Silicate Replacements

Dr George Paffenbarger (Figure 2-1, B) joined the scientists at the Research Unit in 1929 and quickly became the unit’s lead scientist. In 1985 the ADA renamed the Research Unit in Dr Paffenbarger’s honor. Dr Paffenbarger did considerable research in silicates and was strongly interested specifically in silicate cements. In the 1950s and early 1960s, silicates were the only good anterior restorative materials available. Dr Rafael Bowen (Figure 2-2) (the primary developer of dental composites) was a dentist in private practice in Southern California who had a hobby interest in chemistry. A major frustration he faced in his practice was that every time he placed silicate cement he had to explain to the patient that it would last for only a short time and would eventually have to be replaced. Bowen’s passion was to find a better material to replace the silicate cements. In his “back porch chemistry lab” he explored new technologies, such as other types of composites and other polymers such as epoxies. He made the first composites using porcelain by grinding up denture teeth with a mortar and pestle and mixing them into epoxy resins, forming the early prototype of dental composites. He tried these on a patient or two but quickly realized that the epoxy resin was not a good matrix material because the setting was slow, it was very difficult to work with, and the toxicity of some of its components was a concern. He then began synthesizing other types of restorative resins to use in place of epoxy. The first one he developed was bisphenol A glycidyl methacrylate (BIS-GMA). Paffenbarger took notice of this development through Bowen’s presentations and publication of his work and invited him to PRC.

FIGURE 2-2 Dr Rafael Bowen, best known for the development of dental composites based on dimethacrylate (BIS-GMA) resin chemistry. Dr Bowen’s long career at the American Dental Association Foundation’s Paffenbarger Research Center was entirely devoted to the development of improved resin-based adhesives and filling materials.

At that point Bowen began developing and perfecting the BIS-GMA–based composite, for which he is probably best known. He continued to work with the filler components of composite, even studying developments in other aggregate industries and how, for instance, concretes and asphalts were made. He viewed these as just a larger-scale composite and focused on how to make fillers efficiently compact together and bind to the matrix resins. He then applied his findings to dental materials. The materials further evolved through the development of a rapidly reacting catalyst and initiator system so these materials would be clinically convenient enough to use. The BIS-GMA matrix components have low toxicity as well as the necessary strength and toughness. The glass fillers and silane coupling systems that bind the filler to the matrix give the material its optical properties and additional strength and durability.

Evolution of the Components

Bowen’s original research has been improved on through many subsequent developments. The resin matrix in composite is being replaced with more and more filler material packed even more closely together. This requires both the ability to densely pack fillers with a range of particle sizes and the ability to make very small filler particles.

On the matrix side, less has changed until recent times, other than adding a few more resins to choose from in formulating. One major development related to initiators was the use of photoinitiators so that light-cured materials could be applied (Figure 2-3).

FIGURE 2-3 Examples of cordless curing lights. A, Radii Plus cordless curing light (SDI Limited, Victoria, Australia). B, Dentlight (Richardson, Texas).

Physical Properties of Importance

A number of physical properties are commonly measured in composites, some via the ISO Standards testing (Figures 2-8 and 2-9). Some of these properties have direct clinical relevance. The first one is strength. The most common strength measured is flexural strength (Figure 2-10), referring to how much the material will bend or flex before it breaks. It is probably the most relevant property clinically because, when considering how fillings support the forces of mastication, marginal ridges and occlusal tables must be able to support contact stresses. Flexural strength reflects the tensile strength of the material, and this refers to how much one can pull or bend a material before it will break. Most composites have flexural strengths in the 100- to 150-megapascal range. This appears to be the strength required for success from a clinical standpoint for composite restorations.

FIGURE 2-8 Examples of directions of loading. A, Uniaxial loading of cylinder. B, Uniaxial loading of a mesio-occlusal amalgam restoration.

(From Roberson TM, Heymann HO, Swift EJ: Sturdevant’s art and science of operative dentistry, ed 5, St Louis, 2006, Mosby.)

FIGURE 2-9 Measuring hardness.

FIGURE 2-10 Flexural bend strength measurement.

(Courtesy Instron, Norwood, Massachusetts.)

Another important property for polymers and other composites is how stiff they are—the elastic modulus (Figure 2-11). Stiffness measures indicate how much load a material will support before it starts to deform or how much it will resist deformation and bending. Restorations under loading should not flex significantly. All materials deform under load, but this deformation should not be great enough to break bonds and marginal seals. Ideally the restoration should be stiff enough to resist breaking the bond or the seal around the edges, a property that is also clinically relevant.

FIGURE 2-11 Measuring elastic modulus.

(Courtesy RTD, Saint Egreve, France.)

The third property measured in composites is toughness (Figure 2-12), which is really a measure of how well the material resists crack growth. All dental restorations sustain damage over time. They undergo loading with chewing, they fatigue, they form cracks, and they form surface defects. Toughness is the ability to resist the growth of those cracks or defects. Filling materials should have a fairly high degree of toughness. Most composites perform fairly well in this area, and all are much tougher than ceramics. If a ceramic has a small crack or defect, its lack of fracture toughness will allow that crack to grow quite easily, whereas a composite will better resist crack growth.

FIGURE 2-12 Measuring toughness using ElectroForce® 3200 test instruments.

(Courtesy Bose Corporation, ElectroForce Systems Group, Eden Prairie, Minnesota.)

Another relevant property is expansion (Figure 2-13). This includes both thermal expansion and hydrolytic expansion. The thermal expansion values of composites are usually about three times those of tooth structure. With hot and cold, composites expand and contract more than the surrounding tooth structure, putting stress on the adhesive interface. This does not affect the composite material per se, but it does alter the bonded interfaces and result in breakdown of adhesion.

FIGURE 2-13 Thermohydraulic expansion measurement performed on an ElectroPuls™ E10000 linear motor test instrument.

(Courtesy Instron®, Norwood, Massachusetts.)

Another type of expansion to consider is what happens during hydrolysis, as the composite material absorbs water. Hydrolytic expansion can be especially detrimental if the composite is being used as a cement. In some of the early resin-improved glass ionomer cements, too much expansion fractured restorations such as ceramic crowns. This property is commonly measured in standards testing.

Another important composite property is polymerization shrinkage (Figure 2-14). One of the inherent properties of a composite that uses methacrylate-based resins that undergo chain polymerization is that they transform from what are called van der Waals spacing to covalent bonds between the molecules. During this polymerization process the molecules actually move closer together, so the material physically shrinks. Composites each have a characteristic curing shrinkage, but shrinkage of the pure matrix resin is 8% to 10%. Typically, by the time all the filler is packed in, much of that resin has been displaced, which brings the shrinkage down to about 2% to 2.5%. As the material shrinks, it puts stress on bonds or can open up margins. Over the years much effort has been expended trying to develop materials with lower and lower shrinkage. When more of the resin is displaced, less shrinkage can result. More filler and less resin results in a lower-shrinking material. Shrinkage and the stress produced during shrinkage are also affected by the volume of material, the shape of the cavity preparation, how fast the materials cure, and how much they cure. Considerable effort has also gone into reducing shrinkage stress by controlling the curing rate of composites. Many factors enter into the curing process, but the lowest shrinking materials are generally in the 1% shrinkage range. No resin matrix composite has yet been able to totally eliminate polymerization shrinkage.

FIGURE 2-14 Polymerization shrinkage.

(Courtesy Dr Ray Bertolotti.)

Classes of Composite Materials

The classes of composites are generally based on the size of the filler particles. The earliest class of composites is referred to as macrofilled materials (Figure 2-15). These are not used much anymore and consist generally of composites made with large filler particles. Most of the early chemically cured macrofills used ground glass filler particles measuring up to 10 µm or more in average diameter.

FIGURE 2-15 Macrofill material.

Today hybrid or smaller-particle materials are most common (Figure 2-16). A hybrid contains different-sized filler particles ranging from very small submicron size to 2 or 3 µm in average diameter. This broad range of sizes allows hybrids to achieve extremely dense particle packing. Hybrids tend to be the most highly filled systems available today.

FIGURE 2-16 Hybrid material.

Microfill materials also have a mixture of particle sizes, but across a narrower range of diameters (Figure 2-17). The particles are 0.5 µm or less and can achieve a relatively high packing density. Because the particles are very small, it is also possible to finish and polish them very smoothly. They exhibit good optical characteristics, transmitting light quite well, and a/>