EP1610730A1

EP1610730A1 - Prosthetic joint of cervical intervertebral for a cervical spine

Info

Publication number: EP1610730A1
Application number: EP04707931A
Authority: EP
Inventors: Helmut D. Link; Arnold Keller; Paul C. Scoliosis and Spine Center MCAFEE
Original assignee: Cervitech Inc
Current assignee: Cervitech Inc
Priority date: 2003-04-07
Filing date: 2004-02-04
Publication date: 2006-01-04
Also published as: CA2519925A1; RU2005134220A; CN1802133A; CA2519926A1; MXPA05010766A; ZA200508010B; WO2004089258A1; AU2004228908A2; AU2004228908A1; RU2307624C2; KR100754570B1; CN100594855C; AR048120A1; AU2004228908B2; RU2325875C2; JP4617294B2; KR101134264B1; AR048119A1; BRPI0409091A; CN1771013A

Abstract

The invention relates to a prosthesis of a cervical intervertebral for the intervertebral space of a cervical spine bound by the ending plates (12, 13) of adjacent vertebral bodies whose surfaces are provided with very mach bent external lateral areas (4) adjacent to a central area (2) substentially flat on the front plane thereof. Said external areas are more mineralised than the central area and consequently are particularly stable. At least one of the prosthesis surfaces, (10, 11) which are provided for to be based upon the vertebral body surface, has a lateral extension in the external areas (4). The convex curvature of the prosthesis surface (9, 11) at least is equal on the front plane thereof to the corresponding curvature of the ending plates (12, 13). The inventive prosthesis is also supported by said external particularly stable areas which do not require any significant removal of material

Description

.wisc enwirfoβl Glensteck prosthesis for the cervical spine

Joint prostheses for replacing an intervertebral disc of the cervical spine are known, which consist of two cover plates and a joint core. The cover plates, which are arranged approximately parallel to each other on both sides of the core, have 0 surfaces which are intended for connection to the end plates of the adjacent vertebral bodies. Known prostheses of this type (FR-A-2718635, EP-B-699426,. WO 03063727, WO 0211650, EP-A-1166725, EP-A-820740) are limited in a circle. Since the end plates of the vertebral bodies are much wider than they are deep in the AP direction, these known prostheses do not take advantage of the size of the naturally given surfaces for the force transmission. As a result, higher forces occur between the prosthesis surfaces and the vertebral bodies than would be the case with better use of space. For intervertebral disc prostheses that are intended for the lumbar spine, the best use of space is achieved by using an oval outline of the prosthesis (WO 0101893, EP-B-471821, EP-A-747025) or kidney-shaped shape (EP-A-747025). Rectangular prosthesis shapes are also known (US-A-5425773). Previously registered inventions by the same applicant or their legal predecessor (EP-A-1344508, EP-A-1344507, WO 03075803, WO 03075804) disclose a prosthesis outline that approximates a rectangle with rounded corners and the essentially flat area of the vertebral end plates covered. You achieve a much better use of space and a safer long-term connection with the vertebral bodies than circular prostheses. In addition, they have a low height and therefore only require a small amount of natural bone material to be prepared in preparation for the implantation space. In some cases, they make it possible to retain all or part of the hard bone cortex, which is very thin in the case of cervical vertebrae.

Unlike cervical joint prostheses, cages are used to immobilize adjacent vertebral bodies for the purpose of their fusion. Since they aim at the vertebrae growing together, the quality of their own long-term connection to the bone is less important. The preservation of the natural bone substance also plays no role, since it is replaced by homologous material stored in the cage (EP-B-179695, WO 9720526, US 2001/0016774, WO 0191686, WO 9000037).

The invention seeks to further develop the type of prosthesis disclosed in its earlier applications mentioned (WO 03075804) with the aim of better power transmission between the prosthesis and the vertebral end plates, while largely maintaining the natural bone substance.

The invention is based on the knowledge that the vertebral end plates of the cervical spine are regionally a have different degrees of mineralization. The higher the mineralization, the denser the bone material and the more suitable it is for the absorption of force. It has been shown that the highest degree of mineralization is present in the lateral edge zones of the vertebral end plates, where the essentially flat central region of these end plates changes into a greater curvature in the frontal section, which leads to the uncovertebral joints. The basic idea of the invention is to use these edge zones for the force transmission between the prosthesis and the bone. The prosthesis surfaces intended for contact with the vertebral body surface are extended laterally into the more mineralized and at least partially more curved lateral edge zones of the vertebral body surface. So that the higher strength of these edge zones of the vertebral end plates can be used, they should be retained, even if the prosthesis height or the adaptation of the bone to the shape of the prosthesis requires a certain milling of the vertebral end plates. According to the invention, this milling is essentially limited to the central region of the vertebral end plates, where the bone strength is in any case lower, while the firmer edge zones are wholly or partly preserved. The shape of the prosthesis according to the invention makes this possible by the extent of its convex curvature. This curvature is chosen to be at least as large as the counter-curvature of the associated end plate surface. Generally it is larger. This means that the central areas of the prosthesis surface protrude further up or down with respect to the surface of the vertebral bodies than the edge zones. The height of the prosthesis is limited in the peripheral zones in such a way that there is a milling of the bone is dispensable. Only the cartilage is removed and, if necessary, the bone surface is slightly refreshed for a better connection to the prosthesis. If milling is necessary at all, it can mainly be limited to the central area. The shape relationships according to the invention can also be characterized in such a way that the surface of the prosthesis is complementarily similar to the shape of the vertebral end plates in the frontal section, but protrudes more strongly in the central area - based on the average shape of the end plates. Another alternative characterization of the prosthesis shape means that the height of the prosthesis in the caudal-cranial direction in the lateral marginal areas is approximately equal to the height of an average intervertebral space, which is required as a norm, while it is larger in the central area. The dimensions are chosen so that when used in an average shaped intervertebral space, there is little milling in the central area, but not in the edge zones of the front section under consideration. In some cases it is also not necessary to mill the central area.

The greater flexibility of the bone substance in the central area - regardless of whether it is milled or not - forms a good prerequisite for a positive connection to the surface of the prosthesis if it is provided with suitable elevations and depressions, which are designed in particular as teeth. It can also be provided with a coating which promotes the connection to the bone. The more mineralized edge zones of the vertebral end plates are inclined as a transition to the unovertebral joints in the frontal section. The edge zones of the prosthesis surface, which are intended for installation there, also expediently have a corresponding inclination. On the underside of the prosthesis, the angle of inclination with respect to the main direction of extension of the prosthesis is expediently 20 °. On the top of the prosthesis, this inclination is advantageously at least 0 ° and preferably 10 to 30 ° ^' .

In order for the surface of the prosthesis to reach the more mineralized edge zones of the vertebral body end plates, the width of the prostheses should be at least 1.5 times as large as their depth in the anteroposterior direction, which lies in the intervertebral space. This factor is preferably greater than 1.63.

It is not necessary that the shape characteristics described above apply to the entire depth of the prosthesis. This is possible; In many cases, however, it is more expedient if only the dorsal half of the prosthesis is designed according to the invention. This is because the greatest degree of mineralization of the vertebral end plates is achieved in their dorsolateral corner areas.

The invention is explained in more detail below with reference to the drawing, which illustrates advantageous exemplary embodiments of the invention. Show it:

1 is a plan view of a cervical vertebra, 2 shows a frontal section through the vertebral body according to one of the dash-dotted lines in FIG. 3,

3 is a plan view of a vertebral body with indicated frontal planes,

4 shows the outline of a prosthesis within a frontal plane according to FIG. 3,

5 to 7 different caudal prosthesis contours in comparison with the contours of the associated end plate of a lower vertebral body in a frontal section,

8 to 13 different cranial prosthesis contours in comparison with the contours of the associated end plate of an upper vertebral body in a frontal section,

14 is a representation illustrating the height differences of prostheses and end plate surfaces,

15 to 20 three rasps for preparing the installation space for the prosthesis,

Fig. 21 in comparison the outline of the three rasps and

22 and 23 are perspective views of a prosthesis from different directions.

If one looks at the upper end plate of a vertebral body 1, it is found that it is thin and porous in a central region 2. It is surrounded by an edge zone 3, which is more mineralized, hardly porous and much thicker than the end plate in the central area 2. The lateral sections 4 of this edge zone 3 rise to the steep flanks 5 of the uncovertebral joints. The same thing is repeated on the underside of the vertebral body with reverse sense of curvature. It was found that there is a particularly high degree of mineralization in the marginal zones 4 and the flanks 5, specifically in the dorsolateral areas 6, which are indicated by hatching in FIG. 1. The more mineralized areas have a higher load-bearing capacity and are also better supported by the underlying cancellous bone tissue, as indicated by dotted lines. In many cases, the lateral edge zones 4 merge into the flanks 5 with a continuously increasing inclination, without an anatomical boundary being clearly recognizable. Nevertheless, a limit is shown in FIG. 1 for better understanding. It is the line below which the lateral zones -4 lie, which are suitable for supporting the prosthesis according to the invention, while the flanks 5 above them are too steep, namely steeper than a limit value to be selected, which is in the Usually lies between an inclination of 20 and 40 °.

This support of the prosthesis in the lateral edge zones 4 is clear from FIG. 4, which shows a section along one of the frontal planes indicated by dash-dotted lines in FIG. 3. The prosthesis is shown with solid lines, the end plates of the vertebral bodies are shown with dash-dotted lines. A prerequisite is a prosthesis 7, the lower surface 9 of which has an approximately flat central region 8, which cooperates with the central region 2 of the upper end plate 12 of the lower vertebral body, and has bevelled zones 10 to the sides, which correspond to the lateral edge zones 4 of the lower Interacts with the vertebral body. The prosthesis shape corresponds approximately to the shape of the upper end plate in the section shown of the lower vertebral body, so that no or only slight millings on the vertebral body are necessary to adapt the prosthesis. It is desirable that in the edge zones 4 of the end plate only the cartilage resting on the bone substance is removed, while the bone substance itself remains unaffected or is only slightly refreshed in order to adapt sufficiently to the shape of the prosthesis and to connect better with it.

In the example shown in FIG. 4, no essential milling is required in the central area 8 of the lower surface of the prosthesis. However, at least refreshment of the bone is desired here so that it connects better to the central prosthesis surface 8. In order to facilitate this, the surface of the prosthesis is equipped in its central area 8 in such a way that an intimate and permanently firm connection with the bone results. In particular, it can be provided with elevations and depressions (see the teeth in FIGS. 21, 22) and a coating which activates the bone growth.

Other forms of prosthesis that take this basic idea of the invention into account are shown in FIGS. 5 to 7. 5 shows in frontal section a uniformly rounded prosthesis lower surface, which requires almost no bone milling in the edge zones 4, while the central region 2 is milled deeper. Instead of the deeper milling of the central area, provision can also be made for the surface of the prosthesis in the central area 8 to be designed so that it plastically sinks into the remaining bone substance without bone milling or after slight bone milling. The same applies to the prosthesis shapes that are in Fig. 6 reversed roof-shaped and in Fig. 7 centrally flat and rising in edge zones 10 (similar to the embodiment of FIG. 4) are shown.

It also applies to the shape of the upper side 11 of the prosthesis in the example in FIG. 4 that the lateral edge zones 4 of the associated vertebral body end plate 13 can largely be obtained, while a slight milling is required in the central region. The lateral edge zones 4 of the vertebral body end plate can therefore effectively participate in the power transmission. A substantial part of the power transmission also takes place in the central area. However, thanks to its intimate interlocking with the surface of the prosthesis, this area also serves for long-term anchoring of the prosthesis in the intervertebral space.

The shape examples shown in FIGS. 8 and 9 show, in frontal section, dome-shaped prosthesis surfaces 8 of different degrees of curvature. It is assumed that the associated end plate 13 is a little concave, ie complementary, in the frontal section. In contrast to this, FIG. 10 presupposes a weakly convex end plate 13 and shows that in this case too it is possible to largely preserve the lateral edge zones of the end plate 13 and the milling of the bone is limited to the central region. FIG. 11 shows an example in which the upper side of the prosthesis in the edge zones 14 is almost flat in order to be able to better fit against the edge zones 4 of the bone, while the central region 15 is conical or roof-shaped. This ensures that the prosthesis is safely located on the bone. In addition, richly - just as in the other embodiments - a small-scale interlocking with the bone substance can be provided. In the exemplary embodiment according to FIG. 12, the entire upper side of the prosthesis is roof-shaped or conical. This also protects the lateral

Edge zone of the bone and limits any milling to the central area. Finally, FIG. 13 shows an upper side of the prosthesis that is flat in the central region 16 and beveled in the lateral edge regions 17. This shape is particularly advantageous because not only in the lateral

Marginal zones, but also in the central area a very low milling of the bone is sufficient.

In all examples, the upper and lower sides of the prosthesis are convex. In other words, the prosthesis has a greater height in the central area than in its peripheral areas. This is favorable for accommodating a lenticular prosthesis core (see for example WO03 / 075804). In contrast, the prosthesis core in the peripheral zone ^' n requires a lower height. In this way, the overall height of the prosthesis can be kept low. In particular, it can be kept so low that routing in the lateral edge zones of the vertebral end plates can generally be avoided.

So that the edge zones 10 of the lower surface of the prosthesis can cooperate with the edge zones 4 of the end plate surface, they should be inclined approximately like this.

This inclination α (FIG. 5) is determined with respect to the main plane 14 of the prosthesis or the intervertebral space and should be at least 20 ° for the lower surface of the prosthesis. It is preferably of the order of 30 ° or above. The further the prosthesis extends laterally into the marginal zone of a higher degree of mineralization, the greater the angle of inclination it must reach.

The corresponding angle β (FIG. 10) at the top of the prosthesis can be flatter because the prosthesis is not constricted there by the rising flank of the uncovertebral joint. It can go down to 0 ° and preferably reaches 10 to 30 °.

The preferred height ratios of the prosthesis in relation to the associated end plate can be seen with reference to FIG. 14. Compared to an imaginary central plane 20 (or any parallel plane to it), the end plate surface 17 has a height indicated by the arrow 18 in its central region 2 and an average height according to arrow 19 in its lateral edge zone 4. The heights of the prosthesis surface compared to a corresponding one Middle plane 20 'are indicated by arrows 21 and 22. According to the invention, the difference 23 between the heights 21 and 22 of the prosthesis should be at least as large as the difference 24 between the heights 18 and 19 of the end plate surface. If this condition is met, the goal can be achieved that the lateral edge zones 4 of the end plate need only be subjected to less material removal compared to their central area. This also applies accordingly to the top of the prosthesis.

If in this description of predetermined shapes and

Dimensions of the vertebral bodies and their end plates is assumed, this always means that standardized Shapes and dimensions are meant that have been obtained and standardized on the basis of a large number of measurements on natural vertebral bodies in order to find suitable prosthesis shapes and dimensions on the basis thereof. Usually, a provider of cervical intervertebral prostheses will provide a number of prostheses with different shapes or dimensions so that the doctor can choose the most suitable one for the respective application.

In the context of the invention, only the shape of the prosthesis in frontal section was discussed. The prosthesis can have any shape in the sagittal section. For example, their upper and lower surfaces can be essentially straight or arched in a central sagittal section.

A set of rasps is provided to give the bone surfaces exactly the shape they need to practice the invention. They are shown in FIGS. 15 to 21. They are designed to prepare the surface shape of the vertebrae for receiving the prosthesis. The examples shown are matched to the exemplary embodiment of the prosthesis, which is shown in FIGS. 22 and 23. It has a rectangular outline with rounded corners, which is suitable for making extensive use of the extent of the intervertebral space, including the lateral marginal zones. It is so flat that it can be used without deep milling of the vertebral end plates. It turns the vertebral bodies towards outer surfaces, the majority of which are roughly flat and toothed. Your dorsolateral corners 51 are chamfered so that in 3, frontal sections shown in FIG. 3 show approximately the outline shape of the prosthesis shown in FIGS. 7 and 13.

The outline shape is prepared by the set of rasps 52, 53 and 54 in the vertebral space shown in Figs. 15 to 20. The graded size relationships of the rasps are shown in FIG. 21. After the vertebral bodies involved have been set to the distance by instruments (not shown) that they should be after the prosthesis is inserted, the smallest rasp 52 is inserted into the Intervertebral space pushed in to open access. Their depth of penetration is limited by the stop 56. Accordingly, it does not reach deeper into the intervertebral space than FIG. 21 shows. This is followed by rasp 53, which has a trapezoidal shape, approximately corresponding to the trapezoidal shape of the flat surface portion 50 of the prosthesis surface. Finally, rasp 54 shapes the intervertebral space substantially in accordance with the shape of the prosthesis to be inserted. The height of the rasps is the same as that of the prosthesis.

The rasps are toothless in those areas which correspond to the flat portion 50 of the prosthesis. This means that they only carry out slight abrasion with their leading edge 55. If, on the other hand, the prosthesis is designed in such a way that it requires more milling of the vertebral bodies in the central region, teeth can also be provided in these surfaces of the rasp. In the areas 57 of the rasp 54, which are assigned to the dorsolateral areas of the edge zones of the vertebral end plates, there is a toothing around which free the areas of the lateral marginal zones of cartilage and adjust them to the shape of the prosthesis if necessary.

After the vertebral end plates have been refreshed in their central area for receiving the toothed, central area 50 of the prosthesis, the tips of the prosthesis sink into the comparatively compliant surface of the bone until the beveled edge zones 51 of the prosthesis on the lateral edge zones 4 of the vertebral end plates rest.

Claims

Expectations

1. Intervertebral joint prosthesis for an intervertebral space of the cervical spine, that of the end plates

(12, 13) of the adjacent vertebral body is limited, the surfaces of which in a frontal plane laterally adjacent to a substantially flat, central region (2) have more curved edge zones (4), at least one of the prosthesis surfaces (10, 11) has a lateral extension into the edge zones (4) and the convex curvature of this prosthesis surface (9, 11) in the frontal plane is at least as large as the corresponding curvature of the surface of the end plates (12, 13).

2. Prosthesis according to claim 1, characterized in that its height in the caudal-cranial direction in the lateral marginal zones (10, 14, 17) is approximately the same as the height of the intervertebral space at this point and its height in the central area (8) is larger than that of the intervertebral space at this point.

3. Prosthesis according to claim 1 or 2, characterized in that the prosthesis surface in the central area (8) is provided with elevations and depressions and not in the edge area.

4. Prosthesis according to one of claims 1 to 3, characterized in that the surface of the prosthesis in the central Area (8) is serrated.

5. A prosthesis according to claim 1 or 2, characterized in that the angle of inclination (α) of the edge zones (10) of the lower surface of the prosthesis (9) in the frontal plane relative to the main direction of extent (14) of the prosthesis reaches at least 20 °.

6. A prosthesis according to one of claims 1 to 5, characterized in that the angle of inclination (β) of the edge zones (10) of the upper prosthesis surface (11) with respect to the main direction of extension (14) of the prosthesis is at least 0 ° and preferably 10 to 30 ° reached.

7. A prosthesis according to one of claims 1 to 6, characterized in that the width (15) of the prosthesis is at least 1.5 times as large as its depth (16) to lie in the intervertebral space.

8. Prosthesis according to one of claims 1 to 6, characterized in that the specified shape of the prosthesis is limited to its dorsal area.

9. Intervertebral joint prosthesis, in particular according to one of the claims 1 to 8, for an intervertebral space in which an end plate surface (17) in frontal section has a first distance (18) in its central, less mineralized area (2) and more in its lateral one mineralized lateral edge zones (4) has a second distance (19) from a central plane (20) of the intervertebral space, the prosthesis being The same frontal section shows a central surface area (8) intended for contact with the central area (2) of the end plate surface (17) with a third distance (21) and with its edge areas intended for contact with the lateral edge zones (4) of the end plate surface (17) 10) has a fourth distance (22) from the corresponding central plane (20 '), the third distance (21) being greater than the fourth (22) and the difference (23) between them being at least as large as the difference (24 ) between the first and the second distance (18, 19).

10. Intervertebral joint prosthesis esp. According to one of claims 1 to 9, characterized in that the surface of at least one of its cover plates, the size of which is dimensioned for extensive use of the naturally given area of the intervertebral space, a central area which is approximately parallel to The main extension plane of the cover plate extends and has a dorsolaterally adjoining transition surface which is raised with respect to the central area.

11. Instrument set for inserting a prosthesis according to one of claims 1 to 10 with at least one rasp reflecting the shape of the prosthesis (54) for adapting the vertebral body surfaces to the prosthesis shape, which is designed so that it includes the central region and the peripheral zones and at least the dorsal part of the marginal zones is essentially spared from material removal.