Dental materials impression materials Lec 2

Impression materials: The impression is a negative replica of the dental structure, used to create accurate replicas (models or casts) of teeth and other oral tissues. In dentistry, we take impressions of teeth and their supporting structures (gingiva, alveolar bone or residual ridge, hard and soft palate, and frenum). These impressions are used to create restorations and other dental appliances. Sometimes impression materials are used to duplicate a cast or model that has been formed when more than one positive reproduction is required (Such impression materials are referred to as duplicating materials).
The impression is a negative reproduction, whereas the replica (model or cast) is a positive reproduction.

1. Impression Materials Dr. Rebaz Tahsin Kakayi BDS. PhD.

2. Impression Materials Ideal requirements of impression materials : 1- Biocompatible , non toxic and non irritant 2- Should have acceptable taste, color and smell. 3- Adequate shelf life for storage. 4- Easy to manipulate with suitable working and setting time. 5- Should be elastic to allow removal from undercuts without permanent deformation. 6- Adequate strength so that will not tear or break on removal from the mouth.

3. 7- Should give an accurate reproduction of tissue details. 8- Dimensionally stable. 9- Can be disinfected without any changes in properties. 10- Economic or not expensive.

4. Classification of Impression Materials

5. Impression Materials for Complete Denture   Primary Impression - Impression compound - Alginate Secondary Impression - Rubber base - Zinc oxide Eugenol - Plaster of paris

6. Primary Impression A primary impression is an impression made for the purpose of diagnosis or for the construction of a tray.

7. Impression Compound As primary impression of edentulous mouth, they are thermoplastic materials softened to their working consistency by immersion in hot water or by warming over the flame (reversible impression material sets by physical change) Rigid after setting and has poor elastic properties supplied as sheets, sticks, cakes and cones.

8. Advantages of Impression compound 1- Cheap and reusable 2- Does not produce irritation to the patient 3- Can be remodifying and resoften again till an accurate impression is obtain. Disadvantages of Impression compound 1- It is difficult to record details 2- Distortion due to poor dimensional stability 3- Difficult to remove from severe undercuts 4- Does not have a pleasant taste

9. Alginate Alginate is an elastic, irreversible hydrocolloid impression material that transfer from the solution phase to the sold phase by chemical reaction. It is one of the most frequently used dental materials.

10. Advantages of alginate 1- Ease of mixing and manipulation 2- Minimum equipment 3- Flexibility of the set impression material 4- Accuracy if properly handled 5- Cost effectiveness. Alginate Disadvantages 1- It cannot be corrected 2- Poor dimension stability 3- Poor tear strength 4- Distortion may occur if not properly handled

11. Secondary Impression A final impression is an impression for making the master casts. The master casts are used in constructing the dentures.

12. Elastomers Often called rubber materials since they have properties similar to rubber. Four types of elastomers are in general use: • Polysulphides. • Silicone rubbers (condensation curing type). • Silicone rubbers (addition curing type). • Polyethers .

13. Advantages of Elastomers 1- Dimensional stability 2- Accuracy 3- Shorter setting time 4- Automix available Disadvantages of Elastomers 1- Set material very stiff 2- Allergic hypersensitivity in some cases 3- Short working time

14. Zinc oxide Eugenol Z inc oxide eugenol is popular material for making impression of edentulous arches , It is classified as a rigid, irreversible , impression material. It cannot be used for recording impression of dentate arches and in the areas of severe undercut.

15. Advantage of Zinc oxide Eugenol 1-It has enough working time to complete border moulding . 2-It can be checked in mouth repeatedly without deforming. 3-It registers accurate surface details. 4-It is dimensionally stable. 5-Minor defects can be corrected. Disadvantages of Zinc oxide Eugenol 1-Requires special tray for impression making. 2-Sticky in nature and adheres to tissues. 3-Burning sensation of eugenol causes tissue irritation.

16. Plaster of Paris  Impression plaster, also known as 'plaster of Paris', is a rigid impression material. Its chemical composition consists of calcined hemihydrate of calcium sulphate . It reacts with water to form a hard mass of calcium sulphate dihydrate that is used to make impressions of the teeth or any other part of the body.

17. Advantage of Plaster of Paris 1- It is cheap 2- It is easily available 3- It is comfortable 4- It is easy to mould 5- It is strong 6- It is non inflammable Disadvantage of Plaster of Paris 1- Heavy 2- Significantly weakened if cast is wet 3- Partially radio-opaque