Impression materials Definition :- (GPT): A negative likeness or copy in reverse of the surface or object. 2) An imprint of the teeth and adjacent structures for use in dentistry
Impression material:- Any substance or combination of substances used for making an impression or negative reproduction. Impression technique:- A method and manner used in making a negative likeness.
Classification Based on rigidity : Rigid egPlaster,Zincoxideeugenol impression compound Elastic egAlginate,polyether,silicone Based on setting of the material: 1.) Chemical reaction(irreversible) eg plaster of paris,zincoxideeugenol alginate,additon and condensation silicones 2 ) Physical change of state(reversible) eg agar
Based on viscosity : Mucostatic eg impression plaster,agar, light body elastomer Mucocompressive eg Impression compound,alginates Pseudoplastic eg addition silicones
Based on interaction with saliva and water : Hydrophobic eg polysulfide ,condensation silicones Hydrophilic egputty,impression plaster Based on use : Primary impression material eg impression compound,alginate Secondary Impression material egZoe,medium bodied elastomers Duplicating material eg agar
Based on Chemical composition : impression plaster impression compound metal oxide(zinc oxide eugenol) pastes agar hydrocolloid alginate hydrocolloid polysulfides condensation silicones addition silicones polythers light curing polyether (urethane dimethacrylate)
Desirable qualities Pleasant odor taste and color Absence of toxic irritants Adequate shelf life Economical Easy to use Setting that meet clinical requirements Satisfactory consistency
Readily wet oral tissues Elastic Adequate strenght Dimensional stability Compatiblity with cast and die materials Accuracy Disinfection
Zincoxideeugenol impression material Non eugenol pastes- One of the chief disadvantages of Zoe pastes is the possible burning sensation caused by eugenol.orthoethoxybenzoic acid (EBA) is a valuable substitute for eugenol in this regard.
Recent advances in alginates Dust free alginates- De-dusting agent (glycerin or glycol) Siliconized alginates- silicon polymers are added to strengthen the material Alginates containing disinfectants- eg quaternary ammonium salts or chlorohexamine are added to alginate powder. Hard and soft set alginates- by adjusting the amount of fillers
Agar Gelation- Gelation is accomplished by circulating cold water of approximately 18-23 degree for not less than 5 min.
Elastomeric impression materials Classification :- Based on viscosity- Class 1 heavy body heavy viscosity Class 2 regular body or medium body Class 3 light body or low viscosity Based on chemistry- Polysulfides Condensation silicones Additional slilicones Polythers Visible light curable polyether
polysulfides Polysulfide is one of the least stiff or highly flexible of elastomeric material For accurate polysulfide impressions- The cast should be poured immediately after taking impression Minimise the amount of material used to take impression by using custom made acrylic trays
Condensation silicones It is also called room temperature vulcanising silicones(RTV) Uses – It is widely used in recording dentulous impressions in preparation of crowns,bridges,inlays,onlays and to some extent for partial dentures It is also used to record edentulous impressions in preparation of complete denture
Putty type Advantages – No custom tray required No special equipment required Moderate strength in deep sulcus Pleasant odor Adequate shelf life Disadvantages – Should be poured shortly after removal Hydrophobic Difficult to electroplate Easily distorted
Addition silicones Addition polysilicones are the most stable of all the existing materials It has an excellent recovery of 99.93% It exhibits pseudoplasticproperties.it can be used both as syringe and tray materials
Putty type Advantages – No custom tray No special equipment Good shelf life Can be poured more than once Disadvantages – Extremely hydrophilic More difficult to pour Most expensive
Light cured polyether It has excellent elasticity It has infinite working time and short setting time The impression can be stored upto two weeks
Disinfection Most manufacturers recommend a specific disinfectant,such as iodophor,bleach,orglutaldehyde,for disinfection. Certain disinfectants may result in gypsum casts that have a lower surface hardness or diminished surface detail. The current protocol for disinfecting hydrocolloid impressions recommended by the centre for disease control is to use house hold bleach(1 to 10 dilutions),iodophor or synthetic phenols as disinfectants An alternate disinfection method is by immersion,but this should not exceed 10 min.
Duplicating materials Duplicating is required for two reasons:- 1, The cast on which the wax pattern of the metal frame work is to be formed must be made from refractory investment because it must withstand the casting temperatures required for gold or base metal alloys 2, The original cast is needed for checking the accuracy of the metal frame work and for processing the denture base portion of the partial denture The most common duplicating materials are agar hydrocolloid compounds, The primary advantage of agar is their set is reversible.the disadvantage is it is subject to dimensional change.The best storage condition is 100% humidity.
Tissue conditioners Tissue conditoners are soft elastomers used to treat an irritated mucosa supporting a denture.it is used to provide time for healing of soft tissues Tissue conditioners are composed of powder containing poly(ethyl methacrylate)and a liquid containing an aromatic ester – ethyl alcohol. The properties that make tissue conditioners effective are- 1,viscous behavior which allows adaptation to irritated denture bearing mucosa 2,viscoelastic behavior which cushions the cyclic forces of mastication and bruxism