Prosthesis (i.e. – artificial body members) – parts thereof – or ai – Implantable prosthesis – Bone
Reexamination Certificate
1999-12-14
2001-06-12
Willse, David H. (Department: 3738)
Prosthesis (i.e., artificial body members), parts thereof, or ai
Implantable prosthesis
Bone
C623S020320, C623S020350
Reexamination Certificate
active
06245110
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a knee joint endoprosthesis having a femur element and a tibia element.
Such systems are known, for example, from DE-A 41 41 757. There, the femur element, viewed from medial to lateral, has an essentially U-shape, wherein one horizontal, two diagonal and two vertical joint surfaces pointing toward the femur are provided for installation on a resected femur bone. The femur element is provided with two runners which reproduce the natural condyles and condyle rollers.
The tibia element of known systems usually has two sliding tracks, on which the runners of the femur element can execute a rolling away motion and, optionally (depending upon the type of endoprosthesis), sliding movements. The tibia element has a horizontal support surface pointing toward the tibia for supporting the resected tibia.
The tibia element as well as the femur element of known systems respectively have a conical pin, which is insertable into a conical taper socket in a respective shank element of the modular system. Between pin and socket a conical friction (clamping) connection can be produced, which should ensure a durable connection between femur element and femur shank element or tibia element and tibia shank element.
The shanks of modular systems are implanted either by means of a bone cement, for example PMMA, into the marrow space of the respective tubular bone, or without cement, whereby the shank element is then provided with a special surface into which and through which bone material can grow for durable secondary fixation of the implant (see, for example, German Patent DE-C-195 435 30).
The following problems are associated with the known systems:
The shanks (generally made of metal), which are installed in the marrow space of the respective tubular bone, have a completely different elasticity modulus than the bone material (spongiosa) surrounding the shank. This often leads (especially with older persons, whose bones are diseased with osteoporosis) to fatigue fractures above the shank end in the marrow space precisely because of the different elasticity moduli.
The second problem concerns the natural valgus angle, which lies in the range of 7.5° with most people. With the artificial endoprosthesis systems, however, only valgus angles of at most 5° have been producible up until now, and to be sure owing to the previously mentioned conical clamping usually used between the shank element and the femur element or tibia element, since the desired conical clamping no longer occurs with an angle >5°.
Moreover, finding sufficient hold for a new shank endoprosthesis, so that this can be implanted sufficiently firmly, is problematic with the known shank endoprostheses in the event that a revision intervention should become necessary. This is especially critical in younger patients since, according to previous experience, implanted endoprostheses have a limited lifetime of 10 to 15 years, assuming that an unforeseen revision intervention is not necessary before this. With younger patients, it is then in any case necessary to replace the originally implanted endoprosthesis with a new one. This is not always possible, without further measures, for the reasons mentioned.
SUMMARY OF THE INVENTION
Against this background, it is now the object of the present invention to provide a completely new type of knee joint endoprosthesis, in which the problems of known endoprostheses with respect to different elasticity moduli of the implant material and the surrounding bone material do not arise, and in which the restriction with respect to reproducibility of the valgus angle does not exist.
This object is accomplished with a knee joint endoprosthesis with the features mentioned at the beginning by the combination of the features presented below:
1. The femur element and the tibia element are constructed shanklessly. This means that neither the femur element nor the tibia element is to be connected with a modular shank element. The restriction of the valgus angle to a maximum of 5° is thereby absent, since no conical clamp connection must be produced between a respective shank element and the tibia or femur element. The problem in respect to the abutting different elasticity moduli in the marrow space of the tibia or femur also disappears, since no shank is inserted into the marrow space. Basically, the femur element therefore grips around the resected femur end without a shank penetrating into the marrow cavity of the femur. The tibia element, in contrast, lies on the resected bearing surface of the tibia, without it having the known shank which would have to be coupled with it through a conical clamp connection.
With respect to the valgus position, as well as with respect to avoiding the different elasticity moduli impinging upon each other, the shanklessness feature mentioned accomplishes the object previously mentioned. The long term in situ stability of an endoprosthesis constructed in this manner is attained through the following feature.
2. At least the horizontal and the two diagonal joint surfaces of the femur element and the horizontal support surface of the tibia element are provided with an open-mesh, three-dimensional spatial network structure, which is an integral component of the base structure of the femur element and the tibia element. The latter should be stressed in particular, since the three-dimensional spatial network structure is the only component of the endoprosthesis which provides for a stable secondary fixation of the femur element and the tibia element in situ. Any sintered-on metal network would not do the stresses justice. It is contemplated to use of a spatial network structure as disclosed, for example, in German patents DE-C-41 06 971 or DE-C-195 43 530, where so-called tripods are produced as an integral component of the base structure of the femur element and the tibia element by an investment casting process.
In order that the bone material be integrated into the open-mesh spatial network structure in the shortest time possible, the two features reproduced below are provided:
3. Medially and laterally, two latches angled toward the femur are formed on the femur element with respectively at least one through hole formed therein, into which respectively a bone screw can be inserted, and
4. Medially and laterally, two latches angled toward the tibia are formed on the tibia element with respectively at least one through hole formed therein, into which respectively a bone screw can be inserted.
The angling of the mentioned latches is of particular importance in the present case. Namely, by screwing in the respective bone screws, which are inserted through the mentioned through holes in the latches, a diagonal bracing of the femur element or the tibia element is hereby attained, wherein the screws can extend through the spongiosa tissue up to the opposite cortical bone and further beyond, so that they can even penetrate the opposite cortical bone. By the diagonal bracing respective compressive forces with horizontal and vertical components are exerted on the interface between the spatial network structure and the resected bone. The bone material, which comes into contact with the spatial network structure, is hereby stimulated to grow, so that the integration in the open-mesh proceeds in an accelerated manner. The screws screwed into the bones therefore have a smaller effect in respect to a stable secondary fixation, but rather much more in respect to a primary fixation and in relation to the exertion of diagonally acting compressive forces. After complete integration of bone material into the spatial network structure, the bone screws cease to exercise any function at all. Theoretically, they can be removed at this time, which is not conducted in practice, however, as this would mean a new operation.
With the intended secondary fixation as described, an osteosynthesis between the artificial system of the (metallic) spatial network structure and the natural bone is basically replicated.
The femur element and the tibia ele
Grundei Hans
Thomas Wolfram
Akin Gump Strauss Hauer & Feld L.L.P.
ESKA Implants GmbH & Co.
Jackson Suzette J.
Willse David H.
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