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After studying this chapter, the student will be able to do the following:
1. Summarize the reasons why a dental hygienist should be knowledgeable in the science of dental materials.
2. Explain the difference between biomaterials and dental materials.
3. Discuss some of the conditions that make the oral cavity a hostile environment.
4. Identify four characteristics or properties a dental material must possess to survive in the oral environment.
5. Explain how the following organizations evaluate and/or classify dental drugs, materials, instruments, and equipment:
- American Dental Association (ADA)
- U.S. Food and Drug Administration (FDA)
- International Standards Organization (ISO)
6. Name three ways dental materials may be classified, and discuss each.
7. Specifically discuss the locations of all six cavity classifications and the appropriate restorative material to be used for each. Include the following in your discussion:
- Anterior and/or posterior
- Involvement of incisal angle
- Involvement of proximal surface
- Smooth surfaces versus pit and fissures
direct restorative materials
fixed partial denture
indirect restorative material
removable partial denture
What is “dental materials”? It is a subset of materials science, an applied science that combines chemistry, physics, and engineering with a little biology. Other important examples of materials science include food science, parts of pharmacology, and textiles. After all, we want our chips to be crunchy, our time-release medications to slowly dissolve, and our colored fabrics to stay bright. In dentistry, we do not want our fillings to break, our molars to dissolve, nor our incisors to “yellow.” Understanding what makes one material strong and rigid while making another stretchy and snap back to its original shape will help us utilize the vast array of dental products to care for our patients.
I. Rationale for Studying Dental Materials
“Dental materials” is one of many required courses in the dental hygiene curriculum. It focuses on those items and products used in the prevention and treatment of oral disease and the promotion of health. The scope of practice of a dental hygienist includes the delivery of therapeutic, educational, and preventive patient services. Materials used in the practice of dental hygiene include instruments made from common industrial materials, therapeutic agents, and dental biomaterials used to prevent disease. At times, therapeutic dental products and preventive materials overlap.
The preventive aspects of dental hygiene include primary prevention, which attempts to reduce the occurrence of disease, and secondary prevention, which attempts to limit the destruction caused by disease. Both aspects of preventive dentistry involve the use of instruments (made from materials) and dental materials.
A dental hygienist should be knowledgeable in the science of dental materials for the following four reasons:
A. To Understand the Behavior of Materials
This will aid in the delivery of quality patient care. The dental hygienist must understand why specific materials behave as they do and why they are used for certain functions in certain locations to replace missing oral tissues. For example, the proper placement of sealants will prevent dental caries. The proper restoration of teeth and maintenance of restorations will limit the destructive effects of caries and periodontal disease. The proper care and maintenance of instruments (prevention of corrosion) are important when sterilizing and disinfecting. The proper use of dental materials is fundamental to the art and science of dentistry.
B. To Handle Materials Properly
Both preventive and restorative dentistry rely heavily on the proper use of biomaterials. While most of us can boil water, reheating pizza that does not result in a mushy crust requires the proper application of heat (try a low-temperature frying pan with a good dose of patience).
Biomaterials are man-made materials that are used to replace tissues or that function in intimate contact with living tissues. Dental materials are biomaterials used in or around the oral cavity. The hygienist may or may not be involved in the placement of restorations, but he or she plays a significant role in the placement of preventive materials and the maintenance of restorations. The proper handling of dental materials is important because improper handling will likely adversely affect their physical, chemical, and mechanical properties. In turn, this could affect the overall service to the patient.
Therefore, handling a dental material properly is a primary factor in the success or failure of its use. The goal of this text is to present dental materials and their manipulation from a clinical perspective. If materials are properly mixed and placed, improved patient care will result.
C. To Assess and Treat the Patient
The dental hygienist must be able to recognize all dental materials present in the mouth. These may be visible clinically and/or radiographically. Proper identification is important so that they are not mistaken for caries (radiographically) or improperly maintained. An example would be the clinical recognition of an all-ceramic crown. Acidulated phosphate fluoride (APF) gels can etch the surface of some ceramic materials. Using an APF gel is contraindicated for patients with ceramic restorations; instead, a neutral fluoride gel should be used.
D. To Educate the Patient
In many instances, patients may ask the dental hygienist to discuss the characteristics and properties of one dental material compared to another, both of which may be a reasonable option for the patient. Patients may also ask the hygienist to describe the steps involved in the fabrication of a certain type of restoration, or they may also inquire about home care regimens (“How will I take care of my new bridge?”). Knowledge of dental materials is critical so that the patient is given professional, complete, and correct answers.
II. Biomaterials and the Oral Environment
A. Oral Tissues as Biologic Materials
Whether a material is used for preventive or restorative purposes, the oral environment places great restrictions on which materials can be used and the manner in which those materials are used. When one realizes that oral tissues are themselves biologic materials, a variety of properties and functions are evident. All oral tissues must function in the hostile environment of the oral cavity.
Enamel is a hard, wear-resistant surface material. It is able to resist the compressive forces of biting, but it is weak in its resistance to bending and other forces that occur when food is ground by molars. Enamel is well supported by dentin. Enamel will dissolve in oral fluids if the pH is too acidic; dental caries is the result of such an acidic attack. Enamel is also responsible for the tooth’s pleasing esthetic appearance.
Dentin makes up the bulk of the tooth. It acts as a cushion for the brittle enamel, and it provides strength to resist the complex forces that occur when biting. Dentin is more susceptible than enamel to acidic attack.
Pulp is connective tissue that contains nerves and blood vessels. It provides nutrients to the dentin and responds to stimuli with pain or sensitivity.
Periodontium supports the tooth in a stable but dynamic position, and it provides feedback regarding the force placed on the tooth. Periodontium includes the periodontal ligament, cementum, and alveolar bone.
5. Gingival Tissue
A very important function of gingival tissue is to seal out the many noxious agents of the oral cavity. Gingival tissue prevents chemicals and microbes from gaining access to the periodontium and deeper tissues in the body. Gingival tissues surround and attach to teeth, forming a barrier. Although the oral cavity is considered to be inside the body, in many ways it is more like the outside. Biomaterials placed in the oral cavity have very different requirements from those of devices implanted inside the body.
B. Replacement Materials for Oral Tissues
1. Restriction on Materials Use
When oral tissue is lost, dental professionals attempt to replace it with a dental material. The replacement material mimics the function of the oral tissue, and it must withstand the same harsh environment. The biologic nature of the oral environment and the size of the oral cavity restrict the use of materials. These restrictions include the following:
a. Biting forces that may fracture teeth and replacement material
b. Degradation of:
- Materials, such as corrosion of metal
- Teeth, such as dental caries
c. Temperature changes that cause restorations to contract and expand differently than teeth, causing leakage around the restoration as well as tooth sensitivity
d. Biocompatibility (the lack of harmful effects to the patient)
e. Esthetic demands of the patient
2. Effects of Dental Materials and the Oral Environment on Each Other
The dental hygienist must understand the characteristics and properties of dental materials. This knowledge will provide insight regarding how a dental material may affect the oral environment. An orthodontic appliance makes oral hygiene difficult and increases the patient’s susceptibility to gingival inflammation and caries. The oral environment affects the dental materials as well. Yeast or other microbes may colonize on a denture, causing it to become foul-smelling. These characteristics and properties may also limit the selection and use of a dental material.
III. History and Selection of Dental Materials
Why are certain materials chosen instead of others to serve as dental restorative materials? Actually, much has been learned throughout history by trial and error. Paraphrasing C.S. Lewis, “Experience is that most brutal of teachers.” In ancient times, gold was used not only for its corrosion resistance but also for its “workability” or ease of processing. For centuries, humans have attempted to improve their appearance with adornments, such as jewelry and makeup. The replacement of lost teeth is an ancient practice. First, it was more likely for esthetics than for function. As dentistry developed throughout the ages, function became important. Some of the materials used included ivory, which was carved, and porcelain, which was fired into tooth shapes. By the 1800s, dentistry was becoming a scientifically based discipline. The pace of development of new materials quickened. Amalgam, a silver filling material, was frequently used. Porcelain could be crafted into inlays and crowns.
In the 20th century, dental materials science had developed into its own discipline. Numerous materials and techniques were developed. Precise casting techniques were developed for a variety of metals. Polymers and composites were adapted for nearly every dental material need. In the 21st century, new ceramic materials and processing technologies have been adapted by dentistry. Computer-aided design and manufacture are common along with other digital technologies. The pace of dental materials development is so fast that some of this text will be outdated before it is published.
Luckily, the basic concepts of materials science and their use do not change. Both the student and the practitioner need to understand the behavior of the materials they use. After all, they are the ones who must select a product from a rather long list of possibilities.
B. Selection of Dental Materials and Products
The knowledge gained in a dental materials course will aid in the selection of products. Manufacturers readily provide data regarding strength and a variety of other properties. At times, they also provide the results of short-term clinical trials. How reliable is that information? More importantly, how useful is that information? It has been a goal of dental materials scientists to predict the performance of a material from its strength and other mechanical properties. Unfortunately, success has been elusive. Clinical trials are the most reliable source of information for most products. The clinician must evaluate the product information, but he or she must also consider the source of that information.
IV. Standards for Dental Materials
Standards for dental materials have been developed in dentistry in the same manner as in other industries. Standards describe the properties of a product so that a user may select the proper material for a particular use. Standards are common in everyday life. Examples include the octane rating of gasoline, DVD formats, the size of nuts and bolts, computer communications protocols, and even the size of eggs. In the United States, standards are published and administered by the American National Standards Institute (ANSI). Many industries have organizations that work under the guidance of ANSI to develop and administer the standards for the products of that industry; the American Dental Association (ADA) is the institution that represents dentistry.
A. Council on Scientific Affairs of the American Dental Association
In the United States, standards and guidelines for evaluating dental products are developed and administered by the Council on Scientific Affairs of the ADA. The Council evaluates dental drugs, materials, instruments, and equipment. A successful evaluation culminates in awarding of the ADA’s Seal of Acceptance. The applicant (e.g., a toothpaste company or any manufacturer of a dentally related product) submits data for their product following the ADA guidelines. On approval of the product, the applicant is permitted to use the ADA’s Seal of Acceptance. The Seal is illustrated in Figure 1.1. It is commonly seen on accepted brands of toothpaste and toothbrushes. The ADA Seal is awarded for a period of 3 years, after which the applicant must resubmit the product. In addition, advertising for products that have been awarded the ADA Seal is reviewed by the ADA.
FIGURE 1.1. Seal of Acceptance of the American Dental Association. (Courtesy of the American Dental Association, Chicago, IL.)
Some of the ADA guidelines have very specific requirements for physical and mechanical properties that are measured in the laboratory and are called specifications. Specifications have been developed for many (but not all) dental materials. Unfortunately, researchers have not been able to develop a series of tests that adequately predict the clinical performance of many dental materials. As a result, the Acceptance Program relies on clinical data for the evaluation of many dental products. If a product is shown to be safe and effective, it can be given the Seal of Acceptance.
The Acceptance Program of the ADA is voluntary. Manufacturers are not required to have the Seal to market dental products in the United States. Although products might be approved for sale by the U.S. Food and Drug Administration (FDA), some products fail to meet the ADA specifications when tested. The ADA Professional Product Reviews are an excellent summary of dental materials and their properties.
B. Medical Device Amendments of 1976
The federal government, under the auspices of the U.S. FDA, has the authority under the Medical Device Amendments of 1976 to ensure the safety of all medical devices. The U.S. FDA considers dental materials to be medical devices. Medical devices are grouped into three categories:
1. Class I
These devices are the least regulated. Only good manufacturing practices are required. Prophy paste or brushes are two examples of such products.
2. Class II
Class II devices gain approval from the FDA after being shown to be equivalent to products currently in use. Equivalency is demonstrated by meeting performance standards, such as the ADA’s Seal of Acceptance Program. Some dental products have been “grandfathered in,” because they were marketed before 1976. Composite and amalgam restorative materials are two examples of Class II products.
3. Class III
These devices are the most regulated; they require premarket approval. Clinical data must be submitted to the U.S. FDA for evaluation before Class III devices are sold. If safety and efficacy of the device is supported by the data, the U.S. FDA then gives approval to market the product. Bone graft materials are common examples of such products.
C. International Standards Organization
Many other countries have dental specifications or standards and governmental regulations. To simplify the mass of regulations, the International Standards Organization (ISO) attempts to unify standards throughout its member countries. The ISO standards for many dental materials have been developed (and continue to be developed) under the guidance of the Fédération Dentaire Internationale, the international equivalent of the ADA.
Many dental products carry the “CE” marking symbol of the European Union on their packaging, as shown in Figure 1.2. CE stands for Conformité Européenne, and the marking is required for sales in most of Europe. The CE symbol indicates compliance with ISO standards and European Union marketing requirements for dental products.
FIGURE 1.2. Photograph of dental products from several companies displaying the CE marking symbol.
D. Selecting Products
Dentists are fortunate because several products usually will meet the needs of a particular clinical situation. It is important to select and use materials that result in quality service to the patient. The same product may not do so for all practitioners. It is acceptable to select products based on handling characteristics, a company’s reputation and service, or packaging. If two products have been shown to have excellent clinical performance, the ill-defined characteristics of “feel” or “handling” may be the final criteria that result in its selection. A product with the “right feel” will likely result in superior use and service to the patient. In the words of Dr. Karl Soderholm of the University of Florida, “the material must be your friend.”
It is important to realize that most products require some time to learn to use them properly. If the clinician is always changing products to have the latest and greatest “widget bonder,” he or she may be spending so much time learning to use new products that patient care may be affected. Of less consequence, that clinician will also have drawers, closets, and refrigerators filled with expensive, partially used dental products.
V. Classifications of Dental Materials
Like oral tissues, dental materials serve a variety of functions. Some materials replace lost tooth structure and restore the function of the teeth. These materials must withstand biting forces and therefore be strong and wear-resistant. Other materials are used to make impressions of oral tissues from which replicas are made. Many impression materials are soft and stretch a great deal when removed from the mouth. In dentistry, as in other disciplines, properties of a material must be matched to the use of that material. Dental materials can be classified in a number of ways but are typically classified by their use or function. Restorative materials are also classified by the location of fabrication or by the longevity of use.
A. Classification by Use
Materials used to replace lost oral tissues are called restorative materials. As mentioned earlier, those that replace lost tooth structure and restore the function of the teeth must be strong and hard (Figs. 1.3–1.5). Some restorative materials simulate the appearance of the tissues that are being replaced (see Figs. 1.6 and 1.7). Tissues simulated by restorative materials include the enamel of teeth (fillings and crowns), the mucosa of the periodontium (dentures), and even the skin of the face (maxillofacial prostheses). Materials that are tooth-colored are often called esthetic materials.
FIGURE 1.3. Photograph A. and radiograph B. of inlay (tooth #20) and amalgam (tooth #19) restorations. Drawings show the convergence and divergence of preparations for the two materials. The amalgam C and D has several convergent walls (undercuts) at 1 and 3. The inlay E and F has only divergent walls at 1′, 2′, and 3′. Arrows indicate bevels for a metallic inlay.