L1b-Impression materials Flashcards
What are impression materials used for?
- To record the details of intraoral structures in order to fabricate a reproduction of hard (teeth) and soft tissues for the construction of dental prosthesis.
Ideal properties of impression materials:
- Produce an accurate replica or the intra oral structure.
- Prevent deformation.
- Be atraumatic when removing from undercuts.
- Have proper setting time and biocompatibility.
What is an impression?
A negative reproduction of the tissues.
How is a positive cast made?
By filling the impression with dental stone or other model material, a positive cast is made.
impression materials Classification:
- elastic or
- non-elastic
Which are the non-elastic impression materials?
- Impression plaster
- Impression compound
- Impression waxes
elastic impression-subclassification:
- Synthetic elastomers
- Hydrocolloids
Hydrocolloids:
- Reversible (Agar)
- Irreversible (Alginate)
Synthetic elastomers:
- Polysulphides
- Polyether
- Silicones
Subgroups of silicones:
- Additional
- Condensation
Desirable qualities of an impression material:
- A pleasant odor, taste, and acceptable color.
- Absence of toxic or irritant constituents.
- Adequate shelf life for requirements of storage and distribution.
- Economically commensurate with the results obtained.
- Easy to use with the minimum of equipment.
- Setting characteristics that meet clinical requirements.
- Satisfactory consistency and texture.
- Readily wets oral tissues.
- Elastic properties that allow easy removal of the set material from the mouth and good elastic recovery.
- Adequate strength to avoid breaking or tearing upon removal from the mouth. Good dimensional stability over temperature and humidity, long enough to permit the production of a cast or die.
- Compatibility with cast and die materials.
- Accuracy in clinical use.
- Readily disinfected without loss of accuracy.
- No release of gas or other byproducts during the setting of the impression or cast and die materials.
What is a colloid?
- Colloid is a mixture in which one substance of microscopically dispersed insoluble or soluble particles is suspended throughout another substance.
Which is the form in which colloids can exist?
In the form of:
- viscous liquid known as a sol
- or a jelly like elastic semi-solid described as a gel
When is a colloid called hydrocolloid?
If the particles are suspended in water, the suspension is called hydrocolloid.
How can a sol change into gel?
Characteristic of hydrocolloids:
- They are hydrophilic with very good wettability
- have very limited dimensional stability because they are composed of 85% water.
What is agar?
- It is a polysaccharide (a sulfuric ester of a linear polymer of galactose) extracted from certain types of seaweed.
- Although it is an excellent impression material and yields accurate impressions, presently it has been largely replaced by alginate hydrocolloid and rubber
What can agar be used for?
- For cast duplication (during fabrication of cast metal removable partial denture)
- For full mouth impressions without deep undercuts
- For crown and bridge impressions (before elastomers came to the market)
- As tissue conditioner
What is alginate?
- It is a mucous extract yielding from certain brown sea weeds.
Alginate-chemical reaction:
- They change from the sol phase to the gel phase because of a chemical reaction.
- Once gelation is completed, the material cannot be re-liquefied to a sol.
Alginate-Setting reaction:
- Sodium/Potassium alginate powder (soluble) dissolves in water to form asol.
- The sol reacts with calcium sulfate (reactor) to form calcium alginate (insoluble gel).
- This reaction is delayed by addition of a retarder (sodium phosphate), to give more working time.
Alginate impression powder-Ingredients
Alginate-Advantages & disadvantages:
Advantages:
- Easy to mix and manipulate, need minimum equipment
- If properly handled, gives accuracy and good surface details even in presence of saliva.
- Low cost
- Hygienic and comfortable to the patient
Disadvantages:
- Poor tear strength and dimension stability (poured with in 15 minutes).
- Not recommended when high level of accuracy is required (crown and bridge).
- May occur distortion if the material is not properly handled.
- It cannot be corrected.
Properties of Alginate:
- Syneresis
- Imbibition
What is Syneresis?
- Alginate impressions may lose water by expulsion of liquid molecules.
What is Imbibition?
- Gain water when immersed into it.
What can both phenomenon of Syneresis and Imbibition cause?
Both the phenomenon can cause dimensional changes and produce inaccurate casts.
- To avoid this, impression should be poured immediately (within 15 min).
Alginate indication:
- fabrication of diagnostics casts
What is Polysulfide?
- elastomer that is also known as mercaptan, or simply as rubber base
- What is Polysulfide packaged in?
- What does this result into?
- What is the reaction of the two pastes?
- in two tubes, a base and an accelerator (reactor)
- When the two pastes are mixed equally, it results in an increased viscosity and finally in an elastic material.
- exothermic
Polysulfide:
Which is the composition of base paste and reactor paste?
Polysulfide-Properties:
- Stability.
- Low contraction (shrinks 0.3- 0.4% during first 24h).
- High tear strength.
- Hydrophobic.
- Excellent reproduction of details.
- Excellent shelf life.
- Highly compatible with stone and plaster.
Polysulfide:
Working time - Setting time:
Mixing and working time:
- 4-6 min.
Setting time:
- 12-16 min.
Polysulfide indication:
- Mostly for partial and complete dentures
Polysulfide:
Advantages and disadvantages:
Advantages:
- Good stability and accuracy
- It comes in different viscosities.
- Long working (4-7 min) and setting time (7-10 min)
Disadvantages:
- Lead oxide in the base paste can be toxic.
- Mixing procedure can become messy.
- Unpleasant odor and color
- Requires the use of custom tray.
Condensation silicones-Components:
Base:
- Polydimethyl siloxane (Hydroxy terminated).
- Colloidal silica or micro-sized metal oxide (filler).
- Color pigments.
Accelerator:
- Ortho-ethyl silicate (cross linking agent).
- Tin (Sn) Octoate (catalyst).
Condensation reaction-polymerization reaction:
- Ethyl alcohol eliminates from the reaction, the evaporation of which is responsible for shrinkage and dimensional instability.
Condensation silicones-Properties:
- High permanent deformation (1-3)% due to shrinkage.
- Tear strength is lower than polysulfides.
- Hydrophobic.
- Requires dry areas of mouth.
Condensation silicones:
Working time-Setting time:
Working time:
- 2-4 min.
Setting time:
- 6-8 min.
Condensation silicones-Indications:
Mostly for lab work (duplication of casts and wax –ups)
Condensation silicones:
Advantages and disadvantages:
Advantages:
- Clean and pleasant.
- Custom tray not required.
- Good working time.
- Ease of use.
- Relatively economic.
Disadvantages:
- High polymerization shrinkage.
- Low tear strength.
- Second pour is not so accurate.
- Volatile (evaporation of by-product). Hydrophobic.
Additional silicones-Composition:
Base paste:
- Terminal silane hydrogen groups
- Fillers
Catalyst paste:
- Terminal vinyl groups
- Platinum salt (catalyst)
- Palladium (hydrogen absorber)
- Filler
release hydrogen gas
additional silicones-Properties:
- Pleasant odor and color.
- Excellent reproduction of surface details.
- Good tear strength.
- Hydrophobic material.
- Wettability can be improved with surface active agents.
Additional silicones:
Working time-Setting time:
Working time:
- 1-4 min.
Setting time:
- 4-6 min.
Additional silicones- Indications:
Fixed prosthesis and implant restorations.
Additional silicones-Advantages & Disadvamtages
Advantages:
- Ease of use.
- Fast setting.
- Excellent dimensional stability.
- Can be re-poured
- Good shelf life (1-2 years).
Disadvantages:
- Hydrophobic (although improved). No flow if sulcus is moist.
- Sometimes is difficult to remove from mouth (undercuts).
- High cost.
Polyether-Composition:
Base paste:
- Poly ether polymer.
- Silica (filler).
- Glycol ether or phthalate (plasticizer)
Catalyst paste:
- Aromatic sulfonate ester (cross-linking agent).
- Silica (filler).
- Phthalate or glycol ether (plasticizer).
Polyether-Properties:
- Pleasant odor and color.
- Very good dimensional stability.
- Adequate tear resistance and elastic properties, approaching those to addition silicones.
- Hydrophilic material.
Polyether:
Working time-Setting time:
Working time:
- 3 min.
Setting time:
- 6 min.
Polyether- Indications:
Fixed prosthesis and implant restorations.
Polyether:
Advantages and Disadvantages:
Advantages:
- Ease of use.
- Excellent dimensional stability.
- Accuracy.
- Multiple pouring.
- Can be poured hours after impression is taken.
Disadvantages:
- May cause allergic reaction due to the sulphonic acid.
- Stiff set material (could be difficult to remove from mouth).
- High cost.
