unicomposition causes removal of the gamma2 phase. advantages of high copper compared to low copper alloy better corrosion resistance. less susceptible to creep. greater strength. less tarnish and corrosion. greater longevity. amalgam vs. polymer resins amalgam is tolerant to a wide range of clinical placement conditions and moderately tolerant to the presence of moisture during placement. in contrast, the techniques for composite resin placement are more sensitive to many factors. mercury has properties of a bacteriostatic agent whereas certain methacrylate polymers (for example tegma, triethylene glycol methacrylate) composing the matrix of resin composites "encourages the growth of microorganisms". in the casa pia study in portugal (1986–1989), 1,748 posterior restorations were placed and 177 (10.1%) of them failed during the course of the study. recurrent marginal decay was the main reason for failure in both amalgam and composite restorations, accounting for 66% (32/48) and 88% (113/129), respectively. polymerization shrinkage, the shrinkage that occurs during the composite curing process, has been implicated as the primary reason for postoperative marginal leakage. however, there is low-quality evidence to suggest that resin composites lead to higher failure rates and risk of secondary caries than amalgam restorations. several reviews have been made by using database in the cochrane library where randomized controlled trials of few studies comparing dental resin composite with dental amalgams in permanent posterior teeth were compared. this review supports the fact that amalgam restorations are particularly useful and successful in parts of the world where amalgam is still the material of choice to restore posterior teeth with proximal caries. though, there is insufficient evidence to support or refute any adverse effects amalgam may have on patients, new research is unlikely to change opinion on its safety and due to the decision for a global phase-down of amalgam (minamata convention on mercury) general opinion on its safety is unlikely to change. these are some of the reasons why amalgam has remained a superior restorative material over resin-base composites. the new england children's amalgam trial (necat), a randomized controlled trial, yielded results "consistent with previous reports suggesting that the longevity of amalgam is higher than that of resin-based compomer in primary teeth and composites in permanent teeth. compomers were seven times as likely to require replacement and composites were seven times as likely to require repair. there are circumstances in which composite serves better than amalgam. for example, when a more conservative preparation would be beneficial, composite is the recommended restorative material. these situations would include small occlusal restorations, in which amalgam would require the removal of more sound tooth structure, as well as in "enamel sites beyond the height of contour". for cosmetic purposes, composite is preferred when a restoration is required on an immediately visible portion of a tooth. bonded amalgam dental amalgam does not by itself bond to tooth structure. this was recognized as a shortcoming by early practitioners such as baldwin. he recommended that the prepared cavity be coated with zinc phosphate cement just prior to filling with amalgam, in order to improve the seal and retention. the practice did not become universally accepted and eventually fell into disuse. until the 1980s, most amalgam restorations placed worldwide were done without adhesives, although in the 1970s a polycarboxylate-based adhesive liner was formulated specifically for this purpose in the mid-1980s the first reports of the use of resins to bond amalgam to etched tooth structure, much like is done for composite resins, appeared in the literature. since then, a number of papers have been published on laboratory as well as clinical studies of the technique. for large cavity restorations, features such as pins, slots, holes and grooves can be used for the retention of large amalgam restorations, but they do not reinforce the amalgam or increase its strength. there is no current scientific evidence to justify the extra cost and effort associated with the use of adhesively bonded amalgam restorations in comparison with nonbonded amalgam restorations. in view of the lack of evidence on the additional benefit of adhesively bonding amalgam compared with nonbonded amalgam, it is important that clinicians are mindful of the additional costs that may be incurred. liners and bases the placement of amalgam restorations can potentially cause sensitivity post-operatively. according to r. weiner, a protective layer or liner should be placed prior to the placement of amalgam to act as a buffer, helping to reduce sensitivity to the tooth. there are different liners that can be used in dental practices today, many of which contain zinc. examples of lining materials include zinc oxide eugenol, zinc phosphate, glass ionomer cement, zinc poly-carboxylate and resin. sealing amalgam restorations a varnish can be applied to the cavity wall to provide a good marginal seal. the varnish should be insoluble in water and is usually composed of a resin in a volatile solvent. when applied to the cavity, the solvent evaporates, leaving the resin behind to seal the dentinal tubules. the amalgam can then be packed into the cavity. dental amalgam toxicity main article: dental amalgam controversy concerns have been raised about the potential for mercury poisoning with dental amalgam when used in a dental filling. major health and professional organizations regard amalgam as safe but questions have been raised and acute but rare allergic reactions have been reported. critics argue that it has toxic effects that make it unsafe, both for the patient and perhaps even more so for the dental professional manipulating it during a restoration. a study by the life sciences research office found that studies on mercury vapor and dental amalgam "provided insufficient information to enable definitive conclusions." they identified several "research gaps", including: "well-controlled studies using standardized measures that evaluate whether low level produce neurotoxic and/or neuropsychological effect", studies on "co-exposure to hgo and methylmercury", studies on "in utero exposure to hgo", "occupational studies on with well-defined hgo exposure", studies on the absorption of hg2+ by the "human neonatal gut from breast milk", studies on "whether dental professionals have increased incidences of kidney disease, emotional instability, erethrism, pulmonary dysfunction, or other characteristics of occupational hgo exposure", studies on whether there exist "potential gender differences" or "genetic basis for sensitivity to mercury exposure." the removal of amalgam fillings is not recommended for reasons other than a true hypersensitivity to mercury. mercury levels in blood and urine have been shown to rise for a short period of time following the removal of amalgam restorations and no studies have demonstrated any health gain from restoration removal. removal involves exposure to mercury vapor released during the removal process. amalgams also contribute to mercury toxicity in the environment. with regard to amalgam placement and removal in pregnancy, research has not shown any adverse effects for the mother or fetus. however, research is inadequate to determine the chance of harm occurring and therefore placement and removal should be avoided during pregnancy if possible. in response to the minamata convention on mercury, the european commission has confirmed its position that individual nations should work to gradually scale down the use of dental amalgam. environmental impact and prevention of amalgam poisoning dental amalgam is thought to be relatively safe to be used as a restorative material as it is used in low doses. amalgam vapour can be released through chewing but this is minimal. however, there is an increased release of mercury following the exposure of electromagnetic fields generated by mri machines, although the small amount released is not thought to pose a risk to health. some patients may develop allergic reactions to it. resin composite, glass ionomer cements and ceramic or gold inlays can be used as alternatives to amalgam. u.s. amalgam disposal regulation amalgam separator in the united states, dental offices have typically disposed of amalgam waste down the drain. the wastewater is sent to the local sewage treatment plant, which is not designed to treat or recycle mercury or other heavy metals. the mercury contaminates the sludge processed at the treatment plant, and thereby can spread the mercury in surrounding communities, if the sludge is land-applied for disposal. dental amalgam is the largest source of mercury received by u.s. treatment plants. the u.s. environmental protection agency (epa) promulgated an effluent guidelines regulation in 2017 which prohibits most dental practices from disposing dental amalgam waste down the drain. most dental offices in the u.s. are required to use an amalgam separator in their drain system. the separator captures the waste material, which is then recycled. eu amalgam disposal regulation the european commission has issued a waste directive that classifies amalgam waste as a hazardous waste. the waste should be separated from other waste by fitting amalgam separators in all dental practices. avoidance in pregnant women mercury can cross the placenta leading to stillbirths and birth defects. although there is no evidence linking amalgam use and pregnancy damage, it is advisable to delay or avoid dealing with amalgam fillings in pregnant patients. awareness among dentists the dental operating team should deal with amalgam with proper use of personal protective equipment to protect themselves. a popular methodology for removal and replacement is the safe mercury amalgam removal technique or s.m.a.r.t. protocol. oral lesions some individuals have a sensitivity to amalgam and may develop oral lesions in which case a change of filling type is recommended.