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Crowns page 2

 

 

 

Crowns; Page 2

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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. 

Before Caries on x-ray

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. 

Excavated Caries restored

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.

For dental professionals and those studying to become dental professionals,  I have prepared an entire course to help explain how and why dental casting metals are formulated the way they are.

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 hypo allergenic (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 (i.e.. a reaction to dental alloy does not make people sick or cause cancer, or any generalized illness).

Allergy allergy

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?

Ivoclar crown prep

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.

tooth before prep Pre-op x-ray

The images above show a tooth that will be prepared for a crown preoperatively.  The tooth has been endodontically 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.

Crown prepThe 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.

Impression Impression

 

Crown back from lab Post-op

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. 

Crown preps

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.

Metal coping Crowns on model

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

Crown insert

Digital impressions

The newest method of taking an impression for a crown or bridge does not involve using an elastic rubber paste in a tray as shown above.  A digital impression uses a hand held scanning sensor similar to the type used to scan barcodes at the supermarket.  The dentist dries the teeth and sprays a powder over the area to assist the scanner to visualize the teeth.  The scanner is then held over the teeth to obtain a digital three dimensional image, followed by additional scans of the opposing dentition.  Multiple scans are taken, from several different angles.  Once stored in the computer's memory, these images can be merged and manipulated to create a very detailed model of not only the prepared teeth, but also of the adjacent and opposing dentition.

digital impression

Crowns; Page 3==>>

 

 

 

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