denture base resins

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Denture base resins

Presented by:Apurva Thampi

PG 1st year

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Contents Introduction History Requirements of an ideal denture base material Introduction to PMMA Polymerisation reaction Types of denture base resin Heat activated resins Chemically activated resin Light activated resin Physical properties of resins Recent advancements Summary Bibliography

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Introduction

Dentures – mode of replacement of natural teeth since 700 BC

Increased patient awareness lead to increased expectations

Significant advances in development of new materials for replacement of lost teeth

Acrylic resin is the most widely accepted and used Denture base material.

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History

First Dental prosthesis – Egypt in 2500 BC

TORTOISE

SHELL

PORCE-

LAIN

CELLULOID

WOOD

CHEOPLAST

ICALUMINIUM

BONE

GUTTA

PERCHA

BAKELITE

PVCSS & alloys

GOLDIVORY

VULCANITE

PMMA

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WOOD: Readily available and inexpensive Easily carvableX Cracked in moistureX Lacked aestheticsX Degraded in oral environment

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BONE Available at reasonable costs Carvable Better dimensional stabilityX Aesthetic and hygiene concerns

IVORY Stable in oral environment Aesthetic and hygienicX Not readily availableX expensive

First fabricated

by FAUCHARD

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PORCELAIN Shaped easily Ensured intimate contact with

underlying tissue Stable Minimal water sorption Smooth surface Less porosity Low solubilityX BrittleX Difficult in grinding and polishing

ALEXIS DUCHATEAU in 1774 -

first to fabricate porcelain denture

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18 – 20 karat gold alloyed with silver and teeth riveted to it

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TORTOISE SHELL• It was the first thermoplastic denture

base material• Formed by CF HARRINGTON in1850

GUTTA PERCHA• Unstable• First formed by EDWIN TRUMAN in 1851

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CHEOPLASTIC• It is a low fusing alloy of silver, bismuth

and antimony• First formed by ALFRED A BLANDY in 1856

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VULCANITE• First self retaining dentures• Functional• Affordable • Durable• Dark red colour• Unhygienic

NELSON GOODYEAR

in 1864

http://nmhm.washingtondc.museum/collections/archives/agalleries/dental/dentures_lg.jpg

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ALUMINIUM• By Dr Bean in 1867 ( he also invented the

casting machine

CELLULOID• Discolours easily• Has a residual camphor taste• Difficult to repair• Obtained by plasticizing cellulose nitrate

with camphor

J. SMITH HYATT in

1869

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BAKELKITE• Stains easily• Residual phenol taste• Brittle• Difficult to repair• Short shelf life

POLY VINYL CHLORIDE• Pleasing colour but difficult processing

methods

Dr. LEO BAKELAND

in 1909

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STAINLESS STEEL and BASE METAL ALLOYS• Low density• Low metal cost• Higher resistance to tarnishing and

corrosion• High modulus of elasticity• Allergy to nickel

POLYMETHYL METHACRYLATE• Most satisfactory material

tested till date.

Dr. WALTER WRIGHT (1937)

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Requirements of an ideal denture base material

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POLYMETHYL METHACRYLATE

PMMA/Acrylic resin is the material of choice for full denture bases

Chemical model for many other material developments – restorative materials

The most satisfactory denture base material used till date

REFERENCE : Material science for dentistry, B. W. Darwell, 9th Ed

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POLYMERISATION REACTION

Acrylic resins are prepared by a free radical addition polymerisation

chain reaction


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INITIATION RECTION• Vinyl group susceptible to attack by free

radical• Opening of π bond, and formation of σ

bond• Shift of electron takes place

Initiation reaction


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PROPAGATION REACTION• Process of repeated reaction of the same

type - chain propagation• Steric hindrance effects – increased

effects on attack on next double bond• Polymer chains with free radical –

growing or live chains


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TERMINATION REACTION• Not a function of the chain length already

created• Depends on the concentration of free

radicals in the system• Self limitation of the reaction – mutual

annihilation of free radicals


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CHAIN TRANSFER• Hydrogen abstraction - simple transfer of

an H2 atom to attacking radical

• Leaves a free radical residing on attacked species


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TYPES OF DENTURE BASE RESINS

Based on the mode of activation

• Heat activated PMMA High impact resin Rapid heat polymerising resin Microwave – activated PMMA

• Chemical activated PMMA

• Light activated PMMA

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HEAT ACTIVATED DENTURE BASE RESINS

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COMPOSITION:• Polymer:

pre-polymerised spheres of PMMA Benzoyl peroxide – initiator

• Monomer: Hydroquinone – Inhibitor Glycol dimethacrylate – cross linking agent

(1% - 2% by vol)

REFERENCE : Phillips' Science of dental materials, Anusavice, 11th Ed

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BENZOYL PEROXIDE is not

a catalyst!!!

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STORAGE • Specific time and limits for storage• May undergo changes• Causes changes in working properties• Change in Chemical and physical

properties of processed denture


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MANIPULATION

COMPRESSION MOLD

TECHNIQUE

INJECTION MOLD TECHNIQUE


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COMPRESSION MOLDING TECHNIQUE• Preparation of the mold• Selection of separating medium• Polymer -to-monomer ratio• Polymer –monomer interaction• Dough forming time• Working time• Packing• Polymerization procedure• Temperature rise• Internal porosity• Polymerisation cycle REFERENCE : Phillips' Science of dental materials,

Anusavice, 11th Ed

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Three part flask

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Pressure Clamp

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Precautions before flasking

Proper finishing Periphery should be sealed Apply petroleum jelly on the inner surface of

the flask and on the casts Adjustment of the plaster model Plaster models are wetted - soaked with

slurry water


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Preparation of the mold

Flask is filled with freshly mixed stone Place cast on to the mixture Contour the stone coated with separating media (after

initial set) Another mix of stone is poured into the

flask


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Incisal and occlusal surfaces of teeth should be slightly exposed

Allow to set and coat with separating media

Additional increment of stone filled Lid is gently tapped in place Apply pressure with pressure clamp


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Periphery of flask- in level

with the rim of the flask

Occlusal plane – parallel to

the base of the flask

Retromolar pads and

tuberosity- should be

protected

Devoid of undercut


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Place the flask in boiling water for 4 mins

Remove and separate segments Baseplate and softened wax are

removed Prosthetic teeth remain firmly Cleaned with mild detergent and

rinsed in boiling water

Dewaxing


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Selection and application of separating media

Prevent direct contact between denture base resin and the mold

Failure

Water may affect polymerisation rate and alter optical and physical

properties

Presence of Monomer or free polymer may fuse the

investment to denture base


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Water soluble alginate solution – most popular separating agents

Produce thin film of calcium alginate

Water soluble alginate solution

+ Calcium sulphate dihydrate

Calcium alginate


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Application• Applied on the exposed surfaces of a

warm, clean stone mold• Carefully applied in the interdental

surfaces• Should not contact exposed tooth

surfaces


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Monomer-polymer ratio

Polymerisation results in volumetric and linear shrinkage (21% decrease)

Manufactures pre-polymerize – pre shrinking

Powder + liquid = dough like mass 3:1 is accepted monomer : polymer

ratio (0.5% linear shrinkage)


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Polymer – monomer interaction

A workable mass is produced, which passes through 5 stages

Sandy Stringy Dough-like Rubbery Stiff


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SANDY• Coarse or grainy• Polymer beads remain unaltered

STRINGY• Increased viscosity• Monomer attacks polymer beads

DOUGH-LIKE (ideal for compression molding)• Pliable dough• Increased number of polymer chains


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RUBBERY OR ELASTIC• Mass rebounds when compressed or

stretched• Excess monomer is dissipated by

evaporation

STIFF• Due to complete evaporation of free

monomer


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Dough forming time

Time required to reach a dough like stage

According to ADA spec.no.12 , required consistency should be reached in <40mins (clinically - <10min)


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Working time

Defined as the time that a denture base material remains in the dough like stage

According to ADA spec. 12, material should remain in dough like stage for atleast 5 min

Refrigerating increases working time Presence of moisture degrades

physical and aesthetic propertiesREFERENCE : Phillips' Science of dental materials,

Anusavice, 11th Ed

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Packing Placement and adaptation of denture

base resin within the mold cavity

Denture with excessive thickness and resultant mal-positioning

Noticeable denture porosities

OVERPACKING

UNDERPACKING


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Resin should be in dough-like state Bent into an horse-shoe shape and

placed in position Polyethylene sheet placed over resin –

incremental pressure applied


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Excess material – flash Trial closures are repeated till no flash

remains No polyethylene sheet to be placed for

final closure Flask is transferred to a flask carrier –

maintains pressure


50Cross sectional representation of the flask and its contents


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Polymerisation procedure

When heated above 60⁰C, benzoyl peroxide decomposes

Yields free radicals Acts rapidly with monomer – chain

growth polymerisation


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Additional monomer molecules attach to individual polymers – rapid

Heat required - activator Benzoyl peroxide - initiator

Coupling of 2 grouping chains Transfer of H2 ion from one chain to another


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Temperature rise

Temperature - time heating curves for the waterBath, investing plaster and acrylic resin during polymerisation

ofa thick denture base


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Initially heating is slow - resin occupies I the centre of the mold

Temperature >70C – begin to increase rapidly

Decomposition rate of benzoyl peroxide is significantly increased

Resin and dental stone are poor conductors – heat not dissipated

Temperature rises from that of the boiling point of monomer(100.8 C)


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Internal porosity

Porosities are formed when the temperature of the resin exceeds that of the unreacted monomer

resin is poor thermal conductor - heat generated cannot be dissipated


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Polymerisation cycle

Heating process used to control polymerization – curing cycle

Constant temp – 74CFor 8 hrsor longer with no Terminal boiling treatment

Processing at - 74 C for 8hrs and then increasing to 100

At 74 C for 2 hrs and thenIncreasing to 100 C for 1 hr


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Temperature changes in acrylic resin when subjected to various curing schedules


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Denture flask should be bench cooled for 30mins before retrieval

Rapid cooling – warpage – differences in thermal contraction of resin and investing material

Immersed in cool tap water for 15 mins Deflasked Stored in water until delivery


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Injection moulding technique

Half of the flask is filled with stone Contoured and permitted to set Sprues are attached to the wax

denture base


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Investment process is completed. Wax elimination is performed Flask is placed under pressure Resin mix is introduced into the mold polymerised


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MODIFICATIONS OF HEAT ACTIVATED PMMA

High-impact strength resin

• Reinforced with butadiene-styrene rubber.

• Rubber particles are grafted to methyl methacrylate to bond to the acrylic matrix

• Supplied in powder-liquid form • Conventionally processed

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Rapid heat-polymerized resin

• Hybrid acrylics, with both chemical and heat-activated initiators - allow rapid polymerization

• No porosity expected• polymerized in boiling water for 20

minutes

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Microwave-activated PMMA:• Nishii (1968) first used microwave energy

to polymerize denture base resin in a 400 watt microwave oven for 2.5 minutes. This research was later carried on by Kimura et al (1983) and De Clerk.

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CHEMICALLY ACTIVATED DENTURE BASE RESINS

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Chemical activators used to induce polymerisation

Cold curing / self curing/ auto-polymerising resin

Chemical used – dimethyl –para-toluidine (to monomer)

Initiates breakdown of benzoyl peroxide to produce free radicals and

Hence polymerisation


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Degree of polymerisation is not complete – greater amount of unreacted monomer

Less colour stability due to the presence of the amine – susceptible to oxidation

Plasticizer - Results in decreased transverse strength

Potential tissue irritant


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Less shrinkage and greater dimensional accuracy compared to heat activated PMMA


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Technical considerations

Supplied in monomer – polymer form Mixed according to manufacturer’s

instructions to attain dough like consistency

Working time is shorter Refrigerating monomer increases

working time – rate of polymerization decreases


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Processing considerations

Pressure must be maintained throughout

Initial hardening – 30 min Flask should be held under pressure

for min. 3 hrs Low degree of polymerisation –

dimensional instability – soft tissue irritation


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FLUID RESIN TECHNIQUE

Employs a pourable, chemically activated resin

When mixed – low viscosity resin Completed tooth arrangement is

sealed to the underlying cast Flask is filled with reversible

hydrocolloid – allowed to cool


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After gelation – cast is removed and sprues and vents are cut on the external surface

Wax is eliminated using hot water Teeth are carefully retrieved and

placed in position Resin is mixed and poured via sprue

channels


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Placed in pressurised chamber at room temperature

Allowed to polymerize for30 – 45minutes

Denture is retrieved, sprues are removed

Returned to articulator for correction of processing changes


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Advantages• Improved adaptation• Decreased probability to damage to the

teeth• Reduced cost• Simplification of procedure


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Disadvantages• Noticeable shift of teeth• Air entrapment• Poor bonding• Technique sensitive


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LIGHT ACTIVATED DENTURE BASE RESINS

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COMPOSITION:• Matrix : Urethane dimethacrylate,

microfine silica• Filler : acrylic resin beads• Activator : visible light• Initiator : camphorquinone


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Supplied in sheet and rope form Packed in light proof pouches Opaque investing material is required –

no conventional method


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Denture is moulded on an accurate cast

Exposed to a high intensity visible light for a period

Removed from the mold Finished and polished


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PHYSICAL PROPERTIES OF DENTURE BASE

RESINS

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The physical properties will be discussed under the following headings: Polymerisation shrinkage Porosity Water absorption Solubility Processing stresses Crazing


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Polymerisation shrinkage

During polymerization the density of the mass changes from 0.94 g/cm3 to 1.19 g/cm3. thus a volumetric shrinkage of 21%

Linear shrinkage – denture base adaptation and cuspal interdigitation


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Volumetric shrinkage – 7%, hence linear shrinkage 2%

Initial cooling – resin is soft – contraction occurs at the same rate as that of dental stone

At glass transition temperature – contraction occurs at a faster rate than the surrounding stone.


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Glass transition temperature:

It is a thermal change in which the

resin passes from a soft, rubbery

stage to a rigid glassy state

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Decreases in vertical dimension

Increase in overall vertical

dimension

Fluid Resin

technique

Heat/chemicallyactivated


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Porosity

Surface or subsurface voids compromise physical and aesthetic properties

More likely to develop in thicker portions

Due to vapourization of unreacted monomer and low molecular weight polymers

Does not occur equally


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Can also be due to inadequate mixing of powder and liquid

Regions with more monomer, shrink more – resulting in voids

Using proper monomer – polymer ratio is essential


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Also due to insufficient pressure or less material during polymerisation

Assume irregular shape Resultant resin appears lighter


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Final type is associated with fluid resins

Caused due to air inclusions during mixing and pouring


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Water absorption

Absorbs relatively small amounts when placed in water

Water molecules penetrate the PMMA and occupy positions between polymer chains – forces them apart

Slight expansionIn polymerised mass

Water acts asplasticizers


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Water absorption value – 0.69 mg/cm2

Interferes with the polymer chainMaking them more mobile by releasing

stresses

Changes in shape(insignificant)


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Solubility

Insoluble in fluids in the oral cavity Negligible loss


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Processing stresses

Natural dimensional change is inhibited -contains stresses

Stresses relaxed - distortion occurs During polymerization tensile stresses

are sustained Stress is produced during thermal

shrinkage also (cooling < glass transition temperature)


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Additional factors include • improper mixing and handling of the resin• Poorly controlled heating and cooling of

flask assembly Dimensional changes due to small

stresses - 0.1 to 0.2mm


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Crazing Stress relaxation may produce flaws -

CRAZING Hazy or foggy appearance Tensile stresses most often responsible

and may result the denture to crack.


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Produced due to mechanical separation of individual polymer chains – tensile stresses

Also due to solvent action Begins at surface of the resin and

oriented to right angles to the tensile forces.


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Strength

Load application produces stresses within the resin – change in shape

Strength of the resin is directly proportional to the degree of polymerisation shrinkage

Heat activated resins display lower degree of polymerisation

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Creep

Act as rubbery solids that recover from elastic deformation once stresses are eliminated – viscoelastic behaviour

If load is not removed additional plastic deformation occurs – creep

Rate at which this deformation occurs – creep rate


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RECENT ADVANCES IN DENTURE BASE

MATERIALS

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Radio-opaque dentures

The radiolucent nature of PMMA is one of its disadvantages as a denture base material.

Denture wearers can endure serious complications if their dentures fractures and a portion is inhaled or ingested.

Use of sophisticated ultrasound techniques also prove to be difficult for detection.

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The most promising material - Silanated barium fluoride impregnated powdered glass. (kasim 1998)

Barium sulphate (BaSO4) has also been added to denture base resins to improve radiopacity.

Lang et al. (2000) investigated the potential for triphenylbismuth incorporated into injection moulded heat cure resins to improve radiopacity.

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Cytocompatible anti-fungal resin

Inhibition of Candida albicans on denture resins could play a significant role in preventing the development of denture stomatitis

PMMA-silver nanoparticle discs were formulated, with the commercial acrylic resin

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The inner surface of the prosthesis is rough, and in addition to local (eg, poor hygiene, local trauma, tissue integrity loss) and systemic factors (eg, malnutrition, diabetes mellitus, human immunodeficiency virus infection, xerostomia), contributes to the proliferation of C. albicans

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Spherical silver nanoparticles were synthesized and added to a PMMA formulation, resulting in successful reduction of adherence of C. albicans

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Titanium denture base

Commercially pure (CP) titanium has appropriate mechanical properties

Lightweight (low density) compared with conventional dental alloys

Outstanding biocompatibility that prevents metal allergic

reactions

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Flexible dentures

Flexible denture material is available in the form of granules in cartridges of varying sizes.

It was first introduced by the name of valplast and flexiplast to dentistry in 1956.

These are superpolyamides which belong to nylon family

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Advantages • Soft inherent flexibility• Will not warp• Clinically unbreakable• No porosity• Less bulky• Biocompatible• Better esthetics • Better chewing efficiency

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Disadvantages• De-bonding of acrylic teeth• Discolouration• High surface roughness• Cannot be relined• Difficult to polish• Technique sensitive• Cannot be repaired

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Contraindications :• Insufficient inter-arch space (< 4mm

space for placement of teeth)

• Prominent residual ridges

• Flat, flabby ridges

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Applications:

Management of xerostomia patients - soft and adapt well to the gums - comfortable for wearing.

retain moisture and give better lubrications than acrylic dentures

biocompatible - safe for patients with carcinoma.

lighter in weight, are not brittle, do not warp suitable in conditions of inadequate vertical

dimension

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NESBIT RPD

One modification of the Valplast partial denture is called the Nesbit.

The Nesbit is used to replace one to three teeth on the same side of the mouth and is much smaller than a conventional partial denture.

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The procedure can be completed in two short visits, requires no anesthesia or drilling of teeth (in most cases), and the cost is substantially less than either a permanent bridge or dental implants.

A Valplast Nesbit is generally easy to get used to, and has a very realistic appearance

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High impact acrylic

Incorporation of a rubber phase butadiene styrene

Improved impact strength

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High impact acrylic resin reinforced with zirconia

The transverse strength of high-impact denture base resin can be increased significantly by a factor of 29% and 76% when reinforced with zirconia in a concentration of 5% and 15% respectively

In this process, expansion of ZrO2 crystals occurs and places the crack under a state of compressive stress and crack propagation is arrested

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Fibre reinforced dentures

To improve the physical and mechanical properties of acrylic resin, it was reinforced with fibres

1. Carbon fibres2. Kevlar fibres 3. Glass fibres

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CARBON FIBRES:• The use of Carbon fibres as denture base

strengtheners 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 on treating with silane coupling agent(Yazdanie-1985)

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KEVLAR FIBRES:• These fibres are resistant to chemicals,

are thermally stable, and have a high mechanical stability, melting point, and glass transitional temperature

• Studies conducted by Berrong et al(1990) have shown to significantly increase the impact strength and the modulus of elasticity of the resin but they are also unesthetic

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GLASS FIBRES:• Different types of glass fibres are

produced commercially; these include E-glass, S-glass, R-glass, V-glass, and Cemfil.

• E-glass fibre - 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

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Summary

No denture base material has yet been developed which completely fulfils all the criteria for success and conversely does not posses any of the above noted problems. Since PMMA was introduced, most dental material research has focused upon developing materials with higher strength, lower levels of residual methacrylate monomer after processing, improved dimensional stability, increased radiopacity and improved resistance to candidal infiltration

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Bibliography Phillip’s Sciences of dental materials,

Anusavice, 11th Ed Material science for dentistry, B. W.

Darwell, 9th Ed Young, Beth C. (2010) A comparison of

polymeric denture base materials. Cytotoxicity of denture base acrylic resins:

A literature review http://www.iosrjournals.org/iosr-jdms/papers/Vol13-issue3/Version-2/C013320709.pdf. Denture base resins : From past to future

http://ijds.in/article-pdf-RENU_TANDON_SAURABH_GUPTA_SAMARTH_KUMAR_AGARWAL-63.pdf

http://www.iosrjournals.org/iosr-jdms/papers/Vol13-issue3/Version-2/C013320709.pdf




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THANK YOU!!!

denture base resins

Health & Medicine

material science

satisfactory material

material of choice

carvable x

hygienic x

moisture x

aesthetics x

porcelain denture