Master impressions Flashcards
What is a master impression
An accurate imprint of the remaining natural dentition, the tissues of the denture-bearing area and the functional width and depth of the sulci taken so that an accurate master cast can be poured upon which a prosthesis can be fabricated either by traditional methods or by computer assisted design and manufacture
What is another name for master impressions
Secondary impressions or working impressions
What are the key features of a good master impression
-Accurate recording of dentition
-Denture bearing area recorded accurately
-Functional width & depth of sulcus including fraenal and other muscle attachments recorded
-Sulci usually reasonably symmetrical
-Reasonably even thickness of material
-Paucity of air blows
-No air blows in critical areas eg: where you want to put a rest seat
Why are master impressions taken
To allow for an accurate master or working cast to be poured so that a prosthesis can be made to fit
When can you take master impressions
The patient’s mouth is healthy and free of dental disease
You have a special tray made by the laboratory
You have a finalised denture design on surveyed mounted casts
When all tooth preparation and restorative procedures are complete
What is a specialtray made of
Usually light cure acrylic
What is a special tray
A tray made on the primary cast customised to the patients mouth
How is a special tray made
Spacer (wax) placed over the primary cast first (around 3mm)
Acrylic is shaped over the wax to form the tray
Periphery of the tray stops 2mm short of the depth of the sulci
Why is a spacer used in making special trays
To prevent over seating and distortion allowing even distribution of material
When might a spacer of 1.5mm be used
When the impression material is light silicone
What are the most common problems when a special tray is made
Overextended special tray
The periphery of the tray should stop 2mm short of the depth of the sulci and/or the periphery of the impression
Underextended special tray
The periphery of the tray does not extend to 2mm short of the depth of the sulci and/or the periphery of the impression
Can Overextension and
Underextension occur on the same tray
Yes
How can overextension be fixed
Acrylic bur
How is underextension fixed
Add to the peripheries using greenstick impression compound and border mould in the mouth
What might cause underextenion aside from less than 2mm depth
Tray doesn’t extend fully into saddle or post dam areas
The impression material will not be supported and may distort when removed from the mouth
Tray does not capture areas where denture components are planned
What is the texhnique used with greenstick impression compound
Border mould
What is border moulding
A technique for shaping the border areas of a special tray and/or impression by manual or functional manipulation of the tissues adjacent to the proposed denture borders (peripheries of the tray)
How do you take a master impression
- Disinfect and try in special tray
- Adjust tray if required
- Place adhesive on tray
- Mix impression material and load tray
- Insert impression
- Border moulding
- Remove impression
- Assess acceptability
How is tearing and distortion due to large interdental spaces in contact points prevented
large interdental spaces beneath contact points should be blocked out in the mouth using soft red ribbon wax or other suitable material, prior to taking the impression
Why do we manipulate the lips and cheeks to mimic function eg facial expressions when taking a master impression
This imprints the functional sulcus, midline fraenal and other muscle attachments on the impression
What is an example of a non-elastic impression compound
Greenstick
When are non-elastic impression compounds used
Do not use for full arch impressions in dentate patients
Useful in free end saddle areas (primary (red) & master (greenstick) impressions)
How are non-elastic materials softened
Heat in bowl of hot water (red impression compound – primary impressions)
Bunsen burner/hot air blower (“greenstick” impression compound)
What is irreversible hydrocolloid
Alginate
Why should alginate impression casts be poured ASAP
Syneresis: In dry conditions loses water & shrinks
Imbibition: If immersed in water, uptakes water & swells
What impression material is often used in denture construction
Medium bodied addition silicone is most commonly used in denture construction
What are the benefits of silicone impression materials
Good dimensional stability, accurate, good tear strength, but expensive
What is the brand name for polyether
Impregum
What are the benefits of polyether
Good flow and very accurate, dimensionally stable, useful in implant prosthodontics
Automatic mixing and dispensing system
What are the negatives to using polyether/impregum
Sets rigidly, do not use in severe undercuts or where big gaps between teeth e.g. Treated perio disease
What can be used to accurately capture free end saddles in impressions
“Greenstick” impression compound can be used to take an initial impression of these areas
A wash of a flowable material such as alginate or silicone can then be used in the final impression
How are mucostatic impressions with flabby ridges recorded
light body silicones
Alginate imp; cut away over flabby ridge;
Reinsert alginate with silicone wash
Silicone wash goes through perforations so no distortion of ridge
What is a mucostatic impression
Material does not displace the mucosa
Records the resting position of the tissues whilst not under load
What are muco-compressive impressions
Material causes displacement of the mucosa, allowing the recording of the oral tissue in a functional state
Pressure applied to the mucosa whilst taking the impression
What are the adv of mucostatic impressions
Advantages
Dentures conform closely to the shape of the mucosal surface
Physical retention is optimal
Disadvantages
The occlusal loads will be uneven
What are the adv and disadv of mucocompressive impressions
Advantages
More even distribution of occlusal load
Disadvantages
Less retention to be obtained when the teeth are apart
What are the impression materials to choose from
Poly vinyl siloxane - Light/medium/monophase/heavy bodied silicones
Polyether
Irreversible hydrocolloid - Alginate
Impression compounds – Greenstick; Red compound
A combination
What should be written on the prescription when sending to the lab
What information does the dental technician need to know to carry out tasks you prescribe?
What do you want the technician to do with the master impression?
What do you want the technician to make for next visit?
When do you want the work back for?
What should a laboratory prescription include fora Co/Cr denture
Cast the master impression in improved stone
Construct chrome framework as per design shown
Clear design signed by clinician
What needs to be on a prescription for acrylic dentures
Cast the impression in 100% stone
If you cannot hand articulate:
Construct record blocks – design after that stage
If you can hand articulate:
Clear design signed by clinician
Mount on articulator to hand articulation; set up for tooth trial (You must provide a shade and mould)
