LECTURE I
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Titanium in dental prosthetics
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Lecture 1: Titanium in dental prosthetics (I) - Potential for
improvement
Good morning ladies and gentlemen, colleagues in the world of
dentistry. My experience of Danes is that your mastery of my
mother tongue is exemplary. But this is the morning after the
night before for some of you. I’ll be happy to answer questions
even in Danish, but remember that my ear is tuned to Norwegian!
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Titanium - the biocompatible metal. How often have we heard
that?
Not as often lately as the manufacturers of advanced ceramics
have begun to place their wares on the dental market.
Nonetheless it remains a strong brand. The implant industry
relies on titanium, even though there is a continuing need to
improve osteo-integration and to avoid wear in contact with
harder materials. For superstructures of dental implants titanium
has the obvious advantage of being free from galvanic issues.
The next step was titanium casting as a route to making crowns
and bridges. Once low-firing porcelains were developed, this
became feasible if not wide spread.
Why not? Because titanium does in fact have some potential for
improvement.
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To understand why, and where this improvement may be
achieved we need a short course in titanium metallurgy. We also
need to be aware of titanium’s properties. With this
understanding the limitations of commercially pure titanium
become more apparent. Two means of overcoming some of the
shortcomings can then be discussed. By then we are well into the
second of this morning’s lectures.
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Where does titanium come from? The earth’s crust contains
about 6 ‰, which ranks titanium behind aluminium (8 %)and
iron (5 %) as far as industrial metals go. The important ores are
rutile (TiO2) and ilmenite (FeTiO3)
These ores are very stable so reducing them to titanium metal is
energy-consuming. The initial product is a sponge that is melted
to an ingot. That must be done in a vacuum after flushing with
inert gas or else the metal oxidises rapidly again. We can begin to
perceive why so few dental laboratories have introduced lost-wax
methods for producing crowns and bridges.
The sponge does however provide a straightforward means of
producing alloys. One simply fills the sponge with powders of the
metals one wishes to alloy in. Optimal properties are obtained by
rolling or forging the ingots. Retaining these properties is a major
advantage of milling rather than casting titanium.
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Three basic aspects of titanium’s chemistry must be accepted and
need to be remembered
No dental alloy needs to be heated to the temperatures that
titanium requires. Little can be done, in fact the measures that
improve the metal’s properties generally raise the melting point if
only by a small amount.
The second aspect is something that titanium alone among dental
alloys exhibits – a phase transition, at 882 C, that concerns the
dental technician because it limits the firing temperatures for
porcelain. While porcelains for gold and Co-Cr are fired at about
1000 C, those for titanium are fired just below 800 C.
Finally, we have the aspect that is responsible for much of
titanium’s biocompatibility.
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A distinction between titanium and many other metals and alloys
is the immediate formation of an extremely thin, but dense layer
of oxide, TiO2.
Without this layer, just 20 nanometres thick when it forms
naturally at room temperature, titanium is much more reactive
other metals we use in dentistry, Co-Cr or the noble-metal alloys.
This reactivity re-appears when orthopaedic titanium implants
are subject to wear. Wear debris reacts with body fluid to produce
tissue-damaging wear products.
The virtue of TiO2 compared with the iron oxide (rust) that forms
on steel is that it grows fully dense and compact, forming an
stable, electrically insulating coating similar to the Cr2O3 that
forms on stainless steel and Co-Cr dental alloys. Neither chemical
or biological solutions nor moderate electrical potentials can
break it down. In the body it keeps titanium essentially inert – as
long as it is not subject to mechanical forces. We shall return
to this point later.
8 The -to- phase transition
We must brush up some chemistry from dental school.
Titanium metal occurs with two, quite different crystal structures.
At temperatures below 1670 but over 882 C it forms the same
loosely packed structure as iron and brass. Each metal atom is
surrounded by 8 others.
Below the transition temperature of 882 C, titanium atoms
arrange themselves into a closely packed structure with each
atom surrounded by 12 others and just one prominent set of
dense atomic layers. Magnesium and zinc have the same
structure.
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Other atoms can be introduced into these crystal structures. This
is done on purpose to modify properties, but it also occurs
because molten titanium is so reactive that it dissolves the
crucible or mould into which it is cast. We shall look at some of
these elements as we examine attempts to improve dental
titanium. At this stage we can just note which metals are used.
They are mostly closely related to titanium.
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The result of alloying, quenching or working is to vary the
amounts of the phases and . One classifies titanium alloys
accordingly. Note that casting alone cannot achieve the optimal
properties of an alloy. Heating and deformation by rolling or
forging are required.
In pure titanium, the transition from to cannot be stopped so
we always have purely -structure. Alloying slows down the
transition to varying degrees so that we can retain progressively
more .
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We return to the third aspect of titanium chemistry, its reactivity.
Molten titanium reacts so aggressively with oxygen that it
dissolves the investment material to get oxygen. As the
investment is reduced chemically, its metallic elements are
liberated and they dissolve in the metal, while the protective
oxygen forms on the surface. My colleague Morten Syverud at
NIOM examined titanium that had been cast into various
investment materials and found the metallic elements from the
investment up to 50 microns under the surface. This is the
notorious, brittle -case that must be removed before applying
porcelain. Investment materials for titanium contain the mineral
spinel because magnesium is the one common element with a
greater affinity for O2.
12 Physical properties
There are some physical properties that impinge directly on the
dental use of titanium.
You must excuse me if I, as a physicist, claim elasticity as a
physical property. An engineer might call it a mechanical
property, “a rose by any other name”. I also maintain that it is the
most important property of any dental material and I have a mild
hope that by the end of these lectures you will be inclined to
agree with me.
The obvious reason for paying attention to the elastic modulus
that is quoted on information sheet of a dental alloy is that an
alloy’s stiffness determines how rigid a framework or coping will
be. More flexible alloys have to be compensated by greater
thickness.
A second, less obvious, matter is a need to match elasticity to
both porcelain and tooth. As my compatriot Michael Swain has
pointed out, nature avoids abrupt changes in the transition from
dentin to enamel stiffness, and restoration dentistry should too.
Finally, when milling relatively flexible alloys, one must allow for
them flexing away from the grinding tool and springing back after
it has passed.
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Let us look at the numbers for elastic modulus. We remind
ourselves that enamel has a stiffness around 80 GPa and
veneering porcelains around 65 GPa.
Is the unit gigapascal understood? The diagrams compare
bending of test specimens of resin-base polymer and an alloy
under a 1 kg weight.
-titanium is relatively stiff compared to enamel and porcelain,
but only half as stiff as zirconia and Co-Cr alloys. -titanium
alloys are found to have an elastic modulus comparable with both
tooth enamel and porcelain. An + alloy that we have
experimented with at NIOM shows a slight reduction in stiffness.
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Is there an ideal elastic modulus?
We seek a compromise between the need for adequate rigidity and
the need to match with tooth and porcelain. The importance of
the latter is exemplified by high-gold alloys. They have a modulus
around 80 GPa. Neither bonding to porcelain nor cementing to
teeth give problems. The reason is that the unavoidable stress
concentration at the sharp interfaces we create are proportional
to the difference in modulus. One purpose of porcelain bonding
agents is to de-concentrate this stress by introducing a material
with intermediate properties.
All-in-all experience suggests we should strive for an E-modulus
near 80 GPa.
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Porcelain-fused-to-metal requires that the porcelain’s thermal
expansion is slightly less than the alloy to ensure that the
porcelain is under compression after cooling, at least in two
directions. Titanium’s low expansion is a challenge to porcelain
manufacturers, but they have found formulations that work. If
alloy development changes the expansion coefficient substantially
new porcelains will be needed.
Density is one of titanium’s selling points. A light-weight
prosthesis is nearly always possible even if the framework
thickness has to be increased to achieve rigidity.
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Finally, thermal conductivity is an issue for milling titanium.
With conductivity similar to the stainless steel in the flask that we
use to keep our coffee hot, pure titanium is manageable. Alloying
reduces titanium’s conductivity dramatically with definite
consequences for milling.
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Mechanical properties are the basis for much pre-clinical alloy
approval. ISO 22674 sets requirements for yield strength and
ductility. While an issue exists as to whether alloy type 5 ought to
exist, it can never include titanium-based alloys.
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CP titanium barely fulfils the requirements for type 4 and casting
does not give a consistent result. It is hard to recommend CP
titanium for any of the applications above type 3: removable
partial dentures, thin-walled crowns or wide span bridges.
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What about Ti alloys then? There is a standard grade V alloy used
for large orthopaedic prosthetics that is satisfies all the strength
requirements for a type 5 dental alloy. NIOM has experimented
with a simple binary alloy nicknamed Tiqq that from our own
preclinical testing and clinical trials in Bergen, might be fully
adequate.
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What are we looking for? Look again at a bend specimen which is
a simplified form of the load on a dental bridge. The largest stress
occurs in the middle of the unsupported section, under the load.
Once in place, we do not want the yield stress to be exceeded for
that would mean a permanently deformed bridge. However every
dentist and dental technician wants to be able to adjust the
framework to fit, so too great a yield stress means difficult
adjustment. Like the baby bear in Goldilocks, we want things
“just right”.
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We have noted that titanium requires considerable capital
expense to acquire adequate casting equipment with inert gas,
and vacuum arc melting. With some labs embarking on milling of
zirconia and other core ceramics, milling of titanium becomes an
attractive idea.
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There are definitely some traps to be avoided. Titanium’s
moderate elastic modulus compared with the metals and
ceramics that one normally employs in a milling machine means
that it will flex away from the tool only to spring back when it has
passed. Furthermore milling blocks are cut from metal that has
been rolled, forged or perhaps even extruded. The single set of
crystal planes in -titanium interacts with this direction
processing to create a block with slightly different elastic modulus
in the direction of processing. Good CAD/CAM systems cope with
these subtleties, cheap ones don’t.
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We have mentioned low thermal conductivity and how it causes
temperatures to rise. The result is that the titanium, which is
already only moderately hard at room temperature, becomes soft
locally where it meets the tooling, so soft that it flows rather than
breaks off. The result is a pockmarked surface and clogged
tooling, an effect in English we call “galling”. In Norwegian (and I
suspect in Danish) we speak of “rivning på overflaten” og “klining
på verktøyet”
The result is that milling must be slowed down. From experience,
one has found that longer times must be allowed for milling.
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We can now summarise CP titanium’s familiar pro’s and con’s
when employed as a casting alloy.
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The advantages are retained when we start from milling blocks
and we can add one more advantage. Some of the strength
attained by special processing of the block can be retained in the
finished framework. This should ensure that the metal actually
comes safely into Type 4 of the ISO classification giving it a much
wider range of applications
The disadvantages inherent in casting a highly reactive metal
with a high melting point are replaced by the slow machining
characteristics and a need for special coolants and lubricants.
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Ideally we would like a metal with the elasticity of gold, the
strength of Co-Cr and the milling characteristics of aluminium or
at least milling techniques that compensated for titanium’s
special attributes.
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We’ll break there and return in quarter of an hour to look at
progress towards the ideal alloy.
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Pause
TiO2 is a ceramic. It has two crystal forms rutile and anatase.
These are quite rare minerals so that much titanium is refined
from ilmenite, an iron-titanate.
Given that it is a ceramic that imparts the virtues to titanium, it
is natural that ceramics based on similar chemistry have been
considered as biomaterials. Rutile itself exhibits so special
properties but its closest relative zirconia ZrO2 has very attractive
properties provided it is mixed with a little yttria Y2O3 or
magnesia MgO. In many ways yttria-stabilized zirconia (Y-TZP) is
titanium’s nearest competitor.
Titanium’s other competitor is Co-Cr in its many varieties. This
well established casting alloy and now that the Swedish
authorities have begun to consider materials cost it is now in use
across the whole of Nordic region.
Let us then compare titanium with zirconia and Co-Cr in their
traditional forms before we look at the newer ways in which
prostheses are produced from these materials.
Which properties do we require for the framework of a prosthesis?
Rigidity. The rigidity of the substructure largely determines how
the prosthesis responds on a whole-of-mouth scale. For a given
design, stiffness is a product of two quantities, elastic stiffness of
the material and the cross-section. A framework can within limits
be made thicker to compensate for lower material stiffness, but
that means 1) lots of metal, 2) extra weight and 3) less room for
an aesthetic veneer.
A material’s stiffness is measured by its elastic modulus
Zirconia Titanium
Co-Cr
Stiffness 185 GPa 110 GPa 210 GPa
But is it always best to be as stiff as possible. Here we face a
conundrum. Dental tissues are much less stiff than these
restorative materials. Typical modulus values are 20-30 GPa for
dentine and 80-90 GPa for enamel. Furthermore the materials
with which we veneer a framework vary from polymers with
modulus from 2 GPa (soft-lining) up to 25 GPa (composite) to
porcelain with elastic modulus around 65 GPa.
Why is stiffness important? Under a given load, the less stiff a
material is, the more it deforms. When we make a compact unit
such as a crowned tooth or pontic, every constituent is subject to
the load. If the elastic moduli differ then the materials deform by
different amounts. At the interface between two materials, the
difference in deformation leads to considerable stress. The big
differences are between the veneer and the substrate and the
substrate and the tooth. That is why chipping of veneer and
loosening of cement are the likeliest failures. In this situation
titanium’s low elastic modulus is an advantage.
Manufacturing frameworks is performed in two fundamentally
different ways:
casting into a mould that has been formed around a wax model.
This is still the standard method for Co-Cr alloys and a number of
dental laboratories also cast titanium.
or
milling out of a blank by following a template. Today a digitally
recorded template is obtained by first scanning a stone model
then adding the thickness needed to provide stiffness. This is
standard for zirconia and is used increasingly for titanium and
even for Co-Cr
What happens with the titanium in each case?
Let’s look at what happens when a patient bites on something
hard with a multi-unit bridge? The pontics take all the load in a
combination of bending and downward pressure. The more rigid
the prosthesis, the less the bending which is a good thing since
the load is directed down through the prepared supporting tooth.
Bending tends to crush the cement eventually causing it to fail.
For the cement is the weak link in a prosthetic restoration.