Endoprosthesis with anchoring shaft

Details Endoprosthesis with anchoring shaft Endoprosthesis with anchoring shaft

1999-03-24

2001-02-20

Isabella, David J. (Department: 3738)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Implantable prosthesis

Bone

Reexamination Certificate

active

06190418

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns an endoprosthesis with an anchoring shaft, which is intended for use in a recess produced in a long bone, wherein the anchoring shaft has a cross section that is at least approximately polygonal on at least one part of its length.
BACKGROUND OF THE INVENTION
An example of this type of endoprosthesis is the part of an artificial hip joint that is used in the femur and is supposed to replace the head and neck of the femur. To seat the prosthesis firmly in the bone, the recess for the shaft must be precisely machined according to the anchoring philosophy (“formfit” or “pressfit”). In practice, it is not possible to produce the recess in the bone so precisely that the desired seat is achieved along the whole peripheral surface of the shaft.
Therefore, a new procedure is now being used in practice in which the recess is cut out using a program-controlled robot. This makes it possible to produce the recess with high precision.
However, when today's usual straight shaft prostheses, which frequently have a rectangular cross section to achieve rotational stability, are used, the problem is that no corners can be cut with a rotary cutting head. This is mainly due to the fact that the cutting head cannot have less than a certain minimum radius for reasons of stability. The cutting head is on a relatively long shaft and its diameter must be greater than the shaft diameter, so that the shaft can follow the cutting head into the recess. If the shaft diameter is too small, the shaft bends in the lateral forces, so the desired precision cannot be achieved. As a rule, the minimum cutting head radius is therefore approximately 5 mm.
If the bone is allowed to stand in the corners corresponding to the radius of the cutting head, there is a risk that the bone will split when the prosthesis is inserted with its sharp square edges. On the other hand, if the corners are completely cut out, there are hollow spaces on both sides of the corner line, in which the prosthesis shaft is not adjacent to the bone and in which not only does the prosthesis have no stop, but there is also a danger that the bone will recede there, since there is no stress on it.
The problem of the invention is to produce an endoprosthesis of the type mentioned at the beginning so that the shaft can lie on the wall of the recess on at least part of its length with its entire peripheral surface.
SUMMARY OF THE INVENTION
The invention solves this problem by having the cross sectional outline of the shaft composed of straight and curved line sections and having the minimum radius of curvature of the curved line sections always be larger than or equal to the radius of a cutting head used to produce the recess in the bone. This eliminates the danger that either hollow spaces are created in the corner areas between straight line sections or splitting forces are exerted on the bone by sharp edges of the prosthesis shaft.
In the preferred form of embodiment, the cross section of the shaft is basically rectangular on at least part of its length, and the midpoint of the curve of the curved line section in the corner areas lies on the respective median line of two side lines contiguous one another.
The curved line section must therefore not transition tangentially into the respective straight line section or lateral surfaces of the shaft. Rather, the respective curvature midpoint on the median line can also be shifted so that the corresponding curved line runs through the intersection of the two straight side lines contiguous to one another or even outside them.
A curved line section can also consist of several subsections with different radii of curvature. A curved line section can also consist of curved subsections whose curvature midpoints are some distance apart.
Another design possibility is that the distances of the curvature midpoint of a curved line section be different from the side lines connected by them, i.e., that the respective curvature midpoint not lie on the median line between the two side lines contiguous to one another.
Basically, the hold of the shaft in the bone is improved the greater the contact surface between the shaft and the bone is. This contact surface can be enlarged by providing—seen in cross section—in the area of at least one side line, an additional curved line section, whose radius of curvature is not smaller than the minimum radius of curvature.


REFERENCES:
patent: 5549705 (1996-08-01), Michielli et al.
patent: 8213101 (1982-08-01), None
patent: 8318360 (1985-09-01), None
patent: 3822153 (1989-03-01), None
patent: 3740438 (1989-06-01), None

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