Impression materials general considerations for undergrads

1. Impression materials, introduction,
classification and requirements
ASST PROFESSOR
Dr Mumtaz ul Islam
B.Sc. BDS. MHR. M.Phil.

2. Impression
 Negative likeness or copy in reverse of the surface or object
 Dental impression
 An imprint of the teeth and adjacent structures for use in dentistry
 Impression material
 Any substance or combination of substances used for making an impression
 Impression technique
 A method and manner used in making a impression

3. Why impression
 Procedures can not be performed in oral cavity (mouth)
 Model
 A positive replica of any object
 Dental model
 Accuracy of an appliance depends on model
 Accuracy of a model depends on its impression

4. How to take dental impression
 Impression material transferred to patients mouth
 Material is soft initially it is loaded (carried) in to a tray
 Impression tray holds the soft material in position
 After setting of the material impression tray removed from the mouth
 Model prepared by dental plaster or stone

5. Classifications
 Chemical type
 Alginates, silicones, polysulphides, etc
 Properties of the materials either before or after setting
 Before setting property involved is viscosity
 Tissue compression or displacement (mucocompressive)
 Mucostatic initially very fluid

6. Classifications cont,
 According to set material
 Rigidity & elasticity
 Depends mainly on recording capability of an undercut
 Any surface which restricts the insertion or removal of a material
 Flexible to withdraw elastic in recovery
 Elastic and non elastic

7. Classification according to chemical type and
elastic properties

8. Stress
 Plays important role in viscosity
 Increase in stress produces fluidity in material (pseudoplastic)
 Space also puts more pressure on the tissues and displace it
 A close fitting impression tray

9. Classification according to viscosity

10. Requirements
 Factors which effect the
 Accuracy
 Dimensional stability
 Manipulative variables
 Additional factors
 Cost taste colour etc

11. Accuracy
 Low viscosity
 Interaction with saliva and other liquids
 Hydrophobic
 Hydrophillic
 Blow holes
 Wetting

12. Contact angel and setting reaction
 Angel of contact of an aqueous solution with any surface
 Low the angel more will be its wettability
 Resulting model lacks fine details
 Blow holes
 During setting contraction occurs
 contraction towards tray
 Results an expansion of impression and more space
 overs sized die loose fitting crown

13. Over sized die

14. Setting expansion
 During setting expansion occours
 Expansion away from tray (material expands)
 Results an contraction of impression and less space
 Under sized die
 Fitting will be difficult and impossible

15. Temperature
 Temperature of the mouth is 32° - 37° C
 Room temperature is 23° C
 Thermal contraction occurs
 Depending on the coefficient of thermal expansion (CTE)
 Impression material and tray both shows different directions and values
 Kept minimized through materials of same or less difference in CTE

16. Contraction of tray and impression

17. Adhesion of impression material with tray
 Good adhesion is necessary
 Firm attachment if not present detachment occurs
 Only a partial detachment from the tray causes gross defect
 Defective impression ultimately gives a defective cast or die
 Resultant restoration or appliance will be ill or non fitting
 Tray adhesive where applicable is necessary otherwise perforated trays used

18. Tray adhesive

19. Perforated trays for complete dentures

20. Perforated trays for partial dentures

21. What is this assignment

22. Recording of undercut area
 That area which restricts any object from withdrawal or insertion
 To record that area material must have
 Good elasticity and tear resistance and
 low rigidity which facilitates easy recovery
 Compressive and tensile stresses are also produced during removal
 Different materials shows different results at recovery

23. Removal from undercut area

24. Recovery with different materials

25. Distortion
 Degree of undercut
 Thickness of impression material
 Time elapsed in compressed state
 Viscoelastic properties of impression material

26. Ideal impression material for undercut
 Elastic enough
 For viscoelastic material distortion can be minimized by early recovery
 Early recovery should be soon after setting
 Material should show a good tear strength to compensate tensile stress
 Should not tear even if in the form of thin sections

27. Impression of an undercut area

28. Dimension stability
 Accuracy of impression material
 During insertion into the patient’s mouth, setting, and withdrawal
 After taking the impression delay in model or cast making
 Now accuracy depends upon its dimension stability
 Impression should retain its original shape indefinitely

29. Factors affecting dimensional stability
 Continuation of setting reaction beyond setting time
 This happens in viscoelastic and elastic materials
 Allow sufficient time for full recovery
 Delay before pouring the gypsum cast
 Internal stresses
 Thermo plastic Impression materials like compound and waxes
 Material tends to relieve internal stresses
 Distortion occours

30. Factors affecting dimensional stability
contd,
 Effects of volatile substances
 Materials may contain volatile substances as a main ingredient
 After setting it produces a volatile substance
 These easily evaporates and distort the impression
 Mainly distortion resulted by shrinkage
 For most of the materials except elastic and viscoelastics
 gypsum cast should be poured immediately

31. Manipulative variables
 Dispensation
 Mixing
 Mixing through proportioning Streak free mix
 Contrast color material mixed on paper pads or glass slab
 Powder liquid and paste liquid systems are difficult
 Shows no setting at certain areas due to incomplete mixing
 Setting starts as the material started to mix retarders delay setting

32. Manipulative variables Contd,
 Non mixing
 Non mixing are thermoset material
 Require heat for softening and re-hardens above mouth temperature
 No induction time period immediately use
 Working time
 Start of mixing till material becomes not manageable to use at room temperature
 Setting time
 After mixing to initial setting

33. Manipulative variables Contd,
 Setting often goes beyond its prescribed time
 Wiser approach is to leave impression in the mouth for some extra time
 Especially for elastic materials
 Convenient material for patient and operator shows
 Long working time
 Short setting time
 Chemically setting impression materials are convenient

34. Viscosity and time

35. Clinical considerations
 Analysis of the problem, consideration of requirements, available material and choice
 Undercut area present elastomeric material
 Rigid materials can be used at soft tissue undercut areas
 No undercut area rigid material
 Impression plaster is exception it can be used for undercut area
 Due to its reassembling after fracture property
 Both materials are available with accuracy for both conditions
 Selection depends on cast and accuracy more accuracy increase the cast

36. Impression trays
 Impression tray give rigid support to the impression material
 Facilitate introduction into the mouth
 Custom made
 Made on casts obtained from patients impression with low accuracy material in stock tray
 Rigid enough with a uniform space well extended to record the impression
 Stock
 Metallic and plastic
 Available in variety of shapes to provide best fit
 Under or over extended which can be corrected by addition of thermoplastic material or cutting

37. Impression trays Contd,
 Perforated, non perforated and rimlock, rimlock trays may include in both
 Perforated by means of holes, wires interlocks at the base
 Non perforated through contact adhesives needs drying before use
 Trays with water cooling system for reversible hydrocollides
 Stock trays can also be used
 Plastic trays when used with high viscosity material may be problematic

38. Tissue management
 A complete impression not only contain the teeth but the soft tissue around it
 For removable appliances achieved by careful impression or syringing
 Produces a complete impression which is impossible on the other hand
 Fixed appliances requires fine details of gingival margins or beneath
 Impression under the gingival margin area requires gingival displacement
 Three means are available for displacement

39. Troughing
 Gingival displacement through any mean
 Packing a retraction card
 Electrosurgical cauterization
 Soft tissue Laser

40. Retraction cord
 Suture material, braided, knitted cords custom made thin copper filament
 Wide range of diameter impregnated with hemostatic agents
 Epinephrine, aluminum and ferric chloride etc.
 Epinephrine containing cords should be used with great care contraindicated for cardiac patients
 Acidic hemostatic agents can cause dissolution of enamel induce sensitivity

41. Packing of retraction cord
 Main object is to displace the gingival tissue laterally not apically
 Flat thin plastic instrument or a custom designed cord packer
 Cord must be packed laterally

42. The controversy
 How many cords should be used single or multiple
 At base hemostatic cord and above expanding cord is used
 Outer cord is removed the fine one left in crevice
 Not more than 20 minutes a cord should be packed
 On removal dry cord should not be removed
 Cords may induce gingival recession

43. Electro cauterization and soft laser
 Widen the gingival crevice also produce hemostasis
 Careful technique is essential for both
 If properly performed no recession occurs
 For cauterization a fine single wire ensures less damage
 Marginal deficiencies and overgrowths can be corrected

44. Technique for impression
 Variable for each material
 Moisture control and cleanliness is essential
 Salivary control usually attained through cotton rolls and absorbent pads (dry guards)
 Rubber dam can also be used for isolation of teeth
 Contra indicated with addition silicones
 Plasticizer reacts with platinum catalyst and acts as retarder

45. Placement of material
 Material should be placed on occlusal surface of teeth if present
 Smeared or syringed around teeth and loaded in the tray
 This will prevent air entrapment on occlusal surface of teeth
 Entrapped air produces bubbles on the occlusal surface of the tooth
 Cast will give occlusal inaccuracy
 This problem exists in teeth having steep cuspal angel and tortuous fissures

46. Consistency of material
 Material inserted in the tray and mouth in a flow able state
 Retained till setting
 Tray should be kept still as possible
 Prevent distortion of the impression
 Manufacturers recommendations should be followed
 Elastomeric materials if removed before setting results in distorted impression

47. Points to remember
 Material in close contact with lips, tongue and mucosa sets rapidly
 Recovery of impression must be delayed to avoid differential setting
 A thorough examination of teeth and impression is necessary
 Adhesion of material with tray must be checked especially in the molar area
 Extended material beyond the distal ends of tray should be trimmed

48. Cross infection control
 Disinfection is a mandatory protocol for all procedures
 Impression sent to lab or received from lab must be disinfected
 Impression must be washed with running water to remove debris blood and saliva
 Immersion in 1:5 or 1:10 house hold bleach solution for 10 minutes
 Ratios of 5 and 10 depends upon the concentration of original solution
 Inactivate HIV and HBV without any distortion

49. Disinfectants
 Aerosol disinfectants not reliable
 Gluteraldehydes and sodium dichloroisocyanates toxic and needs to replace daily
 Sodiumperoxymonosulphate 2% for 10 minutes except in reversible hydrocolloides
 Impressions of patients with contagious diseases like AIDS, HCV and HBV
 Needs sterilization rather disinfection
 Immersion in 2% gluteraldehyde solution for 10 hours or in phenolic buffer 6.75 hours
 Silicone rubbers are the only impression material kept immersed

50. Guidelines
 For patients with contagious diseases
 Multiple models should be prepared
 Especially where multiple insertions of appliance required
 Where applicable autoclave sterilization is the safest method
 Gypsum casts can not be sterilized so multiple casts should be prepared
 Updated guidelines on cross infection control must be followed