Plastic and nonmetallic article shaping or treating: processes – Dental shaping type – Tooth forming
Reexamination Certificate
1999-05-13
2002-03-26
Derrington, James (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
Dental shaping type
Tooth forming
C264S016000, C106S035000, C433S223000
Reexamination Certificate
active
06361721
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention pertains to restoration of teeth. More specifically, the present invention pertains to materials and methods for forming porcelain tooth restorations for dental restoration of worn, damaged or malformed teeth.
2. Description of the Prior Art
There are many methods of restoring or repairing teeth. Practitioners in the field of restorative dentistry have developed several methods and materials for restoring worn, damaged or malformed teeth with porcelain restorations. Porcelain is attractive and relatively inexpensive. Porcelain restorations may be used for inlays, onlays, crowns and veneers to correct structural and/or cosmetic deficiency of teeth. Such porcelain restorations are custom made for bonding to an underlying or adjacent tooth structure.
In most recent times, there have been two basic methods for producing an all-porcelain restoration: the foil or “indirect” method and the refractory or “direct” method. In the foil method, a mold is made of the tooth structure on which the restoration is to be placed and a positive replication of the tooth structure is formed from a material poured into the mold. A platinum foil matrix is applied to and burnished over the tooth structure replication. Then, dental porcelain, in a water-based slurry, is applied over the foil matrix, baked in a furnace, ground and glazed to produce a restoration which can be bonded to the original tooth structure.
Though the foil method of producing a porcelain restoration has been proven, there are problems associated with such a method. Foil, by its nature, is difficult to completely form and adapt to the surface of a tooth structure replication and due to the fact that porcelain must be built up on the foil and must be removed from the tooth replication for subsequent firings and with the final peeling out of the foil from the finished restoration, the porcelain restorations frequently are deficient in accuracy of fit. This requires filling in with other materials so that the tooth restoration may be bonded to the original tooth structure.
The second and more accurate method of producing porcelain restorations requires the use of high heat resistant refractory investment materials molded in the shape of the tooth structure on which the tooth restoration is to be placed. The refractory investment replicates the original tooth structure and allows for direct application and subsequent firings of successive layers of dental porcelain thereto. When completed, the refractory investment is divested or removed from the finished porcelain restoration. This method, referred to as the refractory or “direct” method results in a porcelain restoration with far greater accuracy of fit.
Since porcelain has been the choice restorative for more than fifty years to replace natural teeth, it has become a common practice among dental practitioners to utilize the refractory investment method. In order to fabricate a porcelain restoration via the refractory method, a compatible porcelain must be applied to the surface of the refractory tooth replication. The commonly used porcelains conventionally used throughout the dental industry are those known as “regular” firing (1800° F.) porcelains or “low” firing (1250°-1500° F.) porcelains. Their coefficient of thermal expansion (CTE) ranges between 12 and 15×10
−6
. These conventional porcelains are made up of crystalline materials such as feldspar, silica and kaolin. Feldspar is the major ingredient making up about 80% of the composition.
The commercially available refractory investments have worked well with these porcelains due to their matched CTE and bonding characteristics. These refractory investments contain varying grades and compositions of silica (quartz), sand, cristobalite, zirconium, magnesium and phosphate. Since the CTE between these conventionally used refractories and porcelains are suitably matched, they represent the only materials used in the refractory method throughout the industry.
There is, however, another type of porcelain available to the dental industry known as “aluminous” or high-alumina based porcelain. This type of porcelain possesses an increased strength (approx. 20,000 PSI) over conventional porcelains (approx. 10,000 PSI) due to the high percentage (50% or more) of alumina in its composition. However, the CTE of high-alumina based porcelains is approximately 6 to 7×1
−6
. This prevents the use of conventional refractory investment materials for fabricating these high alumina based restorations.
With high-alumina porcelains, technicians have been limited to a foil or “indirect” method in which the high-alumina porcelain is applied to a platinum foil matrix which is burnished over a tooth replication made of gypsum. The foil containing the applied porcelain must then be removed from the gypsum before firing in a furnace. This procedure is time consuming, limiting in its scope of restorative types and expensive.
More recently, two other procedures have been developed to produce high-alumina dental restorations: 1) a computerized scanning machine that mills a high-alumina matrix from a block made of high-alumina material, and 2) a process that utilizes a computerized scan of a tooth to construct a tooth replication to which high-alumina particles are pressed and sintered under extreme temperatures. Both of these procedures are available but at very high expense to the industry.
Since high-alumina based porcelains possess a measurably higher strength than conventional porcelains, it would be desirable if a high-alumina restoration could be fabricated in a low cost approach via the refractory method. The reason that this has not yet occurred is due to the complete mismatch of CTE between high-alumina porcelains and existing refractory investments. This mismatch results in cracking, peeling and lifting of the porcelain off the refractory surfaces.
If refractory tooth replication materials and methods could be developed to which high-alumina porcelains would be adaptable and which would properly fire to produce an exact fitting restoration, it should be widely accepted in the industry. It's ease of production and cost effectiveness would greatly enhance the strength advantage of high-alumina porcelain restorations.
SUMMARY OF THE PRESENT INVENTION
The present invention provides materials and methods of forming a porcelain tooth restoration via the refractory method utilizing high-alumina based porcelains. By applying the high-alumina porcelain on a properly formulated refractory investment tooth replication, the tendency for the porcelain to crack or debond will be eliminated or greatly reduced.
In the method of forming a porcelain tooth restoration of the present invention, a negative impression of the tooth structure on which a tooth restoration is to be placed is prepared and a positive replication of the tooth structure is formed of the material of the present invention, an alumina-silicate based refractory investment. The material of this investment includes silica (quartz), magnesium oxide, phosphate and alumina. As stated, all prior art dental porcelain refractories contain varying amounts of silica, magnesium and phosphate, but they do not contain alumina. By adding a powdered alumina-silicate calcine in combination with silica, magnesium and phosphate, the resulting powder, when mixed with an aqueous colloidal silicate, produces a hardened positive tooth replication that will accept and match the CTE of high-alumina based porcelains.
A mixture is prepared of the alumina-silicate based refractory and aqueous hardener and poured into the negative impression of the tooth replication. After degassing and cooling the resulting positive tooth replication, a mixture of alumina-based porcelain materials may be applied and fired to finish forming the tooth restoration. Upon final firing of the tooth restoration, the alumina-silicate refractory material of the positive replication of the tooth structure may be removed, leaving the tooth restorati
Berryhill Bill B.
Derrington James
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