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One of the newest developments
in dentistry is porcelain fused to zirconia crowns. Prior to this, metal
formed the inner layer of most crowns, resulting in that unsightly gray line you
see on so many people.
This was known as a porcelain-fused-to-metal, or
PFM. If your dentist used cheaper metals, the crown actually turned your
gumline black--permanently. And gums inevitably recede from where the
dentist prepared the crown, so the gray line of metal that was intially below
your gumline becomes plainly visible down the road. The one good thing is
that the metal formed a very flush, tight seal with the tooth, minimizing plaque
buildup and crown re-decay.
The first solution to this problem was the porcelain margin, (margin is the
seam where the crown meets the tooth) which looked good but sealed relatively
poorly. They are rarely as flush as a metal margin, so plaque accumulates,
and the gums often get puffy and red around them. Higher plaque buildup
increases the chance of crown re-decay.
Next was all-ceramic crowns, which looked good because they removed the metal
substructure. Brands of pressed all-ceramic crowns include Empress and
Procera. They are pressed under pressure for strength, or milled from a
block of pressed ceramic. Sirona invented a machine called the CEREC 3D
which actually mills the crown while you wait, ending the need for temporary
crowns. (If you're wondering why we don't have it, it's a $100,000
machine) Pressed ceramics have made progress in esthetics, but cannot
match the best hand stacked porcelains...yet. Their weaker strength
required more thickness, sometimes meaning drastic reduction of tooth.
About the same time, companies started developing reinforced all-ceramic
crowns. They use a strong Zirconia substructure as the foundation for
pressed ceramic. Zirconia reinforced porcelain crowns can have better
esthetics than the traditional porcelain-fused-to-metal crowns because zirconia
is white--no gray lines, with a very good marginal seal. Tooth reduction
is comparable to traditional PFMs. Strength is not quite as good, but more
than sufficient for single crowns. Doesn't solve the need for esthetic
bridges, but that's what we have implants for.
We cemented our first zirconia crowns today, and I was very impressed with
the results. We ordered some front teeth units; I'll post pictures in a
couple of weeks when they're ready.
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