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EP-4473935-B1 - MILL BLANK FOR DENTAL CUTTING AND METHOD FOR PRODUCING SAME

EP4473935B1EP 4473935 B1EP4473935 B1EP 4473935B1EP-4473935-B1

Inventors

  • NAGASAWA, YUKO
  • IIDA, Shuntaro

Dates

Publication Date
20260506
Application Date
20230313

Claims (5)

  1. A mill blank for dental cutting comprising a section to be cut (1) that has a shape of a columnar body whose bottom surface (2) and upper surface are in a substantially identical shape and that is formed of a hybrid resin, the section to be cut having a gradation structure (10) in which color appearance changes in stages in a height direction, where the height direction is a direction from the bottom surface of the columnar body being a base end (11) toward the upper surface which is a tip (13), wherein the columnar body is formed of a hybrid resin that is a composite material in which an inorganic filling material is dispersed in a resin matrix, wherein, in the gradation structure, a region having one color appearance forms each of "color units" into which the columnar body is divided in the height direction along a plane horizontal to the bottom surface and that each have a predetermined thickness, and five to nine "color units" (14,15,16,17,18,19) different in the color appearance are continuously arranged in the height direction, and wherein, when the five to nine "color units" are each assigned a unit number: n (where n is a natural number of 1 to m) out of 1 to m (where m is a natural number representing the number of the units and is within a range of 5 to 9) to be denoted by a "color unit n" such that a value of the number increases in order from the tip side toward the base end side, and a color of the "color unit 1" located at the tip is represented by a "tip region color: A" and a color of the "color unit m" located at the base end is represented by a "base end region color: B", a color difference: ΔE AB between the tip region color: A and the base end region color: B is 5 to 20, colors of the "color unit 2" to the "color unit m-1" are all intermediate colors: C between the colors A and B and change from an intermediate color close to A to an intermediate color close to B in stages from the "color unit 2" toward the "color unit m-1", a color difference: ΔE C between the colors of the adjacent units is 1 to 7, wherein the color difference (ΔE) is an index representing a degree of difference between two colors and is defined by the equation (1), which is expressed by using numerical values (L* values, a* values, and b* values) in the coordinate axes in the L*a*b* color space, ΔE = ΔL * 2 + Δa * 2 + Δb * 2 1 / 2 wherein in the equation (1), ΔL* is represented by the difference between lightness L* of a first color and lightness L* of a second color, Δa* is represented by the difference between chromaticity a* of the first color and chromaticity a* of the second color, and Δb* is represented by the difference between chromaticity b* of the first color and chromaticity b* of the second color, characterized in that the columnar body is formed of a single layer not having a laminate interface.
  2. The mill blank for dental cutting according to claim 1, wherein, with the thickness in the height direction of each of the "color units" being defined as a height of each of the "color units", the heights of the "color unit 1" (19) and the "color unit m" (14) are each 15 to 35% of a whole height of the columnar body, and the height of the "color unit 1" and the height of the "color unit m" are optionally different.
  3. A method for producing the mill blank for dental cutting according to claim 1 or 2, the method comprising: a raw material preparation step of preparing a hybrid resin raw material composition (40) in paste form containing a polymerizable monomer, an inorganic filler, a thermal polymerization initiator, and a coloring agent; a filling step of filling the hybrid resin raw material composition in a mold (30) corresponding to the shape of the section to be cut (1); and a curing step of curing the hybrid resin raw material composition filled in the mold by thermal polymerization, wherein the raw material preparation step prepares, as the hybrid resin raw material composition, a "color unit 1" (19) raw material composition A giving a hybrid resin of the color: A and a "color unit m" (16) raw material composition B giving a hybrid resin of the color: B, and by using tanks capable of separately holding the "color unit 1" raw material composition A and the "color unit m" raw material composition B and a discharge device capable of uniformly mixing, at a desired ratio, the two types of hybrid resin raw material compositions held in the tanks and discharging a mixture from a discharge port, mixes the "color unit 1" raw material composition A and the "color unit m" raw material composition B with a mixture ratio of A and B being varied, to prepare the (m-2) types of intermediate region hybrid resin raw material compositions giving the colors: of the "color unit 2" to the "color unit (m-1), wherein the filling step includes: a tip region filling step of extruding the "color unit 1" raw material composition A from the discharge port (26) to fill, in the mold, a necessary amount of the "color unit 1" raw material composition A in order to form the "color unit 1"; a base end region filling step of extruding the "color unit m" raw material composition B from the discharge port to fill, in the mold, a necessary amount of the "color unit m" raw material composition B in order to form the "color unit m"; and an intermediate region filling step of extruding the intermediate region hybrid resin raw material compositions prepared using the discharge device from the discharge port to fill, in the mold, necessary amounts of the intermediate region hybrid resin raw material compositions in order to form the "color unit 2" (18) to the "color unit m-1" (15), wherein the filling step is performed in either pattern out of: 1) a pattern 1 in which the tip region filling step, the intermediate region filling step, and the base end region filling step are performed in the order mentioned; and 2) a pattern 2 in which the base end region filling step, the intermediate region filling step, and the tip region filling step are performed in the order mentioned, and wherein, in the pattern 1 and the pattern2, the filling steps are performed by continuously extruding the hybrid resin raw material composition while varying, in stages, a constitution of the hybrid resin raw material composition that is to be extruded from the discharge port.
  4. The method for producing the mill blank for dental cutting according to claim 3, wherein, when a position in the mold is expressed by a coordinate system whose origin is a given point on a bottom surface in the mold and whose x-axis and y-axis represent a lateral direction and a longitudinal direction of the bottom surface respectively and whose z-axis represents a height direction, and (m-1) pieces of z-coordinates corresponding to heights of boundaries between m pieces of the "color units" that are to be formed in the mold are denoted by z 1 to z m-1 from the bottom surface side, the filling of each of the types of "color unit" raw material compositions in the mold in the filling step is performed using the discharge device capable of freely controlling a position of the discharge port (26) in a front-rear direction, a left-right direction, and the height direction, by: (i) setting the position of the discharge port when the filling starts, at the origin; (ii) moving the position of the discharge port in the x-axis direction and/or the y-axis direction to scan a whole region on the bottom surface once while discharging the hybrid resin raw material composition at a predetermined flow rate from the discharge port, to form a mold lowest filling region having a uniform thickness corresponding to the one scanning; (iii) moving the position of the discharge port in the z-axis direction to a height of an upper surface of the mold lowest filling region or higher after finishing the scanning in (ii); (iv) moving the position of the discharge port in the x-axis direction and/or the y-axis direction to scan a whole region on the upper surface of the mold lowest filling region as a foundation once while discharging the hybrid resin raw material composition at a predetermined flow rate in the same manner as in the (ii), to form a grown filling region resulting from addition of a uniform thickness corresponding to the one scanning; and (v) repeating the operations (iii) and (iv) continuously with the grown filling region as the foundation, and wherein the varying of the ratio in the hybrid resin raw material composition to be discharged is performed in the operations (ii) to (iv) such that, at an instant when the thickness of the formed grown filling region becomes equal or substantially equal to each of the heights z 1 to z m-1 , the raw material composition for the next "color unit" is discharged from the discharge port.
  5. The method for producing the mill blank for dental cutting according to claim 3 or 4, wherein the filling step alternately performs the filling of the hybrid resin raw material composition into the mold in the pattern 1 and the filling of the hybrid resin raw material composition into the mold in the pattern 2

Description

{Technical Field} The present invention relates to a mill blank for dental cutting and a method for producing the same. {Background Art} Typically, a dental prosthesis used for dental treatment is molded from a metal material such as gold, silver, titanium, or a palladium alloy by casting, is shaped through the working of a nonmetal material such as ceramic or a hybrid resin (hereinafter, sometimes abbreviated as "HR"), or is formed using a metal material for forming a core part and using a nonmetal material for forming a surface layer part such as an anterior part and so on. Note that the hybrid resin means a composite material in which an inorganic filling material (hereinafter, sometimes referred to also as an "inorganic filler") is dispersed with a high density in a resin matrix, and is typically obtained by polymerization-curing a polymerization-curable composition in paste form containing a polymerizable monomer, an inorganic filler, and a polymerization initiator, by pressurizing and heating or the like. The demand for a dental prosthesis entirely formed of a nonmetal material is rapidly increasing because it has the merits of not involving a risk of metal allergy and being aesthetically excellent and in addition, its working has become easy owing to the recent development of digital image technology, computer processing technology, and the like. A CAD/CAM system, as is disclosed in, for example, PTL1, has come into wide use that forms a dental prosthesis by cutting a blank for dental cutting made of a nonmetal material based on a photographed image of an oral cavity using a CAD/CAM device that is based on the techniques of CAD (Computer Aided Design) and CAM (Computer Aided Manufacturing). The blank for dental cutting here means an object to be cut (also called a mill blank) made attachable to a cutting machine in the CAD/CAM system, and typically has a section to be cut and a holding section such as a holding pin with which the section to be cut can be attached to the cutting machine. As the section to be cut, a (solid) block formed in a rectangular parallelepiped shape or a columnar shape, a (solid) disk formed in a plate shape or a disk shape, or the like is generally known, and a section to be cut formed of a hybrid resin (HR) is also known. Note that, in the present invention, a mill blank, including one in the form in which a section to be cut made of HR is integrated with a holding section, is called a HR-based mill blank for dental cutting or a HR-based mill blank. Incidentally, in recent years, treatment for tooth crown and dentition restoration has been increasingly facing scenes where a dental prosthesis is required to have aesthetic properties such as color tone and appearance identical or close to those of natural tissues. To attain such aesthetic properties, the production not from a HR-based mill blank having one color tone but from a HR-based mill blank with a multilayer structure in which a plurality of layers with different color tones are integrated has been in practice, and for example, PTL2 describes a method for producing a HR-based mill blank with a multilayer structure, using fluid pastes, which are HR raw materials, and a plurality of formwork units. It describes that, according to this method, it is possible to produce the blank for dental cutting with the multilayer structure by filling the fluid pastes (curable compositions) with different color tones in the respective formwork units and stacking them, and even in a case where low-fluidity pastes which are usually difficult to pour into formworks are used, it is possible to produce a blank for dental cutting with a multilayer structure having a plurality of color tones and constitutions while reducing the occurrence of defects such as a disordered interface between the pastes and voids in the pastes. Further, as a tooth mimicking technique different from the above-described configuration of stacking the layers, PTL3 proposes a HR-based mill blank whose color tone continuously varies as it goes from one end to the other end of the blank and a method for producing the same. Specifically, PTL3 discloses a method that, when filling a mixture of two types of fluid pastes (curable compositions) (which are HR raw materials) with different color tones from one corner to the opposite corner of a mold (for example, from a lower corner to an upper opposite corner of the mold) using an extruder such as a dispenser, continuously varies a mixture ratio of the two pastes in the mixture and cures the filled paste, thereby producing a mill blank for dental cutting (HR-based mill blank) having the above-described gradation structure (where color tone continuously varies). Further prior art is exemplified by PTL6. {Citation List} {Patent Literature} {PTL 1} JP 2016-535610 A{PTL 2} JP 2020-151339 A{PTL 3} Specification of US Patent Application Publication No. 2019/247168{PTL 4} JP 6285909{PTL5} JP 6349480{PTL6} WO 2018/074605 A1 {Summary of