Non elastic impression materials

Non elastic impression materialsASST PROFESSOR

Dr Mumtaz ul IslamB.Sc. BDS. MHR. M.Phil.

Rigid impression materials and types• Incapable to record undercut area • Impression plaster fractures on recovery others distorts• Impression plaster• Impression compound • Impression waxes• Zinc oxide eugenol pastes system

Impression plaster• Similar in composition of dental plaster• Beta calcium sulphate hemihydrate • Reacts with water form calcium sulphate di hydrate• Water powder ratio is very high (0.60)• Mucostatic • Fluid consistency

Control of properties• Accelerator potassium sulphate• Retarder borex • Antiexpansion agent • Alizarine red dye • Accelerator, retarder and dye mixed in a solution can be used • Too fluid stock tray cant hold it

Impression recording• Two methods direct and wash • Direct with special tray • Puddling the impression tray • Wash in a preliminary (primary) compound impression • Simply seat the impression home• Gentle movement from side to side and anterio posterior

Removal of impression • Very fluid records fine details• Hemihydrates absorb moisture a more closer contact• Dimensionally stable after setting • Removal is more difficult often fractures in undercut area• Dry sensation after impression and bad taste complaints• Disinfection with sodium hypochlorite for 10 minutes

Casting the impression• If impression of an undercut area taken it fractures• Fractured pieces can be joined with each other• Impression must be coated with a separating media • Impossible to separate the impression

Properties

Properties Contd,• Rapid setting• Low strength • Excellent for recording flabby ridge (mobile soft tissues)

Requires a special technique Impression of sound ridges taken in ZnO Eugenol A window is framed over the flabby ridge Impression plaster is painted over the exposed tissue

Impression compound• Thermoplastic resembling waxes• Two types

Lower fusing (type I) impression Higher fusing (type II) construction of impression trays

• Presentation Type I sheet (cake) or stick forms Type II …………………

• Uses Sheets for impression of edentulous ridges Stick for border extension of impression and trays etc for single tooth impression with copper ring technique

Composition

Manipulation • Softened in a water bath (conditioning)• Type I at 47° C flows well 37° C or below minimum flow

Stick direct on flame and tempered in warm water

• Effects of time of immersion Less not softens More material becomes sticky leaching of lubricating substance

• Ideal softening temperature is 55- 60° C • Poor conductor takes more time to soften from the center

Cake or sheet should be broken in to pieces

Copper ring technique

Difference between both types• Type I flows above 37°C (mouth temperature)

Sheet shows 20% and stick shows 6% or less flow

• Type II not flows at or near mouth temperature 2% or less flow

• Type I should record the impression of groves 0.02 – 4 mm

Viscosity • The most viscous impression material• Fine details can not be recorded• Makes it muco compressive• Records full depth of sulcus• Displace lingual and buccal soft tissue

Internal stresses• High coefficient of thermal expansion• Poor thermal conductivity• Large temperature drop from mouth to room • Distortion occors gradually • Model should be poured as soon as possible

Clinical considerations • Border seal & dynamic shape of tissues• Localized mucocompression for post dam area marking• Survival is questionable• Chemical disinfection is uncertain• If not tempered can burn the soft tissue (common)

Impression waxes• Low melting paraffin wax and bees wax in 3:1• Readily flow at mouth temperature and relatively soft at room

temperature• Rarely used to record complete impression• Corrective impressions especially for ZnO Eugenol • Small imperfections of impression are corrected by painting• Impression of lower free end saddle partial denture

Applegate technique • Melted wax applied to the faulty or short impression • Where needed loaded in the try• Tray seated in to the mouth with firm pressure• Allow the wax to flow well at mouth temperature under pressure

Zinc oxide eugenol impression material• Presentation is in the form of pastes - base and catalyst• Zinc oxide base paste usually white • Contrast in color facilitates thorough mixing streak free• Phenolic – OH of the eugenol weak acid acid – base reaction • 2C10H12O2 + ZnO → Zn(C10H11O2)2 + H2O• Two molecules of Eugenol reacts with zinc oxide • Ionic bond and two co-ordinate bonds donation of pairs of electrons

Methoxy oxygens to zinc

Composition

Setting reaction

Chelate compound (claws)

Ionic reaction• Needs an ionic medium & ionizable salts • Water acts as ionic medium and accelerator• Some manufacturers not add water • Water added materials starts reaction soon after mixing

Properties • Thickness around 1 mm insignificant dimensional change• Low initial viscosity pseudoplastic fine details• Defects can be corrected with impression -------------• Major disadvantage lack of elasticity• Considered as non-irritant• Ortho ethoxy benzoic acid is used in sensitive patients