Impression materials

produce an accurate 3D negative replica of the surface and shape of hard/soft oral tissues

material used to fill the impression and produce a positive replica (stone cast), recording the position, size, shape and orientation of each tooth used for: * evaluating occlusal/position problems in orthodontic cases * producing restorations (inlays, crowns) * fabricating prosthesis (partial denture framework)

• clinical - mucostatic, mucocompressive • properties - elastic, non-elastic

• mucostatic - fluid materials that displace the soft tissues slightly (undisplaced mucosa) e.g. ZOE, low viscosity alginates • mucocompressive - viscous materials that record an impression of the mucosa under load (displaced soft tissues) e.g. impression compounds, high viscosity alginates/elastomers

• IM in its initial paste form flowed around the tooth and made **close contact ** • once hardened and on removal, the IM bulges out due to strain to overcome the bulbous aspect of the tooth • after removal, the IM undergoes elastic recovery and changes shape temporarily

• at time=0, a force is applied to the IM, causing the material to undergo a change in dimensions • IM reaches its maximum strain instantly (B) - length = L+dL • for as long as the load is applied, the strain on the IM remains constant • when the load is released (time TL), the IM instantly recovers to 0% strain and the IM has the same dimensions

• when a load is applied at time=0, the IM does not reach its maximum strain instantly • there is a rapid increase in strain initially but tapers off so the maximum strain is reached after some time (TL) • when the load is released, the IM gradually returns towards its original dimensions but never reaches full elastic recovery • there is some permanent strain and the IM is not 100% accurate

• the strain of the IM increases the same way over time • when disengaging the impression tray quicker than normal (Tg), the IM has not yet reached maximum strain (Smax) • the IM gradually recovers more of its original shape so there is less permanent strain • technique: short, sharp, pulling force

• elastic materials recover its original shape e.g. hydrocolloids, elastomers • non-elastic materials end up deformed (poor fit and fracturing) e.g. impression compound, impression paste

• preparation - mixing, chemical reaction • insertion - tray type? adhesive? • setting - chemical, polymerisation, cooling • removal - short, sharp pull • storage - how long? • cast preparation - gypsum

• flow - surface detail, low enough viscosity? • no dimensional change on setting • complete elastic recovery on removal - no dimensional change • does not change dimensionally over time (storage) • dimensions unaltered during decontamination • compatible with cast material

• viscosity - affects level of detail recorded • setting mechanism - can cause dimensional contraction • thermal expansion coeficient - ideally low to cope with mouth to room temp transition • hydrophobic/hydrophilic - tooth may need to be dried/wet for good contact • elasticity • tear strength - withstand tearing when removed especially with undercuts • storage • ease of removal

• non-toxic and non-irritant • acceptable taste and smell • short setting time • removable without damage to oral tissues

• quick and simple technique • convenient working and setting times • easily decontaminated • inexpensive

a two phase system of fine particles (1-200nm) of one phase dispersed in another phase

a two phase system where one phase of fine particles is dispersed in water

• as a SOL - viscous liquid state • as a GEL - jelly-like consistency where fine particles form a framework that encapsulates a liquid

sodium alginate + calcium sulfate -> sodium sulfate + calcium alginate

trisodium phosphate * preferably reacts with calcium ions in calcium sulfate to delay gel formation * when all trisodium phosphate has been used, the calcium ions react with sodium alginate

• use correct powder/liquid ratio • use water at 18-24 degrees • use perforated tray and adhesive • insert tray into pts mouth and wait for set • remove impression tray with a sharp pull

• use large bulk of material (5mm) • pull sharply

• flows to record fine detail :) • acceptible setting changes but needs storage ASAP :/ • nearly elastic - always some slight permanent strain :/ • poor tear strength - avoid deep undercuts :( • syneresis (shrinkage) or imbibition (expansion) during storage may occur adn affect dimensional accuracy :( • avoid patient movement during setting - can distort shape and affect internal stresses :(

• non-toxic :) • non-irritant :) • comfortable to patient :) • acceptable taste and smell :) • ideal setting time :)

• relatively easy to use :) • reasonable setting time :) • storage can be difficult - avoid moisture :( (expands), syneresis (release of water), imbibition (uptake of water)