WO2007007284A2

WO2007007284A2 - Composite biomimetic total intervertebral disc prosthesis

Info

Publication number: WO2007007284A2
Application number: PCT/IB2006/052365
Authority: WO
Inventors: Luigi Ambrosio; Filippo Causa; Roberto De Santis; Luigi Nicolais
Original assignee: Consiglio Nazionale Delle Ricerche
Priority date: 2005-07-13
Filing date: 2006-07-12
Publication date: 2007-01-18
Also published as: WO2007007284A3; ITTO20050481A1

Abstract

A disc prosthesis, suitable for replacing an intervertebral disc, comprises a pair of endplates (11, 21) , and an intermediate body (31) , interposed or suitable for being interposed between the endplates. The intermediate body- comprises a nucleus (32) made of hydrogel, around which is wound a peripheral structure (35) made of fibre-reinforced hydrogel .

Description

Composite biomimetic total 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.

The degeneration of the intervertebral disc, an inevitable process of ageing, is one of the main causes of lumbar pain.

At present there are two principal surgical interventions for treating the conditions relating to the "degenerate" disc: discectomy and fusion.

Although discectomy and fusion may produce relatively good clinical results in the short term, both surgical approaches alter the biomechanics of the column, probably leading to a more rapid degeneration of the surrounding tissues and of the discs at an adjacent level (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; 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 (Qi-Bin Bao et al. , ibid. ) .

In recent years, a tremendous effort has been made 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 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 5 314 477, and Patent Application US 2004/0243240) .

The principal advantage of producing a disc totally made 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 turn out to be the principal caμse 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 of 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-Bin 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 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, having 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 came up with 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 of 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.; Patent US 5 556 431). 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. 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 plates; 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.

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 (Qi-Bin Bao et al., ibid.) .

More recently, Marnay has developed an articulating device, comprising 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.

In order to design an alternative artificial intervertebral disc having suitable transport, mechanical and biological properties, research was focused on the concept of reproducing the natural structure of the tissues of the disc.

The natural intervertebral disc consists principally of collagen fibres immersed in a gel of proteoglycanes and water. This gel develops a high swelling pressure which imparts to the disc the characteristic of resisting compression.

The type and the orientation of the collagen in the disc has an.important influence on how the load is distributed.

In the natural- structure there is a distribution of type and orientation of the collagen from the nucleus to the fibrous ring; the orientation of the collagen fibres decreases from 62° to 45° going from the outside towards the inside of the disc (J. J. Cassidy, A. Hiltner, E. Baer, The response of the hierarchical structure of the intervertebral disc to uniaxial compression. Journal of Materials Science: Materials in Medicine 1 (1990) 69-80) . Water is the principal constituent of the disc and occupies from 65 to 85% of the entire volume, depending on the age and on the region of the column (Qi-Bin Bao et al., ibid.) .

The structure of the intervertebral disc, like its biomechanical and transport properties, is unique and very- complex.

In order to simulate, in the development of prostheses, the characteristics of the natural structure, hydrogels are the subject of study, not only for the substitution of the nucleus, but also for the total replacement of the disc, where high mechanical properties are required.

However, the mechanical properties of these materials in the hydrated state are not sufficient for biomedical applications, where a high mechanical resistance is required.

The role of polymers as materials for biomedical prostheses has become increasingly important in recent years; precisely hydrogels, of which polyhydroxyethylmethacrylate (PHEMA) is an example, are used in a large number of biomedical applications, inasmuch as they are characterized by a structure, a high water content and a softness similar to those of the natural tissues.

The porosity and permeability of hydrogels moreover permit easy extraction of unwanted chemical substances used during the polymerisation process, and once implanted they allow the diffusion of metabolites necessary for the vitality of the surrounding tissues (Allan S. Hoffman, Hydrogels for biomedical applications. Advanced Drug Delivery Reviews 43 (2002) 3-12) ; unfortunately, however they exhibit poor mechanical properties, which significantly reduce the possibility of application as materials for artificial implants .

On the other hand, hydrophobic polymers such as polymethylmethacrylate (PMMA.) or polycaprolactone (PCL) , exhibiting a high degree of rigidity and hardness, may be used when a specific mechanical performance is required of the prosthesis; however, precisely the high degree of rigidity and often fragility alongside their low permeability may cause instability in the region between the prosthetic implant and the surrounding tissue.

The aim of the present invention is to produce a disc prosthesis capable of remedying the drawbacks present in the state of the art, and which has, in particular, characteristics similar to those of the natural system of the tissues that is present in an intervertebral disc .

The subject of the invention is therefore a disc prosthesis having the characteristics defined in claim 1.

The disc prosthesis according to the present invention comprises a composite hydrogel-based intermediate body reinforced with fibres, disposed in such a way as to form a peripheral structure which surrounds a nucleus.

Such a material used in the disc prosthesis makes it possible to obtain characteristics similar to those of the natural system, which exhibits, from the mechanical point of view, a viscoelastic behaviour (L. Ambrosio, R. De Santis, L. Nicolais, Composite hydrogels for implants. Proclnstn Mech Engrs 212 (1998) , Part H, 93-99) .

Preferably, for the purpose of simulating more precisely the architecture of the natural system, the production of the prosthesis is carried out by means of a helical winding of the fibres, immersed in the prepared reactive solution, about a rotating mandrel, suitably inclined with respect to the axis of rotation, therefore using the technology of filament winding and, subsequently, that of forming.

Even more preferably, the fibres used are surface-treated in a suitable manner so as to improve adhesion to the polymeric matrix.

Preferred embodiments of the invention are defined in the dependent claims .

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 partially sectional diagrammatic perspective view of an embodiment of a disc prosthesis according to the invention, and

Figure 2 is a diagram of an apparatus usable for the production of a disc prosthesis according to the invention.

With reference to Fig. 1, a disc prosthesis according to the invention comprises a pair of endplates 11, 21. The endplates 11, 21 are suitable for being anchored to respective vertebrae (not illustrated) , for example by means of . screws or a cement. For the purpose of favouring, right from the start, the anchorage to the vertebral bodies, the endplates 11, 21 have teeth 12 (for the sake of simplicity, only the teeth on the lower endplate 11 are illustrated) on the side intended to face towards the vertebral bodies .

Between the endplates 11 and 21 an intermediate body 31 is interposed. The intermediate body 31 comprises a nucleus 32 made of hydrogel, around which is wound an annular structure 35 made of hydrogel reinforced with fibres, preferably- polymeric .

In particular, the annular structure 35 comprises a plurality of radially superposed layers 35a, 35b, 35c, 35d constituted by polymeric fibres immersed in the same hydrogel matrix which forms the nucleus 32.

In Pig. 1, the fibres of the different layers of the annular structure have an angle of winding of the fibres which varies from 45° for the radially innermost layer 35a to 65° for the radially outermost layer 35d. It is understood, however, that the outer annular structure 35 is produced, in general, by winding the fibres helically at an angle of from 0 - 90° with respect to an axial direction x of the device.

The fibres of the annular structure 35 may be Kevlar fibres, or polyester fibres or, in any case, polymeric fibres.

The hydrogel material constituting the nucleus 32 and also the matrix in which the fibres of the annular structure 35 are immersed may be produced, for example, with polyhydroxyethylmethacrylate (PHEMA) , or with PHEMA reinforced with polymethylmethacrylate (PMMA) or with polycaprolactone (PCL) (P.A. Davis, L. 'Nicolais, L. Ambrosio, S.J. Huang, "Poly (2- hydroxyethylmethacrylate) /Poly (caprolactone) Semi- interpenetrating Network". Journal of Bioactive and Compatible Polymers, Vol. 3, pp. 205-218, 1988) .

A device produced experimentally provides for a hydrogel nucleus 32 of PHEMA/PMMA and an outer^" ring 35 of PHEMA/PMMA hydrogel reinforced with fibres of polyethyleneterephthalate

(PET) wound helically, covered above and below by endplates 11, 21 produced with PHEMA/PMMA hydrogel reinforced with particles of hydroxyapatite.

For the production of the device, a reactive solution was used consisting of hydroxyethylmethacrylate (HEMA) in which was dissolved a predetermined percentage of PMMA, of 0.5% by weight of . ethylene glycol dimethacrylate (EGDMA) , as crosslinking agent, and of 0.1% of α,α*-azoisobutyronitrile (AIBN) ; the percentages of such agents are referred to the weight of the monomer (HEMA) alone. The solution was placed on a thermal agitator at 25°C until a homogeneous mixture was obtained. Subsequently, the fibres F of PET optionally pre- treated (for example with plasma process or with ozone) are impregnated in a tank B with the reactive solution and wound on a mandrel M of predetermined geometry, as illustrated in Fig. 2. Once this stage is completed, the annular structure produced is placed in a suitably shaped die, having a shape and dimensions equal to the final shape and dimensions of the prosthesis, filled with the reactive solution and placed in an oven at 80⁰C, finally carrying out 'an after-cure at 90⁰C,^" the whole within a time span necessary for obtaining complete polymerisation .

During this process the hydroxyapatite is added locally to the reactive solution in order to produce the interface endplates. The endplates are produced in such a way as to favour osteo-integration and, consequently, anchorage to the vertebral bodies.

Another possible method of manufacture of the prosthesis according to the invention provides for the manufacture of the intermediate body 31 separately from the endplates 11 and 21. The composite which constitutes the intermediate body 31 may be prepared according to the method previously illustrated, without the stage of production of the endplates. In the intermediate body 31 thus produced, anchorage means (not illustrated) are provided, for example protuberances or projections, in order to allow anchorage to the endplates 11 and 21, which will therefore have corresponding cavities (not illustrated) . In such a case, the endplates 11, 21 are of composite polymeric material, for example polyethylene reinforced with calcium phosphate. The two endplates are preferably of polyethylene reinforced with hydroxyapatite . Even more preferably, the two endplates are made of HAPEX™ (Patent Application GB 2 085 461, Patents US 5 017 627 and 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.

Independently of the method of manufacture of the prosthesis, the endplates 11 and 21 may be subsequently shaped, using for example a digitally controlled milling machine.

Static compression tests, carried out at different speeds of deformation (from 1 to 10 mm/miri) of the composite hydrophilic intervertebral disc prosthesis produced have shown a J-shaped stress-deformation curve, like that relating to the natural system, and a modulus of elasticity variable from 84 to 110 MPa, which are typical values of a lumbar intervertebral disc, while torsion tests, carried out at a speed of 0.1 deg/s, highlighted a torsional rigidity of 2.75 N-m/deg, which comes within the range of values for lumbar discs. Since in the human being the worst-case scenario between the spinal segments is represented by the segment L5- Sl, in view of the fact that this is subjected to the highest load and is characterized by a greater inclination with respect to the horizontal plane, compression-shear tests were also carried out according to a configuration at 45° with respect to the horizontal plane at a speed of 1 mm/min, according to ASTM standard F.04.25.05.01, and made it possible to evaluate a rigidity to compressive shear equal to 205 N/mm.

On the other hand, creep tests, fatigue tests and dynamic- mechanical measurements in compression, all carried out within the range of the physiological loads at a temperature of 37+_^0.5⁰C in a saline bath, besides highlighting the viscoelastic behaviour, showed durability, long-term resistance to compressive creep, and a high dimensional stability of the composite structure produced.

As a conclusion to what has been stated, it should be borne in mind that it is always possible to regulate the hydrophilicity and the mechanical properties of the fibre- reinforced composite, by varying the composition of the hydrogel matrix, the angle of winding and the quantity of fibres in order to optimise the properties according to their position, from the cervical to the lumbar position. The invention thus devised is capable of numerous modifications and variants, all coming within the scope of the same innovative concept. For example, the hydrogel material may contain a hydrophilic polymer capable of releasing, in a controlled manner, bioactive molecules and drugs, incorporated initially.

Claims

1. A disc prosthesis suitable for replacing an intervertebral disc, comprising a pair of endplates (11, 21) and an intermediate body (31) , interposed or suitable for being interposed between said endplates, characterized in that said intermediate body comprises a nucleus (32) made of hydrogel, around which is wound a peripheral structure (35) made of the same hydrogel reinforced with fibres .

2. A disc prosthesis according to claim 1, wherein said fibres have a predetermined angle of winding.

3. A prosthesis according to claim 2, wherein said peripheral structure (35) comprises a plurality of radially superposed layers (35a, 35b, 35c, 35d) , wherein the angle of winding of the fibres increases from the radially innermost layer to the radially outermost layer.

4. A prosthesis according to any one of the preceding claims, wherein said hydrogel contains a hydrophilic polymer or a mixture of hydrophilic polymers.

5.. A prosthesis according to claim 4, wherein said hydrogel contains a hydrophilic polymer capable of releasing, in a controlled manner, bioactive molecules and drugs, incorporated initially.

6. A prosthesis according to any one of the preceding claims, wherein said hydrogel contains polyhydroxyethylmethacrylate .

7. A prosthesis according to any one of the preceding claims, wherein said hydrogel contains a mixture of polyhydroxyethylmethacrylate and polymethylmethacrylate .

8. A prosthesis according to any one of claims 1 to 6, wherein said hydrogel contains a mixture of polyhydroxyethylmethacrylate and polycaprolactone, polylactic, glycolpolyethylenic .

9. A prosthesis according to any one of the preceding claims, wherein said fibres are polymeric fibres.

10. A prosthesis according to claim 9, wherein said fibres are polyester fibres.

11. A prosthesis according to claim 9, wherein said fibres are Kevlar fibres.

12. A prosthesis according to any one of the preceding claims, wherein said endplates are produced on the same hydrogel as said intermediate body, by adding hydroxyapatite and/or calcium phosphate locally to the hydrogel .

13. A prosthesis according to any one of claims 1 to 11, wherein said endplates are produced separately from said intermediate body, anchorage means being provided for mounting the endplates on the intermediate body.

14. A prosthesis according to claim 13 , wherein said endplates are made of polyethylene reinforced with calcium phosphate .

15. A prosthesis according to claim 14 , wherein said endplates are made of polyethylene reinforced with hydroxyapatite .

16. A prosthesis according to claim 15, wherein said endplates are made of polyethylene reinforced with HAPEX™.

17. A prosthesis according to any one of the preceding claims, wherein said endplates have, on the outside, anchorage teeth (12) for fixing to the respective vertebrae, being of cylindrical, conical, frustoconical or rectangular shape .