Joint prosthesis

Joint prosthesis (1) with a cup-shaped supporting part (2) in whose interior a ceramic insert (3) is fastened by conical clamping. Clamping surface (6) of insert (3) runs at an angle (.alpha.) to clamping surface (7) of supporting part (2).

BACKGROUND OF THE INVENTION 
The present invention relates to a joint prosthesis with a cup-shaped 
supporting part in which a ceramic insert is secured by a conical clamp. 
Joint prostheses are known that have a metallic supporting part and an 
insert contained therein. The metallic supporting part is anchored in the 
bone of the patient. The insert forms a wear-resistant sliding surface 
against which the joint ball of the bone or another joint prosthesis 
presses. In so-called hard-sliding pairs, at least one part of the joint 
connection, the insert for example, is made of metal. The fastening of the 
metal insert in the supporting part which likewise consists of metal is 
performed with the aid of a polyethylene coupling. The polyethylene 
coupling is subjected to wear by microrelative movements. This wear is not 
so serious as frictional wear but the seat of the insert is loosened and 
abrasion enters the tissue and acetabulum. 
U.S. Pat. No. 5,282,864 describes a joint prosthesis in which a metal 
insert is anchored in a metal supporting part by screws. The insert 
consists of a cobalt-chromium alloy while the supporting part is made of a 
titanium alloy. As a result of this design for the joint prosthesis, there 
is a risk of frictional corrosion and galvanic corrosion. During 
corrosion, the metals give off toxic ions that can harm the patient. 
To overcome these disadvantages, ceramic inserts are used in the metallic 
supporting parts. Since ceramic materials can be machined only with 
difficulty, they are usually fastened in the supporting part by conical 
clamping. An example of this is described in EP 0 649 641 A2. 
During conical clamping, considerable tensile stresses develop in certain 
zones of the insert because the initiation of force cannot be controlled. 
Although ceramic materials are very hard and resistant to compression, 
they can accept tensile forces to only a limited degree. This means that 
thin-walled ceramic inserts in particular cannot be made for technical 
reasons. 
SUMMARY OF THE INVENTION 
The invention is based on the goal of designing the connection between the 
supporting part and the insert in such fashion that a reliable, 
reproducible connection is produced and the insert can also be prevented 
from coming loose. 
This goal is achieved according to the present invention. 
In the joint prosthesis according to the invention, the ceramic insert is 
fastened by conical clamping in the cup-shaped supporting part. The angle 
of the clamping surface of the insert is different from the angle of the 
clamping surface of the supporting part, so that the insert is connected 
to the supporting part over only a relatively small area. This permits a 
controlled transmission of force from the insert to the supporting part. 
The joint force that acts axially is applied to a large extent in 
compressive stresses that act circumferentially so that the development of 
tensile stresses during use is minimized. 
In one preferred embodiment of the invention, the connection between the 
supporting part and the insert is provided in the area of the opening side 
of the joint prosthesis. At the end that faces away from the opening side, 
the clamping surfaces do not touch one another. This has the advantage 
that the force to be accepted by the joint prosthesis is initiated in a 
certain area distributed uniformly around the circumference. 
The load-bearing surface of the supporting part can be roughened so that a 
roughness of 20 .mu.m is produced for example. In this way, irregularities 
in force transmission, resulting for example from variations in shape in 
the micro range, can be avoided. 
To improve the frictional properties, the clamping surface of the 
supporting part can be precision-turned or ground so that the surface has 
a roughness of 0 to 4 .mu.m. 
Preferably, the insert has a radius or a slight additional bevel at the end 
of the clamping surface that faces away from the opening side. As a 
result, tilting of the insert when it is inserted or when subjected to 
load is prevented. This can also be achieved by having a relief groove 
adjoin the end of the clamping surface of the supporting part that faces 
away from the opening side. 
As the insert wears, it must be replaced. In order to minimize the stress 
on the patient, the insert is removed in situ from the supporting part, 
while the supporting part remains in the bone. In order to be able to 
remove the insert easily, in a special design of the invention at least 
one recess is provided that extends from the opening side on the boundary 
surface between the insert and the supporting part, up to the ends of the 
two clamping surfaces away from the opening side. At the inner end of the 
recess that faces away from the opening side there is a tilting lever that 
fits behind the clamping surface of the insert. By means of a tool, such 
as a wedge for example, the end of the tilting lever that is in the recess 
can be pressed against the insert, and the latter removed from the 
supporting part.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a joint prosthesis 1 consisting of a cup-shaped metallic 
supporting part 2 and a ceramic insert 3 that forms the artificial 
acetabulum. Insert 3 is located in an interior chamber 4 of supporting 
part 2. The inner surface of insert 3 is designed as a spherical cup that 
receives the matching part of the joint on opening side 5. Insert 3 is 
secured in supporting part 2 by conical clamping. 
Clamping surface 6 of insert 3 runs at an angle .alpha. to clamping surface 
7 of supporting part 2 (FIG. 2). The angle at which insert clamping 
surface 6 runs to joint axis 8 of the joint prosthesis is larger than the 
angle of the supporting part clamping surface 7. 
This means that insert 3 is pressed against supporting part 2 over only a 
relatively small contact area 9 that is located on opening side 5. The 
size of contact area 9 can be modified by changing angle .alpha.. The 
smaller the value of angle .alpha., the larger contact area 9 will be. 
Angle .alpha. can also be negative. In this case, contact area 9 is 
located in the lower area of clamping surfaces 6 and 7. The gap between 
the two contact surfaces 6 and 7 therefore opens to opening side 5. 
Clamping surface 7 of supporting part 2 has a roughness of about 20 .mu.m. 
As a result, compensation is provided for variations in the shape of 
insert 3. 
In FIGS. 3 and 4, clamping surfaces 6 and 7 are shown parallel to one 
another and not roughened, in order to improve clarity. In fact, however, 
the connection between insert 3 and supporting part 2 is as shown in FIG. 
2. In FIG. 3 a curved surface 10 with radius R abuts the lower end area of 
clamping surface 6 of insert 3. When insert 3 is introduced into 
supporting part 2, this rounding of the lower edge of clamping surface 6 
avoids tilting of insert 3 in supporting part 2. This can also be achieved 
by providing a relief groove 11 in supporting part 2 which is located in 
the vicinity of the lower end of clamping surface 6 of supporting part 3 
(FIG. 4). 
FIG. 5 shows another embodiment of joint prosthesis 1. Supporting part 2 
has an elongated recess 12 that is located in the boundary area between 
supporting part 2 and insert 3 and runs parallel to clamping surface 6 of 
insert 3. Recess 12 is open to opening side 5 of joint prosthesis 1. At 
the lower end of the recess is a tilting lever 13 consisting of two legs 
13a and 13b that are arranged with respect to one another at a shallow 
angle. Leg 13a fits behind clamping surface 6 of insert 13 and is clamped 
between insert 3 and supporting part 2. Second leg 13b extends into recess 
12. The angle between the two legs 13a and 13b is made such that second 
leg 13b runs approximately parallel to recess 12. 
FIG. 6 shows how tilting lever 13 is activated with the aid of a 
wedge-shaped tool 14. Wedge 14 is pushed into recess 12 until the tip of 
the wedge is between supporting part 2 and second leg 13b of tilting lever 
13. Applying additional pressure to wedge 14 presses second leg 13b in the 
direction of insert 3. This also causes first leg 13a to move, pushing 
insert 3 out of supporting part 2.