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
Ideal properties of an impression material (3)
Accuracy
Patient factors
Operator factors
Features of impression material accuracy (6)
Flow Setting changes Removal Storage Decontamination Compatible with cast material
Effect of accurate flow
Surface detail recorded
Effect of accurate setting changes
No dimensional change
Effect of accurate removal (3)
No effect on oral tissues
No change dimensionally (no thermal contraction, etc.)
Complete elastic recovery
Effect of accurate storage
Does not change dimensionally over time (moisture control)
Effect of accurate decontamination
Dimensions and surfaces unaltered
Properties that affect accuracy (6)
Viscosity Setting mechanism Thermal expansion coefficient Hydrophilic/hydrophobic Elasticity Tear strength
Effect of viscosity on accuracy
Determines ability to flow over surface and so therefore, the level of detail recorded
Effect of eating mechanism on accuracy
Some physical or chemical processes may cause dimensional contraction
Ideal thermal expansion coefficient of impression materials
Ideally low (to cope with mouth to room temperature transition)
Effect of hydrophilic/hydrophobic on accuracy
Affects surface contact
Effect of elasticity on accuracy
Elastic/viscoelastic behaviour
Effect of tear strength on accuracy
Withstand tearing when remove, especially from undercuts
Ideal patient properties (5)
Comfortable Non-toxic, non-irritant Acceptable taste and smell Short setting time Removable without damage to oral tissues
Ideal operator properties (3)
Quick, simple technique
Convenient working/setting times
Must be able to be decontaminated
Definition of colloid
A two-phase system of fine particles (1-200um) of one phase dispersed in another phase (water, etc.)
Definition of hydrocolloid
A colloid dispersed in water
Hydrocolloids can be in two states (2)
SOL - viscous liquid state
GEL - jelly-like consistency
Features of GEL state hydrocolloids (2)
Agglomeration of particles
Entangled framework of solid particles enclosing liquid by capillary forces
Definition of alginate
Irreversible hydrocolloid
Composition of alginate (5)
Salt of alginic acid (Na alginate) CaSO4 Trisodium PO4 Filler Modifiers, flavourings, chemical indicators
Function of salt of alginic acid (Na alginate) in alginate
Reacts with Ca ions
Function of CaSO4 in alginate
Provides Ca ions
Function of trisodium PO4 in alginate
Delays gel formation
Function of filler in alginate
Cohesion, strength
Function of modifiers, flavourings and chemical indicators in alginate
Improve surface, taste, pH colour change
Features of alginate setting
Involves long cross-linked fibrils entangling undissolved particles
Features of alginate delay
Trisodium phosphate preferentially reacts with calcium ions in calcium sulfate
Setting reaction of alginate (2)
2Na3PO4 + 3CaSO4 –> Ca3(PO4) + 3Na2SO4
Sodium alginate then reacts with Ca ions
Components of accurate alginate manipulation (4)
Use correct powder/liquid ratio
Use water 18-24C
Use perforated tray and adhesive
Remove impression with a sharp pull
Relationship between temperature and setting time of alginate
Increased temperature speeds up setting (sets nearest tissue first)
Importance of removing impression with a sharp pull (2)
Elastic recovery; maximum tear strength
Crosslinking continues after apparent set – greater elastic properties if you wait a further minute or two
How is permanent distortion and tearing of alginate reduced
Slightly reduced by using a large bulk of material (typically 0.5mm)
Properties of alginate (9)
Non-toxic, non-irritant
Acceptable taste, smell
Adequate setting time
Relatively easy to use
Storage - avoid moisture, prepare cast ASAP to avoid dimensional change
Flow – fine detail - adequate
Setting changes - adequate but movement leads to stresses and distortion
Elastic - nearly elastic but not 100% elastic recovery
Tear strength - poor – avoid deep undercuts
Factors that affect dimensions and/or accuracy (2)
Syneresis
Imbibition
Definition of syneresis
Release of water
Definition of imbibition
Uptake of water
