(51) Int.Cl.: A 61 f, 1/00

DR. BERG DIPL. ING. STAPF

PATENT ATTORNEYS

MUNICH 80, MAUERKIRCHERSTR. 45

Attorney file 23 257 December 28, 1972

Dr. med. Siegfried Hoffmann-Daimler,

7407 Rottenburg-Seebronn, Gartenstrasse 24

"Intervertebral Disk Prosthesis"

The invention relates to an intervertebral disk prosthesis.

It is well-known that a plurality of physical injuries and impairments to one's health stem from the inability of an intervertebral disk - thus, one of the elastic cushions between two vertebrae of the spinal column - to fulfill any more its task to the full extent.

In many cases the results of this failure can be alleviated or at least ameliorated, according to the invention, by replacing the intervertebral disk with a prosthesis. The intervertebral disk prosthesis of the invention is characterized in that it is constructed as a spacer, which can be inserted between two vertebrae. This spacer permits a tilting and/or the vertebrae to tilt relative to each other by means of convex sliding surfaces.

VI/d ORIGINAL INSPECTED

One design, which is advantageous owing to its simplicity, is characterized by constructing the prosthesis as a disk, which can be inserted between the base plate of a top vertebra and the cover plate of a bottom vertebra and which exhibits convexed face surfaces, which are at least approximately lenticular at the top and the bottom. At the periphery the face surfaces of the disks are spaced apart in such a manner that on bending the spinal column, the two vertebrae do not touch each other.

In the simplest case the intervertebral disk prosthesis consists of a rigid body, which is made, for example, of one of the standard metal alloys, which have withstood the test for endoprostheses, or a plastic material, which has also proven to be suitable. Suitable materials are known from prosthesis technology. If the prosthesis exhibits lens surfaces, which are exactly rotationally symmetrical - thus, exhibits, for example, face surfaces that are in the shape of a spherical cap -, then prior to the insertion of the prosthesis between two vertebrae the cover plate of the bottom vertebra and the base plate of the top vertebra in the region of the cartilaginous layer, which is located between the actual vertebra and the natural intervertebral disk, may be machined to the extent that the bearing surface of the vertebra, resulting from the machining, is at least approximately identical to the corresponding lens surface of the prosthesis. In other embodiments of the invention, which are superior in a variety of relationships and which shall

be explained below, the corresponding surface of the prosthesis may also be designed in conformity with the natural surface of the vertebra, so that in this case it is not necessary to mill or perform other machining operations on the bearing surface between the vertebrae and the prosthesis.

The prosthesis, according to the invention, has to fulfill simultaneously a plurality of functions. First of all, it holds apart the two adjacent vertebrae in the axial direction, so that even if these two vertebrae are distorted when the spinal column bends, mutual contact between the two adjacent vertebrae is avoided with certainty. This feature is very important, because such a contact leads to an extremely undesired change in the two adjacent vertebrae. In order to avoid this type of contact with certainty, the intervertebral disk prosthesis must absorb all of the axial forces, occurring between the vertebrae, without excessive compression. Furthermore, the intervertebral disk prosthesis must exhibit adequately sufficient elastic strength, so that when such axial forces arise, it does not move too far in the radial direction out of the gap between two vertebrae. In order to enable the adjacent vertebrae to bend relative to each other, the prosthesis in the simplest case exhibits at both face surfaces the exact shape of a

spherical cap and is also preferably smoothly polished, so that the vertebrae themselves may slide with their cartilaginous face surfaces, which in this case are to be milled as little as possible, on the intervertebral disk prosthesis. As a result of the lenticular or spherical cap-shaped design of the two face surfaces of the intervertebral disk prosthesis, a displacement of the two adjacent vertebrae, which are mutually supported by the prosthesis, is also prevented in the lateral direction, because the vertebrae are clamped relative to each other in the axial direction subject to the action of the musculature. Even if it is desired that the vertebrae, which are kept spaced apart owing to the prosthesis, exhibit a good capability of bending relative to each other, the other two requirements concerning alignment and the prevention of mutually impinging on each other during a bending movement are, however, more important. The height of the intervertebral disk prosthesis of the invention depends largely on the size of the vertebrae to be separated by the prosthesis. At the edge the thickness of the prosthesis, which ranges advantageously from 10 to 14 mm in the center, should range from approximately 5 to 10, preferably from approximately 6 to 8 mm. At the minimum thickness of the prosthesis, the diameter of the same is advantageously in a magnitude of 30 mm, whereas at the maximum thickness, it is advantageously in the magnitude of approximately 40 mm.

The essential feature in all of the embodiments of the prosthesis of the invention is that this prosthesis may not be

compressible to any arbitrary extent, but rather that at least in the center only slight variations in the distance between the two vertebrae may be tolerated, whereas even at the edge an optionally existing compressibility should not exceed a certain amount and, in addition, should not result in large variations in the diameter of the prostheses. If the prosthesis is constructed as one piece, it may be, as stated above, a rigid body. However, it exhibits preferably at least one elastic intermediate layer, which extends in a normal plane to the axis of the prosthesis (which coincides with the axis of the vertebrae). The elastic intermediate layer may be made, for example, of a silicone rubber, which is suitably stable to ageing and is environmentally friendly. If one makes sure that in the middle of the intermediate layer there is, for example, an elastic spacer, which is slightly or not at all elastic, or even just a harder region of the elastic intermediate layer, then the edge regions may exhibit a correspondingly higher elasticity, which is in the context of the ability of the vertebrae, separated by the prosthesis, to bend relative to each other without the convexed face surfaces of the prosthesis sliding in the correspondingly concaved face surfaces of the vertebrae. This is a significant advantage over the rigid design.

The elastic intermediate layer may be formed, for example, by a

cylindrical disk, which is vulcanized in-between two bodies, which have the shape of spherical caps. However, said elastic intermediate layer may also have even lenticular face surfaces, which are provided with rigid covers.

The prosthesis may also be made entirely of a rubber elastic material. In this case the Shore hardness should not be less than what would be selected, for example, for the treads of rubber tires of motor vehicles. If one wants to select a softer rubber elastic material for a solid or complete prosthesis, this can be done, if the prosthesis is given a suitably rigid insert, for example, in the shape of a thin vulcanized metal plate, which prevents the prosthesis from expanding unduly in the radial direction and, thus, also prevents the prosthesis from being unduly compressed in the axial direction.

The prosthesis, according to the invention, consists preferably of at least two superimposed parts, which can be shifted relative to each other. Each of the parts rests against one of the two vertebrae, which are connected by the prosthesis. In the simplest case such a construction may be realized in the above described manner. In this case then the elastic intermediate layer carries harder plates, which are at least externally convex, at the top and the bottom. If at this point displaceable parts are mentioned, then these parts should not be arbitrarily displaceable relative to each other.

Rather, on the one hand, they should be bendable or tiltable relative to each other by a certain amount (which is limited by the aforementioned requirement that the two vertebrae may not touch when they are bent relative to each other). However, they should not, if possible, be able to move or they should be able to move only slightly relative to each other in a normal plane to the longitudinal axis of the spinal column. Of course, it is permitted that the two parts are somewhat compressible in the axial direction. However, this compressibility should stay quite low. The less the two parts can be compressed in the axial direction, the more they can tilt relative to each other without touching the adjacent vertebrae. The major advantage of the above described displaceable arrangement lies in the fact that the two parts, which can be displaced relative to each other, perform the relative movement, which is necessary during the forward inclining motion of the spinal column, between themselves and no longer relative to the vertebrae.

With respect to the question, whether one should construct the lenticularly convexed face surfaces rotationally symmetrical or so as to diverge therefrom, it is important whether the prosthesis can be rotated or cannot be rotated by a defined amount in itself. If the prosthesis cannot be rotated or cannot be sufficiently rotated in itself, in order to permit the spinal column to perform the natural rotational motions, then the face surfaces should be rotationally symmetrical, because then during rotation the relative movements must take place between the face surfaces and the two vertebrae.

If, in contrast, the prosthesis can be rotated sufficiently in itself, then the face surfaces of the vertebrae may deviate from the rotationally symmetrical shape and may be adapted to the natural shape of the cover plate or the base plate of the corresponding vertebra; or it may be given a different shape that is securely anchored in the vertebra.

A prosthesis, consisting of two superimposed parts, which can be displaced relative to each other, as explained above, may be produced, for example, in that it composes a biconvex lens body and a concave-convex lens body. In this case the former is mounted so that it can slide. In this case, the edge of at least one of the two lens bodies has advantageously a defined thickness, in order to hold apart the two vertebrae even in a bent state. Therefore, the two lens bodies should be made of an optimal pairing of materials - for example, a plastic and a metal -, which exhibit good emergency running properties relative to each other. Suitable pairings of material are known, for example, from the hip prosthesis technology, where the actebulum is often made of a suitable plastic material, whereas the acetebular is made of metal. The above described design has the crucial advantage that it permits not only the adjacent vertebrae to perform bending motions relative to each other, but also the design can be rotated in itself.

That is, a relative movement between the prosthesis and the vertebrae is not necessary. The above described construction can also be designed so as to be elastic by inserting, for example, an elastic intermediate layer into the biconvex lens body. Not only this design, but also other embodiments may provide, instead of an elastic intermediate layer, a flexible cavity that is filled with liquid, provided that the prosthetic parts are prevented from moving relative to each other in the direction of the sides of the cavity.

A prosthesis, which is somewhat more expensive in terms of design, but, on the other hand, satisfies the biological requirements to a fairly large extent, is achieved if the design of the prosthesis provides two superimposed parts, which can be displaced relative to each other and which are constructed as supporting plates, which are connected together with a swivel bearing element and are designed preferably as split lens bodies. In this case split lens bodies are defined as two relatively flat disks, each of which exhibits an external lens area. In this case the external lens area may be designed rotationally symmetrical and may also be adapted better to the exact shape of the face surfaces of the vertebrae. For example, the supporting plates may be flat on their surfaces facing the vertebrae and may bear anchoring projections in the shape of thorns. In the simplest case the swivel bearing element may be formed, for example, by a short silicone rubber body,

which holds the two supporting plates sufficiently apart. At the two face surfaces the swivel bearing element is vulcanized together with the areas of the supporting plates that face each other. However, the preferred swivel bearing is a central body with two spherical cup-shaped surfaces, on which are mounted the supporting plates with the respective sliding surfaces. The sliding surfaces are preferably spherical central recesses of at least approximately the same radius of curvature as the spherical cup-shaped surfaces. The two spherical cup-shaped surfaces complement each other so as to form preferably a spherical surface. In this case, too, the materials are selected in such an advantageous manner that there exist optimal sliding properties between the central body and the supporting plates. The latter may be made, for example, of metal; and the central body may be made of a suitable plastic material or even vice versa.

In the design of the prosthesis comprising two supporting plates, which are connected together by means of a swivel bearing element, the interstitial space between the central swivel bearing element and the two edges of the supporting plates is filled advantageously with an elastic displacement body. This displacement body may be, for example, a ring, which totally fills this interstitial space and is made of a closed pore silicone rubber. In this case, however, a body is chosen that is made preferably of silicone rubber or the like. This body exhibits, for example, an approximately double T-shaped profile, where the leg of the double T-shaped profile lies in a normal plane to the axis of the prosthesis; and the areas of the two supporting plates that face each other

are supported on the free edges of the two flanges of the double T-shaped profile. For the latter purpose the two free edges of the flange may exhibit expansions, which rest against the supporting plates. The fill body is preferably rubber elastic. It may be vulcanized, if desired, together with the two supporting plates. If said fill body is vulcanized together, the ability of the prosthesis to rotate in itself is reduced. If the rotatability of the prosthesis is to be maintained, nevertheless, then it can be achieved in that the flanges and the legs of the fill body are kept relatively thin. In general, however, one will prefer a design that is not vulcanized together. In this case only the inner flange of the double T-shaped profile engages, for example, with a corresponding ring groove of the respective supporting plate, and, in so doing, also holds together the parts of the prosthesis.

It is clear that the various features of the different embodiments, described above, may also be combined if such a combination is practical. For example, a displacement ring body made of sponge rubber may also be vulcanized together with two split lens bodies, if one is willing to accept a lower rotatability of the two split lens bodies.

The subject matter of the invention is explained in detail below as a variety of embodiments with reference to the drawings.

Figure 1 is a schematic cross sectional view of the plane of symmetry of the skeleton of two superimposed vertebrae, which are mutually supported by means of a prosthesis, according to the invention.

Figure 2 is a top view of the front part of the bottom vertebra from Figure 1 and the inventive prosthesis, situated on the vertebra.

Figure 3 is a side view of the prosthesis from Figures 1 and 2 in its approximately natural size.

Figure 4 is a top view from Figure 3.

Figure 5 is an axial cross sectional view of a second embodiment of an inventive prosthesis, which is also formed rotationally symmetrical.

Figure 6 is a view of an additional embodiment that is identical to the view from Figure 5.

Figure 7 is an axial cross sectional view of an optimal embodiment of the inventive prosthesis on a significantly enlarged scale.

Figure 8 depicts a slightly modified embodiment of the prosthesis from Figure 7 in its approximately natural size.

Figure 9 is a view of a modified embodiment of the prosthesis that is identical to the view from Figures 5 and 6.

Figure 1 depicts a prosthesis 1, according to the invention, in its position between the base plate of a top vertebra 2 and the cover plate of a bottom vertebra 3. The top and the bottom cartilaginous layers of the vertebrae 2 and 3 are indicated by the thickened boundary lines 4.

First of all, the drawing shows that all of the illustrated embodiments of the prosthesis, according to the invention, are, as viewed in the longitudinal direction of the vertebral spine, at least approximately and preferably circular, in contrast to a natural intervertebral disk. In principle, it is also possible for the various designs, such as the design in Figures 6 and 7, to be not round. However, since such a non-circular design would necessitate considerably more technical complexity, the circular design, depicted, for example, in Figure 2, is preferred.

The inventive prosthesis, depicted in Figures 1 to 4, constitutes the simplest embodiment of a prosthesis.

It may be made rigid, for example, of metal or plastic or even of a rubber elastic material, such as silicone rubber of adequate Shore hardness. The conditions, which determine the hardness, have already been explained above. In this case the prosthesis consists of a solid body, which has a rotationally symmetrical top lenticular face surface 5 and a bottom face surface 6, which is also rotationally symmetrical. The radius of curvature of the face surfaces 5 and 6 is, on the one hand, small enough to prevent the vertebrae 2 and 3 from moving sideways in a normal plane to the axis 7 of the spine. On the other hand, said radius of curvature is so large that no significantly strong separation forces are exerted on the vertebrae 2 and 3. The curvature of the face surfaces 5 and 6 does not have to be shaped exactly like a spherical cap. In this case other rotational curves, such as parabolas, are also suitable. The decisive factor is that the rotational curves, defining the surfaces 5 and 6, are as close as possible to the natural shape of the base plate and the cover plate of the vertebrae.

The prosthesis 1 has to be designed, just like the prosthesis 5, rotationally symmetrical, because in both cases when the vertebral spine twists, a sliding movement takes place between the prosthesis and the vertebrae.

The prosthesis 1 has rounded edges 8. The peripheral area 9 is designed in a crowned manner so that all areas pass continuously into each other. The height of the peripheral area 9 is chosen in such a manner that in the event that the vertebrae, which are separated by the prosthesis, perform, as expected, the maximum bending motion, the opposing edges of the vertebrae do not impinge on each other.

The prosthesis may be made of an elastic material. However, the elasticity may not be too high so that the prosthesis cannot be compressed in the axial direction. Therefore, in many cases this type of prosthesis is made preferably of a rigid material.

Figure 5 depicts an additional prosthesis 12, according to the invention. This prosthesis has a top rotationally symmetrical plate 13 and a bottom plate 14, which is arched in the opposite direction. Both plates may be made, for example of a correspondingly suitable metal alloy. Said plates have preferably a constant wall thickness, as indicated in the drawing. Said plates are held apart by an elastic plastic insert 15, which is vulcanized together with the two plates 13 and 14. In order to keep the plastic insert 15 as elastic as possible in the edge regions in order to minimize the sliding of the plates 13 and 14 on the separated vertebrae when said vertebrae tilt relative to

each other, the axial compressibility of this prosthesis must be low in the center. For this purpose the central core 16 of the rubber elastic insert 15 is provided with a significantly harder Shore hardness. The exterior shape of the prosthesis 12 matches advantageously that of the prosthesis 1.

The intervertebral disk prosthesis 18, depicted in Figure 6, is designed as two parts. This intervertebral disk prosthesis consists of a relatively rigid, rotationally symmetrical plastic body 19, which interacts with a metal shell 20, which is also rotationally symmetrical. The bottom surface of the plastic body 19 and the top surface of the metal shell 20 exhibit the same radius of curvature, which is constant everywhere, so that the two parts can be rotated and tilted relative to each other and, in so doing, slide flawlessly on each other. This feature has the advantage that the top surface 21 of the body 19 may also be designed asymmetrically in order to enable a better adaptation to the base plate of the top vertebra. The same applies correspondingly to the bottom surface 22 of the bottom part 20 of the prosthesis. In the event that the two prosthetic parts 19 and 20 are unduly shifted relative to each other, the peripheral edge 23 of the prosthesis impinges on the edge of the other prosthetic part 20. Thus, in this case, too, it is guaranteed that on bending the vertebrae, the edges of the vertebrae do not touch

each other.

The prosthesis, depicted in Figure 7, consists essentially of two of the parts, referred to above as the split lens bodies. Both split lens bodies 25 of the prosthesis 26, depicted in Figure 7, are identical. The two face surfaces of this prosthesis are lenticular, and in particular are rotationally symmetrical or are adapted even more precisely to the corresponding bearing surface of the vertebra. The middle of each of the two split lens bodies 25 exhibits a spherical step bearing, with which said split lens bodies rest against a central joint ball 26, so that said split lens bodies can be swivelled relative to each other about the center point of this ball. Furthermore, said split lens bodies can be rotated relative to each other. The parts of the split lens bodies 25 that project inwardly in order to form flawless bearing surfaces for the ball 26 are spaced so far apart from each other that they do not impede the swivelling motion of the split lens bodies 25 relative to each other. These protruding parts exhibit on both sides annular recesses 27, with which a rubber elastic intermediate body 28 having lips 29 engages, so that this intermediate body pushes the two split lens bodies 25 with slight prestress against the ball 26, which is made preferably of plastic in the case of metal split lens bodies or vice versa. The rubber elastic intermediate ring 28 exhibits a double T-shaped profile. The lips 29 are part of the radially inner flange of this double T-shaped profile, whose leg

lies in the axis of symmetry (which is horizontal in Figure 7) of the prosthesis. The radially external flange of the double T-shaped profile of the body 28 is supported by its corresponding edges 30 on the internal areas of the split lens bodies 25. In this way the more the split lens bodies 25 swivel relative to each other, the more force the fill body 28 exerts on any additional swivelling motion of said split lens bodies relative to each other. In order to make it easier for the two split lens bodies 25 to twist relative to each other about the axis 31, the fill body 28 is not vulcanized together with the two split lens bodies 25. However, the latter is also possible. In the latter case, however, for the above described reasons the face surfaces of the prosthesis are given advantageously a rotationally symmetrical shape.

The major distinction between the prosthesis 35, depicted in Figure 8, and the prosthesis 26 is in essence only that, instead of the central ball, there is a double lens body 36, where the position of the exact swivel point of the two split lens bodies 37 and 38 relative to each other is undefined, but, therefore, the bearing forces between the split lens bodies and the central double lens body 36 are better.

Figure 9 depicts an additional prosthetic shape 40, which also

exhibits a top split lens body 41 and a bottom split lens body 42. In this case one of the two split lens bodies is made preferably of a plastic material, whereas the other one of the two is made of a metal material, which has together with the plastic material good sliding properties. For example, the top split lens body 41 exhibits a central projection 43, which has the shape of a spherical cap. The center point of curvature of this projection coincides preferably with the center point of the rest of the rotationally symmetrical prosthesis. The bottom split lens body 42 also exhibits a central projection 44, which exhibits a central recess. The radius of curvature of this recess is equal to that of the spherical projection 43, so that in this way the two split lens bodies 41 and 42 can be mounted in such a manner that they can be swivelled relative to each other and rotated in each other about the center point of the prosthesis. If the two split lens bodies swivel too much relative to each, this excessive swivelling may be prevented, for example, by suitably dimensioning the thickness of the external edges of the same. However, there is also the possibility of inserting in this case a fill body, such as the fill ring 45, which is made of a closed pore silicone foam rubber. If the fill ring 45 is not vulcanized together with the split lens bodies 41 and 42, then the latter may also have non-rotationally symmetrical surfaces, in order to enable a better adaptation to the surfaces of the vertebrae.

Not only the prosthesis itself, but also the parts of the same may also be modified as a function of the circumstances. Thus, for example, the fill ring or the intermediate ring 28 may also have, instead of the double T-shaped profile, a U-shaped profile. For this purpose it suffices if the leg of the double T-shaped profile is moved so far to the side that on this side it connects the ends of the two flanges of the double T-shaped profile. Another possibility lies in the option of omitting the leg of the double T-shaped profile. In this case, however, the external flange must also engage in a corresponding annular groove of the two split lens bodies 25, so that said external flange cannot move radially.

The intermediate body may also exhibit holes, which in this case should be so large that they permit the body fluids to enter and leave easily. In this way an air or gas filled space in the prosthesis is avoided. Such passage holes may be provided either only in the external flange or both in the external flange and in the internal flange of the intermediate body or fill body. It is often the case that just the arrangement of these holes in the external flange will suffice.

Patent Claims:

1. Intervertebral disk prosthesis, characterized in that it is constructed as a spacer, which can be inserted between two vertebrae and which permits a tilting and/or the vertebrae to tilt relative to each other by means of convex sliding surfaces.

2. Intervertebral disk prosthesis, as claimed in claim 1, characterized in that it is designed as disks, exhibiting convexed face surfaces, which are approximately lenticular at the top and the bottom; and that at the periphery the face surfaces of said disks are spaced apart in such a manner that on bending the spinal column, the two vertebrae do not touch each other.

3. Intervertebral disk prosthesis, as claimed in claim 1 or 2, characterized in that it exhibits at least one elastic intermediate layer, which extends in a normal plane to the axis of the prosthesis.

4. Intervertebral disk prosthesis, as claimed in claim 1, 2 or 3, characterized in that it is made of a rubber elastic material.

5. Intervertebral disk prosthesis, as claimed in any one of the claims 1 to 4, characterized in that it consists of at least two superimposed parts, which can be shifted relative to each other; and that each of the parts rests against one of the two vertebrae, connected by means of the prosthesis.

6. Intervertebral disk prosthesis, as claimed in claim 5, characterized in that it composes a biconvex lens body and a concave-convex lens body, in which the former is mounted so that it can slide.

7. Intervertebral disk prosthesis, as claimed in claim 5, characterized in that it exhibits two supporting plates, which are connected together with a swivel bearing element and are designed preferably as split lens bodies.

8. Intervertebral disk prosthesis, as claimed in claim 7, characterized in that a central body with two spherical cup-shaped surfaces serves as the swivel bearing; and that the supporting plates with spherical central recesses of at least approximately the same radius of curvature rest on said spherical cup-shaped surfaces.

9. Intervertebral disk prosthesis, as claimed in claim 7 or 8, characterized in that the supporting plates are braced against each other in the region radially outside the swivel bearing element via a rubber elastic intermediate body, which covers in the outward direction the gap between the split lens body.