Abstract:
A spinal disc prosthetic for replacement of cervical and/or lumber discs, that in each case allows a limited degree of rotational articulation, as well as durability and resistance to fatigue. The prosthetic is formed with three primary layers, including a superior (upper) plate, inferior (lower) plate, and intermediate layer, in a sandwiched configuration. The superior plate member is adapted to be secured on one side to an upper vertebra in a spinal column, and has a formed concave side exposed downwardly. The inferior plate member is adapted to be secured on one side to a lower vertebra in the spinal column, and has a substantially flat side exposed upwardly. The intermediate member is sandwiched between the superior and inferior plate members with conforming sides, and a short cylindrical post protrudes downward from the intermediate member and is seated inside a circular recess in the inferior plate member to center it and to maintain a predetermined spacing there between. This allows a degree of rotational freedom. A variety of locking assemblies lock the post in the recess to prevent withdrawal, thereby increasing durability without obstructing the degree of freedom.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the invention  
         [0002]     The present invention relates to prosthetics and, more particularly, to prosthesis for use as a replacement for a lumbar and/or cervical disc in the spinal column of a human that allows both rotational and angular mobility.  
         [0003]     2. Description of the Background  
         [0004]     Intervertebral discs are subject to various forms of damage and degeneration due to mechanical stresses and aging. A variety of designs for artificial discs and disc prostheses have been proposed over the years. Cervical disc prosthetics pose a more difficult challenge in that the intervertebral spacing of the cervical vertebrae is smaller than for the lower vertebrae, etc., on the order of 8-9 mm for the cervical vertebrae, as compared with approximately 11 mm for the intervertebral discs. Moreover, the average age of patients undergoing spinal disc replacement is 42 years. Consequently, the life span of such a device should exceed 40 years. This requires an extremely high fatigue limit. Existing examples of disc prosthetics are shown in U.S. Pat. Nos. 6,517,580; 5,562,738; 5,676,701; 6,063,121; 6,162,252; 5,071,437; 5,522,899, and 6,132,465. While these designs purport to provide performance mimicking the function of the original disc, in practice, they do not articulate naturally and, consequently, do not function appropriately in the place of the original disc. Still other attempts have been made to replicate the natural action of both lumbar and cervical discs, including the following.  
         [0005]     U.S. Pat. No. 4,759,766 to Buettner-Janz et al. (Humbolt University) issued Jul. 26, 1988 shows an intervertebral disc endoprosthesis with two end plates and a spacing piece, the spacing piece having opposing concave-convex surfaces with pins  13  that extend either into circular recesses  14 , as shown in FIGS.  12  to  16 , or into slot-shaped recesses  15 , as shown in FIGS.  17  to  21 .  
         [0006]     U.S. Pat. No. 6,019,792 to Cauthen issued Feb. 1, 2000 shows an articulating spinal implant for intervertebral disc replacement that relies on an articulating ball-and-socket joint between two replacement disc elements that resists compression and lateral movement between the vertebrae, but allows pivotal movement, thereby preserving mobility. In this case the two replacement discs are hemispherical shapes.  
         [0007]     U.S. Pat. No. 6,740,118 to Eisermann et al. (SDGI Holdings, Inc.) issued May 25, 2004 shows an intervertebral prosthetic joint with two plates formed with abutting convex and concave articular surfaces that cooperate to permit articulating motion. One of the articular surfaces has a surface depression that traps and allows removal of matter disposed between abutting portions of the articular surfaces.  
         [0008]     U.S. Pat. No. 6,723,127 to Ralph et al. (Spine Core, Inc.) issued Apr. 20, 2004 shows an intervertebral disc that uses an intermediate “wave washer” between two plates that allows the plates to compress, rotate and angulate freely relative to one another, enabling the artificial disc to mimic a healthy natural intervertebral disc.  
         [0009]     U.S. Pat. No. 5,401,269 to Buttner-Janz et al. (Waldemar Link) issued Mar. 28, 1995 shows an intervertebral disc endoprosthesis with two concave prosthesis plates sandwiching a convex prosthesis core. In one embodiment, the core has a nib ( FIG. 8 ) that cooperates with at least one prosthesis plate to make possible a rotational movement.  
         [0010]     U.S. Patent Application 20030040802 by Errico shows an artificial intervertebral disc having limited rotation using a captured ball and socket joint. The artificial disc has a pair of opposing baseplates, for seating against opposing vertebral bone surfaces. The base plates are separated by a ball and socket joint, the ball being secured by a post extending from one of the baseplates. The ball is captured within a socket formed in the other of the baseplates. The ball and socket joint therefore permits the baseplates to rotate relative to one another through a limited range and also angulate relative to one another.  
         [0011]     U.S. Patent Application 20040158328 by Eisermann filed Aug. 12, 2004 shows a mobile bearing articulating disc with a plate having a concave recess, a second component having a second recess, and a projection adapted to engage the second recess surface to permit articulating motion between the first and second components. The projection  56  is shown to be a ball-and-socket type mechanism, with a notch  76  for removal of matter.  
         [0012]     U.S. Patent Application 20040049280 by Cauthen filed Mar. 11, 2004 shows an articulating spinal implant for interverteral disc replacement formed from three elements (see  FIG. 5 ), two engaging adjacent vertebra. An articulating disc between the two elements resists compression and lateral movement between the vertebra, but allows the adjacent vertebra to articulate about an instantaneous axis of rotation.  
         [0013]     U.S. Patent Application 20040176851 by Zubok et al. filed Sep. 9, 2004 shows a cervical disc replacement with first and second articulation plates having concave/convex surfaces sized and shaped to engage one another when the first and second members are disposed in the intervertebral disc space to enable the first and second vertebral bones to articulate in at least one of flexion, extension and lateral bending.  
         [0014]     Although the above-described prosthetic discs as well as others have furthered technological development, none have fully solved the disc replacement problem. They pursue articulation, but lack durability and resistance to fatigue. It would be greatly advantageous to provide a fully-articulating spinal disc prosthetic having a high-wear capability and integrally-joined components that are extremely durable.  
       SUMMARY OF THE INVENTION  
       [0015]     Accordingly, it is an object of the present invention to provide a prosthetic disc with fully articulating capability, and also high-durability and resistance to fatigue.  
         [0016]     It is another object to provide a fully-articulated prosthetic disc as above that makes use of integrally-joined components that cannot come apart.  
         [0017]     In accordance with the foregoing object, the present invention comprises a lumbar and/or cervical disc prosthetic formed with three primary layers, including a superior (upper) plate, inferior (lower) plate, and intermediate layer, in a sandwiched configuration. The superior plate member is adapted to be secured on one side to an upper vertebra in a spinal column, and has a formed concave side exposed downwardly. The inferior plate member is adapted to be secured on one side to a lower vertebra in the spinal column, and has a substantially flat side exposed upwardly. The flat side is interrupted by a central cylindrical recess. The intermediate member has a convex side conforming to the concave side of the superior plate member, a flat downside conforming to the flat side of said inferior plate member, and is sandwiched between the superior and inferior plate members. The flat upside of the inferior member is interrupted by a circular recess. A short cylindrical post protrudes downward from the intermediate member and is seated inside the central recess of the inferior plate member to center it, lock it in place, and maintain a predetermined spacing there between. Both the post and the recess within which it resides have flat bearing surfaces for better wear. The post is coupled into the recess by a locking assembly of which two examples are herein disclosed: 1) compression-fit locking rings; and 2) lateral locking tabs for a keyhole fit into the recess. In these and equivalent configurations the locking assemblies prevent withdrawal of the intermediate member from the inferior plate member, thereby increasing reliability and durability. The particular configurations described herein allow a limited degree of rotational articulation, and also afford the durability and resistance to fatigue necessary for a 30-40 year lifetime. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     Other objects, features, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiment and certain modifications thereof, in which  
         [0019]      FIG. 1  is a side view of the spinal disc prosthesis  10  according to a preferred embodiment of the present invention.  
         [0020]      FIG. 2  is a bottom plan view of the intermediate member  108  as in  FIG. 1 , with composite side enlargement.  
         [0021]      FIG. 3  is an exploded view of the spinal disc prosthesis  10  as in  FIGS. 1-2 .  
         [0022]      FIG. 4  is a bottom plan view of an intermediate member  108  having an alternate locking arrangement.  
         [0023]      FIG. 5  is an exploded view of the spinal disc prosthesis  10  as in  FIG. 4  with alternate locking arrangement.  
         [0024]      FIG. 6  is a perspective view of the intermediate member  108  as shown in  FIGS. 1-3 .  
         [0025]      FIG. 7  is a side view of the intermediate member  108  as in  FIG. 6 .  
         [0026]      FIG. 8  is a top view of the intermediate member  108  as in  FIGS. 6-7 .  
         [0027]      FIG. 9  is a side view of the intermediate member  108  as in  FIG. 5  but rotated  90  degrees.  
         [0028]      FIG. 10  is a perspective view of the inferior (lower) plate member  102  as shown in  FIGS. 1-5 .  
         [0029]      FIG. 11  is a top view of the inferior (lower) plate member  102  as in  FIG. 10 .  
         [0030]      FIG. 12  is a side view of the inferior (lower) plate member  102  as in  FIGS. 10-11 .  
         [0031]      FIG. 13  is a side view of the inferior (lower) plate member  102  as in  FIG. 12  but rotated 90 degrees.  
         [0032]      FIG. 14  is a side view of the superior (upper) plate member  100 .  
         [0033]      FIG. 15  is a top view of the superior (upper) plate member  100  as in  FIG. 14 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]      FIGS. 1-3  are a side view, bottom plan view, and exploded view, respectively, of the spinal disc prosthesis  10  (lumber or cervical) according to a preferred embodiment of the present invention. Referring to all of  FIGS. 1-3 , the spinal disc prosthesis  10  as illustrated includes an upper, or superior, plate member  100 , and a lower, or inferior, plate member  102 , which are adapted to be secured to upper and lower vertebra  104 ,  106 , respectively, in a spinal column. An intermediate member  108  is provided, and is disposed between the upper and lower plate members  100 ,  102 , once the prosthesis is assembled in the spinal column.  
         [0035]     It is to be noted that the reference to the plate members as upper and lower members is for the purpose of identifying these members in the drawings. It may well be possible that the positions of the two plate members can be reversed.  
         [0036]     Each of upper and lower plate members  100 ,  102  is provided with means for securement to the upper and lower vertebra. Many types of securement means are known in the art, and could be used with the present invention. For purposes of illustration, the plates  100 ,  102  are respectively provided with one or more tabs  110 ,  112  extending from the periphery of the plates  100 ,  102 , and extending approximately laterally from the surface of the plates which will face the vertebra. As is known in the art, screws  114 ,  116  can be used to fasten the tabs to the vertebra  104 ,  106 , through bores extending through the tabs. Plate members  100 ,  102  may further have their bone-contacting surfaces  118 ,  120  manufactured and/or treated or modified to facilitate or improve bonding to the bone. Again, several such approaches are known in the art and should be suitable for use with the present invention.  
         [0037]     The upper plate member  100  preferably has a lower mating surface  132  that is concave and complementary in shape to the upper surface  130  of intermediate member  108 . The intermediate member  108  is formed with a slightly smaller diameter than upper plate member  100 , is sandwiched between the plate members  100 ,  102 , and is formed with surfaces generally conforming to the opposing plate members  100 ,  102 . Specifically, the intermediate member  108  is formed with a convex or domed upper surface  130  conforming to the concave lower mating surface  132  of the upper plate member  100 . By providing such mating surfaces, the upper vertebrae  104  may shift either laterally or in a front or rearward direction, relative to the intermediate member  108  and lower vertebrae  106 . The material or materials from which the intermediate member  108  is made, or the surface treatment thereof, can be selected to provide a desired degree of frictional engagement between the upper plate and intermediate member. While it is envisioned that the domed surface would preferably have a low coefficient of friction, it is possible to control or restrict movement between the upper plate  100  and intermediate member  108  by proper selection of the coefficient of friction of these two mating surfaces.  
         [0038]     The lower surface  134  of intermediate member  108  is generally planar across the majority of the surface. The surface may, alternatively, be slightly convex. A post  136  protrudes from the lower surface  134 . An upper mating surface  138  of lower plate member  102  is preferably generally planar, and has a recess formed centrally therein to receive the post  136 .  
         [0039]     The post  136  and recess  140  are particularly configured for purposes of the present invention. The recess  140  is generally cylindrical, conforming to the cylindrical post  136 , but with a slightly larger diameter, and a slightly smaller depth. This way, the post  136  is free to rotate within recess  140  (allowing the intermediate member  108  to rotate as well), and the longer post  136  imposes a predetermined clearance between the intermediate member  108  and upper mating surface  138  of lower plate member  102 . Thus, when assembled, the intermediate member  108  and the lower plate member  102  will not come into complete contact. This configuration presents a degree of freedom for rotation of the intermediate member  108  that more closely replicates that of a natural spinal joint. The intermediate member  108  is capable of a small degree of flexion relative to the lower plate member  102 . In addition to the foregoing, the post requires a flat face  113 . The post  136  rests upon the flat face  113 , and the latter serves as a bearing surface to support the intermediate member  108  and maintain an even keel, and yet to allow the foregoing articulation. This permits the relief of at least a portion of any compressive forces exerted on prosthesis  10  by the upper and lower vertebra, as well as permitting the upper and lower vertebra to rotate relative to each other through a small angle, as forces on the two vertebrae are transmitted to the prosthesis. The small circular and flat area of contact between post  136  and recess  140  also permits intermediate member  108  to rotate about a vertical axis relative to the lower plate  102 , in the event that the spinal column experiences twisting forces.  
         [0040]     The upper and lower plate members  100 ,  102  are preferably made of a material that is compatible with the bone and, as noted previously, will preferably facilitate or promote bone grafting. The intermediate member  108  is preferably made of a material that is essentially non-resilient or of low resiliency, such as, for example, a metal, a ceramic, or a polymer having a low degree of resiliency. The function or operation of the prosthesis  10  in approximating the function or operation of a natural disc comes essentially from the degrees of freedom of movement provided between the upper and lower plates, and the intermediate member  108  disposed there between.  
         [0041]     The illustrated embodiment can be constructed with dimensions that are small enough to enable use of the prosthesis as a cervical disc prosthesis, where the intervertebral spacing is small, e.g., on the order of 8-9 mm. Thus, it is envisioned that the prosthesis  10  will be especially suitable for use in replacing damaged cervical discs. However, the prosthesis can be sized as required to serve as a lumbar disc prosthesis, as well.  
         [0042]     The present invention also comprises a locking feature by which the post  136  is inserted into the recess  140  in such a way as to lock into it without detracting from the partial rotation capabilities described above. This may be accomplished with a variety of alternative and equally well-suited configurations, two examples of which are described in detail herein. The first, shown in detail in  FIGS. 1 and 2 , comprises a compressible wire ring  117  that is seated in a shallow annular groove around post  136  near the distal end. Ring  117  cooperates with a lip  119  around the mouth of recess  140 . The ring  117  is of a slightly larger diameter than the groove around post  136  within which it sits, and is thereby given ample space to constrict inward when compressed. The lip  119  around the mouth of recess  140  is ramped inward, and is of a slightly smaller diameter than the uncompressed wire ring  117 . This tends to center the post  136  when inserted therein, and when pushed harder the ramped lip  119  will compress the ring  117  on post  136 . The ring  117  and post  136  will slip past the lip  1   9 , whereupon the ring  117  will expand within the confines of the recess  140  (trapped behind the lip  117 ) for the purpose of locking the post  136  therein and securing the intermediate member  108  together with the lower plate member  102 , and yet without interfering with the ability for limited relative rotation. The post  136  can be removed from recess  140  by forcible pulling.  
         [0043]     A second example of a locking feature is shown in detail in  FIGS. 4 and 5 , and involves forming the post  136  with a variety of locking tabs  127 , and the recess with a lip  119  (as before) this time interrupted with corresponding keyhole slots  161  for receiving the locking tabs  127 .  
         [0044]     For example, in the illustrated embodiment the post  136  is formed with a plurality of opposing tabs  127  adapted for insertion downward through slots  161  in the lower plate member  102 . Once inserted, a slight rotation locks the intermediate member  108  in place and yet allows free rotation and pivoting. The locking engagement of the post  136  in recess  140  operates to maintain the intermediate member  108  in its desired position in the prosthesis assembly, and substantially prevents shifting of this element out of position.  
         [0045]      FIGS. 6-9  are a perspective view, side view, top view, and side rotated view ( 90  degrees), respectively, of the intermediate member  108  as shown in  FIGS. 1-5  (using the locking feature of  FIGS. 4-5 ) with an exemplary set of dimensions stated in millimeters. The intermediate member  108  is an oval-shaped disc having a convex top surface  182 , a substantially flat bottom surface  184 , and a downwardly protruding cylindrical post  136  extending centrally from the bottom surface  184 . As seen in  FIG. 9  post  136  is formed with two opposing tabs  127  adapted for insertion through corresponding keyhole slots  161  in the lip  119  leading into the recess  140  of lower plate member  102  (see  FIGS. 1 &amp; 10 ). The tabs  127  are two opposing integrally-molded tabs extending 1-2 mm at the distal end of the post  136 , although three or four tabs will also suffice.  
         [0046]      FIGS. 10-13  are a perspective view, top view, side view, and side rotated view ( 90  degrees), respectively, of the inferior (lower) plate member  102  as shown in  FIGS. 1-5  (using the locking feature of  FIGS. 4-5 ), with an exemplary set of dimensions stated in mm. The inferior (lower) plate member  102  is an oval-shaped disc having a substantially flat top surface  138 , a slightly contoured bottom surface  139 . The inferior plate member  102  is formed with one or more tabs  110  extending from the periphery orthogonally from the lower surface  139  to face the vertebra. As is known in the art, a screw can be used to fasten the tab(s)  110  to the vertebra through the bore-hole shown extending through the tab  110 . The upper mating surface  138  of lower plate member  102  is generally planar, and has a recess  140  formed centrally therein to receive the post  136 . The recess  140  is a cylindrical recess with a flat bottom surface, and opposing grooves  161  formed to receive the opposing tabs  127  of the post  136  in the intermediate member  108 . The opposing grooves  161  open into the larger recess  140  and may have dimples or ramped protruberances formed along the inside lips to prevent the post  136  from being withdrawn. Thus, the post  136  is inserted by a simple push-and-twist operation which locks the tabs  127  therein, preventing inadvertent withdrawal.  
         [0047]      FIGS. 14 and 15  are a side view and a top view, respectively, of the superior (upper) plate member  100  with an exemplary set of dimensions stated in mm. The superior (upper) plate member  100  is an oval-shaped disc having a contoured top surface  103 , and one or more tabs  112  extending from the periphery orthogonally from the upper surface  103  to face the vertebra. Again, a screw can be used to fasten the tab(s)  112  to the vertebra through the bore-hole shown extending through the tab  112 . Upper plate member  100  preferably has a mating surface  132  which is concave, and which is complementary in shape to the domed surface  130  of intermediate member  108 . By providing such mating surfaces, the upper vertebrae  104  may shift either laterally or in a front or rearward direction, relative to the intermediate member  108  and lower vertebrae  106 .  
         [0048]     Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiments herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that the invention may be practiced otherwise than as specifically set forth in the appended claims.