elastomeric impression materials

• polyether - addition silicones

• poly vinyl siloxanes

• material characteristics - clinical performance = patient acceptance and ease of use

• 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?

• 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

• international standard organisation

• designed to assess relevant properties of a product to see whether it meets acceptable requirements for safe and effective use

• produces a whole series of requirements and rests that materials being used on humans should satisfy before they are classified as safe and reliable

• is that grooves/indentations of either 20µm or 50µm (depending on viscosity) are replicated

• if wettability is good then impression material will not be affected by things on the tooth such as saliva

• 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

• 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

• 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

• 20µm • 50µm • 75µm

• apply impression material and see what size of notches it fills

• this means that the material meets the ISO standard if 50µm limit

• reach maximum strain • stays at maximum strain until removed • then returns to normal

• gradually strains • reaches load • releases load • gradually relaxes back but not to normal = there is a permanent strain/deformation

• 100% elastic recovery

• 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

• if load time is less then there is less overall permanent strain • lower deformation if remove impression faster and with a sharp pull

• low = small deformation

• the stress a material will withstand before fracturing

• undercut regions = want a high tear strength in these areas