Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING A DENTAL PROSTHESIS
The invention relates to a process for the preparation of dentures.
Furthermore
the invention relates to pre-sintered blanks of zirconium oxide ceramic which
have a raw breaking resistance in a selected range.
= Ceramic dentures are normally prepared by grinding of densely sintered
ceramic
blanks.
.
Thus in EP-B-O 160 797 for example a blank and its use for the preparation of
dental shaped parts using a grinding tool is described. Furthermore a process
for
the preparation of ceramic dental prostheses is known from EP-A-0 630 622 in
which a blank of a certain composition is ground using a rotating tool.
A disadvantage of the processing of densely sintered blanks is in particular
their
high hardness which leads to long processing times and high wear of tools. The
costs of the processing of these blanks are thereby very high.
A disadvantage of grinding processes for the processing or preparation of
ceramic dentures is furthermore that a high-precision shape of the ground
blanks
cannot be ensured due to the lack of defined cutting edges.
The processing of blanks pre-sintered to a certain degree of hardness is
mentioned in principle in EP-A-O 630 622 on page 3, column 3, lines 13 ff.,
but
the processing of the blanks by grinding processes is retained.
Pre-sintered blanks have a lower hardness than those which are densely
sintered and show a higher hardness than those which are not sintered. It is
therefore desirable in principle, in order to guarantee easy processing or to
first
make processing possible, to use pre-sintered blanks.
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Thus for example the processing tools are less severely worn, which leads to
longer service lives of the tools and thus to considerably reduced costs. The
preparation of very fine microstructures also first becomes possible as the
predictable shrinkage of the ceramic during dense sintering leads to a further
reduction in size of the produced microstructures. The frequent microscopic
damage to the ceramic during processing can be cured in the case of pre-
sintered blanks within the framework of the dense sintering process.
In order to be able to prepare dentures by processing in the non-densely
sintered
state, a completely homogeneous distribution of the strength and hardness and
the density within each spatial direction of the ceramic blank is necessary,
which
in particular is also retained after the pre-sintering of the blank. It is
advantageous to avoid deviations in the density and hardness distribution of
the
ceramic when filigree structures or multi-section bridges are to be prepared,
as
even the slightest inhomogeneities can lead to breaking points which
considerably impair the durability of these complex structures during the
processing or can lead to a different sintering behaviour, behaviour which can
be
recognized from the distortion of the workpiece during sintering. Such a
distortion
leads however to poor fitting accuracy and thus to unusability of the denture.
For the following reasons the processing of pre-sintered blanks has up until
now
not led to a technical realisation:
The dense sintering of a pre-sintered blank after processing is associated
with
changes in dimensions which are difficult to calculate and can be applied to
the
actual milling parameters only by means of complicated processes. Thus
subsequent adjustments are necessary on non-accurately-fitting denture parts
after dense sintering. Due to the higher hardness of the densely sintered
denture
parts these have to take place using removal processes and are to be evaluated
as very critical, as a self-healing of injuries to the surface structures,
such as
takes place during the dense sintering process, can no longer be made good.
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In summary, there is a considerable demand for methods for the preparation
of accurately-fitting dentures using pre-sintered ceramic blanks.
Accordingly, the present invention provides an improved process for the
preparation of accurately-fitting, highly-precise dentures.
The present invention involves a process for the preparation of dentures,
comprising the steps: a) preparation of a blank comprising a pre-sintered
material having a raw breaking resistance from 15 to 30 MPa, b) processing
of the blank by milling methods, and c) dense sintering of the blank in a
temperature range from 1200 to 1650 C.
The present invention also provides use of a blank of pre-sintered material
with a raw breaking resistance of 15 to 30 MPa in a process for the
preparation of dentures, the blank being processed before dense sintering.
By blanks is meant within the framework of this invention a non-processed
material block or moulding which is subsequently passed to a shaping stage
through the processing. These blanks can consist of the most varied
materials, in particular ceramics.
By dentures are meant within the framework of this invention in particular
crowns and bridges having three or more sections. The blanks according to
the invention are particularly suitable for the preparation of bridges having
three or more sections.
By processing is meant within the framework of this invention milling
measures for shaping a blank, which lead to the blank being convened into a
shape coming as close as possible to a natural tooth. Not meant by
processing is cleaning of the blank processed in the above sense or the
removal of support and holding
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structures which result from the embedding of the blank in a blank holder,
even if
this cleaning can be carried out by milling methods.
The terms "comprise" and "containing" within the meaning of the present
invention introduce a non-limitative list of features.
= Customary raw breaking resistances known from the state of the art for
ceramic
dental blanks are in the higher strength range, for example from 75 to 110
MPa;
such blanks cannot be used for the invention.
It was found that the processing of pre-sintered blanks, the raw breaking
resistance of which lies outside the range according to the invention, does
not
lead to usable results. In the case of lower raw breaking resistances, the
resulting blanks are too soft, and can fracture during milling, whereas in the
case
of higher raw breaking resistances the resulting blanks are too hard, and
neither
can be processed with the normal processing procedures.
The processing of the blanks pre-sintered according to the invention is
carried
out with milling methods. Very fine microstructures can be produced by the
extremely sharp cutting edges of the milling tools. The cutting edges of the
tool
remain sharp over a long use period, as the blank in its pre-sintered state
has
only a low hardness and strength. During the milling of the blank the tool of
the
processing machine operates during rough working for example at a speed of
5,000 to 40,000 rpm, preferably 15,000 to 25,000 rpm with a feed rate of 20 to
5,000 mm/min, preferably 500 to 3,500 mm/min. The fine processing takes place
for example at a speed of 5,000 to 50,000 rpm, preferably 18,000 to 35,000 rpm
with a feed rate of 20 to 5,000 mm/min, preferably 500 to 3,500 mm/min. In
both
processing steps a milling diameter of 0.8 to 4 mm is used for example.
The blanks are particularly preferably processed without a supporting
structure
as described for example in the example of EP-A2-0 824 897. The processing
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step takes place from the side of the fully processed denture part in contact
with
the tooth stump and from the side not in contact with the tooth stump. It is
of
particular advantage that the blank need not be surrounded or supported by a
high-temperature investment compound during the dense sintering process.
During the course of the dense sintering process, the processed blank can be
= held by means of carrier devices which adapt independently to the
contraction
dimensions occurring during the baking process, such as are known for example
from the patent application DE-199 04 534, to avoid a distortion during the
sintering process.
The blanks can consist of normal dental ceramics. By dental ceramics are meant
within the framework of this invention compositions which, along with the
normal
ceramic constituents, can also optionally contain small amounts of other
constituents (additives), such as sintering auxiliaries. Data relating to
formulations in the form of components and wt.-% always relates to a product
which no longer contains additives. Small traces of additives, also in the pre-
or
post-sintered ceramics are of course also possible for kinetic, thermodynamic
or
chemical reasons and are therefore to be understood as also contained within
the scope of protection of this invention.
In particular the presence of impurities encourages the formation of glass
phases
or glass. Blanks which do not form any glass phases or glass during the dense
sintering are therefore preferred.
The blanks according to the invention furthermore display a preferred
deviation
from the linearity of the shrinkage per spatial direction which is less than
0.05 %,
particularly preferably less than 0.01 %.
The blanks according to the invention preferably consist of aluminium oxide or
zirconium oxide ceramic. Zirconium oxide ceramic is particularly preferred.
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It is known that the strength of nonmetallic-inorganic systems in general
depends
on the critical stress intensity factor Kic. This factor is clearly lower with
amorphous materials, for example glasses, than with purely crystalline systems
(D. Munz/T. Fett: Mechanisches Verhalten keramischer Werkstoffe [Mechanical
Behaviour of Ceramic Materials], Springer-Verlag). Thus the strength of
ceramics
also decreases if amorphous phases form at the grain boundaries. The ceramics
preferably usable according to the invention therefore display for example a
Kic
value of 5 to 10, preferably 8 to 10, measured according to EN 843.
Surprisingly it was ascertained that ceramics based on zirconium oxide with a
sinter addition of 0.1 to 0.50 wt.-% of at least one of the oxides of the
elements
aluminium, gallium, germanium, indium have a particularly favourable and
uniformly distributed hardness and strength. They are therefore particularly
suitable for the preparation according to the invention of complex dentures
and
filigree structures. It is an advantage if the oxides of the abovementioned
elements are added in an amount as defined above with homogenous
distribution and these are not, like say impurities, distributed non-uniformly
and
with varying concentration. This homogeneous distribution can be achieved for
example by co-precipitation as is described in the embodiment of this
invention.
In addition a uniform distribution of the particles formed during the pre-
sintering
process is an advantage. The granular form of the particles is preferably
equiaxial with an average grain diameter less than 1 pm, particularly
preferably
less than 0.7 pm.
The blanks used for the invention normally have a pore volume of 50 to 65 %.
The average pore size is normally in the range from 3 pm to 0.1 pm, the range
preferably being from 2 pm to 0.2 pm.
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In the case of this ceramic, the pre-sintering process is carried out in a
preferred
temperature range of 850 C to 1000 C, particularly preferably between 950 C
and 995 C, in order to achieve the raw breaking resistance according to the
invention. The pre-sintering process is carried out for example over a time
period
of 30 to 55 hours.
Such ceramic systems are known to have a tendency to shrink anisotropically,
i.e. have a shrinkage which is different in the three spatial directions. As
this
shrinkage is linear in itself in each spatial direction, these ceramics are
surprisingly extremely suitable for the preparation of extremely accurately-
fitting
and complex dentures.
The use of zirconium oxide ceramics in the medical field is generally known.
Pure
zirconium oxide cannot be used for mechanical applications as its volume
changes too much through modification changes during the cooling process after
sintering. Through the addition of magnesium, cerium or yttrium oxides,
however,
this process can be checked. A detailed discussion can be found in "Aluminium-
und Zirkonoxidkeramik in der Medizin" [Aluminium and Zirconium Oxide
Ceramics in Medicine], reprint from Industrie Diamanten Rundschau, IDR 2/1993
and also in EP-A-0 634 149.
The addition of 0.1 to 0.50 wt.-%, preferably 0.15 to 0.50 wt.-%, particularly
preferably 0.20 to 0.50 wt.-%, quite particularly preferably 0.25 to 0.50 wt.-
% of at
least one of the oxides of the elements aluminium, gallium, germanium, indium
to
such ceramics leads to the lowering of the sintering temperature and the
increasing of the stability and the hydrolytic resistance during use. This
situation
can also be found for aluminium oxide in "Zirconia Powder" 09/97, product
information from the company Tosoh. The ceramic is however not suitable for
the
preparation of accurately-fitting dentures according to the present invention,
as
without the maintenance of the raw breaking resistance according to the
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invention, milling to form highly-accurate dentures is not possible due to the
previously discussed effects.
Likewise a subject of the present invention is a pre-sintered blank made from
zirconium oxide ceramics of the composition (1), containing:
(A) 91 to 98.45 wt.-%, preferably 91 to 97.25 wt.-% zirconium oxide,
(B) 0 to 3.5 wt.-%, preferably 0 to 2.5 wt.-% hafnium oxide,
(C) 1.5 to 6.0 wt.-%, preferably 2.5 to 6.0 wt.-% yttrium oxide,
(D) 0.05 to 0.50 wt.-%, preferably 0.15 to 0.50 wt.-%, particularly preferably
0.20 to 0.50 wt.-%, quite particularly preferably 0.25 to 0.50 wt.-% of at
least one of the oxides of the elements aluminium, gallium, germanium,
indium,
(E) 0 to 1.9 wt.-%, preferably 0.0005 to 1.5 wt.-% coloring additives,
the wt.-% having to add up to 100 and the blank having a raw breaking
resistance of 15 to 30 MPa, preferably 23 to 28 MPa.
By component (E) of the composition (1) are meant coloring oxides from
elements of the group Pr, Er, Fe, Co, Ni, Ti, V, Cr, Cu, Mn, with Fe203, Er203
or
Mn02 preferably being used.
A further subject of the invention is a process for the preparation of ceramic
dentures, a blank of the composition (1) being processed by suitable
processing
measures into a shrinkage-matched enlarged model of the end denture and then
densely sintered to its end dimensions. By shrinkage-matched model is meant a
model of the desired denture enlarged according to part of the theoretically
expected shrinkage.
The composition (1) according to the invention is industrially prepared by
dissolving the components (A) and (B) of the composition (1) contained in
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commercially available zirconium sand with HCI, mechanically separating the
low-soluble impurities and combining them with the additives (C) and (D)
likewise
present as oxichlorides or chlorides after treatment with HCI as an aqueous,
strongly acid solution.
Additives according to component (E) acting as colorants are then added
likewise as chlorides, obtained through dissolution in HCI.
There follows a co-precipitation of the dissolved components by hydrolysis,
calcination of the precipitation product, grinding of the calcinate to the
desired
end fineness and also a spray-drying process using temporary slip and binding
agents.
The thus-obtained granules can be converted into the desired preform with
known compression processes. These compressed blanks are separated by a
binder-dependent heat treatment and pre-sintered at a temperature between
850 C and 1000 C, preferably between 950 C and 995 C, for example with 0.5
to 4 h holding time.
Ceramic powders containing the components (A) to (D) are also commercially
available (Tosoh, Tokyo, Japan).
The bianks processed with customary processes, for example CAD/CAM or
copy-milling, are densely sintered at 1200 C to 1650 C, particularly
preferably
1350 C to 1550 C, for example with 1 to 3 h holding time.
Preferably before the dense sintering, aesthetic measures such as individual
coloring can be carried out. Usable are for example processes according to the
patent application DE-199 04 522, the use of ionic solutions of at least one
of the
salts of the rare earth elements, of the lanthanides or the elements of the
group
Fe, Co, Ni, Ti, V, Cr, Cu, Mn being preferred.
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Optionally, after dense sintering, the ceramic blanks processed to form a
dental
prosthesis are removed from a blank-holding device, a holding device from the
utility model DE-298 154 86 for example being able to be used during
processing. After the removal from a blank-holding device, the blank can
optionally be re-processed for the purpose of removing holding pins or
connection points between the blank-holding device and the processed blank.
Furthermore, the blank can be faced using customary measures. For this
purpose, a facing compound which has the same coefficient of thermal
expansion as the blank can be burned onto the blank. Blanks which are suitable
for the present invention can for example have a coefficient of thermal
expansion
between 9.0 and 10.5 ppm/K, preferably between 9.4 and 9.8 ppm/K.
The invention is explained in more detail in the following by means of
examples
without thereby being limited in any way.
Strength data, in particular breaking resistances within the framework of
these
statements, relate to the "piston-on-three-ball test" according to ISO 6872.
To prepare the blanks according to the invention, preforms obtained while
applying pressure are taken as a basis. When preparing these preforms,
examples of starting materials are pure chlorides, oxichlorides or nitrates,
chlorides being used in the examples.
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Preparation examples 1 and 2
Zirconium oxide ceramic containing aluminium oxide
To obtain approx. 200 g of ready doped compressed granules, the components
are dissolved in distilled water according to the following table:
No. M(ZrCI4) M(YCI3=6H20) M(AICIa) M(FeCI3) M(ErCI3)
[9] [s] [9] [9] [g]
1[coloured] 355.6 33.4 0.65 0.77 0.29
(% as oxide) (94.0) (5.17) (0.25) (0.2) (0.38)
2[uncoloured] 357.66 33.36 0.65 0 0
(% as oxide) (94.55) (5.20) (0.25)
Component (A) (C) (D) (E) (E)
There follows a co-precipitation of the dissolved components by hydrolysis,
the
aforementioned solution being reacted with 32 I 6-molar aqueous NH4OH
solution. An at least 30-times excess of OH" concentration relative to the
stochiometric requirement is recommended. The precipitation product must then
be washed free of CI". The calcination of the precipitation product is carried
out at
700 C over 0.75 hours, followed by a grinding of the calcinate to an end
fineness
of D50 = 0.6 pm and also by a spray-drying process using temporary slip and
binding agents (here: 2.0 wt.-% PVA, 0.15 wt.-% oleic acid relative to oxide).
Using an isostatic press, for example at 1500 to 2500, preferably 1700 to 2200
bar, the granules obtained are made into preforms measuring d = 31 mm and I
150 mm.
The preforms are released from the binder by a heat treatment (heating rate: 4
K/min to 650 C, 1 hour holding time) and pre-sintered at a temperature of 970
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with 0.5 hours holding time to produce the blanks that can be used according
to
the invention.
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Process examgles
To prepare accurately-fitting bridges, blanks prepared according to the
preparation examples 1 and/or 2 are worked with a CAD/CAM system by milling
and densely sintered under the following parameters:
Heating rate: 10 K/min to end temperature: 1500 C
Holding time at end temperature: 2 h
The result is in both cases extremely accurately-fitting dentures with a high
strength (6 > 1000 MPa).
