impression materials

• produce a negative replica of the surface and shape of hard and soft oral tissues • negative replica of tissues • accuracy depends on material and technique

• gypsum • used to fill the impression and produce a positive replica • it is poured into the impression

• the shape, position, size and orientation of each tooth

• for evaluating the dentition when orthodontic, occlusal or other problems involved • in laboratory fabrication of restorations and prosthesis = lab needs study cast to produce RPD framework

• single tooth • whole dentition • edentulous area/ mouth

• good properties - clinician with good technique

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

• zinc oxide eugenol, low viscosity alginates • fluid materials that displace soft tissues slightly • give an impression of the undisplaced mucosa

• impression compound, high viscosity alginates/elastomers • viscous material that record an impression of the mucosa under load • give impression of displaced soft tissue

• because material has to strain/change to get over bulbosity of tooth during removal and you want it to be able to fully recover afterwards • no impression material is truly elastic though and there will always be permanent deformation from removal

• needs to be able to snap back to original shape after going under strain • needs to react in an elastic manner • would return to 0% strain if truly elastic

• no materials are truly elastic, they are all viscoelastic • lower viscoelasticity = more truly elastic • there is a permanent strain, deformed, doesn’t recover to original dimensions • doesn’t truly reflect patients dentition

• if load time is less - impression removed with a sharp pull

• hydrocolloids - elastomers

• agar = reversible (not used) - alginate = irreversible (used in DH)

• polysulphides • polyethers • silicones (conventional and addition cured) • even these aren’t truly elastic

• does not recover well at al - can get worse deformation or even fracturing

• preparation • insertion • setting • removal • storage • cast preparation

• mixing = 2 pastes, paste and liquid, powder and liquid • chemical reaction • heat • material flows and takes up shape/space available

• tray type needed - do you need adhesive

• chemical • polymerisation • cooling • forms solid replica

• storing can have significant dimensional changes that may even mean having to redo the impression

• flow = needs to flow over dentition to make good contact to get good detail • setting changes = want no dimensional change • removal = want no effect on oral tissues and no dimensional change (can be difficult due to change in temp out of mouth) • storage = no dimensional change • decontamination = dimensions and surfaces unaltered • compatible with cast material = gypsum

• viscosity = determines ability to flow over dentition • setting mechanism = does it cause dimensional contraction • thermal expansion coefficient = ideally low to cope with change • hydrophobic/hydrophilic = affects surface contact, does saliva need removed before impression • elasticity = elastic/viscoelastic behaviour • tear strength = withstand tearing when removed especially under undercuts as most fragile