Catherine Draper, RDH, MS

Abstract

Biomaterials science is one of the most rapidly changing areas of dentistry today. Dental materials science began more than 100 years ago with experiments on dental amalgam. Today, it is at a crossroads with new developments in the area of biological or tissue engineering as well as in synthetic materials engineering. While the day-to-day creation of tooth replacements using stem cell pathways is still a number of years away, improvements are constantly being made in the area of synthetic dental biomaterials. New products are developed and brought to the marketplace at a rapid pace and oral health care providers must stay abreast of the ever-changing array of options for tooth structure replacement. As dental hygienists, our background in dental materials is often limited to experiences as dental assistants or from a basic dental materials course in dental hygiene school. Our primary focus is on disease prevention rather than restorative procedures. However, dental hygienists not only must be able to recognize the restorative materials used in dentistry and apply the appropriate instrumentation procedures, but also must be able to help educate patients on the various restorative treatment options available.

Dental restorations can be divided into 2 categories, direct and indirect. Restorations fabricated within the cavity preparation are categorized as direct whereas restorations made outside of the mouth are classified as indirect. This article will focus on the materials available for direct dental restorations—dental amalgams, composites, glass ionomers, and compomers.

Amalgam

The use of dental amalgam dates back to the 1800s when it was first developed in France. Dr. G. V. Black experimented and standardized the material's preparation, in essence, beginning the field of dental material science.¹ Amalgam, by definition, is an alloy of mercury with any other metal.^2,3 Silver, tin, copper, and zinc are used in varying proportions in dental amalgam, which is made by mixing equal parts of the powdered metal alloy with liquid mercury.

Particle shape affects amalgam's handling properties. The metal components can be irregularly lathe cut, spherically cut, or a combination of both cuts. After trituration, the alloy and mercury react while being condensed into the cavity preparation. The initial setting reaction begins during mixing and continues through condensation and carving, becoming fully set 24 hours after initial placement.⁴ Amalgam restorations are held in place via mechanical retention, facilitated by the undercuts or retention grooves incorporated into the cavity preparations, which are typically larger than the preparations needed for similar bonded restorations.

Amalgam polishing is recommended to produce smooth, nonplaque-retentive restorations that resist tarnish and corrosion. After the amalgam has fully set, the restoration can be finished with a succession of finishing burs and stones followed by polishing with rubber points and cups or with slurries of pumice and tin oxide. Care must be taken during the finishing and polishing process to avoid excessive generation of heat, which can damage the pulp and also cause mercury to be released from the amalgam surface. While amalgam restorations can usually be finished to extend their service, restorations with open contacts, excessive corrosion, fracture, open margins, or recurrent decay need to be replaced.²

Several physical properties of amalgam are of importance to the clinician when evaluating amalgam restorations. Amalgam restorations may demonstrate delayed expansion caused when moisture contaminates the cavity preparation. Creep, a slow change in the shape of the amalgam from constant stress, compromises the marginal integrity of the restoration. Marginal breakdown leads to failure and provides an opportunity for secondary decay to develop. The coefficient of thermal expansion in amalgam is higher than the surrounding tooth structure. Repeated expansion and contraction of an amalgam restoration can lead to fracture of otherwise healthy tooth structure.⁵ Dimensional changes can be caused by a number of factors including the mercury-to-alloy ratio along with trituration and condensation techniques.

Figure 1—Tarnished, fractured amalgam restoration.

Corrosion is another common feature seen in amalgam restorations. An electrochemical or galvanic reaction occurs when 2 metals exist in a wet environment. The various metal phases in dental amalgam, such as silver–mercury and tin–mercury, when exposed to the wet and often acidic environment of the oral cavity, can experience a galvanic reaction, causing surface as well as internal corrosion. The corrosion behavior of amalgam restorations is influenced by the metal composition of the alloy, the morphology of metal particles in the material, and the electromechanical behavior of adjacent metal restorations. Corrosion can be accelerated in a new amalgam if there is galvanic current from an adjacent restoration. Surface corrosion can actually serve to improve the marginal integrity by filling the tooth–amalgam interface with corrosion by-products, whereas internal corrosion over time leads to marginal breakdown and restoration failure^2,3 (Figure 1).

Dental amalgam has been the material of choice for direct posterior restorations for more than 100 years.⁶ It is inexpensive, tough, wear-resistant, and said to be the least technique-sensitive of all the direct restorative materials. Yet, amalgam has been controversial ever since its introduction to the United States in 1833.² Mercury toxicity always has been at the root of the issue, originally with pro- and anti-amalgam dentists and currently with anti-amalgam consumers.

The body of research on amalgam safety, as well as the American Dental Association (ADA), continues to support its use as a safe and effective option to treat dental decay. Publication of the New England Children's Amalgam Trial in April 2006 provided further evidence that there are no differences in IQ or kidney function in children with amalgam restorations when compared with those with composite restorations.^7,8 The US Food and Drug Administration, responsible for regulating the safety and efficacy of dental amalgam, continues to evaluate amalgam safety for specific population groups, including pregnant women, small children, and sensitive individuals.⁹

While the mercury in amalgam restorations has not been proven to be harmful to patients, the detrimental effects of improperly handled mercury on dental health care workers and the environment are well established.^2,3 Mercury vapor poses the most significant health risk to clinicians, with toxic exposure affecting kidney and central nervous system function. Mercury safety should begin with well-ventilated work areas and proper handling of amalgam from storage to placement to disposal of scrap products. Amalgam waste can have a detrimental effect on the water supply and food chain if it is not properly recycled. The ADA's Best Management Practices guidelines should be followed to ensure safety and reduce environmental contamination.¹⁰

Composite Restorations

Although amalgam has been the material of choice for restoring posterior teeth for well over 100 years, significant advances have been made in the last 25 years in resin composite materials and bonding systems for retaining restorations in cavity preparations.^1,5 While amalgam restorations are not likely to disappear, composite materials have become a reliable treatment option for restorations in the esthetic zone, as well as occlusal surfaces.^1,5

Composites are defined as the combination of 2 materials. Dental composites consist of a polymer matrix, usually bis-GMA, and filler material.² Filler size and content are critical factors in determining the physical properties of a composite material. Early filler materials were relatively large natural quartz (sand) particles, resulting in restorations with rough outer surfaces and marginal defects caused by shrinkage of the resin matrix. The first generation composite materials, developed in the 1960s, were categorized as macrofilled. These materials are rough to an explorer and become grey when scaled with an instrument. Macrofilled composites proved to be inappropriate for Class 1 and 2 restorations because of their low strength and high incidence of shrinkage. Microfilled composites were developed in the late 1970s. Although the small size filler produced a smooth and lustrous surface, the low overall percentage of filler (40% to 50%) resulted in increased expansion of the resin matrix and reduced strength.² Hybrid composites, developed in the 1980s, contain a range of filler particle sizes and shapes and a higher overall filler concentration (75% to 80%). Hybrids exhibit a smooth surface finish in addition to being durable and abrasion-resistant. Hybrid composites are used for esthetic restorations, as well as Class 1 and 2 restorations.² Filler technology continues to improve the properties of composite materials. Nanofilled composites contain a combination of nanofiller and larger fillers, which improves the wear resistance of the restoration and produces a coefficient of thermal expansion that mimics tooth structure.^1,2,11

Composite restorations adhere to the tooth structure via micromechanical bonding.^1,2,12 Originally composite restorations were held in place without any adhesive agents, but since the development and use of enamel and dentinal bonding systems, microleakage, postoperative sensitivity, and secondary decay have been significantly reduced.² Dentin bonding systems have evolved from first generation, time-consuming, 3-step systems that required applying a separate acid etchant, resin primer, and resin bonding agent to the current, seventh generation systems that require applying a single solution that etches, primes, and bonds the restoration to the prepared surface.^1,11

Composite materials are placed in the cavity preparation in layers. Materials manufactured with flowable viscosities have reduced filler content and are used to build up the base of the restoration. A hybrid or nanofilled material is then placed at the occlusal or incisal surface to mimic the strength and wear of enamel. A variety of color shades is available, from opaque to translucent, to enable the clinician to match the existing tooth structure. Condensable or compactable composite materials also are available for preparations requiring a stiffer, thicker material.²

Composite materials are polymerized, or cured, by several mechanisms. Some products are cured via a chemical reaction while others are initiated via high-intensity visible light.^1,2 Visible-light curing systems have the advantage of operator control of the curing process. However, variations in light intensity and various factors affecting the overall depth of penetration of the light can cause curing imperfections. Some composite materials, most often used for core build-up procedures, cure via both chemical and light activation. This dual-cure mechanism allows for more complete polymerization throughout the composite. Newer light emitting diode systems have the advantage of reliable light output intensities as well as being lightweight and portable.¹

Composite restorations are finished at the time of placement. A scalpel blade is used to trim the interproximal contacts. The use of finishing strips and aluminum-coated discs give the restoration smooth surfaces and a high luster.

While composite materials do not elicit the controversy that amalgam does in regard to the effects of mercury on patients, practitioners, and the environment, concerns have been expressed regarding the bisphenol-A (BPA) that may be produced as the composite matrix slowly degrades in the presence of esterases commonly found in saliva.¹³ Because these effects only have been demonstrated in laboratory animals, more research needs to be conducted on the biocompatibility of composite materials. In a March 2007 press release, the ADA stated that there is no cause for concern at this time regarding potential BPA exposure from dental composites and sealants. The ADA, however, supports additional research on human exposure to BPA. The US Department of Health and Human Services convened an expert panel in March of 2007 to review and assess over 500 scientific studies on the potential reproductive and developmental hazards of BPA. A follow-up meeting is scheduled for later this year to report the final conclusions and identify future research needs.¹⁴

Glass Ionomers and Compomers

Figure 2—The new generation composite materials are radiopaque. Note that all restorations in the radiograph are composites except tooth No. 3.

Glass ionomer cements appeared on the market in the early 1970s and are considered to be the first truly adhesive restorative material.^2,15 Glass ionomers contain aluminum, calcium, fluoride, phosphorous, and silica, in addition to an acid component. The first glass ionomer products were chemically cured, setting via an acid–base reaction. New generations of glass ionomers are dual-cure, initiating with a chemical bond followed by light curing. Glass ionomers are not sensitive to moisture during placement and release fluoride ions over time. These features make glass ionomer restorations an ideal choice for caries-prone individuals. Their opaque appearance may not be as esthetic as composites.^1-3 The latest generation of products are radiopaque, wear-resistant, have a good marginal seal, and demonstrate the same coefficient of thermal expansion as tooth structure. While glass ionomers can be used for all types of restorations, they are primarily used for Class 2 and 5 restorations, cavity bases, and liners. Glass ionomers also can be used as sealants and cements.^1,2

Compomers combine composite materials with glass ionomer in an attempt to create a material that has the esthetics and physical properties of a composite and the ease of use of a glass ionomer.² Unlike glass ionomers, compomers require a separate bonding agent and are sensitive to moisture. Although there is an initial release of fluoride from compomers, a sustained fluoride release is questionable.²

Dental Hygiene Instrumentation

Dental hygienists must be able to recognize the various direct restorative dental materials and use appropriate instrumentation techniques to prolong the life of each restoration. Many restorative materials, such as amalgam, are easily identifiable; however, it is often difficult to distinguish a well-finished Class 5 composite from the adjacent tooth structure. Early filler materials were radiolucent, but the new generation of composite restorations feature radiopaque fillers that are easily identified radiographically (Figure 2). Visual cues (color and visible margins) along with tactile differences are the primary ways to identify restorative materials.² New generation, nanofilled, hybrid composites feel smooth to the explorer, whereas glass ionomers feel dull or rough. A sharp explorer and experienced tactile sensitivity are necessary to detect and evaluate restorations. Table 1 provides additional tips for detecting various direct restorations.

Proper cleaning and polishing of restorations can prolong their service and enhance their esthetics. Caution must be exercised when using high-speed instrumentation around restorations. The vibrating tip of the ultrasonic scaler can alter the margins of amalgam restorations in addition to scratching and damaging glass ionomers and composites. Although care must be taken to use the appropriate setting to minimize potential damage to restorations, the use of ultrasonic scaling devices is not contraindicated.^2,16,17 Improper use of an air polisher can pit or even remove existing composite restorations and should be used with caution.¹⁶ Repeated hand instrumentation has been shown to contribute to substance loss in Class 5 amalgams and composite and glass ionomer restorations. While instrumentation is not contraindicated, care should be taken not to use excessive pressure or heavy strokes¹⁷ Table 2 provides additional recommendations for the proper maintenance of direct esthetic restorations.

Composite and glass ionomer restorations require routine polishing only when they become stained. Cleaning agents rather than abrasive polishing pastes are recommended for esthetic restorations.³ In addition to cleaning pastes, several manufacturers have formulated low-abrasive polishing products that are formulated for esthetic restorations. Table 3 lists cleaning and polishing pastes formulated for esthetic restorations.

Conclusion

Consumers now have more choices of and control over dental treatment options than ever before. Dental hygienists are not only in the position of evaluating the integrity of restorations and the surrounding tooth structure, but also play a key role in educating patients about their treatment options. Appropriate dental hygiene instrumentation techniques will add to the longevity of dental restorations and maintain their esthetics. Dental materials science is constantly evolving. Ongoing continuing education is essential for hygienists to remain current in understanding the properties of restorative dental materials as well as how they are optimally maintained.

Acknowledgment

Clinical photographs courtesy of Patrick McEvoy, DDS, FAGD.

References

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Catherine Draper, RDH, MS
Cathy has been a dental hygienist for more than 30 years. She is past president of the California Dental Hygienists' Association and a 2005 recipient of the Sunstar Butler RDH Award of Distinction. She currently works in a general, cosmetic, and implant dental practice in Mountain View, California, and is a member of the adjunct faculty at Foothill College in Los Altos Hills. In addition to being an active member of CDHA and ADHA, Cathy has been a library reference associate, assisting patients and their families in accessing health information, at the Stanford Health Library for over 10 years. Cathy may be contacted at .