Patent Publication Number: US-2011054617-A1

Title: Intervertebral disc prosthesis having ball and ring structure

Description:
CROSS REFERENCE TO PRIOR APPLICATIONS 
     This application is filed under 35 U.S.C. §120 and §365(c) as a continuation of International Patent Application PCT/CA/2009/000233, filed Feb. 27, 2009, which application claims priority from U.S. Patent Application No. 61/067,545, filed Feb. 28, 2008, which applications are incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of spinal implants and, more particularly, to intervertebral disc prostheses, or artificial intervertebral discs. 
     BACKGROUND OF THE INVENTION 
     The spine is a complicated structure comprised of various anatomical components, which, while being extremely flexible, provides structure and stability for the body. The spine is made up of vertebrae, each having a ventral body of a generally cylindrical shape. Opposed surfaces of adjacent vertebral bodies are connected together and separated by intervertebral discs (or “discs”), comprised of a fibrocartilaginous material. The vertebral bodies are also connected to each other by a complex arrangement of ligaments acting together to limit excessive movement and to provide stability. A stable spine is important for preventing incapacitating pain, progressive deformity and neurological compromise. 
     The anatomy of the spine allows motion (translation and rotation in a positive and negative direction) to take place without much resistance but as the range of motion reaches the physiological limits, the resistance to motion gradually increases to bring the motion to a gradual and controlled stop. 
     Intervertebral discs are highly functional and complex structures. They contain a hydrophilic protein substance that is able to attract water thereby increasing its volume. The protein, also called the nucleus pulposis, is surrounded and contained by a ligamentous structure called the annulus fibrosis. The main function of the discs is load bearing (including load distribution and shock absorption) and motion. Through their weight bearing function, the discs transmit loads from one vertebral body to the next while providing a cushion between adjacent bodies. The discs allow movement to occur between adjacent vertebral bodies but within a limited range thereby giving the spine structure and stiffness. 
     Due to a number of factors such as age, injury, disease, etc., it is often found that intervertebral discs lose their dimensional stability and collapse, shrink, become displaced, or otherwise damaged. It is common for diseased or damaged discs to be replaced with prostheses and various versions of such prostheses, or implants, are known in the art. One of such implants comprises a spacer that is inserted into the space occupied by the disc. However, such spacers have been found to result in fusion of the adjacent vertebrae, thereby preventing relative movement there-between. This often leads to the compressive forces between the vertebrae in question to be translated to adjacent vertebrae, thereby resulting in further complications such as damage to neighboring discs and/or damage to facet joints and the like. 
     More recently, disc replacement implants that allow various degrees of movement between adjacent vertebrae have been proposed. Examples of some prior art implants are provided in the following: U.S. Pat. No. 5,562,738 (Boyd et al.), U.S. Pat. No. 6,179,874 (Cauthen), and U.S. Pat. No. 6,572,653 (Simonson). 
     Unfortunately, the disc replacement, or implant, solutions taught in the prior art are generally deficient in that they do not take into consideration the unique and physiological function of the spine. For example, many of the known artificial disc implants are unconstrained with respect to the normal physiological range of motion of the spine in the majority of motion planes. Although some of the prior art devices provide a restricted range of motion, such restrictions are often outside of the normal physiological range of motion; thereby rendering such devices functionally unconstrained. Further, the known unconstrained implants rely on the normal, and in many cases diseased structures such as degenerated facets, to limit excessive motion. This often leads to early or further facet joint degeneration and other collateral damage to spinal components. 
     In addition, many of the artificial discs known in the art, such as U.S. Pat. Nos. 5,562,738 (mentioned above) and 5,542,773, and United States Patent Application Nos. 2005/0149189 and 2005/0256581, generally comprise a ball and socket joint that is implanted between adjacent vertebral bodies. One of the issues associated with such devices is the difficulty in designing constraints to motion. Quite often, such constraints are provided by the soft tissue adjacent to the implant, thereby resulting in a limited degree of constraint and/or damage to such tissue structures. Where constraints are provided, typical ball and socket implants are not easily adapted to for providing various types and degrees of constraint as may be required depending on the need. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, the present invention provides an artificial disc or implant comprising a ball and ring combination, which generally combines the features of known ball and socket designs but which includes at least some degree of versatility in terms of the type and degree of constraint that can be built into the device. The implant of the invention also provides for variations in the type of motion and center of rotation. 
     In one aspect, the invention comprises an artificial disc having two main sections or components, each being adapted to be positioned against opposed vertebral body surfaces of adjacent vertebrae. One of the two sections including a “ball” structure comprising a convex bearing surface. The other of the sections including a “ring” structure comprising a ring adapted to receive and constrain at least a portion of the convex surface. 
     In another aspect, one or both of the aforementioned sections may include one or more “stops” or restrictive structures for limiting the range of relative movement between the two sections. 
     Thus, in one aspect, the invention provides an artificial intervertebral disc for implantation between adjacent superior and inferior vertebrae of a spine, the disc comprising first and second cooperating shells, each of the shells having opposed inner surfaces and oppositely directed outer surfaces, the outer surfaces being adapted for placement against the vertebrae; the inner surface of the first shell including a convex protrusion; and, the inner surface of the second shell including an articulation surface and a motion constraining ring adapted to receive the convex protrusion when the first and second shells are combined, wherein, when in use, the articulation surface of the second shell contacts and bears against the convex protrusion, and the ring constrains relative movement between the convex protrusion and the second shell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The nature and mode of operation of the present invention will now be more fully described in the following detailed description of the invention in view of the accompanying drawing figures, in which: 
         FIG. 1  is a schematic illustration of the range of motion of vertebrae; 
         FIG. 2   a  is a sagittal cross sectional view of the artificial intervertebral disc of the invention according to one embodiment; 
         FIG. 2   b  is a transverse cross sectional view of the disc of  FIG. 1 ; 
         FIG. 3  is a front coronal cross sectional view of the artificial intervertebral disc of the invention according to another embodiment; 
         FIGS. 4 to 8  are sagittal cross sectional views of the artificial intervertebral disc of the invention according to other embodiments; 
         FIG. 9  is a front coronal cross sectional view of the artificial intervertebral disc of the invention according to another embodiment; 
         FIGS. 10 and 11  are sagittal cross sectional views of the artificial intervertebral disc of the invention according to other embodiments; 
         FIGS. 11   a ,  12   a  and  13   a  are sagittal cross sectional views of the artificial intervertebral disc of the invention according to other embodiments; 
         FIGS. 11   b ,  12   b  and  13   b  are transverse cross sectional views of the artificial intervertebral discs of  FIGS. 11   a ,  12   a  and  13   a , respectively; 
         FIGS. 14 and 15  are sagittal cross sectional views of the artificial intervertebral disc of the invention according to other embodiments; 
         FIGS. 16   a ,  17   a  and  18   a  are sagittal cross sectional views of the artificial intervertebral disc of the invention according to other embodiments; and, 
         FIGS. 16   b ,  17   b  and  18   b  are side perspective views of the rings of the discs shown in  FIGS. 16   a ,  17   a  and  18   a , respectively; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset, it should be appreciated that like drawing numbers on different drawing views identify identical, or functionally similar, structural elements of the invention. It also should be appreciated that figure proportions and angles are not always to scale in order to clearly portray the attributes of the present invention. 
     While the present invention is described with respect to what is presently considered to be the preferred aspects, it is to be understood that the invention as claimed is not limited to the disclosed aspects. The present invention is intended to include various modifications and equivalent arrangements within the spirit and scope of the appended claims. 
     Furthermore, it is understood that this invention is not limited to the particular methodology, materials and modifications described and, as such, may, of course, vary. It is also understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to limit the scope of the present invention, which is limited only by the appended claims. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this invention belongs. In the following description, the terms “superior”, “inferior”, “anterior”, “posterior” and “lateral” will be used. These terms are meant to describe the orientation of the implants of the invention when positioned in the spine and are not intended to limit the scope of the invention in any way. Thus, “superior” refers to a top portion and “posterior” refers to that portion of the implant (or other spinal components) facing the rear of the patient&#39;s body when the spine is in the upright position. Similarly, the term “inferior” will be used to refer to the bottom portions of the implant while “anterior” will be used to refer to those portions that face the front of the patient&#39;s body when the spine is in the upright position. With respect to views shown in the accompanying figures, the term “coronal” will be understood to indicate a plane extending between lateral ends thereby separating the body into anterior and posterior portions. Similarly, the term “laterally” will be understood to mean a position parallel to a coronal plane. The term “sagittal” will be understood to indicate a plane extending anteroposterior thereby separating the body into lateral portions. The term “axial” will be understood to indicate a plane separating the body into superior and inferior portions. It will be appreciated that these positional and orientation terms are not intended to limit the invention to any particular orientation but are used to facilitate the following description. Although any methods, devices or materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods, devices, and materials are now described. 
       FIG. 1  illustrates the complexity of vertebral movement by indicating the various degrees of freedom associated with a spine. In the normal range of physiological motion, vertebrae extend between a “neutral zone” and an “elastic zone”. The neutral zone is a zone within the total range of motion where ligaments supporting the spinal bony structures are relatively non-stressed; that is, the ligaments offer relatively little resistance to movement. The elastic zone is encountered when the movement occurs at or near the limit of the range of motion. In this zone, the visco-elastic nature of the ligaments begins to provide resistance to the motion thereby limiting same. The majority of “everyday” or typical movements occurs within the neutral zone and only occasionally continues into the elastic zone. Motion contained within the neutral zone does not stress soft tissue structures whereas motion into the elastic zone will cause various degrees of elastic responses. Therefore, a goal in the field of spinal prosthetic implants in particular, is to provide a prosthesis that restricts motion of the vertebrae adjacent thereto to the neutral zone. Such restriction minimizes stresses to adjacent osseous and soft tissue structures. For example, such limitation of movement will reduce facet joint degeneration. 
     In general terms, the present invention provides artificial discs or implants for replacing intervertebral discs that are damaged or otherwise dysfunctional. The implants of the present invention are designed to allow various degrees of motion between adjacent vertebral bodies, but preferably within acceptable limits. In one embodiment, the invention is designed to permit relative movement between the vertebrae adjacent to the artificial disc of the invention, such movement including various degrees of freedom but preferably limited to a specified range. In one embodiment, the artificial disc, or prosthesis, of the invention is provided with one or more “soft” and/or “hard” stops to limit motion between the adjacent vertebrae. In particular, the artificial disc of the invention provides for rotation, flexion, extension and lateral motions that are similar to normal movements in the neutral and elastic zones (i.e., the movements associated with a normal or intact disc). In addition, the device of the invention also allows various combinations of such motions, or coupled motions. For example, the disc of the invention can be subjected to flexion and translation, or lateral flexion and lateral translation, or flexion and rotation. Various other motions will be apparent to persons skilled in the art given the present disclosure. 
       FIG. 2   a  illustrates an artificial intervertebral disc  10  according to an embodiment of the invention. As shown, disc  10  includes superior shell  12  and inferior shell  14 . Each of shells  12  and  14  comprise a bone contacting surface for placement against the bony structures of vertically adjacent vertebral bodies in a region where the natural intervertebral disc has been excised. As discussed above, such discecotomy may be necessary in cases where the natural disc is damaged or diseased. Superior shell  12  includes superior surface  16  for placement against the inferior surface of one vertebra while inferior shell  14  includes inferior surface  18  for placement against the superior surface of an adjacent and vertically lower vertebra. It will be understood that the terms “upper” and “lower” are used in conjunction with a spine in the upright position. Although the term “shell” is used herein, it will be understood that such term is not intended to limit the present invention to any shape or configuration. Other terms that may apply to the shells would be plate, etc. The term “shell” will be understood by persons skilled in the art to apply to the structures shown and/or described herein as well as any equivalent structures. 
     In the embodiment shown in  FIG. 2   a , inferior surface  20  of superior shell  12  includes ring  22  attached thereto. In the embodiment shown, ring  22  may comprise a downward depending convex or generally toroidal structure. Ring  22  may be affixed to superior shell  12  or may be formed integrally therewith. 
       FIG. 2   b  illustrates ring  22  of  FIG. 2   a . In the embodiment shown, ring  22  comprises a generally ovoid structure with a longer anteroposterior length and a shorter lateral length. In other embodiments, ring  22  may have a circular or any other shape as may be needed in view of the following discussion of the purpose of the ring. 
       FIG. 2   a  also illustrates superior surface  24  of inferior shell  14 , which is provided with a convex structure, or “ball”  26 , generally extending in the superior (or upward) direction. Although the term “ball” is used herein, it will be apparent to persons skilled in the art that this term is not intended to refer to a full or partial spherical structure. In one embodiment, as shown in  FIG. 2   a , ball  26  may comprise a hemispherical structure. In other embodiments, ball  26  may comprise an ovoid or other shape in plan view. 
     When implanting artificial disc  10  into an intervertebral disc space, two shells  12  and  14  are first aligned with inferior surface of superior shell  12  facing the superior surface of inferior shell  14 . In this alignment, ball  26  and ring  22  are engaged with ball  26  being positioned within the lumen of ring  22 . In this orientation, disc  10  is then inserted within the intervertebral space, between the adjacent vertebral bodies. In this position, the outer surfaces of shells  12  and  14  are in contact with the respective vertebral bodies. Once so implanted, the normal compressive force exerted by one vertebra against the other will serve to maintain disc  10  in position. It will be understood that any other artificial means may be used to prevent dislodging of the disc. For example, the outer surfaces of the shells may be provided with an adhesive or bone cement, etc., to ensure proper positioning. 
     Once in position, superior surface of ball  26  would contact inferior surface  20  of superior plate  12 . This contact provides the desired separation between the adjacent vertebral bodies. Relative movement between ball  26  and surface  20  provides the essential articulation between the vertebral bodies. Further, ring  22  serves to constrain the relative movement between ball  26  and inferior surface  20 . That is, ring  22  limits the amount of movement of the ball over surface  20  to a defined articulation region. Surface  23  of ring  22  that contacts ball  26  is referred to herein as the articulation surface of the ring. It will be understood that ring  22  is dimensioned to be of sufficient height (as measured inferiorly from the inferior surface of the superior shell) to provide the required limit, or “stop”, for ball  26 . In a typical application, ring  22  would have a height of 1 to 5 mm. However, it will be understood that various other sizes may be used or needed depending, for example, on the associated anatomy. The invention is not limited to any specific dimensions as may be mentioned herein, and may be modified to fit within any disc space of the human spine, i.e., the cervical, thoracic, or lumbar regions. Further, as mentioned above, and as discussed further below, ring  22  can be sized to limit or constrain various movements of ball  26  including translation, lateral bending, flexion, extension and any coupled movements involving one or more of such specific movements. This flexibility in design will therefore allow the artificial disc of the invention to function similarly to naturally occurring discs while also allowing correction or prevention of any malformations. 
     In one embodiment, as shown in  FIG. 2   a , ring  22  is sized so that the smallest length in its lumen is larger than the diameter of ball  26 . This arrangement allows ball  26  to contact surface  20  and also allows some degree of travel of the ball before being limited by ring  22 . As mentioned above, in one embodiment, ring  22  is dimensioned to have an ovoid shape (as shown in  FIG. 2   b ). This would, therefore, allow ball  26  to travel in one direction more than the other. In the example discussed above, ring  22  is provided with a longer anteroposterior length than a lateral length. This therefore allows further travel of ball  26  in the anteroposterior direction. In turn, this translates to a vertebral joint that allows greater flexion and extension as compared to lateral flexion. It will also be understood that by allowing movement of ball  26  in these directions, it is possible to allow for coupled movement such as flexion in conjunction with lateral flexion. 
     As indicated above, in one embodiment, the ball may be hemispheric in cross section but the shape may be varied in size in any direction. Thus, ball  26  may comprise a hemisphere or a convex shape that is elongated in the anteroposterior and/or lateral directions. In general, ball  26  may comprise any convex shape that provides the desired amount and type of intervertebral movements. This variability in structure of ball  26  would allow for a variety of different movements to occur within the physical constraints of ring  22 . As discussed further below, further motion constraints may be provided on ball  26  itself. 
     Although  FIG. 2   a  shows ball  26  being located centrally on superior surface  24  of inferior shell  14 , it will be understood that this is not intended as a limitation. In other embodiments, ball  26  may be positioned at any variety of locations on surface  24  depending on the desired movement. As will be appreciated, varying the position of ball  26  over surface  24  would result in a variation in the center of rotation of disc  10 . For example, in one embodiment the ball may be positioned posteriorly on inferior shell  14 . By varying the position of ball  26  with respect to inferior shell  14 , it is possible to provide disc  10  with a variety of movement, or articulation options. 
     In other embodiments, inferior shell  14  may be adapted to provide resistance to the movement of ring  22 . In one embodiment, inferior shell  14  may be provided with one or more hard stops or bumpers to limit the movement of ring  22  over ball  26 . The term “hard stops” is understood to mean a physical motion limiter. In particular, a “hard stop” would serve to limit motion so as not to exceed the aforementioned elastic zone. A “soft stop”, on the other hand would serve to commence limitation of motion once the elastic zone is entered. According to an embodiment of the invention, such stops may be built into the shell around the ball, at any distance, or may be formed as part of the ball itself. In one aspect, the hard stops may be of a height that is only a few millimeters below the maximum height of ball  26 . 
     An example of such hard stops is illustrated in  FIG. 3 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “a” added for clarity. As shown, hard stops  28  may be positioned laterally on either side of ball  26   a  to limit lateral flexion. That is, hard stops  28  provide a barrier for lateral (i.e., coronal) movement of ring  22   a  over the surface of ball  26 . Stops  28  shown in  FIG. 3  may be of any length to serve the aforementioned purpose. 
     In another embodiment, hard stops  28  may be located anteriorly to limit flexion in the anteroposterior direction and in still another embodiment, they would be located posteriorly. Any combination could be used to provide hard stops to constrain motion. The stops could be any manner of shapes from rectangular with rounded edges to domes and of variable height. It will be understood that in one embodiment, hard stops  28  may be provided to restrict movement in all directions if such limited movement is required. “Bumpers”  28  may be of various shapes for example linear or curved. Similarly, it will be understood that in other embodiments, no such hard stops may be needed. 
     Another embodiment of the above mentioned hard stop function is shown in  FIG. 4 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “b” added for clarity. As shown in  FIG. 4 , instead of “bumpers”  28  provided on inferior shell  14  as shown in  FIG. 3 , one edge, in the illustrated case, the anterior edge, of ball  26   b  may be provided with a hard stop, which, in the embodiment shown, is formed as raised extension  30  on the ball. As shown, extension  30  includes a superior surface having concave portion  32  adjacent ball  26   b , which serves as a “soft stop”, as discussed further below. Concave portion  32  extends from the anterior edge of ball  26   b , at a height between the lowermost and uppermost height of ball  26   b , and curves upward towards the anterior end of disc  10   b . Anterior of concave portion  32 , extension  30  includes edge  34 , which acts a hard stop. The arrangement shown in  FIG. 4  may be used in situations where flexion of the spine at the region of the implant, is to be limited. As will be understood, during flexion, the anterior edge of ring  22   b  will traverse anteriorly over the superior surface of ball  26   b  and first encounter concave portion  32 . Concave portion  32 , due to its upwardly curved surface, acts to slowly restrict the movement of ring  22   b , thereby acting as a soft stop for the flexion movement. As movement of the anterior edge of ring  22   b  continues, edge  34  is encountered and further movement is prevented. Thus, edge  34  serves as a hard stop for the flexion movement as well as limiting any tendency for the device to take on an abnormal or perhaps undesired alignment. 
     In another embodiment, hard stops may be placed laterally on either side of ball  26  to a height only a few millimeters below the maximum height of the ball to limit lateral flexion. 
     Another embodiment of the invention is shown in  FIGS. 13   a  and  13   b  (collectively referred to as  FIG. 13 ), wherein elements similar to those described above are identified with the same reference numeral but with the letter “c” added for clarity. In this embodiment, hard stop  36  is provided on superior surface  24   c  of inferior shell  14   c  wherein such hard stop is positioned immediately adjacent to ball  26   c  or may be formed as part of ball  26   c . Hard stop  36  is similar in function to that shown in  FIG. 3  but, is positioned only at anterior edge of ball  26   c . As with the hard stop shown in  FIG. 4 , hard stop  36  of  FIG. 13  serves to limit flexion and prevent abnormal or perhaps undesired alignment. In this case, hard stop  36  does not offer a gradual reduction to the flexion motion. As such, the arrangement shown in  FIG. 13  may be used in cases where it is desired to limit flexion and correct and/or limit kyphosis. 
     In a similar manner, a further embodiment of the invention would have hard stop  36  (or extension  30  of  FIG. 4 ) located posteriorly on inferior shell  14  so as to limit extension. In a further embodiment, a combination of such hard stops could be located in any direction or even circumferentially with respect to the ball and used to constrain motion in any or all directions. Thus, the stops associated with the ball may be varied in many ways to limit motion in one or more planes. The stops could be of any shape such as rectangular or convex such as dome-shaped. The stops may be of the same or different materials amongst themselves, or of similar or different materials compared to the shells. Further, the stops may be provided with rounded edges or any other required shape. In addition, the stops may be of any height as will be understood by persons skilled in the art. In yet another embodiment, disc  10  may include no stops associated with ball  26 , thereby allowing the ring to articulate over a maximum surface area of the ball. 
     Another embodiment of the invention is illustrated in  FIG. 5 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “d” added for clarity. As shown in  FIG. 5 , superior shell  12   d  may be provided with well  38 , which comprises a concave region that is adapted to receive a portion of ball  26   d . As will be understood, well  38  would serve as a location means for positioning ball  26   d  and/or as a further means of constraining the ball. In conjunction with ring  22   d , the provision of well  38  would increase the surface area contacted by ball  26   d  for the purpose of constraining its movement. As such, it will be understood that well  38  would further serve to reduce the wear effects on ring  22   d . Although well  38  in  FIG. 5  is shown as being somewhat complementary in shape to ball  26   d , it will be understood that such complementarity is not a limitation of the invention. That is, well  38  may be of various shapes and sizes to provide a variety of constraint options. 
     Another embodiment of the invention is shown in  FIG. 6 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “e” added for clarity.  FIG. 6  illustrates an embodiment wherein disc  10   e  is provided with a means of absorbing axial forces, that is, forces that are transmitted axially along the spine. To provide such force absorption, disc  10   e  may be provided with one or more resilient elements one or both of inferior and superior shells,  12   e  and  14   e , respectively. In the embodiment shown in  FIG. 6 , ball  26   e  is separated from superior surface  24   e  of inferior shell  14   e  by nucleus  40 . Nucleus  40  may comprise any known resilient material such as hydrogel, silicone, rubber, etc. or may comprise a mechanical device such as a spring, etc. As will be understood, as an axial force is applied to disc  10   e , nucleus  40  would absorb some of such force, thereby offering some cushioning and preventing or minimizing pressure between ball  26   e  and ring  22   e  and/or superior shell  12   e . In one embodiment, as shown in  FIG. 6 , ball  26   e  may be partially hollow to accommodate a greater volume of nucleus  40 . In such arrangement, nucleus  40  would include a raised portion or section adapted to be located within hollow ball  26   e . Such a structure may be advantageous for positively locating ball  26   e  with respect to inferior shell  14   e . That is, as with the embodiment shown in  FIG. 6 , nucleus  40 , having a protruding portion extending away from inferior shell  14   e , may be secured to superior surface  24   e  of inferior shell  14   e . Ball  26   e , having a central cavity adapted to receive the protruding portion of nucleus  40 , would be positioned over nucleus  40  such that the protruding portion is inserted into the cavity of the ball. In such case, ball  26   e  would not need to be secured or attached directly to inferior shell  14   e  since the nucleus would serve to prevent or limit any relative movement between the ball and inferior shell  14   e . In this way, ball  26   e  may be described as “floating” on nucleus  40 . 
     A further embodiment of a resilient force absorbing means is illustrated in  FIG. 10 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “f” added for clarity. In  FIG. 10 , ball  26   f  of disc  10   f  is secured to superior surface  24   f  of inferior shell  14   f  as described previously. In this case, spring  42  is provided, which bears against inferior surface  18   f  of shell  14   f . It will be understood that the opposite side of spring  42  may bear against the bony portion or portions of the adjacent vertebra or against any surface or structure (such as a plate or the like) attached to such vertebra. Spring  42  would function in a manner similar to nucleus  40  described above. However, as shown in  FIG. 10 , a further advantage may be realized with the arrangement shown. Specifically, since the spring may be positioned only against one edge of disc  10   f , the disc may be provided with a pre-set positioning to align the adjacent vertebrae in any desired manner. For example, in the embodiment shown in  FIG. 10 , spring  42  is located at the anterior edge of disc  10   f  thereby causing the superiorly adjacent vertebra (not shown) to be angled posteriorly. As will be understood, such an arrangement, in addition to providing the aforementioned cushioning function, will also serve to correct or prevent kyphosis. In the above description of  FIG. 10 , spring  42  has been described as being located between inferior shell  14   f  and the inferiorly adjacent vertebra. However, in another embodiment, spring  42  may be equally positioned between ball  26   f  and inferior shell  14   f  while achieving the same function. In addition although the term “spring” is used to describe element  42 , it will be understood that any similarly functioning device may be used with disc  10   f . For example, spring  42  may comprise a mechanical device such as a coil spring or a leaf spring. Alternatively, spring  42  may comprise a wedge shaped or similarly angulated resilient nucleus. Although  FIG. 10  illustrates inferior shell  14   f  angled posteriorly, it will be understood that such angulation may also be in the anterior direction in situations where kyphosis is required or to be encouraged (such as a region where lordosis is to be prevented or corrected such as the thoracic spine). 
     Another position adjusting means is illustrated in  FIG. 7 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “g” added for clarity. In  FIG. 7 , disc  10   g  has inferior shell  14   g  which is provided with angled superior surface  24   g  with respect to superior shell  12   g . Due to such angulation, ball  26   g  is similarly angularly disposed in relation to superior shell  12   g  and ring  22   g . As will be understood, such a structure serves to prevent or correct kyphosis as described above in relation to  FIG. 10 . However, unlike  FIG. 10 , disc  10   g  of  FIG. 7  does not necessarily include a force absorbing device. To achieve the desired angulation in inferior shell  14   g , the inferior shell may be formed as a wedge, as depicted in  FIG. 7 . Alternatively, the inferior shell may be formed in two segments thereby separating inferior surface  18   g  and superior surface  24   g  by means of a separating element (not shown). It will be understood that such separating element may comprise a spring such as described above with reference to  FIG. 10 . In such case, disc  10   g  of  FIG. 7  would also include a force absorbing means as well. It will also be understood that ball  26   g  of  FIG. 7  may include a nucleus as described above with respect to  FIG. 6 , thereby also providing disc  10   g  of  FIG. 7  with a means of absorbing axial forces. Although  FIG. 7  illustrates inferior shell  14   g  angled posteriorly, it will be understood that such angulation may also be in the anterior direction in situations where kyphosis is required or to be encouraged (such as a region where lordosis is to be prevented or corrected such as, for example, in the thoracic spine). 
     Much of the above discussion has focused on variations that may be implemented to inferior shell  14  and/or ball  26  of the invention. However, in a similar manner, superior shell  12  and/or ring  22  may also be varied to achieve a variety of positions and functions. For example, in one embodiment, the ring may be formed in various sizes and shapes. These would include variations in the height of the limiting edge of ring  22  and variations in its shape, including circular, ovoid and rectangular forms etc. For example, by varying the shape of ring  22 , it will be understood that the shape and area for articulation with the ball would also be varied thereby allowing the ball&#39;s constraint of motion to be tailored as needed. Similarly, the location of ring  22  may also be varied on superior shell  12  so as to match the position of the ball  26 . In addition, superior shell  12  may be provided with one or more “stops”, such as hard stops and/or soft stops, similar to those described above, for constraining or limiting the relative movements between the superior and inferior shells. Such stops may comprise separate elements attached to the superior shell or may form part of ring  22  itself. For example, in one embodiment, the stops may comprise raised edges of the ring. Further examples and aspects of the invention are discussed further below. 
     An embodiment of the invention showing variations in the superior shell are illustrated in  FIGS. 11   a  and  11   b  (collectively referred to as  FIG. 11 ), wherein elements similar to those described above are identified with the same reference numeral but with the letter “h” added for clarity. In  FIG. 11 , ring  22   h  is sized to be larger than ball  26   h . In this embodiment, it will be understood that articulation of disc  10   h  involves contact mainly between inferior surface  20   h  of superior shell  12   h . In other words, ball  26   h  would be capable of translation movement over a portion of inferior surface  20   h  without hindrance by ring  22   h . Such translation movement may comprise, for example, movement within the neutral zone. However, ring  22   h  would serve to constrain ball  26   h  from travelling beyond such region, thereby acting as a “hard stop”. 
     A variant of ring  22   h  described above is illustrated in  FIGS. 12   a  and  12   b  (collectively referred to as  FIG. 12 ), wherein elements similar to those described above are identified with the same reference numeral but with the letter “j” added for clarity. In this embodiment, disc  10   j , is provided with ring  22   j  on superior shell  12   j  that is narrower in size and designed to be in contact with at least a portion of ball  26   j  during all movement, i.e., articulation of disc  10   j . As will be understood, such an arrangement would assist in minimizing wear on inferior surface  20   j  of superior shell  12   j  caused by constant contact with ball  26   j . In addition, such an arrangement would limit lateral flexion while allowing for a full range of flexion and extension. 
       FIG. 12   b  illustrates a further feature of ring  22   j , namely a larger anteroposterior dimension as compared to a lateral dimension. As will be understood, such an arrangement serves to allow ball  26   j  a greater degree of freedom in movement in the sagittal plane and a restricted amount of movement in the coronal plane. In another embodiment, ring  22   j  may be elongated in the coronal plane thereby achieving the opposite effect. Thus, it will be understood that any combination of movements can be tailored by adjusting the dimensions of ring  22 . 
     Further embodiments of the invention are illustrated in  FIGS. 14 and 15 , wherein elements similar to those described above are identified with the same reference numeral but with the letter “m” or “n” added, respectively, for clarity. In the embodiments discussed above, ring  22  has been described as having a convex outer surface, particularly the articulating surface, that is the surface contacting ball  26 . However, as shown in  FIGS. 14 and 15 , rings  22   m  and  22   n , respectively, may alternatively include a concave articulating surface thereby changing the interaction between the ring and the ball. In both cases, rings  22   m  and  22   n  have an articulation surface contacting balls  22   m  and  22   n , respectively, which is concave in shape. Such concavity may be provided around the entire perimeter of the ring or only on certain locations. Similarly, the degree of curvature provided on the ring may be varied. For example, as shown in the two embodiments illustrated,  FIG. 14  depicts ring  22   m  that includes an articulation surface having a greater degree of curvature than that of ring  22   n  shown in  FIG. 15 . The concave articulation surface of the ring would allow movements such as flexion, extension, lateral bending or any combination thereof to be controlled by varying the degree of curvature provided. That is, the concave articulation surface would also allow for a graduated resistance to the movement of the ball thereby, for example, allowing for initial easy movement within the neutral zone but greater or increasingly greater resistance to movement in the elastic zone. Such resistance will be understood as a resistance provided against the ball. In another embodiment, the degree of curvature provided on the ring may be varied as between locations. For example, a greater degree of curvature may be provided at the lateral regions than in the anterior and posterior regions. This would, therefore, provide greater resistance to lateral bending than to flexion or extension. In another embodiment, the curvature of the ring can be varied to, for example, inhibit flexion by increasing the degree of curvature at the anterior edge of the ring. In another embodiment, the ring may be provided with both a constant or variably curved articulation surface as well as a non-circular shape. For example, the ring may comprise an oval geometry with a large axis generally parallel to the sagittal plane. The anterior and posterior articulation surfaces of such a ring may include a lesser degree of curvature than the lateral articulation surfaces. Further discussion of such variability is provided below with respect to  FIGS. 16 to 18 . 
       FIGS. 8 and 9  illustrate another embodiment of the invention. Where elements similar to those described above are identified, the same reference numerals are used but with the letter “p” added for clarity. As shown in  FIGS. 8 and 9 , superior shell  12   p  is provided with a convex curvature wherein the outer edges thereof are curved inferiorly. It will be understood that the degree of curvature of superior shell  12   p  may vary from the depicted in  FIGS. 8 and 9 . Such curvature of superior shell  12   p  would serve to correspond with the natural curved shape of the endplate on the vertebra. It will be understood that although the superior shell is shown in  FIGS. 8 and 9  as having such curvature, inferior shell  14   p  may similarly be provided with such complementary curvature corresponding to curvatures in the adjacent end plate. As shown in  FIGS. 8 and 9 , superior shell  12   p  would still include ring  22   p  for constraining movement of ball  26   p . Ring  22   p  may therefore also be designed to assume the curvature of superior shell  12   p . Thus, according to this embodiment, ball  26   p  may be constrained to motion over the gently sloping curvature of superior shell  12   p , in either or both of the sagittal or coronal planes. 
       FIGS. 16   a ,  17   a  and  18   a  illustrate other embodiments of the invention. Where elements similar to those described above are identified, the same reference numerals are used but with the letters “r”, “t” and “u” added, respectively, for clarity.  FIGS. 16   a ,  17   a  and  18   a  are shown with inferior shell  14 , ball  26  and stop  36  provided at the anterior edge of ball  26 , in a manner similar to that described above with reference to  FIG. 13 . As described above, although stop  36  is shown as being provided on the anterior edge of ball  26 , such stop may in fact be located in any position depending on the need and in more than one location if necessary. It will be assumed that this structure of the inferior shell is not intended to limit the embodiments illustrated in  FIGS. 16   a  to  18   a.    
       FIG. 16   a  illustrates superior shell  12   r  that is similar to that shown in  FIGS. 14 and 15 . That is, superior shell  12   r  includes ring  22   r  that is provided on generally flat inferior surface  20   r  of superior shell  12   r . Ring  22   r  of this embodiment includes articulation surface  23   r  that is concavely curved for the purposes discussed in reference to  FIGS. 14 and 15 .  FIG. 17   a  illustrates a variation of the disc of  FIG. 16   a . In  FIG. 17   a , disc  10   t  includes superior shell  12   t  having concavely curved inferior surface  20   t . That is, the outer edges of inferior surface  20   t  are curved inferiorly. As with  FIG. 16   a , ring  22   t  also includes a concavely curved articulation surface  23   t . Similarly,  FIG. 18   a  illustrates a variation wherein disc  10   u  includes superior shell  12   u  having convexly curved inferior surface  20   u . As with  FIG. 16   a , ring  22   u  also includes concavely curved articulation surface  23   u.    
     As shown in  FIGS. 16   a  to  18   a , as inferior surface  20  is curved, ring  22  is also allowed to assume a similar curvature. Such overall curvature of ring  22  along with the curvature of articulation surface  23  will be understood to assist in directing and controlling the amount and degree of constraint offered for movement of ball  26 . For example, as shown in  FIG. 17   a , the curvature of inferior surface  20   t  is shown as being concave in the sagittal plane. Thus, this orientation would serve to gradually resist movement of the ball in the anteroposterior directions, i.e., during flexion and extension. As discussed above, optional stop  26   t  (or stops, in the situation where more than one stop is provided) would pose a hard stop to prevent movement in a given direction. Similarly, a concave curvature of inferior surface  20   t  in the coronal plane would inhibit lateral bending. 
     In the case of  FIG. 18   a , it will be understood that the convex curvature would serve to assist motion. As a corollary to the above discussion, it will be understood that the convex curvature of inferior surface  20   u  shown in  FIG. 18   a  may be in either the sagittal or coronal planes. Moreover, the concave or convex curvature of inferior surface  20  discussed in reference to  FIGS. 17   a  and  18   a  will be understood to be provided in one or more directions. In one embodiment, for example, such surface may be partially spherical, thereby providing a respectively curved surface in all directions. 
       FIGS. 16   b ,  17   b  and  18   b  illustrate rings  22   r ,  22   t  and  22   u  depicted, respectively, in  FIGS. 16   a  to  18   a.    
     Although  FIGS. 16   a  to  18   a  illustrate ring  22  having convexly curved articulation surface  23 , it will be understood that such surface may also be convexly curved as discussed above in relation to other embodiments. 
     The structural components of the disc of the invention, in particular the ball and ring, may be formed of from any medically suitable material such as titanium, titanium alloys, nickel, nickel alloys, stainless steel, nickel-titanium alloys (such as Nitinol™ brand), cobalt-chrome alloys, polyurethane, porcelain, plastic and/or thermoplastic polymers (such as PEEK™ brand), silicone, rubber, carbothane or any combination thereof. In addition, it will be understood that the ball and ring may be made from materials that are the same or different from the remainder of the respective shells. For example, the ball may be made of titanium while the ring and both shells may be made of PEEK™ brand. Various other materials and combinations of materials will be known to persons skilled in the art. 
     As will be understood, and as explained above, the present invention may be adapted in various ways to meet any number of desired motion characteristics. That is, the shape, position, and size of the ball and/or ring may be chosen for various intervertebral joints of the spine and may be tailored for providing or restricting the degree and direction of motion. Various features and embodiments of the invention have been described and/or shown herein. It will be understood by persons skilled in the art that various combinations of such features and embodiments can be used depending on the need and requirements of the artificial disc. Further, although the figures illustrate various embodiments for the purposes of describing embodiments of the present, the relative or absolute dimensions shown are not intended to limit the scope of the invention in any way. 
     It will be apparent to persons skilled in the art that although the above discussion has focused on the superior shell being provided with the ring and the inferior shell being provided with the ball, the reverse may also be used. That is, in other embodiments, the superior shell may include the ball and the inferior shell may include the ring. 
     It will be apparent to persons skilled in the art that any bone contacting surfaces of the discs discussed above (such as the external surfaces of the shells) may be provided with a texture, treatment or coating to encourage or enhance bone ingrowth and/or adhesion to the adjacent bony structure. For example, such surfaces may be provided with a roughened or grooved texture and/or may be coated with a bone growth enhancing agent. 
     In addition, although the present invention has been described with reference to intervertebral joints, the present invention may equally be used in other joints such as, for example, knee joints. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the purpose and scope of the invention as outlined in the claims appended hereto. Any examples provided herein are included solely for the purpose of illustrating the invention and are not intended to limit the invention in any way. Any drawings provided herein are solely for the purpose of illustrating various aspects of the invention and are not intended to be drawn to scale or to limit the invention in any way. The disclosures of all prior art recited herein are incorporated herein by reference in their entirety.