Hip-joint endoprosthesis

Details Hip-joint endoprosthesis Hip-joint endoprosthesis

1999-09-23

2001-05-01

Philogene, Pedro (Department: 3732)

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

Implantable prosthesis

Bone

C623S023150, C623S023360

Reexamination Certificate

active

06224634

ABSTRACT:

BACKGROUND OF THE INVENTION
In the early years of prosthetic surgery of the hip-joint, attempts were made to replace only the joint head with an implant held by a straight stem, which was held by the femoral neck and the spongy substance of the adjoining epiphysis of the femur. It was found that this arrangement was inadequate for transmitting the forces to the femur. Hip prostheses are therfore mostly used whose stem reaches into the diaphysis of the bone (FR-A-2,636,837, U.S. Pat. No. 5,413,610, GB-A-2,069,340; EP-A-477,113, FR-A-2,706,283, EP-A-711,534, FR-A-2,629,707, FR-A-2,429,010). To obtain sufficient access for this, the femoral neck is completely resected. In recent years, hip prostheses have been proposed which allow the femoral neck to be preserved. The resection is carried out immediately below the joint head (EP-A-666,069, col. 2, l. 47; EP-A-579,868, col. 8, l. 51; WO 87/00033, p. 6, col. 19). A collar-like neck bearing is provided which is supported on the resection surface of the femoral neck. The distance of the support surface of the neck bearing from the center of the joint head of the prosthesis is unchangeable because variations are ruled out as a result of the resection immediately below the joint head. By contrast, in the case of conventionally implanted hip prostheses, it is known (FR-A-2,429,010, FR-A-2,629,707, U.S. Pat. No. 5,413,610, FR-A-2,636,837) to keep-in stock different neck lengths of the prosthesis for the purpose of compensating for different neck lengths of the bone that is to be replaced.
SUMMARY OF THE INVENTION
The invention starts from that type of prosthesis which seeks to preserve the natural femoral neck (EP-A-666 069). The object of the invention is to achieve greater versatility of use, which, if appropriate, can be decided upon even during the operation.
The solution according to the invention lies in the features disclosed herein. In a hip-joint endoprosthesis (hereinafter hip-joint prosthesis) with a stem which is to be anchored in the femur, is curved substantially uniformly in a circular arc and has a length of not more than 150 mm, and with a neck which adjoins the upper end of the stem approximately in the same direction and whose length is smaller than the length of an average natural femoral neck and supports a joint head, a stem shape is provided whose geometric conditions in the projection on the LM plane (lateral-medial plane) are:
the medial contour in the proximal section follows a circular arc with radius K·7,2 and in the distal section follows a contiguous circular arc with radius K·27;
the lateral contour in the proximal section follows a circular arc with radius K·12,4 and in the distal section follows a contiguous circular arc with radius K·21;
the circular arcs determining the two contours have, at the crossover point from the proximal to the distal section, which is the same for the lateral and medial contour, a distance of 1.9 cm;
the radii of the lateral and medial circular arcs form an angle of 3.8° to one another at the crossover point;
K is a constant of between 0.8 and 1.1 cm uniform for each prosthesis;
the deviations of the actual contours from the said circular arcs are not greater than 3 mm.
The contours of different prosthesis sizes are linked to one another by the scaling factor K.
This combination of features allows the physician, depending on the conditions he finds during the operation, to preserve the femoral neck in its entirety, partially, or not at all. Depending on the circumstances of the case, he implants the prosthesis according to the invention at a greater or lesser depth and is thus able to find the best possible compromise between preserving as much bone material as possible, on the one hand, and creating a stable prosthetic support, on the other hand. Especially in the case of younger patients, it is important, during the first prosthesis implantation, to preserve as much bone substance as possible with a view to an anticipated second prosthesis fitting or even a third prosthesis fitting.
By virtue of the said features, it is possible, depending on the position of the resection plane, for the stem to be inserted parallel to itself on a correspondingly curved path through the femoral neck into the adjoining part of the femur.
Although the tip of the stem can project as far as the diaphysis, according to the invention this is possible only over a limited length; nevertheless, the prosthesis stem reaches into that area of the femur which ensures good support by virtue of the thickness of its cortical bone. The stem length is preferably still considerably smaller than 150 mm, namely of the order of 100 to 140 mm, more preferably up to 130 mm.
The length is to be measured from the center of the upper end of the stem (below the neck bearing, if present) in a straight line as far as the tip of the stem. The depth at which the prosthesis sits in the bone thus depends on the degree of resection of the femoral neck. To ensure that the joint head nevertheless sits at the desired location, independently of the degree of resection of the femoral neck, different neck lengths are provided according to the invention. The available neck lengths are for that reason shorter than the average natural femoral neck length, since in general at least part of the femoral neck length is left in place.
While the known prostheses all require a predetermined resection plane, namely either directly below the joint head or at the distal end of the femoral neck, this is not the case with the prosthesis according to the invention. Instead, the uniform curvature of the stem has the effect that it can be implanted at different depths, with the different position of the resection plane in relation to the joint head being compensated for by the choice of a suitable neck length. This is most easily done by equipping the neck with a plug cone for connection to the joint head and by making available joint heads which have different attachment lengths.
It can be expedient for the stem in the projection on the plane including the lateral and medial contours to lie between two concentric circles whose spacing is equal to the greatest thickness of the stem.
In the view from the lateral direction, the contour of the stem follows the course of the bone. It is also particularly expedient to accurately simulate the anteversion of the neck and to provide correspondingly different stem configurations for the two sides of the body.
The stem preferably has a multiplicity of deep grooves exclusively on its front and its rear, the course of the grooves being parallel to the curvature of the stem. These grooves take up bone material and thus contribute to supporting the stem in cases of cementless implantation. By virtue of their course being parallel to the curvature of the stem, they assist, by means of their guiding properties, in the correct introduction of the prosthesis into the bone upon a curved trajectory parallel to themselves.
The lower section of the prosthesis stem, which takes up approximately a third of its overall length, can have a smaller curvature than the remaining upper part of the stem. This curvature can be smaller, the more the thickness of the stem decreases towards its lower end.


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patent: 0 477 113A1 (1992-03-01), None
patent: 0 579 868 A2 (1994-01-01), None
patent: 0 666 069 A1 (1995-08-01), None
patent: 0 711 534 A1 (1996-05-01), None
patent: 2 429010 (1980-01-01), None
patent: 2 629 707 (1989-10-01), None
patent: 2 636 837 (1990-03-01), None
patent: 2 706 283 (1994-12-01), None
patent: 2 069 340 (1981-08-01), None

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