FOUNDATIONS Lost Wax Casting Technique for Metal Crown Fabrication Todd McCoy, DVM

Lost wax casting is an ancient technique for making a precise replica of an object by casting it in molten metal. In dentistry, this process involves taking a wax pattern of a dental restoration and converting it into a dental casting alloy or ceramic.1 At the dental laboratory, a vinyl polysiloxane impression of the prepared tooth and adjacent teeth is received and a replica of the hard and soft tissues is made by mixing and pouring a super fine ground, high strength, gypsum dental stone into the impression. When the poured stone has set, it is removed from the impression material as a hard and smooth working model.2 This positive reproduction is termed a cast when it involves large areas or the oral tissues, or a die when a single tooth is repli-

Figure 1 Photographs showing an impression and study model for submission to the dental laboratory. A vinyl polysiloxane impression is obtained of the prepared tooth and adjacent teeth (A). A stone gypsum study model is created, and a bite registration is acquired (B). A red pencil mark on the study model identifies the margin of the prepared tooth.

Figure 2 Photographs demonstrating the formation of the stone cast at the dental laboratory. After pouring over a vibrator, a high strength, super fine ground, gypsum based dental stonea is mixed under a vacuum to remove air (A). Because of the hydrophobicity of vinyl polysiloxane, a surfactant sprayb is applied to render the surface of the impression material hydrophilic (B). The surfactant enhances the ability of the gypsum product to flow into the impression and capture maximum detail. The mixed stone is poured into the impression and allowed to set to form a cast (C).


J VET DENT Vol. 31 No.2 Summer 2014

cated.1 The accuracy of the cast depends on the quality of the materials used and the accuracy of the impression.3 Accuracy and dimensional stability are critical because models, casts, and dies must reproduce an intraoral structure accurately and must remain the same shape and size if they are to be useful.1 The core of the cast material stone must be strong, and the surface should be hard, smooth, and abrasion resistant.2 Hardness

and abrasion resistance is necessary to allow a wax pattern to be made on the die using instruments, and to allow adjustment of the resulting prosthesis for fit without wearing away of the material.1 Dental waxes are important in the fabrication and success of the final restoration, and they are simple to manipulate, versatile, and inexpensive.1 Dental waxes are a mixture of vari-

Figure 3 Photographs showing the set cast material that is shaped with a model trimmer (A) and router (B).

Figure 4 Photographs showing further preparation of the stone cast. Holes are drilled into the bottom of the cast (A). Dowel pins, sleeves, and boots are assembled and secured into the holes (B). A plaster separating agentc is applied to the bottom of the cast (C) to prevent the cast from bonding to the basing material that will be poured next..

J VET DENT Vol. 31 No. 2 Summer 2014


ous components including synthetic waxes and natural waxes from minerals, plants, or animals.1 Pigments are added for color. Manufacturers of dental waxes create a wide variety of properties useful in dentistry by mixing or adding gums, fats, fatty acids, oils, and various resins.1 Pattern waxes are casting waxes that are used to create a model of a dental restoration such as a crown. The lost wax technique uses a wax pattern to define a space within a stone-like material.1 The wax pattern is eliminated by melting or burning the pattern and then casting the space into metal. It is important that the wax material

not leave a residue, which would affect the quality of the final restoration.1 Metals have been used for thousands of years as a replacement for missing tooth structure. Pure metals, including gold, generally are not strong enough to be used for crowns and many dental restorations. For this reason metals and nonmetals are commonly mixed together to form an alloy.1 Dental casting alloys have several properties important to their performance which include density, strength, corrosion resistance in the mouth, and hardness.1 Predominantly nickel-based al-

Figure 5 Photographs showing the formation of the cast base. The cast is placed in a basing tray, and a gypsum-based basing stoned (also mixed over a vibrator then under a vacuum) is poured into the tray (A). Once the stone has set, the model is removed from the basing tray (B). The cast and base are then manually separated (C).

Figure 6 Photographs showing the creation of a single tooth segment termed a “die”. The cast is cut with a circular disc to separate the prepared tooth from the adjacent teeth (A). The die is reassembled into the model (B).


J VET DENT Vol. 31 No.2 Summer 2014

loys are commonly used for metal crown fabrication because of their strength, biocompatibility, resistance to wear, and reasonable cost.1 Lost wax casting allows the practitioner and dental laboratory to custom-make precision restorations to restore dam-

aged teeth.1 Veterinary dentists should understand the casting process if they are to communicate effectively with pet owners and the dental laboratory. This article provides an overview of the lost wax casting process to fabricate full-metal crowns.

Figure 7 Photographs demonstrating the application of dental waxes. A prosthetic denture sealante is brushed onto the die to harden and seal the surface and to create a “space” that mimics cement space (A). The sealant is then light cured, and a liquid separating agent is brushed onto the surface to provide the ability to manually remove the dental wax pattern from the die. The die is dipped in a heated dental wax (B), and a pattern wax is applied to the die to create a replica of the prepared tooth (C).

Figure 8 Photographs showing sprue attachment and casting ring placement. A wax sprue is attached with heated wax (A) to act as a channel by which the molten metal travels to create the restoration. The wax pattern is removed from the die, inverted, and attached to a sprue base also made of wax (B). The wax pattern, sprue, and sprue base are placed inside a casting ring (C). The wax pattern model is now ready for investing.

J VET DENT Vol. 31 No. 2 Summer 2014


Figure 9 Photographs demonstrating investing and the “burn out” process. The wax pattern, sprue, and sprue base are filled within the casting ring with a gypsum based investment material poured slowly to avoid air bubbles (A). The “investment” is placed in a pressure container to remove air (18 lbs, no heat) and allowed to set for 15-minutes (B). The investment is placed in an oven for 1hour at 1540° F to “burn out” the wax (hence “lost wax”) [C]. Once the wax is completely eliminated, a space will remain where the sprue and wax pattern were.

Figure 10 Photographs showing casting of the metal alloy. Predominantly nickel-based alloys (A) are commonly selected in veterinary medicine because of their strength, resistance to wear, biocompatibility, and reasonable cost. The hot casting ring is removed from the oven and immediately placed in a centrifugal casting machine behind a crucible (B). The metal alloy is placed in the heat-resistant ceramic crucible and heated until it becomes molten (C). The centrifuge is immediately activated and the crucible-ring assembly is spun rapidly. The molten metal is cast by centrifugal force into the space once occupied by the wax sprue and pattern.


J VET DENT Vol. 31 No.2 Summer 2014

Figure 11 Photographs demonstrating divesting. The investment is removed from the centrifuge and allowed to cool (A). After the investment has cooled and the metal solidified, the investment material is broken away from the casting. This step is referred to as “divesting” (B). The restoration is microblasted in a blasting chamber using ultrafine aluminum oxide particles to remove remaining gypsum and slightly roughen the interior surface for better bonding to the luting cement (C).

Figure 12 Photographs showing sprue removal and contouring of the restoration. The sprue is cut away from the casting (A). Calipers check the thickness at various points of the casting (B). Gross contouring of the restoration is accomplished with a coarse stone (C). Care is taken not to deform the shape of the restoration or make the wall too thin.

J VET DENT Vol. 31 No. 2 Summer 2014


Figure 13 Photographs showing final contouring and finishing the restoration. Contouring is continued with a fine textured bur (A). The restoration is polished using a rubber wheel and a polishing paste containing an extremely fine mild abrasive (B). Once finished and polished, the restoration should have a high luster and be exceptionally smooth. After assuring a firm fit on the die the restoration is ready for delivery (C).

_________________________________________________ Fujirock IMP, GC America Inc, Alsip, IL b Wax-Wet™, Gresco Products Inc, Stafford, TX c Super-Sep Dental Stone Separating Agent, Kerr Corporation, Washington, DC d FlowStone FS Gypsum, Whip Mix Corporation, Louisville, KY e Palaseal prosthetic denture sealant, Heraeus Medical Inc, Livermore, CA a

Author Information

From Veterinary Dental Services, 11001 Fondren, Houston , TX, 77096. Email: [email protected]


J VET DENT Vol. 31 No.2 Summer 2014


Dental Designs and Services, 6140 Highway 6 #123, Missouri City, TX, 77459.

References 1. Craig RG, Powers JM, Wataha JC. Dental Materials, properties and manipulation. St.Louis: Mosby, 2004; 157-267. 2. Anusavice KJ. Gypsum products. In: Anusavice KJ. Phillips’ science of dental materials. St. Louis: Saunders-Elsevier, 2003; 255-281. 3. Shen C. Impression materials. In: Anusavice KJ. Phillips’ science of dental materials. St. Louis:Saunders-Elsevier, 2003; 207- 225.

Lost wax casting technique for metal crown fabrication.

Lost wax casting technique for metal crown fabrication. - PDF Download Free
11MB Sizes 6 Downloads 20 Views