The conical pin end of the male endoprosthesis part is provided with a deformable surface while the conically recessed articular ball is provided with inwardly directed lugs for penetrating into the deformable surface during mounting on the pin end. The lugs have a maximum height of 0.5 millimeters and a maximum width of three millimeters. The deformable pin surface may be formed by a screw thread, a plurality of circumferential grooves of wavy configuration or a plurality of circumferential ribs.

This invention relates to a joint prosthesis and, more particularly, to a 
joint endoprosthesis for a femur head. 
As is known, endoprostheses have frequently been made with an articular 
ball, for example for a femur head prosthesis, joined to an anchoring 
shank by means of a cone plug connection. For example, as described in 
German O.S. No. 2548 077 it has been known to mount an articular ball onto 
a conical pin of an anchoring shank in a self-locking manner. However, in 
the case of relatively large articular balls which are subject to 
increased torsion moments, for example as used for the so-called fracture 
prosthesis, undesirable relative rotations will often occur between the 
articular ball and the pin after implantation. 
It has also been known from Swiss Pat. No. 507,704 and German PS No. 22 20 
304 to use a conical plug connection to fasten an articular ball on a pin 
where the pin axis does not coincide with the axis of symmetry of the 
articular ball. This permits the position of the articular ball relative 
to the femur axis to be approximated to the individual differences in the 
skeletal build of the various patients. However, for such prosthesis 
having an "oblique" position of the two axes, the articular ball must be 
prevented from becoming twisted relative to the pin during the "life" of 
the prosthesis. For this reason, it has been known to provide the base of 
the pin with an anti-twist construction, for example in the form of ribs, 
lugs, teeth or the like to engage in matching radial grooves in the 
surface of a female conical surface in the articular ball. However, apart 
from the fact that the making of the grooves in the conical female surface 
requires much additional cost, the anti-twist constructions have the 
disadvantage that they do not permit a continuous adjustment of the 
articular ball relative to the pin. Instead, these constructions only 
permit a relative rotation corresponding to the number of grooves in order 
to fix the ball on the pin. Hence, the operating surgeon must consider the 
specific angular position between the articular ball and pin during an 
operation. 
Accordingly, it is an object of the invention to provide a means by which 
an articular ball can be fastened on a pin by an operating surgeon without 
consideration of the specific angular position between the pin and 
articular ball. 
It is another object of the invention to be able to continuously adjust the 
position of an articular ball to a pin on which the ball is to be mounted 
during an operation. 
It is another object of the invention to provide a joint endoprosthesis 
which can be manufactured in a simple manner to protect against relative 
twisting of the joints parts on each other. 
Briefly, the invention provides a joint endoprosthesis which is comprised 
of a first part having a conical end with a deformable surface and a 
second part which includes a conical recess receiving the conical end of 
the first part. In addition, the second part is provided with at least one 
inwardly projecting lug at an inner end of the conical recess which is 
able to penetrate into the deformable surface of the first part in order 
to prevent a relative turning between the two parts. 
The joint prosthesis may be constructed, for example as a femur head 
prosthesis. To this end, the first part of the joint includes an anchoring 
shank at one end for implanting in a bone while the conical end extends 
from an opposite end in the form of a pin. The second part is constructed 
in the form of an articular ball having an internal conical recess for 
mounting on the conical pin. 
As the articular ball is placed on and driven in, or pressed in, the lug 
which may be in the form of a spike or a cylindrical or spherical 
projection penetrates into the deformable surface on the pin and thus 
brings about a desired antitwist protection. As the possible shearing 
forces that might cause a twisting of the ball are relatively small, a 
single lug, in principle, should be sufficient to impart the anti-twist 
protection. Of course, several lugs may be distributed over the 
circumference of the conical recess of the articular ball in order to 
protect against larger shearing forces. 
Since the conical pin of the first part has a uniform surface about the 
entire circumference without any privileged areas, the recessed second 
part, i.e. the articular ball, can be placed on and secured in any desired 
position of the lug relative to the circumference of the pin. Thus, the 
operating surgeon need not pay attention to a specific position of the 
ball relative to the pin. Further, for an "oblique" axes situation, a 
continuous adjustability exists. 
As described, the lug of the recessed joint part is able to penetrate into 
the structure of the surface of the pin. Naturally, the force required 
must not be excessive in order to avoid damage to either part. 
Accordingly, it is advantageous if the maximum height of the lug is 0.5 
millimeters with a maximum width of 3 millimeters. 
The anti-twist construction is applicable to all materials which are 
customarily used for the manufacture of a prosthesis, provided the 
material for the articular ball is less deformable than the surface 
regions of the pin. Naturally, the deformability of the surfaces of the 
pin may, if desired, be produced by the selection of a softer material for 
the conical pin. 
The deformable surface on the pin may be formed in various manners. For 
example, the deformable surface may be formed by a screw thread, by a 
plurality of circumferential grooves of wavy configuration or by a 
plurality of circumferential ribs. In addition, the surface may have 
circumferential recesses of decreasing depth in a direction towards the 
anchoring shank in order to implant greater resistance to deformation in 
that direction.

Referring to FIG. 1, the joint endoprosthesis includes a male part having 
an anchoring shank (shown in part) for implanting in a femur bone and a 
pin 1 which forms a conical end opposite the shank. In addition, the 
prosthesis has a female part in the form of an articular ball 2 which is 
secured on the pin 1 via a self-locking coical plug connection. As 
indicated, the ball 2 has a neck-type projection 5 which is made in one 
piece with the ball 2 and in which a conical recess 4 is formed for 
receiving the conical end of the pin 1. The ball 2 also has an undercut 6 
which is contiguous to the conical recess 4 and an internal cavity 7 which 
is spaced from the undercut 6. Of note, the undercut 6 is necessary for 
the production of the conical recess 4 while the cavity 7 serves to save 
weight or, in the case of a cast or poured articular ball 2, is necessary 
for the casting operation. 
The pin 1 is further provided with a deformable surface 3 which is 
sufficient to increase the deformability of the pin 1 relative to the ball 
2. In addition, the ball 2 is provided with a plurality of inwardly 
projecting lugs 8 at the tapered end of the recess 4. These lugs 8 are 
distributed over the periphery of the recess 4 and are formed so as to 
penetrate into the deformable surface 3 of the pin 1 upon pressing on or 
hammering on of the ball 2 onto the pin 1. These lugs thus serve to 
provide an anti-twist protection against a relative twisting between the 
ball 2 and the pin 1. 
Referring to FIG. 2, each lug 8 may have a maximum height of 0.5 
millimeters and a maximum width of 3 millimeters. 
Referring to FIG. 3, the surface 3 of the pin 1 may be formed by means of a 
screw thread. Alternatively, as shown in FIG. 4, the deformable surface 3 
may be formed by a plurality of circumferential grooves of wavy 
configuration or, as shown in FIG. 5, by a plurality of circumferential 
ribs. Advantageously, the deformable surface 3 may be formed of a 
plurality of circumferential recesses of decreasing depth in a direction 
towards the anchoring shank of the male part in order to impart greater 
resistance to deformation in this direction. Thus, the deformability of 
the conical end of the pin 1 may decrease inwardly so that the pin offers 
greater resistance to deformation as the articular ball 2 is moved onto 
the pin 1 a farther distance. 
Of note, the conicity of the recess 4, while being adapted to the conicity 
of the end of the pin 1, need not necessarily coincide with the coinicity 
of the pin 1. 
The invention thus provides an endoprosthesis with a relatively simple 
means for resisting relative rotation between an articular ball and a pin 
end of an anchoring shank.