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
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
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
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
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
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
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
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
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
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