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composite resin spectrum of restorative materials used in dentistry. Towards the GIC end of the spectrum, there is increasing fluoride release and increasing acid-base content; towards the composite resin end of the spectrum, there is increasing light cure percentage and increased flexural strength.
Dental restorative materials are used to replace tooth structure loss, usually due to
dental caries (cavities), but also
tooth wear and
dental trauma. On other occasions, such materials may be used for
cosmetic purposes to alter the appearance of an individual's teeth. There are many challenges for the physical properties of the ideal dental restorative material. The ideal material would be identical to natural tooth structure in strength, adherence, and appearance. The properties of such material can be divided into four categories: physical properties,
biocompatibility, aesthetics and application. • Physical properties of good restorative materials include low thermal conductivity and expansion, resistance to different categories of forces and wear such as
attrition and
abrasion, and resistance to chemical erosion. There must also be good bonding strength to the tooth. Everyday
masticatory forces and conditions must be withstood without material fatigue. • Biocompatibility refers to how well the material coexists with the biological equilibrium of the tooth and body systems. Since
fillings are in close contact with
mucosa, tooth, and pulp, biocompatibility is very important. Common problems with some of the current dental materials include chemical leakage from the material, pulpal irritation and, less commonly, allergic reactions. Some of the byproducts of the chemical reactions during different stages of material hardening need to be considered. • Radiopacity in dental materials is an important property that allows for distinguishing restorations from teeth and surrounding structures, assessing the absorption of materials into bone structure, and detecting cement dissolution or other failures that could cause harm to the patient. Cements, composites, endodontic sealers, bone grafts, 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 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. Root canaled (endodontically) treated teeth have AFRs 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 Amalgam is a metallic filling material composed from a mixture of
mercury (from 43% to 54%) and a 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 with increasingly comparable properties have been developed. Due to the known toxicity of 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 may increase mercury levels in the human body, these levels are below safety threshold levels established by the
World Health Organization and the U.S.
Environmental Protection Agency. However, there exist subpopulations whose members, because of inherited genetic variabilities, are more sensitive to mercury than these threshold levels. They may experience adverse effects caused by amalgam restoration, including neural defects caused by impaired
neurotransmitter processing.
Composite resin 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, there is no added risk of
kidney or
endocrine injury in choosing composite restorations over amalgams. or when control of moisture cannot be achieved. Generally, RMGICs can achieve a better aesthetic result than conventional GICs, but not as good as pure composites.
Compomers Another combination of composite resin and GIC technology, compomers are essentially made up of filler, dimethacrylate monomer, difunctional resin, photo-activator and initiator, and hydrophilic monomers. The filler decreases the proportion of resin and increases the mechanical strength, as well as improving the material's appearance. Although compomers have better mechanical and aesthetic properties than RMGIC, they have some disadvantages which limit their applications: • Compomers have weaker wear properties. • They cannot adhere to tooth tissue due to the presence of resin, which can make it shrink on polymerisation. They therefore require bonding materials. • They release low levels of fluoride, so cannot act as a fluoride reservoir. • They 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 Dental cermets, also known as silver cermets, were created to improve the wear resistance and hardness of glass ionomer cements by adding
silver. Their other advantages are that they adhere directly to tooth tissue, and are
radio-opaque, which helps with identification of
secondary caries when future
radiographs are taken. However, cermets have poorer aesthetics, appearing metallic rather than white. They also have a similar
compressive strength,
flexural strength, and
solubility as GICs, some of the main limiting factors for both materials. In addition, their
fluoride release is poorer than that of GICs. Clinical studies have shown cermets perform poorly. All these disadvantages led to the decline in the use of this restorative material.
Indirect restorative materials An indirect restoration is one where the teeth are first prepared, then an
impression is taken and sent to a
dental technician who fabricates the restoration according to the dentist's prescription.
Porcelain Porcelain fillings are hard, but can cause wear on opposing teeth. Their hardness and rigidity enables them to resist abrasion forces, and are good aesthetically as they mimic the appearance of natural teeth.
Nano-ceramic particles 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; at the same time they are more resistant to wear than all-resin indirect fillings. They are available in blocks for use with
CAD/CAM systems.
Gold fillings 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. There are two categories: cast gold fillings (gold inlays and onlays) made with 14 or 18 kt gold, and gold foil made with pure 24 kt gold that is burnished layer by layer. For years, they have been considered the benchmark of restorative dental materials. However, recent advances in dental porcelains and a consumer focus on aesthetic results have caused the demand for gold fillings to drop. Gold fillings are sometimes quite expensive, but they last a very long time, meaning that gold restorations are less costly and painful in the long run. It is not uncommon for a gold crown to last 30 years.
Other historical fillings Lead fillings were used in the 18th century, but became unpopular in the 19th century because of their softness. This was before
lead poisoning was understood. According to
American Civil War-era dental handbooks, since the early 19th century metallic fillings had been made of
lead,
gold,
tin,
platinum,
silver,
aluminum, or
amalgam. A pellet was rolled slightly larger than the cavity, condensed into place with instruments, then shaped and polished in the patient's mouth. The filling was usually left "high", with final condensation—"tamping down"—occurring while the patient chewed food. Gold foil was the most popular filling material during the Civil War. Tin and amalgam were also popular due to lower cost, but were held in lower regard. One survey of dental practices in the mid-19th century catalogued dental fillings found in the remains of seven
Confederate soldiers from the Civil War. They were made of: • Gold foil: preferred because of its durability and safety. • Platinum: rarely used because it was too hard, inflexible and difficult to form into foil. • Aluminum: failed because of its lack of malleability but has been added to some amalgams. • Tin and iron: believed to have been a very popular filling material during the Civil War. Tin foil was recommended when a cheaper material than gold was requested by the patient, but it wore down rapidly; even if it could be replaced cheaply and quickly, there was a concern, specifically from
Chapin A. Harris, that it would
oxidise in the mouth and cause a recurrence of caries. Due to blackening, tin was only recommended for
posterior teeth. •
Thorium: the element's radioactivity was unknown at that time, and the dentist probably thought he was working with tin. • Lead and tungsten mixture: probably from shotgun pellets. Lead was rarely used in the 19th century, as it is soft and quickly worn down by mastication, and had known harmful health effects.
Acrylic polymers Acrylics are used in the fabrication of
dentures, artificial teeth, impression trays, maxillofacial /
orthodontic appliances and
temporary (provisional) restorations. They cannot be used as tooth filling materials because they can lead to
pulpitis and
periodontitis, as they may generate heat and acids during setting, and in addition they shrink.
Failure of dental restorations Fillings have a finite lifespan; composites appear to have a higher failure rate than amalgam over five to seven years. How well people keep their teeth clean and avoid cavities is probably a more important factor than the material chosen for the restoration.
Evaluation and regulation of dental materials The
Nordic Institute of Dental Materials (NIOM) performs several tests to evaluate dental products in the
Nordic countries. In the European Union, dental materials are classified as medical devices according to the
Medical Devices Directive. In USA, the
Food and Drug Administration is the regulatory body for dental products. == References ==