The outside of each tooth is made of enamel, which is extremely hard. Although teeth are strong a crown is recommended if there is:
2. Tooth decay may also severely weaken a tooth, with a high risk that the tooth may fall apart.
3. Root canal treatment , where the pulp of the tooth containing nerves and blood vessels is removed, can also weaken a tooth.
4. Large fillings can reduce a tooth's strength by more than 70% as shown below diagrammatically and eventually the tooth can break.
5. Tooth wear over time can dramatically reduce the size of a tooth and even expose the nerve.
In the above five cases, a crown is often the best way to save a tooth and strengthen it.
6. Cosmetic needs can also treated with crowns, restoring tooth strength, function and enhancing the overall health and beauty of your mouth
7. Very crooked teeth can sometimes be dramatically improved with crowns if braces or porcelain veneers are not suitable.

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, they rarely chip or break. The metallic colour is the main drawback. Metal crowns are a good choice for out-of-sight molars.

Porcelain-fused-to-metal dental crowns can be colour 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 ceramic or all porcelain crowns look every bit as natural, leaving no black line at the gum level like traditional crowns. Porcelain crowns are as strong as natural teeth and are also recommended for replacing old crowns on front teeth. This type of crown is known as a 'Procera or Zirkon crown'.

        

  

A model of a prepared tooth that requires a Procera crown is scanned on the Procera Scanner shown above and stored into a powerful computer. This computerised image is sent to Sweden and processed in the Procera Mill. An extremely high strength, non-metal ceramic foundation (coping) is produced in Sweden and sent back to the Ukrainian Dental Clinic. Special porcelain is then fired onto the Procera Foundation to create a truly lifelike and natural Procera crown.

To date more than 2 million Procera crowns have been produced world wide.

Procera type ceramics are also used for other applications such as implants for hip joints

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.

What steps are involved in preparing a tooth for a crown?

The process of fitting crowns and bridges requires two visits to the dentist and an approximate 7 days stay or visiting twice for 1 or 2 days

First visit: Examining and preparing the tooth

At the first visit, 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 .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 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 5-8 days. If your crown is made of porcelain, your dentist will also select the shade that most closely matches the colour of the neighbouring 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 temporary cement.

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

Useful links http://www.dentalhealth.org.uk/faqs/leafletdetail.php?LeafletID=7