Week 4- Making Better Impression Flashcards
What are characteristics of a good impression?
- Uniform, homogeneous mix of material
- Tray is sufficiently filled with impression material
- Thoroughly applied tray adhesive (Strong bond between impression and tray)
- Rigid, sturdy impression tray
- No voids, pulls or tears on margin detail
- No tray show-through of the impression material and no tooth contact with tray.
- Good blend between heavy body and light body materials
- Complete information about the impression material used provided to the dental laboratory
What should you assess in pt clinically and on study models before placing crown or bridge?
What is occlusion like?
Any overerrupted teeth?
Space for tooth (large or small)?
Wear facets on teeth?
Excessive grinding?
How should you use light body material?
- Wait to inject material until DA is walking over to you with tray material.
- Need to rotate tip towards you until you hear click.
- Inject light body with purpose around tooth prepared (working time- 25 seconds).
- Ensure tip doesn’t come out of material or you’ll get air bubble. Wiggle and stir while syringing.
What is this impression material?
Polyether
What is light, medium and heavy body material?
- Light body: syringe material dispensed around teeth
- Medium-heavy body: tray material
Why is metal tray not ideal for silicone impression?
Metal tray and silicone impressions are not ideal as you don’t use adhesive. Material stays in tray due to rim lock and is difficult to get out of tray
How long should you wait for adhesive to set on tray before placing tray material?
5-10 minutes
What are causes of inhibited or slow setting?
PVS: latex gloves with traces of sulphur, residues from custom temporary cements present.
Polyether: substances with pH<4 (ferric sulphate, epinephrine)
Both: expired materials
What are causes of lack of impression details?
- Material stored at elevated or low temp
- Thick blood/saliva around prep
- Inadequate retraction of sulcus around prep
- Exceeding working time
- Inadequate disinfection
- Use of disposable tray rather than special tray
- Substances pH<4
What are causes of voids on the margin?
- Air trapped in syringe
- Air trapped while filling tray
- Improper syringe technique
- Inadequate coverage of marginal area with light body impression material
- Blood/saliva contamination
- Tray not seated straight
- Exceeding working time
- Impression stored at elevated temp
What are causes of tearing at the margin?
- Expired impression material
- Inadequate mix
- Insufficient retraction
- Substances with pH<4 contacting polyether
- Sulphur inhibition due to contact of latex gloves with PVS.
- Residue from custom temporary cements present.
What are the causes of delamination?
- Exceeding working time
- Impression material stored at elevated temp
- Sulphur or acrylic contamination of pre-set heavy body material in 2 step technique
- Delay when placing tray material on light body material
What are causes of poor bond of impression material to tray?
- Incompatible or no tray adhesive used
- Inadequate drying time for tray adhesive
- Thin, plastic tray allow deflection, which can cause rebound upon removal
What are causes of stone model discrepancies?
- Hydrogen gas emission
- Tooth contact with tray causes water to leach out of tray, dehydrating the stone.
- Cast not made according to model preparation guidelines
- Leaving alginate impression too long before pouring up models.
Why do stone model cusp tips sometimes become powdery?
Alginate impressions left too long before pouring up
What trays don’t use adhesive?
Never use adhesive for metal trays
How should you assess crown margin on impression?
Use loupes to identify crown and gingival margin. Need to ensure you can visualise crown margin all the way around.
What is the issue in this photo and how can it be avoided?
Inadequate mix
- Bleed mix before application
- Follow manufacturers instructions
- Ensure mix tip is attached correctly.
- When hand-mixing materials, ensure correct mixing ratio and thorough mix of catalyst and base paste.
What causes inhibition or slow setting of material?
- PVS: latex gloves with traces of sulphur, residues from custom temporary cements present.
- Polyether: substances with pH<4 (ferric sulphate, epinephrine)
- Both: expired materials, inadequate mix