Indications and clinical technology of manufacture artificial crowns

Indications and clinical technology of manufacture artificial crowns.

A dental crown is a tooth-shaped “cap” that is placed over a tooth — covering the tooth to restore its shape and size, strength, and/or to improve its appearance.

The crowns, when cemented into place, fully encase the entire visible portion of a tooth that lies at and above the gum line.

A dental crown may be needed in the following situations:

1.     To protect a weak tooth (for instance, from decay) from breaking or to hold together parts of a cracked tooth

2.     To restore an already broken tooth or a tooth that has been severely worn down

3.     To cover and support a tooth with a large filling when there isn’t a lot of tooth left

4.     To hold a dental bridge in place

5.     To cover misshapened or severely discolored teeth

6.     To cover a dental implant

Permanent crowns can be made from all metal, porcelain-fused-to-metal, all resin, or all ceramic.

 

1.     Metals used in crowns include gold alloy, other alloys (for example, palladium) or a base-metal alloy (for example, nickel or chromium). Compared with other crown types, less tooth structure needs to be removed with metal crowns, and tooth wear to opposing teeth is kept to a minimum. Metal crowns withstand biting and chewing forces well and probably last the longest in terms of wear down. Also, metal crowns rarely chip or break. The metallic color is the main drawback. Metal crowns are a good choice for out-of-sight molars.

2.     Porcelain-fused-to-metal dental crowns can be color matched to your adjacent teeth (unlike the metallic crowns). However, more wearing to the opposing teeth occurs with this crown type compared with metal or resin crowns. The crown’s porcelain portion can also chip or break off. Next to all-ceramic crowns, porcelain-fused-to-metal crowns look most like normal teeth. However, sometimes the metal underlying the crown’s porcelain can show through as a dark line, especially at the gum line and even more so if your gums recede. These crowns can be a good choice for front or back teeth.

3.     All-resin dental crowns are less expensive than other crown types. However, they wear down over time and are more prone to fractures than porcelain-fused-to-metal crowns.

4.     All-ceramic or all-porcelain dental crowns provide the best natural color match than any other crown type and may be more suitable for people with metal allergies. However, they are not as strong as porcelain-fused-to-metal crowns and they wear down opposing teeth a little more than metal or resin crowns. All-ceramic crowns are a good choice for front teeth.

5.Temporary versus permanent. Temporary crowns can be made in your dentist’s office whereas permanent crowns are made in a dental laboratory. Temporary crowns are made of acrylic or stainless steel and can be used as a temporary restoration until a permanent crown is constructed by the dental laboratory.

 

 Preparing a tooth for a crown usually requires two visits to the dentist, the first step involves examining and preparing the tooth, the second visit involves placement of the permanent crown.

 

1.     First Visit: Examining and preparing the tooth. At the first visit in preparation for a crown, your dentist may take a few X-rays to check the roots of the tooth receiving the crown and surrounding bone. If the tooth has extensive decay or if there is a risk of infection or injury to the tooth’s pulp, a root canal treatment may first be performed. Before the process of making your crown is begun, your dentist will anesthetize (numb) your tooth and the gum tissue around the tooth. Next, the tooth receiving the crown is filed down along the chewing surface and sides to make room for the crown. The amount removed depends on the type of crown used (for instance, all-metal crowns are thinner, requiring less tooth structure removal than all-porcelain or porcelain-fused-to-metal ones). If, on the other hand, a large area of the tooth is missing (due to decay or damage), your dentist will use filling material to “build up” the tooth to support the crown. After reshaping the tooth, your dentist will use impression paste or putty to make an impression of the tooth to receive the crown. Impressions of the teeth above and below the tooth to receive the dental crown will also be made to make sure that the crown will not affect your bite. The impressions are sent to a dental laboratory where the crown will be manufactured. The crown is usually returned to your dentist’s office in 2 to 3 weeks. If your crown is made of porcelain, your dentist will also select the shade that most closely matches the color of the neighboring teeth. During this first office visit your dentist will make a temporary crown to cover and protect the prepared tooth while the crown is being made. Temporary crowns usually are made of acrylic and are held in place using a temporary cement.

2.     Second Visit: Receiving the permanent dental crown. At your second visit, your dentist will remove your temporary crown and check the fit and color of the permanent crown. If everything is acceptable, a local anesthetic will be used to numb the tooth and the new crown is permanently cemented in place

Onlays and ¾ crowns are variations on the technique of dental crowns. The difference between these crowns and the crowns discussed previously is their coverage of the underlying tooth. The “traditional” crown covers the entire tooth; onlays and ¾ crowns cover the underlying tooth to a lesser extent.

 

Artificial crowns. Clinical and technological aspects.

Teeth do not possess the regenerative ability found in most other tissues. Therefore, once enamel or dentin is lost as a result of caries, trauma, or wear, restora­tive materials must be used to reestablish form and function. Teeth require preparation to receive restora­tions, and these preparations must be based on fun­damental principles from which basic criteria can be developed to help predict the success of prosthodontic treatment. Careful attention to every detail is im­perative during tooth preparation. A good prepara­tion will ensure that subsequent techniques (e.g., provisionalization, impression making, pouring of dies and casts, waxing) can be accomplished.

The principles of tooth preparation may be divided into three broad categories:

1.  Biologic considerations, which affect the health of the oral tissues

2  Mechanical considerations, which affect the integrity and durability of the restoration

3.  Esthetic considerations, which affect the appearance of the patient

Successful tooth preparation and subsequent restoration depend on simultaneous consideration of all these factors. Often improvement in one area will adversely affect another, and striving for perfection in one may lead to failure in another. For example, in the fabrication of a metal-ceramic crown, sufficient thickness of porcelain is necessary for a lifelike appearance. However, if too much tooth structure is removed to accommodate a greater thick­ness of porcelain for esthetic reasons, the pulpal tis­sue may be damaged (biologic consideration) and the tooth unduly weakened (mechanical considera­tion). An in-depth knowledge and understanding of the various criteria are prerequisites to the devel­opment of satisfactory tooth preparation skills. Predictable accomplishment of optimum tooth preparation (Fig. 2-1) often entails finding the best combination of compromises among the prevalent biologic, mechanical, and esthetic considerations.

Indications

 

1. The only option remaining.

 

2. Usually reserved for molar and lower premolar teeth.

 

3. Flame-shaped diamond

 

4. Excavator

 

 

 

Procedure

 

If the problem is such that a partial crown will not function, the only option is the full crown restoration, and it is the treatment court of last appeal. Full crowns are most frequently placed on molar teeth and occasionally on lower bicuspids.

 

The situations that require a complete crown include teeth that have exten­sive but questionable restorations or extensive carious involvement. A significant contributor to this difficulty is a large defective Class V amalgam restoration, and the only way a casting will perform is to make a complete crown. Another situation requiring this type of crown is presented by teeth that are very short and do not provide resistance unless a full crown is utilized.

 

Many times the locations of the gingival margins are predetermined by the existing restoration or caries and decalcification that must be enclosed. This often dictates that these margins, with emphasis on the interproximal and facial, will be in the gingival sulcus. If adequate resistance for the restoration and effec­tive coverage can be met without placing the margin in the sulcus, that is the rec­ommendation. Many times the lingual surface will be intact and it is easy to ter­minate the margin above the level of the gingiva, which eases the maintenance problems of the marginal gingiva.

 

The initial procedure is the occlusal reduction. The reduction may be done using a wheel diamond or a tapered round-end diamond. The same amount of reduction is required as for other posterior teeth, with a minimum of 1 mm wherever function occurs. The occlusal reduction should show correlation with occlusal morphology of the tooth and not be an arbitrary flattening of the occlusal surface.

 

Followin;g the occlusal preparation, proceed to the buccal and lingual reduction. A tapered round-end diamond is useful for this purpose. The gingival finish line will have been predetermined by the needs of retention, defective restorations, or caries. The reduction should reflect a chamfer effect at the gingi­val cavosurface. The buccal and lingual morphology is observed, and the reduc­tion is done to produce an even thickness of metal in the completed restoration consistent with the shape of the tooth.

 

The buccal and lingual walls are structured to provide good resistance to displacement. There is a danger of over-angulating the walls and losing the potential resistance for the casting. This reduction is carried toward the adjacent teeth. To avoid overcutting at the interproximal, a thin flame-shaped diamond is selected.

 

The finish line must be located gingival to the contact and be on healthy enamel. If an amalgam core or restoration  is part of the prepa­ration, it is expected that the finish line for the casting will terminate gingival to the restoration. The proximal walls must relate to each other in the same man­ner as to the buccal and lingual to provide optimal resistance. They will be slightly inclined toward each other.

 

This should complete most of the reduction, and now with a fine grit fin­ishing diamond or a No. 1170 bur the preparation is made smooth. All sharp corners or angles are slightly rounded. For some of the smoothing it is best that the bur run at a reduced speed to avoid excess cutting.

 

The occlusal clearance is checked in the same manner as discussed for pre­vious posterior cast preprations.

Consideration must be given to clearance when the patient goes through eccentric movements, and the preparation must have adequate reduction to allow the restoration to have a acceptable function in all movements. This information must be placed into a suitable articulator to allow for a reliable wax-up and casting.

 

The final segment of the preparation is the placement of a groove on the buccal surface, and usually this will be in the anatomical buccal groove. This groove is of value, as it helps to provide positive orientation during the placement of the casting. It will tend to limit the potential for slight rotation of the crown during seating, as the groove will guide or key it into the specific placement. At times it will prove of value by providing a degree of supplemental retention, which may be helpful to a restoration with marginal retention. On occasion when the buccal segment of the tooth is badly destroyed this groove may be placed on the lingual portion of the preparation. If the preparation  requires a pin-supported amalgam or resin buildup, the groove may be placed m the amalgam or resin.

 

This groove may be formed by using a thin, tapered round-end diamond, or a No. 171 bur may be used effectively for this purpose. If the bur is used, it is advised to use it at reduced speed to avoid overcutting. This groove extends just short of the gingival finish line.

 

Resume of Partial and Full Crown Preparation

 

1. Gingival finish line when possible should be located occlusal to the gingiva.

 

2. Grooves with adequate length are preferred over proximal boxes for resistance form.

 

3. Do not terminate a finish line through a wear facet.

 

4. Occlusal reduction should occur first using wheel or football-shaped diamond. With a wax bite check for a minimum clearance of 1mm.

 

5. Lingual reduction with bullet-nose diamond 2D-T.

 

6. Interproximal reduction with thin or flamed-shaped diamond to avoid cutting adjacent tooth.

 

7. Proximal grooves are located for maximum length and will line up toward the buccal cusp tip.

 

8. Place buccal bevel toward facial to complement esthetics and function.

 

9. A V-shaped connector may be used between grooves.

 

10. With a seven-eighths crown one groove is located on the facial surface.

 

11. For a full crown preparation the facial reduction is done with the same instrument as for the lingual. A definite chamfer is needed for the buccal finish line.

 

12. Place a groove on the facial surface of the preparation for positive seating and use of tapered diamond or a No. 171 bur.

 

 

 

Indications of Dental Crowns

 

Dental crowns are also called “caps”. They are called as such because like caps they cover the teeth and protect them. They are fabricated to fit the crowns of the teeth, and they are cemented onto the teeth so that they are functional as a normal crown.

 

Dental crowns, whether given in single units or constructed as a bridge work may be prescribed for a lot of reasons:

 

– In lieu of lengthy orthodontic treatments that will straighten teeth and bring the bite to proper alignment and occlusion, dental crowns can be provided to a patient so that you can achieve the results almost instantly. Orthodontic treatments are completed after month or years of enduring the pain and discomfort involved with braces. Dental crowns will take away the need for that, because you can enjoy the results almost immediately.

 

– If you are looking for teeth whitening that is more permanent than what laser teeth whitening and other traditional bleaching methods can achieve, you can fabricate dental crowns on your teeth and have them in a shade that is obviously whiter than the original color of your teeth. Teeth whitening results can be quite impressive, but just like your natural teeth, the results fade. If you are looking for a more permanent answer, crowning will be it.

 

– When they are constructed as a bridge, dental crowns can replace a lost tooth. The crown is installed on the teeth adjacent to the empty space and they can be designated to support or carry the artificial tooth or teeth. In this case, your dental crown will fill a gap to replace teeth; and restore the function and appearance of the mouth and teeth.

 

– Stains and discolorations can be a very ugly problem. A tooth that has received a root canal will discolour because it is no longer vital. Teeth that have been exposed to antibiotics during their development stages may also be badly stained. There are all kinds of aesthetic issues that patients may have with their teeth that can only be resolved by the fabrication of dental crowns. Crowns can be given to one or more teeth that need masking.

 

– A root canal treated tooth does not only have a problem with discoloration. A root canal treated tooth is no longer vital so they are more brittle compared to the natural tooth, so they need to be protected by a dental crown. The reinforcement afforded by the crown will strengthen and protect the tooth so that it does not fracture and break easily.

 

– Small fractures, chips and breaks on the teeth may be resolved by a dental filling. When the damage and decay is so much bigger than what dental fillings are able to handle, dental crowns will be the perfect answer. A crown will restore the appearance, form and shape of the tooth to how it normally looks.

 

If you are familiar with any of the conditions mentioned above, understand that dental crowns can resolve your issues perfectly. Dental crowns are commonly made with porcelain and are reinforced by metal, but they may be constructed using ceramic and may be made without any metal component.

A dental crown is a tooth-shaped “cap” that is placed over a tooth — to cover the tooth to restore its shape and size, strength, and improve its appearance.

 

The crowns, when cemented into place, fully encase the entire visible portion of a tooth that lies at and above the gum line.

 

A dental crown may be needed in the following situations:

 

To protect a weak tooth (for instance, from decay) from breaking or to hold together parts of a cracked tooth

To restore an already broken tooth or a tooth that has been severely worn down

To cover and support a tooth with a large filling when there isn’t a lot of tooth left

To hold a dental bridge in place

To cover misshapened or severely discolored teeth

To cover a dental implant

To make a cosmetic modification

 For children, a crown may be used on primary (baby) teeth in order to:

Save a tooth that has been so damaged by decay that it can’t support a filling.

Protect the teeth of a child at high risk for tooth decay, especially when a child has difficulty keeping up with daily oral hygiene.

Decrease the frequency of sedation and general anesthesia for children unable because of age, behavior, or medical history to fully cooperate with the requirements of proper dental care.

In such cases, a pediatric dentist is likely to recommend a stainless steel crown.

 

Permanent crowns can be made from stainless steel, all metal (such as gold or another alloy), porcelain-fused-to-metal, all resin, or all ceramic.

 

Stainless steel crowns are prefabricated crowns that are used on permanent teeth primarily as a temporary measure. The crown protects the tooth or filling while a permanent crown is made from another material. For children, a stainless steel crown is commonly used to fit over a primary tooth that’s been prepared to fit it. The crown covers the entire tooth and protects it from further decay. When the primary tooth comes out to make room for the permanent tooth, the crown comes out naturally with it. In general, stainless steel crowns are used for children’s teeth because they don’t require multiple dental visits to put in place and so are more cost- effective than custom-made crowns and prophylactic dental care needed to protect a tooth without a crown.

Metals used in crowns include gold alloy, other alloys (for example, palladium), or a base-metal alloy (for example, nickel or chromium). Compared with other crown types, less tooth structure needs to be removed with metal crowns, and tooth wear to opposing teeth is kept to a minimum. Metal crowns withstand biting and chewing forces well and probably last the longest in terms of wear down. Also, metal crowns rarely chip or break. The metallic color is the main drawback. Metal crowns are a good choice for out-of-sight molars.

Porcelain-fused-to-metal dental crowns can be color matched to your adjacent teeth (unlike the metallic crowns). However, more wearing to the opposing teeth occurs with this crown type compared with metal or resin crowns. The crown’s porcelain portion can also chip or break off. Next to all-ceramic crowns, porcelain-fused-to-metal crowns look most like normal teeth. However, sometimes the metal underlying the crown’s porcelain can show through as a dark line, especially at the gum line and even more so if your gums recede. These crowns can be a good choice for front or back teeth.

All-resin dental crowns are less expensive than other crown types. However, they wear down over time and are more prone to fractures than porcelain-fused-to-metal crowns.

All-ceramic or all-porcelain dental crowns provide better natural color match than any other crown type and may be more suitable for people with metal allergies. However, they are not as strong as porcelain-fused-to-metal crowns and they wear down opposing teeth a little more than metal or resin crowns. All-ceramic crowns are a good choice for front teeth.

Temporary versus permanent. Temporary crowns can be made in your dentist’s office where as permanent crowns are made in a dental laboratory. Temporary crowns are made of acrylic or stainless steel and can be used as a temporary restoration until a permanent crown is constructed by a lab.

Preparing a tooth for a crown usually requires two visits to the dentist — the first step involves examining and preparing the tooth, the second visit involves placement of the permanent crown.

 

First Visit: Examining and preparing the tooth.

 

At the first visit in preparation for a crown, your dentist may take a few X-rays to check the roots of the tooth receiving the crown and surrounding bone. If the tooth has extensive decay or if there is a risk of infection or injury to the tooth’s pulp, a root canal treatment may first be performed.

 

Before the process of making a crown begins, your dentist will anesthetize (numb) the tooth and the gum tissue around the tooth. Next, the tooth receiving the crown is filed down along the chewing surface and sides to make room for the crown. The amount removed depends on the type of crown used (for instance, all-metal crowns are thinner and require less tooth structure removal than all-porcelain or porcelain-fused-to-metal ones). If, on the other hand, a large area of the tooth is missing (due to decay or damage), your dentist will use filling material to “build up” the tooth to support the crown.

 

After reshaping the tooth, your dentist will use a paste or putty to make an impression of the tooth to receive the crown. Impressions of the teeth above and below the tooth to receive the dental crown will also be made to make sure that the crown will not affect your bite.

 

The impressions are sent to a dental lab where the crown will be manufactured. The crown is usually returned to your dentist’s office in two to three weeks. If the crown is made of porcelain, your dentist will also select the shade that most closely matches the color of the neighboring teeth. During this first office visit your dentist will make a temporary crown to cover and protect the prepared tooth while the crown is being made. Temporary crowns usually are made of acrylic and are held in place using a temporary cement.

 

 

Second Visit: Receiving the permanent dental crown.

 

At the second visit, your dentist will remove the temporary crown and check the fit and color of the permanent crown. If everything is acceptable, a local anesthetic will be used to numb the tooth and the new crown is permanently cemented in place.

A dental crown is a tooth shaped restoration that covers the tooth thus improving the size, shape, strength, and appearance of the tooth. A crown is a cap that completely encases the visible tooth surface including the gum line when cemented over the tooth structure. In general, crown is an artificial, synthetic tooth that is designed from wide range of materials in order to improve an esthetic appearance as that of a natural tooth.

 

Crowns are usually designed from ceramic, porcelain or metals like gold and also various material combinations which are basically fused between porcelain casing and various metals. Permanent crowns are usually made of porcelain-fused-to-metal (PFM), metal, resin, or a complete ceramic. In general, crowns can vary based on the used type of material.

 

Some of the different types of crowns include All-porcelain or All-ceramic crowns- which caaturally match with the tooth color, indicated for front teeth and metal allergic patients; Porcelain-fused-to-metal (PFM) crowns- which can match best with the adjacent teeth thus providing a natural look, indicated for both front and back teeth; Metal crowns that are made of palladium alloy, gold alloy, or a base-metal alloy like chromium or nickel- which can last longer but not esthetic, indicated for out-of-sight molars; All-resin crowns- are cost effective and prone to fractures; temporary crowns made of stainless steel or acrylic- are designed in dental practice; and permanent crowns- are designed by dental laboratory.

 

Porcelain fused to metal dental crown (PFM)

 

Porcelain fused to metal crown (PFM) is a full cast crown that is constructed by fusing the porcelain to all or most of the metal surfaces.

Indications of porcelain fused to metal crown

 

A PFM dental crown is indicated in case of minimal inter-occlusal clearance that requires a stronger restoration. It is also used on any tooth that requires a full crown for preventive and restorative purposes.

 

Contraindications of PFM dental crown

 

PFM dental crown is contraindicated when the size of the tooth pulp is negligibly smaller thus compromising the tooth preparation process. It is also contraindicated when the clinical tooth crown is very short and lacks the required stability including retention that is enough to provide the space for porcelain and metal.

 

Advantages of PFM dental crown

 

The major advantage of PFM crown is its metal substructure that provides adequate strength.

 

Disadvantages of PFM crown

 

PFM crown requires removal of 1.5 mm of buccal tooth substance to place the porcelain and metal layers. It lacks esthetic look due to its opaque metal layer. An e-max or a zirconium crown can be a best option when one is looking for an aesthetic appearing tooth crown.

 

E-Max Crown

 

The E-Max dental crown is considered to be a best inclusion in dental crown technology. It is a form of all-ceramic crown which provides best aesthetic appearance by complimenting with the natural teeth and it lasts longer. It has an appealing translucent color and is highly durable with extra strength.

Emax crown is generally constructed by 4 techniques: Pressed, milled Yttrium Zirconia, pressed to Zirconia, and a milled esthetic glass.

 

Indications of E-Max crown

 

E-max crown is indicated for stained or damaged teeth with poor quality and to provide an aesthetic look.The E-max crown consists of a block of top grade material, lithium di-silicate ceramic which assures correct fitting, durability, toughness, and opaqueness thus it is priced more. The crown is also made of a glass ceramic crown that is endurable with a higher toughness but still providing a delicate look.

 

Advantages of E-max crown

 

The E-max crown has a lifelike shape and transparent color that provides an exact color match with the patient’s owatural teeth. It has no metal alloy base thus does not affect the aesthetics around the gum line. The crown is long lasting, strong, and less prone to fractures when compared to other traditional crown types. When compared to zirconia crowns, an E-Max crown is less likely to get chipped.

 

Disadvantages of E-max crown

 

The only disadvantage of E-max crown is its high cost especially during tooth preparation. It requires a premium crown placement without tooth preparation and hence it is more expensive when compared to its other crown counterparts. In addition, e-max crown is not suitable for all patients and one should discuss it with dentist about availability and whether it is the appropriate one.

 

Zirconia crown

 

A zirconia crown is made of a strong, biocompatible material, zirconia which is also used in medical applications like artificial joints. It is a well-known, type of all-ceramic crown which significantly improves the tooth appearance that has been previously disfigured or stained for years.

 

 

Zirconia is a crystal form that is indestructible and lasts longer. It has adequate strength and durability. In fact, its crown cannot be spotted amongst natural teeth. Patients are least likely to develop an allergic reaction to zirconia and thus considered a safe option when compared to porcelain fused to metal crowns.

 

Zirconia crown is long lasting, easy to wear and durable when compared to other crowns. The translucent appearance of zirconia crowns blends well with the natural teeth color and thus provides the most pleasing appearance. It requires minimal tooth preparation thus preserves most of the original tooth substance which is also favorable while removing the crown. Hence, zirconia crown is an ideal option to cover-up the damage including stains and to restore the tooth function.

 

Disadvantages of zirconia crown

 

The zirconia crown is tough, durable and possesses abrasive quality thus imposing the friction against the tooth root including neighboring teeth. Hence, an E-max crown could be a favorable option for cases that require preservation of the root structure.

Dental Crown

 

A dental crown is a cap-like restoration used to cover a damaged tooth. Crowns can give support to misshapen or badly broken teeth and permanently replace missing teeth to complete a smile or improve a bite pattern. They may be molded from metal, ceramic, plastics, or combinations of all three. They are cemented in place and coated to make them more natural looking. Historically, a variety of materials have been used as tooth replacements. The ancient Egyptians used animal teeth and pieces of bone as primitive replacement materials. More recently, artificial teeth have been fabricated from substances such as ivory, porcelain, and even platinum. With modern technology, high quality tooth replacements can be made from synthetic plastic resins, ceramic composites, and lightweight metal alloys.

 

Design

 

There are several key factors to consider in the design of dental crowns. First, appropriate raw materials with which to make the crown must be identified. These materials must be suitable for use in the oral cavity, which means they must be acceptable for long term contact with oral tissues and fluids. Crown components must have a good safety profile and must be non-allergenic and non-carcinogenic. The American Dental Association/ANSI specification #41 (Biological Evaluation of Dental Materials) lists materials which have been deemed safe for use. In addition to safety considerations, these materials must be able to withstand the conditions of high moisture and mechanical pressure, which are found in the mouth. They must be resistant to shrinkage and cracking, particularly in the presence of water. Metal is preferred for strength but acrylic resins and porcelain have a more natural appearance. Therefore the selection of crown material is, in part, dependent on the location of the tooth being covered. Acrylic and porcelain are preferred for front teeth, which have higher visibility. Gold and metal amalgams are most often used for back teeth where strength and durability are required for chewing but appearance is less critical.

 

The second factor to consider when designing a crown is the shape of the patient’s mouth. Dental restorations must be designed to mimic the bite properties of the original tooth surface so the wearer does not feel discomfort. Since every individual’s mouth is different each crown must be custom designed to fit perfectly. Successful crown design involves preparation of an accurate mold of the oral cavity.

 

Raw Materials

 

There are four main types of materials used in crown construction: The plasters used to create the mold, the materials from which the crown itself is made (e.g., metal, ceramic, plastic), the adhesives used to cement the crown in place, and the coatings used to cover the crown and make it more aesthetically appealing.

 

Molding plasters

 

Plaster molds are made from a mixture of water and gypsum powder. Used for dental applications since the 1700s, gypsum is finely divided calcium sulfate dihydate. Different types of plasters are used depending on application: impression plaster is used to

 

An impression of the tooth to be crowned is taken to record its shape. The impression plaster is mixed and then placed in a tray that is fitted over the teeth. The tray is held still in place until the plaster hardens. When the tray is removed from the mouth, it retains a three dimensional impression of the tooth that is to be covered. This impression is a negative, or reverse, image of the tooth.

An impression of the tooth to be crowned is taken to record its shape. The impression plaster is mixed and then placed in a tray that is fitted over the teeth. The tray is held still in place until the plaster hardens. When the tray is removed from the mouth, it retains a three dimensional impression of the tooth that is to be covered. This impression is a negative, or reverse, image of the tooth.

record the shape of the teeth, model plaster is used to make durable models of the oral cavity, and investment plaster is used to make molds for shaping metal, ceramics and plastics. Waxes are also sometimes used in this regard.

Crown construction materials

 

Metals are frequently used in crown construction because they have good hardness, strength, stiffness, durability, corrosion resistance, and bio-compatibility. Metals formulated as mixtures of mercury have been historically used. In fact, one source notes that metal amalgam was used as a dental restorative as early as 1528. Common alloys used in crowns are based on mixtures of mercury with silver, chromium, titanium, and gold. These mixtures form a blend than can be easily shaped and molded, but which hardens in a few minutes.

 

Ceramics are well suited for use in crowns because they have good tissue compatibility, strength, durability and inertness. They can also be made to mimic the appearance of real teeth fairly closely. However, the tensile strength of ceramic is low enough to make it susceptible to stress cracking, especially in the presence of water. For this reason, ceramic is most often used as a coating for metal-structured crowns. The two primary types of ceramics used in crowns are made from potassium feldspar and glass-ceramic.

 

The first resin used in denture materials was vulcanized rubber in 1839. Since then, a number of other resins have been developed which are more suitable for dental applications. Today, acrylic polymer resins are commonly used in dentures and crowns. Specifically, polymethyl methacrylate is most often used. This type of resin is made by mixing together chemical entities known as monomers with activating chemicals which cause the monomers to react and link together to form long chains called polymers. Some of these resins harden at room temperature as this reaction progresses. Others require heat or ultraviolet light to catalyze the change.

 

Special dental adhesives, or dental cements, are used to hold the crown in place. These can be classified as either aqueous or nonaqueous. The aqueous type include zinc phosphates, polycarboxylate cements, glassionomer cements, and calcium phosphate cements. The nonaqueous type include zinc oxide-eugenol, calcium chelates, and acrylic resins such as polymethyl methyacrylate.

 

Coatings are used to make the crown appear more natural. Porcelain is used in this regard, but it is difficult to work with and hard to match to the tooth’s natural color. Resins similar to the ones used in tooth construction are also used to create tooth-colored veneers on crowns. These resins have an advantage over other veneers in that they are inexpensive, easy to fabricate, and can be matched to the color of tooth structure. However, acrylic coatings may not adhere to the crown’s surface as well as porcelain or other materials. Therefore, the prosethedontist may design the crown’s surface with mechanical undercuts to give the coating a better grip. Resin coatings also have relatively low mechanical strength and color stability and poor abrasion and stain resistance as compared to porcelain veneers.

 

The Manufacturing

Process

 

Creating the mold

 

1 Before beginning, the dentist may need to prepare the area where the crown is to be

Two types of crown preparation: anterior and posterior.

Two types of crown preparation: anterior and posterior.

 This may require the removal of 2-3 millimeters of tooth structure from the four sides and the biting edge. Then, an impression of the tooth is taken to record its shape. This step uses impression plaster which is the softest and fastest setting type of dental plaster. The impression plaster is mixed with a small amount of water until it is fluid. This slurry is placed in a tray that is fitted over the teeth. The tray is held still in place until the plaster hardens. When the tray is removed from the mouth, it retains a three dimensional impression of the tooth that is to be covered. This impression is a negative, or reverse, image of the tooth.

2 The next step is to prepare another type of plaster, known as model plaster. This type of plaster is harder than the impression plaster. Once again the plaster is mixed with the appropriate quantity of water. Then the slurry is poured into the impression mold. In this way a positive model of the tooth can be made. This positive model made from the negative impression mold is called a cast. The cast is used by the dentist for study purposes.

3 The impression is also used to make a mold, called an investment, which is capable of withstanding high temperatures. This is an important consideration because some metals and ceramics require temperatures higher than 2,372° F (1300° C) for molding. These investments are made from calcium phosphate mixed with silica and other modifying agents.

Fabrication

 

4 Fabrication of the crown is done by filling the investment with the appropriate material. In the case of metals, this is done at a high temperature so the metal is molten. For ceramics and plastics, the mixture is initially fluid but may require the addition of heat to cause the materials to cure and harden. A vertical vise may be used to help pack the casting investment tightly. The process also requires the mold first be treated with a release agent to ensure the crown can be easily removed after it has hardened. Some acrylic resins must be heated for up to eight hours to make sure they are fully cured. After the processing is done and the investment has cooled, the mold is broken apart and the crown is removed.

Installation

 

5 After the crown has been successfully completed, it is ready for installation. The prosethedontist applies cement to the inside of the crown surface and then fits it into place over the tooth. Because of the number of processing steps there may be a slight discrepancy in the fit and the crown may require minor grinding and smoothing of its surface to ensure it fits correctly.

Finishing

 

6 The crown may require a finishing coat to seal it and improve its natural appearance. Such coatings are typically acrylic polymers. The polymer can be painted on as a thin film, which hardens to a durable finish. Some polymers require a dose of ultraviolet light to properly cure.