Abstract:
An artificial functional spinal unit is provided comprising, generally, an expandable artificial intervertebral implant that can be placed via a posterior surgical approach and used in conjunction with one or more artificial facet joints to provide an anatomically correct range of motion. Expandable artificial intervertebral implants in both lordotic and non-lordotic designs are disclosed, as well as lordotic and non-lordotic expandable cages for both PLIF (posterior lumber interbody fusion) and TLIF (transforaminal lumbar interbody fusion) procedures. The expandable implants may have various shapes, such as round, square, rectangular, banana-shaped, kidney-shaped, or other similar shapes. By virtue of their posteriorly implanted approach, the disclosed artificial FSU&#39;s allow for posterior decompression of the neural elements, reconstruction of all or part of the natural functional spinal unit, restoration and maintenance of lordosis, maintenance of motion, and restoration and maintenance of disc space height.

Description:
PRIORITY CLAIM  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/634,950 entitled “ARTIFICIAL FUNCTIONAL SPINAL UNIT ASSEMBLIES” filed on Aug. 5, 2003, the disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND  
       [0002]     1. Field of the Invention  
         [0003]     The present invention generally relates to functional spinal implant assemblies for insertion into the intervertebral space between adjacent vertebral bones and reconstruction of the posterior elements to provide stability, flexibility and proper biomechanical motion. More specifically, the present invention relates to artificial functional spinal units comprising an expandable artificial intervertebral implant that can be inserted via a posterior surgical approach and used in conjunction with one or more artificial facet joints to provide a more anatomically correct range of motion.  
         [0004]     2. Description of Related Art  
         [0005]     The human spine is a complex mechanical structure composed of alternating bony vertebrae and fibrocartilaginous discs that are connected by strong ligaments and supported by musculature that extends from the skull to the pelvis and provides axial support to the body. The intervertebral discs primarily serve as a mechanical cushion between adjacent vertebral segments of the spinal column and generally comprise three basic components: the nucleus pulposus, the anulus fibrosis, and two vertebral end plates. The end plates are made of thin cartilage overlying a thin layer of hard cortical bone that attaches to the spongy, cancellous bone of the vertebral body. The anulus fibrosis forms the disc&#39;s perimeter and is a tough outer ring that binds adjacent vertebrae together. The vertebrae generally comprise a vertebral foramen bounded by the anterior vertebral body and the neural arch, which consists of two pedicles and two larninae that are united posteriorly. The spinous and transverse processes protrude from the neural arch. The superior and inferior articular facets lie at the root of the transverse process. The term “functional spinal unit” (“FSU”) refers to the entire motion segment: the anterior disc and the posterior facet joints, along with the supporting ligaments and connective tissues.  
         [0006]     The spine as a whole is a highly flexible structure capable of a high degree of curvature and twist in nearly every direction. However, genetic or developmental irregularities, trauma, chronic stress, and degenerative wear can result in spinal pathologies for which surgical intervention may be necessary.  
         [0007]     It is common practice to remove a spinal disc in cases of spinal disc deterioration, disease or spinal injury. The discs sometimes become diseased or damaged such that the intervertebral separation is reduced. Such events cause the height of the disc nucleus to decrease, which in turn causes the anulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the anulus begin to buckle and separate, either circumferential or radial anular tears may occur. Such disruption to the natural intervertebral separation produces pain, which can be alleviated by removal of the disc and maintenance of the natural separation distance. In cases of chronic back pain resulting from a degenerated or herniated disc, removal of the disc becomes medically necessary.  
         [0008]     In some cases, the damaged disc may be replaced with a disc prosthesis intended to duplicate the function of the natural spinal disc. U.S. Pat. No. 4,863,477 discloses a resilient spinal disc prosthesis intended to replace the resiliency of a natural human spinal disc. U.S. Pat. No. 5,192,326 teaches a prosthetic nucleus for replacing just the nucleus portion of a human spinal disc.  
         [0009]     In other cases it is desired to fuse the adjacent vertebrae together after removal of the disc, sometimes referred to as “intervertebral fusion” or “interbody fusion.” 
         [0010]     Many techniques and instruments have been devised to perform intervertebral fusion. There is common agreement that the strongest intervertebral fusion is the interbody (between the lumbar bodies) fusion, which may be augmented by a posterior or facet fusion. In cases of intervertebral fusion, either structural bone or an interbody fusion cage filled with morselized bone is placed centrally within the space where the spinal disc once resided. Multiple cages or bony grafts may be used within that space.  
         [0011]     Such practices are characterized by certain disadvantages, most important of which is the actual morbidity of the procedure itself. Placement of rigid cages or structural grafts in the interbody space either requires an anterior surgical approach, which carries certain unavoidable risks to the viscous structures overlying the spine (intestines, major blood vessels, and the ureter), or they may be accomplished from a posterior surgical approach, thereby requiring significant traction on the overlying nerve roots. The interval between the exiting and traversing nerve roots is limited to a few millimeters and does not allow for safe passage of large intervertebral devices, as may be accomplished from the anterior approach. Alternatively, the anterior approach does not allow for inspection of the nerve roots, is not suitable alone for cases in which the posterior elements are not competent, and most importantly, the anterior approach is associated with very high morbidity and risk where there has been previous anterior surgery.  
         [0012]     Another significant drawback to fusion surgery in general is that adjacent vertebral segments show accelerated deterioration after a successful fusion has been performed at any level. The spine is by definition stiffer after the fusion procedure, and the natural body mechanics place increased stress on levels proximal to the fused segment. Other drawbacks include the possibility of “flat back syndrome” in which there is a disruption in the natural curvature of the spine. The vertebrae in the lower lumbar region of the spine reside in an arch referred as having a sagittal alignment. The sagittal alignment is compromised when adjacent vertebral bodies that were once angled toward each other on their posterior side become fused in a different, less angled orientation relative to one another. Finally, there is always the risk that the fusion attempt may fail, leading to pseudoarthrosis, an often painful condition that may lead to device failure and further surgery.  
         [0013]     Conventional interbody fusion cages generally comprise a tubular metal body having an external surface threading. They are inserted transverse to the axis of the spine, into preformed cylindrical holes at the junction of adjacent vertebral bodies. Two cages are generally inserted side by side with the external threading tapping into the lower surface of the vertebral bone above, and the upper surface of the vertebral bone below. The cages include holes through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior of the cage to incite or accelerate the growth of the bone into the cage. End caps are often utilized to hold the bone graft material within the cage.  
         [0014]     These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. As previously discussed, however, cages that would be placed from the safer posterior route would be limited in size by the interval between the nerve roots. It would therefore, be a considerable advance in the art to provide a fusion implant assembly which could be expanded from within the intervertebral space, thereby minimizing potential trauma to the nerve roots and yet still providing the ability to restore disc space height.  
         [0015]     Ultimately though, it is important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient&#39;s spine as additional stresses of motion, normally bone by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. Thus, it would be an even greater advance in the art to provide an implant assembly that does not promote fusion, but instead closely mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.  
       SUMMARY  
       [0016]     Accordingly, an artificial functional spinal unit (FSU) is provided comprising, generally, an expandable artificial intervertebral implant that can be placed via a posterior surgical approach and used in conjunction with one or more artificial facet joints to provide an anatomically correct range of motion. Expandable artificial intervertebral implants in both lordotic and non-lordotic designs are disclosed, as well as lordotic and non-lordotic expandable cages for both PLIF (posterior lumber interbody fusion) and TLIF (transforaminal lumbar interbody fusion) procedures. The expandable implants may have various shapes, such as round, square, rectangular, banana-shaped, kidney-shaped, or other similar shapes. By virtue of their posteriorly implanted approach, the disclosed artificial FSU&#39;s allow for posterior decompression of the neural elements, reconstruction of all or part of the natural functional spinal unit, restoration and maintenance of lordosis, maintenance of motion, and restoration and maintenance of disc space height.  
         [0017]     The posterior implantation of an interbody device provides critical benefits over other anterior implanted devices. Placement of posterior devices that maintain mobility in the spine have been limited due to the relatively small opening that can be afforded posteriorly between the exiting and transversing nerve roots. Additionally, placement of posterior interbody devices requires the removal of one or both facet joints, further destabilizing the spine. Thus conventional posteriorly placed interbody devices have been generally limited to interbody fusion devices.  
         [0018]     Since a properly functioning natural FSU relies on intact posterior elements (facet joints) and since it is necessary to remove these elements to place a posterior interbody device, a two-step procedure is disclosed that allows for placement of an expandable intervertebral implant and replacement of one or both facets that are necessarily removed during the surgical procedure. The expansile nature of the disclosed devices allow for restoration of disc height once inside the vertebral interspace. The expandable devices are collapsed prior to placement and then expanded once properly inserted in the intervertebral space. During the process of expansion, the endplates of the natural intervertebral disc, which essentially remain intact after removal or partial removal of the remaining natural disc elements, are compressed against the device, which thereby facilitates bony end growth onto the surface of the artificial implant. Once the interbody device is in place and expanded, the posterior element is reconstructed with the disclosed pedicle screw and rod system, which can also be used to distract the disk space while inserting the artificial implant. Once the interbody device is in place and expanded, the posterior element is further compressed, again promoting bony end growth into the artificial implant. This posterior compression allows for anterior flexion but replaces the limiting element of the facet and interspinous ligament and thereby limits flexion to some degree, and in doing so maintains stability for the anteriorly located interbody device.  
         [0019]     The posterior approach avoids the potential risks and morbidity of the anterior approach, which requires mobilization of the vascular structures, the ureter, and exposes the bowels to risk. Also, the anterior approach does not offer the surgeon an opportunity to view the posterior neural elements and thereby does not afford an opportunity for decompression of those elements. Once an anterior exposure had been utilized a revision procedure is quite risky and carries significant morbidity.  
         [0020]     The artificial FSU generally comprises an expandable intervertebral implant and one or more artificial facet joints. The expandable intervertebral implant generally comprises a pair of spaced apart plate members, each with a vertebral body contact surface. The general shape of the plate members may be round, square, rectangular, banana shaped, kidney shaped, or some other similar shape, depending on the desired vertebral implantation site. Because the artificial intervertebral implant is to be positioned between the facing surfaces of adjacent vertebral bodies, the plate members are arranged in a substantially parallel planar alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the vertebral body contact surfaces facing away from one another. The plate members are to mate with the vertebral bodies so as to not rotate relative thereto, but rather to permit the spinal segments to axially compress and bend relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of an expandable joint insert, which is disposed between the plate members. The securing of the plate members to the vertebral bone is achieved through the use of a osteoconductive scaffolding machined into the exterior surface of each plate member. Alternatively, a mesh of osteoconductive surface may be secured to the exterior surface of the plate members by methods known in the art. The osteoconductive scaffolding provides a surface through which bone may ultimately grow. If an osteoconductive mesh is employed, it may be constructed of any biocompatible material, both metal and non-metal. Each plate member may also comprise a porous coating (which may be a sprayed deposition layer, or an adhesive applied beaded metal layer, or other suitable porous coatings known in the art, i.e. hydroxy appetite). The porous coating permits the long-term ingrowth of vertebral bone into the plate member, thus permanently securing the prosthesis within the intervertebral space.  
         [0021]     In more detail, the expandable artificial implant of the present invention comprises four parts: an upper body, a lower body, an expandable joint insert that fits into the lower body, and an expansion device, which may be an expansion plate, screw, or other similar device. The upper body generally comprises a substantially concave inferior surface and a substantially planar superior surface. The substantially planar superior surface of the upper body may have some degree of convexity to promote the joining of the upper body to the intact endplates of the natural intervertebral disc upon compression. The lower body generally comprises a recessed channel, having a rectangular cross section, which extends along the superior surface of the lower body in the medial-lateral direction and substantially conforms to the shape of the upper and lower bodies. The lower body further comprises a substantially planar inferior surface that may have some degree of convexity to promote the joining of the lower body to the intact endplates of the natural intervertebral disc upon compression. The expandable joint insert resides within the channel on the superior surface of the lower body. The expandable joint insert has a generally flat inferior surface and a substantially convex superior surface that articulates with the substantially concave inferior surface of the upper body. Prior to expansion of the artificial implant, the generally flat inferior surface of the expandable joint insert rests on the bottom surface of the channel. The expandable joint insert is raised above the bottom of the channel by means of an expansion screw, an expansion plate, or other similar device, that is inserted through an expansion hole or slot. The expansion hole or slot is disposed through the wall of the lower body formed by the channel. The expansion hole or slot gives access to the lower surface of the channel and is positioned such that the expansion device can be inserted into the expansion hole or slot via a posterior surgical approach. As the expansion device is inserted through the expansion slot, into the channel, and under the expandable joint insert, the expandable joint insert is raised above the floor of the channel and lifts the upper body above the lower body to the desired disc height. The distance from the inferior surface of the lower body and the superior surface of the upper body should be equal to the ideal distraction height of the disk space. As the artificial implant is flexed and extended, the convex superior surface of the expandable joint insert articulates with the concave inferior surface of the upper body.  
         [0022]     After the insertion and expansion of the expandable intervertebral implant, the posterior facet joints may be reconstructed by employing the disclosed artificial facet joints. One embodiment of the artificial facet joint generally comprises a lower and upper multi-axial pedicle screw joined by a rod bridging the vertebral bodies above and below the artificial implant. The rod comprises a washer-type head at its lower (caudad) end. The rod fits into the heads of the pedicle screws and a top loaded set screw is placed in the pedicle screw heads. The disclosed pedicle screw system may employ different types of pedicle screws so that the top loaded set screw may or may not lock down on the rod depending on surgeon preference. If a non-locking pedicle screw is used the caudad end remains fully multi-axial. The upper (cephalad) end of the rod is held within the head of the upper pedicle screw with a set screw which locks down on the rod and eliminates any rod movement at the cephalad end, which by nature has limited multi-axial function. In an alternative embodiment of an artificial facet joint, the rod may comprise washer-type heads on both ends (caudad and cephalad) so that both pedicle screws can be of the non-locking variety. In the event of a two level surgical procedure, three pedicle screws would be employed with a single rod, which would have washer-type heads at both ends. The middle pedicle screw would be a locking-type and the upper most and lower most pedicle screws would be of the non-locking variety.  
         [0023]     In addition, another embodiment of the artificial facet joint is disclosed that generally comprises two locked pedicle screws joined by a rod having a ball and socket joint centrally located on the rod between the two pedicle screws. The locking of the pedicle screws prevents the screw head from swiveling, but allows rotation and translation of the rod.  
         [0024]     In instances where a fusion procedure is unavoidable, a PLIF and TLIF cage are disclosed that utilize the expansion principal of the functional artificial intervertebral implant. The cage generally comprises three parts: An external body, an internal body, and an expansion device. The external and internal bodies will have substantially the same shape and will be shaped accordingly to the procedures for which they will be used, more specifically, a rectangular cage is employed for a PLIF procedure and round or banana shaped cage is employed for the TLIF procedure. Both the external and internal bodies comprise a mesh structure in which an osteoconductive substance can be placed (i.e., morsilized autograph or an osteobiologic substitute). The external body of the cage contains an internal void space that houses the internal body. The external body further comprises an expansion window on its superior surface through which the internal body is raised upon expansion of the cage. The internal body comprises a planar plate member that is slightly larger than the expansion window in the superior surface of the external body such that when the cage is expanded the planar plate member secures itself against the interior side of the expansion window, thereby interlocking the external and internal bodies and eliminating mobility between the two bodies. Similar to the functional expandable implant, an expansion device is placed through an expansion slot. The expansion device lifts the internal body relative to the external body, interlocking the planar plate member of the internal body against the interior of the expansion window, and pushing the mesh structure of the internal body through the expansion window and above the superior surface of the external body. Varying the height of the expansion device and the dimensions of the external and internal bodies allows for various distraction heights to regain disc space. As with the functional intervertebral implant, the PLIF and TLIF cages may take the form of either an expandable lordotic cage or a non-lordotic cage. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]     Advantages of the present invention will become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which:  
         [0026]      FIG. 1  is a top view of a round, expandable intervertebral implant of the present invention.  
         [0027]      FIG. 2  is a side cross-sectional view of the round, expandable intervertebral implant shown in  FIG. 1 .  
         [0028]      FIG. 3   a  is a top view of a banana-shaped, expandable intervertebral implant of the present invention.  
         [0029]      FIG. 3   b  is a side cross-sectional view of the banana-shaped, expandable intervertebral implant shown in  FIG. 3   a.    
         [0030]      FIG. 4   a  is a cross-sectional illustration of an expandable intervertebral implant in compression.  
         [0031]      FIG. 4   b  is a cross-sectional illustration of an expandable intervertebral implant in flexion.  
         [0032]      FIG. 5   a  is a top view of a banana-shaped, expandable intervertebral implant, illustrating the insertion of expansion screws to expand the joint.  
         [0033]      FIG. 5   b  is a top view of a banana-shaped, expandable intervertebral implant, illustrating the insertion of a non-threaded expansion device to expand the joint.  
         [0034]      FIG. 6   a  is a top view of a banana-shaped, expandable intervertebral implant, illustrating the insertion of an expansion plate to expand the joint.  
         [0035]      FIG. 6   b  is a side cross-sectional view of a banana-shaped, expandable intervertebral implant, illustrating the insertion of an expansion plate to expand the joint.  
         [0036]      FIG. 6   c  is a side cross-sectional view of an expandable intervertebral implant, featuring retaining pegs.  
         [0037]      FIG. 6   d  is a side cross-sectional view of an expandable intervertebral implant in flexion, featuring retaining pegs.  
         [0038]      FIG. 7   a  is a cross-sectional view of an expandable intervertebral implant, prior to expansion.  
         [0039]      FIG. 7   b  is a cross-sectional view of an expandable intervertebral implant, following expansion.  
         [0040]      FIG. 8  is a side perspective view illustrating placement of an expandable intervertebral implant within an intervertebral space.  
         [0041]      FIG. 9   a  is a side view of an artificial facet joint of the present invention, featuring a rod with two washer-type heads.  
         [0042]      FIG. 9   b  is a side view of an artificial facet joint of the present invention, featuring a rod with a single washer-type head.  
         [0043]      FIG. 9   c  is a cross-sectional view of a pedicle screw featuring a locking screw head.  
         [0044]      FIG. 10  is a side view of an artificial facet joint of the present invention, featuring a rod having a ball joint.  
         [0045]      FIG. 11  is a posterior view of the spine after reconstruction and implantation of an artificial functional spinal unit including an expandable intervertebral implant and an artificial facet joint.  
         [0046]      FIG. 12   a  is a top view of an expandable PLIF cage in accordance with the present invention.  
         [0047]      FIG. 12   b  is a side cross-sectional view of an expandable PLIF cage in accordance with the present invention prior to expansion.  
         [0048]      FIG. 12   c  is a side cross-sectional view of an expandable PLIF cage in accordance with the present invention following expansion.  
         [0049]      FIG. 12   d  is a side cross-sectional view of an expandable TLIF cage in accordance with the present invention prior to expansion.  
         [0050]      FIG. 12   e  is a side cross-sectional view of an expandable TLIF cage in accordance with the present invention following expansion.  
         [0051]      FIG. 13   a  is a posterior view of a banana-shaped lordotic expandable intervertebral implant.  
         [0052]      FIG. 13   b  is a top view of a banana-shaped lordotic expandable intervertebral implant.  
         [0053]      FIG. 14   a  is a lateral view of a banana-shaped lordotic expandable intervertebral implant prior to expansion.  
         [0054]      FIG. 14   b  is a lateral view of a banana-shaped lordotic expandable intervertebral implant following expansion.  
         [0055]      FIG. 15   a  is a side cross-sectional view of an expandable lordotic cage prior to expansion.  
         [0056]      FIG. 15   b  is a side cross-sectional view of an expandable lordotic cage following expansion.  
         [0057]      FIG. 16   a  is a lateral view of a banana-shaped lordotic expandable intervertebral implant featuring an inclined expansion plate.  
         [0058]      FIG. 16   b  is a side cross-sectional view of an expandable lordotic cage featuring an inclined expansion plate. 
     
    
       [0059]     While the invention may be susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. The drawings may not be to scale. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but to the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.  
       DETAILED DESCRIPTION  
       [0060]     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings, which form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.  
         [0061]      FIGS. 1 and 2  show a round, expandable artificial intervertebral implant designated generally at  10 . The device is implemented through a posterior surgical approach by making an incision in the annulus connecting adjacent vertebral bodies after removing one or more facet joints. The natural spinal disc is removed from the incision after which the expandable artificial intervertebral implant is placed through the incision into position between the vertebral bodies. The implant is preferably made of a biocompatible metal having a non-porous quality and a smooth finish; however, it may also be constructed of ceramic or any other suitable inert material.  
         [0062]     The expandable artificial intervertebral implant  10  generally comprises an upper body  12  and a lower body  14  in a substantially parallel planar configuration. The superior surface  2  of the upper body  12  and the inferior surface  4  of the lower body  14  comprise a machined osteoconductive scaffolding  13  through which the bone may ultimately grow. Osteoconductive scaffolding  13  may also include spines or barbs that project into and secure against the bony endplates of the adjacent bony vertebral bodies upon expansion of the joint and minimize the possibility of sublaxation and/or dislocation. The upper body  12  has a substantially concave inferior surface  16 . The lower body  14  has a channel  15  in superior surface  17 . Channel  15  has a rectangular cross-section that extends along the lower body  14  in the medial-lateral direction and substantially conforms to the shape of the upper  12  and lower  14  bodies. An expandable joint insert  19  resides within the channel  15  on the lower body. The expandable joint insert  19  has a generally flat inferior surface  20  and a substantially convex superior surface  21  that articulates with the substantially concave inferior surface  16  of the upper body  12 . The expandable joint insert  19  is lifted from the bottom of channel  15  by means of an expansion screw  21 , or other device, that is inserted between the generally flat inferior surface  20  of the expandable joint insert  19  and the bottom of the channel  15  extending along the lower body  14  through an expansion slot  18 . A void space is created between the expandable joint insert  19  and the floor of the channel  15  in cross sections not including the expansion device. A securing means, such as the cables  25 , may be employed to ensure the upper body  12  and the lower body  14  remain intact during flexion and extension of the FSU. Alternative means for securing the upper body  12  and lower body  14  may also be employed, such as retaining pegs, torsion springs, or similar devices.  
         [0063]      FIGS. 3   a  and  3   b  show a banana-shaped expandable artificial intervertebral implant  50 . As with the round implant  10  shown in  FIG. 1 , the banana-shaped implant also comprises an upper body  52  and a lower body  54  in a substantially planar configuration, each having an external osteoconductive scaffolding  53 . Note that the channel  55  and the expandable joint insert  59 , which is disposed within the channel  55 , substantially conforms to the shape of the upper  52  and lower  54  bodies. Whereas the round expandable implant may comprise a single expansion device, the banana-shaped implant may contain one or more expansion devices  61  that are inserted into expansion slots  60 . Otherwise, the cross-section of the banana-shaped implant is substantially similar to  FIG. 2 .  
         [0064]     Turning to  FIGS. 4   a  and  4   b , an expandable artificial intervertebral implant is shown in flexion and extension, respectively. The concave inferior surface of  16  of upper body  12  articulates with the convex superior surface  21  of expandable joint insert  19 . As stated above, securing means  25  may be employed to prevent dislocation of the implant.  
         [0065]      FIGS. 5   a  and  5   b  illustrate the insertion of expansion devices into a banana-shaped implant. The artificial intervertebral implant  50  in  FIG. 5   a  employs expansion screws  70  to expand joint insert  19 . One or more expansion screws  70  may be inserted through one or more threaded expansion slots  71 . Alternatively, as shown in  FIG. 5   b , artificial implant  55  may employ a non-threaded expansion device  72  inserted through a non-threaded expansion slot  73  to accomplish the expansion of joint insert  19 . The non-threaded expansion slot  73  preferably has an arcuate shape to facilitate insertion after the artificial disc prosthesis has been properly placed within the intervertebral space. The non-threaded expansion device  72  has substantially the same shape as expansion slot  73 . A threaded end cap  74  may be employed to retain the expansion device  72  inside the expansion slot  73 .  
         [0066]      FIGS. 6   a  and  6   b  illustrate an alternative embodiment of a non-threaded expansion device. As shown in  FIG. 6   a , a banana-shaped artificial intervertebral implant  80  having a wide expansion slot  81  on either the medial or lateral side of the implant  80 . Expansion plate  82  is impacted into place through expansion slot  81  after artificial implant  80  has been properly placed within the intervertebral space. Similar to the previously described embodiments, the artificial implant comprises an upper body  83  and a lower body  84  in a substantially planar configuration, each having an osteoconductive scaffolding  85  machined on their superior and inferior surfaces, respectively. Note that the channel  86 , as well as expansion plate  82 , substantially conforms to the shape of the upper  83  and lower  84  bodies. Joint insert  87  also generally conforms to the shape of the upper  83  and lower  84  bodies, however, an advantageous shape for the banana-shaped implant  80  is more oval to provide improved biomechanical motion of the implant. The bottom floor of channel  86  may also employ a locking lip  88  to ensure that the expansion plate  82  is properly installed and to minimize the potential for dislocating expansion plate  82 .  
         [0067]      FIGS. 6   c  and  6   d  illustrate another embodiment of an expandable intervertebral implant featuring retaining pegs  91  to ensure against dislocation of upper body  83  from lower body  84  during flexion, extension and torsional motion. A plurality of retaining pegs  91  project substantially upward from the superior surface of lower body  84 . On the inferior surface, upper body  83  comprises a plurality of holes, or containment wells  90 , dimensionally larger than captive pegs  91  and arranged such that when upper body  83  is properly positioned upon lower body  84 , captive pegs  91  are housed within containment wells  90 . As shown in  FIG. 6   d , when the intervertebral implant is flexed or extended, captive pegs  91  prohibit dislocation of upper body  83  from lower body  84 . While the pegs and containment wells may be any shape, captive pegs  91  are preferably round and containment wells  90  are preferably oval in shape, which gives limited torsional mobility as well.  
         [0068]      FIGS. 7   a  and  7   b  illustrate the expansion of joint insert  19  in more detail. As shown in  FIG. 7   a  and prior to expansion of joint insert  19 , upper body  12  rests upon lower body  14  and the generally flat inferior surface  20  of joint insert  19  rests upon the bottom of channel  15 , which extends along the lower body  14 . Disposed along the generally flat inferior surface  20  of expandable joint insert  19  and adjacent to expansion slot  18 , is a lifting notch  17  that engages with the expansion screw  70 . Lifting notch  17  facilitates the lifting of expandable joint insert  19  and allows expansion screw  70  to come into contact with the generally flat inferior surface  20  of joint insert  19 . Once inserted, as shown in  FIG. 7   b , the generally flat inferior surface  20  of expandable joint insert  19  rests upon expansion screw  70  and the upper body  12  is lifted above lower body  14  to the desired intervertebral disc height  71 .  
         [0069]      FIG. 8  shows an expandable artificial intervertebral implant  10  inserted into the spinal column. Note that the expandable artificial implant  10  is posteriorly inserted and expanded through void space  90 , which is created by removal of a facet joint.  
         [0070]     The disclosed techniques of expanding an artificial implant by inserting an expansion plate or similar device may also be employed to expand a PLIF or TLIF cage. As shown in  FIGS. 12   a ,  12   b  and  12   c , a PLIF cage  300  is disclosed comprising a substantially rectangular external cage element  301  housing an internal expandable element  302 . The PLIF cage element  301  has an osteoconductive mesh structure  303 , in which an osteoconductive substance can be placed, on its inferior surface  304  and an expansion window  305  located on its superior surface  306 . The internal expandable element  302  comprises a generally planar plate member  307  having an inferior  308  and superior surface  309 . A second osteoconductive mesh structure  310  is secured upon the superior surface  309  of the planar plate member  307  of the internal expandable element  302 . The inferior surface  308  of the planar plate member  307  has a lifting notch  311  to facilitate the expansion of the device upon installation of the expansion plate 312 . The expansion plate  312  is inserted into the posteriorly located expansion slot  313  of the PLIF external cage element  301  and engages the lifting notch  311  of the planar plate member  307  of the internal expandable element  302 . Locking lip  314  located within expansion slot  313  minimizes the potential of expansion plate  312  dislocation.  
         [0071]      FIGS. 12   d  and  12   e  show a TLIF cage similar to the PLIF cage described above. The primary difference between the TLIF cage and the PLIF cage is that the TLIF cage comprises a t-shaped cross-sectional osteoconductive mesh structure  310  secured upon the superior surface  309  of the planar plate member  307  of the internal expandable element  302  such that the osteoconductive mesh structure  310  overhangs the superior surface  306  of the external cage element  301 . Thus providing more surface area between the osteoconductive mesh structure  310  and the bony endplates within the intervertebral space.  
         [0072]     One embodiment of an artificial facet joint  100  is shown in  FIG. 9   a . Artificial facet joint  100  comprises an upper pedicle screw  101  and a lower pedicle screw  102 . Rod  103  is retained within the head  104  of upper pedicle screw  101  and the head  105  of lower pedicle screw  102 . Rod  103  has washer-type ends  106  that allows for posterior compression, but not extension.  
         [0073]     Another embodiment of an artificial facet joint  110  is shown in  FIG. 9   b . Rod  113  comprises a single washer-type end  116  on its lower end  117 . The head  115  of upper pedicle screw  112  has a threaded locking screw  118 , as shown in  FIG. 9   c , that holds rod  113  in place and prohibits the head  115  of pedicle screw  112  from swiveling, but allows rod  113  to rotate and translate through the head  115  of pedicle screw  102 .  
         [0074]     Another embodiment of an artificial facet joint  200  is shown in  FIG. 10 . Artificial facet joint  200  generally comprises an upper pedicle screw  201  and a lower pedicle screw  202  and rod  203  retained within the heads of pedicle screws  201 ,  202 . Both pedicle screws  201 ,  202  are secured with locking screws  218  that prevent the heads  204 ,  205  of pedicle screws  201 ,  202  from swiveling, but allow rotation and translation of rod  203 . Rod  203  comprises two rod members  206 ,  207  connected via a ball joint  208 . Ball joint  208  allows for a generally upward rotation, away from the bony surfaces of the vertebrae to which they are secured, but prohibit a generally downward rotation, which would bring the ball joint in contact with the vertebrae to which they are secured.  
         [0075]      FIG. 11  shows the artificial facet joint  200  of  FIG. 10  in place on the spinal column. Note that artificial intervertebral implant  10  has been posteriorly placed within the intervertebral space through the void created by the surgical removal of the natural facet joint. In addition, ball joint  208  generally rotates in the posterior (upward) direction during posterior compression to prevent impact upon the bony surfaces of the spine.  
         [0076]      FIGS. 13   a ,  13   b ,  14   a  and  14   b  illustrate a lordotic, banana-shaped expandable artificial intervertebral implant  400 . The lumbar spine is lordotic, thus the anterior disc height is naturally larger than the posterior disc height. Therefore, an expandable artificial intervertebral implant for the lumbar spine must be capable of expanding into a lordotic position.  FIG. 13   a  shows the lordotic expandable artificial intervertebral implant  400  from a posterior view. Lordotic expandable artificial intervertebral implant  400  generally comprises an upper body  412  and a lower hinged body  414  in a substantially planar configuration prior to expansion. The superior surface  402  of the upper body  412  and the inferior surface  404  of the lower hinged body  414  comprise an osteoconductive scaffolding  413  through which the bone may ultimately grow. The upper body  412  has a substantially concave inferior surface  416 .  
         [0077]     The lower hinged body  414  comprises a lower portion  420  and an upper portion  430 . Lower portion  420  and upper portion  430  are posteriorly hinged via hinge  440 . Hinge  440  effectively fixes the posterior disk height  460  (shown in  FIG. 14   b ). Upper portion  430  of hinged body  414  has a generally flat inferior surface  431  and a substantially convex superior surface  432 . The lower portion  420  has a substantially planar configuration prior to expansion. Located at the anterior end  421  of lower portion  420  is a rotational lifting mechanism  422 . Once placed in the intervertebral space, the rotational lifting leg is rotationally engaged, thus lifting the anterior end  421  of the expandable artificial intervertebral implant  400  to achieve the desired anterior disc height  470  and proper lordosis. Securing notch  425  is located on the anterior end  421  of the upper portion  430  of hinged body  414 . Securing notch  425  engages with rotational lifting mechanism  422  once the expandable artificial intervertebral implant  400  has been expanded. The height of rotational lifting mechanism  422  is determined by the desired proper lordosis when the intervertebral implant  400  is under neutral load.  
         [0078]     Upper body  412  has a substantially concave inferior surface  416  that articulates with the substantially convex superior surface  432  of upper portion  430  of lower hinged body  414 . When viewed in the medial or lateral direction, as shown in  FIGS. 14   a  and  14   b , upper body  412  has a downwardly projecting lobe  450  for the attachment of safety bar  452 . Safety bar  452  secures upper body  412  to upper portion  430  of lower hinged body  414  and minimizes the possibility of dislocation.  
         [0079]      FIG. 13   b  is a top view of lordotic expandable artificial intervertebral implant  400  illustrating the placement of posterior hinge  440 , rotational lifting mechanism  422 , and safety bar  452  affixed through upper body  412  and upper portion  430  of lower hinged body  414 .  
         [0080]     The rotational lifting mechanism described above may also be employed to achieve proper lordosis with an expandable PLIF and TLIF cage, as shown in  FIGS. 15   a  and  15   b . Cage  500  is shown prior to expansion in  FIG. 15   a  and expanded in  FIG. 15   b . Cage  500  comprises an upper body  502  and a lower body  504 . Hinge  506  posteriorly connects upper body  502  to lower body  504  and effectively fixes posterior disc height  510  upon expansion of cage  500 . The superior surface  512  of upper body  502  and the inferior surface  514  of lower body  504  may include an osteoconductive scaffolding or mesh  520  as previously described. Expansion of cage  500  is accomplished via rotational lifting mechanism  530 , which engages with securing notch  525 , located on the anterior end  528  of the inferior surface  513  of upper body  502 , and minimizes the potential for dislocation. The height of rotational lifting mechanism  530 , which effectively fixes anterior disc height  540 , is determined by the desired proper lordosis.  
         [0081]     Another embodiment of an expandable lordotic artificial intervertebral implant is illustrated in  FIGS. 16   a and  16   b . Lordotic expandable intervertebral implant  600  and lordotic cage  700  both utilize an inclined expansion plate  650  to achieve proper lordosis. Both devices are similar to those described above with the exception of the expansion device and reference is made to  FIGS. 14   a  and  14   b  for lordotic expandable intervertebral implant  600  and  FIGS. 15   a  and  15   b  for lordotic cage  700  for elements of the intervertebral implants already identified. Expansion plate  650  is generally wedged-shaped and comprises a lifting notch  620  on its posterior end  622  to facilitate expansion. As shown in  FIG. 16   a , expansion plate  650  is installed between the upper portion  430  and lower portion  420  of lower hinged body  414 . Located on the superior surface  630  at the anterior end  624  is securing ridge  635 . Securing ridge  635  engages with securing notch  625  similar to the rotational lifting mechanism described above. Located on the anterior superior surface of lower portion  420  of lower hinged body  414  is a locking lip  637 , which minimizes the potential of dislocating inclined expansion plate  650 .  FIG. 16   b  illustrate the use of expansion plate  650  in conjunction with lordotic cage  700 .  
         [0082]     Although the present invention has been described in terms of specific embodiments, it is anticipated that alterations and modifications thereof will no doubt become apparent to those skilled in the art. It is therefore intended that the following claims be interpreted as covering all alterations and modifications that fall within the true spirit and scope of the invention.