Note: Descriptions are shown in the official language in which they were submitted.
Dental alloy of high gold content
Background of the Invention
The present invention belongs to the field of
dental art and is related to a new and useful dental
alloy having a high gold content and a golden yellow
colour, and which is destined and appropriate for the
facing with commercially available dental ceramic com-
pounds and for the manufacture of dental prosthesis
parts as well which remain unfaced or are to be faced
with synthetic or other materials.
Noble metal dental alloys having a high gold con-
tent are widely spread for the use in metallic, solid-
ly fastened dentures such as crowns, bridges etc., in-
ter alia because of their good biological compatibili-
ty and high corrosion resistance in the mouth milieu.
Furthermore, they are technically easy to process.
Silver and copper containing gold casting alloys
are successfully used since a long time in restorative
dentistry. In the conventional alloys, the high mecha-
nical resistance required for these materials when un-
faced parts or parts to be faced with synthetic mate-
rials are to be made, is adjusted through the silver
to copper ratio.
In view of optimal aesthetics of solidly fastened
dental reconstructions, especially in the visible re-
gion, an at least partial facing of a metallic base by
a ceramic compound has proven to be particularly ap-
propriate since the advantages of the ceramics, namely
hardness, aesthetics and outstanding biological compa-
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tibility, can optimally be combined in this material
composite with the advantages of the metallic materi-
al, namely tensile strength and better precision of
fit.
The use of a ceramic facing requires special pro-
perties of the alloy. Thus, the melting interval of
the alloy should markedly be higher than the baking
temperature of the ceramics which is about 980 °C, and
the alloy must furthermore present a sufficient burn-
ing stability so that the metallic base to be faced
remains dimensionally stable during the burning or
baking operation.
In order to guarantee a durable adhesion between
alloy and ceramics, no tensile stress should build up
during the manufacture process. This is achieved, in
an already known manner, by selecting the thermal ex-
pansion coefficient of the alloy slightly higher than
that of the ceramics. During the cooling process, a
compressive strain is produced in the ceramic coating
due to the somewhat higher shrinking alloy.
The requirements cited above have resulted in the
development of special alloys called burn-on alloys
which form an own class besides the conventional gold
casting alloys and which are standardized by the stan-
dards ISO 9693 and DIN 13927.
In order to achieve the above discussed required
properties of burn-on alloys, the elements platinum
and/or palladium have been added by alloying to the
alloys for metal-ceramic on the base of gold. Fur-
thermore, other metals than noble ones are added by
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alloying in order to improve the strength of the al-
loy, such as copper, indium, gallium, tin and/or zinc.
The alloy strength could basically be improved by add-
ing higher proportions of silver and copper; however,
this possibility cannot be used because of undesired
reactions of such alloys with the ceramic and of a too
high oxidation of the alloy.
In particular, additions of palladium, but also
of platinum lead to a sensible reduction of the de-
sired yellow colour of the alloy which is felt by the
patient as aesthetically agreeable and desirable.
Recently, some non-noble metals used in the noble
metal alloys have been suspected to cause troubles and
pathological reactions in some patients. Reference is
especially made to indium which is contained in nearly
all burn-on alloys; see the article of J. Wirz: "Scha-
digung des Paradontes durch zahnarztliche Werkstoffe"
(Damage of the paradontium by dentistry materials),
zahnarztliche Welt 102, 146 (1993). Palladium too is
suspected to provoke toxic or allergic reactions if it
is contained in higher proportions in the noble metal
alloy and is therefore able to be liberated by corro-
sion. Furthermore, the need for a universally applic-
able dental alloy becomes more and more relevant, i.e.
an alloy which is suitable both for conventional den-
tal prostheses which remain unfaced or are faced with
synthetic material, respectively, and for the techni-
que of metal-ceramic. Such alloys have the advantage
of excluding the danger of forming a galvanic element
due to the use of different alloys in the mouth cavi-
ty, accompanied by corrosion processes. Recently,
such universal alloys are promoted which, however, are
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on the base of Au-Ag-Pt-Cu with the addition of the
non-noble metals indium and zinc. These alloys pre-
sent the shortcomings that they are relatively suscep-
tible to corrosion, in particular due to the surface
oxides formed during burning which are not necessarily
covered by ceramic in the region of the crown edges
and are therefore accessible to saliva and its corro-
sive action. Furthermore, a special, low melting ce-
ramic is required for this alloy and which, dependent
on the manufacturing method, presents a higher corro-
sion rate than the known higher melting facing cera-
mics.
In addition,, alloys of high gold content gene-
rally present a bad high temperature creeping resis-
tance so that metallic, long span bridge structures
are generally deformed during the burning process and
loose their fitting precision. Therefore, long brid-
ges that are to be faced with ceramic must be made
from alloys having a higher palladium content, but
they do no longer have the desired old gold colour
appreciated by the patient, and they present the
biological drawbacks mentioned above.
On the other hand, the non-noble metals titanium
and tantalum have proven to be strongly biologically
compatible materials in dentistry. For example, im-
plants of titanium will heal in the bone without any
defense reaction due to the superficial formation of
titanium oxide which is very corrosion resistant and
inert, and allergic reactions on this material are
occurring extremely seldom, if at all. Therefore,
these metals are to be considered from a clinical and
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biological sight as ideal alloy partners for gold
which is known to be extremely corrosion stable too.
Titanium containing dental alloys of high gold
content which are suited for the facing with ceramics
have already become known from the patent literature.
DE-A-2,302,837 discloses a titanium alloy of high gold
content which further contains a relative high propor-
tion of platinum or a metal of the platinum group as
well as palladium which may provoke, as already des-
cribed above, allergic reactions. Furthermore, the
succeeding application published under DE-A-2,357,552
describes a titanium alloy of high gold content which
also contains an element of the platinum group, but no
quantitative indications can be found.
In addition, titanium alloys of high gold content
for the use in jewelry have become known, for example
from EP-A-0,190,648. These ornamental alloys must
respond to other criteria than those of dental alloys,
and compositions of ornamental alloys that are dis-
closed cannot be transferred automatically, see above,
to dental alloys.
Summarv of the Invention
Based upon this prior art, it is a first and ma-
jor object of the present invention to provide new and
useful dental alloys which not only present an out-
standing biological compatibility but also can be used
in a universal manner.
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There is another object of the invention to pro-
vide dental alloys which fulfil the standards ISO 1562
and ISO 9693 as well as DIN 13927.
A further object of the invention is to provide
dental alloys having a high gold content which pre-
sent, in spite of other metals alloyed thereto, the
pleasant and most desired yellow colour of genuine
gold.
All these objects are attained by the high gold
content dental alloys of the invention which comprise,
on a weight basis, 91 to 99.4 % of gold, 0.5 to 3 % of
at least one metal selected from titanium and tanta-
lum, 0 to 5 % of silver, and 0 to 1 % of at least one
element selected from the group comprising iridium,
rhodium, ruthenium, platinum, osmium, tungsten, iron,
molybdenum, niobium and rhenium.
An alloy which is particularly suited for the
intended purposes of this invention comprises 97.5 to
98.5 % of gold, 1.4 to 2.4 % of titanium and 0.05 to
0.15 % of iridium.
Detailed Description of the Invention
Tests have shown that an alloy composed of 91 to
99.4 % of gold, 0.5 to 3 % of titanium and/or tanta-
lum, 0 to 5 % of silver and 0 to 1 % of iridium, rho-
dium, ruthenium, platinum, palladium, osmium, tung-
sten, iron, molybdenum, niobium and/or rhenium, sur-
prisingly presents universal properties and can there-
fore also be used in conventional techniques; the al-
loy fulfils the requirements of the standards ISO 1652
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and ISO 9693 and of DIN 13927 as well. Since gold as
well as titanium are extremely biologically compati-
ble, an alloy of these two components is extremely
biologically compatible too and is furthermore very
attractive from an aesthetical viewpoint since the
titanium or the tantalum, respectively, do not sen
sibly influence the golden colour. Furthermore, this
alloy only contains a very small proportion of non
noble metals so that it does not loose its corrosion
stability.
Furthermore, it has surprisingly been found that
alloys having the mentioned composition display a very
good high temperature creeping stability. For exam-
ple, an alloy having the following composition: 97.5
to 98.5 % of gold, 1.4 to 2.4 % of titanium, and 0.05
to 0.15 % of iridium has a high temperature creeping
stability better than all of the noble metal casting
alloys used so far in the dental field. For the first
time, this alloys allow without problems to face long
metallic bridge structures, produced by casting from a
golden yellow alloy of a high gold content, with cera-
mics. It could be shown that on the addition of more
than 1 % of platinum to this alloy, as it has for
example been described in DE-A-2,302,837, the high
temperature creeping resistance is markedly reduced.
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