Impression Materials Flashcards
What is the function of impression materials
To produce an accurate replica of the surface and shape of hard and soft oral tissues
What is an impression
a negative reproduction of tissues
What is dental stone used to do
fill impressions to produce a positive replica (stone cast) which represents the position, shape, size and orientation of each tooth
What are dental stone casts used for
evaluating dentition when orthodontic, occlusal or other problems involved
in laboratory fabrication of restorations and prostheses
What does the treatment properties hinge on
the quality and accuracy of the initial impression
What does the impression material require
Impression materials with the right properties
clinician with good technique
What are the classifications of impression materials based on
Clinical
Properties
What are the clinical classification
mucostatic
mucocompressive
What are the properties
elastic
non elastic
What are mucostatic impression materials
zinc oxide eugenol, low viscosity alginates
What do mucostatic impression materials do
fluid materials that displace the soft tissues slightly
give an impression of the undisplased mucosa
What are mucocompressive impression materials
impression compound
high viscosity alginates
elastomers
What do mucocompressive materials do
viscous materials that record an impression of the mucosa under load i.e give an impression of displaced soft tissues
What happens to the impression material during removal
it undergoes elastic strain and has to change shape to overcome the bulbosity of the tooth
What do we want the impression material to do after removal
change back to its original shape making it truly elastic
What is it called when the impression material never returns to its original shape
permanent strain
it does not recover
making the impression inaccurate
What should be done to try and decrease the permanent deformation
keep the load time less by removing with a sharp pull so there is less overall permanent strain
What are the two types of elastic materials
hydrocolloids
elastomers
What are the elastomers
polysulphides
polyethers
silicones
What are the hydrocolloids
agar (reversible (lab duplicating) but no longer used
alginate - irreversible
What are the elastomers
polysulphides
polyethers
silicones (conventional and addition cured)
What happens to non elastic material on removal
don’t recover well
either deform/fracture
What is the impression process
preparation insertion setting removal storage cast prep
What happens in preparation
mixing
chemical reaction
heat
to result in a material that slows and takes up shape/space available
What happens in insertion
pick a tray
need adhesive maybe
What happens in setting
chemical
polymerisation
cooling (only some)
to form a solid replica
What happens in storage
need to see how quickly the impression needs to be poured before the dimensional changes become significant
but should wait a bit to allow elastic recovery
What are the ideal properties needed for
accuracy
patient
operator
What are the properties for accuracy
flow setting changes removal storage decontamination compatible with cast material viscosity setting mechanism thermal expansion coefficient hydrophobic/hydrophilic elasticity tear strength
What is the ideal property for flow
that it can flow over the dentition and make intimate contact with the surfaces and record surface detail
What is the ideal property regarding setting changes
we don’t want it to shrink nor expand, we want it to remain the same dimension
What is the ideal property for removal
no effect on oral tissues
no change dimensionally
complete elastic recovery
What are the ideal properties for storage
does not change dimensionally over time (moisture) - a weak point for alginates
What are the ideal properties for decontamination
dimensions and surfaces unaltered
Why is viscosity important
determines the ability to flow over surface and so level of detail recorded
What is the ideal property for setting mechanism
does physical/chemical process = dimensional contraction
What is the ideal property for thermal expansion coefficient
ideally low
to cope with mouth to room temperature transition
What is the ideal property for hydrophobic/hydrophilic
affects surface contact because saliva will be present on the tooth
What is the ideal property for elasticity
elastic/viscoleastic behavior
for undercuts so that it won’t fracture
What is the ideal property of tear strength
withstand tearing when removed especially from undercuts
What are the ideal properties for patient comfort
non-toxic
acceptable taste and smell
short setting time
removable with 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 (inexpensive) is a consideration BUT not a property
What is a colloid
two phase system of fine particles (1-200nm) of one phase dispersed in another phase
What is a hydrocolloid
colloid dispersed in water
What are the two states of a hydrocolloid
sol
gel
What is the sol phase
viscous liquid state
What is the gel phase
jelly like consistency
What is the gel phase due to
agglomeration of particles
entangled framework of solid particles enclosing liquid by capillary forces
What is the composition of alginate
salt of alginic acid calcium sulphate trisodium phosphate filler modifiers flavourings chemical indicators
What is the function of the salt of alginic acid
it reacts with calcium ions
What is the function of calcium sulphate
provides calcium ions
What is the function of trisodium phosphate
delays gel formation
What is the function of filler
for cohesion and strength
What is the function of modifiers, flavourings and chemical indicators
improve the surface
taste
pH color change (to show transition from sol to gel)
How does trisodium phosphate delay the setting reaction
calcium ions interact with the phosphate ions and it is only when the phosphate ions are used up that the remaining calcium ions react with sodium alginate
What happens in the setting reaction
there is long cross linked fibril entangling undissolved particles
What is the setting reaction
2NanAlg + nCaSO4 –> nNa2SO4 + CanALg
How does calcium ions result in cross linking
because they’re divalent
What is the alginate delay
trisodium phosphate preferentially reacts with Ca ions in CaSO4 then 2NanAlg reacts with Ca ions
How can alginate be manipulated
use correct powder/liquid ratio use water at 18-24 degrees use perforated tray and adhesive remove impression with a sharp pull use in bulk
Why does alginate have greater elastic properties if a minute or two is waited
cross linking continues after apparent set
Why does alginate set quicker on soft tissues
because an increased temp speeds up setting
Why should a bigger bulk of alginate be used (5mm)
as permanent distortion and tearing is slightly reduced
What features of alginate allow for patients comfort
non toxic, non irritant
acceptable taste and smell
setting time is ok
What features of alginate allow for operator convenience
easy to use
okay setting time
storage - need to avoid moisture but should be left with damp gauze. prepare cast ASAP
what is syneresis
release of water
what is imbibition
uptake of water
What is the issue with storage of alginate
it can release water but also take on water
could distort it
How is the flow for alginate
okay
How is the setting changes for alginate
ok
What happens if the patient is moving during setting
can result in internal stresses that lead to distortion
How is the elastic property of alginate
not 100% elastic recovery
How is the tear strength of alginate
poor
avoid deep undercuts
use in bulk
How is the storage of alginate
syneresis or imbibition may effect dimensions/accuracy
