Looking at the same tooth in cross section, you can see that the enamel is a thick glass-like substance that covers the sensitive internal portion at the top of the tooth. This sensitive internal part is really just an extension of the root above the gum line.  If the enamel is defective because of discoloration, decay, or is misshapen, the ideal solution would be to remove just the enamel and replace it with a new covering which would have a more ideal form and color.  And in fact, that is what we try to do.  The new covering is called (appropriately) a crown, and it is usually made of porcelain, or porcelain covering an internal metal or ceramic coping which gives the restoration  extra strength. (By the way, there is no such thing as a "cap" in dentistry.  This term is misused by the public to describe numerous dental services such as fillings, crowns, veneers and fixed bridges.  Its lack of a fixed definition makes it a useless term in the profession of dentistry.)

When a tooth is reduced in size to accept a crown, the portion that will underlie the crown is called the core.  The condition of the core affects the retention and stability of the crown which will be placed on it.  If the core is too small due to damage caused by old decay or fillings, then the crown may not retain well and could loosen or come off and need to be re-cemented  frequently.  If it is too thin, then it could snap off inside the crown when the tooth suffers serious trauma.

The full metal crown--The full metal crown is most frequently cast from jewelry grade gold (about 18 karate), however it may be cast from other alloys as well.  There are several categories of metal including high noble (jewelry gold--made from gold mixed with platinum or palladium), noble, (made from gold, palladium and silver), and base metal alloys (made mostly from nickel and beryllium).  These metals are discussed in detail on my course on dental alloys, and all make serviceable crowns.  This type of crown is used almost exclusively on back teeth, except when the patient requests a gold front tooth.  All-metal crowns have the chief virtue of being nearly indestructible, since they have no porcelain veneer which could chip or break.   Preparations for this type of restoration generally have a bevel around the cervical margin (the place where the crown ends and the tooth begins) in order to make for a tighter fit.  The bevel also serves a special function.  The metal bevel on the crown serves as a ferrule which adds greatly to the stability of the crown.  This ferrule-effect is one of the major advantages of all crowns that have a metal substructure.  A metal bevel engages the chamfer (the shelf around the neck of the preparation at the gum line) more efficiently than a flat chamfer finish line without a bevel.  It also is the best finish line to prevent leakage at the crown margin.

The all-porcelain crown (pictured on the left) allows  the color of the original tooth underneath to shine through.  These crowns have the advantage of being truly translucent, and show no metal at the gum line.  They are the closest thing to the ideal enamel replacement mentioned above that modern dentistry has yet invented. While they are not as strong as porcelain fused to metal crowns, their technology has improved over the years to the point where they are many times stronger than they used to be.  The strength of modern porcelain crowns derives partly from the bonding techniques used to attach them to the core.  The glass inside the crown is etched, and bonding agents are used to bond them directly to the tooth.  This causes the underlying core to act as a stabilizing substructure in much the same way that the metal coping does on a porcelain fused to metal crown.  Most all-porcelain crown systems require greater tooth reduction than the corresponding porcelain to metal coping restoration.  This is especially necessary on surfaces of the preparation facing the opposing dentition (the "biting surface") in order to provide sufficient thickness of porcelain to prevent breakage.  Greater tooth reduction is also necessary to provide enough bulk of the translucent porcelain to mask out any underlying discoloration in the remaining tooth preparation.  All-porcelain preparations are not cut with the bevel used in porcelain fused to metal restoration preparations.  These crowns look quite natural since they transmit some of the color of the underlying dentin.  This can sometimes be a disadvantage if the underlying core is seriously discolored.

The newest type of crown is one made with a zirconium oxide (zirconia) ceramic coping underlying a porcelain "veneer".  The zirconia coping is made on a computerized milling machine and is very strong.  It has the added advantage of being fabricated to match the shade of the ideal tooth core.  In other words, this type of crown looks quite natural, no matter how discolored the original tooth was.  The zirconia ceramic is nearly as strong as the metal coping used in the porcelain fused to metal crowns shown above, but with no metal underneath the porcelain, these crowns are more translucent and have better light transmitting properties.  The strength of the coping makes this type of restoration suite suitable for posterior teeth as well as anterior teeth.

The advantages and disadvantages of these three types of crowns

All-metal crowns--All-metal crowns have only one major disadvantage.  Their esthetics are poor.  Otherwise, they are the most stable and waterproof crowns available.  There is no porcelain to chip or break and they are nearly indestructible.  They can be made quite thin in areas in which there is little room, such as in posterior areas in which there is little clearance between the crown prep and the opposing tooth.  They require less tooth reduction than any of the porcelain crowns, and their bevel makes them more stable and less likely to leak than any of the porcelain crowns with the exception of the porcelain fused to metal crown.

Porcelain fused to metal--PFM crowns are the workhorses of restorative dentistry.  Their esthetics is quite satisfactory, especially if the dentist has provided enough room to accommodate an adequate porcelain thickness.  They can be built with no metal collar in esthetic areas, but even so, there is sometimes a noticeable dark line in the gum area where the porcelain meets the tooth surface.  Porcelain fused to metal crowns do not transmit light like a real tooth since a bright,  opaque layer of porcelain must overlie the metal coping so it does not "shine through" causing the porcelain to look gray.  PFM's are the strongest esthetic crowns in dentistry, and have the advantage of the added stability provided by the bevel at the finish line.   The bevel acts as a ferrule and greatly increases the retention and stability of the crown.  In other words, a PFM crown does not rely exclusively on a sometimes thin core to retain it.  The ferrule effect engages the full diameter of the tooth at the gum line.  Because of this, crowns with metal copings are less likely to snap off with the core still inside the crown if the tooth suffers a blow.  In addition to its ferrule effect, the bevel is a simple mechanical device for reducing any discrepancy that might occur at the seating appointment, and is quite effective at preventing leakage during service.

All-Porcelain crowns--All-porcelain crowns are esthetically the best choice for anterior teeth.  They are not considered strong enough for posterior applications. They transmit light like a real tooth, and are quite strong due to the bonding technique used to secure them to the core.  Note however in the diagram above that a larger share of the core must be removed in order to provide a fairly wide chamfer at the base of the preparation.  This is necessary because the porcelain cannot sustain thicknesses less than about a millimeter and a half without risk of fracture during service.  Since there is no bevel on this type of preparation, there is no ferrule effect. Even though this type of tooth is bonded to the underlying core and to the chamfer, due to inevitable seating discrepancies, the chamfer does not always serve as a stabilizing element.  This has the effect of making the core somewhat more likely to snap off inside the crown if the tooth suffers a blow.  Breakage of this sort happens rarely.  Of more import is the inherent weakness of the porcelain itself.  Even so, because they are bonded to the underlying tooth structure, these crowns are fairly strong and rarely break when used on canines and incisor teeth.

Zirconium based crowns--Crowns with zirconia copings are nearly as strong as porcelain fused to metal crowns.  They are suitable for posterior, as well as anterior teeth, and they have nearly the same excellent esthetics as all-porcelain crowns.  The tooth is prepared in the same manner as it would be for an all-porcelain crown, with a wide chamfer and no bevel.  Thus, while the porcelain rarely breaks off the coping on this type of crown, the core can become fairly thin during the preparation of an anterior tooth.  These crowns cannot be bonded to the underlying core, and are generally cemented in place with the same cements used to lute a porcelain fused to metal crown.  This means that the chamfer is somewhat less involved in supplying stability to the crown than it would be on an all-porcelain crown.  In other words, zirconia crowns offer no ferrule effect at all.  In addition, since the porcelain veneer is applied by hand, the marginal fit of the crown margin to the chamfer is less perfect than either the fit of a metal coping with a bevel, or an all-porcelain crown without a coping.

Which teeth need crowns?

If this were an ideal world, no one would would ever need an artificial crown placed on any tooth.  Unfortunately, people use sugar (especially as children) and get decay.  Or they break their teeth due to bruxing (grinding) or trauma.  Or sometimes the teeth are naturally crooked, discolored, or malformed.  All these situations might call for a crown instead of a filling.  Here is a list of situations in which placing a crown is the best, and sometimes the only solution:

• All back teeth that have had root canals should have crowns placed to prevent breakage of the tooth.  These teeth have lost their natural hydrating mechanism and tend to be brittle.  Even if it is adequately filled, the tooth surrounding the filling is likely to break sooner or later.  When the tooth breaks, since there is no nerve in the tooth, it frequently does not hurt, and is easily ignored by the patient.  But the broken tooth is subject to increased decay, and in a year or two, it may have rotted away to the point where it is not restorable.  
• Teeth that are more than half filling material should be crowned.  Remember that fillings are supposed to be surrounded by tooth structure.  As the filling gets larger, the tooth structure that supports the filling gets smaller, weaker and more brittle.  While the filling material is tough stuff, the tooth around it keeps breaking off over the years necessitating ever larger fillings until, sooner or later,  there is nothing left to fill.  By that time, there is little tooth left upon which to place a crown, and the dentist may need to perform a root canal so he can place a post and core inside what is left of the tooth above gum line in order to retain a crown.  
• Teeth that have "circumferential gingival decay" should be crowned.  This means that the decay is at the gum line and surrounds the tooth so that it "turns the corner" between the surface that faces the lips or cheeks to encompass the surfaces between the teeth.  Circumferential decay is nearly impossible to repair properly using standard filling techniques.
• Teeth with bad esthetics (poor appearance) generally benefit from well made crowns.  Many times, a patient's front teeth are heavily filled, misshapen, crowded, twisted,  and poorly colored.  This is an ideal situation for esthetic crowns on the top front teeth. These cases are quite rewarding for both the dentist and the patient because the patient walks away with a beautiful new smile.  Instant teeth! (Well almost instant anyways.)
• Patients who wear their teeth down because of bruxing can benefit from placing crowns (with metal substructures) on all their teeth.  This is an expensive option, but frequently is the only way to restore the original esthetics and function the patient had before the damage took place.

   

  Consider doing it! When the dentist says you need a crown, you really ought to think twice before rejecting the advice! Even though a good dentist can repair almost any tooth with a filling, he or she may recommend a crown instead.  Lots of people choose the filling anyways since it is always cheaper.  This is often a bad choice.   Very large fillings are technically very difficult to do.  You may leave the office with what looks and feels like a tooth only to find that a year or two down the line, there is recurrent decay under the filling.  It may be near impossible for the dentist to make the filling contact the tooth next to it leaving a gap which jams food between the teeth.  Pieces of the tooth or the new filling may break off over time.  The filling may even have required just enough removal of tooth structure to cause the nerve to die which will lead to a root canal followed by a crown, or even an extraction.  These problems are not the fault of the dentist.  There is a limit to what even the best and most conscientious dentist can accomplish with a very large, difficult filling.  Opting for a filling on a tooth that the dentist feels needs a crown may be opting for an extraction a year or two later.

The journey of a tooth from decay to crown

This is the complete story of the back tooth shown on the tooth-decay page.  The images of the tooth before and after restoration are impressive, but do not tell the whole story. 

The images above show what the tooth looked like on the day it was restored.  The hole you can see in the tooth does not tell the whole story.  The x-ray on the right comes a bit closer since it shows that the area of decay is much larger than is apparent from the clinical appearance, in this case almost touching one of the pulp horns of the nerve.  Decay is also creeping into the tooth from the other side as well.  In fact, the actual extent of the decay is generally larger than even is apparent on the x-ray.  In this case, the decay was excavated with no exposure of the nerve.  This often means that the nerve will survive after the filling is is done, but not always. 

The image on the left above shows what the tooth looked like after the decay was excavated, but before the filling was inserted.  It is apparent that this tooth has been weakened considerably by the amount of tooth that had to be removed in order to excavate the decay.  Since a filling does not actually "glue" the pieces of the tooth together, but rather sits passively in the preparation cut for it, the remaining tooth structure remains inherently weak and prone to fracture.  Had the nerve in this tooth remained in health after the procedure, one or more of the cusps would, in all probability have eventually broken in normal function.  For this reason, the treatment plan called for a crown to be placed over this tooth within about a year.  (We wanted to wait for a year to see if the nerve would die before crowning the tooth.)  As it happened, the nerve in the tooth became inflamed and the patient suffered a serious toothache necessitating a root canal procedure, a post and core and, finally, a crown.

What can go wrong?

It is normal for a vital tooth (one with a live nerve) to remain slightly sensitive to cold for as long as 6 weeks after a new crown is inserted.  However, the fact that a crown is necessary in the first place is an indication that the tooth was in fairly bad condition to begin with.  If the nerve in the tooth is still present (in other words, if a root canal has NOT been done) then there is always a chance that the nerve inside the tooth will begin to die after the crown preparation is done.  This often means that the tooth will become extremely sensitive to hot, or cold or even that the nerve could abscess.  Most dentists will recommend that a root canal procedure be done prior to crowning a tooth if he thinks that the nerve is at risk.  It is always a difficult call, and the decision whether or not to do a root canal is not always correct.  After all, root canal procedures add to both the cost and the risk of treating the tooth, and the decision to do one is not taken lightly. 

If, after a crown is done, the tooth should become "hot", it is still possible to perform a root canal in the tooth right through the top of the crown.  This is especially true if the crown is of a type that has a metal or zirconium substructure under the porcelain.  A hole in the top of a metal or zirconium coping does not seriously weaken the crown, and it still fulfills its structural functions.  The hole will eventually be closed with a standard filling.  If the crown is of a type that does not have a metallic or ceramic substructure (all porcelain), the crown will be seriously weakened by the access preparation.  This does NOT mean that the root canal procedure cannot be done through the crown, but MAY mean that the crown will have to be redone after the root canal if it later breaks apart.

Can a person be allergic to the porcelain or metal in a crown? I know of no situation in which a person has exhibited an allergic sensitivity to all-porcelain restorations.  Porcelain consists of metal oxides which are fused into glass.  Since dental glass is formulated to be insoluble in water (or any fluid that can safely enter the mouth), the metallic molecules are not biologically available to interact with the patient's immune system. On the other hand, it is possible to be allergic to nearly any metallic dental alloy.  Even gold can prove to be an allergen occasionally, but since it does not dissolve in oral fluids under most circumstances allergy to gold is very rare (on the order of 1 in a million).  Porcelain cannot be adhered to pure gold, and gold, by itself is too soft to be of practical use as a framework under porcelain.  Thus, other alloys must be used under porcelain-fused-to-metal restorations.  Gold is frequently used as a major component of these alloys.  High gold content metals used in the fabrication of Porcelain-fused-to-metal crowns are often called "porcelain-fused-to-gold" restorations.  The metal most likely to cause allergic sensitivity is nickel.  This is especially true of women, since most women wear jewelry, and inexpensive jewelry is often made with nickel.  This sensitizes the wearer to this metal.  Labs generally offer four choices of alloy to the dentist: gold, high noble, noble and non precious.  Noble metals contain a high percentage of palladium, which tends to be hypoallergenic (generally does not cause allergic reaction).  Some non precious alloys contain nickel while all the other classifications do not.  Very few dentists prescribe nickel containing alloys for use under metal to porcelain crowns.  Very few labs even stock alloys that contain nickel for this purpose.  As a consequence, we find that very few patients suffer from allergies to dental alloys.  Nickel is used in the formulation of stainless steel which is used to make prefabricated stainless steel crowns.  Stainless steel crowns are used on decayed baby teeth.  However, the stainless steel in these temporary crowns is quite hard and insoluble, and the nickel used in its formulation appears to be unavailable for allergic reactions.   An allergic reaction to dental alloys tends to develop over the course of several years and appears as a red (or magenta), inflamed line in the gums around the margins of the restoration.  The allergic reaction stays localized in the gums.  Upon rare occasion, the patient may experience a localized lichenoid reaction.  Systemic effects are unknown (ie. a reaction to dental alloy does not make people sick or cause cancer, or any generalized illness). The image on the left above shows the reaction of the gingival margin (gums) surrounding the margins of an older formulation (no longer manufactured) of porcelain-fused-to-metal crowns in a person who developed a metal sensitivity.  The image on the right shows the same patient several years after the crowns were replaced with all-porcelain crowns.

How are crowns done?

The above graphic shows some of the technical aspects of preparation for an all-porcelain crown.   It is reprinted with the permission of Ivoclar Viadent, the manufacturer of IPS Empress® which is a popular porcelain system used by dental labs to fabricate all-porcelain crowns and veneers.

The images above show a tooth that will be prepared for a crown preoperatively.  The tooth has been endodonticly treated (had a root canal) and has subsequently been restored with a post and core.  These posts are titanium and have the same x-ray density as the rubber root filling used to finish the root canal, but they are visible on the x-ray as the two bulky "fillings" that reach down into two of the roots.  The photo on the left shows the top of one of the posts at the surface of the composite filling that serves as a core.

The patient is anesthetized, and the tooth is prepared by "grinding" it down to what we call a "core" with a diamond milling bur on a high speed handpiece.  Notice how the tooth is prepared below the gum line so the metal margins of the crown will not be visible  when the final crown is inserted.  After preparing the tooth, both titanium posts are visible at the surface of the composite core.  

After cutting the preparation, an impression is taken of the patient's upper teeth, as well as the teeth in the arch where the prepared tooth lies.  Notice that the margins seen on the impression of the prepared tooth seem to "stick up" well above the level of the other teeth in the arch.  This is because the margins were intentionally placed below the gum line and the impression recorded their shape and position.

This impression was sent to a lab which specializes in making dental crowns.  In about three weeks, they sent back a finished crown (shown above) built to fit this tooth.  It is made of porcelain fused to a metal coping with the shade of the porcelain chosen to match the surrounding teeth as closely as possible. 

Above is the image of two crown preparations.  If you look closely, you can see that both have posts.  The front of the mouth is to the right.  The images below show how the crowns are received back from the lab.  They are on the models which were made by pouring a refined type of plaster (called dental stone) into the impression that the lab received from me.  The image on the left shows the gold coping inside one of the crowns.

The image below is how the crowns look when first inserted on the preparations in the patient's mouth.

Cementation of crowns

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, 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.

Can a crown or bridge that is permanently cemented be removed?

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.

Prefabricated crowns (quick and dirty--and cheap)

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.

Stainless steel crowns 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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