lecture 7- impression materials and tech Flashcards
used to make initial impressions from mounted casts PRIOR to fixed proths work
alginate
alginate is ___based and
water based
irreversible when water is added
softens under heat and solidifies when cooled
reversable hydrocolloid
chemical reaction that cannot revert to preset state
irreversible hydrocolloid
There is no chemical change making the
changes permanent. Therefore, this is
reversible
reversable hydrocolloid
➢ Chemical change is permanent. Therefore,
this is non-reversible
➢ Alginate is in this category
irreversible hydrocolloid
Alginate is ____, so moist tissue surfaces are not a hindrance to the
impression unlike PVS materials.
hydrophilic
any substance consisting of particles
substantially larger than atoms or ordinary molecules but too small to be visible to the unaided eye
-agar or alginate dissolved in water
colloidal materials
a process that converts a colloidal solution into a gel-like network:
agar: reversible due to _______
alingate: irreversible due to:____-
sol to gel
agar: temp change
alginate: chemical rxn
3 properties related to gel network
- evaporation (shrinkage)
- syneresis: get relaxation; water extruded from gel (shrinkage)
- imbibition: absorption of water causing expansion
this reacts with calcium ions and is the powder
Na alginate (or potassium alginate)
reacts with Potassium Alginate to form a dihydrate insoluble alginate gel
-it is the gel
calcium alginate
powder to liquid rato is used. add what first?
avoid incorporating what into mix?
water first
avoid mixing in air
alginate is provided as a powder, which contains the following reacting ingredients:
Potassium or Sodium Alginate(15-20%)
and
Calcium Sulphate Dihydrate (14-20%):
what is alginate mixing time
45-60 seconds
based on fast set or regular set so dont be slow!
what is the thickness of alginate should be
3mm in tray
After pouring the impression, the stone cast
should not be separated for:
with yellow stone
60 mins with yellow stone
CDC recommend _____ dilution of household
bleach for disinfection
1:10 ratio
Impression placed in small what for transfer and so not dry out
Ziploc baggie
or head rest cover with a few drops of
water in the bottom of the baggie and
is sealed.
the
two major factors that affects the shelf
life of the impression
storage temp and moisture
If you remove your impression from the
mouth, rinse it off, and then wrap the
impression with a wet paper towel, what
will happen?
alginate absorbs more water as it continues to harden and the material swells
now anything made from that impression will NOT fit because the model has distorted as it absorbed water
placing in bag with 2-3 drop sof water sealed allows
alginate to not dehydrate as it sets, but does
not provide so much water as to cause further
absorption and distoration
(alginate is hydrophilic- water loving)
The diagnostic cast should be removed from the impression approximately
_______ after its final set.
one hour
What is a Dental Impression for
Fixed Prosthodontics?
analogue impression
-A negative likeness or copy in reverse of the surface of an object
-An imprint of the teeth and adjacent structures for
use in dentistry
-Made from soft semi-fluid material allowed to set
analogue impression
method and manner used in making a negative likeness
impression technique
device that is used to carry, confine,
and control impression material while making an
impression
impression tray
What do we look for in a dental impression?
-Exact duplication of the prepared teeth
-Includes uncut tooth structure beyond finish line
-Accurate reproduction of the other teeth and tissues
-Free of air bubbles specifically in the finish line and occlusal surfaces
-Extended enough to capture the vestibule,
tuberosities, and retromolar pad is ideal
Impression Tray Selection Criteria
1. Extended slightly beyond the _____ surfaces of the teeth
2. Extended approximately_____beyond the 3rd molar, retromolar, or tuberosity
3. Sufficiently deep to allow _____ of material between the tray and the occlusal/incisal
edges
- facial/buccal
- 2 -3 mm
- 2 -3 mm
Tray must be sufficiently rigid to:
- Carry the impression material into the oral cavity
- Hold the material in close proximity to the teeth
- Avoid breaking during removal
- Prevent wrapping of the completed impression
- Plastic or metal; perforated or rimmed
- Complete arch, quadrant, or dual arch
- Dentate or edentulous arch
stock trays
- Made in advance
- Light cured, auto cured, or thermoformed
- Better fit than stock trays
- Less material waste
- Controlled thickness of material
- Increased accuracy
custom trays
used to prevent impression
distortion due to material pulling away from
tray
adhesive
provides chemical adhesion
adhesive
adhesive materials:
1
2
3
- Specific for each material; including Alginate
- Should always be used
- Apply and dry
what is the goal of impression technique for single crown
fully or slightly overextended impression of
the arch, with all anatomical landmarks
what is the purpose (or what we want to see) for good impression technique for single crown
- Evaluate arch form
- Anatomical landmarks
- Musculature
- Hard and soft tissue anomalies
- Restorative space
The material selected is dependent on the type of
impression required for the procedure
tissue retraction for impression technique of single crown
single or double cord technique; cord/s removed right before impression making
for single crown, the custom tray with dried adhesive is loaded with medium or heavy body material and seated with some downward pressure for how long
once set, it is remove quickly how
impression is than disinfected and inspected
4-6 mins
gently and straight as possible
what impression material is
1.rigid, cannot be removed from undercuts
2.best for edentulous impressions
nnonelastic
what impression material
1. can be removed from undercuts without distortion
2. best for dentate impressions
elastic
The first impression materials were popular in the previous century
* Plaster of Paris
* Impression plaster
* Gutta percha
* Modeling compound
* Zinc Oxide Eugenol
* Various waxes
Less commonly used now with the exception of recording atrophic edentulous ridges or flappy ridges
nonelastic/rigid
Elastic Materials examples
Reversible Hydrocolloid (AGAR) – Circa 1925
Irreversible Hydrocolloid (Alginate) – Circa 1940
Polysulfide Rubber – Circa 1950
Vinyl Poly Siloxanes – Circa 1970
Polyether – Circa 1975
- Water is the major component
- Hydrocolloid
- Reversable and irreversible
- Reversable is not commonly used today
aqueous elastomeric (hydrocolloids)
(elastic)
- Polysulfide
- Vinyl Polysiloxane (silicone)
- Polyether
none-aqueous elastomeric (polymers)
(elastic)
ability of liquid to flow over a solid surface
wettability
wettability is related to surface energy of the ____ and surface tension of ____
energy= solid
tension= liquid
contact angle of wetting:
1: hydrophobic:
2: hydrophilic:
- hydrophobic: contact angle >90 degrees [surface not wetted]
- hydrophilic: contact angle <90 degrees
[surface wetted]
non-aqueous elastomers provide:
1
2
3
4
- accuracy
- dimensional stability
- tear resistance
- viscoelasticity
Ability to replicate the intra-oral surface details
accuracy
Ability to retain its absolute dimensional size over time
dimensional stability
ability to resist tearing in thin sections, such as through the feather edge
material within the gingival sulcus
tear resistance
Characteristic of a solid that behaves as an elastic,
solid, and a viscous liquid
* This term relates to the set impression
viscoelasticity
the mechanical properties for viscoelasticity depend on LOADING RATE:
1
2
- viscus nature improves tear strength and elastic limit
- quick removal, decreased distortion
Elastic polymers: come into different consistencies;
light medium heavy and putty
3 elastic polymers
PS- polysulfide or rubber base
VPS- vinyl polysiloxane
PE- polyether
non-aqueous elastomers chemical setting reaction:
- condensation polymerization
by product: H2O or OH - addition polymerization
no by product
these materials are synethetic rubber and are distinguished from hydrocolloids which are water based (aqueous)
non aqueous elastomeric
Non aqueous elastomeric materials offer potential solutions to the two main problems associated with hydrocolloids (alginate):
poor tear resistance and poor dimensional stability
It is known also as Mercaptan or simply rubber base
and provided as two tubes of base and accelerator
polysulfide
polysulfide requires this type of tray and negatives of it:
custom tray
longer setting time, smells and stains
polysulfide base:
polysulfide polymer mixed with inert fillers
polysulfide catalyst
lead dioxide mixed with small amounts of sulfur and act as oxidation initiator
Elastomeric Impression material we use now
vinyl polysiloxane
vinyl polysiloxane uses special what:
and provided as an _____unit, syringe, and mixing tips
special tray adhesive
auto-mix unit
overtime this happens with vinyl polysiloxane
shrinkage
vinyl polysiloxane is (more/less) flexible
more so more chance for distoration
Best dimensional stability among impression materials (addition silicon)
vinyl polysiloxane
Wait 20 – 30 min. before pouring for stress relaxation. It can be
delayed up to 7- 10 days
-Stiffness makes removal of the tray more difficult
vinyl polysiloxane
vinyl polysiloxane composition of addition______
silicon
base of vinyl polysiloxane:
Poly Dimethyl Siloxane
catalyst of vinyl polysiloxane
Polyvinyl siloxane - Platinum salt
Vinyl Polysiloxane: addition reaction, NO____
by-product
vinyl polysiloxane auto-mix:
-uniform mixing
-less air
-less mixing time
vinyl polysiloxane hydrophobic:
-dry field needed
-surfactant added; improve wettability and less cast bubbles
in vinyl polysiloxane, sulfur inhibits the polymerization so
later gloves retards the setting
this impression material is elastic and recovers from undercut- can flex and return to shape
vinyl polysiloxane
this is released during setting as secondary rxn, NOT a by product
in vinyl polysiloxane
H2
- Hydrophilic
- High elastic recovery
- It is provided as hand, auto-mix unit, or electronic mixing
- Light, medium, and heavy body consistencies
- Difficult removal from mouth or cast
polyether
Need to block out undercuts
more stiff than VPS
which elastomeric impression material
polyether
this elastomeric impression material has
1. high elastic recovery
2. hydrophilic
3. hand, automix unit, or electronic mixing
polyether
in polyether, when removing impression,
break the seal and rock slightly to prevent
tearing
Water, saliva, and blood affects the material; increase the marginal
discrepancy
* Increased water absorption occurs if a thinning agent is used
what material?
polyether
polyether composition
base
catalyst
cross-linking agent
Base: Polyether
Catalyst: Glycol based plasticizers
Cross-linking agent: Sulfate
Filler: Silica
polyether is addition reaction (with/without) by product
without (no)
this Elastomeric impression material has short setting time, excellent accurancy and dimensional stability (multiple pours and delayed pours)
polyether
The preferred elastomeric impression materials on the market are
PVS and PE
known for their hydrophilic properties and good flowability
polyethers
are known for their excellent elasticity, high
tear strength and stability.
pvs
have a high resistance to tearing but stretch and do
not recover completely elastically
PS
lends itself to
taking final denture impressions or full-arch impressions of multiple crown
preparations more easily
PS
voids unacceptable
caused by
lifting intra-oral tip while syringing or presence of moisture
tacky unset VPS material in the rep area
cause:
early removal or Sulfur contamination from gloves
Disinfectant soaked paper towel for 10 min.:
* No adverse effect on
VPS
better to rinse and spray impression
Potential expansion with increased exposure time for
PS and PE
to atraumatically displace gingival tissues to allow access for impression material
to record the finish line and provide sufficient thickness of the impression
tissue retraction
tissue retraction techniques
- mechanical
- chemo-mechanical (impregnated cord)
- surgical (electrosurgery, rotary curettage handpiece, laser)
classification of retraction cords
- depending on the configuration: twisted, knitted, plain
- depending on surface finish: waxed or unwaxed
chemo-mechanical technique:
Pack the retraction cord, soaked in hemostatic agent, then lightly dried; use cord packing instrument to place
double vs single cord technique
double: two knitted cords with different diameters are used. smaller is deeper
single: one cord
remove cords after_____. impression material is injected into sulcus once cords are removed
4 mins
capturing the optical image directly of
the patient’s anatomy or indirectly of a definitive
cast of the anatomy
digital scan
