Stock material or miscellaneous articles – Composite – Of silicon containing
Reexamination Certificate
1999-11-12
2002-01-29
Copenheaver, Blaine (Department: 1775)
Stock material or miscellaneous articles
Composite
Of silicon containing
C428S697000, C428S699000, C428S701000, C428S702000, C501S006000, C501S068000, C106S035000, C433S206000, C433S212100, C433S223000
Reexamination Certificate
active
06342302
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a ceramic dental restoration based on a leucitic glass-ceramic which, in particular, can be obtained from a dental-ceramic material according to the invention being press-moulded in the viscous state into a shape corresponding to the dental restoration.
Dental restorations, such as artificial teeth, crowns, partial crowns, bridges, inlays, onlays, facets, stump reconstructions, tooth-root constructions etc., are predominantly manufactured from metal alloys, in particular from alloys based on precious metals. These metal alloys are usually faced for aesthetic reasons with ceramic, in order to give the restoration, particularly in the visible region, an appearance corresponding as far as possible to the natural tooth.
Facing-ceramics have to be very carefully matched as regards melting-point and coefficient of thermal expansion (CTE) to the material of the basic framework. In the case of metal-ceramic systems, ordinarily the CTE of the ceramic is chosen so that the coefficient lies slightly below that of the metallic base material. In this way, tensile stresses in the facing-ceramic are prevented and compressive stresses are induced instead, so that in the course of firing-on and cooling and also in the event of later loads as a result of fluctuation in temperature the occurrence of fissures is avoided. Facing-ceramics that are matched to the high gold containing dental alloys which have been dominant for a great many years by reason of better compatibility typically possess a linear CTE &agr;
(20-500° C.)
from about 12.5·10
−6
to about 16.5·10
−6
K
−1
. Such ceramic materials are described in EP 0 478 937, for example. A facing-ceramic pertaining to this category which is frequently employed in practice exhibits a CTE &agr;
(20-500° C.)
of 15.0·10
−6
K
−1
.
As an alternative to metal-ceramic dental restorations, fully ceramic systems are being employed on an increasing scale. By way of base materials, use is predominantly made of glass-ceramics in this case. Materials are designated as glass-ceramics in which at least one crystalline phase is present in a distributed manner in a glass phase. Glass-ceramic may be obtained from an amorphous primary glass being subjected to a controlled partial crystallization process.
In U.S. Pat. No. 4,798,536, for example, fully ceramic dental restorations consisting of glass-ceramic based on feldspar are described, said restorations containing at least 45 wt.-% leucite by way of crystalline phase. In this case the dental product is shaped using slip technology and then sintered.
A technology for the manufacture of fully ceramic dental restorations has proved particularly advantageous in which a suitable dental-ceramic material is converted into the viscous state under the influence of high temperature and under pressure and is press-moulded into a shape corresponding to the dental restoration. Such a material, as well as the dental product produced therefrom, is also frequently designated in dental technology as pressed ceramic. This technology and a pressing furnace suitable for it are described in EP 0 231 773 A1.
In DE 44 23 793 C1 a dental glass-ceramic which is capable of being processed as pressed ceramic is described which contains leucite and at least one other crystalline phase.
In DE 44 23 794 C1 a dental glass-ceramic which is also capable of being processed as pressed ceramic is described which contains, by way of crystalline phase, ZrO
2
and at least one other crystalline phase.
In DE 196 47 793 A1 a dental glass-ceramic which is capable of being processed as pressed ceramic is described which contains lithium disilicate by way of crystalline phase.
The known glass-ceramics based on leucite that are employed in practice as pressed ceramic exhibit a CTE &agr;
(20-500° C.)
which, as a rule, lies in the range from about 14·10
−6
to about 20·10
−6
K
−1
, and should therefore be capable of being combined with the conventional facing-ceramics having a low CTE.
In practice, however, it has been shown that the manufacture of fully ceramic dental restorations by combining these known glass-ceramics by way of framework material with the facing-ceramics that are used in metal-ceramic systems leads to problems, despite the similar matching of the coefficients of thermal expansion. For instance, in the course of facing work a high reject-rate is observed as a result of cracking. Fluctuating-temperature loads result, to an unacceptable degree, in further cracking and failure as a result of fracture. Furthermore, in the course of the multiple firings which are usually required in practice a drifting of the CTE of the glass-ceramic parent substance towards higher values is observed, which again favours formation of fissures and fracture.
An object of the invention was therefore to develop a glass-ceramic for the manufacture of ceramic dental restorations with which the problems that have been described do not arise in the course of facing with currently available facing-ceramics.
The above and other objects of the invention can be achieved if, in the case of ceramic dental restorations by way of base material, a leucitic glass-ceramic is employed which is characterized in that it comprises
40-95
wt.-%
SiO
2
5-25
wt.-%
Al
2
O
3
5-25
wt.-%
K
2
O
0-25
wt.-%
Na
2
O
0-20
wt.-%
CaO
0-8
wt.-%
B
2
O
3
0-0.5
wt.-%
P
2
O
5
0-3
wt.-%
F,
and, as the sole crystalline phase, leucite in a total proportion of 20-45 wt.-%, at least 80% of the theoretically producible quantity of leucite being present, and which exhibits a linear coefficient of thermal expansion &agr;
(20-500° C.)
from 12.5·10
−6
to 15.5·10
−6
K
−1
.
The present invention consequently provides a ceramic dental restoration based on a leucitic glass-ceramic which is described above.
In a further aspect, the invention provides a process for manufacturing a ceramic dental restoration based on leucitic glass-ceramic wherein a glass-ceramic which is described above is press-moulded in the viscous state into a shape corresponding to the dental restoration.
The invention is based on the surprising discovery that in the case of fully ceramic dental restorations it is advantageous, with a view to avoiding cracking and consequential damage, to have the CTE of the base ceramic, in contrast with the tried and tested situation in the case of metal-ceramic systems, to be lower than that of the facing-ceramic. In the case of leucitic glass-ceramic the CTE is dependent on the content of leucite, the pure glass phase exhibiting a CTE &agr;
(20-500° C.)
of about 10·10
6
K
−1
and the pure tetragonal leucite exhibiting a CTE &agr;
(20-500° C.)
of about 20·10
−6
K
−1
. Leucite, K[AlSi
2
O
6
], may be formed by crystallization in a primary glass if the latter contains the components SiO
2
, Al
2
O
3
and K
2
O.
The glass-ceramic which is provided for manufacturing the ceramic dental restorations according to the invention comprises by way of components
40-95
wt.-%
SiO
2
5-25
wt.-%
Al
2
O
3
5-25
wt.-%
K
2
O
0-25
wt.-%
Na
2
O
0-20
wt.-%
CaO
0-8
wt.-%
B
2
O
3
0-0.5
wt.-%
P
2
O
5
0-3
wt.-%
F.
The glass-ceramic may contain by way of further components
0-10
wt.-%
La
2
O
3
0-10
wt.-%
Sb
2
O
3
0-10
wt.-%
Li
2
O
0-20
wt.-%
MgO
0-20
wt.-%
BaO
0-20
wt.-%
SrO
0-3.5
wt.-%
ZnO
0-30
wt.-%
TiO
2
0-14
wt.-%
ZrO
2
0-30
wt.-%
CeO
2
0-30
wt.-%
SnO
2
.
The glass-ceramic preferably comprises:
50-80
wt.-%
SiO
2
12-25
wt.-%
Al
2
O
3
7-18
wt.-%
K
2
O
0.5-25
wt.-%
Na
2
O
0.1-2.5
wt.-%
CaO.
By virtue of this composition it is established that the glass-ceramic which has been processed into the dental product contains, as sole crystalline phase, leucite in a total proportion from 20 to 45 wt.-% and that it consequently exhibits a linear coefficient of thermal expansion &agr;
(20-500° C.)
from 12.5·10
−6
to 15.5·10
−6
K
−1
. In this case at least 80% of the theoretically producible quantity of leucite is present in the glass-ceramic. By virtue of is the virt
Assmann Steffen
Steidl Jürgen
Copenheaver Blaine
Degussa - AG
McNeil Jennifer
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