DENT 165: LO2- Fabricate Models

gypsum products

-supplied as fine powder

-mixed with water to form mass that can be shaped

-used primarly for positive reproductions or replicas of oral structures

-casts, dies or models

calcination

-process of removing water from gypsum

3 types of gypsum products

-model plaster (most water)

-dental stone (moderate water)

-high strength stone (least water)

model plaster type I

-requires water to mix

-weakest of the 3 gypsum products

-particles are rough, randomly shaped; therefore, porous

-white in colour

model plaster type II

-used mainly when strength is not critical

-attach stone casts/models to an articulator

dental stone type II

-stronger and more resistant to abrasion than model plaster

-particles are smooth, more regularly shaped (less porous than model plaster)

-used primarily for casts of full arch imps

-mouth protectors, whitening trays, full or partial denture construction

cast

-a working model

-used for fabrication of an appliance or restoration

-poured in dental stone

model/study model

-used to plan tx and to observe progress

-poured in stone, plaster, ortho stone

-an imp can be used to produce a model/cast or die

high strength stone: type III

-particles smoothest, least porous; therefore strongest

-most resistant to abrasion

-most accurate; therefore material of choice for dies used in fabrication of C&B

dies

-a working replica of a single tooth

-typically removable part of a cast

-used for crns, bridges, onlays and veneers

-poured in high strength stone

plaster

-rough

-randomly shaped

-porous

dental stone

-smooth

-more regularly shaped

-less porous

high strength stone

-smoothest

-less porous

prepare the imps

-create an artificial floor on mand imp

-rinse imps under cool water

-spray w/ surface disinfectant

-wrap in moistened, disinfected paper towel

-place in a labelled ziploc bag marked w/ biohazard label

-imps to be poured within 30 mins

when are imps to be poured up?

-within 30 min

prepare dental stone: what are the factors related to dental stone that contributes to the success of the cast?

-dental stone

-water

-spatulation

how to store dental stone

-store in air tight container

-close after use

-keep powder scoop clean & dry

water

-controls the strength, rigidity and hardness of the cast

cooler water=

-slower set

warmer water=

-faster set

any change in the recommended water/powder ratio will affect which 3 properties?

-setting time

-strength

-setting expansion (swelling)

water/powder ratio setting time: more water

-longer (slower) setting time

water/powder ratio setting time: less water

-shorter (faster) setting time

water/powder ratio strength: more water

-thinner mix; decrease in strength

water/powder ratio strength: less water

-thicker mix; increase in strength

water/powder ratio swelling: more water

-decreases expansion

water/powder ratio swelling: less water

-increases expansion

wet strength

-condition after setting

-may feel wet to the touch for hours

dry strength

-2-3 times stronger than wet strength after complete setting has occured

spatulation: undermixing

-produces an uneven chemical reaction

proper spatulation

-mix should be homogenous, workable and free of air bubbles

initial setting time/ working time

-where you mix the material and pour it into the imp

-when the material can no longer flow or be manipulated

-"loss of gloss" approx 10 min

final setting time

-when the material can be separated without fracture or distortion

dental stone set up

-impression/dental stone

-flexible bowl/spatulas

-room temp water

-water/powder measurers

-acrylic slab/rubber base former

-dental vibratior

dental vibrator

-flat table set on a motor base

-vibrates at rapid rates w/ varying speeds

-allows entrapped air bubbles to rise to the surface

anatomical portion of model

-contains the registration of the oral anatomy

how much water should be used for pouring up?

-30ml for impression only

-60ml for impression & base

how much powder should be used for pouring up?

-100g for impression only

-200g for impression & base

3 methods to form the base when pouring up

-rubber base former

-paddy base

-boxing w wax (commonly used for casts for dentures/restos)

art portion of model

-serves as a base (top or bottom) of study model

pouring up: impression should be placed how

-tray edges not embedded in base

-impression is parallel to the floor

how long it takes for model to set

-45 mins to an hour

-depends on manufacturer

separating cast from the impression

-use light pressure

-push down & forward on the anterior portion of tray

-carefully pull up on the handle to expose the anterior and posterior portion of cast

-use lab knife to trim excess stone away from edges

evaluating the diagnostic cast: check for

-fractured teeth

-anatomical detail

-adequate trimming area

-air bubbles (voids)