Impression: It is the negative reduction of denture bearing border seal and stabilizing areas obtained in a plastic or semiplastic material. Preliminary impression: A negative likeners made for the purpose of diagnosis, treatment planning or the fabrication of a tray. Final impression: Negative likeners made for the purpose of fabricating a prosthesis. I.
IMPRESSION MATERIALS
ELASTIC
HYDROCOLLOID
AGAR
NON ELASTIC
RUBBERS
ZOE
IMPRESSION COMPOUND
ALGINATE POLYSULPHIDE
SILICONES
CONDENSATION
POLYETHER
ADDITION
II.
ACCORDING TO THE MANNER IN WHICH THEY HARDEN a) Set by clinical reaction (Irreversible) -
Plaster of paris.
-
Zinc oxide eugenol.
-
Alginate.
-
Non-aqueous elastomers.
b) Set by temperature change (reversible)
III.
-
Impression compound.
-
Agar.
-
Alginate.
-
Zinc oxide eugenol.
ACCORDING TO VISCOSITY OR TISSUE DISPLACEMENT
c) Mucostatic materials. -
Impression plaster.
-
Agar.
-
Alginate.
-
Zinc oxide eugenol.
BASIC REQUIREMENTS OF IMPRESSION MATERIALS 1.
Accurate Reproduction of surface detail
(i)
Rheological properties a) A low viscosity is desirable but it should not be so low that the material is not easily contained in the tray. b) Material must be in a fluid or plastic state on insertion into the mouth.
(ii)
Ability of the impression materials to adapt closely to both the soft and hard tissues.
The way in which the material interacts with saliva affects fine detail reproduction. Impression material Hydrophobic Impression material will be repelled by moisture (saliva) surface detail lost. Dry field is essential for such materials.
2. i)
Dimensional accuracy and stability Impression tray. a) Flexible trays are likely to bend during impression making and relax after removal of the impression producing a distorted impression. b) A good bond between the tray and the impression material.
ii)
Shrinkage of the impression material
Whether the impression material sets by a chemical reaction or change in physical state both result in shrinkage of the impression material. The coefficient of thermal expansion of the impression material should be small. Ideally an impression material should show very small contraction (<0.5%). iii)
Impression material must be sufficiently flexible to allow removal from the undercut regions without causing distortion.
iv)
Impression material should have sufficient mechanical strength not to tear or undergo permanent deformation during removal.
v)
Storage stability: There is usually a delay between recording of an impression and pouring of the cast. It is important that the impression material neither shrinks, expands or distorts during this time period.
3.
Manipulative Variables i)
Setting time: a) Setting time should be short with a long working time. b) Setting time should be such that the impression need not be in the mouth too long.
ii)
Setting temperature: a) Impression material should set or harden near the mouth temperature.
4.
Additional factors i)
Should be non toxic and non irritant to the tissues.
ii)
Should have an acceptable odour, taste and appearance.
iii)
Unused impression should be stable on storage over a period of time.
iv)
Should be reasonably inexpensive.
v)
Impression material should be capable of being disinfected without compromising its accuracy or stability.
vi)
Must be easy to use with minimum of equipments.
FINAL IMPRESSION MATERIALS 1. Plaster of paris. 2. Zinc oxide-eugenol paste. 3. Silicone. 4. Polysulphide. 5. Polyether and 6. Tissue conditioning materials. One must not depend on a material to provide good impressions. Any material is only as good as the dentist using it. I. 1.
Zinc oxide Eugenol impression paste Classified as rigid or inelastic materials that harden by chemical action.
2.
Used as a corrective lining (wash) in a secondary impression tray (custom tray).
3.
Two types Eugenol Non-eugenol pastes
4.
ADA specification No. 16: Initial setting time
Final setting time
Type I (Hard)
(3-6) minutes
Within 10 minutes
Type II (Soft)
(3-6) minutes
within 15 minutes
5.
Composition
Tube No. 1. a.
Zinc oxide â&#x20AC;&#x201C; 80-87%
Reactive ingredient which takes part in setting reaction. It should be finely divided and contain very slight amount of water. b.
Fixed vegetable or mineral oil â&#x20AC;&#x201C; 13-15%
Plasticizer and masks the action of eugenol as irritant. Tube No. 2
a)
Oil of clove or eugenol – 12%
Some times oil of clove is used as it reduces the burning sensation by otherwise use of eugenol. b)
Gum or polymerized Rosin – 50%
Gives body and coherence to the mixed material and imparts thermoplastic properties to the set material. It also facilitates the speed of the reaction. c)
Filler – 20%
Inert material, may contain Kaolin, talc or wax or diatomaceous earth. d)
Resinous Balsam – 10%
Used to increase the flow, Canada balsam and Peru balsam are used. e)
Accelerator: MgCl2 or CaCl2 (can be incorporated in one or both the pastes). Zinc salts like zinc stearate.
6.
Setting reaction: Typical acid-base reaction ZnO + H2O
Zn(OH)2
Zn(OH)2 + 2HE
ZnE2 + 2H2O
Base
Acid
Salt
7. II. 5.