Abstract:
A spinal fixation device is described. The device can safely promote fusion across one or more vertebral fusing levels while simultaneously supporting vertebral motion at other levels.

Description:
REFERENCE TO PRIORITY DOCUMENT  
       [0001]     This application claims priority of the following co-pending U.S. Provisional Patent Applications: (1) U.S. Provisional Patent Application Ser. No. 60/763,047, filed Jan. 26, 2006; and (2) U.S. Provisional Patent Application Ser. No. 60/800,959, filed May 17, 2006. Priority of the aforementioned filing dates is hereby claimed and the disclosure of the Provisional Patent Applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND  
       [0002]     Spinal disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. Currently, resection of the painful disc and fusion of the adjacent vertebral bodies has emerged as the most common surgical treatment of degenerative disc disease.  
         [0003]     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 the spinal segments immediately above and below the fused level. Further, as a consequence of fusion, each adjacent disc will experience a displaced center of rotation and produce an aberrant motion profile. The increased load and abnormal movement experienced by the adjacent discs will synergistically act to accelerate the rate of degeneration at these levels. Consequently, the number of patients who require extension of their fusion to the adjacent, degenerating levels has increased with time.  
         [0004]     In the cervical spine, many individuals have degenerative changes of varying severity at multiple discs. When pain, weakness and other symptoms arise, it is not uncommon to find that the symptomatic disc is surrounded by diseased but less-degenerated adjacent disc levels. Unfortunately, resection and fusion of the symptomatic disc will increase the load on the adjacent segments and accelerate the rate of degeneration at those levels. With time, the adjacent disc levels will also require resection and fusion. The second procedure necessitates re-dissection through the prior, scarred operative field and carries a greater risk of complications than the initial procedure. Further, extension of the fusion will increase the load on the motion segments that now lie at either end of the fusion construct and will accelerate the rate of degeneration at those levels. Thus, spinal fusion begets additional, future fusion surgery.  
         [0005]     It would be advantageous to treat the symptomatic level while minimizing the negative biomechanical consequences of fusion on the adjacent disc levels. Clearly, a device that can promote fusion at the desired level(s) while maintaining and supporting vertebral motion at other level(s) is needed. U.S. Pat. Nos. 6,293,949 and 6,761,719 illustrate a method of dynamic vertebral stabilization. In that invention, bone screws are placed into each of two vertebral bodies and a malleable member is used to connect them. The malleable member dampens movement between the vertebral bodies and returns the vertebrae to the neutral position after the force acting upon the construct has dissipated.  
         [0006]     Unfortunately, the devices illustrated cannot accommodate vertebral fusion. During fusion, bone re-absorption at the bone/graft interface is the first step in the healing process. After re-absorption, the fusing bones must settle and reestablish contact with one another in order for the fusion to progress. Since the devices illustrated in the referenced patents are designed to return the vertebrae to the neutral position, they will actively oppose bone settling and forcefully separate the vertebral bodies as they try to re-establish bony contact. Thus, placement of these devices across a disc level that is to be fused would inhibit bone healing, preclude formation of the fusion mass and insure failure of the bony fusion.  
         [0007]     The vertebral bodies immediately adjacent to a fused disc space will exhibit abnormal motion characteristic and this motion profile will accelerate the degenerative process. The disc space above the fused level, for example, will experience a downward migration of the center of rotation so that the upper vertebral body will follow a substantially spherical path of greater radius (i.e., lesser curvature) in the sagittal (anterior-posterior) plane relative to the lower body. The alteration in trajectory will produce greater translational movement of the upper vertebral body in the anterior-posterior plane and subject the intervening disc to a significant increase in shear forces. The devices illustrated in the referenced patents do not correct the aberrant motion seen adjacent to a fused segment and, in fact, make no attempt to favor any particular motion pattern. Since the devices are attached to bone at each end and have an intervening malleable member of uniform design and resistance, it is impossible for them to simultaneously support the widely divergent motion requirements of a fusion at one level and a mobile segment at another level.  
         [0008]     The referenced devices have a bellows-like design that may entrap, pinch and injure the surrounding soft tissues within the expanding and contracting folds of the moving implant. The use of super-elastic materials for device manufacture will only add to the extent of travel and further risk tissue entrapment. Since the device is placed onto the anterior aspect of the cervical spine, it is positioned immediately adjacent to the esophagus and the pharynx and may injure these structures with movement. The vast experience gained with bone plate fixation of this region has unequivocally shown that injury of the pharynx and/or esophagus is among the most feared surgical complications. Should injury occur, serious infection with significant risk of long term morbidity or even mortality will almost certainly develop. Further, the malleable member may fracture with repetitive movement. With failure, these devices can fragment and produce sharp subsegments that can injure the critical tissues contained within the intended area of implantation. In short, the placement of an uncontained bellows-like mechanism immediately behind these critical soft-tissue structures is dangerous.  
       SUMMARY  
       [0009]     There remains a need in the art for a device that can safely promote fusion across one or more fusing levels while simultaneously supporting vertebral motion at other levels.  
         [0010]     In one embodiment, a hybrid fixation device is illustrated. In one segment, the device is adapted to span and accommodate a fusing segment while at another segment the device has a malleable member that supports vertebral motion. The malleable segments is contained within a biocompatible sheath or membrane that serves to contain implant ware debris, keep the soft tissue out of the mobile implant sub-segments, contain implant fragments in case of failure and, if desired, allow placement and containment of a biocompatible lubricant. Multiple embodiments of the fusion fixation segment are provided. A modular device is also provided.  
         [0011]     In an additional embodiment, a multi-segmental device that limits vertebral motion to a spherical path is illustrated. The device can be used to define the center of rotation and correct aberrant motion patterns. In another embodiment, the device is fitted with a malleable member so that it can support vertebral motion. Additional versions illustrate the addition of an intra-disc attachment that can also define the center of rotation for the motion segment. Multiple embodiments are illustrated where the motion of the attached vertebral bodies is supported by malleable members of various designs. Any of the disclosed dynamic implants may be coupled to segments that accommodate fusion so that the hybrid assembly can support fusion at one level and dynamic motion at another level. Finally, the addition of a biocompatible sheath or membrane is illustrated for a rod-based dynamic stabilization implant.  
         [0012]     The implants described in this application can safely promote fusion across one or more fusing levels while simultaneously supporting vertebral motion at other levels. 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  
       [0013]      FIG. 1  is a perspective view of a dynamic bone fixation device configured to retain bone portions such as vertebra of a spinal column in a desired spatial relationship.  
         [0014]      FIG. 2A  shows an assembled view of the device.  
         [0015]      FIG. 2B  shows an exploded view of the bone fixation device.  
         [0016]      FIG. 3  shows the device attached to the vertebrae V 1 , V 2 , and V 3 .  
         [0017]      FIGS. 4 and 5  show top plan views of alternate embodiments the device that permits limited movement between the vertebrae V 2  and V 3 .  
         [0018]      FIGS. 6 and 7  show another alternate embodiment of the device.  
         [0019]      FIG. 8  shows another embodiment of a bone fixation device in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  
         [0020]      FIG. 9  shows a first perspective view of the device of  FIG. 8  in an exploded state.  
         [0021]      FIG. 10  shows a second perspective view of the device of  FIG. 8  in an exploded state.  
         [0022]      FIG. 11  shows a cross-sectional view of the assembled device attached to the vertebrae V 1  and V 2 .  
         [0023]      FIG. 12  shows another embodiment of a dynamic bone fixation device in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  
         [0024]      FIG. 13  shows a first perspective view of the device of  FIG. 12  in an exploded state.  
         [0025]      FIG. 14  shows a perspective view of the device of  FIG. 12  in an exploded state.  
         [0026]      FIG. 15  shows various view of a malleable member  1420 .  
         [0027]      FIGS. 16   a - 16   c  show alternate embodiments of the device.  
         [0028]      FIGS. 17 and 18  show perspective and cross-sectional view of an embodiment of the device.  
         [0029]      FIGS. 19-20  show perspective and cross-sectional views of an embodiment of the device that is similar to the device shown in  FIGS. 17 and 18 .  
         [0030]      FIG. 21  shows a perspective view of an embodiment of a dynamic bone fixation device in an assembled state and attached to a pair of vertebrae V 2  and V 3 .  
         [0031]      FIG. 22  shows an exploded view of the device of  FIG. 21 .  
         [0032]      FIGS. 23 and 24  show the device having different embodiments of the plate member that permits movement between the vertebrae V 2  and V 3 .  
         [0033]      FIG. 25  shows a perspective view of an embodiment of a dynamic bone fixation device in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  
         [0034]      FIG. 26  shows a perspective view of the device of  FIG. 25  in an exploded state.  
         [0035]      FIG. 27  shows various views of the bone screw receiver.  
         [0036]      FIGS. 28 and 29  show perspective and cross-sectional views of the device of  FIG. 25 .  
         [0037]      FIGS. 30 and 31  show another embodiment of the device of  FIG. 25 .  
         [0038]      FIG. 32  shows a perspective, assembled view of another embodiment of a dynamic: bone fixation device.  
         [0039]      FIG. 33  shows an exploded view of the device of  FIG. 32 .  
         [0040]      FIG. 34  shows various views of an articulating member that movably links the plate components of the device.  
         [0041]      FIG. 35  shows a cross-sectional view of the device of  FIG. 32  attached to vertebrae.  
         [0042]      FIG. 36  shows another embodiment of a bone fixation device.  
         [0043]      FIG. 37  shows an enlarged view of a receiver and coupler of the device.  
         [0044]      FIG. 38  shows various views of the coupler and receiver in an assembled state.  
         [0045]      FIGS. 39 and 40  show top and bottom perspective views of another embodiment of a device.  
         [0046]      FIG. 41  shows a top view of the device of  FIG. 39 .  
         [0047]      FIG. 42  shows yet another embodiment of a bone fixation device.  
         [0048]      FIG. 43  shows an exploded view of the device of  FIG. 42 .  
         [0049]      FIG. 43   a  illustrates a partial view of another embodiment of a dynamic bone fixation device.  
         [0050]      FIG. 44  is a perspective view of another embodiment of a dynamic bone fixation device.  
         [0051]      FIG. 45  shows the device of  FIG. 44  in an exploded state.  
         [0052]      FIGS. 46 and 47  show the device of  FIG. 44  attached to vertebrae V 1  and V 2 . 
     
    
     DETAILED DESCRIPTION  
       [0053]      FIG. 1  is a perspective view of a dynamic bone fixation device  105  configured to retain bone portions such as vertebra of a spinal column in a desired spatial relationship.  FIG. 1  shows the device  105  attached to three vertebrae V 1 , V 2 , and V 3 .  FIG. 2A  shows an assembled view of the device and  FIG. 2B  shows an exploded view of the bone fixation device  105 . With reference to  FIGS. 1-2B , the device  105  includes a first connection portion  110   a  malleably linked to al plate  112  that includes connection portions  110   a  and  110   b . The connection portions are adapted to attach to respective vertebrae, such as via bone screws, or other fasteners, that are positioned through boreholes. The device  105  also includes at least one malleable portion  115  that malleably links the connection portions  110   a  to the plate  112 . The connection portions  110  and malleable portion  115  shown in  FIGS. 1 and 2  are exemplary and it should be appreciated that the portions can have other shapes and structures that are adapted to be attached to vertebrae and permit relative motion therebetween. In one embodiment, the device is curved to conform to the surface contour of the vertebral bodies at the site of implantation. In the cervical spine, for example, the posterior surface of the device that abuts the anterior surface of the vertebral bodies can be concave in both the longitudinal and horizontal planes.  
         [0054]     With reference to  FIG. 2B , the device  105  includes attachment structures such as annular seats  305  that enable a sheath  205  to be attached thereto. In this regard, the sheath  205  mates with the annular seats  305 . The device  105  can include clamps  405  that encircle the seats  305  and exert a compressive force to removably fix the sheath  205  to the device  105 . Other structures or mechanisms can be used to attach the sheath  205  to the device  105 .  
         [0055]     With reference still to  FIG. 2B , the malleable portion  115  includes at least one malleable member  202  covered by a malleable sheath  205 . The malleable member  202  and sheath  205  are adapted to alter in shape so as to permit at least limited, relative displacement between the connection members  110   a  and the plate  112 . The malleable member  202  can be an undulated rod that loops back and forth about a longitudinal axis of the device  105 . The undulating shape permits the malleable member  202  to flex, move or otherwise change shape along a direction parallel to the midline M and/or a direction transverse or cross-wise to the midline M. It should be appreciated that the malleable member  202  can have any of a variety of shapes that are adapted to be malleable. In the illustrated embodiment, the malleable members  202  are integral with the plate  112  and connection portion  110   a  so that they form a unitary structure or construct. However, the connection portion and the plate can be formed separate from the malleable members  202  and attached by any method known to one of ordinary skill in the art, such as, for example, by fastening or welding.  
         [0056]     The sheath  205  encloses or encapsulates respective malleable member(s)  202 . The sheath  205  is sized relative to the malleable member  202  such that there is sufficient space to permit movement of the malleable member  202  inside the sheath  205 . The sheath  205  functions to contain shed particulate debris and keep adjacent tissues and scar out of the mobile core and, if desired, permit placement of a biocompatible lubricant within the space  505 . Should the malleable member  202  fail and fracture, the sheath  205  would also serve to contain the fragments and keep the device ends attached to one another.  
         [0057]     For the embodiment shown in  FIGS. 1 and 2 A/ 2 B, each of the connection portions  110  is adapted to be attached to a respective vertebra. This is accomplished by inserting at least one bone screw through a borehole in the connection portion and fixating the bone screw and connection portion to the vertebra.  FIG. 3  shows the device  105  attached to the vertebrae V 1 , V 2 , and V 3 . The device  105  can have various structural shapes and configurations. For example, the device can include a central aperture that provides access to the disc space between the vertebrae V 2  and V 3  (as shown in  FIG. 5 ). The device can also include a slot that provides a location where a distraction screw can be attached to the underlying vertebra (as shown in  FIG. 5 ).  
         [0058]     Each borehole  110   c  that overlies vertebra V 3  has a largely spherical configuration and a bottom aperture that has a diameter greater than that of the bone screws. Because of the size differential between the bottom aperture of the borehole and the diameter of the screw, the spherical head of the screw can rotate within the borehole. This mechanism permits movement of vertebra V 3  towards vertebra V 2 . Conversely, each borehole  110   a  and  110   b  has a bottom aperture with a diameter minimally larger than that of the bone screw so that, once seated, the screws are constrained and relatively immobile within the plate. Movement between vertebrae V 1  and V 2  is provided by the action of the malleable portion  115 . In this way, the device allows vertebra V 1  to move from a first position to a second position relative to vertebra V 2  in reaction to an applied force and then to substantially return to the first position when the force has dissipated. When attached to the vertebrae V 1 , V 2  and V 3 , this hybrid device will fixate vertebrae V 2  and V 3  relative to one another so as to promote fusion at this level while malleable portion  115  will support motion at the non-fused level between vertebrae V 1  and V 2 . As an alternative embodiment, each borehole  110   c  has a bottom aperture with a diameter minimally larger than that of the bone screw so that, once seated, the screws are constrained and relatively immobile within the plate. In this way, the device accommodates fusion by rigidly affixing vertebra V 2  to vertebra V 3 .  
         [0059]      FIGS. 4 and 5  illustrate top plan views of alternate embodiments. The devices include a first connection portion  310  and a malleable portion  115  that are similar to that described with reference to  FIG. 3  and a modified plate portion  112 . When fully seated, the bone screws at all levels provide no significant movement between the screw and the device&#39;s boreholes. With reference to the embodiment of  FIG. 4 , the plate  112  is formed of two components  405  and  410  that are movably coupled to one another. Component  405  includes a slot that slidably receives a tongue of the component  410  and the tongue slides along the slot so as to permit movement between the vertebrae V 2  and V 3 . Movement between vertebrae V 1  and V 2 , however, is provided by the action of the malleable portion  115 . In this way, the device allows vertebra V 1  to move from a first position to a second position relative to vertebra V 2  in reaction to an applied force and then to return to the first position when the force acting upon the vertebrae has dissipated. The device also allows unopposed boney subsidence between vertebrae V 2  and V 3 .  
         [0060]     With reference now to the embodiment of  FIG. 5 , the plate  112  includes a pair of bore holes  505  for receipt of bone screws  320  that attach the plate  315  to the vertebra V 3 . The boreholes  505  are oversized relative to the size of the heads of the bone screws  320 . In one embodiment, the boreholes  505  are elongated such that the boreholes  505  are slot-like. The relative size between the screw heads and the boreholes  505  permits the plate  112  to move relative to the bone screws  320  along the length of the boreholes and thereby permits unhindered boney subsidence between the V 2  and V 3  vertebrae. As before, movement between vertebrae V 1  and V 2  is provided by the action of the malleable portion  115 .  
         [0061]      FIGS. 6 and 7  illustrate an alternate embodiment of the device  105 . In this embodiment, the plate  112  is formed of a single plate member with elongated boreholes  505  as in the embodiment of  FIG. 5 . The malleable portion  115  includes a coupler  605  that removably mates with a complementary-shaped coupler  610  on the plate  112 . The coupler  605  comprises a protrusion and the coupler  610  comprises a cavity that is sized and shaped to lockingly receive the protrusion. In this manner, the couplers  605  and  610  can be mated to modularly attach the malleable portion  115  to the plate  112 . While the fusion portion of the embodiment in  FIG. 5  is illustrated, it should be appreciated that the configuration of either  FIG. 3  or  FIG. 4  could be alternatively used. Further, alternative modular couplers are known in the art and could be alternatively employed. U.S. Pat. Nos. 6,645,208; D505,205 and Pub. No. 2003/0074001 illustrate some of these alterative coupling methods.  
         [0062]      FIG. 8  shows another embodiment of a dynamic bone fixation device  805  in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  FIG. 9  shows a first perspective view of the device of  FIG. 8  in an exploded state.  FIG. 10  shows a second perspective view of the device of  FIG. 8  in an exploded state. The device  805  includes a first connection member  810  that attaches to the first vertebra V 1  and a second connection member  815  that attaches to the second vertebra V 2 . The connection members are fixed to the respective vertebrae using bone screws that extend through boreholes in the connection members. An articulation assembly  820  interconnects the first and second connection members to permit relative movement therebetween, as described below. The relative movement can follow the contour of a sphere or globe. While a threaded screw/borehole configuration is shown as the one embodiment, it should be appreciated that any appropriate screw/borehole/device-to-screw locking mechanism may be alternatively employed.  
         [0063]     With reference to  FIGS. 9 and 10 , the articulation assembly  820  includes a first articulation member  905  that extends outwardly from the first connection member  810 . The first articulation member  905  movably mates with a second articulation member  910  and the inferior surface  8152  of member  815 . The articulation surfaces include complementary dome-shaped portions that couple to one another and have a common center of rotation. This is described in more detail with reference to  FIG. 11 , which shows a cross-sectional view of the assembled device  805  attached to the vertebrae V 1  and V 2 . The first articulation member  905  has upper and lower surfaces that are dome-shaped (i.e., sphere-shaped). First articulation member  905  sits between member  815  and second articulation member  910 . The upper surface of member  905  articulates with the complementary lower surface of member  815  while the lower surface of member  905  articulates with the complementary dome-shaped upper surface of the second articulation member  910 . In an embodiment, the dome-shaped surfaces are defined by radii of curvature that originate at a common point R ( FIG. 11 ), which corresponds to the physiological center of rotation (also referred to as the Instantaneous Axis of Rotation (IAR)) between vertebrae V 1  and V 2 . The complementary shapes of the domed surfaces permit members  810  and  815  to move relative to one another and follow a physiological trajectory that is defined by the common center point. In this manner, the device  805  permits physiological movement between the vertebrae VI and V 2  while fixating the vertebrae to one another. The assembled device  805  will permit unhindered boney subsidence and can be used as a fixation device at the fusion level.  
         [0064]      FIG. 12  shows another embodiment of a dynamic bone fixation device  1305  in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  FIG. 13  shows a first perspective view of the device of  FIG. 12  in an exploded state.  FIG. 14  shows a second perspective view of the device of  FIG. 12  in an exploded state. The device  1305  includes a first connection member  1310  that attaches to the first vertebra V 1  and a second connection member  1315  that attaches to the second vertebra V 2 . The connection members are fixed to the respective vertebrae using bone screws that extend through boreholes in the connection members. An articulation assembly  1320  interconnects the first and second connection members to permit relative movement therebetween, as described below.  
         [0065]     With reference to  FIGS. 13 and 14 , the articulation assembly  1320  includes a first articulation member  1405  that extends outwardly from the first connection member  1310 . The lower surface of articulation member  1405  movably mates with the upper surface of second articulation member  1410  while the upper surface of member  1405  articulates with the spherical lower surface of member  1315 . The articulation members are movably coupled to one another and travel in a spherical trajectory about a common center point as described above for the previous embodiment. In addition, a malleable member  1420  is located within a cavity inside the first articulation member  1405 .  
         [0066]      FIG. 15  shows various view of the malleable member  1420 . Member  1420  includes an annular outer member  1502  that surrounds a series of diamond-shaped cells  1505 . The diamond shape of the cells  1505  permit the cells to flex or otherwise change shape in response to a force applied to outer member  1502 . The cells are biased toward a default shape so that the malleable member  1420  is also biased toward the default shape shown in  FIG. 15  and member  1420  will return to the default shape after a force acting upon it has dissipated. In this manner, the device  1305  ( FIG. 12 ) allows vertebra V 1  to move from a first position to a second position relative to vertebra V 2  in reaction to an applied motive force and then to return to the first position when the force acting upon the vertebrae has dissipated. Since the device restricts vertebral motion to that allowed by the spherical articulating surfaces, it also corrects aberrant vertebral movement and re-establishes a more physiological motion profile. A hybrid device that employs one or more of the current device embodiments at the non-fusion levels and one or more of the preceding device embodiments at the fusion levels can be used to accommodate both fusion and dynamic movement at different disc levels. As illustrated in  FIGS. 16   a - 16   c , the fusion device embodiments shown in  FIGS. 3, 4 , and  5  may be alternatively used with the current embodiment shown in  FIG. 12  to produce additional hybrid device embodiments.  
         [0067]      FIGS. 17 and 18  show perspective and cross-sectional views of an embodiment of the device  1305  that also includes articulation members  1605  and  1610  that are positioned within the disc space between the vertebrae V 1  and V 2 . The articulation member  1605  extends outward from the first connection member  1310  into the disc space while the articulation member  1610  extends outward from the second articulation member  1410  into the disc space. The articulation members  1605  and  1610  include complementary domed surfaces that permit rotational movement therebetween.  
         [0068]     As in the previous embodiment, the dome-shaped surfaces of the articulation members  1405 ,  1410 ,  1605 ,  1610  are defined by radii of curvature that originate at a common point R ( FIG. 18 ), which can correspond to the physiological center of rotation between vertebrae V 1  and V 2 . The complementary shapes of the domed surfaces permit the articulation members to move relative to one another in a spherical trajectory. In this manner, the device  805  permits physiological rotational movement between the vertebrae V 1  and V 2  while securing the vertebrae to one another.  
         [0069]      FIGS. 19 and 20  show perspective and cross-sectional views of an embodiment of the device  1305  that is similar to the device shown in  FIGS. 17 and 18 . However, the embodiment shown in  FIGS. 19 and 20  does not include the articulation member  1605 . Rather, the articulation member  1610  extends outward from the second articulation member  1410  into the disc space such that the domed surface directly abuts the vertebra V 1 , as shown in  FIG. 20 . As in the previous embodiment, the dome-shaped surfaces of the articulation members are defined by radii of curvature that originate at a common point R ( FIG. 20 ), which corresponds to the physiological center of rotation of the vertebrae V 1  and V 2 .  
         [0070]     As alternative embodiments, the articulating surfaces of the embodiment in FIGS.  8  to  11  and/or the embodiments in FIGS.  12  to  20  may be altered so as to produce a device with a variable center of rotation. This can be produced most easily by producing a “loose” articulation at each of the upper and lower bearing surfaces of these two members. In an embodiment, the loose articulation is created by slightly increasing the radius of the bearing surface that contacts and interacts with the superior bearing surfaces of members  905 / 1405  while also decreasing the radius of the bearing surface that contacts and interacts with the inferior surfaces of members  905 / 1405 . The variable center of rotation can be similarly created by a multitude of other member modifications that would be apparent to one of ordinary skill in the art.  
         [0071]      FIG. 21  shows a perspective view of another hybrid device  2105  that features a fixation device at the fusion level and an articulating surface placed within the disc space at the mobile level. The device is shown in an assembled state and attached to vertebrae V 1 , V 2  and V 3 . The device fixates vertebrae V 2  and V 3  relative to one another so as to promote fusion while articulating surface  2205  permits movement of the vertebra V 1  relative to the vertebrae V 2  and V 3 .  FIG. 22  shows an exploded view of the device of  FIG. 21 . The device  2105  includes a plate member  2110  that attaches to the vertebrae V 2  and V 3 . In this regard, the plate member  2110  includes boreholes that couple to at least one bone screw per vertebral level. The inferior boreholes that overlie vertebra V 3  have a largely spherical configuration and a bottom aperture that have a diameter greater than that of the bone screws. Because of the size differential between the bottom aperture of the borehole and the diameter of the screw, the spherical head of the screw can rotate within the borehole. This mechanism allows the unhindered movement of vertebra V 3  towards vertebra V 2 . Conversely, the superior boreholes that overlie vertebra V 2  have a spherical inner surface and a bottom aperture with a diameter minimally larger than that of the bone screws so that, once seated, the screws are constrained and immobile relative to the device. The plate member  2110  can have various structural shapes and configurations. For example, the plate member  2110  can include a central aperture that provides access to the disc space between the vertebrae V 2  and V 3 . The plate member  2110  can also include a slot that provides a location where a distraction screw can be attached to the underlying vertebra.  
         [0072]     With reference to  FIG. 22 , an articulation member  2205  extends from one end of the plate member  2110 . The articulation member  2205  is sized and shaped to extend into the disc space between the vertebrae V 1  and V 2 . The articulation member  2205  has a dome-shaped bearing surface that rests against the inferior aspect of vertebra V 1  and permits vertebra V 1  to move relative to vertebra V 2 .  FIGS. 23 and 24  show the device  2105  with different embodiments of the plate member  2110  that fixates vertebrae V 2  and V 3 . These alternative members are similar to those illustrated in  FIGS. 4 and 5 .  
         [0073]      FIG. 25  shows a perspective view of an embodiment of a dynamic fixation device  2005  in an assembled state and attached to a pair of vertebrae V 1  and V 2 .  FIG. 26  shows a perspective view of the device of  FIG. 25  in an exploded state. With reference to  FIGS. 25 and 26 , the device  2005  includes a member  2010  that is sized and shaped to extend between the two vertebrae V 1  and V 2 . Member  2010  includes a pair of openings  2015  that are each movably coupled to a bone screw receiver  2020  and sized and shaped to receive the screw receiver  2020 . The receiver  2020  is coupled to the member  2010  in a manner that permits relative movement between the receiver  2020  and the plate member  2010  and thereby permit movement between the plate member  2010  and the screw.  
         [0074]      FIG. 27  shows various views of the bone screw receiver  2020 . With reference to  FIGS. 26 and 27 , receiver  2020  has a spherical outer wall that is configured to fit within the complementary spherical walls  2017  of opening  2015  of the member  2010 . Each receiver  2020  includes a central bore  2155  that is threaded for receipt of a correspondingly-threaded shank of a bone screw. One or more malleable couplers  2110  are adapted to secure the receiver  2020  to the member  2010 . Each malleable coupler  2110  is an elongate member that fits through a bore  2115  in the member  2010 . The couplers  2110  have a first end  2125  that attaches to the member  2010  via an attachment pin  2130 . A second end  2135  of the coupler  2110  fits within a receptor hole  2140  in the receiver  2020  to movably :secure the receiver  2020  to the member  2010 .  
         [0075]      FIGS. 28 and 29  show cross-sectional views of the device  2005 . As shown in  FIG. 29 , the ends of the couplers  2110  extend into the receivers  2020  to thereby secure the receivers  2020  to the member  2010 . The couplers  2110  are adapted to flex or otherwise articulate to permit relative movement between the receivers  2020  and the member  2010 . The interaction of the spherical outer wall of receiver  2020  with complementary spherical walls  2017  determines the overall motion trajectory of receiver  2020  relative to member  2010 . As shown in  FIG. 29 , the bone screws  2310  extend through the receivers  2020  into the vertebrae V 1  and V 2 . Because the receivers  2020  can move relative to the member  2010 , the bone screws  2310  can also move relative to the member  2010  while remaining attached to the plate  2010 .  
         [0076]      FIGS. 30 and 31  show another embodiment of the device  2005  of  FIGS. 25-29 . This embodiment is similar to the previous embodiment except the receivers  2020  are positioned within holes  2016  in the member  2010  rather than within open-ended slots  2015 .  
         [0077]      FIG. 32  shows a perspective, assembled view of another embodiment of a dynamic bone fixation device  2405 .  FIG. 33  shows an exploded view of the device of  FIG. 32 . The device  2405  has a configuration that is substantially similar to the embodiment of  FIGS. 30-31 . However, the plate member includes two plate components  2410  and  2415  that are movably attached to one another, as described below. The receivers  2020  are positioned within holes in the plate components and are movably attached to the plate components in the manner described with respect to the previous embodiment.  
         [0078]      FIG. 34  shows various views of an articulating member  2510  that movably links the plate components of the device. With reference to  FIGS. 33 and 34 , the plate components  2410  and  2415  are connected to one another via the articulating member  2510 , which is positioned within boreholes in the plate components. The articulating member  2510  is formed of a plurality of sections. The articulating member is a flexure based bearing, utilizing internal flat crossed springs, capsuled in a cylindrical housing, to provide precise rotation with low hysteresis and little frictional losses. The bearing is relatively friction-free, requires no lubrication, and is self-returning. The articulating member can resist rotational movement away from a neutral state and the extent of resistance to rotation is directly related to the extent of rotation. The extent of resistance to rotation can be a pre-determined property of the device. In one embodiment, the articulation member has high radial stiffness, high axial stiffness and is frictionless (hence, no particle wear debris). An exemplary articulating member  2510  of the type shown in  FIGS. 33 and 34  is distributed by Riverhawk Company of New York under the name FREE FLEX PIVOT.  
         [0079]     The articulating member  2510  includes a first portion  2515  that is positioned inside the plate component  2415  while the hinge portions  2520  are positioned inside the plate component  2410 . In this manner, the components  2415  and  2410  can rotate relative to one another via the articulating member  2510 .  FIG. 35  shows a cross-sectional view of the device of  FIG. 32  attached to the vertebrae.  
         [0080]      FIG. 36  shows another embodiment of a bone fixation device  3610 . The device  3610  includes a plate member  3615  having bone screw receivers  3620  that movably reside within elongated slot  3627 . Movement of receiver  3620  within slot  3627  is resisted by malleable member  3625  so that the receiver  3620  may move from a first position to a second position relative to slot  3627  in reaction to an applied force and then substantially return to the first position when the force has dissipated.  FIG. 37  shows an enlarged view of a receiver  3620  and malleable member  3625 . Member  3625  includes a knob  3710  that fits within a complementary-shaped hole  3715  in the receiver  3620 . While not depicted for diagrammatic simplicity, a pin or small screw may be driven from the top surface of receiver  3620 , through hole  3715  and the knob  3710  retained within, and into a portion of the bottom surface of the receiver  3620  in order to more rigidly affix malleable member  3625  to receiver  3620 .  FIG. 38  shows various views of the coupler  3625  and receiver  3620  in an assembled state. Bone fixation spikes (or other texturing or protrusions) can be situated along the inferior, bone-contacting surface of receiver  3620  in order to increase the extent of bone contact and fixation. The spherical outer walls of receiver  3620  are adapted to fit within slot  3627  and provide a bearing surface with the complementary spherical inner walls of slot  3627 .  
         [0081]      FIGS. 39 and 40  show top and bottom perspective views of another embodiment of a device  3905 .  FIG. 41  shows a top view of the device of  FIG. 39 . The device  3905  includes a plate member  3910  that movably couples to at least one bone screw receiver  3920 . The bone screw receiver  3920  has an internal bore that is sized to receive a shank portion of a bone screw. The bore is threaded to mate with corresponding threads in the screw. With reference to  FIG. 41 , the receiver  3920  has a pair of pin-shaped protrusion  4110  that mate with corresponding couplers  4120  of the malleable assemblies  4130  of the plate  3910 . The couplers  4120  are fan-shaped and are adapted to move relative to the plate such as within spaces  4125  on either side of the couplers  4120 . This permits the couplers  4120  to move or articulate relative to the remainder of the plate such as in response to forces exerted on or by the attached vertebrae. In this way, movement of each receiver  3920  relative to plate  3910  is resisted by each malleable assembly  4130  so that the receiver  3920  may move from a first position to a second position in reaction to an applied force and then substantially return to the first position when the force has dissipated.  
         [0082]      FIG. 42  shows yet another embodiment of a bone fixation device  4210 .  FIG. 43  shows an exploded view of the device of  FIG. 42 . The device  4210  includes two components members  4215  and  4220  that are movably attached to one another, as described below. Each component member includes one or more boreholes for receipt of bone fasteners. The component member  4215  includes a protrusion  4225  that is positioned within a slot  4230  in the plate component  4220 . Protrusion  4225  can have a partially circumferential convex spherical wall  42255  that interacts with complementary concave spherical walls  42305  of slot  4230 .  
         [0083]     With reference to  FIG. 43 , one or more malleable couplers  4310  are adapted to secure the protrusion  4225  to the component  4220 . Each coupler  4310  is an elongate, flexible member that fits through a bore  4315  in the plate component  4220 . The couplers  4310  have a first end  4330  that attaches to the plate member  2010  via an attachment pin  4335 . A second end  4340  of the coupler fits within a receptor hole  4345  in the protrusion  4225  to secure the protrusion to the plate component  4220 . The movement of one component member  4215  relative to the other component member  4220  is resisted by each flexible malleable coupler  4310  so that one component member may move from a first position to a second position relative to the other component member in reaction to an applied force and then substantially return to the first position when the force has dissipated.  
         [0084]     The embodiments illustrated in FIGS.  25  to  43  disclose various devices that can be used to support and maintain movement at the non-fused disc level. Removal of the malleable members from any of these devices would permit unhindered subsidence at the fixated level and render the respective embodiment suitable for use at the fused level. (Complete immobilization of the device components (with or without removal of the malleable member) would also produce a device that can be used at the fused levels. However, these rigid devices are less preferable than those that would accommodate boney subsidence.) A hybrid device that employs one or more of the device embodiments illustrated in  FIG. 25  to  43  and one or more of a fusion device can be made to simultaneously accommodate both fusion and dynamic movement at different disc levels. While a number of fusion devices have been disclosed and illustrated in this application, it is understood that any of the numerous fusion fixation devices that are currently known in the art may be alternatively used with the malleable embodiments to produce the desired hybrid fixation devices. In addition to the illustrated plate fixation devices, known fixation devices include rid-based embodiments such as those disclosed in U.S. Pat. Nos. 5,800,433; 5,713,900, Pub. No. 2005/0004573 and others. Further, embodiments that can support and maintain movement at the non-fused disc level may be generated from the rod-based devices by the addition of a malleable member that would oppose movement of the device components away from a pre-set neutral position and then substantially return the device to that neutral position upon dissipation of the motive force. Lastly, a malleable member can be made to act directly at the bone screw/borehole interface. In  FIG. 43   a , the borehole has a largely spherical configuration and a bottom aperture that has a diameter greater than that of the bone screws. Because of the size differential between the bottom aperture of the borehole and the diameter of the screw, the spherical head of the screw can rotate within the borehole. A cap is threaded onto the top opening of the borehole so that a space is formed between the bottom of the cap and the top of the bone screw. The formed space can contain an elastic material(s), fluids, spring device(s), Belleville washers, magnets or any other appropriate materials/devices that will resist movement between the head of bone screw and the bottom of the cap. The material/device within the space will apply a force to the head of the screw and resist any bone screw movement away from the neutral position. In this way, the screw will move within the borehole in response to a deflecting force and will return to the neutral position when the applied force has dissipated.  
         [0085]      FIG. 44  is a perspective view of a dynamic bone fixation device  2905  configured to retain bone portions such as vertebra of a spinal column in a desired spatial relationship.  FIG. 45  shows the device of  FIG. 44  in an exploded state. With reference to  FIGS. 44 and 45 , the device  2905  includes first and second connection portions  2910   a  and  2910   b  that are adapted to attach to respective vertebrae. In the illustrated embodiment, the connection portions  2910  are elongated and rod-like and are adapted to attach to bone screws, as described in detail below. The device  2905  also includes a malleable portion  2915  that malleably links the connection portions  2910 . The malleable portion  2915  includes a malleable member  3005  that is covered by a sheath  3010  ( FIG. 30 ). The malleable member  3005  and sheath  3010  are adapted to alter in shape so as to permit at least limited relative displacement between the vertebrae.  
         [0086]     With reference still to  FIG. 44 , the malleable member  3005  is an undulated rod that loops back and forth about a longitudinal axis of the device. The undulating shape permits the malleable member flex, move or otherwise change shape along a direction parallel to the midline M and/or a direction transverse or cross-wise to the midline M. The device  2905  can include clamps, such as c-clamps  3050  that encircle the attachment regions edges of the sheath  3010  and exert a compressive force on the sheath to removably fix the sheath  3010  to the connection portions  2910 . Other structures or mechanisms can be used to attach the sheath to the device  105 .  
         [0087]     In the assembled state, the sheath  3010  encloses or encapsulates the malleable member  3005 . The sheath  3010  is sized relative to the malleable member  3005  such that there is sufficient space to permit movement of the malleable member  3005  inside the sheath  3010 . The sheath  3010  functions to contain shed particulate debris, keep adjacent tissues and scar out of the mobile core and, if desired, permit placement of a biocompatible lubricant within the space. Should the malleable member fail and fracture, the membrane would also serve to contain the fragments and keep the device ends attached to one another.  
         [0088]      FIGS. 46 and 47  show the device  2905  of  FIG. 44  attached to vertebrae V 1  and V 2 . Each connection portion  2910  is sized and shaped to fit within a receiver of a bone screw assembly that has been attached to a vertebra. In this manner, the device  2905  interconnects the vertebrae V 1  and V 2  with the malleable portion permitting movement therebetween.  
         [0089]     While many of the disclosed embodiments featured a specific screw/borehole configuration (such as the threaded screw and threaded bore hole), it should be appreciated that these configurations are exemplary and do not limit the scope of the invention. Numerous screw/borehole configurations and screw-to-borehole locking mechanisms are well known in the art and any of these may be alternatively employed to fasten the disclosed devices onto the underlying bone.  
         [0090]     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, 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 demineralized 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 carbon nanotube-based coating) 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 deformable materials.  
         [0091]     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. Therefore the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.