CA2128395C - Use of gold-palladium alloys for dental castings - Google Patents

Abstract

Gold-palladium alloys with a high gold content for dental applications should, for reasons of biocompatibility, not contain any toxically dubious components. For particularly carrosion-resistant and biocompatible Type-4 alloys, tin is needed as the only base-metal component in amounts between 0.7 and 5.8 wt-% if one remains within defined limits in the palladium-tin diagram. Such alloys contain, in addition to gold and tin, 6 to 25 wt-% palladium, 0 to 12 wt-% platinum and 0 to 2 wt-% iridium, rhodium and/or ruthenium.

Description

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The invention relates to the use of gold-palladium alloys with a high gold content for dental castings faced with ceramic and for unfaced dental castings.
Permanent and removable dentures are frequently produced from corrosion-resistant, biocompatible precious-metal alloys, whereby the cast object is subsequently faced with dental ceramic so as to attain an appearance corresponding to the natural tooth. The suitability of alloys for this purpose is associated with a number of properties which have to be matched to the dental ceramic, such as coefficient of thermal expansion, melting range and adhesion between ceramic and alloy. A basic prerequisite is also good corrosion resistance and sufficient strength, in order to withstand the loads 25 arising .in the chewing process. Depending on their mechanical load-carrying capacity, dental alloys are divided into various classes designated as Types 1 to 4. Type-4 alloys possess the greatest strength and therefore the broadest range of application.
The traditional alloy systems that are used for this purpose are precious-metal alloys with a high gold content.
Such alloys have proved their worth clinically over many years.
With regard to corrosion resistance and biocompatibility these alloys remain unequalled. But hitherto i~t has only been possible to meet the numerous demands, mentioned above, that are made on these alloys with alloy systems which as a rule are constructed in very complicated manner.

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The bake-on alloys with a high gold content are characterized by a gold content upwards of about 70 wt-%. With a view to increasing the high-temperature stability during the ceramic baking, palladium and platinum are as a rule added to the alloy. Since platinum widens the melting range of a gold alloy significantly more than palladium, alloys which are particularly stable at high temperature can be obtained by the use, in particular, of palladium as alloy element. Such alloys as a rule have a palladium content of at least 8 wt-%. With a view to increasing the hardness and the mechanical strength, a number of different base metals are added to the alloy.
Additional elements are added to the alloy in order to ensure the fine-tuning of additional data that are relevant from the point of view of tooth technology, such as coefficient of thermal expansion, ceramic adhesion, oxide color or sufficient ductility at high temperature. Common additional alloy elements, therefore, are silver, copper, indium, zinc, tin, iron and gallium. It is known that a number of these elements can in turn also have undesirable properties, so that attempts are made to avoid such elements or to employ them only in small amounts. For instance, silver can result in green discoloration in the case of sensitive ceramics, and copper, especially where crevice-corrosion effects occur, can result in discolorations.
Known dental alloys with a high gold content mostly contain two or more base-metal alloy elements in order to adjust all the alloy properties that are necessary for dentures.
In US Patent 3,716,356 a dental alloy with a high gold content is described which contains 5.5 to 40 wt-o palladium and 0.03 to 1.0 wt-% rhenium. In addition, up to to wt-% platinum, up to 2 wt-o silver, up to 1 wt-% iron, up to 1.5 wt-% zinc, up. to 2 wt-% tin and up to 1 wt-o indium may also be present.
Hardness values for these alloys are not given, but they attain the hardness of Type-4 alloys only ~~.~~ii~~~ a if additional base-metal components are present, the toxic effects of which are still largely unknown.
Dental alloys with a high gold content according to DE-OS 30 19 276 contain, besides palladium, up to 10 wt-o indium and additionally ruthenium and tin in order to attain sufficient hardness values.
DE-PS 24 24 575 describes dental alloys with a high gold content containing 5 to 15 wt-o platinum, 0.1 to 2 wt-o indium, 0.05 to 0.5 wt-% iridium and 0.5 to 3 wt-% rhodium.
These alloys are palladium-free.
The dental alloys with a high gold content of Type 4 which are employed in practice all contain two or more base-metal components in order to attain suitable hardness values.
Within the context of a generally raised health-consciousness and a higher susceptibility to allergies and incompatibilities which is generally to be observed in people who live in modern industrial states, the biocompatibility of dental alloys has been the subject of increased discussion. Previous studies have shown that the type and amount of the components of an alloy which go into solution as a result of corrosive processes are of decisive importance for biocompatibility. The causes of the corrosion and the possible effects of the corrosion products on the organism are very complex. Studies indicate in particular that the thermal loading and oxidation of the bake-on alloys taking place during the ceramic baking is a significant factor reducing the corrosion resistance of the alloys. In general alloys should be aimed for in which the proportion of precious metal is as high as possible for good corrosion resistance and the number of alloy components, especially of base metals, is as low as possible in order to keep the probability of an allergic reaction to a particular component as low as possible. Of course, use should only be made of elements that are not known to have any toxic effects.
The present invention provides gold-palladium alloys with a high gold content for dental castings faced with ceramic and for unfaced dental castings, which, with a view to attaining a hardness necessary for Type-4 alloys, require only a single base metal, the toxic effect of which is known. In addition, these alloys exhibit a better corrosion resistance than the alloys previously known and possess all the other properties necessary for bake-on alloys, such as strength, ductility, coefficient of thermal expansion, ceramic adhesion and high-temperature stability.
The single Figure shows the Pd-Sn diagram for the present alloys.
More particularly, in accordance with the invention alloys are described which contain 6 to 25 wt-% palladium, 0 to 12 wt-platinum, 0 to 2 wt-% iridium, rhodium and/or ruthenium and 0.7 to 5.8 wt-% tin, the remainder being gold, whereby a) .the tin content for platinum contents below 2 wt-% lies within a range which in the palladium-tin diagram is bounded by the points A, B, C and D, where A = 6 Pd, 1.3 Sn, B = 6 Pd, 2.8 Sn, C = 25 Pd, 5.8 Sn, D = 25 Pd 2.2 Sn, b) the permitted tin content for platinum contents above 2 wt-% is decreased by 0.12 wt-% tin for every 2 wt-%
platinum content, and c) the sum of the contents of palladium and platinum does not exceed 30 wt-%.
Preferably, the alloys contain 12 to 25 wt-% palladium, 0 to 10 wt-o platinum and 0 to 2 wt-~ iridium, rhodium and/or ruthenium by way of grain-reducing agent, 2.1 to 5.0 wt-s tin, the remainder being gold, and w the tin content lies within a range which in the palladium-tin diagram is bounded by the points A', B', C' and D, where A' -12 Pd, 2.1 Sn, B' - 12 Pd, 3.0 Sn, C' - 25 Pd. 5.0 Sn and D
has the meaning given above.

Alloys have proved particularly useful which in the palladium-tin diagram are bounded by the points A', B', C " and D ", where C " - 16 Pd, 3.5 Sn, D " - 16 Pd, 2.2 Sn, and A' and B' have the meaning given above.
Alloys have also proved particularly useful which in the palladium-tin diagram are bounded by the points A, B, C " 'and D" ' , where C" ' - 10 Pd, 3. 4 Sn, D" ' - 10 Pd, 1 . 5 Sn, and A
and B have the meaning given above, whereby the sum of palladium and platinum may not exceed 12 wt-o.
In further aspects, the invention also provides a dental casting comprising the alloys described herein; optionally faced with ceramic, as well as a dental prosthesis comprising such dental castings, and a method of using said alloys comprising forming such dental castings.
Surprisingly it has been shown that all the demands made on dental alloys can be met by alloys that contain tin as the only base metal. These alloys possess excellent corrosion resistance lying clearly above the corrosion resistance of the Sa alloys currently known. A prerequisite for these properties is that the tin content of these alloys is matched in defined manner to the palladium content and, in the event of platinum being present, is also modified with respect to the platinum content. By virtue of their very good corrosion resistance and the fact that the base metal tin has been demonstrated to be harmless by its diverse use in the food industry as tinware or tin plate, these alloys possess extraordinary biocompatibility.
Alloys of this system consist, in the simplest case, of gold, palladium and tin and can furthermore also contain 0 - 2o iridium, rhodium and/or ruthenium by way of grain-reducing agent. Very good corrosion resistance and sufficient hardness for alloys of Type 4 are attained if the tin content is precisely adjusted with respect to the palladium content.
Specifically, the higher the palladium ~,~..~~~3;3~5 content of the alloy, the more tin is necessary. The possible tin/palladium ratio is represented in Figure 1.
Accordingly the permissible tin content for the perm:Lssible palladium content of 6 - 25o is defined by a quadrangular area in a Pd-Sn diagram, the corners of which, for a Pd content of 60, are situated at 1.3 and 2.8% tin and, for a Pd content of 25%, between 2.2 and 5.8a tin.
The addition of platinum to the alloy has the advantage that the proportions of the base metal tin can be further reduced. The corrosion resistance and, consequently, also the biocompatibility are thereby improved still further.
The addition of platinum to the alloy is limited to 12%, whereby in total no more than 30 wt-o palladium and platinum should be contained in the alloy. Fox a platinum content of 2 wt-o upwards the necessary tin contents can be reduced on average by 0.12 wt-% for every 2 wt-% platinum.
In Table 1 a number of alloys are listed in accordance with their composition. Alloys 1-6 are alloys which correspond to the state of the art. They contain at least two base metals. Alloys 7-12 represent test alloys, which in fact only contain tin by way of base metal but in which the tin content lies outside the range sketched in Fig. 1. Alloys 13-20 conform in their composition to the demands according to the invention as regards Pd, Sn and Pt contents.

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Table 1: Alloy compositions:
Allay Au Pd Pt Ag Sn In Othars 1 77.3 8.9 9.8 <2.0 <2.0 <2.0 Cu, Fe, Re,Ir 2 84.4 5.0 8 - - 2.5 Ta 3 72 9.7 13 2.8 1.2 1.2 Ir 4 74.8 15 6 - 2 2 Ir 5 86 - 10.4 - - <2.0 Rh,Ta 6 77.7 - 19.5 - - - Zn,Ta 7 64.9 25 4 - 6.0 - 0.1 Ru 8 77.8 20 - - 1.8 - 0.4 Ru 9 83.3 15 - - 1.5 - 0.2 Ir 10 81 7.5 - - 1.3 - 0.2 Ir 11 88.8 7.5 - - 3.5 - 0.2 Ir 12 75.4 14 6 - 4.5 - 0.1 Ir 13 72.8 23 - - 4.0 - 0.2 Ir 14 81.5 15 - - 3.0 - 0.5 Ir 15 77.8 14 6 2.1 - 0.1 Ir 16 81.8 14 2 2.0 - 0.2 Ir 17 76.8 13 8 2.2 -18 86.8 10 - 2.0 - 1.2 Rh 19 89.1 7 2 1.8 0.l Ir 20 81.3 14.5 - - 4.0 - 0.2 Ir The results of corrosion trials are compiled in Table 2.
In order to determine the corrosion resistance, corrosion tests were carried out in accordance with Draft DIN 13927.
To this end, test bodies are stored for 7 days in a solution of 0.1 m lactic acid and 0.1 m common salt at 37° C. Then the corrosive solution is analysed qualitatively and quantitatively by means of suitable analytical procedures with regard to the corrosion products released. In order to exclude surface effects and the influence of oxidation, subsequent to a previously simulated ceramic baking the test bodies are abraded before they are placed in the corrosive solution. In the trials carried out, besides this 'standard corrosion test" more stringent test conditions were additionally chosen, so that precisely the influence of oxidation on the corrosion reaction could be examined. This is necessary, since it has to be assumed that under real conditions it is not possible for the whole set of dentures to be reworked mechanically after the ceramic baking in such a way that the preceding oxidative damage to the unfaced regions is completely removed. In order to simulate these conditions, selected alloys were sandblasted and oxidised and, without subsequent removal of the oxide layer, suspended in the corrosive solution.
In Table 2 the concentrations of the dissolved alloy components are listed which were analysed in each case.
The final column lists, in addition, the sum of the total ion concentrations, which constitutes the essential criterion in Draft DIN 13927. Specifically the sum of all dissolved fans after 7 days of corrosion may not exceed the limiting value of 100 ug/cm2. The rates of corrosion of the elements which lie below the particular detection limit are--.not taken into account in the total value. The detection limit is 0.13 ~g/cm2.

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r e-iH e-i ~~.,~~~3x3~~~ a The results prove that the bake-on alloys with a high gold content all possess corrosion values which lie far below the prescribed limiting values. It is striking, however, that only with the alloys according to the invention do all analysed elements lie below or just above the particular detection limit. Under the more stringent corrosion conditions this difference between the alloys representing the state of the art and the alloys according to the invention~becomes even more apparent. Whereas the alloys according to the state of the art exhibit a significant increase in the corrosion data, the alloys according to the invention exhibit, even under these conditions, only very low rates of corrosion. The analysed elements even lie mostly below the particular detection limit.
The outstanding stability of the alloys according to the invention with respect to oxidation, to which the good corrosion resistance can probably be attributed, is also substantiated by metallographic and thermogravimetric investigations. On metallographic microsections in the case of the alloys according to the state of the art it is possible to detect pronounced internal oxidation zones, whereas in the case of the alloys according to the invention the strips of oxide are so thin that they are almost undetectable under an optical microscope. The oxidation of the alloys results in a weight gain which can be determined thermogravimetrically. To this end, from a number of alloys thin rings were cast so as to present as large a surface as possible to the corroding oxygen.
Suspended on a thermobalance, these rings were heated, at a definite heating rate of 30 K/min, up to 950° C, maintained at this temperature for 120 min and then cooled, again at a constant cooling rate of 30 K/min. During the test the weight gain was measured continuously. Table 3 lists the total weight gains, which represent a measure of the oxidation that has taken place. The alloys according to --w f ~.iVs~e.~~J

the invention (Nos. 14, 15, 17, 19) are characterised by the lowest weight gains.
Table 3: Weight gains through oxidation Alloy No. Weight Gain [mg/cm2]
1 0.58 2 0.69 3 0.38 5 0.66 4 0.46 14 0.29 0.26 15 17 0.24 19 0.21 With a view to characterising the strength at room temperature, Table 4 lists the hardness values subsequent to casting, in the hardened state and after kiln treatment, as well as the yield point, the tensile strength and the elongation at break. The tensile test samples were heat-treated in accordance with Draft DIN 13927, so that a structure was available such as that existing after the ceramic baking.

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The test alloys (8, 9, 10) with tin contents lying below the range according to the invention exhibit only low hardness and strength values. Although the test alloys (Nos. 7, 11, 12) which exhibit tin contents that are too high possess high hardness values and tensile strengths, their ductility is too low. As metallographic trials show, the formation of a second phase is responsible for this.
The measurements with regard to the high-temperature stability of the alloys are compiled in Table 5. The high-temperature stability of the alloys during the ceramic baking is defined by the so-called sag resistance - ie, the resistance to deformation at high temperature on the basis of the dead weight. In order to determine the sag resistance, test rods with dimensions 50 mm x 3 mm x 1 mm were produced by investment casting and were cleansed of potting medium by sandblasting. To simulate the ceramic baking, the test samples were subjected to a 20-minute heat treatment at 980° C, whereby they were stored on two ceramic supports, lying horizontally on the flat side. The ceramic supports were spaced by 40 mm, so that the possibility existed that the test samples would bend under their own weight. The extent of the bending was ascertained by measuring the test rods before and after the ceramic baking by means of an inductive displacement-sensing system. The difference in the bending before and after the ceramic baking represents a measure of the high-temperature stability. As can be seen from Table 5, the alloys without any palladium content or with small palladium contents exhibit strong bending (alloy Nos. 5 and 6). Alloys with relatively high palladium contents are significantly more stable at high temperature.
Particularly good high-temperature stabilities are exhibited by the alloys (Nos. 13, 15 and 17) with palladium contents higher than .120.

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Table 5: Results of sag resistance test Alloy No. Bending in dam (average values) Alloys with palladium contents below 12o do not have quite such good strength properties at high temperatures (alloy Nos. 18 and 19), but these alloys have the advantage that they still exhibit a yellow or yellowish colour, which is preferred for aesthetic reasons.

Claims (8)

1. A gold-palladium alloy with a high gold content for dental castings faced with ceramic and for unfaced dental castings, said alloy consisting of 6 to 25 wt % palladium, 0 to 12 wt % platinum, 0 to 2 wt % of at least one member of the group consisting of iridium, rhodium, and ruthenium, and 0.7 to 5.8 wt-% tin, the remainder being gold, wherein (a) said tin content of said alloy, where said alloy contains less than 2 wt % platinum, lies within a range defined in the palladium-tin diagram according to FIG. 1 wherein the quadrangle is bounded by the points A, B, C and D, wherein point A is at 6 wt % Pd and 1.3 wt % Sn, point B
is at 6 wt % Pd and 2.8 wt % Sn, point C is at 25 wt % Pd and 5.8 wt % Sn, and point D is at 25 wt % Pd and 2.2 wt % Sn;
(b) said tin content of said alloy, where said alloy contains more than 2 wt % platinum, is decreased from the Sn range defined in the palladium-tin diagram according to FIG. 1 by 0.12 wt % tin for every 2 wt % increase in platinum content; and (c) the sum of the contents of palladium and platinum does not exceed 30 wt %.

2. The gold-palladium alloy according to claim 1, comprising 12 to 25 wt % palladium, 0 to 10 wt % platinum, and 0 to 2 wt % of at least one member of the group consisting of iridium, rhodium, and ruthenium, 2.1 to 5.0 wt % tin, the remainder being gold, wherein said tin content of said alloy, where said alloy contains less than 2 wt % platinum, lies within a range defined in the palladium-tin diagram according to FIG. 1 wherein the quadrangle is bounded by the points A', B', C' and D, wherein point A' is at 12 wt % Pd and 2.1 wt % Sn, point B' is at 12 wt % Pd and 3.0 wt % Sn, point C' is at 25 wt % Pd and 5.0 wt % Sn, and point D is at 25 wt % Pd and 2.2 wt %
Sn.

3. The gold-palladium alloy according to claim 1, wherein the tin content of said alloy, where said alloy contains less than 2 wt % platinum, lies within a range defined in the palladium-tin diagram according to FIG. 1 wherein the quadrangle is bounded by the points A', B', C" and D", wherein point A' is at 12 wt % Pd and 2.1 wt % Sn, point B' is at 12 wt % Pd and 3.0 wt % Sn, point C" is at 16 wt % Pd and 3.5 wt % Sn, and point D" is at l6 wt % Pd and 2.2 wt % Sn.

4. The gold-palladium alloy according to claim 1, comprising 6 to 10 wt % palladium, 0 to 6 wt % platinum, 0 to 2 wt % of at least one member of the group consisting of iridium, rhodium, and ruthenium, 1.3 to 3.4 wt % tin, the remainder being gold, wherein the tin content of said alloy, where said alloy contains less than 2 wt % platinum, lies within a range defined in the palladium-tin diagram according to FIG. 1 wherein the quadrangle is bounded by the points A, B, C''' and D''', wherein point A is at 6 wt % Pd and 1.3 wt % Sn, point B is at 6 wt % Pd and 2.8 wt % Sn, point C''' is at 10 wt % Pd and 3.4 wt % Sn, and point D''' is at 10 wt % Pd and 1.5 wt % Sn, and the sum of palladium and platinum does not exceed 12 wt %.

5. The gold-palladium alloy according to claim 1, wherein said tin content of said alloy where said alloy contains 12 wt % platinum lies within a range defined in the palladium-tin diagram according to FIG. 1 wherein the quadrangle is outlined by ----x---- and is bounded by the points 6 wt %
Pd, 0.7 wt % Sn; 6 wt % Pd, 2.2 wt % Sn; 25 wt % Pd, 5.2 wt % Sn; and 25 wt % Pd, 1.6 wt % Sn.

6. A dental casting comprising the gold-palladium alloy according to any one of claims 1 to 4, optionally faced with ceramic.

7. A dental prosthesis comprising the dental casting according to claim 6.

8. A method of using the gold-palladium alloy according to any one of claims 1 to 4, comprising forming a dental casting with said gold-palladium alloy optionally faced with ceramic.