Impression materials

1. Impression materials أحمد عبد العزيز الحسين أخصائي تركيبات سنيه في وزارة الصحة الأردنية محاضر غير متفرغ في جامعة العلوم والتكنولوجيا الأردنية Ahmad Abdul Aziz Alhussien Prosthodontist at M.O.H Part time lecturer at J.U.S.T BSc , MSc, Jor. Board/ prosthodontics

2. The function of an impression material is to make a negative copy of the oral tissue … that is poured to produce a positive replica for these tissues

3. Requirement of an impression material • Must be a semi-liquid material that will flow and adapt itself around the structure of interest. • It must set and harden into a solid ?? that is rigid enough “not too rigid” to be removed from the mouth without becoming deformed. • Copy details accurately • Dimensional stability after removal from the mouth • Appropriate working time (from the start of mix) • Appropriate time to harden in the mouth (setting time) • Bio-compatibility/Aesthetic, odor/taste • Chemically compatible with material used to pour cast or dies

4. Impression trays • Used to carry the impression material into the oral cavity • Can be • metal or plastic • Perforated or non perforated • for edentulous or dentate • pre-manufactured (stock trays) or custom-made (made in the lab. From resin). • Trays need to be rigid enough to support impression material. • For impression material to be retained in tray, tray maybe perforated, and adhesive spray can be used to enhance retention.

5. Variety of trays

6. Cast replica of dental arch • Die replica of a single tooth (cut from a cast)

7. Classification of dental impression materials • Inelastic / Elastic • Mechanism of hardening

8. Impression material (rigid)

9. Impression materials (aqueous)

10. Impression materials (rubber)

11. Inelastic impression materials (rigid) • Plaster of Paris (Type I gypsum) • Dental compound/impression compounds • Softened by heat and pressed against tissue before it hardens. Thermoplastic softens at 60°C, remains plastic at 45°C and becomes firm at mouth temperature. Used for primary impression • ZOE (Zinc oxide eugenol) • Other uses ??? • Impression wax Elastic impression materials (non-rigid) • Hydrocolloids • Elastomers (Rubber)

12. Hydrocolloids

13. Hydrocolloids • The colloid state represents a highly dispersed phase of fine particles within another phase, some where between solution and a suspension. • The colloid can exist as a viscous liquid known as a sol or a solid known as gel. • If the particles are suspended in water, the suspension is called a hydrocolloid (hydrosol, hydrogel • Hydrocolloids come in two forms: • Reversible agar • Irreversible alginate

14. Agar Composition of agar:

15. Properties • Flows well and adapts readily to hard and soft tissue contours • Accurate reproduction (hydrophilic nature), however, the impression need to be poured immediately. • Need to be stored in 100% humidity for short period of time if not poured immediately • Distortion is more likely to occur if impression not poured within an hour.

16. Tear strength is not high. • If compressed it might rebound if compression is not too sever otherwise deformation occurs. • Material suffers from loss or gain of water: • Syneresis: a process by which the gel contracts and some of the liquid is squeezed out, forming an exudate on the surface. • Evaporation: loss of water which causes the material to shrink, and impression is distorted. • Imbibition: uptake of water, this will swell and distort the impression.

17. Clinical application • The use of agar requires special equipment consisting of hydrocolloid conditioner (has 3 water bath chambers) and water -cooled tray connected to a rubber hose that delivers water. • Usually used in lab for cast duplication • Agar is supplied in two viscosities, thick and thin (depending on the amount of agar).

18. Preparing the material for impression: • Gel heated to 71-100°C, becomes liquid (sol) • Tray and agar syringe are liquefied in 8-12 minutes in boiling water in 1st chamber • 2nd chamber is used to store the agar at 60-66°C. • 3rd camber kept at 45-47°C to temper agar before use to prevent burns. • Agar exhibits hysteresis, melting at 85 °C and solidifying from 32-40 °C

19. Alginate • Inexpensive • Easy to manipulate • Requires no special equipment • Reasonably accurate Common uses: • Diagnostic cast (study model) • Preliminary impression for complete denture • Partial denture framework • Custom trays for fluoride or bleaching

20. Composition/setting rxn/working time

21. Setting rxn/working time • Setting rxnoccurs when the powder is mixed with water: Ca sulfate dihydrate + NaAlginate Ca alginate • Working time:total time from start of mixing to the final time at which an impression tray can be fully seated without distortion • Regular set: 2-3 minutes • Fast-set: 1.25-2 minutes • Setting time:elapsed time from the start of mixing until impression material becomes firm enough to resist permanent deformation. Regular set: 2-5 minutes, Fast set: 1-2 minutes

22. Setting rxn/working time • Controlling water temperature shortens or lengthens setting time • Changing P:L has adverse effect on materials property and strength • Allow extra 1-2 minutes, tear strength increases, and rebound from undercuts w/out deformation improves

23. Important considerations to ensure accurate impression • 2-4 mm bulk material in tray • Snap action removal from mouth • Allow extra 1-2 minutes after setting to improve tear strength • Stored in a moist environment to avoid loss of water and deformation • Disinfect in less than 10 minutes to avoid dimensional instability

24. Impression making • Fluff powder, wait 30 seconds for the dust to settle • Measure powder and water according to manufacturer instruction • Use spatula to stir water into powder to mix • Stir vigorously to create homogenous mix (45 seconds for regular, 30 seconds for fast setting alginate)

25. Tray should cover all anatomical features • Utility wax can be added on tray borders to extend its coverage • Adhesive spray is used to prevent separation of alginate from tray. • Tray is loaded from posterior towards anterior • Moisten and smooth alginate • Posterior part of tray is seated first then anterior. • *tips to control gag reflex*

26. Alginate impression

27. Elastomers

28. Elastomers • Often called rubber materials since they have properties similar to rubber. • Clinical uses: • Bridges • Implants • Partial dentures complete dentures • Indirect esthetic restorations

29. Polymerization reaction • Involves formation of long-chain polymers and cross-linking of chains • General properties: • Not as sensitive to water as hydrocolloids • Have a certain amount of rebound, the highest is for addition silicon and polyethers, medium for condensation silicon and poorest for Polysulfides • Not wet well by water (hydrophobic) possible solution: surfactants

30. PolysulphidesDispensing & Composition • Supplied in two tubes as base and catalyst, equal lengths are mixed. Light, regular, heavy viscosities • Chemical composition of base: • 80% low-molecular-weight organic polymer containing mercaptan reactive groups (-SH), and 20% reinforcing agents: titanium dioxide, silica, zinc sulfide • Chemical composition of catalyst: • Lead dioxide or copper hydroxide

31. Setting reaction Mercaptan + lead dioxide polysulfide+H2O • The reaction is sensitive to temperature and moisture so increase in any will accelerate the setting. • Also sensitive to correct mixing ratio

32. Uses • Crown and bridge impressions • Partial and complete denture impressions Clinical considerations when used • Used with custom trays • Allow 2mm thickness • Use tray adhesive • Dry field

33. Properties of clinical interest • Setting time: 8-14 minutes • Higher tear strength than hydrocolloids • Accuracy improves if impression is poured within 30 minutes • very unpleasant taste and odor • Messy (orange solvent to remove stains) • Can be irritant to oral mucosa For all these reasons, Polysulfides have been largely replaced by other rubber materials

34. Silicon rubber Addition (A) silicon Condensation (C) silicon

35. Condensation silicon • Developed as alternative to Polysulfides • Has more desirable qualities in comparison: • Easy mix • Better taste and odorless • Shorter setting time (5-7 minutes)

36. Dispensing & Composition • Two pastes, base and catalyst. • Comes as light, medium, or heavy viscosity • Base: dimethylsiloxane + filler (silica) • Catalyst: suspension of stannous octoate + alkyl silicate

37. Setting rxn • Condensation reaction that produces ethyl alcohol as by product. The alcohol evaporates which causes dimensional instability. • The material continues to contract with time, so needs to be poured within minutes.

38. Addition silicon (PVS) • Desirable clinical qualities: • Dimensional stability • Accuracy • Clean • Easy to mix • No foul odor or taste the most expensive

39. Dispensing & composition • Light, regular and heavy viscosities and also putty • Dispensed as cartridge with 2 chambers (pastes), or two putty- system • Base: low-molecular-weight silicon with vinyl groups (paste system), or low-molecular-weight silicon with silane hydrogens (putty) + silica filler • Catalyst: chloroplatinic acid

40. Setting rxn • Polymerization rxn of chain lengthening and cross-linking with reactive vinyl groups, producing a stable silicon rubber. • No ethyl-alcohol by product • Some addition silicons produce hydrogen as by product, manufacturers incorporated palladium powder that absorbs hydrogen • setting time: 3-7 minutes

41. Impression making techniques • Putty/wash technique • One step • Two step

42. Polyethers • Also used for crown and bridge work since they are very accurate and also more hydrophilic than other silicons

43. Dispensing & composition • Dispensing same as other rubber materials (2 tubes) • In addition it’s supplied in pouches of base and catalyst placed in mechanical mixer. • Composition: • Base: low- molecular-weight polyether with cation reactive group • Catalyst: aromatic sulfonic acid *clinical tip: mix well to avoid irritation from unmixed catalyst.

44. Properties • Stiff, difficult to remove from undercuts • Short working and setting times • Setting time 3-5 minutes • Sensitive to moisture and temperature • More hydrophilic (must not be stored in water or disinfectant) • accurate

45. Inelastic impression materials

46. Impression compound: • Impression compound: softens with heat, hardens in the mouth. • 2 main forms: • Cakes (sheets) /low fusing • Sticks /high fusing • Clinical uses: • Sheets: • Primary impression in metal trays • To make custom trays • Sticks are used for border molding

47. Composition & properties • Composition • Thermoplastic resin and waxes • Fillers to reduce flow • Plasticizers • Organic acids or oils • Pigments • Properties • Softened at 60°C, remains plastic at 45°C, firm at 37°C • Heated in water not by flame • Should be poured ASAP to avoid distortion • Flame used to heat sticks

48. Impression plaster • Impression plaster: seldom used • Composed of plaster gypsum (Ca hemihydrae) • Used for primary impression (high P:L) • Scored with a knife in the mouth then removed and reassembled in the lab (distortion?) • Wash impression • Easy to use • Inexpensive

49. ZNO eugenol • Secondary impression for complete dentures, or wash impression (mucostatic impression) • 2 tube paste system, different colors Zinc oxide (80%) and fillers, eugenol (15%) with oils, resin, fillers. In addition to chemical accelerator (zinc acetate) • Dispensed in 2 equal lengths and mixed to homogenous color forming zinc eugenolate • Initial set:3-6 minutes, final set:10 minutes • To accelerate the setting, a drop of water or zinc acetate is added.

50. Properties • Brittle, so not suitable for areas with undercut • Flows readily • Accurate • Eugenol can be irritant (burning sensation) • Once set, dimensionally stable