Elastic Impression Materials

1. Elastic Impression Materials Hydrocolloids Prof AmalEzzeldin Abdel Aziz Professor of Dental Biomaterials

2. Agar (reversible) Aqueous Hydrocolloids Alginate (irreversible) Elastic Polysulfide Condensation Non-aqueous Elastomers Silicones Addition Polyether O’Brien Dental Materials & their Selection 1997

3. Alginate Impression material ِIrreversible Hydrocolloids

4. Elastic Impression Materials Hydrocolloids • Solution: homogenous mixture: one phase system. • Suspension:hetrogenous mixture: two phase system. • Colloids: hetrogenous, particle size of the particles are smaller than in suspension: two phase system. • Dispersed phase. • Dispersion medium  water  Hydrocolloid

5. Sol state: viscous liquid. • Gel state: jelly like • sol •  fibrils in a network pattern. • Brush heap structure • SOL • reversible hydrocolloid • Or by: chemical reaction: • irreversible hydrocolloid. gel Agglomeration of molecules Gel reduction in temperature

6. Irreversible Hydrocolloids Alginate Impression Material Uses Sol  gel by chemical reaction • Advantages: • Easy to manipulate. • Does not require special equipment. • Relatively inexpensive. • Comfortable for the patient.

7. Composition: • Potassium alginate.(soluble sol) • Calcium sulphatedihydrate.(reactor) • Potassium sulphate. • Sodium phosphate: retarder. • Diatomaceous earth: filler. • Glycols: dustless. • Pigments. • Flavours. • Quaternary ammonium salts: Disinfectants

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

9. 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 alginate gel.

10. Reaction: • CaSO4.2H2O + Na3PO4 • CaSO4.2H2O + kalginateK2SO4 + calcium alginate (insoluble gel) • Types: amount of Na3PO4 • Fast set: less amount of retarder. • Regular set. + H2O Ca3PO4 + Na2SO4

11. Gel Structure • The final structure cross-linked calcium alginate gel. Cross-linking of Sodium alginate

12. Concentration of filler: • Controls flexibility of set impression. • Soft set: less amount of filler. • Hard set. Manipulation: • Proportioning. • Mixing: vigorous mix – against sides of the rubber bowl. • Mixing time: 45 seconds (fast set). One minute (regular set).

13. Manipulation of Alginate impression

14. Gelation time: • Timefrom the start of mix till material is no longer tacky when touched with finger. • Avoid movement of the tray during setting. • Remove the tray after additional/2-3 minutes as elasticity is improved with time and permanent deformation is .

15. 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. • Fast setting = 1.5 -3 mins. • Normal setting = 3 – 4.5 mins • Best regulated by the amount of retarder added during manufacturing.

16. Control of the S.T: • Do not reduce powder/H2O ratio as this affects consistency, accuracy and  strength. • To prolong S.T: reduce temperature of H2O *not below 18°C. • temperature of water should be between 18-24°C. • Better to choose the suitable type of alginate. • Fast setting = 1.5 -3 mins. • Normal setting = 3 – 4.5 mins

17. Removal of the impression: one sudden pull-single stroke. As the material is Visco Elastic: Mechanical properties are time dependant and depends upon rate of load application. Rapid rate of removal: Tear Strength  4 times by sudden pull removal.

18. Avoid torquing or twisting when remove the impression • Pouring impression: washing under running tap H2O, to remove blood, saliva. • No: separating medium.

19. How to avoid suchDimensional changes.(synersis –imbibition –evaporation) • Immediate pouring of the impression. • Storing in a humidor: 100% relative humidity or wrapping in paper towel saturated with water. • This does not prevent synersis. • Done only for one hour maximum.

20. Sealed plastic bag

21. Compatibility with Gypsum • Surface Hardness Of The Cast ( • Gypsum casts poured against hydrocolloids sometimes have rough or chalky surface which causes inaccuracy or lack of sharp details. • Causes: • 1-Impression not washed from blood or saliva after removal from the mouth as blood and saliva retard the setting of gypsum.

22. 2-Excess H2O left in impression surface before pouring  dilute surface of the model. • 3-Impression stored in humidor for ½ or 1 hour, not washed to remove exudate. • 4-Removal of the model after a long time (several hours)…contact of the gel containing H2O with gypsum which is slightly soluble.CaSO4.2H2O chalky surface. • 5-Premature removal of the model: not beforeone hour.

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

24. Agar-Alginate Combination Impression Syringed agar: boiled for 8 minutes cooled to 65°C for 10 minutes, injected around preparation. Alginate: mixed in tray over agar. Setting time: 4 minutes.

25. Advantages: • Simple heating equipment. • No water cooling system trays. • Fine details of preparation. • Agar: more compatible with gypsum model materials than alginate, & has higher tear strength than algiante