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Crowns

 

 

 

Crowns; Page 1

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Relationship of crowns to rootsWhat is a crown?

The crown of a natural tooth is that portion which is covered with enamel and is generally above the gum line.  In the schematic on the right, the roots of the teeth are colored yellow, and the crowns are covered with white enamel.  This is the ideal state of nature, and this shape, color and relative proportions are about what dentists aim to recreate with artificial materials whenever the natural crowns of the teeth are deficient in some way.

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.

Porcelain fused to metal crown in xross sectionThe porcelain fused to metal crown

The image to the right is a schematic of a prepared tooth covered with a crown made of porcelain covering a metal coping.  The one on the left below shows the same preparation with an all porcelain crown.  The difference between the two types of crown involve strength and appearance.  The porcelain that is reinforced with the metal coping is stronger and usually shows at least a tiny strip of metal at the gum line.  This is generally hidden under the gum line so the appearance of the finished tooth is not adversely effected.  This type of crown is generally used on back chewing teeth, and on front teeth in the case of people who habitually grind and clench (brux).  In spite of the fact that they have a layer of opaque metal underneath the porcelain, these crowns look very nice because the method of applying the porcelain to the metal coping has been perfected over the years.  The newest types use the reflective properties of gold to enhance the translucency of the final result. 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 is always made in metal, the reason that the margins of this type of crown are generally hidden under the gum line where they do not show.   This type of crown has many of the stability advantages of the full metal crown.  The metal bevel on the crown serves as a ferrule which adds greatly to the stability of the crown, especially in cases in which the core is small, or fragile owing to the greater tooth reduction necessary for a sufficient thickness of porcelain to mask out the color of the underlying metal coping.  This ferrule-effect is one of the major advantages of porcelain fused to metal crowns not found in crowns made of all-poprcelain or with a zirconia coping.  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, and makes the crown less likely to leak.  You can learn more about how this type of crown is made by clicking here.

All-Porcelain Crown in cross sectionThe all-porcelain crown

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.

Porcelain fused to zirconium core in cross sectionCrowns made of porcelain fused to a ceramic coping

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 quite suitable for posterior teeth as well as anterior teeth.

Zirconium crown in cross sectionAll-Ceramic Crowns

Finally, a crown may be made entirely from a ceramic material such as zirconia or alumina, with no porcelain veneer.  These all-ceramic crowns have the advantage of great strength and ability to resist wear while being tooth colored.  They have the disadvantage of being opaque to light, which means that they are not translucent like crowns with porcelain veneers.  They do not shift their color properties in the same way as natural teeth when seen in different lighting conditions.  Even if they match the surrounding teeth fairly well in incandescent lighting, they may differ considerably from their surrounding neighbors in sunlight or fluorescent lighting.

All-ceramic crowns are very strong and resist wear even better than cast gold crowns.  they have become the new alternative to gold since the price of gold has risen to all time highs. 

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.

Porcelain fused to Ceramic copings --Crowns with zirconia or alumina copings under a porcelain veneer 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.

All Ceramic crowns--Crowns made entirely from zirconia have become the new "full-gold" crown.  They are as strong as gold and protect the crown of the tooth in the same way as gold, while requiring less tooth reduction than crowns made with porcelain.  They can be built with bevels at the margin to further protect and retain the crown of the tooth.  They are tooth colored, but not as esthetic as crowns built of porcelain, or those with porcelain veneers over a stronger core.  They are ideal for posterior teeth or for teeth worn by patients who are severe bruxers (grinders). 

For a thorough understanding of glass and porcelain, students and dental professionals should consult  my five page course "Dental Ceramics for the beginner"

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 anyway.)
  • 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 any ways 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.

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