A crown is one way to restore damaged teeth by either capping or surrounding the remaining tooth structure or dental implants. Its most commonly used to seal off and protect a root canal or tooth structure where a cavity has wreaked havoc, and also to restore health and look of damaged tooth. The need for one is determined by the dentist, and is usually only recommended with the current condition of the tooth is not sufficient for regular use. They can be for a single tooth or implant, as an abutment in a bridge (see Bridge above), on a partial, removable denture, complete dentures, or for prosthetic mouth pieces. They are bonded to the available tooth structure with cement.
Preparing the mouth for a crown has many facets. Firstly, the enamel will be removed, leaving only dentin. Additional structure may be removed depending on what type of crown will be placed upon the remainder, or if the tooth structure is inadequate for crown placement it will be built up with composite resin, amalgam, or in extreme cases, a post and core. The remaining structure needs to have a 3-5 degree taper and the dentist also needs to take into consideration the margin, or visible original tooth structure line, as well as the ferrule effect, the 2mm height of tooth structure the crown should cover. Then either an oral impression or 3D scan is taken before the crown can be created. Often, multiple visits are required to complete these preparations before the crown is ready to be placed.
Materials Used to Create Crowns
Crowns can be created from many different kinds of materials. Indirect methods, where the crowns are cast or fired outside of the mouth, are more common than the direct method.
Gold crowns are often favored for their durability and relatively similar costs of other materials. These caps are made from a single piece of alloy, and must contain at least 40% gold and 20% more of noble metals, that is platinum, gold, or palladium. The remaining composition of the alloy must be silver, copper, or tin. In order to replicate the patient’s mouth, their dentist will remove tooth tissue and make an impression of the tooth and send it to a laboratory where the gold will be shaped.
PFMs, or porcelain-fused-to-metal crowns is often used for front teeth restoration. The shell of the crown is metal, providing strength. A porcelain front is fused to the visible parts of the shell in a high heat oven, and is for aesthetic purposes. Any type of alloy with precious or base metals can be used and the veneer front is matched to surrounding teeth or gums. Again, impressions are taken and then sent to a laboratory for this type of crown.
A leucite reinforced crown, also known as the Empress Crown, are cast by injecting a ceramic re-enforced with a mineral composed of potassium and aluminium tectosilicate into a mold. It is then stained or glazed to match the patients tooth color. These types of crowns are a bit more costly, but have a very low crack rate and high reliability rate.
Zirconia crowns are the strongest full ceramic crowns available. Additional ceramic layers can be added to zirconia crowns for aesthetic purposes, but the facades won’t last as long as the zirconia will, so it is not often used for front teeth restorations. However, zirconia allows light to pass through it better than other metal options, offering a more natural appearance. The hot/cold sensations felt with other metal dental restorations are not felt as much with zirconia because reduced thermal conductivity. A 3D scan of the patient’s mouth or impressions is taken and then the zirconia core is milled from a block of the super ceramic in a hot-but-not-melted state. Then it is placed in a mild-heated oven, where it shrinks and hardens. Additional layers are added to create the perfect fit.
The only crown that is created immediately for a patient is a CAM/CAD chairside crown (Computer-Aided Manufacturing / Computer-Aided Design). A photographic image of the patient’s current or lack of tooth is used to create a 3D version of the proposed crown or inlay/onlay. The dentist tweaks and adjusts the image and sends it to an in-office ceramic mill machine. The machine chooses a piece of ceramic that matches the patient’s teeth color and uses its diamond burs to create the crown. The dentist can then remove any unnecessary ceramic on the piece, fit and cement the crown into the patient’s mouth in the same visit This type of crown has very similar strength, durability, and crack/failure rates as a natural tooth.
There are two types of alumina crowns, one infiltrated with glass and ones without. Glass infiltrated alumina crowns are slip casted with electrical currents and then filled with glass and have a higher bond strength to tooth dentin then CAD/CAM zirconia or their non-glassed filled counterparts. Non-glass filled alumina are milled from softened blocks similarly to CAD/CAM techniques. However, cores milled this way must be enlarged because they shrink more when they are fired. Just like zirconia, they are layered with additional ceramic layers to match color and shape of patient’s teeth.
Costs and Longevity
Our numbers reflect the costs reported on http://health.costhelper.com/dental-crown.html: Porcelain-fused-to-metal crowns can cost $500-$1,500or more per tooth. CostHelper readers with insurance report out-of-pocket expenses of $282-$1,000, with an average payment of $618. Metal crowns of gold alloy or of base metal alloys (non-noble) can cost $600-$2,500 or more per tooth. CostHelper readers with insurance report out-of-pocket expenses of $519-$1,140, for an average payment of $882. All-porcelain crowns require a higher level of skill and take more time to install than metal or porcelain-fused-to-metal crowns, and can cost $800-$3,000 or more per tooth.
Single crowns have a minimum life expectancy of 10 years so many dentists quote this age, but if cared for properly they can last up to 50 years or more with proper oral hygiene. Full gold crowns last the longest with solid ceramics take up second place, while PFMs tend to be the weakest (but still enduring) due to the thin, beautifying layers of porcelain that are prone to cracking. Most U.S. insurance plans sanction crown replacement every 5 years.