Femoral twin-stem for a hip endoproshesis

Abstract: This invention describes a femoral twin-stem for a hip endoprosthesis. The lateral stem portion (2,3) and the medial stem portion (4,7) are connected by a centrally located bridge (1). The bridge (1) is designed in such a way that the lateral and the medial stem portions (2,3;4,7) are separated from each other by an antero-posterior proximal slot (5) and an antero-posterior distal slot (6) both extending away from the bridge (1). The neck (11) of the femoral twin-stem is connected to the proximal section (2) of the lateral stem portion (2,3). The proximal prong (4) of the medial stem portion (4,7) and the distal prong (3) of the lateral stem portion (2,3) allow for a gradual load transfer to the bone since they can be individually designed with a proper geometry all the way to their respective tips. Gradual load transfer in turn reduces stresses (18,19) in the bone cement mantle (10) and improves the long term results of the hip replacement effected with this femoral twin-stem.

Description: Field of the Invention This invention relates to femoral twin-stem for a hip endoprosthesis. Background of the Invention Replacement surgery for artificial joints, in particular of the femoral component of hip joints, entails removal of the bone end-section. The end of the bone and the corresponding joint component are replaced with a prosthesis which must assume the load supporting function. Joint forces are transferred from the prosthesis to the bone. From the very early days of joint replacement till today the preferred mode of anchoring the prosthetic component in the tubular bone has been by means of a stem, i.e. a rod-shaped extension of the prosthesis properly adapted to the shape and size of the medullary cavity. Since the early sixties the standard procedure has included the use of so-called bone cement. Bone cement is a room temperature curing acrylate (usually in the form of a powder component consisting of polymethylmethacrylate with a catalyst and a liquid component consisting of methylmethacrylate with an accelerator, the two components polymerizing upon mixing with each other) with additives such as zirconium oxide (for obtaining radiographic density) and antibiotics. When cured, bone cement forms a relatively soft (about five to ten times softer than cortical bone) mantle between the prosthesis stem and the bone. Stresses induced in this cement mantle by load transfer from the stem to the bone depend on the compliances of the stem and of the bone. In general, a stiffer stem will transfer loads closer to its tip, i.e. more distally in the case of the femoral component of the hip joint; a more compliant stem will transfer loads closer to the neck, i.e. more proximally. Gradual change of the stem cross-section (smaller cross-section distally) results in more gradual load transfer. This works well for the distal part of the stem, but it moves the load transfer more proximally, where the cross-section of the stem cannot be sufficiently reduced since it must support the joint loads transferred to it via the neck of the prosthesis. In addition, the bone tube is opened at this side and thus rendered even softer. To achieve a truly uniform load transfer the stem should reduce its stiffness all the way to zero at its proximal end. A femoral twin-stem according to the features of the preamble of Claim 1 is known from US-A-45 16 277. Summary of the Invention The invention as claimed is intended to remedy these drawbacks. It solves the problem of how to design a femoral twin-stem for a hip endoprosthesis which allows for a truly uniform load transfer. The joint load on the hip joint prosthesis produces a bending moment on the stem which in turn generates compressive stresses on the medial proximal and the lateral distal aspect of the cement mantle. This is where most of the load transfer occurs. Lateral proximal aspect of the femur is the area of soft cancellous bone and does not support much load. Thus the neck of the prosthesis is connected to the proximal aspect of the lateral stem where there is very little load transfer between the prosthesis and the bone. High stiffness of the stem in this region does not induce high stress concentration in the cement. Two stem portions are connected at their mid-section. From there the medial stem portion gradually reduces its stiffness in both the proximal and the distal directions. Proximal direction is more important for the overall load transfer. The lateral stem portion also reduces its stiffness in the distal direction. With this configuration a truly uniform load transfer can be achieved in the frontal plane where the highest joint loads occur. Gradual load transfer reduces peak loads in the cement mantle. As it is well known, bone cement is the weakest and thus the most critical component of the total joint replacement. Reducing the stresses in the bone cement thus improves long term results of this very important surgical treatment of the joint diseases. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. For the better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which are illustrated and described preferred embodiments of the invention. Brief Description of the Drawings In the drawings: Fig. 1 is a sectional view of the femoral twin-stem according to the invention cemented in the proximal part of the femur; Fig. 2 is a sectional view of a preferred embodiment of the invention according to Fig. 1; Fig. 3 is a side view of a preferred embodiment of the invention in which the cuts are filled with a polymer; Description of the Preferred Embodiments Fig. 1 represents the femoral twin-stem according to the invention inserted in the proximal femur. The lateral stem proximal section 2 is shaped much as a conventional stem proximal section with the neck of the prosthesis 11 supporting the head of the prosthesis 12. Antero-posterior cuts 5 and 6 separate the lateral stem portion 2,3 from the medial stem portion 4,7. Stiffness of the distal prong 3 of the lateral stem portion 2,3 reduces in distal direction to facilitate uniform load transfer to the bone through the cement mantle 10. Medial stem portion 4,7 is connected to the lateral stem portion 2,3 by bridge 1 located in the central region of the stem. Both the distal prong 7 and proximal prong 4 of the medial stem portion 4,7 have a reduced stiffness towards their respective tips. The lateral proximal aspect 13 of the femur 14 is made of soft cancellous bone and does not support much of the joint force 15. Thus the proximal section 2 of the lateral stem portion 2,3 serves basically as a neck extension, transferring the joint load 15 down to the bridge 1 level of the twin-stem. At this level the joint force 15 reduces to the force 16 (equal to the joint force 15 in magnitude and direction) and the moment 17. The moment 17 is balanced mostly by compressive stresses 18 on the proximal prong 4 of the medial stem portion 4,7 and compressive stresses 19 on the distal prong 3 of the lateral stem portion 2,3. The force 16 is balanced by shear and compressive forces in the cement mantle 10, which are generally of lower magnitude and less concern. As shown in Fig. 2, to maintain a minimum thickness of the cement mantle 10, especially at the lateral distal and medial proximal aspects, the respective tips may be shaped so as to form thin protrusions 8. Fig. 3 shows a preferred embodiment of the invention in which the antero-posterior cuts 5 and 6 are filled with a polymer 9, preferably polymethylmethacrylate (PMMA) and/or polybutylmethacrylate (PBMA).

Claims: Femoral twin-stem for a hip endoprosthesis comprising a lateral stem portion (2,3) and a medial stem portion (4,7) separated from each other by an antero-posterior distal open slot (6) said lateral stem portion (2,3) and said medial stem portion (4,7) being connected by a bridge (1) and said antero-posterior distal open slot (6) extending away from said bridge (1), characterized in that said bridge (1) is approximately centrally located along the length of said lateral stem portion (2,3), said bridge (1) being constructed so that said lateral and medial stem portions (2,3;4,7) are further separated from each other by an antero-posterior proximal open slot (5) extending away from said bridge (1). Femoral twin-stem according to claim 1, characterised in that said bridge (1) is located in the proximal half of the femoral twin-stem height. Femoral twin-stem according to claim 1 or 2, characterised in that said proximal slot (5) has a length of 1/5 to 1/2 of the total twin-stem height. Femoral twin-stem according to one of the claims 1 to 3, characterised in that said medial stem portion (4,7) is divided by said bridge (1) into a proximal prong (4) and a distal prong (7). Femoral twin-stem according to one of the claims 1 to 4, characterised in that said lateral stem portion (2,3) is divided by said bridge (1) into a proximal section (2) and a distal prong (3). Femoral twin-stem according to claim 5, characterised in that said distal prong (7) of said medial stem portion (4,7) is shorter than the distal prong (3) of said lateral stem portion (2,3). Femoral twin-stem according to claim 6, characterised in that said medial stem portion (4,7) etends only proximally into proximal prong (7) . Femoral twin-stem according to one of the claims 1 to 7, characterised in that said distal slot (6) has a length of 1/5 to 3/5 of the total twin-stem height. Femoral twin-stem according to one of the claims 4 to 8, characterised in that said distal prong (3) of said lateral stem portion (2,3) has a lateral protrusion (8), preferably at the distal end of said distal prong (3), acting as a bone cement spacer. Femoral twin-stem according to one of the claims 4 to 9, characterised in that said proximal and distal prongs (4,7) of said medial stem portion (4,7) have medial protrusions (8) acting as bone cement spacers. Femoral twin-stem according to one of the claims 1 to 10, characterised in that the space within the antero-posterior cuts (5,6) are filled with a polymer, preferably polymethylmethacrylate (PMMA) and/or polybutylmethacrylate (PBMA).

Inventors: TEPIC SLOBODAN, TEPIC, SLOBODAN
Applicants: AO FORSCHUNGSINST, AO-FORSCHUNGSINSTITUT DAVOS
Application year: 1991
Main classification: A61F2
IPC codes: A61F2