US20090054989A1 - Intervertebral implant - Google Patents

Abstract

Disclosed is an intervertebral implant (1) comprising a central axis (2), a bottom cover plate (3) and a top cover plate (4), which are respectively provided with an exterior surface (7; 8) that extends transversal to the central axis (2), and a central part (10). Said central part (10) is located between the cover plates (3; 4) and is provided with a sleeve (12) encompassing a fiber system (5) that is connected to the cover plates (3; 4) and is embedded in an enveloping body (25) made of a homogeneous material. In analogy with the anatomic structure of the natural disk, the inventive intervertebral implant (1) can transfer occurring compressive forces onto the cover plates (3, 4) thereof as tensile forces that are applied to the individual fibers of the fiber system (5) thereof.

Description

CROSS-REFERENCE TO RELATED APPLICATION
• This application is a continuation of U.S. patent application Ser. No. 10/553,495, which is a National Stage application of PCT/CH03/00247, filed Apr. 12, 2003, the entire disclosures of which are incorporated herein by reference.
FIELD OF THE INVENTION
• The invention concerns an intervertebral implant.
BACKGROUND OF THE INVENTION
• An intervertebral disc prosthesis of the generic type is known from U.S. Pat. No. 4,911,718 Lee. This known intervertebral disc prosthesis comprises a central core, that is so formed from a biocompatible elastomer, that it is almost corresponds to that of the nucleus pulposus of a natural intervertebral disc, as well as from a multi-layer laminate from fibres bound in an elastomer, arranged around the core. Each laminate layer has its own yarn system, so that a plurality of fibre groups are present. The fibres of the individual layers have various orientations, whereby the angles of the fibres relative to the central axis of the intervertebral disc are in the range of ±20° and ±50°, preferably 0°, +45° and −45°.
• From WO 90/00374 Klaue a hip prosthesis is known, the shaft of which is made from a tubular mesh, i.e. a structure, that comprises at least two series of fibres crossing one another. In this application the interior of the tubular mesh remains empty as the shaft of the femur component.
• In the case of the prosthesis disclosed in U.S. Pat. No. 4,911,718 Lee, although the individual fibres are integrated in the laminate that is made from an elastomer or another type of synthetic material, their ends are, however, adhered only to the end plates, so that they do not surround the core and consequently, in the case of a radial expansion of the core, cannot accept any tensile force. When adhering the lateral walls, cut out from the fibrous matrix compound, to the end plate, a fixing of the integrated fibres on the end plate is quite difficult, only the cross-section of the fibre offers a contact surface for the chemical joint. Therefore increased stresses occur especially on these joining places of the fibres on the end plate.
• Furthermore, in the case of Lee the length of the individual fibres is only from the bottom cover plate to the top cover plate, what corresponds to the sheathing height or a diagonal of the projected sheathing height. Thus the forces occurring can be reduced only along these lengths due to the transfer of the shearing force of the fibres to the elastomer. Thus positions of increased stresses result at the fixings, i.e. on the ends of the fibres.
• The prosthesis disclosed in WO 90/00374 Klaue comprises a system of fibres, the individual fibres of which are not fixed on both ends, as well as there is no deformable core. Therefore in the case of an axial compression of the prosthesis the axial compression forces occurring cannot be transferred as tensile forces to the fibres.
• From U.S. Pat. No. 3,867,728 Stubstad et al. an intervertebral disc prosthesis is known, that has an elastomeric sandwich structure with a fibre system. A disadvantage of this known prosthesis is that the fibre system, joined with the cover plates, is either not embedded in the sheathing body or in another embodiment is embedded in a multi-layer laminate of an elastomer.
SUMMARY OF THE INVENTION
• This is where the invention wants to provide remedy. The object of the invention is to produce an intervertebral implant, that comprises a fibre system joined with the cover plates, by virtue of which a sheathing body, surrounding the central part and made from a homogeneous material, will be reinforced.
• The inventions achieve this objective with an intervertebral implant having a central axis including a bottom cover plate and a top cover plate, each with an external surface extending transversely to the central axis, a central part with a sheathing that surrounds a fibre system provided between the cover plates, wherein the fibre system is joined with the cover plates at least partially, characterized in that the fibre system is guided over the external surfaces of both cover plates and surrounds at least partially the central part as well as both cover plates, and the sheathing comprises an elastic sheathing body that surrounds the central part on the periphery and is made from a homogeneous material and is passed through by the fibre system.
• The basic advantages, achieved by the invention, are that with the intervertebral implant according to the invention
• the fibre system can be first wound around the central part and following this poured into an elastomer forming the elastic sheathing body, so that the sheathing, enveloping the central part, can be easily produced,
• by applying the elastic material around the fibre system after its winding, the anchoring of the fibre system is possible by various means, for example also on the opposing inner surfaces of the cover plates,
• the central part allows a movement of both adjacent bodies of the vertebra in the case of a compression, flexion or extension, lateral bending and torsion,
• the momentary centre of rotation or the momentary axes of rotation are not determined by the intervertebral implant itself, and they can position themselves according to the rule of minimum forces or moments occurring,
• by varying the number of fibres in the circumferential direction, the cross-section of the fibres and the choice of material, the behaviour of the intervertebral implant can be so adjusted, that under varying loads the movements occur as in the case of the natural intervertebral disc, and
• by varying the arrangement and the execution of the fibre system certain movement limitations can be placed on the intervertebral implant, and from a certain deformation a limit region occurs, where despite the further increasing forces no deformation takes place or in the case of moments occurring the implant will no longer tilt.
• The axial compression forces occurring under a load on the spinal column are transmitted to the central part via the two end plates. The compression forces deform the central part situated between the two end plates, in particular an elastic formed body situated therein, in such a manner that the central part radially buckles. This expansion of the central part is restricted by the fibre system surrounding the central part and the radial compression forces arising can be absorbed by the fibre system as a tensile force. Thus a further, disadvantageous buckling of the central part can be limited. By anchoring the fibre system in both cover plates, the intervertebral implant remains stable even under the greatest loads and the fibre system is capable to withstand even considerable tensile forces.
• In a preferred embodiment the entire fibre system is embedded in the elastic sheathing body, so that the fibre system does not necessarily need to be made from a biocompatible material.
• In a further embodiment the fibre system is only partially embedded in the elastic sheathing body, while the fibre system has a radial thickness δ relative to the central axis and the elastic sheathing body has a radial thickness d, and the δ/d×100% ratio is in a range of 80% and 350%. By virtue of this the advantages can be achieved, that the large relative movements in the peripheral region of the cover plates occurring during a flexion/extension movement or a lateral movement of the adjacent bodies of the vertebra are not subjected to a great resistance by the elastic sheathing body and due to this the danger of a fissure formation in the sheathing body is slighter.
• The embedding of the fibre system in the elastic sheathing body can be carried out various embodiments in such a manner, that
• a) the fibre system can be moved relative to the elastic material of the sheathing body, or
• b) the fibre system cannot be moved relative to the elastic material of the sheathing body.
• In yet another embodiment the entire fibre system is anchored on the cover plates, so that greater tensile forces can be accepted by the fibre system, and consequently the intervertebral implant obtains a great torsional rigidity.
• In another embodiment the sheathing body, accommodating the fibre system, is made from an elastic, biocompatible material, preferably an elastomer, produced in particular based on polyurethane (PUR). However, silicone rubber, polyethylene, polycarbonate urethane (PCU) or polyethylene terephthalate (PET) may also be used.
• In yet another embodiment the central part is filled at least partially with an incompressible medium, preferably a liquid.
• In another embodiment the central part comprises an incompressible liquid core and an elastic formed body provided around it, while the liquid can be accommodated, for example, in a cavity provided in the formed body. This brings with it the advantage, that by virtue of the liquid core a mechanical behaviour of the intervertebral implant is similar to that of a physiological intervertebral disc. The axial deformation of the elastic central part will result in the radial expansion of the incompressible liquid and consequently in the radial expansion of the wall of the central part containing the fibre system. The tensile forces, occurring due to the radial expansion and/or the buckling of the wall of the central part, are basically absorbed by the fibres.
• The anchoring of the fibres on the cover plates can be carried out, for example, in the following manner:
• a) Mechanically by guiding the endless fibres through grooves and over the external surfaces of the cover plates from one groove to another one. Thus the fibres surround the central part together with the cover plates. By guiding the fibres in the grooves the fibre system can be so anchored on the cover plates, that in the case of tensile forces acting on the fibres no slipping of the fibres on the lateral sides is possible because the fibres can absorb only tensile forces,
• b) Mechanically by a wedge-shaped construction of the grooves, so that the fibres extending from cover plate to cover plate can be firmly clamped in the grooves, and/or
• c) By adhering the fibre system on the cover plates.
• In yet another embodiment of the intervertebral implant according to the invention each cover plate comprises on its periphery a lateral surface and grooves distributed on the circumference and radially penetrating into the lateral surfaces. The fibres, part of this fibre system, are guided through these grooves.
• In a further embodiment the central part and the fibre system are joined with the cover plates in a form-locking manner.
• In yet a further embodiment the fibre system is guided over the external surfaces of both cover plates, so that it will surround the central part as well as the cover plates.
• When using an endless fibre, that covers the entire implant, the stresses preferably are distributed on the entire circumference of this winding. The fibre system is preferably in the form of a woven material, fabric or is knitted.
• In another embodiment channels are mortised in the external surfaces of the cover plates to accommodate the fibre system.
• In yet another embodiment the central part is essentially hollow-cylindrical, hollow-prismatic or is a body of rotation, an ellipsoid, a partial sphere or barrel-shaped with an axis of rotation that is coaxial with the central axis. By virtue of such configurations the advantage, that the positions of the axes of rotation of the adjacent intervertebral discs correspond, as far as possible, to that of the natural intervertebral disc, can be achieved.
• The fibre system can be made, for example, from UHMWPE (ultra high molecular weight polyethylene) or from PET (polyethylene terephthalate).
• In a further embodiment of the intervertebral implant according to the invention a closing plate is fastened on each cover plate for placing the adjacent bodies of the vertebra on the base plate or cover plate, each of the said closing plate having an external surface at right angles to the central axis with a macroscopic structure. The structure may be, for example, in the form of teeth. The macroscopic structure allows a primary stabilisation of the intervertebral implant immediately after the operation. Thus a mechanical anchoring of the intervertebral implant at a time when the growing of the bone on the intervertebral implant has not yet taken place, can be achieved.
• In yet a further embodiment the woven material is formed from first and second fibres, wherein the first fibres include an angle α with the central axis and the second fibres include an angle β with the central axis. The angles for α or β are preferably between 15° and 60°.
• In another embodiment the first and second fibres are interwoven with one another.
• In yet another embodiment the elastic formed body has at right angles to the central axis a cross-sectional surface FF, while the central part has at right angles to the central axis a cross-sectional surface FM and the FF/FM ratio of these two cross-sectional surfaces is between 30% and 65%.
• In a further embodiment the elastic formed body is surrounded by a semi-permeable membrane, while in the interior of the elastic formed body preferably physiological table salt solution is present.
• With regard to the central axis the fibre system may be single-layered or multi-layered, preferably 2-6 layered. Furthermore, the fibre system can be wound on the elastic formed body. The winding on the elastic formed body can be in two different directions, preferably rotationally symmetrically.
• In yet another further embodiment a closing plate can be fastened on each cover plate, the closing plate having at right angles to the central axis an external surface with a macroscopic structure, preferably in the form of teeth.
• The diameter of the fibres is in a range of 0.005 mm and 0.025 mm. A yarn (roving) is preferably produced from a plurality of fibres, whereby 500-2000 fibres form a yarn with a cross-sectional surface of 0.5 mm²to 2 mm².
• In those embodiments, wherein the fibre system has fibre sections crossing one another, in the case of flexion movements (flexion, extension, lateral flexion) of the patients some fibre sections will be unilaterally clamped and in case of shearing the fibre sections extending tangentially to the shearing direction absorb the forces.
BRIEF DESCRIPTION OF THE DRAWINGS
• The invention and developments of the invention are explained in detail in the following based on partially schematic illustrations of several embodiments. They show in:
• FIG.1—a side view of an embodiment of the intervertebral implant according to the invention,
• FIG.2—a top view on the embodiment of the intervertebral implant according to the invention, illustrated inFIG. 1,
• FIG.3—a side view of another embodiment of the intervertebral implant according to the invention,
• FIG.4—a section through the embodiment of the intervertebral implant according to the invention, illustrated inFIG. 3,
• FIG. 5a—a perspective illustration of the fibre system of an embodiment of the intervertebral implant according to the invention,
• FIG. 5b—a top view on the fibre system illustrated inFIG. 5a,
• FIG. 6a—a perspective illustration of the fibre system of an embodiment of the intervertebral implant according to the invention,
• FIG. 6b—a top view on the fibre system illustrated inFIG. 6a, and
• FIG.7—a section through a further embodiment of the intervertebral implant according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
• FIGS. 1 and 2 illustrate an embodiment of theintervertebral implant1 according to the invention, that comprises atop cover plate3 and abottom cover plate4, each with anexternal surface7,8 extending at right angles to thecentral axis2 and having alateral surface21,22 on the periphery. Between thecover plates3,4 there is acentral part10 provided with acentral cavity11 and asheathing12, that surrounds thefibre system5. For the purpose of anchoring thefibres6 of thefibre system5 on thecover plates3,4, each of the peripheral lateral surfaces21,22 hasgrooves18, distributed on the circumference and radially protruding into the lateral surfaces21,22, so that thefibre system5 can be anchored in thesegrooves18. In thecentral cavity11 there is an elastically deformable formedbody9 with an incompressible core, preferably aliquid core13. Due to the incompressibility of theliquid core13 during a compression of thecover plates3,4 parallel to thelongitudinal axis2, for example, the elastic formedbody9 and thesheathing12 with thefibre system5 will buckle radially, i.e. at right angles to thelongitudinal axis2, consequently thefibres6 will be under tension.
• FIGS. 3 and 4 illustrate an embodiment of theintervertebral implant1 according to the invention, that comprises twocover plates3,4, provided at right angles to thecentral axis2, and an elastically deformablecentral part10 situated between them. Thecentral part10 comprises a hollow-cylindrical sheathing12 that is coaxial with thecentral axis2 and acentral cavity11. In thecentral cavity11 an elastic formedbody9 with an incompressible core is provided, preferably aliquid core13. The formedbody9 is surrounded by a semi-permeable membrane, whereas thesheathing12, that surrounds thefibre system5 and anelastic sheathing body25 passed through by thefibre system5, is made from a synthetic material. Theclosing plates14,15 are firmly joined with thecover plates3,4 and have axially protrudingsurfaces16,17, which can be brought to rest on the end plates of two adjacent bodies of the vertebra. Thefibre system5 is anchored on thecover plates3,4 and is integrated in thesheathing12 and its purpose is to absorb the forces on thecentral part10, said forces acting on theintervertebral implant1 via the bodies of the vertebra adjacent to theclosing plates14,15, i.e. torsional forces due to the rotation of the bodies of the vertebra about thecentral axis2 relative to one another or bending moments due to lateral bending and/or flexion/extension of the spinal column. For example, a compression force, acting on theintervertebral implant1 parallel to thecentral axis2, is transferred by both closingplates14,15 via bothcover plates3,4 to thecentral part10, while as the result the elastic formedbody9 will buckle at right angles to thecentral axis2. This expansion movement of the elastic formedbody9 is transferred to thesheathing12 with thefibre system5 and contained by this. Since thefibre system5 is anchored on thecover plates3,4, the compression force, acting transversely to thecentral axis2, generates tensile forces in the fibres of thefibre system5. Thefibre system5 in this case is made from synthetic fibres, preferably from UHMWPE-fibres (ultra high molecular weight polyethylene) or from PET (polyethylene terephthalate) and comprises a mesh from first andsecond fibres6a,6b, that are interwoven with one another. By doing so, thefirst fibres6ainclude an angle α and thesecond fibres6ban angle β with thecentral axis2. In the embodiment of theintervertebral implant1 according to the invention illustrated here, the angles α and β are equal and are between 15° and 60°. Thefibres6a,6bare anchored on thecover plates3,4 by means ofgrooves18 that are arranged on the circumference of thecover plates3,4 parallel to thecentral axis2, so that thefibres6a,6bare passed through thegrooves18 and can be guided to thenext groove18 over thesurfaces7,8 in achannel19. Thecover plates3,4 are made from synthetic material, whereas theclosing plates14,15, arranged externally, are made from titanium or a titanium alloy. The externally arrangedclosing plates14,15 are joined with thecover plates3,4 either by form-locking or frictional locking. In particular they can be adhered or welded to one another.
• InFIGS. 5aand5bafibre system5 is illustrated according to an embodiment of theintervertebral implant1 according to the invention, wherein thefibres6 extending over theend plates3,4 form chords on thecircular surfaces7,8 of thecover plates3,4.
• InFIGS. 6aand6bafibre system5 is illustrated according to an embodiment of theintervertebral implant1 according to the invention, wherein thefibres6 extending over theend plates3,4 cross at the point of intersection of thecentral axis2 and theend plates3,4.
• When compared with the arrangement of the fibres6 (FIGS. 6a,6b), the guiding of thefibres6 as chords (FIGS. 5a,5b) over thesurfaces7,8 of theend plates3,4 has the following advantages:
• due to the better distribution of the crossing points of thefibres6 no concentration will occur, especially between theexternal surfaces7,8 of thecover plates3,4 and theclosing plates14,15 (FIGS. 3 and 4), and
• with the aid of a winding technique thefibre system5 can be symmetrically produced relative thecentral axis2 while theintervertebral implant1 can be clamped in at the points of intersection between thecentral axis2 and thecover plates3,4.
• FIG. 7 illustrates an embodiment, that differs from the embodiment illustrated inFIGS. 3 and 4 only by that the periphery of thesheathing12 provided on thecentral part10 comprises anelastic sheathing body25 only partially passed through by thefibre system5, the thickness of the sheathing body d being smaller than theradial thickness6 of the fibre system.

Claims (18)

1. An intervertebral implant for implantation between upper and lower vertebrae, the implant having a central axis, the implant comprising:

an upper closing plate having an external surface extending generally transversely to the central axis for contacting at least a portion of the upper vertebra;

a lower closing plate having an external surface extending generally transversely to the central axis for contacting at least a portion of the lower vertebra;

a top cover plate operatively joined to the upper closing plate, the top cover plate including a plurality of first grooves;

a bottom cover plate operatively joined to the lower closing plate, the bottom cover plate including a plurality of second grooves;

a central part located between the top and bottom cover plates;

a fibre system at least partially surrounding the central part, the fibre system at least partially received within the plurality of first and second grooves; and

a sheathing at least partially surrounding the fibre system and the central part, the sheathing being operatively coupled to the top and bottom cover plates.

2. The intervertebral implant ofclaim 1, wherein the fibre system is independently moveable relative to the sheathing.

3. The intervertebral implant ofclaim 1, wherein the fibre system is multi-layered.

4. The intervertebral implant ofclaim 1, wherein the fibre system is formed from at least first and second fibres interwoven with one another.

5. The intervertebral implant ofclaim 4, wherein the first fibres are disposed at a first angle α with respect to the central axis and the second fibres are disposed at a second angle β with respect to the central axis.

6. The intervertebral implant ofclaim 5, wherein the first angle α and the second angle β are between 15 and 60 degrees.

7. The intervertebral implant ofclaim 1, wherein the fibre system is constructed of an ultra high molecular weight polyethylene (UHMWPE) material.

8. The intervertebral implant ofclaim 1, wherein the fibre system is secured to the top and bottom cover plates by weaving the fibre system into and out of the plurality of first and second grooves.

9. The intervertebral implant ofclaim 1, wherein the upper and lower closing plates are constructed of one of a titanium and a titanium alloy.

10. The intervertebral implant ofclaim 9, wherein the external surface of the upper and lower closing plates include a macroscopic structure for adhering the implant to the upper and lower vertebrae, respectively.

11. The intervertebral implant ofclaim 1, wherein the sheathing is constructed of a material selected from a group consisting of polyurethane, silicone rubber, polyethylene, polycarbonate urethane and polyethylene terephthalate.

12. The intervertebral implant ofclaim 1, wherein the central part is substantially cylindrical.

13. The intervertebral implant ofclaim 12, wherein the central part includes a top surface and a bottom surface, the top surface of the central part contacting the top cover plate and the bottom surface of the central part contacting the bottom cover plate.

14. An intervertebral implant for implantation between upper and lower vertebrae, the implant comprising:

an upper substantially rigid closing plate having an external surface and an internal surface, the external surface of the upper closing plate contacting at least a portion of the upper vertebra in an implanted position;

a lower substantially rigid closing plate having an external surface and an internal surface, the external surface of the lower closing plate contacting at least a portion of the lower vertebra in the implanted position;

a top cover plate mechanically joined to the upper closing plate, the top cover plate having a top surface and a bottom surface;

a bottom cover plate mechanically joined to the lower closing plate, the bottom cover plate having a top surface and a bottom surface;

an elastic core located between the bottom surface of the top cover plate and the top surface of the bottom cover plate;

a fibre system surrounding the elastic core and joined to the top and bottom cover plates; and

a sheathing at least partially surrounding the fibre system and joined to the top and bottom cover plates, wherein the fibre system and the sheathing are independently moveable with respect to one another.

15. The intervertebral implant ofclaim 14, wherein the fibre system is independently moveable with respect to the core.

16. The intervertebral implant ofclaim 14, wherein the fibre system includes a plurality of interwoven fibres.

17. The intervertebral implant ofclaim 16, wherein the top and bottom cover plates include a plurality of grooves for receiving at least a portion of the interwoven fibres.

18. The intervertebral implant ofclaim 17, wherein the fibre system is at least partially adhered to the top and bottom cover plates by weaving the fibre system into and out of the plurality of grooves.

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