[0001]    This application is a Continuation-In-Part of provisional application 60/788,720 filed on Apr. 4, 2006, for which priority is claimed under 35 U.S.C. §120, and which claims the benefit under 35, U.S.C. §119(e) of copending applications Ser. No. 11/019,351, filed on Dec. 23, 2004 and Ser. No. 10/964,633, filed on Oct. 15, 2004; this application also claims priority under 35 U.S.C. §120 of U.S. provisional applications 60/578,319 filed on Jun. 10, 2004; 60/573,346 filed on May 24, 2004; 60/572,468 filed on May 20, 2004; 60/570,837 filed on May 14, 2004; and 60/570,098 filed on May 12, 2004; the entire contents of all the above identified patent applications are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a posterior placed total lumbar artificial disc (“PTTLAD”) without supplemental instrumentation, that uses removable bi-functional screws, sliding expansile plates, and interchangeable cores which enhance individualized custom-fitting. In addition, oblique plate traction spikes are used for enhanced vertebral endplate penetration and incorporation. The present invention also relates to artificial total lumbar discs which can be posteriorly introduced into the lumbar spinal intervertebral disc space, unilaterally, from either left or right side. 
         [0004]    2. Description of the Relevant Art 
         [0005]    Cervical and lumbar total artificial discs are entering the clinical neurosurgical and orthopedic markets. The benefits of these artificial discs are well known. They replace diseased discs, and preserve motion segment mobility. Discogenic and radicular pain are relieved without forfeiting segmental mobility, which is typical of traditional anterior or posterior lumbar fusions. Thus it is currently rational to place prosthetic discs anteriorly where access can be easily obtained, and they can be secured by a variety of anterior screw fixations. This technology is adequate for single level disc replacement in the cervical spine. However based on the current anterior cervical prosthetic disc screw fixation methodology its implantation is periodically complicated by screw failures e.g. partial or complete screw pullouts or breaks, and in most designs it is limited to single level replacement. Furthermore, for lumbar total artificial discs, placement is limited to only the L4/5 and L5/S1 disc spaces, and not above, secondary to aortic and vena caval anatomical restraints. Likewise, for the thoracic spine. Thus far no type of thoracic prosthetic disc device has been reported or described. Furthermore, despite the purported safety of placement of the current anterior total lumbar artificial discs, there is a significant risk of retrograde ejaculations in males, and the risk of vascular injury, which although small, is potentially catastrophic if it occurs. 
         [0006]    The design of total artificial discs, which began in the 1970&#39;s, and in earnest in the 1980&#39;s, consists essentially of a core (synthetic nucleus pulposus) surrounded by a container (pseudo-annulus). Cores have consisted of rubber (polyolefin), polyurethane (Bryan-Cervical), silicon, stainless steel, metal on metal, ball on trough design (Bristol-Cervical, Prestige-Cervical), Ultra High Molecular Weight Polyethylene (UHMWPE) with either a biconvex design allowing unconstrained kinematic motion (Link SB Charite-Lumbar), or a monoconvex design allowing semiconstrained motion (Prodisc-Lumbar). There is also a biologic 3-D fabric artificial disc interwoven with high molecular weight polyethylene fiber, which has only been tested in animals. Cervical and lumbar artificial discs are premised on either mechanical or viscoelastic design principles. The advantages of mechanical metal on metal designs including the stainless steel ball on trough design and the UHMWPE prostheses include their low friction, and excellent wear characteristics allowing long term motion preservation. Their major limitation is the lack of elasticity and shock absorption capacity. The favorable features of the viscoelastic prosthetics include unconstrained kinematic motion with flexion, extension, lateral bending, axial rotation and translation, as well as its cushioning and shock absorption capacity. On the other hand, their long term durability beyond ten years is not currently known. Containers have consisted of titanium plates, cobalt chrome or bioactive materials. This history is reviewed and well documented in Guyer, R. D., and Ohnmeiss, D. D. “Intervertebral disc prostheses”, Spine 28, Number 15S, S15-S23, 2003; and Wai, E. K., Selmon, G. P. K. and Fraser, R. D. “Disc replacement arthroplasties: Can the success of hip and knee replacements be repeated in the spine?”, Seminars in Spine Surgery 15, No 4: 473-482, 2003. 
         [0007]    It would be ideal if total lumbar artificial discs could be placed posteriorly allowing access to all levels of the lumbar spine. Also one could place these devices posteriorly in thoracic disc spaces through a transpedicular approach. Similarly if these devices can be placed anteriorly particularly in the cervical spine without anterior screw fixation, and custom-fit it for each disc in each individual, the ease of placement would reduce morbidity and allow for multi-level disc replacement. Placement of an artificial disc in the lumbar spine if inserted posteriorly through a unilateral laminotomy by using a classical open microscopic approach or by using a minimally invasive tubular endoscopic approach would significantly reduce the possibility of recurrent disc herniation. If placed without facet joint violation, or with only unilateral mesial facetectomy, and the device can purchase the endplates with spikes there would be no need for supplemental posterior pedicle screw fixation, thus obviating the associated morbidity associated with pedicle screws and bone harvesting. To take it one step further, if artificial lumbar discs can be posteriorly placed successfully and safely throughout the entire lumbar spine, every routine lumbar discectomy could be augmented by artificial disc placement which would simultaneously eliminate discogenic and radicular pain while preserving flexibility. Furthermore by so doing, the probability of recurrent herniation plummets, and subsequently the need for posterior pedicle instrumentation plummets, thereby diminishing overall spinal morbidity, expenditure, and leading to the overall improvement in the quality of life. 
         [0008]    Presumably up to now, technology is not focusing on posterior placement of total lumbar prosthetic discs because of inadequate access to the disc space posteriorly. To circumvent this problem others have been working on the posterior placement, not of a total prosthetic disc but of a prosthetic disc nucleus (PDN), or essentially a core without a container (pseudo annulus). PDNs, which are considered post-discectomy augmentations, have consisted of one of the following materials: 1) hydrogel core surrounded by a polyethylene jacket (Prosthetic Disc Nucleus). Two of these devices have to be put in. There is a very high, 38% extrusion rate, 2) Polyvinyl alcohol (Aquarelle), 3) polycarbonate urethane elastomer with a memory coiling spiral (Newcleus), 4) Hydrogel memory coiling material that hydrates to fill then disc space, 5) Biodisc consisting of in-situ injectable and rapidly curable protein hydrogel, 6) Prosthetic Intervertebral Nucleus (PIN) consisting of a polyurethane balloon implant with in-situ injectable rapidly curable polyurethane and 7) thermopolymer nucleus implant. (See the two publications identified above). The approach of posteriorly placing artificial disc cores appears to be flawed in that: 1) there is a high extrusion rate, 2) it lacks good fixation as does total prosthetic devices that are placed anteriorly, 3) it is restricted only to early symptomatically disrupted discs which have only nucleus pulposus but not annulus or endplate pathology, and 4) are contraindicated in discs with an interspace height of less than 5 mm. 
         [0009]    The primary advantages of artificial disc placement include the replacement of diseased discs with prosthetic devices which mimic as much as possible healthy natural discs thereby relieving axial and radicular pain without forfeiting segmental mobility. There are currently in the orthopedic and neurosurgical markets FDA approved anteriorly placed artificial total lumbar discs. The major disadvantages of anterior placement of these devices include vascular injury, blood loss, and retrograde ejaculation in males. 
         [0010]    In our previous copending patent application Ser. No. 11/019,351, filed on Dec. 23, 2004 and Ser. No. 10/964,633, filed on Oct. 15, 2004, which are herein incorporated by reference, we have described artificial expansile total discs for placement throughout the entire spine. The relevant history and prior art of artificial discs are summarized and reviewed there. The artificial discs described in our previous patent applications expand in two or three dimensions, and have internal expanding mechanisms which necessitate a bilateral surgical approach for posterior placement into the lumbar spine. In one embodiment of the present invention, we have simplified the design by omitting an internal expansion mechanism, and by having the one-pieced disc plates expand in only one direction. These modifications make it technically easy to place with minimal disruption of the normal spinal anatomy and with minimal morbidity. Currently in the spinal market there exist only anteriorly placed total artificial lumbar discs. The risks of the anterior placement of these discs are well known and documented, and include but are not limited by vascular injury and retrograde ejaculation. Their surgical removal if warranted is technically challenging and potentially fatal in extreme circumstances. Our design retains all the benefits of the anterior artificial disc with respect to motion preservation, and has none of the above mentioned risks. In addition we introduce an additional novel safety feature, ball limiters, which prevent extrusion of the ball from the artificial disc, and limit complete unrestrained motion. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  illustrates an isometric exploded view of the posteriorly placed total lumbar artificial disc (“PPTLA”). 
           [0012]      FIG. 2  illustrates an isometric view of the closed unexpanded PPLTA device. 
           [0013]      FIG. 3  illustrates an isometric view of the PPLTA device with anterior plate expansion (extension). 
           [0014]    FIGS.  4 A( 1 ) and  4 A( 2 ) illustrate isometric and front views of the insertable core ball (Embodiment I). 
           [0015]    FIGS.  4 B( 1 ) and  4 B( 2 ) illustrate exploded and cross-sectional views of an alternative ball/trough system (Embodiment II). 
           [0016]      FIGS. 5A ,  5 B and  5 C illustrate the components which act in unison to allow width and height device expansion. They include the bi-functional (height/width) adjustment (BFA) screw ( 5 A), the width adjustment screw ( 5 B), and the intervening slotted worm nut ( FIG. 5C ). 
           [0017]      FIGS. 6A and 6B  illustrates the external ( FIG. 6A ), internal ( FIG. 6B ), and top ( FIG. 6   c ) views of the dorsal plate. 
           [0018]      FIGS. 7A and 7B  illustrate the external ( FIG. 7A ) and internal ( FIG. 7B ) views of the ventral plate. 
           [0019]      FIG. 8  illustrates an orthographic view of the uni-dimensional expanding artificial disc, embodiment I (UDEAD I). 
           [0020]      FIG. 9  illustrates an exploded view of the artificial disc (UDEAD I) 
           [0021]      FIGS. 10A and 10B  illustrate the external and internal views of the external plates of the artificial disc (UDEAD I) 
           [0022]      FIGS. 11A and 11B  illustrate the side and orthographic views of the ball of the artificial disc UDEAD I). 
           [0023]      FIGS. 12A and 12B  illustrate the orthographic and exploded views of the ball limiters (UDEAD I) 
           [0024]      FIG. 13  illustrates the ball with the limiters (UDEAD I) 
           [0025]      FIG. 14  illustrates a sample position of the entire artificial disc and how the ball limiters affect range of motion (UDEAD I) 
           [0026]      FIGS. 15  A, B, and C illustrate the orthographic, frontal and exploded views of yet another embodiment of the UDEAD (embodiment II) which employs a ball with raised edges instead of ball limiters. 
           [0027]      FIGS. 16A and 16B  illustrate side and orthographic views of the ball employed in UDEAD II. 
           [0028]      FIGS. 17A and 17B  illustrate cross-sections of UDEAD (embodiment II) during lateral bending and flexion/extension. 
           [0029]      FIGS. 18A ,  18 B and  18 C illustrate the front, back and exploded views of the external insertion device used for UDEAD embodiments I and II. 
           [0030]      FIGS. 19  A and  19 B illustrate a detailed view of the plate insertion section of the external insertion device and the motion of the wedged separator expanding the disc plates. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The Medical Device of FIGS. 1-7 
       [0031]    Referring now to  FIGS. 1-7 , the above described problems can be solved in the lumbar spine by the posterior insertion of a closed PPLTAD in the discs space after the performance of a discectomy. After insertion it is expanded in height (the anterior-posterior direction in a standing patient), and in width (disc space height in a standing patient). 
         [0032]      FIG. 1  illustrates an isometric exploded view of the PPLTAD. It consists of two opposing plates  101 ,  102  which are preferably titanium or cobalt chromium, each of which is comprised of a dorsal component  101   a ,  102   a  and ventral component  101   b ,  102   b . Sandwiched in between the opposing plates  101 ,  102  is a removable ball  103  which contacts a trough  104  on the inner aspects of both opposing plates. 
         [0033]    The mechanical crux to the PPLTAD height and width expandability are based on the interaction of a bi-functional (height/width) adjustment (BFA) screw  105  with a slotted worm nut  106 , and a width adjustment screw  107  and their unified interactions with the dorsal and ventral aspects of each the opposing plates  101 ,  102 , and with their unified interaction with both opposing plates  101 ,  102 . 
         [0034]    Located on the outer aspects of the plates  101 ,  102  are a series of obliquely oriented spikes  108 . The obliqueness of the spikes  108  hinders extrusion by orientation as well as by traction. We believe that this is a unique design which is not found in other prosthetic disc devices. 
         [0035]      FIG. 2  illustrates the PPLTAD in its closed position prior to its insertion into the empty disc space. 
         [0036]      FIG. 3  illustrates the PPLTAD with an extended (expanded) ventral plate  102   b.    
         [0037]    FIGS.  4 A( 1 ) and  4 A( 2 ) illustrate isometric and frontal views respectively of the ball insert  103  (Embodiment I). It consists of an ellipsoid core  401  surrounded by a raised edge  402 . Upon its insertion into the PPLTAD when both surfaces of the ball  103  contact the troughs  104  of the opposing plates  101 ,  102  and moves within them, the raised edge  402  prevents ball extrusion with patient movement. 
         [0038]    FIGS.  4 B( 1 ) and  4 B( 2 ) illustrate a different ball/trough embodiment (II). In this embodiment it is the ensconcing trough protrusions  404  surrounding the ball  403  and ball overhangs  405  which prevent ball extrusion as opposed to the ball rim (Embodiment I) preventing ball extrusion. This preferably allows for the same degree of lateral flexion and rotation as Embodiment I. 
         [0039]      FIGS. 5A ,  5 B and  5 C illustrate close up views of the key components of the expansile mechanism.  FIG. 5A  illustrates a close-up of the bi-functional (height/width) adjustment (BFA) screw  105 . It is composed of a screw body  501  with threads  502 , a hex slot  503 , a neck  504  and a collar  505 . This screw  105  is inserted into the open bearings  601  of the inner aspect dorsal plate  102   a  ( FIGS. 6B and 6C ) and the height adjustment threaded nut  704  and slot  703  of the inner aspect of the ventral plate  102   b  ( FIG. 7B ). 
         [0040]    The BFA threads  502  of screw  105  are in direct contact with the external slots  509  of the slotted worm nut  106  ( FIG. 5B , and  FIG. 1 ). The slotted worm nut  106  in turn has internal threadings  506  ( FIG. 5B ) which accommodate the external threading  507  of the width adjustment screw  107  ( FIG. 5C ,  FIG. 1 ). The countersunk head  510  of the width adjustment screw  107  ( FIG. 5C ), and the head of the slotted worm nut  106  fit into corresponding slots  602  on the inner aspect of the opposing dorsal plates  102   a  ( FIG. 6B ) 
         [0041]      FIGS. 6A ,  6 B,  6 C,  7 A and  7 B illustrate a variety of views of the dorsal and ventral plates  102   a ,  102   b . They illustrate their interrelationship, and their connectivity. The external view of the dorsal plate  102   a  (FIG.  6 A)illustrates a large mid-line flange, and positioning flanges  603  on its left and right, which insert into the ventral plate slots  701  for the dorsal flanges ( FIG. 7A ).  FIG. 6B  illustrates the internal aspect of the dorsal plate  102   a . This has the trough  104  in a fixed position, and the open bearings  601  for insertion of the BFA screws  105 . It also illustrates the slots  602  for either the width adjustment screws  107  or the worm nuts  106  in the opposing plates.  FIG. 6C  is a top view of the dorsal plate  102   a  illustrating the open bearings  601  for the BFA screws  105 , the spikes  108  and the trough  104 . 
         [0042]      FIG. 7B  illustrates the threaded nuts  704  into which the BFA screws  105  are inserted as well as their slots  703  which the bottom aspect of the BFA screws  105  rest upon. 
         [0043]    Another possible embodiment of the opposing plates includes making the opposing plates different sizes, and decreasing the sizes of the screws, thus allowing even more lateral flexion. 
         [0044]    We will now describe the mechanism of height and width expansion. The closed PPLTAD is inserted into the emptied disc space ( FIG. 2 ). The height is expanded by turning each of the four BFA screws  105  ( FIG. 1 ). These screws  105  by virtue of being inserted into the height adjustment threaded nuts  704  of screws of the ventral plate  102   b  ( FIG. 7B ), and the open bearings  601  of the dorsal plate  102   a  ( FIG. 6C ), allow graded sliding of the ventral plate slots  703  vis-á-vis the dorsal plate flanges  603  hence achieving graded separation from each other, i.e. height expansion ( FIGS. 1 ,  3 ,  6 A and  7 A). When maximum desired height is achieved, further turning of the BFA screws  105  rotate the worm nut  106  which then drives the width adjustment screw  107  against the opposing plate thereby leading to opposing plate separation thus driving the opposing plates  101 ,  102  into the opposing vertebral endplates via the spikes  108  ( FIG. 1 ). Once the plates  101 ,  102  are engaged in the vertebral endplates via spike  108  penetration and incorporation, the BFA screws  105  are turned counter-clockwise thereby disengaging them from the inner aspects of the plates  101 ,  102 , and the slotted worm nuts  106 . The BFA screws  105 , slotted worm nuts  106  and width adjustment screws  107  are now removed, having performed their jobs of height and width expansion. It is necessary to remove these objects so that the inner ball core  401  may interact with the inner troughs  104  and achieve complete and unhindered flexibility of motion. Different sized ball inserts  401  accommodate for differences in disc space height. Thus once the plates  101 ,  102  of the PPLTAD are inserted and driven into the endplates, the disc height is measured, and the appropriately fitted ball  401  is inserted to precisely fit the distance of separation between the opposing troughs  104 . This maximizes function, and minimizes extrusion. 
       The Surgical Method of FIGS. 1-7 
       [0045]    The method of posterior insertion of the PPLTAD into the posterior interspace can be performed open microscopically, or closed tubularly, using endoscopic and/or fluoroscopic guidance. 
         [0046]    After the adequate induction of anesthesia the patient is positioned in the prone position. A midline incision is made, bilateral lamina are exposed, and bilateral hemi-laminotomies are performed preserving bilateral facet joints so as not to incur instability. 
         [0047]    A complete discectomy is performed and the superior and inferior endplates exposed. The closed PPTLA without the core ball  401  is inserted. The four BFA screws  105  are turned clockwise leading to height extension of the opposing plates  101 ,  102  via downward sliding of the ventral segments  101   b ,  102   b  of the plates. The screws  105  are turned further clockwise thereby turning the width adjustment screws  107  via the turning of the slotted worm nut  106 . This drives the opposing plates  101 ,  102  with their outer plate spikes  108  into the ventral endplates securing their attachment to the vertebral endplates. Fluoroscopic guidance is used to verify placement of the troughs  104  of the inner aspect of the plates  101 ,  102  at the center of the endplates so that they are at the center of gravity. Once the plates are secured into position the BFA screws are turned counterclockwise, thereby disengaging from the plates  101 ,  102  and the worm nuts  106 . Once disengaged, the BFA screws  105  are removed from their slots, and the slotted worm nuts  106  and widening screws  107  are disengaged from their inserts. We now have two opposing plates  101 ,  102  with their opposing inner troughs  104  engaged in two opposing vertebral endplates. The size between the opposing troughs  104  is measured, and a custom-sized ball  401  is now inserted in between the troughs  104 . The size of the ball  401  is such that it will fit substantially perfectly, and hence not dislodge. The patient is now closed in routine manner. 
         [0048]    This device and method of insertion offer safe posterior lumbar placement with equal motion preservation compared to anteriorly placed lumbar discs. This PPLTAD can also be adopted for anterior lumbar placement, and for posterior and anterior placement into thoracic disc interspaces. In our previous patent we have a modified plate shape for anterior cervical disc placement. The mechanism described herein is easily adapted for cervical artificial discs that do and don&#39;t expand in height. We believe this PPLTAD treats disc disease with significantly decreased morbidity compared to other current devices, whilst preserving spinal segmental flexibility, and enhancing quality of life. 
       The Medical Device of FIGS. 8-19 
       [0049]    Referring now to  FIGS. 8-19 , the above described problems can also be solved by inserting a total artificial disc  800  which consists of three separate components; two opposing bean shaped plates  801 ,  802  and an interposing ball  803  which has ball limiters  804  which prevent ball extrusion (embodiment I), or raised edges which prevent extrusion (embodiment II).  FIGS. 8 and 9  illustrate orthographic and exploded views of the artificial disc  800  (embodiment I).  FIGS. 15A-C  illustrate the orthographic, frontal and exploded views of Embodiment II.  FIGS. 16A  and B illustrate the side and orthogonal views of the ball of embodiment II. 
         [0050]      FIG. 10A  illustrates the external view of either the superior or interior plates  801 ,  802  (embodiments I and II). On the external surface of the plate  801  there are three types of spikes  808  to facilitate penetration and integration into the vertebral endplates. There is one conical center spike  808   a . Around the peripheral edge of the plate  801  are multiple pyramidal spikes  808   c . Surrounding the conical spike are right angled lateral spikes  808   b . Each of the three types of spikes  808  is designed to facilitate penetration contoured to the shape of the plate  801  with respect to the vertebral endplate. We are not aware of any other artificial disc designs which have this feature. Also illustrated are the alignment slots  805  which align with an external insertion/spreading device  1500  ( FIGS. 18-19 ). 
         [0051]      FIG. 10B  illustrates the internal view of the superior or inferior plate  801 . Centrally located is a trough  806  which will interact with the ball  803  of this ball/trough designed artificial disc. At the center of the trough  806  are radial grooves  807  which interact with similar radial grooves  1100  of the ball  803  ( FIG. 11B   15 C) facilitating ball/trough contact. 
         [0052]      FIGS. 11A and 11B  illustrate the ball  803  design (embodiment I) It has superior and inferior domes  1102 ,  1103 . In between the domes  1102 ,  1103  is a groove  1100  for the ball limiters  804 . The ball limiter  804  inserts into the ball groove  1100  ( FIGS. 11 and 13 ).  FIG. 12  illustrates that the ball limiter  804  is composed of superior and inferior leaflets  1201 ,  1202 . At the periphery of these leaflets  1201 ,  1202  there are raised barriers  1203  which limit ball motion and extrusion. After the plates  801 ,  802  are inserted, when the ball  803  and limiters  804  are introduced, the superior and inferior leaflets  1201 ,  1202  are aligned with each other. The inferior leaflet  1201  preferably includes a ball groove insertion ring  1204 . After the ball  803  is inserted the ball limiters  804  are rotated such they are at approx 45-90 degrees angled with respect to each other ( FIGS. 12A and 13 ).  FIG. 14  illustrates a sample position of the artificial disc  800 . It should be noted that with flexion and translation of the device  800 , the raised barriers  1203  of the ball limiters  804  are in contact with the superior and inferior plates  801 ,  802  thereby limiting unrestrained motion of the ball  803 , and prevents ball extrusion. 
         [0053]      FIGS. 15  A, B and C illustrate orthographic, frontal and exploded views of embodiment II. In  FIGS. 15A , B and C, a ball  1503  is disposed between superior plate  181  and inferior plate  802 . 
         [0054]      FIGS. 16A and 16B  illustrate the side and orthographic views of the ball of UDEAD (embodiment II). The ball  1503  preferably includes a groove  1507  for radiographic material, superior dome  1506 , and inferior dome (not shown). The ball  1503  also includes superior raised edge  1504 , inferior raised edge  1505  and radial grooves  1508 . This ball has raised edges instead of limiters which prevent its extrusion and unrestrained motion. 
         [0055]      FIGS. 17A and 17B  illustrate the motion of the ball insert during lateral bending, and flexion/extension. 
         [0056]      FIGS. 18A ,  18 B,  18 C,  19 A and  19 B illustrate the insertion device  1800 . The superior separator  1801  and inferior separator  1802  ( FIGS. 18C and 19  A and B) have extensions which are shaped exactly like the artificial disc plates  801 ,  802  and their cradles fit into the alignment slots  805  of the plates  801 ,  802  ( FIGS. 10A  and B and  19  A and B). The lateral manipulator  1804  and medial manipulator  1803  ( FIGS. 18A-19  A and B) when opened lead to superior plate  801  and inferior plate  802  separation, and cause substantially parallel alignment of superior and inferior plate  801 ,  802  penetration into opposing vertebral bodies. The medial and lateral manipulators  1803 ,  1804  are attached by a transmission linkage  1805  ( FIG. 18C ). The action wedge  1806  upon manual opening of the instrument  1800  by the surgeon inserting his fingers into the manipulator digit insert  1807  ( FIG. 18A ) forces the wedge  1806  down in between the superior and inferior separators  1801 ,  1802  leading to superior disc plate  801  and inferior disc plate  802  separation, expansion and penetration into the superior and inferior vertebral bodies. 
       The Surgical Method of FIGS. 8-19 
       [0057]    The surgical steps necessary to practice the present invention will now be described. 
         [0058]    After the adequate induction of anesthesia the patient is positioned prone on a fluoroscopically amenable table. A unilateral hemi-laminotomy is performed. The procedure can be performed microscopically, endoscopically or tubularly in routine manner. A routine discectomy is performed. The superior and inferior disc plates alignment slots  805  are inserted into the cradles of the insertion device  1800 . The nerve root is gently retracted and the disc plates  801 ,  802  are inserted into the disc space attached to the inserting/spreading device  1800 . Under fluoroscopic guidance the plates  801 ,  802  are then placed at the center of gravity of the vertebral plates i.e. at the anterior-posterior and dorsal-ventral centers. When confirmed radiographically, the surgeon spreads the spreader  1800  which drives the wedge  1806  between the separators  1801 ,  1802  ( FIG. 10 ) until the plates  801 ,  802  have penetrated and incorporated into the superior and inferior vertebral bodies. The inserter/spreader  1500  is then removed. The opposing plates  801 ,  802  are now substantially perfectly opposed to each other. The distance between the superior and inferior troughs  806  are now measured, and the surgeon selects from a selection of balls  803  of different heights to fit between the plates  801 ,  802 , depending on patient size, etc. Using a forceps or similar instrument the ball  803  with the ball limiters  804  (embodiment I), or the ball with raised edges (embodiment II) are inserted in-between the superior and inferior troughs  806 . During insertion of the ball  803  the superior and inferior leaflets  1202 ,  1201  of the limiters  804  are aligned with each other. After the ball  803  is inserted, the superior and inferior leaflets  1202 ,  1201  using a forceps are separated to effectively prevent ball extrusion and prevent completely unrestrained motion. After inserting the ball of embodiment II of  FIGS. 15A-17B , the correct sized ball is simply inserted in between the two plates. The wound is then closed routinely. 
         [0059]    The current device can easily be adapted for placement in cervical and thoracic discs. It may also be suitable for multiple level placements. This current device enables the restoration of motion of diseased discs with minimal anatomical destruction and invasiveness, and avoids the serious complications of anteriorly placed discs. Furthermore when an anteriorly placed lumbar disc is removed, it is extremely technically challenging. Furthermore the artificial disc is then replaced by a fusion device limiting motion. The posterior unilateral placement of this device obviates all the above mentioned risks. The device presented here is safely implanted avoiding anterior vascular structures and nerves which control ejaculation. It is also easily and safely explanted if necessary. The ease and safety of the insertion of this device heralds in a new era of safe and simple artificial lumbar disc technology.
A total artificial expansile disc and a method for posterior insertion between a pair of vertebral endplates are disclosed. The total artificial expansile disc includes at least one pair of substantially parallel plates that move apart along a first axis, in order to occupy a space defined by the vertebral endplates. In another embodiment, each of substantially parallel plates includes a first plate and a second sliding plate. An expansion device or tool is used to move the substantially parallel pair of plates apart along the first axis. A core is disposed between the pair of plates, and the core permits the vertebral endplates to move relative to one another. A ball limiter or ball extender prevents the core from being extruded from between the substantially parallel plates.