Dr. Hanan Zraikat
Denture Base Resins Part II
Aug 9, 2009
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Denture Base Resins Part II
Today’s lecture will continuation for Denture base resins, we talked first about the
composition, the setting stages, flasking and a little bit about the characteristics of
dental material, whether it’s cold cure or heat cure .
Now we are going to talk about manipulation, of course as a rule, as is said you
should always follow the manufacturer instructions, you have to be careful not to
add too much powder or too much monomer.
If we add too much monomer, it will most likely evaporate and lead to more
polymerization shrinkage or porosity, and if we add too much powder, then the
mixing will be ineffective because there won’t be enough monomer to make the
powder particles, and this also can lead to opaque or black appearance of the
acrylic resin in the end.
So too much powder will lead to inappropriate mixing, bubbled incorporation in
the mix, and too much monomer can increase the chance of evaporation of the
excess monomer, can lead to excess porosity or excess polymerization shrinkage,
so always follow the instruction of the manufacturer.
Important to know: usually when you mix, we put the liquid in a container, the
powder will be in another bottle, and we start adding the amount that’s needed to
the liquid and you start mixing, after you mix ideally, we talk about heat cure, you
should put this container aside and cover it for a period of time until it reaches the
dough stage, you keep chick it until you fell its like a dough and then work with it.
If you leave the container open, then some of the monomer might evaporate and
this will lead to a type of porosity in the mix which called Granular porosity, why?
Because you lost the monomer, so you have to be careful when you mix the acrylic
and cover the container that has the monomer and powder inside until it reaches
the dough stage, to avoid evaporation of the monomer and to avoid the porosity in
the result denture which called granular porosity, its spread all over the denture,
and of course the presence of such porosity will weaken the denture, so this
important thing to know when you mix the acrylic.
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The color of the denture of the acrylic resin is affected by pigments, and usually
when they do flasking of the denture they put a separator or a material to cover the
stone to block any pore in the stone, so that when you put your resin (dough) there
will be no penetration of the resin inside the plaster, because you want the surface
of your denture whether its complete or partial smooth, if you don’t use the liner or
this material to cover the surface of plaster and stone, when you pack your acrylic
it start to penetrate inside the pore or opening inside the plaster and stone, and
there will be like protrusions on the surface of the denture, and it will take you
more time to finish and polish your denture, so this why they use this material
which mold lining, to prevent penetration of resin into plaster or stone, as I said the
color is controlled by pigments.
Porosities: there are different types of porosities inside the denture. You have to
know what causes them in order to avoid it in the end result.
The different types are:
Gaseous porosity: due to evaporation of the monomer, because the
temperature during processing is very high, we said before the boiling
temperature of the monomer is 100.3 ˚C, so if your processing temperature
is much higher than that, it will lead to boiling of the monomer, evaporation
and Gaseous porosity.
This type of porosity is usually concentrated in thick areas of the denture,
because in these areas usually the temperature is very high, which will cause
evaporation of the monomer that leads to Gaseous porosity, usually in the
areas of the palate, palatal surface of the teeth(palatal gingival) more
susceptible to Gaseous porosity.
Another type of porosity is:
Contraction porosity: and it’s caused by the following, when you pack your
acrylic inside the flask, you have to pack enough material and you have to
pack while it’s in the dough stage, why do you need to pack excess material?
To make sure that if there will be shrinkage in acrylic during processing,
you’ll have extra material to flow and cover the areas you want to cover, so
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there will be no defect, no shortage of material, and this has to be done under
pressure, usually when you flask you advance to put a cover on top of your
flask and apply a little bit of pressure during processing, so the acrylic
material will flow under this pressure, and as the temperature goes up during
processing to compensate for any shrinkage that happened, you have to pack
enough dough material(acrylic material) to ensure that will compensate for
the shrinkage and the material will cover all the necessary area. If that
doesn’t happen, if the pressure is low, if you don’t pack enough dough
material, then this type of porosity will happen.
So always when you pack your acrylic resin, don’t use it while it is in the
stringy (sticky) stage, because there will not be enough pressure during packing,
because the material is very fluid, but when it is in the dough stage the material
will a little bit of resistance when you pack it so there will be enough pressure
to allow it to flow, and to avoid the Gaseous porosity always control the
temperature of processing, don’t let it go up too high.
Because of the increase or decrease of the temperature, because of the
manipulation of the material, stress will be created inside the material, if this
stress try to relax this might change the shape of the denture, so it can
distortion, ideally we should minimize stress formation and this can be done by
slowly cooling the flask to avoid stress formation inside acrylic resin, and we
should also use acrylic teeth with the acrylic dentures, we have two types of
teeth use for complete or partial dentures: Porcelain teeth and Acrylic teeth,
now if you have the same type of teeth similar to acrylic resin then they will
contract and expand at the same rate, so this will not lead stress created inside
the acrylic resin, but if we use porcelain teeth, they will contract and expand at
different rates (CTE are different) and this can lead to stress created inside the
acrylic resin, and this stress if it relaxes it can lead to distortion, it can also
cause defect inside the denture called crazes (tiny defects), any defect will
weaken your denture, so we avoid stress by slowly cooling the flask and using
acrylic teeth instead of porcelain teeth, so that the shrinkage and expansion is
compatible between the teeth and the denture base material, acrylic resin we
call it denture base material because we use it to make the denture.
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Properties of resin, resin is usually well tolerated by patients, except for some
cases where patients are allergic to the monomers, that’s why sometimes they
replace this type of material with polycarbonate, to avoid these allergic
reactions, and usually there will more reactions toward the cold cure, because
we said that in cold cure resin there will be more residual monomer, on the
other hand in heat cure resin the excess monomer will usually evaporate
because of high temperature, so instead of using the normal resin they use the
polycarbonate to avoid the allergy in some patients.
Dimension stability and accuracy, we need our denture to be retained (retended)
and stable inside the patient mouth, SOO:
Retention: is the ability to resist displacement in a vertical direction.
Stability: is the ability to resist displacement in a horizontal direction.
So we want our denture to be stable in a vertical direction and stable in a
horizontal direction, and why is that? For the patient to use it successfully it has
to be retented and stable, we don’t want it to be ill fitting that will lead later into
trauma and it can lead to fracture of the denture, and if the denture is broken, if
it keeps moving then will be more stress on the denture and it will break, and
usually after the material is cured and hard it is stable dimensionally, but the
patient needs to keep it in a humid environment, we can’t place it in air or dry
environment without being kept it in a wet environment, because this will lead
to distortion and changing the shape of the material, so it is dimensionally
stable if you keep it in a humid environment.
Mechanical properties: it is susceptible to fracture, because it has low
toughness, low impact rate, and if there are crazes inside (small defect) this also
can lead to weakening of the material and creep (creep: change in the shape of
the material due to forces applied on it), and usually this minimized, because
the material has crosslinking agents, as I said before in the composition the
crosslinking agents increases the stability of the material in term of dimension
To overcome the weakness of the material they try to make it stronger by
adding certain fibers, carbon fibers, glass fibers, some of these materials need to
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be treated in order to bond to the acrylic and they increase its taste of the acrylic
resin, in some cases they also include rubber agents to make a little bit more
flexible, so if the patient drop it on the floor it will not break as easily, but
because its rubbery, that means that the dentures will easily flex and this might
lead to excessive flexure and fatigue, fracture by fatigue.
So two ways to Improve the impact strength of the material which is Rubber
including rubber in the material and Incorporating Fibers to increase the
strength of the material, to reinforce it and make it stronger, we said that
thermal conductivity is low that’s why patients can easily burn their palate or
tissue because they can’t feel that what they eating or drinking is hot, and
usually the material can absorb a little bit liquid about 1-2% by weight, but
usually when it’s in the patient’s mouth it is hard and it is insoluble, there will
an exchange of fluids, that’s mean that it will absorb a little bit of liquid at the
beginning but then that’s it, as for the coefficient of thermal expansion, as I said
usually it is recommended that we use acrylic teeth so that the thermal
expansion of the teeth is similar to that of the denture base material and they
will be compatible, otherwise if we use porcelain teeth the difference between
the two materials can lead to loosening of the teeth because they expand and
contract at a different rates.
Instruction for the patients : what should be done is that you have to instruct the
patient to take care of the denture in term of cleaning, always the fitting surface
which the surface that will touch the tissue should not be treated aggressively,
he always should use a brush to clean the teeth and the rest of the denture
except for the fitting surface, he should not use bleaching agents, and just to
soak the denture in water, there are some tablets (disinfectant) to keep the
denture clean, free of bacteria, sometimes the patients can bring the denture to
the clinic for the dentist to remove any calculus or plaque, and after you take
out of the(I couldn’t understand the word) and clean and polish it, it should be
disinfected , sometimes they can use water with a little bit of mouth wash to
keep it fresh and keep it clean, so all these instruction must be given to the
patient so he can maintain his denture clean for a long period of time.
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Artificial teeth, as a requirement they need to have good appearance, they have
to bond to the acrylic base, it need to be strong and tough enough and easy to
adjust (the occlusion need to be adjusted) so there are two types: Acrylic and
Porcelain, both of them are made in mold, but they are retained in the denture in
different way, porcelain teeth need to be retained mechanically because they
can’t chemically bond to the denture material, but the acrylic teeth can bond
chemically and there is no need for mechanical retention, and usually the part of
the acrylic tooth which bond to denture base (which is the neck of the tooth) is
made from lightly cross-linked acrylic which means that the material can be
dissolved a little bit when it is attached to the dough material (acrylic resin) so it
enhance the chemical bonding, so usually the base of the acrylic tooth is made
from softer acrylic, so that when you pack it with the acrylic resin it will
chemically bond and inter-locked with the denture base material, as a
comparison the porcelain teeth look much better, looks more natural than
acrylic teeth, but they are heavier and they are difficult to adjust. At the same
time if they are opposing a natural teeth the can lead to wear of the natural
teeth, so these are some of the disadvantages of the porcelain teeth, but acrylic
teeth if they oppose a natural it will cause wearing or damaging of the natural
teeth, but usually with time their color will change so they need more
Denture lining material: sometimes the patient needs a new denture, but he
found it difficult to adjust to this new denture and he wants to keep using his
old denture for sometimes, in some cases I made a denture for a patient, there
were a bone lock and the denture become loose, so in these cases you need to
reline the whole denture to make the effect better, In some cases there will be
tissue injury if the patient for example uses the denture for the first time or
sometimes there might be sharp areas on the fitting surface of the denture so it
causes pain, and we need to use second material to act as a cushion to relieve
this pain, this is why we use tissue conditioner, if you need to reline the denture
for sometimes to improve the its effect we can use permanent hard reline
materials, sometimes they soft relining materials, and the hard relining material
last for longer period of time but the soft lining materials you have to change it
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after a period of time and make a new denture, so for the
Hard relining materials: 1. Poor retention and stability.
2. Degradation and damage to the base itself.
3. Loss of vertical dimension caused by bone lock.
If the patient find it hard to adjust to a new denture you can use the hard relining
materials which it can be Heat cure or Cold cure, the heat cure you use it in the lab
but the cold cure you use it in the clinic contain powder and liquid, the
composition is similar to the acrylic resin it means PMMA and MMA, sometimes
to avoid the irritation of the MMA they use another a different monomer Butyl.MA
it causes disirrataion, the cold cure resin has disadvantages because you us e it in
the clinics, it’s hard to control the material you using and you are treating on the
denture, it has poor taste and it can lead to irritation of the tissue because the
reaction is exothermic, so it might burn the patient or be uncomfortable to the
Soft relining material: because it’s soft it can be used in cases where there is
soreness or pain caused by the denture, the denture is fine and it has full extension
and functioning well, but for some reasons the patient feels bad, so in that cas es we
need something a little bit soft and usually it can last for up to 6 months,
sometimes part of the composition of the liner is plasticisers to make it flexible,
after sometime these plasticizer will be lost, it will leach out and the material
instead of being soft it will become hard, so it become hard it will lose its purpose
so it is permanent by any need so it can lasts up to 6 months until the patient get
used to the denture or until you find that you need to make a new denture.
About the procedure just for us to know, usually what happened is you have the
denture you release the fitting surface to make space for the liner then the liner is
applied on the fitting surface of the denture and it placed on the patient’s mouth
and we asked him to bite so that the material will spread all inside the denture,
after some time to avoid the temperature from the fitting reaction we will take out
as it is and put in hot water, so the setting will after the patient’s mouth so he will
not be burned by the exothermic reaction.
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For a soft liner to be used you need it to be rubbery and a little bit flexible and
resilient and soft so that it can relieve any pain, and as I said plasticizers will act as
lubricant between the chains so that the material gain toughness and flexibility,
different material used some of them are silicon rubber and the other are acrylic so
two types of materials, silicon rubber is made from rubber not form acrylic so you
need to apply a bonding agent between the liner and the denture so that it can stick
there and bond, so disadvantages weak bonding, it can be separated by any reason
from the denture base because it is not made from the same material so you need a
bonding agent, and another disadvantage is encouraging Candidal infection.
Acrylic material, the problem is it got plasticizer inside it, and the composition also
like any acrylic resin is PMMA and MMA in addition to a plasticizer, as I said the
plasticizer after some time it will leach out and the material will become stiff and
hard, that’s why it need to be replaced, there are certain examples of soft liners:
silicon rubber is similar to the impression material.
Just for comparison between the acrylic and silicon rubber material:
Silicon Rubber Acrylic Resin
Weak bond Permanently bond
Prone to candidal infection Resistance to the material
Remain soft for long time Acrylic & plasticizers will leach out after
Finally the Tissue conditioner compared to Temporary soft liners:
They have similar composition and similar use, the difference is that the tissue
conditioner you need to replace it every 2-3 days, in the other hand the soft liner
can last for a few weeks, week or two or more, so if your patient can’t come every
two or three days to replace the conditioner you can apply a temporary soft liner
that will last for a longer period of time, both can be used for the same purpose, if
there are trauma to the tissue caused by the denture any soreness, first you have to
remove the cause of the pain, if there is an extra part of the denture that damages
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the tissue or hitting the bone, we need to release this area and then we apply the
conditioner so it will act as a cushion, now if you don’t place the conditioner still
the denture could cause pain and he can’t wear it, totally it’s not acceptable for the
patient to go on without teeth, so we can help him by placing a tissue conditioner
because it will prevent pain and it will allow the tissue to heal, at the same time
that the tissue will keep wearing the denture.
The composition of Tissue Conditioner is PMMA and Ethyl alcohol and
plasticizers, as I said it needs frequent replacement of tissue conditioner, why?
Because the plasticizers will leach out and it will become hard and strong, so it has
to be replaced, but the temporary soft liner can stay there for a few weeks without
needing to replace it, so another disadvantage in addition to frequent replacement
is Microorganism colonization: candidal or bacterial or whatever ..
And in some denture cleansers they might double the tissue conditioner and again
you need to replace again and again, so the maintenance of the denture is very
important to protect the fitting surface.
Denture Repair: if a denture is broken we can use the acrylic material to repair it,
for example if we have two pieces of the upper denture, what we do is usually we
stick it together with sticky wax for example then we put plaster and then the
fracture line need to be drilled till reaches the acrylic and they start adding a little
bit of a monomer and then a little bit of a powder like salt and pepper until you
completely block the fracture area, and then you put it in hot water or under
pressure to set, so in denture repair we can use the acrylic resin usually in cold cure
in order to repair the fracture.
End of the Lecture & Course
Sorry for any mistakes!!
....بخوٌى الٌجاح باالهخحاًاث لكل الشباب والصباٌا بالدفعت وإى شاء اهلل هٌشوفكن السٌت الجاي
....رهضاى هبارك .....وعٍد فطز سعٍد إى شاء اهلل
....ًوأ ُدي ُذٍ الوحاضزة إلى والدي رحوت اهلل علٍَ, لوالٍ لوا كٌج حقق ج أحالهً وألًَ دائوً كاى بجاًب
. أحلى ححٍت الى األصدقاء عبد ألزحوي عزاقً و وسام أبو سزٌس
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