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. The main body is formed from a material having a gradual variation in modulus.

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 extremitiesi 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 10000 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. This 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 indentified 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. 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 to provide dynamic stabilization. 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 bottom wall of the upper 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 upper saddle is configured to receive and support the spinous process of the L5 vertebra therein. 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. The lower saddle is not intended to engage and is not supported by the spinous process of the S1 vertebra. Rather the lower saddle merely provides a space into which that spinous process may extend when the implant is properly located in place. 
         [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 fix the implant in the desired location. These channels flare outwardly so the fixation device can extend to the pedicles of the S1 vertebra. For example, the channels may extend at an angle of about 60 degrees away from the sagittal plane toward the rear of the implant and at an angle of about 5 degrees toward the top of the implant in a direction from the front of the implant toward the rear of the implant. 
         [0014]    The spinal implant described herein may also define a passage that extends completely through the implant from one side of the implant to the other side of the implant. The passage may have a concavely curved trajectory when viewed from the top of the implant such that the openings on either side of the implant are generally aligned with a bottom portion of the upper saddle and the nadir of the passage is below and generally aligned along the sagittal plane with the lowest portion of the upper saddle bottom wall and the highest portion of the lower saddle top wall. A tether may extend through this passage. The curve of the passage facilitates a tether being threaded through the passage. 
         [0015]    The spinal implant described herein may be formed as a unitary body of a polymeric material having a gradual variation in modulus along and/or across its cross-section. For example, the modulus may vary along the height of the body such that the upper portion of the body is more flexible than the lower portion of the body. Alternatively, the modulus may vary along the width of the body such that the outer lateral portions of the body are stiffer than the central portion of the body. The modulus may also vary along the width and height of the body such that the outer lateral portions and the lower portion of the body are stiffer than the central and upper portion of the body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a front perspective view of one embodiment of a lumbar-sacral implant; 
           [0017]      FIG. 2  is a rear perspective view of the embodiment of a lumbar-sacral implant shown in  FIG. 1 ; 
           [0018]      FIG. 3  is a bottom perspective view of the embodiment of a lumbar-sacral implant shown in  FIG. 1 ; 
           [0019]      FIG. 4  is a rear elevation view of the embodiment of a lumbar-sacral implant shown in  FIG. 1 ; 
           [0020]      FIG. 5  is a cross-sectional view of the embodiment of a lumbar-sacral implant shown in  FIG. 1  taken along line V-V in  FIG. 3 ; 
           [0021]      FIG. 6  is a cross-sectional view of the embodiment of a lumbar-sacral implant shown in  FIG. 5  located between the L5 spinous process and the sacrum; 
           [0022]      FIG. 7  is a cross-sectional view of the embodiment of a lumbar-sacral implant shown in  FIG. 1  taken along line VII-VII  FIG. 3 ; 
           [0023]      FIG. 7A  is cross-sectional view similar to the view shown in  FIG. 7  showing an alternate embodiment of the lumbar-sacral implant shown in  FIG. 1 ; 
           [0024]      FIG. 8  is a side elevation view of the lumbar-sacral implant shown in FIG. 
           [0025]      FIG. 9  is a front elevation schematic view of the lumbar-sacral implant shown in  FIG. 1  depicting one embodiment of the varying durometer of the implant; 
           [0026]      FIG. 10  is a front elevation schematic view of the lumbar-sacral implant shown in  FIG. 1  depicting another embodiment of the varying durometer of the implant; 
           [0027]      FIG. 11  is a front elevation view of the lumbar-sacral implant shown in  FIG. 1  mounted on a spine; and 
           [0028]      FIG. 12  is a side elevation view of the lumbar-sacral implant shown in  FIG. 1  mounted on a spine. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    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. 
         [0030]    As used in this specification and the appended claims, the terms “upper”, “top”, “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. 
         [0031]    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 show in the FIGS., the sagittal plane is defined by the Y axis and the Z axis. 
         [0032]    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 spinal disorders, 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. 
         [0033]    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. t 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. 
         [0034]    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. 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. 
         [0035]    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 process 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. 
         [0036]    Implant  10  includes an upper saddle  20  defined by a pair of sidewalls  21   a  and  21   b  joined by a bottom wall  22 . Upper saddle  20  receives and supports the spinous process of the L5 vertebra therein. Upper saddle sidewalls  21   a  and  21   b  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. Implant  10  may have a variable radius (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. As shown in  FIG. 5 , upper saddle  20  may be oriented at about a 10 degree angle in the sagittal plane. The angle could be as large as about degrees. The surfaces forming upper saddle sidewalls  21   a  and  21   b  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 and support the spinous process of an L5 vertebra therein. The height of upper saddle sidewalls  21   a  and  21   b  should be chosen so that upper saddle sidewalls  21   a  and  21   b  prevent the upper portion of implant  10  from moving laterally out of engagement with the spinous process of the L5 vertebra. Upper saddle sidewalls  21   a  and  21   b  may extend between ⅓ and ½ of the base of the spinous process so they engage the lamina by about 2 to 3 mm. Upper saddle sidewalls  21   a  and  21   b  may not have a constant cross-section. This would allow 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  31   a  and  31   b  joined by a top wall  32 . As described in more detail below, 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. Lower saddle sidewalls  31   a , and  31   b  flare outwardly away from the sagittal plane toward the bottom of implant  10 . 
         [0037]    Upper saddle sidewalls  21   a  and  21   b  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 . Lower saddle sidewalls  31   a  and  31   b  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 and about 35 degrees in the sagittal plane. 
         [0038]    Implant  10  has outer sidewalls  11   a  and  11   b  that extend on either side of implant  10  from the upper portion of implant  10  to the lower portion of implant  10 . Outer sidewalls  11   a  and  11   b  flare outwardly away from the sagittal plane from the upper portion of implant  10  to give implant  10  a generally triangular shape. 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 sidewalls  11   a ,  11   b . 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   a  and  34   b . 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 and does not engage 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. The wider bottom portion of implant  10 , and indeed the overall configuration of implant  10 , also allow implant  10  to withstand higher forces being placed on it and helps to ensure compression forces placed on implant  10  are evenly distributed throughout the body of implant  10 . 
         [0041]    Each lower lobe  33   a  and  33   b  defines a channel  34   a  and  34   b  extending through implant  10 . Channels  34   a  and  34   b  allow a fixation device  60 , such as a cortical screw or similar device, to extend therethrough to fix implant  10  in the desired location on the spine. As such, the internal diameter of channels  34   a  and  34   b  should be sufficient to allow passage of fixation device  60  therethrough, but should not be so large as to allow too much “play”, or too big of a gap, between fixation device  60  and channels  34   a  and  34   b . For example, channels  34   a  and  34   b  could have an internal diameter that is about 0.5 mm to about 1 mm greater than the outer diameter of fixation device  60 . Channels  34   a  and  34   b  flare outwardly from about the mid-line of implant  10  and adjacent to the top of the bottom portion of implant  10  so that fixation device  60  can be located therein and extend to the pedicles of the S1 vertebra. For example, channels  34   a  and  34   b  may extend at an angle α of about 60 degrees away from the sagittal plane toward the rear of implant and at an angle β of about 5 degrees toward the top of implant  10  in a direction from the front of implant  10  toward the rear of implant  10 . Alternatively, angle α could be between about 45 degrees and about 60 degrees, while angle β could be between about 5 degrees and about 10 degrees. The wider bottom portion of implant  10 , and indeed the overall configuration of implant  10 , also allow implant  10  to withstand higher forces being placed on it and helps to ensure compression forces placed on implant  10  are evenly distributed throughout the body of implant  10 . 
         [0042]    Implant  10  may be formed as a unitary body of a polymeric material having a gradual variation in the modulus of elasticity along and/or across its cross section. For example, the modulus may vary along the height of the body such that the upper portion of the body is more flexible than the lower portion of the body. See for example  FIG. 9 , which shows this arrangement schematically. Alternatively, the modulus may vary along the width of the body such that the outer lateral portions of the body are stiffer than the central portion of the body. The modulus may also vary along the width and height of the body such that the outer lateral portions and the lower portion of the body are stiffer than the central and upper portion of the body. See for example  FIG. 10 , which shows this arrangement schematically. 
         [0043]    A material that may be formed into a product having a gradual variation in the modulus of elasticity, and the process for making the material, are disclosed in pending published U.S. Patent Application No. 2007/0050038, the entire contents of which are hereby expressly incorporated herein by reference. The material and process disclosed in the &#39;038 Publication would allow the inferior portion of implant  10 , or lower lobes  33   a  and  33   b , to be relatively stiff, i.e. have a higher modulus of elasticity, so that fixation device  60  can firmly affix implant  10  to the spine while ensuring that the inferior portion, or lower lobes  33   a  and  33   b , will not be pulled from fixation device  60  during flexion or other movement of the spine. Such pulling through of the implant is more likely if the inferior portion, or lower lobes  33   a  and  33   b , were formed from a flexible material. Conversely, the material and process disclosed in the &#39;038 Publication would allow the superior portion to be more elastic and flexible, i.e. have a lower modulus of elasticity. Having the superior portion be more flexible allows implant  10  to act as a shock absorber in extension while providing adequate stabilization to the L5/S1 level and allowing a more normal range of motion. In addition to having a gradual increasing of the modulus of elasticity from the top of implant  10  to the bottom of implant  10 , the modulus of elasticity could vary from the surface of implant  10  to the inner core of implant  10  such that the inner core would have a lower modulus of elasticity than the surface. In this way, the inner core would be more flexible than the surface. In addition, the modulus of elasticity could vary from the top and central portions of implant  10  to the sides and bottom of implant  10  such that the sides and bottom portion of implant  10  would have a higher modulus of elasticity than the top and central portions. In this way, the sides and bottom would be stiffer than the top and central portions. 
         [0044]    Implant  10  may also define a curved passage  80  that extends between outer sidewalls  11   a  and  11   b  of implant  10 . The curve of passage  80  may be defined by a radius of curvature of about 20 millimeters where the openings  85   a  and  85   b  to passage  80  are closer to the top of implant  10  than the nadir of passage  80 . Openings  85   a  and  85   b  are generally perpendicular to outer sidewalls  11   a  and  11   b . Other radii of curvature may also be used to define passage  80 . The nadir of passage  80  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 passage  80 . The curve of passage  80  facilitates tether  90  being threaded through passage  80  with a standard curved surgical needle. As shown in  FIGS. 11 and 12 , tether  90  may extend across the superior portion of the superior spinous process when implant  10  is located in the interspinous space. Tether  90  thus helps to maintain implant  10  in the proper position in the patient&#39;s anatomy during extension and flexion. It is to be understood that other fixation devices may be used instead of a tether  90 . For example, a pin, rod, screw or other similar mechanical device may be used and would extend through upper saddle  20  and into the upper spinous process. 
         [0045]    While various embodiments of the flexible interspinous process device 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 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.