Master impressions Flashcards
ILO 2.6a: be familiar with the design and choice of materials used in the production of partial dentures, along with the knowledge of laboratory procedures
what is a master impression (secondary impression/working impression)?
an accurate impression of the remaining natural dentition, tissues of the denture-bearing area and the functional width and depth of the sulci taken with a special tray so an accurate master cast can be produced
what are the key features of a master impression?
- accurate recording of dentition
- accurate recording of denture bearing area
- functional width and depth of sulcus (incl. fraenal and other muscle attachments)
- sulci usually reasonably symmetrical
- reasonably even thickness of material
- paucity of air blows
- no air blows in critical areas e.g. area for rest seat
when is a master impression taken?
- patient needs to have a healthy mouth and free from disease
- after a special tray is produced from laboratory from the primary impression
- after denture design is finalised on surveyed mounted casts
- after all tooth preparation and restorative procedures are complete
why are master impressions taken?
- primary casts are fairly inaccurate (functional sulcus / full denture bearing area)
- so the prosthesis can be fabricated on an accurate master cast with all tooth preparation complete
what is a special tray (custom / individual tray)? how is it made?
- a tray made on the primary cast customised to fit the patient’s mouth
- a spacer of 3mm wax is moulded over the primary cast uniformly
- acrylic is shaped over the wax spacer to form a tray and a handle is made
- the periphery of the tray should stop 2mm from the sulci depth
why do you need a 3mm spacer when making master trays?
- to accomodate the impression material used - need to decide material before designing the tray
- prevents over seating and distortion
- allows even distribution of material
what are common problems with special trays?
- overextended special tray - periphery stops under 2mm from depth of sulci
- underextended special tray - periphery stops over 2mm from depth of sulci
how do you fix an overextended tray?
trim back peripheries with an acrylic bur in a straight handpiece
what are the problems with an overextended tray?
- tissue distortion - tray presses too far into tissues and distorts them
- discomfort/pain - if impinges on fraena, muscle attachments or mucosa
- restricted movement - affecting muscles like buccinator or mentalis
- inaccurate master impression - denture won’t fit properly
- poor denture retention and stability - denture can dislodge when speaking/eating
what are the problems with an underextended tray?
- incomplete sulcus capture - misses full depth of vestibule so poor border moulding
- poor peripheral seal - reduced suction and retention of final denture
- lack of support or coverage - doesn’t fully use the available support area, compomising stability
- unstable denture fit - denture may rock or feel use
how do you fix an underextended tray?
add greenstick impression compound and border mould in the mouth - melt material and apply to underextended areas
what is border moulding?
- technique for shaping the border areas of a special tray by manual or functional manipulation of the tissues adjacent to the denture borders (peripheries of the tray)
- aim is to mimic facial features
- important for capturing free end saddles - Kennedy class I and II
how do you take a master impression?
- disinfect special tray then rinse before trying it in
- try in the special tray
- adjust if required (over or under extension)
- use soft red ribbon wax to block out large interdental spaces
- place adhesive on the tray
- mix impression material and load the tray - dont overload
- insert the impression tray
- border mould - cheeks and lips to mimic facial expressions, stick tongue out for lowers (lingual fraenum and floor of mouth)
- remove impression and inspect
what kind of impression material is alginate?
irreversible hydrocolloid
what kind of impression material is impression compound?
non-elastic
what kind of impression material is impregum?
polyether - synthetic elastomer
what kind of impression material are silicones?
poly vinyl siloxane (PVS) - synthetic elastomer
what is a mucostatic impression? what are the advantages and disadvantages?
a material that does not displace the mucosa and records the resting position of the tissues when not under load (little pressure) i.e. silicone
* ✓ dentures conform closely to the shape of mucosal surface
* ✓ physical retention is optimal
* ✖ uneven occlusal loads
what is a mucocompressive impression? what are the advantages and disadvantages?
a material that causes displacement of the mucosa and records the shape of mucosa in a functional state / under load (pressure applied) i.e. alginate
* ✓ more even distribution of occlusal load
* ✖ less retention obtained when the teeth are apart
