Prosthodontic

• Impression materials are used to register or reproduce the form and relationship of the teeth and oral tissues
• A dental impression is a negative likeness of the teeth and/or other tissues of the oral cavity
• Impression materials must be in a plastic or fluid status which become hard or set while in contact with oral tissues
• An impression tray is a device used to carry and control impression material while making an impression
• Ideal requirements of impression materials

  • Taste, colour, and odour
  • Adequate shelf life
  • Biocompatible, non-toxic, and non-irritant
  • Economically viable
  • Accurate
  • No gas release
  • Suitable working and setting time
  • Simple and easy to use
  • Compatible with casting and die materials
  • Dimensionally stable with time, temperature, and humidity
  • Adequate tear strength
  • Wets oral tissue
  • Can be disinfected without any changes of properties
  • Adhere to the impression tray
• Classification of impression materials

  • Non-elastic/Rigid materials
  • Elastic materials
• Non-elastic/Rigid materials

  • Edentulous ridge - Plaster of Paris
  • ZOE - interocclusal records
  • Impression compound - preliminary impression
  • Must not have undercuts
• Elastic materials

  • Hydrocolloids
  • Reversible (agar)
  • Irreversible (alginate)
  • Elastomers: Polysulfide, Silicon rubber, Polyethers
• Classification of impression materials based on setting mechanism

  • No chemical change taking place
  • Reversible (thermally induced reaction)
  • Irreversible (chemical reaction)
• Non-elastic/Rigid materials classification

  • Impression compound
  • Zinc Oxide Eugenol (ZOE)
  • Impression plaster (gypsum)
• Impression compound properties

  • Thermo-plastic or thermally reversible material
  • Softened to working consistency by immersion in hot water or by warming over a flame
  • Softens above 55-60°C and hardens at mouth temperature
  • Composition varies from company to company: Resins, Waxes, Plasticisers, Fillers, Colour
• Impression compound has low thermal conductivity and takes time to soften throughout
• Materials to reduce brittleness

  • Fillers: talc, kaolin, calcium carbonate
• Properties of Impression Compound

  • Thermal conductivity: low
  • Softening and flow: soften at just above mouth temperature, harden at mouth temperature, adequate flow to adapt closely
  • Accuracy: viscosity too high to record fine details, non-wetting because of wax, thermal expansion coefficient too high (wax) resulting in potential dimensional change upon cooling to lab/clinic temperature, deformable by undercuts etc.
• Pressure applied during the formation of the impression

  Residual stress occurs in the material storage in warm environments or extended storage results in warpage
• Contraction of Impression Compound from oral temp. to room temp. is 0.3%
• Primary impression of edentulous mouth is made using a close-fitting special tray with zinc oxide eugenol wash
• Single crown impression is made using the copper ring technique
• Zinc oxide-eugenol has been widely used since the 1890s as cement for hard and soft dental tissue, temporary cement, endodontic sealer, and impression material
• Properties of Zinc Oxide-Eugenol

  • Accuracy: viscosity low and able to record fine details, hydrophilic so wets and adapts to surface of tissue, thermal expansion coefficient not high (wax) resulting in dimensionally stable impressions (<0.1%), rigid and not deformed by undercuts etc., not influenced by humidity, drying, etc., non-toxic but may cause a burning sensation to the patient due to eugenol
• Zinc oxide-eugenol is used in dental applications for secondary impressions for complete dentures, temporary luting of fixed appliances, and as a wash impression mixed in a 1:1 paste ratio and used in thin sections only (2-3mm)
• Advantages of Zinc Oxide-Eugenol

  • Dimensional stability, good surface detail, can be added to, mucostatic or mucocodisplacive
• Disadvantages of Zinc Oxide-Eugenol

  • Cannot be used in very deep undercuts, only sets quickly in thin sections, eugenol allergy in some patients
• Materials for Non-Elastic/Rigid Dental Plaster

  • Additives to colour and flavour of the plaster, other additives incorporated such as gum to improve handling or starch to create soluble plasters when placed in boiling water
• Impression Plaster is composed of CaSO4 . ½ H2O with various additives to modify the reaction
• Setting reaction of Plaster involves a hydration reaction that is quick and exothermic
• Mineral gypsum heating

  CaSO4 · 2 H2O heated at 110-130 °C becomes dry → β-hemihydrate (plaster of Paris, model stone) or autoclave → α-hemihydrate (model stone, die stone)
• Setting reaction

  Hydration reaction, quick, exothermic, setting expansion, CaSO4 · ½ H2O + 1 ½ H2O crystallization to CaSO4 · 2 H2O
• Mixing plaster

  100g powder + 60 ml water mixed in a flexible bowl with a spatula
• Visible signs of plaster setting

  • Dissolution - light surface → fluid
  • Colloidal state - matt surface → paste
  • Crystal formation - solid → hardening
  • Breaks with a special sound
• Influence on setting time and expansion

  Water/powder ratio, spatulation time, Temperature
• Properties of plaster

  • Good working time typically 2 to 3 minutes
  • Excellent registration of detail as plaster is hydrophilic and has low viscosity
  • Good mechanical properties and excellent long term dimensional stability
  • Needs a separating agent applied to surface before taking model from plaster
  • Non-toxic, long shelf life, cheap and low coefficient of thermal expansion
• Impression plaster is rarely used these days as it is brittle and fractures very easily
• Impression plaster is mucostatic impression of edentulous ridges with high W/P ratio and can be combined with impression compound
• Impression compound, Zn Oxide-Eugenol, and Dental stone are three rigid impression materials for edentulous patients
• Hydrocolloids are materials used to obtain preliminary and final impressions
• Alginate hydrocolloid is non-reversible, while Agar hydrocolloid is reversible
• Agar hydrocolloid can convert between colloid (sol) and gel states
• Alginate is the most widely used irreversible hydrocolloid for preliminary impressions
• Alginate composition includes sodium or potassium alginate salts of alginic acid