1. Impression materials and procedures for R.P.D

2. Impression materials Rigid materials
1.Impression plaster ( plaster of Paris)
2. Impression compound
3. Zinc oxide- eugenol
4.impression waxes
Elastic
1. Hydrocolloids
- Reversible ( agar)
- Irreversible ( alginate)
2. Elastomers
- polysulfide
- silicone
- polyetherr

3. Impression Materials Plaster Compound Non-elastic Waxes ZnO - Eugenol
Agar (reversible)
Aqueous Hydrocolloids
Alginate (irreversible)
Elastic
Polysulfide
Silicones
Polyether
Non-aqueous Elastomers

4. Plaster of Paris ( IMPRESSION PLASTER)
Impression plaster, used for taking final impression for completely edentulous patient.

5. Zinc Oxide-eugenol

6. Zinc oxide- eugenol impression paste
Supplied in two tubes , equal lengths of the two pastes are mixed with a spatula
Indications
1-secondary impression material for complete dentures.
2-impression for extension base edentulous ridge areas of R.P.D.
3-impression material for relining distal extension denture bases.

7. Impression Compound

8. Indication 1-Making primary impression for edentulous ridge
Indication 1-Making primary impression for edentulous ridge. 2-Border molding of special trays for Kennedy Class I and II removable partial denture bases. Two basic forms of impression compound: cake form and stick form

9. impression waxes
The mouth temperature waxes used in recording the edentulous ridge also used to correct the borders of the impression made of more rigid material. Also used to correct small imperfection (e.g. airblows) in other impressions, especially zinc oxide impressions

10. Elastic materials
Elastic materials are the only materials that can be withdrawn from tooth and tissue undercuts without permanent deformation and are therefore most generally used for making impressions for R.P.D , fixed P.D when tooth and tissue undercuts and surface details must be recorded with accuracy.

11. Irreversible (Alginate)
Hydrocolloids
Reversible (Agars)
Irreversible (Alginate)

12. Properties of hydrocolloids
They exhibit poor dimensional stability caused by:
Syneresis : formation of an exudates on the surface of the gel.
Imbibition : absorption of water, impression swells as water is absorbed by osmosis.
Therefore hydrocolloid materials must be poured immediately .

13. Agar-Agar
They are fluid at high temperature and gel upon reduction in temperature.
Reversible (agar-agar) hydrocolloids, are used primarily as impression materials for fixed restorations.
Agar is accurate but has poor dimensional stability and as water bath is required , it has been largely replaced by elastomer impression material.

14. Irreversible Hydrocolloid (Alginate)

15. Irreversible Hydrocolloid (Alginate)
Most widely used impression material
Indications
diagnostic casts , orthodontic treatment casts, and master casts for R.P.D .

16. Irreversible hydrocolloids ( alginate)
Advantages: 1-hydrophilic, used in the presence of moisture (saliva). 2-have pleasant taste and odor. 3-nontoxic, 4- nonstaining, and 5-inexpensive. Disadvantages 1.These materials have low tear strength 2. provide less surface details than other materials. 3. they are Dimensionally unstable ,therefore they must be poured immediately.

17. Important precautions to be observed in the handling of hydrocollo id impressions:
impression should not be exposed to air because some dehydration will occur and result in shrinkage.
impression should not be immersed in water or disinfectants because some imbibition will result which lead to expansion.
impression should be protected from dehydration by wrapping it in a damp gauze until pouring the impression with stone.
pouring the impression immediately to decrease the effect of imbibition and evaporation.

18. Possible causes of inaccurate or weak cast of the dental arch
1.Distortion of hydrocolloid impression by:
a. using impression tray that is not rigid
b. partial dislodgement from the tray.
c. shrinkage caused by dehydration
d. expansion caused by imbibition
e. pouring the impression with stone that has begun to set.
2. water / powder ratio that is too high causing weak cast.
3.improper mixing
4.trapping of air (porosity) caused by insufficient vibration.
5.chalky cast surface result from retarding action of hydrocolloids or absorption of water by the dehydrating hydrocolloids.
6.premature separation of cast from impression.
7.failure to separate cast from impression for an extended period.

19. Elastomers Properties: 1-accuracy Indication: crown and bridge work
2-good tear resistance
3- dimensional stability

20. Polysulfide
Indications 1-R.P.D Impression especially for secondary altered cast impressions. 2- Cobalt-chromium dental impression 3- Crown and bridge impressions 4- Secondary impression for edentulous ridge.

21. Polysulfide
Properties: accurate , high tear strength, unpleasant odor.
Pour within 1 hour

22. Silicone impression material
The silicone impression materials are classified according to the type of chemical reaction by which they set.
1.Addition silicone
2.Condensation silicone

23. Addition Silicones Indications Advantages
cobalt-chromium denture impressions.
Crown and bridge impressions
Implant impressions
Master edentulous impression
Advantages
1-Highly accurate
2-High dimensional stability
3-pour up to one week

24. Condensation Silicone
Indications
crown and bridge work partial dentures, implants and overdentures.
Used in stock trays or special trays. Although dimensionally stable the impression should be poured within 24 hours. Supplied as a paste and liquid or two pastes, in light, medium, heavy or very heavy bodied (putty).

25. Polyether
Supplied in two pastes catalyst and base Properties 1-short working and setting time 2-good wettability 3-accurate 4-low tear strength

26. Impression procedures
Tooth-borne partial dentures 1.Single stage anatomic impression –irreversible hydrocolloid Tooth-Mucosa Borne Partial Dentures 2.DUAL STAGE SELECTIVE PRESSURE IMPRESSION (ALTERED CAST IMPRESSION) .

27. Single stage impression –irreversible hydrocolloid
The step- by step procedure for making a hydrocolloid impression is the following: 1. select a suitable perforated stock tray which allows for adequate thickness of impression material. 2. Perforated stock trays usually demonstrate greater accuracy due to the increased mechanical retention of the irreversible hydrocolloid.

28. 3. Stock trays may be modified through the addition of wax to the periphery or palatal vault.
4-. Mix the irreversible hydrocolloid impression material following the manufacturer's instructions and load the tray.
5. Place the impression material into critical areas manually or with an impression syringe (e.g. rest seats, palatal vault and areas of framework). The surface of the impression material may be smoothed with a finger moistened with water to minimize surface voids.

29. 6. Seat the tray in the mouth, being careful to center it accurately.
7. For mandibular impressions, the patient should be instructed to lightly place the tip of the tongue against the palate.
8. To achieve optimum strength , hold the tray in position for several minutes after the impression material reaches its initial set.
9. Retract the cheek to break the peripheral seal and remove the set impression with a quick "snap.“
10. Inspect the impression. water rinse, may be used to remove the saliva.
11. Pour the Impression immediately. using an improved dental stone .
12. If the impression cannot be poured immediately, it must be stored in an environment of 100% humidity.

30. DUAL STAGE SELECTIVE PRESSURE IMPRESSION (ALTERED CAST IMPRESSION) .
Impression procedures for tooth-mucosa borne partial dentures should be designed to record teeth and mucosal tissues in a manner that maximizes support. The purpose is to accurately record the anatomic form of the teeth and the form of the soft supporting tissues under selective pressures. Selective pressure refers to the recording of the tissues with varying degrees of displacement. This requires an altered or modified cast impression.

31. ALTERED CAST IMPRESSION
This is accomplished by attaching a small custom tray over the meshwork of the edentulous ridge, and then making a new impression of the residual ridge. The altered cast method of impression is commonly used for the mandibular distal extension partial edentulous arch(Kennedy class I and II arch forms) .

32. The steps in this procedure are as follows:
1. The metal framework is constructed on a cast produced by anatomic impression procedure using alginate impression material.
2. Fabricate custom trays on the framework over the muco-osseous denture supporting areas. Be certain that the primary supporting areas are covered (e.g. buccal shelf)

33. 3-Cut the cast in correspondence with the internal finish line of the framework or slightly closer to the abutment teeth .9
4-Seat the framework with the impression onto the sectioned cast

34. 5-Box the impression, and pour the altered cast.

35. Custom Tray Fabrication
Technique for making individual (special) acrylic resin impression tray
Custom Tray Fabrication
Block-out Soft, Hard Tissue Undercut Areas.

36. Fabrication of Custom Trays
Apply Spacer for Impression Material.
Elastomeric Material: 2-4 mm
Alginate: minimum 3 mm

37. Tray Resin Manipulation
Rubber Mixing Bowls.
Less Material Used.
Dough Stage Does Not Stick to Bowl.

38. Custom Tray Fabrication
Carefully remove all wax from the tissue surface of the tray with hot water. The impression materials we use will not stick to the wax and there is risk of the impression material separating from the tray distorting the impression.

39. Alginate with Custom Tray

40. Silicone and Rubber Base Impressions

41. Box and Pour Master Cast
Carefully remove salivary residues and dry the cast. Polysulfide casts need to be poured within 30 minutes.

42. Separate the Casts

43. Summary
Alginate most widely used inexpensive displaces moisture lower detail reproduction dimensionally unstable Addition silicones most popular accurate dimensionally stable user friendly expensive

44. Thank you