A crown, sometimes known as dental cap, is a type of dental restoration which completely caps or encircles a tooth or dental implant. Crowns are often needed when a large cavity threatens the ongoing health of a tooth. They are typically bonded to the tooth using a dental cement. Crowns can be made from many materials, which are usually fabricated using indirect methods. Crowns are often used to improve the strength or appearance of teeth. While inarguably beneficial to dental health, the procedure and materials can be relatively expensive.
The most common method of crowning a tooth involves using a dental impression of a prepared tooth by a dentist to fabricate the crown outside of the mouth. The crown can then be inserted at a subsequent dental appointment. Using this indirect method of tooth restoration allows use of strong restorative materials requiring time-consuming fabrication methods requiring intense heat, such as casting metal or firing porcelain which would not be possible to complete inside the mouth. Because of the expansion properties, the relatively similar material costs, and the cosmetic benefit, many patients choose to have their crown fabricated with gold.
As new technology and materials science has evolved, computers are increasingly becoming a part of crown fabrication, such as in CAD/CAM dentistry.
Maps, Directions, and Place Reviews
Clinical aims of dental crowns
As with any prosthodontic restoration, the aim of crowning a tooth is to restore function or to prevent further dysfunction. Generally, one of these requirements should be fulfilled when restoring a tooth with a crown:
- Maintain occlusal stability (i.e. preventing drifting, tiliting or over-eruption of teeth)
- Maintaining masticatory (or chewing) ability
- Aesthetic reasons (especially if it affects self-esteem or other psychological factors)
- Maintain or reinstate correct phonetics
Indications for dental crowns
Crowns are normally used to:
- To 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.
- To improve the aesthetics of unsightly teeth which cannot be managed by simpler cosmetic procedures.
- To maintain the structural stability and reduce the risk of fractures of extensively restored teeth including like teeth which have been root filled, especially posterior teeth which are subjected to higher occlusal forces.
- To restore a dental implant
As there is still no strong evidence in current literature that crowns are better than other routine restorations to restore root-filled teeth, dentists are still advised to use their clinical experience in view of the patient's preferences when making the decision of using a crown.
Crown materials
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 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 seldomly 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 oppose to 0.5mm) however they are harder to adjust and are more likely to cause excessive wear on real opposing teeth. Furthermore, there can be a problem with people with 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
See also: Dental porcelain
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 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 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 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. Today, porcelain fused to alumina crowns set the standard for tooth-like appearance.
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 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 3d 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 850MPa to 1000MPa.
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, "monolithic" zirconia crowns are often made entirely of the zirconia ceramic with no tooth tissue-like porcelain layered on top. Zirconia is the hardest known ceramic in industry and the strongest material used in de
ntistry. 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.
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 1000MPa.), but these crowns are not usually considered to be natural enough for teeth in the front of the mouth; though not as strong, some of the newer zirconia materials are of better appearance, but they are still not generally as good as porcelain fused crowns. When porcelain is fused to the zirconia core, these crowns are more natural than the monolithic zirconia crowns but they are not strong. 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. Another monolithic material, lithium-disilicate, produces extremely translucent leucite-reinforced crowns that often appear to be too gray in the mouth, and to overcome this, the light shade polyvalent colorants take on a distinctly unnatural, bright white appearance. 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.
Zinconia crowns are said to be less abrasive to opposing teeth than metal-ceramic crowns.
Metal-ceramic crowns
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)
Principles of 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 are 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 sacrificing 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 contain 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 and loss of alveolar bone crest height. In these cases, crown lengthening surgery should be considered.
Special considerations for endodontically treated teeth
Ferrule effect
A very important consideration when restoring with a crown is the incorporation of the ferrule effect. As with the bristles of a broom, which are grasped by a ferrule when attached to the broomstick, the crown should envelop a certain height of tooth structure to properly protect the tooth from fracture after being prepared for a crown. This has been established through multiple experiments as a mandatory continuous circumferential height of 2 mm; any less provides for a significantly higher failure rate of endodontically-treated crown-restored teeth. When a tooth is not endodontically treated, the remaining tooth structure will invariably provide the 2 mm height necessary for a ferrule, but endodontically treated teeth tend to be decayed and are often missing significant solid tooth structure. Because they are weaker after the additional removal of tooth structure that occurs during a root canal procedure, endodontically treated teeth require proper protection against vertical root fracture. Some have speculated that a shoulder preparation on an all ceramic crown that will be bonded in place may have the same effect as a ferrule.
Temporization
See also: Temporary crowns
Temporization is important after preparing a tooth for receiving a crown for several reasons:
- Protect from and prevent bacterial invasion of newly exposed dentinal tubules, leading to pulpal inflammation and necrosis
- Prevent ginigival growth in the area created by the tooth preparation
- Allows area to be cleaned more effectively, decreasing the incidence of bleeding and gingival inflammation at the time of fitting definitive restoration.
- Maintain occlusal and approximal contacts therefore preventing over-eruption, rotation and closing of spaces
Stages in crown manufacture
Dimensions of preparation
When preparing a tooth for a traditional crown, the enamel may be totally removed and the finished preparation should, thus, exist primarily in dentin. As elaborated on below, the amount of tooth structure required to be removed will depend on the material(s) being used to restore the tooth. If the tooth is to be restored with a full gold crown, the restoration need only be .5 mm in thickness (as gold is very strong), and therefore, a minimum of only .5 mm of space needs to be made for the crown to be placed. If porcelain is to be applied to the gold crown, an additional minimum of 1 mm of tooth structure needs to be removed to allow for a sufficient thickness of the porcelain to be applied, thus bringing the total tooth reduction to minimally 1.5 mm.
If there is not enough tooth structure to properly retain the traditional prosthetic crown, the tooth requires a build-up material. This can be accomplished with a pin-retained direct restoration, such as amalgam or a composite resin, or in more severe cases, may require a post and core. Should the tooth require a post and core, endodontic therapy would then be indicated, as the post descends into the devitalized root canal for added retention. If the tooth, because of its relative lack of exposed tooth structure, also requires crown lengthening, the total combined time, effort and cost of the various procedures, together with the decreased prognosis because of the combined inherent failure rates of each procedure, might make it more reasonable to have the tooth extracted and opt to have an implant placed.
In recent years, the technological advances afforded by CAD/CAM dentistry offer viable alternatives to the traditional crown restoration in many cases. Where the traditional indirectly fabricated crown requires a tremendous amount of surface area to retain the normal crown, potentially resulting in the loss of healthy, natural tooth structure for this purpose, the all-porcelain CAD/CAM crown can be predictably used with significantly less surface area. As a matter of fact, the more enamel that is retained, the greater the likelihood of a successful outcome. As long as the thickness of porcelain on the top, chewing portion of the crown is 1.5mm thick or greater, the restoration can be expected to be successful. The side walls which are normally totally sacrificed in the traditional crown are generally left far more intact with the CAD/CAM option. In regards to post & core buildups, these are generally contraindicated in CAD/CAM crowns as the resin bonding materials do best bonding the etched porcelain interface to the etched enamel/dentin interfaces of the natural tooth itself. The crownlay is also an excellent alternative to the post & core buildup when restoring a root canal treated tooth.
Adequate and appropriate restoration of tooth structure
As crowns are fabricated indirectly (outside of the mouth) free of the encumbrances of saliva, blood, and tight quarters, they can be made to fit more precisely than restorative materials placed directly (inside the mouth). With regard to marginal adaptations (the circumferential seal which keeps bacteria out), anatomically correct contacts (touching adjacent teeth properly so food will not be retained), and proper morphology, the indirect fabrication of the restorations are unprecedented. Indirectly fabricated crowns may be fabricated one of two ways. In the traditional sense, the tooth in question is prepared, a mold is taken, a temporary crown is placed and then the patient leaves. The mold is then sent to a dental laboratory whereby a model is constructed from the mold, and a crown is created on the model (usually out of porcelain, ceramic, gold, or porcelain/ceramic fused to metal) to replace the missing tooth structure. The patient returns to the dental office a week or two later and then the temporary is removed and the crown is fitted and cemented in place. Alternatively, a crown may be indirectly fabricated utilizing technology and techniques relating to CAD/CAM dentistry, whereby the tooth is prepared and computer software is used to create a virtual restoration which is milled on the spot and bonded permanently in place an hour or two later.
3/4 and 7/8 crowns
There are even restorations that fall between an onlay and a full crown when it comes to preservation of natural tooth structure. In the past, it was somewhat common to find dentists who prepared teeth for 3/4 and 7/8 crowns. Such restorations would generally be fabricated for maxillary second premolars or first molars, which might only be slightly visible when a patient smiled. Thus, the dentist would preserve healthy natural tooth structure that existed on the mesiobuccal corner of the tooth for the sake of its natural appearance, the remainder of the tooth would be enclosed in restorative material. Even when porcelain-fused-to-metal and all-ceramic crowns were developed, preserving any amount of tooth structure adds to the overall strength of the tooth. Some dentists feel that the structural benefits of retaining some of the original tooth structure are more than offset by the potential problems of having a significantly longer marginal length (the "seam" on the surface between the crown and the tooth).
Longevity
Although no dental restoration lasts forever, the average lifespan of a crown is around 10 years. While this is considered comparatively favorable to direct restorations, they can actually last up to the life of the patient (50 years or more) with proper care. One reason why a 10-year lifespan is quoted is because a dentist can usually provide patients with this figure and be confident that a crown that the dental lab makes will last at least this long. Many dental insurance plans in North America will allow for a crown to be replaced after only five years.
The most important factor affecting the lifespan of any restorative is the continuing oral hygiene of the patient. Other factors are the skill of the dentist and their lab technician, the material used and appropriate treatment planning and case selection.
Full gold crowns last the longest, as they are fabricated as a single piece of gold. PFMs, or porcelain-fused-to-metal crowns possess an additional dimension in which they are prone to failure, as they incorporate brittle porcelain into their structure. Although incredibly strong in compression, porcelain is terribly fragile in tension, and fracture of the porcelain increases the risk of failure, which rises as the number of surfaces covered with porcelain is increased. A traditional PFM with occlusal porcelain (i.e. porcelain applied to the biting surface of a posterior tooth) has a 7% higher chance of failure per year than a corresponding full gold crown.
When crowns are used to restore endodontically treated teeth, they reduce the likelihood of the tooth fracturing due to the brittle devitalized nature of the tooth and provide a better seal against invading bacteria. Although the inert filling material within the root canal blocks microbial invasion of the internal tooth structure, it is actually a superior coronal seal, or marginal adaptation of the restoration in or on the crown of the tooth, which prevents reinvasion of the root canal.
Types of dental crowns and materials used
There are many different methods of crown fabrication, each using a different material. Available evidence suggests that all-ceramic crowns last about the same length of time or less than metal-ceramic crowns. Gold crowns are desirable because they require less reduction of tooth tissue than other types of crowns and they are the most long-lasting type of crown.
Porcelain-fused-to-metal crowns
Porcelain-fused-to-metal dental crowns (PFMs) have a metal shell on which is fused a veneer of porcelain in a high heat oven. The metal provides strong compression and tensile strength, and the porcelain gives the crown a white tooth-like appearance, suitable for front teeth restorations. These crowns are often made with a partial veneer that covers only the aspects of the crown that are visible. The remaining surfaces of the crown are bare metal. A variety of metal alloys containing precious metals and base metals can be used. The porcelain can be color matched to the adjacent teeth or gingivae.
Chairside CAD/CAM dentistry
The CAD/CAM method of fabricating all-ceramic restorations is by electronically capturing and storing a photographic image of the prepared tooth and, using computer technology, crafting a 3D restoration design that conforms to all the necessary specifications of the proposed inlay, onlay or single-unit crown; there is no impression. After selecting the proper features and making various decisions on the computerized model, the dentist directs the computer to send the information to a local milling machine. This machine will then use its specially designed diamond burs to mill the restoration from a solid ingot of a ceramic of pre-determined shade to match the patient's tooth. After about 20 minutes, the restoration is complete, and the dentist sections it from the remainder of the unmilled ingot and tries it in the mouth. If the restoration fits well, the dentist can cement the restoration immediately. A dental CAD/CAM machine costs roughly $100,000, with continued purchase of ceramic ingots and milling burs. Because of high costs, the usual and customary fee for making a CAD/CAM crown in the dentist's office is often slightly higher than having the same crown made in a dental laboratory.
Typically, over 95% of the restorations made using dental CAD/CAM and Vita Mark I and Mark II blocks are still clinically successful after 5 years. Further, at least 90% of restorations still function successfully after 10 years. Advantages of the Mark II blocks over ceramic blocks include: they wear down as fast as natural teeth, their failure loads are very similar to those of natural teeth, and the wear pattern of Mark II against enamel is similar to that of enamel against enamel.
Leucite reinforced
Popularly known as the "Empress crown," the leucite reinforced system is superficially similar to a gold crown technique in that a hollow investment pattern is made, but the similarities stop there. A specially designed pressure-injected leucite-reinforced ceramic is then pressed into the mold by using a pressable-porcelain-oven, as though the final all-ceramic restoration has been "cast." The crown that is constructed can be stained and glazed or cut-back and layered with feldspathic ceramic to match the patients natural color and shape.
A study by the Umeå University in Sweden, led by Göran Sjögren, sought to study the effectiveness of leucite-reinforced crowns. Titled "Clinical examination of leucite-reinforced glass ceramic crowns (Empress) in general practice: a restrospective study", it found Empress crowns cracked at approximately only a 6% rate, with the integrity of 86% of the remaining samples being called "excellent."
Source of the article : Wikipedia
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