Alginate Impression Materials
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what is the main function of impression materials?
to produce an accurate replica of the surface and shape of hard and soft oral tissues
what type of reproduction of the tissues does an impression give?
a NEGATIVE reproduction (indentations not protrusions)
what material is used to produce a positive replica of the teeth after the impression stage?
dental stone (stone casts)
why is a stone cast produced?
to evaluate dentition for; • orthodontic uses • occlusion problems • fabrication of restorations • fabrication of prostheses
what are the clinical classifications of impression materials?
• mucostatic - mucocompressive
what are the property classifications fo impression materials?
• elastic - non-elastic
give examples of mucostatic materials & what they are used for:
• zinc oxide eugenol & low viscosity alginates • fluid materials that displace the soft tissues slightly • give an impression of the undisplaced mucosa
give examples of mucocompressive materials and their uses?
• impression compounds & high viscosity alginates/elastomers • viscous materials that record an impression of the mucosa • give impression of displaced soft tissue
what is good about elastic impression materials?
when removing the material form the mouth, they stretch around the bulbous part of the tooth and then recovery back to the original shape of the tooth quickly (almost 100%)
in what way can the dentist prevent a high amount of permanent strain on the impression material?
remove the impression tray with a sharp and quick pull
what type of impression material is alginate?
an irreversible hydrocolloid made from agar
what are some examples of elastomer impression materials?
• polysulphides • polyethers • silicones
what would happen if a non-elastic material was used to take an impression?
on removal of the plate from the mouth, the material will become deformed or fractured
how are impression materials prepared?
• mixing of the material • chemical reaction • heat
what setting reactions can impression materials undergo & what is this dependent on?
• chemical • polymerisation • cooling DEPENDENT ON THE MATERIAL USED
what are the ideal accuracy properties required for an impression material?
• appropriate flow • setting changes • removal • storage • decontamination • compatible with cast material
when removing the material from the mouth, what is important?
that the material; • has NO effect on oral tissues • has NO dimensional change (eg thermal contraction) • has COMPLETE ELASTIC RECOVERY
what is meant by viscosity?
determines the ability to flow over surface & so good level of detail is recorded
in terms of viscosity, what is ideal for a material?
low viscosity
what should the thermal expansion coefficient be of an impression material?
ideally low (to cope with mouth to room temperature transition)
why is tear strength important when considering impression materials?
determines a materials ability to withstand tearing during removal (especially from undercuts)
what are ideal properties for an impression material in terms of patient comfort?
• non-toxic/non-irritant • acceptable taste and smell • short setting time • removable without damage to oral tissues
what are the ideal properties for an impression material in terms of operator convenience?
• quick, simple technique • convenient working/setting times • must be able to be decontaminated (cost = inexpensive… not as important)
what is a hydro colloid?
• a colloid is a TWO phase system of fine particles of one phase dispersed in another phase • therefore hydrocolloid is fine particles dispersed in water!