Thursday, June 4, 2015

All-Porcelain Crowns

All-Porcelain Crowns

There have been a multitude of advances in esthetic dentistry in the last 10 years.  Materials have changed significantly to now have greater strength and more lifelike appearance, as well as more flexural forgiveness and versatility than we have had in the past when making tooth colored crowns.  For 60-odd years, the standard procedures used by dentists consisted of either gold crowns, or porcelain fused to metal (baking porcelain onto the outside of a gold crown.  While there are many new products from dozens of companies, I would like to look at the two that I have found to be the most usable of the alternative materials available.

First is the use of Lithium Disilicate, IPS e-max being the most recognizable of these brands.  This crown offers significantly increased flexure strength (basically, how much you can compress something before the sides break/shear), as well as an increased esthetic option due to the lack of metal under the porcelain.  Lithium Disilicate can be carved from a solid block, pressed into form from smaller grains (monolithic, which exhibits greater strength), and/or veneered with porcelain for increased esthetics.  Personally, I think there are three ways that this crown best used:

1.     Use as a partial crown on posterior (back) teeth
2.     Use as a full crown alternative on posterior teeth where you don't want to remove a lot of tooth structure
3.     Use as a veneer material when attempting to make front teeth look perfect, but also want strength

While we can technically use either standard cement, or bonding cement, I greatly prefer the use of bonding cement.  While it necessitates getting numb a second time for insertion of the crown, long term studies on PFM (older style porcelain to metal crowns) from the past have shown that using the stronger cement leads to a significantly longer life span for the crown.  The use of this cement also allows for the partial 1/2 crowns that can be made conservatively, and then attached with the resin cement, where the stress point for the tooth is then born on the cement and bonding itself.  This means that if a conservative e-max crown comes off in the future, it is more likely to be cleaned up, and re-cemented without being remade.

The other material that has shown wonderful performance is Zirconium (ZrO2).  This material has been improved in appearance just in the last few years from the 3M company when they introduced Lava Plus, which is a customizable colored restoration (the original were significantly monochromatic, like piano keys).  Zirconium restorations have a huge advantage in pretty much every strength category over any other kind of crown (Lithium Disilicate included).  The reduction for this material to make a crown is almost exactly the same as a gold crown, which can be made very thin.  In my opinion, the best use of their crowns is either as a base to which porcelain can be veneered on front teeth, or without any porcelain on either second or first molars, where teeth tend to be short.
There are a few areas where I feel questions remain about Zirconium.  While it has been shown to be relatively non-abrasive against natural teeth, there is little information about what happens when you place it biting against an older porcelain-metal crown, or a Lithium Disilicate crown.  Until proven otherwise, I believe the likelihood is that it would be too strong, and wear the other crown heavily.  They are very difficult to take off if there is a problem or a cavity around the crown later in life.  They are so strong, that they often wear out our drill bits.  It can be done, it just takes longer.  I personally ask for these crowns to be polished, rather than stained to match existing teeth.  While they may appear slightly different than natural teeth, they should be very close.  Statistically, glaze will tend to last only 6-18 months in an individual’s mouth before saliva and normal wear eventually take it off (imagine running a porcelain cup through the dishwasher 100,000 times and what the surface texture looks like).

My opinion on porcelain crowns has changed significantly over the last five years due to the advent of these new technologies and materials.  I grew up in the dental industry, and was trained that gold is the 'gold standard' of restorative materials.  Regardless of the position or placement of the tooth, gold crowns WILL last longer than any other kind of crown.  While I still believe that this is true, I am asked on a daily basis about esthetic options that will allow teeth to look like....teeth.  To quote a patient recently who was trying to decide which kind of crown to use on a lower molar: "So I can have the crown for 20 years, and dislike it (gold), or I can have it for 15 years and love it."  That seemed to sum up the esthetic revolution in dentistry pretty well.

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