Glass ceramic dental crown and method of manufacturing the same

A dental crown is prepared from a dental crown material including 7 to 11% by weight of Li.sub.2 CO.sub.3, 18 to 28% by weight of MgO, 9 to 15% by weight of Al.sub.2 O.sub.3, 38 to 48% by weight of SiO.sub.2 and 10 to 15% by weight of Na.sub.2 SiF.sub.6. The dental crown material is melted and the molten material is molded in a lost wax mold. The molded material is crystallized to form the dental crown.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
This invention relates to a glass ceramic dental crown and a method for 
manufacturing the same. 
(2) Description of the Prior Art 
The lost wax process has hitherto been known as the technique of 
fabricating a dental crown. To describe briefly, the process involves 
applying wax on a damaged tooth to prepare a wax pattern, investing the 
wax pattern with a mold material or an investing material, and hardening 
the investing material. The wax pattern is then thermally removed to 
provide a mold. A dental crown material is cast in the mold to fabricate 
the predetermined dental crown. 
Hitherto, metallic materials including alloys have been used as a dental 
crown material. However, the metallic materials constitute a chemical 
battery in the oral cavity, and dissolve out metal ions within the oral 
cavity, tending to harmfully affect the human body. Moreover, the metallic 
material has a less attractive appearance, and, what is worse, frequently 
imparts an unpleasant feeling to the user due to different thermal 
conductivity between the natural tooth and the metallic dental crown. 
Recently, a new artificial dental crown has been proposed which is prepared 
from crystallized glass of the calcium phosphate series. The crystallized 
glass of the calcium phosphate series has a composition resembling that of 
a natural tooth as described in Japanese Patent Disclosure No. 51-73019, 
and is regarded as adaptable as a dental crown material. At present, 
clinical research work is undertaken on the proposed new dental crown 
material. The proposed artificial dental crown is fabricated by adding a 
nucleating material to relatively low melting glass of the calcium 
phosphate series (for example, CaO-P.sub.2 O.sub.5, CaO-Al.sub.2 O.sub.3 
-P.sub.2 O.sub.5), melting the mixture, molding the molten mass in the 
lost wax mold, and thermally treating the resultant transparent glass 
dental crown at a temperature approximating the crystal-precipitating 
level, thereby crystallizing the glass. 
However, the conventional dental crown material prepared from the 
crystallized glass of the calcium phosphate series reacts with the 
investing material such as cristobalite during the heat treatment for 
crystallization. As a result, the surface of the fabricated dental crown 
is roughened to adversely affect its color and transparency. Further, the 
glass of the calcium phosphate series constituting the conventional dental 
crown has one-dimensional or two-dimensional structure, namely, chain-like 
structure, and consequently has a lower chemical durability than the 
silicate glass having a three-dimensional structure. Moreover, the 
conventional dental crown has further drawbacks that when subjected to 
thermal crystallization, the conventional dental crown presents 
difficulties in attaining a uniform and compact crystallized structure, 
and the crown is reduced in mechanical strength. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a 
mechanically strong dental crown whose surface is not roughened, and a 
method of manufacturing such a crown. 
According to a first aspect of the present invention, a glass ceramic 
dental crown is provided which is prepared from a dental crown material 
comprising 7 to 11% by weight of Li.sub.2 CO.sub.3, 18 to 28% by weight of 
MgO, 9 to 15% by weight of Al.sub.2 O.sub.3, 38 to 48 % by weight of 
SiO.sub.2 and 10 to 15% by weight of Na.sub.2 SiF.sub.6. 
According to a second aspect of the invention, there is provided a method 
of manufacturing a glass ceramic dental crown, comprising the steps of: 
melting the above-mentioned dental crown material according to this 
invention; 
molding the molten mass in a crown-shaped mold fabricated by the lost wax 
process; and 
thermally crystallizing the molded mass within or out of said mold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As mentioned above, the glass ceramic dental crown according to this 
invention is prepared from a dental crown material including 7 to 11% by 
weight of Li.sub.2 CO.sub.3, 18 to 28% by weight of MgO, 9 to 15% by 
weight of Al.sub.1 O.sub.3, 38 to 48% by weight of SiO.sub.2 and 10 to 15 
% by weight of SiF.sub.6. The dental crown material may further include 
from 1 to 7% by weight of ZnO, CaF.sub.2, etc. and/or from 0.1 to 2% by 
weight of coloring material such as Fe.sub.2 O.sub.3, CeO, MnO, NiO etc. 
Particularly when the dental crown material is blended with from 2 to 6% 
by weight of ZrO.sub.2 and/or Y.sub.2 O.sub.3, the resultant dental crown 
is elevated in resistance to water and acids. 
The dental crown material according to this invention may be made suitable 
for anterior teeth. The dental crown composition most suitable for 
anterior teeth comprises 7 to 8% by weight of Li.sub.2 CO.sub.3, 21 to 22% 
by weight of MgO, 10% by weight of Al.sub.2 O.sub.3, 43.5% by weight of 
Si.sub.O.sub.2, 12 to 13% by weight of Na.sub.2 SiF.sub.6, 2 to 3% by 
weight of ZnO, 2 to 3% by weight of ZrO, 0.5% by weight of CeO, 0.01% by 
weight of Fe.sub.2 O.sub.3 and 0.06% by weight of NiO. 
The dental crown material of the invention may further be made suitable for 
molar teeth. The dental crown composition most suitable for molar teeth 
comprises 10 to 11% by weight of Li.sub.2 CO.sub.3, 20 to 21% by weight of 
MgO, 14 to 15% by weight of Al.sub.2 O.sub.3, 40 to 41% by weight of 
Si.sub.O .sub.2, 10 to 11% by weight of Na.sub.2 SiF.sub.6, 3 to 4% by 
weight of ZrO, 0.5% by weight of TiO.sub.2, 0.1% by weight of Y.sub.2 
O.sub.3 and 0.5% by weight of CeO. 
The known lost wax process is adapted for the preparation of a glass 
ceramic dental crown from the dental crown material according to this 
invention. First as shown in FIG. 1A, wax is applied to a damaged tooth to 
fabricate a wax pattern 11. Wax sprue line 12 is attached to wax pattern 
11. The whole assembly is allowed to stand at the apical point of crucible 
former or conical sprue base 13 by being fixed at the bottom end of sprue 
line 12. Sprue base 13 is then surrounded by casting ring 14, as shown in 
FIG. 1B. Investing material 15 of cristobalite series is filled in casting 
ring 14 and investing material 15 is hardened. Later as shown in FIG. 1C, 
wax pattern 11 and sprue line 12 are thermally removed, thereby providing 
lost wax mold 16. Later as illustrated in FIG. 1D, dental crown material 
17 according to this invention is molten at a temperature ranging between 
1350.degree. and 1400.degree. C. The molten mass is cast into lost wax 
mold 16. After hardened in lost was mold 16, the dental crown material is 
again thermally treated for crystallization at a temperature ranging 
between 720.degree. and 750.degree. C. in the mold 16 or a separate 
furnace. The time of thermal treatment generally ranges between 2 hours 
and 2.5 hours. At the time of the heat treatment, mica crystal phase and 
spodumene crystal phase settle out in the glass ceramics, thus providing a 
uniform and dense crystalline structure. The more voluminous the spodumene 
crystal phase which has settled out, the greater the mechanical strength 
of the resultant dental crown. It is preferred that the spodumene crystal 
phase be contained in the dental crown at the rate ranging between 5% by 
weight and 10% by weight. Further, the dental crown is more improved in 
machinability and has a more dense structure, as it contains a larger 
amount of mica crystal phase which has settled out. It is preferred that 
the mica crystal phase be contained in the dental crown at the rate 
ranging between 10% by weight and 20% by weight. 
After the thermal treatment is finished, the molded mass is taken out of 
the mold, and that portion of the molded mass which corresponds to the 
sprue line 12 is cut off. Thus is formed the dental crown according to 
this invention. 
The dental crown material used in this invention does not react with 
cristobalite investing material, thus offering the advantages that the 
heat treatment of the glass crystallization is conveniently performed in 
the lost wax mold and the resultant dental crown has a smooth surface. 
The present invention will become more apparent with reference to the 
examples which follow. 
EXAMPLE 1 
Three samples having compositions shown in Table 1 below were respectively 
molten at a temperature of 1400.degree. C. for one hour, providing 
transparent glass. 
TABLE 1 
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Sample Li.sub.2 CO.sub.3 
MgO Al.sub.2 O.sub.3 
SiO.sub.2 
Na.sub.2 SiF.sub.6 
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No. 1 8.6 28.0 11.8 37.1 14.5 
No. 2 9.4 24.2 13.4 39.5 12.6 
No. 3 10.8 21.2 14.9 42.1 11.0 
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Note: The numbers given above represent % by weight. 
Heating peak temperature was determined by differential thermal analysis in 
order to define a temperature required for the thermal crystallization of 
the respective samples. Curve A in FIG. 2 denotes the spectrum of Sample 
No. 1 having the largest content of MgO and Na.sub.2 SiF.sub.6. Curve B 
represents the spectrum of sample No. 2 having a medium content of MgO and 
Na.sub.2 SiF.sub.6. Curve C shows the spectrum of sample No. 3 containing 
least MgO and Na.sub.2 SiF.sub.6. 
Molten samples Nos. 1-3 were cast into a mold fabricated by the lost wax 
process, and molded into shape, using a centrifugal casting machine. After 
the molding, the glass dental crown embedded in the mold was heat treated 
in tact for crystallization. The crystallization treatment involved 
maintaining the glass dental crown at 750.degree. C. for 2 hours, raising 
the temperature up to 950.degree. C. and then allowing the dental crown to 
cool to room temperature. At this time, substantially no reaction took 
place between the embedded glass dental crown and the cristobalite 
investing material, providing a dental crown having a smooth surface. 
Table 2 gives the comparison of the various properties of the dental crowns 
of this invention, a porcelain dental crown and a dental crown prepared 
from CaO-P.sub.2 O.sub.5 crystallized glass. 
TABLE 2 
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Dental 
crown pre- 
pared from 
Samples of the Porcelain 
CaO--P.sub.2 O.sub.5 
Physical present invention 
dental crystallized 
properties 
No. 1 No. 2 No. 3 crown glass 
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Compression 
Good Good Good Good Good 
strenghth 
Indirect Slight- 
Low Slightly 
tensile ly good good 
strength 
Bending Good Slightly 
Good 
strength good 
Knoop Good Good Slight- 
Low Good 
hardness ly good 
Machinability 
Good Good Slight- 
Low Slightly 
ly good 
Biocompa- 
Good Good Good Slightly 
Good 
tibility good 
Chemical Good Good Good Good Slightly 
durability* good 
Color Same Same Same Same as a 
Same as a 
as a as a as a natural 
natural 
nat- nat- nat- tooth tooth 
ural ural ural 
tooth tooth tooth 
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*Chemical durability: Sample glass powder pulverized to 420-590 .mu.m was 
put in a platinum vessel and the vessel was immersed in a test liquid 
contained in a roundbottomed flask. The flask was heated ina boiling wate 
bath for 60 minutes. The weight decrease of the glass powder was measured 
The test liquid used was distilled water (pH: 6.5-7.5) in the case of 
water resistance test, and was 0.01 N nitric acid in the case of acid 
resistance test. resistance test, and was 0.01N nitric acid in the case 
of acid resistance test. 
Table 2 above shows that the samples of the present invention have 
properties the same as or higher than those of the porcelain dental crown 
and the dental crown prepared from CaO-P.sub.2 O.sub.5 crystallized glass 
in respect of compression strength, indirect tensile strength, bending 
strength, Knoop hardness, machinability, biocompatibility, chemical 
durability and color. Referring to mechanical strength, it has been found 
that the sample of this invention in which the spodumene crystal phase has 
noticeably settled out indicates a correspondingly great mechanical 
strength. It has also been discovered that the larger content of mica 
crystal phase ensures the better machinability and greater compactness of 
the resultant dental crown. 
The dental crown of this invention is possessed of not only affinity to 
living body, but also, when heat treated, presents mica crystal phase and 
spodumene crystal phase, and consequently indicates great machinability 
and mechanical strength after molded. Moreover, this present invention can 
manufacture a glass ceramic dental crown having a proper mechanical 
strength suitable for the anterior tooth or molar tooth by varying the 
relative proportions of the mica crystal phase and spodumene crystal 
phase. 
EXAMPLE 2 
The glass of sample No. 1 of Example 1 was heated at 750.degree. C. for 2 
hours and later heated up to 900.degree. C., and later allowed to cool, 
thus providing crystallized glass. Further, the glass of sample No. 1 of 
the present invention was mixed with 4 % by weight of zirconia (Zro.sub.2 
O) and 0.1% by weight of yttria (Y.sub.2 O.sub.3), thus providing glass 
sample No. 4. The sample No. 4 was treated in the same manner as mentioned 
above to provide crystallized glass. These two crystallized glass were 
pulverized into particles whose diameter ranged between 500 and 900 
microns. 5 grams of each powder were taken into separate flasks, each of 
which contained 100 cc of a 0.01 normal solution of nitric acid or ion 
exchange water having pH of about 7. The flasks were dipped for 1 hour in 
a water bath boiling at 100.degree. C. Measurement was made of a decrease 
in the weight of the respective crystallized glass powders, the results 
being set forth in Table 3 below. 
TABLE 3 
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Decrease in weight (% by wt.) 
Sample Ion exchange water 
Nitric acid 
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No. 1 0.09 0.37 
No. 4 0.04 0.19 
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Table 3 above shows that the crystallized glass to which zirconia and 
yttria were added was improved in the resistance to water and acid. 
As described above, this invention provides a dental crown which has a 
satisfactory biocompatibility, smooth surface and great mechanical 
strength, by a relatively simple method.