Dentistry – Prosthodontics – Tooth construction
Reexamination Certificate
1998-06-12
2001-03-13
Jones, Deborah (Department: 1775)
Dentistry
Prosthodontics
Tooth construction
C428S432000, C428S699000, C428S701000, C428S702000, C065S021200, C065S134100, C427S002290, C427S376200, C501S003000, C501S010000, C501S032000, C501S059000, C501S067000, C501S072000
Reexamination Certificate
active
06200137
ABSTRACT:
The invention relates to a chemically stable, translucent apatite glass ceramic which is particularly suitable for use in restorative dentistry and above all for coating or veneering of dental restorations, such as bridges or crowns.
Apatite glass ceramics are known from the prior art. They are usually employed as bioactive materials for replacing bone in human medicine, or as the main component of glass ionomer cements in dentistry.
In the case of bioactive materials for bone replacement, they have very high CaO and P
2
O
5
contents, however, in order to achieve bioactivity, i.e. the direct growing together of glass ceramic and living bone.
A glass ceramic implantation material is known from DE-A-40 20 893 which has apatite crystals but also contains very large quantities of CaO in order to achieve bioactivity.
Glass ceramics for glass ionomer cements also have high CaO contents and mostly also high fluoride ion contents, in order to obtain the desired high level of ion release in the oral medium.
These two types of apatite glass ceramics are white-opaque, however, and have a high level of ion release and/or bioactivity, so they are not suitable for restorative dentistry.
An apatite glass ceramic for restorative dentistry must have optical properties such as translucence and colour which are similar to those of the natural tooth. A material which is impervious to light, i.e. opaque, is not suitable for this purpose. Moreover, bioactivity or a high level of ion release is undesirable; rather, a high degree of chemical stability is required which should even exceed that of the natural tooth.
In known apatite-containing glass ceramics for restorative dentistry, the main crystal phase is regularly formed not by apatite but by leucite or mullite. This is undesirable, however, since these types of crystals make it difficult, inter alia to imitate the optical properties of the natural tooth material composed primarily of needle-shaped apatite.
EP-A-0 690 030 discloses leucite-containing phosphosilicate glass ceramics which may be used in dental engineering. In view of the leucite content, however, they have very high thermal expansion coefficients so they are not -suitable for the coating of materials with low expansion coefficients, such as lithium disilicate glass ceramics.
Moreover, an apatite glass ceramic containing mullite as a further crystal phase is described by A. Clifford and R. Hill (Journal of Non-Crystalline Solids 196 (1996) 346-351). The high mullite content brings about only low translucence.
Apatite-containing glass ceramics are disclosed by S. Hobo et al. (Quintessence International 2 (1985) 135-141) and Wakasa et al. (J. Oral Rehabil. 17 (1990) 461-472 and J. Mat. Sci. Lett. 11 (1992) 339-340) for restorative tooth replacement. Said glass ceramics have high CaO and P
2
O
5
contents, however, so they show only poor chemical stability. Moreover, the apatite crystals in these glass ceramics do not have a needle-shaped morphology.
Moreover, DE-A-34 35 348 describes apatite-containing glass ceramics for the production of dental crowns. The glass ceramics, however, contain no Al
2
O
3
at all and very large quantities of CaO, for which reason they have a high tendency to ion exchange and consequently only poor chemical stability. In addition, the apatite crystals do not have the needle-shaped morphology which is characteristic of apatite crystals of natural tooth material.
Glass ceramics with good chemical stability are disclosed in EP-A-0 695 726 as alkali-zinc-silicate glass ceramics. The disadvantage of said glass ceramics, however, is that they contain leucite rather than apatite as the crystal phase. As a result of the high expansion coefficient of leucite, the glass ceramics are therefore usually unsuitable as coatings for substrates with low expansion coefficients, such as, in particular, lithium disilicate glass ceramics. The glass ceramic also necessarily contains ZnO in order to achieve good chemical stability.
The object of the invention, therefore, is to provide an apatite glass ceramic which resembles natural tooth material in terms of its optical properties and, in particular, its high translucence, and contains apatite crystals which have a greater chemical stability than the carbonate-apatite crystals of natural tooth material and hence confer excellent chemical stability on the glass ceramic. Moreover, the apatite glass ceramic should preferably contain apatite crystals with a similar morphology to that of the apatite crystals of natural tooth material and have a low thermal expansion coefficient and should therefore be particularly suitable as a dental material and above all as a coating or veneer for dental restorations, such as crowns or bridges, made of lithium disilicate.
Surprisingly, said object is achieved by the chemically stable, translucent apatite glass ceramic according to the present invention.
The invention also provides the process for the production of the apatite glass ceramic, a dental material as well as dental uses and shaped dental products.
The apatite glass ceramic according to the invention is characterised in that it contains CaO, P
2
O
5
and F in a molar ratio of:
CaO:P
2
O
5
:F 1:0.020 to 1.5:0.03 to 4.2
and contains apatite crystals as the main crystal phase.
The molar ratio of CaO:P
2
O
5
:F in the glass ceramic is preferably 1:0.1 to 0.5:0.1 to 1.0 since this leads to particularly stable glass ceramics.
Surprisingly, it has become apparent that by adjusting the molar ratio of CaO to P
2
O
5
to F in the stated manner, apatite glass ceramics are obtained which show an increased chemical stability compared with conventional apatite glass ceramics.
In order to quantify the chemical stability, the test method according to ISO specification 6872:1995 described for glass ceramics was carried out, in which the resistance to aqueous acetic acid solution is determined by measuring the loss of mass in &mgr;g/cm
2
that occurs. The essential stages of this method are given in the Examples.
The apatite glass ceramic according to the invention usually exhibits a loss of mass of less than 200 and preferably less than 100 &mgr;g/cm
2
. Particularly preferred glass ceramics have a loss of mass of less than 60 &mgr;g/cm
2
.
Advantageous apatite glass ceramic according to the invention is also characterised in that it contains at least one of the following components:
Component
Wt. %
SiO
2
45.0 to 70.0
Al
2
O
3
5.0 to 22.0
P
2
O
5
0.5 to 6.5
K
2
O
3.0 to 8.5
Na
2
O
4.0 to 13.0
CaO
1.5 to 11.0
F
0.1 to 2.5
The glass ceramic according to the invention may additionally contain at least one of the following components:
Component
Wt. %
B
2
O
3
0 to 8.0
La
2
O
3
0 to 5.0
Li
2
O
0 to 5.0
BaO
0 to 5.0
MgO
0 to 5.0
ZnO
0 to 5.0
SrO
0 to 7.0
TiO
2
0 to 4.0
ZrO
2
0 to 4.0
CeO
2
0 to 3.0
The lower limits for those additional components are usually 0.05 wt. %.
Preferred quantity ranges exist for the individual components of the apatite glass ceramic according to the invention. Unless otherwise specified, these may be chosen independently of one another and are as follows:
Component
Wt. %
SiO
2
50.0 to 68.0
Al
2
O
3
7.0 to 21.0
P
2
O
5
0.5 to 4.0
K
2
O
4.0 to 8.0
Na
2
O
4.0 to 11.0
CaO
2.0 to 8.0
F
0.2 to 2.0
B
2
O
3
0.2 to 4.0
La
2
O
3
0 to 3.0
Li
2
O
0 to 3.0
BaO
0 to 4.0
MgO
0 to 4.0
ZnO
0 to 4.0
SrO
0 to 5.0
TiO
2
+ ZrO
2
0.2 to 5.0
CeO
2
0 to 2.0
Particularly preferred quantity ranges for the individual components of the apatite glass ceramic according to the invention are as follows and these may be chosen independently of one another:
Component
Wt. %
SiO
2
54.0 to 65.0
Al
2
O
3
8.0 to 21.0
P
2
O
5
0.5 to 3.5
K
2
O
5.0 to 8.0
Na
2
O
6.0 to 11.0
CaO
2.0 to 6.0
F
0.3 to 1.5
B
2
O
3
0.2 to 3.0
La
2
O
3
0 to 2.0
Li
2
O
0 to 2.0
BaO
0 to 3.0
MgO
0 to 3.0
ZnO
0 to 3.0
SrO
0 to 4.0
TiO
2
0.5 to 2.0
ZrO
2
0.5 to 3.0
CeO
2
0.1
Drescher Helga
Frank Martin
Holand Wolfram
Rheinberger Volker
Ivoclar AG
Jones Deborah
Nixon & Peabody LLP
Stein Stephen
LandOfFree
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