Abstract:
A bone prosthesis device is comprised of an upper and lower abutment surfaces and an intervening malleable member. The device is sufficiently small so that implantation into an inter-vertebral disc space can be performed from a substantially posterior approach without significant impingement upon the neural elements.

Description:
REFERENCE TO PRIORITY DOCUMENT 
       [0001]    This application is a continuation of and claims priority to co-pending U.S. patent application Ser. No. 12/894,507 issuing as U.S. Pat. No. 8,500,814 on Aug. 6, 2013, which is incorporated herein by reference in its entirety, and which is a continuation of co-pending U.S. patent application Ser. No. 11/675,597, filed Feb. 15, 2007, which issued as U.S. Pat. No. 7,828,847 on Nov. 9, 2010, and which claims priority of U.S. Provisional Patent Application Ser. Nos. 60/773,584 filed Feb. 15, 2006, 60/850,473 filed Oct. 10, 2006, and 60/874,195 filed Dec. 11, 2006. Priority of the aforementioned filing dates is hereby claimed and the disclosures of the Patent Applications are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to devices and methods that permit stabilization of the bony elements of the skeleton. The devices and methods permit adjustment and maintenance of the spatial relationship(s) between neighboring bones. 
         [0003]    Spinal disease is a major health problem in the industrialized world and the surgical treatment of spinal pathology is an evolving discipline. The current surgical treatment of abnormal vertebral motion and low back pain is the complete immobilization and bony fusion of the involved spinal segment. An extensive array of surgical techniques and implantable devices has been formulated to accomplish this goal. 
         [0004]    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. This second procedure necessitates re-dissection through the prior, scarred operative field and carries significantly greater risk than the initial procedure while providing a reduced probability of pain relief. 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]    There is a growing recognition that segmental spinal fusion and complete immobilization is an inadequate solution to degenerative disc disease. Replacement of the degenerated and painful disc with a mobile prosthesis is a more intuitive and rational treatment option. This approach preserves spinal mobility in a majority of spinal segments and reserves fusion and complete immobilization for those disc spaces where the degenerative disease is advanced and beyond surgical restoration. 
         [0006]    U.S. Pat. Nos. 4,759,769; 4,997,432; 5,674,294; 5,674,296; 5,676,701; 5,888,226; 6,001,130; 6,019,792; 6,162,252; 6,348,071; 6,368,350; 6,419,706; 6,520,996; 6,540,785; 6,607,558; 6,645,249; 6,673,113; 6,749,635 and others have illustrated various artificial disc prosthesis. Despite the number of proposed designs, each device is sized to substantially occupy the majority of the disc space and replace the entire disc. Since the neural elements are anatomically positioned immediately posterior to the disc space, these large devices can be implanted only through an anterior or lateral surgical approach. 
         [0007]    The spine is situated at the most posterior aspect of the body cavities and it can be most readily reached through a posterior approach. Anterior and lateral surgical approaches must dissect around and through the many vital organs and blood vessels that lie anterior to the spine and these approaches add to the risk and morbidity of the procedure. In addition, spine surgeons are more familiar with and technically versed in the posterior approach, further increasing the risks of the more difficult non-posterior approaches. Finally, the posterior approach allows the surgeon to advantageously remove the bone spurs that compress the neural elements at the same time they access the disc space. 
         [0008]    The use of a posterior surgical approach to implant a mobile disc prosthesis has numerous advantages. Unfortunately, the intervening nerve elements limit the size of the posterior corridor that can be used to access the anterior disc space and a posteriorly-placed mobile disc prosthesis (i.e. “artificial disc”) must be small enough to fit within that limited implantation corridor. Consequently, a posteriorly-placed artificial disc can only provide partial coverage of the disc space and partial replacement of the inter-vertebral disc. Attempts to overcome this problem by placing several implants within the disc space is limited by the significant difficulty in producing coordinated movement of separate implants about a specified center of rotation. 
       SUMMARY 
       [0009]    In view of the preceding, there remains a need in the art for a prosthesis that can be safely placed into the disc space via a posterior surgical approach and used to replace the natural function of an inter-vertebral disc. Disclosed are devices and methods for the implantation of a mobile prosthesis within the disc space that can replace the function of a natural disc. 
         [0010]    In one aspect, a prosthesis is comprised of an upper and lower abutment surfaces and an intervening malleable member. The device is sufficiently small so that implantation into an inter-vertebral disc space can be performed from a substantially posterior approach without significant impingement upon the neural elements. 
         [0011]    In other aspects, the prosthesis contains two or more bearing members wherein one set of one or more bearing members provide rotational and/or translational movement between the upper and lower abutment surfaces of the prosthesis. A second set of bearing members allow the abutment surfaces to reversibly move towards one another so that the device is endowed with a shock-absorptive capability. The second set of bearing members also allow the device to be compressed into a secondary configuration of lesser volume so as to allow placement through a smaller implantation portal. Bearing surfaces of fixed and variable centers of rotation are illustrated. 
         [0012]    In another aspect, there is disclosed a spinal implant device for the maintenance of relative motion between two adjacent vertebral bodies, comprising: a first member having an lower abutment surface adapted to contact an upper surface of a first vertebral body; a second member having an upper abutment surface adapted to contact a lower surface of a second vertebral body; and at least one malleable member between the first and second members that permits relative movement between the first and second members, wherein the device is adapted to be implanted within a disc space between the two vertebral bodies, and wherein the device is sufficiently small to be implanted into the disc space via a posterior approach to the disc space. 
         [0013]    In another aspect, there is disclosed an orthopedic implant device for the maintenance of motion between two adjacent bones, comprising: a first member having an lower abutment surface adapted to contact an upper surface of a first bone; a second member having an upper abutment surface adapted to contact a lower surface of a second bone; and a coupler between the first and second members and movably attaching the first member to the second member, the coupler including (a) at least a first bearing mechanism comprising a first bearing surface that includes a malleable member that reversibly opposes a load on the implant so as to return the implant to a predetermined configuration after dissipation of the load; and (b) at least a second bearing mechanism comprising a second bearing surface adapted to permit the implant to permit motion between the first and second bones when positioned between the first and second bones. 
         [0014]    In another aspect, there is disclosed a method for the placement of an orthopedic device within a disc space between two vertebral bodies, comprising: applying a distraction force to the two vertebral bodies to provide a corridor for the placement of the orthopedic device in the disc space between the two vertebral bodies, wherein a distractor device at least partially attaches to a spinous process or lamina of one of the vertebral bodies; and implanting the orthopedic device in the disc space using a substantially posterior placement corridor. 
         [0015]    Placement methods are disclosed. In some placement protocols, vertebral distraction is incorporated in order to limit the necessity of bone and joint resection. In a novel application, the distractors are attachment the spinous processes or lamina of the adjacent vertebrae. 
         [0016]    In yet another aspect, an orthopedic device assembly is disclosed. In one embodiment, the assembly includes: (i) a first member comprising at least a first bearing surface, a second bearing surface, and a bone abutment surface, the first and second bearing surfaces and the bone abutment surface configured to interconnect such that each of the first and second bearing surfaces diverges rigidly away from a segment of the bone abutment surface, and (ii) a second member comprising at least a first bearing surface, a second bearing surface, and a bone abutment surface. In one variant, the first and second bearing surfaces and the bone abutment surface are configured to interconnect such that at least one of the first and second bearing surfaces is movably coupled to the bone abutment surface, and the first and second bearing surfaces are separated by a first distance which is configured to vary between a first value and a second value greater than the first value, the first distance being biased towards the second, greater value. The first bearing surfaces of each of the first and second members form a movable articulation with one another. The second bearing surfaces of each of the first and second members form a movable articulation with one another. The orthopedic device assembly when assembled further comprises a top surface comprising the bone abutment surface of either the first or second member and a bottom surface comprising the bone abutment surface of a remaining one of the first or second member. A decrease in the first distance between the first and second bearing surfaces of the second member reduces a vertical distance between the bone abutment surfaces of the first and second members. 
         [0017]    In another embodiment, the device includes: (i) a first implant member comprising at least: a bone abutment surface configured to abut a first vertebral bone, and a second surface, and (ii) a second implant member. In one variant, the second implant member includes at least: a bone abutment surface configured to abut a second vertebral bone, and at least two biasing members configured to form a movable articulation with respective at least two portions of the second surface of the first implant member, the at least two biasing members configured to exert a force away from the second implant member and toward the at least two portions of the second surface of the first implant member. A distance between the bone abutment surfaces of the first and second implant members is configured to vary in proportion to the force exerted by the at least two biasing members on the second surface of the first implant member. 
         [0018]    In another aspect, an orthopedic device is disclosed. In one embodiment, the device includes: (i) a first member comprising at least a first bearing surface, a second bearing surface, and a bone abutment surface, the first and second bearing surfaces and the bone abutment surface configured to interconnect such that each of the first and second bearing surfaces diverges rigidly away from a segment of the bone abutment surface, and (ii) a second member comprising at least a first bearing surface and a second bearing surface, the first and second bearing surfaces being movably coupled to a bone abutment surface of the second member. In one variant, the first and second bearing surfaces are separated by a variable first distance and biased away from one another. The first bearing surfaces of each of the first and second members form a movable articulation with one another. The second bearing surfaces of each of the first and second members form a movable articulation with one another. The bone abutment surfaces of the first and second members form a top and a bottom surface of the orthopedic device when assembled. An increase in the first distance between the first and second bearing surfaces of the second member increases a vertical distance between the bone abutment surfaces of the first and second members. 
         [0019]    The implants described in this application can be safely placed into the disc space via a posterior surgical approach and used to replace the natural function of an inter-vertebral disc. 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 
         [0020]      FIG. 1  shows a perspective view of implant  105  that is sized and shaped to be positioned within a disc space. 
           [0021]      FIG. 2  shows various views of the implant of  FIG. 1 . 
           [0022]      FIG. 3  shows a spinal motion segment composed of two adjacent vertebral bodies V 1  and V 2  and the intervening disc space. 
           [0023]      FIG. 4  shows the spinal motion segment after a surgical procedure wherein the left facet joint of vertebral body V 1  has been removed. 
           [0024]      FIG. 5  shows the implant positioned adjacent the disc space. 
           [0025]      FIG. 6  shows the implant positioned within the disc space between the vertebral bodies V 1  and V 2 . 
           [0026]      FIG. 7  shows a spine axial view of the implant positioned in the disc space. 
           [0027]      FIG. 8  shows a spine axial view of a pair of implant positioned in the disc space on either side of the spinal midline. 
           [0028]      FIG. 9  shows a perspective view of another embodiment of an implant that is sized and shaped to be positioned within a disc space. 
           [0029]      FIG. 10  shows various view of the implant of  FIG. 9 . 
           [0030]      FIG. 11  shows the implant in an exploded state. 
           [0031]      FIG. 12  shows a side cross-sectional view of the implant. 
           [0032]      FIG. 13  shows a perspective view of the hinge member of the implant. 
           [0033]      FIGS. 14 and 15  show perspective and side view of the implant positioned within a disc space between vertebral bodies V 1  and V 2 . 
           [0034]      FIG. 16  shows an exemplary distractor device that couples to a pair of distractor screws. 
           [0035]      FIG. 17  shows the distractor device coupled to the distractor screws. 
           [0036]      FIG. 18  shows the vertebral bodies after being distracted such that the disc space is accessible. 
           [0037]      FIG. 19  shows the implant adjacent the disc space prior to implantation into the disc space. The implant is transitioned into a compact configuration. 
           [0038]      FIG. 20A  shows an embodiment of a distractor device that does not utilize distraction screws to engage the vertebrae. 
           [0039]      FIGS. 20B and 20C  show an implant positioned in the disc space between the vertebral bodies V 1  and v 2  with supporting dynamic bone screws and rods. 
           [0040]      FIGS. 20D and 20E  show embodiments of dynamic screws and rods. 
           [0041]      FIGS. 20F and 20G  show alternative applications of the implant. 
           [0042]      FIG. 21  shows another embodiment of an implant. 
           [0043]      FIGS. 22 and 23  show another embodiment of an implant. 
           [0044]      FIG. 24  shows another embodiment of an implant that is sized and shaped for implantation into the disc space. 
           [0045]      FIG. 25  shows perspective and cross-sectional views of the implant of  FIG. 25 . 
           [0046]      FIGS. 26 and 27  show partially exploded views of the implant of  FIG. 25 . 
           [0047]      FIG. 28  shows another exploded view of the implant. 
           [0048]      FIG. 29  shows the implant in a compressed state. 
           [0049]      FIG. 30  shows the implant with a malleable member positioned in an internal cavity. 
           [0050]      FIG. 31  shows the implant under load. 
           [0051]      FIG. 32  shows another embodiment of an implant. 
           [0052]      FIG. 33  shows another embodiment of an implant. 
           [0053]      FIG. 34  shows another embodiment of an implant. 
           [0054]      FIGS. 35 and 36  show cross-sectional views of another embodiment of an implant. 
           [0055]      FIGS. 37 and 38  show a spring member of the implant of  FIGS. 35 and 36 . 
           [0056]      FIGS. 39-42  show another embodiment of an implant. 
           [0057]      FIG. 43  shows another embodiment of an implant. 
       
    
    
     DETAILED DESCRIPTION 
       [0058]    Disclosed are devices and methods for the implantation of a mobile prosthesis within the disc space between two vertebrae. The mobile prosthesis is adapted to replace the function of a natural disc. Various implants are described herein. 
         [0059]      FIG. 1  shows a perspective view of a first embodiment of an implant  105  that is sized and shaped to be positioned within a disc space between a pair of vertebrae in a spine.  FIG. 2  shows various views of the implant of  FIG. 1 . The implant  105  includes an upper component  110  and a lower component  115 . An elastic middle component  120  is interposed between the upper and lower components. It should be appreciated that the terms “upper” and “lower” are for reference purposes and use of such terms should not be limiting with respect to placement orientation. 
         [0060]    The middle component  120  is adapted to deform or change shape in response to loads on the upper and/or lower components. The middle component  120  is elastic and biased toward a default shape such that the implant returns to an initial configuration or shape after the force acting upon the implant has dissipated. In this regard, the middle component  120  has a leaf spring-like configuration that is formed, for example, of a pair of inclined walls that meet at a connection location  130 . The walls can flex about the connection location to permit the middle component  120  to change shape&#39; while being biased toward the default shape. 
         [0061]    The middle component  120  is depicted in  FIG. 1  as a spring-like member such that the structural shape of the middle component provides spring-like qualities. The middle component  120  can be alternatively or in combination made of any visco-elastic material(s) such as to compliment or enhance the spring-like qualities of the middle component  120 . Further, the middle component  120  can be fluid based and resist motion by the use of hydrodynamic forces or it can employ magnetic fields that repel/attract various implant components and produce the desired motion characteristics. 
         [0062]    Middle compartment  120  may be at least partially made of shape memory materials that exhibit a stress-induced martensitic transformation. Shape memory materials plastically deform from a first configuration into a second configuration and then return to the first “memorized” configuration in response to a stimulus. The ability of the material to reversibly change shape is secondary to a phase transformation so that the material essentially exists in either an austenitic state or a martensitic state. A phase shift secondary to a temperature change is called a thermoplastic martensitic phase transformation while a shift due to the imposition of load is termed a stress-induced martensitic transformation. Shape-memory materials include a number of shape-memory alloys and shape-memory polymers. The former include a variety of alloys of known metals such as, for example, nickel and titanium, copper and zinc as well as copper, aluminum and nickel. Shape memory polymers have also been described and usually consist of a plastic polymer with two or more components that have different thermal characteristics. These components include, for example, oligo (e-caprolactone) diol and oligo (p-dioxanone) diol. Additional materials exist that reversibly alter shape in reaction to PH, moisture and magnetic and electrical fields. Shape memory alloys that respond to a load change are particularly suitable for this application. 
         [0063]    The upper and lower components  110  and  115  each have an abutment surface  125  that is adapted to abut against a vertebra when the implant  105  is positioned in a disc space. The abutment surfaces  125  of the upper and lower components are preferably configured to promote interaction with the adjacent bone and affix the implant to the bone. While depicted as having pyramidal protrusions, the abutment surfaces may have any of a variety of configurations for promoting such interaction. For example, the abutment surfaces may be alternatively textured, corrugated or serrated. The surfaces may be also coated with substances that promote osteo-integration such as titanium wire mesh, plasma-sprayed titanium, tantalum, and porous CoCr. The surfaces may be further 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, helical rosette carbon nanotubes or other carbon nanotube-based coating may be applied to the surfaces to promote implant-bone interaction. Lastly, a portion of components  110  and  115  could be also incorporated with bone fragment or a bone graft substitute (as illustrated in the embodiment of  FIG. 39 ) to fuse onto the vertebral surfaces. 
         [0064]    A method of positioning or implanting the implant  105  is now described.  FIG. 3  shows a spinal motion segment composed of two adjacent vertebral bodies V 1  and V 2  and the intervening disc space. For clarity of illustration, certain anatomical details are not shown in  FIG. 3  or the accompanying figures. Preferably, a portion of at least one of the vertebrae is removed in order to facilitate device placement.  FIG. 4  shows the spinal motion wherein at least a segment of the left facet joint of vertebra V 1  &amp; V 2  is removed thereby forming a pathway for implantation of the implant  105  into the disc space. 
         [0065]    Next, the implant  105  is positioned for implantation into the disc space, as shown in  FIG. 5 .  FIG. 5  shows the implant  105  positioned adjacent the disc space. The implant  105  is positioned such that the abutment surfaces can be positioned adjacent the bone.  FIG. 6  shows the implant  105  positioned within the disc space between the vertebral bodies V 1  and V 2 . The abutment surfaces of the upper and lower components of the implant are positioned to contact the bone.  FIG. 7  shows an axial view of the implant positioned in the disc space. In another embodiment, the procedure is performed bilaterally so that at least one implant  105  is placed on each side of the spinal midline as shown in  FIG. 8 . 
         [0066]      FIG. 9  shows a perspective view of another embodiment of an implant  905  that is sized and shaped to be positioned within a disc space.  FIG. 10  shows various view of the implant  905 .  FIG. 11  shows the implant  905  in an exploded state. The implant  905  includes an upper component  910 , a lower component  915 , and a middle component  920 . As in the previous embodiment, the upper and lower components each have an abutment surface  925  that is adapted to abut against a vertebra when the implant  905  is positioned in a disc space. The abutment surfaces  925  of the upper and lower components are preferably configured to promote interaction with the adjacent bone and affix the implant to the bone. The lower component  915  has an interior surface with a cavity or seat  930  that is sized to receive at least a portion of the middle component, as described below. 
         [0067]    The middle component  920  is adapted to deform or otherwise yield in response to loads on the upper and/or lower components. In this regard, the middle component is biased toward a default shape or position such that the implant returns to an initial configuration or shape after the force acting upon the implant has dissipated. With reference to the exploded view of  FIG. 11 , the middle component  920  includes a lever member  1105  that is pivotably coupled about a hinge member  1110 . The hinge member  1110  mounts within a shaft  1115  in the lower component  915 . The lever can be at least a first bearing mechanism comprising a first bearing surface that includes a malleable member that reversibly opposes a load on the implant so as to return the implant to a predetermined configuration after dissipation of the load. 
         [0068]      FIG. 12  shows a side cross-sectional view of the implant  905 . The lever member  1105  includes a head  1205  that abuts into a seat in an interior surface of the upper component  910 . In a default state, the lever member  1105  is biased toward the position shown in  FIG. 12 . The lever member  1105  is adapted to pivot about an axis defined by the hinge member  1110  such that the lever member  1105  can move about a curvilinear pathway, as represented by the arrow P in  FIG. 12 . In this manner, the lever member  1105  can change position in response to loads on the implant  905  such that the upper and lower components can move toward one another in a manner that is limited by movement of the lever member  1105 . 
         [0069]    The lever member  1105  is coupled to the hinge member  1110 , which is adapted to deform or articulate in response to loads thereon.  FIG. 13  shows a perspective view of the hinge member of the implant  905  with a wall segment removed. The hinge member  110  includes an outwardly extending tooth  1305  that mates with complimentary-shaped slot in the lever member  1105 . The hinge member  1110  is formed of a plurality of sections. The hinge 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 hinge 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 hinge member has high radial stiffness, high axial stiffness and is frictionless (hence, no particle wear debris). An exemplary hinge member of the type shown in Figure is distributed by Riverhawk Company of New York under the name FREE FLEX PIVOT. 
         [0070]    A second bearing surface is provided by the spherical head  1205  of member  1105  and complimentary cut out  9105  of component  910 . The interaction forms a ball-and-socket type joint. Cut-out  9105  is preferably of slightly larger diameter than head  1205  so that the articulation forms a loose fitting joint and permits additional translational movement between the components. Bore holes  9110  ( FIG. 10 ) are preferably threaded and permit the device to interact with a placement instrument. The latter can also function to compress the device during implantation. The spherical head can be a second bearing mechanism comprising a second bearing surface adapted to permit the implant to permit motion between the first and second bones when positioned between first and second bones. 
         [0071]      FIGS. 14 and 15  show perspective and side view of the implant  905  positioned within a disc space between vertebral bodies V 1  and V 2 . The implant  905  is positioned such that the abutment surfaces of the upper and lower components abut the adjacent bone within the disc space. The lever member  1105  can pivotably move in the manner discussed above in response to loads on the vertebral bodies. 
         [0072]    While the current embodiment may be implanted using a placement procedure similar to that of the first embodiment, alternative placement protocols may be also used. An exemplary method of implant  905  placement is now described. In an initial step, a distractor device is coupled to the vertebral bodies for distracting the vertebral bodies.  FIG. 16  shows an exemplary distractor device  1605  that couples to a pair of distractor screws  1610 . The distractor screws  1610  are fastened onto the vertebral bodies such that they extend outwardly therefrom. The distractor device  1605  includes a pair of sheaths  1615  that are configured to couple to the distractor screws  1610 , such as by sliding over the distractor screws.  FIG. 17  shows the distractor device  1605  coupled to the distractor screws  1610 . The distractor device  1605  includes an actuator  1615  that can be actuated to exert a distraction force onto the vertebral bodies such that the vertebral bodies are distracted.  FIG. 18  shows the vertebral bodies after being distraction such that the disc space is accessible. 
         [0073]      FIG. 19  shows the implant  905  adjacent the disc space prior to implantation into the disc space. During the implantation procedure, the implant  905  can be in a compressed state of reduced size. When in this state, the upper and lower components are compressed toward one another such that the lever member  1105  is sitting within the seats on the interior surface of the upper and lower components. Thus, the implant  905  has a reduced sized profile when in the compressed state. As discussed, the implant  905  is biased toward the uncompressed state shown in  FIG. 12 . Once the implant  905  is positioned within the disc space, the implant  905  will tend to move toward the uncompressed state as limited by the interaction with the vertebral bodies. 
         [0074]    During implantation, the decrease in implant size by folding and the increase in the placement corridor by vertebral distraction will advantageously permit device placement without significant removal of the facet joint and other bony elements. It should be appreciated that the manner in which the vertebral bodies are distracted can vary. For example,  FIG. 20A  shows an embodiment of a distractor device  1605  that does not utilize distraction screws that fasten onto the spinous processes of the vertebrae. Rather, the distractor device  1605  includes a pair of clips  1705  that are each shaped to rest against a portion of a spinous process. Alternatively, a distractor with clip-like attachments similar to  1705  may be positioned against the lower edge  1725  of the upper lamina ( FIG. 20A ) and the upper edge  1727  of the lower lamina ( FIG. 20A ) and used to distract the vertebrae. Any other means of vertebral distraction can also be used. 
         [0075]      FIGS. 20B and 20C  show the implant  905  positioned in the disc space between the vertebral bodies V 1  and v 2 . The vertebral bodies are linked to one another via a pair of screw assembles  2050  and a rod  2055 . The screw assemblies  2050  and rod assembly  2055  are dynamic in that they are adapted to permit at least some movement in response to loads. For example,  FIG. 20D  shows the dynamic rod assembly  2055 , which includes dynamic terminii. The rod  2060  has a pair of heads  2663  that can each be positioned within housing members  2665   a  and  2665   b.  The members  2665   a  &amp;  b  are joined to form the assembled inner housing member using threaded screws, but ratchets, clips, adhesives, or any other well-known technique for segment assembly may be alternatively used. The inner aspect of housing members  2665  contains a space that is positioned above the head  2663 . The space within the housing members  2665  preferably contains a material or structure that resists movement of the head  2663  of the rod relative to the inner aspect of the inner housing members. With movement of head  2663  away from the predetermined neutral position within the inner housing members, the material/device in space applies a force to the head and resist any movement away from the neutral position. 
         [0076]    With reference to  FIG. 20D , the screw assemblies  2050  also have comparable dynamic arrangements. The bone screw assembly  2050  is dynamic in that it pet wits relative movement between the bone screw  2070  and the receiver  2072 . When the assembly is locked by the advancement of locking nut  2075 , an inner housing member  2077  is immobilized relative to the receiver  2072  and the contained rod  2055  while the bone screw is rigidly attached to the vertebral body. However, the head of the screw can move in a ball and socket manner within the inner housing member  2077  so as to permit continued movement between the bone screw and the interconnecting rod  2055 . 
         [0077]    When the screw head is moved out of a predetermined neutral position within the inner housing members, a material/device in space  2080  applies a force to the head of screw and resist any movement away from the neutral position. The assembly will return the screw and the attached bone to the neutral position once the deflecting force has dissipated. 
         [0078]      FIGS. 20F and 20G  illustrate potential alternative applications of the device.  FIG. 20F  shows a deformity in the alignment of two vertebrae such that the vertebral bodies are misaligned in the coronal plane. The condition, termed scoliosis, can be corrected by placing a device into the disc space at the site of height loss—as shown in  FIG. 20G . 
         [0079]      FIG. 21  shows another embodiment of an implant  2105 . The implant  2105  includes upper and lower components  2110  and  2115  and a middle component  2120 . The middle component is similar to the middle component of the previous embodiment in that it permits controlled movement between the upper and lower components. However, the middle component  2120  of the implant  2105  includes a pair of lever members  2125  and  2130 . Each of the lever members  2125  and  2130  is pivotably coupled to the lower component  2115  via a respective hinge member  2122 . The hinge members  2120  are substantially identical to the hinge member shown in  FIG. 13 . 
         [0080]    The lever member  2125  is substantially the same as the lever member of the previous embodiment although the lever member  2125  includes a seat  2135  that receives a bearing tip  2140  of the lever member  2130 . The tip  2140  can slide within the confines of the seat  2135 . Thus, the hinge members  2125  and  2130  collectively provide for relative movement between the upper and lower components of the implant  2105 . 
         [0081]      FIGS. 22 and 23  show another embodiment of an implant  2205 . The implant  2205  includes upper and lower components  2210  and  2215  and a middle component  2220  that permits relative movement of the upper and lower components. The middle component comprises a pair of legs  2225  that are pivotably linked to a hinge member  2230 . The legs  2225  include bearing ends that are slidably positioned within seats in the upper surface of the lower component  2215 . The seats limit the amount of movement of the ends of the legs  2225 . The arrangement of the legs  2225  and the hinge member  2230  impart a shock-absorbing quality to the device. Rotation may be prevented or preserved depending on the interaction of the ends of legs  2225  with the complimentary seats of the upper surface of the lower component  2215 . 
         [0082]    As shown in  FIG. 23 , the legs  2225  can move relative to the hinge member  2230  to permit controlled movement of the upper and lower components relative to one another. In addition, motion is further enhanced by the availability of a second bearing surface. Bearing surface  2239  is positioned atop hinge member  2230  and is affixed to the middle member of hinge  2230 . Each leg member  2225  is affixed onto a side member of hinge  2230 . The bearing surfaces  2239  may be of any known configuration such as, for example, the ball-and-socket arrangement of the prior embodiment ( FIG. 10 ) or the bearing arrangement of the following embodiment ( FIG. 24 ). 
         [0083]      FIG. 24  shows another embodiment of an implant  2405  that is sized and shaped for implantation into the disc space.  FIG. 25  shows perspective and cross-sectional views of the implant  2405 . The implant  2405  includes an upper component  2410  that is movably attached to a lower assembly  2415 . The components of the lower assembly  2415  are described below. The upper component  2410  includes an abutment surface that is configured to abut against bone. The upper component includes an indentation or seat  2510  ( FIG. 25 ) that movably mates with a protrusion  2515  on an upper portion of the lower assembly  2415 . The seat  2510  and protrusion  2515  mate in such a way that the upper component  2410  can articulate relative to the lower assembly  2415  such as in response to loads. 
         [0084]    The lower assembly  2415  is described in more detail with reference to  FIGS. 26 and 27 , which show partially exploded views of the implant  2405 . The lower assembly  2415  includes an outer frame  2610  and an inner frame  2615  that removably attach to one another to define an internal cavity  2620 . The outer frame and inner frame attach to one another in a manner that permits some movement therebetween such as along an upward and downward direction, as represented by the arrow U in  FIGS. 25 and 27 . In this regard, the inner and outer frame˜include shoulders  2520  ( FIG. 25 ) that define the amount of relative movement. Thus, the upper component  2410  can articulate relative to the lower assembly while the lower assembly can itself change shape by virtue of the relative movement between the inner and outer frames. 
         [0085]      FIG. 28  shows another exploded view of the implant  2405 . The lower assembly  2415  can further include a malleable member  2805  that is sized and shaped to fit within the cavity  2620  defined by the inner and outer frames. The malleable member  2805  is made of a material that deforms in response to loads and returns to its original shape upon removal of the load. The malleable member  2805  is attached to a clip  2810  that can be attached to the inner and outer frames to secure the malleable member  2805  within the cavity  2620 . 
         [0086]    In use, the implant  2405  is initially implanted with the malleable member  2805  unattached to the lower assembly  2415 .  FIG. 29  shows the implant  2405  without the malleable member such that the inner frame  2615  is fully seated in the outer frame  2610 . This permits the total implant height to be reduced. Once the implant  2405  is in a desired position in the disc space, the malleable member  2805  is placed within the cavity  2620  and locked into position. The presence of the malleable member raises the inner frame  2615  relative to the outer frame  2610  and increases the height of the implant.  FIG. 30  shows the implant  2405  with the malleable member  2805  in the cavity  2620  and without any load on the implant. After a load is placed on the implant  31 , the malleable member  2805  deforms in response to the load such that the height of the implant  2405  is reduced, as shown in  FIG. 31 . 
         [0087]      FIG. 32  shows another embodiment of an implant that is substantially similar to the implant shown in  FIG. 24 . In this embodiment, the lower assembly  2415  and upper component  2410  are movably coupled to one another in a ball-and-socket manner. The upper component  2410  has a socket that mates with a spherical protrusion on the lower assembly  2415 . In yet another embodiment shown in  FIG. 33 , an implant includes a middle assembly that is similar to the lower assembly described above for the previous embodiment. The middle assembly includes inner and outer frames  3315  and  3320  that move relative to one another. A malleable member  2805  is positioned within the inner and outer frames. Upper and lower components  3325  and  3330  are movably attached to the middle assembly in a manner that permits articulation of the upper and lower components relative to the middle assembly. 
         [0088]      FIG. 34  shows yet another embodiment of an implant  3405 . The implant  3405  includes an upper component  3410  that movably mates with a lower component  3415 . The lower component  3415  has a protrusion  3420  that movably sits within a seat  3425  on an interior surface of the upper component  3410 . The seat  3425  includes three protrusions  3430 . The protrusion has a toroid shape. The interaction of the toroid protrusion  3420  and the protrusions  3430  of the seat forms an articulation that has non-stationary center of rotation. 
         [0089]    In the intact spine, the extent of rotation between adjacent vertebrae is limited and excessive rotation will significantly increase the stress forces applied to the facet joint. For this reason, it is desirable to limit the amount of rotation permitted by the disc prosthesis regardless of the specific design of the articulation surfaces. In the natural motion segment, the range of rotation varies with the amount of flexion between adjacent vertebrae. That is, the amount of rotation permitted by the motion segment is significantly greater in flexion than it is in extension. Recreation of this property within the disc prosthesis is desirable since a fixed range of rotation will likely produce an insufficient rotational range in flexion and an excessive range in extension. 
         [0090]      FIGS. 35 and 36  show cross-sectional views of another embodiment of an implant. The implant  3505  has an upper component  3510  and a lower component  3515  that are adapted to articulate relative to one another. Each of the upper and lower components has an abutment surface that abuts bone when the implant is positioned in a disc space. When the upper component and lower component are attached to one another, a space  3517  exists where the upper and lower components can move relative to one another. An articulating spring member  3520  is positioned within the space  3517 . The spring member  3520  couples the upper and lower components together in a manner that permits relative movement but biases the upper and lower components toward default positions relative to one another. 
         [0091]      FIGS. 37 and 38  show the spring member of the implant  3505 . The spring member  3520  includes a main body  3705  that removably attaches to a cap  3710 . As shown in the cross-sectional views of  FIGS. 35 and 36 , the cap  3710  has a ledge that abuts the lower component  3515  and the main body  3705  has a shoulder that abuts the upper component  3510  to retain the spring member therein. 
         [0092]    FIGS.  39  and  40 A- 40 B show exploded views of another embodiment of an implant. The implant  3905  includes an upper component  3910  and a lower component  3915 . Both the upper component and lower component have outer abutment surfaces for abutting against bone when in the disc space. A bone segment  3917  (preferably allograft), bone graft substitute, and/or a growth factor-soaked (such as BMP, etc.) material is positioned within a cavity in the lower component  3915  and, with implantation, will fuse with the adjacent vertebral surface so as to increase the device anchoring onto the vertebrae. While illustrated in the lower component  3915 , this feature may be applied to either or both components. 
         [0093]    The upper component defines a cavity  3920  ( FIG. 40B ) in which the lower component is movably positioned. As shown in the crosssectional views of  FIGS. 41 and 42 , the cavity  3920  has a pair of slopes walls  4110  that incline inwardly moving upward within the cavity. Member  4115  is formed of two blocks  3925  ( FIG. 39 ) that are biased away from one another using at least one spring  3930 . The interaction of the spherical tips of blocks  3925  and sloped walls  4110  will allow the upper component to rotate and translate relative to the lower component in various planes. In addition, vertical load applied to the implant will cause the upper component and lower components to move toward and away from one another. This load is opposed by the action of horizontally-placed springs  3930 . Because of the action of the springs, member  4115  is biased outward and towards the sloped walls  4110 . Member  4115  exerts a force against the sloped walls  4110  that forces the implant toward a default shape wherein the biasing member  4115  is positioned at the bottom portion of the cavity  3920 , as shown in  FIG. 42 . 
         [0094]    In another embodiment, shown in  FIG. 43 , an implant  4305  has a similar configuration as the implant  3905 . However, the lower member includes a pair of biasing members  4305  that are biased outwardly toward the sloped walls  4110  via springs  4305 . 
         [0095]    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.