Abstract:
In an intervertebral implant with a top contacting element, a bottom contacting element and arranged between these a core which with an upper, spherical bearing surface engages a spherical bearing socket, having the same radius, of the top contacting element and with a lower, spherical bearing surface engages a spherical bearing socket, having the same radius, of the bottom contacting element, in order to improve the kinematics and the endurance, it is proposed that the radius of the bearing surface and of the bearing socket receiving it differ on opposing sides of the core at least by the factor 5.

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to an intervertebral implant with a top contacting element, a bottom contacting element and arranged between these a core which with an upper, spherical bearing surface engages a spherical bearing socket, having the same radius, of the top contacting element and with a lower, spherical bearing surface engages a spherical bearing socket, having the same radius, of the bottom contacting element. 
   Such an intervertebral implant is known, for example, from U.S. Pat. No. 5,401,269. Herein the bearing surfaces and the associated bearing sockets on the upper side and on the underside of the core have the same radius, i.e., the core is lens-shaped. Pivoting movement of the two contacting elements relative to each other automatically results in a translational movement of the two contacting elements towards each other. Moreover, no defined kinematics are achievable with such an arrangement. For, upon pivoting movement of the contacting elements relative to each other, there are various possibilities of relative movement, depending on whether upon pivoting movement of the two contacting elements relative to each other, the core remains unpivoted in one contacting element and upon pivoting movement only the other contacting element moves relative to the core, whether conversely the core pivots only relative to the one contacting element and remains unpivoted relative to the other contacting element or whether a situation between these arises where both contacting elements are displaced relative to the surface of the core. If the core remains at rest relative to one of the two contacting surfaces during this pivoting movement, the entire movement of the contacting element takes place exclusively around the center point of the bearing surface relative to which the contacting element pivots, i.e., in one case around the center point of the top contacting surface, in the other case around the center point of the bottom contacting surface, and if the core moves relative to both bearing surfaces, an undefined movement results, which is made up of pivoting movements around both center points. 
   In particular, in order to keep the undesired translational movement as low as possible, it is also known to allow the core to engage on one side only with a spherical bearing surface in a corresponding bearing socket of a contacting element and to provide on the opposing side a plane bearing surface between the core and the other contacting element (U.S. Pat. No. 5,507,816). The core can then deviate laterally on the plane bearing surface and compensate the undesired translation. Owing to the relatively high forces acting by way of the ligaments and muscles on the adjacent vertebral bodies, the vertebral bodies are clamped strongly against one another and, upon pivoting movement of the contacting elements, this may result in the core being driven laterally out of the space between the contacting elements, i.e. the core will possibly move on the plane bearing surface further out of the position of rest than is expedient for the physiological bearing of the vertebral bodies. 
   The object of the invention is to so design an intervertebral implant of the kind described at the outset that, on the one hand, a defined movement sequence can be ensured and that, on the other hand, the undesired translational movement during the pivoting of the contacting elements is kept as small as possible. 
   SUMMARY OF THE INVENTION 
   This object is accomplished in an intervertebral implant of the kind described at the outset in accordance with the invention in that the radius of the bearing surface and of the bearing socket receiving it differs on opposing sides of the core at least by the factor 5. 
   A core is thus used, which has spherically outwardly curved bearing surfaces on both sides, which engage in corresponding bearing sockets, having the same radius, of the contacting elements, but on one side the radius of this bearing surface is selected so as to be very large, whereas on the other side it is designed in a way similar to that in previously known cores and ensures good pivoting of the contacting elements. On the side on which the bearing surface and the bearing socket have a very large radius the core moves on the surface of the bearing socket similarly to on a plane surface, but a localization of the core in the area of this bearing socket nevertheless results, as the core is pressed into the bearing socket by the high forces clamping the vertebral bodies against one another. A position of rest is thus obtained for the core. However, a pivoting of the core in this bearing socket with a very large radius does not result in any great angular changes between top and bottom implant end plates as the radius is very large. 
   The factor between the radii of the two bearing surfaces and the two corresponding bearing sockets may be between 4 and 10, in particular, in the range of between 6 and 8. 
   While the smaller radius lies in the order of magnitude of between 5 and 10 mm, the radii for the bearing surface with the larger radius will lie between 80 and 120 mm. This results in very flat bearing sockets, with the maximum depth of such a bearing socket lying, for example, between 0.2 and 1.0 mm. 
   It is advantageous for the bearing surface of the core with the larger radius to lie on its underside, but, in principle, the reverse arrangement would also be possible. 
   In a preferred embodiment of the invention it is provided that the bearing socket with the larger radius is machined in the plane bottom of a recess in the adjacent contacting element. 
   Use of a spherical bearing socket also has the great advantage that with a conventional turning tool a surface of this bearing socket can be produced, which is machined substantially more precisely than is possible with a milling tool such as normally used for producing a plane bearing surface. The grinding work required during the manufacture is thus reduced, and, in addition, this improved surface structure results in less abrasion during use and, therefore, in higher durability. 
   It can be provided that the recess in the contacting element is delimited by side walls, and the side walls preferably extend perpendicularly to the bottom of the recess. 
   In a preferred embodiment the core comprises on two opposing sides lateral guiding surfaces which rest on the parallel side walls of the recess and thereby secure the core against pivoting transversely to the longitudinal direction of the guiding surfaces. By way of such a design the pivoting movement of the core in the bearing socket with a larger radius can be reduced to a pivoting movement which occurs in one direction only, which is preferably the anterior-posterior direction. 
   Furthermore, stops may be provided for delimiting the pivoting movement of the core in the longitudinal direction of the guiding surface. It is advantageous for these stops to be formed by the side walls of the recess. 
   When plane bearing surfaces are used between core and contacting element it is also known to delimit the displacing movement by stops. In these cases, the core can be moved in an unbraked manner towards the stops during such displacement, and this results in increased wear. By providing a bearing socket with a large radius as bearing for the core, this movement of the core towards the stops is braked owing to the high forces between the adjacent vertebral bodies during deflection of the core, i.e., in this case the core strikes the stops in a damped manner, and this reduces the wear and thus increases the lifetime of the implant. 
   It is advantageous for the center point of the bearing socket with the larger radius to lie closer to the anterior side wall of the recess than to the posterior side wall. In the normal state, the core thus rests against the stop of the anterior side wall and upon pivoting of the contacting elements is at the most displaced in posterior direction, but not in anterior direction. This corresponds to the physiological behavior of the vertebral bodies during a pivoting movement. 
   The spacing of the stops can be between 1 mm and 3 mm larger than the dimensions of the core between its outer sides which come to rest against the stops. The pivoting movement of the core in the bearing socket of larger radius is thus limited to a relatively narrow pivoting range. In this way an undesirably large translation is avoided, but, on the other hand, owing to the pivoting of the core relative to the top and bottom contacting elements it is made possible for the implant to adapt optimally to the respective kinematic requirements. 
   The spherical bearing surface with the larger radius can extend over the entire cross-sectional area of the core. This can also be the case for the spherical bearing surface of smaller radius. 
   It is advantageous for the core to have between its two bearing surfaces an outer surface which passes over at an angle into the two bearing surfaces. 
   The outer surfaces can extend parallel to a connection line between the highest point and the lowest point of the two bearing surfaces. 
   In a modified embodiment it can be provided that the core has at its posterior end lateral projections which engage under a projection of the recess. A securing of the position of the core in the recess is thereby obtained. In extreme implanting situations and under unphysiological stress, for example, during a fall, the core is thereby secured against snapping out of the recess. The projections on the core can be formed by a stepped formation of the core. 
   In particular, it can be provided that the core comprises at the posterior end a plate-shaped bottom part whose underside forms the bearing surface with the larger radius, and a disc-shaped top part whose upper side forms the bearing surface with the smaller radius, and that the bottom part projects laterally over the top part and thereby forms the projection on the core. 
   The bottom part can have a rectangular cross-section with rounded off corners. 
   The top part preferably has a circular cross-section. 
   In a particularly preferred embodiment it is provided that the bottom part has a square cross-section with an edge length equal to the radius of the top part. The bottom part thus projects on all four sides in the shape of a crescent over the top part and forms corresponding projections there. 
   The bottom part can be thicker at the anterior side than at the posterior side. 
   Bottom part and top part of the core are preferably of one-piece design. 
   The projection of the recess can preferably be formed by the side wall of a groove which is machined in the recess along the posterior side wall thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description of preferred embodiments of the invention serves in conjunction with the drawings for further explanation. The drawings show: 
       FIG. 1 : a side view of an intervertebral implant inserted between two vertebral bodies with contacting elements sectioned in anterior-posterior direction; 
       FIG. 2 : a perspective view of the bottom contacting element and the core to be inserted therein; 
       FIG. 3 : a schematic side view of a modified embodiment of an intervertebral implant with the contacting elements pivoted to an extreme degree; and 
       FIG. 4 : a perspective view of the intervertebral implant of  FIG. 3  prior to insertion of the core into the recess of the bottom contacting element. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The intervertebral implant  1  shown in the drawings comprises a top contacting plate  2  and a bottom contacting plate  3 , between which a core  4  is arranged. The two contacting plates  2  and  3  preferably consist of a biocompatible metal, the core  4  of a biocompatible plastic material, for example, of polyethylene. The contacting plates  2  and  3  have on their outer sides facing away from each other contacting surfaces  5  and  6 , respectively, which come to rest against the end faces of two vertebral bodies  7 ,  8 , between which the intervertebral implant  1  is inserted instead of the previously removed intervertebral disc. 
   The one-piece core  4  comprises a plate-shaped central portion  9  with vertical outer surfaces  10 . Adjoining the upper side of the central portion  9  is a spherical cap-shaped bearing surface  11 , which engages a spherical cap-shaped bearing socket  12  having the same radius on the side of the contacting plate  2  opposing the contacting surface  5 , thereby producing a pivotal bearing in this area. 
   In the side of the contacting plate  3  opposing the contacting surface  6  of the bottom contacting plate  3  a recess  13  is machined, which is delimited on all sides by a vertical side wall  14  and has a flat bottom  15 . There is machined in this flat bottom  15  a spherical cap-shaped bearing socket  16 , in which a likewise spherical cap-shaped bearing surface  17  of the core  4 , having the same radius R and adjoining the central portion  9  on the underside, engages. 
   The radius of the upper bearing surface  11  and the upper bearing socket  12  is substantially smaller than the radius of the lower bearing surface  17  and the lower bearing socket  16 . The ratio of the dimensions is at least 1:5, and preferably lies in the range of between 1:6 and 1:8. For example, the radius of the upper bearing surface  11  and the upper bearing socket  12  may be in the order of magnitude of 15 mm, the radius of the lower bearing surface  17  and the lower bearing socket  16 , in contrast, in the order of magnitude of 100 mm. In this way, the depth of the bearing socket  16  is very low, with this depth lying, for example, in the order of magnitude of between 0.2 mm and 1.0 mm. 
   The side wall  14  of the recess  13  comprises on opposite sides of the recess  13  two parallel, straight-lined sections  18 ,  19 , which extend in anterior-posterior direction of the intervertebral implant  1  and form a guiding surface for the core  4 , whose outer surface  10  likewise comprises on opposite sides two flat sections  20 ,  21  extending parallel to each other. The spacing of these sections  20 ,  21  corresponds to the spacing of the sections  18 ,  19 , so that the core  4  is received in the recess  13  so as to be displaceable in anterior-posterior direction, but not displaceable transversely thereto. A pivoting of the core transversely to the anterior-posterior direction is, therefore, excluded. A pivoting is only possible in anterior-posterior direction, whereas the top contacting plate  2  is freely pivotable in all directions of the core  4 . 
   This free pivoting movement is limited by the outer surface  10  of the core  4  striking the side wall  14  of the recess  13 . Both the side wall  14  and the side wall  10  comprise for this purpose on the anterior side of the intervertebral implant  1  straight-lined sections  22  and  23 , respectively, which extend perpendicularly to the sections  18 ,  19 ,  20  and  21 . On the side opposing the sections  22  and  23 , the outer surface  10  and the side wall  14  are of circular design and pass continuously into the sections  18 ,  19  and  20 ,  21 . The bearing socket  16  in the recess  13  is not arranged exactly at the center in the recess  13 , but is slightly displaced in the direction towards the section  22 , so that when the section  23  rests against the section  22  the core  4  is in the physiologically correct position between the two vertebral bodies  7  and  8 , as shown in  FIG. 1 . This is the position of rest of the core  4 . 
   Upon pivoting of the two contacting plates  2 ,  3  relative to each other, the top contacting plate  2  slides substantially on the upper contacting surface  5 , with the core  4  initially remaining in its position of rest and only being pivoted in the bearing socket  16  upon stronger pivoting of the contacting plate  2 . The bearing socket  16  thus exercises a restoring force, which pivots the core  4  into the position of rest. Nevertheless, the core  4  may, if required, be slightly pivoted in the bearing socket  16  and thereby reduces the translatory displacement of the contacting plates  2 ,  3  when these are pivoted relative to each other. 
   The spacing between the section  22  of the bottom contacting plate  3  and the opposing side of the side wall  14  of the recess  13  is slightly larger than the dimensions of the core  4 , so that the core  4  may be pivoted slightly out of the position of rest in the bearing socket  16 , but the pivoting movement is limited. For example, the spacing between the core  4 , on the one hand, and the side wall  14  of the recess  13 , on the other hand, may lie in the position of rest of the core  4  between 1 mm and 3 mm. 
   The intervertebral implant of  FIGS. 3 to 4  is of similar design to that of  FIGS. 1 and 2 . Therefore, corresponding parts bear the same reference numerals. 
   In the embodiment of  FIGS. 3 and 4 , the core is divided into two portions, namely a plate-shaped bottom part  24  and a disc-shaped top part  25 . The plate-shaped bottom part  24  has a rectangular cross-section with rounded off corners and is thinner in the posterior portion than in the anterior portion. The top part  25  has a circular cross-section. The radius of the top part  25  corresponds to the edge length of the bottom part  24 , so that the disc-shaped top part  25  touches the edges of the square bottom part  24  on all four sides at the center thereof. 
   In its corner areas, the plate-shaped bottom part  24  therefore protrudes in the shape of a crescent over the top part  25  and forms projections  26  there. The underside of the bottom part  24  is formed by the bearing surface with the larger radius, the upper side of the top part  25  by the bearing surface with the smaller radius. 
   There is machined in the posterior side wall of the recess  13  a groove  27  which extends along this side wall, so that the bottom  15  passes over into a side wall of this groove  27 . The thus enlarged recess  13  is covered by the opposing side wall of the groove  27 , which thus forms a projection  28 . 
   When the core  4  is inserted into the recess  13 , the projections  26  engage the groove  27  and a securing against unintentional lifting of the core  4  off the bottom contacting plate  3  is thereby ensured. Even in the event of an unphysiological movement which may occur, for example, during a fall, it is thereby ensured that the core will not be able to snap out of its bearing position.