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Denture base resinLIGHT-ACTIVATED DENTURE BASE RESINS

It consists of urethane dimethacrylate matrix with an acrylic copolymer, microfine silica fillers, and high molecular weight acrylic resin monomers, acrylic resin beads as organic fillers. Camphoroquinone as initiator. Visible light is the activator.

Dispensing The single component denture base resin is supplied in sheet & rope forms & is packed in light proof pouches to prevent inadvertent polymerization.

Opaque investing media prevents the passage of light, therefore light-activated resins cannot be flasked in a conventional manner.The denture base is moulded on an accurate cast while it is still pliable and is polymerized in a light chamber without teeth and is used as a base plate.

The teeth are added to the base with added material & anatomy is sculptured while the material is still soft.Photons from a light source activate the initiator to generate free radicals that, in turn, initiate the polymerization process.In the visible light-cured material, camphorquinone and an organic amine (e.g. dimethylaminoethyl methacrylate) generate free radicals when irradiated by light in the blue to violet region.Light with a wavelength of about 400-500nm is needed to trigger this reaction.Then the denture base is exposed to a high-intensity visible light source.

Advantages These resins display less porosity than chemically activated denture base resins, facilitate fabrication and final adjustment in mouth, These are lighter, Free of methyl methacrylate, Show decreased polymerization shrinkage,Are non toxic.Disadvantages They cannot be flasked in a conventional manner, as opaque investing media prevents the passage of light.

Flexible denture base materialsThermoplastic resins have been used in dentistry for many years.Injection molding technique has been in use since 1954 but has of late become popular. Thermoplastic resins are used with injection molding technique to construct flexible dentures.Acetal began to be used for the same in 1971. but the material used now a days is nylon based plastic-polyamide which is stronger & more flexible than acrylic thus can be used as viable alternative to acrylic in patients allergic to it.Polymerization shrinkage encountered in conventionally cured PMMA led to the development of a special injection moulding technique.Elastomeric resins can be added to resin polymer formulas to create greater flexibility and can be strengthened with glass fibres.

Properties Stable in natureHigh creep resistanceNo porosity, no biologic material build upodors/stainHigh dimensional & color stabilitySpecific gravity = 1.14. denser than acrylic & therefore has lower water sorption.Mold shrinkage = 0.014 in/inTensile strength = 11000 psiFlexural strength = 16000 psi

Other uses for the flexible nylon partial dentures are: 1. TMJ splint 2. Gum veneers 3. Space maintainers 4. Immediate dentures 5. Replacement of missing teeth in pediatric cases 6. Precision attachment 7. Combination with metal framework 8. Transitional bridgework 9. Periodontally compromised cases 10. Patients with allergy to acrylic 11. Patients prone to breaking dentures

They can also be used in combination with metal framework as combo dentures where clasps & saddle is flexible & rest of the components are in metal. This provides esthetic advantage in clasp region while adhering to all principles of cast partials.

Flexible dentures The key benefits of the flexible removable dentures are:Retention - flexes into a retentive position, below the undercut. Comfort - thin, lightweight and flexible Aesthetics - pink shades that allow the patient's natural tissue tone to appear through the material.Contd..

Strength - clinically unbreakable, more durable than acrylic and won't absorb stains or odors. Ease - no tooth or tissue preparation is required so you can offer patients a conservative and pain-free solution. Their minimal porosity prevents the buildup of biologic material, odors, and stains, increasing color stability. Disadvantages: Flexibility is not an advantage in complete dentures as the retentive peripheral seal can be broken in function. Insertion: Denture is placed in very hot water (150 degrees F) for a minute prior to insertion and allowed to cool to tolerable temperature. This makes the partial as flexible as it would be at body temperature. Adjustment of clasps is done by heating in very hot water and bending it severely.

But thermoplastics have few contraindications. Because thermoplastic nylon appliances are inherently flexible, they must be tissue-supported rather than tooth-supported.

They could also be difficult to repair, add teeth to, or reline.

Commercially available thermoplastic materials are:

Flexite

ThermoFlex .. Acetal resin material.

Valplast Lucitone FRS .. Nylon based resin material.

FIBER-REINFORCED DENTURE BASE RESINS:To improve the physical and mechanical properties of acrylic resin, it was reinforced with EMBEDDED METAL FORMS FIBRES.

Fibres have been used in three forms,Continuous parallel, Chopped and Woven.Carbon fibresThe use of Carbon fibres as denture base strengthners have been investigated by Larson et al and Sonit (1991).Carbon fibres have been shown to improve flexural and impact strength, prevent fatigue fracture and increased fatigue resistance. However, carbon fibres have an undesirable dark color.Kevlar fibres (synthetic aramid fibres)Aramid is a generic term for wholly aromatic fibres.These fibres are resistant to chemicals, are thermally stable, and have a high mechanical stability, melting point, and glass transitional temperature. They also have pleated structure that makes aramid weak as far as flexural, compression, and abrasion behaviour are concerned. This explains why aramid fibre-reinforced demonstrate a lower flexural strength than PMMA reinforced with glass fibre. but they are unesthetic.

Glass fibresGlass is an inorganic substance that has been cooled to a rigid condition without crystallization. Different types of glass fibres are produced commercially;E-glass, S-glass, R-glass, V-glass, and Cemfil.E-glass fibre, which has high alumina and low alkali and borosilicate,is claimed to be superior in flexural strength. Because the modulus of elasticity of glass fibres is very high, Most of the stresses are received by them without deformation.

Polyethylene fibresHave increased impact strength. Polyethylene fibres increase modulus of elasticity and flexural strength and they are almost invisible in denture base acrylic resins. Polyethylene fibres in woven form are more effective than carbon fibres in enhancing impact strength and flexural strength.MISCELLANEOUS RESINS AND TECHNIQUESRepair Resins:-Despite the favourable physical charactersitics of denture base resins, denture bases sometimes fractures.These repair may be repaired.Repair resins may be light, heat, or chemically activated. Relining Resin denture basesSoft tissue contours changes, it is sometime necessary to alter tissue surfaces of prosthesis to ensure proper fit and function.Relining involves replacement of tissue surface of an existing denture.Heat activated, chemically activated, microwave and light activated are used.Chemically activated material may be used intraorally, but these material generate heat which can cause the oral tissues.

Rebasing resin denturesRebasing involves replacement of entire denture base.Heat activated, chemically activated, microwave are used.Short Term And Long Term Soft Denture LinersThe purpose of soft denture liner is to absorb some of the energy produced by masticatory impact.Soft liner serves as shock absorber between occlusal surface of a denture and underlying soft tissues.Most commonly use acrylic are plasticized acrylic resins.These resin may be heat activated or chemical activated.Chemically activated soft liners contains poly(methyl methacrylate) or poly(ethyl methacrylate) as principal structural components.Supplied as powder form and mixed with liquid containing 60-80% of plasticizer (dibutyl phthalate).These are large molecular species.The distribution of large plasticizer molecules minimizes entanglement of polymer chains, so individual chain slip past one another.

This slipping motion permits rapid changes in shape of soft liner and provides cushioning effect for underlying tissues.These are called short term soft liners or tissue conditioners.Heat activated materials generally more durable and considered as long term soft liners.Supplied as powder liquid systems.Powder contains acrylic resin polymers or copolymers, liquid consist of appropriate acrylic monomers and plasticizers.Plasticizers impart flexibility, present difficulties.Plasticizers are not bound with resin mass and leached out from the soft liner, and soft liner becomes more rigid.Poly(methyl methacrylate) is replaced by higher methacrylates( ethyl n propyl and n butyl).Vinyl resins are used in soft liner application but poly vinyl chloride and poly vinyl acetate are leached out and becomes more rigid.Successful materials for soft liner applications are silicone rubbers.These are not dependent on leachable plasticizers, so elastic properties are retained for longer period.But they have poor adhesion to denture bases. They may be chemical activated or heat activated.Chemical activated silicones are supplied as two component system.Placement of chemically activated soft liner is relatively uncomplicated.Heat activated silicones are one component system supplied as pastes or gels.To promote adhesion between silicone soft liners and rigid denture base materials, rubber poly(methyl methacrylate) cements are used.

Other polymers used as soft liners include polyurethane and polyphosphazine.All the liners have certain shortcomings-Silicone liners are poorly adherent to denture base resins.Many softliners bond well with denture base but leaching of plasticizer makes it rigid. Allergic ReactionsPossible toxic or allergic reactions to poly(methyl methacrylate) have been postulated.These reaction occurs due to contact with polymer, residual monomer, benzyl peroxide, hydroquinone.Residual monomer is the component most often cited as an irritant.The residual monomer in properly processed denture is less than 1%.Surface monomer is completely eliminated following storage in water for 17 hours.Toxicology No evidence that dental resins produces systemic toxic effects.To enter circulatory system residual monomer must pass through the oral mucosa and underlying tissues.If residual monomer reaches the blood stream is rapidly hydrolyzed to methacrylic acid and excreted.Physical properties of denture base resins A. Polymerization shrinkage :During polymerization, the density of the monomer changes from 0.94gm/cc to 1.19gm/cc. This change in density results in volumetric shrinkage of 21%. So, prepolymerise poly methyl methacrylate is used.It reduces the volumetric shrinkage to 7% However, inspite of the high shrinkage, the fit of the denture is not affected because the shrinkage is uniformly distributed all over the surface of the denture. Thus, the actual shrinkage observed is low.Volume shrinkage 8 %Linear shrinkage 0.53 %Self cure type has lower shrinkage. Its linear shrinkage is 0.26 %.Dimensional changes do occur in the denture bases constructed from various resins as illustrated in the fig.

The illustration reveals that chemically activated resin processed with pour type technique gives least dimensional changes.Polymerization shrinkage of posterior of maxillary denture baseMaterial Shrinkage (%)Conventional acrylic0.43High impact acrylic0.12Vinyl acrylic0.33Rapid heat cure acrylic0.97Pour type0.48B. Dimensional stability & accuracyThe dimensional stability of the denture during processing & in service is important for fit of the denture & satisfaction of the patient.Recent studies indicate that dentures processed by injection molding methods are more accurate than standard compression molding. The increase in vertical dimension was very small for injection molded acrylic when compared with conventional compression molded. ( J Prosthet Dent 1999;82:291-300 )C. Porosity

Properly polymerized, no porosity.& C. rapid heating, relatively small subsurface voids.D. Insufficient mixing of monomer & polymer, large voids resulting from localized polymerization shrinkage.E. Insufficient pressure during polymerization, relatively large irregular voids.D. Water absorptionThe mechanism primarily responsible for the ingress of water is diffusion.The introduction of water molecuale produces two important effects:Cause a slight expansion of polymerized mass.Interfere with the entanglement of polymer chains & thereby acts as plasticizers.PMMA exhibits the water sorption value of 0.69 mg/cm2. it has been estimated that for each 1 % increase in the weight produced by water absorption, acrylic resin expands 0.23 % linearly.The linear expansion caused by water absorption is approximately equal to thermal shrinkage encountered as a result of polymerization process. Hence, these processes almost offset one another.E. SolubilityDenture base resins are virtually insoluble in the fluids commonly encountered in the oral cavity.According to ADA specification no.12, weight loss following test should not be more than 0.04 mg/cm2.Crazing is the formation of the surface cracks on the denturea base resin. These may be microscopic or macroscopic in size. In some cases it has hazy or foggy appearance than cracks.Crazing has weakening effect on the resin & reduces the esthetic qualities. The cracks formed can cause fracture.Causes of crazing1. Mechanical stresses: in PMMA crazing occurs when tensile stresses are present.F. Crazing Crazing is produced due to mechanical separation of individual polymer chains on application of tensile stresses.Crazing generally begins at the surface of the resin & is oriented at right angles to tensile forces.2. Crazing may also be produced as a result of solvent action. Micro-cracks produced in this manner are more randomly produced.3. Incorporation of water. G. Creep Denture resins exhibit viscoelastic behavior. These materials acts as rubbery solids. When denture base resin is subjected to sustained load, the material exhibits an initial deflection or deformation. If this load is not removed, additional deformation may occur over time. This additional deformation is termed as creep.The creep rate for heat-activated & chemically activated resins are similar at low stresses, however, creep rates for chemically activated resins increases more rapidly as stresses are increased.H. Cytotoxicity.Autoploymerized resins are the most cytotoxic denture base material.Acrylic resins polymerized by microwave irradiation are less cytotoxic, probably because of greater conversion of monomers into polymer.In addition, water storage may reduce the level of residual monomer, resulting in decreased cytotoxicity of acrylic denture base materials. (J Prosthet Dent 2003;90:190-3.)Thank you