In dentistry, a crown or a dental cap is a type of dental restoration that completely caps or encircles a tooth or dental implant. A crown may be needed when a large dental cavity threatens the health of a tooth. Some dentists will also finish root canal treatment by covering the exposed tooth with a crown. A crown is typically bonded to the tooth by dental cement. They can be made from various materials, which are usually fabricated using indirect methods. Crowns are used to improve the strength or appearance of teeth and to halt deterioration. While beneficial to dental health, the procedure and materials can be costly.
The most common method of crowning a tooth involves taking a dental impression of a tooth prepared by a dentist, then fabricating the crown outside of the mouth. The crown can then be inserted at a subsequent dental appointment. This indirect method of tooth restoration allows use of strong restorative material requiring time-consuming fabrication under intense heat, such as casting metal or firing porcelain, that would not be possible inside the mouth. Because of its compatible thermal expansion, relatively similar cost, and cosmetic difference, some patients choose to have their crown fabricated with gold.
Computer technology is increasingly employed for crown fabrication in CAD/CAM dentistry.
Indications for dental crowns
Crowns are indicated to: Replace existing crowns which have failed. Restore the form, function and appearance of badly broken down, worn or fractured teeth, where other simpler forms of restorations are unsuitable or have been found to fail clinically. Improve the aesthetics of unsightly teeth which cannot be managed by simpler cosmetic and restorative procedures. Maintain the structural stability and reduce the risk of fractures of extensively restored teeth including those which have been endodontically treated. Restore the visible portion of a single dental implant.
Restoration of endodontically treated teeth
Traditionally, it has been proposed that teeth which have undergone root canal treatment are more likely to fracture and therefore require cuspal protection by providing occlusal coverage with an indirect restoration like crowns. This led to routine prescribing of crowns for root-treated teeth. However, recent review of literature reveals that there is no strong evidence to show that crowns are better than other routine restorations to restore root-filled teeth. The general advice is that dentists should use their clinical experience in view of the patient's preferences when making the decision of using a crown.Fedorowicz, Z., Carter, B., de Souza, R. F., Chaves, C. A., Nasser, M., & Sequeira-Byron, P. (2012). Single crowns versus conventional fillings for the restoration of root filled teeth. Cochrane Database Syst Rev, 5. As a rule of thumb, the use of crowns and other indirect restorations for root treated teeth is justified when the surface area of the access cavity exceeds one third of the occlusal surface of the tooth, when the lingual or buccal walls are undermined or when the mesial and distal marginal ridges are missing.
Clinical stages of dental crown provision
Assessment Choice of restoration Tooth preparation Construction and fit of temporary restoration Tooth preparation impressions Fit of definitive restoration Short-term follow up Long-term follow up
Assessment In order to ensure optimum condition and longevity for the proposed crowns, several factors need to be explored by conducting a thorough and targeted patient history and clinical dental examination. These factors include: Patient factors Patient expectations Patient motivation to adhere to the treatment plan and maintain results Financial and time costs to the patient Biological factors Periodontal health status and periodontal disease risk Pulpal health and endodontic disease risk Caries and caries risk Occlusion and occlusal problems risk Mechanical factors Amount of remaining tooth structure Height and width of tooth to be prepared Attachment levels of the tooth to be prepared Root shape and length of the tooth to be prepared Aesthetic factors
Choice of restoration
The choice(s) of crown restoration can be described by: The dimensions and percentage coverage of the natural crown Full crowns 3/4 and 7/8 crowns Material to be used Metal Metal-ceramic crowns Full ceramic crowns
3/4 and 7/8 crowns These restorations are a hybrid between an onlay and a full crown. They are named based on the estimated wall coverage of the walls of the tooth; e.g. the 3/4 crown aims to cover three out of the four walls, with the buccal wall being usually spared, thus reducing sound tooth tissue to be prepared. They are normally fabricated in gold. Grooves or boxes are normally added to the preparations as close to the unprepared wall as possible to increase retention of the crown. Despite its advantages of reducing sound tooth preparation, these crowns are not commonly prescribed in practice because they are technically difficult and have poor patient acceptability due to the metal showing through in their smile.
Full metal crowns As the name suggests, these crowns are entirely cast in a metal alloy. There are a multitude of alloys available and the selection of a particular alloy over another depends on several factors including cost, handling, physical properties, biocompatibility. The American Dental Association categorizes alloys in three groups: high-noble, noble and base metal alloys.
High-noble and noble alloys Noble and high-noble alloys used in casting crowns are generally based on alloys of gold. Gold is not used in its pure form as it is too soft and has poor mechanical strength. Other metals included in gold alloys are copper, platinum, palladium, zinc, indium and nickel. All types of gold casting alloys used in prosthodontics (Type I - IV) are categorised by their percentage content of gold and hardness, with Type I being the softest and Type IV the hardest. Generally, Type III and IV alloys (62 - 78% and 60 - 70% gold content respectively) are used in casting of full crowns, as these are hard enough to withstand occlusal forces. Gold crowns (also known as gold shell crowns) are generally indicated for posterior teeth due to aesthetic reasons. They are durable in function and strong in thin sections, therefore require minimal tooth preparation. They also have similar wear properties to enamel, so they are not likely to cause excessive wear to the opposing tooth. They have good dimensional accuracy when cast which minimises chair-side/appointment time and can be relatively easy to polish if any changes are required. Palladium based alloys are also used. These were introduced as a cheaper alternative to gold alloys in the 1970s. Palladium has a strong whitening effect giving most of its alloys a silverish appearance.
Base-metal alloys Cast base-metal alloys are rarely used to make full metal crowns. They are more commonly used as part of metal-ceramic crowns as bonding alloys. When compared to high-noble and noble alloys, they are stronger and harder; they can be used in thinner sections (0.3mm as opposed to 0.5mm) however they are harder to adjust and are more likely to cause excessive wear on real opposing teeth. Furthermore, there may be problems with people who have a nickel allergy.
Common base-metal alloys used in dentistry are: Silver-palladium Silver-palladium-copper Nickel-chromium Nickel-chromium-beryllium Cobalt-chromium Titanium
Titanium Titanium and titanium alloys are highly biocompatible. Its strength, rigidity and ductility are similar to that of other casting alloys used in dentistry. Titanium also readily forms an oxide layer on its surface which gives it anti-corrosive properties and allows it to bond to ceramics, a useful property in the manufacture of metal-ceramic crowns.
Full ceramic crowns
Dental ceramics or porcelains are used for crowns manufactured primarily for their aesthetic properties compared to all metal restorations. These materials are generally quite brittle and prone to fracture. Many classifications have been used to categorise dental ceramics, with the simplest, based on the material from which they are made, i.e. silica, alumina or zirconia.
Silica
Silica-based ceramics are highly aesthetic due to their high glass content and excellent optical properties due to the addition of filler particles which enhance opalescence, fluorescence which can mimic the colour of natural enamel and dentine. These ceramics, however, suffer from poor mechanical strength, and therefore often used for veneering stronger substructures.
Examples include aluminosilicate glass, e.g. feldspathic, synthetic porcelain, and leucite reinforced ceramics.
Mechanical properties can be improved by the addition of filler particles, e.g. lithium disilicate, and are therefore termed glass ceramics. Glass-ceramics can be used alone to make all-ceramic restorations either as a single form (termed uni-layered) or can act as a substructures for subsequent veneering (or layering) with weaker feldspathic porcelain (restorations termed bi-layered).
Alumina Alumina (aluminium oxide) was introduced as a dental substructure (core) in 1989 when the material was slip cast, sintered, and infiltrated with glass. More recently, glass-infiltrated alumina cores are produced by electrophoretic deposition, a rapid nanofabrication process. During this process, particles of a slip are brought to the surface of a dental die by an electric current, thereby forming a precision-fitting core greenbody in seconds. Margins are then trimmed and the greenbody is sintered and infiltrated with glass. Glass-infiltrated alumina has significantly higher porcelain bond strength over CAD/CAM produced zirconia and alumina cores without glass.
Alumina cores without glass are produced by milling pre-sintered blocks of the material utilizing a CAD/CAM dentistry technique. Cores without glass must be oversized to compensate for shrinkage that occurs when the core is fully sintered. Milled cores are then sintered and shrink to the correct size.
All alumina cores are layered with tooth tissue-like feldspathic porcelain to make true-to-life color and shape. Dental artists called ceramists, can customize the "look" of these crowns to individual patient and dentist requirements. Alumina cores have better translucency than zirconia, but worse than lithium disilicate.
Zirconia
Yttria-stabilized zirconia, also known simply as zirconia, is a very hard ceramic that is used as a strong base material in some full ceramic restorations. Zirconia is relatively new in dentistry and the published clinical data is correspondingly limited. The zirconia used in dentistry is zirconium oxide (ZrO2) which has been stabilized with the addition of yttrium oxide. Yttria-stabilized zirconia is also known as YSZ.