Method of fabricating high light transmission zirconia blanks for milling into natural appearance dental appliances

[0001] 1. Field of the Invention

[0002] The present invention relates generally to the manufacture of ceramic dental appliances such as crowns and bridges. The invention herein is related more particularly to the milling of zirconia blanks into such appliances where the blanks are fabricated by a vacuum and pressure assisted slip-casting method to achieve superior physical properties.

[0003] 2. Background Art

[0004] Dental prosthetic devices or appliances must satisfy a number of different criteria. They need to have excellent physical properties (Flexural Strength to 1600 MPa) to resist wear and mechanical deformation for very long periods of time. They should also provide good aesthetic characteristics which means that they have a natural appearance in color, texture and shape so that they are not readily distinguishable from the original natural teeth. They should also be readily fabricated at reasonable cost which means that the cost of materials used and the time required in their manufacture, should be within reasonable limits.

[0005] Dental appliances made of zirconia can meet these criteria. It is a ceramic material which can be made to be extremely hard and fracture resistant. Applicant has discovered that if provided in pre-fabricated pre-sintered blanks, they can be readily machined in CAD/CAM systems and then sintered with highly predictable shrinkage to conform to virtually any desired dental appliance shape with high yield and minimal manual intervention. A key aspect of the aesthetic appearance of zirconia dental appliances would be achieved if it were possible to provide such prosthetic devices with a substantial degree of light transmissivity. Translucent appliances would exhibit the color of the underlying natural dental material and thus go a long way toward achieving a highly desirable appearance, i.e., that is matched to the color of adjacent natural teeth. Thus, if it were possible to provide pre-fabricated, pre-sintered zirconia blanks and yet still retain the advantageous mechanical and easy fabrication properties of the ceramic, that would be a significant accomplishment in the art of dental appliances. Even though Applicant is not the first to consider slip-casting zirconia for the dental appliance art (see U.S. Pat. No. 4,772,436 to Tyszblat or U.S. Pat. No. 5,975,905 to Kim et al), there is no known relevant prior art which discloses all of the process steps of the present invention for fabricating a pre-sintered zirconia blank for the dental appliance arts. Moreover, Tyszblat teaches the interlacing of fitted metal oxide particles in the solid phase and glass. Kim et al discloses the creation of a ceramic sheet of thickness 0.1 mm to 1.0 mm and then coating the sheet onto a gypsum mold under heat and pressure and then, after sintering, coating the resultant body with glass powder. No other known partially stabilized zirconia material can be made to reach a stiffness of 1600 MPa mark when slip cast. No other known partially stabilized zirconia material reaches the high degree of light transmission in the 500-800 nm wavelength, which is needed to transmit the warm colors of the mouth. The partially stabilized zirconia is normally very hard to slip cast with a high density of 6.03 g/cc. Aluminum oxide for example is readily slip cast because of its lower density 3.98 g/cc. Any ceramic with density between 2.0 and 4.0 g/cc is more readily made by a slip cast process. This is because the lower density allows the particulate to stay dispersed longer which results in better green density and strength after final sintering. When densities rise above 4 g/cc, high strength is never reached in colloidal processing. Very high density materials normally cannot be cast at all, such as tungsten carbide at 14.6 g/cc.

[0006] The present invention comprises a process for fabricating pre-sintered zirconia blanks that are then computer machined and sintered to form translucent dental appliances having highly advantageous features. The principal steps of a preferred embodiment of that process comprise a) preparing a ceramic slurry of binderless zirconia powder; b) subjecting the slurry to attrition milling for an extended time of up to 24 hours to achieve a crystallite size in the range of about 5 nm to about 29 nm; c) preparing a vacuum assisted and optional pressure assisted slip casting mold and pouring the milled slurry into the slip-casting mold; d) after casting, excess slurry is poured from the mold and a consolidated zirconia blank is removed; e) drying the blank and pre-sintering it to form solid blanks ready for CAD/CAM machining and sintering to net shape. In the preferred embodiment of the process herein, the zirconia powder is dispersed in deionized water using a dispersant chemical solution such as tetramethyl ammonium hydroxide to adjust the pH of the slurry and thus homogeneously disperse the zirconia powder. Other dispersants such as polyisobutylene, various acid salts, bases and certain oils, may also serve as dispersant chemicals. The present invention achieves extremely high flexural strength and unusually high translucency by employing zirconia fabricated by a slip cast method in which the crystallite size is in the range of about 5 nm to about 29 nm to overcome the inherent high density of zirconia. The uniquely achieved flexural strength and translucency are particularly important in the field of dental restorations such as crowns and bridges where resistances to bite pressure and material appearance are critical.

[0017] Referring to the accompanying drawings and initially to FIGS. 1 to 6, it will be seen that the present invention is a process comprising the following steps:

• • [0018] a) The zirconia powder is dispersed into de-ionized water using either polymer dispersants or by controlling pH with base/acid. The slurry is preferably ultra-sonicated to remove soft agglomerates and for better mixing (see FIG. 1).
  • [0019] b) Attrition milling the prepared slurry, preferably in a zirconia bowl (see FIG. 2) until the crystallite size is in the range of about 5 nm to about 29 nm.
  • [0020] c) Slip-casting the milled slurry by preparing a porous plaster of paris mold. An organic polymer mold may be substituted for the plaster of paris mold. Placing the plaster of paris mold within a surrounding silicone mold enclosure (or enclosure of a rubber or other elastic polymer material, see FIGS. 11A and 11B) and softly tightening the enclosure around the plaster of paris to form a leak-proof system (see FIGS. 7 and 8). In the preferred embodiment, the enclosure is secured by rubber bands or pipe ties (see FIG. 9). Attaching to a bottom surface aperture of the enclosure, a vacuum line to suck out excess liquid (i.e., water) (see FIG. 10). A vacuum pressure of −5 to −40 psi is preferred. Pouring the slurry into the enclosure above the plaster of paris mold. Eventually a slip-cast zirconia blank will form above the plaster of paris mold. This blank is removed by pouring out the excess slurry and opening the silicone mold to permit removal of the plaster of paris mold and zirconia blank. An optional positive pressure may be applied above the enclosure to further promote the process. A positive pressure of 1 to 30 psi is preferred.
  • [0021] d) Each removed zirconia blank is allowed to dry at room temperature preferably using gently circulating air at low humidity (see FIG. 4). The zirconia blocks are then pre-sintered at 850° C. to 1200° C. for about two hours (see FIG. 5). The fully dried zirconia blanks are ready for computer-controlled milling and sintering to net shape (see FIG. 6).

[0022] As shown in FIGS. 9A and 9B, the plaster of paris mold may be protected from handling damage by enclosing its radial surface in a plastic or polymer sleeve.

[0023] FIG. 10 shows graphically the favorable light transmission characteristics of zirconia restorations made according to the preferred embodiment hereof and compares such characteristics to more conventionally fabricated zirconia restorations as a function of wavelength.

[0024] It will now be understood that the present invention comprises a process for preparing zirconia blanks for milling into dental appliance shapes using CAD/CAM control and then sintering the shaped zirconia appliances to net shape.

[0025] Although a preferred embodiment has been disclosed herein, it will now be apparent that various modifications may be made to the various steps hereof without deviating from the principal inventive features hereof. Accordingly, the scope of the invention is to be limited only by the appended claims and to their equivalents.