IMPRESSION Definition. : a dental impression is a negative replica of the tissues of the oral cavity.
CLASSIFICATION OF IMPRESSION MATERIALS.
I) Based on mode of setting & elasticitya) Rigid
i) Reversible (thermoplastic) E.g. impression compound ii) Irreversible (thermosest) E.g. Zoe impression paste Impression plaster.b) elastic i) Hydrocolloid a) reversible E.g. Agar b) Irreversible E.g. Alginate
ii) Elastomeric impression materials E.g. Polysulfides Polyether
Addition silicones Condensation silicones
II) Based on type of impression & area of use A) Dentulous i) Primary E.g. alginate
ii) Secondary E.g. Elastomers Agar
B) Edentulousi) Primary
E.g. Impression compoundImpression plasterAlginate
ii) Secondarye.g. ZOE impression paste
Elastomers for CD
III) based on their use in dentistryA) edentulous
For complete denture prosthesise.g. Impression compound
zoe impression pastealginateElastomers
B) dentulousi) FPDii) RPD
IV) based on the amount of pressure applied i) Mucocompressive e.g. Impression compound ii) mucostatic
e.g. Impression plaster
V) based on the manipulation A) hand mixing
i) kneading e.g. Impression compound wet kneading putty consistency Elastomers
ii) circular motion (with glass slab & spatula) eg.zoe impression paste polysulfide
iii) vigorous mixing e.g. Alginate (figure of 8 motions)
B) mechanical mixing
VI) based on the tray used for impression A) stock tray i) types of tray a) rim locked perforated e.g. Alginate Elastomers
b) water cooled e.g. Agar
c) plastic e.g. Orthodontic tray
ii) based on type of perforationa) perforated
e.g. Alginate Elastomers
putty wash impression
b) non perforatede.g. Impression compound
B) special tray i) base plate wax
e.g. Zoe impression paste medium body
IDEAL REQIUREMENTS OF IMPRESSION MATERIAL
1. have a pleasant taste & odor
2. not contain toxic or irritant constituents.
3. adequate shelf life for requirements of storage & distribution.
4. economically commensurate with the results obtained.
5. easy to use with the minimum of equipment.
6. setting characteristics that meet clinical requirements.
7. satisfactory consistency texture.
8. readily wets oral tissues
9. elastic properties with freedom from permanent deformation after strain.
10. adequate strength so it will not break or tear on removal from the mouth.
11. dimensional stability over temp. & humidity ranges normally found in clinical &
lab. Procedures for a period long enough to permit the production of a cast or die.
12. compatibility with cast & die materials.
13. accuracy in clinical use.
14. readily disinfected without loss of accuracy.
15. no release of gas or other by products during the setting of the impression or cast
& die materials.
It is a rigid, reversible impression material which sets by physical change.
Classification:Acc to ADA sp. No.3:Type I:low fusing compound green stick compound.
medium fusing compound impression compound.Type II: high fusing compound tray compound.
Supplied as:1. as sticks, cakes, & cones.2. colors green, brown, red.
For making primary impressions.For individual tooth impressionFor peripheral tracing or border moulding.To check undercuts in inlay preparation.
Type II:To make a special tray.
Requirements of an ideal impression compound:1. harden at or little above mouth temp.2. be plastic at a temp. not injurious or harmful to oral tissues.3. not containing irritating or toxic ingredients.4. harden uniformly when cooled without distortion.5. have a consistency when softened which will allow it to reproduce fine details.6. be cohesive but not adhesive.7. not undergo permanent deformation or fracture while withdrawing the impression
from the mouth.8. be dimensionally stable after removal from mouth & during storage.9. exhibit a smooth glossy surface after flaming.10. withstand trimming with sharp knife without flaking or chipping after hardening.
Rosin 30 Copal resin 30 Carbuna wax 10Stearic acid 5Talc 25Coloring agent app.
Thermal propertiesi) thermal conductivity: has low thermal conductivity. Significance:
during softening of the material , the outside will soften first & inside last. So to ensure uniform softening the material should be kept immersed for a long time in water bath.
the layer adjacent to the mouth tissue swill remain soft. Therefore Its imp to cool the compound thoroughly before removing the impression.
ii) Coefficient of linear expansion. (CTE) high CTE , 0.3% acceptable. iii) Flow high fusing compounds < 2% at 37 .C & < 70-85 % at 45.C low fusing compounds - < 3% at 37 C & < 80-85% at 45 C impression compound to be manipulated at least 8 .C for adequate flow.
iv) Glass transition temp it is the temp. at which the material loses its hardness or brittleness on heating or
forms a rigid mass. App. 39.C Material removed from the mouth after glass transition temp. is reached.
v) Fusion Temp. its the temp. at which the impression compound becomes plastic. Its 43.5.C
vi) Dimensional stability. Poor dimensional stability. To prevent distortion pour the cast within 1 hr.
vii) surface detail reproduction less because of high viscosity & low flow.
Small amt. softened over flameCakes
Softened in a thermostatically controlled water bath.
Precautions:o prolonged immersion in a water bath is not indicatedo overheating In water makes the compound sticky & difficult to handle.o avoid incorporating water while kneading.
Advantages:1. cheap2. reusable3. does not produce irritation to the patient4. impression can be remodified & resoftned again till an accurate impression is
Disadvantages:1. difficult to record details accurately.2. Soft tissues are compressed due to pressure applied while taking the impression.3. Distortion.4. Difficult to remove undercuts.5. Does not have an pleasant taste.6. Can be uncomfortable in patients because of the rigidity.
ZINC OXIDE EUGENOL PASTE
Introduction Produce a rigid impression. High degree of accuracy & good surface reproduction of details. Is irreversible, sets by chemical change.
Classification:ADA specification No. 16
Type I or hardType II or soft
Available as In paste form in 2 tubes
Base paste (white) Accelerator/ reactor/catalyst paste (red)
Base Paste: Zinc oxide 87 %Olive oil /linseed oil 13%
Accelerator paste: Oil of cloves 12%
Gum or polymerized rosin 50%Filler (silica type) : 20%Lanolin 3%Resinous balsam 10%Accelerator solution 5 %
Setting Reaction : ZnO + H2O Zn (OH)2
Zn (OH)2 + 2HE ZnE2+ 2 H2O
Manipulation: Dispense equal length of the 2 pastes. Variation in length alters setting time. Paper pad is used as a mixing slab & a stiff stainless steel spatula with an 8-10 cm
blade should be used. Mixing is done for 30-40 sec. until no color streaks in the mix & a uniform
consistency is obtained.
Initial setting time final setting time Type I 3-6 min. 10 min. Type II 3-6 min 15 min.
Factors controlling setting time:
Decreasing setting time: Small amt. of zinc acetate Accelerators Small drop of water mixed in the paste containing eugenol Increasing the mixing time. Increasing accelerator paste while mixing.
Increasing setting time: Cooling the spatula & mixing slab Addition of inert oils & waxes during mixing to reduce hardness like:
olive oil. Increasing base paste while mixing.
Properties: i) consistency & flow
thick consistency compress the tissues.
ii) dimensional stability quite satisfactory, less than 0.1 % shrinkage occurs during hardening.
iii) reproduction of details good.
Biologic considerations:Some patients experience burning sensation in the mouth due to eugenol & can also cause tissue irritation. This maybe overcome by using a non eugenol paste.
Uses:1. cementing & insulating medium2. temporary filling3. root canal filling material4. surgical pack in periodontal surgical procedures.5. bite registration pastes6. temporary relining material for dentures.7. impression material for edentulous patients.
Advantages:1. Good dimensional stability2. good detail reproduction3. impression surface can be modified by adding in deficient areas4. easy to manipulate5. long working time, so border moulding can be done while relining
Disadvantages1. burning sensation due to eugenol.2. messy to work with3. sticks to skin & instruments , difficult to clean.4. rigid so tends to break off in areas of undercuts.
Introduction: Comes from the term algin coined by Scottish chemist. It is an