Primary Impressions and the Role of Study Casts Flashcards
What are stock trays and when are they used?
Stock trays are commercially made trays to average dimensions and as such are only of use when precision is not a primary requirement.
They will be sufficient (they can be modified by the addition of another impression material such as impression compound) for study cast impressions.
What are the three products obtained from gympsum and what are their qualities?
Dental Plaster
Large, irregular particle structure
Dental Stone
Small, regular particle structure
Improved Dental Stone (Densite)
Small regular particle structure with chemicals added for reduced expansion
What are the factors that should be considered when picking a material and that differ from material to material?
Adequate flow properties
Fill impression
Reproduction of surface detail
Water / Powder ratio
Dimensional accuracy
Adequate working time
Dimensional accuracy
no expansion or contraction on setting
Strength
Hardness and abrasion resistance
No flaking or chipping in thin section
What is the powder to water ratio for each material?
100g plaster - 50ml water
100g dental stone - 25 ml water
100g improved stone - 20 ml
When is hand mixing used and what does it cause?
Used when precision is not paramount, e.g. primary impressions or mounting casts
Air is included in the mix
More expansion
Less dense mix
Low strength cast
When is mechanical mixing used and what does it cause?
Used when precision is required, e.g. study casts and all definitive and working casts
Vacuum pump removes air while mixing
Less expansion
Greater accuracy
Dense mix
Harder cast
What temperature of water acclerates the setting of the impression and what temperature retards it?
warm = acclerates
cold = retards
In the video ‘Pouring Primary impressions to produce study casts’ what was done to reduce the risk of trapping air at the incisal edge/occlusal surface of the teeth, when adding the dental stone
dental stone is slowly vibrated into the teeth, one at a time
What is used to check occlusion of the study casts and partial dentures?
articulator
Agar and alginate are both;
hydrocolloids
