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Mill blank library and computer-implemented method for efficient selection of blanks to satisfy given criteriaRelated Patent Categories: Dentistry, Prosthodontics, Dental Implant ConstructionMill blank library and computer-implemented method for efficient selection of blanks to satisfy given criteria description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060204932, Mill blank library and computer-implemented method for efficient selection of blanks to satisfy given criteria. Brief Patent Description - Full Patent Description - Patent Application Claims
[0001] This application is based on and claims priority from Provisional Patent Application Ser. No. 60/485,935, filed Jul. 9, 2003. BACKGROUND OF THE INVENTION [0002] 1. Technical Field [0003] This invention generally relates to a system for preparing dental prostheses. In particular, the invention relates a smart mill blank library and preparing dental prostheses for use as crowns, onlays, inlays, veneers, bridges, and other restorations from a mill blank selected from a mill blank library. [0004] 2. Related Art [0005] The art of fabricating custom-fit prosthetics in the dental field is well-known. Prosthetics are replacements for tooth or bone structure. They include restorations, replacements, inlays, onlays, veneers, full and partial crowns, bridges, implants, posts, and the like. Typically, a dentist prepares a tooth for the restoration by removing existing anatomy, which is then lost. The resultant preparation may be digitized or a dental impression is taken, for the purpose of constructing a restoration. The restoration may be constructed through a variety of techniques including manually constructing the restoration, using automated techniques based on computer algorithms, or a combination of manual and automated techniques. In one known technique, the prosthetic is fabricated using a computer-assisted (CAD/CAM) system, such as a computer-aided milling machine. One such machine is the CEREC 3D system from Sirona Dental Systems. Computer-aided machines of this type work by shaping the prosthetic from mill blanks. A mill blank is a solid block of material from which the prosthetic is shaped by a shaping apparatus whose movements are controlled by the computer. Under computer control, the size, shape, and arrangement of the restoration may be subject to various physical parameters, including neighboring contacts, opposing contacts, emergence angle, and color and quality of the restoration to match the neighboring teeth. [0006] A common restoration includes a porcelain-fused-to-metal (PFM) crown. The crown typically comprises a cap of porcelain material overlayed on a thin metal coping. The metal coping forms an interface between the preparation and the porcelain material. Common restorations typically include a coping formed from precious or semi-precious metals, including gold or a gold alloy. The material may be selected based on the color and various other properties to optimize a long-lasting natural looking restoration. [0007] The copings or full metal crowns typically are formed from a lost wax casting process. The process may include placing several wax copings on a wax tree, which is connected to a wax base. The structure is placed in a cylinder with investing material, and the wax is melted out after the investing material has set. A molten metal, typically a gold alloy, is then poured into the remaining structure, and the entire cylinder is placed into a centrifuge to distribute the molten material to a uniform distribution. Preferably, the alloy base and the tree are recovered for use in a future casting process. The continued re-melting of the gold alloy along with other contaminants, however, introduces oxidation and other tarnishing agents into the gold alloy. [0008] Other methods for forming the coping may be used, including milling or machining with some kind of block or blank, but these techniques may waste much of the metal material. The ratio of the volume of the final metal coping to the volume of a typical enclosing mill blank (a symmetric block or cylinder) is often very small such that much of the material may be wasted. As noted above, a common milling process includes forming the coping from a mill blank using a computer-assisted milling machine. The blank includes a sufficiently large rigid attachment so that it may be held solidly while the machining process is underway. A rectangular or cylindrical blank is commonly used, and the vast majority of material is removed via the machining process. U.S. Pat. No. 4,615,678 to Moermann et al. discloses a conventional mill blank of this type made of ceramic silica material. There are, of course, numerous other types of mill blanks available commercially. [0009] The cost of recovering the wasted material often exceeds the cost of the material sought to be recovered. The object may be milled using a wet milling process, which typically results in the discarded material (including fine particles) being mixed with water or other cutting fluids. This is not a significant concern when the restoration is being formed using inexpensive materials; however, when utilizing expensive materials, such as gold, the issue of dealing with the recovery of the machined material may make the process prohibitively expensive. Indeed, the cost of the discarded materials in the case of precious or semi-precious materials is the single most important reason that prior art techniques have proven to be undesirable or cost prohibitive. Additional concerns are the time required to cut through the discarded material, as well as the additional wear and tear on the tools. [0010] There have been a few incidental suggestions in the art to address this problem. Thus, for example, U.S. Pat. No. 4,615,678 teaches that the body portion of a mill blank can be formed in a way to minimize wear on and run time of the milling machine by being shaped initially to more closely resemble the final implant. An illustrative example is a blank for use in forming a two lobed inlay that includes a transverse groove in one side thereof. U.S. Published Patent Application 2003/0031984 to Rusin et al. illustrates a similar blank construction, and it further notes that blanks can come in a variety of shapes and sizes. [0011] While these suggestions are useful, there remains a need in the art to provide improved mill blank configurations and assemblages that facilitate prosthetic milling operations in a manner to reduce material waste, reduce machining time, and to increase value. BRIEF SUMMARY OF THE INVENTION [0012] It is an object of the present invention to provide improved mill blank constructions to facilitate the manufacture of dental restorations. In general, this object is achieved by providing a given mill blank in a shape (i.e. with a given geometry) that has been predetermined to reduce material waste when the mill blank is machined into the final part. A mill blank that has been intelligently pre-configured into a form that more closely resembles the final dental part is sometimes referred to as a "smart" blank. [0013] It is a further object of the invention to provide such mill blanks in a collection or "assemblage." A set of two or more smart blanks each having such characteristics is also sometimes referred to as a smart blank "library." In a preferred embodiment, it is desirable to provide a smart blank library that includes a sufficient number of unique blanks such that, when the geometry of the designed restoration is known, the smart blank with a highest yield can be selected for use in milling the restoration. The "yield" of a given smart blank represents the amount of material of the smart blank that is actually used in the final restoration, with the higher the yield value meaning the closer the "fit" of the smart blank to the designed restoration. In a particular embodiment, a smart blank library is maintained with a given number of unique blanks so as to balance an average yield per smart blank with a goal of satisfying an inventory requirement for the library (e.g., the smallest possible library size necessary to meet anticipated production requirements over a given time period). In this embodiment, it is desirable to have a sufficient number of unique smart blanks in the library such that the smart blank with a highest average yield can be selected and is available for use while ensuring that the number of blanks remains within a given inventory production factor. [0014] According to a more specific embodiment, an assemblage of blanks comprises at least first and second smart blanks, with each smart blank adaptable for producing a formed part that can be used for replacement or restoration of one or more teeth by removing as little material from the blank as possible (i.e., an optimize yield). The first blank has a first geometry, and the second blank has a second geometry that differs from the first geometry other than by mere scaling. The first blank is configured to resemble a first given restoration, and the second blank is configured to resemble a second given restoration. Each of the blanks further includes a holder (a sprue) for mounting the blank in a shaping apparatus. The blank comprises a precious or semi-precious material, a ceramic silica material, or other material suitable for the substructure or final restoration. [0015] It is another more general object of the invention to provide a smart mill blank library that comprises multiple smart mill blanks having a variety of predetermined shapes, sizes, and arrangements. Preferably, a given smart mill blank in the library is pre-formed to a target size, shape and arrangement so that the library as a whole is useful across for a particular set of applications. Thus, depending on the type and nature of the restoration, a particular smart mill blank is selected from the library and used in the milling operation. As a result, the amount of material needed to be removed from the mill blank is reduced greatly. This is especially desirable and cost-effective when precious or semi-precious materials (such as gold) are being used in the restoration. Indeed, use of a smart blank pre-formed from gold significantly reduces the amount of gold to be recovered, in many cases reducing it to less than that in a common lost wax casting process. In addition, the amount of time to machine the restoration is reduced due to a relatively small amount of material that needs to be removed from the smart mill blank. The use of such blanks provides further process advantages including, without limitation, reducing spoiling effects such as gold alloy tarnishing, eliminating trace metal oxidation, and the like. [0016] Another more general object of the present invention is to provide a smart blank library that achieves maximum yield, so as to minimize material waste. [0017] According to a specific feature of the present invention, the smart blank library comprises a set of copings or full contour crowns. A coping is the substructure of a crown. The general shape of a coping has an upper surface and a lower surface. The upper surface is generally a convex surface and the lower surface is generally a concave surface. The lower surface is configured to be able to be affixed to a dental preparation and to form a tight seal at a margin having a small but definite gap for cement. The general shape of the lower surface may mirror or correspond to the shape of a typical preparation. The general shape of the upper surface of the coping may correspond to an occlusal surface of a particular dental item. A selection of a smart mill blank from the library provides a more effective way to prepare a dental prosthesis and dental item to maintain optimal porcelain or other surface material on top of the metal coping. [0018] In a common restoration, such as a porcelain-on-metal crown, it is desirable for longevity of the restoration to provide a substantially constant thickness of the porcelain material. Maintaining the constant thickness may reduce a risk of fracturing the material. Accordingly, in one embodiment, the smart mill blanks in the library may have a generally concavo-convex shape, with the top surface having a shape that allows the porcelain-sculpted anatomy to exhibit a near constant thickness [0019] Other methods, features and advantages of the invention will be, or will become, apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the following claims. BRIEF DESCRIPTION OF THE DRAWINGS [0020] The invention may be better understood with reference to the following drawings and its accompanying description. Unless otherwise stated, the components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like referenced numerals designate corresponding parts throughout the different views. [0021] FIG. 1 illustrates a smart blank library according to an embodiment of the present invention; Continue reading about Mill blank library and computer-implemented method for efficient selection of blanks to satisfy given criteria... 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