Machinable glass-ceramics

Details Machinable glass-ceramics Machinable glass-ceramics

2000-03-17

2002-04-23

Koslow, C. Melissa (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Dental

C264S016000, C264S019000, C264S020000

Reexamination Certificate

active

06375729

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to a glass powder, which is crystallizable and forms a sintered micaceous glass-ceramic in a plurality of shades and more specifically to micaceous glass ceramics, which are machinable into various dental articles by conventional tools. This material is especially useful for the fabrication of dental restorations using computer assisted design/computer assisted milling (CAD/CAM) devices.
BACKGROUND OF THE INVENTION
Micaceous glass-ceramic materials (i.e. glass-ceramics comprising a crystalline phase that belongs to the mica family such as tetrasilic fluormicas or fluorophlogopite micas) are known to exhibit excellent machinability. However, their use as CAD/CAM materials for dentistry is limited by the inability to produce the required range of shades and translucency paramount for esthetically sound restorations. This severely inhibits widespread use of micaceous materials as dental restoratives considering that the driving force for all-ceramic restorations is esthetics superior to that of porcelain fused-to-metal (PFM) restorations. For example, Dicor MGC, available from Dentsply International Inc., Caulk Division, (located in Milford, DE) is a commercially available micaceous dental ceramic for use in CAD/CAM devices, but it is supplied in only two modifications, Dicor MGC—Light and Dicor MGC—Dark. Other limitations of micaceous glass-ceramics include high solubility and low strength in comparison to other dental ceramics. One such example is ProGlass™ ceramic available from CAD/CAM Ventures LLC, (located in Irving, Tex.), which is a sugary-white mica-containing material exhibiting a flexure strength of about 100 to about 150 MPa and a solubility of about 1 mg/cm
2
(1000 &mgr;m/cm
2
).
At the same time, micaceous glass-ceramics exhibit far superior machinability compared to other CAD/CAM ceramics such as sanidine-based Vita Mark II, available from Vita Zahnfabrik (Germany) and leucite-based Pro-Cad from Ivoclar (Lichtenstein), as set forth in “Mechanical Properties of a New Mica-Based Machinable Glass Ceramic For CAD/CAM Restorations” by J. Y. Thompson et al., The Journal of Prosthetic Dentistry, 1996, Vol. 76, No. 6,619-623 and “Machinable Glass-Ceramics Based on Tetrasilicic Mica” by D. G. Grossman, Journal of Am.Cer.Soc., 1972, Vol. 55, No. 9. The latter two above-mentioned ceramics can be machined by diamond tools only and require wet processing in contrast to micaceous glass-ceramics such as ProGlass™ which can be machined by carbide tooling using dry processing which is much more cost-effective. In addition, micaceous glass-ceramics can be much more translucent than very opaceous sanidine glass-ceramics.
U.S. Pat. Nos. 4,652,312, 4,431,420 and 5,246,889 are each directed to mica-containing ceramics that are formed from glass compositions and are shaped as glass and converted into micaceous glass-ceramics by conventional volume crystallization techniques. Each process involves melting glass batches, casting the glass melts into molds, and crystallizing the glass into micaceous glass-ceramics. There is no discussion providing how to achieve adequate colors and shades to accurately match the color of a person's tooth or how to control the shading of mica containing glass-ceramics. Any mention of colorants appears to be directed to adding the colorants to the glass batch prior to melting. Such process does not effectively control the color of the resulting glass-ceramic. Furthermore, each of the processes appears to effect crystallization by performing bulk or volume crystallization. It is difficult to control the color of the micaceous glass-ceramics when utilizing volume crystallization. None of the prior art is concerned with the need to provide a variety of colors and shades to adequately match the color and shade of a patient's teeth.
It is desirable to provide a variety of shades of micaceous glass-ceramics in order to fabricate restorations that closely and accurately match the teeth in a patient's mouth. It is preferable to provide an efficient and effective method of producing a variety of shades of micaceous glass-ceramics. It is beneficial to provide micaceous glass-ceramics that are machinable and that come in a variety of shades.
SUMMARY OF THE INVENTION
These and other objects and advantages are accomplished herein by the micaceous glass-ceramics comprising silica, magnesium oxide and fluorine in addition to other components listed below. The glass-ceramics are useful in the fabrication of single and multi-unit dental restorations including but not limited to orthodontic appliances, bridges, space maintainers, tooth replacement appliances, splints, crowns, partial crowns, dentures, posts, teeth, jackets, inlays, onlays, facing, veneers, facets, implants, abutments, cylinders, and connectors by machining the glass-ceramic using CAM/CAM devices. The micaceous glass-ceramics are provided in a shade and color selected from a plurality of shades and colors to adequately match the colors and shades of teeth found in 95% or more of the human population.
In accordance with one embodiment directed to the process of making the glass-ceramics, the batch ingredients of the compositions are melted at a temperature in the range of about 1200° to about 1650° C., for a time in the range of about 0.5 to about 8 hours, thereafter it is quenched, and pulverized into powder. Pigments, opacifiers, fluorescing agents and the like are mixed with the powder. The powder is then used to form net-shaped or block-shaped pre-forms or blanks to be used in CAD/CAM devices. Blanks are dry-pressed and sintered using a one- or two-step heating cycle at a temperature in the range of about 600° to about 1200° C. and for a time in the range of about 0.5 to about 4 hours for each step in the cycle. The sintering is preferably conducted in a vacuum. Occurring simultaneously with sintering, surface crystallization of the starting glass powder yields the amount of mica phase of at least thirty volume percent (30 vol. %) required for machinability as well as strength.
DETAILED DESCRIPTION OF THE INVENTION
As will be appreciated, the present invention provides glass-ceramic compositions comprising a glassy matrix and one or more micaceous phases (e.g., tetrasilic flourmica, fluorophlogopite mica and the like). The glass-ceramics are useful in the fabrication of dental restorations. The micaceous glass-ceramic compositions contain inter alia, silica, magnesium oxide and fluorine in the ranges given in Table 1 below. The glass-ceramic compositions have a combination of properties including high strength and chemical durability useful for dental restorations. The glass-ceramics have good machinability, i.e., the ability to be cut or milled by a cutting tool into a dental restorative shape that accurately depicts the original shape of the tooth to be restored or replaced.
In an important aspect herein, the micaceous glass-ceramics are provided in a shade and color selected from a plurality of shades and colors to adequately match the colors and shades of teeth found in 95% or more of the human population. The shades and colors of the glass-ceramics provide the dental technician with the ability to closely and effectively match the color and shade of the patient's tooth or teeth abutting or adjacent to the tooth or teeth that is/are being restored or replaced.
TABLE 1
Compositions of the starting glass powder
Wt %
Wt %
Oxide
Mole %
Range 1
Range 2
SiO
2
30-65
43-72
43-72
Al
2
O
3
0-7
 3-14
0-3
B
2
O
3
0-3
0-3
0-3
ZnO
0-3
0-3
0-3
CaO
0-5
0-7
0-3
MgO
15-33
10-30
10-30
TiO
2
0-3
0-3
0-3
BaO + SrO
0-3
0-5
0-5
Li
2
O
0-3
0-3
0-3
K
2
O
 0-10
0-7
 7-19
Na
2
O
0-7
0-3
0-3
CeO
2
+ La
2
O
3
+
0-1
0-2
0-2
Tb
4
O
7
ZrO2
0-4
 0-10
 0-10
F
14-25
 5-10
 5-10
In accordance with one embodiment of the method of the invention, the shaded micaceous glass-ceramics are manufactured by admixing pigments and other additives to the starting glass powder. The powder is formed into pre-forms or blanks a

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