Abstract:
An artificial intervertebral implant including a first baseplate having a top surface, a bottom surface, an aperture extending therethrough and a strap attached to the bottom surface of the first baseplate and underlying the aperture. The implant further includes a second baseplate juxtaposed with the first baseplate. The second baseplate includes a top surface with a cavity exposed therein. An articulating element is attached to a pair of opposing sidewalls of the cavity for retaining the strap within the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/546,230 filed Feb. 20, 2004, the disclosure of which is hereby incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The bones and connective tissue of an adult human spinal column consist of more than twenty discrete bones coupled sequentially to one another by a tri-joint complex, which consists of an anterior disc and two posterior facet joints, the anterior discs of adjacent bones being cushioned by cartilage spacers referred to as intervertebral discs. These more than twenty bones are anatomically categorized as being members of one of four classifications: cervical, thoracic, lumbar, or sacral. The cervical portion of the spine, which comprises the top of the spine up to the base of the skull, includes the first seven vertebrae. The intermediate twelve bones are the thoracic vertebrae, and connect to the lower spine comprising the five lumbar vertebrae. The base of the spine comprises the sacral bones (including the coccyx). The component bones of the cervical spine are generally smaller than those of the thoracic spine, which are in turn smaller than those of the lumbar region. The sacral region connects laterally to the pelvis.  
         [0003]     The spinal column is highly complex in that it includes these more than twenty bones coupled to one another, housing and protecting critical elements of the nervous system having innumerable peripheral nerves and circulatory bodies in close proximity. In spite of these complications, the spine is a highly flexible structure, capable of a high degree of curvature and twist in nearly every direction.  
         [0004]     Genetic or developmental irregularities, trauma, chronic stress, tumors, and degenerative wear are a few of the causes that can result in spinal pathologies for which surgical intervention may be necessary. A variety of systems have been disclosed in the art that achieve immobilization and/or fusion of adjacent bones by implanting artificial assemblies in or on the spinal column. The region of the back that needs to be immobilized, as well as the individual variations in anatomy, determine the appropriate surgical protocol and implantation assembly. With respect to the failure of the intervertebral disc, the interbody fusion cage has generated substantial interest because it can be implanted laparoscopically into the anterior of the spine, thus reducing operating room time, patient recovery time, and scarification. Referring now to  FIGS. 6   a  and  6   b , in which a side perspective view of an intervertebral body cage and an anterior perspective view of a post implantation spinal column are shown, respectively, a more complete description of these devices of the prior art is herein provided. These cages  101  generally comprise tubular metal body  102  having an external surface threading  103 . They are inserted transverse to the axis of the spine  104 , into preformed cylindrical holes at the junction of adjacent vertebral bodies (in  FIG. 6   b  the pair of cages  101  are inserted between the fifth lumbar vertebra (L 5 ) and the top of the sacrum (S)). Two cages  101  are generally inserted side by side with the external surface threading  103  tapping into the lower surface of the vertebral bone above (L 5 ), and the upper surface of the vertebral bone (S 1 ) below. The cages  101  include holes  105  through which the adjacent bones are to grow. Additional materials, for example autogenous bone graft materials, may be inserted into the hollow interior  106  of the cage  101  to incite or accelerate the growth of the bone into the cage. End caps (not shown) are often utilized to hold the bone graft material within the cage  101 .  
         [0005]     These cages of the prior art have enjoyed medical success in promoting fusion and grossly approximating proper disc height. It is, however, important to note that the fusion of the adjacent bones is an incomplete solution to the underlying pathology as it does not cure the ailment, but rather simply masks the pathology under a stabilizing bridge of bone. This bone fusion limits the overall flexibility of the spinal column and artificially constrains the normal motion of the patient. This constraint can cause collateral injury to the patient&#39;s spine as additional stresses of motion, normally borne by the now-fused joint, are transferred onto the nearby facet joints and intervertebral discs. It would therefore, be a considerable advance in the art to provide an implant assembly which does not promote fusion, but, rather, which mimics the biomechanical action of the natural disc cartilage, thereby permitting continued normal motion and stress distribution.  
         [0006]     It is, therefore, an object of the invention to provide an intervertebral spacer that stabilizes the spine without promoting a bone fusion across the intervertebral space.  
         [0007]     It is further an object of the present invention to provide an implant device that stabilizes the spine while still permitting normal motion.  
         [0008]     It is further an object of the present invention to provide a device for implantation into the intervertebral space that does not promote the abnormal distribution of biomechanical stresses on the patient&#39;s spine.  
         [0009]     It is further an object of the present invention to provide an artificial intervertebral disc that provides limited rotation of the baseplates transverse to the axis of the spine.  
         [0010]     It is further an object of the present invention to provide an artificial disc that provides limited angular rotation of the baseplates relative to a centroid of motion centrally located within the intervertebral space.  
         [0011]     It is further an object of the present invention to provide an artificial intervertebral disc that supports compression loads.  
         [0012]     It is further an object of the present invention to provide an artificial intervertebral disc that permits the baseplates to axially float toward and away from each other.  
         [0013]     It is further an object of the invention to provide an artificial intervertebral disc that supports tension loads.  
         [0014]     It is further an object of the present invention to provide an artificial intervertebral disc that prevents lateral translation of the baseplates relative to one another.  
         [0015]     It is further an object of the present invention to provide an artificial intervertebral disc that provides a centroid of motion centrally located within the intervertebral space.  
         [0016]     It is further an object of the present invention to provide artificial intervertebral disc baseplates having outwardly facing surfaces that conform to the concave surface of adjacent vertebral bodies.  
         [0017]     Other objects of the present invention not explicitly stated will be set forth and will be more clearly understood in conjunction with the descriptions of the preferred embodiments disclosed hereafter.  
       SUMMARY OF THE INVENTION  
       [0018]     The proceeding objects are achieved by the present invention, which is an artificial intervertebral disc or intervertebral spacer device having a pair of support members (e.g., spaced-apart baseplates), each with an outwardly-facing surface. Because the artificial disc of the present invention is to be positioned between the facing endplates of adjacent vertebral bodies, the baseplates are arranged in a substantially parallel planer alignment (or slightly offset relative to one another in accordance with proper lordotic angulation) with the outwardly-facing surfaces facing away from one another. The baseplates are to mate with the vertebral bodies so as not to rotate relative thereto, but rather to permit the spinal segments to bend (in some embodiments, actually compress) relative to one another in manners that mimic the natural motion of the spinal segment. This natural motion is permitted by the performance of a ball-and-socket-type joint using a spherical member disposed between the secured baseplates, and the securing of the baseplates to the vertebral bone may be achieved through the use of a vertebral body contact element attached to the outwardly-facing surface of each baseplate.  
         [0019]     Preferably, vertebral body contact elements include, but are not limited to, one or more of the following: a convex mesh, a convex solid dome and one or more spikes, as disclosed in U.S. patent application Ser. No. 10/256,160, the disclosure of which is hereby incorporated by reference herein.  
         [0020]     The ball and socket joint of the present invention permit rotation between the two elements by capturing a strap integrally formed with one of the baseplates within a groove of the other baseplate. The strap, preferably, has an inner surface having a curvature which is substantially equal to the curvature of a ball also disposed between the two baseplates, thereby permitting rotation and angulation of the strap about a central point of the ball. This further permits angulational movement and rotational movement of one baseplate relative to the other baseplate.  
         [0021]     The groove of the other baseplate, i.e., second baseplate, has a wider dimension than the strap so as to permit the strap to move freely about the central point of the ball at least with a desired angulation and rotation range. Additionally, the groove has a depth, which, in conjunction with the space between the first baseplate and the ball, limits the ability of the strap to come into contact with a bottom surface of the groove, even during axial movement of the two baseplates.  
         [0022]     In one preferred embodiment, the ends of the groove are angled relative thereto so as to reduce wear and tear between the strap and groove as the strap angulates and rotates about the central point of the ball within the groove.  
         [0023]     In one embodiment of the present invention, the artificial intervertebral disc includes a first baseplate having a top surface, a bottom surface and an aperture extending therebetween. The first baseplate further includes a strap having a top surface, a bottom surface, a first end and a second end. The ends of the strap are remote from one another and are attached to the bottom surface of the first baseplate such that a portion of the strap underlies the aperture.  
         [0024]     The artificial intervertebral disc of the present invention also includes a second baseplate having a top surface, a bottom surface and a cavity exposed at the top surface of the second baseplate. The cavity preferably includes a groove having a first sidewall and a second sidewall, with the sidewalls being remote from each other. A spherical element having a central point is disposed within the aperture of the first baseplate and overlies the strap. The spherical element is preferably attached to the first sidewall and second sidewall of the second baseplate such that the strap is positioned and captured within the groove, thereby permitting the first baseplate and the second baseplate to move in an angulational direction and a rotational direction relative to one another with the strap translating about the central point of the spherical element.  
         [0025]     The first sidewall and second sidewall may each include an indent such that the spherical element is attached to the first sidewall and second sidewall at respective indents. Additionally, the first sidewall and second sidewall may have a plurality of ends that are angled, such that during rotational movement of the first baseplate or second baseplate the strap has an increased range of motion within the groove.  
         [0026]     The artificial intervertebral implant of the present invention may also include a cover having a bottom surface. The cover is preferably designed to be at least partially disposed within the aperture such that the cover overlays the spherical element, thereby capturing the spherical element between the cover and strap.  
         [0027]     In certain embodiments of the present invention, the cover may include a cap and post with the top surface of the first baseplate further including a recess circumferentially extending about the aperture such that the post of the cover is compression fit within the aperture and the cap of the cover is compression fit within the recess.  
         [0028]     The groove of the second baseplate may have a bottom surface and the spherical element may have an apex. Additionally, a distance between the bottom surface of the groove to the apex of the spherical element is preferably greater than a distance between the bottom surface of the cover to the bottom surface of the strap. More preferably, the distance between the top surface of the strap to the bottom surface of the cover is greater than a diameter of the spherical element, such that the combination of the two permits the first baseplate and the second baseplate to move in an axial direction relative to one another.  
         [0029]     In one preferred embodiment of the present invention, the bottom surface of the cover and top surface of the strap have a radius of curvature substantially equal to a radius of curvature of the spherical element, such that the strap and the cover pivot about the central point of the spherical element as the first baseplate moves relative to the second baseplate in both an angulational direction and a rotational direction.  
         [0030]     In one aspect of the present invention a distance between the top surface of the strap and the bottom surface of the cover is greater than a length of the articulating element, such that the first baseplate and the second baseplate may move in an axial direction relative to one another.  
         [0031]     The bottom surface of the cover and the top surface of the strap may have a radius of curvature substantially equal to a radius of curvature of the articulating element, such that the strap and the cover may translate about the articulating element.  
         [0032]     In another aspect of the present invention the aperture may be partially defined by the strap and not be included within the first baseplate.  
         [0033]     In another aspect of the present invention the articulating element may be stationary relative to a first element, with the strap being captured between the articulating element and the first element. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  illustrates a perspective exploded view of a device according to the present invention;  
         [0035]      FIG. 2  illustrates an exploded cross-sectional view of the device of  FIG. 1 ;  
         [0036]      FIG. 3  illustrates an assembled cross-sectional view of a device of  FIG. 1  taken along the Y axis;  
         [0037]      FIG. 4A  illustrates an assembled perspective cross-sectional view of the device of  FIG. 1  taken along the Y axis;  
         [0038]      FIG. 4B  illustrates an assembled cross-sectional view of the device of  FIG. 1  taken along the Y axis;  
         [0039]      FIG. 5  illustrates a top view of a lower baseplate used in the present invention;  
         [0040]      FIG. 6A  illustrates a prior art embodiments of an artificial intervertebral disc; and  
         [0041]      FIG. 6B  illustrates prior art embodiments of an artificial intervertebral disc. 
     
    
     DETAILED DESCRIPTION  
       [0042]     The present invention will now be described with reference to the accompanying figures. The embodiments described herein are meant to be illustrative of the present invention and in no way should be thought of as limiting the present invention.  
         [0043]     As shown in  FIG. 1 , an artificial intervertebral disc  1 , according to the present invention, preferably includes an upper baseplate  10 , a lower baseplate  12 , a ball  14  and a cover  16 . Upper baseplate  10  is provided with a top surface  20  and a bottom surface  22 . Disposed within the boundary of top surface  20  is a recess  24 . Recess  24  includes a circular skirt  26  positioned adjacent top surface  20  and defining the outer boundary of recess  24 . Recess  24  further includes a shoulder  28  defining a lower limit of the recess. An aperture  30  is disposed adjacent shoulder  28  and extends from the shoulder to bottom surface  22  of upper baseplate  10 .  
         [0044]     As best shown in  FIG. 2  aperture  30  is defined by circumferential wall  32  which extends adjacent and between shoulder  28  and bottom surface  22 . Also as shown in  FIG. 2 , upper baseplate  10  includes a strap  34 . Strap  34  preferably includes a substantially semispherical inner surface  36  and a substantial semispherical outer surface  38 . Inner surface  36  and outer surface  38  are attached to one another through edges  40  and  40 ′ extending between the two surfaces and defining remote sides of strap  34 . Inner surface  36  and outer surface  38  have ends remote from one another and preferably include a first chamfered end  42  and a second chamfered end  44 . Chamfered ends  42 ,  44  extend from bottom surface  22  of upper baseplate  10  downward toward lower baseplate  12  and connect strap  34  to upper baseplate  10 . Strap  34  may be integral with upper baseplate  10 . As will be described below, aperture  30  as well as semispherical inner surface  36  of strap  34  preferably have a radius which is at least slightly larger than the radius of ball  14 .  
         [0045]     As illustrated in  FIGS. 1 and 2 , lower baseplate  12  preferably includes a top surface  50  and a bottom surface  52 . Top surface  50  preferably includes a cavity  54  exposed near a central portion of lower baseplate  12 . Cavity  54  preferably includes a groove  56  and a pair of indents  58 ,  59  disposed on opposite sidewalls  60 ,  61  positioned about groove  56 . Groove  56  preferably has a generally semicircular shape—when viewing from the direction X—with opposite sidewalls  60 ,  61  positioned adjacent to indents  58 ,  59 , respectively, and extending in the Y direction. Groove  56  is preferably larger in size than strap  34 , so that when the artificial intervertebral disc  1  is assembled and the strap is disposed within the bounds of groove  56 , as will be described below, strap  34  does not touch the bottom or sidewalls  60 ,  61  of groove  56 . Although groove  56  is shown as having a semicircular shape—viewed from the direction X—the shape of groove  56  is not essential to the present invention so long as it is large enough such that strap  34  does not touch the bottom of groove  56  when the artificial intervertebral disc  1  is assembled. For clarity of illustration, it is to be understood that, as described below, the sizing and shaping of strap  34  and groove  56  are such that when the ball  14  is secured to lower baseplate  12 , the strap  34  is freely movable about ball  14  in the space between sidewalls  60 ,  61  of groove  56 . As previously alluded to, indents  58 ,  59  are disposed on opposite sidewalls  60 ,  61  respectively and are preferably semispherical in shape to complementarily support ball  14 , as will be described below.  
         [0046]     Ball  14  is sized so as to be able to fit within aperture  30  and be supported by strap  34 . In a method of assembly, ball  14  is placed into aperture  30  through recess  24  of top surface  20 .  
         [0047]     As best illustrated in  FIGS. 1 and 2 , cover  16  preferably includes a top surface  66  and a bottom surface  68 . Cover  16  further includes a circumferential edge  70  extending between top surface  66  and bottom surface  68 . Top surface  66 , bottom surface  68  and edge  70  define a cap portion  72  of cover  16 . Cover  16  further includes a cylindrical post  74  having a circumferential skirt  76  adjacent to and extending down from bottom surface  68 . Post  74  preferably further includes a concave bottom surface  78 , the concavity of which may extend into cap portion  72  of cover  16 . The radius of curvature of concave bottom surface  78  (best shown in  FIG. 2 ) is preferably configured to approximate the curvature of ball  14 . In a preferred embodiment, cylindrical post  74  has a diameter that is slightly smaller than the diameter of aperture  30  extending through upper baseplate  10 .  
         [0048]     In a method of assembly, ball  14  is placed within aperture  30  so as to be supported by strap  34  of upper baseplate  10 . Subsequently, cover  16  is placed within recess  24  of upper baseplate  10  with cylindrical post  74  preferably being compression-fit or locked within aperture  30 . Additionally, in a preferred embodiment cap portion  72  may also be compression fit to upper baseplate  10  by edge  70  of cover  16  being engaged with skirt  26  of the upper baseplate.  
         [0049]     As best shown in  FIGS. 4A and 4B , strap  34  preferably has a width extending from edge  40  to edge  40 ′ that is smaller than the width of groove  56  defined by sidewalls  60  and  61 . This configuration allows strap  34  and upper baseplate  10  to rotate around a central point of ball  14  about an axis parallel to axis Z ( FIG. 1 ) (angulational and rotational motion). Such a relative rotation in the transverse plane is limited to some extent by the limited space between sidewalls  60  and  61  of groove  56  and edges  40  and  40 ′ of strap  34 .  
         [0050]     In one preferred embodiment, as shown in  FIG. 5 , sidewalls  60  and  61  of groove  56  are angled at their respective ends  80 ,  81 ,  82 , and  83  relative to one another to accommodate desired rotation and angulation ranges and/or limit rotation to within a desired range of angles, without inviting excess wear or line contact endured by edges  40  and  40 ′ of strap  34  against sidewalls  60  and  61 . That is, if sidewalls  60  and  61  were not angled, the edges  40  and  40 ′ will dig into the sidewalls, causing undesirable wear characteristics over multiple articulations of the device; whereas if the sidewalls  60  and  61  are angled to align with the edges  40  and  40 ′ of strap  34  during the maximum desired axial rotation range, edges  40  and  40 ′ will hit flush against sidewalls  60  and  61 , minimizing wear debris and improving the wear characteristics of the device.  
         [0051]     Rotation (or articulation) of upper baseplate  10  about an axis perpendicular to axis Z, (lateral bending articulation and flexion-extension articulations) relative to lower baseplate  20  can be limited by the distance between bottom surface  22  of upper baseplate  10  and top surface  50  of lower baseplate  12 . In other words, such articulation will be stopped when the two surfaces  22  and  50  come to meet each other. This distance can be determined by properly designing the size of ball  14  as well as the position (depth) of indents  58  and  59  on sidewalls  60  and  61 , respectively in lower baseplate  20  and the dimensions of groove  56  in the lower baseplate, which will be further described below.  
         [0052]     Top surface  20  of upper baseplate  10  and bottom surface  52  of lower baseplate  20  are preferably designed to be convex in shape to match the concave shape of endplates of adjoining vertebral bones. Similarly, the top surface  66  of cover  16  preferably has a convex design and is a smooth extension of top surface  20  of upper baseplate  10  as best shown in  FIGS. 1 and 3 .  
         [0053]     To assemble the artificial intervertebral disc  1  of the present invention, as previously mentioned, ball  14  is placed through recess  24  of upper baseplate  10  and into aperture  30  so as to be supported by strap  34 . With ball  14  resting on semispherical inner surface  36  of strap  34 , the strap is placed within groove  56  of lower baseplate  12 , with portions  14 A, and  14 B of ball  14 , contacting respective indents  58  and  59  as best illustrated in  FIGS. 4A and 4B . Portions  14 A and  14 B of ball  14  are then fixed to respective indents  58  and  59  by, for example, welding or an adhesive, whereby the ball is fixed to lower baseplate  12 , and strap  34  is retained in groove  56  by ball  14 . This also prevents upper baseplate  10  from disengaging from lower baseplate  12 . Cover  16  is next disposed within recess  24  of upper baseplate  10 . Preferably, cover  16  is secured to upper baseplate  10  by a compression lock, threading, an adhesive or the like.  
         [0054]     After the assembling is finished, artificial intervertebral disc  1  can be implanted between the adjoining endplates of vertebral bones. Strap  34  and therefore upper baseplate  10 , can articulate and rotate about a center of ball  14  in universal directions relative to lower baseplate  12 . The distance between upper baseplate  10  and lower baseplate  12  limits the articulation about an axis perpendicular to axis Z. Moreover, upper baseplate  10  can move toward and away from (along axis Z) lower baseplate  12  with such a translation being limited by the space between cover  16  and ball  14  as well as the distance between ball  14  and the bottom surface of groove  56 . Angulational and rotation (rotation about an axis perpendicular to the axis Z) are limited by the difference between the width of strap  34  and the width of groove  56  and, preferably, opposing walls  60  and  61  of groove  56  being angled relative to one another to accommodate desired motion ranges, and/or limit motion to within a desired range of angles, without inviting excess wear or line contact of the edges  40  and  40 ′ against the sidewalls  60  and  61 .  
         [0055]     Alternatively, although not shown in the drawings, edges  40  and  40 ′ of strap  34  and/or sidewalls  60  and  61  of groove  56  are not necessarily flat, but can be curved (concave/convex) in shape, which may result in a smoother contact between the strap and the groove.  
         [0056]     Although the present invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.