The metal male joint part has a taper which is less than that of the cavity of the oxide ceramic female joint part. In addition, the surface of the male part has a resistance to deformation which is less than that of the core of the male part. The cone angle .beta. of the female cavity and the cone angle .alpha. of the male joint part are in the relation: EQU tan .beta./2 - 0.001 .ltoreq. tan .alpha./2 .ltoreq. tan .beta./2 - 0.005.

This invention relates to a joint endoprosthesis and particularly to a hip 
joint endoprosthesis. 
As in known, prosthesis such as hip joint endoprostheses have frequently 
been made of two parts, i.e. a female part and a male part. In some cases, 
such as described in German Offenlegungsschrift No. 2,134,316, the female 
joint part which functions as a joint head has been made of an oxide 
ceramic material while the male part which functions as an anchoring shank 
is made of metal. In such cases, the oxide-ceramic joint head is secured 
by a plastic adhesive on a cylindrical pin of the metal anchoring shank. 
However, it has been found that these adhesives are not resistant to the 
body and are reabsorbed in the course of time so that the connections work 
loose. As a result, a frictional movement can occur between the ceramic 
joint head and the metal pin which, in view of the extreme hardness of the 
ceramics, may result in a correspondingly high metal abrasion. Further, 
there is often a risk of toxicity with these adhesives which is not 
tolerated by the body. 
It has therefore been proposed in such a ceramic/metal composite structure 
wherein the two joint parts consist of very different materials relative 
to their properties, to interconnect the parts in a self-locking manner 
with a conical plug-in connection. However, it has been found that this 
plug-in connection may, under certain extreme loading, have the risk that 
the joint head will tear out of the relatively brittle ceramic material 
and break. Such an extreme loading may occur, for example, due to 
different thermal expansions of the ceramic and metal when the prosthesis 
undergoes a high-temperature sterilization which is, of course, carried 
out at a temperature of about 135.degree. C. Very high mechanical stresses 
may also occur, for example, in the event of sudden impacts such as may 
occur on landing after a jump. 
Accordingly, it is an object of the invention to provide a prosthesis of 
composite construction which is able to resist high loads without 
breaking. 
It is another object of the invention to provide a connection between an 
oxide ceramic joint part and a metal joint part of a joint prosthesis 
which is elastically and/or plastically deformable. 
It is another object of the invention to provide a plug-in connection for a 
composite structure of a joint endoprosthesis in which connection elastic 
and/or plastic deformation of the metal joint part prevents any breaking 
of the ceramic part as far as possible without having an adverse effect on 
the other mechanical properties of the metal anchorage part. 
Briefly, the invention provides a joint endoprosthesis which comprises a 
female joint part of oxide ceramic material having a cone shaped cavity 
with a cone angle .beta. and a male joint part of metal material and a 
cone angle .alpha. secured in the female joint part in self-locking 
relation. The male joint part has a cone with a surface layer which is 
disposed on a taper smaller than the taper of the cavity. In addition, the 
surface layer has a resistance of deformation less than the resistance to 
deformation of the core. This resistance of the core of the male joint 
part denotes the resistance to elastic and/or plastic deformations that a 
male joint part has which is made of the same material but with a smooth 
surface and/or homogeneous material properties over the entire 
cross-section. 
The construction of the connection ensures that the contact point of the 
female part always starts at the tapering end and, on any plastic 
deformation of the surface of the male part, progresses step-by-step 
towards the wider open sections of the joint parts. This prevents a 
premature contact or connection at the wide ends of the two parts such as 
would be particularly likely to cause the joint head to break because of 
the relatively long lever arm of a load acting thereon. 
The increased deformability of just the surface layer of the metal male 
part facilitates the flexibility of this surface layer without having an 
adverse effect on the mechanical properties of the metal part as a whole. 
This effect can be improved if the thickness of the surface layer having a 
lower resistance deformation decreases in a direction away from the end of 
the male part. This decrease is advantageously at an angle .gamma. which 
is equal to or greater than the cone angle .beta. of the female cavity. As 
a result of this, the resistance to elastic and/or plastic deformation 
increases the more the joint head is pressed onto the metal male part. The 
deformation of the surface of the metal part will, for example, be mainly 
of an elastic nature under sterilization stresses, and plastic in the 
event of mechanical loads in the form of impacts. 
Advantageously, the following inequalities apply between the individual 
angles .alpha., .beta. and .gamma., according to the invention: 
EQU tan .beta./2 - 0.001 .ltoreq. tan .alpha./2 .ltoreq. tan .beta./2 - 0.005 
and .beta. .ltoreq. .gamma. .ltoreq. 1.15 .beta. 
The required surface layer may, for example, consist of grooves in the form 
of a screwthreading which decrease in depth and between which are formed 
circumferentially extending projections whose contact width increases 
continuously from the tapering end of the male part. Alternatively, this 
layer may be obtained, for example, by a heat treatment, a porous surface 
region, or a coating which is soft and flexible in relation to the core of 
the metal joint part and which consists of a suitable metal alloy whose 
thickness decreases from the tapering end. 
Finally, the elasticity of the metal joint part as a whole can be further 
improved by making the part as a hollow body. 
In addition, it has been found particularly advantageous for the surface of 
the ceramic female part to have a corresponding structure in order to give 
an even better connection to the metal male part. To this end, the surface 
roughness of the female part has an arithmetic mean roughness Ra (in 
accordance with DIN 4760 and VSM 10321) of between 0.5 and 3 microns. This 
effect can be increased by using additionally ground circumferential 
grooves of a corrugation length preferably between 1 and 3 millimeters and 
of a depth of 0.02 millimeters. This gives an optimum bearing pattern for 
the surface. The term "bearing pattern" is used to denote the percentage 
of the conical surfaces in which the two parts are in contact with and 
bear on one another. This proportion can be measured as an area after the 
parts have been disconnected because the surfaces of the two parts are 
changed in an optically detectable manner in the bearing zone. 
These and other objects and advantages of the invention will become more 
apparent from the following detailed description and appended claims taken 
in conjunction with the accompanying drawing in which:

As shown, a hip joint prosthesis has a female joint part 2 which forms a 
joint head 2 and consists of a known bioceramic material. The joint part 2 
has a cone shaped cavity 1 disposed on a cone angle .beta. such that the 
taper of the female cavity is of from 1:20 to 1:10 and, as shown, 0.100 at 
which the angle .beta. is about 5.degree. 43'. 
A male joint part 3 is inserted in the female cavity 1 and is in the form 
of a pin. The male part 3 consists of one of the known metals used for 
implants, or a metal alloy, and is preferably made as a casting or forging 
in the form of a hollow member. This male part 3 has a core and a surface 
layer 5. The surface layer 5 tapers on a cone angle .alpha. which is 
chosen in accordance with the above-mentioned inequalities and is shown on 
a highly exaggerated scale in the drawing to be smaller than the angle 
.beta.. 
The surface 5 of the male part 3 is of a thickness which decreases 
continuously from the tapering end of the male part 3 at an angle .gamma. 
which again is selected in accordance with the above-mentioned 
relationship and, in this example, is at least approximately equal to the 
cone angle .beta. of the female cavity 1. As shown, the male surface 5 
consists of grooves 4 of decreasing depth, which are cut, for example, as 
screwthreads in the male part, their V-shaped cross-section narrowing with 
the decreasing depth so that the screwthread projections 6 left between 
the grooves 4 have a contact width which increases as the depth decreases. 
The effect of the decreasing groove cross-section and simultaneous 
increasing contact width is that the surface structure has an elastic and 
plastic deformability which decreases progressively from the tapering end 
of the surface 5. 
The dimensions of this structure are generally some hundredths of a 
millimeter. For example, in the region of the tapering end, the depth and 
the distance between two adjacent grooves 4 are 0.05 millimeters (mm) and 
their width directly at the surface is about 0.03 millimeters (mm) so that 
in this region the projections 6 have contact widths of about 0.02 
millimeters (mm). 
In order to produce the structure described, it is possible, for example to 
first form the metal male part 3 as a blank with an outer cone angle equal 
to the angle.gamma., cut the screwthread in this blank and then bring the 
cone angle to the required value.alpha. by a metal-removing treatment 
increasing from the tapering end. Optical means can be used, for example, 
to ensure that the required tolerances are maintained in producing the 
surface structure described. 
The two cones for the cavity 1 and surface layer 5 are produced by 
machining the blanks, the tolerances being checked, for example, in each 
case by means of a set of calibrated gauges which are slideable axially in 
relation to one another on and in the cone and by means of which the 
relatively close tolerances of the angles of the cones are converted into 
differential measurements of these axial longitudinal displacements which 
can be checked with satisfactory measurability and reproducibility owing 
to the relatively long taper of the cones. 
The surface of the cavity 1 of the female part 2 may be roughened to 
improve the connection with the male part 3. To this end, the surface of 
the female cavity may be roughened with an arithmetic mean roughness value 
between 0.5 and 3 microns. Additionally for this purpose, the surface may 
have a plurality of corrugated circumferential grooves of a corrugation 
length between 1 and 3 millimeters and a depth of 0.02 millimeters.