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2-20. GOLD ALLOY INLAYS (CROWNS)

a. Wax Pattern Procedures: General. In making a gold alloy inlay, or crown, the dentist first prepares the tooth to receive the restoration. The next step is obtaining a wax pattern. A wax pattern is an exact duplicate in wax of what is desired in the finished restoration. Most wax patterns will then be used to form a mold into which melted gold alloy is cast to form the inlay or crown. Wax patterns are used also in the formation of acrylic resin crowns. Wax patterns may be obtained by either of two methods. The direct method is one in which softened inlay wax is applied directly to the prepared tooth and carved to shape directly in the mouth. The indirect method is one in which an accurate impression is made of the prepared tooth and adjacent teeth, the impression is poured to provide a cast of these teeth, and the wax pattern is developed on the cast. Ways in which the dental assistant may assist vary with the dentist and the technique followed.

b. Direct Method. In some classes of cavities, the dentist may use a matrix to replace the missing walls of the cavity to help confine and shape the wax when it is forced into place. The matrix is lubricated so that wax will not stick to it and distort the pattern. When the matrix is in place, the dentist will soften a piece of wax in the flame or warm water, then force it into place in the cavity. In making a crown or for some inlay techniques, a copper band may be fitted to the tooth, filled with softened wax, and forced into place upon the tooth carrying the wax into all areas of the preparation. After the wax has been allowed to cool, it is shaped and carved, using wax spatulas and carvers, to approximate the anatomy and outline desired in the final restoration. The matrix material or band is removed some time before final finishing of the wax pattern. When carving is completed and the pattern is to be used to produce a cast gold alloy restoration, a sprue is attached to the wax pattern by heating the sprue metal, placing it in contact with the pattern and permitting the wax to harden around it. After the wax pattern is withdrawn from the tooth, the free end of the sprue is inserted into a sprue former. A sprue usually is in the form of a straight piece of metal. A sprue former is a cone-shaped device of rubber or metal. The sprue and crater former are used to support the wax pattern during subsequent procedures in which the pattern is invested in a plaster-like casting investment to form a mold. The sprue and sprue former also form a passage into the mold for later laboratory procedures. The wax pattern will undergo dimensional changes with time so it should be invested as soon as possible. Investing, wax-elimination, and casting are normally done by a dental prosthetic specialist. In a small clinic having no dental prosthetic specialist, the dentist or even the dental assistant may perform these steps.

c. Indirect Method. This technique includes the making of an impression of an individually prepared tooth and, in most cases, several adjacent and opposing teeth. An individual tooth impression is usually made in a copper matrix band that has been selected for size and adapted to the tooth. Either silicone or polysulfide base impression material is normally used for an impression of an individual tooth. An impression of several teeth is usually made in an impression tray with polysulfide base impression material or reversible (agar) hydrocolloid. To secure maximum detail, the dentist normally uses special syringes, which the dental assistant has filled with the impression material of choice, and ejects the material into the prepared areas.

d. Procedures for Impression Material.

(1) Reversible (agar) hydrocolloid. When the material has been conditioned for making the impression, the dental assistant passes the loaded syringe to the dentist who fills the prepared area. While the dentist accomplishes this step, the dental assistant scrapes the water soaked outer layer of agar from the material in the tray and passes it to the dentist who makes the impression. The dental assistant aids in the gelation of material by connecting one of the plastic tubes extending from the tray to the tumbler water supply system and places the other tube in the cuspidor bowl. The tumbler supply valve is then turned on, circulating the cool water through the tray and gelling the hydrocolloid. After the impression is removed from the mouth, the dental assistant should wash the saliva from its surface with cool tap water or cool slurry water (solution of plaster of Paris or artificial stone and water). The dental assistant should take precautions to avoid distortions or damage to the tissue and peripheral border areas of the impression. He should assure that the impression is poured immediately to prevent distortion of the impression, which would result in an inaccurate cast. In a dental clinic that has no dental prosthetic specialist, the pouring of the cast may become the responsibility of the dental assistant.

(2) Polysulfide base impression material.  Normally, a double mix technique is used. The dental assistant mixes the syringe material, loads the syringe, and passes it to the dentist. While the dentist is injecting the material into the prepared area, the dental assistant mixes the heavy or medium bodied material and loads the customized tray. The dental assistant receives the syringe with one hand and passes the loaded tray to the dentist who makes the impression. When the impression is removed from the mouth, the dental assistant washes saliva from the surface with cool tap water or cool slurry water. Dimensional stability is greater in polysulfide base impression material than in the hydrocolloid impression materials. However, the dental assistant should assure that the impression is poured within 1 hour. Pouring the cast may become the responsibility of the dental assistant when no dental prosthetic specialists are assigned to the clinic.

e. Laboratory Procedures. Various types of casts are made from these impressions. These include master casts with removable dies (model of the individual tooth), split casts, and other types. The master cast is made of artificial stone while the dies may be made of "die" stone, electroplated metal, or sometimes amalgam. Wax patterns are made on the dies. The cast formed from an impression of the opposing teeth are used to establish occlusal relationship. The master cast is used to establish proximal relationships between the wax pattern and other teeth. Many dentists will have the patient return for a "try-in" of the wax pattern before it is sprued and invested.

f. Finishing and Cementing Cast Gold Alloy Restorations.

(1) General. When gold alloy crowns and inlays have been cast and removed from the investment, they are ready to be carried to place in or on the prepared tooth where they receive final fitting and polishing.

(2) Technique. The sprue may be cut from the casting before it is placed in the prepared cavity or it may be left in place to aid in handling the casting, then cut off later. Often a few taps with the mallet and orangewood stick are needed to carry the casting to place. Sometimes a small "bubble" of gold on the inner surface of the casting keeps the casting from going into place and must be removed. Initial steps in fitting and finishing the casting include adjustment of occlusion, removal of excess material from margins and contours, refining occlusal anatomy, and establishing proper contacts with adjacent teeth. Occasionally, it is necessary to build up contact areas by the addition of a small amount of gold solder. When these finishing procedures are completed, the restoration is smoothed and polished. When ready for cementing, both the tooth and restoration are thoroughly cleaned and dried, the tooth is isolated with cotton rolls, copalite varnish is applied, zinc phosphate cement (loose mix) is prepared, the cement is applied to the tooth surface of the restoration, and the restoration is carried to the tooth and tapped firmly in place using the mallet and orangewood stick. The restoration is held in place with firm biting or other pressure until the cement has reached its initial set.

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