Impression Materials BDS 3

1. Impression MaterialsBDS 3 Dr M Slabbert Dept Prosthodontics Wits

2. First Impressions count

4. Classification of Impressions • Preliminary impressions • Taken either by the dentist or an expanded-function dental assistant. • Used tomake a reproduction of the teeth and surrounding tissues. • Used to make (1) diagnostic models, (2) custom trays, (3) provisional coverage, (4) orthodontic appliances, and(5) pretreatment and post‑treatment records.

5. Classification of Impressions- cont’d • Final impressions • Taken by the dentist. • Used tomake the most accurate reproduction of the teeth and surrounding tissues. • Used to make indirect restorations, partial or full dentures, and implants.

6. Classification of Impressions- cont’d • Bite registrations • Taken by the dentist or dental assistant. • Make a reproduction of the occlusal relationship between the maxillary and mandibular teeth. • Provide an accurate registration of the patient’s centric relationship between the maxillary and mandibular arches.

7. Study model (cast) Oral Examination Primary Impression • Treatment Planning • duplicate model Final Impression Mouth and tooth preparation Master model Denture • Laboratory procedure • investment cast • refractory cast Delivery

8. Definition An impression is a negative record of the tissues of the oral cavity which constitutes the basal seat of the denture. An impression is made in a material which has plasticity and which hardens or sets while in contact with the tissue. Prof Owen Fundamentals of Removable partial dentures 2nd Ed pg. 124 -131 Wits Pros Book Vol 2 Pg. 14 Study pages 14- 31 Vol 2 www.health.wits.ac.za/Prosthodontics Primary impressions in alginate…

9. The Ideal Impression Material • Easy to mix and handle. • Suitable working time. • Suitable setting time. • Compatible with die and stone • Not toxic or allergenic to the patient. • Dimensionally stable on setting. • Accurate to record the fine details of the prepared tooth • Has acceptable odor and taste. • Adequate strength. • Adequate shelf life.

10. The Ideal Impression Material cont. • Economical • Ready to disinfected without loss of accuracy. • Fluid or plastic when inserted into the mouth. • It must be an exact record of all the aspects of the prepared tooth and sufficient unprepared tooth structure immediately adjacent to margins, to allow the dentist and the technician to be certain of the location and configuration of the finish line. • Other teeth and tissue surrounding the abutment tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration. • It must be free from air bubbles especially in the finish line area.

11. High accuracy Biocompatibility • (very small contraction <0.5%) • High dimensional stability Compatibility to stone • High elastic recovery High tear strength • Ease of use Long shelf life • Hydrophilic Pleasant color &taste and Hydrophobic • Proper setting time Cost

12. Elastic recovery The amount of rebound after a cylinder of material is strained 10% for 30 seconds. 98%

13. Hydrophilic VS Hydrophobic

14. Which one of the impression materials we choose? • What’s the job you do? • primary impression • final impression • How accurate do you want? • removable denture • fixed prostheses • What technique do you use?

15. a. Accuracy = ability to replicate the intraoral surface details. b. Dimensional Stability = ability to retain its absolute dimensional size over time. c. Tear Resistance = ability to resist tearing in thin sections (such as through the feather-edged material within the gingival sulcus. Impression Tray Impression Accuracy Dimensional Stability Tear Resistance IMPRESSION MATERIALS Key Properties

16. Prepped Tooth Width 8 mm (=8,000 m) PROBLEM ANALYSIS What are tolerable limits for “error” in indirect procedures? a. Impressions = b. Casts, Dies = c. Waxing = d. Investing = e. Casting = f. Finishing, Polishing = g. Cementation = +/- 0 +/- 0 +/- 0 + 1.5%  1.5% +/- 0 +/- 0 8,000 m x 0.5% = 40 m = 20 m/side Typical clinical error = >100 m/side

17. CLASSIFICATION SYSTEM Based on Properties of Set Materials • Properties:Reaction:Set: • Rigid: • 1. Impression Plaster Rigid Irrev (Chem) • 2. Impression Compound Rigid Rev (Phys) • Zinc Oxide/ Eugenol Rigid Irrev (Chem) Water-Based Gel: 4. Alginate (Irreversible Hydrocolloid) Flexible Irrev (Chem) 5. Agar-Agar (Reversible Hydrocolloid) Flexible Rev (Phys) Elastomers: 6. Polysulfide (Rubber Base, Thiokol) Flexible Irrev (Chem) 7. Silicone (Conventional, Condensation) Flexible Irrev (Chem) 8. Polyether Flexible Irrev (Chem) 9. Polyvinyl Siloxane (Addition Silicone) Flexible Irrev (Chem)

18. Impression materials Plaster Nonelastic ImpressionCompound Zinc oxide Eugenol Impressionwaxes Impression material Agar Reversible Polysulfides Hydrocolloids Alginate Irreversible Elastic Polyethers Condensation silicone Non-aqueous elastomers Addition silicone

19. Type I Nonelastic Gypsum (Plaster) ImpressionCompound Zinc oxide eugenol Impression wax

20. Dental Plaster • Dental Plaster • • Type I – impression plaster • • Type II – model (laboratory) plaster (used for mounting casts) • plaster is composed of the β form of calcium sulfate hemihydrate Crystals • plaster is weaker than dental stone due to: • 1.) porosity of the particles, requiring more water for a plaster mix • 2.) irregular shapes of particles prevent them from fitting together tightly • Dental Stone • • Type III - dental stone • (diagnostic casts) • • Type IV - high strength • dental stone (working • models) • • Type V - high-strength, high-expansion dental stone • Stone is the αhemihydrate form Types of Gypsum Products

22. Calcination • Calcination Hor other means • Mineral gypsum ---------------- > Model plaster + Water (CaSO4 . 2H2O) Dental stone High-strength dental stone (CaSO4 . 1/2H2O) Reverse Reaction When calcium sulphate hemihydrate (dental plaster, stone, etc.) is mixed with water, the reverse reaction takes place, and the hemihydrate is converted back to the dihydrate: CaSO4 . 1/2H2O + 11/2H2O ---> CaSO4 . 2H2O + 3900 cal/g mol

25. Elastomeric Impression Materials A material that is used when an extremely accurate impression is essential. The term elastomeric means having elastic or rubberlike qualities.

26. Elastic Agar Hydrocolloids Alginate

27. Hydrocolloids Reversible and Irreversible • Introduced by Sears 1939 • First elastic • Sears AW. Hydrocolloid impression technique for inlays and fixed bridges. Dent Digest 1937; 43: 230-234. • Lin C, Zeiber G J. Accuracy of impression materials for complete arch fixed partial dentures. J Prosthet Dent 1988; 59: 288-291 • Philips Science of Dental Materials 11th Ed. Part 2 Pg. 231

28. Irreversible Hydrocolloid • Material that cannot return to a solution state after it becomes a gel. • Alginate is the irreversible hydrocolloid most widely used for preliminary impressions.

29. Makeup of Alginate • Potassium alginate (Alginic Acid) (12-15%) • Comes from seaweed; is also used in foods such as ice cream as a thickening agent. • Calcium sulfate (8-12%) • Reacts with the potassium alginate to form the gel. • Trisodium phosphate • Added to slow down the reaction time for mixing.

30. Makeup of Alginate- cont’d • Diatomaceous earth (70%) • A filler that adds bulk to the material. • Controls the stiffness of the set gel • Zinc oxide • Adds bulk to the material. • Potassium titanium fluoride (~10%) • Added so as not to interfere with the setting and surface strength. • Sodium Phosphate (retarder) (2%) • Coloring and flavouring agents (traces)

31. Physical Phases of Alginate • The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semiliquid form. (sol: resembles a solution, but is made up of colloidal particles dispersed in a liquid) • The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert. “gel” entangled framework of solid colloidal particles in which liquid is trapped in the interstices and held by capillary forces (Jello)

32. Packaging and Storing of Alginate • Containers about the size of a coffee can are the most commonly used form of packaging. • Premeasured packagesare more expensive, but save time by eliminating the need for measurement of the powder. • Shelf life of alginate is approximately 1 year.

33. Fig. 46-7 Examples of packaging for alginate.

34. Causes for Distortion and Dimensional Change of Alginate • If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition. • If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

35. ADA Specifications • <3% deformation with a 10% strain

36. Altering the Setting Times of Alginate • Cooler water can increase the setting time if additional time is needed for the procedure. • Warmer water can reduce or shorten the setting time of the procedure.

37. Water-to-Powder Ratio • An adult mandibular impression generally requires two scoops of powder and two measures of water. • An adult maxillary impression generally requires three scoops of powder and three measures of water.

38. Fig. 46-8 Scoop and water measure for alginate.

39. Taking an Alginate Impression • Explain the procedure to the patient: • The material will feel cold, there is no unpleasant taste, and the material will set quickly. • Breathe deeply through your nose to help you relax and be more comfortable. • Use hand signals to communicate any discomfort. • Mouth Preparation • Rinse and dry the patient's teeth • If teeth are too dry, alginate will stick

40. An Acceptable Alginate Impression • The impression tray is centered over the central and lateral incisors. • There is a complete "peripheral roll," which includes all of the vestibular areas. • The tray is not "overseated," which would result in exposure of areas of the impression tray. • The impression is free from tears or voids. • There is sharp anatomic detail of all teeth and soft tissues. • The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression. • The hard palate and tuberosities are recorded in the maxillary impression.

42. Trouble Shooting • Inadequate working or setting time: • temperature of the water, incomplete spatulation • W/P too low • improper storage of alginate powder • Distortion: • Tray movement during gelation or removed from mouth prematurely • weight of tray compressing or distorting alginate • impression not poured up immediately • Tearing: • removing impression from mouth before adequately set • thin mixes (high W/P ratio) • presence of undercuts (blocking out these areas before an impression may help) • inadequate amount of impression material in tray (avoided by minimum 3 mm of impression material between tray and oral tissues) • • Loss of detail: • removed from mouth prematurely

43. Consistency: • preset mix is too thin or thick • The W/P ratio is incorrect (avoid by fluffing powder before measuring; do not overfill powder dispenser) • inadequate mixing (avoided by vigorous spatulation and mixing for recommended time) • using hot water: grainy and prematurely thick mix • • Dimensional change: • delay in pouring • alginate impression stored in air: results in distorted, undersized cast due to alginate impressions losing water when stored in air • Porosity: • whipping air into the mix during spatulation (proper mixing: after initial • wetting of powder by the water, mix alginate so as to squeeze the material between the spatula blade and the side of the rubber bowl) • • Poor stone surface (of cast) • set gypsum remaining in contact with the alginate for too long a period of time

44. Reversible Hydrocolloid • An impression material that changes its physical state from a sol to a gel and then back to a sol.

45. Chemical Makeup of Reversible Hydrocolloid • 85% water • 13% agar • Agar is an organic substance derived from seaweed. • Additional chemical modifiers are added to aid in the handling characteristics.

46. Conditioning Bath for Reversible Hydrocolloid • Three Compartments • The first bath is for liquefying the semisolid material. A special water bath called a “hydrocolloid conditioner” at 212° F liquefies the material. After liquefying, the preset thermostat cools the temperature to 150° F automatically. • The second bath becomes a storage bath that cools the material, readying it for the impression. At this temperature, the tubes are waiting for use. • A third bath is kept at 110° F/44° C for tempering the material after it has been placed in the tray.

47. Types of Reversible Hydrocolloid • Tray material • Packaged in plastic tubes. Each tube has enough material to fill a full arch, water-cooled tray. • Syringe material • Packaged in plastic or glass cartridges that fit a syringe or in preloaded syringe or preformed sticks that refill special hydrocolloid inlay syringes.

48. Application of Reversible Hydrocolloid Impression Material • A stock water-cooled tray is selected, making sure that the tray does not impinge on any of the teeth or soft tissue. • Plastic stops are placed in the tray. • Tubing is connected to the tray and to the water outlet for drainage. • The material is liquefied and moved to the storage bath. • The light-bodied material is placed in the syringe, and heavy-bodied material is placed in the tray. • The light-bodied material is expressed around the prepared tooth, and the dentist seats the tray.