impressions, waxes, casting

Question 1

what is gypsum and it’s use

Answer 1

Gypsum = calcium sulphate dihydrate (CaSO4)2H2O

Gypsum products used for models, dies, moulds.

Gypsum gets converted into the hemihydrate (CaSO4)H2O which (depending on technique) makes plaster, stone, or improved stone.

Question 2

diff between plaster and stone

Answer 2

Plaster consists of large, irregular, porous particles. Plaster is used for models since it is cheap and soft.

Stone/improved stone consist of small, regular, non-porous particles. Stone is used for dies since they are harder. Improved stone is more expensive.

Question 3

Properties of gypsum products:

Answer 3

No dimensional change once set
Compressive strength dependent on using the right amount of water
Great storage
Tensile strength (low for plaster, double that for stones)

Question 4

products used for gypsum setting

Answer 4

Accelerators speed up gypsum setting by providing extra sites for crystal growth
K2SO4
CaSO4.H2O
NaCl (<20%)

Retarders slow down gypsum setting by interfering with crystal formation
NaCl (>20%)
Borax
Potassium citrate

Question 5

how do you manipulate the properties of gypsume

Answer 5

Changing ratio of powder and liquid ->

More water = slower setting time + model will be weaker

More powder = hard to mix + creates lots of porosity

Increasing spatulation time -> faster setting time since it makes more sites for crystal growth to occur

Increasing temperature -> setting time is slower and eventually doesn’t occur if temperature is that high

Question 6

define model, die, mould, refractory investment

Answer 6

Model = replica of fitting surfaces of oral cavity
Dies = replica of individual teeth
Mould = used for denture construction
Refractory investment = mould material for casting of wax/alloys or for creating indirect restorations

Question 7

list impression material options

Answer 7

HYDROCOLLOIDS
- Alginate (irreversible)
- Agar (reversible)

ELASTOMERS
- Sillicons (condensation and addition) (addition comes in different bodies)
- Polysulphides
- Polyethers

NON ELASTIC
- ZOE zinc oxide eugonel
- Impression plaster
- Impression compound

Question 8

compare condensaition and addition sillicons

Answer 8

Condensation silicones
Pros  stronger than alginates, more elastic, good tear strength
Cons  shrink in air, hydrophobic, expand in solution, limited shelf life

Addition silicones
Pros  best dimensional stability, good elastic recovery
Cons  gives a porous model, hydrophobic, setting impaired when using natural rubber gloves

Question 9

strongest elastomer imp material

Answer 9

polysulphides

Question 10

pros and cons of polyethers as imp materials

Answer 10

Pros -> good air dimensional stability, quick setting, not messy

Cons -> permanent deformation, poor dimensional stability with water so disinfection is difficult

Question 11

pros and cons of polysulphides as imp materials

Answer 11

Pros  strongest of all

Cons  slow setting, bad smell, poor elastic recovery, poor dimensional stability so shrinkage

Question 12

describe 3 types of non elastic imp materials

Answer 12

3 types of NON-ELASTIC imps materials:

Impression plaster – mucostatic, no tray needed, only for edentulous cases

Impression compound – mucocompressive, high viscosity, can be used for primary imps for edentulous cases

Zinc oxide eugenol – mucostatic, accurate in thin section, brittle when set, used in edentulous cases

Question 13

diff between mucodisplasive/mucocompresive and mucostatic

Answer 13

Mucodisplasive/mucocompressive = viscous, displaces tissues, records impression of mucosa under load – this gives a denture with wider distribution of load so stable!
E.g. Impression compound, high viscosity alginates/elastomers e.g. polyether

Mucostatic = more fluid, does not displace tissues much, so records an imp of un-displaced mucosa – so gives better retention for denture
E.g. Impression plaster, ZOE, low viscosity alginates/elastomers, light body addition silicones

Question 14

what imp materials do you use on dentate vs edentulous pts

Answer 14

Materials used for
dentate patients:
Alginate
Agar
Non-aqueous elastomers

Materials used for edentulous patients:
Impression compound
Impression plaster
Zinc oxide eugenol
Alginate
Non-aqueous elastomers

Question 15

whats the diff between imp materials for fixed vs rem pros

Answer 15

Materials list is same for imps for removable pros – main difference is techniques.

Question 16

ideal imp material

Answer 16

Ideal imp materials should be hydrophilic, flowable, nice taste/smell, snap setting, high tear strength, good recovery from deformation, dimensionally stable, can be disinfected, compatible with gypsum, good reproduction of details when being cast

Question 17

impression techniques for fixes pros

Answer 17

Impression techniques using different viscosity materials (wash can be light-bodied silicone)
Dual viscosity – general term for using different viscosities

Putty and wash 1 stage
Wash material around tooth, putty into tray, insert tray
Not great since can’t control thickness of wash and putty might move wash so detail is compromised

Putty and wash 2 stage
Take putty imp before preparation of tooth, then once tooth is prepared, put wash on tooth, then insert the putty impression on top
Not great since needs 2 imps and difficult to reposition tray for the second go

Question 18

What are dental waxes

Answer 18

Dental waxes = thermoplastic moulding materials that are solid at RT

room temp

Question 19

list the types of waxes and their use

Answer 19

PATTERN WAX
Inlay wax – used for inlays and crowns (made first in wax then converted to metal)
Casting wax – used in patterns for denture framework
Modelling wax – easily mouldable, easy to carve, no residue after removing via boiling

PROCESSING WAX
Boxing wax – used to build up vertical walls around an impression and as beading wax (adapted around impression borders)
Sticky wax – used for the temporary joining of items e.g. to align fractured parts of a denture

IMPRESSION WAX
Impression wax – used for occlusal register (subtype is corrective wax which is for edentulous impressions)

Question 20

how does casting and investing affect the outcome of an indirect resto

Answer 20

Casting and investing can produce errors in the steps of making indirect restorations.
Casting includes significant shrinkage during cooling, so the investing process must produce an oversized mould to cancel out the shrinkage.

Question 21

describe the mechanism for expansion in the casting process

Answer 21

= setting expansion of binder, thermal expansion, hygroscopic expansion

Question 22

what are investment materials

Answer 22

ceramic materials used to form moulds for dental casting

Question 23

requirments for investment material

Answer 23

fast set, easily break away from casting, not react with alloy, can compensate for shrinkage, cheap since they are single use, can withstand high temps/pressures, porous to allow gases to escape

Question 24

general composition of investment materials

Answer 24

Refractory – form of silica used to withstand high temps and compensate shrinkage

Binder – to hold refractory and other particles together

Other additions – to modify physical properties

Question 25

types of investment material

Answer 25

Gypsum bonded – used for gold alloys up to 700 degrees C
Cheap, strong, and porous but need a metal ring for support

Phosphate bonded – up to 1000 degrees C
Stronger so no metal ring needed, finer particle size so smoother

Silica bonded – up to 1200 degrees C
Used for high melting point metal alloys, no setting expansion, expensive

Question 26

requirments for die materials

Answer 26

accurate, compatible with imps material, dimensionally stable
Die materials are often improved stone, type IV, type V, surface hardened die stones.

Dies = positive reproductions of prepared teeth