CHAPTER 16 Dental Materials
Although the past 30 years have seen a marked reduction in the dental caries rate in children, restorative dentistry remains an important part of pediatric dentistry. Recurrent caries remains a major diagnosis indicating necessity for replacement of existing restorations. Development continues on improved restorative materials and manipulation techniques for restorative dentistry for the general population. As these become available to the pediatric dentist they may displace materials that have been used for many years. Much current effort addresses the growing awareness, expectations, and concerns of the dental consumer. Aesthetics has become a major consideration in choice of materials and procedures. Concern is also increasing about the biocompatibility of materials from the standpoint of the patient and the environment. Research has resulted in an ever-increasing body of knowledge related to the behavior of dental materials and an avalanche of new products. These developments place a continuing responsibility on the dentist, who must critically analyze the literature and the claims of manufacturers to determine which materials and techniques will provide optimal service to the patient. For intelligent and best choices to be made, an appreciation of the clinical significance of the chemical, physical, and biologic properties of dental materials is essential.
The oral cavity is a formidable obstacle to maintenance of the integrity of tooth structure and the materials used in its restoration or replacement. Biting stress on the cusp of a molar tooth may be as high as 207 MPa (30,000 psi). The pH of saliva, dental plaque, foods, and beverages fluctuates daily from very acid to alkaline. The temperature of a meal may vary as much as 150° F (66° C). The warm, moist oral cavity contains a variety of enzymes and debris, providing optimum conditions for the accumulation of surface deposits that can degrade restorations and tooth structure. For these and other reasons, restorative materials are readily subject to fracture, solubility, dimensional change, and discoloration. If these problems are to be minimized, a dental material must possess certain minimum chemical and physical properties. Furthermore, those properties must be maintained during the manipulation and placement of the restoration and for its projected lifetime in service.
The American Dental Association (ADA)/American National Standards Institute Specification Program has contributed greatly to providing the dentist with high-quality dental materials that have been carefully developed to resist the rigors of the oral cavity. Many commonly used restorative materials are encompassed by this certification program.
The website of the ADA (www.ADA.org) should be consulted for listings of materials that meet the ADA specifications. From that list the dentist should be able to select a brand that provides the desired manipulative characteristics.
In a like manner to the ADA, the Fédération Dentaire Internationale has been instrumental in the development of international specifications under the auspices of the International Standards Organization (ISO). The dental materials market has become international. The Medical Devices Amendments of 1976 to the Food and Drug Act gave the U.S. Food and Drug Administration (FDA) regulatory authority to protect the public from hazardous or ineffective medical and dental devices. Some dental products with claims for therapeutic effects (e.g., fluoride products) are considered drugs, but most dental materials used professionally are considered devices and are subject to regulation by the FDA Bureau of Medical Devices. Also included are over-the-counter dental products sold to the public, such as floss and denture adhesives.
In addition to the issue of the safety of dental materials and devices for the patient, other safety issues have been raised in recent years that have dramatically influenced the practice of dentistry. Concern about the transmission of infectious disease during dental procedures has led state and federal agencies to develop rigorous standards for infection control. These standards have placed new demands on dental materials, devices, and equipment. Today, most of these must be capable of being subjected to disinfection procedures and, in some cases sterilization, without loss of properties. Delivery systems for materials have been significantly influenced. Most materials are now packaged for unit-dose delivery at chairside, which makes infection control much easier than when bulk containers of materials are used.
The second safety issue to come to the forefront is the safety of individuals working in the dental office. Federal and state occupational safety and health administrations (OSHAs) are responsible for developing and enforcing standards to ensure safety in the workplace. Dental offices that employ any person besides the dentist fall within the regulatory scope of these agencies. Many of the dental materials commonly used in restorative dentistry are considered to present an occupational hazard. One of the federal OSHA regulations requires the dentist to have on file a Material Safety Data Sheet for every material—from impression materials to the cleaning fluids used in the office. All employees must be informed about the nature of any hazardous materials to which they may be exposed and must receive training in the safe handling of these materials. Information to assist the dentist/employer is available from sources such as the ADA Regulatory Compliance Manual, first published in 1989 and continuously updated as new regulations are developed.
Environmental concerns also have affected the practice of dentistry. State and federal environmental protection agencies are giving scrutiny to all waste discharged from the dental office, whether solid, liquid, or gas. Disposal of hazardous materials, such as toxins or biohazards, is regulated and the level of such regulation can be expected to grow. In most cases the generator of waste material remains legally responsible even if another party has been paid to dispose of it.
All of the concerns mentioned can seriously affect the practice of dentistry and require on the part of the dentist an increasing body of knowledge about dental materials and a constant vigilance to stay current with developing regulations.
Possibly the greatest deterrent to the development of an “ideal” restorative material is the leakage that occurs along the restoration-tooth interface. There is as yet no truly adhesive dental material. No restorative dental material exactly duplicates the physical properties of tooth structure. Changes in temperature and mechanical stress can result in the development of gaps at the toothmaterial junction. Moreover, no matter how proficient the dentist may be, no restoration exactly fits the prepared space in the tooth. Overwhelming evidence shows that all restorative materials permit ingress of deleterious agents, such as acid, food debris, and microorganisms, between the walls of the prepared cavity and the restoration. A certain incidence of the clinical failure of materials can be associated with this phenomenon. Microleakage may be the precursor of secondary caries, marginal deterioration, postoperative sensitivity, and pulp pathology. With restorations, microleakage often results in unsightly marginal discoloration and necessitates replacement of the restoration. Microleakage poses a particular problem for teeth in the pediatric patient because the floor of the cavity preparation may be close to the pulp. The added insult to the pulp caused by the seepage of irritants that penetrate around the restoration and through the thin layer of dentin, or a microscopic pulpal exposure, may produce irreversible pulp damage.
One method of bonding substances together is entirely mechanical. A liquid adhesive is used that will flow into irregularities in the surfaces being bonded and then solidify. In dentistry, acid etching is commonly used to accomplish bonding of a restorative resin to enamel by the formation of resin tags into the etched enamel.
For true adhesion to occur, bonding must take place at a molecular level and must involve a chemical interaction between the molecules of the adhesive and the adherend. The only dental materials currently in use that have the potential for true adhesion to tooth structure and an established clinical record for success are based on polyacrylic and other polyalkenoic acids. These materials are the polycarboxylate and glass ionomer cements (GICs).
With the advancements that have been made in surface chemistry and the development of adhesives for all types of unusual applications, the dentist may ask why other adhesive dental materials have not been developed. Availability of a truly adhesive restorative material would greatly alter many phases of dental practice. The need for the typical cavity preparation would no longer be a prime consideration because adhesion would eliminate the need for mechanical retention by the cavity preparation. It would no longer be necessary to use auxiliary aids, such as cavity varnishes and etching techniques, to minimize the microleakage around direct filling restorations. The compromised tooth could actually be reinforced by the restorative material, which would reduce the need for full occlusal coverage to protect the tooth.
However, tooth structure possesses numerous undesirable characteristics as a substrate for bonding of an adhesive. It is rough, inhomogeneous in composition, covered with a tenacious layer of surface debris, and wet. These factors discourage adhesion. Furthermore, the reactivity (surface energy) of enamel is low, and therefore the surface does not easily attract other molecules to it.
It has been shown that topical fluoride applications reduce even further the surface energy of enamel. Although this may be beneficial because it can reduce plaque accumulation and hence caries, it also interferes with the wetting of the tooth surface by a liquid adhesive.
On the other hand, the surface energy of most restorative materials, particularly metallic ones, is higher than that of normal intact tooth structure. Therefore, debris accumulates on the surface of restorations more than on the adjoining enamel. This could, in part, account for the surprisingly high incidence of secondary caries associated with most restorative materials, except for those that release fluoride ion. Debris accumulation can promote marginal deterioration by the loss of tooth structure or restorative material at the interface. Such deterioration would normally increase microleakage.
Cavity varnishes have been used empirically for many years as a liner for cavity preparations. When varnish is painted onto the cavity preparation, the solvent evaporates and leaves a thin resin film. The varnish may reduce microleakage when it is used with certain restorative materials. Cavity varnish may reduce initial microleakage around amalgam. Because varnish films are very thin and do not adhere to tooth structure, some clinicians have replaced cavity varnish with one of the dental adhesives currently marketed. In theory, if such a material adheres to dentin and enamel, any leakage should occur between the amalgam and the adhesive. The results of laboratory studies provide limited support for this theory, although a controlled clinical study by Mahler and colleagues showed no reduction in postoperative sensitivity with the use of dentin adhesives as liners for amalgam restorations. Application of cavity varnish before applying a dental adhesive negates any potential for adhesion or sealing on the part of the adhesive system.
The function of the cement base is to promote recovery of the injured pulp and to protect it against further insult. The base serves as a thermal insulator and replaces missing dentin when it is used under the metallic restoration. The base must be of sufficient thickness to provide effective thermal insulation. A minimum of approximately 0.5 mm is required for this purpose.
A base must be able to support the condensation of the restorative material placed over it. If the strength of the base is inadequate, it may fracture during condensation and permit amalgam to penetrate and come into contact with the dentin floor, which thereby compromises the thermal protection afforded by the base. Zinc phosphate, hard-setting calcium hydroxide, zinc oxide–eugenol, and GICs have sufficient strength to serve effectively. In certain cases, such as a class II preparation that involves the restoration of an angle or of a deep depression, it may be necessary to cover a calcium hydroxide base with a layer of stronger zinc phosphate or GIC.
Controversy regarding the safety of the dental amalgam restoration has existed since the material was introduced to the profession more than 150 years ago. Periodically this controversy surfaces in the news media and becomes a matter for public, as well as professional, debate. As a result, the dentist who uses dental amalgam can expect questions to be raised by patients and their guardians and can expect requests for replacement of intact amalgam restorations with other materials.
Amalgam is no longer the most commonly used material for restoring posterior carious lesions. Tooth-colored restorative materials, such as composite resins and resin-modified glass ionomers, are increasingly being used. The popularity of dental amalgam likely will continue to decline as these other materials demonstrate their longevity and their suitability as general amalgam replacements in the permanent dentition.
The unique clinical success of amalgam during 150 years of use has been associated with many characteristics. It is likely that its excellent clinical service, even under adverse conditions, is attributable to the tendency for its microleakage to decrease as the restoration ages in the oral cavity. Although amalgam does not bond to tooth structure and the margins of an amalgam restoration may appear open, the restoration-tooth interface immediately below the exposed margin becomes filled with relatively insoluble corrosion products that inhibit leakage. Amalgam is unique from this standpoint. The microleakage around other restorative materials usually increases with time. Amalgam is the least technique sensitive of all current direct restorative materials. One of the factors slowing the acceptance of posterior composite resin restorations has been the very exacting clinical technique and time required for placement. Another unique property of amalgam as a direct filling material is its lack of dimensional change during hardening. The ADA specification for dental amalgam limits maximum acceptable dimensional change to ±0.2%. If this is compared with a typical value of 2% or higher for the polymerization shrinkage of a resin matrix composite material, the potential impact on microleakage is obvious.
Nevertheless, failures of amalgam restorations are observed. These may occur in the form of recurrent caries, fracture (either gross or severe marginal breakdown), dimensional change, or involvement of the pulp or periodontal membrane. More significant than the type of failure is its cause. Two factors that lead to such clinical failures are improper design of the prepared cavity and faulty manipulation. In other words, the deterioration of amalgam restorations often can be associated with neglect in observing the fundamental principles of cavity design or abuse in preparing and inserting the material. One other factor also is involved, and that is the choice of the alloy used.
Several criteria are involved in the selection of an amalgam alloy. The first criterion is that the alloy should meet the requirements of the ADA Specification No. 1 or the corresponding ISO specification for dental amalgam alloys.
The manipulative characteristics of dental amalgam are extremely important and a matter of subjective preference. Rate of hardening, smoothness of the mix, and ease of condensation and finishing vary with the alloy. For example, the resistance felt with lathe-cut amalgams during condensation is entirely different from that with spherical amalgams. The alloy selected must be one with which the dentist feels comfortable, because the operator variable is a major factor influencing the clinical lifetime of the restoration. Use of alloys and techniques that encourage standardization in the manipulation and placement of the amalgam enhances the quality of the service rendered. Coincident with this is the delivery system provided by the manufacturer—its convenience, expediency, and ability to reduce human variables.
Obviously the physical properties should be reviewed in the light of claims made for the superiority of one alloy over competing products. Ideally such a list of properties should be accompanied by documented clinical performance in the form of well-controlled clinical studies. Although the cost of the alloy is a factor, this criterion should not be overemphasized when balanced against the alloy’s ability to render maximum clinical service. The dentist should always consider the fractional costs of any material to the overall total charges for a dental procedure when making price comparisons between brands, particularly when comparing a brand with documented clinical performance against a generic brand of material.
Dental amalgam alloys generally are available as either small filings called lathe-cut alloys or spherical particles called spherical alloys. Spherical alloys tend to amalgamate readily. Therefore amalgamation can be accomplished with smaller amounts of mercury than required for lathe-cut alloys, and the material gains strength more rapidly. Also, the condensation pressure and technique employed by the dentist in placing the restoration are somewhat less critical in achieving the same properties of the amalgam. This is an advantage in difficult clinical situations in which optimal access for condensation is limited. Spherical amalgam alloys have a somewhat different feel during condensation and require less condensation pressure than lathe-cut alloys. The dentist and auxiliary should familiarize themselves with the handling characteristics of a new alloy before clinical restorations are placed.
The original dental amalgam alloys were alloys of silver and tin with a maximum of 6% copper. When significantly more copper is available, improved laboratory properties and clinical performance have been demonstrated. This improvement has been attributed to the displacement of the tin-mercury reaction product with a copper-tin phase during the amalgamation reaction. Alloys that contain enough copper to eliminate the formation of the tin-mercury phase (11% to 30%) are called high-copper amalgam alloys. The first such alloy of this type was an admixed system. Small spherical particles of a silver-copper alloy were added to filings of a conventional silver-tin alloy. High-copper alloys also can be made using single composition particles. Each of these alloy particles has the same chemical composition, usually silver, copper, and tin. Amalgams made from high-copper alloys have low creep. Creep is the tendency of a material to deform continuously under a constant applied stress. This property has been associated with the marginal breakdown (ditching) commonly noted with amalgam restoration. Although the ADA specification for dental amalgam permits a maximum of 3% creep, creep of a modern high-copper amalgam alloy should not exceed 1%.
Most of the properties of amalgam restorations have been shown to depend on the relative amount of mercury contained in the finished restoration (the residual mercury). One of the variables that control the final mercury content is the amount of mercury used to mix the amalgam.
Although dental amalgam alloy still may be available in the form of powder or preweighed compressed pellets and bulk mercury can be dispensed by volume, most of the amalgam alloy sold today is in the form of prefilled, disposable mixing capsules containing the proper amounts of alloy and mercury. This delivery system should be used for several reasons. The alloy/mercury ratio is accurately preproportioned. The need for disinfection procedures is minimized because the capsule system is discarded after use. Most importantly, exposure of dental personnel and environmental contamination by mercury vapor is minimized. These prefilled capsules are usually available for different size mixes, often called single- or double-spill capsules.
The second manipulative variable that controls the residual mercury content is trituration. Trituration time can significantly influence both consistency and working time of the mixed amalgam. These in turn relate to the ability to bring excess mercury to the surface during condensation. The correct trituration time varies depending on the composition of the alloy, the mercury/alloy ratio, the size of mix, and other factors. The best practice is to acquire an appreciation for the appearance of a proper mix and then to adjust the trituration time accordingly. The most serious error in amalgamation generally is undertrituration. An undertriturated mix appears dry and sandy and does not cohere into a single mass. Such an amalgam will set too rapidly, which results in a high residual mercury content, reduced strength, and the increased likelihood of fracture or marginal breakdown. Properly mixed amalgam is a shiny, coherent mass that can be readily removed from the capsule.
When first introduced, mechanical amalgamators for dental amalgam operated at a single speed that was usually below 3000 cpm. High-copper alloys in prefilled, self-activating capsules are designed for shorter trituration times at higher trituration speeds. Failure to activate these capsules reliably results in undertrituration and is a common problem with the use of older single-speed amalgamators. Because amalgamators also deteriorate with time, replacement of an older unit with a new high-speed amalgamator is desirable. A unit that allows multiple speeds of operation should be selected, because numerous other products such as dental cements are now marketed in capsules to be mixed in a dental amalgamator. The trituration times suggested by the amalgam alloy supplier are starting points. Amalgamators may vary in operating speed even within the same brand, and a unit’s performance may vary with line voltage or the number of times it is used in rapid succession. Trituration speed, as well as time, significantly influences the rate at which some amalgams harden(Fig. 16-1).
Figure 16-1 The influence of amalgamator speed (lowmedium-high) on the hardening rate of a high-copper amalgam alloy as measured by the Brinell Hardness test (BHN). BHN = 1.0 indicates the working time, and BHN = 4.5 indicates the carving time.
(Redrawn from Brackett W. Master’s thesis. Indianapolis, Indiana University School of Dentistry, 1986.)
The purpose of condensation is to adapt the amalgam to the walls of the cavity preparation as closely as possible, to minimize the formation of internal voids, and to express excess mercury from the amalgam. Within reasonable limits, the greater the condensation pressure, the lower the amount of residual mercury left in the restoration and the greater the strength of the restoration. The selection of the condenser and the technique of “building” the amalgam should be designed to achieve those objectives, as described in detail in textbooks of operative dentistry, and should be tailored to the handling characteristics of the type of amalgam alloy chosen.
Moisture contamination of an amalgam restoration can promote failure. If zinc is present in the alloy, it will react with water, and hydrogen gas will be formed. As this gas builds up within the amalgam, a significant delayed expansion can occur and may cause protrusion of the amalgam from the cavity preparation, which enhances the possibility of fracture at the margins.
Such moisture contamination can result from failure to maintain a dry field during the placement of the restoration. Exposure to saliva after the amalgam has been completely condensed is not harmful. It is only moisture incorporated within the amalgam as it is being prepared or inserted that must be avoided.
Zinc-free alloys are available, and their physical properties are generally comparable to those of their counterparts that contain zinc. A zinc-free, high-copper alloy should be used when the dentist operates in a field where moisture control is difficult.
Because dental amalgam is a brittle material, a commonly observed type of amalgam failure is the restoration in which the marginal areas have become severely chipped. The exact mechanisms that produce this breakdown of the amalgam or the adjoining tooth structure are not established, but it is likely that the deterioration is precipitated by manipulation and technique of finishing rather than by dimensional changes during setting.
If the restoration is improperly finished by the dentist, a thin ledge of amalgam may be left that extends slightly over the enamel at the margins. These thin edges of such a brittle material cannot support the forces of mastication. In time they fracture, leaving an opening at the margins.
Bulk fracture of amalgam is much less common with high-copper amalgam alloys. Those cases that do occur likely have one of two causes. Poor cavity design resulting in an insufficient bulk of material across the isthmus can lead to failure of even a high-strength alloy, as illustrated in Fig. 16-2. The other reason for bulk fracture is premature loading of the restoration. Unlike a resin matrix composite, amalgam gains strength slowly over the first 24 hours. Premature loading can result in minute fractures that are not apparent for weeks or even months. The use of a rapid-setting amalgam with a high 1-hour compressive strength should be considered when treating a pediatric patient in whom compliance with instructions to refrain from biting down hard on the freshly placed amalgam is in question.
Because dental amalgam does not adhere to tooth structure, it must be retained mechanically by the design of the cavity preparation and/or mechanical devices such as pins. The placement of an amalgam does not strengthen the compromised remaining tooth structure and subsequent fracture may occur, particularly in molar teeth with relatively large mesiodistocclusal amalgam restorations. The use of dental adhesive systems, as described in detail in the section related to resin composites, as lining materials for amalgam to create a “bonded amalgam restoration” has been suggested. Several products are marketed specifically for this purpose. In general, they are chemically activated dentin-bonding systems over which the amalgam is condensed before the resin adhesive has hardened. This results in an intermixing of the unset resin and the plastic amalgam at the interface and forms a mechanical bond as both materials harden. It is important to distinguish this application from the use of a dental adhesive to seal the dentin surface and reduce early microleakage as previously discussed. When dental adhesives are used to seal the dentin surface, the adhesive should be polymerized before the amalgam is placed. Bond strengths reported in laboratory studies between amalgam and dentin are lower than the maximum reported for resin composite bonded to dentin. In vitro studies also show that teeth restored with bonded amalgams are more resistant to fracture than those in which amalgam is placed without a bonding adhesive.
These are relatively short-term laboratory studies. Even though longer-term clinical data are available, little is known about the potential influence of embedding the resin into the bulk of the amalgam on the long-term properties of the restoration. At the present time, amalgam bonding should />