WO2008092192A1

WO2008092192A1 - An intervertebral disk prosthesis

Info

Publication number: WO2008092192A1
Application number: PCT/AU2008/000093
Authority: WO
Inventors: Philip Boughton; Andrew Ruys; Gregory Roger
Original assignee: The University Of Sydney
Priority date: 2007-01-29
Filing date: 2008-01-29
Publication date: 2008-08-07

Abstract

An intervertebral disk prosthesis (10) comprising an elastomeric composite member comprising a core member (11), a first endplate (12), and a second endplate (13) spaced from the first endplate (12) by the core member (11). The elastomeric composite member is functionally graded axially from the core member (11) to at least one of the first or second endplates (12, 13). The elastomeric composite member can also be functionally graded radially from the centre of the core member to the radial outer surface (14) of the core member (11). One or both endplates (12, 13) can have a bone anchor fin (17) extending outwardly therefrom, with part or all of the bone anchor fins (17) being bioabsorbable. The prosthesis (10) can be placed using a milling tool (20) for milling the opposed surfaces of adjacent vertebrae. The tool (20) can comprise a first drill bit (21), a second drill bit (22), and an oscillating saw blade (23) having a distal cutting edge (24) and positionable between the first and second drill bits (21, 22). An implantable device (40) is also described and is formed from a bioabsorbable carbohydrate glass matrix (41) surrounding a plurality of bioabsorbable glass fibres (42).

Description

An intervertebral disk prosthesis

Cross-Reference to Related Applications

The present application claims priority from Australian Provisional Patent

Application No 2007900411 filed on 29 January 2007, the content of which is incorporated herein by reference.

Field of the Invention

This specification is firstly directed to an intervertebral disk prosthesis. The specification is also directed to a bioabsorbable orthopaedic fixation device.

Background of the Invention

The human spine is an intricate and unique structure. It facilitates an upright position for standing and walking and unlike most anthropoids that have a flexed lumbar region, the human spine has a characteristic "double-S curve" providing an extended lumbar position referred to as the lumbar lordosis.

The spine consists of rigid osseous vertebrae connected by pliant cartilaginous spinal disks and a system of resilient ligaments. Spinal disks account for about one quarter of the height of the vertebral column. The size of vertebrae gradually increase toward the lumbar region, the coccyx, and also from the cervical region. The sacrum and the coccyx are fused bones without articulations.

Intervertebral disk (FVD) joints aTe essential for the proper growth, movement and balance of the spine. The IVD permits spinal segments to mobilize with six degrees of freedom, whilst imparting substantial constraint, isolating shock, and providing overall stability to the spine.

The IVD is a resilient cartilaginous structure that firmly connects yet tenaciously separates the hyaline cartilage endplates of adjacent vertebral bodies. The central soft gelatinous portion of the disk is named nucleus pulposus. The tough fibrous laminate surrounding the nucleus is termed annulus fibrosus. Low back pain affects a large portion of adults in the industrialized world.

Increasingly sedentary lifestyles and aging populations are recognized as contributing trends. Intervertebral disk (FVD) pathology is strongly implicated as being a primary cause of chronic back pain due to the important role it plays in spine biomechanics and its proximity to major nerves.

Research suggests that the treatment for lower back pain recommended to a sufferer can vary significantly depending on the medical specialist that is consulted. Initially, non-surgical management is often recommended with bed rest, lifestyle changes, physiotherapy and medication all possible treatments. Surgical techniques are also available with common methods of treating lower back pain including discectomy, laminectomy, chemonucleosis and intradiscal electrotherapy.

The dynamic intervertebral disk (FVD) replacement or total disk arthroplasty aims to restore both spinal stability and mobility to a compromised intervertebral disk joint. The scope of this challenge has been reflected in the number and variety of prosthesis designs that have been developed over the past several decades.

Several configurations for dynamic FVD implants have been proposed and include:

• flexible elastomeric disks;

• gaseous/fluid/gel containing polymeric bladders;

• metal spring arrangements (such as compression springs and wave washers);

• polymer, metal and ceramic sliding joints; • bioinert scaffolds, 3D woven fabrics, and Nitinol® meshes; and

• bioabsorbable and bioactive hydrogel tissue scaffolds.

Bioabsorbable screws, pins and plates are being used in the field of orthopaedics. Absorption of such devices has the potential advantage of allowing bone or soft tissues to heal and also preferably minimise the likelihood of future problems such as pain, stress-shielding and metal sensitisation. Many such devices at present lack the necessary stiffness and/or strength at implantation to be used widely, and further are impractical or difficult to manufacture and/or are not approved for use.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for the purpose of providing a context for the present invention. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.

Summary of the Invention

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

According to one aspect, the present invention is an intervertebral disk prosthesis comprising an elastomeric composite member comprising: a core member; a first endplate; and a second endplate spaced from the first endplate by the core member; wherein the elastomeric composite member is at least functionally graded axially from the core member to at least one of the first or second endplates.

One interpretation of the term "functionally graded" is that, in the elastomeric composite member there can be a graded transition of the materials making up the composite and/or the properties of the materials making up the composite. In one embodiment, the functionally graded material can be a form of composite material that is optimized on both microscopic and macroscopic scales simultaneously. It is to be understood that the functionally graded material can be a material that contains a gradual variation in chemical bonding (e.g. degree of cross-linking), or a gradual variation in physical properties (e.g. porosity or hydration). It can also result from a change in amounts of the materials making up the composite.

In one embodiment of this aspect, the elastomeric composite member can be formed in a single piece. In another embodiment, the first and/or second endplates can be formed separately to the core member and connected thereto.

In another embodiment, one or both of the endplates can have a diameter greater than that of the core member. The endplates can have the same diameter as each other or have respectively different diameters. The endplates can be engageable with surfaces of respective opposed adjacent vertebrae of an implantee.

The elastomeric composite member can have a longitudinal axis. The member can be functionally graded along this axis. In one embodiment, the properties of the member can gradually vary from the centre of the core member along the longitudinal axis. In one embodiment, the modulus and/or bioactivity of the member can increase along the longitudinal axis from the core member towards and including at least one of the endplates, more preferably both endplates. The modulus and/or bioactivity can increase from the centre of the core member to the respective outer surfaces of the two endplates. In one embodiment, the change in modulus and/or bioactivity can be uniform along the longitudinal axis. In another embodiment, the change in modulus and/or bioactivity can be non-uniform along the longitudinal axis. For example, the member can have no or only a relatively small increase in modulus and/or bioactivity for a first distance from the centre of the core member and then a relatively large increase in modulus and/or bioactivity within the thickness of the endplate. In yet another embodiment, the change in property, such as modulus and/or bioactivity, can be the same along the longitudinal axis in both directions from the centre of the core member.

In a further embodiment, at least the core member can be radially functionally graded. As such, the properties of the core can gradually vary from the centre of the core member laterally to an outer radial surface of the core member that extends between the first and second endplates. In one embodiment, the modulus and/or bioactivity of the core member can radially increase. In one embodiment, the change in modulus and/or bioactivity can be uniform away from the centre of the core member. In another embodiment, the change in modulus and/or bioactivity can be laterally nonuniform. For example, the member can have no or only a relatively small increase in modulus and/or bioactivity for a first distance from the centre of the core member and then a relatively large increase in modulus and/or bioactivity relatively close to its outer radial surface.

In one embodiment, the core member can be shaped to match the requirements of the disk prosthesis and the spacing that is available between the vertebrae of the implantee. In one embodiment, the core member can have a kidney-shaped or elliptical lateral cross-section. The prosthesis can be dimensioned so as to traverse the span of the vertebral body. In another arrangement that is currently anticipated by the inventors to be less desirable, the prosthesis can reside within the existing annulus.

The most distal outer surfaces of the respective endplates, relative to each other, can be formed parallel with respect to each other. In another embodiment, they can be formed in a non-parallel arrangement. The outer surfaces can be substantially planar or planar. The properties of the elastomeric composite member can be such that the outer surfaces can undergo a change in angle relative to each other during implantation of the prosthesis and/or while in use in vivo.

In the prosthesis, those regions that are subject in vivo to relatively high strains can be void of all reinforcement or have relatively less reinforcement than other regions of the prosthesis. The kinematics and kinetics of the prosthesis are modifiable by altering the gradients, materials combinations, and/or the size and geometry of the core member. For example, a relatively narrow middle section of the core member can serve to reduce resistance to axial rotation, translation, lateral bending, flexion- extension and axial compression whereas a softer wider disk increases the locus for the axis of rotation.

hi one embodiment, the elastomeric composite member consists of a polycarbonate-urethane (PCU) elastomer. This PCU elastomer can be laterally functionally graded by the addition of a fibre material. The PCU elastomer can instead or also be functionally graded towards the relatively stiff endplates by the addition of a fibre material. In one embodiment, the fibre material can be a relatively fine hydroxyapatite (HA) fibre.

A relatively compliant 80A Shore hardness PCU can be used for the core member of the prosthesis, while a relatively strong and stiff 75D Shore hardness PCU can be used for the region at and adjacent the radial outer surface of the core member and the endplates. Still further, the endplates can be reinforced in a functionally graded manner using a relatively fine HA chopped fibre and/or Bioglass® fibre. Where used, the relatively fine HA chopped fibre can constitute about 15 vol% of the endplates. The concentration of the fibres in the endplates can increase moving progressively away from the core member and also radially outward to the radially outer surface of the respective endplates. The relatively fine HA fibres and/or Bioglass® fibres that are embedded within the endplates can form relatively fine networks that assist with bone attachment and ingrowth while also increasing the compressive modulus of the endplates to at least substantially match that of the interfacing bone.

The elastomeric composite member can be formed by blending and moulding.

In yet another embodiment, one or both endplates can have a bone anchor fin extending outwardly therefrom. In one embodiment, the respective bone anchor fins can be integrally formed in the endplates.

In another embodiment, the bone anchor fins can be theπno-mechanically or otherwise integrated into or mounted to the endplates.

In a further embodiment, part or all of the bone anchor fins can be bioabsorbable. Bone fixation devices, such as bone screws can be used to engage the bone anchor fins in place in the respective vertebrae. Such bone fixation devices can also be bioabsorbable. The bioabsorbable material making up part or all of the bone anchor fins and/or the bone fixation devices can be selected from a group of materials comprising bioabsorbable polymers. These polymers can include polycaprolactone (PCL), poly-L-D-Lactide (PLDLA), and polylactide acid (PLA). These bioabsorbable polymers can in turn be reinforced with fibrous Bioglass^®, CaP₅ and/or HA.

While each endplate can have a single bone anchor fin extending outwardly therefrom, in another embodiment, the anchor fin can comprise two or more portions.

In one embodiment, the respective fins can mechanically engage a complementary groove milled into the respective opposed vertebrae of the implantee during or prior to the prosthesis implantation surgery.

The respective bone anchor fins can incorporate an enlarged end or bulb distal the core member that serves to mechanically key with the vertebrae. This arrangement is understood to further improve fixation by transferring tensile loads and forces resulting from a combined extension and anterior-posterior shear regime, axial rotation, and lateral translation. In this embodiment, the bulb portion only of the fin can be bioabsorbable. To use the bone anchor fins, the vertebrae have to be milled in order to provide two aligned keyhole shaped grooves. To do this, a milling tool can be used as described below. In one embodiment, use of the milling tool results in milling of the two grooves simultaneously subsequent to a discectomy and neural decompression to relieve the back pain. It is envisaged that use of the milling tool would prescribe an anterior approach requiring resection of the anterior longitudinal ligament with the intervertebral disk joint slightly flexed so as to allow correct alignment of the vertebrae endplates. Further alignment of the drill pieces could also be achieved through use of the milling tool.

In yet another embodiment, fine perforations can be formed, such as by milling, in the outer surfaces of the endplates to expose the fibre networks, for example the HA fibre networks, and also further increase the surface area of the interface formed between the bone and outer surfaces of the respective endplates.

In one embodiment, one or more holes and/or grooves may be formed in the fin.

Additionally, Bioglass^® chopped fibre, CaP fibres or blades, or other suitable bioabsorbable materials may be present in at least the outer surfaces of the endplates to assist in establishing initial attachment between the prosthesis and the bone followed by gradual absorption and final displacement by bone.

According to another aspect, the present invention is an intervertebral disk prosthesis comprising an elastomeric composite member comprising: a core member; a first endplate; and a second endplate spaced from the first endplate by the core member; wherein extending outwardly from one or both endplates is a bone anchor fin, and further wherein at least a part of said bone anchor fin is bioabsorbable.

In this aspect, the bone anchor fin(s) are engageable in a complementary groove milled into the respective opposed vertebrae of the implantee during the prosthesis implantation surgery.

In this aspect, the composite member and the bone anchor fins can have one, some or all of the features of the prosthesis defined herein above. According to a further aspect, the present invention is a milling tool for milling the opposed surfaces of adjacent vertebrae, the tool comprising: a first drill bit; a second drill bit; and an oscillating saw blade having a distal cutting edge and positionable between the first and second drill bits.

In this aspect, the first drill bit and the second drill bit can extend forwardly from respective chucks mounted on a support holder. The support holder can be movable relatively forwardly and backwardly relative to the saw blade. In one embodiment, the position of the holder can be controlled using one or more linear piezoelectric actuators.

In one embodiment, the chucks can turn the first and second drill bits simultaneously. The rotation of the drill bits can be reversible. In another embodiment, the chucks can be controllable individually.

In one embodiment, the first and second drill bits are parallel with respect to each other. The alignment of the first and second drill bits relative to each other can be adjustable from a parallel alignment. In one embodiment, the angle of alignment of each drill bit can be adjusted between plus and minus 10° from parallel thereby allowing use of the tool in situations where the opposed vertebral surfaces are at an angle of up to 20° relative to each other. If necessary, tools having a capability to be adjusted by greater or smaller amounts can be envisaged. The drill bits can have a diamond, diamond-like or other suitable coating.

In one embodiment of this aspect, the amplitude of oscillation of the saw blade is adjustable. Still further the saw blade is mounted on a saw blade holder that is also movable relatively forwardly and backwardly relative to the drill bits.

According to yet another aspect, the present invention is a method of milling the opposed surfaces of adjacent vertebrae following excision of the intervertebral disk using the milling tool as defined herein, the method comprising: aligning and anteriorly mounting the milling tool relative to the vertebrae to be milled; adjusting, if necessary, the angle of the respective drill bits to suit the inclination of the opposed surfaces; moving the drill bits relatively forwardly to form respective lateral holes in the adjacent vertebrae, the holes being spaced inwardly from the opposed surfaces; at least partially withdrawing the drill bits; moving the oscillating saw blade relatively forwardly to resect bone in each vertebrae between the opposed surfaces and the lateral hole to form a groove; and withdrawing the saw blade and the drill bits.

In this aspect, the alignment of the milling tool can be made with aid of computer tomography (CT) imaging. Anterior mounting of the milling tool can be made using a bracket The respective dill bits can be moved relatively forwardly and rearwardly simultaneously. The saw blade can remain retracted until the drill bits have been at least partially retracted.

On at least partial retraction, at least the distal tips of the drill bits can remain in the holes they have formed and so serve to support and retain the alignment of the adjacent vertebrae.

The oscillating saw blade preferably oscillates at a relatively high speed under amplitude control. If, after its use, any bone remains bridging the formed grooves, for example at the anterior of the vertebra near the stationary partially retracted drill bits, this bone can be removed using a bone chisel.

Once the grooves are formed, the milling tool can be removed and guide arms can be mounted thereon, the guide arms supporting a prosthesis having bone anchor fins as defined herein. The tool can be used to gradually insert the prosthesis into the space between the vertebrae with the fins anchored within the grooves as formed by the milling tool. The guide arms can be adjustable relative to each other to pre-stress the prosthesis ready for insertion into the space. Still further, the guide arms can be retractable during the insertion step.

It is envisaged by the present inventors that the elastomeric composite member can act as a total replacement of the native lumbar disk and can at least partially replicate the structural gradients, range of motion, degrees of freedom, axis of rotation loci, joint stiffness, and/or shock absorption that are characteristic of the native lumbar disk.

By not necessarily having metallic components, metal ions are not released and wear debris is not generated. The prosthesis can also be formed to at least substantially prevent soft tissue ingrowth into the prosthesis that might hamper motion.

According to another aspect, the present invention comprises an implantable device formed from a bioabsorbable carbohydrate glass matrix surrounding a plurality of bioabsorbable glass fibres.

In one embodiment of this aspect, the glass matrix can have anti-bacterial properties. Still further, the glass matrix can have wound healing properties by providing nutrition to the wound site. In one embodiment, the glass matrix can be a caramelized sucrose. The caramelized sucrose can serve to mechanically hold, adhere to and/or protect the glass fibres. Sucrose is a disaccharide formed when the two monosaccharides glucose and fructose combine through dehydration synthesis.

Sugar can be crystallized to form a thermoplastic carbohydrate glass through the caramelisation of sucrose.

In a further embodiment, some or all of the glass fibres within the matrix can be oriented. In one arrangement, at least some of the glass fibres can be parallel to the longitudinal axis of the device.

In yet another embodiment, the device can have one or more layers that act as a relatively thin moisture barrier, at least for a time, for the glass matrix. The barrier can be formed of a suitable biocompatible polymer, for example, polycaprolactone (PCL). PCL is a linear, partially crystalline, non-cytotoxic, thermoplastic, synthetic polymer. In a further embodiment, the polymer can be reinforced, for example, with a biaxial bioactive glass fabric, hydroxyapatite (HA) fibres and/or CaP. One of these layers can surround some or the entire outside surface of the device. Still further, one or more layers can be disposed through the device at various spacings from the outside surface. These one or more additional layers can serve to control the rate of softening and degradation of the device. PCL can also be copolymerized with other materials to obtain desired degradation properties. In a still further embodiment, the device can have a layer formed at least in part from a bioabsorbable metal or metal alloy. In one embodiment, the layer can be formed from iron including pure iron or an iron alloy, a magnesium alloy or a calcium alloy. Such a layeτ can be a foil or be added to the device using a vapour deposition coating process. In one embodiment, the layer can cover a majority, or all, of the device. The layer can degrade in vivo at a predetermined rate. The layer preferably slows the rate of moisture ingress to the device. For example, the layer can be adapted to prevent or at least slow moisture ingress for at least 2 weeks, more preferably a month, yet more preferably 4 months, and even at least 6 months. Where a metal or metal alloy layer is used, the layer can have a further coating to at least substantially or fully isolate the layer from contact with surrounding tissue following implantation. This further coating can comprise a suitable biocompatible polymer, such as polycaprolactone (PCL).

In a still further embodiment, the glass matrix can absorb in vivo at a rate different to that of the glass fibres. In one embodiment, the glass matrix can absorb more quickly than the glass fibres so leaving a fibrous glass scaffold in place in vivo for a time. The glass scaffold can have antibacterial properties and/or encourage wound healing at a fracture site at which the device is used.

In one embodiment, the implantable device can be an orthopaedic fixation device, such as an intramedullary nail, a screw, a pin, an anchor, a bone plate or any other device that is affixable to or with tissue such as bone. In one embodiment, the device can have a thread extending along some, the majority or all of its length. The device can taper for some or all of its length from a head member. The head member can be formed separately and joined to the remainder of the device. The head member can be a hexagonal head or other appropriate shape.

In a further embodiment, the device can be cylindrical. In another, the device can have a frusto-conical outer surface. Still further, the device can have a bore extending some or all of the length of the device. The bore can extend along the longitudinal axis of the device and be cylindrical in form. The bore can have a screw thread extending along some or all of its length. One or more end caps can be formed separately and mounted to one or both of the ends of the devices. In a still further embodiment, two or moτe of the devices can be assembled together to form a multi-layered device. For example, a series of individual cylindrical devices can be formed and then placed one inside the other to build-up the multi- layered device. Each of the devices can have a screw thread on its outer and inner surface to allow engagement of the devices. Respective devices can have alternating thread directions to ensure that the multi-layered device has fibres arrayed in each direction.

An intramedullary nail can be formed by using a plurality of tubular fixation devices as defined herein and a K- wire stabilisation technique for placement. In one embodiment, metal or metal alloy K-wires can be used to position the device at the site of fracture. Titanium and nickel-titanium alloy K-wires can be used.

A titanium shaft can have a relatively narrow central shaft with an enlarged threaded head that provides distal fixation. A series of bioabsorbable multilayered composite tubular devices can then be threaded onto the central shaft. Two forther K- wires follow channels between the shaft and the tubes then splay outward at the bulbous distal end of the shaft to engage with the cortical bone to provide fixation. The tubes are held in compression by a tightening bolt, effectively placing the central shaft in tension and increasing the flexural stifϊhess of the assembled modular nail. As the tubes begin to absorb and gradually soften over a desired time frame (for example a 6 week period) the flexural stiffness of the assembly decreases, while the K-wires and central shaft ensure axial and rotational stability. This prevents stress shielding while avoiding risk of bone shortening or misalignment problems. Although the K-wires and central shaft remain within the bone, they occupy substantially less volume than a typical IM nail and do not contribute to stress-shielding.

In another aspect, the present invention comprises a method of forming an implantable device as defined herein comprising: injecting the molten glass matrix/glass fibre mixture into a mould.

In this aspect, the mixture can be heated to a temperature suitable to allow the mixture to be injected into the mould. This temperature can be up to 180⁰C but other suitable temperatures can be envisaged. In this aspect, the mould can be a tube. The bioabsorbable glass fibres can be at least substantially aligned with the injection flow. The tube can be formed from a metal or metal alloy, such as iron or an iron alloy, a magnesium alloy or a calcium alloy.

The tube can be formed from pure iron. Once filled to the desired extent, the tube can be sealed with a cap, for example an iron cap. The cap can be cold welded to the tube. One end of the tube can be crimped together.

The tube can be coated on its outer surface with an additional layer of biocompatible material, such as polycaprolactone or a polycaprolactone/bone graft mixture. The coating can act to at least substantially, if not wholly, prevent bodily tissue being exposed to undesirable levels of iron (or other ions) and rust whilst also encouraging osteointegration and fixation.

In one embodiment, the additional layer for the tube can cover a majority, or all, of the tube.

The tube (and the additional layer if present) can degrade in vivo at a predetermined rate. The tube preferably slows the rate of moisture ingress to the glass matrix/glass fibre mixture. For example, the tube can be adapted to prevent or at least slow moisture ingress for at least 2 weeks, more preferably a month, yet more preferably 4 months, and even at least 6 months.

If required, the tube could be formed with a thread or texture in its outer surface.

The tube could also incorporate a number of pleats to allow the tube wall to expand if required following implantation. The tube would be appropriately sterilized following manufacture and/or prior to use.

Still further, the tube can be placed in a sock member prior to implantation.

According to a still further aspect, the present invention comprises an implantable device comprising: a bioabsorbable polymeric member; and an additional layer for at least a portion of the polymeric member; wherein the additional layer is a pliable metal or metal alloy that degrades following implantation.

In this aspect, the implantable device can have one or more of the features of the implantable device described herein. For example, the bioabsorbable polymeric member can have one, some or all of the features of the implantable device described in the preceding aspect.

In one embodiment of this aspect, the additional layeτ can comprise a foil, a tube or coating as described herein. For example, the additional layer can be formed from iron or an iron alloy, a magnesium alloy or a calcium alloy. These layers can be added to the device using vapour deposition.

In one embodiment, the additional layer can cover a majority, or all, of the polymeric member. The additional layer can degrade in vivo at a predetermined rate.

The additional layer preferably slows the rate of moisture ingress to the polymeric member. For example, the additional layer can be adapted to prevent or at least slow moisture ingress for at least 2 weeks, more preferably a month, yet more preferably 4 months, and even at least 6 months. Where a metal or metal alloy layer is used, the additional layer can have a further coating to at least substantially or fully isolate the layer from contact with surrounding tissue following implantation. This further coating can comprise a suitable biocompatible polymer, such as polycaprolactone (PCL).

In yet another embodiment of these aspects, the glass mixture/glass fibre mixture can act as a chelating agent for iron or other metal ions liberated by degradation of the tube, coating or layer surrounding the mixture following implantation. An additional chelating agent can also be used if required.

According to a still further aspect, the present invention is a method of implanting an implantable device as defined herein to bodily tissue, the method comprising: heating the implantable device such that the glass matrix/glass fibre mixture becomes at least partially molten; inserting the device into the tissue; driving an expanding member through the device to expand the device into contact with the surrounding tissue; and after a predetermined time, withdrawing the expanding member.

In this aspect, the mixture can be contained within a tube such as the tube described herein with respect to other aspects. The tube can expand without breaking. The expanding member can be a bone anchor screw. Prior to insertion, the tube can be placed in a biocompatible sock after being heated. The sock can serve to prevent heat damage to the surrounding tissue on implantation of the heated implantable device. The sock can be relatively chilled, for example to 0⁰C, prior to placement of the implantable device within the sock. The sock can be formed from glass fibre.

In this aspect, the predetermined time is preferably such that implantable device will have cooled sufficiently such that it retains its expanded configuration following withdrawal of the expanding member. The time can be less than 1 minute, less than 40 seconds, or even less than 30 seconds, for example about 20 seconds.

Brief Description of the Drawings

By way of example only, embodiments of the inventions are described with reference to the attached drawings, in which:

Fig. 1 is a perspective view of one embodiment of an intervertebral disk prosthesis according to the present invention;

Fig. 2A is a longitudinal sectional view of the intervertebral disk prosthesis of Fig. 1;

Fig. 2B is a cross-sectional view of the intervertebral disk prosthesis of Fig. 1;

Fig. 3 is an end portion of one embodiment of a milling tool according to a further aspect of the present invention;

Figs. 4A to 4E depict steps in the use of the tool depicted in Fig. 3 for use in placement of an intervertebral disk prosthesis according to the present invention; Fig. 5 depicts use of a modified embodiment of a tool according to the present invention for use in placement of an intervertebral disk prosthesis according to the present invention;

Fig. 6 is a perspective view of one embodiment of a bioabsorbable orthopaedic fixation device according to the present invention;

Fig. 7 is a perspective view of another embodiment of a bioabsorbable orthopaedic fixation device according to the present invention;

Fig. 8 is illustrative of how a plurality of cylindrical tubular bioabsorbable elements can be brought together to form another embodiment of an orthopaedic fixation device according to the present invention; and

Figs. 9A and 9B depict use of the elements of Fig. 8 to form an intramedullary nail assembly.

Preferred Mode of Carrying out the Invention

One embodiment of an intervertebral disk prosthesis according to the present invention is depicted as 10 in the drawings. The prosthesis 10 as depicted comprises a one-piece elastomeric composite member comprising a core member 11, a first endplate 12, and a second endplate 13 spaced from the first endplate 12 by the core member 11.

Each of the endplates 12,13 have the same diameter, being a diameter that is greater than that of the core member 11. The endplates 12,13 are engageable with respective opposed adjacent vertebrae of an implantee as discussed below.

As depicted in Figs. 2A and 2B, the prosthesis 10 has both a longitudinal and lateral axis and is functionally graded along these axes. As such, the properties of the member gradually vary from the centre of the core member 11 outwardly along the longitudinal axis in both directions. In the depicted embodiment, the modulus and bioactivity of the prosthesis 10 increases non-uniformly along the longitudinal axis from the core member 11 towards and including both of the endplates 12,13. The core member 1 1 has no or only a relatively small increase in modulus for a first distance from the centre of the core member 11 and then a relatively large increase in modulus within the thickness of the endplate. As discussed below, the endplates 12,13 aie also significantly more bioactive than the core member 11.

In addition, the properties of the core member 11 gradually vary from the centre of the core member 11 laterally to its outer radial surface 14 that extends between the two endplates 12,13. Again, the modulus of the core member radially increases in a non-uniform manner away from the centre of the core member 11, with a relatively large increase in modulus relatively close to its outer radial surface.

In the embodiment depicted in Fig. 1, the core member 11 is kidney-shaped and dimensioned so as to traverse the span of the vertebral body of the implantee receiving the prosthesis 10.

As depicted in Fig. 2, the outer surfaces 15,16 respectively of the endplates

12,13 can be formed parallel with respect to each other and are planar. As depicted in Figs. 4 and 5 and discussed in more detail below, the properties of the prosthesis 10 can be such that the outer surfaces 15,16 can undergo a change in angle relative to each other during implantation of the prosthesis and/or while in use in vivo.

In the depicted embodiment, the prosthesis 10 is formed of an elastomeric composite member that consists of a polycarbonate-urethane (PCU) elastomer. This PCU elastomer is both laterally functionally graded and also functionally graded towards the relatively stiff endplates 12,13 by the addition of a relatively fine hydroxyapatite (HA) fibre into the elastomer.

A relatively compliant 80A Shore haidness PCU is used for the core member 11 of the prosthesis 10, while a relatively strong and stiff 75D Shore hardness PCU is used for the region at and adjacent the radial outer surface 14 of the core member 11 and the endplates 12,13. Still further, the endplates 12,13 are reinforced in a functionally graded manner using about 15 vol% of relatively fine HA chopped fibre. The concentration of the fibres in the endplates 12,13 increases moving progressively away from the core member 11 and also radially outward to the radially outer surface 14 of the respective endplates 12,13. The relatively fine HA fibres that are embedded within the endplates 12,13 also form relatively fine networks that assist with bone attachment and ingrowth while also increasing the compressive modulus of the endplates 12,13 to at least substantially match that of the interfacing bone. As such, the bioactivity of the prosthesis 10 does increase along the longitudinal axis in both directions moving away from the core member 11.

As depicted, both endplates 12,13 have a bone anchor fin 17 extending outwardly therefrom. In the embodiment depicted in Figs. 1 and 2, the respective bone anchor fins 17 are integrally formed in the endplates 12,13. In another embodiment, and as depicted in Fig. 5, the bone anchor fins 17a can be thermo-mechanically or otherwise integrated into or mounted to the endplates 12,13. The embodiment depicted in Fig. 5 is discussed in more detail below.

The respective fins 17 are adapted to mechanically engage within a complementary groove milled into the respective opposed vertebrae of the implantee during the prosthesis implantation surgery.

As depicted, the respective bone anchor fins 17 incorporate an enlarged end or bulb 18 distal the core member U that serves to mechanically key with the groove formed in the vertebrae. This arrangement is understood to further improve fixation by transferring tensile loads and forces resulting from a combined extension and anterior- posterior shear regime, axial rotation, and lateral translation.

To use the bone anchor fins 17, the vertebrae have to be milled in order to provide two aligned keyhole shaped grooves. To do this, a milling tool 20 as depicted in Fig. 3 as discussed further below can be used. Appropriate use of the depicted milling tool 20 results in simultaneous milling of the two grooves. This milling can be undertaken subsequent to a discectomy and neural decompression to relieve the back pain presumably being suffered by the implantee. It is envisaged that use of the milling tool 20 would prescribe an anterior approach requiring resection of the anterior longitudinal ligament with the intervertebral disk joint slightly flexed so as to allow correct alignment of the vertebrae endplates. Further alignment of the drill pieces could also be achieved through use of the milling tool. Fine perforations can also be formed, such as by milling, in the outer surfaces 15,16 of the endplates 12,13 to expose the HA fibre networks (where present) and also further increase the surface area of the interface formed between the bone and the outer surfaces 15,16.

One or more holes and/or grooves may also be formed in the fins 17.

Additionally, Bioglass^® chopped fibre, CaP fibres or blades, or other suitable bioabsorbable materials may be present in at least the outer surfaces 15,16 of the endplates 12,13 to assist in establishing initial attachment between the prosthesis 10 and the bone followed by gradual absorption and final displacement by bone.

The depicted milling tool 20 for milling the opposed surfaces of adjacent vertebrae comprises a first drill bit 21, a second drill bit 22, and an oscillating saw blade 23 having a distal cutting edge 24. The blade 23 is positionable between the first and second drill bits 21 ,22 and is adapted to oscillate between the drill bits 21 ,22.

Both the first drill bit 21 and the second drill bit 22 extend forwardly from respective chucks mounted on a support holder. Both the chucks and support holder are not visible in Fig. 3 as they are positioned within the housing 25 of the tool 20. The support holder is movable relatively forwardly and backwardly relative to the saw blade 23. The position of the holder can be controlled using one or more linear piezoelectric actuators.

The chucks of the tool 20 can turn the first and second drill bits 21,22 simultaneously and the rotation of the drill bits is also reversible if required.

While the first and second drill bits 21,22 are depicted as parallel with respect to each other in Fig. 3, in use the alignment of the first and second drill bits 21,22 relative to each other can be adjustable from this parallel alignment between plus and minus 10° from parallel thereby allowing use of the tool in situations where the opposed vertebral surfaces are at an angle of up to 20° relative to each other. If necessary, tools having a capability to be adjusted by greater or smaller amounts can be envisaged. The drill bits can have a diamond, diamond-like or other suitable coating. The amplitude of oscillation of the saw blade 23 is also adjustable. Still further the saw blade 23 is mounted on a saw blade holder (not visible within the housing 25) that is also movable relatively forwardly and backwardly relative to the drill bits 21,22.

A method of milling the opposed surfaces of adjacent vertebrae following excision of the intervertebral disk using the milling tool 20 is depicted in Fig. 4. The method firstly comprises a step of aligning and anteriorly mounting the milling tool 20 relative to the vertebrae 31,32 to be milled and the void 33 left by the excised intervertebral disk. The alignment of the milling tool 20 can be made with the aid of computer tomography (CT) imaging and anterior mounting of the milling tool 20 can be made using a bracket (not depicted).

As depicted in Fig. 4A, the angle of the drill bits 21,22 can be adjusted, if necessary, to suit the inclination of the opposed surfaces of the vertebrae 31,32.

The rotating drill bits 21,22 are then moved relatively forwardly to form respective lateral holes in the adjacent vertebrae 31,32, with the holes being spaced inwardly from the respective opposed surfaces of the adjacent vertebrae.

Once the holes are formed, the drill bits are relatively partially withdrawn (see

Fig. 4B) in order to preserve the alignment of the vertebrae 31,32 and guide the subsequent cutting of the grooves.

The oscillating saw blade 23 is then moved relatively forwardly to resect bone in each vertebra between the opposed surfaces and the lateral hole to form the desired grooves. The saw blade 23 and the drill bits 21,22 are then relatively withdrawn.

The depicted oscillating saw blade 23 oscillates at a relatively high speed under amplitude control. If, after its use, any bone remains bridging the formed grooves, for example at the anterior of the vertebra near the stationary partially retracted drill bits, this bone can be removed by the surgeon using a bone chisel.

Once the grooves are formed, the milling tool 20 can be removed and guide arms 26, as depicted in Fig. 4C, can be mounted thereon. The guide arms 26 can support a prosthesis, such as the prosthesis 10 having bone anchor fins 17 as defined above. The tool 20, with the guide arms 26 mounted thereon, can be used to gradually insert the prosthesis 10 into the space between the vertebrae 31,32 with the fins 17 anchored within the grooves as formed by the milling tool. A sectional view of the prosthesis IO in place between the vertebrae is provided in Fig. 4D and an anterior view is provided by Fig. 4E. The guide arms 26 can also be adjustable relative to each other to pτe-stress the prosthesis 10 ready for insertion into the space. Still further, the guide arms can be retractable during the insertion step.

Another embodiment of a prosthesis and a method of placement is provided by Fig. 5. Here, each endplate 12,13 has two separate anchor fin portions 17a, with the bulbs 18a of the fin portions 17a being bioabsorbable. Bioabsorbable bone screws 34 can be used to engage the bone anchor fins 17a in place in the respective vertebrae 31,32, The bioabsorbable material used in the bulb portions 18a and the bone screws 34 can be selected from a group of materials comprising resorbable polymers including polycaprolactone (PCL), PLDLA and polylactide acid (PLA). These bioabsorbable polymers can be reinforced with fibrous Bioglass^®, CaP, and/or HA.

If desired, the prosthesis 10 depicted in Fig. 5 can be removed from the spine of the implantee by using a blade to cut the fins 17a at the bone-endplate interface so leaving the lodged fin 17a to be adsorbed over time.

As also depicted in Fig. 5, a tool 20a (for example a modified milling tool 20) can be used to drive spindles 35 that in turn serves to turn the screws 34 into place. The spindles 35 also serve to apply a pre-stress to the prosthesis so that it is positioned and aligned with the milled grooves in the vertebrae 31,32.

The prosthesis 10 acts as a total replacement of the native lumbar disk and is anticipated to at least partially replicate the structural gradients, range of motion, degrees of freedom, axis of rotation loci, joint stiffness, and/or shock absorption that are characteristic of the native lumbar disk.

One embodiment of an implantable device, in this case an orthopaedic fixation device, according to the present invention is depicted generally as 40 in Fig. 6. The device 40 is formed from an extruded bioabsorbable carbohydrate glass matrix 41 surrounding a plurality of oriented bioabsorbable glass fibres 42.

In this embodiment, the glass matrix 41 is an extruded caramelized sucrose. The caramelized sucrose serves to mechanically hold, adhere to and/or protect the glass fibres 42.

Apart from the above, the caramelized sucrose glass matrix is also understood to have anti-bacterial properties and still farther, gives wound healing properties by providing nutrition to the wound site.

As depicted in Fig. 6, the fixation device 40 further has a relatively thin outer moisture barrier sheath formed of a number of layers. Here, the sheath is formed of a layer of biaxial glass fibre reinforced glass 44, a layer 43 of biaxial woven glass fabric embedded within a polycaprolactone (PCL) matrix and a further layer 45 of magnesium alloy film on the inner surface of the layer 43.

While the device 40 is depicted with these layers on the outer surface of the device 40, it will be appreciated that multiple layers can be disposed at varying spacings from the outer surface through the device 40 to provide greater control of the rate of softening and degradation of the device 40 following implantation.

Following implantation within an implantee, the glass matrix 41 can absorb relatively more quickly than the glass fibres 42 so leaving a fibrous glass scaffold in place at least for a time. The glass scaffold can also have antibacterial properties and/or encourage wound healing at a fracture site at which the device 40 is used.

Another embodiment of a device is depicted generally as 50 in Fig. 7. Here, the device 50 has an outer tapering thread 51 extending along its length from a hexagonal head member 52. The device 50 can have one, some or all of the features of the fixation device 40 described above.

The device 50 can be formed by injecting heated or molten glass matrix/glass fibre mixture into a metal tube that acts as a mould. The bioabsorbable glass fibres present in the mixture can be at least substantially aligned with the injection flow into the tube. The tube can be formed from a metal or metal alloy, such as iron or an iron alloy, a magnesium alloy or a calcium alloy.

The tube used to form the device can be formed from pure iron. Once filled to the desired extent, the tube can be sealed with a cap, for example an iron cap. The hexagonal head member 52 could constitute the cap. The cap can be cold welded to the tube. One end of the tube can also be crimped together.

As depicted, the tube could be formed with a thread 51 or texture in its outer surface. The tube could also incorporate a number of pleats to allow the tube wall to expand if required following implantation. The tube would be appropriately sterilized following manufacture and/or prior to use. In this regard, the tube could comprise a cylinder having a bore extending therethrough. The bore can extend along the longitudinal axis of the tube. Such a bore can allow an expanding device, such as a bone screw to be inserted into the bore. Where the tube has been heated so that the mixture has softened and the expanding device has a greater diameter than the bore, it will be appreciated that the device will expand as the expanding device is driven into the bore.

Still further, the tube can be placed in a sock member prior to implantation. The glass mixture/glass fibre mixture is also advantageous in that it can act as a chelating agent for iron or other metal ions liberated by degradation of the tube, coating or layer surrounding the mixture following implantation. An additional chelating agent can also be used if required.

While a device using glass matrix/glass fibre injected into an iron tube is depicted in Fig. 7, the tube could be filled with any bioabsorbable polymer, with the outer tube serving to provide a pliable layer that degrades at a suitable rate following implantation.

In use, a modified version of the implantable device depicted in Fig. 7 can be heated such that the glass matrix/glass fibre mixture becomes at least partially molten. It can then be inserted into an orifice in a bone of an implantee, for example, for use in suitable affixing an anterior cruciate ligament. Once inserted, an expanding member can be inserted through the device 50 to expand the device into contact with the surrounding tissue and so affix the ligament. After a predetermined time that is sufficient to allow the device to cool, the expanding member can be withdrawn leaving the expanded device in place.

In this method, the mixture can be contained within a tube such as the tube described herein. The tube can expand without breaking. The expanding member can be a bone anchor screw. Prior to insertion, the tube can also be placed in a biocompatible sock after being heated. The sock can serve to prevent heat damage to the surrounding tissue on implantation of the heated implantable device. The sock can be relatively chilled, for example to 0⁰C, prior to placement of the implantable device within the sock. The sock can be formed from glass fibre.

The predetermined time is preferably such that implantable device will have cooled sufficiently such that it retains its expanded configuration following withdrawal of the expanding member. The time can be less than 1 minute, less than 40 seconds, or even less than 30 seconds, for example about 20 seconds.

Figs. 8 and 9 depict yet another cylindrical embodiment of the fixation device as 60. Fig. 8 depicts how a one device can be made up of a number of separate components disposed one within the other. Each component has a threaded outer surface and a threaded bore extending all of the length of the device. Alternate components, e.g. 60a and 60c, can have a screw thread direction different to that the other components, i.e. 60b, to ensure that the multi-layered device 60 has fibres arrayed in each direction.

As depicted in Fig. 9, if necessary, one or more end caps 61 can be formed separately and mounted to one or both of the ends of the device 60. The end caps 61 can be formed of polycapro lactone with a relatively thin iron or magnesium alloy layer to help prevent moisture ingress. Still further, each device 60 can be formed of anisotropic glass fibre reinforced glass matrix 66 having polycaprolactone moisture barrier layers 67 disposed therein.

As depicted by Fig. 9, an intramedullary nail assembly 70 can be formed and used at a site of fracture 72 in a bone 73. The depicted assembly 70 is made up of a plurality of devices 60 and relies on a K-wire stabilisation technique for placement.

This technique uses a titanium shaft 62 and two nickel-titanium alloy K-wires 63. The shaft 62 has a relatively narrow central shaft with an enlarged threaded head 64 that provides distal fixation. A series of the tubular devices 60 are then threaded onto the central shaft 62. Two K-wires 63 follow channels between the shaft 62 and the tubular devices 60 then splay outward at the bulbous distal end of the shaft to engage with the cortical bone to provide fixation.

The tubular devices 60 aie held in compression by a tightening bolt 65, effectively placing the central shaft 62 in tension and increasing the flexural stiffness of the assembled modular nail 70. As the tubes begin to absorb and gradually soften over a desired time frame (for example a 6 week period) the flexural stiffness of the assembly decreases, while the K-wires and central shaft ensure axial and rotational stability. This prevents stress shielding while avoiding risk of bone shortening or misalignment problems. Although the K-wires 63 and central shaft 62 Temain within the bone 73, they occupy substantially less volume than a typical IM nail and do not contribute to stress-shielding. While an intramedullary nail assembly is depicted, the fixation device can comprise a screw, a pin, an anchor, a bone plate or any other device that is affixable to or with bone.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. An intervertebral disk prosthesis comprising an elastomeric composite member comprising: a core member; a first endplate; and a second endplate spaced from the first endplate by the core member; wherein the elastomeric composite member is at least functionally graded axially from the core member to at least one of the first or second endplates. .

2. The intervertebral disk prosthesis of claim 1 wherein the elastomeric composite member is formed in a single piece.

3. The intervertebral disk prosthesis of claim 1 wherein the endplates have a diameter greater than that of the core member and are engageable with respective opposed adjacent vertebrae of an implantee.

4. The intervertebral disk prosthesis of claim 1 wherein elastomeric composite member has a longitudinal axis and is functionally graded along this axis.

5. The intervertebral disk prosthesis of claim 4 wherein the modulus and/or bioactivity of the member increases along the longitudinal axis from the core member towards and including both endplates.

6. The intervertebral disk prosthesis of claim 1 or 5 wherein the properties of the core gradually vary from the centre of the core member laterally to an outer radial surface of the core member that extends between the two endplates.

7. The intervertebral disk prosthesis of claim 6 wherein the modulus and/or bioactivity of the core member radially increase away from the centre of the core member.

8. The intervertebral disk prosthesis of claim 1 wherein the core member is kidney- shaped or elliptical in lateral cross-section.

9. The intervertebral disk prosthesis of claim 1 wherein the outer surfaces of the endplates are formed parallel with respect to each other.

10. The intervertebral disk prosthesis of claim 5 wherein the elastomeric composite member consists of a polycarbonate-urethane (PCU) elastomer.

11. The intervertebral disk prosthesis of claim 10 wherein the PCU elastomer is laterally functionally graded and also functionally graded towards the relatively stiff endplates by the addition of a fibre material.

12. The intervertebral disk prosthesis of claim I l wherein the fibre material is a relatively fine hydroxyapatite (HA) fibre.

13. The intervertebral disk prosthesis of claim 12 wherein the endplates are reinforced in a functionally graded manner using relatively fine HA chopped fibre and/or Bioglass® fibre with the concentration of the fibres in the endplates increasing progressively away from the core member and also radially outward to the radially outer surface of the respective endplates.

14. The intervertebral disk prosthesis of claim 1 wherein one or both endplates have a bone anchor fin extending outwardly therefrom.

15. The intervertebral disk prosthesis of claim 14 wherein part or all of the bone anchor fins are bioabsorbable.

16. The intervertebral disk prosthesis of claim 14 wherein the respective fins are engageable in a complementary groove milled into the respective opposed vertebrae of the implantee during prosthesis implantation surgery.

17. An intervertebral disk prosthesis comprising an elastomeric composite member comprising: a core member; a first endplate; and a second endplate spaced from the first endplate by the core member; wherein extending outwardly from one or both endplates is a bone anchor fin, and further wherein at least a part of said bone anchor fin is bioabsorbable.

18. A milling tool for milling the opposed surfaces of adjacent vertebrae, the tool comprising: a first drill bit; a second drill bit; and an oscillating saw blade having a distal cutting edge and positionable between the first and second drill bits.

19. A method of milling respective opposed surfaces of adjacent vertebrae following excision of the intervertebral disk using the milling tool of claim 18, the method comprising: aligning and anteriorly mounting the milling tool relative to the vertebrae to be milled; adjusting, if necessary, the angle of the respective drill bits to suit the inclination of the opposed surfaces; moving the drill bits relatively forwardly to form respective lateral holes in the adjacent vertebrae, the holes being spaced inwardly from the opposed surfaces; at least partially withdrawing the drill bits; moving the oscillating saw blade relatively forwardly to resect bone in each vertebrae between the opposed surfaces and the lateral hole to form a groove; and withdrawing the saw blade and the drill bits.

20. An implantable device formed from a bioabsorbable carbohydrate glass matrix surrounding a plurality of bioabsorbable glass fibres.

21. The device of claim 20 wherein the glass matrix is a caramelized sucrose.

22. The device of claim 21 wherein some or all of the glass fibres within the matrix are oriented with respect to each other.

23. The device of claim 20 wherein the fixation device has one or more layers that act as a relatively thin moisture barrier, at least for a time, for the glass matrix.

24. The device of claim 23 wherein the barrier is formed from polycaprolactone (PCL) reinforced with a biaxial bioactive glass fabric, hydroxyapatite (HA) fibres and/or CaP.

25. The device of claim 20 wherein the device has a layer formed at least in part from a bioabsorbable metal or metal alloy.

26. The device of claim 20 wherein the glass matrix absorbs at a rate different to that of the glass fibres.

27. The device of claim 20 wherein the device is an intramedullary nail, a screw, a pin, an anchor, a bone plate or any other device that is affixable to or with bone.

28. A method of forming an implantable device as defined in claim 20 comprising: injecting the molten glass matrix/glass fibre mixture into a mould.

29. The method of claim 28 wherein the mould is a tube.

30. The method of claim 29 wherein the tube is formed from pure iron.

31. An implantable device comprising: a bioabsorbable polymeric member; and an additional layer for at least a portion of the polymeric member; wherein the additional layer is a pliable metal or metal alloy that degrades following implantation.

32. A method of implanting an implantable device as defined herein to bodily tissue, the method comprising: heating the implantable device such that the glass matrix/glass fibre mixture becomes at least partially molten; inserting the device into the tissue; driving an expanding member through the device to expand the device into contact with the surrounding tissue; and after a predetermined time, withdrawing the expanding member.