|
CROWNS: WHAT YOU NEED TO
KNOW
BEFORE YOU HAVE ONE
MADE
Written By Dr. Jeff Shnall
www.beachdentistry.com
This article can be heard on episode 6 of the Dental
Health Podcast if you click ton the following link: Crown Podcast: Episode 6
In this article we will discuss how to improve the appearance
and strength of your teeth with the use of crowns.
What is a crown? A crown is a thin cover made
of either metal and/or porcelain that is cemented or bonded onto a
tooth or an implant.
A crown covers your tooth the way that a hat covers your head or
the way a glove fits on your hand.
Crowns can be made to look like a natural tooth. The
cosmetic gallery at my website beachdentistry.com
has before/after photos of crowns that I have made
for my patients.
I think you will see the nice cosmetic improvement that crowns
can make to your smile.
What kinds of crowns are
there?
The most commonly used crown in dentistry is the porcelain
bonded to metal crown. This crown has an outer layer
of porcelain baked onto an inner metal core.
When this crown is made the outer porcelain layer is
bonded or fused to the inner metal core at high
temperatures in a special furnace.
Another common type of crown is made entirely out of
porcelain, hence the name ALL-PORCELAIN CROWN.
This crown does not have a metal core.
A third type of crown that we may use has an outer layer of
porcelain bonded to an inner layer of the strong ceramic material
Zirconia. We will call this a porcelain bonded to
zirconia crown.
The final kind of crown we use in dentistry is the
all-metal crown, which is made of either gold, a
semi-precious or non-precious metal.
We will discuss advantages and disadvantages of these different
kinds of crowns in a moment, but first,
When would you require a
crown?
Crowning a tooth can improve its appearance and can strengthen a
tooth that has been weakened by large fillings, large cavities, or
previous root canal treatment. Crowns can help rebuild
broken-down teeth and as well can help strengthen teeth that have
cracks.
What is involved in
having a crown made?
- At your first appointment, we
freeze your tooth. We remove any old filling material
and decay and refill the tooth with a bonded filling.
- Then we remove a small
amount of tooth material around the entire tooth and from the
top of the tooth. This creates room for the
crown.
- We make an impression or mold
of the tooth.
- This impression is then
sent to a dental lab that makes the crown.
- A digital photo is taken of
the tooth and is sent to the lab so they know the colour they
need to make the crown in order to match it to the neighbouring
teeth.
- We cement a temporary
crown on your tooth.
- The dental lab will make the
permanent crown and return it to our office in one to two
weeks.
- At your next appointment I
usually freeze the tooth.
- The temporary crown is
removed.
- The permanent crown is tried
on and I check its fit and appearance.
- We then either cement or bond
the crown to the tooth. We’ll talk more about this step
later.
If you need a crown what kind should be placed?
I find that most people do not want an all-metal or gold
crown placed on their teeth.
However, sometimes patients will choose a metal crown for
an upper back second molar which, in many patients, is completely
hidden from view when they talk or smile.
Metal crowns are virtually unbreakable and require removal
of less tooth material than any other type of crown.
Usually though, patients prefer a crown that looks like a
natural tooth. So that leaves us with essentially three
materials to choose from
Common types of Crowns
- Porcelain bonded to metal
- All-porcelain
- Porcelain bonded to zirconia crown.
- All-metal
So how do we choose what is best to
use?
In an article published in the Journal of the American Dental
Association in 2008, Dr. Terence Donovan indicates
that:
Many dentists are somewhat confused” aboutwhat
various kinds of crowns offer, regarding appearance, in
what situations they are best used,
as well as the potential life span of these various
types of crowns.
Dr. Terence Donovan is a professor of Biomaterials at the
School of Dentistry at the University of North Carolina. I
have included a link to his article on the show page for this
podcast as well as on my website.
Why would Dr. Donovan, a clinician who has lectured world-wide
on the topic of crowns feel many dentists are confused regarding
the particular strengths and weaknesses of different kinds of
crowns?
It has much to do with the way that dentists learn about new
materials and new technologies. This situation is no
different from that in medicine. Let me explain it this
way….
As a society we have benefited greatly from advances in
technology. Both in medicine and dentistry we are able to
offer treatments that were almost unimaginable a mere century
ago.
However new products and technologies are brought to market at a
staggering rate, and often the way that physicians and
dentists learn about new products is through direct advertising
from the manufacturer of the new product
Some dental magazines distributed to dentists publish
research reports about new products describing their benefits
or effectiveness, but these research studies are often paid
for by the manufacturers of the product.
Some dentists base their decisions on which products to use
based on these reports rather than on independent research done by
investigators that do not have a conflict of interest or bias
regarding the result of the research.
And some dental schools don’t teach students how to critically
read research studies to determine if they are carefully controlled
studies that actually prove the claims they are making.
Sometimes manufacturers will provide studies that claim to show
that the new product they are selling is worth using. However
when you actually read the raw research data in the study,
published results may not be as impressive as the
manufacturer claims.
Unfortunately sometimes manufacturers exaggerate the
effectiveness of their products.
As well, many new drugs and materials enter the marketplace
every year with little or no long term data on how safe or
effective they actually are. Google the drug Vioxx for an
example of this.
So with all this in mind let’s get back to our question, what is
the best type of crown for you? To answer this question let’s
look at what two prominent dental educators have to say on the
topic. I am referring to Dr. Terry Donovan who I
mentioned a little earlier and Dr. Gordon
Christensen.
Dr. Christiansen is a well known
researcher and dentist, and is adjunct professor of dentistry
at both BrighamYoung University and the Universityof
Utah.
These two respected clinicians wrote articles that
appeared in the Journal of the American Dental Association, a long
established and reputable dental journal.
These articles were published in 2007 and 2009. Links to
all of these articles are in my show notes.
These clinicians are basing their recommendations
regarding choice of crowns on what sound research studies conclude
on the performance various kinds of crowns.
Let’s first discuss what they had to say
about:
Porcelain bonded to metal crowns
- These have been the most common kind of crowns used in
dentistry for the last 50 years.
- They have a good record of longevity and if properly made,
seldom break.
- Porcelain bonded to metal crowns can be placed anywhere in the
mouth, front or back, with a high chance of long term
survival.
- This last point is significant. We put heavy biting force
on our back teeth when we chew and grind and not all types of
crowns can withstand these strong biting forces, as we shall soon
see.
- It is the metal core that gives strength to the
porcelain bonded to metal crown.
- A good lab technician can get a very good cosmetic result with
porcelain bonded to metal crowns, however in some situations, a
better cosmetic result can be achieved with an all-porcelain
crown.
- the problem with porcelain-bonded-to-metal crowns is sometimes
there can be a grayish tone to the crown if the underlying metal
isn’t properly masked by the porcelain.
- This is not always the case and I have had good success with
porcelain bonded to metal crowns in my practice for many
years.
Research studies have shown that porcelain
bonded to metal crowns will often outlast all-porcelain crowns, as
a crown made entirely out of porcelain is more
fragile.
Without the strong metal core, cracks can start and spread
through the all-porcelain crown until the crown eventually
fails.
• If an all- porcelain crown
cracks or breaks it will need to be removed and replaced with a new
crown.
• In my experience, many
patients would rather have a porcelain- bonded-to-metal crown
placed, once they learn that more likely to be troeuble-free
as compared to the all-porcelain crown.
Drs. Donovan and Christianson recommend that for our front
six teeth, the teeth that are front and centre when we smile,
porcelain fused to metal crowns should be the first choice due
to their strength.
However, Dr. Donovan states that bonded all-porcelain crowns can
be used in the front six teeth in patients for whom the
appearance of the crown is more
important than the longevity of the crown.
While the all-porcelain
crown may be fine for our front six teeth both Drs. Donovan and
Christianson make the point that they should not be placed any
further back than the six front teeth, and all-porcelain crowns
especially should not be placed on the molars, which are our large
teeth at the back of our mouths. That is due to the
increased risk of the all-porcelain crown breaking
the further back you go in the mouth. This is due to
higher biting forces that we exert on our teeth the further back in
the mouth you go, forces which can measure several hundreds of
pounds of pressure in some patients.
This brings up the issue
of all-porcelain crowns being made by dentists right in their
offices on a computerized machine. Some dentists advertise
“crowns made in one appointment while you wait”.
In this procedure
the dentist reduces the size of your tooth the way I described
earlier. The dentist then scans the tooth and sends image of the
tooth into a computerized milling machine. This machine then
carves a crown out of a block of porcelain while you
wait.
This crown can then
be cemented or bonded on your tooth in one appointment. As
stated above, this all-porcelain crown made in one appointment is
not recommended from a strength point of view by Drs. Donovan and
Christiansen if placed on a back tooth, especially a
molar.
Let’s now talk about
Porcelain bonded to Zirconia crowns. These crowns have an
outer layer of porcelain bonded to an inner zirconia
core.
These crowns
are stronger than the all-porcelain crown and can be placed on both
front and back teeth with less chance of breaking than
an
all- porcelain
crown.
The appeal of these
crowns to some is that these crowns look like they are entirely
made of a tooth coloured cermamic material. However the
question is: are porcelain bonded to zirconia crowns as
strong as a porcelain bonded to metal crowns and is there any
advantage with going with the Porcelain- Zirconia crown rather than
the traditional porcelain-metal crown?
Let’s look at what a
zirconia bonded to porcelain crown actually is.
Zirconia is actually
composed of a white metallic powder known as zirconium
oxide.
When this white powder is combined with
Yttrium and heated to a high temperature it transforms into a
soft white
metal we call zirconia.
Zirconia can be then carved into the
shape of the desired crown. This crown is can
then be heated in a furnace
to a high temperature to form a very hard ceramic.
Porcelain can then be
layered onto this zirconia core and the crown can then be fired
again to
result in the finished porcelain bonded to zirconia
crown.
Porcelain-Zirconia crowns
have been used in dentistry since the early 2000’s.
So what do Drs. Donovan and Christianson have to say about
porcelain bonded to zirconia crowns?
They both point out research that states porcelain bonded
to zirconia crowns chip and fracture at a higher rate than
porcelain bonded to metal crowns.
Theses failures are in the outer porcelain
layer but not in the zirconia core itself.
The chipping of the outer porcelain layer was a problem when
porcelain bonded to metal crowns were first introduced
to dentistry many years ago but no longer. Scientists got the
bugs worked out of porcelain bonded to metal crown construction
50 years ago.
However, scientists still have not found the best
way to get the outer layer of porcelain to stay tightly bonded to
the underlying zirconia core with the same strength and reliability
that exists with porcelain bonded to metal crowns.
These authors conclude that Zirconium oxide–basedcrowns need
more time to be fully proven in terms their long term
reliability.
Another point that should be kept in mind is zirconia can not
be bonded to tooth material, only cemented.
This can be a disadvantage of zirconia based crowns
compared to traditional porcelain bonded to metal crowns.
There are situations where it would be nice to have that extra
grip that bonding a crown to a tooth gives you rather than just
cementing the crown provides.
Bonding the crown means chemically attaching the crown to the
tooth surface. When you bond a crown to a tooth the crown is
fastened much tighter to the tooth than if it were merely
cemented.
One such situation is when you are placing a crown on a
relatively short tooth. Bonding a crown to a short tooth can
make the difference between success and failure of the crown.
All-porelain crowns as well as porcelain bonded to metal crowns
can be chemically bonded to teeth.
So why would a patient want a porcelain bonded to zirconia
crown if porcelain bonded to metal crowns are
less likely to fail?
Crowns made from Zirconia appeal to patients who
say they don’t want any metal their body. This is
ironic as Zirconia is referred to as a ceramic but is, as I stated
earlier, a metal.
Chemically, zirconium is similar to titanium which is a metal
used in medicine and dentistry..
Zirconium is chemically similar to titanium and it is well
tolerated by the body, as is titanium . In fact, Zirconium
has been a component in artificial hips for the past several
years.
For patients who are sensitive to certain metals they may be
less likely to react less to a zirconia based crown that they
would to a porcelain bonded to metal crown metal.
Note that porcelain bonded to z irconia crowns cannot be made
in-office and certainly can not be made while you wait in one
appointment.
They have to be in a dental lab with specialized equipment
that many labs do not have. The process is also time
consuming.
For a few final words on the performance of porcelain
bonded to zirconia crowns I will mention some facts raised in a
reaearch paper written by dental students at the University of
Toronto in 2009 that I found through an internet
search. These students examined all research studies
published in the English language that reported
on the performance of these crowns.
In their paper, Zina Alwash and her co-authors found
that there were relatively properly designed studies that compared
the performances of porcelain bonded to zirconia crowns to the more
traditional porcelain bonded to metal crowns. Of the studies
that had reliable data, they found:
- Most of the failures in porcelain bonded to zirconia crowns
occur where the outer layer of porcelain meets the inner zirconia
core.
- Although zirconium was introduced to dentistry more than
a decade ago years ago, few well conducted clinical studies are
available that determine the longevity of porcelain bonded to
zirconia crowns
- They also mention that there are no long term studies that
compare the performance of porcelain bonded to zirconia crowns to
porcelain bonded to metal crowns.
- They also site research that showed porcelain bonded to
zirconia crowns have a higher rate of breakage and chipping
when placed on the back teeth than when placed on the front
teeth.
As we near the end of this article, I thought I would add one
more piece of information to round off this discussion.
There is yet one more kind of crown now available to you:
this is the all–zirconia crown, that is a
solid zirconia crown without the added porcelain layer.
There is little is known about the performance or longevity of
pure Zirconia crowns as they are so new to dentistry.
They are being marketed to dentists for patients who grind their
teeth heavily. This is a relatively small subset of the
patients the typical dentist will see.
The thought is that if you have a patient who grinds their teeth
so severely that they break the porcelain off their crowns, an all
zirconia crown will offer a crown that looks like porcelain but is
as strong as an all-metal because it zirconia is a metal
.
In the past, severe tooth grinders would best be
given crowns with a gold or metal biting surface to eliminate the
chance of porcelain fracture. Time will tell how all-zirconia
crowns perform.
One final thoughts if you are considering a crown:
Patients who habitually grind and/or clench their teeth or who
compete in contact sports should think twice about choosing crowns
mad entirely of porcelain, due to their increased risk of fracture
as compared to porcelain bonded to metal crowns.
References and Further
Reading
Below is a list of articles I used as source material for this
podcast. I did place hyperlinks to these articles but I
recently found out that the american ental Association does not
allow this. You can "Google" the articles below. I was
able to access these articles for free over the internet when i
prepared this article and I hope you will be able to as well.
1. Choosing an
all-ceramic restorative material: Porcelain-fused-to-metal or
zirconia-based? Gordon J. Christensen, DDS, MSD,
PhD,J Am Dent
Assoc, Vol 138, No 5, 662-665.
© 2009, American Dental Association.
2.
Porcelain-Fused-to-Metal Versus Zirconia-Based Ceramic
Restorations, 2009,Gordon J. Christensen, DDS, MSD, PhD
,J Am Dent Assoc,
Vol 140, No 8, 1036-1039.
© 2009 American Dental Association.
3. The Efficacy of All ceramic Zirconium crowns by:
Zina Alwash, Showkat Ali, Haleh Hedayatian,
Humaira Hassan, Sarune Leskauskiene, Mandana Tehrani,
Oksana Vozna
University of Toronto. Dental Student Research Paper,
2009.
4. Factors Essential
for Successful All-Ceramic Restorations, Dr.
Terence E.
Donovan, DDS , J
Am Dent Assoc, Vol 139, No suppl_4, 14S-18S.
© 2008, American Dental Association |
