CA1263201A - Endroprosthesis for intervertebral discs - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The present invention relates to a multipart endopro-thesis of the Nucleus pulposus which is inserted between base plate and cover plate of two adjacent vertibral bodies and is for re-establishing and maintaining the distance in the intervertbral space and permits a mobility in the physiological range. Such an endoprothesis of the Nucleus pulposus has biochemical and bio-mechanical tolerance, natural stability on absorbing compression and can be inserted without substantial treatment of the adjacent vertebral bodies while excluding a mechanical destruction of the adjoining vertebral bodies. Three embodiments are provided.
First, the vertebral disc endoprothesis comprises a pair of sym-metrical concave end plates with intervening convex distance piece. End plates and distance piece have a plane guide rim.
The end plates have either a bent rim or an annular tee-slot for a torus on the distance piece. Second, the vertebral disc endo-prothesis comprises a pair of asymmetrical end plates and a distance piece but the two regions of motion are staggered by 90° relative to each other and are jointing planes of a cylinder.
In the third embodiment the two symmetrical end plates are convex and the distance piece is cylindrical and concave at both ends.
Furthermore, it has a strong sheath.

Description

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The present invention relates to a multipart endopros-thesis of -the Nucleus pulposus which is inserted between the base plate and cover plate of two adjacent vertebral bodies.
A number of devices which replace or are at least inten-ded to replace damaged or destroyed in-tervertebral discs are known. There exist devices, like thosedescribed in U.S. Paten-t No. 4,401,112 which merely assure an absorption of pressure with-out restoring the physiological mobility. It is known -to replace a pathalogically changed lntervertebral disc by filling the hollow space formed by operatively removing the Nucleus pulposus, with silicone rubber which polymerizes in situ. The U.S. Patent No.
4, 349,921 described a dowel-shaped intervertebral disc prosthesis.
Multipart interver-tebral disc prostheses of metal or metal-p-las-tics coupling or or plastics are also known, each prosthesis consisting of an upper part and a lower part wi-th press-button-like, disc-or ball-shaped distance piece (see German Patent No.
3,023,353, DE-OS 2263842, and Swiss Pa-tents 624575 and 640131).
U.S. Patent 4,309,777 and Russian Paten-t895433 disclose special constructions.
These devices have the disadvan-tage that they replace the functions of a Nucleus pulposus ei-ther not all or only par-tially or insufficiently. ~n incorporation of bushings into the Spongiosa of the intervertebral bodies has the disadvantage that somewhat lengthlymaniPUlatins on -the pa-tient are required.
Furthermore, it is not desirable that the outer layer of the interver-tebral bodies which is relatively hard as compared with the Spongiosa is destroyed or tapped. Because of the large con-tact loads, the non-uniform pressure distribu-tion and/or -the rigidity of the materials not only does there exis-t the danger of necroses or bone resorptions but also the possibility of a mechanical destruction of the ver-tebral bodies. Further dif-ficulties result from the opera-tion techniques applied and from the fact that there does not exist the require reliability with reyard to changes in position in the use of the intervertebral d:LSC endoprosthf~si.s.
The present invention provides an endoprostehsis of the Nucleus [~]posus as perfect as possible, i.e., a prosthesis which assures the maintainence and/or restoration distance and a phy-siological mobility in the affected spinal section. Furthermore, the ability to function is to be assured over a long period of -time wi-th a maximum preven-tion of changes in position.
Io Thus the present invention provides an endoprosthesis of the Nucleus pulposus which has biochemical and biomechanical -tolerance, natural stability when absorbing pressure and can be inser-ted without substantial trea-tment of the adjacent vertebral bodies while excluding a mechanical destruc-tion of the adjoining vertebral bodies.
According to -the present invention the intervertebral endoprosthesis comprises a pair of end plates wi-th intervening distance piece. In their interaction the end plates and the dis-tance piece permit an inclination of the vertebral bodies in approximately physiological limits. In a first embodiment the dis-tance piece has a partially spherical surface and is lenti-cular in shape. I-t has a flat guide rim and is externally pro-vided with a torus, which prevents sliding and slipping out of the end plates, -the height of the distance piece can be varied cor-responding to the height of the intervertebral space.
In their midportion the symmetrical end plates are con-cave; they also have a flat guide rim and in junction with a bend -they are provided with a serration. The degree of bend of the end plates corresponds -to the sagi-t-tal spine curvature so that the ventrodorsal vertical difference of the intervertebral space can be taken into account. A ventral marking serves for simple inter-operative orientation. The radius of curvature of the concave 3;~

recess exactly corresponds to the radius Of curvature in the sphe~rL(a~ porLion of the d~stance piece.
It is possible to provide tl~e end plates on the two lateral sides wi-th flat extensions, which are so selected that they assume as large as possible an area of the base plate and cover plate of the vetebral body. This results in a reduction of the compressive load per uni-t area. This is particularly desirable for vertebral bodies with reduced carrying capacity.
Addi-tionally or even separately -the contact surface of the end pla-tes can be enlarged in -that the hollow space between the rear of the concave midportion and the bent guide rim is filled with a disc of alloplas-tic material, which can also be bioactive. The use of bone cement also is conceivable. It is also possible to so design the end plates that they are compact, i.e., without a hollow space on the rear.
Finally, the use of compact ma-terial for the entire end plate is also possible. For this purpose the concave mid-por-tion and an annular -tee-slo-t for receiving the torus of a distance piece are incorporated in a compact end plate, which lies against -the base pla-te and cover plate of the vertebral body. The end plates and the dis-tance piece are made of material which has proved itself in -the implan-t technique. For example, the end plates are made of a non-corroding material and the dis-tance piece is made of medical polye-thylene or polyurethane having high tensile and compressive strength. A reverse metal combina-tion is conceivable. The use of other alloplastic materials, which can also be bioactive, is also possible. The end plates and -the distance piece are polished on the contact surfaces to a high luster in order to minimize abrasion (low-friction prin-ciple)-A serration (on the edge and~or flat) on the bottomside of -the end plates serves for reliably anchoring the implant in the :Lntervertebral space. It is possible to coat the serra-tion or the entire bottom side of the end plates bioactively.
Ilowever, it is also possible to anchor the end plates with bone cemen-t.
An intervertebral disc endoprosthesis in which the upper and the lower surEaces of the distance piece as well as the adjoining surfaces of the end plates correspond to jointing planes of a cylinder with identical radius of curvature of the portions articulating with one another and with the plane of mo-tion of one articulation region rotated by 90 relative to the other articula-tion region can also be achieved. The end plates are ei-ther circular or slightly oval and -the distance piece is symme-trically smaller rela-tive there-to.
The intervertebral disc endoprosthesis functioning according to the low-friction principle can be manufactured in four different designs depending on -the posi-tion of the concavely or convexly curved planes of motion. The interver-tebral disc endlprosthesis can be so designed tha-t is ver-tically variable corresponding to -the cranicocaudal or ven-trodorsal extent of the ~ intervertebral disc space. As additional protection against undesired change in position of the midpor-tion it is possible to install thereon a concentric pin extending cranially and caudally.
Said pin extends into the recess disposed in -the end pla-tes. The recesses advantageously are circular or groove-shaped. Conver-sely, end pla-te pins pointing towards the segment centre and boreholes in the midpor-tion or combined variants are also pos-sible.
In the case of stress of the latter embodiment, a rolling motion is carried out by the -two regions of motion in one region and a following slip motion in the other. During the motion the distance piece changes its position and the end plates remain rigidly anchored -to the vertebral bodies, for example, i2~

via a serrat:ior).
Fillally, an intervertebral disc endoprosthesis in which the two symmetrical end planes are convex in their midportion and the di.stance piece is cylindrical and concave at both ends is also possi.ble. ~he radil of curvature of the concave and convex sur-faces are identical. The cylindrical distance piece which con-sists of polyethylene is provided with a s-trong sheath so that the cold flow of the polyethylene which can occur due to the effect of a high compressive load is prevented or res-tricted.
The heigh-t of the shea-th is such that it is less -than -the distance piece in order to exclude direct contact with the end plates.
It is also possible to provide the midpor-tion of the end pl.ates additionally with pins which extend into -the recesses of the distance piece. In -their diame-ter, -the recesses are larger than the pin diame-ters. This kind of construction limits the maximal mobili-ty of -the intervertebral. disc endoprosthesis and constitutes an additional saEeguard against an unintentional change in position of the distance piece.
For the radiological representation plastic parts which usually are no-t visibl.e can be marked correspondingl.y.
The present invention permits, Eor the first time, an al.most comple-te substitute for the Nucleus pulposus and guarantees a physiological mobili-ty in the spine section concerned.
The present invention will be fur-ther illustrated by way of the accompanying drawings, in which:-Figures 1 show end plates;
Figure 2 shows a distance piece;
Figure 3 is a plan view of an end plate;
Figure 4 shows an end plate having lateral extensions;
E'igure 5 is a plan view of an end plate having lateral expansions;

Figure 6 shows a compact end plate;
Figure 7 shows an intervertebral disc endoprosthesis with planes of motion rotated by 90;
F`igure 8 shows an intervertebral disc endoprosthesis with planes of rnotlon rotated by 90;
Figure 9 shows an intervertebral disc endopros-thesis like that in Figures 7 and 8 bu-t with additionalproteCtion by a pin and with circular recesses;
Figure lO shows an intervertebraldisc endoprosthesis 1ike that in Figures 7 and 8 bu-t with additional protection by a pin and with groove-shaped recesses;
Figure 1l shows an intervertebral disc endopros-thesis having convex end plates and a cylindrical distance piece which is concave at both ends;
Figure 12 shows an intervertebral disc endoprosthesis 1ike -tha-t in Figure ll bu-t with additional pro-tec-tion by a pin;
Figure 13 shows a section -through a cylindrical dis-tance piece; and Figure 14 shows an interver-tebral disc endoprosthesis 1ike that in Figure ll but with addi-tional la-teral expansions.
In a first embodiment the end plates are made of a non-corroding metal and have a concave midportion l and an annu-lar flat guide rim 2. The bent edges of the end plates are pro-vided with a serration 3, which guarantee a reliable anchoring ln the vertebral bodies. The anchoring can be carried out with a rim serration in a compact variant with a fla-t serration or with bone cement. The serration can also be biocactive. To maintain the lordosis of the spine -the fron-t edge of -the bent portion is higher than the rear edge. The distance piece has a convex midportion 4 whose radius of curvature exactly corresponds to the concave midportion l of the end plates. Said concave midportion l also is provided with an annular flat guide rim 5 3~
and has a torus 6 to prevent it from slipping out. The height of the di.stance piece can be shaped corresponding to the indi~/idual condiLions of the intervertebral space. It consists of a phy-siologically compatible material and just like the end plates it is polished to a bright luster to minimize abrasion.
rrhe distance piece can be provided with a radiologi-cally visi.ble marking.
In a second embodiment end pl.ates are like those des-cribed in the first embodiment but are additionally provided with flat ex-tensions on the two lateral sides. Said flat extensions can be adap-ted -to -the curvature of the base plate and cover plate of the vertebral body.
The hollow space under -the fl.at guide rim 2 is filled with a disc 8 of alloplastic material. It is also possible -to fill said holl.ow space wi-th bone cement or -to use a compact mid-portion from the outset. The end pl.ate is made of physiologically compatible material.
In the third embodiment end plates for a multipart intervertebral disc endoprosthesis are so-called compact end plates 9. The concave midportion 1 and an annular tee-slot 10 with intervening guide rim 2 are incorporated in said compact end plats. A serration 3 is also present. The curvature of the com-pact end plate 9 can also correspond -to the curvature of the base plate and cover pl.ate of the ver-tebral body.
In a fourth embodiment the intervertebral disc endo-prosthesis comprises sligh-tl.y oval or circul.ar end plates 11 having the same circumference and dis-tance piece 12 which is symmetrically smaller as compared -therewith. The upper and lower surfaces of the distance piece 12 and the adjoining surfaces of the end pl.ates ll correspond to the jointing planes of a cylinder with always identical radius of curvature of the portions articu-lating with one another and in their planes of motion they are rotated by 90 relative to one another.
When this intervertebral disc endoprostehsis functions a rollil-lg mot:ion is carried out in one reyion of motion and a fol1owing slip motion in -the other. During the motions, -the distance piece changes its position and -the end planes remain rigidly anchored on the vertebral bodies.
In a fifth ernbodimen-t the intervertehral disc endopros-thesis is as in the fourth embodiment. Additionally it has con-centric pins 13 extending cranially and caudally. Said pins extend either into circular recesses 1~ or into groove-shaped recesses 15~ A reverse arrangement of pins 13 and recesses 14,15 is possible.
In a six-th embodiment the in-tervertebral disc endo-prosthesis comprises convex end plates 16 and a cylindrical dis-tance piece having concave ends 17. The dis-tance piece is en-compassed by a solid sheath 18. Theradii of curvature of the convex and concave surfaces are identical. The shea-th is smaller in height that the distance piece in order -to avoid con-tac-t with the end pla-tes.
In the seventh embodiment the intervertebral disc enco-prosthesis is as in the sixth embodimen-t. The end plates 16 additionally have pins 19 extending in the cranial and caudal directions. Said pins engage circular recesses 20. The diameters of the recesses 20 are larger than the pins 19.
In an eighth embodiment the intervertebral disc endo-prosthesis is either like that in the sixth or seventh embodiments.
The convex end plates 16 additionally have lateral extensions 21 on the two sides.

Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An intervertebral disc endoprosthesis, comprising two symmetrical end plates having a concave midportion, and a vertically adjustable biconcave distance piece fitting thereto having the same radius of curvature as the concave midportion, said end plates having an annular flat guide rim which is bent at its outer edge and provided with anchoring means in the form of rim serrations, and the distance piece having an annular flat guide rim encompassed by a torus which prevents it from sliding or slipping out of the end plates.

2. An intervertebral disc endoprosthesis as claimed in claim 1 wherein the outer edges of the end plates are bent at varying heights corresponding to the curvature of the vertebral column.

3. An intervertebral disc endoprosthesis as claimed in claim 1, wherein the flat guide rim of the end plates is provided with flat extensions which are adapted to the curvature of the vertebral body.

4. An intervertebral disc endoprosthesis as claimed in any of claims 1 to 3, wherein the hollow space between end plate and vertebral body is filled with a disc of alloplastic material and/or with bone cement.

5. An intervertebral disc endoprosthesis as claimed any of claims 1 to 3, wherein comprising compact said end plates having lateral flat extensions with an annular groove surrounding said concave midportion and an intervening guide rim.

6. Intervertebral disc endoprosthesis comprising first and second end plates and a spacing piece, said spacing piece having top and bottom surfaces, said first end plate having a first surface adjacent the top surface of said spacing piece and said second end plate having a second surface adjacent the bottom surface of said spacing piece, said top, bottom, first and second surfaces each being partial surfaces of a cylinder, said top surface and said first surface adjacent the top surface being congruent and defining a first cylinder-shaped movement surface and said bottom surface and said second surface adjacent the bottom surface being congruent and defining a second cylinder-shape movement surface, said first and second movement surfaces being rotationally-offset 90° with respect to each other about a central axis passing through said spacing piece and said end plates.

7. Intervertebral disc endoprothesis according to claim 6 further comprising respective pins cranially and caudally extending from the spacing piece and a recess in each of the end plates each receiving a respective one of the pins.