Because of its esthetic properties, resin can be used for the reproduction of lost tooth structure. The dental officer prepares the cavity in the tooth in the usual manner, inserts the plastic mixture directly into the cavity, and allows it to polymerize at mouth temperature. These materials are often referred to as direct-filling resins. Certain shortcomings in the physical properties of resins restrict their use when esthetics, such as the restoration of anterior teeth, are the principal concern. There are two types of direct filling resins. The older type is an unfilled polymethyl methacrylate. Such a resin is often referred to as an acrylic resin. The newer type is the composite resin.
a. Clinical Use. Acrylic (unfilled) resins are used as temporary crown material. Temporary crowns are placed to protect the crown preparation and provide patient comfort during the time the permanent crown is being constructed.
b. Composition of Acrylic Resins.
(1) Powder. The powder is composed of a polymethyl methacrylate, a polymer, and contains some inert coloring pigments.
(2) Liquid. The liquid is a monomethyl methacrylate, a monomer, and is the same material as the polymer except in a different molecular form.
c. Properties of Acrylic Resins.
(1) Desirable properties. Acrylic resins are available in several shades to match tooth shading (esthetics). They have a low thermal conductivity. These resins are not easily washed out by the acids of the oral cavity (low solubility). Acrylic resins are also resilient, which allows them to be used in stress-bearing areas.
(2) Undesirable properties. Acrylic resins exhibit a moderate shrinkage of from 3 to 8 percent. This shrinkage and low marginal strength can lead to marginal leakage. Acrylic resins have a low resistance to wear. Acrylic resins cannot be used over a zinc oxide and eugenol-type base because eugenol interferes with the acrylic curing process.
d. Mixing. Always follow the manufacturer's directions when mixing any material. Each brand of acrylic resin requires a slightly different mixing procedure. Insufficient mixing will cause an uneven color or streaks in the mixture. Overmixing will cause the material to harden before it can be placed.
a. Clinical Use. Composite resins are the most commonly used material for all permanent anterior restorations. These resins make excellent restorative materials because of their good resistance to wear and their excellent esthetics. Silar, Adaptic, and Concise are some of the trade names of composite resins.
b. Chemical Composition. Composite resins are composed of universal paste with filler and catalyst paste. All composite resins use quartz as a filler.
c. Properties. Composite resins have excellent esthetic properties. In fact, the universal paste will match 89 percent of all tooth shades. Tints are available for the other 11 percent. Composite resins have good resistance to wear because of the filler. They also have an acceptable compressive strength of 35,000 psi. Thermal expansion is at a minimum. (The manufacturer claims that the thermal expansion is close to the normal expansion of tooth structure.) Solubility and shrinkage are low. Refrigeration of the composite resin is required to prevent deterioration.
d. Mixing. As with unfilled resins, the manufacturer's directions should be followed when mixing composites. There are several types available and each requires a specific mixing procedure. The two-paste system is the most common form used. To mix this system, equal parts of the pastes are mixed (with a folding motion) to a uniform color within 20 to 30 seconds. The average working time from start of mixing to insertion of product is 1 1/2 to 2 minutes. It is important to avoid cross-contamination between jars of universal and catalyst pastes. The double-ended spatula provided with the kit has differently shaped ends, each clearly labeled "uni" and "cat." Improper mixing could cause failure of the restoration. Also, improper ratio of pastes will decrease the strength as will insufficient spatulation. Use of a metal spatula will result in discoloration of the material, giving poor esthetics. Some composite resins are activated by the application of ultraviolet or visible light. The material is placed in the cavity preparation, then exposed to a source of light for a specified time to produce polymerization or setting of the composite.
ACID ETCH TECHNIQUE
Cavities requiring added retention (to hold firmly) are treated with an acid etching technique. This technique improves the seal of the composite resin to the cavity wall. The enamel adjacent to the margins of the preparation is slightly decalcified with a 40 to 50 percent phosphoric acid solution. This etched enamel enhances the mechanical retention of the composite resin. In addition, the acid etch technique is used to splint unstable teeth to adjacent teeth. The acid is left on the cut tooth structure only 15 seconds, in accordance with the directions for one common commercial brand. The area is then flushed with water for a minimum of 30 seconds to remove the decalcified material. Etched tooth structure will have a chalky appearance.
PIT AND FISSURE SEALANTS
Pit and fissure sealants are similar to the unfilled resin portion of acid etch composite filling materials. This plastic resin is used as a prophylactic seal of occlusal pits and fissures. The purpose is to prevent carious destruction of tooth structure. The sealant is used when there is a deep occlusal pit or fossa or a lingual pit, when there is an intact occlusal surface with a carious or restored contralateral tooth surface, and where there is high carious activity, poor oral hygiene, or newly erupted posterior teeth.
INTERMEDIATE RESTORATIVE MATERIAL
Intermediate restorative material (IRM) is a zinc oxide and eugenol cement that has been reinforced for increased strength. It is used as an intermediate base beneath a metallic restoration and also as a temporary restoration.
(1) Powder. The powder is composed of 80 percent zinc oxide and 20 percent polymethyl methacrylate (the powder used for acrylic resin).
(2) Liquid. The liquid is 99 percent eugenol and 1 percent acetic acid.
b. Dispensing. The first step in dispensing IRM is to fluff the powder for uniform density. The measuring scoop is then filled but not packed with powder and leveled with the spatula. The powder is then placed on the mixing pad. Finally, one drop of liquid is added for each level scoop of powder (at a 1 to 1 ratio). After using the liquid, immediately recap to prevent evaporation and contamination.
c. Mixing. Spatulate quickly with a stainless steel spatula, combining half the powder with all the liquid. Add the remaining powder in 2 or 3 increments and spatulate thoroughly. The mix will be stiff and should be stropped (whipped vigorously) each time with the spatula for 5 to 10 seconds. This type of mixing results in a smooth and adaptable working consistency. Mixing the IRM should be completed in approximately 1 minute (60 seconds).