Medical device

Abstract

The invention relates to an implantable medical device having a body comprising a composite material. The body has a variable cross section along a length, a first portion which forms a part of a surface of said body, and a packing portion. An insert is provided in the packing portion for providing an increased thickness to at least a part of the body.

Claims

1. An implantable medical device having: a body comprising a composite material comprising polyetheretherketone (PEEK) and a lay-up configuration having a plurality of plies of composite material, each of the plies being oriented in a specific fiber direction with respect to an axis, the body having: a coating comprising: barium sulphate in the amount of 6 wt % to 60 wt %; and a PEEK polyaryletherketone, a variable cross section along a length, a first portion which forms a part of a surface of said body, and a packing portion, wherein an insert is provided in the packing portion, the insert providing an increased thickness to at least a part of the body, wherein the PEEK has a melt viscosity (MV) of at least 0.06 kNsm.sup.−2, and less than 0.5 kNsm.sup.−2, wherein the melt viscosity is measured using capillary rheometry operating at 400° C. at a shear rate of 1000 s.sup.−1 using a tungsten carbide die, 0.5 mm×3.175 mm, wherein the composite material comprises carbon fibers, wherein an amount of carbon fibers in the composite material is ≤70 vol %, the composite material comprising barium sulphate wherein barium sulphate in the composite material is in the amount of ≤15 wt %.

2. A device as claimed in claim 1, wherein the first portion comprises a first ply and a second ply of the plurality of plies, said plies forming substantially the whole of the surface of the body.

3. A device as claimed in claim 1, wherein the packing portion is adjacent the first portion, the packing portion comprising at least one packing ply of the plurality of plies.

4. A device as claimed in claim 3, wherein each packing ply is adjacent at least a part of the insert.

5. A device as claimed in claim 1, wherein the insert is substantially wholly encapsulated within the packing portion.

6. A device as claimed in claim 1, wherein the insert comprises at least one insert ply of the plurality of plies.

7. A device as claimed in claim 6, wherein each insert ply is of a different length to a neighboring insert ply.

8. A device as claimed in claim 6, wherein a plurality of insert plies is arranged in a randomly stacked configuration, wherein such configuration has no line of symmetry about the longitudinal axis of the body.

9. A device as claimed in claim 1, wherein the coating is adjacent at least a part of the first portion.

10. A device as claimed in claim 1, wherein a first coating layer is remote from a second coating layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

(2) FIG. 1a shows a schematic perspective view of a typical composite lay up arrangement;

(3) FIG. 1b shows a schematic sectional side view of a further typical composite lay up arrangement;

(4) FIG. 2 shows a schematic side view of a medical device according to the present invention;

(5) FIG. 3 shows a schematic sectional side view of a part of the medical device of FIG. 2;

(6) FIG. 4 shows an embodiment of a lay-up arrangement of part of a device according to the present invention;

(7) FIG. 5 shows an embodiment of a lay-up arrangement of a device according to the present invention;

(8) FIGS. 6a and 6b show schematic sectional side views of a device according to the present invention;

(9) FIGS. 7a and 7b are X-ray images of a 2 mm plate and a 5 mm plate respectively, in accordance with the present invention;

(10) FIGS. 8a and 8b show SEM images of part of a device according to the invention; and

(11) FIGS. 9a and 9b show SEM images of part of a prior art device.

DETAILED DESCRIPTION OF THE INVENTION

(12) Figures 1a and 1b show typical lay-up arrangements as previously discussed. FIG. 2 shows a device 2 according to the present invention. The device 2 is an implantable medical device that comprises a body 4 having a head 6 and a tail 8. It can be seen that the cross sectional area of the device 2 varies along the longitudinal length of the body 4, with the head 6 being much greater in thickness compared to the tail 8.

(13) The device 2 comprises a first portion 10, a packing portion 12 and an insert 14. The first portion 10 includes a first ply 16 and a second ply 18. The first and second plies 16,18 together form the exterior surface of the device 2.

(14) The packing portion 12 comprises a plurality of packing plies 20 which are adjacent the inner face of the first portion 10. As shown in FIG. 3, the packing plies 20 are located either side of the insert 14, and adjacent the first and second plies 16,18. The packing plies 20 are symmetrical about the longitudinal axis of the body 4. In an alternative embodiment, packing plies are provided between insert plies.

(15) The insert 14 comprises at least one insert ply 22. In the embodiment shown in FIG. 3, a plurality of insert plies 22 (represented by dashed lines) is provided in the device 2. The or each insert ply 22 is shorter in length when compared to the packing plies 20. In so doing, the packing plies 20 surround the insert 14, encasing it within the packing portion 12 as shown most clearly in FIG. 6b.

(16) An embodiment of a lay-up arrangement of the head 6 of the medical device 2 is shown in greater detail in FIGS. 4 and 5. In FIG. 4, the first ply 16 and second ply 18 form the exterior facing surface of the body 4. Packing plies 20 are located adjacent said first and second plies 16,18. The insert 14 is located between the packing plies 20. It can be seen, that the insert plies 22 are non-symmetrical about an axis A, being of varying lengths to one another, and each being shorter in length than the adjacent packing ply 20. Provision of a non-symmetrical layering of the insert plies 22 ensures a greater degree of consistency in the overall cross section required by the head 6.

(17) FIG. 5 shows the lay-up arrangement of the plies, with respect to the X axis, in the head 6 and tail 8. Specifically, in the tail 8, the first and second plies 16, 18 are placed at 0°, and the packing plies 20 are orientated at +45°, 0°, −45°, 0°, 90°, −45°, 0°, +45°. The head 6 has a greater thickness than the tail 8 and so inserts 14 are provided within the packing portion 20 at locations indicated by the arrows in the figure. Each insert 14 is orientated at 0°.

(18) Such an arrangement achieves both the required cross sectional area required by the varying thickness of the device but also, essentially, ensures that the surface of the device comprises complete plies in contrast to the device shown in FIG. 1b. In so doing, the risk of delamination or shearing occurring at the exterior surface of the device 2 is minimised.

(19) FIG. 6 show an embodiment of a medical device 100 according to the invention. It will be understood by the reader that like reference numerals have been used to reference like parts shown in the previous figures. The device 100 comprises a body 104 having a first portion 110, a packing portion 120, and an insert 114. A coating 130 is located on the first portion 110. The coating 130 comprises a first coating layer 132 and a second coating layer 134. The first coating layer 132 lays adjacent the first ply 116, and the second coating layer 134 lays adjacent the second ply 118. Said coating layers 132, 134 do not extend around the whole surface of the body 104 but cover only an upper surface and a lower surface thereof. FIG. 6a shows that the layup arrangement provided by the invention ensures that the insert 114 is fully encased by the packing plies 120.

(20) The coating 130 comprises a barium sulphate/PEEK composite as will be described in further detail below.

(21) The plies of composite material of PEEK and carbon fibres which make up the body 4, 104 are prepared according to the following method:

(22) General Procedure for Preparing Polyetheretherketone Carbon Fibre Composites

(23) The carbon fibre was an IM7 12K tow from Hexcel Inc. The carbon fibre ply is 62% by weight of carbon fibre, 38% by weight of PEEK-OPTIMA® Natural LT3 obtained from lnvibio Biomaterial Solutions Ltd, with a Tm of 340° C. and MV of 0.15. MV is measured using capillary rheometry operating at 400° C. at a shear rate of 1000 s.sup.−1 using a circular cross-section tungsten carbide die of 0.5 mm capillary diameter×3.175 mm capillary length.

(24) Assembly of Lay-Up Arrangement

(25) Modelling data was generated using Dassault Systems CATIA™ software, to provide an indication of composite lay-up. The plies of the first portion, the packing portion and the insert were then cut. The plies were then assembled according to an embodiment of the invention shown in FIG. 5 wherein a second ply 18, 118 of the first portion 10,110 was orientated and positioned to form a lower surface of the device; a first plurality of packing plies 20,120 were placed on top of the second ply 18,118; the insert plies 22,122 were then positioned within said plurality 20,120 such that the insert plies 22,122 were encapsulated therein; and a first ply 16,116 of the first portion 10,110 was positioned on top of the plurality to form an upper surface of the body 4,104. The lay-up arrangement was compression molded using a PEI Lab Compression Moulder at 370° C.@2 Mpa press force then rapidly cooled under pressure. The laminated device 2,100 was then removed from the mold and machined as required.

(26) General Procedure for Preparing Polyetheretherketone/Barium Sulphate Composites

(27) The coating 130 comprises substantially 20% barium sulphate which is intimately mixed with PEEK polymer having an MV of 0.45 kNsm.sup.−2 commercially available from lnvibio Limited, UK under the trade mark PEEK-OPTIMA Image Contrast. Each layer of the coating is substantially 200 μm in thickness. The barium sulphate used was grade 10175, extra pure for X-ray diagnosis, from Merck.

(28) Barium sulphate was added to PEEK-OPTIMA via an extrusion compounding process. By way of example, the barium sulphate can be gravimetrically metered and fed through a side feeder into a twin screw extruder, where it is combined with plasticized polymer melt and intimately mixed to provide a uniform dispersion of the filler within the polymer. The barium sulphate was added to 20% by weight of the polymer. Extrusion of this mixture through a die generates strands or laces that cool and solidify before being chopped into small granules in preparation for subsequent processing. In the present invention, the composite layer 130 is made into a tape in a conventional manner. For example, the manufacture of tapes is described in, for instance, U.S. Pat. No. 4,626,306 where an aqueous dispersion impregnation method is set out. Other descriptions for the formation of such tapes may be found in “Impregnation Techniques for Thermoplastic Matrix Composites”—A Miller and A G Gibson, Polymer & Polymer Composites 4 (7) 459-481 (1996), in patent application publication EP 0592473 A1 and specifically for melt impregnation in the patent application publications EP 0102158 A2 or EP 0102159 A2.

(29) Addition of Barium Sulphate Coating Tape

(30) A 200 μm tape as prepared above was placed on a base and an upper face of the body so as to sandwich the body therein. The body was placed in a compression mold tool and heated to 360° C. under a pressure of 2 KN. Once the temperature was reached, the mold tool temperature was dropped to 220° C. at a controlled cooling rate using thermocouples to monitor the temperature. The finished device was removed from the mold tool and trimmed to remove unwanted flashing.

(31) FIG. 7a shows an X-Ray images of five 2 mm composite plates made according to the above procedure for preparing PEEK carbon fibre composites. Plate 150 is a reference plate having no barium sulphate film coating. Plate 152 is a PEEK composite having a 100 μm barium sulphate coating layer, made according to the above, deposited on either side of the plate (total film thickness 0.2 mm). Plate 154 has a 200 μm barium sulphate coating layer (total 0.4 mm); plate 156 has a 300 μm coating layer (total 0.6 mm) and plate 158 has a 400 μm coating layer (total 0.8 mm).

(32) It can be seen that plate 158 having the 400 μm film on either side of the plate provides the brightest contrast as would be expected. However, such a contrast removes the advantage of a clinician being able to view a fracture through the device. Further, such a thickness of barium sulphate reduces the mechanical properties of the overall device which has a restricted overall thickness for implantation; the thicker the barium sulphate layers means that less composite layers can be used in the overall structure. The images show a sufficient contrast when using 200 μm coating layer either side of the body—plate 154.

(33) FIG. 7b shows a 5 mm plate with the same layup to those of FIG. 7a. Plate 254 has the 200 μm barium sulphate film either side of the body; this shows sufficient contrast needed for medical use.

(34) FIGS. 8a and 8b show SEMs (taken using a Hitachi TM300 15KV) of a device according to the present invention without a barium sulphate film. As discussed, due to the insert being encased within the packing portion, the exterior surface 360 of the device comprises a complete length of composite ply which extends over the body. The SEMs show a smoothed surface. Consequently, the risk of shearing of the exterior surface is greatly minimised which in turn reduces the risk of carbon fibre exposure. In contrast, FIGS. 9a and 9b show SEMs (taken using a Hitachi TM300 15KV) of a typical prior art device. In particular, it is shown that an exterior surface 380 of a device, formed by graduated plies as shown in FIG. 1b, has a sheared or serrated finish. It can be seen that the carbon fibre in the composite is exposed.

(35) Advantageously, the lay-up arrangement as hereinbefore described allows for the creation of a device of variable cross section, which minimises the risk of shearing at an exposed surface.

(36) Provision of a radiopaque coating enables greater visibility of a medical implant by medical imaging techniques during and following implantation. During implantation, this assists the surgeon in, for example, screw placement. In addition, the surgeon is able to assess location of the device and be alert to possible breaks in the device.

(37) The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

(38) All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

(39) Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

(40) 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.