Impression materials Flashcards
5 necessary requirements of Impression Materials? (5)
- Capable for plastic to rigid transformation
- Acceptable for patient (Non-toxic, non-irritant)
- Adequate working and setting time
- Ability to accurately replicate intraoral surface details
- Dimensionally stable, resistance to mechanical stress, tear resistant
- Resistance to disinfectants
- Compatible with model materials
- Cost effective
4 advantages of using alginate to take impression
Non-toxic and non-irritating Good surface detail Ease of use and mix Cheap Good shelf life
4 disadvantages of using alginate to take impression
Poor dimensional stability
Incompatibility with some dental stones
Setting time very dependent on operator handling
Messy to work with
What will happen if alginate impression is poured by 1-stage technique (1mark)
More susceptible to polymerization shrinkage
What is the setting contraction of polysulphide during the initial 24 hours (1)
- 0.3 - 0.4% over first 24 hrs
Arrange by dimensional stability: Polysulphide, polyether, Additional silicone, hydrocolloids (1)
- Additional silicone > polyether > polysulphide > condensation silicone > hydrocolloid
Is polyvinylsiloxane a suitable impression material for crown preparations? Why?
Yes:
• Dimensionally stable (even when pouring is delayed)
• Good shear strength
• Can record fine details accurately
• Compatible with dental stone & can be poured repeatedly
• Clean & easy to use - Do not produce a volatile by-product during polymerization and therefore minimal dimensional changes occur during setting
- 2 colours (dual phase with heavy body as rigid base & light body for fine details recording) gives contrast & provide easier vision of the impression
- No objectionable odour or taste
No: Expensive Stiff Difficult to remove from undercut Hydrophobic thus the surface detail may not be as fine Heavy body & light body may separate
What are the advantages of the 1-stage impression technique over the 2-stage impression technique?
- Expediency
- Guaranteed correct seating
- Patient comfort
What are the disadvantages of the 1-stage impression technique over the 2-stage impression technique?
- Lower accuracy in MDL (mesial, distal,lingual) regions of the margin compared with two-stage (the hard & stiff heavy body may displace the light-body impression material, preventing the capturing of certain critical areas)
- Poorer dimensional stability – should pour within 2 hours (2 stage can be up to 30 hours)
The manufacturer states that we should wait one hour or more before pouring the cast (PVS). Why?
To ensure that the setting reaction is fully completed (if not fully set, may release hydrogen)
Alginate impression properties compared to elastomeric impression for study cast impression
Elastomers (compared to alginate) o Better tear resistance o Better dimensional stability o Have different range of viscosities o Higher accuracy o Longer setting time
Properties of alginate affecting the timing of pouring the casts
- Poor dimensional stability
- Poor dimensional accuracy
- Shrinkage
Co-Cr framework has porosity. What are the causes?
Gas porosity
Suck back porosity
Shrinkage porosity
Explain why there is gas porosity
o Overheated alloy
o Turbulence in alloy when entering mould (spruing technique)
Explain why there is Suck back porosity
Ken: or called hot spot porosity
o Excessive amount of alloy used (button too large)
o Improper reservoir, sprues and/or wax pattern location
Ken:
o Mold surface not rounded and the metal entering the mold impinges onto the mold surface and create a localised spot at higher temperature
o it becomes the last area to solidify resulting in porosity
o Sprue is at improper position (90o to wax pattern, should be 45o)
