Week 4- Accurate Impressions Flashcards
What factors are involved in taking an accurate impression?
- Material selection
- Tissue and tooth preparation management
- Tray selection
- Retraction
- Haemostasis
- Impression techniques
- Bite registration
- Infection control/disinfection
What details is alginate good for recording?
Alginate is good for recording teeth detail but doesn’t produce compression of tissues.
If you have pt with resorbed lower ridge, what material should be used and why?
If you have resorbed lower ridge, use PVS instead. Thick material pushes tissues out of the way so lab tech can make correct special tray. If first impression is underextended, the final denture will end up being underextended
What is the issue with plastic trays?
Plastic trays flex which can lead to impression distorting.
What adrenaline amount is a ½ inch of cord equivalent to?
Adrenaline in 2.5cm piece of cord contains slightly less than amount of adrenaline in 4 lignocaine cartridges
What are important features to look for in impression?
- Setting time
- Homogenous mix
- Sufficient amount of material
- Complete coverage with adhesive
- Rigid impression tray
- Accurate margins
- High elasticity and dimensional stability
Where is it important to not have voids/pulls in impression material?
Margin of crown prep
How long is required for denture adhesive to stick on tray before placing impression material?
Need to allow 5-10 minutes
What is Hooke’s law?
Extension vs load applied is directly proportional until you exceed elastic limit. At this stage, the material no longer elastically recovers.
What are desirable qualities of impression materials?
- Pleasant odour
- Affordable
- Easy to use
- Adequate strength
- Accurate
- Readily disinfected without loss of accuracy
- Thixotrophic
Why is light body a different colour to tray material?
Give visual contrast to see margins of preparation.
What is thixotrophic?
When squirting material, it flows under pressure.
What is a closed bite impression tray (triple tray or dual arch)?
Prepared arch and opposing arch are taken together so jaw relation and impression can be taken at same time.
What are the disadvantages of triple trays?
- Flexible
- Impression likely to distort.
- No cross-arch stabilization
- May distort the impression material if teeth contact opposing arch or bite side walls of tray, or back plastic of tray
What are tooth requirements for triple trays?
Triple tray must always have teeth on either side of preparation.
What is an open bite impression tray?
Special tray (rigid and reduces distortion and rebound effect)
What is the importance of placing stops for crownwork impressions?
If doing crown work, need to place on lab form that you want stops placed and tell them where you want crown and bridge placed. Stops are little pieces of PMM put inside of tray on fitting surface side to stop tray being pushed all the way onto the teeth. Would see tray showing through if no stops are used.
How much relief if provided around teeth for special trays? Why is this important?
3-4mm relief around prepared teeth. Impression materials need this amount of wall thickness for accuracy
What is the issue with not using adhesive?
The impression shrinks toward the centre of the mass which results in a smaller impression and crowns may not seat completely.
What is the double mix step by step procedure?
- Place light body around tooth
- Load the heavy body material into the tray and seat in mouth
- Hold the tray firmly until the impression has thoroughly set
- Remove the impression from mouth, rinse and disinfect to manufacturer’s recommendation.
What happens if light body material starts setting before placing tray material?
Once light body material starts setting, it won’t bond to tray material.
What is the purpose of light body?
Gives more detail around tooth
What is putty wash procedure?
2-step technique which uses a heavy bodied putty and a light bodied injection material
- Mix 2 putty materials together
- Place the putty into selected tray and cover putty with a separating wafer/plastic sheet
- Place tray in the mouth and seat
- After putty has completely set, remove the impression
- Remove the spacer, rinse and thoroughly dry the impression
using air/water syringe - This impression now becomes a very rigid custom tray
- Syringe the wash material (light body) around the prepared area
- Syringe additional wash material into the putty impression
What is the purpose of impression separating wafer?
Allows space for light body material
What are the 2 purposes of gingival tissue management?
- Haemostasis
- Retraction
What astringent agent is used in Astringident vs Hemodent?
Astringident: ferric sulphate
Hemodent: aluminium chloride
What is the application time for ferric sulphate?
1-3 minutes
What are the pros and cons with aluminium chloride?
- Least irritating
- Disrupts setting of PVS (rinsing with water resolves its inhibitory effect)
What are advantages of 3M Aastringent retraction paste?
- Easy access to tight IP areas
- Long lasting haemostasis (aluminium chloride)
- Gentle on tissue (less risk of tissue trauma and haemorrhaging)
- Better pt comfort
- Effective retraction
What are the issues with ferric sulphate?
- Can cause significant tissue irritation and post operative root sensitivity
- Disrupts setting reaction of PVS (should be rinsed off thoroughly from the tissue before taking an impression)
- Stains gingiva yellow-brown to black for several days (iron)
- More rapid healing than with aluminium chloride
What is the procedure for 3M Astringent retraction paste?
- Place in CR gun and take off red tip.
- Place tip below gingiva and place material into gingival sulcus.
- Leave for 3-4 minutes and wash and dry.
How much retraction cord should be left in when taking crown prep impression?
When you come to take impression, remove cord on top and leave first cord in place
