Compared to plaster, dental stone requires less water in mixing and sets more slowly. When it is set, it is harder, denser, and has a higher crushing strength. These differences make stone the choice to use over dental plaster when using it as a master cast for complete dentures and partial denture construction. Stone is more resistant to scratching and damage and can withstand more pressure in acrylic processing. Stone has many uses, including pouring, mounting casts, and flasking dentures for processing. The initial setting time of a typical stone mixture varies from 8 to 15 minutes. The final set occurs within approximately 45 minutes.
Historically, die stone was only used for making the first pour of a working cast for fixed prosthodontics. Improved die stone now is being used for working casts in removable prosthodontics.
Dental waxes are important in the construction of dental prosthetic appliances. The waxes are supplied in different types, with each designed for specific purposes. Next we describe the waxes with which a chairside prosthetic technician needs to be familiar and be able to use.
Baseplate wax is used to create a spacer over the cast before custom trays can be made. Another use is as a block-out wax for undercuts on casts. It is available in sheet and ribbon form and is pink in color.
Bite registration wax is a metal-impregnated wax in sheet form. It is used to record the occlusal relationships between a patient's opposing arches and to later transfer this relationship to the cast for articulation. Often without this record, it is impossible for the dentist or the laboratory technician to properly occlude the patient's cast.
Indicator wax is usually green in color and is coated with a water soluble adhesive on one side. It is used for registering occlusal contacts on natural teeth, individual restorations, FPDs, RPDs, and CDs. It is sometimes used by the dentist to evaluate high spots on restorations.
Sticky wax is made of beeswax, paraffin, and resin. Its colors are orange and the darker shades of blue, red, and violet. The resin gives the wax its adhesiveness and hardness. An important requirement of sticky wax is that it must break under pressure rather than bend or distort. This property makes it useful for holding the parts of a broken denture together so that it can be repaired.
Utility wax is a red or colorless wax that comes in rope form. It is extremely pliable and tacky at room temperature, making it usable without heating. Its main use is in beading (curbing) impressions before boxing and pouring. It can also be used on the impressions trays to avoid the flow of impression material to the back of the throat and to avoid injury to the soft tissue.
There are a number of acrylic resins that you will use and need to be familiar with in prosthetic assisting. Polymerization is the term used to describe the processing or curing of acrylic resins. Acrylic resins can be classified by its method of curing. Some of the more common acrylic resins include the heat-cured, self-cured, and light-activated types. When handling acrylic resins, you should be sure to read the manufacturer's instructions and safety precautions before using.
Methyl methacrylate is the most widely used synthetic resin used in dentistry. The resin is usually supplied in a fine powder (polymer) and liquid (monomer). They are mixed to form a gel or dough and processed into a rigid solid.
Clear heat-cured acrylic resin is used to construct night guards and surgical templates. As a surgical template (band-aid) it is used after extraction of remaining teeth to show the possible interferences between the alveolar bone and the immediate denture.Continue Reading