L83: What is an impression?
A negative replica/ reproduction of tissues
L83: What is the function of an impression material?
To produce an accurate replica of the surface and shape of hard and soft oral tissues
L83: What material is commonly used in dentistry to create positive replicas?
Dental stone (gypsum) to produce stone cast
L83: To obtain a good impression, the impression material must have the right properties but what else is important?
Clinician’s technique
L83: What are the two main clinical properties an impression material can be?
- Mucostatic;
- Mucocompressive.
L83: What does mucostatic mean?
A material that flows over tissues to represent them in a state of rest
L83: What is an example of a mucostatic impression material?
- Zinc oxide eugenol;
- Low viscosity alginates.
L83: What does mucocompressive mean?
A material that obtains an impression when the mucosa is subject to compression
L83: What is an example of a mucocompressive impression material?
- Impression compound;
- High viscosity alginates/ elastomers.
L83: What are the two main material properties an impression material can be?
- Elastic (not truly);
- Non- elastic (not used anymore).
L83: How do impression materials display elastic behaviour?
- Rigid after setting reaction;
- Undergo strain upon removal over bulbosity of tooth;
- Snap back to same shape after.
L83: Why should you wait (time of Tf-Tl), after taking an impression, before pouring a cast?
Tf represents the permanent strain on the impression material, Tf-Tl is the period of elastic recovery after maximum strain as been exerted
L83: Why are impression materials not considered truly elastic?
Because of permanent strain (slight change in shape) to the material, no full recovery
L83: Why is it better to apply and remove loads quickly from impression materials?
If load time is less, there is less overall permanent strain (i.e. lower deformation)
L83: What ‘elastic’ materials are used in dentistry?
- Hydrocolloids;
- Elastomers.
L83: What hydrocolloid material is used in the dental school, is it reversible?
Alginate, irreversible
L83: What is the process summary of preparation of an impression material?
- Mixing of two materials (2 pastes/ paste + liquid/ powder + liquid etc.);
- Chemical reaction;
- Heat;
- Material flows/ takes up space.
L83: What is the process summary of setting of an impression material?
- Chemical;
- Polymerisation;
- Cooling;
- Formation of solid replica.
L83: What should be considered when storing a taken impression material?
- Depends on material;
- Before dimensional changes become significant).
L83: What are the three main ideal properties an impression material should custom for?
- Accuracy;
- Patient factors;
- Operator factors.
L83: What are the ideal properties of an impression material with regards to maintaining accuracy?
- Surface detail recorded;
- No dimensional change on setting/ removal/ decontamination or storage;
- Complete elastic recovery;
- Compatible with cast material.
L83: What material properties affect accuracy and how?
- Viscosity (ability to flow over surfaces and record detail);
- Setting mechanism (does this lead to dimensional contraction?);
- Thermal expansion coefficient (ideally low, to cope with mouth to room temperature change);
- Hydrophobic/ hydrophilic (surface contact, effect of saliva);
- Elasticity (elastic behaviour);
- Tear strength (to withstand removal, especially from undercuts).
L83: What are the ideal properties of an impression material with regards to maintaining patient comfort?
- Non toxic, non irritant;
- Acceptable taste and smell;
- Short setting time;
- Removable without damage to oral tissues.
L83: What are the ideal properties of an impression material with regards to maintaining operator convenience?
- Quick, simple technique;
- Convenient working/ setting times;
- Ability to be decontaminated;
- Cost (a consideration, not a property).
