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Professor in Dental Materials Science, Head of Unit Faculty of Dentistry, The University of Hong Kong
James Kit Hon Tsoi
Assistant Professor in Dental Materials Science, Assistant Dean of Electives and Experiential Learning Faculty of Dentistry, The University of Hong Kong
The next topic to be discussed is
another modification which is in the mixing liquid.
The first generation materials were supplied in sessions,
which had the powder,
which was made up of the cement and the radiopacifier, and a mixing liquid.
The mixing liquid is usually water.
With the second generation materials,
the mixing liquid has been modified.
In the premixed, there is no mixing liquid.
Such materials are supplied in syringes,
such as sealers and pastes, and they have no mixing liquid.
They are applied in the canal.
Anywhere there is like the root-end,
or perforation repair, and they rely on the moisture, or blood
that is present inside the area where they are placed
for hydration to work here.
This can be slightly problematic
because if the area has been dried
and there is not enough liquid,
there is a risk that the material will not hydrate.
And if the material will not hydrate,
there will be no calcium hydroxide formed.
If no calcium hydroxide is formed,
the material properties will obviously change.
Usually, water is used as a mixing liquid.
To the water, an accelerator like calcium chloride
can be added and also a superplasticizer.
The calcium chloride has the effect
of accelerating the material hyrdation.
And by accelerating the material hydration,
the setting time will be reduced.
If a superplasticizer is added,
less water for the same consistency can be used.
In this way, if the water is reduced,
the compressive strength and the micro hardness
will be enhanced.
The superplasticizer is simply a water soluble polymer,
it will adhere to the cement particles
and with less water, you can get an adequate mix
and no segregation will occur.
Another innovation to the water, or to the mixing liquid,
has been the addition of an anti-washout gel.
One particular brand of tricalcium silicate-based material
has the use of a gel rather than water as the mixing liquid.
The whole idea with using this gel is to avoid washout.
To define washout, washout is basically the material
that will tend to come off the surgical site
when there is irrigation
or when the blood clot is being formed.
If a gel is used, and again the gel used
is a water-soluble polymer, this washout has been shown
to be prevented in freshly set material.
More modifications has been also
that removing the water completely
and the use of light-curable resins,
or salicylate-based resins for various applications.
The light curable resins are used
for pulp capping materials, the water is not used,
but a resin-base is used.
The idea is, obviously using a resin
which is, allows water permeation.
Again, the cement types depend on the surrounding water,
the environmental water for reaction.
If this environmental water does not penetrate inside the resin matrix,
the hydration reaction of the cement will not occur,
the calcium hydroxide is not produced,
and the pulp capping material will obviously be used less.
Salicylate resins are preferred for sealer types.
The salicylate resins are rigid and again,
they should be permeable too.
The environment influence because that is the only way
that the material can form the calcium hydroxide.
Another modification, which is also not present
in the first generation materials in the MTA
is the use of additives.
The second generation materials incorporate additives with them,
which again, enhance the material properties.
One such additive is calcium carbonate,
calcium phosphate, and also silicon dioxide.
Some brands have the addition of calcium carbonate.
This calcium carbonate is added as a nucleating agent.
The nucleating agent,
the scope of using this nucleating agent,
is to enhance the material hydration.
Usually, hydration is disorganised and clumps of hydration material
will form around the cement particle.
When there is this nucleating agent,
the material hydration is more ordered
and this will result in a better microstructure,
and better compressive strength,
and better physical and mechanical properties.
More modifications include the addition
of the calcium phosphate.
The calcium phosphate will provide phosphate ions
for the material.
When there is the formation of calcium hydroxide,
the phosphate ions, which are readily available,
will react with the calcium hydroxide formed,
forming calcium phosphate.
The calcium phosphate has been shown
to be the formation of the hydroxyapatite,
which is responsible for the material bioactivity.
The problem with the formation
of this calcium phosphate is that the pH
of the material will be reduced
because the calcium hydroxide is reacting.
If in the long term,
this calcium hydroxide is not available,
the main function of the material,
which is obviously the formation
of the calcium hydroxide, will be reduced,
if the calcium hydroxide reacts completely
with the phosphate ions in the short term.
The antimicrobial properties are usually a function
of the high pH.
If the pH is reduced because of the formation
of the calcium phosphate, the hydroxyapatite,
or carbonate apatite which is formed
over the material's surface,
the reduction in pH will lead to a reduction
in antimicrobial properties.
This has been shown in a recent study conducted
by my research group where bacteria were shown to grow
inside dentinal tubules when the root canal was irrigated
with a phosphide-based solution.
The reaction of the calcium hydroxide formed
by the root canal sealers,
which were tricalcium silicate-based,
resulted in a reduction in antimicrobial properties.
Reduction in antimicrobial properties is problematic
for root canal sealers because there will be infection
and re-infection of the root canal
with root treatment failure.
Another modification is the addition of the silicon oxide(SiO2).
The silicon oxide(SiO2) is added in some brands
to enhance the material physical properties.
Enhancement of the material physical properties is caused
by reaction of the silicon oxide(SiO2), again,
with the calcium hydroxide formed from the reaction
of the material during hydration, there is again,
another reaction carried out between the calcium hydroxide
and the silicon oxide(SiO2) to form more calcium silicate hydrate
which is the material matrix,
but due to this reaction,
the calcium hydroxide is depleted.
This is another problem because
if the calcium hydroxide is depleted,
the material function may not be well in the clinical sense,
because if there is no calcium hydroxide,
if the material's being used for pulp capping purposes,
Jukka Pekka MatinlinnaProfessor in Dental Materials Science, Head of Unit
James Kit Hon TsoiAssistant Professor in Dental Materials Science, Assistant Dean of Electives and Experiential Learning
