NIOM Webinar – Dental Ceramics

NIOM Webinar – Dental Ceramics

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Dear colleagues, welcome to this webinar
hosted by NIOM on dental ceramics material choice and treatment options. We are so happy to have our
visiting scientist Dr Marit Øilo from the University of Bergen that
will present this webinar to you. Welcome to this webinar about dental ceramics. My name is Marit Øilo and today we’re
going to talk about the different ceramic materials, how to choose between
different materials and what are the limiting factors of the dental ceramics. We have three main groups of dental ceramics:
the porcelains, the glass ceramics and the high-strength ceramics, ceramics
which today are the zirconia materials. They have some similarities, they’re all
stiff and brittle materials, but they have also very large differences among
them, and these differences require a different kind of preparation
technique, different handling and they have different prognosis. The porcelains are mainly feldspathic
based, which also contains a lot of silica, so that they are all
often called silicate-based materials. They are made from a slurry,
which is the solution of powder and liquid baked in an oven, which is
called layering and sintering technique. But the porcelains can also be made
by machining from a set product. There are many different porcelains
on the markets but they all have this in common that they have low
bending strength but high aesthetics. So if you want to make restorations that
are mimicked entirely in the natural dentition, also called dental arts in a
way, these are the materials you want to use, either alone as veneers or
veneering crowns or in a layering technique of metals or core ceramics. Regarding clinical success of porcelain, there are many high-quality studies
with high success rates. Very few complications are noted
and the ones that are, are fractures, as you can see here on these images,
or discolouration of margins. In order to use these materials,
you have to make a preparation that optimizes the use of the porcelain. Even though the material can be
made into very thin crown margins, a very thin crown margin increases
the risk of pores or defects or cracks in the crown module, which can
later lead to total fractures. If you make a shallow chamfer preparation,
you allow the dental technician to have some space to make sufficiently thick crown
margins, which are also easier to seat. The glass ceramics are
a type of material that are made out of a glassy material
or morphos material which can be heated and molded by the lost-wax
technique or milled by CAD/CAM technique. Then you end up with the restoration
that is made out of a glass, but this glass contains minerals that can crystallize
if you heat it in a certain way. During this heating period,
the crystals grow from within the material without any dimensional
changes of the restoration. So you can see here how the material
looks after the pressing or the milling, which is a glassy material, and then
after the crystallization process, you can see that there’s lots and lots
of crystals within the material. The reason we want the crystals within the
material is to increase the fracture resistance. If a crack goes into
the glassy material, or the amorphous phase,
it moves freely through the material, but as soon as it
reaches a crystal, it will require a lot more energy to go either through the
crystal itself or beyond the crystal or around the crystal and thereby
increasing fracture strength. Regarding clinical success of glass-
ceramics, there are many studies with high clinical success rates. In fact, glass-ceramics are
one of the most well- documented prosthetic
treatments you can find. There are relatively few
complications and the ones that are, are a loss of
retention and fractures. And here you can see two examples,
one of a very successful frontal tooth and one not so successful molar that
broke after less than three years of use. Regarding the high-strength ceramic
materials, also called the poly-crystalline materials, zirconia is
the most commonly used today. Zirconia is made up of one zirconium
atom and two oxygen atoms that together form a molecule called zirconium
dioxide, also shortened as zirconia. The zirconia molecules
organize themselves in a lattice structure that can
have different shapes. In natural form, they
organize themselves in a monoclinic structure,
but if you increase the heat and temperature, the lattice structure
would transform into a tetragonal shape, and if you increase the heat and temperature
even further, it will transform into a cubic shape. If you then release the trapped pressure
and temperature, the process will reverse. Unfortunately, a monoclinic shape lattice
structure is not the stable material, it cracks and deteriorates, so we want
to secure it in the tetragonal structure. That we can do by introducing
a stabilizing oxide called yttrium. Then we end up with an yttria stabilized tetragonal
zirconia poly-crystal material shortened by Y-TZP. This material is then kind of forced into
a metastable tetragonal structure. This structure can then be
transformed to monoclinic if it’s exposed to substantial stress, which is
called a t–m phase transformation. What happens, is that when a stress occurs
at the beginning of a crack going through the material, some crystals would then
transform into the monoclinic structure, and the monoclinic structure will then take
up a little bit more space than a tetragonal, creating a compressive seam around the crack-tip
and arresting the crack propagation. If you look at it at a larger scale, you will
see the crack going in through the material. Some of the crystals will transform into
the monoclinic structure, taking up more space and then
making it more difficult for the crack to propagate
through the materials. So the crack doesn’t stop, it just
requires a lot more energy. The crack doesn’t disappear, but it requires
a lot more energy to go further into the material. The clinical success of zirconia is not so
well documented as the previous materials. There are relatively few studies on
single-unit crowns but there is quite a number of studies on three to five
unit fixed partial dentures. And there are beginning to come some
studies on larger restorations as well. These studies show moderate success rates. Especially the early ones had a lot of
chipping problems because of the poor compatibility between the veneering
material and the core material. You can also see increased rates of loss
of retention for fractures and pulpal damages compared to metal ceramics. The interesting thing is that very few
of the studies focus on the aesthetics. Are the material or are the restorations
actually prettier than metal ceramics? We don’t know, based on the
clinical trials, that they are. But zirconia is not only zirconia. There are many different types of zirconia. The first material we had
was the white material, opaque white and opaque material, often
termed the first-generation zirconia. Then we started receiving more tooth-coloured
and semi-translucent material, and now recently the ultra-translucent
material called third-generation zirconia. The traditional or first-generation zirconia
has a 3 volume percent yttria-stabilization. They are primarily in tetragonal
crystals, they are white and they are intended for bi-layered structure
with a veneering ceramic on top. They can be produced by hard
machining and soft machining and these have traditionally very high
strengths, even above 1,200 MPa. Since it requires to be a bi-layered structure,
we need to prepare a bit deeper into the tooth. We still want the chamfer around the
finish line like a chamfer preparation, but it needs to be deeper than
from the previous materials. If we do not give enough
or sufficient room for the material, we risk
having very rugged or chipped margin, so we also even risk having
tiny little chips off the margin like this. The second generation of zirconia, the
tooth-coloured and semi-translucent material, has a bit more of yttria
in it to stabilize it further. Still, it’s primarily
tetragonal crystals, they can come in many
different tooth colours and they are intended for monolithic
and bi-layered restorations. These materials can only be
produced by soft machining, but as soon as you tamper with the
material or the zirconia, you reduce the fracture strength, so these have a
medium strength of 900 to 1,200 MPa. The ultra-translucent material, also called
the third-generation material, has increased the yttria contents even further, so
they have 5–8 volume percent of yttria. They have both tetragonal and cubic crystals
because the cubic crystals are entirely translucent. These materials are intended for
monolithic and to some extent bi-layered restorations,
primarily in the anterior region. They can be produced by soft
machining and they have a medium to low strength and all
the way down to 700 to 900 MPa. These materials are very poorly documented
both in vitro and in vivo. The benefit of using a monolithic
preparation is that you don’t have to prepare so much tooth substance,
you have to don’t have to remove so much tooth substance because
you can use a shallow chamfer. Many of the producers claim that you can
use a knife-edge preparation on these ones, but then if you have a very thin crown margin,
you risk having chipped margin chips like you see on this picture here
of the cubic zirconia crown. So which materials to choose? If you have a very aesthetically demanding
case and limited substance loss, porcelain should be the first
choice if you cannot restore the tooth substance
satisfactorily with composites. If you have a larger substance loss,
you need to have more strength and then you will go to glass ceramics if it’s
still aesthetically demanding or the different zirconia materials
depending on the situation. If you have a large taper
and you have to depend on adhesive bonding, I would
choose the glass ceramics still. Even though you can achieve bonding
to zirconia, as far as we know, the bonding to glass ceramic is more
reliable over time than to zirconia. If you don’t have a discoloured abutment
on the other hand, the zirconias are better at camouflaging discoloration than
the glass ceramics and the porcelains. When there’s not sufficient enamel to
achieve optimal bonding, I would go for zirconia materials, and the same goes if
you have difficulty with humidity control. If you have a dissipation with dysfunctional
bite forces like bruxism or clenching, monolithic zirconia crowns
are the absolutely first choice. So what are the limiting factors? Fractures is the main reason for why the ceramics
have a lower success rates than metal ceramics. And it’s both chipping or fractures of the veneers
and core fractures that contribute to this. The chipping problems that we saw so
much of in the beginning of using zirconia is usually because of poor support
of the veneering ceramic. There are many ways you can increase
the support on veneering ceramic. You can have an individualized core
that gives support to the ceramics, especially in the margin ridges,
so you can build up the zirconia coil like this
and then only have the veneering ceramic in the visible area,
so-called semi-monolithic crown. And if you look at the crown on the side here,
this area within the arrows is entirely zirconia and the rest is veneering ceramic. Another problem that we see as a cause
of chipping is that the dentist has done some adjustment into the bite, into the
occlusion, and then not polished it properly, and the adjustment with a drill
you can see here gives fractures that can function as crack initiated causing the
chipping or the veneering to chip off. Core fractures are usually started
or originated from the margin from different kinds of margin flaws
whether it’s because the machining has caused these flaws or if it’s the fact
that the crown margin is too thin. When it comes to multi-unit restorations
like bridges and fixed partial dentures, it’s usually the dimension of the
connector area that is the problem. And you don’t have to be an engineer to
understand why this bridge here broke when the connector is designed like
this because it’s not sufficient height to support the mastication
forces in the lateral segments. In summary, dental ceramics show good
clinical success when used correctly. Different situations require
different materials and knowledge of the material
is the key to success. The ultra-translucent zirconia has limited
documentation and must be used with caution. Thank you for your attention.

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