Biomaterials- Chapter 5

impression materials are used to:

• obtain an impression of teeth, surrounding oral tissues, or both

impression techniques:

• traditional/material based impressions

• digital impressions

classification of material based impressions:

• preliminary

• final

• occlusal (bite registration)

preliminary impressions

• taken either by the dentist or licensed assistant

preliminary impressions: used for

• diagnostic models/casts

• custom trays

• provisional coverage- (crowns)

• ortho appliances

final impressions

• taken by dentist

• used to produce the most accurate reproduction of the teeth and surrounding tissue

final impressions: used for

• indirect restorations (made outside mouth and put back in)

• partial or full dentures

• implants

bite registrations

• by dentist or EFDA to produce a reproduction of the occlusal relationship of the maxillary and mandibular teeth when the mouth is occluded

• provide an accurate registration of the patients centric relationship between the maxillary and

impression trays

• used to hold impression materials when impression are taken

impression trays: need to be ridged to

• carry the impression material into the oral cavity

• hold material close to the teeth

• avoid breaking during removal

• prevent warping the completed impression

2 basic types of impression trays

• stock trays

• custom trays

stock trays: characteristics

• disposable, start with the bottom

• quadrant, section, full-arch

• perforated or smooth surface (rim lock)

stock trays: sizes

• comfortable

• extends slightly beyond the teeth 2-3 mm

  • mand- retromolar pad

  • max- maxillary tuberosity

stock trays: adaption

• if size is incorrect, utility wax can be used to extend

hydrocolloid materials

• used to obtain preliminary and final impressions

• hydro- water

• colloid- gelatin substance

• depending on the type of hydrocolloid, the physical change from sol (solution) to gel (solid) can be irreversible (changed by chemical factors) or reversable (changed by thermal factors)

alginate is

• the irreversible hydrocolloid most widely used for preliminary impressions

irreversible hydrocolloids: definition

• hydrocolloid impression materials that cannot return to the sol state after they become a gel are

physical phases of alginate

• sol (solution) phase: liquid or semiliquid form

• gel (solid) phase: semisolid

• the gel strength of hydrocolloid is not as great as that od electrometric impression materials

strength of alginate

• must be completely set before removal

• strength increases over time

• leave in the mouth for full recommended time

dimensional change

• can be caused by slight changes in environment

• imbibition

• syneresis

inhibition

• water absorption causes expansion

syneresis

• open air shortage causes shrinkage

packaging and storage of alginate

• containers about the size of a coffee can

• premeasured, individual packages

• will deteriorate very quickly if exposed to elevated temperatures or moisture

alginate shelf life

• 1 year

types of setting and altering setting time

• normal-set alginate

• fast- set- alginate

• working time: time for mixing alginate

• setting time: time required for chemical action to be required

• using water cooler than room temps will lengthen the setting time; warmer water will shorten the setting time

water-to-powder ratio

• accurate measuring is important

  • always use the scoop from the manufacture

how many scoops- from adult mandibular impression

• 2 scoops of powder

• 2 measures of water

how many scoops- from adult maxillary impression

• 3 scoops of powder

• 3 measures of water

alginate rules

• within an hour of it being poured

• 1 year shelf life

• wear PPE

• 1:1 ratio

• don’t pack like brown sugar

• posterior to anterior when placing

liquid or powder first: alginate

• powder into liquid

mixing techniques

• main one: manual spatulation w rubber bowl and beavertail wide spatula

• additional: use of alginate, which is a rubber flexible bowl attached to an electrical component with a low or high speed

elastomeric materials: characteristics

• (when sending to lab)

• used- for accurate impression is essential

• self curing

• supplied as a base and a catalyst

• always supplied together and should be used that way

forms of electrometric impression materials:

• light bodied material

• regular and heavy bodied material

light bodies material

• “syringe type or wash type”

• can flow into and around the details of the prepared tooth

• grown

regular and heavy bodied material

• thicker than light bodied

• used to fill the tray

3 elastomeric materials: curing stages

• initial set

• final set

• final cure

initial set

• first stage results in stiffening of paste without the appearance of elastic properties

final set

• second stage begins with the appearance of electricity and proceeds through a gradual change to a solids, rubber mass

final cure

• last stage occurs within 1-24 hours

types of elastomeric materials

• polysulfide

• polyether

• silicone

• polysiloxane (polyvinyl siloxane)

polysulfide

• rubber base; relatively long working and setting time

• dentures/partials

polyether

• stiff; required a thinner

• (included)

• not used much

silicone

• odor-free

• nonstaining, relatively easy to mix

polysiloxane (polyvinyl siloxane)

• high dimensional stability

• low tear resistance

• no taste or odor

digital impressions

• replacing traditional, material-based impressions

• use lasers or other light sources to scan teeth and surrounding structures

• generates a precise replica of the oral cavity without unpleasant taste

• techniques:

  • digital photograph

  • digital video

digital impression: advantages

• patient comfort/better communication/less anxiety

• improved quality/better fit

• quick and accurate

• electronic storage of scans/efficient record-keeping

digital impression: disadvantages

• high initial investment

• periodic software updates and routine maintenance

• initial learning curve is steep

• some discomfort for patients w small mouths

• records debris, saliva, hanging margins

• may require additional training

digital impressions

• use all the time for assistants

• insurance now asks for these

types of impression materials

• elastic materials

• inelastic materials

elastic materials

• most common

• alginate

• polyvinyl sioxane (PVS)

• polyether

inelastic material

• not used much today anymore

• impression wax etc.

key properties of impression materials

• accuracy

• tear resistance

• dimensional stability