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.

FIGURE 13-1 Chronology of dental composite developments in monomer, filler, bonding, and curing technologies. (Adapted from Figure 5 in Bayne SC: Dental biomaterials: Where are we and where are we going? *J Dent Educ* 69[5]:571−585 2005.)

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.

FIGURE 13-2 Ground quartz filler particles with diameters of about 1 to 30 µm. Such relatively large fillers were used in early formulations of traditional composites. The smaller particles seen in the background contribute to a broad particle size distribution. (Courtesy of K-J.M. Söderholm.)

FIGURE 13-3 Typical particles of a small-particle-filled composite, with sizes in the range of 0.1 to 10 µm. (Courtesy of E.A. Glasspoole and R.L. Erickson.)

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.

Applications

Dental applications for resin-based composites include cavity and crown restoration materials, adhesive bonding agents, pit and fissure sealants, endodontic sealants, bonding of ceramic veneers, and cementation for crowns, bridges, and other fixed prostheses.

Classification

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.

TABLE 13-1

Classification of Resin-Based Composites and Indications for Use

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

^*SPF, small particle−filled.

Composition and Function

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.

Critical Questions

What are the three essential components of resin composite materials?

What are the potential uses of composites in dentistry?

What roles do coupling agents, fillers, bis-GMA, and other high-molecular-weight dimethacrylate monomers play in the function and performance of dental resins?

Matrix

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.

Critical Question

What are the problems that result from shrinkage and cure-induced stress during the polymerization of restorative resins?

TABLE 13-2

Classification of Reinforcing Filler Particles by Size Range

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*

^*5 to 100 nm, nonagglomerated.

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.

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.

FIGURE 13-4 Pyrogenic reaction showing the initial formation of fumed silica particles in the 40-nm size range, as used in microfill resins. (Courtesy of K-J.M. Söderholm.)

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.

FIGURE 13-5 Transmission electron microscope image of fumed silica particles produced by the pyrogenic reaction shown in Figure 13-4. The particle diameters average approximately 0.04 µm or 40 nm. (Courtesy of K-J.M. Söderholm.)

FIGURE 13-6 The pyrogenic silica used in microfilled composites has a very large total surface area because of its extremely small average particle size of about 0.04 µm/40 nm. These particles agglomerate and form long chains, as seen in this transmission electron micrograph. The chains of silica particles act similarly to resin polymer chains, as described in Chapter 6, and dramatically increase the viscosity of the monomer. (Courtesy of K-J.M. Söderholm.)

FIGURE 13-7 The effect of filler specific surface area on the viscosity (log scale) of the uncured resin matrix. The smaller the particle, the larger is the surface area and the greater the viscosity buildup. (Adapted from Darvell BW: *Materials Science for Dentistry*, ed 9, Woodhead Publishing Ltd, Cambridge, UK, 2009.)

FIGURE 13-8 Preparation of resin filler particles for use in microfilled composites. The filler particles in a microfilled composite consist of pulverized “composite filler particles” dispersed in a cured resin matrix. Pyrogenic colloidal silica particles of about 0.04 µm (about 40 nm) are incorporated into both the precured resin filler particles and the curable monomer, with the precured resin containing a substantially higher concentration. (Modified from P. Lambrechts: *Basic Properties of Dental Composites and Their Impact on Clinical Performance*. Thesis. Leuven, Belgium, Katholieke University, 1983.)

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.

FIGURE 13-9 Fractured microfilled composite. The fractured surface shows that the organic filler (composite filler) particles have been plucked out of the matrix resin, suggesting adhesive failure due to a weak interface between the precured microfill particles and the clinically cured resin. (Courtesy of K-J.M. Söderholm.)

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).

TABLE 13-3

Properties of Composite Restorative Materials

Characteristic/Property	Unfilled Acrylic	Traditional	Hybrid (Small-Particle)	Hybrid (All-Purpose)	Microfilled	Flowable Hybrid	Packable Hybrid	Enamel	Dentin
Size (µm)	—	8–12	0.5–3	0.4–1.0	0.04–0.4	0.6–1.0	Fibrous	—	—
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

Critical Questions

Why is the polymerization shrinkage of microfilled composites is not higher than that of conventional composites even though the inorganic filler loading in microfills is substantially lower?

Which properties of microfilled resins are generally inferior to those of other resin-based composite materials, and what are the clinical implications of these deficiencies?

What features of microfilled resins make them the material of choice for certain types of restorations?

Which types of restorations are produced ideally from microfilled composites?

What advantages are conferred by combining filler particles of two or more size ranges?

What are the clinical consequences of this filler combination?

Hybrid Composites.

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/\).

FIGURE 13-10 Fractured maxillary central incisor prior to restoration with a hybrid composite. (Courtesy of William Rose.)

FIGURE 13-11 Class IV restoration made with a hybrid composite. This restoration of the incisal area of the tooth produced a highly aesthetic esthetic result. (Courtesy of William Rose.)

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.