GB2405342A

GB2405342A - Bone plate

Info

Publication number: GB2405342A
Application number: GB0320208A
Authority: GB
Inventors: Robert Andrew Scott; Eric Maylia; Mohammed Imran Khan
Original assignee: Biomet Merck Ltd
Current assignee: Biomet UK Ltd
Priority date: 2003-08-28
Filing date: 2003-08-28
Publication date: 2005-03-02
Also published as: GB0320208D0

Abstract

This invention relates to a fibre reinforced bone fixation plate having a fixation zone for receiving a bone fixing such as a bone screw or nail. The bone plate may comprise a main body 2 in which are fixed a plurality of inserts 4. Alternatively, it may comprise a laminated structure with one or more layers of fibre reinforced material 10 laminated to layers of unreinforced material 12, the fixation zone being formed in the unreinforced material 12, 4.

Description

BONE PLATE

This invention relates to bone plates and particularly, but not exclusively relates to angle stable composite bone plates.

BACKGROUND

In the early days of internal fixation, it was believed that fractures should be rigidly immobilized. However, it soon became recognised that some degree of mobility in the fracture is not only acceptable, but desirable to promote bone healing. One means of achieving this is the use of fibre reinforced polymer matrix composites, most notably for bone plates. These have been used for fracture fixation for more than 30 years. Early models were made from carbon fibre reinforced epoxy. Other matrix materials that have been proposed include thermoplastics such as polysulphone. More recently, polyetheretherketone (PEEK) has emerged as the material of choice for the matrix, since this has excellent hydrolytic stability.

Another more recent shift in opinion concerns contact of bone plates with the underlying bone. With conventional plating, the screws generally have a spherical undersurface to the head, which seats in a spherical recess in the plate. This allows the screws to be angled relative to the plate, so that the surgeon can place the screws precisely where they are needed. In addition, the recess in the plate can be shaped to give compression at the fracture site.

In order to give adequate stability to the fracture, the plate must be screwed down firmly onto the surface of the bone. It has been observed that this can impair the viability of the bone under the plate. An alternative approach to overcome this problem has emerged in recent years. This new concept is called Angle Stable Fixation. Angle Stable devices can be described as internal external-fixators. The devices have limited contact with the bone, the screws that fit them are either mono or bicortical and lock into the plate as well as the bone. Angle Stable fixation is rapidly becoming established as a technique that is likely to replace the conventional screw and plate systems in the treatment of metaphyseal fractures. It is the latest evolution in the fixation of long bone fractures.

In one conventional angle stable composite bone plate system, the plate is provided with threaded holes which are adapted to receive bone screws which have threads on the undersurface of the head. These threads on the screw engage with the threads in a hole. This has the major disadvantage that the screw orientation is fixed, whereas the fracture configuration should always determine screw placement.

Another conventional system develops angular stability by exploiting the friction created by fixing self-tapping screws of harder titanium (type 4) in plates made from softer titanium (type 0/type 1), thus creating a permanently stable bond (cold weld) that also makes various screw angles and positions possible.

It is also known to provide a titanium plate which has PEEK collars which lock into the holes contained with the plate. The screws have a conical head with a threaded undersurface, which self-taps into the PEEK collar. This allows angulation of screws within the plate as dictated by the fracture configuration.

STATEMENT OF INVENTION

According to the present invention there is provided a fibre reinforced bone fixation plate, having a fixation zone for receiving a bone fixing such as a bone screw or nail, the fixation zone comprising a portion of the plate without fibre reinforcement.

Preferably, a hole is formed through the plate in the fixation zone. The hole may be threaded.

Preferably, the plate is reinforced with carbon fibre.

Preferably, the fixation zone is formed from unreinforced polyetheretherketone (PEEK) Preferably, the fixation zone is formed by inserts in the plate. There are several ways in which these inserts can be secured to the plate. For example, they may be screwed in, riveted, fixed by ultrasonic welding, friction welded, shrink fitted, interference fitted or snap fitted. In a preferred embodiment, the inserts are fixed by ultrasonic welding.

Preferably, the insert is cylindrical or conical. For example, the insert may comprise a tapered plug, which is particularly suitable for fixing by ultrasonic welding, as the weld interface can be held in compression.

Preferably, the inset is formed from a material which is less hard than the plate. For example, it may be formed from a material in which a thread may easily be formed by a self tapping screw.

The plates are preferably made from a carbon fibre reinforced PEEK composite material. Alternative polymer matrix materials are possible, as long as they are biocompatible and biostable enough for use in this application.

The reinforcement is preferably made from carbon, either in the form of long (continuous) fibres or chopped fibres. However, alternative fibres are possible, such as molecularly oriented aramid or polyolefin.

The plate is preferably formed with the reinforcing fibres around the holes oriented tangential to the hole perimeter, since cutting a hole through a plate made from longitudinal continuous fibres would seriously weaken it. One means of forming such a plate would be composite flow moulding, such as is described in patent application US 2003/0057590 A1.

To make composite plates suitable for angle stable fixation, it would be possible to make a composite plate, with pre-tapped screw holes. However, this has the significant disadvantage that the screw holes dictate the orientation of the screws.

One means of overcoming this problem would be to manufacture plates with an untapped hole, and using them with self tapping screws. However, tapping a screw into a fibre reinforced composite material is very difficult. This difficulty could be overcome by making the plate from an unreinforced polymer, such as PEEK, into which it would be relatively easy to tap a screw. However, unreinforced polymers do not have adequate strength and stiffness to be used for bone plates other than in very low stress applications.

In a preferred embodiment, the invention overcomes these difficulties, by making the plate predominantly out of fibre reinforced composite, but surrounding the bore of the screw holes with unreinforced polymer. The plate will therefore possess adequate strength and stiffness, as a result of the fibre reinforcement, but it will be relatively easy to insert a self tapping screw into the hole, as a result of the unreinforced polymer fixation zone.

The plugs can be drilled with a hole to accept the screw after being secured to the plate. Alternatively, the holes can be drilled or formed prior to fitting into the plate. The hole in the plate could be cylindrical, conical, or any other shape to mate with a plug.

Similarly, the hole in the plug could be any shape to mate with the thread on the undersurface of the screw head, and allow some angulation of the screw.

An alternative method of construction would be to lay up the fibres in the mould in such a way as to leave an unreinforced layer adjacent the holes. This could be achieved, for instance, by putting sleeves or plugs of unreinforced polymer in the mould.

Another alternative method of construction would be to make the plate in the form of a laminate of fibre reinforced polymer, and unreinforced polymer. The holes in the fibre reinforced layer(s) are larger than the holes in the unreinforced polymer, hence the threads under the head of the screw will engage primarily with unreinforced polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a first embodiment of bone plate incorporating unreinforced inserts (or plugs); Figure 2 shows the construction of a second embodiment of bone plate formed from a layer of reinforced composite material laminated to a layer of unreinforced composite material; and Figure 3 is a cross-section through a portion of the bone plate of Figure 2 when assembled.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Figure 1 shows a first embodiment of bone plate comprising a main body 2 and a plurality of inserts 4. The main body 2 is formed from carbon fibre reinforced PEEK and the inserts 4 are formed from unreinforced PEEK. The main body 2 is formed with a plurality of openings 6, which may for example be moulded, punched or drilled into the main body 2. Each insert 4 is cylindrical and is fitted into a respective opening 6.

The inserts 4 may be screwed into the openings 6 or may be riveted, fixed by ultrasonic welding, friction welded, shrink fitted, inference fitted or snap fitted. In a preferred embodiment, the inserts 4 are fixed by ultrasonic welding. Once the bone plate has been assembled, it can be fixed to a bone (not shown) by bone screws (not shown) inserted through bores 8 formed in the inserts 4. The bores 8 may be threaded or may be made under-size, so that the bone screws self tap into the inserts 4.

Referring to Figures 2 and 3, in an alternative embodiment, the bone plate is formed from layers of material laminated together. In the illustrated embodiment, the bone plate is formed from a first reinforced layer 10 made from carbon fibre reinforced PEEK composite material, and a second unreinforced layer 12 formed from unreinforced PEEK. The second unreinforced layer 12 is provided with a plurality of bores 14 which are adapted to receive bone screws (not shown). The bores 14 may be threaded or may be undersized, so that the bone screws self tap into the second unreinforced layer 12. The first reinforced layer 10 is provided with a plurality of larger openings 16 which are aligned with the bores 14 and provide access, so that the bone screws can pass through the first reinforced layer 10 into the bores 14.

The first reinforced layer 10 is laminated to the second unreinforced layer 12, as best shown in Figure 3. As mentioned above, a bone screw can pass through the opening 16 to engage the smaller diameter bore 14. The opening 16 may be countersunk, so that the bone screw is flush with an upper surface 18 of the first reinforced layer 12 of the bone plate.

Claims

1. A fibre reinforced bone fixation plate, having a fixation zone for receiving a bone fixing such as a bone screw of nail, the fixation zone comprising a portion of the plate without fibre reinforcement.

2. A fibre reinforced bone fixation plate as claimed in claim 1, which is formed from polyetheretherketone (PEEK).

3. A fibre reinforced bone fixation plate as claimed in claim 1, in which the fixation zone comprises a portion of the plate without fibre reinforcement.

3. A fibre reinforced bone fixation plate as claimed in claim 1 or 2, in which a hole is formed through the plate in the fixation zone.

4. A fibre reinforced bone fixation plate as claimed in claim 3, in which the hole is threaded.

5. A fibre reinforced bone fixation plate as claimed in any one of the preceding claims, in which the plate is reinforced with carbon fibre.

6. A fibre reinforced bone fixation plate as claimed in any one of the preceding claims, in which the fixation zone is formed from unreinforced polyetheretherketone (PEEK).

7. A fibre reinforced bone fixation plate as claimed in any one of the preceding claims, which is laminated.

8. A fibre reinforced bone fixation plate as claimed in claim 7, which comprises a layer of fibre reinforced material laminated to a layer of unreinforced material, the fixation zone being formed in the unreinforced material.

9. A fibre reinforced bone fixation plate as claimed in claim 8, in which the fixation zone is bounded by an opening formed through the reinforced material.

10. A fibre reinforced bone fixation plate as claimed in any one of the preceding claims, in which the fixation zone is formed by an insert fixed into the plate.

11. A fibre reinforced bone fixation plate as claimed in claim 10, in which the insert is fixed to the plate by ultrasonic welding.

12. A fibre reinforced bone fixation plate as claimed in any one of the preceding claims, in which the insert is cylindrical.

13. A fibre reinforced bone fixation plate substantially as described herein with reference to and as shown in Figure 1 or Figures 2 and 3 of the accompanying drawings.