non metalic denture base materials

NON METALLIC DENTURE BASE MATERIALS

DENTURE BASE: that part of a denture which rests on the oral mucosa and to which artificial teeth are attached.
• IDEAL PROPERTIES OF DENTURE BASE MATERIALS:
• It should be compatible with absence of odor and taste.
• It should be unaffected by oral fluids (insoluble, non absorbent, and inert)
• It should have adequate mechanical properties; including high modulus of elasticity, high proportional limits, high transverse strength, high impact strength, high fatigue strength, sufficient resilience, hard and with good abrasion resistance.
• It should have other physical properties including; thermal expansion matches that of artificial teeth, high thermal conductivity, low density, and softening temperature higher than the temperature of foods and liquids in the mouth.
•

• It should be esthetically satisfactory.

• It should be easy to process with the minimum of expense and equipments.
• It should be easy to repair.
• It should be radio opaque; if a denture or fragment of a broken denture is accidentally inhaled or ingested, it should be capable of detection by x-ray.
• It should have dimensional stability on processing the denture and in service.
• It should be easy to clean and resistant to microbial growth.
• It should be good shelf life.
•  No denture material that adequately fulfills all these requirements was known.

Denture Base Materials Classified into 3 classifications:

ANSI/ADA Classification(Sp.No.12/ISO 1567)

DENTURE BASE MATERIALS1. metallic: Gold, Ni/Cr, Co/Cr, Ti alloys.2. nonmetallic: acrylic resins, flexible denture base materials.
POLYMERS
Polymers used in dentistry include; vinyl acrylics, polystyrene, epoxies, polycarbonates, polyvinyl acetate polyethylene, cis- and trans-polyisoprene, polysulfides, silicones, polyethers, polyacrylic acids and others.

CHEMICAL COMPOSITION

The term polymer denotes a molecule that is made up of many (poly) parts (mers) . The molecules from which the polymer is constructed are called monomers (one part). Copolymers means polymers contain two or more different types of monomers.
Eg. Monomer : methylmethacrylate
Polymer : polymethylmethacrylate
Copolymer : methylmethacrylate- ethylmethacrylate copolymer


• POLYMERS' USES :
• Denture bases
• Artificial teeth
• Tooth restoratives
• Cements
• Orthodontic space maintainers and elastics
• Facings on fixed metal crowns and bridges
• Obturators for cleft palate
• Impressions
• Dies
• Provisional restorations
• Root canal filling materials
• Athletic mouth protectors.
• Maxillofacial prostheses
• Custom impression trays
• Soft denture lining materials.

Polymers are either thermoplastic or thermosetting.

Thermoplastic refers to polymers that may be softened by heating and solidify on cooling , the process being repeatable. Eg, polyestyrene .
Thermosetting refers to polymers that solidify during fabrication but cannot be softened by reheating .eg, cross-linked polymethylmethacrylate, silicons.
Cross-linked molecules are a network structure that may result in the polymer's becoming one giant molecule .


ACRYLIC RESINS: Acrylic resins is the most widely used and accepted plastic material in dentistry.

Acrylic denture base materials are classified into five groups (types):

Table: Classification of acrylic denture base materials according to ISO 1567 :

• Type

• Description
• 1
• Heat- processing polymers, (powder and liquid or plastic cake)
• 2
• Autopolymerized polymers,( powder and liquid)
• 3
• Thermoplastic ( blank or powder)
• 4
• Light-activated materials
• 5
• Microwave-cured material

Chemical Composition of type 1 and type 2 Acrylic ResinsMost materials are supplied as a powder and liquid.
powder
polymer
• The major component of the acrylic powder is beads of PMMA which are produced by polymerizing methylmethacrylate in specific methods. The PMMA is a clear, glass-like material.


initiator
• it is decomposed either by heating (over than 65ºC), or by an accelerator, or by light. A peroxide such as benzoyl peroxide (approximately 0.5% ) is used.

pigments

• inorganic pigments, usually pink, and small red fibers are incorporated to simulate the natural oral mucosa with its minute blood vessel. Salts of cadmium or iron or organic dyes are used.

Inorganic particles

• to improve acrylic properties

Opacifiers

• to increase the radio opacity of the acrylic
liquid
monomer
• Methylmethacrylate: it is a clear, colorless, volatile liquid with a distinct odor that is toxic if inhaled for a prolong period.

Cross-linking agent

• Ethylene glycoldimethacrylate: to improve the physical properties

inhibitor♦

• Hydroquinone (trace ): to prevent the premature polymerization of the material.


activator
• N N' –DIMETHYL-P-TOLUIDINE (approximately 1% ) ): is added only to self curing material to activate the polymerization at room temperature i.e. without heating.

♦ Note: to prevent the premature polymerization of monomer:

An inhibitor is added to the liquid
The liquid must be stored in a dark- brown, locked bottle.

POLYMERIZATION (CURING )

It is a chemical process which involves the generation of relatively long molecules (polymer) from small component units ( monomer), or;
The process consists of the monomer units becoming chemically linked together to form high molecular weight molecules.

Polymerization stages: Polymerization of acrylics usually occurs in 4 chemical stages summarized as follows:

1. ACTIVATION STAGE : the chemical reaction is activated by one of the following techniques:
1. thermal activation by heating over than 65ºC by either a water bath or microwave oven.
2.chemical activation: the reaction is activated by a chemical agent (accelerator) which is incorporated in the liquid.
3. visible-light activation: by light of a specific wave length.

2.INITIATION STAGE: the reaction is initiated, i.e. the curing starts.

3.PROPAGATION STAGE: the reaction continues and growing long molecules or chains of polymer are produced.

4.TERMINATION STAGE: the molecules reaction is terminated, but the polymerization is ever never be stopped as monomer residue is still present in the final set acrylic material.

MANIPULATION OF CHEMICAL-CURED RESINS:

A powder-liquid ratio of 3:1 is used or according to manufacturer instructions. The powder and liquid are mixed in a glass jar and covered. The mixture passes through the following physical
changes:

1. sand stage: the mixed material.

2.stringy stage: the material is sticking to the spatula and fingers and sides of mixing vessel.
3.dough stage: the mixture is more cohesive and less stickiness and can be separated easily from the side of the mixing jar. The material now is moldable and workable and can be shaped according to use.
4.rubber stage: the material is rubber-like.
5.hard solid stage.

Doughing time: the time taken to reach the dough stage.

Working time of acrylic: the time in which the material remains at the
dough stage and it is moldable and workable.

MANIPULATION OF HEAT-CURED RESINS:

The procedure is same as that of chemical-cured resin, but when the mixture reaches the dough stage, the material is packed into a flask and then cured in a water bath using one of these cycles:
Water bath curing cycles;

1. long cycle: is to heat the water of the bath containing the flask for 7 hours at 700C followed by 3 hours at 1000C.

2. short cycle: tap water is placed in the bath containing the flask. The water is gradually brought to the boil over a period of 1 hour. Then the water is allowed to boil for 1 hour.

Then the flask is cooled slowly on pang, and deflasked. The hard acrylic denture cleaned, finished and polished.

Polymerization of heat cured acrylic resins by microwave

energy:

This process required :

1.a specially formulated resin.
2.non metallic flask
3.a conventional microwave oven to supply the thermal energy.

Advantages of microwave acrylic resins :

1.speed of polymerization
2.the physical properties of microwave resins are comparable to conventional resins
3.the fit of microwave denture bases are comparable with conventional ones.


Light activated denture base resins:

Chemical composition: this material has been described as a composite having the following composition:
1.urethane dimethacrylate
2.microfine silica
3.high-molecular weight acrylic resin monomer
4.organic filler (acrylic resin beads)
5.initiator (camphoroquinone)

Polymerization of Light activated denture base resins:

This type of denture base is supplied as a single component in sheet and rope forms and is packed in light-proof pouches to prevent inadvertent polymerization.

The light activated denture resins cannot be flasked in a conventional manner because opaque investing media prevents the passage of light. Instead teeth are arranged and the light-activated resin is molded on an accurate cast. Then it is exposed to a high-intensity visible light source for an appropriate period. After polymerization the denture is finished and polished in a conventional manner.

Properties of denture base acrylic resins:

• Adequate appearance with various pigments, allowing tissue color matching.
• light weight.
• low thermal conductivity.
• low abrasive resistance.
• They have residual monomer that may develop allergy in some patients.
• Suffecient thickness of the material provides adequate rigidity and strength.
• Insignificant dimensional changes.
• Water absorption followed by drying may cause crazing. Therefore, the dentures should be kept moist at all times.


Flexible denture base materials

Thermoplastic materials for dental prostheses, Valplast and Flexiplast, were first introduced to dentistry in the 1950s. Both materials were similar grades of Polyamides (nylon plastics).

Advantages of nylon flexible denture (1955)

1. Soft and inherent flexibility: Ability to engage undercuts for retention. No need for periodic adjustment of clasp to keep them tight. Low modulus of elasticity
2. Will not warp or become brittle.
3. clinically unbreakable.
4. Good biocompatibility: because it is free of monomer and metal.
5. No porosity, so no bacteria can build up within it.
6. No gingival inflammation
7. More comfortable
8. Absorb small amounts of water to make the denture more soft and tissue compatible.
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9. Less bulky (thinner) and lighter weight
10. Better chewing efficiency
11. Better esthetics: Translucent, so it allows natural gum to show through, making it invisible. Clasps rest on the gums surrounding the natural teeth. They are indistinguishable from the gums. No metal framework
12. More retention and stability
13. Retention depends mainly on the tissue and only a small portion of abutment tooth. No evidence of excessive abutment mobility
14. Ease of fabrication (in comparison with cast RPD)
15. Reduces chair side time (shorter fabrication time)
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Disadvantages of nylon flexible denture

1.Intended only for provisional or temporary applications. Flexible dentures are generally only used when traditional dentures cause discomfort to the patient and cannot be solved through relining.
2. A major drawback is de-bonding of the acrylic teeth from nylon denture base. Nylon polyamide denture base material does not bond chemically with acrylic resin/porcelain, so mechanical undercuts (diatorics) are made in each tooth. It cannot be used with patients having low vertical dimension and closed bite.
3.Tend to absorb the water content and will discolor often.

4.Discoloration and gradual fading of denture base color are reported after 1-2 years.

5.High surface roughness. This may lead to bacterial and fungal colonization. ??!!N.B: Brushing a Valplast appliance is not recommended as this may remove the polish and roughen the surface over time.

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6.The procedure is technique sensitive. Extreme caution is necessary when processing to avoid skin contact with the heated sleeve, cartridge, furnace, hot cartridge, injection insert, hot flasks and heat lamps.
7.Difficult to adjust and polish.
8.When grinding this prosthesis, proper ventilation, masks and vacuum systems should be used.
9.Lower hardness
10.Lacks important elements of RPD, in particular, occlusal rests and a rigid framework, So it won't maintain vertical dimension. It is contraindicated for unilateral distal extension.
11.Usually cannot be relined, so stability is a concern if the alveolar ridge resorbs. Usually cannot be repaired.


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Nylon flexible denture

Conventional rigid acrylic denture
Not brittleClinically unbreakable
Brittleness of PMMA, so frequent fracture occurs
Good biocompatibility: because it is free of monomer and metal
Allergy to MMA monomer
More comfortable, not irritant to mucosa
Irritation of mucosa
Soft and inherent flexibilityAbility to engage undercuts for retention
RigidDifficult to insert in undercut areas
Better esthetics, esthetic clasp
Less esthetics, metal clasp
Shorter fabrication time
Longer fabricaion time
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Nylon flexible denture

Conventional rigid acrylic denture
Temporary only
Permanent
Mechanical retention between acrylic teeth and nylon denture.
De-bonding is a major disadvantage.
Chemical bond between acrylic teeth and acrylic denture
Lower hardness
Higher hardness
High surface roughness.
Brushing increases surface roughness


Usually cannot be relined and repaired
Can be relined and repaired
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