what is the function of impression materials?
produce an accurate 3D negative replica of the surface and shape of hard/soft oral tissues
what is dental stone? what is it used for?
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)
how would you classify impression materials?
- clinical - mucostatic, mucocompressive
- properties - elastic, non-elastic
what is the difference between mucostatic and mucocompressive IMs?
- 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
describe what happens when an IM is removed after setting
- 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
describe the ideal elastic behaviour diagram of an IM
- 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
describe the actual viscoelastic behaviour of an IM
- 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
how would the viscoelastic behaviour of an IM change with a load applied over a shorter time?
- 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
what is the difference between elastic and non-elastic IMs?
- 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
describe the impression taking process
- preparation - mixing, chemical reaction
- insertion - tray type? adhesive?
- setting - chemical, polymerisation, cooling
- removal - short, sharp pull
- storage - how long?
- cast preparation - gypsum
what are the ideal accuracy properties of IMs?
- 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
what properties affect accuracy?
- 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
what are the ideal patient factor properties of IMs?
- non-toxic and non-irritant
- acceptable taste and smell
- short setting time
- removable without damage to oral tissues
what are the ideal operator properties of IMs?
- quick and simple technique
- convenient working and setting times
- easily decontaminated
- inexpensive
what is a colloid?
a two phase system of fine particles (1-200nm) of one phase dispersed in another phase
what is a hydrocolloid?
a two phase system where one phase of fine particles is dispersed in water
what are the two states that hydrocolloids exist in?
- as a SOL - viscous liquid state
- as a GEL - jelly-like consistency where fine particles form a framework that encapsulates a liquid
what is the equation of the setting of alginate?
sodium alginate + calcium sulfate -> sodium sulfate + calcium alginate
what component of alginate delays gel formation? how does this happen?
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
how do you manipulate 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
how can you reduce permanent distortion and tearing?
- use large bulk of material (5mm)
- pull sharply
what are the accuracy properties of alginate?
- 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 :(
what are the patient factor properties of alginate?
- non-toxic :)
- non-irritant :)
- comfortable to patient :)
- acceptable taste and smell :)
- ideal setting time :)
what are the operator properties of alginate?
- relatively easy to use :)
- reasonable setting time :)
- storage can be difficult - avoid moisture :( (expands), syneresis (release of water), imbibition (uptake of water)
