WO2007023399A1

WO2007023399A1 - A novel intervertebral disc prosthesis

Info

Publication number: WO2007023399A1
Application number: PCT/IB2006/052359
Authority: WO
Inventors: Luigi Ambrosio; Elizabeth K. Tanner
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2005-07-13
Filing date: 2006-07-12
Publication date: 2007-03-01
Also published as: ITTO20050480A1; WO2007023399A8

Abstract

A disc prosthesis, suitable for replacing an intervertebral disc, includes a first (1; 1') and a second (2; 2') articulation component. The components comprise respective endplates (11, 21; 11', 21') suitable for being anchored to respective vertebrae. . The first articulation component further comprises a spacer member (13; 13') suitable for being interposed between the endplates, and which is disposed on the endplate (11; 11') of the first component (1; 1') and has a convex articulation surface (19; 19') suitable for being placed in contact with a corresponding concave articulation surface (29; 29') disposed on the endplate of- the second component. The articulation -components are made at least partially of polyethylene reinforced with calcium phosphate, one of same further having a part (17; 27'), facing towards the other articulation component, which is made of ultra-high molecular weight polyethylene, and on which one of the articulation surfaces is provided.

Description

A novel intervertebral disc prosthesis

The present invention relates to a disc prosthesis suitable for replacing an intervertebral disc, of the type defined in the preamble of claim 1.

As is known, the disc is a composite structure, composed of a central nucleus, known as the "pulpous nucleus" (nucleus pulposus) , surrounded by an envelope of fibrous sheets, known as the "fibrous ring" (annulus fibrosis) .

The pulpous nucleus is a semifluid internal mass, composed principally of water, which occupies between 65% and 85% of the entire volume of the disc, depending on the age and on the region of the vertebral column., and of hydrophilic proteins termed proteoglycanes . The water moreover contributes to the control of the feeding of the system and of the mechanical properties; collagen, however, which is a structural protein and one of the fundamental constituents of the soft tissues, is differently organised in the space occupied by the intervertebral disc. The collagen fibres, in fact, are inclined with respect to the axis of the dorsal spine and organised in superposed structures; the angle identified decreases from 62° to 45°, going from the outside to the inside of the disc (Qi-Bin Bao, Geoffry M. McCullen, Paul A. Higham, John H. Dumbleton, Hansen A. Yuan, The artificial disc: theory, design and materials. Biomaterials 17 (1996) 1157-1167) .

The mechanical function of the disc is very precise: the fibrous ring imparts stability to the intervertebral body, by containing the nucleus and resisting tensional states; the nucleus, however, distributes the compressive stresses which are transmitted between the vertebrae of the dorsal spine (Qi-Bin Bao et al., ibid.) .

The collagen fibres are immersed in the gel which develops a swelling pressure, the effect of which is, precisely, that of stabilising the intervertebral body, making it possible to resist the compressive stresses and to transmit same. Above and below, in contact with the vertebrae, there are two thin layers of cartilage, known as "endplates" .

The ageing of the intervertebral disc is a normal, non- pathological process; it commences aproximately around the age of thirty, with a gradual change in the types of proteoglycanes and with a loss of the overall water content. Since the nucleus becomes dehydrated and shrinks, the load on it decreases, while the load on the annulus increases. Like the wheel of an automobile, the multi-layer structure of the fibrous ring behaves well when it is "inflated", while it is subject to delamination and damage when it is "deflated". Radial lacerations, cracks and splits occur first within the annulus; some of these, however, may be repaired. In any case, if the repair does not take place, the nucleus may migrate from the centre of the disc to the periphery, through the laceration (Qi-Bin Bao et al., ibid.).

It is precisely the degeneration of the intervertebral disc which therefore represents one of the principal causes of lumbar pain, among the most common clinical cases in the western world.

Although the two principal surgical interventions for treating the degenerate disc, discectomy and fusion, produce relatively good clinical results in the short term, both approaches alter the biomechanics of the column, composed of a series of co-operating elements, probably leading to a more rapid degeneration of the surrounding tissues and of the discs at an adjacent level {Qi-Bin Bao et al., ibid. ;. Vincent C. Traynelis, M.D., Spinal arthroplasty. Neurosurg Focus 13 (2002) Article 10) . Alternatively, the solution for a degenerate disc involves the use of an artificial one as substitute.

In recent years, a tremendous effort has been made in order to produce artificial discs capable of replacing the degenerate disc (Karin Buttner-Janz, The Development of the Artificial Disc SB Charite. Hundley & Associates (1992) ; Patents US 5,556,431 and US 5,314,477, and patent application US 2004/0243240) .

The current prosthetic systems are^* composed of metals, polymers and metal-polymer combinations (Qi-Bin Bao et al., ibid.); these latter are constituted principally by two metal plates (endplates) which include one or two polymeric components, generally of polyethylene (Qi-Bin Bao et al, ibid.; Karin Buttner-Janz, ibid.; patents US 5,556,431 and US 5,314,477, and patent application US 2004/0243240) .

The principal advantage of producing a disc entirely of metallic material lies in the high resistance to fatigue of such materials; Hedman et al. (Qi-Bin Bao et al., ibid.; Thomas P. Hedman, BME, SMME, John P. Kostuik, MD, FRCS, Geoffrey R. Fernie, PhD, and Wayne G. Hellier, BASc, MASc, Design of an Intervertebral Disc Prosthesis. Spine 16 (1991) 256-260) believed in fact that only metals were long-lived from such a point of view, that is, lasting for up to 100 million cycles at constant amplitude, equivalent to 40 years of life. A "spacer-disc" of metallic material was presented for the first time, in 1954, by Knowles (Qi-Bin Bao et al., ibid.); however, it was a mechanism which was not capable of reproducing the flexibility typical of the natural disc and, consequently, can not be indicated as an "artificial disc".

Numerous attempts were made subsequently, but none was capable of reproducing the complicated movement of the natural disc, with its continual changing of the centre of rotation.

Hedman et al. originated a model characterized by two spiral springs, placed between the plates, with a rear hinge which permitted extension and flexion.

The springs, made of titanium alloy and designed to provide adequate rigidity, at the same time -allowed extension and flexion, as already stated. The remainder of the device was composed of an alloy of cobalt and chromium, isostatically hot-pressed (HIped) , having a high carbon content to limit wear (Qi-Bin Bao et al., ibid.). Vertically protruding fins were positioned in front of the plates and .laterally thereof; moreover, the screws could be fixed through the fins.

Kostuik (Qi-Bin Bao et al., ibid.), experimenting on sheep, succeeded in demonstrating that, at least in the short term, the fibrous tissue did not grow between the pins or around the spiral springs; otherwise, in fact, the eventual internal growth would significantly hinder the mechanical elements of the disc.

Salib and Pettine (Qi-Bin Bao et al., ibid.), mindful of the hip arthroplasty project, produced a system in the shape of a ball and socket, coated externally with ceramic zirconium oxide; in this way, the growth of the fibrous tissue between the components proved less probable. The base plates, which supported either the ball or the socket, were fixed to the vertebrae by means of fins; this system, therefore, proved to be characterized by 6 degrees of freedom. However, the movement accompanied by the compressive load to which the natural system is subjected could lead to an increase in friction between the parts in motion and to the consequent generation of fragments (debris) due actually to wear, and which turned out to be the principal cause of the mobilisation of the prosthetic systems and, therefore, of their failure. Obviously, before the production of models made of metallic material, such as that with the sliding surfaces shaped in the form of a ball and socket, and that with the springs and the pins, the biocompatibility of some alloys (those of titanium, cobalt-chromium) and stainless steel had already been demonstrated in the field of orthopaedic applications.

It should be noted, however, that even if the principal characteristic of the metals is their resistance to fatigue, the great benefit which comes from the use of non-metallic materials, such as polymers and elastomers, lies in their mechanical similarity to the natural disc (Qi-Bin Bao et al., ibid.). With a lower modulus of elasticity, in fact, it is easier to replicate, in the short term, the dynamics of the disc; the difficulties arise however as soon as an attempt is made to produce a long-lasting component which is characterized by a stable interface between structure and vertebrae (Qi-Bin Bao et al., ibid.).

In 1956, van Steenbrugghe (Qi-Bin Bao et al., ibid.) described a multi-component disc, characterized by intermediate cushions which were inlaid with plastic bodies of variable shape.

The method of anchorage was not however included in the specifics of the project.

Stubstad et al., (Qi-Bin Bao et al., ibid.), in 1975, however, used only synthetic materials for the production of a disc prosthesis; elastomeric silicone or other elastomers, such as polyurethane, were modelled according to the "bean" shape of the nucleus, including the core full of fluid, placed between two large elastomeric plates, upper and lower.

Further proposed was a covering weave of Dacron fibres for simulating the annulus; the pores of the weave, by allowing the growth of the tissue towards the inside, could also function as fundamental elements for fixing.

In a similar manner, Downey (Qi-Biή Bao et al . , ibid.) originated a model of a disc characterized by a central core, limited above and below by endplates fixed by means of two screws; the core was filled with a soft polymeric foam, while the endplates were made with a more rigid silicone.

In 1978, Weber (Qi-Bin Bao et al., ibid.) produced a disc prosthesis with three components and which consisted of two support bodies with concave central surfaces which connected with each other and articulated on each of the sides of a central spacer body. As far as the production of the supports is concerned, polyethylene was proposed, while the spacer was produced from a bioceramic material .

Edeland (Qi-Bin Bao et al., ibid.) proposed a model similar to that of Weber, with a nucleus characterized by a porous silicone placed between two base plates made of polyethylene; in reality he suggested a series of systems in which nucleus and annulus were always made of silicone. However, Edeland recognized the need to demonstrate the biomechanical applicability and the biocompatibility of such materials.

Monson (Qi-Bin Bao et al., ibid.) came up again with a model entirely of rubber and which consisted of two hollow parts, held together by means of an adhesive, and a resultant central cavity filled with a saline substance.

The project of Dove et al. (Qi-Bin Bao et al., ibid.), however, required a matrix of plastics material (epoxide resin, polyethylene) reinforced with carbon fibres and shaped like a horseshoe. Staggered holes, arranged on the inner edge, were further produced for positioning the screws.

The disc of Lee et al. and Parsons et al. (Qi-Bin Bao et al., ibid.) was characterized by a central core of soft elastomer and flat layers of fibres with specific alternate orientation of same, disposed in a number of from .six to fifteen sheets, placed in a second elastomer; two rigid endplates made of elastomeric or metallic material or made of hydroxyapatite, limited the whole above and below.

This seems to be the first time that the rigidity to compression-torsion was taken into consideration in the design of an artificial disc; by means of the selection of appropriate materials, this model can reproduce, at the same time, both the modulus of compression and the compressive- torsional rigidity of the natural disc (Qi-Bin Bao et al., ibid. ) .

En route, three different biomaterials were proposed: a silicone rubber, polyurethane and, more recently, a thermoplastic multi-component model, composed of a modified polysiloxane, known as C-Flex (Qi-Bin Bao et al., ibid.).

A "sandwich" model was instead adopted by Tadano et al. This consisted of two 3 mm thick plates, with a semicircular "disc" interposed, characterized by a height of 8 mm and a radius of 40 mm. Owing to their demonstrated ability to form strong bonds with the bony tissue, glass ceramics containing apatite and wollastonite were instead suggested as materials for the production of the endplates (Qi-Bin Bao et al., ibid. ) .

In an attempt to remedy the drawbacks connected with the use solely of metallic or non-metallic materials, some experts brought to the forefront a combination thereof; more commonly, it was a question of a disc in the form of a metal- polymer-metal sandwich (Qi-Bin Bao et al., ibid.).

A metal tray was used to improve the fixing via the use of fins with screws or a porous coating for the growth of the tissue towards the inside; with the whole fixed in such a stable manner, the polymer served to. provide the required flexibility (Qi-Bin Bao et al., ibid.).

Frey and Koch (Qi-Bin Bao et al., ibid.) proposed three different models of artificial disc.

The first consisted of a bean-shaped body entirely of synthetic or metallic material, covered by a network of metal (titanium) wires for the internal growth of the bone.

The second, however, was characterized by a compressible plastics material (polyurethane) , with a central cavity which formed a toroidal ring; an incompressible fluid of variable viscosity could be placed in the space obtained. With the application of the load, this fluid was pushed into the interconnection channels of the ring, permitting the localized modulation of the pressure. As in the Stubstad model, a reinforcing weave, entirely of synthetic hydrocarbon fibres, was used with the addition of an anchorage system constituted by a network of metal wires made of titanium.

Their last model, however, consisted of a disc entirely made of metallic material .

However, the most diffused and lasting clinical trial, among the existing artificial discs, seems to be provided by the LINK SB Charite (Qi-Bin Bao et al . , ibid.; Karin Buttner- Janz, ibid. ) .

This was devised by Buttner-Janz et al . and Zippel around the mid-Eighties, but the prosthesis was subjected to a series of structural and commercial modifications (Qi-Bin Bao et al., ibid.; Karin Buttner-Janz, ibid.; patent US 5,556,431). The present model has two concave endplates, made of an alloy of cobalt and chromium and characterized by hooks or "teeth", which allow fixing to the vertebral body without cement (Qi- Bin Bao et al . , ibid.) . Between the two endplates appears the presence of a bi-convex oval body made of polyethylene.

Steffee (Qi-Bin Bao et al., ibid.), in the United States, improved the initial clinical experiment via the implantation of a particular system in six patients . The model consisted of a core of polyolefinic rubber vulcanized onto two titanium endplates; fixing was possible via a porous coating, which promoted the growth of the tissue towards the inside, and four cone-shaped supports, which extended into the vertebral body. W 2

10

However, failure occurred in two of the six patients into whom said model was implanted, as a result of the breakage of the rubbery core (Qi-Bin Bao et al., ibid.).

After eliminating a chemical compound, 2- mercaptobenzothiazole, probably carcinogenic, which was used in the process of vulcanization of the rubber, the model became the first artificial disc approved by the FDA for clinical use, under the direction of the IDE (Investigational Device Exemption) .

Fuhrmann et al. (Qi-Bin Bao et al., ibid.) used a corrugated circular or elliptical tube, limited on each of the two ends by an endplate and filled with a viscoelastic material, introduced in the form of a liquid. Movement became possible via the flexion of the corrugated tube, with the elastic return provided by the central material.

More recently, Marnay has developed an articulating device, characterized by a mono-convex core of polyethylene and endplates made of titanium which have projections for anchorage to the vertebral bodies (Qi-Bin Bao et al., ibid.; patent US 5,314,477).

However, it has been reported that^' the disc prostheses currently on the market (or in development) are frequently subject to failure owing to the wear and degeneration of the materials, to the surgical techniques used for implantation, or to the difference between the mechanical properties of the devices and those of the natural tissue.

Reproducing the disco-vertebral joint means, above all, providing kinematic and dynamic properties similar to those of the natural structure, avoiding the production of particulate debris and, therefore, dimensional changes due to the wear which occurs between two surfaces in contact and in motion relative to each other,- it was in fact demonstrated that the body reacts firstly to the volume of particles produced and then to any biological incompatibility (Qi-Bin Bao et al . , ibid.; Thomas P. Hedman, ibid.).

It is therefore an aim of the present invention to produce a disc prosthesis capable of achieving such objectives.

This aim is achieved according to the invention by a disc prosthesis having the characteristics defined in claim 1.

The present invention is based on a^' suitable coupling of polymeric and composite elements in contact, capable of being placed in motion relative to one another. According to the invention, a multi-component prosthesis comprises a mono- convex nucleus which, in the implanted state of the prosthesis, is placed between two endplates of polyethylene reinforced with calcium phosphate, and preferably hydroxyapatite, and suitably shaped. One of the contact surfaces is provided on a part made of ultra high molecular weight polyethylene. The choice of materials makes it possible to minimise the wear between the coupled surfaces.

Preferably, for the purpose of favouring right from the start the anchorage to the vertebral bodies, the endplates have teeth on the outer side of the articulation.

The implant must, obviously, be contained within the normal disc space, the dimensions of the natural structure being evaluated, for example, by means of tomographic analysis. A description will now be given of some preferred but non- limiting embodiments of the invention, with reference to the appended drawings, in which:

Figure 1 is a diagrammatic exploded view, in perspective, of a first embodiment of a disc prosthesis according to the invention, and

Figure 2 is a diagrammatic exploded view, in perspective, of a second embodiment of a disc prosthesis according to the invention.

With reference to Fig. 1, a first embodiment of a disc prosthesis according to the invention comprises a first and a second articulation component 1, 2, comprising respective endplates 11, 21. The endplates 11, 21 are suitable for being anchored to respective vertebrae (not illustrated) , for example by means of screws. In order to favour right from the start the anchorage to the vertebral bodies, the endplates 11, 21 have teeth 12, 22 on the side intended to face towards the vertebral bodies. The teeth may be of cylindrical, conical, frustoconical or rectangular shape. In an installed state of the device, the prosthesis is intended to replace an intervertebral disc .

On the endplate 11 of the first component 1 a mono-convex nucleus 13 is arranged, acting as a spacer between the endplate 11 of the first component 1 and the endplate 21 of the second component. In an installed state of the device, the spacer 13 is interposed between the endplates 11, 21. The spacer 13 has a convex articulation surface 19, suitable for being placed in contact with a 'corresponding concave articulation surface 29 disposed on the endplate 21 of the second component 2. In an installed state of the device, the spacer 13 is partially inserted into the cavity delimited by the concave surface 29. The convex and concave surfaces 19 and 29 are therefore capable of sliding one against the other, thus providing an articulation.

The first component 1 is produced partially from polyethylene reinforced with calcium phosphate, and preferably hydroxyapatite . An example of such a composite material is the product HAPEX™, (further described in the patent application GB 2 085 461, and patents US 5,017,627 and US 5,962,549), a polyethylene reinforced with 40% by volume of hydroxyapatite (P. T. Thon That, K. E. Tanner, W. Bonfield, Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. I. Uniaxial fatigue. Journal of Biomedical Materials Research 51 (2000) 453-460; P.T. Thon That, K. E. Tanner, W. Bonfield, Fatigue characterization of a hydroxyapatite-reinforced polyethylene composite. II. Biaxial fatigue. Journal of Biomedical Materials Research 51 (2000) 461-468; Huang J., Di Silvio L., Wang M., Tanner K. E. and Bonfield W. , In vitro mechanical and biological assessment of hydroxyapatite-reinforced polyethylene composite. Journal of Materials Science: Materials in Medicine 8 (1997) 775-779; Di Silvio L., Dalby M. and Bonfield W. , In vitro response of osteoblasts to hydroxyapatite-reinforced polyethylene composites. Journal of Materials Science: Materials in Medicine 9 (1998) 845-848) . Another part 17 of the first component 1 is made of ultrahigh molecular weight polyethylene (UHMWPE) . In more detail, the part 17 made of UHMWPE is disposed so as to face towards the endplate 21 of the second component 2. The thickness of the part 17 of the first component 1 may be uniform, or variable along the side of the component 1 facing towards the second component 2. Moreover, it may also cover that side only partially. In any case, the part 17 made of UHMWPE is disposed in such a way that the convex articulation surface 19 is provided on that part. The concave articulation surface 29, however, is provided on the hydroxyapatite-reinforced polyethylene of the endplate 21 of the second component 2.

The distribution of the two materials on the first component 1 may be such that the endplate 11 is made entirely of hydroxyapatite-reinforced polyethylene, and the spacer 13 is made entirely of UHMWPE. Alternatively, the spacer 13 may be made internally of hydroxyapatite-reinforced polyethylene, therefore forming an extension of the endplate 11, and have a coating of UHMWPE.

With reference to Fig. 2, a second embodiment of a disc prosthesis according to the invention comprises a first and a second articulation component 1 ' , 2 ' , comprising respective endplates 11', 21', suitable for being anchored to respective vertebrae (not illustrated) , for example by means of screws. In order to favour right from the start the anchorage to the vertebral bodies, the endplates II¹, 21' have teeth 12', 22' on the side intended to face towards the vertebral bodies . The teeth may be of cylindrical, conical, frustoconical or rectangular shape .

On the endplate 11' of the first component 1' a mono-convex nucleus 13' is arranged, acting as a spacer between the endplate 11' of the first component I¹ and the endplate 21' of the second component. In an installed state of the device, the spacer 13' is interposed between the endplates 11', 21'. The spacer 13 ' has a convex articulation surface 19 ' , suitable for being placed in contact with a corresponding concave articulation surface 29' disposed on the endplate 21' of the second component 2 ' . In an installed state of the device, the spacer 13' is partially inserted into the cavity delimited by the concave surface 29'. The convex and concave surfaces 19' and 29' are therefore capable of sliding one against the other, thus providing an articulation.

The second component 2 • is produced partially from polyethylene reinforced with calcium phosphate, and preferably hydroxyapatite. An example of such a composite material is the product HAPEX™. Another part 27' of the second component 2 ' is made of ultra-high molecular weight polyethylene (UHMWPE). In more detail, the part 2V made of UHMWPE is disposed so as to face towards the endplate 11 ' of the first component I¹. The thickness of the part 27' of the second component 2 ' may be uniform, or variable along the side of the component 2 ' facing towards the first component I¹. Moreover, it may also cover that side only partially. In any case, the part 27' made of UHMWPE is disposed in such a way that the concave articulation surface 29' is provided on that part. The convex articulation surface 19 ', however, is provided on the hydroxyapatite-reinforced polyethylene of the endplate 11 ' of the first component 1 ' .

In that embodiment, the spacer 13/ forms an integral extension of the endplate 11' of the first component I¹, and is therefore made of hydroxyapatite-reinforced polyethylene.

The UHMWPE-HAPEX™ combination used for the production of the contact surfaces 19 and 29, or 19' and 29', designed for relative motion between them, has produced satisfactory results in terms of gravimetric measurements of wear, as determined by tests carried out according to standard ASTM F 2025-00.

A loss of mass technique was in fact considered for evaluating the wear properties of polymeric and composite materials, such as UHMWPE and HAPEX™, for the purpose of identifying the best combination thereof to be used to reproduce the disco-vertebral joint; to this end, the wear was understood as loss of material from a component as a result of a tangential motion against another component, under a compressive load.

By imposing a variable load cyclically, by means of a pin on disk machine, reproducing with close approximation the physiological stresses acting on the lumbar discs during walking, for the UHMWPE-HAPEX_TM combination a wear velocity was estimated, understood as total mass lost per cycle unit, equal to 10^"4 mg/cycle, which is an order of magnitude smaller than those of the homologue combinations UHMWPE- UHMWPE and HAPEX™-HAPEX™.

The preparation of the different components and couplings (UHMWPE-HAPEX™) was carried out by means of the process of hot-pressing of the respective materials, and in particular in the temperature range of 140-160⁰C, in dies having the desired shape. In the components formed by the UHMWPE-HAPEX™ combination, the materials were placed in the die according to a predetermined distribution, and then subjected to a single pressing stage. The components may alternatively be obtained for example by injection moulding or co-extrusion. The shape of the various parts, and in particular of the articulation surfaces, may further be optimised by means of machining.

The invention thus devised is capable of numerous modifications and variants, all coming within the scope of the same innovative concept.

Claims

1. A disc prosthesis suitable for replacing an intervertebral disc, which includes a first (1; I¹) and a second (2; 2') articulation component, comprising respective endplates (11, 21; 11", 21') suitable for being anchored to respective vertebrae, wherein the first articulation component (1; I¹) further comprises a spacer (13; 13') suitable for being interposed between said endplates, said spacer being disposed on the endplate (11; Il ') of the first component (1; 1') and having a convex articulation surface (19; 19') suitable for being placed in contact with a corresponding concave articulation surface (29; 29') disposed on the endplate (12; 12') of the second component (2; 2¹)_/ characterized in that said articulation components are made at least partially of polyethylene reinforced with calcium phosphate, one of same further having a part (17; 27'), facing towards the other articulation component, which is made of ultra-high molecular weight • polyethylene, and on which one of said articulation surfaces is provided.

2. A prosthesis according to claim 1, wherein said articulation components are made at least partially of hydroxyapatite-reinforced polyethylene .

3. A prosthesis according to claim 2, wherein said articulation components are made at least partially of HAPEX™.

4. A prosthesis according to any one of the preceding claims, wherein the part (17) made of ultra-high molecular weight polyethylene is disposed on the first component (1) and the convex articulation surface (1^*9) is provided on that part.

5. A prosthesis according to claim 4, wherein the part (17) made of ultra-high molecular weight polyethylene covers the spacer (13) , said spacer being made of the same material as the endplate (11) of the first component (1) .

6. A prosthesis according to claim 4 , wherein the spacer (13) is made entirely of ultra-high molecular weight polyethylene, the endplate (11) of the first component (1) being of polyethylene reinforced with calcium phosphate.

7. A prosthesis according to any one of claims 1 to 3, wherein the part (27') made of ultra-high molecular weight polyethylene is disposed on the second component (2 ' ) and the concave articulation surface (29') is provided on that part.

8. A prosthesis according to any one of the preceding claims, wherein said endplates have, on the outer side with respect to the articulation, anchorage teeth (12, 22; 12', 22 ') for fixing to the respective vertebrae, and being of cylindrical, conical, frustoconical or rectangular shape.

9. A prosthesis according to any one of the preceding claims, wherein said articulation components are produced by means of a compression, injection-moulding or co-extrusion process.