Impression materials Prof . Dr AmalEzzEl Din Dental Biomaterials ,Faculty of Dentistry Alex _Univ.
Impression Materials • Uses • Make an accurate replica of tissues. • Impression • Single tooth. • Whole dentition. • Partially or completely edentulous mouth.
Desirable Properties of Impression Materials • Biocompatible: non toxic- non irritant. • Acceptable to the patient: pleasant odour, taste, esthetic colour. • Ease of manipulation, reasonable cost. • Reasonable setting time. • Adequate shelf life. • Adequate strength: not tear or break on removal from mouth.
Compatible with model materials. • Dimensionally stable: neither expands nor contracts. • Adequate flow properties: to register fine details. • Readily disinfected without loss of accuracy.
Classification • According to the ability of a set material to be withdrawn over undercuts. • Non Elastic Elastic
According to the manner which they harden(set): • Reversible • Sets by a physical reaction due to temperature changes Irreversible Sets by a chemical reaction.
Impression materials Plaster ImpressionCompound Nonelastic Zinc oxide eugenol Impression wax Impression material Agar Polysulfides Hydrocolloids Alginate Elastic Polyethers Condensation silicone Non-aqueous elastomers Addition silicone
Non Elastic Impression Materials 1- Impression Compound • Thermoplastic material. • Composition: • Natural resins e.g., shellac & carnuba wax. • Fillers: soap or talc. • Lubricants: stearic acid. • Colouring agent: e.g., rouge.
Types Type I: low fusing compound softens at 53-60°C Supplied in the form of sheets, or cakes used as: impression for edentulous patients. Or in the form of sticks or cones or cylinders. Used as: impression for a single tooth preparation as in (crowns), known as Copper Band Impression.
Type II:High fusing compoundsoftens at 70°°C. Used as tray material to hold a secondary impression material Manipulation Heating: • In a water bath-soaking in water in cases of sheets and cakes. • Over a flame in case of sticks. • Avoid over heating. • Kneading: with the fingers. • Importance: assures uniform heating • Increases the flow. • Gives workability.
Manipulation • The compound disc is completely immersed in a water bath at 55-60 C for about 4-5 minutes to ensure complete softening. Gauze is placed at the bottom of the water bath to prevent adherence. However if it is left in too long some of the constituents may be leached out into the water bath, altering the properties of the material (it is often the plasticizerstearicacid that is leached out). If the compound is kneaded water will become incorporated and act as a plasticiser. Kneading of the compound will incorporate water into the mass and thus increase the flow in the material(water = plasticizer). • The compound is loaded on to the tray and firm pressure is used to seat the tray home in the mouth. After the impression has been taken it is carefully examined, a common mistake is not softening the compound enough. In this is the case the impression can be reheated in a separate bowl of water (cross infection control) and the impression is repeated.
Cooling: • Assure uniform cooling by spraying with water at 16-18°C before removal from the mouth.
Properties • Flow type 1: 85%at 45°C less than 6% at 37°C. • Type II: 70% at 45°C less than 2% at 37°C. • Viscous material does not record fine details.
Low thermal conductivity. • High expansion and contraction coefficients. • Linear contraction on cooling from mouth temperature to room temperature (0.3%-0.4%).
Dimensional Changes: due to relief of internal stresses (relaxation) Warpage distortion of the impression. • Model should be poured as soon as possible. • No separating medium. • Separation of model.
PROPERTIES • Poor surface detail • High coefficient of thermal expansion (contraction of up to 0.3% when removed from mouth to room temperature) • Distorts when removed over undercut areas • Poor dimensional stability • Can be modified by re-heating • Non toxic and non irritant • Good shelf life
ADVANTAGES & DisADVANTAGES • ADVANTAGES • 1. Non irritant and non toxic2. Reusable (but with re-use the constituents are leached out)3. Can be reheated and readapted4. Can support other materials for wash impressions • DISADVANTAGES • 1. Poor dimensional stability2. Poor surface detail3. Expansion coefficient4. Will distort if removed from undercuts