WO2007097747A1

WO2007097747A1 - Metal-bearing dental restorations by three-dimensional printing

Info

Publication number: WO2007097747A1
Application number: PCT/US2006/005992
Authority: WO
Inventors: Lawrence J. Rhoades; Howard A. Kuhn
Original assignee: The Ex One Corporation
Priority date: 2006-02-21
Filing date: 2006-02-21
Publication date: 2007-08-30

Abstract

Methods are disclosed for producing metal bases, such as copings and bridge substrates, for dental restorations, in which a preform of the metal base is made by three-dimensional printing using a metal powder and a printing binder to form box-like printing primitives on at least one surface of the preform that is to receive an enamel-like coating. The preforms may be further processed by sintering, infiltration, and/or hot isostatic pressing and have at least one surface characterized by the presence of a plurality of protuberances which act to promote the binding of the enamel-like coating without an oxidation heat treatment. Enamel-like coatings may be applied to the metal bases to form dental restorations such as crowns and bridges.

Description

Title: Metal-Bearing Dental Restorations by Three-Dimensional Printing Inventors: Lawrence J. Rhoades and Howard A. Kuhn

Cross-Reference to Related Applications: This application claims priority to U.S. Provisional Patent Application Ser. No. 60/655,542 filed on February 23, 2005.

Field of the Invention: The present invention is in the field of dental restorations. More particularly, the present invention relates to methods of making the underlying metal base of ceramic/metal dental restorations by three-dimensional printing.

Background of the Invention: The metal base of a ceramic/metal dental restoration is typically made either by hand by skilled technicians or through the use of a CAD-CAM device. For example, a commonly-used conventional process of creating a replacement tooth or crown involves the production of a metal base called a coping. Ceramic layers are attached to the outside of the coping to produce the visible hard white part of the tooth. In the conventional process, an impression may be taken of a ground tooth stub in a patient's mouth. The impression is used to cast a die of refractory material. The die is used for casting a coping. The casting metal may be a precious metal, such as a gold alloy. The use of the ground tooth impression and subsequent die assures that the inside portion of the coping matches the tooth stub. In some cases, an artificial stub is used in place of a tooth stub as the source of the impression. The casting is given an oxidation treatment by heating it in a furnace to a high temperature in an oxidizing atmosphere such as air. The oxidation treatment is important because the resulting oxidized surface of the coping is more adherent to the ceramic layers that will be applied to the outside of the coping. The ceramic layers are applied in the form of painted-on slurry coatings. Each ceramic coating is given a firing to harden it before the application of the next layer. Each of the layers may have a different color or hue in an attempt to match the patient's other teeth. The final coat may be a clear coat. The completed artificial tooth is fitted onto the tooth stub with an adhesive that binds the inner surface of the coping to the tooth stub. There have been efforts to automate the process of making dental restorations. One of the most significant efforts is described in United States Patent No. 6,955,776 (hereinafter referred to as "the '776 patent"). The '776 patent describes the use of a technology known in the art as three-dimensional printing to create complete dental restorations in a layerwise fashion from powder based upon electronic depictions of the restorations. The electronic depictions are created using data about the desired shape and size of the dental restoration. Where the dental restoration is to fit over a tooth stub, data about the shape of the tooth stub may be obtained from an optical scan of either the tooth stub or of an impression of the tooth stub. After the electronic depiction has been created, a preform of the dental restoration is built up, one layer at a time, by printing a binder onto successive powder layers in patterns corresponding to successive cross sections of the desired dental restoration. The powder used may be a ceramic powder or a metal powder or combinations of metal powders and ceramic powders. After the preform has been built, the unbound powder is removed and the preform is subjected to a heat treatment which removes the binder and sinters adjacent powder particles together to form necks at their contacting points. The sintered compact is then infiltrated with a second phase to fill in the interconnecting voids between the powder particles. The infiltrated compact may then be lightly machined or otherwise shaped to complete the dental restoration. More complete information about three-dimensional printing in general is found in United States Patent No. 5,204,055 and in United States Pat. No. 6,036,777. An almost identical process for making all-ceramic complete dental restorations is disclosed in United States Patent No. 6,322,728 (hereinafter referred to as "the '728 patent"). The principle difference between the two processes, according to the prosecution history of the '776 patent, is that the '728 patent contains no teachings about sintering the powder particles together to form necks, but instead teaches that the preform is heat treated to form a cohesive mass.

The teachings of both the '776 patent and the '728 patent confine the use of three- dimensional printing to the fabrication of complete dental restorations. However, these teachings bypass the advantages inherent in dental restorations that comprise a ceramic coated metal base. One such advantage is the cushioning effect provided by a metal base to a dental restoration having the overall appearance of a natural tooth. Another advantage is that a great amount of expertise has been developed in the dental industry for making the appearance of the ceramic coating closely match the color and opalescence of the surrounding teeth. What is needed is a method that takes advantage of the technological advances illustrated by the '776 and the '728 patents, but retains the advantages of ceramic coated metal base dental restorations.

United States Patent Application Publication No. US 2002/0187458 (hereinafter referred to as "the '458 application") discloses an attempt to use of another layerwise fabrication technology to fill this need. The '458 application teaches the use of a laser to selectively melt successive layers of a primarily metal powder to form a dental restoration. The selective melting imparts a density to the dental restoration that may be greater than 98% and provides a surface roughness that is well-suited for veneering. However, the teachings of the '458 application fall well short of filling the aforesaid need because of the greater difficulties and costs inherently attendant to selective laser melting in comparison to three- dimensional printing.

Disclosure of Invention The present invention fills the need in the dental art for a method of making dental restorations that takes advantage of the efficiencies inherent in three-dimensional printing while retaining the advantages attendant to ceramic coated metal base dental restorations. Moreover, the present invention also takes advantage of a surprising discovery made by its inventors that eliminates the step in the ceramic coating process in which the metal base is subjected to a heat treatment to oxidize its surface to promote the adhesion of the ceramic coatings. The inventors have made the surprising discovery that box-like printing primitives created during three-dimensional printing on the surface of the preform provides acceptable levels of ceramic coating adhesion without subjecting the resulting metal base to an oxidation heat treatment. The inventors have also made a similarly surprising discovery that the present invention eliminates the need for certain alloying elements which are conventionally added to gold alloys for the purpose of promoting ceramic adherence to dental restoration metal bases comprised of those gold alloys.

According to the present invention, a method is provided comprising the following steps. A metal powder is provided that is of a composition that is compatible for use as a metal base in a dental restoration. A preform of the desired metal base is formed from the metal powder by three-dimensional printing. The printing conditions used in the three- dimensional printing are chosen so as to cause the metal powder to form printing primitives on the surface of the preform that are box-like in shape. After the metal base has been formed by three-dimensional printing, it is infiltrated with a molten metal to densify and strengthen the preform and to structurally complete the metal base. Conventional ceramic coating may then be applied without the need to subject the metal base to an oxidation heat treatment.

In some preferred embodiments, the infiltrant is chosen so that the overall composition of the infiltrated metal base falls within the compositional limits of an alloy that is approved by the United States Food and Drug Administration for dental use even though one or both of the metal powder and the infiltrant are not such approved alloys.

The present invention also includes dental restoration metal bases having surfaces characterized by the presence of a plurality of protuberances resulting from the aforementioned box-like printing primitives. The present invention also includes finished ceramic coated dental restorations which have such metal bases.

Brief Description of Drawings

The criticality of the features and merits of the present invention will be better understood by reference to the attached drawings. It is to be understood, however, that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the present invention.

FIG. 1 is a photomicrograph showing powder particles agglomerated into printing primitives. FIG. 2 is a schematic perspective illustration of a box-like protuberance on the surface of a dental restoration metal base according to an embodiment of the present invention.

FIG. 3 is a photograph of coping and crown embodiments of the present invention.

FIG. 4 is another photograph of the coping and crown shown in FIG. 3. Modes for Carrying Out the Invention

In this section, some preferred embodiments of the present invention are described in detail sufficient for one skilled in the art to practice the present invention. It is to be understood, however, that the fact that a limited number of preferred embodiments are described herein does not in any way limit the scope of the present invention as set forth in the appended claims.

According to the present invention, a solid free-form fabrication process known in the art as three-dimensional printing is used to make the metal bases of the dental restorations.

The three-dimensional printing process is conceptually similar to ink-jet printing. However, instead of ink, the three-dimensional printing process deposits a binder onto the top layer of a bed of powder. This binder is printed onto the powder layer according to a two-dimensional slice of a three-dimensional electronic representation of the article that is to be manufactured.

Additional powder layers are added so that one layer after another is printed until the entire article has been formed. The binder may comprise at least one of a polymer and a carbohydrate. Examples of three-dimensional printing binders are given in United States Pat.

No. 5,076,869 and in United States Pat. No. 6,585,930.

The printed article typically consists of from about 30 to over 60 volume percent powder, depending on powder packing density, and about 10 volume percent binder, with the remainder being void space. The as-printed article is somewhat fragile. Post-printing processing is conducted to enhance the physical and/or mechanical properties of the printed article. Post-printing processing includes thermally processing the printed article to remove the binder and to sinter together the powder particles to a desired density. The desired density may be one that provides a network of interconnected porosity suitable for receiving an infiltrant or it may be to a higher density, up to and including full density, i.e., a density which indicates that there is no substantial amount of porosity remaining in the sintered printed article.

Post-printing processing may further include introducing an infiltrant material that subsequently hardens or solidifies, thereby producing a highly dense article having the desired physical and mechanical properties. It may also include sintering to full-density.

The three-dimensional electronic representation of the article that is used in the layered manufacturing process is typically created using Computer-Aided Design ("CAD") software. The CAD file of the three-dimensional electronic representation is typically converted into another file format known in the industry as stereolithographic or standard triangle language ("STL") file format or STL format. The STL format file is typically then processed by a suitable slicing program to produce an electronic file that converts the three- dimensional electronic representation of the article into another format file comprising the article represented as two-dimensional slices. The thickness of the slices is typically in the range of about 0.008 cm (0.003 inches) to about 0.03 cm (0.012 inches), but may be substantially different from this range depending on the design criterion for the article that is being made and the particular layered manufacturing process being employed. Suitable programs for making these various electronic files are well-known to persons skilled in the art.

In one aspect of the present invention, there are provided method embodiments for producing ceramic coated metal base dental restorations. These method embodiments include steps of forming a dental restoration metal base through the use of the three- dimensional printing process. Included among such dental restorations are crowns and bridges. Generally, in the art, the metal base portion of a crown is referred to as a "coping," as described above, and the metal base portion of a bridge is called a "substrate." When used herein and in the appended claims these terms are used in accordance with these meanings.

The nature of the three-dimensional printing process is that the surface detail resolution of the part being built differs somewhat in the printing, spreading, and build directions. In the practice of the method embodiments of the present invention, it is preferred that the metal base be oriented in the three-dimensional printing process such that the greatest resolution is achieved on the margin region of the dental restoration.

In method embodiments of the present invention, an electronic representation of the metal base is used to build the metal base on a layer-by-layer basis from metal powder in a three-dimensional printing apparatus. A suitable three-dimensional printing apparatus for practicing some embodiments of the present invention is the Prometal RX-D machine available from The Ex One Company, Irwin, Pennsylvania 15642 U.S.

The metal powder used in practicing embodiments of the present invention may be any metal that is compatible for use in the dental application. Such metals include: (1) all high noble dental alloys, i.e., alloys having greater than 60 weight percent of one of the noble metals (gold, palladium, and platinum) and at least 40 weight percent gold; (2) noble dental alloys, i.e., alloys having at least 25 weight percent of a noble metal; and (3) base dental alloys, i.e., alloys having less than 25 weight percent of a noble metal. Examples of base dental alloys include cobalt-chromium alloys such as ASTM grade F75, nickel-chromium- titanium alloys, and other alloys of titanium. Such metals also include alloys which, when combined with an appropriate infiltrant, provide the metal base with a composition that is approved by the United States Food and Drug Administration.

When an infiltrant is used in the practice of the present invention, it is preferred that the infiltrant be a pure noble metal. However, any infiltrant that is compatible with the dental application and the preform metal powder is within the contemplation of the present invention.

According to embodiments of the present invention, the particle size of the metal powder and the surface tension of the liquid binder that is to be printed onto the metal powder are selected so that each printed binder droplet can agglomerate the majority of the metal powder it contacts into a discrete voxel, i.e., a volume element. These voxels are referred to herein and in the appended claims as "printing primitives." FIG. 1 shows examples of printing primitives, e.g., printing primitive 2, which are made up of metal powder particles 4. The printing conditions used to create the printing primitives shown in FIG. 1 were selected so as to provide easy viewing of the individual printing primitives by causing significant spatial separation between the individual printing primitives. In practice of the preferred method embodiments of the present invention, however, the printing primitives are much more closely packed together.

The formation of printing primitives is well-known in the art of three-dimensional printing. United States Patent No. 6,146,567 (hereinafter referred to as "the '567 patent") discloses the formation of spherical printing primitives and discusses the effect that varying the diameter of these printing primitives has on the surface roughness of the outer surface of the printed part. United States Patent No. 5,490,962 (hereinafter referred to as "the '962 patent") discusses the formation of printing primitives and discloses that the width of line printing primitives formed by the consecutive printing of droplets along a single line in a powder bed is very similar to the dimensions of droplet printing primitives. The '962 patent also discloses that the dimensions of a line printing primitive depend on the powder and the amount of binder printed per unit line length and relates a higher binder viscosity to a narrower line printing primitive width. However, neither the '567 patent nor the '962 patent suggest the formation of the box-like printing primitives employed by the present invention. Nor do they or anything else known in the art suggest the surprising results the inventors have discovered to occur from the presence of the surface protuberances resulting from box-like printing primitives on the preforms of three-dimensional printed metal bases. After the preform of the metal base has been printed in accordance with the present invention, it may be sintered and then infiltrated with a molten metal that wicks into the pores between the printing primitives and between the powder particles. Upon cooling, the infiltrant solidifies and endows the infiltrated metal base with the strength and toughness necessary for the application. The infiltrated metal base is preferably fully dense. In some embodiments of the present invention, as an alternative to infiltration, the metal base preform may be sintered to or near to full density, i.e., to a density of 95% or greater of its theoretical density. The present invention also contemplates that hot isostatic compression may be employed with either an infiltrated or a sintered metal base to achieve further densification. The metal base produced according to methods of the present invention has a surface roughness that is an artifact of the printing primitives that were present on the surface of the preform as a result of the three-dimensional printing step. This surface roughness promotes the adherence of applied ceramic layers to an unexpectedly high degree. To wit, the inventors have discovered the unexpected result that the surface roughness resulting from printing primitives having a box-like shape provides such good adherence of applied ceramic layers that the need for an oxidation heat treatment prior to the application of the first ceramic layer is eliminated. It should be noted that while a person skilled in the art would expect some surface roughness to result from the metal powder itself, tests have shown that factor to be insufficient by itself to eliminate the need for the oxidation heat treatment. In other words, the present invention provides a surprising degree of adherence of ceramic layers to the surface of the metal base that is superior to what would have been expected by a person skilled in the art due solely to the roughness imparted by the profiles of the powder particles.

Moreover, the inventors have also discovered the further surprising result that the present invention also eliminates the need for alloying elements that are conventionally employed to enhance the ceramic-adhesion benefits of the oxidation heat treatment. Small amounts of iron and tin are conventionally added to gold alloys that are used to make metal bases by casting. Typically, the amount of iron added is in the range of about 0.1 to about 1.0 weight percent and the amount of tin is in the range of about 0.1 to about 1.0 weight percent. As is illustrated in Examples 3 and 4 below, the inventors have discovered the surprising result that a metal base made according to the present invention from powder of a gold alloy from which these additions had been eliminated provided satisfactory ceramic coating adherence.

Preferably, in practicing the present invention the printing binder is chosen to have effluents that have low or no toxicity in a dental laboratory environment. Also, the printing binder preferably has a surface tension in the range of about 30 to about 35 dynes/sq. cm. and a viscosity in the range of about 7 to about 9 centipoise. The printing binder droplet size is preferably in the range of about 50 to about 110 micrometers, and more preferably in the range of about 50 to about 70 micrometers. Preferably, the printing primitives are parallelepiped or box-like in shape. FIG. 2 shows a schematic depiction of a single printing primitive 6 on the surface 8 of a preform of a metal base according to an embodiment of the present invention. The printing primitive 6 has a box-like shape with side lengths 12, 14 and thickness 16. Preferably the ratio of the side lengths 12, 14 range from about 1:1 to about 2:1, where the first number in the ratio is the length of the longer side. The thickness 16 is preferably about the same as the printed layer thickness. It is also preferred that the shorter of the side lengths 12, 14 be in the range of between about 30 to about 80 micrometers, and more preferred that it be in the range of between about 30 to about 60 micrometers. The present invention contemplates that distinguishable printing primitives be located on at least a portion of the above-the-margin surface or surfaces to which ceramic coatings are to be applied. It is not necessary, however, that distinguishable printing primitives be located in the interior of the metal base preform, i.e., across the width of the preform. Thus, in some embodiments of the present invention, the three-dimensional printing is set to provide the distinguishable printing primitives at the surfaces to which ceramic is to be applied and is adjusted to print differently across the interior of the metal dental substrate. In all cases, however, the printing conditions are selected so as to form the box-like printing primitives on the preform surfaces that after subsequent processing will be coated with ceramic. This is accomplished by controlling the saturation level of the printed binder within the range of about 70 to about 110%.

The saturation level (in percent) is defined by the following equation: Saturation level = [(1,000 * V_binde_r)/((l-PR/100) * X_spc * Y_spc * Z_thk)] * 100% wherein,

Vbinder is the volume of one droplet of binder measured in picoliters; X_spc is the droplet spacing in the x-direction measured in micrometers;

Yspc is the droplet spacing in the y-direction measured in micrometers; and

Zthk is the layer thickness measured in micrometers.

It is also preferred in practicing the methods of the present invention to form box-like printing primitives on the surfaces of the preform which, after subsequent processing, will be used to attach the metal base to a tooth stub or appliance. The inventors have found this practice to promote the adhesion of the metal base to the tooth stub or appliance.

In practicing method embodiments of the present invention, it is preferable that the maximum powder particle size of the provided metal powder be smaller than the diameter of the printing binder droplet as this aids in obtaining the agglomeration necessary to form the printing primitives. More preferably, the metal powder particle size distribution should minimize the amount of powder that is greater than 30 micrometers, as such large particles do not lend themselves well to forming printing primitives. It is also preferable that the metal powder particle size have a d50 in the range of about 10 to about 15 micrometers. It is also preferred that the amount of sub-5 micrometer powder in the powder distribution be minimized as powder that fine causes powder spreading problems in the three-dimensional printing process. The use of fine particle size within the preferred ranges promotes the formation of printing primitives. Such fine powder also permits thinner build layers to be used in the three-dimensional printing process, thereby enhancing the obtainable surface feature resolution. Preferably, each three-dimensional printing layer is about three particles deep so as to promote good surface detail. In general, it is preferred that the layer thickness be about 0.05 cm (0.002 inches).

In practicing the method embodiments of the present invention, it is also preferred that the creation of the electronic representation of the dental restoration metal base includes taking an electronic image of the tooth stub or other support that the metal base is to be conformingly attached to. It is also preferred that during sintering, the preform of metal base be mounted on a replica of the tooth stub or other support that it is intended to be mounted upon in use. Such mounting aids in conforming the metal base to the actual tooth stub or support. The present invention contemplates the use of conventional ceramics and ceramic application methods with the metal base to provide an enamel-like coating to at least the above-the-margin portion of the metal base. Examples of such conventional ceramics include IPS d.SIGN® and IPS INLINE™, both of which are available from Ivoclar Vivadent Inc., 175 Pineview Drive, Amherst New York 14228 U.S., and DUCERAM® KISS™ and DUCERAM® PLUS™, both of which are available from Dentsply Ceramco, 570 West College Avenue, York, Pennsylvania 17404 U.S. Examples of such conventional application methods include air brushing and other spraying and brushing techniques.

Example 1

In this example, five gold copings were made using a gold alloy powder of the composition (in weight percent) consisting essentially of: 75 gold, 8 platinum, 13 palladium,

2 silver, 1 iron, and 1 tin. This alloy was chosen so that the gross composition of the coping, after being infiltrated with essentially pure gold, would have the composition of the FDA approved alloy Argedent 87. The gold alloy powder was generally spherical and had a size distribution that included a dlO of 4.63 micrometers, a d50 of 11.77 micrometers, and a d90 of 28.68 micrometers.

The five molar copings were three-dimensional printing printed on a modified

Prometal R2 machine, which is available from Prometal division of The Ex One Company, Irwin, Pennsylvania 15642 U.S. The binder used was a PVP-K30 emulsion having a viscosity of about 6 centipoise and a surface tension of about 30 to 32 dynes/square centimeter. After printing, the five copings were sintered at 1150 ⁰C for one hour in a reducing atmosphere of 96% argon and 4% hydrogen. The density of the sintered copings was about 60% of the theoretical density for the gold alloy used.

The sintered copings were infiltrated with a pure gold powder having a d50 of 70 micrometers. The pure gold powder was placed in the concave surface of the copings in amounts that equaled about 50% of the corresponding coping weight. The copings were supported during the infiltration heat treatment upon a ceramic grit within a ceramic crucible. The infiltration heat treatment was done in a reducing atmosphere of 96% argon and 4% hydrogen at a temperature of 1120 ⁰C and a time of 1 hour. Following the infiltration heat treatment, the infiltrated copings were separated into four groups - the first three of which had one coping each and the fourth had two copings. The Group 1 coping was given the normal oxidation heat treatment. This consisted of heating the coping from room temperature in a period of about five minutes to a temperature in the range of about 650 ⁰C to 980 ⁰C in air and then air cooling the coping. The Group 2 coping was sand blasted to roughen its surface. The Group 3 coping was sand blasted like the Group 2 coping and then given the oxidation heat treatment like the Group 1 coping. The two Group 4 copings were given no oxidation heat treatment or sand blasting.

Ceramic layers of a commercially available enamel-like ceramic coating system were then applied to all five of the copings by skilled dental lab technicians in the conventional manner of sequentially spray applying a layer with an airbrush and firing it in a soft vacuum before applying the next layer. Three layers were applied to each coping: an opaque layer, a tinted layer, and a translucent layer.

The integrity of the adhesion of the ceramic layers to the copings was then tested by grinding the ceramic coating at the base of the copings near the margin, i.e., the profile of the dental restoration where it joins the patient's gumline. The skilled dental laboratory technicians, who conducted the testing, determined that all four groups of copings exhibited the same good level of adhesion between the coping and the ceramic layers. This test showed that the present invention eliminates the need for an oxidation heat treatment to achieve satisfactory adherence of the enamel-like ceramic coating.

Example 2

A bend test was conducted to quantitatively measure the level of adherence of the ceramic. A bend test specimen was fabricated in accordance with the present invention using the same powder, binder, and processing conditions as described in Example 1. The bend test specimen fabricated was a flat bar 25 mm long by 3 mm wide by 0.5 mm thick. The bend test specimen was not given an oxidation heat treatment prior to the application of the ceramic coating. A ceramic coating was applied across the middle third of the bend test specimen in the manner described in Example 1. The bending stress on the bend test bar measured at the fracture of the ceramic was 45 MPa. This meets ANSI/ADA Specification 38, which specifies that the bending stress at the fracture of the ceramic must be greater than 25 MPa to be satisfactory for dental use.

Example 3 A set of five copings were made and processed as in Example 1, except that the gold powder alloy was modified to eliminate the iron and tin content. Iron and tin are normally present in the gold alloy to enhance the response of the alloy to the oxidation treatment and improve the ceramic adhesion. The results of the grinding tests in this case showed that all of the copings had good adherence of the ceramic to the coping. These results show that the present invention not only eliminates the need for the oxidation heat treatment, it also eliminates the need for alloy additions that are meant to enhance the ceramic adherence of the coping surface.

Example 4

A bend test specimen as described in Example 2 was made using the alloy described in Example 3. The bending stress on the bend test specimen measured at the fracture of the ceramic was 30 MPa. Although this value is below the value measured for the alloy used in Example 2, it exceeds the level required by the adherence standard.

Comparative Example

Comparative tests similar to those described in Example 1 were conducted to determine if the surface roughness caused by powder metal alone was responsible for providing the improved adherence of the ceramic material. In these tests, five copings were made by the Captek fabrication process using materials that are available from The Precious Metals Company, Inc., Altamonte Springs, Florida, U.S. The Captek fabrication process involves the use of thin strips consisting of generally spherical powder metal in a wax-like binder. In this process, strips containing metal powder of the same alloy that was used in Example 1 for three-dimensional printing were placed upon a support. The support and strips were heat treated to sinter the metal powder and form a coping. After cooling, similar strips containing essentially pure gold powder were applied to the outside of the copings. These were heat treated to melt and infiltrate the pure gold powder into the sintered coping. The five copings were then processed in the four groupings as in Example 1. Groups 1 through 3 were found to have satisfactory ceramic adhesion. However, the copings of Group 4, i.e., the copings which received no oxidation or sand blasting, had unsatisfactory ceramic adhesion.

Another aspect of the present invention includes the dental restoration metal bases produced by the methods of the present invention. These metal bases are distinguishable from those made by other processes by the presence on their surfaces that are to be coated with ceramic of a plurality of protuberances resulting from the box-like printing primitives. The shapes of these protuberances differ somewhat from those of their printing primitive precursors due to the effects of the sintering and/or infiltration processes. These processes tend to cause a rounding of the edges and surfaces of the printing primitive precursors so that the protuberances have more of a bread loaf shape. These processes also tend to interconnect some adjacent printing primitive precursors into a single protuberance. Preferably, the protuberances have lengths in the range of about 50 to about 70 micrometers, widths in the range of about 70 to about 100 micrometers, and heights in the range of about 50 to about 100 micrometers. In some preferred embodiments, a plurality of protuberances is also provided to a surface of the metal base by which the metal base is to be attached to a tooth stub or appliance. Yet another aspect of the present invention includes the finished dental restorations that include metal bases produced by the method of the present invention. These dental restorations also comprise an enamel-like coating covering at least a part, and preferably all, of the above-the-margin portion of the metal base. These dental restorations can be distinguished from those made by other methods by the presence of a plurality of protuberances on the metal base surfaces that are coated by the ceramic. These protuberances can be observed by examination of a cross-section of the dental restoration or, when sufficient resolution is obtainable, x-ray images of the dental restoration. In some preferred embodiments, at least one surface by which the dental restoration is to be attached to a tooth stub or appliance has a plurality of protuberances. Such protuberances can be observed by simple visual inspection.

Examples of metal bases and dental replacements according to the present invention are illustrated in FIGS. 3 and 4. Referring to FIG. 3, there is shown a photograph of a coping 20 having protuberances 22. Also shown in FIG. 3, is a photograph of a side perspective view of a crown 24 that consists of an enamel-like coating 26 covering a coping of the present invention that is similar to coping 20. Photographs of coping 20 and crown 24 also appear in FIG. 4. While coping 20 is similarly situated in FIG. 4 as it was in FIG. 3, crown 24 is positioned in FIG. 4 to show the coping which underlies the enamel-like coating 26. The underside surface 28 of the coping portion of crown 24 has protuberances 30 and defines a recess for mounting crown 24 upon a tooth stub or appliance post.

While only a few embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that many changes and modifications may be made thereunto without departing from the spirit and scope of the present invention as described in the following claims. All United States Patents and patent applications identified herein are incorporated herein by reference in their entireties. The terms used in the appended claims are meant to be understood in view of the teachings herein and of the meanings afforded to said terms herein. Furthermore, in the event that a claim term is expressly defined by the applicants during the prosecution of this application before a patent office, that definition is to be used in construing the claim term during all proceedings before that patent office and in the patent granted or issued on this application by that patent office and that definition also hereby is expressly incorporated herein as the applicants' definition for the claim term.

Claims

Claims We claim:

1. A method comprising the steps of: a) providing a metal powder compatible with use in a dental application; and b) forming by three-dimensional printing a preform of a metal base of a dental restoration from said metal powder and a printing binder, wherein said printing binder causes at least some particles of said metal powder to agglomerate into box-like shaped printing primitives on at least one surface of said preform that is to receive a ceramic coating, and said printing binder is applied during said three-dimensional printing at a saturation level of between about 70 % and about 110%.

2. The method described in claim 1, further comprising the step of selecting said metal powder to be at least one from the group consisting of a high noble dental alloy, a noble dental alloy, and a base dental alloy.

3. The method described in claim 1, further comprising the step of selecting said metal powder to have a d50 in the range of about 10 to about 15 micrometers.

4. The method described in claim 1, wherein the ratio of the length of the longer to the shorter of the sides of at least one of said box-like shaped printing primitives is in the range of about 1:1 to about 2:1.

5. The method described in claim 1, further comprising the step of selecting said printing binder to have a surface tension in the range of between about 30 to about 35 dynes/sq. cm.

6. The method described in claim 1, further comprising the step of selecting said printing binder to have a viscosity in the range of about 7 to about 9 centipoise.

7. The method described in claim 1, further comprising the step of providing said printing binder with a droplet size in the range of about 50 to about 110 micrometers during said three-dimensional printing.

8. The method described in claim 1, further comprising the step of infiltrating said preform with a molten metal.

9. The method described in claim 8, further comprising the step of selecting the compositions of said metal powder and of said infiltrant to provide the overall composition of the infiltrated preform to be a composition approved by the United States Food and Drug Agency for dental alloys.

10. The method described in claim 8, further comprising the step of applying a ceramic coating to said infiltrated preform to form a dental restoration.

11. The method described in claim 1, further comprising the step of sintering said preform to a density of at least 95% of its theoretical density.

12. The method described in claim 1, further comprising the step of hot isostatic pressing the preform to a density of at least 95% of its theoretical density.

13. The method described in claim 1, wherein said dental restoration is one selected from the group consisting of a crown and a bridge.

14. The method described in claim 1, further comprising the step of forming a plurality of printing primitives on a surface of said preform that corresponds to a surface by which said dental restoration is to be attached to a tooth stub or an appliance.

15. A metal base of a dental restoration comprising an outer surface having a plurality of protuberances, wherein said protuberances have lengths in the range of about 50 to about 70 micrometers and widths in the range of about 70 to about 100 micrometers and heights in the range of about 50 to about 100 micrometers.

16. The metal base described in claim 15, wherein said metal base has an overall composition of one selected from the group consisting of a high noble dental alloy, a noble dental alloy, and a base dental alloy.

17. The metal base described in claim 16, wherein said metal base has the form of one selected from the group consisting of a coping and a substrate.

18. The metal base described in claim 17, further comprising a plurality of protuberances on a surface by which said dental restoration is to be attached to a tooth stub or an appliance.

19. A dental restoration comprising: a) a metal base having an outer surface, said outer surface including an above-the- margin portion having a plurality of protuberances, wherein said protuberances have lengths in the range of about 50 to about 70 micrometers and widths in the range of about 70 to about 100 micrometers and heights in the range of about 50 to about 100 micrometers; and b) an enamel-like coating, said enamel-like coating covering at least a part of the above-the-margin portion of said outer surface.

20. The dental restoration described in claim 19, wherein said dental restoration has the form of one selected from the group of a crown and a bridge.

21. The dental restoration described in claim 19, wherein said metal base has an overall composition of one selected from the group consisting of a high noble dental alloy, a noble dental alloy, and a base dental alloy.

22. The dental restoration described in claim 19, further comprising a plurality of protuberances on a surface that is to be attached to a tooth stub or an appliance.