Master impressions Flashcards

1
Q

What is a master impression

A

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

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2
Q

What is another name for master impressions

A

Secondary impressions or working impressions

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3
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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4
Q

Why are master impressions taken

A

To allow for an accurate master or working cast to be poured so that a prosthesis can be made to fit

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5
Q

When can you take master impressions

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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6
Q

What is a specialtray made of

A

Usually light cure acrylic

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7
Q

What is a special tray

A

A tray made on the primary cast customised to the patients mouth

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8
Q

How is a special tray made

A

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

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9
Q

Why is a spacer used in making special trays

A

To prevent over seating and distortion allowing even distribution of material

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10
Q

When might a spacer of 1.5mm be used

A

When the impression material is light silicone

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11
Q

What are the most common problems when a special tray is made

A

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

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12
Q

Can Overextension and
Underextension occur on the same tray

A

Yes

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13
Q

How can overextension be fixed

A

Acrylic bur

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14
Q

How is underextension fixed

A

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

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15
Q

What might cause underextenion aside from less than 2mm depth

A

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

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16
Q

What is the texhnique used with greenstick impression compound

A

Border mould

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17
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)

18
Q

How do you take a master impression

A
  1. Disinfect and try in special tray
  2. Adjust tray if required
  3. Place adhesive on tray
  4. Mix impression material and load tray
  5. Insert impression
  6. Border moulding
  7. Remove impression
  8. Assess acceptability
19
Q

How is tearing and distortion due to large interdental spaces in contact points prevented

A

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

20
Q

Why do we manipulate the lips and cheeks to mimic function eg facial expressions when taking a master impression

A

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

21
Q

What is an example of a non-elastic impression compound

A

Greenstick

22
Q

When are non-elastic impression compounds used

A

Do not use for full arch impressions in dentate patients​

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

23
Q

How are non-elastic materials softened

A

Heat in bowl of hot water (red impression compound – primary impressions)​

Bunsen burner/hot air blower (“greenstick” impression compound)

24
Q

What is irreversible hydrocolloid

A

Alginate

25
Q

Why should alginate impression casts be poured ASAP

A

Syneresis: In dry conditions loses water & shrinks​

Imbibition: If immersed in water, uptakes water & swells

26
Q

What impression material is often used in denture construction

A

Medium bodied addition silicone is most commonly used in denture construction

27
Q

What are the benefits of silicone impression materials

A

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

28
Q

What is the brand name for polyether

A

Impregum

29
Q

What are the benefits of polyether

A

Good flow and very accurate, dimensionally stable, useful in implant prosthodontics
Automatic mixing and dispensing system

30
Q

What are the negatives to using polyether/impregum

A

Sets rigidly, do not use in severe undercuts or where big gaps between teeth e.g. Treated perio disease

31
Q

What can be used to accurately capture free end saddles in impressions

A

“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

32
Q

How are mucostatic impressions with flabby ridges recorded

A

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

33
Q

What is a mucostatic impression

A

Material does not displace the mucosa

Records the resting position of the tissues whilst not under load

34
Q

What are muco-compressive impressions

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

35
Q

What are the adv of mucostatic impressions

A

Advantages​

Dentures conform closely to the shape of the mucosal surface
Physical retention is optimal​

Disadvantages​

The occlusal loads will be uneven

36
Q

What are the adv and disadv of mucocompressive impressions

A

Advantages​

More even distribution of occlusal load

Disadvantages​

Less retention to be obtained when the teeth are apart

37
Q

What are the impression materials to choose from

A

Poly vinyl siloxane - Light/medium/monophase/heavy bodied silicones​

Polyether​

Irreversible hydrocolloid - Alginate​

Impression compounds – Greenstick; Red compound ​

A combination

38
Q

What should be written on the prescription when sending to the lab

A

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?

39
Q

What should a laboratory prescription include fora Co/Cr denture

A

Cast the master impression in improved stone​

Construct chrome framework as per design shown​

Clear design signed by clinician

40
Q

What needs to be on a prescription for acrylic dentures

A

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)