and acrylic resins all benefit from the addition of radiopaque materials. examples of these materials include zinc oxide, zirconium dioxide, titanium dioxide, barium sulfate, and ytterbium(iii) fluoride. ideally, filling materials should match the surrounding tooth structure in shade, translucency, and texture. dental operators require materials that are easy to manipulate and shape, where the chemistry of any reactions that need to occur are predictable or controllable. direct restorative materials direct restorations are ones which are placed directly into a cavity on a tooth, and shaped to fit. the chemistry of the setting reaction for direct restorative materials is designed to be more biologically compatible. heat and byproducts generated cannot damage the tooth or patient, since the reaction needs to take place while in contact with the tooth during restoration. this ultimately limits the strength of the materials, since harder materials need more energy to manipulate. the type of filling (restorative) material used has a minor effect on how long they last. the majority of clinical studies indicate the annual failure rates (afrs) are between 1% and 3% with tooth colored fillings on back teeth. note that root canaled (endodontically) treated teeth have afr's between 2% and 12%. the main reasons for failure are cavities that occur around the filling and fracture of the real tooth. these are related to personal cavity risk and factors like grinding teeth (bruxism). amalgam main article: amalgam (dentistry) amalgam is a metallic filling material composed from a mixture of mercury (from 43% to 54%) and powdered alloy made mostly of silver, tin, zinc and copper, commonly called the amalgam alloy. amalgam does not adhere to tooth structure without the aid of cements or use of techniques which lock in the filling, using the same principles as a dovetail joint. amalgam is still used extensively in many parts of the world because of its cost effectiveness, superior strength and longevity. however, the metallic colour is not aesthetically pleasing and tooth coloured alternatives are continually emerging with increasingly comparable properties. due to the known toxicity of the element mercury, there is some controversy about the use of amalgams. the swedish government banned the use of mercury amalgam in june 2009. research has shown that, while amalgam use is controversial and may increase mercury levels in the human body, these levels are below safety threshold levels established by the who and the epa. however, there are certain subpopulations who, due to inherited genetic variabilities, exhibit sensitivity to mercury levels lower than these threshold levels. these particular individuals may experience adverse effects caused by amalgam restoration. these include neural defects, mainly caused by impaired neurotransmitter processing. composite resin main article: dental composite enamel and dentin shades of composite. other a2 universal shade for direct and indirect restorations, and flowable composite. composite resin fillings (also called white fillings) are a mixture of nanoparticles or powdered glass and plastic resin, and can be made to resemble the appearance of the natural tooth. although cosmetically superior to amalgam fillings, composite resin fillings are usually more expensive. bis-gma based resins contain bisphenol a, a known endocrine disrupter chemical, and may contribute to the development of breast cancer. however, it has been demonstrated that the extremely low levels of bis-gma released by composite restorations do not cause a significant increase in markers of renal injury, when compared to amalgam restorations. that is, there is no added risk of renal or endocrine injury in choosing composite restorations over amalgams. pex-based materials do not contain bisphenol a and are the least cytotoxic material available. most modern composite resins are light-cured photopolymers, meaning that they harden with light exposure. they can then be polished to achieve maximum aesthetic results. composite resins experience a very small amount of shrinkage upon curing, causing the material to pull away from the walls of the cavity preparation. this makes the tooth slightly more vulnerable to microleakage and recurrent decay. microleakage can be minimized or eliminated by utilizing proper handling techniques and appropriate material selection. in some circumstances, less tooth structure can be removed compared to preparation for other dental materials such as amalgam and many of the indirect methods of restoration. this is because composite resins bind to enamel (and dentin too, although not as well) via a micromechanical bond. as conservation of tooth structure is a key ingredient in tooth preservation, many dentists prefer placing materials like composite instead of amalgam fillings whenever possible. generally, composite fillings are used to fill a carious lesion involving highly visible areas (such as the central incisors or any other teeth that can be seen when smiling) or when conservation of tooth structure is a top priority. the bond of composite resin to tooth, is especially affected by moisture contamination and cleanliness of the prepared surface. other materials can be selected when restoring teeth where moisture control techniques are not effective. glass ionomer cement main article: glass ionomer cement the concept of using "smart" materials in dentistry has attracted a lot of attention in recent years. conventional glass-ionomer (gi) cements have a large number of applications in dentistry. they are biocompatible with the dental pulp to some extent. clinically, this material was initially used as a biomaterial to replace the lost osseous tissues in the human body. these fillings are a mixture of glass and an organic acid. although they are tooth-colored, glass ionomers vary in translucency. although glass ionomers can be used to achieve an aesthetic result, their aesthetic potential does not measure up to that provided by composite resins. the cavity preparation of a glass ionomer filling is the same as a composite resin. however, one of the advantages of gi compared to other restorative materials is that they can be placed in cavities without any need for bonding agents (4). conventional glass ionomers are chemically set via an acid-base reaction. upon mixing of the material components, there is no light cure needed to harden the material once placed in the cavity preparation. after the initial set, glass ionomers still need time to fully set and harden. advantages: glass ionomer can be placed in cavities without any need for bonding agents . they are not subject to shrinkage and microleakage, as the bonding mechanism is an acid-base reaction and not a polymerization reaction.(gics do not undergo great dimensional changes in a moist environment in response to heat or cold and it appears heating results only in water movement within the structure of the material. these exhibit shrinkage in a dry environment at temperature higher than 50c, which is similar to the behavior of dentin. glass ionomers contain and release fluoride, which is important to preventing carious lesions. furthermore, as glass ionomers release their fluoride, they can be "recharged" by the use of fluoride-containing toothpaste. hence, they can be used as a treatment modality for patients who are at high risk for caries. newer formulations of glass ionomers that contain light-cured resins can achieve a greater aesthetic result, but do not release fluoride as well as conventional glass ionomers. disadvantages: the most important disadvantage is lack of adequate strength and toughness. in an attempt to improve the mechanical properties of the conventional gi, resin-modified ionomers have been marketed. gics are usually weak after setting and are not stable in water; however, they become stronger with the progression of reactions and become more resistant to moisture. new generations: the aim is tissue regeneration and use of biomaterial in the form of a powder or solution is to induce local tissue repair. these bioactive materials release chemical agents in the form of dissolved ions or growth factors such as bone morphogenic protein, which stimulates activate cells. glass ionomers are about as expensive as composite resin. the fillings do not wear as well as composite resin fillings. still, they are generally considered good materials to use for root caries and for sealants. resin modified glass-ionomer cement (rmgic) a combination of glass-ionomer and composite resin, these fillings are a mixture of glass, an organic acid, and resin monomers that harden when light cured (light-activated polymerization besides the acid-base reaction of conventional glass-ionomer). the cost is similar to composite resin. it holds up better than glass ionomer, but not as well as composite resin, and is not recommended for biting surfaces of adult teeth, or when control of moisture cannot be achieved. generally, resin modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites. compomers main article: dental compomer another combination of composite resin and glass ionomer technology, with focus lying towards the composite resin end of the spectrum. compomers are essentially made up of filler, dimethacrylate monomer, difunctional resin, photo-activator and initiator, and hydrophilic monomers. the primary reason of the addition of filler is to decrease the proportion of resin and increase the mechanical strength besides improving the material's appearance. although compomers have better mechanical and aesthetic properties than rmgic, they have few disadvantages which limit their applications. compomers have weaker wear properties. compomers are not adhesive, therefore they require bonding materials. compomers themselves cannot adhere to the tooth tissue due to the presence of resin which can make it shrink on polymerisation. as a result, any bonding attempted will be disrupted at this stage. compomers release fluoride at low level, so they cannot act as a fluoride reservoir. compomers have high staining susceptibility. uptake of oral fluid causes them to show staining soon after placement. due to its relatively weaker mechanical properties, compomers are unfit for stress-bearing restorations but can be used in the deciduous dentition where lower loads are anticipated. cermets main article: dental cermets dental cermets, also known as silver cermets, were created to improve the wear resistance and hardness of glass ionomer cements (mentioned above) through the addition of silver. while the incorporation of silver achieved this, cermets have poorer aesthetics, appearing metallic rather than white. cermets also have a similar compressive strength, flexural strength, and solubility as glass ionomer cements, some of the main limiting factors for both materials. clinical studies have shown cermets perform poorly. all these disadvantages led to the decline in the use of this restorative material. below is a summary of the advantages and disadvantages of dental cermets. advantages: radio-opaque - this helps with identification of secondary caries when future radiographs are taken as there will be a greater contrast between the cermet and tooth tissue on the radiographic image adheres directly to tooth tissue higher wear resistance than glass ionomer cements (gics) greater hardness than gics disadvantages: low compressive strength low flexural strength soluble poor aesthetics poorer fluoride release than gics poor clinical performance indirect restorative materials a fabricated indirect restoration (inlay) made of porcelain indirect restorations are ones where the tooth or teeth to receive the restoration are first prepared, then a dental impression is taken and sent to a dental technician who fabricates the restoration according to the dentist's prescription. porcelain fillings are hard, but can cause wear on opposing teeth. they are brittle and are not always recommended for molar fillings.: 91–92 they are hard and rigid thus able to resist abrasion forces, are brittle due to surface irregularities, porosities, tendency to undergo static fatigue, and is good aesthetically as it mimics appearance of natural teeth due to various levels of shades.: 91–92 porcelain materials can be strengthened by soaking fired material in molten salt to allow exchange of sodium and potassium ions on the surface as this successfully create compressive stresses on the outer layer, by controlling cooling after firing, and by the use of pure alumina inserts, a core of alumina or alumina powder, as they act as crack stoppers and are highly compatible to porcelain.: 91–92 tooth colored dental composite materials are either used as direct filling or as construction material of an indirect inlay. it is usually cured by light. nano-ceramic particles embedded in a resin matrix, are less brittle and therefore less likely to crack, or chip, than all-ceramic indirect fillings; they absorb the shock of chewing more like natural teeth, and more like resin or gold fillings, than do ceramic fillings; and at the same time more resistant to wear than all-resin indirect fillings. these are available in blocks for use with cad-cam systems. gold fillings have excellent durability, wear well, and do not cause excessive