Presentation on theme: "Prof. Dr Amal EzzEl Din Dental Biomaterials,Faculty of Dentistry Alex _Univ."— Presentation transcript:
Prof. Dr Amal EzzEl Din Dental Biomaterials,Faculty of Dentistry Alex _Univ.
oUses Make an accurate replica of tissues. oImpression Single tooth. Whole dentition. Partially or completely edentulous mouth.
oBiocompatible: non toxic- non irritant. oAcceptable to the patient: pleasant odour, taste, esthetic colour. oEase of manipulation, reasonable cost. oReasonable setting time. oAdequate shelf life. oAdequate strength: not tear or break on removal from mouth.
oCompatible with model materials. oDimensionally stable: neither expands nor contracts. oAdequate flow properties: to register fine details. oReadily disinfected without loss of accuracy.
Classification 1.According to the ability of a set material to be withdrawn over undercuts. Non ElasticElastic
According to the manner which they harden(set) : Reversible Sets by a physical reaction due to temperature changes Irreversible Sets by a chemical reaction.
By Setting Mechanism By Application or Mechanical Properties Non-elasticElastic Chemical reaction (irreversible) Plaster of Paris ZOE Alginate hydrocolloid Elastomers Temperature changes (reversible) Compound Wax Agar hydrocolloid
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 compound softens at 70°°C. Used as tray material to hold a secondary impression material Heating: o In a water bath-soaking in water in cases of sheets and cakes. o Over a flame in case of sticks. o Avoid over heating. o Kneading: with the fingers. o Importance: assures uniform heating Increases the flow. Gives workability.
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 plasticizer stearic acid 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: o Assure uniform cooling by spraying with water at 16-18°C before removal from the mouth.
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.Low thermal conductivity. High expansion and contraction coefficients.High expansion and contraction coefficients. Linear contraction on cooling from mouth temperature to room temperature (0.3%-0.4%).Linear contraction on cooling from mouth temperature to room temperature (0.3%-0.4%).
o Dimensional Changes: due to relief of internal stresses (relaxation) Warpage distortion of the impression. o Model should be poured as soon as possible. o No separating medium. o Separation of model.
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 1. Non irritant and non toxic 2. Reusable (but with re-use the constituents are leached out) 3. Can be reheated and readapted 4. Can support other materials for wash impressions DISADVANTAGES 1. Poor dimensional stability 2. Poor surface detail 3. Expansion coefficient 4. Will distort if removed from undercuts