Femoral hip prosthesis

Abstract: A prosthetic femoral component for location in a prepared socket in a femur which has been resected at a position on the proximal side of its neck (2) and which includes a tapered insert portion (4) and a proximal head portion (5), the proximal end (6) of said insert portion being adapted for location in said prepared socket and having a maximum dimension in a plane normal to the distal-proximal axis of the neck (2) which is larger than the minimum dimension of the neck (2) in a parallel plane.

Description: This invention relates to a prosthetic femoral component of the type which is applied without a stem in the medullary canal, which is considered to be conservative and bone sparing. For present purposes a conservative femoral hip prosthesis is a prosthesis which leaves sufficient bone in place for it to be eventually replaced by a more conventional femoral hip prosthesis with a medullary stem normally intended for primary (non-revision) application. A bone-sparing femoral hip prosthesis is one which limits the removal of viable bone by conserving some of the femoral head, removing only sufficient bone to resect the diseased tissue and to effect a satisfactory anchorage. The use of femoral hip prostheses which function without a stem in the medullary canal date from the first total hip prosthesis reported by Wyles in 1937. This hip prosthesis was fitted following a high resection of the femoral head and was stabilised with a straight stem which passed along the femoral neck and out below the greater trochanter, where it was attached to a bone plate secured on the lateral cortex of the femur. The Wyles hip restored the femoral head with a bearing diameter deliberately smaller than the natural femoral head it was replacing. Only six cases were ever performed using this device since the clinical outcome was not impressive. Another femoral hip prosthesis design following that of Wyles was the Judet prosthesis, developed in France and used in the period 1945-55. A high neck resection was used with this prosthesis, which attempted to restore the femoral head to its natural diameter for use as a hemiarthroplasty. The prosthesis comprised an acrylic (low modulus) head and a short straight stem which passed along the femoral neck. The prosthetic head included a trough around the stem attachment to the head, which was used to seat and locate the prosthesis on the prepared proximal end of the femoral neck. Early breakage caused the stem to be given a stainless steel core support. Later failures saw the device breaking out through the inferior femoral neck. All versions of this prosthesis suffered from premature wear of the acrylic head. High neck resections, i.e. those conserving the femoral neck, were also used by femoral hip prostheses with stems passing into the medullary canal, notably the designs of Pipino (1978) and Freeman (1985). These hip prostheses were implanted both cement-free and with cement, but did not attempt to restore the femoral head to its natural diameter, being used as total hip replacements with a head of smaller dimensions. Since these femoral hip replacements do place a stem in the medullary canal, they are not considered to be conservative, although the stem on the Pipino design was very short. Designs of femoral hip prostheses which have attempted to secure the replacement of the femoral head without a stem in the medullary canal follow the design of Vincent and Munting reported in 1982, which is still in clinical use. With this design, a portion of the femoral neck is preserved and shaped with a notch to provide seating for the implant. The prosthesis is used as a total hip replacement and replaces part of the femoral neck and the femoral head with a head of smaller diameter than the natural head. The prosthesis is used uncemented and is fixed with a large screw through the lateral cortex into the body of the prosthesis. The prosthesis is intended to sit on the remaining cortex of the neck and is stabilised by fins parallel to the axis of the neck which pass into the remaining diaphyseal cancellous bone. The bone engaging surfaces are provided with a hydroxyapatite coating to promote bone ongrowth to augment fixation. The Vincent-Munting prosthesis is considered to be conservative but not bone sparing, according to the definitions given above. The only type of femoral hip prosthesis which has been developed which is conservative and bone sparing is the femoral cap used in prostheses such as the ICLH (Freeman, 1973), the THARIES (Amstutz, 1976), the Wagner (Wagner, 1973), the Zephyr (Aubriot, 1977) and the Gérard (Gérard, 1975). This type of prosthesis comprised a metal cap with a part-spherical external form and different internal forms and was used both cemented and uncemented. The bearing surface of the femoral cap was always near to anatomical size, therefore the cap could be used as a hemiarthroplasty. Mechanical loosening through stress concentration at the bone interface were reported as well as resorption of epiphyseal bone beneath the cap. The cause of the bone resorption was associated with disruption of the blood supply to regions of bone as a result of the surgical technique. Often the cap was used to articulate with a polyethylene liner in the acetabulum, and with this an additional failure mode of osteolysis at the bone interface with the prosthesis was caused by the ingress of polyethylene debris. A development of the femoral cap design was the inclusion of a short stem to the cap. Examples of such designs include the TARA hip (1970's) and, more recently the McMinn hip (1990's). An alternative design approach for the femoral cup is presented in European Patent Specifications EP 0 094 829 and EP 0 176 188, which describe a stemless femoral hip prosthesis intended to load the bone naturally. The first design required the resection of most the femoral head and part of the neck, the later design required only the resection of the proximal portion of the femoral head up to the epiphyseal scar plate. In the later design, a low modulus material between the bone and the femoral cap was used to transfer load with a more physiological force distribution onto the trabecular structure of the proximal femur. In practice, too little bone was removed for adequate surgical exposure of the acetabulum without excessive soft tissue damage. Furthermore, controlled exposure of the three-dimensional epiphyseal scar plate proved to be too complex and the design was never developed into an implant. Cemented intramedullary fixation of femoral hip prostheses has now approximately 30 years successful clinical results and is the benchmark against which new designs of hip implants are assessed. Early problems of implant fracture, corrosion, cement mantle integrity and excessive bearing wear have now been largely resolved and the main problem which limits the life expectancy of conventional femoral hip prostheses is aseptic loosening. Nevertheless, since premature failure of the reconstruction may occur due to loosening, eventual revision of the prosthesis, particularly when used for younger patients (under 65), must be considered. The revision of cemented stemmed femoral hip prostheses is challenging, particularly a as result of needing to remove all the cement. In fact, cementless stems with intramedullary fixation have been developed to simplify the revision procedure. Such devices require increased surgical precision compared with cemented hip prostheses and have their own failure modes such as pain, loosening and subsidence. It is the likelihood of subsequent revision for the younger and more active patient which makes a conservative, and indeed bone sparing, femoral hip prosthesis an attractive option. In theory, such a device may be revised with a conventional primary stemmed hip prosthesis without the need for bone grafting or other augmentation. Indeed, there is no reason why conservative hip designs could not be at least as safe and efficacious as intramedullary stemmed hip designs. However, attempts so far to develop a conservative, bone sparing femoral hip prosthesis have encountered significantly worse results due to premature loosening of the femoral component (and acetabular component). The present design seeks to provide a conservative, bone sparing femoral hip prosthesis that addresses the problems encountered by previous designs. The prosthesis includes an insert portion which is designed to control the transfer of load to the femur so as to avoid stress concentration at the bone interface. The insert portion is sized so that it replaces all the epiphyseal bone thereby minimising the risk of bone resorption due to disrupted blood supply. It is also tapered so as to self seal under load so as to restrict the ingress of debris leading to osteolysis. In addition to addressing the deficiencies of previous designs, the present design seeks to simplify the surgical technique so as to achieve better reproducibility of results and to minimise the trauma (e.g. loss of blood, post-operative infection) associated with the procedure. Hip replacement is usually performed with a large exposure. Early post-operative infection is no longer a significant problem, but the time to heal such a major wound is significant. Some surgeons now implant conventional stemmed devices with as small an incision as they possibly can. After the femoral head and neck have been removed, only narrow tools are needed to prepare the femoral canal and there is easy access to the acetabulum. However, the bone sparing femoral hip prosthesis designs generally necessitate reverting to a wider exposure for two reasons. Firstly, preparation of the outside of the femoral head involves bulkier instruments. Secondly, the femoral head obstructs access to the acetabulum. More cutting of soft tissues attaching the femur to the pelvis is needed to manoeuvre the femoral head out of the way. The present invention is intended to provide a femoral hip prosthesis which can be employed in a method of fitting which is not part of the claimed invention, and which includes cutting away the natural femoral head to expose the circular cross- section of the neck at the base of or at a mid point of the head. This allows much improved access to the acetabulum, thereby reducing the length of the required incision and minimising the soft tissue dissection necessary to allow the remaining femoral head to be levered out of the way. The shape of the insert portion of the prosthesis is designed so as to allow it to be fitted to the bone accurately following a simple, non-bulky, reproducible reaming operation. As such, the close fit will resist micromotion and act in support of the self-sealing taper design to impede the ingress of debris. The fact that non-bulky instruments may be used allows a less-invasive surgical technique to be employed. WO 98/07393 shows a joint replacement prosthesis which has a conical base and a hemispherical or part hemispherical top which provides a hemispherical or part hemispherical replacement bearing surface. The prosthesis is shaped and sized to be a wedge fit in a conical recess formed in the head or neck of the femur without removal of all of the neck of the femur. Due to the use of a conical base or insert this construction will tend to be unstable in the bone and it is unlikely that there will be sufficient contact area to maintain it in place. FR-A-1 107 877 also shows a prosthetic femoral component but this has an elongated insert which is aligned with a bearing head portion. The specification is not clear as to precisely how it is located in a resected femur but the stem portion is symmetrical about a general proximal/distal axis so that the stem cannot take advantage of the general curved shape of the upper part of a femur where the stem extends through the neck and the insert portion is not designed so as to allow it to be fitted to the bone accurately. According to the present invention a prosthetic femoral component for location in a prepared socket in a femur which has been resected at a position on the proximal side of its neck includes a tapered insert portion and a proximal head portion, the proximal end of said insert portion being adapted for location in said prepared socket and having a maximum dimension in a plane normal to the distal-proximal axis of the neck which is larger than the minimum dimension of the neck in a parallel plane, characterised in that said head portion has a distal rim which contacts an outer surface of the femur with which it is to be used, the insert portion extends radially away from the distal rim towards the central axis of the head and the tapered insert portion has a general axis which is inclined to the central axis of the head portion in a plane radial thereto. Thus, the component according to the present invention takes advantage of the bone at the periphery of the socket which enables the insert to be accurately and firmly located in the bone. The presence of the bone at the outer edges of the socket helps to stabilise the component. Preferably the tapered insert portion is flared outwardly in the proximal direction. The tapered insert portion can be dimensioned to pass through the neck of the femur with which it is to be used or it can stop short of it depending upon the requirements. In one preferred embodiment the tapered insert portion has a smooth finish. This can enable it to sink into the bone if it is inserted with the use of cement. A void centring arrangement can be provided in the manner shown in EP 0 427 444. In any case, the proximal end of the head portion can be of generally spherical shape and have a bearing surface for co-operation with an acetabular socket. Alternatively the proximal end of the head portion can be adapted to receive a substantially part-spherical bearing element. The proximal end of the head portion can be provided with a male taper to receive a matching female taper on the part-spherical bearing element. The bearing element can have a spigot adapted to engage in a bore provided in the head portion and the spigot and bore can be tapered to provide an engaging fit. In one preferred embodiment the spigot is elongated and extends through the head portion and into the tapered insert portion. Alternatively the proximal end of the head portion can be substantially hemispherical as is the inner wall 26 of the bearing element which has a tapered elongate spigot adapted to engage in a tapered bore in a head portion to provide an engaging fit and the spigot can extend through the head portion and into the tapered insert portion to provide stability. In any of the preceding constructions any of the parts can be made of metal, a synthetic plastics material or a ceramic material. In an alternative construction the prosthetic femoral component can be formed as a single component. The tapered insert portion can be non-circular and be adapted to prevent rotation relative to the bone. Thus, the cross-section of the tapered insert portion can be elongate in a plane extending normal to the central axis. With this type of construction the cross-section of the tapered insert portion can be substantially rectangular, oval or figure-of-eight shaped or any other desired cross-section. In another preferred arrangement the distal side of the head portion is formed as a trough which extends around part of the tapered insert portion and the distal portion of the insert portion can be formed with a concave taper. If desired the proximal end of the tapered insert portion can be provided with a series of radially outwardly extending steps or fins. The invention can be performed in various ways but one embodiment will now be described by way of example and with reference to the accompanying drawings in which : Figure 1 is a diagrammatic view of the proximal end of a femur showing the general construction of the bone and the trabecular fibres; Figure 2 is an isometric view of a prosthetic femoral component according to the invention and which is formed as a single metal component; Figure 3 is a cross-sectional side elevation of a component of similar shape to that shown in Figure 2 but which is of a multiple construction; Figure 4 is a cross-sectional side elevation of a similar construction to that shown in figure 3 in place in a bone; Figure 5 is a cross-sectional view on the line V-V of Figure 4; and, Figure 6 is a side view of an alternative construction in place with part of the bone removed. As shown in Figure 1 the natural construction of a femur consists of an outer hard bone, usually referred to as the cortex, which in the region of the ends of the femur encases a spongy interior. The cortex extends over the head of the femur, indicated by reference numeral 1, but is very thin at the junction of the head 1 and the neck 2. Trabecular fibres, indicated by reference numeral 3, sprout from the cortex upwardly and through the head 1, as shown in Figure 1. It has been observed that, if the bone is cut, that these fibres are best able to reform around sharp surfaces. As shown in the drawings the prosthetic femoral component is for location in a prepared socket in a femur which has been resectioned at a position on the proximal side of its neck 2 and which includes a tapered insert portion 4 and a proximal head portion 5. The tapered insert portion 4 has a proximal end 6 which is adapted for location in the prepared socket 7 and which is flared outwardly, as indicated by reference numeral 8, so that it has a maximum dimension in a plane normal to the distal-proximal axis 9 of the neck 2 which is larger than the minimum dimension of the neck 2 in a parallel plane. This will be seen most clearly from Figure 5. From Figure 4 it will also be seen that the femoral head has been transected at a point on the proximal side of the neck 2 and about half way through the femoral head. The general axis of the tapered insert 4 is indicated by reference numeral 10 in Figures 3 and 4 and is inclined to the central axis 11 of the head portion 5 in a plane radial to the axis 10. The general axis 11 is substantially aligned with the distal-proximal axis 9 of the neck 2 when the component is in position. Figure 2 shows how the tapered insert portion is of non-circular cross-section and is shaped to prevent rotation relative to the bone. Thus, the cross-section of this insert portion 4 is elongate in a plane extending normal to the central axis 11 of the head 5 and in this construction is substantially rectangular in cross-section. The insert portion 4 extends radially away from the distal rim 12 of the head portion 5 towards the central axis 11 and the distal side of the head portion 5 is formed as a trough 13 where it surrounds the tapered insert portion 4. The distal portion 14 of the insert portion 4 is formed as a regular or irregular concave taper 15. Figure 2 shows a construction which is made from metal, a synthetic plastics material or a ceramic material and is formed as a single component. Figures 3 to 6 show alternative embodiments similar to that shown in Figure 2 but of multiple construction. In the arrangement shown in Figure 3 a substantially part-spherical bearing element 20 is provided and the proximal end of the head portion 5 is provided with a male taper 23 to receive a matching female taper on the part-spherical bearing element 20. This bearing element 20 has a spigot 21 which is provided with a morse taper adapted to engage in a co-operating morse tapered bore 22 provided in the head portion 5. This provides an engaging fit. Alternatively the spigot 21 and bore 22 could be cylindrical. In the construction shown in Figure 4 however the bearing element 20 has an elongated morse tapered spigot 25 which extends through the head portion 5 and into the tapered insert portion 4 in an extended morse tapered bore 22. In the construction shown the proximal end of the head portion 5 is substantially hemispherical as is the inner wall 26 of the bearing element 20. In these constructions the tapered insert portion may conveniently be made of a synthetic plastics material and the bearing element 20 of any other suitable material, for example metal. If desired however both the tapered insert portion and the outer bearing element 20 can be made of the same material. Again the tapered insert portion 4 and head portion 5 can be made from a ceramic material, for example alumina zirconium or zirconium toughened alumina. Figure 4 indicates how the bone is cut to receive this type of prosthetic femoral component. This procedure does not form part of the claimed invention. The natural head of the femur is resectioned immediately above the neck 2 and is cut to provide a bore on the axis 11. A second bore is then cut at an angle to the first on the line of the axis 10. The opening provided by the bores is enlarged and tapered outwardly to provide a tapering opening which is substantially rectangular in cross-section. To carry out this procedure the preparation is as follows: Step 1 : the femoral head is cut around the equator using an oscillating saw Step 2 : a guide rod is placed along the axis of the femoral neck Step 3 : nibblers are used to clean around the femoral head to create a cylindrical shape Step 4 : an external conical reamer is used to ream the head to correct height and prepare the contact surface for the prosthesis using a trial cap to gauge the approximate depth of cut Step 5 : the trial cap is used to protect the femoral head while the acetabulum is prepared Step 6 : when the acetabulum is in place a trial reduction is performed with the trial cap and the trial head to ensure that the femur has been prepared to correct height before completing preparation of the internal cavity in the femur Step 7 : the trial head is removed and the two bores are reamed in the bone through the trial cap which can also act as a reaming guide The shape of the reamer gives the flared shape to the bone. Step 8 : the trial cap is removed and the cavity is finished with either an osteotom or a rasp. The prosthesis is now fitted as described above. If desired the proximal end of the cut opening can be cut to provide a series of radially inwardly extending steps or fins 27 as shown in Figure 6. Similar steps, or fins 28 can be provided on the flared part 8 of the insert portion 4 to encourage growth of the trabeculum fibres to reform around the sharp corners of the steps or fins. The particular shape of the stem shown in Figures 3, 4 and 5 prevents rotation of the insert portion 4 in the bone and makes it easy to implant with enhanced stability. The advantage of the invention is that it involves minimally invasive surgery. The insert portion 4 and the head portion 5 can be made of any suitable materials, for example a synthetic resin and carbon fibres, typical examples being PEEK (polyetheretherketone) or PBT (polybutalieneterephthalate) resin into which a chopped carbon fibre can be incorporated. Preferably the material is of a similar compressive modulus as cancellous bone. In all the above constructions the surface finish of parts which abut bone can be in the form of a cut-away honeycomb. The insert portion and proximal head portion can be made from any of the materials referred to above and be coated with plasma sprayed hydroxyapatite (HA) which is osteo conductive and stimulates bone growth. If desired the parts can be made from metal, for example, titanium, with a porous coating. In the constructions described above the insert portion 4 is driven into the bone but it could be held by cement. Thus a small amount of cement could be applied at the proximal end of the insert portion, bone growth being relied upon towards the distal end. In the construction described above the tapered insert portion extends through the neck 2 of the femur but if desired the arrangement could be such that it is only of short length and does not pass through the neck portion.

Claims: A prosthetic femoral component for location in a prepared socket in a femur which has been resected at a position on the proximal side of its neck (2) said prosthetic femoral component includes a tapered insert portion (4) and a proximal head portion (5), the proximal end (6) of said insert portion being adapted for location in said prepared socket and having a maximum dimension in a plane normal to the distal-proximal axis of the neck (2) which is larger than the minimum dimension of the neck (2) in a parallel plane, characterised in that said head portion (5) has a distal rim (12) which contacts an outer surface of the femur with which it is to be used, insert portion (4) extends radially away from the distal rim (12) towards the central axis (11) of the head (5) and the tapered insert portion (4) has a general axis (10) which is inclined to the central axis (11) of the head portion (5) in a plane radial thereto. A prosthetic femoral component as claimed in claim 1 characterised in that said tapered insert portion (4) is flared outwardly in a proximal direction. A prosthetic femoral component as claimed in claim 1 or claim 2 characterised in that said tapered insert portion (4) is dimensioned to pass through the neck (2) of the femur with which it is to be used. A prosthetic femoral component as claimed in any one of preceding claims 1 to 3 characterised in that the tapered insertion portion (4) has a smooth finish. A prosthetic femoral component as claimed in any one of preceding claims 1 to 4 characterised in that the proximal end of the head portion (5) is of a generally spherical shape and has a bearing surface for co-operation with an acetabular socket. A prosthetic femoral component as claimed in any one of preceding claims 1 to 4 characterised in that the proximal end of the head portion (5) is adapted to receive a substantially part-spherical bearing element (20). A prosthetic femoral component as claimed in claim 6 characterised in that the proximal end of the head portion (5) is provided with a male taper (23) to receive a matching female taper on the part-spherical bearing element (20). A prosthetic femoral component as claimed in claim 6 or claim 7 characterised in that the bearing element (20) has a spigot (21) adapted to engage in a bore (22) provided in the head portion (5). A prosthetic femoral component as claimed in claim 8 characterised in that the spigot (21) and bore (22) are tapered to provide an engaging fit. A prosthetic femoral component as claimed in claim 9 characterised in that the spigot (25) is elongated and extends through the head portion (5) and into the tapered insert portion (4). A prosthetic femoral component as claimed in claim 6 characterised in that the proximal end of the head portion (5) is substantially hemispherical as is the inner wall (26) of the bearing element (20) which has a tapered elongate spigot (25) adapted to engage in a tapered bore (22) in the head portion (5) to provide an engaging fit and the spigot (25) extends through the head portion (5) and into the tapered insert portion (4). A prosthetic femoral component as claimed in any one of the preceding claims characterised in that the tapered insert portion (4) is made of metal. A prosthetic femoral component as claimed in any one of preceding claims 6 to 11 characterised in that the tapered insert portion (4) is made of a synthetic plastics material. A prosthetic femoral component as claimed in claim 13 characterised in that the tapered insert portion (4) and head portion (5) are made from a synthetic resin and carbon fibres. A prosthetic femoral component as claimed in claim 14 characterised in that the synthetic resin is PEEK (polyetheretherketone) or PBT (polybutalieneterephthalate) into which chopped carbon fibre is incorporated. A prosthetic femoral component as claimed in claim 15 characterised in that the material is of a similar compressive modulus to cancellous bone. A prosthetic femoral component as claimed in any one of claims 1 to 11 characterised in that the tapered insert portion (4) and head portion (5) are made from a ceramic material. A prosthetic femoral component as claimed in claim 17 when dependent on claims 6 to 11 characterised in that the bearing element (20) is made from a ceramic material. A prosthetic femoral component as claimed in claims 17 and 18 characterised in that the ceramic material is alumina zirconium or zirconium thoughened alumina. A prosthetic femoral component as claimed in any one of the preceding claims characterised in that the surface finish of parts which abut bone are in the form of a cut-away honeycomb. A prosthetic femoral component as claimed in any one of the preceding claims characterised in that the parts which abut bone are coated with plasma sprayed hydroxyapatite (HA). A prosthetic femoral component as claimed in any one of the preceding claims characterised in that the tapered insert portion (4) is adapted to be driven into the bone and held without cement A prosthetic femoral component as claimed in any one of the preceding claims characterised in that the tapered insert portion (4) is adapted to be held by cement at its proximal end. A prosthetic femoral component as claimed in any one of preceding claims 1 to 5 and claims 17 and 19 characterised in that it is formed as a single component. A prosthetic femoral component as claimed in any one of preceding claims 1 to 25 characterised in that the tapered insert portion (4) is non-circular and is adapted to prevent rotation relative to the bone. A prosthetic femoral component as claimed in claim 25 characterised in that the cross-section of the tapered insert portion (4) is elongate in a plane extending normal to the central axis (11). A prosthetic femoral component as claimed in claims 25 or 26 characterised in that the cross-section of the tapered insert portion (4 is substantially rectangular, oval, or figure-of-eight shaped. A prosthetic femoral component as claimed in any one of preceding claims 1 to 27 characterised in that the distal side of the head portion (5) is formed as a trough (13) which extends around part of the tapered insert portion (4). A prosthetic femoral component as claimed in any one of preceding claims 1 to 28 characterised in that the distal portion of the tapered insert portion (4) is formed with a rectangular or irregular concave taper. A prosthetic femoral component as claimed in any one of preceding claims 1 to 29 characterised in that the proximal end of the tapered insert portion (4) is provided with a series of radially outwardly extending steps or fins.

Inventors: FIELD RICHARD EDDY, RUSHTON NEIL, STORER JOHN ANDREW, FIELD, RICHARD EDDY, RUSHTON, NEIL, STORER, JOHN ANDREW
Applicants: BENOIST GIRARD SAS
Application year: 2001
Main classification: A61F2
IPC codes: A61F2