GB2042897A

GB2042897A - Prosthesis Sheath for Femoral Prosthesis

Info

Publication number: GB2042897A
Application number: GB7849857A
Authority: GB
Original assignee: Charnley Surgical Inventions Ltd
Current assignee: Charnley Surgical Inventions Ltd
Priority date: 1978-12-22
Filing date: 1978-12-22
Publication date: 1980-10-01

Abstract

A protective sheath for the neck of a femoral prosthesis has a plurality of parts which mate with one another about mating surfaces lying in planes substantially radial to the longitudinal axis of the sheath. At least one of these parts has, at its "shaft end" a flange extending radially outwardly of the sheath, the flange width being at least 8 mm. The flange helps to ensure that, during insertion of the prosthesis, bone cement is retained in the medullary cavity. <IMAGE>

Description

SPECIFICATION Sheath for Femoral Prosthesis This invention relates to sheaths for femoral prosthesis which are used in the technique of arthroplasty of the hip joint.

Femoral prosthesis of the type with which this invention is concerned have a shaft which is located in the femoral canal which has been reamed to accept the shaft, and is often secured therein by bone cement, a head which is shaped as part of a sphere and which seats in an artificial socket implanted into the pelvis, and neck joining the shaft and the head. In order to ensure the correct range of flexion for the prosthetic joint, a special curved shape may be employed for the neck of the formal prosthesis. Such a prosthesis is hereinafter referred to as "a formal prosthesis of the type described".

In order to locate the femoral prosthesis in its correct position in the femur a so-called neck punch may be used, the neck punch comprising two arms having head sections which are pivoted together, the head sections being closable round the neck of the femoral prosthesis. As described in Patent Specification No. 1,402,557, a removable protective sheath may be provided about the neck of the femoral prosthesis so that the head parts of the neck punch can be made with gripping parts complementary to the outer surface of the sheath rather than to the complex surface of the neck. In this way the manufacture of the neck punch is considerably simplified and, furthermore, by fitting the sheath immediately after manufacture of the prosthesis the neck is protected from damage during packing and transit.The prosthesis and punch are normally made of metal and the sheath is preferably of plastics material so preventing metal-to-metal contact of the neck punch and avoiding any metallic pick-up of the neck.

Femoral prosthesis are supported within the femur by cement located between the prosthesis and the bone of the femur. It is important to provide adequate support by the cement and it is believed that inadequate support by cement in the region of the concave aspect of the upper part of the prosthesis can lead to fatigue fracture of the metal of the stem.

According to the present invention there is provided a protective sheath for the neck of a femoral prosthesis, the sheath having a plurality of parts, the part having surfaces which mate with one another, said mating surfaces lying in planes substantially radial to the longitudinal axis of the sheath, at least one of said parts having, at its shaft end, a substantially flat surface in a plane substantially perpendicular to the longitudinal axis of the sheath, the radial width of said surface being at least 8 mm.

The term "shaft end" of the sheath refers to that end which, when the sheath is positioned on a prosthesis, lies closer to the shaft of the prosthesis.

Preferably said flat surface is in the form of a flange which extends outwardly from the shaft end of at least one of the parts of the sheath. The presence of such a flange helps to ensure that, during insertion of the prosthesis into the cementfilled medullary cavity of the femur, the cement which tends to bulge out of the medullary cavity is driven in by the flange. The minimum width of the flange will depend on the relative sizes of the femur and the prosthesis but a minimum width of 8 mm should be sufficient for small femurs. A width of about 12 to 18 mm is reasonable for normal size and large femurs. Preferably the width of the flange is about 1 5 mm.

Preferably the protective sheath comprises two parts of substantially equal size, with one or both of the two parts being provided with a flange extending about substantially the whole of the periphery of the shaft end of said part. If desired, only one part can be provided with a flange in the case where the prosthesis is provided with a dorsal flange such as the Charnley Flanged Prosthesis In such a case the substantially semi-circular flange of the protective sheath is orientated in the neck punch so that it lies on the medial aspect of the prosthesis. Then during the last part of the insertion of the prosthesis the cement is trapped laterally by the dorsal flange of the prosthesis and medially by the flange of the sheath. Accordingly the cement is effectively pressure injected into the region of the medial femoral neck.However, in the case where the prosthesis is not provided with a dorsal flange the sheath may have a flange on both parts so that the flange extends around substantially the whole of the periphery of the mouth portion thereof. Even where the prosthesis is provided with a dorsal flange it may be preferred to include a flange on both parts of the sheath.

The present invention also provides a femoral prosthesis of the type described in which the neck is surrounded by a removable protective sheath of the invention.

Preferably the outer surface portion of the sheath is of circular cross section.

Preferably the sheath is extended to cover the head of the prosthesis as well as the neck. In this case the inner surface of the sheath includes a portion shaped to be complementary to the part of spherical shaped head. Preferably the head is in the shape of a hemisphere and the further portion is of hemispherical shape. It is customary to supply the femoral prosthesis with the head and neck protected from damage by a cover, and a sheath embodying the invention may replace this cover. It is found that effective sterilisation of the neck and head can be carried out with the sheath in position. To aid such sterilisation the sheath may be formed with passages such as grooves or holes from the outer to the inner surface thereof to facilitate penetration of the sterilisation medium to the neck and head surfaces.

The sheath is preferably made of any suitable plastics material such as polyvinylchloride or a polyamide.

In order that the sheath may be removable it is made in two or more parts having mating surfaces lying in planes radial to the longitudinal axis of the sheath. Each mating surface may be provided in a single plane, the two or more parts of the sheath being held together by for instance, adhesive tape or a rubber band. Alternatively the sheath portions may be provided with integral retaining means. In one possible arrangement each part has one or more pegs on one mating surface thereof which is or are a press or snap fit into a corresponding hole or holes on the respective mating surface on the or another said part.

After location of the prosthesis in the femur and removal of the neck punch, the sheath is removed by cutting the rubber band or by simply pulling it apart, depending on its construction.

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings, in which: Figure 1 is an elevation showing the outer surface of part of a sheath in accordance with the present invention; Figure 2 is an elevation showing the inner surface of part of the sheath of Figure 1; Figure 3 is a section through the part of the sheath of Figure 1 taken on line I-I of Figure 1; Figure 4 is a cross-section through a femoral prosthesis with the head and neck enclosed in a sheath of the present invention, the sheath shown in longitudinal section; and Figure 5 shows a femoral prosthesis being inserted into the medullary cavity of the femur.

Referring to the drawings, a femoral prosthesis 1 is made of metal and comprises a shaft 2, a neck 3 and a head 4. The neck 3 is specially shaped to allow sufficient freedom of movement in an artificial socket. As described in patent specification No.1,510,968, the prosthesis is provided with a dorsal flange 2a. Fitted over the head and neck of the prosthesis is a sheath 5 made of plastics material which is substantially incompressible and which is softer than the prosthesis 1.

Sheath 5 comprises two parts 14 and 15, each part having radial surfaces 16 and 17. The internal surface of each part 14 and 1 5 is shaped so as to have a part 20 substantially complementary to the neck 3 of the prosthesis and a part 21 complementary in shape to the head 4 of the prosthesis. The neck portion of each part of the sheath is provided with a groove 22 which extends through the sheath and opens at each end into longitudinal recesses 23a and 236.

Recess 23a opens at its other end, in the head portion of the sheath, into a circular cut-out portion 23c extending through the sheath.

The outer surface of each part 14 and 15 has a head portion 24, a section 6 which is substantially circular cylindrical although tapering slightly outwardly in a direction away from the head portion 24, and a shoulder 6a. Extending outwardly from shoulder 6a at the mouth of the sheath is a flange 25a. The width of the flange 25a, measured radially from the inner surface of the sheath to the outer extremity of the flange, is approximately 15 mm. As shown in Figure 4, both parts of the sheath are provided with flanges 25a so that, in effect, a flange extends around the whole of the shaft end of the sheath. However, in another embodiment flange 25a may be omitted from one of the two parts of the sheath. Such an embodiment may be used, for instance, where the prosthesis is itself provided with a dorsal flange such as dorsal flange 2a of the prosthesis shown in Figure 4.

Both parts 14 and 1 5 of the sheath are moulded in polyamide, polyvinylchloride or some other suitable plastics material.

After manufacture of the prosthesis the two parts 14 and 1 5 of the sheath are fitted round the neck and head and are held together by means of adhesive tape. The dimensions of the internal surfaces of the parts 14 and 1 5 are such that when held together by rubber bands the assembled sheath is a loose fit over the neck 3.

During packing and transit the head and neck are at all times protected by the sheath from damage.

As the head in particular is ground and polished to a finish of very small tolerance, such protection is very important.

Before use, the prosthesis is sterilised, the grooves 22, recesses 23a and 23b and cut-out portions 23c assisting penetration of sterilising medium between the sheath and the prosthesis so that the neck and head become fully sterile. A sterile wrapping is then placed over the prosthesis. During surgery the sterile wrapping is removed, the prosthesis inserted in a neck punch similar to that described in Patent Specification No. 1,402,557 with the shaft 2 of the prosthesis and the arms of the neck punch substantially parallel. Where the sheath is provided with a flange on only one of its parts the flange should lie on the medial aspect of the prosthesis.

The neck of the femur is sawn so that its end 31 is substantially planar. The prosthesis is inserted into the cement filled medullary cavity.

Figure 5 shows the insertion of a prosthesis being of the type having a dorsal flange and the sheath being provided with a flange on only one of its parts. As the prosthesis enters the medullary cavity, excess cement extrudes medially, the cement being prevented from extruding laterally by the surgeon's thumb and by the dorsal flange 2a of the prosthesis.

In the last few millimeters of insertion of the prosthesis the large diameter flange 25a of the protective sheath 5 meets the bulging cement 33 and drives it into the medullary cavity of the medial femoral neck. The cement is trapped laterally by the dorsal flange 2a of the prosthesis and medially by the flange 25a of the sheath.

Thus the cement is injected under pressure into the region of the medial femoral neck.

Hammering on the neck punch ensures that the bottom surface, including flange 25a, of the protective sheath comes into close contact with the whole area of the cut surface of the femur.

Once the cement has reached the rubbery stage the neck punch is detached and the two halves of the protective sheath removed.

Claims

1. A protective sheath for the neck of a femoral prosthesis, the sheath having a plurality of parts, the parts having surfaces which mate with one another, said mating surfaces lying in planes substantially radial to the longitudinal axis of the sheath, at least one of said parts having, at its shaft end, a substantially flat surface in a plane substantially perpendicular to the longitudinal axis of the sheath, the radial width of said surface being at least 8 mm.

2. A protective sheath according to claim 1 wherein the said flat surface is in the form of a flange which extends upwardly from the shaft end of at least one of the parts of the sheath.

3. A protective sheath according to claim 1 or claim 2 wherein said width is from 12 to 1 8 mm.

4. A protective sheath according to any of the preceding claims wherein said width is about 15 mm.

5. A protective sheath according to any of the preceding claims wherein the sheath comprises two parts of substantially equal size, with one or both of the parts being provided with a flange extending about substantially the whole of the periphery of the shaft end of said part.

6. A protective sheath according to any of the preceding claims wherein the outer surface portion of the sheath is of a circular cross-section.

7. A protective sheath according to any of the preceding claims wherein the sheath is adapted to cover the head portion of a prosthesis of the type described.

8. A protective sheath according to claim 7 wherein the sheath is formed with passages from the outer to the inner surface thereof to facilitate penetration of sterilisation medium to the neck and head surfaces.

9. A protective sheath according to any of the preceding claims wherein the sheath is made of plastics material.

10. A protective sheath according to claim 1 and substantially as described herein.

11. A protective sheath for the neck of a femoral prosthesis, substantially as described with reference to the accompanying drawings.

1 2. A femoral prosthesis of the type described in which the neck is surrounded by a removable protective sheath as claimed in any of the preceding claims.