Once the tooth is prepared, and the crown is received back from the lab, it must be cemented (“glued”) onto the tooth. This is done with one of a variety of cements. As a rule, any crown with cast metal coping inside (like the porcelain fused to gold crowns above) may be cemented with virtually any cement available to the dentist including temporary cements: ZOE, zinc phosphate, glass ionomer, zinc polycarboxylate, compomeres, resin modified glass ionomer, or filled resin cements. All-porcelain crowns may be bonded using only filled resin cements. Each cement has its own characteristics, and each situation may call for a different cement. For more information on these cements, as well as how they relate to other composite filling materials, please click on the dental materials button below.
Unfortunately, once a crown or bridge is permanently cemented to the prepared tooth, it is difficult, or often impossible to remove it without breaking the crown, or worse, the tooth itself. Every dentist owns an instrument called a crown and bridge remover. This is sort of like a spring loaded jack hammer. It contains a spring attached to a small internal hammer which releases when the dentist presses a trigger. This instrument has a hook on one end which is placed under the margin of the crown or under the pontic of the bridge to be removed. When the trigger is depressed, the instrument delivers a sharp shock to the crown in hopes of separating the cement/crown interface. This procedure generally needs to be applied several times to produce results.
Unfortunately the results may be catastrophic. The prepared part of the tooth often breaks off and comes out with the crown. Often, the porcelain on the crown is broken off. This instrument is successful only about half of the time in removing a crown from a tooth. Bridges are even less likely to be successful.
A number of companies produce other types of products for the removal of crowns and bridges. They all require that a hole be drilled someplace in the crown to allow entry of the instrument to the tooth/crown interface. Click here for a link to one such instrument. Note that crowns removed with such an instrument are often quite salvageable as the hole can be closed with an appropriate composite restoration.
The safest way to remove a crown or bridge is to sacrifice it and simply cut it off. The dentist makes one or more slices through the crown structure and uses an instrument to pry the fragments apart breaking the cement/crown interface. This is the method least likely to damage the tooth. A new crown is less expensive than an implant or a bridge. if the tooth must be replaced due to fatal fracture while removing the crown.
Properly done, standard crowns are tailor made to fit the teeth for which they were fabricated. As a rule, most dentists set aside a minimum of an hour (often more) to prepare the tooth and take the impressions necessary to send to the lab for the finished product. The time he or she spends doing this, plus the fees charged to the dentist by the lab for fabricating the crown out of gold and porcelain generally push the cost of a custom crown to anywhere from $800 to $2500 per tooth. (Click here to find out why there is such a spread in fees from office to office and area to area.)
A dentist does, however, have several alternatives to doing this type of expensive custom crown. These are called prefabricated crowns. In general, teeth requiring a crown are very badly damaged, and repair with filling material is not practical, impossible, or ill advised. In some cases, prefabricated crowns are a reasonable temporary alternative.
Prefabricated crowns are purchased by the dentist in assortment kits. They come in a variety of materials and graduated sizes, and are simply thin, tooth shaped shells.
Polycarbonate crowns are tooth colored and are enerally called “Ion crowns” in deference to the company that first manufactured them. Polycarbonate crowns are used to restore front teeth and can look quite good (they come in only one color, however), but tend to be somewhat fragile. Their life expectancy depends upon how much the dentist has to modify them from their original state to make them fit, and how careful the patient is with them.
Stainless Steel Crowns (SSC’s) are most frequently used to restore back teeth, and are affectionately referred to in the profession as “tin cans”. They are bulky and generally do not fit the tooth very well. The extra space inside between the stainless steel shell and the actual tooth structure is taken up by a heavy mixture of ZOE (temporary filling material). Because of poor marginal fit, stainless steel crowns are used mostly on baby teeth, but they can be used on adult teeth too, in a pinch. Unlike the polycarbonate crown, a stainless steel crown can last for many years. Prefabricated crowns cost between $150 and $300 because they require minimum preparation time by the dentist, and, since they are mass produced, they are inexpensive for the dentist to buy.
The reason that prefabricated crowns are not used all the time is because they are of an inferior quality to custom crowns, and since they are mass produced, they must be custom fitted by the dentist at the time of insertion. This custom fitting is generally quite imperfect. This means that the margins where the crown meets the tooth are always ill fitting, and this leads to gum problems if the crown is worn for more than a year. It also means that the contact that the crown makes with adjacent teeth may be poor allowing for food impaction between the teeth. These crowns frequently make excessive contact with the opposing teeth causing difficulties in biting, and adjustments to correct for excessive vertical height can cause considerable weakening of the finished prefabricated crown, especially in the case of polycarbonate. In the case of stainless steel crowns, it is usually necessary to adjust the opposing tooth (the one that bites on the stainless steel crown) which may do considerable harm to an otherwise healthy tooth. Finally, the appearance of these crowns is very poor in comparison to the the appearance of a crown that is custom made to fit the circumstances. In other words, a prefabricated crown is better than nothing, or better than an extraction, but it is nowhere near as good as the “real thing”.
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