Patent Publication Number: US-2005143824-A1

Title: Artificial intervertebral disc

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This patent application is a Continuation-In-Part application of U.S. patent application Ser. No. 10/700,748, filed Nov. 3, 2003, which is a Continuation-In-Part application of U.S. patent application Ser. No. 10/653,540, filed Sep. 2, 2003, which is a Continuation-In-Part application of U.S. patent application Ser. No. 10/430,861, filed May 6, 2003, which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Technical Field  
      The present invention relates generally to a spinal implant assembly for implantation into the intervertebral space between adjacent vertebral bones to provide stabilization and continued postoperative flexibility and proper anatomical motion. More specifically, the present invention relates to an artificial intervertebral disc, sometimes referred to as an intervertebral spacer device, for functioning as a load sharing and bearing device for replacement of the damaged, decayed, or otherwise nonfunctioning intervertebral disc.  
      2. Background of the Invention  
      The spine is a complex structure consisting of multiple flexible levels. Each level consists of a system of joints defined by adjacent vertebral bones. The system of joints includes intervertebral discs, which are a two-part structure. The disc consists of a nucleus and an annulus. The system allows motion while the facet joints add posterior stabilization to the spinal column. The disc allows motion and cushioning to the joint.  
      The complex system of the joint is subjected to varying loads and problems over time, including disc degeneration due to a variety of reasons. Disc degeneration can be attributed to aging, damage due to excessive loading, trauma, and other anatomical issues. Facet joints of the structure can be compromised due to the same reasons, as well as due to arthritic changes. Severe joint degeneration and failure can often cause sufficient pain to require surgical intervention.  
      The current standard method of treatment for severe pain caused by spine joint problems is fusion at the damaged level of the spine. The treatment, when successful, fuses the damaged section into a single mass of bone. The fusion of the joint eliminates motion of the joint, thereby reducing or eliminating pain at that level. Success rates for pain elimination are very high for this method of treatment. However, since the entire spine works as a system, fusion results in complications.  
      Elimination of motion at the spine alters the biomechanics of the spine at every other level. If one level is fused, then loads are absorbed by one less disc into a system not designed for such change. Thus, the remaining discs must redistribute loads, each disc absorbing a greater load. In addition, the spine flexes to absorb loads. A fusion alters the means by which the spine flexes, which also increases the loads on the remaining healthy discs. In turn, it is well understood that a complication of fusion is that additional fusions may be required in the future as the other discs deteriorate due to the altered biomechanics of the spine. In other words, short-term pain relief is exchanged for long-term alterations of the spine, which, in turn, usually require further surgery.  
      There are numerous prior art patents addressing the issue of disc replacement. The U.S. Pat. Nos. 6,443,987 B1 and 6,001,130, both to Bryan, disclose polymer composite structures for cushioning intervertebral loads. The U.S. Pat. No. 5,258,031 to Salib, et al. and U.S. Pat. No. 5,314,477 to Marnay disclose ball and socket type implants addressing the issue of intervertebral mobility. These patents are exemplary of a first approach using an elastomer as a motion and dampening structure and a second approach utilizing a ball and socket joint to create a moving pivot joint. There are many variations on these concepts, which include mechanical springs and more complex structural mechanisms. A significant portion of the prior art addresses the issues of intervertebral motion but do not address anatomical loading considerations.  
      The current state of prior art artificial intervertebral discs are associated with various problems. For example, a number of implants constructed from polymers are of insufficient strength to work effectively in the higher loading areas, such as the lumbar spine. Such polymers often take compressive sets so that the original height of the implant decreases over time. A surgeon must either compensate for the compression by initially using a larger polymer prosthesis and estimate compression or use the appropriately sized polymer prosthesis and later surgically replace the same once the irreversible compression of the prosthesis is unacceptable.  
      Implants constructed with ball and socket joints severely restrict or eliminate shock cushioning effect of a normal disc. This implant can provide motion, but biomechanically, the ball and socket joint negatively affects other healthy discs of the spine. The result can be long-term problems at other levels of the spine, as seen with the current treatment of fusion.  
      Other implants, not discussed above, utilize bearing surfaces usually having polyethylene bearing against metal interfaces. Polyethylene as a bearing surface is problematic in large joint replacement due to the wear properties of the material. Since artificial discs are intended to be implanted over long periods of time, such wear can be highly damaging to surrounding tissue and bone.  
      An additional problem with the implants of the prior art is the manner in which the implants are inserted. Most current techniques require an anterior surgical approach to the spine in order to properly access the intervertebral space. The primary difficulty with such techniques is that the techniques require an incision in the abdomen. The surgeon must then acquire visualization of the spine utilizing either a transperitoneal or retroperitoneal approach. When access to the spine is achieved, implantation of a large, single disc unit requires considerable surgical skill and patient risk because blood vessels, generally known as the Great Vessels, run down the anterior spinal column. The Great Vessels must usually be retracted in order to create a space sufficient for implanting the disc. The entire approach creates substantial scar tissue and thus creates further problems with regard to revision procedures.  
      U.S. Pat. No. 6,572,653, to Simonson discloses a vertebral implant adapted for posterior insertion and designed to replace the fibrocartilage between the facing surfaces of adjacent superior and inferior lumbar vertebrae. The implant additionally includes a pair of springs. Each spring is positioned between the side edges of opposing superior and inferior supports with the position of the spring being fixed by the opposing retainers. Each spring has an axial force under compression that drives the teeth of the opposing superior and inferior supports into the facing surfaces of the adjacent vertebrae. However, there are still significant problems associated with the insertion of the implant because of the size of the implant.  
      A posterior approach has not been utilized successfully in the prior art because access to disc space tends to be limited due to the fact that the dura and spinal cord run through the space created between the lamina and vertebral body. Because of the lack of disc space it is virtually impossible to insert a single artificial disc into the disc space.  
      In view of the above, it is desirable to provide a solution to intervertebral disc replacement that restores motion to the damaged natural disc area while allowing for motion as well as cushioning and dampening, similar to the naturally occurring disc. In addition, it is preferable to allow such motion, cushioning, and dampening while preventing a polymer or elastomeric material from experiencing the relatively high compressive loads seen in the spine. It is also preferable to allow a bearing surface to share the spinal loads with the polymer and elastomeric material. Finally, it is preferable to develop a method of implanting the disc that avoids moving the Great Vessels and provides the surgeon with easier access to the intervertebral space, for example a method that enables a posterior approach.  
     SUMMARY OF THE INVENTION  
      According to the present invention, there is provided an artificial intervertebral disc comprising at least two individual disc units that create a single center of rotation within an intervertebral space. An artificial intervertebral disc including housing members including spaced inner surfaces facing each other and oppositely facing outer surfaces for engaging spaced apart intervertebral surfaces; self-adjusting bearing mechanisms operatively disposed between the inner surfaces for moving relative to the housing members to adjust and compensate for vertebral disc motion; and a flange formed on an outer surface of said housing members for aligning the disc in an intervertebral space is also provided. Also provided is a method for posteriorly inserting an artificial disc assembly by inserting at least two artificial disc assemblies around a spine and into an intervertebral space.  
    
    
     DESCRIPTION OF DRAWINGS  
      Other advantages of the present invention can be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:  
       FIG. 1  is a side perspective view of a preferred embodiment of the present invention;  
       FIG. 2  is a side exploded view of the embodiment shown in  FIG. 1 ;  
       FIG. 3  is a side perspective view of a second embodiment of the present invention;  
       FIG. 4  is a perspective view of a lower disc constructed in accordance with the present invention;  
       FIG. 5  is a side view of an upper disc constructed in accordance with the present invention;  
       FIG. 6  is a top perspective view of an upper housing member made in accordance with the present invention;  
       FIG. 7  is a top plan view of a lower housing member made in accordance with the present invention;  
       FIG. 8  is a side perspective view of a third embodiment of the present invention;  
       FIG. 9  is a perspective view of the present invention with the top housing member removed;  
       FIG. 10  is a perspective view of an alternative pad configuration of the present invention;  
       FIG. 11  is a perspective view of a further alternative embodiment of the pad member;  
       FIG. 12  is a further alternative embodiment of the present invention;  
       FIG. 13  is an exploded side perspective view of the embodiment shown in  FIG. 12 ;  
       FIG. 14  shows an alternative embodiment of the housing members of the present invention;  
       FIG. 15  shows a further alternative embodiment of the housing members of the present invention;  
       FIG. 16  is an exploded view of a further embodiment of the present invention demonstrating a bayonet type locking of a disc member to a housing member;  
       FIG. 17  is a perspective view of the disc member utilizing the bayonet locking mechanism to lock the disc member within a housing member;  
       FIG. 18  is an exploded view of a disc member and housing member showing a further embodiment of a locking mechanism for locking the disc member within the housing member;  
       FIG. 19  is a perspective view showing the disc member locked within the housing member;  
       FIG. 20  is a perspective view of the a further embodiment of the housing member;  
       FIG. 21  is a cross sectional view taken along line  21 - 21  in  FIG. 20 ;  
       FIG. 22  is a perspective view of a load sharing pad member including flanges for locking engagement in the recesses of the housing member shown in  FIGS. 20 and 21 ;  
       FIG. 23  shows a further embodiment of a locking mechanism made in accordance with the present invention;  
       FIG. 24  is a top view of the mobile bearing of the present invention;  
       FIG. 25  is a top view of the artificial disc including a mobile bearing with no load sharing pads;  
       FIG. 26  is a top view of the multidirectional mobile bearing of the present invention;  
       FIGS. 27A  and B are side views of the mobile bearing of the present invention;  
       FIG. 28  is a side perspective view of the mobile bearing of the present invention resting in a seat;  
       FIG. 29  is a top perspective view of the seat and bearing combination in a housing having recesses for load sharing pads;  
       FIG. 30  is a side perspective view of a third embodiment of the present invention;  
       FIG. 31  is a perspective view of the base plate of a third embodiment of the present invention;  
       FIG. 32  is a side view of a third embodiment of the lower housing of the present invention;  
       FIG. 33  is a perspective view of the third embodiment of the present invention wherein a spherical surface is incorporated on the bearing;  
       FIG. 34  is a perspective view of the third embodiment of the present invention wherein a spherical surface is incorporated on the bearing;  
       FIG. 35  is a side view of the third embodiment of the present invention;  
       FIG. 36  is a side view of the third embodiment of the present invention;  
       FIG. 37  is a side perspective view of an alternative embodiment of the present invention;  
       FIG. 38  is a perspective view of the base plate of the third embodiment of the present invention wherein the bearing is either convex or concave;  
       FIG. 39  is a perspective view of the base plate of the third embodiment of the present invention wherein the bearing is either convex or concave;  
       FIG. 40  is a top perspective view of the bumpers of the present invention;  
       FIG. 41  is a perspective view of an embodiment of the housing members of the present invention, wherein the housing members include apertures for bone screws and a positioning ring;  
       FIG. 42  is a perspective view of an embodiment of the housing members of the present invention, wherein a recess is shown for accommodating the positioning ring and bearing discs;  
       FIG. 43  is a perspective view of an embodiment of the housing member that is oval-shaped;  
       FIG. 44  is a perspective view of an oval-shaped positioning ring;  
       FIG. 45  is a perspective view of the oval-shaped positioning ring, bearing disc, and housing member;  
       FIG. 46  is a side view of an upper housing member including a fixed bearing disc;  
       FIG. 47  is a cut away view of the disc of the present invention showing engagement of the bearing surfaces and engagement of the oval positioning ring, wherein the bearing disc is oval shaped and the recess on the housing member is oval-shaped;  
       FIG. 48  is a perspective view of the disc assembly of the present invention;  
       FIG. 49  illustrates the insertion of a trial into the disc space;  
       FIG. 50  illustrates a drill guide for use in drilling pilot holes at a guide plate locations;  
       FIG. 51  illustrates securing the guide plate with self-tapping guide plate screws;  
       FIG. 52  illustrates inserting reaming discs matching the trial number into the disc assembly;  
       FIG. 53  illustrates engagement of the trial with the disc assembly;  
       FIG. 54  illustrates removal of guide plate screws and guide plate;  
       FIG. 55  illustrates insertion of disc holder with holes in plate;  
       FIG. 56  illustrates insertion of screws into threaded holes to secure disc to the vertebral bodies;  
       FIG. 57  illustrates attached disc assembly;  
       FIG. 58  illustrates grooves formed on the vertebral body. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An artificial intervertebral disc constructed in accordance with the present invention is generally shown at  10  in the Figures. Similar structures of various embodiments are indicated by primed numerals in the Figures. The invention is an artificial intervertebral disc, sometimes referred to by other terminology in the prior art such as intervertebral spacer device, or spinal disc for replacement of a damaged disc in the spine. The invention restores motion to the damaged natural disc that allows for motion as well as cushioning and dampening. As described below in more detail, the present invention also allows changes to the artificial disc motion intraoperatively to adjust for specific anatomical conditions.  
      Referring to the Figures, the disc  10  includes an upper housing member generally shown at  12  and a lower housing member generally shown at  14 . The housing members  12 ,  14  include spaced inner surfaces  16  and  18  facing each other and oppositely facing outer surfaces  20 ,  22  for engaging spaced apart vertebral surfaces. A pair of bearing surfaces  24 ,  26  extend from each of the inner surfaces  16 ,  18  for engaging each other while allowing for low friction and compression resistant movement of the housing members  12 ,  14  relative to each other while under compression. As shown in the various Figures, the bearing surfaces are integral with disc members  28 ,  30 . The housing members  12 ,  14  can be made from various materials known to those of skill in the art. The materials include, but are not limited to, steel, titanium, surgical alloys, stainless steel, chrome-molybdenum alloy, cobalt chromium alloy, zirconium oxide ceramic, non-absorbable polymers and other anticipated biocompatible metallic or polymeric materials such as polyethylene, polyamide, polypropylene, polyester, polycarbonate, polysulfone, polymethylmethylacrylate, or alternatively fibrous hydrogel, glass, or plastics. Additionally, the housing members  12 ,  14  can include ceramic fibers for reinforcement. Additionally, the housing members  12 ,  14  can be coated with materials to reduce friction between the components of the disc  10 , specifically between the housing members  12 ,  14  and bearing disc members  28 ,  30 . Coating materials include, but are not limited to, TiN (Titanium Nitride), diamond, diamond-like materials, synthetic carbon-based materials, chromium-based materials, and any other similar coating materials known to those of skill in the art. If integral with the bearing surfaces  24 ,  26 , the housing members  12 ,  14  can be made from the preferred material for the bearing discs  28 ,  30  as discussed above. Based on this teaching, various other configurations can be made by those skilled in the art incorporating the present invention. The bearing surfaces  24 ,  26  preferably form a mobile bearing  23  that is capable of automatically adjusting the position of the bearing  23  within a housing  14  as needed. The mobile bearing  23  is shown in  FIGS. 24 through 29 . The bearing  23  is preferably made of any material that slides along the surface of the housing  14  in which it is placed, with minimal to no wear, on either the bearing  23  or the housing  14 . Examples of such materials include ceramic, metal, or other suitable materials that do not negatively react with the housing  14 .  
      The bearing  23  of the present invention is disposed within a slot  35  of a housing  14 . The bearing  23  is able to freely move or float within the slot  35  in response to movement of the housing  14 . The bearing  23  is designed to provide proper cushioning and support of the housing  14  as is required by the specific system in which the bearing  23  is placed. The bearing  23  can be used in any joint for providing proper support of the joint. For example, if the bearing  23  is used in an artificial intervertebral disc assembly, the bearing  23  provides cushioning so as to prevent the plates that are housing the disc from touching and wearing on one another. When the bearing  23  is utilized within the knee, the bearing also provides cushioning for the housing  14  during movement of the housing  14 .  
      The bearing  23  disclosed herein can move freely under load conditions while maximizing the contact area of the upper and lower bearing surfaces  20 ,  24 . In other words, within the slot  35  that the bearing  23  is disposed, the bearing  23  can move in any direction necessary to provide the proper support for the housing  14 . The bearing  23  is able to move in this manner because the bearing  23  is a floating bearing, thus it is not attached or affixed to the housing  14  in which it is placed. Instead the bearing  23  “floats” within the housing  14 , thus enabling the bearing  23  to be mobile and free to move in any direction necessary to provide proper support.  
      The housing  14  limits the “floating” motion of the bearing  23 . In other words the movement of the bearing  23  can be limited based upon the size of the housing  14  and more specifically the slot  35  in which the bearing  23  is disposed. The slot  35  in which the bearing  23  is disposed dictates the range of movement of the bearing  23 , i.e. movement can be constrained such that the bearing  23  can only move from an anterior to a posterior position. More specifically, the slot includes side walls  37 , which define the size and shape of the slot  35 , and a seat  39  on which the bearing is disposed. The movement of the bearing  23  is restricted based upon the shape of the walls  35  of the slot  35  in which the bearing  23  sits. For example, the slot  35  can be in the shape of a circle, an oval, or any other round-sided shape. The slot  35  must be shaped to have rounded sides so as to prevent the bearing  23  from lodging in a corner of the slot  35 . The slot  35  can be formed such that the seat  39  does not have a uniform depth, such that there are peaks or angles within the slot  35 , as shown in  FIG. 27 . The lack of uniformity restricts movement of the bearing  23  within the slot  35  because the bearing  23  would require additional force in order to slide in the direction of the peak or angle.  
      A removable insert  33 , as shown in  FIGS. 28 and 29 , can also be disposed within the housing  14  for holding the bearing  23  in place. The insert  33  includes an upper surface  29  for engaging the bearing surfaces  24 ,  26 . The insert  33 , can be made of any material that enables the bearing  23  to functionally “float” across the insert  33  without excessive friction. The benefit of including the insert  33  in a housing  14  is that the insert  33  can be made of a different material than that of the housing  14 . Accordingly, the housing  14  can be made from a first composition that is advantageous for the functionality of the housing and provides other strength characteristics while the insert  33  can be made from a more lubricious material to allow for more efficient friction-free movement of the bearing  23  thereon.  
      The movement of the bearing  23  is restricted based upon the shape of the insert  33  into which the bearing  23  is placed. The insert  33  includes side walls  41 , which define the size and shape of the insert  33 , and an insert seat  29  on which the bearing is disposed. The movement of the bearing  23  is restricted based upon the shape of the walls  41  of the insert  33  in which the bearing  23  sits. For example, the insert  33  can be in the shape of a circle, an oval, or any other round-sided shape. The insert  33  must be shaped to have rounded sides so as to prevent the bearing  23  from lodging in a corner of the insert  33 . The insert  33  can be formed such that the insert seat  29  does not have a uniform depth, such that there are peaks or angles within the insert  33 , as shown in  FIG. 27 . The lack of uniformity restricts movement of the bearing  23  within the insert  33  because the bearing  23  would require additional force in order to slide in the direction of the peak or angle.  
      The housing  14  can also include load distributing dampening and cushioning pad recesses  32 ,  58 . Load sharing pads  32 ,  34  generally shown at  31  and specifically indicated as pads  32  and  34  in  FIGS. 1 and 2  are disposed between the inner surfaces  16 ,  18  and about at least a portion of the bearing surfaces  24 ,  26  for sharing absorption of compressive loads with the bearing surfaces  24 ,  26  while limiting relative movement of the housing members  12 ,  14 . More specifically, under in vivo loading conditions, the centralized bearing surfaces  24 ,  26  and the floating bearing surfaces not only provide for three-dimensional movement relatively between the housing members  12 ,  14 , but also share with the load sharing pads  32 ,  34  the function of distributing compressive loads on the device  10  to provide a system for motion and effective load distribution. The centralized low friction and compression resistant bearing surfaces  24 ,  26  allow full motion in multiple planes of the spine while the load distributing damper and cushioning pads  32 ,  34  simultaneously share the load. Critical is the function of the pads  32 ,  34  sharing the load with the bearing surfaces  24 ,  26 . Although the pads  32 ,  34  can be compressible, the compression is limited by the noncompressibility of the bearing surfaces  24 ,  26 . Likewise, although the bearing surfaces allow for motion in multiple planes, the pads  32 ,  34  are fixedly secured to the housing members  12 ,  14 , thereby allowing for a degree of flexibility and therefore movement of the housing members  12 ,  14  relative to each other, yet limiting such movement. In total, each element, the bearing surfaces  24 ,  26 , and pads  32 ,  34 , allow for movement, yet limit such movement, whether it is the sliding movement of the bearing surfaces  24 ,  26  or the cushioning movement allowed by the pads  32 ,  34 . Each element allows for relative movement, yet each element limits the movement of the other element of the system.  
      The disc  10  of the present invention is preferably formed as at least two separate units. The two units are placed at the same location in the spine and are able to work in conjunction with one another, such that the two units are able to function as a single unit. In other words, the two units can be placed on separate sides of the spinal column but create a single point of rotation and thus function in a manner equivalent to a single unit. The single point of rotation, or center of rotation, enables both posterior and anterior translation to occur. The single center of rotation can be altered based upon the placement of the two units relative to one another in the disc space, thereby enabling the surgeon to control the alignment of the units. The units are angled relative to one another within the disc space at angles determined by the surgeon to be appropriate for creating proper spinal support. The angle at which the units are inserted is dependent upon the number of units inserted, such that the angle is sized to create the proper center of rotation. Alternatively, the two units can be placed within the disc space parallel to one another, while maintaining a center of rotation between the two units. The two units can be in contact with one another or can have at least one portion of each unit that engages at least one portion of the other unit, within the intervertebral space.  
      The two units are smaller in size than the single unit and thus need a smaller incision and enable the units to be inserted in a posterior approach. The two units are sized such that each singular unit is large enough to create sufficient bone contact, thereby avoiding implant subsidence, but small enough to be inserted past obstacles and to fit within the disc space. Each unit includes the elements disclosed herein.  
      In view of the above, the system allows restoration of normal motion while maintaining load cushioning capabilities of a healthy disc. This is particularly apparent with motion of the spine. Any rotation of the upper and lower housing members  12 ,  14  causes the load distributing dampening and cushioning pads  32 ,  34  to absorb some of the load.  
      As shown in the various Figures, the bearing surfaces  24 ,  26  can include a concave surface portion on one of the upper or lower disc members  28 ,  30 , and a convex surface portion on the other. The bearing surfaces  24 ,  26  can each have an identical radius that can be adjusted to allow for optimal contact. Additionally, the bearing surfaces  24 ,  26  can be spherical in shape. The concave surface is seated within the convex surface for sliding movement relative thereto effectively resulting in relative pivoting motion of the housing members  12 ,  14 , which compresses at least a portion of the load sharing pads  32 ,  34  while extending at least a portion of the oppositely disposed load bearing pad  32 ,  34 . Alternatively, either one of the top and bottom disc members  28 ,  30  can have either of the convex or concave surfaces. The bearing surfaces  24 ,  26  can also include a fluid bearing layer  27  insertable between the bearing surfaces  24 ,  26 . The fluid bearing layer  27  enables optimal contact between the bearing surfaces  24 ,  26 . When multiple units are inserted, the creation of a single center of rotation is beneficial because force exerted from the upper bearing surface to the lower bearing surface is directed at an angle, named the force vector, either towards the center for a convex surface or away from the center for a concave surface. By directing the force vector the units can be locked into proper position. This prevents undesirable movement of the units relative either to one another or to other implanted body plates.  
      The disc members  28 ,  30  can be made from a composition that is noncompressible. Such compositions can be selected from the group including ceramics, plastics, and metal bearing materials, such as cobalt and chrome. Alternatively, the housing members  12 , 14  can include projections wherein the disc members  28 ,  30  are effectively integral with the housing members  12 ,  14 . In this situation, the entire housing, including the projections having the bearing surfaces  24 ,  26  thereon, can be made from the noncompressible material, preferably a ceramic. As stated above, alternative configurations can be made by those skilled in the art once understanding the present invention.  
      The load sharing pads  32 ,  34  can be in various configurations shown in the Figures, such as paired pads  32 ,  34  shown in  FIGS. 1-3 . Alternatively, the device  10  can include four oppositely disposed pads  38 ,  40 ,  42 ,  44  as shown in  FIG. 10 . A further embodiment of the invention is shown in  FIG. 11 , wherein a single pad  46  substantially covers the surface  18 ′″″ of the housing member  14 ′″″. The pads can contour to the shape of the housing members such as shown in  FIGS. 12, 13 , wherein the pad member  48  is an annular pad member disposed with a annular housing  12 ″″″,  14 ″″″. The selection of such housing members  12 ,  14  and pad members  31  can be determined based on the location of the placement of the device  10  as well as the spacing conditions between the vertebrae and load bearing necessities depending on the level of the spine being addressed. In other words, different shaped devices, such as the round shaped housing members shown in  FIG. 12  can be used for placement between smaller discs, such as cervical spines whereas more rectangular shapes, such as the housing members shown in  FIGS. 1-11  can be used in between lumbar vertebrae.  
      The load sharing pads  31 , in which ever shape they are configured, are elastic for allowing relative twisting movement between the housing members  12 ,  14  effecting relative three-dimensional movement between the housing members  12 ,  14 , while limiting the movement and preventing contact between the housing members  12 ,  14  except for the contact between the bearing surfaces  24 ,  26 . By elastic, it is meant that the pad members  31  are compressible and stretchable, yet provide a self-centering effect on the assembly with specific regard to the housing members  12 ,  14 , as well as the bearing surfaces  24 ,  26 . Deflection or rotation of the forces created due to relative movement of the bearing surfaces  24 ,  26 , and likewise the housing members  12 ,  14 , forces the pads  31  to act in such a way to counter the force, thus allowing a unique self-centering capability to the assembly  10 . While in an ideal situation, wherein the patient&#39;s facets are uncompromised and ligamental balances are intact, this self-centering aspect may not be completely necessary. In other words, the patient&#39;s anatomy may still provide stabilization and specifically, ligaments may provide self-centering. However, ligamental imbalance, and damaged facets would normally make an artificial disc questionable, at best, with use of the current technology that is available. In such cases, having the ability to self-center and restrict motion (the pads  31  of the present invention are elastic and thus restrict motion by stretching and returning to rest), the possibility of extending indications to patients currently considered outside of the scope of artificial disc technology will be highly advantageous.  
      The pads  31  of the present invention provide further advantages to the invention. A key advantage is the ability to adjust the pads  31  to patient and surgeon requirements. In such cases wherein range of motion needs to be restricted due to compromised facets, a harder, less elastic pad can be inserted between the housing members  12 ,  14 . Since this less elastic pad would move and stretch less, the disc would be automatically restricted in motion. This method of adjusting pads can be done intraoperatively to compensate for surgical and patient conditions. To one skilled in the art, one can fine-tune the assembly  10  to a patient and surgeon&#39;s needs with multiple pads of different properties or materials.  
      The pads  31  are made from a polymer or elastomer that allows deflection under load. Examples of such polymers and elastomers are silicone, polyurethane, and urethane composites. As discussed above with regard to flexibility or elasticity, the content and composition of the pads  31  are adjustable. A highly dense material creates a very rigid disc, while a very soft material creates a very free moving disc. The motion would be restricted in all planes of the pad depending upon these factors. Rotation is also restricted, as well as flexion or movement of the disc. The amount of compression possible is restricted or allowed according to the pads material properties. This is true of motion towards the back or side-to-side motion. Thus, the pads  31  are always in contact and always share the load, under any adjustment of relative positioning of the housing members  12 ,  14 . Since motion forces the pads to be in contact, the pads  31  automatically damper loads imposed by the artificial disc construct  10 .  
      With specific regard to the flexibility or elasticity of the polymer or elastomer composition of the pads  31 , the pads can be selected from a composition having a durometer from 20 to 98 on the Shore OO Scale. Alternatively, the pads  31  can be selected from a composition having a durometer from 10 to 100 on the Shore A Scale. A further alternative is for the pads  31  to be selected from a composition having a durometer from 22 to 75 on the Shore D Scale. In any event, the pad members  31  can be selected during the operation and procedure by the clinician to suit a specific situation. Although the pad members  31  can be pre-inserted between the housing members  12 ,  14  prior to insertion of the device  10  in situ, the various configurations of the present invention can allow for in situ replacement of the pad members  31  so as to custom select the flexibility or elasticity of the members. In this manner, the pad members  31  are custom designed for the individual environment of the intervertebral space into which the device is being disposed.  
      The disc members  28  and  30 , and pads  31  can be contained or locked in position in between the housing members  12 ,  14  by various means. The disc  28 ,  30  can be locked to the housing members  12 ,  14  by a press fit taper, retaining ring, or other means. The key aspect of such locking mechanisms is to prevent the disc members  28 ,  30  from moving against the upper or lower housing members  12 ,  14  once installed in order to prevent additional wear.  
       FIGS. 1 and 2  show disc members  28 ,  30  disposed in recesses (only the lower recess  50  is shown in  FIG. 2  in an exploded view) in each of the inner surfaces  16 ,  18  of the housing members  12 ,  14 .  FIGS. 6 and 7  show plan views of a second embodiment of the housing member  12 ′,  14 ′, wherein each recess  50 ′,  52  includes a ramped surface  54 ,  56  leading from an outer edge to the inwardly tapered recess portion  50 ′,  52 . The ramping  54 ,  56  allows access of the disc members  28 , 30  in between the housing members  12 ′,  14 ′ after placement of the housing members  12 ′,  14 ′ in the intervertebral space. This intraoperative access of the disc members  28 ,  30  allows the surgeon to test different size disc members under load conditions to perfectly fit the disc members in place. Such an advantage is not obtainable with any prior art device.  
      An alternative mechanical mechanism for locking the disc members within the housing members is shown in  FIG. 16 . The representative housing member  12 ′″ includes recess  52 ′. The recess  52 ′ includes a substantially arcuate peripheral undergroove  70 . The groove is defined by a lip portion  72  including at least one and preferably at least two openings  74 ,  76 . The disc member  28 ′″ includes bayonet style flanges  78 ,  80  extended radially outwardly therefrom, the flanges  78 ,  80  being shaped so as to be received through recess  74 ,  76 . In operation the disc member  28 ′″ can be disposed within the recess  52 ′ such that the flanges  78 ,  80  align with recesses  74 ,  76 . Once the disc member  28 ′″ can be rotated thereby providing a bayonet style locking mechanism of the disc member  28 ′″ within the housing  12 ′″, as shown in  FIG. 17 .  
      A further alternative embodiment of the locking mechanism is shown in  FIGS. 18 and 19 . The housing member  12 ′″ includes a substantially arcuate recess  52 ″ having an open end portion  82  extending to an edge  84  of the housing member  12 ′″. The recess  52 ″ includes a lip portion  86  extending about a substantial portion thereof defining an inner groove  88  between the seating surface  90  of the recess  52 ″ and the lip portion  86 . Arm portions  92 ,  94  are extensions of the lip portion  86  but extend from and are separate from peripheral ends  96 ,  98  of the housing member  12 ′″. The arm portions  92 ,  94  have a spring-like quality such that they can be deflected outwardly from the arcuate circle defined by the recess  52 ″. Each of the arms  92 ,  94  has an elbow portion  100 ,  102  extending from each arm portion  92 ,  94  towards the seating surface  90 , respectively. The disc member  28 ′″ includes a substantially arcuate peripheral, radially outwardly extending flange portion  104 . The flange portion  104  includes two abutment edges  106 ,  108 . In operation, the flange  104  and disc member  28 ′″ are disposed within the annular recess or groove  88 , deflecting outwardly the arms  92 ,  94 . Once disposed in the recess  52 ″, as shown in  FIG. 19 , the elbows  100 ,  102  engage the abutment surfaces  106 ,  108  of the disc member  28 ′″ thereby locking the disc member  28 ′″ in place. Outward deflection of the arms  92 ,  94  can selectively release the disc member  28 ′″ from locked engagement to provide for further adjustment of the selection of the disc member during an operation procedure.  
      Also, as best shown in  FIGS. 6 and 7 , the pads members  31  can be disposed in recesses  58 ,  60  in the lower and upper housing members  12 ′,  14 ′ respectively. It is preferable to permanently adhere the pad members  31  to the housing members  12 ′,  14 ′ by use of mechanical mechanisms and/or various adhesives, such as cyanoarylates, urethanes, and other medical grade adhesives. This list of adhesives, as with other listings of ingredients in the present application, is merely exemplary and not meant to be exhaustive.  
      Examples of mechanical mechanisms for locking the pad members  31  into recesses in the housing members are shown in  FIGS. 20-23 . One such mechanism is an undercut locking mechanism shown in  FIGS. 20-22 . Housing member  12 ″″ includes a central recess  52  such as shown in  FIG. 6  having a ramp portion  56 . The ramp portion  56  includes a centrally located tongue groove  57  allowing for the insertion of a spatula type device under a disc member disposed within the recess  52  for releasing the disc member from the recess, similar to the use of a shoehorn type mechanism. Recesses  60 ′ include undercut recesses  110 ,  112  for locking engagement with a peripheral flange portion  114  extending from an edge  116  of a pad member  31 ′. Since the pad member is made from a deflectable material, the flange portion  114  can be force-fit into and seated within the undercut  110 ,  112 . The undercut locking mechanism effectively prevents the pad member  31 ′ from disengagement with the housing member  12 ″″ in situ. Of course, the upper flange  118  would be locked within a similar undercut locking detail of recesses within the opposing housing member (not shown).  
      An alternative locking mechanism between the pad member and housing member can be a tongue-and-groove relationship as shown in  FIG. 23 . Either the pad or the housing can include the tongue portion  122  and the other pad and housing members can include the groove  124 . In other words, either of the locking members can include the tongue  122  and the other of the members being locked would include the groove  124 . An alternative of this or the other locking mechanism shown is that the recess and/or pad can include multiple grooves or slots as well as multiple tongues.  
      The various recesses or pockets  50 ′,  52 ,  58 ,  60  can be of different relative sizes and shapes. For example, the upper housing member  12 ′ may have a larger recess or pocket for seating a relatively larger one of said discs  28  and the lower housing member  14 ′ may be include a smaller (larger and smaller referring to diameter of the annular recess) of the recesses or pockets for seating a relatively smaller one of the lower disc  30 , thereby providing for an increased range of motion at the bearing surface interface.  
      The various Figures show that the outer surfaces  20 ,  22  of the various embodiments of the housing members  12 ,  14  can include flanges generally indicated at  60 . The flanges  60  or fins, as they are sometimes referred to in the art, provide a mechanism for fixation to the intervertebral surfaces. Various embodiments, such as those shown in  FIGS. 1 and 2  are dual fin constructs. Other embodiments, such as those shown in  FIGS. 8, 12 , and  13  are single fin or single flange constructs. Depending upon the nature of the surfaces to which the outer surfaces  20 ,  22  are to abut, the surgeon can select various flange or fin configurations. Additionally, the fins  60  can be located in alternative positions, either centrally as shown in many of the Figures, or peripherally, as shown in  FIG. 14 , for a specific use with anterior extension plates, as with screw fixations. The flanges, such as flange  60 ′″″″ can include a bore  62  therethrough, which can be either a smooth surface or threaded depending on its intended use.  
      Preferably, the location at which the housing members  12 ,  14  are inserted in the intervertebral space has an alignment groove  63 . The function of the groove  63  is to provide proper alignment of the housing  12 ,  14  within the disc space. The number of grooves  63  created at the site of insertion corresponds to the number of flanges  60  present on the housing members  12 ,  14 . Further, the grooves  63  are aligned such that when the units are inserted in the grooves  63 , the units function as a single device.  
      The outer surfaces  20 ,  22  can be smooth, which allows for easier revision as it allows for minimal to no ingrowth or they can be textured. Texturing of the outer surfaces  20 ,  22  allows ingrowth for long-term fixation of the assembly  10 . Porous coatings, plasma spray, grit blasting, machining, chemical etching, or milling are examples of techniques for creating ingrowth capable surfaces. Additionally, surface roughening can be accomplished by way of, for example, acid etching, knurling, application of a bead coating, or other methods of roughening known to one of ordinary skill in the art. Coatings that enhance bone growth can also be applied. Examples of such coatings are hyroxyapatite and bone morphogenic proteins. Preferably, the hydroxyapatite coating is formed of calcium phosphate.  
       FIGS. 20 and 21  provide structure for further rotational stability of the device in situ. The housing member  12 ″″ includes pointed portions  126 ,  128  extending from the outer surface  20 ′ thereof. The point members  126 ,  128  function in conjunction with the flange portion  61 ′ to engage an opposing vertebral surface. The point portions  126 ,  128  being disposed radially peripherally from the centrally disposed flange  61 ′ provide at least a three-point engagement of the vertebral surface thereby preventing rotation of the housing member  12 ″″ relative thereto. Of course, the point portions  126 ,  128  can be in made in various configurations and extend various amounts from the outer surface  20 ′ to be custom suited to a specific vertebrae surface shape.  
      Alternatively and preferably, as shown in  FIGS. 30-40 , the disc  10 ″″″″ can be formed as at least two separate pieces that are inserted into an intervertebral space, generally shown as  146  in  FIG. 30 . The benefit of this formation of the disc  10 ″″″″ is that the discs  10 ″″″″ can be inserted during a posterior insertion. The two discs  10 ″″″″ function so that the units work in tandem and effectively become one artificial disc assembly. The arrangement of the two discs  10 ″″″″ enables each disc  10 ″″″″ to be inserted on either side of the spinal column into the intervertebral space  146  and work in conjunction as a single artificial disc assembly  10 ″″″″. The two discs  10 ″″″″ are angled toward the mid-line of the vertebral body  146 . While two disc assemblies  10 ″″″″ are described herein, more than two discs  10 ″″″″ can also be utilized without departing from the spirit of the present invention.  
      Each of the discs  10 ″″″″ include an upper housing member  12 ″″″″ and a lower housing member  14 ″″″″. The housing members  12 ″″″″,  14 ″″″″ each include a slot  35 ′ within the housing member  12 ″″″″,  14 ″″″″. The slot  35 ′ enables the bearing  23  to move freely or “float″within the slot  35 ′ in response to movement of the housing  14 . As shown in FIGS.  31 ,  33 - 34 , and  38 - 39 , the slot  35 ′ can be formed in any shape that enables proper movement of the bearing  23 , however, preferably the slot  35 ′ is an open-ended u-shaped slot with a seat  39 ′ and side walls  37 ′. The side walls  37 ′ maintain the bearing  23  in proper alignment within the housing  12 ″″″″,  14 ″″″″. As disclosed above, the bearing  23  is capable of floating within the slot  35 ′, thus enabling the bearing  23  to be mobile and free to move in any direction necessary to provide proper support for the housing  12 ″″″″,  14 ″″″″. The housing  12 ″″″″,  14 ″″″″ limits the motion of the bearing  23 . The size of the housing  12 ″″″″,  14 ″″″″ and, more specifically, the slot  35 ′ in which the bearing  23  is disposed limits the motion of the bearing  23 . Further, bumpers  130 ,  132  can also be included in the slot  35 ′ to further limit the motion of the bearing  23 , provide dampening of the motion of the bearing  23  and prevent the bearing from being displaced from the housing  12 ″″″″,  14 ″″″″. The bumpers  130 ,  132  can be of any size sufficient to provide the necessary limitations on the bearing  23 . For example, a single bumper can be used for both housings  12 ″″″″,  14 ″″″″. Alternatively, each housing  12 ″″″″,  14 ″″″″ can incorporate separate bumpers  130 , 132 . The bumpers  130 , 132  are also useful for load sharing and thereby preventing the housing members  12 ″″″″,  14 ″″″″ from contacting one another. The bumpers of the present invention  130 ,  132  are shaped to conform to the shape of the slot  35 ′. In other words, the bumpers  130 ,  132  are shaped to precisely fit the slot  35 ′in which the bumpers  103 ,  132  are displaced. Preferably, the bumpers  130 ,  132  do not extend beyond the length of the housing  12 ″″″″,  14 ″″″″. The bumpers  130 ,  132  have walls  134 ,  136  respectively that engage the wall  37 ′ of the slot  35 ′. This enables the bumpers  130 ,  132  to be maintained in alignment and prevents the bumpers  130 ,  132  from moving.  
      The upper housing  12 ″″″″ can either include a slot  35 ′ identical to that of the lower housing  14 ″″″″ or can include a single piece having a matching bearing that complements that of the bearing  23 . In other words, the upper housing  12 ″″″″ can either have a slot  35 ′ that is identical to the shape of the slot  35 ′ of the lower housing  14 ″″″″, such that the bearing  23  moves both in both housings  12 ″″″″,  14 ″″″″ equally or the upper housing  12 ″″″″ can be formed such that only a single piece is utilized and there is no movement within the top plate of the bearing  23 .  
      The bearing  23 ′ includes side arms  138 ,  140  that slidably engaged the wall  37 ′ of the slot  35 ′. The bearing  23 ′ is therefore held in position within the slot  35 ′ via the side arms  138 ,  140  and the bumpers  130 ,  132 .  
      The bearing  23  of the present invention can also have incorporated on the bearing surface  24  various shapes as shown in the figures. Specifically,  FIG. 32  shows the bearing surface  24 ′, wherein the surface  24 ′ is a spherical surface. The spherical surface  24 ′ enables the center of rotation of the bearing  23 ′ to exist at the center of the sphere. Therefore, the pair of discs  10 ″″″″ functions as a single artificial disc with one center of rotation. Alternatively, the bearing  23  can have a surface that is either convex  24 ″ or concave  24 ′″. This embodiment is specifically shown in  FIGS. 9 and 10  wherein the center portion of the bearing  23 ′ is either convex or concave and there is a flat portion  29  of the bearing  23 ′. When a convex or concave surface  24 ″,  24 ′″ respectively, is utilized, the rotation center is not in the center for side-to-side rotation. Thus, the assembly is somewhat resistant to side-to-side bending but is more easily aligned.  
      The housings  12 ″″″″,  14 ″″″″ can be inserted simultaneously without incorporating the floating bearing  23  initially. This enables the disc  10 ″″″″ to be inserted into the intervertebral space and once the disc  10 ″″″″ has been inserted, the bumpers  130 ,  132  and the bearing  23  can be slid into place within the slot  35 ′. In another embodiment of the present invention, the lower housing member  12 ′″″″″ and the upper housing member  14 ′″″″″ include a recess  52 ′″ for seating a positioning ring  15 , or spring mechanism  15 , and bearing discs  28 ″″,  30 ″″ therein (See,  FIGS. 41 and 42 ). Preferably, the recess  52 ′″ includes a substantially arcuate peripheral undergroove  70 ″ or wall  70 ″ and a bottom surface  19  that can be super finished smooth. The recess  52 ′″ accommodates the positioning ring  15  therein and the undergroove  70 ″ secures the positioning ring  15 . The undergroove  70 ″ is defined by a lip portion  72 ″. The housings  12 ′″″″″,  14 ′″″″″ include at least one aperture  17  for insertion of screws therein and to secure the housings  12 ′″″″″,  14 ′″″″″ to a vertebral body. The positioning ring  15  can be fixedly or removably attached to the housings  12 ′″″″″,  14 ′″″″″. Similarly, the bearing discs  28 ″″,  30 ″″ can be fixedly or removably attached to the housings  12 ′″″″″,  14 ′″″″″.  
      The positioning ring  15 , or spring member  15 , is elastomeric and can be made any material including, but not limited to, rubber, silicone, polyurethane, urethane composites, plastics, polymers, elastomers, and any other similar elastomeric material known to those of skill in the art. The positioning ring  15  is illustrated in detail in  FIGS. 41-46 . Preferably, the positioning ring  15  or spring member  15  is a substantially annular body including an axially extended bore therethrough defining a passageway. Although the positioning ring is circular in shape, any similar or appropriate design can be used such as an oval shape. Additionally, the substantially annular body has a seat extending radially inward towards the bore for seating therein the bearing discs  28 ,  30  and has an engaging member extending radially outward from the bore for engaging the recess  52  of the housing member  12 ,  14  and securing the positioning ring within the recess  52 . Preferably, the engaging member can be any portion of the substantially annular body that radially extends from the bore. The engaging member includes, but is not limited to, a tapered edge, flange, and the like. The engaging member is shaped so as to be received by the recess and the recess securely engages the engaging member resulting in securing the positioning ring within the recess.  
      The purpose of the positioning ring  15  or spring member  15  is to absorb compressive loads between the bearing discs  28 ,  30  and the undergroove  70 ″ or wall” of the recess of the housing member, while controlling motion and position of the bearing discs  28 ,  30 . The positioning ring  15  cushions and provides bias to absorb compression and lateral forces, while acting as a spring to re-center the bearing discs  28 ,  30  after being displaced through vertebral function.  
      The bearing discs  28 ″″,  30 ″″ are situated within the opening of the positioning ring  15  or spring mechanism  15 . The bearing discs  28 ″″,  30 ″″ can move within the positioning ring  15  and thus the housings  12 ′″″″″,  14 ′″″″″therein. However, movement within the housings  12 ′″″″″,  14 ′″″″″ is semi-constrained by the positioning ring  15 . The positioning ring acts as a spring to self-center the bearing discs  28 ″″,  30 ″″ and as a shock absorption member. As the bearing discs  28 ″″,  30 ″″ are free to float, the positioning ring  15  acts as a damper and self-centering spring. Therefore, the bearing can translate in any direction, while the positioning ring exerts a force to push the bearing back to center. The further the bearing moves, the more force the positioning ring  15  exerts. Any vertebral or spinal motion allows for load sharing and damping of forces to the spine. As a load is transmitted, the bearing discs  28 ″″,  30 ″″ move and the force is shared by the positioning ring  15  or spring mechanism  15 .  
      In another embodiment of the present invention, the bearing discs  28 ′″″,  30 ′″″ along with the positioning ring  15 ′ are oval shaped. Additionally, the recess  52 ″″ located on each housing member  12 ″″″″″,  14 ″″″″″ is oval-shaped, while the housing members  12 ,  14  can also be oval shaped, circular, or any other suitable shape known to those of skill in the art. The recess  52 ″″ accommodates the positioning ring  15 ′ therein and an undergroove  70 ′″ secures the positioning ring  15 ′. The undergroove  70 ′″ is defined by a lip portion  72 ′″. As shown in  FIGS. 43-48 , the bearing discs  28 ′″″,  30 ′″″ can be fixed within the oval recess  52 ′″ or the bearing discs  28 ′″″,  30 ′″″ can be floating (i.e., mobile bearing discs) within the oval recess  52 ′″ of the housing members  12 ″″″″″,  14 ″″″″″. The bearing discs  28 ′″″,  30 ′″″ have oval circumferential exterior sides  21  and a spherical surface machined into the bearing surface  24 ,  26 .  FIG. 44  illustrates the approximate shape of the positioning ring  15 ′.  FIG. 45  shows the positioning ring  15 ′ in place within the recess  52 ″″ and illustrates the oval shape in greater detail.  FIG. 46  illustrates an upper housing member  14 ″″″″″, wherein the bearing disc  30 ′″″ is fixed onto the upper housing member  14 ″″″″″. The exterior circumference of the bearing discs is oval, with the bearing surface  24 ,  26  being spherical.  
      Under rotational loads, positioning ring  15 ′ engages the oval circumferential exterior sides  21  of the bearing discs  28 ′″″,  30 ′″″ and the undergroove  70 ′″ of the recess  52 ″″ of the housing members  12 ″″″″″,  14 ″″″″″. The greater the rotation, the more compressive force is exerted against the positioning ring  15 ′. Therefore, the disc  10  acts similar to a normal anatomic disc, whereby the annulus allows motion, but also provides constraint of excessive motion. With such a rotation, the positioning ring  15 ′ acts as a spring counteracting the rotational forces to allow rotation, while preventing excess rotation therefrom. The positioning ring  15 ′ can be changed in durometer to create more motion or less motion by altering the effective spring rate of the material. Thus, patient specific positioning rings  15 ′ can be chosen based on patient requirements. In cases where facet joints are deteriorated, the disc  10  can compensate by using a higher durometer positioning ring  15 ′ and allowing the surgeon full optimization at the time of surgery.  
      Under translation loads, the positioning ring  15 ′ acts as a spring to resist excessive motion, while acting as a spring to self-center the disc construct. As shown in Figures, the oval aspect allows the necessary engagement area to permit the combination of benefits. Also, by using such an oval surface, the positioning ring  15 ′ remains in compression at all times, allowing maximum benefit and performance from various polymers. To one skilled in the art, the oval recess  52 ″″ could be any elongated surface that effectively provides some moment arm to exert force on the positioning ring  15 ′.  
      Various methods can be utilized for insertion of the present invention in situ. For example, an assembled device  10  as shown in  FIG. 1 , can be disposed between the intervertebral spaces during surgery, after calculation of space, depth, and height. Alternatively, opposing housing members  12 ,  14  can be disposed between the intervertebral spaces and pads  31  and disc members  24 ,  26  can be tested in situ prior to fixation thereof to allow for custom sizing. Accordingly, the present invention broadly provides a method of assembling an artificial intervertebral disc  10  in vivo by inserting upper and lower housing members  12 ,  14  into an intervertebral space and disposing cushioning pads  31  between the inner surfaces  16 ,  18  of the housing members  12 ,  14 , thereby placing the pads in compression. The pair of disc members  28 ,  30  is inserted between the inner surfaces of the plates  16 ,  18 . The disc members  28 ,  30  have abutting low friction surfaces  24 ,  26  therebetween. The disc members  28 ,  30  are surrounded by the pads  31 , whereby the disc members  28 ,  30  and pads  31  are under compressive forces and share such compressive forces. This step of the bearing surfaces  24 ,  26  and shock absorbing pads  31  sharing absorption of the compressive forces and limiting the relative movement of the housing members  12 ,  14  is an advantage not found in the prior art.  
      One use of the bearing of the present invention is in an artificial intervertebral disc for replacement of a damaged disc in the spine. The artificial disc  10  of the present application includes a mobile bearing  23  that allows for the bearing  23  to move to adjust and compensate for vertebral disc motion. By permitting the bearing to self-adjust, the bearing  23  can more freely move under translation loading conditions while maximizing the contact area of the upper and lower bearing surfaces  20 ,  24 .  
      In applications such as the lumbar spine, the disc upper member and lower member are angled relative to each other to maintain spinal curvature. The load distributing damper and cushioning pads are always under some load when the spine is moving, although they can be adjusted for a neutral no load situation when the spine is not moving.  
      The load distributing damper and cushioning pads also create an elastic means of self-centering the disc construct. Deflection of rotation of the disc forces the pads to act in such a way as to counter the force, thus allowing a unique self-centering capability. In an ideal situation where the patient&#39;s facets are uncompromised and ligamental balance is intact, this is not necessary. However, ligamental balance and damaged facets would normally make an artificial disc questionable at best with the current art. In such cases, having the ability to self-center and restrict motion (the pads are elastic and thus restrict motion by stretching and returning to rest), the possibilities of extending indications to patients currently considered outside the scope of artificial disc technology is highly advantageous. In a floating bearing design, the ability to self-center mixed with the dampening abilities of the pads creates an ideal system for an artificial disc.  
      The pads can also be adjusted according to patient and surgeon requirements. In such cases where range of motion needs to be restricted due to compromised facets, a harder, less elastic pad can be inserted. Since a less elastic pad moves and stretches less, the disc is automatically restricted in motion. This method of adjusting pads can be done interoperatively to compensate for surgical and patient conditions.  
      As described above, any of the above embodiments can be used in a cervical disc surgical procedure. With regard to the embodiment of the housing members  12 ,  14  illustrated in  FIGS. 41 and 42 , the general procedure begins with the removal of the damaged disc ( FIGS. 49-57  illustrate the procedure). Then, a trial handle is attached to the trial and the trial is inserted into the disc space ( FIG. 49 ). The trial is adjusted until the disc height is approximately restored, while being careful not to overstretch the ligaments. Using a drill guide, pilot holes are drilled at the four guide plate hole locations ( FIG. 50 ). The guide plate is secured with self-tapping guide plate screws ( FIG. 51 ). Using the end plate preparation instrument, reaming disks are inserted to match the trial number. The depth of the instrument on the dial to the matching number must then be set. Once set, the instrument is advanced into the disc space with the button engaged ( FIG. 52 ). The fins on the instrument remain engaged in the slot on the guide plate for stability. Once maximum depth is reached, the end plate preparation instrument is removed ( FIG. 53 ). The guide plate screws and guide plate are then removed ( FIG. 54 ). The disc holder with holes in plate aligned with holes in the vertebrae is inserted until fully seated ( FIG. 55 ). Screws are then inserted into threaded holes to secure disc to the vertebral bodies ( FIG. 56 ). Finally, the disc inserter is removed ( FIG. 57 ).  
      Alternatively, once the disc inserter is properly inserted into the disc space and an appropriate space has been created for the insertion of a disc, the grooves  63  can be formed on the surface of the vertebral body. The grooves  63  can be formed of a depth sufficient to retain the flanges of the disc. The grooves  63  guide the disc, via the flanges, into proper alignment within the disc space. The grooves  63  are configured such that upon insertion of the discs into the grooves  63  the discs are properly arranged within the disc space so that the discs can function as a single unit. For example, the grooves  63  can be parallel to one another. Other configurations of the grooves  63  can also be formed provided that upon insertion of the discs, the discs are aligned to function as a single unit.  
      Throughout this application, various publications, including United States patents, are referenced by author and year and patents by number. Full citations for the publications are listed below. The disclosures of these publications and patents in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art to which this invention pertains.  
      The invention has been described in an illustrative manner, and it is to be understood that the terminology that has been used is intended to be in the nature of words of description rather than of limitation.  
      Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention can be practiced otherwise than as specifically described.