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