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)
Explain why there is Shrinkage porosity
o Tapered screws at point of connection to wax pattern
o Improper reservoir or wax pattern location
o Mould temperature too high and/or overheated alloy
o Excessive amount of alloy used
o Length of sprues from wax pattern to reservoir too long
Compare the advantages and disadvantages of using elastomeric material over alginate to take the final (working) impression for removable partial denture. [6]
List advantages first…
Advantages of elastomeric material (vs alginate)
• High tear resistance
• Good dimensional stability
• Good reproduction of surface detail
Compare the advantages and disadvantages of using elastomeric material over alginate to take the final (working) impression for removable partial denture. [6]
Now list disadvantages first…
Disadvantages of elastomeric material (vs alginate)
• Long setting time
• Too accurate – cast produced may not be sufficiently oversized
• May be difficult to remove from patient’s mouth
• Some elastomeric materials can’t be used with vinyl gloves
What are the physical processes that affect the dimension stability of alginate? [6]
• Imbibition: a special type of diffusion that takes place when water is adsorbed by solids-colloids causing an increase in volume
o Affected by time before pouring
• Syneresis: the contraction of a gel accompanied by the separating out of liquid, depends on:
- decrease in entropy, complex osmotic pressure, changes in Gibbs free energy, diffusion kinetics
- osmotic pressure changes (e.g. from the type of disinfectant)
- ratio of calcium to sodium – lower ratio loose less water (higher dimensional stability); higher ratio loose more water
- ratio of filler to [alginic polymer and lower-weight molecular polymer chains]
• Evaporation of water
What is the function of impression compound? (2)
To provide support to reduce bulk of alginate impression material
Modify the tray (e.g. high palatal vault, tray not extensive enough)
Give the constituents of impression compound. What is the range of temperature it melts?
Resin (wax)
Filler (talc or soapstone)
Lubricant (stearic acid or stearin)
50-51 degrees Celsius
Give 2 method to isolate subgingival preparation for impression? (2)
- Placement of retraction cord with/without impreganation of astringent-haemostatic agent by two-cord technique [chemo-mechanical]
- Cordless retraction with non-invasive, atraumatic gingiva-displacing/ haemostatic cordless retraction materials such as GingiTrac (Centrix), Expasyl (Kerr), Racegel (Septodont) or Traxodent (Premier)
- Gingiva removal o Electrosurgery o Laser tissue sculpting
- Copper tube impression
- Hydraulic impression
