For additional ancillary materials related to this chapter, please visit thePoint.
After studying this chapter, the student will be able to do the following:
1. List the uses of acrylic resins in dentistry.
2. Explain the physical and chemical stages of polymerization of acrylic resins.
3. Describe the function of the components of heat activated and chemically activated acrylic resin systems.
4. Describe the steps involved in construction of a denture.
5. Summarize the procedures used to reline a denture.
6. Explain the dental hygienist’s role in maintenance of an acrylic prosthesis.
removable partial denture
As with a fixed bridge, a removable denture is a prosthesis; it replaces missing teeth. This chapter presents materials and procedures that are used to construct removable prostheses: partial and complete dentures. Alloys used for partial denture frameworks are very similar to those presented in Chapter 10, Materials for Fixed Indirect Restorations and Prostheses. This chapter focuses on acrylic materials and their use.
I. Acrylic Resins
A. What Is an Acrylic Resin?
Acrylic resins are hard, brittle, glassy polymers. The commercial plastic called Plexiglas is an acrylic resin product. Acrylic resin is clear and colorless, making it an excellent replacement material for glass in storm doors. Acrylic resins are easily colored. Technically, acrylic resins are classified as thermoplastic materials, and many commercial products are made by injection molding acrylic materials. In dentistry, however, acrylic resins are handled more like a thermoset material; after it sets, it is not heated and molded.
The most common acrylic monomer is methyl methacrylate. The chemical structure is shown in Figure 5.4A. Note the C═C bond in this illustration. Acrylic resins are long chains of such monomers. The chains have side groups that inhibit chain slippage and result in mechanical properties that provide more strength than polymer chains without side groups. Polyethylene (the same material used in “plastic” sandwich bags) is an example of a polymer without side groups.
B. Acrylic Resins as Biomaterials
Acrylic resins were developed in the 1930s and were first used in dentistry in the 1940s. They quickly replaced materials previously used in the construction of dentures. An acrylic resin denture is shown in Figure 1.6. They were also used (without success) as a direct restorative material. They have been adapted for many other uses in dentistry. Their handling characteristics and mechanical properties are quite satisfactory for a variety of applications. These uses include temporary crowns, as shown in Figure 1.10C, custom trays, as shown in Figure 8.2, and baseplates for denture construction, to be presented later in this chapter.
It is interesting to note that a dental biomaterials scientist (Dennis Smith) suggested to an orthopedic surgeon (John Charnley) that acrylic resin would be a good material to cement artificial joints in place. In the 1960s, acrylic resin was introduced as the first bone cement and was used in hip replacements. Today, acrylic resin is still commonly used to cement hip prostheses and other orthopedic devices in bone.
II. Acrylic Resin Systems Used in Dentistry
Acrylic resin systems set by addition polymerization in the same manner as dental composites, which were discussed in Chapter 5, Direct Polymeric Restorative Materials. The same terms and classifications are used. They are based on the method used to initiate polymerization.
A. Cold-Cure or Chemically Activated Acrylic Resins
Cold-cure or chemically activated acrylic resin systems are supplied as a powder and a liquid, as shown in Figure 11.1. These are the same materials used for the “brush-bead” buildup technique for artificial fingernails. The typical components are listed in Table 11.1.
FIGURE 11.1. Photograph of an acrylic resin system, powder, and liquid.
TABLE 11.1. Components of an Acrylic Resin System
The liquid is mostly monomer, methyl methacrylate. A cross-linking agent, such as glycol dimethacrylate, is added. An inhibitor is always added to methyl methacrylate to prevent premature polymerization; hydroquinone is most commonly used. Methyl methacrylate is a powerful solvent. It can dissolve permanent marker (used for labeling things) and even some polymers. The ability of methyl methacrylate to dissolve polymethyl methacrylate resin (acrylic resin) has a significant effect on the mixing and handling properties of acrylic resin systems. After mixing, the liquid dissolves some of the powder forming a workable dough.
The powder is predominantly polymethyl methacrylate resin with added colorants and benzoyl peroxide. It is usually composed of very small beads of acrylic resin, as shown in Figure 11.2. When activated, benzoyl peroxide forms free radicals to initiate polymerization.
FIGURE 11.2. Low-power magnification photograph of acrylic resin powder. Note the fibers and the various sizes of beads. (Courtesy of E. M. Krouse, Department of Anatomy, West Virginia University, Morgantown, WV.)
3. Physical Changes During Setting
When the powder and liquid of an acrylic resin system are mixed, several stages in the setting process occur. These stages can be noticed when a sufficient mass of material is mixed as in the construction of a custom tray. During the initial stages, the changes are physical. The mixed powder and liquid have a “grainy” or “sandy” feel. The powder and liquid are separate phases. As some powder dissolves, the mixed material becomes thicker and less “runny.” As more powder is dissolved, the material reaches the “dough” stage. At this point, the material is easy to handle and mold, and up to this point, the changes are mainly physical.
4. Polymerization Reaction
A cold-cure or chemically activated system has an activator, typically a tertiary amine, added to the liquid. When the powder and liquid are mixed, the benzoyl peroxide and the tertiary amine react to produce free radicals. The inhibitor in the liquid destroys the free radicals that are initially produced, and working time results. This occurs while the material goes from a grainy to a dough stage.
When the inhibitor is used up, typically during the dough stage, chemical changes occur, and the polymerization reaction proceeds. The doughy material thickens and becomes stiffer. The reaction generates heat as well, and the material becomes warm. Many times when a mass of material is mixed as in the construction of a custom tray, the material becomes hot to the touch. The material becomes rigid and solid as polymerization reaches completion.
5. Residual Monomer
Initially, the set material contains some residual monomer. Any monomer that does not polymerize soon evaporates, leaving little or no monomer or unreacted double bonds in the set material.
Cross-linking the resin improves mechanical properties. A linear resin without any cross-linking agent is brittle. Addition of a cross-linking agent improves the toughness of the material.
B. Heat-Activated Acrylic Resins
1. Heat-activated acrylic resin systems are very similar to chemically activated systems. The major exception is that no chemical activator is present in the liquid. A minor difference is that less inhibitor is present in the liquid. The inhibitor is not needed to provide working time; it functions as a preservative, reacting with free radicals to prevent polymerization during storage.
2. Heat-activated systems are supplied as powder/liquid systems similar to those of cold-cure resins. When the powder and liquid are mixed, they go through the same initial stages of the setting process. Because no chemical activator is present, the mixed material stays in the dough stage for an extended period of time. Therefore, working time is much longer than it is for heat-cure acrylic resins. After the material is formed into the desired shape (to be explained later), the material is heated in a water bath. The heat breaks down the benzoyl peroxide, forming free radicals. Polymerization proceeds by changing the dough into a rigid material. Products that are properly heat cured are a bit stronger and tougher than cold-cure acrylic resins.
C. Light-Activated and Dual-Cure Acrylic Resins
Light-activated and dual-cure acrylic resin systems are available, but they are not as popular as light-activated and dual-cure composites. Recently, light-activated and dual-cure composite materials for temporary crowns, custom trays, and other acrylic resin uses have been introduced. Because these composite materials are stronger, they are gaining acceptance. As prices decrease, they may completely replace acrylic resins for some uses.
D. Acrylic Resin Systems and Porosity
Regardless of the type of activation of an acrylic resin system, porosity is a major concern. Methyl methacrylate and other monomers evaporate easily at room temperature. If monomer evaporates during handling or processing, the resulting material will be porous, as shown in Figure 11.3. Porosity weakens the material. Also, the denture is likely to collect debris in pores and develop an offensive odor and taste. A great deal of effort is made to prevent porosity when acrylic resins are processed. Pressure and temperature controls are used to minimize porosity.
FIGURE 11.3. Closeup photograph showing porosity (white spots) in a poorly cured acrylic resin sample.
III. Complete Dentures
A complete denture or full denture replaces an entire arch of missing teeth, as shown in Figure 1.6. A complete denture also replaces alveolar bone, which resorbs when teeth are missing. Dentures are made with acrylic materials that are colored to simulate the missing tissues.
Complete dentures are held in place by suction, which is the result of surface tension and atmospheric pressure. Therefore, a complete denture requires precise adaptation to the supporting tissues and a peripheral “seal” for adequate retention (like that of a suction cup). Saliva helps to achieve the seal and improves suction, just as water improves the effectiveness of a suction cup. An impression for a complete denture involves much effort to determine and record the supporting tissues and the proper extension of the denture borders. The borders of the denture that are recorded by the impression are reproduced in the denture. Proper extension of the borders of a denture determines the seal and much of the success of a denture. Most patients function reasonably well with an upper denture. The same cannot be said for lower complete dentures. The peripheral seal of a lower denture is much less effective than that of an upper denture. In addition, an upper denture has a larger surface-bearing area (the palate), and it usually has a better alveolar ridge to support the denture.
A. Components of a Denture
A complete denture has two major components: the white denture teeth and the pink denture base. Denture teeth, as shown in Figure 11.4, are purchased from a manufacturer. The denture base is made in the dental laboratory following the dentist’s prescription; it is made on the master cast, which is a positive reproduction of the patient’s alveolar ridge.
FIGURE 11.4. Photograph of acrylic teeth for maxillary and mandibular dentures (from top to bottom): maxillary posterior, maxillary anterior, mandibular anterior, and mandibular posterior denture teeth.
B. Denture Teeth
Denture teeth come in a variety of shapes, sizes, and shades. The shape is chosen to match that of the patient’s natural teeth, usually as judged from an old photograph. Another technique is to use the shape of the face to select the tooth shape. The size is determined by the size of the patient’s arch. The shade of the teeth is chosen to match the patient’s natural complexion. Often, the patient desires white teeth and must be counseled as to the true color of natural teeth, because bright white teeth will look artificial.
1. Acrylic Resin Teeth
Today, most denture teeth are made from acrylic resin much like that used to construct the denture base. Denture teeth have more cross-linking agents added. Because the teeth are constructed under tightly controlled conditions at a manufacturing plant, they are stronger than the acrylic material used for the denture base. Acrylic denture teeth are “chemically” bonded to the acrylic denture base during processing of the denture.
2. Porcelain Teeth
Porcelain teeth are made by manufacturers in much the same shapes, sizes, and shades as acrylic teeth. Porcelain teeth are much harder and more stain-resistant compared to acrylic teeth. Porcelain teeth are rarely used, however, both because they excessively wear the opposing teeth and because it is generally believed they cause trauma and bone loss in the supporting and opposing alveolar ridges. Porcelain teeth are held in the denture by the mechanical undercuts of pins that are embedded in the back of the denture tooth, as shown in Figure 11.5.
FIGURE 11.5. Photograph of a porcelain (left) and an acrylic (right) denture tooth. Note the pin protruding from the back of the porcelain tooth. The pin serves to anchor the tooth firmly in the denture base.