Alginate Impresions

Diagnostic Casts

Alginates Alginate

Member of Hydrocolloid impression materials

Hydro – water Colloid – gelatin

substance Creates negative

mold of pt teeth

Common Uses Study models Opposing Arch

model Bleach trays

Alginate - Hydrocolliod

Solution in which water is used as the mixing agent

Physical Phases of Hydrocolloids Sol – (as in solution) material is in

liquid or semi-liquid form Gel – the material is semi-solid (as in

gelatin dessert)

Alginate - Hydrocolliod

Irreversible vs. Reversible Irreversible – Materials not able to

return to the sol state after they become a gel example: cake mix

Reversible – materials that change physical state from sol to gel then back to sol example: Ice cream

Alginate

Irreversible hydrocolliod Not very accurate – not used for

final impressions Recommended when accuracy is

not important Easy to mix/use – little equipment Less expensive

Alginate Ingredients

Potassium Alginate Main ingredient Comes from sea kelp (seaweed) Acts as a thickening agent (also in Ice

Cream) Calcium sulfate

Reacts with potassium alginate to accelerate the set

Alginate Ingredients Potassium titanium fluoride

Added to resist interference of the set and strength of the alginate when making cast

Trisodium phosphate Added as a retarder (slows down time it

takes to form a gel Diatomacious earth Zinc Oxide – these are added as bulk

Impressions

Ideal mixing temperature 70 degrees Alginate dispensment

Coffee can size Premeasured – packages more expensive 1 year shelf life

Alginate Impressions

Working Time Time allowed for mixing the alginate,

loading the tray, positioning in mouth Setting Time

Time required for the chemical action to be completed and when the impression is ready to be removed from pt’s mouth

Alginate Impressions

Regular Set Working time – 2 minutes Setting time – 41/2 minutes

Fast Set Working time – 11/4 minutes Setting time – 1-2 minutes

Alginate Impressions

Fast set vs Regular set There is no difference in final product Use determined by:

Difficulty in seating impression Sometimes easier to use when working

alone Children and pt’s w/gag reflex – faster set

can be easier

Alginate Impressions Dimensional Stability

Serious problem associated w/alginate is loss of accuracy when impression is stored before pouring

Will deteriorate w/elevated temperatures or in presence of moisture

Alginate stored in water or a very wet paper towel will absorb & expand (this absorption is called imbibitions)

Alginate Impressions

Dimensional Stability Exposure to open air – water will

evaporate – causing impression to shrink and distort

Optimum dimensional stability store in a damp paper towel for no more than 1 hour before pouring, your book says _?__ minutes.

MATERIALS Many brands – many

techniques Common Ratio –

1 scoop of powder to 1 level of water 1=1

Maxillary 3=3 Mandibular 2=2 Usually

QUALITY OF MIXQuality of mix – createsQuality impression –

combined w/quality pour creates

Awesome models

IMPORTANT: DO NOT ADD MORE WATER

This takes away from strength of impression

MATERIALS - Trays

Perforated trays Allows alginate to ooze through holes as

it sets Forms a lock and keeps alginate from

pulling away from tray Two types:

Plastic – disposable

Metal – sterilized after use

Selection of Trays

Try in pt’s mouth after rinsing Check for comfort and fit Should extend below facial surfaces Should extend 2-3mm beyond molars Deep enough to allow 2-3mm of

alginate between tray & incisal/occlusal

Utility wax may be added to extend

Procedure – explain to Patient

Plastic drape or bib Material will feel cold Not an unpleasant to taste Material will set quickly Mouth may feel full Hand signals – Gag reflex – tilt

forward and breathe through nose

Evaluating the Impression

Impression centered over centrals Complete peripheral roll to include

vestibular areas Tray was not over seated (pushed

down to far) Tray was seated evenly Impression free of tears &

voids/holes

Evaluating the Impression

Retromolar area, lingual frenum, tongue space – mandibular

Hard palate & tuberosities are recorded – maxillary

IMMEDIATELY after removing impression rinse & disinfect, damp paper towel – until poured

WAX BITE/UTILITY WAX

Impression wax is used to show occlusal relationship between arches

Soften wax – place over occlusal surface of mandibular teeth – have pt close gently but firmly

Utility wax may be used to enlarge impression tray

GYPSUM PRODUCTS

Model plaster Dental stone High strength stone

Model Plaster

Plaster or paris Sets fast More porous White Weakest 20 – 30 minute set

Dental Stone

More durable Yellow Finer grains 40 – 60 minute set

High Strength Stone

Used for dies and final working models

Very hard cast Very fine grains Green, Pink, Blue Takes longer to set Strongest 60 – 80 minute set

GYPSUM

Gives off exothermic reaction as it sets (heat)

Warmer water used to mix faster material will set

PARTS OF DENTALCAST

Anatomic portion Created from the alginate impression Reproduction of teeth, muscle &

frenum attachments Art portion

Forms the base of cast (1/3 of overall height)

POURING METHODS

Stone must be vibrated to remove bubbles

Double Pour Anatomic portion poured first Another batch mixed for base/art portion

Box & Pour One mix for both portions Poured into box made of wax

POURING METHODS

Inverted Pour One mix for both portions then turned

over Anatomical portion filled Base made by continuing pour Common

TRIMMING MODELS

Soak models 5 min. Better for wheel on grinder

Models should have ½ inch base Models should have 1 inch anatomic

portion ALWAYS have gloves, goggles, mask ALWAYS use trim shield

ARTICULATOR

Frame that holds the patients models in occlusion as the TMJ would

Used commonly