Impression Materials Flashcards
what is the function of an impression material?
to produce an accurate replica of the surface and shape of hard and soft oral tissues
what kind of reproduction does an impression give of the soft tissue?
negative
what is dental stone used for?
to fill the impression and produce a positive replica (stone cast) which represents the position, shape, size and orientation of each tooth
what are casts used for?
evaluating dentition when orthodontic, occlusal or other problems involved, in laboratory fabrication of restorations and prostheses
what are the classifications of impression materials?
mucostatic, mucocompressive and elastic/non-elastic
what are mucostatic materials?
fluid materials that displaces the soft tissues slightly and give impression of the undisplaced mucosa
what are mucocompressive materials?
viscous materials that record an impression of the mucosa under load i.e. give impression of displaced soft tissues
what is the ideal elastic behaviour for an impression material?
once the strain is taken off of the material i.e. removing impression material from the tooth, it should go back to the way it was when it covered the tooth making an exact impression
what term is used for the elastic behaviour of impression materials?
viscoelastic
what does the term viscoelastic behaviour mean?
when the impression material is removed from the tooth it never fully recovers and so it has permanent strain
what helps to decrease permanent strain?
if the load time with the impression material on the tooth is less
name a hydrocolloid
alginate
name 3 elastomers
polysulphides, polyethers, silicones
how do you prepare impression materials
mixing 2 pastes either a paste and liquid or powder and liquid which produces a chemical reaction
what are the ways in which an impression material sets?
chemical, polymerisation, cooling
what are the ideal properties for accuracy for an impression material?
surface detail recorded, no dimensional change, no effect on oral tissues, no change dimensionally, complete elastic recovery, does not change dimensionally over time, dimensions and surfaces unaltered, compatible with cast material
what type of factors affect accuracy?
viscosity, setting mechanism, thermal expansion coefficient, hydrophobic, elasticity, tear strength
what should the thermal expansion coefficient be?
low
what does viscosity determine?
the ability to flow over the surface and so the level of detail recorded
what are the ideal properties for patient comfort?
non-toxic, non-irritant, acceptable taste and smell, short setting time, removable without damage to oral tissues
what are the ideal properties for operator convenience?
quick, simple technique, convenient working/setting times, must be able to be decontaminated, cost is a consideration
what are the 2 main types of elastic impression materials?
hydrocolloids and elastomers
what are the 2 types of non-elastic impression materials?
impression compound, impression paste
what is a hydrocolloid?
a colloid is a two phase system of fine particles of one phase dispersed in another phase - the dispersing medium is water
what are the two states of hydrocolloid?
sol and gel
what is the sol state of hydrocolloid?
viscous liquid state
what is the gel state of hydrocolloid?
jelly like consistency, agglomeration of particles, entangled framework of solid particles enclosing liquid by capillary forces
in alginate, what does trisodium phosphate do?
delays gel formation so varying amount of this varies the clinicians working time
what is the basic setting reaction of alginate?
sodium alginate to calcium alginate
what occurs in the setting reaction of alginate?
sodium molecules are swapped over for divalent calcium ions which allow crosslinking to increase viscosity and strength
how does trisodium phosphate work in the setting reaction of alginate?
trisodium preferentially reacts with Ca ions in CaSO4 and then sodium alginate reacts with calcium ions
how can you manipulate alginate?
use correct powder/liquid ratio, use water at 18-24 degrees, use perforated tray and adhesive, remove impression with a sharp pull
what does increased temperature do to alginate setting?
speeds up setting
how do you get greater elastic properties with alginate?
wait further minute or two before removing from mouth
how do you reduce permanent distortion and tearing?
using large bulk of materials
how do you store alginate?
avoid moisture, prepare cast ASAP to avoid dimensional change
what is syneresis?
release of water
what is imbibition
uptake of water
what is the tear strength of alginate?
poor so avoid deep undercuts
what does movement of alginate cause?
internal stresses and distortion
