Crowns (Caps) are usually done when there is not enough tooth structure to hold a filling. A filling is only as strong as the remaining tooth to hold it in. Because a crown goes over the entire remaining tooth, it can be much stronger, more resistant to breakage than a filling. However, there are many different types of crowns, each with their own advantages and disadvantages. Crowns can be full coverage or partial coverage. They can be fabricated by casting, milling, sintering, pressed into a mold, or a combination of the above. Here are some popular types:
- All metal crowns
There are different kinds of metal alloys used in dentistry. Each specific alloy composition is usually a trade secret by each manufacture, but the American Dental Association classifies them into 3 basic categories: Non-precious metal, Semi-precious metal, and Precious metal alloys.
Non-precious alloys are strongest, but are not as biocompatible. There is no gold or other precious metals, but may contain some silver. They may have a large composition of nickel. This can cause problems with allergic reactions, especially with females who wear a lot of nickel containing jewelry. Casting fabrication accuracy can be a challenge, especially with large bridges.
Semi-precious alloys contain precious metals such as Gold, Platinum, Palladium, and Silver. These alloys are more biocompatible with tissues, are easy to fabricate accurately, especially with large bridges, and are stronger than Precious metal crowns.
Precious alloys contain a high amount of Gold, and may be slightly more biocompatible than Semi-precious metals, but are significantly weaker. Because of this, they are not used often with porcelain fused to metal crowns (PFM) or large bridges. They also tend to wear faster. However, plaque and tarter do not adhere to the surfaces well, compared to actual tooth structure, thus it is easier to keep clean.
- Porcelain fused to metal (PFM)
PFMs consists of an inner metal coping, with tooth-colored porcelain baked on top. The primary reason that these crowns were popular, is that they are tooth-colored, not metal colored, as with all-metal crowns. These crowns have been the “go to” crowns for tooth-colored restorations for many years, however, because the metal coping blocks the light, if there is any future shrinkage of the gums, there will be a “black line shadow” appearance at the gumline. Cosmetically, this may not be acceptable.
- All ceramic crowns
The technology for fabricating all ceramic crowns have progressed to the point that for tooth-colored crowns, these are the most lifelike and the most popular. Several types of ceramics are used. Old-fashioned porcelain has the potential of having the best beauty, however, Lithium Disilicate crowns come very close in beauty with 4X the strength. Zirconia crowns are the strongest, with up to 10X the strength of porcelain, but are very opaque, and do not look lifelike. These are usually placed on the back teeth, where it is hard to see them.
- Composite crowns
At one time these crowns were popular, because of the ease of fabrication. They consist of a composite mix of ceramic powder and a plastic binder. Their strengths are below porcelain, and are not as durable. It is important to discuss with your dentist which crown will be right for you. Dental technology is in the midst of a technology boom, and new materials are on the horizon.