CLASSIFICATION OF RESINS

Due to their heterogenous structure and complex nature it

is difficult to classify them.

Based on the thermal behaviour, they are classified as;

Thermoplastic:

Refers to resins that are softened and moulded under heat

and pressure without any chemical changes occurring. They are

cooled after moulding. They are fusible and are usually soluble

in organic solvents. E.g., Polymethyl methacrylate, polyvinyl

acrylics and polystyrene.

Thermoset:

Refers to resins in which a chemical reaction takes place

during moulding. The final product is chemically different from

the original substance. These cannot be softened by reheating

like the thermoplastic resins. They are generally infusible and

insoluble. E.g., Crosslinked poly (methyl methacrylate), silicones

etc.

A more exact means for classification is in terms of its

structural units.

BASIC NATURE OF POLYMERS:

“Polymer”:

Denotes a molecule that is made up of many parts. The

mer ending represents the simplest repeating chemical

structural unit from which the polymer is composed. Poly

(methyl methacrylate) is a polymer having chemical structural

units derived from methyl methacrylate. Usually any chemical

possessing a molecular weight higher than 5000 is considered to

be a polymer.

“Monomer”:

The molecules from which the polymer is constructed are

called monomers (one part).

Polymer molecules may be prepared from a mixture of

different types of monomers and they are called co-polymers.

Molecular weight:

The molecular weight of polymer molecule equals the

molecular weight of the various mers multiplied by the number

of mers. They may range from thousand to millions of molecular

weight units depending on preparation conditions.

The molecular weight of polymers play an important role in

determining its physical properties.

Degree of polymerization: Defined as the total number of

mers in a polymer.

The higher the molecular weight of the polymer made

from a single monomer, the higher the degree of

polymerization.

The strength of the resin increases with increase in the

degree of polymerization until a certain molecular

weight is reached. Above this there is no change.

Molecular weight distribution:

A narrow molecular weight distribution gives the most

useful polymers. However most polymers have a wide range of

molecular weights and so vary widely in their properties. For

example, the higher the molecular weight, the higher the

softening and melting points and the stiffer the plastic.

Polymerization chemistry:

The term polymerization refers to a series of chain

reactions by which a macromolecule or polymer is formed from a

single molecule known as “monomer”.

These structural units are connected to each other within

the polymer molecules by bonds. Polymerization is a repetitive

intermolecular reaction that is functionally capable of

proceeding indefinitely.

Types: Most polymerization reactions fall into two basic types.

1) Condensation polymerization:

Condensation resins are divided into two groups:

a) Those in which polymerization is accompanied by

repeated elimination of small molecules: The primary

compounds react with the formation of by products such

as water, halogen acids, and ammonia. The process can

repeat itself and forms macromolecules.

b) Those in which functional groups are repeated in the

polymer chains: The mers are joined by functional

groups (like amide, urethane, ester or sulfide linkages).

Formation of a by-product is not necessary. E.g.,

polyurethane.

In the past several condensation resins have been used

to make denture bases. E.g., bakelite (phenol-

formaldehyde resin).

The formation of polymers by the condensation method

is rather slow. Also, it tends to stop before the

molecules have reached a truly giant size. Thus, building

molecules with high molecular weights is very difficult.

At present, condensation resins are not widely used in

dentistry.

2) Addition polymerization:

All resins employed extensively in dental procedures are

produced by addition polymerization. The word polymerization

when used alone means addition polymerization. Here there is

no change in chemical composition and no by products are

formed during the formation of macromolecules. In this type of

polymer the structure of the monomer is repeated many times in

the polymer.

Giant molecules of almost unlimited size can be produced

in this manner. Starting from an active center, one molecule at a

time is added and a chain rapidly builds up, which can grow

almost identifinitely as long as the supply of building blocks is

available. This process is simple but not easy to control.

ACRYLIC RESINS

The acrylic resins are derivatives of ethylene and contain a

vinyl group in their structural formula. The acrylic resins used in

dentistry are the esters of;

1) Acrylic acid, CH2=CHCOOH

2) Methacrylic acid, CH2=C(CH3)COOH

95 percent of the complete dentures made today use one

of the acrylic resins. These are available as methylmethacrylate

(liquid) and poly (methyl methacrylate) – (powder).

Poly(Methylmethacrylate) resins:

These are widely used in dentistry to fabricate various

appliances. One of the reasons for its wide popularity is the ease

with which it can be processed. Although, it is a thermoplastic

resin, in dentistry is not usually molded by the thermoplastic

means. Rather, the liquid (monomer) methylmethacrylate is

mixed with the polymer (powder). The monomer plasticizes the

polymer to a doughlike consistency which can be easily

moulded.

Types: Based on the method used for its activation;

i) Heat activated resins

ii) Chemically activated resins

iii) Light activated resins

Heat-activated denture base resins:

Mode o supply:

Available as: 1) Powder and liquid

2) Gels – sheets and cakes

Composition:

LIQUID

Methyl methacrylate dibutyl phthalate - Plasticizer

Glycol dimethacrylate (1-2%) - Crosslinking agent

Hydroquinone (0.006%) - Inhibitor

The liquid (monomer) is supplied in tightly sealed amber

coloured bottles (to prevent evaporation and premature

polymerization by light or ultraviolet radiation on storage).

POWDER

Poly (methyl methacrylate)

Other copolymers – 5%

Benzoyl peroxide - Initiator

Compounds of mercuric sulfide, cadmium sulfide - Dyes

Zinc or titanium oxide - Opacifiers

Dibutyl phthalate - Plasticizer

Dyed organic filler

Inorganic particles like glass fibers of beads

The powder is in the form of beads or small spherical

particles. The high molecular weight poly (methyl methacrylate)

dissolves in the monomer very slowly. So the following methods

are used to increase the solubility.

1) By incorporating an additive. E.g., a copolymer of methyl

methacrylate and ethyl acrylate.

2) By adding a plasticizer such as dibutyl phthalate, either

by ball milling it with the pearls or by adding it to the

monomer (8 to 10%).

3) By blending the high molecular beads with poly (methyl

methacrylate) of lower molecular weight, which is more

soluble in the monomer.

Polymerization reaction:

Polymerization is achieved by application of heat and

pressure. The simplified reaction is outlined below:

Powder Liquid

(Polymer) (Monomer)

+ + + + Heat --- Polymer + Heat

Initiator) Inhibitor) (External)

(Reaction)

Technical consideration:

1) Compression moulding technique

2) Injection moulding technique

I. Compression Moulding Technique:

This is the most commonly used technique in the

fabrication of acrylic resin dentures.

Steps:

1. Preparation of the wax pattern

2. Preparation of the split mould

i) The pattern is invested in a dental flask using

dental stone or plaster.

ii) After the stone or plaster sets, it is dewaxed by

placing the flask in boiling water for not more than

5 minutes.

3. Application of separating medium:

The resin must be protected from contact with gypsum

surface for the following reasons:

i) To prevent water from the mould entering into the

acrylic resin. This may affect the rate of

polymerization and colour of the resin.

ii) To prevent monomer penetrating into the mould

material, causing plaster to adhere to the acrylic

resin and producing a rough surface.

Types of separating media:

The various separating media used are:

a. Tinfoil

b. Cellulose lacquers

c. Solution of alginate compounds

d. Evaporated milk

e. Soap

f. Sodium silicate

g. Starches

Tin foil was the material used earlier. This is a time

consuming and difficult process. It has been largely replaced by

other separating media known as tinfoil substitutes.

4. Mixing of powder and liquid:

The correct proportions of polymer and monomer are

mixed.

Proportion: Polymer – Monomer = 3 : 1 by volume

OR

2 : 1 by weight

Mixing is normally carried out by placing a suitable volume

of liquid into a clean, cry mixing vessel followed by slow addition

of powder, allowing each powder particle to become wetted by

monomer. The mixture is then stirred and allowed to stand in a

closed container.

Physical stages of polymerization:

After mixing polymer & monomer, it is allowed to stand in a

closed mixing vessel jar. The material goes through various

physical stages. No polymerization reaction takes place during

these stages. But the plasticization takes place by a partial

solution of the polymer in the monomer.

Stage 1 : Wet sand stage: The polymer gradually settles into

the monomer forming a fluid, incoherent mass

Stage 2 : Sticky stage: The monomer attacks the polymer by

penetrating into the polymer. The mass is sticky and

stringy (cobweb like) if the mixture is touched or

pulled apart.

Stage 3 : Bough or gel stage: As the monomer diffuses into

the polymer, the mass becomes more saturated with

polymer in solution. It becomes smooth and dough

like. It does not adhere to the walls of the jar. It

consists of undissolved polymer particles suspended

in a plastic matrix of monomer and dissolved

polymer. The mass is plastic and homogenous and

packed into the mould at this stage.

Stage 4 : Rubbery stage: The monomer disappears by further

penetration into the polymer and evaporation. The

mass is cohesive, rubber like, non plastic and

cannot be moulded.

Stage 5 : Stiff.

5. Packing: The powder – liquid mixture should be packed into

the flask at the dough consistency for several reasons:

i) If it is packed at the sandy or stringy stages, too

much monomer will be present between the polymer

particles, and the material will be of too low a

viscosity to pack well and will flow out of the flask too

easily. Packing too early may also result in porosity in

the final denture base.

ii) If packed at the rubbery to the stiff stage, the

material will be too viscous to flow, and metal-to-

metal contact of the flask halves will not be obtained.

Delayed packing will result in loss of detail in the

denture, movement or fracture of the teeth and

increase in the contact vertical dimension of the

denture.

6. Curing: After the final closure of the flasks, they should

remain at room temperature for 30 to 60 minutes.

Sometimes this is called as “Bench curing”.

Curing cycle:

The curing or polymerization cycle is the technical name

for the heating process employed to control the initial

propagation of polymerization in the denture mould. The curing

cycle selected should depend on the thickness of the resin.

Following are the recommended curing cycles.

i) Heat the flask in water at 600 to 700C for 9 hours.

ii) Heat the flask at 650C for 90 minutes, then boil the

water for 1 hour for adequate polymerization in thinner

portions.

7. Cooling: The flask should be cooled slowly i.e.

bench cooled. If it is placed directly into tap water,

warpage of the denture takes place due to differential

thermal contraction of the resin and gypsum mould.

Cooling overnight is ideal.

8. Deflasking: This has to be done with care to avoid

flexing and breaking of the acrylic dentures.

9. Finishing and polishing: A suspension of finely

ground pumice in water is commonly used for polishing.

II. Injection moulding technique:

The mould space may be filled by injecting the resin under

pressure before it hardens. A sprue hole and a vent hole are

formed in the gypsum mould and the metal flask is constructed

such that it will adapt to the injection moulding equipment. The

soft resin is contained in the injector and is forced into the

mould space as needed. The resin is injected in dough stage and

kept under pressure until it has hardened.

In the case of the polystyrene resin, the polymer is first

softened under heat and injected while hot. The thermoplastic

resin then solidifies in the mould upon cooling.

No trial closures are required with this technique. There is

no difference in accuracy or physical properties as compared to

compression moulding technique.

Advantages Disadvantages

1. Dimensional accuracy 1. High capital costs

2. Low free monomer content 2. Difficult mould design problems

3. Good impact strength 3. Less craze resistance

4. Less creep resistance

5. Special flask is required

Polymerization procedure

Polymerization:

When the temperature of the dough increases above 600C

the molecules of benzoyl peroxide decompose to form free

radicals. A free radical reacts with a monomer molecule, and a

new free radical is formed, which in turn gets attached to

another monomer molecule. The chain reaction is thus

propagated until a termination occurs.

The rate of polymerization depends upon the rate at which

the free radicals of benzoyl peroxide are released. This is

dependant on the temperature.

Lower temperature of polymerization results in greater

molecular weight of the polymer.

Temperature rise:

The polymerization reaction is exothermic. As the

temperature of water and plaster are increased from room

temperature to 1000C in 60 minutes, the temperature of acrylic

increases to the same rate until the temperature rises slightly

above 700C.

At this time the temperature of the resin begins to rise

rapidly. At this temperature more number of benzoyl peroxide

molecules are activated producing a chain reaction. Because of

this the temperature of the interior of the resin rises above the

temperature of boiling water at which the resin was polymerized.

Curing cycle:

The curing cycle is the technical name for the heating

process employed to control the initial propagation of

polymerization in the denture mould.

The recommended curing cycles are;

i) Heat the flask in water at 600 to 700C for 9 hours.

ii) Heat the flask at 650C for 90 minutes, then boil water

for 1 hour for adequate polymerization in thinner

portions.

Apart from water bath, a variety of other methods of

supplying the necessary heat to accelerate the polymerization

function have been used. They include;

1. Steam 4. Infrared heating

2. Dry air oven 5. Induction or dielectric heating

3. Dry heat (electrical) 6. Microwave radiation

Results of various processing studies have shown that

equally satisfactory, but not superior, results may be obtained

with any of the methods compared to water bath method if

adequate temperature control and pressure are maintained.

Polymerization by microwave energy: Poly (methyl

methacrylate)

Resin can also be polymerized by microwave energy.

Advantages:

1. It is cleaner and faster than polymerization with the

conventional hot water.

2. The fit of the denture is comparable or superior.

3. Acrylic resins formulated for microwave polymerization

are less prone to porosity.

Advantages

1. Good appearance

2. High glass transition

3. Ease of fabrication

4. Low capital costs

5. Good surface finish …Is2

Disadvantages:

1. Radiolucency

2. Free monomer content or formaldehyde may cause

sensitization.

3. Fatigue life too short

4. Low impact strength

Chemically activated denture base acrylic resins:

The chemically activated acrylic resins polymerize at room

temperature. They are also known as ‘self-curing’, ‘cold cure’ or

‘autopolymer’ resins.

In cold cured acrylic resins, the chemical initiator benzoyl

peroxide is activated by another chemical. (dimethyl-para

toluidine) which is present in the monomer.

Therefore, the fundamental difference between heat cure

and self cure resins is the method of activation of benzoyl

peroxide.

Composition Liquid

Methyl methacrylate

Dimethyl-p-toluidine - Activator

Dibutyl phthalate - Plasticize

Glycol dimethacrylate 1 to 2% - Cross linking agent

Hydroquinone 0.006% - Inhibitor

Powder

Poly (methyl methacrylate)

Other copolymers – 5%

Benzoyl peroxide - Initiator

Compounds of mercuric

sulfide, cadmium sulfide

- Dyes

Zinc or titamium oxide - Opacifiers

Dibutyl phthalate - Plasticizer

Dyed organic fillers

Inorganic particles like glass fibres or beads

Manipulation:

1. Sprinkle on technique

2. Adapting technique

3. Fluid resin technique

4. Compression moulding technique

5. Injection moulding technique

Fluid resin technique: (Pour-type acrylic resins)

The chemical composition of the pour type of denture resin

is similar to the poly (methyl methacrylate) materials that are

polymerized at room temperature. The principal difference is

that the pour type of denture resins have high molecular weight

powder particles that are much smaller and when they are mixed

with monomer, the resulting mix is very fluid. Therefore they are

referred as ‘fluid resins’. They are used with significantly lower

powder liquid ratio, i.e., it ranges from 2:1 to 2.5:1. This aids to

prevent undue increase in viscosity during mixing and pouring

stages.

The main difference with these materials lies in the

methods of flasking and curing. This technique most commonly

involves the use of agar hydrocolloid for the mould preparation

in place of usual gypsum investment. The fluid mix is quickly

poured into the mould and allowed to polymerize under pressure

at 0.14 MPa (20 psi).

Light-Activated Denture Base Resins

This denture base material consists of a urethane

dimethacrylate matrix with an acrylic copolymer, microfine silica

fillers, and a photoinitiator systems. (Camphoroquinone amine

photoinitiator).

It is supplied in premixed sheets having a claylike

consistency. It is provided in opaque tight packages to avoid

premature polymerization. The denture base material is adapted

to the cast while it is in a plastic state. The denture base can be

polymerized without teeth and used as a base plate. The teeth

are added to the base with additional material and the anatomy

is sculptured while the material is still soft. It is polymerized in a

light chamber (curing unit) with blue light of 400-500 nm from

high intensity quartz halogen bulbs. The denture is rotated

continuously in the chamber to provide uniform exposure to the

light source.

Properties of denture resins:

Methyl methacrylate monomer:

It is a clear, transparent, volatile liquid at room

temperature. It has a characteristic sweetish odour. The physical

properties of monomer are:

Melting point : - 480C

Boiling point : 100.80C

Density : 0.945 gm/ml at 200C

Heat of polymerization : 12.9 Kcal/mol

Volume shrinkage during polymerization : 21%

Poly (methyl methacrylate):

1. Taste and odour: Completely polymerized acrylic resin is

tasteless and odourless.

2. Esthetics: It is a clear transparent resin which can be

pigmented (coloured) easily to duplicate the oral

tissues. It is also compatible with dyed synthetic fillers.

3. physical and mechanical properties:

Density: The polymer has a density of 1.19 gm/cm3

Strength:

Compressive and tensile strengths:

These materials are typically low in strength. However,

they have adequate compressive and tensile strength for

complete or partial denture applications.

Compressive strength - 75 MP

Tensile strength - 52 MP

Hardness:

Acrylic resins are materials having low hardness. They can

be easily scratched and abraded.

Heat cured acrylic resin : 18.20 KHN

Self cured acrylic resin : 16 – 18 KHN

Modulus of elasticity:

Acrylic resins have sufficient stiffness (modulus of

elasticity – 2400 MPa) for use in complete and partial dentures.

However, when compared with metal denture bases it is low.

Self cured acrylic resins have slightly lower values.

4. Dimensional stability:

A well processed acrylic resin denture has good

dimensional stability. The processing shrinkage is balanced by

the expansion due to water sorption.

a) Shrinkage: Acrylic resins shrink during processing due to

two reasons:

1. Thermal shrinkage on cooling

2. Polymerization shrinkage

Polymerization shrinkage:

During polymerization, the density of the monomer

changes from 0.945 gm/cc to 1.19 gm/cc. This results in a

shrinkage in the volume of monomer-polymer dough.

However, inspite of the high shrinkage, the fit of the

denture is not affected because the shrinkage is uniformly

distributed over all surfaces of the denture. Thus, the actual

linear shrinkage observed is low.

Volume shrinkage - 8%

Linear shrinkage - 0.53%

Self cured resins have a lower shrinkage (linear shrinkage –

0.26%).

b) Water sorption:

Acrylic resins absorb water (0.6 mg/cm2) and expand. This

partially compensates for its processing shrinkage. This process

is reversible. Thus, on drying they lose water and shrink.

(However, repeated wetting and drying should be avoided as it

may result in warpage of the denture).

Solubility:

Poly (methyl methacrylate) is virtually insoluble in water

and oral fluids.

They are soluble in ketones, esters and aromatics and

chlorinated hydrocarbons. Alcohol causes crazing in some resins.

6. Thermal properties:

a. Stability to heat: Poly(methyl methacrylate) is chemically

stable to heat. It softens at 1250C. However, above this

temperature i.e., between 1250C and 2000C it begins to

depolymerize. At 4500C, 90% of the polymer will

depolymerize to monomer.

b. Thermal conductivity: They are poor conductors of heat

and electricity.

c. Coefficient of thermal expansion: These materials have a

high coefficient of thermal expansion (81 x 10 -6/0C).

Addition of fillers reduces the coefficient of expansion.

7. Colour stability: Heat cured acrylic resins have (greater)

colour stability. The colour stability of selfcure resins is

slightly lower (yellows very slightly).

8. Biocompatibility: completely polymerized acrylic resins are

biocompatible.

Pure monomer if inhaled over a long period is toxic. It may

also cause allergic manifestations in some individuals.

Precautions to be taken are;

i. Minimize residual monomer content by using proper

processing techniques.

ii. Avoid direct handling of acrylic dough with bare hands.

iii. Work in well ventilated areas to avoid inhalation of the

monomer vapour and the usage of mouth mask.

Residual monomer:

During the polymerization process the amount of residual

monomer decreases first rapidly and then later more slowly.

The highest residual monomer level is observed with

chemically activated denture base resins at 1% to 4% shortly

after processing. When they are processed in less than one hour

in boiling water the residual monomer is 1% to 3%. IF they are

processed for 7 hours at 700C and then boiled for 3 hours the

residual monomer content may be less than 0.4%.

In heat cured acrylic before the start of curing the residual

monomer is 26.2%. In 1 hour at 700C it decreased to 6.6% and at

1000C it was 0.29%.

In order to reduce the residual monomer in heat cured

dentures it should be processed for a longer time in boiling

water. The processing temperature should be raised to boiling

when most of the polymerization is completed otherwise porosity

may result.

9. Adhesion: The adhesion of acrylic to metal and porcelain is

poor, so mechanical retention is required. Adhesion to

plastic (denture) teeth is good.

10. Shelf life: The shelf life varies considerably. The acrylic

resins dispensed as powder/liquid have the best shelf life.

The gel type has a lower shelf life and has to be stored in a

refrigerator.

Porosity

Porosity is a processing error in acrylic resins. When

porosity is present on the surface, it makes the appearance of

denture base unsightly. Proper cleaning of the denture is not

possible, so the denture hygiene and thus the oral hygiene

suffer. Even when it appears as internal porosity in the form of

pores or blebs, it weakens the denture base and the pores are

areas of stress concentration, thus the denture warps as the

stresses relax.

Porosity may be:

1. Internal porosity

2. External porosity

1. Internal porosity: Is in the form of voids or bubbles within

the mass of the polymerized acrylic. It is usually not

present on the surface of a denture. It is confined to the

thick portions of the denture base and it may not occur

equally throughout the affected area.

2. External Porosity: It can occur due to two reasons:

i) Lack of homogeneity: If the dough is not

homogenous at the time of polymerization, the

portions containing more monomer will shrink more

than the adjacent areas. This localized shrinkage

results in voids. The resin appears white.

It can be avoided or minimized by using proper

powder-liquid ratio and mixing it well. The mix is

more homogenous in the dough stage, so packing

should be done at the dough stage.

ii) Lack of adequate pressure: During polymerization

or due to lack of dough in the mould during final

closure. Bubbles appear which are not spherical

and the resin appears white. A pigmented resin

appears lighter in colour due to lack of pressure

during polymerization.

Crazing

Crazing is formation of surface cracks on the denture base

resin. These cracks may be microscopic or macroscopic in size.

In some cased it has a hazy or foggy appearance raphe than

cracks.

Crazing has a weakening effect on the resin and reduces

the esthetic qualities. Cracks formed on crazing are indicative of

the beginning of a fracture.

Causes: Crazing is due to

1. Mechanical stresses or

2. Attack by a solvent of water

Recent advances:

Several modified poly (methyl methacrylate) materials

have been used for denture base applications. These include:

Pour type of denture resins, hydrophilic polyacrylates. High

impact strength resins, Rapid heat polymerized acrylic and

Light-activated denture base material.

High impact strength materials:

These materials are butadiene styrene rubber-reinforced

poly (methyl methacrylate). The rubber particles are grafted to

methylmethacrylate so that they will bond well to the heat

polymerized acrylic matrix. These materials are supplied in a

powder-liquid form and are processed in the same way, as other

heat-accelerated methyl methacrylate materials.

Rapid heat-polymerized resins:

These are hybrid acrylics that are polymerized in boiling

water immediately after being packed into a denture flask. After

being placed into the boiling water, the water is brought back to

a full boil for 20 minutes. After the usual bench cooling to room

temperature, the denture is deflasked, trimmed, and polished in

the usual manner. The initiator is formulated to allow for rapid

polymerization without the porosity that one might expect.

METALLIC DENTURE BASE MATERIALS

Chrome castings, gold castings and aluminum castings are

still in use. Gold cast bases have some drawbacks. The recent

prices are extremely high.

Gold is very heavy when used as a base; of course its

accuracy is outstanding and adjustment factor causes no

problems. A maxillary denture with a gold base can be self

defeating because of weight.

Chrome metal bases are economical to cast and fit

acceptably. They are used as lower full cast lower bases. They

are difficult to adjust. Chrome also used to reinforce acrylic

denture for added strength and for less dimensional change in

denture resin.

Cast aluminum bases are always acceptable they are used

as they are easy to adjust with simple burs and rubber wheels.

Aluminum can be destroyed by soaking in denture cleansers with

chloride ions. They can be used as palatal cover as chrome

castings or a full casting veneered with denture resin.

Advantages of metal bases:

Metal base prevents warpage during processing while

acrylic resin does not.

Metal base is stronger than acrylic resin and is less

subject to breakage.

The fit of metal is more accurate and tissue detail in

reproduced more faithfully than acrylic resin bases.

Less tissue change seems to occur metal bases than

under those of acrylic resin.

A metal base is less porus than organic material.

Metal is a better thermal conductor than organic

material.

Dentures made with metal bases show less lateral

deformation in function than do others.

Patients appear to master the use of dentures made with

metal bases more quickly than do with others.

Problems of patients with poor ridges have been treated

more successfully with metal bases than others.

A snugness of fit, attainable with metal base dentures,

seems to be absent in acrylic resin denture bases.

Metal denture bases

Metal denture bases may be made from a number of

different materials such as gold, aluminum – manganese,

platinum, satellite (cobalt-chromium) alloys and stainless steel.

Some disadvantages of metal bases are;

Greater initial cost and greater restorative cast.

Difficulty and expense of rebasing and regrinding

occlusion of metal dentures.

Less margin of error permissible in post palatal seal on a

metal denture.

Advantages of cast metal bases:

The metal base prevents warpage during processing

while acrylic resin does not.

Metal bases is stronger than acrylic resin and is less

subject to breakage.

The fit of aluminum is more accurate and tissue detail is

more.

Less tissue change seems to occur under aluminum or

other metal bases than under those of acrylic resin.

A metal base is less porus than organic material.

Classification of dental casting alloys:

In 1932, dental materials group at the National Bureau of

Standards surveyed the alloys being used and roughly classified

them as;

Type I - Soft (VHN 50-90)

Type II - Medium (VHN 90-120)

Type III - Hard (VHN 120-150)

Type IV - Extrahard (VHN ≥ 150)

According to ADA Specification No. 5 a dental casting alloy

is classified as;

Type I (Soft): For restorations subjected to very slight stress

such as inlays.

Type II (Medium): For restoration subjected to moderate stress

such as inlays.

Type III (Hard): For high stress situations, including onlays,

crowns, thick veneer crowns and short span fixed partial

dentures.

Type IV (Extra hard): for extremely high stress such as

endodontic posts and cores, thin veneer crowns, long span fixed

partial dentures and removable partial dentures.

Desirable qualities of casting alloys:

Metal must exhibit biocompatibility.

Ease of melting, casting, brazing (or soldering and

polishing.

Little solidification shrinkage, minimal reactivity with

mold material.

Good wear resistance, high strength and sag resistance.

Excellent tarnish and corrosion resistance.

Alloys can also be classified as;

Alloys for all metal restorations

Metal ceramic restorations

Removable partial dentures

Alloys for removable partial dentures:

Highly noble - Au – Ag – Cu – Pd

Noble – Ag – Pd – Au – Cu

Ag – Pd

Base metal - Pure Titanium

Titanium aluminum vanadium

Ni-Cr-Mo-Be

Ni-Cr-Mo

Co-Cr-Mo

Co-Cr-W

Requirements of cast metal bases:

Bases should be;

As thin as possible and at same time rigid depending on

alloy used.

Dense

Accurate that is having a positive fit on master cast

without rocking.

Of a biologically acceptable alloy.

Design principles of metal bases:

Although some aspects of construction of cast metal bases

depend entirely on alloy chosen, the principles of design remain

essentially the same the design of cast metal base always must

be a clinical decision. A mount of coverage, placement of finish

lines and type of resin retention used has to be determined.

Maxillary bases:

Maxillary bases can cover;

Only the palate

The palate and ridge crest

Entire denture bearing area

The most satisfactory design covers complete palate and

ridge crest, but leaves denture borders in resin. An acceptable

variation in this design also places the posterior palatal seal area

in metal and begins resin periphery at pterygomaxillary notch

area.

If posterior palatal seal is to be in metal, it is essential that

dentist establish this critical area accurately.

Mandibular bases:

The mandibular cast base is of only two types;

With crest of ridge coverage.

With complete coverage of mandibular denture space.

Since exact determination and registration of mandibular

denture space is difficult clinically. The crest of ridge coverage

with resin peripheries is preferable, especially when using chrome

base alloys because adjustment and subsequent repolishing are

much more difficult than when using resin peripheries.

Resin retention:

Resin retention for maxillary or mandibular bases is of four

types;

Raised (relieved) retention mesh.

Non-relieved retention of beads.

Nail beads

Loops.

Since relieved retention places thin resin adjacent to

denture bearing tissues, it is inferior to non-relieved types which

permit a butt joint of thicker resin. In addition non-relieved

retention uses less inter-ridge space.

GOLD DENTURE BASES

Pure gold is a soft and ductile metal and so is not used for

casting dental restorations and appliances dental casting golds

are alloyed commonly with copper, silver, platinum, palladium,

nickel and zinc, alloying with these elements not only improves

its physical and mechanical properties but also reduces its cost.

Type IV (Extra hard) gold alloys are used for fabrication of

removable partial denture frameworks. The composition of type

IV gold alloy is;

Gold - 69%

Copper - 10%

Silver - 12.5%

Palladium - 3.5%

Platinum - 3%

Traces of indium, tin, iron, zinc and gallium function of

alloying elements.

Gold: Provides tarnish and corrosion resistance and has a

desirable appearance it also provides ductility and malleability, it

has low strength. Gold melts at 10640C and has a density of

19.329/cc.

Copper: Is the principal hardener, it reduces melting point and

density of gold, it present in sufficient quantity , it gives reddish

color, it also helps to age harden alloys in greater amounts it

reduces the resistance to tarnish and corrosion of gold alloy.

Hence it should not exceed 16%.

Silver: It whitens the alloy, thus helping to counteract the

reddish colour of copper to a slight extent it increases harden and

strength in large amounts it reduces tarnish resistance.

Platinum: It increases the strength and corrosion resistance, it

also increases the melting point and has a whitening effect on

alloy it helps to reduce grain rise.

Palladium: It is similar to platinum in its effect, it hardens as

well as whitens alloy it also raises the fusion temperature and

provides tarnish resistance it is less expensive than platinum thus

reducing the cost.

Trace elements:

Zinc: It acts as a scavenger for oxygen without zinc the silver in

the alloy causes absorption of oxygen during melting. Later

during solidification, the oxygen is rejected producing gas

porosities in the casting.

Indium tin and iron: They help to harden the metal ceramic

gold-palladium alloys, iron being the most effective.

Gallium: It is added to compensate for the decreased coefficient

of thermal expansion that results when the alloy is made silver

free. The elimination of silver reduces the tendency for green

stain at the margin of metal porcelain interface.

Iridium, ruthenium, rhenium: They help to decrease the grain

size they are added in very small quantities.

Desirable properties in a gold base dentures:

It has weight which is very important in retention of lower

denture.

It has the closest possible adaptation to underlying

tissues. Since it is cast directly to a refractory duplicate

cast.

It provides bracing which prevents the acrylic resin from

contracting in horizontal dimension during processing and

ensures better over all adaptation of denture base.

It is kind to underlying tissue because of excellent

tolerance of tissues to gold.

Chrome cobalt alloy bases in comparison with gold:

Chrome cobalt alloys cannot be cast to same degree of

accuracy.

Being lighter, the base has to be cast to such a thickness

to obtain desired weight that it might interfere with proper

tooth placement in some degrees.

Cobalt chromium alloys are so hard that they are difficult

to adjust.

A cobalt-chrome alloy framework is wholly covered by

acrylic resin is a highly desirable adjust to any lower denture.

If desired weight cannot be obtained excessive bulk

section of tungsten rods can be spot welded to frame or at

points of least interference.

Characteristics of gold bone lower dentures:

Gold bases was thin and appeared to be utilized primarily

to provide good tissue adaptation.

Should not employed for a patient having appreciable

undercuts existing in lower ridge.

Weight and bracing are most important factors provided

by lower gold base close tissue adaptation and tissue

tolerance are secondary.

Possible relining.

To fulfill above requirements, gold base should;

Cover as much of basal surface of lower denture as

possible and still allow at least 3 mm of acrylic resin

around the entire border.

Cover about one half of retromolar pad.

Should have enough finger like extension into denture to

insure good retention of acrylic.

Be cast in a hard, partial denture type of gold.

Extent of gold base:

Gold base should not be extended into undercut as it may

require trimming of final cast to seat the casting.

Gold base extension in cast of undercut height of contour

should be marked with pencil held vertically, if tip of

pencil touches the cast beyond undercut and is outside

the border extension then undercut should not be

covered.

It tip of pencil touches the cast beyond the undercut and is

inside the border extension, then undercut can be covered by

gold. This is permissible because the casting will contact the cast

firmly beyond the area trimmed and prevent the acrylic resin

from flowing under the casting and displacing it during

processing.

Indications of cast gold base:

Can be used in all types of edentulous mandibular ridges, it

is especially indicated for patients who has a resorbed residual

ridge and has worn small under-extended dentures. This is

because the tendency of lip and cheek to return to their habitual

position will result in vertical displacement of the usual light

weight lower denture. This tendency will not result in

displacement of a gold base denture because of its added weight.

Contraindications of lower gold base dentures:

Lower gold bases are not indicated for dentures for very

old patients who have suffered a great loss of muscle tone

and have no residual ridge. These patients are often

stooped and head in a forwardly inclined position. In these

patients weighted lower denture slides forward on body of

mandible, and lower lip is too weak to hold it in place, the

result is a constant protrusion and soreness of oral surface

of lower lip.

Patients who fear of oral cancer, these patients will not

permit the use of gold base for fear the hard metal would

cause cancer.

Some patients have a fixed idea that even a denture with

a plastic base is too heavy. Such patients cannot tolerate

the added weight of gold base.

Weight of the base:

Gold bases varies in weight from10 to 24 dwt. According to

clinical results the weight should be between 12 to 20 dwt. A

weight of 16 dwt was considered to be ideal.

Weight of denture base and vertical dimension of rest

position:

According to clinical analysis, the vertical dimension

established at rest will be increased by about 1.5 mm after 30

days of insertion of denture with gold base.

Relining:

Gold base lower dentures can be relined satisfactorily

providing there is sufficient extension of acrylic resin beyond the

gold around the border.

Comparison of weight of gold and weight of lost tissue

(teeth and bone):

The weight of teeth and bone lost through extraction and

extensive resorption will be around 29 dwt and 23 grains. The

weight of lower denture with porcelain teeth and gold base is 16

dwt.

Summary:

Most lower dentures weigh less than half as much as the

teeth and supporting tissues have been lost. This reduction in

weight might contribute to improper muscle function and a

reduction in normal rest V.D. This is an important factor in

adequate extension and retention of lower dentures. These

deficiencies can be overcomed by a cast gold lower denture base

and adequate expansion of the bone.

ALUMINUM DENTURE BASES

First casting of aluminum complete denture base – by

Bean (US).

Few years later Caroll presented a method for casting the

aluminum bases under pressure.

Advantages: Thermal conductivity, malleability and rigidity.

Disadvantages: Warpage and imperfect density of casting.

Review

Campbell

Aluminum is best base upon which a denture can be

fabricated. Because of thermal conductivity it provides favorable

patient response and also promotes a normal pink, physiologic

condition of mouth.

Sizeland-Coe

First to discuss super-pure aluminum alloy for

construction of denture base. Purity of newer alloy

eliminated much of intraoral corrosion that existed with

earlier alloy.

Anodizing aluminum alloy:

This process adds an oxide layer to surface of alloy which

helps to prevent tarnish and corrosion and it enables the

aluminum to be colored as desired. This process is called as

“Alumilite process”. Alumilite process was recommended by

the aluminum company of America (“ALCOA”). Alumilite

gives maximum corrosion resistance.

Granger

Recommended use of metal bases particularly aluminum.

According to him they provide greater surface detail and

more accurate fit which causes both stability and retention

of his bases.

Neill

Compared physical properties of aluminum with other alloys.

PropertiesAluminum

alloy

Acrylic

resin

Chrome

cobaltGold alloy

Density 2.66 1.18 8.2-8.6 15

Hardness (Brinell) 60.68 23.29 280 Soft-138

Hard -210

Ultimate tensile

strength

9.6 3 49 Soft-26

Hard -49

Melting range 580.640 - 1270-1305 870-985

% of elongation 4 - 5 Soft-4.25

Hard -1-6

Landquist: In region of midpalatine

Average discrepancy – 0.020 inch for heat cured resin under

light pressure.

- 0.0039 inch for aluminum bases.

Primary disadvantage:

Grayish discoloration rather than shiny silver finish may be

due to inadequate cleaning.

Barosoum et al.

Accuracy of aluminum is better than cold cure and heat

cure resin and hence they will be less irritating and provide

tissue health.

Swartz: Compared the retention of various bases (One

hook in mid-palatal region).

Aluminum (most retentive – 15.46 pounds)

Porcelain (13.24 pounds)

Cold cure acrylic resin (12.89 pounds)

Heat cured acrylic resin (least retentive 12.26 pounds)

Defurio and Gehl

Compared retention of various bases (Seven hooks in

periphery of base as well as in mid-palatal region).

Aluminum, acrylic resin, gold and chrome cobalt alloy.

Chrome cobalt alloy – resisted displacement to greatest

degree).

Aluminum – second in retentive ability.

Regli and Kydol

Lateral deformation in horizontal plane gold metal base with

28 gauge – 8.5 times more resistance to lateral deformation

than acrylic resins.

Jha

Tissue response of aluminum bases;

After 8 weeks- Decrease in thickness of stratumcorneum.

- No connective tissue changes.

- Mild increase in vascularity and fibrosis along

with small number of chronic inflammatory

cells.

Technical considerations:

Several techniques can be used for making aluminum

denture bases depending on the impression method, duplication

method and counting procedures utilized.

One of main reasons for using cast aluminum alloy as a

denture base material is its accurate fit. Therefore impression

techniques should be one which accurately records tissue detail

and has as great a tissue coverage as possible.

The problems most frequently encountered are pitting and

porosity of aluminum casting. These are largely due to passivity

of aluminum because of aluminum’s affinity for oxygen, it

oxidizes rapidly in air to form a strong adherent oxide layer at

higher temperatures, oxidation occurs much more rapidly, if the

alloy is overheated during casting, or if it is heated for a longer

period of time than necessary, an excessive amount of oxygen is

incorporated into molten alloy, upon solidification, oxygen is

retained, resulting in pitting and porosity.

COBALT CHROMIUM ALLOYS

Cobalt chromium alloys have been available since 1920’s.

These alloys are hard, rigid and corrosion resistant. Because of

their corrosion resistance at high temperature they are also used

for car sparking plugs and turbine blades. They are also known as

“Stellites” because they maintain their shiny, star-like

appearance under different conditions.

Composition:

Cobalt - 35-65%

Chromium - 20-35%

Nickel - 0-30%

Molybdenum - 0-7%

Carbon upto 0.4%

Tungsten, manganese, silicon and iron may also be present

in small quantities.

Effect of constituents:

Cobalt: Principal element, cobalt is hard, strong and rigid metal

high melting point.

Chromium: Forms a solid solution with cobalt it renders the alloy

corrosion resistant, due to a passivating effect. Chromium

content is directly proportional to tarnish and corrosion resistant,

it reduces melting point.

Nickel: It replaces some of cobalt-nickel and cobalt are

interchangeable, it decreases hardness, strength, MOE and fusion

temperature, increases ductility. Molybdenum, tungsten,

manganese and silicon harden and strengthen the alloy.

Molybdenum reduces the grain size.

Manganese and silicon primarily act as oxide scavengers

and prevent oxidation of other metals during melting. They also

function as hardners.

Carbon: It is invariably present and it reacts with many of other

metals or constituents to form carbides. These solidify lost during

cooling after casting, so appear at grain boundaries, the carbon

content of cobalt chromium depends on;

The quantity of carbon initially present before casting

and

Pick-up carbon from a heating flame, if this technique of

melting is used.

Control of carbon content of these alloys is most important.

The carboides that are formed embrittle the alloy with the

consequent danger of for example, partial denture clasp fracture.

Properties:

Cobalt-chromium alloys have replaced type IV gold alloys

because of their lower cast and adequate mechanical properties.

Chromium is added for tarnish resistance since chromic oxide

forms an adherent and resistant surface layer.

Physical properties:

Density: The density is half that of gold alloys, so they are

lighter in at 8 to 9 gms/cm2.

Fusion temperature: The casting temperature of this alloy is

considerably higher than that of gold alloys 12500C to 14800C.

ADA specification No. 14 divides it into two types based on

fusion temperature, which is defined as liquidus temperature.

Type I (High fusing): Liquidus temperature greater than

13000C.

Type II (Low fusing): Liquidus temperature not greater than

13000C.

Technical properties:

Yield strength: It is higher than that of gold alloys 710 MPa.

Elongation: Their ductility is lower than that of gold alloys

depending on compression rate of cooling and the fusion and

mold temperature employed it ranges from 1 to 12%.

These alloys work harden easily so care must be taken while

adjusting clasp arms of partial denture.

Modulus of elasticity: They are twice as stiff as gold alloys.

Thus, casting can be made more thinner, thus decreasing the

weight of RPD adjustment of clasp is not easy 225 X 103 MPa.

Hardness: These alloys are 50% harder than gold alloys. Thus

cutting grinding and finishing is difficult.

Tarnish and corrosion:

Formation of a layer of chromium oxide on surface of these

alloys prevents tarnish and corrosion in oral cavity. This is called

“Passive effect”.

Solutions of hypochlorite and other chlorine containing

compounds that are present in some denture cleansing agents

will cause corrosion in such base metal alloys. Even the

oxygenating denture cleansers will stain such alloys. Therefore,

these solutions should not be used for cleaning chromium base

alloys.

Casting shrinkage: Casting shrinkage is much greater than that

of gold alloys, so limited use in crown and bridge 23%. High

shrinkage is due to their high fusion temperature.

Porosity: As in gold alloys, porosity is due to shrinkage and

release of dissolved gasses. Porosity is affected by compression

of alloy and its manipulation.

Comparison with gold alloys:

Materials ConditionMOE

GN/m2

Proportiona

l limit

Ultimate

tensile

strength

MN/m2

Elongatio

nHardness M.R.

Densit

y

Gold alloy

type IV

Soft 95 360 480 15 130-

150

850-

950

15

Gold alloy

type IV

Hard 100 585 790 10 210-

230

210-

230

15

Cobalt

chromium

alloy

As Cast 250 515 690 4 370 370 8

Silver

palladium

alloy

Soft 95 345 480 9 140-

170

950-

105

0

12

The proportional limit of cobalt-chromium alloys is less

than that of the hardened gold alloys, and the ultimate

tensile strength of former is slightly lower than that of

latter material.

The cobalt-chromium alloys have a modulus of elasticity

about twice that of gold alloys that is they are stiffer.

This is very desirable for connectors and it means the

sections of cobalt chromium alloys of about half the

thickness of gold alloys can be used to achieve that same

degree of rigidity. Gold alloy clasps, however, are more

flexible and can be withdrawn over a greater degree of

undercut than clasps of cobalt chromium.

Cobalt chromium alloys are more brittle (lower

percentage elongation).

Higher melting range and greater hardness of cobalt

chromium alloys.

Casting shrinkage of cobalt-chromium alloys is greater

than that of gold containing materials but the available

investment materials appear to give satisfactory

compensation for this contraction.

The density of cobalt-chromium is about half that of gold.

This together with fact that cobalt-chromium dentures

can be made of thinner cross section.

Manipulation:

The casting technique for these alloys is similar to that of

gold alloys, but the following differences in manipulation should

be noted.

The melting point of these alloys is on range of 12500C –

14500C, hence gypsum bonded investment should not be

used. A silica bonded or phosphate bonded material

should be chosen.

Because of high melting range, gas/air torches cannot

raise the alloy temperature sufficiently to melt it. There

is a choice of using either.

Oxy-acetylene flame: This requires careful control use the

correct ratio of oxygen to acetylene. Too much of the former gas

may result in oxidation of alloy: too much acetylene will result in

carbon pick-up by alloy which must be avoided.

Induction heating, where the alloy is heated electrically.

This method is usually preferred, as it avoids the problems

mentioned above.

Because of great hardness of alloy special polishing and

finishing techniques are required.

Sand blasting is used to smooth the surface of

casting and remove adherent investment materials.

Electrolytic polishing is then applied the principle is

the same as for electroplating, except that the

appliance is made the anode of on electrolytic cell.

When a current is applied the surface layer is

dissolved.

STAINLESS STEEL DENTURE BASE

Stainless steel has been occasionally used or a denture base

material since about 1921. of particular importance is the 18/8

austenitic type material.

Composition:

Steel is an alloy of iron and carbon, with upto 2% carbon

alloys with greater quantities of iron are cast iron or pig iron.

Chromium may be added in (12-30%) small quantities to

improve tarnish resistance. When chromium is added the alloy is

called as stainless steel. Other than chromium it may also contain

other elements such as nickel which also helps in corrosion

resistance and strength of alloy.

Passivating effect:

Stainless steels are resistant to tarnish and corrosion,

because of passivating effect of chromium, a thin, transparent

but tough and impervious oxide layer forms on surface of alloy

when it is subjected to an oxidizing atmosphere (air), which

protects against tarnish and corrosion, it loses its protection if

the oxide layer is ruptured by mechanical or chemical factors.

Conventional method of swaging:

A stainless steel sheet is pressed between a die and a

counter die in a hydraulic press. Dies and counter-dies are mode

of low fusing alloys, such as zinc, copper-magnesium-aluminum,

tin-antimony-copper, lead antimony tin and lead bismuth alloys.

Some problems associated with this conventional swaging

procedure are;

Possible dimensional in accuracy, particularly if

contraction of die metal or alloy is not matched by

expansion of model.

Loss of fine detail, since many stages are involved

between recording the original impression and obtaining

the final product.

Dies and counter dies can be damaged under hydraulic

pressure. It was usually customary to use more than one

die and counter-die.

It was difficult to ensure a uniform thickness of finished

plate.

Uneven pressure on die and counter die could cause

wrinkling of steel.

Properties:

Despite difficulties in swaging mentioned above, stainless

steel has some merit as a denture base material.

Very thin denture base can be produced. Figures of as

low as 0.11 mm have been quoted compared to about

1.52 mm of acrylic denture.

Steel is fracture resistant.

Such dentures are not heavy, because of the thinness of

material, and the fact that the density of steel is not high

compared to some other metallic materials.

The corrosion resistance is good.

The thermal conductivity of stainless steel is such that

the sensation of temperature is rapidly transmitted to the

palate. This is an advantage not shared by polymeric

denture base materials.

Newer methods of swaging:

To overcome problems with previous method newer

methods of swaging have been investigated.

Explosion forming:

A die made using an epoxy resin is prepared from the dental

impression. A stainless steel plate is placed on top of the die,

with a layer of “plasticine” on it. A pressure wave is produced by

a small charge of high explosives. The pressure is transmitted

through plasticine on to the steel, forcing it into required shape.

Explosion hydraulic forming:

This is similar to the above, except that water is used as

medium for transmitting pressure wave.

Hydraulic forming:

The apparatus for this technique is as follows;

A die is placed in a metal cone and located in pressure

vessel.

A sheet of stainless steel of required thickness is placed

in position over the die.

A rubber diaphragm is placed over the stainless steel,

and a cover plate inserted in place and held in position

by high tensile bolts.

Oil is pumped into chamber upto a pressure of around

70mn/M2.

After the pressure has been released, the chamber is

opened and the work-piece is removed and cleaned.

The denture base is cut to size, and retentive tags are

resistance welded into position.

After polishing denture, it can, it necessary be reformed

on the die to eliminate distortion that may have occurred

during welding.

The stainless steel can be annealed by heating at 1050 0C

for two minutes, followed by quenching in water.

TITANIUM AND TITANIUM ALLOYS

Features:

Resistant to electrochemical degradation.

Stable oxide layer

Repassivating effect

Biocompatibility

Low weight, low density, low MOE and high strength.

Characteristics of titanium:

Titanium is attractive for its low-weight to volume.

High strength to weight.

Fatigue resistance

Corrosion resistance

Biocompatibility: Titanium is hypoallergic and posses

many of clinically favorable properties of type III and IV

dental gold alloys

Casting requirements of titanium:

Melting point of pure titanium is 17200C and is usually

achieved by electric arc. Melting method not used in

dentistry.

Molten titanium is extremely reactive with other

elements such as nitrogen and oxygen and with

compounds such as silica used in casting investments.

When cooling from a molten state, titanium crystallizes in

an alpha phase below 8830C. Alpha phase mechanical

properties are similar to those of type III and IV dental

gold alloys.

Above the critical 8830C temperature, crystallization

occurs in β-phase characterized by brittleness and

increased strength.

Titanium light weight presents another formidable

obstacle for common centrifugal force casting methods.

Atomic weight of titanium is 47.90 making it one half as

heavy as nickel chromium alloys and one fourth as heavy

as high gold alloys.

Two systems for casting of titanium:

One system uses centrifugal force generated by a

powerful motor wound spring and an argon gas melting

and casting environment.

Other system uses a vacuum/pressure casting machine

with electric arc melting in an argon gas environment.

TEMPORARY DENTURE BASE MATERIALS

Elder (1955) gave the following requirement of temporary

denture base.

The temporary denture base should adopt to basal seat

area as finished denture.

The temporary denture base should have the same

border form as the finished denture base.

The temporary denture base should be sufficiently rigid

to resist biting forces.

The temporary denture base should be dimensionally

stable.

The baseplate as constructed should permit its use as a

base for setting up teeth.

It should be possible to construct baseplate quickly,

easily and inexpensively.

Baseplate should have no undesirable color.

(Tucker, 1966) baseplate should not abrade the cast

during removal and replacement.

Shellae recording base material:

Shellae is a commonly used material for recording bases. It

is supplied commercially in forms shaped to correspond to

general shapes of maxillary and mandibular arches. It is

inexpensive and can be easily and quickly adapted accurately,

strengthened and handled carefully. It can be effectively utilized

both for maxillary and mandibular recording bases, if not

adequately strengthened, shellac tends to warp when subjected

to repeated changes in temperature being a brittle material it is

also subjected to breakage. Wires of 12 to 14 gauge should be

used to increase strength and rigidity and thus reduce distortion

of shellac bases. For the maxillary cast, the wire is placed across

the posterior palatal seal area, while for the mandibular cast it is

adapted within lingual flange.

This material is similar to the one used for special tray

construction but is thinner, generally pink in color and contains

no filler. Shellac is more stable than wax at mouth temperature

but is more difficult to adjust at the chairside.

Shellac is a thermoplastic material supplied in shapes

suitable for upper and lower arch forms. Some materials contain

on aluminum powder which is said to increase the strength and

decrease the brittleness of shellac.

Composition:

Shellac derived from resinous exudate of scale insect, is the

base of this base plate material. Other materials such as

powdered talc or mica, serve or fillers and increase the strength.

Properties:

Shellac baseplate will adapt to intimate contact with cast,

they often warp when rewarmed as a result of release of stresses

baseplates of shellac, a thermoplastic material, also warps when

warm wax is added while forming the occlusion rim and setting

denture teeth.

Advantage:

The principal advantage of shellac baseplate is minimal

amount of time required to adopt and make them.

Disadvantage:

The main disadvantage is the chance of losing initial

adaptation, but it is equally true that shellac baseplates readily

readapt.

METHODS:

1st method:

Shellac should finish just below the crest of ridge and at

the junction of hard and soft palate. This position is

drawn on cast which is then dusted lightly with French

chalk to prevent shellac sticking to it.

Shellac is softened and adopted to palate care should be

taken to prevent thinning

Its periphery is resoftened and trimmed to a general

shape by use of scissors. Final shaping is accomplished

by use of a file, the edge of shellac being chamfered to

blend into the ridge. It may need further heating and

adaptation before edges are smoothened with sand

paper. Retentive form for attachment of rim is made by

roughening the shellac with a hot wax knife in the form of

a 5 mm wide band and coating it lightly with sticky wax.

A strip of modeling wax, the length of sulcus area, is

softened and adapted to sulcus. Outer surface of alveolar

ridge and on roughened area of shellac. It is sealed to

shellac by use of a wax hot knife.

Muscle attachments are relieved and fitting surface of

bone checked for accuracy of detail.

2nd method:

All undercuts must be blocked out prior to adapting the

base. Wet asbestos is recommended, since shellac requires heat

to soften and this heat could distort wax and other plastic

materials. Cast should be dusted with talcum powder or soaked in

water for a short period of time until the surface of cast is moist.

As an alternative, tin foil (0.001 inch) can be adapted to the cast.

Using a Bunsen flame shellac is first adapted on palatal or lingual

side than on the ridge crest and later on the buccal side firm

pressure is applied with wet fingers or wet cotton to accurately

adapt the shellac to cast. While material is still warm and soft, it

is removed from the cast and trimmed with scissors. Leaving

approximately 5 mm beyond the edge of the cast, the shellac is

repositioned and reheated and then carefully readapted the

trimmed edges are heated using a hanau torch, elevated from the

cast, and folded into themselves, and burnished with No. 7 wax

spatula to form a smooth, rounded border.

Care must be taken not to overheat the shellac.

Overheating will cause the molten shellac to penetrate the pores

of stone and adhere to surface of cast on coding attempts to

remove the shellac base can result in a fracture of cast surface.

Bubbling or smoothing of shellac indicates overheating, the

shellac also turns black if overheated and is esthetically

unacceptable.

Stabilized shellac baseplates:

As shellac baseplate tends to undergo warpage. Many

materials were recommended for stabilizing them. They were;

ZOE impression paste

Elastomeric impression material

Autopolymerizing resin

Baseplate stabilized with ZnOE impression paste:

Shellac baseplates reinforced with ZnOE paste exhibit better

adaptation and dimensional stability. This adapts well to the cast

and can improve the dimensional stability of shellac baseplate.

Their disadvantages are that the baseplates are thicker

because of the thickness of impression paste liner, their

construction requires additional time, and block out of undercuts

on cast is essential as rigid, stabilized baseplates cannot extend

into undercuts.

Baseplate stabilized with elastomeric impression

materials:

The advantages of using these materials as stabilizers are

their inherent flexibility and smooth surfaces. The flexibility of

material permits baseplate extensions into moderate undercuts

and minimizes the need for blockout of cast. The principal

disadvantage of this procedure is added thickness of baseplate

and it costs more because of materials and increase in

construction time.

Baseplate stabilized with autopolymerizing resin:

Autopolymerizing resin improves both adaptation and

rigidity of baseplate. The disadvantages of this method are

possibility of warping baseplate as a result of internal stresses

being released in resin liner and additional time required for

fabrication.

Baseplate wax:

Baseplate wax recording bases are inexpensive, easily

formed and esthetic. But they lack rigidity, dimensional stability

and can easily be distorted. A strengthening wire adapted in

posterior palatal seal area of maxillary base or incorporated into

lingual flange of mandibular bore will increase both rigidity of

mandibular bone will increase both rigidity and the resistance to

distortion. Talcum powder is applied to cast to prevent wax

stiching to the cast. As an alternative, the cast may be increased

in water for a short period until moist, the wax is softened over a

Bunsen flame and adapted. Excess wax is removed with a sharp

instrument and borders rounded and smoothed.

Impression compound base:

Impression compound may be used as an alternative to

shellac but it is normal to confine its use to cases where the rim

is to be of the same material. It is reasonably stable at mouth

temperature and can be adjusted at the chairside by use of a

warm wax knife.

Impression compound is softened in a water bath at a

temperature of 600C and flattened into an arch form about 1.5

mm thick and large enough to cover the whole cast. It is

resoftened and adapted to the cast as described for wax bases.

The edge may be resoftened in water or over a Bunsen burner

flame to allow for trimming a file is used for final trimming.

Impression compound is easily thinned and distorted during

construction of the base but is reasonably stable once shaped.

Reference:

*

CONTENT