10. Impression Materials Flashcards
Definition of an impression
A negative reproduction of tissues
Function of impression materials
To produce an accurate replica of the surface and shape of hard and soft oral tissues
Function of dental stone
Used to fill the impression and produce a positive replica (stone cast)
Function of dental stone cast (2)
For evaluating dentition when orthodontic, occlusal or other problems are involved
In laboratory fabrication of restorations and prostheses
Impression materials can be used for (3)
Single tooth
Whole dentition
Edentulous mouth
Impressions are important, as treatment outcome hinges on (3)
Quality and accuracy of the initial impression
Requires a material with the correct properties
Requires a clinician with a good technique
Impression materials can be classified as (2)
Clinically
Properties
Clinical classification of impression materials (2)
Mucostatic
Mucocompressive
Definition of mucostatic impression material
Fluid materials that displace the soft tissues slightly (give an impression of the undisplaced mucosa)
Examples of mucostatic impression materials (2)
Zinc oxide eugenol
Low viscosity alginates
Definition of mucocompressive impression material
Viscous materials that record an impression of the mucosa under load (give an impression of displaced soft tissues)
Examples of mucocompressive impression materials (2)
Impression compound
High viscosity alginates and elastomers
Classification of impression materials based on properties (2)
Elastic
Non-elastic
Ideally, elastic strain during removal leads to
Elastic recovery after removal
Ideal elastic behaviour (3)
Upon removal, the material reaches the maximum amount of strain almost instantly
The maximum strain is ‘held’ during removal
When fully removed, the material instantly returns to the original strain and returns to its pre-removal shape
Actual elastic behaviour (3)
Upon and during removal, the material gradually increases to just below the maximum amount of strain
When fully removed, the material quickly (instantly) returns to almost the initial strain
This results in a permanent strain/deformation and a permanent change in dimension
Types of elastic materials (2)
Hydrocolloids
Elastomers
Types of hydrocolloids (2)
Reversible (agar)
Irreversible (alginate)
Types of elastomers (3)
Polysulphides
Polyethers
Silicones (conventional and addition cured)
Types of non-elastic impression materials (2)
Impression compound
Impression paste
Actual non-elastic behaviour
Upon removal, if undercuts are present, non-elastic impression materials may become deformed (permanent strain) or fracture
Impression process (6)
Preparation Insertion Setting Removal Storage Cast preparation
Impression preparation process (3)
Mixing (2 pastes/paste and liquid/powder and liquid, etc.) – chemical reaction
Heat
A material that flows and takes up shape is required
Impression insertion process (2)
Into standard/custom tray
Need for adhesive
Impression setting process (2)
Chemical, polymerisation, cooling
Forms solid replica