MEDICAL IMPLANT

Abstract

An IM nail is positioned within a medullary canal and secured in position by bone screws. The nail includes bores which engage insert assemblies and engage the screws. Each insert assembly is in two parts. Part includes a head and a narrower neck; and similarly, part includes a head and a narrower neck. A bore extends through the assembly from one side to the other. Part of the assembly is engaged with a nail by insertion into the nail via mouth; and part is engaged with the nail by insertion into the nail via mouth. When in position the parts abut one another and define bore which can receive screws. The nail is made from a polyetheretherketone (PEEK)/carbon fibre composite. Parts of the insert assemblies are produced separately from the nail. They may be produced by injection moulding a polymeric composition comprising PEEK and barium sulphate.

Claims

1. A medical implant, the implant comprising a body which comprises a thermoplastic polymer and at least 10 wt % of a fibrous filler, wherein said body includes a first opening for engagement with a fixing means for fixing the body in position in a human or animal body, wherein said implant includes a protection means which is associated with (e.g. engages) said opening, wherein said protection means comprises a thermoplastic polymer and includes no more than 5 wt % of fibres which have a length of greater than 3 mm.

2. An implant according to claim 1, wherein said implant comprises an assembly for stabilising a bone in a human or animal body.

3. An implant according to claim 1, wherein said implant is selected from an intra-medullary nail assembly and a bone plate assembly.

4. Any implant according to claim 1, wherein said protection means includes at least 70 wt % of a first polymeric material and at least 4 wt % of a radiopaque material.

5. An implant according to claim 1, wherein said protection means includes a first polymeric material which includes a repeat unit of formula ##STR00003## wherein t and v independently represent 0 or 1.

6. An implant according to claim 5, wherein t=1 and v=0.

7. An implant according to claim 5, wherein said first polymeric material includes at least 90 mol% of repeat units of formula I.

8. An implant according to claim 1, wherein said thermoplastic polymer of said protection means is polyetheretherketone.

9. An implant according to claim 1, wherein said protection means is secured relative to said first opening.

10. An implant according to claim 1, wherein said protection means is substantially immovably secured in the opening.

11. An implant according to claim 1, wherein said first opening in said body extends from one side of the body to an opposite side.

12. An implant according to claim 1, wherein said protection means includes less than 3 wt % of fibres which have a length greater than 3 mm.

13. An implant according to claim 1, wherein said protection means includes less than 5 wt % of carbon fibres which have a length greater than 3 mm.

14. An implant according to claim 1, wherein said protection means includes no carbon fibres.

15. An implant according to claim 1, wherein said protection means provides a lining of the first opening.

16. An implant according to claim 1, wherein said protection means comprises a spacer for spacing the fixing means in use away from the material which defines the first opening.

17. An implant according to claim 1, wherein the body of said medical implant comprises a thermoplastic polymer and a fibrous filler, wherein said thermoplastic polymer includes a repeat unit of formula ##STR00004## wherein t and v independently represent 0 or 1.

18. An implant according to claim 1, wherein said fibrous filler of said body is carbon-based.

19. An implant according to claim 1, wherein said body includes at least 30 wt % of said fibrous filler which includes fibres of length greater than 5 mm.

20. An implant according to claim 1, wherein said body includes 25 to 75 wt % of said fibrous filler and 25 to 75 wt % of thermoplastic polymer.

21. An implant according to claim 1, wherein said body is an intra-medullar nail and said first filler is continuous.

22. An implant according to claim 1, wherein the fibrous filler is discontinuous.

23. A kit comprising a medical implant according to claim 1 and a fixing means for fixing the body of the implant in position.

24. A method of assembling a medical implant according to claim 1, the method comprising: (i) selecting a body which comprises at least 10 wt % of a fibrous filler, wherein said body includes a first opening for engagement with a fixing means for fixing the body in position in a human or animal body; (ii) selecting a protection means which comprise a thermoplastic polymer and includes no more than 5 wt % of fibres which have a length of greater than 3 mm; (iii) securing said protection means within said first opening, preferably so said protection means lines a mouth of the first opening and/or lines at least 50% (preferably at least 90%) of the surface area of the first opening.

Description

[0080] Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which:

[0081] FIG. 1 is a cross-section through part of a bone showing an intra-medullary (IM) nail fixed in position by bone screws;

[0082] FIG. 2 is a cross-section through part of the IM nail of FIG. 1;

[0083] FIG. 3 is a side view of an insert assembly for engagement with the IM nail of FIG. 3;

[0084] FIG. 4 is a view of the assembly of FIG. 3 in the direction of arrow IV;

[0085] FIG. 5 is a plan view of a bone plate assembly;

[0086] FIG. 6 is a perspective view of an alternative insert assembly for an IM nail; and

[0087] FIG. 7 is a cross-section along line VI-VI of FIG. 6.

[0088] Referring to FIG. 1, an IM nail 2 is positioned within a medullary canal 3 and secured in position by bone screws 4, 6. The nail 2 includes bores 8 (FIG. 2) which engage insert assemblies 10 to define an IM nail assembly 11. The insert assemblies engage the screws 4, 6. The bores 8 include respective mouths 12, 14 (FIG. 2) separated by a narrower portion 16. Each bore is arranged to engage a respective insert assembly 10 (shown in detail in FIGS. 3 and 4) which corresponds in shape to that of the bore 8 with which it is engaged. The insert assembly 10 as shown in FIG. 3 is in two parts, 32, 34. Part 32 includes a head 36 and a narrower neck 38; and similarly, part 34 includes a head 40 and a narrower neck 42. A bore 44 extends through the assembly from one side to the other.

[0089] Part 32 of the assembly 10 is engaged with a nail by insertion into the nail via mouth 12; and part 34 is engaged with the nail by insertion into the nail via mouth 14. When in position the parts 32, 34 abut one another and define bore 44 which can receive screws 4, 6.

[0090] In one embodiment parts 32, 34 may be spin welded (for symmetrical assemblies) within respective mouths 12, 14; may be an interference fit therewithin; may be adhered in position; or may be secured by welding, for example by ultrasonic, induction, laser or thermal welding. Alternatively, solvent or chemical bonding may be used to secure parts 32, 34 in position. Alternatively, in each case, the insert assembly 10, defined by parts 32, 34, lines the bore 8.

[0091] The nail 2 is suitably hollow (although this is not depicted in the figures in the interests of clarity) and is made from a polyetheretherketone (PEEK)/carbon fibre composite by known means. Mouths 12, 14 and narrower portion 16 are suitably defined in the nail during manufacture. Parts 32, 34 of the insert assemblies 10 are produced separately from the nail. They may be produced by injection moulding a polymeric composition comprising PEEK and barium sulphate. After manufacture, the IM nail and insert assemblies may be engaged to define an IM nail assembly.

[0092] As described, the IM nail is made from a composite material comprising polyetheretherketone and carbon fibre. Such a material is very stiff and strong. In contrast, the inserts assemblies 20 are made from PEEK and barium sulphate. The screws 4, 6 are suitably made from titanium, although they may be made from other materials.

[0093] The IM nail assembly 11, including IM nail 2 and parts 32, 34 is introduced into a medullary canal in a conventional manner to stabilise a fractured bone. Then, screws 4, 6 are used to secure the nail in position. Since parts 32, 34 incorporate barium sulphate, a radiopaque material, the parts are visible under X-rays. In contrast, since the nail 2 comprises PEEK and carbon fibre it is generally not readily visible under X-rays. By including parts 32, 34, it is possible for a surgeon to view the opening (e.g. the axis and location) into which a screw 4, 6 is to be inserted and to accurately position the screw. Furthermore, should a screw 4, 6 be slightly misaligned as it is contacted with the nail assembly, the screw will contact an associated part 32, 34 of an insert assembly. Consequently, any abraded particles generated will comprise PEEK and barium sulphate rather than black carbon fibre (since nail 2 itself should not be contacted). Thus, use of parts 32, 34 minimises and/or eliminates generation and/or release of black carbon fibre particles from the IM nail assembly.

[0094] Referring to FIG. 5, the bone plate assembly 40 comprises a bone plate (which may be of any known shape and/or size) which includes openings 42. The openings have a cross-section as for the opening shown in FIG. 2 and include an insert assembly 10 (which is substantially as described in FIGS. 1 to 4) positioned within a recess (not shown) which is as described in FIGS. 1 to 4. The advantages in using the assembly in terms of accurately positioning screws in the bone plate via openings 32 and minimal or zero release of carbon fibre-containing debris may be as described for the IM nail assembly.

[0095] An alternative insert 60 for an IM nail 2 is shown in FIGS. 6 and 7. The insert 60 is in the form of a wire spiral which includes a tantalum core 62 coated with a PEEK layer 64 which provides a sheath for the core. The insert 60 is arranged to be wound (mechanically prepared) to define a reduced diameter loaded state. It may then be introduced into a bore in an IM nail to define an IM nail assembly. The bore may be parallel sided and/or may include a step, for example at one end, to facilitate insertion and/or retention of the insert 60 within the bore. After introduction into the bore, the insert expands radially to fill the bore and become an interference fit therewithin. Advantageously, the tantalum core is visible under X-rays and so can effectively mark the opening into which screws (e.g. 4 or 6) may be inserted to facilitate accurate positioning of the screws. In the case of misalignment of a screw during its insertion, the misaligned screw will contact the PEEK coating (which does not contain any carbon fibre) so only PEEK particles will be released.

[0096] The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.