Master Impressions Flashcards
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 is a master impression taken?
Master impressions are taken so that an accurate master or working cast can be poured upon which a prosthesis can be fabricated either by traditional methods or by computer assisted design and manufacture
When do you take a master impression?
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 special tray and how is it made?
An individual tray made on the primary cast
It is customized to fit the patient’s mouth
It is usually made of light cure acrylic and has a handle
A spacer (usually 3mm wax) is moulded uniformly over
the primary cast
The acrylic is then shaped over the wax spacer to form the tray
At what distance should the periphery of the tray stop relative to the depth of the sulci?
2mm
What does a wax spacer material do and how large should it be?
The laboratory will use a spacer material between the cast and the special tray
3mm will usually accommodate most materials
Prevents over seating and distortion
Allows an even distribution of material
What are the two common problems with special trays?
overextension
underextension
How should the periphery of the tray be?
The periphery of the tray should stop 2mm short of the depth of the sulci and/or the periphery of the impression
How to solve an overextended tray?
Trim back the peripheries with an acrylic bur in a straight handpiece
How to solve an underextended tray?
Add to the peripheries using greenstick impression compound and border mould in the mouth
What material can be used to correct under extension and how is the material used?
Greenstick impression compound can be used to correct special tray under extension
Heat the material (hot air burner/ Bunsen burner) until it flows and apply to areas of the special tray which are underextended
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)
to mimic facial expressions
When is border moulding important?
Particularly Important when making complete dentures or partial dentures with large saddles
Particularly Important for capturing free end saddles (Kennedy Classes I & II)
How should the special tray be spaced?
The special tray should be spaced appropriately for the impression material that you are going to use – usually alginate so 3mm spacer
What should you do before trying the special tray in the patient’s mouth?
Disinfect the tray, then rinse off the disinfectant before trying it in the patient’s mouth
How should you try in the upper and lower tray?
From the right side of the patient
For the upper stand
behind the patient
and rotate the tray
into the mouth
For the lower stand in front of the patient and rotate the tray into the mouth
How should you fix large interdental spaces and why?
triangles
To prevent tearing and distortion of the impression material (alginate), 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.
Remember to remove the wax after an acceptable impression
Before placing impression material and loading tray, what should be applied and how?
Place adhesive on special tray
a) Pour correct adhesive from bottle into a disposable
gallipot
b) Use disposable brush to paint adhesive onto fitting
surface of special tray
Whose responsibility is it to load the tray?
dentist
Why should bordr moulding be done?
This imprints the functional sulcus, midline fraenal and other muscle attachments on the impression
to avoid impingement on the muscle attachments
How should impression be removed?
Once you have finished border moulding - hold tray in place until material has set
Then
Both: Use non dominant hand to break seal
Upper: Push slightly up, rotate out
Lower: Push slightly down rotate out
How should you inspect the impression?
Each impression should be examined for defects and the surface should exhibit clear detail.
No part should be detached from the tray.
Any excess unsupported bulk of material should be removed with a sharp instrument to prevent distortion.
What should you reflect?
Have I recorded what I wanted to record?
Are there errors present? Or is it fit for purpose?
Can I accept this impression?
Do I need to repeat it?
If the impression is viable, what should be done?
Discuss with supervisor, if supervisor agrees disinfect before sending to the laboratory with signed lab prescription
What are the 3 categories of impression materials?
non-elastic
synthetic elastomers
hydrocolloids
What is the most common impression material?
hydrocolloid irreversible alginate
What material is used to extend the tray and get difficult areas?
non-elastic impression compounds
greenstick
When should impression compounds not be used?
Do not use for full arch impressions in
dentate patients
When are impression compounds useful?
Useful in free end saddle areas (primary (red) & master (greenstick) impressions)
How is red impression compound prepared and used for?
Heat in bowl of hot water
primary impressions
How should greenstick be heated?
bunsen burner
hot air blower
What are characteristics of irreversible hydrocolloid alginate?
Easy to mix
Good if undercuts present
Careful with air blows in key areas
Why should alginate casts should be poured as soon as possible?
minimise dimensional change
Syneresis: In dry conditions loses water & shrinks
Imbibition: If immersed in water, uptakes water & swells
What are qualities of silicone?
Good dimensional stability, accurate, good tear strength, but expensive
What silicone is used most commonly in denture construction?
medium bodied addition
What are qualities of polyether (impregum)?
Automatic mixing and dispensing in the PentamixTM System
Sets rigidly, do not use in severe undercuts or where big gaps between teeth e.g. Treated perio disease
Good flow and very accurate, dimensionally
stable, useful in implant prosthodontics
Apply appropriate adhesive
What is polyether most commonly used for in dentistry?
crown/bridgework
How can materials be combined in master impressions?
Free end saddle areas (Kennedy classes I & II) can sometimes be difficult to capture accurately
“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
Remember to use appropriate adhesive on top of fit surface of tray and greenstick
What are mucostatic impression materials?
Material does not displace the mucosa
Records the resting position of the tissues whilst not under load
Most common use is with flabby ridges eg: light body silicones
What are muco-compressive impression materials?
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
Records the shape of the mucosa when under load e.g. Impression compounds and silicone putty very mucocompressive – most materials e.g. alginate are mucocompressive
What are the advantages and disadvantages of mucostatic impressions?
Advantages
Dentures conform closely to the shape of the mucosal surface
Physical retention is optimal
Comfortable for patient
Disadvantages
The occlusal loads will be uneven
What are the advantages and disadvantages of mucocompressive impressions?
Advantages
More even distribution of occlusal load
Disadvantages
Less retention to be obtained when the teeth are apart
Why does the load change on mucosa?
Mucosa is more compressible than tooth So loads on denture bearing tissues differ
For a Cr-Co denture, what material is used to cast the master impression?
improved stone
For acrylic denture, what material is used to cast the master impression?
100% stone
If you cannot hand articulate, what should you do?
construct record blocks
What does the technician do to the master impression to make a Co/Cr denture?
The impression is poured in improved stone to make a master or “working” cast:
Option 1:
The technician produces an accurate refractory model made from phosphate- bonded investment material and builds a wax pattern of the Co/Cr on this model. Via a lost wax technique the Co/Cr framework is cast
Option 2:
The technician scans the cast and via digital workflow the design is transferred onto a design software package. From the data a Co/Cr framework is made by a process called sintering.
What two papers need to be filled out after the patient?
design sheet (before lab prescription)
lab prescription (with indication that impressions have been disinfected)
How many strips of wax for the spacer?
2
How big should the wax spacer be to accommodate the alginate
3mm roughly
What should you do before placing the spacer on the cast?
wet the cast
How is the wax removed from the tray?
using hot water to soften the wax
