Process for the production of ceramic prostheses

A process for the production of a ceramic prosthesis by forming a blank from a mixture of zirconium oxide and hafnium oxide of between 94.8% and 95.3% by weight, yttrium oxide of between 4% and 5.2% by weight, and any further oxide as a balance of less than 1% by weight, with a monoclinic proportion of below 5% by volume. The blank is put into the shape of the required prosthesis by means of a rotating tool of metallically bound diamond grains, using specific operating parameters for the tool.

BACKGROUND OF THE INVENTION 
Prostheses, whether in the form of endoprostheses or exoprostheses, can be 
produced from metallic and ceramic materials, using processing procedures 
which are specifically matched to those materials. The production of 
prostheses of a complicated configuration, with a high level of precision 
in terms of shape, dimensions and fit predominantly involves the use of 
metal in conjunction with machining procedures which are known from metal 
machining. Titanium with its alloys and chromium-cobalt steels have proved 
to be successful for metallic prostheses. The advantage of using metals of 
the specified kind is due to the fact that prostheses can be produced by 
machine with very close tolerances in regard to precision in respect of 
shape, dimensions and fit. However metals suffer from the disadvantage 
that their properties are not entirely satisfactory for endoprostheses and 
exoprostheses. Chromium-cobalt steels are distinguished by having high 
levels of strength but they are on the other hand susceptible to 
fluctuations in the pH-value of body fluids. The situation is precisely 
the reverse in the case of titanium alloys. 
While the use of ceramic materials for prostheses makes it possible to 
achieve high levels of strength and a high degree of chemical resistance, 
the production of ceramic pros theses on the other hand involves 
considerably greater difficulties than the production of metallic 
prostheses so that the use of ceramic materials has been generally limited 
to load-bearing prostheses of an uncomplicated shape, such as ball joints, 
without matching contouring operations using simple machining procedures 
after dense sintering, such as ball grinding and polishing. The area of 
high-precision ceramic prostheses which are of a complex three-dimensional 
shape and which are also shaped individually for a patient, for example 
finger, knee and vertebral prostheses, is largely closed to ceramic 
prostheses. That is due to the need for subjecting pros theses to an 
unavoidable dense sintering or infiltration operation so that the 
prostheses are chemically resistant, while enjoying high strength values, 
and also assume further properties which they do not enjoy in the porous 
and therefore non-sintered condition. Dense sintering or infiltration 
alters dimensions and contours when pre-produced in a porous condition, so 
that after the sintering operation the prosthesis has to be subjected to a 
further working operation in order to restore the prosthesis to the 
required precision in terms of shape and dimensions. That further working 
operation is a very difficult one due to the level of hardness of the 
prosthesis after the sintering or infiltration operation. Added to that is 
the fact that ceramic prosthesis materials are required to have properties 
which are not encountered in all ceramics but only a few thereof, in 
relatively narrow ranges of composition. Endoprostheses such as for 
example finger, knee and vertebral prostheses and the like of ceramic 
materials, and it is to endoprostheses of that kind that the invention is 
also directed besides comparable exoprostheses apart from those in the 
mouth region, can only be used as prostheses when their material is 
physiologically completely harmless, the ceramic therefore being bioinert 
and thus resistant to body fluids, and biocompatible. Further 
requirements, to prevent the absorption of body fluids, is good dense 
sinterability, and in the densely sintered condition a high level of 
strength and a high degree of resistance to abrasion wear. Ceramic 
materials have to meet those requirements overall, as otherwise they 
cannot be considered for ceramic prostheses in both of the areas referred 
to above. 
For larger load-bearing endo- and exoprostheses, for example ball joints, 
two ceramic materials have proven themselves to be suitable, more 
specifically aluminum oxide (Al.sub.2 O.sub.3) with an Al.sub.2 O.sub.3 
proportion of 99.85%, with the balance being other constituents, and 
zirconium oxide (ZrO.sub.2) of predominantly tetragonal structure, 
stabilized by magnesium oxide (MgO.sub.2) or by an oxide of the rare 
earths, preferably yttrium oxide (Y.sub.2 O.sub.3) or cerium oxide 
(CeO.sub.2). The above-mentioned ceramic materials are thought not to be 
suitable for small or very small prostheses of a complicated configuration 
and involving a very high level of accuracy in terms of dimensions and 
shape, as a result of the dense sintering operation with the subsequent 
machining difficulties resulting therefrom, so that metals dominate for 
production of prostheses of the above-indicated kind. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a process for the 
production of ceramic pros theses with which it is possible to produce 
ceramic prostheses involving the same variety in respect of contour and 
accuracy in respect of shape, dimensions and fit as metallic prostheses. 
Another object of the present invention is to provide a process for the 
production of ceramic prostheses which by virtue of the operating 
procedure involved can at least extensively satisfy the requirements made 
in respect of endo- and exoprostheses. 
Still another object of the present invention is to provide a process for 
the production of ceramic prostheses which involves using materials of 
specific compositions and working steps employing specific parameters to 
produce prostheses enjoying a high degree of accuracy. 
In accordance with the principles of the present invention the foregoing 
and other objects are attained by a process for the production of ceramic 
prostheses comprising forming or shaping a blank from, in percent by 
weight, zirconium oxide (ZrO.sub.2) and hafnium oxide (HfO.sub.2), of 
between about 94.8% and 95.3%, yttrium oxide (Y.sub.2 O.sub.3) of between 
about 4% and 5.2%, and further oxides as a balance of less than about 
0.1%, with a monoclinic proportion of below about 5% by volume. The blank 
is subjected to a working-over operation to form a prosthesis, by means of 
a rotating tool comprising metallically bound diamond grains, at a speed 
of rotation of between about 10,000 and 50,000 revolutions per minute, at 
infeed rates (that is to say towards the blank to remove material 
therefrom) of between about 0.1 and 0.7 millimeters per minute, feed 
rates, that is to say along the workpiece, of between about 0.3 and 3.0 
centimeters per second, and with surface speeds for the tool of between 
about 0.5 and 9.0 meters per second. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
It was surprisingly found that with the operating procedures of the process 
in accordance with the invention, it is possible to produce densely 
sintered ceramic prostheses, that is to say which in accordance with the 
invention comprise for example a zirconium oxide, enjoying the same 
accuracy in regard to shape and dimensions as metallic prostheses. By 
virtue of the high level of hardness of the densely sintered ceramic, it 
was not to be expected that the required levels of accuracy in terms of 
shape and dimensions could be achieved with the process parameters in 
accordance with the invention. The for example zirconium oxide in 
accordance with the invention is bioinert and biocompatible and thus 
satisfies all further requirements to be made in respect of ceramic 
prosthesis materials, so that the present invention makes it possible to 
gain entry with ceramic materials into the part of prosthetics which 
hitherto was predominantly reserved for metals as prosthesis materials, 
with the advantages deriving therefrom. 
In accordance with the invention, for the purposes of producing a 
prosthesis, the procedure can start from a densely sintered or infiltrated 
semimanufactured article, for example a round blank or disk of zirconium 
oxide, by the prosthesis being machined out of the semi-manufactured 
article in accordance with a pattern, although zirconium oxide in the 
densely sintered state is substantially more difficult to machine than 
aluminum oxide so that aluminum oxide would tend to present itself for 
prosthetic purposes in this area. In an alternative procedure the process 
can initially start from a porous blank, whereafter the blank is subjected 
to a working operation to afford a prosthesis, under dimensional control, 
the porous prosthesis then being subjected to dense sintering or 
infiltration with dimensional control to constitute an intermediate 
product, whereupon finally the intermediate product is machined to afford 
the final shape and dimensions by means of the operating procedure 
according to the invention. 
It is also in accordance with the invention to produce a ceramic prosthesis 
blank member by means of a mold and slip casting, whereupon the blank 
member is dried, roasted or fired, subjected to hot isostatic 
post-compacting and then subjected to oxidizing post-treatment. The shaped 
blank corresponding to the prosthesis can then be subjected to finishing 
machining in accordance with the invention.

EXAMPLES 
The invention will be described in greater detail hereinafter by means of 
Examples. 
Example I: Finger Joint 
A plastic model of an insert for a finger joint (proximal phalanx) is 
measured by means of a 3D-measuring machine and the measurement data are 
read into a control apparatus for processing the measurement data. A blank 
in the form of a plate of dimensions 20.times.10.times.32 mm is produced 
from a material of the composition ZrO.sub.2 -TZP with a ZrO.sub.2 
-HfO.sub.2 -proportion of between 94.8 and 95.3% by weight, with a Y.sub.2 
O.sub.3 content of between 4.8 and 5.2% by weight, and a maximum 
concentration of further oxides (impurities) of less than 0.1% with 
demonstrated biocompatibility, with a menoclinic proportion of below 5% by 
volume, and a radioactivity level of less than 10 Bq per kg, corresponding 
to a maximum of 0.03 microsievert/year of radiation loading. The shape of 
the proximal phalanx is ground out of that plate, using a machining 
apparatus which is controlled in three directions by the control 
apparatus. For that purpose the machining apparatus uses tools comprising 
metallically bound diamond grains, which are 6 mm in diameter. The 
selected speed of tool rotation is 22,000 rpm while the surface speed is 
0.69 meter per second. The infeed rate, that is to say the rate of feed of 
the tool towards the blank to be ground and thus the rate of blank 
material removal is 0.2 millimeters per minute while the tool feed rate, 
that is to say the rate of feed along the blank, is 0.7 centimeter per 
second. 
After the termination of the machining operation the implant is polished to 
a roughness depth of better than Ra=0.08 .mu.m in the region of the joint 
face by multi-stage polishing. 
Example II: Middle Phalanx 
A plastic model of an insert for a finger joint (middle phalanx) is 
measured by means of a 3D-measuring machine and the measurement data are 
read into a control apparatus for processing same. A blank in the form of 
a plate measuring 15.times.10.times.28 mm is produced from a material of 
the composition ZrO.sub.2 -TZP with a ZrO.sub.2 HfO.sub.2 -proportion of 
between 94.5 and 95.3% by weight, a Y.sub.2 O.sub.3 content of between 4.8 
and 5.2% by weight and with a maximum concentration of further oxides 
(impurities) of less than 0.1%, with proven biocompatibility and with a 
monoclinic proportion of below 5% by volume and a radioactivity level of 
less than 10 Bq per kg corresponding to a maximum of 0.03 
microsievert/year of radiation loading. The shape of the middle phalanx is 
ground out of that plate using a machining apparatus controlled in three 
directions by the control apparatus. The grinding operation is effected 
using tools comprising metallically bound diamond grains, being of a 
diameter of 5 mm. The selected speed of tool rotation is 28,000 
revolutions per minute, the surface speed is 0.73 meter per second, the 
infeed rate is 0.3 millimeter per minute and the feed rate along the 
workpiece is 0.85 centimeter per second. After termination of the 
machining operation the sliding surface of the implant is brought to a 
roughness depth of better than 0.05 .mu.m. 
Example III: Knee Joint 
A plaster mold or cast is made from a three-dimensional metal model, which 
is enlarged linearly by 30%, of a left femoral knee joint portion. That 
mold is then used to produce a ceramic blank member by a suitable shaping 
procedure such as slip casting or an equivalent process. The composition 
of the raw material used for the blank is: ZrO.sub.2 +HfO.sub.2 =94.9% by 
weight; Y.sub.2 O.sub.3 =5.02% by weight; and impurities=0.08%. The 
ceramic blank is dried and roasted at 1465.degree. C. After the roasting 
operation it is subjected to a hot isostatic post-compacting operation 
treatment, which is referred to as a HIP-treatment, at a pressure of 900 
bars, at a temperature T=1375.degree. C., for a period of 1 hour, in an 
argon atmosphere. The material is then subjected to an oxidizing 
post-treatment for 1 hour at 1100.degree. C. The resulting white ceramic 
is distinguished by proven biocompatibility, a monoclinic proportion of 
below 5% by volume and a radioactivity level of less than 10 Bq per kg, 
corresponding to a radiation loading of a maximum of 0.03 microsievert per 
year (averaged over 50 years). 
The blank measuring 82.6.times.66.4.times.63.5 mm is fixed in a rectangular 
holder by means of a suitable receiving means. The external shape of the 
femoral knee joint is machined by means of a machining apparatus in two 
stages of CIM (Computer Integrated Manufacturing). That operation is 
effected using metal-bound diamond tools (grinding points) of diameters of 
12 and 14.5 mm with a hollow structure and a hard metal or carbide holding 
means. The selected grain size is stepped from D91 through D126 to D151. 
The speed of tool rotation is 45,000 rpm, the surface speed for the tool is 
2.8 meters per second, the infeed rate towards the blank is 0.4 mm per 
minute and the advance rate along the blank is 0.8 centimeter per second. 
The articulated joint surfaces are then contour-matched by means of a 
vibrating polishing stage and diamond pastes to polish them to a surface 
quality of better than 0.09 .mu.m (Ra). 
Example IV: Cervical Vertebra Stabilizer 
A plate measuring 18.times.26-94 mm and of a thickness of 0.5-1.5 mm is 
axially pressed by means of a suitable pressing tool from ZrO.sub.2 -TZP 
material of the composition set forth in Example III. In the longitudinal 
direction the plate has a cylindrical curvature corresponding to a radius 
of between 50 and 250 mm, preferably 150 mm. 
The blanks of that kind are roasted or fired in an oxidizing atmosphere 
under conditions as set forth in Examples I through III. After the 
sintering operation the plate is gripped in a holder by means of a 
suitable receiving means thereof and machined in accordance with a 
previously input ted program of 3D CIM (Three-dimensional Computer 
Integrated Manufacturing). In that procedure the external contours are 
machined by means of diamond milling cutters and the inner openings are 
machined by boring or milling by means of suitable diamond tools. The 
diamond cutters or tools comprise metallically bound diamond grains. The 
machining conditions are as follows: 
--tool diameter 4 mm 
--speed of tool rotation 50,000 rpm 
--surface speed 8.3 meters per second 
--diamond grain size D126 
--feed rate along the blank 1.5 centimeters per second 
--infeed rate 0.6 millimeter per minute. 
After machining of the contours the workpiece is subjected to a surface 
smoothing operation by sliding grinding, the grinding bodies comprising 
92% Al.sub.2 O.sub.3. The surface quality which can be achieved is Ra 
=0.86 .mu.m. 
It will be appreciated that the above-described processes for the 
production of ceramic prostheses have been set forth solely by way of 
example and illustration of the principles of the present invention and 
that various modifications and alterations may be made therein without 
thereby departing from the spirit and scope of the invention.