Dental fillings are inserted as restorations in the treatment of dental cavities, after drilling out the cavities. The purpose of drilling is to remove the enamel and dentin that has had its structural integrity compromised by the invasion of acid-producing bacteria. However, once the infected hard tissues have been removed, the resulting cavity preparation must be filled in order to restore structural integrity to the tooth. This will prevent further damage to the tooth and hopefully avoid the eventual need for the tooth to be extracted.
Amalgam fillings are an alloy of mercury (from 43% to 54%) along with silver, tin, and copper. Mercury-based fillings were apparently first used by French dentists in the 1810s. They continue to be used in some countries because of their hardness and durability and because they are inexpensive. Mercury vapor is toxic, and the use of amalgam fillings is therefore controversial, as the fillings do emit mercury as a minute amount of vapor. Some government agencies, including the UN's World Health Organization and Centers for Disease Control and Prevention, claim that amalgam fillings are safe, even for pregnant women, children, and diabetics, except in rare cases of allergy. The Food and Drug Administration has never approved amalgam for use in dental fillings and is meanwhile opposing its use but amalgam fillings are legally considered "devices" and therefore outside the regulatory control of the FDA. See also: Dental amalgam controversy  Composite resin
Composite resin fillings are a mixture of powdered glass and plastic resin, and can be made to resemble the appearance of the natural tooth. They are strong and durable and cosmetically superior to silver or dark grey colored amalgam fillings. Composite resin fillings are usually more expensive than silver amalgam fillings. They contain Bisphenol A a known endocrine disrupter chemical. Most modern composite resins are light-cured, causing a polymerization reaction to occur within the material. Once the composite hardens completely, the filling 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. With proper technique and material selection, microleakage can be minimized or eliminated altogether. Besides the aesthetic advantage of composite fillings over amalgam fillings, the preparation of composite fillings requires less removal of tooth structure to achieve adequate bond strength. 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 composite over amalgam fillings when 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.  Glass Ionomer Cement
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; it is considered a fairly conservative procedure as the bare minimum of tooth structure should be removed. 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. Glass ionomers do have their advantages over composite resins: 1. They are not subject to shrinkage and microleakage, as the bonding mechanism is an acid-base reaction and not a polymerization reaction. 2. 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. 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.
A combination of glass-ionomer and composite resin, these fillings are a mixture of glass, an organic acid, and resin polymer that harden when light cured. (The light activates a catalyst in the cement that causes it to cure in seconds.) 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. In general, resin-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.
Porcelain fillings are hard, but can cause wear on opposing teeth. They are brittle and are not always recommended for molar fillings.
Porcelain Fused to Metal
These are metal shells with porcelain "enameled" on top, and is used for crowns. They are very durable.
Nickel or Cobalt-Chrome Alloys
These are mixtures of nickel and chromium and are used for crowns and bridges. They can be abrasive to opposing teeth and do conduct heat and cold, but have excellent durability.
Gold fillings have excellent durability, wear well, and do not cause excessive wear to the opposing teeth, but they do conduct heat and cold, which can be irritating. For years, they have been considered the gold standard as a restorative dental material. Recent advances in dental porcelains and consumer focus on aesthetic results have caused demand for gold fillings to drop in favor of advanced composites and porcelain veneers and crowns. Gold fillings are usually quite expensive, although they do last a very long time. It is not uncommon for a gold crown to last 30 years in a patient's mouth.
Other historical fillings
Lead fillings were used in the 1700s, but became unpopular in the 1800s because of their softness and because lead poisoning was understood. According to U.S. Civil War-era dental handbooks from the mid-1800s, since the early 1800s metallic fillings had been used, made of lead, gold, tin, platinum, silver, aluminum, or amalgam. A pellet was rolled slightly larger than the cavity, condensed into place with instruments, and then shaped and polished in the patient's mouth. The filling was usually left "high", with final condensation ? "tamping down" ? occurring through the patient's chewing of food. Gold was the preferred filling material during the Civil War, with amalgam being the most common due to cost. Tin was also popular due to cost, but was held in lower regard. One survey  of dental practices in the mid-1800s catalogued dental fillings found in the remains of seven Confederate soldiers from the U.S. Civil War; they were made of: Thorium ? radioactivity was unknown at that time, and the dentist probably thought he was working with tin Lead and tungsten mixture, probably coming from shotgun pellets Tin and iron Mercury amalgam Three soldiers had gold fillings
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