A crown, as the name implies, does not sit on top of the tooth; it covers three-quarters, seven-eighths or the entire tooth. Crowns that cover three-quarters or seven-eighths of the tooth are made out of metal or porcelain, and are used when a portion of the tooth is still healthy and does not need to be covered. This article focuses on full-coverage crowns. There are a number of different materials used to construct full-coverage crowns.
Your tooth is prepared for a crown by reducing the size of the tooth, usually under local anesthesia. A small cord is then packed under your gum around the preparation to allow the crown margin to be seen (the “finish line” where the crown stops and the tooth continues: where the crown and tooth come together). The instructions for constructing the crown are made by the dentist and sent by written prescription to a laboratory by one of two methods:
- By far the most common method today involves an impression of your prepared tooth. The dentist uses a syringe to place a putty-like material around your tooth; the material then hardens around the tooth, creating an impression of the shape of the tooth preparation.
- Technology now provides dentists with the ability to scan the prepared tooth into a computer. The image is sent electronically to the laboratory for construction of the crown.
- Today’s technology also allows the crown to be scanned and milled from a block of material in the dental office and placed on the tooth the same day.
Why do you need a crown?
A crown is usually needed:
- When a portion of your tooth fractures off
- When a fracture line (a crack) runs deep into the tooth structure, causing pain when you chew (see Cracked Tooth Syndrome)
- When a filling is too wide or when more than 50% of the biting surface is covered with a filling
- When a root canal treatment has been performed, it is widely accepted that a tooth that has gone through root canal treatment is more brittle (because it is dead). To prevent fracture, a crown is recommended.
- When you or your dentist want to improve the positioning of a tooth, without moving the tooth/teeth orthodontically. (see Braces)
- When a tooth is needed to anchor a bridge (see Bridges)
- When a tooth has tipped over time and a partial denture needs to be placed; a tooth can be prepared for a crown to upright it,, improving the placement of the removable partial
- When a tooth has no opposing tooth to stop it and, if an upper tooth, has dropped down too far, or, if a lower tooth, has drifted up too far, a crown can be placed to bring the tooth back into the proper position
- When a tooth has a large, old filling with defective margins (the area where the tooth and the filling come together), there is a high probability that bacteria has created decay under the filling and the filling is simply too large to be replaced, a crown is indicated to protect the life of the tooth
What are the different types of crowns?
* Zirconium crowns:
- A zirconium core with porcelain over the top (strong) or a full zirconium crown (stronger and recommended for patients who grind their teeth; milled from a computer image)
- Tooth-colored crowns designed to restore any of your teeth
- Usually more expensive because they are more costly to manufacture than porcelain fused to metal crowns
* Pressed porcelain crowns:
- Designed to be used in the “aesthetic zone” or the smile-line.
- Although very beautiful, do not possess the strength of zirconium crowns.
- Most closely resemble the enamel layer of natural teeth; their translucency and color capabilities are aesthetically outstanding.
- Usually more expensive because they are more costly to manufacture than non-porcelain crowns
Composite with fiber-reinforcement crowns
- Tooth-colored composite resin with fibers added to strengthen the resin
- Do not possess the strength of zirconium or porcelain crowns
- Not usually the most popular crowns to use to restore a tooth
- Metal-free, making them very translucent and aesthetically pleasing
Milled Porcelain Crowns
The prepared tooth is scanned into a computer.
- The crown is designed and a block of porcelain is placed in a milling machine and the crown design is milled in the dental office.
- Within minutes the crown can be bonded onto the prepared tooth.
- This is the only crown that eliminates having to place temporary crown while the lab constructs and returns the finished product.
Porcelain fused to metal crowns (PFM)
* Porcelain fused to high noble or precious metal and covered in porcelain:
A metal substructure with a higher concentration of gold
- Designed especially for individuals who have a nickel or other metal allergy but want a tooth-colored, less expensive crown
- May become more expensive depending on the market value of gold
* Porcelain fused to noble or semi-precious metal and covered in porcelain:
- A metal substructure with a lower concentration of gold
- May cause an allergic reaction in individuals allergic to some metals
- Visually look no different than a high noble PFM crown
* Porcelain fused to non-precious or base metal and covered with porcelain:
- Have a substructure made of non-precious metal, which contains no gold
- Visually look no different, to the untrained eye, than crowns made with precious or semi-precious metal
* Captek crowns
- These crowns contain gold in the substructure that is very yellow, giving the porcelain a much warmer tone
- Most dentists and dental technicians feel the porcelain has a warmer color to it when there is a greater percentage of gold in the metal substructure.
* Full gold crowns (FGC):
- As the name implies, fully cover the tooth and are all gold in color
- Kinder to the teeth they oppose because metal is softer than porcelain and it creates less wear; more appropriate for patients who grind their teeth because metal is softer against natural teeth.
- More appropriate for patients with metal allergies because they have a higher concentration of gold and fewer base metals.
* Full semi-precious crowns:
- Contain less gold and range in color from silvery to gold
- Not as aesthetically pleasing as an all-porcelain crown, a PFM crown, or an FGC
- Sometimes more cost-effective
* Full non-precious crowns:
- Silvery in color
- Made of base metal (no gold)
* Stainless steel crowns:
- Not custom-made by a dental laboratory to fit your prepared tooth, but instead are a stock crown placed in one visit to your dentist.
- Used as a short-term solution to your dental problems and are meant to be replaced at some point by a laboratory-constructed or office milled crown.