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Dental appliance and method of manufactureVIDEO ON THE TOPIC: How It’s Made Fridge
A tooth crown or the like fixed, custom fitted dental appliance made of a fired ceramic which undergoes substantially no shrinkage when fired, the appliance being made by forming a compact of the raw prefired ceramic with an undersurface shaped for a precise fit onto the prepared tooth on which the appliance is to be used, and thereafter firing the compact to a dense hard monolithic structure.
This invention relates to a method for the fabrication of dental appliances such as crowns, bridges and the like, and to dental appliances made by such method. For simplicity, the invention and its background will herein be principally with reference to tooth crowns and their manufacture, though it will be understood that the invention has application to other dental appliances such, for example, as dental bridges. A dental crown is one of the most important restorations in dentistry in that it affords the restoration of deteriorated teeth to a state of health and function.
One of the prime requisites for a successful dental crown is that it adapt as perfectly as possible to the prepared tooth structure i. Anything less than substantially perfect fit, especially at the crown margin, can result in mechanical failure such as crown fracture or loosening or biologic failure such as decay, periodontal disease, or occlusal problems.
At the present state of the art the materials available for the fabrication of crowns include the noble metals such as gold base alloys, acrylics such as polymethyl methacrylate, dental porcelain, or a composite of a metal and a compatible porcelain or acrylic.
At the present date the most widely employed method of fabricating dental crowns is to cast a metal substrate which is subsequently covered with a dental porcelain veneer. The metal substrate is obtained by an indirect process known as the "lost wax" technique.
Specifically, the substrate is first formed and shaped in wax. This involves making an impression of the prepared tooth, casting dental stone against the impression to form a master die, molding a body of wax against the master die and then shaping the upper portions of the wax body to duplicate the external shape of the tooth crown desired.
The wax form must then be accurately converted to the metal. To accomplish this the wax form is invested into a refractory material, thus forming a mold. The wax is then eliminated by melting or burning, thus creating a cavity into which the metal, heated to a molten state, is cast. Hence, the wall of the cavity intended to be an exact replica of the prepared tooth can suffer inaccuracies because of the three-step technique needed to prepare it--preparation of the master die, preparation of the wax form from the master die, and preparation of the mold from the wax form.
This renders the process technique-sensitive. The process is even further technique-sensitive in that each metal alloy possesses a specific casting shrinkage for which there must be accurate compensation. This compensation is critically dependent upon the proper selection and skillful manipulation of all materials employed. The technique is sensitive in that failure can occur at any of the numerous steps involved, such as distortion of the wax form, improper investing procedure, inadequate expansion of the mold, improper burn-out procedure, or improper melting and casting of the metal.
A principal drawback of this technique is that many hours are consumed before a failure in the casting stage can be detected. The success or failure of the composite ceramic-metal crown is also greatly dependent on the proper finishing and handling of the surface of the metal to which the ceramic is applied.
Proper finishing which is difficult and time consuming, is essential to the successful bonding of the ceramic to the metal substrate. Still further, the inherent physical differences between the porcelain veneer and the metal substrate, such as differences in thermal expansion, give rise to numerous potential avenues for failure.
An alternative technique, which eliminates the need for a metal casting, is to form the crown of a dental porcelain with a thin underlayer of platinum foil. The thin platinum foil is carefully shaped to a replica of the prepared tooth and the dental porcelain is then overlaid onto the foil which supports the porcelain during the subsequent firing cycles necessary. This procedure is extremely technique-sensitive, requiring skillful and meticulous processing.
Proper and accurate shaping of the platinum foil over the replica is extremely difficult and frequently leads to poorly fitted crowns. Additional problems of fit arise due to the shrinkage of the dental porcelain which often distorts the thin platinum matrix during firing. It is therefore more difficult to obtain the desired fit with this crown as compared to the cast metal porcelain composite.
The present invention provides a dental appliance which has excellent strength, durability, density, appearance and fit but yet which can be made at relatively low cost both by reason of the material used and, even more significantly, by reason of simplified processing.
In accordance with the invention, the tooth crown or other dental appliance has a tooth-engaging substrate formed of a shrink-free ceramic. Where, as is generally the case, it is desired that the appliance have greater translucence and closer color match to that of the natural tooth, the substrate is coated on its outer surfaces with a porcelain or glaze veneer having the color and degree of translucency desired. A preferred shrink-free ceramic for the practice of the invention will be described in detail hereinafter; however, suffice it to say at this point that by the term "shrink-free ceramic" is meant a ceramic made from a ceramic powder formulation which, when compacted and then fired to maturity, converts to a dense, hard, monolithic body which has the same volume and shape as were the volume and shape of the ceramic powder compact prior to firing.
Because the ceramic does not undergo shrinkage when fired to maturity, i. And because the ceramic powder can be readily molded at relatively low temperature into a compact of any desired shape and with the resulting compact itself being relatively soft such that portions thereof can be easily removed for any further shaping as might be required, the invention provides much simplified processing for dental crowns or other appliances having the required perfect fit along with high strength and all other desired characteristics.
Fundamentally, the method of the present invention involves only three essential steps, namely: 1 preparation of a die which is an exact replica of the prepared tooth to which the crown or other dental appliance is to be fitted; 2 molding or otherwise forming a compact of the raw particulate shrink-free ceramic against the die and simultaneously, or thereafter, shaping the remaining surface portions of the compact to the shape and size desired for the appliance; and 3 firing the so-shaped compact to its maturing temperature.
As a last step, generally necessary for purposes of matching the appearance of the natural tooth, the fired shrink-free ceramic substrate, so manufactured, is coated with a suitable porcelain veneer or glaze thereby completing preparation of the appliance which is thereafter secured to the prepared tooth.
As will be discussed in greater detail hereinafter, the die is prepared by taking an impression of the prepared tooth and then preparing the die by use of the impression, the die being of a material having sufficient strength and other physical characteristics to render it suitable for molding the raw ceramic thereagainst in the molding step which follows. Then, a mold is prepared with the die as a wall portion thereof. To this end wax which can be a wax such as paraffin or some other material which can be easily melted or vaporized can be molded against the die, after which the remaining surfaces of the resultant wax body can be shaped to duplicate the external crown shape desired.
Dental stone or the like is then cast against the wax body thereby to form a mold in which the die serves as the mold wall intended to be an exact replica of the prepared tooth. Mold preparation is completed by melting or burning out the wax. Such mold is then used for fabricating the shrink-free ceramic to the crown shape desired.
Hence, the surface portion of the crown which fits against the prepared tooth is formed by shaping it against the die rather than against a mold wall consisting of an investment surface prepared from a wax body shaped by molding the wax against the die. This greatly increases the closeness and accuracy of fit of the crown to the prepared tooth and it is enabled by the fact that the shrink-free ceramic can be shaped at low temperature, as compared with the temperatures required for casting metal.
In the preferred method as just described, the entire final shape of the shrink-free ceramic compact is accomplished simultaneously with the shaping of that portion of the compact which, after firing, is to fit against the prepared tooth.
However, it is within the purview of the invention to form the non-tooth engaging portions of the compact to the final desired shape subsequent to forming the compact against the die. That is, the wax body formed against the die can, if desired, be shaped on its external surfaces only roughly to the external shape desired of the compact, leaving it for further and final shaping of such surfaces of the compact after it has been molded.
As has already been mentioned, such final shaping of the powder compact is relatively simple, since the compact is soft and hence easily sculptured prior to firing. The above and other features and advantages of the invention will appear more clearly from the detailed description of preferred embodiments which follows. As has already been made clear, a key feature of the present invention is the use of shrink-free ceramic as the material for the dental appliance substrate.
Various shrink-free ceramics have been proposed for use as industrial ceramics but before further discussing them, it is appropriate to mention briefly some of the characteristics of more conventional ceramics and and their manufacture.
Conventional ceramic bodies, typically alumina ceramics, are manufactured by forming a raw batch of the desired ceramic ingredients in particulate form, e. Also, because shrinkage is never exactly uniform throughout the body, there is always a certain amount of warpage or distortion--though with good quality control it is generally possible to maintain warpage or distortion within the bounds of normal tolerances for electrical insulators, mechanical components and the like industrial ceramic bodies.
Shrinkage occurs for one of a combination of two reasons. First, no matter that even high pressure is used to mold or otherwise form the compact, the density of the compact is not as high as theoretically possible--there is always some porosity--to the end that there is some shrinkage when the compact fires to a high density non-porous monolith.
The use of an organic binder in the compact, which is generally desirable to add green strength, may contribute to the porosity of the compact since it is vaporized or burned out during the subsequent firing operation to mature the ceramic to dense monolithic structure.
Secondly where the ceramic ingredients undergo chemical or crystalline transformation during the firing, if the ceramic formed is of greater density and lesser volume than the raw ceramic ingredients, then this also contributes to the shrinkage.
Generally, reason 1 cannot be entirely eliminated even for the manufacture of shrink-free ceramic bodies. However, in the case of most shrink-free ceramic bodies, the formulation of raw particulate ceramic ingredients is one wherein one or more of the ingredients undergo either chemical or crystalline transformation during the firing and with the resultant ceramic formed by way of the chemical or crystalline transformation occupying a greater volume than that of the raw ingredients, to the end that this increase in volume exactly compensates for the decrease in volume caused by reason 1.
Hence, for example, it is known that kyanite converts to mullite and silica upon firing, the combined volumes of the mullite and silica being greater than the volume of the kyanite. Accordingly, it is known to formulate a ceramic batch of kyanite, generally along with other ceramic ingredients, the amount of kyanite being such as to produce, during firing, a volume increase sufficient to compensate for the shrinkage which would otherwise occur by reason of the inclusion of binder and the relatively high porosity of the compact prior to firing.
Various and diverse examples of shrink-free ceramics are discussed in the article entitled "Porcelains Having Low-Firing Shrinkage", Volume 43, No. To provide the desired strength and hardness it is preferred that the ceramic be an alumina ceramic, i. The preferred shrink-free ceramic, and processing thereof, for the practice of the present invention is that set forth in U.
Starling, James E. Stephan, and Robert D. Such shrink-free ceramic body utilizes the fact that when aluminum oxide and magnesium oxide combine, during firing, to form magnesium aluminate spinel MgO Al. The variated particle size of the ceramic batch is to provide high compaction density. The calcium stearate and Acrawax function as lubricants during the compaction operation. When the above body is fired to a temperature of about During the firing there is no shrinkage, and in this regard it is desirable to fire the ceramic on a gradual schedule, for example, by first gradually heating to about By way of the gradual heating, escape of SiO groups from the silicone resin is inhibited to the end that substantially all of the SiO groups of the silicone resin remain in the fired body as silicate.
As discussed in the aforementioned U. It will be also understood that whereas the aforedescribed ceramic has, to date, been found to be best and hence is most preferred, other shrink-free formulations can be used for the practice of the invention. The first step involves preparation of the die to the precise shape of the prepared tooth.
It is necessary that the die be formed of a material having sufficient strength and other physical characteristics to be suitable for molding thereagainst the raw ceramic in the molding operation in which the die is used as a wall of the mold. The preferred material for the die is an epoxy resin which cures to high strength and hardness and which either undergoes no shrinkage during cure or, if it does, can be restored to its as-cast dimensions.
Also, if the raw ceramic is to be molded in a heated condition, the die material should have a coefficient of thermal expansion the same or quite similar to that of the raw ceramic. The preferred material for the die, and the preferred manner of using such material, is disclosed in U. Stephan, Paul A. Boduch and John A. Elverum and assigned to the assignees of the present invention.
In the preferred embodiment a tertiary amine catalyst is also included in an amount up to about 0. These components are mixed, just before use, in a ratio of from 30 parts by weight Component B to each parts by weight Component A and the mixture then cast in the impression and allowed to cure to a solid body while in the impression.
Slight shrinkage occurs during curing; however by thereafter heating the resulting die preferably at a rate of from about 5. Such epoxy resin is particularly desirable as the die material where the shrink-free ceramic used is of the preferred formulation hereinafter disclosed wherein silicone resin is an ingredient and where the molding of the raw ceramic is by the preferred molding operation hereinafter disclosed wherein the raw ceramic is molded at a temperature sufficient to soften the silicone resin so that it provides the raw ceramic thermoplastic properties during molding and functions as a binder.
To commence the dental procedure, the dentist prepares the tooth to be crowned by removing enough of the tooth structure to allow proper thickness of the final crown, after which the dentist secures a negative of the prepared tooth with a suitable impression material, all of which can be in accordance with conventional practice.
The die which is to be used as the mold wall for molding the raw ceramic is then prepared, preferably by casting the epoxy or other material of which the die is to be constructed in the impression of the prepared tooth. As an alternative method: a a model of the prepared tooth is made by casting a material such as dental stone into the impression; b after the tooth model hardens and is removed from the impression, an impression is made of the tooth model; and c the material of which the die is to be constructed is then cast in this impression of the tooth model.
Though the alternative method is quite satisfactory for the practice of the invention the preferred method is advantageous in that it best assures that the die will be an exact replica of the prepared tooth. However, the preferred method cannot be used if the impression material used by the dentist for securing the impression of the prepared tooth is not suitable for casting thereagainst the material of which the die is to be formed.
Specifically, if the die is to be formed of an epoxy resin such as the preferred epoxy formulation indicated above, and if the dentist uses a water-containing hydrocolloid type impression material well known in the dental art for securing the impression of the prepared tooth, then the preferred method cannot be used since such type of impression material is not satisfactory for casting thereagainst such an epoxy resin formulation for forming the die.
In that case the alternative method can be used since such an impression material is satisfactory for casting a material such as dental stone for making a tooth model, an impression then being taken of the tooth model in an impression material which is satisfactory for casting the epoxy formulation for forming the die.
An impression material also well known in the dental art of the synthetic elastomeric type whether it be of polyether, silicone, polysulfide or vinyl polysiloxane elastomer, all commonly used is excellent for casting such an epoxy formulation. Hence, if the dentist uses a synthetic elastomeric type impression material to secure the impression of the prepared tooth, the preferred method of preparing the die can be used.
After the die is prepared it is related to a model of the opposing arch upper to lower teeth on an articulator which simulates jaw movements. When the die at this stage is of multiple teeth, the dies for the individual teeth to be crowned are then separated by fine saw blades to allow easy manipulation of the dies and then trimmed, i.
Next the die, duplicative of the tooth to be crowned, is secured to form a portion of a mold for the molding of the shrink-free ceramic raw batch.
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