Patent Publication Number: US-11382760-B2

Title: Method for inserting and positioning an artificial disc

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/648,501, filed on Jul. 13, 2017, which is a continuation of U.S. patent application Ser. No. 14/721,095, filed on May 26, 2015, now U.S. Pat. No. 9,724,204, which is a continuation of U.S. patent application Ser. No. 12/466,772, filed on May 15, 2009, now U.S. Pat. No. 9,066,809, which are incorporated by reference herein in their entireties for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a prosthetic spinal disc for fully or partially replacing a damaged disc between two vertebrae of a spine. The present invention also relates to a method for implanting a prosthetic spinal disc via posterior or posterior lateral implantation, although other implantation approaches may also be used. 
     BACKGROUND OF THE INVENTION 
     The vertebrate spine is the axis of the skeleton on which a substantial portion of the weight of the body is supported. In humans, the normal spine has seven cervical, twelve thoracic and five lumbar segments. The lumbar spine sits upon the sacrum, which then attaches to the pelvis, and in turn is supported by the hip and leg bones. The bony vertebral bodies of the spine are separated by intervertebral discs, which act as joints but allow known degrees of flexion, extension, lateral bending, and axial rotation. 
     The typical vertebra has a thick anterior bone mass called the vertebral body, with a neural (vertebral) arch that arises from the posterior surface of the vertebral body. The center of adjacent vertebrae are supported by intervertebral discs. Each neural arch combines with the posterior surface of the vertebral body and encloses a vertebral foramen. The vertebral foramina of adjacent vertebrae are aligned to form a vertebral canal, through which the spinal sac, cord and nerve rootlets pass. The portion of the neural arch which extends posteriorly and acts to protect the spinal cord&#39;s posterior side is known as the lamina. Projecting from the posterior region of the neural arch is the spinous process. 
     The intervertebral disc primarily serves as a mechanical cushion permitting controlled motion between vertebral segments of the axial skeleton. The normal disc is a unique, mixed structure, comprised of three component tissues: the nucleus pulpous (“nucleus”), the annulus fibrosis (“annulus”) and two vertebral end plates. The two vertebral end plates are composed of thin cartilage overlying a thin layer of hard, cortical bone which attaches to the spongy, richly vascular, cancellous bone of the vertebral body. The end plates thus act to attach adjacent vertebrae to the disc. In other words, a transitional zone is created by the end plates between the malleable disc and the bony vertebrae. 
     The annulus of the disc is a tough, outer fibrous ring which binds together adjacent vertebrae. The fibrous portion, which is much like a laminated automobile tire, measures about 10 to 15 millimeters in height and about 15 to 20 millimeters in thickness. The fibers of the annulus consist of fifteen to twenty overlapping multiple plies, and are inserted into the superior and inferior vertebral bodies at roughly a 40 degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotates in either direction, relative to each other. The laminated plies are less firmly attached to each other. 
     Immersed within the annulus is the nucleus. The healthy nucleus is largely a gel like substance having high water content, and like air in a tire, serves to keep the annulus tight yet flexible. The nucleus-gel moves slightly within the annulus when force is exerted on the adjacent vertebrae while bending, lifting, and other motions. 
     The spinal disc may be displaced or damaged due to trauma, disease, degenerative defects, or wear over an extended period. A disc herniation occurs when the annulus fibers are weakened or torn and the inner tissue of the nucleus becomes permanently bulged, distended, or extruded out of its normal, internal annulus confines The mass of a herniated or “slipped” nucleus tissue can compress a spinal nerve, resulting in leg pain, loss of muscle control, or even paralysis. Alternatively, with disc degeneration, the nucleus loses its water binding ability and deflates, as though the air had been let out of a tire. Subsequently, the height of the nucleus decreases causing the annulus to buckle in areas where the laminated plies are loosely bonded. As these overlapping laminated plies of the annulus begin to buckle and separate, either circumferential or radial annular tears may occur, which may contribute to persistent or disabling back pain. Adjacent, ancillary spinal facet joints will also be forced into an overriding position, which may create additional back pain. 
     Whenever the nucleus tissue is herniated or removed by surgery, the disc space will narrow and may lose much of its normal stability. In many cases, to alleviate back pain from degenerated or herniated discs, the nucleus is removed and the two adjacent vertebrae are surgically fused together. While this treatment alleviates the pain, all disc motion is lost in the fused segment. Ultimately, this procedure places a greater stress on the discs adjacent to the fused segment as they compensate for lack of motion, perhaps leading to premature degeneration of those adjacent discs. 
     As an alternative to vertebral fusion, various prosthetic discs have been developed. The first prosthetics embody a wide variety of ideas, such as ball bearings, springs, metal spikes and other perceived aids. These prosthetics are all made to replace the entire intervertebral disc space and are large and rigid. Beyond the questionable applicability of the devices is the inherent difficulties encountered during implantation. Due to their size and inflexibility, these devices require an anterior implantation approach as the barriers presented by the lamina and, more importantly, the spinal cord and nerve roots are difficult to avoid during posterior or posterior lateral implantation procedure. 
     Anterior implantation, however, can involve numerous risks during surgery. Various organs present physical obstacles as the surgeon attempts to access the damaged disc area from the front of the patient. After an incision into the patient&#39;s abdomen, the surgeon is forced to navigate around interfering organs and carefully move them aside in order to gain access to the spine. One risk to the patient from an anterior approach is that these organs may be inadvertently damaged during the procedure. 
     In contrast, a posterior approach to intervertebral disc implantation avoids the risks of damaging body organs. Despite this advantage, a posterior approach also raises other difficulties that have discouraged it use. For instance, a posterior approach can introduce a risk of damaging the spinal cord. Additionally, vertebral body geometry allows only limited access to the intervertebral discs. Thus, the key to successful posterior or posterior lateral implantation is avoiding contact with the spinal cord, as well as being able to place an implant through a limited special area due to the shape of the vertebral bones. Because an anterior approach does not present the space limitations that occur with a posterior approach, current prosthetic disc designs are too bulky to use safely with a posterior approach. Therefore, a need exists for a method of surgically implanting a prosthetic spinal disc into the intervertebral disc space through a posterior approach with minimal contact with the spinal cord. 
     SUMMARY OF THE INVENTION 
     In general, the present invention is directed toward prosthetic disc designs. In one particular embodiment, an intervertebral artificial disc is provided with a first endplate having a plurality of protrusions for attaching to an adjacent vertebrae and an extension portion extending towards a second adjacent vertebrae. A second endplate is provided with a plurality of keels for attaching to a second adjacent vertebrae and an extension portion extending towards the first adjacent vertebrae. A flexible member having an upper portion and a lower portion and a slider plate positioned within the upper portion of the flexible member is also provided. The extension portion of the first endplate is adapted to fit within a first cavity in the upper portion of the flexible member and the extension portion of the second endplate is adapted to fit within a second cavity in the lower portion of the flexible member. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a side view of sequentially aligned vertebral bodies, such as are found in the cervical, thoracic and lumbar spine, and a posterior prosthetic spinal disc located between the vertebral bodies; 
         FIG. 1B  is a top view of one embodiment of a prosthetic spinal disc of the present invention; 
         FIGS. 2A-2B  illustrate a surgical approach that may be used for inserting the prosthetic spinal disc of  FIG. 1B ; 
         FIG. 3  is a view of a collapsed posterior prosthetic spinal disc that can be opened via scissor action; 
         FIGS. 4A-4B  are views of a segmented posterior prosthetic spinal disc and its assembly between vertebral bodies; 
         FIGS. 5A-5D  depict various expandable posterior prosthetic spinal discs; 
         FIGS. 6A-6B  show open-sided or C-shaped disc implants having a spring; 
         FIGS. 7A-7B  show open-sided or C-shaped discs having a flexible portion, curved end plates and stops; 
         FIGS. 8A-8B  show open-sided or C-shaped discs having slots that provide flexibility; 
         FIG. 9  shows a flat, generally rectangular or O-shaped disc having two slotted side columns; 
         FIG. 10  shows a flat, generally rectangular or O-shaped disc having an additional column in the center portion of the disc and slots in the outer columns; 
         FIG. 11  is an open-sided or C-shaped disc having a coil slot; 
         FIGS. 12A-12B and 13-14  illustrate the use of compressed elements in the present invention; 
         FIGS. 15-26  illustrate the use of varying types interfacing surfaces in the present invention to achieve or restrict movement in different directions; 
         FIGS. 27-29  illustrate one embodiment of the invention using oblong inserts; 
         FIGS. 30-35, 36A-36B, 37A-37C, 38-41, 42A-42D, and 43-45  illustrate the use of stiffness mechanisms, torsion bars, tension and compression springs that may be used in the present invention; 
         FIGS. 46-47  show one embodiment of the present invention utilizing a braided reinforcing material around a balloon or bladder; 
         FIGS. 48-49 and 50A-50B  show one example of the present invention; 
         FIGS. 51-54  illustrate an embodiment of the present invention having a fixed IAR; 
         FIGS. 55A-55B, 56, 57A-57C, 58A-58C, and 59-61  show an example of the present invention having two articulating surfaces; 
         FIGS. 62A-62H, 63A-63B, and 64-67  further illustrate prosthetic disc designs of the present invention and the use of a trial and chisel for preparing the treated area for insertion of disc assemblies; 
         FIGS. 68-81  illustrate steps used for preparing a treated area for insertion of a prosthetic spinal disc using a posterior approach; 
         FIG. 82  is an illustration of one embodiment of a prosthetic disc of the present invention; 
         FIGS. 83A-83B  illustrate two optional methods for distracting the treated area during insertion of a prosthetic disc; 
         FIGS. 84A-84B  illustrate selective interaction between a free end of an angled guide and a keyed recess of a trial; 
         FIGS. 85A-85B  show one embodiment of a disc assembly holder selectively engaged with a disc assembly; 
         FIG. 86  illustrates one embodiment of a tool used in the methods of the present invention; 
         FIG. 87  illustrates one embodiment of a tool used in the methods of the present invention; 
         FIGS. 88-111  illustrate various tools used in an embodiment of the methods of the present invention; 
         FIG. 112  illustrates an intervertebral disc space in which paths have been cut in vertebral bodies using the methods of the present invention; 
         FIGS. 113-116  illustrate various tools used in an embodiment of the methods of the present invention; and 
         FIGS. 117-118  illustrate prosthetic disc assemblies after implantation according to the methods of the present invention. 
         FIG. 119  illustrates yet another embodiment of the prosthetic disc assembly according to the present invention. 
         FIG. 120  illustrates an exploded view of the of the prosthetic disc assembly according to the present invention. 
         FIG. 121  a illustrates cross sectional views of the prosthetic disc assembly according to the present invention. 
         FIG. 122  illustrates a cross sectional view of another embodiment of the prosthetic disc assembly according to the present invention. 
         FIG. 123  illustrates a cross sectional views of yet another embodiment of the prosthetic disc assembly according to the present invention. 
         FIG. 124  illustrate two disc assemblies according to the present invention. 
         FIGS. 125 and 126  illustrates the disc assemblies of  FIG. 124  positioned in the spine. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates generally to a posterior prosthetic spinal disc for replacing a damaged disc between two vertebrae of a spine. The present invention also relates to a method for implanting a prosthetic spinal disc via posterior or posterior lateral implantation. In particular, the present invention encompasses a method for implanting the prosthetic spinal disc while avoiding or minimizing contact with the spinal cord. 
     As described in detail below, the prosthetic spinal disc may be articulating or non-articulating. In addition, the prosthetic disc may be formed of one, two, three or more units. For example, two units may be disposed in the medial-lateral direction at spaced apart locations, and the upper and lower portions of each unit have interfacing surfaces that forms an arc in the anterior-posterior direction. 
     If multiple units are used, they may be spaced apart from each other or connected to prior to insertion in the patient or as they are being positioned within the body. The ability to connect the units together may allow the prosthetic disc to be inserted using a posterior approach with less risk of injuring the spinal cord, nerve rootlets, lamina or the like. In addition, using a plurality of units, either connected or disposed in spaced apart locations, allows individual units to be interchangeable with a multiplicity of designs or configurations that allow the physician to address different physical conditions of the treated area of the spine and to custom tailor the range of motion that the prosthetic disc will permit. 
     Several embodiments of the invention illustrate different examples of how the interfacing surfaces of an articulating prosthetic disc may be formed. For instance, articulation may be accomplished with one interfacing surface, such as a ball and joint (see e.g.,  FIG. 21 ), or alternatively may be accomplished with two or more interfacing surfaces such as a core disposed between an upper and lower seating surface (see, e.g.,  FIG. 1A ). The configuration of the surface contact may vary to permit or restrict different types and ranges of motion of the treated area. Thus, the contact profile of the interfacing surface may be an area (such as with a ball and socket configuration), a line (such as with a roller or sleeve bearing), or a point (such as with a ball bearing). 
     The materials used for different embodiments of the invention will depend to some extent upon the type of surface contact being used as well as the type and extent of wear that may result. Examples of materials that may be used include, but are not limited to polyehylene (or other elastomeric material) on metal, metal on metal, or ceramic on ceramic. 
     The present invention also allows for customization of the instantaneous axis of rotation (IAR) and/or the center of rotation (COR) of one vertebral body with reference to another. The IAR and COR of a healthy vertebral body with respect to another is constantly changing in all planes because of pushing, pulling, and tethering of the segment through its range of motion by the ligaments, annulus, muscles, facets and other portions of the spine. Often, prosthetic disc replacement designs fail to mimic this varying IAR and COR. For example, a fixed ball and socket has a fixed IAR and COR. One potentially adverse result from using a prosthetic disc having a constrained implant is that the device may cause damage to facet joints due to anatomical interferences that may occur with a fixed axis of rotation. On the other hand, in general constrained JAR systems have been more stable than past designs utilizing a moving JAR. One example of a prosthetic disc having a fixed JAR is described in U.S. Pat. No. 5,314,477. 
     Conversely, past devices utilizing a moving IAR have provided the advantage of allowing for shear translation and of at least partially mimicking of the moving IAR of a healthy spine. These advantages, however, typically have been achieved in the past at the expense of a loss of stability of the device. Some examples of prosthetic disc designs having a moving IAR are described in U.S. Pat. Nos. 4,759,766, 5,401,269, and 6,414,551. 
     In contrast, the present invention allows for an implant design that can mimic or partially mimic this varying IAR and COR to the extent desired by a physician while also preserving stability of the device. For example, one embodiment of the invention is a prosthetic disc that provides a moving IAR substantially in the sagittal plane so that a patient can more easily flex and extent the spine while limiting the movement of the IAR under lateral bending. It is believed that this configuration provides the best of both worlds by allowing a moving IAR for the predominant or more common motion a patient may undertake in day-to-day life while limiting lateral bending to achieve greater stability to the device. Several embodiments of the invention permit translation of one vertebral body with respect to another. By allowing one of these members to translate in the transverse plane results in the IAR and COR also translating in the transverse plane. As explained further below, one additional way of achieving a varying IAR and/or COR in three dimensional spaces is by combining two articulating surfaces opposing one another. 
     The interfacing surfaces of articulating and non-articulating embodiments of the present invention also allow for varying degrees of rotational and linear translation, and several embodiments of the present invention likewise permit a similar range of rotation and linear translation. Rotational translation is the movement of the intervertebral segment as a result of movement such as flexion, extension, and lateral bending. There are two components in this translation: one in the cranial/caudal direction and one in the transverse plane. Linear translation is translation in the transverse plane as a result of shear forces applied to the intervertebral segment. Thus, a ball and socket mechanism fixed in one location relative to the intervertebral segment would allow only rotational translation but would not permit linear translation. As illustrated in many of the embodiments that follow, however, linear translation of a ball and socket configuration could be achieved if the ball and socket were able to move in the transverse plane. 
     Endplates are used to associate the prosthetic disc with the vertebral bodies neighboring the disc. The endplates may be configured in several ways to help ensure a desired endplate-bone interface. For instance, the endplates may have one or more keels that extends into the bony portion of the vertebral body. Over time, bony ingrowths will surround the endplate and further help secure the endplate to the vertebral body. 
     In addition to keels, the endplate may have other or additional geometry that helps securely hold the endplate in place. For example, the end plate may have one or more teeth, rails, ribs, flanges, or other configurations that can help provide a surface that can secure the endplate more readily to the bone. Other short-term fixation may include screws or other fasteners that hold the end plate to the vertebral body. In some embodiments, these fasteners may be removed once a more long-term interface has been established, or alternatively the fasteners may remain in place indefinitely or until the prosthetic disc needs adjustment and/or replacement. 
     In addition to providing an endplate surface geometry or configuration that may promote bony ingrowths to hold the interfacing surfaces together securely over the long term, these configurations also may help provide short term fixation of the endplate to the vertebral body. For example, a keel may have a wedge shape so that the width of a first end of the keel near the endplate is narrower than the width of the distal end. Once installed, the inverted wedge of the keel helps prevent separation of the endplate from the vertebral body at least until bony ingrowths can more securely hold the endplate in place. 
     To help accelerate and to further promote bony ingrowths at the interface between the vertebral body and the end plate, the end plate may be coated with an osteoconductive material and/or have a porous or macrotexture surface. For example, the end plate may be treated with a coating that promotes bone growth. Examples of such coatings include, without limitation, hydroxyl appetite coatings, titanium plasma sprays, sintered beads, or titanium porous coatings. 
     Figure IA is a side view of a posterior prosthetic spinal disc I located between sequentially aligned vertebral bodies  2  and  3 , such as are found in the cervical, thoracic and lumbar spine. Posterior prosthetic spinal disc I conforms in size and shape with the spinal disc that it replaces and restores disc height and the natural curvature of the spine. Posterior prosthetic spinal disc I comprises two opposite end plate  5  and  7  which are disposed in two substantially parallel horizontal planes when it is at rest, i.e., when it is not subjected to any load, either moderate or heavy. 
     The outer faces of end plates  5  and  7  are in direct contact with vertebral bodies  2  and  3  and may be textured or have a plurality of teeth to ensure sufficient contact and anchoring to the vertebral bodies  2  and  3 . The outer faces of end plates  5  and  7  may also have a porous or macrotexture surface that facilitates bone ingrowth so that the posterior prosthetic spinal disc I is firmly affixed to vertebral bodies  2  and  3 . Attached to the inner faces of end plates  5  and  7  are seating members  9  and  11  and a core  13  is securely placed between seating members  9  and  11 . A stop member  15  is formed around the equator of the core  13 , which functions to limit the motion of vertebral bodies  2  and  3  beyond a predetermined limit that is deemed unsafe to the patient. 
     As shown in Figure IA, the stop member may be formed from a ridge of material found on the core  13 . As the end plates move relative to the core in response to movement of the spine, the stop member may approach or engage with one or both of the end plates to restrict further motion in a particular direction. The stop member may be formed of a relatively rigid material so that additional motion is substantially prevented once engaged against an end plate. Alternatively, the stop material may be made of resilient material that provides some cushioning or flex from deformation of the stop material before the range of motion is fully limited. 
     While the stop member is shown in Figure IA as being on the core  13 , it also may be disposed on one or more of the end plates. For instance, the end plates may be configured with raised areas or ridges on its perimeter that engage with either the core or the opposing end plate in order to limit further motion in a particular direction. As mentioned above, the stop member on the end plate may limit motion to a greater degree in one direction than in another. Thus, the stop member may have various shapes and thicknesses to provide a variable range in motion that favors or disfavors movement in particular planes. For example, the stop member may have increased thickness on the side portion of the core to provide a more limited range of lateral motion of the spine while still allowing motion in the posterior/anterior direction. 
     The motion segment comprises a posterior prosthetic spinal disc  1  and adjacent upper and lower vertebral bodies  2  and  3 . The exact contours of the core  13 , seating members  9  and  11  and stop member  15  determine the range of motion allowed in flexion and extension, side bending, shear and rotation. 
     Figure IB is a top view of a posterior prosthetic spinal disc  1 , showing the top end plate  5  and top seating member  9 . The end plates may have various shapes that accommodate posterior insertion which avoids contact with the spinal cord. As shown in  FIG. 1B , end plates  5  and  7  may have a substantially irregular elliptical shape or curved convex portion that resembles a kidney-shape.  FIG. 2A  is a top view of a posterior prosthetic spinal disc  1  being inserted between sequential vertebral bones. The posterior prosthetic spinal disc I is guided in place with a first implant holder  17  via an angled posterior approach that ensures that contact with the spinal cord  19  is avoided. The posterior prosthetic spinal disc  1  is generally oriented in line with the longitudinal axis of the first implant holder  17 . Once the posterior prosthetic spinal disc  1  safely is maneuvered past the spinal cord  19  and in the desired position over the vertebral body  21 , the implant may be turned or rotated, such as from 60.degree. to 120.degree., so that it is oriented at about 90.degree. to the first implant holder  17 , as shown in  FIG. 2B . Reorienting the implant may be accomplished in many ways. For example,  FIG. 2B  shows that a second implant holder  23  may be attached on the contra lateral side of the spinal cord to reposition and distract the implant into its final implanted position. Once the posterior prosthetic spinal disc  1  is in place, the first implant holder  17  and the second implant holder  23  is detached from posterior prosthetic spinal disc  1 . 
     It is preferred that the posterior prosthetic spinal disc  1  closely mimics the mechanical functioning and the various physical attributes of the natural spinal disc that it replaces. In some instances, however, the prosthetic spinal disc may permit a more limited range of motion in one or more directions in order to prevent further spinal injury. In general, the prosthetic spinal disc can help maintain the proper intervertebral spacing, allow for proper range of motion, and provide greater stability. It can also help transmit physiological stress more accurately. 
     End plates  5  and  7 , seating members  9  and  11 , core  13  and stop  15  may be composed of a variety of biocompatible materials, including metals, ceramic materials and polymers. Such materials include, but are not limited to, aluminum, alloys, and polyethylene. The outer surfaces of the end plates  5  and  7  may also contain a plurality of teeth, maybe coated with an osteoconductive material, antibiotics or other medicament, or may have a porous or macrotexture surface to help rigidly attach the end plates to the vertebral bodies by promoting the formation of new bony ingrowth. Such materials and features may be used in any of the posterior prosthetic spinal discs described herein. 
       FIG. 3  is a collapsed posterior prosthetic spinal disc  30  that can be opened via scissor action, in which top end plate  32  and bottom end plate  34  are rotated along a pivot point  36  so that the longitudinal axes of top end plate  32  and bottom end plate  34  are substantially perpendicular. Accordingly, the surface area of the posterior prosthetic spinal disc  30  is increased to facilitate greater spinal support. The posterior prosthetic spinal disc  30  in collapsed form is sufficiently small enough to allow for posterior insertion while avoiding contact with the spinal cord. 
       FIGS. 4A-B  illustrate a posterior prosthetic spinal disc having two segments for each end plate. The segments may be inserted separately between vertebral bones and assembled or joined together. The first segment  40  is inserted between the vertebral bones while avoiding contact with spinal cord  19 . The second segment  42  is subsequently inserted between the vertebral bones while avoiding contact with spinal cord  19 , and assembled or joined with first segment  40 , forming an end plate having larger surface area. The first and second segments may be joined in any suitable manner to form an end plate. In one embodiment, the first segment has one or more protrusions and/or ridges that correspond to depressions, notches, or teeth in the second segment. The joining of the protruding regions of the first segment into the depressions of the second helps secure the two segments together. The same procedure is carried out for the second end plate. The size of the assembled end plates may otherwise be too large to insert between vertebral bones while avoiding contact with spinal cord  19 . 
       FIG. 5A  is an expandable posterior spinal disc  50  that comprises expandable end plates  52  and  54  that can slide open or expand to increase the perimeter or contact area of the end plate with the vertebral body onto which it resides. In its collapsed state, the expandable end plate  52  is small enough to insert between vertebral bodies while avoiding contact with the spinal cord. In its expanded state, the expandable posterior spinal disc  50  has a larger surface area on upper and lower surfaces  52  and  54 , which increases the contact area between the expandable posterior spinal disc  50  and the vertebral bones, or at least distributes loading over a greater surface of the vertebral bodies. 
     The expandable end plate may be formed of two or more segments that provide a low profile when in a collapsed state in order to facilitate a posterior approach during insertion. Once it is positioned over the vertebral body, however, it maybe expanded to increase the surface area of the end plate. The increased surface area helps provide greater stability of the end plate. Expansion of the end plate may be accomplished in several ways. In one embodiment, shown in  FIG. 5A , a first segment and second segment may be selectively expanded or slid open along a substantially linear edge or surface. Thus, when fully extended the end plate will have a substantially linear slot defined by the edges of the first and second segment edges. 
     Alternatively, a portion of the edge of the first and second segments may be curved or rounded as shown in  FIG. 5B . In this embodiment, the first and second segments may provide more balanced peripheral support of the core along its edges or sides. For instance, a curved or rounded portion of the first and second segments may help form a lip  66  that provides extended support of the core on one side than may be achieved from a linear slot. This configuration may help avoid cantilever loading of the core over the slot or opening between the edges of the first and second segments. In other words, lip  66  helps ensure that the connecting portion of the end plate  68  provided more evenly distributed support to the seat member  70 . 
     The additional lip of expandable posterior spinal discs can have other shapes, preferably being configured to reduce or minimize the occurrence of cantilever loads. For example,  FIG. 5C  shows an expandable posterior spinal disc  72  that comprises expandable end plates  74  and  76  that can expand along the latitudinal axis and comprises an additional lip  78  having a rectangular shape on end plate  74  and/or end plate  76 . In another example,  FIG. 5D  shows an expandable posterior spinal disc  80  that comprises expandable end plates  82  and  84  that can expand along the latitudinal axis and comprises an additional lip  86  having a triangular shape on end plate  82  and/or end plate  84 . Additionally, a posterior spinal disc may comprise expandable end plates that can expand along the latitudinal axis and comprise an additional lip having a convex curve. In both  FIG. 5C  and  FIG. 5D , additional lips  78  and  86  have sufficient overlap with seating members  79  and  88  respectively that facilitates reduction of cantilever loads. 
       FIGS. 6A-B  illustrate a non-articulating posterior prosthetic spinal disc  90  comprising a top end plate  92  and a bottom end plate  94  that are joined together at one end to form a C-shaped disc. A spring  96  is located where top end plate  92  and bottom end plate  94  meet or are joined at one end of each plate  92  and  94  and allow for flexible motion of vertebral bones. The spring can be modified to have various tensions depending on the desired range of motion. The portion that joins the top end plate  92  and bottom end plate  94  also may be flexible itself and, in conjunction with spring  96 , facilitates motion of the end plates  92  and  94 .  FIG. 6B  shows two separate non articulating posterior prosthetic spinal discs  90 , both of which can be inserted between the same two vertebral bones. The small size of non-articulating posterior prosthetic spinal discs  90  allows for easy insertion while avoiding contact with the spinal cord, and further provides greater freedom of motion because each non-articulating posterior prosthetic spinal disc  90  functions independently of one another. In general, the non articulating posterior prosthetic spinal discs encompassed by the invention have a C-shaped design, where openings, slots or springs create flexibility in the material to allow motion. 
       FIG. 7A  shows a C-shaped disc  100  having convexly curved end plates  102  and  104 , flexible portion  106 , and stops  108 . The outer surface of end plates  102  and  104  may contain a plurality of teeth, may be coated with an osteoconductive material, antibiotic, or other medicament, or may have a porous or macrotexture surface to rigidly attach the C-shaped disc to the vertebral bodies and promote formation of new bone. The flexible portion  106  is tapered and the amount of taper controls the flexibility of the C-shaped disc. For example, increasing the amount of taper increases the flexibility of the C-shaped disc. Flexibility may further be controlled by providing a slot  109  located at the flexible portion  106 . The slot may be cut in any shape and oriented in any manner within the flexible portion. The size of the slot may be varied to fine tune flexibility. For example, larger slot sizes provide flexibility of C-shaped discs. In another embodiment, more than one slot may be provided to increase flexibility. The stops  108  are located at the end opposite of the flexible portion  106  and limit the motion of the C-shaped disc  100 . The size of the stops  108 , as well as the amount of curvature of end plates  102  and  104  may be varied to control the range of motion of the end plates before the stops  108  touch. Once the stops  108  touch under moderate loads, the curved end plates  102  and  104  provide another range of motion under heavy loads that flatten and decrease the curvature of end plates  102  and  104 . 
       FIG. 7B  shows a C-shaped disc having stops  110  that are convexly curved to provide lateral flexibility. Once the stops  110  touch under moderate load, the curved surface allows the stops  110  to roll in order to facilitate some lateral spinal movement. The curvature of the stops can be varied to provide more or less lateral flexibility. In one embodiment, both stops  110  may be curved. In another embodiment, one stop may be curved while the other stop may be flat, convex, or have a different curvature. The stops also can have other surface shapes that allow for lateral flexibility, such as angled edges. In addition, slots may be formed on the lateral sides of the flexible portion to facilitate movement of end plates  102  and  104  in the lateral plane. The stops also may be curved or shaped to allow a greater degree of lateral movement in one direction than in another. 
       FIG. 8A  shows a C-shaped disc  120  having end plates  121  and  122 , stops  124 , and a flexible portion having an opening  126  and slots  128 . Stops  124  are located at the end opposite of the flexible portion and limit the motion of the C-shaped disc  120 . The size of the stops  124 , as well as the amount of curvature of end plates  121  and  122  may be varied to control the range of motion of the end plates before stops  124  touch. Once stops  124  touch under moderate loads, the curved end plates  121  and  122  provide another range of motion under heavy loads that flatten and decrease the curvature of end plates  121  and  122 . The flexible portion contains slots  128  running through the lateral axis and can have any shape. The flexible portion also contains an opening  126  that is bored out along the longitudinal axis and helps provide flexibility. The number of slots, the size and shape of the slots, and the size and shape of the opening enable fine tuning of flexibility, where, for example, increasing the number of slots, as well as increasing the size of the slots or opening, provides for greater flexibility. In one embodiment, the flexible portion may be located closer to the middle of the disc, forming a skewed H-shaped disc, such as illustrated in  FIG. 8B . The H-shaped disc allows for greater flexibility in the anterior and posterior directions. The outer surface of end plates  121  and  122  may contain a plurality of teeth or be coated with an osteoconductive material, have a porous or macrotexture surface to rigidly attach the C-shaped disc to the vertebral bodies, as well as to promote formation of new bone. 
       FIG. 9  shows a generally oval-shaped or O-shaped disc having end plates  131  and  132  and two flexible portions joining end plates  131  and  132  at the longitudinal ends. Each flexible portion contains slots  136  running through the lateral axis and can have any shape. Each flexible portion also contains an opening  134  that is bored out along the longitudinal axis and helps provide flexibility. The number of slots, the size and shape of the slots, and the size and shape of the opening enable fine tuning of flexibility, where, for example, increasing the number of slots, as well as increasing the size of the slots or opening, provides greater flexibility. Each flexible portion may have the same or different configuration of slot shapes, numbers and sizes, positioning, as well as size and shape of the opening. The flexible portions can also be placed near the midline of the disc. In addition, the end plates can have convex curvature such that at heavy loads, the O-disc can flex by decreasing the curvature of end plates  131  and  132 . The amount of curvature can be varied to provide different flexibilities. The outer surface of end plates  131  and  132  may contain a plurality of teeth or be coated with an osteoconductive material, have a porous or macrotexture surface to rigidly attach the C-shaped disc to the vertebral bodies, as well as to promote formation of new bone. 
       FIG. 10  shows a relatively flat double oval or O-shaped disc having an additional column in the center portion of the disc and slots in the outer columns. The disc has end plates  141  and  142 , and columns  144  having slots  146  that provide flexibility. With the additional column in the center of the disc, end plates  141  and  142  will have a lesser degree of flex when compared to the O-disc described in  FIG. 9 . Such a configuration is desirable in applications where a more rigid disc is required. The slots  146  may any shape, size or positioning and as shown, slots  146  are rectangular notches having a cylindrical hole formed at the inside end of each notch. The outer surface of end plates  141  and  142  may contain a plurality of teeth or be coated with an osteoconductive material, have a porous or macrotexture surface to rigidly attach the C-shaped disc to the vertebral bodies, as well as to promote formation of new bone. 
     As shown in  FIG. 10 , the central column may have a gap or opening where the lower portion of the column terminates below the terminus of the upper column. This gap, which in one embodiment can be from about 0.5 mm to about 5 mm, allows the end plates  141  and  142  to have some ability to flex initially until the upper and lower columns meet to prevent further compression. In another embodiment, one or more columns may be formed from a highly resilient material that can provide some limited motion followed by cushioning that increasingly resists further displacement as loading on the prosthetic disc increases. 
       FIG. 11  illustrates another embodiment of the present invention where a C-shaped disc has two end plates  151  and  152 , the posterior ends of which are connected by a flexible portion, and the flexible portion, and the flexible portion contains a coil slot  156  and an opening  154  that is formed along the longitudinal axis of the disc. The coil slot  156  and opening  154  provide flexibility and can be controlled by varying the size of the coil slot, number of spirals in the coil slot, as well as the size and shape of the opening  154 . The outer surface of end plates  151  and  152  may contain a plurality of teeth or be coated with an osteoconductive material, have a porous or macrotexture surface to rigidly attach the C-shaped disc to the vertebral bodies, as well as to promote formation of new bone. Thus, the end plates and flexible portion may be integrally formed from one material. 
     In another embodiment of the invention, illustrated in  FIGS. 12-14 , utilizes a combination of tensioned and compressed elements disposed between the upper and lower end plates. The tensioned and compressed elements may be springs, as shown in  FIG. 13 , or may be made of resilient material that provides suitable resistance to stretching or compression. The compression element helps support axial loading along the treated vertebral bodies so that their relative positions approximate a healthy vertebral body supported by a natural disc. Additionally, at least one tension element helps provide controlled bending or movement of the vertebral bodies relative to each other. 
     The tensioned or compressed elements may likewise be configured and adapted to allow for compression and translation as shown in  FIG. 12 . Referring to  FIGS. 13 and 14 , the compression element can be pivotally connected to the upper and lower end plates, thereby allowing translation of the end plates in at least one direction by rotating the compressed element about the pivots.  FIG. 14  shows that additional translation can also be provided in a second direction by configuring the pivoting connection such that the compressed element may slide along a rod or bar connected to one or more of end plates. As shown in  FIG. 13 , the first and second direction of translation can be generally orthogonal to each other. In this manner, a limited degree of translation permitted in any direction can be accomplished without affecting the range of translational motion in the second direction. 
       FIGS. 15-20  illustrate another embodiment of the invention including two or more implants that complement each other to form an arced or curved surface in the medial-lateral direction and in the anterior-posterior direction. Figure IS illustrates the curvature created in the medial-lateral direction, while  FIG. 16  shows the curvature created in the anterior-posterior direction. As shown in  FIGS. 17 and 18 , the complementary curved surfaces of the upper and lower portions of the implants allows the upper vertebral body to move relative to the lower vertebral body while also maintaining a distance between the bodies that approximates the height of a natural disc. In one embodiment it is preferred that the curvature of the implant components is spherical so that they cooperate and function similarly to a ball and socket. 
     The implants may be space close together or far apart according to factors such as the size of the vertebral bodies, the loading that the implants will undergo, and the range of motion desired. As the implants are moved either closer together or farther apart, however, the curvature of the sliding surfaces may be changed. For instance, in the embodiment shown in  FIG. 18 , the curvature of the upper and lower portions of the implants in the lateral-medial direction is based on a radius R 1  or R 2 . For implants separated further apart, the radius R 2  is larger to account for the increased space between the implants. Changing the radius R according to the spacing between the implants helps maintain a relatively uniform radius of curvature across the full length of the implants. 
     Referring to  FIGS. 19 and 20 , which are similar in orientation to  FIGS. 15 and 16 , the upper and/or lower portions of the implants may have stops to help limit motion in one or more directions. As shown in  FIG. 19 , for example, medial-lateral movement can be controlled or limited by including a stop on one or more sides of an upper or lower portion of the implant. As the stop engages with the opposing surface of the implant, further movement in that direction is restricted. Alternatively, a resilient material may be disposed between the stop and the opposing surface in order to provide cushioning and to allow resistance to further movement to increase progressively.  FIG. 20  illustrates that stops may be similarly used on one or more sides of the implant to limit the range of motion in the anterior-posterior direction. While the stops in  FIGS. 19 and 20  are illustrated protruding upwards or downwards, other configurations also may be used to create a stop or to limit motion. For instance, the sliding surface of the portions of the implants may be prevented from further movement simply by contacting the end plate of the opposing portion. 
       FIGS. 21-26  illustrate one embodiment of the invention where different surfaces of the prosthetic disc provide for different types of movement. For instance, upper portion indicated as B in  FIG. 22  may be configured so that the interfacing surface permits only lateral bending, while the lower portion A may have an interfacing surface that is a ball or rail having a radius that can translate for axial rotation. 
     Normally, during lateral bending the space between one side of neighboring vertebral bodies becomes larger while the space between the opposite side of the neighboring vertebral bodies gets smaller. One embodiment of the present invention helps mimic this characteristic of lateral bending by using a plurality of implants with upper and lower portions separated by oblong inserts. 
     As shown in  FIG. 28 , the oblong inserts are configured within the upper and lower portions of the implants at an angle so that during bending one insert rotates to help raise one lateral side while the other insert rotates in the same direction to help lower the opposing lateral side. To accomplish this combination of rising and lower of opposing sides of the vertebral body during lateral bending, the oblong inserts are positioned such that the upper ends of the insert are further apart than the lower ends 
     Preferably, the oblong inserts are positioned such that they are angled from abut 5.degree. to about 20.degree. from a vertical axis when the vertebral bodies are in a neutral position, i.e., under conditions when there is no lateral bending. More preferably, the oblong inserts are positioned such that the axis from the upper end to the lower end is from about 70 to about 130 off of a vertical axis when the vertebral bodies are in a neutral position. As shown in  FIG. 27 , the insert on the opposing side of the vertebral body is positioned at approximately the same angle, but at a mirror image of the first insert. In this manner, one side will become lower during lateral binding while the opposing side increases in height. 
     The amount of increase or decrease in height from rotation of the inserts during lateral bending can be controlled in part by the length of the inserts from the upper end to the lower end. Thus, a longer insert will permit a greater range of lifting or lowering than a shorter insert. In one embodiment, the length of the insert is from about 3 mm to about 15 mm. In another embodiment, the length of the insert is from about 5 mm to about 10 mm. 
     Additionally, the angle at which the inserts are initially positioned when the vertebral bodies are in a neutral position will also affect the degree to which there is a rise or fall in height from rotation of the inserts during lateral bending. For example, inserts that are angled only slightly off of a vertical axis will only be able to slightly raise or lower the height of the sides, whereas increasing the initial angle off of the vertical axis will allow more significant differences in height to occur. Thus, it is possible to control the degree of increase or decrease of height during lateral bending at least by either changing the length of the inserts or by changing the angle at which the inserts are positioned. For example, for the configuration shown in  FIG. 29 , the inserts may be positioned such that they are about 100 off of a vertical axis when the vertebral bodies are in a neutral position. In another embodiment, the angle may be from about 3.degree. to about ISO. 
     As discussed previously, the contacting surfaces of the upper and lower portions of an insert may be configured to have curved surfaces that allow varying degrees of lateral-medial movement or posterior-anterior movement. Stops also may be used to help further control or restrict movement. In addition to these features, stiffness mechanisms also may be used to provide greater resistance to movement.  FIG. 30 , for example, illustrates an upper and lower portion of an insert. A ring of elastomer is disposed in the space where the surfaces of the upper and lower insert meet. When compressed, the ring of elastomer adds non-linear resistance. 
     The use of elastomer to provide non-linear resistance to compression may be used in a wide variety of configurations in addition to a ring. In  FIG. 31 , for example, a plurality of elastomer protrusions or nubs  158  may be used to add stiffness or non-linear resistance to compression. Skilled artisans would likewise appreciate those other materials or structures may be used to increase resistance to compression. For example, one or more of elastomer nubs or protrusions in  FIG. 31  may be replaced with springs. Further illustration of this embodiment is shown in  FIG. 32 , where springs and/or elastomer  160  can be placed in tension at various locations between the upper and lower portions of the insert. 
     Yet another variation of this embodiment is to use one or more flexible cantilevers to provide increased stiffness or resistance to compression. Referring to  FIG. 33 , one or more rods  162  may extend from one portion of an insert, i.e., an upper or lower portion, toward the surface of the opposing portion of the insert. In one embodiment, one end of each rod is fixed to a portion of the insert, but is not fixed to the other portion of the insert. 
     Thus, one end is fixed to one portion of the insert while the other end is free to move or bend in response to loading. The free end may be in contact with the surface of the opposing portion of the insert or alternatively may be preloaded by pressing it against the surface of the opposing portion of the insert. In another embodiment, the free end does not contact the surface of the opposing portion of the insert until a predetermined amount of movement of one portion relative to the other has already occurred. 
     Once the free end contacts the opposing surface, the bar or rod will begin to bend in response to additional movement. As the bar bends, the bending forces resist any further movement or compression, and as the movement in a particular direction increases, the resistance increases as well. 
     As shown in  FIG. 33 , the free end may be curved, bent, or otherwise shaped to prevent or minimize wear of the surface of the opposing portion. The flexibility of each cantilever rod may be altered or adjusted to allow greater or more rapid resistance to motion in one direction than in another. For instance, cantilever rods placed to resist lateral bending may be more flexible or less resistant to movement than a cantilever rod used to resist anterior-posterior movement. 
     Cantilever rods also may be used to provide controlled resistance to rotational movement of the vertebral bodies.  FIG. 34  shows a top view of an insert having this embodiment of the invention. Mechanical stops may be disposed near the free ends of the cantilever rods so that once rotation increases beyond a certain point the free end engages with one of the stops and causes the cantilever bar to bend or resist further rotational movement. The torsional resistance created from the stops increases as rotation continues. 
     Another embodiment of the invention utilizes a flexible rod or shape memory metal rod near the center of the insert to provide a stop or to generate progressive resistance to flexing, extension, lateral bending, or rotation. One example of this embodiment is shown in  FIG. 35 , which illustrates a rod connected to a lower portion of an insert and extending upwards into a cavity of the upper portion of the insert. As with any of the embodiments described herein, the upper and lower portions of the insert may be configured to have a ball and socket configuration or a simple radius protruding portion and corresponding simple radius receiving portion, thereby permitting lateral medial movement, anterior-posterior movement, and rotational movement. 
     As the upper portion  164  of the insert moves relative to the lower portion  166 , the cavity wall eventually will contact the free end of the rod. If the rod is very stiff, contact with the cavity wall will stop further movement. In contrast, if the rod is flexible, it may bend in response to contact with the cavity wall, thereby providing progressive resistance to further movement in that direction. 
     The cross-sectional profile of the cantilever rods described herein may be any shape, and are not limited to circular cross-sections. For instance, the cantilever bars may have a generally rectangular cross-section, such as in  FIGS. 37A-C , so that it is more resilient to bending loads in one direction than in another. 
     Different cross-sectional shapes also may be used to provide resistance to rotational movement in the embodiment illustrated in  FIG. 35 . For instance, if the cantilever rod has a rectangular cross-section as illustrated in  FIGS. 37A-C , and extends into a non-circular cavity, rotational movement can cause the free end of the cantilever to contact the cavity wall. Once again, the stiffness of the cantilever can be varied to either prevent further rotation beyond a certain point (i.e., the cantilever acts as a full stop to further rotation), or the cantilever can flex or twist to provide progressively increasing resistance to further rotation. 
     In an alternative embodiment (as depicted in  FIG. 36A-B ), two or more rods may be disposed within the central portion at spaced apart locations so that rotation causes the plurality of rods to bend and impart torsional resistance to further rotation.  FIG. 38  illustrates another socket and ball compression mechanism according to the invention. The hinges may be placed at A to allow the socket and ball to “float”. Under compressive axial loading of the spine, torsion bars  168  may bend or flex to cushion the spine. 
       FIGS. 39-41  show a non-articulating insert according to the invention having two endplates attached to springs, preferably at least 2 or more independent springs. The springs allow for motion (translation), compression, and a combination of both (flexion/extension and lateral bending). As illustrated in  FIG. 42A  (showing an axial view of the spine), a single insert may be used with posterior or posterior lateral implantation. In addition, two or more inserts may be used, jointly or independently of each other. For example,  FIGS. 42B-C  shows two inserts, which may be oriented generally in an anterior-posterior direction or in a medial-lateral direction, whereas  FIG. 42D  depicts three inserts. Multiple inserts may have the ability to attach to one another after implantation. 
       FIGS. 43-45  illustrate an insert where pivots  170  are added to the non-articulating insert of  FIGS. 39-41 . The pivots allow motion, whereas the springs act as shock absorbers and restore the implant to a neutral position. The endplates may have teeth, a textured surface, chemical treatment, or other means to secure the implant to the vertebral body. 
     A hollow braid  172  may also be used to make the insert of the invention. As shown in  FIGS. 46-47 , the braid may be reinforced with metal struts for strength and fixation. In addition, the insert may have a hollow pocket  174  filled with a balloon or a bladder of a gel, fluid, elastomer, gas, or other material to mimic the annulus or nucleus. The balloon may be filled with air or fluid and can have various shapes, e.g., cylindrical, oval, circular, etc. 
     The following three examples further illustrate how several of the features described above may be implemented in a prosthetic disc. 
     The first example, shown in  FIGS. 48-49 and 50A -B, describes a prosthetic disc that may be designed to have an IAR that is either substantially fixed one location or alternatively may be configured to move in the axial plane. As shown in  FIG. 49 , a plurality of upper and lower portions may be inserted at spaced apart locations. Preferably, one upper and one lower portion forms an assembly that can be inserted at the same time. By forming, the disc from two assemblies as shown in the figures one assembly can be inserted on each side of the spinal cord, thereby greatly reducing the space needed in order to insert the disc. In this manner, many of the risks commonly associated with a posterior approach can be avoided or minimized. 
     As explained in detail below, the upper and lower portions may have segments that can be repositioned after the assembly has been positioned inside the patient in order to bring the interfacing surfaces of the upper and lower portions into their final position. 
     For example, the upper and/or lower portions may be configured with a movable segment that allows repositioning of the interfacing surface once the portion has been inserted into the patient&#39;s body. In this manner, the overall size of the assembly can be made more compact when inserting it into the body while also allowing the components of the assembly to be reconfigured once inside the body in order to achieve optimal positioning of the interfacing surfaces of the prosthetic disc. This, while  FIG. 49  illustrates the final positioning of two assemblies after the segments have been repositioned, the segments initially may be inserted into the body in a low-profile configuration, such as illustrated in  FIG. 50A , and then reconfigured to a second position, such as shown in Figure SOB, once in the treated area. The second position allows the implant to perform its intended function, while the first position provides a low-profile insertion of the assembly. As shown in  FIG. 48 , one way to allow repositioning of the segments is to provide a track on which the segments may slide. 
     The segments may be configured such that a first assembly may be inserted independently and then interlock with corresponding segments of a second assembly, as shown for example in  FIG. 49 . Alternatively, the segments may be configured such that even after repositioning they do not contact a corresponding segment. In any of these embodiments, a locking mechanism may be used to fix the position of the segment relative to the portion it is associated with in order to prevent unintended repositioning of the segment after the surgical procedure is completed. One example of such a locking mechanism is the use of a protrusion or detent. 
     To help minimize the profile of the assembly during insertion, one segment may be configured such that the assembly has a lower overall height during insertion than when all of the components of the assembly are in their final position within the patient.  FIGS. 50A-B  illustrate this feature of the invention. In particular, the segment associated with the upper portion of the assembly is configured such that it slides along the interfacing surface of the segment associated with the lower portion of the assembly. The upper portion may be configured with one or more tracks or channels that guide a corresponding number of protrusions or keels on the upper portion of the upper segment. Thus, the upper segment is able to rotate and slide down the surface of the lower segment in order to lower the height of the assembly during insertion. Once inside the body, however, the segment can be slid into its final position. As this occurs, the overall height of the assembly will be increased. In one embodiment, the overall height of the assembly may be increased from about 0.1 mm to about 3 mm, and in another, the distraction caused by repositioning the segment may be from about 0.5 mm to about 1.5 mm. 
     The second example of the present invention, illustrated in  FIGS. 51 to 54 , also uses two assemblies and is configured to have a fixed IAR. The upper and lower portions of the assembly may have interfacing surfaces that are substantially spherical in curvature and that have substantially the same radius of curvature so that the overall configuration of the sliding surfaces provides a surface contact over an area as opposed to a line or point. In this example, the assemblies of the upper and lower portions are not configured with slidable segments as described in the example above. Because the sliding surfaces in this example are substantially spherical in curvature, proper alignment of each portion of each assembly is important to achieve a desired surface contact over an area instead of a line or point. 
     The third example of the present invention is shown in  FIGS. 55-61 . This example uses two articulating surfaces in a three component assembly to provide a moving IAR in the anterior-posterior direction only. As described in the examples above, two assemblies may be used to provide a low profile during insertion. Each assembly is formed of three components: an upper portion  176 , a lower portion  178 , and a central element  180  having upper and lower surfaces that interface with corresponding surfaces of the upper and lower portions. It should be understood that the orientation of the surfaces described below may be placed on an upper or lower component and that the invention is no restricted or limited to only the orientation described below. One interfacing surface is configured in a similar manner as provided in Example 2, above. That is, the interfacing surface is substantially spherical in curvature such that the surface contact is over an area instead of over a line or a point.  FIGS. 58A-C  illustrate the spherical surface interface  184  that may be disposed between the upper portion and the central element. 
     The second interfacing surface is formed of two cylindrical surfaces  182  that permit rotational sliding essentially in one direction (i.e., about one axis). As shown in  FIGS. 57A-C , the lower surface of the central element has a generally cylindrical shape  182  protruding downward, while the lower portion has a corresponding cylindrical shaped groove  182  formed therein that receives the cylindrical shape of the central element. Preferably, the radii of curvature of both cylindrical shapes are approximately the same such that the surface contact is over an area instead of a line. In this manner, the cylindrical surfaces can be configured to permit bending while restricting rotation. Thus, during flexion or extension both interfacing surfaces permit movement, while only one interfacing surface may permit lateral bending or axial rotation. 
     In an alternative embodiment, however, a second cylindrical interfacing surface can be substituted for the spherical surface. This second cylindrical interfacing surface may be disposed orthogonally to the direction of the first cylindrical interfacing surface. In this manner, one surface will permit motion in one direction, such as flexion and extension, while the second will permit lateral bending. 
       FIGS. 59-61  illustrate the types of motion that may be achieved using a first interfacing surface that is generally spherical with a second interfacing surface that is generally cylindrical.  FIG. 59  illustrates a disc disposed in a neutral position having a disc height H. During extension and flexion, the disc can provide rotational translation in the axial and in the anterior-posterior direction. Under these conditions, the overall height of the disc can change. Additionally, however, the disc also permits linear translation without changing the height of the disc. As shown in  FIG. 61 , the upper and lower portions can translate with respect to each other without also having to rotate. 
     As shown in  FIGS. 62A-H , a pair of disc assemblies may be used to form a prosthetic disc of the present invention. One advantage of using multiple assemblies is that a posterior approach may be used to position them into a treated area. A plurality of disc assemblies having varying heights, widths, lengths, and ranges of translation and rotation capability may be provided in a kit to a physician so that the final selection of the proper disc assembly can be made during the surgical procedure. For instance, a plurality of disc assemblies may be provided having disc heights varying from about 10 mm to about 20 mm. In one embodiment, the disc heights may differ by a uniform increment, such as differing by about I mm or by about 1.5 mm within a range. 
     Likewise, the length of the disc assembly may be varied to accommodate different anatomies. For instance, disc assemblies may have longitudinal axes that range from about 20 mm to about 28 mm. Incremental changes in the length of the assemblies may also be provided in a kit, such as by providing disc assemblies of different lengths in 2 mm increments. In another embodiment, a plurality of assemblies may have at least 2 different lengths that differ by more than about 3 mm. For instance, one set of disc assemblies may have a length of about 22 mm, while another set is about 26 mm in length. The length of the disc assembly preferably may be selected to maximize implant/endplate contact area. 
     A plurality of assemblies may also be provided with differing ranges of axial rotation. For instance, one or more assemblies may have no restriction on rotational movement, or may have stops or other devices that prevent rotation only after the rotation has exceeded the range of motion of a natural, healthy spine. Some assemblies may limit a range of axial rotation to .+−.15.degree., .+−.10.degree., .+−.5.degree., or .+−.2. degree. 
     Other disc assemblies of the present invention may permit a range of axial rotation in one direction, but restrict it in the opposite direction. In other words, a disc assembly of this embodiment may permit limited disc rotation so that a patient may rotate or turn their body to one side or in one direction, but not in the other. For example, a disc assembly may allow rotation or movement between a 0.degree. position, where the spine is not rotated or turned, to up to about 5.degree., up to about 8.degree., up to about 10.degree., or up to about 15.degree. in one direction only. 
     As described above, a cylindrical surface may be provided in a disc assembly in addition to a second, curved surface corresponding to a portion of a sphere. One feature of this combination of surfaces is that the disc can permit translation between the upper vertebral body and the lower vertebral body neighboring the treated area. 
     In one embodiment, the disc is capable of permitting translation of up to about 3.0 mm in the anterior-posterior direction, while in another embodiment the disc is capable of translation of up to about 5 mm. Some disc assemblies may permit even more translation, such as up to about 7 mm or even up to about 10 mm. As illustrated in  FIGS. 62A-H  and described in depth above, mechanical stops  186  may be provided to limit the range of motion of the disc assembly.  FIG. 62C  also illustrates that spacing of multiple assemblies may be important for providing a generally spherical surface, if one is desired. For instance, it may be desirable for the central longitudinal axes of the assemblies to be approximately 9-16 mm apart, and more preferably from 11-14 mm apart. 
     The upper and lower portions of a disc assembly may be configured with a keel  188  that can engage with or contact a neighboring vertebral body. One advantage of providing a keel is that it may be used to guide the assembly into position during insertion into a treated area of the spine. For instance, as illustrated in  FIGS. 63A-B  and  64 - 65 , a channel or groove may be cut out of a vertebral body next to the treated area. Then, a physician may insert the assembly into the vertebral body so that the keel slides in the groove or channel. The keel and grove or channel may be substantially linear or straight, or alternatively, may be curved or arched so that the assembly rotates and slides into position. 
     The use of one or more keels may also increase bone to implant surface contact, thereby decreasing the likelihood that the assembly will shift or move about of position. In one embodiment, the increase in surface contact may be about 5% or more, which in another embodiment the increase may be about 15% or more. 
     The cross-sectional profile of the keel may have different shapes. For instance, the cross-sectional profile of the keel may have the shape of a wedge, a truncated wedge, a rectangle, or a square. As shown in  FIG. 63A , the channel or groove may be cut to have a cross-sectional profile corresponding approximately to the shape of the keel. One advantage of the keel having a truncated wedge cross-section is that a similarly shaped channel or groove may ensure that the keel engages with the bony surface. This configuration may also provide increased resistance to expulsion of the disc assembly. 
     Over time, it is believe that the stability of the disc assembly in the treated area will further increase as bone growth engages with outer surfaces of the disc assembly. To facilitate this growth and increased stability, all or part of the surfaces of the disc assembly that engages or otherwise contacts bone may be treated to promote bony on growth. For instance, titanium plasma may be provided on the keel or other portions of the assembly to provide a matrix for bone growth. In addition, the keel may be configured with notches, slots, or openings formed along its length. As bone grows into these openings, the disc assembly will become more securely anchored in place. 
     As a disc assembly is first inserted into a treated area, it may need to be repositioned, rotated or otherwise moved. For instance, repositioning the disc assembly may be needed so that the keel can properly engage with the channel or groove. As shown in  FIG. 62G , the leading edge Le of the disc assembly may be configured without a keel. Thus, in one embodiment the assembly can be partially inserted into the treated area without the keel engaging with or contacting the vertebral body. In one embodiment, the length of the leading edge is from about 1 mm to about 10 mm, while in another embodiment the leading edge is from about 2 mm to about 5 mm. Alternatively, the length of the leading edge may be from about 1% to about 20% of the length of the component on which it is disposed, or may be from about 2% to about 10%. The length of the component may be determined by measuring the longitudinal central axis of the portion or component on which the leading edge is disposed. 
     In addition, referring again to  FIG. 620 , the keel may have an initial portion that is sloped or gradually increases in height. Providing a ramped portion may aid in aligning and inserting the keel into a groove or channel formed in a vertebral body. 
     The present invention also encompasses a method for implanting a posterior prosthetic spinal disc. In particular, the method comprises removing a defective vertebral disc using conventional methods and instruments; separating or distracting adjacent vertebral bodies to permit insertion of the posterior prosthetic spinal disc; inserting and positioning the posterior prosthetic spinal disc using a posterior or posterior lateral insertion that avoids contact with the spinal cord; and relieving the separation or distraction of the adjacent vertebral bodies. 
     As will be explained in detail below, there are several variations in which the present invention may be used to provide a replacement or prosthetic disc for a patient that restores or maintains a more natural range of motion. While a single disc assembly may be used to establish the artificial disc within a patient, it may be preferred in some cases to provide more than one artificial disc assembly. Vertebral bodies having larger sized endplates, for instance, may benefit from using two or more disc assemblies, or subassemblies to create an artificial disc in a treated area. For example, a disc assembly that is from about 9 mm wide may only need an insertion window that is from about 9 mm to about 11 mm of wide. In one embodiment, the insertion window needed to deploy a disc assembly is from about 7 mm to about 15 mm wide, and more preferably is from about 9 mm to about 12 mm wide. 
     Several benefits may be realized from using multiple disc assemblies. For instance, one result of using multiple assemblies may be that the smaller insertion windows may not require as significant motion or retraction of the aorta or vena cava. For example, in one embodiment, movement of the aorta in the present invention for inserting one of a plurality of disc assemblies is less than half the distance of repositioning that would be required if the prosthetic disc were made of a single, full size assembly. In addition, using multiple disc assemblies may allow a shorter duration of time during which the aorta, vena cava or other anatomy is moved out of its natural position. In one embodiment, for example, the duration of time that the aorta or vena cava is moved for inserting one or a plurality of disc assemblies is less than half of the duration of time normally required to insert a prosthetic disc made of only one assembly or unit. In addition, the smaller insertion windows that can be achieved from using multiple disc assemblies will likely make it easier to access the disc space from as well as allow for greater options in the approaches that may be used. 
     Furthermore, the use of multiple assemblies may reduce the frequency and/or the amount of retraction needed during insertion and positioning of the assemblies. For example, if two disc assemblies are used in a posterior approach, a central region of the treated area in the anterior-posterior direction may have sufficient space for placing a distractor. As a result, other benefits from this configuration may also be achieved. For instance, in many embodiments of the invention it may be useful to ensure that the prosthetic disc is positioned properly along the midline of the vertebral body in the anterior-posterior direction. By using a distractor in the central region of the treated area, the present invention may allow a physician to select a midline of the prosthetic disc with respect to the vertebral body, distract the vertebral bodies with the distractor in the central region, conduct an x-ray or other procedure to confirm that the selected midline of the prosthetic disc is approximately the same as the midline of the vertebral body, and make any desired adjustments of the distractor location before inserting a disc assembly. In one embodiment, the physician&#39;s selected location of the midline of the prosthetic disc differs from the midline of the vertebral body by less than about 3 mm, and more preferably differs by less than about 1 mm at any point along the length of the part of the distractor located between the vertebral bodies. If the difference between the selected location of the midline of the prosthetic disc and the confirmed midline of the vertebral body falls outside an acceptable tolerance, the physician may then reposition the distractor and either reconfirm its new position or continue with inserting the disc assemblies after the adjustment is made. Once the distractor is in an acceptable or desired position, the disc assemblies may then be placed within the treated areas. The distractor location may be used with or without other tools or devices to help ensure correct placement of the assemblies with respect to the anterior-posterior midline of the vertebral bodies. 
     A disc assembly may comprise tree component parts: an upper rigid plate, a lower rigid plate, and a central core or core element. The core element is disposed generally between seating surfaces of the upper and lower plates. The seating surfaces of each plate may be contoured to provide a desired range of motion. For example, one or more of the seating surfaces may have a substantially spherical curvature. In this manner, the seating surface may generally correspond to a portion of a ball or a socket. The central element may likewise have a contoured surface that generally has the same curvature as the seating surface it contacts. Thus, a spherical-shaped seating surface can receive or contact a portion of the central element having a spherical contour having a similar radius of curvature. The contact between the two surfaces may therefore correspond to a portion of a ball and socket. 
     Providing a spherical surface allows the two components to rotate and slide across the contacting surfaces in a manner that would permit bending and rotation ozone vertebral body relative to another. If these two contacting surfaces were the only elements allowing movement, the IAR of the disc would be constant. Providing a second contacting surface allows the disc to mimic a variable JAR of a healthy disc. For example, a second contacting surface between the second rigid plate and the central element may have a cylindrical contour, preferably allowing the core element to provide rotation in the anterior-posterior direction. Thus, it is preferred that the cylindrical surfaces of the second rigid plate and core element have an axis of rotation that extends approximately in a lateral direction. 
     The combination of a spherical shaped surface contact between one plate and a portion of the core element with a second generally cylindrical contacting surface between another plate and another portion of the core element allows the disc to have a variable JAR. This configuration also allows for translation ozone vertebral body relative to another vertebral body without requiring either vertebral body to rotate and without requiring the distance between the vertebral bodies to increase or decrease. 
     The curvature of the seating surfaces of the plates may be concave and the corresponding contoured portions of the core element may be convex to provide contact between the surfaces. Alternatively, one or more of the contoured surfaces of the core element may be concave and the seating surface for which it engages likewise may be inverted. For example, in one embodiment the core element may have a contoured convex surface that it semi-spherical or generally corresponds to a portion of a spherical surface, and a contoured concave surface that is semi-cylindrical or generally corresponds to a portion of a cylinder. One advantage of this configuration is that is may be capable of achieving a lower overall height than a core element having two convex contoured surfaces. 
     As described previously, more than one assembly may be used to form a disc. For example, a second assembly may be provided having a similar arrangement of plates and a core element. When disposed in a treated area, one or more components of an assembly may contact or even interlock with a corresponding component of another assembly. For instance, the seating surfaces of plates disposed on the bottom of two assemblies may be independently inserted into the treated region and subsequently joined. Conversely, the assemblies may be disposed at a predetermined distance from the other. For example, if two or more assemblies have contoured semi-spherical surfaces with a large radius of curvature, the assemblies may be separated by a predetermined distance so that the two contacting surfaces operate as component parts of a ball and socket configuration. 
     The configuration of the contacting surfaces of the disc may be varied depending upon the surgical approach used to insert the assembly. For instance, in one embodiment a facet capsule may be removed from one side of a vertebral body to provide access to the treated area from a transforaminal approach. The endplates of the vertebral bodies in the treated area may then be cut or otherwise prepared for receiving an assembly. Preferably, the bony anatomy of the vertebral body that defines the vertebral foramen still encloses this region after the removal of the facet capsule. Once the treated area is prepared, an assembly may be inserted. In addition to a posterior or transforaminal approach, other approaches can be used with the present invention, including, but not limited to posterior-lateral, lateral, or anterior approaches. 
     With a transforaminal approach, the direction or path in which the assembly is inserted may form an angle with an axis extending in the anterior-posterior direction. Because the approach to the treated area is at an angle, the seating surfaces may be configured to provide a desired functionality. For example, as described above, the assembly may have a cylindrical seating surface having an axis that extends generally in a lateral direction of the spine. Thus, the plates of the assembly may have a longitudinal axis that generally corresponds to the path in which the assembly is inserted, and the axis of rotation of the cylindrical contoured surface of the core element may form an angle from about 20.degree. to about 70.degree. of the longitudinal axis. More preferably, the angle between the longitudinal axis of the plate and the core element axis of rotation forms an angle from about 30.degree. to about 60.degree. 
     When a facet capsule is removed, the rotational stability of the vertebral body may be compromised. Since anatomy that helps prevent excessive rotation of the vertebral body is removed, it may be beneficial to provide a mechanical stop that prevents rotation in the compromised direction. In one embodiment, the stop only permits rotation of less than 10 degrees in one direction, and more preferably prevents rotation greater than 7 degrees. In other embodiments, the stop only permits rotation from about I to about 7 degrees or from about I to about 5 degrees in one direction. If the facet capsule on the opposing side of the vertebral body is still intact, it may not be necessary to provide a mechanical stop for rotation in the opposite direction. In this manner, a rotational stop may be provided only when anatomy aiding in this functionality has been removed. 
     It is preferred that the contact between the seating surface of a plate and a contoured surface of a core element extends over an area rather than a line or a point. More preferably, all contact surfaces of the invention extend over an area. However, if a convex surface semi-spherical surface were formed with a smaller radius of curvature than the corresponding concave surface, it would be possible to have the contact between the two surfaces correspond to a point contact. Likewise, a convex cylindrical surface may be formed to be smaller than the concave cylindrical surface it engages with in order to form a contact surface corresponding to a line. 
     The plates also may be configured to engage more securely with the vertebral bodies that they contact. For instance, one or more raised ridges or keels may extend at least partially into the endplate of the vertebral body. The vertebral body likewise may be prepared by cutting a similar number of grooves or channels that will receive the keels. The grooves or channels may help guide the assembly into proper position in the treated area. This feature may be particularly beneficial when a certain orientation of the assembly relative to the vertebral body is desired. 
     The ridges or keels and corresponding channels or grooves also may be straight or curved to match the desired insertion path of the assembly. In one embodiment, the cross-section of a ridge or keel may be triangular or have a truncated triangular shape. As mentioned above, if more than one assembly is being used, it may be desirable for the assemblies to be separated by a predetermined distance. The grooves or channels formed in a vertebral body may help achieve the proper orientation and distance of the assemblies. 
     To date, no tool or device has been developed that can provide these features to ensure proper insertion of a multi-assembly artificial disc. As shown in  FIGS. 63A-B  and  64 - 65 , a trial  190  may be used to accurately form channels or grooves at a predetermined distance. Turning to  FIG. 64 , a trial  190  may be used to aid in cutting upper and/or lower channels in facing endplates of two vertebral bodies. Additionally, the trial may smooth portions of the endplate surfaces where an assembly may travel or ultimately be disposed. The trial may be inserted in a direction that corresponds to the path that will be used to insert the assembly. As mentioned above, the insertion path of the assembly may not always correspond to anterior-posterior axis of the vertebral bodies. For instance, an angle formed between the direction of the insertion path for the assemblies and the anterior-posterior axis may be from about 20.degree. to about 70.degree., or may be from about 30.degree. to about 60.degree. The path also may form a circular arc having a radius of curvature corresponding to the curvature of the ridges or keels of the plates. In this manner, the assembly may be rotated or turned into its final position as it moves along the channels or grooves. 
     Once the first channel and groove or plurality of channels and grooves has been formed, a guide  192  may be used to determine where a second set of channels or grooves may be formed. In general, the guide  192  is in communication with and extends from the first trial  190 . As shown in  FIGS. 63B and 64-65 , the guide  192  may be disposed within a central portion of the trial  190 . Once the trial is in its proper position, the guide may then be deployed a predetermined distance. Turning to  FIG. 65 , a portion of the free end of the guide may have a configuration that can receive a second cutting tool  194 . The second cutting tool  194  may then be used to form a second plurality of grooves or channels and to prepare a second region of the treated area to receive a second assembly. The guide  192  and trial  190  may then be removed and the assemblies inserted into the treated area. 
     The plates used to contact with the endplates of the upper or lower vertebral bodies of the treated area should have sufficient size to distribute loading over an area of the vertebral body to prevent failure of the endplates. Thus, one or more of the rigid plates may have a length from about 25 to about 32 mm, and more preferably from about 28 to about 30 mm. Likewise, the width of one or more plates may be from about 10 to about 18 mm, and more preferably is about 12 to abut 14 mm. 
     In another embodiment illustrated in  FIGS. 66-67 , a trial may be capable of connecting with a handle having a detachable grip. In one embodiment, the trial may have a chisel guide  196  and keyed recess  198 . This tool, among others may be used to facilitate installation of one or more disc assemblies from a posterior approach in the following exemplary manner. 
     As shown in  FIG. 68 , a physician may first perform a discectomy in the treated area. In one embodiment, the discectomy is performed so that a perimeter region of the annulus is not removed. For instance, 1 mm to 7 mm, and more preferably 3 mm to 5 mm, wide region along the perimeter of the anterior side of the vertebral body may remain after the discectomy is completed. 
     When viewed from the posterior side, the spinal cord may obstruct the view of a central portion of the vertebral bodies thereby leaving two posterior sides of the vertebral body for inserting disc assemblies. If desired, a distractor may be used on the contralateral side while a trial is inserted on the other side. When a posterior approach is used, a preferred embodiment of the invention is to use 2 disc assemblies where one is placed in the treated area from one side of the spinal cord and the other is inserted from the other side. 
     In another embodiment, the trial itself may be used to distract the vertebral bodies. The physician may assess the treated area and select a suitable disc a suitable disc assembly from a plurality provided in a kit. Factors that may be considered when selecting a disc assembly may include, among others, the footprint of the disc assembly, lordosis, disc assembly height, and size. 
     As shown in  FIG. 62C , if one or more sliding surfaces of the prosthetic disc is substantially spherical in curvature, it is desirable to position the disc assemblies a predetermined distance apart from each other and in proper alignment to allow portions of the 2 disc assemblies that form the sliding surface to cooperate. Providing a keel on each disc assembly may be useful for properly separating (if needed) and aligning each assembly with respect to each other and possibly also with respect to the treated area. For instance, 2 disc assemblies may be configured such that a keel on one assembly should be approximately 13 mm from the center of a keel on the second disc assembly. The distance between keels may be varied to account for differences in the radius of curvature of the sliding surfaces, the location of the keel on each disc assembly, the condition of the anatomy in the treated area, and the like. 
     While the precise distance between keels does not need to be specified, the physician should understand how to align and position the disc assemblies. For instance, the distance between keels for proper alignment may be selected from a range from about 5 mm to about 20 mm, or from about 10 mm to about 15 mm, and the selected distance may then be provided to the physician or accounted for in the tools provided to the physician. 
     In one embodiment, each of the two disc assemblies is positioned and aligned a predetermined distance from the midline of the vertebral body in the anterior-posterior direction. For instance, as shown in  FIG. 71 , the trial may be inserted into the treated area on one side of the spinal cord such that the center of the chisel guide, when properly positioned, is from about 3 mm to about 10 mm from the midline of the vertebral body. More preferably, the center of the chisel guide when properly positioned is from about 4 mm to about 8 mm from the midline of the vertebral body. 
     Once the trial is in its proper position, the grip of the handle may be removed. Preferably, the handle is formed oat least a detachable grip and a shaft in communication with the trial. When the grip is removed, the shaft may then be used as a guide rod for additional tooling and instruments. 
     For example, once the grip is removed, the shaft may be used as a guide for applying a chisel to form grooves or channels in the treated area. More specifically, with reference to  FIGS. 70-72 , a chisel  200  may be provided that can slidingly engage with the shaft of the handle to help ensure that the chisel is positioned properly for forming a channel or groove in one or both vertebral bodies adjacent to the treated area. In one embodiment, a portion of the chisel  200  forms a tube  206  or aperture having a cross-section corresponding approximately in the cross-section of the handle shaft. The tube or aperture may be slightly larger to allow the chisel to move more easily along the length of the shaft. 
     As shown in  FIG. 70 , the end of the chisel that impacts against, cuts or otherwise contacts the vertebral bodies has chisel blades  202  that may be shaped and configured to form grooves or channels in the vertebral bodies of a desired shape. Thus, in one embodiment, the cross-sectioned shape of the chisel blade is a truncated wedge. In one embodiment, the cross-section of the chisel blade may be approximately the same as the cross-section of the keel of the disc assembly. The end of the chisel opposite the chisel blade may have an enlarged impaction face  204 . Thus, the physician may align and position the chisel blades  202  against one or more vertebral bodies neighboring the treated area and strike the impaction face  204  to drive the blades into the vertebral bodies. As the chisel blades are worked into the treated area, the blades may be guided and maintained in proper position by slidingly engaging with the chisel guide  196  formed on the trial. Preferably, the length of the chisel may be selected such that the chisel blades have progressed to their desired position when the impaction face is flush with the handle shaft. 
     In one embodiment, the chisel blade may be selectively detached from the chisel. As shown in  FIGS. 72-74 , for example, the impaction face  204  and chisel tube  206  may be separated from the chisel blades and removed. Likewise, the trial may be selectively detached from the handle shaft. Thus, it is possible to remove these components of the instruments and leave the trial and chisel blade in the treated area, as shown in  FIG. 74 . 
     Turning to  FIG. 75 , with the trial and chisel blades remaining in position, the spinal cord may be repositioned or moved slightly to provide access to the contra-lateral side of the treated area. As previously discussed, the trial may be configured with a keyed recess  198 . The keyed recess  198  is positioned so that it faces toward the contralateral side of the treated area (i.e., toward the midline of the vertebral body in the A-P direction). Alternatively, the trial may be configured with two keyed recesses  198  formed on opposing lateral faces of the trial. This configuration would permit the trial to be inserted on either side of the spinal cord. As shown in  FIG. 76 , an angled guide  208  may then be inserted into the treated area on the contra-lateral side of the area from the trial and chisel blade. Preferably, the angled guide comprises an angled head  210  and a shaft  212  that is substantially straight. Thus, the shaft  212  may be substantially parallel to the longitudinal axis  214  of the chisel blades when the angled guide is properly connected into the keyed recess. 
     The angled guide may be selectively engaged with a keyed recess of the trial so that is may be attached or removed as desired. Preferably, the angled guide is only capable of engaging with the keyed recess at one angle and orientation. In other words, the angle with which the angled guide is inserted into the keyed recess is predetermined and known. In some embodiments, the angled guide and the keyed recess may have complementary surfaces that allow a surgeon to determine when the angled guide has been fully inserted into the keyed recess. Once the angled guide is in communication or proper registration with the keyed recess, the shaft extending outward of the treated area may then be used to insert a second chisel blade into the treated area. As shown in  FIG. 76 , the cross section of the shaft of the angled guide may be generally oval, but it also may be rectangular, square, triangular, oblong, elliptical, or have some other shape that helps prevent rotation of a chisel blade as it is being inserted. Of course, it is preferable that the second chisel blade has a tube or aperture corresponding generally to the shape of the cross-section of the angled guide. Preferably, the second chisel is substantially the same size as the first. The blade may then be placed on the shaft of the angled guide and positioned near or adjacent to the vertebral bodies. A chisel tube and impaction face may once again be employed to drive the chisel blade into the treated area. 
     One advantage of engaging the angled guide with the keyed recess is that the chisel blades into the contra-lateral side of the vertebral body may be inserted at a known distance away from the first set of chisel blades. Another advantage of using the angled guide may be that the chisel blades on the contra-lateral side of the vertebral body may be inserted substantially parallel to the first set of inserted chisel blades. In one embodiment, the angled guide is preferably configured and dimensioned such that the chisel blades on the contra-lateral side are inserted between about 8 mm and about 16 mm away from the first set of chisel blades. More preferably, the chisel blades on the contra-lateral side are inserted between about 10 and about 15 mm away, and most preferably, the chisel blades on the contra-lateral side are inserted between about 12 mm and about 14 mm away from the first set of chisel blades. 
     Once the chisel blade has been fully placed or inserted into in the contra-lateral side of the treated area, it may then be removed from the treated area along with the angled guide. In one embodiment, both the chisel blade and angled guide are removed at the same time (i.e., the angled guide may be removed with the chisel blade still disposed on the shaft). As shown in  FIG. 78 , a disc assembly  216  may then be deployed into the contra-lateral side. To facilitate insertion of the disc assembly  216 , an implant holder  218  may be used to securely grip the assembly until its keels are inserted into the grooves or channels formed by the chisel.  FIG. 78  illustrates one embodiment where the implant holder may selectively engage with the rear-most or posterior side of the disc assembly. As shown in  FIGS. 62A and 78 , rearward ends of the upper and lower portions of the assembly may have receptacles  220  that allow the holder to securely grip the assembly. In addition, hooked tips  222  formed on the holder may selectively engage with the assembly components. Configuring the disc assembly and implant holder in this manner allows the overall height and width needed for insertion of the disc assembly to remain at a minimum. 
     Alternatively, the implant holder  218  may engage with the outermost upper and lower surfaces of the disc assembly, on either side of the keels. However, this configuration may require the vertebral bodies to be distracted during insertion, thereby potentially causing the first chisel blade and/or the trial to become dislodged from their positions. Additionally, an implant holder may grip the disc assembly from the lateral sides; however, this too may require an increase in the overall size of the window or opening needed in order to insert the disc assembly. Thus, while the use of these alternative embodiments may fall within the scope of the invention, some may have disadvantages. 
     Once the keels of the disc assembly have begun to be positioned on over the channels or grooves, the implant holder may be used to push the disc assembly into the treated area. As the disc assembly nears its final position, resistance between the vertebral bodies and the surfaces of the disc assembly may significantly resist further progress. If desired or needed, gentle impact forces may be applied to the implant holder to aid in moving the disc assembly into position. 
     The first chisel blade and trial may then be removed and a second disc assembly inserted in a similar manner. In particular, the chisel blade may be operatively connected with the chisel tube or another instrument and then withdrawn from the body. Likewise, the handle shaft, and optionally the grip, may be reconnected to the trial so that it too can be withdrawn. The removal of the trial and chisel blade can be performed at the same time or sequentially. Once the trial and chisel blades have been removed, the second disc assembly may be inserted.  FIGS. 68-80  generally illustrates how two disc assemblies may be inserted from a posterior approach into their desired positions. 
     As mentioned previously, the keel of a disc assembly may be configured to promote or permit bony ingrowth that may help hold the disc assembly in place more securely.  FIG. 82  illustrates one embodiment of a keel having a plurality of slots or cuts formed in it.  FIGS. 62E and 62H  also show other examples of slotted keels. Returning to  FIG. 82 , the slots or cuts may extend at an angle, such as from about 5.degree. to about 40.degree. off from a vertical direction, and more preferably from about 10.degree. to about 30.degree. A keel may have two or more, or even three or more slots or cuts. One skilled in the art would appreciate that other configurations may also be used to promote bony ingrowth that might help further secure the disc assembly in place. For instance, the keel may have holes or apertures drilled into it, longitudinal or horizontal slots may be formed, and the sidewalls of the keel may be textured with one or more grooves or channels that does not extend fully through the keel to the opposing sidewall. 
     In addition, the face of the keel that first inserted into a groove or channel may have a taper or chamfer. One potential advantage of configuring a keel with a taper or chamfer on its face is that it may assist in aligning the keel with the opening of the channel or groove. In addition, a chamfered or tapered face may help reduce drag forces and undesired cutting or gouging of the channel or groove as the keel is pushed toward its final position. 
     One advantage of providing multiple assemblies to form the artificial disc is that it allows the assemblies to be placed into position without significant vessel retraction. Thus, insertion from the anterior of the vertebral bodies can be achieved with minimal repositioning of the vena cava or aorta. Because the wall of the vena cava is a thin, it punctures or tears more readily than other vessels. 
     Conversely, the wall of the aorta is thicker than the vena cava, and therefore more resistant to tearing or punctures, but the pressure of the blood supply is considerably higher. As a result, damage to the aorta can result in significant blood loss. Therefore, one benefit of a multi-assembly artificial disc is the reduced need to disturb or move these major blood vessels. 
     Another advantage to using a multi-assembly configuration is that it permits a physician to adjust or replace one or more assemblies from a different approach than used during the original insertion of the disc. When an implant is placed in a region of the spine, a region surrounding the area of insertion can become obscured or blocked by scar tissue that gradually forms after the procedure. This scar tissue can also bind to neighboring anatomy, including the major blood vessels so that it is extremely difficult to reuse the insertion window again without substantial risk to the patient. 
     When multiple assemblies are used, however, it is possible to use a second approach to adjust, remove, or replace the artificial disc. For instance, if disc assemblies are inserted into position from the anterior side of the vertebral body, it would be possible to remove or adjust the assemblies using a posterior approach using the methods, tools, and techniques described herein. Likewise, a multi-assembly artificial disc can be inserted from a posterior direction, thereby leaving the anterior side available for future access to the disc. 
     In some instances, it may be desirable to use a second approach to adjust, remove, or replace an artificial disc at a later time. For example, a disc may be inserted during a first surgery. Normal body movement over a period of time may then necessitate adjustment of the artificial disc. The present invention allows a surgeon to re-enter the vertebrae using a second approach. The second approach may be done at any desired time. For example, a second surgery using a second approach may be performed about six months or more after the first surgery. More preferably, a second surgery using a second approach may be performed about one year or more after the first surgery. Most preferably, a second surgery using a second approach may be performed about five years or more after the first surgery. 
     As discussed previously, where more than one implant or assembly is inserted into the intervertebral space, precise placement of each insert may be desired. Because the articulating surface of each assembly relies or cooperates with the articulating surface of its corresponding assembly, precise placement of the assemblies is preferable. More particularly, precise placement refers to the placement of one assembly such that it is aligned and spaced apart from the other assembly or assemblies in such a way that each articulating surface of the respective assemblies may cooperate with each other to form an effective range of movement as if there were a unified articulating surface. For example, with respect to the embodiment disclosed in  FIGS. 86 to 110 , precise placement means that each keel of each assembly lie parallel to each other. Additionally, each assembly would be spaced apart from the other assembly such that each independent articulating surface allows the articulating surface of its corresponding assembly to act as or mimic one complete surface, i.e. one semispherical articulating surface. In one particular embodiment, each assembly should be inserted at a proper predetermined depth within the intervertebral space, and each assembly should be inserted generally to the same depth. In assuring such precise placement, methods and tools have been developed in accordance with the present invention for ease of implantation and accuracy. Furthermore, in controlling each of these positioning variables, a surgeon should be careful during the entire procedure to avoid contact with the spinal cord while working within confined spaces. 
     In an exemplary embodiment of the present invention, methods and tools are provided for inserting more than one assembly of a prosthetic disc. The methods and tools relate to positioning a second implant based on the position of a first implant. As discussed previously, any number of methods may be used to position a second implant based on the position of the first implant. As used herein, “based on” means the positioning or placement of a second object, path, cut, or other item as determined by a position or placement of a first item. In the embodiment described below, the positioning of a second implant may be accomplished by using a path cut by a first chisel to determine the path cut by a second chisel. 
     In general, prior to insertion of the prosthetic disc, the intervertebral space is prepared. In one variation, a surgeon performs a lamenectomy or laminotomy to remove all or part of the lamina. This procedure is used to create a “window” through which the surgeon may access the intervertebral space. In some instances, a surgeon may perform a total discectomy, in which the disc between two vertebrae is removed. Alternatively, a surgeon may perform a partial discectomy, in which only a portion of the disc is removed. Partial discectomies typically leave a portion of the annulus of the disc intact on the anterior portion of the interverterbral disc space. The present invention is not limited to any particular type of disectomy, whether complete, partial or otherwise. 
     In one embodiment, another step in the process of inserting a prosthetic disc according to the present invention may include preparation of the upper and lower surfaces of the vertebral bodies. In this step, a surgeon may scrape the upper and lower surfaces of the vertebral bodies. Scraping the surfaces may cause some bleeding, which may improve the chances of bony growth into the inserted assemblies. 
     In another embodiment, the disc space is prepared by inserting various tools to help loosen the muscles and ligaments that keep the disc space together. In this embodiment of the present invention, a paddle distractor may be used. With reference to  FIG. 86 , a paddle distractor  250  is provided. Paddle distractor may have a handle  252  that is attached to an elongated shaft portion  254 . At the end of elongated shaft portion  254  the paddle distractor has an area shaped as a paddle  256 , or an area that is wider than its thickness. The length of paddle area or paddle head  256  may vary, although one of skill in the art would recognize that said length would be sized appropriately so as to enter the intervertebral space without interfering with surrounding tissue or other internal body parts. The shape of the paddle area allows a surgeon to insert the instrument in one direction, Le. generally fiat or so that face  258  of paddle area  256  is generally parallel to the upper and lower surfaces of the upper and lower surfaces of the vertebral bodies between which it is being inserted. Once inserted, a surgeon may rotate handle  252  in turn causing paddle area  256  to rotate within the vertebral space. As sides  260  and  262  contact the upper and lower surfaces of the vertebral bodies, the disc space is distracted and the muscles and ligaments are loosened. This helps prepare the disc space for insertion of the trial. 
     As one of skill in the art would understand, the size and shape of the paddle distractor may vary and various sized instruments may be provided to accommodate different areas of the spine or distraction preferences by the surgeon. Furthermore, distractors of various sizes may be used to within the same space to distract the space in a step wise fashion. As seen in  FIG. 86  handle  252  may be releasably attached to shaft  254 . In this fashion, a single handle may be provided with a set of paddle distractors with various sizes of paddle heads such that a surgeon may select different paddle distractors according to surgeon preferences and patient anatomy. While any number of sizes may be used, in one embodiment a set of paddle distractors with paddle heads having widths of 10 mm to 17 mm, with a separate paddle distractor for each different millimeter width, is provided. In some embodiments, a set of paddle distractors are provided wherein there is a paddle distractor, with an appropriately sized paddle head, for each trial in the set or kit. 
     In an alternate embodiment, a dilator may be provided. With reference to  FIG. 87 , a dilator is shown. As seen in  FIG. 87 , dilator  264  is similar to paddle distractor  250  except that the end of dilator  264  is configured as a generally regular pyramid. As one of skill in the art would understand, the shape and configuration of the dilator can aid the surgeon in preparing the disc space by serving as a wedge as the dilator is inserted into the intervertebral space. As further seen in  FIG. 87 , demarcations  266  along the generally pyramid area  268  correspond to the thickness or width of the area at that point. Thus, a dilator with widths or thickness between 8 mm and 12 mm may be used and the surgeon may distract the intervertebral space by a desired amount by inserting the dilator to the appropriate depth. In an embodiment of the present invention, a set of dilators may be provided that correspond to a range of distraction sizes. For example, in one embodiment, a surgeon may be provided with three dilators that contain a range of sizes of about 6-12 mm, 8-14 mm, and 10-16 mm. As with the paddle distractor, handle  252  may be releasably attachable such that a surgeon may use the same handle for different dilators. As one of skill in the art would understand, a kit may be provided that contains both dilators and paddle distractors. In this embodiment, a single handle may be used for both the different dilators and different paddle distractors. 
     After preparing the intervertebral space the next step performed according to one embodiment, a surgeon determines the appropriate size of the assembly to use in the procedure as well as the desired position of the assembly. The present invention contemplates tools and assemblies of various sizes to help a surgeon determine the appropriate prosthetic disc to implant. Trials, of various sizes, are commonly used in this type of surgery to “test fit” items inserted into intervertebral spaces. Specially configured trials may be provided to aid in the positioning of a second assembly based on the position of a first assembly. 
     As seen in  FIG. 88 , a first trial  300  is shown. In  FIG. 88 , only lower vertebral body  30  I is shown. Also, in the following figures, the various instruments and assemblies are shown offset from the vertebral body. This view is provided to show more of the various tools and assemblies and one of skill in the art would understand that in practice, the assemblies and tools would lie within the intervertebral space. As one of skill in the art would also understand, an upper vertebral body is also present but, for the sake of visual clarity, not shown. In operation, first trial  300  may be inserted into the intervertebral space  303  and the surgeon may then orient the trial  300  to determine the best position for the implant. 
     As discussed previously, a number of different trials of varying sizes may be provided. The surgeon may test different trials to determine the size, angle, and length of the implant appropriate for the patient. The surgeon may select the appropriate size trial based on a number of criteria including restoring disc height to the appropriate level or implanting assemblies based on the structural characteristics of the upper and lower vertebral bodies, including for example, surface area and strength of the surfaces. 
     As seen in  FIG. 88 , trial  300  comprises a first portion  305  that includes a trial head member  307  and a trial shaft member  309  and a second portion  311  having a shaft member  313  and a handle member  315 . Trial head member  307  is configured to mimic the size and shape of a prosthetic disc assembly. Trial head member  307  is further configured with keyed recesses  317 ,  319 . Keyed recesses  317 ,  319 , as discussed in more detail below, are configured to receive and help guide chisel blades. 
     Trial shaft member  309  of first portion  305  of trial  300  extends along an axis. Trial shaft member  309  of first portion  305  is connected to trial head member  307 . As seen in  FIG. 88 , in one embodiment, trial shaft member  309  is connected at a position offset from the centerline of trial head  307  in the direction away from the spinal cord (not shown). As one of skill in the art would understand, the offset connection imparts increased functionality to trial  300 . The offset connection decreases the amount of distraction of the spinal cord to access the intervertebral space. The offset further reduces the risk that the trial might contact the spinal cord, thus reducing the chance of injury to the nervous system of the patient. 
     Trial head member  307  is further configured with a flat face  321  on the proximal side of trial head  307 . Flat face  321  is configured as a stop when trial  300  is used with a chisel, which is described in more detail below. As seen in  FIG. 88 , trial shaft member  309  of first portion  305  may also be shaped to accommodate the spinal cord. Area  323  of trial shaft member  309  of first portion  305  is carved out on the side facing the spinal cord to further reduce the potential contact between trial shaft member  309  and the spinal cord. This area may be carved out from a portion of the shaft such that there is a reduced thickness of the shaft in the area of the spinal cord. The shaft  309 , as seen in  FIG. 88 , has other areas that are thicker to maintain rigidity and add strength to the tool. As one of skill in the art would understand, variations could be employed to achieve similar results. 
     At the proximal end, trial shaft member  309  of first portion  305  connects to second portion  311  of trial  300 . Second portion  311  of trial  300  also includes a handle member  315 , which is connected to shaft member  313  of second portion  311 . As seen in  FIG. 88 , trial shaft member  309  and shaft member  313  can be connected to each other. In one embodiment, handle member  315  may include a mechanism by which it may be operatively attached and removed from first portion  305 . In one variation, the mechanism is a lever  325  that may be actuated by a user. Shaft member  313  of second portion  311  is configured with a hollow area to receive the proximal end of trial shaft member  309  of first portion  305 . Trial shaft member  309  of first portion  305  may also be configured to engage lever  325  (hidden). One of skill in the art would understand that any number of different connection types including but not limited to threaded connections, pins and slots, friction fits, or others may be used to connect trial shaft member  309  of first portion  305  with shaft member  313  of second portion  311 . 
     As further seen in  FIG. 88 , handle portion  315  is configured for a user to grasp. Handle  315  is designed to provide a surgeon with control over the insertion and positioning of trial  300  within the intervertebral space. In one embodiment, handle  315  may also be configured with a flat face  327 . Flat face  327  of handle  315  is designed to give a surgeon an impaction surface. Surgeons or other users may strike the impaction surface with a hammer or other tool to drive or insert the trial into the intervertebral space, if desired. 
     As further seen in  FIG. 88 , trial  307  is configured with through holes  318  and  320 . Through holes  318  and  320  may be used by the surgeon to position the trial within the intervertebral space. As one of skill in the art would understand, a surgeon may take radiological or other images of the trial position during surgery. Through holes  318  and  320  provide the surgeon with a reference point. For example, in one embodiment, through hole  318  is placed at the center of the trial head. Thus, during implantation, the surgeon may use through hole  318  to position the center of the trial. This may be useful to a surgeon to allow him or her to determine the point at which the assemblies of the prosthetic disc will be implanted, and hence the location(s) of the instantaneous axis of rotations. Similarly, through hole  319  may be used to measure whether the trial head is sufficiently within the intervertebral space. For example, by taking a lateral image of the intervetrebral space, the surgeon can determine whether the trial has been placed at a sufficient depth such that the assemblies will be fully within the intervertebral space when implanted. 
     In one embodiment, the trial has been positioned according to the preferences of a surgeon, second portion  311  may be detached. Referring to  FIG. 89 , first portion  305  of trial  300  is shown after second portion  311  of trial  300  has been detached. Referring to  FIG. 89 , the proximal end of shaft  309  may be configured with an engagement area  329 . Engagement area  329  interacts with lever  325  of second portion  311  of trial  300  (not shown). 
     Referring to  FIG. 89 , trial shaft member  309  of first portion  305  of trial  300  may be configured with a particular shape. In an exemplary embodiment, shaft  309  is configured with a generally rectangular shape. The shape of shaft  309  is designed to cooperate with other tools and parts used in the method. Accordingly, the hollowed out area of the second portion  311 , and more particularly shaft  313 , may be configured to match the generally rectangular shape of shaft  309 . Thus as should be readily apparent, the shaft  313  may only be inserted over shaft  309  in two orientations. This provides the tools with orientation preferences, which when applied to other parts of the method prove useful. As one of skill in the art would understand, the particular shape may be changed and may include no limitations on the orientation of the various pieces (such as in circular shapes) or only one orientation allowed (such as a scalene triangle shape). 
     In one embodiment, a laminectomy centering guide is provided. Referring to  FIG. 90 , lamenectomy centering guide  312  comprises a template piece  314 . Template piece  314  is sized and dimensioned to approximate the needed window or space that needs to be created in the lamina. Template piece  314  may be keyed to trial shaft portion  309 , as seen in  FIG. 90 . Template piece may also be connected to a handle  316  so a surgeon may position the template piece  314  over the lamina to use said template piece  314  as a guide and then excise the required tissue/bone as desired. In an embodiment of the present invention, the handle is angled with respect to the front face of the laminectomy centering guide to provide the surgeon with a better line of sight. In an embodiment of the present invention, the handle is angled by 10.degree. In an embodiment of the present invention, the handle is angled by between about 3.degree. and 20.degree. A kit may be provided wherein there is more than one laminectomy centering guide. In such an embodiment, various sizes of a guide are provided. Accordingly, a kit may contain a guide for each of the differently sized implants. In some embodiments, the handle may be detachable from the template guide. In this fashion, various templates may be provided and only one handle is needed. As one of skill in the art would understand, the precise configuration of the template piece and handle may vary, and the methods of the present invention contemplate providing template pieces of varying sizes to accommodate different patient requirements or surgeon preferences. 
     In one variation of an embodiment of a method according to the invention, after detaching second portion  311  of trial  300 , paths may be cut in the upper and lower surfaces of the vertebral bodies within the intervertebral space. The paths cut into the surfaces of the intervertebral space are generally configured and dimensioned to correspond to accommodate the keels on the endplates of the prosthetic disc assemblies. Accordingly, the paths cut, their size, their angle, etc. each relate to the specific type of keel (and their configurations) used in the prosthetic disc being inserted. 
     Referring to  FIGS. 89 and 91 , trial head  307  of trial  300  is configured to aid in the alignment and guidance of the chisel. In  FIG. 91 , chisel  329  is shown after insertion onto first portion  305  of trial  300 . Chisel  329  has a blade portion  331 , a shaft portion  333 , and a handle portion  335 . Blade portion  331  is forked having two blades  337  and  339  connected to a central member  341 . Blade portion  331  is configured and dimensioned such that upon insertion, blades  337 ,  339  partially extend above and below trial head  307 . As best seen in  FIG. 91 , trial head  307  is configured with two recesses  317  and  319 , which are keyed into the upper and lower surfaces of trial  307 . Blades  337  and  339  partially ride within keyed recesses  317  and  319  as the chisel blades are driven into the vertebral bodies to create or clear a path in the bone. As one of skill in the art would understand, trial head  307  is configured to receive and guide blades  337  and  339  of chisel  329 , however, alternate chisels and blade designs may be employed to achieve similar results. 
     Referring to  FIG. 91 , chisel portion  331  is connected to shaft portion  333 . As described above, in one embodiment, the chisel portion  331  may be offset from the shaft portion  333  of chisel  329 . This design feature maintains the offset configuration discussed with respect to the trial to maintain the path-cutting tool away from the spinal cord. 
     Shaft portion  333  of chisel  329  is configured with a hollow section so that it may fit over and slidingly engage shaft  309  of trial tool  300 . Similar to shaft  309  of the trial tool  300 , shaft  333  of the chisel tool  329  may be cut away on the side facing the spinal cord, as seen in  FIG. 91 . This feature (as the similar feature does in the trial tool) creates additional space on the side of the spinal cord to minimize potential contact or injury with the spinal cord. In one embodiment, the hollow section of shaft portion  333  of chisel  329  is shaped to match the shape of shaft  309  of trial tool  300 . As discussed previously, this feature requires insertion of chisel  329  over trial  300  at a particular orientation that ensures that the blades will be positioned correctly, i.e. with the blades cutting a pathway into the upper and lower surface of the vertebral bodies. As one of skill in the art would understand, a two-orientation configuration is adequate where the upper and lower keels of the prosthetic disc are similar, equivalent, or the same. Where the keels of the prosthetic disc assembly have different keels (and hence the paths that need to be cut need to be different), a single orientation device may be desired. 
     Handle portion  335  is connected to shaft portion  333  of chisel  329 . In one embodiment, handle portion  335  contains a shaft member  343  connected to an impact member  345 . Shaft member  343  is hollow and shaped to receive shaft  309  of trial  300 . In one variation, impact member  345  is cylindrical in shape and has a flat face  347 , which serves as an impact area. Flat face  347  may have a through hole  349 . As chisel  329  is driven into position (guided by trial  300 ), central member  341  will contact trial head  307  at the final insertion point, i.e. the insertion point determined by the trial. Similarly, the length of shaft  309  and chisel  329  are configured such that when chisel  329  reaches its final insertion point, the end of shaft  309  is flush with the flat face  347  of handle portion  345 . Accordingly, the end of shaft  309  fits through bore hole  347  and, if chisel  329  is driven by impacting flat face  347  including bore hole  349 , the impaction tool will stop driving chisel  329  into bone. This combination of features provides a guide for the surgeon to indicate when the appropriate path has been cut into the vertebral bodies as well as acting as a stop to safeguard against creating longer pathways than required by the keels of the prosthetic disc assemblies. 
     According to one embodiment of a method of the present invention, once the appropriate paths have been cut, handle portion  335  of chisel  329  may be removed. Referring to  FIGS. 91 and 92 , handle portion  335  has been removed from shaft portion  333  of chisel  329 . As seen in  FIG. 92 , handle portion  335  and shaft portion  333  of chisel  329  are engaged with respect to each other at area  351 . As handle portion  335  slides over shaft  309  of trial  300 , handle portion  335  engages shaft  333  of chisel  329 . As best seen in  FIG. 92 , handle portion  335  is configured to interface with shaft  333  of chisel  329 . One of ordinary skill in the art would understand that any variety of configurations, including tongue and groove, threaded connections, or other configurations could be employed to achieve similar results. 
     According to one embodiment of a method of the present invention, after removing the handle portion  335  of chisel  329 , an outrigger or positioning member may be slid onto trial shaft  309 . As seen in  FIG. 93 , positioning member  353  slides onto shaft  309  until it engages with shaft portion  333  of chisel  329  at engagement area  351 . Positioning member  353  is configured with a hollow section that is shaped to match the shape of shaft  309  of trial  300 . In this embodiment, due to the rectangular configuration, the positioning member  353  is placed in a particular orientation, which will not move radially with respect to shaft  309  of trial  300 . 
     In one embodiment, shaft  333  and blade portion  331  may remain inserted. Such a feature may provide a stop against which positioning member  353  may contact (at area  351 ) and may serve to lock trial  300  in place as a result of a friction fit between blades  337  and  339  and the vertebral bodies with which they may contact. Such a feature may secure the position of trial  300  and make it less likely that trial  300  will move during the remaining steps. 
     In one embodiment, positioning member  353  comprises an attaching portion  355 , i.e. the portion that slides over shaft  309  of trial  300 , and a guiding portion  357 . Guide portion  357  is attached or connected to attaching portion  355  by a linking member  359  as seen in  FIG. 93 . Guide portion  357  serves as a hitch or post onto which a second chisel tool may be placed. While guide portion  357  is shown as generally cylindrical, any number of different shapes and configurations could be used. For example, guide portion could be generally rectangular, pyramidal, or square. The invention contemplates a guide portion shaped and configured to mate or key with the shape and configuration of the elongated shaft. Accordingly, one of skill in the art would understand that guide portion  357  is shaped to act as a guide and direct a chisel tool along a path parallel to trial  300 . 
     With reference to  FIG. 94 , second chisel tool  361  is shown. Second chisel tool  361  has a chisel portion  363 , shaft portion  365 , and handle portion  367 . As seen in  FIG. 91 , chisel portion  363  of second chisel  361  is configured similar to first chisel portion  331  of first chisel  329 , except that the chisel portion  363  of second chisel  361  is not forked as it is in first chisel  329 . In this regard, chisel portion  363  of second chisel  361  does not accommodate a trial head and accordingly is designed or constructed from one piece. The upper and lower surfaces  369 ,  371  of chisel portion  363 , however, have blade portions or sharp edges configured similar to blades  337 ,  339  of chisel portion  331  of first chisel  329 . 
     A shaft portion  365  is attached to the second chisel portion  363  of second chisel  361 . Second chisel portion  363  may be similarly offset from shaft portion  365  as in previous descriptions to accommodate the spinal cord. Shaft portion  365  extends from chisel portion  363  and may comprise a sleeve or hollow body. As one of skill in the art would understand, the interior walls of shaft portion  365  may be shaped to match the external shape of guiding portion  357 . In one embodiment, an opening may be formed along shaft portion  365  to provide access to the interior of shaft portion  365 . Said opening is sized to accommodate guiding portion  357 . Accordingly, second chisel  361  may be placed onto guiding portion  357  and slid towards the vertebral bodies. As guiding member  357  rides within the hollow body, which is shaped to match the hollow body of shaft portion  365  of second chisel  361 , the second chisel is directed along a path which is parallel to trial  300  and spaced apart a set distance from trial  300 . 
     In alternate embodiments, guiding portions and attaching members may be integral to the second chisel, and thus slidably engage or attach with the shaft of the first chisel. In alternate embodiments, a central member may be used that selectively engages the shafts of both the trial, first chisel, or second chisel. Accordingly, as one of skill in the art would understand, the precise mechanism by which the pathways are created may be any number of means. 
     According to one embodiment of a method of the present invention and as seen in  FIG. 95 , the second chisel  361  may be driven into position by the surgeon. Shaft portion  365  is attached to handle portion  367 . A surgeon may grip handle portion  367  and use it to position second chisel  361 . In one embodiment, handle portion  367 , as seen in  FIG. 95 , is a cylindrical body attached to shaft portion  365 . Handle portion  367  may have a plurality of bore holes to decrease the weight of the overall tool. Handle portion includes a flat face  369 , which serves as an impaction surface. Accordingly, a surgeon may use an impact tool to drive the second chisel  361 . In alternate embodiments, shaft portion  309  is configured with a length that abuts underside face  370  of handle portion  367  when chisel  361  is in its final position. In these embodiments, flat face  370  may act as a stop. At this point in the method, four pathways have been made, two pathways in the upper and lower vertebral bodies each of the disc area being treated. 
     In an embodiment of the method presented herein, after cutting both paths into the vertebral bodies, a surgeon may remove only the second chisel  361 . With reference to  FIG. 96 , a surgeon would remove the second chisel  361  and outrigger  353  from first trial  305 , thus leaving first trial  305  in place. First trial  305  and first chisel  329  are left in place to both keep the intervertebral space separated as well as allow shaft  309  of trial  305  to serve as a guide for assembly implantation. 
     As seen in  FIG. 96 , an implant holder  390  attached to one assembly  383  is shown. Implant holder  390  is releasably attached to assembly  383 . Implant holder  390  also comprises an implant guide  392 . Implant guide  392  is rigidly attached to said implant holder and comprises a mating portion  394  that is configured to mate with shaft portion  309  of first trial  305 . In an embodiment, mating portion  394  is configured as an elongated rectangle with a groove sized to accommodate the shape and configuration of shaft portion  309  of first trial  305 . As one of skill in the art would understand, mating portion  394  attached to implant holder  390  keys implant holder  390  to shaft portion  309 . Accordingly, a surgeon may place assembly  383  in an approximate position by introducing keels  396  and  398  into paths  371  (not shown) and  377 . The surgeon may then swing implant holder  390  and key mating portion  394  onto shaft portion  309  of first trial  305 . At that point, a surgeon may then drive assembly  383  into position. As one of skill in the art would understand, mating portion helps guide assembly  383  into position by maintaining a proper spacing between implant holder  390  and first trial  305 . Additionally, mating portion  394  ensures that the assembly is inserted parallel to first chisel  305 . With reference to  FIG. 97 , implant holder  390  is shown with assembly  383   60  inserted. As seen in  FIG. 97 , mating portion  394  is configured to provide a keyed mating connection before assembly  383  is inserted as well as allow implant holder  392  to drive assembly  383  along a path parallel to first trial  305  until assembly  383  has been inserted to a proper depth. 
     With reference to  FIG. 98 , implant holder  390  is comprised of a shaft portion  396  and handle portion  398 . As seen in  FIG. 98 , handle portion  398  may be detached from shaft portion  396 . The attachment mechanism may be any number of means although in this embodiment, handle portion contains an internally threaded rotatable cylinder  400  attached to handle portion  398  of implant holder  390 . Internally threaded rotatable rod  400  is configured to mate with one end of shaft portion  396 , which comprises an externally threaded cylindrical portion  402 . Accordingly, rotation of internally threaded cylinder  400  can thus serve to either attach or release handle portion  398  from shaft portion  396  of implant holder  392 . 
     In an embodiment of the present invention, releasing handle  398  from implant holder  392  exposes a proximal end  404  of shaft  396  of implant holder  390 . With reference to  FIG. 99 , shaft  396  of implant holder  390  may be releasably attached to prosthetic disc assembly  383 . To release shaft  396  from assembly  383 , a surgeon may use a driver  406  as seen in  FIG. 99 . Driver  406  may be configured with a head  408 . Shaft  396  has a first internal rod or elongated screw  410  comprising a threaded end (hidden) and a shaped receiving end  412 . As one of skill in the art would understand, head  408  of driver  406  is shaped to mate with receiving end  412  of first internal rod  410  and driver  406  may rotate first internal rod  410 , which in turn rotates the threaded end of said rod. Threaded end of rod  410  engages assembly  383 , and more particularly, an internally threaded bore hole in a stabilizing member  414  attached to assembly  383 . Threaded end of shaft  410  provides the attachment and release mechanism of the implant holder to assembly  383 . As one of skill in the art would understand, the attachment can be by any number of different mechanisms. Accordingly, as seen in  FIG. 100 , shaft  396  is seen released from assembly  383  after assembly  383  has been inserted into the intervertebral space. 
     In an embodiment of the present invention, assembly  383  has a stabilizing member  414  attached to assembly  383 . With reference to  FIG. 101 , stabilizing member  414  is shown attached to assembly  383 . Stabilizing member  414  is configured to prevent the articulating surfaces of assembly  383  from moving. Stabilizing member  414  is attached to assembly  383  but may disengage or release from the assembly by the surgeon as described in more detail below. Accordingly, during implantation of a two assembly artificial prosthetic disc design, stabilizing member  414  locks the articulating surfaces of assembly  383  until stabilizing member is released. 
     With reference to  FIG. 102 , after insertion of assembly  383 , first trial  305  is removed from the intervertebral space. Second assembly  381  is then inserted using an implant holder as described above and as seen previously except that no guiding member is used in this step. In alternative embodiments, a guiding member may be used if a shaft or other elongated structure is connected to assembly  383 . With reference to  FIG. 103 , handle  398  of implant holder  390  may be separated from shaft portion  396 . With reference to  FIG. 104 , driver  406  may be used to release shaft  396  of implant holder  390  from assembly  381 . As seen in both  FIGS. 103 and 104 , second assembly  381  has a stabilizing member attached. In alternative embodiments, only assembly  383  may have a stabilizing member as stabilizing member  414  attached to first assembly  383  may be sufficient to maintain stability and the need for immobilization of the articulating surfaces of the assemblies may no longer be needed once both assemblies have been implanted. As seen in  FIG. 104 , shaft  396  has a second internal rod or elongated screw  411  comprising a threaded end (hidden) and a shaped receiving end  413 . As one of skill in the art would understand, head  408  of driver  406  is shaped to mate with receiving end  413  of second internal rod  411  and driver  406  may rotate first internal rod  411 , which in turn rotates the threaded end of said rod. Threaded end of rod  411  engages a screw in stabilizing member  414 , which in turn connects stabilizing member  414  to assembly  381 . Threaded end of shaft  411  provides the attachment and release mechanism of the implant holder and stabilizing member  414  to assembly  381 . As one of skill in the art would understand, the attachment can be by any number of different mechanisms. 
     With reference to  FIG. 105 , shaft  396  may have a third internal rod or elongated screw  415  comprising a threaded end (hidden) and a shaped receiving end  417 . As one of skill in the art would understand, head  408  of driver  406  is shaped to mate with receiving end  417  of third internal rod  415  and driver  406  may rotate third internal rod  415 , which in turn rotates the threaded end of said rod. Threaded end of rod  415  engages a screw in stabilizing member  414 , which in turn connects stabilizing member  414  to assembly  381 . Threaded end of shaft  415  provides the attachment and release mechanism of the implant holder and stabilizing member  414  to assembly  381 . As one of skill in the art would understand, the attachment can be by any number of different mechanisms. 
     With reference to  FIG. 106 , assembly  381  is shown implanted into the intervertrabal space after shaft  396  of implant holder  390  and stabilizing member  414  has been released. As seen in  FIG. 106 , threaded bore holes  420  and  422  are configured to interact with screws  424  and  426  (hidden) of stabilizing member  414 , said screws having been rotated by the action of driver  408  on rods  41  I and  413  of shaft  396 . As further seen in  FIG. 106 , disc assembly  383  still contains stabilizing member  414  attached to assembly  383 . 
     With reference to  FIG. 107 , stabilizing member  414  of assembly  383  may be released after implantation of assembly  381 . As seen in  FIG. 107 , shaft  396  of implant holder  390  may be reattached to stabilizing member as described above. With reference to  FIGS. 108 and 109 , driver  406  may then be used to engage mated receiving ends of rotatable internal shafts  411 ,  413 . The opposite ends of rotatable internal shafts (not shown) are configured to engage screws (hidden) in stabilizing member  414 , said screws connecting stabilizing member  414  to assembly  383 . Accordingly, driver  406  may be used to rotate internal rods  411 ,  413 , which actuate screws in stabilizing member  414  to either attach or release stabilizing member  414  from assembly  383 . 
     In an embodiment of the present invention, shaft  396  may be connected to stabilizing member  414  through one rod prior to the disengagement of stabilizing member  414  from assembly  396 . Accordingly, in an embodiment implant holder connects to stabilizing member via a threaded rotatable rod and stabilizing member connects to an assembly of the prosthetic disc via a screw. In an alternate embodiment, the stabilizing member is connected to the implant holder by more than one rotatable shaft. In an alternate embodiment, the stabilizing member is connected to an assembly of the prosthetic disc by more than one screw. Where the stabilizing member is connected to an assembly by only one screw, locking of the assembly may be accomplished by physical interference of the stabilizing member with the endplates or other structure of the assembly to physically limit rotation of the assembly. Where the stabilizing member is connected to the assembly by more than two screws, the rigidity of the stabilizing member locks the assembly in place. As stabilizing member is used to lock or prevent articulation of the first assembly implanted into the intervertebral space, any number of mechanisms may be used to prevent said articulation or movement. Accordingly one of skill in the art that screws, interference fits, prongs, tabs, or other configurations can be used on the stabilizing member to prevent articulation of the assembly. With reference to  FIG. 110 , assemblies  381  and  383  are shown in their final position (offset). 
     With reference to  FIG. 111 , a cross section of shaft  396  of implant holder  390  is shown. As seen in  FIG. 111 , in this embodiment, there are three elongated rods  430 ,  432 , and  434  that act to either connect stabilizing member to shaft  396  or stabilizing member to an assembly. As revealed by the cross section view of shaft  396 , the elongated rods may be configured to allow longitudinal movement within the shaft. This movement allows the shafts to individually engage either the threaded bore or screws of the stabilizing member and further allows for proper threading or mating between the end of an elongated shaft and its mating portion. Elongated shafts  430 ,  432 , and  434  may be retained within the shaft and provided a limited range of movement by using retaining clip  436 . As one of skill in the art would understand, elongated shafts  430 ,  432 , and  434  may be formed with a portion having a smaller diameter than the rest of the shafts. Clip  436  may then be formed with two arms  438  and  440 , which may contact or abut inner walls of the elongated rod but otherwise not contact or interfere with the elongated rod. In this manner, depending on the length of the portions of the rod having a smaller diameter, the elongated rods may translate within the shaft. The length of translation may vary but in an embodiment of the present invention, the elongated rods may translate between about 1 mm and 4 mm. 
     As described above, methods and tools are provided that allow a surgeon to implant a prosthetic disc from the posterior approach. The methods allow a surgeon to implant more than one assembly, which may have articulating surfaces. In general, the methods provide a means by which the surgeon can properly align each prosthetic disc with respect to each other. In operation, the methods also generally minimize distraction and injury to the spinal cord. 
     In an alternative embodiment, after cutting both pathways or grooves, the second chisel, first chisel, attaching member, and trial may be removed. Referring to Figure I  12 , the intervertebral disc space is shown after it has been prepared by the methods and tools of an embodiment of the present invention. As seen in  FIG. 112 , four separate pathways  371 ,  373 ,  375 ,  377  have been cut into the surfaces of the vertebral bodies. As one of skill in the art would understand, the paths cut by the methods and tools of the present invention provide spaces for the insertion of keels spaced apart by a predetermined distance. Moreover, each pathway is generally cut parallel to each other. The upper and lower paths are aligned within the same plane. As one of skill in the art would understand, the present methods and tools provide a surgeon with the ability to insert multiple assemblies into the intervertebral space from a posterior approach. 
     According to one embodiment of a method of the present invention, after removing the chisels and trial, an assembly may be inserted into the intervertebral space. Referring to  FIG. 113 , a disc holder  379  is shown attached to a prosthetic disc assembly  381  according to the present invention. Preferably, disc holder  379  maintains assembly  381  in a neutral position. Disc holder  379  may be of any variety of designs and the methods of the present invention are not limited to any particular disc holders. As one of skill in the art would understand, disc holder is used to insert assembly  381  into the intervertebral space. In one embodiment, a first prosthetic disc  381  has an upper and lower keel that rides within the paths cut by the first chisel. 
     Once the first disc assembly is inserted, the disc holder may release the first prosthetic disc. As seen in  FIG. 114 , the disc holder has released assembly  381  and assembly  381  is shown in its final or implanted position in the intervertebral space. Referring to  FIG. 115 , the upper vertebral body is not shown for sake of clarity. Next, the second assembly may be inserted. As seen in  FIG. 115 , a second assembly  383  is inserted into the intervertebral space. The upper and lower keels of the second assembly ride within the paths cut by the second chisel. Once inserted, the disc holder releases the second assembly ( FIG. 116 ).  FIGS. 117 and 118  show the two assemblies inserted into the disc space. The prosthetic disc assemblies are shown in their neutral position. As one of skill in the art would understand, after implantation the spacing between the assemblies is such that the endplates of the assemblies can articulate as if they were a single articulating surface. 
     Now, turning to  FIG. 119 , another embodiment of a posterior disc assembly is illustrated. In this particular embodiment a posterior spinal disc  400  which contains multiple elements according to the present invention is shown.  FIG. 120  illustrates an exploded view of the disc assembly  400 , which more clearly shows the individual elements of the spinal disc  400 .  FIG. 120  illustrates an upper endplate  402 , a lower endplate  404 , a flexible core element  406  and a slider plate  408 . The endplates  402 ,  404 , flexible core element  406 , and the slider plate  408  may be composed of a variety of biocompatiable materials, including metals, ceramic materials and polymers. Such materials include, but are not limited to, aluminum, alloys, and polyethylene. The outer surfaces of the endplates  402 ,  404  may also contain a plurality of teeth which may be maybe coated with osteoconductive material, antibiotics or other medicament, or may have a porous or macrotexture surface to help rigidly attach the endplates  402 ,  404  to the vertebral bodies by promoting the formation of new bony ingrowth. Each of these elements and the interconnections between the associated elements will be discussed in greater detail with reference to  FIGS. 121-126 . 
     In the present embodiment, the upper endplate  402  is configured with a plurality of keels  414  that engage with an interverterbal body and an extension portion  403  which is configured to be in contact with the slider plate  408 . The extension portion  403  is configured with a curvature which corresponds with a curvature associated with the slider plate  408 . The extension portion  403  and the plate  408  being in communications allows rotational and translational motion to occur. As mentioned previously, the present disc assembly can mimic or partially mimic the varying IAR and COR to the extent desired by a physician while also preserving the stability of the device. 
     Although the present embodiment utilizes a keel  414  so that it may be used to guide the disc assembly into position during insertion into the treated area of the spine, ridges, teeth or any other type of mechanism to attach the endplate  402 ,  404  to the vertebral body may also be used. Also, as mentioned previously, the use of one or more keels  414  may also increase bone to implant surface contact, thereby decreasing the likelihood that the assembly will shift or move out of position. 
     The extension portion  403  of the upper endplate  402  is spaced apart from the base of the upper endplate  402  and fits into a cavity  418  in an upper portion of the flexible core element  406 . The extension portion  403 , in the present embodiment, is provided with a flat surface having a length of 17 mm, a width of 7 mm and a height of 2.5 mm. However, in other embodiments the contact surface of the extension portion  403  may be configured with a curvature or dimensioned for optimizing the contact surface. Although in the present embodiment, the extension portion  403  of the upper endplate  402  is configured with a flat surface, any geometry may be used that adapts within the flexible core element  406  and communicates with the slider plate  408 . The lower endplate  404  is also configured with multiple keels  414  to engage with an adjacent vertebral body. The lower endplate  404  is also configured with an extension portion  416  which is adapted to fit within a cavity in a lower portion of the flexible core element  406 . The extension portion  416  is spaced apart from the base of the lower endplate  404  and is configured to conform to the cavity formed in the lower portion of the flexible core element  406 . In this embodiment, the extension portion  416  is configured as an elongated and curved plate having a length of 17 mm, a width of 7 mm and a height of 1.5 mm. It should be noted that the extension portion  416  may be configured to be in any geometry to conform and fit within the cavity formed within the lower portion of the flexible core element  406 . The upper endplate  402  and the lower endplate  404  are also provided with screw holes  412 ,  414  for receiving an instrument for positioning the disc  400  within the intervertebral space. 
     The flexible core element  406  is composed of a flexible material that may be tensioned, compressed or be a combination of tensioned and compressed elements. The flexible core element  406  may be made of resilient material that provides suitable resistance to stretching or compression. The compression element helps support axial loading along the treated vertebral bodies so their relative positions approximate a healthy vertebral body supported by a natural disc. The flexible core element  406  also helps in controlling the bending or movement of the vertebral bodies relative to each other. The flexible core element  406  is configured and adapted to allow for compression and translation by providing a cavity in the upper portion and a cavity in the lower portion which receives the extension portions of the upper  402  and lower endplates  404 . The flexible core element  406  is composed of a resilient material to allow for rotation to occur. 
     The flexible material in one particular embodiment is composed of Polycarbonate Urethane. However, any flexible material that is bio-mechanical and biologically compatible may be used. As mentioned above, the core element  406  has an upper portion and a lower portion. The upper portion is configured to receive and retain the slider plate  408  within the cavity  418 . The lower portion is configured to receive and retain the extension portion  416  of the lower endplate  404  within the cavity  420 . 
     The slider plate  408  is a metal plate having an upper and lower surface. The upper surface of the slider plate  408  is generally a flat surface however a curvature that corresponds to the curvature of the extension portion  403  of the upper endplate  402  may also be utilized. Additionally, the slider plate may be configured to be of any geometric shape which is capable of communicating with an extension portion of the upper endplate. In the preferred embodiment the slider plate  408  is composed of metal, however the slider plate  408  may be comprised of any element that allows a friction coefficient that enables the extension portion  403  translate or axially rotate relative to the slider plate  408 . 
       FIG. 121  illustrates a cross sectional view of one particular embodiment of the disc assembly  400 . In this embodiment of the disc assembly  400 , the upper endplate  402  is in communication with the slider plate  408  through the extension portion  403 . The extension portion  403  is adapted into the flexible core member  406 . The extension portion  403  is slightly smaller than the space provided between one end of the flexible core element  406  to the other end. As a result, the extension portion  403  may translate from 0.5 mm to 1 mm along the slider plate  408 . Additionally, the upper endplate  402  may translate and axially rotate relative to the flexible core element  406 . Thus, the upper endplate  402  can translate in one direction or in a second direction by configuring the extension portion  403  to communicate with the slider plate  408 . 
     The lower endplate  404  is also provided with an extension portion  416  that is fitted into the flexible core element  406 . In this particular embodiment, the lower endplate  404  is adapted to fit tightly within the flexible core element  406  to limit the translational and axial motion of the lower endplate  404  with respect to the flexible core element  406 . However, the lower endplate  404  is adapted to extend and flex within the flexible core element  406 , as the flexible core element  406  is compressed or stretched. In this embodiment, the length, height, and width of the cavity in the lower portion of the flexible core element  406  is substantially equal to the length, height, and width of the extension portion  416  of the lower endplate  404 , thereby limiting translational and rotational motion. As explained above, the lower endplate  404  is configured to compress and flex on the flexible core element  406 , providing flexion and extension. The upper and lower endplates  402  and  404  are also provided with screw holes  410 ,  412  for receiving an instrument which is utilized to position the disc assembly within the intervertebral space of the spine. The upper and lower endplates  402  and  404  are also configured with a base portion that have a spherical contact surface which increases the surface area for contacting with the adjacent vertebral body. 
       FIG. 122  illustrates a cross-sectional view of another embodiment of a disc assembly  420  according to the present invention. In this particular embodiment, the disc assembly  420  is comprised of an upper endplate  422 , a lower endplate  424 , a flexible core element  426  and a slider plate  428 . The extension portion  430  of the upper endplate  422  is adapted to fit within a cavity in an upper portion of the flexible core element  426 . The extension portion  430  of the lower endplate  424  is adapted to fit within a cavity in the lower portion of the flexible core element  426 . The flat surface of the extension portion  434  of the lower endplate  424  is configured with a curvature for enabling flexion and extension with respect to the flexible core element  426 . As illustrated in  FIG. 122 , the cavity  438  in lower portion of the flexible core element  426  is larger than the extension portion  434  of the lower endplate  424 , which allows greater amount of flexion and extension to occur when the flexible core element  426  is either compressed or stretched. 
     The slider plate  428  allows for axial rotation and translation (anterior/posterior sliding motion) within the device. Flexion, extension and lateral bending motions are a hybrid combination of movement available due to the spherical feature provided on the inside of the endplate  422  that mates to the flexible core element  426 . As in the previous embodiments, the upper and lower endplates  422 ,  424  are provided with multiple keels  432 ,  436  to attach the endplates  422 ,  424  to adjacent vertebral bodies. The disc assembly  420  is also provided with threaded holes  442 ,  444  on the posterior faces of the endplates  422 ,  424  for receiving an instrument which is used to position the disc assembly within the disc space of the spine. A locating groove that helps to align the screw holes as well as resist twisting forces while being inserted is also provided on the posterior portion of the disc assembly. 
       FIG. 123  illustrates yet another embodiment of the invention. In this embodiment, the disc assembly  450  is comprised of an upper endplate  452 , a lower endplate  454 , a flexible core element  456 , a slider plate  458 , and cord  460 . 
     The upper endplate  452  and lower endplates  454  are configured with keels  462 ,  464  which engage with the corresponding adjacent vertebral bodies. The upper and lower endplates  452 , and  454  are also provided with threaded screw holes  466 ,  468  on the posterior faces of the upper and lower endplates  452 ,  454  for receiving an instrument to position the disc assembly within the spine. 
     The disc assembly  450  further includes the flexible core element  456  which is situated between the upper and lower endplates  452 ,  454 . The inner portion of the flexible core element  456  is configured with a cavity in the upper portion that receives and retains the slider plate  458  and the extension portion  470  of the upper endplate  452 . The lower portion of the flexible core element  456  is also configured with a cavity which receives and retains the extension portion  472  of the lower endplate  454 . The extension portion  470  of the upper endplate  452  is adapted to communicate with the slider plate  458  to allow for translation since the extension portion  470  is slightly smaller in length than the slider plate  458 . The extension portion of the upper endplate is configured to be slightly smaller than the slider plate  458  and fits within a first cavity in the inner portion of flexible core member, allowing for some space between the edges of the extension portion and the edge of the inner portion. The upper endplate is translated or rotated axially, the extension portion moves along the slider plate thereby providing translations and axial motion in the spinal segment. The amount of translation which occurs between the extension portion  470  and the slider plate  458  can be adapted by lengthening or shortening the extension portion  458 . 
     The extension portion  472  of the lower endplate  454  is configured with as spherical surface and is positioned tightly within a second cavity in the inner portion of the flexible core element  456 . The configuration of the extension portion  472  provides flexion and extension motion when the lower endplate  454  is moved by the natural movements of the spine. 
     The disc assembly  450  is also provided with a central shaft in which a cord  460  is utilized to attach the assembly  450  as a single structure. The cord  460  may be composed any elastic or flexible material that is biocompatible. The cord  460  has a first end  476  and second end  478  that configured to be slightly larger than the body of the cord  460 . This configuration allows the cord  460  to be retained within the disc assembly  450 . However, any structural or mechanical configuration that retains the cord  460  within the disc assembly may  450  be utilized. For instance, cord  460  may be attached to the endplates  452 ,  462  through a clip or a set screw. 
     The cord  460  may also be tensioned so that the assembly  450  is firmly held together. The tension of the cord  460  may vary depending on how much motion is required for the specific patient requiring the disc assembly implant  450 . The characteristics of the cord  460  may be changed depending on what is required, for instance, the cord may be composed of a solid flexible material or a be a hollow member. The cord  460  may also be positioned in various locations through the disc assembly. For example, a single cord may be placed on the perimeter of the disc assembly or in position along the surface of the endplates. In another embodiment, the disc assembly may use a plurality of cords to attach the assembly as a single structure and to restrict the rotational motion of the disc assembly. When multiple cords are used, the cords may spaced apart in any location along the perimeter of the endplates to achieve the optimal results for either limiting or allowing motion in the disc assembly as well as maintaining the structural integrity of the disc assembly. In another embodiment, the core may be comprised of various flexible elements having different flexibilities and/or durometers. 
       FIG. 124  illustrates two disc assemblies  480  and  482  according to the present invention. As described above, two posterior disc assemblies  480 ,  482  may be inserted into the intervertebral space to provide an optimal solution for disc replacement. The positioning of two disc assemblies allows the motion of the spine to mimic the actual motion of the disc nucleus.  FIG. 125  illustrates the two disc assemblies  480  and  482  positioned in the intervertebral space between adjacent vertebral bodies  484  and  486 .  FIG. 126  illustrates a posterior view of the two disc assemblies  480  and  482  positioned in the intervertebral space of adjacent vertebral bodies  484  and  486 . 
     The various features and embodiments of the invention described herein may be used interchangeably with other feature and embodiments. Finally, while it is apparent that the illustrative embodiments of the invention herein disclosed fulfill the objectives stated above, it will be appreciated that numerous modifications and other embodiments may be devised by one of ordinary skill in the art. Accordingly, it will be understood that the appended claims are intended to cover all such modifications and embodiments which come within the spirit and scope of the present invention.