Dental restorations

Abstract: A dental restoration which is formed from a glass-ceramic material having the following composition (by weight percent): Li 2 O 8 - 15 Al 2 O 3 1.5 - 5.0 Si O 2 60 - 85 Na 2 O 0 - 2 K 2 O 0 - 2 P 2 O 5 1.5 - 5 ZrO 2 0 - 3 CaO 0 - 1 BaO+SrO+LaO 0 - 12 Coloring Oxide 0 - 5

Description: Related Applications This application is a divisional of Application No. 0711133A (WO 95/32678). Field of the Invention The present invention relates to dental restorations, such as crowns, bridges, inlays, onlays etc., formed from glass-ceramic materials. Background of the Invention In the prior art various methods have been used to form metal free dental restorations from glass-ceramic materials. Glass-ceramic materials because of their strength, translucency, non-toxicity and other physical properties are ideal materials for use in forming dental restorations. Because of their suitability, glass-ceramic materials have been used to form dental restorations for at least eighty years. In the prior art the most widely used means for forming dental restorations from mixtures of glass and ceramic materials is a process which utilises a slurry of glass and ceramic particles. In this process a die is formed which is an exact replica of the remaining portion of the tooth to which the restoration is to be secured. It is understood by one skilled in the art that the remaining portion of the tooth has been prepared by the dentist in such a manner that the attachment of the restoration is facilitated. To start the process the dentist takes an impression of the prepared tooth to create a negative impression of the prepared tooth or teeth. This negative impression is then packed with a material to form a positive impression of the prepared tooth or teeth. This positive impression is called a die. Platinum foil is then pressed over the die to from a matrix which is essentially a foundation on which the dental restoration is built. In order to form up the dental restoration many layers of a slurry of a particulate glass-ceramic material are applied to the platinum foil matrix. As multiple layers of the slurry are built up and dried a semi-solid structure is formed which can be carved into the shape of the desired dental restoration. Once the desired shape is achieved the structure is then removed from the die . At this stage the structure is referred to as a green structure. The structure is then fired, during the firing process the particulate glass-ceramic material fuses into a solid mass. Because the green structure is formed from multiple layers of the dried glass-ceramic material uneven fusing may result. As a result of this uneven fusing the physical properties of the finished restoration may be detrimentally affected. As a result an inferior dental restoration results. Further, as can be seen from the above description, the overall process is very labor intensive. Other methods for forming the green restoration have been considered in the prior art, for example, in U.S. Patent no 2,196,258, a mixture of particulate glass and ceramic materials which incorporates a binder is packed into a flexible mold to form a green structure which is then fired to form a finished structure. Again, because the process entails the fusing of particulate material, uneven fusing may result and hence a weakened and inferior dental restoration may result. To overcome the problems as described above the trade has recognized that in order to produce strong, translucent metal free dental restorations it would be desirable to form these restorations directly from a homogenous molten glass-ceramic material. It was realized that it may be possible to produce a satisfactory restoration by forcing a molten or plastic glass-ceramic material into a mold having a cavity in the form of the desired dental restoration. The prior art further recognized that the glass-ceramic material could be introduced into the cavity when the glass-ceramic material was in the liquid or in the plastic state. A constant goal of the prior art as described, was to effect the molding process in a quick and efficient manner and in a manner that produces a dental restoration that has excellent definition and fit. In dental restorations definition is extremely important, as in order to have a satisfactory restoration the finest details of the original tooth must be reproduced. For example, for a dental restoration to be successful the margins must be sharp and well defined. It is in this area that the prior art molding processes are deficient in that it was not possible to achieve the desired degree of definition. Further, it is desirable to produce a dental restoration in a short period of time in order to efficiently utilize the overhead of the dental laboratory and in order to minimize the labor content of the dental restoration. Dental laboratories are not typically well funded operations. Therefore, in order to keep cost to a minimum it is highly desirable that a suitable process for forming dental restorations utilize equipment which is relatively inexpensive. While the above described process fits this requirement the below described DICOR process does not. As is discussed above, there are several prior art processes for the manufacture of dental restorations from glass-ceramic materials. A recent addition to the prior art is the DICOR process as sold by the Dentsply International,Inc. of York Pennsylvania. In this process a dental restoration is formed by centrifugal casting of a molten glass-ceramic material. This process is further described in U.S. Patent No. 4,431,420 issued February 14, 1984 and related patents. Centrifugal casting has been extensively used in the casting of metals principally by the lost wax process. Further, this process has been imminently successful for hundreds of years for use in conjunction,with metals. This success results from the fact that molten metals have very low viscosity and high density in the molten state, hence, they function very well in centrifugal casting processes. That is, because molten metals have a high density and a very low viscosity in the molten state centrifugal force is adequate for purposes of injecting the molten metal into a preformed mold cavity. In an attempt to produce dental restorations which have high definition the above mentioned DICOR process uses centrifugal force to form the desired dental restorations from a molten glass-ceramic material. Molten glass-ceramic materials have a much higher viscosity and a much lower density when compared to molten metals. For this reason, it is not possible to consistently drive a molten glass-ceramic material, by centrifugal force alone into a mold in order to produce a satisfactory dental restoration. That is, a molten glass-ceramic material cannot be driven by centrifugal force into a mold cavity with sufficient force in order to always get the required definition, necessary to form a satisfactory dental restoration. It is well recognized by one skilled in the art that in order to have a satisfactory dental restoration, excellent definition must be achieved in order to recreate the desired margins which are needed for the proper fit of a dental restoration into the human mouth. Further, the DICOR process, is deficient as to the coloration of the glass-ceramic material utilized. The resulting Dicor dental restoration had an undesirable white color and must be glazed in order to produce satisfactory human coloration. As a result the coloration is only on the surface of the dental restoration, If adjustment by grinding is needed in the final installation of the restoration into the human mouth the glazing is removed thereby exposing the whitish base which contrasts with the glaze. This contrast is very unsatisfactory from an esthetic point of view. In contrast to this deficiency, the restoration of the subject application is adapted to utilize glass-ceramic materials wherein the coloration of the resulting dental restoration, throughout, approximates human tooth coloration. Hence, if grinding is necessary in final fitting contrast between the surface of the dental restoration and the underlying base is not observed. Undesirable contrast can also result from normal wear, where as a result of the grinding action of one tooth against another the glaze is worn away. Again, this is not a problem in this invention as the preferred glass-ceramic material has a uniform natural coloration throughout. It should be noted that the restoration of the subject invention may be glazed to achieve the exact shade desired. In contrast to the above discussed prior art processes, the process of the subject invention may utilize a positive mechanically applied force for purposes of injecting the molten dental glass-ceramic material into the preformed mold cavity. For a dental glass-ceramic material to be satisfactory for use in the formation of dental restorations the material should incorporate many or all of the following properties: 1. It must be inert and non-toxic in an oral environment. 2. It must have sufficient structural integrity to resist the forces of mastication and generally must have a 3-point MOR of at least 30,000 PSI. 3. Should be capable of being formed into forms which are compatible with the human anatomy using simple equipment. 4. Should have esthetic qualities (coloration similar to human teeth with a slightly translucent appearance) which are compatible with human teeth and hence should be monolithic or glazable. 5. Further, the glass-ceramic material must not absorb moisture or stain and it must be stress corrosion resistant. 6. Likewise the glass-ceramic material should have wear characteristics which are similar to natural human teeth and should be compatible with other dental materials. 7. The glass-ceramic likewise must have dimensional stability and resist thermal shock during processing and in particular it must have dimensional stability during subsequent heat treating processes wherein recrystalization is effected. 8. Further, the glass-ceramic material should be compatible from a thermal expansion point of view with metals, stains, glazes etc. as are conventionally used to form dental restorations. 9. In order to create an esthetically pleasing dental restoration it may be necessary to alter the final dental restoration to the exact shape and shade desired. In order to effect these alterations the dental restoration must be heated to a temperature of about 950°C for each operation. Therefore, a satisfactory glass-ceramic material must be capable of withstanding multiple heat cycles to about 950°C. 10. A suitable glass-ceramic material must be capable of retaining its structural integrity during heat treating. 11. In summary a suitable glass-ceramic material should have; A. Coefficient thermal expansions (C.T.B.) of 5 to 145X10 -7 /°C B. Translucency of 2.5 to 4.0 on a visible scale of 0 (clear) to 5 (opaque) and overall beauty. C. M.O.R. of at least 30 K.S.I. average. D. Ability to be heat treated to 925-950° C E. Structural integrity during heat treat F. Meltability and formability G. Chemical durability in an oral environment The subject invention includes glass-ceramic materials which meet the above set forth criteria. Summary of the Invention According to the present invention there is provided a dental restoration which is formed from a glass-ceramic material having essentially the following constituents (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 8 - 15 Al 2 O 3 1.5 - 5.0 SiO 2 60 - 85 Na 2 O 0 - 2 K 2 O 0-2 P 2 O 5 1.5 - 5.0 ZrO 2 0-3 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0 - 12 (total proportion) Colouring Oxide(s) 0 - 5 In preferred embodiments the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 10 - 13.5 Al 2 O 3 2-3 SiO 2 70 - 84 Na 2 O 0-2 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0.5 - 4.0 (total proportion) Colouring Oxides 0 - 5 In further preferred embodiments the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 9 - 13 Al 2 O 3 1.5 - 4.0 SiO 2 65 - 84 Na 2 O 0 - 1 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0 - 12 Colouring Oxide(s) 0 - 5 In a particularly preferred embodiment the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 10.5 Al 2 O 3 2.5 SiO 2 78.5 K 2 O 0.5 P 2 O 5 2.0 ZrO 2 0.5 CaO 0.4 BaO 3.0 Colouring oxides comprising: TiO 2 0.1 CeO 0.35 NiO 2 0.02 Preferred compositions of the subject invention incorporate Na 2 O in the range of 0.2 to 2.0 by weight percent; K 2 O in the range of 0.5 to 2.0 by weight percent; ZrO 2 in the range of 0.5 to 3.0 by weight percent; CaO in the range of 0.4 to 1.0 by weight percent; and/or a softening temperature of at least 850°C, and preferably about 950°C. Preferred compositions of the subject invention are highly advantageous in that they produce aesthetically pleasing dental restorations which are chemically inert in the human mouth and have outstanding strength properties. Further, such compositions are advantageous in that when dental restorations formed from these compositions are heat treated they maintain their structural integrity. Further, these dental restorations are capable of withstanding multiple heat cycles to about 950°C, and possess thermal expansions which are compatible with the existing porcelains so that these dental restorations can be readily altered using conventional porcelain materials. Furthermore such dental restorations have acceptable coloration after heat treating and can be used at that stage, without further cosmetic treatment, in the mouth. In order to enhance the aesthetic properties of the resulting dental restoration, the dental restoration can be readily altered using porcelain materials to achieve any desired effect. The glass-ceramic compositions are suitable for the formation of dental restorations which have a high degree of micro-structural control during the development of the crystals, thereby permitting great flexibility in the forming of these dental restorations. Such preferred compositions are intended to maintain their structural integrity during heat treating so that, in particular, they will not slump or sag during heat treating outside of the investment. Preferably such compositions are capable of maintaining their translucency during multiple firing cycles to approximately 950°C. Lastly, preferred compositions have excellent chemical durability and as such will not deteriorate when exposed to the fluids in the human mouth. In order that the invention may be more fully understood, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a flow chart showing steps in the production of the preferred embodiment of this invention. Figure 2 shows a sectioned lost wax mould for use in such production. Figure 3 shows a sectioned structure illustrating the heating step of this process. Figure 4 shows a sectioned structure illustrating the initial contact of the crucible with the mould. Figure 5 shows a sectioned structure illustrating the partial sealing of the crucible against the mould. Figure 6 shows a sectioned structure illustrating the complete sealing of the crucible against the mold and the injection of the molten glass-ceramic into the mould cavity. Figure 7 is a sectioned side view showing composite apparatus for use in such production. Figures 7a and 7b are sectional side views showing the preferred composite apparatus. Figure 8 shows a sectioned alternate mould. Figure 9 shows a sectioned alternate mould. Figure 10 shows the mould of Figure 9 being filled with a molten glass-ceramic. It should be noted that Figures 2 to 10 are schematic representations of apparatus which may be useful in this invention. Figure 1 generally describes the overall process wherein dental restorations in accordance with the present invention are formed by the molding of a glass-ceramic material. As can be seen in Figure 1, the first step is the formation of a mold having a suitable mold cavity therein. To start the process whereby the mold cavity is formed the tooth or teeth in the human mouth are prepared by the dentist by procedures which are commonly known in the prior art. By using impressions which are provided by the dentist a positive wax form is produced of the desired dental restoration. This positive wax form is then positioned in a mold usually called a ring. A semi-liquid investment material is then poured around the positive wax form. Once the investment material sets up and cures the resulting mold is placed in a furnace and heated thereby causing the wax to melt and run out of a sprue hole which is integral with the mold. The finished mold is then complete. This constitutes the formation of a mold by the lost wax process. In the second step of Figure 1 a button of a glass-ceramic material is placed in deformable crucible 8, as is illustrated in Figure 3. The button of glass-ceramic material constitutes a small cylinder of the glass-ceramic material which usually weighs about six grams and has a diameter of about two cm. and a thickness of about one cm. These buttons of glass-ceramic materials are a convenient shape whereby these materials may be manufactured, sold and used. It is understood by one skilled in the art that a sufficient amount of the glass-ceramic material must be used in order to form the desired dental restoration. In the third step of Figure 1 crucible 8 and the button of glass-ceramic material 7 is uniformly heated. The glass-ceramic material is heated to a temperature above its liquidus temperature as will be described in greater detail herein below. Further the heat-pressure deformable crucible 8 will be described in greater detail herein below. In this process the preformed mold can incorporate multiple cavities in order that more than one dental restoration can be produced during one cycle of the subject process. In steps four, five and six of Figure 1 the heated crucible and glass-ceramic material are moved into contact with the mold by mechanical means in a manner which will be described in greater detail relative to the discussion of Figures 3 to 6 herein below. In accordance with steps 7 and 8 of Figure 1 the mold is cooled and the solidified glass-ceramic casting is removed from the investment by mechanical means. The dental restoration is then cut off the sprue and finished by heat treating polishing and/or glazing in order to achieve the desired esthetic effect. The process of this invention may utilize both glass and glass-ceramic materials for purposes of forming the desired dental restoration. For strength and esthetic reasons, it is preferred that a glass-ceramic material be utilized. When a glass-ceramic material is utilized, the button of glass-ceramic material 7 is initially placed in crucible 8 in a glass phase. As heat is applied to crucible 8, glass-ceramic material 7 is heated. As the heating of the glass-ceramic material 7 continues, crystal formation is effected whereby glass-ceramic material 7 becomes a ceramic via the formation a of crystalline phase. As the heating is continued, the glass matrix slowly dissolves the crystalline phase thereby causing the glass-ceramic material 7 to reenter the glass phase. The temperature at which the crystal phase completely dissolves in the glass matrix is defined as the liquidus temperature of the glass-ceramic material. In accordance with this invention, it is found desirable to heat the glass-ceramic material to a temperature above the liquidus temperature in order to eliminate any possibility of the existence of a crystalline phase in the molten material. Further at this temperature the glass-ceramic material has a viscosity of about log 3 to about log 4 P which is suitable to allow the molten glass-ceramic material to be readily injected into the mold cavity 5. The viscosity at this temperature is defined as the working range. Throughout this application, the viscosity of the molten glass-ceramic materials and of the soften crucible will be defined as the log of the respective viscosity in poises. Further, the unit poise will be abbreviated as P. For example, if the viscosity of a molten glass-ceramic glass material is 10 4 poises the viscosity will be indicated by log 4 P. While Figure 1 illustrates the broad process for producing dental restorations in accordance with the subject invention. The individual steps as illustrated in Figure 1 are defined in greater detail in the description of Figures 2 through Figure 10. In Figure 2, it can be seen that mold 4 has been formed by the placement of an investment material 10 in ring 12. Prior to the placement of investment material 10 in ring 12, ring 12 is positioned around wax form 14, the upper extremity of which is shaped in the form of a desired dental restoration 16. Wax form 14 is only shown in an outline form, as Figure 2 illustrates a finished mold 4 after wax form 14 has been burnt out. The procedure whereby mold 4 is formed is generally known in the prior art, and has been used for centuries to form lost wax molds for use in metal casting procedures. As is stated above in the preparation of mold 4, wax form 14 is positioned in ring 12 an investment slurry 10 is then slowly poured into ring 12 usually with the aid of vibratory techniques to insure that investment 10 completely fills ring 12, and in particular, completely encases wax form 14. Again, these procedures are commonly known in the prior art. Figures 3, 4, 5, and 6 show the general process for producing dental restorations is accordance with the subject invention in a schematic form. Referring to Figure 3, it can be seen that mold 4 having cavity 14 is positioned in the vicinity of crucible 8. Positioned in crucible 8 is glass-ceramic material 7, the details of which will be described herein. Crucible 8 is further positioned on a ceramic base 21 which is positioned on ram 22 whereby crucible 8 can be moved in relation to mold 4. An electric heating element 24 is further provided whereby heat can be applied to crucible 8. Details of the heating of glass-ceramic material 7 will be discussed herein below. During the heating of crucible 8 glass-ceramic material 7 is converted from the solid state to the liquid state, and crucible 8 converted from a solid brittle state to a plastic state wherein it is heat-pressure deformable. Once the glass-ceramic material 7 reaches the desired working range, ram 22 is actuated as is illustrated in Figure 4, thereby causing crucible 8 to move upwardly and to come into contact with the lower extremities of mold 4. The actuation of ram 22 is effected by a power source not shown. As can be seen in Figure 5 as the upward movement of ram 22 continues, crucible 8 continues to deform against mold 4 causing the upper extremities of crucible 8 to seal against mold 4 at interface 28. Further, it can be seen that molten glass-ceramic material 7 has started to flow into mold cavity 14. Referring to Figure 6, as the travel of ram 22 continues the deformation of crucible 8 against mold 4 is completed, and further cavity 14 is completely filled with glass-ceramic material 7 as a result of the applied pressure of ram 22. Subsequent to the procedures as are illustrated in Figures 3 through 6, mold 4 is then cooled and the desired dental restoration is removed from investment 10. Once the dental restoration is removed from investment 10, the dental restoration is cut apart from the sprue and is heat treated and finished by polishing and glazing in order to achieve the desired esthetic effect. Further, after formation dental restoration 26 is heat treated in such a manner that its strength and other properties are enhanced by the formation of crystals in a glass matrix. The details of heat treating are described below. Figure 7 illustrates apparatus 32 which may be utilized to effect the composite process which is illustrated in the flow chart of Figure 1. This apparatus generally comprises support frame 30, heat source 33, mold retaining means 34 and a plunger 36. Support frame 30 generally comprises the outside frame of apparatus 32. Mold retaining means 34 comprises a bar 50 into which is threaded mold clamp 35. As can be seen, bar 50 permits mold 4 to be locked to support bar 44. Support bar 44 further incorporates an aperture 56 which is slightly smaller than the diameter of mold 4 and slightly larger than the diameter of crucible 8. By the actuation of ram 36, crucible 8 is moved up into contact with mold 4 in a manner which is similar to that as is described above in conjunction with Figures 4, 5 and 6. The apparatus of figure 7 further incorporates a heat source 33, which in the illustrated embodiment comprises an electric resistance heating element 37. In the preferred embodiment the heating element is molybdium disilicide. Heat source 33 is controlled by a power control source 58. As is illustrated in Figures 3 and 7, an electric resistance heater may be utilized to effect the heating of crucible 8 and, hence, glass-ceramic material 7. In addition to the electric resistance heating, as illustrated heating may be effected via induction heating, gas torch heating, or any other appropriate means. The apparatus further may incorporate rotating means, not shown, whereby ram 36 may be rotated during the heating process in order to effect a more uniform heating of crucible 8 and glass-ceramic material contained therein. The rotating means may be an electric, pneumatic or hydraulic motor. Ram 36 is further provided with means for effecting its upward movement, which in the preferred embodiment is a pneumatic cylinder 39. Further, referring to Figure 7, it can be seen that apparatus 32 incorporates a plurality of structural insulating members 41, 43, 45, 47 and 49 which support and contain heating element 37, and contain the heat created during the operation of the element 37. These structural insulating members are formed from ceramic fiber board. Likewise, it can be seen that support bar 44 incorporates an aperture 56 which has a waist, section 55. Waist section 55 is advantageous in that when crucible 8 deforms against mold 4 the deforming upper extremities of crucible 8 are prevented from downward movement at the constriction of waist 55 thereby causing the molten glass-ceramic material to be efficiently injected into the preformed cavity 14 in mold 4. That is, because upper extremities of crucible 8 are caused to solidify in the vicinity of waist 55, the downward movement of the molten glass-ceramic material is prevented, thereby causing the glass-ceramic material to be injected into mold cavity 14. Figure 7a and 7b represent the preferred apparatus 31 for use in producing dental restorations in accordance with this invention. Most of the components of the preferred apparatus 31 are identical or similar to the components of apparatus 32 as discussed herein above. Apparatus 31 differs from apparatus 32 primarily in that a movable arm 38 is provided for wherein a preheated mold 4 may be brought into position for the molding process and positioned against furnace base 41. Moveable arm 38 may be connected to a central pivot point whereby it may be swung into position or it can be laterally moved into position. Prior to moving arm 38 into position, as is shown in figure 7 a ram 22 is extended in order to place crucible 8 in heat source 33, whereby glass-ceramic material 7 may be melted. Once this melting is effected ram 22 is withdrawn and moveable arm 38 is swung into place to the position as is illustrated in figure 7b. In this position the forming sequence as is shown in Figures 2 to 6 can be completed. Relative to apparatus 31 and 32 it is understood by one skilled in the art that crucible 8 may be placed in position on ceramic base 21 either manually or by automatic means. The above described Figures 1 to 7b illustrate the formation of a dental restoration. In addition to being useful in the formation of essentially complete dental restorations, the composition and process described above may be used to form copings over which porcelain materials may be applied to alter and shade the dental restoration. In the prior art, metal copings are extensively utilized. These copings are covered with layers of porcelain materials which are applied to the metal copings for purposes of forming a composite dental restoration which comprises a metal base and a porcelain exterior portion. Composite structures are advantageous in that a metal coping significantely enhances the strength of the resulting dental restoration. Metal copings are disadvantagous in that they are opaque and they have a color which contrasts with the color of a natural tooth and, further, they have toxicity problems in some instances. The application of the subject invention to an all glass-ceramic dental restoration is advantagous in that the coping is essentially the same color as a natural tooth and, in particular, it is essentially the same color as the porcelain from which the exterior portion of the dental restoration is formed. Further, the all ceramic structure is advantagous in that problems resulting for certain patients who are allergic to certain metals are eliminated. In this procedure, metal free crowns and bridges of outstanding strength and esthetic properties are achieved. The outstanding esthetic properties results from the fact that these glass-ceramic copings can be utilized as a base over which a variety of different porcelains can be fired in order to achieve the most delicate coloration and esthetic properties. As is stated above, the process described may utilize both glass and glass-ceramic materials. Preferred glass-ceramic compositions for use in accordance with the invention are listed in Tables I to IV. All components listed in Tables I to IV are in weight percent. The subject invention can utilize any suitable glass-ceramic materials As is shown in Table I to IV preferred glass-ceramic materials for use in this invention are lithium disilicate glass-ceramic materials. In these materials Li 2 O 2(SiO 2 ) constitutes the crystalline phase, of the heat treated glass ceramic material. Lithium disilicate glass-ceramic materials are particularly suited for use in the invention in that they are non-toxic, they resist thermal shock, have excellent strength, are corrosion resistant and they produce dental restorations which approximate humane coloration and are translucent and esthetically pleasing. Further, lithium disilicate glass-ceramic materials are advantageous in that they maintain their structural integrity during heat treating, in that they do not slump or sag during proper heat treating. Other lithium disilicate glass-ceramic materials which may be used in this invention are disclosed in U.S. patent 5,219,799 issued June 15, 1993. Lithium disilicate glass-ceramic materials may utilize P 2 O 5 as a nucleating agent. Other nucleating agents are TiO 2 and ZrO 2 . Glass-ceramic compositions which may be useful in this invention are as per Table I. The percentages of Table I and the other tables herein are in weight per cent. Li 2 O 8 - 15 Al 2 O 3 1.5 - 5.0 SiO 2 60 - 85 Na 2 O 0 - 2 K 2 O 0 - 2 P 2 O 5 1.5 - 5.0 ZrO 2 0 - 3 CaO 0 - 1 BaO + SrO + LaO 0 - 12 Coloring Oxides 0 - 5 More specific glass-ceramic compositions which may be useful in the invention are as per Table II. In particular the compositions of Table II are useful in conjunction with either high or low fusing porcelains which may be used to alter, shade or glaze the dental restorations of this invention. Li 2 O 10 - 13.5 Al 2 O 2 2 - 3 SiO 2 70 - 84 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 BaO+SrO+La 2 O 3 .5 - 4.0 CaO 0 - 1 Coloring Oxides 0 - 5 In some instances it is desirable to form a dental restoration which can be altered, or glazed with a low fusing porcelain. For purposes of this application a low fusing porcelain is defined as a porcelain which fuses at a temperature of about 700°c Specific glass-ceramic compositions which are useful in conjunction with low fusing porcelains are as per Table III Li 2 O 9 - 13 Al 2 O 3 1.5 - 4.0 Si O 2 65 - 84 Na 2 O 0 - 1 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 BaO+SrO+La 2 O 3 0 - 12 CaO 0 - 1 Coloring Oxides 0 - 5 Specific glass-ceramic compositions which may be useful in this invention are as per Table IV. As to a general range of compositions which are preferred for use in conjunction with this invention the glass-ceramic compositions as defined in Table II are preferred. The most preferred glass-ceramic composition for use in accordance with this invention is the glass-ceramic composition as is defined by composition 18 of Table IV. The glass-ceramic composition as defined by Table I to IV above are particularly advantageous over the prior art in that; 1. They are stronger than the prior art glass-ceramic compositions. 2. Those compositions which incorporate barium and cerium oxides are florescent in ultra violet light. 3. The expansion rates of many of these glass ceramic compositions are compatable with the expansion rate of existing altering porcelain materials. 4. They retain their structural integrity at higher temperatures than those of the prior art, thereby permitting altering at higher temperatures. As is listed in Tables I to IV, coloring oxides may be added to the glass-ceramic material in order to achieve the desired coloration of the glass-ceramic dental restoration. Suitable coloring oxides for use in glass-ceramic compositions which can be used in this invention, some of which are illustrated in Table I to IV are SnO 2 , MnO, CeO, Fe 2 O 3 , Ni 2 O, V 2 O 3 , Cr 2 O 3 , TiO 2 etc. These coloring oxides may be used singularly or in combination. After formation of a glass-ceramic restoration in accordance with this invention, the resulting dental restoration is heat treated in order to effect the formation of crystals in the dental restoration. This crystal formation is generally referred to as heat treating and enhances the physical and esthetic properties of the dental restoration. A suitable heat treating sequence for use with the glass-ceramic compositions of Tables I to IV is as per Table V below. For the glass compositions which may be used in this invention the best nucleating temperatures are about 25° to 50°C above the upper anneal point of the glass-ceramic. It has also been determined that a slow increase in temperature from just above the upper anneal point to some 50°C. higher produces the best results in achieving maximum nucleation. It is also well known that the temperature then must be raised to a higher temperature to affect crystallization, this temperature depends upon the composition of the particular glass-ceramic material being used. If the heating sequence in the heat treating stage is improper or is incorrectly controlled the dental restoration may slump or deform. It is understood by one skilled in the art that an optimum heat treating procedure should be developed for each particular glass-ceramic material in order to allow the dental restoration to maintain its structural integrity during the heat treating process. In the preferred embodiment the heat treat cycle is such that in the crystalline phase the glass-ceramic material incorporates a large number of fine crystals which are evenly dispersed through out the glass matrix. It has been found that when the crystalline phase is very fine and evenly dispersed a dental restoration of maximum strength results. Further a fine crystalline structure tends to produce translucent dental restorations. From the above discussion it is obvious that dental restorations produced in accordance with this invention can be heat treated after the dental restoration is removed from the investment material and the sprue is cut away. In accordance with an alternate preferred embodiment of this invention the heat treating of the dental restoration may be effected while the dental restoration is still in the invested state. That is heat treating can be effected while the dental restoration is still encased in the investment material. In accordance with still another embodiment, the dental restoration can be removed from the investment material in which it was formed and finished or partially finished prior to heat treating. For heat treating the material is then reinvested in an investment material which enhances the heat treatment process. After reinvestment the heat treating is carried out in accordance with the procedure described above. Whenever the dental restoration is heat treated in the invested state the shrinkage of the dental restoration is minimized if not eliminated. In heat treating and in particular in the formation of a crystalline phase shrinkage of the dental restoration occurs. This shrinkage can amount to 3 percent. Naturally, this shrinkage is undesirable as it adversely affects the fit of the dental restoration back into the patients mouth. As a way of overcoming the shrinkage problem, it is within the scope of this invention to invest the wax form in an investment material which expands when it hardens and is heated. That is the investment material expands when it hardens to produce an oversize mold cavity. This oversize mold cavity naturally produces an oversize dental restoration. This oversize dental restoration is then shrunk back to the correct size during the heat treating process. For example since the glass-ceramic material shrinks about 3 percent during the heat treating process the original wax form may be invested using an investment material which expands about 3 percent when it hardens and is heated. Using this procedure a dental restoration which is 3 percent oversize results. This oversize dental restoration is then heat treated whereupon it shrinks about 3 percent to produce a finished dental restoration of the correct size. In order to achieve proper fit, if the restoration is to be heat treated after removal from the investment, an investment material which expands on hardening and heating by about 3 percent must be used. Investment materials which are useful with metals are not particularly suited for use in the invention as they only expand about 1.75 percent. Investments which expand about 3 percent upon hardening and heating and hence permit this type of process are manufactured by Whipmix Corporation of Louville, Ky. and sold under the designation GIJM 3-23-94; 1 The preferred heat treating process for use with the glass-ceramic compositions of Table I to IV is as per Table V Heat Restoration To 500 Ramp or Soak at 500 - 600°C for 2 - 6 hours Ramp at 600 - 925°C for 2 - 3 hours Soak at 925°C for 1 - 2 hours Cool to Room temperature After heat treating the resulting dental restoration may be further finished. This finishing may include a step wherein the dental restoration is altered with one or more porcelains in order to achieve the precise shape, shade and shading desired. For those glass-ceramic materials of Tables I to IV which have a C.T.E. (Coeficient of Thermal Expansion) of from about 135 to about 145 at the set point temperature, it is desired that the C.T.E. of the altering porcelain material be from about 125 to about 135 and always less the C.T.E. of the glass ceramic material. The process, wherein the C.T.E. is measured is defined in the description of the Examples herein below. Deformable crucible 8 is a critical part of the apparatus used. Crucible 8 in its preferred embodiment has a circular base and hence is generally cylindrical. However it is understood by one skilled in the art that the crucible can have configurations other that circular. The crucible is formed by heat sintering a particulate mixture of components, such as, fused silica, aluminum oxide, zirconium oxide, magnesium oxide with a glass such as borosilicate glass, soda lime glass, bottle glass, window glass, etc., clay or other materials that those skilled in the art of crucible making might use, to form a crucible suitable for use with a glass-ceramic material. Broadly, one forms the crucible of a mixture of materials, such that, at the temperature where the glass-ceramic material is in the working range of about log 3 to log 4 P the crucible has a viscosity of about log 5 to log 7 P. Crucibles for use in this process are formed by slip casting. The formation of crucibles by slip casting is well known to those skilled in the art and is described in greater detail in the Examples below. In the description, as set forth above, the compositions which are used to form crucibles which are useful in this process may utilize a wide range of materials. While many glasses can be used to form crucibles which are useful in the process, because of their toxicity, glasses which contain heavy metals such as lead, cadmium etc. should not be used. An alternate method for the formation of crucibles for use in this process is by powder pressing and then sintered. The composition from which the crucible is formed contains materials such that it will be heat-pressure deformable at the desired temperature i.e. about log 3 to log 4 P working temperature of the glass-ceramic. The heat-pressure deformation properties of the crucibles used in this process is to be contrasted with the crucibles of the prior art which are designed to be rigid at the working temperature of the glass-ceramic material which is contained therein. In the alternate structure, as is shown in Figures 8, 9, and 10, the sealing of the deformed crucible against the mold is enhanced. In the structure as shown, mold 64 incorporates a circular depression 66 having vertical walls 68 and 70 to which crucible 8 may seal. Vertical walls 72 and 74 of mold 65 may further incorporate a plurality of annular grooves 76. As is further shown in Figure 10, the sealing of crucible 8 against mold 65 is enhanced by the flowing of the upper extremities of crucible 8 into annular grooves 76 during the deformation process. EXAMPLES The present invention is illustrated by the following Examples however, these examples are not to be construed as limiting the invention. Dental restorations were prepared in accordance with the below listed examples. In these examples wax models of a tooth were prepared. A sprue was then attached to the wax model. The wax model with the sprue attached was then placed in an investment ring. An investment material was then prepared by mixing 90 gram of Kerr Thermovest with 17 ml of a mixture of 2 parts of Thermovest liquid with 1 part water. The resulting mixture was then mixed to a uniform consistency. The mixed investment was then vibrated into the investment ring and around the wax model. The mold was allowed to dry and harden overnight. The investment ring was then placed in a burn out furnace at room temperature and the temperature raised to 600°C where upon the wax model was burnt out of the investment material. The crucibles as used in these examples were prepared by slip casting. In this process a female plaster of paris mold was prepared by mixing 1247.4 grams of plaster of paris with 946 ml of water to a uniform consistency. The resulting mold was allowed to harden for 36 hours. A slurry of a particulate crucible formulation mixture, as is identified in the examples below was then placed in the preformed plaster of paris mold. The green crucible was removed from the mold and fired in a furnace for 15 minutes at 1100°C to sinter the crucible to a hardened more durable form. The slurry used to form the crucible was formed by mixing 1 pound of particulate material with 160 ml water. Because of the hydroscopic nature of the plaster of paris mold the particulate crucible formulation slurry coagulates in the plaster of paris mold. When the desired crucible wall thickness was achieved the remainder of the slurry was poured out of the mold resulting in a green crucible structure which was allowed to dry. In the listed examples and in this application, the coefficient of thermal expansion (C.T.E.) was measured from room temperature to 250°C and is reported in units of x10 -7 /°C. The translucency of the resulting dental restoration was measured by visual inspection given a value of 0 to 5 wherein value of 0 was deemed to be perfectly clear and 5 was deemed to be opaque. In the below listed Examples for purposes of injecting the molten glass and glass ceramic material into the mold a pressure of 206,8 kPa (30 P.S.I) was utilized, with the exception of Examples 26, and 27 wherein a pressure of about 345 kPa (50 P.S.I.) was utilized. The referenced heat treating sequence for all Examples except Examples 26, and 27 is in accordance with Table IV above. In the below Examples the glass-ceramic test rods for measuring modules of rupture (MOR) and thermal expansion (C.T.E) were formed along with color tab test samples. These test rods and color tab samples were subjected to the heat treatment process of Table VI whereby the glasses were crystallized in situ to glass-ceramics. The test rod samples were 0.3 cm x 3.18 cms (.120" X 1.25"). The Examples also record the visual appearance of each glass-ceramic and values of various properties exhibited by the glass-ceramic, such as, linear coefficient of thermal expansion (C.T.E.), reported in terms of X 10 -7 /°C, modules of rupture (MOR), cited in terms of K.S.I. (thousands of pounds per square inch) as determined in accordance with measurement techniques conventional in the art. K.S.I. may be conversted to its metric equivalent MPa by dividing K.S.I. by .145. The components used in these Examples are as follows: Thermovest and Therovest Liquid as sold by Kerr Manufacturing Co. Romulus, Mi. 48174. 3I Fused Silica as sold by Harbison Walker Refactories division of Indresco Inc., Pittsburg, Pa. 15223. SP921 TF (borosilicate glass) as sold by Specialty Glass Inc. of Oldsmar, Florida and has the following compositions in weight percent. SiO 2 78 B 2 O 3 15 Al 2 O 3 2.5 Na 2 O 4.5 Example 1 An attempt was made to form a glass-ceramic dental restoration in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 90.8 grams of SP921TF glass in 363.2 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The crucible cracked and hence no dental restoration was formed. Example 2 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 136.2 grams of SP921TF glass and 317.8 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting dental restoration had excellent definition and a M.O.R. of 2.82 MPa (41 K.S.I.), a C.T.E. of 148, a translucency of 3.5 and a softening temperature of 975° c. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Additional tests were conducted wherein it was determined that the crucible of this Example deformed and sealed well at 1375 and 1425°C Example 3 A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 181.6 grams of SP921TF glass and 272.4 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The preheated crucible without glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. During the heating process, the crucible lost its structural integrity and was not capable of effecting a seal with a mold. Therefore, no molding process was carried out. Example 4 A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 227 grams of SP921TF glass and 227 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. During the heating process, the crucible melted and hence was not capable of forming an effective seal with the mold. Therefore, no molding process was carried out. Example 5 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 118 grams of SP921TF glass and 336 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400° C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting dental restoration had excellent definition and physical properties as are reported with Example 2. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform. Further tests were conducted on crucibles having the composition as set forth above wherein it was determined that the crucible cracked and hence was not functional at 1375°C. An additional test demonstrated that the crucible of this example deformed and sealed at 1425°C. Example 6 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 127.1 grams of SP921TF glass and 336.9 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400° C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting dental restoration had excellent definition and physical properties as are in accordance with Example 2. During the forming process, the crucible retained its structural integrity and formed an effective seal for the mold. Further, during the heat treating process, the dental restoration retained its structural integrity and did not slump or deform. Additional tests were conducted on the crucible of this example wherein it was determined that the crucible would deform and seal at both 1375 and 1425°C. Example 7 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure described above by mixing 145.3 grams of SP921TF glass and 308.7 grams of 3I fused silica with 160 ml of water. After slip casting and drying, the crucible was sintered at a temperature of 1100° C. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The composition of the glass-ceramic material used is in accordance with composition #1 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400° C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting dental restoration had excellent definition and physical properties as are reported in accordance with example 2. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the dental restoration retained its structural integrity and did not slump or deform. Additional tests were conducted on the crucible of this Example wherein it was determined that the crucible functioned at 1375°C. However at 1425°C the crucible slumped and hence could not be used in the process of this invention. Example 8 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #2 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heated treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 322 MPa (46.7 K.S.I.), a C.T.E. of 138 a translucency of 2.75 and a softening temperature of 975° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 9 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #3 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heated treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 296 MPa (43 K.S.I.), a C.T.E. of 141 a translucency of 3.0 and a softening temperature of 975°C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 10 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and description described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #4 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heated treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 331 MPa (48 K.S.I.), a C.T.E. of 133 a translucency of 2.5 and a softening temperature of 975°C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 11 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #5 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then heat treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 290 MPa (42 K.S.I.), a C.T.E. of 140 a translucency of 2.5 and a softening temperature of 975°C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 12 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #6 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then heat treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 286 MPa (41.5 K.S.I.), a C.T.E. of 162 a translucency of 4 and a softening temperature of 950°C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. . Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 13 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material is in accordance with a composition #7 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heated treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 369 MPa (53.5 K.S.I.), a C.T.E. of 136 a translucency of 4.0 and a softening temperature of 950° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. Example 14 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The glass-ceramic material used was Di-Cor as sold by the Densply Corporation and as is described above. Di-Cor is thought to be flouro-mica glass-ceramic. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then heat treated in the investment. While the glass-ceramic properly filled the mold the resulting dental restortion cracked as a result of shrinkage during the heat treating process. This cracking is thought to result from the fact that the forming investment was not suitable for use in the heat treating sequence. Example 15 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 2. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass used in weight percent was; SiO 2 50.2 B 2 O 3 8.6 AlF 3 .7 Al 2 O 3 16.0 BaO 4.75 ZnO 19.75 The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1400°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. A satisfactory dental restoration resulted which was then removed from the investment material cut apart from the sprue. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 27.6 MPa (4 K.S.I.), a C.T.E. of 56 a translucency of 0. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Because the material used was a glass the resulting dental restoration was not heat treated. Example 16 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 8 of Table IV. After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 209 MPa (30 K.S.I.), a C.T.E. of 157, a translucency of 40 and a softening temperature of 925°C. The test samples cracked during heat treating therefore the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 17 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 9 of Table IV. After melting, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 165 MPa (24 K.S.I.), a C.T.E. of 145, and a softening temperature of 975°C. Because the test samples had a M.O.R. of only 24 and the glass-ceramic was hard to melt and form the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 18 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 10 of Table IV. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 285 MPa (41.4 K.S.I.), a C.T.E. of 129, a translucency of 2.5 and a softening temperature of 975°C. Because the test samples had a C.T.E. of less than 130 the composition may not be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 19 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 11 of Table IV. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 214 MPa (35 K.S.I.), a C.T.E. of 80, a translucency of 3.5 and a softening temperature of 975°C. Because the test samples had a C.T.E. of 80 the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 20 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 12 of Table IV After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 214 MPa (31 K.S.I.), a C.T.E. of 127, and a translucency of 2.5. Because the test samples had a C.T.E. of 127 therefore the composition may not be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 21 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 13 of Table IV. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 221 MPa (32 K.S.I.), a C.T.E. of 256, and a translucency of 5. Because the test samples had a C.T.E. of 256 the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 22 A glass-ceramic dental composition was prepared and heat treated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 14 of Table IV. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a M.O.R. of 89.6 MPa (13 K.S.I.), a C.T.E. of 126, and a translucency of 4.5. Because the test samples had a low C.T.E. and a low M.O.R. the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 23 A glass-ceramic dental composition was prepared and elevated in accordance with this invention. The composition of the glass-ceramic material is in accordance with the composition No. 15 of Table IV. After preparation, test samples were prepared in accordance with the procedure described above. The resulting test samples had a translucency of 5.0. Because the test samples turned opaque during heat treating therefore the composition was deemed to be unsuitable as a material for use in preparation of dental restorations, accordingly, further tests were not carried out and a dental restoration was not formed. Example 24 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #16 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1425°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 241 MPa (35 K.S.I.), a C.T.E. of 148 translucency of 3.5 and a softening temperature of 950° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. The investment material utilized was Whipmix GTJM 3-23-94; As described herein above. The investment material had an expansion rate of about 3%. As a result of this expansion rate the resulting dental restoration had excellent fit. Example 25 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650 °C. The composition of the glass-ceramic material used is in accordance with composition #17 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1425°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then heat treated in the investment material. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 276 MPa (40 K.S.I.), a C.T.E. of 138 a translucency of 3.5 and a softening temperature of 950° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. The investment material utilized was Thermovest and after heat treating was removed and the resulting dental restortion had excellent fit. Example 26 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible. The composition of the glass-ceramic material used is in accordance with composition 18 of Table IV. The crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figures 7a and 7b and heated to a temperature of 1425° C for a period of 5 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 324 MPa (47 K.S.I.), a translucency of 4.0 and a softening temperature of 950° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. The investment material utilized was Whipmix GTJM 3-23-94; As described herein above. The investment material had an expansion rate of about 3%. As a result of this expansion rate the resulting dental restoration had excellent fit. Example 27 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and not preheated . The composition of the glass-ceramic material used is in accordance with composition 19 of Table IV. The crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figures 7a and 7b and heated to a temperature of 1425° C for a period of 5 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then removed from the investment material cut apart from the sprue and heat treated. The resulting test samples and dental restoration had excellent definition and a M.O.R. of 359 MPa (52 K.S.I.), a translucency of 3.5 and a softening temperature of 950 ° C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. The investment material utilized was Whipmix GTJM 3-23-94; As described herein above. The investment material had a total expansion rate of about 3%. As a result of this expansion rate the resulting dental restoration had excellent fit. Examples 26 and 27 represent the preferred process and glass ceramic compositions in accordance with this invention. As is mentioned the resulting dental restorations of these Examples were heat treated the heat treating procedure used for Examples 26 and 27 is per Table VIII. Heat Dental Restoration to 450° C at furnace rate Ramp or soak at 450-550° C for 6 hours Ramp at 550-850° C for 2 hours Hold at 850° C for 45 minutes Ramp at 850-900° C at furnace rate Soak at 900° C for 15 minutes Cool to Room Temperature After heat treating the dental restorations of Examples 26 and 27 were further finished by altering the shape and shade with a high fusing porcelain sold under the trademark Ceramco II and a low fusing porcelain sold under the trademark Final Touch. Both of the high and low fusing porcelains fired and adhered well. Likewise, both porcelains had thermal expansions which were compatible with the glass-ceramics from which the dental restorations were formed. The Ceramco II and Final Touch porcelains as used herein are manufactured and sold by Ceramco, Inc. of six Terri Lane, Burlington, N.J. 08016. Relative to the dental restorations as were produced in accordance with Examples 26 and 27 above it should be noted that these restorations were exceptionally strong in that they had M.O.R. ratings of about 345 MPa (50 K.S.I.). It should be noted that the M.O.R. ratings of Examples 26 and 27 do not correlate with the M.O.R. ratings as are specified relative to the other examples in that a different test protocol was used for mearusing the M.O.R. of Examples 26 and 27. The M.O.R. for Examples 26 and 27 were tested on a Lloyd Instrument type No.TG 18 as manufactured by John Chatillon & Sons Inc. of Greensboro, N.C.. These measurements were made in accordance with I.S.0 test No. 6872 using cylindrical bars of 0.318 cm (0.125") diameter with a 2.54 cms (1") span, 3 point bending with a crosshead speed of 1 m.m. per minute. In contrast the M.O.R. measurements of the other Examples were made on a piece of equipment which was manufactured by the applicant. This piece of equipment has characteristics which are similar to those of the above described Lloyd instrument type No. TG 18 and used the same test bars. Generally it could be said that the M.O.R. rating of those Examples other than Examples 26 and 27 appear to be low. For a corelation between the M.O.R. test protocol of Examples 26 and 27 and the test protocol of the other Examples Table IX should be referred to Example No. Reported M.O.R. As per test used for Examples 1-25 M.O.R. as per. I.S.O. Test No. 6872 26 290 MPa (42 KSI) 324 MPa (47 KSI) 27 310 MPa (45 KSI) 359 MPa (52 KSI) Example 28 A glass-ceramic dental restoration was prepared in accordance with the following procedure. A mold was prepared in accordance with the procedure described above. By use of slip casting techniques, a crucible was prepared in accordance with the procedure and composition described above in connection with Example 6. A glass button weighing 6 grams was then placed in the crucible and preheated to a temperature of 650°C. The composition of the glass-ceramic material used is in accordance with composition 10 of Table IV. The preheated crucible and glass-ceramic material was then placed in an apparatus which is similar to that shown in Figure 7 and heated to a temperature of 1425°C for a period of 10 minutes. The ram was then actuated, the crucible was brought into contact with the mold in accordance with the general procedure as is illustrated in Figures 3 through 6. The dental restoration was then heat treated. The resulting test samples and dental restoration had excellent definition and a C.T.E. of 111 a translucency of 3.5 and a softening temperature of 850°C. During the forming process, the crucible retained its structural integrity and formed an effective seal with the mold. Further, during the heat treating process, the finished dental restoration retained its structural integrity and did not slump or deform during heat treating. It should be understood that the invention is not limited to the embodiments shown and described in Figures 1-10 and examples 1 to 28 since the process and composition parameters can be varied and the configuration of the apparatus can be altered without departing from the scope of the invention. The above description and drawings are illustrative only since modifications could be made without departing from the present invention, the scope of which is to be limited only by the following claims.

Claims: A dental restoration which is formed from a glass-ceramic material having essentially the following constituents (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 8 - 15 Al 2 O 3 1.5 - 5.0 SiO 2 60 - 85 Na 2 O 0 - 2 K 2 O 0 - 2 P 2 O 5 1.5 - 5.0 ZrO 2 0 - 3 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0 - 12 (total proportion) Colouring Oxide(s) 0 - 5 A dental restoration according to claim 1, wherein the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 10 - 13.5 Al 2 O 3 2 - 3 SiO 2 70 - 84 Na 2 O 0 - 2 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0.5 - 4.0 (total proportion) Colouring Oxide(s) 0 - 5 A dental restoration according to claim 1, wherein the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 9 - 13 Al 2 O 3 1.5 - 4.0 SiO 2 65 - 84 Na 2 O 0 - 1 K 2 O 0 - 1 P 2 O 5 1.5 - 4.0 ZrO 2 0 - 1 CaO 0 - 1 BaO and/or SrO and/or La 2 O 3 0 - 12 Colouring Oxide(s) 0 - 5 A dental restoration according to claim 3, wherein the following constituents are present in the glass-ceramic material in the following proportions (by weight percent), the percentages of such materials adding up to approximately 100%: Li 2 O 10.5 Al 2 O 3 2.5 SiO 2 78.5 K 2 O 0.5 P 2 O 5 2.0 ZrO 2 0.5 CaO 0.4 BaO 3.0 Colouring oxides comprising: TiO 2 0.1 CeO 0.35 NiO 2 0.02 A dental restoration according to any one of claims 1 to 4, which is altered with a porcelain material which has a C.T.E. which is slightly less than the C.T.E. of the glass-ceramic material. A dental restoration according to any one of claims 1 to 5, wherein the resulting dental restoration has a M.O.R. of at least 207 MPa (30 K.S.I.). A dental restoration according to any one of claims 1 to 6, wherein the glass-ceramic material is produced in a crucible formed by the heat sintering of a particulate mixture of from about 68 to 74 percent of fused silica and from about 26 to about 32 percent of a borosilicate glass having the following composition (by weight percent): SiO 2 78 B 2 O 3 15 Al 2 O 3 2.5 Na 2 O 4.5 the glass-ceramic material being heat treated to effect crystal formation. A dental restoration according to any one of claims 1 to 7, wherein the finishing of the dental restoration includes the altering of said dental restoration with one or more porcelains, having C.T.E.'s of from about 125 to about 135 at their set point. A dental restoration according to any one of claims 1, 2, and 5-8 when not dependent upon claims 3 and 4, wherein the glass-ceramic material incorporates Na 2 O in the range of 0.2 to 2.0 by weight percent. A dental restoration according to any one of claims 1, and 5 to 9 when not dependent upon claims 2 to 4, wherein the glass-ceramic material incorporates K 2 O in the range of 0.5 to 2.0 by weight percent. A dental restoration according to any one of claims 1 and 5-10 when not dependent upon claims 2 to 4, wherein the glass-ceramic material incorporates ZrO 2 in the range of 0.5 to 3.0 by weight percent. A dental restoration according to any one of claims 1 to 3 and 5-11 when not dependent upon claim 4, wherein the glass-ceramic material incorporates CaO in the range of 0.25 to 1.0 by weight percent. A dental restoration according to any one of claims 1 to 12, wherein the glass-ceramic material has a softening temperature of at least 850°C. A dental restoration according to claim 13, wherein the glass-ceramic material has a softening temperature of about 950°C.

Inventors: PETTICREW RICHARD, PETTICREW, RICHARD
Applicants: TEC VENTURES INC, TEC VENTURES, INC.
Application year: 1995
Main classification: A61C13
IPC codes: A61C13, B28B1, B28B5