Intervertebral implant

[0001] This application is related to and claims the benefit of German Utility Model No. 203 13 183.5 entitled Intervertebral Implant issued on Oct. 16, 2003, and German Patent Application No. 103 39 170.3 filed Aug. 22, 2003.

[0002] The present invention pertains to an intervertebral implant with which the original height of the intervertebral disk can be restored, e.g., in case of degeneratively altered intervertebral disks, and the function can be preserved at the same time.

[0003] Intervertebral implants may be based on a ball and socket joint, i.e., they make pivotability possible in the same way in all directions. For example, an intervertebral disk prosthesis with two metallic end plates and an intermediate part made of polyethylene, in which a convex bearing surface slides on a concave surface, is described in WO 01/01893. In this prosthesis, the rotation center is located centrally in the middle between the anterior and posterior edges of the support bodies designed as metal plates. An intervertebral disk prosthesis in which the rotation center is displaced in the dorsal direction is described in U.S. Pat. No. 5,258,031.

[0004] There remains a need for an improved intervertebral implant of the type described such that it is optimized in terms of wear, kinematics, and load distribution.

[0005] The present invention pertains to an intervertebral implant with an upper support body and a lower support body, which are supported at each other via a joint in such a way that they are pivotable in relation to one another. This implant is used as a replacement for an intervertebral disk, and the original height of the intervertebral disk can be restored by means of this implant while the function is preserved at the same time.

[0006] The joint is designed as a saddle joint, in which two saddle-shaped joint surfaces are in contact with one another, rotated by 90° in relation to one another.

[0007] Consequently, each of the two saddle-shaped joint surfaces has a convex contour in one direction and a concave contour in the direction extending at right angles thereto, and the vertex of the convex contour and the lowest point of the concave contour coincide. The joint surfaces are consequently curved in one direction opposite the curvature in a direction extending transversely thereto. The two saddle-shaped joint surfaces thus designed are rotated by 90° in relation to one another around a perpendicular axis of the intervertebral implant, so that pivoting or rotation of the joint surfaces around two axes that are at right angles to one another, which are in the plane of the intervertebral space, is possible.

[0008] While the centers of the pivoting movement around all axes coincide in a ball and socket joint, the centers of the pivoting movement around mutually perpendicular axes in a saddle joint are different. Thus, the fulcrum for flexion/extension is in the vicinity of the lower support body in a preferred embodiment, whereas the fulcrum for the lateral flexion is in the vicinity of the upper support body. Thus, such a saddle joint comes closer to the physiological conditions of the normal intervertebral disk than a ball and socket joint that has only one center for all rotary movements.

[0009] It is favorable if one joint surface is directed in the anterior-posterior direction and the other in the lateral direction.

[0010] Furthermore, it is advantageous if the saddle joint is arranged between the middle of the support bodies and the dorsal edge of the support bodies, so that the pivoting movement takes place around centers that are displaced in the dorsal direction.

[0011] It is especially advantageous if the joint surfaces consist of a ceramic, because such articulating surfaces are not subject to wear of any significance. The problem of creep under load, which cannot be avoided in case of the use of polyethylene as a joint surface material, is also eliminated with such joint surfaces.

[0012] It is also advantageous in the case of the use of ceramic that smaller dimensions can be selected for the joint components because of the very high compressive strength of the ceramic, and during the flexion/extension movement, these smaller radii reduce the translational motion superimposed to this movement. It is advantageous, for example, if the radius of the joint surfaces is between 4 mm and 7 mm and preferably equals about 5 mm. Translational motion[s] amounting to less than 2 mm during a full extension of about 14° can thus be achieved.

[0013] Provisions may be made in a preferred embodiment for the support body and the joint surface to be made of ceramic in one piece.

[0014] Provisions are made in another embodiment for the joint surfaces to be part of a joint body, which is inserted into the support body. The joint body can be inserted into the support body without clearance in this case.

[0015] For example, the joint body may be held in the support body by means of a conical clamping, and the support body can be shrunk onto the joint body, or the joint body can be fixed at the support body by means of locking screws or by means of elastic intermediate elements.

[0016] It is especially advantageous if at least one of the joint bodies is rotatable in relation to the support body receiving it around an axis of rotation extending at right angles to the two pivot axes of the saddle joint. There is a limitation to two pivot axes in a saddle joint, and the joint surfaces are only in a punctiform contact with one another during rotation around the axis of rotation extending at right angles to these two pivot axes, and they lead as a result to an increase in the distance between the two support bodies. To avoid this, at least one of the joint bodies is designed such that it is rotatable in relation to its support body, so that rotation around all three mutually perpendicular pivot axes or axes of rotation becomes possible in this embodiment.

[0017] Provisions are made in a first preferred embodiment for the rotatable joint body to be received rotatably in a rotationally symmetrical recess of the support body.

[0018] In another embodiment, the rotatable joint body has a multipart design with a bearing part held rigidly in the support body and with a joint surface part rotatable around the axis of rotation.

[0019] It is advantageous in these embodiments if a layer made of a low-friction and/or wear-reducing material, for example, a ceramic layer, is arranged between the joint body and the support body (designated by reference numeral 24 in FIG. 9) and between the bearing part and the joint surface part (designated by reference numeral 26 in FIG. 9). It is ensured hereby that the friction is reduced and the wear is diminished in the contact area of the parts that are rotatable in relation to one another.

[0020] In a preferred embodiment, the bearing part and the joint surface part may be connected with one another rotatably via a central bearing journal.

[0021] Provisions are made in another preferred embodiment for the joint surface part to be received rotatably in a bearing shell of the bearing part.

[0022] In a modified type of embodiment, the support body may consist of a metal, especially titanium, a titanium alloy, or a chromium-cobalt alloy.

[0023] The use of special plastics for the support body, e.g., the use of polyether ether ketone (PEEK™ manufactured by Victrex® PLC of the United Kingdom), is also favorable; such a plastic is transparent to X-rays and makes possible a trouble-free observation of the operating area by means of X-rays.

[0036] Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.

[0037] The intervertebral implant 1 shown in the drawings comprises two plate-like support bodies 2, 3, which preferably consist of titanium or a titanium alloy, or another metal that is compatible with the body, and carry respective, perpendicularly projecting anchoring surfaces 4 and 5 on their sides facing away from each other. As shown in FIG. 1, the plate-like support bodies 2, 3 are placed on the underside and the top sides of two adjacent vertebral bodies 6 and 7, and the anchoring surfaces 4, 5 immerse into the vertebral bodies 6 and 7, respectively.

[0038] On their sides facing each other, the two support bodies 2, 3 carry a respective joint body 8 and 9, which are supported at each other and permit a pivoting movement of the two support bodies 2, 3.

[0039] The joint bodies 8, 9 consist of ceramic in the exemplary embodiment shown, and are inserted into a respective recess 10 and 11 of the corresponding support body 2, 3, as shown in FIG. 3. The recesses 10, 11 are not arranged in the middle of the support bodies 2, 3, but are displaced in the direction of the dorsal edge 12, 13 of the support bodies 2, 3, so that the centers of the recesses 10, 11 are at a distance from the ventral edges 14, 15 of the support bodies 2, 3 that is twice the distance from the dorsal edge 12, 13.

[0040] Both joint bodies 8, 9 form a joint surface 16 and 17, respectively, and the two joint bodies 8, 9 are supported at each other via these joint surfaces 16, 17. Each of these joint surfaces 16, 17 is designed as a saddle-shaped joint surface, i.e., this joint surface is bent convexly in one direction and concavely in the direction extending at right angles to that direction, and the lowest point of the concave contour coincides with the highest point of the convex contour. The joint bodies 8, 9 are rotated by 90° in relation to one another, so that the saddle joint 18 formed by these joint bodies 8, 9 permits pivoting in two mutually perpendicular directions that are parallel to the lower support body 3. The arrangement is selected here to be such that these pivot axes extend in parallel to the anterior-posterior direction and in parallel to the lateral direction.

[0041] The saddle joint 18 permits the pivoting of the two support bodies 2, 3 in relation to one another, so that both lateral flexion and extension/flexion of the adjacent vertebral bodies 6, 7 are possible, but such a saddle joint 18 prevents the rotation of the two support bodies 2, 3 around an axis of rotation extending at right angles to the plate-like support body 2, 3 if the two joint bodies 8, 9 are rigidly connected with their respective support bodies 2, 3.

[0042] Such a rigid connection may be provided, for example, the joint bodies 8, 9 may be fixed in the support bodies 2, 3 by additional clamping elements not shown in the drawings or by shrinking the support bodies onto the joint bodies or according to other fixing methods.

[0043] However, provisions are made in a preferred exemplary embodiment for at least one of the joint bodies 8, 9 to be rotatable in relation to its support body 2 or 3 around a perpendicular axis of rotation, i.e., around an axis of rotation that extends transversely to the pivot axes that are formed by the saddle joint 18. This can be achieved simply by the joint body being freely rotatable in the corresponding recess of the support body, but provisions may also be made for the use of special embodiments of the joint body for this, as is shown, for example, in FIGS. 7 and 8. The lower joint body 9 has a two-part design in the exemplary embodiment according to FIG. 7, comprising a bearing part 19 inserted into the recess 11, rotating in unison, with a central bearing journal 20 and a joint surface part 21 placed flatly on this bearing part 19 with a central bearing opening 22 adapted to the bearing journal 20. The joint surface part 21 is thus rotatable on the bearing part 19 around the axis of rotation defined by the bearing journal 20.

[0044] A central bearing journal is missing in the exemplary embodiment according to FIG. 8, in which parts that correspond to each other are designated by the same reference numbers, but a rotationally symmetrical bearing shell 23, which receives the joint surface part 21 and supports it rotatably around the axis of rotation, is formed in the bearing part 19, instead.

[0045] Biocompatible metals, especially titanium alloys or chromium-cobalt alloys, are preferably used as the material for the above-described parts. As an alternative, components may be made of plastic, especially from PEEK™, which is a polymer (polyether ether ketone) manufactured by Victrex® PLC of the United Kingdom. PEEK™ is transparent to X-rays, which leads to a great advantage in postoperative X-ray diagnostics with CTs or nuclear spin tomography, because, unlike metals, the plastic does not cause any artifacts (i.e., obstructions) in the X-ray image.

[0046] To minimize wear, components may be made of ceramic. Such ceramic components are manufactured with corresponding precision such that the wear nearly equals zero. A further advantage of a ceramic-on-ceramic bearing is that the problem of creep under load, which is peculiar to polyethylene, is absent. Since ceramic material has a substantially higher compressive strength and dimensional stability than polyethylene, dimensions may be reduced. The forced translational motion superimposed to the flexion/extension movement decreases as a result.

[0047] The components described above may be mounted substantially without clearance, because abrasion may otherwise occur at ceramic/metal interfaces because of the hardness of the ceramic material. This clearance-free mounting/assembly can be achieved, e.g., by means of a conical clamping, as illustrated in FIG. 10 (α helps depict the conical shape of joint body 9). However, other possibilities of the clearance-free mounting/assembly can be exhausted as well, such as: shrinking of the parts onto bearing surfaces by means of thermal expansion, as illustrated in FIG. 4; use of elastic intermediate elements (as illustrated in FIG. 11), which compensate a clearance between components due to their intrinsic elasticity/deformation; and additional locking screws 30, as illustrated in FIG. 12.

[0048] The intervertebral disk prosthesis can be inserted with the aid of navigated instruments. In use, components are assembled prior to implantation, and the intervertebral disk prosthesis is implanted in the assembled state, thereby significantly simplifying the implantation procedure.

[0049] While preferred embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.