Master impressions

Question 1

What is a master impression

Answer 1

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

Question 2

What is another name for master impressions

Answer 2

Secondary impressions or working impressions

Question 3

What are the key features of a good master impression

Answer 3

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-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

Question 4

Why are master impressions taken

Answer 4

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

Question 5

When can you take master impressions

Answer 5

The patient’s mouth is healthy and free of dental disease​

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You have a special tray made by the laboratory​

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You have a finalised denture design on surveyed mounted casts​

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When all tooth preparation and restorative procedures are complete

Question 6

What is a specialtray made of

Answer 6

Usually light cure acrylic

Question 7

What is a special tray

Answer 7

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

Question 8

How is a special tray made

Answer 8

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

Question 9

Why is a spacer used in making special trays

Answer 9

To prevent over seating and distortion allowing even distribution of material

Question 10

When might a spacer of 1.5mm be used

Answer 10

When the impression material is light silicone

Question 11

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

Answer 11

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​

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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

Question 12

Can Overextension and
Underextension occur on the same tray

Answer 12

Yes

Question 13

How can overextension be fixed

Answer 13

Acrylic bur

Question 14

How is underextension fixed

Answer 14

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

Question 15

What might cause underextenion aside from less than 2mm depth

Answer 15

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

Question 16

What is the texhnique used with greenstick impression compound

Answer 16

Border mould

Question 17

What is border moulding

Answer 17

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)

Question 18

How do you take a master impression

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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

Question 21

What is an example of a non-elastic impression compound

Answer 21

Greenstick

Question 22

When are non-elastic impression compounds used

Answer 22

Do not use for full arch impressions in dentate patients​

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

Question 23

How are non-elastic materials softened

Answer 23

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

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

Question 24

What is irreversible hydrocolloid

Answer 24

Alginate

Question 25

Why should alginate impression casts be poured ASAP

Answer 25

Syneresis: In dry conditions loses water & shrinks​ Imbibition: If immersed in water, uptakes water & swells

Question 26

What impression material is often used in denture construction

Answer 26

Medium bodied addition silicone is most commonly used in denture construction

Question 27

What are the benefits of silicone impression materials

Answer 27

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

Question 28

What is the brand name for polyether

Answer 28

Impregum

Question 29

What are the benefits of polyether

Answer 29

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

Question 30

What are the negatives to using polyether/impregum

Answer 30

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

Question 31

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

Answer 31

“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

Question 32

How are mucostatic impressions with flabby ridges recorded

Answer 32

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

Question 33

What is a mucostatic impression

Answer 33

Material does not displace the mucosa Records the resting position of the tissues whilst not under load

Question 34

What are muco-compressive impressions

Answer 34

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

Question 35

What are the adv of mucostatic impressions

Answer 35

Advantages​ Dentures conform closely to the shape of the mucosal surface Physical retention is optimal​ Disadvantages​ The occlusal loads will be uneven

Question 36

What are the adv and disadv of mucocompressive impressions

Answer 36

Advantages​ More even distribution of occlusal load Disadvantages​ Less retention to be obtained when the teeth are apart

Question 37

What are the impression materials to choose from

Answer 37

Poly vinyl siloxane - Light/medium/monophase/heavy bodied silicones​ Polyether​ Irreversible hydrocolloid - Alginate​ Impression compounds – Greenstick; Red compound ​ A combination

Question 38

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

Answer 38

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?

Question 39

What should a laboratory prescription include fora Co/Cr denture

Answer 39

Cast the master impression in improved stone​ Construct chrome framework as per design shown​ Clear design signed by clinician

Question 40

What needs to be on a prescription for acrylic dentures

Answer 40

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)