Purpose Of Impression Materials

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Purpose of Impression Materials

Dental Impression Materials Silicone

Reversible hydrocolloids

• To make an accurate replica of the oral tissues • To give a negative reproduction of the tissues • To be filled with model material to make a positive cast

Irreversible hydrocolloids

Positive cast

Desirable properties 1. A pleasant odor, taste, and esthetic color 2. No toxic or irritant constituents 3. Adequate shelf life 4. Economically commensurate with the results obtained 5. Easy to use with the minimum of equipment

Desirable properties (con’t) 6. Setting characteristics that meet clinical requirements 7. Satisfactory consistency and texture 8. Readily wets oral tissues 9. Elastic properties with no permanent deformation after strain 10. Adequate strength so that it will not break or tear on removal from the mouth

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Desirable properties (con’t) 11. Dimensional stability over temperature and humidity ranges met in clinic and lab 12. Compatibility with cast and die materials 13. Accuracy in clinical use 14. Readily disinfected without loss of accuracy 15. No release of gas during the setting of the impression or cast and die materials

Classification By Setting Mechanism

By Application or Mechanical Properties non-elastic Elastic

Chemical Plaster of Paris Alginate reaction hydrocolloid ZOE (irreversible) Elastomers Temperature Compound Agar changes hydrocolloid Wax (reversible)

Hydrocolloid Colloidal Solution (Sol.) Solution Suspension (emulsion)

•Colloid has two phase: dispersed phase + dispersion phase Adequate Semisolid material concentration = Gel Sol-Gel transformation

“ No impression material

fulfills all the requirements” the selection of the material best suited for a particular clinical situation and technique rests with the dentist.

Classification by application or mechanical properties 1. Elastic 2. Non-elastic 1.1 Hydrocolloid

• Reversible hydrocolloid (Agar) • Irreversible hydrocolloid

(Alginate) 1.2 Elastomeric Polysulfide Silicone Polyether

• Zinc oxide eugenol • Impression wax (ZOE) • Impression compound • Impression plaster

Hydrocolloid Dipersion phase = water

Hydrocolloid

Reversible hydrocolloid (Agar) • Irreversible hydrocolloid (Alginate) •

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Overview

Reversible Hydrocolloid (Agar)

Composition • Potassium sulfate

• Agar (8-15%) – Polysaccharide from seaweed – Provides matrix phase which forms fibrils in the “gel” stage.

• Water (>80%) – Reaction medium, controls flow properties

• Borax

– to accelerate the setting of gypsum

• Fillers – to control the strength, viscosity, and rigidity

• Thymol and glycerine – bactericidal and plasticizer

– to strengthen the gel – retarder for the setting of gypsum

• Alkyl benzoate – anti-fungal

• Pigments and flavors

Manipulation Heat in water at 100o C for 8-12 m Store in water at 65o C Place in tray at 65o C Temper in 45o C in water for 2 m Impression making: making: water cooling until gelation occurs (Water circulated~ 20o C)

• Setting = “Gelation” ( sol gel states) • Phase change is induced by Temp. change • A colloid forms with water and the material liquefies between 70 and 100°C (liquefaction temp) and sets to a gel-like state between 37 to 50°C (gelation temp).

Agar manipulation • Three stationary tempering • Boiling temperature • Storing temperature 65 C • Tempering in water-filled container (~45 C ) Final temperature < 55 C

Impression with reversible hydrocolloid • Accuracy - exellent impression of high accuracy and fine detail • Require careful control and more expensive appartus than alginate • Dimensional unstable on standing: model should be made as soon as possible

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Impression with reversible hydrocolloid • Agar lose moisture and shrink in air • Should be poured immediatly • Store in 100%relative humidity not more than 1 hour. • After initial setting of model material should be store in humidor

Clinical significance • To reduce the amount of permanent deformation attributed to dashpot D1, the impression should not be removed by a teasing or weaving method, but it should be removed suddenly in a direction as nearly parallel as possible to the long axes of the teeth.

Viscoelastic Properties • The stress-strain relationship of the hydrocolloid impression materials changes as the rate of loading changes.

Permanent Deformation • Deformation should be < 1.5% following a 10% total compression on the material and a 30 second recovery. • It is important to lower the amount of compressive loading on the material in undercuts; therefore, (again!) one should remove the material as fast as possible and have more thickness in areas of severe undercuts.

Irreversible Hydrocolloid (Alginate)

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Overview

Composition

• Substance is alginic acid, which was extracted from seaweed. • Potassium salt form of alginic acid (Potassium alginate) react with calcium sulfate dihydrate and water to form a calcium alginic gel.

Gelation Process (Sol) (Gel) Soluble Alg + CaSO4 Insoluble Alg (rapid reaction) Sodium or Potassium alginate

Calcium alginate

Add water-soluble salt e.g. sodium phosphate to react with CaSO4 --> prolong the working time.

Gel Structure

• Potassium alginate (15%) • – Soluble alginate • Calcium sulfate dihydrate (16%) – Reactor • Diatomaceous earth (60%) • – Fillers – Strength, Stiffness, Texture • Potassium titanium fluoride (3%) • – Accelerator for the setting of the stone • Zinc Oxide • – Filler

Sodium phosphate (2%) – Retartder - reacts with calcium ions to slow the working time Glycols – Decrease dust formation Quaternary ammonium salts – Disinfectants Pigments, taste

Gelation Process (con’t) Calcium sulfate Potassium alginate Trisodium phosphate

+ Water

2Na3PO4 + 3 CaSO4 --> Ca3(PO4)2 + 3 Na2SO4 Na3PO4 used up

K2nAlg + nCaSO4 --> nK2SO4 + CanAlg

Gelation Time • Once gelation starts, the impression material must not be disturbed because the growing fibrils will fracture and the impression would be significantly weakened.

• The final structure Sodium alginate crosslinked with calcium ions.

– Fast setting = 1.5 -3 mins. – Normal setting = 3 – 4.5 mins

• Best regulated by the amount of retarder added during manufacturing. Cross-linking of Sodium alginate

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Strength

Gelation Time (con’t) • Water Temperature – 1 min. of reduction in gelation time for each 10°C of temp. decrease

• Gel strength doubled during the first 4-min period. • Water amount – Weakened, less elastic

• Spatulation – Insufficient spatulation --> failure of the ingredients to dissolve sufficiently

• Overmixing – Breaks the CaAlg gel network --> decrease strength

Viscoelasticity

Accuracy

• Strain-rate dependent • Avoid torquing or twisting when remove the impression.

• Not as good as elastomeric impression materials – Reproduction of fine detail – Dimension stability – Roughness of the impression surface

• Can be used for Removeable Partial Denture (RPD)

Dimensional Stability • Syneresis, evaporation, imbibition • The impression should be exposed to air for as short a time as possible. • Immediately constructing the stone cast

air

Dimensional Stability

water

• 100% relative humidity is the best storage environment to preserve the normal water content of the impression. Percentage of change in water content by weight

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Compatibility with Gypsum

Disinfection

• The surface of the gypsum cast may be too soft for waxing procedures.

• Certain disinfectants my result in gypsum casts that have a lower surface hardness or diminished surface detail

– Immerse the impression in the accelerator for gypsum product – Incorporate a plaster hardener or accelerator in the impression material (fluoride)

• Current protocol – Impression --> thoroughly rinsed --> spray disinfectant --> wrap in a disinfectant-soaked paper towel --> place in a sealed plastic bag for 10 minutes – Iodophor, Sodium hypochlorite, Glutaraldehyde

Shelf Life • Storage temperature and moisture contamination • Materials stored for 1 month at 65°C are unsuitable to use, either failing to set or setting too rapidly. • Should not stock more than one year’s supply.

Effects of Mishandling • Examples of common causes for failures encountered with hydrocolloid impression materials – Distortion – Tearing – Rough stone model

Distortion • • • • •

Impression not poured immediately. Movement of tray during gelation Premature removal form mouth Improper removal from mouth Tray held in mouth too long (only with certain brands)

Tearing • • • •

Inadequate bulk Moisture contamination Premature removal from mouth Prolonged mixing

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Rough stone model • • • • •

Inadequate cleaning of impression Excess water left in impression Premature removal of model Leaving model in impression too long Improper manipulation of stone

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