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Crowns

A crown is a type of restoration which completely covers or encircles a tooth. They are sometimes called a cap.

Crowns are often needed when a large cavity threatens the ongoing health and strength of a tooth. They are typically bonded to the tooth using a cement.

Crowns can be made from different materials, which are usually fabricated using indirect methods (created in a lab by a technician).
Crowns are often used to improve the strength or appearance of a tooth.

The most common method of crowning a tooth involves taking an impression of the prepared tooth to fabricate the crown outside of the mouth.
The crown can then be fitted and cemented at an appointment approximately 2 weeks later. In the meantime a temporary crown keeps the tooth comfortable and protected.
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.

A crown may be indicated for a tooth after a root canal.
The endodontically treated tooth (root filled tooth) can become brittle as the nerve and living tissue is no longer there to provide hydration that helps the tooth maintain a resilience to chewing forces.
Posterior teeth (molars and premolars) can be loaded with up 70-90kg of muscular force which is approximately 9 times the loading on anterior (front) teeth.
These teeth are far more likely to fracture in the years after a root canal as they become more brittle over time yet are still loaded with the same forces. In almost all situations, posterior teeth should be crowned to protect against this.
Although the inert filling material within the root canal blocks against 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.

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 mark is given is because a dentist can usually provide patients with this number and be confident that a crown that the dental lab makes will last at least this long.

The most important factor affecting the lifespan of any restorative is the continuing oral hygiene performed by the patient.
]Other factors depend on 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
The main disadvantage of restoring a tooth with a crown, is the irreversible tooth preparation (grinding away) and higher costs than for direct restorations such as amalgam or composite.
However, crowns are normally only chosen for teeth that already have suffered significant tooth loss, and the advantages of a crown for the ongoing function of that tooth far outweigh the disadvantages.

GOLD CROWNS

Restorations made of gold alloy. Generally used for full crowns and overlays rather than traditional fillings.
Full gold crowns (FGCs) consist entirely of a single piece of alloy.
Although referred to as a gold crown, this type of crown is actually composed of many different types of elements, including but not limited to gold, platinum, palladium, silver, copper and tin. FGC’s do not contain any mercury.

Advantages

• Gold doesn't corrode and it is inert in the mouth so does not deteriorate over time.
• Gold alloy is the same hardness as enamel so does not wear the opposite tooth enamel. This is particularly important in a patient who grinds their teeth.
• Gold can be very thin but still be strong enough, so the tooth can be prepared less therefore conserving as much enamel and dentine as possible. Only about 0.7-1mm thickness is needed.

Disadvantages

• Gold is not aesthetically pleasing, so may be limited to use on teeth that are predominantly ‘out of sight’.
• The cost is high because of the high cost of gold.

PORCELAIN CROWNS

A ceramic material made with traditional porcelain techniques (the same as making a china plate or cup). It is commonly used for crowns and overlays, rather than for traditional fillings.

Advantages

• Ceramics are tooth-coloured.
• Ceramics are more resistant to staining and abrasion than composite resin.
Disadvantages

• Porcelain is not strong. It usually needs to be reinforced with a metal alloy lining, or a pressed porcelain lining (porcelain strengthened by pressure). This means it needs twice the thickness of gold, 1.5-2mm thickness. This is less conservative and may mean more tooth has to be removed to create space.
• Porcelain is harder than enamel which may mean the opposing tooth enamel is damaged and worn over time, especially in the mouth of a bruxist (someone who grinds their teeth).
• The cost is high because of the extra technician work involved with the two layers.
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
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.

METAL-FREE CROWNS

Porcelain crowns usually require a reinforced lining of high strength to take the place of the metal alloy.

Reinforced porcelain has been developed to try and achieve even better aesthetics than can be achieved with a PFM crown.

Empress Crowns are made from 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" just like metal is.
These crowns have great aesthetics, but have a high rate of cracking compared to other crowns, so the tooth selection for a low stress, low loading area, is important.

Zirconia is a very hard ceramic that is used as a strong base material in some full ceramic restorations. On the core structure a dental technician can layer an aesthetic ceramic to create the final colour and shape of the tooth. These crowns tend to be dense in appearance with a high value and can lack translucency and fluorescence.

Chairside Crowns

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
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 un-milled ingot and tries it in the mouth. If the restoration fits well, the dentist can cement the restoration immediately.

These chairside crowns depend heavily on excellent operator expertise to creat a crown with a good fit and great aesthetics.

At Thorndon Dental we choose to use the traditional method of taking an impression of the prepared tooth and sending it to a dental technician who constructs the crown. We have long standing relationships with the technicians we use and feel we can get the best results with regards to fit, function and aesthetics.

Thorndon Dental • 246 Tinakori Rd Thorndon Wellington • 04 472 8353
Dentists and staff include: Parm Gill Andrew McKenna Sophie McKenna Colleen Loo Hannah Kelly Michelle Rochelle hygienists Vanessa and Michelle and administration Anne