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2 - Prosthodontics (Removable) > Master Impressions > Flashcards

Master Impressions Flashcards

1
Q

What are the key features of a good master impression?

A

-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

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2
Q

Why is a master impression taken?

A

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

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3
Q

When do you take a master impression?

A

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

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4
Q

What is a special tray and how is it made?

A

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

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5
Q

At what distance should the periphery of the tray stop relative to the depth of the sulci?

A

2mm

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6
Q

What does a wax spacer material do and how large should it be?

A

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

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7
Q

What are the two common problems with special trays?

A

overextension
underextension

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8
Q

How should the periphery of the tray be?

A

The periphery of the tray should stop 2mm short of the depth of the sulci and/or the periphery of the impression

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9
Q

How to solve an overextended tray?

A

Trim back the peripheries with an acrylic bur in a straight handpiece

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10
Q

How to solve an underextended tray?

A

Add to the peripheries using greenstick impression compound and border mould in the mouth

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11
Q

What material can be used to correct under extension and how is the material used?

A

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

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12
Q

What is border moulding?

A

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

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13
Q

When is border moulding important?

A

Particularly Important when making complete dentures or partial dentures with large saddles

Particularly Important for capturing free end saddles (Kennedy Classes I & II)

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14
Q

How should the special tray be spaced?

A

The special tray should be spaced appropriately for the impression material that you are going to use – usually alginate so 3mm spacer

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15
Q

What should you do before trying the special tray in the patient’s mouth?

A

Disinfect the tray, then rinse off the disinfectant before trying it in the patient’s mouth

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16
Q

How should you try in the upper and lower tray?

A

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

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17
Q

How should you fix large interdental spaces and why?

triangles

A

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

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18
Q

Before placing impression material and loading tray, what should be applied and how?

A

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

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19
Q

Whose responsibility is it to load the tray?

A

dentist

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20
Q

Why should bordr moulding be done?

A

This imprints the functional sulcus, midline fraenal and other muscle attachments on the impression

to avoid impingement on the muscle attachments

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21
Q

How should impression be removed?

A

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

22
Q

How should you inspect the impression?

A

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.

23
Q

What should you reflect?

A

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?

24
Q

If the impression is viable, what should be done?

A

Discuss with supervisor, if supervisor agrees disinfect before sending to the laboratory with signed lab prescription

25
Q

What are the 3 categories of impression materials?

A

non-elastic
synthetic elastomers
hydrocolloids

26
Q

What is the most common impression material?

A

hydrocolloid irreversible alginate

27
Q

What material is used to extend the tray and get difficult areas?

A

non-elastic impression compounds

greenstick

28
Q

When should impression compounds not be used?

A

Do not use for full arch impressions in
dentate patients

29
Q

When are impression compounds useful?

A

Useful in free end saddle areas (primary (red) & master (greenstick) impressions)

30
Q

How is red impression compound prepared and used for?

A

Heat in bowl of hot water

primary impressions

31
Q

How should greenstick be heated?

A

bunsen burner
hot air blower

32
Q

What are characteristics of irreversible hydrocolloid alginate?

A

Easy to mix
Good if undercuts present
Careful with air blows in key areas

33
Q

Why should alginate casts should be poured as soon as possible?

A

minimise dimensional change

Syneresis: In dry conditions loses water & shrinks

Imbibition: If immersed in water, uptakes water & swells

34
Q

What are qualities of silicone?

A

Good dimensional stability, accurate, good tear strength, but expensive

35
Q

What silicone is used most commonly in denture construction?

A

medium bodied addition

36
Q

What are qualities of polyether (impregum)?

A

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

37
Q

What is polyether most commonly used for in dentistry?

A

crown/bridgework

38
Q

How can materials be combined in master impressions?

A

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

39
Q

What are mucostatic impression materials?

A

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

40
Q

What are muco-compressive impression materials?

A

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

41
Q

What are the advantages and disadvantages of mucostatic impressions?

A

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

42
Q

What are the advantages and disadvantages of mucocompressive impressions?

A

Advantages
More even distribution of occlusal load

Disadvantages
Less retention to be obtained when the teeth are apart

43
Q

Why does the load change on mucosa?

A

Mucosa is more compressible than tooth So loads on denture bearing tissues differ

44
Q

For a Cr-Co denture, what material is used to cast the master impression?

A

improved stone

45
Q

For acrylic denture, what material is used to cast the master impression?

A

100% stone

46
Q

If you cannot hand articulate, what should you do?

A

construct record blocks

47
Q

What does the technician do to the master impression to make a Co/Cr denture?

A

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.

48
Q

What two papers need to be filled out after the patient?

A

design sheet (before lab prescription)

lab prescription (with indication that impressions have been disinfected)

49
Q

How many strips of wax for the spacer?

A

2

50
Q

How big should the wax spacer be to accommodate the alginate

A

3mm roughly

51
Q

What should you do before placing the spacer on the cast?

A

wet the cast

52
Q

How is the wax removed from the tray?

A

using hot water to soften the wax

2 - Prosthodontics (Removable) (16 decks)

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