elastomers Flashcards
2 types of elastomers
polyethers
addition silicones
elastic behaviour is when
material Fits around tooth and on removal changes shape to overcome bulbous part of the tooth to allow removal
why are clinical trials often not carried out for dental materials
expensive to conduct
how should you evaluate a product information brochure
list information manufacturer thinks dentists are interested in and/or what they want the dentists to hear and focus on
- Tells you a positive story as want you to buy it
Need evidence
Pick out key points
Look at the source and evidence - Where they independent or paid to assess - Questions left unanswered Unheard % were they unhappy with product - How did they measure accuracy - Reliability
name 8 common material properties that can be assessed
flow /viscosity
surface detail (reproduction)
contact angle / wettability
elastic recovery (%)
stiffness (flexibility)
tear strength only for IM
mixing time (min)
working time (min)
2 different methods for preparing elastomers for use
Two cartridges – catalyst and base paste, mixed in the spiral tips
OR
Two tubs of PUTTY like materials, one the base, the other the catalyst putty
- Mix putties until homogeneous colour
ISO standards
International Standard Organisation
Dental Standard – designed to assess relevant properties of a product to see whether it meets acceptable requirements for safe and effective use
industries use ISO for their produce as a badge, sign of quality
- But really just saying it is safe to use
ISO standard for impression materials
is that grooves/indentations of either 20um or 50um (depending on material viscosity) are replicated
how could good wettability of an IM be described
“they spread easily and adapt smoothly to dentine and moist oral tissue”
- The IM will not be deturbed from foreign material on tooth surface
- E.g. saliva
- Will not leave a void between them
ideal qualities of impression materials
Quality of surface interaction between material & tooth/soft tissue surfaces
- Viscosity
- Surface wetting (wettability)
- Contact angle (BDS3)
Accuracy
- surface reproduction (ISO)
- visco-elasticity / elastic recovery
Dealing with removal & undercuts
- Tear/tensile strength
- Rigidity
Dimensional stability
- Setting shrinkage
- Thermal expansion/contraction
- Storage
contact angle
Determines how well the material envelops the hard/soft tissue surface (to record fine detail)
viscosity
a measure of material’s ability to flow
determines a material’s potential for making close contact with hard/soft tissue surfaces
- so how well it records surface detail
Globules of impression materials spread out so minimal gaps between modules
- Big gaps = poor surface wetting, so not reliable
range: low, medium, high
surface wetting (wettability)
surface reproduction (ISO4823) few notches in substrate with standard notch dimensions A = 20um B = 50um C = 75um
Apply force after applying IM Push into notches - Flows readily into gap C and B ------B is the ISO norm Doesn’t flow into A
visco-elasticity/ elastic recovery
ideal: Material should relax back to normal once removed
- 100% elastic recover
Or recovery from deformation
- No permanent strain
IDEAL – low viscoelasticity (small deformation)
when impression material, having been stretched/compressed on removal from mouth, fails to return to its original dimensions/shape
- there is permanent deformation (permanent strain)
- not truly elastic
- always some deformation or permanent strain
Elastomers are not ideal
- there is gradual increase in strain over time; gradual recovery which is incomplete
impact on visco-elasticity is load time is less
Viscoelastic Behaviour
- influences tray removal method
If LOAD time is less -
- impression removed with a sharp pull
- less overall permanent strain (lower deformation)
2 material properties which influence how material deals with removal and undercuts
tear/tensil strength
rigidity
tear/tensile strength
Stress material will withstand before fracturing
Especially in undercut regions
– tear readily here
rigidity
Stress/strain ratio
RIGID - large stress needed to cause material to change shape by 1%
- Composite resins
ideally impression material is FLEXIBLE
- has low rigidity, to ease its removal from undercut /interdental regions
what is a material property which is critical for accurate impressions
elastic recovery
if a dentition has many undercuts what standard of tear strength is it advisable to use
high tear strength
5 factors to go through when deciding what material to use
Know the key properties of the material
Review product specification data
Know typical values expected for specific properties
Identify properties not mentioned
- What have they missed out
- Is it important/of note
REJECT claims not supported with scientific and/or clinical data
