beneficial for differentiating the material between tooth tissues on radiographs for diagnosing secondary caries. The cost of the restoration is typically cheaper than composite restorations. Disadvantages of amalgam include poor aesthetic qualities due to its colour. Amalgam does not bond to tooth easily, hence it relies on mechanical forms of retention. Examples of this are undercuts, slots/grooves or root canal posts. In some cases this may necessitate excessive amounts of healthy tooth structure to be removed. Hence, alternative resin-based or glass-ionomer cement-based materials are used instead for smaller restorations including pit and small fissure caries. There is also a risk of marginal breakdown in the restorations. This could be due to corrosion which may result in "creep" and "ditching" of the restoration. Creep can be defined as the slow internal stressing and deformation of amalgam under stress. This effect is reduced by incorporating copper into amalgam alloys. Some patients may experience local sensitivity reactions to amalgam. Although the mercury in cured amalgam is not available as free mercury, concern of its toxicity has existed since the invention of amalgam as a dental material. It is banned or restricted in Norway, Sweden and Finland. See Dental Amalgam Controversy. Direct gold Direct gold fillings were practiced during the times of the Civil War in America. Although rarely used today, due to expense and specialized training requirements, gold foil can be used for direct dental restorations. Composite resin Dental composites, commonly described to patients as "white fillings", are a group of restorative materials used in dentistry. They can be used in direct restorations to fill in the cavities created by dental caries and trauma, minor buildup for restoring tooth wear (non-carious tooth surface loss) and filling in small gaps between teeth (labial veneer). Dental composites are also used as indirect restoration to make crowns and inlays in the laboratory. These materials are similar to those used in direct fillings and are tooth-colored. Their strength and durability is not as high as porcelain or metal restorations and they are more prone to wear and discolouration. As with other composite materials, a dental composite typically consists of a resin-based matrix, which contains a modified methacrylate or acrylate. Two examples of such commonly used monomers include bisphenol A-glycidyl methacrylate (BISMA) and urethane dimethacrylate (UDMA), together with tri-ethylene glycol dimethacrylate (TEGMA). TEGMA is a comonomer which can be used to control viscosity, as Bis GMA is a