Activator—Source of energy used to activate an initiator and produce free radicals. Three energy sources are currently used to dissociate an initiator into free radicals: (1) heat, which supplies thermal energy; (2) an electron-donating chemical such as a tertiary amine, which forms a complex and reduces the necessary thermal energy to that available at ambient temperature; and (3) visible light, which supplies energy for photoinitiation in the presence of a photosensitizer such as camphorquinone (CQ).
C-factor—Configuration factor. This represents the ratio between the bonded surface area of a resin-based composite restoration to the nonbonded or free surface area. The greater the C-factor, the greater the stress that develops at the restoration margin, which can lead to gap formation, marginal breakdown and leakage, and other problems, such as secondary caries.
Chemically activated resin—Resin system consisting of two pastes—(one containing an initiator (e.g., benzoyl peroxide) and the other an activator (e.g., an aromatic tertiary amine)—which, when mixed together, release free radicals that initiate polymerization.
Chemically cured composite—Particle-reinforced resin that is polymerized through a chemical activation process wherein two components are blended together just prior to placement of the composite. Also known as self-cure composite. See chemically activated resin.
Composite—In materials science, a solid formed from two or more distinct phases (e.g., filler particles dispersed in a polymer matrix) that have been combined to produce properties superior to or intermediate to those of the individual constituents; also a term used in dentistry to describe a dental composite or resin-based composite.
Coupling agent—A bonding agent applied to the surfaces of reinforcing particles (filler) to ensure that they are chemically bonded to the resin matrix. Organo-silane compounds are the more common class of dental composite coupling agents.
Degree of conversion (DC)—Percentage of carbon-carbon double bonds (–C=C–) converted to single bonds (–C—C–) during curing to form a polymeric resin. Also known as degree of cure and degree of monomer-to-polymer conversion.
Dental composite—Highly cross-linked polymeric materials reinforced by a dispersion of amorphous silica, glass, crystalline, mineral, or organic resin filler particles and/or short fibers bonded to the matrix by a coupling agent.
Dual-cure resin—Dental composite that contains both chemically activated and light-activated components to initiate polymerization and potentially overcome the limitations of either the chemical- or light-cure systems when used alone.
Estrogenicity—Potential of synthetic chemicals with a binding affinity for estrogen receptors to cause reproductive alterations. Bisphenol-A, a precursor of certain monomers such as bis-GMA, is a known estrogenic compound that is considered to have possible effects on fetal and infant brain development and behavior.
Filler—Inorganic, glass, and/or organic-resin particles that are dispersed in a resin matrix to increase rigidity, strength, and wear resistance, to decrease thermal expansion, due to water sorption, and reduce polymerization shrinkage.
Gel point/gelation—The point in the polymerization reaction where sufficient cross-links have formed to produce a rigid, glassy state in which internal flow among the developing polymer chains has stopped. All but highly local molecular motion has stopped. After the gel point, stresses cannot be relieved but instead continue to increase and concentrate near the bonded interfaces. Thus, reducing the polymerization rate provides more time for adjacent polymer chain segments to slip among themselves, rearrange to lower energy configurations, and relieve developing stresses before the gel point is reached.
Hybrid composite—A particle-filled resin that contains a graded blend of two or more size ranges of filler particles to achieve an optimal balance among the following properties: ease of manipulation, strength, modulus (relative rigidity), polymerization shrinkage, wear resistance, appearance, and polishability. Nanohybrids contain at least one dispersed filler with particle sizes of 100 nm or less (see nanofilled, below).
Initiator—A free radical−forming chemical used to start the polymerization reaction. It enters into the chemical reaction and becomes part of the final polymer compound; thus it is not a catalyst although often incorrectly labeled as such.
Lute/luting/cementing agent—Viscous material placed between two components, such as tooth structure and a restoration, that hardens to bind the two components together primarily through micromechanical interlocking, similar to the way mortar holds bricks together. Resin cements (Chapter 14) are luting agents, although some also provide chemical bonding
Light-cured/photocured/photoinitiated composite—Particle-filled resin consisting of a single paste that becomes polymerized through the use of a photosensitive initiator system (typically camphorquinone and an amine) and a light-source activator (typically visible blue light). See also activator and initiator.
Microfilled composite—Composite reinforced with colloidal silica filler particles, approximately 40 nm in size, which can be polished to a highly smooth surface. Microfillers are nondiscrete nanometer-sized particles that are agglomerated into large three-dimensional chainlike networks that drastically increase monomer viscosity.
Nanofilled composite/nanocomposite—Composites with the same-size particles as microfilled composites, but the particles have been surface-treated before they have agglomerated into large three-dimensional chainlike networks. Instead, they form either isolated and/or loosely bound spheroidal agglomerates (called clusters) of primary nanoparticles. The surface treatment allows an increase in filler loading by reducing the viscosity that develops when the particles are added to the monomer. Cluster size may exceed 100 nm. All of the nanocomposites currently being marketed have average primary particle sizes in the 40-nm range and so are of the same size as microfilled composites.
Ormocer—Ormocer is an acronym for organically modified ceramics, which are inorganic-organic copolymers. The organic, reactive monomers are bound to an inorganic Si-O-Si network to reinforce the resin, reduce polymerization shrinkage, and improve abrasion resistance.
Oxygen-inhibited layer—The thin surface region of a polymerized resin containing unreacted methacrylate groups arising from dissolved oxygen, which acts to inhibit the free-radical polymerization curing reaction; also known as the air-inhibited layer.
Packable composite—A hybrid resin composite designed for use in posterior areas where a stiffer consistency facilitates condensation into a cavity form in a manner similar to that used for lathe-cut amalgams, also known as condensable composite.
Polyhedral oligomeric silsesquioxane (POSS)—A molecular-sized hybrid, organic-inorganic resin containing 12-sided silicate cage structures that impart nanoparticle-like reinforcement. See also Figure 13-27.
Dental resin-based composites are structures composed of three major components: a highly cross-linked polymeric matrix reinforced by a dispersion of glass, mineral, or resin filler particles and/or short fibers bound to the matrix by coupling agents. Such resins are used to restore and replace dental tissue lost through disease or trauma and to lute and cement crowns and veneers and other indirectly made or prefabricated dental devices.
The gold standard of reference for these materials is amalgam. However, amalgam has its own disadvantages, such as (1) poor esthetics, (2) unfounded concerns about health hazards from the leakage of mercury, and (3) waste disposal concerns. Because resin-based composites can be made to match the natural appearance of teeth, they have become the most popular of the esthetic or tooth-colored filling materials and are widely used for a variety of dental applications.
Another key advantage of resin materials is that they can be made in a range of consistencies, from highly fluid to rigid pastes, which allows them to be conveniently manipulated and molded, to a custom-made form and then converted through a polymerization curing reaction to a hard, strong, attractive, and durable solid.
There have been remarkable developments in filler, bonding, and curing technologies in esthetic restorative materials over the past 55 years, as shown in Figure 13-1. During the first half of the twentieth century, silicates were the tooth-colored material of choice for cavity restoration. Silicates release fluoride and are excellent for preventing caries, but they are currently used almost exclusively for deciduous teeth because they become severely eroded within a few years (see Chapter 14). Acrylic resins, similar to the materials used to make dentures and custom impression trays (polymethylmethacrylate [PMMA], see Chapter 20), soon replaced silicates because of their tooth-like appearance, insolubility in oral fluids, ease of manipulation, and low cost. Unfortunately, these acrylic resins had relatively poor wear resistance and tended to shrink severely during curing, which caused them to pull away from the cavity walls, thereby producing crevices or gaps that facilitiate leakage within these gap. Excessive thermal expansion and contraction caused further stresses to develop at the cavity margins when hot or cold beverages and foods were consumed.
These problems were reduced somewhat by the addition of quartz powder particles to form a composite structure. The filler occupies space, but it does not take part in the setting reaction. In addition, commonly used fillers have an extremely low coefficient of thermal expansion, approaching that of tooth structure, thus greatly reducing thermal expansion and contraction. However, these early PMMA-based composites were not very successful, in part because the filler particles simply reduced the volume of polymer resin without being bonded (coupled) to the resin. Thus defects developed between the particles and the surrounding resin, which led to leakage, staining, and poor wear resistance.
In 1962, Bowen developed a new type of composite material that largely overcame these problems. Bowen’s main innovations were bisphenol-A glycidyl dimethacrylate (bis-GMA), a monomer that forms a cross-linked matrix that is highly durable (see Chapter 6), and a surface treatment utilizing an organic silane compound called a coupling agent to bond the filler particles to the resin matrix. Current tooth–colored restorative materials continue to use this technology, but many further innovations have been introduced since 1962.
Many of these advances have occurred through developments in the filler component. The filler has evolved to ever smaller sizes, mainly in order to improve appearance and polishability. Barium and other specialty glass and inorganic mineral fillers have been developed to impart radiopacity, enhance manipulation and handling, offset curing shrinkage, and improve mechanical properties. In the 1970s a category now known as traditional composites (also known as conventional or macrofilled composites) was developed; this contained very large particles of ground amorphous silica and quartz (Figure 13-2), which imparted significant improvements in mechanical properties, water sorption, polymerization shrinkage, and thermal expansion compared with unfilled acrylic. However, these composites suffered from roughening of the surface as a result of the selective abrasion of the softer resin matrix surrounding the harder filler particles. To improve surface smoothness and retain or improve the physical and mechanical properties of traditional composites, small-particle–filled composites were developed, using inorganic fillers ground to a size range of about 0.5 to 3 µm but with a broad size distribution (Figure 13-3), allowing a higher filler loading (80% to 90% by weight and 65% to 77% by volume). This resulted not only in smoother surfaces but also greater wear resistance and some decrease in curing shrinkage. Today, further advances in the filler component have resulted in microfilled composites and nanocomposites, hybrid composites, and packable and flowable composites, just to name a few.
Other advances involved the monomer component, providing better chemical and mechanical properties, reduced shrinkage, color and storage stability, biocompatibility, and other features. Today the monomer system is highly complex with the use of a variety of monomers and monomer blends of various molecular weights and functions. Many added innovations were later introduced in both the filler/reinforcing systems and the resin matrix-forming monomers.
Finally, advances in curing technology have yielded light-cured systems that make it possible to harden resins on demand as well as to improve working time and ease of manipulation. Initially an ultraviolet (UV) curing system was used, but it had several drawbacks and was soon replaced by visible blue–light curing systems, which require less than 1 minute of exposure, and they have many other advantages. These advances were soon followed by further developments in curing-lamp technology as well as in clinical techniques and training aids to optimize the advantages that light-cured resins offer.
The history and achievements in dental composites are outlined in Figure 13-1 and are further discussed in detail in this chapter.
A useful classification system for composites is one based on filler particle size and size distribution, as shown in Table 13-1. Subgroups and overlaps exist, particularly for the hybrid category, which combines filler from either the small or the traditional category with micro- and nanofillers. Any resin with elongated fillers (fibers, “whiskers,” filaments), with spheroidal particle fillers or fillers from two or more size ranges can in principle be considered a hybrid. Thus a single classification of hybrid composite is not very meaningful. Many modern dental composites have particle sizes less than 0.5 to 1.0 µm in combination with 10% or more by weight of micro- or nanofiller to adjust the paste to a desired viscosity/rheology so as to provide clinically useful manipulation and handling characteristics. Filler technology is discussed in detail below.
|Class of Composite||Particle Size||Clinical Use|
|Traditional (large particle)||1- to 50-µm glass or silica||High-stress areas|
|Hybrid (large particle)||(1) 1- to 20-µm glass||High-stress areas requiring improved|
|(2) 40-nm silica||polishability (Classes I, II, III, IV)|
|Hybrid (midfilled)||(1) 0.1- to 10-µm glass||High-stress areas requiring improved|
|(2) 40-nm silica||polishability (Classes III, IV)|
|Hybrid (minifilled/SPF*)||(1) 0.1- to 2-µm glass||Moderate-stress areas requiring optimal polishability (Classes III, IV)|
|(2) 40-nm silica|
|Nanohybrid||(1) 0.1- to 2-µm glass or resin microparticles||Moderate-stress areas requiring optimal Polishability|
|(2) ≤100-nm nanoparticles||(Classes III, IV)|
|Packable hybrid||Midfilled/minifilled hybrid, but with lower filler fraction||Situations where improved condensability is needed (Classes I, II)|
|Flowable hybrid||Midfilled hybrid with finer particle size distribution||Situations where improved flow is needed and/or where access is difficult (Class II)|
|Homogeneous microfilled||40-nm silica||Low-stress and subgingival areas that require a high luster and polish|
|Heterogeneous microfilled||(1) 40-nm silica
(2) Prepolymerized resin particles containing 40-nm silica
|Low-stress and subgingival areas where reduced shrinkage is essential|
|Nanofilled composites||<100-nm silica or zirconia
Homogeneous independent nanoparticles or nanoclusters
|Anterior and noncontact posterior areas|
Dental composites are made up of three major components: a highly cross-linked polymeric resin matrix reinforced by a dispersion of glass, silica, crystalline, metal oxide or resin-reinforcing filler particles or their combinations and/or short fibers, which are bonded to the matrix by silane coupling agents. In addition, dental composites contain a number of other components, including an activator-initiator system that is required to convert the resin paste from a soft, moldable filling material to a hard, durable restoration. Pigments help to match the color of tooth structure. Ultraviolet (UV) absorbers and other additives improve color stability, and polymerization inhibitors extend storage life and provide increased working time for chemically activated resins. Other components may be included to enhance performance, appearance, and durability.
The resin matrix in most dental composites is based on a blend of aromatic and/or aliphatic dimethacrylate monomers such as bis-GMA (see Figure 7-16) and urethane dimethacrylate (UDMA, Figure 7-18) to form highly cross-linked, strong, rigid, and durable polymer structures (see Figures 7-3 and 13-16). This matrix forms a continuous phase in which the reinforcing filler is dispersed. Because of the large molecular volume of these monomers, polymerization shrinkage can be as low as 0.9% (average of 1.5% compared with a range of 2 to 3% for most composites) when combined with inorganic particulate fillers at levels of up to 88% by weight. However, UDMA and bis-GMA are highly viscous (800,000 centipoise, similar to honey on a cold day) and are difficult to blend and manipulate. Thus, it is necessary to use varying proportions of lower-molecular-weight highly fluid monomers such as triethylene glycol dimethacrylate (TEGDMA, 5 to 30 centipoise, Figure 7-17) and other lower-molecular-weight dimethacrylates to blend with and dilute the viscous components to attain resin pastes sufficiently fluid for clinical manipulation and for incorporating enough filler to reinforce the cured resin. For example, a blend of 75% bis-GMA and 25% TEGDMA by weight has a viscosity of 4300 centipoise, whereas the viscosity of a 50% bis-GMA/50% TEGDMA blend is 200 centipoise (similar to thin syrup). Unfortunately, these smaller, diluent monomers undergo greater polymerization shrinkage, partially offsetting the advantage of using large monomers such as bis-GMA. Generally the greater the proportion of these “diluting” monomers, the greater the polymerization shrinkage and the greater the risk of eventual leakage in marginal gaps and the problems that may result.
Various transparent mineral fillers are employed to strengthen and reinforce composites as well as to reduce curing shrinkage and thermal expansion (generally between 30% to 70% by volume or 50% to 85% by weight of a composite). These include so-called “soft glass” and borosilicate “hard glass”, fused quartz, aluminum silicate, lithium aluminum silicate (beta-eucryptite, which has a negative coefficient of thermal expansion), ytterbium fluoride, and barium (Ba), strontium (Sr), zirconium (Zr), and zinc glasses. The latter five types of fillers impart radiopacity because of their heavy metal atoms.
Quartz had been used extensively as a filler in the early versions of dental composites. It has the advantage of being chemically inert but it is also very hard, making it abrasive to opposing teeth or restorations as well as difficult to grind into very fine particles; thus, it is also difficult to polish. So-called amorphous silica has the same composition and refractive index as quartz; however, it is not crystalline and not as hard, thus, greatly reducing the abrasiveness of the composite surface structure and improving its polishability.
For acceptable esthetics, the translucency of a composite restoration must be similar to that of tooth structure. Thus, the index of refraction of the filler must closely match that of the resin. For bis-GMA and TEGDMA, the refractive indices are approximately 1.55 and 1.46, respectively, and a mixture of the two components in equal proportions by weight yields a refractive index of approximately 1.50. Most of the glasses and quartz used for fillers have refractive indices of approximately 1.50, which is adequate for sufficient translucency.
• Reinforcement. Increased filler loading generally increases physical and mechanical properties that determine clinical performance and durability, such as compressive strength, tensile strength, modulus of elasticity (i.e., stiffness or rigidity), and toughness. As the volume fraction of filler approaches approximately 70%, abrasion and fracture resistance are raised to levels approaching those of tooth tissue, thereby increasing both clinical performance and durability.
• Reduction of polymerization shrinkage/contraction. Increased filler loading reduces curing shrinkage in proportion to filler volume fraction. Although shrinkage varies from one commercial composite to another, it typically ranges from slightly less than 1% up to about 4% by volume.
• Reduction in thermal expansion and contraction. Increased filler loading decreases the overall coefficient of thermal expansion of the composite because glass and ceramic fillers thermally expand and contract less than do polymers. As the overall expansion coefficient decreases with filler loading and approaches that of tooth tissue, less interfacial stress is produced because of differential volumetric changes while an individual consumes hot and cold foods and beverages.
• Control of workability/viscosity. Fluid liquid monomer + filler → a paste. The more filler, the thicker is the paste. Filler loading, filler size, and the range of particle sizes and shapes all affect markedly the consistency of a composite paste, which in turn strongly affects clinical manipulation and handling properties, such as ease of mixing and sculpting, tackiness, and the tendency to either hold a shape or to minimize slumping. Consistency and handling are more than just convenience-related properties. They determine the ease of operation, the skill and time required, and also how reliably a cavity can be restored free of errors and with the proper interproximal contact, occlusal anatomy, smoothness, and appearance.
• Imparting radiopacity. Resins are inherently radiolucent. However, leaking margins, secondary caries, poor proximal contacts, wear of proximal surfaces, and other problems cannot be detected unless adequate radiographic contrast can be achieved. Thus, radiopacity is an important property. Radiopacity is most often imparted by adding certain glass filler particles containing heavy metal atoms, such as Ba, Sr, or Zn, and other heavy-metal/heavy-atom compounds such as YbF3, which strongly absorb x-rays. For optimal diagnostic contrast, the restoration should have a radiopacity approximately equal to that of enamel, which is about twice that of dentin. A wide range of radiopacity values have been considered to be adequate, but exceeding the radiopacity of enamel by a large degree will have the effect of obscuring radiolucent areas caused by gap formation or secondary caries.
The most commonly used glass filler is barium (Ba) glass. Although glass fillers containing metals of a high atomic number provide radiopacity, they are not as inert as quartz and amorphous silica and are slowly leached and weakened in acidic liquids such as citrus juices, high pH solutions, and other oral fluids. The glass filler is also attacked over time by caries-protective acidulated phosphate fluoride solutions or gels. Because of differences in the composition of saliva among patients, it is difficult to predict the clinical effects of exposure to saliva. However, the implication is that glass-filled composites will gradually become more susceptible to abrasive wear and hence they will have a shorter functional lifetime compared with silica-reinforced resins.
Most important properties are improved by increased filler loading. A distribution of particle sizes is used to maximize loading (volume fraction of filler). If particle size is uniform, no matter how tightly packed the particles are, spaces will exist among them, as illustrated by the example of the void spaces in a box filled with spheres of the same size. The maximal theoretical packing fraction for close-packed spherical structures of uniform-size is approximately 74% by volume, as shown in Figure 13-7. However, if smaller particles are inserted among the larger spheres, the void space can be reduced. By extending this process, a continuous distribution of progressively smaller particles can yield maximal filler loading.
Another advantage of using small particles is that they improve esthetics (appearance) and smoothness to the tongue (polishability). The traditional inorganic filler particles had average diameters of about 8 to 40 µm. Currently, small particles range from 0.005 µm to 2 µm. Particles larger than the wavelength of visible light cause light scattering. Scattering increases opacity and produces a visibly rough texture when the particles are exposed at the surface. A roughened surface also tends to accumulate stains and plaque. Curing produces an initially smooth surface, but finishing operations remove the resin matrix from the perimeter of filler particles, exposing particles that protrude from the surface. Oral wear mechanisms complement this process, such that the ultimate smoothness that can be maintained is highly dependent on particle size.
However, the smaller the filler particle size, the higher the surface-to-volume ratio available to form polar or hydrogen bonds with monomer molecules to inhibit their flow and increase viscosity (resistance to mixing and manipulation) thus, the less filler that can be added. The proportion of filler is limited to about 80% by volume. Therefore, there is always a tradeoff among the requirements for workability, durability, and esthetics. The clinician’s challenge is to make an informed judgment concerning the claims for the many products offered and to select one that is well suited to a particular clinical application.
Two general classifications are used to categorize resin-based composites, one based on the size and combination of sizes of the reinforcing filler particles and another is based on the manipulation characteristics of the filled monomer paste.
|Class of Filler||Particle size|
|Macrofillers||10 to 100 µm|
|Small/fine fillers||0.1 to 10 µm|
|Midfillers||1 to 10 µm|
|Minifillers||0.1 to 1 µm|
|Microfillers||0.01 to 0.1 µm (agglomerated)|
|Nanofillers||0.005 to 0.1 µm*|
Small (Fine) Particle Composites. Small-particle composites have mean particle diameters between 0.1 and 10 µm (minifiller and midifiller). These composites, while more polishable than traditional macrofilled composites (i.e., 10 to 100 µm), cannot be polished to a high gloss. However, filler loadings are as high as or higher (77% to 88%) than those of macrofilled composites, which provides a high degree of hardness and strength but also brittleness. Its excellent balance among polishability, appearance, and durability make this category suitable for general anterior use.
Microfilled composites are agglomerates of 0.01- to 0.1-µm inorganic colloidal silica particles embedded in 5- to 50-µm resin filler particles. The problems of surface roughening and low translucency associated with traditional and small-particle composites can be overcome through the use of colloidal silica particles such as the inorganic filler component, with a mean particle diameter about one tenth of the wavelength of visible light (i.e., about 40 nm). Such a filler is made by a pyrolytic precipitation process where a silicon compound such as SiCl4 is burned in an oxygen/hydrogen atmosphere to form macromolecular chains of colloidal silica (see Figure 13-4) resulting in amorphous silica (colloidal, noncrystalline SiO2), which produces highly polishable esthetic composite restorations.
However, these particles, because of their extremely small size, have extremely large surface areas ranging from 50 to 400 m2 per gram. In addition, the pyrolytic process results in particle “agglomeration” into long, molecular-scale chains (see Figures 13-5 and 13-6). These nondiscrete three-dimensional chainlike networks drastically increase monomer viscosity and make clinical manipulation difficult. Filler incorporation at high enough loading to adequately reinforce the resin is also difficult with these fillers. In fact, when incorporated directly in “homogeneous” microfill composites, only about 2% by weight produces a stiff paste that is much too viscous for clinical manipulation (see Figure 13-7). However, in the manufacturing process, a resin with a very high loading (60% to 70% by weight, about 50% by volume) of silane-treated colloidal silica microfiller can be added to the monomer at a slightly elevated temperature to lower its viscosity. This is cured and then pulverized to make a filled resin powder consisting of 5- to 50-µm particles. This amorphous colloidal silica-containing resin is then used as an “organic” filler (“heterogeneous” microfill composite), which is incorporated into the monomer with additional silane-treated colloidal silica to form a workable paste. In this way the overall inorganic filler content of the final, cured composite is increased to about 50% by weight. However, if the composite particles are counted as filler particles, the filler content is closer to 80% by weight (approximately 60% by volume). Thus, the resin particles are themselves composites that contain 0.01 to 0.1 µm of amorphous silica (see Figure 13-6). A diagram representing the preparation of the filler in microfilled resins of this type is shown in Figure 13-8.
One should note that because these composite particles do not shrink when the composite is cured, a microfilled composite, despite having a much lower inorganic filler loading than the traditional or small-particle composites, will not shrink as much as expected based on the total resin volume. However, a major shortcoming of these materials is that the bond between the composite particles and the clinically cured matrix is relatively weak, facilitating wear by a chipping mechanism (Figure 13-9). Because of this deficiency, with some notable exceptions, microfilled composites are not generally suitable for use as stress-bearing surfaces.
Whereas microfilled composites are among the more highly polishable restorative composites, their physical and mechanical properties are generally inferior to those of traditional composites (Table 13-3). This is to be expected, because 40% to 80% by volume of the restorative material is made up of resin, resulting in greater water sorption, a higher coefficient of thermal expansion, and decreased elastic modulus. In addition, the weak bond of the prepolymerized particles to the clinically cured resin matrix results in decreased tensile strength, similar to that of composites with nonsilanized filler particles. In addition, in the longer term, if microfilled composites are placed in wear-prone areas, they eventually break down and wear at a rate too fast for acceptable clinical performance. If placed in areas of proximal contact, anterior tooth “drifting” may occur. The wear process has also been related to fracture propagation around the poorly bonded “organic” filler particles. Thus, diamond burs, rather than fluted tungsten-carbide burs, are recommended for trimming microfilled composites so as to minimize the risk of chipping. Microfilled composites are the resins of choice for restoring teeth with carious lesions in smooth surfaces (classes III and V) but not in stress-bearing situations (classes II and IV).
|Characteristic/Property||Unfilled Acrylic||Traditional||Hybrid (Small-Particle)||Hybrid (All-Purpose)||Microfilled||Flowable Hybrid||Packable Hybrid||Enamel||Dentin|
|Inorganic filler (vol%)||0||60–70||65–77||60–65||20–59||30–55||48–67|
|Inorganic filler (wt%)||0||70–80||80–90||75–80||35–67||40–60||65–81||—||—|
|Compressive strength (MPa)||70||250–300||350–400||300–350||250–350||—||—||384||297|
|Tensile strength (MPa)||24||50–65||75–90||40–50||30–50||—||40–45||10||52|
|Elastic modulus (GPa)||2.4||8–15||15–20||11–15||3–6||4–8||3–13||84||18|
|Thermal expansion coefficient (ppm/°C)||92.8||25–35||19–26||30–40||50–60||—||—||—||—|
|Water sorption (mg/cm2)||1.7||0.5–0.7||0.5–0.6||0.5–0.7||1.4–1.7||—||—||—||—|
|Knoop hardness (KHN)||15||55||50–60||50–60||25–35||—||—||350–430||68|
|Curing shrinkage (vol%)||8–10||—||2–3||2–3||2–3||3–5||2–3||—||—|
|Radiopacity (mm Al)||0.1||2–3||2–3||2–4||0.5–2||1–4||2–3||2||1|
As the name implies, hybrid composites are formulated with mixed filler systems containing both microfine (0.01 to 0.1 µm) and fine (0.1 to 10 µm) particle fillers in an effort to obtain even better surface smoothness than that provided by the small-particle composites while still maintaining the desirable mechanical properties of the latter. Thus, they are a general utility class of composite that are also suitable for restoring certain high-stress-sites where esthetic considerations dominate—for example, incisal edges and small non-contact occlusal cavities. They are widely used for anterior restorations, including class IV sites (Figures 13-10 and 13-11/\).
Most modern hybrid fillers consist of colloidal silica and ground particles of glasses containing heavy metals, constituting a filler content of approximately 75% to 80% by weight (see Table 13-1). The glasses have an average particle size of about 0.4 to 1.0 µm, with a trend to steadily reduce this size range as improvements are made. In a typical size distribution, 75% of the ground particles are smaller than 1.0 µm and colloidal silica represents 10% to 20% by weight of the total filler content. The smaller microfiller sizes increase the surface area, which generally increases the viscosity and requires a decrease in overall filler loading as compared with small-particle composites. A polished surface is shown in Figure 13-12.
More recently, nanoparticles (1 to 100 nm) have been fabricated by a different method from the pyrolytic precipitation process used for colloidal silica. This allows the individual, primary particles to be surface-coated (with γ-methacryloxypropyltrimethoxysilane, for example) prior to becoming incorporated into three-dimensional macromolecule chains, thereby preventing or limiting particle agglomeration into large networks and driving up viscosity. In essence, the particle size is similar to that in microfilled composites, but the difference is that the particles in microfilled composites are in three-dimensional agglomerates or networks that increase viscosity whereas those in nanofilled composites are mostly discrete and have a minimal effect on viscosity. Thus, these composites have optical properties and superior polishability like those of microfilled composites, but the surface treatment reduces the increase in viscosity when incorporated into the monomer, which allows an increased filler loading of upwards of 60% by volume and 78% by weight. Filler loading in this range is expected to lead to the necessary mechanical properties for use in posterior, stress-bearing restorations, but these properties have not yet been confirmed.
One of the reasons that increased mechanical properties are not observed is that no commercial products have as yet been reported to contain only isolated, discrete, homogeneously dispersed nanoparticles as the sole filler component. In fact, few if any dental resins currently designated as nanocomposites meet the strict requirement of having essentially all filler less than 100 nm. This is because some of the nanoparticles exist as loosely bound “clusters” (loose agglomerates, such as in Filtek Supreme, 3M ESPE) of primary nanoparticles, which are sometimes reported to extend into the micron size range (e.g., 60 nm to 1.4 µm). Above 100 nm, clusters, like any particles, begin to scatter visible light and thereby reduce the translucency and the depth of cure of the composite. In addition, these clusters are not chemically bound to each other and act to decrease mechanical properties. Thus, while these nanocomposites with clusters have increased filler loading and hence better mechanical properties than a true homogeneous nanocomposite, they are not as strong as a hybrid composite or a microfilled composite.
To combat this deficiency, larger particles of either finely ground glass or precured nanoparticle-filled resin organic filler particles (essentially the same as those found in the microfilled composites) are combined with the monomer-dispersed nanoparticles. Thus, most, if not all, of these products are more accurately designated as “hybrid” nanocomposites or nanohybrids consisting of a blend of two or more size ranges of filler particles, one or more of which is in the nanoparticle range (see Table 13-1).