Abstract:
Devices and methods are adapted to permit fixation and stabilization of the bony elements of the skeleton. The devices permit adjustment and maintenance of the spatial relationship between neighboring bones. The motion between adjacent skeletal segments may be maintained, limited or completely eliminated.

Description:
REFERENCE TO PRIORITY DOCUMENTS 
     This application is a continuation of co-pending U.S. patent application Ser. No. 12/072,695, filed Feb. 26, 2008, which claims priority of U.S. Provisional Patent Application Ser. No. 60/903,486 filed Feb. 26, 2007, U.S. Provisional Patent Application Ser. No. 60/921,570 filed Apr. 3, 2007, and U.S. Provisional Patent Application Ser. No. 60/926,839 filed Apr. 30, 2007. Priority of the aforementioned filing dates is hereby claimed and the disclosures of the Applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to devices and methods that permit fixation and stabilization of the bony elements of the skeleton. The devices permit adjustment and maintenance of the spatial relationship(s) between neighboring bones. Depending on the specifics of the embodiment design, the motion between adjacent skeletal segments may be maintained, limited or completely eliminated. 
     Spinal degeneration is an unavoidable consequence of aging and the disability produced by the aging spine has emerged as a major health problem in the industrialized world. Alterations in the anatomical alignment and physiologic motion that normally exists between adjacent spinal vertebrae can cause significant pain, deformity, weakness, and catastrophic neurological dysfunction. 
     Surgical decompression of the neural tissues and immobilization of the vertebral bones is a common option for the treatment of spinal disease. In addition to mechanical fixation, a bone graft or comparable bone-forming material is used to connect the vertebral bones and, with ossification of the graft material, the vertebral bodies are fused together by the bony bridge. Currently, mechanical fixation is most frequently accomplished by anchoring bone screws into the pedicle portion of each vertebral body and then connecting the various screw fasteners with an interconnecting rod. The screw/rod construct produces rigid fixation of the attached bones. 
     The growing experience with spinal fusion has shed light on the long-term consequences of vertebral immobilization. It is now accepted that fusion of a specific spinal level will increase the load on, and the rate of degeneration of, the spinal segments immediately above and below the fused level. As the number of spinal fusion operations have increased, so have the number of patients who require extension of their fusion to the adjacent, degenerating levels. The rigidity of the spinal fixation method has been shown to correlate with the rate of the degenerative progression of the adjacent segments. In specific, implantation of stiffer instrumentation, such as rod/screw implants, produced a more rapid progression of the degeneration disease at the adjacent segment than use of a less stiff fixation implant. 
     An additional shortcoming of the traditional rod/screw implant is the large surgical dissection required to provide adequate exposure for instrumentation placement. The size of the dissection site produces unintended damage to the muscle layers and otherwise healthy tissues that surround the diseased spine. A less invasive spinal fixation implant would advantageously minimize the damage produced by the surgical exposure of the spine. 
     Fixation of the spinous process segment of adjacent vertebrae provides a less rigid and less invasive method of vertebral fixation. Kapp et al. in U.S. Pat. No. 4,554,914 issued Nov. 26, 1985 disclosed a device of two elongated plates that are adapted to clamp onto adjacent spinous process. The plates are disadvantageously connected by locking bolts that transverse the substances of each spinous process. Bolts placed in this configuration will necessarily weaken the bony elements and lead to spinous process fractures and construct failure. Howland et al in U.S. Pat. No. 5,496,318, issued Mar. 5, 1996 disclosed the placement of an inter-spinous process spacer and encircling tension band to reduce vertebral motion. While the device can reduce vertebral flexion and extension, it can not effectively resist vertebral movement in the other motion planes. In U.S. Pat. No. 6,312,431 issued Nov. 6, 2001, Asfora disclosed a device comprised of two opposing plates that are interconnected by a malleable tether and adapted to capture the adjacent spinous processes between them. As with the Howland device, the fixation strength of this implant is limited by the mobile interconnecting tether. As such, neither implant can effectively immobilize the vertebral bones in all relevant motion planes. The lack of fixation significantly increases the possibility that the bone graft will not heal, the vertebral bones will not fuse, the construct will fail and the patient will develop chronic pain. 
     Superior immobilization devices were disclosed by Robinson et al. in U.S. Pat. No. 7,048,736 issued May 23, 2006 and by Chin et al. in U.S. Pub. Nos. 2007/0179500, 2007/0233082 and 2007/0270840. Each of these documents disclosed plates (or segments thereof) that engage each side of two adjacent spinous processes, wherein the plates are interconnected by a rigid member that resides within the interspinous space. Mechanical testing of the Robinson device was recently published by J C Wang et al. in the Journal of Neurosurgery Spine (2006 February; 4(2):160-4) and the text is hereby incorporated by reference in its entirety. The device was found to be weaker than conventional fixation techniques in all modes of vertebral movement and particularly lacking in fixation of rotational motion. Because of its limited stabilization properties, the device should be used in conjunction with additional implants. (See Wang J C et al. in the Journal of Neurosurgery Spine. 2006 February; 4(2):132-6. The text is hereby incorporated by reference in its entirety.) 
     As an additional shortcoming, the Robinson device can not be used to fixate the L5 vertebral bone to the sacrum. The spinous process of the first sacral vertebra is simply too small to permit adequate bone purchase and fixation with either the Robinson or Chin device. Since the L5/S1 level is a frequent site of spinal disease, the inapplicability of these devices at this level is a significant limitation of these implants. 
     In U.S. Pub. Nos. 2006/0036246, Carl and Sachs disclose a fixation device adapted to fixate the spinous process of one vertebral level to bone screws anchored into the pedicle portion of an adjacent vertebral level. While this invention would permit application at the L5/S1 level and circumvent one disadvantage of the aforementioned spinous process fixation plates, it relies on direct screw fixation into the distal aspect of the spinous process. This technique disadvantageously replicates the inadequate fixation characteristics of the Kapp device previously discussed (U.S. Pat. No. 4,554,914) and carries a high likelihood of spinous process fracture and complete construct failure. Indeed, the inventors try to address this design flaw by augmenting the strength of the spinous process through the use of an internal bone filler or an external brace. Regardless of these efforts, however, the disclosed device provides a cumbersome implant that carries a high likelihood of spinous process fracture and complete loss of vertebral fixation. 
     SUMMARY 
     The preceding discussion illustrates a continued need in the art for the development of a spinous process device and method that would provide superior vertebral fixation than existing spinous process implants. The device should be amenable to placement through a minimally invasive surgical approach. When vertebral fusion is desired, the device desirably provides adequate fixation in all movement planes so that the probability of bone graft healing is maximized. The implant would desirably provide less rigid fixation than traditional rod/screw fixation. 
     In the treatment of spinal disease, it is sometimes desirable to limit vertebral motion in one or more axis while maintaining movement in other motion planes. Vertebral segments that are treated using these motion preservation techniques will not be fused and a bone graft spanning the space between the vertebral bones is not employed. When motion preservation is desired, the device provides adequate fixation onto each attached vertebral bone while controlling the motion between them. Moreover, a hybrid device would advantageously provide fusion at one or more vertebral levels and motion preservation at other vertebral levels. 
     This application discloses novel implants and methods of implantation that address current deficiencies in the art. In an embodiment, there is disclosed an orthopedic device adapted to fixate the spinous processes of one vertebral bone to bone fasteners anchored into the pedicle portion of an adjacent vertebral body. The implant may capture the spinous process by using an encircling contoured rod or hooks. Alternatively, the implant may contain at least one barbed bone engagement member located on each side of the spinous process and adapted to forcibly abut and fixate into the side of the spinous process. The device further contains a locking mechanism that is adapted to transition from a first unlocked state wherein the device components are freely movable relative to one another to a second locked state wherein the device is rigidly immobilized and affixed to the bone. 
     Alternative embodiments of the aforementioned device are disclosed. In one embodiment, the device is adapted to fixate at least three vertebral bones. In that embodiment, the device captures the spinous processes of one vertebral bone and fixates it onto an elongated rod that is adapted to engage bone fasteners anchored into the pedicle portion of at least two additional vertebral bodies. In another embodiment, the device is adapted to attach onto the rod portion of an existing screw/rod construct and functions to extend the level of vertebral fixation. 
     In other embodiments, there is disclosed a series of orthopedic devices that are adapted to fixate onto the spinous processes of one vertebral bone and onto bone fasteners anchored into the pedicle portion of an adjacent vertebral body. The device provides controlled movement between the two attached vertebral bones. Multiple iterations of this device are illustrated. In some embodiments, bone graft or bone graft substitute may be used to fixate and fuse the device onto each of the anchored vertebral bones while still permitting movement between them. 
     In an alternative embodiment, the device also contains an elongated rod that is adapted to engage bone fasteners anchored into the pedicle portion of at least two additional vertebral bodies. This design feature produces a hybrid device that provides controlled motion between at least a first pair of vertebral bones and rigid immobilization between at least a second pair of vertebral bones. 
     In an additional embodiment, a implant is used to fixate onto the spinous process of each of two adjacent vertebral bone. The implant contains at least one barbed bone engagement member located on each side of the spinous process and adapted to forcibly abut and fixate into the side of the spinous process at each level. The implant allows controlled movement between the two attached spinous processes. The implant may further contain a cavity adapted to accept a bone graft or bone graft substitute so that, with bone formation, the device members may fuse onto the spinous processes and provide superior device adhesion to the vertebral bone. In another embodiment, a bone containment device is disclosed that is adapted to span the distance between the lamina of neighboring vertebrae. The device contains an internal cavity adapted to accept a bone graft or a bone graft substitute so that, with bone formation, the lamina of neighboring vertebral bones are fused together. 
     In one aspect, there is disclosed an orthopedic device adapted to fixate at least two vertebral bones, comprising: at least one bone engagement member located on each side of a spinous process of a first vertebra wherein the bone engagement member are each forcibly compressed and affixed onto the sides of the spinous process; a connector member adapted to interconnect each bone engagement members on one side of a spinous processes of a first vertebra with at least one bone fastener affixed to a second vertebra; a cross member extending across the vertebral midline and adapted to adjustably couple the bone engagement member and connector member on one side of the vertebral midline with the bone engagement member and the connector member on the other side of the vertebral midline; and a connection between the bone engagement members the connection comprising a connector member, and a cross member wherein the connection is capable of reversibly transitioning between a first state where the orientation between the bone engagement member, the connector member and the cross member is changeable in at least one plane and a second state where the orientation between the bone engagement member, the connector member and the cross member is rigidly affixed. 
     In another aspect, there is disclosed an orthopedic device adapted to fixate at least two vertebral bones, comprising: at least one bone engagement member located on each side of a spinous process of a first vertebra wherein the bone engagement member is forcibly compressed and affixed onto the sides of the spinous process; a connector member adapted to inter-connect each bone engagement members on one side of a spinous processes of a first vertebra with at least one rod that is used to inter-connect at least two bone fastener affixed to additional vertebral bones; a cross member extending across the vertebral midline and adapted to adjustably couple the bone engagement member and connector member on one side of the vertebral midline with the bone engagement member and connector member on the other side of the vertebral midline; and a connection between a bone engagement members, the connection comprising a connector member and a cross member wherein the connection is capable of reversibly transitioning between a first state where the orientation between the engagement member, the connector member and the cross member is changeable in at least one plane and a second state where the orientation between the engagement member, the connector member and the cross member is rigidly affixed. 
     In another aspect, there is disclosed an orthopedic device adapted to fixate at least two vertebral bones, comprising: at least one contoured rod that contacts at least one surface of the spinous process of a first vertebra; a connector member adapted to interconnect one end of the contoured rod that is located on one side of a spinous processes of a first vertebra with a bone fastener affixed to a second vertebra; and a device body member extending across the vertebral midline and adapted to adjustably couple at least one end of the contoured rod with the connector members wherein the device body member further contains at least one locking mechanism that is capable of reversibly transitioning between a first state wherein the orientation between the contoured rod and at least one connector member is changeable in at least one plane and a second state wherein the orientation between the contoured rod and at least one connector member is rigidly affixed. 
     In another aspect, there is disclosed an orthopedic device adapted to fixate at least two vertebral bones, comprising: at least one hook member that contacts at least one surface of the posterior aspect of a first vertebra; and a connector member adapted to interconnect one end of the hook member attached to the posterior aspect of a first vertebra with a bone fastener affixed to a second vertebra; a device body member extending across the vertebral midline and adapted to adjustably couple at least one hook member attached to the posterior aspect of a first vertebra the connector members wherein the device body member further contains at least one locking mechanism that is capable of reversibly transitioning between a first state wherein the orientation between the hook member and at least one connector member is changeable in at least one plane and a second state wherein the orientation between the hook member and at least one connector member is rigidly affixed. 
     In another aspect, there is disclosed an orthopedic device adapted to control motion between at least two vertebral bones, comprising: at least one bone engagement member located on each side of a spinous process of a first vertebra wherein the bone engagement member is forcibly compressed and affixed onto the sides of the spinous process; a connector member adapted to interconnect each bone engagement members on one side of a spinous processes of a first vertebra with at least one bone fastener affixed to a second vertebra, wherein the engagement member contains a channel adapted to accept an end of the connector member and wherein the motion permitted by the interaction of each of the two channel and connector member surfaces determines the motion profile permitted by the device; a cross member extending across the vertebral midline and adapted to adjustably couple bone engagement member and connector member on one side of the vertebral midline with the bone engagement member and connector member on the other side of the vertebral midline; and a connection between the bone engagement members and cross member wherein the connection is capable of reversibly transitioning between a first state where the orientation between the engagement member and the cross member is changeable in at least one plane and a second state where the orientation between the engagement members and the cross member is rigidly affixed. 
     Other features and advantages will be apparent from the following description of various embodiments, which illustrate, by way of example, the principles of the disclosed devices and methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows perspective views of an orthopedic implant adapted to fixate the spinous process of a first vertebral bone to screw fasteners affixed to the pedicle portion of a second vertebral bone. 
         FIG. 2  illustrates multiple views of the implant. 
         FIG. 3  shows an exploded view of the implant. 
         FIG. 4  shows a section view through the locking mechanism of the implant. 
         FIG. 5  shows a perspective view of the implant attached onto a segment of the spine. 
         FIGS. 6 and 7  illustrate multiple views of a second device embodiment. 
         FIG. 8  shows a partly exploded view of the second device embodiment. 
         FIGS. 9 and 10  illustrate multiple views of another device embodiment. 
         FIG. 11  illustrates a perspective view of a preferred embodiment of the current invention. 
         FIG. 12  shows the device of  FIG. 11  in multiple orthogonal planes. 
         FIG. 13  shows an exploded view of the implant. 
         FIGS. 14 and 15  illustrate cross-sectional views of the locking mechanism of the implant. 
         FIGS. 16 through 18  illustrate devices and methods for vertebral distraction in preparation for device placement. 
         FIG. 19  shows a method of vertebral and nerve decompression. 
         FIGS. 20 a -20 c    show the device of  FIG. 11  attached to the spine. 
         FIG. 21  illustrates the addition of a second device at an adjacent spinal level. 
         FIG. 22  shows an additional device embodiment that is adapted to fixate multiple vertebral levels. 
         FIG. 23  shows a perspective view of an alternative embodiment of the device shown in  FIG. 11 . 
         FIG. 24  shows the device of  FIG. 23  in multiple orthogonal planes. 
         FIG. 25A  shows the device of  FIG. 23  attached to a spine model. 
         FIG. 25B  illustrates a cross-sectional view wherein the spinous process fixation screw is shown. 
         FIG. 26A  shows another embodiment of the device of  FIG. 11  wherein the rods are replaced with paddle attachment members. 
         FIG. 26B  shows an exemplary embodiment of a paddle attachment member. 
         FIGS. 27 and 28  illustrate additional device embodiments. 
         FIG. 29  shows another device embodiment used to fixate three or more vertebral bones. 
         FIG. 30  shows the device of  FIG. 29  attached to the spine. 
         FIGS. 31 to 33  illustrate a device adapted to attach onto existing rod/screw instrumentation. 
         FIG. 34  shows a perspective view of a device embodiment adapted to preserve motion between the vertebral bodies. 
         FIG. 35  shows the device of  FIG. 34  in multiple orthogonal planes. 
         FIG. 36  illustrates an exploded view. 
         FIG. 37A  shows a cross-sectional view through the articulation mechanism. 
         FIG. 37B  shows a cross-sectional view through the locking mechanism. 
         FIG. 38  illustrates a perspective view of an additional device embodiment. 
         FIG. 39  shows the device of  FIG. 38  in multiple orthogonal planes. 
         FIG. 40  illustrates an exploded view. 
         FIGS. 41, 42 and 43  illustrate cross-sectional views at different points within the device. 
         FIG. 44  shows a perspective view of an alternate embodiment of the motion preservation device. 
         FIG. 45  illustrates a perspective view of an additional device embodiment. 
         FIG. 46  shows the device of  FIG. 45  in multiple orthogonal planes. 
         FIG. 47  illustrates an exploded view. 
         FIG. 48  shows a cross-sectional view through the locking mechanism. 
         FIG. 49  illustrates a perspective view of an alternative embodiment. 
         FIG. 50  shows an exploded view of the device of  FIG. 49 . 
         FIG. 51  shows an alternative embodiment of the device in  FIG. 49 . 
         FIGS. 52 and 53  illustrate additional device embodiments. 
         FIG. 54  illustrates another device embodiment. 
         FIG. 55  shows the device of  FIG. 54  in multiple orthogonal planes. 
         FIG. 56  shows an exploded view. 
         FIG. 57  illustrates an additional device embodiment. 
         FIG. 58  shows exploded views of the device. 
         FIG. 59  shows a sectional view through the locking mechanism and articulation surface. 
         FIG. 60A  shows the posterior aspect of a spine. 
         FIG. 60B  shows a bone containment implant in place at the L4/5 level. 
         FIG. 61A  shows a perspective view of a bone containment implant 
         FIG. 61B  illustrates the device of  FIG. 61A  in multiple orthogonal views. 
         FIG. 62  shows another embodiment of the bone containment implant in place at the L4/5 level. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1-3  show various views of an orthopedic device adapted to fixate the spinous process of a first vertebral bone to screw fasteners affixed to the pedicle portion of a second vertebral bone. The device includes a central member  110  having a pair of movably attached rods  115  extending outwardly therefrom. A central threaded bore  112  is contained in member  110  and serves as an attachment point for the device placement instruments. Each of the rods  115  has a ball-shaped head that is positioned inside a complimentary shaped seat inside the central member  110 . The spherical head is positioned into the seat inside member  110  and retained in place by collapsible “C” ring  116 . In the unlocked state, the spherical head of rod  115  is freely movable within the seat of member  110 . 
     A U-shaped rod  120  is also attached to the central member  110 . The rod  120  can be fixated to the central member  110  by tightening a pair of lock nuts  125  downwardly onto ends of the rod  120 . As shown in  FIG. 2 , the lock nuts  125  are positioned atop the heads of the rods  115 . This permits the lock nuts  125  to provide a downward force onto both the U-shaped rod  120  and the heads of the rods  115 . In this manner, the lock nuts  125  serve as a locking member that simultaneously locks the U-shaped rod  120  and the rods  115  to the central member  110 . The U-shaped rod  120  is adapted to fit around a spinous process of a vertebral bone. The rod  120  can have various shapes and configurations beside a U-shape that permits the rod to be fit around a spinous process. 
       FIG. 3  shows an exploded view of the device and  FIG. 4  shows a cross-sectional view of the device through the locking mechanism. A locking plug  305  is interposed between each of rods  120  and the spherical heads of rods  115 . As the locking nuts  125  are tightened downward onto the rod  120 , the locking plugs  305  are advanced onto the spherical heads of rods  115 , locking and immobilizing the rods  115  relative to the central member  110 . 
       FIG. 5  shows a perspective view of the device attached onto a segment of the spine. The vertebrae are represented schematically and those skilled in the art will appreciate that actual vertebral bones may include anatomical details that differ from those shown in  FIG. 5 . The U-shaped rod  120  is shaped such that it can wrap around or otherwise secure onto the spinous process of a vertebral body. The central member  110  is also positioned to contact the spinous process. The foot plate  118  of member  110  is preferably positioned beneath the lamina of the upper vertebral bone. The U-shaped rod  120  can be adjusted relative to the central member  110  prior to the actuation of the lock nuts. The rod  120  can adjustably slide relative to the central member  110  to accommodate spinous processes of various sizes. Preferably, the rod  120  is positioned around the spinous process in a manner that tightly captures the top surface of the spinous process against the central rod bend and the bottom surface of the spinous process or lamina against member  110 . After appropriate positioning of rod  120 , the free end of each rod  115  is rotated and placed into the rod-receiving seat of the previously placed bone fasteners  122 . The fastener lock nuts are tightened and the ends of rods  115  are immobilized relative to the fasteners. Subsequently, tightening of lock nuts  125  immobilizes rod  122 , rods  115  and central member  110  relative to one another and produce a rigid implant. As illustrated, the device fixates the spinous processes of a first vertebral bone to bone fasteners anchored into the pedicle portion of a second vertebral bone. 
       FIGS. 6 and 7  show another device embodiment. An exploded view is shown in  FIG. 8 . While similar to the previous embodiment, the current device uses rods  120  with terminal hooks  805  to attach onto the upper aspect of the spinous process or upper edge of the lamina of the upper vertebral bone. As shown in the exploded view of  FIG. 8 , the end  805  of each rod  120  is configured as a hook wherein the two hooks  805   a  and  805   b  can interfit with one another. The cylindrical end of each rod  120  is adapted to fit within complimentary bores  126  of member  110 . 
     As in the previous embodiment, central member  110  has a cavity adapted to accept the spherical head of each rod member  115 . “C” ring  116  retains the spherical heads attached to member  110  after device assembly. The locking mechanism of the device is similar to that of the previous embodiment. Advancement of lock nuts  125  immobilizes rods  120 , rods  115  and central member  110  relative to one another. The placement protocol is similar to that of the previous embodiment. However, as noted, hook member  805  may be alternatively attached onto the superior edge of the lamina of the upper vertebral bone. 
       FIGS. 9 and 10  illustrate multiple views of another device embodiment that fixates the spinous processes of one vertebral bone to bone fasteners anchored into the pedicle portion of an adjacent vertebral body. The vertebrae are represented schematically and those skilled in the art will appreciate that actual vertebral bones may include anatomical details that differ from those shown in these figures. In this embodiment, a U-shaped rod  120  is sized and shaped to wrap around the spinous process of a first vertebral body. Opposed ends of the rod  120  are coupled to bone fasteners such as bone screw assemblies  810 . The bone fasteners are attached to the pedicles of an adjacent vertebral body. Unlike the previous embodiments, this device does not include a central member. 
       FIGS. 11-13  show another embodiment of a device that fixates the spinous processes of one vertebral body to bone fasteners anchored into the pedicle portion of an adjacent vertebral body. The device includes a pair of central members  1105   a  and  1105   b  (collectively central members  1105 ) with opposed interior surfaces. Fixation members such as barbs  1107  are positioned on the interior surfaces such that the barbs face inward for attaching to a spinous process positioned between the central members  1105 . The central members  1105  are slidably mounted on a rod  1110  such that the central members  1105  can move toward and away from one another. In this manner, the size of the space between the central members  1105  can be adjusted to accommodate spinous processes of various sizes. Further, the orientation of members  1105  relative rod  1110  is adjustable in multiple planes. 
     Each rod  115  is coupled to a central member  1105  such that it extends outwardly therefrom. Rod  115  has a spherical head that is positioned inside a complimentary shaped seat inside a respective central member  1105  and retained in position collapsible “C” ring  116 . In the unlocked state, the spherical head of rod  115  is freely movable within member  1105  in a ball and socket manner. The end of each rod  115  can be attached to a bone fastener, such as pedicle screw assemblies  810 , that is anchored to the pedicle portion of a vertebral bone. 
     The top surface of each member  1105  contains a bore  1127 , which extends from the top surface to the cavity adapted to receive the spherical head of rod  115 . The upper aspect of bore  1127  is threaded. Bore  1127  is crossed by bore  1129 , wherein the latter bore extends from the lateral to the medial wall of member  1105 . A cross sectional view through the locking mechanism is shown in  FIGS. 14 and 15 . Spherical member  1410  has central bore  1412  and full thickness side cut  1414 , thereby forming a compressible “C” ring that can be compressed onto the contents of bore  1412 . In the assembled device, rod  1110  is positioned within central bore  1412  and can translate relative to it. With the application of a compressive load onto the outer surface of member  1410  by threaded locking nut  1125 , member  1410  is compressed onto rod  1110  and the latter is immobilized within bore  1412 . Retention pins  1145  are used to retain rod  1110  within member  1410  in the assembled device. 
     Advancement of each of lock nuts  1125  immobilizes rod  1110 , rod  115  and central member  1105  relative to one another and renders the device rigid. With reference to the cross-sectional views of  FIGS. 14 and 15 , tightening lock nut  1125  downwardly onto spherical member  1410  produces a compressive load onto rod  1110  and a downward force onto locking plug  1405 . The latter is pushed towards the spherical head of rod  115 , thereby immobilizing rod  115  within central members  1105 . In this manner, advancement of each lock nut  125  provides a downward force onto both rod  1110  and the spherical head of rod  115  contained with each member  1105 . Thus, each lock nut  125  serves as a locking member that simultaneously locks rod  1110  and rod  115  to the central member  1105 . 
     The spinal level to be implanted has an upper and a lower vertebral bone and the device is attached onto the posterior aspect of these vertebral bones. Prior to device placement, the upper and lower vertebral bones are distracted to facilitate decompression of the nerve elements.  FIG. 16  shows a perspective, assembled view of a distractor device. For clarity of illustration, the vertebral bodies are represented schematically and those skilled in the art will appreciate that actual vertebral bodies include anatomical details not shown in  FIG. 16 . The device generally includes a pair of anchors that include elongate distraction screws  1610  coupled to a platform  1615 . Each of the distraction screws  1610  is advanced into the posterior surface of a spinous process and follows a posterior to anterior trajectory along the long axis of the spinous process. The distal end of each screw includes a structure for attaching to the spinous process, such as a threaded shank. The proximal ends of the distraction screws  1610  are attached to the platform  1615 . The screws  1610  are axially positioned within sheaths  1619  that surround the screws and extend downwardly from the platform  1615 . 
     The distraction actuator  1622  is actuated to cause one of the distraction screws to slide along the rail  1621  such that it moves away form the other distraction screw. This applies a distraction force to the vertebral bodies to distract the vertebral bodies—as shown in  FIG. 17 . (In another embodiment, shown in  FIG. 18 , the distraction screws are replaced by clip members  1805  that couple to the spinous processes or lamina of the vertebral bodies. Other known methods of vertebral distraction may be alternatively used.) The decompression of the nerve elements is performed under distraction and it is schematically illustrated in  FIG. 19 . The bony and ligament structures that are compressing the nerves are removed from the lower aspect of the lamina of the upper vertebra and the upper aspect of the lamina of the lower vertebra (regions  1152 ). 
     Prior to device implantation, bone fasteners  810  had been placed into the pedicel portion of the lower vertebra on each side of the midline. A bone graft or bone graft substitute is packed with the facet joints and used to span the distance between the lamina of each of the upper and lower vertebra. The implant is positioned at the level of implantation such that opposing central members  1105  are disposed on either side of a spinous process of a the upper vertebral body. A compression device (not shown) attaches onto the lateral wall of each opposing central member  1105  at indentation  11055 . The compression device forcefully abuts the medial aspect of each central member  1105  against a lateral wall of the spinous process and drives spikes  1107  into the bone. Spikes  1107  provide points of device fixation onto the each side of the spinous processes. 
     With the compression device still providing a compressive force, the distal ends of rods  115  are positioned into the rod receiving portions of bone fasteners  810 . The locking nuts of the fasteners are actuated so that each rod  115  is locked within the respective fastener. Lock nuts  1125  are actuated, locking the device&#39;s locking mechanism and immobilize opposing central members  1105 , the interconnecting rod  1110  and rods  115  relative to one another. The compression device is removed, leaving the device rigidly attached to the upper and lower vertebral bones. 
       FIGS. 20 a -20 c    show the device of  FIG. 11  attached to the spine. As mentioned, the central members  1105  are spaced apart with a spinous process of an upper vertebra positioned in the space between them. The rods  115  are oriented so that they extend toward respective bone fasteners that are anchored to the pedicle portion of a lower vertebra. In this manner, the device fixates the spinous processes of one vertebral body to bone fasteners anchored into the pedicle portion of an adjacent vertebral body.  FIG. 21  illustrates the addition of a second device at an adjacent spinal level. Note that device can be used to fixate the L5 vertebra to the sacrum. 
       FIG. 22  shows another embodiment of a device that is similar to the device of  FIG. 11 . In this embodiment, the rods  115  have a length that is sufficient to span across multiple vertebral levels. This permits the device to be used to fixate multiple vertebral bodies across multiple levels to a spinous process of a single vertebral body. 
       FIGS. 23 and 24  show an alternative embodiment. In this device, at least one of the central members  1105  has a portion  2305  that extends outwardly and overhangs the space between the central members  1105 . The portion  2305  is sized, shaped, and contoured such that it can fit around the spinous process that is positioned between the central members  1105 . A bore  2310  extends through the portion  2305 . The bore receives a bone fastener, such as a bone screw, that can be driven into the posterior surface of the spinous process and having a posterior to anterior trajectory that substantially follows the long axis of the spinous process.  FIG. 25A  shows the device of  FIG. 23  attached to a spine model.  FIG. 25B  illustrates a cross-sectional view wherein the spinous process fixation screw  2510  is shown extending through the portion  2305  and into the spinous process. 
       FIG. 26A  shows another embodiment of the device of  FIG. 11  wherein rods  115  are replaced with paddle attachment members  2605 .  FIG. 26B  shows an exemplary embodiment of a paddle attachment member  2605 . The paddle attachment member  2605  is used in place of a rod  115 . The attachment member  2605  has a head that fits into the central member  1105  and also has an opening  2610  that can be coupled to a bone fastener, such as a pedicle screw assembly. 
       FIGS. 27 and 28  show additional embodiments of the device of  FIG. 11 . In these devices, a portion  2705  is sized and shaped to capture the inferior surface of the lamina of the upper vertebral bone. In the embodiment of  FIG. 27 , the portion  2705  extends outward from the rod  1110 . In the embodiment of  FIG. 28 , the portion  2705  extends outward from each of the central members  1105 . 
       FIG. 29  shows another device embodiment used to fixate three or more vertebral bones. In this embodiment, the central members  1105  are sufficiently long such that the spinous processes of one or more vertebral bodies can fit between the central members  1105 . The central members  1105  have barbs or other attachment means that are adapted to secure to the spinous processes. One end of each of the central members has a rod  115  movably attached thereto while the opposed end has another rod  117  movably attached thereto. The rods  115  and  117  can extend outward at any of a variety of orientations and angles relative to the central members. The rods  115  and  117  can be attached to pedicle screw assemblies for attaching the device to adjacent vertebral bodies. Thus, the device is adapted to fixate the spinous process of a middle vertebra to screw fasteners attached to the pedicle portions of an upper and a lower vertebra.  FIG. 30  shows the device of  FIG. 29  attached to a schematic representation of the spine. 
       FIGS. 31 to 33  illustrate a device adapted to attach onto existing rod/screw instrumentation and extend the fusion to a additional level. Each of two rods  3110  is attached to a pair of vertebral bodies in a conventional screw/rod fixation arrangement. Each rod  3110  is attached to two pedicle screw assemblies  3115 —as shown in  FIG. 31 . The extension device has a pair of central members  1105  that are positioned on opposed sides of a spinous process of an upper vertebra. Rods  115  extend outwardly from the device. The rods  115  movably attach to the rods  3110  via a pair of brackets  3120 . Perspective views of bracket  3120  are shown in  FIG. 33 . Each bracket is sized to receive a spherical end of rod  115  while also receiving a cylindrical segment of rod  3110 . Actuation of the locking screw  3130  of bracket leads to the upward movement of member  3150  and the immobilization of rod  3110  and the special head of rod  115  within bracket  3120 . A cross-sectional view of the locking mechanism is shown in  FIG. 32 . 
       FIG. 34  illustrates a device embodiment  605  adapted to fixate onto the spinous processes of one vertebral bone and bone fasteners anchored into the pedicle portion of an adjacent vertebral body. The device provides controlled movement between the two attached vertebral bones.  FIG. 35  shows the device in multiple orthogonal planes and  FIG. 36  shows the device components in an exploded view. Each of opposing body members  612  has a top surface, bottom surface, an outer side surface, an inner side surface and a front and back surface. Each medial surface contains spike protrusions  617  that are adapted to be driven into the side surface of a spinous process and serve to increase device fixation onto bone. The lateral surface contains opening  622  of channel  624  that is intended to receive the spherical head  632  of rod  634 . Movement of head  632  within channel  624  forms the mobile bearing surface of the implant. A cross-sectional view of head  632  contained within channel  624  is illustrated in  FIG. 37A . As shown, head  632  can move unopposed within channel  624 . In an alternative embodiment, a spring member is placed within channel  624  so that the position of head  632  is biased against movement away from a default position. Preferably, in the default position, head  632  is positioned at the end of channel  624  that is adjacent to bore  628 —as shown in  FIG. 34 . 
     The top surface of each body member  612  contains bore  628  adapted to accept a bone fastener  629 . Preferably, but not necessarily, bores  628  of the opposing body members  612  are angled in one or more planes so that the seated bone fasteners are not parallel. Non-parallel bore trajectories provide a crossed screw configuration and increased resistance to screw pull-out. As previously discussed, the seated screws may engage any portion of the lamina or spinous process bone but are preferably targeted and placed to engage the junction of the lamina and spinous process. 
     The top surface of each body member  612  contains a cavity  636  with full thickness bore holes  638  within the medial cavity wall. The cavity is adapted to accept a segment of bone graft or bone graft substitute and to function as a bone containment cage. With time, the graft material within cavity  636  of an implanted device  605  will fuse with the lateral wall of the spinous process and provide an additional attachment point with the underlying bone. Since it contains living bone tissue, ossification of the fusion mass will provide a stronger and more enduring bridge between the implant and vertebral bone than any mechanical fastener. 
     The top surface of each body member  612  contains a second bore  642 , wherein partial thickness bore  642  does not extend through to the bottom surface of the body member. The upper aspect of bore  642  is threaded. Bore  642  is crossed by bore  646 , wherein the full thickness bore  646  extends from the lateral to the medial wall of body member  612 . Bores  642  and  646  contain the device&#39;s locking mechanism. (A cross sectional view through the locking mechanism is shown in  FIG. 37B .) Spherical member  652  has central bore  654  and full thickness side cut  655 , thereby forming a compressible “C” ring that can be compressed onto the contents of bore  654 . In the assembled device, longitudinal member  658  is positioned within central bore  654  and can translate relative to it. With the application of a compressive load onto the outer surface of member  652  by threaded locking nut  656 , spherical member  652  is compressed onto longitudinal member  658  and the latter is immobilized within bore  654 . Retention pins  645  are used to retain longitudinal member  658  in the assembled device. In the assembled configuration, retention pins  647  are positioned within side cut  655  of spherical member  652  so as to limit the extent of rotation of opposing body members  612 . 
     The spinal level to be implanted has an upper and a lower vertebral bone and the device is attached onto the posterior aspect of the vertebral bones. Prior to device placement, bone fasteners  660  had been placed into the pedicel portion of the lower vertebra on each side of the midline. In addition, each side of the spinous process of the upper vertebra is gently decorticated in order to maximize the likelihood of bone (fusion) mass formation. Each of opposing body members  612  is placed on an opposite side of the spinous process of the upper vertebra. A compression device (not shown) is used to compress each body member  612  onto a side of the spinous process and drive the spike protrusions  617  into the bone surface. With the compression device still providing a compressive force, the distal ends of rods  634  are positioned into the rod receiving portions of bone fasteners  660 . Preferably, each head  632  is positioned at the end of channel  624  immediately adjacent to bore  628  prior to locking bone fasteners  660  onto rods  634 . This configuration assures that vertebral extension is limited to the position set at the time of surgery. The locking nuts of the fasteners are then actuated so that each rod  634  is locked within the respective fastener  660 . Locking nuts  656  of device  605  are then actuated, locking the device&#39;s locking mechanism and immobilize opposing body member  612  and the interconnecting longitudinal member  658  relative to one another. The compression device is removed, leaving the device rigidly attached to the upper and lower vertebral bones. Preferably, but not necessarily, cavity  636  is packed with bone graft or bone graft substitute so that, with time, a bone fusion mass connects the device to the side wall of the spinous process. If desired, a bone fastener  629  can be placed through each bore hole  628  into the underlying bone and further increase device fixation onto bone. 
     It is important to note that spike protrusions  617  and fastener  629  provide immediate device fixation to the upper vertebral level. With time, these fixation points may weaken from the cyclical device loading that invariably results during routine patient movement. Formation and ossification of the bone fusion mass contained within cavity  636  provides long-term fixation for the device. In contrast to spike and screw fixation, the fusion mass will increase in strength with time and provide a more permanent attachment point for the device. In this way, the immediate fixation of the spike and fasteners and the long-term fixation of the fusion mass compliment one another and provide optimal fixation for the device. 
     After device implantation, certain movements between the upper and the lower vertebras are permitted while other movements are limited. For example, the illustrated embodiment permits forward flexion of the upper vertebra relative to the lower vertebra. However, extension is limited by the position set at the time of implantation (that is, the position of head  632  within channel  624 ). Anterior translation of the upper vertebral bone relative to the lower vertebral bone is significantly limited so that aberrant motion resulting in spondylolisthsis is prevented. Lateral flexion between the vertebral bones is permitted but to a lesser degree than that of normal physiological vertebral motion. Vertebral rotation is substantially eliminated. 
     These limitations are determined by the interaction of heads  632  with channels  624  and can be varied by the shape and/or orientation of one or both of these structures. For example, extending the diameter of channel  624  in a medial to lateral direction will permit an increase in vertebral rotation. Further, a channel with lesser medial to lateral diameter at one end and a greater medial to lateral diameter at another end will permit a variable degree of rotational movement, wherein the extent of rotation depends of the extend of anterior flexion. This configuration can simulate physiological vertebral motion, wherein grater vertebral rotation is permitted in anterior flexion than in extension. As can be easily seen, numerous alternative motion characteristics can be produced by one of ordinary skill in the art through the simple manipulation of the shape and/or orientation of heads  632  and/or channels  624 . In addition, malleable members can be placed within channel  624  so that the position of head  632  is biased towards a default position and movement away from that position is opposed. 
     An alternative embodiment is shown in  FIG. 38 . While similar to the preceding embodiment, this device provides a cross-member that inter-connects the bone fasteners  660  so as to obviate the possibility of fastener rotation (along its long axis) within the pedicle portion of the bone. The cross member also increases the resistance to fastener pull-out from the lower vertebral bone.  FIG. 39  shows the device in multiple orthogonal planes. An exploded view is shown in  FIG. 40  and multiple cross-sectional views are shown in  FIGS. 41, 42 and 43 . 
     Device  685  is adapted to fixate onto the spinous processes of one vertebral bone and bone fasteners anchored into the pedicle portion of an adjacent vertebral body. As before, each of opposing body members  612  has side spikes  617 , a central cavity  636  adapted to accept a bone forming graft, and a locking mechanism adapted to immobilize body members  612  to interconnecting longitudinal member  658 . (A section view through the locking mechanism is shown in  FIG. 41 .) The top surface of each body member  612  contains bore  628  adapted to accept a bone fastener  629 . Side indentations  662  receive the compression device during device implantation. 
     The inferior surface of each body  612  contains opening  682  of channel  686 . Head  692  of rod  690  travels within channel  686  and forms the mobile bearing surface of the implant. Retention pin  681  ( FIG. 40 ) is used to retain head  692  within channel  682  and prevent device disassembly. As before the motion characteristics permitted by the implant are determined by the interaction of heads  692  with channels  686  and can be varied by the shape and/or orientation of one or both of these structures. (A section view through the bearing surface is shown in  FIG. 42 .) Examples of the possible configuration changes were previously discussed. In addition, malleable members can be placed within channel  682  so that the position of head  692  is biased towards a default position and movement away from that position is opposed. 
     Interconnecting rod  702  is used to attach the device onto the bone fasteners imbedded within the pedicel portion of the lower vertebral body. Rod  702  is comprised of telescoping segments  704  and  706  so that the rod length may be varied. Segment  704  contains rectangular protrusion  704  that, in the assembled state, is housed with a complimentary bore within segment  706 . A cross-sectional view through rod  702  is shown in  FIG. 43 . A side rod  690  with head  692  (bearing surface) is contained in each of segments  704  and  706 —as illustrated. The procedure for placement of device  685  is similar to the placement procedure previously described for device  605 . 
     An alternative device embodiment is illustrated in  FIG. 44 . While the portion of the device that attaches onto the spinous process of the upper vertebral bone is largely identical to that of device  605 , the current embodiment contains two contoured rods  712  that are adapted to attach bone fasteners at multiple vertebral levels. In use, bodies  612  attach onto the spinous process segment of an upper vertebral while contoured rod  712  attaches onto bone fasteners that are attached onto a middle and a lower vertebral level. As before, the bone fasteners are preferably, but not necessarily, anchored into the pedicle portion of the middle and lower vertebral bones. In this way, the current embodiment provides a hybrid device that permits vertebral movement between a first and second vertebral bones and complete immobilization (and fusion) between a second and third vertebral bone. Clearly, additional fasteners can be attached to contoured rod  712  to immobilize additional vertebral levels. This device is particularly adapted for use within the lower lumber spine where it is frequently desirable to immobilize and fuse the S1 and L5 vertebral levels and preserve motion between the L5 and L4 vertebral levels. 
       FIG. 45-48  show another embodiment of a device. The device includes central members  4510  that are slidably attached to a rod  4515  that extends through a bore  4513  in both of the central members  4510 . Each of the central members  4510  has a u-shaped slot  4517  that is sized to receive a contoured rod  115 . As in the previous embodiments, the central members are positioned on opposed sides of a spinous process and engaged thereto via spikes or barbs on the interior surface of the central members. 
     A pair of locking nuts  125  are positioned within boreholes of central members  4510  and adapted to produce a compressive force onto “C” ring  119  and interconnecting rod  4515 . A cross-sectional view of the locking mechanism is illustrated in  FIG. 48 . As illustrated in prior embodiments, each ember  4510  can move relative to rod  115  in one or more planes while in the unlocked state. With actuation of locking nuts  125 , members  4510  and rod  4515  are immobilized relative to one another. Rod  115  is affixed to fasteners that are attached to the pedicle portion of the lower vertebral level. Rod is freely movable within slot  4517 . In use, the device will preserve vertebral motion but prevent abnormal translational movement that produces spondylolisthesis. 
       FIG. 49  shows perspective views of an additional device embodiment while  FIG. 50  illustrates an exploded view. The present embodiment is similar to the preceding embodiment with the exception of placement of malleable members  131  between the interconnecting rod  4515  and rod  115 . The malleable member biases movement between the vertebral bones towards a default position and resists vertebral movement away from that position.  FIG. 51  illustrates an embodiment in which a cavity  242  is placed within each spinous process abutment member in order to accept a bone forming substance. As noted in pervious embodiments, this feature would permit device fusion onto the spinous process of the first vertebral bone. Further, a bone graft or bone graft substitute  252  is positioned so that rod  115  transverses a bore within member  252 . This feature permits the establishment of a bony fusion between rod  115  and the lamina or spinous process of the second vertebral bone. 
     Alternative device embodiments are shown in  FIGS. 52 and 53 . In either embodiment, the device is adopted to fixate three vertebral bones. In the embodiment of  FIG. 52 , the device anchors onto the spinous process of the middle vertebral level. Rod  890  is attached to bone fasteners that are anchored into the pedicle portion of the lower vertebral level. Rod  890  is freely movable within slot  892  of the spinous process attachment member. Rod  902  is attached to bone fasteners that are anchored into the pedicle portion of the upper vertebral level. Rod  902  is freely movable within slot  904  of the spinous process attachment member. In the embodiment of  FIG. 53 , rod  902  is freely movable within slot  904  whereas arms  888  rigidly attach onto the spinous process attachment member using the same mechanism as that shown in  FIG. 11 . In use, the embodiment of  FIG. 53  provides rigid fixation between the middle and lower vertebral levels while permitting movement between the upper and middle vertebral levels. 
     A perspective view of an additional embodiment is illustrated in  FIG. 54 . Multiple orthogonal views are shown in  FIG. 55  while an exploded view is shown in  FIG. 56 . Interconnecting rod  2012  has articulation member  2014  on each end. The spinous process engagement members and the locking mechanism of the device are similar to prior embodiments, such as that of  FIG. 45 . Rod  2022  is attached to bone fasteners anchored into the pedicle portion of the lower vertebral bone. Rod  2022  has triangular projections  2024  that articulate with articulation members  2014  of rod  2012 . The embodiment provides controlled movement between the two vertebral bones. 
     A perspective view of an additional embodiment is shown in  FIG. 57 . Exploded views are shown in  FIG. 58  and a cross-sectional view through the articulation surface is illustrated in  FIG. 59 . While similar to the prior embodiment, this device employs a different articulation mechanism. Spherical members  2106  are contained at the end of interconnecting rod  2102 . Two complimentary articulation surfaces  2112  are attached to rod  2114 . As shown in the cross-sectional view, the complimentary articulation surface  2112  contains a depression adapted to accept spherical member  2106  and, preferably, the depression is larger spherical member  2106  so as to permit some additional translational movement. That is, the articulations form a “loose” joint. 
       FIG. 60A  illustrates the posterior aspect of spine model whereas  FIG. 60B  shows the placement of bone forming material between the lamina of the L4 and L5 bones. The bone forming material may be an actual bone graft that is cut to the shape illustrated or a device adapted to contain bone graft or bone graft substitute.  FIGS. 61A  and B show perspective and orthogonal views of an exemplary graft containment device. As shown, the device preferably has a solid bottom that keeps the contained bone forming material form impinging upon the nerve elements. The sides may be open or solid. The top is preferably open and contains side protrusions  2302  that prevent anterior migration of the device into the spinal canal. An alternative device configuration is shown in  FIG. 62 . The latter device is intended to cross the vertebral midline, whereas the former is placed on either side of the vertebral midline. 
     The disclosed devices or any of their components can be made of any biologically adaptable or compatible materials. Materials considered acceptable for biological implantation are well known and include, but are not limited to, stainless steel, titanium, tantalum, shape memory alloys, combination metallic alloys, various plastics, resins, ceramics, biologically absorbable materials and the like. Any components may be also coated/made with osteo-conductive (such as deminerized bone matrix, hydroxyapatite, and the like) and/or osteo-inductive (such as Transforming Growth Factor “TGF-B,” Platelet-Derived Growth Factor “PDGF,” Bone-Morphogenic Protein “BMP,” and the like) bio-active materials that promote bone formation. Further, any surface may be made with a porous ingrowth surface (such as titanium wire mesh, plasma-sprayed titanium, tantalum, porous CoCr, and the like), provided with a bioactive coating, made using tantalum, and/or helical rosette carbon nanotubes (or other nanotube-based materials) in order to promote bone in-growth or establish a mineralized connection between the bone and the implant, and reduce the likelihood of implant loosening. Lastly, the system or any of its components can also be entirely or partially made of a shape memory material or other deformable material. 
     Although embodiments of various methods and devices are described herein in detail with reference to certain versions, it should be appreciated that other versions, embodiments, methods of use, and combinations thereof are also possible. At a minimum, any feature illustrates in one device embodiment may be alternatively incorporated within any other device embodiment. Therefore the spirit and scope of the appended claims should not be strictly limited to the description of the embodiments contained herein.