle  20 . The lower saddle sidewalls flare out and have a constant angle between about 25 degrees and about 35 degrees. Lower saddle top wall  32  may be concavely curved or may have another configuration that allows the lower portion of implant  10  to be fixed to the S1 pedicles and minimizes any interference between the S1 spinous process and the rear of implant  10 . Lower saddle top wall  32  is inclined between about 30 degrees to about 35 degrees in the sagittal plane. 
         [0038]    Implant  10  has outer sidewalls that extend on either side of implant  10  from the upper portion of implant  10  to the lower portion of implant  10 . Right outer sidewall  11   a  is shown in the FIGS. It is to be understood that the left outer sidewall may be a mirror image of right outer sidewall  11   a . The outer sidewalls flare outwardly away from the sagittal plane from the upper portion of implant  10  to give implant  10  a generally triangular-like shape when viewed from the front or the back. See e.g.  FIG. 6 . In addition, the overall shape of implant  10  transfers load from the L5 spinous process to the S1 pedicles instead of to the S1 spinous process or the S1 laminae. This is especially helpful where implant  10  is used in the L5-S1 level since the small size and shape of the S1 spinous process may not provide adequate support for an implant. 
         [0039]    The front face  12  of implant  10  may have a curved profile that tapers from about 0 degrees along the middle of front face  12  to about 35 degrees adjacent to the outer sidewalls. Implant  10  may have a curvature radius of between about 20 mm and about 30 mm. The generally triangular shape, where the base is larger than the top, results in a constant pressure applied along the cross-sectional area of implant  10 . The shape of implant  10  also provides a better fit in the L5/S1 space and therefore offers stability for implant  10 . The rear of implant  10  has a stepped configuration and includes a shelf  40  separating the rear of implant  10  into an upper portion and a lower portion. Shelf  40  may be curved and is located so it is generally aligned with or above channels  34  and  35 . Shelf  40  acts as a transition between the upper and lower portions of the rear of implant  10  and ensures that implant  10  will fit properly in the patient&#39;s anatomy. The upper rear portion of implant  10  is defined by the rear wall  45 , which flares outwardly from the top of implant  10 . Rear wall  45  is curved such that it does not compete for engagement with upper saddle  20  but rather allows implant  10  to rest freely on the L5 lamina. This allows for easy implantation on the L5 level. The thickness of implant  10  gradually increases from the top of implant  10  to shelf  40 . This taper may be between about 30 degrees and about 50 degrees. The bottom rear portion of implant  10  has a thinner profile and provides clearance so that lower saddle  30  does not engage the inferior spinous process. This results in practically no load being transferred from implant  10  to the inferior spinous process. Indeed, lower saddle  30  may be configured such that it is spaced from the inferior spinous process when implant  10  is implanted in the patient. 
         [0040]    The wider bottom portion of implant  10  allows two lower lobes  33   a  and  33   b  to be defined along the bottom portion of implant  10  adjacent to either side of lower saddle  30  and provides an area through which implant  10  may be fixed to the spine. Each lower lobe  33   a  and  33   b  defines a channel  34  and  35 , respectively, extending through implant  10 . The wider bottom portion of implant  10 , and indeed the overall configuration of implant  10 , also allows implant  10  to withstand higher forces being placed on it and helps to ensure that the compression forces placed on implant  10  are evenly distributed throughout the body of implant  10 . 
         [0041]    Each of channels  34  and  35  allows a fixation device  60 , such as a cortical screw or similar device, to extend through each of them to engage the pedicles of the inferior vertebra and thus fix implant  10  in the desired location on the spine. The internal diameter of channels  34  and  35 , at a minimum, should be sufficient to allow passage of fixation device  60  therethrough. In addition, each channel  34  and  35  may each have a converging tapered portion  34   a  and a diverging tapered portion  34   b  that are joined at a narrowed waist portion  34   c  along a medial portion of each channel. See  FIG. 3 . Although the right side of implant  10  is not shown, it is to be understood that right channel  35  has the same general configuration as left channel  34 . Waist portion  34   c  acts as a pivot location for fixation device  60 . The combination of converging portion  34   a  adjacent to the front of implant  10 , narrowed waist portion  34   c  and diverging portion  34   b  adjacent to the rear of implant  10  allows each fixation device  60  to extend through each channel  34  and  35  and allows the surgeon to maneuver fixation device  60  along varying trajectories to engage the target pedicle at the desired location. For example, converging portion  34   a  and diverging portion  34   b  may be configured to allow fixation device to extend along a lateral, i.e. side to side, angle α, which may be between about 30 degrees and about 60 degrees, and along a superior-inferior, i.e. up and down, angle β, which may be between about 5 degrees and about 10 degrees. Thus in one embodiment, converging portion  34   a  and diverging portion  34   b  may have an asymmetrical cone-like cross-section where the angle of the side walls taper at an angle of between about 30 degrees and about 60 degrees and the angle of the top and bottom walls taper at an angle of between about 5 degrees and about 10 degrees. These angles of course would smoothly transition together around the circumference of these portions of channels  34  and  35 . Alternatively, the cross-sections of converging portion  34   a  and diverging portion  34   b  could be symmetrical and taper at an appropriate angle between about 15 degrees and about 75 degrees. 
         [0042]    As shown in  FIGS. 5A through 5F , bushings  700   a  and  700   b  may be disposed in respective channels  34 ′ and  35 ′ through which respective fixation devices  60  may extend. Bushings  700   a  and  700   b  may be formed of any biocompatible material and may be relatively rigid, such as PEEK or titanium. Bushings  700   a  and  700   b  may be generally spherical, i.e. have a generally circular cross-section, with a passage  710   a  and  710   b  therethrough, to allow fixation device  60  to pass through bushings  700   a  and  700   b . Similarly, channels  34 ′ and  35 ′ may have a generally spherical cross-section in the area where bushings  700   a  and  700   b  are to be located to hold bushings  700   a  and  700   b  in place. The spherical configuration of bushings  700   a  and  700   b  and channels  34 ′ and  35 ′ allow bushings  700   a  and  700   b  to freely rotate within channels  34 ′ and  35 ′. This in turn allows fixation device  60  to be rotated within channels  34 ′ and  35 ′ with some degree of control because bushings  700   a  and  700   b  provide a bearing surface in the gap between the respective fixation device  60  and the inner surface of the respective channel  34 ′ and  35 ′. 
         [0043]    If desired, the rear portion of channels  34 ′ and  35 ′ may flare outwardly so that the cross-section of the channels flare toward a larger diameter at the rear of the implant. See element  34   b ′ in  FIG. 5B . It is to be understood that the right channel may have the same general configuration. In other words, the diameter of channels  34 ′ and  35 ′ increases from the spherical portions to the rear of the implant. This configuration allows room in channels  34 ′ and  35 ′ for fixation device  60  to have wide latitude in rotating within these channels. The particular angle of the taper for this portion of channels  34 ′ and  35 ′ may be as described in connection with the embodiment shown in  FIG. 3 . 
         [0044]    In a further embodiment, the fixation device used with the implant described herein may have a tapered shaft portion  601   a  that tapers to a smaller diameter toward its distal end. Bushing  700   a ′ may also define a tapered passage therethrough to mate with tapered shaft portion  601   a . See  FIGS. 5C and 5D . Although only the left portion of the implant is shown, it is to be understood that the right channel, right bushing and right fixation device have the same configuration as the left channel, left bushing and left fixation device. During implantation, the surgeon will locate implant  10  in the interspinous space between the L5 vertebra and the S1 vertebra. Once the surgeon locates implant  10  in the desired location, fixation device  600   a  may be inserted through channels  34 ′ and  35 ′. The trajectory of fixation device  600   a  may be adjusted during this phase of the operation to ensure that the distal end of fixation device engages the desired location of the target pedicle. Fixation device  600   a  is able to rotate with respect to the implant in this orientation because bushing  700   a  has not been squeezed between the channel sidewalls and the fixation device since the narrower portion of the shaft of fixation device  600   a  is in engagement with passage  710   a . See  FIG. 5C . Once fixation device  600   a  is properly aligned, it is driven into the pedicle. Continued movement of fixation device  600   a  into the pedicle forces tapered portion  601   a  into passage  710   a ′ so that tapered portion  601   a  fully engages bushing  700   a  thus squeezing bushing  700   a  within channel  34 ′ by fixation device  600   c . See  FIG. 5D . In this configuration, fixation device  600   a  is locked in position with respect to the implant. As shown in  FIGS. 5C and 5D , the implant may include a grommet  70  disposed about the channels. Such a grommet  70  may have different hardness characteristics to ensure that the bushings are locked in place with respect to the implant. Alternatively, the bottom portion of the implant may be formed from a separate material that is more rigid than the top of the implant. See  FIGS. 6 and 6A . 
         [0045]    Another embodiment of a fixation device that may be used with the implant described herein is shown in  FIGS. 5E and 5F . Fixation device  600   b  may have a tapered shaft portion and a nut to force the tapered shaft portion into cooperation with bushing  700   a ″ and the channel to lock the fixation device with respect to the implant. Although only the left portion of the implant is shown in the FIGS., it is to be understood that the right channel, right bushing and right fixation device have the same general configuration as the left channel, left bushing and left fixation device. As shown in  FIGS. 5E and 5F , fixation device  600   b  includes a tapered portion  615  located between a proximal portion  610  and a distal portion  620 . The taper of tapered portion  615  is reverse to the taper shown in  FIGS. 5C and 5D  such that the taper of tapered portion  615  increases in diameter in the distal direction. Bushing  700   a ″ also defines a passage  710   a ″ that has a taper that increases toward the rear of each bushing. Proximal portion  610  is threaded to receive a nut  800  thereon. Distal portion  620  is threaded so fixation device  600   b  can be driven into a pedicle. During implantation, the surgeon inserts fixation device  600   b  into the pedicles first. Once, the fixation devices are properly located, the surgeon may place implant  10  in the interspinous space between the L5 vertebra and the S1 vertebra over the fixation devices through the channels. When the implant is partially placed over the fixation devices, the bushings are able to rotate with respect to the implant in this orientation because the bushings have not been squeezed within the channel by the fixation device since the larger tapered portion of the shaft of the fixation device is not in engagement with the bushing passage. See  FIG. 5E . Once the implant is properly aligned, it is driven completely over the fixation devices so that the larger tapered portions fully engage the bushings thus squeezing the bushings in the channels. See  FIG. 5F . Nut  800  is disposed over proximal portion  610 , threaded over the threads and rotated to engage the front of bushing  700   a ″. In this configuration, fixation device  600   b  is locked in position with respect to the implant. 
         [0046]    While various embodiments of the flexible interspinous process device and delivery system have been described above, it should be understood that they have been presented by way of example only, and not limitation. Many modifications and variations will be apparent to the practitioner skilled in the art. The foregoing description of the flexible interspinous process device and delivery device is not intended to be exhaustive or to limit the scope of the invention. It is intended that the scope of the invention be defined by the following claims and their equivalents.