Steps In Making Denture Base

STEPS IN MAKING DENTURE BASE: •

IMPRESSION TAKING o

PRELIMINARY 

o

By constructing an INDIVIDUAL TRAY (working cast)

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Position the upper half of the flask over the lower portion

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Pour a second mix of investing medium

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Remove excess so that there would be a good metal to metal contact

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Allow the stone to set

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TWO METHODS OF POURING THE PLASTER OF PARIS TO THE UPPER HALF OF THE METAL FLASK

CAST CONSTRUCTION o

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Using modeling compound to make STUDY CAST (Plaster of Paris)

SECONDARY 

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(COLORGARD) then wait for it to dry

Construction of working cast using dental stone

WAX PATTERN CONSTRUCTION o

Use pink wax then heat it over open flame

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Place the softened wax over the cast and

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SINGLE TECHNIQUE o

press it hard to get the exact shape of the cast then place another wax over the 1st wax to seal the periphery of melted wax

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o

Pour melted wax (using carver) into the periphery of wax pattern and allow it to flow to prevent Plaster of Paris from going inside wax pattern during investing

o

Wax pattern should be stable to the working cast

•

INVESTING OF WAX PATTERN o

DOUBLE POUR TECHNIQUE/ TWO CAPPING TECHNIQUE o

Pouring is done twice

o

First, pouring of the investing medium is only to the level of the wax pattern. This is good for the presence of PONTICS

Materials needed: 

Metal flask, plaster of paris/dental stone (investing medium), separating medium (for easy separation of the upper half from the lower half)

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The lower half of the metal flask is half-filled with Plaster of Paris

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Place the cast with wax pattern (both coated with separating medium) into the plaster exposing only the wax pattern

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Invest only up to the level of the base of w.c.

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Wait for the plaster of Paris to set (initial setting time)

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During initial setting time, coat the stone with a separating medium

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•

ADVANTAGES OF DOUBLE POUR TECHNIQUE o

Easy location of teeth during deflasking

o

Less danger of healing or moving teeth during deflasking

WAX ELIMINATION/BURNOUT PROCEDURE o

1

Mixing the plaster then pouring it to the upper half then place the cover

Done to create a mold space

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o

Immerse the flask in a water bath (60C, 5,mins)

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Remove the flask; separate the upper and lower halves

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Remove the softened wax pattern

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In case of overheating, flush out melted wax with boiling water

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Make sure there are no melted wax on the cast so that all details will be copied Paint separating medium for easy deflasking and to prevent color change of the resin

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PACKING OF RESIN DOUGH

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Resin  powder (polymer)  liquid form/gel form (monomer)

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o

o

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Heating process of resin to allow complete polymerization

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Mixture is cloudy and sandy

o

No reaction yet between polymer and monomer

STICKY/STRINGY STAGE o

With threadlike projections

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Monomer attacks the surface of polymer

o

The mixture becomes viscous and sticks to the stirring rod

GEL/DOUGH STAGE o

Mixture is easily molded into different forms or shapes

o

Ideal for compression molding

o

Needed in packing of resin into the flask

o

No longer sticky

RAPID CURING 

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DAMP/SANDY STAGE

CURING o

74C water bath for two hours then increase to 100C for one hour

SLOW CURING 

74C water bath for 8 hours or overnight



Better than rapid curing because polymerization is more thorough

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DEFLASKING o

Removal of finished denture base from the metal flask and from the investing medium

o

After curing, allow the metal flask to cool for 30mins (bench cooling)

o

If metal flask will be put immediately to cold water, it will cause distortion

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RUBBERY STAGE o

Mixture is rubber-like in character and no longer flows freely

o

Cannot be molded anymore

STIFF STAGE o

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Mixture is dry and resistant to mechanical deformation

*Cover the mixture while waiting for the dough stage to prevent premature evaporation of monomers  produces porous base

POLISHING THE DENTURE BASE o

It is important to remove the residual monomer (due to low curing temperature and shorter time of polymerization) from the finished denture base because it can cause irritation to the oral tissues

STAGES OCCURING ONCE THE RESINS ARE MIXED

SEPARATION OF MOLD SPACE o

•

o

Trim excess resin, make surface shiny using wheel raq and acryluster

*When packing is done during the sticky stage, the denture base will be porous

2

*When mixture is in dough stage, collect the dough using moist cellophane  for upper arch: roll the dough into a ball then place over the mold space then spread it

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Trapped within the finished denture base

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Caused by too rapid curing process and improper manipulation of resin and packing too soon

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Caused by premature evaporation of monomer

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Not usually seen clinically

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Weaker denture base

 for lower arch: form a horse shoe shaped dough

*Assemble again the upper and lower halves of the flask. Use the presser to remove excess resin(flash)

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FLASH •

Excess resin

TRIAL CLOSURE •

Alternate opening and closing of the flask to remove excess resin

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Turn the handle slowly to pack the resin better

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CRAZING o

Small linear cracks

o

Caused by too much pressure exerted during deflasking

o

Caused by accidental fall during deflasking

FRACTURE o

Caused by too much pressure

DIMENSIONAL CHANGES DURING CURING •

THERMAL EXPANSION o

•

Polymerization shrinkage of resin

THERMAL CONTRACTION o

Change from temperature of water bath to room temperature

DEFECTS OF DENTURE BASE: •

HEAT – CURED

SELF - CURED

DIMENSIONAL STABILITY

Less dimensionally stable

more dimensionally stable

STRENGTH

Higher

lower

COLOR STABILITY

More color stable

Less color stable

HARDNESS VALUE

20KHN

16 – 18KHN

ACTIVATOR

Heat

Dimethyl paratoluene

RESIDUAL MONOMER

0.2 – 0.3%

3 – 5%

CAN WITHSTAND ABRASIONS

Can withstand

CONTRACTION o

•

Change from room temperature to temperature of water bath

PROPERTIES

PRESENCE OF POROSITIES o

o

EXTERNAL POROSITIES 

Caused by improper manipulation of investing medium



Weaker, more retentive to food debris (difficult to clean)

INTERNAL POROSITIES

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-Rosette Go 020109 

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