US 20040073307 A1
Intervertebral prosthesis (10) with at least one cover plate (11) which bears on a vertebral body (1) and is to be firmly connected thereto. In order to ensure that the prosthesis (10), during its insertion or during its lifetime, does not contribute to a protruding bone margin (5) breaking off, the cover plate (11) is provided with a bulge (12) corresponding to the depression (4) and/or with a bevel, a shortened end or rounded end (13).
1. Intervertebral prosthesis with at least one cover plate (11, 19) which bears on a vertebral body (1, 2) and is to be connected thereto, characterized in that the cover plate surface directed toward the vertebral body (1) has a bulge (12) protruding beyond the level of adaptation to the normal shape of the vertebral body.
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8. Intervertebral prosthesis according to one of
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14. Set of intervertebral prostheses which comprise at least one cover plate (11, 19), with a substantially flat cover plate surface, which bears on a vertebral body (1) and is to be connected thereto, characterized in that it comprises at least one intervertebral prosthesis whose cover plate surface has a bulge (20).
15. Set of intervertebral prostheses which comprise at least one cover plate (11, 19) which has a dorsal margin and which bears on a vertebral body (1) and is to be connected thereto, characterized in that it comprises at least one intervertebral prosthesis on whose cover plate the dorsal margin is shortened.
 The cover plates of the vertebral bodies between which the intervertebral disks of the spinal column are situated are generally flat or slightly concave. A pronounced concavity of the cover plates sometimes occurs, particularly in the lower region of the spinal column. Although this is not necessarily caused by injury, it can be. For example, it can sometimes happen that, after an intervertebral disk prolapse, the margin of a vertebral body impinges on the cover plate of the adjacent vertebral body and grinds out a depression there. Regardless of how the pronounced concavity has arisen, it leads to inadequate support of the dorsal margin, from which osteophytes may also project in some cases. If a prosthesis is now introduced into the intervertebral space, it can easily happen that the dorsal edge of the prosthesis impacts against the inadequately supported margin of the vertebral body or against osteophytes and thus breaks off part of the bone in the direction of the medullary canal, where it poses a danger to the spinal cord. The same thing can also happen if the prosthesis is implanted and, because of its insufficient support in the area of the depression, places too great a load on the margin of the vertebral body.
 Even in normally structured vertebral bodies, it can happen that, when an intervertebral prosthesis is inserted, the margin of the cover plate of the vertebral body is subjected to such stresses by the edge of the intervertebral prosthesis that it is damaged.
 It is the object of the invention to ensure that in such cases parts of the vertebral body cannot break off. To solve this problem for the first time, the invention provides various possibilities, namely those set forth in claims 1, 5, 8, 13, 14 and 15 and in their dependent claims.
 The first solution lies in the fact that, on its cover plate surface directed toward the vertebral body, the intervertebral prosthesis has a bulge which positions itself in a depression in the bone of the vertebral body in order on the one hand to obtain an improved support for the prosthesis there and on the other hand to protect the projecting bone margin next to the depression. This bulge rises above the usually flat basic shape, or, in adaptation to the natural shape of the vertebral, at the very most sligtly concave basic shape, of the cover plate surface. The bulge is compared to the flat or only slightly concave shape of the cover plates of the vertebral bodies or compared to prosthetic cover plates intended for normal cases of use.
 If the pronounced concavity or depression affects only part of the bone surface, then, accordingly, the bulge also advantageously has a smaller extent than the cover plate surface in the A-P and/or L-M direction. It preferably lies nearer to the dorsal margin of the cover plate surface than to its ventral margin. It goes without saying that, in terms of its height, extent and position, the bulge should be approximated as far as possible to the shape of the vertebra found. In this respect, it generally proves expedient if the bulge reaches as far as the dorsal edge of the cover plate surface.
 The bulge should be approximated as far as possible to the shape of the depression which is to receive it. For this purpose, simple geometric shapes often suffice, for example those of a portion of a cylinder with a transversely lying axial direction, a portion of a sphere, or a portion of an ellipsoid.
 According to a related solution of the invention, the margin of the cover plate surface directed toward the vertebral body is beveled or rounded. This ensures that, when the prosthesis is being inserted, the prosthesis margin does not come up sharp and hard against the dorsal bone margin, but instead slides along the latter. In addition, the bone margin is relieved of force transmission, which is taken over by other areas better able to provide support. This embodiment of the prosthesis is used in particular if the depression is not great and/or can be filled with bone cement. This embodiment can also be expedient if the vertebral body is normally structured and only the danger of excessive loading of the margin of the vertebral body by a prosthesis edge is to be avoided. This embodiment can also be combined with the aforementioned bulge. The bulge then forms the bevel or rounding or merges into these. The bevel or rounding is especially of interest on the dorsal side, but it can also be of advantage on the other sides of the prosthesis.
 The invention has further recognized that the teeth or projections which are provided on the cover plate of the prosthesis, in order to penetrate into the bone and anchor themselves there, can contribute to damaging the weakened dorsal margin of the vertebral body. They should therefore maintain a distance, from the dorsal edge of the cover plate, of at least 20%, preferably more than 30%, of the greatest dimension of the cover plate measured in the A-P direction. If the teeth are of a self-cutting and planar design, i.e. in the form of small plates whose main plane is transverse with respect to the bone surface, it can also be expedient for their main plane to extend in the sagittal direction. This applies in particular to those teeth which are arranged near the dorsal margin of the cover plate. This ensures that, upon insertion of the prostheses into the intervertebral space, these teeth subject the bone to only slight dorsally directed forces and thus protect the possibly weakened areas of the bone. Finally, it can be expedient for the teeth near the dorsal margin to be made smaller than the teeth arranged further in the ventral direction, for example half the height of the latter. In some cases it may suffice to limit this measure to one or just a few of the dorsal teeth, for example to the middle tooth within a group of three dorsal teeth, which tooth, if appropriate, can also be omitted so that there are only two dorsal anchoring teeth.
 As long as the injuries are typical ones often occuring in the same form, standardized cover plates with different sizes or shapes of the bulge, bevel or rounding can be provided. However, as it will often be necessary to take account of the particular shape and position of the bone depressions, it is preferable, according to the invention, to design at least the bulge, but if appropriate also the bevel or rounding, as a separate attachment part which can be connected to a suitable cover plate. It is then possible to provide quite a large number of different attachment parts. Of these, the one suitable for the specific case can be selected and connected to the cover plate, if appropriate also in a different chosen position. A collection of vertebral prostheses according to the invention can therefore provide a large number of different attachment parts and at least one cover plate, but preferably several different cover plates, connectable to these attachment parts. It may also suffice to have one cover plate which permits various possible arrangements of the attachment parts.
 A set of intervertebral prostheses according to the invention is characterized in that it comprises at least one intervertebral prosthesis whose cover plate surface has a bulge protruding farther than the cover plate surface in the other, normal prostheses.
 The invention is explained in more detail below with reference to the drawing which depicts advantageous illustrative embodiments. In the drawing:
FIG. 1 shows a sagittal section through a portion of the spinal column,
FIG. 2 shows the conditions upon insertion of a conventional prosthesis between a pair of vertebral bodies,
 FIGS. 3-5 show the corresponding configuration with prostheses formed according to the invention,
FIG. 6 shows the plan view of a cover plate surface and its arrangement of teeth, and
FIG. 7 shows the side view of a cover plate with modular bulge attachment.
 The pair of vertebral bodies 1, 2 enclose an intervertebral disk 3 between their mutually facing cover plates. Near its dorsal margin, the upper vertebral body 1 has a depression 4 in the surface of its lower cover plate. This depression can be a phenomenon resulting from degeneration or it can be a natural formation. Behind the depression 4, a bone margin 5 remains which, because of the absence of bone substance on its side toward the depression, is not well supported and is therefore susceptible to fracturing.
 If, in accordance with FIG. 2, a conventional intervertebral prosthesis 6 is inserted into the space between the spread-apart vertebral bodies 1 and 2 in the arrow direction, it can happen that the sharp dorsal edge 7 of the prosthetic cover plate strikes against the sensitive bone margin 5 and completely or partially breaks the latter off in the dorsal direction.
 This happens less readily in the first embodiment of a prosthesis according to the invention shown in FIG. 3. Near its dorsal margin, the cover plate 11 is provided with a bulge 12 which has been selected so that it corresponds approximately to the shape of the depression 4. Its surface can be approximated to the shape of a portion of a cylinder, with an axis lying preferably in the lateral-medial direction, or of a sphere or of an ellipsoid. In contrast to the conventional illustrative embodiment in FIG. 2, its dorsal margin 13 is not designed with a sharp edge, but is instead beveled or rounded. The bevel or rounding merges monotonously (i.e. without any substantial change of direction) into the surface of the bulge 12. Even if the prosthesis, during insertion, were to be upwardly directed obliquely rearward toward the vertebral body 1 so that the cover plate 11 slides along the surface of the vertebral body, there is no danger of the leading, dorsal margin 13 of the cover plate 11 coming up hard against the bone margin 5. It will instead slide gently along the latter without exerting any dangerous force on it. Likewise, in the inserted state, the bevel or rounding 13 ensures that no excessive forces are transmitted in the area of the bone margin 5. Instead, the cover plate 11 is supported, by virtue of its bulge 12, in the depth of the depression 4.
FIG. 4 shows an example in which the cover plate 14 of the prosthesis 15 does not have a bulge, but instead only a bevel 16 at its dorsal margin. During insertion, the bevel ensures that the rear edge of the cover plate 14 slides gently along the bone margin 5, if it touches the latter at all. FIG. 5 shows that, in the inserted state, the bevel 16 avoids direct contact between the cover plate 14 and the bone margin 5 and thus rules out any danger to the latter. Instead of this, the cover plate, whose normal dorsal limit is indicated in FIG. 6 by the line 24, can also be shortened or straightened dorsally, as is shown by the line 25. This embodiment of the invention is comparable to those prostheses of the same order of size which are provided for normal cases of use. It may be necessary to provide smaller prosthesis cores for the cover plates shortened at the dorsal end.
FIGS. 4 and 5 show that the cover plates of the prosthesis 15 are provided with teeth 17, 18 which have the shape of small triangular plates, their plane being perpendicular to the plane of the cover plate. They are made so sharp that they automatically sink into the bone surface when the spreading-apart of the vertebral bodies is reversed after insertion of the prosthesis. It is known to arrange such teeth parallel to the margin (EP-A-1057462). For the teeth 18 arranged along the ventral margin of the cover plate 14, this is also the case in the present illustrative embodiment. However, the teeth 17 arranged nearer the dorsal margin 16 are, on the one hand, at a considerable distance from this margin and, on the other hand, are aligned in the A-P direction, with the result that, during insertion into a narrow intervertebral space or during sinking into the bone surface or, later, during normal functioning of the prosthesis, said teeth cannot apply a damaging, dorsally directed force on the bone. Moreover, their height can be made smaller than that of the teeth 18. These measures can be employed jointly or independently of one another.
FIG. 7 illustrates the possibility of providing a cover plate 19 with differently shaped bulges 20. For this purpose, interacting securing means are provided which, in the simplest case, can consist of screws or pins. In the example shown, they comprise tenons and mortises 21 in the manner of a dovetail joint.
 The prosthetic cover plates are provided for a secure connection to the cover plates of the vertebral bodies. Suitable connecting means can be provided for this purpose. For example, the cover plates 6, 11, 14 are equipped with ventral flanges 22 which in a known manner can be used for screwing to the bone and which safeguard against inserting the prosthesis too far into the intervertebral space. Another example of anchoring the prosthesis to the bone is a biological coating which promotes bone growth and interconnection.
 An explanation of the prosthesis core has not been given here, because this prosthesis core is known in a wide variety of forms, which can also be used in conjunction with the invention.
 What is to be considered as a bulged cover plate and what is to be considered as a normal cover plate surface must be determined by comprison between the normal prostheses and the bulged prostheses of one and the same set of prostheses. The cover plate surfaces which are intended to bear on the vertebral body are in most cases basically flat. In some cases, the normal cover plate surface has a slight convex curve which complements the average curvature of the vertebral body surfaces. The bulge is then the convex area which rises above a correspondingly naturally flat or slightly curved imaginary surface of the cover plate. A bulge according to the invention is at any rate to be regarded as a part of the prosthetic cover plate whose radius of curvature in the sagittal plane is less than 40 mm, preferably 30 mm, or which rises above the arc 26 (see FIG. 7) which in the sagittal plane extends through the edges of the cover plate and has an arc height of 4 mm, preferably 3 mm, over the straight connection line of these edges.
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