Composites generally are classified with respect to the components, amounts, properties of their filler or matrix phases, or by their handling properties. The most common classification method is based on filler content (weight or volume percent), filler particle size, and method of filler addition. Composites also could be defined on the basis of the matrix composition (BIS-GMA or UDMA) or polymerization method (self-curing, ultraviolet light-curing, visible light-curing, dual curing, or staged curing), but these do not communicate as much information about the properties. One of the most often used classification systems is based upon filler particle sizes. That system is extended here to include the particle size by order of magnitude, acknowledging mixed ranges of particle sizes, and distinguishing procured composite pieces as special filler. Composite filler particles are called macrofillers in the range of 10 to 100 |im, midifillers from 1 to 10 |im, minifillers from 0.1 to 1 |im, and microfillers 0.01 to 0.1 |J,m. Very large individual filler particles, called megafillers, also have been used in special circumstances. New ultrasmall fillers are being used that are from 0.005 to 0.01 ^m in diameter and are called nanofillers. Accordingly, composites are classified by particle size as megafill, macrofill, midifill, minifill, microfill, and nanofill. Composites with mixed ranges of particle sizes are called hybrids, and the largest particle size range is used to define the hybrid type (e.g., minifill hybrid) because microfillers are normally the second part of the mixture. If the composite simply consists of filler and uncured matrix material, it is classified as homogeneous. If it includes procured composite or other unusual filler, it is called heterogeneous. If it includes novel filler modifications in addition to conventional fillers, then it is called modified, such as fiber-modified homogeneous. Another consequence of advances in the control of filler particle size, particle size distribution, particle morphology, and monomer technology has been the introduction of composites with specific handling characteristics. These include flowable composites and packable composites. Flowable composites are a class of low-viscosity materials that possess particle sizes and particles size distrubutions similar to those of hybrid composites, but with reduced filler content, which allows the increased amount of resin to decrease the viscosity of the mixture. Packable composites, also referred to as condensable composites, were developed in a direct effort to produce a composite with handling characteristics similar to amalgam, thus the moniker of "packable" or "condensable". These "amalgam alternatives" are intended primarily for Class I and Class II restorations. For posterior composite restorations, it is also possible to place one or two large glass inserts (0.5-to 2-mm particles) into composites at points of occlusal contact or high wear. These pieces of glass are referred to as inserts (or megafillers). Although they have demonstrated improved wear resistance to contact area wear, the techniques are more complicated and do not totally eliminate contact frea area wear. Furthermore, the bonding of the composite to the insert is questionable [7, 16, 55].
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