Diagnostic Casts

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Alginate
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Member of
Hydrocolloid impression materials
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Hydro – water
Colloid – gelatin substance
Creates negative mold of pt teeth
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Common Uses
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Study models
Opposing Arch model
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Bleach trays

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Solution in which water is used as the mixing agent
Physical Phases of Hydrocolloids
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Sol – (as in solution) material is in liquid or semi-liquid form
Gel – the material is semi-solid (as in gelatin dessert)

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Irreversible vs. Reversible
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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

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Irreversible hydrocolliod
Not very accurate – not used for final impressions
Recommended when accuracy is not important
Easy to mix/use – little equipment
Less expensive

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Potassium Alginate
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Main ingredient
Comes from sea kelp (seaweed)
Acts as a thickening agent (also in Ice
Cream)
Calcium sulfate
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Reacts with potassium alginate to accelerate the set

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Potassium titanium fluoride
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Added to resist interference of the set and strength of the alginate when making cast
Trisodium phosphate
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Added as a retarder (slows down time it takes to form a gel
Diatomacious earth
Zinc Oxide – these are added as bulk

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Ideal mixing temperature 70 degrees
Alginate dispensment
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Coffee can size
Premeasured – packages more expensive
1 year shelf life

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Working Time
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Time allowed for mixing the alginate, loading the tray, positioning in mouth
Setting Time
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Time required for the chemical action to be completed and when the impression is ready to be removed from pt’s mouth

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Regular Set
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Working time – 2 minutes
Setting time – 41/2 minutes
Fast Set
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Working time – 11/4 minutes
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Setting time – 1-2 minutes

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Fast set vs Regular set
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There is no difference in final product
Use determined by:
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Difficulty in seating impression
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Sometimes easier to use when working alone
Children and pt’s w/gag reflex – faster set can be easier

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

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Dimensional Stability
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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.

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Many brands – many techniques
Common Ratio –
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1 scoop of powder to 1 level of water 1=1
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Maxillary 3=3
Mandibular 2=2
Usually

Quality of mix – creates
Quality impression – combined w/quality pour creates
Awesome models
IMPORTANT: DO NOT ADD
MORE WATER
This takes away from strength of impression

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Perforated trays
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Allows alginate to ooze through holes as it sets
Forms a lock and keeps alginate from pulling away from tray
Two types:
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Plastic – disposable
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Metal

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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

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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

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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

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Retromolar area, lingual frenum, tongue space – mandibular
Hard palate & tuberosities are recorded
– maxillary
IMMEDIATELY after removing impression rinse & disinfect, damp paper towel – until poured

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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

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Model plaster
Dental stone
High strength stone

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

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More durable
Yellow
Finer grains
40 – 60 minute set

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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

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Gives off exothermic reaction as it sets
(heat)
Warmer water used to mix faster material will set

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Anatomic portion
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Created from the alginate impression
Reproduction of teeth, muscle & frenum attachments
Art portion
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Forms the base of cast (1/3 of overall height)

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Stone must be vibrated to remove bubbles
Double Pour
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Anatomic portion poured first
Another batch mixed for base/art portion
Box & Pour
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One mix for both portions
Poured into box made of wax

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Inverted Pour
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One mix for both portions then turned over
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Anatomical portion filled
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Base made by continuing pour
Common

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Soak models 5 min.
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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

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