final impressions and tissue retraction

lecture given 10/15/2026

impression

a negative likeness or copy in reverse of the surface of an object; an imprint of the teeth and adjacent structures

what are the requirements of an impression?

should be an exact duplicate of the prepared tooth

other teeth and tissue adjacent to the prepared tooth must be accurately reproduced

it must be free of bubbles, especially in the area of the finish line and occlusal surfaces of the other teeth in the arch

what are the types of non-elastic impression materials?

plaster, impression compound, zinc-oxide eugenol

what are the types of elastic impression materials?

hydrocolloids, nonaqueous elastomers

what are the types of hydrocolloids?

agar (reversible), alginate (irreversible)

undergo sol-gel transformation

how can hydrocolloids be distorted?

imbibition, syneresis

what are the types of nonaqueous elastomers?

polysulfides, silicones (condensation, addition), polyethers

1925 in dental impression world

1st reversible hydrocolloid- nogacoll aka denticole

1930 in dental impression world

zinc-oxide eugenol alginate

1950 in dental impression world

1st elastomeric impression material- polysulfide

1955 in dental impression world

condensation silicone

1965 in dental impression world

polyether

1970 in dental impression world

addition silicone

what are the ideal general properties of an impression material?

ease of manipulation and reasonable cost

adequate flow and wetting properties

appropriate setting time and characteristics

sufficient mechanical strength- elastic recovery and resistant to tearing during removal

good dimensional accuracy- stable over time

taste and odor acceptable to pt

safe- non toxic or irritating

no significant degradation of properties as a result of disinfection

compatability with all die and cast material

good shelf life

what is the maximum mixing time required by ADA for all viscosity elastomeric impression materials?

1 min

what is the minimum working time required by ADA for all viscosity elastomeric impression materials?

1 min

shear thinning

the tendency for viscosity to decrease when a force, or shear stress, is applied

pseudoplasticity

the shear-thinning behavior of a fluid where its apparent viscosity decreases as the rate of shear or pressure increases

thixotropy

a time-dependent, reversible property where a gel or fluid becomes less viscous (thinner and easier to flow) when shaken, stirred, or exposed to shear stress, and then gradually returns to its original thicker, more viscous state after resting

newer PVS and polyether materials have been modified to become…

thixotropic

elastic recovery

the ability to return to its original dimension without significant distortion upon removal from the mouth

relative amount of permanent deformation in compression following strain during removal increases in the following order…

additional silicone

condensation silicone

polyether

polysulfide

elastic modulus (youngs modulus)

describes the stiffness or rigidity of a material, indicating its resistance to deformation under stress before it begins to permanently change shape

stiffness of materials of similar viscosity was reported to increase in the following order…

polysulfide

condensation silicone

additional silicone

polyether

tear strength

the ability to resist tearing when subjected to tensile stress during removal from the mouth or when separated from the stone

lowest to highest tear strength…

silicones (addition and condensation)

polyether

polysulfide

dimensional accuracy

the lack of dimensional change during curing and shortly after removal from the mouth

dimensional stability

the ability to maintain the accuracy of the impression over time

t/f if a material is accurate, it is also always stable

false! materials may be accurate but not stable, or stable but inaccurate

what are the 6 major sources of dimensional change?

polymerization shrinkage

loss of condensation reaction by-product (water or alcohol)

thermal contraction from oral temperature to room temperature

absorption of water or disinfectant over a period of time

incomplete recovery of deformation because of viscoelastic behavior

incomplete recovery because of plastic deformation

wetting

commonly characterized by the contact angle, which is defined as the angle formed when a drop of liquid meets a solid surface

if a contact angle of a drop on the surface is over 90 degrees, the surface is…

hydrophobic (poor wetting)

more prone to voids/bubbles

if a contact angle of a drop on the surface is under 90 degrees, the surface is…

hydrophilic (good wetting)

contact angles of distilled water on set silicone impressions are approx ____ degrees, which makes them the most ____ among all elastomeric impression materials

100

hydrophobic

polysulfides

also known as marcaptan, thiokol, or rubber base

2 tubes- base and accelerator- low, medium, and high viscosity

base- liquid polysulfide polymer mixed with a filler

accelerator- lead dioxide mixed with sulfur and oil

moisture and temp can affect polymerization

poor dimensional stability- water by product

hydrophobic nature

bad odor and radiopaque

30-45 min to pour

condensation silicones

supplied as a base and an accelerator

poor dimensional stability- ethyl alcohol produced as a by-product evaporates and contributes to high shrinkage during the first 24 hrs after setting

setting reaction is sensitive to moisure and heat

addition silicones

polyvinyl siloxane (PVS)

extra low, low, medium, heavy, very heavy (putty)

2 pastes- upon mixing equal parts there is an addition of silane hydrogen groups with the formation of no by-products

best fine detail

good dimensional stability

the least affected by pouring delays or by second pours

release of hydrogen gas- pouring should be delayed for 30-60min

hydrophobic- surfactants when pouring to reduce voids

latex gloves can affect the setting

polyether

viscosities- low, medium, and high consistencies

good dimensional stability even when pouring is delayed for prolonged periods of time- no later than 14 days

has affinity for water- hydrophilic

imbibition

excellent reproduction of details

very stiff- undercuts should be blocked out

good tear strength

what are the advantages of polysulfide?

long working time

high tear resistance

margins easily seen

moderate cost

what are the disadvantages of polysulfide?

requires custom tray

stretching leads to distortion

compatible with stone

stains clothing

bad odor

pour within 30 min

what are the advantages of condensation silicone?

clean and pleasant

good working time

margins easily seen

what are the disadvantages of condensation silicone?

high polymerization shrinkage

volatile by-product

low tear strength

hydrophobic

pour immediately

what are the advantages of addition silicone?

clean and pleasant

margins easily seen

ideally elastic

pour repeatable

stable- delay pour

what are the disadvantages of addition silicone?

hydrophobic

no flow if sulcus is moist

low tear strength

least distortion on removal

what are the advantages of polyether?

fast setting

clean

least hydrophobic of all elastomers

margins easily seen

good stability

what are the disadvantages of polyether?

stiff, high modulus

bitter taste

need to block undercuts

absorbs water

high cost

what are the 6 major steps of making a final impression?

preparing a tray

managing tissues

preparing the material

making an impression

removing the impression and disinfection

preparation of the cast

custom trays

reduces quantity of material required

allows uniform distribution of the impression material- improves the accuracy

aids with undercuts

can be more comfortable for the pt

tray adhesive

the most consistently accurate impressions were obtained with adhesive lined resin custom trays

tray surface should be dry before applying adhesive

to ensure optimal adhesion, recommended drying time 3-15 min

tissue management

to ensure access for the tooth preparation and for making the impression, it is necessary to displace the gingival tissues, control gingival hemorrhage, and control sulcular fluids

allows for the finish line to be evaluated

allows impression material to record the finish line plus some tooth structure apically

gingival retraction cord or electrosurgical unit/soft tissue laser

retraction cord

objective is to displace the gingival tissue laterally away from the margin of the preparation

still considerede the gold-standard technique

classified according to composition (cotton, silk, yarn, wool), impregnation (w astringent, hemostatic solution, none), design (twisted, knitted, braided), and size (000, 00, 0, 1, 2, 3)

what are the uses for #000 cord?

use as lower cord in the double cord technique

anterior teeth

what are the uses for #00 cord?

preparing and cementing veneers

restorative procedures dealing with thin, fiable tissue

what are the uses for #0 cord?

lower anteriors

when luting near gingival and subgingival veneers

class III, IV, V restorations

what are the uses for #1 cord?

non-impregnaned #1 and #2 sizes are particularly effective for tissue control and/or displacement when soaked in coagulative hemostatic solution prior to and/or after crown preparations

use as protective pre-preparation cord on anteriors

what are the uses for #2 cord?

upper cord for double cord technique

use as a protective pre-preparation cord

what are the uses for #3 cord?

areas that have fairly thick gingival tissues where a significant amount of force is required

upper cord for use with the double cord technique

cord size will be determined by…

the depth of the sulcus and the amount of retration needed

chemico-mechanical methods of retraction

retraction cords may be impregnated with a hemostatic agent by dipping the cord in the solution prior to placement- minimizes gingival hemorrhage, inhibits formation of the crevicular fluid or absorbs it if already present

aluminum chloride, aluminum sulfate, epinephrine, zinc chloride, ferric sulfate

cord positioning force

strength of epithelial attachment is 1 N/mm

very low pressure of 0.01 N/mm will cause the sulcus to open and quick recovery can happen

pressure of 0.1 N/mm makes sulcus open at 1.5 mm and delays recovery to 2 minutes per 0.5mm opening

pressure applied in periodontal probing 1.31-2.41 N/mm

pressure applied to insert cord 1.5-5 N/mm

instruments for packing cord

fischer’s cord packer

smooth or serrated

what is not the technique for placing cord?

holding the cord packing instrument parallel to the long axis of the tooth pushes the cord against the wall of the gingival crevice and it will cause the cord to rebound

if the cord continues to rebound when placing…

greater pressure should be applied

smaller size cord should be used

or the wrong cord packing technique is being used

single cord technique

involves packing one retraction cord into the gingival sulcus to displace the tissue

to be used when margins are visible, shallow sulcus

area should be dry

double cord technique

uses 2 cords, a smaller one initially to control bleeding and a larger cord on top for more significant retraction

first cord- moisture control and vertical/apical retraction

second cord- horizontal/lateral retraction

tissue retraction paste

15% aluminum chloride astringent paste

easily dispensed right into the sulcus through a composite dispenser

single use capsule with extra-fine tip

can be used alone or with cords

provides constant and non-destructive pressure of 0.1 N/mm

disinfection of impression

all impressions should be disinfected upon removal from the mouth to prevent transmission of organisms to the cast or to the lab personnel

should be EPA registered disinfectant

what chemical disinfectants can be used for the disinfection of impression?

chlorine compounds

combination synthetic phenolic compounds

glutaraldehydes

iodophors

phenol-alcohol combinations

cavicide

alcohol based disinfectant, primarily isopropanol at about 17% and a quartery ammonium compound at approx 0.28%

to prevent expansion of the material spray impression with disinfectant (no more than 10 minutes), rinse and dry immediately

what can you do if you have incomplete reproduction of preparation margins?

have good moisture control

displace gingival tissue

use wash material liberally on preparation

follow manufacture’s working time specs

choose material with longer working time

use impression material with sufficient tear resistance

what can you do if you have voids on the margin?

keep syringe tip immersed in wash material to avoid entrapping air

wiggle and stir while syringing, push material forward

have good moisture control/hemostatic agent

use wash material liberally on preparation

properly bleed filled elastomer syringe

insert tray properly

what can you do if you have tearing at the margin?

displace tissue properly

use impression material with sufficient tear resistance

clean area after fabricating temp/latex gloves

check setting of impression material in peripheral areas before removal

bleed before applying mixing tip to ensure even dispensing

use recommended mixing tip

do not use expired impression material

what can you do if you have preparation margins that are complete but not sharp?

make sure there is no blood or saliva

use good retraction technique

follow manufacturer’s working time specs

use recommended disinfectants

store impression material at room temp

store final impression at room temp

what can you do if you have wash material displaced from preparation area?

make sure tray is seated within the working time of the tray material

use wash material liberally on preparation and abutments

rinse and dry the prepared area before making the impression

use good moisture control; stop bleeding by using appropriate retraction technique

what can you do if you have distortions

support tray until impression material is sufficiently hardened

balanced impression tray removal- avoid unilateral rotation causing hard distortion forces

use impression material with excellent elastomeric properties

what can you do if you have facial-oral flow defects?

use a tray that supports the flow of the material or fabricate a custom tray

use more material for it to flow

do not move tray after seating

follow manufacturer’s working time specifications

what can you do if you have show through of tray?

use proper size of tray or custom tray

fill tray adequately

what can you do if you have impression material is not completely set?

wear gloves proven to not inhibit setting

fabricate temporary restoration after making the impression

bleed before applying mixing tip to ensure even dispensing

use mixing tip according to manufacturer’s recommendations

ensure mixing tip is correctly attached

do not use expired impression material

what can you do if you have poor bond btwn tray and wash material?

follow working time specifications

store impression material at room temp

avoid relining- better to remake

avoid using latex gloves

what can you do if you have poor bond of impression material to the tray?

use tray adhesive for all types of impression trays

follow manufacturer’s instructions for application and drying time of adhesive

remove smear layer of custom trays

use stiff and rigid trays

make sure tray fits well

what can you do if you have stone cast discrepancies (powdery cusp tips, voids)?

if using PVS, wait at least 1 hr to pour cast