Use of palladium-silver alloys for the manufacture of dentures

Gold-free palladium-silver alloys for dentures [dental prostheses] which alloys can be veneered without color changes with dental ceramics and can be easily removed from the mold after the casting contain 45 to 80% by weight palladium, 7 to 50% by weight silver, 0 to 5% by weight gold, 0 to 5% by weight tin, 0 to 5% by weight indium, 0 to 3% by weight zinc, 0 to 2% by weight copper, 0 to 1% by weight ruthenium, iridium and/or rhenium, 0 to 7% by weight gallium, 0 to 5% by weight cobalt and 0 to 3% by weight germanium, whereby of the elements gallium, cobalt and germanium at least two must be present with contents totalling 2 to 9% by weight.

INTRODUCTION AND BACKGROUND 
The present invention relates to the use of palladium-silver alloys for the 
manufacture of fixed or removable dentures, which can be veneered with 
dental ceramics. 
Fixed or removable dentures are manufactured primarily from 
corrosion-resistant, biocompatible precious-metal alloys with the 
so-called precision investment casting, also referred to as the lost-wax 
process in which the cast object is frequently veneered with dental 
ceramics in order to achieve an appearance which corresponds to the 
natural tooth. To be compatible with the dental ceramics the alloys must 
exhibit special properties, such as coefficient of thermal expansion, 
melting range or adhesion. 
Alloys with a high gold content such as those described, for example, in 
German patents 11 83 247 and 15 33 233 are especially well-suited for this 
purpose. However, due to the high and very fluctuating price of gold, more 
of an effort has been made recently to find more economic alternatives to 
the alloys with a high gold content. Palladium presents itself from the 
group of noble metals as an appropriate substitute on account of its 
relatively economic price, its density, which is considerably less in 
comparison to that of gold, and its resistance to corrosion in the mouth 
(oral environment), which is comparable to that of gold. 
It is to make a distinction between alloys containing silver and those not 
containing silver in the previously known palladium-based alloys in the 
field of dentistry. 
Silver-free palladium-based alloys contain copper, tin, indium, cobalt and 
gallium as main alloying elements. Typical silver-free palladium-based 
alloys are described for example in German patents 33 16 595; 33 04 183; 
33 14 657 or 35 22 523. Compared with alloys with a high gold content, 
these alloys react more sensitively to manufacturing errors and are 
difficult to solder. They absorb considerable amounts of carbon in the 
liquid state, so that they should only be melted in a ceramic crucible. 
The dark oxides which form during the fixing of the dental ceramics at 
approximately 980.degree. C. adversely affect the aesthetic appearance of 
the denture by the formation of dark margins in the edge area of the 
veneering. 
Silver-containing palladium-based alloys fall between the alloys with a 
high gold content and the silver-free palladium-based alloys as concerns 
their working behavior. As a result of the silver portion, they are easier 
to melt and to cast, have a brighter oxide and a good soldering behavior. 
In addition,they are more economical than the silver-free palladium-based 
alloys. 
The typical composition of such alloys can be found in the "Survey of 
Dental Noble-Metal Alloys and Dental Base Metal Alloys in the Federal 
Republic of Germany" [in German] published by the Research Institute for 
Dental Care (FZV), Vol 1., (July 1986), pp. 31-2. In addition to palladium 
and silver, these alloys contain primarily tin, indium and zinc, in 
individual instances also copper or gallium as further alloying elements. 
The disadvantage of these alloys is the fact that they discolor the dental 
ceramic to a yellow or a yellowish green during the firing process. This 
discoloration is caused by the silver, which passes by diffusion or via 
the vapor phase into the ceramics. 
DE-PS 25 23 971 describes palladium-silver alloys which contain 0.1 to 0.5 
% titanium in order to suppress the discoloration of the ceramics. As a 
result of the reactivity of titanium with the atmospheric oxygen and/or 
the crucible materials, the melt becomes impoverished of this element 
relatively rapidly so that the reducing action on the tendency toward 
discoloration is lost when using old material (runners, casting funnels) 
and under unfavorably selected melting conditions. Moreover, the titanium 
brings about a strong adhesion of the investment material to the surface 
of the cast object, which makes it more difficult and more time-consuming 
to divest and to finish it. 
U.S. Pat. No. 4,350,526 describes palladium-silver alloys which exhibit no 
discoloring action on dental ceramics due to the addition of 0.1-1.0% 
silicon. Silicon is insoluble both in palladium and in silver. Moreover, 
palladium and silicon form intermetallic phases, so that a strong 
embrittlement of the alloy and fracture after casting can occur. 
Silicon favors, similar to titanium, a reaction with ceramic materials, so 
that a strong adhesion of the material to the cast object also occurs in 
the case of these alloys. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide palladium-silver alloys 
for the manufacture of fixed or removable dentures which can be veneered 
with dental ceramics and onto which alloys discoloration-sensitive dental 
ceramics can be burned without recognizable changes in color and which can 
be easily removed from the customary inestment material without the other 
properties of the known palladium-silver alloys significantly changing.

In achieving the above and other objects, one feature of the invention 
resides in using palladium-silver alloys consisting of 45 to 80% by weight 
palladium, 7 to 50% by weight silver, 0 to 5% by weight gold, 0 to 2% by 
weight platinum, 0 to 5% by weight tin, 0 to 5% by weight indium, 0 to 3% 
by weight zinc, 0 to 2% by weight copper, 0 to 1% by weight tungsten, 
molybdenum and/or tantalum, 0 to 1% by weight ruthenium, iridium and/or 
rhenium, 0 to 7% by weight gallium, 0 to 5% by weight cobalt and 0 to 3% 
by weight germanium and further, wherein at least two of the elements 
gallium, cobalt and germanium must be present in the alloys with contents 
totalling 2 to 9% by weight. 
It is preferable to use alloys which contain 55 to 75% palladium and 20 to 
45% by weight silver in addition to the other components. Moreover, it has 
proven to be advantageous if 0.5 to 4% by weight gallium, 1 to 4% by 
weight cobalt and 0.1 to 1.5% by weight germanium are present in the 
alloys, whereby the sum of these contents must be between 2 and 7% by 
weight. 
Especially advantageous alloys contain 60 to 68% by weight palladium, 28 to 
32% by weight silver, 0.1 to 0.5% by weight ruthenium, iridium and/or 
rhenium, 1.5 to 2.5% by weight gallium, 1.5 to 2.5% by weight cobalt and 1 
to 1.5% by weight germanium. 
Palladium-silver alloys become corrosion resistant the mouth above a 
palladium content of approximately 25-30%. Alloys with a silver content of 
more than 50% by weight exhibit very high coefficients of thermal 
expansion (&gt;16.0.times.10.sup.-6 /K), so that they are no longer 
compatible with conventional veneering ceramics. If the silver contents 
are too low, the palladium-silver alloys behave similarly to silver-free 
palladium-based alloys. For this reason, palladium-silver alloys in a 
range of 45-80% palladium and 7-50% silver were selected, which exhibit an 
excellent corrosion resistance in the mouth and offer the prerequisite for 
a crack-free veneering of the alloy with dental ceramics. 
The elements tin, indium, zinc, copper and gold serve to set the mechanical 
properties of the alloys, such as for example strength, hardness, melting 
and casting properties, coefficient of thermal expansion and melting range 
Ruthenium, rhenium and/or iridium are alloyed therewith in concentrations 
between 0.1 and 1% as grain refinement additives. 
However, palladium-silver alloys which contain only the previously 
mentioned alloying elements discolor the dental ceramics distinctly 
yellowish-green during the firing process. 
It was surprisingly found that the addition of gallium, cobalt and geranium 
in the concentration ranges in accordance with the invention distinctly 
reduces or eliminates the discoloring action of the alloys designated 
above on the dental ceramics. This reduction or elimination of the 
tendency to discolor is only observed, however, if at least two of these 
three elements are present in the alloy. If the sum of the contents of at 
least two of the three elements gallium, germanium and cobalt is less than 
2%, the discoloring action on the ceramics is still relatively strong. If 
this sum exceeds 9%, the other properties of the alloy are negatively 
influenced, so that a concentration range between 2 and 9% must be 
maintained. 
Table 1 shows the composition and the properties of a few alloys in 
accordance with the invention. They have excellent melting and casting 
properties bright oxide and can be veneered without difficulties with the 
known dental ceramics. The high yield strength paired with a high 
elongation permits these alloys to also be used for removable partial 
dentures 
The determination of the color of the dental ceramics on the alloys after 
being fired on was performed with a color-measuring device 
(datacolor--Light Colour Systems GmbH, light type D 65, angle of 
observation 10.degree.) and visually by several persons in comparison to a 
"standard specimen". A gold-reduced, silver-free alloy of the composition 
52.0% by weight gold, 37.6% by weight palladium, 8% by weight indium, 2.0% 
by weight gallium and 0.4% by weight iridium was used as standard alloy 
and the brightest color of a commercial veneer ceramics assortment was 
used as ceramics. 
Table 2 summarizes the results of the color measurement of known reference 
alloys and of a few alloys in accordance with the invention according to 
table 1 (color values according to the CIELab-System, DIN 6174). The L 
value indicates the position on the bright-dark axis (L=0) black, L=100 
white ) The "a" value describes the position on the green/red axis 
(negative=green, positive red) and the "b" value the position on the 
blue/yellow axis (blue negative, yellow positive). 
Table 2 does not show the absolute color values but rather the deviation of 
the color values for the ceramic veneer on the different alloys from the 
standard specimen (.DELTA. values). The color values of the alloys in 
accordance with the invention are in the range of the known, silver-free 
alloys or alloys with a high gold content. Even a visual observation of 
veneered crowns and bridges was not able to detect any deviation in color 
of the alloys in accordance with the invention from the standard specimen. 
Further variations and modifications of the foregoing will be apparent to 
those skilled in the art and are intended to be encompassed by the claims 
appended hereto. 
German priority application P 38 06 343.3 is relied on and incorporated 
herein. 
TABLE 1 
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Hardness 
Coeffic. 
after of th. 
Melting 
ceramic 
exp. RT 
Alloying 
Composition in % by weight interval 
firing cycle 
600.degree. C. 
elongation 
No. Au 
Pd Ag Ru 
Sn In Cu Zn Ga 
Ge 
Co 
W [.degree.C.] 
HV5 [10.sup.-6 K.sup.-1 
(MPa) 
[%] 
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1 -- 
64.5 
30 0.5 
-- -- -- -- 3 1 -- 1291-1150 
200 15.15 n.b. 
n.b. 
2 -- 
63.3 
30 0.5 
2 -- -- -- 2 -- 
2 02 
1291-1192 
220 14.9 n.b. 
n.b. 
3 2 60.5 
30 0.5 
4 -- -- -- -- 
1 2 1307-1213 
210 n.b. n.b. 
n.b. 
4 -- 
64 30 0.5 
-- 2 -- -- 2 1.5 
-- 1287-1140 
205 15.0 n.b. 
n.b. 
5 -- 
64 30 0.5 
-- -- 1.5 
1.0 
2 1 -- 1296-1185 
210 15.0 n.b. 
n.b. 
6 -- 
63.5 
30 0.5 
2 -- -- -- 2 1 1 1270-1170 
210 15.0 559 23.6 
7 -- 
65.1 
30 0.5 
-- -- -- -- 1 1.4 
2 1298-1192 
230 n.b. 552 25.7 
8 -- 
62.5 
30 0.5 
10 10 -- -- 2 1 2 1249-1155 
210 15.3 n.b. 
n.b. 
9 -- 
62.5 
30 0.5 
-- 1 -- -- 2 1 3 1245-1148 
240 14.9 n.b. 
n.b. 
10 -- 
77.5 
10 0 
0.5 
4 -- -- -- 4 -- 
4 1301-1204 
190 n.b. n.b. 
n.b. 
11 -- 
55.5 
40 0.2 
1 -- -- -- 2 1 -- 1266-1175 
180 15.8 n.b. 
n.b. 
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TABLE 2 
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Alloy .DELTA.L 
.DELTA.a 
.DELTA.b 
______________________________________ 
1 Au 52 Pd 37 6 In 8 Ga 2 0 Ir 0 4 (Standard) 
0 0 0 
2 1 (according to Table 1) 0 0 0.5 
3 2 (according to Table 1) -0.5 0.3 0.4 
4 4 (according to Table 1) -2.9 0.1 0.8 
5 6 (according to Table 1) -0.6 0.2 0.5 
6 7 (according to Table 1) -1.9 0.2 -0.1 
7 Au 77.3 Pd 8.9 Pt 9 8 -1.6 0.2 0.2 
8 Au 53.2 Pd 35.1 Sn 6.6 Co 2.8 
-1.4 0.5 0.3 
9 Pd 79 Ga 9 Cu 9 5 0.9 0.3 1.0 
10 Pd 79.7 Sn 6.5 Ga6 Cu 5 -3.1 0.1 -0.7 
11 Pd 76.5 Cu 11.6 Ga 7.2 -1.7 0.5 0.3 
12 Pd 57.8 Ag 30 Sn 6 In 4 0.7 0.2 3.1 
(In reference alloys 7 to 12, only 
those components are indicated which are 
over 2% by weight.) 
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