impression matsone102208xhandout
TRANSCRIPT
1
Impression Materials (One)
2
Required Properties of Dental Impression Materials
Properties of AlginateImpression Materials as Examples
3
The Importance of Impression Materials
Dental appliances can be made outside the patient’s mouth
They must have accuracy of ‘fit’ and functional efficiency
That depends on the model closely replicating the natural
oral tissues
Accuracy of models depend on accuracy of impressions
4
5
Classification of Impression Materials
Chemical types allow dentist to quickly select for particular applications
Properties either before or after setting can also be usedMost used property before setting is Viscosity
Effects ability to recording of fine detail of hard tissues
Degree of soft tissue compression or displacement
6
7
8
Mucostatic and Mucocompressive Properties
Materials initially fluid are termed MinimallyDisplacive (Previously called Mucostatic)
More viscous materials are termed Mucocompressive
Note that some materials have pseudoplastic properties(become more fluid on mixing)
Initially fluid, all impression materials need trays for support
Thickness will also affect pressure transmitted to the tissues
Thinner sections of fluid materials can act as if mucocompressive
9
10
Set Properties of Impression Materials
Important factors are rigidity (non-rubbery) and elasticity
Flexibility for withdrawal from hard and soft tissue undercuts
Elasticity to recover original form for accuracy
Note some materials are plastic (pastes) and some rigid
(plaster)
11
Introduction to Hydrocolloids in Dentistry
Colloidal suspensions of polysaccharides in water
Behave neither as solutions nor as true
suspensions
Small electrical charges repel the colloid molecules
Sol forms (fluid) random polysaccharide chains
12
13
Setting of Hydrocolloids in the Mouth by Cooling
Conversion from sol form to gel form with elasticity
Alignment of chains due to one of two mechanisms
Cooling allows bonding between adjacent chains
Mostly Van der Waals forces but also hydrogen bonds
These are be reversed to gel form by heating (Agar Gel)
14
Irreversible Hydrocolloids (Alginates)
Supplied as powders which are mixed with water
Composition of supplied powers is varied
Some have reduced filler to give more fluidity
Some set faster due to lower trisodium phosphate content
15
16
Rheological Properties:Study of Flow and Deformation
P = Shear StressQ = Shear Rate
17
Rheological Properties:Study of Flow and Deformation
Viscosity
18
Requirements of Impression Materials
A.Factors which affect the accuracy of the impression
B. Factors which affect the dimensional stability (time)
C. Manipulative variables - handling,
setting characteristics, etc
D. Additional factors such as cost, taste, colour etc
19
Accuracy (One)
Low viscosity or a degree of pseudoplasticity on insertion
Ideally hydrophilic; Hydrophobic materials require dry tissues
Wettable by freshly mixed dental plaster of stone
Minimal small change of dimension on setting (contraction)
20
21
22
23
Setting reaction of Alginate Impression Materials
Mixing and spatulating forms an aqueous alginate sol
Gypsum is only sparingly soluble (solubility about 0.2%)
Sodium in alginate readily exchanges with calcium ions
Cross-links of the alginate chains result producing a gel
24
Extension of Setting Time of Alginate Material
Sodium phosphate is dissolving readily in the water
Reacts with calcium ions being released from gypsum
When all the sodium phosphate is used up setting occurs
The effect is to extend the period of low viscosity
25
Accuracy (Two)
Minimal thermal contraction from 32-37oC to about 23oC
Also minimal thermal contraction of impression tray
Must remain attached to impression tray (adhesive)
Sufficient rigidity to allow removal of material
26
27
Accuracy (Three)
Adequate elastic propertiesNot so much that removal is prevented or teeth damaged
Viscoelastic materials removed rapidly to reduce deformation
Adequate tear resistance – tensile strengthEspecially in thin section
28
29
Dimensional stability
Best if casting into impression can be when convenient
Ideally the impression retains its original accuracy indefinitely
Setting reaction may continue and cause change
Slow elastic recovery of viscoelastic materials (? Beneficial to wait before casting?)
Loss of volatile substances either primary or products
30
Dimensional Stability of Alginates
Poor due to syneresis and imbibition of water
Cast model as soon as possible
31
End of
IMPRESSION MATERIALS (ONE)