Indirect restorative material—A ceramic, metal, metal-ceramic, or resin-based composite used extraorally to produce prostheses, which replace missing teeth, enhance esthetics, and/or restore damaged teeth.
Preventive dental material—Cement, coating, or restorative material that either seals pits and fissures or releases a therapeutic agent such as fluoride and/or mineralizing ions to prevent or arrest the demineralization of tooth structure.
The science of dental materials covers a broad range of terminology, composition, microstructure, and properties used to describe or predict the performance of preventive and restorative biomaterials. Previous courses in mathematics, chemistry, and physics should have prepared you to read this book and understand the terms and principles involved in describing the behavior of these materials as they are used in the testing laboratories of academia, governmental facilities, and industry. Of greatest importance is the potential of this information to predict clinical performance and to allow us to analyze the causes of structural degradation and failure of these materials when they no longer serve their intended functions.
Although many properties of biomaterials can be grouped into one of the broadest categories, i.e., physical properties, this book has been designed to separate these properties into subcategories that allow a clearer visualization of the variables that are most likely to influence the success or failure of preventive and restorative dental materials. Chemical properties generally comprise the behavior of materials in a chemical environment with or without any other external influences. Mechanical properties are related primarily to the behavior of materials in response to externally applied forces or pressures. Of course, in a clinical environment, the behavior of dental materials may be dependent on several variables simultaneously, but a general understanding of a material’s performance will be controlled by our ability to differentiate primary from secondary factors or properties. Lists of the most relevant chemical, manufacturing, mechanical, optical, and thermal properties are presented below. Separate chapters are devoted to more detailed descriptions: Chapter 3, “Chemical and Physical Properties of Solids,” and Chapter 4, “Mechanical Properties of Solids.” Because of the dramatic increase in the use of CAD-CAM technology, a category of processing or manufacturing properties has been introduced in this chapter.
A physical property is any measurable parameter that describes the state of a physical system. The changes in the physical properties of a biomaterial can serve to describe the changes or transformations of the material when it has been subjected to external influences such as force, pressure, temperature, or light. Because these properties may include other properties listed above, a more detailed description of their characteristics is presented in Chapter 3, “Chemical and Physical Properties of Solids.” In contrast to physical properties, chemical properties define the ways in which a material behaves during a chemical reaction or in a chemical environment.
Several properties listed above may fall into more than one category. For example, the optical properties can simply be grouped under physical properties. However, because of the importance of esthetics in dentistry, optical properties have been placed in a separate category. There are many other properties to be considered in a dental setting. However, this book focuses on those most relevant to the biomaterials and auxiliary materials designed for use in dental clinics and dental laboratories.
The directions taken by the dental profession will affect the future of dental materials, although the practice of dentistry will depend on current and future developments in dental materials science. Dentistry will continue to focus on the preservation and enhancement of oral health through the prevention of caries and periodontal disease and the rehabilitation of missing, damaged, and/or destroyed hard and soft tissues. A cure for dental caries will have a dramatic impact on the use of restorative materials to improve the form and function of teeth with cavitated lesions. The need to restore teeth will always exist because of the time-dependent failure or degradation of restorative materials and oral tissues. The decision on which biomaterials to use for a given clinical situation will be controlled by the known benefits of each choice compared with the known risks.
Historically, a wide variety of materials have been used as tooth crown and root replacements, including animal teeth, bone, human teeth, ivory, seashells, ceramics, and metals. Restorative materials for the replacement of missing portions of tooth structure have evolved more slowly over the past several centuries.
Dental materials may fall into any of the following classes: metals, ceramics, polymers, or composites. In general, polymers, cements, and composites are used for preventive as well as restorative applications. Some of these products are capable of releasing diagnostic or therapeutic agents on a controlled-release basis to support the preventive treatments for populations at risk for dental caries.
Pure metals are rarely used for dental applications, although commercially pure titanium can be used to make dental implants, inlays, onlays, crowns, and bridges. Pure gold in a foil form can be used to make dental restorations (“fillings”) directly on teeth, but this technique is used only rarely today. Metals and alloys can also be used to construct orthodontic appliances, partial denture frameworks and clasp arms, and these materials may require auxiliary products such as matrix bands, burs, cutting blades, endodontic files, brooches, and reamers to ensure proper adaptation and placement.
Ceramics can be used to produce inlays, onlays, crowns, and multiple-unit fixed dental prostheses. However, because of the need for high fracture resistance and esthetic appeal, these prostheses are often made of two or more layers, including a strong and tough core ceramic and one or two layers of a less tough but translucent, veneering ceramic. It is also possible to use yttria-stabilized zirconia for implant bodies and endodontic posts and cores.
Despite recent improvements in the physical properties of these materials, none of them is permanent. In the 21st century, dentists and materials scientists will continue to search for the ideal restorative material. Such a material would (1) be biocompatible; (2) bond permanently to tooth structure or bone; (3) match the natural appearance of tooth structure and other visible tissues; (4) exhibit properties similar to those of tooth enamel, dentin, and other tissues; and (5) be capable of initiating tissue repair or the regeneration of missing or damaged tissues.
Dental materials may be classified as preventive materials, restorative materials, or auxiliary materials. Preventive dental materials include pit and fissure sealants; sealing agents that prevent leakage; materials used primarily for their antibacterial effects; and liners, bases, cements, and restorative materials such as compomer, hybrid ionomer, and glass ionomer cement that are used primarily because they release fluoride or other therapeutic agents to prevent or inhibit the progression of tooth decay (dental caries). Table 1-1 summarizes the types of preventive and restorative materials, their applications, and their potential durability. In some cases a preventive material may also serve as a restorative material that may be used for a short-term application (up to several months), for moderately long time periods (1 to 4 years), or for longer periods (5 years or more). Dental restoratives that have little or no therapeutic benefit may also be used for short-term (temporary) use, or they may be indicated for applications requiring moderate or long-term durability. For example, restorative materials that do not contain fluoride can be used for patients who are at a low risk for caries.
|Material Type||Applications of Products||Potential Preventive Benefits||Durability|
|Resin adhesive||A||F (certain products)||M|
|Resin cement||L||F (certain products)||M|
|Compomer||B, L, R||F||M|
|Hybrid ionomer||B, L, R||F||M|
|Glass ionomer (GI)||A, B, L, R, S||F, S||L, M|
|Metal-modified GI||R||F||L, M|
|Zinc oxide–eugenol||B, L, T||– – –||L, M|
|Zinc phosphate||B, L||– – –||M|
|Zinc polycarboxylate||B, L||– – –||M|
|Zinc silicophosphate||B, L||F||M|
|Resin composite||R||F (certain products)||H|
|Dental amalgam||R||– – –||H|
|Ceramic||R||– – –||H|
|Metal-ceramic||R||– – –||H|
|Metal/-resin||R||– – –||M, H|
|Temporary acrylic resin||T||– – –||L|
|Denture acrylic||R||– – –||H|
|Cast metal||R||– – –||H|
|Wrought metal||R||– – –||H|
Restorative dental materials consist of synthetic components that can be used to repair or replace tooth structure, including primers, bonding agents, liners, cement bases, amalgams, resin-based composites, compomers, hybrid ionomers, cast metals, metal-ceramics, ceramics, and denture polymers. Some of these materials can also be designed as controlled-delivery devices for release of therapeutic or diagnostic agents. Restorative materials may be used for temporary, short-term purposes (such as temporary cements and temporary crown and bridge resins) or for longer-term applications (dentin bonding, and indirect inlays, onlays, crowns, removable dentures, fixed multiple-unit, and orthodontic appliances). Restorative materials may further be classified as direct restorative materials or indirect restorative materials, depending on whether they are used intraorally to fabricate restorations or prosthetic devices directly on the teeth or tissues or extraorally, respectively, in which the materials are formed indirectly on casts or other replicas of the teeth and other tissues. Auxiliary dental materials are substances used in the process of fabricating dental prostheses and appliances but that do not become part of these devices. These include acid-etching solutions, impression materials, casting investments, gypsum cast and model materials, dental waxes, acrylic resins for impression and bleaching trays, acrylic resins for mouth guards and occlusion aids, and finishing and polishing abrasives.
Polymers have many uses as both preventive and restorative materials as well as auxiliary materials such as cements, impression materials, impression trays, mouth guards, orthodontic appliances, and interocclusal records. When a monomer resin contains inorganic or polymeric filler particles that are bonded to the matrix resin by means of an organosilane coupling agent, the material is classified as a dental composite or resin-based composite. The term composite resin is technically incorrect unless the microstructure contains only polymeric filler particles (i.e., a composite of resin components).
Temporary restorative materials are a subcategory of restorative materials and include products used for dental restorations and appliances that are not intended for moderate- or long-term applications. Examples include temporary cements used for luting, temporary cements, or other restoratives used for fillings, orthodontic wires, and acrylic resins used for temporary inlays, onlays, crowns, and fixed dental prostheses that span two or more tooth positions. Other auxiliary materials include waxes, gypsum products, dental compounds, and gutta percha.
The overriding goal of dentistry is to maintain or improve the quality of life of the dental patient. This goal can be met by preventing disease, relieving pain, improving the efficiency of mastication, enhancing speech, and improving appearance. Because many of these objectives require the replacement or alteration of tooth structure, the main challenges for centuries have been the development and selection of biocompatible, long-lasting, direct-filling tooth restoratives, and indirectly processed prosthetic materials that can withstand the adverse conditions of the oral environment. Figure 1-1 is a schematic cross-section of a natural tooth and supporting bone and soft tissue. Under healthy conditions, the part of the tooth that extends out of adjacent gingival tissue is called the clinical crown; that below the gingiva is called the tooth root. The crown of a tooth is covered by enamel. The root is covered by cementum, which surrounds dentin and soft tissue within one or more root canals.
Dentistry as a specialty is believed to have begun about 3000 B.C. Gold bands and wires were used by the Phoenicians (after 2500 B.C.). Around 700 B.C. the Etruscans carved ivory or bone for the construction of partial dentures that were fastened to natural teeth by means of gold wires or bands, which were used to position extracted teeth in place of missing teeth.
Although inscriptions on Egyptian tombstones indicate that tooth doctors were considered to be medical specialists, they are not known to have performed restorative dentistry. However, some teeth found in Egyptian mummies were either transplanted human teeth or tooth forms made of ivory. The earliest documented evidence of tooth implant materials is attributed to the Etruscans as early as 700 B.C. (Figure 1-2). Around 600 A.D. the Mayans used implants consisting of seashell segments that were placed in anterior tooth sockets. Hammered gold inlays and stone or mineral inlays were placed for esthetic purposes or traditional ornamentation by the Mayans and later the Aztecs (Figure 1-3). The Incas performed tooth mutilations using hammered gold, but the material was not placed for decorative purposes.