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 sandwich. 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 includes a short cylindrical post protruding downward into a circular recess in the inferior plate member to center it and to maintain a predetermined spacing there between. Two lateral pins on the intermediate member fit into two corresponding slots on the inferior member as guides and allow a degree of rotational freedom, and the post includes snap-in spring fingers that lock into the recess of the inferior member to prevent withdrawal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application derives priority from provisional patent application No. 60/657,755 filed Mar. 1, 2005, and is a continuation-in-part of U.S. patent application Ser. No. 10/997,823 filed Nov. 24, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     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.  
         [0004]     2. Description of the Background  
         [0005]     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.  
         [0006]     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 .  
         [0007]     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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     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.  
         [0011]     United States 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.  
         [0012]     United States 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.  
         [0013]     United States Patent Application 20040049280 by Cauthen filed Mar. 11, 2004 shows an articulating spinal implant for intervertebral 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.  
         [0014]     United States 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.  
         [0015]     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  
       [0016]     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.  
         [0017]     It is another object to provide a fully-articulated prosthetic disc as above that makes use of integrally-joined components that cannot come apart.  
         [0018]     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 that uses central snap-in spring fingers located at the bottom of the post as well. Additionally the locking assembly involves two lateral pins, one located at each end of the intermediate member, that fit into two corresponding slots located at each end of the inferior plate member. In this and equivalent configurations the locking assemblies prevent withdrawal of the intermediate member from the inferior plate member, thereby increasing reliability and durability. The particular configuration described herein limits rotational articulation to 20 degrees (10 degrees on each side), and also affords the durability and resistance to fatigue necessary for a 30-40 year lifetime.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     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  
         [0020]      FIG. 1  is a side view of the spinal disc prosthesis  10  according to a preferred embodiment of the present invention.  
         [0021]      FIG. 2A  is a perspective view of intermediate member  108 .  
         [0022]      FIG. 2B  is a side view of the intermediate member  108 .  
         [0023]      FIG. 3  is an exploded view of the spinal disc prosthesis  10  as in  FIG. 1 .  
         [0024]      FIG. 4  is a perspective view of the intermediate member  108  as shown in  FIGS. 1-3 .  
         [0025]      FIG. 5  is a side view of the intermediate member  108  as in  FIG. 4 .  
         [0026]      FIG. 6  is a side view of the intermediate member  108  as in  FIGS. 4-5  but rotated 90 degrees.  
         [0027]      FIG. 7  is an exploded view of the cylindrical post  136  with central snap-in spring fingers  137  attached at the end of the post  136 .  
         [0028]      FIG. 8  is a perspective view of the inferior (lower) plate member  102  as shown in  FIGS. 1 and 3 .  
         [0029]      FIG. 9  is a side view of the inferior (lower) plate member  102  as in  FIG. 8 .  
         [0030]      FIG. 10  is a side view of the inferior (lower) plate member  102  as in  FIGS. 8-9 .  
         [0031]      FIG. 11  is a top view of the inferior (lower) plate member  102  as in  FIG. 10 .  
         [0032]      FIG. 12  is a perspective view of the superior (upper) plate member  100 .  
         [0033]      FIG. 13  is a top view of the superior (upper) plate member  100  as in  FIG. 12 .  
         [0034]      FIG. 14  is a side view of the superior (upper) plate member  100  as in  FIGS. 12 and 13  but rotated 90 degrees.  
         [0035]      FIG. 15  is a side view of the superior (upper) plate member  100  as in  FIGS. 12-14 .  
         [0036]      FIG. 16  is a sectional view of the view of the superior (upper) plate member  100  as in  FIGS. 12-15 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]      FIGS. 1 and 3  are a side 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  FIGS. 1 and 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.  
         [0038]     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.  
         [0039]     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.  
         [0040]     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.  
         [0041]     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  140  formed centrally therein to receive the post  136 .  
         [0042]     The post  136  and recess  140  are particularly configured for spacing the intermediate member  108  from the lower plate member  102 , and for maintaining a rotating engagement therewith for purposes of the present invention. Thus, the recess  140  is generally cylindrical, conforming to the cylindrical post  136 , with a conforming diameter and depth. This way, the intermediate member  108  is in complete contact with the top side of the lower plate member  102 . If desired, the post  136  may be made slightly longer than the recess  140  in the lower plate member  102  with which it mates so as to allow for slight “flexion” of the intermediate member  108  (since the middle member will not be in “complete contact” with the bottom member). Generally, however, full contact is desired.  
         [0043]     The post  136  remains free to rotate within recess  140  (allowing the intermediate member  108  to rotate as well), and the post  136  imposes a weight-bearing point of contact between the intermediate member  108  and upper mating surface  138  of lower plate member  102 . 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 post is preferably formed with a substantially planar 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. 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. 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.  
         [0044]     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, one example of which is described in detail herein.  
         [0045]     The locking feature shown in  FIGS. 1-3  entails forming the post  136  as a hollow annular member with side walls interrupted by a plurality of vertical notches, these notches defining snap-in spring fingers  137  located at the end of the post  136 . The post  136  fits into the recess  140  via the snap-in spring fingers  137 , the snap-in spring fingers  137  retracting a bit to allow insertion down past a slight lip in the recess  140  of lower plate member  100 . As the snap-in spring fingers  137  clear the lip they spring outward in the recess  140  to lock the post  136  in place, and yet allow 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.  
         [0046]      FIG. 2A  is a perspective view of intermediate member  108  with post  136 , and  FIG. 2B  is a side enlarged view of the intermediate member  108  and post  136  located in the center. As can be seen in both  FIGS. 2A-2B , the post  136  is formed with a distal lip or flange, and the side walls of hollow annular post  136  (including the lip) are interrupted by four evenly-spaced notches to define four snap-in spring fingers  137  located at the end of the cylindrical post  136 .  
         [0047]     In addition to the foregoing, the present invention also comprises a rotation-limiting feature to restrict relative rotation of the intermediate member  108  and lower plate member  100 .  
         [0048]     One example of an embodiment to accomplish this is best seen in  FIGS. 2A &amp; 2B , and comprises two flanking pins  117  protruding downwardly from the intermediate member  108  at opposite ends thereof. When the above-referenced post  136  is locked in place, the lateral pins  117  fit into two corresponding arcuate slots  119  located at each end of the inferior plate member  102 . The slots  119  both comprise approximately 20 degrees of arc to give +/−10 degrees of rotation of the intermediate member  108  relative to the lower plate member  100  either clockwise of counterclockwise (10 degrees on each side).  
         [0049]      FIGS. 4-6  are a perspective view, side view, and a side rotated view (90 degrees), respectively, of the intermediate member  108  as shown in  FIGS. 1-6  (using the locking feature of  FIGS. 1-3 ) 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  FIGS. 4-6 , the central snap-in spring fingers  137  are located in the center of intermediate member  108 .  FIGS. 4 and 6  show two lateral pins  117  which are located at opposite ends of intermediate member  108 .  FIG. 7  is an exploded view of the central snap-in spring fingers  137  which are located at the bottom of post  136 , and which snap into recess  140 .  
         [0050]      FIGS. 8-11  are a perspective view, side view, side view, and a top view, respectively, of the inferior (lower) plate member  102  (using the locking feature of  FIGS. 1-3 ), 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  112  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)  112  to the vertebra through the bore-hole shown extending through the tab  112 . 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. There are two slots  119  located on either side of the recess  140  of the lower plate member  102 , to receive the two corresponding lateral pins  117  located on the intermediate member  108 . The central snap-in spring fingers  137  at the end of the post  136 , snap into the recess  140  and thus prevent the post  136  from being withdrawn. Therefore, the post  136  is inserted by a simple snap-in operation which locks the pins  117  therein, preventing inadvertent withdrawal.  
         [0051]      FIGS. 12-16  are a perspective view, top view, side rotated view (90 degrees), side view, and sectional 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  110  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)  110  to the vertebra through the bore-hole shown extending through the tab  110 . 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 .  
         [0052]     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.