Abstract:
A cervical intervertebral disk prosthesis has two cover plates, at least one of which is provided with a wedge-shaped connection surface for connection to a vertebral body and which is wider than it is deep. The dorsal edge of the connection surface may be set back from the dorsolateral edge of the cover plate having the connection surface and be connected to the dorsal edge, protruding above it, via a rounding or bevel.

Description:
BACKGROUND OF THE INVENTION 
     Endoprostheses for replacement of an intervertebral disk of the cervical spine are known (FR-A-2 718 635, EP-B-699 426) which consist of two cover plates and a joint core. The cover plates, which are arranged approximately parallel on both sides of the core, have connection surfaces which are intended for connection to the adjacent vertebral bodies. The cranial vertebral body cover plates, which are to be connected to a lower prosthesis cover plate, have a roughly rectangular shape. They are approximately flat and are delimited at the sides by edge serrations. The caudal vertebral body cover plates have extensive edge serrations at the ventral edge. These have to be removed before the prosthesis is fitted. It is also expedient for the vertebral body surfaces which are intended to bear on the prosthesis to be worked in order to adapt them to the connection surfaces of the prosthesis. 
     The connection surfaces of the known prostheses mentioned are circularly delimited. Since the end plates of the vertebral bodies have approximately the shape of a rectangle whose width is substantially greater than its dimension in the anteroposterior direction, they do not exploit the size of the naturally occurring surfaces for force transmission. Accordingly, between the connection surfaces of the prosthesis and the bearing surfaces on the vertebral bodies, greater forces occur than would be the case if the surfaces were better utilized. In the case of intervertebral disk endoprostheses intended for the lumbar spine, the best utilization of space is achieved by using an oval prosthesis contour (WO 0 101 893, EP-B-471 821, EP-A-747 025) or a kidney-shaped configuration (EP-A-747 025), because the cover plates of the vertebral bodies have an oval to kidney-shaped configuration. Rectangular prosthesis shapes are also known (U.S. Pat. No. 5,425,773). 
     SUMMARY OF THE INVENTION 
     The object of the invention is to make available an endoprosthesis for replacement of the intervertebral disk in the cervical region of the spine which permits the best possible utilization of space. Since the vertebral body attachment surfaces are approximately rectangular, a similarly approximately rectangular or oval configuration of the connection surfaces of the prosthesis would at first sight seem most appropriate. However, the invention has recognized that better and more reliable results are achieved by using a prosthesis in which at least one of the two connection surfaces, but preferably both connection surfaces, has a configuration in which the connection surface has side edges, a dorsolateral edge and an anteriolateral edge, the side edges inclining toward an imaginary mid-line extending from the ventral edge to the dorsal edge of the cover plate so as to form a wedge shape. The distance from the side edges to their corresponding adjacent dorsal corners of an imaginary rectangle circumscribing a circumferential contour of the cover plate is greater than the distance of the anteriolateral edge from corresponding adjacent ventral corners of the imaginary rectangle. 
     When shaping the vertebral body bearing surfaces for the prosthesis from the ventral direction, the operating surgeon must keep a safe distance from the spinal canal. This applies in particular when shaping the dorsolateral corners of the bearing surfaces, where visual control is more difficult than in the central dorsal region. This means that in the dorsolateral corners of the vertebral body cover plates bone material may be left which protrudes beyond the worked bearing surface of the vertebral body. If one were to use a rectangular prosthesis, this would not be able to be inserted fully into the intervertebral space because its dorsal corners would strike against the bone material which has been left. If the prosthesis corners are rounded, the circumstances are not much more favorable. The prosthesis then protrudes ventrally beyond the vertebral bodies and may cause irritation of the esophagus or the large blood vessels located there. The set-back, according to the invention, of the prosthesis connection surfaces spares the dorsolateral corners and thereby avoids this disadvantage. By contrast, the surface area of the connection surfaces in their anterolateral regions remains unchanged. 
     The measure according to the invention can be best described by reference to the distance of the limit of the connection surface from the corner points of an imaginary rectangle which circumscribes the cover plate. It is assumed here that the sides of the rectangle which are tangential to the circumferential contour of the cover plate extend parallel to the sagittal plane or perpendicular thereto. In this context, only those parts intended for accommodation in the intervertebral space are considered as the cover plate. If, for example, a ventral flange is connected to the cover plates and, in the implanted state, lies in front of the ventral end face of the vertebral body, this is not counted as part of the cover plate in the construction of the imaginary rectangle. 
     The minimum distance of the limit of the connection surface from the dorsal corners of the imaginary rectangle is preferably at least 1.3 times as great, more preferably at least 1.5 times as great, still more preferably at least twice as great as the minimum distance from the ventral corners. It is preferably assumed here that the width of the cover plate is greater than its anteroposterior dimension, generally by a factor of 1.1 to 1.6, preferably by a factor of 1.2 to 1.4. 
     In general, the connection surfaces of both prosthesis cover plates are configured in accordance with the invention. There are cases, however, where it is sufficient to do this on one of the two connection surfaces, specifically in particular the lower one. 
     In a particularly preferred embodiment of the invention, the edge of the prosthesis cover plate extends beyond the dorsolateral limit of the associated connection surface. In this way, the cover plate, the prosthesis core and the connection or slide surfaces interacting on the cover plates and the prosthesis core can have an optimum size despite the set-back nature of the connection surface. These parts can therefore be designed, for example, roughly as a rectangular shape with rounded corners. 
     The transition between the dorsolateral set-back limit of the connection surface to the farther extended edge takes place in the form of an inclined transition surface at both the dorsolateral regions of the cover plate. This can directly adjoin the connection surface. The advantage of this is that the transition surface too can if appropriate participate in the transmission of forces, namely in contact with the bone parts which have been left in the dorsolateral region and protrude beyond the worked bearing surface of the vertebral body. The cross section of the transition surface can be straight, convexly rounded or even stepped. 
     The term dorsolateral designates the transition region from the lateral aspect to the dorsal aspect of the cover plate. The set-back, according to the invention, of the connection surface is accordingly not restricted only to a dorsal region or a lateral region but instead includes portions of both regions. The term set-back relates to a standard contour of the cover plate which to the front and rear is approximately symmetrical to the middle transverse axis. The limit of the connection surface is set back relative to this standard contour. In case of doubt, the standard contour is the anterolateral contour of the cover plate mirrored about the middle transverse axis. If the cover plate has an edge protruding beyond the limit of the connection surface, which edge does not protrude farther than the standard contour, the set-back of the limit of the connection surface can also be related to the contour of this edge. 
     Viewed from the lateral direction, the portion in which the limit of the connection surface is set back extends generally over at least one third of the antero-posterior dimension of the connection surface. Viewed from the dorsal direction, this portion generally extends over at least approximately one quarter of the width of the connection surface respectively on both sides. It may be expedient if the connection surface, in a central portion of the dorsal plate edge, is not set back in relation to the latter or is set back less than in the dorsolateral region. This middle portion expediently extends over at least one quarter of the width of the connection surface. 
     In the ventral third of the lateral sides of the cover plate, the limit of the connection surface is preferably not set back. Slight roundings of the edges of the connection surface are not taken into account here. The set-back of the connection surface limit is preferably at its greatest where it is nearest to the dorsal corner of the imaginary circumscribing rectangle. The set-back limit of the connection surface then has an approximately rectilinear course. 
     At the place where the difference in height between the set-back limit of the connection surface and the edge protruding above it is at its greatest, the difference in height should be at least approximately 1 mm. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below with reference to the drawing which depicts an advantageous illustrative embodiment, and in which: 
         FIGS. 1 and 2  show perspective views of the whole prosthesis, 
         FIG. 3  shows a dorsal side view of the prosthesis, 
         FIG. 4  shows a lateral view of the prosthesis, 
         FIG. 5  shows a top view of the outer face of the prosthesis, 
         FIGS. 6 to 8  show different profiles of the transition surface, 
         FIG. 9  shows a plan view of an alternative embodiment, and 
         FIG. 10  shows an enlarged representation of the connection surface and the edge contour. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The prosthesis shown in  FIGS. 1 to 8  consists of two cover plates  1  and of a prosthesis core  2 . The inner face of one of the two cover plates can be provided as a mounting for the prosthesis core  2 , while the other cover plate forms, with the prosthesis core, a spherical slide surface  3 , for example. At the ventral edge of each cover plate there is a flange  4  whose dorsal face  5  is intended to bear on the ventral face of a vertebral body. The flange can have securing means, for example screw holes  6 . Each prosthesis cover plate has a connection surface  10  for bearing on the associated vertebral body cover plate and on the bearing surface created by shaping the vertebral body, said connection surface  10  extending approximately parallel to the main plane of the cover plate. It is substantially flat, but can also have a slight curvature. It is equipped with teeth  13  and/or other means for connecting it securely in position to the bone, for example with a biologically active coating. 
     Each cover plate is surrounded by a circumferential surface  14  which is approximately perpendicular to the connection surface  10  and which in the present context is designated as an edge. The edge  14  defines the circumferential contour of the cover plate, which has an oblong shape, which can be designated as oval or as rectangular with rounded corners. It is also designated by reference number  14  in  FIG. 10 . The prosthesis core  2  has the same circumferential contour. The anteroposterior depth dimension  11  of the cover plates  1  is smaller than their width dimension  12 , specifically for example in the ratio of three to four. An example which has proven useful has a depth of 15 mm and a width of 20 mm. Its extent is adapted to the vertebral body cover plates and is slightly smaller than these, so that the prosthesis fits into the intervertebral space which is available and has been shaped between the vertebral bodies. If appropriate, it is possible to provide different size categories of prostheses. The cover plates  1  are preferably made of metal, and the core  2  of polyethylene. 
     The cover plate  1  or the contour of the edge  14  has a ventral face  15 , a dorsal face  16  and lateral faces  17  and  18 . Between these there extend the anteroposterior depth dimension  11  and the width dimension  12 , which in  FIG. 10  are indicated as sides of an imaginary rectangle  21  circumscribing and touching the cover plate or the contour  14  of the edge. Only the part of the cover plate  1  lying behind the flange face  5  is taken into consideration here, since it is only the dimension of the part which will lie in the intervertebral space which is relevant here. 
     On the ventral face  15  and in the ventral half of the lateral faces  17 ,  18 , the limit of the connection surface  10  follows the contour of the edge  14 . They do not need to correspond exactly, because slight deviations in the form of roundings or bevels can be present. The width of these is generally less than 3% of the width dimension  12 . 
     In the transition region from the lateral faces  17 ,  18  to the dorsal face  16  of the edge, which in the present context is designated as the dorsolateral region, a portion  23  of the limit  24  of the connection surface  10  does not follow the contour  14  but is instead set back relative to the latter. The greatest extent of the set-back, in a plan view, lies between approximately 2 and 5 mm, generally of the order of 2.5 to 3.5 mm. The minimum distance  28  from the corner  29  of the circumscribing rectangle  21  is greater than 13% and preferably greater than 15% and preferably greater than 18% of the width dimension  12 . In an illustrative embodiment which has proven useful, it is slightly more than 20% of this width. 
     The set-back limit  24  relative to the contour  14  is obtained by providing an oblique transition surface  25  between the limit  24  and the edge. Since the transition surface is raised in relation to the plane of the connection surface, a space is obtained below it for receiving the bone parts which have been left in the dorsolateral region upon shaping of the vertebral body surface. It will be appreciated that this space must have an adequate height for this purpose. It should be at least 1 mm at the place of the maximum height difference between the connection surface and the edge  14 . The transition surface  25  can, for example, be convexly rounded ( FIG. 6 ) or have a straight profile ( FIG. 7 ). It can also be cut with a step ( FIG. 8 ). 
     In a side view of the example illustrated, the portion  23  in which the limit  24  is set back relative to the contour  25  of the edge  14  takes up approximately two thirds of the depth dimension  11  and is preferably not less than one third thereof. In the front third, the contour of the cover plate does not deviate from the limit of the connection surface. Slight roundings of the edges are not taken into consideration here. 
     Viewed from the dorsal direction ( FIG. 3 ), the set-back of the limit  24  of the connection surface decreases toward the center. In a central portion  22  of the dorsal face  16 , the edge  14  in practice does not protrude beyond the limit  24 . The length of this central portion is preferably between one tenth and one third of the width dimension  12 . 
     The set-back portion  23  of the limit  24  of the connection surface  10  in the dorsolateral region has an approximately rectilinear extent. If one compares it to a rectilinear course connecting the end points of this portion to one another, it scarcely deviates from this course. Any deviation is well below 10% of the length of this course. The end points of said course lie at those points where the deviation of the limit  24  of the connection surface from the circumferential contour  14  goes beyond the extent of the edge roundings otherwise provided. 
     The deviation of the limit  24  from the circumferential contour  14  in the dorsolateral region is approximately 10% of the cover plate width  12  and should be not less than 5%. 
     A set-back of the limit of the connection surface in the dorsolateral region—corresponding to the explanations given above—is also shown in the embodiment according to  FIG. 9 , which differs from that of  FIGS. 1 to 8  in that the edge  14  does not extend beyond the limit of the connection surface. The description of the illustrative embodiment in  FIGS. 1 to 8  and  FIG. 10  correspondingly applies to this illustrative embodiment, except that the course of the limit of the connection surface in the dorsolateral region is not compared to the circumferential contour  14  but instead to the course  14 ′, mirrored about the central transverse axis  21 , of the anterolateral circumferential contour  14  of the cover plate. 
       FIG. 10  illustrates the different course of the limit of the connection surface in the anterolateral region and dorsolateral region, by reference to the corners of the imaginary rectangle  30 . It will be noted that the minimum distance  28  of the dorsal corner point  29  from the limit line  24  is almost three times as great as the corresponding minimum distance  26  of the front corner point  27 . It should normally be at least 1.5 times as great.