Crown (dentistry)

In dentistry, a crown most commonly refers to a dental cap, a type of dental restoration that completely caps or encircles a tooth or dental implant. A crown may be needed when a large cavity threatens the health of a tooth. 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. 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
# Review

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 categories 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 manufacture 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 nanofabricating 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.

The zirconia substructure (core) is usually designed on a digital representation of the patient's mouth, which is captured with a three-dimensional digital scan of the patient, impression, or model. The core is then milled from a block of zirconia in a soft pre-sintered state. Once milled, the zirconia is sintered in a furnace where it shrinks by 20% and reaches its full strength of 850–1000 MPa. Recently, the strength of zirconia for dental restorations reaching 1200 MPa is reported. The zirconia core structure can be layered with tooth tissue-like feldspathic porcelain to create the final color and shape of the tooth. Because bond strength of layered porcelain fused to zirconia is not strong; chipping of the conventional veneering ceramic frequently occurs, crowns and bridges are nowadays increasingly made with monolithic zirconia crowns produced from a color and structure graded zirconia block, and coated with a thin layer of glaze stains. Esthetic prosthetic restorations, with natural reflection, color from within and color gradients influenced by the internal dentinal core anatomy can best be accomplished by veneered zirconia, rather than with crowns of monolithic zirconia. In the production of dental restorations specifically made for one patient, dental technicians with their problem-solving skills, dexterity and cognitive skills are until recently the only way to provide the required esthetics, individuality and artistry with porcelain. Fear for chipping of conventional mono glass component zirconia porcelains on the longer term and price pressure on manual application of porcelain, are possible drivers for the monolithic zirconia restorations. However, by the application of multi-glass component porcelain chipping is no longer an issue, especially with prosthetic mimetic restorations where the crown follows a model of the natural tooth in two layers: a histo-anatomic dentin layer mimicking the dentin shape of the dentition of the patient and an enamel layer. These restorations that mimic the structure of natural teeth by cognitive design of the dentin core present a new production paradigm to fabricate natural restorations of veneered zirconia using a high strength porcelain with CAD/CAM. These crowns are produced with a core of tooth-colored tetragonal zirconia, on which a high strength translucent porcelain layer has been applied and subsequently milled to size. In the subtle cooperation between the dentin-colored zirconia and the veneering porcelain, the zirconia shines through the translucent porcelain layer, all the more as the porcelain layer is thinner. This creates the natural color dynamics with color "from the inside" as found in natural elements, instead of color "on the outside", with monolithic zirconia. As a result, the natural tooth, in terms of esthetics and hardness, is approached closer than crowns made from solid monolithic zirconia. This implies that the histo-anatomic dentin core is the key to esthetic crowns.

Zirconia is the hardest known ceramic in industry and the strongest material used in dentistry, it has to be fabricated using a CAD/CAM process but not the conventional manual dental technology. Because of this monolithic zirconia does not wear itself as the normal vertical wear of 25-75 microns of natural enamel and porcelain, there are no clinical data on the fact whether as a consequence too high zirconia crowns will damage opposing dentition on the longer term. Although in two body wear testing of monolithic, veneered and glazed zirconia and their corresponding enamel antagonists showed similar wear, at least twice as much extensive, and branched enamel microcracks were observed in the samples opposing monolithic zirconia.

Monolithic zirconia

Monolithic zirconia crowns tend to be opaque in appearance with a high value and they lack translucency and fluorescence. For the sake of appearance, many dentists will not use monolithic crowns on anterior (front) teeth. Monolithic zirconia crowns are produced from a color- and structure-graded zirconia block and coated with a thin layer of glaze stains which also provides some kind of fluorescence. The "graded" zirconia crown has a darker cervical area consisting of tetragonal zirconia, a main tooth color in the buccal area, and a translucent incisal edge consisting of cubic zirconia. The only thing a dental technician has to do is to use the proper height of the zirconia block so that the crown fits in all the different color zones. Although on the outside the color gradient mimics natural teeth, they are still far from the optical, physical, biomimetic and esthetic properties of natural teeth.

To a large extent, materials selection in dentistry determine the strength and appearance of a crown. Some monolithic zirconia materials produce the strongest crowns in dentistry (the registered strength for some zirconia crown materials is near 1200 MPa), but these crowns are not usually considered to be natural enough for use in the front of the mouth. Although not as strong, some of the newer zirconia materials are better in appearance but generally still not as good as porcelain-fused crowns. By contrast, when porcelain is fused to glass-infiltrated alumina, crowns are very natural-looking and very strong, though not as strong as monolithic zirconia crowns.

Zirconia crowns are said to be less abrasive to opposing teeth than metal-ceramic crowns.

Other crown material properties to be considered are thermal conductivity and radiolucency. Stability/looseness of fit on the prepared tooth and cement gap at the margin are sometimes related to materials selection, though these crown properties are also commonly related to system and fabricating procedures.

Lithium-disilicate

Another monolithic material, lithium disilicate, produces extremely translucent leucite-reinforced crowns that often appear to be too gray in the mouth. To overcome this, the light shade polyvalent colorants take on a distinctly unnatural, bright white appearance.

Metal-ceramic crowns (P-F-M Crown)

These are a hybrid of metal and ceramic crowns. The metal part is normally made of a base metal alloy (termed bonding alloy). The properties of the metal alloy chosen should match and complement that of the ceramic to be bonded otherwise problems like delamination or fracturing of the ceramic can occur. To obtain an aesthetic finish which is able to be functional with normal mastication activity, a minimal thickness of ceramic and metallic material is required, which should be planned for during tooth preparation stage.

Ceramic bonds to the metal framework by three methods:
* Compression fit (via ceramic shrinkage on firing)
* Micro-mechanical retention (via surface irregularities)
* Chemical union (via oxide formation)

Tissue control and gingival retraction

Gingival retraction refers to the displacement of the free gingivae. For crowns with margins which are supragingival, there is no need for gingival retraction, provided there is good moisture control.

For crown preparations which have subgingival margins, tissue control is necessary at the preparation stage and impression stage to ensure visibility, good moisture control and ensure enough bulk of impression material can be placed to accurately record the marginal areas.

Options available are gingival retraction cord, Magic Foam cord, and ExpaSyl.

Another method to expose the margins of a subgingival preparation is to use electrosurgery or crown lengthening surgery.

Tooth preparation

The design of a preparation for a tooth to accept a crown follows five basic principles:

# Retention and resistance
# Preservation of tooth structure
# Structural durability
# Marginal integrity
# Preservation of the periodontium

Aesthetics can also play a role in planning the design.

Retention and resistance

As there are currently no biologically compatible cements which are able to hold the crown in place solely through their adhesive properties, the geometric form of the preparation is vital in providing retention and resistance to hold the crown in place. Within the context of prosthodontics, ''retention'' refers to resistance of movement of a restoration along the path of insertion or along the long axis of the tooth. ''Resistance'' refers to the resistance of movement of the crown by forces applied apically or in an oblique direction which prevents movement under occlusal forces. Retention is determined by the relationship between opposing surfaces of the preparation (e.g. the relationship of the buccal and lingual walls).

Taper

Theoretically, the more parallel the opposing walls of a preparation, the more retention is achieved. However this is almost impossible to achieve clinically. It is standard for preparations for full coverage crowns to slightly taper or converge in an occlusal direction. This allows the preparation to be visually inspected, prevent undercuts, compensate for crown fabrication inaccuracies and allow, at the cementation stage, for excess cement to escape with the ultimate aim of optimising the seating of the crown on the preparation. Generally axial walls prepared using a long tapered high speed burs confer a 2 - 3° taper on each wall and an overall 4 - 6° taper to the preparation. As taper increases, retention decreases so taper should be kept to a minimum whilst ensuring elimination of undercuts. An overall taper of 16° is said to be clinically achievable and being able to fulfil the aforesaid requirements. Ideally, the taper should not exceed 20 degrees as will negatively impact retention.

Length

Occluso-gingival length or height of the crown preparation affects both resistance and retention. Generally, the taller the preparation, the greater the surface area is. For the crown to be retentive enough, the length of the preparation must be greater than the height formed by the arc of the cast pivoting around a point on the margin on the opposite side of the restoration. The arc is affected by the diameter of the tooth prepared, therefore the smaller the diameter, the shorter the length of the crown needs to be to resist removal. Retention of short-walled teeth with a wide diameter can be improved by placing grooves in the axial walls, which has the effect of reducing the size of the arc.

Freedom of displacement

Retention can be improved by geometrically limiting the number of paths along which the crown can be removed from the tooth presentation, with maximum retention being reached when only one path of displacement is present. Resistance can be improved by inserting components like grooves.

Preservation of tooth structure

Preparing a tooth to accept a full coverage crown is relatively destructive. The procedure can damage the pulp irreversibly, through mechanical, thermal and chemical trauma and making the pulp more susceptible to bacterial invasion. Therefore, preparations must be as conservative as possible, whilst producing a strong retentive restoration. Although it may be seen as contradictory to the previous statement, at times, sound tooth structure may need to be sacrificed in order to prevent further more substantial and uncontrolled loss of tooth structure.

Structural durability

In order to last, the crown must be made of enough material to withstand normal masticatory function and should be contained within the space created by the tooth preparation, otherwise problems may arise with aesthetics and occlusal stability (i.e. high restorations) and cause periodontal inflammation. Depending on the material used to create the crown, minimal occlusal and axial reductions are required to house the crown.

Occlusal reduction

For gold alloys there should be 1.5mm clearance, whilst metal-ceramic crowns and full ceramic crowns require 2.0 mm. The occlusal clearance should follow the natural outline of the tooth; otherwise there may be areas of the restorations where the material may be too thin.

Functional cusp bevel

For posterior teeth, a wide bevel is required on the functional cusps, palatal cusps for maxillary teeth and buccal cusps for mandibular teeth. If this functional cusp bevel is not present and the crown is cast to replicate the correct size of the tooth, bulk of material may be too little at this point to withstand occlusal surfaces.

Axial reduction

This should allow enough thickness for the material chosen. Depending on the type of crown to be fitted, there is a minimum preparation thickness. Generally, full metal crowns require at least 0.5mm, whist metal-ceramic and full ceramic crowns require at least 1.2mm

Marginal integrity

In order for the cast restoration to last in the oral environment and to protect the underlying tooth structure, the margins between cast and tooth preparation need to be as closely adapted. The marginal line design and position should facilitate plaque control, allow for adequate thickness of the restorative material chosen therefore providing enough strength for the crown at the margin. Several types of finish line configurations have been advocated, each having some advantages and disadvantages (see the table below). Chamfer finish are normally advocated for full metal margins and shoulders are generally required to provide enough bulk for metal-ceramic crowns and full ceramic crown margins. Some evidence suggests adding a bevel to margins, especially where these are heavy, to decrease the distance between the crown and the tooth tissue.

Preservation of the periodontium

Linked to marginal integrity, placement of the finish line can directly affect the ease of manufacturing the crown and health of the periodontium. Best results are achieved where the finish line is above the gum line as this is fully cleanable. They should also be placed on enamel as this creates a better seal. Where circumstances require the margins to be below the gum line, caution is required as several problems can arise. First, there might be issues in terms of capturing the margin when making impressions during the manufacturing process leading to inaccuracies. Secondly, the biologic width, the mandatory distance (roughly 2 mm) to be left between the height of the alveolar bone and the margin of the restoration; if this distance is violated, it can result in gingival inflammation with pocket formation, gingival recession

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