Biocompatible dental restoration system using layers of high strength ceramic, gold, and porcelain

The present invention is a multilayer dental system for the production of restorations which incorporates a high strength bridging ceramic core, a layer of biocompatible metal for aesthetics and encasing the abutment and a conventional porcelain top layer. The restoration is made by producing the high strength bridging ceramic core, electroforming the 24 K gold upon the core, and then completing the restoration by applying outer layers of conventional dental porcelain. A restoration of the present invention has a natural appearance, is lightweight, is highly biocompatible, does not impart thermal sensitivity, and is strong enough to be used for posterior bridgework. The system of the present invention eliminates the use of toxic alloys and will not result in unsightly oxidation, as is seen with other cast metal restorations. The high strength ceramic core does not slump with repeated firings the way cast alloys do when fired. The completed units can be bonded or cemented within the patient's mouth, and are easily handled by the dentist.

FIELD OF THE INVENTION 
This invention relates to a system for producing biocompatible restorations 
using layers of high strength ceramic, gold or other biocompatible metal, 
and porcelain, and the restorations, or other objects, thus produced. 
BACKGROUND OF THE INVENTION 
Electroforming is the electrolytic deposition of dimensionally stable metal 
layers on a substrate. Invented in 1838 by Jacobi in Russia, this 
technique was adapted for use in dentistry in 1856 by Newell. The use of 
this technique in dentistry was greatly expanded by the work of O. W. 
Rogers, who patented an apparatus and method of electroforming dental 
crowns in U.S. Pat. No. 4,288,298. The development of nontoxic plating 
solutions having high penetration power made it possible to produce 
porcelain fused to gold dental restorations where the gold deposition is 
of a precise and uniform thickness. This application was limited to a 
single crown, however, until the development of the cast metal pontic 
technique for bridges, and the 24 K gold was electroformed over the metal 
and abutments. The result was a multilayer metal bridge having sufficient 
stability for multiple units, that is, a bridge which spans multiple 
teeth. Two articles which discuss the use of electroforming in this way 
are "Electroforming as an Alternative to Full Ceramic Restorations and 
Cast Substructures" by Ronald M. Stewart, published in Trends and 
Techniques in the Contemporary Dental Laboratory, April 1994:42-47 and 
"Electroforming Technology for Ceramometal Restorations" by Tonino Traini, 
published in Quintessence of Dental Technology, 1995: 21-28. Gramm 
Technology, of Woodbridge, Va. and Tiefenbronn, Germany both manufacture 
dental systems for electroforming gold and related products. The founder 
of Gramm Technology, Gerhard Gramm, has several U.S. and foreign patents 
in this area, including U.S. Pat. No. 5,173,161, which describes a device 
for electroforming work pieces and WO 9207977, which describes a device 
for coating workpieces used in the dental field. 
Electroformation is also used in dentistry as a casting method to produce 
accurate dental prostheses, as described in U.S. Pat. No. 5,316,650, to 
Ratzker et al. This technique uses a metallic glass alloy containing 
cobalt and phosphorus. U.S. Pat. No. 4,451,639, to Prasad describes 
special metallic ceramics which have been developed for use in the 
electroforming process. The ceramic of Prasad contains many metals, 
including palladium, cobalt, gallium, gold, aluminum, copper, zinc and 
ruthenium or rhenium. Additionally, electroforming is used in other 
manufacturing methods beyond dentistry. For example, U.S. Pat. No. 
5,393,405, to lacono et al., discusses the production of jewelry using 
electroformation of multiple layers of gold where the layers differ in 
hardness. 
Ceramic restorations have long been an alternative to metal structures, 
having vastly superior aesthetics and perfect biocompatibility. 
Developments in high strength ceramics, that is, ceramics having a 
flexural strength greater than 300 megapascals (MPa), have made 
all-ceramic bridges possible. Such high strength ceramic is necessary for 
producing the coping portion of the all-ceramic restoration. In 
particular, as described in U.S. Pat. No. 5,695,337 to Sadoun, a ceramic 
formulation containing multiple metal oxides displays the desired 
strength. The amount of Al.sub.2 O.sub.3 has been increased to 85% by 
weight in a commercial product known as In-Ceram, produced by VITA 
Zahnfabrik H. Rauter GmbH & Co., Bad Sackingen, Germany. This ceramic has 
a strength of 600 MPa and functions well in single units and anterior 
bridges. The general method of using this material is reported in "Working 
with the In-Ceram Porcelain System" by Harry Levy and Xavier Daniel, 
published in Prothese Dentaire 44-45 45: June-July 1990. A further 
discussion of this technique can be found in "A renaissance of ceramic 
prosthetics?" by Norbert Futterknecth and Vanik Jinoian, published in 
Quintessence of Dental Technology, special reprint, 1992. The inceram 
zirconia high strength ceramic (AL.sub.2 O.sub.3 --ZrO.sub.2) has 
considerably improved mechanical characteristics. This makes posterior 
bridges possible. Due to the increased opacity, however, there are 
aesthetic limitations. 
Each of these dental systems have serious drawbacks. Metal-base 
restorations have biocompatibility problems, often reflected as gingival 
degeneration, which reveals a dark metal margin on the restored tooth 
which ruins the aesthetic appearance of the restoration. Metal has poor 
thermal conductivity qualities and can corrode over time. The preparation 
of metal copings often involves toxic metals which present environmental 
problems for the dental office personnel and during disposal. Ceramic 
restorations, when of sufficient strength for posterior bridgework, are 
too opaque to provide good aesthetics. The present invention has solved 
these problems by providing a restoration which does not contain materials 
which cause reactions in the body, do not promote temperature sensitivity, 
and are relatively non-toxic. Additionally, the present invention provides 
vivid aesthetics with the warm color of the 24 K gold deposition, and of 
all the alloys, 24 K gold is highly biocompatible. 
SUMMARY OF THE INVENTION 
Accordingly, the present invention is a multilayer dental system for the 
production of restorations which incorporates a high strength bridging 
ceramic core, a layer of 24 K gold for aesthetics, and a conventional 
porcelain top layer. The restoration is made by producing the high 
strength bridging ceramic core, electroforming the 24 K gold upon the core 
and abutments, and then completing the restoration by applying outer 
layers of conventional dental porcelain. A restoration employing the 
present invention has a natural appearance, is lightweight, is highly 
biocompatible, does not impart thermal sensitivity, and is strong enough 
to be used for posterior bridgework. The system of the present invention 
eliminates the use of toxic alloys and will not result in unsightly 
oxidation, as is seen with other cast metal restorations. The high 
strength ceramic core does not slump with repeated firings the way cast 
alloys do when fired. The completed units can be bonded or cemented within 
the patient's mouth, and are easily handled by the dentist.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is a multilayer dental system that can be used to 
produce restorations that have excellent aesthetics and are biocompatible. 
A preferred embodiment of the system utilizes three layers. The bridging 
core of the system is high strength ceramic, preferably a ceramic high in 
aluminum oxide or zirconia content. The second layer a biocompatible 
metal, preferably 24 K gold, which is electroformed onto the high strength 
bridging ceramic core and abutments after application of a bonder to the 
ceramic core. Those of skill in the art will realize that the 
biocompatible metal may include platinum. The final layer is one or more 
applications of conventional dental porcelain, which are baked onto the 
metal surface after a second application of bonder to the surface of the 
electroformed metal. Thus, the bonder is used both to attach the core 
ceramic to the metal and the metal to the outer porcelain layer. Once all 
three layers are present, the restoration undergoes a final contour stain 
and/or glaze to achieve the desired appearance, and is then ready for the 
dentist to cement or bond the restoration in the patient's mouth. 
In order to produce the high strength bridging ceramic core for the system 
of the present invention, an impression and a master cast are produced in 
the manner well known in the art. For example, the impression can be taken 
using the putty/wash technique or any non shrink silicone material or 
reversible hydrocolloid. Whatever method is used, the impression should be 
clean without air bubbles, free from distortion, and the margins of the 
prepared tooth should be visible in the impression. The impression is used 
to produce the master cast using standard techniques and duplicate models 
are produced from the master. Specifically, once the master cast dies have 
been sectioned, the margins are exposed and marked with a pencil. The 
undercuts should be blocked out with wax, particularly if a bridge 
abutment is being produced. If the model is cast in stone, two coats of 
dies spacer should be applied. Epoxy resin models require three coats. The 
die spacer should be easily removable and should remain on the die only 
during duplication. 
An impression of the sectioned dies of the master are taken with high 
quality silicone. Optec-Exact (Generic Pentron, Wallingford, Conn.) or 
Elite vinyl polysiloxane (Zhermack, Rovigo, Italy) are suitable for this 
purpose. This impression is then cast with plaster specific for this use, 
In-Ceram plaster, produced by VITA, Bad Sackingen, Germany. The plaster 
must be mixed according to an exact water to powder ratio to insure that 
the core will be the correct size after sintering. Once the stone has 
hardened fully, the cast is removed from the impression, the margins are 
marked with pencil, and a sealer is applied to the entire cast. 
The next step in the process is preparation of the Al.sub.2 O.sub.3 or 
Al.sub.2 O.sub.3 --ZrO.sub.2 slip. Mixing fluid is combined with a small 
amount of binder, i.e. one drop binder to 5 ml mixing fluid. This mixture 
is placed in an ultrasonic unit, and the Al.sub.2 O.sub.3 powder--38 g for 
5 ml of slip--is sprinkled into it. The mixing and ultrasonic unit 
transforms the paste into liquid slip, similar in consistency to oil 
paint. The slip is placed under vacuum for several seconds before use. 
Slip can be applied using a brush with synthetic bristles, or by dipping. 
Once the cast has been built up with the slip, the margins should be 
reduced with a scalpel-like instrument until the previously placed pencil 
line is visible. This shaping process is not difficult, because the slip 
is similar to wax in consistency and feel. 
The built up cast with the slip is then fired two times, but can be fired 
at least once. During the first firing, the temperature is slowly raised 
from 20.degree. C. to 120.degree. C. over 6 hours. During this time, the 
stone gives off moisture and shrinks away from the core. The firing 
temperature is then raised from 120.degree. C. to 1120.degree. C. at a 
rate of 10.degree. C. per minute and this temperature is held for 2 hours. 
This firing results in a solid phase sintering, where the Al.sub.2 O.sub.3 
crystals fuse. The core is removed from the cast and transferred to the 
master dies in order to check the precision of the fit. Adjustments to the 
fit of the core can be made using a diamond burr. 
The final step in the production of the high strength bridging ceramic core 
is a glass filtration firing. The infiltration of glass provides the 
strength to the core. The glass powder is mixed with water and a thick 
coating is applied to the core, which is placed on a piece of platinum 
foil. The core is then fired where the temperature is raised quickly to 
1100.degree. C., where it remains for a time dependent on the thickness of 
the core wall. During this time the glass liquefies and infiltrates the 
Al.sub.2 O.sub.3 or Al.sub.2 O.sub.3 --ZrO.sub.2 of the core. Once the 
firing is complete, a sandblaster can be used to remove the excess glass. 
For example, the sandblaster can be loaded with 50 micron Al.sub.2 O.sub.3 
and run at a pressure of 6 kg/cm.sup.2 or 80 lb/in.sup.2. 
The completed high strength bridging ceramic core is then brushed over with 
a bonder, preferably Bonder A, produced by Gramm Technology, of 
Woodbridge, Va. and Tiefenbronn, Germany. Bonder is put on the core 
wherever metal deposition is desired. Generally, the entire outer surface 
of the restoration requires a layer of metal, and the inner surface, that 
is, the surface ultimately in contact with the patient's prepped teeth is 
not coated. Partial coverage is also possible, where only a pontic is 
created with approximal rests. The bonder is fused to the core by baking 
in an oven at 950.degree. C. for approximately six minutes. A stone 
duplicate is glued to the core, and a copper wire is attached to the 
duplicate by drilling a small hole and gluing the tip of the wire to the 
stone with cyanoacrylate glue. Silver lacquer is also applied to complete 
the electrical path between abutments and the bonder on the ceramic core 
and the wire. Standard electroforming equipment, such as the Gammat Dent, 
produced by Gramm Technology, of Woodbridge, Va. and Tiefenbronn, Germany 
is used to electroform the metal onto the core surface. The use of 24 K 
gold is preferred, as this metal is biocompatible and lends a warm, 
natural color to the final restoration. The equipment has internally 
programmed processes which automatically checks the temperature and 
intensity of the current flow. Desired thickness for this application is 
about 0.2 mm and is generally achieved in about 8-11 hours, using a 
standard 24 K gold electroplating solution. When deposition is complete, 
the outer surface is sandblasted and a coating of Bonder A is applied to 
the gold, and baked by firing at 950.degree. C. for six minutes. 
Standard porcelain layering techniques well known in the art are used to 
complete the restoration of the present invention. Only a thin layer of 
opaque is necessary because the layer of gold has an ideal warm tone for 
the finished restoration. After a final contour, stain and glaze, the 
completed restoration is ready for the dentist to cement or bond to the 
patient's prepared teeth. 
Non-dental applications, such as the creation of jewelry, which require the 
strength or aesthetic characteristics of the present system are also 
contemplated. All disclosures of the references described herein are 
hereby incorporated by reference. 
Other variations and modifications of the present invention will be 
apparent to those of ordinary skill in the art, and it is the intent of 
the appended claims that such variations and modifications be covered. The 
particular values and configurations discussed above can be varied, are 
cited to illustrate particular embodiments of the present invention and 
are not intended to limit the scope of the invention. It is contemplated 
that the use of the present invention can involve components having 
different characteristics as long as the principle, the presentation of a 
biocompatible dental restoration system using layers of high strength 
ceramic, gold, and porcelain, is followed.