Dental composite resins are types of synthetic resins which are used in dentistry as restorative material or adhesives. Synthetic resins evolved as restorative materials since they were insoluble, aesthetic, insensitive to dehydration, easy to manipulate and reasonably inexpensive. Composite resins are most commonly composed of Bis-GMA monomers or some Bis-GMA analog, a filler material such as silica and in most current applications, a photoinitiator. Dimethacrylates are also commonly added to achieve certain physical properties such as flowability. Further tailoring of physical properties is achieved by formulating unique concentrations of each constituent. Unlike Amalgam which essentially just fills a hole and requires retention features to hold the filling, composite cavity restorations when used with dentin and enamel bonding techniques restore the tooth back to near its original physical integrity.
As with other composite materials, a dental composite typically consists of a resin-based oligomer matrix, such as a bisphenol A-glycidyl methacrylate (BISGMA) or urethane dimethacrylate (UDMA), and an inorganic filler such as silicon dioxide (silica). Compositions vary widely, with proprietary mixes of resins forming the matrix, as well as engineered filler glasses and glass ceramics. The filler gives the composite wear resistance and translucency. A coupling agent such as silane is used to enhance the bond between these two components. An initiator package (such as: camphorquinone (CQ), phenylpropanedione (PPD) or lucirin (TPO)) begins the polymerization reaction of the resins when external energy (light/heat, etc.) is applied. A catalyst package can control its speed.
How long have materials scientists sought a dental composite that doesn't shrink during the polymerization process? Bowman: From day one. Dr. Rafael Bowen introduced the first effective resin-based dental composite in the 1960s. It consisted of a methacrylate monomer system and silica powder as the filler material that bonds to and stabilizes the polymer. Methacrylate is a resin that is derived from methacrylic acid. Today, although the filler materials have evolved considerably, most dentists still use commercial dental composites that utilize Bowen’s original methacrylate monomer system or a close derivative thereof. There certainly are other fine monomer systems on the market or under development. But the problem of matrix shrinkage remains unsolved.
What causes the shrinkage? Stansbury: The laws of chemistry. As the monomers link, they become less frenetic and fit together more densely. This process is known as “double bond conversion,” or just “conversion” for short. What happens biochemically is the monomer’s reactive vinyl groups form covalent bonds, meaning they share one or more pairs of electrons and enjoin like links in a chain. This biochemical transformation causes the monomers to lose volume and shrink slightly.
How much does a composite shrink? Bowman: That’s a little difficult to answer, and here’s why. Nearly all dentists today aim a light source onto the soft composite to initiate the polymerization process inside the tooth. We call this “curing (curing light),” or hardening, the composite. Some of the most debated questions in the field today are: What is the ideal length of the cure? Or what is the ideal depth of cure and how much polymerization, or double-bond conversion, is ideal. So there are many technique-driven variables in play that influence the degree of shrinkage.