impression materials chapter 8 dae/dhe 203. impression materials: used to make replicas of oral...
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Impression Materials:
Used to make replicas of oral structures “Negative” replica made by impression Cast material is placed into impression to
yield a “positive” replica of the oral tissue Allows for the indirect restoration or dental
appliance to be fabricated without the patient present
Variety of materials for a variety of applications
Uses of Impression Materials:
Used in the fabrication of:
Indirect restorations: Crowns & bridges Dental prosthetics Temporary crowns
Orthodontic & Dental appliances
Acrylic trays Bite registration Study models
Characteristics of Impression Materials:
Fluid enough to flow around area of interest Must “set” in reasonable amount of time Can remove from mouth without distortion No harmful effects on tissues Relatively tasteless & odorless Dimensionally stable until a cast is created Give detailed reproduction Compatible with cast materials
Impression Trays:
Used to carry the material to pt’s mouth Must be sturdy enough to support the material Need to be disposed of or sterilized after use Metallic trays are autoclavable Stock trays available: full arch, quadrant Custom trays made of acrylic for most accuracy Triple trays takes impression of opposing teeth,
occlusal registration, and prep site Perforated trays have holes for retention of
material
Categories of Impression Materials:
INELASTIC
Rigid Cannot be used
with undercuts For edentulous or
bite registration
ELASTIC
Flexible & “rubbery” Used with undercuts Generally used today Two subcategories:
Hydrocolloids (Aqueous) Elastomers
Inelastic Impression Materials:
Impression Compound: Resins + wax Thermoplastic, solid material
Heat to soften material Cool to set material Physical change only; NO chemical reaction
Forms: plates and sticks Soften in water bath (10° warmer than body) Applied to tray and inserted in mouth to cool Used as a custom tray
Inelastic Impression Materials:
ZOE Impression Paste: Similar formulation as ZOE cements Pastes mixed together – chemical
reaction BASE + CATALYST
Applied in thin layer in custom tray for edentulous arch; or to occlusal surfaces
Sets in 4 – 5 minutes
Elastic Impression Materials:
Hydrocolloids: Reversible Hydrocolloids Irreversible Hydrocolloids
Elastomers: Polysulfides Polyethers Condensation Silicones Addition Silicones - Polyvinylsiloxanes
Elastic Impression Materials:
HYDROCOLLOIDS – “water-based” impression materials Colloid – liquid suspension of particles Particles derived from seaweed & kelp Liquid = water GEL – the rubber-like form of the
material SOL – the material in liquid solution Not dimensionally stable – must be
“poured” immediately
Hydrocolloids:
Reversible Hydrocolloid: Agar – a gelatinous material + water Thermoplastic – NO chemical reaction “Reversible” –
Goes from gel, to sol, and back to gel, by raising and lowering temperature
Dispensed in tubes & syringes Very accurate final impression material
Hydrocolloids:
Handling Reversible Hydrocolloid:Special water baths with 3 chambers are used
1) Material is immersed in boiling tank (212°F) for 10-20 minutes; turns from gel to sol
2) Place in storage bath (150°F); 10 minutes to few days; remains sol
3) Load tray; immerse in the tempering bath (110°F); 5-10 minutes; lowers temp for pt comfort
Hydrocolloids:
Handling Reversible Hydrocolloid:4) Load syringe; extrude onto prep site5) Seat tray in mouth6) Attach cooling hoses to tray7) Hold firmly in pt’s mouth until cooled
and transformed to gel state8) Remove from pt’s mouth and pour
immediately
Reversible Hydrocolloid:
Advantages:
Very accurate Excellent
compatibility with gypsum
Disadvantages:
Labor-intensive Poor tear
resistance Poor stability Need equipment
space and very organized staff
Hydrocolloids:
Irreversible Hydrocolloid: ALGINATE impression material Formed by chemical reaction Powder + water; mixed NOT reversible; SOL to GEL only Protect from inhalation Dispensed in cans or bulk packages Used when less detail is required
Hydrocolloids:
“What are indications for alginate impressions?”
Study models Opposing models of final casts Casts for mouthguards, whitening trays,
custom trays, orthodontic appliances, provisional crowns, etc.
To fabricate a direct provisional crown
Hydrocolloids:
Handling Alginate: Fluff powder and
measure accurately Measure water – temp! Mix in a flexible,
rubber bowl Stir to wet powder Strop mixture against
side of bowl to eliminate air bubbles
Mix until creamy & homogeneous; 60 sec.
Hydrocolloids:
Handling Alginate: Load alginate into tray
from posterior Press material into tray
to eliminate voids Smooth & indent
alginate with wet finger
Use extra alginate to wipe onto occlusal surfaces of teeth
Taking an Alginate Impression:
Bead the tray as necessary Stand behind patient for maxillary (11:00) Center tray above teeth and seat posterior Press tray down & toward anterior and allow
lip to cover tray; material should fill vestibule
Hold tray in patient’s mouth until set (2 - 3 min)
Loosen cheeks and lips with finger Remove occlusally with a firm snap
Evaluating an Alginate Impression:
Full coverage, including retromolar area Tray centered Clear & sharp impression No voids, air bubbles, or tears Has a “peripheral roll” (vestibular area)
If all is OK, rinse & disinfect impression, pour-up in stone/plaster, or store in humid
bag.
Irreversible Hydrocolloid:
Advantages:
Easy & economical Many applications Patient comfort Excellent wetting
by gypsum Disposable tray
Disadvantages:
Not enough detail for final impression
Must be poured immediately
Hydrocolloids:
SYNERESIS – shrinkage in impression due to loss of water from heat or exposure to air.
IMBIBITION – swelling of impression due to taking up moisture
To avoid these conditions (dimensional distortion), pour-up immediately!
Elastomers:
Two-paste systems: Two tubes – pastes mixed by hand Two putties – mixed/kneaded by hand Cartridge & Extruder Gun – automix tips used Cartridge & Motor-driven Mixer – automix tips
used Set by chemical reaction Warmth and moisture may slow setting Must use a tray adhesive (if tray not perforated)
Latex may inhibit set of polyvinylsiloxanes
Elastomers:
Elastomers are used in a two-step process: Preliminary Impression - the material used to
form the base or the “tray” material used in an impression; usually more viscous or heavy-body
Secondary or “Wash” Impression – the material applied through a syringe around the prep site for detail; usually less viscous;light or medium -body.
Elastomers:
POLYSULFIDE: Oldest elastomeric used in dentistry Two pastes – mix base with catalyst Liquid polymer with “sulfhydryl” group –
contains sulfur “rubber-base” impression material Used best with custom trays Other materials have replaced this one
Elastomers:
Handling Polysulfides:1. Equal lengths of pastes on pad2. Pastes are “swirled” together, then stropped3. Material placed in custom tray4. Take impression – hold in place for up to 15
min.5. Remove from pt’s mouth slow & steady force6. Rinse & disinfect7. Pour-up impression within several hours
Polysulfides:
Advantages:
Economical Good tear
resistance Good
compatibility with gypsum
Disadvantages:
Malodor Stains clothing Long setting time Fair stability Less accurate
Elastomers:
Condensation Silicones: Similar to silicone rubber products (but
non-toxic)
Setting by-product of alcohol “Hydrophobic” – results in voids in stone Shrinkage occurs as it sets Must be poured immediately Accurate, but slow setting time Replaced by improved products
Elastomers:
POLYETHERS: Developed in 1960’s No reaction by-product produced Shorter working and setting time Only come in a single viscosity Stiff material – can use a triple tray Very popular – “Impregum” No need to pour-up immediately
Elastomers:
Handling Polyether:1. Mix equal lengths of paste; or extrude
through the automix cartridge tip2. Load syringe and apply to tooth thru
tip3. Load tray and invert over area for
impression4. Allow to set; 4 – 5 minutes5. Remove from mouth6. Rinse & disinfect
Polyether:
Advantages:
Short setting time Single viscosity Good stability Good tear strength Clean & easy to
use
Disadvantages:
Bad taste Most difficult to
remove from mouth
Elastomers:
ADDITION SILICONES: “Polyvinylsiloxanes” – silicone polymer Two pastes or two putties Hydrophobic by nature – manufacturers
adding components to increase wettability Very accurate & fast setting Avoid contact with latex (gloves, rubber dams)
Low setting shrinkage & very stable
Elastomers:
Handling Addition Silicone: Mix equal lengths of
pastes, or automix Apply light-body
material to tooth thru syringe
Load tray with heavy-body
Set tray over prep site Set in 4 – 5 minutes Rinse & disinfect
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