3. Impressions Flashcards
Is tissue management needed with CAD CAM technology
yes
What are the issues with taking impressions for a crown with stock trays
- Often lack extension to support the impression material
- Lack of stops stops makes positioning and stabilizing more challenging
- Flexible (rebound on removal) –> distorted impression
Pros of custom trays
- Less impression material wasted
- Controlled uniform thickness of impression material more accurate impressions
- Effective extension
- Better positioning and stabilization
- Rigid no distortion **
What are the 3 different materials that can be used to make a custom tray
- Auto-polymerizing resin
- Thermoplastic resin
- Photo-polymerized resin
If making the custom tray out of autopolymerizing resin the tray should be fabricated how soon before the impression needs to be taken
9hrs
What is the material we use to make custom trays
photo-polymerizing resin
Which custom try material is the best to use
no difference
The custom tray should extend how far into the vestibule past the gingival margins
3-5 mm
How many layers of baseplate wax should be used to make a custom tray
2
Possible contaminants of a custom tray are _ and can be removed how
- MRA (model releasing agent)
- ABC (Air Barrier Coating)
- Baseplate wax
Removed by washing with soap and water
Critical (important) features of a custom tray are
- Rigidity **
- Proper extension
- Stops (optional)
- Adequate thickness
- No sharp edges
- Uncontaminated
- Effective handle
What impression materials are and aren’t used for custom trays
Not
- Reversible hydrocolloid
- Condensation silicone
Are
- Polysulfide polymer
- Polyether
- Addition Silicone PVS
Recommended use for reversible hydrocolloid is
multiple preps
Problems with moisture control
Why is reversible hydrocolloid good to use in situations with poor moisture control
material is hydrophilic
Hallmark of polysulfide is
high tear strength
What impression material is the stiffest
polyether
Condensation silicone is (hydrophilic/hydrophobic)
hydrophobic
What impression material is the most dimensionally stable
addition silicone
What are the three different impression techniques and which one is most commonly used
- Monophase (one vistosity)
- Single step (2 viscosities at the same time) **most common*
- Double step (use material of different viscosity at each step)
Advantages of polyvinyl siloxane (PVS) aka addition silicone
- Easy to handle
- Automix system
- Favorable setting time (5 min regular set)
- High dimensional stability
- Excellent elastic recovery
Disadvantages of addition silicone
- hydrophobic
- Some materials release gas (hydrogen) H2
- Expensive
- Set inhibited by sulfur compounds in latex glove
How can the release of H2 from addition silicone be reduced
Pt or Pd as hydrogen scavengers
Before taking an impression what should you look for in order to avoid locking the tray in the patient’s mouth
- Bridges (always block out out under the pontic)
- Black triangles
- block out with periphery wax or oraseal
What is important in selecting your adhesive for the impression tray
must be compatible with the impression material
How can you avoid trapping air bubbles in the impression material when dispensing
-Keep tip at sulcus opening and in the impression material
PVS takes how long to set
5 min
Problems to look for when evaluating the impression
- Major distortions
- Voids or creases
- Incomplete areas
- Pulls and tears
- Unset material
- Syringe material not fused with tray material
- Adhesive failure/separation from the tray
Describe the protocol you should follow for impression analyzation
- Rinse
- Dry
- Inspect
- Make judgement
- Show faculty
- Disinfect
Cavicide requires _ contact time
3 min
T/F Cavicide slightly affects the accuracy of the PVS impression
F- no effect
What should you make sure you make sure you do before the patient leaves
make sure all the cords are out
