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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.[1] 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.

Crown (dentistry)
Frontzahnkronen Unterkiefer 20100304 007.JPG
Dental crown
ICD-9-CM 23.41
MeSH D003442

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.

Contents

Indications for dental crownsEdit

 
Crown used as part of implant restoration

Crowns are normally used to:[2][3]

  • 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 procedures.
  • Maintain the structural stability and reduce the risk of fractures of extensively restored teeth including like teeth which have been root filled as in Root Canal Treatment, especially posterior teeth which are subjected to higher occlusal forces.
  • 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/ root canal treated 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.[4]

Crown materialsEdit

Full metal crownsEdit

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.[5] The American Dental Association categories alloys in three groups: high-noble, noble and base metal alloys.[6]

High-noble and noble alloysEdit

 
Gold shell crown

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.[7] They also have similar wear properties to enamel, so they are not likely to cause excessive wear to the opposing tooth.[8][9][10] 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.[8]Palladium based alloys are also used. These were introduced as a cheaper alternative to gold alloys in the 1970s.[5] Palladium has a strong whitening effect giving most of its alloys a silverish appearance.

Base-metal alloysEdit

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.[8] Furthermore, there can be a problem with people with a nickel allergy.[5]

 
Stainless Steel Preformed Crowns

Common base-metal alloys used in dentistry are:

  • Silver-palladium
  • Silver-palladium-copper
  • Nickel-chromium
  • Nickel-chromium-beryllium
  • Cobalt-chromium
  • Titanium
TitaniumEdit

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.[8][11]

Full ceramic crownsEdit

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.

SilicaEdit

 
Feldspathic porcelain crowns fabricated on a dental model, then cemented on the upper central anterior teeth using luster paste of glass ionomer.

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 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).

AluminaEdit

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.[12] 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.[12] 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.

ZirconiaEdit

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.

 
High translucent Zirconia bridge layered by porcelain and stained with luster paste

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 veneering ceramic usually occurs [13], "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 dentistry, it has to be fabricated using a CAD/CAM process but not the conventional manual dental technology.[14] 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.[15]

Metal-ceramic crownsEdit

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)

Objectives of tooth preparationEdit

The design of a preparation for a tooth to accept a crown follows five basic principles:[16][3]

  1. Retention and resistance
  2. Preservation of tooth structure
  3. Structural durability
  4. Marginal integrity
  5. Preservation of the periodontium

Aesthetics can also play a role in planning the design.

Retention and resistanceEdit

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 structureEdit

 
Fractures of endodontically treated teeth increase considerably in the posterior dentition when cuspal protection is not provided by a crown.[17]

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.[18] 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.[16]

Structural durabilityEdit

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 reductionEdit

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 bevelEdit

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 reductionEdit

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 IntegrityEdit

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.

Advantages and disadvantages of different marginal finish lines. Table adapted from Fundamentals of Fixed Prosthodontics 4ed[16]
Name Advantages Disadvantages
  Knife edge Minimal tooth destruction Poor aesthetics with ceramic crowns

Weaker crown margins

  Chamfer Minimal tooth destruction

Minimal stress

Inadequate crown strength and aesthetics if ceramic used
  Deep chamfer Moderate tooth destruction

Minimal stress

Potential lip formation
  Radial shoulder Best aesthetics

Best crown strength

Less stress than classical shoulder

Destructive

More stress than a chamfer

  Radial shoulder with a bevel Excellent crown strength

Less stress than a classic shoulder

Destructive

More stress than a chamfer

  Classic shoulder Best aesthetics

Maximal crown strength

Prevents over-contouring

Most destruction and tooth stress

Preservation of the periodontiumEdit

 
Biologic width

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.[16][3]

Special considerationsEdit

 
Crown with post and core system displaying a root fracture

Ferrule effectEdit

Endodontically-treated teeth, especially those with little sound tooth tissue, are prone to fractures. The successful clinical outcome for these teeth relies not only on adequate root canal treatment, but also on the type of restorative treatment used, including the use of a post and core system and the type of extra-coronal restoration selected.Some evidence advocates the use of a ferrule to optimise the bio-mechanical behaviour of root-filled teeth, especially where a post and core system needs to be used.[19][20]

In dentistry, the ferrule effect is, as defined by Sorensen & Engelman (1990), a "360° metal collar of the crown surrounding the parallel walls of the dentine extending coronal to the shoulder of the preparation".[21] Like the ferrule of a pencil which encircles the junction between the rubber and the pencil shaft, the ferrule effect is believed to minimise the concentration of stresses at the junction of post and core, ultimately providing a protective effect against fractures. It also reduces stress transmission to the root due to non-axial forces applied by the post during placement or during normal function. The ferrule can also help preserve the hermetic seal of the luting cement. It has been suggested that protection acquired by the use of a ferrule occurs due to the ferrule resisting functional lever forces, wedging effect of tapered posts and lateral forces during post insertion.[21] To make full use of the ferrule effect, the preparation needs to allow for a continuous band of dentine which should be at least 2 mm in height from the level of the preparation margin and with the band being at least 1 mm in thickness.[19][20]

It has been shown, however, that whilst the absence of a 360° ferrule can increase the risk of fracture of root-filled teeth restored with fiber post & cores and crowns, having insufficeient coronal walls poses an even greater one.[20][22]

Stainless steel crowns for posterior primary dentitionEdit

Stainless steel preformed metal crowns are the treatment of choice for the restoration of posterior primary teeth. A systemic review found that it has the highest success rate (96.1%).[23] In order to accept a stainless steel crown, the entire occlusal surface should be reduced by 1 - 1.5 mm and interproximally contacts should be cleared by cutting a thin mesial and distal portion or slice subgingivally by holding the tip of a thin high speed bur at 15-20° relative to the long axis of the tooth, to avoid the creation of a shoulder. No preparation of the buccal or lingual/palatal surfaces is required.[24]

Hall TechniqueEdit

The Hall Technique is a non-invasive treatment for decayed posterior primary teeth where caries are sealed under a preformed stainless steel crown. This technique requires no tooth preparation.[25][26]

Temporisation and temporary crownsEdit

See also: Temporary crowns

It is very likely that once a tooth has been prepared and whilst waiting for the definitive restoration, the tooth preparation is fitted with a temporary crown.

Need for temporisationEdit

Temporisation is important after preparing a tooth for receiving a crown for several reasons:[27][28]

  • Protect from and prevent bacterial invasion of newly exposed dentinal tubules, leading to pulpal inflammation and necrosis
  • Prevent gingival 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
  • Aesthetic reasons

Temporary crowns can also play a diagnostic role in treatment planning where there is a need for occlusal, aesthetic or periodontal changes.[27]

Types of temporary crownsEdit

There are many different types of temporary crowns available. There also exist several ways to classify crowns. One way is to classify temporary crowns by the predicted or planned length of temporisation. Temporary crowns can be described as short-term, if used for a few days, medium-term, if their planned use for several weeks and long-term if their planned use is for several months. The choice in length of temporisation often relates to the complexity of restorative work planned. Short-term temporary crowns are generally appropriate for simple restorative cases whilst complex cases involving more that one tooth often require long-term temporary crowns.[8][27][29]

Temporary crowns can also be described by the way they are manufactured or fitted on the crown preparation. Temporary crowns can either be direct, if constructed by the dentist in the clinic, or indirect if they are made off-site, usually in a dental laboratory. Generally direct temporary crowns tend to be for short-term use. Where medium-term or long-term temporisation is required, the use of indirect temporary crowns should be considered.[8]

MaterialsEdit

There are several materials that can be used to construct temporary crowns. Direct temporary crowns are either made using metal or plastic pre-formed crowns, chemically-cured or light-cured resins or resin composites. Indirect restorations are either made of chemically-cured acrylic, heat-cured acrylic or cast in metal.[27]

Materials used in crown temporization
Material Advantages Disadvantages Indications
Preformed crowns
Tooth coloured
Polycarbonate
  • Good aesthetics
  • Needs relining with self-cured resin
Direct restorations for all teeth, especially anteriors
Acrylic
  • Good aesthetics
  • Needs relining with self-cured resin
Direct restorations for all teeth, especially anteriors
Metal
Aluminum
  • Strong
  • Poor aesthetics
  • Needs relining with self-cured resin
Direct restorations for posterior teeth
Stainless steel
  • Strong
  • Poor aesthetics
  • Needs relining with self-cured resin
Direct restorations for posterior teeth
Nickel Chromium
  • Strong
  • Poor aesthetics
  • Needs relining with self-cured resin
Direct restorations for posterior teeth
Self-cured and light-cured resins
Polymethyl methacrylate

(Self or heat cured)

  • Strong
  • High wear resistance
  • Good aesthetics
  • Easy to modify
  • Polymerisation shrinkage - can affect fit
  • Setting reaction exothermic - can cause damage to dental pulp
  • Unreacted monomer can damage gingivae and pulp
Indirect temporary crowns, all teeth
Polyethyl methacrylate
  • Setting produces less heat and shrinkage than polymethyl methacrylates - can be used in the mouth
  • Poorer strength, wear resistance, aesthetics and colour stability than polymethyl methacrylates
Reliner for pre-formed crowns
Bisacryl composite
  • Setting produces less heat and shrinkage than other resins
  • Better marginal fit
  • OK aesthetics
  • Better colour stability than polyethyl methacrylates
  • Stains easily if unreacted surface layer not removed
  • brittle in thin sections
  • Difficult to modify/add to
Urethane dimethacrylate

(Light cured)

  • Expensive
Restorative composite
Cast metal
Long-term temporisation

CementationEdit

Unlike cementation of definitive crowns, temporary crowns should be relatively easy to remove. For this reason softer cements are used when cementing temporary crowns. These tend to be zinc oxide eugenol cements.

Clinical stages of crown manufactureEdit

  1. Tooth preparation
  2. Impression making
  3. Temporary crown fabrication
 
A full-arch Polyvinyl siloxane impression of the teeth prepared for the 5-unit PFM bridge shown in the photographs below. The salmon-colored impression material used near the crown preparations is of a lower viscosity than the blue, allowing for the capture of greater detail.

Dental laboratory stages of crown manufactureEdit

Crown manufacture using CAD/CAMEdit

  • CERAC

Chairside CAD/CAM dentistryEdit

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.[30][31] Further, at least 90% of restorations still function successfully after 10 years.[30][31] Advantages of the Mark II blocks over ceramic blocks include: they wear down as fast as natural teeth,[31][32] their failure loads are very similar to those of natural teeth,[31][33] and the wear pattern of Mark II against enamel is similar to that of enamel against enamel.[31][34][35]

In recent years, the technological advances afforded by CAD/CAM dentistry offer viable alternatives to the traditional crown restoration in many cases.[36][37] 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.

 
Use of CAD/CAM in dental clinic
 
The stone model die for the same PFM crown on tooth No. 29. Notice how much tooth structure has been removed in order to facilitate placement of a crown. The original dimensions of the tooth approach if not duplicate the contours of the restoration in the photo above. The silvery paint on the stone die of tooth No. 29 is a die spacer, placed to allow for a minute amount of space between the tooth structure and the internal surface of the crown, which will later fill with cement upon final insertion of the crown into the mouth.

3/4 and 7/8 crownsEdit

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).

LongevityEdit

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. However , if marginal gaps occur between the crown and the endodontically filled tooth, it will lead to the dissolution of the cement (luting agent) and eventually to the failure of restoration.[38]

Types of dental crowns and materials usedEdit

 
Comparison between a porcelain-metal dental crown, an all-porcelain dental crown and a porcelain veneer laminate

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.[39][40] Gold crowns are desirable because they require less reduction of tooth tissue than other types of crowns[7] and they are the most long-lasting type of crown.[39][41]

Porcelain-fused-to-metal crownsEdit

 
Side view of porcelain-fused-to-metal crown for left mandibular first molar
 
Unaesthetic display of metal in a porcelain fused to metal crown in a first premolar tooth

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.

Leucite reinforcedEdit

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.[42]

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."[42][43]

History of dental crownsEdit

There is some evidence of gold dental prosthesis dating back to the Etruscans.[44]

See alsoEdit

ReferencesEdit

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External linksEdit