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Master impressions Flashcards

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1
Q

What is a master impression?

A

an accurate imprint of the remaining dentition, denture-bearing tissues and sulci which allows the pouring of an accurate master cast for the fabrication of the prosthesis

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2
Q

What may master impressions also be known as?

A

secondary impressions or working impressions

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3
Q

What is required for a master impression to be taken?

A

special tray

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4
Q

What are the key features of a good master impression?

A
  • accurate recording of dentition
  • recording of functional width and depth of sulcus
  • recording frenal and other muscle attachments
  • usually symmetrical sulci
  • even thickness of material
  • no air bubbles in critical areas
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5
Q

What is used to make the special tray?

A

primary cast

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6
Q

Why is it important to avoid damage to the master cast?

A

master cast is required to fabricate the prosthesis (either by traditional methods or computer assisted)

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7
Q

When / under which conditions do you take a master cast?

A
  • patient mouth is healthy and free of disease
  • you have a special tray
  • denture design has been finalised on surveyed mounted casts
  • all tooth preparation and restorative procedures are complete
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8
Q

Which exception may you take a master impression without the patient’s mouth being healthy?

A

When an immediate / temporary denture is required for aesthetics in the case of trauma for example

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9
Q

What is a special tray?

A

a tray made on the primary cast and therefore is customised to fit the patient’s mouth

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10
Q

What may a special tray also be known as?

A

custom tray / individual tray

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11
Q

How is a special tray fabricated?

A

a wax spacer (~3mm) is moulded uniformly over the primary cast. Light cure acrylic is then shaped over the wax spacer to form the tray and handle

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12
Q

How far should the light cure acrylic (special tray) extend into the sulci?

A

tray periphery should stop 2mm short of the depth of the sulci

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13
Q

What is the function of the wax spacer in the fabrication of a special tray?

A

allows even distribution of the impression material and prevents over seating and distortion

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14
Q

What material is usually used to form the tray spacing?

A

sheet of modelling wax

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15
Q

Why is it important to decide the impression material for the master impression prior to the fabrication of the special tray?

A

the partial denture impression material determines the thickness of the wax spacer

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16
Q

How thick must the wax spacer be for an alginate impression?

A

3mm (accommodates most materials e.g. polyether = 3mm, light/medium silicone 1.5-3mm)

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17
Q

What are some common problems with special trays?

A

special tray may be overextended and / or underextended (can co-exist on the same tray) - tray periphery should stop 2mm short of sulcus depth

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18
Q

Why must you inspect the special tray when it is returned from the lab?

A

identify any errors you need to fix e.g. over/under extension

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19
Q

Under what circumstances may a special tray be overextended and therefore require trimming?

A

tray periphery encroaches on sulcus (does not stop 2mm short of sulcus depth), tray impinges on midline frenal attachments / other muscle attachments and tissues hence restricting their movements

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20
Q

How can you fix an overextended tray?

A

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

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21
Q

What may be the features of an underextended tray?

A

tray periphery does not extend to 2mm short of the sulci depth / impression periphery, tray doesn’t extend fully into saddle or post dam areas, impression material will not be supported and may distort when removed from mouth, tray doesn’t capture areas where denture components are planned

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22
Q

What may be a reason for the special tray returning from the lab overextended?

A

stock tray (for primary impression) may have been overextended itself leading to trapped muscle attachments

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23
Q

What could be a reason for the special tray returning from the lab underextended?

A

primary impression may not have been posterior enough

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24
Q

What material can be used to correct under extension of special trays?

A

greenstick impression compound

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25
Describe how you would use greenstick impression compound to correct an underextended special tray?
heat the greenstick (Bunsen burner / hot air burner) until it flows and apply to the underextended areas of the special tray. Insert into the patient's mouth and border mould
26
What is border moulding?
technique for shaping the border areas of a special tray / impression by manual and functional manipulation of the tissues adjacent to the proposed denture borders (tray peripheries)
27
When is border moulding particularly important?
making complete dentures / partial dentures with large saddles / capturing free end saddles (Kennedy class I and II)
28
What are the stages for taking a master impression?
1. Disinfect the tray (Perform) and rinse off disinfectant 2. Try in special tray 3. Adjust special tray (if required) (3. if required block out large interdental spaces) 4. Place adhesive on special tray 5. mix impression material and load tray 6. Insert the impression 7. Border moulding 8. Remove impression 9. Check the impression to see if it's acceptable
29
What must you do before trying in the special tray in the patient's mouth?
disinfect the tray in Perform then rinse off the disinfectant
30
What position / movements should you use when trying in the upper special tray or taking an upper impression?
upper - stand behind patient and rotate the tray into the mouth. patient in seated position slightly lower down (easier to push up)
31
What position / movements should you use when trying in the lower special tray or taking a lower impression?
lower - stand in front of patient and rotate the tray into the mouth. Patient in seated position slightly higher up (easier to push down)
32
What additional step may be required when taking a master impression of a patient with large interdental spaces?
block out large interdental spaces beneath contact points using soft red ribbon wax
33
Why should large interdental spaces beneath the contact points be blocked out prior to taking an impression?
to prevent tearing and distortion of the impression material
34
What material is used to block out large interdental spaces?
soft red ribbon wax (remember to remove after acceptable impression)
35
Example of a condition that may result in patients having large interdental spaces
treated periodontal disease
36
Do you use the same adhesive for all impression materials?
use different adhesives for different impression materials
37
How do you apply adhesive to the special tray?
1. pour correct adhesive from the bottle into disposable gallipot 2. use disposable brush to paint adhesive onto fitting surface of special tray
38
Why should you apply the adhesive from a disposable gallipot instead of directly from the bottle?
for IP+C as the tray has been tried in the patient's mouth so don't want to put brush from the tray back into bottle
39
Whose responsibility is it to load the (special) impression tray?
dentist responsibility not nurse's
40
What is a common error when it comes to loading the special tray?
overloading - only a 3mm spacer
41
How should the seating position of the patient vary between taking an upper and lower impression?
For an upper impression, patient should be lower (easier to push up). For a lower impression, patient should be higher (easier to push down)
42
At what stages of taking a master impression is border moulding required?
when correcting an underextended special tray using greenstick and when taking the master impression
43
What should you do during border moulding?
manipulate the lips and cheeks to mimic function e.g. facial expressions, for a lower impression ask patient to stick tongue forward
44
What is the function of border moulding?
imprints the functional sulcus, midline frenal, lingual frenum and floor of mouth (lower) and other muscle attachments on the impression.
45
What is a possible consequence of not border moulding?
tray impinges of muscle and frenal attachments and dislodges / is less retentive when patient speaks / laughs / eats
46
When should you remove the impression?
wait another ~30sec after you think it's set before removing (although temp in mouth 37 degree, teeth are not)
47
How should you remove the impression?
Use non-dominant hand to break seal and then for the upper push slightly up and rotate out, for the lower push slightly down and rotate out. If difficult to remove, don't panic, retry.
48
What should you do once you have removed the impression?
check the patient is okay. Then inspect and reflect on the impression
49
What should you inspect the impression for?
clear surface detail, no part should be detached from the tray, remove any excess unsupported bulk of material with sharp instrument to prevent distortion
50
What should you reflect on when checking an impression?
whether you have recorded what you wanted to, ant errors present, whether it's fit for purpose, whether you can accept this impression or need to repeat it
51
What should you do once you think you have achieved an acceptable impression?
discuss with supervisor and if the agree then disinfect before sending to laboratory with signed lab prescription
52
What are the 3 types of impression materials used in removable prosthodontics?
non-elastic, synthetic elastomers, hydrocolloids
53
Examples of non-elastic impression materials for removable prosthodontics
zinc oxide eugenol (complete dentures not PD) and compounds (used as adjunct to correct under extension / in difficult areas)
54
Examples of synthetic elastomers impression materials for removable prosthodontics
polyvinyl siloxane (Silicones), polyethers, polysulphides
55
When may polyvinyl siloxane (silicones) and polyethers be used?
as an adjunct or entire impression sometimes - when high degree of accuracy required
56
What are the 2 types of hydrocolloids?
reversible (agar - rarely used) and irreversible hydrocolloids (alginate)
57
What is the most common impression material used?
irreversible hydrocolloid alginate
58
What are the properties of impression compounds?
non-elastic, set rigidly (becomes mouldable at higher temp)
59
When are impression compounds used?
to add to free end saddle areas on stock tray (red) and to correct under extension of special tray (greenstick). Used as an adjunct
60
Which impression compound is useful in free end saddle areas of stock trays (primary impressions)?
red impression compound
61
Which impression compound is used to correct underextended special trays (master impressions)?
greenstick impression compound
62
How is red impression compound heated?
bowl of hot water
63
How is greenstick impression compound heated?
Bunsen burner / hot air blower (requires higher temp than red to become elastic)
64
What is the advantage of using irreversible hydrocolloid (alginate)?
easy to mix and good if undercuts present due to elastic nature
65
What must you be mindful of when using irreversible hydrocolloid / alginate?
avoiding air bubbles in key areas and casts should be poured as soon as possible to minimise dimensional change (therefore avoid taking alginate impression on Friday afternoon - use more dimensionally stable material e.g. silicon
66
Why is it important to not leave an alginate impression dry (wrap in damp paper towel)?
syneresis - loses water and shrink in dry conditions
67
Why is it important not to leave an alginate impression in Perform too long?
imbibition - uptakes water and swells if immersed in water
68
What forms is poly vinyl siloxane (PVS) / silicone available in?
light / medium / monophase / heavy bodied addition (and putty) silicone
69
What type of poly vinyl siloxane (PVS) / silicone is most commonly used in denture construction?
medium bodied addition silicone
70
What must you do before applying impression material to the tray +/ greenstick?
apply appropriate adhesive
71
What are the properties of poly vinyl siloxane (PVS) / silicone?
good dimensional stability, accurate, good tear strength but expensive
72
What is the brand name of polyether at GDH?
Impregum
73
What machine is used to mix and dispense Impregum (polyether)?
Pentamix system
74
Why should Impregum / polyether not be used in severe undercuts / large interdental spaces?
sets rigidly therefore would be difficult to remove
75
What are the advantages of using Impregum / polyether?
good flow, very accurate, dimensionally stable - useful in implant prosthodontics (crown and bridge too)
76
What are mucostatic impressions?
the material does not displace the mucosa
77
What is the purpose of mucostatic impressions?
recording the resting position of the tissues whilst not under load
78
When are mucostatic impressions most commonly used for?
flabby ridges (alginate impression is cut away over flabby ridge and replaced with silicone
79
What is an example of a mucostatic impression material?
light body silicones
80
What impression tray should be used when taking a mucostatic impression?
special perforated tray (use for alginate master imp then cut way area and replace with silicone) - so minimal pressure is applied to prevent distortion of flabby ridge
81
What are muco-compressive impressions?
material causes displacement of the mucosa, allowing recording of oral tissue in a functional state (under load)
82
How does taking a muco-compressive impression differ from taking a mucostatic impression?
pressure is applied to the mucosa for a muco-compressive impression (records under load) but little pressure for mucostatic impression (requires perforated tray) (records at rest)
83
What are the advantages of mucostatic impressions?
dentures conform closely to shape of mucosal surface, optimal physical retention
84
What is the disadvantage of mucostatic impressions?
occlusal loads will be uneven
85
What is the advantage of mucocompressive impressions?
more even distribution of occlusal load
86
What is the disadvantage of mucocompressive impressions?
less retention obtained when teeth apart (mucosa more compressible than tooth so loads on denture bearing tissue differ)
87
What do you need to include in a laboratory prescription?
- info technician needs to carry out tasks prescribed - what you want the technician to do with the master impression - what you want the technician to make for next visit - when you want work back for
88
What do the lab prescriptions at master impressions depend on?
whether denture will be acrylic or Co/Cr, whether denture can be hand articulated
89
What should the laboratory prescription at master impressions be for a Co/Cr denture?
- cast master impression in improved stone - construct chrome framework as per design shown - clear design by clinician (must have prepared occlusal rests / guide planes before master impression)
90
What should the laboratory prescription at master impressions be for an acrylic denture?
- cast impression in 100% stone - if cannot hand articulate: construct record blocks (design after this stage) - if can hand articulate: clear design signed by clinician, mount on articulator to hand articulation; set up for tooth trial (must provide shade and mould)
91
What material is used to make a master / working cast for a Co/Cr denture?
improved stone
92
What does the technician do to the master impression to make a Co/Cr framework?
pours impression in improved stone to make a master / working cast and then does one of two options
93
What is the traditional option used to fabricate the Co/Cr framework from a master/working cast?
technician produces accurate refractory model made from phosphate-bonded investment material and builds a wax pattern of the Co/Cr on the model. Via a lost wax technique, the Co/Cr framework is cast
94
What is the modern method used to fabricate the Co/Cr framework from a master/working cast?
technician scans 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
95
What should you do before writing the denture design on the laboratory prescription?
Write on design sheet first so you can then copy it more accurately onto the prescription
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