Prosthetic intramedullary femoral prosthesis with preformed sheath

Abstract: A prosthetic intramedullary femoral prosthesis (1) provided with a separate preformed sheath (9) made from an acrylic material which is dimensioned to enclose the prosthesis stem (4) from the distal tip (10) to a location on the stem (4) which will be adjacent the proximal cut end (6) of the femur (5) with which it will be used.

Description: This invention relates to a prosthetic intramedullary femoral prosthesis provided with a separate preformed sheath and to a preformed sheath for use with such a prosthesis. German Patent Application DE-A-3802214 shows an anchor for anchoring a prosthesis in a bone. The adaptor is in the form of a sheath preferably made from a plastics material and the outer surface has a sawtooth profile to anchor the sheath in the intramedullary canal of the femur. The purpose of this construction is to achieve adequate boney fixation without resorting to the use of bone cement. British Patent Application GB-A-2162753 also discloses a prosthetic intramedullary femoral prosthesis provided with a preformed sleeve the outer surface of which is again shaped to provide good keying with the bone without the use of bone cement. Many kinds of prosthetic intramedullary femoral prostheses are held in position in the medullary canal of the femur in which the prosthesis is planted by bone cement In the type of prosthesis in which there is no neck collar the prosthesis stem can sink further into the cement after fitting but this can cause difficulties in re-engagement of the tapering stem. There have been cases where the cement has been squeezed away totally during operative implantation such that there is direct contact between the metal stem and the bone forming a window in the cement mantle. Such a window is highly undesirable, it can be the source of cracks progressing through the cement mantle, and it can be a route by which debris of polyethylene, metal or bone cement can travel down the interface between the stem and the cement and out through the window and there it lodges in the bone and can be a source of bonelysis (bone death or loss of substance). The present invention is intended to overcome some of the disadvantages referred to above. According to the present invention a prosthetic intramedullary femoral prosthesis is provided with a separate preformed sheath made from an acrylic material which is dimensioned to enclose the prosthesis stem from the distal tip to a location on the stem which in the implanted state will be adjacent the proximal cut end of the femur with which it will be used, characterised in that the assembled stem and sheath are for insertion into the bone cavity of said femur and attachment thereto by bone cement the sheath being made from an acrylic material similar to the bone cement. By preforming such a sheath there is reduced adhesion between the cement which holds the stem in place and the polished surface of the stem and thus preferably the sheath is dimensioned and constructed to allow the stem to move further into it under load. The chances of a window or cracks being formed in the cement mantle is therefore reduced. Preferably the distal end of the sheath is in the form of a cup, the inner end surface of which is spaced away from the distal end of the prosthesis stem to provide a void when initially located in position to accept subsequent inward movement after fitting. This ensures that the end of the sheath is not pierced by the stem by subsequent movement. The sheath is made from an acrylic material similar to bone cement material, for example polymethylmethacrylate. The sheath therefore forms what is, in effect, another layer of cement but it is not secured to the stem and it does ensure that the stem is completely covered. The thickness of the sheath can be as small as possible provided it is strong enough not to crack during implantation and, for example, it can be between ½ mm and 5 mm and is preferably about 1 mm for practical use. It will be appreciated that the sheath is moulded separately and is shaped to fit a particular size of prosthesis stem. In a preferred embodiment the sheath is provided with means for centralising the prosthesis stem in the intramedullary canal and such centralising means may include outwardly projecting resiliently deformable abutments. These abutments can be in the form of resilient wings, fins or arms which can deform inwardly and may be arranged to deform substantially radially inwardly or, deform circumferentially inwardly in the manner shown in the Applicants' co-pending British Patent Application No. 8924985.8 filed 6 November 1989 (Applicants' reference PCT 7713). The invention also includes a preformed sheath for use with an intramedullary femoral prosthesis having a prosthesis stem, said sheath being dimensioned to enclose the prosthesis stem from the distal tip to a location on the stem which in the implanted state will be adjacent the proximal cut end of the femur with which it will be used characterised in that the assembled stem and sheath are for insertion into the bone cavity of said femur and attachment thereto by bone cement and the sheath is made from an acrylic material similar to the bone cement. 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 drawing in which : Figure 1 is a side elevation of an intramedullary femoral prosthesis provided with a sheath according to the invention; and, Figure 2 shows the distal end of a sheath according to a different embodiment. As shown in Figure 1 an intramedullary femoral prosthesis 1 has a head 2 of known type which is connected through a neck 3 to an intramedullary stem 4. The prosthesis is shown in position in a femur indicated by broken lines 5 the proximal cut end of which is indicated by reference numeral 6. In Figure 1 the prosthesis 1 is shown in position in the intramedullary canal 7 and is held by bone cement 8. The stem 4 is enclosed in a sheath 9 which covers the stem from its distal tip 10 to a location adjacent the proximal cut end 6 of the femur 5. In the arrangement shown this end 11 of the sheath is shown slightly protruding from the cement mantle 8. The sheath is made from a material similar to bone cement material, for example polymethylmethacrylate, with a filler to make it more flexible, and its distal end 12 is somewhat longer than the end of the stem 10 to create a void 13. The thickness of the sheath is about 1 mm and can be made as a separate moulding. Moulded into the sheath are four thin outwardly projecting abutments 14 in the form of four equally spaced tangentially projecting wings or arms. These abutments act as a centraliser and to hold the stem 4 away from the wall of the medullary canal during implantation. The projections can resiliently deform circumferentially inwardly as the stem, together with the sheath, is pushed into place in the canal 7. With the sheath in position the centraliser provided by the abutments becomes well integrated with the cement, as does the remains of the sheath thus avoiding any weakening or hole creation. If there is now a tendency for the stem to move further into the cement mantle this is accommodated by movement within the sheath which, it will be appreciated, has become integral with the cement mantle itself but, because it is not connected to the stem adhesion between the cement and the stem is reduced and therefore the frictional resistance to the subsiding action or re-engagement of the stem is reduced. An alternative construction of the sheath is shown on Figure 2 and in which similar reference numerals are used to indicated similar parts. With this arrangement the void 13 is again created when the sheath 9 is placed on the stem 4 (not shown in Figure 2). In this construction however the abutments 14 are provided by three radially and upwardly extending arms 15 spaced around the sheath and which again act to centralise the stem in the medullary canal. When the prosthesis is to be implanted the surgeon fits the appropriate size of preformed sheath to the stem of the prosthesis, the sheath including the integrally formed centraliser with a void creator on the distal tip. The assembled stem and sheath are then inserted into the bone cavity which has already been filled with normal bone cement in the normal way. The preformed cement sheath bonds directly with the bone cement inserted by the surgeon. The material of the sheath and the cement are identical or compatible so that they are bonded together. It is important that the preformed sheath is separate from the stem, it cannot be provided merely by a coating applied to the stem as this will not give the same low friction characteristics at the surface of the stem which are required. It is possible to provide a lubricant between the preformed sheath and the stem in order to reduce friction still further, a suitable sterile lubricant being used, for example sterile liquid paraffin. The invention is intended to provide an optimisation of the interface conditions between the cement on an intramedullary canal and the surface of the stem. The avoidance of windows in the cement and direct contact between metal and bone is also an advantage.

Claims: A prosthetic intramedullary femoral prosthesis (1) provided with a separate preformed sheath (9) which is dimensioned to enclose the prosthesis stem (4) from the distal tip (10) to a location on the stem which in the implanted state will be adjacent the proximal cut end (6) of the femur (5) with which it will be used, characterised in that the assembled stem (4) and sheath (9) are for insertion into the bone cavity of said femur (5) and attachment thereto by bone cement (8) the sheath (9) being made from an acrylic material similar to the bone cement (8). A prosthetic intramedullary femoral prosthesis as claimed in claim 1 in which the sheath (9) is dimensioned and constructed to allow the stem (4) to move further into it under load. A prosthetic intramedullary femoral prosthesis as claimed in claim 2 in which the distal end (12) of the sheath (9) is in the form of a cup the inner end surface of which is spaced away from the distal end (10) of the prosthesis stem to provide a void (13) when initially located in position, to accept subsequent inward movement after fitting. A prosthetic intramedullary femoral prosthesis as claimed in claims 1 to 3 in which the sheath (9) is made from polymethylmethacrylate. A prosthetic intramedullary femoral prosthesis as claimed in claims 1 to 4 in which the sheath (9) has a thickness of between 1/2 mm and 5 mm. A prosthetic intramedullary femoral prosthesis as claimed in claim 5 in which the sheath (9) has a thickness of substantially 1 mm. A prosthetic intramedullary femoral prosthesis as claimed in claims 1 to 6 in which said sheath (9) is provided with means (14,15) for centralising the prosthesis stem (4) in the intramedullary canal (7) of the femur (5) with which it is to be used. A prosthetic intramedullary femoral prosthesis as claimed in claim 7 in which the centralising means (14,15) include outwardly projecting resiliently deformable abutments (14,15). A prosthetic intramedullary femoral prosthesis as claimed in claim 8 in which the resilient abutments (14,15) are in the form of resilient wings, fins or arms which can deform inwardly. A prosthetic intramedullary femoral prosthesis as claimed in claim 9 in which the abutments (15) can deform substantially radially inwardly. A prosthetic intramedullary femoral prosthesis as claimed in claim 9 in which the abutments (14) can deform circumferentially inwardly. A preformed sheath for use with an intramedullary femoral prosthesis having a prosthesis stem (4), said sheath being dimensioned to enclose the prosthesis stem (4) from the distal tip (10) to a location on the stem which in the implanted state will be adjacent the proximal cut end of the femur (5) with which it will be used characterised in that the assembled stem (4) and sheath (9) are for insertion into the bone cavity of said femur (5) and attachment thereto by bone cement (5) and the sheath (9) is made from an acrylic material similar to the bone cement (8).

Inventors: LAWES PETER, VANDERLINDEN JACQUES, LAWES, PETER, VANDERLINDEN, JACQUES
Applicants: HOWMEDICA, HOWMEDICA INTERNATIONAL INC.
Application year: 1991
Main classification: A61F2
IPC codes: A61F2