Сraig. Dental Materials

Resin composites are used to replace missing tooth structure and modify tooth color and contour, thus enhancing facial esthetics. Composites discussed in this chapter include allpurpose, microfilled, packable, flowable, and laboratory types. In addition, core composites, provisional composites, and compomers are described. Direct filling materials such as glass and hybrid ionomers, used primarily because of their fluoride release, are discussed in Chapter 8. Light-curing units are also discussed in this chapter, because most composites and compomers

are light activated.

Of the direct restorative materials, silicates were developed first, followed by acrylic resins, then resin composites. Silicates were introduced in 1871 and were prepared from alumina-silica glass powder and phosphoric acid liquid. Although silicates provided an anticariogenic feature, early clinicalfailure was noted and was most frequently related to dissolution in oral fluids, loss of translucency, surface crazing, and a lack of adequate mechanical properties. These deficiencies caused the demise of silicates in the 1960s.

Acrylic restorative resins were unfilled, lowmolecular weight polymers and lacked the reinforcement provided by the ceramic filler particles

used in composites. Early clinical failure of acrylics was related directly to dimensional instability, resulting in unsightly stains and often recurrent caries.

The development of composites about 1960 has resulted in higher mechanical properties, lower thermal coefficient of expansion, lower dimensional change on setting, and higher resistance to wear, thereby improving clinical performance. Later development of bonding agents for bonding composites to tooth structure (see Chapter 10) has also improved the quality of composite restorations.

Compomers were introduced in 1995 to provide improved handling and fluoride release as compared to composites.

* h:

ALL-PURPOSE COMPOSITES -. .;2;

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Composites were initially developed for anterior Class 3 to Class 5 restorations, in which esthetics were crucial, and for Class 1 restorations, in which moderate occlusal stresses occur. In the 1990s, modifications of materials and techniques extended their application to Class 2 and Class 6 (MOD) posterior restorations. Laboratory-

processed composites are used for crowns and even bridges, when reinforced with fibers, and can be bonded to an alloy substructure. Types of restorations and recommended composites are listed in Table 9-1.Characteristics of these composites are summarized in Table 9-2.

COMPOSITION

Overview A resin composite is composed of four major components: organic polymer matrix, inorganic filler particles, coupling agent, and the initiator-accelerator system. The organic polymer matrix in most composites is either an aromatic or urethane diacrylate oligomer. Oligomers are viscous liquids, the viscosity of which is reduced to a useful clinical level by the addition of a diluent monomer.

The dispersed inorganic particles may consist of several inorganic materials such as glass or quartz (fine particles) or colloidal silica (microfine particles). Two-dimensional diagrams of fine and microfine particles surrounded by polymer matrix are shown in Fig. 9-1.

The coupling agent, an organosilane (silane), is applied to the inorganic particles by the manufacturer before being mixed with the unreacted oligomer. The silane contains functional groups (such as methoxy), which hydrolyze and react with the inorganic filler, as well as unsaturated organic groups that react with the oligomer during polymerization. Silanes are called coupling

agents, because they form a bond between the inorganic and organic phases of the composite.

Composites are formulated to contain accelerators and initiators that allow self-curing, light curing, and dual curing. IS0 4049 for polymerbased filling, restorative, and luting materials

Fig. 9-1 Two-dimensional diagrams of composites with, A, fine and, B, microfine particles.

(From Cra~gRG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed 7, St Louis, 2000, Mosby.)

Handlincr Characteristics and Pro~erties

(ANSVADA No. 27) describes two types and three classes of composites, as shown by the following:

Type 1: Polymer-based materials suitable for restorations involving occlusal surfaces

Type 2: Other polymer-based materials Class 1: Self-cured materials

Class 2: Light-cured materials

Group 1: Energy applied intra-orally

Group 2: Energy applied extra-orally

Class 3: Dual-cured materials

Oligomers The two most common oligomers that have been used in dental composites are dimethacrylates (Bis-GMA) 2,2-bis[4(2- hydroxy-3 methacryloyloxy-propy1oxy)-phenyll propane and urethane dimethacrylate (UDMA). These oligomers were described in Chapter 7. Both contain reactive carbon double bonds at each end that can undergo addition polymerization. A few products use both Bis-GMA and UDMA oligomers.

The viscosity of the oligomers, especially Bis-GMA, is so high that diluents must be added, so a clinical consistency can be reached when they are compounded with the filler. Lowmolecular weight compounds with difunctional carbon double bonds, usually triethylene glycol dimethacrylate (TEGDMA), shown below, are added by the manufacturer to reduce and control the viscosity of the compounded composite.

New monomers are being developed to reduce shrinkage and internal stress build-up resulting from polymerization in an effort to improve clinical durability.

Fillers A helpful method of classifying dental composites is by the particle size, shape, and distribution of filler. Early composites contained large (20 to 30 pm) spherical particles, followed by products containing large irregularly shaped particles, microfine particles (0.04 to 0.2 pm), fine particles (0.4 to 3 pm), and finally blends (microhybrids) containing mostly fine particles with some microfine particles. Based on the type of filler particles, composites are currently classified as microhybm'd and microfilled products.

Microhybrid composites contain irregularly shaped glass (borosilicate glass; lithium or barium aluminum silicate; strontium or zinc glass) or quartz particles of fairly uniform diameter. Typically, composites have a distribution of two or more sizes of fine particles plus microfine filler (5% to 15%). This distribution permits more efficient packing, whereby the smaller particles fill the spaces between the larger particles. Microhybrid composites may contain 60% to 70% filler by volume, which, depending on the density of the filler, translates into 77% to 84% by weight in the composite. Most manufacturers report filler concentration in weight percent (wt %). A micrograph of a typical, fine glass filler is shown in Fig. 9-2, A.

Microfilled composites contain silica with a very high surface area (100 to 300 m2/g) having particle diameters of 0.04 to 0.2 pm. Because of the high surface area, only 25% by volun~eor 38% by weight can be added to the oligomer to keep the consistency of the paste sufficiently low for clinical applications. Fillers consisting of microfine silica in polymerized oligomers are prepared and ground into particles 10 to 20 pm in diameter. These reinforced fillers may be added to the oligomer in concentrations, so the inorganic content can be increased to 32% to 50% by

volume or about 50% to 60% by weight. A variation of this modification is used in which most of the filler is reinforced filler,with smaller amounts of silica added to the oligomer. Another modification (homogeneous microfill) has no reinforced filler, but rather microfine silica dispersed in the oligomer. A typical microfine silica filler is shown in Fig. 9-2, B, and a reinforced filler containing microfine silica is shown in Fig. 9-2, C.

For a composite to have successful properties, a good bond must form between the inorganic filler and the organic oligomer during setting. Bonding is accomplished by the manufacturer, who treats the surface of the filler with a coupling agent before mixing it with the oligomer. The most common coupling agents are organic silicon compounds called silanes. A typical silane is shown in the following equation:

During the deposition of the silane on the filler, the methoxy groups hydrolyze to hydroxy groups that react with adsorbed moisture or -OH groups on the filler.They can also condense with -OH groups on an adjacent hydrolyzed silane to form a homopolymer film on the surface of the filler. During the setting reaction of the oligomer, the carbon double bonds of the silane react with the oligomer, thus forming a bond from the filler through the coupling agent to the polymer matrix (see the schematic sketch on p. 236). This coupling reaction binds the filler and the oligomer, so when a stress is applied to a composite, the stress can be transferred-from one string filler particle to another through the rather low-strength polymer. As a result, the strength of the composite is intermediate to that of the filler and the polymer separately. This bond can be degraded by dater absorbed by the composite during clinical use.

Fig. 9-2 Scanning electron micrographs of types of

Initiators and Accelerators Composites are light cured or self-cured, with the former being more common. Light activation is accomplished with blue light with a peak wavelength of about 470 nm, which is absorbed usually by a photo-activator, such as camphoroquinone, added by the manufacturer in amounts varying from 0.2% to 1.0%. The reaction is accelerated by the presence of an organic amine containing a carbon double bond (see Chapter 7). The amine and the camphoroquinone are stable in the presence of the oligomer at room temperature, as long as the composite is not exposed to light. Although camphoroquinone is the most common photo-activator, others are sometimes used to accommodate special curing conditions, such as the use of plasma-arc lights with rapid-cured composites.

Chemical activation is accomplished at room temperature by an organic amine (catalyst paste) reacting with an organic peroxide (universal paste) to produce free radicals, which in turn attack the carbon double bonds, causing polymerization. Once the two pastes are mixed, the polymerization reaction proceeds rapidly.

Some composites, such as core and provisional products, are dual cured. These formulations contain initiators and accelerators that allow light activation followed by self-curing or selfcuring alone.

Pigments and Other Components Inorganic oxides are usually added in small amounts to provide shades that match the majority of tooth shades. Numerous shades are supplied, ranging from white bleaching shades to yellow to gray. An ultraviolet (W)absorber may be added to minimize color changes caused by oxidation.

SETTING REACTION

The basic chemistry and general setting reactions for free-radical addition polymerization are presented in Chapter 7. The polymerization reaction of self-cured composites is chemically initiated with a peroxide initiator and an amine accelerator. Polymerization of light-cured composites is initiated by visible blue light. Dual-cured products use a combination of chemical and light activation to carry out the polymerization reaction. Light-curing units are described in detail later in this chapter.

The polymerized resin is highly cross-linked because of the presence of the difunctional carbon double bonds. The degree of polymerization varies, depending on whether it is in the bulk or in the air-inhibited layer of the restoration. The polymerization of light-cured composites varies according to the distance from the composite to the light and the duration of light exposure. The percentage of the double bonds that react may vary from 35% to 80%. The degree of polymerization is higher for laboratory composites that are post-cured at elevated temperatures.

PACKAGING OF COMPOSITES

Light-Cured Composites These composites are supplied in various shades in spills, syringes, and compules, examples of which are shown in Fig. 9-3. The syringes are made of opaque plastic to protect the material from exposure to light and thus provide adequate shelf life. If packaged as a compule, the compule is placed on the end of a syringe, and the paste is extruded after removal of the protective tip. The advantages of compules are ease of placement of the composite paste, decrease in cross infection,

Fig. 9-3 Single-paste,visible light-initiated composite.

(From Craig RG, Powers JM, Wataha JC: Dental materials: properties and manipulation, ed 7, St Louis, 2000, Mosby.)

Fig. 9-4 Self-cured, two-paste core composite with single-bottle bonding agent.

(From Craig RG, Powers JM, Wataha JC: Dental materials: propertiesand manipulation, ed 7, St Louis, 2000, Mosby.)

and protection of the paste from exposure to ambient light.

Self-CuredlDual-CuredComposites

Selfand dual-cured composites are typically packaged in syringes or tubs of paste and catalyst and require mixing. An example of a two-paste core composite is shown in Fig. 9-4.

PROPERTIES OF COMPQS

OVERVIEW

Important properties of composites include working and setting times, polymerization shrinkage, thermal properties, water sorption and solubility, mechanical properties, color stability,

For lightcured composites, initiation of polymerization is related specifically to the application of the light beam to the material; about 75% of the polymerization takes place during the first 10 minutes. The curing reaction continues for a period of 24 hours. Not all of the available unsaturated carbon double bonds react; studies report that about 25% remain unreacted in the bulk of the restoration. If the surface of the restoration is not protected from air by a transparent matrix, polymerization is inhibited; the number of unreacted carbon double bonds may be as high as 75% in the tacky surface layer. Although the restoration can be finished with abrasives and is functional after 10 minutes, the optimum physical properties are not reached until about 24 hours after the reaction is initiated.

For most composites that are initiated by visible light, there is a critical time period after dispensing of the paste onto a paper pad during which fresh composite flows against tooth structure at an optimum level. Within 60 to 90 seconds after exposure to ambient light, the surface of the composite may lose its capability to flow readily against tooth structure, and further work with the material becomes difficult. Fluorescent lights labeled "gold" can be substituted to provide unlimited working time for light-cured composites.

The setting times for chemically activated composites range from 3 to 5 minutes. These short setting times have been accomplished by controlling the concentration of initiator and accelerator.

Polymerization Shrinkage The linear polymerization shrinkage of typical composites are listed in Table 9-3. Free volumetric polymer-

Property

Working time (min, sec)

Setting time (max, min)

Depth of cure (min, mm)

Opaque shades

Other shades

Water sorption (max, yg/mm3)

Solubility (max, yg/mm3)

Flexural strength (MPa)

Type 1

Type 2

*Group 1: cured intraorally. tGroup 2: cured extraorally.

ization shrinkage is a direct function of the amount of oligomer and diluent, and thus microhybrid composites shrink only 0.6% to 1.4%, compared with shrinkage of microfilled composites of 2% to 3%.This shrinkage creates polymerization stresses as high as 13 MPa between the composite and tooth structure. These stresses severely strain the interfacial bond between the composite and the tooth, leading to a very small gap that can allow marginal leakage of saliva. This stress can exceed the tensile strength of enamel and result in stress cracking and enamel fractures along the interfaces. The potential for this type of failure is even greater with microfilled composites, in which there is a much higher volume percent of polymer present, and polymerization shrinkage is greater. The net effect of polymerization shrinkage can be reduced by incrementally adding a light-cured composite and polymerizing each increment independently, which allows for some contraction within each increment before successive additions.

Thermal Properties The thermal expansion coefficient of composites ranges from 25 to

particles. The values for composites are considerably less than the average of the values for the polymer matrix and the inorganic phase, however, the values are higher than those for dentin (8.3x C) and enamel (11.4x C). The higher values for the microfilled composites are related mostly to the greater amount of polymer present. Certain glasses may be more effective in reducing the effect of thermal change than are others, and some resins have more than one type of filler to compensate for differential rates.

Thermal stresses place an additional strain on the bond to tooth structure, which further compounds the detrimental effect of the polymerization shrinkage. Thermal changes are also cyclic in nature, and although the entire restoration may never reach thermal equilibrium during the application of either hot or cold stimuli, the cyclic effect can lead to material fatigue and early bond failure. If a gap were formed, the difference between the thermal coefficient of expansion of composites and teeth could allow for the percolation of oral fluids.

The thermal conductivity of composites with fine particles (25 to 30 x cal/sec/cm2 [" C/cm]) is greater than that of composites with microfine particles (12 to 15 x 10-* cal/sec/cm2 [" C/cm]) because of the higher conductivity of the inorganic fillers compared with the polymer matrix. However, for highly transient temperatures the composites do not change temperature as fast as tooth structure and this difference does not present a clinical problem.

Water Sorption The water sorption of composites with fine particles (0.3 to 0.6 mg/ cm2) is greater than that of composites with microfine particles (1.2 to 2.2 mg/cm2), because of the lower volume fraction of polymer in the composites with fine particles. The quality and stability of the silane coupling agent are important in minimizing the deterioration of the bond between the filler and polymer and the amount of water sorption. It has been postulated that the corresponding expansion associated with the uptake of water from oral fluids could relieve polymerization stresses. In the measurement of hygroscopic expansion starting 15 minutes after the initial polymerization, most resins required 7 days to reach equilibrium and about 4 days to show the majority of expansion. Because composites with fine particles have lower values of water sorption than composites with microfine particles, they exhibit less expansion when exposed to water.

Solubility The water solubility of composites varies from 0.01 to 0.06 mg/cm2. Adequate exposure to the light source is critical in light-cured composites. Inadequate polymerization can readily occur at a depth from the surface if insufficient light penetrates. Inadequately polymerized resin has greater water sorption and solubility, possibly manifested clinically with early color instability.

During the storage of microhybrid composites in water, the leaching of inorganic ions can be detected; such ions are associated with a breakdown in interfacial bonding. Silicon leaches into the water bath in the greatest quantity (15 to

17 pg/ml) during the first 30 days of storage and decreases with time of exposure. Microfilled composites leach silicon more slowly and show a 100% increase in amount during the second 30-day period (14.2 pg/ml). Boron, barium, strontium, and lead, which are present in glass fillers, are leached to various degrees (6 to 17 pg/ ml) from the various resin-filler systems. Breakdown and leakage can be a contributing factor to the reduced resistance to wear and abrasion of composites.

Color and Color Stability The color and blending of shades for the clinical match of esthetic restorations are important. The characteristics of color are discussed in Chapter 3, and these principles can be applied specifically to composites for determining appropriate shades for clinical use. Universal shades vary in color among currently marketed products.

Change of color and loss of shade-match with surrounding tooth structure are reasons for replacing restorations. Stress cracks within the polymer matrix and partial debonding of the filler to the resin as a result of hydrolysis tend to increase opacity and alter appearance. Discoloration can also occur by oxidation and result from water exchange within the polymer matrix and its interaction with unreacted polymer sites and unused initiator or accelerator.

Color stability of current composites has been studied by artificial aging in a weathering chamber (exposure to W light and elevated temperatures of 70 "C) and by immersion in various stains (coffeehea, cranberry/grape juice, red wine, sesame oil). As shown in Table 7-3, composites are resistant to color changes caused by oxidation but are susceptible to staining.

MECHANICAL PROPERTIES

Flexural Strength and Modulus Values of flexural strength and modulus for composites are listed in Table 7-3. The flexural strengths of the various composites are similar. The flexural moduli of microfilled and flowable composites are about 50% lower than values for all-purpose

and packable composites, which reflects the lower volume percent filler present in the microfilled and flowable composites (see Table 9-2).

Compressive Strength and Modulus

Values of compressive strength and modulus for composites are listed in Table 9 - 3 The compressive strengths of the various composites are similar. The flexural moduli of microfilled and flowable composites are typically lower than values for all-purpose and packable composites, which reflects the lower volume percent filler present in the microfilled and flowable composites (see Table 9-2). For comparison, the modulus of elasticity in compression is 62 GPa for amalgam, 19 GPa for dentin, and 83 GPa for enamel.

Knoop Hardness Values of Knoop hardness for composites (22 to 80 kg/mm2) are low compared with values of 343 kg/mm2 for human enamel and 110 kg/mmz for dental amalgam. The Knoop hardness of composites with fine particles is somewhat greater than values for composites with microfine particles because of the hardness and volume fraction of the filler particles. These values indicate a moderate resistance to indentation under functional stresses for the more highly filled composites, but this difference does not appear to be a major factor in resisting functional wear.

A microhardness measurement such as Knoop can be misleading on composites with large filler particles (>I0 pm in diameter), in which the small indentation could be made solely on the organic or the inorganic phase. However, with most current products, filler particle sizes have become much smaller (<1 pm), and the microhardness values appear more reliable.

Bond Strength to Dental Substrates

Bonding of composites to tooth structure and other dental substrates is discussed in detail in Chapter 10.

Enamel and Dentin The bond strength of composites to etched enamel and dentin is typically between 20 and 30 MPa. Bonding is prin-

of the bonding agent into the etched surfxes of enamel and dentin. In dentin, a hybrid layer of bonding resin and collagen is often formed, and the bonding adhesive penetrates the dentinal tubules (Fig. 9-5).

Other Substrates Composite can be bonded to existing composite restorations, ceramics, and alloys when the substrate is roughened and primed appropriately (see Chapter 10). In general, the surface to be bonded is sandblasted (microetched) with 50-pm alumina and then treated with a resin-silane primer for composite, a silane primer for ceramic, or a special alloy primer. Bond strengths to treated surfaces are typically greater than 20 MPa.

CLINICAL PROPERTIES

Clinical requirements for composites accepted for unrestricted use, including cuspal replacement in posterior teeth as defined by American Dental Association (Proposed) Guidelines for Resin-based Composites for Posterior Restorations, are listed in Table 9-5.

Depth of Cure (Light-curedComposites)

Maximum intensity of the light radiation beam is concentrated near the surface of a light-cured composite. As the light penetrates the material, it is scattered and reflected and loses intensity. A number of factors influence the degree of polymerization at given depths from the surface after light curing. The concentration of photoinitiator or light absorber in the composite must be such that it will react at the proper wavelength and be present in sufficient concentration. Both filler content and particle size are critical to dispersion of the light beam. For this reason, microfilled composites with smaller and more numerous particles scatter more light than microhybrid composites with larger and fewer glass particles. Longer exposure times are needed to obtain adequate depth of cure of microfilled composites.

The light intensity at the resin surface is a