so the crack starts on the surface of the material and as it starts to go down because
this is a hundred percent tetragonal full strength zirconia we get the crystals changing from the
tetragonal to the monoclinic clamping down on that crack to keep it from going any deeper considering
that this has been around for three years now from year five to year eight in this study this should
have been really concerning for another material but again the 100 tetragonal zirconia is just kind
of chugging along uh happily in this module we're going to tackle material selection for crown and
bridge and this certainly of all the modules has gotten the most complicated over the last say 35
years so when i was in dental school um it was two things you had your choice basically of pfms
or cast gold you know the pfm was invented in 1959 and became the workhorse of the dental industry
because it was more aesthetic than what had been the workhorse for the two or three decades or four
decades before that which was cast gold until the introduction in 2007 of lithium disilicate
ips emacs and then of course in 2009 with the introduction of bruxer solid zirconia these were
the first two all-ceramic systems that really uh provided a a challenge for the pfm and so
the landscape definitely changed and we'll take a look at that more specifically and so now it's
gotten more confusing with these different classes that we have to choose from which are now largely
high strength all ceramic materials or certainly all ceramic materials not all of them
are high strength so gone are the simple days of just trying to choose between
a pfm and a cast gold crown things have definitely gotten a little more confusing now
so as we go a little deeper into zirconium dennis will often say to me isn't zirconium a metal and
as you look at the periodic table of elements here i don't know if you've seen this since high
school or college uh but there's zirconium right underneath titanium it's in the group four uh
transitional metals and so zirconium is related to titanium in the same family so yeah zirconium is
in fact a metal so those are some zirconium rods um they're used to cool things like in nuclear
reactors and other things like that so zirconium as on the periodic table is in fact a metal
but this is zirconium oxide this pile of white powder that looks like an outtake from
scarface here this is what all dental crowns are made of so the oxide of zirconium zirconium oxide
is this polycrystalline material that you see on the right so it's not a metal zirconium is
but the oxide is not and that's true for a lot of the group for metals
now i want to show you some things about the crystalline structure of zirconia that helps
explain why it's as strong as it is and why it's important to kind of know the characteristics
of the zirconius that you're using so this is the monoclinic crystal structure this normally occurs
at room temperature so that pile of white powder that you saw on the last slide that is would
typically be this monoclinic crystal structure and what we do with dental zirconia is we actually add
things we add some dopants to it so that at room temperature we will have this tetragonal crystal
structure that you see here so the important difference the note between these two crystals
as you look is the monoclinic and the tetragonal is that the monoclinic looks like it's kind of
shifted off axis a little bit and as a result it's about two to three percent more volume in
size the crystal structure itself compared to the tetragonal which is more compact and usually like
you can see there you'd have to heat the zirconia oxide over 117 degrees celsius to get the material
the crystals in the zirconium oxide to go into this uh tetragonal pattern but by adding things
to the zirconia we can keep dental zirconium oxide at the tetragonal crystalline structure at room
temperature that's very important and it helps to explain what we were just saying about full
strength zirconia as being 100 tetragonal this is what tetragonal means is that 100 of the crystals
in original bruxer for example are these tetragonal crystals more on that in a second
because something important happens when you have 100 tetragonal crystals so all of these blue
crystalline structures that you see here these are all tetragonal crystals and now the toughening
mechanism the transformational toughening takes place when a crack is introduced into
the material this could be from us aggressively doing an occlusal adjustment on the zirconia or
re-contouring the zirconia maybe not you know the patient just biting down on it too hard or
maybe hitting a rock and something that they bite on and a crack will start in the zirconia
and you've got this frontal zone where all the the energy is taking place as the crack moves
its way down into it and here's what happens is that you get this wake the energy actually causes
these tetragonal crystals adjacent to the fracture that's going into the material to change the
monoclinic uh crystalline structure and if you remember from that drawing on the last slide
the monoclinic crystal structure is actually three percent bigger in volume remember it was
like shifted on its axis it looked like you took the tetragonal and squeezed it so it got a little
bit bigger and so what happens is those tetragonal crystals actually switch to the monoclinic and so
those crystals now get two to three percent bigger in volume when that happens you can see this
little compression that happens and you can see it compress along that crack and so really what
happens is it stops the crack it closes the cramps down on it so it can't go any farther down in the
material it doesn't close it all the way you can still see that the beginning of the fracture on
the surface is there but it won't go any deeper now because the monoclinic crystals have changed
uh it's gone from the tetragonal to the monoclinic they increase in volume and they clamp down
on the fracture itself so this is an sem from relic christensen's ongoing study which is now
at the eight year mark of original bruxer and this is an sem of original bruxer crown that's
been in the mouth now for eight years and if you look on the right the yellow arrows pointing to
a defect that occur occurred at year five and that's the one i want to take a closer look at
is when you look at that defect from year five and you see how deep that is and how wide that is
that would have been a failure in any other all-ceramic material this is i mean we would
have never been able to take this sem because a crack like that and everything you know facial to
that just shears off and that is an absolute fracture but this is a good example of the
transformational toughening taking place so the crack starts on the surface of the material and
as it starts to go down because this is a hundred percent tetragonal full strength zirconia we get
the crystals changing from the tetragonal to the monoclinic clamping down on that crack to keep it
from going any deeper it doesn't do it all the way at the surface but that's okay because down where
it matters as it's heading deeper and deeper it's been closed off and that crack can't go
any further and you see it kind of happening in a couple different areas so considering that this
has been around for three years now from year five to year eight in this study this should
have been really concerning for another material but again the 100 tetragonal zirconia is just kind
of chugging along happily even though it's got this defect on the surface which of course is
completely can't be seen by the naked eye are you enjoying this instructional video i hope so
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