what are meow types of elastomers
- polyether
- addition silicones
what are addition silicones also called
- poly vinyl siloxanes
what do you need to judge impression materials by
- material characteristics
- clinical performance = patient acceptance and ease of use
how to assess materials
- look at source
- look at product
- look at the evidence= look at actual data not just claims
- reliability = need to be able to trust material
- ask questions = were the people asked paid or independent, how many people were asked, why were people not satisfied with it?
what properties of the material do you want to know about
- flow/viscosity
- surface detail = reproduction
- wettability = ability to make good contact with tooth
- elastic recovery = %
- stiffness = flexibility (want flexibility for impression materials and rigid for composite)
- tear strength = only applies to impression materials
- mixing time
- working time
what is ISO
- international standard organisation
what is the dental standard for ISO
- designed to assess relevant properties of a product to see whether it meets acceptable requirements for safe and effective use
what do ISO do
- produces a whole series of requirements and rests that materials being used on humans should satisfy before they are classified as safe and reliable
what is the ISO standard for impression materials
- is that grooves/indentations of either 20µm or 50µm (depending on viscosity) are replicated
what is wettability
- if wettability is good then impression material will not be affected by things on the tooth such as saliva
what properties are important for impression materials
- quality of surface interaction between material and tooth tissue = viscosity and surface wetting and contact angle
- accuracy = surface reproduction, viscoelasticity/elastic recovery
- handling with removal and undercuts = tear strength, rigidity
- dimensional stability
what is important with viscosity
- material must be able to flow readily
- viscosity determines a materials potential for making close contact with hard and soft tissue surfaces= how well it records surface detail
- range of viscosity= low, medium and high
what is wanted with wettability
- must make intimate contact with teeth/mucosa
- interface between impression material
- if don’t have good contact then will get a bubbly/rough surface and will be a poor model to work with
- impression material globules sit and spread out on the surface and if there is not a lot of gaps between globules then that is good, but if there is then that is poor wettability
what are the standard notch dimensions
- 20µm
- 50µm
- 75µm
how is the surface detail reproduction determines
- apply impression material and see what size of notches it fills
what is the surface reproduction limit if material flows into notch size 50µm and 75µm but not 20µm
- this means that the material meets the ISO standard if 50µm limit
what is the ideal behaviour of elasticity
- reach maximum strain
- stays at maximum strain until removed
- then returns to normal
what is the impression material elastic recovery
- gradually strains
- reaches load
- releases load
- gradually relaxes back but not to normal = there is a permanent strain/deformation
what is the ideal elasticity score
- 100% elastic recovery
what is viscoelastic behaviour of impression material
- no materials are truly elastic, all are viscoelastic
- when impression material, having been stretched/compressed on removal from mouth, fails to return to its original dimensions
- there is a permanent deformation
how can the permanent deformation be reduced
- if load time is less then there is less overall permanent strain
- lower deformation if remove impression faster and with a sharp pull
what is the ideal viscoelasticity
- low = small deformation
what is the tear strength
- the stress a material will withstand before fracturing
where is the impression material most likely to tear and fracture
- undercut regions = want a high tear strength in these areas
