Dental restorative materials

This page is about types of dental restorative materials. For dental fillings see dental restorations

Dental restorative materials are specially fabricated materials, designed for use as dental restorations (fillings), which are used to restore tooth structure loss, usually resulting from but not limited to dental caries (dental cavities). There are many challenges for the physical properties of the ideal dental restorative material.

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Restorative material development

The goal of research and development is to develop the ideal restorative material. The ideal restorative material would be identical to natural tooth structure, in strength adherence and appearance. The properties of an ideal filling material can be divided into four categories: physical properties, biocompatibility, aesthetics and application

Physical properties

The physical properties include heat insulation, resistance to different categories of forces, and wear, bond strength, and chemical resistance. The material needs to withstand everyday forces and conditions on it without fatiguing.

Biocompatibility

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 allergies, chemical leakage from the material, and pulpal irritation. Some of the byproducts of the chemical reactions during different stages of material hardening need to be considered.

Aesthetics

Filling materials ideally would match the surrounding tooth structure in shade translucency and texture.

Application

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

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.

Amalgam

Amalgam fillings, (also called silver fillings) are a mixture of mercury (from 43% to 54%) and powdered alloy made mostly of silver, tin, zinc and copper commonly called the amalgam alloy.^[1], Due to the known toxicity of the element mercury, there is some controversy about the use of amalgams. see Amalgam controversy

History

The Chinese were the first to use a silver amalgam to fill teeth in the 7th century; in 1816, Auguste Taveau developed his own dental amalgam from silver coins and mercury. This amalgam contained a very small amount of mercury and had to be heated in order for the silver to dissolve at an appreciable rate. Taveau's formula offered lower cost and greater ease of use compared to existing materials such as gold, but had many practical problems, including a tendency to significantly expand after setting. Because of these problems, this formula was abandoned in France. In 1833, however, two untrained Europeans, the Crawcour brothers, brought Taveau's amalgam to the United States under the name "Royal Mineral Succedaneum"^[2]

Aesthetics

Most of the patients don't like the silvery glossy appearance of dental amalgams to be visible. So dental amalgams are mostly not used for the restoration of incisors or canines. Composites are given preference in such cases.

Gamma 2 phase amalgams

After widespread adoption and wildly varying standards, the multitude of formulas for making amalgams were standardised into the gamma-2-phase amalgam formula in 1895.

The gamma-2-phase amalgams contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:

• > 65% silver (Ag)
• < 29% tin (Sn)
• < 6% copper (Cu)
• < 2% zinc (Zn)
• < 3% mercury (Hg)

The resulting amalgam is composed of the gamma phase (the silver-tin eutectic Ag₃Sn, which reacts with mercury, yielding the gamma-1 phase (Ag₂Hg₃) and gamma-2 phase (Sn_7-8Hg). The gamma phase is prone to corrosion and its mechanical strength is low. The alloy tends to undergo crevice corrosion and form local galvanic cells.

Around 1970, the ingredients changed to the new non-gamma-2 form, with lower manufacturing cost, greater mechanical strength, and better corrosion resistance. The reduced-gamma-2 amalgams (sometimes referred to as "high-copper" amalgams) contain approximately equal parts 50% of liquid mercury and 50% of an alloy powder containing:

• > 40% silver (Ag)
• < 32% tin (Sn)
• < 30% copper (Cu)
• < 2% zinc (Zn)
• < 3% mercury (Hg)

The amalgam alloy is strengthened by presence of Ag-Cu particles. The gamma-2 phase reacts with the Ag-Cu particles to form eta phase Cu₆Sn₅ and gamma-1 phase.

The possible difference in toxicology between the two has not been studied conclusively. Amalgams continue to be used today because they are hard, durable and inexpensive.

Galvanic shock

When aluminium foil makes contact with some amalgam fillings, saliva in the mouth can act as an electrolyte. This can generate small electrical currents which are felt through the nerves in the tooth as (often extremely painful) electrical "jolts" or shocks.

Composite resin (also called white or plastic filling)

Main article: Dental composite

Composite resin fillings (also called white 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, durable and cosmetically superior to silver or dark grey colored amalgam fillings. Composite resin fillings are usually more expensive than silver amalgam fillings. Bis-GMA based materials contain Bisphenol A, a known endocrine disrupter chemical, and may contribute to the development of breast cancer. PEX-based materials do not.

Most modern composite resins are light-cured photopolymers. 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 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 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.

Composite resin fillings require a clean and dry surface to bond correctly with the tooth, so cavities in areas that are harder to keep totally dry during the filling procedure may require a less moisture-sensitive filling. The use of a rubber dam is highly recommended.

Glass Ionomer Cement

See main article 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.

Resin modified Glass-Ionomer Cement

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 modified glass-ionomer cements can achieve a better aesthetic result than conventional glass ionomers, but not as good as pure composites.

Indirect Restorative materials

Porcelain (ceramic)

Porcelain fillings are hard, but can cause wear on opposing teeth. They are brittle and are not always recommended for molar fillings.

Gold

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 of gold fillings, 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. 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. This was before 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, 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 and preferred 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-1800s catalogued dental fillings found in the remains of seven Confederate soldiers from the U.S. Civil War; they were made of:

• Gold foil: Preferred because of its durability and safety.
• Platinum: Was rarely used because it was too hard, inflexible and difficult to form into foil.
• Aluminum: A material which 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, however tin wore down rapidly and even if it could be replaced cheaply and quickly, there was a concern, specifically from Harris, that it would oxidise in the mouth and thus cause a recurrence of caries. Due to the blackening, tin was only recommended for posterior teeth.
• 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. Lead was rarely used in the 19th century, it is soft and quickly worn down by mastication, and had known harmful health effects.
• Amalgam: The most popular amalgam was a mixture of silver, tin and mercury. According to the authors of the article " It set very hard and lasted for many years, the major contradiction being that it oxidized in the mouth, turning teeth black. Also the mercury contained in the amalgam was thought at that time to be harmful." as explained in the pre-eminent dental textbook of that century, The Principles and Practice of Dental Surgery by Chapin A. Harris A.M., M.D., D.D.S.. ^[3]

Replacement fillings

Fillings have a finite lifespan: an average of 12.8 years for amalgam and 7.8 years for composite resins ^[4]. Fillings fail because of changes in the filling, tooth or the bond between them.

Amalgam fillings expand with age, possibly cracking the tooth and requiring repair and filling replacement. Composite fillings shrink with age and may pull away from the tooth allowing leakage. As chewing applies considerable pressure on the tooth, the filling may crack, allowing seepage and eventual decay in the tooth underneath.

The tooth itself may be weakened by the filling and crack under the pressure of chewing. That will require further repairs to the tooth and replacement of the filling.

If fillings leak or the original bond inadequate, the bond may fail even if the filling and tooth are otherwise unchanged.

See also

• Anesthesia
• Dental x-ray
• Dental caries
• Dentistry
• Oral hygiene
• Tooth brushing

References

• Dental Materials Fact Sheet, Dental Board of California, May 2004