The majority of resin composites in clinical use today are categorized in the general term of ''hybrid composites.'' This broad category includes traditional hybrids, micro-, and nanohybrids. The ''hybrid'' moniker implies a resin composite blend containing submicron inorganic filler particles (.04 ^m) and small particles (1 |J,m-4 ^m). The combination of various sizes of filler particles corresponds to an improvement in physical properties as well as acceptable levels of polishability. These improvements in wear resistance and fracture strength, along with good polishability, make hybrids the material of choice for Class III and Class IV restorations. In addition, practitioners have used these traditional hybrids in posterior load-bearing surfaces such as Class I and Class II restorations because of their improved strength and wear resistance.
Recent improvements in filler technology by manufacturers have allowed blends of both submicron particles (0.04 ^m) and small particles (0.1 ^m-1.0 ^m) to be incorporated into a composite formulation. These materials are classified as micro-hybrid composites. The mixture of smaller particles distinguishes microhybrids from traditional hybrids and allows for a finer polish, along with improved handling. The desirable combination of strength and surface smoothness offers the clinician flexibility for use in posterior stress-bearing areas as well as anterior esthetic areas. Although microhybrids offer superior strength, their polishability is not better than a traditional microfilled composite resin. The trend in the newer microhybrid materials is to maximize filler loading and minimize filler size. The latest version of microfilled hybrids has used nanofiller technology to formulate what have been referred to as nanohybrid composite resins. Nanohybrids contain nanometer-sized filler particles (.005-.0l microns) throughout the resin matrix, in combination with a more conventional type filler technology. Nanohybrids may be classified as the first truly universal composite resin with handling properties and polishability of a microfilled composite, and the strength and wear resistance of a traditional hybrid. These nanohybrids can be used in any situation similar to the microhybrids, with possibly a slight improvement in polishability because of the smaller particle size [55, 74].
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