GB2181438A - Biocompatible material - Google Patents

Abstract

Biocompatible materials 10 suitable for use in prosthetic devices such as bone plates, ligament fixation arrangements, nails, joints, or the like, comprise a composite of a polymeric plastics matrix 12, within which are embedded plastics reinforcing fibres 14. The matrix and fibres are both thermoplastic and have softening points at different temperatures. The fibres have a softening point at a higher temperature than the matrix. <IMAGE>

Description

SPECIFICATION Biocompatible material THIS INVENTION relates to a biocompatible material for use in prosthetic devices.

According to the invention, broadly, there is provided a biocompatible material for use in prosthetic devices, which biocompatible material may be composite in nature and may comprise a polymeric plastics matrix within which are embedded a plurality of polymeric plastics reinforcing fibres, the matrix being thermoplastic and the fibres acting to reinforce the matrix and also being thermoplastic, the fibres and the matrix having softening points at different temperatures, the temperature of the softening point of the fibres being higher than that of the matrix.

The matrix and fibres are preferably chemically of substantially the same polymeric plastics material, eg they may both be polyethylene or polyester, but will be different from each other as far as their melt indices and, optionally, other physical properties, are concerned. The biocompatible matrix and fibres of the material of the present invention will preferably be resistant to bioabsorption.

The temperature of the softening point of the fibres may be above 1 200C and the temperature of the softening point of the matrix may be above body temperature and below 100 C. Thus, the temperature of the softening point of the fibres may be between 120 and 1 300C and the temperature of the softening point of the matrix may be between 50 and 70 C.

Typically the fibres and matrix will be more or less flexible at body temperature, or more or less rigid, depending on the intended function of the prosthetic device. Thus, for use in ligament prosthesis, repair or replacement, the composite may be flexible, intended only to be load bearing in tension. On the other hand, when it is intended for bone prosthesis, replacement or repair, the composite may be more rigid, so that it is at least partially load bearing as regards flexion or compression and/or torsion as well as tension.

The fibres may be selected to be those in which the molecules of the polymeric or plastics material from which they are formed are elongated in shape and are as long as practicable, and in which these molecules are aligned as far as possible in parallel alignment with the longitudinal direction of the fibres. A suitable polyethylene fibre of the type in question is available under the trade mark "Dyneema" from DSM, Geleen, The Netherlands, and in accordance with the invention will typically be used to reinforce a polyethylene matrix of lower softening point and lower melt index.In composite applications, Dyneema is described by DSM as having the following properties: Density 0,97 (g/cm3) Tensile Strength 2,0-3,5 (GPa) Specific Tensile Strength 2,0-3,5 (105m) Modulus 50-125 (GPa) Specific Modulus 50-125 (105m) Elongation to Break 3-6% Instead, use may be made of polyester fibres in a polyester matrix.

The Applicant contemplates that the basic form of the material of the invention will be a ribbon or strip of matrix material in which the fibres are embedded, although the matrix can, if desired, be moulded into more complex shapes other than ribbons. Such ribbons can have the matrix material and fibres selected so that the ribbons can be cut or otherwise shaped into ligament prostheses for bearing tension loads; or they can have their matrix and fibres selected so that they can be sandwiched and welded or moulded together into laminated composites for use as bone plates or similar prostheses intended for use bearing loads in flexion or compression as well as tension.

The reinforcing fibres, which will generally have elongated molecules as described above aligned with the longitudinal axis of the fibre, will be aligned and arranged in the matrix material, and the matrix material itself will be arranged and aligned in any prosthesis made therefrom, such that the fibres, and hence the molecules therein, perform an effective reinforcing function in the prosthetic device, to counteract anticipated stressing of the device. The composite material of the invention, and prosthetic devices made therefrom, may contain between 5% and 75% by mass of the fibres, i.e. the fibres may form from 5-75% by mass of the material, the remainder being matrix material.

The invention proposes, in addition to flexible prosthetic devices such as ligaments, more rigid devices intended to bear loads in flexion, compression and/or torsion such as bone plates or intramedullary nails. These more rigid prosthetic devices can in accordance with the invention be made with a general or standardised shape, and can then be shaped during a surgical procedure by heating, into their final form. They can accordingly be customised or tailor-made by the surgeon for a particular use, while still being manufactured originally by standardized mass production techniques.The melt index or softening point of the matrix will accordingly be chosen so that their is no softening of the matrix at any temperatues which can be encountered in a living human or animal body, but so that the matrix will soften at as low a temperature as is practicable (with a safety margin if appropriate) above said maximum temperature which can be encountered in a body in use. As a rule of thumb a softening temperature for the matrix could be selected which is of the order of 60 C.

During a surgical procedure, a fixation device such as a bone plate or intramedullary nail for bone repair can be softened and moulded while still soft against the bone which is to be repaired (or against a flexible template which has been shaped in accordance with said bone).

The low temperature to which the matrix is heated for softening, coupled with the low specific heat of the plastics material, then allows the prosthetic device to be fastened into position with a minimum, if any, delay occasioned by cooling, although the device can, if desired, be cooled after it has been softened, and prior to insertion into place.

The material may include carbon dispersed therein for facilitating heating thereof. The carbon may be in the form of carbon fibres, for resistance heating of the material, or it may be in the form of granular particles, for microwave heating of the material.

Thus, for the purpose of heating, the matrix may be electrically insulating but may include electrically conducting fibres such as heating fibres in addition to reinforcing fibres. Such heating fibres may be carbon fibres or the like, capable of carrying a heating current, to permit resistance heating of the matrix by the application of an electrical potential to opposite ends of the fibres. These heating fibres can be incorporated in the matrix during manufacture of the device, suitably interspersed throughout the device to permit rapid and effective heating, and the heating fibres will provide an additional reinforcing function.

Instead, a suitable granular material such as carbon may be dispersed throughout the matrix, to facilitate microwave heating and softening of the matrix. In use, whether resistance heating or microwave heating is used, softening energy can be supplied to the matrix in pulsed bursts, to allow dispersion of heat into the matrix between pulses, and to permit heating to softening temperature without local or other overheating.

Naturally, heating may also take place in a more conventional fashion, for example by means of hot air or hot liquid, eg in a fluidized bed, the heat thus being supplied externally to the device.

A further feature of the invention is that its surface can be textured, eg by the formation thereon of a multiplicity of indentations such as grooves, pits or cavities, or protrusions such as bosses or pyramids. When subjected to external heating, the textured surface will soften more rapidly than the interior of the matrix, because of the relatively low thermal conductivity of the matrix materials in question. If the prosthetic device is then urged into place against a bone to be repaired, its textured surface can be deformed to conform readily with the surface of the bone against which it abuts, while said surface is soft.The invention thus provides for gross softening and shaping of the prosthetic device, and also for selective softening of its surface, during its being urged against a bone to be repaired, so that its surface hardens while in contact with the bone, assuming the more or less exact appropriate shape.

Thus, for example, an intramedullary nail can be made suitable for either left hand use or right hand use. The nail as a whole can be softened for shaping into left hand or right hand shape, and it can be forced into position, for example in the medullary canal, while its surface is still soft so that the texture of the surface permits it to take up the exact shape of the medullary canal. The texturing of the surface provides the further feature that bony ingrowth can take place into spaces defined therein, eg in cavities, between pyramids, or the like.

When pyramids on the surface are employed, they may have square (or possibly triangular or the like suitable outline) bases which are side-by-side in the fashion of a tiled surface, each pyramid tapering upwardly to a point. The pyramids will thus be arranged in a grid, and the spacings between the pyramid centres on this grid may be about 0,25-0,75 mm, the heights of the pyramids being at least roughly of the same order of magnitude. If desired, the material of the pyramids may selectively be supplied with the abovementioned material for facilitating microwave heating, to permit selective heating and softening of the pyramids, at the surface of the prosthetic device.

According to a further feature of the invention the textured surface, eg in pyramid mouldings or the like, can include materials which are stable at the moulding temperature but which can subsequently be leached out, using eg an alkaline or acid liquor, to provide said textured surface with a plurality of sub-surface interconnected passages opening out of said surface, thereby promoting subsequent bony or indeed soft tissue ingrowth in use.

Furthermore, if desired a modified or different polymeric material from the bulk of the matrix may be chosen for the texturized surface, so that the physical properties of the surface of the prosthetic device are as close an approximation as possible to the physical properties of the bone against which it in use abuts. The modulus of elasticity of the material may be selected to conform with that of bone.

Apart from the general advantage that prosthetic devices can be customised by low temperature heating, softening and shaping thereof, during surgical procedures and from mass-production artifacts, the formation of prosthetic devices by layering together ribbons reinforced by fibres, and the use of different fibre managements in different parts of a device, permit the formation of prosthetic devices which have variable stiffness at various positions, eg along their lengths.

These devices can then be clamped in position against bones which they are to repair, while still soft, so that they may harden into an overall gross shape which conforms with the shape of the bone being repaired, and in addition by means of a textured surface, can be provided with surfaces, which conform substantially exactly with the bone surface. As an example a hip joint stem can be heated until it softens, and then shaped by a surgeon to conform to a medullary canal in which it is to be inserted. The relatively low weight of such prosthetic devices, and their biological inertness, permits them to be left permanently in position.

Turning to prosthetic devices intended for ligament replacement, these will generally be anchored in holes provided in bone, located so as to meet anatomical requirements for the ligaments being repaired or replaced. In such holes the ligament replacement may be provided with locking devices to clamp it into position, and such locking devices may make provision for subsequent bony or soft tissue ingrowth.

The invention thus extends also to locking devices made of the composite material of the present invention, eg in the form of locking devices for the anchoring of ligament replacements in position; and the invention extends further to the ribbon described above, mounted in a suitable needle such as an atraumatic needle for suturing soft tissue defects; or a soft-bodied needle suitable for use in the augmentation of ligament repair. Furthermore, for ligament repair, the ligament replacement in the form of a ribbon with an anchoring device can be adapted for use for insertion by means of an arthroscope.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings in which, Figure 1 shows a schematic three dimensional representation of part of a ribbon in accordance with the invention; Figure 2 shows a similar view of part of one of the fibres forming part of the ribbon of Fig. 1; Figure 3 shows a similar view of part of a bone plate according to the invention; Figure 4 shows a schematic cross section of a ligament fixation arrangement in accordance with the invention; Figure 5 shows a three dimensional detail of part of the surface of certain components of the arrangement of Fig. 4; Figure 6 shows a schematic three-dimensional view of another ligament fixation arrangement according to the invention; Figure 7 shows a view similar to Fig. 4 of a yet further ligament fixation arrangement according to the invention;; Figure 8 shows a schematic three-dimensional view of an intra-medullary nail according to the invention; and Figure 9 shows a schematic side elevation of a hip joint prosthesis according to the invention.

In Fig. 1 of the drawings, reference numeral 10 generally designates part of a flexible ribbon in accordance with the invention suitable for use as a ligament prosthesis. The ribbon 10 comprises a matrix 12 of polyethylene, reinforced by means of a plurality of parallel longitudinally extending flexible Dyneema polyethylene fibres 14. The matrix has a lower softening point (about 60 Cì and melt index than the fibres 14, whose softening point is at least 120 C. The fibres and matrix each make up between 5 and 75% by mass of the ribbon, and the matrix is flexible.

In Fig. 2, one of the fibres 12 is generally designated 14 and is shown (schematically) to comprise a plurality of elongated molecules 16, arranged at least roughly parallel to one another and extending lengthwise along the fibre 14. The molecules 16 are shown staggered lengthwise relative to one another, so that they overlap one another more or less randomly, along the length of the fibre 14.

To make the ribbon 10, the polyethylene of the matrix 12 is first dissolved in a suitable solvent. Generally as much polyethylene as possible is dissolved, subject to the constraints that the solution must be stable at room temperature and should not separate into its constituent parts, and that its viscocity should not be too high. To form the solution the solvent is heated to a temperature at least as high as the softening point of the solute (polyethylene of the matrix) after which the solute is added to the hpt solvent. This is conveniently carried out under pressure in a pressure vessel, so that solvent is not lost. Once the solvent has been formed it can be cooled to room temperature.

Once the solution is formed, it is used to coat the fibres, eg by dipping at room temperature or above, the amount of solution used being selected to give the appropriate proportions by mass of fibres and matrix (solute) in the final product. After said coating, the solvent is driven off by heating the coated firbes eg to the boiling temperature of the solvent, to evaporate the solvent, which may then be recovered.

The fibres will now be coated with matrix material and the ribbon can be formed by fusing them togetherunder pressure at a temperature above the softening point of the matrix but below the softening point of the fibres. The fusion will take place at a suitable pressure and temperature, and for a suitable time to obtain an adequately fused product. The higher the value of any one of the temperature, pressure or fusion time is, the lower may be the other two, within limits, bearing practical considerations in mind. Thus routine experimentation will be used to find the best set of these parameters for a particular case. In this regard it should be borne in mind that essentially the same fusion method will be used for all the articles in accordance with the invention, and described with reference to Figs. 3-9 hereunder.

In Fig. 3, reference numeral 18 generally designates a bone plate in accordance with the invention, and the same reference numerals designate the same parts as in Fig. 1. In addition to the matrix 12 and fibres 14 (which are electrically insulating polyethylene), the bone plate 18 is shown having a bundle 20 of carbon heating fibres extending along it capable, when an electric potential is applied to its ends, of carrying a heating current which can, by resistance heating to above the softening temperature of the matrix, soften the bone plate to permit it to be shaped by hand.In contrast to the ribbon of Fig. 1, where the matrix 12 is flexible and, apart from encapsulating the fibres 14, has little other function than to supplement the tension-load-bearing ability of the ribbon which is provided primarily by the fibres 14, the matrix 12 of the bone plate 18 is substantially stiffer and more rigid (although still flexible to a limited extent) so that it can, in addition to encapsulating the fibres and providing some tension-load-bearing function, also provide compression-, torsion- and flexion-load-bearing functions.

The fibres 14 will once again be flexible Dyneema fibres and the matrix 12 will again have a softening point of about 60 C, but the polyethylene of the matrix will be selected to provide the bone plate 18 with desired rigidity. The matrix of the bone plate 18 may be of unitary construction; or its matrix may be of composite construction, i.e. formed by welding or moulding together a plurality of thinner strips or ribbons 10 (Fig. 1). Instead of or in addition to the bundles 20 of heating fibres, a particulate material such as particulate carbon (not shown) may be dispersed in the matrix to facilitate microwave heating for softening the bone plate.Furthermore the surface of the bone plate may be textured as described hereunder with reference to Fig. 5, and, if desired, the particulate carbon may only be provided in, and/or in the matrix material immediately underlying, the matrix material of the textured surface.

In a surgical procedure the bone plate will be used to repair a bone fracture, the bone plate being affixed by suitable fixation means (not shown) to the bone on opposite sides of the fracture. The bone plate will be selected to provide the fractured bone with the desired degree of sufficiently rigid but flexible support.

In use the bone plate 18 will have its matrix 12 heated and softened throughout to permit gross shaping thereof to conform with the shape of the bone to which it is to be affixed, prior to implantation. Furthermore, if desired, only its textured surface only may be softened, immedi ately prior to implantation, so that the surface is soft when the bone plate is pressed into place against the bone, and flows sufficiently to conform more or less exactly with the bone surface.

In this regard the softening temperature, thermal conductivity and specific heat of the matrix, and the thickness of the textured surface layer which is softened, will be sufficiently low for the surface layer to cool down sufficiently rapidly on contact with the bone, to cause no harm.

Turning to Fig. 4, reference numeral 22 generally designates a ligament fixation arrangement in accordance with the invention in which, once again, the same reference numerals are used for the same parts as in Figs. 1 to 3, unless otherwise specified. A ligament/ribbon 10 is shown in the form of a radio trace ribbon (having X-ray opaque material dispersed in its matrix) with its one end 24 affixed in a passage 26 in a bone 28. Said one end 24 is in the form of a thickened portion or laterally enlarged elongated block of matrix material 12, integral with the matrix material 12 of the ribbon 10, but containing an additional or higher proportion of fibres 14. One side of said end 24 is provided with transversely extending ribs and valleys to provide undulations as at 30.Said end 24 is held in place by a locking device in the form of an elongated tapered wedge 32 of essentially the same construction (as regards the matrix 12, fibres 14 and relative proportion of fibres). One side of the wedge 32 is provided with transversely extending ribs and valleys to provide longitudinally spaced transverse teeth as at 34, of more angular cross-section than the undulations 30.

In use the end 24 is located outside the month 36 of the passage 26 with the wedge alongside it with the ribbon 10 extending through the passage 26. The end 24 and wedge 32 are then inserted simultaneously into the passage, for example by pulling on the ribbon and/or pushing the end surfaces 24.1, 32.1 of the end 24 and wedge 32, in the direction of arrow 38, so that the wedge enters the passage 26 narrow end first. Said end 24 and wedge 32 will wedge firmly into the passage 26, the undulations 30 and teeth 34 becoming compressed and flattened during the wedging as shown. To assist in this wedging, the hole is slightly tapered in the direction of arrow 38.When the end 24 and wedge 32 are fully wedged in place any parts thereof projecting from the mouth 36 of the passage 26 can be cut off, However, for a non tapering passage 26 the broad end of the wedge 32 may project out of the mouth 36.

If desired, the end 24 and wedge 32 may be provided with the textured surface and/or particulate material (optionally only in the textured surface) for microwave heating, as described for Fig. 3. Essentially the undulations 30 and teeth 34 are interchangeable and are illustrated as possible variations of compressible ribs/valleys.

In Fig. 5, reference numeral 40 shows a detail of the textured surface referred to above. The textured surface 40 comprises a plurality of pyramids 42 having rectangular bases arranged in a square pattern in the fashion of tiles, the bases being eg 0,5 mm on a side, and the apices projecting about 0,25 mm above the bases. The textured surface provides for the ingrowth of bony tissue between the pyramids to improve fixation eg in the passage 26 (Fig. 4)); the particles of leachable material may also be provided in the textured surface material and/or the material underlying the pyramids 42. This material, after it has been leached out, will provide the surface material with a plurality of sub-surface interconnected passages, opening out of the surface and into which additional bony ingrowth can take place.

Turning to Fig. 6, reference numeral 44 generally designates another ligament fixation arrangement, the same reference numerals again referring to the same parts. A ligament/ribbon 10 is shown extending between two bones 28 having passages 26 therethrough. The ribbon 10 is shown extending from its one end through one of the passage 26, thence to and through the other passage 26, around a cylindrical anchor pin or peg 46, back through said other passage 26 and thence to and through said one passage 26, so that the opposite ends of the ribbon 10 are located adjacent each other. A locking device in the form of a tapered wedge 48 is shown about to be inserted into the one passage 26, between the ends of the ribbon 10 to wedge them securely into said one passage. The pin 46 and wedge 48 may be of a similar internal construction and surface texture to that of the wedge 32 of Fig. 4.In use the ribbon 10 will be threaded through the passage 26 and around the pin 46 as shown, and the wedge will be driven home, its broad end projecting somewhat, unless said one passage tapers, in which case the projecting parts of the wedge 48 and ribbon can be cut off.

In Fig. 7 reference numeral 50 generally designates a yet further ligament fixation arrangement and once again the same numerals designate the same parts unless otherwise specified. In this case a locking device in the form of a tubular insert 52 having an inwardly tapering outwardly belled mouth 54 is inserted into the passage 26 in bone 28, and the wedge 32 is used to wedge the ribbon 10 in the interior of the insert 52. The outer cylindrical surface of the insert 52 may be textured as described above. The insert is shown with a central passage 56 which tapers and the wedge 52 is shown with its broad end projecting out of the mouth 54 of the insert 52.It will however be appreciated that this outward projection is only strictly necessary if the passage 56 does not taper, and that the projection can be cut off, with the outwardly projecting end of the ribbon 10, if the passage 56 tapers as shown. In use, the insert 56 is simply inserted in the direction of arrow 58 into the passage 26 (which is drilled to have a diameter the same as the outer diameter of the cylindrical body of the insert), the ribbon is threaded through the passage 56 and the wedge is wedged, narrow end first, into the belled mouth 54 of the insert 52.

The insert is of composite construction, made up of a radially inner portion formed from ribbons 10 (Fig. 1) extending lengthwise and moulded together, and a radially outer portion, moulded to the inner portion, formed from further ribbons 10 (Fig. 1) extending circumferentially around the inner portion and moulded together. The inner portion gives the insert 52 longitudinal tensile strength whereas the outer portion resists the radially outward wedging force of the wedge 32, to combat necrosis caused by outward pressure from the cylindrical body of the insert 52 on the inner surface of the bone passage 26.

In Fig. 8 reference numeral 60 designates an intra-medullary nail in accordance with the invention. The nail 60 comprises the matrix 12 having the fibres 14 extending both helically along its length and substantially straight and parallel along its length, and includes two bundles of carbon resistance heating fibres extending straight long its length. The nail 60 may be in solid or optionally tubular form and can be a unitary moulding or can be made of a plurality of ribbons 10 (Fig. 1) moulded together. The nail 60 will typically have a textured porous outer cylindrical surface (Fig. 5) capable of separate microwave heating.In use the matrix as a whole can be softened by passing a current along the bundles 20 to resistance heat the nail for gross shaping thereof, and upon insertion only, the surface can be softened by microwave heating for an exact fit against the bone which the nail reinforces.

Finally, in Fig. 9 reference numeral 62 generally designates a hip joint prosthesis in accordance with the invention, the same reference numerals again referring to the same parts as before. The prosthesis 62, as with the nail 60 of Fig. 8, can be a unitary moulding or can be composite, being built up from ribbons 10 (Fig. 1) moulded together. It is shown with its neck 64 provided with a bundle 20 of carbon fibres arranged in a coil 66 for resistance heating, and with its stem 68 provided with a further bundle 20 of carbon fibres for the same purpose. The prosthesis 62 has a textured surface (Fig. 5) capable of separate microwave heating and having subsurface passages for porosity. Gross shaping in use can take place separately for the neck 64 and stem 68 by heating for softening by means of their respective associated bundles 20 of carbon fibres, and microwave heating can be used for surface softening immediately prior to insertion.

It is an advantage of the invention that it provides a useful and versatile novel biocompatible material with a multiplicity of potential implant or prosthesic uses. In particular it is to be noted that, when the matrix 12 and fibres 24 are of the same chemical material, eg polyethylene, problems arising from interface bonding between fibres and chemically dissimilar matrix materials are avoided or at least reduced, leading to artifacts which are durable, stable and resistant to disintegration.

Claims (11)

1. A biocompatible material for use in prosthetic devices, which biocompatible material is composite in nature and comprises a polymeric plastics matrix within which are embedded a plurality of polymeric plastics reinforcing fibres, the matrix being thermoplastic and the fibres acting to reinforce the matrix and also being thermoplastic, the fibres and the matrix having softening points at different temperatures, the temperature of the softening point of the fibres being higher than that of the matrix.

2. A material as claimed in claim 1, which the matrix and fibres are chemically of substantially the same polymeric plastics substance.

3. A material as claimed in claim 2, in which the matrix and fibres are both members of the group consisting of polyethylene and polyester.

4. A material as claimed in any one of the preceding claims, in which the temperature of the softening point of the fibres is above 1 200C and the temperature of the softening point of the matrix is above body temperature and is below 100 C.

5. A material as claimed in claim 4, in which the temperature of the softening point of the fibres is between 120 and 1300C and the temperature of the softening point of the matrix is between 50 and 70 C.

6. A material as claimed in any one of the preceding claims, in which the fibres form from 5-75% by mass of the material.

7. A material as claimed in any one of the preceding claims, which includes carbon dispersed therein for facilitating heating thereof.

8. A material as claimed in claim 7, in which the carbon is in the form of carbon fibres, for resistance heating of the material.

9. A material as claimed in claim 7, in which the carbon is in the form of granular particles, for microwave heating of the material.

10. A material as claimed in any one of the preceding claims, in which the surface of the material is textured.

11. A biocompatible material for use in prosthetic devices, substantially as described herein.