Artificial hip joint.

The invention relates to an artificial hip joint to replace the natural joint head and / or the natural joint socket, the artificial joint surfaces being designed as rotational surfaces.

It is known to produce artificial hip joints with joint surfaces designed essentially as spherical surfaces both in the joint socket and in the joint head. Although such hip joints were a good substitute for pathologically altered hip joints or those that were bad from birth, there were always walking difficulties and disabilities of the patients after artificial hip joints were implanted.

The object of the invention is to remedy or at least improve the prevailing condition in the field of artificial hip joints. This is achieved if, according to the invention, the joint surface of the joint socket or the joint surface of the joint head is designed as a Pascal curve (worm) in the meridian section. This configuration ensures that a restoring torque occurs with every deviation between the axes of rotation of the joint socket and the joint head, so that any indifferent equilibrium position is avoided, if one disregards a single position in which the axes of the joint socket and the joint head are aligned. The patient, who is provided with an artificial hip joint, can move the joint of the new configuration better than if - as before - each position is practically an indifferent equilibrium position.

It is preferred to design the meridian section of the joint surface of the joint socket as an elongated Pascal curve. The meridian section of the articular surface of the head is then preferably a Pascal curve with a loop.

A further embodiment of the hip joint according to the invention is characterized in that the meridian section of the joint surface of the joint socket and the meridian section of the joint surface of the joint head are Pascal curves with interchanged parameters, i.e. the meridian section of the femoral head follows the relationship r = a + b cos φ, so follows the meridian section of the acetabulum of the relationship r ′ = b + a cos φ, where r or r ′ and φ are the polar coordinates of a point of the meridian section of the femoral head or acetabulum and φ is the angle between the axis of symmetry of the Pascal curve and the radius vector in a point of the curve, and a and b are the specified parameters.

The ratio of the two parameters <maths id="math0001" num=""><math display="inline"><mrow><mfrac><mrow><mtext>a</mtext></mrow><mrow><mtext>b</mtext></mrow></mfrac></mrow></math><img file="EP0302850A2_D0001.tif" /></maths> is preferably carried out &lt;1. Characteristic values ​​for the parameters are 18.75 mm for parameter a and 21.5 mm for parameter b.

Another invention provides that there is an angular deviation between the axis of the femoral neck and the axis of rotation of the joint head (seen in plan view), which is preferably 10.7 ° +/- 3 °. This configuration ensures that dead positions between the pelvis and thigh are avoided and a restoring moment always acts on the thigh. The inventive design of the artificial hip joint ensures that the axis of rotation of the joint head runs through the axis of the femur shaft, but the axis of the femoral neck runs past the axis of the thigh shaft (ventrally). Although this design according to the invention is particularly expediently used in artificial hip joints in which the joint surface of the joint socket or the joint surface of the joint head in the meridian section is formed as a Pascal curve (snail), in particular with the above-mentioned features, it can also be used with the advantage of avoiding dead positions between the pelvis and thigh in artificial hip joints, in which joint head and / or joint socket are designed in a conventional manner (spherical).

The invention is explained below with reference to the drawing, for example. Show it,<ul id="ul0001" list-style="none"><li>1 shows an axial section through an inventive hip joint consisting of a joint head and an acetabular cup,</li><li>Fig. 2 is a graph for explaining the mathematical relationship, and</li><li>Fig. 3 is a plan view corresponding to Fig. 1 with a cut joint socket.</li></ul>

In the drawing, 1 denotes the joint head and 2 the socket. The joint head 1 and the socket 2 are designed as rotating bodies or surfaces, both the joint surface 3 of the socket 2 and the joint surface 4 of the joint head 1 having a meridian cut formed by a Pascal curve (Pascal screw). The meridian section of the joint surface 4 of the joint head 1 is designed as a Pascal curve with a loop, as can be seen from FIG. 1. Of course, only that part of the Pascal curve is used that is arranged above the fillet circle 5, ie a circle that is perpendicular to the axis of rotation and runs through the colon of the Pascal curve. The meridian section of the articular surface 3 of the socket 2 is an elongated Pascal curve, ie a curve which has a dent on the axis of symmetry. The meridian section of the joint surface 3 of the socket 2 and the meridian section of the joint surface 4 of the joint head 1 are Pascal curves with parameters interchanged.

2 shows a Pascal snail with a loop for a more detailed illustration. The points of this Pascal curve follow the equation r = a + b cos φ in polar coordinates.

If you put a ζ / η coordinate system through the Pascal curve such that the η axis coincides with the symmetry axis of the Pascal curve and the origin of the coordinate system coincides with the colon of the Pascal curve, then the parameters a and a + b are defined as axis sections of the Pascal curve, the angle φ being measured as the angle of the respective radius vector r against the axis of symmetry (η axis).

If the meridian section of the acetabulum 2 now follows the relationship r ′ = b + a cos.φ, the meridian section of the femoral head follows the relationship r = a + b cos.φ. The result is that both the meridian section of the acetabulum 2 and the meridian section of the femoral head 1 have the same section size, namely a + b, on the axis of symmetry. On the axis perpendicular thereto, the meridian section of the femoral head 1 has the axis section a, but the meridian section of the hip socket 2 has the axis section b. The choice is made so that b&gt; a. Preferred values ​​are 18.75 mm for a and 21.5 mm for b.

In the illustrated embodiment, the socket 2 consists of plastic. Several grooves 6 are arranged in the circumferential direction perpendicular to the axis of rotation for the purpose of anchoring the joint socket in the pelvis by means of a corresponding cement (plastic adhesive). The joint head 1 is shown in the illustrated embodiment as a piece with a neck 7 and the shaft 8, which is implanted in the thigh bone (femur). However, it is also possible to push the joint head 1 onto a pin, the axis of which coincides with the axis of rotation of the joint head 1 and consists of one piece with the shaft 7. In such a case, the joint head 1 can also consist of plastic.

3, there is an angular deviation between the axis 11 of the swivel neck and the axis of rotation 10 of the joint head 1, which lies in one plane with the axis 9 of the shaft 8 of the thigh or the prosthesis. This deviation is preferably 10.7 ° +/- 3 °. The medullary canal 12 of the femur in which the shaft 8 is inserted is shown in dash-dot lines in FIG. 3. The axis of rotation 10 of the joint head forms an angle of approximately 126 ° with the axis 9 of the shaft 8. In plan view (FIG. 3), the axis of rotation 10 of the joint head 1 encloses an angle of approximately 34 ° with the rear femoral condylar axis 14. The angular positions of the axes 10 and 11 (FIG. 3) or 9 and 10 (FIG. 1) which can be seen in the drawings can also be used in the case of artificial hip joints whose joint head or joint socket in a conventional manner (for example spherical) are formed.