Dental materials sheet # 1 -

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Dental materials sheet # 1 -

Post by Shadi Jarrar on 23/9/2011, 8:48 pm

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Dental Materials

Lecture no. (1)

Doctor: Nadia Ereifej

Date of lecture: 18/9/2011

Done by: Dania Salhab


POLYMERIC DENTURE BASE MATERIALS

Today we will talk about different denture base materials and the techniques that we use in construction of dentures.

** Dental base materials should have the following requirements:

- Biocompatible, Readily cleansable.

- High flexural and impact strength, high modulus of elasticity (higher rigidity) >> it should resist scratching and abrasion.

- High abrasion resistance.

- Long fatigue life >> it has to resist forces for a long period of time.

- High craze and creep resistance >> by creep we mean dimensional strain under constant low stresses must be high.

- High thermal conductivity >> no thermal isolation.

- Low density >> big volume for a low weight, especially the upper denture. Low solubility and sorption of oral fluids >> no leaking or absorption of fluids because if it absorbed fluid it will invite Candida and it won't be hygienic.

- High softening temperature, dimensionally stable and accurate >> no softening under low temperatures.

- Superior aesthetic and color stability, Radiopaque >> there are some kinds of additives that we can add to enhance the radiopacity

- Good adhesion with teeth and denture liners

- Ease of fabrication, inexpensive, easily repair

** The general technique for construction a denture as we all remember, we bring the flasks, make a mould, put the denture, provide investment material around the denture, soften the denture, get rid of the wax and in the end we get a mould inside the flasks and then we fill it with denture base material.

** Most commonly used materials for construction of denture base are Acrylic resins

- Were introduced in the mid – 1940s.

- Mechanical properties appropriate for use intra-orally.

- Can be colored almost to any color and translucency.

- The liquid contains methyl methacrylate.

- The powder contains polymerized resin in the form of beads.

- Upon mixing the liquid and the powder, a workable mass is formed and introduced into the mould.

- We have different shapes of bottles, different manufacturers, but most of the components are the same.

** Polymeric denture base materials, this mean that they set via a polymerization reaction (monomer + monomer = polymer), this reaction has to pass through activation initiation, propagation and termination.

- Most denture bases used today are formed of heat-cured PMMA and copolymers (PMMA with polystyrene-butadiene rubber). Rubber improves impact strength.

- Polymers with chemical bonds between different chains (cross-linked), this improves rigidity, craze resistance and reduces solubility.

- To initiate the reaction we need Benzoyl peroxide that yields free radicals that bind to the monomer and initiate the reaction. Activation of the initiator can be achieved by heat, chemicals (tertiary amines), and light or microwave radiation.

** Heat-cured PMMA

Have favourable working characteristics, acceptable aesthetics, physical and mechanical properties, easy to fabricate, inexpensive, BUT:

- Undergoes water sorption and loss >> colonization of Candida.

- Has low thermal conductivity >> works as insulator.

- Deforms under load with time (low creep resistance); this can be reduced by the addition of cross-linking agents.

- Does not bond chemically with non-acrylic teeth.

- Needs heat to start the polymerization reaction. Heat is transferred to the mix using water bath or microwave oven.

** Composition:

- The powder:

. Pre-polymerized beads of the resin.

. Initiator: benzoyl peroxide (0.5%).

-The liquid:

. Methyl methacrylate monomer.

. Traces of hydroquinone (inhibitor >> inhibit the monomer from acting by itself, so this increase the half life of the material).

. Cross linking agent: Ethylene glycol dimethacrylate (10%).

. Activator: N-N-dimethyl-p-toluidine.

** Storage

Manufacturers recommend storage at certain temperature and for specific time limit. Violations will cause deteriorations of the properties of the denture base or setting of the monomer. Sometimes some of the components may evaporate.

** Compression moulding technique:

- This is the most common technique that is used for construction of dentures.

- Other techniques include injection molding that is claimed to overcome the problem of polymerization shrinkage.

** Preparation of the mould

The doctor is showing us some pictures that describe the technique in details, here are her notes:

- Coating the flasks with Vaseline.

- Place the master cast with the denture.

- First pour >> plaster.

- Second pour >> covers all the teeth except the tips.

- Third pour, close the flasks and the put them under pressure.

** Polymerization shrinkage:

- The unreacted monomer molecules repel each other.

- When they have reacted, the carbon to carbon bonds that attach them together are short and the distance between the molecules becomes shorter >> shrinkage.

- This results in about 21% decrease in the volume of the material

- To reduce shrinkage, the monomer is mixed with prepolymerized material (it is already polymerized so no further shrinkage). This will reduce the shrinkage to 6% by volume (0.5% linear shrinkage), which is much lower and more acceptable.

- When the powder is mixed with the monomer in the proper proportions, dough like mass results that is workable and manageable.

- The proper polymer (powder) / liquid ratio is 3:1 by volume.

- This ratio will provide sufficient monomer to wet the powder that is not in excess to cause unduly shrinkage.

** Polymer monomer interaction

Upon mixing the material passes through 5 physical stages:

1. Sandy stage: wetting the powder with the liquid, it has a grainy mix.

2. Stringy stage: the monomer attacks the polymer beads uncoiling the chains and dissolving them, increasing the viscosity of the mix. The mass becomes sticky when handled with a spatula, still not the proper stage to be packed.

3. Dough like stage: more chains dissolve, increasing the viscosity. The material is no longer tacky or adheres to the walls of the mixing jar, the proper stage to be packed.

If we are late…

4. Rubbery or elastic stage: This happens as a result of more penetration of the monomer, it's too late to be used.

5. Stiff: this is due to evaporation of the monomer.

** Dough forming time

- It is the time required to reach the dough like stage.

- It should not exceed 40 minutes.

- Most materials on the market have a Dough forming time of less than 10 minutes.

** Working time:

- This is the time duration the material remains in the dough like stage; we can shape it and pack it.

- It should not be less than 5 minutes.

- Temperature can affect working time (The higher the temperature, the shorter the working time).

** The doctor again is showing us some pictures describing the procedure:

- The material is mixed inside the jar.

- Roll it.

- Thin it.

- Cut it into pieces.

- Pack it.

** Packing

- It is the placement of the resin in the mould cavity.

- Over packing results in denture base of excessive thickness and increased vertical dimension at occlusion.

- Under packing results in denture porosity (presence of bubble like or void spaces in the body of the denture base).

…so what we do is Trial closure that ensures proper packing of the resin and the amount is enough.

- Trial closure: The placement of resin when in the dough stage into the mould over the artificial teeth and closing the flask after placing a poly ethylene sheet over the mix. The flask is closed under slow build up of pressure from a mechanical press and the extra material will go out.

- The flask is opened and the flash material of the resin is removed. A new poly ethylene sheet is adapted and the flask is closed again, another excess material will go out but in a lesser amount, we have to do this until the upper half of the flask meets the lower edge to edge in full closure.

** Injection moulding technique:

Similar to compression molding technique but here the resin is injected into the mould from outside the closed flask after wax elimination, and the pressure is kept to compensate for the shrinkage of the material. This technique needs special equipments that is why it is expensive and not commonly used as the compression moulding technique. But because of the constant pressure, the material will be more properly packed and the polymerization shrinkage will be less.

** Polymerization procedure:

- Chemical reaction:

.Benzoyl peroxide decomposes when the temperature is brought above 60°C producing free radicals.

.Free radicals react with monomer molecules to initiate chain growth polymerization.

.Growth of the polymer chain continues very rapidly until:

a. The coupling to two growing chains or

b. The transfer of a single hydrogen ion from one chain to another, we put the flasks in water bath at 60°C.

- The activator of the reaction is the heat from the water bath.

- The reaction is exothermic in nature, the temperature will increase.

- Temperature has to be well-controlled as if it rises above 100.8°C, which is the boiling point of the monomer; this will result in gaseous porosity because of the evaporation (instead of reaction).

** Internal gaseous porosity

Occurs due to evaporation of the monomer, usually it is noticeable on the thick parts of the denture like the lingual flanges, so it doesn't occur on the surfaces, only in the internal parts of the denture.

** Porosity

We have different types of porosity:

- Gaseous porosity >> over heating of the denture.

- Granular porosity >> improper mixing.

- Contraction porosity >> insufficient amount of pressure and dough.

- Air inclusions porosities (in fluid resins) >> inclusion of air while packing the material.

** Polymerization cycles

- Long cycle: Controlled constant temperature of 74°C for 8 hours or longer, sometimes followed by 2-3 hours of terminal boil at 100°C, the most commonly used.

- Short cycle: 74°C for 2 hours then boiling for 1 hour at 100°, we only use it when we don't have time.

- Processing at low temperature or short time causes higher amounts of residual monomer, tissue irritation, sensitivity, allergic reactions, reduces strength and dimensional stability.

- Compression moulding causes high processing stresses due to polymerization shrinkage (7% by volume), thermal shrinkage as the resin cools, differences in thermal contraction between gypsum and resin. These do not affect denture fit but cause occlusal inaccuracies.







** Bench cooling

- Considering all the stresses that are generated inside the flasks during packing and polymerization shrinkage, we have to let go the stresses gradually, because if we put the flasks in a cool water suddenly, this will make cracks.

- Cool slowly at room temperature to allow release of internal stresses and minimize warpage.

- Bench cooling should be carried out for 30 minutes, followed by immersion in cool tap water for 15 minutes.

** Chemically-cured denture base resins

- Less commonly used.

- Chemically activated resins (cold-cured, self-cured or autopolymerizing resin) do not require the application of thermal energy and can be completed at room temperature.

- Chemical activator can be tertiary amines that are added to the monomer.

- Upon mixing with the powder, it decomposes benzoyl peroxide.

- As external heat is not used to facilitate consumption of monomer, its reaction will be incomplete. Excess monomer act as plasticizer reducing transverse strength of the material and irritate tissues.

- Polymerization shrinkage is less than heat cured resins.

- Color stability of the resin is less than the heat- cured resin due to the presence of tertiary amines that are prone to oxidation.

- Working time is less. It can be prolonged by refrigerating the liquid.

- To help complete the polymerization, the flask is held under pressure for at least 3 hours.

- Unreacted monomer is 3-5% (vs. 0.2-0.5% in heat-cured resins).

…NOTE: Chemically-cured denture base resins are not preferable, but if we don't have sufficient time or we want to use the denture temporary until the permanent one is ready, we can use them.

** Fluid resin technique

- This technique employs the use of low viscosity resins (pourable) to construct denture bases.

- These resins are not widely used; one of the causes is that whenever the material is more viscous, there will be more voids inside it so it will be weaker.

** Light activated denture base resins

- Easy to use but require special machines in the laboratory, which is why they are not commonly used.

- They are copolymers of urethane dimethacrylcate and high molecular weight acrylic resin monomers. Contain microfine silica and curing is performed with light at 400-500 nm. Photoinitiator is camphoroquinone-amine (the light is the initiator). Because it has lower percentage of acrylic they can be used with monomer-sensitive patients. They have reduced shrinkage, but slightly inferior mechanical properties and poor bonding with teeth (disadvantage).

- Supplied in sheets that are applied to the surface of the cast. The teeth are arranged and the setup is cured in special light curing unit producing high intensity visible light.

** Microwave- activated acrylic resins

- Same composition, reaction in 3 min, comparable properties but more expensive and added to enhance the mechanical properties of the material.

- Mechanical reinforcement of resins by fibres (glass, carbon, kevlar and others) or metal inserts (wires, plates, fillers). But problems in tissue irritation from fibres, poor aesthetics, poor bonding with fibres, stress concentration from inserts, increased working time and handling difficulties.

- We can improve the radiopacity by the addition of salts such as barium sulfate and bismuth.

- There are problems when dealing with these resins such as cytotoxicity, reduced strength and aesthetics.


Best wishes





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Shadi Jarrar
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تاريخ التسجيل : 2009-08-28
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