IMPRESSION MATERIALS- Hydrocolloids
Desirable properties for Impression Materials
Adequate shelf life
Acceptable odour and taste
Non-irritant
Easily manipulated with minimum equipment
Setting Characteristics
Suitable working time
Suitable setting time
Readily wets oral tissues
Not affected by oral fluids
Does not release gas during setting
Properties of the set material
Sufficient mechanical strength
No permanent deformation
Good dimensional accuracy
Accept addition and correction
Satisfactory consistency and texture
Readily disinfected without loss of accuracy
Compatibility with cast and die materials
Economical
Impression Materials
Inelastic
Waxes
Plaster
Zinc Oxide Eugenol
Compound
Elastic
Hydrocolloids
Alginate
Agar
Agar/ Alginate
Rubber Base (Elastomeric)
Polysulfide
Silicone
Polyether
Condensation
Addition
Solid to Gel Transformation
Process: sol --> agglomeration of molecules --> fibrils --> network pattern by adhesion/ capillary attraction
By
Reduction in temp (AGAR)
Chemical Reaction (ALGINATE)
Dimensional Effects
SYNERESIS: LOSS OF WATER --> SHRINKAGE
IMBIBITION: ABSORPTION OF WATER --> EXPANSION
Alginate
Pros and Cons
Uses
Advantages
Easy to mix and manipulate
Minimum equipment
Flexible
Accurate
Low Cost
Disadvantages
Restrict the model and die material to only gypsum
Does not produce fine details (compared to elastomeric)
Poor dimensional stability
Messy to work with
Cannot be corrected
Preparing study cast for RPD
Preparing gymsum models for athletic mouth protector
Setting Reaction
Sodium alginate + calcium sulfate = sodium sulphate + calcium alginate
PROPERTIES
Permanent deformation should be less than 3%
Strength factors
water to powder ratio
Proper spatulation
Rate of deformation
Thickness of impression material
Storage: should be stored in 'free' humidity to avoid synerersis/ imbibition
Clinical Manipulation
Working and setting time
Normal set alginate: WT 2 minutes, ST 5 mins
Fast set alginate: WT 1.5 mins and ST 1-2 mins
Agar
Pros and Cons
Advantages
Highly accurate
Elastic
Disadvantages
Difficult manipulation
No method for obtaining model die
Dimensionally unstable
Patient discomfort from thermal shock
Types
Tray type: stiffer
Syringe type: less agar content
Setting Reaction: converts from solid to gel upon heating and back to solid upon cooling
PROPERTIES
Gelation temperature
37 - 45 degrees
Elastic Recovery
ANSI/ADA requires elastic recovery greater than 96.5%
Depends on
Time under compression
Rate of loading
Flexibility: 4-5%
Strength
Compressive Strength: 0.8 MPa
Tear Strength: 0.8-0.9N/mm
Dimensional Stability
Storage should be in free relative humidity to avoid syneresis and imbibition
Clinical Manipulation
Tray type
- Liquefaction of the material in boiling water for 8-12 minutes
- Immersion of tube in water at 43 to 49 degrees and manipulated to ensure even cooling
- Tray is filled and tempered for a minimum of 2 mins in water at 46 degrees
Syringe type
- Syringe is loaded with a cylinder and placed in boiling water for 10 mins
- No tempering is required before use
- Agar is injected directly into tooth preparation
- After removal, impression is rinsed, disinfected, superficially dried and poured in dental stone
Alginate/ Agar Method
Hydrocolloid in the tray is replaced with a mix of chilled alginate that bonds with the agar expressed from the syringe
Alginate gels by a chemical reaction and agar by means of contact with cool alginate
Pros and Cons
Advantages
Disadvantages
Maximum details are produced
Lower equipment cost and preparation time
The bond between the agar and the alginate is not always sound
The higher viscocity of the alginate material displaces the agar hydrocolloid during setting
The dimensional inaccuracy of the alginate hydrocolloid limits its use to single units
NOTE: this is still the most cost effective way of producing an impression with adequate details
Other applications and Handlings of Hydrocolloids
Other applications
Duplicate dental casts/ models
Construction of Prosthodontic appliances or orthodontic models
Higher water content/ lower agar content influences
Compressive strength
Percentage permanent set
Biocompatibilty
No allergic reactions, only thermal injury and maybe inhalation of fine particles
Disinfection
must disinfection q fast to prevent dimensional change
Compatibility with Gypsum
Agar more compatible with gypsum than alginate
Must wash off all saliva and blood if not setting will be retarded
Before storing must rinse with cool water to remove exudate from syneresis
Effect of mishandling
grainy material
separation of tray and syringe materials
external bubbles
irregularly shaped voids
rough or chalky stone model
distortion
Accuracy of Hydrocolloids
Adequately fluid to record fine details in the mouth
Adequately elastic to reproduce undercuts
They need perforated trays
They are not dimensionally stable during storage after removal form the mouth due to syneresis
To be compatible with dental stones there should be an accelerator for the setting of stones otherwise a model with soft surface may be obtained