Femoral component of a hip joint endoprosthesis

Abstract: A femoral component of a hip joint endoprosthesis is disclosed, which comprises a shaft (2) whose cross-section increases gradually from the distal end (8) of the shaft towards its proximal end (12) and is extended at the proximal end (6) of the shaft (2) in the mediolateral direction to form an oval. To improve the retention of the femoral component within a natural femur bone, the cross-section of the shaft (2) increases in the ventrodorsal direction towards the end (12) of the shaft (2) in several steps (22,28,32), whose mediolateral width increases from the distal step edge (24) towards the proximal end (12) of the shaft (2).

Description: The invention concerns a femoral component of a hip joint endoprosthesis for cementless and cemented implantation, with a shaft whose cross-section increases gradually from the distal towards the proximal end of the shaft, the increase of the cross-section in the ventrodorsal direction at the proximal shaft portion and towards the proximal end of the shaft taking place in several steps. Such a femoral component of a hip joint endoprosthesis is known from FR-A-2693367. The proximal shaft portion of this known femoral component provides a substantially triangular cross-section the width of which increases step-wise from the medial to the lateral side of the shaft. As this triangular cross-section does not conform to the shape of the femur lumen, any cementless implantation will require additional resection of bone material in the femur lumen resulting in undue loading of the adjacent portions of the weakened femur. DE-A-38 29 361 discloses a femoral component designed to be cemented in place which comprises a shaft whose cross-section is longitudinally adapted to the bone lumen of the natural femur, and which has a flange-type supporting collar at the proximal end of the shaft that rests against the resection edge of the natural femur when the femoral component is cemented into the femur. EP 0 112 435 concerns a femoral component which increases in cross-section from the distal to the proximal end of the stem, the increases occurring in steps in the distal part of the stem. Since bone cement penetrates non-uniformly under the supporting collar during the cementing process, direct contact between the supporting collar and the natural femur cannot be reliably prevented, and there is also a risk that in this area thin sections of bone cement may splinter under loading. Furthermore, because of the different material properties, undesired relative movement between the metallic supporting collar and the natural femur can also not be reliably excluded, as a result of which the bone tissue will undergo undesired alteration. In addition, as a rule the supporting collar only makes point contact when resting against the natural femur - sometimes via a layer of bone cement - since the final position of the femoral component and hence that of the supporting collar depends on the course and shape of the bone lumen and cannot be precisely predicted when the resection cut is made. Thus, disadvantages cannot be excluded so far as the transfer of vertical loading via a support collar to the natural femur is concerned. It is therefore the object of the present invention to realize a femoral component of the type described above, which has no supporting collar involved in the transfer of vertical loading and has load transfer in a form-locking way over relatively large support surfaces. In accordance with this invention there is provided a femoral component of a hip joint endoprosthesis for cementless and cemented implantation respectively, with a shaft whose cross-section increases gradually from the distal towards the proximal end of the shaft, the increase of the cross-section in the ventrodorsal direction at the proximal shaft portion and towards the proximal end of the shaft taking place in several steps, characterized in that the cross-section is extended in the mediolateral direction in the proximal portion of the shaft to form a substantially oval shape in cross-section and that the mediolateral width of the steps increases from the distal step edge towards the proximal end of the shaft, at least one of the several steps being located in the proximal portion of the shaft, the ventrodorsal width of the proximal shaft portion increasing in several steps in the mediolateral and the lateromedial direction. The advantages of the invention consist particularly in the fact that in the proximal portion of the shaft, the increase of cross-section - to adapt to the lumen of the natural femur - takes place in several steps that begin a certain distance away from the distal end of the shaft with a smaller mediolateral width, and whose mdiolateral width increases gradually towards the proximal end of the shaft in correspondence with the cross-section of the shaft. Since, when the femoral component according to the invention is to be implanted, the bone lumen is shaped beforehand using a tool in the dorsal and ventral areas to form stepped recesses of corresponding shape, the step edges of the prosthesis together with the corresponding ones of the bone lumen form a form-locking fit which ensures both primary and long-term stability and also makes for vertical load transfer to the bone over a relatively large area. The conforming step-shaped recesses in the internal wall of the femur are produced using a shaping tool adapted to the femoral component, and their volume is then filled completely by implantation of a uniform layer of cement and the femoral component itself. Alternatively, it is also possible to implant the prosthesis according to the invention without any cement. The desired shaping of the step-like recesses to conform with the stepped projections of the artificial femoral component can be made simply with a corresponding tool, a rasp or similar, since the working movement of the tool takes place only in the axial direction and the prosthesis is then inserted into the prepared bone lumen with a movement of the same kind. Any undefined movement of the prosthetic femoral component during insertion is avoided. It is particularly preferable to position at least one step both on the ventral side and on the dorsal side of the proximal portion of the shaft, such that support is achieved symmetrically and in a form-locking way on both sides of the femoral component. The step begins a certain distance away from the distal end with a smaller width, and becomes wider upwards towards the proximal end of the shaft. In accordance with a preferred embodiment of the invention, on the ventral and dorsal sides of the shaft there are in each case several steps towards the proximal end of the shaft, arranged above one another like a staircase. The edge of the next-higher step is preferably offset in each case some distance away from the edge of the step under it, so that there is also some lateral or medial stepping whereby the whole cross-section of the shaft increases towards the proximal end on all sides and therefore approximates to the shape of the bone lumen. This makes for more reliable and firmer seating of the prosthesis in the bone lumen. In accordance with a preferred embodiment of the invention, the edge height of the individual steps is constant in each case and the step surfaces are flat and preferably parallel to one another. This design feature ensures a high strength and favourable load transfer characteristics, with advantageous hardening. Alternatively, the edge height of the steps can gradually increase in the proximal direction, which makes for additional bone compression when the prosthesis is inserted. For preference, a flat basal surface is formed on the ventral and dorsal sides of the proximal portion of the shaft, on which the steps - staggered with respect to one another in the longitudinal direction - are formed on both the dorsal and the ventral sides of the cross-section. The ventral and dorsal basal surfaces can be parallel. At the proximal end of the shaft there is a conical boss to enable the attachment of a prosthesis head by means of a push-on joint. Alternatively, at the proximal end of the shaft there can be a prosthesis neck with a prosthesis head formed on it. In addition, at the proximal end of the shaft a lateral projection can also be formed, which increases the primary stability immediately after implantation of the prosthesis component, especially against rotational loads. For preference, the cross-section of the shaft becomes circular towards the distal end, and along the distal portion of the shaft a wedge surface is formed laterally, which reduces the shaft cross-section towards the distal end to circular sections flat on one side. In accordance with a preferred embodiment of the invention, the distal shaft portion of the prosthesis is bent towards the ventral direction by a specified angle α, preferably in the range 3° to 50°. This makes for better adaptation of the prosthesis shaft to the course of the bone lumen, so simplifying the implantation of the prosthesis. Further advantageous features of the invention are characterized in the dependent claims. In what follows, an embodiment of the invention will be explained with reference to the drawings, which show: Fig. 1: A side view of the femoral component, seen from the ventral side. Fig. 2: A cross-section along the line A-A in Fig. 1, through the proximal portion of the shaft. Fig. 3: A lateral view of the femoral component. Fig. 4: A medial view of the femoral component, and Fig. 5: A perspective view of the femoral component. Figs. 1 to 5 show various views of a femoral component of a hip joint endoprosthesis. The femoral component comprises a shaft 2 whose cross-section increases gradually from the distal end 8 of the shaft towards the proximal end 12 of the shaft. Over its entire length, the cross-section of the shaft 2 is adapted to the bone lumen of the natural femur bone in man. Along the distal portion 4 of the shaft, its cross-section is almost circular, while along the proximal portion 6 the cross-section assumes an oval shape extended in the mediolateral direction, such that it increases more rapidly in the mediolateral direction than in the ventrodorsal direction. At the proximal end 12 of the shaft 2 there is no supporting collar, but there is a lateral projection which increases the rotational stability, especially shortly after the implantation process. At the proximal end 12 of the shaft a conical boss 16 is formed, inclined with respect to the longitudinal axis 3 of the shaft by the so-termed CCD angle. The conical boss 16 serves to allow attachment of a prosthesis head (not shown) with a corresponding conical socket, enabling the latter to be pushed on and attached firmly to the femoral component by adhesive bonding. Alternatively, it is also possible to form a prosthesis head with a neck in one piece at the proximal end of the shaft. The distal portion 4 of the shaft - over a certain length - is offset or bent towards the ventral direction, to achieve better adaptation to the bone lumen of the human femur and thereby enable simpler implantation. Further, the distal portion 4 of the shaft has a wedge-shaped surface 10 formed laterally towards the distal end 8 of the shaft, which reduces the circular cross-section towards the bottom to circular sections flat on one side. Along the proximal portion of the shaft 2 flat basal surfaces 20 are formed on the ventral and dorsal sides, and towards the proximal end 12 of the shaft the shaft cross-section increases in the ventrodorsal direction in several steps 22, 28, 32, which begin at a certain level with a distally facing step edge 24, 30, 34 and then rise towards the proximal end 12 of the shaft with increasing mediolateral width. At least one step 22 is located on both the ventral and dorsal sides of the shaft 2. However, in the embodiment illustrated there are several steps 22, 28, 32 on both the ventral and dorsal sides, staggered with respect to one another towards the proximal end of the shaft and whose step edges are each some distance away from the edge of the step below, so forming a stepped increase of the shaft cross-section. All the steps extend up to the proximal end 12 of the shaft, the first step 22 extending most widely in the distal direction, with the largest mediolateral width. The second step has a smaller distal extension and its mediolateral width is smaller than that of the first step. The third step 32 extends still less in the distal direction and its mediolateral width is smaller than that of the previous, second step 28. In the embodiment illustrated, the steps have a constant edge height all round and their surfaces are flat. As can be seen particularly in Figs. 3 and 4, the step surfaces are thus parallel to one another. The stepping on the ventral side is a mirror-image of that on the dorsal side of the shaft 2.

Claims: Femoral component of a hip joint endoprosthesis for cementless and cemented implantation respectively, with a shaft whose cross-section increases gradually from the distal towards the proximal end of the shaft, the increase of the cross-section in the ventrodorsal direction at the proximal shaft portion and towards the proximal end (12) of the shaft taking place in several steps (22, 28, 32), characterized in that the cross-section is extended in the mediolateral direction in the proximal portion of the shaft to form a substantially oval shape in cross-section and that the mediolateral width of the steps increases from the distal step edge (24) towards the proximal end (12) of the shaft, the distal end of at least one of the several steps being located in the proximal portion of the shaft, the ventrodorsal width of the proximal shaft portion increasing in several steps in the mediolateral and the lateromedial direction. Femoral component according to claim 1, characterized in that there is at least one step (22) both on the ventral and on the dorsal side of the shaft (2). Femoral component according to claims 1 or 2, characterized in that both on the ventral and on the dorsal side of the shaft (2) several steps (22, 28, 32) are arranged over and staggered with respect to one another towards the proximal end (12) of the shaft, and the edge (29) of each next-higher step (28) is some distance away from the edge (23) of the step (22) below it. Femoral component according to any of the preceding claims, characterized in that the edge heights of the individual steps (22, 28, 32) are the same for each step. Femoral component according to any of the preceding claims, characterized in that the edge height of all the steps (22, 28, 32) has the same value. Femoral component according to any of claims 1 to 3, characterized in that the edge height of individual steps increases gradually towards the proximal direction. Femoral component according to any of the preceding claims, characterized in that the surfaces of the steps (22, 28, 32) are flat. Femoral component according to any of the preceding claims, characterized in that the surfaces of the ventral steps (22, 28, 32) are parallel to the surfaces of the dorsal steps (22, 28, 32). Femoral component according to any of the preceding claims, characterized in that in the proximal area of the shaft (2) on both the ventral and the dorsal sides, a flat basal surface (20) is formed, on which the first step (22) is located. Femoral component according to claim 9, characterized in that the ventral and dorsal basal surfaces (20) are parallel to one another. Femoral component according to any of the preceding claims, characterized in that at the proximal end (12) of the shaft (23) a conical plug (16) is provided for the attachment of a prosthesis head. Femoral component according to any of claims 1 to 10, characterized in that at the proximal end (12) of the shaft (2), a prosthesis neck with a prosthesis head formed on it is located. Femoral component according to any of the preceding claims, characterized in that on the proximal portion of the shaft (2) a lateral projection (14) is formed. Femoral component according to any of the preceding claims, characterized in that the cross-section of the shaft (2) changes to a circular section towards the distal end. Femoral component according to any of the preceding claims, characterized in that along the distal portion (4) of the shaft (2) a lateral wedge surface (10) is formed. Femoral component according to any of the preceding claims, characterized in that a distal portion (4) of the shaft is bent towards the ventral direction.

Inventors: HOEPPNER DIRK, KNIPPRATH JOHANNES DR, HOEPPNER, DIRK, KNIPPRATH, JOHANNES, DR., Höppner, Dirk
Applicants: ENDOPLANT GMBH, KNIPPRATH JOHANNES DR, KNIPPRATH, JOHANNES, DR.
Application year: 1996
Main classification: A61F2
IPC codes: A61F2