14.4. Dental Restoration

A dental restoration or dental filling is a treatment to restore the function, integrity, and morphology of missing tooth structure resulting from caries or external trauma. A restoration can be direct and indirect and it is classified by location and size. A root canal filling, for example, is a restorative technique used to fill the space where the dental pulp normally resides.

Tooth preparation

Restoring a tooth to good form and function requires two steps: (1) preparing the tooth for placement of restorative material or materials, and (2) placement of these materials. The process of preparation usually involves removing the decayed material from the tooth with a rotary dental handpiece and dental burrs. If permanent restoration cannot be carried out immediately after tooth preparation, temporary restoration may be performed.

A number of considerations will determine the type and extent of the preparation. The most important factor to consider is decay. The extent of the decay greatly influences the extent of the preparation, and in turn, the subsequent method and appropriate materials for restoration. Another consideration is unsupported tooth structure. When preparing the tooth to receive a restoration, unsupported enamel is removed to allow for a more predictable restoration. While enamel is the hardest substance in the human body, it is particularly brittle, and unsupported enamel fractures easily.

Direct restorations

This technique involves placing a soft or malleable filling into the prepared tooth and building up the tooth. The materials used are most commonly gold, amalgam, dental composites, glass ionomer cement, or porcelain. The material is then set hard and the tooth is restored. The advantage of direct restorations is that they usually set quickly and can be placed in a single procedure. The decision of which material to use is usually made based on the location and severity of the cavity.

Indirect restorations

In this technique the restoration is fabricated outside of the mouth using dental impressions of the prepared tooth. Common indirect restorations include inlays and onlays, crowns, bridges, and veneers. Usually a dental technician fabricates the indirect restoration from records the dentist has provided. The most commonly used materials are gold and ceramics. The finished restoration is bonded permanently with a dental cement. The treatment is often done in at least two separate visits to the dentist because time is needed for the fabrication of the restoration after the impression is obtained. While the indirect restoration is being prepared, a temporary restoration is used to cover the prepared tooth and allow the patient to bite. This usually consists of a softer dental cement that can easily be removed with the dental drill.

Removable dental prostheses (mainly dentures) are a form of indirect dental restoration, as they are made to replace missing teeth. There are numerous types of precision attachments (also known as combined restorations) to aid removable prosthetic attachment to teeth, including magnets, clips, hooks, and implants which may themselves be seen as a form of dental restoration.

Materials used

Casting alloys are mostly used for making crowns, bridges and dentures. Titanium, usually commercially pure but sometimes a 90% alloy, is used as the anchor for dental implants as it is biocompatible and can integrate into bone.

Metals and alloys

• gold (high purity: 99.7%) and gold alloys (with high gold content)

• gold-platinum alloy

• silver alloy

• cobalt-chrome alloy

• nickel-chrome alloy

• Amalgam: 65% Silver, 29% Tin, 6% Copper, 2% Zinc, 3% Mercury

Dental Amalgam is widely used because it is easily made into rigid direct fillings, completed in single appointment, with acceptable strength, hardness, corrosion, and toxicity properties. Its preparation is more forgiving than that of composite resins. Amalgam is now mainly used for posterior teeth (less visible) because of its gray color. Although the mercury in cured amalgam is not available as free mercury, concern about its toxicity has existed since its invention as a dental material. It is banned or restricted in Norway, Sweden and Finland.

Composite resin

Dental composites, also called "white fillings", are a group of restorative materials used in dentistry. In addition to fillings done at the clinic, composite resins are also used to manufacture crowns and in-lays in the laboratory. The greatest advantage of these materials is that they can be made tooth-colored. Their strength and durability is not as high as porcelain or metal restorations and they are more prone to wear and discoloration. Compositions vary widely, with proprietary mixes of resins forming the matrix, as well as engineered filler glasses and glass ceramics.

An initiator substance begins the polymerization reaction of the resin when light is applied at a specific wavelength (460-480 nm blue light) to yield necessary free radicals and start the process. This process is quick and safe, but it require the composites to be applied in thin layers as determined by their opacity. Immediately after light induced curing, the final surface can be be shaped and polished.

Glass ionomer cement

A glass ionomer cement (GIC) is one of a class of materials commonly used in dentistry as filling materials and luting cements. These materials are based on the reaction of silicate glass powder and polyalkenoic acid. These materials can also have the color of the tooth and they were used to restore anterior teeth before composite resins were developed.

As they bond chemically to dental hard tissues and release fluoride for a relatively long period, modern-day applications of GICs have expanded. The desirable properties of glass ionomer cements make them useful in restorations at low-stress areas on primary teeth. They do not need to be applied layer by layer, like in composite fillings.

Porcelain (ceramics)

Various types of dental porcelain (ceramic fired at high temperatures) are used in indirect fillings and crowns. They are also used as in-lays, on-lays, and aesthetic veneers. A veneer is a very thin shell of porcelain that can replace or cover part of the enamel of the tooth. Full-porcelain restorations are particularly desirable because their color and translucency mimic natural tooth enamel. Porcelain can be combined with metal to provide strength to a crown or bridge. These restorations are more durable than porcelain used alone.

Comparison of filling materials

• Composites and amalgam are used mainly for direct restoration. Amalgam is more durable but composites can be made to match the color of the tooth and the surface of the restoration can be polished after the procedure has been completed.

• Amalgam fillings expand with age, possibly cracking the tooth and requiring filling replacement.

• Composite fillings shrink with age and become prone to infiltration. If not noticed early, this results in new decay.

• Fillings are recommended for small to medium size restorations.

• Inlays and onlays are more expensive indirect restoration alternatives to direct fillings. They should be more durable than composite direct fillings, however.

• Porcelain, cobalt-chrome, and gold are used for indirect restorations like crowns and partial coverage crowns (onlays). Traditional porcelains are brittle and are not always recommended for molar restorations.

Complications

Irritation of the pulp

When a deep cavity is filled, the vibration, heat and chemical stimulation that the tooth receives may irritate the pulp and generate pulpitis (inflammation of the pulp). This can result in pain when biting on the treated tooth or sensitivity to cold and hot substances. It may settle down spontaneously with time. If not, root canal treatment is usually conducted to resolve the pain and save the tooth.

Weakening of tooth structure

In situations where a relatively larger amount of tooth structure has been lost or replaced with a filling material, the overall strength of the tooth may be affected. This significantly increases the risk of the tooth fracturing in the future when excess force is placed on the tooth, such as trauma or grinding teeth at night. The standard treatment of large cavities and root canal therapy in premolars and molar includes sealing with a crown because this reduces the risk of fracture.

Summary

Dental restoration involves tooth preparation and filling. The preparation is mostly influenced by the extent of decay and the structural state of the tooth. All decayed tissues and healthy tissues without structural support are removed. Many options are available for filling materials and procedures. Direct restorations are produced in the tooth, whereas indirect restorations are produced in the lab and cemented onto the tooth. Considerations in the choice of materials and techniques include durability, cost, appearance, position, size of the cavity and type of tooth (deciduous or permanent). Composite resin and amalgam are most common in direct fillings whereas gold and ceramic are most common in crowns.

Key terms

Dental filling, tooth preparation, direct filling, burr, temporary restoration, permanent, restoration, indirect filling, amalgam, gold, composite resin, glass ionomer cement, dental, porcelain, dental ceramic, inlay, onlay, dental crown, bridge, veneer, denture, pulpitis.

Figure credits

Figure 1 by DRosenbach - en:File:Undermined4.jpg (originally uploaded to the English Wikipedia), CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=32609065

Figure 2 by Jeffrey Dorfman - http://www.nycdentist.com/dental-photos/40/Bonding-teeth-white-tooth-fillings-dental-bonded-composite-resin, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17746002

Figure 3 by Jeffrey Dorfman - http://www.nycdentist.com/dental-photo-detail/2466/70/Crown-dental-porcelain-ceramic-cap-PFM-also-see-Bridge, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17746159