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