WO2007003437A2

WO2007003437A2 - Cage for the introduction into an intervertebral-disc compartment

Info

Publication number: WO2007003437A2
Application number: PCT/EP2006/006608
Authority: WO
Inventors: Franz Jun. Copf
Original assignee: Copf Franz Jun
Priority date: 2005-07-06
Filing date: 2006-07-06
Publication date: 2007-01-11
Also published as: ZA200801287B; CN101257866A; WO2007003437A3; EP1919403A2

Abstract

A rigid cage (110) to be introduced into an intervertebral-disc compartment between two adjacent vertebrae is assembled from two constituent elements (110a, 110b) along a jointing face which extends parallel to transverse sides of the cage. According to another embodiment, operating means, for example a pair of rods (231, 233), may be fastened to a cage (210) in order to rotate the cage within the intervertebral-disc compartment. Both approaches make it possible to introduce the cage through an access canal having a very omall diameter.

Description

CAGE FOR THE INTRODUCTION INTO AN INTERVERTEBRAL-DISC COMPARTMENT

CROSS-REFERENCE TO RELATED APPLICATIONS

This applications claims benefit of US provisional application US Ser. No. 60/696,889 filed July 6, 2005. The full disclosure of this earlier application is incorpo- rated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a cage to be introduced into an intervertebral-disc compartment formed between two adja- cent vertobrae .

2. Description of the State of the Art

The term "cage" is understood to denote a cage-like body that is surgically positioned within an intervertebral- disc compartment, which is formed between two adjacent vertebrae of the human spine. The cage has the task of replacing an intervertebral disc and fixing the spacing between the two adjacent vertebrae by fusing the latter.

Most cages have a substantially cylindrical overall shape and are screwed into the intervertebral-disc compartment via a ventral access canal. Cages of such a type are known from US 2003/0045938 Al and DE 199 57 339 C2, for example. These known cages can be introduced into the in- tervertebral-disc compartment via ventral access canals having a relatively small diameter. As a result the risk is small that ventral blood vessels will be damaged during the operation.

However, the vertebrae adjoining the intervertebral-disc compartment are stressed very unevenly by the cylindrical cages, which may result in degradations on these verte- brae. Furthermore, with the cylindrical cages it is difficult to fix the vertebrae delimiting the intervertebral-disc compartment exactly in the desired position.

DE 198 27 515 Al discloses a cage which has in its entirety the shape of a wedge. This cage reduce the risk of degradations of the adjacent vertebrae observed with cylindrical cages. However, due to its larger dimensions the access canal has to be larger, too. This considerably increases the risks of damaging blood vessels during the implant surgery.

SUMMARY OF THE INVENTION

For this reason, it is an object of the invention to provide a cage that may, even if it has larger dimensions than known cylindrical cages, be introduced into the intervertebral disc compartment through an access canal having a small diameter. According to a first aspect of this invention this problem is solved by a cage which is assembled from at least two constituent elements which preferably have at least substantially the same size.

This construction of the cage permits the two constituent elements to be introduced into the intervertebral disc compartment in succession via a (ventral or ventrolateral) access canal. Thus the cage is not introduced as a whole, but in parts, and it is only completely assem- bled after the introduction of the parts into the intervertebral disc compartment.

Usually cages have the longest dimension laterally. Therefore it is advantageous to divide the cage in two constituent elements along a plane of symmetry which runs parallel to the transverse sides of the cage. Such a configuration ensures that both constituent parts have the same lateral dimension that determines the diameter of the access canal.

In order to facilitate the assembly of the cage from the two constituent elements, the latter may be provided with guide faces which engage one another when the constituent elements are joined together. The guide faces align the constituent elements relative to each other in at least two orthogonal directions. The guide faces may be de- signed such that it is possible to connect the constituent elements positioned in the intervertebral disc compartment with a sliding movement that can be produced even through a small access canal. However, other means - A -

to connect the constituent elements may be envisaged as well. For example, constituent elements may be connected using screws, brackets or glue.

The subdivision of the cage into two (or more) constitu- ent elements that are assembled within the intervertebral disc compartment also makes it possible to implant cages that are significantly larger than those known in the art. Cages having larger base and cover plates are advantageous because they may then completely rest on the hard bone tissue {substantia compacta) forming the surface of the adjacent vertebrae. In the case of smaller cages used heretofore, significant forces are borne by the softer bone tissue {substantia spongiosa) in the middle of the vertebrae. This conventional approach entails significant risks of bone degradations caused by excessive forces acting on the substantia spongiosa.

On the base plate and/or on the cover plate a fin may be arranged which extends parallel to the transverse sides and which is preferably perpendicular to the plate. The fins are formed by adjacent fin elements provided on each constituent element. Such a fin enables the cage to be fixed in relation to the adjacent vertebrae if, prior to the introduction, a groove for receiving the fin is chiseled into said vertebrae with the aid of a chisel .

On the base plate and/or on the cover plate, on the outward-pointing surface, a corrugation which extends parallel to the longitudinal sides of the plates may furthermore be provided at least partially. The corrugation has the task of increasing the frictional resistance of the cage in relation to the adjacent vertebrae and, in this way, of preventing it from being pressed out. This is particularly useful if the cage has the overall shape of a wedge, or if no fins are provided. For this purpose, the corrugation preferably has a serrated profile with corrugation flanks, the inclines of which in the direction of introduction are smaller than in the opposite direction. In this way, when the cage is being introduced into the intervertebral-disc compartment the corrugation generates a smaller resistance than when it is being pulled out.

According to another aspect of the invention the above stated problem is solved by a cage system comprising a cage and operating means that are separably connectable to the cage. The operating means are configured to rotate the cage when connected thereto upon operation of the operating means .

Since cages are generally oriented in the intervertebral- disc compartment in such a way that their longer longitudinal side is oriented at right angles to a longitudinal axis of the ventral access canal, the ventral or lateral access canal must generally have a diameter that is at least as large as the length of the cage. However, if the cage is introduced through the access canal into the intervertebral-disc compartment with its narrower transverse side to the front and is then rotated into its final position by the operating means within the intervertebral-disc compartment, the diameter of the access canal can be considerably reduced. Accordingly, the risk of damage to blood vessels that have to be displaced in the course of creating the access canal is also reduced.

In the simplest case the operating means may comprise two rods that are separably connectable to the cage in an articulating manner. However, more sophisticated operating means may be envisaged as well. For example, the operating means may be separably, but fixedly attached to the cage, and the operating means may then comprise articu- lating members that make it possible to rotate the fixed cage upon actuation of a lever or rotating wheel .

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and advantages of the present invention may be more readily understood with reference to the fol- lowing detailed description taken in conjunction with the accompanying drawing in which:

FIG. 1 is a perspective representation of a ventral cage according to a first exemplary embodiment of the invention;

FIG. 2 is a side view of the cage shown in FIG. 1 ;

FIG. 3 is a side view of a ventral cage according to a variant of the first exemplary embodiment;

FIG. 4 is a perspective representation, corresponding to FIG. 1, of a ventral cage according to a second exemplary embodiment of the invention, wherein the cage has been assembled from two constituent elements ;

FIG. 5 is a front view of the cage shown in FIG. 4;

FIG. 6 is a side view of a constituent element of the cage shown in FIG. 4;

FIG. 7 is a perspective representation of a ventral cage according to a third exemplary embodiment of the invention, wherein the cage is capable of being rotated about a vertical axis in the interverte- bral-disc compartment by two operating rods;

FIG. 8 is a perspective, merely exemplary representation of a hook connection which is suitable in principle for separable articulation of the operating rods;

FIGS. 9a to 9c show the cage of FIG. 7 in a simplified top view in varying rotary positions;

FIGS. 10a to 10c are representations corresponding to

FIGS. 9a to 9c, but with other points of articu- lation of the operating rods;

FIG. 11 is a perspective view of another embodiment of a cage in accordance with the invention; FIG. 12 is a central cross-section through a plane of symmetry of the cage shown in FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a perspective view of a ventral cage which is denoted in its entirety by 10. The cage 10 has an upward- pointing cover plate 12 which has a substantially rectangular basic shape. The cover plate 12 has a ventral longitudinal side 14 which is convexly curved and which is oriented ventrally after the introduction of the cage 10 into an intervertebral-disc compartment. On the opposite side the cover plate 12 is bounded by a dorsal longitudinal side 16 which includes a right angle with two transverse sides 18, 20.

In the cover plate 12 two window-like openings 22, 24, which define a crosspiece 26 berween them, are provided symmetrically relative to a plane of symmetry which passes perpendicularly through the cover plate 12 and which extends through the vertex of the curved longitudinal side 14.

In the embodiment shown, the crosspiece 26 bears a first fin 28 which is trapezoidal and which extends in the aforementioned plane of symmetry of the cage 10. In the fin 28 three bores are provided which are denoted by 30, 32, 34 in the side view of FIG. 2.

Situated opposite the cover plate 12 is a base plate 36 which is ronstructed just like the cover plate 12. Dis- cernible only in the side view of FIG. 2 is a second fin 38 which is borne by a crosspiece 40 between two openings 42, 44 in the base plate 36.

The ventral longitudinal sides 14 of the cover plate 12 and of the base plate 36 are connected by a front plate 46 which likewise has two window-like openings 48, 50. On the opposite side the cover plate 12 and the base plate 36 are connected to one another via a closed back plate 52 which has no openings. Side plates with opening 53 and 55 extend between the transverse sides of the cover plate 12 and of the base plate 36.

With the exception of the two fins 28, 38, all the outward-pointing corners and edges of the cage 10 are rounded .

- As can best be discerned in FIG. 2, the cover plate 12 and the base plate 36 are not parallel but are arranged at a wedge angle α relative to one another. The longitudinal sides of the cover plate 12 and of the base plate 36 therefore extend parallel to one another, whereas the transverse sides 18, 20 of the cover plate 12 include the wedge angle α with the transverse sides of the base plate 36.

For the purpose of describing the shape of the cage 10, use may be made of the length 1 of the cage 10 which is the length of the dorsal longitudinal side 16, and also of the quantities indicated in FIG. 2. These quantities are the wedge angle α, the maximal height h of the cage 10 and the minimal width b. For the purpose of describing the curvature of the front plate 46, use may be made of the radius of curvature.

The entire cage may be manufactured in one piece from a suitable material, for example titanium. In the case where use is made of varying materials, it should be ensured that the back plate 52 is opaque to X-radiation, in order that the introduction of the cage 10 into the in- tervertebral-disc compartment can be monitored during the operation with the aid of imaging methods. Favorable, furthermore, is a rough coating at least on the outsides of the cover plate 12 and of the base plate 36, in order to prevent the cage 10 from sliding out as a consequence of the wedge shape.

The cage shown in FIGS. 1 and 2 may be introduced into an intervertebral-disc compartment between two adjacent vertebrae in the following way:

Firstly the intervertebral-disc compartment, into which the cage is to be introduced via a ventral access canal, is surveyed with the aid of biometric methods. In this connection, recourse may, for example, be taken to imaging methods such as magnetic-resonance imaging, for instance. With the aid of such methods, it is possible to determine the width, the depth, the maximal height of the intervertebral-disc compartment and also the angle of the intervertebral-disc compartment. Particularly in the case of cervical vertebrae it may become necessary to machine in material-abrading manner such as milling the surfaces of the adjacent vertebrae.

In principle it is possible to produce a cage having the exact dimensions of the intervertebral-disc compartment that has been surveyed in such a way. However, since the accuracy of biometric measurements is limited, individual manufacture will only be sensible in the individual case, since relatively small deviations are unavoidable and not necessarily disadvantageous. Rather, it will be usually sufficient to provide a set of different cages that differ from one another with regard to their aforementioned geometrical parameters.

From such a set a cage is now selected having geometrical parameters that come closest to the biometric data ob- tained in the course of surveying the intervertebral-disc compartment. A ventral access canal leading to the intervertebral-disc compartment in question is now created surgically, and the material that is present there, for example an intervertebral disc or a bone graft replacing said disc, is removed. A material-abrading preliminary treatment of the intervertebral-disc compartment is also required in many cases, particularly in the cervical section of the spine. Subject to constant monitoring by imaging methods, grooves which serve to receive the fins 28, 38 are now chiseled with the aid of a chisel into the two vertebrae that delimit the intervertebral- disc compartment. The selected cage 10 is now introduced into the exposed intervertebral-disc compartment via the access canal in such a way that the back plate 52 points towards the spinal canal .

When the cage 10 is being introduced into the interverte- bral-disc compartment, the fins 28, 38 engage the grooves, which were previously chiseled into the vertebrae, and thus fix the cage 10 laterally. A fixing along the direction of introduction is obtained by the friction of the fins 28, 38 within the grooves. In the introduced state the cover plate 12 and the base plate 36 are sup- ported on the hard bone tissue known as substantia com- pacta. As a result, forces acting in the longitudinal direction of the vertebral column are optimally transferred between the vertebrae adjacent the int_^rvertebral-disc compartment. The forces acting on the softer central re- gion {substantia spongiosa) of the vertebrae are kept small .

After the conclusion of the operation, surrounding spongy tissue is able to grow into the cage 10 through the openings. By virtue of this penetration of the cage 10 with the surrounding tissue an additional fixing in the in- tervertebral-disc compartment is achieved.

FIG. 3 shows, in a side view corresponding to FIG. 2, a variant of the cage 10 shown in FIGS. 1 and 2, wherein a cover plate 12' and a base plate 36' bear on their out- ward-pointing surface a longitudinal corrugation 60 having an approximately sawtooth-shaped profile. As can be discerned in FIG. 3, the profile flanks in this case are inclined to a variably strong degree, so that the cage 10, despite the corrugation 60, can be readily introduced by its rear side 52' into the intervertebral-disc compartment but resists a movement in the opposite direction by virtue of the steps 62 which extend approximately ver- tically.

FIGS. 4 to 6 show a second embodiment on the basis of a perspective representation corresponding to FIG. 1, a front view of the front face, and a side view of a constituent element of a jointing face, respectively.

Parts corresponding to those of the cage 10 are denoted by reference numerals augmented by 100, and in some cases will not be elucidated again in detail.

The essential difference of the cage 110 shown in FIGS. 4 to 6 from the cage 10 shown in FIGS. 1 and 2 consists in the fact that the cage 110 is subdivided along the aforementioned plane of symmetry into two constituent elements 110a, 110b. In FIG. 4 the peripheral jointing seam between the constituent elements 110a, 110b is denoted by 111. In this embodiment the two constituent elements 110a, 110b are aligned relative to one another via grooves and guide ridges, so that the cage 110 has again a smooth surface.

In FIG. 5 the cage 110 is shown in a front view, the two constituent elements 110a, 110b having been moved away from one another for some distance for the sake of clarity. In this representation it can be discerned that each constituent element 110a, 110b is provided with its own fins 128a, 138a and 128b, 138b, respectively. In the joined state of the two constituent elements 110a, 110b the fins 128a, 128b and the fins 138a, 138b bear against one another in planar manner, so that only one groove needs to be chiseled into each of the vertebrae delimiting the intervertebral-disc compartment despite the division of the cage 110 into two parts. Nevertheless, each constituent element 110a, 110b is independently secured in the lateral direction against slipping in the groove.

In FIG. 5 it can further be discerned that in the right- hand constituent element 110b, on the jointing face at the levels of the cover plate 112b and of the base plate 136b, grooves 115 and 117, respectively, of rectangular cross-section have been sunk which, however, as the side view of FIG. 6 shows, do not extend over the entire transverse direction of the cover plate 112b or of the base plate 136b. Rather, the grooves 115, 117 are bounded in the direction towards the back plate 152b by stops 119 and 121, respectively.

The left-hand constituent element 110a comprises, on its jointing face at the levels of the cover plate 112a and of the base plate 136a, guide ridges 123 and 125, respectively. The guide ridges 123, 125 take the form of complements of the grooves 115, 117. In particular, this im- plies that the length of the guide ridges 123, 125 coincides with the length of the grooves 115, 117, as can be seen in FIG. 6. The division of the cage 110 into the two constituent elements 110a, 110b permits the following procedure when the cage 110 is being introduced into an intervertebral- disc compartment:

Firstly, the intervertebral-disc compartment is surveyed biometrically, and a cage is selected from a set of prefabricated cages or is produced having dimensions which coincide as well as possible with the biometrically determined dimensions of the intervertebral-disc compart- ment . Then the intervertebral disc, or a substitute structure that is present in the intervertebral-disc compartment, is exposed via a ventral access canal. It should be noted that this access canal may now be significantly smaller than is the case with a one-piece cage.

In a next step, one half of the intervertebral-disc cci- partment is now exposed and is stiffened with a stiffening element. In a manner similar to that in the case of the embodiment described above with reference to FIGS. 1 to 3, the grooves for the fins 128, 138 are now chiselled into the adjacent vertebrae with the aid of chisel.

In a next step, the other half of the intervertebral-disc compartment is exposed, and the constituent element 110a or 110b associated with this side is introduced via the ventral access canal. The constituent element 110a or

110b which has been introduced now stiffens the intervertebral-disc compartment, so that the stiffening element that was previously inserted can be removed. The half in which the stiffening element was located is now exposed completely, in order to be able to receive the other half of the cage 110. In the course of insertion of the other constituent element, the guide ridges 123, 125 engage the grooves 115, 117 and finally strike the stops 119, 121. As a result, the two constituent elements 110a, 110b are fixed relative to one another in all directions other than the direction of introduction.

This procedure has the advantage that the conventional step of taking out a large stiffening element and introducing a cage practically simultaneously is dispensed with. Instead of this, only one half of the interverte- bral-disc compartment is stiffened, so that a constituent element of the cage can be comfortably inserted into the other half of the exposed intervertebral-disc compartment. This constituent element serves as a stiffening element when the stiffening element that was inserted to begin with is removed in order to create room for the other constituent element of the cage.

FIG. 7 shows, in a representation similar to FIG. 1, a cage 210 according to a third embodiment. Parts corresponding to those of the cage 10 are denoted by reference numerals augmented by 200, and in some cases will not be elucidated again in detail.

The cage 210 differs from the cage 10 shown in FIG. 1 in that operating rods 231, 233 are articulated on the front plate 246 of the cage 210. The joint elements required for this, which cannot be discerned in FIG. 7, for ar- ticulating the operating rods 231, 233 are located on opposite transverse sides of the cage 210 and permit the operating rods 231, 233 to be separably connected to the cage 210 in such a way that the operating rods 231, 233 can be swivelled in a plane which is substantially parallel to the base plate 236. By pulling or pushing the operating rods 231, 233 within this plane, the cage 210 can be rotated about a vertical axis which is denoted by A in FIG. 7.

In the simplest case, the joint elements - which are not discernible in FIG. 7 - for the operating rods 231, 233 may be short vertically extending pegs 237, such as are shown in an isolated manner in FIG. 8. Together with hooks 235 which are fastened to the ends of the operating rods 231, 233, the pegs 237 form joints that enable the swivelling of the operating rods 231, 233, described above. For the purpose of releasing the connection, the hooks 235 are simply unhooked from the pegs 237. Of course, there are a multitude of other fastening elements with which the operating rods 231, 233 or other operating elements can be separably connected to the cage 210 in an articulated manner.

The rotatability of the cage 210 about the vertical axis A using the operating rods 231, 233 allows the cage to be introduced in the longitudinal direction through a narrow ventral access canal, and to be rotated into the definitive transverse position only in, or in the immediate vicinity of, the intervertebral-disc compartment using the operating rods 231, 233. This will be elucidated in the following with reference to FIGS. 9a to 9c, which show a simplified top view of the cage 210 and the operating rods 231, 233. The axes of rotation of the fastening elements for the operating rods 231, 233 are indicated in FIGS. 9a to 9c by 239 and 241, respectively .

In FIG. 9a the cage 210 is shown in its initial position similar to what is illustrated in FIG. 7. Let it be assumed that the cage 210 is to be positioned in the in- tervertebral-disc compartment in this orientation. The back plate 252 of the cage 210 consequently points towards the spinal canal, whereas the convexly curved front plate 246 is aligned ventrally.

If the cage 210 were to be introduced into a ventral ac- cess canal in this orientation, the diameter d of the access canal must not drop below the length of the cage 210. If forces are now exerted - by hand or with the aid of a manipulator - on the operating rods 231, 233 in the directions indicated by arrows 243 and 245, respectively, the cage 210 rotates in the direction indicated by an arrow 247.

As can be discerned in FIG. 9b, the cage 210 appears to be considerably narrower in this rotated orientation. In the course of the introduction of the cage which has been rotated in this way, the ventral access canal consequently merely requires a diameter d' which is somewhat larger than the maximal overall width of the cage 210. For comparison, in FIG. 9b the diameter d of the ventral access canal is indicated such as would be required in the case of the orientation of the cage 210 shown in FIG. 9a.

In this transversely-placed orientation the cage 210 is now introduced through the ventral access canal until it extends partially into the intervertebral-disc compartment. Forces are now exerted on the operating rods 231, 233, as indicated by arrows 249 and 251, respectively, in FIG. 9b. The cage now rotates within the intervertebral- disc compartment in the direction indicated by an arrow 253 and consequently assumes its desired position in the intervertebral-disc compartment, as shown in FIG. 9c. After this position has been attained, the operating elements 231, 233 are separated from the cage 210 and guided out of the ventral access canal. In order to allow rotation of the cage 210 in or close to the intervertebral disc compartment, the cage 210 is not provided with fins. Sufficient friction may be produced by providing a corrugation 60 on the surface of the base and cover plates as has been described above with reference to FIG. 3.

In FIGS. 10a to 10c a variant of this embodiment is shown in which joint elements with axes of rotation 239', 241' are provided at points on a cage 210' which are situated diagonally opposite one another. A diagonal arrangement of such a type has the advantage that the cage 210' is able to rotate still further, namely by exactly 90°, about a vertical axis. This can best be discerned in FIG. 10b. A comparison with FIG. 9b reveals that the ventral access canal needs only to have a diameter d", which corresponds to the maximal width of the cage 210'. The diameter of the ventral access canal can consequently be even smaller than is the case with the cage 210 shown in FIGS. 7 to 9.

FIGS. 11 and 12 show another embodiment of a cage 310 in a perspective view similar to FIG. 1 and a central cross- section through a plane of symmetry.

The cage 310 has a cover plate 312 comprising at its circumference a plane annular abutment area 370 which may be roughened so as to increase its friction coefficient with respect to bone tissue. A crosspiece 326 extends along an X direction between a ventral longitudinal side 314 and a dorsal longitudinal side 316. The crosspiece 326 separates two openings 322, 324 similar to the embodiment shown in FIG. 1. However, in this embodiment an outward surface 372 of the crosspiece 326 is convexly curved. The curvature in the XZ plane can be seen best in the cross- section of FIG. 12. In the YZ plane the crosspiece 326 is also convexly symmetrical to the middle plane of the crosspiece 326. Due to this curvature, the surface 372 has a vertex 374 at its top.

The cross-section shown in FIG. 12 results in a ramp-like shape of the crosspiece 326, having a main flank 376 and a steeper counter flank 378. The shape of the cross-piece thus corresponds significantly, although not completely to the shape of the dome that is formed within the apophyseal ring of the adjacent vertebra. In the cross- section of FIG. 12 such a vertebra 380 is indicated by a dotted line, with its dome 382 formed between the apophyseal ring 384. In this representation it can be seen that the crosspiece 326 is more strongly curved than the dome 382. This is advantageous because the crosspiece 326, and thus the entire cage 310, can be rotated, preferably by at least 10°, with respect to the vertebra 380 around an axis 390, if the cross-piece 326 gets into contact with the dome 382. The rotation may be initiated with the help of operating rods 231, 233, and/or it may be a result of compression forces produced by ligaments surrounding the vertebrae. The ability to rotate greatly facilitates the correct angular orientation of the cage 310 within the intervertebral disc compartment. Furthermore, a self- centering effect may be observed due to the asymmetric shape of the dome 382 and of the crosspiece 324 so that the adjustment process is less critical for the surgeon.

In order to facilitate rotation of the crosspiece 324 within the dome 382, the surface 372 of the crosspiece should have a very low kinetic friction coefficient with respect to the soft bone tissue (substantia spongiosa) which delimits the dome 382. Such a low kinetic friction coefficient may be achieved by polishing of by the application of a smooth coating to the surface 372, for example. A surface made of diamond-like carbon (DLC) can be applied using ion beam deposition or sputter deposition techniques and is not only biologically compatible, but is very hard and provides a smooth surface with very low friction. An arithmetic roughness Ra of less than 10 μm or even less than 1 μm may be easily achieved with such a coating, resulting in a very low kinetic friction coefficient with respect to the adjacent bone material.

Since the shape of the dome 382 generally differs between the different vertebrae, and also from patient to pa- tient, it may be useful for the surgeon to have a supply of cages with different shapes of the crosspiece 324. It is also envisaged to biometrically determine the shape of the dome 382 of the respective vertebra, and to manufacture the cage 310 according to the data obtained in the measurement.

If the compression forces are further increased, the area around the vertex 374 will sink into the soft bone tissue (substantia spongiosa) within the apophyseal ring 384, thereby deforming and displacing this bone tissue to a certain extent, until the plane annular abutment area 370 rests on the hard apophyseal ring 384. Although an intimate connection is finally achieved between the cross- piece 326 on the one hand and the soft bone tissue (substantia spongiosa) within the apophyseal ring 384 on the other hand, the main loads are taken by the annular abutment area 370 and the apophyseal ring 384.

As can be seen in FIG. 12, the base plate 336 is configured in the same manner. All of the above remarks therefore apply correspondingly to the base plate 336 as well.

Claims

1. Cage (110; 210) for the introduction into an in- tervertebral-disc compartment formed between two adjacent vertebrae, characterized in that the cage is assembled from at least two constituent elements (110a, 110b) which preferably have at least substantially the same size.

2. Cage according to claim 1, characterized in that the assembled cage (110) has a base plate (136a, 136b) and a cover plate (112a, 112b) which each have two longitudinal sides and two transverse sides .

3. Cage according to claim 2, characterized in that a dcrsal back plate (152a, 152b) and a ventral front plate (146a, 146b) extend between the base plate (136a, 136b) and the cover plate (112a, 112b) .

4. Cage according to claim 3, characterized in that the ventral front plate (146a, 146b) has at least one opening (148; 150).

5. Cage according to claim 3 or 4, characterized in that the dorsal back plate (152a, 152b) is free of openings . - 2 A -

6. Cage according to any of claims 3 to 5, characterized in that the ventral front plate (146a, 146b) is convexly curved.

7. Cage according to any of claims 2 to 6, character- ized in that the base plate (136a, 136b) has at least one opening.

8. Cage according to any of claims 2 to 7, characterized in that the cover plate (112a, 112b) has at least one opening.

9. Cage according to any of claims 2 to 8, characterized in that a fin element (138a, 138b) extending parallel to the transverse sides is arranged on the base plate (136a, 136b) of each constituent element (110a, 110b), said fin elements (138a, 138b) form- ing a single fin when the cage (110) is assembled.

10. Cage according to any of claims 2 to 9, characterized in that a fin element (128a, 128b) extending parallel to the transverse sides is arranged on the cover plate (112a, 112b) of each constituent ele- ment (110a, 110b), said fin elements (128a, 128b) forming a single fin when the cage (110; is assembled.

11. Cage according to any of claims 2 to 10, characterized in that the base plate (36¹) has on its out- ward-pointing surface at least partially a corruga- tion (60) which extends parallel to the longitudinal sides.

12. Cage according to any of claims 2 to 11, characterized in that the cover plate (12') has on its out- ward-pointing surface at least partially a corrugation (60) which extends parallel to the longitudinal sides (18, 20) .

13. Cage according to claim 11 or 12, characterized in that the corrugation (60) has a serrated profile with steeply inclined corrugation flanks (62) and less steeply inclined counter flanks so as to ensure smaller friction along an introduction direction than along a direction opposite thereto.

14. Cage according to any of claims 2 to 13, character- ized in that: the cage (110) has at least substantially the shape of a wedge.

15. Cage according to claim 14, characterized in that the base plate (136a, 136b) and the cover plate (112a, 112b) are arranged relative to one another in such a manner that the longitudinal sides extend parallel to one another and the transverse sides of the base plate (136a, 136b) include a weάge angle α with the transverse sides of the cover plate (112a, 112b) .

16. Cage according to any of claims 2 to 15, characterized in that the two constituent elements (110a, 110b) are capable of being joined together along a jointing face which extends parallel to the transverse sides of the cage (110) .

17. Cage according to claim 16, characterized in that the jointing face extends in a plane of symmetry of the cage (110) .

18. Cage according to any of the preceding claims, characterized in that the constituent elements (110a, 110b) have guide faces (115, 117, 123, 125) which engage one another when the constituent elements

(110a, 110b) are joined together and align the constituent elements (110a, 110b) relative to each other in at least two orthogonal directions.

19. Cage according to claim 18, characterized in that the guide faces (115, 117, 123, 125) extend parallel to the transverse sides.

20. Cage system, comprising a cage (210) for the introduction into an intervertebral-disc compartment formed between two adjacent vertebrae,

characterized in that

the system comprises an operating means (231, 233) that is separably connectable to the cage, wherein the operating means is configured to rotate the cage when connected thereto upon operation of the operat- ing means.

21. Cage system according to claim 20, characterized in that the cage (210) comprises a base plate (236) and a cover plate that abut the vertebrae when the cage (210) is introduced into the intervertebral-disc compartment, and in that the operating means (231, 233) is configured to rotate the cage about an axis (A) which is at least substantially perpendicular to the base plate (236) .

22. Cage system according to claim 20 or 21, character- ized in that the cage (210) comprises connecting elements (237) adapted to cooperate with the operating means (231, 233) for connecting the operating means to the -Jcige .

23. Cage system according to claim 22, characterized in that the cage (210) comprises a ventral front plate an n a dorsal back plate, and in that the connecting elements (237) are arranged on opposite transverse sides of the ventral front plate of the cage (210) .

24. Cage system according to claim 22, characterized in that the cage (210) comprises a ventral front plate and a dorsal back plate, and in that one connecting element (237) is arranged ε~c a transversal side of the ventral front plate, and in that another connecting element (237) is arranged at an opposite transversal side of the dorsal back plate.

25. Cage system according to any of claims 20 to 24, characterized in that the connecting elements are joint elements enabling articulated movement between the cage and the operating means.

26. Cage system according to any of claims 27 to 32, characterized in that the operating means comprises two rods .

27. Cage (110; 210) for the introduction into an in- tervertebral-disc compartment formed between two adjacent vertebrae, characterized in that the cage is assembled from at least two constituent elements (110a, 110b) which preferably have at least substantially the same size.

28. Method for introducing a cage into an interverte- bral-disc compartment formed between two adjacent vertebrae, characterized by the following steps:

a) exposing an access canal to the intervertebral- disc compartment;

b) exposing a first part of the intervertebral-disc compartment;

c) introducing a stiffening element into the first part of the intervertebral-disc compartment;

d) exposing a second part of the intervertebral-disc compartment; e) providing a cage (210) which is assembled from a first and a second constituent elements (110a, 110b) ;

f) introducing the first constituent element (110a) into the second part of the intervertebral-disc compartment;

g) removing the stiffening element from the first part of the intervertebral-disc compartment;

h) introducing the second constituent element (110b) into the first part of the intervertebral-disc compartment .

29. Method according to claim 27, characterized in that in step h) the second constituent element (110b) is aligned with respect to the first constituent ele- ment (HOa) on guide faces (115, 117, 123, 125) which are formed on the both constituent elements (HOa, HOb) .

30. Method for introducing a cage into an intervertebral-disc compartment formed between two adjacent vertebrae, characterized by the following steps:

a) exposing an access canal to the intervertebral- disc compartment;

b) exposing the intervertebral-disc compartment; c) providing a cage (210) which has at least substantially the shape of a wedge having an elongated base surface with longitudinal sides;

d) introducing cage (210) into the intervertebral- disc compartment in such a manner that the longitudinal sides of the base surface extend at least substantially parallel to a longitudinal axis of the ventral access canal;

e) rotating the cage within the intervertebral-disc compartment with the aid of an operating means

(213, 233) which is separably connected to the cage (210) .

31. Method according to claim 29, characterized in that for the purpose of rotating in accordance with step e) the cage (210) is connected co two operating rods (231, 233) which engage opposite transverse sides of the cage.

32. Method according to claim 29 or 30, characterized in that the cage is rotated in step e) by a rotation angle between 70° and 110°.

33. Method of manufacturing a cage (310) for insertion into an intervertebral disc compartment which is formed between a first and a second vertebra, characterized by the following steps: a) biometrically determining the shape of a first dome formed within an apophyseal ring of the first vertebra;

b) manufacturing a cage having a first convexly curved abutment face which abuts against the first vertebra when the cage (310) is inserted into the intervertebral disc compartment, wherein the first abutment face has a shape that has a vertex (388) being configured such that the cage (310) is allowed to rotate within the first dome when the vertex (388) of the first abutment face contacts a vertex of the dome.

34. Method according to claim 33, characterized in that the cage (310) is allowed to rotate by at least 10°.

35. Cage, for the int\oduction into ai intervertebral- disc compartment formed between two adjacent vertebrae, characterized in that the cage (310) comprises a cover plate (312) having a convexly curved region (326) .

36. Cage according to claim 35, characterized in that the covcvr plate (312) has at least two openings (322, 324) separated by a crosspiece (326), and in that the convexly curved region (326) is formed by the crosspiece (326).

37. Cage according to claim 35 or 36, characterized in that the cover plate (312) comprises a plane annular area (370) surrounding the convexly curved region.

38. Cage according to claim 37, characterized in that the plane annular area (370) has a rougher surface than the curved region.

39. Cage according to any of the claims 35 to 38, characterized in that the surface (372) of the curved region is polished.

40. Cage according to claim any of the claim 35 to 39, characterized in that the surface (372) of the curved region has a kinetic friction coefficient of less than 0.1 with respect to bone material.

41. Cage according to claim any of the claim 35 to 40, characterized in that the surface (372) of the curved region has an arithmetic roughness Ra of less than 10 μm, preferably less than 1 μm.

42. Cage according to claim any of the claim 35 to 41, characterized in that the surface (372) of the curved region is_ formed by a diamond-like carbon coating .

43. Cage according to claim any of the claim 35 to 42, characterized in that the surface (372) of the plane area (370) has a kinetic friction coefficient of more thεn 1.0 with respect to bone material.

44. Cage according to claim any of the claim 35 to 43, characterized in that the curved region has at least substantially the shape of a ramp.