Artificial hip joint.

(42) Date of commencement of the patent: 15. 4.1988 (45) Date of issue: 10.11.1988 (73) Patent owner:

MENSCHIK ALFRED DR.

A-3400 KLOSTERNEUBURG, LOWER AUSTRIA (AT).

(54) ARTIFICIAL HIP JOINT (57) The invention relates to an artificial hip joint in which the joint surfaces of both the joint socket and the joint head are designed as rotation bodies (rotation surfaces) deviating from the spherical shape.

CQ

AT 386 948

DUR 0078018

Nr.386948

The invention relates to an artificial hip joint for replacing the natural joint head and / or the natural joint socket, the artificial joint surfaces being designed as surfaces of revolution.

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 have provided a good substitute for morbidly altered or from birth to poor hip joints, it came again and again after implantation of artificial hip joints to walking difficulties and disabilities of the patients.

The invention has for its object to remedy this situation or at least to 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 in meridian section as Pascal'sehe curve (worm) is formed. By this configuration it is achieved that at each deviation between the axes of rotation of the joint socket and the joint head a restoring moment occurs, so that any indifferent equilibrium position, apart from a single position in which the axes of the joint socket and the joint head are aligned, is avoided. The patient with an artificial hip joint can thus move the joint of the new design better than when - as before - each layer is practically an indifferent equilibrium position.

It is preferred to form the meridian section of the joint surface of the joint socket as a stretched Pascal's 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 cup and the meridian section of the joint surface of the joint head are Pascal curves with mutually reversed parameters, ie follows the meridian section of the femoral head of the relationship r = a + b cos4>, follows the meridian section of the acetabulum of the relationship r '= b + a cos. $, where r resp. r 'and Φ are the polar coordinates of a point of the meridian section of the femoral head and hip socket, respectively, and Φ represents the angle between the axis of symmetry of the Pascal curve and the radius vector at a point of the curve and a and b are the predetermined parameters.

The ratio of the two parameters - is preferably carried out <1. characteristic

Values ​​for the parameters are 18.75 mm for the parameter a and 21.5 mm for the parameter b.

The invention will be explained in more detail below with reference to the drawings, for example. FIG. 1 shows an axial section through a hip joint consisting of joint head and joint socket, designed according to the invention, and FIG. 2 shows a diagram for explaining the mathematical relationship.

In the drawings, -1- the joint head and -2- denotes the pan. Joint head -1- and joint cup -2- are formed as a rotating body or rotating surfaces, wherein both the joint surface -3- of the joint cup -2- and the joint surface -4- of the joint head -1- one of a Pascal curve (Pascal screw) formed meridian section have. The meridian section of the joint surface -4- of the joint head -1- is formed as Pascal'sehe curve with loop, as can be seen from Fig.l. Of course, only that part of the Pascal curve is used, which is located above the throat circle -5-, ie a circle which is perpendicular to the axis of rotation and passes through the colon of the Pascal curve. The meridian section of the articular surface -3- of the joint cup -2- is a stretched Pascal's curve, ie a curve that has a lying on the symmetry axis indentation. The meridian section of the joint surface -3- of the joint cup -2- and the meridian section of the joint surface -4- of the joint head -1- are Pascal curves with mutually exchanged parameters.

In Fig.2 is shown for closer illustration a Pascal screw with noose.

In polar coordinates the points of this Pascal curve follow the equation r = a + b cos Φ.

Inserting through the Pascal curve a ξ / η coordinate system such that the η-axis coincides with the symmetry axis of the Pascal curve and the origin of the coordinate 3

Nr.386948

System coincides with the colon of the Pascal curve, the parameters a and a + b are defined as intercepts of the Pascal curve, with the angle φ as

Angle of the respective radius vector r is measured against the symmetry axis (η-axis).

If the meridian section of the acetabulum follows the relationship r '= b + a cos. $, 5 then the meridian section of the femoral head follows the relation r = a + b οοε.ψ. It can be seen that both the meridian section of the acetabulum -2- and the meridian section of the femoral head -1- on the axis of symmetry have the same section size, namely a + b. On the axis perpendicular to it, the meridian section of the femoral head -1- has the axis section -a-, the meridian section of the acetabulum -2- but the intercept -b-.

The choice is such that b> a. Preferred values ​​for this are a for 18.75 mm and for b 21.5 mm.

In the illustrated embodiment, the joint socket -2- made of plastic. On the outer casing are in the circumferential direction perpendicular to the axis of rotation several grooves -6- for the purpose of anchoring the Gelenkpan in the basin by means of a corresponding cement (Kunststoffkle15 over) arranged. The head of the joint -1- is shown in the illustrated embodiment as a piece with neck -7- and the shaft -8-, which is implanted in the femur (femur). However, it is also possible, the hinge head -1- aufzustecken on a pin whose axis coincides with the axis of rotation of the joint head -1- and consists of one piece with the shaft -7-. In such a case, then the joint head -1- 20 also consist of plastic. The shaft axis is denoted by -9- and the axis of rotation of the joint head -1- and the pan -2- with -10-. The axis of rotation of pan -2- and joint head -1- fall together in the position shown.