lecture 4 - impression materials
TRANSCRIPT
IMPRESSION MATERIALS
Impression Materials
Are used to form replicas or copies of teeth and other oral structures.
The impression is a negative reproduction while the model or cast is the positive reproduction.
Objective is to copy accurately the teeth and other structures in the oral cavity.
Properties of Good Impression Material
Good flow property Good dimensional stability
(don’t shrink or expand after setting)
Reasonable cost Easy to manipulate Biocompatible Should have adhesive
property Compatible with the cast
Good storage life Palatable taste Appropriate setting time No toxic agents Produce creamy
consistency Enough strength to
withstand withdrawal from the patient’s mouth
Setting Mechanism
Impression materials can set by means of reversible or irreversible reactions.
Irreversible Reaction implies that chemical reaction have occurred and that the material cannot revert to its present state, example: alginate, ZOE impression paste, impression plaster and elastomeric impression materials.
Reversible materials softened under heat and solidify when they are cooled, with no chemical change taking place, example reversible hydrocolloid and impression compound.
Impression Trays Are appliance used to reach the patient’s mouth Kinds of trays1. Stock Tray – used for modelling compound for preliminary
impression2. Perforated Tray – used when impression materials do not
adhere to the tray3. Rim-Lock Tray 4. Individual Tray – custom built; self-made, used in final
impression for study cast1. Shellac base plate2. Type II modelling compound3. resin
5. Water-Cooled – used for agar impression6. Disposable Tray – made of plastic
Impression Trays
Stock Tray
Perforated Tray
Rim-Lock Tray
Disposable Tray
Individual Tray
Types of Impression Taking
Single Impression Double impression
Preliminary impression Final impression/secondary “wash”
More accurate Corrects the defect of preliminary impression
Classification of Impression Materials According to Manner of Setting
Thermoset Set with chemical reaction Irreversible Examples are soluble plaster, ZOE, alginate,
irreversible hydrocolloid Thermoplastic
Set with change in temperature Reversible Examples are modelling compound, wax, agar
Classification On Impression Materials According to Mechanical Properties
Rigid/Inelastic (thermoset) Elastic
Rigid/Inelastic (Thermoset)
Used for taking secondary impression Used for edentulous patients (no undercuts) WP ratio is .50-.75 Examples: compound and wax Manipulation
Same as other gypsum products After mixing, pour on impression trays, tap a little to release
air bubbles, then seat the tray inside the patient’s mouth.
Manner of Withdrawal: teasing movement
Elastic
For edentulous patients (for undercuts) Hydrocolloid
Reversible – agar Irreversible – alginate Rubber impression material (elastomers)
Polysulfides, polyethers, silicone:condensation and addition
Manner of Withdrawal: sudden pull, snap jerk Parallel to the long axes of the tooth
After impression taking, wash it in running water to remove saliva, blood (for patients with periodontal problem) and food debris Presence of such substances can alter the
accuracy of the cast After washing, shake off excess water Optional; put some separating medium
before cast construction: varnish, soapsuds, lacquer
Composition of Soluble/Impression Plaster 80% Plaster of Paris ( ß-hemihydrate) k2SO4 – speed up setting time (3-5 mins.) Potato Starch
Makes the plaster “soluble”) Helps in separating cast and impression material when
submerged in hot water (soluble plaster swells when placed in hot water)
Coloring Materials Helps in interpreting the impression Provides ease in reading the impression For easy identification of impression from cast
Flavoring Material For palatability
Methods of Cast Construction
1. Boxing Method
2. Inversion Method
3. Rubber Base Former
Boxing Method
Place a 1” wax strip around the tray Pour a cast material on tray and level it to the
wax Let it set, remove the wax and separate the
cast after setting
Inversion Method
Pour cast material on tray Pour the excess on a glass slab or on the
tiled working table Invert the impression tray on slab or table Scrape flowing cast material towards the tray
to form the base
Rubber Base Former
Instead of inverting on a slab or table, invert on a rubber base former.
Properties of Good Cast
No porosities or no nodules (no bubble formation)
No distortion: copy accordingly the given impression
Dimensionally stable
Impression Compound
Also called modeling compound, is supplied in the form of sheets and sticks.
This compound is softened by heat, inserted in an impression tray, and place against tissue before it cools to a rigid mass.
Its primary indication for use has been making an impression of the edentulous ridge.
Types of Impression Compound
Type I True impression compound Has high flow property For preliminary impression
Type II Tray compound More rigid For individual tray
Components of Modeling Compound
Beeswax and Thermoplastic Resin Responsible for thermoplastic property
Shellac, Gutta Percha, Stearic Acid Act as plasticizer which can improve workability
French Chalk, Talc, Diatomaceous Earth Fillers
Hardening agents, improve the strength
Coloring Pigments and Flavoring Agent
Types of Modeling Compound
Cake Form Stick Form Cone Form
Uses of Modeling Compound Cake Form: used for full arch impression during preliminary
impression. Stick Form: used for single tooth impression known as
impression tube with the se of copper band (matrix band). Cylinder in shape, open on both ends Come in different sizes Should fit the tooth properly
Used to make individual tray construction Used to border molds
Makes the rim higher Serve as a wedge material to hold the matrix in place:
orangewood stick
Manipulation
Moist Heat Method Dry Heat Method
Moist Heat Method
Use of water bath 50-70°C Get the modeling compound (cake form) Immerse in the water bath
Because of poor thermal conductivity or poor heat transfer, the outer surface softens while the inner surface remains hard
Knead with fingers to expose the inner layer Repeat until the modeling compound is
homogenously soft inside and out.
Dry Heat method
Use of open flame for small amount of modeling compound (stick form)
Modeling compound are heated to become soft and not to be melted Important ingredients are lost during melting Overheating – sparks indicate that some
components (plasticizers) are leached out. If during heating, the modeling compound does
not exhibit shiny surface, discard it (dullness indicates that the plasticizers are lost already)
Properties of Modeling Compound
Poor thermal conductivity Dimensional change: shrinks at 0.3-0.4% With good flow property Advantage – enables us to get a more detailed and
accurate impression Disadvantage – if you fail to construct the cast
immediately after withdrawing the impression from the patient’s mouth, the continuous flow property can be a source of error
Cast Construction and Separation
Wash in running water after withdrawal No need for separating medium Mix plaster of paris to make a study cast Immerse in a hot water bath
MC softens, if MC sticks to the cast, soften a piece of MC and allow it to come into contact with the melted MC.
Zinc Oxide Eugenol (ZOE)
The reaction between zinc oxide and eugenol yields a relatively hard mass that possesses certain medicinal advantages, as well as mechanical property benefits, for some dental operations.
This type of material has been involved in a wide range of applications in dentistry, including use as an impression material for edentulous mouths, a surgical dressing, bite registration paste, temporary filling material, root canal filling material, cementing medium, and temporary relining material for dentures.
Uses of ZOE
Secondary impression for edentulous ridge Surgical dressing
After periodontal surgery, open wounds are covered with ZOE to allow healing (medicament covers the wound from debris)
Stabilizes occlusion rims Temporary filling material
When there’s pain When time is not enough When there is rampant caries
Uses of ZOE
Temporary relining material for loose dentures Dentures become loose because the bone
resorbs Remedy: relining or rebasing depends on the
extent of looseness
Relining
Relining – resurfacing the tissue side of a denture in order to compensate for changes in the soft tissue occurring during the wearing of the denture and to achieve an accurate fit
Use ZOE but temporary only because it is soluble by oral fluids
Use Resin – permanent reliner
Rebasing
Heating the entire denture base because of too much looseness
Uses of ZOE Root Canal Sealer
During root canal treatment, ZOE is used to cement gutta percha sticks inserted into the canal
ZOE is a sealer or obtundant material Cementing Medium
To cement crown preparations, onlays, orthobrackets Onlays – with missing cusps, restorations that are fabricated
outside the mouth, they must be cemented on the prepared cavity
Inlays – restorations are confined within the walls of the cavity 2 varieties
Weak – for temporary cementing medium Strong – for permanent cementing medium
Finished Preparation - MOD porcelain onlay preparation
Uses Of ZOE Pulp Capping Agent
When there is excessive cavitation Depth of Cavitation Class A – depth is 0.2-0.5 mm beyond DEJ; ideal depth of cavity Class B – a little beyond 0.5mm Class C – no pulp involvement yet deeper than class B Class D – with pinpoint pulp involvement
Pulp capping is done on Class C and D For class C – use ZOE, indirect P.C. procedure For class D – not ZOE but a cement-based capping agent
(Calcium hydroxide) direct pulp capping
Uses of ZOE
Bite Registration Paste To stimulate the occlusal relationship between the upper
and lower Use of bite wax (yellow wax) and ZOE paste Seat the horse-shoe shaped wax on ZOE paste on the
mouth Allow ZOE paste to set Once set, transfer the bite to the cast
Heat Insulating Base Used to absorb heat coming from outside stimulus before
amalgam estoration
Composition of ZOE
Dispensed as two separate pastes. Tube 1 Universal/ Base Tube 2 Reactor/Catalyst/Accelerator
Tube 1 Universal/Base
87% Zinc Oxide 13% Fixed Vegetable or Mineral Oil
Acts as a plasticizer and aids in offsetting the action of the eugenol as an irritant.
Tube 2 Reactor/Catalyst/Accelerator Oil of Cloves
Contains 70 to 85% eugenol, produces less burning sensation for patients
Gum or Polymerized Rosin Facilitates the speed of the reaction and yields a smoother,
homogenous product Resinous Balsam
Used to increase flow and improve mixing properties Accelerators
CaCl2 – speed p setting Silica & Lanolin (fillers or hardening agent)
Increase the strength of ZOE Olive Oil
Dilute the content
Manipulation
Accomplished on an oil impervious paper or a glass mixing slab.
Squeeze two strips of paste of the same length, one from each tube, onto the mixing slab
Use stainless steel spatula for mixing procedure Combined the two strips of contrasting colors, with
the first stroke of the spatula, and mixing is continued for approximately 1 min, or as directed by the manufacturer, until uniform color is achieved
Put it on a tray and insert it to the patients mouth.