Abstract:
A total artificial expansible disc having at least two pairs of substantially parallel shells, which move in multiple directions defined by at least two axes, is disclosed. Several methods for implanting the total artificial expansile disc are also disclosed. The total artificial expansile disc occupies a space defined by a pair of vertebral endplates. An expansion device, which preferably includes a jackscrew mechanism, moves the pairs of shells in multiple directions. A core is disposed between the pairs of shells, and the core permits the vertebral endplates to move relative to one another.

Description:
[0001]     This application is continuation-In-Part of copending application Ser. No. 10/964,633, filed on Oct. 15, 2004, which claims the benefit under Title 35, U.S.C. §119 (e) of U.S. provisional application 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 which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. § 120. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to artificial discs, and more specifically relates to artificial expansile total lumbar and thoracic discs for posterior placement without supplemental instrumentation, and to anterior placement of artificial discs for the cervical, thoracic and lumbar spine.  
         [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. Total artificial disc replacements aim to cover the entire expanse of the disc space because restoration of range of motion is reportedly greatest when roughly 80% of the vertebral endplate is covered. Thus it is only rational, currently 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 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. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  illustrates a cross-section of the, expansile total lumbar/thoracic implant upon initial posterior insertion into the lumbar (or thoracic) disc space against the background of a vertebral body, i.e., this illustrates a three-dimensional expandable elastic polymer nucleus design (Embodiment I);  
         [0010]      FIG. 2  illustrates ratcheting of the three-dimensional expansile total lumbar/thoracic titanium shells to conform to the length of the vertebral body (Embodiment I);  
         [0011]      FIG. 3  illustrates the dorsal view of the three-dimensional expansile total lumbar/thoracic construct from the surgeon&#39;s perspective (Embodiment I);  
         [0012]     FIGS.  4 A-M illustrate in detail the mechanical cylinder-spur-gear-spring (CSGS) system incorporated into the cross-connecting 8 titanium shells of the three-dimensional Lumbar/Thoracic prosthesis which enable expansion of the device in x, y and z dimensions. A system of springs is incorporated into the y-axis of the CSGS so as not to hinder the flexibility of the inner core.  
         [0013]      FIGS. 5A and 5B  illustrate a second embodiment of the three-dimensional expansile lumbar/thoracic disc invention, i.e., these illustrate an in-situ injection/expansion elastic polymer nucleus design (Embodiment II);  
         [0014]      FIGS. 6A, 6B  and  6 C illustrate a third embodiment of the expansile total lumbar/thoracic artificial disc implant, i.e., this illustrates a mechanical metal on metal, stainless steel, ball on trough design (Embodiment III);  
         [0015]      FIGS. 7A, 7B  and  7 C illustrate a fourth embodiment of the three-dimensional expansile total lumbar/thoracic artificial disc implant, i.e., this illustrates a mechanical metal on metal, biconvex ultra high molecular weight polyethylene (UHMWPE) design (Embodiment IV);  
         [0016]      FIG. 8  illustrates a fifth embodiment of the three-dimensional expansile total lumbar/thoracic artificial disc implant, i.e., this illustrates a mechanical metal on metal, monoconvex UHMWPE design (Embodiment V);  
         [0017]     FIGS.  9 A-E illustrate a sixth embodiment of the expansile total lumbar/thoracic artificial disc implant. This simpler design expands in two not three dimensions (height and width). The mechanism of expansion is based on calibrated ratcheting of corrugated interconnected bars. FIGS.  9 A-E therefore represent a two-dimensional expansile prosthesis, using the elastic polymer nuclear design as the prototype (Embodiment VI);  
         [0018]     FIGS.  10 A-E illustrate a seventh embodiment of the expansile total lumbar/thoracic artificial disc implant which expands in two dimensions using a jackscrew width expansion mechanism and a fixed-screw height expansion mechanism (Embodiment VII).  
         [0019]     FIGS.  11 A-D illustrate the precise mechanism of the jackscrew opening and closing employed in embodiment VII. The figures illustrate four types of mechanisms; mechanical using a screw, electrical-wired control, electrical-wireless control, and a hybrid mechanical-electrical mechanism combining a screw and wired control.  
         [0020]      FIG. 12A  represents an endoscope variant of the present invention inserted unilaterally into the disc space to inspect the disc space circumferentially;  
         [0021]      FIG. 12B  represents a specifically designed pituitary rongeur endoscopic attachment with a light source emanating from the junction of the adjoining dorsal and ventral cup forceps. This significantly aids in performing a complete circumferential discectomy necessary for adequate prosthesis implantation.  
         [0022]      FIG. 12C  illustrates a right-angled ratchet driver integrated into an endoscope to assist in visualization of screws beneath the caudal aspect of the spinal cord or thecal sac, if necessary.  
         [0023]      FIGS. 13A, 13B ,  13 C and  13 D illustrate a cross-section of the prostheses adapted for anterior implantation into the cervical disk space.  FIG. 13A  illustrates the expandable elastic polymer nucleus design (Embodiment I).  FIG. 13B  illustrates the in-situ injection/expansion elastic polymer nucleus design (Embodiment II).  FIG. 13C  illustrates the mechanical, metal on metal, stainless steel, ball on trough design (Embodiment II).  FIG. 13D  illustrates the mechanical, metal on metal, UHMWPE biconvex or monoconvex design (Embodiments IV and V). 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]     The Medical Device  
         [0025]     Referring now to  FIGS. 1-4 , the above described problem can be solved in the lumbar/thoracic spine by the insertion of a total boomerang (bean) shaped prosthetic disc  100  including an expansile disc core  101  surrounded by ratchetable titanium shells (containers) Q 1 -Q 8  that can expand geometrically in all three x, y, and z planes, horizontally, vertically and width wise.  
         [0026]     The outer titanium shells Q 1 -Q 8  themselves when ratcheted width-wise have titanium spikes  103  inserting themselves into and purchasing the endplates, thus securing permanent integration into the vertebral endplates. The outer shell titanium surfaces can be treated with hydroxyappetite to facilitate bone incorporation. There is currently available a vertebral ratcheting corpectomy construct which can be ratcheted up vertically until it purchases the rostral and caudal endplates with spikes. There are currently transpedicular/posterior lumbar interbody fusion (T/PLIF) bean shaped constructs, which can be unilaterally inserted into disc spaces. There are currently static total artificial discs (anteriorly placed). The present invention, however, constructs an expansile disc core within a boomerang (bean) shaped titanium construct which can be unilaterally inserted posteriorly into the lumbar and thoracic disc spaces, and can then be ratcheted in vertical and horizontal dimensions to custom fit the implant with the height and length of the individual vertebral body, and ratcheted width wise to conform to the individual width of the a disc space. This total prosthetic device can be secured to the endplates with spike attachments (teeth).  
         [0027]      FIG. 1  illustrates a cross-section of the implant  100  upon initial posterior insertion into the lumbar (or thoracic) disc space  104  against the background of a vertebral body. Note the expansile elastic polymer nucleus  101 , surrounded by an elastometric sheath, which is molded to (vulcanized) the inner surface of the outer titanium shells. Note the titanium spikes  103 . In  FIG. 1A , it should be noted that the spikes are on four separate titanium plates (shells). Q 1 -Q 4  which currently are adjacent to each other. This represents one of two leaflets of the device (rostral and caudal). Hence there are a total of eight moveable titanium shells Q 1 -Q 8 . Upon ratcheting the height up with a screwdriver, the device  100  can be fine-tuned to the individual vertebral body height (FIG. 1). In  FIG. 1B , an expansion device  102  causes the shells Q 1 , Q 2  to separate from shells Q 3 , Q 4 .  
         [0028]      FIGS. 2A and 2B  demonstrate ratcheting of the titanium shells Q 1 -Q 4  to conform to the length of the lumbar/thoracic vertebral body  104 .  
         [0029]     Once the construct  100  has been fine-tuned to conform to the height and length of the vertebral body  104 , it can now be ratcheted to conform to the width of the disc space and to be secured to the endplates  105 .  FIG. 3A  demonstrates the dorsal view of the construct  100  from the surgeon&#39;s perspective, upon initial placement of the construct. Note the expansile artificial disc core  101 , two leaflets of titanium shells Q 1 , Q 2 , Q 5 , Q 6  which are opposed to each other and the outer titanium spikes  103 . Note the three screws  111 ,  112 ,  113 . One of three screws ratchets the height of the implant (screw  111 ), another screw ratchets the length (screw  112 ), and another screw ratchets the width (screw  1133 ).  FIG. 3B  illustrates the width-expanded construct  100  accommodating to individual disc width, and titanium spike  103  endplate purchase.  
         [0030]     FIGS.  4 A-M illustrate the relationship of the screws to the internally incorporated expansion device  102  or cylinder-spur-gear-spring (CSGS) system allowing expansion of the prosthesis in all three planes. There are a total of three screws.  111 ,  112 ,  113 . Any one screw controls the highly adjustable simultaneous movements of the appropriate titanium shells with respect to one another on both rostral and caudal leaflets, in any one given dimension (x, y or z). This is accomplished by internalizing and embedding within the titanium shells Q 1 -Q 8  the expansion device  102  or the gear mesh, a cylinder-spur-gear spring (CSGS) system. This system is designed such that turning screw  111  adjusts the height of the prosthesis by moving the appropriate titanium shells in the z axis. Turning screw  112  adjusts the length of the prosthesis by simultaneously moving the appropriate shells in the x axis. Finally turning screw  113  leads to simultaneous expansion of the appropriate shells in the y axis. Turning screw  113  ensures the final locking position of the prosthesis by engaging and incorporating the outer spikes into the opposing rostral and caudal vertebral endplates.  
         [0031]     Referring now to  FIG. 4A , a three dimension illustration of the lumbar/thoracic prosthesis  100  is provided, and it illustrates the three axes of motion (x,y, and z) vis-à-vis the interconnections between the superior/dorsal titanium shells (Q 1 , Q 2 , Q 5 , Q 6 ) and the inferior/ventral shells (Q 3 , Q 4 , Q 7 , Q 8 ), as well as the external surfaces of the titanium shells, as well as the external titanium spikes  103 . It should be noted that screw  111  adjusts height; screw  112  adjusts length, and screw  113  adjusts width, and that during surgery, height, length and width screws  111 ,  112 ,  113  are sequentially adjusted. The titanium shells of  FIG. 4A  include rostral leaflet superior shells Q 1 , Q 2 ; rostral leaflet inferior shells Q 3 , Q 4 ; caudal leaflet superior shells Q 5 , Q 6 ; and caudal leaflet inferior shells Q 7 , Q 8 .  
         [0032]     Referring now to  FIG. 4B , an illustration of lumbar/thoracic prosthesis height and adjustment using screw  111  is provided. During a first stage of operation, screw  111  (height) is adjusted by counter-clockwise/clockwise twisting. The amount of turning of this screw  111  determines the device&#39;s resting height (H). It should be noted that in our Cartesian coordinate system, shell Q 1  will always be fixed as a reference point (x=0,y=0,z=0). It should also be that noted the space between the shells Q 1 -Q 8  is to be filed with a core  101  as explained in connections with embodiments I, II, III, IV, V. The core  101  is taken out of  FIGS. 4   a - 4 D to increase mechanical clarity.  
         [0033]     Referring now to  FIG. 4C , a three-dimensional illustration of the lumbar/thoracic prothesis  100  is provided. The length (L) is adjusted with screw  112 . During the second stage of operation, screw  112  (length) is adjusted by counter-clockwise/clockwise twisting. The amount of turning of screw  112  determines the device&#39;s final resting length.  
         [0034]     Referring now to  FIG. 4D , a three-dimensional illustration of lumbar/thoracic prosthesis  100  is provided. The width (W) is adjusted screw  113 . During the third stage of operation, screw  113  (width) is adjusted by counter-clockwise/clockwise twisting. The amount of turning of screw  113  determines the device&#39;s final resting width (W). This step is performed last, because the spikes  103  anchor into the bone.  
         [0035]     Referring now to  FIG. 4E , a side view of the expansion device  102  controlled by the screws  111 ,  112 ,  113  is provided. The expansion device  102  includes a plurality of components. Component S 1 X rotates about the Y axis and moves along the Z axis. Component E 2 Z moves along the X axis. Component E 2 X rotates about the X axis, moves along the Y axis, and has spring connection  114  at its midpoint. Component X 1  rotates about the X axis. Component Y 1  rotates about X, and moves along the Y axis. Component X 2  rotates about the Y axis. Component Y 2  rotates about X, and moves along the Y axis. Component Z 2  moves along the Z axis, and it has a ball-socket joint  115  at its midpoint. Component E 2 Z moves along the X axis.  
         [0036]     Referring now to  FIG. 4F , a side view of the mechanical infrastructure of titanium shells Q 1 , Q 3 , Q 5 , Q 7  is provided. Component Y 2  rotates about X and moves along Y. Component Z 1  moves along Z, has ball-socket joint  115  at its midpoint. Component S 1 Z rotates about Z. Component S 1 X rotates about X, and move along Z. Component S 2 Z rotates about Z. Component E 1  is static with spring (not shown in diagram) connecting at midpoint along the width axis. Component S 2 Y rotates about Y. Component X 1  rotates abut X. Component Y 1  rotates about X and moves along Y. Component X 2  rotates about X.  
         [0037]     Referring now to  FIG. 4G , a dorsal view of the mechanical infrastructure of titanium shells Q 1 , Q 2 , Q 5 , Q 6  is provided. S 1 Z rotates about Z. Component S 2 X rotates about X. Component E 1  is static with a spring (not shown in diagram) connecting at midpoint along the width axis. Component S 3 X rotates about Y. Component Z 1  moves along Z, and it has a ball-socket joint  118  at its midpoint. Component X 2  rotates about X. Component Y 2  rotates about X and moves along Y. Component E 2 Y has rings that rotate about Y to force the structure to move along X. A coil spring is located at its midpoint. Component Z 2  moves along Z and it has a ball-socket joint  117  at its midpoint. E 2 Z: Moves along X. Component S 1 X rotates about Y and moves along Z. Component E 1  is static with spring (not shown in diagram) connecting at midpoint along the width axis. Component S 3 Z rotates about Z. Component S 2 Z rotates about Z.  
         [0038]     Referring now to  FIG. 4H , a side view of the rostral titanium shells Q 1 , Q 2 , Q 3 , Q 4  and the mechanical infrastructure is provided. Component S 1 Z rotates about Z. Component S 2 Z rotates about Z. Component S 3 Z rotates about Z. Component E 1  is static with a spring (not shown in diagram) connecting at the midpoint along the width axis. Component E 2 Z moves along X. Component Y 2  rotates about X and moves along Y. Component X 1  rotates about X. Component Z 2  move along Z. Component S 1 X rotates about X and moves along Z. Component Z 1  moves along Z. Component S 3 Y rotates about Y. Component S 2 Y rotates about Y.  
         [0039]     Referring now to  FIG. 41 , an axial view of caudal titanium shells Q 5 , Q 6 , Q 7 , Q 8  and the mechanical infrastructure is provided. Component E 2 Z rotates about Y and moves along X. Component Z 2  moves along Z. Component Z 1  moves along Z. Component E 1  is static with a spring (not shown in diagram) connecting at the midpoint along the width axis. Component S 3 Z rotates about Z. Component S 2 Z rotates about Z. Component S 1 Z rotates about Z. Component S 1 X rotates about X and moves along Z. S 2 Y rotates about Y. S 3 Y rotates about Y. Component D 1  is satic. Component Z 1  moves along Z. Component Y 2  rotates about X and moves along Y. Component Z 2  moves along Z. Component X 2  rotates about X and moves along Y.  
         [0040]     Referring now to  FIG. 4J , a ventral view of titanium shells Q 3 , Q 4 , Q 7 , Q 8  and the mechanical infrastructure is provided. Component X 1  rotates about X. Component Z 1  moves along Z. Component E 2 Y has rings that rotate about Y to force the structure to move along X. A coil spring is located at its midpoint. Component X 2  rotates about X. Component Y 1  rotates about X and moves along Y. Component Z 2  moves along Z. Component E 1  is static with a spring (not shown in diagram) connecting at the midpoint along the width axis. Component S 3 Y rotates about Y. Component S 2 Y rotates about Y.  
         [0041]     Referring now to  FIG. 4K , the mechnaical infrastructre and height adjustment components S 1 X, E 2 , Z 2 , E 1 , S 1 Z, Z 1 , Z 2  are illustrated. In  FIG. 4K ( 1 ), which provideds an overview of height adjustment system. Q 3 , Q 4 , Q 7 , Q 8  are attached or linked to bottom pins as shown. Shells Q 4  and Q 8  are linked or connected to component Z 2 . Shells Q 3  and Q 7  are linked or connected to component Z 1 .  FIG. 4K ( 2 ), shows how twisting of component S 1 Z (by external screw driver) about Z translates to a twisting of component S 1 X about X (via miter gears  121 ). Component S 1 Z is twisted by an external screw driver. Rotation of component S 1 Z causes rotation of component S 1 X by way of miter gears  121 . In  FIG. 4K  ( 3 ), component S 1 X is held in space by components E 1 , E 2 , yet is allowed to twist about, interacting with the racks on component Z 1  and component Z 2  up or down along Z. Component S 1 X is held in space by a ring (that allows it to twist however) on both ends that are fixed to E 1  or E 2  (also by ring). Component Z 1  is an upside-down letter-U shape that rises or lowers through holes in component E 1 . Component Z 1  (and component Z 2 ) has a rack on the left side of the U, as shown, to accommodate the complementary spurs on component S 1 X (in two locations). Since component S 1 X is held in place by component E 2  (which is relatively static), component Z 1  must move up (or down) when component S 1 X is twisted. Components E 1 , E 2  are spring connected in their respective midpoints along the y(width) axis to allow for the Q 1 -Q 2 -Q 3 -Q 4  complex to have three degrees of freedom with respect to the Q 5 -Q 6 -Q 7 -Q 8  complex. Components Z 1  and Z 2  have ball-socket joints along their y(width) midpoints for this same reason. Components Z 1 , Z 2  use ball sockets rather than aspring-coil in order to allow for uniform z-direction motion.  
         [0042]     Referring now to  FIG. 4L , the mechanical infrasructure length adjustment components E 2 , E 1 , S 2 Z,  2 Y, X 1 , X 2  are illustrated. In  FIG. 4L ( 1 ), an overview is provided of the length adjustment mechanism. Shells Q 2 , Q 4 , Q 6 , Q 8  are attached or linked to component E 2 . In  FIG. 4L ( 2 ), component S 2 Z turns S 2 Y by miter gear (component S 2 Z is turned by an outside screw-driver). Component S 2 Z is twisted by an external screw driver. Rotation of component S 2 Z causes rotation of component S 2 Y by way of a miter gear. In  FIG. 4L ( 3 ), component S 2 Y turns components X 1 , X 2  by a bevel gear. Component S 2 Y interacts with component X 1  through a bevel gear mechanism. Rotation of component S 2 Y causes rotation of component X 1 . In  FIG. 4L ( 4 ), components X 1  and X 2  are threaded at the shown ends; their twisting caues the bevels surrounding E 2  to move along the threading (in X). Components X 1  and X 2  have threaded sections at their ends, as shown, which when twisted force component E 2  to move along the X-axis. Component E 2  does not rotate at all, but has two threaded rings that are allowed to rotate in place in order to move component E 2  along components X 1 , X 2 .  
         [0043]     Referring now to  FIG. 4M , the mechanical infrastructure and adjustment omponents E 2 , E 1 , S 3 Z, S 3 Y, Y 1 , Y 2  are illustrated. In  FIG. 4M ( 1 ), an overview of the widh adjustment mechanism is provided. Shells Q 5 , Q 6  are attached or linked to Y 1 . Shells Q 7 , Q 8  are attached or linked to component Y 2 . In  FIG. 4M ( 2 ), the twisting of component S 3 Z by a screw driver turns component S 3 Y by a miter gear. Component S 3 Z is twisted by an external screw driver. Rotation of component S 3 Z causes rotation of component S 3 Y by way of a miter gear. In  FIG. 4M ( 3 ), component S 3 Y is shown threaded at the end that touches component Y 1  and component T 2 . Components Y 1  and Y 2  are spirally threaded in opposing directions so that both move in parallel along Y, either back or forth. Component Y 1  has two intermediate spured sections, one corresponding to acomplementary rack on component E 1 , and the other to a complementary rack on component E 2 . Likewise, component Y 2  has two spured sections. Both components Y 1  and Y 2  are beveled at their ends, as shown—in opposing diretions—one right-handed, the other left-handed. Component S 3 Y&#39;s rotation causes rotation in components Y 1  and Y 2 , and thereby their parallel movements along the double racked components E 1  (and E 2 ).  
         [0044]     The present invention depends on an expansile disc core  101  that is molded to the titanium shells (Embodiment I). Rubber, silicon, or polyurethane variants are potential candidates for the core. Because there is already well-documented safe experience with elastic polyurethane, this would be the most likely candidate. One skilled in the art would need to select the most appropriate synthetic core, which has the physico-chemical properties of expansion upon release of pressure, while still maintaining elastic resilience  
         [0045]     Depending on the feasibility of finding and adapting a core with such properties,  FIGS. 5A and 5B  illustrate a second alternative embodiment (Embodiment II).  FIG. 5A  illustrates the first three stages of filling the device with an elastometric material, and  FIG. 5B  illustrates the final two stages. This design consists of the same boomerang shaped bi-leaflet with a total of eight ratchetable titanium shells Q 1 -Q 8  in x, y and z planes. An expandable elastometric sheath  109  is molded to the shells, and can enlarge and conform to the disc space. Within this sheath is a coil  130  with pores (micro-catheter) attached to a port  131 . Once the titanium shells Q 1 -Q 8  are fine tuned to the height and length of the vertebral body, and the width fine tuned to the disc space, liquefied material can be injected into this port  131  from a source  132  filling the elastometric balloon  109  until it fills the disc space and conforms to its geometry, as illustrated in  FIG. 5B . It then cures (gels) permanently. This material could include polycarbonateurethane, polyurethane, polyvinyl alcohol, protein hydrogel, or any other material that one skilled in the art might select. The previous safe employment of protein hydrogel and polyurethane in nucleus disc cores makes these materials the most likely candidates. The appropriate selection of cores with specific chemico-physical properties is a significant design choice.  
         [0046]      FIGS. 6A, 6B  and  6 C illustrate a third alternative embodiment (Embodiment III). This embodiment is an expansile, custom-fit, mechanical metal on metal, ball on trough design ( FIG. 6A ). It consists of two leaflets, rostral and caudal,  140  and  141 . Each leaflet  140 ,  141  in turn consists of three shells; 1) An inner stainless steel shell  142  with a trough on the rostral leaflet, or with a protruding steel ball  143  on the caudal leaflet, 2) An intermediate thin titanium plate  144  or  145  which is molded to the outer surface of the stainless steel shells  142 ,  143 , and to the inner surface of the outer moveable titanium shells Q 1 -Q 8 , and 3) Outer titanium shells Q 1 -Q 8 , four on each leaflet, which when ratcheted glide over the intermediate titanium plates  144 ,  145  allowing expansion of prosthetic height and length to conform i.e. custom fit to the individual vertebral endplate.  
         [0047]      FIG. 6B ( 1 ) illustrates an axial composite view of the lumbar/thoracic metal on metal, ball on trough prosthesis  100 . It illustrates the horizontal and vertical movements of the four titanium shells Q 1 -Q 4  per leaflet expanding in height and length conforming to the particular vertebral body dimensions.  
         [0048]     FIGS.  6 B( 1 )- 6 B( 3 ) illustrate different positions of the inner and outer surfaces of each of the separate three shells per leaflet, which mechanically allow expansion of the implant in x, y, and z dimensions while maintaining a static relationship between the metal on metal ball and trough.  FIG. 6C ( 1 ) illustrates the outer surface of the outer titanium shells Q 1 -Q 4 . It has spikes  103  to engage the vertebral endplates, with four moveable shells as mentioned in the two other designs above.  FIG. 6C ( 2 ) illustrates the inner surface of the outer titanium shells Q 1 -Q 4 . It has both horizontal and vertical bar elevations which fit into horizontal and vertical grooves  150 ,  151  of the outer surface of the intermediate titanium shells  144 ,  145 . This is the mechanism, which allows height and length prosthesis extension. Also note the cross section of four ratchetable bars  152  which extend from the intermediate titanium shell  144  to the outer surface of the inner stainless steel shell  142  allowing expansion of prosthetic disc width, while maintaining static contact between the inner surfaces of the stainless steel ball and trough.  
         [0049]      FIG. 6C ( 4 ), illustrates the inner surface  153  of the intermediate titanium shell  144 . This is molded to the outer surface of the inner stainless steel ball and trough shells  142 ,  143 . Also note the cross-section of the four width expansion bars  152 .  
         [0050]     FIGS.  6 C( 5 ) and  FIG. 6C ( 6 ) illustrates the inner surfaces of the inner stainless steel shells  142 ,  143 . The rostral leaflet has a depression  154 , i.e. a trough, serving as a socket for the steel ball  155  of the opposing leaflet.  FIG. 6C ( 7 ) illustrates the outer surface of the inner stainless steel shells  142 ,  143 . This is molded to the inner surface of the intermediate titanium shell  144 . Also note the cross-section of ratchetable bars  152  allowing width expansion.  
         [0051]     FIGS.  7 A( 1 )- 7 A( 2 ) illustrate a fourth alternative embodiment of the lumbar/thoracic design (Embodiment IV). This embodiment is an expansile, custom-fit, mechanical metal on metal, biconvex ultrahigh molecular weight polyethylene (UHMWPE) design. It consists of two leaflets  161 ,  162 , rostral and caudal. Each leaflet in turn consists of three shells; 1) An inner UHMWPE convex shell  163 , 2) An intermediate thin titanium plate  164  which is molded to the outer surface of the UHMWPE shell  163 , and to the inner surface of the outer moveable titanium shells Q 1 -Q 8 , and 3) Outer titanium shells Q 1 -Q 8 , four on each leaflet which when ratcheted glide over the intermediate titanium plate  164  allowing expansion of the prosthetic height and length to conform, i.e. custom fit to the individual vertebral endplate.  FIG. 7A ( 2 ) illustrates the width adjustment of the fourth alternative embodiment.  
         [0052]     FIGS.  7 B( 1 )- 7 B( 3 ) illustrate an axial composite view of the lumbar/thoracic metal on metal, biconvex UHMWPE prosthesis. It illustrates the horizontal and vertical movements of the four titanium shells Q 1 -Q 4  of one leaflet expanding in height and length conforming to the particular vertebral body dimensions.  
         [0053]     FIGS.  7 C( 1 ) and  7 C( 2 ) illustrate the axial views of the outer and inner UHMWPE biconvex shell  163  surfaces. The axial views of the outer and inner surfaces of the outer titanium shells Q 1 -Q 8  are identical to those illustrated in FIGS.  6 C( 1 ) and  6 C( 2 ). The axial views of the outer and inner surfaces of the intermediate titanium shell is identical to that illustrated in FIGS.  6 C( 3 ) and  6 C( 4 ).  
         [0054]      FIGS. 8A and 8B  illustrate a fifth alternative embodiment of the lumbar/ thoracic disc (Embodiment V). This embodiment is an expansile, custom fit, mechanical metal on metal, monoconvex UHMWPE design. It consists of two leaflets  171 ,  172 , rostral and caudal. The rostral leaflet consists of two shells; 1) an inner thin titanium shell  173  which is molded to the inner surface of the 2) outer titanium shells Q 1 -Q 4 . The caudal leaflet has three shells; 1) An inner monoconvex UHMWPE shell  174 , 2) An intermediate thin titanium plate  175  which is molded (vulcanized) to the outer surface of the UHMWPE shell and to the inner surface of the outer moveable titanium shells Q 1 -Q 4 , and 3) Outer titanium shells Q 1 -Q 8 , four on each leaflet, which when ratcheted glide over the intermediate titanium plates  173 ,  174  allowing expansion of the prosthetic height and length to conform, i.e. custom-fit to the individual vertebral endplate. The composite axial view of the total lumbar/thoracic metal on metal, monoconvex UHMWPE disc (Embodiment V) is identical to that illustrated in FIGS.  7 B( 1 )- 7 B( 3 ) (UHMWPE biconvex embodiment, IV). The cross-sectional axial views of the outer and inner surfaces of the outer titanium shells for both rostral and caudal leaflets (Embodiment V) is identical to that illustrated in FIGS.  6 C( 1 ) and  6 C( 2 ). The axial views of the outer and inner surfaces of the intermediate titanium shell of both rostral and caudal leaflets for Embodiment V is identical to that illustrated in FIGS.  6 C( 3 ) and  FIG. 6C ( 4 ). The axial views of the inner and outer surfaces of the monoconvex UHMWPE shell of the caudal leaflet are identical to FIGS.  7 C( 1 ) and  7 C( 2 ). There is no equivalent UHMWPE shell on the rostral leaflet.  
         [0055]      FIG. 9A  illustrates a total Lumbar/Thoracic prosthetic disc  200 , which expands in two instead of three dimensions. The prototype used to illustrate this design is a variant of the psedoannulus three dimensional designs employed in embodiments I-V. In this embodiment (VI) there are a total of four titanium shells Q′ 1 -Q′ 4 . There are two dorsal shells Q′ 1 , Q′ 2  (rostral and ventral), and two caudal (rostral and ventral) shells Q′ 3 , Q′ 4 . There is a single widened bar  220  attaching the dorsal and ventral shells Q′ 1 -Q′ 4  which expands the height by ratcheting two screws  221  for either rostral or caudal height control.  
         [0056]     FIGS.  9 B( 1 ) and  9 B( 2 ) illustrate the dorsal surgeon&#39;s view. Note the central width bar  224  which connects the rostral and caudal titanium shells Q′ 1 -Q′ 4  dorsally and ventrally. Ratcheting the central screw  225  expands the dorsal width driving the dorsal rostral and caudal shell titanium spikes  203  into the vertebral bodies. Ratcheting a ventral central width screw (not shown) widens the ventral rostral and caudal shells Q′ 3 , Q′ 4  leading to engagement of spikes  203  into the bone. This screw can be accessed endoscopically as will be described below.  
         [0057]      FIG. 9C ( 1 ) illustrates an oblique view of the rostral and caudal shells Q′ 1  and Q′ 2  and the width expansion bar  224  and ratchet screw  225 .  FIG. 9C ( 2 ) illustrates an oblique view of the rostral and caudal shells Q′ 3  and Q′ 4  and the width expansion bar  228  and ratchet screw  229 .  
         [0058]      FIG. 9D ( 1 )- 9 D( 3 ) illustrates an enlargement of the width widening bar  224 . It consists of an inner bar  231  with corrugations, which is in contact with inner grooves of the outer bar  232 . By ratcheting the screws  233 ,  234  in the clockwise direction, width expansion is achieved. By ratcheting the screws  233 ,  234  counter clockwise, width contraction is achieved. Prosthesis width expansion allows incorporation of the spikes  203  into the bone. Once the spikes have engaged the bone to achieve maximal expansion, screws  233 ,  234  can now be turned counter clockwise. This will lead to the contraction of the inner width bar  231  within the outer width bar  232 , enabling the removal of these bars from the construct. Now that the spikes  203  have engaged the bone, removal of the bars are important for allowing complete and uninhibited flexibility of the prosthesis in this most important dimension.  
         [0059]      FIG. 9E  illustrates the axial view of the inner surfaces of the dorsal and ventral titanium shells Q′ 1  and Q′ 3  revealing the indented grooves  239  into which the width bars  224 ,  228  is inserted, and expanded. When the width bars  224 ,  228  are contracted, the bars fall away from these grooves  239  facilitating their removal.  
         [0060]     This two-dimensional pseudo annulus variant (embodiment VI) can be combined with the same cores described for embodiments I, I, III, IV and V. Thus the cores used in embodiments I-V can be adapted and combined with a pseudo annulus which can expand in two or three dimensions. To compensate for the lack of length expansion of the two-dimensional design, it would become necessary to design this variant with the appropriate range of differing length options.  
         [0061]      FIG. 10A   1  illustrates a perspective of a total lumbar/thoracic prosthetic disc pseudo annulus  1000  (Embodiment VII) which expands in two dimensions. The prosthetic disc  1000  includes four shells  1111 ,  1112 ,  1113 ,  1114 , and a plurality of spikes  1115 .  FIG. 10A   2  illustrates a simplified perspective view of the prosthetic disc  1000  and a pair of jackscrew width expansion mechanisms  1022 . Screws  1005 ,  1006  control the jackscrews  1021 ,  1022  respectively.  FIG. 10A   3  is a perspective view that illustrates the jackscrews  1021 ,  1022  and two fixed-screw height expansion mechanisms  1023 ,  1024  which are controlled by screws  1001 ,  1002 ,  1003 ,  1004 . This Embodiment VII can also be combined with either of the cores of embodiments I, II, IIl, IV or V.  
         [0062]      FIG. 10B  illustrates a side view of the prosthetic disc  1000  that sequential turning of screws  1001 ,  1002 ,  1003 ,  1004  leads to height expansion of the rostral and caudal shells  1111 ,  112  by widening the distance between their superior and inferior shells.  
         [0063]      FIG. 10C  illustrates that turning screws  1005 ,  1006  leads to width expansion of the prosthesis  1000  by widening the distance between the rostral and caudal superior and inferior shells  1111 - 1114 .  
         [0064]      FIG. 10D  illustrates an enlarged simplified perspective view of the jackscrew width expansion mechanisms  1021  and  1022 .  
         [0065]      FIG. 10E  illustrates that once maximum width expansion of prosthetic disc  1000  and purchasing of spikes have been achieved, the jackscrews  1021  and  1022  can be removed by counter turning screws  1005  and  1006 . Removal of these screws  1005 ,  1006  allows unconstrained or semi constrained motion of the prosthetic device  1000  depending on which core is selected to be inserted into this pseudo annulus embodiment.  
         [0066]     FIGS.  11 A 1 - 11 A 6  illustrate the basic jackscrew  1021  opening and closing mechanism of embodiment VII. IIlustrated is the geometric conformation the jackscrew  1021  assumes enabling expansion of the rostral and caudal shells  1111 - 1114 , and the conformation it assumes allowing its removal. As illustrated in  FIG. 11A   3 , the more horizontally aligned the four arms  1031 - 1034  of the jackscrew  1021  are the greater the expansion. As illustrated in  FIG. 11A   5 , the more vertically oriented the four arms  1031 - 1034  of the jackscrew  1021  are the greater the contraction.  
         [0067]      FIG. 11B   1 - 11 B 6  illustrate the mechanism of mechanical opening and closing of the jackscrew  1021  (embodiment VII). A straight screw  1005  is attached to the superior and inferior central apices of the jackscrew  1021 . Turning the screw  1005  counterclockwise leads to horizontally oriented expansion/lengthening of the jackscrew  1021  arms which are attached to the titanium shells  1111 - 1114  achieving device expansion. Turning the straight screw  1005  clockwise leads to vertically oriented contraction/shortening of the jackscrew arms  1031 - 1034  allowing the jackscrew  1021  to be detached from the titanium shells  1111 - 1114  upon final engagement of the shells&#39; titanium spikes.  
         [0068]      FIG. 11C   1 - 11 C 6  illustrates a mechanism for sequential electrical opening and closing of the jackscrew  1021  employed in embodiment VII. As illustrated in  FIG. 11C   2 , two partially insulated wires  1041 ,  1042  (e.g. nitinol) are embedded into the jackscrew  1021 . One vertically oriented wire  1041  is attached to the superior and inferior jackscrew apices  1043 ,  1044 . Another horizontally oriented wire  1042  is attached to the rostral and caudal jackscrew apices  1045 ,  1046 . When power is applied to the vertical wire  1041  from the power supply  1050  by activating the switch the four arms of the jackscrew  1021  contract achieving more horizontally oriented positions thereby expanding the device. When power is applied to the horizontal wire  1042 , the four arms of the jackscrew  1021  achieve a more vertical position thereby leading to contraction of the device. The jackscrew  1021  can then be removed as illustrated in  FIG. 11C   6 . The electrical jackscrew  1021  can be controlled by an external power source  1050  making it wire controlled. Alternatively the power source can be enclosed in the jackscrew  1021  itself making it a wireless device. If desirable the same electrical jackscrew mechanism  1021  may be employed for vertical height expansion of the embodiments as well.  
         [0069]     FIGS.  11 D 1 - 11 D 6  illustrates a hybrid mechanical-electrical mechanism to expand and contract the jackscrew  1021  of embodiment VII. This design employs the placement of a vertical screw  1005  as well as a vertically oriented insulated wire  1041  attached to the superior and inferior apices of the jackscrew  1021 . It also has a horizontally oriented insulated wire  1042  attached to the rostral and caudal apices of the jackscrew  1021 . The purpose of this design is to have a mechanical backup in the event of an electrical failure/malfunction. The power from a power supply  1050  applied can be external to the jackscrew  1021  making it wire controlled, or it could be enclosed in the jackscrew  1021  making it wireless controlled.  
         [0070]     a. The Surgical Method  
         [0071]     The surgical steps necessary to practice the present invention will now be described.  
         [0072]     For posterior lumbar spine prosthetic implantation there are two embodiments of the surgical approach: A) Classic microscopic open lumbar hemilaminotomy and discectomy, and B) Minimally invasive microendoscopic hemilaminotomy and discectomy.  
         [0073]     Classic Microscopic Open Hemilaminotomy/Discectomy (Approach A):  
         [0074]     Step 1. After the adequate induction of general anesthesia, the patient is placed prone on a radiolucent Jackson table. The patient is prepped and draped and using x-ray or fluoroscopic guidance, the correct disc space is identified. The microscope is brought in for appropriate magnification of the operative site. A routine hemilaminotomy is performed as fully described elsewhere.  
         [0075]     Step 2. A complete discectomy is performed, and the endplates are curetted thereby preparing the disc space for prosthesis implantation. An endoscope can be employed to verify from a unilateral hemilaminotomy a successful circumferential discectomy.  FIG. 12A  illustrates an endoscope variant of the present invention having an endoscope  301  and a monitor  302  that specifically looks into the disc space  104  (discoscope) to verify an adequate circumferential discectomy.  FIG. 12B  illustrates a specifically lightweight design pituitary rongeur endoscopic  304  attachment, which can also be used to assist in complete and adequate discectomy for prosthesis implantation. A specifically designed right-angled screw ratcheter endoscopic attachment  305  can be used to aid in visualization and ratcheting of screws if partially hidden by the spinal cord orthecal sac as in  FIG. 12C .  
         [0076]     Step 3. The boomerang shaped artificial lumbar/thoracic disc, embodiment I, II, III, IV, V or VI is unilaterally inserted into the disc space by gently retracting the thecal sac and nerve root, and using a forceps or a similar specifically designed instrument to grab and secure the device. Once the edge of the boomerang is introduced into the disc space, it is then curvalinearly further inserted into the disc space underneath the thecal sac and nerve root, aligning the horizontal axis of the prosthesis with the horizontal axis of the vertebral endplate.  
         [0077]     Step 4. Once the construct is beneath the thecal sac, using live fluoroscopy, adjust (ratchet) the construct height (Screw  1 ) until the outer titanium shells conform to the individual vertebral endplate height. Specifically designed screwdrivers, straight or right-angled, of appropriate length and screw fittings are employed.  
         [0078]     Step 5. Now again using live fluoroscopy adjust (ratchet) the length of the titanium shells until the prosthesis conforms to the desired individual length of the vertebral body (Screw  2 ). In embodiments VI and VII there are no length adjustments. Measurements of the length of the vertebral bodies will determine the selection of prefabricated prostheses of different lengths.  
         [0079]     Step 6. Now under direct microscopic or endoscopic visualization adjust (ratchet) the prosthesis width screw. As the width is expanded conforming to the precise disc width, the titanium outer spikes will engage, and then penetrate the bony endplates. Once total spike-bone penetration has occurred with complete purchase of the spikes, the implant is now safely secured in its position, and custom fit with respect to all x, y and z planes. Final lateral and anterior-posterior fluoroscopic images are then obtained to verify the precision fit. Verification of prosthetic purchase to the endplates can be performed by grasping and testing the prosthesis with a forceps verifying lack of motion. For Embodiment VI, once width expansion has been achieved, and the spikes  200  incorporated into the bone, the width bars  224 ,  228  are removed by turning screws  225 ,  229  counter clockwise (see FIGS.  9 C( 1 ) and  9 C( 2 )). For embodiment VII once width expansion has been achieved the jackscrews are removed (see  FIGS. 10E  and  FIGS. 11A, 11B ,  11 C AND  11 D)).  
         [0080]     B) Posterior Lumbar Minimally Invasive Microendoscopic Lumbar Hemilaminotomy (Approach B).  
         [0081]     Step 1. After the adequate induction of general anesthesia the patient is placed prone on a radiolucent Jackson table. The patient is prepped and draped. Then using fluoroscopic guidance a minimally invasive microendoscopic approach is used to gain access to the appropriate disc space as described in detail elsewhere. In brief, serial tubular dilators are sequentially placed in the dorsal musculature and fascia through which a working channel is created and an endoscope is docked on the appropriate laminar landmark.  
         [0082]     Step 2. Through the working channel a discectomy is performed. The endoscope can be repositioned to verify complete discectomy.  
         [0083]     Steps 3-6 are then performed identically to steps 3-6 mentioned above (Approach A; Posterior lumbar placement using an open classical microscopic technique).  
         [0084]     The steps for anterior implantation of lumbar/thoracic prosthetic devices, embodiments I, II, IIl, IV, V, VI or VII into the lumbar L4/5 and L5/S1 disc interspaces will now be outlined.  
         [0085]     Step 1. After the adequate induction of general anesthesia the patient is placed supine on the radiolucent Jackson table. The patient is prepped and draped. A General abdominal surgeon creates an infraumbilical incision, and then using a retroperitoneal or transperitoneal approach as described in detail elsewhere, the L4/5 or L5/S1 disc spaces are identified using fluoroscopic or x-ray guidance.  
         [0086]     Step 2. The microscope is now brought in and a complete discectomy is performed which can be verified under direct microscopic vision.  
         [0087]     Step 3. The prosthetic disc, embodiment I, II, III, IV, V, VI or VII is placed with forceps directly into the disc space with the horizontal axis of the device aligned with the horizontal axis of the vertebral body. The convex lower aspect of the device is placed above the ventral surface of the thecal sac. The dorsal upper aspect of the prosthesis with its ratcheting screws is in the surgeon&#39;s field.  
         [0088]     Step 4. The prosthesis is secured with a forceps, and using fluoroscopic guidance, the prosthesis height is expanded to custom fit the vertebral endplate by ratcheting screw  1  with the appropriately designed screwdriver.  
         [0089]     Steps 5 and 6 are now identical to steps 5 and 6 used for posterior lumbar placement (Approach A).  
         [0090]     The surgical steps necessary to practice the present invention for posterior implantation of the lumbar/thoracic artificial disc, embodiment I, II, III, IV VI or VII into the thoracic disc interspace will now be described.  
         [0091]     Step 1. After the adequate induction of general anesthesia the patient is positioned prone on a radiolucent Jackson table. The patient is prepped and draped. Then utilizing fluoroscopic or x-ray guidance, the correct and relevant spinous processes and posterior elements are identified. Then using a standard extracorporeal, transpedicular approach, the rib head is removed and the pedicle drilled overlying the appropriate disc space as fully described in detail elsewhere.  
         [0092]     Step 2. The microscope is now brought in for appropriate magnification. The discectomy is performed routinely. An endoscope can be employed to verify complete circumferential discectomy.  
         [0093]     Step 3. The lumbar/thoracic prosthesis, embodiment I, II, III, IV, V, VI or Vllis introduced into the disc space using a forceps aligning the horizontal axis of the device with the horizontal axis of the vertebral body. The concave surface of the prosthesis is now placed several millimeters under the spinal cord. Specifically designed right-angled screwdrivers, are introduced ventral to the cord, and custom fit the prosthesis to the vertebral endplate by ratcheting the length, height and width of the prosthesis.  
         [0094]     Steps 4-6 are now identical to steps 4-6 of the posterior lumbar surgical approach A.  
         [0095]     The surgical steps necessary to practice the present invention for the anterior implantation of artificial discs, embodiment I, II, III, IV, V or VI into the thoracic spine will now be described.  
         [0096]     Step 1. After the adequate induction of general anesthesia, the patient is positioned on a beanbag in the lateral position, with the right or left side up depending on the side of the disc herniation. A thoracic surgeon now performs a thoracotomy, the lung is deflated, and using fluoroscopic or x-ray guidance, the appropriate disc space is identified and visualized as described in detail elsewhere.  
         [0097]     Step 2. The microscope is now brought in and a discectomy is performed routinely. Complete and adequate discectomy is confirmed under direct visualization.  
         [0098]     Step 3. The lumbar/thoracic prosthesis, embodiment I, II, III, IV, V, VI or VII is placed with forceps directly into the disc space aligning the horizontal axis of the device with the horizontal axis of the vertebral body. The convex lower surface of the device is placed under the ventral surface of the cord. The concave dorsal surface is facing the surgeon such that there is direct visualization of the ratchetable screws.  
         [0099]     Steps 4-6 are now identical to steps 4-6 of the posterior lumbar placement approach A.  
         [0100]     FIGS.  13 A( 1 )- 13 D( 3 ) illustrate an alternative cervical disc embodiment or prosthetic disc  400  which includes a core similar to embodiments I, II, III, IV and V, and it is specifically configured for anterior implantation. These figures illustrate the axial views of this cervical disc alternative embodiment. They illustrate expansion of prosthetic cervical disc height and length. For the cross-sectional axial views of all outer and inner surfaces of the multiple shells, and for the mechanical infrastructure (CSGS), and for the dorsal surgeons view illustrating prosthetic width expansion refer to the corresponding figures illustrating these views for lumbar/thoracic disc alternative embodiments, embodiments I, II, III, IV and V ( FIGS. 3, 4 ,  5 B,  7 A, and  8 ). These views are virtually identical. Because the cervical spine is less rectangular than the lumbar or thoracic spine, and placement is via an anterior approach, the prosthetic implant  400  takes on a more square/rectangular design (FIGS.  13 A( 1 )- 13 D( 3 ) as opposed to a boomerang bean-shaped design. The. two-dimensional expansile variant for cervical embodiments I-V, i.e. embodiments VI and VII are identical to that illustrated for the Thoracic-Lumbar prosthesis, except for the rectangular design of the cervical titanium shells.  
         [0101]     The surgical steps necessary to practice the present invention for anterior implantation of prosthetic discs, embodiment I, II, III, IV, V, VI or VII into the cervical spine will now be described.  
         [0102]     Step 1. After the adequate induction of general anesthesia, the patient is positioned supine on the radiolucent Jackson table. An interscapular roll is placed and the patient&#39;s neck is prepped and draped. A horizontal incision is made overlying the appropriate disc interspace with the aide of fluoroscopy or x-ray. The platysma is divided, the esophagus and trachea retracted, the anterior spine exposed, and the appropriate disc space verified radiographically as described in detail elsewhere.  
         [0103]     Step 2. The microscope is brought in for appropriate magnification of the operative site. A complete discectomy is performed under direct visualization.  
         [0104]     Step 3. The cervical prosthesis embodiment, embodiment I, II, III, IV, V or VI is placed with a forceps directly into the disc space aligning the horizontal axis of the device with the horizontal axis of the vertebral body. The dorsal surface of the prosthesis with ratchetable screws is facing the surgeon. The ventral surface of the prosthesis is above the cervical spinal cord.  
         [0105]     Step 4. Once the prosthesis is in the disc space above the spinal cord, using fluoroscopic guidance screw  1  is rotated thereby ratcheting the outer titanium shells until they conform to the individual vertebral plate height.  
         [0106]     Steps 5 and 6 are now identical to steps 5 and 6 of posterior placement of lumbar/thoracic artificial discs-approach A.  
         [0107]     Because the current embodiment of cervical prosthetic discs lack anterior screw fixation, multi-level disc replacements can now be entertained. Furthermore with respect to all the lumbar, thoracic and cervical prosthetic disc embodiments surgically implanted via posterior or anterior approaches, it is unnecessary to have multiple sizes of each prosthesis. For embodiments I-V one lumbar/thoracic prosthesis of each of the five design embodiments can be custom fit for the individual lumbar or thoracic disc space. One cervical prosthesis of each particular design embodiment can be custom fit for the individual cervical disc space. For embodiments VI and VII only two or maximum three length options can be chosen. The ease of placement diminishes operating room time and decreases morbidity and mortality. This feature of Embodiments I II III IV, V, VI and VII is unique compared to all other designs to date.  
         [0108]     The present invention may provide an effective and safe technique that overcomes the problems associated with current techniques, and for most degenerative disc conditions it could replace pedicle screw instrumentation and fusions.