Plugging and centralising device for locating the stem of a prosthesis

Abstract: A device (1) for plugging the medullary canal (2) of a bone (3) to prevent the passage of locating cement (4) and for centralising or guiding the stem (5) of a prosthesis (6) comprising a main body portion (7) adapted to engage the wall (9) of the canal (2) to act as a plug and having a centralising or guide opening (10) to receive the stem (5) of the prosthesis (6).

Description: This invention relates to a plugging and centralising device for locating the stem of a prosthesis. When using cement to locate the stem of a prosthesis in an medullary canal it is usual to plug the lower end of the canal to prevent cement extending along it beyond the stem of the prosthesis. Such plugs are well known, for example as shown in GB 2 017 503 which shows a plug with ranks of resilient lugs which jam against the side wall of the cavity to prevent the bone cement from escaping downwards. Another type of plug is shown in GB 2 047 541 which has a mid-portion which serves as a seal against the walls of the canal and an upper portion which is open ended to define a recess formed by a series of petals which are intended to prevent extrusion of the plug back out of the open canal. It is also known to provide a centraliser for the distal end of the stem of a prosthesis, for example as shown in EP 0 427 444 in which there are resiliently deformable fins which engage the wall of the canal and an annular body portion which locates the tip end of the stem of the prosthesis. Another type of centraliser is shown in GB 1 409 053 which has an annular base member with a number of spring members extending therefrom. The annular portion receives the free end of the stem and slides only part way along it and the spring members act to centraliser the stem in the canal. Plugs and centralisers are frequently used together. FR-A-2 704 423 discloses a device for plugging the medullary canal of a bone to prevent the passage of a locating cement and for centralizing or guiding the stem of a prosthesis which has a main body portion adapted to engage the wall of the canal to act as a plug and having centralizing or guide opening to receive the stem of the prosthesis. This French specification, similar to those referred to in GB 2 017 503 and EP 0 427 444 all have unprotected central openings and when the cement is loaded into the intramedullary canal prior to insertion of the stem of the prosthesis and during insertion thereof cement can be lost through the opening before the stem is located in it. The present invention provides a construction in which such cement loss is prevented. According to the present invention a device for plugging the medullary canal of a bone to prevent the passage of locating cement and for centralising or guiding the stem of a prosthesis comprises a main body portion adapted to engage the wall of the canal to act as a plug and having a centralising or guide opening to receive the stem of the prosthesis characterised in that said centralising or guide opening (10) is provided with a piercable membrane (14) which closes the opening (10) prior to use. Thus, this device can replace the combination of known types of plugs and centralisers. In one preferred embodiment the centralising or guide opening is adapted to engage and locate the stem at a position up to one half of its length from its distal end. In another alternative embodiment the centralising or guide opening is adapted to engage and locate the distal end of the stem of the prosthesis. The opening may be constructed to extend through the main body portion and be adapted to allow the stem, when in place, to protrude beyond it. In another construction the main body portion can include a cup, the open end of which provides the opening, the open end being adapted to engage the stem and the cup being deep enough to accept subsequent further inward movement of the stem. The open end or ends of the opening can be reinforced. The device can be made from any convenient material, for example metal but is preferably made from a synthetic material, for example polymethylmethacrylate (PMMA). In any case, the material used is preferably resilient. The invention can be performed in various ways and two embodiments will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a cross-sectional side elevation of the device according to the invention located on the stem of a prosthesis in an medullary canal; Figure 2 is an isometric view of the device according to the invention as used in Figure 1; Figure 3 is a cross-sectional side elevation of another construction according to the invention in place on a prosthesis in an medullary canal; Figures 4 and 5 are side elevations of further alternative construction according to the invention; and, Figures 6 and 7 are elevations of other alternative constructions. As shown in Figure 1 a device 1 for plugging the medullary canal 2 of a bone 3 to prevent the passage of locating cement 4 and for centralising the stem 5 of a prosthesis 6 comprises a main body portion 7 having a circumferential outer wall 8 which is shaped and adapted to engage the wall 9 of the medullary canal 2. The main body portion 7 is provided with an opening 10 which extends right through the main body portion and is dimensioned and adapted to allow the stem 5, when in place, to protrude beyond it as shown in Figure 1. The opening 10 is adapted to engage the stem 5 at a point spaced away from its tip 11. A reinforcing collar 12 surrounds each of the open ends of the opening 10 to provide reinforcement. The device can be made from any convenient material and is preferably made from a synthetic material, for example polymethlyl-methacrylate (PMMA). The combined plug and centraliser can be made from a resilient material so that it allows a good sealing fit against the walls 9 of the canal 2 and against the stem 5 of a prosthesis 6. When the cement is loaded into the canal 2 prior to insertion of the stem 5 and during insertion thereof a small amount of cement may be lost through the opening 10 before the stem is located in it. In order to prevent the cement loss the opening 10 can be provided with a membrane 14 which closes the opening and which is pierced by the tip 11 of the stem 5 when it is inserted. In the construction shown in Figures 1 and 2 the opening 10 is substantially circular to accept a circular stem 5 but in constructions where the lower end of the stem is some other shape, for example it may have flat sides, the opening 10 is shaped appropriately. The opening can initially be straight sided if the material is resilient enough to accept the deformation of the tapered end of the stem. Alternatively the opening 10 can be preformed in a tapered shape. Figure 3 shows another construction according to the invention and in which the same reference numerals are used to indicate similar parts. In this construction the body portion 15 includes a cup 16, the opening end 17 of which provide the entry opening 10. This open end 17 is adapted to engage the stem 5 and the cup 16 is deep enough to provide a void 18 beneath the stem 5 and which can accept subsequent further inward movement of the stem if it is of the collarless type. In order to assist location the outer sides of the cup are provided with retention flanges 19 which can act to assist in gripping the sides 9 of the canal 2. This construction can, of course, be made solid provided it is resilient enough to accept the deformation as it is placed in position in the canal 2. Once again the shape of the cup will be appropriate for use with the stem 5 of the prosthesis in which it is intended to be used. With this construction no cement is lost through the opening 10 when the stem is inserted. In an alternative construction (not shown) but which is generally similar to that shown in Figure 3 the engagement wall of the device could be rigid rather than resilient and the canal wall could be appropriately prepared to accommodate it. Figure 4 shows another construction according to the invention in which the main body portion 20 includes a cup 21 the open end 22 of which provides the entry opening 10 as with the construction shown in Figure 3 but this open end is adapted to engage the stem 5 and the cup 21 is deep enough to provide a void beneath the stem 5 and which can accept subsequent further inward movement of the stem if it is of the collarless type. In this construction the proximal end of the plug is formed with an engagement rim 23 adapted to engage the wall 9 of the canal 2. Beneath this rim the general diameter of the main body 20 is reduced but is provided with four projecting wings 24 which are dimensioned so that they act to centralise the stem at its distal end. The wings 24 can be rigid to engage a prepared portion of the canal 2 or can be made to accommodate the narrowing in the canal 2 by compression or flexing. The rim 23 provides sufficient plugging effect. Figure 5 shows a construction in which the main body portion 30 includes a cup 31 the open end 32 of which provides an entry opening 33 which is adapted to engage the stem 5 and the cup 31 is again deep enough to accommodate inward movement of the stem in a similar manner to the construction shown in Figure 4. With this arrangement however the spacer is formed as an annulus 34 on which are mounted a number of centralising arms 35. The arms can be constructed so that they will flex to centralise the plug as it is moved down the canal 2. The annulus 34 can be made of the same material as the main body portion 30 or it can be a different material and may be metal or a plastics material. The centralising arms 35 can be replaced by, for example, wings or any other element which will provide the necessary centralising effect. The material and dimensions can be arranged so that the annulus 34 can either be fitted immediately prior to use or it could be assembled during initial manufacture. Figure 6 shows another construction of plug and centraliser in which the same reference numerals are again used to describe similar parts but in this construction the upper part of the opening 40 is tapered which assists entry of the stem 5. The sides of the main body 7 are provided with corrugations 41 to assist in holding the plug against the wall of the canal 9. In the construction shown the tip 11 of the stem 5 is exposed but if desired a cup, similar to that shown in Figures 3, 4 and 5, could be provided to accommodate downward movement of the stem 5 after fitting and for the same reasons. Figure 7 shows a construction in which the dimensions of the plug and centraliser or guide are determined so that the device will fit onto the stem 5 at a point up to half the length of the stem. Thus, the arrangement is such that only the proximal two thirds, or even the proximal half of the stem, is engaged within the cement 4. The construction of the plug could be as set out in any of the preceding examples but it would of course be impractical to provide a cup in the opening but the methods of locating the plug in position could take any convenient form. In the construction shown in Figure 7 the outer surface of the main body is provided with corrugations 42 and the opening 43 is again tapered to assist in locating the stem 5 when it is entered. A diaphragm (not shown) could again be employed across the opening 43 with any of the constructions described. There are advantages with the construction shown in Figure 7 as far as load transmission is concerned. The problem with regard to making certain that the surgeon can get the tip of the stem to engage in the central part of the device and thus pass through, for example, the membrane is solved by making the device in construction rather deeper and including the tapered entrance to the opening so that the taper will automatically guide the tip of the stem into the aperture in the device. It will be obvious that the aperture has to be appropriately shaped according to the rough level in the canal that the device is to be placed. A close fit between the margins of the stem and the walls of the device is not particularly necessary because the acrylic cement 4 would itself seal any potential gaps, particularly if there is a tapering entrance to the device. As tapered polished stems are used on this type of prosthesis it is unlikely that debris will find its way down around the stem between the stem and the cement and to the distal part of the medullary canal which is not filled with cement. This type of arrangement has the added advantage of potentially significantly reducing the amount of cement that is needed for a given femur. It will be clear that in all the constructions described above the shape of the opening that is covered by the membrane or even which is open can be varied according to the stem geometry and the rough site of the canal in which it is proposed to place the plug. The plug and centraliser can be made in various sizes suitable for the requirements of the bone and canals concerned.

Claims: A device for plugging the medullary canal (2) of a bone to prevent the passage of locating cement (4) and for centralising or guiding the stem (5) of a prosthesis (6) comprising a main body portion (7) adapted to engage the wall (9) of the canal (2) to act as a plug and having a centralising or guide opening (10) to receive the stem (5) of the prosthesis, characterised in that said centralising or guide opening (10) is provided with a piercable membrane (14) which closes the opening (10) prior to use. A device as claimed in claim 1 characterised in that the opening (10) extends through the main body portion (7) and is adapted to allow the stem (5) when in place to protrude beyond it. A device as claimed in claim 2 characterised in that the centralising or guide opening (10) is adapted to engage and locate the stem (5) at a position up to one half of its length from its distal end (11). A device as claimed in claim 2 characterised in that the centralising or guide opening (10) is adapted to engage and locate the distal end (11) of the stem (5) of the prosthesis. A device as claimed in any of claims 1 to 4 characterised in that the outer sides of the main body are provided with projections (24). A device as claimed in claim 5 characterised in that said projections are provided as corrugations (41). A device as claimed in claim 4 characterised in that the main body portion can include a cup (16), the open end (17) of which provides the opening (10), said open end (17) being adapted to engage the stem (5) and the cup (16) being deep enough to accept subsequent further inward movement of the stem (5). A device as claimed in claim 7 characterised in that the cup is provided with projections (24) on its outer surface. A device as claimed in claim 8 characterised in that said projections (24) are flexible. A device as claimed in claim 6 or claim 7 characterised in that said projections are formed as centralising arms (35). A device as claimed in claims 1 to 10 characterised in that the open end or ends (10) of the opening are reinforced. A device as claimed in any one of the preceding claims 1 to 12 characterised in that the centralising or guide opening (40) is tapered downwardly from its proximal end. A device as claimed in claims 1 to 13 characterised in that it is made from a synthetic material. A device as claimed in claim 14 characterised in that the synthetic material is polymethylmethacrylate (PMMA). A device as claimed in claims 1 to 15 characterised in that it is made from a resilient material.

Inventors: GIE GRAHAM ALLAN, LING ROBIN SYDNEY, PICHON DENIS, STORER JOHN ANDREW, TIMPERLEY ANDREW JOHN, GIE, GRAHAM ALLAN, LING, ROBIN SYDNEY MACKWOOD, PROF., PICHON, DENIS, STORER, JOHN ANDREW, TIMPERLEY, ANDREW JOHN
Applicants: BENOIST GIRARD SAS
Application year: 1999
Main classification: A61F2
IPC codes: A61F2